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Lab experiments on phyllosilicates and comparison

with CRISM data of Mars Mario Parente, Janice L. Bishop and Javier Cuadros

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Outline

• Observation 1: diversity in clay spectroscopy in the NIR not mirrored by equal diversity in visible range

• Hypothesis: dust coating and mixtures dust / clays • Observation 2: the ferrous slope of Mawrth Vallis• Hypothesis: hydrothermal alteration of glass into

smectite as a possible spectroscopic evidence for slope

• Comparison with CRISM• Conclusions

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Phyllosilicate diversity in VNIR

• In CRISM images, we detect different clays using Metal-OH bands near 2.2 µm (Al-smectite, blue) and 2.3 µm (Fe/Mg-smectite, red)

• In Mawth Vallis, phyllosilicates often accompanied by hydrated silica (opal and amorphous Al/SiOH) ~ green

• also detected using 2.2 µm band depth Ferrous phase + Fe/Mg-smectite ~ yellow

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Little spectral variation in the visible range

Lab spectra of montmorillonite and nontronite show differences in metal-OH band positions (2.2 vs 2.3 µm) + variation in the extended visible region due to Fe electronic transitions in the nontronite.

CRISM spectra of Mawrth Vallis (from McKeown et al 2008) show variation in metal-OH bands but almost identical spectra from 0.4 to 1.2 µm

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Little spectral variation in the visible range

• Further investigation showed subtle differences in the position of the peak near 0.7-0.8 µm

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Dust coating experiments• We simulated dust deposition by

sprinkling progressively increasing amounts of dust on top of nontronite and montmorillonite rocks using a spatula.

• We measured the spectra from 0.3 to 2.5 mm of the rock surfaces and the rocks with different amounts of both nanophase ferrihydrate dust and fine-grained Haleakala ash on a black Teflon dish using an ASD spectrometer under ambient conditions.

• In order to qualitatively assess the amount of dust present on the rocks, we captured images of the samples with a microscope at multiple magnifications after dust deposition

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Nontronite with Haleakala coating

• Number of stars represents level of dust.

• Effect on NIR region is minimal.• Significant and gradually increasing

attenuation of absorptions near 600 and 900 nm.

• Position of nontronite peak near 780 nm shifts only slightly.

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Nontronite with Ferrihydrate coating

• Effect on NIR region is minimal.

• Significant attenuation of absorption near 600 nm.

• Position of nontronite peak near 780 nm does not shift

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Montmorillonite with Haleakala coating

• Primary effect of Haleakala ash coating is darkening of the spectra.

• Haleakala coating produces change in slope between 500 and 700 nm and a slight shift of the montmorillonite peak around 800 nm

• Note- montmorillonite has strong bands due to adsorbed water.

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Montmorillonite with Ferrihydrate coating

• Dominance of ferrihydrite coating in visible region.

• Slight shift of montmorillonite peak towards longer WL

• Note- montmorillonite has strong bands due to adsorbed water.

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Comparing coatings with mixtures

• Fine-grained mixture of nontronite and ferrihydrate (prepared by Alicia Muirhead).

• In both coating and mixture we observe the dominating effect of the iron-oxide before 0.7 µm

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The ferrous component in spectra of Mawrth Vallis

• Ferrous slope observed at transition from nontronite to Al-clay/hydrated silica layers.

• Likely to be mica (could also be carbonate, sulfate, olivine).

• Extremely unusual on Earth without microbes.• On Mars implies active aqueous chemistry:

– Rapid input of Fe2+ into system that converted to mineral quickly before Fe could be oxidized.

– Closed system where Fe3+ in nontronite was reduced by unknown source at upper boundary.

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• Red is Fe/Mg smectite (Fe3+)

• Green is ferrous phase (Fe2+)

• Blue is Al-phyllosilicate and/or hydrated silica

The ferrous component in the spectra of Mawrth Vallis

• Fe/Mg-smectite found in phyllosilicate deposits at lower elevations.

• OH bands indicate FeMg-OH in octahedral layers.• Fe2+ slope typically observed at boundaries of

Fe/Mg-smectite deposit.

43EC

CRISM image 43EC draped over MOLA with 20X vertical enhancement

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Alteration of volcanic glass: spectral features

• De la Fuente, Cuadros et al., explored hydrothermal alteration as possible scenario for formation of smectites and preservation of ferrous features.

• The study used volcanic tuff samples primarily composed of rhyolitic glass and were exposed to hydrothermal alteration in the lab at temperatures ranging from 60 to 160 C for up to a year

• Most prominent effect of abundant smectite present in the matrix is a shift in Fe band position from 1.1 µm towards shorter WL.

• Ferrous slope in spectra of these sample is less than that observed at Mawrth Vallis; however, other ferrous-bearing glasses may have a stronger ferrous absorption.

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Alteration of volcanic glass: spectral features

•The alteration does not affect the spectrum before 0.7 µm

•The band center near 1.1µm shifts towards lower WL for more smectite formed

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Direct comparison with CRISM

• CRISM spectra have the band around 1.2 µm removed

• We compare with selected spectra from the studies described in the previous slides

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Direct comparison with CRISM (close-up)• Subtle changes observed in

extended visible region (max near 0.8 and min near 1 µm) for montmorillonite and nontronite rocks implying changes in Fe mineralogy with these clay minerals.

• The concave feature near 0.5 µm is better explainred by the presence of iron oxides

• The band minimum around 1.0 µm could be due to a mixture

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Conclusions

• Coatings: only a few grains of ferrihydrate or altered volcanic ash coating a rock surface are sufficient to alter the spectral properties of the rock in the visible region. At the same time these coatings have only a minor effect on the glass bands from 1.9 to 2.5.

• Mixtures: fine grains of nontronite mixed with ferrihydrite show only a hint of 0.6 µm nontronite shoulder with 10% ferrihydrite (work by REU intern Alicia

Muirhead).• Chemical alteration: hydrothermally altered ferrous

glass mixed with phyllosilicates in rocks could account for the presence of the ~ 1 μm band in the clay units at Mawrth Vallis

• There are possibly multiple causes of the spectral features observed