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Haroardottir, S, Halldorsson SA, Hiltons DR.  2018.  Spatial distribution of helium isotopes in Icelandic geothermal fluids and volcanic materials with implications for location, upwelling and evolution of the Icelandic mantle plume. Chemical Geology. 480:12-27.   10.1016/j.chemgeo.2017.05.012   AbstractWebsite

The distribution of helium isotope ratios (He-3/He-4) in Icelandic geothermal fluids, volcanic glasses and phyric lavas is investigated. Along with presenting a new helium isotope dataset using phyric lavas largely from off-rift regions, we compiled published data and constructed a database of all available helium isotope data from Iceland. The new dataset reveals an exceptionally high He-3/He-4 ratio from a phyric lava in NW-Iceland (47.5 R-A, where R-A is the He-3/He-4 ratio of air), which is among the highest values measured in any mantle-derived magma to date. Modifications of primary (i.e., mantle-derived) helium isotope ratios, due to additions of air-derived helium and He from radiogenic ingrowth, were evaluated and the database was filtered accordingly. The geographical information system ArcGIS (ESRI) was used to perform spatial analysis on the filtered database and the interpolation method, Natural Neighbor, was used to calculate representative helium isotope ratios for all parts of Iceland, including off-rift regions. The results show that helium isotope ratios for the whole of Iceland vary from 5.1 to 47.5 R-A. However, this study allows for a fine-scale distinction to be made between individual rift segments and off-rift regions. The results clearly reveal that each rift zone has its own distinctive mean isotope signature: 12-17 R-A in the Western Rift Zone, 8-11 R-A in the Northern Rift Zone and 18-21 R-A in the Eastern Rift Zone. Our isoscape map places new constraints on a previously inferred high-helium plateau region in central Iceland (Breddam a al., 2000). The plateau continues southward along the propagating Eastern Rift Zone and through to the South Iceland Seismic Zone and the Mid-Iceland belt. Its location coincides with many geological features, e.g., eruption rates, location of abandoned rift segments, seismic velocity and gravity anomalies. Such high helium isotope ratios have been associated with undegassed and primordial mantle sources that have been isolated in the lower mantle over Earth's history. Thus, high-helium domains throughout Iceland are interpreted to mark the loci of present and past plume conduits which help explain the considerable spatial variation in the sampling of a primordial mantle He component beneath the Iceland hotspot.

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.