The end of the Neoproterozoic is punctuated by glacial deposition, but the chronology of these deposits is hindered presently by the paucity of geochronological data. Here, we present new radiometric dating for the basal Sete Lagoas cap carbonate deposits that overlie glacial units in the São Francisco craton. Six samples from aragonite‐pseudomorph crystal‐rich facies, showing pristine textures and constant 87Sr/86Sr ratios around 0.7075, yielded a Pb–Pb isochron age of 740 ± 22 Ma, which is interpreted as the depositional age for these remarkably preserved rocks. This age can be used to infer a low‐to‐moderate palaeolatitude of 20–30° for carbonate (and glacial) deposition. In addition, as it overlaps the ages obtained for the oldest Neoproterozoic glacial successions, our result reinforces the idea of a long‐standing ‘Sturtian’ interval, suggesting that this event represents either different discrete glaciations or a protracted event encompassing almost 80 Ma.

A sedimentological and C-O isotopic study has been carried out in nine sections of the Sete Lagoas Formation at its classical outcropping area, in the southern tip of the Sa ˜o Francisco craton (central Brazil), with the objective of refining its stratigraphic position within the Neoproterozoic. At the study area, the Neoproterozoic Sete Lagoas Formation comprises two shallowingupward megacycles, corresponding to more than 200 m in thickness. Each cycle is limited by a flooding surface amalgamated with a third-order sequence boundary. The first megacycle presents deep-platform deposits with abundance of crystal fans (aragonite pseudomorphs). These deposits are characterized by negative C-isotope values (-4.5%). They grade upward to storm-wave and tide-influenced layers with d 13 C values around 0%. In the second megacycle, a new transgression drowned the platform, depositing a thick, mixed sub-storm wave-base succession. This megacycle comprises deposits of lime mudstone-pelite rhythmite, which grade to crystalline limestone rich in organic matter, both with unusually positive d 13 C values (up to + 14%). Regional correlation of Sete Lagoas deposits indicate that they rest atop glaciomarine rocks of the Macau ´bas Group and basal strata show seafloor precipitates with negative d 13 C values. Therefore, it is possible to characterize the Sete Lagoas carbonate as a cap carbonate sequence. The very high d 13 C in the second megacycle together with geochronologic data suggest that this unit correlates better with post-Sturtian sequences. Some differences in the depositional record are observed between Sete Lagoas and the other post-Sturtian units previously described in North America, Australia, and Namibia. Those differences may in part be due to deposition in shallower settings of the Sete Lagoas carbonates, thus preserving a thick record of storm-and wave-influenced sedimentation not found elsewhere. Alternatively, they may also be attributed to diachronic deposition of the so-called post-Sturtian cap carbonate sequences.

International audienceAlthough Ni isotopes have been shown to be significantly fractionated in terrestrial samples, their use in continental environmental studies has not yet been evaluated. The present study focuses on an ultramafic (UM) massif (Barro Alto, Goias, Brazil) because such areas are naturally rich in Ni. We present developed lateritic weathering profiles. The goal of the study is to evaluate the potential of using Ni isotopes in environmental continental studies by combining its isotopic signature with mineralogy, in order to better understand the geochemical cycling of Ni in UM settings during weathering. As such, Ni isotope values were measured in samples from the Barro Alto UM complex in the main stages of the lateritic weathering profile of UM rocks, including bedrock, ores (saprolitic and lateritic samples) and soil. The mineralogical composition of the samples, with a focus on the different Ni-bearing minerals, was also determined to decipher the potential links between isotopic fractionation and weathering dynamics. Isotopic signatures (delta Ni-60) from the natural Ni geochemical cycle include: bedrock samples (delta Ni-60 = 0.28 +/- 0.08%), ore samples (saprolitic and lateritic, delta Ni-60 from - 0.60 to 0.30%) and soil samples (delta Ni-60 from - 0.19 to - 0.02%). An overall trend of heavier isotope depletion was observed in the solid phase during weathering (Delta Ni-60(Soil-Bedrock) = -0.47%). The mineralogical results were consistent with the literature and showed that the mineralogy of the lateritic part and soil was dominated by Fe-oxides, whereas clay minerals were the primary Ni phase scavengers in the saprolitic part of the profile. Thus, the formation of Ni-bearing clay minerals and Fe-oxides appeared to lead to depletion in heavier isotopes, which indicates preferential export of heavy isotopes in the dissolved phase. This result is consistent with isotopic signatures measured in the exchangeable pool of the solid phase (Delta Ni-60(exch-total) up to 0.47%), and Ni isotopes appear to be a promising tracer to better understand the biogeochemical Ni cycling on the Earth's surface

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Resumo Estudos detalhados de fáceis sedimentares e empilhamento estratigráfico de 12 seções em minas e corte de estrada, ao longo de um perfil de 80,5 km, conduziram ao reconhecimento de 11 fácies sedimentares da Formação Sete Lagoas, 2 fácies do Conglomerado Carrancas e 3 fácies na porção basal da Formação Serra de Santa Helena. Essas fácies compõem 9 associações de fácies, as quais correspondem ao registro de 3 seqüências deposicionais. As seqüências estratigráficas descritas são caracterizadas por tratos de sistema transgressivo e mar alto, com preservação secundária de um trato de sistema de mar baixo. Duas das três seqüências reconhecidas estão associadas à sucessão carbonática da Formação Sete Lagoas, enquanto a última seqüência, sobreposta aos carbonatos, corresponde ao registro siliciclástico da Formação Serra de Santa Helena. A distribuição estratigráfica das associações de fácies revela uma transição de ambientes de águas rasas a Oeste da área estudada para ambientes de águas mais profundas em direção a Leste. Um modelo de rampa carbonática, mostrando os diferentes estágios de evolução das seqüências, é apresentado como proposta de interpretação para os dados faciológicos e estratigráficos obtidos para os depósitos da Formação Sete Lagoas na região homônima e proximidades.