Spinel to garnet lherzolite transition in relation to high temperature palaeogeotherms, eastern Australia∗

Table Cape vent xenolith suite, northwest Tasmania: Mineralogy and implications for crust-mantle lithology and Miocene geotherms in Tasmania

“…The Ellis & Green thermometer, however, has proved unreliable in its application. It generally overestimates T relative to experimental results for the TP range for such xenolith suites (see Sutherland et al 1994, Ai 1994, Taylor 1998 or is erratic where Ca exceeds a concentration of O.IS atoms in the garnet (Alaoui et al 1997). The Table Cape Ellis & Green (Fe5+) and Wells (Fe3+) for Table Cape vent grt-websterites is 100S-1220°C and 10SS-107SoC respectively, which leads to corresponding differences in P estimates when inserted into the Wood barometer (e.g., 0.74-1.86 GPa, Sutherland et al 1994, cf1.23-1.36 GPa, Sutherland et al 2004.…”

“…It generally overestimates T relative to experimental results for the TP range for such xenolith suites (see Sutherland et al 1994, Ai 1994, Taylor 1998 or is erratic where Ca exceeds a concentration of O.IS atoms in the garnet (Alaoui et al 1997). The Table Cape Ellis & Green (Fe5+) and Wells (Fe3+) for Table Cape vent grt-websterites is 100S-1220°C and 10SS-107SoC respectively, which leads to corresponding differences in P estimates when inserted into the Wood barometer (e.g., 0.74-1.86 GPa, Sutherland et al 1994, cf1.23-1.36 GPa, Sutherland et al 2004. Wood (1974) barometry, however, is retained for the Table Cape vent study as a suitable barometer for Fe-rich grt-metawebsterites, particularly as some other potential barometers were experimentally calibrated against more Mg-rich assemblages.…”

“…Some grains resemble Fe-hydroxide altered olivine (5%) in one assemblage, which has pervasive fluid inclusion trails and alterations along grain boundaries. These garnet (± spinel)-two pyroxene assemblages allow TP estimates for the Table Cape vent suite, but are notably more Fe-enriched cpx (FeO total> 5wt%) than in most garnet-websterite suites (Sutherland et al 1994).…”

“…Previously reported couplings (Ellis &Green 1974 T andWood 1974 P;Brey & Kohler 1990 TP) for Table Cape (Sutherland et al 1994) are not further considered because of apparent compositional and experimental incompatibilities, discussed later. The Wells (1977) two-pyroxene thermometry, with appropriate allocations ofFe 2 + and Fe3+, provides good results at T < 1300°C, with uncertainties in the precision within ± 50°C (Tribaudino & Bruno 1993, Taylor 1998).…”

“…The pyroclastics include irregular sill-like bodies of chilled tachylytic basalt that form a more resistant protection to the included fragments. The deeper origin xenoliths are important in reconstructing the underlying Tasmanian lithosphere and its geothermal state at the time of volcanism, as they not only include garnet-two pyroxene assemblages suitable for geothermobarometry but also diverse mantle peridotites and pyroxenites (Sutherland et al 1994, Beyer 1998, Beyer et al 2001. The Table Cape suite is one of only two known garnetiferous mantle xenolith sites in Tasmania, the other being at Bow Hill, west of Oatlands in the central midlands ( fig.…”