Comments on “Evaluation of thermobarometers for garnet peridotites” by A. A. Finnerty and F. R. Boyd

Abstract

In order to evaluate the accuracy of a given combination of thermo-barometer, Finnerty and Boyd (1984) calculated the P-T conditions of two control samples of garnet lherzolite xenoliths, namely BD 2125 and PHN 1569. The importance of the samples lies in the fact that BD 2125 is diamond bearing, whereas PHN 1569 is graphite bearing. The alumina solubility in orthopyroxene (OPx) coexisting with garnet (Gt) is sensitive to both pressure and temperature changes and has thus been used widely, in combination with various geothermometers, for the thermo-barometry of garnet lherzolite xenoliths. Finnerty and Boyd (1984) concluded that the experimental calibrations of alumina solubility in OPx by Akella (1976) and Lane and Ganguly (1980) are as precise as, but probably less accurate than MC74 barometer,' where MC74 referred to the experimental calibration of alumina solubility in OPx by McGregor (1974) in the system MgSiO{sub 3}-Al{sub 2}O{sub 3}-SiO{sub 2} (MAS) based on synthesis experiments from glass of appropriate compositions. This conclusion on the accuracy of the above barometers was based on their observation that the use of only MC74 placed the calculated P-T conditions of the control samples in the right field with respect to the diamond-graphite equilibrium boundary, while those ofmore » Akella (1976; AK76) and Lane and Ganguly (1980; LG80) yielded P-T conditions that did not exactly satisfy the latter constraint, but were within 2 kb of the phase boundary. While it is clear from thermodynamic considerations that an unambiguous test of the accuracy of the calibrations cannot be carried out without making corrections for the effects of the additional components which are present in the natural samples but not in the experimental charges, the calculations of Finnerty and Boyd (1984) using LG80 are grossly erroneous.« less