Abstract
Near-infrared remote sensing data of Mars have revealed thousands of ancient deposits of Fe/Mg-rich smectitic clay minerals within the crust with relevance to past habitability. Diagnostic metal–OH infrared spectroscopic absorptions used to interpret the mineralogy of these phyllosilicates occur at wavelengths of 2.27–2.32 μm, indicating variable Fe/Mg ratios in the clay structures. The objective of this work is to use these near infrared absorptions to constrain the mineralogy of smectites on Mars. Using Fe/Mg-rich seafloor clay minerals as mineralogical and spectroscopic analogs for Martian clay minerals, we show how crystal–chemical substitution and mixed layering affect the position of the diagnostic metal–OH spectral feature in smectitic clay minerals. Crystal-chemistry of smectites detected on Mars were quantitatively constrained with infrared data and categorized into four mineralogical groups. Possible alteration processes are constrained by comparisons of clay chemistry detected by remote sensing techniques to the chemistry of candidate protoliths. Of the four groups identified, three of them indicate significant segregation of Fe from Mg, suggestive of alteration under water-rich and/or oxidizing conditions on Mars. The fourth group (with low Fe/Mg ratios) may result from alteration in reducing or water-limited conditions, potentially in subsurface environments. Some samples are interstratified di–trioctahedral clay minerals that have characteristics of dioctahedral clay minerals but clear chemical evidence for trioctahedral sheets. Approximately 70% of smectite deposits previously detected on Mars are classified as Fe-rich (FeO/MgO > 10). Only 22% of detections are trioctahedral and relatively Mg-rich. An additional ∼8% are difficult to characterize, but might be very Fe-rich. The segregation of Fe from Mg in Martian clay minerals suggests that Mg should be enriched in other contemporaneous deposits such as chlorides and carbonates.
Highlights
Near-infrared remote sensing data of Mars collected by two instruments, OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) and CRISM (Compact Reconnaissance Imaging Spectrometer for Mars), have revealed thousands of detections of ancient Fe/Mg-rich smectite clay minerals within the Martian crust (Poulet et al, 2005; Murchie et al, 2009; Ehlmann et al, 2011; Carter et al, 2013)
The precise mineralogy of the Fe/Mg-rich smectites is not well understood, limiting the ability to connect these deposits to their protoliths through their geochemistry, or to understand the nature of aqueous processes from whence they formed on ancient Mars
The geographic origin, geologic context, mineralogy, and chemistry of most of the samples discussed here were reported previously (Cuadros et al, 2013). All of those samples were extensively characterized by X-ray diffraction (XRD), including quantitative modeling of interstratified clay minerals from XRD data, chemical analysis, thermal gravimetryevolved gas analysis (TG-EGA), Mössbauer spectroscopy, scanning electron microscopy (SEM), and transmission infrared spectroscopy
Summary
Near-infrared remote sensing data of Mars collected by two instruments, OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) and CRISM (Compact Reconnaissance Imaging Spectrometer for Mars), have revealed thousands of detections of ancient Fe/Mg-rich smectite clay minerals (in addition to other clay minerals) within the Martian crust (Poulet et al, 2005; Murchie et al, 2009; Ehlmann et al, 2011; Carter et al, 2013) These materials likely formed through aqueous chemical alteration of pyroxene, olivine, and mafic glass within volcanic and impact-fragmented materials (Christensen et al, 2001; Poulet et al, 2007). Michalski et al / Earth and Planetary Science Letters 427 (2015) 215–225
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