WHAT CAN WE LEARN FROM APPARENTLY DIVERGENT THERMAL HISTORIES OF MANTLE PERIDOTITES FROM OPHIOLITIC AND AMAGMATIC MID-OCEAN RIDGE SPREADING CENTERS?

Mantle peridotites are sampled at ophiolites with supra subduction zone and mid-ocean ridge type tectonic affinities, amagmatic mid-ocean ridge spreading centers, and more rarely, from anomalous exposures at mid-ocean ridge spreading centers with igneous crustal sections, e.g., Hess Deep. Here we assess similarities and differences in the thermal histories of peridotites from these geologic settings using a REE-based thermometer sensitive to high temperature cooling, and major element thermometers sensitive to lower temperature subsolidus cooling. Our compilation includes new data from Masirah, a mid-ocean ridge type ophiolite with a thin but complete igneous crust. Samples from all settings record magmatic or near-magmatic REE temperatures, consistent with fast high temperature cooling. Efficient hydrothermal cooling of the oceanic crust and uppermost mantle is invoked to explain fast cooling of samples from spreading centers with overlying crustal sections, whereas peridotites from amagmatic centers may be tectonically emplaced in unroofing events that produce quenching. Through lower temperature intervals, samples from ophiolites apparently diverge from abyssal peridotites. Ophiolitic peridotites exhibit systematically lower Ca-in-olivine and olivine-spinel closure temperatures than abyssal peridotites, suggesting at face value that their cooling rates are ~2 orders of magnitude slower. Here, we evaluate potential explanations for the apparent difference in low temperature thermal histories, including grain size and sampling bias, and consider what the divergent low temperature thermal histories reveal about cooling mechanisms that operate beneath spreading centers.