Sunday, October 3, 2010

Radiogenic Osmium in Ganga river sediments of India.

Ganga river Sediments are the major source of radiogenic Osmium.
by
Dr. Nitish Priyadarshi



The Ganga sediments are characterized by high Re/Os (rhenium-osmium) ratios and are extremely radiogenic as evident from their 187 Os/ 188 Os isotopic ratios. Samples from the tributaries Alaknanda, Bhagirathi, Gandak and Ghaghra show pronounced 187 Os/ 188 Os. High Os concentrations combined with sediments flux makes the Ganga an important source for soluble Os isotopic evolution in oceans.

Interest in rhenium-osmium systematics in rivers has risen sharply in recent years due to the revelation of changes associated with sea water Os isotopic compositions during the past 70 Ma. Radiogenic 187 Os is produced from the β- decay of 187 Re. Osmium isotopic composition in sea water is derived from the weathering of basaltic and peridotitic oceanic crust, hydrothermal solutions, additions from cosmic dust and continental weathering products.

Os isotopes in the oceans convey the then prevalent continental weathering processes. The Osmium isotopic composition of the present day sea water is markedly higher since the past 70 Ma. This enhancement in radiogenic Os in sea water is largely attributed to the Himalayan tectonics with its accompanying increased silicate weathering and in particular, chemical weathering of the extremely radiogenic black shales in the lesser Himalayan region.

Marine black shales have been proposed as a source of very radiogenic Os because of their large enrichments in Re and because they can even at trace amounts weather very fast due to oxidative processes. If true, this then has the possibility of tracking exposure and weathering of organic carbon in the form of black shales in the past, e.g. uplift of the Himalayas or the Andes.

According to the article published in Geochim. Cosmochim. Acta in year 1999, Indus and Brahmaputra are less radiogenic than the Ganges, presumably due to the erosion of ophiolitic assemblages exposed along the Indus-Tsangpo suture zone.

The tributaries in the upstream, Alaknanada and Bhagirathi flow through predominantly silicates (shales, phyllites, quartzites, crystalines) and show high Re/ Os ratios of 24.9 and 19.8 respectively. Re is preferentially incorporated into the silicate minerals relative to Os.

Rivers draining the Canadian Shield have radiogenic values at low concentrations (24 to 35 fmol/kg). They are more radiogenic than the Zaire draining the Congo shield (1.4) or the Tapajos draining the Amazon shield (1.33) in the tropics.

Recently osmium concentrations and isotopic compositions in groundwater samples from the Bengal plain have been reported. Groundwaters have Os concentrations (16.9–191.5 pg/kg), about 5–10 times higher than those published for most rivers or seawater. 187Os/188Os varies widely (from 0.96 to 2.79) and is related to the isotopic signatures of the sediments constituting local aquifers.

Table-1 Osmium in Ganga River and its tributaries (in ppt).

Bhagirathi- 32.8
Alaknanda- 21.1
Yamuna- 64.3
Chambal-33.8
Gandak-55.7
Ganga- 37.7 to 51.7

Reference:

Chakrapani, G.J. et.al 2002. Osmium isotopic compositions in Ganga river sediments. Current science, Vol.83, no. 10.
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