&xhrmrca er Cosmochimica Acla Vol. 56, pp. 841-842 Cop+ghto 1992 Pergamon Pmplc. Printed zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA in U.S.A. 0016-7037/92/$5.00 t .I0 COMMENT Comments on “Evaluation of thermobarometers for garnet peridotites” by A. A. Finnerty and F. R. Boyd J. GANGULY Department of Geosciences, The University of Arizona, Tucson, AZ 8572 I, USA zyxwvutsrqponmlkjihgfedcba (Received January 11, 199 1; acceptedin revised form November27, 199 1) FINNERTY AND BOYD ( 1984) undertook an evaluation of the precision and accuracy of mineralogical thermometers and barometers that are applicable to the determination of P-T conditions of garnet peridotites. They have also developed a computer program TEMPEST, which calculates P-T con- dition of equilibration of these rocks from the compositional properties of coexisting minerals using various combinations of thermometers and barometers available at that time. The program has been used by a number of workers interested in the calculation of P-T condition of mantle xenoliths, and was later updated ( FINNERTY and BOYD, 1987) to incorporate subsequent calibrations of thermometers and barometers applicable to garnet peridotites. 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 2 125 and PHN 1569. The importance of the samples lies in the fact that BD 2 125 is diamond bear- ing, whereas PHN 1569 is graphite bearing. Thus, if there is no significant kinetic problem in the transformation of graphite to diamond and vice versa, then the relative accu- racies of different combinations of thermo-barometers can be tested by noting how closely the calculated P-Tconditions of these control samples satisfy the experimentally determined diamond-graphite equilibrium boundary of KENNEDY and KENNEDY ( 1976). This test, however, does not provide any idea of the relative accuracies of the thermo-barometers at P-T conditions significantly removed from those of the con- trol samples. Nonetheless, the approach of FINNERTY and BOYD ( 1987) provides a reasonable initial test of the accuracy of the thermo-barometers. The precision of the thermo-ba- rometers was evaluated by noting the scatter of P-T estimates of four xenoliths “from a wide range of P and T when many point analyses of the constituent minerals are used for P-T estimation.” 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 cali- bration of alumina solubility in OPx by MCGREGOR ( 1974) in the system MgSi03-A1203-Si02 (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 of AKELLA( 1976; AK76) and LANE and GANGULY ( 1980; LGSO) 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 unambigious 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 er- roneous. If the positions of the alumina isopleths are not corrected for the effects of the additional components, which the authors did not, then the solution for P-T condition for a given sample must lie on the alumina isopleth (i.e., a line of constant alumina solubility) corresponding to the mea- sured alumina content of orthopyroxene coexisting with gar- net in the sample. This, unfortunately, is not the case with Finnerty and Boyd’s calculations. For example, for the graphite-bearing sample PHN 1569, in which there is 0.92 wt% alumina in OPx coexisting with garnet, FINNERTY and BOYD ( 1987) calculated P-T condition of 29 kb, 900°C (their Fig. 4F), whereas according to the alumina isopleth in LG80, the pressure defined at 900°C is 35 kb, and the temperature defined at 29 kb is 800°C. Unlike the calibration of MCGREGOR ( 1974), the LANE and GANCULY ( 1980) alumina isopleths were based on re- versed experimental determinations at selected P-T condi- tions and thermodynamic analysis, which satisfied the re- versed experimental determination of DANCKWERTH and NEWTON ( 1978) of the equilibrium transition between spinel- and garnet-peridotite in the MAS system. Since then, PER- KINS,HOLLAND, and NEWTON( 198 1, henceforth referred to as PEHN81) have carried out an exhaustive study of the alumina solubility in orthopyroxene in equilibrium with gar- net in the MAS system. For the small amount of alumina (<2 wt%) commonly found in orthopyroxene from garnet peridotite xenoliths, LG80 and PEHN8 1 yield essentially the same results. It is found (also see GANGULY and BHATTA- CHARYA, 1987; Fig. 123a) that the P-T conditions of the 841