Hydrous species geospeedometer in rhyolite: improved calibration and application

Abstract

The hydrous species geospeedometer is based on the homogeneous interconversion reaction between molecular H 2O species and OH species in silicate melts and glasses. Here we report new data for the calibration of the geospeedometer in rhyolitic glass, extending the coverage of quench rate to 94 K/s and of H 2O t to 7.7 wt.% by using a piston-cylinder apparatus at low pressures (200–500 MPa) to prevent bubble growth and to generate high and monitored quench rates. The new experimental data at pressure are highly reproducible and consistent with earlier data at 0.1 MPa, indicating negligible pressure effect on the relation between speciation and quench rate at P ≤ 500 MPa. In order to avoid calibration uncertainties, the original infrared data are used to represent species concentrations and the equilibrium constant. Ā 523 and Ā 452 (absorbances of the 523 and 452 mm −1 bands in terms of peak height per mm sample thickness) are used to represent concentrations of molecular H 2O and OH groups, respectively, and Q′ (= Ā 452 2/Ā 523) is used to represent the quotient of the species interconversion reaction, since there is rough proportionality between the corresponding parameters ( Ā 523 and molecular H 2O, Ā 452 and OH, Q′ and the quotient Q). Zhang et al. [Geochim. Cosmochim. Acta 61, 3089–3100 (1997a)] showed that for a given quench rate ( q), there is an excellent linear relation between ln Q′ and ln( Ā 523 + Ā 452) when total H 2O is ≤3.0%. With new data at higher total H 2O, the linear relation does not hold anymore. Furthermore, the new data show that the linear relation between ln Q′ and ln q does not hold at high q. Hence, the geospeedometry model of Zhang et al. can be used for interpolation, but extrapolation may lead to large errors. A new geospeedometry model using the combined data set is presented in this work and applied to natural rhyolitic glasses. The new geospeedometer can be used to quantify cooling rates in a quench medium or an experimental apparatus. Furthermore, it can be used to determine the cooling rates of individual pyroclasts, different parts of a lava flow, and melt inclusions in phenocrysts, thus allowing inference of rich details of volcanic processes.