U-SERIES AGE EQUATIONS FOR URANIUM ASSIMILATION BY FOSSIL BONES

Because of the general abundance of fossil bone in archeological deposits, U-series chronologists are often forced to resort to bone studies in the absence of more reliable material such as authigenic carbonate. Bone is less reliable than authigenic phases because living bone contains no uranium, and fossil bone readily acquires uranium post-depositionally, In the ideal case, uranium is assimilated into bone from soil fluids soon after burial in early uptake (EU), and the bone remains a closed system thereafter. At the other extreme is a gradual, linear uptake of uranium (LU) in which the bone assimilates uranium more or less continuously with time. We derive age equations and nomographs for both the LU process and for the special case of EU followed by linear uptake of U-234 to provide semi-quantitative upper limits on the age of many fossil bones. The equations show that LU ages are about twice the EU ages for EU ages up to about 50 kyrs, are about 2.5 times mid-range EU dates, and approaching about 3 times EU as isotopic equilibrium is approached. Thus the Limit of LU dates is on the order of 1 my compared to about 350 kyrs for EU. Ages calculated for the linear uptake of U-234 increases the EU dates by only 10% for EU dates up to about 50 kyrs, increasing to 50% in mid-range, and approaches 100% as isotopic equilibrium is approached.