Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions
Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions
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DOI：
10.1007/s00410-014-1081-8
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发表时间：
2014-11
影响因子：
3.5
通讯作者：
A. Thomson;S. Kohn;G. Bulanova;C. Smith;D. Araújo;M. Walter;Eimf
中科院分区：
文献类型：
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作者：
A. Thomson;S. Kohn;G. Bulanova;C. Smith;D. Araújo;M. Walter;Eimf
对来自巴西南部Juina-5金伯利岩管的41颗钻石进行了综合研究，其中含有光学可识别的包裹体。这些钻石含有20ppm的氮(N)，完全聚集成B中心。用阴极发光(CL)揭示的几颗钻石的内部结构与通常在岩石圈样品中观察到的结构不同。大多数钻石由同位素轻碳组成，该系列具有单峰分布，严重偏向δ13C~−25~‰。单个钻石可以表现出高达~15‰的大碳同位素不均一性，并且主要具有同位素较轻的核心显示蓝色CL，较重的边缘显示绿色CL。轻碳同位素组成被解释为金刚石是在热液蚀变过程中添加到洋壳中的非生物有机碳生长的证据。洋壳的整体同位素组成，碳酸盐加有机质，等于地幔碳(−5，‰)的组成，我们认为，俯冲物质的再循环/混合将随着地质时间的推移补充这个储集层。几个暴露在外的同生包裹体具有与以前榴辉硅酸镁钙钛矿、硅酸钙钙钛矿以及在折返到地表过程中重新平衡的NaL或CF相一致的主体成分。主晶石榴石与辉石出溶、柯石英与蓝晶石出溶、单斜辉石、铁或铁碳化物、硫化物矿物及橄榄石和铁方镁石单生。作为一个群体，包裹体具有榴辉亲和力，并为钻石的形成提供了证据，压力延伸到地球深部过渡带，可能还包括下地幔。观察到包裹体的主要元素组成与寄主Juina-5钻石的同位素组成没有相关性。钻石和包裹体成分与俯冲物质密切相关，记录了从下地幔到岩石圈底部的深部段的多压力生长历史。认为板岩熔体和地幔物质的相互作用，再加上随后在沟道化网络或浮力底辟中的向上输送，解释了Juina-5钻石的形成。我们的结论是，尽管这些样本来自巨大的地幔深处，但它们并没有提供关于周围地幔的直接信息，而是提供了地球深层碳循环的快照。
Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ13C ~ −25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (−5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have re-equilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth’s deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are intimately related to subducted material and record a polybaric growth history across a depth interval stretching from the lower mantle to the base of the lithosphere. It is suggested that the interaction of slab-derived melts and mantle material combined with subsequent upward transport in channelised networks or a buoyant diapir explains the formation of Juina-5 diamonds. We conclude that these samples, despite originating at great mantle depths, do not provide direct information about the ambient mantle, instead, providing a snapshot of the Earth’s deep carbon cycle.