Magnetic properties of aggregate polycrystalline diamond: implications forcarbonado history

Carbonados are aggregate polycrystalline diamonds and are found in placer d eposits of the Central African Republic (CAR) and the Bahia Province of Bra zil. Their origin is uncertain, but several mutually exclusive hypotheses h ave been proposed ranging from extraterrestrial to crustal, and mantle prov enance. In an endeavor to further our understanding of these diamonds, we p erformed a series of magnetic characterization studies on 20 samples from t he CAR. Our results reveal that the carbonados contain material with contra sting magnetic hysteresis behavior and magnetization, Acid leaching permitt ed us to monitor the distribution of magnetic carriers within the samples. An absence of sample size dependence on magnetization loss indicates that t he magnetic carriers are distributed at the vitreous surface; including ope n pores and that the carbonade interior is essentially devoid of the magnet ic carriers. The diamond's isolated nonmagnetic interior of carbonados sugg ests that the initial formation environment was deficient in magnetic parti cles. The formation of the magnetic carriers is thus closely linked with th e origin of the smooth surface, perhaps during the hypervelocity ejection o f carbonados into the Earth's atmosphere. Partial ablation in the Earth's p rimordial, oxygen-poor atmosphere resulted in a fusion of the originally po rous material and a decrease in the original pore density at the surface. S econdary mineralization of the impact-generated melt and/or the products of later diagenetic processes, containing the majority of the magnetic carrie rs, filled the remainder of the open pores. (C) 2000 Elsevier Science B.V. All rights reserved.