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 delta Ba-138/134 (permil deviation of the Ba-138/Ba-134 ratio from the standard SRM3104a) in a suite of Hekla samples and demonstrate that magmatic differentiation does not impact the delta Ba-138/134 values of OIB over MgO concentrations from 2.3 to 8.0 wt%, and higher MgO lavas are unlikely to experience delta Ba-138/134 fractionation. After excluding OIB samples that experienced low-temperature alteration-which may shift the delta Ba-138/134 values of samples away from their mantle sources-the delta Ba-138/134 values of Samoan (including EM2 and high He-3/He-4) lavas range from -0.07 to 0.07 parts per thousand, while for Cook-Austral (HIMU) lavas the delta Ba-138/134 values range from 0.00 to 0.11 parts per thousand, and for St. Helena (HIMU) samples the delta Ba-138/134 values range from 0.02 to 0.05 parts per thousand. High He-3/He-4 (> 20 Ra, ratio to atmosphere) lavas from Vestfir eth ir of Iceland (-0.09 to 0.14 parts per thousand) and Samoa (-0.02 to 0.01 parts per thousand) also exhibit Ba isotopic variability. An extreme EM2 lava examined here (Nd-143/Nd-144 = 0.512582) has lighter Ba isotopic composition (-0.07 +/- 0.02 parts per thousand, 2SE) than the depleted mantle sampled by MORB (0.05 +/- 0.05 parts per thousand, 2SD). The Samoan lavas show a possible positive trend between delta Ba-138/134 values and Nd-143/Nd-144 ratios, and can be modeled as a binary mixing between the high He-3/He-4 (FOZO) mantle and recycled heterogeneous continental crust. In contrast, the extreme HIMU composition (Pb-206/Pb-204 > 21.7) shows heavier Ba isotope signatures (0.11 +/- 0.02 parts per thousand, 2SE); delta Ba-138/134 values positively correlate strongly with Pb-206/Pb-204 ratios in HIMU lavas, consistent with a contribution from recycled oceanic crust. Published by Elsevier Ltd.