Pyroxene xenocryst geotherms: Techniques and application

Abstract

Pyroxene thermobarometry of mantle materials has matured with time, experimental investigation and experience to encompass solutions for both pressure and temperature based on the composition of garnet-facies peridotitic clinopyroxene alone. Application to data sets now available in the public domain show robust compositional, pressure (P) and temperature (T) trends for clinopyroxene as well as orthopyroxene xenocrysts, parallel to matching trends for coexisting pyroxenes in garnet lherzolite xenoliths. Investigation of the unified and well-populated xenolith plus xenocryst data set shows (i) natural pyroxenes in a closer approach to compositional equilibrium than typically obtained by reversed, high P–T experiments, (ii) inappropriate treatment by modern geobarometers of aluminous pyroxene compositions in spinel + garnet peridotite, (iii) model-conductive P–T conditions at mid-lithospheric pressures, and (iv) an effectively universal presence in kimberlite-hosted deep mantle materials of supra-conductive P–T conditions, typically also associated with compositional and/or textural modification. The available clinopyroxene and garnet xenocryst data from cold, high-pressure cratonic interiors (Slave, Yakutia, Sarfartoq, Botswana) and craton-margin settings (southern Superior, south-western Kaapvaal, Lesotho, Gibeon, Minas Gerais) combine with chronologic data to show that Cr/Al-rich, depleted peridotite forms a rheological and chemical layer of remarkably variable depth that must be underlain by a thermal boundary layer to ultimate depths of 200 to 250 km. High-pressure pyroxene xenocrysts faithfully record thermal spikes from conductive to adiabatic conditions, presumably caused by advective breaching of the thermal boundary layer during plume-impact or riftogenic processes that also act to generate kimberlite melts from within the decaying thermal boundary over a 10 to 40 Ma time-scale. The compositions of pyroxene xenocrysts and age of their host constitute a powerful means with which to investigate the thermo-tectonic evolution of garnet-facies depleted lithospheres and their contained diamond reservoir.