Melt Inclusions Indicate Magma Mixing in the Conduits of Persistently Degassing Volcanoes

Abstract:

Degassing processes drive the activities of persistently degassing volcanoes that emit a large amount of gas with little magma. However, without direct observations of the degassing processes at depth, we rely on indirect evidence such as melt inclusions to understand these processes, which is challenging without understanding the physical processes during magma ascent. In this study, we model magmatic flow in volcanic conduits and investigate its effect on volatile concentrations via direct numerical simulations and test the model against melt inclusion and surface gas flux data.

In persistently degassing volcanoes, the degassed magma sinks through the conduit after losing the volatiles at the surface, leading to a bidirectional flow. Previous experimental and numerical studies suggest that the bidirectional flow forms a core-annular flow regime. Using a direct numerical simulation of bubble-bearing conduit flow, we quantify magma mixing and analyze factors controlling the degree of mixing and flow regimes. Hypothesizing that the presence of bubbles favors mixing by destabilizing the interface between the volatile-rich and degassed magma, we resolve gas bubbles in our model.

The CO₂-H₂O concentration recorded by melt inclusions from persistently degassing volcanoes significantly differ from the predictions by either closed-system or open-system degassing path. Based on the idea that magma mixing leads to this discrepancy by Witham (2011), we analyze magma mixing in our simulations and show that gas bubbles disrupt the interface between volatile-rich and degassed melts, causing significant mixing during magma ascent. After quantifying magma mixing via simulations, we incorporate the loss of volatiles to the down-welling magma due to mixing in the calculation of CO₂-H₂O concentration profiles. We analyze the joint effect of mixing and the total amount of CO₂ in the system. We find that the concentration profiles recorded by melt inclusions from persistently degassing volcanoes indicates magma mixing, while the scatter of melt inclusion data and the variation in the surface gas flux reflect the variation in the amount of CO₂. Our findings indicate that melt inclusion data can help assess the significance of magma mixing during magma ascent and track the variability in the amount of CO₂.