SPECTRAL, MINERALOGICAL, AND TEXTURAL CHANGES ASSOCIATED WITH OLIVINE ALTERATION IN NATURALLY WEATHERED DUNITE

Olivine is highly susceptible to aqueous alteration but is widely observed in spectra of the Martian surface. It is important to understand whether alteration prohibits detection of olivine in spectral data, and conversely, if the olivine detected on Mars could be altered and to what extent. We examined naturally weathered dunite stream cobbles from the Twin Sisters, Washington. The dunite is comprised mostly of olivine, with minor chromite and accessory phases, effectively isolating the effects of weathering on olivine. We measured visible to near-infrared (VNIR) spectra of dunite specimens and their weathering rinds, and examined those with optical microscopy, scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS), and X-ray diffraction (XRD) to assess the physical and mineralogical effects of weathering. Compared to the unaltered rocks, VNIR spectra of weathering rinds were redder, more reflective, and displayed a ferrous iron absorption feature that was narrowed and better defined. The pale green dunite has an orange-brown weathering rind that optical microscopy showed results from a ferric iron phase deposited in fractures near the rock exterior. SEM imaging showed small fractures in the weathering rind and EDS microanalysis revealed small but significant amounts of Al in rind fractures. EDS also showed elevated Al and Si content on rock surfaces, where patchy smooth-textured regions showed characteristics of deposited alteration materials. XRD analysis of the bulk rock and weathering rind were similar and dominated by forsterite peaks. XRD of clay-sized material from the weathering rinds showed no evidence of clay minerals. The VNIR results of brightening and ferrous absorption changes in the weathering rind are consistent with enhanced fracturing or fissuring seen in the weathering rind, which decreased the effective particle size. The observed reddening is due to deposition of ferric alteration phases in those fractures, which also have enhanced Al and Si content. Our results show that weathering of dunite in a temperate stream environment left rock surfaces with VNIR-detectable olivine. This indicates that olivine on Mars could have had significant contact with water while not degrading to the point that it could not be remotely detected.