Schönig, J, von Eynatten, H, Meinhold, G and Lünsdorf, NK (2021) Life-cycle analysis of coesite-bearing garnet. Geological Magazine. ISSN 0016-7568

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Detrital coesite-bearing garnet is the final product of a complex geological cycle including coesite entrapment at ultrahigh-pressure conditions, exhumation to Earth’s surface, erosion, and sedimentary transport. In contrast to the usual enrichment of high-grade metamorphic garnet in 14 medium- to coarse-sand fractions, coesite-bearing grains are often enriched in the very fine-sand fraction. To understand this imbalance, we analyze the role of source rock lithology, inclusion size, inclusion frequency, and fluid infiltration on the grain-size heterogeneity of coesite-bearing garnet based on a dataset of 2100 inclusion-bearing grains, of which 93 contain coesite, from the Saxonian Erzgebirge, Germany. By combining inclusion assemblages and garnet chemistry, we show that mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and felsic garnet contains variable amounts of coesite inclusions per grain, whereby coesite-poor grains are enriched in the coarse fraction and coesite-rich grains extensively disintegrated into smaller fragments resulting in an enrichment in the fine fraction. Raman images reveal that small coesite inclusions <9 µm are primarily monomineralic, whereas larger inclusions partially transformed to quartz, and garnet fracturing, fluid infiltration, and the coesite-to-quartz transformation is a late process during exhumation taking place at ~330°C. A model for the disintegration of coesite-bearing garnet enables explaining the heterogeneous grain27 size distribution by inclusion frequency. High abundances of coesite inclusions cause a high degree of fracturing and fracture connections to smaller inclusions, allowing fluid infiltration and the transformation to quartz, which in turn further promotes garnet disintegration.

Item Type: Article
Additional Information: This is the author accepted manuscript - the final published version (version of record) will be available via Cambridge University Press - please refer to any applicable terms of use of the publisher.
Uncontrolled Keywords: coesite preservation, garnet, inclusions, provenance, grain size, exhumation
Subjects: G Geography. Anthropology. Recreation > GB Physical geography
G Geography. Anthropology. Recreation > GE Environmental Sciences
Divisions: Faculty of Natural Sciences > School of Geography, Geology and the Environment
Depositing User: Symplectic
Date Deposited: 01 Feb 2021 12:56
Last Modified: 12 May 2021 10:24

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