The origin and implications of clay minerals from Yellowknife Bay, Gale crater, Mars

Abstract

The Mars Science Laboratory (MSL) rover Curiosity has documented a section of fluvio-lacustrine strata at Yellowknife Bay (YKB), an embayment on the floor of Gale crater, approximately 500 m east of the Bradbury landing site. X-ray diffraction (XRD) data and evolved gas analysis (EGA) data from the CheMin and SAM instruments show that two powdered mudstone samples (named John Klein and Cumberland) drilled from the Sheepbed member of this succession contain up to ~20 wt% clay minerals. A trioctahedral smectite, likely a ferrian saponite, is the only clay mineral phase detected in these samples. Smectites of the two samples exhibit different 001 spacing under the low partial pressures of H₂O inside the CheMin instrument (relative humidity <1%). Smectite interlayers in John Klein collapsed sometime between clay mineral formation and the time of analysis to a basal spacing of 10 Å, but largely remain open in the Cumberland sample with a basal spacing of ~13.2 Å. Partial intercalation of Cumberland smectites by metal-hydroxyl groups, a common process in certain pedogenic and lacustrine settings on Earth, is our favored explanation for these differences. The relatively low abundances of olivine and enriched levels of magnetite in the Sheepbed mudstone, when compared with regional basalt compositions derived from orbital data, suggest that clay minerals formed with magnetite in situ via aqueous alteration of olivine. Mass-balance calculations are permissive of such a reaction. Moreover, the Sheepbed mudstone mineral assemblage is consistent with minimal inputs of detrital clay minerals from the crater walls and rim. Early diagenetic fabrics suggest clay mineral formation prior to lithification. Thermodynamic modeling indicates that the production of authigenic magnetite and saponite at surficial temperatures requires a moderate supply of oxidants, allowing circum-neutral pH. The kinetics of olivine alteration suggest the presence of fluids for thousands to hundreds of thousands of years. Mineralogical evidence of the persistence of benign aqueous conditions at YKB for extended periods indicates a potentially habitable environment where life could establish itself. Mediated oxidation of Fe²⁺ in olivine to Fe³⁺ in magnetite, and perhaps in smectites provided a potential energy source for organisms.

Keywords: CheMin; Mars; XRD; Yellowknife Bay; clay minerals; habitability.

Figure 1

1D XRD patterns from John Klein and Cumberland converted from 2D patterns, with the d-spacing and phase assignments labeled for major peaks. Diffractogram data were previously presented in Vaniman et al. (2014).

Figure 2

Comparison of John Klein and Cumberland XRD patterns in the clay mineral 001 region. Labeled basal spacings have been corrected to for Lorentz polarization.

Figure 3

02l region of John Klein (a) and Cumberland (b) XRD patterns showing data collected (black dots), and the modeled pattern (solid black line) calculated using Rietveld refinement showing contributions from saponite, pigeonite, and plagioclase (see description in text). Refined b unit-cell parameter of saponite is also shown.

Figure 4

Relative humidity (black) and temperature data (red) from outside the rover (Planetary Data System Data Set ID for the REMS Instrument: MSL-M-REMS-5-MODRDR-V1.0). Data are averaged over 5 sol periods from sol 196–201 (a) and sol 485–490 (b) showing seasonal variability. RH inside CheMin, where temperatures are maintained between 5 to 20 °C remain <1%.

Figure 5

Evolved gas analysis of H₂O from a continuous ramp SAM pyrolysis of drill material from John Klein and Cumberland, compared with data from a ferrian saponite. The ferrian saponite tested is sample AMNH 89172 reported in Treiman et al. (2015). EGA data from the ferrian saponite were acquired under SAM-like conditions in the high-fidelity SAM Testbed laboratory instrument suite (McAdam et al. 2014, and in preparation). Because data from the main mass-to-charge ratio of H₂O (m/z 18, H₂¹⁶O) were saturated, counts from H₂O isotopologs and fragments were plotted to illustrate H₂O signal vs. temperature [m/z 20 (H₂¹⁸O) for John Klein and Cumberland and m/z 17 ×0.1 (OH fragment) for Fe-saponite].