J.C. Hadler Neto

Consolidation and break-up of the South American platform in southeastern Brazil: Tectonothermal and denudation histories

The different tectonic stages that occurred at the end of the Proterozoic and during the Phanerozoic have an important bearing on the tectonothermal history of the South American Platform and its consolidation. Geochronological data (U/Pb monazite, 40Ar/39Ar whole rock) and apatite fission-track analysis, from Precambrian rocks of the southeastern Brazilian coastline, permit the modeling of a long-term thermal history of the crust and constrain variable denudation rates. Using these data, a temperature-time diagram reflects a period of accelerated exhumation during the end of the Brasiliano Orogeny, followed by long stability and reactivation of the platform during the Rifting Phase of the South Atlantic Ocean. U/Pb zircon and monazite (blocking temperature of ca. 650° C) data from a series of igneous bodies suggest that a tangential and transpressional tectonic regime occurred between 625 and 610 Ma. During the following escape tectonics, between 610 and 590 Ma the exhumation process indicates cooling rates of ca. 12°C/Ma. 40Ar/39Ar biotite ages between 540 and 510 Ma (ca. 300°C) and a corrected fission-track age on apatites (100°C) of 480 Ma indicate an exhumation event related to block tectonics with huge vertical displacement along shear zones. A long stabilization phase, with low exhumation, and cooling rate around 0.25°C/Ma was recorded from the Cambro/Ordovician to the Mesozoic. At 65 Ma an acceleration of the exhumation through denudation and reworking of the South American surface with cooling rate of 1.5°C/Ma is observed. The uplift of the Mantiqueira and Serra do Mar mountain ranges along the southeast Brazilian coastline works as a climatic barrier provoking lateral erosional processes causing long-term scarp retreat, combined with intense, but progressive denudation towards the continent. A denudation of 2.5 to 4 km was calculated for such processes. This lateral retreat of escarpments and flexural response can provide important insights regarding marginal isostatic uplift and the evolution of offshore sedimentary basins of southeast Brazil.

A reactor intercalibration for fission track-dating and uranium micromapping

Abstract Selected positions of three reactors in France, Italy and Brazil were studied for Fission-Track dating and Uranium distribution micromapping. Spatial gradients of neutron fluxes within the chosen irradiation facilities resulted negligible. Comparison of FT ages calculated by different neutron dosimetries (SRM 962a and SRM 963a standard glass; Fish Canyon Tuff apatite used as age standard) shows that equivalent results are obtained, despite widely differing irradiation conditions in three different reactors. Some observed systematics trends are very probably due to the individual fluence determination.

Interlaboratory fission track dating of volcanic ash levels from eastern Paratethys. A mediterranean-paratethys correlation

Abstract The age of glass shard levels, interlayered in Sarmatian deposits from the southwestern Russia are measured with the fission track method. The results obtained from different laboratories (Moscow, Pisa, Campinas) using their specific techniques are in good agreement so proving a good interlaboratory agreement. The results suggest a new correlation between Paratethys and the Mediterranean area, in better agreement with the global climatic trend of this period. According to the proposed scheme the Chersonian/Maeotian boundary (9–9.5 Ma) is correlatable with the Vallesian/Turolian boundary (continental Mediterranean stratigraphy) and with the Middle Tortonian (marine Mediterranean stratigraphy); the Bessarabian (time span between 12.5 and 11 Ma) is correlatable with the Lower Vallesian and with the Upper Serravallian; the beginning of the Volhynian, that is the beginning of the Sarmatian (14-13.5 Ma), is correlatable with the Upper Astaracian and with the lower boundary of the Serravallian. Due to the large experimental errors of the ages and possible systematic errors the proposed stratigraphic sequence may be regarded as a working hypothesis to be confirmed by further data.

Dating of glass: The importance of correctly identifying fission tracks

Abstract Comparing age determinations by persons with different degrees of experience in FT dating shows that induced track counts are in good agreement but spontaneous track densities determined by beginners disagree with those determined by experienced persons. Proper identification of fission tracks appears to be of prime importance in glass samples; reliable data are the result of experience and careful selection of samples.

Simulating radon daughters diffusion through the air and their depletion on material surfaces

In recent decades, much has been learned about the nature of radon and radon daughters (RD) contamination. Nevertheless, the RD plate-out phenomenon is still not well understood due to environmental conditions, mathematical and detection limitations. When nuclear track detectors are employed to assess the radiation levels of alpha activity, a problem takes place: the self-plate-out effect caused by the detectors surface disturbs the spatial distribution of the surrounding RD and the activity measured is not the real activity present in the air. In this work, we simulate the RD diffusion inside a region where a detector, with shape and size chosen, is exposed. The influence of the detectors size and shape on the RD spatial distribution and on the depletion rate is shown to be in accordance with experimental data.

The thermal stability of fission tracks in the 612 nist glass standard at room temperature

Abstract Two wafers of the NIST (formerly NBS) glass standard SRM 612 recently irradiated have been compared to the pre-irradiated wafers RT3 and RT4 of glass SRM 962-7, stored for 9 years at 5°C, and SRM 962, stored for 20 years at room temperature. Track area densities on internal surfaces of the glass as well as track size measurements suggest that (1) the old SRM 962 and the more recent SRM 962a calibrations are consistent and (2) annealing of the fission tracks in the pre-irradiated wafers is negligible. This last experimental result enables a direct comparison of contemporary and previous fission track age calibrations.

Characterization of two irradiation positions for fission track dating at the Lena Triga Mark II reactor

In the present paper the use of two natural uranium dosimeters is described. Data are compared with fluence values obtained by using NIST glass standards and the traditional activation of metal foils. Finally an example is reported for application of neutron dosimetry to the dating of the Fish Canyon Tuff age standard.