Rare earth element geochemistry of the Antalo Supersequence in the Mekele Outlier (Tigray region, northern Ethiopia)

The Antalo Supersequence (ASS), a Mesozoic carbonate–marl–shale succession, in the Mekele Outlier was sampled for chemical analysis. The samples were analysed by ICP-MS and ICP-OES for rare earth element (REE) and major elements both in the total sample and the acid-soluble fraction. han 80% of the REE are held in the acid-soluble fraction. However, the very good correlation between REE, insoluble residue (IR), Mn and Fe coupled with the three-fold enrichment of REE in the impure compared to the pure limestone suggests that the content of REE in the ASS is related to the non-carbonate constituents. LREE, relatively flat HREE patterns and a negative Eu anomaly characterize the chondrite-normalized pattern of REE. Fractionation of REE in the sedimentary environment is displayed by a vertical increase and a west–east decrease in the LREE/HREE ratio and by a negative Ce anomaly towards the base of the ASS. The latter suggests that the Mesozoic Sea of the region was relatively more oxidizing during its earlier history of transgression. The Ce-depletion displayed by the whole ASS is, however, very small and this is ascribed to the influence of continental clastic materials, shallowness of the basin and dominant reducing conditions in the seawater. Based on the similarity in the ratios and patterns of REE, it is concluded that the ASS and the nearby clastic unit have the same continental source of REE.