Hydroxyl in eclogitic garnet, orthopyroxene and oriented inclusion-bearing clinopyroxene, western Norway

Ten eclogites from the Caledonide orogen in western Norway, whose mineral chemistry records metamorphic conditions of up to 850 °C and 5.5 GPa, were investigated quantitatively for structural hydroxyl (OH). Each rock sample contains clinopyroxene with oriented inclusions of either quartz, albite or quartz + pargasite. Five orthopyroxene-bearing eclogites have relatively homogeneous amounts of structural OH in garnet (13–32 μg g⁻¹ H₂O equivalent), clinopyroxene (119–174 μg g⁻¹) and orthopyroxene (4–17 μg g⁻¹). In contrast, the other five orthopyroxene-free eclogites have variable amounts of OH and it ranges in garnet from 8 to 306 μg g⁻¹ and in clinopyroxene from 58 to 711 μg g⁻¹. Apart from extreme values, the OH content of clinopyroxene in these eclogites is significantly lower than in comparable ultra-high-pressure metamorphic samples, such as metasomatised and pristine eclogite xenoliths from the lithospheric mantle underneath several cratons and coesite- and quartz-eclogites from the Erzgebirge and the Kokchetav massifs. The low structural OH contents, the deficiency of molecular water and the evidence for the preservation of diffusion-sensitive mineral chemistry arising from metamorphism well beyond the stability field of amphibole suggest that oriented inclusions of quartz + pargasite were formed isochemically during eclogite-facies decompression.