Quantifying Carbonate and Serpentine Abundances through VSWIR Microspectroscopy

Visible and shortwave-infrared (VSWIR) reflectance spectroscopy has been used for large-scale mineral mapping on Earth and on Mars. A prototype instrument (UCIS—Ultra Compact Imaging Spectrometer), operating in microscopic mode, applies the same principles over an area the size of a traditional thin section (15-50 cm2), acquiring data over the 0.5-2.5 μm range at a spatial scale of 80 μm/pixel. This technique requires little to no sample preparation and is non-destructive, preserving rock texture. It can be used on future rovers/landers for in-situ petrology and in the lab to link spectral data acquired at large scale with rock mineralogy. We compare microscale mineral maps and abundance results from linear spectral unmixing to other techniques, including XRD, acid dissolution, and EDS/WDS mapping. Samples from the Semail Ophiolite (Oman) are used as an analogue for Martian carbonate and serpentine deposits to assess the capabilities of IR spectroscopy to discriminate carbonate from minerals with absorptions at similar wavelength positions, to determine carbonate composition and the composition of intermixed phases, and to determine their relative abundances. We find that UCIS infrared images can differentiate between carbonate phases not distinguished in XRD results. For example, in a magnesite vein sample, the bulk magnesite is spectrally distinct from another carbonate phase present only in cavities within the sample. Microprobe and EDS analyses of the sample confirm that calcite is present only as a coating within secondary porosity. Similar to SEM-EDS mapping at smaller scales, UCIS can identify rare phases contained within a few pixels (100s μm). For example, aluminum-bearing phyllosilicates in discrete clasts were found among more typical serpentine in a carbonate-cemented breccia. Signals from such rare phases are typically not detectable in XRD but are in spatially resolved microscale IR data. Collectively, data indicate that VSWIR microspectroscopy is an effective, novel way to study carbonate-bearing and other rocks accurately and rapidly and is a means of understanding the accuracy of remote sensing-derived mineralogies.