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A mechanism for chromite growth in ophiolite complexes: evidence from 3D high-resolution X-ray computed tomography images of chromite grains in Harold's Grave chromitite in the Shetland ophiolite.

Published online by Cambridge University Press:  28 February 2018

Hazel M. Prichard ,
Stephen J. Barnes  and
Belinda Godel
Hazel M. Prichard
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, Wales, UK
Stephen J. Barnes*
Affiliation:
CSIRO Mineral Resources, Kensington, WA, 6151, Australia
Belinda Godel
Affiliation:
CSIRO Mineral Resources, Kensington, WA, 6151, Australia
*
*E-mail: Steve.barnes@csiro.au
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Abstract

A fundamental difference exists between the textures of chromite crystals in chromitites in layered complexes and ophiolites. Those in layered complexes generally have euhedral octahedral shapes except where sintered, whereas those in ophiolites generally have rounded shapes accompanied commonly by nodular and more rarely dendritic chromite. Here we describe another texture characteristic of ophiolitic chromitite. The analysis of high-resolution X-ray computed tomography images of chromitite from Harold's Grave in the Shetland ophiolite has revealed 3D hopper structures on chromite grains. In 2D, these hopper structures appear at the surface of the chromite grain as stepped inward facing edges. A study of chromitites in 2D from ten ophiolite complexes has shown that all commonly contain chromite grains displaying these stepped edges. They occur mainly in protected enclaves surrounded by chromite grains that otherwise have rounded edges. The hopper crystals and the often associated clusters of inclusions represent periods of chromite crystal growth in a chromite supersaturated magma due to the presence of a more supercooled and more volatile-rich magma than that present in most layered complexes. Subsequent exposure of chromite crystals to chromite-undersaturated magma caused corrosion, resulting in the characteristic rounded shape of the ophiolitic chromite grains.

Keywords

podiform chromitehigh-resolution X-ray computed tomographyShetlandophiolite
Type
Article
Information
Mineralogical Magazine , Volume 82 , Special Issue 3: Special Issue dedicated to the work and memory of Professor Hazel M. Prichard (1954–2017) , June 2018 , pp. 457 - 470
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: Brian O'Driscoll

†

Deceased Jan 2017

This paper is published as part of a thematic set in memory of Professor Hazel M. Prichard

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