Decoupled He-W isotope systematics in the East African Rift System

Recent studies have identified negative correlations of He and W isotopes in modern plume-derived ocean island basalts (OIB), where samples with high ³He/⁴He are characterized by negative µ¹⁸²W (the deviation of a sample's ¹⁸²W/¹⁸⁴W from that of standards in ppm). These signatures have been interpreted to reflect a primordial lower mantle plume component originating at the core-mantle boundary [1-4]. While combined He-W data from OIB exist for many hotspots around the world, W data from oceanic and continental flood basalts, magmatic rift systems and mantle material remain scarce.

The ~3000 km long East African Rift System (EARS) represents complex geological processes manifested in diverse magmatism. It consists of several volcanic segments and has been active for ~45 Ma. Some samples from the EARS have plume-like atmosphere-normalized ³He/⁴He ratios of up to ~18 R/R_A, distinctly exceeding typical upper mantle values of ~8 R/R_A. This has been suggested to reflect a mantle plume component, potentially representing a contribution from the head of the African Superplume [e.g., 5, 6].

Here, we present newly obtained and previously published W-He-Sr-Nd-Os isotope data, as well as trace and highly siderophile element systematics of mafic lavas sampling several segments of the EARS. The studied samples have ³He/⁴He ratios ranging from 4.5 to 18.3 R/R_A and μ¹⁸²W values between -5.9 and +2.2 ±4, revealing no resolvable ¹⁸²W anomalies. Hence, samples from the EARS do not plot on the previously established negative He-W trends observed in OIB [e.g., 1, 2]. Instead, these samples plot on a flat trend mimicking that of the previously published trend for continental flood basalts from the North Atlantic Igneous Province (NAIP) [4]. Decoupled He and W systematics observed in EARS samples may be explained by a mantle component similar to that of the NAIP, further suggesting a distinct mantle source for mantle plume heads. Alternatively, the decoupling may be the result of minor amounts of assimilated upper mantle or lower crust material strongly affecting W isotope systematics while not significantly altering ³He/⁴He ratios.

[1] Mundl et al., 2017, Science; [2] Mundl-Petermeier et al., 2020, GCA; [3] Rizo et al., 2019, GPL; [4] Mundl-Petermeier et al., 2019, Chem. Geol.; [5] Halldórsson et al., 2014, GRL; [6] Castillo et al., 2014, FiES