Cooling rates in the lower crust of the Oman ophiolite: Ca in olivine, revisited

Debate over the mechanism of accretion of the lower oceanic crust has centered around the gabbro glacier and the sheeted sills models. The thermal profile of the crust, specifically the roles of hydrothermal circulation and cooling rate, is a key component in distinguishing between these two models. Results of this study show no systematic variation of cooling rate with depth in the lower crust of the Khafifah section in the Wadi Tayin massif of the Oman ophiolite. Additionally, very high rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust. While the conclusions presented here do not rule out the possibility of accretion by gabbro glacier, they do contradict the initial tenet of the gabbro glacier model - convective heat loss by hydrothermal circulation can penetrate to Moho depths and crystallization is not, by necessity, constrained to the upper crust. On the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust.