MANTLE TO SURFACE, PASSIVE AND EXPLOSIVE DEGASSING OF EREBUS VOLCANO

Erebus is an intraplate volcano associated with extensional tectonics, mantle upwelling and high heat flow. It is famous for its long-lived phonolite lava lake but one of the puzzling behaviors exhibited by the volcano is the onset of phases of violent Strombolian activity through the lake that typically last a few months. Using open-path Fourier transform infrared (FTIR) spectroscopy to measure the magmatic gas emissions, we have identified remarkably different signatures associated with passive and explosive activity. These contrasting emissions are representative of volatile contents and redox conditions that identify the overlap between shallow and deep degassing sources. We show that this multiple signature of magma degassing provides a unique probe for magma differentiation and transfer of CO₂-rich oxidized fluids from lithospheric roots up to the surface, and show how these processes operate in time and space. Magma deeper than 4 km equilibrates under vapour-buffered conditions, whereas shallower magmas allow deep, CO₂-rich fluids to accumulate prior to release either via open-system degassing conditions with reduced oxidation states, or as volatile-enriched, phonolitic blobs that ascend as a closed-system preserving the deep, oxidized signature, to explode at the surface in Strombolian fashion. Arguably, the principal volcanic risks posed by Erebus are to the field scientists who sporadically work there, but the findings have broader implications for understanding the onset of explosive activity at passively degassing volcanoes, and we note also that Erebus may provide some analogies to Vesuvio in respect of its magmatic evolution and behavior.