CHARACTERIZING PALEOSOLS BY REFLECTANCE SPECTROSCOPY, RAMAN SPECTROSCOPY, AND X-RAY DIFFRACTION

Paleosols are prehistoric soils that have been formed by the interaction at the surface between the atmosphere and the hydrosphere. The search for paleosols is of particular interest as they can be used to reconstruct the evolution of the Earth. Precambrian paleosols are present near Flin Flon, Manitoba, Canada. Mineralogy and chemical composition suggest that the outcrop - a 1800 – 1900 million-year-old paleosol – is remarkably well preserved, with a meters-thick zone of alteration in a pillow basalt complex. Zones of alteration around the individual pillow basalts are visually evident in terms of tonal and textural variations. Fieldwork was conducted at this site to investigate how paleosols could be recognized in the field.

Five geological samples were collected from various transects of the outcrop and were characterized using X-ray Diffraction (XRD), iRaman, and a visible to near-infrared reflectance spectrometer. Samples were brought to the laboratory to analyze the samples as powders and as whole rocks, to search for spectral and compositional differences. The reflectance spectral analysis showed differences between whole rocks vs. powders, indicating slight changes of absorption features for whole rocks than for powders.

The XRD results detected slight trace mineral differences within the various transects of the outcrop (interior to exterior of individual pillows) for each sample. Some samples, however, from different transects, shared similar mineralogy, such as chlorites and silica, but also differed in mineralogy, such as micas, plagioclase feldspar and carbonates. In addition, reflectance spectra indicated measurable OH absorption bands near 1400 nm present in several samples, indicative of OH-bearing phyllosilicates. The reflectance spectra also suggests the presence of hydrated clay minerals with the presence of the metal-OH absorption band at 2200-2400 nm. The Raman spectra showed the presence of carbon (G-band) whereas XRD and reflectance did not. This study indicates that paleosols can be recognized by tonal and textural evidence that correlates with data from spectroscopic and XRD analysis, and that different analytical techniques provide different and complementary information.

This study was supported by grants and contracts from the CSA, the GSA, MRIF, CFI, UWinnipeg, and NSERC.