Incompatible trace element partitioning and residence in anhydrous spinel peridotites and websterites from the Ronda orogenic peridotite

Carbonados are aggregate polycrystalline diamonds and are found in placer deposits of the Central African Republic (CAR) and the Bahia Province of Brazil. Their origin is uncertain, but several mutually exclusive hypotheses have been proposed, ranging from extraterrestrial to crustal, and mantle provenance. In an endeavor to further our understanding of these diamonds, we performed a series of magnetic characterization studies on 20 samples from the CAR. Our results reveal that the carbonados contain material with contrasting magnetic hysteresis behavior and magnetization. Acid leaching permitted us to monitor the distribution of magnetic carriers within the samples. An absence of sample size dependence on magnetization loss indicates that the magnetic carriers are distributed at the vitreous surface; including open pores and that the carbonado interior is essentially devoid of the magnetic carriers. The diamondʹs isolated non-magnetic interior of carbonados suggests that the initial formation environment was deficient in magnetic particles. The formation of the magnetic carriers is thus closely linked with the origin of the smooth surface, perhaps during the hypervelocity ejection of carbonados into the Earthʹs atmosphere. Partial ablation in the Earthʹs primordial, oxygen-poor atmosphere resulted in a fusion of the originally porous material and a decrease in the original pore density at the surface. Secondary mineralization of the impact-generated melt and/or the products of later diagenetic processes, containing the majority of the magnetic carriers, filled the remainder of the open pores.