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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.

Barry, PH, Hilton DR, Day JMD, Pernet-Fisher JF, Howarth GH, Magna T, Agashev AM, Pokhilenko NP, Pokhilenko LN, Taylor LA.  2015.  Helium isotopic evidence for modification of the cratonic lithosphere during the Permo-Triassic Siberian flood basalt event. Lithos. 216–217:73-80.   10.1016/j.lithos.2014.12.001   Abstract

Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record — eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~ 250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here, we present helium isotope (3He/4He) and concentration data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kimberlites: the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3He/4He ratios span a range from 0.1 to 9.8 RA (where RA = air 3He/4He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3He/4He = 0.41 ± 0.30 RA (1σ); n = 7 excluding 1 outlier), and 2) ‘mantle-like’ post plume Obnazhennaya samples (mean clinopyroxene 3He/4He = 4.20± 0.90 RA (1σ); n = 5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3He/4He than clinopyroxenes (or garnet). Helium contents in Udachnaya samples ([He] = 0.13–1.35 μcm3STP/g; n = 6) overlap with those of Obnazhennaya ([He] = 0.05–1.58 μcm3STP/g; n = 10), but extend to significantly higher values in some instances ([He] = 49–349 μcm3STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3He/4He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic fluids. Indeed, the stark contrast between pre- and post-plume 3He/4He ratios in peridotite xenoliths highlights the potentially powerful utility of He-isotopes for differentiating between various types of metasomatism (i.e., crustal versus basaltic fluids).

Barry, PH, Hilton DR, Fischer TP, de Moor JM, Mangasini F, Ramirez C.  2013.  Helium and carbon isotope systematics of cold “mazuku” CO2 vents and hydrothermal gases and fluids from Rungwe Volcanic Province, southern Tanzania. Chemical Geology. 339:141-156.   AbstractWebsite

We report new helium and carbon isotope (3He/4He and δ13C) and relative abundance (CO2/3He) characteristics of a suite of 20 gases and fluids (cold mazuku-like CO2 vents, bubbling mud-pots, hot and cold springs) from 11 different localities in Rungwe Volcanic Province (RVP), southern Tanzania and from 3 additional localities in northern Tanzania (Oldoinyo Lengai Volcano and Lake Natron). At RVP, fluids and gases are characterized by a large range in He-isotope compositions (3He/4He) from 0.97 RA to 7.18 RA (where RA = air 3He/4He), a narrow range in δ13C ratios from − 2.8 to − 6.5‰ (versus VPDB), and a large range in CO2/3He values spanning nearly four orders of magnitude (4 × 109 to 3.2 × 1013). Oldoinyo Lengai possesses upper‐mantle-like He–CO2 characteristics, as reported previously (Fischer et al., 2009), whereas hot springs at Lake Natron have low 3He/4He (~ 0.6 RA), CO2/3He (~ 5–15 × 108) and intermediate δ13C (~−3.7 to − 4.9 ‰). At RVP, fluid phase samples have been modified by the complicating effects of hydrothermal phase-separation, producing CO2/3He and δ13C values higher than postulated starting compositions. In contrast, gas-phase samples have not been similarly affected and thus retain more mantle-like CO2/3He and δ13C values. However, the addition of crustal volatiles, particularly radiogenic helium from 4He-rich reservoir rocks, has modified 3He/4He values at all but the three cold CO2 gas vent (i.e., mazuku) localities (Ikama Village, Kibila Cold Vent and Kiejo Cold Vent) which retain pristine upper-mantle He-isotope (~ 7 RA) and He–CO2 characteristics. The extent of crustal contamination is controlled by the degree of interaction within the hydrothermal system, which increases with distance from each major volcanic center. In contrast, we propose that pristine cold CO2 mazuku gases collected at stratigraphic contacts on the flanks of RVP volcanoes may potentially tap isolated gas pockets, which formed during previous eruptive events and have remained decoupled from the local hydrothermal system. Furthermore, by identifying and utilizing unmodified gas samples, we determine mantle versus crustal provenance of the CO2, which we use to estimate mantle-derived CO2 fluxes at both Rungwe and Lake Natron. Finally, we investigate the origin of the apparent discrepancy in He isotopes between fluids/gases and mafic phenocrysts at RVP (from Hilton et al., 2011), and discuss the tectonic (i.e., rift zone dynamics) and petrogenic conditions that distinguish RVP from other plume-related subaerial rift zones.

Halldórsson, SA, Hilton DR, Troll VR, Fischer TP.  2013.  Resolving volatile sources along the western Sunda arc, Indonesia. Chemical Geology. 339:263-282.   AbstractWebsite

We present the chemical and isotope (HeCN) characterization of active fumaroles and hydrothermal gases and waters from the summits and flanks of 19 volcanic centers along the western Sunda arc, Indonesia. Samples were collected over two field expeditions (1991 and 2010) and cover 13 volcanic centers in Sumatra, 5 in Java and one in Bali. In addition, we present data from three geothermal sites in Sumatra associated with active fault systems in-between volcanic centers (IBVC). The overall aim is to resolve volatiles associated with the sub-arc mantle (subducting slab and mantle wedge) from inputs derived from the over-riding arc crust. The western Sunda arc is a prime locality to assess controls on volatile provenance at subduction zones due to changes in composition and thickness of over-riding crust and variations in sediment input rates along the strike of the arc. The dry gas chemistry of the majority of volcanic samples is dominated by CO2 with inert gas variations (HeArN2) typical of subduction zone gases. However, there is a strong crustal control on the HeCO2 isotope and relative abundance systematics on a number of volcanic centers: this effect is most clearly observed at flank localities and in water phase samples. Filtering the entire database for modifications due to air contamination, degassing-induced fractionation (C-isotopes and CO2/3He ratios) and crustal contamination associated with volatile transport within shallow-level hydrothermal systems allows recognition of the magmatic volatile signature of individual volcanoes along the arc. Helium isotopes at all but two volcanic centers (Talang and Dempo on Sumatra) range from 5.3 RA to 8.1 RA (RA = air 3He/4He) pointing to the mantle wedge as the principal source of He but with a small input of crustal (radiogenic) He at some localities. Samples from Java and Bali span an even more limited range (6.6 to 7.9 RA) implying a relatively smaller input of crustal He. Carbon isotope and CO2/3He ratios vary from − 1.4‰ to − 6.4‰ and 4.38 to 150 (× 109), respectively, with higher and more variable values to the north of Sumatra. This latitudinal effect is not apparent in air-corrected N-isotope values (δ15NC = − 3.91 to + 5.92‰) or various elemental ratios such as N2/Ar and N2/He. The three IBVC sites, all located in Sumatra, have significantly lower 3He/4He ratios (< 3.6 RA) with CO2/3He values both higher and lower than volcanic centers. Their δ13C, δ15NC and gas ratios overlap with the volcanic centers. The elemental and isotopic characteristics of carbon and helium have been modified at IBVC sites due to crustal processes. However, based upon relationships between CH4 and 3He as well as co-variations between HeCN isotopes, the over-riding crust and associated sediments add minimal volatiles to volcanic centers throughout the western Sunda arc. In turn, subducted sediment, particularly the Nicobar Fan which supplies Himalayan-derived sediment to the slab off Sumatra, exerts a strong control on the magmatic CO2 characteristics although it is less influential for N2. In spite of large sections of incoming sedimentary material being off-scraped during subduction, our data suggest that a significant fraction must enter the trench, enhancing fluid/melt production in the mantle wedge. We propose that subduction-related source contamination plays the dominant role over thick/old crustal basement in supplying the major volatile output budget of the western Sunda arc.

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.

Hilton, DR, Ramirez CJ, Mora-Amador R, Fischer TP, Furi E, Barry PH, Shaw AM.  2010.  Monitoring of temporal and spatial variations in fumarole helium and carbon dioxide characteristics at Poas and Turrialba volcanoes, Costa Rica (2001-2009). Geochemical Journal. 44:431-440. AbstractWebsite

We report results of a 9-year monitoring program that took place from 2001 to 2009 of the helium and carbon isotope ((3)He/(4)He, delta(13)C) and relative abundances ratios (CO(2)/(3)He) of fumarole sites at Pods and Turrialba volcanoes, Costa Rica. Over the monitoring period, helium isotopes ((3)He/(4)He), delta(13)((CO2)) and CO(2)/(3)He values varied between 6.7-7.6 R(A) (where R(A) = 1.4 x 10(-6)), -5.5 to -1.3 parts per thousand (vs. PDB) and 8.2-59.5 (x10(9)), respectively, at Pods Volcano. Corresponding values for Turrialba Volcano were 6.9-8.0 R(A), -4.4 to -2.7 parts per thousand and 9.4-19.6 (x10(9)), respectively. Notably, fumarole sites at both volcanoes underwent changes in temperature, intensity and areal extent during the 9-year period, and Pods Volcano experienced hydrophreatic eruptions and structural damage induced by a nearby earthquake. At both volcanoes, there were significant and sympathetic temporal changes involving all three geochemical parameters notably in 2001 and 2006 at Pods and in 2001 and between 2005-2007 at Turrialba. We dismiss increased hydrothermal interaction, magma degassing and calcite precipitation as likely causes of the observed variations. Instead, by ascribing endmember compositions to the three principal contributors to the CO(2) inventory mantle wedge as well as limestone and organic (sedimentary) carbon (both slab and crust derived) we show that changes in observed He-CO(2) relationships mainly reflect enhanced crustal contributions of CO(2) and increased inputs from magma degassing. Such changes in the relative roles of crust and magma as suppliers of CO(2) are readily apparent in the He-CO(2) temporal record. This work supports calls for long-term geochemical monitoring to be included within hazard assessment and mitigation studies at active volcanoes.

Ray, MC, Hilton DR, Munoz J, Fischer TP, Shaw AM.  2009.  The effects of volatile recycling, degassing and crustal contamination on the helium and carbon geochemistry of hydrothermal fluids from the Southern Volcanic Zone of Chile. Chemical Geology. 266:38-49.   10.1016/j.chemgeo.2008.12.026   AbstractWebsite

We report new helium and carbon isotope ((3)He/(4)He, delta(13)C) and relative abundance (CO(2)/(3)He) characteristics of a suite of hydrothermal gases and fluids (fumaroles, hot springs, geothermal wells) from 18 localities in the Central Southern Volcanic Zone (CSVZ) of Chile. The CSVZ is characterized by a wide range of (3)He/(4)He ratios, from 1.50 to 6.47 R(A) (where R(A) = air (3)He/(4)He), delta(13)C (CO(2)) values, from -2.9 to -17.7 parts per thousand (vs. PDB), and CO(2)/(3)He ratios, which vary over 5 orders of magnitude (3.1 x 10(5) to 2.3 x 10(11)). One hydrothermal locality, Aguas Calientes, has combined He-CO(2) characteristics remarkably similar to other arc-related systems worldwide implying that the underlying subduction zone complex (and mantle wedge) supplies volatiles to the volcanic front with little or no modification en route to the surface. The mechanism controlling helium isotope ratios of other hydrothermal systems appears to be mixing between mantle-derived helium and a radiogenic component derived from (4)He-rich country rock. The variable He-CO2 elemental relationships and delta(13)C (CO(2)) values at these localities are consistent with gas separation (gas samples) or temperature-dependent calcite precipitation (water samples) in shallow-level hydrothermal systems. Both processes result in CO(2) loss which exacerbates the effects of contamination by crustal gases. Whereas the Aguas Calientes locality is useful for understanding the role of the underlying mantle wedge and subducting slab in supplying volatiles to the Andean volcanic front, the value of the majority of hydrothermal samples in the present study lies with discerning the potentially complicating effects of degassing and/or crustal contamination on the resulting He-CO(2) record. (C) 2009 Elsevier B.V. All rights reserved.

de Leeuw, GAM, Hilton DR, Fischer TP, Walker JA.  2007.  The He-CO2 isotope and relative abundance characteristics of geothermal fluids in El Salvador and Honduras: New constraints on volatile mass balance of the Central American Volcanic Arc. Earth and Planetary Science Letters. 258:132-146.   10.1016/j.epsl.2007.03.028   AbstractWebsite

We report helium and carbon isotope and relative abundance data of fumaroles, hot springs, water springs, mud-pots and geothermal wells from El Salvador and Honduras to investigate both along and across-arc controls on the release of CO2 from the subducted slab. El Salvador localities show typical volcanic front volcanic gas signatures, with He-3/He-4 ratios of 5.2-7.6 R-A, delta C-13 values of -3.6% to -1.3% and CO2/He-3 ratios of 8-25 x 10(9). In Honduras, we find similar values only for volatiles collected in the Sula Graben region located similar to 200 km behind the volcanic front. All other areas in Honduras show significantly lower He-3/He-4 ratios (0.7-3.5 R-A), lower delta C-13 values (<-7.3%) and more variable CO2/He-3 ratios (6.2 x 10(7)-2.0 x 10(11)): characteristics consistent with degassing-induced fractionation of CO2 and He and/or interaction with crustal rocks. The provenance of CO2 released along the volcanic front is dominated by subducted marine carbonates (L=76 +/- 4%) and organic sediments (S = 14 +/- 3%), with the mantle wedge (M) contributing 10 +/- 3% to the total carbon flux. The L/S ratio of the El Salvador volatiles (average = 5.6) is comparable to volcanic front localities in Costa Rica and Nicaragua [A.M. Shaw, D.R. Hilton, T.P. Fischer, L.A. Walker, G.E. Alvarado, Contrasting He-C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones. Earth Planet. Sci. Lett. 214 (2003) 499-513] but is approximately one-half the input value of sediments at the trench (L. Li, G.E. Bebout, Carbon and nitrogen geochemistry of sediments in the Central American convergent margin: Insights regarding subduction input fluxes, diagenesis, and paleoproductivity, J. Geophys. Res. 110 (2005), doi: 10.1029/2004JB003276). We use the L/S ratio of El Salvador geothermal fluids, together with estimates of the CO2 Output flux from the arc, to constrain the amount and composition of subducted sediments involved in the Supply of CO2 to the arc. For the El Salvador segment of the volcanic front, a similar to 180 m continuous section of the incoming sedimentary pile - with the uppermost similar to 42 in removed by under-plating, is required. Significantly, there is no need for oceanic basaltic basement to Supply CO2 to El Salvador or any other part of the volcanic front. This new approach, combining provenance characteristics of CO2 from the slab (L/S ratio) and CO2 flux estimates of the volcanic output, allows a more realistic estimate of the recycling efficiency of slab-derived sedimentary CO2 through the Central American Volcanic Arc to the atmosphere. Furthermore, the low L/S ratio (4.8) of Sula Graben samples from behind the front in Honduras is inconsistent with continued supply of slab-derived sedimentary CO2 following volatile loss at sub-are depths, thereby pointing to ancient enrichment and/or lateral entrainment processes controlling CO2 in the mantle wedge below Honduras. (C) 2007 Elsevier B.V All rights reserved.

Shaw, AM, Hilton DR, Fischer TP, Walker JA, de Leeuw GAM.  2006.  Helium isotope variations in mineral separates from Costa Rica and Nicaragua: Assessing crustal contributions, timescale variations and diffusion-related mechanisms. Chemical Geology. 230:124-139.   10.1016/j.chemgeo.2005.12.003   AbstractWebsite

We report new He abundance and isotope measurements of phenocryst phases in volcanic tephra and lavas from the Nicaragua-Costa Rica section of the Central American arc, where significant variations in crustal thickness have been inferred. Helium isotope values range from 4.6R(A) to 7.5R(A), with no evidence for crustal thickness influencing measured (3)He/(4)He ratios. A comparison of He abundances and isotopes measured in mafic phenocrysts from tephra vs. lavas from two separate eruptions at Cerro Negro show that both sampling media preserve phenocrysts with high (3)He/(4)He values. (3)He/(4)He ratios measured in phenocryst phases show good agreement with He isotope values of geothermal fluids from the same volcanoes. However, we note that the pyroxenes tend to have lower (3)He/(4)He ratios (4.6-7.0R(A)) than the olivines ((3)He/(4)He=6.1-7.5R(A)) over a range of concentration values and are consistently lower in cogenetic phenocryst pairs at all locations sampled. In order to assess how this difference arises, we explore two alternative mechanisms: (1) diffusion-related isotopic fractionation, and (2) late-stage radiogenic (4)He additions, preferentially influencing pyroxene grains. In the first case, we reject diffusion-related firactionation of He isotopes since lower (3)He/(4)He ratios are not accompanied by a decrease in He concentration values. The second scenario is evaluated on the basis of Mg numbers in cogenetic phenocryst pairs and by petrological modeling of the crystallization sequence. Mg numbers and modeling results at low pressure conditions (= 1 kbar) suggest that olivine crystallization preceded pyroxene crystallization. However, since lavas do not show evidence for extensive crustal contamination, we suggest that the best explanation for the lower (3)He/(4)He ratios in pyroxenes is related to the closure temperatures of the phenocryst phases. Given its lower closure temperatures and higher He diffusion rates, we suggest that pyroxenes would be more susceptible to late-stage He exchange with a low (3)He/(4)He source during ascent, presumably the surrounding crust. (c) 2005 Elsevier B.V. All rights reserved.

Crossey, LJ, Fischer TP, Patchett PJ, Karlstrom KE, Hilton DR, Newell DL, Huntoon P, Reynolds AC, de Leeuw GAM.  2006.  Dissected hydrologic system at the Grand Canyon: Interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine. Geology. 34:25-28.   10.1130/g22057.1   AbstractWebsite

Geochemical study of water and gas discharging from the deeply incised aquifer system at the Grand Canyon, Arizona, provides a paradigm for understanding complex groundwater mixing phenomena, and Quaternary travertines deposited from cool springs provide a paleohydrologic record of this mixing. Geochemical data show that springs have marked compositional variability: those associated with active travertine accumulations (deeply derived endogenic waters) are more saline, richer in CO2, and elevated in Sr-87/Sr-86 relative to springs derived dominantly from surface recharge of plateau aquifers (epigenic waters). Endogenic waters and associated travertine are preferentially located along basement-penetrating faults. We propose a model whereby deeply derived fluids are conveyed upward via both magmatism and seismicity. Our model is supported by: (1) gas analyses from spring waters with high He/Ar and He/N-2 and He-3/He-4 ratios indicating the presence of mantle-derived He; (2) large volumes of travertine and CO2-rich gases in springs recording high CO2 fluxes; and (3) Sr-87/Sr-86 in these springs that indicate circulation of waters through Precambrian basement. Geochemical trends are explained by mixing of epigenic waters of the Colorado Plateau aquifers with different endogenic end-member waters in different tectonic sub-provinces. Endogenic waters are volumetrically minor but have significant effects on water chemistry. They are an important and largely unrecognized component of the hydrogeochemistry and neotectonics of the southwestern United States.

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.

Stimac, JA, Goff F, Counce D, Larocque ACL, Hilton DR, Morgenstern U.  2004.  The crater lake and hydrothermal system of Mount Pinatubo, Philippines: evolution in the decade after eruption. Bulletin of Volcanology. 66:149-167.   10.1007/s00445-003-0300-3   AbstractWebsite

The June 1991 eruption of Mount Pinatubo, Philippines breached a significant, pre-eruptive magmatic-hydrothermal system consisting of a hot (>300degreesC) core at two-phase conditions and surrounding, cooler (<260&DEG;C) liquid outflows to the N and S. The eruption created a large, closed crater that accumulated hydrothermal upwellings, near-surface aquifer and meteoric inflows. A shallow lake formed by early September 1991, and showed a long-term increase in level of &SIM;1 m/month until an artificial drainage was created in September 2001. Comparison of the temporal trends in lake chemistry to pre- and post-eruptive springs distinguishes processes important in lake evolution. The lake was initially near-neutral pH and dominated by meteoric influx and Cl-SO4 and Cl-HCO3 hydrothermal waters, with peaks in SO4 and Ca concentrations resulting from leaching of anhydrite and aerosol-laden tephra. Magmatic discharge, acidity (pH&SIM;2) and rock dissolution peaked in late 1992, during and immediately after eruption of a lava dome on the crater floor. Since cessation of dome growth, trends in lake pH (increase from 3 to 5.5), temperature (decline from 40 to 26&DEG;C), and chemical and isotopic composition indicate that magmatic degassing and rock dissolution have declined significantly relative to the input of meteoric water and immature hydrothermal brine. Higher concentrations of Cl, Na, K, Li and B, and lower concentrations of Mg, Ca, Fe, SO4 and F up to 1999 highlight the importance of a dilute hydrothermal contribution, as do stable-isotope and tritium compositions of the various fluids. However, samples taken since that time indicate further dilution and steeper trends of increasing pH and declining temperature. Present gas and brine compositions from crater fumaroles and hot springs indicate boiling of an immature Cl-SO4 geothermal fluid of near-neutral pH at approximately 200&DEG;C, rather than direct discharge from magma. It appears that remnants of the pre-eruptive hydrothermal system invaded the magma conduit shortly after the end of dome emplacement, blocking the direct degassing path. This, along with the large catchment area (&SIM;5 km(2)) and the high precipitation rate of the area, led to a rapid transition from a small and hot acid lake to a large lake with near-ambient temperature and pH. This behavior contrasts with that of peak-activity lakes that have more sustained volcanic gas influx (e.g., Kawah Ijen, Indonesia; Poas and Rincon de la Vieja, Costa Rica).

Jaffe, LA, Hilton DR, Fischer TP, Hartono U.  2004.  Tracing magma sources in an arc-arc collision zone: Helium and carbon isotope and relative abundance systematics of the Sangihe Arc, Indonesia. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000660   AbstractWebsite

[1] The Sangihe Arc is presently colliding with the Halmahera Arc in northeastern Indonesia, forming the world's only extant example of an arc-arc collision zone. We report the first helium and carbon isotopic and relative abundance data from the Sangihe Arc volcanoes as a means to trace magma origins in this complicated tectonic region. Results of this study define a north-south trend in He-3/He-4, CO2/He-3, and delta(13)C, suggesting that there are variations in primary magma source characteristics along the strike of the arc. The northernmost volcanoes (Awu and Karangetang) have higher CO2/He-3 and delta(13)C (up to 179 x 10(9) and -0.4parts per thousand, respectively) and lower He-3/He-4 (similar to5.4 R-A) than the southernmost volcanoes ( Ruang, Lokon, and Mahawu). Resolving the arc CO2 into component structures (mantle-derived, plus slab-derived organic and carbonate CO2), the northern volcanoes contain an unusually high (> 90%) contribution of CO2 derived from isotopically heavy carbonate associated with the subducting slab ( sediment and altered oceanic basement). Furthermore, the overall slab contribution (CO2 of carbonate and organic origin) relative to carbon of mantle wedge origin is significantly enhanced in the northern segment of the arc. These observations may be caused by greater volumes of sediment subduction in the northern arc, along-strike variability in subducted sediment composition, or enhanced slab-derived fluid/melt production resulting from the superheating of the slab as collision progresses southward.

Shaw, AM, Hilton DR, Fischer TP, Walker JA, Alvarado GE.  2003.  Contrasting He-C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones. Earth and Planetary Science Letters. 214:499-513.   10.1016/s0012-821x(03)00401-1   AbstractWebsite

We report He-3/He-4 ratios, relative He, Ne, and CO2 abundances as well as delta(13)C values for volatiles from the volcanic output along the Costa Rica and Nicaragua segments of the Central American arc utilising fumaroles, geothermal wells, water springs and bubbling hot springs. CO2/He-3 ratios are relatively constant throughout Costa Rica (av. 2.1 X 10(10)) and Nicaragua (av. 2.5 X 10(10)) and similar to arcs worldwide (similar to1.5 X 10(10)). delta(13)C values range from -6.8parts per thousand (MORB-like) to -0.1parts per thousand (similar to marine carbonate (0parts per thousand)). He-3/He-4 ratios are essentially MORB-like (8 +/- R-A) with some samples showing evidence of crustal He additions - water spring samples are particularly susceptible to modification. The He-CO2 relationships are consistent with an enhanced input of slab-derived C to magma sources in Nicaragua ((L+S)/M = 16; where L, M and S represent the fraction of CO2 derived from limestone and/or marine carbonate (L), the mantle (M) and sedimentary organic C (S) sources) relative to Costa Rica ((L+S)/M = 10). This is consistent with prior studies showing a higher sedimentary flux to the arc volcanics in Nicaragua (as traced by Ba/La, Be-10 and La/Yb). Possible explanations include: (1) offscraping of the uppermost sediments in the Costa Rica forearc, and (2) a cooler thermal regime in the Nicaragua subduction zone, preserving a higher proportion of melt-inducing fluids to subarc depths, leading to a higher degree of sediment transfer to the subarc mantle. The absolute flux Of CO2 from the Central American arc as determined by correlation spectrometry methods (5.8 X 10(10) mol/yr) and CO2/He-3 ratios (7.1 X 10(10) mol/yr) represents approximately 14-18% of the amount of CO2 input at the trench from the various slab contributors (carbonate sediments, organic C, and altered oceanic crust). Although the absolute flux is comparable to other arcs, the efficiency Of CO2 recycling through the Central American are is surprisingly low (14-18% vs. a global average of similar to50%). This may be attributed to either significant C loss in the forearc region, or incomplete decarbonation of carbonate sediments at subarc depths. The implication of the latter case is that a large fraction of C (up to 86%) may be transferred to the deep mantle (depths beyond the source of arc magmas). (C) 2003 Elsevier B.V. All rights reserved.

Gulec, N, Hilton DR, Mutlu H.  2002.  Helium isotope variations in Turkey: relationship to tectonics, volcanism and recent seismic activities. Chemical Geology. 187:129-142.   10.1016/s0009-2541(02)00015-3   AbstractWebsite

The distribution of helium isotope ratios in the various tectonic provinces of Turkey is examined through a synthesis of previously published data and the results of a recent survey along the North Anatolian Fault Zone (NAFZ) following the catastrophic 1999 earthquakes. The R/R-A values (where R = sample He-3/He-4 and RA = air He-3/He-4) cover a wide range from 0.05 to 7.87, and the mantle-derived helium is clearly identified in most locations, The mantle-derived component is high (> 50% of total He) in (a) regions of central and eastern Anatolia, both of which are associated with historically active volcanoes, and (b) the seismically active west-to-central segment of NAFZ. While the mantle contribution reaches a maximum at Nemrut volcano in eastern Anatolia-a region of dominantly compressional tectonics and with moderate enthalpy geothermal fields, it is relatively low (< 50%) in the western Anatolian extensional province even though this region has the highest geothermal potential. The average He-3/enthalpy ratios estimated for the different provinces suggest lithospheric stretching and rise of the geotherm as the major mechanism of high heat flow, yet limited intrusive activity in western Anatolia is suggested as the reason for the comparatively low He additions and consequently low (1.7 x 10(6) atoms/J) He-3/enthalpy ratios. A more extensive magmatic activity appears to be responsible for the greater input of both heat and helium in eastern and central Anatolia, with intra-province He-3/enthalpy variations (from 3.1 x 10(6) atoms/J in eastern to 0.25 x 10(6) atoms/J in central Anatolia) reflecting the ageing of hydro-magmatic systems. The relatively high He-3 concentrations in low enthalpy waters of northern Anatolia (2.8 x 10(6) atoms/J) are particularly significant since there is no evidence of volcanic activity associated with the strike-slip motion along the seismically active segment of NAFZ. Continuous monitoring of He-isotope compositions along NAFZ should lead to a better understanding of this apparent anomaly, as well as the relationship (if any) between He-isotope variations and seismic activities in this region. (C) 2002 Elsevier Science B.V. All rights reserved.

Van Soest, MC, Hilton DR, Macpherson CG, Mattey DP.  2002.  Resolving sediment subduction and crustal contamination in the Lesser Antilles island Arc: A combined He-O-Sr isotope approach. Journal of Petrology. 43:143-170.   10.1093/petrology/43.1.143   AbstractWebsite

We report an extensive helium isotope survey of basaltic to andesitic lavas from the Lesser Antilles island arc-an arc system with well-documented evidence of crustal contamination. Given the sensitivity of helium isotopes as a tracer of the effects of crustal additions, our aim is to evaluate the relationship of He-3/He-4 ratios to other indices of contamination processes such as oxygen and strontium isotopes. To this end, we have carried out 53 He-3/He-4 analyses on separated minerals (olivines and pyroxenes) from throughout the arc, which we compare with whole-rock strontium and phenocryst oxygen isotope measurements. We show that the three isotopic tracers show coherent patterns throughout the Lesser Antilles, indicating a regional control on crustal contamination. The southern section of the arc (Grenada to Martinique) shows clear evidence for major crustal contamination in all three isotopic systems with results for our samples in the range He-3/He-4((olivine)) 3.6-7.6R(A), delta(18)O((olivine)) 4.74-5.76parts per thousand, and Sr-87/Sr-86((whole-rock)) 0.703970-0.705463. We suggest that terrigenous sediments incorporated into the arc crust are the principal contaminant. In contrast, there is minimal contamination in the northern part of the arc [Martinique-(Dominica)-Guadeloupe to Saba] with results for the samples in the range He-3/He-4((olivine)) 6.8-8.4R(A), delta(18)O((olivine)) 5.01-5.29parts per thousand, and Sr-87/Sr-86((whole-rock)) 0.703221-0.703843. In this part of the arc, only pyroxene He-3/He-4 values (1.4-6.8R(A)) consistently record evidence of relatively minor, late-stage additions from the arc crust. Martinique appears to represent the transition between the two sections of the arc. The tracer with the greatest sensitivity appears to be helium isotopes, as contamination is seen first and most prominently in this system.

Hilton, DR, Gronvold K, Sveinbjornsdottir AE, Hammerschmidt K.  1998.  Helium isotope evidence for off-axis degassing of the Icelandic hotspot. Chemical Geology. 149:173-187.   10.1016/s0009-2541(98)00044-8   AbstractWebsite

We report a helium and carbon survey of 16 hydrothermal localities from Vestfirdir-the northwest peninsula of Iceland. The region is situated similar to 250 km away from the Icelandic rift-axis, and is characterized by low-temperature (< 100 degrees C) hydrothermal activity. A mantle-derived input dominates the helium systematics: nine localities have He-3/He-4 ratios (R/R-A) values between those typical of MORE (8 +/- 1) and 30-the highest value matches the most extreme magmatic ratios reported worldwide to date. Four other localities have R/R-A similar to MORB ratios with the remaining three showing heavy dilution with crustal radiogenic helium (R/R-A < MORB). Carbon isotope analysis of seven of the localities reveals a wide range in delta(13)C(CO2) (-6 to - 16 parts per thousand PDB), and CO2/He-3 ratios vary over 4 orders of magnitude (4 x 10(6) to 2.9 x 10(10)). There appears no systematic relationship between the volatile systematics and geographic distribution or age of reservoir rocks. Stable isotope (delta(13)O and delta D) variations are consistent with a localized meteoric origin for the hydrothermal waters-the carrier phase of the volatiles. We conclude that release of magmatic volatiles in Vestfirdir occurs predominantly via incipient mantle melting with a negligible input of re-distributed volatiles from the rift zones. It is also highly unlikely that leaching of old reservoir rocks contribute to the mantle volatile flux: however, release of radiogenic helium from the uppermost crust does occur and this is accompanied by addition of isotopically light carbon most likely of organic origin. The light carbon input is most discernible when the mantle-derived carbon signal is reduced-most probably by reaction to form calcite, Although Vestfirdir makes a discernible (off-axis) input to the degassing history of the Icelandic hotspot, its contribution is extremely minor(similar to 0.04%) when compared to the degassing flux associated with rifting and volcanism along the Icelandic spreading zones. (C) 1998 Elsevier Science B.V. All rights reserved.