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

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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.