Alian Wang

An integrated view of the chemistry and mineralogy of martian soils

The mineralogical and elemental compositions of the martian soil are indicators of chemical and physical weathering processes. Using data from the Mars Exploration Rovers, we show that bright dust deposits on opposite sides of the planet are part of a global unit and not dominated by the composition of local rocks. Dark soil deposits at both sites have similar basaltic mineralogies, and could reflect either a global component or the general similarity in the compositions of the rocks from which they were derived. Increased levels of bromine are consistent with mobilization of soluble salts by thin films of liquid water, but the presence of olivine in analysed soil samples indicates that the extent of aqueous alteration of soils has been limited. Nickel abundances are enhanced at the immediate surface and indicate that the upper few millimetres of soil could contain up to one per cent meteoritic material.

Characterization and petrologic interpretation of olivine‐rich basalts at Gusev Crater, Mars

Additional co-authors: PR Christensen, BC Clark, JA Crisp, DJ DesMarais, T Economou, JD Farmer, W Farrand, A Ghosh, M Golombek, S Gorevan, R Greeley, VE Hamilton, JR Johnson, BL Joliff, G Klingelhofer, AT Knudson, S McLennan, D Ming, JE Moersch, R Rieder, SW Ruff, PA de Souza Jr, SW Squyres, H Wnke, A Wang, A Yen, J Zipfel

Water alteration of rocks and soils on Mars at the Spirit rover site in Gusev crater.

Gusev crater was selected as the landing site for the Spirit rover because of the possibility that it once held a lake. Thus one of the rovers tasks was to search for evidence of lake sediments. However, the plains at the landing site were found to be covered by a regolith composed of olivine-rich basaltic rock and windblown ‘global’ dust. The analyses of three rock interiors exposed by the rock abrasion tool showed that they are similar to one another, consistent with having originated from a common lava flow. Here we report the investigation of soils, rock coatings and rock interiors by the Spirit rover from sol (martian day) 1 to sol 156, from its landing site to the base of the Columbia hills. The physical and chemical characteristics of the materials analysed provide evidence for limited but unequivocal interaction between water and the volcanic rocks of the Gusev plains. This evidence includes the softness of rock interiors that contain anomalously high concentrations of sulphur, chlorine and bromine relative to terrestrial basalts and martian meteorites; sulphur, chlorine and ferric iron enrichments in multilayer coatings on the light-toned rock Mazatzal; high bromine concentration in filled vugs and veins within the plains basalts; positive correlations between magnesium, sulphur and other salt components in trench soils; and decoupling of sulphur, chlorine and bromine concentrations in trench soils compared to Gusev surface soils, indicating chemical mobility and separation.

Sulfate deposition in subsurface regolith in Gusev crater, Mars

Excavating into the shallow Martian subsurface has the potential to expose stratigraphic layers and mature regolith, which may hold a record of more ancient aqueous interactions than those expected under current Martian surface conditions. During the Spirit rovers exploration of Gusev crater, rover wheels were used to dig three trenches into the subsurface regolith down to 6-11 cm depth: Road Cut, the Big Hole, and The Boroughs. A high oxidation state of Fe and high concentrations of Mg, S, Cl, and Br were found in the subsurface regolith within the two trenches on the plains, between the Bonneville crater and the foot of Columbia Hills. Data analyses on the basis of geochemistry and mineralogy observations suggest the deposition of sulfate minerals within the subsurface regolith, mainly Mg-sulfates accompanied by minor Ca-sulfates and perhaps Fe-sulfates. An increase of Fe2O3, an excess of SiO2, and a minor decrease in the olivine proportion relative to surface materials are also inferred. Three hypotheses are proposed to explain the geochemical trends observed in trenches: (1) multiple episodes of acidic fluid infiltration, accompanied by in situ interaction with igneous minerals and salt deposition; (2) an open hydrologic system characterized by ion transportation in the fluid, subsequent evaporation of the fluid, and salt deposition; and (3) emplacement and mixing of impact ejecta of variable composition. While all three may have plausibly contributed to the current state of the subsurface regolith, the geochemical data are most consistent with ion transportation by fluids and salt deposition as a result of open-system hydrologic behavior. Although sulfates make up >20 wt.% of the regolith in the wall of The Boroughs trench, a higher hydrated sulfate than kieserite within The Boroughs or a greater abundance of sulfates elsewhere than is seen in The Boroughs wall regolith would be needed to hold the structural water indicated by the water-equivalent hydrogen concentration observed by the Gamma-Ray Spectrometer on Odyssey in the Gusev region. Copyright 2006 by the American Geophysical Union.

Raman spectroscopy of Fe-Ti-Cr-oxides, case study: Martian meteorite EETA79001

Abstract Raman spectral features of chromite, ulvöspinel, magnetite, ilmenite, hematite, and some of their solid solutions are presented. Although most Fe-Ti-Cr-oxides produce relatively weak Raman signals compared to oxyanionic minerals, sufficient information can be extracted from their spectra to identify the end-member mineral phases as well as some information about compositional variations in solid solutions. Correlations between Raman spectral features and mineral chemistry are used to interpret the Raman data of Fe-Ti-Cr oxides found during Raman point-count measurements on rock chips of Martian meteorite EETA79001, as an analog to Mars on-surface planetary investigations. In general, ulvöspinel, magnetite, and chromite end-members are readily distinguished by their Raman spectral patterns, as are ilmenite and hematite. In the low signal-to-noise (S/N) spectra generally obtained from the Raman point-count procedure, the position and shape of the strongest peak of Fe-Ti-Cr oxides in the region 660-680 cm-1 (A1g mode) is the most useful for discriminating Fe3+-Ti-Cr-Al substitutions in the magnetite-ulvöspinel, ulvöspinel-chromite, and chromite-spinel series, but minor peaks in the range 300-600 cm-1 also assist in discrimination. These spectral features are useful for investigating the variability among Fe-Ti-Cr-Al oxide solid solutions in natural samples. In EETA79001, a Martian basaltic meteorite, most of the oxide grains (as measured with the electron microprobe) are ulvöspinel, chromian ulvöspinel, and chromite, but ilmenite, titanian chromite, and titanomagnetite are also observed. The Fe-Ti-Cr-oxides identified by Raman point-count include end-member ilmenite, low-Al chromite-spinel solid solutions, ulvöspinel-magnetite solid solutions, and more complex chromitespinel- ulvöspinel-magnetite solid solutions; the latter exhibit a wide range of main peak positions and broadened peak widths that may reflect structural disorder as well as specific cation contents. One Raman spectrum suggests end-member magnetite, and one spectrum from a different rock chip appears to be that of non-terrestrial hematite, reflecting local oxidizing alteration, which has not been observed previously in this meteorite. These results show that analyses done in an automated mode on the surface of an unprepared Martian rock sample can provide useful constraints on the Fe-Ti-Cr oxide mineralogy present and on compositional variations within those minerals, including an indication of oxygen fugacity

Raman spectroscopy for mineral identification and quantification for in situ planetary surface analysis: A point count method

Quantification of mineral proportions in rocks and soils by Raman spectroscopy on a planetary surface is best done by taking many narrow-beam spectra from different locations on the rock or soil, with each spectrum yielding peaks from only one or two minerals. The proportion of each mineral in the rock or soil can then be determined from the fraction of the spectra that contain its peaks, in analogy with the standard petrographic technique of point counting. The method can also be used for nondestructive laboratory characterization of rock samples. Although Raman peaks for different minerals seldom overlap each other, it is impractical to obtain proportions of constituent minerals by Raman spectroscopy through analysis of peak intensities in a spectrum obtained by broad-beam sensing of a representative area of the target material. That is because the Raman signal strength produced by a mineral in a rock or soil is not related in a simple way through the Raman scattering cross section of that mineral to its proportion in the rock, and the signal-to-noise ratio of a Raman spectrum is poor when a sample is stimulated by a low-power laser beam of broad diameter. Results obtained by the Raman point-count method are demonstrated for a lunar thin section (14161,7062) and a rock fragment (15273,7039). Major minerals (plagioclase and pyroxene), minor minerals (cristobalite and K-feldspar), and accessory minerals (whitlockite, apatite, and baddeleyite) were easily identified. Identification of the rock types, KREEP basalt or melt rock, from the 100-location spectra was straightforward.

Characterization and comparison of structural and compositional features of planetary quadrilateral pyroxenes by Raman spectroscopy

Abstract This study reports the use of Raman spectral features to characterize the structural and compositional characteristics of different types of pyroxene from rocks as might be carried out using a portable field spectrometer or by planetary on-surface exploration. Samples studied include lunar rocks, martian meteorites, and terrestrial rocks. The major structural types of quadrilateral pyroxene can be identified using their Raman spectral pattern and peak positions. Values of Mg/(Mg + Fe + Ca) of pyroxene in the (Mg, Fe, Ca) quadrilateral can be determined within an accuracy of ± 0.1. The precision for Ca/(Mg + Fe + Ca) values derived from Raman data is about the same, except that corrections must be made for very low-Ca and very high-Ca samples. Pyroxenes from basalts can be distinguished from those in plutonic equivalents from the distribution of their Mg′ [Mg/(Mg + Fe)] and Wo values, and this can be readily done using point-counting Raman measurements on unprepared rock samples. The correlation of Raman peak positions and spectral pattern provides criteria to distinguish pyroxenes with high proportions of non-quadrilateral components from (Mg, Fe, Ca) quadrilateral pyroxenes.

Magnesite-bearing inclusion assemblage in natural diamond

Abstract A significant mineral assemblage has been found as an inclusion in a natural diamond from the Finsch kimberlite pipe of South Africa: a euhedral rhombohedron-shaped magnesite (MgCO3) crystal (d ∼ 30 μm) co-exists with several idiomorphic olivine [(Mg1.86Fe0.14)SiO4] grains (d ∼ 80 μm). Many tiny anatase (TiO2) particles (d ∼ 2–5 μm) and microcrystallites (d The occurrence of this syngenetic multiphase inclusion assemblage in a natural diamond provides unambiguous evidence for the existence in the Earths mantle of magnesite, which has been proposed as a major carbon reservoir in most of the mantle. Both the formation and preservation aspects of the assemblage have been investigated. The mineralogy of the assemblage indicates that carbonated peridotite formed the surrounding petrologic environment. The inclusion assemblage suggests two reactions involving the decomposition of carbonates in mantle peridotite during decompression, which, in part, may explain the paucity of magnesite that has been found in other mantle rocks. The chemical inertness and low compressibility of the host diamond must have been critical to the preservation of this magnesite-bearing assemblage. The incorporation of a pure TiO2 phase in a peridotitic diamond inclusion and its occurrence in the anatase structural form further emphasize the unusual conditions that allowed both the formation and preservation of this multiphase inclusion. The P-T-fO2 conditions defined by the inclusion assemblage are represented by the intersection of the graphite-diamond transition curve and the enstatite-magnesite-olivine-diamond buffer. The oxygen fugacity range represented by the inclusion assemblage is below that of the quartz-fayalite-magnetite and CCOCO2 buffers in the P-T range common to most diamonds. However, the co-existence of diamond, graphite, magnesite, olivine and anatase in this inclusion assemblage represents the highest oxygen fugacity at which olivine could be stable with both diamond and magnesite; that is, the highest oxidation state under which a mantle diamond can be stabilized in a peridotite environment. The diamond-carbonate-silicate co-existence wedge is relatively restricted in P-T-fO2 space. Therefore, the P-T-fO2 conditions implied by this and other diamond inclusion assemblages lead to two significant implications for mantle petrology: (1) the conditions for diamond formation are very limited if carbonates are major carbon sources for diamonds; (2) given the low-fO2 conditions inferred for portions of the Earths mantle, carbonates may rarely occur in peridotites, and much of the carbon in the mantle may be locked in reduced phases.

Prototype Raman Spectroscopic Sensor for in Situ Mineral Characterization on Planetary Surfaces

Raman spectroscopy has the potential to provide definitive identification and detailed characterization of the minerals that comprise rocks and soils on planetary surfaces. We have designed a probe head for Raman spectroscopy that is suitable for use on a spectrometer deployed by a rover or a lander on the surface of a planet such as Mars, the Moon, or an asteroid. The probe head is lightweight, low power, rugged, and simple. It is based on a tiny distributed feedback diode laser and volume holographic components. A protective shell surrounds the probe head and serves as a mechanical stop for the mechanical arm of a planetary rover or lander during placement of the probe head onto the surface of a rock or soil. Pressing the shell against the rough surface of a target rock or soil also places the sampling objective of the probe head in rough focus, and the probe head is designed to be tolerant of focusing errors of ∼5 mm. A breadboard version of the probe head gave spectra of high quality on clean crystals of diamond, sulfur, calcite, quartz, and olivine. The results are qualitatively comparable to those obtained by using a conventional micro-Raman spectrometer on fine-grained travertine and on difficult specimens of basaltic lavas and impactites whose original mineralogy had been altered by reaction with water and air.

Characterization of graphite alteration in an uranium deposit by micro-Raman spectroscopy, X-ray diffraction transmission, electron microscopy and scanning electron microscopy

Abstract A series of graphitic samples associated with a uranium deposit has been studied by micro-Raman spectrometry, transmission electron microscopy and X-Ray diffraction. The dependence of the Raman spectrum on the orientation with respect to the laser beam is explained both from the structure of the tensor components associated with the different vibrational modes ( E 2 g 2 and defect bands) and from the analysed volume by the spectrometer. Doubly polished thin rock sections, usually made for classical petrographic observations, are not suitable for Raman analysis because polishing damages the structural order of graphitic compounds. A progressive and continuous but still small loss of structural ordering along the c axis is shown from both second order Raman spectra, interpreted by the model of Lespade and TEM measurements. Comparison with XRD data leads to the conclusion that the degradation of graphite occurs only on the surface and is probably linked with the alteration of its host-rock. In addition to these defects, which were suggested by variations of the reflectance, graphite leaves close to uranium concentrations exhibit hollow points with diameters up to several micrometers. Their characterization by TEM and Raman spectrometry has shown an amorphous structure of the carbon. They probably originate from a higher degree of graphite alteration.