Grain coarsening in contact metamorphic carbonates: effects of second-phase particles, fluid flow and thermal perturbations

Berger, Alfons; Herwegh, Marco (2004). Grain coarsening in contact metamorphic carbonates: effects of second-phase particles, fluid flow and thermal perturbations. Journal of metamorphic geology, 22(5), pp. 459-474. Blackwell Scientific Publications 10.1111/j.1525-1314.2004.00526.x

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Under contact metamorphic conditions, carbonate rocks in the direct vicinity of the Adamello pluton reflect a temperature-induced grain coarsening. Despite this large-scale trend, a considerable grain size scatter occurs on the outcrop-scale indicating local influence of second-order effects such as thermal perturbations, fluid flow and second-phase particles. Second-phase particles, whose sizes range from nano- to the micron-scale, induce the most pronounced data scatter resulting in grain sizes too small by up to a factor of 10, compared with theoretical grain growth in a pure system. Such values are restricted to relatively impure samples consisting of up to 10 vol.% micron-scale second-phase particles, or to samples containing a large number of nano-scale particles. The obtained data set suggests that the second phases induce a temperature-controlled reduction on calcite grain growth. The mean calcite grain size can therefore be expressed in the form D 1⁄4 C2 eQ*/RT(dp/fp)m*, where C2 is a constant, Q* is an activation energy, T the temperature and m* the exponent of the ratio dp/fp, i.e. of the average size of the second phases divided by their volume fraction. However, more data are needed to obtain reliable values for C2 and Q*. Besides variations in the average grain size, the presence of second-phase particles generates crystal size distribution (CSD) shapes characterized by lognormal distributions, which differ from the Gaussian-type distributions of the pure samples. In contrast, fluid-enhanced grain growth does not change the shape of the CSDs, but due to enhanced transport properties, the average grain sizes increase by a factor of 2 and the variance of the distribution increases. Stable d18O and d13C isotope ratios in fluid-affected zones only deviate slightly from the host rock values, suggesting low fluid/rock ratios. Grain growth modelling indicates that the fluid-induced grain size variations can develop within several ka. As inferred from a combination of thermal and grain growth modelling, dykes with widths of up to 1 m have only a restricted influence on grain size deviations smaller than a factor of 1.1.To summarize, considerable grain size variations of up to one order of magnitude can locally result from second-order effects. Such effects require special attention when comparing experimentally derived grain growth kinetics with field studies.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Berger, Alfons and Herwegh, Marco

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0263-4929

Publisher:

Blackwell Scientific Publications

Language:

English

Submitter:

Marco Herwegh

Date Deposited:

04 Aug 2016 16:16

Last Modified:

04 Aug 2016 16:16

Publisher DOI:

10.1111/j.1525-1314.2004.00526.x

BORIS DOI:

10.7892/boris.73889

URI:

https://boris.unibe.ch/id/eprint/73889

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