Preservation of coesite in exhumed eclogite: insights from Raman mapping

Abundant coesite relics have been identified in a single thin section from a carbonate-bearing eclogite from the Saidenbach reservoir, Erzgebirge, German Variscides. The rock is only very slightly retrogressed despite having experienced granulite facies conditions during exhumation. Coesite, mostly 0.1–0.2 mm in long dimension, occurs mainly in garnet but has also been identified in omphacite. Partial transformation of coesite to palisade quartz, the relief contrast between these two phases, and radial cracks around the inclusions allowed an easy optical identification of coesite. Micro-Raman mapping of specific phases from grids of thousands of individual Raman spectra helped show that: (1) a visible palisade quartz corona between coesite and host is in places lacking, and (2) some inclusions contain aragonite. Based on published results from modelling of elastic behaviour of minerals along a pressure–temperature path, a scarcity of palisade quartz suggests that capturing of coesite during garnet growth occurred well into the coesite stability field such that quartz growth at the coesite = quartz univariant curve during decompression was only minimal in unfractured, overpressured inclusions. Another consequence of a high pressure for inclusion capture is that final fracturing of host garnet occurs at lower temperatures. This would explain the presence of aragonite relics in some inclusions. Based on extrapolations from kinetic experiments, aragonite should invert to calcite even faster than coesite to quartz and thus suggests temperatures of 300 °C or less for final fracturing.