Thermochronology Of The South American Platform In The State Of São Paulo, Brazil, Through Apatite Fission Tracks

The fission-track method (FTM) in apatite was applied to 45 samples collected in the Serra da Mantiqueira (Mantiqueira mountain range), the Serra do Mar (Mar mountain range), regions next to these mountain ranges and the coastal region between Ubatuba and Santos in the State of São Paulo, Brazil, to study the thermochronology of the South American Platform in southeast Brazil and its influence on Santos and Campos basins. The data presented in this work complement the previously presented data on the same region (Tello Saenz et al., 2003. J. S. Am. Earth Sci. 15, 765-774) with 31 new samples analyzed. The weighted mean of the corrected ages from high Mantiqueira (around 1000 m), (121±6)Ma, coincides with the South Atlantic opening. The fact that its thermal history starts at a relatively low temperature (∼80°C) suggests that the age of ∼120Ma would be the formation age of Serra da Mantiqueira due to a rapid pulse, in which tracks had no time to be retained at the closure temperature, that is ∼120°C. The Serra do Mar presents a more complicated thermal history, with several reactivations indicated by the changes in the slope of its cooling curve. The thermal histories obtained in the regions next to these mountain ranges are compatible with the results mentioned above. The Santos Basin has unconformities that agree with changes in the slope thermal histories of the studied region. © 2005 Elsevier Ltd. All rights reserved.396635640Almeida, F.F.M., Carneiro, C.D.R., Origem e evolução da Serra do Mar (1998) Rev. Bras. Geociência, 28, pp. 135-150Amaral, G., Born, H., Hadler Neto, J.C., Iunes, P.J., Kawashita, K., Machado Jr., D.L., Oliveira, E.P., Tello Saenz, C.A., Fission track analysis of apatites from São Francisco craton and mesozoic alcaline-carbonatite complexes from central and southeastern Brazil (1997) J. S. Am. Earth Sci., 10, pp. 285-294Fleischer, R.L., Hart, H.R., Fission track dating: Techniques and problems (1972) Proceedings of Burg Wartenstein Conference on Calibration of Hominoid Evolution, pp. 135-170. , W.W. Bishop D.A. Miller S. Colle Scottish Academic Press EdinburghGuedes, S., Hackspacher, P., Hadler Neto, J.C., Iunes, P.J., Paulo, S.R., Ribeiro, L.F.B., Tello Saenz, C.A., Constraints on the evolution and thermal history of the continental platform of southeast Brazil, São Paulo state, using Fission Track Analyses (AFTA) (2000) Rev. Bras. Geociências, 30, pp. 107-109Guedes, S., Hadler Neto, J.C., Iunes, P.J., Paulo, S.R., Zuñiga, A., The spontaneous fission decay constant of 238U using SSNTD (2000) J. Radioanal. Nucl. Chem., 245, pp. 441-442Guedes, S.O., Hadler Neto, J.C., Iunes, P.J., Zuñiga, A.G., Tello Saenz, C.A., Paulo, S.R., The use of the U(n,f) reaction dosimetry in the determination of the λf value through fission-track techniques (2003) Nucl. Instrum. Meth. A, 496, pp. 215-221Guedes, S., Hadler Neto, J.C., Sarkis, J.E.S., Oliveira, K.M.G., Kakazu, M.H., Iunes, P.J., Saiki, M., Paulo, S.R., Spontaneous-fission decay constant of 238U measured by nuclear techniques without neutron irradiation (2003) J. Radioanal. Nucl. Chem., 258, pp. 117-122Hadler Neto, J.C., Paulo, S.R., Iunes, P.J., Tello Saenz, C.A., Balestrieri, M.L., Bigazzi, G., Curvo, E.A.C., Hackspacher, P., A PC compatible Brazilian software for obtaining thermal histories using apatite fission track analysis (2001) Radiat. Meas., 34, pp. 149-154Hadler Neto, J.C., Bigazzi, G., Guedes, S., Iunes, P.J., Oddone, M., Tello Saenz, C.A., Paulo, S.R., Spontaneous U-238 fission half-life measurements based on fission track techniques (2003) J. Radioanal. Nucl. Chem., 256, pp. 155-157Holden, N.E., Hoffman, D.C., Spontaneous fission half-lives ground-state nuclides (Technical report) (2000) Pure Appl. Chem., 72, pp. 1525-1562Iunes, P.J., Hadler Neto, J.C., Bigazzi, G., Tello Saenz, C.A., Guedes, S.O., Paulo, S.R., Durango apatite fission-track dating using length-based age corrections and neutron fluence measurements by thorium thin films and natural U-doped glasses calibrated through natural uranium thin films (2002) Chem. Geol., 187, pp. 201-211Laslett, G.M., Green, P.F., Duddy, I.R., Gleadow, J.W.A., Thermal annealing of fission tracks in apatite 2. The quantitative analysis (1987) Chem. Geol., 65, pp. 1-13Lutz, T.M., Omar, G., An inverse method of modeling thermal histories from apatite fission-track dates (1991) Earth Planet. Sci. Lett., 104, pp. 181-195Pereira, M.J., Feijó, F.J., Bacia de Santos (1994) Bol. Geociências Petrobrás, 8, pp. 219-234Tello Saenz, C.A., Hackspacher, P., Hadler Neto, J.C., Iunes, P.J., Guedes, S.O., Paulo, S.R., Ribeiro, L.F.B., Recognition of Cretaceus, Paleocene and Neogene Tectonic reactivation, through apatite fission-track analysis, in Precambrian areas of the Southeast Brazil: Association with the South Atlantic Ocean opening (2003) J. S. Am. Earth Sci., 15, pp. 765-774Tello Saenz, C.A., Hadler Neto, J.C., Iunes, P.J., Guedes, S., Paulo, S.R., Annealing experiments on induced fission tracks in apatite: I. Measurements of horizontal-confined-track lengths and track densities in basal sections and randomly oriented grains (2004) Am. Mineral., , submitte

Effects Of Etching On Zircon Grains And Its Implications For The Fission Track Method

Studies of zircon grains using optical microscopy, micro-Raman spectroscopy, and scanning electron microscopy (SEM) have been carried out to characterize the surface of natural zircon as a function of etching time. According to the surface characteristics observed using an optical microscope after etching, the zircon grains were classified as: (i) homogeneous; (ii) anomalous, and (iii) hybrid. Micro-Raman results showed that, as etching time increases, the crystal lattice is slightly altered for homogeneous grains, it is completely damaged for anomalous grains, and it is altered in some areas for hybrid grains. The SEM (energy dispersive X-ray spectroscopy, EDS) results indicated that, independent of the grain types, where the crystallinity remains after etching, the chemical composition of zircon is approximately 33% SiO 2:65% ZrO 2 (standard natural zircon), and for areas where the grain does not have a crystalline structure, there are variations of ZrO 2 and, mainly, SiO 2. In addition, it is possible to observe a uniform surface density of fission tracks in grain areas where the determined crystal lattice and chemical composition are those of zircon. Regarding hybrid grains, we discuss whether the areas slightly altered by the chemical etching can be analyzed by the fission track method (FTM) or not. Results of zircon fission track and U-Pb dating show that hybrid and homogeneous grains can be used for dating, and not only homogeneous grains. 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