Abstract The lithospheric mantle should be depleted in base- and precious-metals as these elements are transferred to the crust during partial melting. However, some melt-depleted mantle peridotites are enriched in these ore-forming elements. This may reflect re-fertilization of the mantle lithosphere and/or sequestering of these elements by residual mantle phase(s). Both processes remain poorly understood because of the low abundances of incompatible elements in peridotite and the nugget-like distribution of digestion-resistant mantle phases that pose analytical challenges for conventional geochemical methods. Herein we report new major and trace element concentrations for a suite of mantle peridotite and pyroxenite samples from the Late Permian to Middle Triassic Nahlin ophiolite (Cache Creek terrane, British Columbia, Canada) using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) analysis of nanoparticulate powders and olivine. Compatible to moderately incompatible element concentrations suggest that Nahlin ophiolite peridotites represent residues after ≥20% melt extraction. Pyroxenite dykes and replacive dunite bands are folded and closely intercalated with residual harzburgite. These field relationships, coupled with the presence of intergranular base metal sulphide, clinopyroxene and Cr-spinel at the microscale, point to percolating melts that variably re-fertilized melt-depleted mantle peridotite. Radiogenic Pb (206Pb/204Pb = 15.402–19.050; 207Pb/204Pb = 15.127–15.633; 208Pb/204Pb = 34.980–38.434; n = 45) and Os (187Os/188Os 0.1143–0.5745; n = 58) isotope compositions for a subset of melt-depleted peridotite samples further support metasomatic re-fertilization of these elements. Other ore-forming elements are also implicated in these metasomatic reactions because some melt-depleted peridotite samples are enriched relative to the primitive mantle, opposite to their expected behaviour during partial melting. New LA-ICPMS analysis of fresh olivine further demonstrates that a significant proportion of the highly incompatible element budget for the most melt-depleted rocks is either hosted by, and/or occurs as trapped inclusions within, the olivine-rich residues. Trapped phases from past melting and/or re-fertilization events are the preferred explanation for unradiogenic Pb isotope compositions and Paleozoic to Paleoproterozoic Re-depletion model ages, which predate the Nahlin ophiolite by over one billion years.

中文翻译：

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通过纳米粉末和橄榄石 LA-ICPMS 分析确定的再受精地幔橄榄岩的元素和同位素特征

摘要 岩石圈地幔中的贱金属和贵金属在部分熔融过程中会转移到地壳中，因此这些元素应该是贫化的。然而，一些熔体耗尽的地幔橄榄岩富含这些成矿元素。这可能反映了地幔岩石圈的再肥化作用和/或残余地幔相对这些元素的隔离作用。由于橄榄岩中不相容元素的丰度低以及耐消化地幔相的金块状分布，这对传统地球化学方法的分析构成了挑战，因此这两个过程仍然知之甚少。在此，我们报告了从晚二叠世到中三叠世 Nahlin 蛇绿岩（Cache Creek terrane，不列颠哥伦比亚省，Canada) 使用激光烧蚀电感耦合等离子体质谱 (LA-ICPMS) 分析纳米颗粒粉末和橄榄石。与中等不相容元素浓度相容表明 Nahlin 蛇绿橄榄岩代表了 ≥ 20% 熔体提取后的残留物。辉石岩脉和置换纯晶带被折叠并与残余的菱镁矿紧密夹层。这些场的关系，加上在微观尺度上存在粒间贱金属硫化物、单斜辉石和铬尖晶石，表明渗滤熔体不同程度地重新施肥了熔体耗尽的地幔橄榄岩。放射性铅（206Pb/204Pb = 15.402–19.050；207Pb/204Pb = 15.127–15.633；208Pb/204Pb = 34.980–38.434；n = 487.87Os/50s/4050s–1870s–15.633）n = 58) 熔融耗尽橄榄岩样品子集的同位素组成进一步支持这些元素的交代再受精。其他成矿元素也与这些交代反应有关，因为一些熔融耗尽的橄榄岩样品相对于原始地幔富集，这与它们在部分熔融期间的预期行为相反。对新鲜橄榄石的新 LA-ICPMS 分析进一步表明，对于大部分熔体耗尽的岩石，高度不相容的元素预算中有很大一部分是由富含橄榄石的残渣承载的，和/或作为夹杂物出现在富含橄榄石的残渣中。过去熔化和/或再受精事件的俘获阶段是非放射性铅同位素组成和古生代到古元古代再耗竭模型年龄的首选解释，