西秦岭造山带蛇纹岩的Mg同位素研究

Serpentine is one of the most important H2O-bearing minerals in subduction zones, which is widely developed in subduction oceanic lithosphere and at the bottom of forearc mantle wedge. Previous studies concerning the behavior of Mg isotope during subduction is mainly focused on the metamorphic dehydration of meta-basaltic rocks and metapelites, and few attentions have been paid to serpentinization under different circumstances and the metamorphic dehydration process of serpentine minerals in subduction zones. Serpentinites in orogenic belts are exhumated serpentinites from subduction zones, and preserve abundant information about different serpentinization and metamorphic dehydration processes, which is the ideal samples for studying the behavior of Mg isotope during subduction. We choose the Jianchaling serpentinite, the Liangyazi serpentinites, the Heimulin serpentinites and the Loufanggou serpentinites with different mineral assemblages to carry out our study. We firstly on detailed mineralogical, petrological and geochemical studies to depict the protolith, the serpentinization processes and the metamorphic evolution of these serpentinites, and then analysis Mg isotopes of representative whole rocks and corresponding all minerals aiming at a better understanding of the behavior of Mg isotopes during different serpentinization processes and the transition from lizardite to antigorite in subduction zones. Our study can also be used to explore the nature of fluids in subduction channel, the behavior of Mg isotopes during subduction and the genesis of arc magmas.

蛇纹石是俯冲带重要的含水矿物，并广泛发育在俯冲大洋岩石圈内和弧前地幔楔底部。前人对板块俯冲过程中Mg同位素行为的研究主要针对变基性岩和变泥质岩的变质脱水过程，而对不同条件下的蛇纹石化过程和蛇纹岩变质脱水过程中的Mg同位素研究较为缺乏。造山带蛇纹岩代表了俯冲折返的蛇纹岩，其中保留了丰富的蛇纹石化过程和蛇纹岩变质脱水过程的信息，是研究俯冲带Mg同位素地球化学行为的理想样品。申请人选择西秦岭煎茶岭、亮垭子、黑木林和楼房沟4个地区具有不同矿物组成的蛇纹岩为研究对象，首先通过矿物学、岩石学和地球化学方法来恢复这些蛇纹岩的原岩属性、蛇纹石化过程和变质演化历史；在此基础上，通过测定蛇纹岩全岩和单矿物的Mg同位素组成，来查明不同蛇纹石化反应和蛇纹石矿物相转变过程中的Mg同位素行为，进而探究俯冲通道流体性质、板块俯冲过程中Mg同位素的地球化学行为和岛弧岩浆的起源。

蛇纹石是俯冲带重要的含水矿物，并广泛发育在俯冲大洋岩石圈内和弧前地幔楔底部。前人对板块俯冲过程中Mg同位素行为的研究主要针对变基性岩和变泥质岩的变质脱水过程，而对俯冲通道内的蛇纹石化过程和蛇纹岩变质脱水过程中的Mg同位素研究较为缺乏。本项目首先对西秦岭地区煎茶岭、亮垭子、黑木林和楼房沟4个地区蛇纹岩原岩属性和变质演化过程进行了研究，它们代表了俯冲带弧前位置不同深度形成的弧前地幔楔蛇纹岩，记录了俯冲带弧前不同位置来源板片流体对弧前地幔楔底部橄榄岩的相互作用过程。在俯冲板片脱碳产生的富CO2流体参与下发生蛇纹石矿物相转变时，利蛇纹石中大量占据四面体位置的Fe3+被还原和释放，从而在俯冲通道内形成贫铁的叶蛇纹石；同时，流体较高的氧逸度使得B3+，As5+和Sb5+更多地进入到贫铁叶蛇纹石中。贫铁叶蛇纹石可以有效地将俯冲带浅部释放的这些元素携带到弧下深度，并参与岛弧岩浆的形成。通过对蛇纹岩全岩和单矿物的Mg同位素研究发现，蛇纹石矿物相转变过程不存在显著的Mg同位素分馏，而碳酸盐和共生蛇纹石之间Mg同位素分馏可以作为一个潜在的地质温度计。由于在弧下位置所对应的温压条件下，碳酸盐保持稳定，但是与碳酸盐共生的蛇纹石或滑石会发生脱水分解并释放富Mg流体，这些流体参与岛弧岩浆过程就可以解释部分岛弧岩浆高Mg同位素的特征。本项目的研究结果说明，俯冲板块的弧前脱碳过程对于俯冲带元素的分异和循环过程起到了非常重要的作用，是理解岛弧岩浆的一些特殊地球化学特征的关键。