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U.S. GEOLOGICAL SURVEY
Data Series 280

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Analytical Methods

Core and cutting samples of Paleozoic rock core were obtained in 2006 from material archived at the U.S. Geological Survey (USGS) Core Library and Data Center in Mercury, Nevada. Intervals corresponding to water-producing zones were examined and samples were selected based on availability, representation of macroscopic features, and location in the stratigraphic section. Borehole intervals, sample types, stratigraphic units, and sample descriptions are given in table 3. Where only cuttings were available, samples with the largest fragment size were selected preferentially. In a few cases, special features were targeted, such as secondary calcite veins or altered breccia zones, to evaluate the differences between primary and secondary ⁸⁷Sr/⁸⁶Sr signatures; however, most samples were collected to evaluate unaltered, bulk-rock compositions. Stratigraphic units were assigned to samples using lithologic logs from borehole completion reports to DOE, which have been compiled recently by the U.S. Geological Survey (2006). Other sources include Maldonado and others, (1979) for borehole UE-25 a #3, and Carr and others, (1986) for borehole UE-25 p #1. These assignments were made by correlations with known stratigraphic sections or through biostratigraphic assignments, most commonly using conodonts (typically evaluated and reported as unpublished data by A. Harris, U.S. Geological Survey).

Samples from road cuts or outcrops were collected between 1989 and 1994 as part of a study to evaluate the usefulness of strontium isotopes as indicators of epithermal mineralization in areas surrounding Yucca Mountain, Nevada, the potential site for a geologic repository for high-level radioactive waste (Peterman and others, 1994). Samples of mineralized and unmineralized Paleozoic carbonate rocks were analyzed to represent geologic map units in several areas to the west, south, and east of the NTS (table 4; fig. 1). In addition, core samples from Paleozoic strata underlying the Tertiary volcanic rocks constituting Yucca Mountain (borehole UE-25 p #1) were included.

Samples of NTS core were split and representative fragments were selected for further processing. Samples of cuttings were examined under a binocular microscope and fragments of foreign material likely representing down-hole contamination by rock fragments from overlying lithologic units were removed. Final samples of cuttings and core fragments weighed between 3 and 50 grams (g). Outcrop samples were trimmed as much as possible in the field to remove weathering rinds or coatings of pedogenic carbonate and were subsampled further in the laboratory using a small diamond core drill (1.3-centimeter outside diameter). Resulting unweathered core samples ranged from 10 to 20 g. All samples were crushed and then pulverized to less than 0.075 millimeters using a steel shatterbox. All rock preparation and analyses were done by USGS Yucca Mountain Project Branch personnel in Denver, Colorado, between 1990 and 2006.

Analyses of the recently acquired NTS borehole samples were made on partial digestions (leachates) obtained by adding approximately 20 milliliters of 0.2 molar nitric acid to 0.1 g of rock powder. Acid solutions were allowed to react with rock powders in a 50°C incubator oven for 12 to 24 hours. Acid volumes were sufficient to maintain low pH in the final leachate solutions. Leachates then were analyzed for a selected suite of major and trace constituents (MgO, CaO, SiO₂, Al₂O₃, MnO, Rb, Sr, Th, and U) by inductively coupled plasma mass spectrometry (ICP-MS). Strontium was purified from the same leachates using Eichrom Sr-SPEC™ resin and ⁸⁷Sr/⁸⁶Sr isotopic compositions were measured using a Finnigan MAT 262™ multiple-collector thermal-ionization mass spectrometer operating in static mode. The instrument was calibrated using the USGS rock standard EN-1 (calcite from a modern Tridacna shell from Enewetok Lagoon in the western Pacific Ocean; Ludwig and others, 1988). To correct for instrument drift and eliminate small interlaboratory biases (Faure and Mensing, 2005, p. 78), measured ⁸⁷Sr/⁸⁶Sr values for samples in each magazine were normalized to the value obtained from the EN-1 standard measured in the same magazine and adjusted to an assigned value of 0.70920 for modern mean ocean water (Elderfield, 1986, p. 77). Replicate ⁸⁷Sr/⁸⁶Sr analyses of the uncorrected EN-1 standard varied by less than ±0.00005 (0.007 percent) at the 95-percent confidence level (2-sigma). Results for the National Institute of Standards and Technology strontium carbonate standard, SRM 987, (accepted ⁸⁷Sr/⁸⁶Sr value of 0.71025; Faure and Mensing, 2005, p. 79) analyzed between 1997 and 2006 yielded a weighted average ⁸⁷Sr/⁸⁶Sr value of 0.710270 ±0.000011 (95-percent confidence limit) indicating that estimates of accuracy and precision are similar. Sr contributions from the chemical procedure (blank) were approximately 2×10^‑9 g, which are 5–7 orders of magnitude lower than the total amount of Sr contributed from the samples.

Rock powders from outcrop samples were analyzed before ICP-MS capabilities became available. Instead, selected elements (Rb, Sr, Y, Zr, Nb, Ba, La, and Ce) were analyzed by energy-dispersive XRF (X-ray fluorescence) spectroscopy on bulk rock powders. Results for the rock-forming constituents MgO and CaO are not available. ⁸⁷Sr/⁸⁶Sr was analyzed in the carbonate fraction by dissolving subgram powdered aliquots in 1 molar hydrochloric acid followed by purification of Sr using cation exchange resins. ⁸⁷Sr/⁸⁶Sr values were measured using either the instrument described in the previous paragraph, or on one of two single collector instruments (National Bureau of Standards instrument or VG Isomass 54E) operating in peak-hopping mode. The measurements were treated identically to those described above using results of EN-1 to normalize unknown ⁸⁷Sr/⁸⁶Sr values. Samples of drill core obtained in the early 1990’s from UE-25 p #1 (fig. 1B) were processed in the same manner as outcrop samples. Samples of Gap Wash Formation siliciclastic rocks (Eleana Formation equivalent; Trexler and others, 2003) from UE-25 a #3 (fig. 1B) as well as two of the outcrop samples (HD-1107 and HD-1114 with respective Map No. 4 and 5 in table 4) were digested using hydrofluoric and sulfuric acids prior to Sr purification.

To emphasize small but significant variations in ⁸⁷Sr/⁸⁶Sr, data also are reported as per mil (‰) deviations from modern seawater (⁸⁷Sr/⁸⁶Sr = 0.70920) using the equation

(1)

Analytical uncertainties in δ⁸⁷Sr values are typically less than ±0.07 per mil at the 95-percent confidence level. Although modern seawater provides a useful reference point for comparative purposes, it is desirable to know whether present-day ⁸⁷Sr/⁸⁶Sr compositions of the marine carbonate rocks have been modified from seawater ⁸⁷Sr/⁸⁶Sr compositions that were present at the time of deposition. Therefore, δ⁸⁷Sr_t values are computed using stratigraphic age assignments and paleoseawater ⁸⁷Sr/⁸⁶Sr compositions using the equation,

(2)

where ⁸⁷Sr/⁸⁶Sr_{paleoseawater} is derived from figure 3 and the stratigraphic ages listed in table 1. Explicit uncertainties for δ⁸⁷Sr_t are not given in this report because of the difficulty in defining uncertainties to the age assignments. However, they are expected to be as much as an order of magnitude larger than those for δ⁸⁷Sr.

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