Barium isotopes in ocean island basalts as tracers of mantle processes. (1st November 2022) - Bai, Ruixia
Abstract: 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 138/134 Ba (permil deviation of the 138 Ba/ 134 Ba ratio from the standard SRM3104a) in a suite of Hekla samples and demonstrate that magmatic differentiation does not impact the δ 138/134 Ba values of OIB over MgO concentrations from 2.3 to 8.0 wt%, and higher MgO lavas are unlikely to experience δ 138/134 Ba fractionation. After excluding OIB samples that experienced low-temperature alteration—which may shift the δ 138/134 Ba values of samples away from their mantle sources—the δ 138/134 Ba values of Samoan (including EM2 and high 3 He/ 4 He) lavas range from -0.07 to 0.07‰, while for Cook-Austral (HIMU) lavas the δ 138/134 Ba values range from 0.00 to 0.11‰, and for St. Helena (HIMU) samples the δ 138/134 Ba values range from 0.02 to 0.05‰. High 3 He/ 4 He (> 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 ( 143 Nd/ 144 Nd = 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 138/134 Ba values and 143 Nd/ 144 Nd ratios, and can be modeled as a binary mixing between the high 3 He/ 4 He (FOZO) mantle and recycled heterogeneous continental crust. In contrast, the extreme HIMU composition ( 206 Pb/ 204 Pb > 21.7) shows heavier Ba isotope signatures (0.11 ± 0.02‰, 2SE); δ 138/134 Ba values positively correlate strongly with 206 Pb/ 204 Pb ratios in HIMU lavas, consistent with a contribution from recycled oceanic crust.