Mineral replacement reactions take place primarily by dissolution-reprecipitation processes. Processes such as cation exchange, chemical weathering, deuteric alteration, leaching, pseudomorphism, metasomatism, diagenesis and metamorphism are all linked by common features in which one mineral or mineral assemblage is replaced by a more stable assemblage. The aim of this paper is to review some of these aspects of mineral replacement and to demonstrate the textural features they have in common, in order to emphasize the similarities in the underlying microscopic mechanisms. The role of volume change and evolution of porosity is explored both from natural microtextures and new experiments on model replacement reactions in simple salts. It is shown that the development of porosity is often a consequence of mineral replacement processes, irrespective of the relative molar volumes of parent and product solid phases. The key issue is the relative solubility of the phases in the fluid phase. Concepts such as coupled dissolution-precipitation, and autocatalysis are important in understanding these processes. Some consequences of porosity generation for metamorphic fluid flow as well as subsequent crystal growth are also discussed.

Mineral replacement reactions: from macroscopic observations to microscopic mechanisms

Whenever a mineral or mineral assemblage comes into contact with a fluid with which it is out of equilibrium, reequilibration will tend to take place to reduce the free energy of the whole system (i.e., of the solid + fluid). Such fluid-solid interactions span a very wide range of possible reactions, and are responsible for most of the mineral assemblages we see in the Earth’s crust. However, before discussing mineral replacement reactions, we will put them into the broader context of fluid-solid interactions by considering some examples of such reequilibration.

In the simplest case, we could consider the situation where a mineral, thermodynamically stable under some speciflc temperature and pressure conditions, comes into contact with pure water, such as quartz within its stability field (e.g., at T = 100 °C and 1 atmosphere pressure). Clearly, quartz will tend to dissolve until, at equilibrium, the aqueous silica solution, which at neutral pH is H4SiO4(aq), becomes saturated with respect to quartz. The reaction for this equilibration can be written:

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[SiO\_{2\ (qtz)}\ +\ H\_{2}O\ {\leftrightarrow}\ H\_{4}SiO\_{4(aq)}\] \end{document}(1) 

The equilibrium solubility constant for reaction (1) is given by

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathit{K}\_{\mathit{sp}}\ (\mathit{qtz})\ =\ \frac{\mathit{a}(H\_{4}SiO\_{4})\_{\mathit{aq}}}{\mathit{a}(SiO\_{2})\_{\mathit{qtz}}\ {\cdot}\ \mathit{a}(H\_{2}O)}\ =\ \mathit{a}(H\_{4}SiO\_{4})\_{\mathit{aq}}\] \end{document} 

where a ( i ) aq stands for the activity of the parenthetical aqueous species. For the case of pure quartz and pure water where the activity is 1, K sp ( qtz ) ~1.2 × 10−3 at 100 °C and 1 atmosphere pressure. However, if under these conditions the solid silica phase in contact with pure water was cristobalite (the high temperature polymorph of SiO2), the resulting aqueous solution, saturated with respect to cristobalite, would be supersaturated with respect to quartz, since the less stable phase is more soluble. The value of K sp for cristobalite at 100 °C and 1 atmosphere pressure is ~5.1 × 10−3. Thus the thermodynamics would indicate that quartz …

Mineral Replacement Reactions

https://cyberleninka.org/article/n/966601.pdf

Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution.

Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: Implications for element transfer in subduction zones

https://www.geosociety.org/gsatoday/archive/12/9/pdf/i1052-5173-12-9-4.pdf

Strength of the continental lithosphere: Time to abandon the jelly sandwich?

Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitization

Large-scale metasomatic albitization in the albitite terranes of the Bamble sector of southeastern Norway has affected both mafic and granitic lithologies. In partially metasomatized tonalite, the albitization fronts advance normal to fractures and can be recognized in the field by a distinct reddening of the rock in which original plagioclase crystals are replaced by albite. To determine the mechanism of albitization within single crystals of Ca-bearing plagioclase (oligoclase: An 21–23 ), intragranular replacement interfaces have been studied by electron-probe micro-analysis, scanning (SEM) and transmission electron microscopy (TEM). The albite replacement-product (An 2–5 ) has micrometric pores observable by SEM, as well as nanopores imaged by TEM. The albite contains fine-grained white mica commonly associated with the pores, as well as precipitates of hematite. The interface between oligoclase and albite is sharp, with corresponding compositional gradients across no more than tens of nanometers, and the crystallographic orientations of oligoclase and albite are coincident within less than a degree. These are all characteristic features of an interface-coupled dissolution–reprecipitation mechanism of replacement which, in combination with fracturing, is responsible for the fluid infiltration and the mineral-replacement process.

Albitization of granitic rocks: the mechanism of replacement of oligoclase by albite

The Mesoproterozoic crust in the southwest part of the Fennoscandian Shield was reworked during the Sveconorwegian orogeny (Pasteels & Michot 1975; Berthelsen 1980; Demaiffe & Michot 1985; Falkum 1985). Though this idea has been well established in the literature for decades, it is not until recently that the Sveconorwegian age of high-grade metamorphism was established in all lithotectonic units of the Sveconorwegian orogenic belt. Especially controversial was the timing of granulitefacies metamorphism in the Bamble Terrane in south Norway (Kullerud & Dahlgren 1993) and the Eastern Segment in southwestern Sweden (Johansson et al. 1991). Today, the objective is to improve the chronologic and tectonic model for the Sveconorwegian orogenic event itself. In this paper, monazite, zircon and titanite U-Pb data and molybdenite Re-Os data are reported from metamorphic rocks to date high-grade metamorphism in the four main lithotectonic units in south Norway. A pressure-temperature estimate is reported for one critical sample. These data were derived from material collected over several years, in the course of regional geologic investigations, and analysed using different analytical methods. Together, they provide a significantly enlarged field coverage, and point to important differences in the metamorphic evolution of the four lithotectonic units. These data are integrated in an orogen-scale tectonometamorphic model in a companion paper (Bingen et al. 2008).

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Geochronology of high-grade metamorphism in the Sveconorwegian belt, S. Norway: U-Pb, Th-Pb and Re-Os data

The Proterozoic banded granulite facies complex of BArdsholmen, Western Gneiss Region, Norway ( T =815–845°C) was locally transformed to eclogite ( T =455–510°C, P >12 kbar) and amphibolite facies rocks ( T =460°C) during the Caledonian continental collision. The granulite complex consists of mafic two‐pyroxene granulite and leucocratic orthopyroxene+garnet‐bearing layers alternating on a scale from 1 cm to 10 m. The granulite facies rocks change to eclogite facies rocks over centimetre‐scale distances along well defined fluid‐infiltration fronts. The mafic granulite was transformed to omphacite+garnet‐rich eclogites and the leucocratic rocks were converted to quartz+phengite‐rich assemblages with minor garnet and local omphacite. Melange‐like lithologies consisting of mafic lenses of eclogite surrounded by felsic material represent an advanced stage in the process of converting deep crust to eclogite facies. During amphibolitization this melange‐like lithology evolves to a rock where amphibolite lenses and layers are surrounded by granitoid gneiss, a lithology typical of the Western Gneiss Region. The BArdsholmen locality illustrates the profound control exerted by fluids on the timing of metamorphism, the structural make up and petrophysical properties such as density and rheology of crustal root zones. Fluid‐induced metamorphism will therefore exert control on the attributes of orogenic belts such as topography and Moho depth and influence the dynamics of collision zones by controlling the time of orogenic collapse and the buoyancy of the subducted crust. We suggest that orogens may develop differently depending on the fluid budget.

Ane K. Engvik

Papers

Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitization

Albitization of granitic rocks: the mechanism of replacement of oligoclase by albite

Geochronology of high-grade metamorphism in the Sveconorwegian belt, S. Norway: U-Pb, Th-Pb and Re-Os data

Structural, mineralogical and petrophysical effects on deep crustal rocks of fluid‐limited polymetamorphism, Western Gneiss Region, Norway

Intragranular replacement of chlorapatite by hydroxy-fluor-apatite during metasomatism