The {sup 207}Pb/{sup 235}U ages for 14 subsamples of opal or chalcedony layers younger than calcite formed at elevated temperature range between 1.88 {+-} 0.05 and 9.7 {+-} 1.5 Ma with most values older than 6-8 Ma. These data indicate that fluids with elevated temperatures have not been present in the unsaturated zone at Yucca Mountain since about 1.9 Ma and most likely since 6-8 Ma. Discordant U-Pb isotope data for chalcedony subsamples representing the massive silica stage in the formation of the coatings are interpreted using a model of the diffusive loss of U decay products. The model gives an age estimate for the time of chalcedony formation around 10-11 Ma, which overlaps ages of clay minerals formed in tuffs below the water table at Yucca Mountain during the Timber Mountain thermal event

Amelin, Yuri V.

Neymark, Leonid A.

Paces, James B.

Peterman, Zell E.

Whelan, Joseph F.

UNT Digital Library

AGE CONSTRAINTS ON FLUID INCLUSIONS IN CALCITE AT YUCCA MOUNTAIN Leonid A. Neymark Pacific Western Technologies, Ltd. c/o U.S. Geological Survey DFC Box 25046, MS 963 Denver, Colorado 80225, USA Yuri V. Amelin Jack Satterly Geochronology Lab Royal Ontario Museum 100 Queen’s Park, Toronto, Ontario M5S 2C6, Canada (303) 236-7898 (416) 586-5811 James €3. Paces Zell E. Peterman Joseph F. Whelan U.S. Geological Survey U.S. Geological Survey U.S. Geological Survey DFC Box 25046, MS 963 Denver, CO 80225, USA DFC Box 25046, MS 963 Denver, CO 80225, USA DFC Box 25046, MS 963 Denver, CO 80225, USA (303) 236-0533 (303) 236-7883 (303) 236-767 1 I. INTRODUCTION Tuffs of Miocene age in the unsaturated zone (UZ) at Yucca Mountain, Nevada contain sporadic deposits of cal- cite and silica in fractures and on floors of lithophysal cavi- ties. Calcite from older portions of some of these coatings contains two-phase fluid inclusion assemblages (TPFIA) and has low 6l80 values that imply formation temperatures between about 35 and 85 “C 14. Timing of this calcite depo- sition is important for establishing the thermal history of the mountain, which is being evaluated as a potential site for the construction of a high-level nuclear waste repository. Silica (opal and chalcedony) from the UZ at Yucca Mountain is interlayered with the calcite and has large con- centrations of U (commonly 10’ to 10’ pg/g compared to pg/g in pure calcite). It also has small concentra- tions of common Pb making it suitable for U-Pb A general time framework for the formation of the coatings was established by previous geochronological studies 5-8, which showed that the coatings grew in a direction from their bases to outer surfaces at slow long-term average rates (mm/m.y.) over the past 10 m.y. A prominent early stage of chalcedony locally intergrown with drusy quartz commonly postdates TPFIA-bearing calcite. Because TPFIA were not found in the late-stage calcite” 3, the TPFIA-bearing ele- vated-temperature calcite is presumed to be at least several million years old, although more direct geochronological evidence is needed to confirm this conclusion. This paper presents new 207Pb/235U ages for opallchalcedony layers microstratigraphically younger than TPFIA-bearing and l 0 w - 6 ~ ~ 0  calcite and expanded interpretation of previously reported U-Pb isotope results for chalcedony from the early silica stage ’. Assuming that fluid inclusion data give reli- to able temperatures of calcite deposition, these new ages con- strain timing of fluids with elevated temperatures in the UZ at Yucca Mountain. This work was completed in coopera- tion with the U.S. Department of Energy, under Interagency Agreement DE-AI08-97NV 12033. 11. SAMPLES AND METHODS Five samples of mineral coatings containing both TPFIA-bearing calcite and interlayered silica were selected for U-Pb age determinations. The samples were collected from lithophysal cavities and fractures in tuffs exposed in the Exploratory Studies Facility (ESF) from alcove 5, at 28.5 m from the main tunnel; at distances from tRe North Portal of the ESF of 161.9,7165.8, and 7294.5 m, and from the east-west cross drift at 125 m from the ESF. Ten sub- samples of chalcedony and opal were separated using dental burrs and sharp needles from layers microstratigraphically above TPFIA-bearing calcite in the same about 150-pm- thick sections that were used for fluid-inclusion studies ’. This technique produced silica subsamples ranging in weight from 0.3 to 2.3 mg and 207Pb/235U ages with typical 2 0  errors of 3 to 30 percent. In addition, silica Iayers above calcite with low 6l80 values were subsampled using the same technique from four calcite and silica coatings (ESF distances 1215.58, 6600, 7165.8, and 7574.7 m). In all sub- samples 234U/238U was also measured. Analytical techniques used for the U-Pb age determinations and U isotope analysis have been described in detail elsewhere 6,7. Total procedure blank during this study did not exceed 3 picograms of Pb, which allowed analyses of sub-milligram-sized subsamgles. DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor arty of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or use- fulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spc- cific commercial product, process, or %Nice by trade name, trademark, manufac- turer, or otherwise dots not necessarily constitute or imply its endorsement, ream- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. ESF AL#5!, 0+28,5E Fig. I .  Cross-section of sample from Alcove 5 showing 207Pb/235U ages (Ma) for chalcedony (Chal) layers overly- ing quartz (Qtz) and early calcite (Cc, black symbols) hosting two-phase fluid inclusions and underlying late calcite (Cc, white symbols). F 94.5 m Fig. 2. Cross-section of sample HD2250fa, ESF distance 7294.5 m, showing 207Pb/235U ages (Ma) for chalcedony layers (white outline) overlying early calcite (Cc, white symbols) hosting two-phase fluid inclusions and underlying late calcite (Cc, black symbols) without two-phase fluid inclusions. 111. RESULTS AND DISCUSSION Uranium concentrations in the analyzed silica ranged from 1.8 to 120 ppm, with the lower values measured for chalcedony subsamples. These U concentrations in chalced- ony are among lowest previously observed at Yucca Moun- tain. Blank-corrected 206PW204Pb ratios ranged from 19.8 to about 400. Fourteen new U-Pb ages were obtained during this study. Because 206Pb/238U ages for young materials are af- fected by initial radioactive disequilibrium, 207Pb/235U ages mainly are discussed in this paper. The 207Pb/235U ages for ten subsamples of silica collected above TPFIA-bearing calcite ranged between 1.88k0.05 (opal) and 9.0k1.0 Ma (chalcedony). Similar data for four subsamples collected from opalkhalcedony overlying calcite with low 6"O val- ues have 207Pb/235U ages ranging from 3.97k0.05 to 9.7k1.5 Ma. Figures 1 and 2 show cross-sections of two samples where chalcedony layers overlying TPFIA-bearing calcite and underlying TPFIA-free calcite were dated. The data shown in Fig. 1 and 2 indicate that elevated-temperature calcite is older than 6 to 8 Ma. These data are consistent with the younger ages (<2 Ma) determined from silica asso- ciated with the TPFIA-free late-stage calcite 3* 5 ,  '. Addi- tional information on the closed or open system behavior of U and its decay products was obtained by assessing the con- sistency of obtained U-Pb ages with measured 234U/238U for all subsamples. Most measured 234U/238U activity ratios U- Pb ages of the subsamples and indicating that these silica subsamples remained closed relative to U mobility during the past several hundred thousand years. Previously obtained and new U-Pb data from chalced- ony subsamples (samples HD2021, HD2029, and HD2257 in the shallow ESF) representing early silica deposition plot below the equilibrium concordia curve (Fig. 3), indicative of the preferential loss of U decay products from the ana- lyzed subsamples. If chalcedony is formed by a process of progressive crystallographic ordering and transformations (i.e. opal-A + opal-CT -+ chalcedony), U-decay products may migrate out of the original system, complicating inter- pretations of U-Pb ages. These transformations may be caused by changes in thermal conditions and aging of opal 9- The data points in Fig. 3 follow a pattern for U-Pb sys- tems with continuous leakage of both radiogenic Pb and radioactive daughters of 238U such as Rn 12. The discordia path (curved line) that best fits the data was calculated for the case of simultaneous Pb and Rn loss when the ratio of their diffusion coefficients DRn/DPb = 5 * 10'. Because 234U/238U initial activity ratios (Ui) for these subsamples are not known, their ages can be constrained only by the inter- sections of the diffusion discordia line with two concordia curves corresponding to Ui = 1 (equilibrium) and U, = 10 (disequilibrium, maximum value observed in UZ opals of Pleistocene age at Yucca Mountain ') (Fig. 3). This discor- dia line intersects concordia curves at ages of about 9.8 and 11.2 Ma (average value of 10.5 Ma, Fig. 3). These ages of chalcedony formation overlap the K-Ar ages (8.7k0.4 to 11.7r0.7 Ma, average 10.4 Ma) determined from clay min- erals formed in the saturated zone and attributed to wide- spread alteration of tuffs at Yucca Mountain that occurred as a result of Timber Mountain thermal event 13. IV. CONCLUSIONS The 207Pb/235U ages for 14 subsamples of opal or chal- cedony layers younger than calcite formed at elevated tem- perature range between 1.88kO.05 and 9.7-1-1.5 Ma with most values older than 6-8 Ma. These data indicate that flu- ids with elevated temperatures have not been present in the unsaturated zone at Yucca Mountain since about 1.9 Ma and most likely since 6-8 Ma. Discordant U-Pb isotope data for chalcedony subsam- ples representing the massive silica stage in the formation of 0.0022 0.0020 0.0018 h 00 * 0.0016 2 0.0014 n c1W 0.0012 :: 0.0010 0.0008 0.0006 0.003 0.005 0.007 0.009 0.011 0.01; Fig. 3. Concordia diagram for samples HD2021, HD2029, and HD2257. Two concordia curves for ini- tial 234U/238U activity ratios Ui=l (equilibrium) and Ui=10 (disequilibrium) are shown. Curved discordia line corresponds to simultaneous Pb and Rn loss when the ratio of their diffusion coefficients DRn/Dpb=5* IO8. The area is shaded where the data points would plot if no loss of U decay products occurred. Error ellipses are the coatings are interpreted using a model of the diffusive loss of U decay products. 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Age constraints on fluid inclusions in calcite at Yucca Mountain

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