Different B[sbnd]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 (f O 2) 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 FeOT/MgO ratios and low K 2 O 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 δ98/95Mo 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 f O 2 values lower than FMQ + 1.5. The limited Mo isotopic fractionation could be induced by the relatively low f O 2 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 f O 2 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. • Mantle wedge beneath Sumatra is metasomatized by melts from subducted sediments. • Structure of subducted slab influences melting of subducted sediments. • Change of f O 2 causes different B Mo isotopic fractionation processes in subduction zones. [ABSTRACT FROM AUTHOR]