The origin of high helium concentrations in the gas fields of southwestern Tanzania

Abstract

Volatile elements are concentrated at Earth's surface, forming a rich atmosphere and oceans which enabled the eventual emergence of life. However, volatiles are also abundant in solid Earth reservoirs, such as the crust and mantle, and these reservoirs play a key role in moderating volatile movement throughout the planet. Continental cratons represent a potentially large, yet under-constrained volatile reservoir. When cratonic regions are catastrophically disrupted by large volcanic and/or rifting events, they release massive amounts of volatiles into Earth's atmosphere on geologically-abrupt timescales (e.g., Lowenstern et al., 2014; Muirhead et al., 2020). Here, we report gas data (He-Ne-N2-Ar-CO2) from seeps along the flanks of the Tanzanian craton, within the western branch of the East African Rift System (EARS) - a region where the stable continental craton is actively being broken apart by rifting and simultaneously heated by plume-induced volcanism. Bulk gas and noble gas isotopic data are reported in seeps from three regions: 1) the Rukwa Rift Basin (RRB), 2) the Lupa Hydrothermal System (LHS) and 3) the Rungwe Volcanic Province (RVP). Seep gases from the RRB are dominantly comprised of N2 and He, with >90% N2 concentrations, high 4He concentrations (2.4–6.9%) and radiogenic He isotopes (0.16–0.20 RA). Seeps in the LHS - located between RRB and RVP - are characterized by little-to-no N2, high CO2 contents (72–84%), relatively low He contents (0.008–0.15%), and higher 3He/4He (0.95–0.99 RA). RVP gases have high CO2 (78%) and low 4He (0.0003%) and more mantle-like He isotopes (3.27–4.00 RA) consistent with previous findings (Pik et al., 2006; Barry et al., 2013). All neon isotopes can be explained by mixing between air, high O/F crust and depleted Mid Oceanic Ridge Basalt (MORB) mantle-like signatures. RVP neon isotope seep data potentially suggest a solar-like deep mantle contribution, consistent with findings in rocks from the area (Halldórsson et al., 2014), however we note that this signal is difficult to discern from mass dependent fractionation (MDF). The largest 40Ar/36Ar anomalies occur in RRB, with resolvable excess 40Ar derived from radiogenic production in the crust. Using a noble gas solubility model, we calculate volumetric gas to water ratios (Vg/Vw) and show that Vg/Vw values are low for RRB (0.1), consistent with longer migration distances, whereas Vg/Vw are higher for LHS (Vg/Vw = 0.1–10) and RVP (Vg/Vw = 3–12), suggesting a more direct conduit for volatiles from source to surface. In summary, these data demonstrate interaction between two distinct helium sources, one of which is crustal in origin (most prominent in RRB) and the other being mantle-derived (enriched in RVP). The extent of mixing between the two is shown to be influenced by proximity to rift-related fault structures, groundwater interaction and magmatic heat.