Rubidium isotopic evidence for sedimentary input to the mantle source of Martinique lavas from the Lesser Antilles arc

The recycling of crustal material through subduction is considered a key driver of chemical heterogeneity in the mantle. Rubidium (Rb) is a highly incompatible element with substantial fluid mobility. Its significant enrichment in the crust relative to the mantle, along with the inherent isotope variability among crustal materials, makes Rb isotopes a promising tracer of crustal recycling. However, the limited current understanding of Rb isotope behavior during subduction-related processes hinders their effective application. Here, we present Rb isotope compositions in a set of Martinique lavas from the Lesser Antilles arc, as well as sediments from DSDP Sites 144 and 543 located in front of the Lesser Antilles trench. Sediments from both sites exhibit Rb isotope compositions similar to continental crust or mantle. In contrast, the δ 87 Rb of the studied arc lavas vary from a mantle-like value at −0.08 ± 0.03‰ to variably lower values (down to −0.49 ± 0.05‰). Strong correlations are observed between their δ 87 Rb values and radiogenic isotopic compositions (e.g., 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, 206 Pb/ 204 Pb, and 176 Hf/ 177 Hf). The Rb isotope variations cannot be explained by post-eruption alteration, mantle partial melting, magmatic differentiation or crustal assimilation during magma ascent. Instead, they reflect a binary mixing between the sub-arc mantle and isotopically light sediment melts. Because Rb isotope signatures of subducting sediments are largely unaffected by prograde metamorphism in cold subduction zones, their mantle-like pre-subduction δ 87 Rb values imply that Rb isotope fractionation may occur during sediment partial melting, with the light Rb isotope preferentially migrating with melts. Overall, this study provides compelling evidence for the addition of isotopically light sediment melts to the mantle wedge, demonstrating that Rb isotopes can serve as a tracer of crustal recycling in the sub-arc mantle.