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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.

Day, JMD, Hilton DR.  2011.  Origin of (3)He/(4)He ratios in HIMU-type basalts constrained from Canary Island lavas. Earth and Planetary Science Letters. 305:226-234.   10.1016/j.epsl.2011.03.006   AbstractWebsite

New helium isotope and abundance measurements are reported for olivine and clinopyroxene phenocrysts from HIMU-type (high-mu=elevated (238)U/(204)Pb) lavas and xenoliths spanning the stratigraphies of El Hierro and La Palma, Canary Islands. Some pyroxene phenocrysts have suffered post-eruptive modification, either by less than 1% assimilation of crustal-derived He, or by closed-system ageing of He. Olivine phenocrysts record mantle source (3)He/(4)He compositions, with the average (3)He/(4)He for La Palma olivine (7.6 +/- 0.8R(A), where R(A) is the atmospheric (3)He/(4)He ratio of 1.38 x 10(-6)) being within uncertainty of those for El Hierro (7.7 +/- 0.3R(A)), and the canonical mid-ocean ridge basalt range (MORB: 8 +/- 1R(A)). The new helium isotope data for El Hierro and La Palma show no distinct correlations with whole-rock (87)Sr/(86)Sr, (143)Nd/(144)Nd, (187)Os/(188)Os, or Pb isotopes, but (3)He/(4)He ratios for La Palma lavas correlate with (18)O/(16)O measured for the same phenocryst populations. Despite limited (3)He/(4)He variations for El Hierro and La Palma, their He-O isotope systematics are consistent with derivation from mantle sources containing distinct recycled oceanic basaltic crust (El Hierro) and gabbroic lithosphere (La Palma) components that have mixed with depleted mantle, and a high-(3)He/(4)He component (>9.7R(A)) in the case of La Palma. The new data are consistent with models involving generation of compositionally and lithologically (e.g., pyroxenite, eclogite, peridotite) heterogeneous mantle sources containing recycled oceanic crust and lithosphere entrained within upwelling high-(3)He/(4)He mantle that has been severely diluted by interaction with depleted mantle. We propose that the noble gas systematics of HIMU-type lavas and ocean island basalts (OIB) in general, are most simply interpreted as being controlled by the most gas-rich reservoir involved in mixing to generate their mantle sources. In this scenario, HIMU and enriched mantle (EM) sources are dominated by depleted mantle, or high-(3)He/(4)He mantle, because recycled crust and lithosphere have low He concentrations. Consequently, high-(3)He/(4)He OIB would predominantly reflect derivation from a less depleted mantle source with sub-equal to higher He contents than depleted mantle. The available coupled He-O isotope systematics measured for OIB lavas are consistent with this hypothesis. (C) 2011 Elsevier B.V. All rights reserved.

Mitchell, EC, Fischer TP, Hilton DR, Hauri EH, Shaw AM, de Moor JM, Sharp ZD, Kazahaya K.  2010.  Nitrogen sources and recycling at subduction zones: Insights from the Izu-Bonin-Mariana arc. Geochemistry Geophysics Geosystems. 11   10.1029/2009gc002783   AbstractWebsite

We report new chemical and nitrogen isotopic data from 29 volcanic and hydrothermal gas samples covering eight centers in the Izu-Bonin-Mariana (IBM) arc to investigate the sources, flux, and mass balance of nitrogen at a "cool" convergent margin. The majority of samples have high N(2)/He (1217-17,300) and low CO(2)/N(2,exc.) (78-937), implying addition of nitrogen from the subducting slab. This inference is supported by positive (i.e., sediment-like) delta(15)N values (up to 5.5%) in most samples. The exception to these trends is Agrigan in the Mariana arc, with low N(2)/He (similar to 200), high CO(2)/N(2,exc.) (similar to 1500), and negative delta(15)N. Mixing calculations suggest an average of 34% of the nitrogen in our samples is derived from subducted sediment, or 75% after correction for atmospheric contamination. Sediment-derived N(2) fluxes estimated by three different methods range from 0.25 x 10(8) to 1.11 x 10(8) mol yr(-1) N(2), representing 4%-17% of the total nitrogen input flux or 11%-51% of the sedimentary nitrogen input flux. The altered oceanic crust is identified as an important contributor to the arc nitrogen budget, and the delta(15)N of the residual nitrogen subducted into the mantle is estimated at approximately -1.9 parts per thousand. Despite similarities in gas chemistry and delta(15)N values, our conclusions regarding nitrogen recycling for IBM are markedly different than those for the Nicaraguan segment of the Central American arc, and we suggest that thermal regime is the major control on nitrogen recycling within subduction zones. The global nitrogen cycle is estimated to be in steady state, suggesting either that subducted sediments are an unlikely source for heavy nitrogen in plume-related rocks or secular variation in the isotopic composition of subducted sediments. Better constraints on nitrogen recycling at other arcs are required to test these conclusions.

Fischer, TP, Takahata N, Sano Y, Sumino H, Hilton DR.  2005.  Nitrogen isotopes of the mantle: Insights from mineral separates. Geophysical Research Letters. 32   10.1029/2005gl022792   AbstractWebsite

We present the first nitrogen (N) isotope measurements determined by in-vacuo crushing of mineral separates from arc lavas, OIBs (Ocean Island Basalts), and mantle xenoliths. Measured OIB delta(15)N values range from similar to - 8 parts per thousand for the northern rift zone in Iceland to + 3.1 parts per thousand for a dunite nodule from Hawaii. Most arc-related olivines show distinctly positive values - up to + 6.2 parts per thousand (Cerro Negro, Nicaragua). The measured N isotope values in olivine separates are similar to gas samples collected at the same localities, suggesting that both media ( olivines and gases) sample volatiles primarily derived from the magma. This observation also implies that N isotope fractionation does not occur during magma degassing, a notion supported by (4)He/(40)Ar* data. Our results indicate a heterogeneous mantle source region, in terms of N isotopic composition, that may have resulted from surface recycling of N at some localities.