Diamonds: how they form where they are

Seismic tomography, lithosphere modeling and trace-element microanalysis of diamonds are providing new insights into the genesis of cratonic diamonds and the tectonic location of kimberlitic magmas. This multidisciplinary approach also helps to understand why these diamonds occur where they do, and how to improve exploration sucess. Cratonic diamonds form by the refertilisation of highly depleted dunite/harzburgite in the deep lithosphere. This process probably involves the oxidation of CH4-bearing fluids and the reduction of Fe in lithospheric rocks. Further refertilisation by silicate melts produces lherzolites, and oxidised fluids that form most fibrous diamonds. A review of data on the alluvial diamonds of Eastern Australia emphasizes how the major Copeton-Bingara population differs from cratonic diamonds, supports their unique subduction origin, and notes some pitfalls for the explorer.