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Truong, TB, Castillo PR, Hilton DR, Day JMD.  2018.  The trace element and Sr-Nd-Pb isotope geochemistry of Juan Fernandez lavas reveal variable contributions from a high-3He/4He mantle plume. Chemical Geology. 476:280-291.   10.1016/j.chemgeo.2017.11.024   Abstract

The Juan Fernandez Islands in the southeastern Pacific are an atypical linear volcanic chain that exhibits a considerable range in 3He/4He ratios (8 to 18 RA, where RA is the 3He/4He ratio of air), but limited ranges of 87Sr/86Sr and 143Nd/144Nd. Here we report new trace element abundance data and Sr-Nd-Pb isotope data for mafic lavas previously analyzed for their 3He/4He and He contents from the two main islands of Robinson Crusoe and Alexander Selkirk. Lavas from these islands have been previously grouped based on geochemical and petrological classification into Group I and III basalts, and Group II basanites. In general, samples have overlapping Sr-Nd-Pb isotope compositions that suggest a common, albeit slightly heterogeneous mantle source. In detail, the Group I and III tholeiitic and alkalic basalts have nearly identical incompatible trace element patterns, whereas the Group II basanites show elevated incompatible trace element abundances. Major and incompatible trace element modeling indicates that Group III basalts (3He/4He = 7.8–9.5 RA) from younger Alexander Selkirk Island were produced by the highest degree of partial melting (> 10%) of a common mantle source, followed by Group I basalts (13.6–18.0 RA) and Group II basanites (11.2–12.5 RA) from older Robinson Crusoe Island. The 206Pb/204Pb of Group I basalts and Group II basanites are slightly more radiogenic and limited in range (19.163 to 19.292) compared with those of Group III (18.939 to 19.221). The Group I and II lavas from Robinson Crusoe are consistent with an origin from the so-called focus zone (FOZO) mantle component, whereas the Alexander Selkirk basalts additionally contain contributions from a less-enriched or relatively depleted mantle component. Juan Fernandez lavas reveal limited ranges of Sr-Nd-Pb isotopes but variable 3He/4He as their parental magmas originated mainly from the FOZO component with high 3He/4He (> 9 RA) and variably polluted with a depleted component with lower 3He/4He (ca. 8 RA). Contributions from high-3He/4He mantle sources to ocean island basalts can therefore vary both spatially and temporally, over meter to kilometer lengths and hundred to million-year time scales, and may not be strongly correlated to radiogenic lithophile isotope systematics.

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.

Day, JMD, Hilton DR.  2011.  Origin of (3)He/(4)He ratios in HIMU-type basalts constrained from Canary Island lavas. Earth and Planetary Science Letters. 305:226-234.   10.1016/j.epsl.2011.03.006   AbstractWebsite

New helium isotope and abundance measurements are reported for olivine and clinopyroxene phenocrysts from HIMU-type (high-mu=elevated (238)U/(204)Pb) lavas and xenoliths spanning the stratigraphies of El Hierro and La Palma, Canary Islands. Some pyroxene phenocrysts have suffered post-eruptive modification, either by less than 1% assimilation of crustal-derived He, or by closed-system ageing of He. Olivine phenocrysts record mantle source (3)He/(4)He compositions, with the average (3)He/(4)He for La Palma olivine (7.6 +/- 0.8R(A), where R(A) is the atmospheric (3)He/(4)He ratio of 1.38 x 10(-6)) being within uncertainty of those for El Hierro (7.7 +/- 0.3R(A)), and the canonical mid-ocean ridge basalt range (MORB: 8 +/- 1R(A)). The new helium isotope data for El Hierro and La Palma show no distinct correlations with whole-rock (87)Sr/(86)Sr, (143)Nd/(144)Nd, (187)Os/(188)Os, or Pb isotopes, but (3)He/(4)He ratios for La Palma lavas correlate with (18)O/(16)O measured for the same phenocryst populations. Despite limited (3)He/(4)He variations for El Hierro and La Palma, their He-O isotope systematics are consistent with derivation from mantle sources containing distinct recycled oceanic basaltic crust (El Hierro) and gabbroic lithosphere (La Palma) components that have mixed with depleted mantle, and a high-(3)He/(4)He component (>9.7R(A)) in the case of La Palma. The new data are consistent with models involving generation of compositionally and lithologically (e.g., pyroxenite, eclogite, peridotite) heterogeneous mantle sources containing recycled oceanic crust and lithosphere entrained within upwelling high-(3)He/(4)He mantle that has been severely diluted by interaction with depleted mantle. We propose that the noble gas systematics of HIMU-type lavas and ocean island basalts (OIB) in general, are most simply interpreted as being controlled by the most gas-rich reservoir involved in mixing to generate their mantle sources. In this scenario, HIMU and enriched mantle (EM) sources are dominated by depleted mantle, or high-(3)He/(4)He mantle, because recycled crust and lithosphere have low He concentrations. Consequently, high-(3)He/(4)He OIB would predominantly reflect derivation from a less depleted mantle source with sub-equal to higher He contents than depleted mantle. The available coupled He-O isotope systematics measured for OIB lavas are consistent with this hypothesis. (C) 2011 Elsevier B.V. All rights reserved.

Hilton, DR, 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.

Goff, F, McMurtry GM, Counce D, Simac JA, Roldan-Manzo AR, Hilton DR.  2000.  Contrasting hydrothermal activity at Sierra Negra and Alcedo volcanoes, Galapagos Archipelago, Ecuador. Bulletin of Volcanology. 62:34-52.   10.1007/s004450050289   AbstractWebsite

Sierra Negra and Alcedo volcanoes are adjacent tholeiitic shields with summit calderas located on Isabela Island in the Galapagos Archipelago. Although basaltic eruptions have characterized the evolution of these volcanoes, Alcedo has produced minor volumes of rhyolite lavas and tephras dated at approximately 100 ka. Fumaroles and ephemeral acid-sulfate seeps occur within the calderas of both volcanoes, whereas neutral-chloride and diluted steam-heated hot springs are absent. Fumarolic activity inside Sierra Negra (Mina Azufral) is fault controlled along the west margin of a horst and discharge temperatures are less than or equal to 210 degrees C (January to February 1995). Water content of the total gas is approximately 75 mol.%, and noncondensible gases consist of approximately 97 mol.% CO2 and approximately 85% SO2 of the total sulfur gas. Relative amounts of He, Ar, and N-2 show a distinct hot-spot signature (He-3/ He-4=17.4+/-0.3 R-A) The delta(13)C-CO2 is approximately -3.6 parts per thousand and delta(34)S(T) is approximately +3.3 parts per thousand. The delta D/ delta(18)O of fumarole H2O indicates steam separation from local meteoric waters whose estimated minimum mean residence time from H-3 analyses is less than or equal to 40 years. Fumarolic activity at. Alcedo is controlled by a caldera-margin fault containing at least: seven hydrothermal explosion craters, and by an intracaldera rhyolite vent. Two explosion craters which formed in 1993-1994 produce approximately 15 m(3)/s of steam, yet discharge temperatures are less than or equal to 97 degrees C. Water content of the total gas is 95-97 mol.%, noncondensible gas is 92-98 mol.% CO2, and sulfur gas is dominated by H2S. Relative amounts of He, Ar, and N-2 show extensive mixing between hot spot and air or air-saturated meteoric water components but the average >He-3/ He-4=15.5+/-0.4 R-A. The delta(13)C-CO2 is approximately -3.5 parts per thousand and delta(34)S(T) is approximately -0.8 parts per thousand. The delta D/ delta(18)O of fumarole steam indicates separation from a homogeneous reservoir that is enriched 3-5 parts per thousand in O-18 compared with local meteoric water. H-3 indicates that this reservoir water has a maximum mean residence time of approximately 400 years and empirical gas geothermometry indicates a reservoir temperature of 260-320 degrees C. The intracaldera hydrothermal reservoir in Alcedo is probably capable of producing up to 150 MW; however, environmental concerns as well as lack of infrastructure and power users will limit the development of this resource.

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.