Varying behaviour of Li in metasomatised spinel peridotite xenoliths from western Victoria, Australia

Abstract

Lithium concentrations have been measured in minerals in a suite of spinel peridotite xenoliths from western Victoria, Australia. Carbonatite metasomatism has previously been documented in most of these samples, while others have evidence of interaction with a mafic silicate melt [Earth Planet. Sci. Lett. 107 (1991) 305; J. Petrol. 39 (1998) 1917]. In addition, all samples contain small, dispersed pockets of siliceous aluminous alkali-rich melt that frequently contain fine-grained phenocrysts. This melt is mostly of local derivation and developed very late, just prior to or during transport to the surface. Equilibrium partitioning of Li is observed between phenocryst minerals and between phenocrysts and melt. In contrast, an apparent disequilibrium Li distribution exists between the olivine and clinopyroxene affected by metasomatism. Olivine exhibits preferential Li enrichment compared to clinopyroxene in the carbonatite metasomatised xenoliths. Two samples with evidence of mafic silicate melt metasomatism contain preferentially enriched clinopyroxene. These results are in accordance with [Chem. Geol. 166 (2000) 47], who suggested that the interphase Li distribution can discriminate different metasomatic agents. An explanation for this varied behaviour could be a change in DLi (ol/cpx) due to a compositional effect in one or other phase, or both. The olivine that exhibits preferential Li enrichment is also P-rich, suggesting a coupled substitution involving P and Li. In contrast, there is an anti- correlation between P and Li in clinopyroxene, suggesting different substitution mechanisms for Li in olivine and clinopyroxene. Coupled Li-P substitution cannot be the only mechanism important for Li incorporation in olivine, however, and a combination of compositional parameters is expected to play a role in determining DLi (ol/cpx).