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2018
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.

2012
Troll, VR, Hilton DR, Jolis EM, Chadwick JP, Blythe LS, Deegan FM, Schwarzkopf LM, Zimmer M.  2012.  Crustal CO2 liberation during the 2006 eruption and earthquake events at Merapi volcano, Indonesia. Geophysical Research Letters. 39   10.1029/2012gl051307   AbstractWebsite

High-temperature volcanic gas is widely considered to originate from ascending, mantle-derived magma. In volcanic arc systems, crustal inputs to magmatic gases mainly occur via subducted sediments in the mantle source region. Our data from Merapi volcano, Indonesia imply, however, that during the April-October 2006 eruption significant quantities of CO2 were added from shallow crustal sources. We show that prior to the 2006 events, summit fumarole gas delta C-13((CO2)) is virtually constant (delta C-13(1994-2005) = -4.1 +/- 0.3 parts per thousand), but during the 2006 eruption and after the shallow Yogyakarta earthquake of late May, 2006 (M6.4; hypocentres at 10-15 km depth), carbon isotope ratios increased to -2.4 +/- 0.2 parts per thousand. This rise in delta C-13 is consistent with considerable addition of crustal CO2 and coincided with an increase in eruptive intensity by a factor of similar to 3 to 5. We postulate that this shallow crustal volatile input supplemented the mantle-derived volatile flux at Merapi, intensifying and sustaining the 2006 eruption. Late-stage volatile additions from crustal contamination may thus provide a trigger for explosive eruptions independently of conventional magmatic processes. Citation: Troll, V.R., D.R. Hilton, E.M. Jolis, J.P. Chadwick, L.S. Blythe, F.M. Deegan, L.M. Schwarzkopf, and M. Zimmer (2012), Crustal CO2 liberation during the 2006 eruption and earthquake events at Merapi volcano, Indonesia, Geophys. Res. Lett., 39, L11302, doi: 10.1029/2012GL051307.

Barry, PH, Hilton DR, Halldorsson SA, Hahm D, Marti K.  2012.  High precision nitrogen isotope measurements in oceanic basalts using a static triple collection noble gas mass spectrometer. Geochemistry Geophysics Geosystems. 13   10.1029/2011gc003878   AbstractWebsite

We describe a new system for the simultaneous static triple-collection of nitrogen isotopes at the < 10 mu cm(3) STP [N-2] (< 1 x 10(-5) cm(3) STP; < 0.5 nmol) level using a modified VG-5440 noble gas mass spectrometer. The system consists of an internal N-2-STD with a delta N-15 value of -0.11 +/- 0.22 parts per thousand (1 sigma) calibrated against an air-standard (Air-STD). The N-2-STD was measured repeatedly with an average uncertainty on an individual delta N-15 measurement being 0.03 parts per thousand (1 sigma) versus an average single day reproducibility of 0.38 parts per thousand (1 sigma). Additional refinements include (1) monitoring of interfering CO contributions at mass 30, allowing a comprehensive CO correction to be applied to all samples, (2) quantification of procedural N-2 blanks (n = 22) in both size (4.2 +/- 0.5 mu cm(3) STP) and isotopic composition (delta N-15 = 12.64 +/- 2.04 parts per thousand), allowing consistent blank corrections to all samples, and (3) independent measurement of N-2/Ar ratios using a quadrupole mass spectrometer (QMS). The new system was tested by measuring nitrogen isotopes (delta N-15), concentrations and N-2/Ar ratios on 11 submarine basalt glasses. Results show that the uncertainty on the delta N-15 data is improved as a consequence of multiple standards being run per day. Reduced analytical times, afforded by triple collection, also minimize sample depletion and memory effects, thus improving measurement statistics. Additionally, we show that CO corrections can be accomplished using mass 30 to monitor CO interferences, leading to substantial improvements in reproducibility and the overall accuracy of results when the contribution of CO is significant.

2011
Tian, LY, Castillo PR, Lonsdale PF, Hahm D, Hilton DR.  2011.  Petrology and Sr-Nd-Pb-He isotope geochemistry of postspreading lavas on fossil spreading axes off Baja California Sur, Mexico. Geochemistry Geophysics Geosystems. 12   10.1029/2010gc003319   AbstractWebsite

Postspreading volcanism has built large seamounts and volcanic ridges along the short axes of a highly segmented part of the East Pacific Rise crest that ceased spreading at the end of the middle Miocene, offshore Baja California Sur, Mexico. Lava samples from Rosa Seamount, the largest volcano, are predominantly alkalic basalts, mugearites, and benmoreites. This lavas series was generated through fractional crystallization and is compositionally similar to the moderately alkalic lava series in many oceanic islands. Samples from volcanic ridges at three adjacent failed spreading axes include mildly alkalic, transitional, and tholeiitic basalts and differentiated trachyandesites and andesite. The subtle but distinct petrologic and isotopic differences among the four sites may be due to differences in the degree of partial melting of a common, heterogeneous source. Postspreading lavas from these four abandoned axes off Baja California Sur together with those from other fossil spreading axes and from seamount volcanoes that grew on the East Pacific Rise flanks define a compositional continuum ranging from normal mid-ocean ridge basalt (NMORB)-like to ocean island basalt (OIB)-like. We propose that the compositional spectrum of these intraplate volcanic lavas is due to different degrees of partial melting of the compositionally heterogeneous suboceanic mantle in the eastern Pacific. A large degree of partial melting of this heterogeneous mantle during vigorous mantle upwelling at an active spreading center produces NMORB melts, whereas a lesser degree of partial melting during weak mantle upwelling following cessation of spreading produces OIB-like melts. The latter melts have a low (<8 R(A)) (3)He/(4)He signature indicating their formation is different from that of OIBs from major "hot spot" volcanoes in the Pacific with high (3)He/(4)He ratios, such as Hawaii and Galapagos.

2010
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.

2009
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.

2008
Shaw, AM, Hauri EH, Fischer TP, Hilton DR, Kelley KA.  2008.  Hydrogen isotopes in Mariana arc melt inclusions: Implications for subduction dehydration and the deep-Earth water cycle. Earth and Planetary Science Letters. 275:138-145.   10.1016/j.epsl.2008.08.015   AbstractWebsite

Water is carried into Earth's mantle at subduction zones via hydrous mineral phases in subducting lithospheric plates. A certain fraction of this water is released during subduction and leads to the generation of arc volcanism. The extent of lithosphere dehydration and associated hydrogen isotope fractionation are critical to understanding the global water cycle. To further understand the origin of subduction-related fluids and how water is exchanged between Earth's reservoirs, we have analyzed volatiles and delta D of magmatic melt inclusions from the Mariana arc. We find high delta D values, ranging from -55 parts per thousand to -12 parts per thousand, indicating release of D-enriched fluids from the subducting plate into the mantle wedge. A consequence of this process is the formation of complementary hydrous mantle components that are D-enriched (mantle wedge) and D-depleted (slab). This implies that ocean island basalts (OIB) with recycled slab components should be characterized by low delta D, while high delta D signatures are expected from OIB containing recycled mantle wedge peridotite. These results have important implications for the hydrogen isotope evolution of terrestrial H2O reservoirs. Based on the magnitude of subduction-related fractionations, we show that terrestrial delta D variations are inconsistent with steady-state exchange of water between the mantle and surface reservoirs. We suggest that the subduction process is responsible for the present-day delta D difference observed between Earth's major water reservoirs. (c) 2008 Elsevier B.V. All rights reserved.

Mutlu, H, Gulec N, Hilton DR.  2008.  Helium-carbon relationships in geothermal fluids of western Anatolia, Turkey. Chemical Geology. 247:305-321.   10.1016/j.chemgeo.2007.10.021   AbstractWebsite

We investigate the helium, carbon and oxygen-hydrogen isotopic systematics and CO(2)/(3)He ratios of 8 water and 6 gas samples collected from 12 geothermal fields in western Anatolia (Turkey). (3)He/(4)He ratios of the samples (R) normalized to the atmospheric (3)He/(4)He ratio (R(A) = 1.39 x 10(-6)) range from 0.27 to 1.67 and are significantly higher than the crustal production value of 0.05. Fluids with relatively high R/R(A) values are generally found in areas of significant heat potential (Kizildere and Tuzla fields). CO(2)/(3)He ratios of the samples, ranging from 1.6 x 10(9) to 2.3 x 10(14), display significant variation and are mostly higher than values typical of an upper mantle source (2 X 109). The delta(13)C (CO(2)) and delta(13)C (CH(4)) values of all fluids vary from -8.04 to +0.35 parts per thousand and -25.80 to -23.92 parts per thousand (vs. PDB), respectively. Stable isotope values (delta(18)O-delta D) of the geothermal waters are conformable with the Mediterranean Meteoric Water Line and indicate a meteoric origin. The temperatures calculated by gas geothermometry are significantly higher than estimates from chemical geothermometers, implying that either equilibrium has not been attained for the isotope exchange reaction or that isotopic equilibration was disturbed due to gas additions en route to the surface. Evaluation of He-CO(2) abundances indicates that hydrothermal degassing and calcite precipitation (controlled probably by adiabatic cooling due to degassing) significantly fractionate the elemental ratio (CO(2)/(3)He) in geothermal waters. Such processes do not affect gas phase samples to anywhere near the same extent. For the gas samples, mixing between mantle and various crustal sources appears to be the main control on the observed He-C systematics: however, crustal inputs dominate the CO(2) inventory. Considering that limestone is the main source of carbon (similar to 70 to 97% of the total carbon inventory), the carbon flux from the crust is found to be at least 20 times that from the mantle. As to the He-inventory, the mantle-derived component is found to vary up to 21% of the total He content and is probably transferred to the crust by fluids degassed from deep mantle melts generated in association with the elevated geotherm and adiabatic melting accompanying current extension. The range of (3)He/enthalpy ratios (0.000032 to 0.19 x 10(-12) cm(3) STP/J) of fluids in western Anatolia is consistent with the release of both helium and heat from contemporary additions of mantle-derived magmas to the crust. The deep faults appear to have facilitated the deep circulation of the fluids and the transport of mantle volatiles and heat to the surface. (C) 2007 Elsevier B.V. All rights reserved.

2007
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.

2006
Elkins, LJ, Fischer TP, Hilton DR, Sharp ZD, McKnight S, Walker J.  2006.  Tracing nitrogen in volcanic and geothermal volatiles from the Nicaraguan volcanic front. Geochimica Et Cosmochimica Acta. 70:5215-5235.   10.1016/j.gca.2006.07.024   AbstractWebsite

We report new chemical and isotopic data from 26 volcanic and geothermal gases, vapor condensates, and thermal water samples, collected along the Nicaraguan volcanic front. The samples were analyzed for chemical abundances and stable isotope compositions, with a focus on nitrogen abundances and isotope ratios. These data are used to evaluate samples for volatile contributions from magma, air, air-saturated water, and the crust. Samples devoid of crustal contamination (based upon He isotope composition) but slightly contaminated by air or air-saturated water are corrected using N(2)/Ar ratios in order to obtain primary magmatic values, composed of contributions from upper mantle and subducted hernipelagic sediment on the down-going plate. Using a mantle endmember with delta(15)N = -5 parts per thousand and N(2)/He = 100 and a subducted sediment component with delta(15)N = +7 parts per thousand and N(2)/He = 10,500, the average sediment contribution to Nicaraguan volcanic and geothermal gases was determined to be 71%. Most of the gases were dominated by sediment-derived nitrogen, but gas from Volcan Mombacho, the southernmost sampling location, had a mantle signature (46% from subducted sediment, or 54% from the mantle) and an affinity with mantle-dominated gases discharging from Costa Rica localities to the south. High CO(2)/N(2exc.) ratios (N(2 exc.) is the N(2) abundance corrected for contributions from air) in the south are similar to those in Costa Rica, and reflect the predominant mantle wedge input, whereas low ratios in the north indicate contribution by altered oceanic crust and/or preferential release of nitrogen over carbon from the subducting slab. Sediment-derived nitrogen fluxes at the Nicaraguan volcanic front, estimated by three methods, are 7.8 x 10(8) mol N/a from (3)He flux, 6.9 x 10(8) mol/a from SO(2) flux, and 2.1 x 10(8) and 1.3 x 10(9) mol/a from CO(2) fluxes calculated from (3)He and SO(2), respectively. These flux results are higher than previous estimates for Central America, reflecting the high sediment-derived volatile contribution and the high nitrogen content of geothermal and volcanic gases in Nicaragua. The fluxes are also similar to but higher than estimated hernipelagic nitrogen inputs at the trench, suggesting addition of N from altered oceanic basement is needed to satisfy these flux estimates. The similarity of the calculated input of N via the trench to our calculated outputs suggests that little or none of the subducted nitrogen is being recycled into the deeper mantle, and that it is, instead, returned to the surface via arc volcanism. (c) 2006 Elsevier Inc. All rights reserved.

Gulec, N, Hilton DR.  2006.  Helium and heat distribution in western Anatolia, Turkey; relationship to active extension and volcanism. Special Paper Geological Society of America. 409:305-319.   10.1130/2006.2409(16)   Abstract

Western Anatolia, one of the world's best-known extensional terrains, is characterized by the presence of several moderate- to high-enthalpy geothermal fields. Geo-thermal fluids have helium isotope compositions reflecting mixing between mantle and crustal helium components, the former ranging between 0.58% and 45% of the total helium in a given sample. Regarding the distribution of heat and mantle He and their correlation with tectonic structure and volcanism in western Anatolia, the prominent features are as follows: (1) the association between highest heat and highest (super 3) He lies along the eastern segment of the Buyuk Menderes graben, (2) the high heat and high (super 3) He occur in the vicinity of the Quaternary Kula volcanism, (3) high-enthalpy fields exist in close vicinity to the young alkaline volcanics, (4) relatively high mantle He contributions occur in areas of not only the young alkaline, but also the old calc-alkaline volcanics, and (5) there is a lack of volcanic exposures along the Buyuk Menderes graben (except at its western and southeastern terminations), where the highest values are recorded for both heat and helium. The first three features collectively suggest that the transfer mechanism for both heat and helium is probably mantle melting accompanying the current extension in western Anatolia, yet the latter two further indicate that this may be accomplished via subsurface plutonic activities. The large range observed in the helium isotope compositions may be linked with differential (local) extension rates and associated melt generation in the respective areas. This suggestion can be substantiated by He isotope data from more of the region.

2005
Day, JMD, Hilton DR, Pearson DG, Macpherson CG, Kjarsgaard BA, Janney PE.  2005.  Absence of a high time-integrated He-3/(U+Th) source in the mantle beneath continents. Geology. 33:733-736.   10.1130/g21625.1   AbstractWebsite

Volcanic rocks from ocean island and continental flood basalt provinces can exhibit He-3/He-4 ratios greatly in excess of those of mid-oceanic-ridge basalts (MORB). High He-3/He-4 ratios must indicate derivation from a mantle source with high time-integrated He-3/(U+Th) relative to depleted MORB-source mantle. The location of the high He-3/He-4 mantle reservoir is a poorly resolved but important issue because of the constraints it places upon the structure and convective style of Earth's mantle. It has been proposed that the high He-3/He-4 reservoir resides in the upper mantle, rather than the lower mantle, because Earth should be volatile poor and highly differentiated, with incompatible elements (such as He) concentrated in the upper mantle and crust. This hypothesis can be tested using continental intraplate alkaline volcanics (CIAV) that are generated at or near the boundary between the conducting lithospheric and convecting asthenospheric mantle. Olivine and clinopyroxene phenocrysts from Cretaceous to Miocene CIAV from Canada, South Africa, and Uganda have He-3/He-4 ratios more radiogenic than MORB, strongly arguing against a widespread high He-3/He-4 source in the continental lithosphere or the underlying convecting upper mantle. Combined with a global data set of CIAV and continental lithosphere mantle xenoliths, these results provide no evidence for high He-3/He-4 in any samples known to originate from this environment. Therefore, volcanic rocks with He-3/He-4 greater than MORB He-3/He-4 are likely to sample a mantle source with high time-integrated He-3/(U+Th) that cannot exist within or below the continents. This reservoir is also unlikely to exist within the upper mantle as defined by the He-3/He-4 distribution in MORB.

Macpherson, CG, Hilton DR, Day JMD, Lowry D, Gronvold K.  2005.  High-He-3/He-4, depleted mantle and low-delta O-18, recycled oceanic lithosphere in the source of central Iceland magmatism. Earth and Planetary Science Letters. 233:411-427.   10.1016/j.epsl.2005.02.037   AbstractWebsite

New helium and oxygen isotope data and trace element concentrations are reported for volcanic rocks from central Iceland. Basalts that are depleted in the most incompatible trace elements possess a wide range in He-3/He-4 but most ratios are similar to or higher than those of mid-ocean ridge basalt (MORB:similar to 8R(A)[1] [D.W. Graham, Noble gas geochemistry of mid-ocean ridge and ocean island basalts: characterisation of mantle source reservoirs, in: D.P. Porcelli, C.J. Ballentine, R. Wieler (Eds.), Noble gases in Geochemistry and Cosmochemistry, Rev. Mineral. Geochem., vol. 47, 2002, pp. 247-317]). The low concentrations of helium in these rocks suggest that significant degassing has made them susceptible to contamination by low-He-3/He-4 crust, therefore all measured He-3/He-4 are considered minimum estimates for their sources. Elevated helium isotope ratios in the source of these rocks result from interaction with high-He-3/He-4 mantle. The highest oxygen isotope ratios in the depleted rocks are similar to those in melts from typical depleted upper mantle and the range of delta(18)O values is consistent with variable, limited amounts of contamination by Icelandic crust. Most of the incompatible trace element-enriched rocks possess He-3/He-4 ratios that are similar to or lower than those in MORB. These rocks were erupted close to the postulated centre of the Iceland plume. This observation contradicts models in which high-He-3/He-4 characterises the focus of mantle upwelling. A source with MORB-like He-3/He-4 ratios may also be common to other parts of the North Atlantic Igneous Province. The highest delta(18)O values in the enriched rocks are lower than those in MORB and do not appear to have been affected by interaction with low-delta(18)O Icelandic crust. Recycling of hydrothermally altered oceanic crust that has been subducted into the mantle provides a plausible mechanism for generating an O-18-poor source with the trace element and isotopic characteristics of the enriched lavas. (C) 2005 Elsevier B.V All rights reserved.

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.

2004
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.

2003
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.

2002
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.

Fischer, TP, Hilton DR, Zimmer MM, Shaw AM, Sharp ZD, Walker JA.  2002.  Subduction and recycling of nitrogen along the central American margin. Science. 297:1154-1157.   10.1126/science.1073995   AbstractWebsite

We report N and He isotopic and relative abundance characteristics of volatiles emitted from two segments of the Central American volcanic arc. In Guatemala, delta(15)N values are positive (i.e., greater than air) and N-2/He ratios are high (up to 25,000). In contrast, Costa Rican N-2/He ratios are low ( maximum 1483) and delta(15)N values are negative ( minimum -3.0 per mil). The results identify shallow hemipelagic sediments, subducted into the Guatemalan mantle, as the transport medium for the heavy N. Mass balance arguments indicate that the subducted N is efficiently cycled to the atmosphere by arc volcanism. Therefore, the subduction zone acts as a barrier to input of sedimentary N to the deeper mantle.

2001
Hulston, JR, Hilton DR, Kaplan IR.  2001.  Helium and carbon isotope systematics of natural gases from Taranaki Basin, New Zealand. Applied Geochemistry. 16:419-436.   10.1016/s0883-2927(00)00045-7   AbstractWebsite

The chemical and isotopic compositions of gases from hydrocarbon systems of the Taranaki Basin of New Zealand (both offshore and onshore) show wide variation. The most striking difference between the western and south-eastern groups of gases is the helium content and its isotopic ratio. In the west, the Maul gas is over an order of magnitude higher in helium concentration (up to 190 mu mol mol(-1)) and its He-3/He-4 ratio of 3.8 R-A (where R-A = the air He-3/He-4 ratio of 1.4 x 10(-6)) is approximately half that of upper mantle helium issuing from volcanic Vents of the Taupo Volcanic Zone. In the SE, the Kupe South and most Kapuni natural gases have only a minor mantle helium input of 0.03-0.32 RA and low total helium concentrations of 10-19 mu mol mol(-1). The He-3/C ratio (where C represents the total carbon in the gas phase) of the samples measured including those from a recent study of on-shore Taranaki natural gases are generally high at locations where the surface heat flow is high. The He-3/CO2 ratio of the Maui gases of 5 to 18 x 10(-9) is higher than the MORB value of 0.2 to 0.5 x 10(-9), a feature found in other continental basins such as the Pannonian and Vienna basins and in many high helium wells in the USA. Extrapolation to zero CO2/He-3 and CO2/C indicates delta C-13(CO2) values between -7 and -5 parts per thousand close to that of MORB CO2. The remaining CO2 would appear to be mostly organically-influenced with delta C-13(CO2) c.-15 parts per thousand. There is some evidence of marine carbonate CO2 in the gases from the New Plymouth held. The radiogenic He-4 content (He-rad) varies across the Taranaki Basin with the highest He-rad/C ratios occurring in the Maul field. delta C-13(CH4) becomes more enriched in C-13 with increasing He-rad and hydrocarbon maturity. Because 3He/4He is related to the ratio of mantle to radiogenic crustal helium and He-3/C is virtually constant in the Maul held, there is a correlation between R-C/R-A (where R-C = air-corrected He-3/He-4) and delta C-13(CH4) in the Maui and New Plymouth fields, with the more negative delta C-13(CH4) values corresponding to high 3He/4He ratios. A correlation between He-3/He-4 and delta C-13(CO2) was also observed in the Maul field. In the fields adjacent to Mt Taranaki (2518 m andesitic volcano), correlations of some parameters, particularly CO2/CH4, C2H6/CH4 and delta C-13(CH4), are present with increasing depth of the gas reservoir and with distance from the volcanic cone. (C) 2001 Elsevier Science Ltd. All rights reserved.

2000
Hilton, DR, Thirlwall MF, Taylor RN, Murton BJ, Nichols A.  2000.  Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox. Earth and Planetary Science Letters. 183:43-50.   10.1016/s0012-821x(00)00253-3   AbstractWebsite

To consider the He-3 characteristics of plume-related lavas, we report a detailed survey of helium isotope (He-3/He-4) and concentration ([He]) variations along an 800-km transect of the Reykjanes Ridge (RR). He-3/He-4 ratios vary from 11.0 to 17.6 R-A (where R-A = air He-3/He-4) whereas [He] ranges over three orders of magnitude from > 5 mu cm(3) STP/g-in the range of most mid-ocean ridge basalts (MORB) - to lows of 4 ncm(3) STP/g. The lowest [He] and intermediate He-3/He-4 ratios occur along the northern RR (closest to Iceland) where eruption depths are shallow (<1000 m) and water contents of lavas are high (0.3-0.4 wt%). We suggest that low-pressure, pre-eruptive magmatic degassing is extensive in this region with degassed magmas susceptible to addition of radiogenic helium thereby lowering He-3/He-4 ratios. Along the southern RR, [He] reaches maximum values, and He-3/He-4 ratios display strong correlations with lead isotopes (Pb-206/Pb-204) consistent with binary mixing. These correlations indicate that the high-He-3/He-4 plume component has higher absolute abundances of the primordial isotope He-3 compared to the source of depleted MORB mantle. This finding implies that the so-called 'helium paradox' - the observation that plume-derived oceanic glasses apparently have lower He-3 contents than MORB glasses - may be an artifact related to considering lavas (e.g. from Loihi seamount, Hawaii) which have not retained their source volatile inventory as well as those erupted along the southern RR. (C) 2000 Elsevier Science B.V. All rights reserved.

Macpherson, CG, Hilton DR, Mattey DP, Sinton JM.  2000.  Evidence for an O-18-depleted mantle plume from contrasting O-18/O-16 ratios of back-arc lavas from the Manus Basin and Mariana Trough. Earth and Planetary Science Letters. 176:171-183.   10.1016/s0012-821x(00)00002-9   AbstractWebsite

Back-are basin glasses from the Mariana Trough and Manus Basin display contrasting oxygen isotope characteristics that require differences in their mantle sources. In both basins, the lavas that are most depleted in high field strength elements possess delta(18)O values of around 6.0 parts per thousand, that are elevated with respect to mid-ocean ridge basalt (MORB). This characteristic is consistent with a mantle source that has been infiltrated by fluids released from subducted oceanic lithosphere. The nature of the more fertile mantle component differs between the two basins, The lowest delta(18)O values in the Mariana Trough are similar to MORB and suggest that the ambient upper mantle interacts with a subduction-modified mantle to produce Mariana Trough back-arc basin basalts. Oxygen isotope ratios of basaltic glasses from the Manus Basin display a negative correlation with helium isotope ratios. The subduction-modified component is associated with He-3/He-4 ratios typical of the upper mantle, Glasses with He-3/He-4 ratios greater than average MORB, characteristic of a deep mantle plume, have delta(18)O values that are lower than expected for upper mantle melts. This suggests that the Manus Basin plume taps a reservoir that is O-18-depleted relative to the source of MORB. Two mechanisms are identified that might generate this reservoir. Deep recycling of oceanic crust that has been hydrothermally altered at high temperature may provide large O-18-depleted domains in the deep mantle. Alternatively, we propose that interactions between silicate and iron alloy during the segregation of the Earth's core may have the potential to generate such reservoirs. Resolution between these mechanisms:requires further experimental investigation of oxygen partitioning between silicates and iron alloy. Each of these mechanisms has distinct implications for the origins and dynamics of the Manus Basin plume. (C) 2000 Elsevier Science; B.V, All rights reserved.

1999
Hilton, DR, Gronvold K, Macpherson CG, Castillo PR.  1999.  Extreme He-3/He-4 ratios in northwest Iceland: constraining the common component in mantle plumes. Earth and Planetary Science Letters. 173:53-60.   10.1016/s0012-821x(99)00215-0   AbstractWebsite

Olivine and clinopyroxene phenocrysts contained in late Tertiary basalts from Selardalur, northwest Iceland, carry volatiles with the highest helium isotope ratio yet reported for any mantle plume. He-3/He-4 ratios measured on three different samples and extracted by stepped crushing in vacuo fall consistently similar to 37 R-A (R-A = air He-3/He-4) - significantly higher than previously reported values for Iceland or Loihi Seamount (see K.A. Farley, E. Neroda [Annu. Rev. Earth Planet. Sci. 26 (1998) 189-218]). The Sr, Nd and Ph isotopic composition of the same sample places it towards the center of the mantle tetrahedron of Hart et al. (S.R. Hart, E.H. Hauri, L.A. Oschmann, J.A. Whitehead [Science 256 (1992) 517-520]) - in exactly the region predicted for the common mantle endmember based on the convergence of a number of pseudo-linear arrays of ocean island basalts worldwide (E.H. Hauri, J.A. Whitehead, S.R. Hart [J. Geophys. Res. 99 (1994) 24275-24300]). This observation implies that Selardalur may represent the best estimate available to date of the He-Sr-Nd-Pb isotopic composition of the 5th mantle component common to many mantle plumes. (C) 1999 Elsevier Science B.V. All rights reserved.

Macpherson, CG, Hilton DR, Newman S, Mattey DP.  1999.  CO2, C-13/C-12 and H2O variability in natural basaltic glasses: A study comparing stepped heating and FTIR spectroscopic techniques. Geochimica Et Cosmochimica Acta. 63:1805-1813.   10.1016/s0016-7037(99)00124-6   AbstractWebsite

A comparison of two independent techniques was used to assess the homogeneity of CO2 and H2O concentrations in a number of natural basaltic glasses. Variations in carbon concentration and isotopic ratio were determined by comparison of stepped heating data obtained in two different laboratories. Dissolved volatile concentrations were also obtained by stepped heating and Fourier Transform Infrared (FTIR) spectroscopy. Replicate stepped heating analyses of a mid-ocean ridge basaltic glass show that the concentration and C-13/C-12 of bulk magmatic and dissolved CO2 vary by less than +/-10% and +/-0.5 parts per thousand, respectively. A similar degree of correlation is observed for replicate stepped heating analyses of Mariana Trough glasses conducted in two different laboratories. Dissolved CO2 concentrations determined by stepped heating also correlate well with concentrations measured by FTIR spectroscopy. The correspondence of results obtained in these experiments provide an upper limit to the degree of natural variation in concentrations and isotopic ratios of these volatiles in basaltic glasses and suggest that intrinsic, magmatic carbon has a relatively homogeneous distribution in these glasses. Water concentrations determined through extraction by heating and FTIR also show excellent agreement. Copyright (C) 1999 Elsevier Science Ltd.

1994
Gasparon, M, Hilton DR, Varne R.  1994.  Crustal Contamination Processes Traced by Helium-Isotoped - Examples from the Sunda Arc, Indonesia. Earth and Planetary Science Letters. 126:15-22.   10.1016/0012-821x(94)90239-9   AbstractWebsite

Helium isotope data have been obtained on well-characterised olivine and clinopyroxene phenocrysts and xenocrysts from thirteen volcanic centres located between central Sumatra and Sumbawa in the Sunda arc of Indonesia. Olivine crystals in mantle xenoliths (lherzolite) from Bukit Telor basalts are primitive (Mg# = 90), and their He-3/He-4 value (R/R(A) = 8.8) indicates that the Sumatran mantle wedge is MORB-like in helium isotope composition. All other samples have lower He-3/He-4 ratios ranging from 8.5R(A) to 4.5R(A), with most (thirteen out of eighteen) following a trend of more radiogenic He-3/He-4 values with decreasing Mg#. The only exceptions to this trend are phenocrysts from Batur, Agung and Kerinci, which have MORB-like He-3/He-4 values but relatively low Mg# (Mg# = 70-71), and two highly inclusion-rich clinopyroxenes which have He-3/He-4 values lower than other samples of similar Mg#. The results indicate that crustal contamination unrelated to subduction in the Sunda arc is clearly recorded in the He-3/He-4 characteristics of mafic phenocrysts of subaerial volcanics, and that addition of radiogenic helium is related to low-pressure differentiation processes affecting the melts prior to eruption. These conclusions may have widespread applicability and indicate that helium isotope variations can act as an extremely sensitive tracer of upper crustal contamination.