Rates of hydrothermal cooling of new oceanic upper crust derived from lithium-geospeedometry

Episodic emplacement and cooling of lavas and dikes at mid-ocean ridges leads to large fluctuations in hydrothermal fluxes and biological activity. However, the processes operating beneath the seafloor during these transient events such as permeability creation and dike cooling are poorly understood. We have developed a new approach to determine the cooling rate of the sheeted dike complex based on the extent of diffusion of lithium from plagioclase into clinopyroxene during cooling. We have calibrated this Li-geospeedometer using new high-temperature experiments to determine both the temperature dependence of the partitioning of Li between plagioclase and clinopyroxene and the diffusion coefficient for Li in clinopyroxene. Application of this method to lavas and dikes from ODP Hole 504B shows that cooling rates vary dramatically with depth in the upper oceanic crust. Extremely rapid cooling rates (>450 degrees C hr(-1)) in the upper part of the sheeted dike complex are sufficient to power hydrothermal megaplume formation within the overlying water column. (C) 2005 Elsevier B.V All rights reserved.