Helium in hot spring gases from the East-African Rift System

A total of 51 high temperature (30 to 87°C) hot spring localities have been sampled in Zambia (n=28), Botswana (n=1) and Malawi (n=20), resulting in the most extensive compilation of helium ratios from hot spring gases in these countries. ³He/⁴He ratios in the gases range from0.008 to 2.65 (+/- 0.03) times atmosphere (R_A). Neon abundances and isotope ratios were also measured to rule out the role of atmospheric mixing. Of the 51 hot springs, 29 have ³He/⁴He <0.1 R_A, consistent with helium signatures of old continental crust. An additional, 14 samples have helium isotope values between 0.2 and 0.1 R_A, which likely indicate slightly higher signatures of continental crust in the region.   However, 8 hot springs have ³He/⁴He >0.3, perhaps indicating the presence of a magmatic source at depth.

There is no simple north-south gradient in helium isotope compositions observed along either Malawi Rift or the Luangwa Rift in Zambia. The highest ³He/⁴He are observed in two hot springs in northern Malawi (~3 R_A), with the second highest values (1–1.4 R_A) occurring in two springs at the southern tip of the Malawi Rift. These hot spring gases require at least 35% and 20% mantle helium, respectively, assuming two component mixing of a continental source (³He/⁴He of 0.002 Ra) and a depleted upper mantle asthenosphere ³He/⁴He of ~8 R_A. Slightly elevated values (0.2–1 R_A) are observed in southern Malawi, and in northern Botswana and northeastern Zambia, where there is no surficial expression of rifting. The highest 3He/4He values in northern Malawi correlate well with areas of thinner continental crust, as inferred from gravity and seismic studies. Although the ³He/⁴He values are lower than hot springs gases in Tanzania, where there are active volcanoes, they indicate a mantle component is present during incipient continental rifting.