Harold Wollenberg

A Sr-isotopic comparison between thermal waters, rocks, and hydrothermal calcites, Long Valley caldera, California

Abstract The 87 Sr/ 86 Sr values of thermal waters and hydrothermal calcites of the Long Valley caldera geothermal system are more radiogenic than those of young intracaldera volcanic rocks. Five thermal waters display 87 Sr/ 86 Sr of 0.7081-0.7078 but show systematically lighter values from west to east in the direction of lateral flow. We believe the decrease in ratio from west to east signifies increased interaction of deeply circulating thermal water with relatively fresh volcanic rocks filling the caldera depression. All types of pre-, syn-, and post-caldera volcanic rocks in the west and central caldera have ( 87 Sr/ 86 Sr) m between about 0.7060 and 0.7072 and values for Sierra Nevada granodiorites adjacent to the caldera are similar. Sierran pre-intrusive metavolcanic and metasedimentary rocks can have considerably higher Sr-isotope ratios (0.7061-0.7246 and 0.7090-0.7250, respectively). Hydrothermally altered volcanic rocks inside the caldera have ( 87 Sr/ 86 Sr)m slightly heavier than their fresh volcanic equivalents and hydrothermal calcites (0.7068–0.7105) occupy a midrange of values between the volcanic/plutonic rocks and the Sierran metamorphic rocks. These data indicate that the Long Valley geothermal reservoir is first equilibrated in a basement complex that contains at least some metasedimentary rocks. Reequilibration of Sr-isotope ratios to lower values occurs in thermal waters as convecting geothermal fluids flow through the isotopically lighter volcanic rocks of the caldera fill.

Sources and fractionation processes influencing the isotopic distribution of H, O and C in the Long Valley hydrothermal system, California, U.S.A.

Abstract The isotopic ratios of H, O and C in water within the Long Valley caldera, California reflect input from sources external to the hydrothermal reservoir. A decrease in δD in precipitation of 0.5‰ km −1 , from west to east across Long Valley, is caused by the introduction of less fractionated marine moisture through a low elevation embayment in the Sierra Nevada Mountain Range. Relative to seasonal fluctuations in precipitation (−158 to −35‰.), δD ranges in hot and cold surface and groundwaters are much less variable (−135 to −105‰.). Only winter and spring moisture, reflecting higher precipitation rates with lighter isotopic signatures, recharge the hydrological system. The hydrothermal fluids are mixtures of isotopically heavy recharge (δD = − 115‰, δ 18 O = − 15‰) derived from the Mammoth embayment, and isotopically lighter cold water (δD = −135‰, δ 18 O = −18‰). This cold water is not representative of current local recharge. The δ 13 C values for dissolved carbon in hot water are significantly heavier (− 7 to − 3‰) than in cold water (−18 to −10‰) denoting a separate hydrothermal origin. These δ 13 C values overlie the range generally attributed to magmatic degassing of CO 2 . However, δ 13 C values of metamorphosed Paleozoic basement carbonates surrounding Long Valley fall in a similar range, indicating that hydrothermal decarbonization reactions are a probable source of CO 2 . The δ 13 C and δ 18 O values of secondary travertime and vein calcite indicate respective fractionation with CO 2 and H 2 O at temperatures approximating current hydrothermal conditions.

DOE Thermal Regimes Drilling Program through 1988

To investigate the thermal, chemical, and mechanical behavior of magma during its ascent toward the surface, four holes were continuously cored into an igneous system young enough to be essentially unchanged since its emplacement. The system investigated was the 600-year-old Inyo chain of rhyolitic domes and flows in Long Valley, California (Figure 2). Three holes were drilled in or adjacent to Obsidian Dome at the north end of the chain, just outside the Long Valley caldera. One was drilled beneath the phreatic south Inyo Crater, at the south end of the chain in the west moat of the caldera. The first hole was drilled in 1983 and penetrated the side of the lava dome, and continued into the underlying Tertiary basalt. The second hole, drilled in 1984, intersected and passed through the domes vent and into Sierran granitic rock. The third hole, also drilled in 1984, sampled the rhyolitic feeder dike 1 km south of the domes vent, where the magma did not reach the surface. The dike is part of the shallow dike system that underlies the Inyo chain.

Mobility and depositional controls of radioelements in hydrothermal systems at the Long Valley and Valles calderas

Abstract The loci and abundance of U and Th were examined in tuffaceous rocks encompassing hydrothermal systems at the Long Valley caldera, California and the Valles caldera, New Mexico. Aspects of these systems may be analogous to conditions expected in a potential site for a high-level waste repository in welded tuff. Examination of radioelements in core from scientific drill holes at these sites was accomplished by gamma-ray spectrometry and fission-track radiography. In the lateral-flowing hydrothermal system at the Long Valley caldera, where temperatures range from 140 to 200 °C, U is concentrated to 20 ppm in Fe-rich zones of varved tuff and to 50 ppm with Fe-rich mineral phases in tuff fragments of a calcite-cemented breccia. U-series disequilibrium in some of these samples suggests mobilization/deposition of parent U and/or its daughters. In the vapor zone of the Valles calderas hydrothermal system (temperature ~ 100 °C), the concordance of high U, low Th/U and decreasing whole-rock O-isotope ratios suggests that U was concentrated in response to hydrothermal circulation when the system was formerly liquid-dominated. In the underlying present-day liquid-dominated zone (temperature to 210 °C), U, up to several tens of parts per million, occurs with pyrite and Fe-oxide minerals, and in concentrations to several percents with a Ti-Nb-Y-rare earth mineral. In the Valles systems outflow zone, U is also concentrated in Fe-rich zones as well as in carbonaceous-rich zones in the Paleozoic sedimentary rocks that underlie the Quaternary tuff. Th, associated with accessory minerals, predominates in breccia zones and in a mineralized fault zone near the base of the Paleozoic sedimentary sequence. Relatively high concentrations of U occur in springs representative of water recharging the Valles calderas hydrothermal system. In contrast, considerably lower U concentrations occur in hot waters (> 220 °C) and in the systems outflow plume, suggesting that U is concentrating in the hotter part of the system. The Long Valley and Valles observations indicate that U and Ra are locally mobile under hydrothermal conditions, and that reducing conditions associated with Fe-rich minerals and carbonaceous material are important factors in the adsorption of U, and thus can retard its transport in water at elevated temperature.

A Natural Analogue for Storage of Radwaste in Crystalline Rocks

The Bryan-Eldora stock (Colorado) intruded the Precambrian Idaho Springs Formation metamorphic rocks 58 million years ago. Geochronologic-geochemical work by Hart et al. (1) has demonstrated that the heat from the cooling intrusive rocks was sufficient to affect mineral isotopic systematics up to 2,000 m. from the contact, and the nature of these isotopic perturbations can be explained by a simple diffusion model in turn based on various heat flow models. Our new studies are focused on elemental exchange between stock and intruded rock as a function of distance from the contact; the assumption is made that the stock is a very large, high heat source analogous to a waste form emplaced in the metamorphic rocks without benefit of canister or engineered backfill. Data for U, Th and the REE indicate actinide and lanthanide immobility except perhaps in the 0–2m. contact zone where some infiltration of the country rocks by stock-derived fluids occurred. Beyond 4 m. no stock-derived U, Th, REE or *Pb are noted. Further, whole rock Rb-Sr and stable 0 isotopic data indicate conductive cooling as opposed to convective, water-induced cooling. The intruded rocks possess low porosity and permeability; this helped prevent elemental migration during the 10 5 − 10 6 years of stock crystallization. The petrographic and geochemical studies show that the Idaho Springs (or equivalent) metamorhpic rocks are well suited for radwaste storage.

Radiogeological Assessment of Candidate Sites For Nuclear Waste Repositories, Exemplified by Studies of The Stripa Pluton, Sweden

Investigation of candidate sites for nuclear waste isolation will require an assessment of their radiogeologic settings. Studies at the Stripa research facility in granitic rock of central Sweden incorporated the distribution and abundance of naturally occurring radioelements in rocks encompassing the underground exp?riments and in the accompanying fracture-controlled groundwater system. These studies showed that besides defining the natural radioactivity baseline upon which the effects of radioactive waste will be superimposed, radioelement distributions can be used to determine the apparent age of the groundwater and its flow paths. In crystalline rocks, where the groundwater systems are confined to the joints and fractures, the uranium daughter element, radon-222 in the water serves as a natural tracer to locate fractures along which significant flow is occurring and to measure the flow rates. The heat production from radioactive decay of uranium-238, thorium-232, and potassium-40 in the rock, combined with measurements of regional and local geothermal heat flow, permit calculation of the apparent size of the rock mass that will encompass the repository. This method is especially useful in terranes such as at Stripa where the contacts between plutons and older rocks are concealed. 4 INTRODUCTION To properly characterize candidate sites for radioactive waste isolation, it will be necessary to obtain a good understanding of their radiogeologic settings. The distribution and abundance of the naturally-occurring radioelements, 238U, 232Th, their daughters and 40K in the rock mass encompassing the repository and in neighboring rocks, comprise the baseline upon which the effects of the radioactive waste are superimposed. The distribution of these radioelements is also a good indicator of the geochemical homogeneity of the rock mass. At the Stripa experimental facility in an inactive iron mine in central Sweden (1), radiogeologic studies included gamma spectrometric surveys on the surface and underground of the U, Th and K contents of the quartz monzonite pluton encompassing the experiments, high-grade metamorphic rocks surrounding the pluton, and neighboring larger granitic plutons (2). A geological cross section through the experimental workings comprises Figure 1. DISTRIBUTION OF RADIOELEMENTS The abundance of the radioelements, K, U and Th, was measured, both on the surface and underground by a portable gamma-ray spectrometer to obtain a preliminary indication of the geochemical homogeneity of the Stripa pluton and to calculate its radiogenic heat production. Gamma readings were made at both surface and underground locations with the detector held in contact with the rock. A hand specimen for subsequent laboratory analyses was collected at each sample location. The field gamma spectral measurements were calibrated by laboratory analyses of hand specimens and drill cores, permitting calculations of radioelement concentrations from field counting rates. OA

Radionuclides in Hydrothermal Systems as Indicators of Repository Conditions

Hydrothermal systems in tuffaceous and older sedimentary rocks contain evidence of the interaction of radionuclides in fluids with rock matrix minerals and with materials lining fractures, in settings somewhat analogous to the candidate repository site at Yucca Mountain, NV. Earlier studies encompassed the occurrences of U and Th in a ``fossil`` hydrothermal system in tuffaceous rock of the San Juan Mountains volcanic field, CO. More recent and ongoing studies examine active hydrothermal systems in calderas at Long Valley, CA and Valles, NM. At the Nevada Test Site, occurrences of U and Th in fractured and unfractured rhyolitic tuff that was heated to simulate the introduction of radioactive waste are also under investigation. Observations to date suggest that U is mobile in hydrothermal systems, but that localized reducing environments provided by Fe-rich minerals and/or carbonaceous material concentrate U and thus attenuate its migration. 11 refs., 6 figs., 1 tab.

Rush to judgment at Yucca Mountain

The Department of Energy (DoE) published a revised Civilian Radioactive Waste Management Program Plan late last year calling for an evaluation of “whether Yucca Mountain appears to be technically suitable for development as a geological repository” by the end of 1998. Almost concurrently, the DoE halted investigations that are critical for such an evaluation. These investigations concern the Calico Hills tuffs, the volcanic rocks that underlie the Topopah Springs welded tuffs that comprise the proposed repository.

Strontium Isotopic Study of Fracture Filling Minerals in the Grande Ronde Basalt, Washington

In the licensing process attention is given to many topics, including radionuclide migration and to hydrologic integrity of the repository rocks. The following conclusions were reached: (1) Strontium isotopic signatures of fracture-filling minerals are essentially identical to their host basalt, within analytical detection limits. The model for Sr mixing between reservoirs of different total and isotopic Sr is a powerful tool for investigating the origin of such fracture filling. (2) REE/CHON plots indicate that the rare earth elements (REE) in the fracture-filling minerals have been derived from their host basalts. (3) The INAA data for numerous trace elements indicate that the fracture-filling minerals have obtained their chemistry from a basalt source. (4) If water flowing vertically through the basalt had caused the precipitation of the secondary minerals noted, the chemistry of the minerals would reflect this water. This is not the case. (5) Studies of similar fracture-filling minerals from all proposed candidate repository horizons at the Hanford Site should be undertaken. 8 references, 3 figures, 4 tables.