Investigating magma storage histories at flank cones of Nyiragongo and Nyamulagira with olivine-hosted melt inclusions and diffusion chronometry

Nyiragongo and Nyamulagira are the two most active volcanoes in the East African Rift, posing considerable risk to the citizens of Goma and surrounding communities. While these volcanoes are known for their alkalic, low viscosity lava flows, scoria cones with relatively primitive compositions preserve evidence of explosive magmatism and phreatomagmatism in the region. Over 100 cones are present along the flanks of these volcanoes, yet little is known about their pre-eruptive storage conditions or connection to the central vents at these two volcanoes. To better understand magma storage conditions preceding flank eruptions, we are studying melt inclusions (MI) contained in high-Mg tephra samples from flank cones on Nyiragongo (olivine melilitites) and Nyamulagira (basanites). Olivine-hosted MI contain vapor bubbles with high CO2 densities and evidence of carbonates along the bubble walls. At Nyiragongo, median CO2 densities are 0.20-0.24 g/cm3 for the four samples studied, and at Nyamulagira, are 0.22-0.23 g/cm3 for three samples. Using the vapor bubble CO2 densities only, estimated minimum melt inclusion CO2 concentrations are 900-5000 ppm at Nyiragongo and 1400-4900 ppm at Nyamulagira. Preliminary FTIR measurements of MI glasses show CO2 values up to 2000 ppm. Using a solubility model for alkalic compositions (Allison et al., 2019) and assuming vapor saturation, we calculate maximum MI entrapment pressures of 522 MPa, equivalent to ~20 km depth. To understand the history of magma storage and mixing before eruption, we measured major (Mg, Fe) and trace (Ni, Ca, Mn, Al, Cr) element concentration profiles across crystallographically-oriented olivine phenocrysts. Olivines are dominantly normally zoned, although reverse and complexly zoned olivines were also measured. Core compositions of Fo80-89 indicate crystallization from high-Mg melts, and rim compositions are Fo74-84. Best fits of zoning profiles were modeled to estimate timescales required to produce diffusion gradients in our dataset. For normally-zoned phenocrysts, modeled timescales range from days to weeks. These short timescales indicate rapid transitions between storage, triggering processes, and eruption, which would have likely been accompanied by seismic and geodetic signals of unrest.