Application of the Ti-in-quartz thermobarometer to rutile-free systems. Reply to: a comment on: ‘TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz’ by Thomas et al.

The premise of the Wilson et al. comment is that the Ti-in-quartz solubility calibration (Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010) is fundamentally flawed. They reach this conclusion because P–T estimates using the Ti-in-quartz calibration differ from their previous interpretations for crystallization conditions of the Bishop and Oruanui rhyolites. If correct, this assertion has far-reaching implications, so a careful assessment of the Wilson et al. reasoning is warranted. Application of the Ti-in-quartz calibration as a thermobarometer in rutile-free rocks requires an estimation of TiO₂ activity in the liquid ( (liquid–rutile); referenced to rutile saturation) and an independent constraint on either P or T to obtain the crystallization temperature or pressure, respectively. The foundation of Wilson et al.’s argument is that temperature estimates obtained from Fe–Ti oxide thermometry accurately reflect crystallization conditions of quartz in the two rhyolites discussed. We maintain that our experimental approach is sound, the thermodynamic basis of the Ti-in-quartz calibration is fundamentally correct, and our experimental results are robust and reproducible. We suggest that the reason Wilson et al. obtain implausible pressure estimates is because estimates for T and they used as input values for the Ti-in-quartz calibration are demonstrably too high. Numerous studies show that Fe–Ti oxide temperature estimates of some rhyolites are substantially higher than those predicted by well-constrained phase equilibria. In this reply, we show that when reasonable input values for T and (liquid–rutile) are used, pressure estimates obtained from the Ti-in-quartz calibration are well aligned with phase equilibria and essentially identical to melt inclusion volatile saturation pressures.     

Reply to the comment by J. R. Ali on “Illawarra Reversal: the fingerprint of a superplume that triggered Pangean breakup and the end-Guadalupian (Permian) mass extinction” by Yukio Isozaki

The following four major questions were raised about my recent proposal for the possible link between the end-Guadalupian extinction and a unique geomagnetic event called the Illawarra Reversal (Isozaki, 2009a); 1) timings of extinction, cooling, and the Illawarra Reversal (end of the Kiaman Superchron), 2) geomagnetic intensity during superchrons, 3) ascent rate of mantle plume, and 4) age constraints of LIP volcanism in east Pangea. The latest research results on the Permian biodiversity change, numerical modeling of plume, and single-crystal measurement of geomagnetism support that the timings of extinction and the Illawarra Reversal, high field intensity during the Kiaman superchron, and ascent rate of plume are reasonably explained in accordance with the integrated “plume winter” scenario (Isozaki, 2009b). The onset ages of LIP volcanism need further refinement for identifying the impingement of a plume head.