Yu. S. Genshaft and I. P. Ilupin
, 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).