REFLECTED LIGHT IMAGE ANALYSIS METHOD: APPLICATION TO MAGNETIC MINERAL SPO MEASUREMENTS

Accurate magnetic mineral shape preferred orientation (SPO) measurements, possibly identifying composite fabrics, can be critical to the interpretation of aeromagnetic maps and the anisotropy of magnetic susceptibility (AMS) in rocks. Traditional microstructural SPO analysis methods have to be modified since the main magnetic carriers are often optically opaque and volumetrically minor, and can be both clustered and spaced at the thin section scale, and fine grained. The image analysis method presented here solves these problems using reflected light photomicrographs from a standard petrographic microscope, and commonly used soft-/freeware to process the images and measure the abundance, grain size distribution, and SPO for grains as small as 10 microns. Phase(s) are classified by reflected light properties and checked by microprobe. Image mosaics proceed semi-automatically using the Adobe Photoshop® “Photomerge” tool, which are reduced by indexing into 5-10 colors and bicubic sharpener resampling. We have optimized settings for the Adobe Illustrator® “Live Trace” tool for reflected light images for initial automatic grain tracing. Trace editing, and classification of grain clusters and mineral phases are manual in conjunction with the microscope. Traces are imported into SPO2003 (Launeau and Robin, 2005) for SPO analysis. Starting with high resolution images at 4x magnification, this method identifies grains down to 10 microns. This method can be applied to a wide variety of rock types as reflected light image content is less variable than plane and polarized light images. This method has been tested on two-pyroxene granulites (~780°C and ~8kb) with almost identical bulk compositions from a ~1730 Ma shear zone with a well-developed and steeply-inclined foliation and lineation (Mt. Hay block of central Australia; Waters-Tormey et al., 2009). Prior work showed that the granulites' very different bulk susceptibilities and complex AMS fabrics cannot be explained entirely by variations in abundance of the magnetic minerals alone (magnetite ± ilmenite ± phyrrotite). Analysis of up to 48 photomicrographs per thin section (~2/3) resulted in up to 1500 magnetic mineral matrix grains. Results show that the major and minor magnetic phases in the same thin section have different SPO intensities and are oblique by up to 20°.