Howard V. Lauer

A simple inorganic process for formation of carbonates, magnetite, and sulfides in Martian meteorite ALH84001

Abstract We show experimental evidence that the zoned Mg-Fe-Ca carbonates, magnetite, and Fe sulfides in Martian meteorite ALH84001 may have formed by simple, inorganic processes. Chemically zoned carbonate globules and Fe sulfides were rapidly precipitated under low-temperature (150 °C), hydrothermal, and non-equilibrium conditions from multiple fluxes of Ca-Mg-Fe-CO2-S-H2O solutions that have different compositions. Chemically pure, single-domain, defect-free magnetite crystals were formed by subsequent decomposition of previously precipitated Fe-rich carbonates by brief heating to 470 °C. The sequence of hydrothermal precipitation of carbonates from flowing CO2-rich waters followed by a transient thermal event provides an inorganic explanation for the formation of the carbonate globules, magnetite, and Fe sulfides in ALH84001. In separate experiments, kinetically controlled 13C enrichment was observed in synthetic carbonates that is similar in magnitude to the 13C enrichment in ALH84001 carbonates.

Evidence for exclusively inorganic formation of magnetite in Martian meteorite ALH84001

Abstract Magnetite crystals produced by terrestrial magnetotactic bacterium MV-1 are elongated on a [111] crystallographic axis, in a so-called “truncated hexa-octahedral” shape. This morphology has been proposed to constitute a biomarker (i.e., formed only in biogenic processes). A subpopulation of magnetite crystals associated with carbonate globules in Martian meteorite ALH84001 is reported to have this morphology, and the observation has been taken as evidence for biological activity on Mars. In this study, we present evidence for the exclusively inorganic origin of [111]-elongated magnetite crystals in ALH84001. We report three-dimensional (3-D) morphologies for ~1000 magnetite crystals extracted from: (1) thermal decomposition products of Fe-rich carbonate produced by inorganic hydrothermal precipitation in laboratory experiments; (2) carbonate globules in Martian meteorite ALH84001; and (3) cells of magnetotactic bacterial strain MV-1. The 3-D morphologies were derived by fitting 3-D shape models to two-dimensional bright-field transmission-electron microscope (TEM) images obtained at a series of viewing angles. The view down the {110} axes closest to the [111] elongation axis of magnetite crystals ([111]⋅{110} ≠ 0) provides a 2-D projection that uniquely discriminates among the three [111]-elongated magnetite morphologies found in these samples: [111]-elongated truncated hexaoctahedron ([111]-THO), [111]-elongated cubo-octahedron ([111]-ECO), and [111]-elongated simple octahedron ([111]-ESO). All [111]-elongated morphologies are present in the three types of sample, but in different proportions. In the ALH84001 Martian meteorite and in our inorganic laboratory products, the most common [111]-elongated magnetite crystal morphology is [111]-ECO. In contrast, the most common morphology for magnetotactic bacterial strain MV-1 is [111]-THO. These results show that: (1) the morphology of [111]-elongated magnetite crystals associated with the carbonate globules in Martian meteorite ALH84001 is replicated by an inorganic process; and (2) the most common crystal morphology for biogenic (MV-1) magnetite is distinctly different from that in both ALH84001 and our inorganic laboratory products. Therefore, [111]-elongated magnetite crystals in ALH84001 do not constitute, as previously claimed, a “robust biosignature” and, in fact, an exclusively inorganic origin for the magnetite is fully consistent with our results. Furthermore, the inorganic synthesis method, i.e., the thermal decomposition of hydrothermally precipitated Fe-rich carbonate, is a process analogue for formation of the magnetite on Mars. Namely, precipitation of carbonate globules from carbonate-rich hydrothermal solutions followed at some later time by a thermal pulse, perhaps in association with meteoritic impact or volcanic processes on the Martian surface

Hematite, pyroxene, and phyllosilicates on Mars: Implications from oxidized impact melt rocks from Manicouagan Crater, Quebec, Canada

Visible and near-IR reflectivity, Mossbauer, and X ray diffraction data were obtained on powders of impact melt rock from the Manicouagan Impact Crater located in Quebec, Canada. The iron mineralogy is dominated by pyroxene for the least oxidized samples and by hematite for the most oxidized samples. Phyllosilicate (smectite) contents up to ∼15 wt % were found in some heavily oxidized samples. Nanophase hematite and/or paramagnetic ferric iron is observed in all samples. No hydrous ferric oxides (e.g., goethite, lepidocrocite, and ferrihydrite) were detected, which implies the alteration occurred above 250°C. Oxidative alteration is thought to have occurred predominantly during late-stage crystallization and subsolidus cooling of the impact melt by invasion of oxidizing vapors and/or solutions while the impact melt rocks were still hot. The near-IR band minimum correlated with the extent of aleration (Fe3+/Fetot) and ranged from ∼1000 nm (high-Ca pyroxene) to ∼850 nm (bulk, well-crystalline hematite) for least and most oxidized samples, respectively. Intermediate band positions (900–920 nm) are attributed to low-Ca pyroxene and/or a composite band from hematite-pyroxene assemblages. Manicouagan data are consistent with previous assignments of hematite and pyroxene to the ∼850 and ∼1000 nm bands observed in Martian reflectivity spectra. Manicouagan data also show that possible assignments for intermediate band positions (900–920 nm) in Martian spectra are pyroxene and/or hematite-pyroxene assemblages. By analogy with impact melt sheets and in agreement with observables for Mars, oxidative alteration of Martian impact melt sheets above 250°C and subsequent erosion could produce rocks and soils with variable proportions of hematite (both bulk and nanophase), pyroxene, and phyllosilicates as iron-bearing mineralogies. If this process is dominant, these phases on Mars were formed rapidly at relatively high temperatures on a sporadic basis throughout the history of the planet. The Manicouagan samples also show that this mineralogical diversity can be accomplished at constant chemical composition, which is also indicated for Mars from analyses of soil at the two Viking landing sites.

Mineralogy of three slightly palagonitized basaltic tephra samples from the summit of Mauna Kea, Hawaii

Certain palagonites from Hawaii are considered to be among the best analogs for Martian fines, based upon similar spectral properties. For this study, three distinctly colored layers were sampled from slightly palagonitized basaltic tephra just below the summit of Mauna Kea at 4145 m elevation. The mineralogy of size fractions of these samples was examined by diffuse reflectance (visible and near-IR) and far-IR spectroscopy, optical microscopy, X ray diffraction, Mossbauer spectroscopy, magnetic analysis, electron microprobe analysis (EMPA), and transmission and scanning electron microscopy. For the 20–1000 μm size fraction, sample HWMK11 (red) is essentially completely oxidized and has a hematite (Ti-hematite) pigment dispersed throughout the silicate matrix. The alteration is present throughout particle volumes, and only a trace amount of glass is present; no palagonitic rinds were detected. In addition to ferric Fe-Ti oxides, other phases detected were plagioclase feldspar and a trace of olivine. Sample HWMK12 (black) has the lowest proportion of ferric-bearing phases and is thus least weathered. It consists mostly of unaltered glass with embedded plagioclase and minor amounts of pyroxene, olivine, and Ti-magnetite. In some grains, a thin palagonitic rind is visible, indicating some surface alteration. The mineralogy for sample HWMK13 (yellow) is the same as that for HWMK12, except that it has distinct, well-developed palagonitic rinds consisting of erionite and smectite. For all samples, the amount of glass and plagioclase decreases and the amount of smectite increases with decreasing particle size for size fractions <20 μm. For HWMK11, the amount of hematite is essentially constant, and mica is present only in the coarse clay-sized fraction; smectites are low in structural Fe. For HWMK12 and HWMK13, the zeolite erionite is present along with smectites and nanophase ferric oxides (np-Ox). Erionite abundance decreases and np-Ox abundance increases with decreasing particle size. The smectite in both black and yellow samples contains some Fe3+ in octahedral layers. There were only two mineral phases containing iron in the fine clay fraction, namely, smectites and iron oxides. For HWMK11, relatively large iron oxide particles (0.1 to 0.4 μm) were dispersed on clay surfaces; for HWMK12 and HWMK13, much finer np-Ox particles were present in lesser concentrations. Formation of the zeolite erionite is consistent with the arid climate zone where these samples were collected. However, transient hydrothermal processes that occurred during the eruption of Mauna Kea volcano under its permanent ice cap during the Pleistocene may have resulted in minerals such as zeolites and smectites which may persist as relicts over a long period of time. Most of the iron released during weathering of basaltic tephra precipitated as poorly crystalline iron oxides and some of the Fe has substituted for the octahedral cations in the structure of authigenic smectites. The Ti-hematite in HWMK11, however, is the result of high-temperature oxidation of Ti-magnetite and exsolution from ironbearing silicate phases. Visible and near-IR reflectivity spectra for the <20 μm size fraction of HWMK11 is dominated by well-crystalline Ti-hematite. Corresponding spectra for HWMK12 and HWMK13, whose ferric mineralogy is dominated by np-Ox particles, are more similar to Martian bright-region spectra.

REFLECTIVITY (VISIBLE AND NEAR IR), MOSSBAUER, STATIC MAGNETIC, AND X-RAY DIFFRACTION PROPERTIES OF ALUMINUM-SUBSTITUTED HEMATITES

Because aluminum substitution for iron occurs in polymorphs of Fe2O3 and FeOOH in terrestrial (and by inference Martian) environments, it is important for the mineralogical remote sensing of both planets at visible and near-IR wavelengths to know the effects of Al substitution on their reflectivity spectra. Diffuse reflectivity (350–2200 nm), Mossbauer, static magnetic, and X ray diffraction data are reported for a series of aluminum-substituted hematites α-(Fe,Al)2O3 for compositions having values of Als. (mole ratio Al/(Al+Fe)) up to 0.61. Samples were prepared by oxidation of magnetite, dehydroxylation of goethite, and direct precipitation. Unit cell dimensions decrease with Als but at a rate less than that predicted by the Vegard rule. At 293 K, Mossbauer spectra are sextets (negative quadrupole splitting, QS) for Als up to ∼0.5 and doublets for larger Als. At 21 K, all compositions are sextets; however, there is a discontinuity of ∼0.5 T in the magnetic hyperfine field (Bhf) and QS changes sign at Als = 0.06(2) (Morin transition). Al-poor compositions have positive QS and higher Bhf. Negative and positive quadrupole splittings are indicative of the weakly ferromagnetic and antiferromagnetic states of hematites, respectively. The position of the least energetic crystal-field transition (6A1→4T1g) of ferric iron shifts to longer wavelengths with increasing Als. The magnitude of the shift is a linear function of (1/a)5, where a is the hexagonal unit cell dimension. For geologically reasonable amounts of Al substitution (Als < 0.33), the magnitude of the shift is small (∼20 nm), so that it is problematical (based on reflectivity data alone) to uniquely ascribe shifts in 4T1g band positions to different degrees of Al substitution. On the basis of Martian spectral data, the range in Als for Martian hematites is 0 < Als < 0.19.