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Stefansson, A, Hilton DR, Sveinbjornsdottir AE, Torssander P, Heinemeier J, Barnes JD, Ono S, Halldorsson SA, Fiebig J, Arnorsson S.  2017.  Isotope systematics of Icelandic thermal fluids. Journal of Volcanology and Geothermal Research. 337:146-164.   10.1016/j.jvolgeores.2017.02.006   AbstractWebsite

Thermal fluids in Iceland range in temperature from <10 degrees C to >440 degrees C and are dominated by water (>97 mol%) with a chloride concentration from <10 ppm to >20,000 ppm. The isotope systematics of the fluids reveal many important features of the source(s) and transport properties of volatiles at this divergent plate boundary. Studies spanning over four decades have revealed a large range of values for delta D (-131 to +3.3%o), tritium (-0.4 to +13.8 TU), delta(18) O(-20.8 to + 2.3%o), He-3/He-4 (3.1 to 30.4 R-A), delta B-11 (-6.7 to+25.0%o), delta C-13 Sigma co(2) (-27.4 to+ 4.6%o), C-1 Sigma co(2), (+0.6 to + 118 pMC), delta C-l3(CH4) (-523 to-17.8%o), delta N-15 (-10.5 to+3.0%o), 8(34)C Sigma s(-ll) (-10.9 to (+)3.4%o), delta S-34(SO4) (-2.0to + 21.2%) and delta Cl-37 (-1.0 to + 2.1%o) in both liquid and vapor phases. Based on this isotopic dataset, the thermal waters originate from meteoric inputs and/or seawater. For other volatiles, degassing of mantle-derived melts contributes to He, CO2 and possibly also to Cl in the fluids. Water-basalt interaction also contributes to CO2 and is the major source of H2S, SO4, Cl and B in the fluids. Redox reactions additionally influence the composition of the fluids, for example, oxidation of H2S to SO4 and reduction of CO2 to CH4. Air water interaction mainly controls N-2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as delta C-13 Sigma co(2)and(34)S Sigma S-u indicate heterogeneity of the mantle and mantle-derived melts beneath Iceland. In contrast, the large range of many reactive isotopes, such as delta C-13 Sigma co(2), and delta S-34 Sigma S-u, are heavily affected by processes occurring within the geothermal systems, including fluid-rock interaction, depressurization boiling, and isotopic fractionation between secondary minerals and the aqueous and vapor species. Variations due to these geothermal processes may exceed differences observed among various crust and mantle sources, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements. (C) 2017 Elsevier B.V. All rights reserved.

Furi, E, Hilton DR, Halldorsson SA, Barry PH, Hahm D, Fischer TP, Gronvold K.  2010.  Apparent decoupling of the He and Ne isotope systematics of the Icelandic mantle: The role of He depletion, melt mixing, degassing fractionation and air interaction. Geochimica Et Cosmochimica Acta. 74:3307-3332. AbstractWebsite

We present new He Ne data for geothermal fluids and He-Ne-Ar data for basalts from throughout the Icelandic neovolcanic zones and older parts of the Icelandic crust. Geothermal fluids, subglacial glasses, and mafic phenocrysts are characterized by a wide range in helium isotope ratios ((3)He/(4)He) encompassing typical MORB-like ratios through values as high as 36.8 R(A) (where R(A) = air (3)He/(4)He). Although neon in geothermal fluids is dominated by an atmospheric component, samples from the northwest peninsula show a small excess of nucleogenic (21)Ne, likely produced in-situ and released to circulating fluids. In contrast, geothermal fluids from the neovolcanic zones show evidence of a contribution of mantle-derived neon, as indicated by (20)Ne enrichments up to 3% compared to air. The neon isotope composition of subglacial glasses reveals that mantle neon is derived from both depleted MORB-mantle and a primordial, 'solar' mantle component. However, binary mixing between these two endmembers can account for the He Ne isotope characteristics of the basalts only if the (3)He/(22)Ne ratio of the primordial mantle endmember is lower than in the MOR B component. Indeed, the helium to neon elemental ratios ((4)He/(22)Ne* and (3)He/(22)Ne(s) where (21)Ne* = nucleogenic (21)Ne and (22)Ne(s) = 'solar'-derived (22)Ne) of the majority of Icelandic subglacial glasses are lower than theoretical values for Earth's mantle, as observed previously for other Of B samples. Helium may be depleted relative to neon in high-(3)Her(4)He ratio parental melts due to either more compatible behavior during low-degree partial melting or more extensive diffusive loss relative to the heavier noble gases. However, Icelandic glasses show higher (4)He/(40)Ar* ((40)Ar* = radiogenic Ar) values for a given (4)He/(21)Ne* value compared to the majority of other OIB samples: this observation is consistent with extensive open-system equilibrium degassing, likely promoted by lower confining pressures during subglacial eruptions of Icelandic lavas. Taken together, the He Ne Ar systematics of Icelandic subglacial glasses are imprinted with the overlapping effects of helium depletion in the high-(3)Hef(4)He ratio parental melt, binary mixing of two distinct mantle components, degassing fractionation and interaction with atmospheric noble gases. However, it is still possible to discern differences in the noble gas characteristics of the Icelandic mantle source beneath the neovolcanic zones, with MORB-like He Ne isotope features prevalent in the Northern Rift Zone and a sharp transition to more primitive 'solar-like' characteristics in central and southern Iceland. (C) 2010 Elsevier Ltd. All rights reserved.

Crossey, LJ, Karlstrom KE, Springer AE, Newell D, Hilton DR, Fischer T.  2009.  Degassing of mantle-derived CO2 and He from springs in the southern Colorado Plateau region-Neotectonic connections and implications for groundwater systems. Geological Society of America Bulletin. 121:1034-1053.   10.1130/b26394.1   AbstractWebsite

Groundwaters of the southern Colorado Plateau-Arizona Transition Zone region are a heterogeneous mixture of chemically diverse waters including meteoric (epigenic) fluids, karst-aquifer waters, and deeply sourced (endogenic) fluids. We investigate the composition of travertine-depositing CO2-rich springs to determine the origin, transport, and mixing of these various components. The San Francisco Mountain recharge area has little surface flow. Instead, waters discharge through major springs hundreds of kilometers away. About 70% (9340 L/s) of the total recharge (13,500 L/s) discharges 100 km to the north in the incised aquifer system at Grand Canyon. Most of this water (85%; 8070 L/s) emerges through two travertine-depositing karst spring systems: Blue Springs (6230 L/s) and Havasu Springs (1840 L/s). About 30% of recharge (4150 L/s) flows to the south and discharges along NW-striking faults in the Arizona Transition Zone, forming the base flow for the Verde River. Geochemical data define regional mixing trends between meteoric recharge and different endogenic end members that range from bicarbonate waters to sulfate waters. Water quality in the region is dictated by the percentage and character of the endogenic inputs that cause a measurable degradation of groundwater quality for water supply. Sources for the high CO2 include dissolution of limestone and dolostone (C-carb) and "external carbon" (C-external). C-external is computed as the bicarbonate alkalinity (dissolved inorganic carbon [DIC]) minus the C-carb (C-external = DIC-C-carb). C-external is deconvolved using carbon isotopes into biogenically derived sedimentary carbon (C-organic) and deep CO2 inputs (C-endogenic). Measured delta C-13 values are -17 parts per thousand to +3 parts per thousand versus Pee Dee Belemnite (PDB). Assuming delta(13) C-carb = +2 parts per thousand, delta C-13(organic) = -28%, and delta(13) C-endogenic = -5 parts per thousand, water chemistry mixing models indicate that an average of 42% of the total DIC comes from dissolution of carbonate rocks, 25% from organic carbon, including soil-respired CO2, and 33% from deep (endogenic) sources. Helium isotope values (He-3/He-4) in gases dissolved in spring waters in the southern Colorado Plateau region range from 0.10 to 1.16 R-A (relative to air) indicating that a significant component of the deeply derived fluid is from the mantle (mean of 5% asthenospheric or 10% subcontinental lithospheric mantle source). Measured CO2/He-3 ratios of 2 x 10(9) to 1.4 x 10(13) are adjusted by removing the proportion of CO2 from C-carb and C-organic to give values < 5 x 10(10) for all but four samples. Various mixing models using CO2/He-3 suggest that the mantle-derived components of the CO2 load are highly variable from spring to spring and may make up an average of similar to 10% of the total CO2 load of the regional springs. Fluid-rock interactions involving endogenic fluids are suggested by Sr-87/Sr-86, delta O-18, and other tracers. The endogenic CO2 component, multiplied by discharge for each spring, yields an integrated annual flux of deeply derived CO2 to the groundwater system of similar to 1.4 x 10(9) mol/yr. This CO2 emission from the Colorado Plateau region reflects a complex tectonic evolution involving Laramide hydration of the lithosphere above the Farallon slab, addition of fluids from mid-Tertiary mantle tectonism during slab removal, and ongoing fluid movement induced by neotectonic small-scale asthenospheric convection.

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.

Clor, LE, Fischer TP, Hilton DR, Sharp ZD, Hartono U.  2005.  Volatile and N isotope chemistry of the Molucca Sea collision zone: Tracing source components along the Sangihe Arc, Indonesia. Geochemistry Geophysics Geosystems. 6   10.1029/2004gc000825   AbstractWebsite

Volcanic gases are sensitive indicators of subduction processes and are used to evaluate the contributions from various source components. Nitrogen isotope systematics in particular are a valuable tool for determining the fate of organic matter in subduction zones. We present the first arc-wide survey of trace gas chemistry and nitrogen isotope variations from the Sangihe Arc of northeastern Indonesia, where the narrow Molucca Sea Plate subducts beneath the Sangihe Arc to the west and the Halmahera Arc to the east. Relative volatile abundances and N isotopic compositions of volcanic gases show systematic along-arc variations. Northern volcanoes exhibit low N(2)/He ratios and delta(15)N values ( northern minima 542 and -7.3 parts per thousand, respectively), indicating minimal addition of sediment to source magmas. In contrast, the southern part of the arc is characterized by high N(2)/He and delta(15)N values ( southern maxima 2000 and +2.1 parts per thousand, respectively), consistent with greater sediment contributions in the formation of the magmas. These observations can be correlated with the complex tectonic setting of the region whereby oblique collision between the two arcs has caused sediment obduction, decoupling the accretionary wedges from the underlying oceanic plate. In the north, where the collision is more developed, the lack of trace gas and N isotope evidence of sedimentary inputs to the source of arc magmas is consistent with enhanced sediment decoupling. In the south, where collision and accretionary wedge decoupling are not yet taking place, sediments would presumably subduct normally, in agreement with higher N(2)/He and delta(15)N values. Awu volcano, at the northernmost extension of the arc, is anomalous and exhibits high N(2)/He (2852) coupled with low delta(15)N (-3.3 parts per thousand). These values are suggestive of increased slab contribution in the northernmost arc, possibly by slab melting as collision stalls the progress of the subducting plate and allows it to become superheated.

Forrest, MJ, Ledesma-Vazquez J, Ussler W, Kulongoski JT, Hilton DR, Greene HG.  2005.  Gas geochemistry of a shallow submarine hydrothermal vent associated with the El Requeson fault zone, Bahia Concepcion, Baja California Sur, Mexico. Chemical Geology. 224:82-95.   10.1016/j.chemgeo.2005.07.015   AbstractWebsite

We investigated hydrothermal gas venting associated with a coastal fault zone along the western margin of Bahia Concepcion, B.C.S., Mexico. Copious discharge of geothermal liquid (approximate to 90 degrees C) and gas is occurring in the intertidal and shallow subtidal zones (to a depth of 13 m) through soft sediments and fractures in rocks along a similar to 750 m linear trend generally sub-parallel to an onshore fault near Punta Santa Barbara. Hydrothermal activity shows negative correlation with tidal height; temperatures in the area of hydrothermal activity were up to 11.3 degrees C higher at low tide than at high tide (measured tidal range approximate to 120 cm). Gas samples were collected using SCUBA and analyzed for chemical composition and stable isotope values. The main components of the gas are N-2 (approximate to 53%; 534 mmol/mol), CO2 (approximate to 43%; 435 mmol/mol), and CH4 (approximate to 2.2%; 22 mmol/mol). The delta(13)C values of the CH4 (mean= -34.3 parts per thousand), and the ratios of CH4 to C2H6 (mean= 89), indicate that the gas is thermogenic in origin. The carbon stable isotopes and the delta(15)N of the N-2 in the gas (mean= 1.7 parts per thousand) suggest it may be partially derived from the thermal alteration of algal material in immature sedimentary organic matter. The He isotope ratios (He-3/He-4= 1.32 R-A) indicate a significant mantle component (16.3%) in the gas. Here, we suggest the name El Requeson fault zone for the faults that likely formed as a result of extension in the region during the late Miocene, and are currently serving as conduits for the observed hydrothermal. activity. (c) 2005 Elsevier B.V. All rights reserved.

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.

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

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

Hoke, L, Hilton DR, Lamb SH, Hammerschmidt K, Friedrichsen H.  1994.  HE-3 Evidence for a Wide Zone of Active Mantle Melting Beneath the Central Andes. Earth and Planetary Science Letters. 128:341-355.   10.1016/0012-821x(94)90155-4   AbstractWebsite

We report results of a regional survey of helium isotopes measured in water and gas samples in volcanic sulfataras and geothermal springs from the Central Andes of northern Chile and Bolivia between the latitudes 15 degrees S and 23 degrees S. The highest He-3/He-4 ratios (reported as R/R(A) ratios: R = sample 3He/ 4He, R(A) = air He-3/He-4) are associated with the active volcanic are of the Western Cordillera (0.92 < R/R(A) < 5.52) and approach ratios found at other convergent margins in the circum-Pacific region. A significant He-3 component is also present in fluid and gas samples from the high Altiplano plateau (0.48 < R/R(A) < 3.56) and the Eastern Cordillera (0.03 < R/R(A) < 1.2), up to 300 km east of the active are and more than 300 km above the subducting slab. This wide zone of He-3 anomalies is delineated both to the east and the west by regions with low He-3/He-4 ratios (less than or equal to 0.2R(A)), typical of radiogenic helium production in the crust. Studies of the regional groundwater regime suggest that the wide zone of elevated He-3/He-4 values away from the active volcanic are is unlikely to be caused by lateral and shallow transport of magmatic helium and there is no evidence for significant crustal sources of He-3. The high He-3/He-4 ratios are interpreted as reflecting degassing of volatiles from mantle-derived magmas emplaced over an area 400 km wide beneath and into crust up to 75 km thick. The subducting slab is at depths of 100-350 km in this region. In the west, underneath the active volcanic arc, mantle melting is probably largely controlled by mantle hydration and dehydration and the helium isotope data can be used to delineate the extent of the asthenospheric mantle wedge at depth. In contrast, mantle melting behind the are, beneath the Altiplano and Eastern Cordillera, may be a result of convective removal of the base of the lithosphere. The sharp cut-off in the mantle helium signal in the east is interpreted as marking the western edge of thick and relatively cold lithosphere, devoid of mantle melts, which could transport mantle volatiles towards the surface. This may coincide with the limit of underthrusting of the Brazilian shield beneath the eastern margin of the Central Andes.