绿片岩-低角闪岩相变质条件下磁铁矿与黄铁矿间的Fe同位素分馏

Fe isotope fractionation between magnetite and pyrite during green schist-lower amphibolite facies metamorphism

对辽宁省鞍山-本溪地区经历了绿片岩-低角闪岩相变质的新太古代条带状铁建造中磁铁矿和黄铁矿矿物对的Fe同位素分析结果显示：相对于标准IRMM-014，所有样品的磁铁矿和黄铁矿均显示Fe的重同位素富集；且黄铁矿的Fe同位素比值均大于磁铁矿的Fe同位素比值（ε^57 Fe黄铁矿〉ε^57 Fe磁铁矿），两种矿物的Fe同位素比值之差为△^57 Fe黄铁矿-磁铁矿=2．23-5．13。黄铁矿富集铁的重同位素表明矿物的Fe同位素组成并不代表其原始沉积的特征，而是在区域变质作用过程中Fe同位素发生了交换的结果。由同位素平衡判别图解可知，在绿片岩-低角闪岩相变质作用中，磁铁矿-黄铁矿间的Fe同位素基本达到了平衡，且在平衡条件下黄铁矿比磁铁矿更富集Fe的重同位素，二者之间的Fe同位素平衡分馏系数口黄铁矿-磁铁矿≈1．0004‰±0．06‰（2σ）。这一研究成果是对变质作用过程中Fe同位素的地球化学行为认识的重要进展。

We present in this paper Fe isotope compositions for magnetite and pyrite mineral separates from banded iron formations in the Anshan-Benxi area, where green schist-lower amphibolite facies metamorphism has occurred. Both pyrite and magnetite show heavy Fe isotope enrichment relative to IRMM-014; furthermore, Fe isotope compositions of pyrite are systematically heavier than those of corresponding magnetite from the same samples（△^57 Fe pyrite-magnetite=2.23-5. 13 5） . These features cannot be explained if they represent the primary signatures inherited from their sedimentary precursors, but can be reconciled if Fe isotope exchange and reequilibrium have been taking place during regional metamorphism. The equilibrium mass fractionation factor of 57 Fe/S4Fe in green schist-to lower-amphibolite facies conditions is αpyrite-magretite≈l. 000 49‰±0. 069‰（2σ）. This study represents a significant advance in understanding the behaviour of Fe isotopes during metamorphism.