The role of clinopyroxene in generating U-series disequilibrium during mantle melting

被引:126
作者
Wood, BJ [1 ]
Blundy, JD [1 ]
Robinson, JAC [1 ]
机构
[1] Univ Bristol, Dept Earth Sci, CETSEI, Bristol BS8 1RJ, Avon, England
关键词
D O I
10.1016/S0016-7037(98)00302-0
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
We develop recent models of crystal-liquid partitioning (Blundy and Wood, 1994; Wood and Blundy, 1997) to predict the effects of crystal chemistry on the partitioning of U-series elements between clinopyroxene and coexisting silicate melt. With increasing pressure along the mantle solidus, clinopyroxene becomes richer in Na2O and Al2O3 and these components are predicted to reduce the size of the large M2 site. This means that, because U4+ is Smaller than Th4+, clinopyroxene should progressively discriminate in favour of U4+ with increasing pressure. We find that the effect is sufficiently great that D-U/D-Th should change from <1.0 at pressures up to 1.0 GPa to >1.0 above 1.5 Cpa. Thus, melting in the stability field of clinopyroxene should produce excess U-238 relative to Th-230 at low pressure and excess Th-230 above 1.5 GPa. Observed excesses of Th-230 in MORB may not, therefore, require the presence of garnet in the source region. We performed experiments at 1.5-1.9 Cpa on compositions doped with U and Th to test the calculations. Crystals and quenched melts were analysed by ion microprobe. On the mantle solidus at 1.5 GPa, D-U/D-Th for clinopyroxene was found to be 1.07 +/- 0.08, while a value of 1.19 +/- 0.04 was obtained at 1.9 GPa. These results confirm the prediction that mantle clinopyroxenes have D-U/D-TH > 1.0 at high pressure. Absolute values of D-U for clinopyroxene are 0.029 at 1.5 GPa and 0.023 at 1.9 GPa in good agreement with earlier measurements on aluminous clinopyroxenes. Corresponding values for orthopyroxene at 1.5 GPa are 1.7 x 10(-3) for D-U and 8 x 10(-4) for D-Th while olivine gave D-U of 1.8 x 10(-5) and D-Th of 1.2 x 10(-5) under similar conditions. When the new data are used in dynamic melting calculations we find that Th-230/U-238 activity ratios of up to about 1.23 in the liquid are consistent with melting in the spinel Iherzolite field provided porosity is approximately 10(-3) and melting rate less than or equal to 3 x 10(-5) kg. m(-3) y(-1). This confirms our model prediction that residual garnet is not required to generate excess Th-230 in mantle melts. Solid-liquid partition coefficients for other members of the U decay series, Ra and Ac may be calculated from the model. For clinopyroxene we obtain D-RA of about 10(-6) and D-Ac of 5 x 10(-4). The value for Ac is large enough that Ac-227 activities may provide information on melt extraction processes. The M2 site of orthopyroxene is found to be elastically similar to the corresponding clinopyroxene site, which enables us to calculate for this phase, D-Ra of 4 x 10(-10) and D-Ac of 2 x 10(-6). Copyright (C) 1999 Elsevier Science Ltd.
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页码:1613 / 1620
页数:8
相关论文
共 34 条
[1]  
[Anonymous], 1988, MINERALS-BASEL
[2]   DETERMINING THE COMPOSITION OF HIGH-PRESSURE MANTLE MELTS USING DIAMOND AGGREGATES [J].
BAKER, MB ;
STOLPER, EM .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1994, 58 (13) :2811-2827
[3]   URANIUM THORIUM DISEQUILIBRIA AND PARTITIONING ON MELTING OF GARNET PERIDOTITE [J].
BEATTIE, P .
NATURE, 1993, 363 (6424) :63-65
[4]   THE GENERATION OF URANIUM SERIES DISEQUILIBRIA BY PARTIAL MELTING OF SPINEL PERIDOTITE - CONSTRAINTS FROM PARTITIONING STUDIES [J].
BEATTIE, P .
EARTH AND PLANETARY SCIENCE LETTERS, 1993, 117 (3-4) :379-391
[5]   ACTINIDE CRYSTAL-LIQUID PARTITIONING FOR CLINOPYROXENE AND CA3(PO4)2 [J].
BENJAMIN, T ;
HEUSER, WR ;
BURNETT, DS ;
SEITZ, MG .
GEOCHIMICA ET COSMOCHIMICA ACTA, 1980, 44 (09) :1251-1264
[6]   PREDICTION OF CRYSTAL-MELT PARTITION-COEFFICIENTS FROM ELASTIC-MODULI [J].
BLUNDY, J ;
WOOD, B .
NATURE, 1994, 372 (6505) :452-454
[7]   SODIUM PARTITIONING BETWEEN CLINOPYROXENE AND SILICATE MELTS [J].
BLUNDY, JD ;
FALLOON, TJ ;
WOOD, BJ ;
DALTON, JA .
JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 1995, 100 (B8) :15501-15515
[8]   Heavy REE are compatible in clinopyroxene on the spinel lherzolite solidus [J].
Blundy, JD ;
Robinson, JAC ;
Wood, BJ .
EARTH AND PLANETARY SCIENCE LETTERS, 1998, 160 (3-4) :493-504
[9]   Constraints on mantle melting at mid-ocean ridges from global U-238-Th-230 disequilibrium data [J].
Bourdon, B ;
Zindler, A ;
Elliott, T ;
Langmuir, CH .
NATURE, 1996, 384 (6606) :231-235
[10]   SOME THERMODYNAMIC ASPECTS OF GROWTH OF STRAINED CRYSTALS [J].
BRICE, JC .
JOURNAL OF CRYSTAL GROWTH, 1975, 28 (02) :249-253