Fe-Mg同位素在蛇绿岩中铬铁矿床成因研究中的应用潜力
Potential Applications of Fe and Mg Isotopes in Genesis of Chromite Deposits in Ophiolites

蛇绿岩中铬铁矿床成因一直存在较大争议，其主要原因可归结为：寄主蛇绿岩存在成因争议、产出状态不清、矿石及围岩矿物组合单一以及主要矿物成分简单但矿物包裹体复杂多样.针对这些研究瓶颈，率先对西藏普兰和罗布莎、土耳其Kzldag和Kop蛇绿岩中的地幔橄榄岩和铬铁岩进行了全岩和单矿物Fe-Mg同位素的探索性研究工作.结果表明：(1)蛇绿岩中的地幔橄榄岩具有较均一的Fe-Mg同位素组成，与世界上其他地区的地幔橄榄岩相似；(2)铬铁岩中铬铁矿和橄榄石之间存在明显的Fe-Mg同位素分馏，铬铁矿多具有比共存橄榄石轻的Fe同位素组成，与地幔橄榄岩中的尖晶石和橄榄石相反，Mg同位素变化较大；(3)铬铁矿和橄榄石的Fe-Mg同位素主要受控于结晶分异和Fe-Mg交换，且这两个过程造成的同位素变化趋势明显不同.因此，Fe-Mg同位素在揭示铬铁矿母岩浆来源、性质及成矿过程方面具有较大的应用潜力.
Genesis of chromittie deposit in ophiolites has been debated for a long time. It is commonly related to host ophiolite, chromite occurrence, simple mineral assemblage and simple mineral chemistry, and unusual and complicated mineral inclusions. To constrain the genesis of chromite, a study on Fe and Mg isotopes of mantle peridotite and chromitite from Purang and Luobusa ophiolites in Tibet and Kzldag and Kop ophiolites in Turkey is conducted. The results reveal that (1) mantle peridotite of ophiolites has uniform Fe and Mg isotope compositions, falling in the same range of worldwide peridotite; (2) chromite and olivine in chromitite show large Fe and Mg isotope fractionations with overall lower δ56Fe and higher δ26Mg in chromite than olivine; (3) Fe and Mg isotope fractionations between chromite and olivine are mainly controlled by fractional crystallization and subsolidus Fe and Mg exchanges in two opposite processes. Therefore, Fe and Mg isotopes have great potentials to trace origin and nature of parental magmas of chromitite and crystallization and precipitation of chromite