The preservation potential of microstructures during static grain growth

Abstract

Grain growth simulations using the microstructure simulation system Elle have been performed in materials with a pre-existing grain shape foliation. As might be expected, the foliation is destroyed by the end of the experiment, and grain areas have increased by a factor of seven. The area of material swept by the migrating grain boundaries was monitored, and it was found that at every stage, virtually all of the grains which survived the grain growth process contain one and only one core of ÔunsweptÕ material. Remarkably these remnant unswept cores preserve a useable record of the initial grain size and the orientation of the grain shape foliation. This work suggests that, even for samples where no equivalent protolith can be found, it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the rock, and perhaps even reconstruct the grain boundary kinematics. In addition the identification of unswept cores has the potential to help unravel the evolution of grain boundary chemistry in rocks during metamorphism.As an example of a natural system showing these microstructures, we describe a peridotite from Almklovdalen, Norway. This peridotite was infiltrated by aqueous fluids at several stages during late Caledonian exhumation and retrogressive metamorphism. Grain boundary migration associated with the last of these infiltration events swept off abundant intragranular fluid inclusions in the original chlorite-peridotite. At the grain scale, microstructural mapping of the fluid inclusion rich areas shows that, as with the numerical simulations, many of the grains retain exactly one core of unswept material. Examples of other natural systems discussed include dislocation density distributions and trace element zoning.

Zebra pattern in rocks as a function of grain growth affected by second-phase particles

“…Particle numbers and maximum grain size relationships (Equation 16) are in good agreement with the general theory of grain boundary motion in the presence of impurities [23,25,49]. Our simulation can show that during progressive grain boundary migration the impurities will line up along grain boundaries [38,39,49] and that surviving grains comprise of a core which is not affected by grain growth [50]. Both simulated processes comprise of some simplification but reflect the natural processes to the extent needed to show how a layered pattern can be generated by combining these two mechanisms.…”

The effect of different second-phase particle regimes on grain growth in two-phase aggregates: insights from in situ rock analogue experiments

“…CoveyCrump 1997; Jessell et al 2001Jessell et al , 2003. Associated with the formation of foam structures is the evolution of isometric grains, as well as straight to slightly curved grain boundaries typical of static grain growth.…”

“…Similar to their study, we started with grain-size analysis in our experiments not before foam textures with equidimensional grains developed, which we interpret to result from surface energy as dominant driving force for grain growth. Nonetheless, we cannot fully exclude that a small portion of stored deformation energy survived in the cores of the grains (Jessell et al 2003). The influence on bulk grain growth, however, will continuously decrease with increasing grain size and become of subordinate importance.…”

“…6). In nature, this has important consequences for the interpretation of grain internal 'mineral inclusions' or 'cores' (Jessell et al 2003). In the case of a formerly pinned and then released grain boundaries, such grain internal features can be trapped quite rapidly.…”

“…Initially, research on grain growth behaviour and grain scale processes was experimentally conducted in the field of material sciences on monomineralic metals (e.g., Andrade and Aboav 1966;Simpson et al 1971) as well as on two-phase metallic systems and alloys (Smith 1948;Dai et al 1999;Ferry et al 2005). Meanwhile, experimental approaches are expanded by numerical modelling (Jessell et al 2001(Jessell et al , 2003Solomatov et al 2002;Moelans et al 2005;El-Khozondar et al 2006;Zheng et al 2006;Becker et al 2008). In industrial applications, micro-to nano-scale grain sizes are important for superior physical and mechanical properties of high-quality materials and, thus, pinning of the matrix grains by second-phase particles is an effective tool to control the grain growth process and the final grain size of the aggregate (Andersen and Grong 1994;Manohar et al 1998).…”