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Geiger, H, Troll VR, Jolis EM, Deegan FM, Harris C, Hilton DR, Freda C.  2018.  Multi-level magma plumbing at Agung and Batur volcanoes increases risk of hazardous eruptions. Scientific Reports. 8   10.1038/s41598-018-28125-2   AbstractWebsite

The island of Bali in Indonesia is home to two active stratovolcanoes, Agung and Batur, but relatively little is known of their underlying magma plumbing systems. Here we define magma storage depths and isotopic evolution of the 1963 and 1974 eruptions using mineral-melt equilibrium thermobarometry and oxygen and helium isotopes in mineral separates. Olivine crystallised from a primitive magma and has average delta O-18 values of 4.8%. Clinopyroxene records magma storage at the crust-mantle boundary, and displays mantle-like isotope values for Helium (8.62 R-A) and delta O-18 (5.0-5.8%). Plagioclase reveals crystallisation in upper crustal storage reservoirs and shows delta O-18 values of 5.5-6.4%. Our new thermobarometry and isotope data thus corroborate earlier seismic and InSAR studies that inferred upper crustal magma storage in the region. This type of multi-level plumbing architecture could drive replenishing magma to rapid volatile saturation, thus increasing the likelihood of explosive eruptions and the consequent hazard potential for the population of Bali.

Furi, E, Hilton DR, Murton BJ, Hemond C, Dyment J, Day JMD.  2011.  Helium isotope variations between Reunion Island and the Central Indian Ridge (17 degrees-21 degrees S): New evidence for ridge-hot spot interaction. Journal of Geophysical Research-Solid Earth. 116   10.1029/2010jb007609   AbstractWebsite

We report new helium abundance and isotope results for submarine basaltic glasses from the Central Indian Ridge (CIR) between the Marie Celeste (16.7 degrees S) and Egeria fracture zones (FZ) (20.6 degrees S); the adjacent Gasitao, Three Magi, and Rodrigues ridges; and for olivine separates from lavas and cumulate xenoliths from the Mascarene Islands (Reunion, Mauritius, and Rodrigues). Helium isotope ratios in basaltic glasses range from 7.1 to 12.2 R(A) (where R(A) = air (3)He/(4)He) and lie between values of Mid-Ocean Ridge Basalt (MORB) (8 +/- 1 R(A)) and samples from Reunion Island (11.5 to 14.1 R(A)). The highest (3)He/(4)He values (up to 12.2 R(A)) are found in glasses recovered off axis from the Three Magi and Gasitao ridges. Along the CIR axis, MORB-like (3)He/(4)He ratios are found near the Egeria FZ, and there is a marked increase to values of similar to 11 R(A) between similar to 19 degrees and 20 degrees S. The lowest (3)He/(4)He values (< 8 R(A)) are found immediately south of the Marie Celeste FZ, where incompatible trace element ratios (e. g., La/Sm) are highest. These low (3)He/(4)He ratios can be explained by closed system radiogenic (4)He ingrowth in either (1) a "fossil" Reunion hot spot mantle component, embedded into the subridge mantle when the CIR migrated over the hot spot at similar to 34 Ma or (2) trace element enriched MORB mantle. In contrast, the high (3)He/(4)He ratios observed on the CIR axis adjacent to the Gasitao Ridge, and along the off-axis volcanic ridges, are consistent with flow of hot spot mantle material from Reunion (similar to 1100 km to the west) toward the CIR.

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

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

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

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

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

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

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.

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.