Volcanic SO2 emissions and magmatic volatile budgets

Modeling volcanic eruptions and understanding their atmospheric and climate effects requires knowledge of magmatic volatile contents. The volatile budget includes the mass fraction of major volatiles dissolved in magma before eruption and the mass fraction present in a separate vapor or fluid phase. It is well demonstrated that intermediate to silicic magmas are typically vapor saturated before eruption (e.g., Scaillet & Pichavant, 2003; Shinohara, 2008). The large SO2 discharges detected by satellite remote sensing for eruptions spanning several orders of magnitude in volume indicate that gas accumulation is required to generate explosive andesitic to rhyolitic eruptions. As part of a NASA Interdisciplinary Research in Earth Science (IDS) project on volatiles in volcanic systems, we are investigating the volatile budgets of andesitic to rhyolitic systems. We consider three components: (1) addition of volatiles to the magmatic system by input of magma from depth (recharge), (2) loss of volatiles by leakage out of an upper crustal reservoir, and (3) enrichment of volatiles due to crystallization and buoyant vapor segregation. The composition of recharge magma is variable due to evolution of mantle-derived basalt within transcrustal magmatic systems. Data for repose times and erupted volumes (Trial & Spera, 1990; White et al., 2006; Passarelli & Brodsky, 2011) show that eruptible magma accumulates in the upper crust at rates of ~5 x 10-4 to 5 x 10-3 km3/yr. Estimates of intrusive to extrusive ratios for individual systems are poorly constrained and highly variable, but values of ~5:1 are common. Published melt inclusion data indicate magmatic S contents of ~1000-3000 ppm for basaltic compositions to ~100-1700 ppm for andesitic compositions. These data provide a means of comparing the flux of mantle-derived volatiles with the flux of SO2 released from explosive eruptions. The results demonstrate that the fluxes of S are approximately balanced if the mantle-derived basaltic supply rate is ~0.001-0.01 km3/yr, broadly consistent with thermal models. However, if extensive differentiation occurs in the lower crust, then andesitic melts ascending from the lower to upper crust must contain dissolved S contents at the high end of the range measured for melt inclusions.