Contrib Mineral Petrol (1998) 131: 323-346 © Springer-Verlag 1998

Glenn A. Gaetani . Timothy L. Grove

The influence of water on melting of mantle peridotite

Received: 7 April 1997 / Accepted: 9 January 1998

Abstract This experimental study examines the effects ofvariable concentrations of dissolved H20 on the com-positions of silicate melts and their coexisting mineralassemblage of olivine + orthopyroxene ± clinopyrox-ene ± spinel ± garnet. Experiments were performed atpressures of 1.2 to 2.0 GPa and temperatures of 1100to1345 °C, with up to ~12 wt% H20 dissolved in theliquid. The effects of increasing the concentration ofdissolved H20 on the major element compositions ofmelts in equilibrium with a spinel lherzolite mineral as-semblage are to decrease the concentrations of Si02,

FeO, MgO, and CaO. The concentration of Ah03 isunaffected. The lower Si02 contents of the hydrousmelts result from an increase in the activity coefficientfor Si02 with increasing dissolved H20. The lowerconcentrations of FeO and MgO result from the lowertemperatures at which H20-bearing melts coexist withmantle minerals as compared to anhydrous melts. Thesecompositional changes produce an elevated Si02/

(MgO + FeO) ratio in hydrous peridotite partial melts,making them relatively Si02 rich when compared toanhydrous melts on a volatile-free basis. Hydrousperidotite melting reactions are affected primarily by thelowered mantle solidus. Temperature-induced composi-tional variations in coexisting pyroxenes lower the pro-portion of clinopyroxene entering the melt relative toorthopyroxene. Isobaric batch melting calculations in-dicate that fluid-undersaturated peridotite melting ischaracterized by significantly lower melt productivitythan anhydrous peridotite melting, and that the

G.A. Gaetani (1:8])1 . T.L. GroveDepartment of Earth, Atmospheric and Planetary Sciences,Massachusetts Institute of Technology,Cambridge, MA 02139, USA

Present address:'Division of Geological and Planetary Sciences,California Institute of Technology,Pasadena, CA 91125, USAE-mail: gaetani@gps.caltech.edu

Editorial responsibility: J. Hoefs

peridotite melting process in subduction zones isstrongly influenced by the composition of the H20-richcomponent introduced into the mantle wedge from thesubducted slab.

Introduction

Water-bearing partial melts of mantle peridotite havebeen the subject of experimental investigations and sci-entific debate for the past 30 years. Our understandingof the mantle melting process has increased significantlyover that time through the development of various directand indirect experimental approaches for determiningthe compositions of anhydrous peridotite partial melts(Ito and Kennedy 1967; Kushiro 1968; Presnall et al.1979; Stolper 1980; Jaques and Green 1980; Falloonand Green 1987; Kinzler and Grove 1992a,b; Hiroseand Kushiro 1993; Baker and Stolper 1994;Walter andPresnall 1994; Baker et al. 1995;Kinzler 1997). Experi-mental data bearing on hydrous peridotite partial meltsremain sparse. Water plays a central role in peridotitemelting at subduction zones, and accurate experimentaldeterminations of the effects of H20 on the composi-tions and liquidus temperatures of peridotite partialmelts are necessary to advance our understanding of thephysical processes involved in melt generation at con-vergent plate margins.

Much of our understanding of hydrous peridotitemelting comes from the pioneering experimentalstudies of Kushiro and co-workers on simplified ana-log systems such as Mg2Si04-SiOrH20 (Kushiro et al.1968; Kushiro 1969, 1972). The results from thesestudies suggested that hydrous peridotite partialmelting generates SiOrrich melts, and appeared tosupport the hypothesis that subduction-related ande-sites represent primary partial melts of hydrous mantleperidotite (Poldervaart 1955; O'Hara 1965). Subse-quent experimental work carried out on both analog

324

and natural compositions was focused on the origin ofandesite (Kushiro et al. 1972; Nichols and Ringwood1972, 1973; Nichols 1974; Green 1973, 1976; Nehruand Wyllie 1975; Mysen and Boettcher 1975a,b). Someof the H20-bearing glasses produced in these studieswere characterized by high Si02 and low FeO andMgO contents, and were cited as further evidence infavor of the primary andesite hypothesis. These ex-perimental results were controversial, however, and itwas suggested that the glass compositions had beenmodified by the growth of amphibole and pyroxeneduring the quench (e.g., Green 1973, 1976). Recentexperimental studies have produced H20-bearing par-tial melt compositions that are broadly similar tothose produced by anhydrous peridotite partial melt-ing (Kushiro 1990; Hirose and Kawamoto 1995).

Here we present the results from experiments per-formed using natural mineral and rock compositionsthat provide data on the nature of H20-bearing sili-cate melts saturated with an upper mantle peridotitemineral assemblage of olivine (Oliv) + orthopyroxene(Opx) ± clinopyroxene (Cpx) ± spinel (Sp) ± garnet(Gt). Comparisons of hydrous and anhydrous meltcompositions in both simple and natural systems dem-onstrate that the defining characteristic of hydrous par-tial melts of mantle peridotite is not an elevated Si02

concentration but an elevated Si02/(MgO + FeO)ratio. The variation in this ratio with increasing dis-solved H20 produces an apparent increase in the Si02

content of H20-bearing peridotite partial melts whencompared to anhydrous melts on a volatile-free basis.Batch melting calculations show that although H20increases the extent to which peridotite partially melts ata given set of pressure-temperature conditions, meltproductivity at H20-undersaturated conditions issignificantly lower than during anhydrous peridotitemelting.

Table 1 Compositions of starting materials. Units in parenthesesrepresent 1 standard deviation of least units cited on the basis ofreplicate analyses; thus, 47.7(2) should be read as 47.7 ± 0.2. Re-ported composition for PUM is the nominal composition of the

Experimental and analytical methods

Starting materials

Three natural basalts, a synthetic basalt analog, and 3 syntheticperidotite compositions were used as starting materials for thisstudy (Table 1). The basalts were chosen to be close to saturationwith a lherzolite or harzburgite mineral assemblage at upper mantleconditions, and to cover a range of incompatible element concen-trations. Bulk compositions consisting of 60-70 wt% basalt and30-40 wt% peridotite were melted to produce experimental chargesconsisting of silicate melt saturated with a mantle peridotite min-eral assemblage (Oliv + Opx ± Cpx ± Sp ± Gt) (Table 2). Theproportion of melt relative to crystals resulted in large areas (~100-700 11m)of pristine glass in each experimental charge.

The starting material for each experiment was prepared byweighing out the desired proportions of synthetic peridotite andbasalt, then grinding them in an agate mortar and pestle underethanol for 10 min. The bulk H20 content was controlled byadding the basalt as either an anhydrous powder or as a H20-bearing basaltic glass powder and, for some experiments, by addingsome proportion of a H20-bearing synthetic peridotite powder.The consistent partitioning behavior of total Fe as FeO amongminerals and silicate melt in hydrous and anhydrous experimentssuggests that this method of adding H20 to the experiments resultsin oxygen fugacity (f02) conditions comparable to those in anhy-drous experiments performed in graphite crucibles (at or more re-ducing than the C-CO-C02 buffer). Previous high-pressureexperimental studies have bracketed the f02 conditions for experi-ments that initially contained liquid H20 to be between the Ni-NiOand magnetite-hematite oxygen buffers, considerably more oxidiz-ing than is the case for anhydrous experiments (Kushiro 1990;Kawamoto and Hirose 1994).

Natural rock powders were prepared by crushing chips of clean,fresh, aphyric lava in a WC shatterbox. Peridotite FP1 was syn-thesized from mantle minerals separated from a Kilbourne Holespinel lherzolite xenolith (KH-5-4). A second peridotite composi-tion (PUM), corresponding to the primitive upper mantle compo-sition of Hart and Zindler (1986), and a synthetic basalt analogcorresponding to the composition of a small extent anhydrousperidotite partial melt calculated using the method of Kinzler(1997), were prepared from Johnson-Matthey high purity Si02,Ti02, A1203, Cr203, Fe203, MnO, MgO, NiO, and previouslyprepared mixes of CaSi03, Na2Si03, and K2Si409. These com-

synthetic mix. The composition and associated uncertainties for FP1were calculated on the basis of electron microprobe analyses of mi-neral separates from the Kilbourne Hole spinel lherzolite xenolith. (nnumber of electron microprobe analyses included in average)

Kilbourne Hole xenolith minerals

82-72f 85-44 93-26 DM151 PUM FP1 Cpx Opx Oliv Sp

n 25 25 25 25 27 25 21 25Si02 47.7(2) 51.6(2) 50.9(2) 49.9(2) 45.97 44.9(2) 51.8(4) 54.8(4) 40.32(14) 0.06(2)Ti02 0.62(3) 0.59(3) 1.28(4) 1.25(3) 0.18 0.18(1) 0.58(2) 0.12(2) 0.06(1) 0.11 (3)Al203 19.05(9) 15.95(9) 17.7(2) 19.71(9) 4.06 4.27(5) 7.11(13) 4.7(2) 0.02(1) 57 .3(4)Cr203 0.12(4) 0.16(4) 0.06(2) 0.14(4) 0.47 0.57(1) 0.72(2) 0.37(3) 0.08(2) 9.9(2)FeO* 7.82(12) 8.08(12) 8.11(12) 6.80(11) 7.54 7.58(6) 2.70(6) 6.39(7) 9.80(11) 10.3(2)MnO 0.15(5) 0.11(4) 0.15(3) 0.17(4) 0.13 0.11(1) 0.08(3) 0.10(3) 0.12(2) 0.10(3)MgO 10.49(8) 11.30(9) 8.63(10) 9.33(7) 37.79 38.04(10) 15.5(2) 32.8(2) 49.8(2) 22.4(2)CaO 11.75(9) 9.43(11) 8.89(6) 8.75(6) 3.21 4.28(2) 20.41(9) 0.71(2) 0.06(2) 0.02(1)Na20 2.35(12) 2.59(12) 3.36(11) 3.4(2) 0.33 0.38(1) 1.76(5) 0.10(2)K20 0.08(1 ) 0.31 (2) 0.74(2) 0.49(2) 0.03P20S 0.08(2) 0.15(3) 0.31(3) 0.00NiO 0.28 0.18(1 ) 0.32(2) 0.40(2)Total 100.21 100.27 100.13 99.94 99.99 100.49 100.66 100.09 100.58 100.59Mg# 70.5 71.4 65.5 71.0 89.9 89.9 91.1 90.1 90.1 79.5

Table 2 Experimental conditions and phase assemblages. Units in parentheses are 1a uncertainties

Experiment P(GPa)

T(DC)

Duration(h)

Bas.perid''ratio

Run products Phase proportions(wt%)

KDCpx Opx Oliv Sp Gt

Feex-change"

82-72f ExperimentsB333 1.2 0.02

B305 1.2

B304 1.2

B330 1.2

B329 1.2

B326 1.2

B310 1.2

B303 1.2

B292 1.2

B287 1.2

B359 1.6

B277g 1.6

B348 1.6

B366 1.6

B394 1.6

B302 1.6

B365 2.0

B399 2.0

1245

1230

1215

1200

1185

1115

1100

1345

1330

1315

1260

1255

1245

1230

1370

1355

1290

1275

85-44 ExperimentsB384 1.2 1200B388 1.2 1200B392 1.2 1170

93-26 ExperimentsB408 1.2 1200DM151 ExperimentsB432 1.2 1185

25

24

24

292628

24

24

24

24

2010

202224

24

15

15

302830

24

26

3.3(3)e

4.5(5/

5.99(5)e

5.06(4)e

6.26(10)e

~12

~12

0.71 (17)[

0.98(24/

4.9(11 )e

5.0(6/

5.3(5)e

4.8(6)e

5.3(6)[

6.8( 4)e

5.19(7)e6.1(7)e7.87(12)e

5.6(8)e

6.2( 4)e

60:40

64:36

69:31

70:30

70:30

70:30

70:30

65:35

70:30

70:30

70:30

69:31

70:30

70:30

70:30

70:30

70:30

70:30

70:3060:4060:40

70:30

70:30

Gl, Cpx, Opx,Oliv, SpGl, Opx, Oliv, Sp

Gl, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, SpQch, Cpx, Opx,Oliv, SpQch, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, Sp, GtGl, Cpx, Opx,Oliv, Sp, GtGl, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, GtGl, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, SpGl, Cpx, Opx,Oliv, GtGl, Cpx, Opx,Oliv, Gt

Gl, Opx, OlivGl, Opx, OlivGl, Cpx, Opx,Oliv

Gl, Opx, Oliv

Gl, Cpx, Opx,Oliv, Sp

68.7(8): 1.5(6):9.4(7):20.2(5):0.2(2)74.7(7):6.5(11): 18.5(6):0.4(2)68.6(10):3.9(6): 10.8(6):16.5(6):0.2(2)71.2(9):6.8(6):4.4(9):16.9(6):0.7(2)59.0(15): 14.4(9): 10.6(14):13.8(9):2.2(3)

61.0(19): 12.4(17): 12.2(15):12.7(10): 1.7(3)59.5(10): 18.9(8):8.3(9):11.2(5):2.1 (2)58.6(11): 16.9(10): 14.4(10):7.7(6):2.4(2)67.1 (24):4.7(9):8.7(26):1O.4(8):tr:9.1 (53)66.6(7):8.2(4): 11.4(5):12.7(3): 1.1(l):tr73.0(9):3.9( 6):14.1(7):9.0(4):tr56.5(26):12.0(11):11.1(12):8.3(9):12.1(18)64.1 (15): 14.1(13): 15.2(12):4.8(7): 1.8(2)57.9(19):20.2(19): 15.1(15):4.2(9):2.6(3)60.6(29):9.0(15): 13.0(14):3.3(8):14.1(18)63.6(10):7.9(5):8.5(5):6.1(3):13.9(6)

67.7(8):23.0(13):9.3(8)61.6(8):21.1 (12): 17.3(8)46.0(4): 11.5(4):24.5(7):18.0(4)

75.2(5): 14.0(9): 10.8(5)

57.5(21 ):6.5(13):29.3(23):4.1(15):2.3(4)

0.35

0.35

0.32

0.29

0.34

0.32

0.33

0.35

0.35

0.34

0.33

0.34

0.34

0.34

0.31

0.33

0.28

0.34

0.33

0.34

0.33

0.32

0.31

0.30

0.30

0.33

0.34

0.33

0.34

0.31

0.30

0.35

0.32

0.310.340.33

0.31

0.28

0.34

0.34

0.35

0.35

0.34

0.32

0.32

0.32

0.34

0.35

0.34

0.36

0.32

0.32

0.36

0.34

0.330.360.36

0.33

0.31 0.58

0.75

0.88

0.66

0.76

0.58

0.49

0.47

0.49

0.56

0.64

0.59

0.48

0.46

-0.88

+2.5

-0.39

+ 1.2

+1.1

+ 1.2

+0.65

+0.90

0.59 -0.52

0.59 +0.39

-0.61

0.66 +2.0

+0.89

+ 1.2

0.63 +2.1

0.59 0.0

+2.3 0.13-0.85 0.12-1.1 0.02

-1.8

+2.8

0.06

0.02

0.02

0.05

0.07

0.02

0.03

0.03

0.008

0.01

0.08

0.05

0.08

0.11

0.01

0.06

0.11

326

.2:<5

r ~~ ~o 00':: ;t:~ ~M ~~ ~M ~N MM M~ ~0\ ,-..,r- ,-..,a:~[::::'~~c:~r-:.,......;("'1.,......;.,......;

00~o

pounds were weighed out and ground automatically for 5 h, underethanol, using an agate mortar and pestle, after which FeD spongewas added and the mix was ground for an additional h (Lindsleyet al. 1974). The resulting powder was pressed into 300 mg pellets,using elvanol as a binder, and conditioned by sintering in a gas-mixing furnace for 24 h at 1050 DC,with the f02 controlled at 1 logunit below the fayalite-magnetite-quartz (FMQ) oxygen bufferusing a mixture of CO2 and H2 gases. A H20-bearing PUMperidotite composition was synthesized from Johnson-Mattheyhigh purity Si02, Ti02, A1203, Cr203, MnO, NiO, and previouslyprepared mixes of Fe2Si04, Mg(OHh, CaSi03, Na2Si03, andK2Si409. These compounds were weighed out and ground auto-matically for 6 h, under ethanol, using an agate mortar and pestle.The Mg(OHh was synthesized following the methods described byJohnson and Walker (1993) .

Hydrous glasses were prepared by melting natural rock powder,or a synthetic basalt, in a titanium-zirconium-molybdenum (TZM)rapid quench cold-seal pressure vessel at H20-saturated conditions.Twenty-five III of distilled H20 were placed into a thick-walled Aucapsule, which was then packed with ~200-300 mg of rock powderand welded shut. The charge was held at 200 MPa and 1050 DCfor24 h, after which the experiment was terminated using the rapidquenching technique of Sisson and Grove (1993a). The experi-mental products were analyzed by electron microprobe, the relativeproportions of glass and crystals (Oliv + Sp ± Cpx ± Plag) weredetermined by materials balance (Bryan et al. 1969), and the bulkH20 content was calculated. The remaining material was groundunder ethanol using an agate mortar and pestle for 1 h for use asstarting material in piston-cylinder experiments.

Capsule materials

Maintaining a constant bulk composrtion is a prerequisite forproducing an equilibrated experimental product. Capsules fabri-cated from Au are a good choice for hydrous melting experimentsbecause they minimize loss of Fe from the charge to the capsule andthey are less permeable to hydrogen than Pt, Ag, or AgPd alloys(Chou 1985). Because the majority of our experiments were per-formed at temperatures in excess of the melting point of Au, AuPdalloys were used as capsule materials.

Iron loss from the silicate to unconditioned AU90PdlO capsuleswas determined to be 30% relative after 24 h at 1.2 GPa and1200 DC in experiments performed using hydrous glass startingmaterial, while in unconditioned AusoPd2o capsules Fe loss fromthe silicate was determined to be 38% relative. Hirose and Kawa-moto (1995) performed hydrous peridotite melting experiments inAU7sPd2s capsules and reported no significant loss of Fe from thesilicate. A plausible explanation for the difference between our re-sult and that of Hirose and Kawamoto (1995) is that the f02 in theirexperimental charges was significantly higher than in our assem-blies. The solubility of Fe in AuPd alloys is a strong function of theambient f02 conditions (Kawamoto and Hirose 1994), and there isexperimental evidence to indicate that the f02 conditions in ex-periments that initially contain liquid H20, as opposed to hydrousglass, tend to be oxidizing (Kushiro 1990; Kawamoto and Hirose1994), The relatively low olivine/melt exchange K~e/Mg values[K~e/Mg= (FeOcrystalx Mgomelt)/(Feomeit x MgocrYSt1H)](0.28 ± 0.01 versus 0.34 ± 0.01 for our hydrous experiments) andmolar olivine/silicate melt partition coefficients for total Fe as FeO(~35% low relative to our experiments at a given temperature)reported by Hirose and Kawamoto (1994) are consistent with rel-atively high concentrations of Fe203 in their silicate melts. Fromthe above comparisons we conclude that unconditioned AuPd al-loys do not behave as inert containers at the redox conditions oftypical anhydrous piston-cylinder experiments performed ingraphite crucibles.

Maintaining a constant bulk composition over the course of anexperiment required that each inner capsule be conditioned tominimize Fe exchange with the silicate. For experiments in whichthe AusoPd2o alloy was used the capsule was presaturated with abasaltic liquid (82-72f) at 0.1 MPa and 1250 DC in a vertical gas-mixing furnace for 48 hat anf02 1 log unit below the FMQ buffer.

Following the presaturation step, the silicate glass was removedfrom the AuPd capsule using a warm HF bath. The ternary alloy inthe capsule from experiment B304, which experienced negligible Feexchange with the silicate after 24 h at 1.2 GPa and 1215 DCcon-sists of 78.2 ± 0.6 wt% Au, 19.9 ± 0.2 wt% Pd, and1.35 ± 0.05 wt% Fe. An andesitic liquid containing 7.23 wt%FeO was used for presaturating the AU90PdlOalloy. These condi-tioning runs were performed for 72 h at 1155 DC, with f02 con-trolled at 2 log units below the FMQ buffer. The ternary alloy inthe capsule from experiment B359, which experienced negligible Feexchange with the silicate after 20 h at 1.6 GPa and 1260 DCcon-sists of 88.5 ± 0.6 wt% Au, 10.0 ± 0.1 wt% Pd, and0.68 ± 0.01 wt% Fe.

Experimental methods

Anhydrous and H20-undersaturated experiments were performedusing a 1.27 em solid-medium piston cylinder device (Boyd andEngland 1960). Experiments were first pressurized to 1.0 GPa atroom temperature, then the temperature was raised to 865 DCat100 DC/min. The pressure was then increased to the desired value,and the experiment was held at these conditions for 6 min. Finally,the temperature was increased at a rate of 50 DC/min to the desiredexperimental conditions. The pressure medium consisted of sin-tered BaC03, which was found to have a friction correction of300 MPa through calibration against the pressure-dependentmelting point of Au (Akella and Kennedy 1971), and the Ca-Tschermakite breakdown reaction (Hays 1966). This correction hasbeen applied to the pressures reported in Table 2. Pressures arethought to be accurate to within ± 50 MPa. The temperature wasmonitored and controlled using W97RerW7sRe2s thermocoupleswith no correction for the effect of pressure on thermocouple EMF.Temperatures reported in Table 2 are corrected for a 20 DCtem-perature difference between the position of the thermocouple beadand the hotspot, determined using offset thermocouples. Temper-atures are thought to be accurate to ± 10 DC.

The assemblies for hydrous melting experiments were preparedby packing ~ 15 mg of powder into a capsule made from either Au(Experiments B310 and B326) or conditioned AU90Pd10 orAUsoPd2oalloy, and welding it shut. The sealed inner capsule wasthen placed into a graphite sleeve which was, in turn, placed into anouter capsule made from either AUsoPt2oalloy or Pt, depending onthe temperature of the experiment. Graphite powder was packedinto the outer capsule, and it was welded shut. The sealed outercapsule was placed into a high-density Al203 sleeve and centered inthe hotspot of a straight-walled graphite furnace using MgOspacers.

The assemblies for anhydrous experiments were prepared bypacking ~6 mg of powder into a graphite crucible and placing agraphite lid on top. The crucible was then placed into a Pt capsule,and held at 120 DCin a drying oven for 14 to 65 h. Upon removalfrom the oven, dried graphite powder was packed on top of thecrucible lid and the Pt capsule was welded shut. The sealed outercapsule was then placed into an assembly identical to that used forthe hydrous melting experiments.

Analytical methods

All experimental run products were analyzed using either a 4- or 5-spectrometer JEOL 733 electron microprobe at the MassachusettsInstitute of Technology. A 10 nA beam current and 15 kV accel-erating potential were used for all analyses. Beam diameters were20 11mfor hydrous glasses, 10-20 11mfor anhydrous glasses, and2 11mfor crystalline phases. The hydrous glasses were analyzed for° using a W /Si multilayer crystal (Nash 1992), with corundum asan oxygen standard. Forty second peak counting times resulted in a1o uncertainty due to counting statistics of ± 0.50% relative for 0.Four hydrous basaltic glass secondary standards with H20 con-tents of 2.11 to 6.17 wt% (Muenow et al. 1990; Sisson and Grove1993b) were carbon-coated along with the unknowns, and analyzed

327

at the beginning of each microprobe session to determine the ac-curacy and reproducibility of the ° analyses. On-line data reduc-tion was accomplished using the phi-rho-z correction procedure.Migration of Na from the excitation volume during analysis of thehydrous glasses was minimized through use of a broad beam andby measuring Na for 5 s prior to measuring the other elements(Sisson and Grove 1993a). The maximum peak counting time forthe other elements was 40 s.

Replicate analyses of basaltic glass from an anhydrous, 0.1MPa melting experiment (Experiment 839b-23 of Gaetani et al.1994) were used to estimate analytical precision. Standard devia-tions calculated on the basis of the distribution of 390 replicateanalyses performed over 37 months expressed as percent relativeare 0.52% for Si02, 0.90% for A1203, 0.98% for CaO, 1.3% forMgO, 2.1% for FeO, 5.0% for Ti02, 6.5% for Na20, 6.7% forK20, 31% for MnO, 38% for Cr203, and 65% for P20S. The meansum for the 390 analyses is 99.98 wt% (Table 3). Replicate analysesof natural basalt glass 70-002 were used to determine whether useof updated standards and the phi-rho-z correction procedure re-sults in systematic deviations from previous analyses performed atMIT using the correction procedure of Bence and Albee (1968),with the modifications of Albee and Ray (1970). A comparison ofthe mean of 30 replicate analyses with the analysis of the same glassreported by Kinzler and Grove (1992a) is given in Table 3.

Glasses from five of the experiments were analyzed for bothH20 and CO2 by Fourier transform infrared (FTIR) spectroscopy,following the general methods of Dixon et al. (1991) and modifiedby the use of a NicPlan microscope (Table 4). For the high-Fl-Oglasses, each experiment was analyzed four times with the redun-dant aperturing system set to 115 11mx 115 11m,while the nomi-nally anhydrous experiments were analyzed twice each with a20 11mx 30 11m aperture, using 1000 to 16000 scans. Samplethicknesses ranged from 15 to 30 11m.

The H20 contents of the remaining experimentally producedglasses were determined using a modified Cameca 3f ion probe atLawrence Livermore National Laboratory, using a 2 nA primarybeam (15-20 11m diameter) of 0- ions accelerated at 12.5 kV.Sputtered ions were accelerated at a nominal 4.5 kV through adouble-focusing mass spectrometer, and H+, 160+, and 30Si+ ionbeams were measured with an electron multiplier. Samples weremeasured at a mass-resolving power of ~500, and in order tosuppress isobaric molecular species and minimize matrix effects, anenergy offset of -80 ± 30 eV was used. Measured H+ /30Si+ ratioswere converted to wt% H20 by a calibration using 2 of the ex-perimentally produced glasses that had been analyzed by FTIR(B277 and B365), as well as natural and synthetic glasses providedby S. Newman (Caltech) and J. Dixon (University of Miami).

A liquid nitrogen-filled cold trap was used to reduce the H20background and the sample chamber vacuum. BackgroundH+ /30Si+ ratios measured on crystalline Oliv were in the range~0.02-0.04, equivalent to ~0.1-0.2 wt% H20, and H+ /30Si+ ra-tios measured on glasses were corrected for this background con-tribution. Reported H20 contents represent averages of 3-5individual measurements and reported uncertainties are the stan-dard deviation of these measurements. For samples that were notobviously heterogeneous this error is typically ± 10% relative (2Cl).The reproducibility of an individual measurement, based on re-peated standard measurements, is ± 15% relative (2Cl).

Experimental results

Hydrous experiments

Eighteen hydrous experiments produced silicate meltssaturated with a mantle peridotite mineral assemblage ofOliv + Opx ± Cpx ± Sp ± Gt. Experiments per-formed using the more aluminous basalt compositions,82-72f and DM151, produced silicate melts saturated

wIV00

Table 3 Electron microprobe analyses of run products from anhydrous experiments and anhydrous glass secondary standards. Units in parentheses as in Table 1. Analyses indicatenumber of individual electron microprobe analyses included in average. (FeO* total Fe as FeO)

Experiment Phase Analyses Si02 Ti02 Al203 Cr203 FeO* MnO MgO CaO Na20 K20 P20S Total

Anhydrous experimentsB303 Gl 10 47.5(3) 0.66(3) 17.82(13) 0.17(4) 8.1(2) 0.10(5) 11.80(10) 10.98(7) 2.17(13) 0.08(2) 0.12(3) 99.50

Cpx 15 51.5(8) 0.18(4) 8.0(11 ) 0.49(8) 4.9(2) 0.10(4) 20.9(11 ) 13.8(10) 0.49(3) 100.36Opx 10 53.6(7) 0.12(2) 6.6(11 ) 0.35(7) 6.4(2) 0.10(3) 30.3(5) 2.3(2) 0.12(2) 99.89Oliv 7 40.0(3) 0.05(3) 0.12(1) 0.06(4) 10.71(10) 0.10(3) 48.3(4) 0.31 (2) 99.65Sp 5 0.53(10) 0.07(7) 62.1 (7) 4.5(3) 8.13(11) 0.11 (2) 24.0(3) 0.03(3) 99.47

B292 Gl 10 48.04(13) 0.68(2) 18.3(2) 0.13(3) 8.3(2) 0.16(3) 11.16(7) 10.57(7) 2.5(2) 0.12(1) 0.11 (3) 100.07Cpx 15 51.7(7) 0.26(5) 8.9(9) 0.35(7) 4.8(3) 0.10(3) 20.1 (9) 14.3(10) 0.52(4) 101.03Opx 10 53.2(6) 0.11 (3) 7.8(8) 0.28(3) 6.56(10) 0.15(3) 29.7(4) 2.11(14) 0.11(2) 100.02Oliv 7 40.2(3) 0.05(2) 0.12(1) 0.14(3) 11.42(8) 0.10(2) 47.6(2) 0.27(1) 99.90Sp 5 0.42(7) 0.14(1) 63.6(3) 3.2(3) 8.3(2) 0.08(2) 23.9(3) 0.07(2) 99.71

B287 Gl 10 46.9(2) 0.66(3) 17.90(10) 0.07(2) 8.41(11) 0.12(4) 11.41(11) 10.71(6) 2.50(11) 0.12(1) 0.13(2) 98.93Cpx 15 51.7(6) 0.25(5) 8.4(9) 0.44(5) 4.8(2) 0.14(2) 19.9(8) 15.0(7) 0.56(3) 101.19Opx 10 53.8(7) 0.12(3) 7.3(12) 0.35(6) 6.75(7) 0.13(2) 30.4(4) 2.08(7) 0.10(2) 101.03Oliv 7 40.3(3) 0.02(2) 0.13(4) 0.09(3) 11.25(12) 0.12(2) 48.3(3) 0.27(2) 100.48Sp 3 0.58(2) 0.08(3) 62.7(2) 4.64(9) 8.6(2) 0.09(0) 23.97(5) 0.08(2) 100.74

B394 Gl 10 46.3(2) 0.60(3) 17.08(14) 0.18(3) 8.62(8) 0.16(4) 12.99(12) 10.66(8) 2.22(5) 0.12(1 ) 0.14(2) 99.07Cpx 15 51.5(5) 0.20(4) 8.7(6) 0.31 (3) 4.6(2) 0.11 (3) 20.6(4) 13.9(4) 0.63(3) 100.55Opx 10 54.1(4) 0.12(3) 7.4(6) 0.35(2) 6.30(8) 0.11 (3) 30.6(4) 2.13(3) 0.12(1) 101.23Oliv 6 40.2(3) 0.11(1) 0.03(8) 0.14(2) 10.41(7) 0.10(3) 49.5(2) 0.29(2) 100.78Sp 2 0.38(3) 0.14(1) 64.2(5) 2.93(11 ) 7.92(11) 0.10(3) 25.0(2) 0.07(0) 100.74

B302 Gl 10 46.4(3) 0.68(4) 17.2(2) 0.13(3) 8.9(2) 0.11(4) 12.5(2) 10.75(13) 2.29(12) 0.07(2) 0.15(5) 99.18Cpx 15 51.9(7) 0.20(7) 8.8(8) 0.29(4) 5.1(3) 0.11(4) 20.9(12) 12.8(12) 0.62(6) 100.72Opx 10 53.0(5) 0.10(3) 7.3(9) 0.25(3) 6.50(5) 0.10(3) 30.1 (3) 2.16(10) 0.13(3) 99.64Oliv 7 40.2(4) 0.08(3) 0.12(1) 0.10(3) 10.9(2) 0.07(5) 48.2(4) 0.31 (2) 99.98Sp 4 0.7(2) 0.13(5) 64.4(4) 3.02(12) 8.00(11) 0.10(3) 24.33(3) 0.13(1) 100.81

B414 Cpx 15 52.3(6) 0.48(7) 6.7(8) 0.7(2) 4.0(2) 0.06(5) 18.8(7) 17.4(9) 0.88(8) 101.32Opx 10 54.7(4) 0.20(3) 5.8(6) 0.55(7) 6.39(9) 0.10(5) 31.3(4) 1.66(8) 0.16(4) 100.86Oliv 7 40.7(3) 0.09(2) 0.3(2) 0.14(5) 9.90(10) 0.10(3) 49.0(5) 0.3(2) 100.53Sp 2 0.9(2) 0.34(1 ) 52.5(13) 14.5(12) 9.7(4) 0.09(6) 22.5(2) 0.22(2) 100.75

B412 Cpx 15 52.1(6) 0.44(4) 6.8(4) 0.68(8) 4.0(2) 0.05(6) 18.8(7) 16.8(8) 0.87(5) 100.54Opx 10 54.4(4) 0.20(4) 6.1(5) 0.57(5) 6.28(6) 0.08(5) 32.0(2) 1.52(4) 0.16(5) 101.31Oliv 7 40.7(3) 0.02(2) 0.2(2) 0.17(2) 9.97(13) 0.12(6) 49.1(2) 0.16(1) 100.44Sp 2 0.6(3) 0.33(1 ) 53.7(3) 14.0(4) 9.97(12) 0.10(1) 22.73(12) 0.09(1 ) 101.52

Anhydrous glass secondary standards839b-23a 390 52.7(3) 0.65(3) 15.19(14) 0.12(5) 8.8(2) 0.15(5) 9.16(12) 11.36(11 ) 1.46(10) 0.26(2) 0.13(8) 99.98839b-23b 10 53.3(2) 0.60(3) 15.16(8) 0.05(3) 8.85(8) 0.22(2) 9.36(5) 11.3(1) 1.48(5) 0.30(2) 0.11 (3) 100.7370-002a 30 49.6(3) 1.28(3) 16.3(2) 0.10(5) 8.9(2) 0.16(4) 8.60(8) 11.88(13) 2.63(11 ) 0.09(2) 0.22(3) 99.7670-002c 368 49.6(1) 1.20(2) 15.8(2) 8.98(7) 0.17(3) 8.66(6) 11.9(1) 2.67(6) 0.10(1) 0.12(4) 99.20

"This studyb Analysis from Gaetani et al. (1994)c Analysis from Kinzler and Grove (1992a)

Table 4 Water and carbon dioxide concentrations of experi-mentally produced silicate melts as determined by FTIR. ForFTIR analyses, units in parentheses represent uncertainty esti-mates calculated by propogating uncertainties associated withmeasurement of sample thickness (± 10% for high-HjO samples;

329

± 20% for low-Hjf) samples), goodness of spectra (negligible forhigh-HyO samples; ± 13% for low-H-O samples), and molarabsorbance coefficient (± 5%). For electron microprobe ana-lyses, units in parentheses are as in Table 1

Experiment H20 (wt%) CO2 (wt%) ° in H20 + CO2 ° in H20 + CO2

(wt% by FTIR) (wt% by EMP)

B305 4.5(5) 1.27(14) 4.9(5) 4.9(2)B277 5.0(6) 1.19(13) 5.3(6) 5.7(4)B365 5.3(6) 1.25(14) 5.6(6) 5.1(2)B287 0.98(24) 0.07(2)B303 0.71(17)

with a spinel and/or garnet lherzolite mineral assem-blage at pressures of 1.2 to 2.0 GPa and temperatures of1100to 1275 °C. Experiments performed using the loweralumina compositions, 85-44 and 93-26, produced hy-drous melts saturated with Oliv + Opx ± Cpx at1.2 GPa and 1170 to 1200 0c.

Experimental charges contained large amounts ofsilicate melt (46-75 wt%; Table 2) that, with the ex-ception of the experiments described below, quenched toproduce homogeneous glasses containing 3.3 ± 0.3 to7.87 ± 0.12 wt% H20 and ~1.25 wt% CO2 (Tables 4and 5). The molar Mg/(Mg + Fe) ratio (Mg#) of theglasses (calculated assuming all Fe as FeO) are 0.69 to0.76. Mineral compositions are similar to those found innatural peridotite. The forsterite (Fo) contents of theOliv range from 0.87 to 0.91, while the Mg# of the Opxare 0.88 to 0.92. The Cpx have Mg# of 0.88 to 0.92 andCaO contents of l7.3±0.3 to 21.2±OA wt% (all un-certainties are 10").Detailed descriptions of the sizes andmorphologies of the crystalline phases in both hydrousand anhydrous experiments are given by Gaetani (1996).

The silicate melt did not quench to a glass in theexperiments designed to produce melts with high(~12 wt%) dissolved H20 contents. The melts in theseexperiments underwent extensive modification due tothe growth of amphibole during the quench, and weconsider the glass compositions to be unreliable. Thecompositions of the crystalline phases from 2 of theseexperiments (B3l0 and B326) are reported in Table 5.

Ion probe analyses of several of the experimentallyproduced glasses (B359; B366; B388) indicate significantheterogeneity with respect to H20 (Table 6). The originof this heterogeneity is unknown. A comparison of theion probe analyses with electron microprobe analyses ofthe 0 contents of the glasses requires unreasonably largeconcentrations of dissolved CO2 (4-5 wt%) in order toexplain the lower H20 concentrations. Given the goodagreement between the 0 concentrations associated withH20 and CO2 in experiments B305, B277, and B365determined by FTIR and by electron microprobe(Table 4) we used the results of the electron microprobeo analyses to determine preferred H20 contents for theexperiments listed in Table 6. The preferred values rep-resent averages of the individual ion probe analyseswhich indicate the highest H20 contents. The standard

deviations for H20 reported for these experiments werecalculated using all of the ion probe measurements inorder more accurately to reflect the variability foundamong the individual analyses.

Anhydrous experiments

Anhydrous experiments were performed to provide abaseline for interpreting the results of the hydrous ex-periments. Five anhydrous experiments produced sili-cate melts saturated with a mantle peridotite mineralassemblage of Oliv + Opx + Cpx + Sp at pressures of1.2 to 1.6 GPa and temperatures of 1315 to l370°C.These experiments produced large amounts of silicatemelt (58-64 wt%; Table 2) that quenched to producehomogeneous glasses with Mg# (calculated assuming allFe as FeO) of 0.71 to 0.73 (Table 3). The Fo content ofthe Oliv range from 0.88 to 0.89, while the Mg# of theOpx are 0.89 to 0.90. The Cpx have Mg# of 0.88 to 0.89,and CaO contents of 12.8 ± 1.2 to 15.0 ± 0.7 wt%.Two experiments performed on the PUM compositionat sub-solidus conditions produced Oliv + Opx +Cpx + Sp mineral assemblages.

Approach to equilibrium

Several lines of evidence can be used to evaluate theapproach to equilibrium represented by our experi-ments. First, a reversal of mineral compositions wasperformed. Experiment B359 was carried out at 1.6 GPaand 1260 °C using hydrous 82-72f glass and PUMperidotite, which was synthesized from oxides, whileExperiment B277 was carried out at 1.6 GPa and1255 °C using hydrous 82-72f glass and FPl peridotite,which was synthesized from mantle minerals. The use ofdifferent synthetic peridotites means that the minerals inExperiment B359 crystallized from the hydrous glassand synthetic oxide mix starting materials, while those inExperiment B277 (with the exception of Gt) reequili-brated through exchange among the melt and minerals,as well as through the growth of new crystalline mate-rial. There is good agreement between the compositionsof the crystalline phases in the two experiments

Table 5 Electron microprobe analyses of run products from H20-undersaturated melting experiments. Units in parentheses as in Table 1. Analyses indicate number of individual ww

electron microprobe analyses included in average. (FeO* total Fe as FeO) 0

Experi- Phase Analyses Si02 Ti02 Ah03 Cr203 FeO* MnO MgO CaO Na20 K20 P20S H2O Totalment

B333 Gl 10 45.8(3) 0.61(2) 17.06(10) 0.09(3) 6.87(11 ) 0.14(2) 11.44(7) 10.91(11) 2.03(13) 0.13(1) 0.09(4) 3.3(3) 98.47Cpx 9 52.4(6) 0.14(5) 6.0(11 ) 0.9(2) 4.1 (2) 0.08(5) 19.4(8) 17.6(5) 0.49(5) 101.11Opx 10 55.0(6) 0.09(3) 6.0(9) 0.8(1 ) 6.4(1 ) 0.13(4) 31.2(4) 1.7(1) 0.05(3) 101.37Oliv 7 40.4(2) 0.01 (1) 0.07(1 ) 0.04(2) 10.03(8) 0.13(1) 49.9(2) 0.18(2) 100.76Sp 2 0.17(2) 0.15(2) 49.4(17) 18.6(23) 10.0(2) 0.12(5) 22.20(8) 0.05(5) 100.69

B305 Gl 10 45.05(11) 0.54(3) 15.79(9) 0.17(3) 7.09(12) 0.12(4) 12.33(9) 10.33(11 ) 1.94(11 ) 0.05(2) 0.16(3) 4.5(5) 98.07Opx 10 53.6(8) 0.08(4) 5.2(10) 1.0(2) 6.0(1 ) 0.09(4) 31.4(3) 1.6(1) 0.08(2) 99.05Oliv 7 40.3(3) 0.10(2) 0.05(3) 0.08(3) 9.6(1) 0.14(3) 48. 7(4) 0.17(3) 99.14Sp 2 0.31(1) 0.10(1) 41.1(17) 27.3(21) 10.6(4) 0.12(3) 21.0(2) 0.03(2) 100.56

B304 Gl 10 44.63(13) 0.59(4) 17.45(8) 0.05(5) 6.85(10) 0.08(6) 10.72(9) 10.72(13) 2.21(10) 0.08(3) 0.16(2) 5.99(5) 99.53Cpx 15 51.3(5) 0.19(5) 7.2(7) 0.50(6) 3.9(1) 0.09(3) 17.6(3) 19.1(3) 0.50(3) 100.38Opx 10 52.9(4) 0.09(6) 6.9(4) 0.50(6) 6.72(6) 0.12(6) 30.6(3) 1.63(9) 0.07(2) 99.53Oliv 7 40.2(4) 0.05(3) 0.07(2) 0.12(3) 10.7(2) 0.17(5) 48.0(2) 0.21(2) 99.52Sp 5 0.15(1) 0.11 (3) 58.9(16) 7.9(15) 9.6(2) 0.09(2) 23.0(4) 0.08(8) 99.83

B330 Gl 10 45.1 (2) 0.60(2) 17.63(8) 0.11 (2) 6.92(12) 0.12(2) 10.26(9) 10.22(9) 2.15(13) 0.10(1) 0.10(3) 5.06(4) 98.37Cpx 15 52.0(3) 0.18(2) 6.2(5) 0.8(1 ) 3.9(1) 0.09(4) 18.0(3) 18.9(3) 0.43(2) 100.50Opx 10 55.0(4) 0.09(2) 5.4(5) 0.71 (9) 7.0(1 ) 0.11 (3) 31.1 (2) 1.50(9) 0.04(2) 100.95Oliv 7 40.5(2) 0.02(1) 0.06(1 ) 0.12(2) 11.3(2) 0.13(3) 48.3(3) 0.17(1) 100.60Sp 5 0.3(2) 0.16(1) 54.0(8) 14.0(9) 10.9(1 ) 0.09(3) 21.3(4) 0.15(8) 100.90

B329 Gl 10 44.78(13) 0.63(2) 17.99(11) 0.08(3) 7.21 (13) 0.14(2) 9.39(11) 9.80(8) 2.31(13) 0.12(1) 0.08(2) 6.26(10) 98.79Cpx 15 51.4(3) 0.22(3) 7.1 (5) 0.31(6) 3.9(2) 0.09(4) 17.5(4) 19.3(4) 0.51(3) 100.33Opx 10 54.1 (7) 0.10(2) 6.5(7) 0.42(4) 7.41(7) 0.18(2) 30.5(3) 1.35(6) 0.06(1 ) 100.62Oliv 7 40.3(1 ) 0.00(1) 0.06(2) 0.12(2) 12.2(1) 0.15(3) 47.3(2) 0.17(2) 100.30Sp 5 0.3(3) 0.12(2) 63.0(5) 4.2(8) 10.0(2) 0.09(1 ) 22.5(2) 0.3(2) 100.51

B326 Cpx 15 52.3(5) 0.18(4) 4.6(6) 0.9(1 ) 2.8(2) 0.05(4) 17.5(4) 21.2(4) 0.37(4) 99.90Opx 4 55.0(5) 0.07(1) 4.6(16) 0.8(2) 5.37(8) 0.09(4) 33.1(4) 1.2(1) 0.1(1) 100.33Oliv 7 40.9(4) 0.05(3) 0.03(2) 0.06(4) 9.0(2) 0.12(4) 49.5(4) 0.13(2) 99.79

B310 Cpx 15 51.9(6) 0.32(3) 7.1 (7) 0.4(1 ) 3.6(2) 0.09(4) 16.6(3) 20.6(4) 0.42(5) 101.03Opx 10 54.0(4) 0.13(3) 7.3(4) 0.33(5) 7.2(1 ) 0.12(4) 31.0(3) 1.2(1) 0.02(2) 101.30Oliv 7 40.6(3) 0.02(2) 0.11(4) 0.08(2) 11.3(2) 0.13(2) 48.5(5) 0.18(2) 100.92Sp 1 0.21 0.11 60.6 7.5 10.6 0.07 21.6 0.18 100.87

B359 Gl 10 43.9(2) 0.66(3) 17.19(13) 0.08(4) 7.36(11 ) 0.08(5) 11.79(8) 10.40(8) 2.37(12) 0.12(1 ) 0.15(4) 4.9(11 ) 99.00Cpx 15 51.0(5) 0.17(3) 8.0(5) 0.37(8) 3.9(2) 0.10(5) 17.8(4) 18.1(6) 0.66(6) 100.10Opx 10 53.6(5) 0.10(4) 7.1 (6) 0.33(5) 6.39(6) 0.12(3) 31.0(3) 1.55(7) 0.08(3) 100.27Oliv 7 40.0(2) 0.02(2) 0.07(2) 0.04(5) 10.4(2) 0.10(4) 49.1(4) 0.18(2) 99.91Sp 5 0.24(6) 0.05(6) 63.1(9) 4.2(8) 8.5(2) 0.07(4) 24.4(2) 0.11(3) 100.67Gt 7 42.3(5) 0.18(6) 23.6(6) 0.66(7) 7.24(8) 0.27(4) 19.6(5) 6.5(4) 0.01(2) 100.36

B277 Gl 10 43.7(3) 0.59(3) 16.8(2) 0.08(2) 7.35(14) 0.16(2) 11.92(8) 10.48(8) 2.17(11) 0.09(1 ) 0.08(2) 5.0(6) 98.42Rims Cpx 15 51.7(3) 0.19(7) 7.4(5) 0.6(1 ) 3.8(2) 0.08(4) 17.7(5) 18.8(4) 0.6(1) 100.87

Opx 10 54.3(5) 0.08(2) 6.5(6) 0.50(7) 6.6(1 ) 0.10(4) 30.9(4) 1.5(2) 0.06(2) 100.54Oliv 7 40.7(2) 0.02(2) 0.06(1 ) 0.04(3) 10.7(2) 0.09(3) 48.8(3) 0.17(1) 100.58Sp 5 0.16(3) 0.06(4) 60.1(13) 7.4(11) 9.2(2) 0.05(4) 23.3(4) 0.11(4) 100.38Gt 7 42.8(2) 0.17(2) 24.0(3) 0.82(8) 7.1(1) 0.26(2) 19.4(4) 6.5(5) 0.02(2) 101.07

Cores Cpx 3 52.5(4) 0.52(5) 7.5(3) 0.81(5) 3.1(2) 0.11 (2) 15.3(4) 19.7(4) 1.5(2) 101.04Opx 2 56.1 (1) 0.13(2) 4.81(2) 0.35(1 ) 7.2(4) 0.12(4) 32.1 (3) 0.76(1 ) 0.08(2) 101.65

B348 Gl 10 43.5(2) 0.64(4) 16.84(9) 0.09(3) 7.30(7) 0.15(4) 11.78(8) 10.52(12) 2.26(9) 0.05(3) 0.06(4) 5.3(5) 98.49Cpx 15 51.5(3) 0.15(4) 7.7(4) 0.48(9) 3.8(1) 0.07(5) 18.1(3) 18.3(5) 0.60(3) 100.70Opx 10 53.7(5) 0.03(3) 7.3(5) 0.38(5) 6.46(6) 0.09(4) 31.3(2) 1.55(8) 0.08(2) 100.89

Table 5 (continued)

Experi- Phase Analyses Si02 Ti02 Al203 Cr203 FeO* MnO MgO CaO Na20 K20 P20S H2O Totalment

Oliv 7 40.4(2) 0.03(2) 0.02(2) 0.11(5) 10.4(2) 0.12(4) 49.2(5) 0.13(4) 100.41Sp 2 0.20(3) 0.08(3) 63.0(3) 4.9(4) 8.65(1) 0.10(0) 23.76(9) 0.11 (3) 100.80

B366 Gl 10 44.2(3) 0.66(3) 16.82(9) 0.12(2) 7.77(8) 0.13(3) 11.43(12) 9.86(13) 2.42(12) 0.11 (2) 0.16(2) 4.8(6) 98.48Cpx 15 51.6(5) 0.19(6) 7.1(10) 0.32(7) 4.1(1) 0.06(5) 18.2(4) 18.4(4) 0.45(3) 100.42Opx 10 54.5(8) 0.03(6) 6.1(7) 0.3(1) 7.0(3) 0.10(5) 30.7(10) 1.8(10) 0.07(3) 100.60Oliv 7 40.3(2) 0.06(4) 0.04(3) 0.04(2) 11.7(2) 0.13(2) 48.3(2) 0.17(4) 100.74Gt 9 41.8(2) 0.18(5) 23.3(2) 0.7(1) 8.3(1) 0.32(4) 18.6(2) 6.8(2) 0.02(2) 100.02

B365 Gl 10 43.88(13) 0.64(2) 15.41(5) 0.15(3) 8.21(13) 0.12(5) 13.36(10) 10.25(10) 2.2(2) 0.10(2) 0.09(2) 5.3(6) 99.71Cpx 15 52.7(4) 0.12(4) 6.2(2) 0.26(6) 4.1(1) 0.09(4) 19.4(2) 17.3(3) 0.48(2) 100.65Opx 10 55.2(4) 0.11 (3) 5.6(4) 0.29(5) 6.7(1 ) 0.10(4) 31.3(2) 1.71(7) 0.06(3) 101.07Oliv 7 40.5(3) 0.01(2) 0.05(2) 0.13(4) 10.75(6) 0.09(4) 48.6(3) 0.20(2) 100.33Gt 10 42.1 (4) 0.11(4) 23.7(3) 0.6(1) 7.54(9) 0.27(2) 19.5(2) 5.9(3) 0.00(1 ) 99.72

B399 Gl 10 43.6(2) 0.65(2) 15.03(9) 0.07(3) 7.74(10) 0.11 (5) 12.70(6) 9.84(12) 2.41(10) 0.12(2) 0.21(2) 6.8(4) 99.28Cpx 15 53.1(4) 0.17(2) 6.0(4) 0.18(5) 3.6(1 ) 0.10(3) 19.0(3) 18.0(4) O.76(4) 100.91Opx 10 55.9(4) 0.09(2) 4.7(7) 0.21(5) 6.24(9) 0.07(3) 32.2(2) 1.65(7) 0.09(2) 101.15Oliv 6 40.5(3) 0.11 (2) 0.10(3) 0.02(2) 10.2(1 ) 0.16(2) 49.6(2) 0.27(3) 100.96Gt 10 42.2(2) 0.14(3) 23.9(2) 0.5(1) 7.1(2) 0.25(2) 19.9(2) 6.1(1) 0.01 (1) 100.10

B384 Gl 10 45.90(10) 0.57(3) 15.75(11) 0.12(3) 7.39(14) 0.11 (3) 10.25(9) 9.47(13) 2.71(13) 0.35(2) 0.21(3) 5.19(7) 98.02Opx 10 55.4(3) 0.10(4) 4.1 (2) 0.66(9) 7.2(1 ) 0.13(5) 32.0(2) 1.43(7) 0.06(2) 101.08Oliv 7 40.4(2) 0.02(1) 0.08(4) 0.15(2) 11.6(1 ) 0.15(4) 48.6(2) 0.15(1) 101.15

B388 Gl 10 45.8(2) 0.57(3) 15.6(2) 0.15(3) 6.65(10) 0.10(2) 10.41(6) 9.55(7) 2.66(9) 0.35(2) 0.15(2) 6.1(7) 98.09Opx 10 55.7(2) 0.10(3) 4.2(2) 0.71(7) 7.0(1 ) 0.16(2) 32.0(2) 1.31(7) 0.04(2) 101.22Oliv 7 41.1(2) 0.01 (1) 0.08(2) 0.03(1) 11.1(1) 0.10(3) 48.7(2) 0.13(1) 101.25

B392 Gl 10 46.2(3) 0.68(3) 18.0(3) 0.06(3) 6.4(2) 0.08(3) 8.48(9) 8.82(4) 3.0(2) 0.44(2) 0.22(3) 7.87(12) 99.88Cpx 15 51.9(4) 0.28(3) 6.7(7) 0.65(9) 4.2(1 ) 0.12(2) 17.2(3) 19.2(4) 0.62(6) 100.87Opx 10 54.6(3) 0.10(2) 6.3(3) 0.59(9) 7.7(1 ) 0.14(3) 30.6(2) 1.37(7) 0.07(1) 101.32Oliv 7 40.7(2) 0.11 (2) 0.09(2) 0.07(2) 12.7(2) 0.17(2) 47.3(2) 0.18(2) 101.32

B408 Gl 10 46.8(2) 1.20(3) 16.18(6) 0.12(3) 6.97(12) 0.10(4) 9.74(7) 8.90(9) 3.06(11) 0.70(3) 0.32(3) 5.6(8) 99.69Opx 10 55.3(2) 0.19(2) 4.4(2) 0.75(4) 7.04(7) 0.13(3) 31.8(2) 1.53(4) 0.07(2) 101.21Oliv 7 40.3(1 ) 0.05(2) 0.05(3) 0.10(3) 11.5(1 ) 0.13(3) 48.9(1 ) 0.17(1) 101.20

B432 Gl 10 45.8(2) 1.26(4) 18.13(10) 0.02(1 ) 6.97(11) 0.17(3) 8.76(8) 8.91(8) 3.59(9) 0.48(1) 6.2(4) 100.29Cpx 15 51.6(4) 0.53(6) 7.7(5) 0.61(18) 3.8(2) 0.06(5) 17.2(4) 18.9(6) 0.80(4) 101.20Opx 10 54.3(3) 0.20(3) 7.0(3) 0.48(11 ) 6.9(3) 0.16(5) 30.7(2) 1.43(11 ) 0.08(2) 101.25Oliv 7 40.4(2) 0.10(3) 0.05(3) 0.07(3) 12.0(2) 0.15(6) 48.3(2) 0.15(1) 101.22Sp 3 0.29(16) 0.17(1) 61.2(2) 6.2(2) 10.68(5) 0.10(4) 23.0(3) 0.09(3) 101.73

332

Table 6 Water contents of experimentally produced silicate meltsthat appear to be heterogeneous with respect to H20 as determinedby ion microprobe

Experiment Range of H2O Preferred H2Ocontents (wt%) content (wt%)

B359 1.45-4.25 4.19B366 3.14-4.96 4.81B388 3.66-6.07 6.07

(Table 5), indicating that diffusion rates are fast enoughunder hydrous conditions to reequilibrate the rims of themineral grains in 10 h. The cores of the larger pyroxenegrains in Experiment B277 have compositions similar tothe starting material (Tables 1 and 5), indicating thatbulk equilibration of the charge was not achieved.

The maintenance of constant sample bulk composi-tion is essential for equilibration, and has been demon-strated by materials balance (Table 2). The achievementof consistent mineral/melt exchange equilibrium indi-cates a close approach to equilibrium. The average ex-change K~e/Mgvalues for olivine (0.34 ± 0.01 versus0.33 ± 0.03), clinopyroxene (0.33 ± 0.02 versus0.36 ± 0.04), orthopyroxene (0.32 ± 0.02 versus0.33 ± 0.04), spinel (0.60 ± 0.13 versus 0.54 ± 0.06),and garnet (0.61 ± 0.03 versus 0.54 ± 0.06) are inagreement with those from the longer duration (18-112 h, with a mean duration of 66 h), anhydrous ex-periments of Kinzler (1997).

Discussion

The compositions of hydrous melts of mantle peridotite

Evaluating the effects of dissolved H20 on the compo-sitions of peridotite partial melts requires a comparisonof hydrous and anhydrous partial melts of the samesilicate bulk composition. Although recent experimentalwork has provided compositional data for hydrous sili-cate melts in equilibrium with mantle peridotite (Ku-shiro, 1990; Hirose and Kawamoto 1995; Hirose 1997;Kawamoto and Holloway 1997), comparisons of thecompositions of anhydrous and hydrous partial meltshave been carried out only on a volatile-free basis. Thistype of comparison is useful for interpreting the com-positions of subduction-related lavas, but it does notallow an evaluation of thermodynamic controls on thecompositions of H20-bearing peridotite partial melts. Inthe discussion that follows, a comparison of the com-positions of hydrous and anhydrous silicate liquids inequilibrium with a spinel lherzolite mineral assemblageis carried out in 2 ways: (1) with H20 included as acomponent to identify thermodynamic controls on thecompositions of hydrous peridotite partial melts; (2)with melt compositions normalized on a volatile-freebasis (hereafter referred to as apparent compositions) toidentify the compositional characteristics of degassedprimitive arc and back-arc lavas, and to place con-

straints on the composition of material transferred fromthe mantle to the crust by island arc magmatism.

The ternary system Mg2Si04-SiOrH20 provides auseful starting point for discussing the effects of H20 onthe compositions of peridotite partial melts. Kushiroet al. (1968) demonstrated that the incongruent meltingbehavior of enstatite (En), which disappears at pressuresof ~0.5 GPa under anhydrous conditions (Boyd et al.1964), persists to at least 3.0 GPa when H20 is presentin the system. Experimental work on the system CaO-MgO-AhOrSiOrNa20-H20 further demonstrated thatH20-bearing silicate melts saturated with Fo and En arequartz normative to pressures of at least 2.5 GPa (Ku-shiro 1969, 1972).A reexamination of the experiments ofKushiro et al. (1968) demonstrates that when H20 isincluded as a melt component the liquids produced bythe incongruent melting of En have lower Si02 contentsthan anhydrous En melts formed at the same pressure.The elevated Si02 concentrations that are commonlyassociated with hydrous En + Fo-saturated melts arean artifact of comparing liquid compositions on a vol-atile-free basis.

Under anhydrous conditions En melts congruently at1.0 GPa to form a liquid containing 60 wt% Si02. Thebulk composition of an H20-undersaturated experimentperformed by Kushiro et al. (1968) at 1.0 GPa is rep-resented by the open circle in Fig. 1 (91 wt%En + 9 wt% H20). The composition of the hydrousmelt produced in this experiment could not precisely be

P= 1.0 GPaT = 1375°C

Weight Units

Fig. 1 Ternary system Mg2Si04-H20-Si02 illustrating phase relationsrelevant to determining the compositions of liquids produced bymelting MgSi03 + H20 at 1.0 GPa. Open circle represents the bulkcomposition (91 wt% MgSi03 + 9 wt% H20) of a fluid-undersat-urated melting experiment performed by Kushiro et al. (1968) at1.0 GPa and 1375 "C. Possible compositions of the experimentallyproduced silicate liquid coexisting with forsterite and enstatite at theseconditions, as determined from three-phase triangles, are representedby the black triangle. The shaded region represents all possible liquidcompositions with Si02 contents equal to or greater than a liquidproduced by melting anhydrous MgSi03 at 1.0 GPa. The field of fluidcompositions (vertical lines) is approximate.

determined, but the coexistence of En + Fo + liquid,with no fluid phase, requires that it lie within the blacktriangle shown in Fig. 1. The melt composition is con-strained by the coexistence of Fo and En to lie on theSiOrrich side of the MgSiOrH20 join, above an ex-tension of the line connecting the F0 apex of the ternaryto the bulk composition (dashed line labeled En out).The lack of a fluid phase requires the H20 content of themelt to be less than the saturation value of ~ 11 wt%.This geometric analysis demonstrates that when H20 isincluded as a melt component, the range of possiblehydrous liquid compositions have Si02 contents (~53-57 wt% Si02) lower than 60 wt%. Although the absol-ute concentration of Si02 is lower in the hydrous Enpartial melts they are characterized by an elevated Si02/

MgO ratio, making them quartz normative. When pro-jected from H20 onto the Fo-Si02 join and consideredon an anhydrous basis, this increased Si02/MgO ratioproduces a range of possible volatile-free liquid com-positions that have Si02 contents (60-64 wt%) equal toor greater than the anhydrous En melt.

Our experiments on natural starting compositionsdemonstrate that these observations also hold for melt-ing of mantle peridotite. Figure 2 contains plots of wt%Si02 (Fig. 2a) and wt% Ah03 (Fig. 2b) versus molarMg/(Mg + Fe) ratio comparing the compositions ofour experimentally produced hydrous (3.3 ± 0.3 to6.26 ± 0.10 wt% H20) and nominally anhydrous(:'::1wt% H20) silicate melts in equilibrium with a spinellherzolite assemblage at 1.2 GPa. When H20 is included

Fig. 2a, b Plots of molar Mgj(Mg + Fe) ratio versus the weightconcentrations of: a Si02; b Al203, comparing the compositions ofexperimentally produced anhydrous (filled symbols) and hydrous(open symbols) silicate melts saturated with a spinel lherzolite mineralassemblage at 1.2 GPa and 1185-1345 "C. Water has been included asa component in all liquid compositions. Experiments performed usingmixtures of 82-72f basalt and PUM peridotite are represented bycircles. Experiment performed using a mixture of DM151 basaltanalog and PUM peridotite is represented by a triangle. (Vertical errorbars 1o values from Tables 3 and 5, horizontal error bars 1o valuescalculated by propagating the standard deviation of the mean for FeOand MgO from Tables 3 and 5)

49 ,-...,---r---r,-....,-,---.---r----,P= 1.2 GPa

~ 48~-Qi:2 47

.~~ 46

-+Anhydrous +

+

+ +.f:

-+-® . .

+3.3 - 6.3 wt%Hp+

440.68 0.70 0.72 0.74 0.76

Molar Mg/(Mg+Fe) of Silicate Melt

333

as a component, the Si02 contents of the hydrous meltsare lower than those of the anhydrous melts by ~2 to3 wt%, just as in the system Mg2Si04-SiOrH20. Theconcentration of Ah03 is largely unaffected by thepresence of dissolved H20. The Si02 content of the meltfrom the experiment performed using the DM15l syn-thetic basalt (open triangle) is ~ 1 wt% higher than thelower alkali H20-bearing melt with a comparable Mg#,but the Si02 versus Mg# trend formed by the 82-72fexperiments is poorly defined. The A1203 content of thehigher alkali hydrous melt is comparable to the Ah03contents of the lower alkali melts.

Thermodynamic controls on the Si02 contents ofH20-bearing peridotite partial melts can be determinedby calculating the activity of Si02 (a~:6) in each of theexperimentally produced silicate liquids. The coexistenceof Oliv, Opx and liquid fixes a~:6 for a given set ofpressure-temperature conditions, aJd the reaction:

Mg2SiO~liv + SiO~iq = Mg2Si20~PX

can be used to calculate a~i62values on the basis of thecompositions of coexisting Oliv and Opx (Carmichaelet al. 1970;Morse 1979;Ghiorso et al. 1983;Grove andJuster 1989). The Si02 activity coefficients (y~i6 ) canthen be determined by dividing a~:62by the mol~ frac-tion of Si02 in the melt. Calculations were carried outusing two different combinations of standard statethermodynamic properties and mixing models for Olivand Opx (Table 7). Although the 2 sets of calculationsresulted in different a~:62values, both indicated the sameY~:6dependencies. The compositions of anhydrous liq-uid~ saturated with a minimum assemblage of Oliv +Opx at 1.2 GPa have been included from the study ofKinzler and Grove (l992a) to extend the database.

The results from the a~i62calculations are presentedin Table 7 and in Fig. 3. There is a weak temperaturedependence for a~i62so that, if y~i62remained constant,the concentration of Si02 in the hydrous liquids wouldbe slightly lower than in the anhydrous liquids due totheir lower liquidus temperatures. A plot of lnY~:62versus inverse temperature (Fig. 3a) demonstrates thaty~i62is not constant. Some of the y~i62variability found

19 .----..---r----r-....,-,...---,--'.--,P= 1.2GPa

+-+-:+Anhydrous++~+

3.3 - 6.3 wt% +H20

. . , .160.68 0,70 0.72 0.74 0.76

Molar Mg/(Mg+Fe) of Silicate Melt

334

Table 7 Data used to calculate Si02 activity coefficients for experimentally produced silicate melts

Experiment T(°C) XMe1t,a XMe1t,a Oliv,b Opx,c Melt,d Melt,e Oliv,f Opx,g Melt,h Melt,iH2O Si02

aFo aEn aSi02 YSi02 aFo aEn aSi02 YSi02

B333 1245 0.114 0.469 0.819 0.570 0.255 0.544 0.809 0.765 0.289 0.616B305 1230 0.146 0.445 0.821 0.587 0.260 0.585 0.812 0.784 0.293 0.659B304 1215 0.191 0.428 0.803 0.548 0.247 0.576 0.792 0.765 0.291 0.681B330 1200 0.167 0.449 0.797 0.565 0.254 0.567 0.785 0.756 0.283 0.630B329 1185 0.202 0.435 0.782 0.542 0.247 0.568 0.768 0.747 0.270 0.622B384 1200 0.168 0.447 0.794 0.584 0.264 0.590 0.782 0.762 0.286 0.639B388 1200 0.202 0.430 0.801 0.587 0.263 0.612 0.790 0.766 0.285 0.661B392 1170 0.241 0.426 0.776 0.538 0.246 0.577 0.760 0.741 0.256 0.601B408 1200 0.179 0.446 0.796 0.579 0.261 0.585 0.784 0.759 0.284 0.636B432 1185 0.197 0.435 0.788 0.541 0.245 0.563 0.775 0.757 0.272 0.625356 1250 0.158 0.462 0.804 0.619 0.282 0.611 0.793 0.752 0.290 0.629357a 1200 0.205 0.439 0.774 0.594 0.275 0.627 0.759 0.791 0.306 0.696357b 1200 0.298 0.393 0.816 0.642 0.282 0.718 0.807 0.788 0.287 0.729358 1150 0.148 0.477 0.792 0.551 0.244 0.512 0.778 0.754 0.236 0.495B303 1345 0.026 0.523 0.801 0.563 0.268 0.512 0.791 0.745 0.297 0.568B292 1330 0.543 0.790 0.541 0.260 0.478 0.778 0.741 0.299 0.551B287 1315 0.036 0.514 0.794 0.549 0.260 0.507 0.783 0.743 0.297 0.577B29 1315 0.536 0.810 0.563 0.262 0.489 0.801 0.784 0.306 0.572B32 1285 0.546 0.703 0.499 0.264 0.484 0.679 0.721 0.329 0.603B30 1300 0.537 0.744 0.529 0.266 0.495 0.727 0.736 0.315 0.587B52 1315 0.539 0.771 0.546 0.267 0.495 0.757 0.759 0.314 0.582B54 1285 0.553 0.782 0.522 0.249 0.450 0.769 0.762 0.307 0.555B55 1270 0.551 0.780 0.513 0.243 0.442 0.766 0.750 0.302 0.548B59 1285 0.551 0.767 0.527 0.256 0.464 0.752 0.776 0.320 0.581

a Mole fraction of oxide component in silicate melt calculated using e Calculated using values from columns 4 and 7the components of Bottinga and Weill (1972) f Calculated relative to a standard state of pure forsterite at the Pb Calculated relative to a standard state of pure forsterite at the P and T of interest using standard state properties from Davidsonand T of interest using standard state properties from Berman and Lindsley (1989) and the mixing model of Davidson and Mu-(1988) and the mixing model of Hirschmann (1991) khopadhyay (1984)C Calculated relative to unit activity of pure clinoenstatite in or- g Calculated relative to pure enstatite at the P and T of interestthopyroxene at the P and T of interest using standard state prop- using standard state properties from Davidson and Lindsley (1989)erties from Berman (1988) and the mixing model of Sack and and the mixing model of Davidson and Lindsley (1985, 1989)Ghiorso (1994) h Calculated relative to a standard state of pure Si02 melt at the Pd Calculated relative to a standard state of pure Si02liquid at the P and T of interest using a~~v and a~~x from columns 9 and 10, withand T of interest using a~~v and a~~x from columns 5 and 6, with liquid standard state properties calculated using data from Helge-liquid standard state properties calculated using data from Berman son et al. (1978), Richet et al. (1982), Lange and Carmichael (1990),(1988), Richet et al. (1982), Lange and Carmichael (1990), and and Ghrioso and Sack (1995)Ghrioso and Sack (1995) I Calculated using values from columns 4 and 11

among the anhydrous experiments (filled circles) is at- are larger than would be predicted on the basis of thetributable to temperature dependence (solid line), cal- L'temperature dependence of the anhydrous Ys:6 values,culated by assuming that the partial molar excess free indicating that there is a significant dependence ~n liquidenergy of mixing is independent of temperature (Ryer- composition (Fig. 3a).son 1985; Hirschmann et al. in press). The Y~:6 values When all of the Y~:6values in Fig. 3a are corrected tocalculated from the hydrous experiments (open2 circles) an intermediate tempdrature (1250 "C), there is a posi-

~ON -.55:.:JU5?-

.E -.65

o 8oo 8 o

-.25 , ,

c H~drous Mells "-.35 ( ushiro, 1990)

-.45 ":'2 (\J0

"5-0 00 0~ <ii-.55 o 0 qp.E

0

"-.65

••-.75 P= 1.2 GPa

® T = 1250°C

-.35

-.45

o Hydrous Mells (This Study)• Anhydrous Melts (This Study;

Kinzler & Grove, 1992)

P=1.2GPa 0

o

• TemperatureDependence

......._of Iny.§~u~d-.75

® • •-0.85

6.0 62 6A 6~ 6B10,0001T (K)

-0.85 L.-----I._-'-_...L..._.L----'_-l.._-'7.0 0.0 0.050.100.150.200.250.300.35

H20 in Silicate Melt (Mole Fraction)

P= 1.2 GPa

-=t= Anhydrous

++++3.3 -6.3 wt%

H<!O

® ,60.68 0.70 0.72 0.74 0.76

Molar Mg/(Mg+Fe) of Silicate Melt

Fig. 4a, b Plots of molar Mgj(Mg + Fe) ratio versus the weightconcentrations of: a FeO; b MgO, comparing the compositions ofnominally anhydrous (filled symbols) and H20-bearing (open symbols)silicate melts saturated with a spinel lherzolite assemblage at 1.2 GPaand 1185-1345 DC.Water has been included as a component in allliquid compositions. Symbols are the same as in Fig. 2 (Vertical errorbars 1o values from Tables 3 and 5; horizontal error bars 1o valuescalculated by propagating the standard deviation of the mean for FeOand MgO from Tables 3 and 5)

tive correlation between lny~f6 and the mole fraction ofH20 dissolved in the liquid (Fig. 3b). This correlation isalso evident in the hydrous basalt-peridotite sandwichexperiments of Kushiro (1990) (open squares), and thecombined database suggests that y~t62 increases con-tinuously up to ~30 mol% H20. The positive correla-tion in Fig. 3b indicates a progressively smallerdeviation from ideal mixing for Si02 with increasingH20, and is the opposite of what would be expectedfrom the formation of hydroxyl groups through the in-teraction of H20 with bridging oxygens (e.g., Kushiro1975; Stolper 1982). Although it is possible that this

••Fig. 3 a Plot illustrating the relationship between InY~:6 and inversetemperature for experimentally produced anhydrous (jilled circles)and hydrous (open circles) silicate melts saturated with a mantleperidotite mineral assemblage (Oliv + Opx ± Cpx ± Sp) at1.2 GPa and 1170-1345 DC. Data are from this study and fromKinzler and Grove (l992a). The Iny~i6 values and the temperaturedependence of Iny~i6 from an anhydrous liquid (solid line) werecalculated as discussed in the text. b Plot illustrating the relationshipbetween the mole fraction of H20 dissolved in a silicate melt andIn y~i6 for nominally anhydrous (filled symbols) and H20-bearing(open Iymbols) silicatemelts saturated with a mantle peridotite mineralassemblage (Oliv + Opx ± Cpx ± Sp) at 1.2 GPa and 1250 DC.Activity coefficients have been corrected to a single intermediatetemperature (1250 DC)as discussed in the text. Open squares representexperimentally produced H20-bearing silicate liquids from Kushiro(1990). Circles represent data from this study and from Kinzler andGrove (1992a). The standard state free energy of pure SiCh liquid atthe pressure and temperature of interest was determined by firstcalculating the Gibbs free energy of ~-cristobalite at its 0.1 MPamelting point (1999 K) using thermochemical data from Berman(1988). The Gibbs free energy of pure SiCh liquid at the pressure andtemperature of interest was then calculated using enthalpy of fusionand heat capacity data from Richet et al. (1982) and molar volumedata from Richet et al. (1982), Lange and Carmichael (1990), andGhiorso and Sack (1995). The heat capacity was assumed to betemperature dependent below the glass transition (1206.85 DC)

335

12 ,Anhydrous ~

+ --9-

i 11--+-

-+(j) +-:22 10C1lg 3.3 - 6.3 wt%

Ui -+- H<!O.s 90Cl --<>-:2

P= 1.2 GPa@8L---..L_-'----''---L_...J....---''---.L.---l0.68 0.70 0.72 0.74 0.76

Molar Mg/(Mg+Fe) of Silicate Melt

trend results directly from interactions between Si02 andH20 in the melt, it may also be produced indirectlythrough variations in melt composition or the speciationof melt components related to increasing H20. For ex-ample, there is a tendency for y~t62 to decrease with in-creasing alkalis in anhydrous silicate melts (e.g., Kushiro1975;Ryerson 1985;Hirschmann et al. in press), and it ispossible that H20 affects the speciation of alkalis in sucha way as to reduce this effect. It is also possible that thehigher concentrations of CO2 in our hydrous melts have

Lan effect on Yst62, but the agreement between our results

and those of Kushiro (1990) argue against it beingsignificant.

Figure 4a and b are plots ofwt% FeO and wt% MgOversus molar Mgj(Mg + Fe) comparing the composi-tions of our experimentally produced hydrous and an-hydrous silicate melts in equilibrium with a spinellherzolite assemblage at 1.2 GPa. As was the case withSi02, the presence of dissolved H20 produces significantdecreases in the concentrations of both FeO and MgO.Thermodynamic controls on the MgO and FeO contentsof hydrous peridotite partial melts can be identified byconsidering the molar Olivjmelt partition coefficients forthese elements (DOlivjMelt= XOlivjXMelt). Figure 5 con-

MO'j MO MO 'jtains plots of (a) log D~~~Meltand (b) log D~~~Meltversusinverse temperature, showing Oliv-melt pairs from bothanhydrous and H20-bearing experiments performed atpressures of 0.9 to 2.0 GPa and temperatures of 1170 to1370 "C. The hydrous and anhydrous experiments fallalong the same trend for the partitioning of both FeO andMgO between Oliv and liquid. This suggests that thedominant control on the FeO and MgO contents of thesemelts is temperature, and that because hydrous melts arein equilibrium with olivine of a given composition at sig-nificantly lower temperatures than anhydrous melts, theirOlivjsilicate melt partition coefficientsare larger and theyhave lower FeO and MgO contents.

The relative importance of temperature versus dis-solved H20 in controlling the FeO and MgO contents ofperidotite partial melts can be quantified by consideringa formation reaction of the form:

2

336

0.30 o Hydrous Melts (This Study)• Anhydrous Melts (This Study;

Kinzler & Grove, 1992) -¢-

0; 0.20

~ aB ~0010Ol ..Q

0.00

®-0.10

6.0 6.2 6.4 6.61O,OOOfT (K)

6.8

Fig. Sa, b Plots of: a log D~~~/Melt;b log D~i~Melt versus inversetemperature comparing nominally anhydrous (/!tIed circles) and H20-bearing (open circles) silicate melts saturated with a mantle peridotitemineral assemblage of Oliv + Opx ± Cpx ± Plag ± Sp ± Gt.Experimental conditions are pressures of 0.9-2.0 GPa and tempera-tures of 1170-1370 DC.Data are from this study and from Kinzler andGrove (1992a). Partition coefficients were calculated on a molar basisusing the liquid components of Bottinga and Weill (1972), with allH20 treated as molecular H20. The data from Kinzler and Grove(1992a) included in this comparison are limited to experimentallyproduced liquids with Na20 contents (1.35-3.87 wt%) similar tothose of the silicate liquids produced in this study (1.94-3.59 wt%).The experiments of Kinzler and Grove (1992a) with anomalously lowOlivjmelt exchange K~e/Mg values (:0:0.26) have been excluded.(Vertical error bars 2ITvalues calculated by propagating the standarddeviation of the mean for FeO and MgO values from Tables 3 and 5,horizontal error bars ± 10 DC for experiments from this study and± 15 DCfor experiments from Kinzler and Grove 1992a). Where errorbars are not shown, they are smaller than symbols

where Mis Mg or Fe. Longhi et al. (1978) showed thatthe equilibrium constant for this reaction (Keq) can berearranged to give the molar Oliv/melt partition coeffi-cients for MgO and FeO:

X01iv 1/2 LiqDOliv/Melt= MO = 0.667(K aLiq) ~

MO XLiq eq S102 yMO MSio,S02

where Xko is the mole fraction of either FeO or MgO inOliv or silicate melt, and MSio.s02 is either forsterite orfayalite. Equation 3 shows that the FeO and MgOcontents of a silicate melt in equilibrium with mantleolivine of a given composition will be dependent on Keq,

on a~:6' and on the ratio of the activity coefficients forFeO or

2MgO in the liquid and for fai;alite or forsterite in

the Olivo Of these variables, only y~'bcould directly beaffected by the presence of H20 dissolved in a melt inequilibrium with a mantle peridotite mineral assem-blage.

In the same way that Eq. I was used to calculateLiq h .ySi02' t e reaction:

Mg2SiO~liv = 1Mg2Si20~PX + MgOLiq 4

was used to calculate the activity coefficient for MgO(y~io) in each of the experimentally produced liquidsshown in Fig. 5. The results from these calculations areshown in Fig. 6. In contrast with the results from the

0.80

P=0.9 -2.0 GPa0.70

~~ 0o ~0.60oOl..Q

0.50

0.40

7.0 6.0 6.2 6.4 6.610,000fT (K)

6.8 7.0

Si02 calculations, which show that Y~:62varies stronglyas a function of the concentration of dissolved H20, thecalculations for MgO indicate that the dominant controlon Y~io is temperature. If any dependence of Y~io on

-1.5

o Hydrous Melts (This Study)

• Anhydrous Melts (This Study;Kinzler & Grove, 1992)

~o:3" ~ -2.5<-.£:

3-3.5

6.0 6.8 7.06.2 6.4 6.6

10,OOOfT (K)

Fig. 6 Plot illustrating the relationship between InY~io' calculated asdiscussed in the text, and inverse temperature for nominallyanhydrous (filled circles) and H20-bearing (open circles) silicate meltssaturated with a mantle peridotite mineral assemblage (Oli-v + Opx ± Cpx ± Plag ± Sp ± Gt) at 0.9-2.0 Gpa and 1170-1370 DC.Data are the same as in Fig. 5 and are from this study andfrom Kinzler and Grove (1992a). The standard state free energy ofpure MgO liquid at the pressure and temperature of interest wasdetermined by first calculating the Gibbs free energy of periclase at its0.1 MPa melting point (3105 K) using thermochemical data fromBerman (1988). The Gibbs free energy of pure MgO liquid at thepressure and temperature of interest was then calculated using theenthalpy of fusion and heat capacity from the JANAF tables (Chaseet al. 1985). Due of the lack of volume data for molten MgO, thefollowing values were adapted on the basis of the results of Cohen andGong (1994), Lange and Carmichael (1987), and Kress andCarmichael (1991): V = 1.626 Jjbar at 0.1 MPa and 3105 K;dV/dT = 2.62 x 10-4 Jjbar K; dV/dP = -2.00 x 10-6 Jjbar2 at1673 K; (dV/dP)/dT = -1.30 x 10-8 Jjbar2 K. The heat capacityof molten MgO was assumed to be independent of temperature

Table 8 Anhydrous basis comparison of variations in spinel lher-zolite-saturated melt compositions as a function of varying pressureand dissolved H20. (I increases, D decreases, C constant)

Oxide Constant F Constant MgO

Increasing H20 at constant PSi02 I IAl203 I IFeO D DMgO DCaO C INa20 C DIncreasing P at constant H2OSi02 D DAl203 C IFeO I IMgO ICaO C DNa20 C IIncreasing P, anhydrous"Si02 D DAl203 D CFeO I IMgO ICaO C DNa20 C I

"Determined on the basis of comparison of batch melt composi-tions calculated using the method of Kinzler (1997)

dissolved H20 exists, it is negligible. Therefore, tem-perature represents the dominant control on the MgO,and by analogy FeO, contents of moderate extent an-hydrous and hydrous peridotite partial melts. The dif-ference in the mixing behaviors of Si02 and MgO withrespect to H20 is plausibly related to the role of theformer as a network former and of the latter as a net-work modifier in silicate melts.

Having identified some of the important thermody-namic controls on the compositions of H20-bearingperidotite melts, we now examine their compositionalsystematics using liquid compositions that have beennormalized to 100% on a volatile-free basis. The ap-parent compositional variations in lherzolite-saturatedmelts as a function of H20 content are summarized inTable 8 and in Figs. 7 and 8. For comparison, Table 8also lists the melt compositional variations associated

337

with increasing pressure under anhydrous conditionsand at a constant concentration of H20 dissolved in themelt. From the systematics presented in Table 8 it isevident that, at a given extent of partial melting, in-creasing the amount of H20 dissolved in the melt at asingle pressure, and increasing the pressure at whichmelting occurs under anhydrous conditions have oppo-site effects on normalized melt compositions. The ap-parent composition of a hydrous partial melt formed at1.6 GPa is nearly the same as that of an anhydrouspartial melt formed at 1.2 GPa. Kushiro (1990) reacheda similar conclusion on the basis of basalt-peridotitesandwich experiments.

Figure 7 contains plots of wt% Si02 versus totalFeO + MgO, in wt%, comparing the compositions ofhydrous and anhydrous 1.2 GPa spinel lherzolite-satu-rated melts with H20 included in the compositions(Fig. 7a), and on an anhydrous basis (Fig. 7b). WhenH20 is included in the composition there is a positivecorrelation between Si02 and total FeO + MgO. Thistrend is opposite to that expected from an expansion ofthe Oliv stability volume, as shown by the FOs9 Olivcontrol line in Fig 7a. When the melt compositions arenormalized on an anhydrous basis, however, the ele-vated Si02/(MgO + FeO) ratio of the hydrous meltsproduces an apparent expansion of the Oliv stabilityfield, as illustrated by the relative positions of hydrousand anhydrous melt compositions along the FOs9 Olivcontrol line shown in Fig. 7b. This is also seen in nor-mative projection schemes, such as that shown in Fig. 8.The lowest normative Oliv is found in the melt produced

Fig. 7a, b Plot of the weight concentration of Si02 versus totalweight concentration of FeO + MgO, comparing experimentallyproduced anhydrous (filled symbols) and hydrous (open symbols)silicate melts saturated with a spinel lherzolite assemblage at 1.2 GPaand 1185-1345 DC: a with H20 included in the composition; b on avolatile-free basis. Solid line illustrates the effect of addition orsubtraction of F089 Oliv from the volatile-free composition of theliquid from experiment B303. Experiments performed using mixturesof 82-72f basalt and PUM peridotite are represented by circles.Experiment performed using a mixture of DM151 basalt analog andPUM peridotite is represented by a triangle. Error bars are lIT fromTables 3 and 5

21 21~ Comparison of 1.2GPa 'l Comparison of 1.2GPa~0 Melt Compositions Melt ComJr0sitions +!. 20 on a Hydrous Basi~ !. 20 n an Anhy reus Basis- -iii iii Anhydrous:2 19 Anhydrous :2 19.S! .S!g18 + g

18i:75 i:75.s -9-+ .s0 17 0 17Ol i 3.3 -6.3wt% Ol:2 :2+ H20 F089 Olivine +0 16 + Control Line 0 16(])

® (])LL LL @

15 1544 45 46 47 48 49 44 45 46 47 48 49

Si02 in Silicate Melt (wt'%) Si02 in Silicate Melt (wt%)

338

Cpx Cpxso

PlagAo1"

Plag Olivso

Projected FromSi02 and H20

(Oxygen Units)

Melts Saturated WithOliv + Opx + Cpx + Sp

at 1.2 GPa

3.3 to 6.3 wt% ~H20 A. cro Anhydrous

Fig. 8 Pseudoternary projection from Si02 and H20 onto the Plag-Cpx-Oliv plane comparing the compositions of nominally anhydrous(filled symbols) and H20-bearing (open symbols) experimentallyproduced silicate liquids saturated with a spinel lherzolite assemblageat 1.2 GPa and 1185-1345 "C. Experiments performed using mixturesof 82-72f basalt and PUM peridotite are represented by circles.Experiment performed using a mixture of DM151 basalt analog andPUM peridotite is represented by a triangle. Projection scheme is fromTormey et al. (1987)

in the experiment performed using the synthetic basalt(DM151) with a higher total alkali content (open tri-angle). Once again it is evident that the change in theapparent composition of lherzolite-saturated silicatemelts associated with increasing dissolved H20 is thesame as that associated with decreasing pressure.

Our experimental results indicate that the apparentincrease in partial melt Si02 content associated with 3.3-6.3 wt% H20 is only ~1 wt%, significantly less thanwould be necessary to produce andesitic magmas di-rectly from partial melting of hydrous mantle peridotite.A similar conclusion can be drawn from an examinationof the liquid compositions produced in the hydrousbasalt-peridotite sandwich experiments of Kushiro(1990), although the apparent Si02 contents of Ku-shiro's 1.2 GPa melts (~50-51 wt%) are systematicallyhigher than ours (~48-49 wt%) at comparable H20contents. The apparent compositions of hydrouslherzolite-saturated melts produced at 1.0 GPa by Hi-rose and Kawamoto (1995) are also basaltic (~50-53 wt% Si02), with higher Si02 occurring in the moresodic melts. Recent experimental studies are, therefore,consistent with the idea that the material added to thecrust through subduction zone magmatism is domi-nantly basaltic in composition (Grove and Kinzler1986). The one exception to this is the recent study ofHirose (1997), in which normalized hydrous melt com-positions contain ~54-60 wt% Si02. Despite compara-ble concentrations of alkalis and dissolved H20, themelts produced in our experiments have apparent Si02

concentrations that are ~6-7 wt% lower than an H20-undersaturated melt composition (54.4 wt% Si02;

~6.3 wt% H20) reported by Hirose (1997). When cor-

rected to an intermediate temperature (1198 "C), theY~:62value calculated for this melt composition is ~20%lower than expected on the basis of the combined ex-perimental database presented in Fig. 3b. Although theexperiments of Hirose (1997)were performed at 1.0 GPaand those in Fig. 3b were carried out at 1.2 GPa, a de-crease in pressure should produce an increase in Y~:62'The elevated Si02 contents reported by Hirose (1997),therefore, do not appear to be consistent with our ex-perimental results or those of Kushiro (1990), and thereason for this discrepancy is unknown.

Peridotite melting reactions under hydrous conditions

A quantitative understanding of the stoichiometry ofperidotite melting reactions can be used to place limitson the compositions of partial melts and their mantleresidues (Kinzler and Grove 1992a), on the behavior oftrace elements during partial melting (Walter et al.1995), and on the modal mineralogy of partial meltingresidues (Kinzler et al. 1993). The stoichiometries ofanhydrous peridotite melting reactions have been de-termined in several experimental studies (Kinzler andGrove 1992a; Walter and Presnall 1994; Baker andStolper 1994; Walter et al. 1995; Kinzler 1997). Ourexperiments illustrate the important control exercised bythe presence of dissolved H20 on the stoichiometries ofperidotite melting reactions.

Melting of anhydrous spinel lherzolite at moderatepressures (~1.0 to 1.7 GPa) occurs through a peritecticreaction in which Cpx, Opx, and Sp are consumed toproduce melt + Oliv (e.g., Kinzler and Grove 1992a;Baker and Stolper 1994).At higher pressures, changes inthe compositions of coexisting Cpx and melt lead toprogressive changes in the reaction stoichiometry (Stol-per 1980). First, at pressures of ~1.7 to 2.0 GPa, Opxswitches from being consumed during lherzolite meltingto being produced. At still higher pressures Oliv, Cpx,and Sp are consumed during anhydrous peridotitemelting, producing melt + Opx (Kinzler 1997).

The experimentally determined stoichiometric coeffi-cients for spinel lherzolite melting reactions under hy-drous and anhydrous conditions are compared inTable 9. High-Ca clinopyroxene dominates the mass ofmaterial entering the melt under anhydrous conditionsat 1.0 and 1.2 GPa. The spinel lherzolite melting reac-tions determined at 1.0 to 1.2 GPa from our experimentsand from those of Hirose and Kawamoto (1995)indicatethat under hydrous conditions the proportion of Opxentering the melt increases relative to Cpx. Figure 9aand b provide a graphical representation of this throughthe use of pseudo-ternary projections from Sp onto theOliv - Cpx - Si02 plane. The 1.2 GPa spinel lherzolitemelting reaction is represented by a tie-line connectingthe compositions of Oliv and silicate melt that pierces acompositional plane defined by Cpx, Opx, and Sp. Thelocation of the piercing point relative to the apices ofthis compositional triangle represents the relative pro-

Table 9 Comparison of experimentally determined stoichiometriccoefficients for spinel lherzolite melting reactions under anhydrousand hydrous conditions. Uncertainties reported for coefficients

339

calculated in this study were determined by propagating the stan-dard errors from the mass balance results reported in Table 2

Experiments P !1T XCpx

(GPa) (DC)

Anhydrous20_24a,c 1.0 1270-1330 0.71(5)B30-B52b,d 1.2 1300-1315 1.08(7)H179-H200b,d 1.6 1320-1340 1.34(2)Ll16- Ll20a,e 1.9 1401-1416 1.16(11 )Hydrous38-41 a.f 1.0 1200-1300 0.64(5)B329-B330a,g 1.2 1185-1200 0.62(9)

XOpx XOliv XSp

0.38(5) -0.22(6) 0.13(1)0.17(10) -0.36(4) 0.07(2)

-0.15(1 ) -0.25(2) 0.07(1)-0.38(5) 0.14(3) 0.09(3)

0.50(11) -0.16(8) 0.02(3)0.51(14) -0.25(9) 0.12(3)

a Coefficients determined using change in phase proportionsb Coefficients estimated using 2 liquid methodc Baker and Stolper (1994)dKinzler and Grove (1992a)

portions of Cpx, Opx, and Sp involved in the meltingreaction. The compositional differences between Cpxand Opx coexisting across the two-pyroxene phase re-gion under hydrous and anhydrous conditions producesa shift of the Oliv-melt tie-line piercing point away fromthe Cpx apex, decreasing the proportion of Cpx enteringthe melt. The stoichiometric coefficients for Cpx andOpx in the hydrous spinel lherzolite melting reactionsare sub-equal (Table 9). This change is in response to thelower temperatures at which hydrous peridotite meltingtakes place relative to anhydrous melting. The effects oftemperature on the compositions of Cpx and Opx co-existing across the pyroxene two-phase region are illus-trated in Fig. 9c, where pyroxene compositions from ananhydrous experiment performed at 1.2 GPa and1315 °C are compared with those from experimentsperformed at 1.2 GPa and 1200 °C in which the meltcontained 5.06 ± 0.04 wt% H20, and at 1.2 GPa and1115 °C with ~12 wt% H20 dissolved in the melt. Atlower temperatures the pyroxene two-phase region iswider, the CaO content of the Cpx is greater, and theOpx is less calcic, changing the stoichiometry of thespinel lherzolite melting reaction.

At 1.6 GPa, anhydrous, the compositions of coexistingCpx and melt are such that Opx is beginning to appear onthe product side of the melting reaction (Table 9). Ourexperimental results indicate that under hydrous condi-

Cpx

Oliv

"Kinzicr (1997)f Coefficients calculated from phase compositions reported byHirose and Kawamoto (1995)gThis study

tions the Cpx is more calcic and the nature of the meltingreaction is the same as that determined at 1.2 GPa (i.e.,only Oliv + liquid are produced), although the Cpx co-efficient is larger at 1.6 GPa. At 2.0 GPa and 1275-1290 °C, the hydrous melting experiments produced sili-cate melts saturated with a Gt lherzolite assemblage. Theinferred hydrous peridotite melting reaction at this pres-sure is one in which Cpx + Opx + Gt react to produceOliv + melt. This differs from the anhydrous Gt lherzo-lite melting reaction determined by Kinzler (1997) at2.3 GPa and 1430-1440 °C, in which Cpx + Oliv + Gtreact to produce Opx + melt.

Two of the hydrous melting experiments performedat 1.6 GPa produced silicate melts in equilibrium with alherzolite assemblage in which Sp and Gt coexist. Theseexperiments allow the nature of hydrous peridotite

Fig. 9a-e Pseudoternary projections from Sp onto the Oliv-Cpx-Sit),plane illustrating the stoichiometry of spinel lherzolite meltingreactions at 1.2 GPa under: a hydrous; b anhydrous conditions; andc the systematic change in the compositions of experimentallyproduced pyroxenes with increasing H20 dissolved in the silicatemelt and decreasing temperature. The experiment at 1315 DC isnominally anhydrous, while the experiment at 1200 DC has5.06 ± 0.4 wt% H20 dissolved in the melt, and the experiment at1100 DChas ~ 12 wt% H20 dissolved in the melt. Projection scheme isfrom Kinzler (1997)

Cpx Cpx

/\ 1115DC

C/ ~~200DCpx \

1315DC

Projected FromSpinel

(Oxygen Units)

Opx

Effect of Ton PyroxeneCompositions

340

melting reactions at pressures corresponding with thespinel to garnet transition to be inferred. Such alherzolite melting reaction, involving both Sp and Gt, isshown schematically in Fig. 10. The phases entering themelt are represented by filled circles, while the opencircles are the phases being produced. The coexistence ofSp and Gt produces a compositional tetrahedron withthe Cpx-Opx-Sp plane as its base and Gt as its apex. Thepoint at which the Oliv-melt tie-line pierces the base ofthe tetrahedron defines a Sp-lherzolite melting reactionsimilar to those listed in Table 9. The point at which theOliv-melt tie-line pierces Cpx-Opx-Gt compositionalplane defines a Gt-lherzolite melting reaction similar tothe one described above. The overall lherzolite meltingreaction is a combination of these two reactions in whichCpx, Opx, Sp, and Gt react to form Oliv and melt withincreasing temperature.

The spinel to garnet transition

The pressure at which ascending mantle peridotite that isat or above its solidus intersects the Sp to Gt transitioninfluences both the major and trace element composi-tions of partial melts by controlling the amount ofmelting that occurs within the Gt stability field (Kay andGast 1973; Salters and Hart 1989; Kinzler 1997). Thepressure of intersection of the anhydrous peridotitesolidus with the Sp to Gt transition is poorly constrainedby existing experimental data, but has been estimated tobe ~2.5 GPa (Hirschmann and Stolper 1996). Becauseof the pressure-temperature dependence of the transi-tion, Gt should be stable to lower pressures in tectonicenvironments where the mantle potential temperature isrelatively low, such as subduction zones. Given the sol-idus-lowering effect of H20 on mantle peridotite, it islikely that the amount of melting that occurs within the

Oliv

Hydrous Sp + GtLherzolite Melting

at 1.6 GPa

Fig. 10 Schematic illustration of melting reaction for hydrousperidotite containing coexisting Sp and Gt at 1.6 GPa. Filled circlesare solid phases being consumed during melting and open circles arephases being produced. Lightly-shaded triangle is compositional planeformed by coexisting Cpx + Opx + Sp. Darkly-shaded triangle iscompositional plane formed by coexisting Cpx + Opx + Gt

Gt stability field is greater in subduction zones than it isbeneath mid-ocean ridges. Our experimental resultsprovide an estimate of the magnitude of this effect.

A comparison of the mineral assemblages in our hy-drous and anhydrous experiments shows that, for aconstant silicate bulk composition, Gt joins the mantleresidual assemblage at a lower pressure under hydrousconditions. Spinel and Gt coexist at 1.6 GPa and 1255-1260 °C in hydrous experiments, while only Sp occurs at1.6 GPa and 1355-1370 °C under anhydrous conditions.Garnet is the stable aluminous phase in the hydrousexperiments performed at 2.0 GPa and l275-1290°C.Although the relative stabilities of Sp and Gt are sensi-tive to the Cr content of Sp and the Fe content of theminerals (O'Neill 1981), the compositional similaritiesbetween the phases in the hydrous and anhydrous1.6 GPa experiments suggest that temperature is theimportant variable in this case. This possibility was in-vestigated using the results of Kinzler (1997), who per-formed melting experiments on bulk compositionssimilar to ours (i.e., intermediate to basalt and perido-tite) and found that Gt and Sp coexist with Oliv, Cpx,Opx, and anhydrous silicate melt at 1.9 GPa and1396 0C. Extrapolating this to 1.6 GPa using the dTjdPslope for the Sp to Gt transition in natural peridotite(~500 °CjGPa) (O'Hara et al. 1971)gives a temperatureof 1246 °C, in reasonable agreement with our experi-mental conditions (1255-1260 0C). Although it is pos-sible that at a given temperature the Sp to Gt transitionoccurs at a higher pressure for a peridotite bulk com-position than in experiments such as ours, the solidus-lowering effect of H20 will stabilize Gt in the meltingregime to pressures that are several hundred MPa lowerthan under anhydrous conditions.

Isobaric batch melting of hydrous mantle peridotite

Various subduction zone thermal models have beenproposed, and although they all predict a downwarddeflection of mantle isotherms in the vicinity of the slab,model mantle wedge temperatures are strongly depen-dent on assumed heat sources and sinks (McKenzie1969; Oxburgh and Turcotte 1970; Toksoz et al. 1971;Anderson et al. 1976; Davies and Stevenson 1992). In-dependent estimates of temperatures in the mantlewedge from experimental studies of primitive arc basaltsare much higher than those predicted by thermal mod-els, even if the effects of H20 are taken into account(e.g., Tatsumi et al. 1983; Baker et al. 1994). A quanti-tative understanding of the influence of H20 on meltgeneration has the potential to place new constraints onmodels of the dynamics of subduction zones and toreconcile petrologic observations with subduction zonethermal models. Although melting of anhydrous mantleperidotite beneath mid-ocean ridges is thought to be apolybaric, near-fractional process (e.g., Klein and Lan-gmuir 1987;Johnson et al. 1990),isobaric batch meltingof hydrous mantle peridotite provides basic insights into

melt generation processes at subduction zones. Here weinvestigate the effects of H20 on the temperature atwhich batch melts coexist with mantle peridotite at1.5 GPa, on isobaric melt production rates, and on theextent to which peridotite partially melts at a givenpressure and temperature. .

Taking advantage of the observation that y~io is notdirectly affected by the presence of dissolved H20, wedeveloped an approach to modeling the liquidus tem-peratures of isobaric batch melts of hydrous mantleperidotite using the Oliv/melt partitioning of MgO. Thisapproach allowed us to exploit an extended database ofexperimentally produced Oliv-saturated melt composi-tions from the literature consisting of 69 hydrous and 65anhydrous melts coexisting with olivine at temperaturesof 940 to 1511 °C, and pressures of 0.1 MPa to 2.3 GPa(Kinzler and Grove 1992a; Sisson and Grove 1993a,b;Gaetani et al. 1994;Hirose and Kawamoto 1995;Wag-ner et al. 1995; Kinzler 1997; Grove et al. 1997). Theconcentrations of K20 (0.04-2.04 wt%) and of Na20(0.78-4.7 wt%) in the hydrous melts are broad enoughto allow meaningful liquidus temperature calculations tobe carried out for low extent partial melts. Details of thecalculation procedure are presented in an appendix.

The results from liquidus temperature calculationsfor an anhydrous peridotite (solid curve), for a peridotitecontaining 0.15 wt% H20 (dashed curve), and for aperidotite containing 0.32 wt% of an H20-Na20-K20subduction component (SC) (dotted curve) are shown inFig. 11. The peridotite composition used in these cal-culations is the depleted MORB source of Kinzler(1997), and the SC composition was derived from theH20-rich Mariana component of Stolper and Newman(1994)by normalizing the concentrations of H20, Na20,and K20 to 100%. The liquidus temperatures of theanhydrous partial melts increase systematically from1330 °C at 2% partial melting to 1400 °C at 20%, de-fining a temperature-melt fraction (T-F) curve that isconcave-downward. The temperature of the H20-bear-ing peridotite must be raised by ~120 °C, and that of theSC-bearing peridotite by ~200 °C, in order to increasethe extent of partial melting from 2 to 5%. These tem-perature increases are ~4-7 times larger than is requiredin the anhydrous case (~30 0C) and result primarilyfrom a dilution of the concentration of H20 dissolved inthe melt with increasing extent of partial melting. In-creasing the partial melting from 5 to 20% for the H20-bearing peridotite requires that the temperature beraised by ~90 °C and that of the SC-bearing peridotitebe raised by ~ 130°C, while the temperature increaserequired under anhydrous conditions is only ~40 "C.

The large temperature increases required to partiallymelt H20- and SC-bearing peridotite are a reflection oftheir relative isobaric melt production rates, (oF joT)p,or the change in melt fraction for a given change intemperature. Isobaric batch melting rates were calcu-lated for each of the T-F curves in Fig. 11and are shownin Fig. l2a and b, plots of log(oF joT)p versus F and oflog(oF joT)p versus temperature. For the anhydrous

341

891500

Forsterite in Olivine (Mole%)

90 91

1400 AnhydrousPeridotite - -

f) 1300/

// ,:

Peridotite +0.15wt% Hp

1100Peridotite + 0.32 wt%Mariana Subduction

Component

1000

o 0.15 0.200.05 0.10

Melt Fraction

Fig. 11 Plot of melt fraction versus temperature comparing theliquidus temperatures of anhydrous batch melts of a depleted MORBmantle source at 1.5 GPa (solid curve) calculated using the methods ofKinzler (1997) with the liquidus temperatures of hydrous batch meltsfrom peridotite containing 0.15 wt% H20 (dashed curve) and0.32 wt% of simplified Mariana subduction component (dottedcurve). Liquidus curves for H20-bearing batch melts were calculatedas described in the text

peridotite, the isobaric melt production rate is 0.05%;oCat 2% partial melting and increases continuously withincreasing extent of melting to 0.8%;oC at 20% partialmelting. The H20-bearing peridotite has a (oF joT)p at2% partial melting (0.01%;oC) that is a factor of 5 lowerthan the anhydrous case, while the melt production ratefor the SC-bearing peridotite (0.007%;oC) is lower by afactor of 7. The melt production rates for both the H20-and SC-bearing peridotites increase faster as functionsof F than the anhydrous case, so that at 20% partialmelting they differ from the anhydrous (oF joT)p byapproximately a factor of 2. If (oF joT)p is considered asa function of temperature, the melt production rates forthe H20- and SC-bearing peridotites are higher than forthe anhydrous peridotite at a given temperature(Fig. l2b). This is due to the increase in (oF joT)p withincreasing F, and reflects the increased extent of meltingfor the H20- and SC-bearing peridotites at a giventemperature relative to the anhydrous peridotite.

Asimow et al. (1997) showed that the most significantfactors for controlling the polybaric melt productionrate during isentropic (adiabatic) ascent of mantleperidotite are the isobaric productivity, (oF joT)p, andthe pressure-temperature slopes of constant melt frac-tion contours, (oT joP)F' We calculated (oT joP)F valuesat 1.5 GPa for the 5% melt contour of anhydrousperidotite (1.9 °C/GPa) and of peridotite containing0.15 wt% H20 (1.6 °CjGPa), and combined them with

342

Forsterite in Olivine (Mole%)89 90 91

-1 -1

Peridotite +0.15wt%H20

-2

LLII-'o..~'b~ -3.Q / .: '

/ ...""Peridotite + 0.32 wt%Mariana Subduction

Component

-4

®-5

o 0.05 0.10 0.15Melt Fraction

Fig. 12 a Plot of 10g(aF jaT) p versus melt fraction comparing theisobaric batch melt production rates of anhydrous peridotite (solidcurve) with peridotite containing 0.15 wt% H20 (dashed curve) and0.32 wt% simplified Mariana subduction component (dotted curve) at1.5 GPa. Curves were calculated by fitting a polynomial of the formT = Lc;F±n to the temperatures from Fig. 11, taking its derivativewith respect to F, then inverting to get (aF jaT)p. b Plot of10g(aF jaT)p versus temperature comparing the isobaric batch meltproduction rates of anhydrous peridotite (solid curve) with peridotitecontaining 0.15 wt% H20 (dashed curve) and 0.32 wt% simplifiedMariana subduction component (dotted curve) at 1.5 GPa

the (oF joP)p values from Fig. 12 to calculate the pro-ductivity, (-oF joP)s, of H20-undersaturated polybaricmelting relative to anhydrous melting using the pro-ductivity equation of Asimow et al. (1997). Thermo-chemical and volume data for diopside were used in thecalculation (see Table 1 of Asimow et al. (1997) andreferences therein); thus the absolute productivities areapproximate, but the relative values provide an estimateof the magnitude of the effect of H20 on polybaric meltgeneration rates. The results indicate that at 5% partialmelting the H20-undersaturated melting rate is ap-proximately half that for anhydrous melting. This dif-ference will increase at lower extents of melting anddecrease at higher melt fractions. Polybaric melt pro-duction is, therefore, an inefficient process for closed-system melting at fluid-undersaturated conditions. Theamount of melt that can be produced in a subductionzone through the breakdown of amphibole followed byadiabatic ascent of mantle peridotite is extremely limitedunless H20 is periodically added to the zone of meltgeneration.

The increase in melt fraction produced by increasingthe concentration of H20 in the peridotite,(oF jOCH20)p T' at 1.5 GPa and temperatures of 1200 to1350 °C is shown in Fig. l3a, a plot of melt fractionversus H20 in the peridotite. From this plot it can beseen that (oF jOCH20)p T increases with increasing tem-perature. For example, 'the addition of 0.25 wt% H20 toa peridotite at 1200 °C increases the extent of meltingfrom 0.5 to 3%, while at 1350 °C the melt fraction in-creases from 3 to 12%. Despite this temperature de-

-2

Peridotite + 0.32 wt% ~'Mariana Subduction .' ' Anhydrou~

Component . 'j Peridotite

~.,';........./ \-4

0.20-5

1000

Peridotite +0.15wt%H20. ,

1100 1200 1300 1400Temperature (DC)

1500

pendence, the (oF jOCH20)P T values indicated by ourcalculations are significantly lower than the value in-ferred by Stolper and Newman (1994) from a suite ofbasaltic glasses from the Mariana Trough, in which theaddition of 0.25 wt% H20 to the peridotite increases theextent of melting by ~ 15%. The difference between ourcalculations and the increase in extent of melting ob-served in the Mariana basalts may reflect several factors.As noted by Stolper and Newman (1994), in addition tothe effect of H20, the range of extents of melting rep-resented by the Mariana basalts could reflect a system-atic variation in temperature or the effect of the additionof alkalis from the H20-rich component to the perido-tite. We investigated the influence ofrefertilization of theperidotite on extent of melting, and the results are shownin Fig. l3b. At 1330 °C, the addition of any of the majorconstituents of the Mariana SC (H20; K20; Na20) isnot by itself sufficientto explain the observed increase inextent of melting. The curves in Fig. l3b show that theaddition of H20 (long dashes) has the strongest effect onmelt fraction, while the addition of K20 (short dashes)produces a smaller melt fraction increase, and Na20(dash-dot) has least effect on extent of melting. If,however, a H20-K20-Na20 SC is added to the perido-tite, the match between our calculated increase in extentof melting (dotted curve) and that inferred by Stolperand Newman (1994) (solid line) is very good.

There are two important conclusions concerningsubduction zone melt generation to be drawn from thesecalculations. First, the Mariana basalts can be explainedby a melt generation process in which they equilibratedwith mantle peridotite at nearly identical pressure-tem-perature conditions. We chose 1.5 GPa to carry out ourcalculations because it represents a reasonable minimumdepth for melt generation in the wedge, and the resultsdo not place rigorous constraints on the conditions ofmelt generation beneath the Mariana back arc. Thecalculations do, however, indicate that the combinedeffects of H20 and alkalis can account for the inferredmelting extents and that a systematic temperature in-crease is not required. Second, the influence of the ad-

343

0.30 ~ 0.60 Kp0.252 I

I

~ ~ N~I I /H 0 P=1.5 GPa ;g~ '1: I 2 \'3300C 0.20 !-

0.20 cf. 0.40 //2 2~ .!::

0.15 ~C I"0'1: (]) I / / Slope From '1:(]) I: (])Q. 0 Stolper & Newman Q.

0.

// 0.10 .s.s 0.10 E 0.20 (1994)0 0 0

or 0 . I '":r: I: II. : Mariana Subduction 0.05 :r:0

® is I .• ' Component@:J

"0.0 0 00.05 0.10 0.15 ~ 0 0.05 0.10 0.15 0.20 0.25

Melt Fraction Melt Fraction

Fig. 13 a Plot of melt fraction versus H20 in peridotite showing therelationship between these variables at temperatures of 1200-1350 DCand a pressure of 1.5 GPa. Curves were determined by calculatingtemperature versus melt fraction curves for peridotites containingvarious amounts of H20, then determining isotherms. b Plot of meltfraction versus subduction component in peridotite illustrating theeffects of adding H20 (dashes), Na20 (dash-dot), K20 (small dashes),and simplified Mariana subduction component to peridotite (dots).Curves for addition of K20 and Na20 were calculated using themethod of Kinzler (1997). Shown for comparison is the subductioncomponent versus melt fraction slope determined for basalts from theMariana back are by Stolper and Newman (1994)

dition of a subduction-related component to the mantlewedge on the melt generation process will be stronglydependent on the composition of the Sc. If it is alkali-poor, the amount of melt produced at a given pressureand temperature will be much less than would be gen-erated by the addition of a high-alkali Sc. The details ofthe melt generation process may, therefore, vary fromarc to arc, with the composition of material transferredfrom the subducted slab to the mantle wedge exercisingan important control on the amount of melt produced.

Acknowledgments The authors would like to thank M. Hirschmannand T. Sisson for thoughtful and constructive reviews. We are alsograteful to S. Bowring, F. Frey, P. Hess, and G. Hirth for com-ments on an earlier version of the paper, and to M. Baker, I. Eiler,M. Hirschmann, G. Hirth, R. Kinzler, S. Newman, and T. Sissonfor helpful discussions. We would like to thank 1. Kushiro forgenerously providing unpublished experimental data. N. Chatterjeeand M. Iercinovic kept the electron microprobe facility at MITrunning smoothly over the course of this study. We are grateful toS. Newman for performing the FTIR analyses, and to A. Kent forperforming the ion probe analyses. M. Ghiorso provided advice oncalculating Si02 activities, and timely assistance with the MELTS

Table Al Mineral/melt partition coefficients and weight-to-molarconversion factors used in liquidus temperature calculations. Cpx/melt and Opx/melt partition coefficients for Na20 are from this

supplemental calculator. The first author gratefully acknowledgesthe financial support of EM Stolper and PI Wyllie during finalmanuscript preparation. This research was supported by NSFgrants EAR-9406177 and EAR-9706214, and by an O.K. Earl PrizePostdoctoral Fellowship from the Division of Geological andPlanetary Sciences at the California Institute of Technology.

Appendix

The temperature-melt fraction calculations presented in this paperwere carried out using expressions describing the Oliv/melt parti-tioning of MgO:

X01iv 5404 ± 139InX~i~O = 7.53 ± 0.95 + T(K)

MgO

( L' )2- 1.76 ± 0.24 1 - XN~\I02

( L')2- 8.25 ± 0.97 1 - XK;(I02 Al

and of FeO:

X01iv 5559 ± 350In ;~~ = 7.29 ± 1.26 + T(K)

FeO

(P(bars) - 1)+ 0.017 ± 0.006 T(K)

( L' )2- 0.95 ± 0.29 1 - XN~\I02

( L,)2- 10.09 ± 1.20 1 - XK;(I02 A2

that were calibrated by performing stepwise multiple linear re-gressions to determine the significant variables (regression param-eter uncertainties are 10} Melt compositions in the calibrationdatabase were recalculated, on a molar basis, into the componentsof Bottinga and Weill (1972), molecular H20, and molecular CO2,

study; other partition coefficients are from Kinzler and Grove(1992a). (CH20 weight concentration of H20 in silicate melt, XH20molar concentration of H20 in silicate melt)

Mineral/melt partition coefficientsCpx0.250.001

Opx0.0310.001

Weight percent-to-mole fraction conversion factorsH20 Na200.037-0.0009CH20 0.022-0.017XH2o

K200.014-0.011XH2o

Oliv0.0010.001

Sp0.0010.001

344

Equation Al has an r2 value of 0.98 and, when rearranged topredict temperature, recovers the calibration database with a meanuncertainty of ± 24°C. Equation Al predicts the liquidus tem-peratures for the experimentally produced anhydrous spinellherzolite partial melts of Baker and Stolper q994) with a meanuncertainty of ± 20°C. Equation A2 has an r value of 0.96 andrecovers the Oliv /melt partition coefficients in the calibration da-tabase with a mean uncertainty of ± 7% relative.

Calculating the liquidus temperatures of hydrous peridotitepartial melts using Eq. Al requires knowledge of the equilibriumolivine composition, and the molar MgO, NaAI02, and KAI02contents of the melt. Equilibrium Oliv compositions were determinedby first calculating, at 1% intervals using the methods of Kinzler(1997), the compositions and liquidus temperatures of anhydrousbatch melts, ranging from 2 to 20% partial melting, generated at1.5 GPa from a depleted MORB source. Equations Al and A2 werethen used to calculate the compositions of Oliv in equilibrium witheach of these melts (F089.1-91.1). Because H20 dissolved in the meltdoes not have a significant effect on the Oliv /melt exchange K~e/Mg,itwas assumed that the variation in Oliv composition as a function ofextent of partial melting is identical for both hydrous and anhydrousperidotite melting. The large proportion of olivine in spinel lherzolite(~50 wt%) buffers its composition, so that the composition of oli-vine in hydrous and anhydrous peridotite should not be significantlydifferent between 2 and 20% partial melting.

The mole fraction of MgO in a given H20-bearing partial meltwas estimated from the MgO content of the anhydrous melt inequilibrium with the same Oliv using the approximation:

xHydrousLiq "'" (1 _ xHydrousLiq) XAnhdrousLiqMgO - H20 X MgO

When Eq. Al and A3 are used to calculate the liquidus tempera-tures for the H20-bearing melts from experiments B304, B330, andB359 from the MgO contents of the anhydrous melts from Ex-periments B303, B287, B394, the calculated temperatures are ingood agreement with the measured temperatures (1219 °C versus1215 DC; 1225 °C versus 1200 DC; 1267 °C versus 1260 0C). TheNa20, and K20 contents of the hydrous partial melts were calcu-lated using the batch melting equation and mineral/melt partitioncoefficients given in Table AI. Weight concentrations of H20,Na20, and K20 were converted to molar concentrations usingconversion factors determined on the basis of regressions of ourexperimental database (Table AI). Water was assumed to behaveas a perfectly incompatible element during partial melting.

The melting of amphibole lherzolite under fluid-absent condi-tions could not rigorously be incorporated into our calculations dueto the lack of experimental data on natural compositions. Themaximum stability of pargasitic amphibole, and thus the maximumtemperature at which it will be lost during partial melting, was es-timated from the experimental results of Holloway (1973). Themaximum stability of amphibole occurs when the H20 content ofthe coexisting melt is ~0.5 times the saturation value, or ~ 7-8 wt%at 1.5 GPa. The maximum temperature stability for endmemberpargasite at 1.5 GPa is ~ 1125 "C. We estimate, therefore, thatamphibole will be exhausted by ~ 1-2% partial melting for the bulkcompositions that we investigated, and calculations were begun at2% partial melting. Amphibole-bearing peridotite melts by a reac-tion in which amphibole and Opx are consumed to produce liquid,Cpx, Oliv, and Sp (Sisson et al. 1997). Because this reaction willproduce only ~1-2% melt in the bulk compositions investigated,the presence of amphibole at the solidus should not have a signifi-cant effect on the residual mode relative to melting of spinelperidotite.

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