The science of Single Grain Provenance
Full caption:Â This probability density plot shows the most likely ages for zircon ages in four of the biggest source terrains in the western Himalaya. Young grains are unique to the Karakoram and Transhimalaya, while even here differences do occur. Grains younger than 10Ma or older than 110Ma (insert) are only found in the Karakoram. Although the Lesser and Greater Himalaya have significant overlap, there are nevertheless ages that are typical of each range and allow a first-order sediment budget to be made.
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.Â
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.Â
Lower temperature thermochronometers, such as fission track dating apatite or zircon, can be used to determine when crystals cooled below 110Â°C and 200Â°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.Â
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 grainâ ™s origin from even an approximate age.
The Indus Valley Civilisation (mature period 2600â “1900 BCE) flourished around the Indus River basin and encompassed most of what is now Pakistan (mainly the provinces of Sindh, Punjab and Balochistan), as well as Indian states Gujarat, Haryana, Punjab and Rajasthan. IVC remains have been found in Afghanistan, Turkmenistan and Iran. The mature phase of this civilisation is technically known as the Harappan Civilisation, after the first of its cities to be unearthed - Harappa in Pakistan.Â
The civilisation is sometimes referred to as the Indus Ghaggur-Hakra civilisation or the Indus-Sarasvati civilisation. The appellation Indus-Sarasvati is based on the possible identification of the Ghaggur-Hakra River with the Sarasvati River mentioned in the Rig Veda, but this usage is disputed on linguistic and geographical grounds.
The full paper of Peter Clift (pdf) can be read at
Excerpt of 5 page article at: http://www.scribd.com/doc/19997044/geoscientistharappa2009 Please email me for a copy. Kalyan97@gmail.com (Geoscientist 19.1 September 2009)