Publications

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1993
Jenden, PD, Hilton DR, Kaplan IR, Craig H.  1993.  Abiogenic hydrocarbons and mantle helium in oil and gas fields. U. S. Geological Survey Professional Paper, Report: P 1570. ( Howell DG, Wiese K, Fanelli M, Zink LL, Cole F, Eds.).:31-56., Reston: U. S. Geological Survey Abstract
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1997
Taylor, RN, Thirlwall MF, Murton BJ, Hilton DR, Gee MAM.  1997.  Isotopic constraints on the influence of the Icelandic plume. Earth and Planetary Science Letters. 148:E1-E8.   10.1016/s0012-821x(97)00038-1   AbstractWebsite

Thermally buoyant mantle, in the form of a plume, rises beneath Iceland creating a major topographic anomaly along the Mid-Atlantic Ridge and in the surrounding ocean basin. However, the influence of the Iceland plume on the composition of lavas erupted on adjacent ridges remains a contentious issue. Trace element systematics and radiogenic isotope ratios of Sr, Nd and Pb suggest that the plume influences a region 1200 km in length. In contrast, the He-3 anomaly associated with Iceland closely corresponds to the 2400 km ridge section affected by thermal uplift. We present evidence that the Sr, Nd and Pb isotope signature of the Iceland plume is in fact as widespread as its thermal and He-3 anomalies. Results imply that much of the source of North Atlantic ridge basalts has been contaminated by lateral outflow of asthenosphere from the Icelandic plume. Consequently, estimates of the average composition of mid-ocean ridge basalt (MORB) sources are likely to be biased by including data from plume-contaminated regions. True MORB values, and perhaps upper mantle geochemistry, can be constrained only by considering data untainted by plume asthenosphere.

1998
Macpherson, CG, Hilton DR, Sinton JM, Poreda RJ, Craig H.  1998.  High He-3/He-4 ratios in the Manus backarc basin: Implications for mantle mixing and the origin of plumes in the western Pacific Ocean. Geology. 26:1007-1010.   10.1130/0091-7613(1998)026<1007:hhhrit>2.3.co;2   AbstractWebsite

Helium isotope ratios in oceanic glasses provide a high-integrity tracer of contributions from mantle plumes. Despite a diverse array of petrogenetic affinities, glasses from the central part of the Manus Basin-a backarc basin in the western Pacific-have typical plume (or hotspot) He-3/He-4 ratios that cluster around 12.2R(A) (+/-1.0R(A), n = 18, where R-A = He-3/He-4 of air), a value significantly higher than the range found in most mid-ocean-ridge basalts (MORB) ([8 +/- 1]R-A). Lavas in other parts of the basin have MORE-like or lower He-3/He-4 values. A wide range of He concentrations characterizes the Manus Basin glasses: This is considered to reflect the high water content of some lavas, which promotes He loss through volatile degassing, For the most part, it is the degassed lavas that do not show the plume He isotope signature. Results of the present study, together with He-3/He-4 data for lavas and gases from islands to the south and east of the Bismark Sea, indicate that the focus of mantle plume upwelling is either the center of the Manus Basin or possibly the region to the northwest beneath the volcanic islands of the St. Andrew Strait, This region of plume or hotspot He-3/He-4 ratios coincides with a domain of anomalously low seismic velocities at the underlying core-mantle boundary, and indicates that the provenance of high-He-3/He-4 magmas in the Manus Basin land possibly elsewhere) is linked to this boundary layer-either by plume entrainment of lower mantle or, more speculatively, through addition of material from the core-mantle boundary.

Hilton, DR, McMurtry GM, Goff F.  1998.  Large variations in vent fluid CO2/He-3 ratios signal rapid changes in magma chemistry at Loihi seamount, Hawaii. Nature. 396:359-362.   10.1038/24603   AbstractWebsite

Loihi seamount, an active submarine volcano situated about 30 km south of the island of Hawaii, is the youngest manifestation of the hotspot responsible for the Emperor-Hawaiian seamount chain and Hawaiian islands. This seamount has been the focus of numerous studies characterizing the geophysical, geochemical and biological features of an active intraplate volcano(1-14). In July-August 1996, Loihi seamount experienced the most intense period of seismic activity yet recorded for any Hawaiian volcano(1). Within two months of the 'seismic crisis: summit and flank hydrothermal vent fluids were collected using a manned submersible. Here we report data from these samples that indicate large and systematic changes in the CO2/He-3 ratios of the vent fluids compared to pre-seismic-crisis values(2,3). These changes are consistent with an abrupt transition from alkalic to tholeiitic basaltic magma having supplied volatiles to the vents. This rapid change in magma Chemistry has been discernible only through CO2/He-3 monitoring, and suggests that the anticipated evolution of the Hawaiian plume to a phase of shield-building tholeiitic magmatism is highly episodic at Loihi and not yet complete.

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.

2000
Hoke, L, Lamb S, Hilton DR, Poreda RJ.  2000.  Southern limit of mantle-derived geothermal helium emissions in Tibet: implications for lithospheric structure. Earth and Planetary Science Letters. 180:297-308.   10.1016/s0012-821x(00)00174-6   AbstractWebsite

The isotopic composition of helium emitted from geothermal springs in the southern Tibetan plateau, reported as R-c/R-A (R-c = air corrected sample He-3/He-4, R-A = air He-3/He-4), ranges from 0.013 to 0.38, and defines two principal domains. In southernmost central Tibet, helium isotope ratios are typical of radiogenic helium production in the crust (R-c/R-A < 0.05, crustal helium domain). Further north, there is a resolvable He-3 anomaly consistent with a mantle contribution (R/R-A > 0.1, mantle helium domain). The highest values of 0.27-0.38 R-A occur at the southern end of the Karakoram fault. The boundary between the two domains lies 50-100 km north of the Indus-Zangpo suture zone. There seems to be no association between the He-3 anomaly and zones of active normal faulting and litho-tectonic crustal units, such as the ultramafic rocks of the Indus-Zangpo suture zone and the Gangdese intrusive belt. Although scavenging of mantle-derived helium, stored in large ultrabasic and basic intrusions in the crust, cannot be ruled out entirely, we argue that the He-3 anomaly most plausibly reflects degassing of volatiles from young (Quaternary) mantle-derived melts intruded into the crust. As such, it defines the southern limit of recent mantle melting and mantle melt extraction beneath the Tibetan plateau. The southern limit of the He-3 anomaly coincides with the junction between the Indian and Asian plates, in the region where the Indian lithospheric slab steepens and is subducted beneath Tibet as suggested by seismic studies. Recent mantle melting and melt extraction is confined to the Asian mantle, but the southern limit of the melt zone may have migrated northwards during the last 10 Ma as the Indian lithosphere has progressively underthrust the Himalayas and Tibet. (C) 2000 Elsevier Science B.V. All rights reserved.

2001
Shaw, AM, Hilton DR, Macpherson CG, Sinton JM.  2001.  Nucleogenic neon in high He-3/He-4 lavas from the Manus back-arc basin: a new perspective on He-Ne decoupling. Earth and Planetary Science Letters. 194:53-66.   10.1016/s0012-821x(01)00539-8   AbstractWebsite

We report new neon isotope data obtained for well-characterised basaltic glasses from the Manus back-are basin where helium studies have identified a mantle plume component (mean He-3/He-4 similar to 12 R-A). In three-isotope neon space. seven of the Manus samples lie along a trajectory between air and an endmember more nucleogenic than MORB i.e., compared to typical MORB, samples have a higher Ne-21/Ne-22 ratio for a given Ne-20/Ne-22 ratio. Thus the slope of the Manus Basin line is less than that of the MORB line [Sarda et al., Earth Planet. Sci. Lett. 91 (1988) 73-88]. This is the first observation of lavas with high He-3/He-4 ratios having nucleogenic neon isotope systematics, indicating a unique decoupling of lie from Ne. We evaluate five possible explanations for the observed trend. We discount: (1) crustal contamination, (2) devolatisation of the subducting Solomon Sea plate and (3) addition of neon from an ancient recycled slab component - based upon mass balance considerations of the availability of nucleogenic Ne. Two possibilities remain - both of which must produce an elevated He/Ne ratio in the Manus Basin source region to account for the nucleogenic neon: (4) a previous degassing event which would leave a Ne-depleted residual reservoir, or (5) a deep mantle source heterogeneity preserving a unique signature inherited from Earth's accretion. We find that isolation times as short as 10 Ma for a previously degassed source are sufficient to grow in the nucleogenic Ne without significantly altering the plume-like He-3/He-4 ratios. Alternatively, solubility-controlled outgassing/ingassing of a magma ocean in contact with a proto-atmosphere may have produced the requisite high He/Ne ratio, although an open-system style of equilibration is necessary. At present, insufficient evidence is available to discriminate between these alternatives. (C) 2001 Elsevier Science B.V. All rights reserved.

2002
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.

Kulongoski, JT, Hilton DR.  2002.  A quadrupole-based mass spectrometric system for the determination of noble gas abundances in fluids. Geochemistry Geophysics Geosystems. 3   10.1029/2001gc000267   AbstractWebsite

[1] We present details of the operation and performance of a quadrupole mass spectrometer-based system developed for the extraction, separation, and high-precision measurement of noble gases dissolved in fluid samples. The relatively inexpensive quadrupole mass spectrometer system, employed with the isotope dilution technique, achieves a precision for He, Ne, Ar, Kr, and Xe abundances of +/-0.4%, +/-0.7%, +/-0.1%, +/-0.8%, and +/-0.9% respectively, as determined from analyses of mixed spike-air aliquots. The system has an overall accuracy of +/-0.6degreesC on the noble gas temperature of a water sample, as determined by analyses of water samples air-equilibrated under controlled conditions. We describe improvements in calibration procedures and sample processing protocols that have led to these high-quality measurements.

2004
Kulongoski, JT, Hilton DR, Selaolo ET.  2004.  Climate variability in the Botswana Kalahari from the late Pleistocene to the present day. Geophysical Research Letters. 31   10.1029/2003gl019238   AbstractWebsite

Noble gas concentrations and oxygen isotopes in groundwaters from the Kalahari Desert in Botswana contain a similar to45 ka paleotemperature record indicating that mean annual temperatures during the period from 24 ka BP to 45 ka BP were 5.2 +/- 1.5degreesC cooler than from similar to16 ka to the present day. In addition to enrichments in oxygen-18, groundwaters recharged after 16 ka BP also contain higher concentrations of excess-air (entrained air bubbles). Taken together, the noble gas, oxygen isotope, and excess-air based paleoclimate archives provide evidence that the high interior desert of southern Africa was both cooler and dryer prior to the Last Glacial Maximum (LGM). Cool, dry conditions also suggest a weaker hydrological cycle in southern Africa. This is contrary to the predicted enhancement of monsoon strength by increased summer insolation in the southern tropics induced by orbital precession during the LGM.

2005
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.

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.

2006
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.

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.

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.

2008
Newell, DL, Jessup MJ, Cottle JM, Hilton DR, Sharp ZD, Fischer TP.  2008.  Aqueous and isotope geochemistry of mineral springs along the southern margin of the Tibetan plateau: Implications for fluid sources and regional degassing of CO(2). Geochemistry Geophysics Geosystems. 9   10.1029/2008gc002021   AbstractWebsite

[1] Springs issuing from different faults and shear zones along the crest of the Himalayas tap three different levels of crust beneath the Tibetan Plateau. From structurally highest to lowest these are the Tingri Graben, the South Tibetan Detachment System (STDS), and the Ama Drime massif (ADM). The aqueous chemistry reflects water-rock interactions along faults and is consistent with mapped rock types. Major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the N-S trending faults that bound the Tingri Graben and Ama Drime detachment (ADD) compared to the STDS, suggesting that these structures penetrate to greater depths. Springs have excess CO(2), N(2), He, and CH(4) compared to meteoric water values, implying addition from crustal sources. The (3)He/(4)He ratios range from 0.018 to 0.063 RA and are consistent with a crustal source for He. The delta(13)C values of dissolved inorganic carbon (DIC) and CO(2) gas range from -5.5 to + 3.8% and -13.1 to -0.3% versus Peedee belemnite, respectively. Sources of carbon are evaluated by calculating isotopic trajectories associated with near-surface effervescence of CO(2). Positive delta(13)C values of the Tingri graben and STDS springs are consistent with decarbonation of marine carbonates as the source of CO(2). Negative values for the ADD springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon. The delta(15)N values of N(2) range from -2.2 to +2.1% ( versus AIR) and are explained by mixtures of air-derived nitrogen, metamorphic devolatilization of sedimentary nitrogen, and nitrogen from near-surface biogenic processes. CO(2) flux is estimated by scaling from individual springs (similar to 10(5) mol a(-1) per spring) to extensional structures across the southern limit of the Tibetan Plateau and likely contributes between 10(8) and 10(11) mol a(-1) (up to 10%) to the global carbon budget.

Suer, S, Gulec N, Mutlu H, Hilton DR, Cifter C, Sayin M.  2008.  Geochemical monitoring of geothermal waters (2002-2004) along the North Anatolian Fault Zone, Turkey: Spatial and temporal variations and relationship to seismic activity. Pure and Applied Geophysics. 165:17-43.   10.1007/s00024-007-0294-4   AbstractWebsite

A total of nine geothermal fields located along an 800-km long E-W transect of the North Anatolian Fault Zone (NAFZ), Turkey were monitored for three years (2002-2004 inclusive; 3-sampling periods per year) to investigate any possible relationship between seismic activity and temporal variations in the chemistry and isotope characteristics of waters in the fields. The geothermal fields monitored in the study were, from west to east, Yalova, Efteni, Bolu, Mudurnu, Seben, Kursunlu-Cankiri, Hamamozu, Gozlek and Resadiye. The chemical (major anion-cation contents) and isotopic (O-18/O-16, D/H, H-3) compositions of hot and cold waters of the geothermal sites were determined in order to both characterize the chemical nature of the individual fields and identify possible temporal variations associated with localized seismic activity. The geothermal waters associated with the NAFZ are dominantly Na-HCO3, whereas the cold waters are of the Ca-HCO3 type. The oxygen- and hydrogen-isotope compositions reveal that the hot waters are meteoric in origin as are their cold water counterparts. However, the lower delta O-18, delta D and H-3 contents of the hot waters point to the fact that they are older than the cold waters, and that their host aquifers are recharged from higher altitudes with virtually no input from recent (post-bomb) precipitation. Although no major earthquakes ( e. g., with M C 5) were recorded along the NAFZ during the course of the monitoring period, variations in the chemical and isotopic compositions of some waters were observed. Indeed, the timing of the chemical/ isotopic changes seems to correlate with the occurrence of seismic activity of moderate magnitude (3 < M < 5) close to the sampling sites. In this respect, Cl, H-3 and Ca seem to be the most sensitive tracers of seismically-induced crustal perturbations, and the Yalova and Efteni fields appear to be the key localities where the effects of seismic activity on the geothermal fluids are most pronounced over the monitoring period. The present study has produced a 'baseline' database for future studies directed at characterizing the effects of moderate-major earthquakes on the composition of geothermal waters along the NAFZ. Future work involving longer monitoring periods with more frequent sampling intervals should lead to a better understanding of the underlying mechanism(s) producing the observed chemical and isotopic variations.

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.

Hahm, D, Hilton DR, Cho M, Wei H, Kim KR.  2008.  Geothermal He and CO(2) variations at Changbaishan intra-plate volcano (NE China) and the nature of the sub-continental lithospheric mantle. Geophysical Research Letters. 35   10.1029/2008gl035955   AbstractWebsite

We report new He and CO(2) data for geothermal fluids from Changbaishan Volcano located on the border of China and North Korea. Helium isotope ratios reach a maximum of 5.4 R(A) (where R(A) = (3)He/(4)He in air) whereas carbon isotope values (delta(13)C) fall between -2 and -7%(vs. PDB). The ratio of CO(2)/(3)He varies between 0.5-7.1 (x 10(9)) for gas samples but is considerably higher (similar to 10(12)) in waters. The combined He-CO(2) systematics reveal the clear imprint of a dominant subducted slab contribution to the total CO(2) inventory. We conclude that these geothermal volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events. Citation: Hahm, D., D. R. Hilton, M. Cho, H. Wei, and K.-R. Kim (2008), Geothermal He and CO2 variations at Changbaishan intra-plate volcano (NE China) and the nature of the sub-continental lithospheric mantle, Geophys. Res. Lett., 35, L22304, doi:10.1029/2008GL035955.

2009
Hahm, D, Castillo PR, Hilton DR.  2009.  A deep mantle source for high He-3/He-4 ocean island basalts (OIB) inferred from Pacific near-ridge seamount lavas. Geophysical Research Letters. 36   10.1029/2009gl040560   AbstractWebsite

One of the most contentious issues in the debate on the origin of volcanic island and seamount chains is the significance of high He-3/He-4 ratios at such locations. The contemporary hotspot hypothesis calls for the high He-3/He-4 signature to be derived from a distinct source reservoir that lies deep in the mantle. The competing plate stress hypothesis claims that extreme isotopic signals, such as the high He-3/He-4, come from dispersed crustal lithologies in the upper mantle. Here, we show that lavas from the East Pacific Rise-the ridge axis and near-ridge seamounts, which have radiogenic isotope compositions overlapping with other Pacific OIB, do not have high He-3/(4) He ratios. This suggests that high He-3/He-4 is not associated with dispersed, heterogeneous lithologies embedded in the upper mantle. We conclude that the mantle source of high He-3/He-4 OIB is unique to volcanic island and seamount chains and likely resides at depth in the mantle. Citation: Hahm, D., P. R. Castillo, and D. R. Hilton (2009), A deep mantle source for high He-3/(4) He ocean island basalts (OIB) inferred from Pacific near-ridge seamount lavas, Geophys. Res. Lett., 36, L20316, doi: 10.1029/2009GL040560.

2010
de Leeuw, GAM, Hilton DR, Gulec N, Mutlu H.  2010.  Regional and temporal variations in CO2/He-3, He-3/He-4 and delta C-13 along the North Anatolian Fault Zone, Turkey. Applied Geochemistry. 25:524-539.   10.1016/j.apgeochem.2010.01.010   AbstractWebsite

New He and C relative abundance, isotope and concentration results from nine geothermal locations situated along an 800-km transect of the North Anatolian Fault Zone (NAFZ), Turkey, that were monitored during the period November 2001-November 2004, are reported. The geothermal waters were collected every 3-6 months to study possible links between temporal geochemical variations and seismic activity along the NAFZ. At the nine sample locations, the He isotope ratios range from 0.24 to 2.3R(A), delta C-13 values range from 4.5 to +5.8 parts per thousand, and CO2/He-3 ratios range from 5 x 10(9) to 5 x 10(14). The following geochemical observations are noted: (1) the highest He-3/He-4 ratios are found near the Galatean volcanic region, in the central section of the NAFZ, (2) at each of the nine sample locations, the He-3/He-4 ratios are generally constant; however, CO2/He-3 ratios and He contents both show one order of magnitude variability, and delta C-13 values show up to similar to 4 parts per thousand variability, and (3) at all locations (except Resadiye), delta C-13 values show positive correlations with CO2 contents. The results indicate that at least three processes are necessary to account for the geochemical variations: (1) binary mixing between crustal and mantle-derived volatiles can explain the general characteristics of He-3/He-4 ratios, delta C-13 values, and CO2/He-3 ratios at the nine sample locations; (2) preferential degassing of He from the geothermal waters is responsible for variations in CO2/He-3 values and He contents at each sample location; and (3) CO2 dissolution followed by calcite precipitation is responsible for variations in CO2 contents and delta C-13 values at most locations. For each of the geochemical parameters, anomalies are defined in the temporal record by values that fall outside two standard deviations of average values at each specific location. Geochemical anomalies that may be related to seismic activity are recorded on June 28, 2004 at Yalova, where a M = 4.2 earthquake occurred 43 days earlier at 15 km distance from the sample location, and on April 7, 2003 at Efteni, where a M = 4.0 earthquake occurred 44 days later at a distance of 12 km. At both locations, the sampling periods containing geochemical anomalies were preceded by an increase in M >= 3 earthquakes occurring within 60 days and less than 40 km distance. (C) 2010 Elsevier Ltd. All rights reserved.

2015
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).

2017
Peters, BJ, Day JMD, Greenwood RC, Hilton DR, Gibson J, Franchi IA.  2017.  Helium–oxygen–osmium isotopic and elemental constraints on the mantle sources of the Deccan Traps. Earth and Planetary Science Letters. 478:245-257.   10.1016/j.epsl.2017.08.042   Abstract

The Deccan Traps, a 65 million-year-old continental flood basalt province located in western India, is the result of one of the largest short-lived magmatic events to have occurred on Earth. The nature and composition of its mantle source(s), however, have been difficult to resolve due to extensive assimilation of continental crust into the ascending Traps magmas. To circumvent this issue, using high-precision electron microprobe analysis, we have analyzed olivine grains from MgO-rich (up to 15.7 wt.%) lavas that likely erupted before substantial crustal assimilation occurred. We compare olivine, pyroxene and plagioclase mineral chemistry and He–O–Os isotope compositions with bulk rock major- and trace-element abundances and 187Os/188Os for both bulk-rocks and mineral separates. Helium isotope compositions for the olivine grains generally show strong influence from crustal assimilation (<3 RA), but one ankaramite from the Pavagadh volcanic complex has a 3He/4He ratio of 10.7 RA, which is slightly lower than the range of 3He/4He measured for present-day Réunion Island volcanism (∼12–14 RA). Olivine-dominated mineral separates span a more restricted range in 187Os/188Os (0.1267 to 0.1443) compared with their host lavas (0.1186 to 0.5010), with the separates reflecting a parental magma composition less affected by lithospheric or crustal interaction than for the bulk-rocks. Despite significant He–Os isotopic variations,Δ17O is relatively invariant (−0.008±0.014‰) and indistinguishable from the bulk mantle, consistent with high-3He/4He hotspots measured to-date. Compositions of olivine grains indicate the presence of up to 25% of a pyroxenite source for Deccan parental magmas, in good agreement with ∼20% predicted from isotopic data for the same samples. Modeled pyroxenite signatures are similar to geochemical signatures expected to arise due to other types of mantle differentiation or due to assimilation of continental crust; however, we show that crustal assimilation cannot account for all of the compositional features of the olivine. Weak correlations exist between a global compilation of Xpx (Deccan: 0.2–0.7) and 3He/4He, δ18O (Deccan olivine: 4.9–5.2‰) and 187Os/188Os. Robust relationships between these parameters may be precluded due to a lack of two-reservoir source mixing, instead involving multiple mantle domains with distinct compositions, or because Xpx may reflect both source features and crustal assimilation. Notwithstanding, geochemical similarities exist between Deccan Traps olivine (3He/4He = 10.7 RA; 187Os/188Osi = 0.1313 ± 45, 2σ) and Réunion igneous rocks (3He/4He = 12–14 RA; 187Os/188Osi = 0.1324 ± 14). These relationships imply that a characteristic geochemical ‘fingerprint’ may have persisted in the mantle plume that fed the Deccan Traps, since its inception at 65 Ma, to ongoing eruptions occurring on Réunion up to the present-day.