Cooling Rate Constraints on Accretion of the ower oceanic crust

Mechanisms of crustal accretion at mid-ocean ridges depend upon the balance between the rate of magma supply and the rate of cooling. The former can be estimated if integrated over long time intervals but the episodicity is very unclear. The latter depends principally on the efficiency of hydrothermal cooling which is poorly constrained. The cooling rate of samples of the lower oceanic crust provide insight into this balance of heat supply and heat extraction. Cooling rates in the lower oceanic crust can be determined using two standard metamorphic approaches - geospeedometry and thermochronology. The former is based on the down-temperature diffusive exchange of elements between phases and is of wide applicability in mafic rocks with low abundances of radiogenic parent elements. Using two geospeedometry approaches - the down temperature diffusion of Ca from olivine to clinopyroxene and the down-temperature exchange of Mg and Fe between olivine and spinel the cooling rate of the lower oceanic crust has been investigated. Cooling rates are fast, and show no variation with depth in the crust in samples from the Mid-Atlantic Ridge (ODP Hole 923A) or the Southwest Indian Ridge (ODP Hole 735B). Cooling rates determined using the geospeedometry approach range between 1000 and 5000°C per Myr consistent with those derived by John et al. (2004) using a thermochronology approach of >800°C per Myr. In contrast cooling rates from the Oman ophiolite and the EPR (Hess and Pito Deeps), fast- spreading ridges, vary dramatically with depth from much faster to much slower than the rates derived from slow-spreading ridges. Mantle samples from the mid-Atlantic ridge also cool far more rapidly than those from the Oman ophiolite. The rapid cooling rates in the crust at slow-spreading ridges, and lack of any systematic variation in cooling rate with depth are most readily interpreted as indicating crustal accretion through numerous small magma bodies, that cool separately, with little thermal gradient in the surrounding rocks. The size and episodicity of these magma batches would have to be relatively constant at the MAR and SWIR suggesting a regulated mechanism of magma supply to the crust. Rapid cooling in the mantle suggests rapid uplift towards the surface. Coogan, L.A., G.R.T. Jenkin, and R.N. Wilson, EPSL 199, 127-146, 2002. John, B.E. et al., EPSL 222, 145-160, 2004