Origin of Diamonds and Their Host Xenoliths

University Of Tennessee Knoxville, Knoxville TN

Investigators

Abstract

9909430 Taylor The mantle, although making up over 75% by volume of the Earth, remains relatively unstudied in hand sample. Fortunately, kimberlites (the source-rock of diamonds, named after Kimberley, So. Afr.) bring precious mantle samples to the surface. These eclogite and peridotite xenoliths (Gr. foreign rocks) were incorporated into the kimberlite at great depths and can contain diamonds. The origin of these diamond-bearing xenoliths, particularly eclogites, and their diamonds from deep within the mantle of the Earth is the subject of our research. There is some controversy over the reliability of diamond-bearing rocks as indicators of the conditions under which the diamonds formed, because these rocks may have been subjected to post-diamond metasomatism and/or partial melting. Mineral inclusions within diamonds are considered pristine samples of the mantle because these inclusions were isolated from post-diamond-formation mantle processes. It appears that each diamond can indicate a possible different set of P-T-x conditions for its formation, all of which may not be represented by the host mineralogy. The host eclogite may only record the most recent residence locale for the diamond and not the original conditions of its formation. The proper manner with which to study diamonds and their inclusions is together on polished surfaces, involving sophisticated micro-instrumental chemical techniques, coupled with isotopic analyses of the diamond, all within the context of the intricate crystal growth patterns of the diamonds. A comprehensive geochemical and petrologic study of inclusions within diamonds is proposed, with the key distinction that the growth-histories and chemical characteristics of the diamonds will also be documented, as placed within the 'stratigraphy' of the diamonds. Diamonds will be used as recorders of the evolution of the Earth's mantle. To this end, an international consortium of scientists has been assembled with expertise in most aspects of diamond genesis. This constitutes the first truly integrated study of the petrogenesis of diamonds, their mineral inclusions, and their host xenoliths. This integrated approach to the study of diamonds, their inclusions, and host xenoliths, will make a significant contribution to the knowledge of mantle conditions necessary for diamond formation and the evolution of the mantle