CO2 contents of basaltic arc magmas from the southern Cascades: Corrections for shrinkage bubble effects and implications for crustal storage

Abstract:

Volatiles such as H₂O and CO₂ play an important role in a variety of magmatic processes from magma generation to eruption, and melt inclusions (MI) – small volumes of melt trapped inside phenocrysts – have been used to measure their pre-eruptive concentrations. In particular, the volatile contents of MI from basaltic arc magmas have been used to track the role of dehydrating subducted oceanic lithosphere in magma formation in subduction zones. However, recent studies have shown that MI are imperfect storage containers and can lose H by diffusion through the mineral host and CO₂ due to formation of a vapor bubble in the inclusion. Such results suggest that even the least degassed melt inclusions from a volcano may have volatile concentrations that underestimate the initial volatile contents of the magma. Thus, recognizing pre- and post-entrapment processes that influence MIs is important for interpreting magmatic processes at depth. Recent studies have developed methods that can be used to distinguish and correct for H diffusive loss (Bucholz et al., 2013) and CO₂ loss to vapor bubbles (Wallace et al., 2015). Here, we focus on MI from eight cinder cones that erupted primitive basaltic magmas in the Lassen region of the Cascade arc, where H₂O and Cl concentrations have been shown to relate to the amount of a subduction component added to the mantle wedge (Walowski et al., 2015). Using methods of Aster (2015), we correct for the loss of CO₂ to a vapor bubble formed within a melt inclusion as the result of post-entrapment crystallization and thermal contraction. The results of the CO₂ restoration calculations suggest that ~25-75% of the initial dissolved CO₂ in the melt inclusions at the time of trapping was lost to a vapor bubble after entrapment. Trapping pressures for the restored CO₂ and maximum H₂O contents calculated using methods of Iacono-Marziano et al. (2012) range from ~2-5 kbar, equivalent to entrapment depths of ~7-18 km below the surface. The results indicate that primitive magmas in the southern Cascades stall in the middle crust, where they crystallize olivine and trap melt inclusions. Lastly, we consider the relationships of restored CO₂ contents to highly incompatible elements like Nb and compare our results to data for basaltic magmas from other tectonic environments.