GEMOC ARC National Key Centre

COEXISTING ANDESITIC AND CARBONATE MELTS IN A LHERZOLITE XENOLITH FROM MT. SHADWELL, VICTORIA

Marc D. Norman and Norman J. Pearson, GEMOC Macquarie

Metasomatism is a frequently invoked but poorly understood means for creating compositional diversity within the mantle. Metasomatic agents may include hydrous fluids, silicate melts, and carbonate-rich fluids and melts. We have discovered assemblages of coexisting carbonate melt and andesitic glass in a lherzolitic xenolith that provides a snapshot of carbonate-related metasomatism in the continental lithospheric mantle.

The xenolith is a fertile, protogranular spinel lherzolite. Silicate glasses and carbonate globules occur in melt pockets and veins. Phenocrysts of olivine, cpx and spinel, and minor sulfide globules are present in the glasses, which are intermediate in bulk composition with 55.5-60.5% SiO2, high Al2O3 (19-20.5%), Na2O (4.7-6.2%) and K2O (0.7-2.1%). The carbonate is calcic, ranging from nearly pure CaCO3 to ~10% MgCO3. It occurs as globules with curved boundaries against the glass and as the major phase filling cavities. Internally, most of the carbonate is homogeneous under BSE imaging, with a subtle striated appearance which we interpret as a primary quench texture. The bulk compositions of the glass and the carbonate are distinct from those of equilibrium immiscible liquids, but the form of these globules strongly suggests that they coexisted as poorly miscible melts prior to entrainment.

 The host lherzolite was reacting strongly with the melts. Spinels have spongy rims against the glass. Olivine and cpx phenocrysts have magnesian compositions comparable to those of the host lherzolite, but with distinctive minor element abundances. Olivine phenocrysts have CaO contents more typical of magmatic values (0.15-0.20% CaO vs. 0.05%). New cpx ranges to markedly higher Al2O3 (up to 11%) and TiO2 (up to 2.2% TiO2), and lower Na2O (0.3-1.5%) compared to the host lherzolite.

CaO contents of an olivine grain in the lherzolite adjacent to a large patch of carbonate show a clear trend of increasing Ca toward the contact, suggesting diffusion of Ca into the olivine. Modelling of the diffusion profile shows that the residence time of the carbonate was extremely short, on the order of days to tens of days, indicating a temporal link to the magmatism that transported the xenolith to the surface. Diffusion profiles were calculated using diffusion data of Jurewicz and Watson (1988) and the thermal evolution model of Lasaga (1983), with values for *T/t of 100-1000 °C/my and a grain size of 1 mm. The core of the olivine is assumed to represent the initial composition, which is justified by similar compositions in unzoned grains.

Trace element compositions of the andesitic glass as determined by laser ablation ICPMS preclude an origin as decompression breakdown of amphibole, and are more consistent with an origin as a small degree melt, perhaps induced by carbonate fluid fluxing of the lithosphere associated with the basaltic magmatism.