High fO2 During Sillimanite Zone Metamorphism of Part of the Barrovian Type Locality, Glen Clova, Scotland |
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Authors: | AGUE JAY J; BAXTER ETHAN F; ECKERT JAMES O JR |
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Institution: | DEPARTMENT OF GEOLOGY AND GEOPHYSICS, YALE UNIVERSITY, PO BOX 208109, NEW HAVEN, CT 06520-8109, USA |
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Abstract: | The redox state of sillimanite zone (650700°C, 56kbar) metasediments of the Barrovian type area, Scotland, wasinvestigated using estimates of metamorphic oxygen fugacity(fO2), sulfur fugacity (fS2), and fluid chemistry based on newdeterminations of mineral and rock compositions from 33 samples.A total of 94% of the samples lack graphite, contain both ilmenitehematitesolid solutions (RHOMOX) and magnetite, and had metamorphicfO2 about 2 log10 units above the quartzfayalitemagnetite(QFM) buffer. The regional variation in metamorphic fO2 forthese rocks was minimal, about ±0·3 log10 units,reflecting either a protolith that was homogeneous with respectto redox state, or an initially variable protolith whose redoxstate was homogenized by metamorphic fluidrock interaction.RHOMOX inclusions in garnet porphyroblasts that become richerin ilmenite from the interior to the edge of the host porphyroblastsuggest that at least some syn-metamorphic reduction of rockoccurred. Significant variations in bulk-rock oxidation ratio(OR) that are probably inherited from sedimentary protolithsare found from one layer to the next; OR ranges mostly between |
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