Divergent subduction of a tearing slab controls deep carbon recycling efficiency: Helium and carbon isotopic evidence from the southeast Tibetan Plateau

Abstract

Variations of helium isotopic compositions across tectonic belts and at regional scales provide critical insights into crust–mantle interactions and the transport of heat and volatiles from the Earth’s interior. In this study, we report geochemical data from 26 thermal springs, including 51 newly analyzed samples, and integrate a broader dataset comprising 854 samples from 350 sites distributed along the Xianshuihe and Red River fault zones of southeastern Tibetan Plateau. Distinct helium isotopic anomalies and heterogeneous contributions of recycled carbon sourced from altered oceanic crust (AOC) were identified using combined analyses of ³He/⁴He ratios and δ¹³C–CO₂ signatures, supported by an updated helium–carbon coupling model. Clear spatial contrasts were observed across the helium boundary zone (HBZ), with δ¹³C–CO₂ and CO₂/³He ratios revealing marked differences between the northwestern and southeastern sectors. The mantle-derived helium-3 flux reaches approximately 1.7 × 10⁴ atoms m⁻² s⁻¹ in the northwest, compared with 1.04 × 10⁴ atoms m⁻² s⁻¹ in the southeast. These disparities align closely with geophysical evidence indicating the presence of a slab tear in the Indian lithosphere beneath the Lijiang–Xiaojinhe fault (LXF). In addition, a decarbonation boundary linked to slab subduction was delineated near 26°N with higher recycled carbon fluxes in the northwest (∼35 %) than those in the southeast (∼22 %). This asymmetry is attributed to variations of subduction depth and angle associated with tearing of the Indian slab. The results of this study highlight the pivotal role of slab tearing and divergent subduction processes in modulating the efficiency and spatial heterogeneity of deep carbon recycling along convergent plate margins.