Molybdenum Isotope Systematics of the Kamchatka Subduction Zone System

Molybdenum (Mo) isotopes in magmatic rocks are a promising tool in high-temperature isotope geochemistry. In particular, basalts from subduction zones that are geochemically controlled by mass transfer through slab-fluid addition have systematically higher δ98Mo values (i.e. measured 98Mo/95Mo ratio in a sample relative to that in a standard) than the depleted mantle (δ98Mo = –0.21‰). In these rocks, the elevated δ98Mo values are linked to high Pb/Ce and high (238U/230Th) ratios and can be reconciled by the addition of isotopically heavy Mo via a slab fluid component1,2. So far, these systematics are best expressed in subduction zone basalts from the Mariana and Izu arc systems that also form coherent mixing trends between fluid-enriched mantle domains in δ98Mo versus 143Nd/144Nd and 176Hf/177Hf space1,2.The Kamchatka arc system represents the northernmost expression of the W-Pacific convergent margin. Volcanic front lavas are dominated by slab-to-mantle mass transfer through fluid transport, whereas subduction of the Emperor seamount ridge gives rise to back-arc basalts with a geochemical and isotopic affinity to within-plate basaltic rocks3.Here, we report δ98Mo data for 47 basalts from an E-W transect across the Kamchatka peninsula that have previously been analysed for their major, trace element, radiogenic and stable isotope data. The δ98Mo data extent the trend defined by samples from the Marianas and Izu arcs starting from moderately high δ98Mo and Pb/Ce values towards sub-depleted mantle δ98Mo and mantle-like Pb/Ce ratios that indicate the presence of a source component formed by partial melts of a rutile-bearing mafic crust4.The common geochemical and isotopic trends formed by the combined Mariana – Izu – Kamchatka datasets suggest a surprisingly uniform Mo isotope composition of a subduction zone fluid endmember for more than 5000 km along-strike of the Circum-Pacific subduction zone system. Our data also confirm the presence of an enriched source component in the Kamchatka mantle wedge, possibly originating from the subducted Emperor seamount chain5.1Freymuth, H., et al., EPSL 432, 176-186 (2015). 2Villalobos-Orchard, J., et al., GCA 288, 68-82 (2020). 3Churikova, T., et al. JPet 42, 1567-1593 (2001). 4Chen, S., et al., Nat. Comm. 10, 4773 (2019). 5Shu,Y., et al.,Nat. Comm. 13, 4467 (2022).