Crystal chemistry and evolution of the clinopyroxene in a suite of high pressure ultramafic nodules from the Newer Volcanics of Victoria, Australia

A clinopyroxene suite from lherzolite inclusions associated with the Victorian (Australia) “Newer Volcanics” has been investigated with the aim of understanding the clinopyroxene crystal-chemical response to increasing temperature (e.g. a melting model and/or crystallization processes prevailing at high pressure). The M1 clinopyroxene polyhedron dominates the intracrystalline physical-chemical variations, essentially given by the triple substitution AlVIFe M1 2 Ti4+⇌Cr3+ Fe3+Mg M1 2+ corresponding to an increase in the volume of M1 with increasing Mg/Mg+Fe2+ (mg) for the clinopyroxene. A relative Ca2+ increase in M2 ensures the necessary charge balance. However, Na+ occupancy of M2 persists to the highest mg values, i.e. maximum thermal stability, where the volume of M2 is the largest due to Fe M2 2+ depletion. The variations of M1 and M2 volumes are greater than, and opposite to, the variations in the volume of T (tetrahedron) by factors of ca. 3 and 1.5, respectively. Inclusions with relatively low clinopyroxene content (Mt. Porndon specimens) show distinct intracrystalline variations, essentially reflecting lower AlVI, i.e. higher volume of M1, and implying a lower pressure regime compared to clinopyroxene-rich analogues (Mt. Leura specimens). The intracrystalline relationships of the Mt. Porndon clinopyroxene suggest that the host peridotite inclusions survived larger degree of mantle melting at shallower depths relative to the Leura analogues.