Single clinopyroxene thermobarometry for garnet peridotites. Part I. Calibration and testing of a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer

Abstract

Experimental clinopyroxenes synthesized at 850-1500 °C and 0-60 kbar in the CMS and CMAS-Cr systems and in more complex lherzolitic systems have been used to calibrate a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer for Cr-diopsides derived from garnet peridotites. The experiments cover a wide range of possible natural peridotitic compositions, from fertile pyrolite to refractory, high-Cr lherzolite. The barometer is based on the Cr exchange between clinopyroxene and garnet. Pressure is formulated as a function of temperature and clinopyroxene composition: where a(CaCrTs)(Cpx) = Cr - 0.81 · Cr · (Na + K) and Cr = ((Cr+Al)/(Cr)), with elements in atoms per 6 oxygens. This formulation reproduces the experimental pressures to ± 2.3 kbar (1σ) and has a temperature dependence (1.2-2.4 kbar/50 °C, varying with composition) that is weaker than that of the widely used Al-in-Opx barometer (2-3 kbar/50 °C). The enstatite-in-Cpx thermometer includes corrections for the effect of minor components and is formulated as where a(en)(Cpx) = (1 - Ca - Na - K) · (1 -1/2(Al + Cr + Na + K)). The thermometer reproduces the experimental temperatures to ± 30 °C (1σ). The uncertainties of the present formulations are comparable to, or better than, those of the most widely used thermobarometers for garnet peridotites. P-T estimates obtained for diamond-bearing and graphite-bearing lherzolite xenoliths and peridotitic clinopyroxene inclusions in kimberlitic and lamproitic diamonds confirm the reliability of the thermobarometer. Cr-diopside thermobarometry appears to be a potential tool for obtaining information on the thermal state of the upper mantle and the extent of mantle sampling by deep-seated magmas. We consider the Cr-in-Cpx barometer to be the best alternative to the Al-in-Opx barometer for the evaluation of pressure conditions of equilibration of natural garnet lherzolites. P-T conditions of equilibration can be directly retrieved from the composition of Cr-diopside alone, thus allowing application to partially altered xenoliths, inclusions in diamonds, and loose grains from sediments. We foresee application of the present thermobarometer to evaluation of the diamond potential of kimberlite and lamproite provinces and in diamond exploration where Cr-diopside from deep mantle sources is preserved in the surficial weathering environment. Show more