High temperature hydrothermal activity in the deep oceanic crust: Experiments and investigations on natural rocks in the Oman ophiolite
Final Report Abstract
Hydrothermal circulation within the oceanic crust at fast-spreading ridges is one of the key processes controlling the cooling of the oceanic crust. During hydrothermal circulation, the crust undergoes extensive chemical exchange with seawater. If during the accretion of the lower crust in-situ crystallization of deep gabbros plays a significant role (so-called "sheeted sill model"), thermal models suggest that extensive hydrothermal circulation within the lower section of the oceanic crust must exist. However, the extent and temperature of hydrothermal circulation within the lower oceanic crust and related petrological record are still poorly constrained. This project presents a complementary approach of partial melting experiments on oceanic gabbros and petrological, geochemical and isotopic data from hydrothermal dikes and veins crosscutting a layered olivine gabbro from the Wadi Wariyah in the Southern Oman ophiolite, forming the deepest parts of the crustal series in the Oman ophiolite. The results show that hydrothermal activity within the lower crust starts at very high temperature and very low water/rock ratios (0.4-6) with the production of amphibole-bearing dikes and veins. Complementary experimental results, characteristic mineral assemblages, and trace element modeling suggest partial melting of the layered gabbro triggered by aqueous fluids at temperatures of about 1000°C (based on amphibole thermometry). Rims of amphibole extremely enriched in chlorine (up to 4 wt%) indicate the influence of very Cl-rich brines (vapour) at the transition between the magmatic and metamorphic regime (temperatures: 500-800°C). At lower temperatures, hydrothermal circulation produced veins consisting of greenschist facies minerals (epidote, chlorite, and actinolite). Isotopic data indicate an interaction between gabbro and hydrothermal fluid at temperatures between 300-500°C and water/rock ratios between 0.7-15.2. In a final stage, cm-thick monomineralic prehnite veins indicate low temperature hydrothermal circulation at temperatures around 250°C and the highest water/rock ratios of up to 18.8. The results suggest that the hydrothermal alteration within the lower oceanic crust proceeds in multiple stages from “magmatic” to very low temperatures with increasing water/rock ratios and decreasing temperature. The study highlights that the same pathways were used at different temperatures for hydrothermal circulation. Conclusively, hydrothermal circulation at magmatic temperatures needs to be considered for thermal models regarding cooling of the deep oceanic crust via fluid circulation.
Publications
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The reaction mechanism of fluid-induced partial melting of gabbro in the oceanic crust, Eur J Mineral, 25(3), 279-298, 2013
Wolff, P. E., J. Koepke, and S. Feig
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Partial melting of oceanic gabbro triggered by migrating water-rich fluids: a prime example from the Oman Ophiolite, in Tectonic Evolution of the Oman Mountains, edited by H. Rollinson, M. Searle, I. A. Abbasi, A. Al-Lazki and M. H. Al-Kindi, Geological Society, London, Special Publications, 392, 195-212, 2014
Koepke, J., J. Berndt, I. Horn, J. Fahle, and P. E. Wolff
