DIAMOND CRYSTALLIZATION DURING THE INTERACTION OF Ca,Mg,Fe-CARBONATES WITH SULFUR AT THE P-T PARAMETERS OF THE LITHOSPHERIC MANTLE
O.V. Furman1, Yu.V. Bataleva1, Yu.M. Borzdov1, Yu.N. Palyanov1,2
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
2Novosibirsk State University, Novosibirsk, Russia
Keywords: High-pressure experiment, mantle metasomatism, diamond genesis, carbonate, diamond micromorphology, sulfur, magnetite, inclusions in diamond
Abstract
The paper is concerned with experimental studies aimed at assessing the possibility of diamond crystallization during interactions of alkaline-earth and transition metal carbonates with sulfur and at characterizing the obtained diamond crystals and inclusions in them. The experiments were carried out on a multi-anvil high-pressure “split sphere” apparatus (BARS) at constant P - T - t parameters: 6.3 GPa, 1550 °C, and duration of 20 h, in the Mg,Ca-carbonate-sulfur system with different CaO/MgO ratios and in the Fe,Mg,Ca-carbonate-sulfur system with variable FeO/CaO/MgO proportions. It has been experimentally established that in the alkaline-earth carbonate-sulfur systems, a Ca,Mg-carbonate melt with dissolved sulfur (1.2-6.0 wt.%) forms, which is a diamond growth medium. An octahedron is the stable growth form of diamond crystals, and an increase in the Ca# value of the system is accompanied by an increase in the growth rate of the {111} faces. In the Fe,Mg,Ca-carbonate-sulfur system, Fe,Mg,Ca-carbonate melt with dissolved sulfur is shown to be the crystallization medium and the source of diamond carbon, and diamond growth occurs as a result of the redox interaction of the sulfide and carbonate melts. An increase in the Fe# value of the system from 0.41 to 0.78 is accompanied by: (1) an increase in the growth rate of the {100} faces; (2) a change in the predominant elements of the octahedron face microrelief from triangular growth layers to hexagonal vicinals; and (3) an increase in the number and size of inclusions and the evolution of their phase composition. Inclusions in diamond form as a result of the melt preservation on the {100} faces during the transformation of a cuboctahedron into an octahedron. The inclusions contain quenched carbonate melt (quenching phases are carbonate, magnetite, and graphite), sulfur melt, and Fe-S-O melt. The revealed carbonate + magnetite + graphite assemblage in the inclusions testifies to a siderite redox dissociation reaction at the P - T parameters of the lithospheric mantle. These results shed light on the possible origin of magnetite inclusions in upper-mantle diamonds and suggest that the presence of magnetite in syngenetic inclusions is an indicator of the participation of iron-bearing carbonates in natural diamond formation processes.
|
