Cl-rich amphiboles as record for hydrothermal processes at very high temperatures in the deep oceanic crust : brine/rock interaction experiments and investigation on natural rocks

Interactions between rock and high temperature seawater-derived fluids are recorded in hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Wadi Wariyah, Sultanate of Oman. Here we present a study – using petrographic, microanalytical, isotopic, and structural methods – of amphiboles found in the aforementioned veins and dykelets, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (with up to 5.4 wt% Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal at varying water/rock ratios and salinities, while the formation of super Cl-rich amphibole suggests the occurrence of phase separation, and 87Sr/86Sr and stable δ18O isotope analyses confirm the influence of a hydrothermal fluid in a rock-dominated environment. A parallel experimental study was conducted at hydrothermal (500 – 750 °C) and magmatic (900 °C) conditions at pressures of 2 kbar, and fO2 close to NNO, with an amphibole-containing natural olivine gabbro and saline fluid (6, 20 and 50 wt% NaCl). Results in subsolidus experiments demonstrate the growth of newly-formed amphibole with a wide range of compositions comparable to those seen in the lithologies sampled in Wadi Wariyah, and Cl-bearing amphiboles with Cl up to 0.47 wt. %. Our findings highlight the heterogeneities in fluid infiltration and Cl activity that account for the complexity of hydrothermal fluid/rock interactions in deep oceanic geological systems, providing insight into the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid