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 degrees 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 Iherzolite. The barometer is based on the Cr exchange between clinopyroxene and garnet. Pressure is formulated as a function of temperature and clinopyroxene composition: P(kbar) = -T(K)/126.9 . ln[a(CaCrTs)(Cpx)] + 15.483 . ln(Cr#(Cpx)/T(K)) + T(K)/71.38 + 107.8, where a(CaCrTs)(Cpx) = Cr - 0.81 . Cr# . (Na + K) and Cr# = (Cr/Cr+Al), with elements in atoms per 6 oxygens. This formulation reproduces the experimental pressures to +/-2.3 kbar (la) and has a temperature dependence (1.2-2.4 kbar/50 degrees C, varying with composition) that is weaker than that of the widely used Al-in-Opx barometer (2-3 kbar/50 degrees C). The enstatite-in-Cpx thermometer includes corrections for the effect of minor components and is formulated as T(K) = 23166 + 39.28 . P(kbar)/13.25+15.35 . Ti+4.50 . Fe-1.55 .(Al+Cr-Na-K)+(ina(en)(Cpx))(2), where a(en)(Cpx) = (1 - Ca - Na - K) . (1 - 1/2(Al + Cr + Na + K)). The thermometer reproduces the experimental temperatures to +/-30 degrees C (1 sigma).