DIAMONDIFEROUS METEORITES AND THEIR GENESIS

The paper summarizes results obtained during comparative studies of di amondiferous mineralization in iron meteorites, ureilites, and chondri tes. In all types of meteorites, diamond is restricted to the metallic taenite-kamacite phase, in which it occurs together with moissanite ( SiC), daubreelite, troilite, and spinel. The diamond is specifically h igh in its abundances of hydrogen, nitrogen, and noble gases, with a u niform ''spectrum'' of species. The diamond of ureilites is magmatic; it was the first phase to crystallize from the nickel-iron fluid-beari ng melt, from which graphite and kamacite crystallized later. The latt er mineral occurs as drop-shaped inclusions in graphite. The diamondif erous melt of ureilites fills interstitions and veinlets. As a result of the fluid effect of this melt, the olivine iron mole fraction decre ases from 17 - 20 to 0, and native iron and enstatite form: Mg1.7Fe0.3 SiO4 + 0.3H(2) = 0.7Mg(2)SiO(4) + 0.3MgSiO(3) + 0.3Fe + 0.3H(2)O. The high-pressure conditions of this effect are evident from the occurrenc e of both diamondiferous mineralization and pyrope, which we first dis covered in ureilites. The diamond of chondrites is restricted to the t aenite-kamacite melt in the matrix, which actively affects the pyroxen e-olivine chondrules. The paper describes the proposed two-stage genet ic model for the evolution of parental meteoritic planets: stage I, un der high pressure of the hydrogen envelopes and progressively more red uced conditions, and stage II, after the migration of the gas envelope s in a progressively more oxidized environment. The model explains the relict nature of diamondiferous mineralization in chondrites, which o riginated exclusively during stage I of planetary evolution. The progr esively stronger effect of hydrogen on the molten meteoritic material during this stage accounts for the coexistence of diamondiferous miner alization and the isotopic anomalies of the chondrites. The metallic c ompounds that formed during this stage show anomalously low oxidation states (SiO, SiH2, SiCl2, etc.) due to reactions such as SiO2 + H-2 = SiO + H2O. Such reactions result in that the silicates in assemblage w ith diamond are anomalously enriched in isotopically light oxygen, for example: 2MgO + 2Si0 + CH2 + CO = Mg2SiO4 + SiC + C + H2O. The chondr itic assemblage of diamond and moissanite (SiC) and the occurrence of native silicon provide evidence for the operation of such reactions du ring diamond-forming processes.