Publication Date:
2023-11-21
Description:
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., 3He/4He, d13C, and CO2/3He) 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-CO2 systematics of hydrothermal
gases, which extends from India-Asia continental convergent margin to intra-continent extensional region. d13C-CO2
( 11.8‰ to 3.1‰) and CO2/3He (1.7 108 to 7.1 1011) 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 3He/4He dataset (0.01–5.87 RA) 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 4He), defining a transitional
level He degassing (13–33% mantle He). The 3He/4He values (8.16–8.48 RA) 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 entire study area,
and can be attributed to (i) degassing of sub-continental lithospheric mantle, and/or (ii) higher degrees of crustal contamination
than those of the transitional level He degassing near volcanic centers. Combined with olivine 3He/4He data reported in
this study and whole-rock 87Sr/86Sr data of host basalts from literature, source components of the mantle-derived magmas are
suggested to include the MORB-type convective mantle, subducted Indian slab materials, and less degassed mantle materials,
which can account for the possible decoupling between He and Sr isotope systematics. These findings delineate the origin and outgassing of mantle volatiles controlled by the India-Asia continental convergence, and would contribute to a better understanding
of the deeply-sourced volatile emissions in these tectonic settings.
Description:
Published
Description:
61-78
Description:
9T. Geochimica dei fluidi applicata allo studio e al monitoraggio di aree sismiche
Description:
JCR Journal
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
