Partial Melting Event Research Articles

Garnet-peridotite, primary ultrabasic magma and eclogite; Interpretation of upper mantle processes in kimberlite

The ultramafic nodules in kimberlite are little altered samples of the upper mantle. Source mantle (garnet-lherzolite), residual mantle (garnet-harzburgite and harzburgite), primary magma (ultrabasic) and high-pressure cumulates (eclogite) are all represented. Twenty-seven new analyses of ultramafic rocks and 24 new analyses of their constituent minerals are presented and used in conjunction with published data to demonstrate that the upper mantle sampled by kimberlite displays a mineralogical and chemical variation reflecting very deep seated partial melting events which produced from the rocks affected about 15% of ultrabasic primary magma at a temperature not much more than 50°C above the solidus. The majority of the subcontinental mantle section sampled by kimberlite is not, however, a totally depleted residuum, but is potentially fertile source mantle. An upper mantle model based upon the inclusions in kimberlite has conspicuously higher Mg Mg + Fe , and differs in other important respects from hypothetical “pyrolite” models and from models based on ultramafic nodules in basalts, althought it has some similarities with a model based on alpine-type garnet-peridotites. Primary magmas in equilibrium with the magnesian source rocks indicated by the kimberlite model are themselves so magnesian and poor in incompatible elements that extensive fractionation of eclogite and/or olivine is required in order to account for the chemistry of apparently parental tholeiite magmas erupted at the earth's surface. The nickel contents of these erupted magmas do not constitute a bar to previous extensive olivine fractionation.

Origin and Provenance of Fossil and Recent Monazite Deposits in Brazil

The study of monazite beach deposits and the underlying basement along the Brazilian continental margin indicates that monazite is first concentrated in pegmatite and augengneiss which have crystallized from a water-rich mobilizate phase during partial melting events within a migmatite-charnockite Archean terrain. Erosion of such rocks has reconcentrated the monazite in the late Precambrian quartzites of the Sepetiba group, now metamorphosed in the granulite facies. Recrystallization of monazite took place during progressive metamorphism simultaneously with the muscovite-sillimanite reaction. The monazite deposits of Guanabara were derived directly from the Sepetiba quartzites whereas those of Rio de Janeiro, Espirito Santo, and Bahia have the Tertiary clayish sediments of the Barreiras formation as an additional host.

Uranium and boron distributions in some oceanic ultramafic rocks

From mica fission-track maps the serpentinized and weathered portions of four ultramafic rocks from oceanic ridge systems contained 0.5 to 2.4 ppm U compared to only 2.5 ppb in clinopyroxene, 0.6 ppb in chromite and less than 7.0 ppb in olivine. Orthopyroxene grains contained 0.4 ppb U which is three orders of magnitude lower than had previously been reported. Long thin tracks from (n, α) reactions with boron were recorded in cellulose nitrate plastic and were counted like fission tracks. The track density from boron was 2×10 4 times higher than that from uranium fission alone. Boron in serpentine was variable on a 50-μm scale attaining 155 ppm concentrations. Orthopyroxene grains, in contrast, had maximum concentrations of 0.8 ppm. Most of the uranium and boron in the rocks is believed to have been introduced during serpentinization. From known crystal-melt partitioning ratios the uranium and boron distributions are consistent with the ultramafic rocks being cumulates or residues from partial melting events.