Bernhard Spettel

EXPERIMENTAL PETROCHEMISTRY OF SOME HIGHLY SIDEROPHILE ELEMENTS AT HIGH TEMPERATURES, AND SOME IMPLICATIONS FOR CORE FORMATION AND THE MANTLE'S EARLY HISTORY

The highly siderophile elements (HSEs: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) and those elements with distribution coefficients between Fe-rich metal and silicate phases which exceed 104. The large magnitude of these distribution coefficients makes them exceedingly difficult to measure experimentally. We describe a new experimental campaign aimed at obtaining reliable values of DMmets/sil melt for selected HSEs indirectly, by measuring the solubilities of the pure metals (or simple HSE alloys) in haplobasaltic melts as a function of oxygen fugacity. Preliminary results for Pd, Au, Ir and Re indicate that the HSEs may dissolve in silicate melts in unusually low valence states, e.g., 2+ for Ir and 1+ for the others. These unusual valence states may be important in understanding the geochemical properties of the HSEs. Inferred values of DMmet/sil melt from the solubility data at 1400°C and IW −1 are ∼107 for Pd and Au, and 109−1012 for Ir. Metal/silicate partition coefficients are thus confirmed to be very large, and are also different for the different HSEs. A review of the abundance of the HSEs in the Earths upper mantle shows that they are all present at ∼0.8% of chondritic, i.e. they have the same relative abundance, and the ratios of their concentrations are chondritic (e.g., Re/Os). Both the low degree of depletion (compared to the high values of DMmet/sil melt) and the chondritic relative abundances support the idea that the mantles HSEs were added in a “late veneer” after the cessation of core formation. Sulfur is even more depleted in the mantle relative to CI chondrites than the HSEs: this implies a late veneer which was depleted in volatile elements, and which was added to a mantle stripped of S. Since considerable S dissolves in silicate melt, this further implies that core formation in the Earth either occurred under P−T conditions below the solicate solidus, or, if the process occurred over a range of temperatures in a cooling Earth, then the process continued down to conditions below the silicate solidus. The chondritic relative abundances of the HSEs in the upper mantle argue for a chemically unstratified primitive mantle, unless the late veneer was mixed only into the upper mantle.

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.

Acfer 182 and paired samples, an iron-rich carbonaceous chondrite: Similarities with ALH85085 and relationship to CR chondrites

Three samples of a new, Fe-rich chondrite were found in the Sahara in 1990 and 1991 (Acfer 182, Acfer 207, Acfer 214). The samples are paired and the meteorite will be designated as Acfer 182. The chondrite is chemically, texturally, and mineralogically similar to the Allan Hills meteorite ALH85085. One important difference between the two meteorites is the smaller average chondrule size in ALH85085. The major components of Acfer 182 (in decreasing abundance) are 1. (1) highly altered (by terrestrial weathering) matrix 2. (2) mineral and polymineralic silicate fragments and aggregates 3. (3) chondrule fragments 4. (4) chondrules 5. (5) metal 6. (6) fine-grained, dark inclusions. The abundance of chondrules is lower and the average chondrule size (

Chemical systematics of the shergotty meteorite and the composition of its parent body (Mars)

90 μm) smaller than in most other chondrites. Chondrule fragments are often so large that they cannot be derived from the present chondrule population. Apparently, size sorting has prevented accumulation of the larger parent chondrules. Several spectacular Ca,Al-rich inclusions were found, rich in Ca-dialuminate, hibonite, or Zr-, Y-, Sc-bearing phases. The chemical composition of Acfer 182 and of ALH85085 are almost indistinguishable. Major chemical signatures are 1. (1) uniform enrichment of Fe and other nonvolatile metals relative to CI-chondrites by about 70% 2. (2) absence of enrichment in refractory lithophiles, characteristic of most type 2 and 3 carbonaceous chondrites 3. (3) strong depletion of volatile and moderately volatile elements. Based on the oxygen isotopic composition, the chemical composition, and the abundances of chondrules and matrix, Acfer 182 should be classified as a carbonaceous chondrite. Considering their affinity to carbonaceous chondrites and their high bulk iron content the two meteorites, Acfer 182 and ALH85085, are designated as CH-chondrites. There are mineralogical and chemical similarities among Acfer 182, ALH85085, and CR chondrites which distinguish these meteorites from other types of carbonaceous chondrites: 1. (1) low FeO contents of olivine and pyroxene and correspondingly high metal contents 2. (2) high Cr-content in olivine 3. (3) abundant fine-grained dark inclusions 4. (4) abundant Ca-dialuminate (CaAl4O7) in CAIs 5. (5) similarities in oxygen isotopic composition 6. (6) low contents of moderately volatile elements 7. (7) low refractory element contents 8. (8) presence of a unique component of subsolar rare gases. These observations suggest similar conditions of formation for the components of these meteorites. A single common parent body is unlikely in view of the differences in chemical composition and in the size distribution of individual components.

Paired Renazzo-type (CR) carbonaceous chondrites from the Sahara

Abstract We report chemical data for 60 elements by INAA and RNAA in two bulk samples, for 30 elements in various mineral separates of Shergotty, and results of leaching experiments with 1M HCl on powdered aliquots of Shergotty and BETA 79001, lithologies A and B. Shergotty is homogeneous in major element composition but heterogeneous with respect to LIL trace elements (~20%). The heterogeneity is even greater for volatile and siderophile trace elements. The mineral data, including three clinopyroxene fractions with variable FeO contents, maskelynite and minor phases (Ti-magnetite, ilmenite, quartz, K-rich phase), show that major minerals do not account for the rare earth elements (REE) in the bulk meteorite. Instead, the REE are to a large extent concentrated in accessory whitlockite and apatite (shown by leaching with 1M HCl): together with the majority of REE (La, 96%, Yb 70%), Cl and Br are quantitatively dissolved by leaching. The REE patterns of the leachate of Shergotty and EETA 79001 are different. The Shergotty leachate may consist of two components. Component l is similar to that of EETA 79001 leachate (whitlockite), component 2 is enriched in light REE and may be responsible for the higher LREE contents of Shergotty in comparison to the other shergottites. There is some evidence that Shergotty was an open system and component 2 was introduced after crystallization. The REE patterns of the residues of Shergotty and EETA 79001 are identical indicating that the parent magmas of both meteorites are compositionally similar. Based on cpx separates with the lowest REE content, the REE pattern in the Shergotty parent magma was calculated. It is enriched in LREE and has a subchondritic Nd Sm ratio. The negative Eu anomaly in the phosphates indicates that at least some plagioclase crystallized before phosphate. Based on several element correlations in SNC meteorites, it was suggested (Dreibus and Wanke, 1984) that both the Shergotty parent body (SPB, very probably Mars), and the Earth accreted from the same two chemically different components: component A, highly reduced and devoid of volatile elements and an oxidized component B containing also volatile elements. The SPB (Mars) mantle is 2–4 times richer in volatile and moderately siderophile elements than the Earth, indicating a higher portion of component B in the SPB. The concentrations of chalcophile elements in the SPB mantle are low, reflecting equilibration with a sulfide phase and subsequent segregation of sulfide into the core. Unlike the Earth (Wanke, 1981), the SPB (Mars) may therefore have accreted almost homogeneously.

The Acapulco meteorite: Chemistry, mineralogy and irradiation effects

Ten chondrites with chemical and mineralogical similarities to the carbonaceous chondrite Renazzo were recovered at two locations of the Sahara: Acfer 059, 087, 097, 114, 139, 186, 187, 209, 270 and El Djouf 001. Although the El Djouf location is more than 500 km away from the Acfer location, all samples appear to result from a single fall based on chemical and petrographic similarities and supported by light element stable isotope geochemistry, noble gas record, and similar 26Al contents. The Acfer-El Djouf meteorite is classified as a CR (Renazzo-type) carbonaceous chondrite. This group presently comprises three non-Antarctic members (Al Rais, Renazzo, Acfer-El Djouf) and five Antarctic meteorites. The major lithological components of the Acfer-El Djouf meteorite are large chondrules (up to 1 cm in size; mean diameter: 1.0 ± 0.6 mm), chondrule and mineral fragments, Ca,Al-rich inclusions, FeNi-metal (about 8–10 vol%) and dark inclusions embedded in a fine-grained fragment-bearing groundmass. Mineral compositions of the ten Acfer-El Djouf samples are similar to those of other CR chondrites. Most of the Ca,Al-rich inclusions are below 300 μm in size and rich in melilite and spinel. In some CAIs the rare phase CaAl4O7 is dominant. Fo-rich, Cr-bearing olivine (Fa0–4) and enstatite (Fs0–4) are the major phases of the chondrite. The meteorite is mildly shocked with a shock stage of S2 indicating a peak shock pressure of 5–10 GPa for the bulk meteorite. The oxygen isotopic compositions and carbon and nitrogen stable isotope geochemistry of the Acfer-El Djouf samples are very similar to those of the other CR-type chondrites. The major element composition of the Acfer-El Djouf meteorite is indistinguishable from CR chondrites. When compared to Renazzo the Acfer-El Djouf samples, however, have systematically lower contents of the moderately volatile elements Zn, Ga, As, Au, Sb, and Se and the highly volatile elements Br, C, and N. This is thought to reflect primary differences between Renazzo and the Acfer-El Djouf meteorite.

Thermal and impact metamorphism on the HED parent asteroid

Abstract The Acapulco meteorite fell in August, 1976, at El Quemado, near Acapulco, Mexico. It is a unique object with chondritic composition but achondritic texture. High degree of recrystallisation and mineral chemical data indicate formation of the meteorite under redox conditions intermediate between those of H- and E-chondrites at ~ 1100°C, from which it cooled at a rate > 10°C/Myr. The major element composition is within the range of H-chondrites. Troilite and metal, and associated trace elements, are inhomogeneously distributed. Chromium is a factor of two higher than in H-chondrites. Enrichments of P and U indicate high phosphate content. Limited extent of partial melting may explain the light REE enrichment. However other incompatible elements have normal H-chondritic abundances or are even depleted like K and Rb. Moderately volatile or volatile elements (e.g. Mn, Ga, Ge, Zn) are enriched nearly to the level of C1-chondrites. Planetary noble gases are also significantly higher than in equilibrated ordinary chondrites. High temperature recrystallisation has not affected volatile element abundances. Compared to H-chondrites Acapulco is enriched in refractory siderophile elements. The distribution of W and other siderophile elements between metal and silicate phases are indicative of the higher temperature and lower oxygen fugacity of the assemblage. However, contrary to previous claims, the distribution of W cannot be used to calculate the equilibration temperature. Low K and high U contents are also reflected in the anomalous amounts of 40 Ar and 4 He. The old K-Ar age (4.7 ± 0.3 Gyr) and high 244 Pu track densities indicate mobilisation of U and Pu-rich phases shortly after formation of the parent material. This and other evidence suggests that Acapulco may represent a rock formed in the early stages of incipient melting of a chondritic parent body. However, since compositional differences between Acapulco and H-chondrites cannot be explained by fractionation processes on the Acapulco parent body. Acapulco must have originated from a different parent body. Lack of depletion of volatile elements, absence of chondrules and reduced mineral composition indicate some relationship of Acapulco to silicate inclusions in iron meteorites and to other unusual meteorites. Oxygen isotopes and chemical data suggest that there are at least three different groups of reduced chondritic meteorites: 1. (a) Acapulco, Lodran, and probably Allan Hills A 77081; 2. (b) Pontlyfni, Mount Morris, Winona and silicate inclusions in IAB iron meteorites, and 3. (c) Kakangari. An exposure age of 5 × 10 6 yr is deduced from spallogenic rare gas data.

The solubility of rhenium in silicate melts: Implications for the geochemical properties of rhenium at high temperatures

Abstract The bulk texture and composition of four monomict eucrites, five polymict eucrites, and one howardite, as well as those of 16 separated clasts and lithological units from these samples were analyzed by optical and scanning electron microscopy and by electron microprobe. Bulk chemical compositions were obtained by INAA. The monomict eucrites Stannern, Millbillillie, Camel Donga, and Juvinas are recrystallized monomict breccias that probably originate from brecciated crater floors or ejecta blocks. The texture of igneous clasts from Juvinas can be explained by interactions of impact and igneous activity that led to disturbance of magma crystallization. Due to the presence of lithic clasts with highly variable chemical compositions and the occurrence of both equilibrated and unequilibrated pyroxenes, the monomict eucrite Pasamonte is redescribed as a polymict eucrite. Three clasts of impact-related lithologies in the polymict eucrite Pasamonte and the howardite EET 87503 contain considerable amounts of chondritic projectile contaminations. The textures of the investigated meteorites reflect a complex post-igneous history dominated by multistage thermal and impact metamorphism. The chronological sequence of thermal and impact events comprises up to six evolutionary phases. Phase I represents crystallization of primary magmas that led to the formation of unequilibrated basalts and other igneous rocks. Phase II represents slow subsolidus cooling or a period of reheating during which pyroxene equilibrated. Phases III and V represent periods of impact brecciation during which the rocks were brecciated in situ or, in the case of polymict HED breccias, mixed with various other rock types. During Phases IV and VI the breccias suffered annealing and recrystallization due to thermal metamorphism. The thermal events that caused recrystallization and equilibration of HED lithologies were active prior, during, and after the formation of impact breccias, indicating that the thermal input by impact might be responsible for thermal overprinting.

Lunar highland meteorites and the composition of the lunar crust

The solubility of rhenium (Re) in a haplobasaltic melt (anorthite-diopside eutectic composition) has been experimentally determined using the mechanically assisted equilibration technique at 1400°C as a function of oxygen fugacity (10−12 < fO2 ≤ 10−7 bar), imposed by CO-CO2 gas mixtures. Samples were analysed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). This is a true microanalytical technique, which allows small-scale sample heterogeneity to be detected, while providing a limit of detection of 2 ppb Re. Time-resolved LA-ICP-MS spectra revealed the presence of suboptically sized micronuggets of Re in all samples, which, because they are present at the 0.5 to 10 ppm level, dominate the true solubilities of Re (<1 ppm at the conditions of the experiment) in bulk analyses of the samples. Nevertheless, the micronuggets could be filtered out from the time-resolved spectra to reveal accurate values of the true Re solubility. A number of time series of samples were taken at constant fO2 to demonstrate that the solubilities converge to a constant value. In addition, solubilities were measured after increasing and decreasing the imposed fO2. The results show that Re dissolves in the silicate melt as ReO2 (Re4+) and ReO3 (Re6+) species, with the latter predominating at typical terrestrial upper-mantle oxygen fugacities. The total solubility of Re is described by the following expression (fO2 in bars): [Re/ppb] = 9.7(±1.9) × 109 (fO2) + 4.2 (±0.3) × 1014 (fO2)1.5Assuming an activity coefficient for Re in Fe-rich metal of 1, this gives a value of DRemet/sil of 5 × 1010 at log fO2 = IW-2, appropriate for metal-silicate partitioning in an homogenously accreting Earth. Thus, Re is indeed very highly siderophile, and the mantle’s abundance cannot be explained by homogenous accretion.

Halogens in meteorites and their primordial abundances

Abstract Major, minor, and trace element data obtained by neutron activation techniques and by spark source mass spectrometry (SSMS) on two lunar meteorites MAC88104 and MAC88105 are reported. Both MAC samples were also analysed for their contents and isotopic compositions of rare gases. Additional SSMS-data were obtained on four lunar highland meteorites previously found in Antarctica: ALHA81005, Y791197, Y82192, and Y86032. MAC88104 and MAC88105 are very similar in chemistry, suggesting that they are pieces of a single fall event. The bulk chemical composition of MAC88104/5 is not very different from the other lunar highland meteorites: highly aluminous with relatively low contents of REE and siderophile element concentrations slightly above 1% of a CI-chondritic level. However, mafic element concentrations (Mg, Cr, Mn, etc.) are slightly lower in MAC88104/5 than in the other lunar highland meteorites. The contents of solar rare gases in the two MAC samples are low, indicating only a small regolith contribution in agreement with rare petrographically identifiable regolith components. The MAC samples and also Y82192 and Y86032 are classified as fragmental breccias with negligible regolith components, in contrast to ALHA81005 and Y791197 which are regolith breccias with high solar wind derived rare gas contents. There is no correlation among lunar meteorites between peak shock pressures and solar gas contents, indicating that peak shock pressures of up to 25 GPa do not lead to gas loss. A low 26Al activity ( Vogt et al., 1990) and high contents of cosmogenic rare gases in MAC88104/5 suggest a long exposure (400,000 years) in the lunar sub-surface. K-Ar ages are in excess of 3.9 by. Lunar highland meteorites and compositionally similar granulitic rocks from the Apollo 16 and 17 landing sites contain about 1% of a CI-chondritic component, according to siderophile and volatile element contents, but independent of the amount of regolith components. Apparently, the major fraction of meteoritic elements in these rocks was not provided by micrometeorites impacting the regolith. The abundances of siderophile (e.g., Ir) and volatile elements may therefore reflect the last spike of accretion of the Moon after the formation of the anorthositic crust. Lunar meteorites of highland origin are chemically different from the bulk of the Apollo 16 highland samples in having higher Fe Mg ratios and lower contents and less fractionated patterns of incompatible and siderophile elements. Since lunar highland meteorites are associated with at least three but probably four different fall events, and since they are not derived from chemically exotic front-side terranes, they may represent a better sampling of the average chemical composition of the lunar crust than previous estimates based on returned lunar samples and remote sensing data. A comparison between an average highland composition derived by Taylor (1982) and an estimate based on lunar highland meteorites shows that the Taylor composition contains higher concentrations and more fractionated incompatible elements mainly because of a substantial amount of KREEP (a trace element rich, highly fractionated component from the front side of the Moon not present at the sites from which the lunar meteorites come).