Paradoxical co-existing base metal sulphides in the mantle: The multi-event record preserved in Loch Roag peridotite xenoliths, North Atlantic Craton
Paradoxical co-existing base metal sulphides in the mantle: The multi-event record preserved in Loch Roag peridotite xenoliths, North Atlantic Craton
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DOI：
10.1016/j.lithos.2016.09.035
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发表时间：
2017-04
期刊：
影响因子：
3.5
通讯作者：
H. Hughes;I. McDonald;M. Loocke;I. Butler;B. Upton;W. John;Faithfull
中科院分区：
文献类型：
--
作者：
H. Hughes;I. McDonald;M. Loocke;I. Butler;B. Upton;W. John;Faithfull
次大陆岩石圈地幔作为镁铁质岩浆中贵金属来源的作用存在争议，而且由于铂族元素(PGE)、Se、Te、Re、Cu和Au等金属的亲黄(亲铁)特征，这些金属的活动性与硫的活动性密切相关。因此，寄主阶段(S)的性质以及它们在次大陆岩石圈地幔中的年龄和稳定性可能是至关重要的。研究了苏格兰西北部(路易斯岛罗格湖)尖晶石二辉橄榄岩地幔包体中贱金属硫化物(BMS)的硫化物矿物学和硫化物原位微量元素组成。该地区位于被古元古代造山带叠加的北大西洋克拉通的边缘，发生在从古元古代到始新世经历了岩浆事件的地区。我们在单个尖晶石二辉橄榄岩捕虏体(LR80)中识别出两个共存的BMS，它们也可以作为一个整体在橄榄岩包体中被识别出来。这两个群体都是由Fe-Ni-Cu硫化物矿物混合而成，我们根据BMS结构、岩相环境(即，X-射线计算机显微断层扫描显示的‘间隙’位置或长尖晶石-尖晶石混合矿物中的位置)和原位微量元素成分来区分它们。A组BMS粗，交代，总PGE含量低(约40ppm)，高(Re/Os)N(1~400)。B组BMS严格赋存于尖晶石和长石的包裹体中，为微米级的圆形微滴，PGE总量高(15-800 ppm)，Re/Os低(0.04-2)。B组BMS有时与磷灰石共生，B组BMS和磷灰石均能保存指示硫化物-碳酸盐-磷酸盐不混溶的微米级圆形碳酸钙包裹体。这种碳酸盐-磷酸盐交代作用似乎对形成富含PGE的硫化物液体很重要，尽管其确切机制仍不清楚。由于它们在共晶岩中的位置，B组BMS特别容易被合并到上升的地幔来源的岩浆中(无论是通过熔融还是物理夹带)。基于交界点的交叉关系，有可能推断出每个交代BMS种群的相对年龄。通过与记录在地壳和地幔岩石中的碳酸盐岩事件的对比，我们将这些事件与北大西洋地区的主要构造-岩浆事件相吻合，并认为铂的富集期与前石炭纪碳酸盐岩事件有关。这种地幔捕虏体贱金属硫化物记录方法最终可能允许绘制岩石圈地幔亲银成矿预算的时空图，为评估区域成矿潜力提供蓝图。
The role of the subcontinental lithospheric mantle as a source of precious metals for mafic magmas is contentious and, given the chalcophile (and siderophile) character of metals such as the platinum-group elements (PGE), Se, Te, Re, Cu and Au, the mobility of these metals is intimately linked with that of sulphur. Hence the nature of the host phase(s), and their age and stability in the subcontinental lithospheric mantle may be of critical importance. We investigate the sulphide mineralogy and sulphide in situ trace element compositions in base metal sulphides (BMS) in a suite of spinel lherzolite mantle xenoliths from northwest Scotland (Loch Roag, Isle of Lewis). This area is situated on the margin of the North Atlantic Craton which has been overprinted by a Palaeoproterozoic orogenic belt, and occurs in a region which has undergone magmatic events from the Palaeoproterozoic to the Eocene.We identify two populations of co-existing BMS within a single spinel lherzolite xenolith (LR80) and which can also be recognised in the peridotite xenolith suite as a whole. Both populations consist of a mixture of Fe-Ni-Cu sulphide minerals, and we distinguished between these according to BMS texture, petrographic setting (i.e., location within the xenolith in terms of ‘interstitial’ or within feldspar-spinel symplectites, as demonstrated by X-ray Computed Microtomography) and in situ trace element composition. Group A BMS are coarse, metasomatic, have low concentrations of total PGE (< 40 ppm) and high (Re/Os)N(ranging 1 to 400). Group B BMS strictly occur within symplectites of spinel and feldspar, are finer-grained rounded droplets, with micron-scale PtS (cooperite), high overall total PGE concentrations (15–800 ppm) and low (Re/Os)Nranging 0.04 to 2. Group B BMS sometimes coexist with apatite, and both the Group B BMS and apatite can preserve rounded micron-scale Ca-carbonate inclusions indicative of sulphide-carbonate-phosphate immiscibility. This carbonate-phosphate metasomatic association appears to be important in forming PGE-rich sulphide liquids, although the precise mechanism for this remains obscure. As a consequence of their position within the symplectites, Group B BMS are particularly vulnerable to being incorporated in ascending mantle-derived magmas (either by melting or physical entrainment). Based on the cross-cutting relationships of the symplectites, it is possible to infer the relative ages of each metasomatic BMS population. We tally these with major tectono-magmatic events for the North Atlantic region by making comparisons to carbonatite events recorded in crustal and mantle rocks, and we suggest that the Pt-enrichment was associated with a pre-Carboniferous carbonatite episode. This method of mantle xenolith base metal sulphide documentation may ultimately permit the temporal and spatial mapping of the chalcophile metallogenic budget of the lithospheric mantle, providing a blueprint for assessing regional metallogenic potential.