Reconstructing CO2 concentrations in basaltic melt inclusions from Cascade cinder cones using Raman analysis of vapor bubbles

Abstract:

Because melt inclusions (MIs) trap melt prior to eruptive degassing, they are useful probes of melt volatile concentrations. However, during post-entrapment cooling and crystallization, the melt in the inclusion contracts more than the mineral host, causing depressurization and nucleation of a vapor bubble. This causes pressure-soluble volatiles, particularly CO₂, to exsolve from the melt into the bubble. To explore the extent of CO₂ loss, CO₂ densities in bubbles were estimated using data from Raman analysis of olivine-hosted melt inclusions from two cinder cones in the southern Cascade Arc (Basalt of Round Valley Butte [BRVB]; Basalt of Old Railroad Grade [BORG]). In BRVB, bubble vol. % (bubble vol./MI vol.) and measured CO₂ densities ranged from 0.9 – 6.7 vol. % and 0.05– 0.24 g/cm³, respectively. In BORG, bubble vol. % and CO₂ densities ranged from 1.4 - 9.2 vol. % and 0.07 – 0.29 g/cm³, respectively. To eliminate MI containing bubbles that were co-entrapped with the melt, we used a model from Riker (2005) to predict bubble vol. % as a function of the difference between eruption and formation temperatures. This suggested that bubbles larger than ~3.3 vol. % should be eliminated from CO₂ reconstructions. Using average values of MI H₂O and CO₂ measured by FTIR, we added CO₂ from the bubbles back into the MIs to obtain estimates of dissolved CO₂ at the time of trapping. Analyzed concentrations were 933 ppm CO₂ and 2.8 wt. % H₂O for BORG (2.7 kbar, or ~9 km depth), with a reconstructed CO₂ concentration of 1860 ± 612 (2s) ppm. Analyzed concentrations for BRVB were 426 ppm CO₂ and 1.6 wt. % H₂O (1.1 kbar, or ~3.7 km depth), with a reconstructed CO₂ concentration of 2320 ± 1688 (2s) ppm. Using the reconstructed CO₂ concentrations, VolatileCalc estimates place both BORG and BRVB MI entrapment at 4.3 kbar, or ~14 km depth. Thus, adding the CO₂ in bubbles back to the CO₂ in the glass of MIs is essential for determining accurate depths of magma crystallization.