Abundances of Ag and Cu in mantle peridotites and the implications for the behavior of chalcophile elements in the mantle
Abundances of Ag and Cu in mantle peridotites and the implications for the behavior of chalcophile elements in the mantle
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DOI：
10.1016/j.gca.2015.04.006
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发表时间：
2015-07
影响因子：
5
通讯作者：
Zaicong Wang;H. Becker
中科院分区：
文献类型：
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作者：
Zaicong Wang;H. Becker
地幔橄榄岩中银的丰度以及高温地幔过程中银的行为很少受到关注，因此，块状硅酸盐地球（BSE）中银的丰度几乎没有受到限制。为了更好地了解地幔中银和其他亲铜元素的分馏过程，通过同位素稀释 ICP-MS 方法获得了来自不同地质环境的地幔橄榄岩 (n= 68) 中银和铜的丰度。在橄榄岩构造岩和几套橄榄岩捕虏体中，银和铜丰度与硫、硒、碲和金等中度不相容元素呈正相关，这些橄榄岩捕虏体显示出不同程度的熔体消耗和硅酸盐熔体再肥化的证据。肥沃橄榄岩 (3500 ± 1200, 1s,n= 38) 中的平均 Cu/Ag 与大洋中脊玄武岩 (MORB, 3600 ± 400, 1s,n= 338) 和 MORB 硫化物液滴的平均 Cu/Ag 无法区分。恒定的平均 Cu/Ag 比率表明 Ag 和 Cu 在地幔部分熔化、再肥化和岩浆分馏过程中具有相似的行为，因此应该代表地球的上地幔。橄榄岩和玄武岩中 Cu、Ag、Au、S、Se 和 Te 的系统分馏与硫化物熔体-硅酸盐熔体分配一致，表观分配系数为铂族元素 (PGE) > Au ⩾ Te > Cu ≈ Ag > Se ⩾ S。由于二次过程的影响，亲铜元素的丰度，特别是 S、Se，但也 许多橄榄岩捕虏体中的 Cu 和 PGE 是可变的，并且低于橄榄岩地块中的 Cu 和 PGE。橄榄岩的再肥化可能会改变橄榄岩地块中亲铜元素和亲石元素的丰度，然而，这似乎主要以系统方式发生。与亲石元素和亲铜元素的相关性以及橄榄岩和洋脊玄武岩的重叠平均 Cu/Ag 比率用于将 BSE 中 Ag 和 Cu 的丰度分别限制在 9 ± 3 (1s) ng/g 和 30 ± 6 μg/g (1s)。如果应用目前可用的金属-硅酸盐分配数据，则 BSE 中银和铜的消耗程度差异很大，不能用低压-温度核心的形成来解释。
Silver abundances in mantle peridotites and the behavior of Ag during high temperature mantle processes have received little attention and, as a consequence, the abundance of Ag in the bulk silicate Earth (BSE) has been poorly constrained. In order to better understand the processes that fractionate Ag and other chalcophile elements in the mantle, abundances of Ag and Cu in mantle peridotites from different geological settings (n= 68) have been obtained by isotope dilution ICP-MS methods. In peridotite tectonites and in a few suites of peridotite xenoliths which display evidence for variable extents of melt depletion and refertilization by silicate melts, Ag and Cu abundances show positive correlations with moderately incompatible elements such as S, Se, Te and Au. The mean Cu/Ag in fertile peridotites (3500 ± 1200, 1s,n= 38) is indistinguishable from the mean Cu/Ag of mid ocean ridge basalts (MORB, 3600 ± 400, 1s,n= 338) and MORB sulfide droplets. The constant mean Cu/Ag ratios indicate similar behavior of Ag and Cu during partial melting of the mantle, refertilization and magmatic fractionation, and thus should be representative of the Earth’s upper mantle. The systematic fractionation of Cu, Ag, Au, S, Se and Te in peridotites and basalts is consistent with sulfide melt–silicate melt partitioning with apparent partition coefficients of platinum group elements (PGE) > Au ⩾ Te > Cu ≈ Ag > Se ⩾ S.Because of the effects of secondary processes, the abundances of chalcophile elements, notably S, Se, but also Cu and the PGE in many peridotite xenoliths are variable and lower than in peridotite massifs. Refertilization of peridotite may change abundances of chalcophile and lithophile elements in peridotite massifs, however, this seems to mostly occur in a systematic way. Correlations with lithophile and chalcophile elements and the overlapping mean Cu/Ag ratios of peridotites and ocean ridge basalts are used to constrain abundances of Ag and Cu in the BSE at 9 ± 3 (1s) ng/g and 30 ± 6 μg/g (1s), respectively. The very different extent of depletion of Ag and Cu in the BSE cannot be explained by low pressure–temperature core formation if currently available metal–silicate partitioning data are applied.