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

在活跃的边缘裂谷环境中，由离轴浅基底侵入体驱动的热液系统的寿命受到了很差的限制。然而，这些系统为海底元素通量提供动力，这可能是回答裂谷岩浆作用在全球元素循环中的作用以及深层微生物栖息地演化等基本问题的关键。岩浆侵位过程中发生的岩浆-沉积物混合作用是这类体系的一个重要控制过程。国际海洋探索计划第385远征队（Exp. 385）在加利福尼亚湾瓜伊马斯盆地（GB）的浅层、柔软、松散的沉积物中钻探。这个年轻的裂谷边缘嵌套着浅层侵入岩，为研究和量化岩浆-沉积物混合作用的影响提供了前所未有的活跃环境。该提案介绍了由PI （Exp. 385的岩石物理团队负责人）领导的dfg资助的为期两年的研究项目的初步结果，并证明了第三年的资金需求。我们选择了两个IODP位点，分别代表一个同步到后冷（中温）热液系统（U1547-U1548）和一个后冷，绝灭热液系统（U1545-U1546）。我们结合了岩石学和地球化学方法，分析了IODP样品，并对基岩微结构进行了高分辨率x射线μ-CT测量，再加上创新的实验室实验，重现了岩浆-沉积物混合过程中高流动性流体相进入岩浆的吸收和分馏过程。初步结果表明，基岩异常高的原生孔隙度是岩浆-沉积物混合作用的直接产物。我们的研究结果表明，由此产生的储层孔隙度可能是建立持久的后冷却热液系统的关键因素，通过将深层地热流体沿着已建立的岩浆管道系统引导。在此，我们申请了为期12个月的延期资金，进行专门设计的实验室实验，以推断GB沉积物的原位渗透率-孔隙度关系，并将其用于我们的数值模拟。此外，我们打算模拟液化管的形成过程，在缸罐实验。数值模拟的初步结果表明，由于寄主沉积物中富含水和有机碳的岩石物理变化与加速成岩相变有关，因此热量传递到寄主沉积物的效率较低，无法引起显着的热成因脱气。我们估计，在初始TOC含量相等的情况下，与硬沉积岩中的基台侵入相比，在软多孔沉积岩中的浅基台侵入只动员了1 - 4%的热成因气体。我们的发现预示着一个新的范例，在一个年轻的裂谷系统的背景下，仍然的就位可能会促进生命而不是抑制生命。

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.