Physics
Scientific paper
Apr 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987pepi...45..280k&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 45, Issue 3, p. 280-293.
Physics
3
Scientific paper
Two unique mantle-derived fragments demonstrating intensive fluid action have been found in 1.5 Ma-old basanitic breccia of Shavaryn-Tzaram maar, Khangai highland, northern Central Mongolia, known for abundant mantle xenoliths (dominant spinel therzolites and various spinel and garnet spinel peridotites and pyroxenites). One is a composite nodule in which common spinel lherzolite grades into black websterite with a druse of large cuhedrally terminated garnet and clinopyroxene crystals; the latter mineral contains macroscopic vugs. The Ti and Fe content of minerals increases from the lherzolite to the druse in which accessory phlogopite contains as much as 11.4 wt.% TiO2. Minerals in the druse are practically identical in composition to those found as megacrysts in the same pipe. The other sample is a vesicular clinopyroxene megacryst with tubular vugs up to 5 mm across lined with fine-grained glassy aggregate, stretching parallel to the cleavage plane and making up at least 1/4 of the sample. `Normal' clinopyroxene megacrysts in this occurrence also frequently contain some subspherical macroscopic vugs. Clinopyroxene of both the druse and megacrysts contains microinclusions grouped into 4 types: (1) fluid, (2) sulphide-fluid, (3) silicate-sulphide-fluid, and (4) silicate-fluid, the first three types being primary. Composition of minerals in the inclusions is similar for the druse and megacrysts. They are inferred to have been formed from a multicomponent fluid system represented by an emulsion of silicate-sulphide melt drops (~30%) in high-density CO2 (~ 70%). By combining the measured composition of major phases with estimates of their proportions in inclusions the chemical composition of the initial fluid system has been estimated.
These observations suggest that, in areas of alkali basaltic magmatism, subcontinental upper mantle may trap small reservoirs of fluidized CO2-silicate-sulphide mixtures capable of producing profound metasomatic transformations of the enclosing mantle peridotites leading to the formation of veins and pockets of very coarse-grained iron-rich pyroxenites and eclogite-like rocks. Disintegration of the latter during their entrainment in and subsequent transportation by rapidly ascending alkali basalt magmas will lead to the formation of mineral fragments (discrete nodules) indistinguishable from garnet, clinopyroxene and mica megacrysts, that is, at least some of the megacrysts are of metasomatic origin.
Bogatikov O. A.
Ionov Dmitri A.
Kovalenko V. I.
Naumov V. B.
Ryabchikov D. I.
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