Ultra-high temperatures from oxygen isotope thermometry of a coesite-sanidine grospydite

The oxygen isotope compositions of coesite, sanidine, kyanite, clinopyroxene and garnet were measured in an ultra-high pressure-temperature grospydite from the Roberts Victor kimberlite, South Africa. The ¹⁸O values (per mil v. SMOW) of each phase and (1 ) are as follows: coesite, 8.62 (0.31); sanidine, 8.31 (0.02); kyanite, 7.98 (0.08); pyroxene, 7.63 (0.11); garnet, 7.53 (0.03). In situ analyses of the coesite with the laser extraction system are ¹⁸O=9.35 (0.08), n=4, demonstrating that the coesite is homogeneous. The coesite has partially inverted to polycrystalline quartz and the pyroxene is extensively altered during uplift. The larger scatter for the mineral separate coesite and pyroxene data may be due to partial reequilibration between the decompression-related breakdown products of these two phases. The anomalously high ¹⁸O value of the grospydite (¹⁸O_wholerock=7.7) is consistent with altered oceanic crust as a source rock. Temperature estimates from a linear regression of all the data to three different published calibrations correspond to an equilibrium temperature of 1310±80°C. The calculated isotopic pressure effect is to lower these estimates by about 40°C at 40 kb. The estimated temperature based on Al–Si disorder in sanidine is 1200±100°C and that from Fe–Mg exchange thermometry between garnet and clinopyroxene is 1100±50°C. Given the large errors associated with thermometry at such high temperatures, it is concluded that the xenolith equilibrated that 1200±100°C. Pressure estimates are 45±5 kb, based on dilution of the univariant equilibria albite = jadeite + coesite and 2 kyanite + 3 diopside = grossular + pyrope + 2coesite. Zoning in the outer 20 m of the feldspar from Ab_0.8 to Ab₁₆ indicates rapid decompression to 25 kb or less. The isotopic temperature estimates are the highest ever obtained and combined with the high degree of Al–Si disorder in sanidine require rapid cooling from ultra-high temperatures. It is inferred that the xenolith was sampled at the time of equilibration, providing a point on the upper Cretaceous geotherm in the mantle below South Africa.