Subducted ophicarbonates as source for B-bearing diamond formation

Abstract

This study presents in-situ B isotope compositions (δ¹¹B) of antigorite from well-characterized ophicarbonates that underwent prograde to high-pressure (high-P) metamorphic evolution during the Alpine orogeny (300–500 °C and 0.6–2.3 GPa). The δ¹¹B values in antigorite are highly variable, ranging from ca. -8 to +7 ‰. Coupled with in-situ ⁸⁷Sr/⁸⁶Sr ratios of carbonates (ranging from 0.7065 to 0.7085), this B isotope variability may reflect either inherited oceanic imprints or re-equilibration with metamorphic fluids during subduction processes. These interpretations are further corroborated by in-situ REE and incompatible trace element data on antigorite. The devolatilization and densification of slab lithologies during subduction drive the recycling of slab and wedge materials into the Earth's mantle, contributing to its chemical heterogeneity. Remarkably, the δ¹¹B signatures of antigorite reported here overlap with the variability observed in B-bearing diamonds formed at transition zone/lower mantle depths, as well as in carbonatites and ocean island basalts (OIBs), suggesting a geochemical connection. Together with literature data, this study proposes that deeply subducted ophicarbonates may transform into carbonate-bearing secondary peridotites with geochemical features compatible with B-bearing diamonds. Deeply subducted ophicarbonates, with enriched trace element and isotopic signatures, may also act as reservoirs influencing mantle dynamics and the geochemical diversity of OIB and carbonatite mantle sources. These findings provide new constraints into the recycling of B and volatiles at convergent margins, extending to depths compatible with the lower mantle region.