Linking subsurface to surface degassing at active volcanoes: A thermodynamic model with applications to Erebus volcano

AbstractVolcanic plumbing systems are the pathways through which volatiles are exchanged between the deep Earth and the atmosphere. The interplay of a multitude of processes occurring at various depths in the system dictates the composition and quantity of gas eventually erupted through volcanic vents. Here, a model is presented as a framework for interpreting surface volcanic gas measurements in terms of subsurface degassing processes occurring throughout a volcanic plumbing system. The model considers all possible sources of fluid from multiple depths, including degassing of dissolved volatiles during crystallization and/or decompression as recorded in melt inclusions plus any co-existing fluid phase present in a magma reservoir. The former is achieved by differencing melt inclusion volatile contents between groups of melt inclusions saturated at discrete depths. The latter is calculated using a thermodynamic model, which computes the composition of a C–O–H–S fluid in equilibrium with a melt given a minimum of five thermodynamic parameters commonly known for natural systems (T, P, fO2, either fH2 or one parameter for H2O, and either fS2 or one parameter for CO2). The calculated fluids are thermodynamically decompressed and run through a mixing model, which finds all possible mixtures of subsurface fluid that match the chemistry of surface gas within ±2.0 mol%. The method is applied to Mount Erebus (Antarctica), an active, intraplate volcano whose gas emissions, which emanate from an active phonolitic lava lake, have been well quantified by FTIR, UV spectroscopy, and multi-gas sensors over the last several decades. In addition, a well-characterized suite of lavas and melt inclusions, and petrological interpretations thereof, represent a wealth of knowledge about the shallow, intermediate, and deep parts of the Erebus plumbing system. The model has been used to calculate the compositions of seven C–O–H–S fluids that originate from four distinct regions within the Erebus plumbing system and in the lava lake (deep basanite, intermediate, shallow phonolite, and lava lake phonolite equilibrium fluids, plus crystallization-induced degassing of deep, intermediate, and shallow melts). A total of 144 possible mixtures were found. In all cases, ∼60% of the surface gas is sourced from deep degassing. The remaining ∼40% is made up primarily of fluid in equilibrium with the lava lake (∼20%) plus intermediate (∼10%) and phonolite (∼5%) equilibrium fluids and minor to no contribution from all other fluid sources. These results, whereby the surface gas signature is dominated by fluids originating from deep mafic melts, could be representative of any volcanic system comprised of a deep mafic member and shallow evolved fractionates as has been inferred at Yellowstone, Etna, and many others. At Erebus, results of this modeling demonstrate that the degassing of stagnant magma can contribute significant fluid and energy to the system such that the continuous convection and degassing of volatile-rich magma is not necessary to explain the volcano's persistently active nature or the composition of its gas emissions.The C++ model code is open source and is hosted as a github repository at https://github.com/kaylai/Iacovino2015_thermodynamic_model/