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Experimental studies on the relationship between kimberlite magmas and partial melting of peridotite

1979; American Geophysical Union; Linguagem: Inglês

10.1029/sp015p0330

David H. Eggler, Richard F. Wendlandt,

earthquake and tectonic studies

The melting relations of an average Lesotho kimberlite composition have been investigated at 30 and 55 kbar pressure. The content of CO2 in all experiments was 5.2 wt %, but amounts of H2O were varied from 0 to about 10 wt %. It can be shown from a theoretical analysis that for nearly all these volatile contents, the solidus temperature of kimberlite composition is not a function of the volatile composition, because at subsolidus conditions, compositions of vapor are buffered by reaction with a carbonate mineral (dolomite at 30 kbar, magnesite at 55 kbar). The subsolidus phase assemblage—olivine + clinopyroxene + orthopyroxene + garnet + carbonate + phlogopite + vapor — melts at 1075°± 25°C at 30 kbar and 1225°±25°C at 55 kbar. At both 30 and 55 kbar, olivine is the liquidus phase at the conditions studied, followed closely in temperature by the crystallization of clinopyroxene and orthopyroxene. At 30 kbar, garnet is consumed immediately above the solidus, but at 55 kbar it persists well into the melting range. Extrapolation of the phase boundaries, in P-T projection, indicates that garnet, olivine, and two pyroxenes crystallize near the liquidus at 60 kbar, a phase field configuration consistent with the hypothesis that kimberlite composition is a primary magma, i.e., is a product at the beginning of melting of a parental peridotite. A comparison of the experimentally-determined P-T projection of phase equilibria with the Lesotho pyroxene geotherm reveals that beneath Lesotho a liquid of kimberlitic composition co-existed with garnet-olivine-clinopyroxene-orthopyroxene (and perhaps with magnesite or phlogopite as well) at a pressure of 50–60 kbar. Because those phases constitute the Lesotho megacryst assemblage, which equilibrated at about 50–60 kbar, megacrysts are interpreted to be cognate to kimberlite magma. Inasmuch as partial melting of peridotite containing CO2 and H20 will produce liquids of kimberlitic composition at pressures of 50–60 kbar, such liquids may be common in the upper mantle. The rarity of kimberlites as rocks may be attributed to the rarity of tectonic settings conducive to the ascent of appropriate magmas.