ANOMALOUS PHYLLOSILICATE-BEARING OUTCROP SOUTH OF COPRATES CHASMA: A STUDY OF POSSIBLE EMPLACEMENT MECHANISMS

High along the southern wall of Coprates Chasma, aluminum smectites are observed directly above a iron-magnesium smectite layer. These smectites were included as part of the “Plateau Phyllosilicates” formation by Le Deit et al [2012], which was proposed to be formed by pedogenesis, a process of weathering basaltic soils by continued exposure to meteoric water percolating down from the surface. However, a short distance south of Coprates Chasma there is an outcrop in which Al-smectites appear to be below, not above, Fe/Mg-smectites. Finding Al-phyllosilicates below Fe/Mg-phyllosilicates suggests that some process other than, or in addition to, pedogenesis must be at work.

HiRISE imagery shows the outcrop at 16°S, 305.6°E to be a low-albedo circular feature surrounded by a ring of high-albedo material. The circular shape suggests that it was an impact structure, but MOLA topography shows that it is now effectively flat. In THEMIS daytime IR data the entire circular feature appears dark, suggesting that it is indurated. CRISM hyperspectral targeted observations (20-40 m/pixel) indicate that the bright ring is Al-smectite, while the central material is Fe/Mg-smectite located below a stripped-back cap material. HiRISE observations show that the polygonally fractured Al-smectites are just below the Fe/Mg smectites, which have a sculpted appearance consistent with the cap rock having been stripped off.

One possible explanation for the observed sequence of phyllosilicates is that the overlying Fe/Mg-smectites represent a second pedogenic profile, forming in newer soils that were deposited over an older surface that had already undergone pedogenesis. In this scenario, two separate layers of Fe/Mg-smectites bracket the Al-smectite layer, but the underlying Fe/Mg-smectites are not exposed at this location.

A second hypothesis is that subsurface groundwater flowing through brecciated basaltic rock caused Fe/Mg-phyllosilicates to form in the crater rim. Later exposure to meteoric water enabled further weathering of the rim to Al-phyllosilicates while material filling the crater altered to Fe/Mg-phyllosilicates, leaving an Al-smectite ring surrounding a circular Fe/Mg-smectite deposit. Determining between these scenarios has broad implications toward our understanding of aqueous alteration on early Mars.