Slow cooling of the lowermost oceanic crust at the fast-spreading East Pacific Rise

The cooling history of the lowermost ocean crust formed at the East Pacific Rise and exposed at the Hess Deep Rift is constrained using fresh primitive olivine gabbro and troctolite cores recently recovered by Integrated Ocean Drilling Program Expedition 345. The Mg-in-plagioclase and Ca-in-olivine geospeedometers constrain the initial subsolidus cooling rates, down to closure temperatures in the range of 900-750 degrees C. These independent geospeedometers yield similar slow cooling rates, 0.005-0.0001 degrees C yr(-1), with a mean value of 0.0011 degrees C yr(-1), making these results robust despite the uncertainties inherent in the methods. The slow cooling rates, derived using natural samples, provide a crucial test of predictions of the thermal structure of the lower crust based on remote imaging techniques and thermal modeling. The cooling rates are most consistent with thermal models that are dominated by conductive cooling within the plutonic complex. This implies that lower crust formed at fast spreading rates remains hot in the near-axis environment, indicating limited hydrothermal fluid ingress and reaction.