Hydrogen distribution in plagioclase from the Atlantis Bank gabbros (IODP Hole U1473) : preliminary data

This contribution reports preliminary data on the H distribution in plagioclase from abyssal gabbros recently drilled from an oceanic core complex (Hole U1473A, SW Indian Ridge). Our project is aimed to reassess models for the rheology of the lower ocean crust by focussing on porphyroclastic and neoblastic plagioclase in plastically-deformed abyssal gabbros. Current models hold that the lower crust is fundamentally strong, consistent with dry-plagioclase flow laws. However, geochemical and structural evidence suggest that the crust can be fundamentally weak near-axis, controlled primarily by the behaviour of wet plagioclase in near-pervasive shear zone networks (MacLeod et al., 2017). A quantification of H in deformed gabbros is thereby fundamental to demonstrate the link between water and deformation in order to upscale the appropriate wet flow laws gaining a very different understanding of ocean crust rheology. We selected gabbroic samples at variable degree of deformation where shear zones were very probably wet under hyper-solidus to high-temperature sub-solidus (~granulite facies) conditions. FT-IR maps were acquired with a spot diameter from 100 to 30 µm2 on plagioclase ranging from large porphyroclasts to neoblasts. Our preliminary FT-IR results undoubtedly show that H concentrations in plagioclase is extremely heterogeneous and in many cases clearly concentrated along fractures or grain boundaries. A preliminary knowledge of the distribution of H in “anhydrous” mineral is thereby fundamental to obtain a quantification of the water incorporated into the mineral lattice at high temperature, avoiding any effect related to low temperature alteration and/or subsolidus diffusion during cooling. When the contribution of this late-stage H enrichment in plagioclase is removed, our maps clearly show that the H contents increase within the shear zones. If confirmed, a link between water content and deformation intensity would allow to use the appropriate wet flow laws gaining a very different understanding of ocean crust rheology. MacLeod, C.J., Dick, H.J.B., Blum, P., and the Expedition 360 Scientists, 2016. Southwest Indian Ridge Lower Crust and Moho. Proceedings of the Integrated Ocean Drilling Program. Volume 360. DOI: 10.14379/iodp.pr.360.2016