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Stefansson, A, Hilton DR, Sveinbjornsdottir AE, Torssander P, Heinemeier J, Barnes JD, Ono S, Halldorsson SA, Fiebig J, Arnorsson S.  2017.  Isotope systematics of Icelandic thermal fluids. Journal of Volcanology and Geothermal Research. 337:146-164.   10.1016/j.jvolgeores.2017.02.006   AbstractWebsite

Thermal fluids in Iceland range in temperature from <10 degrees C to >440 degrees C and are dominated by water (>97 mol%) with a chloride concentration from <10 ppm to >20,000 ppm. The isotope systematics of the fluids reveal many important features of the source(s) and transport properties of volatiles at this divergent plate boundary. Studies spanning over four decades have revealed a large range of values for delta D (-131 to +3.3%o), tritium (-0.4 to +13.8 TU), delta(18) O(-20.8 to + 2.3%o), He-3/He-4 (3.1 to 30.4 R-A), delta B-11 (-6.7 to+25.0%o), delta C-13 Sigma co(2) (-27.4 to+ 4.6%o), C-1 Sigma co(2), (+0.6 to + 118 pMC), delta C-l3(CH4) (-523 to-17.8%o), delta N-15 (-10.5 to+3.0%o), 8(34)C Sigma s(-ll) (-10.9 to (+)3.4%o), delta S-34(SO4) (-2.0to + 21.2%) and delta Cl-37 (-1.0 to + 2.1%o) in both liquid and vapor phases. Based on this isotopic dataset, the thermal waters originate from meteoric inputs and/or seawater. For other volatiles, degassing of mantle-derived melts contributes to He, CO2 and possibly also to Cl in the fluids. Water-basalt interaction also contributes to CO2 and is the major source of H2S, SO4, Cl and B in the fluids. Redox reactions additionally influence the composition of the fluids, for example, oxidation of H2S to SO4 and reduction of CO2 to CH4. Air water interaction mainly controls N-2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as delta C-13 Sigma co(2)and(34)S Sigma S-u indicate heterogeneity of the mantle and mantle-derived melts beneath Iceland. In contrast, the large range of many reactive isotopes, such as delta C-13 Sigma co(2), and delta S-34 Sigma S-u, are heavily affected by processes occurring within the geothermal systems, including fluid-rock interaction, depressurization boiling, and isotopic fractionation between secondary minerals and the aqueous and vapor species. Variations due to these geothermal processes may exceed differences observed among various crust and mantle sources, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements. (C) 2017 Elsevier B.V. All rights reserved.

Elkins, LJ, Fischer TP, Hilton DR, Sharp ZD, McKnight S, Walker J.  2006.  Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front. Geochimica Et Cosmochimica Acta. 70:5215-5235.   10.1016/j.gca.2006.07.024   AbstractWebsite

We report new chemical and isotopic data from 26 volcanic and geothermal gases, vapor condensates, and thermal water samples, collected along the Nicaraguan volcanic front. The samples were analyzed for chemical abundances and stable isotope compositions, with a focus on nitrogen abundances and isotope ratios. These data are used to evaluate samples for volatile contributions from magma, air, air-saturated water, and the crust. Samples devoid of crustal contamination (based upon He isotope composition) but slightly contaminated by air or air-saturated water are corrected using N(2)/Ar ratios in order to obtain primary magmatic values, composed of contributions from upper mantle and subducted hernipelagic sediment on the down-going plate. Using a mantle endmember with delta(15)N = -5 parts per thousand and N(2)/He = 100 and a subducted sediment component with delta(15)N = +7 parts per thousand and N(2)/He = 10,500, the average sediment contribution to Nicaraguan volcanic and geothermal gases was determined to be 71%. Most of the gases were dominated by sediment-derived nitrogen, but gas from Volcan Mombacho, the southernmost sampling location, had a mantle signature (46% from subducted sediment, or 54% from the mantle) and an affinity with mantle-dominated gases discharging from Costa Rica localities to the south. High CO(2)/N(2exc.) ratios (N(2 exc.) is the N(2) abundance corrected for contributions from air) in the south are similar to those in Costa Rica, and reflect the predominant mantle wedge input, whereas low ratios in the north indicate contribution by altered oceanic crust and/or preferential release of nitrogen over carbon from the subducting slab. Sediment-derived nitrogen fluxes at the Nicaraguan volcanic front, estimated by three methods, are 7.8 x 10(8) mol N/a from (3)He flux, 6.9 x 10(8) mol/a from SO(2) flux, and 2.1 x 10(8) and 1.3 x 10(9) mol/a from CO(2) fluxes calculated from (3)He and SO(2), respectively. These flux results are higher than previous estimates for Central America, reflecting the high sediment-derived volatile contribution and the high nitrogen content of geothermal and volcanic gases in Nicaragua. The fluxes are also similar to but higher than estimated hernipelagic nitrogen inputs at the trench, suggesting addition of N from altered oceanic basement is needed to satisfy these flux estimates. The similarity of the calculated input of N via the trench to our calculated outputs suggests that little or none of the subducted nitrogen is being recycled into the deeper mantle, and that it is, instead, returned to the surface via arc volcanism. (c) 2006 Elsevier Inc. All rights reserved.

Gulec, N, Hilton DR.  2006.  Helium and heat distribution in western Anatolia, Turkey; relationship to active extension and volcanism. Special Paper Geological Society of America. 409:305-319.   10.1130/2006.2409(16)   Abstract

Western Anatolia, one of the world's best-known extensional terrains, is characterized by the presence of several moderate- to high-enthalpy geothermal fields. Geo-thermal fluids have helium isotope compositions reflecting mixing between mantle and crustal helium components, the former ranging between 0.58% and 45% of the total helium in a given sample. Regarding the distribution of heat and mantle He and their correlation with tectonic structure and volcanism in western Anatolia, the prominent features are as follows: (1) the association between highest heat and highest (super 3) He lies along the eastern segment of the Buyuk Menderes graben, (2) the high heat and high (super 3) He occur in the vicinity of the Quaternary Kula volcanism, (3) high-enthalpy fields exist in close vicinity to the young alkaline volcanics, (4) relatively high mantle He contributions occur in areas of not only the young alkaline, but also the old calc-alkaline volcanics, and (5) there is a lack of volcanic exposures along the Buyuk Menderes graben (except at its western and southeastern terminations), where the highest values are recorded for both heat and helium. The first three features collectively suggest that the transfer mechanism for both heat and helium is probably mantle melting accompanying the current extension in western Anatolia, yet the latter two further indicate that this may be accomplished via subsurface plutonic activities. The large range observed in the helium isotope compositions may be linked with differential (local) extension rates and associated melt generation in the respective areas. This suggestion can be substantiated by He isotope data from more of the region.