基性岩浆分离结晶过程中矿物间Fe、Mg同位素分馏研究：以四川白马层状岩体为例

It is paid more and more attentions for geological scientists to study on Fe and Mg isotopes, because Fe and Mg are two major elements and their isotopes can be widely applied to trace numerous geological processes. Recent studies have suggested that the Fe isotope would largely fractionate during mafic magma differentiation, while fractionation of the Mg isotope is very limited. However, the fractionation of Fe and Mg isotopes between minerals in mafic rocks are still poorly understood due to very lack of isotopic compositions of these minerals. Particularly, the mechanisms of Fe and Mg isotopic fractionation between silicate minerals, magnetite and ilmenite, together with the key factors that control these fractionation are not well constrained. On the basis of our preliminary studies on Fe isotope of a small amount of samples from the Baima layered intrusion, Sichuan province, we will use MC-ICP-MS to measure the Fe and Mg isotopic systematics of co-existing magnetite, ilmenite, olivine, and clinopyroxene from all petrographical zones of this intrusion in this project. By doing these analyses, we further attempt to illustrate the regularities, mechanisms and critical control factors of the Fe and Mg isotopic fractionation between minerals during mafic magma differentiation. Meanwhile, we will utilize UV femtosecond LA-MC-ICP-MS to determinate the Fe isotopic variations of the magnetite and ilmenite in-situ, and then to reveal the influencing factors on their Fe isotopes during the process of sub-solidus equilibration on cooling. This project will highlight significantly the fundamental theory of the non-traditional stable isotope fractionation during mafic magma differentiation.

Fe和Mg作为两种常量元素，其同位素的地球化学示踪意义受到日益广泛的关注。已有研究表明基性岩浆演化过程会导致Fe同位素发生显著分馏，而Mg同位素分馏微弱。但是，部分矿物Fe、Mg同位素数据的缺乏使得基性岩矿物间Fe、Mg同位素分馏研究十分薄弱，特别是硅酸盐矿物与磁铁矿和钛铁矿间Fe、Mg同位素的分馏机制和控制及影响因素尚不明确。本项目拟在申请人对四川白马层状岩体部分样品已有的Fe同位素研究基础上，利用MC-ICP-MS技术，测定不同岩相带中磁铁矿、钛铁矿、橄榄石和单斜辉石的Fe、Mg同位素组成，阐明基性岩浆演化过程中矿物间Fe、Mg同位素的分馏规律、机制及其关键控制因素。与此同时，借助飞秒激光LA-MC-ICP-MS对磁铁矿和钛铁矿微区的Fe同位素原位分析，深入揭示亚固相线再平衡对二者Fe同位素组成的影响。本项目对进一步完善基性岩浆分离结晶过程中非传统稳定同位素分馏基础理论具有重要意义。

近十几年来，Fe、Mg等金属稳定同位素地球化学成为地球科学中最活跃、发展最快的前沿领域。然而，目前对镁铁-超镁铁质侵入岩的Fe、Mg同位素研究非常薄弱。本项目以四川白马含钒钛磁铁矿层状岩体为研究对象，对其中的磁铁矿、钛铁矿、橄榄石和单斜辉石开展了系统的Fe、Mg同位素研究。结合矿物学、岩石学、以及岩石地球化学，阐明了幔源镁铁质岩浆演化过程中矿物间Fe、Mg同位素的分馏规律、机制及其关键控制因素，取得了一些创新性认识，包括：（1）层状岩体堆晶矿物之间会发生显著的Fe同位素分馏：δ57Fe磁铁矿>δ57Fe橄榄石≈δ57Fe单斜辉石>δ57Fe钛铁矿，且分馏受Fe的价态和矿物结构的制约；磁铁矿和钛铁矿的Fe同位素组成不仅受分离结晶的控制，而且受Fe-Ti氧化物亚固相线再平衡的改造；（2）层状岩体中橄榄石和单斜辉石的Mg同位素组成变化较小（δ26Mg = -0.33~+0.05‰），Fe-Ti氧化物的变化非常大（δ26Mg = -0.50~+1.90‰）。橄榄石和单斜辉石的δ26Mg值及其相关关系表明二者主要受母岩浆分离结晶过程Mg同位素平衡分馏的控制，Fe-Ti氧化物非常大的δ26Mg值变化范围是由亚固相线阶段Fe-Ti氧化物与硅酸盐矿物之间Mg-Fe互扩散导致的；（3）岩体中堆晶矿物的δ57Fe值随岩体深度的变化指示了Fe-Ti氧化物是母岩浆侵入到白马岩浆房内较早阶段（而非岩浆演化晚期）结晶的产物；橄榄石和单斜辉石矿物“正常的Mg同位素”暗示成岩时围岩大理岩同化混染作用不明显，而Fe-Ti氧化物大的δ26Mg值变化表明高温地质作用过程（金属）稳定同位素可能产生显著的“动力学分馏”。这些研究结果不仅大大加深了对基性岩浆演化过程中矿物间Fe、Mg同位素分馏规律及其机制的理解，完善了金属稳定同位素地球化学基本理论，也为利用这些同位素示踪基性岩浆作用及其成矿效应奠定了良好的基础。