Aqueous and isotope geochemistry of mineral springs along the southern margin of the Tibetan plateau: Implications for fluid sources and regional degassing of CO2

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 CO2, N2, He, and CH4 compared to meteoric water values, implying addition from crustal sources. The 3He/4He ratios range from 0.018 to 0.063 RA and are consistent with a crustal source for He. The δ13C values of dissolved inorganic carbon (DIC) and CO2 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 CO2. Positive δ13C values of the Tingri graben and STDS springs are consistent with decarbonation of marine carbonates as the source of CO2. Negative values for the ADD springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon. The δ15N values of N2 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. CO2 flux is estimated by scaling from individual springs (∼105 mol a−1 per spring) to extensional structures across the southern limit of the Tibetan Plateau and likely contributes between 108 and 1011 mol a−1 (up to 10%) to the global carbon budget.

[1]  D. Newell,et al.  Travertine deposits from along the South Tibetan Fault System near Nyalam, Tibet , 2008, Geological Magazine.

[2]  J. Spotila,et al.  Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal , 2008 .

[3]  T. Holland,et al.  Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs , 2008 .

[4]  M. Searle,et al.  Structural insights into the early stages of exhumation along an orogen-scale detachment: The South Tibetan Detachment System, Dzakaa Chu section, Eastern Himalaya , 2007 .

[5]  Z. Sharp,et al.  Principles of Stable Isotope Geochemistry , 2006 .

[6]  T. Fischer.  Subduction and Recycling of Nitrogen Along the , 2006 .

[7]  M. Searle,et al.  Structural evolution and vorticity of flow during extrusion and exhumation of the Greater Himalayan Slab, Mount Everest Massif, Tibet/Nepal: implications for orogen-scale flow partitioning , 2006, Geological Society, London, Special Publications.

[8]  D. Hilton,et al.  Dissected hydrologic system at the Grand Canyon: Interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine , 2006 .

[9]  D. Hilton,et al.  Continental-scale links between the mantle and groundwater systems of the western United States: Evidence from travertine springs and regional He isotope data , 2005 .

[10]  B. Kjarsgaard,et al.  Absence of a high time-integrated 3He/(U+Th) source in the mantle beneath continents , 2005 .

[11]  J. Selverstone.  Preferential embrittlement of graphitic schists during extensional unroofing in the Alps: the effect of fluid composition on rheology in low‐permeability rocks , 2005 .

[12]  N. Varley,et al.  Nitrogen isotopes in thermal fluids of a forearc region (Jalisco Block, Mexico): Evidence for heavy nitrogen from continental crust , 2004 .

[13]  F. Gaillard,et al.  Evidence for present-day leucogranite pluton growth in Tibet. , 2004 .

[14]  D. Hilton,et al.  Nitrogen systematics and gas fluxes of subduction zones: Insights from Costa Rica arc volatiles , 2004 .

[15]  Carlo Cardellini,et al.  Carbon dioxide Earth degassing and seismogenesis in central and southern Italy , 2004 .

[16]  M. Searle,et al.  Strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab, Everest Massif, Tibet , 2004, Journal of the Geological Society.

[17]  D. Hilton,et al.  Contrasting He–C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones☆ , 2003 .

[18]  F. Tilmann,et al.  Seismic Imaging of the Downwelling Indian Lithosphere Beneath Central Tibet , 2003, Science.

[19]  M. Searle,et al.  The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal–South Tibet , 2003, Journal of the Geological Society.

[20]  J. Saul,et al.  Seismic Images of Crust and Upper Mantle Beneath Tibet: Evidence for Eurasian Plate Subduction , 2002, Science.

[21]  D. Hilton,et al.  Subduction and Recycling of Nitrogen Along the Central American Margin , 2002, Science.

[22]  B. N. Upreti,et al.  Records of the evolution of the Himalayan orogen from in situ Th–Pb ion microprobe dating of monazite: Eastern Nepal and western Garhwal , 2002 .

[23]  D. Graham.  Noble Gas Isotope Geochemistry of Mid-Ocean Ridge and Ocean Island Basalts: Characterization of Mantle Source Reservoirs , 2002 .

[24]  P. Burnard,et al.  Production, Release and Transport of Noble Gases in the Continental Crust , 2002 .

[25]  B. Marty,et al.  Noble gases and volatile recycling at subduction zones , 2002 .

[26]  D. Kerrick.  Present and past nonanthropogenic CO2 degassing from the solid earth , 2001 .

[27]  P. Bedrosian,et al.  Detection of Widespread Fluids in the Tibetan Crust by Magnetotelluric Studies , 2001, Science.

[28]  K. Whipple,et al.  Neotectonics of the Thakkhola graben and implications for recent activity on the South Tibetan fault system in the central Nepal Himalaya , 2001 .

[29]  D. Hilton,et al.  Southern limit of mantle-derived geothermal helium emissions in Tibet: implications for lithospheric structure , 2000 .

[30]  K. Hodges.  Tectonics of the Himalaya and southern Tibet from two perspectives , 2000 .

[31]  K. Caldeira,et al.  WAS THE HIMALAYAN OROGEN A CLIMATICALLY SIGNIFICANT COUPLED SOURCE AND SINK FOR ATMOSPHERIC CO2 DURING THE CENOZOIC , 1999 .

[32]  Y. Makovsky,et al.  Measuring the seismic properties of Tibetan bright spots: Evidence for free aqueous fluids in the Tibetan middle crust , 1999 .

[33]  H. Wakita,et al.  Helium and carbon isotopic compositions of hot spring gases in the Tibetan Plateau , 1999 .

[34]  D. L. Parkhurst,et al.  User's guide to PHREEQC (Version 2)-a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations , 1999 .

[35]  P. Philippot,et al.  Precambrian ammonium biogeochemistry: a study of the Moine metasediments, Scotland , 1998 .

[36]  G. Bebout.  Nitrogen isotope tracers of high-temperature fluid-rock interactions: Case study of the Catalina Schist, California , 1997 .

[37]  Xianwen Liu,et al.  Bright Spots, Structure, and Magmatism in Southern Tibet from INDEPTH Seismic Reflection Profiling , 1996, Science.

[38]  Handong Tan,et al.  Partially Molten Middle Crust Beneath Southern Tibet: Synthesis of Project INDEPTH Results , 1996, Science.

[39]  Eric J. Fielding,et al.  Tibet uplift and erosion , 1996 .

[40]  D. Hilton.  THE HELIUM AND CARBON ISOTOPE SYSTEMATICS OF A CONTINENTAL GEOTHERMAL SYSTEM : RESULTS FROM MONITORING STUDIES AT LONG VALLEY CALDERA (CALIFORNIA, U.S .A.) , 1996 .

[41]  Ken Caldeira,et al.  Convective hydrothermal C02 emission from high heat flow regions , 1995 .

[42]  W. C. Evans,et al.  Forest-killing diffuse CO2 emission at Mammoth Mountain as a sign of magmatic unrest , 1995, Nature.

[43]  B. Marty,et al.  Origin of carbon in fumarolic gas from island arcs , 1995 .

[44]  K. Caldeira,et al.  Paleoatmospheric consequences of CO2 released during early Cenozoic regional metamorphism in the Tethyan orogen , 1993 .

[45]  Thomas W. Trull,et al.  C-He systematics in hotspot xenoliths: implications for mantle carbon contents and carbon recycling , 1993 .

[46]  Yuji Sano,et al.  Isotopic composition of helium, and CO2 and CH4 contents in gases produced along the New Zealand part of a convergent plate boundary , 1993 .

[47]  N. Rogers,et al.  Timing of Tibetan uplift constrained by analysis of volcanic rocks , 1993, Nature.

[48]  C. Pillinger,et al.  The measurement of δ15N in crustal rocks by static vacuum mass spectrometry: Application to the origin of the ammonium in the Cornubian batholith, southwest England , 1993 .

[49]  M. Fogel,et al.  Nitrogen-isotope compositions of metasedimentary rocks in the Catalina Schist, California: Implications for metamorphic devolatilization history , 1992 .

[50]  S. J. Schaefer,et al.  Global carbon dioxide emission to the atmosphere by volcanoes , 1992 .

[51]  B. Burchfiel,et al.  The South Tibetan Detachment System, Himalayan Orogen: Extension Contemporaneous With and Parallel to Shortening in a Collisional Mountain Belt , 1992 .

[52]  J. Ferry.  Dehydration and decarbonation reactions as a record of fluid infiltration , 1991 .

[53]  R. C. Newton.  METAMORPHIC FLUIDS IN THE DEEP CRUST , 1989 .

[54]  W. Giggenbach.  Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators , 1988 .

[55]  Francis T. Wu,et al.  Nature of the upper crust beneath central Tibet , 1987 .

[56]  Bernard Marty,et al.  C3He in volatile fluxes from the solid Earth: implications for carbon geodynamics , 1987 .

[57]  Michael J. Whiticar,et al.  Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation—Isotope evidence , 1986 .

[58]  Ian Wright,et al.  Carbon isotopes in submarine basalts , 1984 .

[59]  R. Folk,et al.  Travertines: Depositional Morphology and the Bacterially Constructed Constituents , 1984 .

[60]  C. Jaupart,et al.  High heat flow in southern Tibet , 1984, Nature.

[61]  A. H. Truesdell,et al.  An empirical NaKCa geothermometer for natural waters , 1973 .

[62]  J. Hoefs.  Stable Isotope Geochemistry , 1973 .

[63]  J. Troughton,et al.  Carbon isotope fractionation by plants , 1972 .

[64]  J. Vogel,et al.  Carbon isotope fractionation during the precipitation of calcium carbonate , 1970 .

[65]  H. Craig,et al.  Excess 3He in the sea: Evidence for terrestrial primodal helium , 1969 .

[66]  R. Fournier,et al.  Estimation of underground temperatures from the silica content of water from hot springs and wet-steam wells , 1966 .

[67]  H. E. Moore,et al.  The isotopic composition of the nitrogen in natural gases and associated crude oils , 1958 .

[68]  Arthur M. Piper,et al.  A graphic procedure in the geochemical interpretation of water-analyses , 1944 .