Termocronologia por traços de fissão em monazita aplicada no Domínio Ceará Central, Brasil

Abstract

Fission Track Thermochronology (FT) constitutes one of the principal geochronological tools for reconstructing the low-temperature thermal history of rocks, being crucial for understanding geotectonic evolution and the surface dynamics of relief. Based on the counting of microscopic tracks produced by the spontaneous fission of ²³⁸U, this technique records geological events associated with low thermal regimes with high sensitivity. In this context, monazite emerges as a particularly promising thermochronometer for ultralow-temperature ranges (25–45 °C), with the capacity to record subtle processes such as shallow exhumation and recent erosion. Despite its widespread occurrence and high uranium content, the application of FT in monazite remained limited for decades by methodological constraints, mainly due to the dependence on equations requiring the use of nuclear reactors. In this work, Monazite Fission Track Thermochronology (MFT) was implemented in an exploratory manner within the TRACKs Group based on an internal laboratory calibration equation combined with a chemical etching protocol described in the specialized literature. Analyses performed on international reference samples from Japan and Australia yielded ages consistent with previously published data, demonstrating the robustness and analytical feasibility of the adopted protocol. In addition, a chemical etching curve was constructed relating surface track density and confined track length to etching time. The revelation of tracks during chemical etching proved congruent with previously reported results, while small observed variations may be associated with the degree of accumulated radiogenic damage. These results highlight the superiority of the step-etch approach compared to the use of fixed chemical etching times. The application of MFT to Brazilian samples from the Central Ceará Domain resulted in ages ranging from 2.10 ± 0.25 to 12.15 ± 1.40 Ma, associated with ultralow-temperature conditions. This interval points to a cooling episode subsequent to the youngest magmatic events in the region, active since the Miocene. Thermal modeling corroborates these results, indicating progressive cooling until the Pleistocene, followed by a period of thermal stability. Taken together, the data reinforce the potential of MFT as an innovative and robust tool for investigations into the recent thermal evolution of the continental crust.