U/Th dating of carbonate deposits from Chafarinas Islands, Spain

Abstract

The Chafarinas Islands are a group of volcanic outcrops off the north coast of the African continent. Quaternary deposits and calcrete formations cover most of the islands’ planed surface, mainly on Rey Francisco I Island. Uranium-series ages are presented for a geological section with 18 m of detrital deposits and calcrete layers. Calcretes consist of impure carbonate; the leach/leach (L/L) method was therefore used to obtain 234U/238U and 230Th/234U activity ratios in the pure authigenic carbonate fraction to obtain ages of the samples. Resulting ages range from 14 to 80 ka. Except for one sample at the bottom of the age sequence, with the youngest age, all others have ages that agree with their stratigraphic location. Ages obtained using the L/L method were also compared with the method of Allegre and Condomines (Earth Planet Sci Lett 28:395–406, 1976) and with a more recent method (Ludwig and Paces Geochim Cosmochim Acta 66:487–506, 2002, Sharp et al. Quat Res 59:139–150, 2003, Haase-Schramm et al. Geochim Cosmochim Acta 68:985–1005, 2004). The latter two methods were also used to estimate isotopic composition of the detrital com ponent in the impure carbonate and model a more realistic age for the anomalous sample. Minimal uranium isotope activity ratio differences imply that environmental condi tions between 25 and 60 ka remained relatively uniform and that a major change may have occurred *80 ka ago.