Magnesium and boron isotope evidence for the generation of arc magma through serpentinite-mélange melting

Serpentinites play a crucial role in mass transport and volatile recycling in subduction zones, yet the mechanism for their contribution to the formation of arc magma remains elusive. Here, we investigate this issue by examining the magnesium (Mg) and boron (B) isotope compositions of volcanic rocks and forearc serpentinites from the South Sandwich Island arc. The volcanic rocks display δ²⁶Mg values ranging from -0.25‰ to -0.06‰ and δ¹¹B values ranging from +9.6‰ to +16.5‰, while the forearc serpentinites exhibit δ²⁶Mg values of -0.21‰ to -0.02‰ and δ¹¹B values of +5.2‰ to +9.8‰. Given the substantial contrast in both Mg and B contents between mantle rocks and fluids, the combined heavy Mg-B isotope compositions of volcanic rocks pose a challenge to traditional arc formation models, i.e. flux melting of depleted subarc mantle metasomatized by slab-derived fluids. Although an alternative model involving flux melting of dehydrated serpentinites can partly account for the heavy Mg isotope compositions of arc magmas, it is difficult to simultaneously explain the B isotope and trace-element compositions. Instead, these distinct compositions can be adequately explained by partial melting of a serpentinite-dominated mélange beneath the volcanic arc. Given that arc magmas exhibiting coupled heavy Mg-B isotope compositions are increasingly reported, we propose that serpentinite-mélange melting represents an effective and geochemically self-consistent mechanism for transferring signatures of subducted slabs to the overlying mantle source. This process can be significant in subduction zones with prominent forearc mantle erosion or those involving considerable amounts of slab-hosted serpentinite.