Shallow upwelling of the upper mantle is a key process in plate tectonics, allowing seafloor spreading, and enabling the formation of the ocean crust by decompression melting. Normally, the upper mantle is inaccessible beneath the crustal layer, so we rely on exposure along major faults, where dredge and submersible samples, and IODP drilling enables study of this important layer in the Earth.
IODP Expedition 399, in 2023, set a new record for drilling to depth of more than 1.2 km into upper mantle lithologies at the Atlantis Massif, 800 metres north of the Lost City vent field. Recovering mostly serpentinised depleted harzburgites, with dunite veins and gabbroic intrusions this hole provides new insights into the magmatic, hydrothermal and tectonic evolution of the upper mantle along slow-spreading ridges.. We expect this session to showcase initial results from Expedition 399, as well as providing an opportunity for comparison with other locations, particularly from near-ridge environments.
We encourage contributions from scientists working on abyssal peridotites including previous IODP holes, and on the chemistry of peridotite-hosted hydrothermal systems. Topics could include (1) the mechanisms for exhumation of mantle rocks, including deformation processes; (2) partial melting and melt transport in the upwelling mantle, and the relationship to crustal melts and gabbroic intrusions; (3) alteration of peridotites and gabbros during exhumation, including mineral alteration mechanisms and kinetics, and the impact of fluid-rock interaction on vent fluid chemistry, rheology and rock mechanics; (4) the diversity and extent of the shallow and deep biosphere associated with fluid-rock interaction in the exposed upper mantle.