SOME PETROLOGICAL ASPECTS OF DIAMOND GENESIS

Analysis of mineral inclusions in discrete growth zones of diamond cry stals provides evidence for the magmatic genesis of both ultrabasic an d pyroxenite-eclogitic diamond types. Changes in the K2O, Na2O, CaO, F eO, Fe2O3, and Cr2O3 proportions in the garnet-clinopyroxene, enstatit e-spinel, and spinel-garnet pairs indicate that the pressure declined discretely and regularly during the diamond growth in the diamond-bear ing facies due to a series of rapid magma ascents to higher levels. Th e persistent coexistence of different nodules-xenoliths of abyssal ecl ogites and peridotite and their contrasting chemistries provide eviden ce for an early immiscible splitting of the initial mantle melts into ultrabasic and basic magmas. The latter magmas evolved, after immiscib le splitting, independently due to their different physical properties and the various compositions of volatiles contained in them. The basi c (eclogitic) magmas evolved in the presence of more oxidized fluids. Differences in their fluid chemistries and evolution predetermine diff erent carbon isotope compositions of ultrabasic and eclogitic diamonds . The former show uniform carbon compositions, delta(13)C = -5 parts p er thousand; the eclogitic diamonds consist of carbon that is enriched to a different degree in the light or the heavy isotope. This phenome non was caused by the fractionation of the heavy carbon isotope in CO2 during the crystallization of diamond and the isotope exchange with t he transmagmatic fluids. The differences in mineral assemblages of inc lusions in discrete zones of diamond crystals are coupled with the mod al variability of nodules bearing intratelluric diamond. These feature s provide evidence for a polyfacial diamond growth and the polyfacial genesis of the rocks that inherited these diamonds. This polyfacial ch aracter is most conspicuously pronounced in kimberlites, lamproites, a nd metamorphic rocks, which crystallize or are transformed far beyond the diamond stability field. The diamond of these rocks is a relict mi neral, inherited from some precursor material. Diamond-bearing structu res reach deeper into the mantle and exist from the Precambrian to Mes ozoic. They serve as conduits for abyssal fluid flows, which probably ascend from the Earth's core. The high fluid pressure causes endogenou s explosions at different depths, which range from old mantle magma ch ambers to recent hypabyssal diatremes. The sequential explosions assis t rapid uplifts of the liquid, partly crystallized magmas into new lit hospheric levels. The explosions are also favorable for the interactio n of the fluids with ultrabasic igneous rocks, which leads to the orig in of compositionally intermediate alkalic-basalt magmas and the final highly alkalic kimberlite and lamproite melts rich in fluid component s.