Noble gas and volatile geochemistry of geothermal gases at Yellowstone National Park

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Noble gas and volatile geochemistry of geothermal gases at Yellowstone National ParkD. J. Byrne1*, M. W. Broadley2, D.V. Bekaert3, A. M. Seltzer4,R. L. Tyne2, K.G. Lloyd5, M. G. Almayrac2, C. J. Ballentine6, B. Marty3 and P. H. Barry41Wairakei Research Centre, GNS Science, Taupō 3384, New Zealand2 Department of Earth and Environmental Science, University of Manchester, UK3 Université de Lorraine, CNRS, CRPG, 54000 Nancy, France4 Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA5University of Southern California, Los Angeles, CA, USA6 Department of Earth Sciences, University of Oxford, OX1 3AN, UK*Corresponding author: d.byrne@gns.cri.nzAbstract The study of geothermal gases can provide a window into the volatile geochemistry of the deep mantle, and allow us to unravel the degassing and cycling processes that have shaped the composition of our oceans and atmosphere over Earth history. As inert tracers, noble gas isotopes have been a primary tool for investigating these physical processes, but must be considered alongside major volatile elements such as C and N. Yellowstone National Park is a unique area that has featured centrally in the development of geothermal gas geochemistry. Here, we provide a historical overview of gas geochemistry studies at YNP and how they helped to shape the thinking of the field. We also present a new dataset from a recent field campaign alongside a detailed discussion of the neon isotope composition of the Yellowstone mantle source. Furthermore, we discuss a selection of recent studies employing cutting edge analytical techniques. Finally, we discuss some wider implications of the work to related fields and thoughts on a future outlook.IntroductionVolatile Geochemistry and Yellowstone National ParkThe importance of Yellowstone to the field of volatile geochemistry results from the confluence of a number of factors. The geological setting of the area is unique, with volcanic activity driven by the imposition of a deep-sourced mantle plume beneath the thick ancient crust of the North American craton, which is made up of igneous and meta-igneous rocks that formed between 3.5 and 2.5 Ga (Smith & Braile, 1994). The effects of this interaction are seen today through the generation of the Yellowstone Plateau Volcanic Field (YPVF), commonly known as Yellowstone National Park (YNP). Since the Yellowstone plume settled under its current position on the North American Plate devastating eruptions that are some of the largest in geological history. The most recent large eruption took place at 631.3 ± 4.3 ka, forming the present-day Yellowstone Caldera (Matthews et al., 2015). Today, evidence of the Yellowstone mantle plume is limited to the geothermal areas, comprising geysers, hot springs, and mud pots, which fill the Yellowstone Caldera and the surrounding areas. These geothermal areas cover around 65 km2 within the park making it the Earth’s largest active continental hydrothermal system.The heat driving the Yellowstone hydrothermal system originates from an extensive silicic magma chamber with seismic tomography suggesting a depth of ~3 to 8 km and a partial melt fraction of between 16 and 20% (Maguire et al., 2022). Whilst the shallow silicic magma chamber is directly supplying heat and volatiles to the Yellowstone hydrothermal system, the ultimate source must reside deeper, since the prodigious CO2 flux from the Yellowstone hydrothermal system would exhaust the present magma chamber of CO2within around 1,000 years (Lowenstern & Hurwitz, 2008). A constant deeper supply of basaltic magma arising from the plume is therefore required to continually supply heat and volatiles to drive the Yellowstone hydrothermal system. Seismic tomography suggests that this deeper zone of basaltic melt may lie to the west of the present day caldera, underneath the Snake River Plain (Huang et al., 2015; Maguire et al., 2022). See Hurwitz & Lowenstern, (2014) for a comprehensive overview of the work that has shaped our understanding of the Yellowstone hydrothermal system.Over the last 50 years, noble gas geochemistry has provided key evidence supporting the mantle plume origin for the volcanism that has occurred over a 700 km long track across North America, culminating at the present-day site of the YPVF. Pioneering work by Craig et al. (1978) demonstrated that CO2 gas emissions from within the park had elevated 3He/4He isotopes ratios (up to 16 Ra, where 1 Ra =3He/4He of atmospheric air = 1.39 x 10-6). These values are significantly higher than that typically measured in the convecting upper mantle (~8 Ra) and are consistent with a less degassed mantle plume source similar to the plume that feeds Iceland and Hawaii (Kurz et al., 1985, 1987). Corroborating evidence from seismic tomographic studies have revealed deep-seated magma bodies that link the surface manifestations at Yellowstone to the deep crust and the mantle (Huang et al., 2015; Nelson & Grand, 2018). Noble gases have also been key to linking many of the volcanic products along the Yellowstone hotspot track to the Yellowstone mantle plume. For example, high3He/4He measured in Columbia River Basalts samples suggest that the Yellowstone mantle plume was a key source of material and heat which drove the formation of the Columbia River flood basalts (Dodson et al., 1997).The importance of noble gases as geochemical tracersNoble gas isotopes underpin some of the most fundamental discoveries in the geochemical structure of Earth (e.g. Porcelli et al., 2002). Due to their inert nature, they are unaffected by chemical reactions and are therefore ideal tracers to directly observe the provenance of different volatile sources, as well as physical processes such as mixing, degassing, diffusion (Burnard et al., 2013) and temperature (e.g. Byrne et al., 2021). Furthermore, the noble gases often exhibit large variations in isotopic composition between different volatile endmembers, allowing for even minor contributions from different volatile sources to be readily resolved (Porcelli & Ballentine, 2002). Their lack of participation in chemical reactions means that isotope signatures within geochemical reservoirs are preserved indefinitely without being overprinted by isotope exchange between species. In general, the noble gas isotopic composition of a subsurface domain is determined simply by its primordial composition (acquired during Earth accretion), with the addition of radiogenic isotopes (those produced by radioactive decay) over time (Pepin & Porcelli, 2002). As Earth’s crust is enriched compared to the mantle in radioisotopes such as238U, 235U, 232Th and 40K, it is also enriched in daughter products such as 4He and 40Ar, as well as fissiogenic isotopes of Xe (131,132,134,136Xe), and nucleogenic by-products of radioactivity such as 21Ne and 22Ne (Ballentine & Burnard, 2002). Recycling of atmospheric gas into the mantle via subduction is believed to be a significant source of heavier noble gases (i.e., Ar, Kr, and Xe) in the mantle (Holland & Ballentine, 2006; Parai & Mukhopadhyay, 2018). However for Ne, and especially He, it is typically assumed that there exists a “subduction barrier”, with no atmospheric He or Ne residing in the deep Earth (Staudacher & Allègre, 1988).The history of noble gas geochemistry at YellowstoneHague (1911) and Allen and Day (1935) were the first geochemists to study gases and fluids from YNP, and their work showed that gases were primarily composed of CO2, with minor traces of hydrogen, methane, nitrogen, argon, and H2S. In the 1950s, the first isotopic analyses were conducted on YNP gases to determine the origins of the geothermal waters and gases. Craig (1953) measured C isotopes throughout the park and found that the δ13C value of CO2 in Yellowstone gases averaged –2.8 ‰ (vs VPDB), which was closer to typical limestone values than mantle δ13C values of ~-5 ‰. Early gas chromatography analyses suggested a pr

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