H. W. Weber

Solubility of He, Ne, Ar, Kr and Xe in a basalt melt in the range 1250–1600°C. Geochemical implications

Abstract The solubility constant of Henrys law has been experimentally determined in a tholeiitic basalt melt. Equilibration with air, a noble gas mix, and various mixtures of the two permitted us to assess the validity of Henrys law under 1 bar ambient pressure in the temperature range 1250–1600°C. Mass spectrometric analyses of quenched products yield solubilities decreasing with increasing atomic size as: 56 (11.), 25 (3.), 5.9 (0.9), 3.0 (0.7) and 1.7 (0.4) in units of 10−5 cm3 STP/g-bar (with standard deviation) for He, Ne, Ar, Kr and Xe, respectively (1400°C). Partial pressures were varied by actors between 100 and 2 × 104. The dependence on temperature is very moderate. Comparison with earlier results confirms a significant dependence of solubility on melt composition. The data permit calculation of the distribution of noble gases between a melt and coexisting vesicles. Comparison with data obtained on MORB glasses shows that He, Ne and Ar display an equilibrium distribution while Kr and Xe in the vesicle-free glass are probably below analytical detection. The strong fractionation effects implied by the very different solubilities can explain most of the variations observed in MORB-glasses for He Ne and He Ar ratios.

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 (

Rare gases and 36Cl in stony-iron meteorites: cosmogenic elemental production rates, exposure ages, diffusion losses and thermal histories

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

Metal and silicate portions from 13 mesosiderites, one pallasite, Bencubbin (“unique”) and Udei Station (‘iron with silicate inclusions’) have been analysed for their content of He, Ne and Ar; in most cases 36Cl could be determined as well. 36Cl-36Ar cosmic ray exposure ages fall between 10 and 160 Myr. Half of the metal samples show a deficit of spallogenic 3He (up to 30%) which we ascribe to a loss of tritium. The observed depletion of 3He in the silicates is correlated with their mineralogical composition: feldspar has lost its 3He in all cases, pyroxene definitely in one and possibly in five others, while olivine has been affected in only two meteorites. The thermal histories during their exposure to the cosmic radiation have been different for different meteoroids. Nevertheless, with the exception of Veramin, the data are compatible with the assumption of a continuous diffusion loss during a considerable fraction of the exposure era. For Veramin, however, an episodic event late in the exposure history is required. The exceptionally high 39Ar36Cl ratio in the metal, which is due to a high 39Ar activity, indicates that the event occurred during the last 500,000 years or so and resulted in an extremely excentric orbit (large aphelion). Production rates of 38,39Ar from Ca and 21,22Ne from Mg are given. The ratio P38CaP21Mg is close to unity. The ratios P38CaP38Fe vary between 20 and 50, and are not correlated with the absolute production rate of 38Ar from metal. The 22Ne21Ne production ratio from Mg is found to be close to but below unity. Of the mesosiderites only Veramin shows unambiguous evidence for primordial rare gases with larger amounts and a higher 20Ne36Ar ratio in the olivine, suggesting in situ fractionation to have at least been partly responsible for the abundance pattern found. Bencubbin contains large amounts of strongly fractionated primordial gases, but again part of the fractionation may have occurred in situ. Udei Station shows an excess of (3.5 ± 0.6) × 10−10 cm3 STP 129Xe/g in the non-magnetic portion.

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.

Helium and argon from an Atlantic MORB glass: concentration, distribution and isotopic composition

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.

Lunar highland meteorites and the composition of the lunar crust

Data are reported for detailed He and Ar measurements on a single MORB glass from the Atlantic (CH 98-DR 11; 30°41′N, 41°49′W; depth ≈ 3500 m). Grain-size fractions prepared under ambient conditions are strongly affected by diffusive loss of He from grains and by adsorption of atmospheric argon on grain surfaces. Both effects could be controlled by crushing gram-sized chunks of glass under vacuum and analyzing the powder without any further handling, in particular without its exposure to the atmosphere. Gases from vesicles, released upon crushing, are characterized by a4He/40Ar ratio of6 ± 1 and a40Ar/36Ar ratio of up to 22,600. In dissolved gases the4He/40Ar ratio was found to be (7.2 ± 1.6) times higher and the40Ar/36Ar ratio to be about ten times lower than in vesicles. The difference of theelemental ratio He/Ar is as anticipated from the ratio of the solubilities (ca. nine) in the investigated basalt of He and Ar. Thus, whileelemental abundance ratios are compatible with equilibrium between vesicles and basalt the grossly different argonisotopic ratios are not. It is proposed that two basalts, chemically very similar but with different40Ar/36Ar ratios, were mixed shortly before eruption. The overall4He/40Ar ratio in the vesiculated basalt is12 ± 2 so that, for the ratio in the primary magma, we find6 ⩽4He/40Ar⩽ 12 which is considerably higher than the radiogenic production ratio in all conceivable sources of MORB. Preferential removal from a melt of argon via vesicles is suggested to be the most likely explanation, whereas any metasomatic transfer seems unrealistic.

Noble gases and the history of Jilin meteorite

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).

Allan Hills 77081—an unusual stony meteorite

Abstract Potassium and noble gases have been determined in more than twenty specimens from the largest known stone meteorite, the H5 chondrite Jilin. Thirteen specimens came from the surface of the present main mass, the remainder from various locations in the strewn field. The average K content is 802 ppm (29 samples from 23 specimens), maximum deviations from the mean are −7% and +11%. Whole-rock gas retention ages of different specimens are distinctly different; they vary between 2.22 and 3.90 AE for40Ar 40K and between 0.44 and 2.0 AE for4He U/Th. Severe losses of4He and40Ar (up to 95% and 80%, respectively) must have occurred (up to) less than 440 Ma ago; they cannot have happened during the fall of the meteorite, however. Differences in concentration of cosmic-ray-produced3He,21Ne, and38ArMe by factors of five, six, and seven, respectively, reflect a complex irradiation history; they are compatible with a short 4π irradiation and an extended one in 2π geometry at shallow depth (top samples were most probably located near the transition maximum of nuclear-active particles at around 15 cm depth; they definitely cannot have been buried deeper than 4 m).3He/21Ne ratios in bulk samples are lowered by diffusion losses of3He (25–61%) while22Ne/21Ne ratios appear to be unaffected.22Ne/21Ne values range between 1.060 and 1.086 (with a mean of 1.069) which is at variance with predictions for the particular irradiation conditions of Jilin. Low36Ar/38Ar ratios (down to 0.553) in clean metal samples are interpreted as the combined effect of large size and the transient lowering of this ratio because of a sudden increase in production rates upon going from 2π to 4π irradiation.

26Al and10Be production in iron meteorites

Abstract Allan Hills (ALHA) 77081 is achondritic in texture while the mineral composition and the chemistry are chondritic with the exception of a few elements. An assignment to one specific group of ordinary chondrites is therefore difficult. In many respects this meteorite is similar to the unusual stone meteorite Acapulco. The REE pattern of ALHA 77081 is essentially flat and the distribution ratios of siderophile elements between metal and silicates are high compared to ordinary chondrites. Gas retention ages are 3.5±0.5 AE for U, Th-He and 4.50±0.15 AE for K-Ar. In spite of the high degree of recrystallisation the meteorite contains trapped noble gases in amounts comparable to type 4 chondrites. Cosmic ray tracks and spallogenic noble gases indicate a small preatmospheric radius of about 2–3 cm. Spallogenic nuclides produced by solar cosmic rays or stopped solar flare ions may be present.