SEM CHARACTERIZATION OF NATURALLY WEATHERED BASALT SURFACES FOR UNDERSTANDING PHOTOMETRY OF WEATHERING RINDS

Reflectance spectra collected from naturally weathered Columbia River Basalt (CRB) surfaces at varying geometries have shown variations in forward scattering and backscattering behavior (Hoza et al., 2019, LPSC, Abstract #2958). In this study, we investigate how differences in weathering contribute to such photometric variability in visible to near-infrared wavelengths. Weathering changes a rock’s chemistry, mineralogy, and texture. Focusing on the textural effects of weathering, we explore Scanning Electron Microscope (SEM) imaging techniques to characterize the naturally weathered surfaces of Columbia River Basalts.

Under low vacuum conditions, we acquired backscattered electron images of uncoated specimens. Through a combination of tilt series, shadow imaging, and image processing, we produced 2- and 3-D visualizations describing surface topography and morphology on the scale of 10’s to 100’s of micrometers. 3D imagery has revealed an abundance of planar surfaces, at times oriented to create box-like structure. Regions of low topography appear to be characterized by flat lying planar surfaces, highly fractured rubble, and voids, while regions of high topography are dominated by edges. From shadow imaging, we have identified textures on the planar surfaces that we describe as etched, flaked, and pitted. Preliminary results suggest that surface roughness at the scale of 100’s of micrometers is controlled by the orientation of the planar surfaces. Energy dispersive X-ray spectroscopy of these surfaces indicate that they may be sodium and calcium bearing feldspars. We interpret the abundance of these planar surfaces as a product of weathering. Of the mineral phases identified within the interior of CRB’s, feldspars are more stable and more resistant to weathering than their more mafic counterparts. The identified textures on the planar surfaces, on the scale of 10’s of micrometers, may be indicative of feldspar dissolution and interaction with biologic materials. Next steps include further textural and chemical characterization of the planar surfaces and an investigation into the frequency in which they appear on CRBs with backscattering behavior. Ultimately, we aim to connect the characteristics of the surface morphology to the photometric behavior of the complex, natural basalt surfaces.