CRYSTAL-CHEMISTRY AND INTRACRYSTALLINE RELATIONSHIPS OF ORTHO-PYROXENE IN A SUITE OF HIGH-PRESSURE ULTRAMAFIC NODULES FROM THE NEWER VOLCANICS OF VICTORIA, AUSTRALIA

A suite of orthopyroxenes from spinel lherzolite xenoliths associated with basanites occurring in the Victorian (Australia) post-Pliocene 'Newer Volcanics' province was investigated by means of a crystal chemical methodology which provides accurate site occupancy and site configuration parameters. The M1 configuration is essentially constrained by Al(IV) rather than Fe2+. In addition, Fe3+, Cr3+ and Ti4+ are confined to M1 (Molin, 1989) and Al(IV) to TB. M2 is controlled by Fe(M2)2+ half arrow right over half arrow left constrained by (Fe2+ + Ca)M2 > 0.14 atoms per formula unit (p.fu.). Cation substitution in TB and M2 constrains the sum of the volumes of the respective polyhedra V(TB) + V(M2) to remain essentially constant. Therefore, M2 favours the retention of the large Fe2+ up to melting-point, causing nonideality of this iron-depleted orthopyroxene. As a consequence, the investigated orthopyroxene can be considered an ultimate Fe2+ carrier during partial mantle melting.