Abstract
Abstract Mechanical twins form by the simple shear of the crystal lattice during deformation. In order to test the potential of narrow twins in monazite to record the timing of their formation, we investigated a ca. 1700 Ma monazite grain (from the Sandmata Complex, Rajasthan, India) deformed at ca. 980 Ma, by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT). APT 208Pb/232Th ages indicate that the twin was entirely reset by radiogenic Pb loss during its formation at conditions far below the monazite closure temperature. The results are consistent with a model where Pb is liberated during rupture of rare earth element–oxygen (REE-O) bonds in the large [REE]O9 polyhedra during twinning. Liberated Pb likely migrated along fast diffusion pathways such as crystal defects. The combination of a quantitative microstructural investigation and nanogeochronology provides a new approach for understanding the history of accessory phases.
Highlights
The radiogenic decay of U and Th into different isotopes of Pb enables a series of widely used geochronometers
Pb mobility has been suggested to occur in highclosure-temperature minerals during crystal-plastic deformation (Moser et al, 2009) and deformation-induced grain boundary migration (Erickson et al, 2015), with radiogenic Pb accumulation continuing after deformation
We integrate electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT) analyses to investigate Pb mobility associated with micrometer-scale deformation twins formed within tectonically deformed monazite
Summary
The radiogenic decay of U and Th into different isotopes of Pb enables a series of widely used geochronometers. Pb mobility has been suggested to occur in highclosure-temperature minerals (zircon and monazite) during crystal-plastic deformation (Moser et al, 2009) and deformation-induced grain boundary migration (Erickson et al, 2015), with radiogenic Pb accumulation continuing after deformation. We integrate electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT) analyses to investigate Pb mobility associated with micrometer-scale deformation twins formed within tectonically deformed monazite.
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