INCLUSION THERMOBAROMETRY: BEYOND QUARTZ

Numerical elastic difference models have been developed for mineral inclusions in garnet to evaluate the application of inclusion barometry to common inclusion minerals. For encapsulation at 20 kbar and 400 °C, the percent pressure retention (%P_r) for various inclusion phases are as follows: 42% for quartz, 39% for albite, 31% for anorthite, 38% for graphite, 2.5% for zircon, and 0% for magnetite and rutile (ilmenite is not considered due to instability at high pressures). Increasing pressure by 10 kbar has little effect on the proportion of pressure retained. Although these results appear promising for application to several minerals, complications with these phases may impose certain limits. For example, the extremely strong anisotropy of graphite requires a complex, three-dimensional elastic model for rigorous evaluation. In addition, some of these phases lack a robust Raman calibration. Sphene, rutile and magnetite, for example, have broad-band Raman spectra that may not be amenable to pressure determination.

When extrapolating the elastic model to metamorphic temperatures, some phases deviate significantly from quartz-like behavior. For example, zircon inclusions are not very sensitive to pressure changes, owing to the low compressibility of zircon. Thus, zircon may be better suited as a geothermometer (in which over-pressuring of the inclusion is a function of temperature variation). The %P_r of zircon increases from ca. 0% to 15% when temperature is increased from 300 °C to 800 °C (at 20 kbar). Although the uncertainties of formation pressure estimates are relatively large (±1.6 kbar, compared to < ±0.5 kbar for quartz), the expected temperature uncertainty is surprisingly small (ca. ±20 °C, with known pressure). If both quartz and zircon inclusions are analyzed in a single host crystal, our results suggest that the analyses can be combined to provide an independent thermobarometer, giving new insight into P-T conditions at the time of formation.