USGS Home
Contact USGS
Search USGS

U.S. GEOLOGICAL SURVEY
Data Series 280

Back to Table of Contents

Strontium Isotopic Compositions

Values of ⁸⁷Sr/⁸⁶Sr for the carbonate fractions of borehole (tables 5 and 7), and outcrop (table 6) samples range from 0.70720 to 0.72543 and have distributions that are skewed toward radiogenic values (fig. 9). For both sets of samples, approximately 54 percent of the ⁸⁷Sr/⁸⁶Sr analyses are within the range of Paleozoic seawater values between 0.7070 and 0.7092. The remaining analyses have larger ⁸⁷Sr/⁸⁶Sr values, typically between 0.7093 and 0.7150. Data from Yucca Flat core and cutting samples do not form linear trends on plots comparing ⁸⁷Sr/⁸⁶Sr to reciprocal Sr concentration (fig. 10A) where binary mixing trends would plot as straight lines. Therefore, there is no indication that low Sr carbonates were more susceptible to addition of radiogenic Sr from secondary fluids than high Sr samples. Furthermore, there are no clear indications that elevated ⁸⁷Sr/⁸⁶Sr values in carbonate fractions are related to the presence of Rb-rich silicate components leached from the samples (fig. 10B). Although the two clay-rich samples with the highest SiO₂ concentrations also have the highest ⁸⁷Sr/⁸⁶Sr values, similar trends are not present in samples with lower SiO₂ concentrations.

Values of δ⁸⁷Sr_t calculated with respect to the estimates for Paleozoic seawater ⁸⁷Sr/⁸⁶Sr at the time of deposition range widely from -1.40 to 24.0 (tables 5–7; fig. 11). Estimates for the original seawater ⁸⁷Sr/⁸⁶Sr were made using the lithostratigraphic age assignments (table 1) and the Paleozoic seawater ⁸⁷Sr/⁸⁶Sr curve of McArthur and others (2001; fig. 3). In many cases, specific age and Paleozoic seawater ⁸⁷Sr/⁸⁶Sr values for individual samples were chosen from the ranges permissible in table 1 to obtain a δ⁸⁷Sr_t value closer to 0.0. Although the Paleozoic seawater ⁸⁷Sr/⁸⁶Sr evolution curve is well established, error bars are not shown on figure 11 because of the difficulty in defining uncertainties for rock ages based on stratigraphic intervals that can span ranges of 10–50 m.y. or more. Values of δ⁸⁷Sr_t close to 0.0 indicate that the carbonate rocks likely have retained their original Paleozoic marine Sr isotopic signatures. In some cases, these data may help corroborate age assignments based on biostratigraphic information (Banner, 2004, p. 167). However, it is preferable to obtain ⁸⁷Sr/⁸⁶Sr data from low-magnesium calcite constituting marine skeletons instead of from bulk rock to avoid post-depositional alteration (Veizer and others, 1999, p. 62). Because all analyses reported here were done on bulk-rock samples, no attempts are made to refine depositional ages.

Samples with measured ⁸⁷Sr/⁸⁶Sr values greater than 0.7093 in tables 5–7 have undergone obvious post-depositional addition of radiogenic Sr (Peterman and others, 1994). Samples with elevated δ⁸⁷Sr_t are present throughout the Paleozoic stratigraphic section in borehole and outcrop samples (fig. 11), indicating that ⁸⁷Sr/⁸⁶Sr alteration was not restricted to rocks formed during a narrow interval of Paleozoic time. Peterman and others (1994) documented that Paleozoic carbonates at Bare Mountain have elevated δ⁸⁷Sr_t values compared to samples from the Striped Hills, Specter Range, Spring Mountains, and mountain ranges east of the NTS (fig. 12). These anomalous δ⁸⁷Sr_t values were obtained on samples from mineralized areas including the Stirling Gold Mine on the eastern flank of Bare Mountain. Peterman and others (1994, p. 1,321) postulated that hydrothermal mineralization was associated with Tertiary volcanic activity and involved localized plumes of thermal solutions which introduced radiogenic Sr derived from the Precambrian basement into carbonate rocks present at shallower crustal levels. Therefore, the spatial distribution of Paleozoic carbonate rocks with nonmarine ⁸⁷Sr/⁸⁶Sr compositions at and around the NTS (fig. 12) could be related to major crustal structures that provide fluid pathways and magmatic activity that provided thermal sources.

Paleozoic carbonates present in the northeastern part of the NTS have the potential to be affected by Tertiary magmatic activity constituting the southwestern Nevada volcanic field centered at Timber Mountain and Pahute Mesa (Byers and others, 1989; Sawyer and others, 1994). Results from this study indicate that Paleozoic limestone and dolomite beneath Rainier Mesa, Yucca Flat, and Frenchman Flat tend to have δ⁸⁷Sr_t values intermediately between those measured at Bare Mountain and ranges southeast of the NTS (fig. 1; fig. 12A). Carbonate rocks with elevated δ⁸⁷Sr_t are present near the bottom of borehole UE‑15d (Rainstorm Member of the Johnnie Formation or Noonday Dolomite) underlying a thick section of Neoproterozoic siliciclastic rocks constituting the Lower Clastic Confining Unit in northern Yucca Flat. In western Yucca Flat and the Calico Hills, boreholes ER‑16‑1, ER‑12‑1, ER‑6‑2, UE‑16d, and UE-25 a#3 penetrate thick sections of Eleana Formation, Gap Wash Formation, and Chainman Shale consisting of abundant litharenite, siltstone, and shale (fig. 1; fig. 12B). Clay minerals with elevated Rb/Sr in these siliciclastic rocks constituting the upper clastic confining unit are a possible source for radiogenic Sr in carbonate rocks present in these boreholes. Rocks constituting the upper clastic confining unit are not present in eastern Yucca Flat or Frenchman Flat, and carbonates from ER‑3‑1, ER‑7‑1, ER‑6‑1#2, and HTH #3 have retained their Paleozoic seawater signatures (fig. 12B). Other boreholes having LCA rocks directly underlying Tertiary volcanic or alluvial rocks may or may not have Paleozoic carbonate with elevated δ⁸⁷Sr_t values.

In several cases, Sr isotopic compositions were obtained from macroscopically unaltered and altered core samples [Army 1(859) and Army 1(862), TW‑C(1458) and TW‑C(1463.5)]. For both pairs, samples with brecciated textures, iron staining, and clay mineralization (table 3) had higher ⁸⁷Sr/⁸⁶Sr values than the sample lacking obvious alteration (table 5). The difference is most dramatic for leachates of samples Army 1(859) and Army 1(862) where the highly altered dolomite has Sr concentrations more than twice the value present in the unaltered dolomite and a δ⁸⁷Sr_t value of 3.84 compared to the near seawater value of ‑0.06 in the unaltered dolomite. Differences are not as large between samples TW‑C(1458) and TW‑C(1463.5) with negligibly higher Sr concentrations in the altered limestone (261 compared to 251 µg/g) and δ⁸⁷Sr_t values of 2.82 compared to 1.87. However, addition of radiogenic ⁸⁷Sr is not always accompanied by obvious evidence for secondary alteration. Samples UE15d(5994), ER12‑4(2580), ER6‑2(1830), and ER5‑3#2(4780) have δ⁸⁷Sr_t values between 3.45 and 5.78, but do not show obvious macroscopic evidence of alteration.

Finally, one pair of samples representing unaltered host dolomite [Army 1(982)A] and coarse secondary dolomite spar [Army 1(982)B] filling a cross-cutting vein were analyzed to investigate the degree of isotopic disequilibrium between the two materials. Leachates of both samples have similar Sr concentrations (30.7 compared to 37.9 µg/g; table 5) and analytically indistinguishable ⁸⁷Sr/⁸⁶Sr values (0.71027 compared to 0.71023). These data are consistent with a local source of Sr for the younger dolomite spar. Although the age and petrogenesis of the secondary dolomite vein are not known, the thick filling of dense, coarsely crystalline dolomite is not likely to have formed under modern ground-water flow conditions.

Back to Table of Contents