DEGASSING OF MAFIC ALKALINE MAGMAS AT NYAMURAGIRA VOLCANO, DR CONGO

There are few constraints on volatile behavior in alkaline magmas, despite their involvement in several globally significant, S-rich volcanic eruptions (e.g., Tambora 1815). Nyamuragira volcano, located in the East African Rift in D.R. Congo, is one of the most active volcanoes in Africa, and an exceptional locality for volcanic degassing studies. Its sulfur dioxide (SO₂) emissions significantly contribute to the global volcanic sulfur budget; ~25 Mt of SO₂ was released into the atmosphere by 15 effusive eruptions in a 24-year period. In addition, Nyamuragira erupts mafic alkaline lavas (low-Si, high-K), which results in fluid lava flows that can inundate populated areas quickly.

We use satellite-based SO₂ measurements and corresponding erupted lava volumes for 15 Nyamuragira eruptions (1980-2006) to estimate the dissolved S required to explain the observed SO₂ emissions by syn-eruptive degassing of erupted melt. Results show that degassing of ~1,200-8,000 ppm S would account for the measured SO₂ emissions. These estimates were compared to existing data on S contents of mafic alkaline magmas (olivine-hosted melt inclusion data from alkali basalts, basanites, minettes and tephrites from various tectonic settings). The data show that mafic alkalic melts commonly contain 500-2,700 ppm S, but high-K magmas in arc settings can have even higher S (3,000-5,400 ppm). These values are consistent with our lower estimates for syn-eruptive S release (1,200-3,000 ppm S), but our higher estimates (6,400-8,000 ppm S) suggest that either Nyamuragira erupts unusually S-rich magmas, or that gas segregation or accumulation might have occurred in the system prior to eruption. To determine if Nyamuragira magmas contain such high S concentrations, we have analyzed olivine-hosted melt inclusions (MI) from a suite of Nyamuragira tephras and lavas.

Our preliminary results show a maximum dissolved S content of 2,300 ppm in MIs from the 1938, 1948, 1986, and 2006 Nyamuragira eruptions. Comparing SO₂ emission estimates derived from the petrologic method to satellite-based SO₂ measurements, the remotely-sensed SO₂ emissions were up to 3x greater for the 1986 and 2006 Nyamuragira eruptions. We are further assessing the information that Nyamuragira MIs give us regarding the degassing of SO₂ and other volatiles from this alkaline volcano.