Crystal Chemistry and Intracrystalline Relationships of Orthopyroxene in a Suite of High Pressure Ultramafic Nodules from the ‘Newer Volcanics’ of Victoria, Australia

Gianmario Molin and Marilena Stimpfl
Dipartimento di Mineralogia e Petrologia, Universita' di Padova, Corso Garibaldi 37, 35100 Padova, Italy

Abstract: A suite of orthopyroxenes from spinel Iherzolite 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 AlVI rather than Fe2+. In addition, Fe3+, Cr3+ and Ti4+ are confined to M1 (Molin, 1989) and AlIV to TB. M2 is controlled by Fe M2 2+ ⇌ Mg M2 , constrained by (Fe2+ + 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 Fe2+ up to melting-point, causing non-ideality of this iron-depleted orthopyroxene. As a consequence, the investigated orthopyroxene can be considered an ultimate Fe2+ carrier during partial mantle melting.

Keywords: orthopyroxene • crystal chemistry • spinel Iherzolite

Mineralogical Magazine; June 1994 v. 58; no. 391; p. 325-332; DOI: 10.1180/minmag.1994.058.391.15
© 1994, The Mineralogical Society
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