Gateway to the subarc mantle: volatile flux, metal transport, and conditions for early life1
Published June 2017
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Almost two-thirds of Earth’s submarine volcanism is expressed in the ocean basins along the ~65,000 km long mid-ocean-ridge (MOR) system, but the remaining third takes place along intraoceanic arcs and seamounts. The 22,000 km long intraoceanic arc system appears to surpass the MOR system in terms of both the frequency of hydrothermal activity, with several sites per 100 km of arc, and the range in chemical composition of the fluids being discharged.
Hydrothermal systems hosted by submarine arc volcanoes differ substantially from those in spreading environments in that they commonly contain a large component of magmatic fluid. Our primary scientific goal is to discover the fundamental underlying processes that develop as a consequence of this difference. A magmatic-hydrothermal signature, coupled with the shallow depths of arc volcanoes and their high-volatile contents, strongly influences the chemistry of fluids and the resulting mineralization and likely has important consequences for the biota associated with these systems. Because of the high metal contents and very acidic fluids, these hydrothermal systems are also thought to be important analogs of many porphyry copper and epithermal gold deposits mined today on land.
Drilling at Brothers volcano on the Kermadec arc will provide the missing link (i.e., the third dimension) in our understanding of mineral deposit formation along arcs, the subseafloor architecture of these volcanoes and their related permeability, as well as the relationship between the discharge of magmatic fluids and the deep biosphere. Expedition 376 will drill and log two caldera sites, one on the rim of the caldera and a second inside the caldera, and a third site at the summit of a volcanic cone, located inside the caldera at Brothers. These sites represent discharge zones of geochemically distinct fluids that are variably affected by magmatic volatile input, allowing us to directly address the consequences of magma degassing for metal transport to the seafloor and its effect on the functioning of microbial communities. Drilling will provide access to critical zones dominated by magma degassing and high-temperature hydrothermal circulation over depth intervals regarded as crucial, not only in the development of multiphase mineralizing systems but also in identifying subsurface microbially habitable environments. The specific objectives of Expedition 376 are
- To characterize the subvolcano, magma chamber–derived volatile phase to test model-based predictions that this is either a single-phase gas or two-phase brine-vapor;
- To determine the subseafloor distribution of base and precious metals and metalloids and the reactions that have taken place along pathways to the seafloor;
- To quantify the mechanisms and extent of fluid-rock interaction and consequences for mass transfer of metals and metalloids into the ocean and the role of magmatically derived carbon and sulfur species in mediating these fluxes; and
- To assess the diversity, extent, and metabolic pathways of microbial life in an extreme, metal-toxic, and acidic volcanic environment.