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Gao, J, Liu J, Hilton DR, Meng F, Li Z, Zhai L, Sun C, Zhang L.  2017.  The chemical and isotopic compositions of volatiles in magmatic hydrothermal fluids beneath the Songliao Basin, northeastern China. Chemical Geology. 465:11-20.   10.1016/j.chemgeo.2017.05.015   Abstract

It is controversial whether deep fluids can make contributions to natural gas accumulations in the Songliao Basin, NE China. The occurrence of magmatic hydrothermal quartz veins in crystalline basement of the basin provides a way to understand the characteristics of deep fluids and their significance for natural gas accumulation. Three types of fluid inclusions were identified in hydrothermal quartz: aqueous inclusions, H2O-CO2-rich inclusions and H2O-CO2-CH4-rich inclusions. The primary inclusions show high homogenization temperatures (280–433°C) which are typical of magmatic hydrothermal fluids. The oxygen and hydrogen isotopic compositions (δ18O and δD) of water are 2.0‰ to 4.6‰ and −91.6‰ to −75.7‰, respectively. The OH isotopic compositions suggest that hydrothermal fluids associated with the quartz veins were derived from the primary magmatic system and mixed by small amounts of meteoric water. Two types of magmatic hydrothermal volatiles were identified in the study. Both types of volatiles are dominated by CO2. CO2 in type I volatiles shows more negative δ13C values (−13.8‰ to −9.7‰) than typical magmatic CO2, which likely resulted from carbon isotopic fractionation during magma degassing. Type II volatiles have higher δ13C values (−5.5‰ to −3.3‰) and are similar to typical magmatic CO2. Small amounts of hydrocarbons were observed in both types of magmatic hydrothermal fluids. Hydrocarbons in type I volatiles have relatively high δ13CCH4 values (−30.6‰ to −24.1‰) and reversed carbon isotopic trends while hydrocarbons in type II volatiles have more negative δ13CCH4 values (−49.7‰ to −37.6‰) and orderly carbon isotopic trends. CO2 in hydrothermal fluids is a potential source for CO2 gas accumulations. Hydrocarbons in hydrothermal fluids, however, can hardly make significant contributions to the commercial gas reservoirs due to their low concentration.

de Moor, JM, Fischer TP, Sharp ZD, Hilton DR, Barry PH, Mangasini F, Ramirez C.  2013.  Gas chemistry and nitrogen isotope compositions of cold mantle gases from Rungwe Volcanic Province, southern Tanzania. Chemical Geology. 339:30-42.   AbstractWebsite

We report the first complete bulk gas chemistry and nitrogen isotope data for geothermal volatiles from the Rungwe Volcanic Province, located in the western branch of the East African Rift north of Lake Malawi. Temperatures of springs and gas emissions at Rungwe vary from 13 °C to 65 °C with the highest temperatures observed at the springs in the northern and southern lowlands. The vigorously degassing cold CO2 vents and springs have temperatures between 13 °C and 36 °C and are located at higher elevation than the hot springs. The gas compositions are ~ 99% CO2, 0.0008 to 0.0078 mmol/mol H2, 0.0004 to 0.062 mmol/mol He, 0.08 to 0.77 mmol/mol Ar, 3.1 to 28.5 mmol/mol N2, 0.4 to 3.73 mmol/mol O2, < 0.002 to 1.541 mmol/mol CH4, < 0.001 to 0.009 mmol/mol CO, and are poor in H2S (0.045 to 0.201 mmol/mol). The CO2 flux at a local gas collection plant is estimated to be 1.6 × 105 mol/year. Gas geothermometry indicates a range of equilibration temperatures from > 250 °C (from CO2–Ar) to ~ 60 °C (from H2–Ar), which is interpreted to reflect deep equilibration with hot saline fluids and shallow re-equilibration of kinetically fast gas geothermometers with cold meteoric recharge from the highlands. N2–He–Ar systematics show that the gases fall on a well-defined mixing line between upper mantle or sub-continental lithospheric mantle and air saturated water endmembers. Details of an improved method for analyzing nitrogen isotope compositions in gas samples are presented. Nitrogen isotope compositions (δ15N values) range between + 2‰ and − 5.9‰, overlapping with the upper mantle range, with only one sample location displaying δ15N values greater than air (0‰). The results emphasize the importance of the East African Rift as a potential, but poorly constrained, contributor of sub-continental lithospheric mantle volatiles to the Earth's surface even in regions that are currently volcanically dormant, but are seismically active.

Suer, S, Gulec N, Mutlu H, Hilton DR, Cifter C, Sayin M.  2008.  Geochemical monitoring of geothermal waters (2002-2004) along the North Anatolian Fault Zone, Turkey: Spatial and temporal variations and relationship to seismic activity. Pure and Applied Geophysics. 165:17-43.   10.1007/s00024-007-0294-4   AbstractWebsite

A total of nine geothermal fields located along an 800-km long E-W transect of the North Anatolian Fault Zone (NAFZ), Turkey were monitored for three years (2002-2004 inclusive; 3-sampling periods per year) to investigate any possible relationship between seismic activity and temporal variations in the chemistry and isotope characteristics of waters in the fields. The geothermal fields monitored in the study were, from west to east, Yalova, Efteni, Bolu, Mudurnu, Seben, Kursunlu-Cankiri, Hamamozu, Gozlek and Resadiye. The chemical (major anion-cation contents) and isotopic (O-18/O-16, D/H, H-3) compositions of hot and cold waters of the geothermal sites were determined in order to both characterize the chemical nature of the individual fields and identify possible temporal variations associated with localized seismic activity. The geothermal waters associated with the NAFZ are dominantly Na-HCO3, whereas the cold waters are of the Ca-HCO3 type. The oxygen- and hydrogen-isotope compositions reveal that the hot waters are meteoric in origin as are their cold water counterparts. However, the lower delta O-18, delta D and H-3 contents of the hot waters point to the fact that they are older than the cold waters, and that their host aquifers are recharged from higher altitudes with virtually no input from recent (post-bomb) precipitation. Although no major earthquakes ( e. g., with M C 5) were recorded along the NAFZ during the course of the monitoring period, variations in the chemical and isotopic compositions of some waters were observed. Indeed, the timing of the chemical/ isotopic changes seems to correlate with the occurrence of seismic activity of moderate magnitude (3 < M < 5) close to the sampling sites. In this respect, Cl, H-3 and Ca seem to be the most sensitive tracers of seismically-induced crustal perturbations, and the Yalova and Efteni fields appear to be the key localities where the effects of seismic activity on the geothermal fluids are most pronounced over the monitoring period. The present study has produced a 'baseline' database for future studies directed at characterizing the effects of moderate-major earthquakes on the composition of geothermal waters along the NAFZ. Future work involving longer monitoring periods with more frequent sampling intervals should lead to a better understanding of the underlying mechanism(s) producing the observed chemical and isotopic variations.

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.