Interaction of highly saline fluid and olivine gabbro: Experimental simulation of deep hydrothermal processes involving amphibole at the base of the oceanic crust

Abstract

Natural occurrences of Cl-bearing amphibole indicate crystallization in the presence of a highly saline fluid. Amphibole with varying Cl contents (0.1 to 5 wt% Cl) found in lower oceanic crustal rocks demonstrates the activity of saline hydrothermal fluids at depth. Here we present an experimental study done in cold seal pressure vessels (CSPV) and internally heated pressure vessels (IHPV) to illustrate the process by which gabbro-hosted amphibole-rich parageneses evolve in the presence of a hydrothermal saline fluid. The starting material used was a natural gabbroic rock containing high-Ti magnesio-hastingsite with the addition of a moderately to highly saline fluid (NaCl-H2O) with XNaCl = 0.02, 0.07 and 0.24 (6, 20 and 50 wt% NaCl). We evaluated a range of conditions from hydrothermal (500–750 °C) to magmatic (900 °C) at pressures of 200 MPa and fO2 close to NNO. Fluid/rock mass ratios used were 0.2 and 1 in subsolidus (hydrothermal) experiments, and 0.07 in partial melting (magmatic) experiments. New amphibole was formed on rims of the starting high-Ti magnesio-hastingsite, with product compositions corresponding to magnesio-hastingsite, high-Si magnesio-hornblende, tschermakite, edenite, hastingsite and ferro-pargasite with varying Cl contents up to 0.47 wt% Cl. Our results from subsolidus experiments demonstrate a decrease in olivine, plagioclase and clinopyroxene in the starting rock, and formation of new amphibole, with decreased IVAl and Ti with respect to starting amphibole. Product amphibole does not display any correlation between Cl and Fe2+, IVAl and K, in contrast to natural highly Cl-rich amphibole, suggesting that these correlations hold true only at Cl contents higher than those attained in our experiments. In addition, we found that increasing NaCl in the fluid correlates with increased maximum Cl contents in amphibole. Compositional variations found in product amphibole highlight the heterogeneities in fluid infiltration and Cl activity that account for the complexity of hydrothermal fluid/rock interactions in deep oceanic geological systems.