Análise das razões isotópicas de 87sr/86sr em mesossaurídeos do Membro Assistência da Formação Irati – SP
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2023-12-05
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Universidade Estadual Paulista (Unesp)
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A Formação Irati é a unidade basal do Grupo Passa Dois, Permiano da Bacia do Paraná. Esta Formação é caracterizada por sua grande continuidade lateral, e exibe em seu registro geológico dois membros: Taquaral (inferior) e Assistência (superior). Neste trabalho, a porção alvo foi o Membro Assistência, característico por seu conteúdo fossilífero e camadas de folhelhos intercaladas com rochas dolomíticas. A unidade apresenta um contexto paleogeográfico restrito, definido como mar epicontinental. Apesar de a Formação já ter sido alvo de diversos estudos, o caráter anômalo de seu contexto paleoambiental e paleogeográfico ainda apresenta lacunas. No intuito de discutir sobre esses aspectos e obter novas informações acerca do contexto paleogeográfico e do paleoambiente, foram feitas análises geoquímicas isotópicas de estrôncio (isótopos 87Sr/86Sr) em fósseis de mesossaurídeos e em dolomitos do Membro Assistência. Foram analisadas 8 amostras no total, 6 de fósseis e 2 de rochas. As amostras foram analisadas no espectrômetro de massa multicoletor por termo-ionização. A partir destas análises, foi notada uma heterogeneidade isotópica entre as amostras, tanto em cada fóssil como em cada amostra de rocha. Além dessa heterogeneidade, as razões isotópicas obtidas se apresentam muito mais elevadas do que o esperado para o Permiano global, variando de 0,709682 a 0,711087. A partir desta análise foram discutidas quais as hipóteses para estes valores se mostrarem tão altos. A principal hipótese sugere que o contexto paleogeográfico e ambiente restrito da Formação Irati são os principais responsáveis pelos valores excedentes para o Permiano, devido ao fato de que ambientes restritos ou de conexão parcialmente limitada com o oceano podem apresentar uma pré-disposição para concentração de Sr radiogênico no ambiente deposicional, a depender da composição química das rochas que alimentaram o sistema deposicional. Por outro lado, discute-se a hipótese de que as amostras analisadas apresentam valores alterados por processos diagenéticos e pós-deposicionais. Desta forma, o presente trabalho buscou abordar, da maneira mais ampla possível, o significado dos valores elevados de 87/86Sr da Formação Irati e contribuir para o conhecimento acerca dessa unidade.
The Irati Formation is the basal unit of the Passa Dois Group, Permian of the Paraná Basin. This formation is characterized by its significant lateral continuity and displays two members in its geological record: Taquaral (basal) and Assistência (upper). In this study, the target portion was the Assistência Member, distinguished by its fossil content and layers of shales interbedded with dolomitic rocks. The unit exhibits an restricted paleogeographic context, defined as an epicontinental sea. Despite the Formation having been the subject of various studies, the anomalous nature of its paleoenvironmental and paleogeographic context still presents gaps.In an effort to discuss these aspects and obtain new information about the paleogeographic and paleoenvironmental context, geochemical analyses of strontium isotopes (87Sr/86Sr) were conducted on fossils of mesosaurs and dolomites from the Assistência Member. A total of 8 samples were analyzed, 6 of fossils and 2 of rocks. The samples were examined using a multi-collector thermal ionization mass spectrometer. From these analyses, isotopic heterogeneity was observed among the samples, both within each fossil and each rock sample. In addition to this heterogeneity, the obtained isotopic ratios are much higher than expected for the global Permian, ranging from 0.709682 to 0.711087. This analysis led to a discussion of hypotheses to explain why these values are so high. The primary hypothesis suggest that the paleogeographic context and restricted environment of the Irati Formation are the main contributors to the excess values for the Permian, due to the fact that restricted (or with partially limited connection to the ocean) environments may predispose to the concentration of radiogenic Sr in the depositional environment, depending on the chemical composition of the rocks that fed the depositional system. On the other hand, it is also discussed a hypothesis in which the analyzed samples have altered values due to diagenetic and post-depositional processes. In this way, the present work sought to address, as broadly as possible, the significance of the elevated values of 87/86Sr in the Irati Formation and contribute to the understanding of this unit.
The Irati Formation is the basal unit of the Passa Dois Group, Permian of the Paraná Basin. This formation is characterized by its significant lateral continuity and displays two members in its geological record: Taquaral (basal) and Assistência (upper). In this study, the target portion was the Assistência Member, distinguished by its fossil content and layers of shales interbedded with dolomitic rocks. The unit exhibits an restricted paleogeographic context, defined as an epicontinental sea. Despite the Formation having been the subject of various studies, the anomalous nature of its paleoenvironmental and paleogeographic context still presents gaps.In an effort to discuss these aspects and obtain new information about the paleogeographic and paleoenvironmental context, geochemical analyses of strontium isotopes (87Sr/86Sr) were conducted on fossils of mesosaurs and dolomites from the Assistência Member. A total of 8 samples were analyzed, 6 of fossils and 2 of rocks. The samples were examined using a multi-collector thermal ionization mass spectrometer. From these analyses, isotopic heterogeneity was observed among the samples, both within each fossil and each rock sample. In addition to this heterogeneity, the obtained isotopic ratios are much higher than expected for the global Permian, ranging from 0.709682 to 0.711087. This analysis led to a discussion of hypotheses to explain why these values are so high. The primary hypothesis suggest that the paleogeographic context and restricted environment of the Irati Formation are the main contributors to the excess values for the Permian, due to the fact that restricted (or with partially limited connection to the ocean) environments may predispose to the concentration of radiogenic Sr in the depositional environment, depending on the chemical composition of the rocks that fed the depositional system. On the other hand, it is also discussed a hypothesis in which the analyzed samples have altered values due to diagenetic and post-depositional processes. In this way, the present work sought to address, as broadly as possible, the significance of the elevated values of 87/86Sr in the Irati Formation and contribute to the understanding of this unit.
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Paleogeografia, Geoquímica, Isótopos de estrôncio, Permiano, Paleogeography, Geochemistry, Strontium isotopes, Permian