Pressure solution in rocks: focused ion beam/transmission electron microscopy study on orthogneiss from South Armorican Shear Zone, France

Abstract

In order to characterize the µm-to-nm structures related to operation of pressure solution on phase boundaries in naturally deformed rocks, we have performed a detailed focused ion beam/transmission electron microscopy study in ultramylonite samples from South Armorican Shear Zone (France) that focused on grain boundary scale. We have studied phase boundaries between quartz, K-feldspar and white mica both in 2 and 3D and compare our evidences with theoretical dissolution-precipitation models in the current literature. The dissolution (re)precipitation processes lead to the development of different features at different phase boundaries. In both quartz-white mica and quartz-K-feldspar phase boundaries, voids were ubiquitously observed. These voids have different shapes and the development of some of them is crystallographically controlled. In addition, part of these voids might be filled with vermiculite.Amorphous leached layers with kaolinite composition were observed at the boundaries of Kfeldspar-quartz and K-feldspar-white mica. The development of different features along the phase boundaries is mainly controlled by the crystallography of the phases sharing a common interface, together with the presence of fluids that either leaches or directly dissolve the mineral phases. In addition, the local dislocation density in quartz may play an important role during pressure solution.We suggest that the nano-scale observations of the quartz -white mica phase boundaries show direct evidence for operation of island-and-channel model as described in Wassmann and Stöckhert (2013), while K-feldspar -quartz phase boundaries represents amorphous layers formed via interface-coupled dissolution-reprecipitation as described by Hellmann et al. (2012).

Mineral dissolution and reprecipitation mediated by an amorphous phase

“…The role of an amorphous transport agent needs to be clarified in future experiments and natural observations. Our results indicate that cluster-attachment-driven crystallization mechanisms, as well as amorphous element transport materials seem to be common in Nature and together with several individual observations in natural rocks 28 , 29 , 51 , 52 demonstrate that the mechanisms described in this paper may be of far greater relevance to dissolution–reprecipitation processes than previously envisaged.…”

“…Several experimental and natural examples show, however, that dissolution–reprecipitation processes can involve the formation of a silica-rich amorphous phase. The conditions under which amorphous phases are observed as reaction products range from weathering at ambient temperatures 24 – 27 to metamorphic conditions 28 , 29 . The role of these amorphous phases as element transport agents and pre-nucleation products are not fully resolved.…”

Replacement reactions and deformation by dissolution and precipitation processes in amphibolites

“…The textural features of amphibole suggest a similar deformation history to plagioclase (Figure b). Amp1 (high Mg#) grains are preserved mostly as relict cores and display embayments, lobate edges, and truncated chemical zoning patterns (Figures f and c; Bukovská, Wirth, & Morales, ; Gratier et al., ; Hyppolito, García‐Casco, Juliani, Meira, & Hall, ; Passchier & Trouw, ; Rutter, ; Stokes et al., ; Wassmann & Stöckhert, ; Wintsch & Yi, ). These textural features suggest that Amp1 underwent coupled dissolution and Amp2 precipitated on Amp1.…”

The influence of microscale lithological layering and fluid availability on the metamorphic development of garnet and zircon: insights into dissolution–reprecipitation processes

“…The formation of quartz during this process might assist in the transport of alkalis between reactant and product phases (Konrad-Schmolke et al , 2018). Dissolution–reprecipitation of pyroxene and amphibole has been suggested to involve the formation of a silica-rich amorphous phase (Keller et al , 2006; Bukovská et al , 2015; Konrad-Schmolke et al , 2018). This amorphous phase allows the repolymerisation of product phases in addition to promoting fluid flow through the creation of grain boundaries (Konrad-Schmolke et al , 2018).…”

“…Dissolution-reprecipitation of pyroxene and amphibole has been observed to involve the formation of a silica-rich amorphous phase (Keller et al, 2006;Bukovská et al, 2015;Konrad-Schmolke et al, 2018). This amorphous phase allows the repolymerisation of product phases as well as promoting fluid flow through the creation of grain boundaries (Konrad-Schmolke et al, 2018).…”