Oxygen isotope studies of illite/smectite and clinoptilolite from Yucca Mountain: implications for paleohydrologic conditions

Illite/smectite (I/S) and clinoptilolite mineral separates from drill holes in 11 Ma old altered volcanic tuffs of Yucca Mountain, Nevada were analyzed for δ18O values. We reconstruct the diagenetic and paleohydrologic conditions using the isotopic composition of clay minerals combined with other independent geological observations. Our isotopic data show that the δ18O values of clay minerals preserve paleohydrologic information from a Middle to Late Miocene episode of hydrothermal alteration at Yucca Mountain. We provide additional evidence for a dual decoupled groundwater circulation system at Yucca Mountain ca. 11 Ma ago. A near-surface system dominated by downward percolation of groundwater is separated from a deeper hydrothermal system by profound thermal and isotopic discontinuities near the R0–R1 I/S transition. Given the set of inferred formation temperatures and the isotopic composition of formation water, we show that the current set of isotopic compositions of clinoptilolite is significantly lower than those at the time of formation, and are not significantly different from the equilibrium values defined by the current groundwater δ18O and geothermal gradient. We conclude that, unlike I/S, clinoptilolite has not preserved its original isotopic composition over the past 11 million years, but we do not know how long is necessary to reset the isotopic signature of clinoptilolite under near-surface conditions. The current groundwater at Yucca Mountain is 4%. more enriched in 18O than the paleogroundwater 10–11 Ma ago as inferred from our reconstruction. This shift may be attributed to a change in the pattern of atmospheric circulation, such as by surficial uplift of the Sierra Nevada, which caused it to become an orographic barrier to moisture penetrating inland from the west.