Three-dimensional cathodoluminescence imaging and electron backscatter diffraction: tools for studying the genetic nature of diamond inclusions |
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Authors: | D F Wiggers de Vries M R Drury D A M de Winter G P Bulanova D G Pearson G R Davies |
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Institution: | (1) Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands;(2) Department of Earth Sciences, Utrecht University, Postbus 80021, 3508 TA Utrecht, The Netherlands;(3) Biomolecular Imaging, Faculty of Sciences, Utrecht University, 3584 CH Utrecht, The Netherlands;(4) Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, UK;(5) Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, UK |
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Abstract: | As a step towards resolving the genesis of inclusions in diamonds, a new technique is presented. This technique combines cathodoluminescence
(CL) and electron backscatter diffraction (EBSD) using a focused ion beam–scanning electron microscope (FIB–SEM) instrument
with the aim of determining, in detail, the three-dimensional diamond zonation adjacent to a diamond inclusion. EBSD reveals
that mineral inclusions in a single diamond have similar crystallographic orientations to the host, within ±0.4°. The chromite
inclusions record a systematic change in Mg# and Cr# from core to the rim of the diamond that corresponds with a ~80°C decrease
of their formation temperature as established by zinc thermometry. A chromite inclusion, positioned adjacent to a boundary
between two major diamond growth zones, is multi-faceted with preferred octahedral and cubic faces. The chromite is surrounded
by a volume of non-luminescent diamond (CL halo) that partially obscures any diamond growth structures. The CL halo has apparent
crystallographic morphology with symmetrically oriented pointed features. The CL halo is enriched in ~200 ppm Cr and ~80 ppm
Fe and is interpreted to have a secondary origin as it overprints a major primary diamond growth structure. The diamond zonation
adjacent to the chromite is complex and records both syngenetic and protogenetic features based on current inclusion entrapment
models. In this specific case, a syngenetic origin is favoured with the complex form of the inclusion and growth layers indicating
changes of growth rates at the diamond–chromite interface. Combined EBSD and 3D-CL imaging appears an extremely useful tool
in resolving the ongoing discussion about the timing of inclusion growth and the significance of diamond inclusion studies. |
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