Brian D. Marshall

Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry

Muhs, D. R., Bettis III, E. A., Aleinikoff, J. N., McGeehin, J. P., Beann, J., Skipp, G., Marshall, B. D., Roberts, H. M., Johnson, W. C., Benton, R. (2008). Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry. GSA Bulletin, 120(11-12), 1378-1407.

Estimation of past seepage volumes from calcite distribution in the Topopah Spring Tuff, Yucca Mountain, Nevada

Low-temperature calcite and opal record the past seepage of water into open fractures and lithophysal cavities in the unsaturated zone at Yucca Mountain, Nevada, site of a proposed high-level radioactive waste repository. Systematic measurements of calcite and opal coatings in the Exploratory Studies Facility (ESF) tunnel at the proposed repository horizon are used to estimate the volume of calcite at each site of calcite and/or opal deposition. By estimating the volume of water required to precipitate the measured volumes of calcite in the unsaturated zone, seepage rates of 0.005 to 5 liters/year (l/year) are calculated at the median and 95th percentile of the measured volumes, respectively. These seepage rates are at the low end of the range of seepage rates from recent performance assessment (PA) calculations, confirming the conservative nature of the performance assessment. However, the distribution of the calcite and opal coatings indicate that a much larger fraction of the potential waste packages would be contacted by this seepage than is calculated in the performance assessment.

Geochemical and PB, SR, and O isotopic study of the Tiva Canyon Tuff and Topopah Spring Tuff, Yucca Mountain, Nye County, Nevada

Yucca Mountain is currently being studied as a potential site for an underground repository for high-level radioactive waste. One aspect of the site characterization studies is an evaluation o the resource potential at Yucca Mountain. Geochemical and isotopic signatures of past alteration of the welded tuffs that underlie Yucca Mountain provide a means of assessing the probability of hydrothermal ore deposits being present within Yucca Mountain. In this preliminary report, geochemical and isotopic measurements of altered Tiva Canyon Tuff and Topopah Spring Tuff collected from fault zones exposed on the east flank of Yucca Mountain and from one drill core are compared to their unaltered equivalents sampled both in outcrop and drill core. The geochemistry and isotopic compositions of unaltered Tiva Canyon Tuff and Topopah Spring Tuff (high-silica rhyolite portions) are fairly uniform; these data provide a good baseline for comparisons with the altered samples. Geochemical analyses indicate that the brecciated tuffs are characterized by addition of calcium carbonate and opaline silica; this resulted in additions of calcium and strontium,increases in oxygen-18 content, and some redistribution of trace elements. After leaching the samples to remove authigenic carbonate, no differences in strontium or lead isotope compositions between altered and unaltered sections were observed. These data show that although localized alteration of the tuffs has occurred and affected their geochemistry, there is no indication of additions of exotic components. The lack of evidence for exotic strontium and lead in the most severely altered tuff samples at Yucca Mountain strongly implies a similar lack of exotic base or precious metals.

Strontium isotope evolution of pore water and calcite in the Topopah Spring Tuff, Yucca Mountain, Nevada

Pore water in the Topopah Spring Tuff has a narrow range of {delta}{sup 87}Sr values that can be calculated from the {delta}{sup 87}Sr values of the rock considering advection through and reaction with the overlying nonwelded tuffs of the PTn. This model can be extended to estimate the variation of {delta}{sup 87}Sr in the pore water through time; this approximates the variation of {delta}{sup 87}Sr measured in calcite fracture coatings. In samples of calcite where no silica can be dated by other methods, strontium isotope data may be the only method to determine ages. In addition, other Sr-bearing minerals in the calcite and opal coatings, such as fluorite, may be dated using the same model.

Lanthanide Element Mobility and Sm-Nd Systematics in a Contact Metamorphic Zone

The effects of a sudden thermal pulse on two contrasting wallrock types are being studied by the detailed examination of a contact metamorphic zone in the Beartooth Mountains, Wyoming. At this locality a late Proterozoic basalt dike intrudes Archean granite gneiss and amphibolite. Modeling of the cooling history of the dike and wall rocks shows that maximum temperatures greater than 500 °C were maintained for « 5 0 years within a few meters of the contact. The dike cooled to less than 200 °C within 1000 years after intrusion. Samples of gneiss and amphibolite from traverses nearly perpendicular to the strike of the dike have been studied petrographically and analyzed for a large suite of elements by INAA, XRF, and isotope dilution. Lanthanide (rare earth) element patterns show enrichment of the light rare earth elements (LREE) and a somewhat flat heavy rare earth element (HREE) pattern for the two wall rock types. The amphibolite samples show slight depletion in LREE near the contact and slight enrichment in HREE, although not as pronounced as in the gneiss. Sm—Nd model ages also show some disturbance, confirming the slight mobility of LREE. Lanthanide mobility adjacent to the dike has been facilitated by mineralogic changes (recrystallization) accompanied by fluid migration along fractures. These data put constraints on the mobility of lanthanide elements in geologic media at high temperatures with low water/rock ratios.