Metasomatism-induced melting in mantle xenoliths from Mongolia

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Authors:Ionov, D. A.; Hofmann, A. W.; Shimizu, N.
Author Affiliations:Primary:
Max-Planck-Institut für Chemie, Mainz, Federal Republic of Germany
Other:
CNRS, Centre Géologique et Géophysique, France
Woods Hole Oceanographic Institution, United States
Volume Title:Journal of Petrology
Source:Journal of Petrology, 35(3), p.753-785. Publisher: Clarendon Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:1994
Note:In English. 45 refs.; illus. incl. 7 tables, geol. sketch map
Summary:Mantle xenoliths from two locations in Mongolia contain patches of glass-phenocryst aggregates (melt pockets) up to 1 cm in diameter, including one composite xenolith which shows a complete transition from unaltered spinel lherzolite to a zone containing melt pockets surrounded by a clinopyroxene and spinel-free peridotite matrix. Petrographic and geochemical studies show that the melt pockets were formed neither by infiltration of the host basalt nor by dehydration melting of such hydrous phases as amphibole; instead, melting was induced by the interaction of metasomatic fluid with clinopyroxene and spinel. This reaction produced melts of varying composition, with SiO2 52-68 and MgO 4.5-0.5%. The glass and clinopyroxene phenocrysts are very rich in the LREE and Sr. The clinopyroxenes in the unaltered zones of the composite xenolith show evidence of an earlier phase of metasomatism which enriched Ce, La and Sr, but did not affect the other REE. Clinopyroxenes from these zones have high εNd values of +14 and +19, indicating a history of low Nd/Sm ratios; at the same time, 87Sr/86Sr ratios are high (≥ 0.704), indicating infiltration of relatively radiogenic Sr during the early stage of metasomatism. No zoning of La/Nd ratios was found, indicating that there was enough time to equilibrate the metasomatic effects in the grain interiors, perhaps ∼ 105 yr. In sharp contrast, the second, or main, metasomatic event that caused the formation of the melt pockets must have been extremely short-lived, probably lasting only hours or days before the xenolith was captured by the magma and erupted at the surface. The isotopic compositions of Sr and Nd are identical between melt pockets and host basalts, indicating that the metasomatic fluids and the basalts were derived from the same source. [R.A.H.]
Subjects:Aggregate; Chain silicates; Clinopyroxene; Electron probe data; Geochemistry; Hydrothermal alteration; Inclusions; Ion probe data; Major elements; Mantle; Mass spectra; Melts; Metals; Metasomatism; Pyroxene group; Rare earths; Silicates; Spectra; Trace elements; X-ray fluorescence spectra; Xenoliths; Asia; Far East; Mongolia
Abstract Numbers:95M/594
Record ID:1994056085
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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