Three-dimensional cathodoluminescence imaging and electron backscatter diffraction: tools for studying the genetic nature of diamond inclusions

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 beamscanning 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.

Archaean and Proterozoic diamond growth from contrasting styles of large-scale magmatism

“…Moreover, epitaxial relationships between inclusions and diamond hosts have also been thought as strong evidence of the control by diamond on the crystallisation of the inclusion 18 – 21 . Recent studies from Siberian diamonds reported that epitaxy played both a strong 22 , 23 and limited control 24 on inclusion formation. Theoretical considerations of adhesion energy and interface energy, however, suggest that there is no energetic benefit for olivine-diamond interfaces to form an epitaxial relationship 25 .…”

FIB-SEM cathodoluminescence tomography: practical and theoretical considerations

“…These changes occur near crystal corners. Some additional applications of FIB‐SEM CL tomography to the origin of inclusions in natural diamonds are presented by Wiggers de Vries et al . (2011).…”

“…To our knowledge the combination of FIB tomography and CL imaging is yet unexplored. This paper addresses the technical details of the 3D CL technique that we have recently used to obtain 3D images of mineral inclusions and growth zoning in a natural diamond (Wiggers de Vries et al ., 2011). A number of applications are found for the FIB, which relate to luminescence properties of materials.…”

Diamonds and the Geology of Mantle Carbon

“…The observation that in many cases diamond growth zones, as revealed by cathodoluminescence studies, do not wrap around the inclusions is consistent with, although it does not prove, syngenesis (Bulanova 1995). An epitaxial relationship between an inclusion and its host would represent a more robust proof of syngenicity (e.g., Futergendler and Frank-Kamenetsky 1961;Harris 1968;Harris and Gurney 1979;Wiggers de Vries et al 2011). Although some apparently recurrent crystallographic orientations with potential epitaxial significance have been found for some inclusions, such orientations are rarely determined and a systematic survey for the different mineral species is lacking or has been restricted to limited sets of samples (see review in Harris and Gurney 1979).…”