Metasomatic PGE mobilization by carbonatitic melt in the mantle: Evidence from sub-μm-scale sulfide-carbonaceous glass inclusion in Tahitian harzburgite xenolith
Metasomatic PGE mobilization by carbonatitic melt in the mantle: Evidence from sub-μm-scale sulfide-carbonaceous glass inclusion in Tahitian harzburgite xenolith
复制标题
地幔中碳酸岩熔体的交代 PGE 动员：大溪地方辉橄榄岩捕虏体中亚微米级硫化物-碳质玻璃包裹体的证据
DOI：
10.1016/j.chemgeo.2017.10.037
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发表时间：
2017
期刊：
影响因子：
3.9
通讯作者：
Kogiso Tetsu
中科院分区：
文献类型：
--
作者：
Akizawa Norikatsu;Miyake Akira;Ishikawa Akira;Tamura Akihiro;Terada Yasuko;Uesugi Kentaro;Takeuchi Akihisa;Arai Shoji;Tanaka Chima;Igami Yohei;Suzuki Katsuhiko;Kogiso Tetsu
铂族元素因其在金属核中的优先分布而成为揭示地球早期分异过程的重要示踪剂之一。与此同时，交代PGE干扰极大地干扰了早期分异的信息证据，这一点已通过天然地幔橄榄岩及其贱金属硫化物和铂族矿物颗粒的大量PGE数据得到证实。因此，要正确估算原始上地幔中铂族元素的丰度，就必须对交代作用下铂族元素的活化机制和过程进行详细的研究。然而，这还没有实现，因为亚微米尺度（即小于一微米的规模）的交代作用印在地幔橄榄岩的描述还没有得到充分的recorded.Here，我们报告了一个亚微米尺寸的硫化物玻璃夹杂物阵列在大溪地哈兹伯拉岩捕虏体。利用同步辐射X射线衍射和透射电镜分析了其结构和化学特征。结果表明，硫化物和玻璃含有可观量的PGE（9.7at.% Ir，4.3原子% Rh和5.8at.% Pt）和碳（21.2原子% C）分别。硫化物玻璃包裹体阵列托管在钠富集的单斜辉石（高达1.8wt%Na2O），显示脉状分布，部分取代斜方辉石。原始地幔标准化的微量元素模式的单斜辉石表现出普遍增加，从重稀土元素（REEs），轻稀土元素的负异常铅和高场强元素，如锆，铪和钛，这表明与富镁碳酸岩熔体平衡。这些结果表明，含钠富镁的碳酸岩熔体参与了方辉橄榄岩的形成，Ir，Rh和Pt通过碳酸岩交代活动，并最终分布在硫化物中。
Platinum-group elements (PGEs) are one of the key tracers to reveal early differentiation processes of the Earth due to their preferential distribution into the metallic core. Meanwhile, informative evidence for early differentiation has been greatly disturbed through metasomatic PGE disturbance, which has been demonstrated through a number of PGE data for natural mantle peridotites as well as base-metal sulfide and platinum group mineral grains therein. The mechanism and process of metasomatic PGE mobilization should be investigated in detail for an appropriate estimation of PGE abundance in the primitive upper mantle. However, this has not yet been achieved, because sub-micrometer-scale (i.e.scale of less than a micrometer) descriptions for metasomatic effects imprinted in the mantle peridotites have not been sufficiently recorded.Here, we report a sub-micrometer-sized sulfide–glass inclusion array in a Tahitian harzburgite xenolith. The textural and chemical characteristics were disclosed with employing synchrotron X-ray and transmission electron microscope analyses. The results demonstrate that the sulfide and glass contain appreciable amounts of PGE (9.7 at.% Ir, 4.3 at.% Rh and 5.8 at.% Pt) and carbon (21.2 at.% C), respectively. The sulfide–glass inclusion array is hosted in sodium-enriched clinopyroxene (up to 1.8 wt% Na2O) that shows vein-like distribution and partly replaces orthopyroxene. Primitive mantle-normalized trace-element patterns of the clinopyroxene show a general increase from heavy rare-earth elements (REEs) to light REEs with negative anomalies in Pb and high-field-strength elements such as Zr, Hf and Ti, which indicate equilibration with Mg-rich carbonatitic melt. These results suggest that Na-bearing Mg-rich carbonatitic melts were involved in the harzburgite formation and that Ir, Rh and Pt were mobilized through carbonatitic metasomatism and eventually distributed in the sulfides.