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

Study on podiform chromitite and related platinum group mineral (PGM):Progress and prospection

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。

Podiform chromitite and related platinum group minerals（ PGM） in ophiolitic melanges are discussed in detail in this review paper. PGM with different origins and multi-generations found in chromitite might represent recycled detrital PGM. Reduction of the melt around crystallizing chromite grain caused saturation in the most easily oxidized platinum group elements（ Os,Ir,Ru： IPGE） in melt,which precipitate as metallic nanoparticles. Some authors have argued that these nanoparticles would be precursors of larger PGM sulfides by reaction with S upon local increases of f（ S_2） in melt. An alternative view suggests that the formation of IPGE-bearing sulfides and alloys is related to the desulfurization of base-metal sulfides（ BMS） during partial melting. In this case,a series of small events of melt extraction and melt-rock reaction produce a progressive but stepped sequence of decreasing f（S_2） in the mantle source region,promoting the breakdown of PGE-bearing BMS into residual PGM sulfides and IPGE-bearing alloys. The reaction of primary PGM with metamorphic fluids usually produces secondary rims and/or nanometer to micrometer-sized inclusions via in situ desulfurization. The multi-stage alteration and re-equilibration,linked with the ability of fluids to infiltrate chromite,may cause resolving and recrystallization of PGM,and seriously disturb the Os isotopic system. The coexistence of different ore-rock types and their recorded different magmatic processes demonstrate that ophiolitic melange represents a mechanic mingling of different rock units.Chromitite blocks could be dragged down into deep mantle and brought up to the surface afterwards. These recycled chromitites could occur in extensional center and be located in an ophiolitic melange with newly formed podiform chromitite finally.