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

A review of the formation of tectonic veins and their microstructures

U–Pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the Earth. Despite the emphasis placed on zircon and monazite ages, the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy. In addition, few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample. Such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner, as per this study. The data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock. Here experimental data for zircon and monazite saturation in melt-bearing rocks are integrated with phase diagram calculations. The calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure, temperature and silicate mineral assemblages in high-grade, melt-bearing, metasedimentary rocks. Several key results emerge from this modelling: first, that in aluminous metapelitic rocks (i.e. garnet + cordierite + sillimanite assemblages), zircon ages are older than monazite ages in the same rock; second, that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures; and third, that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature(s) at which zircon and monazite grow rather than differences in closure temperature systematics. Using our methodology the cooling rate of granulites from the Reynolds Range, central Australia, have been constrained at ∼4 °C Myr−1. This study serves as a first-pass template on which further research in applying the technique to a field study can be based.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.2007.00757.x

Thermobarometric modelling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites

Metamorphism and metasomatism both involve the reequilibration of mineral assemblages due to changes in pressure, temperature and ⁄or chemical environment. Both processes involve material transport but on different length scales, so every metamorphic reaction is metasomatic on a local scale. Fluids provide a transport mechanism which is orders of magnitude faster than solid state diffusion and induce reequilibration through dissolution of parent phases and reprecipitation of products. Chemical weathering (kaolinitization and serpentinization), and albitization are used as examples to describe the coupling between dissolution and precipitation. Albitization of feldspars in nature and in experiments is a pseudomorphic replacement which generates porosity in the albite. Porosity generation associated with interface-coupled dissolution-precipitation allows rapid material transport and together with fluid induced fracturing, is the mechanism of pervasive fluid flow through reacting crystals. Examples of metamorphic reactions in granulite-eclogite rocks illustrate the role of fluids in inducing chemical changes along fluid pathways. Microstructural criteria for a metamorphic event (i.e. change in P, T) are critically reviewed by describing the corona formed by reaction between kyanite and garnet, as well as partial replacement textures. We conclude that both corona structures and partial replacement textures are equally indicative of a metasomatic reaction (driven by a fluid-induced compositional change) as they may be of a metamorphic reaction driven by a change in P and ⁄or T. This raises the question of the extent to which fluids play not only a catalytic role but also a thermodynamic role in determining the course of a metamorphic reaction.

Fluid-induced processes: metasomatism and metamorphism

http://www.ugr.es/~grupo179/pdf/Ruiz%20Agudo%202014.pdf

Coupled dissolution and precipitation at mineral-fluid interfaces

The development of shear zones at mid-crustal levels in the Proterozoic Willyama Supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration. Monazite dating of shear zone fabrics reveal that they formed at 1582 ± 22 Ma, at the end of the Olarian D3 deformational event and immediately prior to the emplacement of regional S-type granites. Two stages of fluid flow are identified in the area: first an albitizing event which involved the addition of Na and loss of Si, K and Fe; and a second phase of calcsilicate alteration with additions of Ca, Fe, Mg and Si and removal of Na. Fluid fluxes calculated for albitization and calcsilicate alteration were 5.56 × 109 to 1.02 × 1010 mol m−2 and 2.57 × 108–5.20 × 109 mol m−2 respectively. These fluxes are consistent with estimates for fluid flow through mid-crustal shear zones in other terranes. The fluids associated with shearing and alteration are calculated to have δ18O and δD values ranging between +8 and +11‰, and −33 and −42‰, respectively, and ɛNd values between −2.24 and −8.11. Our results indicate that fluids were derived from metamorphic dehydration of the Willyama Supergroup metasediments. Fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions. Shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent ‘retrograde’ fluid-flow event. Thus, the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times, prior to regional granite formation, during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing.

Timing and nature of fluid flow and alteration during Mesoproterozoic shear zone formation, Olary Domain, South Australia

The North European Basin hosts mineral deposits like the Kupferschiefer and the Mississippi Valley Type deposits in the Silesian sub-basin in Poland. The basement to this basin, exposed in the Harz Mts and in the Flechtingen and Calvorde Blocks, contains Mesozoic Pb–Zn vein mineralization and barite–fluorite deposits as well as massive hematite veins in the Rotliegend volcanics. A comparison of the mineralizing models of these deposits with results from a basin-wide petrographic, fluid inclusion and stable isotope study shows that the genesis of the mineral deposits can be explained by fluid systems that were active during different stages of basin evolution. These comprise syn- to post-magmatic fluids derived from or mobilized in the course of the Rotliegend magmatism, fluids convecting in the Rotliegend units during the extensional basin subsidence in the Permo-Triassic and originating from progressive devolatilization of the basin sequence and fluids derived from the overlying Zechstein evaporites. Deep-reaching fault systems developing during the Cretaceous tectonic reactivation enhanced fluid percolation from the surface to the deep sections of the basin sequence. Identification and correlation of these fluids across the basin and in the mineralizations provide the base for a basin-wide metallogenetic model.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1468-8123.2004.00090.x

Fluid systems and mineralization in the north German and Polish basin

The Walter-Outalpa shear zone in the southern Curnamona Province of NE South Australia is an example of a shear zone that has undergone intensely focused fluid flow and alteration at mid-crustal depths Results from this study have demonstrated that the intense deformation and ductile shear zone reactivation, at amphibolite facies conditions of 534 ± 20 °C and 500 ± 82 MPa, that overprint the Proterozoic Willyama Supergroup occurred during the Delamerian Orogeny (c 500 Ma) (EPMA monazite ages of 501 ± 16 and 491 ± 19 Ma) This is in contrast to the general belief that the majority of basement deformation and alteration in the southern Curnamona Province occurred during the waning stages of the Olarian Orogeny (c 1610–1580 Ma) These shear zones contain hydrous mineral assemblages that cut wall rocks that have experienced amphibolite facies metamorphism during the Olarian Orogeny The shear zone rock volumes have much lower δ18O values (as low as 1‰) than their unsheared counterparts (7–9‰), and calculated fluid δ18O values (5–8‰) consistent with a surface-derived fluid source Hydrous minerals show a decrease in δD(H2O) from −14 to −22‰, for minerals outside the shear zones, to −28 to −40‰, for minerals within the shear zones consistent with a contribution from a meteoric source It is unclear how near-surface fluids initially under hydrostatic pressure penetrate into the middle crust where fluid pressures approach lithostatic, and where fluid flow is expected to be dominantly upward because of pressure gradients We propose a mechanism whereby faulting during basin formation associated with the Adelaidean Rift Complex (c 700 Ma) created broad hydrous zones containing mineral assemblages in equilibrium with surface waters These panels of fault rock were subsequently buried to depths where the onset of metamorphism begins to dehydrate the fault rock volumes evolving a low δ18O fluid that is channelled through shear zones related to Delamerian Orogenic activity

Up-temperature flow of surface-derived fluids in the mid-crust: the role of pre-orogenic burial of hydrated fault rocks

Stable isotope relationships of mineralisation in theNorth Eastern German Basin

Application of Ti-in-quartz thermometry to Au and Cu–Au mineralising systems

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