Erebus source to surface degassing, thermodynamics, solubilities, and magma storage and evolution (2013-2016)

We present new equilibrium mixed-volatile
(H2O–CO2) solubility data for a phonotephrite from Erebus volcano, Antarctica. H2O–CO2-saturated experiments were conducted at 400–700 MPa, 1,190 °C, and ~NNO + 1 in non-end-loaded piston cylinders. Equilibrium H2O–CO2 fluid compositions were determined using low-temperature vacuum manometry, and the volatile and major element
compositions of the glassy run products were determined by Fourier transform infrared spectroscopy and electron microprobe. Results show that the phonotephrite used in this study will dissolve *0.8 wt% CO2 at 700 MPa and a fluid composition of XH2O *0.4, in agreement with previous experimental studies on mafic alkaline rocks. Furthermore, the dissolution of CO at moderate to high X(super[fluid])(sub[H2O]) in our experiments exceeds that predicted using lower-pressure experiments on similar melts from the literature, suggesting a departure from Henrian behavior of volatiles in the melt at pressures above 400 MPa. With these data, we place new constraints on the modeling of Erebus melt inclusion and gas emission data and thus the interpretation of its magma plumbing system and the contributions of primitive magmas to passive and explosive degassing from the Erebus phonolite lava lake.

Volcanic 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.

We present the results of phase equilibrium experiments carried out on basanite and phonotephrite lavas from Ross Island, Antarctica. Experiments were designed to reproduce the P&#8211;T&#8211;X&#8211;fO2 conditions of deep and intermediate magma storage and to place constraints on the differentiation of each of the two predominant lava suites on the island, which are thought to be derived from a common parent melt. The Erebus Lineage (EL) consists of lava erupted from the Erebus summit and the Dry Valley Drilling Project (DVDP) lineage is represented by lavas sampled by drill core on Hut Point Peninsula. Experiments were performed in internally heated pressure vessels over a range of temperatures (1000&#8211;1150&#176;C) and pressures (200&#8211;400 MPa), under oxidized conditions (NNO to NNO + 3, where NNO is the nickel&#8211;nickel oxide buffer), with XH2O of the H2O&#8211;CO2 mixture added to the experimental capsule varying between zero and unity. The overall mineralogy and mineral compositions of the natural lavas were reproduced, suggesting oxidizing conditions for the deep magma plumbing system, in marked contrast to the reducing conditions (QFM to QFM &#8211; 1, where QFM is the quartz&#8211;fayalite&#8211;magnetite buffer) in the Erebus lava lake. In basanite, crystallization of spinel is followed by olivine and clinopyroxene; olivine is replaced by kaersutitic amphibole below ~1050&#176;C at intermediate water contents. In phonotephrite, the liquidus phase is kaersutite except in runs with low water content (XH2Ofluid < 0.2) where it is replaced by clinopyroxene. Experimental kaersutite compositions suggest that the amphibole-bearing DVDP lavas differentiated below 1050&#176;C at 200&#8211;400 MPa and NNO + 1.5 to NNO&#254;2. Olivine- and clinopyroxene-bearing EL lavas are consistent with experiments performed above 1050&#176;C and pressures around 200 MPa. The plagioclase liquidus at <1&#8211;2 wt % H2O suggests extremely dry conditions for both lineages (XH2Ofluid approaching zero for EL, ~0.25 for DVDP), probably facilitated by dehydration induced by a CO2-rich fluid phase. Our results agree with previous studies that suggest a single plumbing system beneath Ross Island in which DVDP lavas (and probably other peripheral volcanic products) were erupted through radial fractures associated with the ascent of parental magma into the lower crust. The longer travel time of the DVDP lavas through the crust owing to lateral move- ment along fractures and the lack of a direct, sustained connection to the continuous CO2-rich gas flux that characterizes the main central Erebus conduit is probably responsible for the lower temperatures and slightly wetter conditions and hence the change in mineralogy observed.