Reaction‐induced nucleation and growth v. grain coarsening in contact metamorphic, impure carbonates

Abstract

The understanding of the evolution of microstructures in a metamorphic rock requires insights into the nucleation and growth history of individual grains, as well as the coarsening processes of the entire aggregate. These two processes are compared in impure carbonates from the contact metamorphic aureole of the Adamello pluton (N-Italy). As a function of increasing distance from the pluton contact, the investigated samples have peak metamorphic temperatures ranging from the stability field of diopside ⁄ tremolite down to diagenetic conditions. All samples consist of calcite as the dominant matrix phase, but additionally contain variable amounts of other minerals, the so-called second phases. These second phases are mostly silicate minerals and can be described in a KCMASHC system (K 2 O, CaO, MgO, Al 2 O 3 , SiO 2 , H 2 O, CO 2 ), but with variable K ⁄ Mg ratios. The modelled and observed metamorphic evolution of these samples are combined with the quantification of the microstructures, i.e. mean grain sizes and crystal size distributions. Growth of the matrix phase and second phases strongly depends on each other owing to coupled grain coarsening. The matrix phase is controlled by the interparticle distances between the second phases, while the second phases need the matrix grain boundary network for mass transfer processes during both grain coarsening and mineral reactions. Interestingly, similar final mean grain sizes of primary second phase and second phases newly formed by nucleation are observed, although the latter formed later but at higher temperatures. Moreover, different kinetic processes, attributed to different driving forces for growth of the newly nucleated grains in comparison with coarsening processes of the pre-existing phases, must have been involved. Chemically induced driving forces of grain growth during reactions are orders of magnitudes larger compared to surface energy, allowing new reaction products subjected to fast growth rates to attain similar grain sizes as phases which underwent long-term grain coarsening. In contrast, observed variations in grain size of the same mineral in samples with a similar T-t history indicate that transport properties depend not only on the growth and coarsening kinetics of the second phases but also on the microstructure of the dominant matrix phase during coupled grain coarsening. Resulting microstructural phenomena such as overgrowth and therefore preservation of former stable minerals by the matrix phase may provide new constraints on the temporal variation of microstructures and provide a unique source for the interpretation of the evolution of metamorphic microstructures.

Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones

“…Under the presence of fluids, pore fluid pressure in the new voids drops and new phases can be precipitated/ nucleated. Once nucleated, the second phases are capable to influence the microstructure of the matrix phase by pinning of grain boundaries (Berger et al, 2010). The link between subgrain rotation and an increase in grain boundary sliding has been postulated in the past due to CPO weakening with progressive subgrain rotation recrystallization (e.g.…”

Evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation

“…The Zener parameter is defined as the size (d p ) divided by the volume fraction (f p ) of the second phases (d p /f p ; see Zener given by Smith [1948]). For polymineralic rocks with calcite as matrix mineral, the Zener trends have been extensively studied for static [ Berger and Herwegh , 2004; Berger et al , 2010; Brodhag and Herwegh , 2010] and for different deformation conditions (different temperatures and strain rates [e.g., Herwegh and Jenni , 2001; Herwegh et al , 2005; Ebert et al , 2007, 2008]). It proved to be a powerful approach, allowing the reconstruction of grain size evolution maps [ Herwegh et al , 2005] in which the dependence of the matrix grain size on the volume fraction, size, and distribution of second phases can be determined.…”

Short‐duration contact metamorphism of calcareous sedimentary rocks by Neoproterozoic Franklin gabbro sills and dykes on Victoria Island, Canada

“…3). The morphology differs from the usual equant morphology of diopside and coarse marbles in aureoles of large intrusions (Berger et al. , 2010), where elevated temperature regimes can prevail for tens of thousands of years due to large pluton sizes and long durations of pluton growth (Annen, 2011; Nabelek et al.…”

“…Diopside occurs only very close to the gabbroic sheets. In contrast to the typical equant shape of diopside in contact aureoles around large plutons (e.g., Berger et al. , 2010; Nabelek & Morgan, 2012), diopside in these carbonates from Victoria Island typically occurs as radial splays of acicular crystals that resemble the common morphology of tremolite (Fig.…”