GEMOC ARC National Key Centre

Norman, M.D. and Pearson, N.
GEMOC, Macquarie

Metasomatism is a frequently invoked but poorly understood means for creating compositional diversity within the mantle. We have discovered assemblages of coexisting carbonate melt and andesitic glass in a lherzolite xenolith that provides a snapshot of carbonate metasomatism associated with plume-type magmatism in the continental lithospheric mantle of eastern Australia. The xenolith is a fertile, protogranular spinel lherzolite from Mt. Shadwell, Victoria, with mineral compositions indicating temperatures of 850-900 oC. Trace element compositions of diopside show a smooth pattern that is slightly depleted in highly incompatible elements, similar to a fertile MORB source.

Silicate glasses and carbonate globules occur in melt pockets and veins. The orientations of these veins do not resemble in situ decompression breakdown of hydrous phases such as amphibole. The lherzolite was reacting strongly with the melts; diopside and spinel were the least stable phases. Carbonate occurs as globules with pillow forms that are deformed by phenocrysts, and as the major phase filling cavities. Internally, most of the carbonate has a subtle striated appearance that we interpret as a primary quench texture. These forms suggest that the carbonates were fluid at the time of entrainment, and are not rounded crystals of solid calcite. The carbonate is calcic, ranging from nearly pure CaCO3 to ~10% MgCO3. The compositions of the glass and the carbonate are distinct from those of experimentally produced equilibrium immiscible liquids, but the form of these carbonate globules strongly suggests that they coexisted with the silicate melt as poorly miscible phases in the mantle prior to entrainment.

The glasses are andesitic with 55.5-60.5% SiO2, 19-20.5% Al2O3, and 5.4-8.3% Na2O + K2O. The most mafic glasses are olivine normative, but the majority are quartz normative. They would be classified as trachyandesites from total alkalies vs. SiO2, but their low Fe and high Ti contents distinguish them from arc andesites. Euhedral olivine and cpx phenocrysts in the glass have Mg# similar to the host lherzolite but distinctive minor element abundances. Olivine phenocrysts have CaO contents typical of magmatic values (0.15-0.20%) compared to the host lherzolite (0.05%). Cpx phenocrysts range 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 in this assemblage was on the order of days to tens of days, indicating a temporal link to the magmatism that transported the xenolith to the surface.

The high CaO, high Ca/Al, and relatively low total alkali element contents of these glasses are distinct from those of other glasses in lherzolite xenoliths. Trace element compositions of the Mt. Shadwell glasses are enriched in LREE, Nb, Th, and U, in contrast to the depleted patterns in the host diopside. The carbonate is enriched in Sr and Ba but otherwise poor in trace elements. The compositions of these glasses preclude an origin by decompression breakdown of amphibole, and their low alkali element contents argue against interaction with alkali-rich metasomatic fluids. The distinctive compositions of these andesitic glasses suggests that they may have formed as small degree partial melts of anhydrous lithospheric mantle, possibly in response to carbonate fluid fluxing associated with the plume-type basaltic magmatism that brought the xenoliths to the surface.