16-37 Prograde Mineral Assemblages During Cold Subduction: Raman Analyses of Mineral Inclusions in Garnets from Lawsonite Eclogites from Sivrihisar, Türkiye

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

Lawsonite eclogite forms under cold subduction conditions at depths greater than ~45 km. Our samples originate from the Tavşanlı zone of Sivrihisar, Türkiye, which is a previously subducted and then later exhumed slab.  As the garnet grows throughout metamorphism, inclusions of different minerals are preserved from earlier phases in pressure and temperature, and characterizes the prograde path. In this study we identified mineral inclusions in garnets to determine the prograde mineral assemblage. We analyzed thin sections using a Renishaw inVia Raman Microscope with a 532 nm laser. The automated mapping stage was used to select points of interest within garnets in thin section. Spectra were input into a search to be matched with standard mineral spectra reflecting spectrum peaks most similar to the analyzed point. The most abundant minerals are: titanite, rutile, chlorite, quartz, amphibole (glaucophane), mica (muscovite), and calcite. Less abundant minerals are: epidote, apatite, zircon, barite, pyroxene, and lawsonite. Diamonds were found within multiple garnets in one thin section but are likely traces of polishing paste used to create the thin section, as no diamonds were found in a different polished section produced using SiC polishing paper. Chlorite and mica inclusions commonly occur near or along fractures in the garnets, and some may be retrograde in origin. Titanite inclusions are generally found near garnet cores while rutile inclusions occur near garnet rims. Zoning of the mineral inclusion assemblages is consistent with cold subduction and the proposed prograde path from ~425°C and 2.1 GPa to 500°C and >2.6 GPa for lawsonite blueschist by Pourteau et al (2019). Pressure-temperature diagrams reflecting mineral stability can be associated with mineral appearance and sequence within the garnets, to create a model of the P-T changes present during prograde metamorphism of the lawsonite eclogite. 

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