The Constitution of the Transition Zone and Lower Mantle Shown by Diamonds and their Inclusions

Hutchison, 1998, University of Edinburgh

Up until recently, what is known about the constitution of the Earth's mantle below about 200 km has been derived through indirect methods such as geophysical measurement and high pressure experimental petrology. Throughout the past 10 years or so, it has become increasingly apparent that transition zone and lower mantle mineral assemblages can be found in-tact at the Earth's surface, having been transported from depth within diamond. The evidence that some mineral associations within diamond have come from such great depths is compelling and is discussed in the thesis Hutchison (1998) and its accompanying papers. Briefly, the principal evidence revolves aroung the fact that associations of mineral phases have been recovered within single diamonds which, following the results of experimental petrology, can only have formed in equilibrium at pressures equivalent to the Earth's deep mantle. Examples of such associations are enstatite and periclase-wüstite; aluminous enstatite, corundum and periclase-wüstite composition phases and the single phase, majoritic garnet. Other separate lines of evidence include the observation of elevated internal pressure imposed by diamond on periclase-wüstite inclusions, high ferric iron contents in pyroxene inclusions in association with extremely depleted ferric iron in associated periclase-wüstite and the discovery of a high pressure garnet composition phase termed TAPP.

Ninety-five percent of diamonds recorded to contain transition zone and lower mantle inclusions come from the Säo Luiz alluvial diamond deposits in central Brazil. Hutchison (1998) is the major reference to present data from this source and additionally represents the most comprehensive discussion of deep mantle diamonds to date.

Key sections from the thesis are provided below with links to selected figures. Copies of the full 660 page work is available on interactive CDRom; just follow the link "Statistics and Ordering Information". Hard copies of related articles are also avaible on request

The thesis documents all major, minor and trace element compositions, structural and morphological data of Säo Luiz inclusions and their host diamonds and includes much of the data published for deep mantle diamonds from elsewhere. Furthermore, due to the cross-disciplinary nature of this work, reviews of current thinking on the subjects of diamond geochemistry, mantle convection and composition and high pressure experimental petrology are presented. Also included are detailed compilations of relevent phase diagrams and tables of thermoelastic data which are already being utilised for teaching purposes by, amongst others, Prof. Liebermann of SUNY at Stony Brook and Dr. Ian Main of the University of Edinburgh.

Extracts from the Thesis
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Statistics and Ordering Information