Jutta Zipfel

Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer

The Alpha Particle X-ray Spectrometer on the Opportunity rover determined major and minor elements of soils and rocks in Meridiani Planum. Chemical compositions differentiate between basaltic rocks, evaporite-rich rocks, basaltic soils, and hematite-rich soils. Although soils are compositionally similar to those at previous landing sites, differences in iron and some minor element concentrations signify the addition of local components. Rocky outcrops are rich in sulfur and variably enriched in bromine relative to chlorine. The interaction with water in the past is indicated by the chemical features in rocks and soils at this site.

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.

Soils of Eagle Crater and Meridiani Planum at the Opportunity Rover Landing Site

The soils at the Opportunity site are fine-grained basaltic sands mixed with dust and sulfate-rich outcrop debris. Hematite is concentrated in spherules eroded from the strata. Ongoing saltation exhumes the spherules and their fragments, concentrating them at the surface. Spherules emerge from soils coated, perhaps from subsurface cementation, by salts. Two types of vesicular clasts may represent basaltic sand sources. Eolian ripples, armored by well-sorted hematite-rich grains, pervade Meridiani Planum. The thickness of the soil on the plain is estimated to be about a meter. The flatness and thin cover suggest that the plain may represent the original sedimentary surface.

Non-chondritic platinum-group element ratios in oceanic mantle lithosphere: petrogenetic signature of melt percolation?

Abstract The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt and Pd were determined in 11 abyssal peridotites from ODP Sites 895 and 920, as well in six ultramafic rocks from the Horoman peridotite body, Japan, which is generally thought to represent former asthenospheric mantle. Individual oceanic peridotites from ODP drill cores are characterized by variable absolute and relative PGE abundances, but the average PGE concentrations of both ODP suites are very similar. This indicates that the distribution of the noble metals in the mantle is characterized by small-scale heterogeneity and large-scale homogeneity. The mean Ru/Ir and Pt/Ir ratios of all ODP peridotites are within 15% and 3%, respectively, of CI-chondritic values. These results are consistent with models that advocate that a late veneer of chondritic material provided the present PGE budget of the silicate Earth. The data are not reconcilable with the addition of a significant amount of differentiated outer core material to the upper mantle. Furthermore, the results of petrogenetic model calculations indicate that the addition of sulfides derived from percolating magmas may be responsible for the variable and generally suprachondritic Pd/Ir ratios observed in abyssal peridotites. Ultramafic rocks from the Horoman peridotite have PGE signatures distinct from abyssal peridotites: Pt/Ir and Pd/Ir are correlated with lithophile element concentrations such that the most fertile lherzolites are characterized by non-primitive PGE ratios. This indicates that processes more complex than simple in-situ melt extraction are required to produce the geochemical systematics, if the Horoman peridotite formed from asthenospheric mantle with chondritic relative PGE abundances. In this case, the PGE results can be explained by melt depletion accompanied or followed by mixing of depleted residues with sulfides, with or without the addition of basaltic melt.

238U/235U Variations in Meteorites: Extant 247Cm and Implications for Pb-Pb Dating

How to Get a Date Radiometric dating relies on measuring the abundance of a radioactive isotope and/or its decay products. By knowing a decay rate and an isotopic starting abundance—both assumed to be constant—an age is determined. Using high-resolution mass spectrometry, Brennecka et al. (p. 449, published online 31 December; see the Perspective by Connelly) show that the known starting abundance of 238U and 235U isotopes in meteorites, which decay into 206Pb and 207Pb, respectively, is actually quite variable. Trace amounts of 247Cm in the early solar system may have unexpectedly contributed additional 235U, skewing the ratio. Pb-Pb dating, the method commonly used to date early solar system materials, may thus need a correction of up to 5 million years. Variable abundances of meteorite isotopes may require correcting the lead-based age of the solar system by 5 million years. The 238U/235U isotope ratio has long been considered invariant in meteoritic materials (equal to 137.88). This assumption is a cornerstone of the high-precision lead-lead dates that define the absolute age of the solar system. Calcium-aluminum–rich inclusions (CAIs) of the Allende meteorite display variable 238U/235U ratios, ranging between 137.409 ± 0.039 and 137.885 ± 0.009. This range implies substantial uncertainties in the ages that were previously determined by lead-lead dating of CAIs, which may be overestimated by several million years. The correlation of uranium isotope ratios with proxies for curium/uranium (that is, thorium/uranium and neodymium/uranium) provides strong evidence that the observed variations of 238U/235U in CAIs were produced by the decay of extant curium-247 to uranium-235 in the early solar system, with an initial 247Cm/235U ratio of approximately 1.1 × 10−4 to 2.4 × 10−4.

Chemical composition and origin of the Acapulco meteorite

New chemical data for both whole-rock and individual mineral samples from the Acapulco meteorite are reported. Results of instrumental neutron activation analysis (INAA) of bulk samples show large variation in La, Cr, Se, and Fe contents reflecting inhomogeneous distribution of the corresponding host phases: phosphate, chromite, sulfide, and FeNi metal. In contrast, Mn, Sc, and Na contents are uniformly distributed reflecting constant fractions of orthopyroxene, olivine, and plagioclase in bulk samples. INAA data were obtained on single olivine and orthopyroxene grains with numerous tiny metal inclusions. The siderophile elements in these inclusions, in particular the low Ir contents, suggest that the inclusions formed by partial melting of matrix metal. The composition of the metal inclusions and the absence of such inclusions in clinopyroxene suggests an upper limit of about 20% partial melting of Acapulco at around 1200°C. During melting and closed-system crystallization the inhomogeneous distribution of metal, chromite, sulfide, and phosphates was established. Acapulco phosphates are of igneous origin. Most of them were not formed by oxidation of metal. The uniform K contents of bulk Acapulco samples and the comparatively high contents of volatiles (including rare gases) in bulk samples demonstrate a closed-system behavior. Neither loss nor gain of volatile elements has occurred. Ion microprobe data of rare earth elements (REEs) in Acapulco minerals show an equilibrium distribution for heavy REEs and nonequilibrium for light REEs. Bulk Acapulco samples have large excesses of light REEs and U with phosphates being the major carrier. Absolute REE contents of phosphates are variable. The observed enrichments of REEs and U in bulk Acapulco samples cannot be explained by the addition of phosphates, since the approximately chondritic ratios of Ca/Mg and P/Mg exclude major gains of Ca and P through the addition of phosphates. It is suggested that a fluid phase rich in incompatible elements infiltrated Acapulco and that REEs and U were extracted by phosphates. The remaining fluid phase subsequently must have drained away. From temperatures determined by two-pyroxene equilibria, spinel-olivine and Ca zonation in olivine, a thermal history of Acapulco is constructed. Cooling rates at 900°C of about 100 K/Ma are estimated. The concentration profiles of Ca in olivine, showing constant Ca throughout grain interiors and sharp decreases at the rims, are difficult to explain. If the Ca zonation in olivine exclusively reflects cooling, slow cooling must have been followed by fast cooling at temperatures below 650°C. Alternatively, the low Ca contents of olivine rims may reflect other processes, such as, for example, formation of phosphates. A model for the evolution of Acapulco is presented: The parent body of Acapulco accreted earlier than that of the ordinary chondrites (OC), thus more 26Al was available for heating, leading to higher peak temperatures than in OC. Extensive solid state equilibration at 900°C completely erased the 26Mg signature in plagioclase. Acapulco is, despite the absence of chondrules, essentially a chondritic meteorite, attesting to the variety of planetesimals of chondritic composition in the asteroid belt.

The Inventory of Presolar Grains in Primitive Meteorites: A NanoSIMS Study of C-, N-, and O-isotopes in NWA 852

Meteorites as well as interplanetary and cometary dust contain small amounts of mineral grains that formed in the winds of evolved stars or in the ejecta of stellar explosions and survived incorporation into solid bodies of our own solar system. Investigation of the abundance and distribution of these “presolar grains” in primitive solar system matter can shed light on parent body processes as well as possible heterogeneities in the early solar nebula. We investigated presolar silicate and oxide grains in the CR chondrite NWA 852 with a NanoSIMS 50. Abundances of 77 ppm for silicates, 39 ppm for oxides, and 160 ppm for silicon carbide, as well as evidence for N-enriched organic molecular cloud material were observed. Although NWA 852 has the lowest presolar silicate/oxide-ratio observed so far for presolar-grain-rich material, indicating extensive aqueous alteration, a significant fraction of O-anomalous grains (silicates and oxides) remained intact. Thus, this meteorite may be a link between presolargrain-rich, pristine CR chondrites and CRs with lower presolar grain abundances.

The Chemistry of Solar System Materials: Sun, Planets, Asteroids, Meteorites and Dust

In this paper we summarize our knowledge of the chemical composition of solar system materials accessible to analysis. In the Sun the three most important rock forming elements Mg, Si and Fe have about the same number of atoms (Mg/Si = 1; Fe/Si = 0.91); the number of Al atoms is a factor of 10 lower (Al/Si = 0.09). Chondritic meteorites have essentially the same chemical signature with some variability, about 20 % for Mg/Si, 50 % for Al/Si and a factor of two for Fe/Si. These variations can be accounted for by variably mixing components that formed by condensation in a cooling gas of solar composition (Mg-silicates, Ca,Al-rich inclusions, NiFe metal). The bulk Earth composition is within this range and may be considered in a broad sense to be chondritic. The bulk compositions of the other terrestrial planets are less well known. They all have a metal core and basaltic surface rocks. Exceptions are Mercury with too much and the Moon with too little iron for a chondritic bulk composition. Asteroids also seem to have chondritic bulk compositions. S-type asteroids have been confirmed to be the parent bodies of ordinary chondrites. Most of the C-type asteroids appear to represent carbonaceous chondrites. The mm to sub-millimeter sized micrometeorites are debris of asteroids and/or comets. They are largely chondritic in composition but the ratio of cometary to asteroidal material is unclear. If there is a significant fraction of cometary material, comets should have chondritic bulk composition, as approximately inferred from the Giotto data.