• A Mössbauer spectroscopy and X-ray diffraction study of ordinary chondrites: Quantification of modal mineralogy and implications for redox conditions during metamorphism

      Menzies, O. N.; Bland, P. A.; Berry, F. J.; Cressey, G. (The Meteoritical Society, 2005-01-01)
      We present a method that combines Mössbauer spectroscopy and X-ray diffraction to quantify the modal mineralogy of unequilibrated ordinary chondrites (UOCs). Despite being a fundamental tool in the interpretation of geological systems, there are no modal mineralogical data available for these meteorites. This is due to their fine-grained nature, highly heterogeneous silicate mineralogy, and the presence of poorly characterized phases. Consequently, it has not been possible to obtain accurate modal mineralogy by conventional techniques such as point counting. Here we use Mössbauer spectroscopy as a preliminary identification technique and X-ray diffraction provides the quantification for a suite of recent UOC falls. We find the most primitive UOCs to contain a significant amount of phyllosilicate material that was converted during metamorphism to form ferromagnesian silicates. A complete suite of Antarctic samples is analyzed by each method to observe mineralogical trends and these are compared with trends shown by recent falls. The fact that mineralogical relationships shown by finds and falls are in agreement allows us to be confident that we are observing the products of pre-terrestrial alteration. Mössbauer spectroscopy reveals evidence of steadily increasing reduction with metamorphism in the UOCs. Because this technique allows comparisons to be made between UOCs and EOCs, our reduction sequence can be combined with other evidence showing progressive oxidation in the EOCs. This yields an integrated model of changing redox conditions on equilibrating ordinary chondrite parent bodies.
    • Bhawad LL6 chondrite: Chemistry, petrology, noble gases, nuclear tracks, and cosmogenic radionuclides

      Bhandari, N.; Murty, S. V. S.; Shukla, P. N.; Mahajan, R. R.; Shukla, A. D.; Suthar, K. M.; Parthasarathy, G.; Paliwal, B. S. (The Meteoritical Society, 2005-01-01)
      Chemical and mineral analysis of the Bhawad chondrite, which fell in Rajasthan in 2002, suggest that this stone belongs to LL6 group of chondrites. Based on helium, neon, and argon isotopes, it has a cosmic ray exposure age of 16.3 Ma. The track density in the olivines shows a narrow range of 1.7-6.8 106/cm2. The 22Na/26Al ratio of 1.13 is about 25% lower than the solar cycle average value of about 1.5, but is consistent with irradiation of the meteoroid to modulated galactic cosmic ray fluxes as expected for a fall around the solar maximum. The cosmogenic records indicate a pre-atmospheric radius of about 7.5 cm. Based on U/Th-4He and K-40Ar, the gas retention ages are low (about 1.1 Ga), indicating a major thermal event or shock event that lead to the complete loss of radiogenic 4He and 40Ar and the partial loss of radiogenic 129Xe and fission Xe from 244Pu.
    • Call for nominations: J. Lawrence Smith Medal

      The Meteoritical Society, 2005-01-01
    • Evidence from the Rb-Sr system for 4.4 Ga alteration of chondrules in the Allende (CV3) parent body

      Shimoda, G.; Nakamura, N.; Kimura, M.; Kani, T.; Nohda, S.; Yamamoto, K. (The Meteoritical Society, 2005-01-01)
      The timing and processes of alteration in the CV parent body are investigated by the analysis of Sr isotopes, major and trace elements, and petrographic type and distribution of the secondary minerals (nepheline and sodalite) in 22 chondrules from the Allende (CV3) chondrite. The Sr isotopic compositions of the chondrules are scattered around the 4.0 Ga reference line on the 87Sr/ 86Sr evolution diagram, indicating that the chondrules have been affected by late thermal alteration event(s) in the parent body. The degree of alteration, determined for individual chondrules based on the distribution of nepheline and sodalite, is unrelated to the disturbance of the Rb-Sr system, suggesting that the alteration process that produced nepheline and sodalite is different from the thermal process that disturbed the Rb-Sr system of the chondrules. Considering the geochemical behavior of Rb and Sr, the main host phase of Sr in chondrules is likely to be mesostasis, which could be most susceptible to late thermal alteration. As there is a poor connection between the alteration degree determined from abundances of nepheline and sodalite and the disturbance of Rb-Sr isotopic system, we consider the mesostasis to provide a constraint on the late parent body alteration process. From this point of view, 23 mesostasis-rich chondrules, including those from literature data, were selected. The selected chondrules are closely correlated on the 87Sr/86Sr evolution diagram, with an inferred age of 4.36 +/- 0.08 Ga. This correlation would represent an age of the final major Sr isotopic redistribution of the chondrules in the parent body.
    • Fine-grained, spinel-rich inclusions from the reduced CV chondrite Efremovka: II. Oxygen isotopic compositions

      Aléon, J.; Krot, A. N.; McKeegan, K. D.; MacPherson, G. J.; Ulyanov, A. A. (The Meteoritical Society, 2005-01-01)
      Oxygen isotopes have been measured by ion microprobe in individual minerals (spinel, Al- Ti-diopside, melilite, and anorthite) within four relatively unaltered, fine-grained, spinel-rich Ca-Alrich inclusions (CAIs) from the reduced CV chondrite Efremovka. Spinel is uniformly 16O-rich (Delta-17O is less than or equal to -20 ppm) in all four CAIs; Al-Ti-diopside is similarly 16O-rich in all but one CAI, where it has smaller 16O excesses (-15 ppm is less than or equal to Delta-17O, which is less than or equal to -10 ppm). Anorthite and melilite vary widely in composition from 16O-rich to 16O-poor (-22 ppm is less than or equal to Delta-17O, which is less than or equal to -5 ppm). Two of the CAIs are known to have group II volatilityfractionated rare-earth-element patterns, which is typical of this variety of CAI and which suggests formation by condensation. The association of such trace element patterns with 16O-enrichment in these CAIs suggests that they formed by gas-solid condensation from an 16O-rich gas. They subsequently experienced thermal processing in an 16O-poor reservoir, resulting in partial oxygen isotope exchange. Within each inclusion, oxygen isotope variations from mineral to mineral are consistent with solid-state oxygen self-diffusion at the grain-to-grain scale, but such a model is not consistent with isotopic variations at a larger scale in two of the CAIs. The spatial association of 16O depletions with both elevated Fe contents in spinel and the presence of nepheline suggests that latestage iron-alkali metasomatism played some role in modifying the isotopic patterns in some CAIs. One of the CAIs is a compound object consisting of a coarse-grained, melilite-rich (type A) lithology joined to a fine-grained, spinel-rich one. Melilite and anorthite in the fine-grained portion are mainly 16O-rich, whereas melilite in the type A portion ranges from 16O-rich to 16O-poor, suggesting that oxygen isotope exchange predated the joining together of the two parts and that both 16O-rich and 16O-poor gaseous reservoirs existed simultaneously in the early solar nebula.
    • MALDI analysis of presolar nanodiamonds: Mass spectrometric determination of the mass distribution of nanodiamonds from meteorites and a technique to manipulate individual nanodiamonds

      Lyon, Ian C. (The Meteoritical Society, 2005-01-01)
      This paper describes the use of matrix-assisted laser desorption and ionization (MALDI) to measure the mass distribution of nanodiamonds extracted from meteorites. The techniques used to prepare and mass analyze nanodiamond samples from the Murchison (CM2) and Allende (CV3) meteorites are described. The mass spectra of nanodiamonds (peaking at between 1 x 10^(4)-1.5 x 10^4 Daltons) are compared with size distributions obtained by point-counting transmission electron microscopy (TEM) images obtained elsewhere and reasonable agreement is found. The implications of the ability to produce and mass analyze a beam of nanodiamonds are explored.
    • Motion of iron sulfide inclusions inside a shock-melted chondrule

      Uesugi, M.; Akaki, T.; Sekiya, M.; Nakamura, T. (The Meteoritical Society, 2005-01-01)
      We calculated the trajectories of molten spheres of iron sulfide inclusions inside a melted chondrule during the nebular shock wave heating. Our calculations included the effects of highvelocity internal flow in the melted chondrule and apparent gravitational force caused by the drag force of nebular gas flow. The calculated results show that large iron sulfide inclusions, which have radii 0.23 times larger than those of the parent chondrules, must reach the surface of the melted chondrule within a short period of time (<<1 s). This effect will provide us with very important information about chondrule formation by nebular shock wave heating.
    • "New" lunar meteorites: Impact melt and regolith breccias and large-scale heterogeneities of the upper lunar crust

      Warren, P. H.; Ulff-Møller, F.; Kallemeyn, G. W. (The Meteoritical Society, 2005-01-01)
      We have analyzed nine highland lunar meteorites (lunaites) using mainly INAA. Several of these rocks are difficult to classify. Dhofar 081 is basically a fragmental breccia, but much of its groundmass features a glassy-fluidized texture that is indicative of localized shock melting. Also, much of the matrix glass is swirly-brown, suggesting a possible regolith derivation. We interpret Dar al Gani (DaG) 400 as an extremely immature regolith breccia consisting mainly of impact-melt breccia clasts; we interpret Dhofar 026 as an unusually complex anorthositic impact-melt breccia with scattered ovoid globules that formed as clasts of mafic, subophitic impact melt. The presence of mafic crystalline globules in a lunar material, even one so clearly impact-heated, suggests that it may have originated as a regolith. Our new data and a synthesis of literature data suggest a contrast in Al2O3- incompatible element systematics between impact melts from the central nearside highlands, where Apollo sampling occurred, and those from the general highland surface of the Moon. Impact melts from the general highland surface tend to have systematically lower incompatible element concentration at any given Al2O3 concentration than those from Apollo 16. In the case of Dhofar 026, both the bulk rock and a comparatively Al-poor composition (14 wt% Al2O3, 7 micrograms/g Sm) extrapolated for the globules, manifest incompatible element contents well below the Apollo 16 trend. Impact melts from Luna 20 (57 degrees E) distribute more along the general highland trend than along the Apollo 16 trend. Siderophile elements also show a distinctive composition for Apollo 16 impact melts: Ni/Ir averaging ~1.8x chondritic. In contrast, lunaite impact-melt breccias have consistently chondritic Ni/ Ir. Impact melts from Luna 20 and other Apollo sites show average Ni/Ir almost as high as those from Apollo 16. The prevalence of this distinctive Ni/Ir ratio at such widely separated nearside sites suggests that debris from one extraordinarily large impact may dominate the megaregolith siderophile component of a nearside region 2300 km or more across. Highland polymict breccia lunaites and other KREEP-poor highland regolith samples manifest a strong anticorrelation between Al2O3 and mg. The magnesian component probably represents the chemical signature of the Mg-suite of pristine nonmare rocks in its most pure form, unaltered by the major KREEP-assimilation that is so common among Apollo Mg-suite samples. The average composition of the ferroan anorthositic component is now well constrained at Al2O3 ~29-30 wt(implying about 17-19 wt% modal mafic silicates), in good agreement with the composition predicted for flotation crust over a ferroan magma ocean (Warren 1990).
    • Petrography and geochemistry of the LaPaz Icefield basaltic lunar meteorite and source crater pairing with Northwest Africa 032

      Zeigler, R. A.; Korotev, R. L.; Jolliff, B. L.; Haskin, L. A. (The Meteoritical Society, 2005-01-01)
      We report on the bulk composition and petrography of four new basaltic meteorites found in AntarcticaLAP (LaPaz Icefield) 02205, LAP 02224, LAP 02226, and LAP 02436--and compare the LAP meteorites to other lunar mare basalts. The LAP meteorites are coarse-grained (up to 1.5 mm), subophitic low-Ti basalts composed predominantly of pyroxene and plagioclase, with minor amounts of olivine, ilmenite, and a groundmass dominated by fayalite and cristobalite. All of our observations and results support the hypothesis that the LAP stones are mutually paired with each other. In detail, the geochemistry of LAP is unlike those of any previously studied lunar basalt except lunar meteorite NWA (Northwest Africa) 032. The similarities between LAP and NWA 032 are so strong that the two meteorites are almost certainly source crater paired and could be two different samples of a single basalt flow. Petrogenetic modeling suggests that the parent melt of LAP (and NWA 032) is generally similar to Apollo 15 low-Ti, yellow picritic glass beads, and that the source region for LAP comes from a similar region of the lunar mantle as previously analyzed lunar basalts.
    • Re-examining the role of chondrules in producing the elemental fractionations in chondrites

      Alexander, C. M. O'D. (The Meteoritical Society, 2005-01-01)
      The matrices of all primitive chondrites contain presolar materials (circumstellar grains and interstellar organics) in roughly CI abundances, suggesting that all chondrites accreted matrix that is dominated by a CI-like component. The matrix-normalized abundances of the more volatile elements (condensation temperatures <750–800 K) in carbonaceous and ordinary chondrites are also at or slightly above CI levels. The modest excesses may be due to low levels of these elements in chondrules and associated metal. Subtraction of a CI-like matrix component from a bulk ordinary chondrite composition closely matches the average composition of chondrules determined by instrumental neutron activation analysis (INAA) if some Fe-metal is added to the chondrule composition.Measured matrix compositions are not CI-like. Sampling bias and secondary redistribution of elements may have played a role, but the best explanation is that ∼10–30% of refractory-rich, volatile depleted material was added to matrix. If most of the more volatile elements are in a CI-dominated matrix, the major and volatile element fractionations must be largely carried by chondrules. There is both direct and indirect evidence for evaporation during chondrule formation. Type IIA and type B chondrules could haveformed from a mixture of CI material and material evaporated from type IA chondrules. The Mg-Si-Fe fractionations in the ordinary chondrites can be reproduced with the loss of type IA chondrule material and associated metal. The loss of evaporated material from the chondrules could explain the volatile element fractionations. Mechanisms for how these fractionations occurred are necessarily speculative, but two possibilities are briefly explored.
    • Silica as a shock index in shergottites: A cathodoluminescence study

      Chennaoui Aoudjehane, H.; Jambon, A.; Reynard, B.; Blanc, P. (The Meteoritical Society, 2005-01-01)
      Silica in shergottites is a minor phase of great significance. Determining its structural state as either silica glass, quartz, cristobalite, tridymite, coesite, stishovite, or post-stishovite could provide informations about their shock history. The purpose of this work is to assess the shock intensity in shergottites using two spectroscopic methods. On a conventional polished section, a scanning electron microscope (SEM) enables us to study the cathodoluminescence (CL) of silica at variable magnification. The results were crosschecked by systematic Raman spectroscopy of the selected areas. CL spectra differ substantially from one another and enable separating stishovite, high and low pressure silica glass, quartz, and cristobalite. We studied a set of five shergottites: Northwest Africa (NWA) 480, NWA 856, Zagami, Shergotty, and Los Angeles. Stishovite is common in Shergotty, Zagami, NWA 856, and NWA 480 and absent in the studied section of Los Angeles. High-pressure glass is very common, particularly in close association with stishovite. According to the textural relationship, it may be a product of the retromorphosis (amorphization during decompression) of stishovite. Large stishovite areas result from the transformation of preexisting low-pressure silica crystals, while needles result from the high-pressure transformation of pyroxene to glass (melt) and silica. In the latter case, they are found in melt pockets and represent a small fraction of areas of overall pyroxene composition. Needles exhibit square sections of about 1 m. Silica spots identical to those described previously as poststishovite are found in Shergotty, Zagami, NWA 480, and NWA 856. At present, the spectroscopic distinction of post-stishovite from stishovite is difficult. Post-stishovite is destroyed under the Raman beam, and CL spectra are possible mixtures of several phases (e.g., glass and post-stishovite). It is concluded that the shock intensity is highly heterogeneous, and the pressure probably exceeded 60 GPa in all shergottites studied here.
    • The effect of Lagrangian L4/L5 on satellite formation

      Howard, E. (The Meteoritical Society, 2005-01-01)