Steven J. Lambert

Stable-isotope studies of rocks and secondary minerals in a vapor-dominated hydrothermal system at The Geysers, Sonoma County, California

The Geysers, a vapor-dominated hydrothermal system, is developed in host rock of the Franciscan Formation, which contains veins of quartz and calcite whose δ^(18)O values record the temperatures and isotopic compositions of fluids prevailing during at least two different episodes of rock-fluid interaction. The first episode took place at about 200°C, during which marine silica and carbonate apparently interacted with ocean water entrapped in the sediments to form veins of quartz and calcite whose δ^(18)O values were around +19 and +16%, respectively. The calculated water/mineral ratios were less than unity. The water may have profoundly influenced the δ^(18)O values of spilitic basalts during their metamorphism to greenstones. Serpentinization and structural emplacement of ophiolite slabs were isotopically unrelated to this episode, which was essentially a low-grade (post-Cretaceous?) burial metamorphism. D/H ratios of actinolite, chlorite, and micas in host rocks were more profoundly altered during this episode than were ^(18)O/^(16)O ratios. A paleogeothermal gradient of about 53°C/km has been inferred for this episode, from δ^(18)O-depth distributions of vein minerals. The second episode, in part recorded by cogenetic vein quartz and calcite δ^(18)O values of +4 to +6% and +1 to +3%, respectively, began with large quantities of meteoric water circulating in fractures in the rock at temperatures of 160–180°C in response to the initiation of the Pliocene-Pleistocene Clear Lake magmatism. The temperature rose, and with the restricted circulation of fluids the ancestral hot-water system evolved into the presently active vapor-dominated system, which according to the cogenetic vein quartz and calcite δ^(18)O values involved temperatures as high as 320°C and fluid/mineral ratios near unity. The change in the oxygen-isotopic composition of the serpentinite within the host rock during this later activity was negligible. The δ^(13)C values of vein calcite at The Geysers reflect both a marine carbonate and organic component of carbon, but carbon-isotope exchange has been facilitated by the vapor-dominated hydrothermal fluid to a greater degree than in any other episode or in other hot-water systems.

Geochemistry of the waste isolation pilot plant (WIPP) site, southestern New Mexico, U.S.A.

Abstract An extensive geochemical data base, including analyses of major and minor solutes, mineralogical studies of core samples, and isotopic studies of waters, carbonates and sulfates, has been assembled for evaporites and related rocks in the northern Delaware Basin of southeastern New Mexico. These data were compiled for the geological and hydrological characterization of the Waste Isolation Pilot Plant (WIPP), which is excavated in the evaporites of the Salado Formation. These data were evaluated in order: (1) to determine the stability of the evaporite mineralogy over geological time; (2) to compare the aqueous geochemistry with host rock mineralogy; (3) to delineate the nature and timing of water-rock interactions, such as dissolution and recrystallization; (4) to determine the geological and climatic conditions that have governed groundwater recharge. The resulting synthesis of data and current hypotheses concerning the origin, composition and history of waters in the evaporite rocks and related units of the Delaware Basin provides a tentative conceptual model for the behavior of the water-rock system since the deposition of the evaporites in the Permian. Essential components of this model include: (1) widespread Late Triassic/Early Jurassic evaporite recrystallization; (2) accumulation of deep-basin brines isolated from meteoric recharge; (3) evaporite dissolution by meteoric waters flowing in carbonates and sulfates interbedded in the uppermost Permian section and at the basin margin; (4) lateral rather than vertical infiltration of pre-Holocene meteoric waters in the uppermost Permian section; and (5) climatic conditions presently less conducive to recharge than in the Late Pleistocene.

Evaporite Dissolution Relevant to the Wipp Site, Northern Delaware Basin, Southeastern New Mexico

Evaluation of the threat of natural dissolution of host evaporites to the integrity of the Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico has taken into consideration (1) the volume of “missing” rock salt, (2) the occurrence (or not) of characteristic dissolution brines, (3) geomorphic features, some of which are unrelated to dissolution, and (4) the time intervals over which dissolution may have been active. Even under the assumption that all “missing” halite was originally present and has been removed by dissolution, there is no evidence of active preferential removal of the lower Salado Formation halite by any geologically reasonable process. The geologic record contains evidence of dissolution in the Triassic and Jurassic; to constrain all removal of basinal halite to the late Cenozoic yields an unrealistically high rate of removal. Application to the lower Salado of a stratabound mechanism known to be active in Nash Draw, a near-surface feature within the Basin, allows a minimum survival time of 2,500,000 years to be predicted for the subsurface facility for storage of radioactive waste at WIPP. This calculation is based on an analysis of all known dissolution features in the Delaware Basin, and takes into account the wetter (pluvial) climate during the past 600,000 years.

Geochemical evidence for transient karstic water/rock interaction in evaporites of the northern Delaware Basin, New Mexico, USA

Permian evaporites in the northern Delaware Basin (southeastern New Mexico) have experienced varying degrees of water/ rock interaction, locally resulting in karstic landforms such as topographic depressions, sinkholes, and blind valleys. Meteoric groundwaters and the rocks with which they have interacted bear geochemical signatures that delineate such interactions even in the absence of surficial indicators. Based on fresh-water potentiometric heads alone, hydraulically confined groundwater appears to flow north to south, at steady state across the site of the Waste Isolation Pilot Plant (WIPP), but freshens from an ionic strength of 0.3–1.6 to <0.1 molal south of the site. This abrupt change in hydrochemical facies is not caused by younger vertical recharge preferentially occurring in the south; residence times based on radiocarbon (∼14 ka) of less saline Ca-SO4 waters in the south are no shorter than those of more saline Na-Cl waters in the north.234U/238U activity ratios (A.R.’s) monotonically increase from west to east, correlative with a 104 decrease in fracture-permeability, but normal to the that of inferred modern flow. Preservation of high A.R.’s argues against rapid oxic recharge from shallower units. These geochemical constraints suggest that modern groundwater flow is transient, and that the original paleoflow direction at the time of recharge in the late Pleistocene had a significant west-to-east component. The geochemical record of paleoflow has not yet been obliterated by Darcian flow, whereas the potentiometric contours have responded to post-Pleistocene potentiometric changes. The records of interactions between these groundwaters and adjacent evaporite rocks are preserved in the characteristic87Sr/86Sr ratios of gypsum altered from anhydrite and the18O/16O ratios of calcite altered from dolomite. These zones of alteration occurring at less than 300 m depth are interbedded with less altered zones bearing geochemical signatures more characteristic of marine evaporites, suggesting stratabound rather than vertical flow. Combinations of these geochemical parameters provide the basis for distinguishing hydrologic regimes in which the water/rock ratios have been higher (karstic) from those where evaporite dissolution has been less active.

Degradation of Portland Cements Exposed to Evaporite Brine at Hydrothermal Temperatures

The long-term integrity of cementitious materials is of interest where cement is used as part of the isolation system in underground workings sited in evaporite rocks. However, the slow rates of cement/fluid interactions at room temperature provide minimal guidance for predicting long-term cement behavior. To accelerate reactions that degrade cement, hydrothermal brine/cement interaction experiments were carried out at temperatures between 100° and 250° C. The pastes used in these experiments were two batches containing 65 (weight) % Class H cement and 35 % fly ash (one with Class C and one Class F, both with 10 mass equivalent % plaster), and one batch of Type K expansive cement, all with NaCl-saturated mix water. The brine used in testing was based on a composite representation of groundwaters from the Rustler formation of southeastern New Mexico, a mixed evaporite/clastic rock sequence. This brine was sufficiently concentrated that its use afforded an opportunity to study several mechanisms likely to degrade cement performance. At the conclusion of each experiment solid products were characterized and the coexisting fluids analyzed. In general, the cement was extensively altered by the brine at all temperatures assessed. In addition to the expected sulfate attack, it was found that magnesium effectively displaced calcium from the CSH gels that impart strength to cementitious materials. Sodium and potassium did not play a role in altering the cement.

Authigenic Clay Minerals in the Rustler Formation, WIPP Site Area, New Mexico

Transuranic waste is planned for disposal in the Late Permian evaporites of the Delaware Basin, southeastern New Mexico, at the WIPP Site. The disposal horizon is located in the bedded halite of the Salado Formation, which is overlain by the impure halite-anhydrite (gypsum)-siltstone-mudstone of the Rustler Formation. The Rustler Formation also contains two dolomite members, the Magenta and Culebra, which transmit water. The Culebra Member is suspected to have actively interacted with waters at time(s) from the Late Permian to the present, and it is important to assess the reactivity of these waters in conjunction with WIPP stability. We have investigated the Rb-Sr systematics of clay minerals from the Culebra Member and elsewhere in the Rustler Formation. By separating the less than 0.125 μm size material we are able to deal with presumed true authigenic clay minerals. The authigenic fraction is especially sensitive to chemical and isotopic exchange with waters, and an episodic exposure to a large amount of water will re-set the clay minerals to such a time. Our data yield 259 ± 22 Ma Rb-Sr isochron, which is consistent with the Late Permian age of the Rustler Formation. This age demonstrates that age-determining cations in these clay minerals have preserved their isotopic and chemical integrity since the Late Permian.

Radiometric Dating of Ochoan (Permian) Evaporites, Wipp Site, Delaware Basin, New Mexico, Usa

We have attempted radiometric dating of halide-sulfate salts and clay minerals from the Delaware Basin, New Mexico, USA, as part of geochemical study of the stability of the evaporite sequence at the WIPP (Waste Isolation Pilot Plant--a US DOE facility) site. We undertook this dating to determine: (1) primary age of evaporite genesis or time(s) of recrystallization, (2) if previously undated evaporite minerals (leonite, polyhalite, kieserite) give useful data, and (3) if the detrital clay minerals have been radiometrically reset at any time following their incorporation into the evaporite medium. We have shown earlier that polyhalites can indeed be successfully dated by the K-Ar method, and once corrections are applied for admixed halide minerals, dates of 210-230 Ma for the Delaware Basin are obtained. Rb-Sr isochrons from early stage sylvites-polyhalites- anhydrites yield 220 ± 10 Ma, even when some sylvites yield lower K-Ar dates due to l1s of 40 Ar*. K-Ar dates on leonites and kieserites are also low due to 40 Ar* loss, but their Rb-Sr dates are higher. Detrital clay minerals from the Delaware Basin collectively yield a highly scattered isochron (390 ± 77 Ma), but samples from a local area, such as the WIPP Site, give a much better age of 428 ± 7 Ma. These dates show that the interaction between the clay minerals and the evaporitic brines was insufficient to reset the clay minerals Rb-Sr systematics. In a related study, we note that a dike emplaced into the evaporite at 34 Ma had only very limited effect on the intruded rocks; contact phenomena were all within 2 m of the dike. All of our geochemical (radio-metric and trace element) studies of the WIPP site argue for preservation of the isotopic and chemical integrity of the major minerals for the past 200 Ma.