Geochemical study of UHP serpentinites in southwestern Tianshan, China and its implications for trace elements deep recycling

Abstract

Serpentinites play a crucial role in the recycling of elements and volatiles in subduction zones. We studied the geochemistry of Changawuzi serpentinites from Chinese southwestern Tianshan, which underwent ultrahigh-pressure (UHP) metamorphism. The bulk rock compositions of the serpentinites suggest that Changawuzi serpentinites originated from a variably depleted mantle and were subsequently enriched in fluid-mobile elements (FMEs) during initial serpentinization. Integrated analyses of trace element concentrations (Cs, U, As, and Sb), Sr–Nd–Pb isotopic compositions, and geological context collectively indicate that serpentinization likely occurred within the interior of the subducted slab. During the early stages of subduction, the concentrations of rare earth elements (REE), high-field-strength elements (HFSE), and other trace elements (such as Cs, Rb, Ba, Th, and U) decrease from low-grade lizardite to high-grade antigorite, as well as from primary clinopyroxene to metamorphic diopside. FMEs, particularly boron (B), are significantly enriched in lizardite, and the majority are liberated during prograde subduction, resulting in the metamorphic transformation from lizardite to antigorite. The uptake and variation of Sb and As in Changawuzi serpentinites are relatively minor. At peak metamorphic conditions, olivine, Ti-chondrodite and Ti-clinohumite become the main minerals hosting trace elements (such as REE, B, Sb, As and Li) that were released during partial antigorite dehydration and clinopyroxene re-equilibration. Ti-chondrodite/Ti-clinohumite as isolated grains or as lamellae within metamorphic olivine have the potential to carry HFSE, FMEs (particularly B and Li) and H₂O into the deeper mantle beyond the dehydration of serpentinites, potentially impacting arc magmatism and the global cycling of these elements.