Wednesday 6th August

12 – 1 pm

Rm 312.222

Zoja Vukmanovic

Department of Applied Geology
Curtin University, Perth WA

Morphology And Microstructures Of Chromite Crystals From The Merensky Reef: Implications For Inclusion Entrapment Mechanism

Abstract

The Merensky Reef of the Bushveld Complex consists of two chromitite layers separated by coarse-grained melanorite. Microstructural analysis of the chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine grained idiomorphic and large silicate inclusion bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre-existing smaller idiomorphic crystals. Two possible mechanisms have been proposed for sintering of chromite crystals: 1) amalgamation of a cluster of grains with the same original crystallographic orientation; and 2) sintering of randomly orientated crystals followed by annealing into a single grain. The EBSD data show no evidence of recrystallization or clusters of similarly oriented grains among the idiomorphic population. An alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals, formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the Merensky Reef. The upper chromite layer formed from a subsequent magma influx, and hence lacked a mechanism to form dendritic chromite. This accounts for the difference between the two layers.

Biography

Zoja joined Curtin University as a Research Associate in October 2013. She is working on emplacement of the Ivrea Verbano ultramafic pipes in Italy as a part of Core to Crust Fluid System project. She completed her BSc at University of Turin, Italy and MSc at Free University, Amsterdam, the Netherlands. She received her PhD at the University of Western Australia in 2013.