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 ÆT/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.