岩石圈地幔的金伯利岩预处理及其对钻石存活的影响:对科伊杜矿(塞拉利昂)橄榄石和地幔异晶的个案研究

IF 4.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Geoffrey H. Howarth, Andrea Giuliani, Yannick Bussweiler, Merrily Tau, Sinelethu Hashibi, Philip E. Janney, Tom E. Nowicki
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引用次数: 0

摘要

金伯利岩的金刚石潜力一般是通过母岩浆在次大陆岩石圈地幔(SCLM)中上升时夹带的指示矿物(即异晶)来确定的。越来越明显的是,橄榄石也可以用来了解地幔取样深度,利用铝-橄榄石温度计,并制约 SCLM 中金刚石破坏性变质作用的程度。为了进一步了解目前在SCLM中的垂直取样和金刚石保存情况,我们介绍了Koidu矿(塞拉利昂)金伯利岩橄榄石的地球化学结果。我们将橄榄石数据与现有橄榄石金刚石包裹体、倩辉石异晶体和斜长岩侧晶的压力-温度估算值相结合,直观地展示了科伊杜地下 SCLM 中岩性的垂直分布。由于在下层沙壳岩层中没有橄榄石,橄榄石金刚石包裹体的丰度也很低,因此在最下层岩石圈的取样物质中,巨晶似乎占主导地位。在较浅的区域(160-180 千米),可以观察到一个明显的以埃克洛辉石为主的区域,而在 110-150 千米深处的岩石圈则是由贫化的哈兹堡岩/云英岩、蛭石和埃克洛辉石组成的异质区域。金刚石主要是蚀变质岩,在蚀变质岩为主的区域内,含金刚石蚀变质岩的压力-温度估计值为 150-190 千米。在岩石圈-热成层边界附近取样几乎没有发现金刚石,这是因为原金伯利岩熔体的大量渗入导致埃洛ogite 和橄榄岩基质的变质作用以及巨晶的形成,从而破坏了金刚石。其他地方金伯利岩采样的SCLM最深处广泛的变质作用表明,科伊杜并不是一个孤立的案例,金刚石破坏性变质作用的程度可以用橄榄石异晶石来制约。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

Kimberlite pre-conditioning of the lithospheric mantle and implications for diamond survival: a case study of olivine and mantle xenocrysts from the Koidu mine (Sierra Leone)

Kimberlite pre-conditioning of the lithospheric mantle and implications for diamond survival: a case study of olivine and mantle xenocrysts from the Koidu mine (Sierra Leone)

The diamond potential of kimberlites is generally determined using indicator minerals (i.e., xenocrysts), entrained by the parent magma while ascending through the sub-continental lithospheric mantle (SCLM). It is becoming increasingly apparent that olivine can also be used to understand mantle sampling depth, using the Al-in-olivine thermometer, and to constrain the extent of diamond-destructive metasomatism in the SCLM. To further current understanding of vertical sampling and diamond preservation in the SCLM, we present geochemical results for kimberlitic olivine of the Koidu mine (Sierra Leone). We combine our olivine data with pressure-temperature estimates from available olivine diamond inclusions, clinopyroxene xenocrysts, and eclogite xenoliths to visualise the vertical distribution of lithologies in the SCLM beneath Koidu. In agreement with the absence of peridotitic olivine and low abundance of olivine diamond inclusions in the lower SCLM, megacrysts appear to dominate the material sampled from the lowermost lithosphere. At shallower levels a distinct eclogite-dominated region is observed (160-180 km) whereas the SCLM at depths of 110-150 km is heterogeneous comprising depleted harzburgite/dunite, lherzolite, and eclogite. Diamonds are predominantly eclogitic with pressure-temperature estimates for diamondiferous eclogites of 150-190 km within the eclogite-dominated region. The near absence of diamonds sampled from near the lithosphere-asthenosphere boundary is attributed to diamond destruction by extensive infiltration of proto-kimberlite melts leading to metasomatism of the eclogite and peridotite substrate and megacrysts formation. Widespread metasomatism of the deepest reaches of the SCLM sampled by kimberlites elsewhere suggests that Koidu does not represent an isolated case and the extent of diamond-destructive metasomatism can be constrained using olivine xenocrysts.

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来源期刊
Mineralium Deposita
Mineralium Deposita 地学-地球化学与地球物理
CiteScore
11.00
自引率
6.20%
发文量
61
审稿时长
6 months
期刊介绍: The journal Mineralium Deposita introduces new observations, principles, and interpretations from the field of economic geology, including nonmetallic mineral deposits, experimental and applied geochemistry, with emphasis on mineral deposits. It offers short and comprehensive articles, review papers, brief original papers, scientific discussions and news, as well as reports on meetings of importance to mineral research. The emphasis is on high-quality content and form for all articles and on international coverage of subject matter.
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