He, Ne, Ar and CO2 systematics of the Rungwe Volcanic Province, Tanzania: Implications for fluid source and dynamics

Abstract

Volatiles, such as carbon and noble gases, are continuously degassed from Earth's mantle and crust to Earth's surface in continental rifting systems. Here, we present He-Ne-Ar abundances and isotopes as well as major gas chemistry (CO2, He, Ar and CH4) data for (n = 13) naturally-degassing seeps in the Rungwe Volcanic Province (RVP) of the East African Rift System (EARS) in southwestern Tanzania, Africa. Helium isotopes (3He/4He) suggest that trace gases are derived from mantle sources, with moderate crustal additions, in agreement with previous studies from RVP (e.g., Pik et al., 2006; Barry et al., 2013). Samples broadly fall into two groups: 1) dominantly mantle-like, with 3He/4He ranging from 5.2–6.3RA, which can be explained by release of magmatic volatiles, and 2) those with slightly lower 3He/4He (3.0–4.1RA), representing more 4He (i.e., crustal) additions. Furthermore, we report the first Ne and Ar isotopes from RVP, which show 20Ne/22Ne from 9.67–10.0 and 40Ar/36Ar from 301.2–412.2, respectively, which are broadly air-like. We employ a solubility degassing model to show that elemental ratios (4He/40Ar*, CO2/40Ar* and CO2/3He) in the gases can be explained by variable extents of open and closed system degassing from a melt with an initial mantle-like composition. However, we note that CO2/3He observations require additional carbon assimilation, likely derived from the thick nearby cratonic crust.