Mutual relationships among temperatures estimated with the most widely used geothermometers for garnet peridotites and pyroxenites demonstrate that the methods are not internally consistent and may diverge by over 200 degrees C even in well-equilibrated mantle xenoliths. The Taylor (N Jb Min Abh 172:381-408, 1998) two-pyroxene (TA98) and the Nimis and Taylor (Contrib Mineral Petrol 139:541-554, 2000) single-clinopyroxene thermometers are shown to provide the most reliable estimates, as they reproduce the temperatures of experiments in a variety of simple and natural peridotitic systems. Discrepancies between these two thermometers are negligible in applications to a wide variety of natural samples (<=30 degrees C). The Brey and Kohler (J Petrol 31:1353-1378, 1990) Ca-in-Opx thermometer shows good agreement with TA98 in the range 1,000-1,400 degrees C and a positive bias at lower T (up to +90 degrees C, on average, at T (TA98) = 700 degrees C). The popular Brey and Kohler (J Petrol 31:1353-1378, 1990) two-pyroxene thermometer performs well on clinopyroxene with Na contents of similar to 0.05 atoms per 6-oxygen formula, but shows a systematic positive bias with increasing Na(Cpx) (+150 degrees C at Na(Cpx) = 0.25). Among Fe-Mg exchange thermometers, the Harley (Contrib Mineral Petrol 86:359-373, 1984) orthopyroxene-garnet and the recent Wu and Zhao (J Metamorphic Geol 25:497-505, 2007) olivine-garnet formulations show the highest precision, but systematically diverge (up to ca. 150 degrees C, on average) from TA98 estimates at T far from 1,100 degrees C and at T < 1,200 degrees C, respectively; these systematic errors are also evident by comparison with experimental data for natural peridotite systems. The older O'Neill and Wood (Contrib Mineral Petrol 70:59-70, 1979) version of the olivine-garnet Fe-Mg thermometer and all popular versions of the clinopyroxene-garnet Fe-Mg thermometer show unacceptably low precision, with discrepancies exceeding 200 degrees C when compared to TA98 results for well-equilibrated xenoliths. Empirical correction to the Brey and Kohler (J Petrol 31:1353-1378, 1990) Ca-in-Opx thermometer and recalibration of the orthopyroxene-garnet thermometer, using well-equilibrated mantle xenoliths and TA98 temperatures as calibrants, are provided in this study to ensure consistency with TA98 estimates in the range 700-1,400A degrees C. Observed discrepancies between the new orthopyroxene-garnet thermometer and TA98 for some localities can be interpreted in the light of orthopyroxene-garnet Fe(3+) partitioning systematics and suggest localized and lateral variations in mantle redox conditions, in broad agreement with existing oxybarometric data. Kinetic decoupling of Ca-Mg and Fe-Mg exchange equilibria caused by transient heating appears to be common, but not ubiquitous, near the base of the lithosphere.
Internally consistent geothermometers for garnet peridotites and pyroxenites
NIMIS, PAOLO;
2010
Abstract
Mutual relationships among temperatures estimated with the most widely used geothermometers for garnet peridotites and pyroxenites demonstrate that the methods are not internally consistent and may diverge by over 200 degrees C even in well-equilibrated mantle xenoliths. The Taylor (N Jb Min Abh 172:381-408, 1998) two-pyroxene (TA98) and the Nimis and Taylor (Contrib Mineral Petrol 139:541-554, 2000) single-clinopyroxene thermometers are shown to provide the most reliable estimates, as they reproduce the temperatures of experiments in a variety of simple and natural peridotitic systems. Discrepancies between these two thermometers are negligible in applications to a wide variety of natural samples (<=30 degrees C). The Brey and Kohler (J Petrol 31:1353-1378, 1990) Ca-in-Opx thermometer shows good agreement with TA98 in the range 1,000-1,400 degrees C and a positive bias at lower T (up to +90 degrees C, on average, at T (TA98) = 700 degrees C). The popular Brey and Kohler (J Petrol 31:1353-1378, 1990) two-pyroxene thermometer performs well on clinopyroxene with Na contents of similar to 0.05 atoms per 6-oxygen formula, but shows a systematic positive bias with increasing Na(Cpx) (+150 degrees C at Na(Cpx) = 0.25). Among Fe-Mg exchange thermometers, the Harley (Contrib Mineral Petrol 86:359-373, 1984) orthopyroxene-garnet and the recent Wu and Zhao (J Metamorphic Geol 25:497-505, 2007) olivine-garnet formulations show the highest precision, but systematically diverge (up to ca. 150 degrees C, on average) from TA98 estimates at T far from 1,100 degrees C and at T < 1,200 degrees C, respectively; these systematic errors are also evident by comparison with experimental data for natural peridotite systems. The older O'Neill and Wood (Contrib Mineral Petrol 70:59-70, 1979) version of the olivine-garnet Fe-Mg thermometer and all popular versions of the clinopyroxene-garnet Fe-Mg thermometer show unacceptably low precision, with discrepancies exceeding 200 degrees C when compared to TA98 results for well-equilibrated xenoliths. Empirical correction to the Brey and Kohler (J Petrol 31:1353-1378, 1990) Ca-in-Opx thermometer and recalibration of the orthopyroxene-garnet thermometer, using well-equilibrated mantle xenoliths and TA98 temperatures as calibrants, are provided in this study to ensure consistency with TA98 estimates in the range 700-1,400A degrees C. Observed discrepancies between the new orthopyroxene-garnet thermometer and TA98 for some localities can be interpreted in the light of orthopyroxene-garnet Fe(3+) partitioning systematics and suggest localized and lateral variations in mantle redox conditions, in broad agreement with existing oxybarometric data. Kinetic decoupling of Ca-Mg and Fe-Mg exchange equilibria caused by transient heating appears to be common, but not ubiquitous, near the base of the lithosphere.
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