EVALUATING THE INFLUENCE OF COMPLETE CLAY REMOVAL ON THE DD VALUES OF VOLCANIC GLASS AND ITS APPLICATION TO THE LATE CENOZOIC PALEOTOPOGRAPHY IN THE CENTRAL ROCKY MOUNTAINS

The dD values of hydrated volcanic glasses provide a means to reconstruct ancient meteoric water dD values. However, the accuracy of measured dD values of volcanic glasses is hindered by the alteration of glass surfaces to clay minerals. We examined three different methods for effective clay removal. Methods include 1) simple DI water wash, 2) wash with 5% sodium hexametaphosphate solution for two hours, and 3) wash with 5% HF for ~30 seconds. We pretreated eight samples using the three methods, and used a binocular microscope to examine the effectiveness of clay removal, then analyzed the dD values of each treated batch with TC/EA-IRMS. We found that samples pretreated with sodium hexametaphosphate show substantial but not complete removal of clays, while a short 5% HF wash can completely remove adhering clays and intensely altered glasses. The dD values of volcanic glasses pretreated with HF are 5-20 ‰ lower than the ones pretreated with sodium hexametaphosphate, which are then 5-20 ‰ lower than the dD values of samples pretreated with a simple water wash. Our results show that the incomplete removal of clay minerals from volcanic glasses can increase the dD values, which may lead to erroneous interpretations of paleoclimate and paleoelevation estimates than using the dD values of volcanic glasses. We then adopted the pretreatment method of short 5% HF wash for analyzing the dD values of volcanic glasses of late Cenozoic age collected from a regional west-east transect across central Wyoming and western Nebraska. The variation of the late Cenozoic meteoric water dD values along the transect will be used to constrain when the central Rocky Mountains reached their currently high regional elevations.