THE TRANSITION FROM NEUTRAL TO ACIDIC ALTERATION AT MAWRTH VALLIS AS OBSERVED FROM THE MINERALOGICAL RECORD

The Mawrth Vallis region on Mars hosts multiple widespread and diverse aqueous outcrops. We present here on the transition from the lower nontronite and Fe/Mg-smectite beds formed under neutral to slightly basic conditions to a zone of acid alteration. The transition material is characterized by a doublet feature in CRISM spectra at 2.21-2.23 and 2.26-2.28 µm and includes small patches of acid-altered smectite and jarosite. On top of this we observe a thin bed of montmorillonite and hydrated silica, followed by a unit containing poorly crystalline aluminosilicates at the top of the profile. Characterization of these units using HiRISE imagery shows different surface morphologies that directly correlate to the CRISM-derived compositional units.

The mineralogical record at Mawrth Vallis documents a region where water was present long-term and geochemical processes were active and changing over time. Acidic alteration of the Fe/Mg-smectite-rich rock appears to have taken place following formation of that unit and either prior to or coincident with deposition of jarosite, hydrated silica, montmorillonite and kaolinite. The patches of acid-altered smectite and jarosite are typically 100-500 meters across. Identification of these unique materials has benefitted from recent advances in processing of CRISM images as MTRDR files and in the creation of new spectral parameters. We have recently developed a method of overlaying 5 different geologic units identified using CRISM over newly generated HRSC stereo mosaics. This has enabled us to trace the stratigraphy of the acid alteration unit across broader areas and has revealed a similar geochemical history throughout the aqueous outcrops in the Mawrth Vallis region.