ACTA UNIVERSITATIS CAROLINAE — GEOLOGICA 1998, 42 (1) pp. 181-188

Late-stage processes in P- or F-rich granitic magmas

J.D. WEBSTER', R. THOMAS2, I. VEKSLER3 , D. RHEDE2, R. SELTMANN2 AND H.—J. FORSTER2

1 Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79m Street, NY, NY 10024-5192 U.S.A.; E-mail• jdw@amnh.org

2 GeoForschungsZentrum Potsdam, P.O. Box 600751, D-14407 Potsdam, Germany; E-mail: thornas@gfz-potsdam.de, rhede@gfz-potsdam.de, seltm@gfz-potsdam. de, forhj@gfz-potsdam.de

3 Danish Lithosphere Centre, Oster Voldgade, 10, 1350 Copenhagen K, Denmark; E-mail: ivv@d1c.ku.dk

Abstract: To understand magmatic degassing of P, F, and Al, experiments were conducted to determine the solubility of P-, F-, and Al-rich silicate liquids in aqueous fluids at 2 kbar and 800 °C and the solubility of Ca-, Mg-, and F-rich liquids in K- and Al-rich silicate liquids at » 1 bar and 1100 to 1400 °C. The former experiments show significant solubility of P-, F-, and Al-rich liquid in fluid, and the latter experiments demonstrate that immiscible F-rich liquids exsolve from silicic liquids containing 4 wt. % F at these conditions. Integration of these new results with observations on two-phase (P-rich liquid and Si-rich liquid) bearing melt inclusions from P-rich granites suggests that under certain conditions immiscible P- ± F-rich liquids as well as late-stage aqueous fluids may be important in the release of P, F, and Al from granitic liquids which may explain the transport and deposition of these components to form fluorite, topaz, apatite, and triplite in greisens, hydrothermal miaroles, and hydrothermal veins associated with some granites.

INTRODUCTION

Mineralized granites and associated pegmatites and aplites exhibit extreme degrees of geochemical evolution; many, for example, are variably enriched in the fluxing components P, Al, and F. Mineralized granites also exhibit strong evidence of hydrothermal alteration even though it is difficult to know how much of the alteration occurs at magmatic conditions and how much is caused by late-stage hydrothermal fluids (i.e., at subsolidus conditions). Although hydrothermal fluids exert the predominant control during late-stage alteration of granitic magmas, it is theoretically plausible that magma compositions may be altered through interactions with stable, immiscible water-poor F-rich or water-poor P-rich liquids during the final stages of crystallization.

This report describes results of experiments that were conducted to determine the solubility of peraluminous, F-P-enrichcd silicate liquids in aqueous fluids at 2 kbar, and the results of experiments conducted at 1 bar to investigate immiscibility phenomena involving F-rich and Si-rich liquids. These new data are combined with information on immiscibility between P-rich and Si-rich liquids, based on observations of two-liquid bearing silicate melt inclusions, in order to better understand processes that may be involved in the transport of F, Al, and P from late-stage highly-evolved, peraluminous silicate liquids. In this report, the term "fluid" is used to describe a single supercritical aqueous phase, and "liquid" refers to molten aluminosilicate phases, molten water-poor fluoride-rich phases, or molten water-poor phosphate-rich phases. The

stability of aqueous vapour and liquid water relative to the stability of a single supercritical fluid is unknown for multicomponent systems enriched in F, P, and Al. Nevertheless, we believe that only one aqueous fluid phase was stable during our 2 kbar experiments, based on the experimental results of Bodnar et al. (1985) and Chou (1987).

EXPERIMENTAL METHODS

Fluid-silicate liquid experiments

Two peraluminous, P- and F-bearing glasses were prepared by mixing natural topaz rhyolite powder (8009) with reagent grade AIPO4 and by fusing the mixtures in sealed Pd30Ag70 capsules (Tab. 1). Roughly 35 to 45 mg of chunks (not powder) of these starting glasses were loaded into perforated 2.5 mm diameter Pd30Ag70 capsules, and the ends of the capsules were sealed. The perforated capsules had at least 50 holes (with diameters of r-t 100 p,m) through their surface. The glass-bearing 2.5 mm capsules were loaded into 4 5 mm diameter and 3 cm long Pd30Ag70 capsules along with 80 to 100 mg of doubly-distilled H2O. The larger capsules were crimpled shut, sealed with an are welder, and stored in a drying oven to test for leaks.

The capsules were pressurized and heated in an internally heated pressure vessel using experimental methods of Webster (1990). Two experiments (98-5A and 98-5B) were conducted at 2.02 kbar and 800 ± 15 °C for 142 hours; it is not known if these runs achieved an equilibrium distribution of all constituents between liquid

181

8009P 1 h 98-513, 98-3a 8009P2a 98-5Ar Start glass Run glass Fluid opts. Start glass Run glass

Si02 65.8 64.0 66.26 68.2 67.2 Al 20 3 16.3 15.9 13.78 14.8 14.8 Ca0 0.44 0.40 1.12 0.41 0.19 Na 20 4.31 4.11 6.84 4.27 4.27 K20 4.81 4.57 6.33 4.85 4.68 FeOg 0.91 0.98 1.33 1.21 0.62 Mg0 0.06 0.05 n.d. h 0.05 0.07 TiO2 0.03 0.04 n.d. 0.04 0.08 Mn0 0.05 0.04 n.d. 0.07 0.07 F 1,17 0.79 0.08 1.30 0.87 CI 0.13 0.01 n.d. 0.14 0.01 P20 5 4.35 4.27 3.85 2.25 1.85 TOTAL 98.4 95.2 99.66 99.64 94.7 ()MONK) , 1.24 1.19 0.68 1.14 1.19 Dpi 0.08±0.03 n.d. 0.19±0.02 DFk 0.33±0.06 n.d. 0.33±0.04 Wt.% DV 10 n.d. 11

Tab. 1. Compositionsa of starting glasses and run products and run conditions of 2 kbar silicate liquid-fluid solubility experiments.

a Compositions in weight percent; analyses by electron microprobe (see text).

d Composition of starting glass 8009P1 prepared from fused mixture of Spor Mountain topaz rhyolite 8009 and crystalline AIPO4 . Composition of water-saturated run product glass representing liquid phase of experiment 98-5A (using starting glass 8009P1); run conducted at

2.02 kbar and 800 ± 15 °C for 142 hours. d Composition (anhydrous basis) of glass representing fused solid

precipitates recovered from experiment 98-3 (using starting glass 8009P1); run conducted at 2.07 kbar and large temperature gradient of 675 to

800 °C across the length of the capsule for 232 hours. This is the time integrated composition of the fluid for a non-equilibrium run; see text for

discussion. e Composition of starting glass 8009P2 prepared from fused mixture of Spor Mountain topaz rhyolite 8009 and crystalline AIPO4 .

f Composition of run product glass representing liquid phase of experiment

98-5B (using starting glass 8009P2); run conducted at 2.02 kbar and 800 ± 15 °C for 142 hours.

9 Total iron as FeO. h Constituent not determined.

Molar ratio of [Al203/(Ca0 + Na20 + K20)] in glass. r Fluid/liquid distribution coefficient (± one sigma variability) for P computed

by mass balance using method of Webster and Holloway (1988). k Fluid/liquid distribution coefficient (± one sigma variability) for F computed

by mass balance using method of Webster and Holloway (1988). I Concentration (wt.%) of aluminositicate components dissolved in fluid

phase of experiment at run conditions.

and fluid. Another experiment (98-3) was conducted at 2.07 kbar under non-equilibrium conditions for 232 hours. One end of the large capsule for run 98-3 was held at 800 °C and the other end at 675 °C. The internal 2.5 mm capsule was situated at the hot end of the larger capsule and should have been at or near 800 °C during the experiment. All runs were quenched under isobaric conditions.

After each run, the capsules were cleaned, weighed, punctured, heated, and reweighed. The loss in weight observed after puncturing and heating the capsules represents the apparent mass of water in the fluid phase at run conditions. The capsules were cut open longitudinally and the solid precipitates of the fluid were recovered (this includes solids within the larger, outer capsule and solids adhering to the outer surface of the small, inner capsule). The inner capsule was removed, cleaned, and weighed. There was no visible evidence that silicate liquid flowed out of the inner perforated capsule, so all recovered solids are believed to represent fluid precipitates. The final weight of the perforated 2.5 mm capsule plus silicates, minus the weight of the metal capsule, represents the mass of silicate liquid at run conditions. The concentration of silicate liquid (and silicate minerals) that dissolved in the aqueous fluid at run conditions was computed from the mass of fluid precipitates and the mass of water in the fluid (Tab. 1). The fluid precipitates from run 98-3 were collected and analyzed qualitatively by SEM; subsequently, they were mixed and fused to a glass in a Pd30Ag70 capsule at M I bar (Tab. 1).

Fluoride-silicate immiscibility experiments

Nine different starting compositions in the system K-Ca-Mg-Al-Si-F-0 were prepared (Tab. 2) using reagent grade powders of CaF,, MgF,, K2CO3, Al 203, and Si02. The K2CO3, Al203, and Si02 were sintered (and CO2

driven off) in open crucibles at 1200 °C, and these sintered materials were ground and mixed with CaF2 and MgF, using a BN mortar and pestle. Ten to twenty mg of these final mixtures were loaded into 2 to 3 cm long Pt capsules (outer diameter of 2.5 mm), and the capsules were sealed by crimping and arc welding. The sealed capsules were heated at 1 atm in a vertically mounted, tubular Deltech furnace at the Lamont-Doherty Earth Observatory for run durations of 3 to 5 hours. Temperature was monitored with a PtRh thermocouple. The experiments were quenched to room temperature in seconds by dropping the hot capsules into a beaker of water. Subsequently, the capsules were cleaned, weighed, cut open, and chips of the run products were mounted and polished. The sealed capsules were puffed up at the conclusion of each run, and they could have held internal pressures greater than 1 bar without

Tab. 2. Starting bulk compositions for » 1 bar fluoride-silicate liquid immiscibility experiments.

Bk. Comp. 1 2 3 4 5 6 7 8 9 Si02 27.7 27.5 27.4 35.7 35.5 35.4 41.6 41.5 41.4 Al203 7.8 11.7 15.5 5.0 7.5 10.0 2.9 4.4 5.9 K20 14.5 10.8 7.2 9.3 7.0 4.6 5.4 4.1 2.7 MgF2 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 CaF2 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 25.0 Total 100.0 100.0 100.1 100.0 100.0 100.0 99.9 100.0 100.0

Theoretical starting composition (in wt. %) based on quantities of Si02 , Al203, K2CO3, MgF2 and CaF 2 . Individual stoichiometries for K : Al : Si, respectively, in each starting bulk composition: 1 = 1.33 : 0.66: 2, 2 = 1 : 1 : 2, 3 = 0.66: 1.33 : 2, 4 = 1.33 : 0.66 : 4,

5 = 1 : 1 : 4, 6 = 0.66 : 1.33 : 4, 7 = 1.33 : 0.66 : 8, 8 = 1 : 1 :8, and 9 = 0.66 : 1.33 : 8. 1

Fig. 1. Backscattered electron image of run products of n 1 bar and 1350 °C experiment (bulk composition 3) conducted to determine the miscibility of fluoride liquids and aluminosilicate liquids in the system K—Ca—Mg—Al—Si—F-0. Image shows large lighter-coloured regions representing quenched products of Mg- and Ca- fluoride liquid and large, darker regions of K-enriched aluminosilicate glass. Note that lighter material contains dark quench-phase blebs of K-enriched aluminosilicate glass. Scale bar is 50 pm long.

bursting. Thus, run pressures are believed to be approximately 1 bar.

ANALYTICAL METHODS

The run product glasses of the 2000 bar experiments were analyzed for Si, Al, K, Na, Ca, Mn, Mg, Fe, Ti, F, Cl, and P with a Cameca SX-100 electron microprobe at 15 keV and 4 nA beam current. The glasses were analyzed with a defocused beam, and the glasses were moved under the beam during analysis (using methods in Webster et al. 1997).

The run product glasses of the 1 bar experiments were analyzed for K, Ca, Mg, Al, F, and Si with the same microprobe at 10 nA and either 20 or 15 keV. Because of the presence of secondary quench products in the liquid immiscibility runs (Fig. 1), each primary "liquid" phase was analyzed in large rastered areas with a defocused beam (20 um diameter) in order to determine its average composition.

RESULTS

2000 bar experiments

The run products collected from the internal, 2.5 mm perforated capsules, which represent the "liquid" phase, for experiments 98-5A and 98-5B contain vesicular silicate glass plus minor crystals of quartz. The quenched products of the "liquid" phase of experiment 98-3 are void of glass; they are comprised of crystals that include quartz, Cl-

bearing apatite, P-bearing alkali feldspar, and an unidentified Fe- and Mn-phosphate.

The fluid precipitates recovered from run 98-3 were examined microscopically and by SEM. They include rare spheroidal blebs variably enriched in Si, Al, P, Na, K, Ca, and Cl; clear euhedral crystals exhibiting compositions similar to that of alkali feldspar and apatite as well as crystalline phases enriched in P and Al ± Fe; and a white film enriched in alkali chlorides that coats the interior of the outer capsule.

The imposition of a large temperature gradient on experiment 98-3 lead to highly efficient dissolution of the liquid into the fluid at 800 °C as well as precipitation of solids from the fluid at 675 °C, because most of the fluid precipitates were collected from the "cooler" end of the large capsule. The extensive temperature gradient (125 °C over 3 cm) apparently established a strong convection current in the fluid, because 65 wt. % of the silicate liquid was dissolved into the fluid and transported out of the inner, perforated capsule during the course of this experiment. The glass that was prepared from the fused solid precipitates of this run is roughly similar in composition to that of the starting glass, except that the former glass contains comparatively higher abundances of Na, K, and Ca and less Al. This glass is peralkaline in composition, and hence, the fluid was also peralkaline at run conditions. Moreover, the incongruent nature of the dissolution of the silicate liquid into the fluid, induced a composition-driven quench of the liquid because no glass is present in the run products representing the liquid phase. In comparison, the run products representative of the liquid phases from experiments 98-5A and 98-5B consist predominantly of glass, and these experiments involved equivalent compositions and pressures.

The charges for experiments 98-5A and 98-5B were subjected to a much smaller temperature gradient than the charge for run 98-3, but it is not known if the 142 hour run duration resulted in equilibrium between liquid, fluid, and crystals for the former charges. For comparison, the experiments of London et al. (1993) required much longer run times for equilibrium between fluid and P-rich (F-deficient) silicate liquids at temperatures like these. We suggest that experiments 98-5A and 98-5 B may have achieved a close approach to equilibrium, because our experiments show significant levels of dissolution of the silicate liquids into the fluid at 2 kbar, the fluids in both experiments contained at least 10 wt. % of aluminosilicate components at run conditions, and the P contents of the run product glasses appear homogeneous.

Distribution coefficients expressing the partitioning of P and F between aqueous fluid and silicate liquid (element in fluid/element in liquid) were computed by mass balance for experiments 98-5A and 98-5B (Tab. 1). The values of Dp are very consistent with those detennined previously for F-deficient metaluminous and peraluminous P-liquids at similar conditions (London et al. 1993), and the computed values of DF are compatible with those determined for

183

Smallest (Si /A11-10Ca+Mg) largest (Si/Al-FIC+Ca*Mg)

Moderate (SUAII.K+Ca.+Mg)

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6 A 0

8 a 8

25

20

15

I0

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..11A • nu 1 el 0 113. be

(flu

ori

de

liq

uid

/sili

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uid

) P-deficient peraluminous liquids for equivalent pressure and temperature (Webster 1990). Thus, the presence of several wt. % of F and P in a peraluminous liquid does not appear to have a detectable influence on the individual behaviours of F or P for peraluminous granitic systems.

One bar fluoride-silicate immiscibility experiments The run products from most of the fluoride-silicate immiscibility experiments are comprised of blebs of silicate glass that are intimately mixed with the quenched residue of a fluoride-rich liquid; several of the 1100 °C experiments also contain crystals of fluorite. The distinct trends shown by the data, e.g., changes in the distribution coefficient for F (DF* = wt. % F in fluoride liquid/wt. % F in silicate liquid) as a function of varying temperature and composition (Fig. 2), are not consistent with the notion that these liquids represent metastable immiscible liquids, and we conclude that the experiments represent equilibrium conditions. The fast quench rate for these experiments, however, was not sufficiently rapid to prevent secondary quench liquation, because in many runs the silicate glass contains small rounded blebs of a secondary F-rich phase and the quenched residue of the primary fluoride-rich liquid contains secondary blebs of silicate glass. These secondary phases represent liquids that apparently exsolved from the primary liquids during quench.

In general, the silicate glasses in many of the individual runs contain as little as 4 to 7 wt. % F, and the bulk mean F content of the coexisting fluoride phases is 30 to 40 wt. %. It should be noted that additional details on the compositions of the liquids (i.e., the fractionation of Ca, Mg, Al, K, and Si between the two liquids) will be addressed in subsequent publications. In this report, we only address the first-order variables that influence the exsolution of fluoride-rich liquids from aluminosilicate liquids.

DISCUSSION

Dissolution of granitic liquids (± silicate minerals) in magmatic aqueous fluids The number of published phase equilibrium experiments involving aqueous fluids and silicate liquids is far too small to be particularly useful for providing accurate experimental constraints on late-stage processes of volatile release from granitic magmas. This is particularly true for peraluminous magmas; the solubility of peraluminous silicate liquids (± crystals) in aqueous fluids at elevated pressure and temperature is not well established. Burnham (1967) systematically determined the solubility of liquids of Spruce Pine pegmatite (which is metaluminous in composition) in aqueous fluids at magmatic temperatures and pressures up to 10 kbar (Fig. 3). He observed that the silicate liquid dissolves incongruently in the aqueous fluid phase and that liquid solubility in hydrothermal fluid increases with increasing pressure and temperature. At very

3 4 S 6

Bulk Starting Compositions

Fig. 2. Distribution coefficient (D F* = wt. % F in fluoride liquid/wt. % F in silicate liquid) for the partitioning of F between a Ca- and Mg- fluoride liquid and coexisting K-bearing aluminosilicate liquid at

1 bar and temperatures of 1100 °C (cross), 1150 °C (triangle), 1250 °C (circle), 1350 °C (diamond), and 1400 °C (square). As silica content increases relative to abundances of Al, K, Ca, and Mg in the bulk system, the extent of fractionation of F between the two liquids increases which implies the stability of two liquids increases relative to the stability of a single liquid phase. As temperature decreases, the extent of fractionation of F between the two liquids increases which implies the stability of two liquids increases relative to the stability of a single liquid phase. See text for discussion. Numbers for bulk starting compositions relate to compositions reported in Tab. 2.

high pressure and temperature, silicate liquid and fluid exhibit complete miscibility. In the subaluminous system albite—H20, for example, only one phase is stable at » 10.6 kbar and 989 °C (Shen and Keppler 1997).

System composition also controls the dissolution of silicate liquids (i.e., aluminosilicate components) in aqueous fluids; thus, composition is fundamentally important to processes involved in dissolving and transporting granitic components in hydrothermal fluids. In the past decade, experimental studies (London et al. 1988: Webster 1990) have investigated the solubility of mildly peraluminous silicate liquids variably enriched in F f P and B in coexisting fluids. Fluorine strongly enhances the ability of a fluid to dissolve (and transport) aluminosilicate components from the liquid (Fig. 3). A fluid containing 4 wt. % F at 2 kbar and 800 °C, for instance, will dissolve and carry nearly 15 times the abundance of aluminosilicates as that in a F-free fluid at the same conditions. Interestingly, aqueous fluids in equilibrium with liquids of the P-, Al-, F-, and B-bearing Macusani obsidian dissolve similarly high abundances of aluminosilicates. At 750-775 °C and 2 kbar, fluids dissolve up to 15 wt. % of aluminosilicate components (London et al. 1988).

Our new results for strongly peraluminous, P-rich and F-bearing silicate liquids conform with previous work involving P-bearing, F-poor peraluminous liquids and F-bearing, P-poor peraluminous liquids. These new data demonstrate that P-rich and F-bearing liquids are also highly soluble in aqueous fluids at 2 kbar; 1 wt. % F and 2 to 4 wt. % P205 in the starting liquid force at least 10 wt. % aluminosilicate components to dissolve in the fluid. Although this observation may seem obvious, at first appearance, it is not necessarily so given that F (Kohn et al. 1991) and P (Gan and Hess 1992) form complexes with Al in silicate liquids, and hence, the activity—composition

184

Peraluminow granitic liquid with

low P and F contents

tit kbar

6-

4 kbar

0

400 600 800 1000

10-

8 -

A.

Temperature (°C)

Peraluminous, P-deficient granitic liquid at 2 kbar and 800° C

•

Websler(1990)

4 2 0

Fluorine Concentration of Aqueous Fluid Coexisting with Silicate Liquid (wt.%)

Fig. 3. Solubility of (A) mildly peraluminous (P- and F-deficient) silicate liquid in aqueous fluid at 2 to 9.8 kbar and 500 to 900 °C (data from Burnham 1967) and (B) peraluminous (P-deficient) silicate liquids (most are F-bearing) in aqueous fluid at 2 kbar and 800 °C (data from Webster 1990). Solubility of liquid in fluid increases with increasing pressure, F content of system, and temperature. See text for discussion.

relationships for F and P in P-, F-, and Al-bearing liquids may be quite complex and difficult to predict. Thus, the behaviour of Al in P-rich liquids may not be similar to that of Al in F-rich liquids.

The composition of the glass prepared from the fused precipitates for run 98-3 represents the composition of the fluid (on an anhydrous basis) integrated over the time duration of this non-equilibrium experiment; this composition does not represent the abundance of aluminosilicate components in the fluid at any given instant during the experiment. The composition of this glass indicates that the solubility of silicate liquid and crystals in fluid was nearly congruent, except that the fluid was more strongly enriched in Ca and alkalis versus aluminum as compared to the starting glass. The alkaline nature of this P-bearing fluid is consistent with observations of for fluids in equilibrium with F-free, peraluminous P-bearing liquids (London et al. 1993). Moreover, the distribution of F and P between aqueous fluid and the peraluminous liquids of our study is very similar to that determined previously. The fluid/liquid distribution coefficient for F, DF, in our P- and F-bearing peraluminous systems is 0.33 which is compatible with experiments involving F-bearing, P-poor peraluminous liquids at similar conditions (Webster 1990), and Dp ranges from 0.08 to 0.19 in our P- and F-bearing peraluminous systems which conforins with results of experiments involving P-bearing. F-poor subaluminous liquids at similar conditions (London et al. 1993). Our values of Dp for peraluminous liquids, however, are

marginally lower than those observed by Keppler (1994) for subaluminous P-bearing, F-deficient liquids for equivalent pressures and temperatures.

WILL F-RICH LIQUIDS EXSOLVE FROM PERALUMINOUS GRANITIC MAGMAS?

The exsolution of water-poor, F-rich liquids from granitic magmas, in nature, has been addressed previously (see reviews of Kogarko et al. 1974; Bailey 1977). Early solubility studies involving immiscible fluoride and aluminosilicate liquids in the system MeF2—Me0—Si02

Al203 (where Me is Mg, Ca, Sr, or Ba) at 1 bar and temperatures of 1200 to 1565 °C observed: 1) the miscibility gap for fluoride and silicate liquids increases as radius of the alkaline earth cation decreases, 2) fluorine solubility in silicate liquid increases as silica content decreases, and 3) fluorine solubility in silicate liquid increases as Al content increases (Ershova and Olshenskii 1957; Ershova 1957). Moreover, prior experimental studies of aqueous fluid-saturated, F-bearing granitic systems at 1 kbar have reported textural evidence of two liquid phases (silicate and fluoride) in their run products (Glyuk and Anfilogov 1973; Kovalenko 1977). In these experiments, two liquids were stable with as little as 2 wt. % F in the starting bulk composition.

Our experiments involve anhydrous, alkaline earth element-rich silicate-fluoride systems at low pressure, i.e.. conditions obviously different from those involved during the emplacement and crystallization of P- and F-rich granitic magmas, nevertheless these data do provide useful constraints on the conditions required to exsolve fluoride-rich liquids from aluminosilicate liquids. For example, the partitioning of Mg, Ca, K, Al, Si, and F between the two liquids in this system can be used as a measure of the likelihood for liquid immiscibility to occur. As a given parameter (temperature or composition) is changed, for a series of experiments conducted at equilibrium, there should be a strong reduction in the driving force for the constituents to partition unequally between two coexisting liquid phases as the distribution coefficients for all components in the system approach values of one. If all of the distribution coefficients approach one, the liquids become increasingly miscible, and at the critical point only one liquid phase is stable. Conversely, as distribution coefficients increase or decrease to values greater than or less than one, respectively, the driving force (i.e., the free energy) for the coexistence of two liquids increases.

At roughly 1 bar, the distribution coefficient for F between fluoride and silicate liquids, D1,*, increases dramatically to values 15 with decreasing temperature and increasing (Si/A1 + K + Ca + Mg) in the silicate run product glasses. The distribution coefficients for the other elements in this system show departures away from a value of one with similar changes in these parameters. Consequently, since decreasing temperature and increasing

20

15

10

0.

1Q

(Si/A1 + K + Ca + Mg) cause DF* and the other distribution coefficients to vary, the likelihood for liquid immiscibility to occur in the system K-Ca-Mg-Al-Si-F-0 increases with decreasing temperature and increasing (Si/Al + K + Ca + Mg). It should be noted that the silicate run product glasses for starting compositions 7, 8, and 9 exhibit compositions that are roughly similar to those of primitive granitic liquids. Reported as oxides, the glasses contain roughly 80 wt. % Si02, 8 to 9 wt. % Al 203, 7 wt. % K20, and 1 to 4 wt. % CaO + MgO (as well as 3.5 to 6 wt. % F).

Application of these results to granitic systems implies that the tendency for a stable F- and alkaline earth element-rich liquid to exsolve from a granitic magma (at equilibrium conditions) will increase as temperature decreases and as the silica content of the residual fractions of silicate liquid increases. The experiments also show that liquid immiscibility can occur in silicate liquids, like those studied herein, if they contain as little as 4 to 7 wt. % F. These F contents are equivalent to those reported for some silicate melt inclusions collected in evolved granites (Webster and Duffield 1994; Webster et al. 1997; Naumov et al. 1998), and hence, the final fractions of some granitic liquids may evolve to compositions (i.e., sufficiently high F contents) that facilitate the exsolution of F-rich liquid from an aluminosilicate liquid. These experimental results, however, are not exactly equivalent to highly-evolved, P- and F-enriched peraluminous granitic liquids because the latter typically contain lower abundances of Ca and Mg (Bea et al. 1992), higher H2O contents, and because our experiments did not contain P. Moreover, the experiments were conducted at temperatures much greater than those representative of evolved, P-enriched peraluminous granitic liquids; the latter crystallize at temperatures s 700 °C (Thomas 1994; Thomas et al. 1996). Thus, the use of these experimental data is further complicated by the suggestion that Ca- and F-rich liquids are probably unstable relative to crystalline fluorite at these low temperatures. This follows from a prior experimental investigation that demonstrated phenocrystic fluorite is stable at temperatures of 700 to 900 °C in molten topaz rhyolite containing 1.2 wt. % F at pressures 2 kbar (Webster et al. 1987).

EXSOLUTION OF P-RICH LIQUIDS FROM ANHYDROUS SILICATE LIQUIDS

Experiments demonstrate that P-enriched liquids will exsolve from aluminosilicate liquids (summary in Kogarko et al. 1974; Hess et al. 1975; Roedder 1979; Suk 1993). A P-enriched liquid will dissolve relatively greater abundances of Fe, Mg, Ti, and Ca whereas a coexisting Si-enriched liquid dissolves more Al and K (Hess et al. 1975). These results agree with observations on melt inclusions in quartz phenocrysts from peraluminous granites that are enriched in P, F, Al, Sn, W, Rb, and other constituents

Fig. 4. Photomicrograph of transmitted light image of silicate melt inclusion in quartz phenocryst from sample of granite pegmatite from Ehrenfriedersdorf Sn-W mine, Germany. This reheated inclusion contains P-, F-, and Al-bearing silicate glass (clear phase), shrinkage bubble (large dark circle in lower right of inclusion), and coexisting P- and F-rich blebs (small round blebs showing high relief) indicating that two liquids were stable during heating. Inclusion is roughly 60 pm in diameter.

(Thomas and Klemm 1997; Breiter et al. 1997). While attempting to remelt crystal-bearing inclusions (in quartz phenocrysts) to a homogeneous glass, it was discovered that some inclusions reorganize, after heating and quenching, to form a clear silicate glass and coexisting spheres of a dark material (Fig. 4) that is enriched in P and F with or without Fe and Al (Tab. 3). The P contents of the silicate glass in these multi-liquid inclusions are equivalent to those of other silicate melt inclusions from evolved P-rich granites (Thomas and Klemm 1997; Breiter et al. 1997; Webster et al. 1997) which suggests that some final

Tab. 3. Composition,' of aluminosilicate glasses in two-liquid phase silicate melt inclusions.

Fe0-enrichedb inclusions

Al 203-enrichedd inclusions

Si02 76.4 59.1 Al 203 12.3 19.5 CaO 0.02 0.06

Na20 1.49 5.2 K20 6.17 2.6 FeOd 2.71 0.5 MgO 0.00 bdle 1102 0.04 0.1 MnO 0.02 0.03 F 2.16 4.2 CI 0.05 0.21

P205 0.02 6.8 TOTAL 101.38 98.30 (A/C N K)f 1.35 1.70

a Compositions in wt. %; analyses by electron microprobe (see text). b Average composition of silicate glass coexisting with Fe-, F-, and P-rich

immiscible blebs in 2 refused silicate melt inclusions in quartz phenocrysts

from a pegmatite sample collected at Ehrenfriedersdorf, Germany (Webster et al. 1997).

Average composition of silicate glass coexisting with Al- and P-rich

immiscible blebs in 23 refused silicate melt inclusions in quartz phenocrysts

from a pegmatite sample collected at Ehrenfriedersdorf, Germany (data of Thomas et al. 1998).

d Total iron as FeO.

e Below detection limit.

f Molar ratio of [Al203/(CaO + Na20 + K20)] in glass.

186

fractions of highly-evolved silicate liquids could fractionate toward compositions that facilitate the exsolution of stable P-rich liquids from silicate liquids. Such P-rich liquids need not be metastable. This exsolution process is apparently more likely with increasing abundances of Fe in the liquid; in contrast, increasing Al contents increase the mutual solubility of the two liquids (Tab. 3).

THE TRANSPORT OF F, P, AND AL IN LATE- STAGE GRANITIC "FLUIDS"

Mineralized and hydrothennally-altered P-rich granites like those of the Erzgebirge region of Germany provide abundant evidence that Al, P, and F are dissolved and transported within and from the granites by hydrothermal fluids. Topaz. fluorite, apatite, and triplite occur in greisens, (magmatic-) hydrothermal miaroles in granitic pegmatites, and in hydrothermal veins associated with granites at Ehrenfriedersdorf, Annaberg, and the Greifensteine, for example (Klemm 1994; Kumann and Leeder 1994; Seltmann et al. 1995). The compositions of melt inclusions and alkali feldspars from Erzgebirge granites indicate that some fractions of late-stage liquids are extremely enriched in Al, F, and P (London 1992; Fryda and Breiter 1995; Kontak et al. 1996; Webster and Duffield 1994; Webster et al. 1997; Breiter et al. 1997; Thomas and Klemm 1997). The presence of wt. % levels of F and P2O5 in late-stage granitic liquids is at variance however with their abundances in whole-rock granite samples; the latter contain far less F and P. Thus, these evolved granites and granitic magmas have lost significant quantities of Al, F, and P, and a crucial question ensues: how and when are these "non-volatile" fluxing components transported from granites and granitic magmas (?). This issue is fundamentally important because the phase that is responsible for altering the granites and transporting the fluxing components may also be responsible for transporting and depositing Sn, W, and other ore metals.

Aqueous hydrothermal fluids are largely responsible for transporting F, P, and Al in granitic systems. Silicate liquids that are enriched in the fluxing components F, P, and excess Al are quite soluble in aqueous fluids at 2 kbar and 800 °C; these solubilities are significantly greater than those determined for silicate liquids containing low abundances of these fluxing components (Burnham 1967). The small values of DF and Dp determined in the present study, which are compatible with prior results for P-rich and F-poor as well as F-rich and P-poor silicate liquids, are seemingly inconsistent, however, with the suggestion that magmatic fluids will dissolve and remove significant quantities of F and P from silicate liquids. Because of these small distribution coefficients, it appears that magmatic fluids cannot dissolve the bulk of the F and P in a silicate liquid unless large quantities of fluid are

flushed through the final fractions of liquid or unless the fluids operate through a significant gradient in temperature. This is supported by results of the non-equilibrium experiment of the present study which indicate that a fluid phase convecting across a temperature gradient is extremely efficient in dissolving F-. P- and Al-enriched silicate liquids. Thus, although DF and Dp for F-and P-rich peraluminous liquids suggest that these non-volatile fluxing components are more soluble in a liquid phase (relative to their solubility in an aqueous fluid phase), large quantities of fluid may dissolve and transport these fluxing constituents in evolved magmas. Magmatic-hydrothermal fluids released from early-crystallizing fractions of granite bodies may be transported through and react with the more liquid-rich fractions of the magma, and these fluids may be exposed to a significant gradient in temperature which facilitates extraction of Al, F, and P from the granitic liquid—crystal mix and facilitates mass transport of these components from the granite. In summary, rarely-crystallizing portions of a magma may exsolve fluids that react efficiently with later-crystallizing portions of the magma.

Although aqueous hydrothermal fluids are largely responsible for transporting F, P, and Al in granitic systems, aqueous fluids may not be the only additional phase to exsolve and remove these components from silicate liquids. The high concentrations of F and P in some silicate melt inclusions from granites and rhyolites are equivalent to the F and P contents of the silicate liquids generated in the immiscibility experiments, and this implies that stable immiscible, non-aqueous P-rich, and, or, F-rich liquids could exsolve from evolved, peraluminous F- and P-enriched granitic magmas during the final stages of crystallization. Silicate melt inclusions from a variety of evolved granites and rhyolites display F and P contents that are well within those apparently required to generate immiscible liquids. However, although the bulk compositions of the melt inclusions are roughly equivalent to those of the experimentally-generated silicate glasses, the H2O contents can be quite different. The two-liquid phase melt inclusions from Ehrenfriedersdorf and the fluoride—silicate immiscibility experiments involve H20-poor systems, whereas some melt inclusions indicate that late-stage, evolved granitic magmas contain substantially higher abundances of H2O (Thomas 1994; Thomas and Klemm 1997). We suggest that magmas containing H2O contents at or near those required for saturation and exsolution of an aqueous fluid are more likely to release an aqueous fluid phase (Paparoni and Webster 1997) and that H20-poor fractions of P- and/or F-rich silicate liquid may exsolve an immiscible F- or P-rich liquid. We also acknowledge that these data do not provide firm evidence that F-, and, or, P-rich liquids exsolve from granitic magmas in nature. Further experiments should be conducted with system compositions closer to those of natural, highly evolved granitic liquids to settle this issue.

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ACKN OWLEDGEMENTS

We express our appreciation to C. Rebbert for considerable assistance with experiments and analyses, and to D. Walker and J. Longhi for use of the Deltech furnace at Lamont-Doherty Earth Observatory. This material is based upon work supported by the National Science Foundation under Grant No. (EAR-9725072).

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