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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 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.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 grain’s
origin from even an approximate age.
The Indus Valley Civilisation (mature period 2600–1900
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.
Excerpt of 5 page article at:
Please email me for a copy. Kalyan97@gmail.com
(Geoscientist 19.1 September 2009)
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