Contrasting Cooling Rates in the Lower Oceanic Crust at Fast- and Slow-spreading Ridges Revealed by Geospeedometry

Two approaches to determining the high-temperature (1000°Cto 600°C) cooling rate of the lower oceanic crust and uppermantle are presented and critically evaluated. The first isbased on the down-temperature diffusive exchange of Ca betweenolivine and clinopyroxene. The second, less well-constrained,approach is based on the down-temperature diffusive exchangeof Mg and Fe between olivine and spinel. Cooling rates basedon olivine–spinel geospeedometry are approximately anorder of magnitude faster than those from Ca-in-olivine geospeedometry.In contrast, cooling rates derived from thermochronology andremanent magnetism are approximately an order of magnitude slowerthan those derived by Ca-in-olivine geospeedometry; this isprobably because they record cooling at lower temperatures.Using the Ca-in-olivine geospeedometer, the cooling rate ofsamples from the lower oceanic crust and upper oceanic mantleformed in the Oman ophiolite and in the three main ocean basinshas been determined. Samples from the lower oceanic crust formedat fast-spreading ridges show a large decrease in cooling ratebetween the top and base of the gabbroic section, with mostof the variation occurring within the upper kilometre. Thisis consistent with vertical heat loss (within the crustal frameof reference) dominating the thermal evolution at fast-spreadingridges. Samples from Ocean Drilling Program Hole 735B, whichformed at the slow-spreading Southwest Indian Ridge, show novariation in cooling rate over 1500 m depth range and cooledsubstantially faster than rocks from the deeper portion of thegabbros in the Oman ophiolite, where the change in cooling ratewith depth is limited. These observations are consistent withheat loss from small plutons emplaced in cool lithosphere atthe slow-spreading ridge. Alternatively, they could be explainedby cooling through the Ca-in-olivine closure interval duringuplift towards the surface. KEY WORDS: geospeedometry; lower oceanic crust; Hess Deep; Pito Deep; ODP Hole 735B; ODP Leg 153