Hydrous species geospeedometer in rhyolite: Improved calibration and application

The hydrous species geospeedometer is based on the homogeneous interconvers ion reaction between molecular H2O species and OH species in silicate melts and glasses. Here we report new data for the calibration of the geospeedom eter in rhyolitic glass, extending the coverage of quench rate to 94 Ws and of H2Ot 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 q uench 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 less than o r equal to 500 MPa. In order to avoid calibration uncertainties, the origin al infrared data are used to represent species concentrations and the equil ibrium constant. (A) over bar(523) and (A) over bar(452) (absorbances of th e 523 and 452 mm(-1) bands in terms of peak height per mm sample thickness) are used to represent concentrations of molecular H2O and OH groups, respe ctively, and Q' (= (A) over bar(452)(2)/(A) over bar(523)) is used to repre sent the quotient of the species interconversion reaction, since there is r ough proportionality between the corresponding parameters ((A) over bar(523 ) and molecular H2O, (A) over bar(452) and OH, Q' and the quotient Q). Zhan g et al. [Geochim. Cosmochim. Acta 61, 3089-3100 (1997a)] showed that for a given quench rate (q), there is an excellent linear relation between In Q' and 1n((A) over bar(523) + (A) over bar(452)) when total H2O is less than or equal to 3.0%. With new data at higher total H2O, the linear relation do es not hold anymore. Furthermore, the new data show that the linear relatio n between 1n Q' and 1n 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 s et 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, an d melt inclusions in phenocrysts, thus allowing inference of rich details o f volcanic processes. Copyright (C) 2000 Elsevier Science Ltd.