Carbonate-bearing mantle peridotite xenoliths from Spitsbergen: phase relationships,mineral compositions and trace-element residence |
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| Authors: | D A Ionov Suzanne Y O’Reilly Yuri S Genshaft Maya G Kopylova |
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| Institution: | (1) Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, Sydney, N.S.W. 2109, Australia, AU;(2) Schmidt Institute of the Physics of the Earth, Gruzinskaya 10, Moscow 123810, Russia, RU |
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| Abstract: | Carbonates of mantle origin have been found in xenoliths from Quaternary basaltic volcanoes in NW Spitsbergen. The carbonates
range from dolomite to Mg-bearing calcite and have high Mg-numbers Mg/(Mg+Fe)=(0.92–0.99)]. In some samples they occur interstitially,
e.g. at triple junctions of silicate minerals and appear to be in textural and chemical equilibrium with host lherzolite.
Most commonly, however, the carbonates make up fine-grained aggregates together with (Ca,Mg)-rich olivine and (Al,Cr,Ti)-rich
clinopyroxene that typically replace spinel, amphibole, and orthopyroxene as well as primary clinopyroxene and olivine. Some
lherzolites contain amphibole and apatite that appear to have formed before precipitation of the carbonates. In situ analyses by proton microprobe show very high contents of Sr in the clinopyroxene, carbonates and apatite; the apatite is
also very rich in LREE, U, Th, Cl, Br. Disseminated amphibole in carbonate-bearing rocks is very poor in Nb and Zr, in contrast
to vein amphibole and mica from carbonate-free rocks that are rich in Nb and Zr. Overall, the Spitsbergen xenoliths provide
evidence both for the occurrence of primary carbonate in apparent equilibrium with the spinel lherzolites (regardless of the
nature of events that emplaced them) and for the formation of carbonate-bearing pockets consistent with metasomatism by carbonate
melts. Calcite and amorphous carbonate-rich materials occur in com- posite carbonate-fluid inclusions, veins and partial melting
zones that appear to be related to fluid action in the mantle, heating of the xenoliths during their entrainment in basaltic
magma, and to decompression melting of the carbonates. Magnesite is a product of secondary, post-eruption alteration of the
xenoliths.
Received: 6 October 1995/Accepted: 17 June 1996 |
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