No unique temperature of equilibration for mantle peridotites – Thermal histories told by differently diffusing cations based on novel calibrations

We generated novel thermometric formulae for cation exchange reactions between mantle minerals involving species of different diffusivities (FeMg orthopyroxene-spinel; CrAl & CaMg clinopyroxene-orthopyroxene) and used them in conjunction with published expressions (FeMg olivine-spinel; CaMgFe clinopyroxene-orthopyroxene) to decipher the thermal history of peridotites from diverse tectonic settings. When tested on abyssal peridotites, TCrAlopx-cpx vs. TCaMgcpx-opx form tight trends with TCrAlopx-cpx ≥ TCaMgcpx-opx at T<1060oC and TCrAl opx-cpx ≤ TCaMgcpx-opx at T>1060oC in accordance with crossing diffusivities of Cropx and CaMgcpx at about that temperature, with the latter becoming smaller at higher temperatures and blocking higher as the peridotites are quickly emplaced and cooling at the ridge. Both TCrAlopx-cpx and TCaMgcpx-opx are higher than the widely used TCaFeMgcpx-opx in abyssal peridotites by ~100°C on average. On a TCrAlopx-cpx vs. TFeMgopx-sp plot there is clear-cut distinction between peridotite massifs (major ocean basins, oceanic forearcs, oceanic backarcs, ophiolites, ocean-continent transitions, Alpine-type) from peridotite xenoliths (cratons, intracontinental rifts, continental backarcs, active continental margins, oceanic within-plate) with TCrAlopx-cpx being invariably larger than TFeMgopx-sp for the peridotite massifs reflecting the blocking of the slower-diffusing cations at higher temperatures upon cooling. The higher TFeMgopx-sp exhibited by the xenoliths are ascribed to some, probably magmatic, heating event at a time prior to their transport to the surface. The Alpine-type peridotites of Beni Bousera, Morocco, that were exhumed in the footwall of a lithosphere-scale extensional shear zone display increasing TCrAlopx-cpx and TCaMgcpx-opx towards the shear zone but flat TCaFeMgcpx-opx, TFeMgopx-sp and TFeMgol-sp profiles demonstrating the memory of the slower-diffusing cations.