Abstract
Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. For example, the reliable use of the mineral zircon (ZrSiO4) as a U-Th-Pb geochronometer and trace element monitor requires minimal radiogenic isotope and trace element mobility. Here, using atom probe tomography, we document the effects of crystal–plastic deformation on atomic-scale elemental distributions in zircon revealing sub-micrometre-scale mechanisms of trace element mobility. Dislocations that move through the lattice accumulate U and other trace elements. Pipe diffusion along dislocation arrays connected to a chemical or structural sink results in continuous removal of selected elements (for example, Pb), even after deformation has ceased. However, in disconnected dislocations, trace elements remain locked. Our findings have important implications for the use of zircon as a geochronometer, and highlight the importance of deformation on trace element redistribution in minerals and engineering materials.
Highlights
Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes
The size of this zone varies between elements since it depends on the lattice diffusion rate, which is affected by temperature, the relative sizes of the solute and matrix atoms and the bonding type[21]
The zircon selected for this study needed to satisfy three main criteria: (i) a sufficiently high U and Pb content to allow reliable, statistically robust detection of radiogenic isotopes using atom probe tomography (APT), (ii) a known crystal–plastic deformation history to enable an understanding of the link between deformation and element mobility and (iii) a sufficiently long time after the main deformation event(s) to allow an assessment of the importance of pre-existing deformation structures on trace element/isotope mobility in a static regime
Summary
Trace elements diffuse negligible distances through the pristine crystal lattice in minerals: this is a fundamental assumption when using them to decipher geological processes. Correspondence and requests for materials should be addressed to S.P. The recent development of a range of high-resolution, chemically sensitive analytical techniques has enabled routine geochemical characterization of geological samples; this allows the use of trace element distributions in single grains as indicators of the Earth’s large-scale processes[1,2]. The recent development of a range of high-resolution, chemically sensitive analytical techniques has enabled routine geochemical characterization of geological samples; this allows the use of trace element distributions in single grains as indicators of the Earth’s large-scale processes[1,2] Fundamental to these studies is the assumption of element immobility and/or predictable element diffusion. In metals pipe diffusion has been demonstrated to occur along single dislocations[24,25], evidence for pipe diffusion occurring in minerals is so far only indirect[17,18,19,26]
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