An experimental test of some geothermometer and geobarometer formulations for upper mantle peridotites with application to the thermobarometry of fertile Iherzolite and garnet websterite

A series of high pressure (P), temperature (T) experiments (P = 1.0 to 3.5 GPa, T = 1050-1260 degrees C) were undertaken on Na2O-, TiO2- and pyroxene-rich peri dotite compositions under controlled fO(2) conditions. Compositions of olivine, pyroxene and garnet have been used to evaluate the suitability of existing geothermometer and geobarometer formulations in estimating the equilibration conditions of fertile peridotite. The quality of the mineral chemical data reported here represents an improvement over that in many earlier experimental studies due to: the absence of Fe-loss problems by use of graphite inner capsules; better approach to equilibrium because of increased run times and the presence of a fluid or melt phase; and control of fO(2) at low values so that silicate phases contain negligible Fe3+ contents. Of the geothermometers tested, only those of WELLS (1977) [two-pyroxene] and HARLEY (1984) [garnet-orthopyroxene] reproduce experimental temperatures with any accuracy. For the geobarometers, a version of the WOOD (1974) formulation [garnet-orthopyroxene] gave the most satisfactory results. However, somewhat better P estimates were obtained using a modified version of the NICKEL & GREEN (1985) barometer [NG85mod] in which a Ti correction was applied to the activity term for the Mg-tschermaks component: X-MgTs = (Al-Cr+Ti+Na)/2. To extend the P,T, and compositional range of applicability for two-pyroxene thermometers, a new formulation [TA97] has been devised: T(K) = [24,787 + 678.P(GPa)]/[15.67 + 14.37.Ti-cpx + 3.69.Fe-cpx -3.25 . X-ts + (lnKd)(2)] where: lnKd = ln{a(En)(cpx)} - ln{a(En)(opx)}; X-ts = (Al+Cr-Na)(cpx); and a(En) = (1-Ca-Na).(1-Al-vi-Cr-Ti).(1-Al-iv/2)(2) It satisfactorily reproduces experimental T for the present experiments, as well as those of BREY et al. (1990) and WALTER & PRESNALL (1994). A new thermobarometer based the NG85mod-TA97 pair is recommended for fertile lherzolite and websterite compositions. Application to garnet Iherzolite, websterite, and pyroxenite xenoliths from southeastern Australia suggest that they equilibrated under lower P,T conditions, and originated from restricted depth intervals, compared with previous estimates.