Cobalt and precious metals in sulphides of peridotite xenoliths and inferences concerning their distribution according to geodynamic environment: A case study from the Scottish lithospheric mantle
Cobalt and precious metals in sulphides of peridotite xenoliths and inferences concerning their distribution according to geodynamic environment: A case study from the Scottish lithospheric mantle
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DOI：
10.1016/j.lithos.2015.11.007
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发表时间：
2016
期刊：
影响因子：
3.5
通讯作者：
H. Hughes;I. McDonald;J. Faithfull;B. Upton;M. Loocke
中科院分区：
文献类型：
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作者：
H. Hughes;I. McDonald;J. Faithfull;B. Upton;M. Loocke
岩石圈地幔中贵金属和钴的丰度通常是通过对地幔捕虏体进行大量地球化学分析获得的。这些元素具有强亲铜性，必须考虑此类样品中硫化物相的矿物学、结构和微量元素组成。在这项研究中，我们评估了来自四个苏格兰岩石圈地体的尖晶石二辉橄榄岩中硫化物的矿物学、结构和微量元素组成，这为根据地体年龄和地球动力学环境检查硫化物及其贵金属禀赋的变化提供了理想的试验场。具体来说，我们测试了北部地体的太古代-古元古代克拉通次大陆岩石圈地幔 (SCLM) 与南部地体的古生代岩石圈地幔的硫化物成分差异，按大峡谷断层 (GGF) 划分。古生代地体（GGF 南部）的硫化物中钴含量持续升高，Co 含量持续升高。浓度 > 2.9 wt.% 且 Co/Ni 比率 > 0.048（球粒陨石）。相比之下，来自太古宙克拉通地体（GGF 以北）的硫化物具有较低的 Co 丰度（< 3600 ppm）和较低的 Co/Ni 比率（< 0.030）。钴富集的原因尚不清楚，但我们强调，全球重要的钴矿化与蛇绿岩（例如摩洛哥的布阿泽尔和芬兰奥托昆普）或缓慢扩张的山脊的海洋橄榄岩层背景有关。因此，我们认为苏格兰南部地体的钴富集与海洋密切相关，这可能与加里东造山运动期间富钴洋壳的俯冲直接相关。此外，我们还确定了 GGF 中 Pt/Pd 比率之间的区别，例如克拉通 SCLM 中的硫化物具有 Pt/Pd ≥ 球粒陨石，而古生代硫化物具有 Pt/Pd < 球粒陨石。我们观察到，具有离散 Pt 矿物（例如 PtS）的富 Pt 硫化物与 GGF 北部两个捕虏体组中的碳酸盐和磷酸盐相关。这种三向不混溶性（碳酸盐-硫化物-磷酸盐）表明碳酸盐岩交代作用是导致该（边缘）克拉通环境中铂富集的原因。这些 Co 和 Pt 富集可能从根本上反映了克拉通与非克拉通岩石圈地体的地球动力学背景，并为促进岩石圈地幔的地球化学绘图提供了潜在的工具。
Abundances of precious metals and cobalt in the lithospheric mantle are typically obtained by bulk geochemical analyses of mantle xenoliths. These elements are strongly chalcophile and the mineralogy, texture and trace element composition of sulphide phases in such samples must be considered. In this study we assess the mineralogy, textures and trace element compositions of sulphides in spinel lherzolites from four Scottish lithospheric terranes, which provide an ideal testing ground to examine the variability of sulphides and their precious metal endowments according to terrane age and geodynamic environment. Specifically we test differences in sulphide composition from Archaean-Palaeoproterozoic cratonic sub-continental lithospheric mantle (SCLM) in northern terranesvs.Palaeozoic lithospheric mantle in southern terranes, as divided by the Great Glen Fault (GGF).Cobalt is consistently elevated in sulphides from Palaeozoic terranes (south of the GGF) with Co concentrations > 2.9 wt.% and Co/Ni ratios > 0.048 (chondrite). In contrast, sulphides from Archaean cratonic terranes (north of the GGF) have low abundances of Co (< 3600 ppm) and low Co/Ni ratios (< 0.030). The causes for Co enrichment remain unclear, but we highlight that globally significant Co mineralisation is associated with ophiolites (e.g., Bou Azzer, Morocco and Outokumpu, Finland) or in oceanic peridotite-floored settings at slow-spreading ridges. Thus we suggest an oceanic affinity for the Co enrichment in the southern terranes of Scotland, likely directly related to the subduction of Co-enriched oceanic crust during the Caledonian Orogeny. Further, we identify a distinction between Pt/Pd ratio across the GGF, such that sulphides in the cratonic SCLM have Pt/Pd ≥ chondrite whilst Palaeozoic sulphides have Pt/Pd < chondrite. We observe that Pt-rich sulphides with discrete Pt-minerals (e.g., PtS) are associated with carbonate and phosphates in two xenolith suites north of the GGF. This three-way immiscibility (carbonate-sulphide-phosphate) indicates carbonatitic metasomatism is responsible for Pt-enrichment in this (marginal) cratonic setting. These Co and Pt-enrichments may fundamentally reflect the geodynamic setting of cratonicvs.non-cratonic lithospheric terranes and offer potential tools to facilitate geochemical mapping of the lithospheric mantle.