Internally consistent geothermometers for garnet peridotites and pyroxenites

Existing major-element geothermometers for garnet-bearing ultramafic rocks have been evaluated using the composistions of coexisting minerals in over 700, carefully selected, mantle xenoliths and in experiments in relevant systems. Mutual relationships among temperatures estimated with the most widely used geothermometers demonstrate that the methods are not internally consistent and may diverge by over 200°C even in well equilibrated mantle xenoliths. The Taylor (1998, N Jb Min Abh 172: 381-408) two-pyroxene (TA98) and the Nimis and Taylor (2000, Contrib Mineral Petrol 139: 541-554) single-clinopyroxene thermometers show an excellent agreement and reproduce the temperatures of experiments in a variety of simple and natural peridotitic systems. The Brey and Köhler (1990, J Petrol 31: 1353-1378) Ca-in-Opx thermometer shows good agreement with TA98 in the range 1000-1400°C and a positive bias at lower T (up to +90°C, on average, at TTA98 = 700°C). The popular Brey and Köhler (1990, J Petrol 31: 1353-1378) two-pyroxene thermometer performs well on clinopyroxene with Na contents of ~0.05 atoms per 6-oxygen formula, but shows a systematic positive bias with increasing NaCpx (+150°C at NaCpx = 0.25). Among Fe-Mg exchange thermometers, the Harley (1984, Contrib Mineral Petrol 86: 359-373) orthopyroxene-garnet and the Wu and Zhao (2007, J Metamorphic Geol 25: 497-505) olivine-garnet formulations show the highest precision, but systematically diverge (up to c. 150°C, on average) from TA98 and experimental temperatures at T far from 1000°C and at T < 1200°C, respectively. The older O'Neill and Wood (1979, Contrib Mineral Petrol 70: 59-70) version of the olivine-garnet Fe-Mg thermometer and all versions of the clinopyroxene-garnet Fe-Mg thermometer show very low precision, with discrepancies exceeding 200°C when compared to TA98 results for well equilibrated xenoliths. Empirical correction to the Brey and Köhler (1990, J Petrol 31: 1353-1378) Ca-in-Opx thermometer and recalibration of the orthopyroxene-garnet thermometer, using well equilibrated mantle xenoliths and TA98 temperatures as calibrants, ensure consistency with TA98 estimates in the range 700-1400°C. Observed discrepancies between the new orthopyroxene-garnet thermometer and TA98 for some localities can be ascribed to variations in mantle redox conditions and to kinetic decoupling of Ca-Mg and Fe-Mg exchange equilibria caused by transient heating. The latter process appears to be common, but not ubiquitous, near the base of the cratonic lithosphere.