Investigating metasomatic effects on the 187Os isotopic signature: A case study on micrometric base metal sulphides in metasomatised peridotite from the Letlhakane kimberlite (Botswana)
Investigating metasomatic effects on the 187Os isotopic signature: A case study on micrometric base metal sulphides in metasomatised peridotite from the Letlhakane kimberlite (Botswana)
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研究交代对 187Os 同位素特征的影响：Letlhakane 金伯利岩交代橄榄岩中微米级贱金属硫化物的案例研究（博茨瓦纳）
DOI：
10.1016/j.lithos.2015.06.017
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发表时间：
2015
期刊：
影响因子：
3.5
通讯作者：
Pearson DG
中科院分区：
文献类型：
--
作者：
Wainwright AN;Luguet A;Fonseca ROC ;Pearson DG
Letlhakane 金伯利岩（博茨瓦纳）的橄榄岩捕虏体侵入津巴布韦克拉通西缘的元古代马贡迪带，代表高度贫化的熔融残留物。这些残留物随后遭受了可变的交代叠印，尖晶石橄榄岩中神秘的微量元素富集以及石榴石橄榄岩中金云母、单斜辉石和尖晶石的模式添加证明了这一点。为了评估 Re-Os 天文仪在这种高度交代橄榄岩中的稳健性，在三个代表性橄榄岩中对贱金属硫化物 (BMS) 岩相学和单 BMS 颗粒 187Os/188Os 分析进行了详细研究。BMS 表现为 < 10 µm–50 µm 包裹体和间隙颗粒，与交代阶段或交代边缘相关。原生硅酸盐或显示出熔体状形态，所有这些都证明了它们的交代起源。它们的 187Os 同位素组成从 0.1016 到 0.6109 不等，产生从 3.75 ± 0.54 (2se) 到未来的 TRDages。它们的变化独立于橄榄岩上的神秘或模态硅酸盐交代叠印，也独立于 BMS 硅酸盐结构习性（例如孤立的包裹体、假包裹体、粒间熔体状池），这与通常的假设相反。在这种高度贫化的橄榄岩中，在部分熔融事件后必定不含硫化物，太太古代年龄很可能继承自残留的 PGM（铂族矿物；即月桂石和锇合金），这些铂族矿物是由于原始 BMS 的耗尽而形成的，后来在交代 BMS 中重新溶解。相比之下，较年轻的单颗粒 TRDages 代表了交代 BMS 内残余 PGM 信号的日益稀释，单颗粒 187Os/188Os 特征越来越多地以来自交代 BMS 的 Os 为主。整体而言，单 BMS 颗粒太太古代年龄表明 Letlhakane 的岩石圈稳定年龄与津巴布韦克拉通最古老的地壳样本相似，因此支持 Letlhakane 地幔根部是津巴布韦克拉通地幔的向西延伸。单一 BMS 颗粒尺度的 TRD 年龄显示出比其各自的全岩石 TRD 年龄大得多的范围，最显着的是来自两个交代末端成员中的 BMS：隐交代橄榄岩和最模态叠印的橄榄岩，其中 BMS 比其全岩石年龄 1 至 2.5 Ga。因此，无论全岩经历了硅酸盐交代作用如何，贱金属硫化物都是比全岩更好的时间胶囊。此外，远古年龄是从间隙富镍 BMS 中获得的，这表明应该研究 Os 的所有可能宿主，而不仅仅是看起来是“原生”的 BMS（即橄榄石中的富铁 BMS 分离包裹体）。因此，单 BMS 颗粒铼-锇测年应被视为唯一可靠的测年方法，适用于所有经历过广泛交代叠印或再化作用的构造环境（造山带、蛇绿岩、克拉通带）。
The peridotite xenoliths of the Letlhakane kimberlite (Botswana), which intrude the Proterozoic Magondi Belt on the western margin of the Zimbabwe craton, represent highly depleted melting residues. These residues suffered subsequent variable metasomatic overprinting, evidenced by cryptic trace element enrichments in the spinel peridotites to modal addition of phlogopite, clinopyroxene and spinel within the garnet peridotites. In order to assess the robustness of the Re–Os chronometer in such highly metasomatised peridotites, detailed investigations of base metal sulphide (BMS) petrography and single-BMS grain187Os/188Os analyses have been undertaken in three representative peridotites.The BMS occur as < 10 μm–50 μm inclusions and interstitial grains that are associated with metasomatic phases or metasomatised rims of primary silicates or display melt-like morphology, all attesting of their metasomatic origin. Their187Os isotopic compositions vary from 0.1016 to 0.6109 yielding TRDages from 3.75 ± 0.54 (2se) to future ones. They vary independently of the cryptic or modal silicate metasomatic overprinting on the peridotites and independently of the BMS-silicate textural habits (e.g. isolated inclusions, pseudo-inclusions, intergranular melt-like pools), contrary to what is commonly assumed. In such highly depleted peridotites that must have been sulphide-free after the partial melting event, the Eoarchean age is likely inherited from residual PGM (platinum group minerals; i.e. laurite and Os-alloys) that formed in response to the exhaustion of the primary BMS and were later redissolved within the metasomatic BMS. In contrast, the younger single grain TRDages represents an increasing dilution of the residual PGM signals within the metasomatic BMS, with the single grain187Os/188Os signatures becoming increasingly dominated by Os derived from metasomatic BMS.Taken as a whole, the single BMS grain Eoarchean age suggests a lithospheric stabilisation age in Letlhakane similar to the oldest crustal samples of the Zimbabwe craton, which thus supports the Letlhakane mantle root to be a westerly extension of the Zimbabwe cratonic mantle.The TRDages at the single BMS grain scale show a much larger range than their respective whole-rock TRDage, most notably from BMS in the two metasomatic end members: the cryptically-metasomatised peridotite and the most modally overprinted peridotite containing BMS 1 to 2.5 Ga older than their whole-rock. Base metal sulphides are thus better time capsules than the whole-rock, regardless of the silicate metasomatism the whole-rocks have experienced. Furthermore, the ancient ages are obtained from interstitial Ni-rich BMS indicating that all possible hosts of Os, not only BMS which appear to be ‘primary’ (i.e. Fe-rich BMS isolated inclusions in olivine) should be investigated. Single BMS grain Re–Os dating should then be considered as the only reliable dating approach, across all tectonic contexts (orogenic, ophiolitic, cratonic), which have experienced extensive metasomatic overprinting or refertilisation.
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DOI：
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