Multistep evolution of harzburgitic mantle underneath pipe 200 kimberlite, northern Lesotho: a study on xenoliths and their implication on diamond-barren nature of pipe 200 kimberlite

IF 1.8 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
Sahroz Khan, István J. Kovács, Yana Fedortchouk, Monika Feichter, Csaba Szabó, Tivadar M. Tóth
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Abstract

The Pipe 200 kimberlite in northern Lesotho on the southeast margin of the Kaapvaal Craton is a diamond-poor deposit despite its proximity to economically viable kimberlites like the Liqhobong kimberlite. We study harzburgite xenoliths' mineral composition, geochemistry, and pressure–temperature evolution to understand factors influencing diamond destruction. The xenoliths are classified into five types based on their petrography and geochemistry. The diversity in the mineral assemblage correlates with a sampling depth of ~ 100 to 175 km (~ 2.8 to < 5.0 GPa). The signatures of metasomatism are evident in type 3 and 4 xenoliths, where garnet breaks down to form a cluster (henceforth assemblage) of phlogopite, chromite, and diopside. Fine-grained melts associated with an uplift in the mantle to shallower depths of < 90 km (< 2.3 GPa) encompass the minerals in the assemblage, which display resorption at the boundaries. Water contents (structural hydroxyl) of olivine and possibly orthopyroxene are lower in the xenoliths with metasomatism-induced breakdown of garnet. The structural hydroxyl distribution in the nominally anhydrous minerals shows flat distribution profiles of re-equilibration due to residence in the kimberlite magma. It is supported by the disruption of the inter-mineral water partition coefficient due to olivine water diffusion during residence in the kimberlite magma. The barren nature of the Pipe 200 kimberlite is attributed to the signatures of mantle metasomatism and residence in kimberlite magma, which led to the diamond destruction.

Graphical abstract

Abstract Image

莱索托北部 200 号管状金伯利岩下的哈兹堡垒地幔的多步演化:对异岩石的研究及其对 200 号管状金伯利岩金刚石贫瘠性质的影响
位于莱索托北部卡普瓦尔克拉通东南边缘的 Pipe 200 金伯利岩是一个贫钻石矿床,尽管它靠近具有经济价值的金伯利岩,如 Liqhobong 金伯利岩。我们研究了哈兹伯格岩的矿物成分、地球化学和压力-温度演化,以了解影响钻石破坏的因素。根据岩石学和地球化学,我们将这些岩石划分为五种类型。矿物组合的多样性与约 100 至 175 千米(约 2.8 至 5.0 GPa)的取样深度相关。变质作用的特征在 3 型和 4 型斜长石中非常明显,石榴石分解后形成由辉绿岩、铬铁矿和透辉石组成的矿物群(以下简称矿物群)。与地幔上升到较浅深度 90 千米(2.3 GPa)有关的细粒熔体包裹着该集合体中的矿物,这些矿物在边界处显示出再吸附现象。橄榄石的含水量(结构羟基)和正辉石的含水量(结构羟基)在变质作用诱发石榴石分解的异岩石中较低。名义上无水矿物中的结构羟基分布显示出由于在金伯利岩浆中的停留而产生的再平衡的平坦分布曲线。在金伯利岩浆中停留期间,橄榄石的水扩散导致矿物间水分配系数的破坏,这也证明了这一点。Pipe 200 金伯利岩的贫瘠性质可归因于地幔变质作用和在金伯利岩岩浆中的停留,这导致了金刚石的破坏。
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来源期刊
International Journal of Earth Sciences
International Journal of Earth Sciences 地学-地球科学综合
CiteScore
4.60
自引率
4.30%
发文量
120
审稿时长
4-8 weeks
期刊介绍: The International Journal of Earth Sciences publishes process-oriented original and review papers on the history of the earth, including - Dynamics of the lithosphere - Tectonics and volcanology - Sedimentology - Evolution of life - Marine and continental ecosystems - Global dynamics of physicochemical cycles - Mineral deposits and hydrocarbons - Surface processes.
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