-
36. A GEOCHEMICAL STUDY OF 17 SELECTED SAMPLES FROM BASEMENT
CORESRECOVERED AT SITES 447, 448, 449, 450, AND 451, DEEP SEA
DRILLING PROJECT LEG 59
D. A. Wood,1 D. P. Mattey,1 J. L. Joron,2 N. G. Marsh,1 J.
Tarney,1 and M. Treuil2
INTRODUCTION
Deep Sea Drilling Project Legs 59 and 60 drilled 15sites along
an east-west transect at 18°N from the WestPhilippine Basin to the
Mariana Trench (Fig. 1) in orderto study the nature and genesis of
the back-arc,marginal basins and the remnant and active arcs of
theregion. Leg 59 drilled at five sites at the western end ofthe
traverse: Site 447 in the West Philippine Basin; Site448 on the
Palau-Kyushu Ridge; Sites 449 and 450 in theParece Vela Basin; and
Site 451 on the West MarianaRidge. Penetration into basaltic
basement of these siteswas 183.5 meters at 447 (8 basalt flows);
623 meters at448 (46 basalt flows, sills, and dikes and
volcaniclasticunits); 40.5 meters at 449 (2 basalt flows); 7 meters
at450 (1 basalt intrusion); and 4 meters of basalt breccia at451
overlain by 861 meters of volcaniclastic sedimentaryrocks.
The petrology, mineralogy, and geochemistry ofthese cores have
been described by Mattey et al. (thisvolume). They have
distinguished the main igneousunits recovered on the basis of X-ray
fluorescence(XRF), whole rock geochemical data, microprobe datafor
minerals and glasses, and petrography. From thesedata we have
chosen 17 samples which are represen-tative of the main
compositional units and subjectedthem to epithermal neutron
activation analysis (Chaylaet al., 1973; Jaffrezic et al., in
press) to obtain highprecision data for a number of trace elements
(viz. Sc,Co, Cs, La, Ce, Eu, Tb, Hf, Ta, Th, and U) whichcould not
be obtained by XRF. The core intervals of the17 samples are given
in Table 1 together with a summaryof the nature and size of the
compositional units fromwhich they are taken. Major and trace
element analysesof the samples are given in Table 2.
One aim of this study has been to compare the com-position of
these rocks with those erupted in other tec-tonic environments to
elucidate the chemical charac-teristics of basaltic crust formed at
active plate marginswhich consist of a series of remnant island
arcs and in-terarc basins. Data for mid-ocean ridge basalts and
thelava series of Japan have been particularly useful to usin this
respect.
1 Department of Geological Sciences, University of Birmingham,
Birmingham, B15 2TT,England. (Dr. Tarney's present address:
Department of Geology, University of Leicester,Leicester, United
Kingdom.)
2 Laboratoire de géochimie comparée et systématique, UER des
Sciences de la Terre et In-stitut de Physique du Globe, Université
Pierre et Marie Curie, 75230 Paris, France, andGroup des Sciences
de la Terre, Laboratoire Pierre Sue, CNRS Centre d'Etudes
Nucléaires deSaclay, Gif-sur-Yvette, France.
Figure 1. Location of drill sites for DSDP Leg 59.
THE MAIN GEOCHEMICALCHARACTERISTICS OF THE SITES
The basement rocks recovered during Leg 59 can bedivided into
two main compositional groups, on thebasis of major and trace
element chemistry (andpetrography), which correspond to the basin
(447, 449,and 450) and ridge (448 and 451) sites. The basin
sitesare low alkali tholeiites with high Mg numbers (Mg/[Mg+ Fe2+]
from 0.65 to 0.73) and CaO contents but lowabundances of the more
hygromogmatophile (HYG) orincompatible trace elements (e.g., Ba,
Th, Ta, and La).The ridge sites also consist of tholeiitic rocks
but havelower Mg numbers (0.40 - 0.59) and significantly
higherabundances of Sr and the alkali elements than the basinsites.
The rocks from the basin sites show cleargeochemical affinities
with normal (N-type) mid-oceanridge basalts, whereas the rocks from
the ridge site aresimilar to those associated with active plate
margins.
743
-
D. A. WOOD ET AL.
Table 1. Leg 59 samples used in this study, with a summary of
the main chemical and petrographic units.
SampleReference (interval in cm)
Number of FlowsLithologic Unit Chemical and Thickness of
of Sample Unit Chemical Unit (m)Core Intervals (cm) of
Compositional GroupGeneral Petrography ofCompositional Group
447-1 447A-12-1, 138-142 basalt in volcaniclastic breccia with
up to 10% zeolitenot sampled 1 5(28) 14-1 (top of basement)-17-l,
125
447-2 447A-18-2, 116-118
447-3 447A-22-3, 9-11
447-4 447A-24-2, 125-128
447-5 447A-30-4, 91-93
448-1 448-44-1, 103-106
448-2 448-53-2, 59-61
448-3 448-57-2, 120-122
448-4 448-61-3, 55-58
448-5 448A-36-4, 130-133
448-6 448A-41-1, 17-19
448-7 448A-41-4, 4-6
F8
F8
F9
Fll
2
3
4
5
1(15)
3(29)
3(24)
6 (>87)
17-1, 125-19-2, 125
19-2, 125-23-4, 100
23-4, 100-27-2, 15
27-2, 15-base of hole
F8
Fl l
F13
F13
F22
D26
D27
"olivine" basalt group
No chemical units have beendistinguished, but the
623-meterbasement section consists of 46separate basalt (flows,
sills, anddikes) and volcaniclastic units.
"orthopyroxene" basaltgroup
"clinopyroxene"basalt group
aphyric to sparsely PI.,Ol., Sp. phyric basaltPL, Ol., Cpx.,
Sp.phyric basalt with rarexenocrysts of Cpx.sparsely PI., Ol.,
Cpx.phyric basalt with rarexenocrysts of Cpx.sparsely PL, Ol.
phyricbasaltsparsely OL, PL, Sp.phyric basalt with rarexenocrysts
of Sp. and PLand xenoliths ofanorthosite
PL, OL, Cpx. phyricbasaltsparsely PL, Cpx. phyricbasalt with
rare Ol.phenocrystsPL, Cpx., Opx. phyricbasaltPL, Cpx., Opx.
phyricbasaltPL, Cpx., Opx. phyricbasaltsparsely OL, PL, Cpx.phyric
basaltsparsely PL, Cpx., OLphyric basaltsparsely PL, Cpx.,
Ol.phyric basalt (ortho-pyroxene in groundmass)
449 449-15-2, 11-13
450 450-36-3, 7-9
451-1 451-45-1, 29-32451-2 451-46-1, 24-30451-3 451-59-1,
54-59
no units distinguished
no units distinguished
no units distinguished
(40 meters of pillow basalt penetrated, only7 meters
recovered)(7 meters of pillow basalt penetrated, only2 meters
recovered)
basalt and andesite clasts in a volcaniclastic breccia
sparsely PL, OL, Cpx.,Sp. phyric basaltPL, Cpx. phyric
basalt
PL phyric basaltPL, Ol. phyric basaltCpx. and Opx.
phyricandesite
Note: F = flow unit, D = dike unit, C = chemical unit (see
Mattey et al., this volume); PL = plagioclase, Ol. = olivine, Sp. =
sphene,Cpx. = clinopyroxene, Opx. = orthopyroxene.
Figure 2 is an AFM diagram on which the Leg 59rocks have been
plotted, together with rocks from thearc tholeiite and calc-alkalic
series of Japan (Wood etal., in press). It indicates that the ridge
sites are moreFe-rich than the basin sites but also enriched in
alkalisrelative to Fe than is the arc tholeiite series of Japan.The
higher alkali content of the ridge sites relative to thearc
tholeiites of Japan is apparent on an alkali-silicaplot (Fig. 3)
and is due to the systematically lower Na2Ocontents of the Japanese
arc tholeiite series. In bothFigure 2 and Figure 3 rocks from the
ridge sites plot insimilar positions to the Japanese calc-alkali
series.
There are significant geochemical differences be-tween the rocks
from the two ridge sites (Table 2): Site451 rocks have higher
abundances of Sr, Ba, and thealkali elements and higher ratios of
more HYG (hygro-magmatophile)/less HYG elements (e.g., Th/Hf,
La/Tb,etc.) but with lower abundances of Fe and TiO2 thanSite 448.
Core 451-3 is a two-pyroxene andesite. TheSite 451 rocks therefore
have more of the chemical char-acteristics of a calc-alkalic series
than those of Site 448.This indicates that the West Mariana Ridge
represents amore evolved island arc than does the
Palau-KyushuRidge.
744
-
GEOCHEMICAL STUDY OF BASEMENT CORES
Table 2. Major and trace element analyses of 17 selected samples
from the basement cores of DSDP Leg 59.
Component3
SiO2TiO 2Al 2 θ3Fe 2 O 3FeOMnOMgOCaONa 2OK2OP 2 O 5H 2 O +
TotalTotal Fe as FeO
Mg/(Mg + Fe 2 +)ScCrCoNiZnCiaRh
SrYZ r
CsHaLaCeEu
TbHfTaTh
U
447-1
49.00.85
15.71.177.720.168.37
12.932.690.390.06n ri
99.038.77
0.65834.7
332
40.9117
71
185.2
79
2243
0.2061.85.0
0.740.481.170.060.070.06
West Philippine Basin
447-2
50.71.41
14.61.248.200.158.24
12.012.960.390.10n.d.
100.019.32
0.64142.1
199
55.6154
82
184.7
85
3568
0.1172.26.4
1.220.812.180.120.150.57
447-3
50.61.13
14.51.228.080.189.13
12.072.830.060.08n.d.
99.889.18
0.66837.7
25749.3
1436917
6926
51
0.0310
1.4
5.3
0.830.631.620.070.08—
447-4
47.20.83
15.71.167.670.16
11.6111.672.820.050.05n.d.
98.928.71
0.72932.4
265
47.0197
5315
2.3107
20
53
0.0511
1.65.20.760.451.430.050.10-
447-5
49.40.89
15.21.167.350.169.23
12.662.790.480.07n.d.
99.698.39
0.69135.8
34051.0
14662184.2
87
21
48
0.13102.2
5.3
0.790.521.350.160.21-
448-1
49.61.54
14.01.87
12.320.234.639.543.090.570.19n.d.
97.5914.00
0.40047.492
39.412
117
19
5.21602947
0.1322
2.44.51.040.731.760.030.230.11
448-2
49.21.15
14.71.62
10.660.205.19
10.233.000.470.12n.d.
96.5412.12
0.46443.891
39.5199221
4.5181
50
0.0641
2.7
4.00.770.521.640.040.30-
Palau-Kyushu Ridge
448-3
48.90.81
17.11.409.210.225.37
10.562.720.560.03n.d.
96.8810.47
0.53636.86737.51575
214.9
18617
450.05
232.4
5.60.790.381.440.040.41
-
448-4
50.21.12
15.01.479.690.156.308.843.571.410.13n.d.
97.8811.01
0.50937.27936.921
8319
12.6172
2974
0.1541
3.56.00.980.622.170.050.350.19
448-5
50.60.95
13.51.54
10.170.268.18
10.332.970.160.08n.d.
98.7411.56
0.58835.8
32046.7
107
11821
2.1
15727
400.02
14
1.74.1
0.740.451.320.020.250.07
449-6
49.71.25
13.61.61
10.650.216.709.153.141.280.17
n.d.
97.4612.10
0.52838.97837.81799
12.5157
25
390.14
14
1.8
4.50.940.511.340.010.160.23
447-7
50.51.43
13.11.70
11.210.227.268.943.390.500.12n.d.
98.3712.74
0.53540.974
51.415
113183.5
207
28
60
0.0526
3.3
5.91.080.662.000.040.340.18
Parece-VelaBasin
449
49.40.99
14.41.258.240.149.14
12.253.160.460.08n.d.
99.619.36
0.66337.4
205
39.510363
164.7
158
24
610.37
20
2.97.2
0.920.491.490.180.270.17
450
50.01.43
15.31.097.220.177.58
13.083.220.230.11n.d.
99.428.20
0.65135.2
256
49.911171182.4
134
3189
0.0713
3.17.7
1.150.692.250.150.150.10
West Mariana Ridge
451-1.
48.10.59
19.81.207.910.184.86
12.602.550.260.10n.d.
98.158.99
0.52226.554
31.45
68
21
2.3
4951331
0.1318
2.84.2
0.530.290.700.020.380.16
451-2
50.60.77
15.91.348.810.176.709.583.040.620.23n.d.
97.7610.02
0.57429.571
31.91166
20
6.8
533
66
0.30188
8.7
17.30.890.521.820.701.560.22
451-3
57.30.65
17.70.865.690.153.038.603.652.130.14
n.d.
99.906.36
0.48628.043
18.66
6821
39.4520
25
83
0.41467
12.621.4
1.090.562.360.102.300.88
AnalyticalMethod
(XRF)(XRF)(XRF)(XRF)
(XRF)(XRF)(XRF)(NAA)(XRF)(XRF)
(NAA)(NAA)(NAA)(NAA)(XRF)(XRF)(NAA)(XRF)(XRF)(XRF)(NAA)(NAA)(NAA)(NAA)(NAA)(NAA)(NAA)(NAA)(NAA)(NAA)
Note: XRF = X-ray fluorescence, NAA = neutron activation
analysis.a Major element oxides in weight percent, trace elements
in parts per million.
Figure 2. MgO-FeO-Na2O + K2O triangular AFM diagram forDSDP Leg
59 basement samples and lavas from the calc-alkali (in-verted
triangle) and arc tholeiite (open circle) series of Japan (fromWood
et al., 1980). The curved lines represent the general limits
ofJapanese lavas (Kuno, 1968), with the pigeonitic (arc
tholeiite)series plotting above line K' -K' and the hypersthenic
(calc-alkalic)series plotting below it. (Site 447-filled circle,
Site 448-cross, Site449-open square, Site 450-open diamond, Site
451-cross in circle.)
2h
40 50
Figure 3. Alkali-silica diagram for DSDP Leg 59 basement
samplesand lavas from the calc-alkali, arc tholeiite, and alkalic
series ofJapan (from Wood et al., in press). The curved line
represent thegeneral limits of the high alumina basalt series of
Japan (Kuno,1968). (Symbols same as for Fig. 2.)
HYG Element Discrimination Diagrams
Wood, Joron, and Treuil (1979) have recently shownthat the
relative abundances of the elements Th, La, Ta,and Hf can be used
to distinguish between volcanicseries erupted in different tectonic
environments. Nor-
mal N-type MORB mid-ocean ridge basalts and E-typeMORB (enriched
in the more HYG elements and erup-ted at segments of mid-ocean
ridges with positiveresidual heat flow, gravity, and depth
anomalies relativeto N-type MORB) can also be distinguished using
these
745
-
D. A. WOOD ET AL.
elements. The Leg 59 rocks have been plotted in Figures4, 5, and
6 for comparison with well-studied volcanicseries from other
environments.
Figure 4 is a log-log plot of Th (more HYG element)versus Hf
(less HYG element) and shows the progressiveincrease in Th/Hf ratio
from the basin sites to Site 448(Palau-Kyushu Ridge) to Site 451
(West MarianaRidge). Most of the samples from the basin sites
(exceptfor those from Chemical Unit 5, Sites 449 and 477,which show
secondary enrichment of Th—see the fol-lowing) plot close to the
field of N-type MORB, withHf/Th ratios greater than 12.5. The Site
448 rocks plotclose to the field of the Japanese arc tholeiites and
theSite 451 rocks plot close to the field of the
Japanesecalc-alkalic series.
Figure 5 is a log-log plot of La (more HYG element)versus Ta
(more HYG element). These two elements arethought to have very
similar bulk partition coefficientsbetween partial melts and a
mantle residue consisting ofmajor mineral phases—olivine,
orthopyroxene, clino-pyroxene, ±garnet, and ±plagioclase—(Joron et
al.,1978; Wood, Varet, et al., 1979). In order to fractionateLa and
Ta from each other by magmatic processes it isnecessary to involve
minor phases—Ta (similar argu-ments apply to Nb) is strongly
partitioned into titaniummineral phases (e.g., ilmenite, rutile,
and sphene). TheLa/Ta ratio varies significantly between E-and
N-typeMORB and active margin volcanics. The active marginvolcanics
are significantly depleted in Ta (La/Ta ratio> 30) relative to
the other more HYG elements (Sun etal., 1979; Wood, Joron, et al.,
1979; Wood, Joron, andTreuil, 1979; Tarney et al., in press), which
suggeststhat a minor Ti phase is involved at some stage in
their
100
La (ppm)
10
0.1
0.01
Hf (ppm)
1.0 10
Figure 4. Log Th versus Log Hf for DSDP Leg 59 basement
samples.The fields of lava series from other areas are from Wood,
Varet, etal. (1979), Wood, Joron, et al., (1979), and Wood, Joron,
andTreuil (1979). (Symbols same as for Fig 2.)
ECMORB& Ocean Islands
0.1
Ta (ppm)
0.1 10
Figure 5. Log La versus Log Ta for DSDP Leg 59 basement
samples.The fields of lava series from other areas are from Wood,
Varet, etal. (1979), Wood, Joron, et al., (1979), and Wood, Joron,
andTreuil (1979). (Symbols same as for Fig 2.)
genesis. Again the rocks from the basin sites plot closeto the
field of N-type MORB, and Site 448 rocks plotclose to the Japanese
arc tholeiite series. Site 451 rockshave significantly higher La/Ta
ratios than both the Site448 rocks and the arc tholeiite and
calc-alkalic series ofJapan.
Figure 6 is a triangular diagram of Th-Hf/3-Ta inwhich Wood,
Joron, and Treuil (1979) showed that vol-canic rocks from different
tectonic environments (in-cluding silicic differentiates) could be
readily separated.The main distinction between oceanic and active
marginvolcanic series in this diagram is the relative depletion
inTa of the latter series. The rocks from the basin sitesplot in
the field of N-type MORB, although the samplesfrom Chemical Unit 5,
Sites 447 and 449, are more en-riched in Th and Ta relative to Hf,
which will be dis-cussed in more detail later in this chapter. The
rocksfrom Sites 448 and 451 are clearly separated in Figure 6and
group with other island arc and calc-alkalic
series,respectively.
The HYG element compositions of the basalts erup-ted in the West
Philippine and Parece Vela basins areclearly identical to those
erupted at normal mid-oceanridges. Many of the basalts recovered at
DSDP Sites442, 443, and 444 (Shikoku Basin) also had HYG ele-ment
compositions similar to basalts of the major oceanbasin but
included rock types similar to E-type MORBand ocean island basalts
as well as N-type MORB(Marsh et al., in press; Wood et al., in
press). It seems,therefore, that the basaltic crust of the West
Pacificback-arc and interarc basins cannot be
distinguishedgeochemically from oceanic Crustal Layer 2,
particu-larly after alteration. This has important implicationsfor
the tectonic environment of formation of the crustal
746
-
GEOCHEMICAL STUDY OF BASEMENT CORES
Hf/3
Figure 6. Th-Hf/3-Ta triangular diagram for DSDP Leg 59
basementsamples and lavas from the calc-alkalic and arc tholeiite
series ofJapan (Wood et al., in press). The fields of the major
terrestrialbasalt types are slightly modified from Wood, Joron, and
Treuil(1979). (Symbols same as for Fig. 2).
sections exposed in many ophiolite complexes. For ex-ample, the
Late Cretaceous ophiolite complexes of theMiddle East have crustal
sections which include basaltsof N-type MORB composition associated
with activemargin volcanics (e.g., Pearce, in press). A back-arc
orinterarc basin seems a reasonable tectonic environmentfor such a
sequence of igneous rocks. It is perhaps morerealistic that N-type
MORB could be preserved as anophiolite by closure of a
back-arc-interarc basin than byobduction of Crustal Layer 2 from a
major ocean floor.Saunders, Tarney, et al. (in press) have also
concludedthat back-arc basin basalts and ocean floor basalts
areequal contenders, on geochemical grounds, for ophio-lite
precursors.
Detailed HYG Element Chemistry at the Basin Sites
Mattey et al. (this volume) have described thepetrographical and
some of the geochemical effects oflow-temperature sea water
alteration, or halmyrolysis,to which all the basalts recovered from
the basementsites have been subjected to a greater or lesser
extent.The alteration of trace element concentrations in basal-tic
rocks during halmyrolysis has received much atten-tion during
recent years (e.g., Hart, 1971; Hellman andHenderson, 1977;
Humphris and Thompson, 1978;Ludden and Thompson, 1979). These
studies haveshown that the elements Cs, Rb, K, U, and Sr are
highlymobile during halmyrolysis and that Ba, Th, and thelight rare
earth elements (especially La) are also mobile,but to a lesser
extent. These elements are often signifi-cantly enriched in the
altered glassy margins of singlepillows of basalt relative to the
less altered massivepillow interiors. This is due to the
crystallization ofsecondary minerals such as zeolites, calcite, and
smec-tites, which can contain relatively high concentrations
of the mobile HYG elements (e.g., Pritchard et al.,1979).
Mattey et al. (this volume) have compared micro-probe analyses
of fresh glass, glass showing variousstages of devitrification, and
whole rock XRF analysisfrom a single unit at Site 447. The results
show thathalmyrolysis has generally resulted in a decrease inSiO2,
A12O3, MgO, and CaO, with increases in K2O (andpossibly in Na2O,
but this could be due to volatile lossduring the microprobe
analyses) and presumably inH2O + and Fe2O3 (at the expense of FeO).
These changesare typical of those generally described for ocean
floorbasalts. Using immobile trace element data from theXRF whole
rock analyses, Mattey et al. (this volume)have distinguished two
fractionation trends in the basinsite (one with Ti/Zr - 125 and
Zr/Y ~2 and the otherwith Ti/Zr -90 and Zr/Y -2.5) which cannot be
linkedby realistic crystal fractionation processes. We haveused
these two coherent trends on biaxial plots of im-mobile HYG
elements to look more closely at the effectsof alteration on the
mobile HYG elements.
Figure 7 shows a series of biaxial plots involving theimmobile
less HYG element Tb versus other HYG ele-ments for the seven
samples from the basin sites. Inthose plots which involve two
immobile elements—i.e.,Hf versus Tb, Zr versus Tb, and P2O5 versus
Tb—thetwo linear fractionation trends mentioned in the fore-going
can be clearly distinguished. However, when amobile HYG element is
involved, there is a considerablygreater scatter. Nevertheless, the
samples do vary syste-matically in most of the plots, and it is
possible todistinguish between the samples in which the
concentra-tions of the mobile HYG elements have been modifiedand
those samples in which they have not. The samplesfrom Chemical
Units 2 and 3 generally lie along astraight line passing through
the origin on plots of im-mobile versus slightly mobile elements
(i.e., Ta versusTb, Ce versus Tb, La versus Tb), which we interpret
asthe primary characteristics of one fractionation trend.The sample
from Chemical Unit 5 and from the basaltclast from the
volcaniclastic breccia overlying the base-ment at Site 447 (Sample
447-12-1, 138-142 cm) gene-rally plot away from this trend on plots
of immobile ver-sus slightly mobile elements; they are
systematicallyenriched in the slightly mobile elements. We
interpretthe two latter samples as containing significantly
mod-ified concentrations of the slightly mobile elements. Thesame
is true for the other fractionation trend samplesfrom Chemical Unit
4, Sites 447 and 450 which lie alonga straight line passing through
the origin on the plots ofimmobile versus slightly mobile elements;
the samplefrom Site 449 on the other hand is significantly
enrichedin the mobile elements. We interpret the element
ratiosdefined by samples from Chemical Unit 4, Sites 447 and450, as
primary.
Having obtained the primary HYG element ratios inthis way, it is
possible to estimate the primary concen-tration of a mobile element
in an altered sample of agiven fractionation trend and to calculate
the magnitudeby which it has been enriched during alteration of
thesample. We have made this calculation for a number of
747
-
D. A. WOOD ET AL.
0.75
0.5
0.25
Tb (ppm)
12-1
Tb (ppm)
C3
MORB '
C#5 4 3 C4 ‰
-o4 4 9 449
12-1
C4
Hf (ppm) Ce (ppm) K2O (wt. Eu (ppm)
J I L1 8 0.2 0.4 0.5 10
Figure 7. Biaxial plots of HYG elements versus Tb for DSDP Leg
59 samples from Sites 447, 449, and 450. (Symbols same as for Fig.
2 except thatthe cross in circle represents average N-type MORB
[Wood, Varet, et al., 1979]. The bars represent ± 2-sigma
analytical errors [Jaffrezic et al.,in press; Tarney et al.,
1979].)
HYG elements in the three altered samples (see Table
3).Two-sigma analytical error bars have been included inFigure 7 to
indicate the precision of these calculations.The calculations have
also been made for the moremobile elements (K, Rb, and Cs), but
these are less ac-curate because the primary ratios of the elements
withimmobile elements are more difficult to define. Thesamples
which showed no secondary modification of theless mobile elements
contain in some cases modifiedconcentrations of these more mobile
elements. Further-more, the analytical precision for Ba at
concentrationsof less than 20 ppm is too poor to determine
secondaryenrichment factors accurately.
Nevertheless, the enrichment factors for the same ele-ment in
the three samples are consistent and the resultsconfirm that (1)
Cs, Rb, and K are significantly moremobile than the other HYG
elements; (2) Ba, Sr, and Lahave been enriched in the altered
samples to a similar ex-tent (~ × 1.5 to ×2); (3) Ce and Eu are
very slightlyenriched in the altered samples; (4) Th and Ta have
beensignificantly enriched in the two samples that containcalcite
veins (Chemical Unit 5, Sites 447 and 449) but
Table 3. Approximate secondary enrichment factors of HYG
elementsin intensely halmyrolyzed basin samples.
^ ^ ^ ^ SampleTrace \N"™berElement \ ^
CsRbBaThKTaLaCeSrPHfZrEuTiTbY
secondarymineralspresent
447A-12-1, 138-142 cm
×6×5immobileimmobile×4immobile×2×1.25×2immobileimmobileimmobile<
× l . limmobileimmobileimmobile
zeoliteand clays
447A-30-4, 91-93 cm(Chemical Unit 5)
×5×4× 1 . 3× 2 . 5×5×2×2×1.25×1.75mmobilemmobilemmobile< × l
. lmmobilemmobilemmobile
calciteand clays
449-15-2, 11-13 cm
x6×2×2×2.5×3×1.5xl .75×1.2×1.5mmobilemmobilemmobile< × l .
lmmobilemmobilemmobile
calciteand clays
have not been modified in the sample containing
zeolite(447-12-1, 138-144 cm). The primary concentrations ofTa and
Th in N-type MORB are extremely low (less than
748
-
GEOCHEMICAL STUDY OF BASEMENT CORES
0.15 ppm), and ratios involving these elements arechanged
drastically by only very small increases (~O.lppm) in their
concentrations. This is evident in Figure 4,where the samples from
Chemical Unit 5, Sites 447 and449 plot outside the field of N-type
MORB, and in Fig-ure 6, where they plot away from the rest of the
samplesfrom the basin sites (although still in the field of
N-typeMORB). Th and Ta must therefore be considered asslightly
mobile elements in N-type MORB subjected tohalmyrolysis. We suspect
that this mobility is directlyrelated to the presence of calcite
and the ability of Taand Th to form soluble complexes with CO2-rich
fluids.By avoiding samples which contain secondary calcite,the
problem of the slight mobility of these elementscould probably be
overcome. It is somewhat encourag-ing that despite the intense
alteration shown by thesamples from Chemical Unit 5, Sites 447 and
449 andthe slight mobility of Ta and Th, they still plot in
theN-type MORB field in Figure 6. However, it alsostresses the need
to analyze several samples showingvarious stages of secondary
alteration, before identify-ing the tectonic environment in which a
lava series wasformed from the HYG element data.
In Figure 8, HYG element contents of the leastaltered samples
(Chemical Unit 3, Sites 447 and 450)from the two fractionation
trends shown by the basinsites have been normalized to estimated
primordialmantle concentrations (Wood, Varet, et al., 1979).
Theelements in this diagram are arranged approximatelyfrom left to
right in increasing order of bulk partitioncoefficient for
anhydrous partial melting conditions(> 10%) of the upper mantle
with only olivine, orthopy-roxenes, and clinopyroxene ± garnet in
the residue. Thecomposition of average N-type MORB (Sun et al.,
1979;Wood, Varet, et al., 1979) has been plotted for com-parison.
Both the magma types from the basin sites plotclose to the average
N-type MORB composition withevidence to suggest that the K, Rb, and
Cs concentra-
• Average N-type MORB+ 447A-22-3, 9-11 cmo 450-36-3, 7-9 cm
Cs Rb Ba Th U K Ta Nb La Ce Sr Nd P Hf Zr Eu Ti Tb Y
Figure 8. HYG element abundances of the two magma typesrecovered
at the basin sites of DSDP Leg 59, normalized to theestimated
primordial mantle abundances (Wood, Varet, et al.,1979). Average
N-type MORB has been included for comparison.(A primordial mantle
abundance of Ti-1200 ppm has been used in-stead of 1526 ppm used by
Wood, Varet, et al., [1979]. This givesmore acceptable HYG element
ratios of mantle material—e.g., Ti/2 × -110.)
tions in even these least altered samples have been slight-ly
modified. The magma type represented by the samplefrom Chemical
Unit 3, Site 447 has slightly lower abun-dances of the more-HYG
elements (those plottingtoward the left in Figure 8) than the magma
typerepresented by the sample from Hole 450, and averageN-type
MORB. If we assume that the generation of allMORB types involves
significant degrees of partialmelting (> 10%), the HYG element
ratios in the basaltswill be close to those of their respective
mantle sources.When more than 10 per cent partial melting is
involved,it is not easy to change HYG element ratios by
fluctua-tions in the degree of partial melting of a single
mantlesource. The different HYG element ratios in the twomagma
types shown in Figure 8 would require large dif-ferences in the
degrees of batch partial melting of asingle mantle source. Also,
the variation could be pro-duced from a single mantle source by a
continuous melt-ing process (Langmuir et al. 1977; Wood, Joron, et
al.,1979) in which the Hole 450 magma type is extractedfrom the
source first (with some melt remaining in thesource) and the more
depleted Chemical Unit 3, Site 447magma type subsequently extracted
from the residue.Alternatively, the two magma types could be
derivedfrom different mantle sources; the latter originating in
amantle source which was slightly more depleted in themore HYG
elements than the average N-type MORBsource. It is necessary to
obtain radiogenic isotope databefore determining whether or not
different mantlesource compositions are involved.
Detailed HYG Element Chemistry at the Ridge Sites
The samples from Sites 448 and 451 have also suf-fered
significant halmyrolysis, as is evident from the lowmajor element
totals in Table 2. This makes the data onthe more mobile HYG
elements suspect in terms of pri-mary concentrations of the lavas.
Figure 9 shows aseries of biaxial plots of HYG elements in the
samplesfrom the ridge sites, together with data on the Japaneselava
series in some cases. In each plot the more HYGelement (according
to the order in Figure 8) is plotted onthe abscissa. All the
samples from Site 448 conform tosingle straight line trends passing
through the origin inplots of Th versus Ta, Th versus Ba, Th versus
Hf, Laversus Sr, La versus Eu, and La versus Ba, which sug-gests
that the concentrations of these slightly mobileelements have not
been significantly modified duringalteration at this site.
Mattey et al. (this volume) have noted that the ortho-pyroxene
basalt group at Site 448 has lower Ti/Zr ratiosthan the other
basalt groups. This group has lower Ti/Hf (3300 relative to 4800)
and slightly higher Hf/Tb,Ce/Tb, La/Tb, and Zr/Y ratios. It is
possible that thisgroup represents slightly lower degrees of
partial melt-ing (of a similar source) than that involved in the
genesisof the "olivine" and "clinopyroxene" basalt groups,which
would produce a slight enrichment of the moreHYG elements relative
to the less HYG elements with-out changing the ratios of the more
HYG elements (Th/Ta, Ba/La, etc.).
Although there are many similarities between theisland arc
tholeiites erupted on the Palau-Kyushu Ridge
749
-
D. A. WOOD ET AL.
0.1Ta (ppm)
/
0.05 F12.' F 1 2
/.F10
- /F8
./F22•"D26
Hf (ppm)
×'F12I ?D27 ,Fx8/ '
,£ *F12 'D26/*F22? /
/ /-1/ / /
Th (ppm)2
Th (ppm)2
®} Sr(ppm)
»(dacite)
• (andesite)
La (ppm)15
• f 7®» Eu (ppm)
Primordial MantleLa/Eu = 4.8
La (ppm)15
Hf(ppm)
\ \
!
f
×D27
-
GEOCHEMICAL STUDY OF BASEMENT CORES
100
50
x 448A-36-4, 130-133 cmP 451-45-1, 29-32 cm
• O-ShimaOS3+ Omuro-yama (Oliviπe
Figure 10. HYG element abundances of island arc and
calc-alkalilavas normalized to the estimated primordial mantle
abundances(Wood, Varet, et al., 1979).
wedge and induce partial melting under conditions ofhigh PH2O
The HYG element variations in activemargin volcanics of the West
Pacific are certainly con-sistent with this model.
In order to be able to explain the extremely low Taand Nb
abundances in active margin volcanics through-out the world (Joron
and Treuil, 1978; Sun et al., 1979;Wood, Joron, et al., 1979;
Saunders, Tarney, andWeaver, in press) it seems necessary to
hypothesize thatthese elements are held in a mineral phase in the
mantleduring the partial melting (Wood, Varet, et al., 1979;Tarney
et al., in press). Ta and Nb are strongly parti-tioned into
titanium mineral phases—e.g., rutile andsphene. Recent experimental
studies have shown thatthe stabilities of rutile and sphene are
enhanced underhydrous conditions (Hellman and Green, 1979). If
rutileor sphene survive the partial melting event and remainin the
mantle residue, Ti cannot be considered as ahygromagmatophile
element and its concentration inthe melt phase will depend on the
mass fraction of the Tiphase(s), the proportion it contributes to
the melt, andthe degree of partial melting. Several of the lavas
plot-ted in Figure 10 have negative anomalies relative to theother
less HYG elements which would be expected if arefractory Ti mantle
phase were involved in their gen-esis. This might also explain why
the differencesbetween the "orthopyroxene" group and the
otherpetrographic groups recovered at Site 448 are more pro-nounced
on plots involving Ti if they are related to dif-ferent degrees of
partial melting.
CONCLUSIONS
1) The basalts erupted at Sites 447, 449, 450 are geo-chemically
indistinguishable from N-type MORB. Thisindicates that Crustal
Layer 2 in the West Philippine
and Parece Vela Basins cannot be chemically distin-guished from
Crustal Layer 2 of the major oceans.
2) In the intensely halmyrolized samples from Sites447 and 449,
the elements K. Rb, and Cs have been en-riched by approximately a
factor of 5; Sr, La, and Bahave been enriched by approximately a
factor of 2; andCe has been enriched by a factor of 1.25. The
elementsP, Hf, Zr, Ti, Tb, and Y are immobile. In two
samplessubjected to alteration with calcite Th and Ta have
beenenriched by a factor of 2.5 and about 2, respectively,but are
immobile in samples not containing calcite.Even the altered samples
plot in the field of N-typeMORB in the Th-Hf/3-Ta triangular
discriminationdiagram.
3) The basalts recovered from Site 448 on the Palau-Kyushu Ridge
are primitive island arc tholeiites, withcompositions similar to
arc tholeiites from Japan,though with systematically lower Ba and
higher Na2O.
4) The basalts recovered from Site 451 on the WestMariana Ridge
are more evolved island arc tholeiiteswith strong calc-alkalic
tendencies and are similar to thecalc-alkalic series from Japan,
but with higher Sr andlower abundances of Ta and the less HYG
elements.
5) The enrichment of the alkali elements Sr, La, Th,and U in the
active margin volcanics of the West Pacificis consistent with a
petrogenetic model involving meto-somatism of the mantle wedge
above a subducted zone(with hydrous fluids derived from the
subducted crust)prior to or at the time of partial melting in the
mantlewedge. The depletion of Ta and Nb in these volcanicscan be
explained by the presence of a refractory Tiphase in the mantle
residue.
ACKNOWLEDGMENTS
This study was supported by the Natural Environment
ResearchCouncil (U.K.) and the Centre National de la Recherche
Scientifique(France).
REFERENCES
Best, M. G., 1975. Migration of hydrous fluids in the upper
mantleand potassium variation in calc-alkalic rocks. Geology,
3:429-432.
Chayla, B., Jaffrezic, H., and Joron, J. L., 1973. Analyse par
activa-tion dans les mentions épithermiques. Application à la
determina-tion d'éléments en trace dans les roches. Comptes Rend.
Acad.Sci. Paris, 227:273-275.
Hart, S. R., 1971. K, Rb, Cs, Sr and Ba contents and Sr isotope
ratiosof ocean floor basalts. Philos. Trans. Roy. Soc. London Ser.
A,268:573-588.
Hawkesworth, C. J., O'Nions, R. K.( Pankhurst, R. J., et al.,
1978.A geochemical study of island arc and back-arc tholeiites from
theScotia Sea. Earth Planet. Sci. Lett., 36:253-262.
Hellman, P. L., and Green, T. H., 1979. The role of sphene as an
ac-cessory phase in the high pressure partial melting of hydrous
maficcompositions. Earth Planet. Sci. Lett., 42:191-201.
Hellman, P. L., and Henderson, P., 1977. Are rare earth
elementsmobile during spilitisation? Nature, 267:38-40.
Humphris, S. E., and Thompson, G. 1978. Trace element
mobilityduring hydrothermal alteration of oceanic basalts.
Geochim.Cosmochim. Acta, 42:127-136.
Jaffrezic, H., Joron, J. L., Treuil, M., et al., in press. A
study of theprecision obtained by neutron activation analysis using
interna-tional standard rocks GS-N and BCR-1 as examples. A
discussionof a geochemical model accounting for the estimated
errors. J.Radioanalyt. Chem.
Joron, J. L., Bougault, H., Wood, D. A., et al., 1978.
Application dela géochimie des elements en trace a 1'étude des
propriétés et des
751
-
D. A. WOOD ET AL.
processus de genèse de la cròute oceanique et du
manteausuperieur. Bull. Geol. Soc. France, 20:521-531.
Joron, J. L., and Treuil, M., 1978. Utilisation des propriétés
des ele-ments fortement hygromagmatophiles pour 1'étude de la
composi-tion chimique et de 1'hétérogenéité du manteau. Bull. Geol.
Soc.France, 19, 1197-1205.
Kuno, H., 1968. Differentiation of basaltic magmas. In Hess, H.
H.,and Poldervaart, A. (Eds.), Basalts (Vol. 2): New York
(Wiley),623-688.
Langmuir, C. H., Bender, J. F., Bence, A. E., et al., 1977.
Petrogene-sis of basalts from the FAMOUS area: Mid-Atlantic Ridge.
EarthPlanet. Sci. Lett., 36:133-156.
Ludden, J. N., and Thompson, G., 1979. An evolution of the
behav-iour of the rare earth elements during the weathering of
sea-floorbasalt. Earth Planet. Sci. Lett., 43:85-92.
Marsh, N. G., Saunders, A. D., and Tarney, J., in press.
Geochem-istry of basalts from the Shikoku and Daito Basins, DSDP
Leg 58.In Klein, G. DeV., Kobayashi, K., et al., Init. Repts. DSDP,
58:Washington (U.S. Govt. Printing Office).
Pearce, J. A., in press. Geochemical evidence for the genesis
anderuptive setting of lavas from Tethyan Ophiolites. Proceedings
ofthe Symposium on Ophiolite Problems. Geol. Survey: Cyprus.
Pritchard, R. G., Cann, J. R., and Wood, D. A., 1979.
Lowtemperature alteration of oceanic basalts, DSDP Leg 49.
InLuyendyk, B. P., Cann, J. R., et al., Init. Repts. DSDP,
49:Washington (U.S. Govt. Printing Office), 709-714.
Saunders, A. D., Tarney, J., Marsh, N. G., et al., in press.
Ophiolitesas oceanic crust or margin basin crust: A geochemical
approach.Proceedings of the Symposium on Ophiolite Problems.
Geol.Survey: Cyprus.
Saunders, A. D., Tarney, J., and Weaver, S. D., in press.
Transversegeochemical variations across the Antarctic Peninsula:
Implica-tions for the genesis of calc-alkalic magmas. Earth Planet.
Sci.Lett.
Sun, S. S., Nesbitt, R. W., and Sharaskin, A. Y., 1979.
Geochemicalcharacteristics of mid-ocean ridge basalts. Earth
Planet. Sci. Lett.,44:119-138.
Tarney, J., Saunders, A. D., Weaver, S. D., et al., 1979. Minor
ele-ment geochemistry of basalts from Leg 49, North Atlantic
Ocean.In Luyendyk, B. P., Cann, J. R., et al., Init. Repts. DSDP,
49:Washington (U.S. Govt. Printing Office), 657-691.
Tarney, J., Wood, D. A., Saunders, A. D., et al., in press.
Nature ofmantle heterogeneity in the North Atlantic: Evidence from
deepsea drilling. Philos. Trans. Roy. Soc. London Ser. A.
Wood, D. A., 1979a. A variably veined sub-oceanic upper
mantle:Genetic significance for mid-ocean ridge basalts from
geochemicalevidence. Geology, 7:499-503.
, 1979b. Dynamic partial melting and its application to
thepetrogenesis of basalt lava series from Iceland, the Faeroe
Islands,the Isle of Skye (Scotland) and the Troodos Massif
(Cyprus).Geochim Cosmochim. Acta, 43:1031-1046.
Wood, D. A., Joron, J. L., Marsh., N. G., et al., in press.
Major- andtrace-element variations in basalts from the North
Philippine Seadrilled during Deep Sea Drilling Project Leg 58: A
comparativestudy of back-arc basin basalts with lava series from
Japan andmid-ocean ridges. In Klein, G. deV., Kobayashi, K., et al.
Init.Repts. DSDP, 58: Washington (U.S. Govt. Printing Office).
Wood, D. A., Joron, J. L., Treuil, M., 1979. A re-appraisal of
the useof trace elements to classify' and discriminate between
magmaseries in different tectonic settings. Earth Planet. Sci.
Lett., 45:326-336.
Wood, D. A., Joron, J. L., Treuil, M., et al., 1979. Elemental
and Srisotope variations in basic lavas from Iceland and the
surroundingOcean floor: The nature of the mantle source in
homogeneities.Contrib. Mineral. Petrol., 70:319-339.
Wood, D. A., Varet, J., Bougault, H., et al., 1979. The
petrology,geochemistry and mineralogy of North Atlantic basalts: A
discus-sion based on IPOD Leg 49. In Luyendyk, B. P., Cann, J. R.,
etal., Init. Repts. DSDP, 49: Washington (U.S. Govt. Printing
Of-fice), 695-755.
752
