Predominantly recycled carbon in Earth's upper mantle revealed by He-CO2-Ba systematics in ultradepleted ocean ridge basalts

Basalts erupted within intra-transform spreading centers can be valuable probes of geochemical components in Earth's upper mantle, and provide constraints on the proportions mantle carbon that are juvenile (primordial) vs. tectonically recycled. Here we present new results for submarine basalt glasses, Garrett Transform Fault (GTF) to its north south along East Pacific Rise (EPR). Analyses 3He/4He, He CO2 concentrations vesicles glass were performed, through a series crushing melting experiments plus FTIR spectroscopy. Trace elements analyzed by laser ablation ICP-MS. The GTF basalts further tests origin volatile-undersaturated using CO2/Ba CO2/Nb systematics. is highly correlated with Ba Nb transform domain (n = 13, including 5 undersaturated basalts) does not show variability expected from mixing between undegassed variably degassed melts. Rather, melts appear originate heterogeneous source was depleted partial melting, limited involved their generation. weight ratios 106 ± 8 308 27, respectively, similar values determined previously global suite mid-ocean ridge (MORBs). domains have distinct 3He/4He ratios. Along nearby EPR, 8.5 - 9.1 RA, while 9.2 10.1 RA. These two populations also Pb-Sr-Nd isotope compositions based earlier regional studies. result different compositions. Melting containing small amount (∼1 5%) enriched, ancient heterogeneities occurs beneath EPR. ultradepleted (in which been removed EPR) GTF. This explains distinction if enriched U, Th had low as would found recycled material. component sampled EPR has molar CO2/3He =2×109, it dominates ratio generally ascribed basalts. In contrast, MORB =3×108 or less. indicates most originates tectonic recycling.