DI003-01
Decoupled He-W isotope systematics in the East African Rift System

Monday, 7 December 2020: 17:30
Virtual
Andrea Mundl-Petermeier, University of Vienna, Department of Lithospheric Research, Vienna, Austria, Saemundur A Halldorsson, University of Iceland, Institute of Earth Sciences, Reykjavik, Iceland, Paterno Castillo, UCSD-SIO, La Jolla, CA, United States, Richard J Walker, University of Maryland College Park, Department of Geology, College Park, MD, United States, Lukáš Ackerman, Academy of Sciences of the Czech Republic, Institute of Geology, Prague, Czech Republic, Nathalie Tepe, Centre for Microbiology and Environmental Systems Science, Department of Environmental Geoscience, Vienna, Austria and Christian Koeberl, University of Vienna, Vienna, Austria
Recorded Presentation
Abstract:
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/RA, distinctly exceeding typical upper mantle values of ~8 R/RA. 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/RA 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

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