Textural and compositional complexities resulting from coupled dissolution-reprecipitation reactions in geomaterials

Texture encompasses 'the overall appearance a rock has because of the size, shape, and arrangement of its constituent mineral grains'. Textural observations are crucial for deciphering the origin and geological history of rocks and their constituting minerals. In metamorphic and hydrothermal settings, textural observations hence serve to reconstruct the P,T path and the compositions and origins of the parent fluids. Over the past 13 years, a number of studies have emphasized the role of 'coupled dissolution reprecipitation reactions' (CDR) in geological systems. In these fluid-driven reactions, the replacement of one phase by another occurs via coupling between the dissolution of the parent and the precipitation of the product. In this paper we review the diversity of textures that arise from the CDR mechanism. The great diversity of textures relates to the diversity of mechanisms responsible for the coupling between dissolution and precipitation. Key parameters defining textures include volume change, the rate-limiting process, and the local composition at the mineral-fluid interface. In many of the reviewed examples, reaction mechanisms, rather than intensive properties such as P-T history, control the textures in the products, and far-from-equilibrium or local equilibriums at the mineral-fluid interface play a key role in controlling the final textures and mineral assemblages. These processes can also lead to the scavenging of trace elements from hydrothermal fluids. Because by nature CDR reactions are interface-controlled, many of the products are metastable, which further drives the reactions. These subsequent reactions can add to the textural complexity, or on the contrary obscure the original reaction mechanism. This review emphasizes the need to improve our understanding of reaction mechanisms, especially in systems containing even minor amounts of fluids (ore systems; metasomatic and metamorphic systems). Such a process-driven understanding is vital to supporting the petrological interpretation of textures. (C) 2015 Elsevier B.V. All rights reserved.