A Geochemical and Isotopic Study of the Magmatic-Hydrothermal Transition in the Lower Oceanic Crust

洋中脊系统洋壳热结构受海底岩浆热输入与循环海水热液热排出的平衡控制。然而，我们对岩浆-热液转换（MHT）的认识非常有限。这一建议旨在更好地理解横跨MHT的质量和热传递过程及其地球化学后果。我们将检查从西南印度洋脊的海洋钻探计划（ODP） 735B孔中回收的下地壳岩石。我们有初步证据表明，这个1500米深的下地壳钻孔穿透了MHT，从而为研究岩浆和热液流体之间的质量运输提供了一个令人兴奋的机会。岩石矿物学组成和裂缝充填脉的井下变化表明了不同的矿化条件，从岩石与孔内浅层海水衍生流体的相互作用到岩石与深部凝固岩浆溶解的岩浆流体的浸渍。我们将利用矿物的化学和同位素组成，结合流体包裹体工作，区分晚期岩浆结晶和成矿作用，以响应岩浆流体和海水衍生流体的反应。地壳中矿化类型的分布、矿化温度和压力的变化以及矿化事件相对时间的观测，将为我们了解岩浆-热液转换动力学提供新的认识。

The thermal structure of ocean crust along the mid-ocean ridge system is controlled by the balance between magmatic heat input from below and hydrothermal heat removal through circulating seawater. However, our knowledge of the magmatic-hydrothermal transition (MHT) is extremely limited. This proposal is aimed at providing a better understanding of mass and heat transport processes across the MHT and their geochemical consequences. We will examine rocks from the lower crust recovered from Ocean Drilling Program (ODP) Hole 735B on the Southwest Indian Ridge. We have preliminary evidence that this 1500 m deep borehole in the lower crust penetrates the MHT, and thereby provides an exciting opportunity to investigate mass transport between magma and hydrothermal fluids. Downhole variations in the mineralogical composition of rocks and fracture-filling veins suggest variable mineralization conditions, ranging from interaction of rocks with seawater-derived fluids shallow in the hole to impregnation of the rock with magmatic fluids exsolved from solidifying magmas in the deeper sections. We will use the chemical and isotopic compositions of minerals, in combination with fluid inclusion work, to distinguish between late-stage magmatic crystallization and mineralization in response to reaction with magmatic fluids and seawater-derived fluids. The distribution of mineralization types in the crust, variations in mineralization temperature and pressure, and observations on the relative timing of mineralization events, will provide us with new insights into the dynamics of the magmatic-hydrothermal transition.