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Boletín de la Sociedad Geológica Mexicana

versão On-line ISSN 1405-3322

Resumo

SANCHEZ-BERISTAIN, Francisco  e  LOPEZ-ESQUIVEL KRANKSITH, Laura. Geochemical analyses (major, minor and trace elements, δ18O, and rare earths) of selected microbialites from the San Casiano Formation (Upper Triassic, NE Italy). Bol. Soc. Geol. Mex [online]. 2011, vol.63, n.3, pp. 399-420. ISSN 1405-3322.

The results of the geochemical analysis of 28 major, trace and rare earth elements, as well as carbon and oxygen isotopes, are presented for 17 microbialite samples from the St. Cassian Formation (NE Italy). These results have allowed the characterisation of the mineralogical composition of the microbialites as high-Mg calcite (>4.00 % mol Mg), the determination of their good state of preservation, and the detection of the influence of fine siliciclastic sedimentation during their growth. Furthermore, redox conditions of the palaeoenvironment were determined with the aid of these analyses. Finally, it was possible to compare the geochemical signals of the microbialites with that of skeletal components, allomicrites and cements in order to find possible metabolical relicts and/or evidence of biological activity using trace elements. Two methods were used to perform these analyses: Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), and mass spectrometry to obtain carbon and oxygen isotope values. Microbialitic samples show medium to high contents of silicon and aluminium. These elements reflect the sedimentation of clays during the growth of microbialites. There is no correlation between the values of Mn/Sr and δ18O or between δ13C and δ18O, indicating the absence of meteoric diagenesis. Iron and manganese concentrations show a medium correlation with sulfur, which links the origin of these elements to early diagenetic processes. These processes can be confirmed by the high strontium content and the δ18O values. The microbialites are enriched with a vast array of minor and trace elements. Some elements of unknown biological importance (Rb, Ti, Zr, Nb and Th) are associated with certain major elements (r2 with Si > 0. 5), confirming their terrigenous origin. Other elements (Zn, Sb, Cr, V, Sn, Ni and Co) show no association with silicon or aluminium, yet they cannot be considered as metabolic residues due to their enrichment in non-biological phases. Instead, some of these elements (Cr, V, Ni and Co) were useful in determining the redox conditions of the palaeoenvironment of the St. Cassian Formation as slightly disoxic. Only copper and molybdenum are associated with a microbial origin, and the latter is also useful in determining redox conditions. Rare Earth Element analyses are not conclusive, since phosphorus, silicon and aluminium can significantly alter their precipitation processes. Due to the exceptional preservation of the studied material, it is possible to interpret all geochemical signals of the studied material as primary.

Palavras-chave : Microbialites; Carnian; Triassic; St. Cassian Formation; element analysis; LA-ICP-MS.

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