• The Third International Conference on Magnetism, Metamorphism and Associated Mineralizations "3MA", Casablanca, Morocco 2003 May 8-10

      The Meteoritical Society, 2002-01-01
      Announcement: The Third International Conference on Magnetism, Metamorphism and Associated Mineralizations "3MA"
    • Pre-atmospheric depths and thermal histories of Canyon Diablo spheroids

      Leya, I.; Wieler, R.; Ma, P.; Schnabel, C.; Herzog, G. F. (The Meteoritical Society, 2002-01-01)
      Despite having melted during formation, seven of eight Canyon Diablo spheroids weighing from 0.6 to 13 mg retain cosmic-ray-produced 38Ar (38Arcos) in concentrations [10^(-10) cm^3 STP/g] ranging from 0.35 to 68. The presence of 38Arcos is consistent with pre-atmospheric depths of <2.3 meters and most likely rules out an origin for the spheroids deep within the projectile, which had a radius of ~15 m. Low levels of 21Necos indicate gas loss from these spheroids. Relative to most Canyon Diablo meteorites, the spheroids contain lower concentrations of cosmogenic noble gases. The difference partly reflects diffusion losses from the spheroids, especially for 3He and 21Ne, but also suggests deeper locations on average for the precursor material, consistent with independent results from 59Ni.
    • Aluminum-26 in H4 chondrites: Implications for its production and its usefulness as a fine-scale chronometer for early solar system events

      Zinner, Ernst; Göpel, Christa (The Meteoritical Society, 2002-01-01)
      In order to investigate whether or not 26Al can be used as a fine-scale chronometer for early-solar-system events we measured, with an ion microprobe, Mg isotopes and Al/Mg ratios in separated plagioclase, olivine, and pyroxene crystals from the H4 chondrites Ste. Marguerite, Forest Vale, Beaver Creek and Quenggouk and compared the results with the canonical 26Al/27Al ratio for Ca,Al-rich inclusions (CAIs). For Ste. Marguerite (SM) and Forest Vale (FV) Pb/Pb and Mn-Cr ages have previously been determined (Gpel et al., 1994; Polnau et al., 2000; Polnau and Lugmair, 2001). Plagioclase grains from these two meteorites show clear excesses of 26Mg. The 26Al/27Al ratios inferred from these excesses and from isotopically normal Mg in pyroxene and olivine are (2.87 +/- 0.64) x 10^(-7) for SM and (1.52 +/- 0.52) x 10^(-7) for FV. The differences between these ratios and the ratio of 5 x 10^(-5) in CAIs indicate time differences of 5.4 +/- 0.1 Ma and 6.1 +/- 0.2 Ma for SM and FV, respectively. These differences are in agreement with the absolute Pb/Pb ages for CAIs and SM and FV phosphates but there are large discrepancies between the U-Pb and Mn-Cr system for the relative ages for CAIs, SM and FV. For example, Mn-Cr ages of carbonates from Kaidun are older than the Pb/Pb age of CAIs. However, even if we require that CAIs are older than these carbonates, the time difference between this "adjusted" CAI age and the Mn-Cr ages of SM and FV require that 26Al was widely distributed in the early solar system at the time of CAI formation and was not mostly present in CAIs, a feature of the X-wind model proposed by Shu and collaborators (Gounelle et al., 2001; Shu et al., 2001). From this we conclude that there was enough 26Al to melt small planetary bodies as long as they formed within 2 Ma of CAIs, and that 26Al can serve as a fine-scale chronometer for early solar system events.
    • Noble gas compositions of Antarctic micrometeorites collected at the Dome Fuji Station in 1996 and 1997

      Osawa, T.; Nagao, K. (The Meteoritical Society, 2002-01-01)
      The noble gases He, Ne, Ar, and Xe were measured in 27 individual Antarctic micrometeorites (AMMs) i the size range 60 to 250 micrometers that were collected at the Dome Fuji Station. Eleven of the AMMs were collected in 1996 (F96 series) and 16 were collected in 1997 (F97 series). One of the F97 AMMs is a totally melted spherule, whereas all other particles are irregular in shape. Noble gases were extracted using a Nd-YAG continuous wave laser with an output power of 2.5-3.5 W for ~5 min. Most particles released measurable amounts of noble gases. 3He/4He ratios are determined for 26 AMMs ((0.85-9.65) x 10^(-4)). Solar energetic panels (SEP) are the dominant source of helium in most AMMs rather than solar wind (SW) and cosmogenic He. Three samples had 3He/4He ratios compared to that of SW, showing the presence of spallogenic 3He. The Ne isotopic higher composition of most AMMs resembled that of SEP as in the case of helium. Spallogenic 21Ne was detected in three samples, two of which had extremely long cosmic-ray exposure ages (>100 Ma), calculated by assuming solar cosmic-ray (SCR) + galactic cosmic-ray (GCR) production. These two particles may have come to Earth directly from the Kuiper Belt. Most AMMs had negligible amounts of cosmogenic 21Ne and exposure ages of <1 Ma. 40Ar/36Ar ratios for all particles (3.9-289) were lower than that of the terrestrial atmosphere (296), indicating an extraterrestrial origin of part of the Ar with a very low 40Ar/36Ar ratio plus some atmospheric contamination. Indeed, 40Ar/36Ar ratios for the AMMs are higher than SW, SEP, and Q-Ar values, which is explained by the presence of atmospheric 40Ar. The average 38Ar/36Ar ratio of 24 AMMs (0.194) is slightly higher than the value of atmospheric or Q-Ar, suggesting the presence of SEP-Ar which has a relatively high 38Ar/36Ar ratio. According to the elemental compositions of the heavy noble gases, Dome Fuji AMMs can be classified into three groups: chondritic (eight particles), air-affected (nine particles), and solar-affected (eight particles). The eight AMMs classified as chondritic preserve the heavy noble gas composition of primordial trapped component due to lack of atmospheric adsorption and solar implantation. The average of 129Xe/132Xe ratio for the 16 AMMs not affected by atmospheric contamination (1.05) corresponds to the values in matrices of carbonaceous chondrites (~1.04). One AMM, F96DK038, has high 129Xe/132Xe in excess of this ratio. Our results imply that most Dome Fuji AMMs originally had chondritic heavy noble gas composition, and carbonaceous chondrite-like objects are appropriate candidate sources for most AMMs.
    • Production of krypton and xenon isotopes in thick stony and iron targets isotropically irradiated with 1600 MeV protons

      Gilabert, E.; Lavielle, B.; Michel, R.; Leya, I.; Neumann, S.; Herpers, U. (The Meteoritical Society, 2002-01-01)
      Two spherical targets made of gabbro with a radius of 25 cm and of steel with a radius of 10 cm were irradiated isotropically with 1600 MeV protons at the SATURNE synchrotron at Laboratoire National Saturne (LNS)/CEN Saclay, in order to simulate the production of nuclides in meteorites induced by galactic cosmic-ray protons in space. These experiments supply depth-dependent production rate data for a wide range of radioactive and stable isotopes in up to 28 target elements. In this paper, we report results for 78Kr, 80-86Kr isotopes in Rb, Sr, Y and Zr and for 124Xe, 126Xe, 128-132Xe, 134Xe, 136Xe isotopes in Ba and La. Krypton and xenon concentrations have been measured at different depths in the spheres by using conventional mass spectrometry. Based on Monte-Carlo techniques, theoretical production rates are calculated by folding depth-dependent spectra of primary and secondary protons and secondary neutrons with the excitation functions of the relevant nuclear reactions. The comparison of the model calculation results with experimental data in the thick target experiments performed at LNS and previously at CERN have allowed adjustments of the poorly known excitation functions of neutron-induced reactions. Thus, for the two experiments at SATURNE, excellent agreement is obtained between experimental and calculated production rates for most Kr and Xe isotopes in all investigated target elements. Only Xe production in Ba in the gabbro is underestimated by the calculations by ~25%. This work validates the approach of the thin-target model calculations of cosmogenic nuclide production rates in the attempt of modeling the interaction of galactic cosmic-ray protons with stony and iron meteorites in space as well as with the lunar samples.
    • An experimental and petrographic investigation of Elephant Moraine 79001 lithology A: Implications for its petrogenesis and the partitioning of chromium and vanadium in a martian basalt

      Herd, C. D. K.; Schwandt, C. S.; Jones, J. H.; Papike, J. J. (The Meteoritical Society, 2002-01-01)
      A composition approximating the lithology A groundmasss of the Elephant Moraine (EET) 79001 martian bassalt (Eg; McSween and Jarosewich, 1983) has been used to investigate the petrogenesis of the meteorite and the behavior of Cr and V at different oxygen fugacities. Crystallization experiments were carried out over a range of temperatures, and oxygen fugacities of either iron-wustite (IW) or IW + 2 (i.e., 1.5 log units below the quartz-fayalite-magnetite (QFM) buffer). Comparison of trace element concentrations (obtained by secondary ion mass spectrometry (SIMS) analysis) in experimental silicates with those of natural silicates supports the Fe-Ti oxide-derived oxygen fugacity of QFM -1.8 +/- 0.3 for this basalt (Herd et al., 2001). Experimental distribution coefficients, in conjuction with SIMS analyses of rims from the olivine and pyroxene xenocrysts in lithology A, as well as analyses of lithology A groundmass pigeonite cores, are used calculate coexisting liquid concentrations of V and Cr. Liquid compositions derived from pigeonite xenocryst rims and groundmass pigeonite cores are similar, suggesting that the rims of orthopyroxene xenocrysts are overgrowths, which have not previously been accounted for when reconstructing the groundmass composition. This implies that the Eg composition requires modification. A similar exercise for the ferroan rims on olivine xenocrysts yields very different liquid compositions, indicating that these rims are not overgrowths but are part of the xenocryst assemblage. These results are shown to be consistent with the petrography of lithology A xenocrysts.
    • 433 Eros: Problems with the meteorite magnetism record in attempting an asteroid match

      Wasilewski, P.; Acuña, M. H.; Kletetschka, G. (The Meteoritical Society, 2002-01-01)
      The Magnetometer experiment (MAG) onboard the NEAR-Shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of (2.1x10^(-6) Am^2 kg^(-1)) for asteroid 433 Eros. This is in sharp contrast with the estimated magnetization of other S-class asteroids (Gaspra ~ 2.4 x 10^(-2) Am^2 kg^(-1), Braille ~ 2.8 x 10^(-2) Am^2 kg^(-1) and is below published values for all types of ordinary chondrites. This includes the L/LL types considered to most closely match 433 Eros based on preliminary interpretations of NEAR remote geochemical experiments. The ordinary chondrite meteorite magnetization intensity data was reviewed in order to assess the reasonableness of an asteroid - meteorite match based on magnetic property measurements. NRM (natural remanent magnetization) intensities for the ordinary chondrite meteorites show at least a 2 order of magnitude range within each of the H,L,LL groups, all well above the 2.1 x10^(-6) Am^2 kg^(-1) level for 433 Eros. The REM values (ratio of the NRM to the SIRM (saturation remanent magnetization)) range over 3 orders of magnitude for all chondrite groups indicating no clear relationship between NRM and the amount of magnetic material. Levels of magnetic noise in chondrite meteorites can be as much as 70% or more of the NRM. Consequently published values of the NRM should be considered suspect unless careful evaluation of the noise sources is done. Goddard studies of per unit mass intensities in large (> 10,000gm) and small (down to 1 gm) samples from the same meteorite demonstrate magnetic intensity decreases as size increases. This would appear to be explained by demagnetization due to magnetic vector randomness at unknown scale sizes in the larger samples. This would then argue for some level of demagnetization of large objects such as an asteroid. The possibility that 433 Eros is an LL chondrite cannot be discounted.
    • The oxygen isotopic composition of water from Tagish Lake: Its relationship to low-temperature phases and to other carbonaceous chondrites

      Baker, L.; Franchi, I. A.; Wright, I. P.; Pillinger, C. T. (The Meteoritical Society, 2002-01-01)
      The fall and recovery of the Tagish Lake meteorite in British Columbia in January 2000 provided a unique opportunity to study relatively pristine samples of carbonaceous chondrite material. Measurements of the oxygen isotopic composition of water extracted under stepped pyrolysis from a bulk sample of this meteorite have allowed us to make comparisons with similar data obtained from CI and CM chondrites and so further investigate any relationships that may exist between these meteorites. The much lower yield of water bearing a terrestrial signature in Tagish Lake is indicative of the pristine nature of the meteorite. The relationship between the isotopic composition of this water and reported isotopic values for carbonates, bulk matrix and whole rock, have been used to infer the extent and conditions under which parent body aqueous alteration occurred. In Tagish Lake the difference in Delta-17O isotopic composition between the water and other phases is greater than that found in either CMs or CIs suggesting that reaction and isotopic exchange between components was more limited. This in turn suggests that in the case of Tagish Lake conditions during the processes of aqueous alteration on the parent body, which ultimately controlled the formation of new minerals, were distinct from those on both CI and CM parent bodies.
    • From the Editor

      Rietmeijer, F. J. M. (The Meteoritical Society, 2002-01-01)
      Editorials: F.J. M. Rietmeijer: And just when you thought that the Leonid meteor storm held no more surprises: The 2001 storm.
    • Thermal decomposition pattern and particle size estimation of iron minerals associated with the Cretaceous-Tertiary boundary at Gubbio

      Verma, H. C.; Upadhyay, Chandan; Tripathi, Amita; Tripathi, R. P.; Bhandari, N. (The Meteoritical Society, 2002-01-01)
      Mössbauer studies of the samples from the Cretaceous-Tertiary Boundary (KTB) layer at Gubbio, Italy show that iron appears mainly in two phases, magnetically ordered hematite and a paramagnetic silicate phase. The average particle size of hematite is estimated to be in the range 16-27 nm from the Transmission Electron Micrographs and lack of Morin transition. The hyperfine magnetic field at the iron nucleus is observed to be somewhat less than that of bulk hematite, which may be explained by collective magnetic excitation. Stepwise heating up to 1000 degrees C shows a decomposition pattern of the paramagnetic phase, which suggests it to be a tri-octahedral layer silicate. The iron-bearing phases found in the bulk sedimentary KTB material are different from those found in the spherules separated from this material indicating that the REDOX conditions changed rapidly after the impact, becoming more oxidizing during the period these bulk phases were formed.