Kimberlite Genesis According to Data on Hot Accretion of the Earth

Abstract

Analysis of modern geological and planetological data indicates hot accretion of the Earth. Impact melting during its formation led to origin of layered global magma ocean. Kimberlites were from residual melts of its lower peridotite latyer. This explains high content of meltphylic (lithophylic) components in kimberlites. Prolonged cooling and crystallization of magma ocean from top to bottom in the Earth’s history is the cause of mainly the Phanerozoic age of kimberlites. Moving apart of lithosphere and products of magma ocean fractionation contained therein when oceanic areas are formed, explains absence of kimberlites in these areas and Clifford rule nature. Low temperature of kimberlite magmas and high content of volatile component in them was caused by formation from residual melts. Increased temperature of crystallization of volatile-poor melts and emission of volatile components from magmas at shallow stages of upwelling led to solidification of upper parts of magmatic columns and their explosion during further upwelling under impact of high pressure of gas phase preserved by solidification. This explains the fact that, kimberlites formed mostly explosion pipes, and that kimberlite lavas are lacking. Diamonds were crystallized as a result of carbon accumulation in residual melt. Evolution of its composition during crystallization from peridotite through eclogite to kimberlite was responsible for existence of xenoliths of the same composition in kimberlites and inclusions in diamonds, and decrease of their average isotope age in this sequence. Degree of the saturation of the melt with carbon during its fractionation increased. Due to that, layer growth was followed by radial one, and morphology of originating diamond crystals was evolved from octahedrons to dodecahedrons and then to cubes and aggregates, and sculptures were formed on them. Accumulation of meltphylic components in residual melts is causes increase of nitrogen, light carbon isotope and other admixtures content in later diamonds.