Physics
Scientific paper
Dec 1971
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1971esrv....7..187d&link_type=abstract
Earth Science Reviews, Volume 7, Issue 4, p. 187-214.
Physics
23
Scientific paper
Over the past fifteen years the original data on the kimberlites of South Africa have been supplemented with a wealth of information on the kimberlites of the U.S.S.R., the U.S.A., and various parts of Africa. From their distribution pattern, it can now be shown that kimberlites are virtually confined to the old Precambrian cratons and that the diamond-bearing kimberlites exist on the older cratonic nuclei that have not been deformed for the past 2,500 m years. The kimberlite dykes infill major, deep-seated fractures that cross the cratons on a geometrical pattern. The kimberlite that infilled these fractures was a hot fluid that penetrated to within 2 3 km of the surface before there was explosive breakthrough to the surface with subsequent formation and infilling of the high-level diatremes by a gas-solid fluidisation process. The kimberlites can vary widely texturally, from the fragmental variety found in the diatremes, to massive varieties found within the hypabyssal dykes or skills. Both petrographically and chemically, kimberlite is a hybrid rock, formed by incorporation of crystals mainly derived from fragmentation of upper mantle garnet lherzolite into a matrix that has strong affinities with carbonatite. Kimberlites have carried out extensive sampling of the upper mantle during their ascent to the surface and from these it is possible to propose the structure of the upper mantle under the craton. Although garnet lherzolite and to a lesser extent, eclogite are the main upper mantle xenolith types, numerous variants of these two major rock-types point to both major and minor inhomogeneities within the upper mantle. There are at present two main hypotheses to explain the genesis of kimberlite. The “residual” hypothesis regards kimberlite as the end product of high-pressure fractionation of piprite basalt, that results itself from melting of four-phase garnet lherzolite. The alternative “incipient melting” hypothesis proposes that kimberlite is formed by incipient melting of a parental garnet lherzoline in which phlogopite is an additional phase; the initial melt liquid, plus fragments of the parental five-phase garnet lherzolite, is kimberlite. The latter hypothesis appear to explain more adequately various aspects of kimberlite petrology and geochemistry. However, although the recognition of phlogopite as an additional phase in the deeper parts of the upper mantle where kimberlite originates, explains the source of elements such as water, fluorine, potassium, and rubidium, there is still no adequate explanation for the source of carbon, sulphur, phosphorus and nitrogen within the upper mantle.
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