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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.

de Moor, JM, Fischer TP, Sharp ZD, Hilton DR, Barry PH, Mangasini F, Ramirez C.  2013.  Gas chemistry and nitrogen isotope compositions of cold mantle gases from Rungwe Volcanic Province, southern Tanzania. Chemical Geology. 339:30-42.   AbstractWebsite

We report the first complete bulk gas chemistry and nitrogen isotope data for geothermal volatiles from the Rungwe Volcanic Province, located in the western branch of the East African Rift north of Lake Malawi. Temperatures of springs and gas emissions at Rungwe vary from 13 °C to 65 °C with the highest temperatures observed at the springs in the northern and southern lowlands. The vigorously degassing cold CO2 vents and springs have temperatures between 13 °C and 36 °C and are located at higher elevation than the hot springs. The gas compositions are ~ 99% CO2, 0.0008 to 0.0078 mmol/mol H2, 0.0004 to 0.062 mmol/mol He, 0.08 to 0.77 mmol/mol Ar, 3.1 to 28.5 mmol/mol N2, 0.4 to 3.73 mmol/mol O2, < 0.002 to 1.541 mmol/mol CH4, < 0.001 to 0.009 mmol/mol CO, and are poor in H2S (0.045 to 0.201 mmol/mol). The CO2 flux at a local gas collection plant is estimated to be 1.6 × 105 mol/year. Gas geothermometry indicates a range of equilibration temperatures from > 250 °C (from CO2–Ar) to ~ 60 °C (from H2–Ar), which is interpreted to reflect deep equilibration with hot saline fluids and shallow re-equilibration of kinetically fast gas geothermometers with cold meteoric recharge from the highlands. N2–He–Ar systematics show that the gases fall on a well-defined mixing line between upper mantle or sub-continental lithospheric mantle and air saturated water endmembers. Details of an improved method for analyzing nitrogen isotope compositions in gas samples are presented. Nitrogen isotope compositions (δ15N values) range between + 2‰ and − 5.9‰, overlapping with the upper mantle range, with only one sample location displaying δ15N values greater than air (0‰). The results emphasize the importance of the East African Rift as a potential, but poorly constrained, contributor of sub-continental lithospheric mantle volatiles to the Earth's surface even in regions that are currently volcanically dormant, but are seismically active.

Sapienza, G, Hilton DR, Scribano V.  2005.  Helium isotopes in peridotite mineral phases from Hyblean Plateau xenoliths (south-eastern Sicily, Italy). Chemical Geology. 219:115-129.   10.1016/j.chemgeo.2005.02.012   AbstractWebsite

Fourteen He isotope and abundance analyses have been performed on olivine, orthopyroxene, and clinopyroxene grains from selected spinel-facies peridotite xenoliths collected from the Hyblean Plateau, southeastern Sicily. These peridotites consist of protogranular-textured, spinel-facies lherzolites and lherzolitic harzburgites. Microthermometric data on olivine and pyroxenes reveal that the fluid inclusions (FIs) are nearly pure CO2, with densities ranging from 0.76 to 1.15 g/cm(3). The densest inclusion trail occurs in an orthopyroxene grain and indicates a trapping-pressure of 0.98-1.14 GPa (estimated equilibration temperature of 1000 degrees C for pure CO2 system) corresponding to a depth of similar to 36-41 km (for an average estimated density of the lithospheric column of 2.85 g/cm(3)). These are minimum estimates. The near-unimodal distribution Of CO2 densities indicates that these xenoliths have been stored at the pressure interval of 0.75-0.95 GPa-corresponding to a depth of similar to 27-35 km (crust-mantle boundary or just below). En route to the surface, they have not had any significant interaction with shallower volatile reservoirs. He trapped in CO2-rich fluid inclusions ranges in abundance from 1.8 to 86.6 X 10(-9) cm(3) STP/g, with clinopyroxenes usually showing higher He contents than coexisting olivines and orthopyroxenes. The inter-mineral differences are due to physical properties of minerals. In contrast, He isotope ratios cover a narrow range (-7.3 +/- 0.3R(A), where R-A=air(3) He/He-4), indicating isotopic equilibration between the mineral phases. He composition indicates a MORB-type source for the metasomatic agent(s), influenced by relatively minor radiogenic production in the source. Therefore, in closed-system conditions, the calculated helium residence time is 790 Myr. Geophysical and geochemical studies suggest the existence of a common fluid reservoir in the mantle beneath the Central Mediterranean area. The most pristine He signature for the mantle endmember is found on Pamelleria Island (Sicily Channel): this He ratio is similar to that found in Hyblean peridotite minerals, testifying to a common source history. We conclude, therefore, that Hyblean xenoliths can be used as a powerful tool in defining the geochemical features of this portion of the upper mantle. (c) 2005 Elsevier B.V. All rights reserved.