INVESTIGATIONS INTO THE DEGASSING OF NYAMURAGIRA VOLCANO (D.R. CONGO, AFRICA) THROUGH SYNCHROTRON MICRO-XANES ANALYSIS

Nyamuragira volcano (D.R. Congo, Africa) is located in the western branch of the East African Rift, erupts frequently (every ~2 years), and produces large amounts of SO₂. When comparing its eruptions, magmatic gas content and explosivity vary, sometimes greatly. The driving mechanisms that account for these differences are not well understood, but potential variability in both magmatic redox (ƒO₂) and degassing history are likely important parameters controlling these processes. To constrain the impact of these effects, we analyzed more than 50 MI from different Nyamuragira eruptions by microfocused X-ray absorption near edge structure spectroscopy (μXANES). Iron and vanadium valence state μXANES were used to constrain melt ƒO₂ and sulfur μXANES for evaluating differences in S speciation. Nyamuragira samples investigated in this study were from the 1912, 1986, and 2006 eruptions. Magmatic S concentrations measured in melt inclusions (MI) were highest in the 1912 sample (3700 ppm), while the 1986 and 2006 MI samples contained similar S concentrations (~2000 ppm). Our μXANES studies show that there are systematic differences in melt ƒO₂ and S speciation in glasses from the 1986 eruption compared with those from 1912 and 2006. Fe and V data indicate that all samples are reduced with ƒO₂ ranging from QFM+1 to -1, but the 1986 data are at the lower end of this range, while showing higher and more variable sulfate abundance than the 1912 and 2006. We believe that these differences can be attributed to S degassing in the 1986 magmas. Entrapment pressures determined from MI H₂O and CO₂ data, paired with S data, show variability in S degassing with storage for each eruption; the 1986 magma degassed more strongly with ascent than the 2006 magma prior to eruption, which could explain a reduction of Fe species and an oxidation of S due to electron exchanges. The results also demonstrate the sensitivity of the V redox oxybarometer for measuring the redox evolution of alkalic melt inclusions.