Life-cycle analysis of coesite-bearing garnet

Abstract

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.

Citation

(2021). Life-cycle analysis of coesite-bearing garnet. Geological Magazine, https://doi.org/10.1017/S0016756821000017

• Acceptance Date: Jan 6, 2021
• Publication Date: Feb 22, 2021
• Journal: Geological Magazine
• Print ISSN: 0016-7568
• Publisher: Cambridge University Press
• DOI: https://doi.org/10.1017/S0016756821000017
• Keywords: coesite preservation, garnet, inclusions, provenance, grain size, exhumation
• Publisher URL: https://doi.org/10.1017/S0016756821000017