EM - Escola de Minas

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Strain localization and fluid-assisted deformation in apatite and its influence on trace elements and U–Pb systematics.
(2020) Ribeiro, Bruno Vieira; Lagoeiro, Leonardo Evangelista; Faleiros, Frederico Meira; Hunter, Nicholas J. R.; Queiroga, Gláucia Nascimento; Raveggi, M.; Cawood, Peter Anthony; Finch, M.; Campanha, Ginaldo Ademar da Cruz
This paper presents electron backscatter diffraction (EBSD), trace element and U–Pb data of apatite grains from a granitic mylonite from the Taxaquara Shear Zone (SE Brazil). The mylonite recrystallized under upper-greenschist facies and presents two types of apatite with distinct microstructures. Type1 apatite appears in quartz-rich layers and does not exhibit any microstructural, crystallographic, or chemical evidence of deformation/recrystallization, and resembles the original igneous apatite. Type2 apatite appears in mica-rich layers and exhibits core-and-mantle microstructures, and intragranular subgrain development, suggesting that they have undergone dynamic recrystallization. Recrystallized tails of type-2 apatite grains exhibit a strong c-axis crystallographic preferred orientation parallel to the X-direction (stretching lineation), and lack evidence of dislocation density. This evidence from type-2 apatite grains, combined with REE depletion, high La and a negative Ce anomaly compared to type-1 grains, suggests that type-2 apatite tails underwent recrystallization via dissolution-precipitation creep, whereas parental grains underwent crystal-plastic deformation and subgrain formation through dynamic recrystallization. Phase-equilibrium modelling and quartz CPO opening-angle thermometry are consistent with recrystallization at ∼480 – 530◦C and 2.2 – 5.0 kbar. We were not able to determine precise deformation ages from type-2 apatite because fluid-assisted recrystallization appears to have substantially decreased the U/Pb ratio. We find that preferential fluid flow along high-strain, biotite-rich layers in the mylonite caused type-2 apatite to recrystallise, whereas type-1 apatite in low strain layers was unaffected and retained the characteristics of the protolith.
Determining the conditions for dynamic recrystallization in hot deformation of C–Mn–V steels and the effects of Cr and Mo additions.
(2014) Martins, Charles de Abreu; Poliak, Evgueni; Godefroid, Leonardo Barbosa; Fonstein, Nina
The refinement of microstructure and its homogeneity during controlled hot strip rolling is primarily achieved by controlling the austenite recrystallization before its transformation during accelerated cooling. The paper describes the methodology to determine the deformation conditions favorable for dynamic recrystallization (DRX). Using this methodology it becomes possible to delineate the conditions for postdeformation static and metadynamic recrystallization as well. The work is based on viscoplastic power law formalism applied to steady state flow within wide range of deformation temperatures and strain rates. Two equations of the same form but with different coefficients can be used depending on whether the steady state flow is controlled by dynamic recovery (DRV) or DRX. The transition from DRV- to DRX at the corresponding value of Zener-Hollomon parameter Zt can be viewed as the demarcation between static and metadynamic recrystallization occurring after deformation. The approach is illustrated using low carbon Mn–V steel. Alloying with Cr and especially with Mo suppresses DRX and MDRX as manifested by increasing Zt.
Sheared-bedding parallel quartz vein as an indicator of deformation processes.
(2012) Barbosa, Paola Ferreira; Lagoeiro, Leonardo Evangelista
Monomineralic veins are well known as good recording of filling and precipitation processes. However, they are also able to register the action of deformation in rocks. We used monomineralic, quartz veins from Quadriláterro Ferrífero, Brazil, to represent how the deformation actuated during the transformation of different structures. They were analyzed using a combination of a u-stage and EBSD. Two main types of quartz aggregates are distinguished: single crystals with subgrain development and strain-free grains of quartz. We interpret the microstructures and textures as the result of a combination of concurrent crystal plasticity, microfracturing, solution transfer and recrystallization. The single quartz grains were deformed by dislocation glide. However, as the c-axis orientation of the grain was not favorable for further glide on basal planes, the deformation was accommodated by microfracturing. New grains were formed along the deformed zones with crystallographic orientations suitable for gliding on basal planes. As the deformation proceeded, the new grains continue to develop until the vein was completely recrystallized into an aggregate of granular grains.

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