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Mechanism and kinetics of forsterite formation in metamorphic siliceous dolomites; findings from a rock-sample experiment

Paul Metz and Ralf Milke
Mechanism and kinetics of forsterite formation in metamorphic siliceous dolomites; findings from a rock-sample experiment
European Journal of Mineralogy (January 2012) 24 (1): 59-72

Abstract

A cylindrical sample of a tremolite-dolomite marble was used as starting material for a long-term experiment of 184 days at 581 degrees C and a CO (sub 2) -H (sub 2) O-fluid pressure of 100 MPa. The fluid, which was always present in excess, had a composition in the range of X (sub CO2) =0.21-0.37. During the run forsterite and magnesian calcite formed both within the rock sample as well as on its cylindrical surface. In areas where a tremolite crystal was exposed on the sample surface a 400-500 mu m thick forsterite-magnesian calcite reaction rim evolved with a composition of 58+ or -2 mol% forsterite and 42+ or -2 mol% calcite. The dolomite areas of the surface of the rock cylinder are similarly covered by a reaction rim of forsterite plus magnesian calcite. This ca. 300 mu m thick rim, however, is composed of 28+ or -5 mol% of forsterite and 72+ or -5 mol% of calcite. From the different compositions of the reaction rims we conclude that the well-known overall reaction: 1 tremolite+11 dolomite <--> 8 forsterite+13 calcite+9 CO (sub 2) +1 H (sub 2) O occurs via the following two partial reactions: (1) 1 tremolite+2 CO (sub 2) +21 H (sub 2) O<-->2.5 forsterite+2 calcite+5.5 Si(OH) (sub 4) .2 H (sub 2) O and (2) 11 dolomite+5.5 Si(OH) (sub 4) .2 H (sub 2) O<-->5.5 forsterite+11 calcite+11 CO (sub 2) +22 H (sub 2) O. All three equations are slightly simplified, because they assume the formation of pure instead of magnesian calcite. The derived partial reactions, which occur as expected via dissolution-transport-precipitation mechanisms, are sequential reactions; they are linked by the production, diffusion, and consumption of the component Si(OH) (sub 4) .2 H (sub 2) O. In contrast to the two kinds of rims on the rock cylinder's surface, only one combined forsterite-magnesian calcite reaction rim was formed between tremolite and dolomite in the interior of the sample. The formation of this reaction rim can be explained by a combination of the two partial reactions given above. For the formation of forsterite plus magnesian calcite on the surface of the rock cylinder we come from an interpretation of the experimental results to the conclusion that the diffusion of the component Si(OH) (sub 4) .2 H (sub 2) O through the forsterite-calcite reaction rim that developed on dolomite is the rate-controlling step in the sequence of the two partial reactions given above. For the formation of forsterite plus calcite within the rock sample it is not possible to derive a rate-controlling step for the overall reaction, because of different diffusion paths of the reaction fluid and the components dissolved in it. The observed partial reactions are crucial for deciphering the formation of the various textures of forsterite-calcite-dolomite (-tremolite) assemblages developed in metamorphic siliceous dolomites. The magnesian calcites of the reaction rims have different compositions, which all deviate considerably from the equilibrium composition according to the calcite-dolomite solid solution. This result sheds light on the difficulty to attain equilibrium between the reactant dolomite and the product magnesian calcite. The consequences of the observed disequilibrium for the calcite-dolomite geothermometry are discussed.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 24
Serial Issue: 1
Title: Mechanism and kinetics of forsterite formation in metamorphic siliceous dolomites; findings from a rock-sample experiment
Affiliation: Hinsiekweg 11A, Alfeld/Leine, Germany
Pages: 59-72
Published: 201201
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 38
Accession Number: 2012-030473
Categories: Igneous and metamorphic petrologyMineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Secondary Affiliation: Freie Universitaet Berlin, DEU, Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201216
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