T172 When Water Meets Rock: Aqueous Alteration in the Solar System II

Monday, 20 October 2014
1:00 PM - 5:00 PM
301

MINERALOGICAL EVIDENCE FOR WATER-ROCK INTERACTIONS IN GALE CRATER, MARS: CHEMIN’S PERSPECTIVE

Paper number: T172-5
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

The Mars Science Laboratory (MSL) Rover, Curiosity, has delivered powders of three fine-grained sedimentary rocks to the CheMin XRD instrument. All contain minerals indicative of water-rock reactions, in addition to high-T basalt minerals and a significant proportion of X-ray amorphous material. The first two samples, Cumberland and John_Klein from the Sheepbed mudstone stratum, contain ~20% (weight) of phyllosilicate minerals, recognized by broad 001 diffraction peaks at ~1.3 nm and ~1.0 nm respectively and 02L diffraction bands. These peaks are most consistent with smectite clay minerals with different interlayer cations: Mg for Cumberland, and Na or K for John_Klein. The 02L band locations imply that they are trioctahedral. Detections of phyllosilicates in the central mound of Gale crater from VNIR orbtial reflectance spectra are inferred to represent the dioctahedral smectite nontronite; however, the reflectance spectrum of a terrestrial trioctahedral smectite is similar to that of nontronite, so the clay mineral(s) of the mound may include trioctahedral smectite.

The smectites in Cumberland and John_Klein are likely to be diagenetic because: (1) the samples are from the same stratum, only a few decimeters apart, implying that their difference reflect local processes; and (2) the mineral proportions of both rocks are similar to that of local windblown sand, except for containing: the smectite, more magnetite, and much less olivine. This relationship suggests that the mudstone, as deposited, was like the windblown sand, and that much of its original olivine reacted with water to form smectite + magnetite. Parallel with this reaction, dissolution of olivine can release Fe2+ ions, which (with a CO2 atmosphere) could provide energy for chemolithoautotrophic microbial life.

The third rock sample, Windjana, is a basaltic sandstone rich in K-feldspar. It contains small proportions of phyllosilicates, seen as a broad diffraction rise at ~1.3 nm, and a low sharper peak at ~1.0 nm. The rise is comparable to the diffraction pattern of Cumberland and can be interpreted as smectite; little more can be discerned because its 02L band is obscured by peaks from other minerals. The latter peak is consistent with illite or mica, which could form by alteration of K-feldspar.

Thanks to the whole CheMin and MSL teams!