Aqueous and isotope geochemistry of mineral springs along the southern margin of the Tibetan plateau: Implications for fluid sources and regional degassing of CO(2)

Citation:
Newell, DL, Jessup MJ, Cottle JM, Hilton DR, Sharp ZD, Fischer TP.  2008.  Aqueous and isotope geochemistry of mineral springs along the southern margin of the Tibetan plateau: Implications for fluid sources and regional degassing of CO(2). Geochemistry Geophysics Geosystems. 9

Date Published:

Aug

Keywords:

(3)He/(4)He, beneath central tibet, carbon, catalina schist, CO(2), delta(13)C, delta(15)N, everest massif, flux, helium, high heat-flow, himalayan, middle crust, noble-gases, orogen, springs, subduction zones, tibetan plateau

Abstract:

[1] Springs issuing from different faults and shear zones along the crest of the Himalayas tap three different levels of crust beneath the Tibetan Plateau. From structurally highest to lowest these are the Tingri Graben, the South Tibetan Detachment System (STDS), and the Ama Drime massif (ADM). The aqueous chemistry reflects water-rock interactions along faults and is consistent with mapped rock types. Major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the N-S trending faults that bound the Tingri Graben and Ama Drime detachment (ADD) compared to the STDS, suggesting that these structures penetrate to greater depths. Springs have excess CO(2), N(2), He, and CH(4) compared to meteoric water values, implying addition from crustal sources. The (3)He/(4)He ratios range from 0.018 to 0.063 RA and are consistent with a crustal source for He. The delta(13)C values of dissolved inorganic carbon (DIC) and CO(2) gas range from -5.5 to + 3.8% and -13.1 to -0.3% versus Peedee belemnite, respectively. Sources of carbon are evaluated by calculating isotopic trajectories associated with near-surface effervescence of CO(2). Positive delta(13)C values of the Tingri graben and STDS springs are consistent with decarbonation of marine carbonates as the source of CO(2). Negative values for the ADD springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon. The delta(15)N values of N(2) range from -2.2 to +2.1% ( versus AIR) and are explained by mixtures of air-derived nitrogen, metamorphic devolatilization of sedimentary nitrogen, and nitrogen from near-surface biogenic processes. CO(2) flux is estimated by scaling from individual springs (similar to 10(5) mol a(-1) per spring) to extensional structures across the southern limit of the Tibetan Plateau and likely contributes between 10(8) and 10(11) mol a(-1) (up to 10%) to the global carbon budget.

Notes:

n/a

Website

DOI:

10.1029/2008gc002021

Scripps Publication ID:

Gc002021