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Metamorphic reaction
About: Metamorphic reaction is a research topic. Over the lifetime, 155 publications have been published within this topic receiving 7204 citations.
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TL;DR: Banded iron-formations (BIFs) occur in the Precambrian geologic record over a wide time span as mentioned in this paper and are part of Archean cratons and range in age from about 3.5 until 2.5 Ga.
Abstract: Banded iron-formations (BIFs) occur in the Precambrian geologic record over a wide time span. Beginning at 3.8 Ga (Isua, West Greenland), they are part of Archean cratons and range in age from about 3.5 until 2.5 Ga. Their overall volume reaches a maximum at about 2.5 Ga (iron-formations in the Hamersley Basin of Western Australia) and they disappear from the geologic record at about 1.8 Ga, only to reappear between 0.8 and 0.6 Ga.
The stratigraphic sequences in which BIFs occur are highly variable. Most Archean iron-formations are part of greenstone belts that have been deformed, metamorphosed, and dismembered. This makes reconstruction of the basinal setting of such BIFs very difficult. The general lack of metamorphism and deformation of extensive BIFs of the Hamersley Range of Western Australia and the Transvaal Supergroup of South Africa allow for much better evaluations of original basinal settings. Most Archean iron-formations show fine laminations and/or microbanding. Such microbanding is especially well developed in the Brockman Iron Formation of Western Australia, where it has been interpreted as chemical varves, or annual layers of sedimentation. BIFs ranging in age from 2.2 Ga to about 1.8 Ga (e.g., those of the Lake Superior region, U.S.A., Labrador Trough, Canada, and the Nabberu Basin of Western Australia) commonly exhibit granular textures and lack microbanding.
The mineralogy of the least metamorphosed BIFs consists of combinations of the following minerals: chert, magnetite, hematite, carbonates (most commonly siderite and members of the dolomite-ankerite series), greenalite, stilpnomelane, and riebeckite, and locally pyrite. Minnesotaite is a common, very low-grade metamorphic reaction product. The Eh-pH stability fields of the above minerals (and/or their precursors) indicate anoxic conditions for the original depositional environment.
The average bulk chemistry of BIFs, from 3.8 through 1.8 Ga in age, is very similar. They are rich in total Fe (ranging from about 20 to 40 wt%) and SiO2 (ranging from 43 to 56 wt%). CaO and MgO contents range from 1.75 to 9.0 and from 1.20 to 6.7 wt%, respectively. Al2O3 contents are very low, ranging from 0.09 to 1.8 wt%. These chemical values show that they are clean chemical sediments devoid of detrital input. Only the Neoproterozoic iron-formations (of 0.8 to 0.6 Ga in age) have very different mineralogical and chemical make-ups. They consist mainly of chert and hematite, with minor carbonates.
The rare-earth element profiles of almost all BIFs,with generally pronounced positive Eu anomalies, indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water.
The prograde metamorphism of iron-formations produces sequentially Fe-amphiboles, then Fe-pyroxenes, and finally (at highest grade) Fe-olivine-containing assemblages. Such metamorphic reactions are isochemical except for decarbonation and dehydration.
The common fine lamination (and/or microbanding) as well as the lack of detrital components in most BIFs suggest that such are the result of deposition, below wave base, in the deeper parts of ocean basins. Those with granular textures are regarded as the result of deposition in shallow water, platformal areas. Carbon isotope data suggest that for a long period of time (from Archean to Early Proterozoic) the ocean basins were stratified with respect to δ13C (in carbonates) as well as organic carbon content. In Middle Proterozoic time (when granular BIFs appear) this stratification diminishes and is lost.
The Neoproterozoic BIFs occur in stratigraphic sequences with glaciomarine deposits. These BIFs are the result of anoxic conditions that resulted from the stagnation in the oceans beneath a near-global ice cover, referred to as “Snowball Earth.”
All of the most “primary” mineral assemblages appear to be the result of chemical precipitation under anoxic conditions. There are, as yet, no data to support that BIF precipitation was linked directly to microbial activity. The relative abundance of BIF throughout the Precambrian record is correlated with a possible curve for the evolution of the O2 content in the Precambrian atmosphere.
900 citations
TL;DR: The role of fluids in metamorphic reactions has been discussed in this article, where the authors conclude that both corona structures and partial replacement textures are equally indicative of a metasomatic reaction driven by a fluid-induced compositional change.
Abstract: 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.
351 citations
TL;DR: Abzalov et al. as mentioned in this paper examined the nature and magnitude of these netite compositions at the inner chromite-magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade.
Abstract: Chromite compositions in komatiites are influenced by metamorphic chromite is highly susceptible to modification during processes, particularly above 500°C. Metamorphosed chromite is early hydrous alteration and subsequent prograde metasubstantially more iron rich than igneous precursors, as a result of morphism of host rocks. This modification is the subject Mg–Fe exchange with silicates and carbonates. Chromite metaof this paper. morphosed to amphibolite facies is enriched in Zn and Fe, and Metamorphic modification of chromite has been disdepleted in Ni, relative to lower metamorphic grades. Relative cussed extensively in the literature in the context mainly of proportions of the trivalent ions Cr, Al and Fe are not ophiolitic or ‘alpine ultramafic’ complexes (Onyeagocha, greatly modified by metamorphism up to lower amphibolite facies, 1974; Ulmer, 1974; Evans & Frost, 1975; Hoffman & although minor Fe depletion occurs during talc–carbonate alWalker, 1978; Loferski & Lipin, 1983; Kimball, 1990; teration at low temperature. Significant Al is lost from chromite Burkhard, 1993) and in a few studies of komatiitic rocks cores above 550°C, as a result of equilibration with fluids in (Bliss & MacLean, 1975; Donaldson, 1983; Gole & Hill, equilibrium with chlorite. Elevated Zn content in chromite is 1990), and in a detailed study of the Pechenga intrusions restricted to rocks with low (metamorphic) Mg/Fe ratios, and is (Abzalov, 1998). These studies have highlighted two the result of introduction of Zn during low-temperature alteration, important effects. First, chromites become rimmed and with further concentration and homogenization during prograde progressively replaced by chromian magnetite or ‘fermetamorphism. Cobalt and Mn also behave similarly, except where ritchromit’. Second, chromite core compositions become carbonate minerals are predominant in the metamorphic assemblage. progressively modified during prograde metamorphism Chromite at amphibolite facies is typically extensively replaced by as a result of exchange of components with surrounding magnetite. This is the result of incomplete metamorphic reaction silicate minerals (Evans & Frost, 1975; Abzalov, 1998). between chromite and chlorite-bearing silicate assemblages. MagThis paper examines the nature and magnitude of these netite compositions at the inner chromite–magnetite boundary are effects in komatiites from a variety of localities in the indicators of metamorphic grade. Eastern Goldfields Province of the Archaean Yilgarn Block in Western Australia (Fig. 1). Brief locality descriptions are given in Appendix B.
337 citations
TL;DR: The Liaohe assemblage has experienced three distinct deformational events (D1 to D3) and four episodes of metamorphism (M1 to M4).
313 citations
TL;DR: In this article, the Bence-Albee matrix correction was applied to X-ray intensity maps collected using eclogite samples from northern New Caledonia in order to determine the chemical composition of all parts of the sample.
Abstract: Chemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards. An effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock. To examine this effect on mineral assemblages, the Bence–Albee matrix correction was applied to X-ray intensity maps collected using eclogite samples from northern New Caledonia in order to determine the chemical composition of all parts of the sample. The manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix. A series of calculated equilibrium-volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high-P conditions. Pressure–temperature pseudosections are calculated for these compositions, in the CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O system. Assemblages, modal proportions and mineral textures observed in the New Caledonian eclogites can be closely modelled by progressively ‘removing’ elements forming garnet cores from the bulk rock composition. The pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage, prograde textures and the development of retrograde assemblages.
249 citations
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Performance Metrics
| Year | Papers |
|---|---|
| 2021 | 4 |
| 2020 | 3 |
| 2019 | 5 |
| 2018 | 2 |
| 2017 | 9 |
| 2016 | 2 |