My OperaDiamond-bearing Kimberlite Depth
Thursday, November 15, 2007 4:31:07 PM
Kimberlites are magnesium-rich igneous rocks that are sometimes diamond-bearing. Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. The study of kimberlite can provide valuable information on the composition of the mantle, and melting processes in the mantle.
Kimberlites are generally thought to originate by melting of the mantle layer that forms the lower part of continental plates, the subcontinental lithospheric mantle. A recent geochemical study published in the November 2007 issue of the GSA-journal Geology suggests that these magmas might instead come from the underlying mantle layer, the asthenosphere, perhaps from depths over 400 km.
Samples were studied from two different fields of kimberlitic rocks on the Dharwar Craton in southern India. The remarkable homogeneity in the strontium isotopic composition of crystals within each of the fields is hard to reconcile with derivation of the magmas from the subcontinental lithospheric mantle, which is usually heterogeneous (as I mentioned in my latest post on Magma Chamber Cartoons). Together with the mineralogy of the kimberlites and constraints from models of magma movement, the observed homogeneity is more likely to result from melting upwelling asthenospheric mantle, possibly from great depth.
* http://www.nature.com/ngeo/reshigh/2007/1107/full/ngeo.2007.46.html
The Dharwar Craton is divided into the Western and Eastern Dharwar. In the West, the basement is dominated by gneisses that are mostly 3000 – 3300 million years old; rocks to the East are mainly 2700 – 2900 million years old. Amalgamation of the West and East Dharwar segments is considered to have taken place around 2500 million years ago, The northern part is cover by Deccan Traps, which are ca. 65 million years old.
See also my post on Diamonds in Ukraine.
Kimberlites are generally thought to originate by melting of the mantle layer that forms the lower part of continental plates, the subcontinental lithospheric mantle. A recent geochemical study published in the November 2007 issue of the GSA-journal Geology suggests that these magmas might instead come from the underlying mantle layer, the asthenosphere, perhaps from depths over 400 km.Samples were studied from two different fields of kimberlitic rocks on the Dharwar Craton in southern India. The remarkable homogeneity in the strontium isotopic composition of crystals within each of the fields is hard to reconcile with derivation of the magmas from the subcontinental lithospheric mantle, which is usually heterogeneous (as I mentioned in my latest post on Magma Chamber Cartoons). Together with the mineralogy of the kimberlites and constraints from models of magma movement, the observed homogeneity is more likely to result from melting upwelling asthenospheric mantle, possibly from great depth.* http://www.nature.com/ngeo/reshigh/2007/1107/full/ngeo.2007.46.html
The Dharwar Craton is divided into the Western and Eastern Dharwar. In the West, the basement is dominated by gneisses that are mostly 3000 – 3300 million years old; rocks to the East are mainly 2700 – 2900 million years old. Amalgamation of the West and East Dharwar segments is considered to have taken place around 2500 million years ago, The northern part is cover by Deccan Traps, which are ca. 65 million years old.
See also my post on Diamonds in Ukraine.
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