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Clay mineral formation under oxidized conditions and implications for paleoenvironments and organic preservation on Mars.

ABSTRACT: Clay mineral-bearing locations have been targeted for martian exploration as potentially habitable environments and as possible repositories for the preservation of organic matter. Although organic matter has been detected at Gale Crater, Mars, its concentrations are lower than expected from meteoritic and indigenous igneous and hydrothermal reduced carbon. We conducted synthesis experiments motivated by the hypothesis that some clay mineral formation may have occurred under oxidized conditions conducive to the destruction of organics. Previous work has suggested that anoxic and/or reducing conditions are needed to synthesize the Fe-rich clay mineral nontronite at low temperatures. In contrast, our experiments demonstrated the rapid formation of Fe-rich clay minerals of variable crystallinity from aqueous Fe3+ with small amounts of aqueous Mg2+. Our results suggest that Fe-rich clay minerals such as nontronite can form rapidly under oxidized conditions, which could help explain low concentrations of organics within some smectite-containing rocks or sediments on Mars.

SUBMITTER: Gainey SR

PROVIDER: S-EPMC5663933 | biostudies-literature | 2017 Nov

REPOSITORIES: biostudies-literature

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Clay mineral formation under oxidized conditions and implications for paleoenvironments and organic preservation on Mars.

Gainey Seth R SR Hausrath Elisabeth M EM Adcock Christopher T CT Tschauner Oliver O Hurowitz Joel A JA Ehlmann Bethany L BL Xiao Yuming Y Bartlett Courtney L CL

Nature communications 20171101 1

Clay mineral-bearing locations have been targeted for martian exploration as potentially habitable environments and as possible repositories for the preservation of organic matter. Although organic matter has been detected at Gale Crater, Mars, its concentrations are lower than expected from meteoritic and indigenous igneous and hydrothermal reduced carbon. We conducted synthesis experiments motivated by the hypothesis that some clay mineral formation may have occurred under oxidized conditions ...[more]

PMID: 29089493

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Project description:The emerging field of astropedology is the study of ancient soils on Earth and other planetary bodies. Examination of the complex factors that control the preservation of organic matter and other biosignatures in ancient soils is a high priority for current and future missions to Mars. Though previously defined by biological activity, an updated definition of soil as planetary surfaces altered in place by biological, chemical or physical processes was adopted in 2017 by the Soil Science Society of America in response to mounting evidence of pedogenic-like features on Mars. Ancient (4.1-3.7 billion year old [Byr]) phyllosilicate-rich surface environments on Mars show evidence of sustained subaerial weathering of sediments with liquid water at circumneutral pH, which is a soil-forming process. The accumulation of buried, fossilized soils, or paleosols, has been widely observed on Earth, and recent investigations suggest paleosol-like features may be widespread across the surface of Mars. However, the complex array of preservation and degradation factors controlling the fate of biosignatures in paleosols remains unexplored. This paper identifies the dominant factors contributing to the preservation and degradation of organic carbon in paleosols through the geological record on Earth, and offers suggestions for prioritizing locations for in situ biosignature detection and Mars Sample Return across a diverse array of potential paleosols and paleoenvironments of early Mars. A compilation of previously published data and original research spanning a diverse suite of paleosols from the Pleistocene (1 Myr) to the Archean (3.7 Byr) show that redox state is the predominant control for the organic matter content of paleosols. Most notably, the chemically reduced surface horizons (layers) of Archean (2.3 Byr) paleosols have organic matter concentrations ranging from 0.014-0.25%. However, clay mineralogy, amorphous phase abundance, diagenetic alteration and sulfur content are all significant factors that influence the preservation of organic carbon. The surface layers of paleosols that formed under chemically reducing conditions with high amounts of iron/magnesium smectites and amorphous colloids should be considered high priority locations for biosignature investigation within subaerial paleoenvironments on Mars.

Project description: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.

Project description:Green rusts (GR) are important drivers for trace metal and nutrient cycling in suboxic environments. We investigated whether green rusts would incorporate aluminum (Al) or other elements from naturally-formed clay minerals containing easily-weatherable clay minerals (e.g. mica, interlayered clays). We isolated the clay minerals from a Matapeake silt loam soil by removal of silt and sand, organic matter, and reducible oxides to study mechanisms of interaction between Fe(II) and soil-sourced clay minerals. We conducted batch Fe(II) sorption experiments at multiple near-neutral pHs (6.5-7.5) and reaction times (2 h-365 days). Mineral transformations were characterized by selective extractions, X-ray diffraction (XRD), and Fe X-ray absorption spectroscopy (XAS) analyzed by shell-fitting and linear combination fitting (LCF) with natural and synthetic standards. Clay mineral fraction contained a mixture of quartz, kaolinite, interlayered vermiculite, mica, and chlorite with significant structural Fe (2.6% wt). Uptake of Fe(II) increased with pH and kinetics were rapid until 5 days, followed by slow continuous Fe(II) uptake. Citrate-bicarbonate desorption kinetics from Fe(II) sorbed clay released more Al and silicon (Si) compared with unreacted soil clay fraction whereas magnesium (Mg) and potassium (K) were unaffected. Citrate-bicarbonate extracted Fe contained more Fe(II) than an ideal GR with an Fe(II)/Fe(III) molar ratio of 5.50. Analysis of the Fe EXAFS by both LCF and shell fitting was best modeled as a combination of Fe(III)-clay reduction to Fe(II) and precipitation of GR and Fe(II)-Al LDH. After 7 days of Fe(II) sorption, LCF identified 55.2% total Fe in clay, 33.4% GR(Cl) and 11.4% Fe(II)-Al LDH. These results provide novel evidence of Fe(II)-Al LDHs precipitating on naturally-formed soil clay minerals as a minor phase to GR. The geochemical implications are that GRs formed in soils and sediments should be considered to have Al and Si as well as Mg substitutions affecting their structure and reactivity.

Project description:Abstract A conspicuous biomorphic ovoid structure has been discovered in the Nakhla martian meteorite, made of nanocrystalline iron-rich saponitic clay and amorphous material. The ovoid is indigenous to Nakhla and occurs within a late-formed amorphous mesostasis region of rhyolitic composition that is interstitial to two clinopyroxene grains with Al-rich rims, and contains acicular apatite crystals, olivine, sulfides, Ti-rich magnetite, and a new mineral of the rhoenite group. To infer the origin of the ovoid, a large set of analytical tools was employed, including scanning electron microscopy and backscattered electron imaging, wavelength-dispersive X-ray analysis, X-ray mapping, Raman spectroscopy, time-of-flight secondary ion mass spectrometry analysis, high-resolution transmission electron microscope imaging, and atomic force microscope topographic mapping. The concentric wall of the ovoid surrounds an originally hollow volume and exhibits internal layering of contrasting nanotextures but uniform chemical composition, and likely inherited its overall shape from a preexisting vesicle in the mesostasis glass. A final fibrous layer of Fe-rich phases blankets the interior surfaces of the ovoid wall structure. There is evidence that the parent rock of Nakhla has undergone a shock event from a nearby bolide impact that melted the rims of pyroxene and the interstitial matter and initiated an igneous hydrothermal system of rapidly cooling fluids, which were progressively mixed with fluids from the melted permafrost. Sharp temperature gradients were responsible for the crystallization of Al-rich clinopyroxene rims, rhoenite, acicular apatites, and the quenching of the mesostasis glass and the vesicle. During the formation of the ovoid structure, episodic fluid infiltration events resulted in the precipitation of saponite rinds around the vesicle walls, altered pyrrhotite to marcasite, and then isolated the ovoid wall structure from the rest of the system by depositing a layer of iron oxides/hydroxides. Carbonates, halite, and sulfates were deposited last within interstitial spaces and along fractures. Among three plausible competing hypotheses here, this particular abiotic scenario is considered to be the most reasonable explanation for the formation of the ovoid structure in Nakhla, and although compelling evidence for a biotic origin is lacking, it is evident that the martian subsurface contains niche environments where life could develop.

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Clay mineral formation under oxidized conditions and implications for paleoenvironments and organic preservation on Mars.

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Clay mineral formation under oxidized conditions and implications for paleoenvironments and organic preservation on Mars.

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