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2017
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.

2015
Fischer, TP, Ramirez C, Mora-Amador RA, Hilton DR, Barnes JD, Sharp ZD, Le Brun M, de Moor JM, Barry PH, Furi E, Shaw AM.  2015.  Temporal variations in fumarole gas chemistry at Pods volcano, Costa Rica. Journal of Volcanology and Geothermal Research. 294:56-70.   10.1016/j.jvolgeores.2015.02.002   AbstractWebsite

We report the chemical and isotopic composition of fumarole gas discharges collected at Pods Volcano, Costa Rica from 2001 to 2014, covering a period during which the volcano experienced a series of phreatic eruptions (March 2006 to October 2014). The relative abundances of Poas C-S-H-O gas species are controlled by reactions involving the SO2-H2S and So-SO2 gas buffers indicating magmatic temperatures of up to 800 degrees C. Although fumarole outlet temperatures are <120 degrees C for most samples, SO2 is the dominant sulfur gas and HCl contents are relatively high. Gas compositional changes within the magma-lake-hydrothermal system likely result from a combination of several processes, including: 1) The injection of new and undegassed magma in late 2000-early 2001,2) the heating of the hydrothermal system, accompanied by gas pressure build-up, and 3) hydrofracturing through 2006. These processes culminated in the phreatic eruptions of 2006 and 2008. Since 2005 the lake level has declined and is now (January 2014) at the lowest level (10 m) since the last periods that it dried out completely (April 1984 and April 1994). The most recent data of 2014 show high level of degassing from the dome fumaroles and the release of HCI-rich and CO2-poor gases implies that the magma injected in late 2000 continues to supply volatiles. Our data show that time series sampling of fumarole gases provides important insights to better understand magmatic and hydrothermal processes at active volcanoes and also to potentially forecast phreatic eruptions. (C) 2015 Elsevier B.V. All rights reserved.

2013
de Moor, MJ, Fischer TP, King PL, Botcharnikov RE, Hervig RL, Hilton DR, Barry PH, Mangasini F, Ramirez C.  2013.  Volatile-rich silicate melts from Oldoinyo Lengai volcano (Tanzania): Implications for carbonatite genesis and eruptive behavior. Earth and Planetary Science Letters. 361:379-390.   http://dx.doi.org/10.1016/j.epsl.2012.11.006   AbstractWebsite

This study presents volatile, trace, and major element compositions of silicate glasses (nepheline-hosted melt inclusions and matrix glass) from the 2007–2008 explosive eruption at Oldoinyo Lengai volcano, Tanzania. The bulk compositions of the heterogeneous ash erupted in 2007–2008 are consistent with physical mixing between juvenile nephelinite magma and natrocarbonatite emplaced during the preceding ∼25 years of effusive carbonatite eruption. The melt inclusions and matrix glasses span a wide range of silica-undersaturated compositions, from ∼46 wt% SiO2 and (Na+K)/Al∼3 in the least evolved melt inclusions to 38 wt% SiO2 and (Na+K)/Al up to 12 in the matrix glass. The depletion in SiO2 between melt inclusions and matrix glass is accompanied by strong enrichment in all of the incompatible trace elements measured (Ba, Nb, La, Ce, Sr, Zr, Y), which is consistent with fractional crystallization of a bulk mineral assemblage with SiO2 higher than that of the melt inclusions but inconsistent with silicate melt evolution by assimilation of carbonatite. The melt inclusions are volatile-rich with 2.7 wt% to 8.7 wt% CO2 and 0.7 wt% to 10.1 wt% H2O, indicating that Oldoinyo Lengai is a hydrous system. This is contrary to the long-held assumption that Oldoinyo Lengai is relatively anhydrous, which is based on the observation that natrocarbonatite lavas are water-poor. We argue that natrocarbonatites are derived from hydrous carbonate liquid that degas H2O at low pressure. The silicate glass data show that H2O concentration is negatively correlated with incompatible element enrichment, which we attribute to crystallization of the melt in response to decompression degassing of H2O. The eruptive cycle at Oldoinyo Lengai reflects changes in bulk silicate magma viscosity due to extensive H2O-driven crystallization and explosive eruptions occur when volatiles (i.e. H2O>CO2 gas, and carbonate liquid) cannot separate from the crystal-rich nephelinite magma. Melt H2O content decreases as a function of pressure; however CO2 concentration in the melt inclusions is buffered by the presence of immiscible carbonate liquid. CO2 content increases with melt evolution parameters (e.g. increasing (Na+K)/Al) due to enhanced solubility with alkali enrichment and SiO2 depletion in the melt. The matrix glasses and evolved melt inclusions, on the other hand, experienced low pressure (<50 MPa) CO2 degassing and were not buffered by a coexisting carbonate liquid. Whereas the melt inclusions are the most CO2-rich yet identified, their CO2/Nb ratios are without exception lower than that in MORB, indicating that a volatile-rich mantle source is not required for Oldoinyo Lengai.

2010
Mitchell, EC, Fischer TP, Hilton DR, Hauri EH, Shaw AM, de Moor JM, Sharp ZD, Kazahaya K.  2010.  Nitrogen sources and recycling at subduction zones: Insights from the Izu-Bonin-Mariana arc. Geochemistry Geophysics Geosystems. 11   10.1029/2009gc002783   AbstractWebsite

We report new chemical and nitrogen isotopic data from 29 volcanic and hydrothermal gas samples covering eight centers in the Izu-Bonin-Mariana (IBM) arc to investigate the sources, flux, and mass balance of nitrogen at a "cool" convergent margin. The majority of samples have high N(2)/He (1217-17,300) and low CO(2)/N(2,exc.) (78-937), implying addition of nitrogen from the subducting slab. This inference is supported by positive (i.e., sediment-like) delta(15)N values (up to 5.5%) in most samples. The exception to these trends is Agrigan in the Mariana arc, with low N(2)/He (similar to 200), high CO(2)/N(2,exc.) (similar to 1500), and negative delta(15)N. Mixing calculations suggest an average of 34% of the nitrogen in our samples is derived from subducted sediment, or 75% after correction for atmospheric contamination. Sediment-derived N(2) fluxes estimated by three different methods range from 0.25 x 10(8) to 1.11 x 10(8) mol yr(-1) N(2), representing 4%-17% of the total nitrogen input flux or 11%-51% of the sedimentary nitrogen input flux. The altered oceanic crust is identified as an important contributor to the arc nitrogen budget, and the delta(15)N of the residual nitrogen subducted into the mantle is estimated at approximately -1.9 parts per thousand. Despite similarities in gas chemistry and delta(15)N values, our conclusions regarding nitrogen recycling for IBM are markedly different than those for the Nicaraguan segment of the Central American arc, and we suggest that thermal regime is the major control on nitrogen recycling within subduction zones. The global nitrogen cycle is estimated to be in steady state, suggesting either that subducted sediments are an unlikely source for heavy nitrogen in plume-related rocks or secular variation in the isotopic composition of subducted sediments. Better constraints on nitrogen recycling at other arcs are required to test these conclusions.

2005
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.

Fischer, TP, Takahata N, Sano Y, Sumino H, Hilton DR.  2005.  Nitrogen isotopes of the mantle: Insights from mineral separates. Geophysical Research Letters. 32   10.1029/2005gl022792   AbstractWebsite

We present the first nitrogen (N) isotope measurements determined by in-vacuo crushing of mineral separates from arc lavas, OIBs (Ocean Island Basalts), and mantle xenoliths. Measured OIB delta(15)N values range from similar to - 8 parts per thousand for the northern rift zone in Iceland to + 3.1 parts per thousand for a dunite nodule from Hawaii. Most arc-related olivines show distinctly positive values - up to + 6.2 parts per thousand (Cerro Negro, Nicaragua). The measured N isotope values in olivine separates are similar to gas samples collected at the same localities, suggesting that both media ( olivines and gases) sample volatiles primarily derived from the magma. This observation also implies that N isotope fractionation does not occur during magma degassing, a notion supported by (4)He/(40)Ar* data. Our results indicate a heterogeneous mantle source region, in terms of N isotopic composition, that may have resulted from surface recycling of N at some localities.

2004
Jaffe, LA, Hilton DR, Fischer TP, Hartono U.  2004.  Tracing magma sources in an arc-arc collision zone: Helium and carbon isotope and relative abundance systematics of the Sangihe Arc, Indonesia. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000660   AbstractWebsite

[1] The Sangihe Arc is presently colliding with the Halmahera Arc in northeastern Indonesia, forming the world's only extant example of an arc-arc collision zone. We report the first helium and carbon isotopic and relative abundance data from the Sangihe Arc volcanoes as a means to trace magma origins in this complicated tectonic region. Results of this study define a north-south trend in He-3/He-4, CO2/He-3, and delta(13)C, suggesting that there are variations in primary magma source characteristics along the strike of the arc. The northernmost volcanoes (Awu and Karangetang) have higher CO2/He-3 and delta(13)C (up to 179 x 10(9) and -0.4parts per thousand, respectively) and lower He-3/He-4 (similar to5.4 R-A) than the southernmost volcanoes ( Ruang, Lokon, and Mahawu). Resolving the arc CO2 into component structures (mantle-derived, plus slab-derived organic and carbonate CO2), the northern volcanoes contain an unusually high (> 90%) contribution of CO2 derived from isotopically heavy carbonate associated with the subducting slab ( sediment and altered oceanic basement). Furthermore, the overall slab contribution (CO2 of carbonate and organic origin) relative to carbon of mantle wedge origin is significantly enhanced in the northern segment of the arc. These observations may be caused by greater volumes of sediment subduction in the northern arc, along-strike variability in subducted sediment composition, or enhanced slab-derived fluid/melt production resulting from the superheating of the slab as collision progresses southward.

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.

1996
Hilton, DR.  1996.  The helium and carbon isotope systematics of a continental geothermal system: Results from monitoring studies at Long Valley caldera (California, USA). Chemical Geology. 127:269-295.   10.1016/0009-2541(95)00134-4   AbstractWebsite

This study reports fumarole and hot spring gas chemistry of a 3-year monitoring programme (1986-1988) at the seismically-active Long Valley caldera (LVC) in the Sierra Nevada, eastern California. The focus is on helium and carbon dioxide (isotopes and concentrations) and their variation in response to seismic activity in the region. Within the caldera, both species are predominantly magmatic in origin but their isotopic and elemental characteristics appear to be established prior to shallow-level intrusion and/or are influenced by pre-eruptive degassing. In response to intra-caldera and regional seismicity over the monitoring period, CO2 and helium show markedly different behaviour: the greatest change in various carbon-related parameters (CO2%; delta(13)C(CO2); CO2/He-3) occurred in 1986 and were most likely related to regional seismicity in the nearby Chalfont Valley. Helium did not respond to these events. The largest change (up to similar to 25%) in He-3/He-4 ratios was seen in 1987 with the occurrence of both increases and decreases relative to the almost constant values observed in 1988. The increases are consistent with magma intrusion occurring within the caldera in 1987 whilst the decreases occurred significantly later (>6 months) than any seismic activity. It is suggested that decreases in He-3/He-4 are related to the regional seismicity and that the hydrothermal system exerts a (temporal) control on the release of near-surface He-4. At LVC this is related to the timing of the late spring thaw. Results from previous monitoring programmes (when the level of seismicity in the caldera was higher) are evaluated against variations in the present work. There appears to be a convincing link between higher He-3/He-4 values and the level of seismicity in the caldera although factors related to location, magnitude and frequency of seismic events are difficult to quantify. Because of the inferred small isotopic contrast between new magma and presently-degassing magma at LVC, it is anticipated that large variations in He-3/He-4 within the central caldera are unlikely to occur until the magmatic volatile signal wanes as a function of degassing and time. Alternatively, only at those localities situated at significant distances from the region of magma intrusion (i.e. away from resurgent dome vicinity) are helium and/or carbon likely to respond dramatically to intra-caldera seismicity.