Kimberlite and diamond genesis (Earth hot accretion model)
Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk, Russia
Obtaining satisfactory evidence of hot accretion of the terrestrial planets and existence of global magma oceans there in the past is significant achievement of paleontology in last decades. Existence of such oceans on the earth is testified by determined locate of composition of the major varieties of mantle xenoliths in kimberlites along trends of magmatic fractionation, decrease in average isotope age and temperature of crystallization of these varieties according to the sequence of their formation during fractionation, projection of the fields of P-T conditions of xenolith rock crystallization into the area of very high temperature (up to 11000) on the earth surface (Shkodzinskiy, 2012). These results lead to a new solution of the problem of kimberlite and diamond genesis, which corresponds to all empirical data.
According to the developed model, magma ocean of layered composition, with average depth 240 km existed on the earth after accretion. The deeper, the density increased (from 2,3 to 2,9 g/cm3), which led to the absence of extensive (from bottom to surface) single convection in it and its continuous crystallization from top to bottom. As a result of its solidification, lithosphere of ancient platforms appeared at the end of Proterozoic. Fractionation of the lower peridotite layer ended by formation of kimberlite residual liquids and kimberlites. Such origin of the latter explains their late formation (mainly in the Phanerozoic), their occurrence only on ancient platforms, their richness in components, concentrated mainly in the melts (volatile, rare-earth). Position of the fields of eclogite compositions from xenoliths on the same trends with other mantle rocks indicates their formation during fractionation of peridotite layer.
Areas in lithosphere containing residual melts were the lowest strength. That is why they were extruded during tectonic deformations, solid phases were melted in them, influenced by decompression and frictional heat release. Kimberlite magmas were formed this way. Various quantitative relationship of the phases in different parts of the extruded mixture is the reason of significant variations of kimberlite composition even in the same pipe. A very small amount of kimberlite residual liquids (less than one-thousandth of peridotite layer amount) resulted in small size of kimberlite bodies. Multi-tiered location of residual liquids in peridotite layer – reason for the presence of kimberlites with different composition and diamond content in the same field. Kimberlite-rising zones of tectonic disturbances originated in lithosphere as a result of the rise of lower-mantle plumes, where big amount of tholeiitic magmas were formed. This is a reason for wide development of traps in kimberlite magmatism areas (Shkodzinskiy, 2009).
Formation of kimberlite melt at a later stage of cooling of peridotite layer caused a relatively low temperature of kimberlite magmas. Thereby loss of the hyperfusible, volatile components, influenced by their boiling away at shallow stages of the rise led to decompression solidification of the melt in upper parts in kimberlite magmatic columns and then to their explosion influenced by preserved solidification of high internal pressure fluid phase. This is a reason of occurrence of kimberlites mainly as pipes and their often fragmental texture.
Diamonds and kimberlite residual liquids were formed during fractionation of peridotite layer of magmatic ocean. Diamonds are associated with kimberlites by paragenetic relations. This explains, their seemingly incompatible features – determined correlation of morphology, content, coarseness and impure composition with kimberlite composition, but often much more ancient age of this mineral in contrast with pipes. Diamonds, as other deep-seated minerals of kimberlites, are not alloxenocrysts, but autoxenocrysts (Shkodzinskiy, 2012). That is diamonds not accidently were imprisoned by kimberlite magmas, but were originated at early stages of formation of the latter.
The reason of diamond crystallization is increase of carbon content in residual liquids and supersaturation with these components because of sharp decrease of their amount as fractionating. Initial peridotite magma was undersaturated with carbon, which explains the lack of diamonds in most peridotite xenoliths. About 3,5 – 3,6 billion years ago the magma was supersaturated, it was a reason of the beginning of diamond formation. Graphite firstly was crystallized due to very high temperature. It explains the presence of inclusions of this mineral in centers of growth of some diamonds. Later on, decrease of melt amount supported its supersaturation with carbon and led to very long formation of diamond. Low solubility of carbon in the melt probably is a reason of low content of this mineral (less 1g/t), even in the richest kimberlites. In case of assumed carbon addition to the mantle by hypothetical fluid flows or trough blocks of oceanic crust in subduction zones, there would be high diamondiferous lenses and veins, and diamond contents in kimberlites sometimes could be higher.
Viscosity of residual liquids significantly increased (2-3 orders of magnitude) because of increased content of silicic acid at early and middle stage of fractionation. Due to that carbon diffusion rate decreased and degree of supersaturation of residual liquids with carbon increased. Consequence of this was reducing the role of tangential layer growth of diamond crystals and the growing role of normal radial growth. Reduction of area of the formed layers led to origin of different sculptures in there. As a result, morphology of growing crystals was changed in the following sequence: octahedrons – rhombic octahedron – cubes and octahedrons with smooth faces – with convex laminar faces – with polycentric faces – modular crystals – rounded-step – rounded crystals.
Such origin of these diamond varieties is confirmed by decrease of values of specific intensity of diamond roentgenoluminescence in the mentioned sequence (Argounov, 2005), due to increase of admixture and defect content in them during fractionation. Growth origin of sculptures and rounded diamonds correlates is consistent with determined increase of an average size of their crystals in the mentioned sequence in kimberlite pipes and placers. Early formation of octahedrons is confirmed by presence only of this variety in peridotite xenoliths and cube and rhombic octahedron formation in xenoliths of later eclogites.
Increase of concentration of volatile components, light carbon isotope and rare earth elements in residual liquids during fractionation explains determined increase of their average content from early to late diamonds. As magmas fractionate, part of diamond crystals was buried in cumulates and almost stopped growing, due to decrease of carbon inflow from their melts. When rising of kimberlite magmas, part of these diamonds got into magmas as a result of decompression and friction melting of cumulates. Crystals, remaining in the melt, sometimes continued to grow, that explains presence of small amount of diamonds-giants in kimberlites. Processes of diamond crystal sedimentation into bottom, low differentiated areas of peridotite layer, led to octahedron layer growth on rhombic octahedron and cubic crystals. This led to formation of crystals of complex structure.
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Shkodzinskiy V.S. Kimberlite and diamond genesis. Yakutsk: Yakutia media-holding, 2009. P. 352.
Shkodzinskiy V.S. Origin of mantle, magma, kimberlites and diamond. Earth hot accretion model. Saarbrücken: Palmarium academic publishing, 2012. P. 579.