Crystallographic Orientations of Olivine Inclusions in Diamonds

Constraining the genetic relationships between diamonds and their mineral inclusions is critical to obtain a complete picture of diamond-growth environment and processes in the Earth's mantle. A common assumption is that most of the mineral inclusions are syngenetic with respect to their diamond hosts. One observation that led to this assumption is that a handful of olivine inclusions exhibited a preferred crystallographic orientation with respect to their diamond hosts. To provide a statistically significant dataset the determination of the olivine relative crystallographic orientations has to be performed with a rapid and non-invasive method, in order to not lose further useful information. This is now possible using an in-house single-crystal X-ray prototype diffractometer equipped with an area detector and a micro-source, which enables the measurement of small ( 30 µm) individual mineral inclusions in the same diamond. The measurement of inclusion orientations is performed with a 360° phi scan. The orientation matrices of the minerals, which give the relative orientation of the sample with respect to the holding mount, are determined by indexing the positions of the diffraction peaks from the inclusion and the diamond. Owing to the symmetry of both the olivine and the diamond, and consequent ambiguities in indexing the diffraction patterns, there are different but symmetrically equivalent possible descriptions of the orientation of the inclusion with respect to its host. If these ambiguities are not properly considered, the orientation distribution of the inclusions will look more random than it really is. We have used the newly developed OrientXplot software (Angel et al., 2015, J Appl Crystallogr, 48, 1330-1334) to remove these ambiguities by applying the known symmetry of the crystals to the measured orientation matrices. From these measurements it was observed that olivines did not show a statistical preferred orientation for any specific orientation. The most intriguing part was that some olivine multiple inclusions in the very same diamond showed similar orientation, which places significant constraints on the mode of growth of the diamonds. This work has been supported by the ERC Starting Grant (number 307322) to F. Nestola.