Crystal chemistry and intracrystalline relationships of orthopyroxene in a suite of high pressure ultramafic nodules from the ‘Newer Volcanics’ of Victoria, Australia

Abstract

A suite of orthopyroxenes from spinel lherzolite xenoliths associated with basauites occurring in the Victorian (Australia) post-Pliocene 'Newer Volcauics' province was investigated by means of a crystal chemical methodology which provides accurate site occupancy and site configuration parameters.The.4+M1 configuration is essentially constrained by A1 w rather than Fe 2 +. In additmn,' " Fe 3 +, Cr 3 + and TI are confined to M1 (Molin, 1989) and A1 Iv to TB. M2 is controlled by Fe~2 ~ Mgxt2, constrained by (Fe 2+ + Ca)M2 > 0.14 atoms per formula unit (p.f.u.). Cation substitution in TB and M2 constrains the sum of the volumes of the respective polyhedra VTB+ VM2 to remain essentially constant. Therefore, M2 favours the retention of the large Fe 2+ up to melting-point, causing nonideality of this iron-depleted orthopyroxene. As a consequence, the investigated orthopyroxene can be considered an ultimate Fe 2+ carrier during partial mantle melting.