慢速洋中脊热液成矿过程对围岩渗透性差异的响应：来自Fe和S同位素的制约

Wall-rocks of seafloor hydrothermal systems show permeability discrepancies due to their different rock compostions, thermal cooling histories and tectonic evolutions. Different wall-rock permeabilities will have impact on the mixing ratios between the downward infiltrated water and the raising hydrothermal fluid within the stockwork-forming space, which in sequence will influence hydrothermal mineralization process. However, as of yet little study have been done on the response of hydrothermal mineralization to wall-rock permeabilities. To cast new light on this questionissue, we will focuse on two hydrothermal fields from the southern Mid-Atlantic ridge in this research. Reasons have two: 1) slow-spreading ridges show considerable potential of source; 2) the main difference between these two hydrothermal fields concerns the permeabilities of wall-rocks. It’s expected that, by systematic study on Fe- and S- isotopic geochemistry of massive sulfides from these two areas, the compositional differences and fractionation mechanisms of Fe-S isotopes within minerals ( Py-Cpy-Sph ) and between hydrothermal fields could be analyzed. The physical, chemical and biological processes during the isotopic fractionation process should be determined specifically and the response of hydrothermal mineralization processes to wall-rock permeability differences should be identified from the aspect of isotopic geochemistry.

海底热液系统的围岩由于岩石组成、热冷却史、构造发育程度的不同，表现出渗透性的差异，影响了热液上升区近海底处的下渗海水与热液流体的混合比例，继而影响热液成矿过程。热液成矿过程对围岩渗透性差异的具体响应目前还不清楚，本项目选择慢速扩张的南大西洋中脊两个热液区开展相关的研究，探寻两个热液区硫化物成矿过程对围岩渗透性差异的响应，原因之一是慢速洋中脊具有较大的资源潜力，之二是两个热液区主要差别在于围岩的渗透性差异。通过对两个热液区的采集到的多金属硫化物样品开展系统的Fe-S同位素地球化学研究，分析热液区内Py-Cpy-Sph矿物间及热液区之间Fe-S同位素组成差别及分馏机制，明确影响硫化物同位素分馏具体的物理、化学、生物过程，从同位素地球化学角度探究热液成矿过程对围岩渗透性差异的响应。

本课题通过对南大西洋两处热液区的多金属硫化物样品开展矿物学、元素地球化学、矿物化学、Fe-S同位素组成的同位素地球化学研究，阐明硫化物的矿物学、地球化学及Fe和S同位素对洋壳渗透性差异的响应。同时分析了两处热液区S-Fe同位素组成差异原因，即不同构造环境下海底热液过程中Fe-S同位素的主要分馏机制。彤管区位于脊轴处，洋壳渗透性较差，太极区位于离轴非转换不连续带上，洋壳破碎程度高，渗透性好。通过该课题的研究，发现两处热液区在Fe-S同位素组成上具有显著差异，彤管区Fe同位素表现出动力学分馏，主要受控与喷发过程。太极区也存在动力学分馏，对洋壳渗透性具有显著的反应。此外，太极区较高的重S同位素也对洋壳渗透性具有显著反应，一方面来自于海底下流体混合，另一方面来自于大面积岩石蚀变，提高了热液流体的还原性。研究结果表明，Fe-S同位素组成均对洋壳渗透性差异具有响应。