Fractionation of highly siderophile and chalcogen elements during magma transport in the mantle: Constraints from pyroxenites of the Balmuccia peridotite massif
Fractionation of highly siderophile and chalcogen elements during magma transport in the mantle: Constraints from pyroxenites of the Balmuccia peridotite massif
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DOI：
10.1016/j.gca.2015.03.036
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发表时间：
2015-06
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
硫饱和基性岩浆中硫化物的分凝作用对高亲铁元素（HSE）和硫属元素S、Se、Te等亲硫元素的组成有不同程度的影响。这一过程是否主要发生在下地壳或地幔中，以及液态硫化物的分离和积累如何影响这些元素在不同地幔深度和假定的原始基性岩浆中的浓度和比例仍然不确定。巴尔穆恰橄榄岩中菱白云母和尖晶石单斜辉石的HSE、S、Se、Te和Os同位素丰度（伊夫雷亚-韦尔巴诺区，意大利阿尔卑斯山脉）的岩浆热液注入拉张的Ivrea-Verbano带的大陆岩石圈地幔中形成的Balmuccia二辉橄榄岩和单斜辉石岩，分别钠闪石和单斜辉石的HSE和硫族元素组成反映了硫化物熔体从硫饱和硅酸盐熔体中的分凝和聚集，具有不同的硫族元素丰度和比例，HSE。辉石岩显示铂族元素（PGE）和Te的丰度变化很大，而较少亲铜元素S，Se和Re的丰度变化小得多。地幔辉石岩中PGE与Re/Os、S/Se和Se/Te的分馏与硫化物熔体-硅酸盐熔体分配一致，表观系数顺序为DPGE>DAu DTe>DSe DS≈DRe。大洋中脊玄武岩和下洋壳辉长岩中的浓度也与这种分馏作用相一致。在亏损橄榄岩再受精过程中形成的伟晶岩显示HSE和中等粒晶上初始187 Os/188 Os的比值与再受精橄榄岩的间隙硫化物相似。这些成分与橄榄岩中硫化物包裹体的“残余”成分不同。这些观察结果表明，优先溶解的间隙硫化物从橄榄岩迁移到硅酸盐熔体，从而化学和同位素不平衡，可能是常见的开放系统熔体提取和熔体橄榄岩反应在地幔中。硫化物和辉石从这种迁移熔体沉淀可能导致橄榄岩的再受精和形成的辉石在地幔中的其他地方。PGE和Te在这些堆晶岩中早期沉淀硫化物中的富集解释了许多玄武岩和下地壳辉长岩中PGE和Te相对于橄榄岩的亏损。辉石岩和洋壳中Re/Os和Pd/Ir以及超球粒陨石初始187 Os/188 Os的相似范围表明，这些元素在基性岩浆中的成分变化可能反映了岩浆在地幔中迁移过程中主要是间隙质、橄榄岩托管的硫化物液体同化和随后的液体硫化物分离的重复循环。
Sulfide segregation from sulfur saturated basic magmas affects the compositions of chalcophile elements such as the highly siderophile elements (HSE) and the chalcogens S, Se, Te to variable extent. Whether this process predominantly occurs in the lower crust or in the mantle and how segregation of liquid sulfide and accumulation affects concentrations and ratios of these elements at different mantle depths and in presumed primitive basic magmas remains uncertain. Abundances of the HSE, S, Se and Te and Os isotopes in websterites and spinel clinopyroxenites of the Balmuccia peridotite massif (Ivrea-Verbano Zone, Italian Alps) provide new insight on sulfide segregation and the compositional change of melt and peridotite during magma transport in the mantle.Balmuccia websterites and clinopyroxenites formed from late Paleozoic and Mesozoic melt influx into stretched continental lithospheric mantle of the Ivrea-Verbano Zone, respectively. The HSE and chalcogen element compositions of websterites and clinopyroxenites reflect the segregation and accumulation of sulfide melt from S saturated silicate melts with different abundances and ratios of chalcogens and the HSE. The pyroxenites display large variations in abundances of the platinum group elements (PGE) and Te whereas abundances of less chalcophile elements S, Se and Re are much less variable. The fractionation between the PGE and fractionation of Re/Os, S/Se and Se/Te in the mantle pyroxenites are consistent with sulfide melt–silicate melt partitioning with a sequence of apparent coefficients ofDPGE>DAu⩾DTe>DSe⩾DS≈DRe. Concentrations in ocean ridge basalts and in gabbros of the lower oceanic crust are also consistent with such fractionation.Websterites which have formed during refertilization of depleted peridotites display ratios of the HSE and moderately suprachondritic initial187Os/188Os similar to interstitial sulfides of refertilized peridotites. These compositions are different from ‘residual’ compositions of sulfide inclusions in such peridotites. These observations suggest that preferential dissolution of interstitial sulfides from peridotites into migrating silicate melts, and thus chemical and isotopic disequilibrium, may be common during open-system melt extraction and melt-peridotite reaction in the mantle. Precipitation of sulfides and pyroxenes from such migrating melts may lead to refertilization of peridotites and formation of pyroxenites elsewhere in the mantle. The enrichment of the PGE and Te in early precipitated sulfides in these cumulates explains the depletion of the PGE and Te in many basalts and lower crustal gabbros relative to peridotites. Similar ranges of Re/Os and Pd/Ir and suprachondritic initial187Os/188Os in pyroxenites and in oceanic crust indicate that most of the compositional variation of these elements in basic magmas may reflect repeated cycles of assimilation of mainly interstitial, peridotite-hosted sulfide liquid and subsequent liquid sulfide segregation during magma transport in the mantle.