B. Mack Kennedy

Flow of Mantle Fluids Through the Ductile Lower Crust: Helium Isotope Trends

Heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust. Minimum 3He/4He ratios in surface fluids from the northern Basin and Range Province, western North America, increase systematically from low crustal values in the east to high mantle values in the west, a regional trend that correlates with the rates of active crustal deformation. The highest ratios occur where the extension and shear strain rates are greatest. The correspondence of helium isotope ratios and active transtensional deformation indicates a deformation-enhanced permeability and that mantle fluids can penetrate the ductile lithosphere, even in regions where there is no substantial magmatism. Superimposed on the regional trend are local, high 3He/4He anomalies indicating hidden magmatic activity and/or deep fluid production with locally enhanced permeability, identifying zones with high resource potential, particularly for geothermal energy development.

Tracing and quantifying magmatic carbon discharge in cold groundwaters: Lessons learned from Mammoth Mountain, USA

Abstract A major campaign to quantify the magmatic carbon discharge in cold groundwaters around Mammoth Mountain volcano in eastern California was carried out from 1996 to 1999. The total water flow from all sampled cold springs was ≥1.8×10 7 m 3 /yr draining an area that receives an estimated 2.5×10 7 m 3 /yr of recharge, suggesting that sample coverage of the groundwater system was essentially complete. Some of the waters contain magmatic helium with 3 He/ 4 He ratios as high as 4.5 times the atmospheric ratio, and a magmatic component in the dissolved inorganic carbon (DIC) can be identified in virtually every feature sampled. Many waters have a 14 C of 0–5 pmC, a δ 13 C near −5‰, and contain high concentrations (20–50 mmol/l) of CO 2(aq) ; but are otherwise dilute (specific conductance=100–300 μS/cm) with low pH values between 5 and 6. Such waters have previously escaped notice at Mammoth Mountain, and possibly at many other volcanoes, because CO 2 is rapidly lost to the air as the water flows away from the springs, leaving neutral pH waters containing only 1–3 mmol/l HCO 3 − . The total discharge of magmatic carbon in the cold groundwater system at Mammoth Mountain is ∼20 000 t/yr (as CO 2 ), ranging seasonally from about 30 to 90 t/day. Several types of evidence show that this high discharge of magmatic DIC arose in part because of shallow dike intrusion in 1989, but also demonstrate that a long-term discharge possibly half this magnitude (∼10 000 t/yr) predated that intrusion. To sustain a 10 000 t/yr DIC discharge would require a magma intrusion rate of 0.057 km 3 per century, assuming complete degassing of magma with 0.65 wt% CO 2 and a density of 2.7 t/m 3 . The geochemical data also identify a small (

HELIUM AND NEON ISOTOPES IN THE IMNAHA BASALT, COLUMBIA RIVER BASALT GROUP: EVIDENCE FOR A YELLOWSTONE PLUME SOURCE

Abstract Helium, neon, and argon isotopic compositions were measured in two flows of the Columbia River flood basalt. The Imnaha Basalt has a 3 He/ 4 He ratio of 11.4 times atmospheric and 20 Ne/ 22 Ne and 21 Ne/ 22 Ne ratios characteristic of a plume component. The measured 3 He/ 4 He is a lower limit, due to possible preferential 3 He loss and/or addition of radiogenic 4 He. A Wanapum Basalt flow, erupted approximately 2 Ma later in the waning stages of volcanism, has more MORB-like noble gases. The He, Nd and Sr isotopic compositions of these lavas suggest that the Columbia River basalts were derived from the Yellowstone plume head which contained both ‘high-helium’ plume material and entrained depleted mantle. As the eruptions progressed the plume component in the melting region was gradually diluted or replaced.

Fluid inclusion gas compositions from an active magmatic-hydrothermal system: a case study of The Geysers geothermal field, USA

Abstract Hydrothermal alteration and the active vapor-dominated geothermal system at The Geysers, CA are related to a composite hypabyssal granitic pluton emplaced beneath the field 1.1 to 1.2 million years ago. Deep drill holes provide a complete transect across the thermal system and samples of the modern-day steam. The hydrothermal system was liquid-dominated prior to formation of the modern vapor-dominated regime at 0.25 to 0.28 Ma. Maximum temperatures and salinities ranged from 440°C and 44 wt.% NaCl equivalent in the biotite hornfels adjacent to the pluton to 305°C and 5 wt.% NaCl equivalent at distances of 1730 m from the intrusive contact. The major, minor, and noble gas compositions of fluid inclusions in the hydrothermally altered rocks were integrated with microthermometric and mineralogic data to determine their sources and the effects of mixing and boiling. Major and minor gaseous species were released from the inclusions by crushing or thermal decrepitation; noble gases were released by crushing. The samples were analyzed by mass spectrometry. The analyses document the presence of magmatic, crustal, and meteoric components in the trapped fluids. Hydrothermal fluids present during the liquid-dominated phase of the system contained gaseous species derived mainly from crustal and magmatic sources. At The Geysers, N 2 /Ar ratios greater than 525 and 3 He/ 4 He ratios of 6–10.7 Ra are diagnostic of a magmatic component. Crustal gas has CO 2 /CH 4 ratios less than 4, N 2 /Ar ratios between 45 and 525, and low 3 He/ 4 He ratios (0.5 Ra). Meteoric fluids have CO 2 /CH 4 ratios greater than 4 and N 2 /Ar ratios between 38 (air-saturated water) and 84 (air). However, N 2 /Ar ratios between 15 and 110 can result from boiling. Ratios less than 15 reflect the incorporation of N 2 into NH 3 -bearing clay minerals. In the central Geysers, the incursion of meteoric fluids occurred during the transition from the liquid- to vapor-dominated regime. Variations in the relative CH 4 , CO 2 , and H 2 contents of the gas analyses demonstrate that boiling took place under open-system conditions. The gas data indicate that the inclusions have remained closed to the diffusion of He and H 2 since their formation.

Helium isotopes in lithospheric mantle: evidence from tertiary basalts of the western USA

Abstract The isotopic compositions of He, Sr, and Nd were measured in Tertiary-age basalts from the Basin and Range province of the western USA to evaluate models for the He isotopic character of subcontinental mantle lithosphere (SCML) and assess the role of recycled SCML in models of mantle evolution. Previous isotopic and trace element measurements suggested that most of these basalts were formed by melting of SCML. 3 He/ 4 He ratios, measured by in-vacuo crushing of olivine phenocrysts, vary from 2.9 to 7.8 times the atmospheric value (2.9 to 7.8 Ra) consistently below the MORB value of 8.7 ± 0.5 Ra. The lowest R/Ra values, associated with low e Nd , high 87 Sr/ 86 Sr, and high La/Nb, are attributable to lithospheric mantle, and indicate that SCML is not dominated by MORB-type He, nor by high R/Ra, plume-type He. Consideration of geographic variability indicates there are two, and possibly three, distinct regions of SCML with differing He isotopic characteristics. SCML beneath the eastern Sierra Nevada is inferred to have 3 He/ 4 He of ∼5.5 Ra and a He/Nd ratio slightly less than MORB-type mantle; SCML beneath the central Basin and Range has 3 He/ 4 He of ∼4 Ra and a higher He/Nd ratio than MORB-type mantle. The SCML under southwestern Utah shows less systematic correlation of He isotopes with other geochemical parameters, but also has a lower bound R/Ra value of about 4 Ra. The inferred SCML helium ratios are consistent with retention of radiogenic 4 He over 800 Ma for the eastern Sierra Nevada and 1700 Ma for the other two regions. The results are not consistent with models of He infiltration from the underlying asthenosphere and suggest the lithosphere of the Basin and Range region was not delaminated during the early Tertiary. The He, Sr, Nd, and Pb isotopic compositions inferred for the SCML of the southwestern USA are a reasonably good match to the characteristics of the EMII component of mantle heterogeneity identified in oceanic island basalts. High R/Ra mantle reservoirs identified in these basalts are not likely to represent recycled SCML.

The Northwest Geysers high-temperature reservoir: evidence for active magmatic degassing and implications for the origin of the Geysers geothermal field

Noble gas isotope abundances in steam from the Coldwater Creek field of the Northwest Geysers, California, show mixing between a nearly pure mid-ocean ridge (MOR) type magmatic gas with high 3He/4He and low radiogenic 40∗Ar (R/Ra > 8.3 and 40∗Ar/4He 0.25). The 3He-enriched component is positively correlated with 4He/40∗Ar ratios, total helium to non-condensable gas ratios, and the ratios of total helium to atmospheric noble gases, and is accompanied by mantle-like 3He/CO2 and 4He/36Ar ratios. The steam samples most enriched in this high 3Hecomponent are produced from a high-temperature reservoir (HTR) and are also the most enriched in total gas and HCl. These results support the hypothesis of active magma degassing beneath the Northwest Geysers, suggest that a significant fraction of the non-condensable gases produced with steam from the HTR is magmatic, and add new constraints to genetic models of the system and its evolution. The intensity of the magmatic signal is inconsistent with deep boiling of connate or metamorphic waters and suggests active magma degassing. A correlation between magmatic helium and non-condensable gases implies that the HTR high-gas component is also magmatic and that the formation of the HTR was possibly related to magmatic intrusion. Magmatic input is unlikely to decline on the time scale of production, but injection of water into the HTR would have multiple benefits: (1) pressure and steam flow would increase; (2) gas concentrations would decrease by dilution with low-gas steam from vaporized liquid; and (3) HCl in steam would be removed once a liquid phase is established. High 3He/4He ratios in steam produced from the reservoirs in the southern parts of The Geysers suggest the presence of a similar HTR underlying these portions of the field. Monitoring of 3He content may provide early warning of potential pressure drawdown and entry of corrosive, high-gas HTR steam.

Correlation of gold in siliceous sinters with 3He4He in hot spring waters of Yellowstone National Park

Opaline sinter samples collected at Yellowstone National Park (YNP) were analyzed for gold by neutron activation and for other trace elements by the inductively coupled plasma optical emission spectroscopy (ICP-OES) method. No correlation was found between Au and As, Sb, or total Fe in the sinters, although the sample containing the highest Au also contains the highest Sb. There also was no correlation of Au in the sinter with the H2S concentration in the discharged hot spring water or with the estimated temperature of last equilibration of the water with the surrounding rock. The Au in rhyolitic tuffs and lavas at YNP found within the Yellowstone caldera show the same range in Au as do those outside the caldera, while thermal waters from within this caldera all have been found to contain relatively low dissolved Au and to deposit sinters that contain relatively little Au. Therefore, it is not likely that variations in Au concentrations among these sinters simply reflect differences in leachable Au in the rocks through which the hydrothermal fluids have passed. Rather, variations in [H2S], the concentration of total dissolved sulfide, that result from different physical and chemical processes that occur in different parts of the hydrothermal system appear to exert the main control on the abundance of Au in these sinters. Hydrothermal fluids at YNP convect upward through a series of successively shallower and cooler reservoirs where water-rock chemical and isotopic reactions occur in response to changing temperature and pressure. In some parts of the system the fluids undergo decompressional boiling, and in other parts they cool conductively without boiling. Mixing of ascending water from deep in the system with shallow groundwaters is common. All three processes generally result in a decrease in [H2S] and destabilize dissolved gold bisulfide complexes in reservoir waters in the YNP system. Thus, different reservoirs in rocks of similar composition and at similar temperatures may contain waters with different [H2S] and [Au]. The [H2S] in a subsurface reservoir water is difficult to assess on the basis of analyses of hot spring waters because of uncertainties about steam loss during fluid ascent. However, the same processes that result in low [H2S] in reservoir waters also tend to result in decreases in the ratio of 3He4He(R) dissolved in that water. Values of R relative to this ratio in air (Ra) attain values > 15 in YNP thermal waters. To date, all of the thermal waters at YNP that have RRa values <9 have been found to deposit sinters with relatively low gold concentrations. These include all of the thermal waters that discharge from 180–215°C reservoirs at Upper, Midway, and Lower Geyser Basins within the western part of the Yellowstone caldera, and thermal waters at Norris Geyser Basin, outside the Yellowstone caldera, where some of the waters flow directly to the surface from a reservoir where the temperature is about 300°C. A high 3He4He ratio in thermal water discharged at the surface does not guarantee high gold concentrations in the sinter deposited by this water. Boiling with loss of steam (the gas phase takes a separate route to the surface) during rapid upflow from the shallowest reservoir to the surface decreases the [H2S] and total He dissolved in the residual liquid without appreciably changing the 3He4He ratio. This is because the isotopic composition of the He of the initial bulk fluid is unchanged and there is too little time for much radiogenic 4He to build back into the liquid during this rapid ascent from the near-surface reservoir. However, if boiling with phase separation and loss of steam occurs deep in the system, the 3He4He ratio in the residual liquid, now depleted in H2S and total He, will be susceptible to dilution with radiogenic 4He that is acquired during the longer residence time underground. Some or all of the Au that comes out of solution when an initial gold bisulfide complex breaks down as a result of loss of H2S may be swept up to the surface as solid (probably colloidal) particles, depending on the rate of flow of the mixture of water and steam, and the geometry of the channel. Where colloidal silica also forms as a result of this boiling, free Au apparently becomes attached to the colloidal silica and deposits where the silica deposits.

A History of Geothermal Energy Research and Development in the United States. Reservoir Engineering 1976-2006

This report, the third in a four-part series, summarizes significant research projects performed by the U.S. Department of Energy (DOE) over 30 years to overcome challenges in reservoir engineering and to make generation of electricity from geothermal resources more cost-competitive.

Noble Gases in Geochemistry and Cosmochemistry Reviews in Mineralogy and Geochemistry, v. 47

This 47th volume of Reviews in Mineralogy and Geochemistry succeeds admirably at the very difficult task of summarizing, in a succinct and approachable manner, what is and is not known about noble gases and their roles in geochemistry and cosmochemistry. The list of significant contributions to our understanding of stellar and nebular processes, planetary and lunar science, and geoscience is long and diverse. Noble gases introduced the first evidence for isotopic heterogeneity in the solar nebula, and the presence of live but now extinct radionuclides, the latter leading to a fine-scale chronology of nebular processes and planetary evolution and the first estimates of the age of the elements in the universe. Noble gases have been instrumental in identifying and isolating relict nova and supernova grains, providing a unique insight into stellar nucleosynthesis and chemistry. Additional contributions include evidence for temporal changes in the composition of the solar wind, planetary and lunar cratering histories and time scales, evidence for pre-compaction irradiation of meteoritic material, constraints on processes and time scales of planetary degassing, atmosphere formation, and crust-mantle-core evolution.