Chromite–Ulvöspinel Series of Minerals from Alkali Picrite–Basic Rocks in the North Siberian Platform and Their Oxythermobarometry

The typochemical features of spinels forming continuous series from Al–Cr spinels to ferrispinel and ulvöspinel from basic–ultrabasic rocks of the North Siberian Platform are considered. Their composition features are compared with those of alkali basalts of oceanic islands and lunar marine basalts in the modified Al–Cr–(Fe3+ + 2Ti)–Fe2+ 3D diagram proposed by the author. The identified continuous isomorphous series from Cr-spinel to ulvöspinel is one of the mineralogical indicators of a possible paragenetic relationship between picrite–basalt, alkali olivine–basalt, alkali–ultrabasic, and kimberlite rocks in the North Siberian Platform. The Ballhaus–Berry–Green olivine–Cr-spinel oxythermobarometer used in the rock study in the North Siberian Platform has made it possible to demonstrate that oxygen volatility log fO2 is from two to four orders of magnitude higher than that specified by the fayalite–magnetite–quartz buffer and corresponds to the oxidation state of Pt-bearing dunite–clinopyroxenite rock assemblages of the Ural–Alaskan and Aldan types.