Magma-sediment mingling processes, control and longevity of related hydrothermal systems – Implications for the Earth’s Subseafloor Elements-, Plate-, Life-Cycles

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The longevity of hydrothermal systems powered by shallow off-axis sill intrusions emplaced in active marginal rift settings is poorly constrained. Yet, these systems power subseafloor element fluxes which may hold the key to answer fundamental questions related to the role of rift magmatism in the global elements cycles, and the evolution of deep microbial habitat. An important controlling process of such systems is the magma-sediment mingling that occurs during magma emplacement. The International Ocean Discovery Program Expedition 385 (Exp. 385) drilled through the shallow, soft, unconsolidated sediments of the Guaymas Basin (GB), Gulf of California. This young rifted margin is nested with shallow sill intrusions providing unprecedented access to an active setting to study and quantify the impact of the magma-sediment mingling. This proposal presents the preliminary results of a DFG-funded two-year research project led by the PI (Petrophysics team leader of the Exp. 385), and demonstrates the need for a third year of funding. We have chosen to study two IODP sites representing a syn- to post-cooling (moderate-temperature) hydrothermal system (U1547-U1548), and a post-cooling, extinct hydrothermal system (U1545-U1546). We have combined a petrological and geochemical approach, analyzing IODP samples, with a high-resolution X-ray μ-CT survey of the sill microfabric, paired with innovative laboratory experiments reproducing the uptake and fractionation of highly mobile fluid phases into the magma during magma-sediment mingling. Our preliminary results indicate that the unusually high primary porosity of the sills is a direct product of magma-sediment mingling. Our findings suggest that the resulting sill porosity could be a key factor in establishing longlasting, post-cooling hydrothermal system by channelling deeply sourced geothermal fluids along the established magma plumbing system. Here, we apply for a 12-months extension funding to perform specially designed laboratory experiments that will infer the in-situ permeability-porosity relation of the GB sediment which will be used in our numerical simulations. Moreover, we intend to simulate the liquefaction pipe formation process in cylinder tank experiments. Our preliminary results derived from numerical simulations indicate that heat transfer into the host sediment is inefficient to cause significant thermogenic degassing due to the petrophysical changes associated with the accelerated diagenetic phase transition of in the host sediment, which is rich in water and organic carbon. We estimate that, for equivalent initial TOC content, only a fraction of 1 to 4 % thermogenic gas is mobilized in case of shallow sill emplacement in soft, porous sediments compared to sill intrusion emplaced in hard sedimentary rocks. Our finding heralds a new paradigm that in the context of a young rift system, sill emplacement may power life instead of suppressing it.