MELTING AND METASOMATISM IN THE CONTINENTAL LITHOSPHERE - LASER-ABLATION ICPMS ANALYSIS OF MINERALS IN SPINEL LHERZOLITES FROM EASTERN AUSTRALIA

Olivine, low-Ca pyroxene, diopside, and spinel from a suite of protogr anular lherzolite xenoliths from southeastern Australia have been anal ysed for their major and trace element compositions using electron mic roprobe and laser ablation ICPMS. Bulk compositions of the lherzolites range from fertile (12-13% modal diopside) to depleted (2-3% modal di opside), with equilibration temperatures of 850-900 degrees C indicati ng entrainment of these lherzolites from relatively shallow depths (pr obably less than or equal to 35 km) within the lithosphere. Mineral co mpositions and abundances indicate a primary control by partial meltin g, with decreasing abundance of modal diopside accompanied by increasi ng Mg# of olivine and pyroxene, decreasing Al and Ti contents of diops ide, increasing Ni contents of olivine, and increasing Cr/Al of spinel . HREE, Y, and Ga in diopside also follow melting trends, decreasing i n concentration with increasing Mg#. In contrast, highly incompatible elements such as LREE, Nb, and Th reveal divergent behaviour that cann ot be ascribed entirely to partial melting. Diopsides from the fertile lherzolites have mantle-normalized patterns that are depleted in Th, Nb, and the LREE relative to Y and the HREE, whereas, diopsides from t he cpx-poor samples are strongly enriched in Th, Nb and the LREE, and have elevated Sm/Hf and Zr/Hf, and low Ti/Nb. All diopsides have stron gly negative Nb anomalies relative to Th and the LREE. Trace element p atterns of diopside in the fertile lherzolites can be reproduced by le ss than or equal to 5% batch melting of a primitive source. The negati ve Nb anomalies are a consequence of this melting, and do not require special conditions or tectonic environments. The low concentrations of Y and HREE in diopside from the cpx-poor lherzolites cannot be produc ed by realistic degrees of batch melting, but can be accomplished by u p to similar to 20% fractional melting, suggesting multiple episodes o f melt depletion. Os isotopic compositions of these lherzolites show t hat the melt depletion events occurred in the middle and late Proteroz oic, demonstrating the longterm stability of lithospheric mantle benea th regions of eastern Australia. The LREE-enriched diopsides are well equilibrated and record metasomatic enrichment events that pre-date th e magmatism that entrained these xenoliths. Trace element patterns of these pyroxenes suggest a carbonatitic melt as the metasomatic agent.