We report combined UPb ages, Hf isotopes, and trace-element compositions of 116 mantle zircon megacrysts from the unusual diamond-bearing sedimentary rocks of the northeastern Siberian craton to constrain the age and geochemical affinity of their parental rocks as well as their possible relationship to diamond-rich placers of northern Yakutia. The textures, trace-element systematics, and PbU dates of zircon megacrysts are evidence of their formation from kimberlites or related alkaline mantle melts and are similar to those of zircon from most Siberian kimberlites. The exclusively Triassic ages of zircon (247–202 Ma) fit the Triassic kimberlite eruption ages and those of related ultramafic-mafic magmatism in northern Siberia, which has conventionally been considered to sample the diamond-barren Proterozoic subcontinental lithospheric mantle. The chemical compositions of zircon megacrysts largely overlap with those of mantle zircon from both diamondiferous Paleozoic and diamond-barren Late Triassic and Late Jurassic kimberlite fields and indicate kimberlitic parental melts of significantly variable composition. Zircon megacrysts reveal Hf/Hf heterogeneities (ε(t) of +1.7 to +10.5), which, combined with a range of parental melt compositions, favor a chemically-heterogeneous sublithospheric source, which comprises a mixture of moderately depleted peridotitic and enriched pyroxene-rich (pyroxenitic, lherzolitic, or eclogitic) domains. As favored mechanisms, local incipient melting of variable degrees and carbonate-dominated melt fractionation en route to the surface might produce alkaline melts of contrasting bulk compositions within a spatially-limited mantle section. The zircon observations in this study indicate that the Triassic Bulkur rocks as well as – potentially – all the diamond-rich placers in northeastern Siberian craton may be sourced from one or a few coeval occurrences of unexposed kimberlites, which should have sampled the yet diamondiferous Triassic mantle. Considering the existing data for placer diamonds and specific K-Fe-Ti enriched composition of kimberlites in northeastern Siberian craton, we propose that chemical and LuHf isotopic signatures of the Bulkur zircon, bulk kimberlite compositions and diamond growth and storage in northeastern Siberia might be governed by the same processes of Mesozoic interaction between the hot ascending asthenosphere, oceanic slabs and lower cratonic lithosphere.

中文翻译：

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追踪西伯利亚克拉通东北部三叠纪沉积物中锆石巨晶的起源，对克拉通边缘次大陆岩石圈地幔钻石稀缺的影响

我们报告了来自西伯利亚克拉通东北部不寻常的含金刚石沉积岩的 116 个地幔锆石巨晶的 UPb 年龄、Hf 同位素和微量元素组合，以限制其母岩的年龄和地球化学亲和力以及它们与雅库特北部富含钻石的砂矿。锆石巨晶的结构、微量元素系统学和 PbU 年代是其由金伯利岩或相关碱性地幔熔体形成的证据，并且与大多数西伯利亚金伯利岩中的锆石相似。锆石的独特三叠纪年龄（247-202 Ma）符合三叠纪金伯利岩喷发年龄以及西伯利亚北部相关的超镁铁质-镁铁质岩浆作用的年龄，传统上认为西伯利亚北部是不含钻石的元古代次大陆岩石圈地幔的样本。锆石巨晶的化学成分与来自含金刚石古生代和不含金刚石的晚三叠世和晚侏罗世金伯利岩田的地幔锆石的化学成分大部分重叠，表明金伯利岩母体熔体的成分存在显着变化。锆石巨晶揭示了 Hf/Hf 异质性（ε(t) 为 +1.7 至 +10.5），与一系列母体熔体成分相结合，有利于化学异质的岩石圈下源，其中包括中度贫化的橄榄岩和富集的辉石的混合物-富含（辉石岩、二辉橄榄岩或榴辉岩）区域。作为有利的机制，不同程度的局部初期熔融和在到达地表的途中以碳酸盐为主的熔体分馏可能会在空间有限的地幔部分内产生具有对比整体成分的碱性熔体。这项研究中的锆石观测结果表明，三叠纪大块岩石以及西伯利亚克拉通东北部所有富含钻石的砂矿可能源自一处或几处同时期未暴露的金伯利岩，这些金伯利岩应该采样了尚含钻石的三叠纪地幔。考虑到西伯利亚克拉通东北部砂金钻石和金伯利岩特定 K-Fe-Ti 富集成分的现有数据，我们建议西伯利亚东北部大块锆石、大块金伯利岩成分以及钻石生长和储存的化学和 LuHf 同位素特征可能受到控制热上升软流圈、大洋板片和下克拉通岩石圈之间的中生代相互作用过程相同。