Open AccessAqueous and isotope geochemistry of mineral springs along the southern margin of the Tibetan plateau: Implications for fluid sources and regional degassing of CO 2
Author(s) -
Newell Dennis L.,
Jessup Micah J.,
Cottle John M.,
Hilton David R.,
Sharp Zachary D.,
Fischer Tobias P.
Publication year - 2008
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2008gc002021
Subject(s) - geology , massif , graben , geochemistry , crust , metamorphic rock , meteoric water , mantle (geology) , plateau (mathematics) , isotopes of carbon , sedimentary rock , mineralogy , geomorphology , total organic carbon , paleontology , structural basin , environmental chemistry , chemistry , mathematical analysis , mathematics , hydrothermal circulation
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 R A and are consistent with a crustal source for He. The δ 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 δ 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 δ 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 (∼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.