Exp. 399 Building Blocks of Life, Atlantis Massif


航海名Expedition 399: Building Blocks of Life, Atlantis Massif
Scientific Prospectus
・プロポーザル #937-Full2, #937-Add
Scientific Prospectus
共同首席研究者Andrew McCaig and Susan Lang
掘削船JOIDES Resolution
乗船/下船地Ponta Delgada to Ponta Delgada, Portugal
The Atlantis Massif (AM) Oceanic Core Complex (30°N, Mid-Atlantic Ridge) is one of the earliest sites recognized for the extensive exposure of ultramafic and mafic rocks at the seafloor caused by an oceanic detachment fault, and has been the focus of four IODP expeditions (304, 305, 340T, and 357). The Lost City Hydrothermal Field (LCHF) is hosted in peridotite on its southern wall and vents alkaline fluids rich in H2 as a by-product of serpentinization. The AM is therefore an ideal natural laboratory for studying tectonics, magmatism, and the interaction between the ocean and lithosphere, as well as their combined influence on ocean chemistry and the subseafloor biosphere.
Expedition 399 has three main scientific objectives:
Characterizing the life-cycle of an oceanic core complex, including links among igneous, metamorphic, structural, and fluid flow processes.
Accessing the chemical kitchen preceding the appearance of life on Earth, including the formation of organic molecules of prebiotic interest at high and low temperatures.
Identifying the extent of the deep biosphere and limits for life, including how they are influenced by lithological substrate, porosity and permeability, temperature, fluid chemistry, and reactive gradients.
A principle aim of the expedition is to sample fluids and rocks in a stable regime where active serpentinization may be occurring, creating the conditions where the building blocks for life (H2, CH4, and more complex organic compounds form abiotically. IODP Hole U1309D, located 5 km north of the LCHF, is the deepest (1415 m) hole drilled so far in young (<2 Ma) ocean crust, and recovered a primitive series of gabbroic rocks interpreted in part to be metasomatised peridotite. Expedition 399 will sample fluids in the existing Hole U1309D using newly developed temperature-sensitive sampling tools. It will also deepen Hole U1309D to ~2060 mbsf, where temperatures up to 220°C are predicted, and leave it available for future logging and fluid sampling once thermal equilibrium has returned. The proportion of ultramafic rocks is expected to increase with depth, and at these temperatures serpentinization and hydrogen generation by redox reactions should be actively occurring. Volatiles and organic molecules will be sampled in fluid inclusions to identify the physicochemical conditions that lead to their formation.
A second shallow (~200 mbsf) hole will be cored close to the LCHF to obtain a complete section through a detachment fault zone in serpentinized peridotite, extending the findings of Expedition 357. It targets zones of higher porosity that may facilitate geochemical and microbial processes. A re-entry system will be installed to allow for future deeper drilling, logging, fluid sampling, and a borehole observatory. The thermal structure of this hole will place important constraints on the Lost City circulation system, and there is a possibility of intersecting Lost City fluids pathways.

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阿部 なつ江JAMSTEC研究員Physical Properties Speicalist
/Downhole Measurements
野坂 俊夫岡山大学准教授Metamorphic Petrologist