Tobias P. Fischer

Contrasting He–C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones ☆

We report 3He/4He ratios, relative He, Ne, and CO2 abundances as well as δ13C 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. CO2/3He ratios are relatively constant throughout Costa Rica (av. 2.1×1010) and Nicaragua (av. 2.5×1010) and similar to arcs worldwide (∼1.5×1010). δ13C values range from −6.8‰ (MORB-like) to −0.1‰ (similar to marine carbonate (0‰)). 3He/4He ratios are essentially MORB-like (8±1 RA) 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, 10Be 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 (5.8×1010 mol/yr) and CO2/3He ratios (7.1×1010 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 arc is surprisingly low (14–18% vs. a global average of ∼50%). 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).

Volcanic flux of nitrogen from the Earth

Abstract The global flux of nitrogen from subduction zones is estimated by the elemental and isotopic compositions of nitrogen, argon and helium observed in volcanic gases and hydrothermal fluids in island arcs and in back-arc basin basalt (BABB) glasses. The 3He/4He ratios of island arc samples vary from 4.7 Ratm to 7.5 Ratm, indicating a typical subduction signature. The 40Ar/36Ar ratios are consistent with atmospheric values except for a few samples. The δ15N values range from +0.1‰ to +4.6‰, which is generally higher than those of BABB glasses. Taking into account data distribution in the δ15N–N2/36Ar diagram, we distinguish three nitrogen components (mantle-derived, sedimentary and atmospheric nitrogen) for the island arc samples. Contribution of mantle-derived nitrogen is 9–30% in the samples, which is consistent with that of mantle-derived carbon. It is possible to calculate nitrogen flux based on the 3He flux in the literature and N2/3He ratios corrected for elemental fractionation. The nitrogen flux of 6.4×108 mol/year from island arc is comparable with 5.6×108 mol/year from back-arc basin, but smaller than 2.2×109 mol/year from mid-ocean ridges. In detail, island arcs show a large flux of subducted sedimentary nitrogen, while back-arc basins have a relatively small but measurable subduction component. The nitrogen flux of 4.1×106 mol/year from hot spot region is significantly small, which is consistent with the characteristic of global carbon flux from the Earth. Total volcanic flux of nitrogen amounts to 2.8×109 mol/year by taking mid-ocean ridge, hot spot and subduction values. The global nitrogen flux, if it has been constant for the 4.55 billion years of geological time, leads to an accumulation of 1.3×1019 mol in total, which is one order of magnitude smaller than 1.8×1020 mol of the present inventory of nitrogen at the Earths surface.

Chlorine isotope homogeneity of the mantle, crust and carbonaceous chondrites

Chlorine in the Earth is highly depleted relative to carbonaceous chondrites and solar abundances. Knowledge of the Cl concentrations and distribution on Earth is essential for understanding the origin of these depletions. Large differences in the stable chlorine isotope ratios of meteoritic, mantle and crustal materials have been used as evidence for distinct reservoirs in the solar nebula and to calculate the relative proportions of Cl in the mantle and crust. Here we report that large isotopic differences do not exist, and that carbonaceous chondrites, mantle and crust all have the same 37Cl/35Cl ratios. We have further analysed crustal sediments from the early Archaean era to the Recent epoch and find no systematic isotopic variations with age, demonstrating that the mantle and crust have always had the same δ37Cl value. The similarity of mantle, crust and carbonaceous chondrites establishes that there were no nebular reservoirs with distinct isotopic compositions, no isotopic fractionation during differentiation of the Earth and no late (post-core formation) Cl-bearing volatile additions to the crustal veneer with a unique isotopic composition.

Fluxes and sources of volatiles discharged from kudryavy, a subduction zone volcano, Kurile Islands

Abstract The Kudryavy volcano, a 996-m-high basaltic-andesite cone on the northeastern shore of Iturup Island in the Kuriles, erupted last in 1883 and has since been in a persistent state of high-temperature, >900°C fumarolic activity. Its flux of SO2, measured by COSPEC, is 73±15 t/d, or 416 Mmol/a. In combination with the chemical composition of the parent gas supplying the high-temperature vents and the isotopic compositions of He and C, it allows the evaluation of contributions from major source components, such as the mantle, the crust, and subducted sediments and carbonate. The 3He/4He ratio of 6.7 RA corresponds to a 84% mantle origin and a flux of 2200 mol/a of mantle He. At a He concentration of 2200 mol/Mt, the mass of mantle material required to generate this flux is 1.0 Mt/a. The same mass produces a flux of 0.025 mol/a of 3He and of 50 Mmol/a of mantle CO2 at a CO2/3He ratio of 2·109. In conjunction with the C-isotopic composition of fumarolic CO2 of −7.2‰, about 12% of the CO2 are derived from the mantle, 67% from marine carbonate in subducted, altered oceanic crust, 21% are of subducted organic sedimentary origin. The flux of 280 Mmol/a of carbonate-derived CO2 requires 0.41 Mt/a of oceanic crust with a CO2 content of 3 wt%, and 0.35 Mt/a of sedimentary material to supply the organic CO2 flux of 86 Mmol/a. Nitrogen from the mantle contributes at most 2% to the total N2 flux of 5.4 Mmol/a. Assuming N to be derived from the subducted sediments, its concentration there is 460 mg/kg. The total volume of mantle and subducted material required to maintain the flux of volatiles over the 100 a period of high-temperature fumarolic activity of Kudryavy is 0.07 km3. Steady-state release of volatiles from the depth of arc magma generation to the fumaroles and continuously high heat flow from the mantle are proposed as the main process supporting the long-term high-temperature degassing at Kudryavy. In this steady-state system, the calculated volatile fluxes are balanced over time by volatiles originating from subducted sediments, hydrothermally altered oceanic crust below the Kudryavy volcano and the mantle wedge. This has significant implications for volatile cycling from the Earths crust and mantle to the atmosphere.

Geochemistry of the magmatic–hydrothermal system of Kawah Ijen volcano, East Java, Indonesia

Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemical and isotopic analysis. An extremely low pH (<0.3) lake contains SO 4 -Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce immature solutions. The strong D and 18 O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ratios also modifies the lake D and 18 O content. The large Δ SO4-S D (23.8-26.4‰) measured in the lake suggest that dissolved SO 4 forms during disproportionation of magmatic SO 2 in the hydrothermal conduit at temperatures of 250∼280°C. The lake δ 18 O SO4 and δ 18 O H2O values may reflect equilibration during subsurface circulation of the water at temperatures near 150°C. Significant variations in the lakes bulk composition from 1990 to 1996 were not detected. However, we interpret a change in the distribution and concentration of polythionate species in 1996 as a result of increased SO 2 -rich gas input to the lake system. Thermal springs at Kawah Ijen consist of acidic SO 4 -Cl waters on the lakeshore and neutral pH HCO 3 -SO 4 -Cl-Na waters in Blawan village, 17 km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO 4 /Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO 4 -Cl water and ground water. The lakeshore fumarole discharges (T = 170∼ 245°C) have both a magmatic and a hydrothermal component and are supersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is ∼260°C. The proportions of the oxidized, SO 2 -dominated magmatic vapor anti of the reduced, H 2 S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interaction of SO 2 -bearing magmatic vapors with the summit acidic hydrothermal reservoir. This idea is supported by the lower H 2 S/SO 2 ratio deduced for the gas producing the SO 4 -Cl reservoir feeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid-vapor zone at about 350°C. Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H 2 S in the lake water. This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar to the present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H 2 S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quenched liquid sulfur products show δ 34 S values similar to those found in the banded deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lake bottom.

Dissected hydrologic system at the Grand Canyon: Interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine

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 87Sr/86Sr 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/N2 and 3He/4He 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) 87Sr/86Sr 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 subprovinces. 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.

Degassing of mantle-derived CO2 and He from springs in the southern Colorado Plateau region - Neotectonic connections and implications for groundwater systems

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 CO 2 -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 CO 2 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 CO 2 inputs (C endogenic ). Measured δ 13 C values are −17‰ to +3‰ versus Pee Dee Belemnite (PDB). Assuming δ 13 C carb = +2‰, δ 13 C organic = −28‰, and δ 13 C endogenic = −5‰, 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 CO 2 ,and 33% from deep (endogenic) sources. Helium isotope values ( 3 He/ 4 He) 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 CO 2 / 3 He ratios of 2 × 10 9 to 1.4 × 10 13 are adjusted by removing the proportion of CO 2 from C carb and C organic to give values 10 for all but four samples. Various mixing models using CO 2 / 3 He suggest that the mantle-derived components of the CO 2 load are highly variable from spring to spring and may make up an average of ~10% of the total CO 2 load of the regional springs. Fluid-rock interactions involving endogenic fluids are suggested by 87 Sr/ 86 Sr, δ 18 O, and other tracers. The endogenic CO 2 component, multiplied by discharge for each spring, yields an integrated annual flux of deeply derived CO 2 to the groundwater system of ~1.4 × 10 9 mol/yr. This CO 2 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.

Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites

Carbonatite lavas are highly unusual in that they contain almost no SiO2 and are >50 per cent carbonate minerals. Although carbonatite magmatism has occurred throughout Earth’s history, Oldoinyo Lengai, in Tanzania, is the only currently active volcano producing these exotic rocks. Here we show that volcanic gases captured during an eruptive episode at Oldoinyo Lengai are indistinguishable from those emitted along mid-ocean ridges, despite the fact that Oldoinyo Lengai carbonatites occur in a setting far removed from oceanic spreading centres. In contrast to lithophile trace elements, which are highly fractionated by the immiscible phase separation that produces these carbonatites, volatiles (CO2, He, N2 and Ar) are little affected by this process. Our results demonstrate that a globally homogenous reservoir exists in the upper mantle and supplies volatiles to both mid-ocean ridges and continental rifts. This argues against an unusually C-rich mantle being responsible for the genesis of Na-rich carbonatite and its nephelinite source magma at Oldoinyo Lengai. Rather, these carbonatites are formed in the shallow crust by immiscibility from silicate magmas (nephelinite), and are stable under eruption conditions as a result of their high Na contents.

The chemical and isotopic composition of fumarolic gases and spring discharges from Galeras Volcano, Colombia

Abstract Galeras fumarole discharges have been collected since its reactivation, in 1988, through December 1995. The gases are dominated by H2O, CO2, S (as SO2 and H2S) and HCl. The relative proportions of these gases classify them as ‘magmatic’. Thermodynamic equilibrium temperatures of the gases range from 260 to > 600 °C. The relative abundance of inert gases, N2, Ar and He, can be used as ‘tracers’ to identify the source of the fumarole discharges. At Galeras the majority of the samples have a composition characteristic of gases originating from arc-related magmas, with relatively high N2 contents and minor He and Ar. During 1993, the year of frequent eruptions, the gas composition changed to basaltic or ‘mantle-derived’ gases, with significantly higher He contents. This is interpreted to be the result of injection of volatiles from a basaltic magma body at depth prior to and during the increased eruptive activity of 1993. The δ13C values for CO2 in fumarole discharges are typical of andesitic volcanoes and may indicate addition of MORB-derived CO2. The δ15N values for N2 may indicate significant contribution of N2 from marine sediments and only minor contribution of MORB-derived N2. The δ D and δ18O values of the discharging steam lie on a mixing trend between the isotopic composition of ‘arc-related’ magmatic water and18O-shifted meteoric water. The most magmatic discharges have δ D values of −30 to −35‰; while the most meteoric discharges have values of −70 to −75‰, similar to Galeras thermal spring waters. Galeras thermal water discharges consist of acid sulfate and bicarbonate waters.S/Cl ratios in the acid sulfate waters are similar to fumarole ratios, suggesting direct absorption of magmatic gases into shallow ground waters. This is supported by the essentially meteoric δD and δ18O values of the discharges and by elevated3He/4He ratios of thermal spring waters. The absorption of acid S- and Cl-rich gases yield acid waters which are capable of dissolving rocks. The thermal waters, however, are far from equilibrium with Galeras lavas and pyroclastic rocks, providing evidence of the immaturity of the Galeras hydrothermal system. The SO4 and Cl content, as well as the O and H isotopic composition of Galeras thermal springs vary with the activity of the volcano. The 7-year sampling program at Galeras revealed intriguing results concerning the activity of Galeras, its magmatic-hydrothermal system and the origin of the volatiles. Despite decreasing outlet temperatures since 1992, deep temperatures remain high, implying continued unrest in the Galeras magmatic system.

Geochemical surveillance of magmatic volatiles at Popocatépetl volcano, Mexico

Surveillance of Popocatepetl volcanic plume geochemistry and SO 2 flux began in early 1994 after fumarolic and seismic activity increased significantly during 1993. Volatile traps placed around the summit were collected at near-monthly intervals until the volcano erupted on December 21, 1994. Additional trap samples were obtained in early 1996 before the volcano erupted again, emplacing a small dacite dome in the summit crater. Abundances of volatile constituents (ppm/day of Cl, S total , F, CO 2 , Hg, and As) varied, but most constituents were relatively high in early and late 1994. However, ratios of these constituents to Cl were highest in mid-1994. δ 34 S-S total in trap solutions ranged from 1.5‰ to 6.4‰; lowest values generally occurred during late 1994. δ 13 C-CO 2 of trap solutions were greatly contaminated with atmospheric CO 2 and affected by absorption kinetics. When trap data are combined with SO 2 flux measurements made through November 1996, Popocatepetl released about 3.9 Mt SO 2 , 16 Mt CO 2 , 0.75 Mt HCl, 0.075 Mt HF, 260 t As, 2.6 t Hg, and roughly 200 Mt H 2 O. Near-vent gas concentrations in the volcanic plume measured by correlation spectrometer (COSPEC) and Fourier transform infrared (FTIR) commonly exceed human recommended exposure limits and may constitute a potential health hazard. Volatile geochemistry combined with petrologic observations and melt-inclusion studies show that mafic magma injection into a preexisting silicic chamber has accompanied renewed volcanism at Popocatepetl. Minor assimilation of Cretaceous wall rocks probably occurred in mid-1994.