Carbonate-Bearing Mantle Xenoliths in Alkali Basalts: Phase Relationships, Mineral Compositions and Implications for Mantle Carbonate Metasomatism

Ionov, Dmitri, Griffin, W.L. and O'Reilly, S.Y., GEMOC (ARC National Key Centre), School of Earth Sciences, Macquarie University, Sydney 2109, Australia

Malkovets, Vladimir, Institute of Mineralogy and Petrography, Novosibirsk 630090, Russia

This communication provides new data obtained on carbonate-bearing mantle peridotite xenoliths from basaltic volcanics in NW Spitsbergen using electron and proton microprobe and laser ablation ICP-MS and preliminary results for xenoliths from SW Siberia and SE Mongolia. In Spitsbergen xenoliths, the carbonates range from dolomite to Mg-bearing calcite and have high Mg# (0.90 to 0.99). They occur in coarse spinel lherzolites, most commonly around resorbed spinel, orthopyroxene and amphibole grains, and are accompanied by fine-grained (Al,Cr,Ti)-rich clinopyroxene and (Mg,Ca)-rich olivine. The carbonate-cpx-ol aggregates appear to have been produced by reaction of carbonate-rich melts with the host peridotites. The carbonates also occur as intergranular films or small pockets at triple junctions of silicate minerals that appear to be in textural and chemical equilibrium with host lherzolite. The mineral assemblages and chemical compositions of the Spitsbergen carbonate-bearing peridotites are consistent with experimental data on carbonate stability. Accessory apatite and amphibole occurring in some lherzolites appear to have formed before precipitation of the carbonates; their absense or presense may not be relevant as indicators of carbonate-related metasomatism. Patches and veins of quenched carbonate and silicate glasses are common in xenoliths from Spitsbergen and appear to be related to decompression melting of primary carbonates during the ascent of the xenoliths in their host magma.

Dolomite (Mg# ~0.8) has been recently found in garnet pyroxenite xenoliths from SW Siberia (Minusa); magnesian calcite (Mg# ³0.9) has been found in spinel lherzolites from the same site and from SE Mongolia (Dariganga). In both localities, the carbonates occur as interstitial grains texturally and chemically equilibrated with host rocks. Inclusions of pure calcite have been found in pyroxenes from metasomatised peridotite xenoliths from central Mongolia and southern Hamar-Daban (Baikal region, Siberia).

Relative enrichment in LREE and Sr over HREE and HFSE indicated by an earlier solution ICP-MS study appears to be characteristic of mantle carbonate-bearing rocks. In situ analyses by electron and proton microprobes yield very high contents of Sr in carbonates, clinopyroxene and accessory apatite; amphibole is low in Nb. Laser-ablation ICP-MS analyses provide direct data on the contents of a large number of trace elements in the carbonates and coexisting silicate minerals.

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