深俯冲带锆石溶解和再沉淀过程的流体地球化学效应：高压实验与天然样品对比

The relict magmatic zircons within ultrahigh-pressure metamorphic rocks may occur in-situ and simultaneous (temporally and spatially consistent) resolution-reprecipitation or non-in-situ and non-simultaneous (temporally and spatially inconsistent) resolution and reprecipitation via interaction with metamorphic fluids during continental deep subduction. This would induce U-Pb and O isotopes as well as Lu-Hf isotopes and trace elements being reworked to some degrees, and bring large uncertainties for zircon geochronological data interpretation and geochemical tracing. This project would select the relict magmatic zircons within ultrahigh-pressure metamorphic rocks from the Dabie orogen, which were undisturbed by metamorphic fluids. They are used as the starting materials for high-temperature and high-pressure hydrothermal experiments using piston-cylinder apparatus, in order to recur the interaction between zircon and aqueous fluids with variable chemical compositions under subduction-zone metamorphic temperature and pressure. The quenched products will compare with natural samples (especially that from various metamorphic veins), which have been identified to occur dissolution and reprecipitation processes, on different aspects including morphology, submicron structure and geochemical characteristics. In doing so, the following scientific goals will be issued: (1) understand the requirement and mechanism for the occurrence of zircon dissolution and reprecipitation processes; (2) explore the elemental and isotopic effects induced by zircon dissolution and reprecipitation processes via interaction with metamorphic fluids and their implications for zircon geochronology and geochemical tracing; (3) provide indirect constraints on geochemical recycling of Zr and Hf during continental deep subduction.

深俯冲带变质岩原岩残留锆石在流体作用下可能发生原地即时的(时空一致的)溶解-再沉淀，或者非原地即时的(时空不一致的)溶解和再沉淀，引起锆石U-Pb、Hf-O同位素和微量元素不同程度的重置，为锆石年代学数据解释和地球化学示踪带来极大的不确定性。本项目选取大别造山带超高压岩石中未经历变质流体作用的原岩残留岩浆锆石作为初始物质，利用活塞圆筒压机技术开展高温高压热液实验，重现俯冲带变质温度压力条件下，不同化学组成的富水流体与锆石之间的相互作用场景。实验产物与天然样品中发生溶解和再沉淀作用(特别是各种变质脉中)的锆石进行深度的矿相学和地球化学对比，进而：(1) 认识锆石发生溶解和再沉淀作用的条件和机制；(2) 探讨变质流体作用下锆石溶解和再沉淀过程引起的地球化学效应及其对锆石年代学和地球化学示踪的指示意义；(3) 为高场强元素Zr和Hf的地球化学循环提供间接制约条件。

深俯冲带变质岩原岩残留锆石在流体作用下可能发生原地即时的(时空一致的)溶解-再沉淀，或者非原地即时的(时空不一致的)溶解和再沉淀，引起锆石U-Pb、Hf-O同位素和微量元素不同程度的重置，为锆石年代学数据解释和地球化学示踪带来极大的不确定性。本项目选取大别造山带超高压岩石中未经历变质流体作用的原岩残留岩浆锆石作为初始物质，利用活塞圆筒压机技术开展高温高压热液实验，重现俯冲带变质温度压力条件下，不同化学组成的富水流体与锆石之间的相互作用场景。实验产物与天然样品中发生溶解和再沉淀作用(特别是各种变质脉中)的锆石进行深度的矿相学和地球化学对比，研究发现：(1) 绝大部分受到改造的锆石区域为高U锆石，即U含量 > 1000 ppm，熔流体作用形式多样化，以形成类似马赛克的结构为主，兼有其它不规则的形貌和结构。所在区域富集大量非锆石结构的矿物和流体包裹体，流体成分以富集Si、Al、Na、K为主，部分可含有一定量的Mg、Fe、Ca等，且变化范围较大，暗示了不同变质演化阶段和/或不同期次的熔流体与锆石相互作用；(2) 变沉积岩由于其物源复杂，锆石发生溶解和再沉淀作用时需要以同期同源的锆石作为参考物质，分别进行地球化学深度比对，从而限定变质流体作用程度。(3) 一旦熔流体活动造成亚显微尺度上锆石的交代和溶蚀，其元素和同位素行为不再单纯受到实验体系下元素扩散速率的制约，还与熔流体的化学组成及其中元素的相对活动性有关。