Applications of Isotope Geochemistry to the Reconstruction of Yucca Mountain , Nevada , Paleohydrology Status of Investigations : June 1996

........................................................................................................._ 1 mtroduction................................................................................................_^ 1 Sample Distribution.............................................................................................................................................................^ 3 Sampling and analytical methods............................................................................................................................ 3 Paragenesis of Exploratory Studies Facility Occurrences..................................................................................................... 6 Most recent event: Calcite or opal? Precipitation or dissolution?........................................................................ 8 Dissolution events within the unsaturated zone?..................................................................................................... 8 Isotopic Data.......................................................................................................................................................................... 13 Implications of new Exploratory Studies Facility data to previous interpretations................................................. 13 Carbon-13-enriched calcite from the Exploratory Studies Facility......................................................................... 13 Silica-phase oxygen isotope studies........................................................................................................................ 15 Radiocarbon geochronologic studies....................................................................................................................... 18 Conclusions............................................................................................................................................................................ 21 References Cited.................................................................................................................................................................... 21 Appendixes............................................................................................................................................................................ 23 FIGURES 1. Maps showing location of the Yucca Mountain region, Nevada, the potential repository site, the Bullfrog Mine, sampled boreholes, Trench 14, and the Exploratory Studies Facility..................................................... 4 2. Map showing location of the Exploratory Studies Facility, topographic features, prominent faults, and the locations of sampled secondary mineralization occurrences near Yucca Mountain, Nevada................................... 5 3. Schematic illustrations of the typical paragenetic sequences found (a) in lithophysal cavities and (b) on fracture surfaces..................................................................................................................................................................... 7 4a.-4h. Photomicrographs showing: 4a. Late botryoidal opal with rhombic overgrowths of calcite that are, in turn, locally pimpled with tiny opaline spheres ........................................................................................................................_^ 9 4b. Late botryoidal opal grown on calcite and partially engulfed by a later stage of calcite............................................ 9 4c. Opal with overgrowths of a later opaline stage followed by calcite overgrowth coated with botryoidal opal that is, in turn, partially buried by late calcite.............................................................................................. 10 4d. Bladed calcite crystals with scepter-like terminations................................................................................................ 10 4e. Pyramidal to spire-like calcite forms from the basal zone of lithophysal secondary calcite mineralization........................................................................................................................................................................ 11 4f. Basal dissolution zone, with calcite microspeleothems or meniscus cements............................................................ 11 4g. Unetched surface of calcite crystal............................................................................................................................. 12 4h. Pitted surface of apparently etched calcite crystal face.............................................................................................. 12 5 7. Graphs showing: 5. Distribution of unsaturated-zone delta carbon-13 and delta oxygen-18 values of calcite plotted against depth.............................................................................................................................................................. 14 6. Distribution of delta carbon-13 and delta oxygen-18 values of Exploratory Studies Facility calcite samples plotted against distance from the portal........................................................................................................ 14 7. Preliminary radiogenic strontium isotopic compositions plotted against the stable carbon and oxygen isotopic compositions, and correlation of delta carbon-13 and delta oxygen-18 values of Exploratory Studies Facility calcite................................................................................................................................... 16 8-9. Histograms showing: 8. Yucca Mountainun unsaturated zone secondary silica delta oxygen-18 values.......................................................... 17 9. Distribution of carbon-14 ages from Exploratory Studies Facility calcite occurrences............................................. 20


INTRODUCTION
Yucca Mountain, Nevada, is presently under evaluation as a potential national subsurface repository for high-level radioactive wastes.Current designs for the potential repository would emplace wastes within the sequence of welded and nonwelded tuffs in the thick unsaturated zone of the mountain, several hundred meters below ground surface but still well above the modem water table.Siting within the unsaturated zone is advantageous in that it isolates the wastes from regional ground water; unfortunately, it also introduces climate-driven infiltration and percolation through the unsaturated zone as site-evaluation variables.Although present climate is arid, past climates have ranged to much wetter (R.M. Forester, U.S. Geological Survey, oral commun., 1997), and future climates are likely to be wetter, as well.The effects of future climatic changes on site performance are critical issues for site suitability assessment.
Efforts to reconstruct the hydrologic response of Yucca Mountain to past climatic variations, especially within the unsalurated zone, have focused on textural and geochemical study of secondary minerals deposited from past percolation fluxes and ground water.This report discusses stable carbon and oxygen isotopic data and mineral parageneses determined from secondary calcite and silica mineralization and describes reconnaissance radiogenic strontium (Sr) isotopic and carbon-14 ( 14C) age data and their applicability to paleohydrologic and paleoclimatic studies.Most of the new data reported here were collected from samples of secondary mineral occurrences accessed during construction of the Exploratory Studies Facility (ESF) tunnel.Construction of the tunnel was a fundamental component of site-characterization studies of Yucca Mountain.

Abstract
Evolution of understanding of the origins of secondary minerals within the Yucca Mountain tuffs is reflected in several papers (Whelan andStuckless, 1990 and1992;Whelan and others, 1994;and J.F. Whelan and R.J. Moscati, U.S. Geological Survey, written commun., 1997).The latter report summarized the evolution of studies of the secondary calcite and silica minerals within Yucca Mountain and updated understanding of the processes responsible for deposition of these minerals.It stated that: "investigation of the secondary minerals within the mountain, spurred by the study of the surficial deposits, revealed that the calcite and opal commonly coating fractures and lining open cavities in the unsaturated zone were deposited during downward percolation of meteoric waters ...These fluids reflected the oxygen isotopic signature of precipitation ... and had acquired distinctive carbon and strontium isotopic signatures from the thin overlying soils during infiltration that were recorded by the secondary calcite and opal deposited by the fluids as they percolated through the tuffs."(Emphasis added.) A stage of the secondary calcite, paragenetically early but volumetrically minor, is characterized by uncommonly carbon-13 ( 13C)-enriched isolopic compositions and by delta oxygen-18 (5 18O) values that decrease more rapidly with depth than succeeding calcite stages.That steeper 618O decrease may record a steeper geothermal gradient during formation of the paragenetically early calcite.Data collected from ESF occurrences during 1996 indicate that, at least locally, this paragenetically early calcite is volumetrically more abundant than drill hole studies had indicated.
Other studies of secondary mineralization in the ESF estimated Pleistocene percolation fluxes through the unsaturated zone by measuring secondary calcite and opal volumes and inferring rates of secondary calcite and opal formation from thorium-230/uranium (230Th/U) and 14C age determinations.If that early calcite stage were formed much earlier and under significantly different hydrologic conditions, as is suggested by its atypical 613C systematics, then its inclusion in the mineral record used to estimate past percolation fluxes might result in erroneous flux estimates.
This report describes efforts to map the parageneses of secondary mineral occurrences (appendix 1) within the ESF, discusses the stable carbon and oxygen isotopic compositions of calcite, the stable oxygen isotopic compositions of multiple stages of silica (chalcedony, quartz, and opal) deposition and some provisional 87Sr/86Sr ratios of calcite, and evaluates the apparent timing of latest calcite formation in several occurrences from the ESF based on provisional 14C age determinations.The purpose of this investigation is to use this isotopic characterization of secondary minerals to trace isotopic variations in the water that percolated through the sequence of volcanic rocks at Yucca Mountain and to reconstruct the hydrologic response of Yucca Mountain to past climatic variations.This study focuses mainly on samples obtained in excavations of the ESF and expands interpretations based on earlier studies.Sampling and examination of secondary mineralization within the ESF has increased our understanding of the distribution of secondary minerals and demonstrated a greater extent and thickness of secondary mineral occurrences, the very delicate nature of some of the calcite crystal forms, and a more widespread distribution of opal than had been observed from studies of drill-core occurrences.
Yucca Mountain is located near the western boundary of the Nevada Test Site in southwestern Nevada about 140 km northwest of Las Vegas and lies in the north-central part of the Basin and Range physiographic province.It consists of a series of rugged north-trending fault-block ridges composed of volcanic rocks with a general eastward tilt of 5° to 10° (Scott and Bonk, 1984).A thick section of Tertiary volcanic rocks (table 1) overlies Paleozoic sedimentary strata in the Yucca Mountain region (Byers and others, 1976) with units of the 12.8 to 12.7 Ma Paintbrush Group (Sawyer and others, 1994) forming most exposures (Christiansen and Lipman, 1965;Scott and Bonk, 1984).Two voluminous densely welded ashflow tuffs of the Paintbrush Group underlie Yucca Mountain (the Tiva Canyon and Topopah Spring Tuffs), separated by a much thinner interval of mostly nonwelded pyroclastic rocks (Buesch and others, 1996); the tuffaceous rocks are greater than 3,000 m thick in some places (Snyder and Carr, 1984).Welding and consolidation of Uie thick tuffaceous units produced zones of abundant lithophysal cavities, where lithification around gas bubbles formed open cavities in the rock.High-angle faults locally exhibit displacements of several hundred meters (Carr, 1984;Scott and Bonk, 1984); numerous studies (summarized in Volume 1, Part A, of the Site Characterization Plan for Yucca Mountain; U.S. Department of Energy, 1988, section 1.3) have shown that the tectonic setting of Yucca Mountain is complex, with the area situated at the intersection of contrasting structural zones.
This document was originally a milestone report to the Department of Energy Yucca Mountain Project, submitted in June of 1996.This version has benefited from several thorough editorial reviews, but the content herein is consistent with the original 1996 report.

SAMPLE DISTRIBUTION
Since tunneling began in 1995,145 occurrences of secondary mineralization within the ESF have been collected for the USGS, 88 by the Bureau of Reclamation during fiscal year (FY) 1995 as part of the  Buesch and others (1996).2This informal grouping represents multiple rhyolites of small areal extent erupted from multiple sources at the margins of calderas such as the Timber Mountain caldera complex north of Yucca Mountain.Correlations are incompletely understood for all such identified rhyolites and calderas.
Consolidated Sampling Program and 57 during FY 1996 by the USGS (Yucca Mountain Project) Isotope Hydrology Team.The location of the ESF and the locations of newly sampled boreholes (USW SD-7, USW SD-12, USW VH-1, USW VH-2, USW UZ-14, and UE-25 a #5) are shown on figure 1; the approximate locations of those ESF occurrences that have been sampled are shown on figure 2.

Sampling and analytical methods
Secondary mineral occurrences were examined from drill cores UE-25 a #7, USW UZ-14, USW SD-7, USW SD-12, and USW VH-2 and from underground exposures in the ESF.These samples are tabulated in appendix 2 along with each sample's Hydrogenic Deposits (HD) sample tracking number.Individual occurrences from drill core are designated by borehole name and depth below surface.Position within the ESF is defined by station number (a station is located every 100 m beginning at the North Portal) and incremental distance from the previous station.For instance, an occurrence collected 1456.7 meters from the North Portal would be designated as station 14+56.7.Occurrences within the ESF are listed in appendix 2 by their distance from the North Portal.
Sample textures, morphology, and mineralogy were described from hand specimens and pctrographic thin sections.Hand specimens were examined under a stereomicroscope at magnifications up to 75x at visible and short-wave ultraviolet (UV) wavelengths.Minerals were identified by their habit, reaction to hydrochloric acid, hardness, and UV fluorescence.Thick (100-to 200-micrometers) polished sections were prepared from most samples for examination in plane-polarized light and observation of cathodoluminescence (CL) behavior.Scanning-electron microscopic (SEM) observations were made on gold-coated sections and specimen fragments on a Cambridge StereoScan 250 Mkll instrument at an accelerating voltage of 20 kV and a working distance of 20 millimeters.
Calcite was sampled from hand specimens for stable isotope analysis by milling with dental burs.Sampling of individual mineralization stages, when such were distinct, was accomplished by milling under low-power magnification.Layers as thin as 0.1 millimeter could be milled from calcite crystal faces and from channels as narrow as 0.5 millimeter within mineral sequences.
Silica minerals were sampled by chipping or gouging from hand specimens or by milling with a diamond-impregnated dental bit.Impurities were removed by hand-picking, and the separates were then digested at room temperature in 0.1 N HC1 to remove possible carbonate impurities.

SAMPLE DISTRIBUTION
Procedures based on conventional techniques first described by McCrea (1950) were used to extract CO2 from carbonates.Oxygen was extracted from the silica separates by stepwise fluorination for more highly hydrated opaline separates according to procedures described by Haimson and Knauth (1983), or by reaction with BrF5 for less-hydrated separates according to procedures developed by Clayton and Mayeda (1963), and converted to CO2 by reaction with hot graphite.Stable carbon and oxygen isotopic compositions of the extracted CO2 were determined on Finnigan MAT 251 or 252 mass spectrometers.Stable isotope compositions are reported as the per mil deviations of the samples from the international standards PDB (for carbon) and SMOW (for oxygen).Delta (8) deviations are defined as 5 = ((Rsample ' Rstandard^slandard)) X 1000 (1 where R is the ratio of the heavier isotope (13C or I8O) to the lighter isotope ( I2C or 16O, respectively) in the sample or standard CO2.One-sigma reproducibility of the 813C and 818O values is routinely less than or equal to 0.1 and 0.15 per mil, respectively.
Ninety-eight aliquots of laboratory standard NBS-19 (or its in-house equivalent, TS-1) analyzed during the course of data collection had average 813C and 5I8O values of 1.91±0.02per mil and 28.32 ± 0.15 per mil.respectively, compared to accepted values of 1.92 and 28.65 per mil.All <5 I8O values reported in appendix 2 have been adjusted by +0.30 per mil to correct the measured deviation from the accepted value for NBS-19.
Strontium isotopic compositions (87Sr/86Sr) were measured on samples of calcite from the ESF.Small (about 10-mg) subsamples of calcite were milled or chipped from natural or sawn surfaces.The subsamples were leached with hydrochloric acid to remove calcite from opal and wall rock, and any residues were dried and weighed to determine the actual amounts of calcite that was dissolved.Strontium was separated using standard ion-exchange methods, and isotopic compositions were determined on an automated multi-collector solid-source mass spectrometer.The reported 87Sr/86Sr ratios are within about ± 0.005 per cent of the actual value.
Paragenesis is defined as the order of mineral formation or a characteristic association or occurrence of minerals (Gary and others, 1974).A paragenesis may include a wide variety of minerals or only one or two; it may be simple, or it may be complex with numerous resolvable depositional events.Within a mineralization sequence, many textural relationships may be clear, some may seem clear but are not, and some will be ambiguous.Paragenetic descriptions always contain an element of subjectivity imparted by formulating a genetic framework from empirical observations they are, as such, interpretative.Appendix 1 lists preliminary descriptions of paragenetic sequences of about 70 samples collected in the ESF during FY 1996 by the USGS.That appendix also contains a brief glossary of some of the terminology used in the following discussion.The occurrences described in appendix 1 represent only about half of the occurrences collected to date (February 1996) from the ESF.
Within the brittle and fractured welded tuffs exposed in the ESF, post-vapor-phase secondary minerals are generally restricted to the footwalls of fractures or the floors of lithophysae.This relation indicates that the fluids that precipitated secondary minerals did not fill these open spaces; that is, those tuffs were not within the saturated zone at the time of secondary mineralization.Mineralizing fluids apparently filled only the smallest-aperture fractures and formed, at most, only shallow pools on the floors of lithophysae.
Lithophysal-cavity parageneses differ from those in steeply dipping fractures in that the cavities have thicker accumulations of secondary minerals, they contain larger free-growing crystals of calcite that commonly display unusual bladed habits, and they contain much more opal (figs.3a and b).Lithophysalcavity paragenetic sequences begin with vapor-phase tridymite (± hematite), deposited while the tufs were still hot.Where lithophysae are connected to percolation-flux fracture pathways, those lithophysae may host later calcite and silica minerals.
The earliest secondary mineralization found in such lithophysae consists of massive silica (undiffcrentiated quartz and chalcedony, possibly with minor opal) with, locally, minor calcite that appears to underlie the massive silica.Calcite, with irregularly a.
early vapor-phase minerals quartz of massive silica stage chalcedony/opal of massive silica stage late bubbly or laminated opal blocky calcite stage bladed calcite and sceptor overgrowths

PARAGENESIS OF EXPLORATORY STUDIES FACILITY OCCURRENCES
distributed opal (both in time and space), generally completes the paragcnesis.Defined paragenelic stages are, therefore, early massive silica (± calcile) and later main-stage calcite (± opal).Main-stage calcite and opal, as defined here, are pertinent to model estimations of past flux.
Secondary mineral habits vary somewhat within the main stage of lilhophysae.Early main-stage calcite within the lithophysae is commonly blocky or tabular.Later main-stage calcite, however, commonly occurs as unusual bladed crystals, typically thin at the base and flaring outward at the top, a habit that results in a scepter-like cross section (figure 4d).Opal may be found throughout the main stage although it is typically more common in the later part.Opal most commonly occurs as scattered spheres or botryoidal to colloform masses (figures 4a-c) but locally forms thin laminated coatings.Thin-section observations indicate that opal deposition is commonly preceded or accompanied by dissolution of calcite substrates.
Secondary mineral parageneses in steeply dipping fractures differ in habit and mineral abundance from those in lithophysae.Steeply dipping fractures generally contain only blocky to tabular calcite, locally on patches of early-stage massive silica.The sceptered calcite blades and small-bubbled or laminated opal found in lilhophysal cavities are rare in steeply-dipping fractures.In the more horizontal fractures, however, the paragenetic sequences are similar to those found flooring lithophysae.This structural control indicates that spatial geometries that retard or halt flow, such as lithophysae or flat-lying fractures, promote formation of the coarse-bladed calcite as well as opal, perhaps because such settings allow solutions to pond and solutes to concentrate through evaporation.

Most Recent Event: Calcite or opal? -Precipitation or dissolution?
Dating the most recent record of percolation fluxes is important for correlation with regional climatic reconstructions and to provide bounds for assessment of performance of a potential repository.Observing the outer, free-growth surface of mineralization and identifying whether calcite or opal was the last-formed mineral seems straightforward, bearing in mind that the most recent evidence of percolation flux might be mineral dissolution.Scanning-electron microscope observations, however, have revealed complex cycles of latest calcite/opal formation (figures 4a-c).These observations show that most calcite crystal faces appear fresh (figures 4b, d, and g).Some calcite crystal faces, however, appear pitted (figure 4h) which may indicate calcile dissolution (Vaniman and Chipera, 1996).Dating of materials from these outermost surfaces may, therefore, determine the age of most recent mineral deposition or, if the last event was mineral dissolution, the maximum age of the corrosion event.Either age constraint provides useful information, because correlating climatic states with percolation fluxes (and chemistries) that resulted in secondary mineral dissolution is as important as identifying those climates that produced mineralizing percolation.

Dissolution events within the unsaturated zone?
Many secondary mineral occurrences within the ESF, particularly those flooring lithophysae, appear to have been corroded around their basal attachment to the tuffaceous substrate (noted in appendix 1 as "basal dissolution").This basal dissolution is local, commonly affecting only part of an occurrence that otherwise consists of firmly attached and tightly intergrown calcite crystals with pristine youngest crystal faces.Basal dissolution created voids several millimeters high by centimeters wide, a sort of small-scale cavernous porosity.Some of the calcite remnants in these "dissolution" zones are adorned with tiny, toothlike, calcite forms that appear to be overgrowths postdating dissolution (figures 4e and f).In fracture settings, basal dissolution is expressed as loosening of the attaching layer resulting in coatings that lie loosely on the fracture walls.
In many instances, the secondary mineral substrates are more permeable than the secondary mineralization coatings or growths they host, especially the bleached and altered margins associated with vapor-phase mineralization of lithophysae and fractured tuffs.In the laboratory, water dripped onto the bleached margin of a lithophysal cavity moved through the altered matrix and, within seconds, produced obvious wetting of the matrix 10 to 15 centimeters away.The high permeability of the altered rims of lithophysae provides a means for percolating water, perhaps at low flux rates, to contact and dissolve calcile from the base of a lilhophysal occurrence without corroding the later calcite in the occurrence.The generalizations discussed here and the specifics listed in appendix 1 are preliminary.Further petrographic and scanning-electron microscopic examination of samples collected during completion of the ESF might provide additional textural constraints that would clarify the hydrologic and geochemical histories of calcite and opal deposition.

ISOTOPIC DATA
Determinations of the 6 13C and 6 18O of 310 secondary calcite samples from ESF occurrences and 123 determinations from drill-core occurrences were performed to obtain isotopic characterizations of the secondary calcite (appendix 2).These measurements supplement data from previously published reports (Whelan and Stuckless, 1992;Whelan and others, 1994;Whelan and Moscati, U.S. Geological Survey, written commun., 1997) and are merged with those data on figures 5 and 6.Isotopic data obtained from samples taken from ESF occurrences located between the northern portal and station 40 are listed in appendix 2. Calcite from those ESF occurrences has 8 3C values ranging from -9 to +8.4 per mil and 8 18O values ranging from 10.5 to 20.6 per mil (fig.6).Preliminary determinations of the 87Sr/86Sr ratio of ESF calcite from occurrences near stations 5, 14, 16, 17,19, and 30 ranged from 0.70958 to 0.71250.

Implications of new Exploratory Studies Facility data to previous interpretations
Carbon isotopic compositions of unsaturatedzone calcite reflect exchange between infiltrating fluids and components of the overlying soils and are fundamentally controlled by variations in the proportions of C3 and C4 plants that comprise the surface plant community and which vary as a function of climate.Plants photosynthesize via two pathways, designated C3 and C4, which result in markedly different carbon isotopic compositions of about -26 and -13 per mil, respectively (see, for example, Quade and others, 1989).The C4 plants (such as many grasses) favor drier and hotter climates and are better suited to handle those climatic stresses, whereas wetter and milder climates promote growth of woody, herbaceous C3 plants.The isotopic compositions of soil organic matter and respired CC>2 in the soil are, therefore, closely related to climate, and the 6 13C values of dissolved carbonate in unsaturated-zone percolation (and the calcite formed from it) are a reflection of the climate at the time of infiltration.Oxygen isotopic compositions of unsaturated-zone calcite record the 6 18O of contemporaneous meteoric waters.The newly obtained S I3C and 6 18O values shown in figures 5 and 6 and tabulated in appendix 2 support the conclusions of Whelan and Stuckless (1992) that unsaturated-zone secondary mineralization formed in a vadose setting, largely from percolation of surface infiltration and at near present-day ambient temperatures.The new data reported here, however, demonstrate a much wider distribution of paragenetically early calcite than previous studies of drill-core occurrences had revealed.The early calcite has higher 8 13C values that are incompatible with the carbon isotopic compositions of soil-exchanged percolation.

Carbon-13-enriched calcite from the Exploratory Studies Facility
Local occurrences of early calcite from drillcore samples display atypically heavy ( 13C-enriched) 8 13C values (fig.5).Those occurrences (Whelan and Moscati, U.S. Geological Survey, written commun., 1997) also display 6 18O values that decrease more rapidly with depth, indicating that this early calcite was deposited at warmer temperatures and possibly was subject to a steeper geothermal gradient than prevailed during the formation of later, percolationdeposited calcite.
Exploratory Studies Facility calcite occurrences (fig.6) more commonly display 8 13C values greater than 0 per mil than had been observed in drill-core occurrences (fig.5).Most of that 13C-enriched calcite occurs within the upper half of the Topopah Spring Tuff.Because it is possible to examine and select the most paragenetically complete assemblages when sampling in the ESF, in contrast to the random set of occurrence intersections from drill core, the increased representation of 13C-enriched calcite in the ESF data set could reflect a sampling bias.However, it could as well indicate a real difference between the early hydrologic history of the Topopah Spring Tuff and the other unsaturated zone tuffs.The 13C-enriched calcite is much more common in the non-lithophysal zones than in the upper lithophysal zone of the Topopah Spring Tuff (fig.6), although the cause of this apparent lithologic control is not clear.Provisional measurements of the 87Sr/86Sr ratios of ESF calcite samples (20 analyses) provide further insight into the origin of the paragenetically early 13C-enriched calcite.Peterman and others (1992) and Marshall and others (1993) concluded that most of the secondary calcite in the unsaturated zone reflects pedogenic strontium sources which have an average 87Sr/86Sr ratio of 0.71215.Unsaturated-zone calcite occurrences just above the water table, however, had strontium isotopic compositions (average 87Sr/86Sr value of 0.70909) indicating addition of less-radiogenic strontium, perhaps from the saturated zone.Saturated-zone calcite 87Sr/86Sr ratios are compatible with derivation of strontium either from the host tuffs of Yucca Mountain or from underlying Paleozoic marine carbonate rocks during an incursion of heated Paleozoic-aquifer water (Marshall and others, 1992), possibly during formation of the Timber Mountain caldera (Broxton and others, 1987;Bish and Aronson, 1993).
Paragenetically early calcite has less-radiogenic 87Sr/86Sr, higher 5 13C, and lower 5 18O, whereas paragenetically later calcite exhibits more-radiogenic 87Sr/86Sr, lower 5 13C, and relatively higher 6 18O (figs.7a and b).Whether these trends reflect a continuum of 87Sr/86Sr ratios, or mixtures of early and late calcite created mechanically during sampling, is not yet clear.It is clear, however, that the less-radiogenic 87Sr/86Sr ratios may indicate a tuffaceous strontium source for the early calcite as opposed to the more radiogenic pedogenic calcite that appears to provide strontium for the later unsaturated-zone calcite.Furthermore, the rapid decrease of early calcite 5 18O values with depth indicates a steeper geothennal gradient and warmer fluid temperatures (fig.5).Both of the above observations are compatible with calcite deposition closely following eruptive activity, when geothennal gradients may have been higher and calcareous soils had not yet formed.Although these preliminary data indicate a distinctly different geochemical environment for the paragenetically early calcite, the distribution of that calcite is still spatially restricted to the footwalls of fractures and the floors of lithophysae.Therefore, other differences aside, the earliest and oldest calcite present in these samples still indicate formation under unsaturated conditions.

Silica-phase oxygen isotope studies
Silica minerals occur throughout the paragenesis.Tridymite formed as a vapor-phase mineral within lithophysal cavities and along cooling joints during initial cooling and outgassing of the tuffs.Chalcedony (cryptocrystalline quartz) and both drusy and coarse euhedral quartz appeared early and generally preceded calcite deposition, whereas opal occurs sporadically within and upon later calcite as laminated coats, botryoidal masses, or isolated hemispherical grains.
The fractionation of 18O between a silica mineral and the mineralizing fluid is determined by temperature; 5 18O values can be used, therefore, to infer the formation temperatures of such minerals.Histograms of the 5 18O values of the various silica phases sampled from Yucca Mountain drill cores and 1 0 outcroppings (appendix 3) display increasing 8 O with decreasing temperature (fig.8).Tridymite 5 18O values record deposition at high, nearly syn-volcanic, temperatures and low water-to-rock ratios, with 5 18O values in the range 9 to 17 per mil.Chalcedony, the earliest post-vapor-phase mineral in many occur-1 ft rences, has 8 O values ranging from 5.8 to 17.3 per mil.Quartz, occurring both.asearly drusy crusts within and upon the chalcedony and as sprays of clear, inclusion-free prismatic crystals as long as 1 cm, has 8 18O values ranging from 8.6 to nearly 24 per mil.Higher water temperatures are required by the lowest 6 18O values of both the chalcedony and quartz, likely greater than 120°C 1 (Moscati and Whelan, 1996).Such temperatures are consistent with formation either 1 There are no independent constraints on the temperatures of any unsaturated-zone fluids except fluid inclusion studies of late calcite suggesting formation temperatures less than 100°C (Roedder and others, 1994).A fluid 5 18O of-12.5 per mil is assumed only to permit qualitative comparison of the 8 18O values of the different silica phases.At cooler temperatures, this is a reasonable assumption, although the 8 18O of percolation has likely varied by at least ± 2 per mil around this value.At higher temperatures, water exchanges 180 with the rocks that it passes through which results in increased water 8 18O values.The amount of the increase will be a function of the temperature and the waterto-rock ratio; assuming a value of-12.5 per mil for the heated water produces minimum calculated mineral-formation temperatures.Clayton and others (1972) and Kita and others (1985) are the respective sources of the quartz (and chalcedony)-H2O and opal-H2O fractionation factors used to determine formation temperatures.Silica mineralization records a wide range of depositional conditions and possibly long depositional history, as well.Whether silica phases also have recorded climatic variability is problematic.Silica phases might have been, occasionally, the dominant secondary minerals forming from percolation, perhaps during times or climates when overlying soils were thin or absent and percolating fluids were deprived of that important source of calcium and carbon for calcite formation.
Opal is the latest silica phase in the unsaturated zone, generally occurring only with calcite.Late opal has 8 O values ranging from 18.0 to 27.9 per mil and, assuming water of-12.5 per mil, formed at temperatures of 7 to 45°C (Moscati and Whelan, 1996).Temperatures in the lower part of this range are unlikely.Past variability in 8 18O values of meteoric water and (or) percolation probably contributed to the relatively large range of opal 8 18O values.
Pedogenic opal from the overlying soils has the highest 8 18O values, ranging from 22 to nearly 32 per mil.Assumption of a water 8 18O value of -12.5 per mil produces estimates that the pedogenic opal formed at temperatures ranging from about 30°C down to an improbable -3°C.Such low estimated temperatures indicate that the opal with the highest 8 18O values likely was formed from waters with 8 18O values higher than -12.5 per mil.The 18Oenriched waters might reflect warmer and (or) wetter climates, or evaporative enrichment of the water precipitating pedogenic opal (Moscati and Whelan, 1996).
Silica-mineral 8 O values increase from early to late in the paragenetic sequence, implying an overall decrease of depositional temperatures with time.In addition, with the lowest calculated formation temperatures occurring within the overlying soils, opal 8 O values conform to increasing temperatures with depth in the unsaturated zone.Some of the early chalcedony and drusy quartz seem to have formed at somewhat elevated temperatures.Combined with their early position in the paragenesis, this indicates that they formed before the tuffs had completely cooled.Later quartz euhedra, however, appear seem to have formed at temperatures compatible with the modern geotherm and climate; their water-clear, inclusion-free crystal-Unity and euhedral habit are consistent with slow growth from relatively cool fluids.

Radiocarbon geochronologic studies
The timing of latest calcite formation in ESF occurrences is being studied with 14C and 230Th/U geochronometers.Carbon-14 age determinations can be performed on far smaller amounts of calcite than can the 230Th/U method, an advantage that permits greater sampling resolution.Calcite 14C contents, however, may be susceptible to post-depositional contamination from modern carbon sources such as carbon-bearing fluids or gases in the unsaturated zone.Carbon-14 decays with a half-life of about 5730 years.Although the analytical precision of accelerator mass spectrometry (AMS) is excellent, age determinations begin to lose credibility at ages about 40 ka.At that age, over 99% of the initial 14C has decayed, and minimal contamination by modern 14C can seriously compromise age determinations, especially from small (10-to 30-mg) samples.
Past studies have demonstrated that percolating fluids in the unsaturated zone at Yucca Mountain obtain their 8 13C signature largely through interaction with carbonate and organic matter in the overlying soils (for example, Whelan and others, 1994).This interaction is characterized by the reaction CaCO3 + CO2 + H2O (2) in which, as CO2 is taken into solution, calcite is dissolved.The CO2, derived primarily from the oxidation of soil organic matter but secondarily from diffusion of atmospheric gases, is near 100 percent modern carbon (pmc).The calcite dissolved according to equation 2, however, may be old and have 0 pmc ("dead" C), and the HCO3~ of the resulting solution could have a 14C content as low as 50 pmc; infiltrating waters could, therefore, have an initial 14C age of one half-life, or about 5700 years.It is unlikely, therefore, that either percolating waters, or calcite precipitated from them in the unsaturated zone, ever record the correct timing of infiltration.Furthermore, the 14C clock for secondary calcite is not set until the calcite is precipitated, so estimates of the timing of infiltration must be adjusted for the travel time of the percolation to the site of deposition.
An ancillary undertaking is to evaluate the likelihood of exchange between the outer surfaces of secondary calcite crystals and later, 14C-bearing, gases and fluids in the unsaturated zone.Such exchange might create a 14C-enriched outer layer on the calcite crystals and, therefore, a young age bias in the 14C data.To test this possibility, the crystal faces of freegrowing calcite from several samples were etched briefly with dilute HC1 to remove the outer surface.Those etched surfaces were then sampled, as were comparable unetched free-growth calcite surfaces from the occurrence.A minimal difference between these paired 14C age determinations might indicate that post-depositional interaction with rock gases is not a concern.A large difference would be compatible with a thin surface layer of 14C-contaminated calcite on a 14C-"dead" substrate; such a relationship would also, however, be in accord with extremely low calcite depositional rates and most of the 14C-datable calcite record having been removed by the acid etching.This is a moot point, however, in that if calcite depositional rates are that low, then calculated 14C ages are probably unrealistic anyway.
In spite of the stated caveats, 14C dating of calcite still offers useful constraints on the history of percolation flux in the unsaturated zone, primarily because, in many of the occurrences, calcite was the last or only mineral formed.(This is especially true for the fracture occurrences.)Furthermore, because ages can be determined from calcite subsamples as small as 10 mg, 14C permits finer-scale sampling and, therefore, better age resolution of calcite formation than can other dating techniques.
To date, subsamples submitted for 14C AMS age determination have come from (1) the latest surfaces of free-growth calcite crystals or (2) the basal zones of calcite dissolution present in many samples.All of the subsamples from zones of basal dissolution contain a mixture of the "late" overgrowths and the residual, possibly 14C-dead, calcite substrate.These subsamples should still provide an indication, however, of whether a given dissolution event occurred within the past 40 ka, and the subsamples are therefore significant for reconstruction of hydrology/climate interactions in the recent past.
Provisional results of the 14C geochronometry indicate calculated 14C ages (radiocarbon years) in a range from greater than 16 ka to about 42 ka (fig.9).A significant percentage of the samples showed calculated ages less than 32 ka, indicating a significant component of modem carbon.This relation implies that much of the secondary calcite sampled in the ESF contains at least thin layers deposited during or since the last glacial cycle, and that calcite 14C measurements provide at least a relative tool for spatial and temporal mapping of percolation pathways within the unsaturated zone at Yucca Mountain.
None of the data (shown in appendix 3 and fig.8) discussed here tested the possibility of surfacelayer 14C contamination in ESF samples.Samples of thick mammillary calcite crusts, however, collected from the unsaturated zone of the Sterling Mine on Bare Mountain (fig. 1) west of Yucca Mountain, were tested.Opposite halves of a sawn specimen were ground and polished to reveal the growth banding.One half was briefly etched in dilute (0.2 N) HC1, then both were washed ultrasonically in deionized water, and the youngest band of calcite was milled from each half.The acid-etched sample had an apparent C age of 15.3 ± 0.12 ka, whereas the unetched sample gave an age of 14.7 ± 0.12 ka.The minimal difference between the two ages provides little support for post-depositional exchange between modern carbon and unsaturated-zone gases or fluids during the past 15 ky, at least at Bare Mountain.1. Post-vapor-phase secondary mineralization in the unsaturated zone consisted of an early stage dominated by silica deposition, locally with coeval or possibly precursor calcite, followed by main-stage calcite and opal.Main-stage mineralization, as defined herein, groups all early-stage calcite and opal deposition.This is probably a simplification, and further study might reveal a more complex depositional history.
2. Early calcite is characterized by heavy 8 13C values and 8 18O values that increase more rapidly with depth than those of main-stage calcite.Coupled with its association with formation of early chalcedony and quartz, this relation may indicate that the early calcite formed at somewhat higher temperatures than the main-stage calcite and that early calcite should not be included in estimates of past percolation fluxes based on the secondary mineral record.
3. Preliminary strontium isotope analyses indicate that the early calcite contains less-radiogenic strontium than does the main-stage calcite.Rather than a pedogenic strontium source as is the case for the main stage, the tuffs themselves may have been the source of strontium in the early calcite.
4. Delta-18O values of the silica phases quartz, chalcedony, and opal indicate that some of the early massive-silica-stage phases must have formed from heated water, although parts of the massive-silica stage are compatible with modern or near-modem rock temperatures.Opal 8 18O values argue that evaporative 1 C processes increased the 8 O of the precipitating water in the soil zone.
5. Preliminary radiocarbon age determinations argue that in situ interactions with later carbon-bearing water and gases have not significantly altered the 14C ages of the calcite.Radiocarbon ages of the latest calcite in many of the occurrences may indicate relatively recent formation of the outermost calcite layers.

Figure 3 .
Figure 3. Schematic illustrations of the typical paragenetic sequences found (a) in lithophysal cavities and (b) on fracture surfaces.Descriptive terminology is defined in the glossary included in Appendix 1.

IFFigure 4a .
Figure 4a.Photomicrograph of late botryoidal opal with rhombic overgrowths of calcite that are, in turn, locally pimpled with tiny opaline spheres.Scale bar indicates 400 microns.

Figure 4b .
Figure 4b.Photomicrograph of late botryoidal opal on calcite (not shown) partially engulfed by a later stage of calcite.Note the clean uncorroded surfaces of the calcite and opal.Scale bar indicates 200 microns.

Figure 4c .
Figure 4c.Photomicrograph of opal (lower left in left image) with overgrowths of a later opaline stage (white spheroids) followed by calcite overgrowth coated with botryoidal opal that is, in turn, partially buried by late calcite.Image on the right is an approximately 3X enlargement of the later mineralization stages.Scale bar indicates 100 microns.

Figure 4e .
Figure 4e.Photomicrograph of pyramidal to spire-like calcite forms on the basal zone of lithophysal secondary calcite mineralization apparently corroded by later solutions; the relatively fresh faces of this calcite indicate that it may be neoformed and postdate the dissolution event.Scale bar indicates 100 microns.

Figure 4f .
Figure 4f.Photomicrograph of a basal dissolution zone, with delicate forms representing miniature speleothems or meniscus cements.Image on right is an approximately 3X enlargement.Scale bar indicates 100 microns.

Figure 4h .
Figure 4h.Photomicrograph of pitted surface of apparently etched calcite crystal face.Image on right is any approximately 3X enlargement of the etched surface.Scale bar indicates 40 microns.

Figure 5 .Figure 6 .
Figure 5. Distribution of unsaturated-zone 613C (o symbol) and 618O (+ symbol) values of calcite plotted against depth (in meters).Occurrences of 13C-enriched paragenetically early calcite are designated by (613C) and (818O) symbols.The hatched line shows the predicted 618O of calcite precipitated along a hypothetical geothermal gradient of approximately 100° C/km from a fluid with a 618O of approximately -12.5 per mil.

Figure 7 .
Figure 7. Preliminary radiogenic Sr isotopic compositions plotted against the 613C ( ) and 618O (X) isotopic compositions of the Exploratory Studies Facility (ESF) calcite (A), and the correlation between 613C and 818O values of the same ESF calcite ( ), shown in (B).In general, larger 513C values correlate with older occurrences.

Figure 9 .
Figure 9.Histogram showing the distribution of 14C ages from the Exploratory Studies Facility calcite occurrences.Upper x-axis shows the apparent age calculated from measured percentages of modern carbon in the sample; no attempt was made to correct for incorporation of dead carbon or ground-water travel time.Lower x-axis displays the relationship between percent modern carbon and apparent age, and underscores the large error that trace amounts of 14C contamination cause in older materials.System blanks are the amounts of 1 C measured in samples of Paleozoic calcitic marble subjected to the entire extraction and Accelerator Mass Spectrometer analysis process.
Caicite delta carbon-13 and delta oxygen-18 values from secondary mineral occurences within the Exploratory Studies Facility (ESF) and drill holes near Yucca Mountain, Nevada (Continued) in borehole or distance from ESF portal (in meters) PDB-Pee Dee Belemnite standard SMOW-Standard Mean Ocean Water standard

TABLE 1 .
Major stratigraphic units in the Late Cenozoic volcanic rock sequence of the Yucca Mountian area, Nevada...... 3 In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)-a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Table 1 .
Major stratigraphic units in the Late Cenozoic volcanic rock sequence of the Yucca Mountain area.Ages are given in millions of years (Ma).Modified from Sawyer and others 91994) and
I Figure 2. Location of the Exploratory Studies Facility, topographic features, prominent faults, and the locations of sampled secondary mineralization occurrences, near Yucca Mountain, Nevada.
Appendix 1. Paragenetic sequences of secondary mineral occurrences within the Exploratory Studies Facility (ESF), Yucca Mountain, Nevada AP P E NDI X Paragenetic sequences of secondary mineral occurrences within the Exploratory Studies Facility (ESF), Yucca Mountain, Nevada (Continued)