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Raman spectra were obtained for a series of clinopyroxenes, orthopyroxenes and inverted pigeonites from the Munni Munni layered maÐc-ultramaÐc intrusion in Western Australia. Their composition varies from to mg 48 where mg = mg number = atomic 100 Mg/(Mg + Fe) and Fe = total Fe. The characteristic Raman bands mg 87 , of pyroxene are mainly due to vibrations of the tetrahedra and occur in the 300-400, 650-700 and 980-1020 SiO 4 wavenumber regions of the spectra. Clinopyroxene shows only one intense Raman band in the 650-700 cm-1 region and is readily distinguished from orthorhombic pyroxene which has two intense bands in this region. This di †erence is probably due to the single symmetrically distinct tetrahedral site in the C2/c structure, whereas orthopyroxene has two symmetrically distinct tetrahedral chains. The pyroxenes showed systematic shifts in the wavenumber of the Raman bands with increasing mg number with the orthopyroxene spectra being the most sensitive to variations in mg number. The variations in wavenumber are believed to be related to the straightening of the tetrahedral chains and the corresponding changes in the SiÈO bond lengths and OÈSiÈO angles caused by substitution of Fe for Mg. These changes are greater in the orthorhombic Pbca structure, because the edge-sharing polyhedra enclosing the Fe2' and Mg2' cations form wide bands which are separated by voids or rifts parallel to the c-axis. In the monoclinic C2/c structure, however, these polyhedra are linked in a continuous layer which restricts structural adjustments of the polyhedra and the adjacent tetrahedral chains. Therefore, since Raman SiO 4 spectroscopy is sensitive to changes in crystal structure, it may be used to obtain a semi-quantitative chemical analysis of pyroxene when calibrated with a suitable solid solution series. The number of Raman bands observed and their width also gives an indication of cation order/disorder within the crystal. 1997Crown Copyright ( reserved.

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The Munni Munni Complex/T Na model age 2.85 Ga), located in the west Pilbara block k CHUR of Western Australia, is one of the best preserved layered intrusions in Australia. Exposed over an area of 4 X 9 km, it is composed of a lower 1,850-m-thick ultramafic zone and an overlying gabbroic zone which has a minimum thickness of 3,630 m. The ultramafic zone contains rhythmically layered dunRe, lherzolite, olivine websterRe, clinopyroxenite, and websterRe, with orthopyroxenite, notRe, chromitite, and platiniferous websterRe prominent near the top of the zone. The gabbroic zone consists of gabbronorite, anorthositic gabbro, and minor anorthosite which display a pronounced tho|eiitic fractionation trend. The order of appearance of cumulus mineral assemblages in the complex is olivine, olivine + clinopyroxene, clinopyroxene + olivine, clinopyroxene, clinopyroxene + orthopyroxene, orthopyroxene + chromite, and plagioclase + clinopyroxene + ?orthopyroxene. This sequence is at variance with major platinum-group element-bearing intrusions in which crystallization of orthopyroxene generally precedes that of clinopyroxene.Trace-element data, obtained on samples collected across the entire intrusion to investigate the effects of crystal fractionation and S evolution on the distribution of the platinum-group elements, show that in the sulfide-undersaturated ultramafic zone, Pt, Pd, Au, Cu, S, Se, Cs, Rb, St, and Zr behaved incompatibly and were concentrated in the melt during fractionation. The S content of the melt began to increase above the 700-m stratigraphic level of the ultramafic zone, but Pt, Pd, and Au contents increased above background levels of approximately 3 ppb to 3 ppm Pt q-Pd only with the attainment of sulfide saturation at approximately the 1,830-m stratigraphic level. The concentration trends of Zr, St, Cs, Rb, and Cu paralleled that of S, but Ir and Ni largely partitioned with early crystallizing olivine and decreased in concentration with increasing fractionation. In contrast to the ultramafic zone, Pt, Pd, It, and Au have depletion trends in the sulfide-saturated gabbroic zone. Hence, the evolution of S largely governed the behavior of the platinum-group elements during the fractionation of the Munni Munni magma(s).The platinum-group element mineralization occurs immediately below the ultramafic-gabbroic contact. It resulted from the combined magmatic processes of crystal fractionation (as evidenced by increasing Cu/(Cu q-Ni) ratios and incompatible element trends with stratigraphic height), and magma mixing. Two models are presented. In model 1, a hot, buoyant sulfidesaturated tholeiitic magma (containing 1,700-2,600 ppm S) rose through the density stratified platinum-group element-enriched, sulfide-undersaturated resident ultramaflc magma (containing 530 ppm S) until reaching its own density level near the top of the chamber, where it spread out laterally for a distance of at least 12 km. Due to crystallization of plagioclase and subsequent Fe-enrichment [of the melt], the density of the gabbroic melt i...