Navegando por Autor "Oliveira, Fabio Soares"

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Mineralogical and geochemical signatures of Quaternary pyroclasts alterations at the volcanic Trindade Island, South Atlantic.
(2020) Mateus, Ana Carolina Campos; Varajão, Angélica Fortes Drummond Chicarino; Petit, S.; Oliveira, Fabio Soares; Schaefer, Carlos Ernesto Gonçalves Reynaud
This paper reports the composition and alteration products of pyroclasts in the Holocene Paredão volcano (pyroclast 1) and Late Quaternary Morro Vermelho Formation (pyroclasts 2 and 3) of Trindade Island, South Atlantic, Brazil using combined macromorphological, micromorphological, mineralogical and geochemical techniques. The pyroclasts 1 and 2 are interpreted as volcanic tuff breccia deposits, whereas pyroclast 3 is a lapilli deposit. They are dark gray in color with some altered reddish regions and show vesicles and amygdales structures with small greenish crystals of 2.0 mm scattered throughout the matrix. The eruptions can be regarded as Strombolian-type by producing pyroclastic deposits with coarse fragments with high vesicularity and fluidal shape that indicate magmatic degassing and fragmentation. Petrologic and XRD data revealed a mixture of biotite, goethite, ilmenite, anatase, magnetite, hematite, pyroxene, zeolites, and olivine as their main mineral components. Optical microscopy analysis confirms the vesicular and amygdaloid structures, with a hypocrys- talline texture and a pale brown stained vitreous mass classified as sideromelane, due to its basaltic composition. The sideromelane changes to a reddish brown and yellowish-brown staining material identified as palagonite, clearly indicating a hydrovolcanic eruption that occurs when the ascending magma comes into contact with water. Infrared analyses in the palagonitized regions revealed the presence of halloysite, suggesting alteration of sideromelane to tubular clay minerals. Amygdales and microfractures are partially or totally filled with zeolites, which are formed by the percolation of water that reacts with the palagonite and precipitation of chemical elements of hydrothermal fluid. Reddish dark brown iddingsite and anhedral crystals of titaniferous magnetites occur in the fractures and edges of the olivine. These crystals are also dispersed in the matrix while some of them are zoned with Cr-rich core and Cr-poor edge, suggesting a deep mantle origin of the magma. The high trace elements content can be related to clinopyroxene (diopside) that include these elements. Geochemical data show that the pyroclasts are undersaturated in silica, plotting in the ultrabasic and foidites fields on the TAS classification diagram.
Phosphatized volcanic soils of Fernando de Noronha Island, Brazil : paleoclimates and landscape evolution.
(2020) Silveira, Jonas Carvalho; Oliveira, Fabio Soares; Schaefer, Carlos Ernesto Gonçalves Reynaud; Varajão, Angélica Fortes Drummond Chicarino; Varajão, Cesar Augusto Chicarino; Senra, Eduardo Osório
Volcanic oceanic islands host soils that may hold important paleo-environmental proxies, based on vegetation and faunal evidences coupled with pedological and geomorphological indicators. In this regard, the main Brazilian oceanic island of Fernando de Noronha (FNI) still lacks a deeper and more complete analysis of its climatic history and soil evolution. We select and collect representative FNI soils to interpret their chemical, physical and mineralogical properties and to explain their changing pedogenesis and geomorphic balance, aiming at revealing the environmental and landscape evolutionary history of this island. Ten soil pedons on phonolites, ankaratrites and the associated pyroclastic rocks were collected in June/2018. Six profiles were classified as Inceptisols, one as Entisol, one as Mollisol, one as Vertisol and another as the first Oxisol ever described in a Brazilian oceanic island. The pH ranged between 5.0 and 6.0 in the more developed, dystrophic and Al-saturated soils, whereas alkaline pHs were found in the eutrophic and Na-rich pedons. For the latter group, chemical and physical attributes are closely associated with the parent material composition, whereas in the former, present day soil features overprint paleoweathered regoliths and former pedogenetic processes. High concentrations of available phosphorus (P) were observed in all soils and horizons, even at depths greater than 150 cm, and P forms were detected as amorphous or poorly-crystalline phosphates (Al, Fe, Na, Ca and K). High- and low-activity clay minerals coexist, combined with gibbsite and unstable primary minerals, indicating the polygenetic nature of FNI soils. A combined pedogeomorphological analysis indicated the existence of a high paleoweathered plateau with relict soils, and erosional surfaces hosting younger soils. Results showed the succession of at least three distinct main paleoclimates since the Pliocene and a post-magmatic model for landscape evolution. The unreported, widespread and intense phosphatization of all soils and paleosurfaces is attributed to a past large population of birds nesting all over the island, corroborating old historical records of the island’s early occupation. Report the occurrence of Brazil’s first ornithogenic Oxisol with more than 500 mg/ kg of bioavailable P.
The Cenozoic deposits of the ancient landscapes of Quadrilátero Ferrífero highlands, Southeastern Brazil : sedimentation, pedogenesis and landscape evolution.
(2020) Varajão, Angélica Fortes Drummond Chicarino; Mateus, Ana Carolina Campos; Santos, Maria do Carmo; Varajão, César Augusto Chicarino; Oliveira, Fabio Soares; Yvon, Jacques
The Quadrilátero Ferrífero (QF), Southeastern Brazil, is a very important tropical highland region in the world’s geological context for its large and diverse Archean and Proterozoic rocks with great reserves of gold, iron, manganese, aluminum and industrial rocks, on a apparently stable geological structure. There, Cenozoic deposits perched on highland valleys show unclear genetic relationships with the underlying bedrock, with no apparent regional correlation. We studied five representative Cenozoic deposits (BR356, Água Limpa, Padre Domingos, Pau Branco and Casa de Pedra) on the highlands of the Serra da Moeda syncline, Western QF, to answer the question of their sedimentological origin, and investigate their pedological evolution. Field sampling was complemented by macromorphological, mineralogical and micromorphological analysis supported by X-ray diffraction (XRD), differential thermal analysis (DTA), infrared analysis (IR), scanning electron microscopy (SEM), micropobre and transmission electron microscopy (TEM). The evolution of these isolated highland de- posits comprises a deep-weathered source area, a tectonic activity, besides sedimentological and pedological processes, during and after the deposition. The deposits overlie deep saprolites of Precambrian rocks (Piracicaba and Itabira Groups), representing unconformable contact. The onset of the deposition was marked by torrential, coarse colluvial and large blocks landslides into the small basins generated by reactivating tectonic events during the Oligocene, producing local grabens. These tectonic basins were filled by cohesive debris and mudflow from the adjacent and previously laterized cover, developed under the hot and humid climate in the Eocene. The cover reveals an upside-down lateritic profile where morphology and kaolinite crystal properties (values of size of coherent scattering thickness ranging from 135 Å to 162 Å) in the bottom is related to the pedolith horizons of the former lateritic cover. Later, due to climate changes (during and after the Miocene), renewed weathering on these pre-weathered sediments occurred, characterized by alternating ferruginization and Fe-losses, with the superimposed generation of new pedogenic features such as nodules, ferruginous duricrusts and mottling (redox features). In addition to demonstrating that the Brazilian platform was not stable during the Cenozoic, these deposits reveal the role of polygenetic tropical pedological processes in their formation and transformation.