Different B–Mo isotopic fractionation processes controlled by redox conditions in the subduction zone

The control of redox conditions on the geochemical recycling process in subduction zones is still poorly understood due to large uncertainties in oxygen fugacity (fO₂) for subducted slabs. We present the first systematic geochemical and B-Mo-Sr-Nd-Hf-Pb isotopic data for continental arc basalts (CABs) and back-arc basalts (BABs) from Sumatra to investigate the relationships between oxygen fugacity and B–Mo isotopic fractionation processes during subduction. The Sumatran CABs and BABs yield high FeO^T/MgO ratios and low K₂O contents (0.48–1.42 wt%) and can be classified as tholeiitic. They have variable Sr-Nd-Hf-Pb isotopic compositions, consistent with the input of different amounts of melt from subducted terrigenous sediments into mantle sources. The BAB samples show much lower δ¹¹B (−9.0‰ to −7.3‰) values than the CABs (δ¹¹B = −7.0‰ to +0.17‰), reflecting significant B isotopic fractionation during multistage melting of subducted sediments. This is distinct from the similar δ^98/95Mo values for the Sumatran CABs (−0.21‰ to −0.01‰) and BABs (−0.17‰ to −0.08‰), suggesting limited Mo isotopic fractionation during subduction. The Sumatran CABs show low V/Yb ratios, suggesting that melts from subducted sediments were possibly generated at fO₂ values lower than FMQ + 1.5. The limited Mo isotopic fractionation could be induced by the relatively low fO₂ during melting of subducted sediments, which would significantly decrease the mobility of Mo but have no influence on B. Alternatively, rocks from highly oxidized arcs, such as the Izu arc (e.g., >FMQ + 3), exhibit across-arc lightening trends for both B and Mo isotopes, which is consistent with enhanced Mo isotopic fractionation due to elevation of fO₂ during subduction. Thus, the B–Mo isotopes of arc rocks could provide diagnostic indicators for distinct geochemical recycling processes controlled by changes in redox conditions in subduction zones.