Melting and Metasomatism in the Lithospheric Mantle Beneath SE Australia: Trace Element Studies by Laser Microprobe

Marc Norman and Suzanne O'Reilly

GEMOC, School of Earth Sciences, Macquarie University, North Ryde, NSW 2109 Australia

Trace element abundances in clinopyroxene from southeast Australian mantle xenoliths and megacryst suites have been determined by laser microprobe to investigate the formation and evolution of the continental lithosphere. Abundances of REE, Nb, Y, Zr, Hf, Th, U, Pb, Sr, Ga, Ti, Sc, V, Co, and Ni have been measured in individual mineral grains using a frequency-quadrupled Nd:YAG laser microprobe coupled to an ICPMS. Instrument response was calibrated using the NIST 610 glass standard, and each analysis was normalised using CaO as an internal standard. Four spinel lherzolites from Mt. Shadwell, Victoria, and megacryst suites from The Anakies, Riley's Peak, and Mt. Franklin were studied.

Clinopyroxenes in two of the Mt. Shadwell xenoliths are depleted in LREE, U, Th, Pb, Zr, and Nb relative to Y and the HREE. Primitive mantle normalised patterns of these samples have large negative Nb and Ti anomalies and flat HREE patterns. Assuming that subsolidus redistribution has not modified the pyroxene compositions, trace element characteristics of the melt calculated from published distribution coefficients are very similar to those of continental crust formed at convergent plate margins.

In contrast, clinopyroxenes from the other two lherzolites are enriched in absolute abundances of LREE, U, Th, Pb, and Nb, consistent with metasomatic enrichment within the continental lithosphere. Trace element patterns of these pyroxenes show depletions of Zr and Hf relative to Sm and Nd, as well as negative Nb and Ti anomalies. Enrichments of LREE relative to Zr and Hf have been found in other carbonated mantle xenolith suites (e.g, Spitsbergen, Tanzania), and suggest a carbonate-rich melt or fluid as the metasomatic agent responsible for trace element enrichment in the lithospheric mantle beneath Mt. Shadwell.

Pyroxene megacrysts from Tertiary alkali basalts have compositions that are distinct from the xenoliths in their lack of Nb and Ti anomalies and fractionated HREE patterns. Melt in equilibrium with these megacrysts would have been broadly similar to those of the alkali basalts themselves, which is consistent with previous suggestions that the megacrysts crystallised from earlier batches of plume-related magmatism during the same episode of basaltic volcanism.

These data suggest the following scenario for this region of the continental lithosphere. A primary melt extraction event occurred at relatively shallow levels in the upper mantle, possibly beneath a volcanic arc. This was followed by carbonate-rich metasomatism that may have been related to impingement of a plume at the base of the lithosphere and mobilisation of small degree partial melts or fluids into the lithospheric mantle. Plume-related magmatism followed, with the erupted lavas entraining earlier cumulate phases from similar magmas.