Improved microanalytical methods in recent years have greatly increased the ease of single grain provenance analysis and our ability to reconstruct ancient patterns of erosion.
Although analysis of entire sediment samples for chemical composition and isotopic character are still widely used determining the relative influence of a number of different sources is impossible in complex river system systems without single grain methods. The best method for a given river is not always the same, but will vary depending on how diverse the source regions are, and what the local history of deformation and magmatism has been.
Lower temperature thermochronometers, such as fission track dating apatite or zircon, can be used to determine when crystals cooled below 110°C and 200°C respectively. Unfortunately, these methods alone are usually insufficient to resolve all sources in the Himalaya. Previously, ion probe methods were used to date the crystallisation of zircon grains, or to determine the Pb isotope character of potassium feldspar sand grains, both of which are heterogeneous across the western Himalaya and Karakoram. Now zircon grains can be dated at lower resolution but at much higher speed and lower cost using a LA-ICP-MS. This is important because statistical analysis demands that more than 100 grains need to be dated for a robust result.
Whereas this used to be prohibitive in cost and machine time, it is now practical - and allows at least one sediment sample to be processed each day. Although the crystallisation dates are not as precise as those obtained with ion probe, the huge differences in crystallisation ages that are known from across the Himalaya allow good constraints to be placed on the grain’s origin from even an approximate age.