On the significance of thermal histories of ophiolitic and abyssal peridotites

To understand the geologic significance of their thermal histories, we assessed temperatures and cooling rates recorded by peridotites from abyssal and ophiolitic geologic settings compiled from the literature, and provided by new analyses of peridotites from Masirah, a mid-ocean ridge type ophiolite with a thin but complete igneous crust. Peridotites dredged or drilled from the seafloor at amagmatic or magma starved spreading centers, core complexes, transform faults, and a tectonic window exposing mantle lithosphere formed beneath the East Pacific Rise (Hess Deep) constitute what we refer to as the abyssal peridotites. We additionally consider a smaller number of peridotites recovered from forearc settings. We evaluate temperatures from a rare earth element (REE)-based thermometer sensitive to high temperature cooling, and major element thermometers sensitive to lower temperature subsolidus cooling. Recovered cooling rates are compared to results of simple heat conduction models, which may be used to contextualize the sample-determined thermal histories. Cooling rates within a body conductively cooling into a cold thermal boundary vary as a function of temperature and distance to the boundary, i.e., the conductive cooling lengthscale. Analysis of cooling rates of Masirah peridotites suggests the lower crust and uppermost mantle at Masirah were conductively cooled, and the overlying extrusive crust was hydrothermally cooled. This result is consistent with interpretations of spatial variations in cooling rate from another paleo-spreading center with a thin crustal section, but contrasts with observations from paleo-spreading centers with thick crustal sections, which suggest hydrothermal cooling of the lithosphere to the crust-mantle boundary and conductive cooling of the underlying mantle. Among the global dataset, ophiolitic peridotites record lower temperatures than abyssal peridotites through lower temperature intervals. The significance of a temperature recovered by a thermometer for a sample’s thermal history depends on grain size. We assessed grain sizes reported in our own samples and a literature compilation and found no statistically significant bias between ophiolitic and abyssal settings, such that the lower temperatures recorded by ophiolitic peridotites demonstrate that the ophiolites cooled more slowly than the abyssal peridotites. In the context of conductive cooling models, this difference can be interpreted as reflecting differences in conductive cooling lengthscales between the settings, with ophiolites cooling over lengthscales of hundreds of m to >ten km, and abyssal peridotites cooling over shorter lengthscales compressed by their advection through the geotherm. Crustal sections present at many ophiolites may have restricted the depth of hydrothermal circulation in some cases, producing thicker conductive cooling regimes than magma-starved abyssal settings, and/or the ease of sampling deeper parts of the mantle lithosphere at ophiolites biases the ophiolitic sample record toward longer conductive lengthscales. Evaluation of temperature distributions among abyssal peridotites demonstrates a positive correlation of cooling rate with spreading rate. Cooling rates calculated from the global dataset of temperatures are qualitatively consistent with, but underestimated by, models of simple advection of upwelling lithosphere through a conductive geotherm, implying that shallow hydrothermal circulation cools the mantle in magma-starved abyssal settings to some extent, and that heat conduction and the rate of advection through the geotherm all play a role in determining the thermal histories of peridotites from abyssal settings.