Hydrated minerals in the deposits of Aureum Chaos

Abstract

Hydrated minerals have been detected in many martian chaos regions and chasmata, playing a major role in its past aqueous activity. Based on short wave infrared data from CRISM, imagery and elevation data, we identified and mapped hydrated minerals in Aureum Chaos to shed light on their stratigraphy and geological context.The Interior Layered Deposits (ILDs) display three stratigraphic units: The lowest unit shows massive and also layered, high-albedo monohydrated sulfate (MHS, best matching kieserite; 20–650m thick) with intercalated hydroxylated ferric sulfates (HFSs, best matching jarosite) and ferric oxides. The overlying polyhydrated sulfate (PHS) is commonly layered (20–40m thick), smooth to heavily fractured, of lower albedo and partially contains ferric oxides. Spectrally neutral, distinctly layered, and bumpy cap rock (40–300m thick) forms the top.We found spectral and morphological similarities to Aram Chaos (PHS, MHS, ferric oxides; texture of ILD and cap rock) and Juventae Chasma (HFS). Besides, the phyllosilicate nontronite was found attributed to chaotic terrain as light toned fractured exposure and within dark, smooth mantling. The coexistence of sulfates and phyllosilicates indicates changes in the geochemistry of the aqueous environment.Since sulfates and phyllosilicates could be alteration products, the observed mineralogy presumably is not the original; conversions between PHS and MHS, MHS or PHS into jarosite, jarosite into iron oxides are considered. Due to its occurrence along mantling edges and on flat surfaces of MHS without textural differences, it appears that PHS is an alteration product of MHS, e.g. due to surface exposure. The facies and relative timing of sulfate formation remains undefined. However, two different formation models are considered. The first implies contemporaneous ILD and PHS deposition and diagenetic sulfate conversion (into MHS, iron oxides) due to overburden later on. This model is less conclusive than groundwater evaporation -the second model- due to the lack of a sharp PHS–MHS boundary that would indicate a diagenetic formation.Alternatively, the second model suggests subsequent sulfate formation. Groundwater would have penetrated into pre-existing sulfate-free ILD. The permeability and porosity of ILD material would have defined the rate of water absorption and sulfate precipitation (low in cap rock?), resulting in cementation of probably aeolian deposited ILDs. We think this model is more consistent and could explain ILD stratigraphy with the potential anhydrous cap rock on top.The surface age of chaotic terrain (late Hesperian) and mantling deposits (mid to late Amazonian) limit the ILD age and possibly the emplacement of sulfates. Phyllosilicates in the mantling are presumably allochthonous. Limiting the timing of in situ phyllosilicates is more complicated; they could be Noachian (excavated material, following the phyllosian era), or instead syn- or post-chaotic. A close spatial and temporal association of sulfates and phyllosilicates, in which nontronite represents the deep facies, and sulfates the evaporitic facies is known from Earth and is also possible and would combine groundwater alteration with the observed mineralogy.The preservation of nontronite, HFS and MHS probably reflects a relatively dry environment with intermittent aqueous activity since their emplacement.