G. Kurat

Petrology and geochemistry of Antarctic micrometeorites

Abstract The petrology and geochemistry of twentythree chondritic dust particles with masses of 1–47 μg (sizes 100–400 μm) were recovered from blue ice near Cap Prudhomme, Antarctica, and studied by INAA, ASEM, EMPA, and optical microscopy. Sample selection criteria were irregular shape and (for a subsample) black color, with the aim of studying as many unmelted micrometeorites (MMs) as possible. Of thirteen unmelted MMs, six were phyllosilicate-dominated MMs, and seven were coarsegrained crystalline MMs consisting mainly of olivine and pyroxene. The remaining ten particles were largely melted and consisted of a foamy melt with variable amounts of relic phases (scoriaceous MMs). Thus, of the black particles selected, an astonishing portion, 40% (by number), consisted of largely unmelted MMs. Although unmelted, most phyllosilicate MMs have been thermally metamorphosed to a degree that most of the phyllosilicates were destroyed, but not melted. The original preterrestrial mineralogy is occasionally preserved and consists of serpentine-like phyllosilicates with variable amounts of cronstedtite, tochilinite-like oxides, olivine, and pyroxene. The crystalline MMs consist of olivine, low-Ca pyroxene, tochilinite-like oxides, and occasional Ni-poor metal. Relics in scoriaceous MMs consist of the same phases. Mineral compositions and the coexistence of phyllosilicates with anhydrous phases are typical of CM and CR-type carbonaceous chondrites. However, the olivine/pyroxene ratio (~ 1) and the lack of carbonates, sulfates, and of very Fe-poor, refractory element-rich olivines and pyroxenes sets the MMs apart from CM and CR chondrites. The bulk chemistry of the phyllosilicate MMs is similar to that of CM chondrites. However, several elements are either depleted (Ca, Ni, S, less commonly Na, Mg, and Mn) or enriched (K, Fe, As, Br, Rb, Sb, and Au) in MMs as compared to CM chondrites. Similar depletions and enrichments are also found in the scoriaceous MMs. We suggest that the depletions are probably due to terrestrial leaching of sulfates and carbonates from unmelted MMs. The overabundance of some elements may also be due to processes acting during atmospheric passage such as the recondensation of meteoric vapors in the high atmosphere. Most MMs are coated by magnetite of platy or octahedral habit, which is rich in Mg, Al, Si, Mn, and Ni. We interpret the magnetites to be products of recondensation processes in the high (>90 km) atmosphere, which are, therefore, probably the first refractory aerominerals identified.

Geochemistry of ultramafic xenoliths from Kapfenstein, Austria: evidence for a variety of upper mantle processes

Major, minor, and trace element contents have been determined in seven ultramafic xenoliths, the host basanite, and some mineral separates from xenoliths from Kapfenstein, Austria. Most of the xenoliths represent residues after extraction of different amounts of basaltic liquid. Within the sequence Iherzolite to harzburgite contents of Al, Ca, Ti, Na, Sc, V, Cr and the HREE decrease systematically with increasing Mg/Fe and decreasing Yb/Sc. Although all samples are depleted in highly incompatible elements, the less depleted end of our suite very closely approaches the chondritic Yb/Sc ratio and consequently the primitive upper mantle composition. Chromium behaved as a non-refractory element. Consequently it should have higher abundances in basalts than observed, suggesting that most basalts experienced Cr fractionation by chromite separation during ascent. Several processes have been active in addition to partial melting within the upper mantle beneath Kapfenstein: 1. (1) a hornblendite has been identified as wet alkali-basaltic mobilisate; 2. (2) an amphibole Iherzolite is the product of alkali-basalt metasomatism of a common depleted Iherzolite; 3. (3) two amphibole Iherzolites contain evidence for rather pure water metasomatism of normal depleted Iherzolites; 4. (4) a garnet-spinel websterite was a tholeiitic liquid trapped within the upper mantle and which suffered a subsequent partial melting event (partial remobilization of a mobilisate). 5. (5) Abundances of highly incompatible elements are generally very irregular, indicating contamination of upper mantle rocks by percolating liquids (in the mantle). Weathering is an important source of contamination: e.g. U mobilization by percolating groundwater. Contamination of the xenoliths by the host basanite liquid can only amount to approximately 5.5 × 10−4 parts. Distributions of minor and trace elements between different minerals apparently reflect equilibrium and vary with equilibration temperature.

Properties of Interplanetary Dust: Information from Collected Samples

The properties of hundreds of interplanetary particles have been determined by direct laboratory analysis of recovered samples. The particles that span the 1 μm to 1 mm size range have been collected from the stratosphere, from polar ice, and from deep sea sediments. Typically, these particles are black, somewhat porous and have chondritic elemental compositions. They are rather complex mineral assemblages in that they are mixtures of very large numbers of sub-micrometer-sized components. While the data are not totally representative of small interplanetary meteoroids at 1 AU they provide significant insight into the common physical properties of meteoroids. These properties can be used as guidelines for analysis of spacecraft and astronomical observations and for modeling solar system dust as well as some circumstellar dust in systems around other stars.

Zur genese der Ca-Al-reichen einschlüsse im chondriten von lancé

Abstract Spinel-melilithe-diopside (± perovskite ± alumina) aggregates from the Lancecarbonaceous chondrite (type III) are described. Some simple shaped examples have been investigated in detail and the phases have been analysed with an electron-microprobe. The analyses show that the chemical composition of the phases varies from piece to piece but generally all phases are Fe poor to Fe free. Remarkable are the high ZrO 2 (2.2 wt %) and Y 2 O 3 (1.1 wt %) contents of the perovskite as well as the TiO 2 content of the alumina phase (0.7 wt %). If recalculated to the bulk composition we find very strong enrichments for Y(430×), Zr(123×), Ti(64×), Al(17×), and Ca(6×) as compared with the average chondrite contents. These enrichments as well as the mineral association suggest a very high temperature origin. The extreme chemical composition probably is the result of a “boiling off” of the more volatile components. The relative amounts of enrichments furthermore indicate a formation at pressures below 10 −1 atmosphere.

Accretion of neon, organics, CO2, nitrogen and water from large interplanetary dust particles on the early Earth

Abstract Large interplanetary dust particles (micrometeorites) with sizes of 100– 200 μm , recovered from the Greenland and Antarctica ice sheets, represent by far the dominant source of primitive extraterrestrial material accreted by the Earth today. Comparisons of mineralogical, chemical and isotopic analyses of micrometeorites and meteorites indicate that micrometeorites are mostly related to the relatively rare group (2% of the meteorite falls) of the primitive hydrous-carbonaceous meteorites, and not to the most abundant classes of the ordinary chondrites and differentiated meteorites. But there are differences between these two classes of extraterrestrial objects, such as a high pyroxene to olivine ratio, a strong depletion in chondrules, a much smaller size of the most refractory components, and a much higher AIB (α-isobutyric amino acid) to isovaline ratio in micrometeorites as compared to meteorites. They indicate that micrometeorites represent a new population of solar system objects, not represented as yet in the meteorite collections. The major objective of this work is to predict various effects of the accretion of early micrometeorites on the Earth during the period of heavy bombardment suffered by the Earth–Moon system ⩾3.9 Ga ago. The application of a simple arithmetics of accretion to a selection of measurements (average contents of neon, carbon, nitrogen and water in micrometeorites, and isotopic composition of their Ne and H), shows that during the peak of this cataclysmic epoch (sterilization period) which occurred just after the formation of the young Earth (4.45 Ga ago), the accretion of early micrometeorites did play a major role in the formation of the terrestrial atmosphere and oceans. Later on, during the early life period (around 4 Ga ago), when liquid water and organics could condense and/or survive, micrometeorites were possibly functioning as tiny chemical reactors to synthesize the prebiotic molecules required for the origin of life. Efforts were made to start reducing the number of major speculations in this “early-micrometeorite-accretion” scenario (EMMAC), which is finally extended with some confidence to Mars, where the survival of micrometeorites upon atmospheric entry looks even more favorable than on the Earth.

An ESA study for the search for life on Mars

Similarities in the early histories of Mars and Earth suggest the possibility that life may have arisen on Mars as it did on Earth. If this were the case, early deterioration of the environment on Mars (loss of surface water, decrease in temperature) may have inhibited further evolution of life. Thus, life on Mars would probably be similar to the simplest form of life on Earth, the prokaryotes. We present a hypothetical strategy to search for life on Mars consisting of (i) identifying a suitable landing site with good exobiological potential, and (ii) searching for morphological and biogeochemical signatures of extinct and extant life on the surface, in the regolith subsurface, and within rocks. The platform to be used in this theoretical exercise is an integrated, multi-user instrument package, distributed between a lander and rover, which will observe and analyse surface and subsurface samples to obtain the following information: 1. environmental data concerning the surface geology and mineralogy, UV radiation and oxidation processes; 2. macroscopic to microscopic morphological evidence of life; 3. biogeochemistry indicative of the presence of extinct or extant life; 4. niches for extant life.

Primitive Meteorites: An Attempt towards Unification

A model is developed that recognizes genetic links between almost all meteorites. The model is based on processes that were active in the solar nebula. The most important processes identified are: (1) early part to total evaporation of presolar matter; (2) recondensation beginning with olivine followed by other phases; (3) aggregation of the condensates to millimetre-sized objects during condensation; (4) partial or total compaction of early aggregates by continuing condensation preferentially utilizing the aggregate’s pore space; (5) a second heating event leading to sintering and partial melting of the aggregates (chondrule formation) and mild vapour fractionation; (6) vapour—solid exchange reactions introducing Fe2+ (and other elements) into the silicates (‘equilibration’ via metasomatism) and forming FeS via S-metasomatism; (7) depending on conditions, centimetre-sized aggregates of early aggregates form either before or while processes (4)-(7) are active, or later. This process can continue to form decimetre-sized aggregates; (8) accretion takes place at very low temperatures with condensation of volatile elements still continuing; (9) some matter experienced H2O- and CO2-metasomatism (formation of phyllosilicates and carbonates) and almost total oxidation before accretion (Cl, C2). The typical early condensate is olivine, which displays different growth features depending on physical conditions. Common are ‘barred’ olivines (greater than 200 pm), huge (100 pm) subparallel stacks of olivine platelets and small stacks (10-50 pm) of similar platelets. These olivines have highly defective lattices and contain large amounts of minor elements (including refractory lithophile elements). Depending on growth rate and particle abundance, primitive olivines aggregate to millimetre-sized objects (the protoliths of chondrite constituents) or centimetre-sized aggregates. Formation of large aggregates apparently takes place when the condensation rate is high. This quickly leads to compaction of the aggregates and highly reduced pore space. These aggregates will ultimately end up in ureilites and pallasites. Slower growth rates produce smaller aggregates (millimetre-sized) with abundant pore space which serves as a cold trap for the condensation of the volatile elements. Olivine is partly converted into pyroxene by reaction with the vapour (formation of poikilitic pyroxenes). Pyroxene and olivine abundances will vary depending on reaction time and other parameters. The pore space will also be highly variable. Moderately volatile elements condense into this pore space with their abundance governed by the space available. In this manner, the protoliths for chondrules formed. The chemical fractionations observed in meteoritic constituents can be accounted for mostly by physical parameters: (1) the main condensing phase and the pore space provided by the aggregates; (2) the timing of formation of aggregates of aggregates and of the closure of pore space (physical isolation of aggregate interiors); (3) reprocessing of aggregates in a high temperature event; (4) metasomatic exchange reactions between solid aggregates and vapour; (5) isolation of aggregates from vapour via shielding by condensates (e.g. metal) or physical removal; (6) timing and rate of final accretion; (7) degree of mixing with solar and presolar dust.

Allende Xenolith AF: Undisturbed Record of Condensation and Aggregation of Matter in the Solar Nebula

Abstract Allende-AF (All-AF) is chemically similar to bulk Allende, its texture is. however, completely different. It consists mainly of highly porous silicate-rich aggregates embedded in a largely opaque matrix. Occasionally olivine-sulfide-metal aggregates and large sulfide-andradite objects are found. Chondrules are absent and grain size in matrix and aggregates is the same, quite different from normal Allende. The main mineral in All-AF is olivine (22-41 wt% FeO). low Ca-pyroxene is absent, but a variety of clinopyroxenes were encountered. Rare phases found throughout All-AF are refractory metal nuggets (Ir, Os, Ru etc.), HgS, and Ca-phosphate. Associated with sulfide-andradite objects are native-Cu, Ti-magnetite, perovskite, barite, and calcite. Morphology, crystalline state and minor element contents of olivine strongly suggest an origin by condensation from a gas. Al20 3-contents, for example, range from 0.5 to 3% and Cr2O3-contents reach 2.5%. Texture of All-AF indicates that condensation of olivine continued during formation of aggregates. Relictic low-Fe aluminous diopsides suggest that olivine was initially FeO-poor. Before final accretion into a solid rock exchange reactions between condensed phases and an (increasingly oxidized) vapor established the high FeO-content of olivine. A similar process led to sulfurization of most of the original metal. The exotic sulfide-andradite objects are probably alteration products of unusual, metal-rich calcium-aluminium-rich inclusions. It appears that All-AF has preserved a unique record of condensation, hierarchical aggregation, and metasomatic exchange reactions in the solar nebula.

Zur genese des kohligen materials im meteoriten von Tieschitz

Abstract A carbonaceous inclusion in a chondrule and a carbonaceous fragment from the Tieschitz meteorite are described. Electronmicroprobe analyses of that material and of the carbonaceous matrix are compared with analyses of carbonaceous chondrites. This comparison shows that the carbonaceous material from the Tieschitz meteorite probably is a residual material of the chondrule-forming process. Therefore it is depleted mostly in magnesium and iron if compared with the carbonaceous chondrites. There is also a high probability that the Tieschitz carbonaceous material had suffered some alterations after agglomeration which are thought to be caused by a hydrothermal or pneumatolytic activity on the Tieschitz parent body.

Mass-independent fractionation of oxygen isotopes during thermal decomposition of carbonates

Nearly all chemical processes fractionate 17O and 18O in a mass-dependent way relative to 16O, a major exception being the formation of ozone from diatomic oxygen in the presence of UV radiation or electrical discharge. Investigation of oxygen three-isotope behavior during thermal decomposition of naturally occurring carbonates of calcium and magnesium in vacuo has revealed that, surprisingly, anomalous isotopic compositions are also generated during this process. High-precision measurements of the attendant three-isotope fractionation line, and consequently the magnitude of the isotopic anomaly (Δ17O), demonstrate that the slope of the line is independent of the nature of the carbonate but is controlled by empirical factors relating to the decomposition procedure. For a slope identical to that describing terrestrial silicates and waters (0.5247 ± 0.0007 at the 95% confidence level), solid oxides formed during carbonate pyrolysis fit a parallel line offset by −0.241 ± 0.042‰. The corresponding CO2 is characterized by a positive offset of half this magnitude, confirming the mass-independent nature of the fractionation. Slow, protracted thermolysis produces a fractionation line of shallower slope (0.5198 ± 0.0007). These findings of a 17O anomaly being generated from a solid, and solely by thermal means, provide a further challenge to current understanding of the nature of mass-independent isotopic fractionation.