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Life cycle analysis of coesite-bearing garnet

Schönig, Jan; von Eynatten, Hilmar; Meinhold, Guido; Lünsdorf, N. Keno


Jan Schönig

Hilmar von Eynatten

Guido Meinhold

N. Keno Lünsdorf


Detrital coesite-bearing garnet is the final product of a complex geological cycle including coesite entrapment at ultra-high-pressure conditions, exhumation to Earth’s surface, erosion and sedimentary transport. In contrast to the usual enrichment of high-grade metamorphic garnet in medium- to coarse-sand fractions, coesite-bearing grains are often enriched in the very-fine-sand fraction. To understand this imbalance, we analyse 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 (1) mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and (2) 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 of dimension < 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 c. 330°C. A model for the disintegration of coesite-bearing garnet enables the heterogeneous grain-size distribution to be explained 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.


Schönig, J., von Eynatten, H., Meinhold, G., & Lünsdorf, N. K. (2021). Life cycle analysis of coesite-bearing garnet. Geological Magazine, 158(5), 1421-1440.

Journal Article Type Article
Acceptance Date Jan 6, 2021
Online Publication Date Feb 22, 2021
Publication Date 2021-08
Journal Geological Magazine
Print ISSN 0016-7568
Electronic ISSN 1469-5081
Publisher Cambridge University Press
Peer Reviewed Peer Reviewed
Volume 158
Issue 5
Pages 1421-1440
Publisher URL

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