Deciphering the signature of fluid/rock reactions in the lower oceanic crust: The Oman ophiolite analogue

Abstract

Hydrothermal circulation is a key Earth process, moderating elemental and heat transfers between oceanic lithosphere and the hydrosphere. The characterization and quantification of fluid/rock reactions and fluid fluxes in fast-spreading mid-ocean ridges has been a long-term ambition given these ridges are responsible for formation of two thirds of present day oceanic crust. However, our understanding of fluid pathways and the extent of fluid/rock interactions in the lower crust, particularly associated with faulting, is scarce due to limited sampling. The Oman Drilling Project has sampled three 400 metre boreholes in the lower oceanic crust of the Semail ophiolite. Hole GT1A located in Wadi Gideah targeted a hydrothermal fault zone, recovering multiple deformed intervals throughout the whole section of layered gabbros. This study recognizes two main categories and a further 7 sub-categories of deformation, showing a range of mineral assemblages and variable chemical composition. The time-integrated record of fluid circulation reveals that alteration occurred mostly under greenschist facies conditions and continued down to prehnite-pumpellyite facies. However, the textural characteristics of amphibole-rich fault zones imply an earlier, high temperature stage of deformation. The 87Sr/86Sr and δ18O values of the whole rocks and mineral separates point to formation from evolved fluids and so far no evidence of unaltered seawater-like compositions are found. Similar styles of deformation and geochemical variability are found in Hole GT2A (~2km stratigraphically above Hole GT1A), albeit at varying scales. Previous models that identify large-scale faults with ~1km spacing in the lower crust must be revised to incorporate the higher abundance of faulting. Deformation is uniformly associated with elevated LOI and depletions in all major elements and base metals except for CaO and V, which are enriched in these zones. The elements mobilized by fluids are either incorporated into the alteration halo, vein minerals, or transported away. Relative to the previous estimates of crustal hydrothermal fluxes, the fault zones contribute 155% of Si, 46% of Al and 33% of all Fe extracted from the crust into the fluid. This work presents new data on the contribution of the fault zones to the global hydrothermal fluxes of Sc, V, Cr, Co, Ni and Sr.