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To: Letmebe Frank who wrote (358)	3/17/2002 1:00:36 PM
From: jpthoma1	Read Replies (1) \| Respond to of 613

Hi Frank, Just download «Diamond geology» from this address: debeersgroup.com Clifford's rule is explained. “Clifford’s Rule”, states that diamondiferous kimberlites are almost exclusively found in regions underlain by Archaean craton, that is continental crust older than 2.5 billion years in age. In Africa, Russia and Canada, all of the significantly diamondiferous kimberlites are “on-craton”. The only significant exception to Clifford's Rule is the Argyle lamproite in Australia. It lies "off-craton" in a Proterozoic mobile belt. The reason for the originally empirical association between Archaean basement and diamondiferous kimberlites has been explained theoretically by consideration of the structure of the cratons (elucidated by geophysics and the study of mantle xenoliths brought to surface by kimberlites), and the temperature/pressure relationship between graphite and diamond. Natural diamonds form and are preserved in a high-pressure environment present in nature at depths of over 120 kilometres. In most parts of the Earth, the temperatures at this depth are too high for diamonds to form. However Archaean cratons have relatively cool lithospheric roots in which there exists a downward deflection of isotherms and a corresponding upward deflection of the diamond stability field. This region of high pressure and relatively low temperature (less than about 1200 o C) provides a “window“ in which diamonds can form and be preserved. Kimberlitic magmas are generated at or below these depths (as evidenced by their xenoliths), and may “sample” the lithospheric roots, thus collecting diamonds en route to surface. Kimberlites formed away from the craton do not sample the diamond window, and thus are unlikely to be diamondiferous. JP

To: Letmebe Frank who wrote (358)	3/20/2002 12:45:53 AM
From: VAUGHN	Read Replies (1) \| Respond to of 613

Hello Letmebe Frank It can get very complicated and many geophysicists are still debating it, but here is the simplest definitions I could find for you. The 'Moho' is now recognized as a fundamental geological boundary, and knowledge of its depth and lateral variation has been valuable for unravelling geological models of tectonic history. To date only seismic and density parameters are known to change across the boundary, so the Moho defined by seismologists is used universally as the definition of the crust-mantle boundary. Moho The Mohorivicic discontinuity (seismic reflector) at the base of the crust., Hope that helps. Regards Vaughn