LIFETIME AND AGE SPECTRA OF DETRITAL GRAINS

The observed lifetimes of minerals, as estimated by the average survival time of dated detrital grains compiled from the literature, correlates linearly with their chemical durability estimated from dissolution rates, but cannot be predicted from mechanical durability estimated by mineral hardness. The best predictor of the observed lifetimes is the total durability, an estimator that combines chemical and mechanical metrics. Age spectra of grains tend to be right-skewed, as predicted by the exponential model used to describe the lifetime of mechanical parts or the radioactive decay of atoms. Consistent with the model, the less durable grains produce age spectra that are more right-skewed and leptokurtic and may yield more accurate estimates of depositional age, whereas the more durable grains often depart from the exponential model and their age spectra primarily reflect long-term geological changes in the rate of grain production. The integration of geochronological dates and laboratory mineralogical data indicates that the lifetimes of detrital grains can be predicted from intrinsic properties of minerals and estimated and modeled in a quantitative way.