Abstract The appearance of complex megascopic multicellular eukaryotes in the Ediacaran occurred just when the dynamics of a cooling Earth allowed establishment of a new style of global tectonics that continues to the present as “modern-type plate tectonics”. The advent of this style was first registered in 620 Ma-old coesite-bearing Ultra-High Pressure eclogites within the Transbrasiliano-Kandi mega-shear zone along the site of the West Gondwana Orogeny (WGO). These eclogites comprise the oldest evidence of slab-pull deep subduction capable of inducing continental collisions and producing high-relief Himalayan-type mega-mountains. Life, prior to this time, was essentially microscopic. Yet with increasing Neoproterozoic oxygenation and intensified influx of nutrients to Ediacaran oceans, resulting from the erosion of these mountains, complex macroscopic heterotrophic eukaryotes arose and diversified, taking the biosphere to a new evolutionary threshold. The repeated elevation of Himalayan-type mega-mountains ever since then has continued to play a fundamental role in nutrient supply and biosphere evolution. Other authors have alluded to the influence of Gondwana mountain-building upon Ediacaran evolution, however we claim here to have identified when and where it began.

ABSTRACT: We present new Sm-Nd, U-Pb and Hf isotope geochronological data for the siliciclastic rocks in the southern sector of the São Francisco Basin. An abrupt change in the Sm-Nd data is observed from the Carrancas Formation’s oligomitic conglomerates (TDM ages between 2.7 and 3.3 Ga; εNd(550 Ma) values between -25.09 and -17.98), its finer facies, and the finer facies of the the Moema Laminite (TDM ages from 1.4 to 2.1 Ga; εNd(550 Ma) values between -9.46 and -5.59). No further significant changes in the Sm-Nd data occur farther upwards in the Bambuí Group (Sm-Nd TDM ages within the 1.3-2.0 Ga interval; εNd(550 Ma) values from -9.53 to -4.09), showing a lack of reorganization in the source areas throughout the deposition of the basin. This is yet another argument to dismiss an unconformity within the Bambuí Group. The presence of the index fossil Cloudina sp. in the lower Sete Lagoas Formation makes the glaciation probably Late Ediacaran in age. U-Pb ages for detrital zircons of the Bambuí Group range from the Archean to the early Ediacaran, but the current data is insufficient to distinguish between the contribution from sources in the Brasília and Araçuaí belts.

RESUMO: Dados quimioestratigráficos de alta resolução em carbonatos da Formação Sete Lagoas (Grupo Bambuí) apresentam grandes variações nas composições isotópicas de C, O e Sr. Os calcários impuros na base da unidade mostram valores primários de δ 13C entre -1,0 e 0‰ e de δ 18O, entre -12,0 e -8,0‰. No entanto, alguns níveis de dolomitos indicam valores de δ 13C variando de +2,8 a -6,8‰ e razões 87Sr/86Sr altamente radiogênicas (>0,7111), além de baixas concentrações de Sr (<350 ppm) relacionadas a processos pós-deposicionais. Em contraste, os calcários puros no topo da unidade demonstram valores de δ 13C muito positivos entre +8,3 e +12,8‰, de δ 18O entre -10,0 e -6,0‰ e razões 87Sr/86Sr de 0,7073 a 0,7086, com altas concentrações de Sr (>900 ppm). Esses estão ligados a controles deposicionais na plataforma carbonática, tais como fluxos de água fluvial e/ou submarina, em que os carbonatos depositados no setor proximal exibem variações isotópicas de Sr significativas e os depositados no setor distal não estão sujeitos a tais controles, resultando em perfis isotópicos de Sr homogêneos. No entanto, as razões 87Sr/86Sr dos carbonatos distais são menos radiogênicas do que aquelas esperadas para o Ediacarano tardio (~0,7085). Essa discrepância sugere um ambiente deposicional em uma bacia marinha restrita, sem homogeneização isotópica de Sr com oceanos contemporâneos e, neste caso, as correlações globais baseadas na estratigrafia isotópica de Sr não são confiáveis.

ABSTRACT: High-resolution chemostratigraphic data of carbonates from the Sete Lagoas Formation (Bambuí Group) show large variations on the C, O, and Sr isotope compositions. Impure limestones at the base show primal δ13C values between -1.0 and 0‰, and δ18O values between -12.0 and -8.0‰. However, some dolostones demonstrate δ13C values varying from +2.8 to -6.8‰, highly radiogenic 87Sr/86Sr ratios (>0.7111), and low Sr concentrations (<350 ppm) related to post-depositional processes. In contrast, pure limestones at the top show very positive δ13C values between +8.3 and +12.8‰, δ18O values between -10.0 to -6.0‰, and 87Sr/86Sr ratios from 0.7073 to 0.7086, with high Sr concentrations (>900 ppm). They are linked to depositional controls on the carbonate platform, such as fluvial and/or submarine water influx, in which carbonates deposited on the proximal sector exhibit significant Sr isotopic variations and those on the distal sector were not subject to such controls, resulting in very homogeneous Sr isotope profiles. However, 87Sr/86Sr ratios of the distal carbonates are less radiogenic than carbonates expected for late Ediacaran (~0.7085). This discrepancy suggests a restricted marine basin without Sr isotopic homogenization with contemporary oceans and, in this case, global correlations based on Sr isotope stratigraphy are not reliable.

Precise and accurate d66/64Zn data for environmental reference materials (RMs) including rocks, sediments, soils and plants are presented in order to improve the metrological traceability and analytical control of Zn isotope ratio determinations in future environmental studies. Previously developed ion exchange chromatography protocols were adjusted to enable faster sample throughput and instrumental mass bias processes were investigated. The improved chromatographic protocol yielded precise and quantitative recoveries (99 ± 7%, σ, n = 16), while the mass bias correction using Cu as external dopant provided precisions better than 0.02‰, 2σ, n = 7. Investigations into spectral and non-spectral interferences identified significant formation of Cr and Ti oxides and hydroxide ionic species. Analysis of six RMs (BHVO-2 basalt (USGS), BCR-2 basalt (USGS), AGV-2 andesite (USGS), 2709 San Joaquin soil (NIST), 1646a estuarine sediment (NIST) and 1573a tomato leaves (NIST)) showed good reproducibility (< 0.01‰, 2s, 5 ≤ n ≥ 1).

RESUMO: Este trabalho apresenta os primeiros resultados de composições isotópicas de Pb dos diques mesozoicos com alto titânio do Espinhaço Meridional. As composições isotópicas de Pb dos diques investigados não mostram grandes variações, sendo significativamente mais radiogênicas do que aquelas dos basaltos com alto titânio da Província Basáltica Continental do Paraná. Esses resultados foram integrados a análises geoquímicas e isotópicas de Sr-Nd realizadas nas mesmas amostras, com o objetivo de obter informações adicionais sobre a gênese desses diques. As composições isotópicas de Sr-Nd-Pb não são compatíveis com a ocorrência de processos de contaminação crustal e requerem que a geração desses magmas tenha ocorrido em manto litosférico subcontinental metassomatizado, com a participação de componentes carbonatíticos e manto do tipo HIMU. Esses magmas podem também ter sido originados em fonte mantélica contendo ~4 - 5% de componentes piroxeníticos e ~1% de carbonatíticos, concordando também com as composições isotópicas de Os dos basaltos com alto titânio da Província Basáltica Continental do Paraná. Esses agentes metassomatizantes foram os responsáveis pela refertilização da fonte mantélica, conforme proposto na literatura para explicar as características dos xenólitos que ocorrem na Província Alcalina de Goiás, localizada na borda do Cráton do São Francisco. Com o objetivo de avaliar os riscos de contaminação por Pb durante o processo de preparação de amostras para análise, foram também realizados vários testes experimentais que indicam a necessidade de remoção de superfícies serradas e de uma lavagem cuidadosa da rocha finamente britada antes da pulverização, especialmente para aquelas com [Pb] < 7 µg /g.

ABSTRACT: The first results of Pb isotope compositions of the high-Ti Mesozoic dykes of the Southern Espinhaço are presented. The results do not show large variations and are significantly more radiogenic than the Pb isotope compositions of the high-Ti tholeiites from the Paraná Continental Flood Basalts. The data combined with published geochemical and Sr-Nd isotope results rule out crustal contamination processes in the genesis of the dykes, requiring magma generation in metasomatized subcontinental lithospheric mantle with the involvement of HIMU-type and carbonatite components. The magmas may have been also derived from a mantle source containing ~4 - 5% of pyroxenite and ~1% of carbonatite melts, agreeing with published Os isotope compositions of high-Ti rocks from the Paraná Continental Flood Basalts. These metasomatizing agents could be responsible for mantle source refertilization, as was also proposed in the literature to explain the characteristics of xenoliths of the Goiás Alkaline Province, which also occurs in the border of the São Francisco Craton. Additionally, to evaluate the risks of Pb contamination during sample preparation for analysis, several experimental tests were accomplished, which indicate the need of sawed surface removal and a careful washing of small-sized rock fragments before powdering, especially for rocks with [Pb] < 7 µg/g.

RESUMO: O Grupo Bambuí, mais importante unidade de cobertura do Cráton do São Francisco, tem sido alvo de intensos estudos e debates, entre outros motivos, pela possibilidade de correlação com eventos glaciais globais. A maior parte dos dados quimioestratigráficos disponíveis, no entanto, provém de amostras coletadas em afloramentos, sujeitos a expressivo intemperismo químico. Neste trabalho, é apresentado um levantamento quimioestratigráfico de alta resolução, baseado em análises de C, O e Sr realizadas em 53 amostras coletadas em 175 m de sequência sedimentar contínua, livre de intemperismo. Tal sequência foi obtida a partir de testemunhos de sondagem realizada na porção sul do Cráton do São Francisco, onde ocorrem rochas das formações Carrancas e Sete Lagoas, as mais basais do Grupo Bambuí. Os testemunhos revelaram contato extremamente irregular entre uma camada de diamictito e seu embasamento, um granodiorito foliado de idade arqueana. Dados sedimentológicos e geocronológicos indicam que o diamictito representa um tilito de alojamento, que apresenta contato brusco com a sequência carbonática sobrejacente. Essa sequência se inicia com calcário impuro, que exibe leques de cristais pseudomorfos de aragonita e delgadas camadas de folhelho negro. O calcário passa gradacionalmente para um intervalo argiloso, que por sua vez volta a gradar para uma espessa sequência de calcário com laminação microbiana, sucedido por dolomito estromatolítico no topo da coluna. As assinaturas isotópicas de C e O permitem a identificação de três intervalos distintos. O Intervalo I, basal, corresponde a um carbonato de capa, exibindo valores negativos de δ 13C (c. -4‰), e grande oscilação nos valores de δ 18O (-6 a -15‰). O Intervalo II exibe marcante homogeneidade nos valores de δ 13C e δ 18O, que se situam em torno de 1‰ e -7‰, respectivamente. No topo, o Intervalo III exibe uma grande excursão positiva dos valores de δ 13C (até 8‰) e δ 18O (-8 a -3‰). Razões 87Sr/86Sr variam entre 0,7075 e 0,7077, obtidas exclusivamente em amostras do topo da sequência. A assinatura geoquímica das rochas carbonáticas mostrou-se fortemente controlada pelo conteúdo terrrígeno, concentrado sobretudo nas unidades mais basais. Amostras livres da influência de elementos terrígenos mostram razões Y/Ho variando entre 25 e 50, o que sugere a influência de água doce durante a deposição dos calcários. Com base nos dados obtidos, conclui-se que a porção basal da sequência estudada representa um par "diamictito-carbonato de capa" relacionado a um dos eventos glaciais que ocorreram entre o fim do Neoproterozoico e o início do Paleozoico. Além disso, discrepâncias nas razões 87Sr/86Sr, quando comparadas às curvas globais de evolução isotópica, podem ser atribuídas à influência da água doce em um ambiente marinho restrito. Tendo-se em vista o contexto regional, a Bacia Bambuí representaria, no limite Neoproterozoico/Paleozoico, uma bacia marinha restrita, total ou parcialmente circundada por cadeias de montanhas, no interior de Gondwana. Em seus estágios iniciais, a sedimentação teria ocorrido sob influência de um episódio glacial, cuja fase de degelo foi responsável por significativa entrada de água doce no mar. Com o gradual aumento de temperatura, a atividade biológica no mar teria aumentado progressivamente. Por fim, um súbito aumento na atividade biológica pode ter sido catalisado por mudanças paleogeográficas causadas pela tectônica ativa.

ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.

Lead analyses in bottom sediments from the hydrographic system of Belem (Para) indicated low contents of this metal for the sediments from the Guama river, with no significant anthropogenic contribution. A concentration of 18.1 ± 1.5 mg kg-1 and 206Pb/207Pb isotopic signature of 1.196 ± 0.002 are assigned for Pb from natural sources. On the other hand, the significant increase of Pb contents in the sediments from the Guajará bay, together with the decrease of 206Pb/207Pb ratios (1.172 < 206Pb/207Pb < 1.188) point to an anthropogenic lead contribution, originated by the industrial and urban activities of the city of Belem.

Few articles deal with lead and strontium isotopic analysis of water samples. The aim of this study was to define the chemical procedures for Pb and Sr isotopic analyses of groundwater samples from an urban sedimentary aquifer. Thirty lead and fourteen strontium isotopic analyses were performed to test different analytical procedures. Pb and Sr isotopic ratios as well as Sr concentration did not vary using different chemical procedures. However, the Pb concentrations were very dependent on the different procedures. Therefore, the choice of the best analytical procedure was based on the Pb results, which indicated a higher reproducibility from samples that had been filtered and acidified before the evaporation, had their residues totally dissolved, and were purified by ion chromatography using the Biorad® column. Our results showed no changes in Pb ratios with the storage time.