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加拿大渥太华大学Patricia L. Clay及课题组在研究早期地球环境中的极端的蛇纹石化和脱硫中取得新进展。2023年4月24日出版的《地质学》杂志发表了这项成果。
课题组人员提出了一个地球化学(稀土元素(REE)、卤素和高度亲铁元素(HSE)丰度)和同位素(惰性气体、锌、铬、和稀土-锇)的研究，以阐明这个谜一样的矿体的起源。与原始地幔(PM)相比，Bon Accord富含稀土元素，其模式类似于贫铝的科马提岩。大部分材料有大于原始地幔10倍的锇和铱，大于原始地幔100倍的钌、铂和钯，及放射性的187Os /188Os。铁镍矿、硅酸盐和两个大块岩石样品与形成时的球粒状陨石到亚球粒陨石的初始187Os /188Os一致。
新的稀土元素数据表明Bon Accord形成过程中的科马提岩前体，高度亲铁元素数据显示，其与富镍太古代岩浆硫化物矿床具有惊人的相似性。在蛇纹石化过程中，富集较重的锌同位素有助于硫化物(同位素较轻)矿床的脱硫，从而产生以铁镍矿为主的矿体。该课题组保守估计，这个过程可以动员多达约9.2×104吨的H2S，为硫的来源提供了一个有趣的可能性，这种硫操纵了巴伯顿绿岩带中具有历史意义的金矿化。
据介绍，ca. 3.5 Ga的Bon Accord镍矿床，位于南非卡普瓦尔克拉通的巴伯顿绿岩带，含有独特的富镍矿物组合包括铁镍矿(NiFe2O4)。它最初被解释为一个变质的外星物体，但这个观点最近被Cr同位素数据排除，随后被解释为太古代地核碎片。最近的观点强调了Bon Accord和太古代岩浆硫化物矿化之间的相似性。
附：英文原文
Title: Extreme serpentinization and desulfurization in an early Earth setting
Author: Patricia L. Clay, James M.D. Day, Henner Busemann, Pierre Bonnand, Ray Burgess, Richard A. Hornsey, Richard D. Ash, Frédéric Moynier, Brian ODriscoll
Issue&Volume: 2023-04-24
Abstract: The ca. 3.5 Ga Bon Accord Ni deposit was located in the Barberton Greenstone Belt of the Kaapvaal Craton, South Africa, and contained a unique assemblage of Ni-rich minerals including trevorite (NiFe2O4). It was initially interpreted as a metamorphosed extraterrestrial body, recently ruled out by Cr isotope data, and subsequently as a fragment of the Archean Earth’s core. More recent suggestions have highlighted similarities between Bon Accord and Archean magmatic sulfide mineralization. We present a geochemical (rare earth element [REE], halogen, and highly siderophile element [HSE] abundance) and isotopic (noble gases, Zn, Cr, and Re-Os) study to elucidate the origin of this enigmatic body. Bon Accord is enriched in the REE relative to primitive mantle (PM), with a pattern resembling that of Al-depleted komatiites. The bulk material has >10 × PM Os and Ir, >100 × PM Ru, Pt, Pd, and Re, and radiogenic 187Os/188Os. Trevorite, silicates, and two bulk-rock samples are consistent with chondritic to sub-chondritic initial 187Os/188Os at the time of formation. The new REE data implicate a komatiite precursor in the formation of Bon Accord, and the HSE data bear striking similarities to those of Ni-enriched Archean magmatic sulfide deposits. Enrichment in the heavier Zn isotopes supports desulfurization of a sulfide (isotopically light) deposit during serpentinization to produce the trevorite-dominated body. We conservatively estimate this process could have mobilized as much as ～9.2 × 104 tonnes of H2S, offering an intriguing possibility for sourcing of the sulfur that fixes the historically important gold mineralization in the Barberton Greenstone Belt.
