Assessing SPO techniques to constrain magma flow: Examples from sills of the Karoo Igneous Province, South Africa

•The process of obtaining photomicrographs for the SPO-2003 programme is scrutinised.•Photomicrographs are taken at magnifications defined using a grain size threshold.•Images should not be segmented in SPO-2003 to minimise the Global Compatibility Index.•SPO ellipsoids are used to infer magma flow directions in Karoo dolerite sills.•Foliation and lineation data define petrofabrics using the shape of the ellipsoid.

Shape ellipsoids that define the petrofabrics of plagioclase in Jurassic Karoo dolerite sills in KwaZulu-Natal, South Africa are rigorously constrained using the long axis lengths of plagioclase crystals and ellipse incompatibility. This has been undertaken in order to determine the most effective technique to determine petrofabrics when using the SPO-2003 programme (Launeau and Robin, 2005). The technique of segmenting an image for analysis is scrutinised and as a process is found redundant. A grain size threshold is defined to assist with the varying grain sizes observed within and between sills. Where grains exceed the 0.2 mm size threshold, images should be acquired at a high magnification (i.e., 10 × magnification). Petrofabrics are determined using the foliation and the lineation of the ellipsoid as defined by the maximum and minimum principal axes (respectively) of the resultant ellipsoid. Samples with strongly prolate fabrics are isolated allowing further constraint on the petrofabric to be made. Once the efficacy of the petrofabric determination process has been determined, the resultant foliations (and lineations) then elucidate the most accurate petrofabric attainable. The most accurate petrofabrics will be determined by using the correct magnification when the images are obtained and to run the analyses without segmenting the image. The fabrics of the upper and lower contacts of the Karoo dolerite sills are analysed in detail using these techniques and the fabrics are used as a proxy for magma flow.