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NASA NTRS1986

Trace elements in Antarctic meteorites: Weathering and genetic information

Antarctic meteorite discoveries have created great scientific interest due to the large number of specimens recovered (approximately 7000) and because included are representatives of hitherto rare or unknown types. Antarctic meteorites are abundant because they have fallen over long periods and were preserved, transported, and concentrated by the ice sheets. The weathering effects on the Antarctic meteorites are described. Weathering effects of trace element contents of H5 chondrites were studied in detail. The results are examined. The properties of Antarctic finds and non-Antarctic falls are discussed.

Lipschutz, M. E.↗
NASA NTRS1986

Clay-mineraloid weathering products in Antarctic meteorites

The production of clay mineraloids (CMs) in the weathering of stony meteorites recovered in the Allan Hills and Elephant Moraine areas of Antarctica is investigated, applying electron microbeam analysis, pyrolysis/mass spectroscopy, X-ray diffractometry, and differential scanning calorimetry to whole-rock chips from two eucrites, two diogenites, and an H5 chondrite. The data are presented in tables, graphs, and photomicrographs and characterized in detail. Massive to incipient-vermicular CM formations with smectitelike or micalike compositions and indications of poor crystallization are observed and attributed to hydrocryogenic diagenesis (with little or no liquid water) on time scales of 10-1000 kyr. The need to take the compositional effects of weathering into account before attempting to reconstruct the preterrestrial histories of meteorites is stressed.

Gooding, James L.↗
NASA NTRS1988

Rapid growth of magnesium-carbonate weathering products in a stony meteorite from Antarctica

Nesquehonite, a hydrous magnesium carbonate, occurs as a weathering product on the surface of the Antarctic meteorite LEW 85320 (H5 chondrite). Isotopic measurements of delta(C-13) and delta(O-18) indicate that the nesquehonite formed at near freezing temperatures by reaction of meteoritic minerals with terrestrial water and carbon dioxide. Results from carbon-14 dating suggest that, although the meteorite has been in Antarctica for at least 32,000 to 33,000 years, the nesquehonite formed after AD 1950.

Jull, A. J. T.↗
NASA NTRS1989

The formation of weathering products on the LEW 85320 ordinary chondrite - Evidence from carbon and oxygen stable isotope compositions and implications for carbonates in SNC meteorites

Isotopic analysis of nesquehonite recovered from the surface of the LEW 85320 H5 ordinary chondrite shows that the delta C-13 and delta O-18 values of the two generations of bicarbonate (Antarctic and Texas) are different: delta C-13 = + 7.9 per mil and + 4.2 per mil; delta O-18 = + 17.9 per mil and + 12.1 per mil, respectively. Carbon isotopic compositions are consistent with equilibrium formation from atmospheric carbon dioxide at - 2 + or - 4 C (Antarctic) and + 16 + or - 4 C (Texas). Oxygen isotopic data imply that the water required for nesquehonite precipitation was derived from atmospheric water vapor or glacial meltwater which had locally exchanged with silicates, either in the meteorite or in underlying bedrock. Although carbonates with similar delta C-13 values have been identified in the SNC meteorites EETA 79001 and Nakhla, petrographic and temperature constraints argue against their simply being terrestrial weathering products.

Grady, Monica M.↗
NASA NTRS1991

Terrestrial weathering of Antarctic stone meteorites - Formation of Mg-carbonates on ordinary chondrites

Results are presented on the mineralogy, chemistry, and origin of white efflorescences on the surface of Lewis Cliff (Antarctica) 85320 (H5) chondrite (LEW 85320). Particular attention is given to determining the sources of the cations and anions of the evaporite, in order to establish the relative importance of the meteoritic element distribution and terrestrial contamination in the evaporite formation during the weathering process. The data on Na, K, Ca, and Rb abundances in efflorescence from LEW 85320 suggest that cations in evaporite minerals on Antarctic meteorites are not the products of contamination by terrestrial (marine) salts. It is suggested that the Mg in efflorescence on LEW 85320 originated from weathering of meteoritic olivine.

Velbel, Michael A.↗
NASA NTRS1991

Meteorite mass distributions and differences between Antarctic and non-Antarctic meteorites

Meteorite mass distributions provide important insight into real and apparent differences between Antarctic and non-Antarctic meteorites. Antarctic meteorites are typically smaller than non-Antarctic meteorites and represent a different portion of the mass distribution of infalling meteorites. This results from the different methods employed to acquire the collections and from the much smaller effective collecting area for Antarctic meteorites. Comparisons of the proportions and mass distributions of H and L chondrites for different ice fields suggest that previously reported high H to L ratio among Antarctic meteorites compared to witnessed falls results from an unrecognized H5 shower fall that covered the Allan Hills Main and Near Western ice fields in the relatively recent past. The probable presence of unrecognized shower falls among the Antarctic meteorites means that population statistics, even corrected for presently recognized pairing, cannot be used to support the suggestion by Dennison et al. (1986) of a temporal variation in the mixture of meteorites arriving on earth.

Huss, Gary R.↗
NASA NTRS1991

Evidence for differences in the thermal histories of Antarctic and non-Antarctic H chondrites with cosmic-ray exposure ages less than 20 Ma

Antarctic H chondrites show a different range of induced thermoluminescence properties compared with those of H chondrites that have fallen elsewhere in the world. Recent noble gas data of Schultz et al. (1991) show that this difference is displayed most dramatically by meteorites with cosmic-ray exposure ages less than 20 Ma, and they confirm that the differences cannot be attributed to weathering or to the presence of a great many fragments of an unusual Antarctic meteorite. Annealing experiments on an H5 chondrite, and other measurements on a variety of ordinary chondrites, have shown that induced TL properties are sensitive to the thermal histories of the meteorities. It is concluded that the events(s) that released the less than 20 Ma samples, which are predominantly those with exposure ages of 8 + or - 2 Ma, produced two groups with different thermal histories, one that came to earth several 100,000 years ago and that are currently only found in Antarctica, and one that is currently falling on the earth.

Sears, Derek W. G.↗
NASA NTRS1992

Classification of four ordinary chondrites from the Monnig Meteorite Collection

We classified four ordinary chondrites from the Monnig Meteorite Collection into compositional groups, petrologic types and shock stages, based on optical microscopy in transmitted and reflected light, and electron microprobe and modal analyses. These meteorites are Allen, Texas, H4(S2): May Day, Kansas, H4(S2); Pony Creek, Texas, H4(S3); and Springer, Oklahoma, H5(S3).

Ehlmann, Arthur J.↗
NASA NTRS1993

Magombedze - A new H-chondrite with light-dark structure

Magombedze is a light-dark structured H-chondrite breccia that fell in Zimbabwe on July 2, 1990 at 15:30 GMT. White clasts are moderately shocked and have equilibrated mafic silicates (pyroxene Fs(16-18), olivine Fa(18-19)) together with clear optically-recognizable plagioclase of variable composition (An(9-13) found); chondrules are distinct but contain no trace of preserved glass. The darker surrounding material contains a higher proportion of fine-grained metal and sulfide than the white clasts, and many of its constituent grains show little evidence of shock. Mafic silicates in the dark lithology are distinctly less-equilibrated (pyroxene Fs(5-21), olivine Fa(11-20)) than those in the white clasts, and many chondrules preserve brown devitrified glass; some metamorphic plagioclase of variable composition (An(11-22) found) is present. Some monoclinic pyroxene occurs in both fractions, but it is relatively common in the dark fraction. The white clasts are classified as H5, and the enclosing dark material is H3-5.

Macpherson, Glenn J.↗
NASA NTRS1993

A detailed study of the photo-injection annealing of thermally diffused InP solar cells

A detailed analysis of the annealing of thermally diffused InP solar cells fabricated by the Nippon Mining Co. is presented. The cells were irradiated with 1 MeV electrons, and the induced degradation is measured using deep level transient spectroscopy and low temperature (86 K) IV measurements. Clear recovery of the photovoltaic parameters is observed during low temperature (T is less than 300 K) solar illuminations (1 sun, AMO) with further recovery at higher temperatures (300 less than T less than 500 K). For example, the output of a cell which was irradiated up to a fluence of 1 x 10(exp 16) cm(sup -2) was observed to recover to within 5 percent of the pre-irradiation output. An apparent correlation between the recovery of I(sub sc) and the annealing of the H4 defect and of the minority carrier trapping centers is observed. An apparent correlation between the recovery of VO, and the annealing of the H5 defect is also observed. These apparent correlations are used to develop a possible model for the mechanism of the recovery of the solar cells.

Walters, R. J.↗
NASA NTRS1993

The great 8 MA event and the structure of the H-chondrite parent body

The H-chondrites have been the subject of several recent controversies, including the question of whether Antarctic and non-Antarctic meteorites are or are not the same and whether there or is not evidence for stratigraphic layering in the original parent body. We have identified two distinct groups of H5 chondrites in the Antarctic collection. One group has induced thermoluminescence (TL) peak temperatures less than 190 C and metallographic cooling rates between S to 50 K/Myr, similar to modern falls. It also has a variety of cosmic ray exposure ages, many being greater than 107 years. The other group has TL peak temperatures greater than 190 C, metallographic cooling rates of 100 K/Myr and cosmic ray exposure ages of 8 Ma. The members of this group were generals smaller than those of the greater than 190 C group (including the mode falls) during cosmic ray exposure. Detailed study of the cosmogenic nuclide concentrations of these groups indicates that they are not solely the result of pairing of a few unusual meteorites. It is likely that the greater than 190 C group was an important part of the H-chondrite flux about 1 million years ago, but has since decreased in importance relative to the less than 190 C group. In a previous work, we discussed several possible origins for the greater than 190 C group, including multiple H-chondrite parent bodies, unusual parent body structure, and creation during the 8 Ma event. In this paper, we present new data for H4 chondrites in light of these ideas.

Benoit, P. H.↗
NASA NTRS1993

Recondensation of chondritic material in the early solar system: Results of thermodynamic simulation

We have performed a thermodynamic simulation of the recondensation of evaporated meteoritic material. We suggest that evaporation and recondensation occurred in impact events during the intercollision of planetesimals during the early evolution of the solar system. The source materials adopted for our model are the chondrites CI Orgueil and H5 Richardton. These chondrites are representative examples of the two extremes regarding volatile content and oxidation state. We calculated equilibrium mineral compositions of the closed systems of the Orgueil's and Richardton's elemental composition at the P-T conditions characteristic of the explosion cloud formed at a planetesimal collision. The P-T conditions are as follows: 10(exp -4) bar, and 1500 and 2000 K. The results are presented.

Dorofeyeva, V. A.↗
NASA NTRS1993

Chemical and isotopic compositions in acid residues from various meteorites

We are planning to carry out systematic isotopic investigations of Ru, Mg, etc., in primordial samples. The investigations will be pursued in the context of a study of the pre-history of the solar system. It is hoped that the study will yield direct evidence for processes of nucleosynthesis in the pre-solar stage and detection of extinct radioactive nuclides. In this paper, we present the results of chemical compositions of acid residues obtained from three types of meteorites: Canyon Diablo (IA), Allende (CV3), and Nuevo Mercuro (H5); and the preliminary results of Ru isotopic compositions.

Kano, N.↗
NASA NTRS1993

The Sm-(146,147)-Nd-(142,143) formation interval for the lunar mantle and implications for lunar evolution

Small anomalies in the isotopic abundance of Nd-142 have been measured for two A17 high-Ti basalts, ilmenite basalt 12056, olivine-pigeonite basalt 12039, feldspathic basalt 12038, and two KREEP basalts. These anomalies correlate with Sm-147/Nd-144 for the basalt source regions as calculated from initial Nd-143/Nd-144 ratios in the basalts, and are interpreted to be from decay of Sm-146 (t sub 1/2 = 103 Ma) in distinct lunar mantle reservoirs. A three-stage model for evolution of Nd-143/Nd-144 and Nd-142/Nd-144 yields reservoir Sm-147/Nd-144 ratios which, with the Nd-142/Nd-144 ratios in the basalts, form a 'mantle isochron' giving a lunar mantle formation interval of 94+2230 Ma (2c(rho)). Calculated reservoir Sm/Nd ratios are in the range expected from some earlier models of basalt petrogenesis. The isochron value of Nd-142/Nd-144 at Sm-147/Nd-144 sub CHUR = 0.1967 is within error limits of the average Nd-142/Nd-144 measured for an L6 chondrite, an H5 chondrite, and the Orgueil carbonaceous chondrite. Evolution of Nd-143 and Nd-142 for high-Ti basalt 70135 was modeled precisely, starting from chondritic relative REE and Nd-isotopic abundances and using the initial (Sm-146/Sm-144) sub 0 ratio inferred from a previous study of angrite LEW86010 as the initial solar system value of this parameter. We infer that the initial Sm/Nd ratio in precursor lunar materials was very nearly chondritic (within approximately 8 percent) prior to lunar differentiation.

Nyquist, L. E.↗
NASA NTRS1995

Chemical studies of H chondrites. 4: New data and comparison of Antarctic suites

We report data for the trace elements Au, Co, Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, Ti, and In (ordered by putative volatility during nebular condensation and accretion) determined by neutron activation analysis in 13 H5 chondrites from Victoria Land and 20 H4-6 chondrites from Queen Maud Land, Antarctica. These and earlier results provide Antarctic sample suites of 34 chondrites from Victoria Land and 25 from Queen Maud Land. Treatment of data for the most volatile 10 elements (Rb to In) in these studies by multivariate statistical techniques more robust, as well as more conservative, than conventional linear discriminant analysis and logistic regression demonstrates that compositions differ at marginally significant levels. This difference cannot be explained by trivial (terrestrial) causes and becomes more significant, despite the smaller size of the database, when comparisons are limited to data from a single analyst and when all upper limits are eliminated from consideration. The Victoria Land and Queen Maud Land suites have different mean terrestrial ages (approximately 300 kyr and approximately 100 kyr, respectively) and age distributions, suggesting that a time-dependent variation of chondritic sources with different thermal histories is responsible. As a result, these two Antarctic suites are, on average, chemically distinguishable from each other. Since H chondrites serve as a paradigm for other meteorite classes, these results indicate that the near-Earth populations of planetary materials varied with time on the 10(exp 5)-year timescale.

Wolf, Stephen F.↗
NASA NTRS1995

Chemical studies of H chondrites. 6: Antarctic/non-Antarctic compositional differences revisited

We report data for the trace elements Au, Co, Sb, Ga, Rb, Ag, Se, Cs, Te, Zn, Cd, Bi, T1, and In (ordered by putative volatility during nebular condensation and accretion) determined by radiochemical neutron activation analysis of 14 additional H5 and H6 chondrite falls. Data for the 10 most volatile elements (Rb to In) treated by the multivariate techniques of linear discriminant analysis and logistic regression in these and 44 other falls are compared with those of 59 H4-6 chondrites from Antarctica. Various populations are tested by the multivariate techniques, using the previously developed method of randomization-simulation to assess significance levels. An earlier conclusion, based on fewer examples, that H4-6 chondrite falls are compositionally distinguishable from the Antarctic suite is verified by the additional data. This distinctiveness is highly significant because of the presence of samples from Victoria Land in the Antarctic population, which differ compositionally from falls beyond any reasonable doubt. However, it cannot be proven unequivocally that falls and Antarctic samples from Queen Maud Land are compositionally distinguishable. Trivial causes (e.g., analyst bias, weathering) cannot explain the Victoria Land (Antarctic)/non-Antarctic compositional difference for paradigmatic H4-6 chondrites. This seems to reflect a time-dependent variation of near-Earth meteoroid source regions differing in average thermal history.

Wolf, Stephen F.↗
NASA NTRS1995

High and low energy proton radiation damage in p/n InP MOCVD solar cells

InP p(+)nn(+) MOCVD solar cells were irradiated with 0.2 MeV and 10 MeV protons to a fluence of 10(exp 13)/sq cm. The degradation of power output, IV behavior, carrier concentration and defect concentration were observed at intermediate points throughout the irradiations. The 0.2 MeV proton irradiated solar cells suffered much greater and more rapid degradation in power output than those irradiated with 10 meV protons. The efficiency losses were accompanied by larger increases in the recombination currents in the 0.2 MeV proton irradiated solar cells. The low energy proton irradiations also had a larger impact on the series resistance of the solar cells. Despite the radiation induced damage, the carrier concentration in the base of the solar cells showed no reduction after 10 MeV or 0.2 MeV proton irradiations and even increased during irradiation with 0.2 MeV protons. In a DLTS study of the irradiated samples, the minority carrier defects H4 and H5 at E(v) + 0.33 and E(v) + 0.52 eV and the majority carrier defects E7 and E10 at E(c)- 0.39 and E(c)-0.74 eV, were observed. The defect introduction rates for the 0.2 MeV proton irradiations were about 20 times higher than for the 10 MeV proton irradiations. The defect E10, observed here after irradiation, has been shown to act as a donor in irradiated n-type InP and may be responsible for obscuring carrier removal. The results of this study are consistent with the much greater damage produced by low energy protons whose limited range causes them to stop in the active region of the solar cell.

Rybicki, George↗
NASA NTRS1996

The Cooling History and Structure of the Ordinary Chondrite Parent Bodies

Most major meteorite classes exhibit significant ranges of metamorphism. The effects of metamorphism have been extensively characterized, but the heat source(s) and the metamorphic environment are unknown. Proposed beat sources include Al-26, Fe-60, electromagnetic induction, and impact. It is typically assumed that metamorphism occurred in parent bodies of some sort, but it uncertain whether these bodies were highly structured ("onion skins") or were chaotic mixes of material ("rubble piles"). The lack of simple trends of metallographic cooling rates with petrologic type has been considered supportive of both concepts. In this study, we use induced thermoluminescence (TL) as an indicator of thermal history. The TL of ordinary chondrites is produced by sodic feldspar, and the induced TL peak temperature is related to its crystallographic order/disorder. Ordered feldspar has TL peak temperatures of approx. 120 C, and disordered feldspar has TL peak temperatures of approx. 220 C. While ordered feldspar can be easily disordered in the laboratory by heating above 650 C and is easily quenched in the disordered form, producing ordered feldspar requires cooling at geologic cooling rates. We have measured the induced TL properties of 101 equilibrated ordinary chondrites, including 49 H, 29 L, and 23 LL chondrites. For the H chondrites there is an apparent trend of decreasing induced TL peak temperature with increasing petrologic type. H4 chondrites exhibit a tight range of TL peak temperatures, 190 C - 200 C, while H6 chondrites exhibit TL peak temperatures between 180 C and 190 C. H5 chondrites cover the range between H4 and H6, and also extend up to 210 C. Similar results are obtained for LL chondfiles and most L6 chondrites have lower induced TL peak temperatures than L5 chondrites.

Benoit, P. H.↗