On genetic classification of chromian spinels in deep-seated rocks from continental structures
Yu. S. Genshaft
Schmidt United Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, RussiaI. P. Ilupin
Central Research Institute for Base and Noble Metals, Moscow, Russia
Abstract
[1] A large data base of 1821 chromian spinel compositions (Cr2O3 > 2 wt %) in
ultramafic nodules from basalts, kimberlites, lamproites, ultramafic massifs, and from the
diamond assemblage has been analyzed using mathematical statistics methods. For each
spinel component under study (TiO2, Al2O3, Cr2O3,
FeO, MnO, MgO), a considerable
overlap is observed, spinels with the highest Cr-number (#Cr) being found in the diamond
assemblage and those with the highest Mg-number (#Mg), in nodules from basalts. The
lowest oxidation degree is detected in spinels from nodules in basalts and from the
diamond assemblage. Spinels from ultramafic massifs have the highest oxidation degrees
and Fe-numbers. In ultramafic massifs from kimberlite to lamproite through to the diamond
type, spinel compositions follow the same variation trends. Spinels from the basaltic group
fall away from these trends. Compositional variability of spinels is due chiefly to
isomorphic replacement of Fe+2 for Mg and of Al and Mg for Cr. Kimberlite-type spinels
have high and broadly variable Ti contents. Igneous and metamorphic/metasomatic origins
for spinels of the five groups under study are discussed. Spinel compositions from
ultramafic rocks found in continental structures are shown to indicate that at depth beneath
these structures, there exist physicochemical conditions for crystallization inherent to all
the principal tectonic features of the earth-continents, oceans, and transition zones
(island arcs).
Received 8 February 2002; published 26 March 2002.
Keywords: classification of chromian spinels, deep-seated rocks, continental structures, kimberlites.
Citation: Genshaft, Yu. S., and I. P. Ilupin (2002), On genetic classification of chromian spinels in deep-seated rocks from continental structures, Russ. J. Earth Sci., 4, No.2, 153-162, doi:10.2205/2002ES000086.