利用大别山超高压变质岩和脉体来制约变质流体的Si同位素组成

Metamorphic fluids are critical for transporting materials in subduction zone settings. Metasomatism of such fluids could significantly affect non-traditional stable isotope composition of the mantle wedge and arc lavas. Silicon is important solute in metamorphic fluids and Si contents increase with increasing depths of subduction. Although it is well known that Si isotopes can be fractionated in fluid activities, Si isotope composition of metamorphic fluid is not yet clear. There is no data reported from high temperature-pressure experimental or theoretic studies, Therefore, some important processes, including transporting Si in metamorphic fluids and Si isotope fractionation during fluid activities, are still poorly understood..This project is going to study ultrahigh pressure metamorphic rocks and veins in the Dabie Orogen. Based on Si isotope data of whole rocks and minerals, we can constrain Si isotope composition of metamorphic fluids and how Si isotopes are fractionated during fluid activities. Furthermore, this project is going to apply the first-principles calculations to estimate equilibrium isotope fractionation of Si between important metamorphic minerals in the veins. Our results will be helpful for understanding how materials are transported by metamorphic fluids and provide a new geochemical tool to study the formation of metamorphic veins in subduction zones. Finally, by comparing Si isotope fractionation by fluid process with that by magmatism, this project will provide new insights into isotope fractionation mechanism at high temperature.

变质流体对于俯冲带物质的运移至关重要。流体交代可能对地幔楔和岛弧岩浆岩的非传统稳定同位素组成产生显著影响。Si是变质流体的重要溶质组分，随着俯冲深度的增加，流体的Si含量也逐渐增加。流体作用有可能产生较大的Si同位素分馏，但是关于变质流体的Si同位素组成尚不清楚，也没有高温高压实验和理论计算的工作发表。这限制了我们对变质流体作用中Si的迁移过程、Si同位素分馏机理以及流体Si同位素组成的理解。.本项目拟对大别造山带出露的超高压变质榴辉岩和脉体开展研究。通过测量全岩和矿物的Si同位素，制约变质流体的Si同位素组成以及流体活动造成的Si同位素分馏。本项目还将利用第一性原理计算重要变质矿物之间的Si同位素分馏系数。本项目的实施有助于理解变质流体对物质的迁移和运输作用，为研究脉体的形成提供新的地球化学方法；对比脉体和岩浆演化过程中的Si同位素分馏，可为高温下Si同位素的分馏机制提供了新的认识。

人们认识到变质流体对于俯冲带物质的运移至关重要。Si是变质流体的重要溶质组分，流体的Si含量随着俯冲深度的增加而增加。流体作用有可能产生较大的Si同位素分馏，但是关于变质流体的Si同位素组成尚不清楚。大别山出露了大量的超高压变质脉体，为研究变质流体的同位素组成提供了很好的条件。本项目测量了大别山港河和花凉亭超高压变质岩和脉体中石英、绿辉石、绿帘石、白云母、蓝晶石的Si同位素组成。利用第一性原理计算获得了绿帘石、白云母、蓝晶石之间的Si同位素分馏系数。发现港河和花凉亭榴辉岩中，距离脉体越近，榴辉岩的δ30Si值越高，说明水岩相互作用可以导致流体中富集重的Si同位素。发现变质流体结晶产物中的SiO2含量和δ30Si之间呈线性正相关关系，反映了结晶产物中矿物种类和相对比例的持续变化。港河和花凉亭脉体中单矿物30Si富集的顺序是石英≈白云母>绿辉石≈绿帘石≈蓝晶石>石榴石，和第一性原理计算的结果相符合。这说明脉体中矿物可能经历了从流体中平衡结晶的过程。花凉亭脉体中的矿物随着流体演化，表现出持续δ30Si升高的趋势，反映了流体δ30Si值的变化。根据模型计算结果，花凉亭流体的初始δ30Si值约为-0.22 ± 0.08‰，而到结晶的最后阶段，流体的δ30Si值上升至约-0.07 ± 0.10‰。