3D crystal size distributions: A case study on quantifying olivine populations in kimberlites

Abstract

New advances in textural analysis and imaging allow the possibility for the true distribution of mineral phases in a rock to be quantified to high resolution. Also we are able to explore the relations between 2D to 3D datasets more rigorously. In this study the application of 2D and 3D textural analysis to the quantification of olivine populations in kimberlites is investigated. Crystal size distribution studies (CSD) are carried out on two kimberlite samples: 1) one from the Venetia K2 kimberlite pipe, South Africa; and 2) one from the Dutoitspan kimberlite pipe, Kimberley, South Africa. A 3D X-ray CT study is undertaken to investigate the application of this developing technique in kimberlite research and to test the 2D and 3D data sets. CSD profiles show markedly curved upwards profiles synonymous with fragmented CSDs, supporting an initial crystal population which is modified by fragmentation of crystals on ascent. Raw plots of 2D area size frequency provide a powerful first estimate of the population 3D statistics in terms of volume size frequency, allowing a quick analysis of olivine phases during exploration which can be quality controlled using more detailed 3D studies where necessary. The 3D data set provides accurate analysis over a range of 6-similar to 0.5 mm, below which the separation of grains becomes very difficult. Additionally the 3D data provides valuable information on crystal shape which can further be used to distinguish different olivine populations. Both 2D and 3D textural studies using state of the art textural analysis techniques have been successfully tested on complex kimberite textures, and will prove useful techniques in the routine quantification of key indicator phases in kimberlite analysis.