洋中脊岩石圈地幔的交代变质作用—以西南印度洋“龙旂”热液区为例

The abyssal peridotite exposed along the detachment fault at mid-ocean ridges is a major component of the oceanic lithosphere, which bears strong significance on the tectonic evolution of oceanic lithosphere and associated magmatic and hydrothermal activities. Previous studies mainly focused on the low temperature (<400℃) serpentinization process of abyssal peridotites at slow spreading ridges. Hence, there is an absence of knowledge about the metasomatic process at medium to high temperature (400-700℃) and the deformation process in the deep root of detachment faults. This project focuses on "Longqi" hydrothermal fields at Southwest Indian Ridge (SWIR). We will conduct an integrated study on peridotites sampled along the detachment fault surface with many analytical methods like SEM, EBSD, EMPA, LA-ICP-MS, and Micro-FTIR etc. Combined with the evidences from the metasomatic microtextures, deformation microstructures and in situ mineral chemistry, isotopes and water contents, we are aimed at revealing the processes of multistage serpentinization, and metasomatism at medium to high temperature. It helps to identify the elemental diffusion process and water content variations during different periods of metasomatism. This study will provide valuable information on the perspectives of melt/fluid activities and the hydrothermal circulation process beneath the "Longqi" hydrothermal fields. Additionally, it will provide new insights into the evolution of the detachment fault at ultraslow spreading ridge.

沿洋中脊拆离断层出露的深海橄榄岩是大洋岩石圈地幔的重要组成，对研究岩石圈的构造演化及伴生的岩浆和热液活动等具有重要意义。前人研究集中于慢速扩张洋脊，主要关注橄榄岩的低温（<400℃）蛇纹石化过程，而对拆离断层区深部的中高温（400-700℃）交代变质作用和岩石变形过程缺乏深入认识。本项目聚焦超慢速扩张西南印度洋中脊“龙旂”热液区，选取断层拆离面出露的橄榄岩为主要研究对象，利用扫描电镜、电子背散射衍射（EBSD）、电子探针、LA-ICP-MS和显微傅里叶红外光谱等实验手段，对橄榄岩的变质-变形显微结构特征、矿物原位微区元素和同位素组成及矿物水含量等进行研究，查明橄榄岩经历的中高温交代变质和多期次蛇纹石化过程，揭示不同变质阶段元素的迁移规律及矿物水含量变化响应特征。该研究将为透视“龙旂”热液区的深部熔流体活动和热液循环过程提供重要认识，也为超慢速扩张洋中脊深大拆离断层的构造演化提供流变学约束。

大洋岩石圈地幔中的交代变质作用是洋中脊重要的地质过程。典型拆离断层区出露的蛇纹石化橄榄岩可以保存岩石圈演化不同阶段的记录，是认识大洋岩石圈地幔属性和构造演化的重要研究对象。本项目综合利用多种现代分析测试技术，对SWIR龙角区（49.7°E）的橄榄岩开展精细的变质和显微结构特征研究、全岩和矿物微区元素和同位素组成变化研究。取得了以下重要成果和认识：（1）获得了龙旂热液区热液循环和岩石蚀变作用的响应认识；（2）通过龙角区超基性岩的化学组成揭示了龙角区极度亏损地幔成因；（3）构建了龙角区岩浆旋回模型，提出了岩浆旋回对多期拆离断层的演化控制作用。项目很好的完成了预期目标，首次为超慢速扩张西南印度洋中脊的海底高频热液活动发育提供了岩石地球化学的佐证，揭示了其深部岩石圈地幔高度亏损的属性和成因。本研究也为超慢速扩张洋中脊拆离断层发育机制和演化过程也提供了新的认识。