Experimental study of element partitioning between majorite, olivine, merwinite, diopside and silicate melts at 16 GPa and 2, 000°C
Experimental study of element partitioning between majorite, olivine, merwinite, diopside and silicate melts at 16 GPa and 2, 000°C
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16 GPa、2, 000°C 下镁镁石、橄榄石、汞长石、透辉石和硅酸盐熔体元素分配的实验研究
DOI：
10.2343/geochemj.26.357
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发表时间：
1992
影响因子：
0.8
通讯作者：
E. Ohtani
中科院分区：
文献类型：
--
作者：
Junko Moriyama;I. Kawabe;K. Fujino;E. Ohtani
我们通过实验确定了在球粒状和富含 CaO 的硅酸盐体系中，在 16 GPa 和 2, 000°C 条件下硅酸盐矿物和共存熔体之间的元素分配系数。在富 MgO 球粒陨石体系中，多数石榴石 + 熔体 ± 橄榄石形成，而在富 CaO 体系中，汞锌矿与熔体 ± 单斜辉石共存。通过 EPMA 分析确定了多数石榴石、汞辉石和单斜辉石/熔体对的 La、Ce、Sm、Yb、Sc、Zr、Hf 和其他微量或主要元素的分配系数（D 值）。还使用 EPMA 测定了橄榄石/熔体对的有限数量元素的 D 值。大多数铁矿/熔体的 D（重稀土元素和钪）接近一致。这些值远低于 2-3 GPa 和 1、200-1、500°C 下镁铝榴石/熔体对以及镁铝榴石巨晶/主岩基质对的报告值。在高于 10 GPa 的较高压力下，压力引起的 Al3+ 配位变化是造成 D 值以及对比温度条件的巨大差异的原因：(VIIIM2+, IVSi4+) = (VIIIHREE3+, IVAl3+) 的耦合取代对于将 HREE3+ 容纳到石榴石中很重要，但受到严重限制，因为 IVAl3+ 不太可能存在于多数铁矿/熔体系统中。随着压力增加而减少 D(REE 和 Sc) 的机制也可能在单斜辉石/熔体对之间起作用。对铁铝榴石、镁铝榴石和多数石榴石/熔体对中 VIIIM2+ 和 VIIIM3+ 的 Onuma 图进行系统比较，清楚地表明铁铝榴石和镁铝榴石石榴石中 HREE 的高富集与电荷平衡取代以及或多或少与 IVAl3+ 聚合的硅酸盐熔体的存在有关。这些 Onuma 图还表明，Fe2+ 和 Co2+ 在石榴石和熔体之间的分配行为是异常的。这可能反映了由于晶体场效应，Fe2+和Co2+倾向于熔体中的六个配位点而不是石榴石中的立方位点。研究发现，merwinite/富钙熔体对的 D（REE 和 Sc）值模式与 CaTiO3/熔体和 CaSiO3（钙钛矿）/熔体对的 D（REE 和 Sc）值模式非常相似。鉴于混合 Ca2+ 和 O2- 层包含类似于钙钛矿结构的伪六方致密堆积的 merwinite 子结构，这一点很有趣。
We have determined experimentally the partition coefficients of elements between silicate minerals and coexisting melts at 16 GPa and 2, 000°C in chondritic and CaO-rich silicate systems. Majorite garnet + melt ± olivine forms in the MgO-rich chondritic system, while in the CaO-rich system, merwinite coexists with melt ± clinopyroxene. The partition coefficients (D-values) of La, Ce, Sm, Yb, Sc, Zr, Hf and other minor or major elements for majorite garnet, merwinite, and clinopyroxene/melt pairs have been determined through EPMA analysis. D-values of a limited number of elements were also determined for the olivine/melt pair with EPMA. The D(HREE and Sc) for majorite/melt are close to unity. These values are much lower than reported values for pyrope/melt pairs at 2–3 GPa and 1, 200–1, 500°C and for pyrope megacryst/host rock matrix pairs. The pressure-induced coordination change in Al3+ at higher pressures above 10 GPa is responsible for the enormous difference in the D values as well as the contrasting temperature conditions: The coupled substitution of (VIIIM2+, IVSi4+) = (VIIIHREE3+, IVAl3+), important for accommodating HREE3+ into garnets, is severely limited because IVAl3+ is unlikely to exist in the majorite/melt system. The mechanism to reduce D(REE and Sc) with increasing pressure may also be operative between the clinopyroxene/melt pair. Systematic comparison of Onuma diagrams for VIIIM2+ and VIIIM3+ in almandine, pyrope, and majorite garnet/melt pairs clearly indicates that high enrichments of HREE in almandine and pyrope garnets are related to charge-balanced substitution and the presence of more or less polymerizing silicate melts with IVAl3+. These Onuma diagrams also suggest that the partitioning behaviors of Fe2+ and Co2+ between garnet and melt are anomalous. This may reflect the tendency for Fe2+ and Co2+ to prefer six coordinated sites in melts to cubic sites in garnets due to the crystal field effect. The D(REE and Sc) value pattern for the merwinite/Ca-rich melt pair has been found to be quite similar to those for CaTiO3/melt and CaSiO3 (perovskite)/melt pairs. This is interesting in view of the substructure of merwinite that mixed Ca2+ and O2- layers comprise a pseudo-hexagonal dense packing analogous to the perovskite structure.