Primitive mantle reservoirs investigated using short-lived Hf-W systematics

Resolvable ¹⁸²W deviations from terrestrial standards were recently reported in relatively modern volcanic rocks associated with mantle plume magmatism [1, 2]. The cause of these deviations is however still unresolved. Because of the short half-life of ¹⁸²Hf of 8.9 Ma, variations in ¹⁸²W can be produced by Hf/W fractionation within the first 50 Ma of Earth's history. Therefore, deviations in ¹⁸²W could reveal early geological events, such as magma ocean crystallization or metal-silicate segregation, that are capable of fractionating the Hf/W ratio in chemically differentiated reservoirs. Alternatively, the ¹⁸²W anomalies could reflect incomplete remixing into the mantle of late accreted materials characterized by deficits in ¹⁸²W. The increasing number of ¹⁸²W results suggest that chemical heterogeneities, present in the mantle today, could date back to processes that occurred during Earth's accretion. Interestingly, the distributions of reconstructed LIP eruption sites and current ocean island basalt (OIB) eruptions seem to correlate with the Ultra-Low Velocity Zones (ULVZ) and Large Low Shear Velocity Provinces (LLSVP) imaged through seismic tomography at the base of the mantle. If these deep mantle reservoirs were sampled by LIP and OIB volcanism, then these could contain primitive material that has been preserved from remixing by mantle convection since the first tens of Ma of the Earth's history. Here, we present high-precision W isotopic compositions for several LIP and OIB rocks from the 1976 Ma Onega Plateau, the 100-130 Ma Kerguelen Plateau, the 68 Ma Deccan Province and the associated hotspot in Reunion Island. Preliminary results show that while the samples studied from the Onega Plateau don't show resolvable 182W variations, rock samples from the Kerguelen Plateau, the Deccan traps and the Reunion Island show deficits in ¹⁸²W ranging from -4.8 ± 3.7 down to -19.5 ± 5.1. The potential source regions for these rock samples will be explored using particle tracking in geodynamically constrained convection simulations [3]. Available ¹⁸²W data for LIP and OIB, together with particle tracking simulations, may provide insights into the dynamics of the mantle from the earliest stages of the Earth's formation until present.

[1] Rizo et al., 2016, Science. [2] Mundl et al., 2017, Science. [3] Glisovic & Forte, 2017, Science.