豆荚状铬铁矿以及其中铂族元素矿物的成因问题——进展与展望

发布时间：2018-04-02 17:49

本文选题：地质学　切入点：豆荚状铬铁矿　出处：《矿床地质》2017年04期

【摘要】：铂族元素矿物(Platinum Group Mineral:简称PGM)资料的不断积累,丰富了人们对蛇绿岩中豆荚状铬铁矿成因的认识。文章总结近年来有关PGM的新资料和取得的新认识,探讨豆荚状铬铁矿以及其中PGM的成因问题。幔源岩浆结晶过程中,铬铁矿周边熔体减少将诱发那些易氧化的铂族元素(Os、Ir、Ru)在熔体中达到饱和状态,并结晶形成纳米级PGM。在地幔熔体中,随着硫逸度升高,PGM微粒与熔体中的硫反应并逐渐长大。多期次的熔体抽提和熔体-岩石反应事件,可以在地幔源区通过逐步降低硫逸度、促进含铂族元素的贱金属硫化物分解,形成PGM以及铂族元素合金。低硫逸度环境更有利于PGM的形成和保存。在变质环境或流体环境中,这些PGM往往会与流体反应,造就了PGM矿物的多样性。原生PGM与变质流体反应并发生原地去硫化作用,可以形成次生的PGM环边或者纳米级PGM包体。铬铁矿的多阶段蚀变/再平衡过程可以导致PGM溶解—沉淀—均一化,并扰动Os同位素体系。不同类型矿石在有限空间伴生的现象以及它们所具有显著差异的地球化学特征,说明蛇绿岩是不同地幔组分的机械混杂。随着俯冲板片,铬铁矿团块被拖曳到地幔深部,并通过地幔对流重新出现在扩张中心附近,最终混杂在蛇绿岩中。发生循环的铬铁矿团块因此可以与新生铬铁矿及其围岩伴生在同一蛇绿混杂岩中。
[Abstract]:The accumulation of Platinum Group Mineraldata enriches the understanding of the origin of pod-like chromite in ophiolite.In this paper, the new data and knowledge about PGM in recent years are summarized, and the origin of pod-like chromite and its PGM are discussed.During the crystallization of mantle-derived magma, the decrease of the peripheral melts of chromite will induce the oxidized platinum group elements Osan Ir-Ru) to reach saturation state in the melt and form nanocrystalline PGM.In mantle melts, PGM particles react with sulfur in melts and grow up with increasing sulfur fugacity.Multi-stage melt extraction and melt-rock reaction events can promote the decomposition of base metal sulphide containing platinum group elements and form PGM and platinum alloy in mantle source region by gradually reducing sulfur fugacity.Low sulfur fugacity environment is more conducive to the formation and preservation of PGM.In metamorphic or fluid environments, these PGM often react with fluids, resulting in the diversity of PGM minerals.The primary PGM reacts with the modified fluid and occurs in situ devulcanization to form a secondary PGM ring edge or nanoscale PGM inclusion.The multistage alteration / reequilibrium process of chromite can lead to PGM dissolution-precipitation-homogenization and disturb the Os isotopic system.The phenomena associated with different types of ores in finite space and their distinct geochemical characteristics indicate that ophiolite is a mechanical mixture of different mantle components.With the subduction plate the chromite lump is towed to the deep mantle and reappears in the vicinity of the spreading center through the mantle convection finally mixed in in the ophiolite.The chromite block with cyclic occurrence can be associated with the new chromite and its surrounding rock in the same ophiolite melange.
【作者单位】：造山带与地壳演化教育部重点实验室北京大学地球与空间科学学院;
【基金】：国家重点研发计划项目(编号:2017YFC0601302) 国家自然科学基金项目(编号:41672047)资助
【分类号】：P618.3;P618.53

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