2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 1
Presentation Time: 1:30 PM

CHEMICAL AND U-SERIES ISOTOPE VARIATIONS IN FELSIC TUFFS AT YUCCA MOUNTAIN, NEVADA, AS INDICATORS OF PAST WATER FLOW THROUGH THE UNSATURATED ZONE


NEYMARK, Leonid A., US Geological Survey, Denfer Federal Center, Mailbox 25046, MS 963, Denver, CO 80225 and PACES, James B., U.S. Geol Survey, Box 25046, MS963, Denver Federal Center, Denver, CO 80225-0046, lneymark@usgs.gov

Water percolating through the 500-m-thick unsaturated zone (UZ) at Yucca Mountain resulted in variable chemical modification and U-series radioactive disequilibrium in the 12.8-Ma felsic tuffs. Some block-bounding faults may be preferential pathways, as evidenced by bleaching, iron staining, and alteration to clays. In contrast, chemical and mineralogical alteration of the tuffs elsewhere is minimal.  U-series isotopes are useful for investigating Pleistocene water/rock interaction because water preferentially mobilizes U relative to less soluble Th and 234U relative to 238U via alpha-recoil-related processes, but rocks disturbed > 1 Ma will return to radioactive equilibrium. U, Th, 234U/238U, and 230Th/238U were determined in two sets of bulk-rock samples: (1) core samples from depths of 16 to 300 m in borehole USW SD-9 to evaluate changes in flow along a vertical profile in the UZ, and (2) tunnel-wall samples from a lateral transect across the Solitario Canyon fault at depths of 200 to 370 m to investigate the possibility of greater flow along major faults.  In addition, bulk-rock and leachable-ion geochemical analyses were conducted on the second set of samples.

Thirty bulk-rock samples forming the USW SD-9 vertical profile show systematic variations in U and Th concentrations reflecting lithostratigraphic compositional differences, but only small amounts of 238U-234U-230Th radioactive disequilibrium that are not associated with lithostratigraphic variations.  234U/238U activity ratios (AR) of 0.94 to 1.01 and 230Th/238U AR of 0.93 to 1.10 reflect Pleistocene water/rock interaction in otherwise unaltered tuff.  Most analyses have 234U/230Th AR <1, which indicates U removal. 

Fifteen tunnel-wall samples show systematic chemical and mineralogical changes with distance from the Solitario Canyon fault; however, 234U/238U and 230Th/238U AR are not significantly different from those in the USW SD-9 profile. These analyses commonly have 234U/230Th AR >1 and 234U/238U<1, indicating that U mobility included both U gains and 234U losses.  The small amounts of radioactive disequilibrium in highly tectonized fault rock do not support a Pleistocene age for the observed rock alteration or the presence of substantially greater water fluxes at depth in this fault zone during Pleistocene time.