Deep recycling of volatile elements in the mantle: Evidence from the heterogeneous B isotope in intra-plate basalts

Abstract

Volatiles in the mantle are crucial for Earth’s geodynamic and geochemical evolution. Understanding the deep recycling of volatiles is key for grasping mantle chemical heterogeneity, plate tectonics, and long-term planetary evolution. While subduction transfers abundant volatile elements from the Earth’s surface into the mantle, the fate of hydrous portions within subducted slabs during intensive dehydration processes remains uncertain. Boron isotopes, only efficiently fractionating near the Earth’s surface, are valuable for tracing volatile recycling signals. In this study, we document a notably large variation in δ¹¹B values (−14.3‰ to +8.2‰) in Cenozoic basalts from the South China Block. These basalts, associated with a high-velocity zone beneath East China, are suggested to originate from the mantle transition zone. While the majority exhibit δ¹¹B values (−10‰ to −5‰) resembling the normal mantle, their enriched Sr-Nd-Pb isotope compositions and fluid-mobile elements imply hydrous components in their source, including altered oceanic crust and sediments. The normal δ¹¹B values are attributed to the dehydration processes. Remarkably high δ¹¹B values in the basalts indicate the presence of subducted serpentinites in their mantle source. A small subset of samples with low δ¹¹B values and radiogenic isotope enrichments suggests a contribution from recycled detrital sediments, though retaining minimal volatile elements after extensive dehydration. These findings provide compelling evidence that serpentinites within subducted slabs predominantly maintain their hydrous nature during dehydration processes in subduction zones. They may transport a considerable amount of water into deep mantle reservoirs, such as the mantle transition zone.