Laser40Ar39Ar ages for individual detrital muscovites in the Southern Uplands of Scotland, U.K.

Dating individual detrital muscovite micas in clastic sediments can be achieved using the⁴⁰Ar³⁹Ar laser microprobe technique. The results provide a powerful approach to understanding the relationship between intra-sample age variation and inter-sample age variation, something not possible using “bulk” analytical techniques. A key point in interpreting⁴⁰Ar³⁹Ar ages from detrital minerals is understanding their state of alteration which requires careful sampling and preparation procedures. The insights which this technique can provide, along with some of the pitfalls, are illustrated using data from turbidites in the Southern Uplands of Scotland. The ages of individual micas at the time of deposition, range from zero to 70Ma old. Distributions of ages vary from single populations with small standard deviations to widely distributed multi-mode populations. Intra-flow mica age variation is greater than inter-flow variation in Ordovician rocks, but higher in the sequence, in Silurian flows, the intra-flow variation is greater than inter-flow variation. This change reflects changes in the sedimentary environment with time.