Helium isotopes in geothermal and volcanic gases of the western United States, I. Regional variability and magmatic origin (original) (raw)
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Helium isotopes at Satsuma-Iwojima volcano, Japan
GEOCHEMICAL JOURNAL, 2002
We present isotopic analyses of helium in fluids collected during two field trips at Satsuma-Iwojima and Shin-Iwojima islands in November 1998 and October 2000. These are the first reported helium measurements at Shin-Iwojima. Copper tubes tightly closed by clamps at both ends were used to sample 1) gases from high and low temperature fumaroles in the summit area of Iwodake cone, 2) gases from a fumarole on Shin-Iwojima, 3) gas bubbling from the sea-floor along Shin-Iwojima where an increasing bubbling intensity was observed between 1998 and 2000, 4) waters from Sakamoto and Higashi hot springs. For gas samples, two types of correction for atmospheric contamination are discussed, using either the neon concentration or just the partial pressure of condensable gases at liquid nitrogen temperature; this latter method turned out to be very efficient for many samples. For all gas samples, we found 3 He/ 4 He isotopic ratios between 7.1 and 8.2 times the atmospheric ratio, pointing to a magmatic origin for fumaroles both at Iwodake crater and at Shin-Iwojima island. Our measurements show a decrease of the isotopic ratios at Iwodake with decreasing temperature of the fumarole. Comparing the results of both field trips and those published in previous studies, we suggest this volcanic system is undergoing a recent increase of activity. served both on and offshore , including high (>800°C) and low temperature fumaroles, hot springs and gas bubbling, which provide a good opportunity to sample and study this volcanic system. In this aim, helium has interesting properties: a gaseous state, inertness, and above all, its isotopes show contrasting ratios in the continental crust (<0.1 Ra, Ra being the atmospheric ratio) and in the upper (about 8 Ra) and lower (>30 Ra) mantle (Ozima, 1994; Farley and Neroda, 1998). Therefore it has proven to be a powerful tracer of the origin and possible crustal dilution of mantle-derived fluids (i.e., Hilton et al., 1993b; Van Soest et al., 1998) and of short-term variations induced by active volcanism (i.e., Sano et al., 1997). Although comprehensive studies of the fluid and gas chemistry have been carried out
Geochimica et Cosmochimica Acta, 1990
Early studies of 3HefHe variations in geothermal systems have generally attributed these fluctuations to either differences in the source of the magmatic 'He-rich helium or to local differences in the deep flux of magmatic 'He-rich helium. KENNEDY et al. (1987), however, show that near-surface processes such as boiling and dilution may also drastically affect 3He/4He ratios of geothermal vapors, Helium isotope ratios were determined for several hot springs at Shoshone Geyser Basin of Yellowstone National Park for this study, along with other noble gas data. Stable isotope data and water and gas chemistry data for each spring were also compiled. The water chemistry indicates that there is one deep, hot thermal water in the area which is mixing with dilute meteoric water that has entered the system at depth. Spring HCO$ concentrations correlate with 3He/4He values, as in nearby Lower Geyser Basin. This correlation is attributed to variable amounts of deep dilution of thermal waters with a relatively cool water that inhibits boiling at depth, thus preventing the loss of COr (and therefore HCO;) and magmatic He in the most diluted samples. Oxygen and hydrogen isotope data also support a boiling and dilution model, but to produce the observed fractionations, the boiling event would have to be extensive, with steam loss at the surface, whereas the boiling that affected the helium isotope ratios was probably a small scale event with steam loss at depth. It is possible that deep boiling occurred in the basin and that small amounts of steam escaped along fractures at about 500 m below the surface while all subsequently produced steam was lost near or at the surface.
2005
1] We measure all stable noble gases (He, Ne, Ar, Kr, Xe) in spring waters in the Oregon Cascades volcanic arc and in eastern Oregon, USA. We show that in order to estimate magmatic helium (He) contributions it is critical to simultaneously consider He isotopic ratios, He concentrations, and mixing of He components. Our component mixing analysis requires consideration of all measured noble gases but no other elements and is particularly insightful when strong dilution by air-saturated water has occurred. In addition, this approach can allow distinction between crustal and magmatic He components and facilitates their identification in deep groundwaters that have been diluted by near-surface water. Using this approach, we show that some cold springs on the eastern flanks of the Oregon Cascades exhibit He isotopic ratios that indicate significant magmatic He contributions comparable to those observed in thermal springs on the western flanks. Furthermore, while these magmatic He contributions are largest in deep groundwaters near the Cascades crest, greater magmatic excess He fractions than may be inferred from He isotopic ratios alone are present in all (deep) groundwaters including those at larger distances (>70 km) from the volcanic arc. We also suggest that excess He and heat discharge without dilution by air-saturated water may be restricted to spring discharge along faults.
Helium isotopic variations in volcanic rocks from Loihi Seamount and the Island of Hawaii
Earth and Planetary Science Letters, 1983
Helium isotopic ratios ranging from 20 to 32 times the atmospheric 3 He/4 He (R A ) have been observed in a suite of 15 basaltic glasses from the Loihi Seamount. These ratios, which are up to four times higher than those of MORB glasses and more than twice those of nearby Kilauea, are strongly suggestive of a primitive source of volatiles supplying this volcanism. The Loihi glasses measured span a broad compositional range, and the 3He/4He ratios were found to be generally lower for the alkali basalts than for the tholeiites. The component with a lower 3He/nile ratio appears to be associated with olivine xenocrysts, within which fluid inclusions are probably the carrier of contaminant helium. One Loihi sample has a much lower isotopic ratio (< 5 RA), but a combination of low He concentration, high vesicularity, and presence of cracks lined with clay minerals suggests that the low ratio is due to gas loss and contamination by atmospheric helium.
Chemical Geology, 2003
We report helium isotope and concentration results for groundwaters from the western Mojave River Basin (MRB), 130 km east of Los Angeles, CA. The basin lies adjacent to the NW -SE trending San Andreas Fault (SAF) system. Samples were collected along two groundwater flowpaths that originate in the San Gabriel Mountains and discharge to the Mojave River located f 32 km to the northeast. Additional groundwater samples were collected from Mojave River Deposits underlying the Mojave River. The primary objective of this study is to identify and quantify crustal and mantle helium contributions to the regional groundwater system.
the western United States: Evidence from travertine springs and regional He isotope data
2000
to understand regional mantle degassing, we compiled new and existing helium isotope data measured in hot springs, gas fields, and travertine-depositing cool springs and compared these geochemical data with mantle velocity struc - ture determined from tomographic studies. these data sug - gest heterogeneous mantle degassing, with regions of highest 3 He/4He in groundwaters (hence, highest mantle helium con -