228 Recent Advances in Geochemistry Geology

Wednesday, 3 November 2010
8:00 AM - 12:00 PM
Room 205

MASS INDEPENDENT 238U/235U FRACTIONATION AND WEATHERING PROCESSES

Paper number: 228-6
Program: Technical Sessions
Time: 9:15 AM – 9:30 AM

Authors

Abstract

Recent work shows that 238U/235U varies by ~1.5‰ across a near-surface environments. Understanding controls on the modern variability of this ratio is key to interpretating 238U/235U preserved in soils, speleothems, U ores, and marine and lacustrine sediments. Theoretical work suggests that this fractionation is a result of differences in the ability of individual U nuclei to undergo reduction. Earliest measurements of this fractionation, however, implied that preferential leaching during aqueous weathering of U is a potential mechanism for this fractionation; this assertion is based principally on a correlation between 238U/235U and 234U/238U. The difficulty reproducing this fractionation in the laboratory necessitates examination of modern 238U/235U variability in natural settings to resolve fractionation mechanisms. Here, we examine 238U/235U variability in geologic settings that principally experience either changes in: 1) reduction-oxidation potential (redox); or 2) the degree of chemical leaching.

Water samples were obtained from an U ore “roll front” deposit; 238U/235U from these waters strongly support the assertion that this fractionation is controlled by redox conditions. Samples were obtained from: 1) the oxidizing zone, located up the hydrologic gradient from the ore body; 2) within the ore zone; and 3) in the reducing zone down the hydrologic gradient from the ore body. 238U/235U is highest in the oxidizing zone, lowest in the reducing zone, and intermediate in the ore.

Water samples were also obtained from surface waters at locations where the ratio of chemical to physical weathering in the soils has been previously quantified; preliminary results also suggest a relationship between the degree of selective leaching of local soils and the 238U/235U of surface waters. A decreased ratio of chemical to physical weathering produces higher 238U/235U in the surface waters. This association is further supported by a correlation between 234U/238U and 238U/235U, a correlation with a similar slope in waters within the oxidizing zone of the U roll front deposit, and in a prior study of Quaternary deposits. These results are consistent with redox transformation as the principal mechanism for 238U/235U fractionation, as aqueous leaching of U from minerals generally involves the oxidation of UIV to UVI.