测定超高压榴辉岩及其矿物的同位素组成，能够了解超高压变质下俯冲板片的同位素组成，及获得超高压矿物间的同位素分馏值。但，榴辉岩折返过程中会伴有退变质流体与岩石、矿物的相互作用，研究已表明折返初期的流体交代能在超高压矿物没有发生退变质分解的情况下显著改变其元素成分，但该过程的同位素行为尚未深入研究。本项目计划对碧溪岭榴辉岩退变质流体交代作用前、后的两期石榴石和单斜辉石进行研究，由于前人对该变质区榴辉岩进行矿物Mg，Fe同位素研究时并未对矿物期次加以区分，我们计划通过分别测定流体交代前、后两期矿物的Fe，Mg同位素组成并对比，研究这种交代作用能否引起矿物Fe，Mg同位素分馏，并通过获得的交代前的同位素组成来修正前人研究获得的矿物间Fe，Mg同位素平衡分馏值。此外，两期石榴石Fe3+/ΣFe相差约50%，通过研究Fe同位素将有助于了解Fe的氧化状态和配位数哪个是控制Fe同位素分馏的主导因素。

We can understand the isotopic compositions of subducted slice at metamorphic peak and obtain the equilibrium fractionation factors of UHP minerals by determining the isotopic compositions of UHP eclogite and the minerals therein, separately. However, whether the isotopic compositions of eclogite at the metamorphic peak can be retained in the rock exhumed to the earth’s surface is doubtful, because eclogite undergoes retrogression and has interactions with retrogressive fluids during exhumation. It is shown in our previous study that garnet and omphacite in eclogite can be partly modified on chemical contents rather than decomposed after rock-fluid interactions at the stage of initial exhumation, but the isotopic behaviors during this process are poorly understood. This proposal plans to investigate the magnesium and iron isotopic compositions of such two stage garnets and clinopyroxenes in the Bixiling eclogites after a rock-fluid interaction, to see whether this kind of rock-fluid interaction can result in magnesium and iron isotopic fractionations between minerals. Previous studies on magnesium and iron isotopes of the Bixiling eclogite didn’t distinguish the mineral stages, so that some doubtful results could be obtained. In this study, we can make some significant modifications on the magnesium and iron isotopic fractionation factors of garnet-omphacite obtained in previous studies, because only the isotopic compositions of fresh mineral without modification by the rock-fluid interaction can represent the magnesium and iron isotopic compositions of the metamorphic peak. In addition, iron isotopic investigation of two stage garnets can give some new insights into which one of iron oxidation state and coordination is the primary driver of garnet iron isotopic fractionation, since there is a 50% difference of Fe3+/ ΣFe ratios between the fresh and modified garnets.