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Journal of Petrology | Volume 43 | Number 9 | Pages 1749-1777 | 2002
© Oxford University Press 2002
Metasomatism and Partial Melting in Upper-Mantle Peridotite Xenoliths from the Lashaine Volcano, Northern Tanzania
DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF EDINBURGH, EDINBURGH EH9 3JW, UK
A group of chrome-spinel peridotite upper-mantle xenoliths from the Lashaine volcano, northern Tanzania, differs from other xenoliths at this locality in containing glassy melt pockets. Modal, mineral chemical and isotopic evidence indicates that, before the melting that was coincident with the xenolith entrainment and eruption in the Pleistocene, the sub-Tanzanian mantle lithosphere had a complex history. A major element depletion at 
3·4 Ga gave rise to a high-olivine restite protolith, and this was followed by an episode of K, Fe, Ca, Ti and Rb metasomatism at 2·0 Ga (Metasomatic Event I) resulting in the formation of well-equilibrated Cr-diopside and phlogopite. A later episode of metasomatism (Metasomatic Event II), first reported here, is recognized by texturally and chemically non-equilibrated titanian phlogopite, Ti–Cr-diopside, enstatite–bronzite, ilmenite and rutile, interpreted as due to an influx of K, Fe, Ca, Ti, Nb and Ta. The recognition of two episodes of metasomatism reported here provides an explanation for previously recognized major differences in the isotopic chemistry of Lashaine peridotite diopsides. Immediately before entrainment and eruption in the Pleistocene, some of the peridotites underwent partial melting with the formation of melt pockets and veins containing vesiculated glass, olivine, diopside, phlogopite, spinel (sensu lato), calcite, apatite and zeolite. Compared with most previously reported mantle glasses, the Lashaine glasses are potassic, rather than sodic, contain relatively low amounts of SiO2, Al2O3 and total alkalis, and are not in equilibrium with peridotite olivine. The melting event was accompanied by an influx of K, Ti, Ca, Rb, Sr, Ba, Zr, the rare earth elements (REE), H2O and (inferred) CO2, which may be the culmination of Metasomatic Event II. Hence the melting is interpreted as being metasomatically triggered combined with decompression melting. The computed composition of one of the larger melt pockets is most similar to olivine lamproite. Ion microprobe analyses of glass, diopside and mica in two of the melt pockets show preferential partitioning of Rb, Zr, Nb, Ba, Pb and the light REE (LREE) into the glass with respect to clinopyroxene, but the reverse for Sr and Y, in both mica-bearing and mica-free parageneses. In a mica-bearing melt pocket, Rb and Ba partition preferentially into mica relative to both clinopyroxene and glass. Similar patterns of partitioning exist in a similar xenolith from Labait (another Tanzanian xenolith locality) except that Sr is highly concentrated in the glass; and the glass and mica contain high Ba. Comparison of the chemistry of the melt pockets in the xenoliths from Lashaine and other northern Tanzanian volcanoes (Labait and Olmani) indicates that the melting in each xenolith suite was accompanied by a distinctive metasomatic influx.
KEY WORDS: Tanzania; mantle xenoliths; metasomatism; melting; element partitioning
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