T. D. Shelfer

Mineralogy, composition, and alteration of Mars Pathfinder rocks and soils: Evidence from multispectral, elemental, and magnetic data on terrestrial analogue, SNC meteorite, and Pathfinder samples

Major element, multispectral, and magnetic properties data were obtained at Ares Vallis during the Mars Pathfinder mission. To understand the compositional, mineralogical, and process implications of these data, we obtained major element, mineralogical, and magnetic data for well-crystalline and nanophase ferric minerals, terrestrial analogue samples with known geologic context, and SNC meteorites. Analogue samples include unaltered, palagonitic, and sulfatetic tephra from Mauna Kea Volcano (hydrolytic and acid-sulfate alteration), steam vent material from Kilauea Volcano (hydrolytic alteration), and impactites from Meteor Crater (relithification). Salient results for Mars Pathfinder include: (1) Band depths BD530b and BD600 and the reflectivity ratio R800/R750 are consistent with the dominant ferric mineral being nanophase ferric oxide associated with an unknown amount of H2O and occurring in composite particles along with subordinate amounts of other ferric minerals. Hematite and hematite plus nanophase goethite are most consistent with the data, but maghemite, akaganeite, schwertmannite, and nanophase lepidocrocite are also possible interpretations. Ferric oxides that are consistently not favored by the data as sole alteration products are jarosites and well-crystalline goethite and lepidocrocite. (2) The strength of the ferric adsorption edge (R750/R445) implies the Fe3+/Fe2+ values for Pathfinder rock and soil are within the ranges 0.7–3 and 3–20, respectively. (3) Ferrous silicates are indicated for subsets of Pathfinder rocks and soils. One subset has a band minimum near 930 nm that can attributed to low-Ca pyroxene. Alternatively, the band could be a second manifestation of certain ferric oxides, including nanophase goethite, maghemite, akaganeite, and schwertmannite. Another subset has a negative spectral slope from ∼800 to 1005 nm which could result from the high-energy wing of a high-Ca pyroxene and/or olivine band, a mixture of bright and dark materials, and, for rocks, thin coatings of bright dust on dark rocks. (4) Chemical data on Pathfinder rocks and soils are consistent with two-component mixtures between an “andesitic” rock with low MgO and SO3 concentrations (soil-free rock) and a global, basaltic soil with high MgO and SO3 concentrations (rock-free soil). Pathfinder rock-free soil can be modeled as a chemical mixture of SNC meteorites and the Pathfinder soil-free rock. (5) Pathfinder soil cannot be obtained by chemical alteration of Pathfinder rocks by any of the hydrolytic and acid-sulfate alteration processes we studied. Presumably, global mixing has obscured and possibly erased the elemental signatures of chemical alteration. (6) The strongly magnetic phase in palagonitic and sulfatetic tephra is titanomagnetite and possibly its oxidation product titanomaghemite (Fe-Ti spinels). The saturation magnetization of the tephra samples (0.5–2.0 Am2/kg) is at or below the low end of the range inferred for Martian dust (4±2 Am2/kg), implying that lithogenic Fe-Ti spinels are a possible candidate for the Martian strongly magnetic phase. (7) The predominantly palagonitic spectral signature and magnetic nature of Martian soil and dust are consistent with glassy precursors with imbedded Fe-Ti spinel particles. Comparison with lunar glass production rates suggests that production of sufficient quantities of glassy materials on Mars by volcanic and impact processes is sufficient to account for these observations.

Phyllosilicate-poor palagonitic dust from Mauna Kea Volcano (Hawaii): A mineralogical analogue for magnetic Martian dust?

The mineralogical and elemental composition of dust size fractions (<2 and <5 μm) of eight samples of phyllosilicate-poor palagonitic tephra from the upper slopes of Mauna Kea Volcano (Hawaii) were studied by X-ray diffraction (XRD), X-ray fluorescence (XRF), visible and near-IR reflectance spectroscopy, Mossbauer spectroscopy, magnetic properties methods, and transmission electron microscopy (TEM). The palagonitic dust samples are spectral analogues of Martian bright regions at visible and near-IR wavelengths. The crystalline phases in the palagonitic dust are, in variable proportions, plagioclase feldspar, Ti-containing magnetite, minor pyroxene, and trace hematite. No basal reflections resulting from crystalline phyllosilicates were detected in XRD data. Weak, broad XRD peaks corresponding to X-ray amorphous phases (allophane, nanophase ferric oxide (possibly ferrihydrite), and, for two samples, hisingerite) were detected as oxidative alteration products of the glass; residual unaltered glass was also present. Mossbauer spectroscopy showed that the iron-bearing phases are nanophase ferric oxide, magnetite/titanomagnetite, hematite, and minor glass and ferrous silicates. Direct observation by TEM showed that the crystalline and X-ray amorphous phases observed by XRD and Mossbauer are normally present together in composite particles and not normally present as discrete single-phase particles. Ti-bearing magnetite occurs predominantly as 5–150 nm particles embedded in noncrystalline matrix material and most likely formed by crystallization from silicate liquids under conditions of rapid cooling during eruption and deposition of glassy tephra and prior to palagonitization of glass. Rare spheroidal halloysite was observed in the two samples that also had XRD evidence for hisingerite. The saturation magnetization Js and low-field magnetic susceptibility for bulk dust range from 0.19 to 0.68 Am2/kg and 3.4×10−6 to 15.5×10−6 m3/kg at 293 K, respectively. Simulation of the Mars Pathfinder Magnet Array (MA) experiment was performed on Mauna Kea Volcano in areas with phyllosilicate-poor palagonitic dust and with copies of the Pathfinder MA. On the basis of the magnetic properties of dust collected by all five MA magnets and the observation that the Pathfinder MAs collected dust on the four strongest magnets, the value for the saturation magnetization of Martian dust collected in the MA experiments is revised downward from 4±2 Am2/kg to 2.5±1.5 Am2/kg. The revised value corresponds to 2.7±1.6 wt % magnetite if the magnetic mineral is magnetite (using Js = 92 Am2/kg for pure magnetite, Fe3O4) or to 5.0±3.0 to 3.4±2.0 wt % maghemite if the magnetic mineral is pure maghemite (using Js = 50 to 74 Am2/kg for pure maghemite, γ-Fe2O3). Comparison of the magnetic properties of bulk Mauna Kea palagonitic dust to those for dust collected by MA magnets shows that the MA magnets extracted (culled) a subset (25–34 wt %) of composite magnetic particles from bulk dust. The extent of culling of Martian dust is not well constrained. Because the Mauna Kea palagonitic dust satisfies the essential constraints of the Pathfinder magnetic properties experiment (composite and magnetic particles capable of being collected by five MA magnets), a working hypothesis for the strongly magnetic mineral present in Martian dust and soil is magnetite (possibly Ti-bearing) formed by rapid crystallization from silicate liquids having volcanic and/or impact origins. Subsequent palagonitization of the glass produces the nanophase ferric oxide phases that dominate the spectral properties of Martian bright regions at visible and near-IR wavelengths. Magnetic and phyllosilicate-poor palagonitic dust from Mauna Kea Volcano is thus a spectral and magnetic analogue for magnetic Martian dust.

MOSSBAUER MINERALOGY ON THE MOON: THE LUNAR REGOLITH

A first-order requirement for spacecraft missions that land on solid planetary objects is instrumentation for mineralogical analyses. For purposes of providing diagnostic information about naturally-occurring materials, the element iron is particularly important because it is abundant and multivalent. Knowledge of the oxidation state of iron and its distribution among iron-bearing mineralogies tightly constrains the types of materials present and provides information about formation and modification (weathering) processes. Because Mössbauer spectroscopy is sensitive to both the valence of iron and its local chemical environment, the technique is unique in providing information about both the relative abundance of iron-bearing phases and oxidation state of the iron. The Mössbauer mineralogy of lunar regolith samples (primarily soils from the Apollo 16 and 17 missions to the Moon) were measured in the laboratory to demonstrate the strength of the technique for in-situ mineralogical exploration of the Moon. The regolith samples were modeled as mixtures of five iron-bearing phases: olivine, pyroxene, glass, ilmenite, and metal. Based on differences in relative proportions of iron associated with these phases, volcanic-ash regolith can be distinguished from impact-derived regolith, impact-derived soils of different geologic affinity (e.g., highlands and maria) can be distinguished on the basis of their constituent minerals, and soil maturity can be estimated. The total resonant absorption area of the Mössbauer spectrum can be used to estimate total FeO concentrations.

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.

Extraterrestrial Mössbauer spectrometry

We describe a combined backscatter Mössbauer spectrometer and X-ray fluorescence analyzer (BaMS/XRF) instrument suitable for planetary missions to the surfaces of Mars (MESUR Program), the Moon, asteroids, or other solid solar-system objects. The BaMS/XRF instrument is designed to be capable of concurrent analysis of a sample for its elemental abundances (XRF) and for the mineralogy of its iron-bearing phases (BaMS) without any sample preparation.