Water in the structure of minerals from mantle peridotites as controlled bythermal and redox conditions in the upper mantle

Abstract Hydrous species and the amount of water ($ OH^{−} $ ions and crystal hydrate $ H_{2} $O) in structures of nominally anhydrous rock-forming minerals (olivine, ortho- and clinopyroxenes) were studied with Fourier spectroscopy in peridotite nodules (19 samples) from Cenozoic alkali basalts of...
Ausführliche Beschreibung

Abstract Hydrous species and the amount of water ($ OH^{−} $ ions and crystal hydrate $ H_{2} $O) in structures of nominally anhydrous rock-forming minerals (olivine, ortho- and clinopyroxenes) were studied with Fourier spectroscopy in peridotite nodules (19 samples) from Cenozoic alkali basalts of the Baikal-Mongolia region (Dariganga Plateau, Taryat Depression, and Vitim Plateau). Single-crystal samples oriented relative to the crystallographic axes of minerals were examined with an FTIR spectrometer equipped with an IR microscope at the points of platelets free from fluid inclusions. FTIR spectra were measured in regions of stretching vibrations of $ OH^{−} $ and $ H_{2} $O (3800–3000 $ cm^{−1} $) and deformation vibrations of $ H_{2} $O (1850–1450 $ cm^{−1} $). The water content in mineral structures was determined from integral intensities. To estimate the conditions of entrapment and loss of structural water in minerals, their chemical composition, including $ Fe^{2+} $ and $ Fe^{3+} $ contents, was determined with an electron microprobe analysis and Mössbauer spectroscopy. The bulk chemical composition of some nodules was determined with XRF and ICP MS. The total water content ($ OH^{−} $ + $ H_{2} $O) varies from 150 to 1140 ppm in olivines, from 45 to 870 ppm in clinopyroxenes, and from 40 to 1100 ppm in orthopyroxenes. Both water species in the mineral structures are retained down to a depth of 150–160 km in wide temperature and pressure ranges (1100–1500 °C, 32–47 kbar) at the oxygen fugacity of −1.4 to −0.1 log units relative to that of the quartz-fayalite-magnetite buffer. Ausführliche Beschreibung