Metasomatic processes in the lithospheric mantle beneath Grande Comore, West Indian Ocean, have been inferred through studies of ultrabasic xenoliths in the lavas of La Grille volcano. These consist of protogranular to variably porphyroclastic spinel lherzolites and wehrlites with widespread evidence of pyrometamorphic overprinting. The xenolith suite has been subdivided into three groups on the basis of different pyrometamorphic textures and mineral/glass compositions. Pyrometamorphic features consist of a) development of irregular fine-grained rims of clinopyroxene, and olivine (± glass and spinel) at the expense of primary orthopyroxene, b) elongated partly glassy patches containing secondary clinopyroxene, spinel, and olivine and c) "spongy" clinopyroxenes with recrystallized areas composed of clinopyroxene, olivine, spinel, together with glass. Mineral compositions indicate higher temperatures for the secondary than for the primary assemblage; e.g. the secondary olivines and spinels have, respectively, higher MgO, CaO and Cr2O3 than the primary crystals.
The glass compositions vary widely, from SiO2-oversaturated with up to 24% normative qz (related to orthopyroxene incongruent melting), to undersaturated, with up to 40% normative ne , spanning the whole compositional range described for other glass occurrences in xenoliths both from continental and oceanic mantle (Siena et al., 1991); Siena and Coltorti, 1993). Some compositions are peculiar being characterized by an extremely high Na2O contents (up to 13 %wt). However, within any one xenolith the glasses appear to be homogeneous suggesting high diffusion rates for several elements between percolating fluid or melt and host peridotite.
Ion-probe analyses for Rb, Ba, Sr, Nb, Zr Y, and REE on clinopyroxenes and glasses, together with major element (electron-probe) data and textures, suggest equilibrium between the glasses and clinopyroxenes and, in some cases, with the secondary spinels. Clinopyroxene and glass chondrite-normalized patterns show remarkable negative anomalies for Sr, Zr and Ti. These patterns and the calculated cpx-melt partition coefficients (closely corresponding to recent experimental data - Green et al., 1992; Sweeney et al., 1994; Sweeney et al., 1995) indicate equilibration with a carbonatitic melt. These lines of evidences, together with mass balance calculations, suggest that the observed secondary minerals and glasses were produced by reaction between restitic sub-Comorien peridotites and metasomatizing alkali-rich carbonatitic melts.
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