Annealing in a Natural Laboratory: an EBSD and Cl Study of Calcite and Quartz Growth from Volumes of Rock Heated by a Nearby Melt Intrusion

Abstract

Annealing is an important mechanism of microstructural modification both in rocks and metals. In order to relate directly changes in crystallographic orientation to migrating boundaries the researcher has the option to investigate either samples where the grain boundary motion can be directly tracked or a series of samples exhibiting successively higher degrees of annealing. Here we present results from rock samples collected from two well characterised contact aureoles (a volume of rock heated by the intrusion of a melt in its vicinity): One quartz sample in which patterns revealed by Cathodoluminescence (CL) indicate the movement of grain boundaries and a series of calcite samples of known temperature history. Electron backscatter diffraction (EBSD) analysis is used to link the movement of grain, twin boundaries and substructures with the crystallographic orientation / misorientation of a respective boundary. Results from the quartz bearing rock show: (a) propagation of substructures and twin boundaries in swept areas both parallel and at an angle to the growth direction, (b) development of slightly different crystallographic orientations and new twin boundaries at both the growth interfaces and within the swept area, and (c) a gradual change in crystallographic orientation in the direction of growth. Observations are compatible with a growth mechanism where single atoms are attached and detached both at random and at preferential sites i.e. crystallographically controlled sites or kinks in boundary ledges. Strain fields caused by defects and/or trace element incorporation may facilitate nucleation sites for new crystallographic orientations at distinct growth interfaces but also at continuously migrating boundaries. Calcite samples show with increasing duration and temperature of annealing: (a) systematic decrease of the relative frequency of low angle grain boundaries (gbs), (b) decrease in lattice distortion within grains, (c) development of distinct subgrains with little internal lattice distortion, (d) change in lobateness of gbs and frequency of facet parallel gbs and (e) change in position of second phase particles. These observations point to an increasing influence of grain boundary anisotropy with increasing annealing temperature, while at the same time the influence of second phase particles and subtle driving-force variations decrease. This study illustrates the usefulness of using samples from natural laboratories and combining different analysis techniques in microprocess analysis.

The effect of pre‐tectonic reaction and annealing extent on behaviour during subsequent deformation: insights from paired shear zones in the lower crust of Fiordland, New Zealand

“…Inversely, grain size increase through annealing (temperature‐related coarsening and nucleation; e.g. Covey‐Crump, ; Heilbronner & Tullis, ; Bestmann et al ., ; Piazolo et al ., , ; Bergmann & Piazolo, ), namely grain boundary migration and grain growth (e.g. Urai et al ., ; Drury & Urai, ; Baker, ), will have the opposite rheological effect, thus strengthening the rock (see Jessell et al ., for further discussion).…”

The recognition of multiple magmatic events and pre-existing deformation zones in metamorphic rocks as illustrated by CL signatures and numerical modelling: examples from the Ballachulish contact aureole, Scotland

“…Numerous studies have been conducted to determine the microstructural evolution during annealing, on both geological materials (e.g. Bestman et al 2005;Covey-Crump 1996;Heilbronner and Tullis 2002;Masuda et al 1997;Piazolo et al 2007) and on metals (e.g. Piazolo et al 2004).…”