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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.

Pik, R, Marty B, Hilton DR.  2006.  How many mantle plumes in Africa? The geochemical point of view Chemical Geology. 226:100-114.   10.1016/j.chemgeo.2005.09.016   AbstractWebsite

The association of anomalous topographic swells and widespread Cenozoic volcanism within the African plate (Hoggar, Tibesti, Darfur, Ethiopian highlands, Kenyan dome) may reflect either the involvement of one, or several, deep mantle plumes, or, alternatively, be attributed to tectonic processes involving only the lithosphere or the shallow asthenosphere. We present here new helium isotopic measurements that, added to existing data, allow us to restrict the spatial extent of a high-(3) He component (up to 20 Ra) to the Ethiopia-Afar volcanic province, for places where large volumes of Oligocene pre-rift flood basalts and ignimbrites erupted within a short (1-2 Ma) time interval. All other investigated African volcanic provinces display MORB-type, and/or continental lithosphere-like, He-3/He-4 signatures (7 +/- 2 Ra) often modified by a contribution of crustal He. The distribution of He isotopic signatures in Africa, together with other isotopic (Sr-Nd-Pb) tracers measured in Miocene to Plio-Quaternary alkaline lavas in East-Africa, is fully consistent with the occurrence of two types of mantle plumes: (i) a large, deep-sited mantle plume characterized by a high-(3) He signature, possibly originating from the core-mantle boundary according to seismic mantle tomography, which triggered the flood basalt eruptions 30 Ma ago and which subsequently interacted with shallower mantle sources to produce the syn-rift volcanism of the Ethiopia-Afar province; and (ii) a second-order type of shallow mantle upwelling, presumably originating from depths shallower than 400 km as suggested by seismic wave imaging, distinct from the main Afar plume and disseminated within the African plate under the uplifted and rifted swells. The above conclusions do not support the view of a unique large mantle plume feeding all Cenozoic African volcanic provinces. The fact that high-(3) He signals are associated with the largest lava volumes erupted in Africa since the beginning of the Cenozoic argue against models advocating a shallow origin for high He-3/He-4 signatures. Instead, they confirm that such signatures characterize hot material coming from the deep mantle. (c) 2005 Elsevier B.V. All tights reserved.

Hilton, DR, Macpherson CG, Elliott TR.  2000.  Helium isotope ratios in mafic phenocrysts and geothermal fluids from La Palma, the Canary Islands (Spain): Implications for HIMU mantle sources. Geochimica Et Cosmochimica Acta. 64:2119-2132.   10.1016/s0016-7037(00)00358-6   AbstractWebsite

We report a comprehensive study of He isotope (He-3/He-4) variations on well-characterized lavas from La Palma, the current locus of activity of the Canary hotspot. Cogenetic olivine (OL) and clinopyroxene (CPX) phenocrysts from 11 basaltic lava hows representing all stages of the island's evolution were analyzed by crushing in vacuo. Additionally, the sample set is supplemented by a CO2-rich bubbling cold spring in Taburiente caldera, Phenocryst He-3/He-4 ratios vary between 6.3 to 8.9R(A); both extremes occur in the 2 to 0.6 Ma shield-building Taburiente lavas. Historic flows vary between 7.0 to 7.8R(A), whereas phenocrysts from a single submarine basement sill (4.0-2.9 Ma) have He-3/He-4 ratios of 8.3R(A) (OL) and 8.4R(A) (CPX). He isotopic equilibrium characterizes all the phenocryst pairs except one Taburiente lava. The absence of a correlation between He-3/He-4 and [He] contrasts with other ocean island basalts, e.g., Heard Island (Hilton et al., 1995) and, together with equilibrium in phenocrystic He-3/He-4, suggests that the La Palma sample suite has He-3/He-4 ratios that have suffered minimal crustal contamination. The cold spring (geothermal) sample gives a He-3/He-4 ratio of 9.5R(A), which is higher than any of the phenocrysts and in excellent agreement with a previously reported value for Taburiente caldera (Perez et al., 1996). The geothermal He-3/He-4 values indicate that La Palma is the first HIMU-like ocean island (where HIMU implies a distinctive mantle source with high time-integrated U-238/Pb-204 (mu) ratio) with reported He-3/He-4 ratios higher than the canonical value (8 +/- 1R(A)) characteristic of depleted mid-ocean ridge basalt (MORB) mantle. We present two simple mixing scenarios that are compatible with the He-Pb isotope systematics of La Palma: a) a high-He-3 mantle plume, extensively but variably depleted in its original He content, mixes with a HIMU source characterized by both radiogenic He and lead isotope compositions; and b) mixing between a composite plume, with high He-3/He-4 and radiogenic lead, and depleted mid-ocean ridge basalt mantle. The latter model is favored as it appears to account more readily for the mid-ocean ridge MORB-like He isotope characteristics of the lavas. Addition of radiogenic He-most likely produced in mantle melts frozen into the sub-Canarian mantle lithosphere-to resultant mixtures may subsequently occur in either model. Such a process can explain the absence of correlations between He and Pb isotope ratios in the lavas, and the lower He-3/He-4 values of the phenocrysts with respect to the geothermal fluids. We find no need to invoke a radiogenic growth/diffusion model (Hanyu and Kaneoka, 1998) to explain the He-Pb isotope relationships for La Palma. These mixing models may also be applicable to other HIMU islands: however, any high-He-3 plume signature of St. Helena and Mangaia may be obscured due to the high proportion of the HIMU component at these localities and/or by addition of radiogenic He. Copyright (C) 2000 Elsevier Science Ltd.