Barium isotopes in ocean island basalts as tracers of mantle processes.

The compositions of ocean island basalts (OIB) can record signatures of ancient subducted crust that has been recycled through the mantle. Based on their radiogenic isotope, major and trace element compositions, several geochemically-extreme mantle compositions are suggested to host components of recycled crust. Barium (Ba) stable isotopes can serve as a tracer of crustal recycling into the mantle sources of OIB because 1) Ba isotopes are fractionated in surface crustal reservoirs and 2) Ba is highly enriched in crust relative to the mantle. Therefore, the Ba isotopic compositions of the subducted crust could be detected in OIB. Here, we characterized Ba isotopic compositions of fresh OIB from hotspots sampling a range of mantle compositions. We first explore δ 138/134Ba (permil deviation of the 138Ba/134Ba ratio from the standard SRM3104a) in a suite of Hekla samples and demonstrate that magmatic differentiation does not impact the δ 138/134Ba values of OIB over MgO concentrations from 2.3 to 8.0 wt%, and higher MgO lavas are unlikely to experience δ 138/134Ba fractionation. After excluding OIB samples that experienced low-temperature alteration—which may shift the δ 138/134Ba values of samples away from their mantle sources—the δ 138/134Ba values of Samoan (including EM2 and high 3He/4He) lavas range from -0.07 to 0.07‰, while for Cook-Austral (HIMU) lavas the δ 138/134Ba values range from 0.00 to 0.11‰, and for St. Helena (HIMU) samples the δ 138/134Ba values range from 0.02 to 0.05‰. High 3He/4He (> 20 Ra, ratio to atmosphere) lavas from Vestfirðir of Iceland (-0.09 to 0.14‰) and Samoa (-0.02 to 0.01‰) also exhibit Ba isotopic variability. An extreme EM2 lava examined here (143Nd/144Nd = 0.512582) has lighter Ba isotopic composition (-0.07 ± 0.02‰, 2SE) than the depleted mantle sampled by MORB (0.05 ± 0.05‰, 2SD). The Samoan lavas show a possible positive trend between δ 138/134Ba values and 143Nd/144Nd ratios, and can be modeled as a binary mixing between the high 3He/4He (FOZO) mantle and recycled heterogeneous continental crust. In contrast, the extreme HIMU composition (206Pb/204Pb > 21.7) shows heavier Ba isotope signatures (0.11 ± 0.02‰, 2SE); δ 138/134Ba values positively correlate strongly with 206Pb/204Pb ratios in HIMU lavas, consistent with a contribution from recycled oceanic crust. [ABSTRACT FROM AUTHOR]