Two-Pyroxene Thermometry's Inability to Recover Temperatures of Hydrous Arc Andesites
Abstract
Two-pyroxene thermometry is one of the most commonly used methods for constraining crystallization temperatures of igneous rocks. It is based primarily on the distribution of Ca, Mg, and Fe between orthopyroxene and clinopyroxene. Numerous variants of the two-pyroxene thermometer have been calibrated over a wide range of temperatures (700-1500° C) and compositional conditions (e.g. Davidson and Lindsley, 1985; Bertrand and Mercier, 1985; Brey and Kohler, 1990; Fonarev and Grapchikov, 1991). However, based on comparison with recent experimental results, the applicability of the two-pyroxene thermometer for hydrous arc andesites is refuted, and its use elsewhere is questioned. The run products of several water-saturated phase equilibria experiments on a calc-alkaline basaltic andesite (55.3 wt% SiO2), andesite (62.6 wt% SiO2), and two high-Mg andesites (62.2 and 62.0 wt% SiO2) from western Mexico were fully characterized in detailed microprobe analyses by Moore and Carmichael (1998) and Blatter and Carmichael (2001). Experiments were performed at temperatures between 930 and 1000° C and pressures between 441 and 2280 bars. Experimental equilibration was tested by comparing long duration runs, in which all phases were homogeneous in composition and had euhedral shapes. The starting materials were phenocryst-poor and relict crystals were not observed in the run products. The experimental run temperatures could not be recovered using the compositions of the co-existing pyroxenes and subsequent calculations from QUILF (Andersen et al., 1993) or iterative determinations from MELTS (Ghiorso and Sack, 1995). The crystallization temperatures yielded by QUILF were typically too high by ~100° C, but ranged from -186 to +177° C. The crystallization temperatures obtained from MELTS were more problematic, with several experiments not converging on a unique temperature and others off by up to 800° C. These results suggest that these two-pyroxene thermometers may not be adequately calibrated for hydrous andesites. As seen in Lindsley's (1983) graphical representation of two-pyroxene thermometry, the isotherms for the Ca component in orthopyroxene are very tightly spaced, so the temperature calculation is controlled primarily by the Ca component in the clinopyroxene. However, in the temperature interval of the phase-equilibria experiments (930-1000° C), the Ca component does not appear to be correlated with temperature. This suggests that not only are previous calibrations of two-pyroxene thermometry not applicable for arc magmas, but that the attempt to find a calibration based on Ca content in this temperature range is a flawed approach. The experience of this study leads us to call into question the reliability of two-pyroxene thermometry for any igneous rocks which are not almost identical to those used in experimental calibrations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.V13B0555F
- Keywords:
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- 3612 Reactions and phase equilibria (1012;
- 8412);
- 3620 Mineral and crystal chemistry (1042);
- 3651 Thermobarometry