以我国攀西峨眉山大火成岩省白马层状镁铁超镁铁岩为例探讨高温岩浆过程的Mg同位素分馏

High-temperature fractionation of non-traditional stable isotopes has been a hot topic for years. Its applications in tracing magmatic processes and being potential geothermometer are still in exploration. To further reveal high-temperature magnesium isotope fractionation and the geochemical behaviors of magnesium isotopes during fractionation of high-Fe and Ti basaltic magma, this project will take the Baima layered mafic-ultramafic intrusion in the Emeishan large igneous province, SW China as an example. Through the studies of magnesium isotopes of whole-rock and magnesium-bearing minerals of olivine, clinopyroxene, magnetite and ilmenite from the ~2 km thick Baima mafic-ultramafic rocks, our purposes are 1) to reveal magnesium isotope compositions of silicate and oxide minerals and their magnesium isotope fractionation at high temperatures; 2) to discuss behaviors of magnesium isotopes during fractional crystallization and immiscibility processes combined with previous petrological, mineralogical and geochemical studies; and 3) to constrain the origin of Fe-Ti oxide minerals associated with layered intrusions in the Panxi region through comparisons of magnesium isotopes of silicate and Fe-Ti oxide minerals from the lower and upper orebodies of the Baima intrusion, and try to apply it to other Fe-Ti oxide-bearing layered mafic-ultramafic intrusions worldwide. This study will complement our understanding about magnesium isotope fractionations of the layered mafic-ultramafic intrusions and provide magnesium isotope constraints on magma chamber processes.

非传统稳定同位素的高温分馏一直是颇具争议的问题，其在示踪岩浆过程和地质温度计中的应用仍然处于探索阶段。为进一步揭示高温下Mg同位素的分馏，以及在富Fe、Ti的玄武质岩浆分异过程中Mg同位素的地球化学行为，本项目拟以我国四川攀西峨眉山大火成岩省白马层状镁铁-超镁铁岩为例，通过对近2km厚层状岩体的全岩和橄榄石、单斜辉石、磁铁矿、钛铁矿等含Mg矿物进行Mg同位素分析，探讨高温下不同矿物的Mg同位素组成及矿物之间，尤其是硅酸盐和铁钛氧化物间的Mg同位素分馏；结合已有的岩石学、矿物学和地球化学研究，探讨分离结晶和不混溶过程中Mg同位素的地球化学行为。同时通过岩体底部和顶部两个铁矿带硅酸盐和铁钛氧化物的Mg同位素对比来制约攀西地区与层状岩体有关的铁钛氧化物的成因，进一步应用于世界上其它含铁钛氧化物的层状岩体。本项目可以弥补目前对层状岩体Mg同位素研究的不足，为岩浆房过程提供Mg同位素制约。

非传统稳定同位素在高温下的分馏行为是近年来颇具争议的前沿研究问题，镁同位素在重要的富镁岩石 层状基性-超基性岩体 中的分馏行为对于理解成岩和成矿过程具有重要的意义。本项目对我国四川攀西峨眉山大火成岩省白马层状镁铁-超镁铁质岩体的底部带和顶部带铁矿石中富（含）镁矿物橄榄石、辉石、磁铁矿和钛铁矿进行了详细的镁同位素研究，结果表明，底部带铁矿石的组成矿物发生平衡分馏，镁同位素大小依次为钛铁矿<橄榄石<单斜辉石<磁铁矿，而顶部带的镁同位素呈现出不平衡分馏的特点，表现为靠近岩体的顶部，橄榄石和钛铁矿的镁同位素呈变重的趋势，钛铁矿表现出迄今为止发现的最重的镁同位素组成（δ26Mg=2.34‰)。这些表明，顶部带铁矿石中矿物的不平衡镁同位素分馏是由于矿物和粒间熔体之间的Mg交换引起的，交换过程中，镁分配系数高的矿物和残余熔体表现出更严重的Mg元素交换，Mg从橄榄石和钛铁矿等矿物中扩散到熔体里，轻镁同位素先扩散，因此导致反应之后的橄榄石和钛铁矿镁同位素变重。该研究告诉我们，粒间熔体和矿物的反应是造成侵入岩发生镁同位素动力学分馏的重要原因之一。