hydrothermal fluids

noble gas

mantle source

continental crust

sr nd

magma

thermal waters

hydrothermal

Geochimica et Cosmochimica Acta

• We report the first mantle degassing transect across a continental convergent margin. • Three levels of mantle He degassing are recognized based on 3 He/ 4 He distribution. • Causes for possible decoupled He-Sr isotopes of Quaternary basalts are explored. • Volatile origin and outgassing are controlled by continental convergence processes. Mantle degassing transect across different tectonic units within a plate convergent setting has been well documented for oceanic convergent margins by systematic changes in geochemistry (e.g., 3 He/ 4 He, δ 13 C, and CO 2 / 3 He) of hydrothermal gases. However, little is known about spatial variations in volatile geochemistry across a continental convergent margin. In this study, we identify a mantle degassing transect in the southeastern Tibetan Plateau using He-CO 2 systematics of hydrothermal gases, which extends from India-Asia continental convergent margin to intra-continent extensional region. δ 13 C-CO 2 (−11.8‰ to −3.1‰) and CO 2 / 3 He (1.7 × 10 8 to 7.1 × 10 11 ) values of hydrothermal gases show large variations that are consistent with modification by secondary physico-chemical processes, such as multi-component mixing, hydrothermal degassing, and calcite precipitation. Three levels of He degassing can be recognized based on 3 He/ 4 He dataset (0.01–5.87 R A ) of the hydrothermal gas samples and their distances to volcanic centers. A magmatic level He degassing (35–74% mantle He) is found near active and/or Quaternary volcanoes fed by mantle-derived magmas. With increasing distance to volcanic centers, the outgassed magmatic volatiles are gradually diluted by crustal components (e.g., radiogenic 4 He), defining a transitional level He degassing (13–33% mantle He). The 3 He/ 4 He values (8.16–8.48 R A ) of olivine phenocrysts indicate a MORB (mid-ocean ridge basalts)-type mantle source for the magmatic and transitional levels of He degassing that are localized in Quaternary volcanic fields. In contrast, a background level He degassing (<12% mantle He) dominates the...

Deciphering a mantle degassing transect related with India-Asia continental convergence from the perspective of volatile origin and outgassing