High fO2 during sillimanite zone metamorphism of part of the Barrovian type locality, Glen Clova, Scotland

Abstract

The redox state of sillimanite zone (650-700°C, 5-6 kbar) metasediments of the Barrovian type area, Scotland, was investigated using estimates of metamorphic oxygen fugacity (fO2), sulfur fugacity (fS2), and fluid chemistry based on new determinations of mineral and rock compositions from 33 samples. A total of 94% of the samples lack graphite, contain both ilmenite-hemalite solid solutions (RHOMOX) and magnetite, and had metamorphic fO2 about 2 log10 units above the quartz-fayalite-magnetite (QFM) buffer. The regional variation in metamorphic fO2 for these rocks was minimal, about ± 0.3 log10 units, reflecting either a protolith that was homogeneous with respect to redox state, or an initially variable protolith whose redox state was homogenized by metamorphic fluid-rock interaction. RHOMOX inclusions in garnet porphyroblasts that become richer in ilmenite from the interior to the edge of the host porphyroblast suggest that at least some synmetamorphic reduction of rock occurred. Significant variations in bulk-rock oxidation ratio (OR) that are probably inherited from sedimentary protoliths are found from one layer to the next; OR ranges mostly between 20 and 50 [OR = molecular 2Fe2O3 × 100/(2Fe2O3 + FeO)]. These OR variations are uncorrelated with fO2 and do not indicate that large, order-of-magnitude gradients in fO2 and redox state existed or were preserved between layers during metamorphism. The other 6% of the samples contain ilmenite, lack magnetite, and had low fO2 0-1 order of magnitude below QFM in the stability field of graphite. They are characterized by combinations of the following: large fluid HF/H2O; metasomatic, tourmaline-bearing veins; absence or rarity of primary organic matter; and crosscutting late metamorphic shear zones rich in carbonaceous material. Such observations suggest that locally low fO2 conditions may have been related to the influx of reducing fluids from elsewhere in the area.