Trace element partitioning between garnet lherzolite and carbonatite at 6.6 and 8.6 GPa with applications to the geochemistry of the mantle and of mantle-derived melts
Trace element partitioning between garnet lherzolite and carbonatite at 6.6 and 8.6 GPa with applications to the geochemistry of the mantle and of mantle-derived melts
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DOI：
10.1016/j.chemgeo.2009.02.004
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发表时间：
2009-05-15
期刊：
影响因子：
3.9
通讯作者：
Withers, Anthony C.
中科院分区：
文献类型：
--
作者：
Dasgupta, Rajdeep;Hirschmann, Marc M.;Withers, Anthony C.
本文介绍了在6.6~8.6 Gpa压力和1265~1470℃温度范围内，天然可育二辉橄榄岩与近固相线碳酸盐岩(CBL)之间微量元素分配的实验测定，这些条件与洋脊下最初产生富CO2熔体和大洋岛状玄武岩深源区碳酸盐熔体渗流有关。与以前在较低压力下产生更多铝CPx的实验相比，我们的实验CPx-熔体对显示出较低的三价阳离子进入M2位的分配系数(D-Lu(CPx/cbl)=0.17，D-La(cpx/cbl)=0.006)。D-U(cpx/cbl)(0.002)和D-Th(cpx/cbl)(0.001)也明显低于先前的估计。石榴石-碳酸盐熔体对稀土元素的分配系数从高度不相容的La到中等相容的Lu不等，略低于低压实验的结果。从6.6 Gpa到8.6 Gpa，包括Zr、Hf和Ti在内的HFSE与D-Hf(Gamet/CBL)和D-Ti(Gamet/CBL)和GT；D-Zr(Gamet/CBL)从中度不相容到轻度相容。在两种压力下，U和Th在石榴石中仍然是高度不相容的，在6.6 GPa时D-U(Gamet/CBL)/D-Th(Gamet/CBL)为1.2-1.25，在8.6 GPa时为1.35。根据我们新测量的分配系数对D-gtlherzite/CBL的估计表明，通过从地球深部上地幔有效地提取0.1%的碳酸盐熔体，可以消除30-60%的一些高度不相容的元素，而0.1%的富C熔体提取可以产生残留物，其Rb/Sr、U/Pb和Th/U比值分别比未熔融源低29%、12%和6%，而Sm/Nd值类似于高出11%。我们的数据还表明，古老的碳酸盐熔体提取和交代作用可以解耦Hf和Nd同位素体系。石榴石二辉橄榄岩碳酸盐岩熔体的归一化HREE和Nb/Ta比值均低于喷发的天然碳酸盐岩。因此，喷发碳酸盐岩的微量元素丰度要么需要与模式石榴石含量较低的较浅地幔橄榄岩反应，要么需要来自富集源。深部富碳酸盐熔体的微量元素丰度必须被亚脊玄武岩源区较浅部分产生的大量硅酸盐熔体稀释。但是，观测到的Th-230和Pa-231过量以及喷发的MORBs中CO2/Nb的变化可能是富碳酸盐熔体变化贡献的结果。(C)2009爱思唯尔B.V.保留所有权利。
We present experimental determinations of trace element partitioning between natural fertile lherzolite and near-solidus carbonatitic liquid (cbL) at pressures of 6.6 and 8.6 GPa and temperatures between 1265 and 1470 degrees C, conditions relevant for initial generation Of CO2-rich melt beneath oceanic ridges and for carbonate melt percolation in the deep source regions of ocean island basalts. Compared to previous experiments at lower pressures, which produced more aluminous cpx, our experimental cpx-melt pairs show lower partition coefficients for trivalent cations entering the M2 site (D-Lu(cpx/cbL) = 0.17, D-La(cpx/cbL) = 0.006). D-U(cpx/cbL) (0.002) and D-Th(cpx/cbL) (0.001) also are distinctly lower than previous estimates. The garnet-carbonate melt partition coefficients for REE vary from highly incompatible La to moderately compatible Lu and are slightly lower than those from lower pressure experiments. From 6.6 to 8.6 GPa, HFSEs including Zr, Hf, and Ti range from moderately incompatible to slightly compatible, with D-Hf(gamet/cbL) > D-Ti(gamet/cbL) > D-Zr(gamet/cbL). U and Th remain highly incompatible in garnet at both pressures with D-U(gamet/cbL)/D-Th(gamet/cbL) of 1.2-1.25 at 6.6 GPa and 1.35 at 8.6 GPa. Estimates of D-gtlherzolite/cbL, based on our newly measured partition coefficients, suggest that 30-60% of some of the highly incompatible elements can be removed by efficient extraction of 0.1% carbonatitic melt from the Earth's deep upper mantle, and that 0.1% C-rich melt extraction can create a residue that has Rb/Sr, U/Pb and Th/U ratios, respectively, 29, 12, and 6% lower and Sm/Nd ratio similar to 11% higher than the unmelted source. Our data also suggest that ancient carbonate melt extraction and metasomatism can decouple the Hf and Nd isotope systems. Garnet lherzolite derived carbonatitic melt has lower normalized HREEs and smaller Nb/Ta ratios than erupted natural carbonatites. Therefore, the trace element abundances of erupted carbonatites require either reaction with a shallower mantle peridotite with low modal garnet or derivation from an enriched source. The trace element abundances of deep carbonate-rich melts must become diluted by greater volumes of silicate melt produced in the shallower portions of sub-ridge basalt source regions. But, the observed Th-230 and Pa-231 excesses and variations Of CO2/Nb in erupted MORBs are a likely consequence of variable contributions of carbonate-rich melt. (C) 2009 Elsevier B.V. All rights reserved.