Discriminating fluid sources in Miocene cold seep systems using REEs in authigenic carbonates

Authigenic carbonates are a common feature in cold seep environments, where fluids enriched in methane and other hydrocarbon gases escape from the seafloor. Seep-carbonates have been reported worlwide both in modern and ancient sedimentary deposits. In the Northern Apennines (Italy), numerous outcrops of seep-carbonates are particularly well-preserved. Evidences from paleoecological, sedimentological, geochemical and isotopic (O and C stable isotopes) analyses cleary show that they were derived from the microbial oxidation of methane-rich fluids. REE patterns and abundances in fossil seep-carbonates may provide additional informations for better constraining the origin and the composition of the fluids from which they have precipitated.Here, we report REE data for a series of Miocene carbonate samples recovered from various geological settings in the Northern Apennines. Samples were leached with 5% HNO3, prior to analysis by SF-ICPMS using the Tm addition method [1]. Total REE concentrations (ΣREEN) in our studied carbonates are very similar to those reported for modern authigenic carbonates, suggesting negligible post-depositional diagenetic alteration. The shale-normalized REE patterns vary significantly amongst the different authigenic carbonate samples analysed in this study. These data indicate that they were formed from fluids having distinct REE signatures. These results, coupled with other geochemical, petrographic and mineralogical data, allow us to reconstruct the variation of fluid seepage activity in the Northern Appenines during the Miocene.