Which Clays are Really Present on Mars? or Are you sure about those squiggly lines? Ralph Milliken (JPL/Caltech) 50 m Clays in Shalbatana Vallis (HiRISE)

VIS-NIR reflectance spectra can be used to distinguish between major phyllosilicate groups: - kaolinite, serpentine (1:1, 7Å) - smectites, micas (2:1, 10Å) - chlorites (2:1+1, 14Å) However, there are some potential sources of confusion for distinguishing between different minerals within the groups.

Using CRISM and OMEGA, we should be able to distinguish kaolinite from dickite (high-temp polymorph)… but it may be difficult to tell the difference between halloysite and a mixutre of kaolinite + a hydrated mineral (e.g. zeolite). We can also tell the difference between Al-rich smectites and Mg/Fe-rich smectites… but it may be difficult to tell the difference between the various types of Mg/Fe smectites due to cation substitutions.

Nili Fossae Not all ‘smectite’ spectra look similar. Are the ‘smectite’ deposits actually smectites, or could they be mixed-layer smectite/chlorite?

Burial Diagenesis of Clays On Earth, burial can (and often does) lead to transitions in clay structures and compositions. Observations of smectite changing to illite or chlorite with depth on Mars can inform us about temperature and fluid chemistry. Smectite, mixed-layer S/C, & chlorite have been observed in CRISM data.

opaline silica Chlorite has been detected in the walls of V. Marineris, Nili, and throughout the southern highlands (e.g., Mustard et al., 2008). However, could some of these chlorite detections be confused with Fe-serpentines (e.g. greenalite)? Both chlorite and greenalite have broad features centered at wavelengths longer than 2.3 µm; presence of Al can cause additional features. We need to improve our spectral libraries for Fe-rich clays! CRISM Image West of Juventae What about serpentine?

Central mound of crater in southern highlands Smectite to Illite transition: Geothermometer for Martian Crust? Montmorillonite detections may have significant amounts of interlayered illite. To date, there have been very few detections of illite/muscovite on Mars. - chlorite is more dominant, likely related to low abundance of K + on Mars However, we need to search for possible smectite  illite transitions because illitization can be used as a geothermometer to constrain crustal heat flow.

VIS-NIR spectra can be used to distinguish between major phyllo groups: - smectite, mica/illite, chlorite, kaolinite, serpentine Potential sources of confusion: - saponite & nontronite with Fe +2,+3 -Mg substitutions (trioctahedral vs. dioctahedral) - illite & muscovite (but other micas are spectrally distinct) - physical mixtures of clays versus mixed-layered clays? (TBD) - sepiolite versus Mg-bearing smectites such as saponite Different clays have implications for environment and fluid chemistry, so we must be careful when assigning names and inferred chemistry to orbital clay detections.

Holden Ladon Margaritifer Argyre Uzboi Uzboi-Ladon-Margaritifer System Ritchey

olivine basalt Fe/Mg clay

V.E.=2 Clay signatures are strongest at the bottom of the stratigraphic section & spectrally similar to clays in the crater rim: source to sink V.E. = 2 clays (more H 2 O) clays (less H 2 O)

Ladon Basin: Stratigraphic variations in clay signatures

Physical versus Chemical Weathering Eslinger & Pevear (1988) Chlorite is concentrated at high latitudes where physical, not chemical, weathering is dominant (chlorite present in source rocks). Provided by Joel Hurowitz, JPL Data from the Mars landing sites indicates that there is minimal chemical segregation….evidence that physical weathering is dominant?

Clays are present in Hesperian-age deposits…..but are they authigenic or detrital? - We are looking at alteration products, not primary minerals. - Just because clays are found in Noachian aged units doesn’t mean that they formed in the Noachian. - Noachian crust is heavily cratered, fractured, and materials likely have high surface area to volume ratios, this will favor alteration, especially with low water-to-rock ratios. HiRISE color Clays Formed in the Hesperian?

So what do we know? OMEGA & CRISM have ‘definitively’ detected: nontronite (reducing conditions) montmorillonite chlorite (w/ Al) illite/muscovite kaolin mineral (kaolinite or ‘halloysite’) OMEGA & CRISM have also detected: Mg-clay: saponite, sepiolite, something else? Mixed-layer clays: smectite/chlorite? smectite/illite? Analcime (or some other zeolite?) Clays are most commonly found in the ancient cratered terrains (Noachian). Some clays have been transported by fluids and deposited as sedimentary rocks. Some clays are associated with Hesperian sulfates (V. Marineris, Gale): not acidic! Majority of clays are the Mg/Fe varieties, but many show evidence of Al substitution. ‘Chlorite’ much more common than illite: related to availability of K, Na, etc.?

How do we distinguish between formation and depositional environments? On Earth, the vast majority of clays in sediments are detrital. - plate tectonics, crustal recycling - average crustal composition is granitic - large fraction of clays are derived from pre-existing sedimentary rocks - contribution from soils and the role of organics - clays on Earth are primarily a story of erosion, transport, and (re)deposition. The same is likely true for Mars, with some important differences: - no plate tectonics or crustal recycling - average crustal composition is basaltic - hydrated minerals are commonly Fe/Mg varieties This is consistent with low water-to-rock ratios, but does it require this? - minimal leaching or continuous fluid flow (kaolinite, gibbsite not dominant) - chlorites at the surface suggest physical, not chemical, weathering has been dominant since their exposure (>3 Ga?) How do we reconcile the paucity of end-stage weathering products with the abundant geomorphic evidence for extensive water flow over the surface?

Formation of smectite requires a lot of silica. Clay deposits on Mars are often not associated with other hydrated minerals or alteration products (at least not that we see from orbit). However, dissolution of basalt and precipitation of smectites would result in a significant excess in cations….where is the complementary salt? Possibilities are OH -, Cl -, SO 3, SO 4, CO 3 Determining this phase is the key to understanding the atmospheric chemistry, oxidation state, and fluid chemistry on early Mars. Role of Excess Cations [Figure provided by Joel Hurowitz, JPL/Caltech]

Particle Size - does not have noticeable effect on position of specific bands - spectra of large particles may lose weaker bands at long wavelengths (illite vs mont.) - band strength is not necessarily directly comparable to clay abundance Loss of H 2 O and/or OH µm H 2 O band can disappear; reversible - H 2 O bands can shift during dehydration - can lose metal-OH bands; irreversible - heating can change structure

Al-smectite Fe/Mg-smectite Clays in the Noachian Crust CRISM spectra exhibit Al-OH, Mg/Fe-OH, and H 2 O absorption features,most consistent with smectites (montmorillonite, nontronite, saponite). Mapping the band depths of these features suggests they generally occur in separate locations, but a closer inspection of the spectra suggests some regions are mixtures of these clays. These deposits are mineralogically similar to those in Mawrth Vallis.