Re-visiting barium isotope compositions of mid-ocean ridge basalts and the implications

Abstract

Barium (Ba) isotopes can be used as potential tracers for crustal material recycling in the mantle. Determination of the Ba isotope composition of the depleted mantle is essential for such applications. However, Ba isotope data for mantle-derived basalts are still rare. In this study, we reported high-precision Ba isotope data of 30 oceanic basalts including 25 mid-ocean ridge basalts (MORBs) from geochemically and geologically diverse mid-ocean ridge segments and five back-arc basin basalts. The δ<sup>138/134</sup>Ba values of these samples varied from −0.06‰ to +0.11‰, with no systematic cross-region variation. Together with published data, we constrained the average δ<sup>138/134</sup>Ba of global MORBs to +0.05‰±0.09‰ (2 standard deviation, <i>n</i> = 51). Based on depleted MORBs that have (La/Sm)<sub>N</sub> < 0.8, low <sup>87</sup>Sr/<sup>86</sup>Sr (< 0.70263), and low Ba/Th < 71.3, we estimated the average δ<sup>138/134</sup>Ba of the depleted MORB mantle (DMM) as + 0.05‰ ± 0.05‰ (2SD, <i>n</i> = 16) that is significantly lower than the DMM (≈ 0.14‰) reported previously. If a new estimation of the DMM is applied, it is unreasonable to infer that the Ba isotope signatures of the “enriched-type” MORBs (E-MORBs) could be attributed to pervasive sediment recycling in the upper mantle. We, therefore, conclude that the Ba isotope compositions of the E-MORBs could be sourced from the incorporation of subducted altered oceanic crust and/or sediments depending on the Ba isotope composition and other geochemical information of the local mantle.