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 component^1,2. 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 space^1,2.

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).