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

Abstract. A total of 10 western Norwegian eclogites, whose mineral chemistry records metamorphism of up to 850 °C and 5.5 GPa, were investigated for structural hydroxyl content in nominally anhydrous minerals. Garnet shows pronounced absorption in the wavenumber ranges of 3596–3633, 3651–3694, and 3698–3735 cm−1 and minor absorption centred at about 3560 cm−1. Clinopyroxene with aligned inclusions of either quartz, albite, or quartz + pargasite has major absorption at 3450–3471 and 3521–3538 cm−1 and minor absorption centred at 3350 and approximately 3625 cm−1. The latter band is strongest in a sample with minute lamellar inclusions rich in Al, Fe, and Na and was excluded from hydroxyl quantification. Orthopyroxene has large, narrow absorption peaks centred at 3415 and 3515 cm−1 and smaller peaks at 3555, 3595, and 3625 cm−1. Five orthopyroxene-bearing eclogites exhibit relatively homogeneous amounts of structural hydroxyl in garnet (13–32 µg g−1), clinopyroxene (119–174 µg g−1), and orthopyroxene (4–17 µg g−1). The outer 200 µm wide rims of the orthopyroxene grains illustrate a late hydroxyl loss compared to core values of about 30 %, which is not evident in garnet and clinopyroxene. In contrast, the other five orthopyroxene-free eclogites exhibit variable amounts of hydroxyl in garnet (8–306 µg g−1) and clinopyroxene (58–711 µg g−1). Apart from extreme values, the structural hydroxyl content of clinopyroxene in the eclogites studied is lower than in comparable ultra-high-pressure metamorphic samples, e.g. both metasomatised and pristine eclogite xenoliths from the lithospheric mantle underneath several cratons and coesite- and quartz-eclogites from the Erzgebirge and the Kokchetav massifs, by up to several hundreds of micrograms per gram (µg g−1). The low structural hydroxyl contents, the deficiency of molecular water, and the preservation of diffusion-sensitive evidence from the mineral chemistry for metamorphism well beyond the stability field of amphibole suggest that oriented inclusions of quartz + pargasite were formed isochemically during decompression. In addition, structural hydroxyl content in clinopyroxene is inversely correlated with metamorphic pressure estimates obtained from orthopyroxene of the same samples. Therefore, structural hydroxyl in nominally anhydrous eclogite minerals can serve as an indicator of the effectiveness of retrogression.