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1

Primordial rare gases in unequilibrated ordinary chondrites.  

PubMed

The primordial gases of eight unequilibrated ordinary chondrites are strongly fractionated with respect to" cosmic" proportions. The absolute amounts are roughly proportional to the degree of disequilibration. Apparently, ordinary chondrites originally contained considerably larger amounts of primordial rare gases. PMID:17807955

Heymann, D; Mazor, E

1967-02-10

2

Forsteritic Olivine Grains in Unequilibrated Ordinary Chondrites: Additional Evidence for a Link Between Ordinary and Carbonaceous Chondrites  

NASA Technical Reports Server (NTRS)

We report mineral-chemical and Secondary Ion Mass Spectrometry (SIMS) oxygen-isotopic studies of forsteritic olivine grains from various Unequilibrated Ordinary Chondrites (UOCs). Our studies provide additional evidence for a genetic link between ordinary and carbonaceous chondrites and between various lithologies in these meteorites.

Ruzicka, A.; Hiyagon, H.; Prinz, M.; Taylor, L. A.

2000-01-01

3

Thermoluminescence Sensitivity and Thermal History of Unequilibrated Ordinary Chondrites: Review and Update  

NASA Technical Reports Server (NTRS)

We report on the induced thermoluminescence (TL) data for 102 unequilibrated ordinary chondrites. We discuss these data in terms of pairing, weathering, and parent body thermal history. We identify ten possible meteorites of petrologic types 3.0-3.1.

Benoit, P. H.; Ninagawa, K.; Sears, D. W. G.

2000-01-01

4

Extreme oxygen-isotope compositions in magnetite from unequilibrated ordinary chondrites  

Microsoft Academic Search

Primitive meteorites (such as the unequilibrated ordinary chondrites) have undergone only minor thermal processing on their parent asteroids, and thus provide relatively unaltered isotopic records from the early Solar System. For terrestrial materials, oxygen isotope compositions form a linear array called the terrestrial fractionation line. In meteorites the oxygen isotopic composition commonly deviates from this line, the magnitude of the

Byeon-Gak Choi; Kevin D. McKeegan; Alexander N. Krot; John T. Wasson

1998-01-01

5

Ultrarapid chondrite formation by hot chondrule accretion? Evidence from unequilibrated ordinary chondrites  

NASA Astrophysics Data System (ADS)

Unequilibrated ordinary chondrites (UOCs) of all groups (H, L, LL) contain unique chondrite clasts, which are characterized by a close-fit texture of deformed and indented chondrules. These clasts, termed "cluster chondrites," occur in 41% of the investigated samples with modal abundances between 5 and 90 vol% and size variations between <1 mm and 10 cm. They show the highest chondrule abundances compared with all chondrite classes (82-92 vol%) and only low amounts of fine-grained interchondrule matrix and rims (3-9 vol%). The mean degree of chondrule deformation varies between 11% and 17%, compared to 5% in the clastic portions of their host breccias and to values of 3-5% found in UOC literature, respectively. The maximum deformation of individual chondrules is about 50%, a value which seemingly cannot be exceeded due to geometric limitations. Both viscous and brittle chondrule deformation is observed. A model for cluster chondrite formation is proposed where hot and deformable chondrules together with only small amounts of co-accreting matrix formed a planetesimal or reached the surface of an already existing body within hours to a few days after chondrule formation. They deformed in a hot stage, possibly due to collisional compression by accreting material. Later, the resulting rocks were brecciated by impact processes. Thus, cluster chondrite clasts are interpreted as relicts of primary accretionary rocks of unknown original dimensions. If correct, this places a severe constraint on chondrule-forming conditions. Cluster chondrites would document local chondrule formation, where chondrule-forming heating events and the accretion of chondritic bodies were closely linked in time and space.

Metzler, Knut

2012-12-01

6

Identification of solar nebula condensates in interplanetary dust particles and unequilibrated ordinary chondrites  

NASA Technical Reports Server (NTRS)

Orthopyroxene and olivine grains, low in FeO, but containing MnO contents up to 5 wt percent were found in interplanetary dust particles (IDP) collected in the stratosphere. The majority of olivines and pyroxenes in meteorites contain less than 0.5 wt percent MnO. Orthopyroxenes and olivines high in Mn and low in FeO have only been reported from a single coarse grained chondrule rim in the Allende meteorite and from a Tieschitz matrix augite grain. The bulk MnO contents of the extraterrestrial dust particles with high MnO olivines and pyroxenes are close to CI chondrite abundances. High MnO, low FeO olivines and orthopyroxenes were also found in the matrix of Semarkona, an unequilibrated ordinary chondrite. This may indicate a related origin for minerals in extraterrestrial dust particles and in the matrix of unequilibrated ordinary chondrites.

Kloeck, W.; Thomas, K. L.; Mckay, D. S.

1989-01-01

7

Fe-Mn systematics of type IIA chondrules in unequilibrated CO, CR, and ordinary chondrites  

NASA Astrophysics Data System (ADS)

We have examined Fe/Mn systematics of 34 type IIA chondrules in eight highly unequilibrated CO, CR, and ordinary chondrites using new data from this study and prior studies from our laboratory. Olivine grains from type IIA chondrules in CO chondrites and unequilibrated ordinary chondrites (UOC) have significantly different Fe/Mn ratios, with mean molar Fe/Mn = 99 and 44, respectively. Olivine analyses from both these chondrite groups show well-defined trends in Mn versus Fe (afu) and molar Fe/Mn versus Fe/Mg diagrams. In general, type IIA chondrules in CR chondrites have properties intermediate between those in UOC and CO chondrites. In most UOC and CR type IIA chondrules, the Fe/Mn ratio of olivine decreases during crystallization, whereas in CO chondrites the Fe/Mn ratio does not appear to change. It is difficult to interpret the observed Fe/Mn trends in terms of differing moderately volatile element depletions inherited from precursor materials. Instead, we suggest that significant differences in the abundances of silicates and sulfides ± metals in the precursor material, as well as open-system behavior during chondrule formation, were responsible for establishing the different Fe/Mn trends. Using Fe-Mn-Mg systematics, we are able to identify relict grains in type IIA chondrules, which could be derived from previous generations of chondrules, including chondrules from other chondrite groups, and possibly chondritic reservoirs that have not been sampled previously.

Berlin, Jana; Jones, Rhian H.; Brearley, Adrian J.

2011-04-01

8

Fine, nickel-poor Fe-Ni grains in the olivine of unequilibrated ordinary chondrites  

NASA Astrophysics Data System (ADS)

Nickel-poor Fe-Ni grains smaller than 2.0 microns are common inclusions in ordinary, unequilibrated chondrites' porphyritic chondrule olivine, where the olivine grains seem to be relicts that survived chondrule formation without melting. This 'dusty' metal, whose most common occurrence is in the core of olivine grains having clear, Fe-poor rims, appears to be the product of the in situ reduction of FeO from the host olivine, with H2 or carbonaceous matter being the most likely reductants. H2 may have been implanted by solar wind or solar flare irradiation, but this requires the dissipation of nebular gas before the end of the chondrule formation process. Carbonaceous matter may have been implanted by shock. The large relict olivine grains may be nebular condensates or fragments broken from earlier chondrule generations.

Rambaldi, E. R.; Wasson, J. T.

1982-06-01

9

Petrographic and oxygen-isotopic study of refractory forsterites from R-chondrite Dar al Gani 013 (R3.5-6), unequilibrated ordinary and carbonaceous chondrites  

NASA Astrophysics Data System (ADS)

We have conducted petrographic, chemical and in-situ oxygen isotopic studies of refractory forsterites from unequilibrated ordinary and carbonaceous chondrites as well as an unequilibrated R-chondrite. Refractory forsterites occur in all types of unequilibrated chondrites and all have very similar chemical composition with low FeO and high refractory lithophile element (RLE) contents. Refractory forsterites are typically enriched in 16O relative to 'normal' olivine independent of the bulk O-isotope ratios of the parent meteorites. Analyses of refractory forsterites spread along a ? 17O mixing line with ? 17O ranging from +2 to -10‰. Due to similarities in oxygen isotopes and chemical compositions, we conclude that refractory forsterites of various types of chondrites come from a single common reservoir. Implications of this hypothesis for the chemical and O-isotope evolution of silicates in the early solar nebular are discussed.

Pack, Andreas; Yurimoto, Hisayoshi; Palme, Herbert

2004-03-01

10

Magnetite in unequilibrated ordinary chondrites - Evidence for an O-17-rich reservoir in the solar nebula  

Microsoft Academic Search

To better understand the formation of magnetite and low-temperature processes in the solar nebula and in the parent asteroids of the chondrites, the authors performed in situ measurements of O-isotopic compositions of magnetite in the Semarkona and Ngawi ordinary chondrites. The compositions of magnetite are cvlearly distinguished by larger Delta O-17 values (average = 5.3 per mil) from those of

B.-G. Choi; K. D. McKeegan; A. N. Krot; J. T. Wasson

1997-01-01

11

Oxygen Isotope Composition of Refractory Forsterite in R-Chondrite Dar Al Gani 013, Unequilibrated Ordinary, and Carbonaceous Chondrites  

NASA Astrophysics Data System (ADS)

We have measured O-isotope composition of refractory forsterite in different carbonaceous, ordinary and R-chondrites using SIMS. We will discuss a model for the formation of the population of refractory forsterite pre-dating the formation of the different chondrite types.

Pack, A.; Yurimoto, H.; Palme, H.

2002-03-01

12

Aqueous Alteration and Hydrogen Generation on Parent Bodies of Unequilibrated Ordinary Chondrites: Thermodynamic Modeling for the Semarkona Composition  

NASA Technical Reports Server (NTRS)

Ordinary chondrites are the most abundant class of meteorites that could represent rocky parts of solar system bodies. However, even the most primitive unequilibrated ordinary chondrites (UOC) reveal signs of mild alteration that affected the matrix and peripheral zones of chondrules. Major chemical changes include oxidation of kamacite, alteration of glass, removal of alkalis, Al, and Si from chondrules, and formation of phases enriched in halogens, alkalis, and hydrogen. Secondary mineralogical changes include formation of magnetite, ferrous olivine, fayalite, pentlandite, awaruite, smectites, phosphates, carbonates, and carbides. Aqueous alteration is consistent with the oxygen isotope data for magnetite. The presence of secondary magnetite, Ni-rich metal alloys, and ferrous silicates in UOC implies that H2O was the oxidizing agent. However, oxidation by H2O means that H2 is produced in each oxidative pathway. In turn, production of H2, and its redistribution and possible escape should have affected total pressure, as well as the oxidation state of gas, aqueous and mineral phases in the parent body. Here we use equilibrium thermodynamic modeling to explore water-rock reactions in UOC. The chemical composition of gas, aqueous, and mineral phases is considered.

Zolotov, M. Y.; Mironenko, M. V.; Shock, E. L.

2005-01-01

13

Fayalite-silica association in unequilibrated ordinary chondrites: Evidence for aqueous alteration on a parent body  

NASA Technical Reports Server (NTRS)

We report ten occurrences of high-fayalite (Fa56-99 mol%; four with Fa greater than 82 mol%) olivine in association with silica in type-3 ordinary chondrites. Pyroxene with high Fs contents is much less common; Fs contents do not exceed 66 mol%, and most maxima are less than 50 mol%. In those cases where the amount of fayalite is minor relative to that of silica, the fayalite forms a layer on the silica, and shows textural evidence of formation by reaction of silica with oxidized Fe; the latter seems to have resulted from reaction of metallic Fe-Ni with an oxidant, most likely H2O vapor. The fayalite is generally in contact with pyroxene (and, in one case, olivine) having a much lower Fe/(Fe + Mg) ratios, indicating that lattice diffusion has been minimal. Formation of fayalite from SiO2 explains the low Mg content of this olivine; the Mg was sequestered inside the lattices of mafic minerals and was thus inaccessible. In contrast, the moderately high Mn contents of the fayalite indicate that an appreciable fraction of the Mn in the precursor assemblage was accessible; it was probably sited in the matrix in the form of tiny, poorly crystallized oxide grains produced by nebular condensation at temperatures too low to permit diffusion into forsterite or enstatite. The reaction of SiO2 with FeO produced by oxidation (during metamorphism) of Fe-Ni can also account for fayalitic olivine associated with SiO2 microspherules in the fine-grained matrices of type-3 ordinary chondrites and, because matrix is SiO2 normative, for other occurrences of fayalite in matrix. The presence of Mn in the fayalitic rims on the olivine of carbonaceous chondrites does not require a nebular sign.

Wasson, John T.; Krot, Alexander N

1994-01-01

14

Compositions and textures of relic forsterite in carbonaceous and unequilibrated ordinary chondrites  

NASA Technical Reports Server (NTRS)

The compositions and textures of olivines in equilibrated ordinary chondrites, C1, C2, and C3 meteorites are described. The olivines are classified by color into blue, red, or dark olivines. The characteristic features of blue olivines are examined. It is observed that the texture of the olivine: (1) forms cores in single olivine grains; (2) shows subhedral to euhedral boundaries against rim olivines; (3) rarely contains inclusions; (4) has embayments containing olivines similar to that of the rim; and (5) occurs within chondrules. The mode of formation for blue olivines, the relation between blue olivine and high temperature compounds, and modifications in the features of blue olivines are analyzed. It is noted that the olivine is a product of condensation, has been modified by additions of olivine formed from a liquid, and that these olivines represent relic grains that are similar to olivines found in cosmic particles and in cores of some deep sea particles.

Steele, I. M.

1986-01-01

15

Nitrogen isotopes in bulk ordinary chondrites  

NASA Astrophysics Data System (ADS)

Nitrogen isotopic compositions of twenty-one ordinary chondrites were measured using a stepwise combustion technique. We observed a good correlation between the abundances of cosmogenic 15N and cosmogenic 38Ar. We obtained a relationship between the abundances of these two cosmogenic nuclides as follows: 15N c(atoms/g) = (16.0 ± 3.7) · 38Ar c(atoms/g) Using this relationship, the cosmogenic nitrogen production rate in L-chondrites is calculated to be (5.0 ± 1.2) × 10 -12 g- 15N/g·myr, assuming the cosmogenic 38Ar production rate given by Eugster (1988). This value is consistent with the production rate on the lunar surface estimated by Becker et al. (1976). Isotopic ratios of trapped nitrogen in ordinary chondrites seem to be heterogeneous, among both unequilibrated and equilibrated meteorites. Heterogeneity among the latter are well observed in this study. The isotopic ratios of the trapped components seem to differ by more than 110%, ranging from -18%, observed in Guaren˜a (H6), to +95%, observed in Jilin (H5). A lack of correlation between nitrogen abundances and isotopic ratios indicates that differences in nitrogen isotopic ratios are not produced by mass-dependent isotopic fractionation. An inverse correlation between nitrogen abundances and the degree of thermal metamorphism is observed among most of the unequilibrated chondrites. Nitrogen abundances decrease from 8-25 p.p.m. to <1 p.p.m. with increasing petrographic subtype from 3.4-3.9. The nitrogen abundance in equilibrated chondrites is generally higher than the amount expected by extending the trend observed among unequilibrated ones. This is probably because contaminating nitrogen masked the trend at small amounts of trapped nitrogen.

Hashizume, Ko; Sugiura, Naoji

1995-10-01

16

Cathodoluminescence Spectroscopy of Presolar and Solar Al_2O_3 Grains in Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

We performed cathodoluminescence spectroscopy of presolar and solar Al_2O_3 grains separated from unequilibrated ordinary chondrites. While the CL spectra displayed significant diversity, they are useful to probe crystallinity of presolar Al_2_O3 grains.

Takigawa, A.; Vicenzi, E. P.; Wight, S.; Alexander, C. M. O'D.; Nittler, L. R.; Stroud, R. M.; Huss, G. R.

2013-09-01

17

Roosevelt County 075: A petrologic chemical and isotopic study of the most unequilibrated known H chondrite  

NASA Technical Reports Server (NTRS)

Roosevelt County (RC) 075 was recovered in 1990 as a single 258-gram stone. Classification of this meteorite is complicated by its highly unequilibrated nature and its severe terrestrial weathering, but we favor H classification. This is supported by O isotopes and estimates of the original Fe, Ni metal content. The O isotopic composition is similar to that of a number of reduced ordinary chondrites (e.g., Cerro los Calvos, Willaroy), although RC 075 exhibits no evidence of reduced mineral compositions. Chondrule diameters are consistent with classification as an L chondrite, but large uncertainties in chondrule diameters of RC 075 and poorly constrained means of H, L and LL chondrites prevent use of this parameter for reliable classification. Other parameters are compromised by severe weathering (e.g., siderophile element abundances) or unsuitable for discrimination between unequilibrated H, L and LL chondrites (e.g., Co in kamacite delta C-13). Petrologic subtype 3.2 +/- 0.1 is suggested by the degree of olivine heterogeneity, the compositions of chondrule olivines, the thermoluminescence sensitivity, the abundances and types of chondrules mapped on cathodoluminescence mosaics, and the amount of presolar SiC. The meteorite is very weakly shocked (S2), with some chondrules essentially unshocked and, thus, is classified as an H3.2(S2) chondrite. Weathering is evident by a LREE enrichment due to clay contamination, reduced levels of many siderophile elements, the almost total loss of Fe, Ni metal and troilite, and the reduced concentrations of noble gases. Some components of the meteorite (e.g., type IA chondrules, SiC) appear to preserve their nebular states, with little modification from thermal metamorphism. We conclude that RC 075 is the most equilibrated H chondrite yet recovered and may provide additional insights into the origin of primitive materials in the solar nebula.

Mccoy, T. J.; Keil, K.; Ash, R. D.; Morse, A. D.; Pillinger, C. T.; Wieler, R.; Mayeda, T. K.; Clayton, R. N.; Benoit, P. H.; Sears, D. W. G.

1993-01-01

18

Glass-rich chondrules in ordinary chondrites  

NASA Technical Reports Server (NTRS)

There are two types of glass-rich chondrules in unequilibrated ordinary chondrites (OC): (1) porphyritic chondrules containing 55-85 vol% glass or microcrystalline mesostasis and (2) nonporphyritic chondrules, containing 90-99 vol% glass. These two types are similar in mineralogy and bulk composition to previously described Al-rich chondrules in OC. In addition to Si-, Al- and Na-rich glass or Ca-Al-rich microcrystalline mesostasis, glass-rich chondrules contain dendritic and skeletal crystals of olivine, Al2O3-rich low-Ca pyroxene and fassaite. Some chondrules contain relict grains of forsterite +/- Mg-Al spinel. We suggest that glass-rich chondrules were formed early in nebular history by melting fine-grained precursor materials rich in refractory (Ca, Al, Ti) an moderately volatite (Na, K) components (possibly related to Ca-Al-rich inclusions) admixed with coarse relict forsterite and spinel grains derived from previously disrupted type-I chondrules.

Krot, Alexander N.; Rubin, Alan E.

1994-01-01

19

Actinide abundances in ordinary chondrites  

USGS Publications Warehouse

Measurements of 244Pu fission Xe, U, Th, and light REE (LREE) abundances, along with modal petrographic determinations of phosphate abundances, were carried out on equilibrated ordinary chondrites in order to define better the solar system Pu abundance and to determine the degree of variation of actinide and LREE abundances. Our data permit comparison of the directly measured Pu/ U ratio with that determined indirectly as (Pu/Nd) ?? (Nd/U) assuming that Pu behaves chemically as a LREE. Except for Guaren??a, and perhaps H chondrites in general, Pu concentrations are similar to that determined previously for St. Se??verin, although less precise because of higher trapped Xe contents. Trapped 130Xe 136Xe ratios appear to vary from meteorite to meteorite, but, relative to AVCC, all are similar in the sense of having less of the interstellar heavy Xe found in carbonaceous chondrite acid residues. The Pu/U and Pu/Nd ratios are consistent with previous data for St. Se??verin, but both tend to be slightly higher than those inferred from previous data on Angra dos Reis. Although significant variations exist, the distribution of our Th/U ratios, along with other precise isotope dilution data for ordinary chondrites, is rather symmetric about the CI chondrite value; however, actinide/(LREE) ratios are systematically lower than the CI value. Variations in actinide or LREE absolute and relative abundances are interpreted as reflecting differences in the proportions and/or compositions of more primitive components (chondrules and CAI materials?) incorporated into different regions of the ordinary chondrite parent bodies. The observed variations of Th/U, Nd/U, or Ce/U suggest that measurements of Pu/U on any single equilibrated ordinary chondrite specimen, such as St. Se??verin, should statistically be within ??20-30% of the average solar system value, although it is also clear that anomalous samples exist. ?? 1990.

Hagee, B.; Bernatowicz, T. J.; Podosek, F. A.; Johnson, M. L.; Burnett, D. S.; Tatsumoto, M.

1990-01-01

20

The duration of ordinary chondrite metamorphism inferred from tungsten microdistribution in metal  

Microsoft Academic Search

Precise ratios of W and Re relative to Ir determined by laser ablation ICP-MS reveal that matrix metal in equilibrated ordinary chondrites (EOC) exhibits a correlated variation of these ratios. In unequilibrated ordinary chondrites (UOC), the W\\/Ir ratio varies by over two orders of magnitude (W\\/Ir=0.003–0.6), and shows no correlation with any other siderophile element. Most matrix metal in UOC

Munir Humayun; Andrew J Campbell

2002-01-01

21

Porphyritic Olivine-Pyroxene Clast in Kaidun: First Discovery of an Ordinary Chondrite Clast?  

NASA Technical Reports Server (NTRS)

Kaidun is an enigmatic meteorite showing a micro-brecciated texture composed of variable kinds of lithic clasts and mineral fragments. The constituent components range from primitive chondritic materials to differentiated achondritic materials, and thus believed to have originated from a large parent body accumulating materials from many different bodies in the asteroid belt. One of the interesting observations is that no ordinary chondrite component has been found yet, although C and E chondrites components are abundant. In this abstract, we report mineralogy of the clast (Kaidun #15415- 01.3.13a) showing a porphyritic olivine-pyroxene chondrule-like texture similar to those found in unequilibrated ordinary chondrites.

Mikouchi, T.; Makishima, J.; Koizumi, E.; Zolensky, M. E.

2005-01-01

22

Chainpur-like Chondrites: Primitive Precursors of Ordinary Chondrites?  

PubMed

Chainpur and similar, apparently primitive, chondritic meteorites may be precursors of ordinary chondrites; a variety of evidence supports this working hypothesis. In general, carbonaceous chondrites seem to be related collaterally to this genetic sequence rather than being direct ancestors of ordinary chondrites. Metamorphic processes may be responsible for fractionations of elements such as indium and iodine, and type-II carbonaceous chondrites seem to be more primitive than types I or IIIA. PMID:17757239

Schmitt, R A; Smith, R H; Goles, G G

1966-08-01

23

Incompletely compacted equilibrated ordinary chondrites  

SciTech Connect

We document the size distributions and locations of voids present within five highly porous equilibrated ordinary chondrites using high-resolution synchrotron X-ray microtomography ({mu}CT) and helium pycnometry. We found total porosities ranging from {approx}10 to 20% within these chondrites, and with {mu}CT we show that up to 64% of the void space is located within intergranular voids within the rock. Given the low (S1-S2) shock stages of the samples and the large voids between mineral grains, we conclude that these samples experienced unusually low amounts of compaction and shock loading throughout their entire post accretionary history. With Fe metal and FeS metal abundances and grain size distributions, we show that these chondrites formed naturally with greater than average porosities prior to parent body metamorphism. These materials were not 'fluffed' on their parent body by impact-related regolith gardening or events caused by seismic vibrations. Samples of all three chemical types of ordinary chondrites (LL, L, H) are represented in this study and we conclude that incomplete compaction is common within the asteroid belt.

Sasso, M.R.; Macke, R.J.; Boesenberg, J.S.; Britt, D.T.; Rovers, M.L.; Ebel, D.S.; Friedrich, J.M.; (Fordham)

2010-01-22

24

Oxygen isotopic compositions of chondrules in Allende and ordinary chondrites  

NASA Technical Reports Server (NTRS)

The ferromagnesian chondrules in Allende follow a trend in the oxygen three-isotope plot that diverges significantly from the 16-O mixing line defined by light and dark inclusions and the matrix of the meteorite. The trend probably results from isotopic exchange with an external gaseous reservoir during the process of chondrule formation sometime after the establishment of the isotopic compositions of the inclusions and matrix. The Allende chondrules approach, but do not reach, the isotopic compositions of chondrules in unequilibrated ordinary chondrites, implying exchange with a similar ambient gas, but isotopically different solid precursors for the two types of meteorite.

Clayton, R. N.; Mayeda, T. K.; Hutcheon, I. D.; Molini-Velsko, C.; Onuma, N.; Ikeda, Y.; Olsen, E. J.

1983-01-01

25

Metallic copper in ordinary chondrites  

NASA Technical Reports Server (NTRS)

Metallic Cu of moderately high purity (approximately 985 mg/g Cu, approximately 15 mg/g Ni) occurs in at least 66% of ordinary chondrites (OC) as heterogeneously distributed, small (typically less than or equal to 20 micrometers) rounded to irregular grains. The mean modal abundance of metallic Cu in H, L and LL chondrites is low: 1.0 to 1.4 x 10(exp -4) vol%, corresponding to only 4 - 5 % of the total Cu in OC whole rocks. In more than 75% of the metallic-Cu-bearing OC, at least some metallic Cu occurs at metallic-Fe-Ni-troilite grain boundaries. In some cases it also occurs within troilite, within metallic Fe-Ni, or at the boundaries these phases form with silicates or chromite. Ordinary chondrites that contain a relatively large number of occurrences of metallic Cu/sq mm have a tendency to have experienced moderately high degrees of shock. Shock processes can cause local melting and transportation of metallic Fe-Ni and troilte; because metallic Cu is mainly associated with these phases, it also gets redistributed during shock events. In the most common petrographic assemblage containing metallic Cu, the Cu is adjacent to small irregular troilite grains surrounded by taenite plus tetrataenite; this assemblage resembles fizzed troilite and may have formed by localized shock melting or remelting of a metal-troilite assemblage.

Rubin, Alan E.

1994-01-01

26

Shock metamorphism of ordinary chondrites  

NASA Technical Reports Server (NTRS)

This study proposes a revised petrographic classification of progressive stages of shock metamorphism of 26 ordinary chondrites. Six stages of shock (S1 to S6) are defined on the basis of shock effects in olivine and plagioclase as recognized by thin section microscopy, and the characteristic shock effects of each shock stage are described. It is concluded that shock effects and the sequence of progressively increasing degrees of shock metamorphosis are very similar in H, L, and LL groups. Differences in the frequency distribution of shock stages are relatively minor. It is suggested that the collisional histories of the H, L, and LL parent bodies were similar. Petrologic type-3 chondrites are deficient in stages S4 and S6 and, with increasing petrologic type, the frequency of stages S4 to S6 increases. It is suggested that the more porous and volatile-rich Type-3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type. Stage S3 is the most abundant in nearly all petrologic types.

Stoeffler, Dieter; Keil, Klaus; Scott, Edward R. D.

1991-01-01

27

Oxygen isotopic abundances in calcium- aluminum-rich inclusions from ordinary chondrites: implications for nebular heterogeneity.  

PubMed

The oxygen isotopic compositions of two calcium-aluminum-rich inclusions (CAIs) from the unequilibrated ordinary chondrite meteorites Quinyambie and Semarkona are enriched in 16O by an amount similar to that in CAIs from carbonaceous chondrites. This may indicate that most CAIs formed in a restricted region of the solar nebula and were then unevenly distributed throughout the various chondrite accretion regions. The Semarkona CAI is isotopically homogeneous and contains highly 16O-enriched melilite, supporting the hypothesis that all CAI minerals were originally 16O-rich, but that in most carbonaceous chondrite inclusions some minerals exchanged oxygen isotopes with an external reservoir following crystallization. PMID:9545215

McKeegan, K D; Leshin, L A; Russell, S S; MacPherson, G J

1998-04-17

28

Oxidation during metamorphism of the ordinary chondrites  

NASA Technical Reports Server (NTRS)

It is suggested that some current concepts about the conditions of metamorphism in ordinary chondrites may be flawed. These meteorites display small systematic variations in the oxidation state of Fe. Evidence is presented that oxidation of Fe is linked to metamorphic grade in types 4-6 ordinary chondrites. This conclusion is at variance with a commonly accepted model for chondrite metamorphism that assumes Fe reduction by graphite.

Mcsween, Harry Y., Jr.; Labotka, Theodore C.

1993-01-01

29

Oxidation during metamorphism of the ordinary chondrites  

NASA Astrophysics Data System (ADS)

It is suggested that some current concepts about the conditions of metamorphism in ordinary chondrites may be flawed. These meteorites display small systematic variations in the oxidation state of Fe. Evidence is presented that oxidation of Fe is linked to metamorphic grade in types 4-6 ordinary chondrites. This conclusion is at variance with a commonly accepted model for chondrite metamorphism that assumes Fe reduction by graphite.

McSween, H. Y.; Labotka, T. C.

1993-03-01

30

Semarkona meteorite: first recorded occurrence of smectite in an ordinary chondrite, and its implications  

SciTech Connect

Semarkona is an unshocked unequilibrated ordinary chondrite. Much of the interchondrule matrix and the mesostases of some chondrules and clasts have been hydrothermally altered. Analyses of altered materials and opaque matrix are presented. Ca and Fe have been redistributed from primary, anhydrous phases into calcite and Na, Fe smectite, respectively. In Semarkona the process possibly requires open system behavior inconsistent with the conventional view of metamorphism of the ordinary chondrites. The low petrologic type previously assigned to Semarkona is the result of retrograde metamorphism, under aqueous conditions, of a higher temperature mineral assemblage. Semarkona, and possibly Bishunpur, should be assigned to petrologic type 2.

Hutchison, R.; Alexander, C.M.O.; Barber, D.J.

1987-07-01

31

Pyroxene Equilibration Temperatures in Metamorphosed Ordinary Chondrites.  

National Technical Information Service (NTIS)

Ordinary chondrites are divided into petrographic types based on observed mineralogical and textural properties consistent with progressive thermal metamorphism from low grade (type 3) to high (type 7). Regardless of the exact cause of the metamorphism, h...

R. P. Harvey M. L. Bennett H. Y. Mcsween

1993-01-01

32

Chondrules and their associates in ordinary chondrites  

NASA Technical Reports Server (NTRS)

A theory for the origin of the ordinary chondrites (OC's) must account for the origin(s) of their constituents. If the OC's accreted in a protoplanetary disk, their components represent the variety of materials that coexisted in it, which constrains their possible origins. The following discussion excludes gas-rich meteorites that formed after any disk had dissipated. This discussion entails: (1) ordinary chondrites' physical properties; (2) textural relationships between chondrules; (3) textures within chondrules; (4) chemical composition; (5) igneous fragments; (6) chondrule rims and interchonrule matrix; and (7) ages of chondrites and inclusions.

Hutchison, R.

1994-01-01

33

Survey of Large, Igneous-Textured Inclusions in Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

Ordinary (O) chondrites are a class of primitive stony meteorites, and as a group comprise our most abundant samples of early solar system materials. Unique to O chondrites are igneous-textured inclusions up to 4 cm in diameter; about an order of magnitude larger than the much more abundant chondrules. These inclusions are almost always highly depleted in metal and sulfide relative to their host meteorite, but but otherwise have diverse characteristics. They exhibit a large range of textures, mineralogies, and bulk compositions, suggesting a variety of formation processes. They all crystallized from large melt volumes, the origins of which are poorly understood. Models proposed for their formation include (1) shock melting of ordinary chondrites with an associated loss of metal and sulfide; (2) melting of vapor-fractionated condensate mixture; (3) chondrule formation involving a larger melt production volume than typical for chondrules; and (4) igneous differentiation occurring within planetesimals sampled by ordinary chondrite parent bodies. Polished thin sections of inclusions from several O-chondrites have been examined with optical light microscopy (OLM) using a Leica DM 2500 petrographic microscope. Petrographic data such as texture, grain sizes and shapes were collected for the inclusions and their hosts in order to facilitate comparisons. Texturally, the inclusions were determined to fall into one of three distinct textural categories: porphyritic, fine granular, and skeletal. Mean grain sizes are on the order of 100 um for both microporphyritic and fine granular inclusions, with microporphyritic inclusions showing a much wider range of grain sizes. The largest grains in the microporphyritic inclusions are on average ~0.25 mm, with the grains of the mesostasis <100 microns. Skeletal olivine textures are defined as being dominated by crystals that are an order of magnitude longer across one direction than the other (e.g., 1 mm x 100 um). Five inclusions have been investigated with scanning electron microscopy (SEM) on a Zeiss Sigma FE-VP SEM at Portland State University. Backscatter electron micrographs were obtained in order to provide additional petrographic data, and olivine composition was determined using a silicon-drift energy dispersive X-ray (EDX) detector integrated with an Oxford Instruments AZtec X-ray analytical system. Olivine crystals of six inclusions were investigated with electron microprobe analysis (EMPA), performed with a Cameca SX-100 electron microprobe located at Oregon State University, and remotely operated from Portland State. The instrument will be operated at 15 kV accelerating voltage, with a 10-50 nA sample current and a beam diameter of approximately 1 um. Chemically, some inclusions were found to have equilibrated olivine, while others were uneqilibrated. This was also unrelated to host classification, as well as unrelated to host petrographic type. Of the two unequilibrated hosts studied, one had two unequilibrated inclusions, while the other had one equilibrated inclusion. Several eqilibrated hosts had equilibrated inclusion, while one equilibrated host had an unequilibrated inclusion. Neither texture nor chemistry was determined to be related to host type or related to whether the inclusion is a drop-formed mass or a clast.

Armstrong, K.; Ruzicka, A. M.

2013-12-01

34

Reduction during metamorphism of four ordinary chondrites  

NASA Astrophysics Data System (ADS)

Three type 3 reduced chondrites (Willaroy, Buwah, and Moorabie) and one type 4 find (Cerro los Calvos) were studied by instrumental neutron-activation analysis and petrographic techniques in order to determine if they are in fact ordinary chondrites (OCs) differing from most known OCs only in their lower redox state, as was suggested in earlier investigations. Particular consideration was given to obtaining phase composition data bearing on the nature and timing of the hypothesized reduction event. On the basis of the results obtained, it is concluded that the most plausible explanation of the chondrites' properties is that these OCs were incorporated into and metamorphosed in chemical contact with a reduced host regolith.

Wasson, J. T.; Rubin, A. E.; Kallemeyn, G. W.

1993-04-01

35

Extinct Radionuclides in the Early Solar System: The Initial Solar System Abundance of Iron-60 from Angrites and Unequilibrated Ordinary Chondrites and Aluminum-26-Magnesium-26 Chronology of Ungrouped Achondrites  

NASA Astrophysics Data System (ADS)

The presence of a number of extinct radionuclides in the early Solar System (SS) is known from geochemical and isotopic studies of meteorites and their components. The half-lives of these isotopes are short relative to the age of the SS, such that they have now decayed to undetectable levels. They can be inferred to exist in the early SS from the presence of their daughter nuclides in meteoritic materials that formed while they were still extant. The extinct radionuclides are particularly useful as fine-scale chronometers for events in the early SS. They can also be used to help constrain the astrophysical setting of the formation of the SS because their short half-lives and unique formation environments yield information about the sources and timing of delivery of material to the protoplanetary disk. Some extinct radionuclides are considered evidence that the Sun interacted with a massive star (supernova) early in its history. The abundance of 60Fe in the early SS is particularly useful for constraining the astrophysical environment of the Sun's formation because, if present in sufficient abundance, its only likely source is injection from a nearby supernova. The initial SS abundance of 60Fe is poorly constrained at the present time, with estimates varying by 1-2 orders of magnitude. I have determined the 60Fe-60Ni isotope systematics of ancient, well-preserved meteorites using high-precision mass spectrometry to better constrain the initial SS abundance of 60Fe. I find identical estimates of the initial 60Fe abundance from both differentiated basaltic meteorites and from components of primitive chondrites formed in the Solar nebula, which suggest a lower 60Fe abundance than other recent estimates. With recent improved meteorite collection efforts there are more rare ungrouped meteorites being found that hold interesting clues to the origin and evolution of early SS objects. I use the 26Al-26 Mg extinct radionuclide chronometer to constrain the ages of several recently recovered meteorites that sample previously unknown asteroid lithologies, including the only know felsic meteorite from an asteroid and two other ungrouped basaltic achondrites. These results help broaden our understanding of the timescales involved in igneous differentiation processes in the early SS.

Spivak-Birndorf, Lev

36

An In-Situ Trace Element Study of Refractory Forsterites from Different Types of Unequilibrated Chondrites  

NASA Astrophysics Data System (ADS)

Refractory lithophile elements (RLE) and transition metals were analyzed by LA-ICP-MS in refractory forsterites of carbonaceous ordinary and R-chondrites. High RLE contents require an origin by condensation or crystallization from RLE-enriched melts.

Pack, A.; Shelley, M.; O'Neill, H. St. C.; Palme, H.

2003-03-01

37

Classification of six ordinary chondrites from Texas  

NASA Technical Reports Server (NTRS)

Based on optical microscopy, modal and electron microprobe analyses, six ordinary chondrites from Texas were classified in compositional groups, petrologic types, and shock facies. These meteorites are Comanche (stone), L5c; Haskell, L5c; Deport (a), H4b; Naruna (a), H4b; Naruna (b), H4b; and Clarendon (b), H5d.

Ehlmann, Arthur J.; Keil, Klaus

1988-01-01

38

The Oxidation (Not Reduction) of Ordinary Chondrites During Metamorphism  

NASA Astrophysics Data System (ADS)

Subtle but systematic changes in the compositions and relative abundances of olivine, pyroxene, and metal with increasing petrologic type in equilibrated (types 4-6) H, L, and LL chondrites suggest that metallic Fe in these meteorites was oxidized during metamorphism. Observed changes include increases in the mean Fe contents of olivine and pyroxene and in the mean Ni and Co contents of bulk metal, as well as increases in the olivine:low-Ca pyroxene ratio with decreasing metal abundance. This evidence for oxidation is at variance with the commonly accepted idea that chondritic Fe was reduced by graphite during metamorphism (Ringwood, 1965; Williams, 1971; Brett and Sato, 1984; Rubin et al., 1988). We suggest that graphite activity was lowered by its dissolution in taenite at metamorphic temperatures, so that redox state was determined largely by equilibrium between ferromagnesian silicates and metal. Oxygen fugacities calculated from chondrite mineral equilibria are 2-3 log units below intrinsic fO(sub)2 measurements (Brett and Sato, 1984). The mineralogies of type 3 chondrites do not conform to the oxidation sequence seen in types 4-6 chondrites, and there is some evidence to suggest that Fe in unequilibrated chondrites was actually reduced during mild heating. Apparently, redox conditions in the surficial layers of parent bodies were reducing, but were oxidizing in the hotter interiors. Much of the current confusion over oxidation versus reduction is attributable to comparing unequilibrated and equilibrated chondrites. Progressive oxidation of Fe during metamorphism implies reaction with an oxidizing agent no longer present in the meteorites. We suggest that this oxidant was an aqueous vapor, derived from heating small amounts of ices originally accreted into the parent asteroids. The condensation of this vapor in cooler, outer layers of asteroids could account for aqueous alteration phases documented in some type 3 chondrites (Alexander et al., 1989). Assuming that the vapor was pure H2O, the water:rock weight ratio required to account for the observed Fe oxidation was very modest, less than 3o/oo. However, the presence of even minor amounts of a fluid attending metamorphism has important implications for the interpretation of chemical and isotopic patterns in ordinary chondrites. For example, this vapor may have acted as a carrier gas to facilitate redistribution of volatile trace elements during metamorphism (Sugiura et al., 1984). Open-system metamorphism may also explain oxygen isotopic patterns seen in the data of Clayton et al. (1991). This pattern could have resulted from varying exchange with an isotopically light vapor at different temperatures, or with a vapor whose isotopic composition evolved as it permeated through the parent bodies. References: Alexander C.M. et al. (1989) Geochim. Cosmochim. Acta 53, 3045-3057. Brett R. and Sato M. (1984) Geochim. Cosmochim. Acta 48, 111-120. Clayton R.N. et al. (1991) Geochim. Cosmochim. Acta 55, 2317-2337. Ringwood A.E. (1965) Nature 207, 701-704. Rubin A.E. et al. (1988) In Meteorites and the Early Solar System (eds. J.F. Kerridge and M.S. Matthews), pp. 488-511. Univ. Arizona Press, Tucson, AZ. Sugiura N. et al. (1984) J. Geophys. Res. 89, B641-644. Williams R.J. (1971) Geochim. Cosmochim. Acta 35, 407-411.

McSween, H. Y., Jr.; Labotka, T. C.

1992-07-01

39

Oxygen isotope studies of ordinary chondrites  

NASA Technical Reports Server (NTRS)

Several stages in the evolution of ordinary chondritic meteorites are recorded in the oxygen isotopic composition of the meteorites and their separable components (chondrules, fragments, clasts, and matrix). The whole-rock isotopic compositions reflect the iron-group of the meteorite (H, L, or LL). Isotopic uniformity of H3 to H6 and L3 to L6 are consistent with closed-system metamorphism within each parent body. LL3 chondrites differ slightly from LL4 to LL6, implying a small degree of open-system aqueous alteration and carbon reduction. On the scale of individual chondrules, the meteorites are isotopically heterogeneous, allowing recognition of the solar-nebular processes of chondrule formation. Chondrules for all classes of ordinary chondrites are derived from a common population, which was separate from the population of chondrules in carbonaceous or enstatite chondrites. Chondrules define an isotopic mixing line dominated by exchange between (O - 16)-rich and (O - 16)-poor reservoirs. The oxygen isotopic compositions of chondrites serve as 'fingerprints' for identification of genetic association with other meteorite types (achondrites and iron) and for recognition of source materials in meteoritic breccias.

Clayton, Robert N.; Mayeda, Toshiko K.; Olsen, Edward J.; Goswami, J. N.

1991-01-01

40

Fossil records of high level of 60Fe in chondrules from unequilibrated chondrites  

NASA Astrophysics Data System (ADS)

The short-lived now-extinct nuclide (SLN) 60Fe, which decays to 60Ni with a half-life of 2.62 Ma, is uniquely of stellar origin. Hence, its Solar System initial abundance yields information about the source of SLNs and the astrophysical environment in which the Solar System was born. Only a few chondrules (?19) from unequilibrated ordinary chondrites have reported resolved 60Ni excesses using in situ secondary ion mass spectrometry implying Fe60/Fe56>?0.6×10-7 in the early Solar System, and among these very few (3) have higher excesses implying Fe60/Fe56?7×10-7 (Mishra et al., 2010; Mishra and Goswami, 2014; Telus et al., 2012). At variance, multi-collector inductively coupled plasma mass spectrometer studies of bulk samples and mineral separates from differentiated meteorites, angrites, achondrites, and chondrules suggest a low abundance of 60Fe/56Fe of ?1.4×10-8 which would rule out the need for an external seeding of the early Solar with stellar 60Fe (Quitté et al., 2011; Tang and Dauphas, 2012). Two Semarkona chondrules and one Efremovka chondrule analyzed in the present study have mass fractionation corrected excess of up to ?75 permil (‰) and give 60Fe isochrons with initial 60Fe/56Fe ratios of (7.8±3.7)×10-7, (3.8±1.6)×10-7, and (2.2±1.1)×10-7 (2?), for Efremovka Ch 1, Semarkona Ch 12, and Semarkona Ch J5 respectively. The higher values of 60Fe/56Fe ratios seen in the chondrules of these least altered meteorites samples concur with and lend greater credence to the suggestion of a massive star as the source of 60Fe, and possibly of other short-lived nuclides, to the early Solar System. However, no definitive explanation (e.g. sample bias, effects of metamorphism, 60Fe heterogeneity) to the apparent disagreement with studies of bulk chondrules and chondrule fragments has been found.

Mishra, Ritesh Kumar; Chaussidon, Marc

2014-07-01

41

Primitive ultrafine matrix in ordinary chondrites  

NASA Technical Reports Server (NTRS)

Ultrafine matrix material has been concentrated by sieving and filtering disaggregated samples of six ordinary chondrites of different classes. This component(s), 'Holy Smoke' (HS), is enriched in both volatile, e.g. Na, K, Zn, Sb, and Pb, as well as refractory elements, e.g. W and REE; however, the element ratios vary greatly among the different chondrites. SEM studies show that HS contains fragile crystals, differing in composition, and apparently in gross disequilibrium not only among themselves but also with the major mineral phases and consequently thermodynamic equilibration did not occur. Thus HS must have originated from impacting bodies and/or was inherent in the 'primitive' regolith. Subsequent impact brecciation and reheating appears to have altered, to varying degrees, the original composition of this ultrafine matrix material. Recent 'cosmic dust' studies may indicate that HS still exists in the solar system. Survival of such delicate material must be considered in all theories for the origin of chondrites.

Rambaldi, E. R.; Fredriksson, B. J.; Fredriksson, K.

1981-01-01

42

{sup 60}Fe AND {sup 26}Al IN CHONDRULES FROM UNEQUILIBRATED CHONDRITES: IMPLICATIONS FOR EARLY SOLAR SYSTEM PROCESSES  

SciTech Connect

The presence of about a dozen short-lived nuclides in the early solar system, including {sup 60}Fe and {sup 26}Al, has been established from isotopic studies of meteorite samples. An accurate estimation of solar system initial abundance of {sup 60}Fe, a distinct product of stellar nucleosynthesis, is important to infer the stellar source of this nuclide. Previous studies in this regard suffered from the lack of exact knowledge of the time of formation of the analyzed meteorite samples. We present here results obtained from the first combined study of {sup 60}Fe and {sup 26}Al records in early solar system objects to remove this ambiguity. Chondrules from unequilibrated ordinary chondrites belonging to low petrologic grades were analyzed for their Fe-Ni and Al-Mg isotope systematics. The Al-Mg isotope data provide the time of formation of the analyzed chondrules relative to the first solar system solids, the Ca-Al-rich inclusions. The inferred initial {sup 60}Fe/{sup 56}Fe values of four chondrules, combined with their time of formation based on Al-Mg isotope data, yielded a weighted mean value of (6.3 {+-} 2) x 10{sup -7} for solar system initial {sup 60}Fe/{sup 56}Fe. This argues for a high-mass supernova as the source of {sup 60}Fe along with {sup 26}Al and several other short-lived nuclides present in the early solar system.

Mishra, R. K.; Goswami, J. N.; Rudraswami, N. G. [Physical Research Laboratory, Ahmedabad-380009 (India); Tachibana, S. [Department of Earth and Planetary Sciences, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033 (Japan); Huss, G. R., E-mail: goswami@prl.res.i [Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822 (United States)

2010-05-10

43

Lunar and Planetary Science XXXV: Ordinary and Enstatite Chondrites  

NASA Technical Reports Server (NTRS)

The session "Ordinary and Enstatite Chondrites" included the following reports:The Distribution of Molybdenum in the Indarch EH4 Chondrite; Cosmic-Ray Exposure Age and Heliocentric Distance of the Parent Body of E Chondrites ALH 85119 and MAC 88136; Further Observations of Fe-60-Ni-60 and Isotopic Systems in Sulfides from Enstatite Chondrites; Thermal Metamorphism in L Chondrites: Implications of Percent Mean Deviation in Olivine and Pyroxene; Cooling Rates and the Mn-53-Cr-53 Isotopic System of Yamato 86753, an Equilibrated Ordinary Chondrite; Production Rates of Cosmogenic Nuclides in the Knyahinya L-Chondrite; Preliminary Mineralogical Data from the Saratov (L4) Primitive Ordinary Chondrite; Phosphate Minerals in Semarkona; A Textural Comparison of Chondrules and Smelter-derived Dust: Implications Regarding Formation Conditions; and Modification of the Van Schmus & Wood Petrologic Classification for Lithic Fragments in the Chondritic Breccia Rumuruti.

2004-01-01

44

Igneous Rock from Severnyi Kolchim (H3) Chondrite: Nebular Origin.  

National Technical Information Service (NTIS)

The discovery of lithic fragments with compositions and textures similar to igneous differentiates in unequilibrated ordinary chondrites (UOC's) and carbonaceous chondrites (CC's) has been interpreted as to suggest that planetary bodies existed before cho...

M. A. Nazarov F. Brandstaetter G. Kurat

1993-01-01

45

Carbon in Metal of Primitive Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

Abundant graphite inclusions have recently been found in Fe-Ni metal grains of the Bishunpur (L3.1) chondrite [1]. Their presence in metal is correlated with the concentrations of minor elements (Co, Cr), which has been interpreted as evidence that carbon played a role in redox processes during chondrule formation [2]. The origin of these inclusions is not clear. In a companion paper (Mostefaoui and Perron, this conference) their characterization has been attempted by Raman spectrometry. Here we try i) to ascertain whether they are related to the rest of the carbon in the chondrite, ii) to determine the carbon content of metal grains. Graphite has been occasionally observed in the metal of ordinary chondrites (OC) [3] but no systematic search has apparently been conducted. We surveyed the metal of a number of chondrites and found graphite in 10 OCs, all of type 3 (Bishunpur, Inman, Kohar, Manych, Massenya, Mezo-Madaras, Moorabie, Ngawi, Semarkona, Tieschitz) and in Kainsaz (CO3). We have quantified the abundance of graphite-bearing metal by the number of metal grains with visible graphite inclusions (regardless of the size of the grains and of the inclusions) per cm2 of polished surface, normalized to the metal abundance of the chondrite, taken from [4]. Fig. 1 shows the values obtained, normalized to Bishunpur, plotted as a function of total carbon content, taken from [4] and references therein, for 10 chondrites (the total carbon content of the 11th, Massenya, is unknown). The line is a regression through 6 points. Four points have not been taken into account as i) Kainsaz, a carbonaceous chondrite, may not follow the OC trend; ii) Semarkona metal contains abundant carbides [5], which may result in a lower graphite abundance; iii) Kohar and Moorabie, whose graphite is essentially in veins extending into silicates, show abundant signs of shock effects (the link between graphite and shock is not clear, but there is some evidence of concentration of carbonaceous matter in shock veins [6]). If the correlation shown in Fig. 1 is real, it is a strong argument in favor of a common origin for graphite inclusions in metal and the bulk of carbon. Preliminary values of C concentration have been obtained with the ion probe at Nancy for 7 metal grains in Bishunpur. They range from *0.1wt%C for kamacite in reduced chondrules (FeO-poor silicates, Cr-rich metal)

Mostefaoui, S.; Perron, C.; Chaussidon, M.; Hanon, P.; Robert, F.

1995-09-01

46

A comparison of FeO-rich, porphyritic olivine chondrules in unequilibrated chondrites and experimental analogues  

NASA Astrophysics Data System (ADS)

Experimentally produced analogues of porphyritic olivine (PO) chondrules in ordinary chondrites provide an important insight into chondrule formation processes. We have studied experimental samples with PO textures grown at three different cooling rates (2, 5 and 100 C/h), and samples that have been annealed at high temperatures (1000-1200 C) subsequent to cooling. These are compared with natural chondrules of similar composition and texture from the ordinary chondrites Semarkona (LL3.0) and ALH 81251 (LL3.3). Zoning properties of olivine grains indicate that the Semarkona chondrules cooled at comparable rates to the experiments. Zoning in olivine from chondrules in ALH 81251 is not consistent with cooling alone but indicates that the chondrules underwent an annealing process. Chromium loss from olivine is very rapid during annealing and calculated diffusion coefficients for Cr in olivine are very similar to those of Fe-Mg interdiffusion coefficients under the same conditions. Annealed experimental samples contain an aluminous, low-Ca pyroxene which forms by reaction of olivine and liquid. No similar reaction texture is observed in ALH 81251 chondrules, and this may be evidence that annealing of the natural samples took place at considerably lower temperatures than the experimental analogues. The study supports the model of chondrule formation in a cool nebula and metamorphism of partly equilibrated chondrites during reheating episodes on the chondrite parent bodies.

Jones, R. H.; Lofgren, G. E.

1993-06-01

47

A comparison of FeO-rich, porphyritic olivine chondrules in unequilibrated chondrites and experimental analogues  

NASA Technical Reports Server (NTRS)

Experimentally produced analogues of porphyritic olivine (PO) chondrules in ordinary chondrites provide an important insight into chondrule formation processes. We have studied experimental samples with PO textures grown at three different cooling rates (2, 5 and 100 C/h), and samples that have been annealed at high temperatures (1000-1200 C) subsequent to cooling. These are compared with natural chondrules of similar composition and texture from the ordinary chondrites Semarkona (LL3.0) and ALH 81251 (LL3.3). Zoning properties of olivine grains indicate that the Semarkona chondrules cooled at comparable rates to the experiments. Zoning in olivine from chondrules in ALH 81251 is not consistent with cooling alone but indicates that the chondrules underwent an annealing process. Chromium loss from olivine is very rapid during annealing and calculated diffusion coefficients for Cr in olivine are very similar to those of Fe-Mg interdiffusion coefficients under the same conditions. Annealed experimental samples contain an aluminous, low-Ca pyroxene which forms by reaction of olivine and liquid. No similar reaction texture is observed in ALH 81251 chondrules, and this may be evidence that annealing of the natural samples took place at considerably lower temperatures than the experimental analogues. The study supports the model of chondrule formation in a cool nebula and metamorphism of partly equilibrated chondrites during reheating episodes on the chondrite parent bodies.

Jones, Rhian H.; Lofgren, Gary E.

1993-01-01

48

Penecontemporaneous metamorphism, fragmentation, and reassembly of ordinary chondrite parent bodies  

NASA Astrophysics Data System (ADS)

The thermal histories of ordinary chondrites and the canonical internal heating or onion shell models, which predict an inverse relation between the petrologic type of chondrites and the metallographic cooling rate, are reviewed. The thermal and accretional requirements of the 'metamorphosed planetesimal' model proposed by Scott and Rajan (1981) are analyzed, and an alternative model consistent with the metallographic cooling rate constraints is suggested in which ordinary chondrite parent bodies are collisionally fragmented and then rapidly reassembled before metamorphic heat has been dissipated.

Grimm, R. E.

1985-02-01

49

The evolution of enstatite and chondrules in unequilibrated enstatite chondrites: Evidence from iron-rich pyroxene  

NASA Technical Reports Server (NTRS)

FeO-rich (Fs(sub 6)-34) pyroxene lacking cathodoluminescence (CL), hereafter black pyroxene, is a major constituent of some of the chondrules and fragments in unequilibrated (type 3) enstatite chondrites (UECs). It contains structurally oriented zones of Cr-, Mn-, V-rich, FeO-poor enstatite with red CL, associated with mm-sized blebs of low-Ni, Fe-metal and, in some cases, silica. These occurrences represent clear evidence of pyroxene reduction. The black pyroxene is nearly always rimmed by minor element (Cr, Mn, V)-poor enstatite having a blue CL. More commonly, red and blue enstatites, unassociated with black pyroxene, occur as larger grains in chondrules and fragments, and these constitute the major silicate phases in UECs. The rare earth element (REE) abundance patterns of the black pyroxene are LREE-depleted. The blue enstatite rims, however, have a near-flat to LREE-enriched pattern, approx. 0.5-4x chondritic. The petrologic and trace element data indicate that the black pyroxene is from an earlier generation of chondrules that formed in a nebular region that was more oxidizing than that of the enstatite chondrites. Following solidification, these chondrules experienced a more reducing nebular environment and underwent reduction. Some, perhaps most, of the red enstatite that is common throughout the UECs may be the product of solid-state reduction of black pyroxene. The blue enstatite rims grew onto the surfaces of the black pyroxene and red enstatite as a result of condensation from a nebular gas. The evolutionary history of some of the enstatite and chondrules in enstatite chondrites can be expressed in a four-stage model that includes: Stage 1. Formation of chondrules in an oxidizing nebular environment. Stage 2. Solid-state reduction of the more oxidized chondrules and fragments to red enstatite in a more reducing nebular environment. Stage 3. Formation of blue enstatite rims on the black pyroxene as well as on the red enstatite. Stage 4. Reprocessing, by various degrees of melting, of many of the earlier-formed materials.

Weisberg, Michael K.; Prinz, Martin; Fogel, Robert A.

1994-01-01

50

Crystallization of chondrules in ordinary chondrites  

NASA Astrophysics Data System (ADS)

The process of crystallization and the origin of chondrules are discussed, in terms of the phase relations of the minerals in chondrules in six ordinary chondrites of the Yamato-74 meteorites, especially the Yamato-74191 (L3). Chondrules are classified into six types. The bulk compositions of chondrules projected onto the MgO-FeO-SiO2 system show that the compositions of chondrules vary widely. Investigations by means of the MgO-Al2O3-SiO2 system indicate that porphyritic chondrules can be regarded as products of supercooling crystallization. The difference between types of chondrules is interpreted in terms of the compositions of chondrules and the nucleation temperatures of the supercooled droplets. The impact and dust fusion theories do not appear to be plausible. Only a liquid condensation theory can well explain the characteristic features and the process of the crystallization of chondrules.

Kimura, M.; Yagi, K.

1980-04-01

51

Pore size distribution in an uncompacted equilibrated ordinary chondrite  

SciTech Connect

The extraordinarily uncompacted nature of the ordinary L chondrite fall Baszkowka gives a unique opportunity to investigate the potentially pre-compaction pore size distribution in an equilibrated ordinary chondrite. Using X-ray microtomography and helium pycnometry on two samples of Baszkowka, we have found that on average, two-thirds of the 19.0% porosity resides in inter- and intra-granular voids with volumes between {approx}3 x10{sup 05} and 3 mm{sup 3}. We show the cumulative number density of pore volumes observable by X-ray microtomography obeys a power law distribution function in this equilibrated ordinary chondrite. We foresee these data adding to our understanding of the impact processing of chondrites and their parent asteroids, where porosity and pore size play significant roles in the parameterization of impact events.

Friedrich, J.M.; Macke, R.J.; Wignarajah, D.P.; Rivers, M.L.; Britt, D.T.; Ebel, D.S. (Univ of Central FL); (Fordham Univ); (UofC); (Univ of Central FL); (AMNH); (Fordham Univ)

2008-05-30

52

Oxygen isotopes in R-chondrite magnetite and olivine: links between R chondrites and ordinary chondrites  

NASA Astrophysics Data System (ADS)

Ion-microprobe studies yield ? 17O (=? 17O - 0.52 · ? 18O) values in magnetite from the Rumurti chondrite (RC) PCA91241 (which is paired with PCA91002) of +3.1 to +3.9‰, slightly higher than that in O from whole-rock R samples. Despite ? 17O values in whole-rock RCs that are much (by ca. 1.6‰) higher than in whole-rock LL chondrites, the ? 17O in R magnetite is much lower (by ca. 2‰) than the values (+4 to +7‰) from LL3 Semarkona and Ngawi (Choi et al., 1998). The ? 18O values in PCA magnetite (-15 to -10‰) are the lowest known in meteorites, well below the range in Semarkona (-4 to +9‰). On a ? 17O-? 18O diagram both magnetite data sets form linear arrays with slopes of ca. 0.7, indicating mixing of O from different isotopic reservoirs; the slopes and intercepts of the two arrays are similar enough to permit them to be segments of a single array. This suggests that, in RCs and LL chondrites, magnetite formed from the same raw materials by the same processes, probably by aqueous alteration of metal in an asteroidal setting. We observed ? 17O values in olivines and pyroxene from RCs ranging from -1.2 to +2.9‰ and ? 18O from +1.4 to +9.1‰. These compositions scatter in the same general range observed in chondrules from ordinary chondrites. The similarity in the O-isotopic composition of minerals that preserve a record of formation in the solar nebula supports a model in which RCs formed from nebular components similar to those in H chondrites, but with a matrix/chondrule ratio several times higher in the RCs, and with more extensive aqueous alteration in the RCs than in known H chondrites. We postulate that the matrix in R chondrites has ? 17O higher than whole-rock values. We suggest that the original ? 17O value of H 2O in the RC body was similar to that incorporated into the ordinary chondrites, previously estimated by Choi et al. (1998) to be ca. +7‰ in the LL parent body.

Greenwood, James P.; Rubin, Alan E.; Wasson, John T.

2000-11-01

53

Classification of eight ordinary chondrites from Texas  

NASA Technical Reports Server (NTRS)

Based on optical microscopy and electron microprobe analyses, eight previously undescribed or poorly known chondrites were classified into compositional groups, petrologic types, and degree of shock alteration. These chondrites are: Leander, L4b; Nazareth(a), L6d; La Villa, H4b; Mereta, H4c; Gail, H4d; Shafter Lake, H5a; Uvalde, H5d; and Howe, H5d.

Ehlmann, A. J.; Keil, K.

1985-01-01

54

Classification of eight ordinary chondrites from Texas  

NASA Astrophysics Data System (ADS)

Based on optical microscopy and electron microprobe analyses, eight previously undescribed or poorly known chondrites were classified into compositional groups, petrologic types, and degree of shock alteration. These chondrites are: Leander, L4b; Nazareth(a), L6d; La Villa, H4b; Mereta, H4c; Gail, H4d; Shafter Lake, H5a; Uvalde, H5d; and Howe, H5d.

Ehlmann, A. J.; Keil, K.

1985-06-01

55

The mineralogy of ordinary chondrites and implications for asteroid spectrophotometry  

NASA Astrophysics Data System (ADS)

Published data from bulk chemical analyses of 94 ordinary chondrites are compiled in a table of normative mineralogy and discussed in detail. Significant variations in olivine, pyroxene, and metal abundance ratios are found within each chondrite class and attributed to redox processes superimposed on initial differences in metal/silicate ratios. The use of the diagrams constructed here to predict the mineralogic characteristics of asteroids on the basis of spectrophotometric observations is suggested.

McSween, H. Y.; Bennett, M. E.; Jarosewich, E.

1991-03-01

56

A hydrogen isotope study of CO3 type carbonaceous chondrites; comparison with type 3 ordinary chondrites  

NASA Technical Reports Server (NTRS)

Meteorites of the Ornans type 3 carbonaceous chondrites exhibit a range in degree of equilibration, attributed to differing amounts of thermal metamorphism. These differences have been used to split the CO3 chondrites into petrologic sub-types from 3.0, least equilibrated, to 3.7, being most equilibrated. This is similar to the system of assigning the type 3 ordinary chondrites into petrologic sub-types 3.0 to 3.9 based upon thermoluminescence (TL) and other properties; however, the actual range of thermal metamorphism experienced by CO3 chondrites is much less than that of the type 3 ordinary chondrites. The least equilibrated ordinary chondrites show evidence of aqueous alteration and have high D/H ratios possibly due to a deuterium-rich organic carrier. The aim of this study was to determine whether the CO3 chondrites, which have experienced similar secondary conditions to the type 3 ordinary chondrites, also contain a similar deuterium-rich carrier. To date a total of 5 CO3 meteorites, out of a set of 11 for which carbon and nitrogen isotopic data are available, have been analyzed. Ornans has not been analyzed yet, because it does not appear to fit in with the metamorphic sequence exhibited by the other CO3 chondrites; it also has an extremely high delta-D value of +2150 percent, unusual for such a comparatively equilibrated meteorite (type 3.4). Initial results indicate that the more equilibrated CO3's tend to have lower delta-D values, analogous to the higher petrologic type ordinary chondrites. However this is complicated by the effects of terrestrial weathering and the small data-set.

Morse, A. D.; Newton, J.; Pillinger, C. T.

1993-01-01

57

Ordinary Chondrites: 2. Diffusion Losses of Rare Gases  

NASA Astrophysics Data System (ADS)

Data on the isotopic abundances and ratios of light rare gases (He and Ne) in 600 ordinary chondrites are analyzed. The ratio of cosmogenic isotopes (^3He/^21Ne)_cin ~20% of the ordinary chondrites has been found to lie well below the correlation line that represents the dependence of (^3He/^21Ne)_con (^22Ne/^21Ne)_c. This effect shows up most clearly in ^4He_r-chondrites, particularly in meteorites with diffusion losses of radiogenic ^21Ne_c, and is most likely attributable to the predominant (compared to ^3He_c) diffusion losses of cosmogenic ^3He_cthrough the solar heating of meteorites in orbits with small perihelion distances. This effect is enhanced by periodic variations in orbital parameters (including the perihelion distance) of meteorites throughout their exposure histories. Thermoluminescence data for ordinary chondrites confirm this scenario. The (^3He/^21Ne)_cratio for ~15% of the chondrites was significantly overestimated, which may stem from the fact that such meteorites were heavily shielded in preatmospheric bodies.

Alekseev, V. A.

2001-03-01

58

Redox Controls of Fe-Mn-Mg in Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

If the Fe/Mn ratio of CI chondrites (~95) is an average for the solar system, then the variation of Fe/Mn ratios (50-150) among other chondritic meteorites reflects processing of the prechondritic material. The known, moderate volatility of Mn is often invoked to explain these variations [1], but CI normalized abundance data for O and C chondrites [2] reveals that although carbonaceous chondrites are indeed depleted in Mn, as reflected in their high Fe/Mn ratios, the ordinary chondrites are essentially undepleted. Relative to CI, however, the ordinary chondrites have variable Fe abundances and this is reflected in their bulk Fe/Mn (atom) ratios. The low Fe/Mn ratios of ordinary chondrites are, therefore, not a reflection of Mn enrichment relative to CI but of Fe depletion. In detail, ordinary chondrites also reveal evidence of processing prior to their assembly as O-chondrites. Figure 1 shows the variation of Fe, Mn, and Mg in ordinary chondrites relative to CI. The line shows the trace of iron loss or addition relative to Orgueil [2]. Both bulk and silicate fractions of OCs fall on or close to this line, confirming that they are related to the CI average by Fe loss. The order of increasing Fe loss (H-L-LL) reflects the general classification of these meteorites. However, the silicate-only fraction of the OC [3] shows the opposite trend (LL-L-H) with Fe loss, reflecting the metal/silicate fractionation present in the meteorites. The variation of bulk OC Fe/Mn reflects the removal of Fe metal from the OC precursors by a reduction reaction presumably of the type FeO(silicate) + CO(nebula) = Fe(metal) + C02 (gas). Clearly H-chondrites show little of this metal removal in bulk. L and LL chondrite precursors, however, have been progressively depleted in Fe resulting in lower bulk Fe/Mn ratios. In contrast, the metal rich H-chondrites have the most reduced silicates (lowest silicate Fe/Mn) although much of the metal fraction is still present in the meteorites. The LL chondrites contain the least evidence of in situ reduction, as their silicates have Fe/Mn ratios close to their bulk Fe/Mn. The reaction that reduced the FeO to metal was probably controlled by in situ carbon rather than nebular CO. Since the lowering of Fe/Mn in the bulk OCs reflects nebular reduction by CO, other evidence of that reduction should be sought. The progressive shift of the oxygen isotope ratios in sequence H-L-LL toward heavier oxygen probably reflects this reduction reaction with a gas reservoir enriched in heavy oxygen as suggested by [4]. The removal of Fe metal prior to the assembly of OC meteorites, therefore provides a link to the undifferentiated iron meteorites. The fractionation of the silicates in OCs relative to the bulk meteorites may reflect either in situ reaction of FeO with C in the chondritic host, or incomplete separation of metal formed by nebular reaction. Correlation of the Fe-Mn-Mg results for chondrules from UOCs with their oxygen isotope signature should provide a measure of these competing effects, although the effects of Mn volatility may obscure the data at the scale of a chondrule. References: [1] Ganapathy and Anders (1977) Proc. LPSC 8th. [2] Wasson and Kallemeyn (1988) Phil. Trans. R. Soc. A235. [3] Jarosewich (1990) Meteoritics, 25. [4] Clayton and Mayeda (1992) Ann. Rev. Earth Planet. Sci. Acknowledgment: NAG9-304

Delaney, J. S.

1993-07-01

59

Classification of four ordinary chondrites from the Monnig Meteorite Collection  

NASA Astrophysics Data System (ADS)

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.; Keil, Klaus

1992-09-01

60

Classification of a second group of ordinary chondrites from Texas  

NASA Technical Reports Server (NTRS)

Based on optical microscopy and electron microprobe analyses of mafic minerals, six previously undescribed or poorly known ordinary chondrites from Texas were classified into compositional groups, petrologic types, and shock facies. These meteorites are Junction, L5d; Anton, H5b; Venus, H4d; Dalhart, H5a; Rosebud, H5c; and Cranfills Gap, H6c.

Ehlmann, Arthur J.; Keil, Klaus

1987-01-01

61

Classification of four ordinary chondrites from the Monnig Meteorite Collection  

NASA Technical Reports Server (NTRS)

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.; Keil, Klaus

1992-01-01

62

Elevated initial Sr-87/Sr-86 in ordinary chondrite metal  

NASA Technical Reports Server (NTRS)

The metal phase of ordinary chondrites has been found to contain a complex assemblage of non-metal phases, evidently formed when elements such as Si, Cr and P, originally dissolved in the metal, were subsequently oxidized and exsolved. We have investigated the Rb-Sr isotopic system in samples of H-chondrite metal, finding small but nontrivial amounts of Sr, apparently concentrated toward the surface of metal grains, and characterized by pronounced excesses of Sr-87/Sr-86 in chondritic metal can plausibly account for the characteristically elevated initial Sr-87/Sr-86 found in ordinary chondrite phosphates, but it remains unclear when and where the metal experienced the high Rb/Sr environment needed to account for such high Sr-87/Sr-86. We have previously reported Rb-Sr data for separated metal from Dhajala (H3), Forest Vale (H4) and Estacado (H6). Here we report data for two size fractions (80-160 and 280-450 microns) of separated metal from the H4 chondrite Ste. Marguerite. These samples were etched twice (.5N HCl for one hour) before dissolution (6N HCl). Aliquots of the solutions were spiked for Fe, Ca and Rb + Sr. The results for Ste. Marguerite metal are presented; errors are two-sigma and elemental concentrations are stated in terms of total sample weight.

Podosek, F. A.; Brannon, J. C.; Perron, C.; Pellas, P.

1993-01-01

63

Origin of petrofabrics and magnetic anisotropy in ordinary chondrites  

NASA Technical Reports Server (NTRS)

Three-dimensional finite strain and magnetic susceptibility anisotropy have been determined for 15 ordinary chondrites. The axes of strain and magnetic ellipsoids roughly correlate in both magnitude and orientation. The shapes of these ellipsoids are generally oblate spheroids that define a dominant foliation and a weak lineation. These characteristics suggest deformation involving uniaxial compaction. The degree of uniaxial deformation correlates with intensity of shock, as indicated by optical, TEM and chemical criteria. These data, plus the lack of a relationship between foliation and metamorphic history, indicate that dynamic processes, i.e., impacts, produced planar deformation fabrics in chondrites.

Sneyd, Deana S.; Mcsween, Harry Y., Jr.; Sugiura, Naoji; Strangway, David W.; Nord, Gordon L., Jr.

1988-01-01

64

Ordinary Chondrites Viewed as Reassembled 'Splash Ejecta'  

NASA Astrophysics Data System (ADS)

A case has already been made favouring chondrites as re-assembled "splash ejecta" following low velocity collisions between molten planetesimals[1]. Here I briefly review this hypothesis, then develop further arguments in its support. The scenario envisaged may be summarized as follows. Planetesimals grew to radii greater than 30 km in less than 1 Ma after the formation of CAIs, and they were heated rapidly by the decay of 26Al. By 2 Ma each planetesimal had a molten interior insulated by a cool, dusty carapace. Low velocity collisions at this stage released enormous, turbulent, expanding clouds of incandescent spray mixed with dust and solid grains from the carapace. The cloud constituted a rather special, transient nebular environment; as it cooled the melt droplets became chondrules. Much of the cloud's contents re-assembled under gravity onto the surface of the hot, residual planetesimal and the accumulated debris became re-heated and metamorphosed. Collisions recurred over the few million years that relative velocities remained low and planetesimals remained molten. Thus, the cumulative debris contained many recycled and broken chondrules. This scenario is apparently reconcilable with chondrule cooling rates, the preservation of clasts of "planetary" rock in chondrites, the retention of volatiles in chondrules, the preservation of solar chemistry and more than a dozen other features. Is it reasonable to claim that 30 km radius bodies existed by 1 Ma, and were substantially molten by 2 Ma? Cameron[2] argued that CAIs were saved from drifting into the sun by their incorporation, soon after formation, into planetesimals whose mass was sufficient to hold them in orbits, decoupled from the drag of nebular gas. Wetherill's models [3] show that many bodies >100 km radius may have formed on a timescale of 10^5 years. In these terms, the proposed 30 km by 1 Ma is quite conservative. Regarding 26Al heating, the remarkably constant initial ratio of 26Al/27Al (5 x 10^-5) in CAIs from different classes of meteorite [4] suggests that 26Al was uniformly distributed in the dust which eventually formed the chondrite parent bodies. This amount of 26Al translates to some 7000 J g^-1. A simple finite element calculation was made to assess the likely thermal evolution of planetesimals of different sizes, starting from 300K at different times. The proposed body of 30 km radius at 1 Ma was found to be a limit for substantial internal melting. Its interior would have remained molten for several million years. Earlier accretion, or larger planetesimals, would have generated even more melt. It seems, therefore, that molten planetesimals were abundant in the early solar system. Moreover, they evidently suffered collision and accretion. If their collision products were not chondrules, then what were they? Two further arguments favouring the proposed scenario concern the age difference of CAIs and chondrules, and the existence of macrochondrules. Cameron's Leonard Award address [2] was stimulated by the inferred time interval of several million years between the formation of CAIs and chondrules. Chondrules were interpreted as dust melted by solar flare activity, the dust having been produced by late collisions between planetesimals. If, as is argued here, the planetesimals were already internally molten, chondrules would have been produced directly, without need to invoke a solar flare heat source. A separate issue is the existence of porphyritic olivine macrochondrules up to 4 cm across [5]. Macrochondrules are not easily reconcilable with chondrule formation by radiative heating in a nebular setting. Such a mechanism predicts an inverse relationship between chondrule diameter and temperature rise, which is not observed. However, in the present scenario macrochondrules are interpreted simply as examples of large blobs of frozen melt. References: [1] Sanders I. S. (1994) Meteoritics, 29, 527. [2] Cameron A. G. W. (1995) Meteoritics, 30, 133-161. [3] Wetherill G. W. (1989) in Asteroids II (R. P. Binzel et al., eds.), 661-

Sanders, I. S.

1995-09-01

65

Shock Classication of Ordinary Chondrites: New Data and Interpretations  

NASA Astrophysics Data System (ADS)

Introduction. The recently proposed classification system for shocked chondrites (1) is based on a microscopic survey of 76 non-Antarctic H, L, and LL chondrites. Obviously, a larger database is highly desirable in order to confirm earlier conclusions and to allow for a statistically relevant interpretation of the data. Here, we report the shock classification of an additional 54 ordinary chondrites and summarize implications based on a total of 130 samples. New observations on shock effects. Continued studies of those shock effects in olivine and plagioclase that are indicative of the shock stages S1 - S6 as defined in (1) revealed the following: Planar deformation features in olivine, considered typical of stage S5, occur occasionally in stage S3 and are common in stage S4. In some S4 chondrites plagioclase is not partially isotropic but still birefringent coexisting with a small fraction of S3 olivines. Opaque shock veins occur not only in shock stage S3 and above (1) but have now been found in a few chondrites of shock stage S2. Thermal annealing of shock effects. Planar fractures and planar deformation features in olivine persist up to the temperatures required for recrystallization of olivine (> ca. 900 degrees C). Shock history of breccias. In a number of petrologic types 3 and 4 chondrites without recognizable (polymict) breccia texture, we found chondrules and olivine fragments with different shock histories ranging from S1 to S3. Regolith and fragmental breccias are polymict with regard to lithology and shock. The intensity of the latest shock typically varies from S1 to S4 in the breccias studied so far. Frequency distribution of shock stages. A significant difference between H and L chondrites is emerging in contrast to our previous statistics (1), whereas the conspicuous lack of shock stages S5 and S6 in type 3 and 4 chondrites is clearly confirmed (Fig. 1). Correlation between shock and noble gas content. The concentration of radiogenic argon and of solar-wind implanted noble gases (2) decreases with increasing shock, as does the range of the measured concentrations, particularly above shock stage S3 (>10 GPa). Conclusions. From the present statistics of shock metamorphism of ordinary chondrites it appears that the H and L parent bodies experienced distinctly different collision histories, which also had different effects on the gas retention ages. Taking the distribution of exposure ages and 40Ar-39Ar ages (3) into account, the data may indicate that the unshocked, >4 b.y. old H chondrites result from a major catastrophic collision of one parent body some 6 m.y. ago, whereas the L chondrites and other H chondrites characterized by young Ar ages may be derived from repeated impact processing of their parent bodies over the past 90 m.y. References. (1) Stoffler, D., Keil, K. and Scott, E.R.D. (1991) Geochim. Cosmochim. Acta 55, 3845-3867. (2) Schultz, L. and Kruse, H. (1989) Meteoritics 24, 155-172. (3) Kerridge, J.F. and Matthews M.S., eds. (1988) Meteorites and the Early Solar System, Univ. of Arizona Press, Tucson, 1269 pp.

Stoffler, D.; Keil, K.; Scott, E. R. D.

1992-07-01

66

U-Pb systematics of phosphates from equilibrated ordinary chondrites  

NASA Astrophysics Data System (ADS)

U-Pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites. The high U-238/Pb-204 ratios of thirteen of these phosphate separates lead to extremely radiogenic Pb whose Pb-206/Pb-204 ratios range from 250 up to 3500. The Pb/Pb model ages for these phosphates range from 4.563 to 4.502 Ga, with an analytical precision of 106y and the U-Pb system is apparently concordant. The time interval observed 60 x 106y, reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the Rb/Sr, K/Ar and Pu chronologies. The Pb/Pb ages of the phosphates from the seven H chondrites show a negative correlation versus their metamorphic grade. This is the first clear relationship ever observed between a long-lived chronometer and the intensity of metamorphism as reflected by metamorphic grade. Assuming that the Pb/Pb age indicates the accurate U-Pb closure time in phosphates, the Pb/Pb chronology is compatible with the model of a layered H chondrite parent body. However, this interpretation of the U/Pb systematics is not unique; it postulates a slow cooling of the equilibrated materials at high temperature, in apparent conflict with petrological observations. Except for the H chondrites, which agree rather well with Pu systematics, comparison of the Pb/Pb chronology with published radiochronometric data does not reveal simple correlations. In the present debate concerning the thermal history of chondrites, the chronometric information derived from each isotope system is interpreted as the time of its thermal closure. However, this basic assumption may not be correct for all the radio-chronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4.56 - 4.4 Ga of proto-planetary history.

Gopel, C.; Manhes, G.; Allegre, C. J.

1994-01-01

67

Petrology of nine ordinary chondrite falls from China  

NASA Astrophysics Data System (ADS)

Nine twentieth-century ordinary chondrite falls from China are described and classified. They include: Nantong (H6), Zaoyang (H5), Zhaodong (L4), Qidong (L-LL5), Raoyang (L6), Sheyang (L6), Guangnan (L6), Suizhou (L6) and Nan Yang Pao (L6). Kamacite in Qidong is rare and contains much more Co (15 mg/g) than is characteristic of L-group chondrites; Qidong may be a member of a chondrite group intermediate in its properties between L and LL. Zhaodong, Qidong, Raoyang, Sheyang and Suizhou have several olivine and/or low-Ca pyroxene grains with aberrant Fe/(Fe + Mg) ratios; it is probable that these five chondrites are fragmental breccias. The lack of correlation between shock facies and occurrence of aberrant silicate grains suggests that breccia lithification caused only minimal shock effects in many meteorites. Alternatively, postshock annealing may have resulted in the recrystallization of shock-indicating phases, leading to assignment of a shockfacies that is lower than that present immediately after the shock event.

Wang, D.; Rubin, A. E.

1987-03-01

68

Generation of an Ordinary-Chondrite 'Regolith' by Repetitive Impact  

NASA Technical Reports Server (NTRS)

Analyses of meteorites and remote sensing studies for years have suggested the presence of regolith on asteroids, yet detailed observations of asteroid regoliths have been possible only recently with the flybys of 951 Gaspra, 243 Ida, and 253 Mathilde, and with the orbiting of and landing on 433 Eros by the NEAR Shoemaker spacecraft. Virtually all investigations into the generation and evolution of asteroid regoliths to date have been theoretical in nature. These have been guided mainly by observations of the lunar regolith, using what meager experimental data exist for terrestrial materials as substitutes for their asteroidal counterparts. As part of a program to evaluate the behavior of an ordinary chondrite under impact conditions, about 460 g of the L6 chondrite ALH85017 were subjected to 50 consecutive impacts, sufficient to reduce the target from a mean grain size of 11 mm to 0.5 mm. Some of the details of these experiments are presented here.

Cintala, M. J.; Hoerz, F.; See, T. H.; Morris, R. V.

2004-01-01

69

The onset of metamorphism in ordinary and carbonaceous chondrites  

USGS Publications Warehouse

Ordinary and carbonaceous chondrites of the lowest petrologic types were surveyed by X-ray mapping techniques. A variety of metamorphic effects were noted and subjected to detailed analysis using electron microprobe, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) methods. The distribution of Cr in FeO-rich olivine systematically changes as metamorphism increases between type 3.0 and type 3.2. Igneous zoning patterns are replaced by complex ones and Cr-rich coatings develop on all grains. Cr distributions in olivine are controlled by the exsolution of a Cr-rich phase, probably chromite. Cr in olivine may have been partly present as tetrahedrally coordinated Cr3+. Separation of chromite is nearly complete by petrologic type 3.2. The abundance of chondrules showing an inhomogeneous distribution of alkalis in mesostasis also increases with petrologic type. TEM shows this to be the result of crystallization of albite. Residual glass compositions systematically change during metamorphism, becoming increasingly rich in K. Glass in type I chondrules also gains alkalis during metamorphism. Both types of chondrules were open to an exchange of alkalis with opaque matrix and other chondrules. The matrix in the least metamorphosed chondrites is rich in S and Na. The S is lost from the matrix at the earliest stages of metamorphism due to coalescence of minute grains. Progressive heating also results in the loss of sulfides from chondrule rims and increases sulfide abundances in coarse matrix assemblages as well as inside chondrules. Alkalis initially leave the matrix and enter chondrules during early metamorphism. Feldspar subsequently nucleates in the matrix and Na re-enters from chondrules. These metamorphic trends can be used to refine classification schemes for chondrites. Cr distributions in olivine are a highly effective tool for assigning petrologic types to the most primitive meteorites and can be used to subdivide types 3.0 and 3.1 into types 3.00 through 3.15. On this basis, the most primitive ordinary chondrite known is Semarkona, although even this meteorite has experienced a small amount of metamorphism. Allan Hills (ALH) A77307 is the least metamorphosed CO chondrite and shares many properties with the ungrouped carbonaceous chondrite Acfer 094. Analytical problems are significant for glasses in type II chondrules, as Na is easily lost during microprobe analysis. As a result, existing schemes for chondrule classification that are based on the alkali content of glasses need to be revised. ?? The Meteorological Society, 2005.

Grossman, J. N.; Brearley, A. J.

2005-01-01

70

Stable Isotope Fractionation of Cr in Carbonaceous and Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

Difficulties with chemical separation and mass spectrometry combined with little expectation of isotopic fractionation at high temperature left the stable isotope geochemistry of Cr almost unknown [1]. The search for ^{53}Cr excess resulting from the decay of the radioactive nuclide ^{53}Mn (T1/2 = 3.5 My) was very successful but the small amount of data produced to date attests to the particularly difficult measurement by TIMS. This preliminary report describes evidence of mass-dependent fractionation of the stable Cr isotopes 50, 52, and 53 as measured by MC-ICPMS in meteorites relative to SRM 979 Cr standard. Cr was purified using cation-exchange chemistry. The yield is ~100 %. The samples were run on the Nu- Plasma HR of UC Davis in pseudo high-resolution mode. The absence of isotopic fractionation induced by the chemical purification has been double checked by 1) processing the standard through the column and 2) running the same sample several times through the columns. In both cases, we observe no difference in the measured isotopic ratio. The external reproducibility, estimated from 13 replicates of Bjurbole, is 50 ppm. The range of the fractionation per mass unit among 7 carbonaceous chondrites (CI, CM, CO, CV and CK) and 5 ordinary chondrites (H, L and LL) is 0.3 ‰. Individual chondrules from Chainpur and Bjurbole have a wider range (0.6 ‰). All the chondrites analyzed so far are isotopically lighter than the bulk silicate earth (? ^{50/52}Cr=0, [1]). As for Zn [2], Cr isotopes seem to be heavier in ordinary chondrites (-0.23 ‰) than in carbonaceous chondrites (-0.35 ‰). Also as for Zn and Cu [2-3], Cr stable isotopes in carbonaceous chondrites are correlated with ? 17O, suggesting a mixing between an isotopically heavy component and an isotopically light one. The correlation between mass-dependent fractionation of Cr and non-mass-dependent ? 17O requires attention and adequate interpretation. References: [1] Johnson and Bullen, 2004, Review in Mineralogy and Geochemistry. [2] Luck et al., 2005 GCA. [3] Luck et al., 2003 GCA.

Moynier, F.; Jacobsen, B.; Yin, Q.

2006-12-01

71

An unequilibrated inclusion in the Romero (H3-4) chondrite  

NASA Astrophysics Data System (ADS)

Within the chondritic matrix of Romero (H3-4) is a metal-poor inclusion consisting largely of olivines showing quench textures, minor low-Ca pyroxene and rare metal and sulfide. The olivines within this inclusion have the dominant composition Fa8 with a range Fa(7.5-17). The matrix (Romero host) olivines are, in the main, equilibrated with an average composition of Fa18, but are lower in CaO and Cr2O3 than the inclusion olivines. The high chromium content of the inclusion olivines suggests that they formed under conditions different from those represented by the other olivines in Romero.

Graham, A. L.

1983-03-01

72

Textural variability of ordinary chondrite chondrules: Implications of their formation  

NASA Technical Reports Server (NTRS)

Scanning electron microscopy (SEM) and microprobe examination of the Raguli H3-4, Saratov L3, and Fucbin L5-6 ordinary chondrites and the analysis of preexisted data on other meteorites have shown that the variety of textural types of chondrules depends on the chemical composition of the chondrules. The comparison of bulk-rock chemistries of the chondrules by major components demonstrates that they apparently fall, like basic-ultrabasic rock, into groups of dunitic and pyroxenitic composition. This separation is further validated by the character of zoning in chondrules of the intermediate, peridotitic type. The effect is vividly demonstrated by the 'chondrule-in-chondrule' structure.

Zinovieva, N. G.; Mitreikina, O. B.; Granovsky, L. B.

1994-01-01

73

Matrix and fine-grained rims in the unequilibrated CO3 chondrite, ALHA77307 - Origins and evidence for diverse, primitive nebular dust components  

NASA Astrophysics Data System (ADS)

SEM, TEM, and electron microprobe analysis were used to investigate in detail the mineralogical and chemical characteristics of dark matrix and fine-grained rims in the unequilibrated CO3 chondrite ALHA77307. Data obtained revealed that there was a remarkable diversity of distinct mineralogical components, which can be identified using their chemical and textural characteristics. The matrix and rim components in ALHA77307 formed by disequilibrium condensation process as fine-grained amorphous dust that is represented by the abundant amorphous component in the matrix. Subsequent thermal processing of this condensate material, in a variety of environments in the nebula, caused partial or complete recrystallization of the fine-grained dust.

Brearley, A. J.

1993-04-01

74

Matrix and fine-grained rims in the unequilibrated CO3 chondrite, ALHA77307 - Origins and evidence for diverse, primitive nebular dust components  

NASA Technical Reports Server (NTRS)

SEM, TEM, and electron microprobe analysis were used to investigate in detail the mineralogical and chemical characteristics of dark matrix and fine-grained rims in the unequilibrated CO3 chondrite ALHA77307. Data obtained revealed that there was a remarkable diversity of distinct mineralogical components, which can be identified using their chemical and textural characteristics. The matrix and rim components in ALHA77307 formed by disequilibrium condensation process as fine-grained amorphous dust that is represented by the abundant amorphous component in the matrix. Subsequent thermal processing of this condensate material, in a variety of environments in the nebula, caused partial or complete recrystallization of the fine-grained dust.

Brearley, Adrian J.

1993-01-01

75

Kamacite and olivine in ordinary chondrites: Intergroup and intragroup relationships  

SciTech Connect

The three principal groups of ordinary chondrites (OCs), H (high total Fe), L (low total Fe), and LL (low total Fe, low metallic Fe), constitute {approximately}80% of all meteorites observed to fall. In going from H to L to LL chondrites, the abundances of siderophile elements decrease and the degree of oxidation increases. The proportion of oxidized Fe (i.e., FeO in silicates) increases at the expense of metallic Fe. Because Fe is more readily oxidized than Ni or Co, bulk metal becomes increasingly rich in Ni and Co (e.g., Prior, 1916). Equilibrated LL chondrites are thus characterized by high FeO/(FeO + MgO) ratios in olivine and low-Ca pyroxene, high taenite/kamacite ratios, and the occurrence of Co-rich kamacite. The present study reports high-precision electron microprobe analyses of olivine and kamacite in a large suite of OCs. The purpose of the study was sixfold: (1) rigorously define the compositional ranges of these phases for each OC group, (2) identify anomalous OCs whose olivine and/or kamacite compositions lie outside the established ranges, and hence may not belong to the three main OC groups, (3) characterize the phases in the chondritic clasts of the Netschaevo iron meteorite and determine how closely related Netschaevo is to OCs, (4) determine if there are intragroup variations of olivine and kamacite compositions with petrologic type, (5) identify those OCs as fragmental breccias that contain some olivine and/or kamacite grains with aberrant compositions, and (6) search for new metallic Fe-Ni phases with extreme compositions. An expected by-product of this investigation was that a few meteorites that previously had not been well-characterized would be reclassified.

Rubin, A.E. (Univ. of California, Los Angeles (USA))

1990-05-01

76

Oxygen isotopes in R-chondrite magnetite and olivine: links between R chondrites and ordinary chondrites  

Microsoft Academic Search

Ion-microprobe studies yield ?17O (=?17O ? 0.52 · ?18O) values in magnetite from the Rumurti chondrite (RC) PCA91241 (which is paired with PCA91002) of +3.1 to +3.9‰, slightly higher than that in O from whole-rock R samples. Despite ?17O values in whole-rock RCs that are much (by ca. 1.6‰) higher than in whole-rock LL chondrites, the ?17O in R magnetite

James P Greenwood; Alan E Rubin; John T Wasson

2000-01-01

77

Fe-Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites  

USGS Publications Warehouse

We report the results of our petrological and mineralogical study of Fe-Ni metal in type 3 ordinary and CO chondrites, and the ungrouped carbonaceous chondrite Acfer 094. Fe-Ni metal in ordinary and CO chondrites occurs in chondrule interiors, on chondrule surfaces, and as isolated grains in the matrix. Isolated Ni-rich metal in chondrites of petrologic type lower than type 3.10 is enriched in Co relative to the kamacite in chondrules. However, Ni-rich metal in type 3.15-3.9 chondrites always contains less Co than does kamacite. Fe-Ni metal grains in chondrules in Semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and Ni-rich regions. Metal in other type 3 chondrites is composed of fine- to coarse-grained aggregates of kamacite and Ni-rich metal, resulting from metamorphism in the parent body. We found that the number density of Ni-rich grains in metal (number of Ni-rich grains per unit area of metal) in chondrules systematically decreases with increasing petrologic type. Thus, Fe-Ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites, and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites. Among the known ordinary and CO chondrites, Semarkona is the most primitive. The range of metamorphic temperatures were similar for type 3 ordinary and CO chondrites, despite them having different parent bodies. Most Fe-Ni metal in Acfer 094 is martensite, and it preserves primary features. The degree of metamorphism is lower in Acfer 094, a true type 3.00 chondrite, than in Semarkona, which should be reclassified as type 3.01. ?? The Meteoritical Society, 2008.

Kimura, M.; Grossman, J. N.; Weisberg, M. K.

2008-01-01

78

FeO-rich, porphyritic pyroxene chondrules in unequilibrated ordinary chondrites  

Microsoft Academic Search

A suite of FeO-rich (type II), porphyritic, olivine\\/pyroxene (POP) chondrules has been studied in detail. Data for ten chondrules from Semarkona (LL3.0) are emphasized, and one chondrule from Chainpur (LL3.4) and two from Parnallee (LL3.6) are included as further examples of certain properties. The chondrules contain phenocrysts of pyroxene and olivine in varying proportions, and have Fe(Fe + Mg) >

Rhian H. Jones

1996-01-01

79

Unusual olivine and pyroxene composition in interplanetary dust and unequilibrated ordinary chondrites  

NASA Technical Reports Server (NTRS)

The presence, in both a number of interplanetary dust particles (IDPs) and in meteorite matrices, of olivine and orthopyroxene grains, low in FeO but containing up to 5 wt pct MnO, is reported. The majority of olivines and pyroxenes in meteorites contain less than 0.5 wt pct MnO. The presence of these low-iron, manganese-enriched (LIME) olivines and pyroxenes in IDPs and meteorites may indicate a link between the origin and history of IDPs and the matrix material of primitive meteorites. The origin of the LIME silicates could be explained by condensation from a gas of solar composition. Forsterite is the first major silicate phase to condense from a solar nebula gas, and Mn, which is not stable as a metal under solar nebula conditions, would condense at about 1100 K as Mn2SiO4 in solid solution with forsterite.

Klock, W.; Mckay, D. S.; Thomas, K. L.; Palme, H.

1989-01-01

80

Compositions of three low-FeO ordinary chondrites: Indications of a common origin with the H chondrites  

SciTech Connect

Burnwell, EET 96031, and LAP 04575 are ordinary chondrites (OC) that possess lower than typical olivine Fa content than has been established for the H chondrites (< 17 mol%). Mean low-Ca pyroxene Fs contents are typically lower than mean Fa content, with generally 16 mol% Fs. We have investigated these three low-FeO chondrites by measuring their trace element abundances, oxygen isotopic compositions, and examining their three-dimensional (3D) petrography with synchrotron X-ray microtomography. We compare our results with those established for more common OC. The low FeO chondrites studied here have bulk trace element abundances that are identical to the H chondrites. From bulk oxygen isotopic analysis, we show that Burnwell, EET 96010, and LAP 04757 sampled oxygen reservoirs identical to the H chondrites. Burnwell, EET 96031, and LAP 04575 possess common 3D opaque mineral structures that could be distinct from the H chondrites, as evidenced by X-ray microtomographic analysis, but our comparison suite of H chondrites is small and unrepresentative. Overall, our data suggest a common origin for the low-FeO chondrites Burnwell, EET 96010, and LAP 04757 and the H chondrites. These three samples are simply extreme members of a redox process where a limiting nebular oxidizing agent, probably ice, reacted with material containing slightly higher amounts of metal than typically seen in the H chondrites.

Troiano, Julianne; Rumble III, Douglas; Rivers, Mark L.; Friedrich, Jon M. (CIW); (UC); (Fordham)

2011-11-17

81

A Survey of Large Silicate Objects in Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

We present the results of a survey of large silicate objects in ordinary chondrites (OCs) from the collection of the Natural History Museum, London; 390 H-group, 386 L-group and 57 LL-group meteorites were examined. A total of 61 objects were identified (Table 1). Meteorites with light and dark, brecciated fabrics were excluded from our survey. Following Weisberg et al. [1], large silicate objects are taken to be >= 5mm in size. Macrochondrules have rounded outlines and textures - porphyritic, barred olivine, radiating pyroxene - that are indistinguishable from normal chondrules in OCs [1]. In addition, we also recognise igneous clasts and chondritic clasts. The largest macrochondrule in the collection is 4cm diameter, with a microporphyritic texture [2]. Igneous clasts are those objects whose properties indicate that they originated through melting and differentiation on a planetary body. Examples include a 2cm diameter clast, in Ness County (L6), which contains large (2mm) olivine and enstatite grains set in a plagioclase + olivine groundmass, cristobalite- and tridymite-rich clasts [3] and the FELINE feldspar-nepheline clast [4]. Chondritic clasts comprise a diverse group including a 1cm clast from Barwell (L6) which contains apparently remelted chondrules, microporphyritic clasts with K-rich mesostasis e.g. in Quenggouk (H4) and a 1cm single olivine grain with minor inclusions of anorthite and enstatite, in Julesburg (L3). The K-rich objects are similar to others described from a survey of LL-chondrites and may have an impact origin or have undergone exchange with a K-rich vapor [5]. Abundances of the three types of large silicate objects (Table 1) reflect the relative numbers of H, L and LL meteorite samples in the collection, although LL-group hosted clasts are over-represented as our work concentrated on sections of LL-chondrites. In total, 46% of the objects are macrochondrules, 18% are igneous clasts and 36% are in the indeterminate chondritic clast group. All three types of object occur in petrographic types 3-6, but none was found in H3 meteorites. Mineral compositions in large silicate objects in hosts of petrographic types 4-6 are usually equilibrated, although enstatite in H4-hosted samples and the cristobalite-rich igneous clasts from Farmington (L5) [3, 6] are exceptions to this. Clasts and chondrules from the same host-meteorite sometimes belong to different shock stages. References: [1] Weisberg M. K. et al. (1988) Meteoritics, 23, 309-310. [2] Binns R. A. (1967) Mineral. Mag., 36, 319-324. [3] Bridges J. C. et al. (1995) Meteoritics, submitted. [4] Bridges J. C. et al. (1995) Proc. NIPR Symp. Antarct. Meteorites, 8, in press. [5] Wlotzka F. et al. (1983) GCA, 47, 743-757. [6] Binns R. A. (1967) Science, 156, 1222-1226.

Hutchison, R.; Bridges, J. C.

1995-09-01

82

Chromite and olivine in type II chondrules in carbonaceous and ordinary chondrites - Implications for thermal histories and group differences  

Microsoft Academic Search

Unequilibrated chromite and olivine margin compositions in type II chondrules are noted to differ systematically among three of the chondrite groups, suggesting that type II liquids differed in composition among the groups. These differences may be interpreted as indicators of different chemical compositions of the precursor solids which underwent melting, or, perhaps, as differences in the extent to which immiscible

C. A. Johnson; M. Prinz

1991-01-01

83

Interpretation of Ordinary Chondrites SNOM images: comparison with electromagnetic models  

NASA Astrophysics Data System (ADS)

To clarify the role of iron phase transformation induced in asteroid surfaces, we studied samples of Ordinary Chondrites of different types (different metallic content and shock degree) in laboratory, by means of the nanoimaging analytical technique SNOM (Scanning Near Field Optical Microscopy). In SNOM technique, a light beam is passed through an optical fiber, that ends in a tip. Tip width is smaller than light wavelength: in this way light passing through the aperture is confined by the dimension of the tip and hence a high spatial resolution (hundreds of nanometers) is obtained. Samples, whose dimensions are of micron order, is placed near the tip, in such a way that radiation emerging to the tip is forced to interact with it before diffracting out. Applying this technique, it's possible to retrieve a reflectivity image and a topographic image of the sample, having in this way evidence of npFe presence. Laboratory analysis is supported by theoretical modelling that is performed using a code that allows to obtain the electromagnetic fields distribution in 3D space, after interaction between radiation and sample, and hence a reflectivity image of the sample at a fixed wavelength. In the simulation, we consider a monochromatic plane wave addressed to a tip, 10 microns long and recovered by a thin silver layer. Sample, 700 nm far from the tip, is a 4mm2 square. In preliminary simulations, we chosen materials that are very common in Ordinary Chondrites such as the olivine (forsterite and fayalite) and pyroxenes. In the code every material is described by the following input parameters: isotropy, relative dielectric constant, magnetic permeability, electric, magnetic and thermal conductivity, opacity and density. Wavelengths considered here are 980 nm, 1050 nm, 1300 nm and 1550 nm, which correspond to maximum or minimum of olivine and pyroxenes reflectivity: in this way it's easier to have npFe evidence. The comparison between real and simulated SNOM reflectivity images will help us to interpret the data and possibly to discern the presence and amount of npFe in the sample.

Longobardo, A.; Palomba, E.; Girasole, M.; Longo, G.; Pompeo, G.; Cricenti, A.

2009-04-01

84

Silica-Fayalite-bearing Chondrules in Ordinary Chondrites: Evidence of Oxidation in the Solar Nebula  

NASA Astrophysics Data System (ADS)

Most ordinary chondrite (OC) chondrules have compositions similar to those of bulk OC in terms of lithophile-element abundances. There are only a few rare chondrule classes that deviate significantly from OC-like compositions; these include Al-rich chondrules, chromitic and chromite-bearing silicate chondrules, and silica-rich chondrules. We studied 41 thin sections of unequilibrated OC and found 82 silica-bearing chondrules that can be divided into two major categories: silica-pyroxene chondrules and silica-fayalite- pyroxene chondrules. These chondrules are more common in H (>3/cm^2) than in L and LL chondrites (<1/cm^2). Silica-pyroxene chondrules consist mainly of low-Ca pyroxene and silica and have radial and porphyritic textures. Silica-bearing radial pyroxene (RP) chondrules contain 5-10 vol% silica grains; the low-Ca pyroxene is uniform in individual chondrules but varies from one chondrule to another (Fs(sub)10.2- Fs(sub)31.5). Silica-bearing porphyritic pyroxene (PP) chondrules contain 15- 40 vol% silica; the low-Ca pyroxene varies in composition within individual PP chondrules and tends to be more magnesian than in the silica-bearing RP chondrules (Fs(sub)5.0-Fs(sub)21.1). Petrographic observations suggest that some PP chondrules were not completely molten; they appear to have cooled more slowly than the silica-bearing RP chondrules. Silica-fayalite-pyroxene chondrules consist of silica, fayalite, and low-Ca pyroxene; accessory high-Ca pyroxene, plagioclase mesostasis, troilite, and metallic Fe-Ni are also present. Based on texture and the modal abundances of pyroxene and silica these chondrules can be divided into two types: (1) radial or porphyritic silica-fayalite-pyroxene chondrules containing 5-40 vol% silica and (2) granular silica-fayalite-pyroxene chondrules consisting almost entirely (90-95 vol%) of silica. Silica-fayalite-bearing pyroxene chondrules are texturally and compositionally similar to the silica-bearing pyroxene chondrules described above; the principal difference between them is the presence of fayalite-forming veins within or rims around the silica grains. The continuum between these chondrule categories implies that they are genetically related: We infer that the fayalite veins and rims formed by nebular alteration of the silica grains. Fayalite forms veins along the silica grain boundaries in granular silica-fayalite-bearing chondrules. Fragments of granular silica chondrules occur as relict clasts within two pyroxene chondrules in Sharps. These fragments were altered after chondrule solidification. Conclusions: (1) Silica-bearing chondrules have similar textures to common mafic silicate chondrules and were formed by melting silica-rich precursor material that possibly formed by nonequilibrium condensation. (2) The higher abundance of silica-bearing chondrules in H than in L and LL chondrites may indicate a greater degree of silica condensation in the H-formation region. (3) Silica-fayalite-bearing chondrules formed by alteration of silica-bearing chondrules. The common occurrence of both categories within the same chondrite suggests that oxidation and fayalite formation by nebular gas was an inefficient process.

Krot, A. N.; Wasson, J. T.

1993-07-01

85

Chrome-spinel Inclusions in Ordinary Chondrites: Mineralogy, Chemistry and Petrogenesis  

Microsoft Academic Search

We surveyed 270 ordinary chondrites (115 H, 116 L and 39 L\\/LL and LL) for chrome-spinel (Cr-Sp) chondrules and inclusions and Cr- Sp-rich mafic silicate chondrules. Here we discuss Cr-Sp inclusions. These inclusions are most common among H (52) chondrites and 3-4X less common in L (15) and L\\/LL+LL (5) chondrites. We divide the inclusions into two types chiefly on

A. N. Krot; M. A. Ivanova; J. T. Wasson

1992-01-01

86

Occurrence of Hercynite in the Ordinary Chondrite Yamato-82133 (H3)  

NASA Astrophysics Data System (ADS)

Peculiar hercynite-kamacite aggregates were found in the matrix of the ordinary chondrite Yamato-82133(H3). These objects consist of kamacite, hercynite, and, sometimes an alumina (corundum?). It was shown that these phases crystallized simultaneously.

Zinovieva, N. G.; Mitreikina, O. B.; Granovsky, L. B.

1996-03-01

87

Oxygen Isotope Ratios of Large Cosmic Spherules: Carbonaceous and Ordinary Chondrite Parent Bodies  

NASA Astrophysics Data System (ADS)

Oxygen isotopes measurements of 33 cosmic spherules, using IR-laser fluorination/mass spectrometry, indicate that 30% of them are above the terrestrial fractionation line, i.e., are unrelated to carbonaceous chondrites but rather to ordinary and R chondrites.

Suavet, C.; Alexandre, A.; Franchi, I. A.; Gattacceca, J.; Sonzogni, C.; Folco, L.; Greenwood, R. C.; Rochette, P.

2009-03-01

88

Rhenium-osmium isotope systematics of ordinary chondrites and iron meteorites  

NASA Technical Reports Server (NTRS)

Using negative thermal ionization mass spectrometry, Re and Os abundances were determined by isotope dilution and Os-187/Os-186 measured in 11 ordinary chondrites, and also in 1 IIB and 3 IIIB irons. In addition, Os-186/Os-188 and Os-189/Os-188 ratios were precisely determined for 3 unspiked ordinary chondrites as a means of constraining the intensity of any neutron irradiation these meteorites may have experienced.

Walker, R. J.; Morgan, J. W.; Horan, M. F.; Grossman, J. N.

1993-01-01

89

Chondrules in the Sharps H3 chondrite - Evidence for intergroup compositional differences among ordinary chondrite chondrules  

NASA Technical Reports Server (NTRS)

Bulk compositions of 19 chondrules and one matrix-rich sample from H3.4 Sharps were determined by instrumental neutron activation analysis. Samples were characterized petrographically, and mineral compositions were determined by electron microprobe analysis. There is constancy among ordinary chondrite (OC) groups in the compositional interrelationships of different chondrule types; e.g., in H3 as well as L3 and LL3 chondrites, porphyritic chondrules are more refractory than nonporphyritic chondrules. Precursor components of H3 chondrules are closely related to those of LL3 chondrules. The mean Ir/Ni, Ir/Co, and Ir/Au ratios of H3 chondrules differ from the corresponding ratios of LL3 chondrules at the 99, 90, and 79 percent confidence levels, respectively. The ratios in H3 chondrules exceed those in LL3 chondrules by amounts similar to those by which H whole-rocks exceed LL whole-rocks. These data suggest that there are primary systematic differences in bulk composition between H and LL chondrules. These differences support the inference that chondrule formation occurred after major nebular fractionation events had established the observed bulk compositional differences among OC groups.

Rubin, Alan E.; Pernicka, Ernst

1989-01-01

90

Metamorphism and aqueous alteration in low petrographic type ordinary chondrites  

NASA Technical Reports Server (NTRS)

In order to investigate the relative importance of dry metamorphism and aqueous alteration in the history of chondruies, chondruies were hand-picked from the Semarkona (petrographic type 3.0), Bishunpur (3. 1), Chainpur (3.4), Dhajala (3.8) and Allegan (5) chondrites, and matrix samples were extracted from the first three ordinary chondrites. The thermoluminescence (TL) properties of all the samples were measured, and appropriate subsets of the samples were analyzed by electron-microprobe and radiochemical neutron activation and the water and H-isotopic composition determined. The TL data for chondrules from Semarkona and Bishunpur scatter widely showing no unambiguous trends, although group B1 chondrules tend to have lower sensitivities and lower peak temperatures compared with group A5 chondrules. It is argued that these data reflect the variety of processes accompanying chondrule formation. The chondrules show remarkably uniform contents of the highly labile elements, indicating mineralogical control on abundance and volatile loss from silicates and loss and recondensation of mobile chalcophiles and siderophiles in some cases. Very high D/H values (up to approx. 8000% SMOW) are observed in certain Semarkona chondrules, a confirmation of earlier work. With increasing petrographic type, mean TL sensitivities of the chondrules increase, the spread of values within an individual meteorite decreases, and peak temperatures and peak widths show trends indicating that the TL is mainly produced by feldspar and that dry, thermal metamorphism is the dominant secondary process experienced by the chondrules. The TL sensitivities of matrix samples also increase with petrographic type. Chainpur matrix samples show the same spread of peak temperatures and peak widths as Chainpur chondruies, indicating metamorphism-related changes in the feldspar are responsible for the TL of the matrix. The TL data for the Semarkona and Bishunpur matrix samples provide, at best, only weak evidence for aqueous alteration, but the matrix contains H with approximately terrestrial D/H values, even though it contains much water. Secondary processes (probably aqueous alteration) presumably lowered the D/H of the matrix and certain chondrules. While chondrule properties appear to be governed primarily by formation processes and subsequent metamorphism, the matrix of Semarkona has a more complex history involving aqueous alteration as a meteorite-wide process.

Xie, T.; Lipschutz, M. E.; Sears, D. W. G.; Guimon, R. K.; Jie, Lu; Benoit, P. H.; O'D. Alexander, C. M.; Wright, Ian; Pillinger, C.; Morse, A. D.; Hutchison, Robert

1995-01-01

91

Metal phases in ordinary chondrites: Magnetic hysteresis properties and implications for thermal history  

NASA Astrophysics Data System (ADS)

Magnetic properties are sensitive proxies to characterize FeNi metal phases in meteorites. We present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls. We show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples. Except for the most primitive chondrites, these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite (both in the cloudy zone as single-domain grains, and as larger multidomain grains in plessite and in the rim of zoned taenite) dominates the remanent magnetism, in agreement with previous microscopic magnetic observations. The bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses. We evidence a decreasing metal content with increasing petrologic type in ordinary chondrites, compatible with oxidation of metal during thermal metamorphism. Types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites. This is compatible with lower cooling rates in the 650-450 °C interval for higher petrographic types (consistent with an onion-shell model), but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism. In equilibrated chondrites, shock-related transient heating events above approximately 500 °C result in the disordering of tetrataenite and associated drastic change in magnetic properties. As a good indicator of the amount of tetrataenite, hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites, revealing low cooling rates during thermal metamorphism and high cooling rates (e.g., following shock reheating or excavation after thermal metamorphism). Our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study.

Gattacceca, J.; Suavet, C.; Rochette, P.; Weiss, B. P.; Winklhofer, M.; Uehara, M.; Friedrich, Jon M.

2014-04-01

92

Igneous rock from Severnyi Kolchim (H3) chondrite: Nebular origin  

NASA Technical Reports Server (NTRS)

The discovery of lithic fragments with compositions and textures similar to igneous differentiates in unequilibrated ordinary chondrites (UOC's) and carbonaceous chondrites (CC's) has been interpreted as to suggest that planetary bodies existed before chondrites were formed. As a consequence, chondrites (except, perhaps CI chondrites) cannot be considered primitive assemblages of unprocessed nebular matter. We report about our study of an igneous clast from the Severnyi Kolchim (H3) chondrite. The results of the study are incompatible with an igneous origin of the clast but are in favor of a nebular origin similar to that of chondrules.

Nazarov, M. A.; Brandstaetter, F.; Kurat, G.

1993-01-01

93

Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites  

NASA Technical Reports Server (NTRS)

Ion and electron microprobes were used to examine Mg-26 excesses from Al-26 decay in four Al-rich objects from the type 3 ordinary hibonite clast in the Dhajala chondrite. The initial Al-26/Al-27 ratio was actually significantly lower than Al-rich inclusions in carbonaceous chondrites. Also, no Mg-26 excesses were found in three plagioclase-bearing chondrules that were also examined. The Mg-26 excesses in the hibonite chondrites indicated a common origin for chondrites with the excesses. The implied Al-26 content in a proposed parent body could not, however, be confirmed as a widespread heat source in the early solar system.

Hinton, R. W.; Bischoff, A.

1984-01-01

94

Classification of five new ordinary chondrites (RC 073, 074, 076-078) from Roosevelt County, New Mexico  

NASA Astrophysics Data System (ADS)

Five new ordinary chondrites from Roosevelt County, New Mexico were examined by optical microscopy in transmitted and reflected light. The chondrites were moderately to heavily weathered and were assigned to a petrologic time, chemical group, and shock stage.

McCoy, T. J.; Keil, K.; Wilson, I. E.

1993-03-01

95

Physical propoerties of incompletely compacted equilibrated ordinary chondrites: Implications for asteroidal structure and impact processing  

SciTech Connect

Aside from robotic exploration, meteorites are our primary source of information about the asteroids that they sample. Although there are some discrepancies, there are dynamical, spectral, and compositional evidence for an S-type asteroid connection to the ordinary chondrite meteorites. Reconciling the measured bulk density of chondrites with that of asteroids can yield important inferences about the internal structure of asteroids. For example, the bulk density of S-type asteroids is typically much less than the bulk density of chondrites, leading to the inference that asteroids contain a significant quantity of macroporosity. We have identified several unusual ordinary chondrites that have been incompletely compacted relative to petrologically similar but much less porous chondrites. Although these are equilibrated chondrites, they have extreme amounts of pore spaces between mineral grains. Here, we detail our efforts quantifying the nature of the pore spaces in these chondrites and we examine the implications for the structure and mechanical processing of the asteroids from which these chondrites originate. Our pore size distribution data may also provide constraints for the modeling of heat flow and shock waves within primordial chondritic parent bodies.

Sasso, M.R.; Macke, R.J.; Britt, D.T.; Rivers, M.L.; Ebel, D.S.; Friedrich, J.M. (Central Florida); (AMNH); (UC); (Fordham)

2009-03-19

96

A Possible Ordinary Chondrite Affinity for Metal from the Unique Chondrite NWA 5492  

NASA Astrophysics Data System (ADS)

NWA 5492 metal is compositionally similar to that from the unique chondrite GRO 95551, and both are linked to H-chondrite siderophile-element composition despite the more extreme state of reduction in NWA 5492.

Humayun, M.; Weisberg, M. K.

2012-03-01

97

Ordinary chondrites - Multivariate statistical analysis of trace element contents  

NASA Technical Reports Server (NTRS)

The contents of mobile trace elements (Co, Au, Sb, Ga, Se, Rb, Cs, Te, Bi, Ag, In, Tl, Zn, and Cd) in Antarctic and non-Antarctic populations of H4-6 and L4-6 chondrites, were compared using standard multivariate discriminant functions borrowed from linear discriminant analysis and logistic regression. A nonstandard randomization-simulation method was developed, making it possible to carry out probability assignments on a distribution-free basis. Compositional differences were found both between the Antarctic and non-Antarctic H4-6 chondrite populations and between two L4-6 chondrite populations. It is shown that, for various types of meteorites (in particular, for the H4-6 chondrites), the Antarctic/non-Antarctic compositional difference is due to preterrestrial differences in the genesis of their parent materials.

Lipschutz, Michael E.; Samuels, Stephen M.

1991-01-01

98

Chromite and olivine in type II chondrules in carbonaceous and ordinary chondrites - Implications for thermal histories and group differences  

NASA Technical Reports Server (NTRS)

Unequilibrated chromite and olivine margin compositions in type II chondrules are noted to differ systematically among three of the chondrite groups, suggesting that type II liquids differed in composition among the groups. These differences may be interpreted as indicators of different chemical compositions of the precursor solids which underwent melting, or, perhaps, as differences in the extent to which immiscible metal sulfide droplets were lost during chondrule formation. Because zinc is detectable only in type II chromites which have undergone reequilibration, the high zinc contents reported for chondritic chromites in other studies probably reflect redistribution during thermal metamorphism.

Johnson, Craig A.; Prinz, Martin

1991-01-01

99

Bleached chondrules: Evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites  

USGS Publications Warehouse

We present the first detailed study of a population of texturally distinct chondrules previously described by Kurat (1969), Christophe Michel-Levy (1976), and Skinner et al. (1989) that are sharply depleted in alkalis and Al in their outer portions. These 'bleached' chondrules, which are exclusively radial pyroxene and cryptocrystalline in texture, have porous outer zones where mesostasis has been lost. Bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4-6. They are most abundant in the L and LL groups, apparently less common in H chondrites, and absent in enstatite chondrites. We used x-ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites. We studied bleached chondrules from Semarkona by ion microprobe for trace elements and H isotopes, and by transmission electron microscopy. Chondrule bleaching was the result of low-temperature alteration by aqueous fluids flowing through fine-grained chondrite matrix prior to thermal metamorphism. During aqueous alteration, interstitial glass dissolved and was partially replaced by phyllosilicates, troilite was altered to pentlandite, but pyroxene was completely unaffected. Calcium-rich zones formed at the inner margins of the bleached zones, either as the result of the early stages of metamorphism or because of fluid-chondrule reaction. The mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites, and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types. The ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories, but there is no relationship between the degree of alteration and metamorphic grade. A correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter. Ordinary chondrites were probably open systems after accretion, and aqueous fluids may have carried volatile elements with them during dehydration. Individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching.

Grossman, J. N.; Alexander, C. M. O'D.; Wang, J.; Brearley, A. J.

2000-01-01

100

Restriction of parent body heating by metal-troilite melting: Thermal models for the ordinary chondrites  

NASA Astrophysics Data System (ADS)

Ordinary chondrite meteorites contain silicates, Fe,Ni-metal grains, and troilite (FeS). Conjoined metal-troilite grains would be the first phase to melt during radiogenic heating in the parent body, if temperatures reached over approximately 910-960 °C (the Fe,Ni-FeS eutectic). On the basis of two-pyroxene thermometry of 13 ordinary chondrites, we argue that peak temperatures in some type 6 chondrites exceeded the Fe,Ni-FeS eutectic and thus conjoined metal-troilite grains would have begun to melt. Melting reactions consume energy, so thermal models were constructed to investigate the effect of melting on the thermal history of the H, L, and LL parent asteroids. We constrained the models by finding the proportions of conjoined metal-troilite grains in ordinary chondrites using high-resolution X-ray computed tomography. The models show that metal-troilite melting causes thermal buffering and inhibits the onset of silicate melting. Compared with models that ignore the effect of melting, our models predict longer cooling histories for the asteroids and accretion times that are earlier by 61, 124, or 113 kyr for the H, L, and LL asteroids, respectively. Because the Ni/Fe ratio of the metal and the bulk troilite/metal ratio is higher in L and LL chondrites than H chondrites, thermal buffering has the greatest effect in models for the L and LL chondrite parent bodies, and least effect for the H chondrite parent. Metal-troilite melting is also relevant to models of primitive achondrite parent bodies, particularly those that underwent only low degrees of silicate partial melting. Thermal models can predict proportions of petrologic types formed within an asteroid, but are systematically different from the statistics of meteorite collections. A sampling bias is interpreted to explain these differences.

Mare, Eleanor R.; Tomkins, Andrew G.; Godel, Belinda M.

2014-04-01

101

Hydrogen Isotope Study of CO3 Type Carbonaceous Chondrites; Comparison with Type 3 Ordinary Chondrites.  

National Technical Information Service (NTIS)

Meteorites of the Ornans type 3 carbonaceous chondrites exhibit a range in degree of equilibration, attributed to differing amounts of thermal metamorphism. These differences have been used to split the CO3 chondrites into petrologic sub-types from 3.0, l...

A. D. Morse J. Newton C. T. Pillinger

1993-01-01

102

Multiple impact events recorded in the NWA 7298 H chondrite breccia and the dynamical evolution of an ordinary chondrite asteroid  

NASA Astrophysics Data System (ADS)

The major geologic process that has shaped the asteroids and led to development of their regoliths is impact. Petrofabrics in ordinary chondrites are undoubtedly the result of impact events on their asteroidal parent bodies and the foliation present in a chondrite serves as an enduring record of the magnitude of the most intense compacting event experienced by the material. An overwhelming majority of chondrites have an internally consistent petrofabric contained within the spatial dimensions of the entire rock, including across clasts or different petrographic domains. This indicates that the magnitude of the most recent impact to have affected the assembled chondrite was significant enough to impart a foliation across all lithologies. Information of any previous impacts is largely lost because of the consistent, realigned foliations. We present X-ray microtomography derived 3D petrofabric intensity and orientation data for three lithologies in the NWA 7298 breccia. The internally inconsistent petrofabrics among differing lithologies indicate that the magnitude of the final impact event was smaller than previous ones. This latter case preserves fabric information recorded during previous impacts and allows a more complete interpretation of the impact history of a local region of the asteroidal parent. We used our data to infer the sequence and intensity of distinct impact events affecting the NWA 7298 parent asteroid. We suggest a near-surface impact debris zone on the H chondrite parent asteroid as an origin for NWA 7298. These observations yield new opportunities for investigating and interpreting the dynamic collisional evolution of asteroids.

Friedrich, Jon M.; Weisberg, Michael K.; Rivers, Mark L.

2014-05-01

103

Elemental composition of individual chondrules from ordinary chondrites.  

NASA Technical Reports Server (NTRS)

Sequential nondestructive neutron activation analysis was used to determine the bulk abundance of Fe, Al, Na, Mn, Cr, Sc, Co, and Ir in approximately 300 individual chondrules from 16 chondrites representing the H(3-5), L4, and LL(3-6) compositional and petrologic classes. The histograms indicate that the most probable abundances for lithophilic elements, except Cr, are enriched in the chondrules, while the siderophilic elements are depleted in the chondrules compared to the whole chondrite. Some of the abundance populations, such as Al and Fe, appear to be multimodal. Systematic variations in the composition of the chondrules with increasing petrologic type were observed; most consistent are an increasing Na-Al and Cr-Al correlation, a decreasing Na-Mn correlation, increasing Na abundance, and decreasing Na and Mn dispersions among chondrules.

Osborn, T. W.; Schmitt, R. A.; Smith, R. H.

1973-01-01

104

The chemistry and origin of the ordinary chondrites Implications from refractory-lithophile and siderophile elements  

NASA Astrophysics Data System (ADS)

Thirty-eight ordinary chondrites (17 H, 20 L, and 1 LL) have been analyzed for major and selected trace elements. These data indicate that the lithophile elements Mg, Ca, Al, Cr, and V normalized to Si are in higher abundance in the H than in the L chondrites. The siderophile elements Ni, Co, and Fe show very good correlation within, as well as between, the two major ordinary chondrite groups. Twenty-four of the analyses are of Antarctic finds, while ten are samples of falls. Comparing the Antarctic data with the fall data reveals no evidence that any of the elements studied here have been mobilized by terrestrial weathering processes. Within the H and L chondrite groups there is little chemical variation, indicating that the source of these samples is remarkably homogeneous. Equilibrium condensate fractionation from a nebula of CI composition can result in the observed ordinary chondrite compositions. The fractionation of metal at about 1440 K (and 0.001 atm) into high and low iron groups, followed by a gas-solid fractionation at about 1380 K with the H group losing more solids than the L, will produce the observed H and L compositions and intragroup trends.

Fulton, C. R.; Rhodes, J. M.

1984-02-01

105

Partial melting of ordinary chondrites: Lost City (H) and St. Severin (LL)  

NASA Technical Reports Server (NTRS)

Eucrites and diogenites are examples of asteroidal basalts and orthopyroxenites, respectively. As they are found intermingled in howardites, which are inferred to be regolith breccias, eucrites and diogenites are thought to be genetically related. But the details of this relationship and of their individual origins remain controversial. Work by Jurewicz et al. showed that 1170-1180 C partial melts of the (anhydrous) Murchison (CM) chondrite have major element compositions extremely similar to primitive eucrites, such as Sioux County. However, the MnO contents of these melts were about half that of Sioux County, a problem for the simple partial melting model. In addition, partial melting of Murchison could not produce diogenites, because residual pyroxenes in the Murchison experiments were too Fe- and Ca-rich and were minor phases at all but the lowest temperatures. A parent magma for diogenites needs an expanded low-calcium pyroxene field. In their partial melting study of an L6 chondrite, Kushiro and Mysen found that ordinary chondrites did have an expanded low-Ca pyroxene field over that of CV chondrites (i.e., Allende), probably because ordinary chondrites have lower Mg/Si ratios. This study expands that of both Kushiro and Mysen and Jurewicz et al. to the Lost City (H) and St. Severin (LL) chondrites at temperatures ranging from 1170 to 1325 C, at an fO2 of one log unit below the iron-wuestite buffer (IW-1).

Jurewicz, Amy J. G.; Jones, John H.; Weber, Egon T.; Mittlefehldt, David W.

1993-01-01

106

SXRF determination of trace elements in chondrule rims in the unequilibrated CO3 chondrite, ALH A77307  

NASA Technical Reports Server (NTRS)

The concentrations of Ni, Cu, Zn, Ga, Ge, and Se in five chondrule rims in the CO3 chondrite ALH A77307 (3.0) using the synchrotron x-ray fluorescence (SXRF) microprobe at Brookhaven National Laboratory were determined. The data show that the trace element chemistry of rims on different chondrules is remarkably similar, consistent with data obtained for the major elements by electron microprobe. These results support the idea that rims are not genetically related to individual chondrules, but all sampled the same reservoir of homogeneously mixed dust. Of the trace elements analyzed Zn and Ga show depletions relative to CI chondrite values, but in comparison with bulk CO chondrites all the elements are enriched by approximately 1.5 to 3.5 x CO. The high concentrations of the highly volatile elements Se and Ga and moderately volatile Zn (1.5 to 2 x CO) in rims show that matrix is the major reservoir of volatile elements in ALH A77307.

Brearley, Adrian J.; Bajt, Sasa; Sutton, Steve R.; Papike, J. J.

1993-01-01

107

The histories of ordinary chondrite parent bodies - U, Th-He age distributions  

NASA Astrophysics Data System (ADS)

Age patterns observed in meteorite groups reflect the different thermal or impact histories experienced by their parent bodies. To assess the number of ordinary chondrite (OC) parent bodies, rare-gas data in the Schultz and Kruse (1989) data base were used to calculate U, Th-He gas-retention ages. Most H- and LL-chondrite ages are high; about 81 percent are greater than 2.2 Ga. In contrast, most L-chondrite ages are low; about 69 percent are not greater than 2.2 Ga, and about 35 percent are not greater than 0.9 Ga. The latter fraction is substantially lower than the value of 44 percent given by Heymann (1967). The difference is attributed to the preferential inclusion of shocked L chondrites in early studies. Broad age peaks in the H and LL groups near 3.4 Ga probably reflect thermal loss during metamorphism, but in the H distribution there is a hint of minor outgassing "events" near 1 Ga. The L/LL chondrites have chemical properties intermediate between and unresolvable from L and LL chondrites. The high ages of most L/LL chondrites are evidence against these originating on the L parent body; the L/LL age distribution is consistent with an origin on the LL parent body or on an independent body.

Wasson, John T.; Wang, Sichao

1991-06-01

108

Redox effects in ordinary chondrites and implications for asteroid spectrophotometry  

NASA Technical Reports Server (NTRS)

The sensitivity of reflectance spectra to mean ferrous iron content and olivine and pyroxene proportion enhancements in the course of metamorphic oxidation is presently used to examine whether metamorphically-induced ranges in mineralogy, and corresponding spectral parameters, may explain the observed variations in S-asteroid rotational spectra. The predicted spectral variations within any one chondrite class are, however, insufficient to account for S-asteroid rotational spectra, and predicted spectral-range slopes have a sign opposite to the rotational measurements. Metamorphic oxidation is found unable to account for S-asteroid rotational spectra.

Mcsween, Harry Y., Jr.

1992-01-01

109

Carbon-rich aggregates in type 3 ordinary chondrites - Characterization, origins, and thermal history  

NASA Technical Reports Server (NTRS)

Carbon-rich aggregates from three type 3.4-3.6 ordinary chondrites and two chondritic clasts have been characterized in detail, using TEM techniques. The aggregates in all the meteorites studied range in size from 5-1000 microns and consist of a fine scale intergrowth of poorly graphitized carbon, amorphous carbon, Fe,Ni metal, and minor chromite. Contrary to previous reports, well-crystallized graphite and magnetite are absent. The association of Fe,Ni metal and carbonaceous material suggests that the original carbonaceous material may have formed by Fischer-Tropsch (FT) type reactions at low temperatures (less than 400 K), possibly in the solar nebula. This carbonaceous material probably consisted of a complex mixture of hydrocarbons, kerogen-like material, and other complex organic molecules. The aggregates were subsequently accreted onto the ordinary chondrite parent bodies and underwent planetary thermal processing which resulted in the catalytic graphitization of hydrocarbons, in the presence of Fe,Ni metal, to produce poorly graphitized carbon. None of the meteorites studied experienced temperatures sufficiently high to produce crystalline, ordered graphite. Using the empirical geothermometer of Rietmeijer and Mackinnon (1985), the measured d(002) spacings of poorly graphitized carbon show that graphitization occurred at temperatures between 300 and 450 C. This range of temperatures is significantly lower than the generally quoted metamorphic temperatures for type 3.4-3.6 ordinary chondrites (about 450-500 C).

Brearley, Adrian J.

1990-01-01

110

Non-Destructive Classification Approaches for Equilbrated Ordinary Chondrites  

NASA Technical Reports Server (NTRS)

Classification of meteorites is most effectively carried out by petrographic and mineralogic studies of thin sections, but a rapid and accurate classification technique for the many samples collected in dense collection areas (hot and cold deserts) is of great interest. Oil immersion techniques have been used to classify a large proportion of the US Antarctic meteorite collections since the mid-1980s [1]. This approach has allowed rapid characterization of thousands of samples over time, but nonetheless utilizes a piece of the sample that has been ground to grains or a powder. In order to compare a few non-destructive techniques with the standard approaches, we have characterized a group of chondrites from the Larkman Nunatak region using magnetic susceptibility and Moessbauer spectroscopy.

Righter, K.; Harrington, R.; Schroeder, C.; Morris, R. V.

2013-01-01

111

Ordinary chondrite-related giant (>800 ?m) cosmic spherules from the Transantarctic Mountains, Antarctica  

NASA Astrophysics Data System (ADS)

In order to identify the parent bodies of cosmic spherules (melted micrometeorites) with porphyritic olivine (PO) and cryptocrystalline (CC) textures, we measured the oxygen isotopic composition of 15 giant (>800 ?m) cosmic spherules recovered in the Transantarctic Mountains, Antarctica, with IR-laser fluorination/mass spectrometry, and we conducted a characterization of their petrographic and magnetic properties. Samples include 6, 8 and 1 spherules of PO, CC and barred olivine (BO) textural types, respectively. Eleven spherules (˜70% of the total: 4/6 PO and 6/8 CC, and the BO spherule) are related to ordinary chondrites based on oxygen isotopic compositions. Olivines in ordinary chondrite-related spherules have compositions Fa 8.5-11.8, they are Ni-poor to Ni-rich (0.04-1.12 wt.%), and tend to be richer in CaO than other spherules (0.10-0.17 wt.%). Ordinary-chondrite related spherules also have high magnetite contents (˜2-12 wt.%). One PO and one CC spherules are related to previously identified 17O-enriched cosmic spherules for which the parent body is unknown. One CC spherule has an oxygen isotopic signature relating it to CM/CR carbonaceous chondrites. The majority of PO/CC cosmic spherules derive from ordinary chondrites; this result exemplifies how the texture of cosmic spherules is not only controlled by atmospheric entry heating conditions but also depends on the parent body, whether be it through orbital parameters (entry angle and velocity), or chemistry, mineralogy, or grain size of the precursor.

Suavet, Clément; Cordier, Carole; Rochette, Pierre; Folco, Luigi; Gattacceca, Jérôme; Sonzogni, Corinne; Damphoffer, Dorian

2011-10-01

112

Evidence from the Semarkona ordinary chondrite for Al-26 heating of small planets  

NASA Technical Reports Server (NTRS)

The first observation of radiogenic Mg-26 in nonrefractory meteoritic material, a plagioclase-bearing, olivine-pyroxene clast chondrule in the Semarkona ordinary chondrite, is reported. The inferred initial abundance of Al-26 is sufficient to produce incipient melting in well-insulated bodies of chondritic composition. It is concluded that planetary accretion and diffentiation must have begun on a timescale comparable to the half-life of Al-26 and that, even if widespread melting did not occur, Al-26 heating played a significant role in thermal metamorphism on small planets.

Hutcheon, I. D.; Hutchison, R.

1989-01-01

113

Phosphate minerals in LL chondrites: A record of the action of fluids during metamorphism on ordinary chondrite parent bodies  

NASA Astrophysics Data System (ADS)

Ordinary chondrites contain two phosphate minerals, merrillite and chlorapatite, both of which are secondary minerals that developed in response to metamorphism on the chondrite parent bodies. We have studied the phosphate mineralogy of four LL chondrites, of petrologic types 3.9-6, in order to determine the petrogenesis of the two minerals and interpret the conditions under which they formed. Characterization of merrillite and apatite includes textural observations, mineral compositions determined by electron probe microanalysis, and ion microprobe analyses of trace element and volatile anion elemental abundances. Initial formation of phosphate minerals during mild metamorphism, to petrologic type 4 conditions, resulted in oxidation of P that was originally incorporated in metal, and growth of merrillite as inclusions within metal grains. Subsequent development of both phosphate minerals occurred in response to diffusional equilibration, possible precipitation from fluids as well as replacement reactions resulting from interactions with fluids. Porosity and vein-filling textures in both merrillite and chlorapatite, as well as textures indicating replacement of merrillite by chlorapatite, support a model in which fluid played a significant role and suggest an interface-coupled dissolution-reprecipitation mechanism during metasomatism. Some associations of phosphate minerals with chromite-plagioclase assemblages suggest that phosphate minerals could also be related to impact processes, either as precipitation from an impact melt or as a result of interactions with a fluid or vapor derived from an impact melt. Fluid compositions may have been water-bearing initially, at low temperatures of metamorphism, but later evolved to become halogen-rich and very dry. Late-stage halogen-rich fluids that dominated during cooling of LL chondrite material may have been derived from vaporization of partial melts in the interior of the parent body. Overall, the LL chondrite parent body underwent a complex chemical evolution, in which metasomatism played a significant role.

Jones, Rhian H.; McCubbin, Francis M.; Dreeland, Linda; Guan, Yunbin; Burger, Paul V.; Shearer, Charles K.

2014-05-01

114

Variations of the isotopic composition of sulfur in enstatite and ordinary chondrites  

NASA Technical Reports Server (NTRS)

High-precision sulfur isotopic analyses (delta S-33, delta S-34, and delta S-36) of bulk ordinary and enstatite chondrites demonstrate that systematic variations exist. The average delta S-34 values are -0.26 +/- 0.07, -0.02 +/- 0.06, and 0.49 +/- 0.16 percent for enstatite and ordinary and carbonaceous chondrites, respectively. Isotopic variations of different sample specimens of primitive meteorites, e.g., Qingzhen and Abee, were observed which may be attributed to heterogeneity in the early solar nebula. Sulfur isotopic fractionations in both bulk samples and mineral separates are mass-dependent, and no nuclear isotopic anomalies were detected. The sulfur isotopic compositions of both mineral and density separates were measured. The sulfur isotopic compositions of separated chondrules from Chainpur and Bjurbole are reported. Significant isotopic difference for the chondrules from the bulk meteorite are noted for both meteorites.

Gao, Xia; Thiemens, Mark H.

1993-01-01

115

Experimental Space Weathering of Ordinary Chondrites by Nanopulse Laser: TEM Results  

NASA Technical Reports Server (NTRS)

A set of ordinary chondrite meteorites has been subjected to artificial space weathering by nanopulse laser to simulate the effects of micrometeorite bombardment. Three meteorites, an H (Ehole), L (Chateau Renard - CR), and LL (Appley Bridge - AB) were lasered following the method of Sasaki et al [1]. Near IR spectra were taken before and after exposure to examine the optical changes induced and the samples were examined by scanning and transmission electron microscopy (SEM and TEM) to understand the physical changes.

Noble, S. K.; Hiroi, T.; Keller, L. P.; Pieters, C. M.

2011-01-01

116

Chemical, isotopic and mineralogical evidence for the origin of matrix in ordinary chondrites  

NASA Astrophysics Data System (ADS)

Results are presented of a combined chemical, isotopic, and mineralogical study of fine-grained opaque matrix material from a type-3 ordinary chondrite Allan Hills A77299. It was found that the matrix of this chondrite (in contrast to chondrules) shows no siderophile element depletions and is remarkably unfractionated relative to CI chondrites. It was also found that the matrix lump has a unique oxygen isotopic composition which lies below the terrestrial fractionation line and differs markedly from the composition of fine-grained matrix lumps and chondrule rims from Semarkona chondrules (Grossman and Wasson, 1987; Grossman et al., 1987); the mineralogy and textures of the matrix lump (which consists of rare angular grains of pyroxene and olivine embedded within a ground-mass of rounded fine-grained olivines), as well as its major element composition, are typical of matrix material of other type-3 chondrites studied. It is concluded that the matrix lump in these chondrites consists largely of material that formed by annealing of amorphous presolar dust or nebular condensates.

Brearley, A. J.; Scott, E. R. D.; Keil, K.; Clayton, R. N.; Mayeda, T. K.; Boynton, W. V.; Hill, D. H.

1989-08-01

117

Pressure demagnetization of ordinary chondrites up to 1.8 GPa: new experimental data  

NASA Astrophysics Data System (ADS)

We conducted hydrostatic pressure demagnetization experiments up to 1.8 GPa on H, L, and LL ordinary chondrite samples (FeNi-bearing). We used a new non-magnetic pressure clamp cell made of "Russian" alloy (NiCrAl) together with a liquid pressure transmitting medium (polyethylsiloxane) to ensure purely hydrostatic pressure. This technique allowed to measure magnetic remanence of investigated samples directly under pressure as well as after pressure release. Pressure was always applied in low magnetic field (<5?T). The experiments revealed that under hydrostatic pressure up to 1.8 GPa, ordinary chondrite samples lost up to 51% of their initial saturation isothermal remanent magnetization. Pressure demagnetization degree is directly proportional to ln(Bcr), where Bcr is the coercivity of remanence, similar to what is observed for other ferrimagnetic minerals (Bezaeva et al., submitted). For samples with Bcr>80 mT, no pressure demagnetization effect is observed under 1.8 GPa. Using this new experimental data, we propose a simple model to predict the amount of pressure demagnetization of ordinary chondrites as a function of pressure (up to 1.8 GPa). The physical mechanism of the pressure demagnetization effect and the implications of this new experimental data for extraterrestrial paleomagnetism will be discussed. Acknowledgements This work was partially supported by the CNRS-RFFI PICS program (grant 07-05-92165). The conference attendance by N.S. Bezaeva is funded by Rosnauka (president grant MK-3252.2009.5).

Bezaeva, N. S.; Gattacceca, J.; Rochette, P.; Sadykov, R. A.

2009-12-01

118

A unique high Mn/Fe microgabbro in the Parnallee (LL3) ordinary chondrite - Nebular mixture or planetary differentiate from a previously unrecognized planetary body?  

NASA Technical Reports Server (NTRS)

We studied a unique microgabbro fragment from the Parnallee (LL3) unequilibrated ordinary chondrite. The fragment, which was originally identified by its ophitic to sub-ophitic texture, exhibits features characteristic of lunar and terrestrial tholeiitic basalts (i.e., extreme compositional zoning in clinopyroxene (Wo10En65Fs25 to Wo15En2FS83), a multiply saturated major element composition similar to mid-ocean ridge basalt with 3.1 wt pct Na2O and 0.15 wt pct K2O, and uniformly enriched rare earth elements (c. 10 x C1). A high bulk MnO/FeO ratio (0.064) distinguishes the microgabbro from other basaltic rocks and suggests the precursor material formed or reached equilibrium in a reducing environment. However, the absence of Fe metal and the extreme enrichment of FeO (up to 40 wt pct), in late crystallizing pyroxferroite, requires the last crystallization event to have occurred in a relatively oxidizing environment. We suggest the microgabbro formed by partial melting in a planetary body after removal of metallic Fe.

Kennedy, A. K.; Hutchison, R.; Hutcheon, I. D.; Agrell, S. O.

1992-01-01

119

Raman spectroscopic study of four Spanish shocked ordinary chondrites: Cañellas, Olmedilla de Alarcón, Reliegos and Olivenza.  

PubMed

Shock metamorphism in chondritic parent bodies produces typical textures, visible under the microscope, which are a consequence of structural deformation of the crystals. Such deformations can be studied with Raman spectroscopy. The vibrational characteristics of olivines and pyroxenes, structurally deformed by weak-to-moderate shock metamorphism, have been determined on four Spanish ordinary chondrites (Cañellas, Olmedilla de Alarcón, Reliegos and Olivenza). Such deformations would affect, in principle, the band positions and widths of the Raman spectra peaks. The measured band positions and relative intensities are consistent with chemical composition for olivines and pyroxenes, but show little influence on the degree of shock. However, the full spectral band width of the silicate internal modes shows some dependence on the impact grade, which could be attributed to inhomogeneous effects produced by the impacts. PMID:20529952

Rull, F; Muñoz-Espadas, M J; Lunar, R; Martínez-Frías, J

2010-07-13

120

Itokawa dust particles: a direct link between S-type asteroids and ordinary chondrites.  

PubMed

The Hayabusa spacecraft successfully recovered dust particles from the surface of near-Earth asteroid 25143 Itokawa. Synchrotron-radiation x-ray diffraction and transmission and scanning electron microscope analyses indicate that the mineralogy and mineral chemistry of the Itokawa dust particles are identical to those of thermally metamorphosed LL chondrites, consistent with spectroscopic observations made from Earth and by the Hayabusa spacecraft. Our results directly demonstrate that ordinary chondrites, the most abundant meteorites found on Earth, come from S-type asteroids. Mineral chemistry indicates that the majority of regolith surface particles suffered long-term thermal annealing and subsequent impact shock, suggesting that Itokawa is an asteroid made of reassembled pieces of the interior portions of a once larger asteroid. PMID:21868667

Nakamura, Tomoki; Noguchi, Takaaki; Tanaka, Masahiko; Zolensky, Michael E; Kimura, Makoto; Tsuchiyama, Akira; Nakato, Aiko; Ogami, Toshihiro; Ishida, Hatsumi; Uesugi, Masayuki; Yada, Toru; Shirai, Kei; Fujimura, Akio; Okazaki, Ryuji; Sandford, Scott A; Ishibashi, Yukihiro; Abe, Masanao; Okada, Tatsuaki; Ueno, Munetaka; Mukai, Toshifumi; Yoshikawa, Makoto; Kawaguchi, Junichiro

2011-08-26

121

Distribution of rare earth elements and uranium in various components of ordinary chondrites  

NASA Astrophysics Data System (ADS)

Rare earth elements (REE) and uranium were studied for their distributions in various component phases of four ordinary chondrites. Kesen (H4), Richardton (H5), Bruderheim (L6), and Saint Severin (LL6). A selective dissolution method was applied for the phase fractionation. The REE were analysed by neutron activation analysis, and U was determined by neutron-induced fission tracks. The present study revealed that both REE and U are highly enriched in the Ca-phosphate minerals with different enrichment factors, implying chemical fractionation between them. The phosphates seem to be responsible for more than 80 percent of the light REE in all chondrites. On the other hand, only 20-40 percent of the total U resides in the Ca-phosphates. This difference in enrichments might have been caused through the levels of metamorphic activity on the meteoritic parent bodies.

Ebihara, M.; Honda, M.

1984-06-01

122

Postshock Annealing and Postannealing Shock in Equilibrated Ordinary Chondrites: Implications for the Thermal and Shock Histories of Chondritic Asteroids  

NASA Technical Reports Server (NTRS)

In addition to shock effects in olivine, plagioclase, orthopyroxene and Ca-pyroxene, petrographic shock indicators in equilibrated ordinary chondrites (OC) include chromite veinlets, chromite-plagioclase assemblages, polycrystalline troilite, metallic Cu, irregularly shaped troilite grains within metallic Fe-Ni, rapidly solidified metal-sulfide intergrowths, martensite and various types of plessite, metal-sulfide veins, large metal and/or sulfide nodules, silicate melt veins, silicate darkening, low-Ca clinopyroxene, silicate melt pockets, and large regions of silicate melt. The presence of some of these indicators in every petrologic type-4 to -6 ordinary chondrite (OC) demonstrates that collisional events caused all equilibrated OC to reach shock stages S3-S6. Those type-4 to -6 OC that are classified as shock-stage S1 (on the basis of sharp optical extinction in olivine) underwent postshock annealing due to burial beneath materials heated by the impact event. Those type-4 to -6 OC that are classified S2 (on the basis of undulose extinction and lack of planar fractures in olivine) were shocked to stage S3-S6, annealed to stage S1 and then shocked again to stage S2. Some OC were probably shocked to stage 253 after annealing. It seems likely that many OC experienced multiple episodes of shock and annealing. Because 40Ar-39Ar chronological data indicate that MIL 99301 (LL6, Sl) was annealed approximately 4.26 Ga ago, presumably as a consequence of a major impact, it seems reasonable to suggest that other equilibrated S1 and S2 OC (which contain relict shock features) were also annealed by impacts. Because some type-6 S1 OC (e.g., Guarena, Kernouve, Portales Valley, all of which contain relict shock features) were annealed 4.44-4.45 Ga ago (during a period when impacts were prevalent and most OC were thermally metamorphosed), it follows that impact-induced annealing could have contributed significantly to OC thermal metamorphism.

Rubin, Alan E.

2006-01-01

123

The Cali Meteorite: Luminescence of a recently fallen H/L ordinary chondrite  

SciTech Connect

The Cali meteorite fall occurred on 6 July 2007 at 21h33m+-1m UTC. Some specimens were recovered just after their fall so they are extremely fresh materials. Mineral analysis and bulk chemistry revealed that the measured abundances for most elements closely match the values recorded for other ordinary chondrites classified as H/L. We present here thermoluminescence studies of this recently fallen meteorite in order to get additional information on the radiation environment, and the thermal history of this meteorite. Such information is revealed to be complementary with the range of orbital elements deduced from eyewitness reports of the fireball.

Trigo-Rodriguez, J. M. [Institute of Space Sciences (CSIC-IEEC). Campus UAB, Facultat de Ciencies, Torre C5-p2. 08193 Bellaterra (Spain); Llorca, J. [Institut de Tecniques Energetiques. Universitat Politecnica de Catalunya, Diagonal 647, ed. ETSEIB. 08028 Barcelona (Spain); Sears, D. W. G. [Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, Arkansas 72701 (United States)

2009-08-17

124

57Fe Mössbauer study of the Nurina-003 ordinary chondrite meteorite  

NASA Astrophysics Data System (ADS)

Nurina-003 is a Group LL5/br Ordinary Chondrite meteorite which was recovered in the Nullarbor Desert (Australia) in 1986. The degree of weathering is classified as moderate to severe. We have characterised the Fe-bearing phases in Nurina-003 by 57Fe Mössbauer Spectroscopy over the temperature range 5-295 K. The spectra are dominated by Olivine and also contain Pyroxene, Troilite and a Ferric component that is (super)paramagnetic at 295 K. We clearly see the effects of the magnetic ordering of the Olivine phase in the 5 K spectrum.

Cadogan, J. M.; Devlin, E. J.

2014-04-01

125

Mössbauer studies of Soltmany and Shisr 176 meteorites - comparison with other ordinary chondrites  

NASA Astrophysics Data System (ADS)

Two ordinary chondrites type L6: Soltmany and Shisr 176, were recently investigated in our laboratory. The distribution of iron in the 4 main iron bearing mineral phases (olivine, pyroxene, kamacite and troilite) determined by Mössbauer spectroscopy in Soltmany and Shisr 176 meteorites was very different from that in the Baszkowka meteorite (type L5). To explain the cause of this difference, a comparison of the distribution of iron among the main mineral phases present in other type L and H meteorites was performed. Our studies have shown that the Baszkowka meteorite is not a good point of reference for Soltmany and Shisr 176 meteorites.

Ga?a¸zka-Friedman, J.; Szlachta, K.; Karwowski, ?.; Wo?niak, M.

2014-04-01

126

Mössbauer studies of Soltmany and Shisr 176 meteorites - comparison with other ordinary chondrites  

NASA Astrophysics Data System (ADS)

Two ordinary chondrites type L6: Soltmany and Shisr 176, were recently investigated in our laboratory. The distribution of iron in the 4 main iron bearing mineral phases (olivine, pyroxene, kamacite and troilite) determined by Mössbauer spectroscopy in Soltmany and Shisr 176 meteorites was very different from that in the Baszkowka meteorite (type L5). To explain the cause of this difference, a comparison of the distribution of iron among the main mineral phases present in other type L and H meteorites was performed. Our studies have shown that the Baszkowka meteorite is not a good point of reference for Soltmany and Shisr 176 meteorites.

Ga?ažka-Friedman, J.; Szlachta, K.; Karwowski, ?.; Wo?niak, M.

2014-01-01

127

Origin of SiO 2-rich components in ordinary chondrites  

NASA Astrophysics Data System (ADS)

Silica-rich objects are common minor components in ordinary chondrites (OC), occurring as fragments and as chondrules. Their typical paragenesis is orthopyroxene + SiO 2 (with bulk SiO 2 >65 wt%) and occasionally with additional olivine and/or spinel. Individual silica-rich components (SRC) have previously been studied in various types of OCs, although there is only one comprehensive study of these objects by Brigham et al. [Brigham, C.A., Murrell, M.T., Yabuki, H., Ouyang, Z., El Goresy, A., 1986. Silica-bearing chondrules and clasts in ordinary chondrites. Geochim. Cosmochim. Acta 50, 1655-1666]. Several different explanations of how SRCs formed have been published. The main question is how silica-enrichment was achieved, because CI-chondritic atomic Mg/Si-ratio is 1.07 and as a consequence only olivine and pyroxene, but no free silica should be stable. There are two basic possibilities for the SiO 2-enrichment: (1) a RedOx-mechanism or magmatic fractionation on the parent body and (2) fractional condensation or recycling of chondrule mesostasis in the solar nebula. To better constrain the origin of these objects, we measured major and rare earth elements in SRCs of various types of ordinary chondrites, and in addition, we studied silica polymorphism in these objects using an in situ micro-Raman technique. Bulk chondrule compositions define mixing lines between the compositions of olivine and pyroxene. The SRCs extend these lines to an SiO 2 end member. In contrast, magmatic trends grossly deviate from these mixing lines. Concentrations of CaO, Al 2O 3, and REE in the pyroxenes of the SRCs are low (0.01 to 1× CI) and the CI-normalized REE-patterns are virtually flat, typical of bulk chondrules, but untypical of magmatic trends. We therefore conclude that SiO 2-rich objects are not of magmatic origin. They are the result of fractional condensation in the solar nebula. The silica in SRCs occurs mainly as tridymite and sometimes as cristobalite or—in very rare cases—as quartz. Some SiO 2-phases yielded a yet unknown micro-Raman spectrum, which we were unable to identify. The often chondrule-like shape of SRCs as well as the presence of high-temperature SiO 2-polymorphs lead to the following model for the origin of SRCs: formation of SiO 2-rich precursors in the solar nebula by fractional condensation, reheating to temperatures between 1140 and >1968 K, thereby forming the SRCs,—probably during the chondrule-forming process—followed by rapid cooling.

Hezel, Dominik C.; Palme, Herbert; Nasdala, Lutz; Brenker, Frank E.

2006-03-01

128

An analytical electron microscopy (AEM) investigation of opaque inclusions in some type 6 ordinary chondrites  

NASA Astrophysics Data System (ADS)

A large number of ordinary chondrites contains micron-sized particles of metal and/or troilite dispersed in their silicate grains. Such metallic phases are responsible for the so-called darkening of the silicate grains and might be either precipitates, which formed during reduction of the silicates, or inclusions injected as a melt during a shock event. We have investigated these tiny foreign phases by analytical transmission electron microscopy in three unweathered, metamorphosed ordinary chondrites (Saint Séverin, LL6, Tsarev, L6 and Kernouvé, H6). We also looked for remnant shock indices. Our TEM observations suggest the following sequence of events in the three meteorites. First, a number of relatively strong shock events occurred on the parent body/bodies producing an Fe-FeS melt that was injected into silicate grains along a dense network of open fractures. Most of these shock defects were subsequently erased by high-temperature (700-900 °C) thermal metamorphism. Some remnants of the shock events are the observed trails of tiny metal and/or sulfide inclusions that formed as a result of fracture healing. Chemical homogenization of the silicates and limited oxidation of the metallic blebs also occurred during this high-temperature annealing event, resulting in Ni-rich inclusions. This effect was especially pronounced in the L and LL-chondrites studied. During subsequent cooling of the body/bodies, inclusions of chromite and phosphate precipitated, nucleating preferentially on lattice defects (dislocations, subgrain boundaries) and on the metal and sulfide inclusions. A later shock event of moderate intensity, probably corresponding to the separation of the meteorite from its parent body, produced new shock features in the silicate grains of the Saint Séverin meteorite, including mechanical twins in diopside and straight free screw dislocations in olivine.

Leroux, Hugues; Doukhan, Jean-Claude; Guyot, François

1996-11-01

129

First occurrence of pyrophanite (MnTiO3) and baddeleyite (ZrO2) in an ordinary chondrite  

NASA Technical Reports Server (NTRS)

An examination is conducted of the distinct optical and compositional zoning exhibited by a Mg-Al-chromite fragment in the Raguli H3.8 ordinary chondrite. Two end-member pyrophanite grains occur in the fragment's outer portion; this is noted to be the first occurrence of pyrophanite in conjunction with baddeleyite in an ordinary chondrite. Two alternative models are presented for the formation of the fragment: a multistage nebular melting process, and the fragment's formation on a metamorphosed parent body. The latter alternative is favored.

Krot, Aleksandr N.; Rubin, Alan E.; Kononkova, Natal'ya N.

1993-01-01

130

The Iodine-Xenon System in Outer and Inner Portions of Chondrules from the Unnamed Antarctic LL3 Chondrite  

NASA Technical Reports Server (NTRS)

Alteration processes may affect I-Xe system in unequilibrated ordinary chondrites. It was shown that at the edges, where a contribution is made by matrix material around the rim, *Xe-129/Xe-128 values are generally lower (later apparent ages) than in the main chondrule mass. In this work we attempted to investigate whether thermal metamorphism can affect the I-Xe system in LL3 chondrites which did not experienced aqueous alteration.

Meshik, A. P.; Pravdivtseva, O. V.; Hohenberg, C. M.; Amelin, Y.

2004-01-01

131

The Danebury Iron Age meteorite—An H5 ordinary chondrite "find" from Hampshire, England  

NASA Astrophysics Data System (ADS)

What remains of a 30 g sample, first recognized as a meteorite in 1989 during characterization of metalworking debris from Danebury, an Iron Age hillfort, in Hampshire, England, has been classified as an H5 ordinary chondrite. Its arrival on Earth has been dated as 2350 ± 120 yr BP, making it contemporary with the period of maximum human activity at the recovery site. Despite its considerable terrestrial residence age, the interior of the specimen is remarkably fresh with a weathering index of W1/2. There is, however, no evidence of human intervention in its preservation. Its near-pristine state is explained in terms of its serendipitous burial during the back-fill of a pit dug into chalk by prehistoric people for the storage of grain. This chance discovery has interesting ramifications for the survival of meteorites in areas having a high pH because of a natural lime content arising as a result of the local geology.

Pillinger, C. T.; Pillinger, J. M.; Johnson, D.; Greenwood, R. C.; Tindle, A. G.; Jull, A. J. T.; Allen, D. H.; Cunliffe, B.

2014-06-01

132

The Danebury Iron Age meteorite—An H5 ordinary chondrite "find" from Hampshire, England  

NASA Astrophysics Data System (ADS)

What remains of a 30 g sample, first recognized as a meteorite in 1989 during characterization of metalworking debris from Danebury, an Iron Age hillfort, in Hampshire, England, has been classified as an H5 ordinary chondrite. Its arrival on Earth has been dated as 2350 ± 120 yr BP, making it contemporary with the period of maximum human activity at the recovery site. Despite its considerable terrestrial residence age, the interior of the specimen is remarkably fresh with a weathering index of W1/2. There is, however, no evidence of human intervention in its preservation. Its near-pristine state is explained in terms of its serendipitous burial during the back-fill of a pit dug into chalk by prehistoric people for the storage of grain. This chance discovery has interesting ramifications for the survival of meteorites in areas having a high pH because of a natural lime content arising as a result of the local geology.

Pillinger, C. T.; Pillinger, J. M.; Johnson, D.; Greenwood, R. C.; Tindle, A. G.; Jull, A. J. T.; Allen, D. H.; Cunliffe, B.

2014-05-01

133

Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids  

NASA Astrophysics Data System (ADS)

We use a combination of 2D and 3D petrographic examination and 40Ar-39Ar analyses to examine the impact histories of a suite of seven ordinary chondrites (Baszkówka, Miller, NWA 2380, Mount Tazerzait, Sahara 98034, Tjerebon, and MIL 99301) that partially preserve their ancient, but postaccretionary, porosity ranging from 10 to 20%. We examine whether materials that seem to be only mildly processed (as their large intergranular pore spaces suggest) may have more complex shock histories. The ages determined for most of the seven OCs studied here indicate closure of the 40Ar-39Ar system after primary accretion, but during (Baszkówka) or shortly after (others) thermal metamorphism, with little subsequent heating. Exceptions include Sahara 98034 and MIL 99301, which were heated to some degree at later stages, but retain some evidence for the timing of thermal metamorphism in the 40Ar-39Ar system. Although each of these chondrites has olivine grains with sharp optical extinction (signaling an apparent shock stage of S1), normally indicative of an extremely mild impact history, all of the samples contain relict shock indicators. Given the high porosity and relatively low degree of compaction coupled with signs of shock and thermal annealing, it seems plausible that impacts into materials that were already hot may have produced the relict shock indicators. Initial heating could have resulted from prior collisions, the decay of 26Al, or both processes.

Friedrich, Jon M.; Rubin, Alan E.; Beard, Sky P.; Swindle, Timothy D.; Isachsen, Clark E.; Rivers, Mark L.; Macke, Robert J.

2014-07-01

134

Igneous inclusions from ordinary chondrites: High temperature cumulates and a shock melt  

NASA Technical Reports Server (NTRS)

We report microprobe, instrumental neutron activation analysis, and radiochemical neutron activation analysis data for three large igneous inclusions in the Yamato (Y-)75097, Y-793241, and Y-794046 ordinary chondrites. The inclusions in the first two chondrites are troctolitic cumulates that have undergone appreciable reactions with their hosts either during emplacement and/or cooling. Olivine-spinel Fe-Mg exchange pairs in these two inclusions record equilibration temperatures of about 710 C, and these temperatures are similar to those exhibited by mineral pairs in the Y-75097 and Y-793241 hosts. The inclusion in Y-794046 is texturally unique, consisting of fine-grained, randomly distributed olivines, coarse (approximately 2 mm) fascicular pyroxene laths, and angular pockets of maskelynite/plagioclase feldspar. The phase compositions are readily interpreted as having resulted from extremely rapid, essentially isochemical cooling to temperatures less than 1000 C of a melt with an initial temperature greater than 1670 C. We suggest that this igneous inclusion formed in-situ by shock.

Sack, Richard O.; Ghiorso, Mark, S.; Wang, Ming-Sheng; Lipschutz, Michael E.

1994-01-01

135

Contemporaneous Formation of Chondrules in the Al-26-MG-26 System for Ordinary and CO Chondrites  

NASA Technical Reports Server (NTRS)

Chronometer using the short-lived extinct-nuclide (26)Al has been applied to chondrules in order to obtain of their formation ages. Previous studies were mostly performed on Al-rich chondrules, which constitute only 1% of all chondrules, because of their high Al/Mg ratios. Recently, (26)Al ages of major ferromagnesian chondrules in least equilibrated ordinary chondrites (OC) have been obtained. However, (26)Al ages of ferromagnesian chondrules in least equilibrated carbonaceous chondrites (CC) are very limited. Particularly, age data of FeO-poor (Type I) chondrules in CC have been scarcely obtained, because of their fine textures and lack of phases with high Al/Mg (>100) ratios. In order to clarify the origin and formation processes of chondrules, we started systematic investigations on Type I chondrules in the most pristine CC (CO3.0 Yamato-81020), by examining textures, bulk chemical compositions, (26)Al ages and oxygen isotopic compositions. We find Type I chondrules in CC formed contemporaneously with ferromagnesian chondrules in OC.

Kurahashi, E.; Kita, N. T.; Nagahara, H.; Morishita, Y.

2004-01-01

136

Ancient porosity preserved in ordinary chondrites: Examining shock and compaction on young asteroids  

NASA Astrophysics Data System (ADS)

We use a combination of 2D and 3D petrographic examination and 40Ar-39Ar analyses to examine the impact histories of a suite of seven ordinary chondrites (Baszkówka, Miller, NWA 2380, Mount Tazerzait, Sahara 98034, Tjerebon, and MIL 99301) that partially preserve their ancient, but postaccretionary, porosity ranging from 10 to 20%. We examine whether materials that seem to be only mildly processed (as their large intergranular pore spaces suggest) may have more complex shock histories. The ages determined for most of the seven OCs studied here indicate closure of the 40Ar-39Ar system after primary accretion, but during (Baszkówka) or shortly after (others) thermal metamorphism, with little subsequent heating. Exceptions include Sahara 98034 and MIL 99301, which were heated to some degree at later stages, but retain some evidence for the timing of thermal metamorphism in the 40Ar-39Ar system. Although each of these chondrites has olivine grains with sharp optical extinction (signaling an apparent shock stage of S1), normally indicative of an extremely mild impact history, all of the samples contain relict shock indicators. Given the high porosity and relatively low degree of compaction coupled with signs of shock and thermal annealing, it seems plausible that impacts into materials that were already hot may have produced the relict shock indicators. Initial heating could have resulted from prior collisions, the decay of 26Al, or both processes.

Friedrich, Jon M.; Rubin, Alan E.; Beard, Sky P.; Swindle, Timothy D.; Isachsen, Clark E.; Rivers, Mark L.; Macke, Robert J.

2014-06-01

137

New Evidence for 26Al in CAI and Chondrules from Type 3 Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

We have known since 1976 that 26A1 (tl/2 = 7.2 x 105 yrs) was alive in the early solar system, at a level of (26Al/27Al)o z 5 x 10-5 in calcium-aluminum inclusions (CAI). However, several outstanding questions remain. Little evidence for 26A1 has been found in other chondritic material, and none has been found in differentiated meteorites. These results might imply that 26A1 was heterogeneously distributed in the nebula or by mineralogic site in nebular dust, or they might reflect differences in time of formation. There are strict limitations on finding evidence of 26A1 in normal chondrules with bulk Al/Mg ~ 0.1, since even quenched, perfectly preserved, late-stage glasses would have low Al/Mg. Primary plagioclase crystals provide the only possibility, but these only crystallize rarely in melts within the compositional range of normal chondrules. Also, metamorphism can erase the evidence in high-AI/Mg phases. To address these issues, we have conducted a search for chondrules and CAI with high-Al/Mg phases suitable for ion-probe measurement in type 3 ordinary chondrites. Previous work has revealed evidence for 26Al in a plagioclase bearing, olivine-pyroxene class from Semarkona (LL3.0; (26Al/27Al)o = 7.7+/-2.1 x 10-6)), a plagioclase-rich object from Bovedy (L3.7?; 2.5+/-1.2 x 10-7), in separated plagioclase from St. Marguerite (H4; 2.0+/-0.6 x 10-7), an isolated hibonite grain from Dhajala (H3.8; 8.4+0.5 x 10-6), and in Al2O3 and hibonite grains ((26Al/27Al)o = 2-5 x 10-5; [GRH, unpublished]) from acid residues of Semarkona, Bishunpur (LL3.1), and Krymka (LL3.1). We have identified and measured Al-Mg isotope systematics in two CAI and seven chondrules from ordinary chondrites of low metamorphic grade and have found clear evidence for 26A1 in both CAI and in two chondrules.

Srinivasan, G.; Russell, S. S.; MacPherson, G. J.; Huss, G. R.; Wasserburg, G. J.

1996-03-01

138

Chrome-spinel Inclusions in Ordinary Chondrites: Mineralogy, Chemistry and Petrogenesis  

NASA Astrophysics Data System (ADS)

We surveyed 270 ordinary chondrites (115 H, 116 L and 39 L/LL and LL) for chrome-spinel (Cr-Sp) chondrules and inclusions and Cr- Sp-rich mafic silicate chondrules. Here we discuss Cr-Sp inclusions. These inclusions are most common among H (52) chondrites and 3-4X less common in L (15) and L/LL+LL (5) chondrites. We divide the inclusions into two types chiefly on the basis of Cr/(Cr+Al) in the Cr-Sp: high (>0.84) in type I, low (<0.84) and, in many cases, variable in type II. Type I inclusions are irregularly shaped aggregates of Cr-Sp grains embedded in or surrounded by plagioclase mesostasis (Pl), and having merrillite (Mrl) and/or chlorapatite (Apt) rims. Some contain Mrl/Apt in the cores. Most rims are framed by low-Al and low-Ti clinopyroxene (Px). A few inclusions show core-to-rim changes in modal composition: cores consisting of Pl, Px and/or olivine (Ol) are surrounded by a Cr-Sp-rich zone and a Pl rim. Cr-Sp is uniform in composition with a Cr/(Cr+Al) ratio of 0.84- 0.86. Plagioclase occurs as coexisting Na- and K-rich varieties, the K-rich Pl generally occurring in cores. Type II inclusions are also irregularly shaped; they consist of a Pl core or fine- grained Pl+Cr-Sp with accessory ilmenite (Ilm), a surrounding Cr-Sp-rich zone and a rim of Pl and Px enclosed by Mrl. Some type-II inclusions consist of compact Cr-Sp cores surrounded by Pl and Mrl rims. One inclusion contains a Mg-Ca-Na-Si phase in the core. Cr-Sp has a uniform or slightly variable composition within the individual inclusions but has significant grain-to- grain variability in Cr/(Cr+Al): ~0.29-0.80, increasing from core to rim. Pl is igneously zoned: cores have higher Ca than rims. Cr-Sp inclusions have >10 wt% bulk Al2O3 and differ from Al-rich objects in ordinary chondrites by having high contents of Cr2O3 and FeO, and low contents of SiO2 and MgO. Models for the formation of Cr-Sp inclusions include the following: (1) The similarity in mineralogy and chemistry of Cr- Sp inclusions and chondrules and the occurrence of both within the same meteorite suggest a genetic relationship. The formation of Cr-Sp chondrules was explained by Krot et al. (1992) as having involved melting and incomplete evaporation of presolar lumps, perhaps during infall into the nebula; this resulted in enrichment of the residue in Cr and Al. Subsequent melting and fractional crystallization produced the high Cr/Al ratios. Differences between Cr-Sp chondrules and inclusions indicate that either (a) inclusions formed by lower degrees of melting than chondrules, (b) inclusions formed by chondrule disruption, or (c) inclusions are relicts of nebular precursors. (2) The presence of phosphates within some Cr-Sp inclusions and the presence of inclusions within metal suggest formation of Cr-Sp inclusions from oxidized precursor materials that originally consisted of Cr-P-Si-rich metal. Melting of oxidized metal grains together with an alkali-rich nebular component resulted in the formation of immiscible Cr-P-rich silicate and metal melts. (3) Based on the similarity of textures and mineralogy of Cr-Sp and Sp inclusions in ordinary chondrites, A. Bischoff (pers. comm.) proposed that Cr-Sp objects were formed by alteration of Sp inclusions during parent-body metamorphism; Sp was replaced by Cr-Sp, perovskite by Ilm, and anorthite by Na-Pl. The occurrence of Cr-Sp of variable composition in equilibrated ordinary chondrites implies low diffusion rates in Cr-Sp, inconsistent with this model; nebular reaction between Sp- and Cr-rich phases is still less likely. (4) The presence of shock veins and melt pockets having mineralogy similar to those of Cr-Sp inclusions suggests a shock origin. However, this seems unlikely because (a) Cr-Sp inclusions and melt pockets have different textures, (b) Cr-Sp within melt pockets has high Cr/(Cr+Al) ~0.86, similar to matrix chromite, and (c) most of the chondrites containing Cr- Sp inclusions are of low shock stage (S1-S3). References: Krot A., Ivanova M.A., and Wasson J.T. (1992) Earth Planet. Sci. Lett., submitted.

Krot, A. N.; Ivanova, M. A.; Wasson, J. T.

1992-07-01

139

Chemical and physical studies of chondrites: 10. Cathodoluminescence and phase composition studies of metamorphism and nebular processes in chondrules of type 3 ordinary chondrites  

SciTech Connect

The cathodoluminescence (CL) properties of eight type 3 ordinary chondrites and one L5 chondrite have been determined, and phenocryst and mesostasis compositions have been analyzed in the chondrules of four of them (Semarkona, type 3.0; Krymka, 3.1; Allan Hills A77214, 3.5; and Dhajala, 3.8) in order to investigate their origins and metamorphic history. In the present study, the authors discuss the CL properties of nine ordinary chondrites of a variety of petrologic types with particular emphasis on detailed studies of the compositions of the relevant phases in four of these: Semarkona (3.0), Krymka (3.1), Allan Hills A77214 (3.5), and Dhajala (3.8). They describe a means of classifying chondrules that is based on the composition of their two major components, the mesostasis and phenocrysts. The system is applicable to > 90-95% off the chondrules in a given meteorite and it describes the range of material produced by nebular material and of the effect of metamorphism on the chondrules. They also discuss the relevance of the results for the origin of the nine chondrite classes.

DeHart, J.M.; Lu Jie; Benoit, P.H.; Sears, D.W.G. (Univ. of Arkansas, Fayetteville (United States)); Lofgren, G.E. (NASA Johnson Space Center, Houston, TX (United States))

1992-10-01

140

Coincidental Compositional and Orbital Correspondences Among Some Ordinary Chondrites: No Strong Evidence for Meteoroid Streams  

NASA Astrophysics Data System (ADS)

Previous attempts to assign ordinary chondrites (OC) to meteoroid streams have been unsuccessful because the orbits of the proposed members had different radiants and, in some cases, the meteorites had significantly different cosmic-ray exposure (CRE) ages. Using more conservative criteria, we have identified four pairs of equilibrated OC (L6 Nejo, Salem; L6 Perpeti, Vouillé; L6 Drake Creek, Forsyth; H5 Okabe, Kerilis) wherein each member of the pair could conceivably have been derived from the same immediate precursor body (IPB). The members of each pair are of the same chondrite group and petrologic type; they have similar CRE ages and fell within 1 calendar day of each other (in different years). Because there is a moderate range in oxidation state (represented by mean olivine Fa) among equilibrated OC in each group, similarities in this intrinsic geochemical property between the members of two of the proposed pairs offer some support for the hypothesis that these rocks were derived from the same IPB. If the pairs are genuine, their precursor bodies were probably meter-size near-Earth asteroids (NEAs) with aphelia within or beyond the Main Asteroid Belt. Fragmentation of such NEAs is most likely to have occurred near aphelia; in principle, the ejecta could have spread somewhat along the NEAs’ orbits and collided with Earth on approximately the same calendar date but in different years. However, literature data show that, although ˜670 meteorites with masses ?10 kg reach the Earth’s surface each year, only five or six falls (typically in this mass range) are observed and recovered. This suggests that the chances of recovering more than one meteorite from a disrupted meter-size body in Earth-crossing orbit are small. It thus seems likely that the similar properties of the proposed OC pairs are due to coincidence.

Rubin, Alan E.; Matson, Robert D.

2008-12-01

141

Chromite-rich mafic silicate chondrules in ordinary chondrites: Formation by impact melting  

NASA Technical Reports Server (NTRS)

Chromium-rich chondrules constitute less than 0.1 percent of all ordinary chondrite (OC) chondrules and comprise three groups: chromian-spinel chondrules, chromian-spinel inclusions, and chromite-rich mafic silicate (CRMS) chondrules. Chromian-spinel chondrules (typically 100-300 microns in apparent diameter) exhibit granular, porphyritic and unusual textures and occur mainly in H chondrites. Their morphologies are distinct from the irregularly shaped chromian-spinel inclusions of similar mineralogy. Chromian-spinel chondrules and inclusions consist of grains of chromian-spinel embedded in plagioclase (Pl) or mesostasis of Pl composition. Many also contain accessory ilmenite (Ilm), high-Ca pyroxene (Px), merrillite (Mer), and rare olivine (Ol); some exhibit concentric mineral and chemical zoning. CRMS chondrules (300-1100 microns in apparent diameter) are generally larger than chromian-spinel chondrules and occur in all metamorphosed OC groups. Most CRMS chondrules are nearly spherical although a few are ellipsoidal with a/b aspect ratios ranging up to 1.7. Textures include cryptocrystalline, granular, radial, barred, and porphyritic varieties; some contain apparently relict grains. The chondrules consist of chromite (Chr), Ol and Pl, along with accessory Mer, troilite (Tr), metallic Fe-Ni (Met), Px and Ilm. The mesostasis in CRMS chondrules is nearly opaque in transmitted light; thus, they can be easily recognized in the optical microscope. Based on the similarity of mineralogy and chemistry between CRMS chondrules of different textures (opaque chromite-rich mesostasis, skeletal morphology of Ol grains, similar bulk compositions) we suggest that these chondrules form a genetically related population.

Krot, Alexander N.; Rubin, Alan E.

1993-01-01

142

Coarse-grained rims on magnesium-rich and magnesium-poor chondrules in ordinary chondrites  

NASA Astrophysics Data System (ADS)

Chondrules with igneous rims and enveloping compound chondrules in ordinary chondrites (OC) preserve the record of solids that were present at different times or in different regions of the solar nebula during chondrule formation. These objects demonstrate that OC chondrules experienced multiple episodes of chondrule formation. This conclusion is consistent with the presence of relict grains in chondrules that were probably produced by disaggregation of chondrules of a previous generation, small range in OC chondrule O isotope composition, and interelement correlations in bulk chondrule data that can be interpreted as the random sampling of a previous generation of chondrules. Rims around chondrules can be divided into two major categories: fine-grained rims (FGR), typically opaque and Fe rich and relatively coarse-grained rims. Thirteen CGR on Mg-rich chondrules (type I, Fa/Fs less than 10 mol%) and nine rims on Mg-poor chondrules (type II, Fa/Fs greater than 10 mol%) were studied petrographically, by electron microprobe analysis, and scanning electron microscopy. Many of the CGR on type I chondrules show evidence of significant and, in many cases, complete melting. Similar Fa and Fs contents in mafic minerals of OC igneous rims and their type I chondrule hosts indicate that many OC chondrules experienced multiple heating events during a time short compared to the time necessary for appreciable evolution in the mean Fa or Fs of the nebular solids, and were than withdrawn from the chondrule-forming region. Type II chondrules and their CGR formed from more oxidized material mixed with fragments of type I chondrules and were heated to lower temperatures than type I chondrules and their CGR. Type I and type II chondrules may have formed in different OC nebular subregions or at different times and were mixed together before or during agglomeration to form chondrites.

Krot, A. N.; Watson, J. T.

1994-07-01

143

Thermoluminescence and the shock and reheating history of meteorites. IV - The induced TL properties of type 4-6 ordinary chondrites  

NASA Technical Reports Server (NTRS)

The thermoluminescence (TL) properties were measured in 121 equilibrated H and L ordinary chondrites of which 33 H and 32 L were from Antarctica. It was found that the distribution of TL sensitivities for non-Antarctic L chondrites differs from that of non-Antarctic H chondrites, reflecting the well-known differences in shock history between L and H classes, the greater proportion of the former having suffered postmetamorphic shock. The data also show differences in TL sensitivity between Antarctic and non-Antarctic H chondrites, suggesting nontrivial differences in thermal history of these chondrites.

Haq, Munir; Hasan, Fouad A.; Sears, Derek W. G.

1988-01-01

144

Evidence for a late thermal event of unequilibrated enstatite chondrites: a Rb-Sr study of Qingzhen and Yamato 6901 (EH3) and Khairpur (EL6)  

USGS Publications Warehouse

The Rb-Sr whole rock and internal systematics of two EH3 chondrites, Qingzhen and Yamato 6901, and of one EL6 chondrite, Khairpur, were determined. The internal Rb-Sr systematics of the EH3 chondrites are highly disturbed. Fractions corresponding to sulfide phases show excess 87Sr, while other fractions corresponding to silicate phases produce a linear trend on a Rb-Sr evolution diagram. If these linear relations are interpreted as isochrons, the ages of the silicate phases are 2.12?? 0.23 Ga and 2.05 ??0.33 Ga with the initial Sr isotopic ratios of 0.7112 ?? 0.0018 and 0.7089 ?? 0.0032, for Qingzhen and Yamato 6901, respectively. The Rb-Sr results are interpeted as indicative of a late thermal event about 2Ga ago on the parent bodies of these EH3 chondrites. These ages agree well with previously published K-Ar ages. An older isochron age of 4.481 ?? 0.036 Ga with a low initial Sr isotopic ratio of 0.69866 ?? 0.00038 was obtained for the data from silicate fractions of Khairpur, indicating early petrological equilibrium on the parent body of EL6 chondrites. -from Authors

Torigoye, N.; Shima, M.

1993-01-01

145

Space Weathering of Ordinary Chondrite Parent Bodies, Its Impact on the Method of Distinguishing H, L, and LL Types and Implications for Itokawa Samples Returned by the Hayabusa Mission  

NASA Technical Reports Server (NTRS)

As the most abundance meteorites in our collections, ordinary chondrites potentially have very important implications on the origin and formation of our Solar System. In order to map the distribution of ordinary chondrite-like asteroids through remote sensing, the space weathering effects of ordinary chondrite parent bodies must be addressed through experiments and modeling. Of particular importance is the impact on distinguishing different types (H/L/LL) of ordinary chondrites. In addition, samples of asteroid Itokawa returned by the Hayabusa spacecraft may re veal the mechanism of space weathering on an LLchondrite parent body. Results of space weathering simulations on ordinary chondrites and implications for Itokawa samples are presented here.

Hiroi, T.; Sasaki, S.; Noble, S. K.; Pieters, C. M.

2011-01-01

146

Partial melting of the St. Severin (LL) and Lost City (H) ordinary chondrites: One step backwards and two steps forward  

NASA Technical Reports Server (NTRS)

This study looks at partial melting in H and LL chondrites at nearly one atmosphere of total pressure as part of a continuing study of the origins of basaltic achondrites. Previously, melting experiments on anhydrous CM and CV chondrites showed that, near its solidus, the CM chondrite produced melts having major element chemistries similar to the Sioux County eucrite; but, the pyroxenes in the residuum were too iron-rich to form diogenites. Our preliminary results from melting experiments on ordinary (H, LL) chondrites suggested that, although the melts did not look like any known eucrites, pyroxenes from these charges bracketed the compositional range of pyroxenes found in diogenites. We had used the Fe/Mg exchange coefficients calculated for olivine, pyroxene, and melt in these charges to evaluate the approach to equilibrium, which appeared to be excellent. Unfortunately, mass balance calculations later indicated to us that, unlike our CM and CV charges, the LL and H experimental charges had lost significant amounts of iron to their (Pt or PtRh) supports. Apparently, pyroxene stability in chondritic systems is quite sensitive to the amount of FeO, and it was this unrecognized change in the bulk iron content which had stabilized the high temperature, highly magnesian pyroxenes. Accordingly, this work reinvestigates the phase equilibria of ordinary chondrites, eliminating iron and nickel loss, and reports significant differences. It also looks closely at how the iron and sodium in the bulk charge affect the stability of pyroxene, and it comments on how these new results apply to the problems of diogenite and eucrite petrogenesis.

Jurewicz, A. J. G.; Jones, J. H.; Mittlefehldt, D. W.

1994-01-01

147

Partial melting of the St. Severin (LL) and Lost City (H) ordinary chondrites: One step backwards and two steps forward  

NASA Astrophysics Data System (ADS)

This study looks at partial melting in H and LL chondrites at nearly one atmosphere of total pressure as part of a continuing study of the origins of basaltic achondrites. Previously, melting experiments on anhydrous CM and CV chondrites showed that, near its solidus, the CM chondrite produced melts having major element chemistries similar to the Sioux County eucrite; but, the pyroxenes in the residuum were too iron-rich to form diogenites. Our preliminary results from melting experiments on ordinary (H, LL) chondrites suggested that, although the melts did not look like any known eucrites, pyroxenes from these charges bracketed the compositional range of pyroxenes found in diogenites. We had used the Fe/Mg exchange coefficients calculated for olivine, pyroxene, and melt in these charges to evaluate the approach to equilibrium, which appeared to be excellent. Unfortunately, mass balance calculations later indicated to us that, unlike our CM and CV charges, the LL and H experimental charges had lost significant amounts of iron to their (Pt or PtRh) supports. Apparently, pyroxene stability in chondritic systems is quite sensitive to the amount of FeO, and it was this unrecognized change in the bulk iron content which had stabilized the high temperature, highly magnesian pyroxenes. Accordingly, this work reinvestigates the phase equilibria of ordinary chondrites, eliminating iron and nickel loss, and reports significant differences. It also looks closely at how the iron and sodium in the bulk charge affect the stability of pyroxene, and it comments on how these new results apply to the problems of diogenite and eucrite petrogenesis.

Jurewicz, A. J. G.; Jones, J. H.; Mittlefehldt, D. W.

1994-03-01

148

Study of iron nanophases in ordinary chondrites by means of near field microscopy  

NASA Astrophysics Data System (ADS)

The aim of this work is to study the role, the characteristics and the formation process of iron nanoparticles (npFe) in asteroids. These npFe are considered the main responsible for the reddening (i.e. red-IR reflectance increase at increasing wavelength) observed in the asteroids' spectra and it is believed that they are formed as consequence of the Space Weathering (i.e. the ensemble of processes acting on a body exposed to space environment). \\citet{mo05} discusses a scenario regarding npFe formation, according to which they originate from shock-induced phase transformations of Fe-Ni alloys caused by collisions. We looked for npFe in samples of Ordinary Chondrites (OCs), whose parent bodies are S-type asteroids, and are trying to link the amount of metal and the mechanical shock degree (which would confirm the scenario above mentioned). For this purpose, we have choosen to use SNOM (Scanning Near-field Optical Microscopy). This technique permits to collect at the same time high resolution topography images and optical images of the analyzed sample. For the first time, a multi-colour SNOM experiment (i.e. every sample has been analyzed at different wavelengths) has been performed on extraterrestrial samples: because npFe are more reflective at longer wavelengths, comparison of reflectance images obtained at different wavelength gives a strong aid in npFe detection and identification. Finally, laboratory analysis has been supported by simulation methods.

Longobardo, A.; Palomba, E.; Girasole, M.; Longo, G.; Pompeo, G.; Gori, P.; Cricenti, A.

149

Chemical and physical studies of type 3 chondrites. III Chondrules from the Dhajala H3.8 chondrite  

NASA Technical Reports Server (NTRS)

Thermoluminescence (TL) properties have been measured in 58 chondrules separated from the Dhajala H3.8 chondrite. The pyrolytic chondrules are noted to have higher mass-normalized TL values than nonpyrolytic ones. Significant correlations are noted between log(TL) and the bulk CaO, Al2O3, and MnO content of the chondrules. These, together with correlations of log(TL) with the CaO, Al2O3, SiO2 and normative anorthite content of the chondrule glass, indicate an association of the TL and the abundance and position of mesostasis. It is suggested that the TL level in a given chondrule is governed by its bulk composition and metamorphism, and it is hypothesized that the devitrification resistance of unequilibrated chondrule mesostasis explains the unequilibration of certain chondrules in type 3 ordinary chondrites.

Sears, D. W. G.; Sparks, M. H.; Rubin, A. E.

1984-01-01

150

Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6  

Microsoft Academic Search

Many equilibrated ordinary chondrites contain (besides chromites of constant composition) Cr spinel with a large spread in Cr\\/(Cr + Al) ratios. They occur mainly as large grains in chondrules rich in mesostasis, preventing complete equilibration in Cr\\/Al but not in Fe\\/Mg. This partially equilibrated Cr spinel turned out to be particularly useful for the selection of an appropriate olivine\\/spinel thermometer

Frank Wlotzka

2005-01-01

151

Silica-bearing objects in the Dengli H3.8 and Gorlovka H3-4 chondrites  

NASA Technical Reports Server (NTRS)

Silica-bearing objects are enigmatic components of the olivine-normative ordinary chondrites. Several papers have been devoted to the study of these objects in various chondrite types. While a relatively large body of information has been collected, the origin of these objects is still controversial. Here we report new data on silica-bearing objects in the unequilibrated H-chondrites Dengli and Gorlovka. The crystallization history of these objects could be explained on the basis of the phase diagram of the Q-Ol-Pl (Al2O3) system, but the origin of the silica-rich liquids remains unclear.

Ivanova, M. A.; Kononkova, N. N.; Petaev, M. I.

1993-01-01

152

Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites  

Microsoft Academic Search

Precise determination of REE and Ba abundances in three carbonaceous (Orgueil Cl, Murchison C2 and Allende C3) and seven olivine-bronzite chondrites were carried out by mass spectrometric isotope dilution technique. Replicate analyses of standard rock and the three carbonaceous chondrites demonstrated the high quality of the analyses (accuracies for REE are ±1-2 per cent). Certain carbonaceous chondrite specimens showed small

Noboru Nakamura

1974-01-01

153

Using the U-Pb system's dual decay scheme towards reconstructing the thermal histories and origins of ordinary chondrites (Invited)  

NASA Astrophysics Data System (ADS)

Ordinary chondrites provide a record of planetary formation through the accretion of the Solar System's earliest forming solids. Despite the critical importance these samples have for understanding Solar System history, the origin of chondrites as well as the size and histories of their parent bodies remains unclear. The antiquity of chondrules permit chondrite accretion prior to the extinction of the 26Al short-lived radionuclide and the possibility of parent body melting and differentiation due to radioactive heating. Yet this antiquity and the overall abundance of chondrites has raised questions concerning the origin of chondrules and the conditions leading to the preservation of their primordial geochemical signatures, accretion textures and ancient inclusions. In short, if these bodies accreted early in the history of the solar system, how has this material avoided melting by radioactive heating? Different models for the size and structure of the chondrite parent bodies as well as for chondrule formation exist to explain how chondrites survive or avoid early radiogenic heating. Here we propose to evaluate these scenarios through comparison between modeled thermal histories and thermal histories reconstructed using U-Pb thermochronology of chondritic phosphates. Detailed thermal histories are reconstructed by exploiting the U-Pb system's dual decay scheme, where two parent isotopes, 238U and 235U, decay to two daughter isotopes 206Pb and 207Pb respectively. The difference in decay rates between parent isotopes imposes a time-variant parent and instantaneous daughter isotopic composition for any point in Solar System history. This new thermochronologic methodology works to capture this isotopic evolution using the variation in the time of Pb retention between both: 1) chondrites from different parent body depths or metamorphic grades, where variations in the timescale of cooling result from the thermal gradient within a conductively cooling body, and; 2) crystals of different size, where volume diffusion behavior, in particular for slowly cooled systems, result in larger crystals retaining Pb at higher temperatures and over longer and older timescales in comparison to smaller crystals. The differences in 207Pb/206Pb among samples from different depths and within each sample--between grains of different size -- can be used to reconstruct the long-term chondrite thermal histories. Here we will present 207Pb-206Pb data for phosphates from over 10 ordinary chondrites with samples from: 1) H, L and LL chondrites, 2) metamorphic grades Type 4-6, and 3) a range of shock grades. Preliminary phosphate (U-Th)/He data from the same samples provide a means to understand whether the U-Pb system has been influenced by subsequent heating events. The combined data are used to constrain parent body thermal histories--and in doing so constrain the size and structure of the chondrite parent bodies.

Blackburn, T. J.; Elkins-Tanton, L. T.; Carlson, R. W.; Alexander, C. M.; Hourigan, J. K.

2013-12-01

154

Lithium isotopes as an indicator of primary and secondary processes in unequilibrated meteorites: Chondrule cooling and aqueous alteration in CO chondrites  

NASA Astrophysics Data System (ADS)

Chondrites have escaped planetary scale differentiation and thus represent some of the best examples of early solar system material. However, even the most pristine chondrites have experienced some degree of aqueous alteration and/or metamorphism. Where and when these processes occurred, their nature, duration and extent remains poorly understood (e.g.[1]). During the crystallisation of chondrule phenocrysts, compositional gradients drive the more rapid diffusion of 6Li compared to 7Li, creating distinctive 7Li/6Li profiles [2,3]. This potentially makes Li isotopes a useful tool for the calculation of chondrule cooling rates. Lithium is also highly mobile during the aqueous weathering of silicate material with 7Li preferentially entering the solution, thus fractionating the two isotopes (e.g. [4]); a process already identified in the aqueous alteration of chondritic materials [5]. Lithium isotopes may therefore provide the means to quantify the effects of both primary and secondary processes in chondritic material. We will present new data for intra- and inter-chondrule ?7Li variation, determined by ion microprobe and MC ICP MS, as well as bulk data for Ornans (CO3.3) and Lancé (CO3.4) with the aim to (i) assess the preservation of primary Li isotope diffusion profiles in chondrule phenocrysts (ii) examine the extent and effects of aqueous alteration using the Li isotope systematics of bulk-rock and chondrules, in addition to intra-chondrule ?7Li variations. High Mg# (>0.99) in chondrule cores suggests that primitive geochemical compositions may have been retained. In contrast, lower rim Mg# (?0.80) suggests diffusive exchange with matrix during cooling or subsequent secondary alteration. As variability in Mg# is also observed close to fractures in the interior of chondrule phenocrysts these variations are unlikely to be primary, suggesting that Li isotope fractionation during chondrule cooling may have been overprinted. Bulk-rock ?7Li values for Ornans (4.9 ± 0.5) and Lancé (3.9 ± 0.3) are marginally heavier than previously reported [6] and overlap with values obtained for CM chondrites (e.g. 4.3 ± 0.8) inferred to have experienced more aqueous alteration than CO chondrites. References: [1] Brearley (2006) In: Meteorites and the Early Solar System II (Eds Dante Lauretta, H.Y. McSween Jr and L. Leshin), Arizona University Press, pp. 587-624. [2] Parkinson et al., (2007) Earth Planet Sci. Lett. 257, 609-621. [3] Beck et al., (2004) Geochim. Cosmochim. Acta 68, 2925-2933 [4] Kisakurek et al., (2004) Chem. Geol. 212, 27-44. [5] Sephton et al., (2006) Meteor. Planet. Sci. 41, 1039-1043 [6] Seitz et al., (2007) Earth Planet Sci. Lett. 260, 582- 596.

Sharrock, J. L.; Harvey, J.; Fehr, M.; James, R. H.; Parkinson, I. J.

2010-12-01

155

Chondrules in the Murray CM2 meteorite and compositional differences between CM-CO and ordinary chondrite chondrules  

NASA Astrophysics Data System (ADS)

Thirteen of the least aqueously altered chondrules in Murray (CM2) were analyzed for bulk compositions, by means of a broad beam electron microprobe, to explore the compositional differences between the CM-CO, and the ordinary chondrite OC chondrules. The CO chondrules are richer in refractory lithophiles and poorer in Cr, Mn, and volatile lithophiles than the OC chondrules; much lower refractory lithophile abundances in CM chondrules resulted from aqueous alteration. Evidence is found for two important lithophile precursor components of CM-CO chondrite chondrules: (1) pyroxene- and refractory-rich, FeO-poor, and (2) olivine-rich, refractoryand FeO-poor. It is suggested that the pyroxene- and refractory-rich, FeO-poor lithophile precursor component has formed by an incomplete evaporation of presolar silicates that brought these materials into the enstatite stability field.

Rubin, A. E.; Wasson, J. T.

1986-02-01

156

Porphyritic Olvine-Pyroxene Clast in Kaidun: First Discovery of an Ordinary Chondrite Clast.  

National Technical Information Service (NTIS)

Kaidun is an enigmatic meteorite showing a micro-brecciated texture composed of variable kinds of lithic clasts and mineral fragments. The constituent components range from primitive chondritic materials to differentiated achondritic materials, and thus b...

T. Makiouchi J. Makishima E. Koizumi M. E. Zoelnsky

2005-01-01

157

An 57Fe Mössbauer study of three Australian L5 ordinary-chondrite meteorites: dating Kinclaven-001  

NASA Astrophysics Data System (ADS)

Three L5-type ordinary chondrite meteorites recovered from the Nullarbor Region of Western Australia were studied by 57Fe Mössbauer spectroscopy: Kinclaven-001, Camel Donga-007 and Gunnadorah-002. The relative amounts of the various Fe-bearing phases including the primary minerals (Olivine, Pyroxene, Troilite and Fe-Ni metal) and the ferric alteration products (Goethite, Maghemite/Magnetite) were obtained to determine the percentage of iron converted to Fe3 + by weathering processes. These data allow us to estimate the terrestrial age of Kinclaven-001 at 1,700 ± 1,300 yrs.

Cadogan, J. M.; Rebbouh, L.; Mills, J. V. J.; Bland, P. A.

2013-12-01

158

Crystallization trends of precursor pyroxene in ordinary chondrites: Implications for igneous origin of precursor  

NASA Technical Reports Server (NTRS)

Various observations suggest that the precursor of the fine fragments and the relict pyroxene was formed by an igneous process in a grandparent body of the chondrite. Furthermore, the fact that the precursors appear as fragments in the matrix as well as relicts in the chondrule suggests a shock origin for the chondrite by collision of two or more bodies, which had already to some extent undergone igneous differentiation.

Fujita, T.; Kitamura, M.

1994-01-01

159

Magnetic properties of the LL5 ordinary chondrite Chelyabinsk (fall of February 15, 2013)  

NASA Astrophysics Data System (ADS)

Here we characterize the magnetic properties of the Chelyabinsk chondrite (LL5, S4, W0) and constrain the composition, concentration, grain size distribution, and mineral fabric of the meteorite's magnetic mineral assemblage. Data were collected from 10 to 1073 K and include measurements of low-field magnetic susceptibility (?0), the anisotropy of ?0, hysteresis loops, first-order reversal curves, Mössbauer spectroscopy, and X-ray microtomography. The REM and REM' paleointensity protocols suggest that the only magnetizations recorded by the chondrite are components of the Earth's magnetic field acquired during entry into our planet's atmosphere. The Chelyabinsk chondrite consists of light and dark lithologies. Fragments of the light lithology show log?0 = 4.57 ± 0.09 (s.d.) (n = 135), while the dark lithology shows 4.65 ± 0.09 (n = 39) (where ?0 is in 10-9 m3 kg-1). Thus, Chelyabinsk is three times more magnetic than the average LL5 fall, but is similar to a subgroup of metal-rich LL5 chondrites (Paragould, Aldsworth, Bawku, Richmond) and L/LL5 chondrites (Glanerbrug, Knyahinya). The meteorite's room-temperature magnetization is dominated by multidomain FeNi alloys taenite and kamacite (no tetrataenite is present). However, below approximately 75 K remanence is dominated by chromite. The metal contents of the light and dark lithologies are 3.7 and 4.1 wt%, respectively, and are based on values of saturation magnetization.

Bezaeva, Natalia S.; Badyukov, Dmitry D.; Nazarov, Mikhail A.; Rochette, Pierre; Feinberg, Joshua M.; Markov, Gennadiy P.; Borschneck, Daniel; Demory, FrançOis; Gattacceca, JéRôMe; Borisovskiy, Sergey E.; Skripnik, Anna Ya

2014-06-01

160

Optical Effects of Regolith Processes on S-Asteroids as Simulated by Laser Shots on Ordinary Chondrite and Other Mafic Materials  

Microsoft Academic Search

Laboratory results from a simulation of possible optical effects of impact melting and repeated crystallization on asteroidal surfaces are presented. Quick melting and crystallization of surface materials were simulated by impulse laser treatment of powdered materials in vacuum. The mafic materials used in this study were: ordinary chondrite (OC) ElenovkaL5, carbonaceous chondrite Allende CV3, terrestrial olivine, clinopyroxene, and olivine–clinopyroxene mixture

L. V. Moroz; A. V. Fisenko; L. F. Semjonova; C. M. Pieters; N. N. Korotaeva

1996-01-01

161

Relationships Among Intrinsic Properties of Ordinary Chondrites: Oxidation State, Bulk Chemistry, Oxygen-isotopic Composition, Petrologic Type, and Chondrule Size  

NASA Technical Reports Server (NTRS)

The properties of ordinary chondrites (OC) reflect both nebular and asteroidal processes. OC are modeled here as having acquired nebular water, probably contained within phyllosilicates, during agglomeration. This component had high Ai70 and acted like an oxidizing agent during thermal metamorphism. The nebular origin of this component is consistent with negative correlations in H, L, and LL chondrites between oxidation state (represented by olivine Fa) and bulk concentration ratios of elements involved in the metal-silicate fractionation (e.g., NdSi, Ir/Si, Ir/Mn, Ir/Cr, Ir/Mg, Ni/Mg, As/Mg, Ga/Mg). LL chondrites acquired the greatest abundance of phyllosilicates with high (delta)O-17 among OC (and thus became the most oxidized group and the one with the heaviest O isotopes); H chondrites acquired the lowest abundance, becoming the most reduced OC group with the lightest O isotopes. Chondrule precursors may have grown larger and more ferroan with time in each OC agglomeration zone. Nebular turbulence may have controlled the sizes of chondrule precursors. H-chondrite chondrules (which are the smallest among OC) formed from the smallest precursors. In each OC region, low-FeO chondrules formed before high-FeO chondrules during repeated episodes of chondrule formation. During thermal metamorphism, phyllosilicates were dehydrated; the liberated water oxidized metallic Fe-Ni. This caused correlated changes with petrologic type including decreases in the modal abundance of metal, increases in olivine Fa and low-Ca pyroxene Fs, increases in the olivine/pyroxene ratio, and increases in the kamacite Co and Ni contents. As water (with its heavy 0 isotopes) was lost during metamorphism, inverse correlations between bulk (delta)O-18 and bulk (delta)O-17 with petrologic type were produced. The H5 chondrites that were ejected from their parent body approx.7.5 Ma ago during a major impact event probably had been within a few kilometers of each other since they accreted approx.4.5 Ga ago. There are significant differences in the olivine compositional distributions among these rocks; these reflect stochastic nebular sampling of the oxidant (Le., phyllosilicates with high (delta)O-17) on a 0.1-1 km scale during agglomeration.

Rubin, Alan E.

2006-01-01

162

Relationships among intrinsic properties of ordinary chondrites: Oxidation state, bulk chemistry, oxygen-isotopic composition, petrologic type, and chondrule size  

NASA Astrophysics Data System (ADS)

The properties of ordinary chondrites (OC) reflect both nebular and asteroidal processes. OC are modeled here as having acquired nebular water, probably contained within phyllosilicates, during agglomeration. This component had high ? 17O and acted like an oxidizing agent during thermal metamorphism. The nebular origin of this component is consistent with negative correlations in H, L, and LL chondrites between oxidation state (represented by olivine Fa) and bulk concentration ratios of elements involved in the metal-silicate fractionation (e.g., Ni/Si, Ir/Si, Ir/Mn, Ir/Cr, Ir/Mg, Ni/Mg, As/Mg, Ga/Mg). LL chondrites acquired the greatest abundance of phyllosilicates with high ? 17O among OC (and thus became the most oxidized group and the one with the heaviest O isotopes); H chondrites acquired the lowest abundance, becoming the most reduced OC group with the lightest O isotopes. Chondrule precursors may have grown larger and more ferroan with time in each OC agglomeration zone. Nebular turbulence may have controlled the sizes of chondrule precursors. H-chondrite chondrules (which are the smallest among OC) formed from the smallest precursors. In each OC region, low-FeO chondrules formed before high-FeO chondrules during repeated episodes of chondrule formation. During thermal metamorphism, phyllosilicates were dehydrated; the liberated water oxidized metallic Fe-Ni. This caused correlated changes with petrologic type including decreases in the modal abundance of metal, increases in olivine Fa and low-Ca pyroxene Fs, increases in the olivine/pyroxene ratio, and increases in the kamacite Co and Ni contents. As water (with its heavy O isotopes) was lost during metamorphism, inverse correlations between bulk ? 18O and bulk ? 17O with petrologic type were produced. The H5 chondrites that were ejected from their parent body ˜7.5 Ma ago during a major impact event probably had been within a few kilometers of each other since they accreted ˜4.5 Ga ago. There are significant differences in the olivine compositional distributions among these rocks; these reflect stochastic nebular sampling of the oxidant (i.e., phyllosilicates with high ? 17O) on a 0.1-1 km scale during agglomeration.

Rubin, Alan E.

2005-10-01

163

Distribution of the U, Th-He, and K-Ar Ages of Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

U,Th-He and K-Ar ages of over 700 H, L and LL chondrites were calculated.The noble gas data were taken from [1].The distributions of the ages of all chondrites and,independently, of chondrites having t(sub)3/t(sub)21 <1 within each chemical group were studied (t(sub)3 and t(sub)21 are the 3He and 21Ne cosmic ray exposure ages, respectively). Analysis of the distribution of the 4He and 3He contents in L-chondrites showed the parent body of these meteorites underwent the catastrophic collision in space 350+/-30 Myr ago [2]. This event stipulated the high abundance of L chondrites having low gas-retention ages. As a whole,ages of non-Antarctic finds are less than ages of falls most probably due to losses of noble gases during terrestrial weathering. However,the parameters of the age distributions of Antarctic finds are close to those of falls: storage of meteorites inside ice do not lead to essential losses of gases. Not only L but also H chondrites having low U,Th-He ages tend to have low cosmic-ray exposure ages. Apparently,the most outgassing took place in the events that ejected the meteorites from their parent bodies. The large impacts may produce more outgassing and greater acceleration, leading to a shorter transit time to the Earth [3,4]. The possible difference of thermal history between Antarctic and non-Antarctic meteorites is found. References: [1] Schultz L. and Kruse H. (1989) Meteoritics, 24, 155-172; Schultz L.(1994) personal communication. [2] Alexeev V. A. (1995) LPS XXVI,19-20; Astronom. Vestnik (in Russian), in press. [3] Waenke H. (1966) Fortschr. Chem. Forsch., 7, 322-408. [4] Hintenberger H. et al. (1966) Z. Naturforsch., 21A, 1147-1159.

Alexeev, V. A.

1995-09-01

164

The Isotopic Composition and Mineralogy of Silicon Nitride (Si3N4) Within Ordinary and Enstatite Chondrites  

NASA Astrophysics Data System (ADS)

During stepped combustion studies of primitive meteorite residues, samples with high relative nitrogen abundances and delta^15N values have been encountered (e.g., Grady et al., 1986; Alexander, 1990; Russell et al., 1991). Such characteristics are consistent with the presence of nitrogen-rich minerals such as silicon nitride (Si3N4), which has recently been identified within the enstatite chondrites Indarch (Stone et al., 1991) and Qingzhen (Alexander et al., 1991). In order to explore this possibility more fully, the isotopic composition and mineralogy of acid residues from a number of ordinary, enstatite and carbonaceous chondrites has been determined. The carbon and nitrogen isotopic composition of HF/HCl, Cr(sub)20(sub)7^2-, HClO4 residues were analysed by stepped combustion and simultaneous C/N abundances measured. The delta^15N and C/N ratio over the 800 to 1200 degree C regime, the expected combustion temperature of nitrides and carbides, is shown in the figure. The carbonaceous chondrites contain isotopically light nitrogen (delta^15N = -650o/oo) and have a C/N ratio of ~30-35. This can be attributed to the combustion of SiC. In contrast, the ordinary chondrites and enstatite chondrite Indarch yield a more nitrogen-rich gas at the same temperatures, which can be attributed to the cocombustion of Si3N4 with SiC. Assuming the data in the figure represent two-component mixing and that nitrogen in SiC from all the meteorite classes has the same mean delta^15N and C/N ratio as that from carbonaceous chondrites, then an estimate can be made of the nitrogen isotopic composition of nitride grains. Thus we calculate that Si3N4 in ordinary chondrites has a delta^15N = +336 +- 13o/oo whereas that in Indarch is +12o/oo. The data for Indarch do not conflict with the ion probe study of Alexander et al., (1991). The mineralogical information concerning the acid residues was determined by pipetting samples onto perforated carbon films and examining them using a transmission electron microscope equipped with an ultrathin window X-ray detector. Silicon nitride with a low C/N ratio has so far been identified from the Indarch and Tieschitz residues. Silicon nitride is abundant in Indarch and its electron diffraction patterns are consistent with alphaSi3N4. This mineral forms elongate tabular crystals, ~1.5 micrometers to ~5.0 micrometers long by ~0.2 micrometers to ~0.7 micrometers wide. Silicon nitride grains within the Tieschitz residue are smaller than those from Indarch and ~500 to 1000 times less abundant. The crystals are needle-like in shape, ~0.4 micrometers to ~1.5 micrometers long by ~0.04 micrometers to ~0.07 micrometers wide. Most of the silicon nitride indexes as alphaSi3N4, although one large segmented crystal found was formed of both alphaSi3N4 and betaSi3N4. The contrasting isotopic and mineralogical characteristics of silicon nitrides from enstatite (Indarch) and ordinary (Tieschitz) chondrites may be due to differences in their origin. Whereas the delta^l5N of nitride in Indarch falls within the normal solar system range and could be a solar system condensate, as suggested by Alexander et al., (1991), the nitride component in ordinary chondrites is isotopically heavy and possibly represents a newly identified presolar grain. References Alexander C. M. O'D., Arden J. W., Ash R. D., and Plllinger C. T. (1990) Earth Planet. Sci. Lett. 97, 220-229. Alexander C. M. O'D., Prombo C. A., Swan P. D., and Walker R. M. (1991) Lunar Planet Sci. (abstract) 22, 5-6. Grady M. M., Wright I. P., Carr L. P., and Pillinger C. T. (1986) Geochim. Cosmochim. Acta, 50, 2799-2813. Russell S. S., Ash R. D., Pillinger C. T., and Arden J. W. (1991) Meteoritics, 26, 390. Stone J., Hutcheon I. D., Epstein S., and Wasserburg G. J. (1991) Earth Planet. Sci. Lett., 107, 570-586.

Lee, M. R.; Russell, S. S.; Arden, J. W.; Pillinger, C. T.

1992-07-01

165

Radial Heterogeneity of 53-Mn in the Early Solar System and the Place of Origin of Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

The earlier results of our studies of 53Mn-53Cr isotope systematics (T(sub)1/2 of 53Mn = 3.7 My) in angrites (LEW 86010, Angra dos Reis) and in some eucrites (Chervony Kut [CK], Juvinas [JI3V1) have suggested that the resolution of small age differences between these differentiated objects is possible if the assumption is true that 53Mn was homogeneously distributed at least in that region of the solar nebula where the meteorite parent bodies formed. The fact that the present day and the initial 53Cr/52Cr ratios in both meteorite classes are significantly higher than the terrestrial and lunar values led us to test the 53Mn-53Cr systematics in several ordinary chondrites, the putative building blocks of planetesimals. We found that these chondrites possess uniform and higher than terrestrial bulk 53Cr/52Cr ratios (0.5 epsilon; 1 epsilon = l x 10^-4) but more than a factor of two lower than the bulk eucrites. Thus, it became clear that the original assumption of 53Mn homogeneity on a large scale may not be valid and that the low abundance on earth of the 53Mn decay product may be the consequence of a solar system wide phenomenon rather than, say, simply depletion at 1 AU due to volatility. The idea of a radially heterogeneous 53Mn distribution in the nebula was suggested and tests of this hypothesis were undertaken.

Lugmair, G. W.; Shukolyukov, A.; MacIsaac, Ch.

1996-03-01

166

Weathering of ordinary chondrites from the Atacama Desert, Chile, by Mössbauer spectroscopy and synchrotron radiation X-ray diffraction  

NASA Astrophysics Data System (ADS)

Some terrestrial areas have climatic and geomorphologic features that favor the preservation, and therefore, accumulation of meteorites. The Atacama Desert in Chile is among the most important of such areas, known as DCA. This desert is the driest on Earth, one of the most arid, uninhabitable localities with semiarid, arid, and hyper-arid conditions. The meteorites studied here were collected from within the DCA of San Juan and Pampa de Mejillones, located, respectively, in the Central Depression and the Coastal Range of the Atacama Desert. 57Fe Mössbauer spectroscopy was used for quantitative analysis of the degree of weathering of the meteorites, through the determination of the proportions of the various Fe-bearing phases and in particular the amount of oxidized iron in terrestrial alteration products. The abundance of ferric ions in weathered chondrites can be related to specific precursor compositions and to the level of terrestrial weathering. The aim of the study was the identification, quantification, and differentiation of the weathering products in the ordinary chondrites found in the San Juan and the Pampa de Mejillones areas of the Atacama Desert. The 57Fe Mössbauer spectroscopy study was complemented by synchrotron radiation X-ray diffraction and magnetic susceptibility measurements. The results allow a clear differentiation of the rate of weathering in meteorite samples collected from the San Juan versus the Pampa de Mejillones areas of the Atacama Desert.

Munayco, Pablo; Munayco, Jimmy; Avillez, Roberto R.; Valenzuela, Millarca; Rochette, Pierre; Gattacceca, JéRôMe; Scorzelli, Rosa B.

2013-03-01

167

Optical effects of regolith processes on S asteroids as simulated by laser impulse alteration of ordinary chondrite  

NASA Technical Reports Server (NTRS)

The spectral properties of some powdered chondrites and minerals altered by Isser impulse are studied in order to estimate possible optical effects of regolith processes (micrometeoritic bombardment). Gradual reduction of overall reflectance and spectral contrast, the increase of continuum slope, the increase of spectrally derived olivine/pyroxene ratio and Fs content of orthopyroxene with increasing alteration degree show that regolith processes could affect optical properties of surface material more heavily than has been previously appreciated. Ordinary chondrites (OC's) are known to account for 80 percent of observed meteorite falls, but so far no main belt parent bodies have been identified for these meteorites. S-asteroids resemble OC's spectrally, but are characterized by a steeper red continuum unlike that of OC's and their spectrally derived mineralogies are far outside OC range. Attempts were made to explain the spectral mismatch between OC's and S asteroids by some process, which alters optical properties of uppermost regolith. However, the spectral studies of shocked (black) OC's, gas-rich OC's, melted OC's and synthetic metal-rich regoliths derived from OC's demonstrate that such altered OC materials darken, but do not redden.

Moroz, L. V.; Fisenko, A. V.; Semjonova, L. F.; Pieters, C. M.

1993-01-01

168

Partitioning of Nd, Tb, Lu, and Hf Between Garnet and Ordinary Chondrite Melt at 5 to 9 GPa: Applications to Martian Differentiation  

NASA Technical Reports Server (NTRS)

One explanation for Al and REE depletions in SNC meteorites is early majorite fractionation in a deep martian magma ocean. We report initial results from an experimental investigation of partitioning between majoritic garnet and ordinary chondrite liquid. Additional information is contained in the original extended abstract.

Draper, D. S.; Chabot, N. L.; Xirouchakis, D.; Wasserman, A. A.; Agee, C. B.

2001-01-01

169

Mineralogical characterization of primitive, type-3 lithologies in Rumuruti chondrites  

NASA Astrophysics Data System (ADS)

Rumuruti chondrites (R-chondrites) constitute a new, well-established, chondrite group different to carbonaceous, ordinary, and enstatite chondrites. Many of these samples are gas-rich regolith breccias showing the typical light/dark structure and consist of abundant fragments of various parent body lithologies embedded in a fine-grained, olivine-rich matrix. Unequilibrated type 3 lithologies among these fragments have frequently been mentioned in various publications. In this study detailed mineralogical data on seven primitive fragments from the R-chondrites Dar al Gani 013 and Hughes 030 are presented. The fragments range from about 300 ?m in size up to several millimeters. Generally, the main characteristics can be summarized as follows: a) Unequilibrated type 3 fragments have a well-preserved chondritic texture with a chondrule to matrix ratio of about 1:1. Chondrules and chondrule fragments are embedded in a fine-grained olivine-rich matrix. Thus, the texture is quite similar to that of type 3 carbonaceous chondrites. b) In all cases, matrix olivines in type 3 fragments have a significantly higher Fa-content (44-57 mol%) than olivines in other (equilibrated) lithologies (38-40 mol% Fa). c) Olivines and pyroxenes occurring within chondrules or as fragments are highly variable in composition (Fa0-65 and Fs0-33, respectively) and, generally, more magnesian than those found in equilibrated R-chondrites. Agglomerated material of the R-chondrite parent body (or bodies) was highly unequilibrated. It is suggested that the material that accreted to form the parent body consisted of chondrules and chondrule fragments, mainly having Mg-rich silicate constituents, and Fe-rich highly oxidized fine-grained materials. The dominating phase of this fine-grained material may have been Fa-rich olivine from the beginning. The brecciated whole rocks, the R-chondrite regolith breccias, were not significantly reheated subsequent to brecciation or during lithification, as indicated by negligible degree of equilibration between matrix components and Mg-rich olivines and pyroxenes in primitive, type 3 fragments.

Bischoff, Addi

2000-07-01

170

Lavras do Sul: A New Equilibrated Ordinary L5 Chondrite from Rio Grande do Sul, Brazil  

NASA Astrophysics Data System (ADS)

The new Brazilian chondrite, Lavras do Sul, was found in 1985 at Lavras do Sul, Rio Grande do Sul State-Brazil (33°30'48?S; 53°54'65?W). It consists of a single mass weighing about 1 kg, covered by a black fusion crust with grayish interior. Four polished thin sections were prepared from a slice weighing 67 g on deposit at the Museu Nacional/UFRJ. It consists mostly of chondrules and chondrule fragments dispersed in a recrystallized matrix. Most chondrules are poorly defined and range in size from 300 to 2,000 ?m, although some of them show distinct outlines, particularly when viewed under cross-polarized transmitted and reflected light. The texture of chondrules varies from non-porphyritic (e.g., barred-olivine, radial-pyroxene) to porphyritic ones (e.g., granular olivine as well as olivine-pyroxene). The meteorite contains mainly olivine (Fa24.9), low-Ca pyroxene (Fs22.6) and metal phases, with minor amounts of plagioclase, chromite and magnetite. Mössbauer Spectroscopy studies indicate that the metal phase is kamacite, tetrataenite and antitaenite. Veins of secondary iddingsite crosscut the thin section and some ferromagnesian silicates. The chemical composition indicates that Lavras do Sul is a member of the low iron L chondrite group. The poorly delineated chondritic texture with few well-defined chondrules, the occurrence of rare clinopyroxene and plagioclase (and maskelynite) with apparent diameters ranging from 5 to 123 ?m led us to classify Lavras do Sul as an equilibrated petrologic type 5. The shock features of some minerals suggest a shock stage S3, and the presence of a small amount of secondary minerals such as iddingsite and goethite, a degree of weathering W1. The meteorite name was approved by the Nomenclature Committee (Nom Com) of the Meteoritical Society (Meteoritic Bulletin Nº99).

Zucolotto, M. E.; Antonello, L. L.; Varela, M. E.; Scorzelli, R. B.; Ludka, Isabel P.; Munayco, P.; dos Santos, E.

2012-03-01

171

Ortho- and clinopyroxene compositions in ordinary chondrites and related blander model calculation procedures  

NASA Technical Reports Server (NTRS)

Chemical analyses of the orthopyroxene and clinopyroxene compositions in chondrites are reported. Standard microprobe techniques to 15 kilovolts, 0.03 microamperes, and 40 second counting time were employed. Duplicate analyses were conducted on three grains of each opx and cpx together with two different raw data correction methods as checks on analytical precision and correction procedures. Only those analytical summations of between 99.20 and 100.80 weight percent, and cation summations between 3.980 and 4.020 (based on 6 oxygens) were used.

Bunch, T. E.; Olsen, E.

1973-01-01

172

Silica-merrihueite/roedderite-bearing chondrules and clasts in ordinary chondrites: New occurrences and possible origin  

NASA Technical Reports Server (NTRS)

Merrihueite (K,Na)2(Fe,Mg)5Si12O30 (na less than 0.5, fe greater than 0.5, where na = Na/(Na + K), fe = Fe/(Fe + Mg) in atomic ratio) is a rare mineral described only in several chondrules and irregularly-shaped fragments in the Mezo-Madaras L3 chondrite (Dodd et al., 1965; Wood and Holmberg, 1994). Roedderite (Na,K)2(Mg,Fe)5Si12O30 (na greater than 0.5, fe less than 0.5) has been found only in enstatite chondrites and in the reduced, subchondritic silicate inclusions in IAB irons (Fuchs, 1966; Rambaldi et al., 1984; Olsen, 1967). We described silica-roedderite-bearing clasts in L/LL3.5 ALHA77011 and LL3.7 ALHA77278, a silica-roedderite-bearing chondrule in L3 Mezo-Madaras, and a silica-merrihueite-bearing chondrule in L/LL3.5 ALHA77115. The findings of merrihueite and roedderite in ALHA77011, ALHA77115, ALHA77278 and Mezo-Madaras fill the compositional gap betweeen previously described roedderite in enstatite chondrites and silicate inclusions in IAB irons and merrihueite in Mezo-Madaras, suggesting that there is a complete solid solution of roedderite and merrihueite in meteorites. We infer that the silica- and merrihueite/roedderite-bearing chondrules and clasts experienced a complex formational history including: (a) fractional condensation in the solar nebular that produced Si-rich and Al-poor precursors, (b) melting of fractionated nebular solids resulting in the formation of silica-pyroxene chondrules, (c) in some cases, fragmentation in the nebula or on a parent body, (d) reaction of silica with alkali-rich gas that formed merrihueite/roedderite on a parent body, (e) formation of fayalitic olivine and feerosilite-rich pyroxene due to reaction of silica with oxidized Fe on a parent body, and (f) minor thermal metamorphism, possibly generated by impacts.

Krot, Alexander N.; Wasson, John T.

1994-01-01

173

The iodine-xenon system in clasts and chondrules from ordinary chondrites: Implications for early solar system chronology  

NASA Astrophysics Data System (ADS)

We have studied the iodine-xenon system in chondrules and clasts from ordinary chondrites. Cristobalite bearing clasts from Parnallee (LL3.6) closed to xenon loss 1-4 Ma after Bjurböle. Feline (a feldspar and nepheline rich clast also from Parnallee) closed at 7.04 +/- 0.15 Ma. 2 out of 3 chondrules from Parnallee that yielded well defined initial iodine ratios gave ages identical to Bjurböle's within error. A clast from Barwell (L5) has a well-defined initial iodine ratio corresponding to closure 3.62 +/- 0.60 Ma before Bjurböle. Partial disturbance and complete obliteration of the I-Xe system by shock are revealed in clasts from Julesburg (L3.6) and Quenggouk (H4) respectively. Partial disturbance by shock is capable of generating anomalously high initial iodine ratios. In some cases these could be misinterpreted, yielding erroneous ages. A macrochondrule from Isoulane-n-Amahar contains concentrations of iodine similar to 'ordinary' chondrules but, unlike most ordinary chondrules, contains no radiogenic 129Xe. This requires resetting 50 Ma or more later than most chondrules. The earliest chondrule ages in the I-Xe, Mn-Cr and Al-Mg systems are in reasonable agreement. This, and the frequent lack of evidence for metamorphism capable of resetting the I-Xe chronometer, leads us to conclude that (at least) the earliest chondrule I-Xe ages represent formation. If so, chondrule formation took place at a time when sizeable parent bodies were present in the solar system.

Gilmour, J. D.; Whitby, J. A.; Turner, G.; Bridges, J. C.; Hutchison, R.

2000-05-01

174

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

NASA Astrophysics Data System (ADS)

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, M. A.; Long, D. T.; Gooding, J. L.

1991-01-01

175

An Ordinary Chondrite Impactor Composition for the Bosumtwi Impact Structure, Ghana, West Africa: Discussion of Siderophile Element Contents and Os and Cr Isotope Data  

NASA Technical Reports Server (NTRS)

Osmium isotope data had shown that Ivory Coast tektites contain an extraterrestrial component, but do not allow distinction between chondritic and iron meteorite contamination. PGE abundances of Ivory Coast tektites and impactites and target rocks from the Bosumtwi crater, the source crater of the Ivory Coast tektites, were all relatively high and did not allow to resolve the presence, or identify the nature, of the meteoritic component. However, Cr isotope analyses of an Ivory Coast tektite yielded a distinct 53Cr excess of 0.30+/-0.06, which indicates that the Bosumtwi impactor was an ordinary chondrite.

Koeberl, Christian; Shukolyukov, Alex; Lugmair, Guenter

2004-01-01

176

Investigation of the H7 ordinary chondrite, Watson 012: Implications for recognition and classification of Type 7 meteorites  

NASA Astrophysics Data System (ADS)

Despite the fact that the number of officially classified meteorites is now over 45,000, we lack a clearly defined sequence of samples from a single parent body that records the entire range in metamorphic temperatures from pristine primitive meteorites up to the temperatures required for extensive silicate partial melting. Here, we conduct a detailed analysis of Watson 012, an H7 ordinary chondrite, to generate some clarity on the textural and chemical changes associated with equilibrium-based silicate partial melting in chondritic meteorites. To do this we compare the textures in the meteorite with those preserved in metamorphic contact aureoles on Earth. The most distinctive texture generated by the partial melting that affected Watson 012 is an extensively interconnected plagioclase network, which is clearly observable with a petrographic microscope. Enlarged metal-troilite grains are encapsulated at widenings in this plagioclase network, and this is clearly visible in reflected light. Together with these features, we define a series of other characteristics that can be used to more clearly classify chondritic meteorites as being of petrologic Type 7. To provide comprehensive evidence of silicate partial melting and strengthen the case for using simple petrographic observations to classify similar meteorites, we use high-resolution X-ray computed tomography to demonstrate that the plagioclase network has a high degree of interconnectedness and crystallised as large (cm-scale) skeletal crystals within an olivine-orthopyroxene-clinopyroxene framework, essentially pseudomorphing a melt network. Back-scattered electron imaging and element mapping are used to show that some of the clino- and orthopyroxene in Watson 012 also crystallised from silicate melt, and the order of crystallisation was orthopyroxene ? clinopyroxene ? plagioclase. X-ray diffraction data, supported by bulk geochemistry, are used to show that plagioclase and ortho- and clinopyroxene were added to the Watson 012 sample by through-flowing basaltic melt. Along with the absence of glass and granophyre, this interconnected network of coarse-grained skeletal plagioclase indicates that the sample cooled slowly at depth within the parent body. The evidence of melt flux indicates that Watson 012 formed in the presence of a gravitational gradient, and thus at significant distance from the centre of the H chondrite parent body (the gravitational gradient at the centre would be zero). Our interpretation is that incipient silicate partial melting in Watson 012 occurred when a region of radiogenically heated H6 material located at considerable depth (possibly at ?15-20 km from surface) was heated by an additional ca. 200-300 °C in association with a large shock event. Due to insulation at depth within an already hot parent body, the post-shock temperature equilibrated and remained above the solidus long enough for widespread equilibrium-based silicate partial melting, and for melt to migrate. Although the observed melting may have been facilitated by additional heating from an impact event, this is not an example of instantaneous shock melting, which produces thermal disequilibrium at short length scales and distinctly different textures. A small number of H, L and LL chondrites have been previously classified as being of petrologic Type 7; with our new criteria to support that classification, these represent our best opportunity to explore the transition from high temperature sub-solidus metamorphism through the onset of silicate partial melting in three different parent bodies.

Tait, Alastair W.; Tomkins, Andrew G.; Godel, Bélinda M.; Wilson, Siobhan A.; Hasalova, Pavlina

2014-06-01

177

Impact Melting of Ordinary Chondrite Regoliths and the Production of Fine-grained Fe(sup 0)  

NASA Technical Reports Server (NTRS)

The detailed study of individual lunar soil grains provides evidence that the major optical properties of the lunar surface are primarily related to the production of fine-grained (< 20 nm, super-paramagnetic) Fe-particles in agglutinitic impact melts and to iron-rich vapor deposits on the surfaces of individual grains. These Fe-rich materials are derived from oxidized species due to high post-shock temperatures in the presence of solar-wind derived H2; part of the Fe-rich grain surfaces may also be due to sputtering processes. Identical processes were recently suggested for the optical maturation of S-type asteroid surfaces, the parent objects of ordinary chondrites (OCs). OCs, however, do not contain impact-produced soil melts, and should thus also be devoid of impact-triggered vapor condensates. The seeming disparity can only be understood if all OCs resemble relatively immature impact debris, akin to numerous lunar highland breccias. It is possible to assess this scenario by evaluating experimentally whether impact velocities of 5- 6 km/s, typical for the present day asteroid belt, suffice to produce both impact melts and fine-grained metallic iron. We used 125-250 m powders of the L6 chondrite ALH85017. These powders were aliquots from fines that were produced by collisionally disrupting a single, large (461g) chunk of this meteorite during nine impacts and by subjecting the resulting rubble to an additional 50 impacts. As a consequence, the present shock-recovery experiments employ target materials of exceptional fidelity (i.e., a real chondrite that was impact pulverized). The target powders were packed into tungsten-alloy containers to allow for the potential investigation of freshly produced, fine-grained iron and impacted by stainless-steel and tungsten flyer plates; the packing density varied between 38 and 45% porosity. Peak pressures ranged from 14.5 to 67 GPa and were attained after multiple reverberations of the shock wave at the interface of the silicate powder and metal container. Pressures in the 50 to 70 GPa range should be fairly typical for asteroid impacts at approx. 5-6 km/s, yet we note that these pressures refer to those at the projectile/target interface only and that most crater ejecta on OC parent-bodies will have experienced much lower stresses.

Hoerz, Friedrich; Cintala, Mark J.; See, Thomas H.

2003-01-01

178

A Quantitative NMR Analysis of Phosphorus in Carbonaceous and Ordinary Chondrites  

NASA Technical Reports Server (NTRS)

Phosphorus is important in a number of biochemical molecules, from DNA to ATP. Early life may have depended on meteorites as a primary source of phosphorus as simple dissolution of crustal apatite may not produce the necessary concentration of phosphate. Phosphorus is found in several mineral phases in meteorites. Apatite and other Ca- and Mg phosphate minerals tend to be the dominant phosphorus reservoir in stony meteorites, whereas in more iron-rich or reduced meteorites, the phosphide minerals schreibersite, (Fe, Ni)3P, and perryite, (Ni, Fe)5(Si, P)2 are dominant. However, in CM chondrites that have experienced significant aqueous alteration, phosphorus has been detected in more exotic molecules. A series of phosphonic acids including methyl-, ethyl-, propyl- and butyl- phosphonic acids were observed by GC-MS in Murchison. Phosphorian sulfides are in Murchison and Murray. NMR spectrometry is capable of detecting multiple substances with one experiment, is non-destructive, and potentially quantitative, as discussed below. Despite these advantages, NMR spectrometry is infrequently applied to meteoritic studies due in large part to a lack of applicability to many compounds and the relatively high limit of detection requirements. Carbon-13 solid-state NMR has been applied to macromolecular carbon in Murchison. P-31 NMR has many advantages over aqueous carbon-13 NMR spectrometry. P-31 is the only isotope of phosphorus, and P-31 gives a signal approximately twice as strong as C-13. These two factors together with the relative abundances of carbon and phosphorus imply that phosphorus should give a signal approximately 20 as strong as carbon in a given sample. A discussion on the preparation of the quantitative standard and NMR studies are presented

Pasek, M. A.; Smith, V. D.; Lauretta, D. S.

2004-01-01

179

Esperance: A New Type 3 L/LL Ordinary Chondrite from Western Australia  

NASA Astrophysics Data System (ADS)

The following article is a preliminary commentary on the petrographic characteristics found in our investigation of the recently discovered Esperance chondrite. The material on which this study is based consists of a thin section made from the near-surface portion of the meteorite, combined with a small sample displaying its exterior features. A thin, dull, dark brown fusion crust is present on one side of the small specimen, while the remainder of the sample shows a highly weathered and irregular interior surface with some cracking and numerous exposed chondrules and cavities. In thin section, however, the weathering effects are limited and mainly restricted to a superficially light brown staining on silicates and around metal, close to the penetrating fractures that traverse the sample. These fractures are filled with oxidation products and are a continuation of those seen in the hand sample. The thin section shows a close-packed aggregate of sharply defined chondrules and chondrule fragments set in a dark opaque matrix. A great variety of chondrule types are present with sizes up to approx. 3 mm in diameter, ranging in form from perfectly spherical to rather irregular shapes. Several examples of chondrules within chondrules as well as rimmed, armored, and dark-zoned varieties are well-displayed features seen in thin section. Other prominent characteristics are transparent pale brown glass within many chondrules and abundant polysynthetically twinned pyroxenes and zoned olivines. Metal alloy and iron sulphide, the dominant opaque mineral phases, occur within chondrules, as chondrule rims or in the matrix. Although they occupy the same mode of occurrence they are rarely seen in intimate intergrowth. A large fraction of the metal consists of several up to mm sized interstitial grains. The remaining metal and the iron sulphide both occur finely dispersed throughout the section associated with small but abundant chromite and phosphate grains and rare ilmenite. Chromite is also found within chondrules associated with both olivine and pyroxene. Within the matrix, following a rather straight narrow zone, numerous small globules of iron sulphide and metal result in thin, more or less interconnecting veins. These opaque veins are exclusively confined to matrix regions, although small globules are often seen in connection with aggregates that make up opaque chondrule rims. Quite often small vein-like strings are seen to occur in the immediate vicinity of large interstitial opaque grains. These rapidly thin out in abundance and size, a short distance away from the larger grains. Locally melt pockets are evident, either as completely isolated enclaves or as junctions between melt veins. Both melt pockets and the thin veins can be distinguished from the dark matrix by their slightly lighter colour. These features, combined with many of the larger olivine crystals showing sets of parallel planar fractures and undulatory to mosaic extinction, indicate a degree of shock deformation. According to the petrographic classification of progressive stages of shock metamorphism by Stoffler, Keil, and Scott (1991), the Esperance can at present be placed in the catergories of weakly to moderately shocked chondrites (S3-S4). A microprobe investigation of the silicate fraction of the meteorite has revealed a pronounced chemical zoning of both olivines and pyroxenes in many chondrules. Measured Fa variations in single olivine crystals have, in the most extreme cases, been found to range from Fa=5,9% in the core, to Fa=25,8% at the rim. The highest Fa content encountered at present is 27,6% in a homogeneous matrix crystal. A frequency plot of rim and matrix olivine Fa contents, groups around a peak of 25%. The chemical composition of Fs contents in pyroxenes are seen to range continuously from 0,6% to 29,2%. Several low-Ca pyroxenes show a notable Fe-Mg zoning that, however, does not exceed approx. 10% difference from core to rim. Frequently chemical compositions consistent with augite, endiopside and pigeonite are seen as overgrowths on the low-Ca

Davis, D. E.; Petersen, A. K.

1992-07-01

180

Chemical and physical studies of type 3 chondrites. IV Annealing studies of a type 3.4 ordinary chondrite and the metamorphic history of meteorites  

NASA Astrophysics Data System (ADS)

Samples of a type 3.4 chondrite have been annealed at 400 - 1000°C for 1 - 200 hours, their thermoluminescence properties determined and analyzed for K, Na, Mn, Sc and Ca by instrumental neutron activation analysis.

Guimon, R. K.; Keck, B. D.; Weeks, K. S.; Dehart, J.; Sears, D. W. G.

1985-07-01

181

Chromite and olivine in type 2 chondrules in carbonaceous and ordinary chondrites: Implications for thermal histories and group differences  

SciTech Connect

Analyses of chromite and olivine in type 2 chondrules in CM, CO, CV, CR, H, L, LL, and two ungrouped chondrites indicate that chromites are extremely sensitive indicators of thermal metamorphism. Chromite-olivine pairs in chondrites of petrographic types {le}3.0 have iron-magnesium partitioning characteristic of phsases which crystallized from silicate melts at temperatures > 1,400C; paris in chondrites of types > 3.0 have partitioning characteristic of reequilibration at lower temperatures. With the possible exception of the CR group, chondrite chemical groups underwent either substantial aqueous alteration (CM) or sufficient heating to reset type 2 chromites, but not both. The CR parent body may have been heated following aqueous alteration, although our data are insufficient to support a firm conclusion. Chromite and olivine compositions differ systematically among three of the chondrite groups, which suggest that type 2 melts differed in composition. Whether differences existed among the other groups is uncertain.

Johnson, C.A.; Prinz, M. (American Museum of Natural History, New York, NY (United States))

1991-03-01

182

Chromite-Plagioclase Assemblages as a New Shock Indicator; Implications for the Shock and Thermal Histories of Ordinary Chondrites  

NASA Technical Reports Server (NTRS)

Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL chondrites shows that unshocked chromite grains occur in equant, subhedral and rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted chromite grains are fractured or crushed during whole-rock brecciation. Others are transected by opaque veins; the veins form when impacts cause localized heating of metal-troilite intergrowths above the Fe-FeS eutectic (988 C), mobilization of metal-troilite melts, and penetration of the melt into fractures in chromite grains. Chromite-plagioclase assemblages occur in nearly every shock-stage S3-S6 OC; the assemblages range in size from 20-300 microns and consist of 0.2-20-micron-size euhedral, subhedral, anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition. Plagioclase has a low impedance to shock compression. Heat from shock-melted plagioclase caused adjacent chromite grains to melt; chromite grains crystallized from this melt. Those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in Al2O3, than unmelted, matrix chromite grains in the same meteorite. Chromite veinlets (typically 0.5-2 microns thick and 10-300 microns long) occur typically in the vicinity of chromite-plagioclase assemblages. The veinlets formed from chromite-plagioclase melts that were injected into fractures in neighboring silicate grains; chromite crystallized in the fractures and the residual plagioclase-rich melt continued to flow, eventually pooling to form plagioclase-rich melt pockets. Chromite-rich chondrules (consisting mainly of olivine, plagioclase-normative mesostasis, and 5-15 vol.% chromite) occur in many shocked OC and OC regolith breccias but they are absent from primitive type-3 OC. They may have formed by impact melting chromite, plagioclase and adjacent mafic silicates during higher-energy shock events. The melt was jetted from the impact site and formed droplets due to surface tension. Crystallization of these droplets may have commenced in flight, prior to landing on the parent-body surface. Chromite-plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock-stage S1 OC of petrologic type 5 and 6 that I examined. Although these rocks contain unstrained olivine with sharp optical extinction, most possess other shock indicators such as extensive silicate darkening, numerous occurrences of metallic Cu, polycrystalline troilite, and opaque veins. It seems likely that these rocks were shocked to levels at least as high as shock-stage S3 and then annealed by heat generated during the shock event. During annealing, the olivine crystal lattices healed but other shock indicators survived. Published Ar-Ar age data for some SI OC indicate that many shock and annealing events occurred very early in the history of the parent asteroids. The common occurrence of shocked and annealed OC is consistent with collisions being a major mechanism responsible for metamorphosing OC.

Rubin, Alan E.

2006-01-01

183

Fe-Ni metal and sulfide minerals in CM chondrites: An indicator for thermal history  

USGS Publications Warehouse

CM chondrites were subjected to aqueous alteration and, in some cases, to secondary metamorphic heating. The effects of these processes vary widely, and have mainly been documented in silicate phases. Herein, we report the characteristic features of Fe-Ni metal and sulfide phases in 13 CM and 2 CM-related chondrites to explore the thermal history of these chondrites. The texture and compositional distribution of the metal in CM are different from those in unequilibrated ordinary and CO chondrites, but most have similarities to those in highly primitive chondrites, such as CH, CR, and Acfer 094. We classified the CM samples into three categories based on metal composition and sulfide texture. Fe-Ni metal in category A is kamacite to martensite. Category B is characterized by pyrrhotite grains always containing blebs or lamellae of pentlandite. Opaque mineral assemblages of category C are typically kamacite, Ni-Co-rich metal, and pyrrhotite. These categories are closely related to the degree of secondary heating and are not related to degree of the aqueous alteration. The characteristic features of the opaque minerals can be explained by secondary heating processes after aqueous alteration. Category A CM chondrites are unheated, whereas those in category B experienced small degrees of secondary heating. CMs in category C were subjected to the most severe secondary heating process. Thus, opaque minerals can provide constraints on the thermal history for CM chondrites. ?? The Meteoritical Society, 2011.

Kimura, M.; Grossman, J. N.; Weisberg, M. K.

2011-01-01

184

Chondrules in the bishunpur L13 chondrite  

NASA Technical Reports Server (NTRS)

Twenty-six chondrules, chondrule fragments or clasts were analyzed. An automated wavelength dispersive instrument with a 90 micrometer beam integrated a series of analyses in traverses across each object. Depending on the size of the cross-sectional area, from 1 to 21 analyses were performed for each bulk analysis. Si, Ti, Al, Cr, Fe, Ni, Mn, Mg, Ca, Na, K and S were determined, and an analyzed augite was used as a secondary standard before and after each set of analyses. The work is part of a study of chondrule rims, interchondrule matrix, chondrules and clasts in unequilibrated ordinary chondrites. Only two of the twenty-six objects analyzed have Ca/Al atomic ratios greater than the ordinary chondritic average of 0.74. The bulk meteorite has a normal Ca/Al ratio, so presumably a Ca-rich, Al-poor component must be present to compensate for chondrules and chlasts. This component is unlikely to be rim or matrix, but may be phosphate associated with metal or sulphide. Na/Al ratios range from 1 to almost zero, but there is no hiatus as in a suite of Manych chondrules and glasses.

Hutchison, R.; Alexander, C.; Barber, D. J.

1984-01-01

185

The Search for Meterorites with Complex Exposure Histories Amoung Ordinary Chondrites with Low 3HE/21NE Ratios  

SciTech Connect

In calculating cosmic-ray exposure ages of meteorites it is generally assumed that the meteoroids were expelled from a shielded position within their parent body and then experienced a single stage exposure before colliding with Earth. The combination of noble gas and radionuclide measurements in several large meteorites, such as Jilin and Bur Ghelaui, have revealed complex exposure histories: i.e. an initial exposure on the surface of an asteroid (or within meter-sized meteoroid), followed by a second exposure as a smaller object. In fact, orbital dynamics calculations predicted that at least 30% of the meteorites arriving on Earth experienced two- or multiple-stage exposure histories [1]. More recently, after the recognition that the Yarkovsky effect plays an important role in delivering meteorites from the asteroid belt to Earth-crossing orbits, it was confirmed that complex exposure histories should be common [2]. Nevertheless, despite the ability to measure a wide range of radionuclides with accelerator mass spectrometry (AMS), only a few meteorites with complex exposure histories have been identified [e.g. 3,4]. The question is whether the relatively paucity of complex exposure histories is real or have we simply overlooked complex-exposure histories. In this work we focus on meteorites with low {sup 3}He/{sup 21}Ne ratios, since it is known that most meteorites with complex exposure histories have relatively low {sup 3}He/{sup 21}Ne ratios, i.e. the {sup 3}He/{sup 21}Ne ratio is below the ''Bern-line''. Several hypotheses have been suggested for these low {sup 3}He/{sup 21}Ne ratios, including solar heating in low-perihelion orbits, shock-related diffusion of He during the collision that ejected the meteoroid, or an artifact of high shielding conditions [4]. The first two hypotheses seem to be supported by low radiogenic {sup 4}He concentrations in samples with low {sup 3}He, whereas Monte Carlo calculations have shown that some of the low {sup 3}He/{sup 21}Ne ratios may be due to high shielding conditions in objects with radii > 1m [5]. To elucidate these issues, we selected 15 samples with known noble gas concentrations [6] for radionuclide studies and obtained aliquots of the samples adjacent to those measured for noble gases. The specific goal is the identification of complex exposure histories among samples having low {sup 3}He/{sup 21}Ne ratios. All samples have {sup 3}He deficiencies of >20% relative to the ''Bern-line'' (Table 1). Most of the selected samples also have low {sup 22}Ne/{sup 21}Ne ratios ({le}1.1), indicative of high shielding during most of their cosmic-ray exposure (Table 1), whereas one sample (Suizhou) was selected because of its relatively low {sup 81}Kr concentration [7]. In addition, we selected QUE 93021, for which initial radionuclide results suggested a short exposure age. Here we present cosmogenic {sup 10}Be, {sup 26}Al and {sup 36}Cl in stone and metal fractions for the 16 ordinary chondrites listed in Table 1.

Welton, K C; Nishiizumi, K; Caffee, M W

2001-04-30

186

Carbon-rich Chondritic Clast PV1 from the Plainview H-chondrite Regolith Formation from H3 Chondrite Material by Possible Cometary Impact  

NASA Technical Reports Server (NTRS)

Chondritic clast PV1 from the Plainview H-chondrite regolith breccia is a subrounded, 5-mm diameter unequilibrated chondritic fragment that contains 13 wt% C occurring mainly within irregularly shaped 30-400-micron-size opaque patches. The clast formed from H3 chondrite material as indicated by the mean apparent chondrule diameter (310 micron vs. approximately 300 micron in H3 chondrites), the mean Mg-normalized refractory lithophile abundance ratio (1.00 +/- 0.09 XH), the previously determined 0-isotopic composition (Delta O-17 = 0.66% vs. 0.68 +/- 0.04%0 in H3 chondrites and 0.73 +/- 0.09% in H4-6 chondrites), the heterogeneous olivine compositions in grain cores (with a minimum range of Fal-19), and the presence of glass in some chondrules. Although the clast lacks the fine-grained, ferroan silicate matrix material present in type 3 ordinary chondrites, PV1 contains objects that appear to be recrystallized clumps of matrix material. Similarly, the apparent dearth of radial pyroxene and cryptocrystalline chondrules in PV1 is accounted for by the presence of some recrystallized fragments of these chondrule textural types. All of the chondrules in PV1 are interfused indicating that temperatures must have briefly reached approximately 1100C (the approximate solidus temperature of H-chondrite silicate). The most likely source of this heating was by an impact. Some metal was lost during impact heating as indicated by the moderately low abundance of metallic Fe-Ni in PV1 (approximately 14 wt%) compared to that in mean H chondrites (approximately 18 wt%). The carbon enrichment of the clast may have resulted from a second impact event, one involving a cometary projectile, possibly a Jupiter-family comet. As the clast cooled, it experienced hydrothermal alteration at low water/rock ratios as evidenced by the thick rims of ferroan olivine around low-FeO olivine cores. The C-rich chondritic clast was later incorporated into the H-chondrite parent-body regolith and extensively fractured and faulted.

Rubin, Alan E.; Trigo-Rodriguez, Josep M.; Kunihiro, Takuya; Kallemeyn, Gregory W.; Wasson, John T.

2006-01-01

187

Comparison of Nickel XANES Spectra and Elemental Maps from a Ureilite, a LL3.8 Ordinary Chondrite, Two Carbonaceous Chondrites and Two Large Cluster IDPs  

NASA Technical Reports Server (NTRS)

Nickel in the extraterrestrial world is commonly found in both Fe-Ni sulfide and Fe-Ni met-al forms [1] and in the pure metal state in the interior of iron meteorites where it is not easily oxidized. Ni is also found in olivine, pyroxene and glasses and in some melts the partitioning of Ni between the olivines and glass is controlled by the amount of S in the melt [2]. Its most common valence state is Ni(2+) but Ni also occurs as Ni(0), Ni(+), and Ni(3+) and rarely as Ni(2-), Ni(1-) and Ni(4+) [3]. It's valence state in olivines is Ni(2+) in octa-hedral coordination on the M1 site and rarely on the M2 site.[4]. The chemical sensitivity of X-ray absorp-tion near-edge structure (XANES) spectroscopy is well established and can be used to determine not only va-lence states but also coordination sites [5]. We report here Ni XANES spectroscopy and elemental maps collected from 2 carbonaceous chondrites, 2 large clus-ter IDPs, 1 ureilite and 1 LL3 orginary chondrite.Using XANES it may be possible to find a common trait in the large cluster IDPs that will also be found in mete-orite samples.

Wirick, S.; Flynn, G. J.; Sutton, S.; Zolensky, M. E.

2014-01-01

188

Alkali-granitoids as fragments within the ordinary chondrite Adzhi-Bogdo: Evidence for highly fractionated, alkali-granitic liquids on asteroids  

NASA Technical Reports Server (NTRS)

Adzhi-Bogdo is an ordinary chondrite regolith breccia (LL3-6) that fell October 30, 1949 in Gobi Altay, Mongolia. The rock consists of submm- to cm-sized fragments embedded in a fine-grained elastic matrix. The breccia contains various types of clasts, some of which must be of foreign heritage. Based on chemical compositions of olivine some components have to be classified as L-type. Components of the breccia include chondrules, impact melts (some are K-rich, similar to those found in other LL-chondrites, highly recrystalized rock fragments ('granulites'), pyroxene-rich fragments with achondritic textures, and alkali-granitoidal fragments that mainly consist of K-feldspar and quartz or tridymite. Probably, this is the first report on granitoids from asteroids. It can be ruled out that these fragments represent huge rock assemblages of the parent body like granites do on Earth. Therefore, to avoid misunderstandings, these rocks will be designated as granitoids. In one thin section four granitoids were observed. The main phases within these clasts are K-feldspar and SiO2-phases. Minor phases include albite, Cl-apatite, whitlockite, ilmenite, zircon, Ca-poor pyroxene, and an unidentified Na,Ti-bearing silicate. Based on chemical composition and on optical properties quartz appears to be the SiO2-phase in two fragments, whereas tridymite seems to occur in the other two. The calculated formula of the unknown Na,Ti-rich silicate is very close to (Na,Ca)2.7(Fe,Mg)6(Ti)1.3(Si)7(O)24. Quartz and K-feldspar can reach sizes of up to 700 microns. Thus, the fragments can be described as coarse-grained (by chondritic standards). This is especially the case considering that quartz and K-feldspar are very rare minerals in ordinary chondrites. Representative analyses of minerals from some granitoidal clasts are given. Based on the mineral compositions and the modal abundances the bulk compositions were calculated. Besides these granitoidal rocks, pyroxene-rich fragments occur that show exsolution textures that are similar to those found in eucrites.

Bischoff, A.

1993-01-01

189

Using the Fe/Mn Ratio of FeO-Rich Olivine In WILD 2, Chondrite Matrix, and Type IIA Chondrules to Disentangle Their Histories  

NASA Technical Reports Server (NTRS)

The Stardust Mission returned a large abundance of impactors from Comet 81P/Wild2 in the 5-30 m range. The preliminary examination of just a limited number of these particles showed that the collection captured abundant crystalline grains with a diverse mineralogy [1,2]. Many of these grains resemble those found in chondrite matrix and even contain fragments of chondrules and CAIs [1-3]. In particular, the olivine found in Wild 2 exhibits a wide compositional range (Fa0-97) with minor element abundances similar to the matrix olivine found in many carbonaceous chondrites (CCs) and unequilibrated ordinary chondrites (UOCs). Despite the wide distribution of Fa content, the olivine found in the matrices of CCs, UOCs, and Wild 2 can be roughly lumped into two types based solely on fayalite content. In fact, in some cases, a distinct bi-modal distribution is observed.

Frank, David R.; Le, L.; Zolensky, M. E.

2012-01-01

190

Terrestrial weathering of ordinary chondrites in nature and continuing during laboratory storage and processing: Review and implications for Hayabusa sample integrity  

NASA Astrophysics Data System (ADS)

Abstract- The Hayabusa mission recently returned the first samples from an ordinary chondrite (OC) parent body. Olivine, low-Ca pyroxene, and kamacite compositions fall within the known ranges of minerals from LL4 to LL6 chondrites. Hayabusa samples are being processed and stored in a pure N2 atmosphere. However, during recovery, prior to receiving, and during preliminary examination, some Hayabusa samples were briefly exposed to terrestrial atmosphere. Some of the minerals already identified in the Hayabusa samples (olivine, sulfides) are known to be among the most vulnerable to weathering reactions in moist, oxidizing terrestrial environments. Oxidation of Fe in metal, sulfides, and ferrous silicates is ubiquitous in naturally weathered OC finds, in samples of falls subjected to even a few decades of weathering before recovery, and in OC falls recovered and curated promptly after recovery. All prerecovery oxidation, hydrolysis, hydration, and product-forming phenomena documented to affect OC finds have been documented to continue in OC samples in curatorial and laboratory settings, producing mineralogical and textural effects at scales easily discernable by electron microscopy, on timescales of decades. Hayabusa samples will be exposed to similar terrestrial conditions at times throughout sample processing, allocation, and examination. Maximizing the science yield from these important samples requires thorough understanding of how LL chondrite minerals like those in the Hayabusa samples react with terrestrial moisture and oxidants in support of proper planning for maintaining Hayabusa sample integrity after allocation, and for proper anticipation of the effects of inevitable exposure to Earth's atmosphere during storage and examination in terrestrial analytical laboratories.

Velbel, Michael A.

2014-02-01

191

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  

NASA Technical Reports Server (NTRS)

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.; Wright, I. P.; Pillinger, C. T.; Gibson, E. K., Jr.

1989-01-01

192

Impact experiments of porous gypsum-glass bead mixtures simulating parent bodies of ordinary chondrites: Implications for re-accumulation processes related to rubble-pile formation  

NASA Astrophysics Data System (ADS)

Laboratory impact experiments were conducted for gypsum-glass bead targets simulating the parent bodies of ordinary chondrites. The effects of the chondrules included in the parent bodies on impact disruption were experimentally investigated in order to determine the impact conditions for the formation of rubble-pile bodies after catastrophic disruption. The targets included glass beads with a diameter ranging from 100 ?m to 3 mm and the volume fraction was 0.6, similar to that of ordinary chondrites, which is about 0.65-0.75. Nylon projectiles with diameters of 10 mm and 2 mm were impacted at 60-180 m s -1 by a single-stage gas gun and at 4 km s -1 by a two-stage light gas gun, respectively. The impact strength of the gypsum-glass bead target was found to range from 56 to 116 J kg -1 depending on the glass bead size, and was several times smaller than that of the porous gypsum target, 446 J kg -1 in low-velocity collisions. The impact strengths of the 100 ?m bead target and the porous gypsum target strongly depended on the impact velocity: those obtained in high-velocity collisions were several times greater than those obtained in low-velocity collisions. The velocities of fragments ejected from two corners on the impact surface of the target, measured in the center of the mass system, were slightly dependent on the target materials, irrespective of impact velocity. These results suggest that chondrule-including planetesimals (CiPs) can reconstruct rubble-pile bodies in catastrophic disruptions at the size of the planetesimal smaller than that of planetesimals without chondrules.

Yasui, Minami; Arakawa, Masahiko

2011-08-01

193

Cumberland Falls chondritic inclusions. II - Trace element contents of forsterite chondrites and meteorites of similar redox state  

NASA Astrophysics Data System (ADS)

Mineralogic study of black inclusions in the Cumberland Falls enstatite achondrite revealed that they constitute a highly unequilibrated chondritic suite distinct from other chondrite groups. The authors analyzed these samples and possibly related meteorites for Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn, trace elements known to yield important genetic information.

Verkouteren, R. M.; Lipschutz, M. E.

1983-09-01

194

Origin of iron-rich olivine in the matrices of type 3 ordinary chondrites - an experimental study  

NASA Astrophysics Data System (ADS)

The reaction of metallic iron and enstatite, with and without forsterite and SiO2, is experimentally studied at temperatures between 1150 and 800 C in order to investigate the origin of iron-rich olivine in the matrices of type 3 chondrites. The composition of olivine is shown to become more iron-rich with increasing silica/enstatite ratio. Possible scenarios for the origin of this olivine include: (1) free silica having been present if the iron-rich olivine was formed by solid-state reactions under oxidizing conditions in the solar nebula; and (2) the reaction of silicon-rich gas with metallic iron having taken place under oxidizing conditions in the solar nebula.

Nagahara, H.; Kushiro, I.

1987-10-01

195

The S(IV)-type Asteroids as Ordinary Chondrite Parent Body Candidates: Implications for the Completeness of the Meteorite Sample of Asteroids  

NASA Astrophysics Data System (ADS)

The discrepancy between the abundance of ordinary chondrites (OCs) among the meteorites and the rarity of unambiguously similar assemblages in the asteroid belt has been a major point of discussion within and between the asteroid and meteorite communities. Various resolutions to this apparent paradox have been proposed [e.g., 1-5], including: 1) interpretations of S-type asteroid spectra are incorrect due to space weathering effects; 2) ordinary chondrites derive from a few rare but favorably situated parent bodies; 3) OCs come from a residual population of small unheated mainbelt asteroids; 4) shock effects darken OC parent body surfaces disguising them as C-type asteroids, and 5) OCs come from inner solar system planetesimals ejected to the Oort cloud which have been recently perturbed into Earth-crossing orbits. Although none of these possibilities has yet been rigorously excluded, recent investigations suggest that the resolution of the apparent paradox lies in some combination of the first three options. For option 3, the discovery of a small mainbelt asteroid with an OC-like spectrum indicates OC-assemblages among the smaller mainbelt asteroids [6], although their abundance is still low in the current sample [7]. For option 2, the mineralogical survey indicated that while most S-asteroids could be rigorously excluded on mineralogical criteria, the S(IV) subtype of this class has silicate compositions within the OC range [8]. The S(IV)-objects are concentrated near the 3:1 secular resonance at 2.5 AU providing an efficient escape into Earth-crossing orbits. Unfortunately for a simple resolution of the OC parent body question, S(IV) spectra still exhibit weaker silicate features and redder spectral slopes than OC assemblages. Although significant uncertainties remain, optical alteration of asteroid surfaces interpreted from the Galileo images of Ida and Gaspra may reconcile the mismatch between OC and S(IV) spectra [option 1]. Although only a subset of the S(IV) objects are viable OC-parent bodies [3 Juno, 6 Hebe, and 7 Iris are the leading candidates], their proximity to the 3:1 chaotic zone would allow them to contribute a significant portion of the ordinary chondrites. In particular, dynamical models suggest that Hebe should be a major contributor to the terrestrial meteorite flux [9]. Each leading contender is currently undergoing detailed spectral evaluation as a potential OC source. From both asteroid observational constraints and from chemical and isotopic studies of meteorites, the ordinary chondrites appear to represent an extensive and relatively complete (by meteoritic standards) sample of a few asteroid source bodies. In a similar fashion, the Howardite-Eucrite-Diogenite suite sample a single primary parent body (Vesta) and are over-represented in meteorite collections due to a fortuitous (and temporary on a solar system timescale) emplacement of Vesta ejecta fragments close to the 3:1 resonance. This suggests that the particular value of the ordinary chondrites lies in the good sample provided for each source body rather than as representatives of an abundant asteroid type. Acknowledgments: Various portions of this research were supported by NASA Planetary Geology and Geophysics grant NAGW-642 and NSF Planetary Astronomy grant AST-9012180. References: [1] Wetherill G. W. and Chapman C. R. (1988) in Meteorites and the Early Solar System, pp. 35-67, Univ. of Arizona. [2] Bell J. F. et al. (1989) in Asteroids II, pp. 921-945, Univ. of Arizona. [3] Gaffey M. J. et al. (1989) in Asteroids II, pp. 98-127, Univ. of Arizona. [4] Britt D. T. and Pieters C. M. (1991) LPS XXII, 141-142. [5] Gaffey M. J. (1984) Icarus, 60, 83-114. [6] Binzel R. P. et al. (1993) Science, 262, 1541-1543. [7] Shui X. et al. (1995) Icarus, 115, 1-35. [8] Gaffey M. J. et al. (1993) Icarus, 106, 573-602. [9] Farinella P. et al. (1993) Icarus, 101, 174-187.

Gaffey, M. J.

1995-09-01

196

Ca,Al-rich inclusions in Rumuruti (R) chondrites  

NASA Astrophysics Data System (ADS)

Rumuruti chondrites (R chondrites) constitute a well-characterized chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Many of these meteorites are breccias containing primitive type 3 fragments as well as fragments of higher petrologic type. Ca,Al-rich inclusions (CAIs) occur within all lithologies. Here, we present the results of our search for and analysis of Al-rich objects in Rumuruti chondrites. We studied 20 R chondrites and found 126 Ca,Al-rich objects (101 CAIs, 19 Al-rich chondrules, and 6 spinel-rich fragments). Based on mineralogical characterization and analysis by SEM and electron microprobe, the inclusions can be grouped into six different types: (1) simple concentric spinel-rich inclusions (42), (2) fassaite-rich spherules, (3) complex spinel-rich CAIs (53), (4) complex diopside-rich inclusions, (5) Al-rich chondrules, and (6) Al-rich (spinel-rich) fragments. The simple concentric and complex spinel-rich CAIs have abundant spinel and, based on the presence or absence of different major phases (fassaite, hibonite, Na,Al-(Cl)-rich alteration products), can be subdivided into several subgroups. Although there are some similarities between CAIs from R chondrites and inclusions from other chondrite groups with respect to their mineral assemblages, abundance, and size, the overall assemblage of CAIs is distinct to the R-chondrite group. Some Ca,Al-rich inclusions appear to be primitive (e.g., low FeO-contents in spinel, low abundances of Na,Al-(Cl)-rich alteration products; abundant perovskite), whereas others were highly altered by nebular and/or parent body processes (e.g., high concentrations of FeO and ZnO in spinel, ilmenite instead of perovskite, abundant Na,Al- (Cl)-rich alteration products). There is complete absence of grossite and melilite, which are common in CAIs from most other groups. CAIs from equilibrated R-chondrite lithologies have abundant secondary Ab-rich plagioclase (oligoclase) and differ from those in unequilibrated type 3 lithologies which have nepheline and sodalite instead.

Rout, S. S.; Bischoff, A.

2008-11-01

197

Sahara 03505 sulfide-rich iron meteorite: Evidence for efficient segregation of sulfide-rich metallic melt during high-degree impact melting of an ordinary chondrite  

NASA Astrophysics Data System (ADS)

The Sahara 03505 meteorite is a 65 g sulfide-rich iron found in an undisclosed locality of the Sahara. It consists of roughly equal volumetric proportion of polycrystalline troilite (crystal size 1.5-7.5 mm) enclosing cellular/dendritic metallic Fe-Ni (width of the dendrite arms, ~100 µm). The mineral assemblage is completed by sparse skeletal crystals of chromite, abundant droplets, 5- 100 µm in size, of anhydrous Fe-, Fe-Na-, and Fe-Mn-Mg-Ca-Na-K-phosphates, tiny crystals of schreibersite, and particles of metallic Cu. The medium- to fine-grained quench texture, and cooling modeling suggest that Sahara 03505 formed through crystallization of a sulfur-rich metallic melt under rapid cooling conditions (1- 4 °C s-1). The low troilite/metallic Fe-Ni ratio (~0.6 by weight) shows that this liquid was generated at much higher temperatures (>1300 °C) with respect to the FeS-Fe,Ni cotectic liquids. Based on bulk chemistry and oxygen isotope composition of chromite, we propose that Sahara 03505 formed by extensive impact melting of an ordinary chondrite lithology, followed by the efficient segregation of the immiscible silicate and metallic liquids. The sulfur-rich metallic liquid rapidly cooled either by radiation into space as a small lump, or by conduction to a chondrite country rock as a vein intruded into the walls of an impact crater. Sahara 03505 belongs to a small group of sulfide-rich iron meteorites which are characterized by medium- to fine-grained quench textures and by bulk chemistry that is different from the other iron meteorite groups. We propose here to use the descriptive term “sulfide-irons” for this meteorite group, by analogy with the stony-irons.

Orazio, M. D.; Folco, L.; Chaussidon, M.; Rochette, P.

2009-03-01

198

Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl-36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar  

NASA Astrophysics Data System (ADS)

We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides 10Be, 26Al, and 36Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic 36Cl and 36Ar measured in the metal separates, we determined 36Cl-36Ar cosmic-ray exposure (CRE) ages, which are shielding-independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation 3He/21Ne versus 22Ne/21Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation 3He/21Ne versus 22Ne/21Ne breaks down at high shielding. Using 36Cl-36Ar exposure ages and measured noble gas concentrations, we determine 21Ne and 38Ar production rates as a function of 22Ne/21Ne. The new data agree with recent model calculations for the relationship between 21Ne and 38Ar production rates and the 22Ne/21Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for 21Ne and 38Ar production rates as a function of the shielding indicator 22Ne/21Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the 10Be/21Ne and 26Al/21Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.

Dalcher, N.; Caffee, M. W.; Nishiizumi, K.; Welten, K. C.; Vogel, N.; Wieler, R.; Leya, I.

2013-10-01

199

Petrographic, isotopic, and chemical studies of cristobalite- and tridymite-bearing chondrules and clasts in ordinary chondrites  

NASA Astrophysics Data System (ADS)

The cristobalite in chondrules and clasts is uniquely O-16 depleted for chondritic material. Six alpha-cristobalite-bearing chondrules from Parnallee (LL3.6), one alpha-cristobalite xenolith from Farmington (L5), and one tridymite-bearing clast from Parnallee have been studied. Silica polymorphs were identified by x ray diffraction (XRD). Some of these chondrules have been described briefly. The tridymite-bearing clast is larger, 1.6 cm diameter, and contains clusters of needlelike tridymite grains (40 modal%) enclosed by clinoenstatite (En(88-91.5)). The clinoenstatite shows extreme compositional zonation towards its margins, to Wo20, Fs(75.6), En(4.3). Minor plagioclase (An(71-83)) is present. Bulk compositions of the tridymite-bearing clast and two alpha-cristobalite-rich chondrules were obtained by averaging Electron Probe Microanalysis (EMPA) analyses of 280-300 points in arrays on polished sections. The assemblage protoenstatite or clinoenstatite enclosing cristobalite and tridymite crystallizes at cooling rates of 0.01-0.23 C/s in experimental charges of 65.1 wt% SiO2 (remainder MgO) from starting temperatures of around 1550 C. Typical chondrule cooling rates also lie within this range, suggesting that these samples originated through flash melting of SiO2-rich, alkali- and Rare Earth Element (REE)-depleted solids. During or shortly after the flash heating events, the cristobalite and tridymite exchanged O with an O-16-poor gas. High degrees of O diffusion from an ambient gas may be due to the open structure of tridymite and cristobalite. The Si-isotopic ratios (P7-CONCEPT ion probe), of two alpha-cristobalite-bearing chondrules lie on the terrestrial fractionation line, showing that the chondrules are not derived from an exotic Si reservoir. All the silica-bearing samples analyzed so far are plotted on an O three-isotope plot with o.c chondrules. A least-squares best-fit line of slope 0.76 is defined, showing a marked deviation from the equilibrate chondrite line (ECL) line of slope 1.0. The O-16-poor gas may be an end member at some extension of the cristobalite line (CRIL).

Bridges, J. C.; Franchi, I. A.; Hutchison, R.; Alexander, C. M. O'd.; Morse, A. D.; Pillinger, C. T.; Long, V. P.

1994-07-01

200

Thermal history of the H-group of chondritic meteorites  

NASA Astrophysics Data System (ADS)

The Tieschitz unequilibrated H-group chondrite accreted at 800 + or - 100 C during rapid cooling. From their mineral chemistry, nine other H-group chondrites, exhibiting greater degrees of equilibration, also formed hot. Degree of equilibration is equated with slowness of cooling, especially below about 700 C. If metamorphism is defined as change brought about by an increase in temperature, in the H-group chondrites it is recognized only in the localized effects of transient reheating, probably induced by shock.

Hutchison, R.; Bevan, A. W. R.; Agrell, S. O.; Ashworth, J. R.

1980-10-01

201

Impact experiments of porous gypsum-glass bead mixtures simulating parent bodies of ordinary chondrites: Implications for re-accumulation processes related to rubble-pile formation  

NASA Astrophysics Data System (ADS)

Most of asteroids are expected to be impact fragments produced by collisions among planetesimals or rubble-pile bodies produced by re-accumulation of fragments. In order to study the formation processes of asteroids, it is necessary to examine the collisional disruption and re-accumulation conditions of planetesimals. Most of meteorites recovered on the Earth are ordinary chondrites (OCs). The OCs have the components of millimeter-sized round grains (chondrules) and submicron-sized dusts (matrix). So, the planetesimals forming the parent bodies of OCs could be mainly composed of chondrules and matrix. Therefore, we conducted impact experiments with porous gypsum mixed with glass beads having the spherical shape with various diameters simulating chondrules, and examined the effect of chondrules on the ejecta velocity and the impact strength. The targets included glass beads with a diameter ranging from 100 ?m to 3 mm and the volume fraction was 0.6, similar to that of ordinary chondrites, which is about 0.65-0.75. We also prepared the porous gypsum sample without glass bead to examine the effect of volume fraction. Nylon projectiles with the diameters of 10 mm and 2 mm were impacted at 60-180 m/s by a single-stage gas gun and at about 4 km/s by a two-stage light gas gun, respectively. After the shot, we measured the mass of the recovered fragments to calculate the impact strength Q defined by Q=mpVi^2/2(mp+Mt), where Vi is the impact velocity, and mp and Mt are the mass of projectile and target, respectively. The collisional disruption of the target was observed by a high-speed video camera to measure the ejecta velocity. The antipodal velocity Va increased with the increase of Q, irrespective of glass bead size and volume fraction. However, the Va for low-velocity collisions at 60-180 m/s was an order magnitude larger than that for high-velocity collisions at 4 km/s. The velocities of fragments ejected from two corners on the impact surface of the target Vc-g measured in the center of the mass system, were independent on the target materials. The impact strength of the mixture target was found to range from 56 to 116 J/kg depending on the glass bead size, and was several times smaller than that of the gypsum target, 446 J/kg in low-velocity collisions. The impact strengths of the 100 ?m bead target and the gypsum target strongly depended on the impact velocity: those obtained in high-velocity collisions were several times greater than those obtained in low-velocity collisions. The obtained results of Vc-g were compared to the escape velocity of chondrule-including planetesimals (CiPs) to study the conditions for the formation of rubble-pile bodies after the catastrophic disruption. The fragments of CiPs for catastrophic disruption could be re-accumulated at the radius of a body larger than 3 km, irrespective of chondrule size included in the CiPs, which is rather smaller than that for basalt bodies. Thus, we suggested that there were more parent bodies of OCs having a rubble-pile structure.

Yasui, M.; Arakawa, M.

2011-12-01

202

Intrinsic oxygen fugacity measurements on seven chondrites, a pallasite, and a tektite and the redox state of meteorite parent bodies  

USGS Publications Warehouse

Intrinsic oxygen-fugacity (fO2) measurements were made on five ordinary chondrites, a carbonaceous chondrite, an enstatite chondrite, a pallasite, and a tektite. Results are of the form of linear log fO2 - 1 T plots. Except for the enstatite chondrite, measured results agree well with calculated estimates by others. The tektite produced fO2 values well below the range measured for terrestrial and lunar rocks. The lowpressure atmospheric regime that is reported to follow large terrestrial explosions, coupled with a very high temperature, could produce glass with fO2 in the range measured. The meteorite Salta (pallasite) has low fO2 and lies close to Hvittis (E6). Unlike the other samples, results for Salta do not parallel the iron-wu??stite buffer, but are close to the fayalite-quartz-iron buffer in slope. Minor reduction by graphite appears to have taken place during metamorphism of ordinary chondrites. fO2 values of unequilibrated chondrites show large scatter during early heating suggesting that the constituent phases were exposed to a range of fO2 conditions. The samples equilibrated with respect to fO2 in relatively short time on heating. Equilibration with respect to fO2 in ordinary chondrites takes place between grades 3 and 4 of metamorphism. Application of P - T - fO2 relations in the system C-CO-CO2 indicates that the ordinary chondrites were metamorphosed at pressures of 3-20 bars, as it appears that they lay on the graphite surface. A steep positive thermal gradient in a meteorite parent body lying at the graphite surface will produce thin reduced exterior, an oxidized near-surface layer, and an interior that is increasingly reduced with depth; a shallow thermal gradient will produce the reverse. A body heated by accretion on the outside will have a reduced exterior and oxidized interior. Meteorites from the same parent body clearly are not required to have similar redox states. ?? 1984.

Brett, R.; Sato, M.

1984-01-01

203

Fayalitic olivine in CV3 chondrite matrix and dark inclusions: A nebular origin  

NASA Astrophysics Data System (ADS)

Fayalitic olivine (Fa>32) is the major component of the matrices and Dark Inclusions (DI) of CV3 and other unequilibrated chondrites. It occurs most commonly as rims, veins and halos in and around chondrule silicates in the Allende-type (CV3OxA) chondrites and to a much lesser extent in the reduced (CV3R) and Bali-type (CV3OxB) chondrites. The olivines have distinctive platy, tabular and lath- or irregular-shaped crystals, with the ratio the two types varying widely. In CV3OxB chondrites, matrix fayalitic olivines range up to Fa99.9, whereas in the other CV3 chondrites the range is much smaller. The platy and tabular anisotropic forms of the fayalitic olivines strongly suggest growth from a vapor and the nature of occurrences suggests that CV3 matrices are unequilibrated mixtures of nebular materials. We argue that the parent body hydration/dehydration model has numerous inconsistencies that make this hypothesis highly unlikely. These include: (1) There is no direct evidence linking fayalitic olivine to precursor phyllosilicates. (2) Dehydration of phyllosilicates cannot explain the wide range of morphologies of the fayalitic olivines. (3) Fayalitic olivine clearly predates the formation of the hydrous phases in CV3 chondrites and is one of the phases that breaks down to form phyllosilicates (Keller et al., 1994). (4) The unequilibrated nature of the matrix, including fine scale zoning in 10=B5-sized fayalitic olivine crystals, would not survive the parent body metamorphism required in the dehydration model. (5) A DI in the Ningqiang chondrite contains fayalitic olivine rimmed by glassy and microcrystalline material (Zolensky et al., 1997), which probably formed by radiation damage. This indicates that the fayalitic olivine was exposed to solar radiation in a nebular setting. (6) Some Allende chondrules contain unaltered primary, anhydrous glassy mesostasis in contact with the host matrix (e.g., Ikeda and Kimura, 1995). Chondrule mesostases would not have survived parent body hydration without becoming hydrated and would probably not survive the metamorphic heating required in the dehydration scenario. (7) Single platy and barrel-shaped crystals of fayalitic olivine are present in accretionary rims in CAIs (MacPherson and Davis, 1997), which developed in the nebula. (8) Matrix lumps completely encased in chondrules in ordinary chondrites contain mainly fayalitic olivine (Scott et al., 1984), indicating a nebular origin. (9) Oxygen isotopic compositions of Allende matrix and DIs strongly indicate little or no hydration for Allende and its components (Clayton, 1997). We favor a nebular vaporization/recondensation model in which vaporization of chondritic dust produced a fayalite-rich vapor, followed by formation of the fayalitic olivine by direct recondensation from the vapor, epitactic growth on surfaces of existing forsterite and enstatite in chondrules, and replacement of existing forsterite and enstatite by gas-solid exchange.

Weisberg, Michael K.; Prinz, Martin

1998-09-01

204

Possible Oxygen-Isotopic Heterogeneity in R Chondrites  

NASA Astrophysics Data System (ADS)

In this study, we have analyzed oxygen isotopes of individual olivine and magnetite grains by ion microprobe from an unequilibrated R(?) clast in PCA91241(R3.8) to investigate if further insights can be gained into the origin of elevated d17O in R chondrites.

Greenwood, J. P.; Rubin, A. E.; Wasson, J. T.

1999-03-01

205

The Natural Thermoluminescence Survey of Antarctic Meteorites: Ordinary Chondrites at the Grosvenor Mountains, Macalpine Hills, Pecora Escarpment and Queen Alexandra Range, and New Data for the Elephant Moraine, Ice Fields  

NASA Technical Reports Server (NTRS)

The natural TL (Thermoluminescence) survey of Antarctic meteorites was started in 1987 at the request of the Antarctic Meteorite Working Group in order to provide an initial description of radiation and thermal histories. It was intended to be a complement to the mineralogical and petrographic surveys performed at the Johnson Space Center and the Smithsonian Institution. All ANSMET (Antarctic Search for Meteorites) samples recovered since then, besides those that were heated throughout by atmospheric passage, have been measured. To date this amounts to about 1200 samples. As the data for each ice field reaches a significant level, we have been conducting a thorough examination of the data for that field with a view to identifying pairing, providing an estimate of terrestrial age and residence time on the ice surface, looking for differences in natural TL between ice fields, looking for variations in natural TL level with location on the ice, looking for meteorites with natural TL levels outside the normal range. Pairing is a necessary first step in ensuring the most productive use of the collection, while geographical variations could perhaps provide clues to concentration mechanisms. Samples with natural TL values outside the normal range are usually inferred to have had either small perihelia or recent changes in orbital elements. In addition, induced TL data have enabled us to look for evidence for secular variation in the nature of the flux of meteorites to Earth, and look for petrologically unusual meteorites, such as particularly primitive ordinary chondrites, heavily shocked meteorites, or otherwise anomalous meteorites. To date we have published studies of the TL properties of 167 ordinary chondrites from Allan Hills, 107 from Elephant Moraine and 302 from Lewis Cliff and we have discussed the TL properties of fifteen H chondrites collected at the Allan Hills by Euromet after a storm during the 1988 season. We now have additional databases for a reasonable number of ordinary chondrites from Grosvenor Mountains (39 meteorites), MacAlpine Hills (70 meteorites), Pecora Escarpment (60 meteorites), and Queen Alexandra Range (173 meteorites) and we have data for a further 101 samples from Elephant Moraine. The results are summarized. We also have fairly minimal databases (10-15 meteorites) for Dominion Range, Graves Nunataks, Reckling Peak and Wisconsin Range that will not be discussed here.

Benoit, Paul H.; Sears, Derek W. G.

1999-01-01

206

The Natural Thermoluminescence Survey of Antarctic Meteorites: Ordinary Chondrites at the Grosvenor Mountains, MacAlpine Hills, Pecora Escarpment and Queen Alexandra Range, and New Data New Data for the Elephant Moraine, Ice Fields  

NASA Technical Reports Server (NTRS)

The natural TL survey of Antarctic meteorites was started in 1987 at the request of the Antarctic Meteorite Working Group in order to provide an initial description of radiation and thermal histories. It was intended to be a complement to the mineralogical and petrographic surveys performed at the Johnson Space Center and the Smithsonian Institution. All ANSMET samples recovered since then, besides those that were heated throughout by atmospheric passage, have been measured. To date this amounts to about 1200 samples. As the data for each ice field reaches a significant level, we have been conducting a thorough examination of the data for that field with a view to (1) identifying pairing, (2) providing an estimate of terrestrial age and residence time on the ice surface, (3) looking for differences in natural TL between ice fields, (4) looking for variations in natural TL level with location on the ice, (5) looking for meteorites with natural TL levels outside the normal range. Pairing is a necessary first step in ensuring the @ost productive use of the collection, while geographical variations could perhaps provide clues to concentration mechanisms. Samples with natural TL values outside the normal range are usually inferred to have had either small perihelia or recent changes in orbital elements. In addition, induced TL data have enabled us to (5) look for evidence for secular variation in the nature of the flux of meteorites to Earth, and (6) look for petrologically unusual meteorites, such as particularly primitive ordinary chondrites, heavily shocked meteorites, or otherwise anomalous meteorites. To date we have published studies of the TL properties of 167 ordinary chondrites from Allan Hills, 107 from Elephant Moraine and 302 from Lewis Cliff and we have discussed the TL properties of fifteen H chondrites collected at the Allan Hills by Euromet after a storm during the 1988 season. We now have additional databases for a reasonable number of ordinary chondrites from Grosvenor Mountains (39 meteorites), MacAlpine Hills (70 meteorites), Pecora Escarpment (60 meteorites), and Queen Alexandra Range (173 meteorites) and we have data for a further 101 samples from Elephant Moraine. The results are summarized in Table 1. We also have fairly minimal databases (10-15 meteorites) for Dominion Range, Graves Nunataks, Reckling Peak and Wisconsin Range that will not be discussed here.

Benoit, Paul H.; Sears, Derek W. G.

2000-01-01

207

Highly siderophile elements in chondrites  

USGS Publications Warehouse

The abundances of the highly siderophile elements (HSE), Re, Os, Ir, Ru, Pt and Pd, were determined by isotope dilution mass spectrometry for bulk samples of 13 carbonaceous chondrites, 13 ordinary chondrites and 9 enstatite chondrites. These data are coupled with corresponding 187Re-187Os isotopic data reported by Walker et al. [Geochim. Cosmochim. Acta, 2002] in order to constrain the nature and timing of chemical fractionation relating to these elements in the early solar system. The suite of chondrites examined displays considerable variations in absolute abundances of the HSE, and in the ratios of certain HSE. Absolute abundances of the HSE vary by nearly a factor of 80 among the chondrite groups, although most vary within a factor of only 2. Variations in concentration largely reflect heterogeneities in the sample aliquants. Different aliquants of the same chondrite may contain variable proportions of metal and/or refractory inclusions that are HSE-rich, and sulfides that are HSE-poor. The relatively low concentrations of the HSE in CI1 chondrites likely reflect dilution by the presence of volatile components. Carbonaceous chondrites have Re/Os ratios that are, on average, approximately 8% lower than ratios for ordinary and enstatite chondrites. This is also reflected in 187Os/188Os ratios that are approximately 3% lower for carbonaceous chondrites than for ordinary and enstatite chondrites. Given the similarly refractory natures of Re and Os, this fractionation may have occurred within a narrow range of high temperatures, during condensation of these elements from the solar nebula. Superimposed on this major fractionation are more modest movements of Re or Os that occurred within the last 0-2 Ga, as indicated by minor open-system behavior of the Re-Os isotope systematics of some chondrites. The relative abundances of other HSE can also be used to discriminate among the major classes of chondrites. For example, in comparison to the enstatite chondrites, carbonaceous and ordinary chondrites have distinctly lower ratios of Pd to the more refractory HSE (Re, Os, Ir, Ru and Pt). Differences are particularly well resolved for the EH chondrites that have Pd/Ir ratios that average more than 40% higher than for carbonaceous and ordinary chondrite classes. This fractionation probably occurred at lower temperatures, and may be associated with fractionation processes that also affected the major refractory lithophile elements. Combined, 187Os/188Os ratios and HSE ratios reflect unique early solar system processing of HSE for each major chondrite class. ?? 2002 Elsevier Science B.V. All rights reserved.

Horan, M. F.; Walker, R. J.; Morgan, J. W.; Grossman, J. N.; Rubin, A. E.

2003-01-01

208

Metal phases of L chondrites: Their formation and evolution in the nebula and in the parent body  

NASA Astrophysics Data System (ADS)

Metal phases of six (three equilibrated and three unequilibrated) L chondrites were studied by INAA, SEM, and Mössbauer spectroscopy. Characteristics retained in the bulk metals of unequilibrated chondrites (abundant carbon, high contents of Cr, V, Mn, and low contents of W, Mo, and Ga compared to metals of equilibrated chondrites, less enrichment of W than Mo, and fractionation of Co from Ni) demonstrate that chondrite metals are not nebular condensates. All those characteristics can be well explained by melting. Chondrite metals are not melting remnants of previously condensed metals, rather, they were produced by reduction of CI- or CM-like material during the melting process. The complementarity in composition and the similarity in melting feature suggest that chondritic metals and chondrules are the complementary components of the same melting event. Distribution of trace siderophile elements between taenite and bulk metal indicates that kamacite and taenite can only be the low-temperature diffusion products and must have been developed in the chondrite parent body. The difference in the taenite composition between equilibrated and unequilibrated chondrites reveals that the equilibrated chondrites were located near the surface while the unequilibrated chondrites were in the interior if they were derived from the common parent body. Thus, while the exsolution of chondrite metal into kamacite and taenite was due to the internal thermal activity, the crystallization of EOC silicates resulted from an external heating. The internal metamorphism was mild (400-600°C) and long whereas the external heating was intense (with a maximum temperature in range of 800-950°C) and short. Tetrataenite is present not only in UOCs but also in EOCs, suggesting that the external heating occurred during the internal metamorphism, i.e., within 100 myr of chondrite formation.

Kong, Ping; Ebihara, Mitsuru

1996-07-01

209

The solar nebula redox state as recorded by the most reduced chondrules of five primitive chondrites  

NASA Technical Reports Server (NTRS)

Mafic minerals in the most reduced chondrules of five primitive meteorites were analyzed with an electron microprobe to determine the lower limit on their FeO contents. The accuracy obtained was + or - 0.01 weight percent FeO. The thermodynamic relationship between mole fraction FeSiO3 and pO2 of the ambient nebular gas at the time of mineral equilibration was established, and was used to infer the local O/H ratio of the nebular gas during chondrule formation. The lowest ferrosilite compositions reflected equilibration at 1500 K with a gas 2-18 times more oxidizing than a gas of solar composition. Olivines in low-FeO unequilibrated ordinary chondrites (UOC) chondrules are uniformly more FeO-rich than coexisting pyroxenes. This discrepancy suggests that a significant change in the O/H ratio of the nebular gas occurred between the time of olivine and pyroxene crystallization in the region of the nebula where UOCs formed. Mineral compositions in the chondrules of two C2 chondrites studied suggest they formed in a more homogeneous region of the nebula than the UOCs.

Johnson, M. C.

1986-01-01

210

Chemical and physical studies of type 3 chondrites. IX - Thermoluminescence and hydrothermal annealing experiments and their relationship to metamorphism and aqueous alteration in type below 3.3 ordinary chondrites  

NASA Astrophysics Data System (ADS)

Thermoluminescence (TL) properties were measured in samples of four type-3.0 chondrites annealed at 400-850 C and 0.77-1 kbar in the presence of various amounts of water and sodium disilicate. Several changes recorded in TL characteristics, such as the lowering of TL sensitivity in certain samples, its increase in other samples, and changes in the peak position and peak width of TL suggested the occurrence of metamorphic processes in these samples. It is suggested that, for the chondrite types above 3.2, the observed changes in TL are consistent with the TL-metamorphism trends, being due to the formation of feldspar by the devitrification of chondrule glass during metamorphism. For types below 3.2, the TL data are consistent with the hypothesis that these chondrules experienced lower levels of metamorphism than the higher types or, alternatively, with the hypothesis that the type 3.0 chondrules are being produced from higher types by aqueous alteration.

Guimon, R. K.; Lofgren, G. E.; Sears, D. W. G.

1988-01-01

211

Origin of volatile-rich H chondrites with light/dark structures  

NASA Technical Reports Server (NTRS)

The light and dark fractions of four gas-rich, brecciated H chondrites are studied in terms of petrography and volatile element chemistry in an investigation of the origin of light/dark chondrites. Petrographic constraints on the nature of the light and dark fractions of the meteorites are presented. Light clasts within the brecciated H chondrites are more highly metamorphosed than dark host material, and olivine, pyroxene and metal compositions suggest that dark fractions consist of mixtures of unequilibrated material and pulverized equilibrated H chondrite similar to the light clasts. Volatile element concentrations in the dark fractions are similar to those in the least equilibrated H3 chondrites, and no petrographic evidence for admixture of significant amounts of carbonaceous chondrite was noted. It is concluded that volatile-rich H chondrite breccias may not have incorporated carbonaceous chondrite dust as the volatile element carrier.

Mcsween, H. Y., Jr.; Lipschutz, M. E.

1980-01-01

212

Lunar and Planetary Science XXXV: Concerning Chondrites  

NASA Technical Reports Server (NTRS)

The Lunar and Planetary Science XXXV session entitled "Concerning Chondrites" includes the following topics: 1) Petrology and Raman Spectroscopy of Shocked Phases in the Gujba CB Chondrite and the Shock History of the CB Parent Body; 2) The Relationship Between CK and CV Chondrites: A Single Parent Body Source? 3) Samples of Asteroid Surface Ponded Deposits in Chondritic Meteorites; 4) Composition and Origin of SiO2-rich Objects in Carbonaceous and Ordinary Chondrites; 5) Re-Os Systematics and HSE distribution in Tieschitz (H3.6); Two Isochrons for One Meteorite; 6) Loss of Chromium from Olivine During the Metamorphism of Chondrites; 7) Very Short Delivery Times of Meteorites After the L-Chondrite Parent Body Break-Up 480 Myr Ago; and 8) The Complex Exposure History of a Very Large L/LL5 Chondrite Shower: Queen Alexandra Range 90201.

2004-01-01

213

Lunar and Planetary Science XXXV: Carbonaceous Chondrites  

NASA Technical Reports Server (NTRS)

The titles in this section include: 1) A Quantitative NMR Analysis of Phosphorus in Carbonaceous and Ordinary Chondrites; 2) An Infrared Study of the Matrices of CI1 and CM2 Chondrites; 3) A Study of the Morphology, Composition and Mineral Associations of Fe-Ni Sulphides in CM Carbonaceous Chondrites; 4) Aqueous Alteration Mineralogy in CM Carbonaceous Chondrites; 5) In Situ Location and Characterization of Carbon-bearing Phases in Carbonaceous; 6) Chondrites: Insights from Yamato 791198, a Weakly-altered CM2 Chondrite; 7) Unique Texture in EET 83389: Clues to Formation of Fine Grained Rims in CM Chondrites; 8) Amoeboid Olivine Aggregates in the NWA 760 CV3 Chondrite; 9) Petrologic Type of CV3 Chondrites as Revealed by Raman Spectroscopy of Organic Matter; 10) A First Look at Acfer 324: Evidence for Another CR 3 Chondrite? 11) Hydrogen Isotopic Composition of the Bencubbin Meteorite; 12) The Fountain Hills Meteorite: A New CBa Chondrite from Arizona; 13) Shock Effects in the Metal-rich Chondrites QUE 94411, Hammadah al Hamra 237 and Bencubbin; 14) Mineralogy and Petrology of Al-rich Objects in the CH Carbonaceous Chondrite North West Africa 739.

2004-01-01

214

Gas permeability of shocked chondrites  

NASA Astrophysics Data System (ADS)

The gas permeability of 11 ordinary chondrites was measured at various gas pressures (0.5-2.5 bars) under confining pressures up to 120 bars. The gas permeability ranges from less than a nanodarcy to a few millidarcies. There is a positive correlation between the permeability and the porosity. The permeabilithy decreased by as much as 50 percent when the confining pressure was increased from 10 to 100 bars, suggesting that the permeability of some chondrites is partly due to cracks. A linear relation between gas flow pressure dependence and confining pressure dependence of the gas permeability is observed, suggesting that on average, crack apertures are larger than pore spaces. The permeabilithy of heavily-shocked chondrites is less than of mildly shocked chondrites. Using the measured permeability data the size of a possible shocked-chondrite precursor body is estimated.

Matsui, T.; Sugiura, N.; Brar, N. S.

1986-03-01

215

Young PB-Isotopic Ages of Chondrules in CB Carbonaceous Chondrites.  

National Technical Information Service (NTIS)

CB (Bencubbin-type) carbonaceous chondrites differ in many ways from more familiar CV and CO carbonaceous chondrites and from ordinary chondrites. CB chondrites are very rich in Fe-Ni metal (50-70 vol%) and contain magnesian silicates mainly as angular to...

Y. Amelin A. N. Krot

2005-01-01

216

Mineralogy and bulk chemical composition of Ca,Al-rich inclusions in the Rumuruti (R) chondrites  

NASA Astrophysics Data System (ADS)

The R-chondrites are a relatively new classified group of chondrites, which are distinct from the carbonaceous, ordinary, and enstatite chondrites. The distinctive features of the R-chondrite are the high oxidation state, which is inferred from the high Fa content in olivine (~39-41 mol%) and the very low metal abundance, and the high ?O17 value [1-4].

Rout, S. S.; Bischoff, A.

2008-09-01

217

Osmium isotope evidence for an s-process carrier in primitive chondrites.  

PubMed

Osmium extracted from unequilibrated bulk chondrites has isotope anomalies consistent with an insoluble s-process carrier, termed Os(i) here. Osmium from metamorphosed bulk chondrites does not have isotope anomalies, implying that the Os(i) carrier was destroyed by metamorphism. The isotopic homogeneity of metamorphosed bulk chondrites is consistent with extremely effective mixing of presolar grains from varied sources in the nebula. Osmium in the Os(i) carrier is likely from nucleosynthetic sites with a neutron density about two to four times as high as that of the average solar s-process Os. PMID:16109878

Brandon, A D; Humayun, M; Puchtel, I S; Leya, I; Zolensky, M

2005-08-19

218

Chondrites and the Protoplanetary Disk, Part 1  

NASA Technical Reports Server (NTRS)

The papers discussed the following: The Formation Process of Adhering and Consorting Compound Chondrules Inferred Their Petrology and Major-Element Composition. The Prospect of High-Precision Pb Isotopic Dating of Meteorites. Evolution of UV-Irradiated Protoplanetary Disks. A Model for the Formation of E Chondrites. Oxygen Isotopic Diffusion and Exchange Experiments on Olivine and Chondrule Melts: Preliminary Results. Shock Heating: Origin of Shock Waves in the Protoplanetary Disk. Thermal Structures of Protoplanetary Disks. Meteoritical Astrophysics: A New Subdiscipline. Origin and Thermal History of FeNi-Metal in Primitive Chondrites. The Collisions of Chondrules Behind Shock Waves. Primary Signatures of the Nebular Dust Preserved in Accretionary Rims and Matrices of CV Chondrites. History of Thermally Processed Solids in the Protoplanetary Disk: Reconciling Theoretical Models and Meteoritical. Evidence Evaporation and Condensation During CAI and Chondrule Formation. Shock Heating: Effects on Chondritic Material. Rhounite-bearing Inclusions E201 and E202 from Efremovka: Constraints from Trace. Element Measurements Element Mapping in Anhydrous IDPs: Identification of the Host Phases of Major/Minor Elements as a Test of Nebula Condensation Models. Theoretical Studies of Disk Evolution Around Solar Mass Stars. Chemical Effects of High-Temperature Processing of Silicates. I-Xe and the Chronology of the Early Solar System. The Effects of X-Rays on the Gas and Dust in Young Stellar Objects. Origin of Short-lived Radionuclides in the Early Solar System. On Early Solar System Chronology: Implications of an Initially Heterogeneous Distribution of Short-lived Radionuclides. The Origin of Short-lived Radionuclides and Early Solar System Irradiation. Disequilibrium Melting and Oxygen Isotope Exchange of CAIs and Chondrules in the Solar Nebula. Mineralogy and Chemistry of Fine-grained Matrices, Rims, and Dark Inclusions in the CR Carbonaceous Chondrites Acfer/El Djouf 001 and the Ungrouped Carbonaceous Chondrites Acfer 094 and Adelaide. Oxygen Isotopes of Aluminum-rich Chondrules from Unequilibrated Enstatite Chondrites.

2004-01-01

219

Constraints on the Thermal History of H Chondrites Deduced From the U-Pb Systematics of Phosphates  

NASA Astrophysics Data System (ADS)

Pb/Pb ages of phosphates separated from equilibrated H chondrites vary from 4.563 to 4.502 aeons and show a negative correlation with the metamorphic grade of the host meteorite. This is the first clear relationship in ordinary chondrites ever observed between a long-lived chronometer and the intensity of metamorphism [1]. The range of the Pb/Pb ages, 6 x 10^7 y indicates the period for the thermal processing of equilibrated chondrites. The U/Pb systematics do however not allow to distinguish if this thermal activity has an episodic character (metamorphic event) or if it was a continuous process (internal heating followed by slow cooling). In the case of the second hypothesis the precision of the Pb/Pb chronometer with an age resolution of about 1 myr and the estimate of the temperature for the thermal U/Pb closure deduced from experimental studies of Pb diffusion in apatites [2] allow us to derive sharp and absolute chronological constraints for the thermal evolution of the H-chondrite parent body. The calculated temperature for the U/Pb closure in phosphates in chondrites of grade 5-6 is 710 +- 20 K for a cooling rate of 5 +- 2 K/10^6 y. We consider a classical modelization for the thermal evolution of the H parent body assuming that the accretion of the primitive material occured at low temperatures and in a short time interval relative to the mean life of ^26Al. The inferred value of 1150 K for the peak temperature of type 6 chondrites defines a value for the ^27Al/^26A = 5 +- 1 x 10^-6 at the time of accretion. The Pb/Pb systematics indicate that the accretion of the H chondrite parent body occurred at 4.564 aeons. The time interval between the Pb/Pb age of the Allende refractory inclusions and the accretion of the H chondrite parent body (Delta T = 3.0 +- 2.6 x 10^6 y) is compatible with this ^26Al time constraint. The radius of the H chondrite parent body is about 80 km, similar to that previously defined by [3]. The Pb/Pb chronology of phosphates from equilibrated chondrites supports the model of a ~10^2 km large parent body, which preserved its layered structure and, which was heated by heat furnished from ^26Al in situ decay if the Pb/Pb ages are interpreted as indicating the accurate time of the thermal U/Pb closure. This interpretation is, however, not coherent with the few internal Pb/Pb ages determined in unequilibrated chondrites, which show ages lying around 4.48 aeons [4]. Moreover, this model postulates that the most equilibrated material spent about 3 x 10^6 y at temperatures higher than 1100 K. Such a long period and slow cooling at high temperatures however is incompatible with petrographic observations made in this material, they require a very short time interval and a fast cooling period at temperatures above 1000 K. References: [1] Gopel C. et al. (1990) Meteoritics, 25, 367-368. [2] Cherniak D. et al. (1991) GCA, 55, 1663-1673. [3] Miyamato M. et al. (1981) LPS XII, 1145-1152. [4] Hanan B. B. and Tilton G. R. (1985) EPSL, 74, 209219.

Gopel, C.; Manhes, G.; Allegre, C. J.

1993-07-01

220

The record of cosmogenic, radiogenic, fissiogenic, and trapped noble gases in recently recovered Chinese and other chondrites  

NASA Astrophysics Data System (ADS)

Noble-gas isotopic abundances were determined in 36 recently recovered chondrites including 27 chondrites recovered in China. The comparison of the release patterns of trapped noble gases from ordinary and from carbonaceous chondrites showed that the planetary trapped noble gases in ordinary chondrites were released mainly above 1200 C, whereas more than 85 percent of noble gases trapped in carbonaceous chondrites were released at or below 1200 C, indicating that the carrier phases of the trapped noble gases in ordinary and in carbonaceous chondrites may not be the same. It is suggested that the ordinary chondrites started to retain fission Xe about 48 +/- 30 Ma earlier than Angra dos Reis. No systematic differences were observed between H, L, and LL or type 5 and 6 chondrites with respect to the time of fission Xe retention. Eight chondrites displayed neutron capture effects due to secondary cosmic-ray-produced neutrons.

Eugster, O.; Michel, Th.; Niedermann, S.; Wang, D.; Yi, W.

1993-03-01

221

Establishment of redox conditions during planetary collisions as an origin of chondrites  

NASA Technical Reports Server (NTRS)

Collisions between a 'cometlike' body (mixtures of chondritic materials and ice) and a slightly differentiated body were proposed for shock origin of ordinary chondrites. In this model, chondrules were formed with shock melting, and matrices were formed both by fracturing of materials and by recondensation of evaporated materials. This model can explain different redox conditions of chondrite formations by ice evaporation. Although this model was originally proposed for ordinary chondrites, we assume here that the model can be extended to chondrite formation in general. In this paper, redox conditions during chondrite formation by collisions will be discussed in the light of phase diagrams for solid-vapor equilibria.

Tsuchiyama, A.; Kitamura, M.

1994-01-01

222

Establishment of redox conditions during planetary collisions as an origin of chondrites  

NASA Astrophysics Data System (ADS)

Collisions between a 'cometlike' body (mixtures of chondritic materials and ice) and a slightly differentiated body were proposed for shock origin of ordinary chondrites. In this model, chondrules were formed with shock melting, and matrices were formed both by fracturing of materials and by recondensation of evaporated materials. This model can explain different redox conditions of chondrite formations by ice evaporation. Although this model was originally proposed for ordinary chondrites, we assume here that the model can be extended to chondrite formation in general. In this paper, redox conditions during chondrite formation by collisions will be discussed in the light of phase diagrams for solid-vapor equilibria.

Tsuchiyama, A.; Kitamura, M.

223

Opaque Assemblages in CK and CV Carbonaceous Chondrites  

NASA Technical Reports Server (NTRS)

CK carbonaceous chondrites are the only group of carbonaceous chondrites that exhibit thermal metamorphism. As a result, CKs display features of metamorphism such as silicate darkening, recrystallization and shock veins. Calcium Aluminum Inclusions and Fe-Ni metal are rare. CV carbonaceous chondrites are unequilibrated and have two subgroups; oxidized and reduced. The CV and CK carbonaceous chondrite groups have been compared to each other often because of petrographic similarities, such as overlapping oxygen isotopic ratios. Scientists have suggested the two groups of carbonaceous chondrites formed from the same parent body and CKs are equilibrated CV chondrites [1, 2]. The oxidized CV group has been most closely related to CKs. This study examines the petrology and mineralogy of CKs and CVs focusing on opaque minerals found in the meteorites. Using the oxide, metal and sulfide assemblages, constraints can be placed on the temperature and oxygen fugacity at which the meteorites equilibrated. The temperature and oxygen fugacity of the CK and CV chondrites can be compared in order to help define their formation history.

Neff, K. E.; Righter, K.

2006-01-01

224

Lea County 001, an H5 chondrite, and Lea County 002, an ungrouped type 3 chondrite  

NASA Astrophysics Data System (ADS)

A search of active deflation basins near Jal, Lea County, New Mexico resulted in the discovery of two meteorites, Lea County 001 and 002. Lea County 001 has mean olivine and low-Ca pyroxene compositions of Fa(19) and Fs(17), respectively. These and all other mineralogical and petrological data collected indicate a classification of H5 for this stone. Lea County 002 has mean olivine and low-Ca pyroxene compositions of Fa(2) and Fs(4), and is unequilibrated. Although it is mineralogically most similar to Kakangari and chondritic clasts within Cumberland Falls, the high modal amount of forsterite makes Lea County a unique type 3 chondrite. Oxygen isotope data for Lea County 002 fall on an 0-16-mixing line through those of the enstatite meteorites and IAB irons, a feature shared by Kakangari.

Zolensky, Michael E.; Score, Roberta; Clayton, Robert N.; Mayeda, Toshiko K.; Schutt, John W.

1989-12-01

225

Lea County 001, an H5 chondrite, and Lea County 002, an ungrouped type 3 chondrite  

SciTech Connect

A search of active deflation basins near Jal, Lea County, New Mexico resulted in the discovery of two meteorites, Lea County 001 and 002. Lea County 001 has mean olivine and low-Ca pyroxene compositions of Fa(19) and Fs(17), respectively. These and all other mineralogical and petrological data collected indicate a classification of H5 for this stone. Lea County 002 has mean olivine and low-Ca pyroxene compositions of Fa(2) and Fs(4), and is unequilibrated. Although it is mineralogically most similar to Kakangari and chondritic clasts within Cumberland Falls, the high modal amount of forsterite makes Lea County a unique type 3 chondrite. Oxygen isotope data for Lea County 002 fall on an 0-16-mixing line through those of the enstatite meteorites and IAB irons, a feature shared by Kakangari. 28 refs.

Zolensky, M.E.; Score, R.; Clayton, R.N.; Mayeda, T.K.; Schutt, J.W. (NASA, Johnson Space Center, Houston, TX (USA) Lockheed Engineering and Sciences Co., Houston, TX (USA) Chicago Univ., IL (USA))

1989-12-01

226

The classification and complex thermal history of the enstatite chondrites  

NASA Technical Reports Server (NTRS)

We have carried out instrumental neutron activation analysis of 11 enstatite chondrites and electron microprobe analyses of 17 enstatite chondrites, most of which were previously little described. We report here the third known EH5 chondrite (LEW 88180) and an unusual EL6 chondrite (LEW 87119), new data on four EL3 chondrites (ALH 85119, EET 90299, PCA 91020, and MAC 88136, which is paired with MAC 88180 and MAC 88184), the second EL5 chondrite (TIL 91714), and an unusual metal-rich and sulfide-poor EL3 chondrite (LEW 87223). The often discussed differences in mineral composition displayed by the EH and EL chondrites are not as marked after the inclusion of the new samples in the database, and the two classes apparently experienced a similar range of equilibrium temperatures. However, texturally the EL chondrites appear to have experienced much higher levels of metamorphic alteration than EH chondrites of similar equilibration temperatures. Most of the petrologic type criteria are not applicable to enstatite chondrites and, unlike the ordinary chondrites, texture and mineralogy reflect different aspects of the meteorite history. We therefore propose that the existing petrologic type scheme not be used for enstatite chondrites. We suggest that while 'textural type' reflects peak metamorphic temperatures, the 'mineralogical type' reflects equilibration during postmetamorphic (probably regolith) processes. Unlike the ordinary chondrites and EH chondrites, EL chondrites experienced an extensive low-temperature metamorphic episode. There are now a large number of enstatite meteorite breccias and impact melts, and apparently surface processes were important in determining the present nature of the enstatite chondrites.

Zhang, Yanhong; Benoit, Paul H.; Sears, Derek W. G.

1995-01-01

227

Shock metamorphism of enstatite chondrites  

NASA Astrophysics Data System (ADS)

We have extended the shock-metamorphism classification scheme of Stöffler et al. (1991) to account for shock effects in orthopyroxene and applied it to sixty enstatite chondrites. Orthopyroxene exhibits the following sequence of progressive shock effects: shock stage S1 (unshocked), sharp optical extinction; S2 (very weakly shocked), undulose extinction; S3 (weakly shocked), development of clinoenstatite lamellae parallel to (100); S4 (moderately shocked), weak mosaicism; S5 (strongly shocked), strong mosaicism. As in the Stöffler et al. (1991) scheme, stage S5 is characterized principally by the solid-state transformation of crystalline plagioclase into maskelynite. Most EL3 chondrites exhibit foliations caused by impact deformation of chondrules and metal particles. EL5 and EL6 chondrites are all shock stage S2. Because opaque veins are rare in S2 ordinary chondrites, it is plausible that the centimeter-size kamacite veins in Atlanta and Blithfield and the 1.6-cm-long oldhamite-rich vein in Jajh deh Kot Lalu (all EL6) formed when their hosts were shocked to S3-S5 levels. Because removal of shock-stage S3-S5 features (including repair of shock-damaged orthopyroxene) requires levels of metamorphism comparable to those experienced by petrologic type-5 to -6 chondrites, we infer that proto-EL6 material was shocked to S3-S5 levels prior to peak metamorphism and shocked again to stage S2 after metamorphism. Overall, enstatite chondrites appear to have suffered greater shock damage than ordinary and carbonaceous chondrites.

Rubin, Alan E.; Scott, Edward R. D.; Keil, Klaus

1997-02-01

228

The formation of Ca-, Fe-rich silicates in reduced and oxidized CV chondrites: The roles of impact-modified porosity and permeability, and heterogeneous distribution of water ices  

NASA Astrophysics Data System (ADS)

CV (Vigarano type) carbonaceous chondrites, comprising Allende-like (CVoxA) and Bali-like (CVoxB) oxidized and reduced (CVred) subgroups, experienced differing degrees of fluid-assisted thermal and shock metamorphism. The abundance and speciation of secondary minerals produced during asteroidal alteration differ among the subgroups: (1) ferroan olivine and diopside-hedenbergite solid solution pyroxenes are common in all CVs; (2) nepheline and sodalite are abundant in CVoxA, rare in CVred, and absent in CVoxB; (3) phyllosilicates and nearly pure fayalite are common in CVoxB, rare in CVred, and virtually absent in CVoxA; (4) andradite, magnetite, and Fe-Ni-sulfides are common in oxidized CVs, but rare in reduced CVs; the latter contain kirschsteinite instead. Thus, a previously unrecognized correlation exists between meteorite bulk permeabilities and porosities with the speciation of the Ca-, Fe-rich silicates (pyroxenes, andradite, kirschsteinite) among the CVox and CVred meteorites. The extent of secondary mineralization was controlled by the distribution of water ices, permeability, and porosity, which in turn were controlled by impacts on the asteroidal parent body. More intense shock metamorphism in the region where the reduced CVs originated decreased their porosity and permeability while simultaneously expelling intergranular ices and fluids. The mineralogy, petrography, and bulk chemical compositions of both the reduced and oxidized CV chondrites indicate that mobile elements were redistributed between Ca,Al-rich inclusions, dark inclusions, chondrules, and matrices only locally; there is no evidence for large-scale (>several cm) fluid transport. Published 53Mn-53Cr ages of secondary fayalite in CV, CO, and unequilibrated ordinary chondrites, and carbonates in CI, CM, and CR chondrites are consistent with aqueous alteration initiated by heating of water ice-bearing asteroids by decay of 26Al, not shock metamorphism.

MacPherson, Glenn J.; Krot, Alexander N.

2014-06-01

229

The formation of Ca-, Fe-rich silicates in reduced and oxidized CV chondrites: The roles of impact-modified porosity and permeability, and heterogeneous distribution of water ices  

NASA Astrophysics Data System (ADS)

CV (Vigarano type) carbonaceous chondrites, comprising Allende-like (CVoxA) and Bali-like (CVoxB) oxidized and reduced (CVred) subgroups, experienced differing degrees of fluid-assisted thermal and shock metamorphism. The abundance and speciation of secondary minerals produced during asteroidal alteration differ among the subgroups: (1) ferroan olivine and diopside-hedenbergite solid solution pyroxenes are common in all CVs; (2) nepheline and sodalite are abundant in CVoxA, rare in CVred, and absent in CVoxB; (3) phyllosilicates and nearly pure fayalite are common in CVoxB, rare in CVred, and virtually absent in CVoxA; (4) andradite, magnetite, and Fe-Ni-sulfides are common in oxidized CVs, but rare in reduced CVs; the latter contain kirschsteinite instead. Thus, a previously unrecognized correlation exists between meteorite bulk permeabilities and porosities with the speciation of the Ca-, Fe-rich silicates (pyroxenes, andradite, kirschsteinite) among the CVox and CVred meteorites. The extent of secondary mineralization was controlled by the distribution of water ices, permeability, and porosity, which in turn were controlled by impacts on the asteroidal parent body. More intense shock metamorphism in the region where the reduced CVs originated decreased their porosity and permeability while simultaneously expelling intergranular ices and fluids. The mineralogy, petrography, and bulk chemical compositions of both the reduced and oxidized CV chondrites indicate that mobile elements were redistributed between Ca,Al-rich inclusions, dark inclusions, chondrules, and matrices only locally; there is no evidence for large-scale (>several cm) fluid transport. Published 53Mn-53Cr ages of secondary fayalite in CV, CO, and unequilibrated ordinary chondrites, and carbonates in CI, CM, and CR chondrites are consistent with aqueous alteration initiated by heating of water ice-bearing asteroids by decay of 26Al, not shock metamorphism.

MacPherson, Glenn J.; Krot, Alexander N.

2014-07-01

230

The behavior of actinides, phosphorus, and rare earth elements during chondrite metamorphism  

NASA Astrophysics Data System (ADS)

The mineralogical distribution of U in the Nadiabondi meteorite has been determined, and U and Pu results for a group of unequilibrated H-chondrites are reported. The experimental technique is summarized and results are given for U in Ca-phosphates, U in chondrule mesostasis, and Pu-244 distribution. A discussion is presented of the chemistry of P, U, Pu, and REE in chondrites, the mechanism of Pu-REE formation, chronologies of Pu-244/U-238, Pu-Nd, and Pu/P, and of the solar system Pu/U.

Murrell, M. T.; Burnett, D. S.

1983-11-01

231

Noble gases in E-chondrites  

NASA Astrophysics Data System (ADS)

The combination of noble gas data for 12 E-chondrites with literature data shows K-Ar ages greater than 4 AE for 14 out of 18 meteorites, while U, Th-He ages are often shorter. Cosmic ray exposure ages are found to differ systematically between types E4 and E6, with the respective, below-16 Myr and above-30 Myr values implying that the E-chondrite parent body predominantly contains a single petrologic type on the 1 km scale of individual impacts in contrast to the mixed parent bodies of the ordinary chondrites. Amounts of planetary gas in E4-E6 chondrites fall in the range for ordinary chondrites of types 4-6, but fail to correlate with petrologic type or volatile trace element contents, in contrast to the ordinary chondrites. Analyses of mineral separates show that the planetary gases are concentrated in an HFand HCl-insoluble mineral, similar to phase Q. The subsolar gases are located in an HCl- and HNO3-resistant phase.

Crabb, J.; Anders, E.

1981-12-01

232

Sm-Nd and Lu-Hf isotope composition of chondritic components  

NASA Astrophysics Data System (ADS)

The 146Sm-142Nd, 147Sm-143Nd and 176Lu-176Hf radiogenic isotopic systems are widely used as chronometers and tracers of planetary evolution. These involve refractory lithophile elements and thus it is assumed that the average Sm-Nd and Lu-Hf composition of bulk terrestrial planets should be the same as that of chondrites (CHUR). We previously revised the CHUR compositions with 0.1960 ±0.0004 for 147Sm/144Nd and with 0.0336 ±0.0001 for 176Lu/177Hf using unequilibrated ordinary (OC) and carbonaceous (CC) chondrites [1], and proposed these should apply to the bulk silicate Earth (BSE). Recent studies suggest that BSE may have a super-chondritic Sm/Nd (~5%) and Lu/Hf (~10%) composition and could explain the Nd and Hf isotopic systematics of Earth and planetary materials [2, 3]. Here, we present additional Sm-Nd and Lu-Hf compositions of chondrites and chondritic components to evaluate potential isotopic heterogeneities present in the protoplanetary disk. Isotopic analyses were carried out by Neptune MC-ICPMS at ASU. Analytical details are in [1, 4]. We extend our study to homogenized whole-rock (WR) powders of 4 equilibrated OC to investigate the scale of Lu-Hf isotopic heterogeneities as consequences of thermal metamorphism on the OC parent bodies (PB) [1]. Their 147Sm/144Nd and 176Lu/177Hf vary from 0.1954 to 0.1969, and 0.0298 to 0.0341 respectively indicating that open metasomatism associated with crystallization of phosphate [1] occurred at least at the cm scale on the OCPB. We also present the first Lu-Hf and coupled Sm-Nd isotopic data of 6 single or pooled chondrules, and 2 calcium aluminum-rich inclusions (CAIs) from 5 type 3 OC & CC. The 147Sm/144Nd and 176Lu/177Hf ranges are 0.1956-0.1969, and 0.0331-0.0341 respectively for chondrules, and 0.1947-0.2147, and 0.0392-0.0501 respectively for CAIs. The chondrules are within the range of our earlier Sm/Nd and Lu/Hf CHUR-BSE estimates but the CAIs have significantly higher values, especially for Lu/Hf. Thus, only CAIs have strongly fractionated Sm/Nd and Lu/Hf, likely due to high temperature fractionation of elements in the early solar nebula. Based on evidence from Cr and Ti isotope systematics in CC [9], however, it appears unlikely that the Earth accreted in the CAI-forming region. Basaltic eucrites, which were differentiated early, also have chondritic ratios [10]. A super-chondritic Earth would thus imply accretion from large differentiated parent bodies that significantly fractionated Sm/Nd and Lu/Hf by igneous processes. This would require a late final accretion and homogenization which does not seem consistent with timing and global mixing models proposed in the accretion zone of terrestrial planets [11]. [1] Bouvier et al., 2008. EPSL 273, 48. [2] Kostitsyn, 2004. Petrology 12, 397. [3] Caro et al., 2008. Nature 452, 336. [4] Münker et al., 2001. G3, doi:10.1029/2001gc000183. [5] Bizzarro et al., 2003. Nature 421, 931. [6] Patchett et al., 2004. EPSL 222, 29. [7] Scherer et al., 2001. Science 293, 683. [8] Söderlund et al., 2004. EPSL 219, 311. [9] Trinquier et al., 2009. Science 324, 374. [10] Blichert-Toft et al., 2002. EPSL 204, 167. [11] Chambers, 2004. EPSL 223, 241.

Bouvier, A.; Vervoort, J. D.; Patchett, P. J.; Gopel, C.

2009-12-01

233

Metasomatic Processes in the Early Solar System: The Record from Chondritic Meteorites  

NASA Astrophysics Data System (ADS)

Chondritic meteorites are among the oldest solar system materials available for study in the laboratory. These unique samples contain a complex record of processes that occurred during the earliest stages of solar system evolution, including evidence of metasomatism. The effects of metasomatism are variably among the different chondrite groups, being most well developed in the CV group, with more limited effects in the CO3 and unequilibrated ordinary chondrite groups. Unlike most metasomatized terrestrial rocks, the chondrite metasomatism has not caused detectable changes in the bulk chondrite composition. Instead, it occurs at a highly localized scale and has involved elemental exchange between the different chondrite components - chondrules, matrix and Ca,Al-rich inclusions (CAIs) with vastly differing mineralogies. These components respond in different ways to metasomatism and the resultant alteration assemblages are largely dependent on the local bulk composition/mineralogy of the objects that are being metasomatized. As a consequence, there is remarkable diversity in the alteration mineralogy on the scale of 10-100?m. In CV chondrites, the secondary alteration is characterized by the introduction of alkalis, Cl and Fe into metasomatized chondrules and CAIs and the loss of Ca from these objects into the matrix. Chondrule glass has been replaced by nepheline, sodalite and other minor phases, forsteritic olivine phenocrysts have developed overgrowths of ferroan olivine, clinoenstatite is replaced by ferroan olivine and metal/sulfide nodules have been replaced by magnetite, Ni-rich metal and Fe,Ni-carbides. In CAIs, the main refractory phases, melilite, Al-Ti-rich pyroxene, anorthite, spinel and perovskite show variable degrees of alteration with the formation of a large number of secondary phases including grossular, andradite, nepheline, sodalite, wadalite Di-Hd pyroxenes, kushiroite, wollastonite, monticellite, ilmenite and, in some cases, minor amounts of phyllosilicates such as margarite, clintonite and kaolinite. The fine-grained matrix mineralogy has been extensively modified to form platy ferroan olivine associated with lesser amounts of Di-Hd pyroxenes, andradite, wollastonite, nepheline, sodalite etc. The environment in which metasomatism occurred has been the subject of extensive debate. Several studies have argued for high-temperature (>1000°C) gas-solid interactions in the solar nebula prior to asteroidal accretion, but more recent work has provided a body of evidence favoring fluid-mineral interactions below 300°C in an asteroidal environment. These include (a) in situ oxygen-isotope measurements of secondary minerals showing large mass-dependent fractionation effects of minerals precipitating from aqueous solutions, indicative of low-temperature formation, (2) dating of secondary minerals using 26Al-Mg and 53Mn-53Cr short-lived isotope chronologies suggesting late-stage alteration, several Myr after CAIs, and (3) evidence of mass transfer between CAIs and matrix and lithic clasts (dark inclusion) and Allende host, indicative of in situ alteration.

Brearley, A. J.; Krot, A.

2012-12-01

234

The Iron, Magnesium, and Calcium Distribution in Coexisting Olivines and Rhombic Pyroxenes of Chondrites  

Microsoft Academic Search

Iron, magnesium, and calcium in olivines and pyroxenes of 95 chondritic meteor- ites were analyzed with an electron microprobe. Of these, 86 were 'ordinary' chondrites, each having constant iron-magnesium ratios in olivine and pyroxene. Between different meteorites these ratios vary within three narrow ranges which reflect the It- and L-group chondrites, as well as a third group, designated the LL

Klaus Keil; Kurt Fredriksson

1964-01-01

235

Roedderite in the Qingzhen (EH3) chondrite  

NASA Technical Reports Server (NTRS)

The rare mineral roedderite, (Na1.09 K0.89 Ca0.02)2.00 (Mg4.71 Fe0.27)4.98 (Si11.80 Al0.09)11.89 O30 has been found in accessory amounts in the highly unequilibrated enstatite chondrite, Qingzhen. It occurs in association with minor amounts of albite and SiO2 as inclusions within the metal or sulfide phases of metal-sulfide assemblages. The roedderite crystals are connected through oxide and silicate veins to the surrounding matrix. The presence of glass coated vesicles on the surface of the assemblages strongly suggests that roedderite originated in the presence of a fluid phase, presumably during post-accretional planetary processes.

Rambaldi, E. R.; Rajan, R. S.; Housley, R. M.

1986-01-01

236

Roedderite in the Qingzhen (EH3) chondrite  

NASA Astrophysics Data System (ADS)

The rare mineral roedderite, (Na1.09 K0.89 Ca0.02)2.00 (Mg4.71 Fe0.27)4.98 (Si11.80 Al0.09)11.89 O30 has been found in accessory amounts in the highly unequilibrated enstatite chondrite, Qingzhen. It occurs in association with minor amounts of albite and SiO2 as inclusions within the metal or sulfide phases of metal-sulfide assemblages. The roedderite crystals are connected through oxide and silicate veins to the surrounding matrix. The presence of glass coated vesicles on the surface of the assemblages strongly suggests that roedderite originated in the presence of a fluid phase, presumably during post-accretional planetary processes.

Rambaldi, E. R.; Rajan, R. S.; Housley, R. M.

1986-03-01

237

The Effects of Saharan Weathering on Light Element Contents of Various Primitive Chondrites  

NASA Astrophysics Data System (ADS)

In recent years the Sahara Desert, particularly the Acfer region, has proven itself a rich source of meteoritic material, with over 400 samples from North Africa now residing in research laboratories. Among the samples retrieved has been a number of primitive chondrites, including CVs, CRs, COs, an odd CM/CO specimen, and several fragments similar to the "unique" chondrite ALH 85085 and a type 3.0-3.2 ordinary chondrite. Samples of each of these have been analyzed for carbon content and delta^13C and some for nitrogen and hydrogen content, delta^5N and deltaD. Each of these elements shows a lower concentration in the Saharan samples than those determined for non-Saharan, including Antarctic samples of the same group. Carbon. The carbon content of the Saharan carbonaceous chondrites analyzed were between 25 and 50% of the mean of the non-Saharan members of the group (the one exception is Allende, which is known to have a lower carbon content that any other members of the CV group). Stepped combustion showed that a low organic carbon content of these samples was the cause of the overall carbon depletion. Nitrogen. The nitrogen contents of the Acfer region CR chondrites was substantially lower than that of their non-Saharan equivalents. The nitrogen of the CR chondrites is isotopically distinct from terrestrial samples and from other carbonaceous chondrites in that it is highly enriched in ^15N. The isotopic composition of the Saharan samples shows no gross difference in the delta^15N, but there is some internal variation, due to differential weathering and the rusting of metal leading to the presence of trapped atmospheric nitrogen and consequently the delta^15N becoming variably lighter. Hydrogen. The hydrogen contents of the Saharan CR chondrites and the 3.0-3.2 ordinary chondrite Adrar 003 were found to be lower than the non-Saharan counterparts: The deltaD of the samples were isotopically normal quite unlike their non-Saharan counterparts, which are known to be the most enriched in deltaD known. The stepped combustion of the whole-rock Saharan chondrites shows that organic contamination is minimal, but evaporitic carbonate occurs in some samples with ca. 1200 ppm in the most affected. This is a localized feature as some possibly paired meteorites from the same fall sometimes show no evidence for the presence of terrestrial carbonates. Given the range of meteorites over which the observations are made it it necessary to question whether the difference in abundance and isotopic composition are primary effects. Perhaps the data are more easily explained by the destruction of macromolecular carbon as a result of extreme weathering conditions, a temperature cycle of over 150 degrees C and desiccation followed by rehydration. Such circumstances may lead to the volatilization of side chains, the degradation and removal of the organic material. The most important implication of the effects may be in terms of identifying the site of the heavy hydrogen in the CR (Kolodny et al., 1980) and unequilibrated ordinary chondrites (Robert et al., 1979; McNaughton et al., 1981, 1982) as it appears to be in a form that can be easily exchanged with terrestrial water or destroyed by mild but prolonged heating. It would seem to preclude a phyllosilicate carrier as exchange of waters of hydration occurs only above 200 degrees C, thus supporting the labile side chains of the macromolecular species or soluble organic material as the major carrier of the deuterium anomaly. References: Kolodny, Y., Kerridge, J.F., and Kaplan I.R. (1980) EPSL, 46, 149-158. McNaughton, N.J., Fallick, A.E., and Pillinger, C.T. (1982) J. Geophys. Res. 87, A297-302. McNaughton, N.J., Borthwick, J., Fallick A.E., and Pillinger, C.T. (1981) Nature 294, 639-641. Robert, F., Melivat, L., and Javoy, M. (1979) Nature 282, 785-789.

Ash, R. D.; Pillinger, C. T.

1992-07-01

238

Chondrites: The Compaction of Fine Matrix and Matrix-like Chondrule Rims  

NASA Astrophysics Data System (ADS)

Primitive chondritic meteorites mainly consist of chondrules, sulfide+/-metal, and fine-grained matrix. The most unequilibrated chondrites preserve in their phase compositions and, to a lesser degree, their textures, many details about processes that occurred in the solar nebula. On the other hand, much of the textural evidence records processes that occurred in or on the parent body. I suggest that the low-porosity of chondrule matrix and matrix-like rims reflects compaction processes that occurred in asteroid-size bodies, and that neither matrix lumps nor compact matrix-like rims on chondrules could have achieved their observed low porosities in the solar nebula. Recent theoretical studies by Donn and Meakin (1) and Chokshi et al. (2) have concluded that grain-grain sticking in the solar nebula mainly produces fluffy structures having very high porosities (probably >>50%). If these structures grow large enough, they can provide an aerogel-like matrix that can trap chondrules as well as metal and sulfide grains, and thus form suitable precursors of chondritic meteorites. However, the strength of any such structure formed in the solar nebula must be a trivial fraction of that required to survive passage through the Earth's atmosphere in order to fall as a meteorite. The best evidence of accretionary structures appears to be that reported by Metzler et al. (3). They made SEM images of entire thin sections of CM chondrites, and showed that, in the best preserved chondrites, rims are present on all entitities--on chondrules, chondrule fragments, refractory inclusions, etc. A study by Krot and Wasson (4) shows a more complex situation in ordinary chondrites. Although matrix is common, a sizable fraction of chondrules are not surrounded by matrix-like rims. As summarized by Rubin and Krot (1995), there are reports of small textural and compositional differences between matrix lumps and mean matrix-like chondrule rims, but there is so much overlap in properties between these two classes that I will assume that they can be treated as parts of a single statistical population. Published SEM images of matrix lumps and matrix-like rims show them to be relatively compact. Although some porosity is surely present as indicated by broad-beam electron-probe analysis totals <100%, it never reaches values comparable to those expected from low-velocity collisions in the solar nebula. Most chondrite researchers seem to hold that the low porosities reflect efficient packing of each grain as it accreted to the assemblage (i.e., as micrometer-size grains gradually covered the surface of a chondrule to form the matrix-like rim). I find this process very difficult to envision. If the velocities are low, the fluffy structures of Dodd and Meakin (1) should result; if the velocities are high, then rim erosion would seem to be more probable than growth. A possible scenario that avoids this dilemma is to form cm to m-size fluffy structures in low-turbulence regions of the nebular midplane. During accretion of these larger objects these experienced enough compaction to form tough, low-porosity (but unequilibrated) chondrites. If no chondrules were in a region, matrix lumps formed; if chondrules were widely separated, a matrix-like rim resulted. And, if chondrules were close to other chondrules or chondrule fragments, only small amounts of intervening fine-grained materials now separate them from their neighbors. During the compaction event, gas and dust migration occurred, and matrix filled all interstices, as now observed in the most primitive chondrites. According to this picture there could have been more diversity in the fine-grained nebular component before compaction occurred. These differences would be best preserved in matrix-like rims and matrix lumps. Much of the interchondrule matrix should consist of homogenized dust that was mixed during compaction-induced transport. References: [1] Donn B. and Meakin P. (1989) Proc. LPSC 19th, 577-580. [2] Chokshi A. et al. (1993) Astrophys. J., 407, 806-819. [3] Metzler K. et al. (1992) GCA,

Wasson, J. T.

1995-09-01

239

Aqueous Alteration of Enstatite Chondrites  

NASA Technical Reports Server (NTRS)

The Kaidun meteorite is different from all other meteorites [1], consisting largely of a mixture of “incompatible” types of meteoritic material – carbonaceous and enstatite chondrites, i.e. corre-sponding to the most oxidized and the most reduced samples of meteorite materials, including CI1, CM1-2, CV3, EH3-5, and EL3. In addition to these, minor amounts of ordinary and R chondrites are present. In addition, approximately half of the Kaidun lithologies are new materials not known as separate meteorites. Among these are aqueously altered enstatite chondrites [1], which are of considerable interest because they testify that not all reduced asteroids escaped late-stage oxidation, and hydrolysis, and also because hydrated poorly crystalline Si-Fe phase, which in turn is re-placed by serpentine (Figs 3-5). In the end the only indication of the original presence of metal is the re-sidual carbides. In other enstatite chondrite lithogies (of uncertain type) original silicates and metal have been thoroughly replaced by an assemblage of authi-genic plagioclase laths, calcite boxwork, and occasion-al residual grains of silica, Cr-rich troilite, ilmenite, and rare sulfides including heideite (Fig. 6). Fe and S have been largely leached from the rock (Fig. 4). Again the accessory phases are the first clue to the original character of the rock, which can be verified by O isotopes. It is fortunate that Kaidun displays every step of the alteration process.

Zolensky, M. E.; Ziegler, K.; Weisberg, M. K.; Gounelle, M.; Berger, E. L.; Le, L.; Ivanov, A.

2014-01-01

240

Fayalitic Olivine in Matrix of the Krymka LL3.1 Chondrite  

NASA Astrophysics Data System (ADS)

INTRODUCTION. Matrix persists as one of the most poorly characterized chondritic components. Its aggregational nature makes it an excellent place to search for primitive chondritic components that prevailed in the nebula during and after chondrule formation as well as components recording processes that predated and postdated accretion. In this study we focus on the occurrence and formation of the fayalitic olivine in the matrix of the Krymka LL3.1 unequilibrated ordinary chondrite. RESULTS. We limited our study to matrix areas clearly sandwiched between chondrules and did not include chondrule rims. In Krymka, matrix is coarser-grained and more Fe-rich than the rim material. Matrix is also highly variable in the size, shape and composition of its components, whereas chondrule rims appear more uniform. Krymka matrix is an aggregation of diverse mineral and lithic components. Mineral components include olivine, enstatite, diopsidic pyroxene, Ti-Al-rich Ca-pyroxene, hedenbergite, amorphous silicate material, spinel, oxides, troilite, and metal. Olivine is clearly dominant (~75% normative) and occurs in a variety of textures and compositions. Fayalitic olivine (Fa(sub)(58-94), avg.=Fa(sub)(72)) is ubiquitous throughout the matrix and occurs as (1) Isolated platelets (typically 1-3 micrometers x 3-5 micrometers, with some up to 10 micrometers in length), (2) Platelet clusters, which include randomly oriented platelets and/or intergrown platelets, (3) Platelet overgrowths which are overgrowths of parallel platelets on surfaces of larger (10-300 micrometers), more magnesian (Fa(sub)(4-34), avg.=Fa(sub)(19)) olivine fragments, (4) Euhedral-subhedral crystals (1-10 micrometers) which are often associated with and compositionally similar to platelets, and (5) Fluffy aggregates - irregularly shaped porous aggregates of submicron crystals. TEM study of the overgrowths reveals that the direction of elongation of the fayalitic platelets is along the c axis corresponding with the c direction of the larger olivine substrate. The larger, more magnesian olivine fragments are generally single crystals, but in some cases are associated with pyroxene (Fs(sub)(5-27),Wo(sub)(0.5-2)) or high-Ca pyroxene. These larger olivines are compositionally similar to chondrule olivines, whereas the fayalitic platelets are texturally and compositionally unlike olivines in chondrules. Fayalitic olivine with morphologies similar to those in Krymka matrix occur in the Chainpur LL3.4 and Ngawi LL3 matrix, but are much less common. These textures may have been characteristic of all primitive ordinary chondrite matrix, but were generally overprinted by metamorphic recrystallization. DISCUSSION: Fayalitic olivine in Krymka matrix records a process that has important implications for understanding the evolution of ordinary chondrites. It may form under oxidizing nebular conditions through solid state reactions in the presence of free silica, or vapor-solid reactions in a gas with a high silica activity [1,2]. Textural observations have been used to support a nebular origin for similar fayalitic olivine in CV3 matrix, and thermodynamic calculations indicate it could form in a nebula with a supersolar H2O/H2 ratio [3]. Vaporization experiments show that at ~10^(-6) bar and ~1650K olivine evaporates incongruently to produce a fayalitic vapor [4]. However, textural arguments favoring post-accretion formation of the fayalitic olivine in ordinary chondrite matrix and in CV3 dark inclusions have also been presented [5,6]. Thus, we consider three hypotheses for the formation of the fayalitic olivine in Krymka matrix: (1) vapor-solid reactions between a silica-rich vapor and metallic Fe degrees in the nebula, (2) vaporization of olivine-rich material to produce a fayalite vapor, followed by recondensation, or (3) parent body heating/dehydration of pre-existing phyllosilicates. Although the platy layered structure of some of the fayalitic olivine is suggestive of the layered structures of phyllosilicates and some fayalitic olivine texturally resembles saponite replacing ol

Weisberg, M. K.; Zolensky, M. E.; Prinz, M.

1995-09-01

241

Chondrites and their Components  

Microsoft Academic Search

What are Chondrites?Chondrites are meteorites that provide the best% clues to the origin of the solar system. They are the oldest known rocks - their components formed during the birth of the solar system ca. 4,567 Ma - and their abundances of nonvolatile elements are close to those in the solar photosphere. Chondrites are broadly ultramafic in composition, consisting largely

E. R. D. Scott; A. N. Krot

2003-01-01

242

Simulation of possible regolith optical alteration effects on carbonaceous chondrite meteorites  

Microsoft Academic Search

As the spectral reflectance search continues for links between meteorites and their parent body asteroids, the effects of optical surface alteration processes need to be considered. We present the results of an experimental simulation of the melting and recrystallization that occurs to a carbonaceous chondrite meteorite regolith powder upon heating. As done for the ordinary chondrite meteorites, we show the

Beth E. Clark; Fraser P. Fanale; Mark S. Robinson

1993-01-01

243

The Pasamonte unequilibrated eucrite: Pyroxene REE systematic and major-, minor-, and trace-element zoning. [Abstract only  

NASA Technical Reports Server (NTRS)

We are evaluating the trace-element concentrations in the pyroxenes of Pasamonte. Pasamonte is a characteristic member of the main group eucrites, and has recently been redescribed as a polymict eucrite. Our Pasamonte sample contained eucritic clasts with textures ranging from subophitic to moderately coarse-grained. This study concentrates on pyroxenes from an unequilibrated, coarse-grained eucrite clast. Major-, minor-, and trace-element analyses were measured for zoned pyroxenes in the eucritic clast of Pasamonte. The major- and minor-element zoning traverses were measured using the JEOL 733 electron probe with an Oxford-Link imaging/analysis system. Complemenatry trace elements were then measured for the core and rim of each of the grains by SIMS. The trace elements analyzed consisted of eight REE, Sr, Y, and Zr. These analyses were performed on a Cameca 4f ion probe. The results of the CI chondrite normalized (average CI trace-element analyses for several grains and the major- and minor-element zoning patterns from a single pyroxene grain are given. The Eu abundance in the cores of the pyroxenes represents the detection limit and therefore the (-Eu) anomaly is a minimum. Major- and minor-element patterns are typical for igneous zoning. Pyroxene cores are Mg enriched, whereas the rims are enriched in Fe and Ca. Also, Ti and Mn are found to increase, while Cr and Al generally decrease in core-to-rim traverses. The cores of the pyroxenes are more depleted in the Rare Earth Elements (REE) than the rims. Using the minor- and trace-element concentrations of bulk Pasamonte and the minor- and trace-element concentrations from the cores of the pyroxenes in Pasamonte measured in this study, we calculated partition coefficients between pyroxene and melt. This calculation assumes that bulk Pasamonte is representative of a melt composition.

Pun, A.; Papike, J. J.

1994-01-01

244

Occurrence of oxidized components in Qingzhen enstatite chondrite  

NASA Astrophysics Data System (ADS)

It is shown that the most unequilibrated enstatite chondrite, Qingzhen, contains a population of enstatite grains which appear to have formed under more oxidizing nebular conditions than the bulk of the meteorite, which is highly reduced. These grains are black in transmitted light because of the presence of micron-sized inclusions of Ni-poor, Cr-rich metal, and occur either isolated within the matrix or in chondrule interiors. The textural occurrence of these grains argues against their having been introduced during collision of Qingzhen with an oxidized meteorite. Most likely, they originated in the same general nebular neighborhood of the reduced bulk Qingzhen material and were subsequently transported into the reducing environment either before or during the process of chondrule formation. The discovery of this once-oxidized material in Qingzhen posed significant constraints on existing models of formation of reduced matter in the solar system.

Rambaldi, E. R.; Housley, R. M.; Rajan, R. S.

1984-09-01

245

Dynamic Crystallization Experiments Using Conventional and Solar Furnace Techniques--implications For The Formation of Refractory Forsterite In Chondrites  

NASA Astrophysics Data System (ADS)

A distinct generation of forsteritic olivine (Mg2SiO4) grains (RF) with unusually high concentrations of refractory components including CaO (0.7 wt.%), Al2O3 (0.4 wt.%), V, Sc, and REEs occurs in unequilibrated chondrites, including ordinary, carbonaceous, and the highly oxidized Rumurutiites. Contents of siderophile elements like FeO (1 wt.%), Ni, or Mn are extremly low in RF. It is inferred that formation of RF pre-dates the formation of chondrules and matrix as well as formation of the different types of parent bodies (CCs, OCs, oxidized R-chondrites). Hence, RF can help us to better understand the processes in the early stage of the solar system in the time between formation of CAIs and Si,Mg-chondrules. However, formation of RF is not well understood. Crystallization of RF in chon- drules requires chondrule melts with ca. 20 wt.% CaO. Most chondrules have typically <4 wt.% CaO (max. ca. 10 wt.% CaO). We have conducted dynamic crystallization experiments using a conventional furnace (1.5...1000 K·min-1) and the DLR solar furnace (approx. 100000 K·min-1) in order to test if rapid cooling of a chondrule- like melt would produce high CaO in RF. We demostrate that Ca-partitioning between olivine and silicate melt is only weakly influenced by rapid cooling even at extremly high cooling rates in the range of 105 K·min-1 as obtained in the solar furnace ex- periments. At a bulk composition of the starting melt of 7.5 wt.% CaO, no deviation from equilibrium fractionation was observed (T/t = 1.5 . . . 105 K·min-1). At a ol/melt bulk content of 17.5 wt.% CaO, in increase in DCa of approximately 10...20% was observed. Chondrules with CaO contents in the range of 10 wt.% (upper limit of CaO in chondrules) thus cannot be regarded to be the host of RF. Hence, there must have been an early generation of extremly CaO-rich chondrules (20 wt.% CaO). Alternatively, RF may have formed by other processes, e. g. direct condensation from the solar nebula.

Pack, A.; Sauerborn, M.; Klerner, S.; Palme, H.; Neumann, A.; Seboldt, W.

246

Rhodium, gold and other highly siderophile element abundances in chondritic meteorites  

NASA Astrophysics Data System (ADS)

The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and 187Os/ 188Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir. Differences in HSE abundances and ratios such as Re/Os, 187Os/ 188Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07. Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale. Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide-liquid sulphide fractionation processes during impact induced melting.

Fischer-Gödde, M.; Becker, H.; Wombacher, F.

2010-01-01

247

Oxygen isotopic composition of chondritic interplanetary dust particles: A genetic link between carbonaceous chondrites and comets  

NASA Astrophysics Data System (ADS)

Oxygen isotopes were measured in four chondritic hydrated interplanetary dust particles (IDPs) and five chondritic anhydrous IDPs including two GEMS-rich particles (Glass embedded with metal and sulfides) by a combination of high precision and high lateral resolution ion microprobe techniques. All IDPs have isotopic compositions tightly clustered around that of solar system planetary materials. Hydrated IDPs have mass-fractionated oxygen isotopic compositions similar to those of CI and CM carbonaceous chondrites, consistent with hydration of initially anhydrous protosolar dust. Anhydrous IDPs have small 16O excesses and depletions similar to those of carbonaceous chondrites, the largest 16O variations being hosted by the two GEMS-rich IDPs. Coarse-grained forsteritic olivine and enstatite in anhydrous IDPs are isotopically similar to their counterparts in comet Wild 2 and in chondrules suggesting a high temperature inner solar system origin. The small variations in the 16O content of GEMS-rich IDPs suggest that most GEMS either do not preserve a record of interstellar processes or the initial interstellar dust is not 16O-rich as expected by self-shielding models, although a larger dataset is required to verify these conclusions. Together with other chemical and mineralogical indicators, O isotopes show that the parent-bodies of carbonaceous chondrites, of chondritic IDPs, of most Antarctic micrometeorites, and comet Wild 2 belong to a single family of objects of carbonaceous chondrite chemical affinity as distinct from ordinary, enstatite, K- and R-chondrites. Comparison with astronomical observations thus suggests a chemical continuum of objects including main belt and outer solar system asteroids such as C-type, P-type and D-type asteroids, Trojans and Centaurs as well as short-period comets and other Kuiper Belt Objects.

Aléon, J.; Engrand, C.; Leshin, L. A.; McKeegan, K. D.

2009-08-01

248

The primitive matrix components of the unique carbonaceous chondrite Acfer 094: a TEM study.  

PubMed

The mineralogical and chemical characteristics of the fine-grained matrix (< or = 3 micrometers) of the unique primitive carbonaceous chondrite Acfer 094 have been investigated in detail by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM). Generally, the fine-grained matrix represents a highly unequilibrated assemblage of an amorphous material, small forsteritic olivines (200-300 nm), low Ca-pyroxenes (300-400 nm), and Fe,Ni-sulfides (100-300 nm). The matrix is basically unaffected by secondary processes. Only minor amounts of serpentine and ferrihydrite, as products of hydrous alteration, are present. Texturally, the amorphous material acts as a groundmass to olivines, pyroxenes, and sulfides, mostly exhibiting rounded or elongated morphologies. Only very few clastic mineral grains have been found. The texture and chemical composition of the amorphous material are consistent with an origin by disequilibrium condensation in either the cooling solar nebula or a circumstellar environment. As such, the amorphous material may be considered as a possible precursor of matrix materials in other types of chondrites. The non-clastic matrix olivines (Fo98-99) and pyroxenes (En97-100) are suggested to have formed either by condensation in the solar nebula under highly oxidizing conditions or by recrystallization from the amorphous material. The formation of these grains by fragmentation of chondrule components is unlikely due to chemical and microstructural reasons. Rapid cooling caused the observed intergrowths of clino/orthoenstatite in the Mg-rich matrix pyroxenes. Although some similarities exist comparing the fine-grained matrix of Acfer 094 with the matrices of the unequilibrated CO3 chondrite ALHA77307 and the unique type 3 chondrite Kakangari, Acfer 094 remains unique. Since it contains the highest measured concentrations of circumstellar SiC and the second highest of diamond (highest is Orgueil), it seems reasonable to suggested that at least parts of the amorphous material in the fine-grained matrix may be of circumstellar origin. PMID:11539920

Greshake, A

1997-01-01

249

Transmission electron microscopy of an interplanetary dust particle with links to CI chondrites  

NASA Technical Reports Server (NTRS)

The majority of hydrated interplanetary dust particles (IDPs) have compositions that resemble CI and CM chondrites, however, their mineralogies are most similar to the fine grained material in certain altered type-3 carbonaceous and ordinary chondrites. During the transmission electron microscope studies of hydrated IDPs, a unique particle was discovered whose mineralogy is very similar to that reported from CI chondrites. W7013F5 is the first IDP whose mineralogy and chemistry approximates that of CI chondrites. The similarity in mineralogy and mineral chemistry suggests that W7013F5 was altered under conditions similar to those that existed on the CI parent bodies.

Keller, Lindsay P.; Thomas, Kathie L.; Mckay, David S.

1991-01-01

250

Chondrite chronology by initial Sr-87/Sr-86 in phosphates?  

NASA Technical Reports Server (NTRS)

New data are presented on Rb-Sr isotope analyses of phosphates from nine ordinary chondrites, including accurate identification of initial Sr-87/Sr-86. The initial Sr-87/Sr-86 ratios found in this study were generally significantly higher than the more primitive initial Sr-87/Sr-86 ratios inferred for carbonaceous chondrite refractory inclusions, basaltic achondrites, or bulk ordinary chondrites. Such elevation of initial Sr-87/Sr-86 is generally considered to reflect isotopic redistribution during metamorphism. However, in this study, no evident correlation was found between the phosphate initial Sr-87/Sr-86 compositions and the metamorphic grade. Two possible alternative hypotheses for high initial Sr-87/Sr-86 ratios are considered.

Podosek, Frank A.; Brannon, Joyce C.

1991-01-01

251

The Olton, Texas, H chondrite regolith breccia - Paired with Dimmitt  

NASA Technical Reports Server (NTRS)

The Olton ordinary chondrites (two stones found in 1948) are H group, solar wind-bearing regolith breccias. The proximity of the recovery site to the Dimmitt strewnfield, and the similarity in texture, composition, noble gas contents and isotopic ratios to Dimmitt indicate that Olton is paired with Dimmitt.

Keil, Klaus; Ehlmann, A. J.; Wieler, Rainer

1990-01-01

252

The Origin of Chondrules and Refractory Inclusions in Chondritic Meteorites  

Microsoft Academic Search

Examples of calcium-aluminum-rich inclusions (CAIs) surrounded by thick chondrule mantles have been found in chondritic meteorites and cast doubt on the conventional belief that CAIs and chondrules possessed different spacetime origins in the primitive solar nebula. We study specific processes by which such objects, and the more common ordinary CAIs and chondrules, might have formed by flare heating of primitive

Hsien Shang; Matthieu Gounelle; Alfred E. Glassgold; Typhoon Lee

2001-01-01

253

Feldspar in Chondrites.  

PubMed

Whereas most chondrites contain sodic plagioclase (or occasionally maskelynite, its glassy equivalent), a few are free of feldspar. Absence of plagioclase is correlated with the presence of calcium-poor clinopyroxene (pigeonite sensu lato) instead of orthopyroxene. Olivine in feldspar-free chondrites is frequently variable in composition; in feldspar-bearing chondrites this mineral has essentially uniform composition. It appears that the silicate material of most chondrites was initially an association of olivine and pigeonite (or perhaps olivine and glass), and recrystallization has produced the usual olivine-orthopyroxene-sodic plagioclase association. PMID:17751041

Mason, B

1965-05-14

254

The stable carbon isotopes in enstatite chondrites and Cumberland Falls  

NASA Astrophysics Data System (ADS)

The carbon-isotopic composition (CIC) of the total carbon in the enstatite chondrites Indarch, Abee, St. Marks, Pillistfer, Hvittis and Daniel's Kuil and the enstatite achondrite Cumberland Falls has been measured. The empirical relationship between CIC and total carbon content is distinct from that of carbonaceous and ordinary chondrites. Within the enstatite chondrite group the average C-13 content increases with petrographic type: E4 less than E5 less than E6. Daniel's Kuil shows the largest C-13 enrichment in the bulk carbon of any meteorite. The CIC is most clearly correlated with the abundance of the elements Zn, Cd, and In. Insofar as these elements may hold the key to the understanding of enstatite chondrites, more detailed combined CIC and trace-element studies of these meteorites will play an important role in the deciphering of their history.

Deines, P.; Wickman, F. E.

1985-01-01

255

Two new chondrite-falls in Japan  

NASA Astrophysics Data System (ADS)

Circumstances of two recent chondrite falls in the northern part of Honshu, Japan, are reported, the first (Aomori) at 1:50 p.m. on June 30, 1984 and the second (Tomiya) at 1:35 p.m. on August 22, 1984. Results of chemical analyses of major elements and composition ratios indicate that the meteorites are typical L- and H-group ordinary chondrites, respectively. In the Aomori, chondrules are present as relicts in the well-crystallized matrix. Olivine and pyroxene are homogeneous in composition, and coarse clear feldspar, up to 100 micrometers in size, is wll developed in the chondrules and matrix. Though the Aomori is a petrologic type 6 based on its texture and mineralogy, it includes a few grains of multiple twinned clinbronzite which is rarely observed in high equilibrated ordinary chondrites. In the Tomiya, chondrules possess a fine-grained mesostasis, and both orthopyroxene and clinobronzite are noticeable in thin sections; this meteorite was classified to be petrologic type between 4 and 5. The deformation texture of olivine, pyroxene and plagioclase indicates that both meteorites were shocked by 0.2-0.25 Mb. In conjunction with the discussion of the frequency of meteorite falls, all observed falls of meteorites in Japan are tabulated.

Shima, M.; Murayama, S.; Wakabayashi, F.; Okada, A.; Yabuki, H.

1986-03-01

256

Magnetic Properties of LL6 Ordinary Chondrite St. Severin  

NASA Astrophysics Data System (ADS)

The LL6 St. Severin possesses an ancient stable natural remanent magnetization (NRM), whose higher unblocking fractions (520-560°C) are randomly oriented yet lower unblocking fractions (<520°C) form a potential magnetic foliation. The St. Severin contains paramagnetic troilite, Cr-Fe-Ti-oxide and potentially ferromagnetic Fe-Ni system. A sharp unblocking around 560°C during thermal demagnetization strongly indicates that tetrataenite is the sole remanence carrier in the St. Severin. However, the absence of ferromagnetic signal in kamacite and the presence of a randomly oriented higher unblocking fraction (520-560°C) of NRM for tetrataenite indicate a multi-stage shock-induced metamorphism of the St. Severin rather than a thermal origin of NRM. More than an order difference in NRM/ARM (anhysteretic remanent magnetization) and NRM/SIRM (saturation isothermal remanent magnetization) with respect to a terrestrial signal can be a manifestation of taenite conversion to tetrataenite.

Doh, S.; Yu, Y.; Kim, W.; Min, K. K.

2010-12-01

257

Accumulation mechanisms and the weathering of Antarctic equilibrated ordinary chondrites  

NASA Astrophysics Data System (ADS)

Induced thermoluminescence (TL) is used to quantitatively evaluate the degree of weathering of meteorites found in Antarctica. We find a weak correlation between TL sensitivity and descriptions of weathering in hand specimens, the highly weathered meteorites having lower TL sensitivity than unweathered meteorites. Analysis of samples taken throughout large meteorites shows that the heterogeneity in TL sensitivity within meteorite finds is not large relative to the range exhibited by different weathered meteorites. The TL sensitivity values can be restored by minimal acid washing, suggesting the lower TL sensitivities of weathered meteorites reflects thin weathering rims on mineral grains or coating of these grains by iron oxides produced by hydration and oxidation of metal and sulfides. Small meteorites may tend to be more highly weathered than large meteorites at the Allan Hills ice fields. We find that meteorite fragments >150 g may take up to 300,000 years to reach the highest degrees of weathering, while meteorites <150 g require <40,000 years. However, at other fields, local environmental conditions and variability in terrestrial history are more important in determining weathering than size alone. Weathering correlates poorly with surface exposure duration, presumably because weathering occurs primarily during interglacial periods. The Allan Hills locality has served as a fairly stable surface over the last 100,000 years or so and has efficiently preserved both small and large meteorites. Meteorites from Lewis Cliff, however, have experienced extensive weathering, probably because of increased surface melt water from nearby outcrops. Meteorites from the Elephant Moraine locality tend to exhibit only minor degrees of weathering, but small meteorites are less weathered than large meteorites, which we suggest is due to the loss of small meteorites by aeolian transport.

Benoit, P. H.; Sears, D. W. G.

1999-06-01

258

Further Observations of Fe-60-Ni-60 and Mn-53-Cr-53 Isotopic Systems in Sulfides from Enstatite Chondrites  

NASA Technical Reports Server (NTRS)

Recent studies have shown that short-lived Fe-60 (t(sub 1/2) = 1.5 Ma) was present in some components of ordinary and enstatite chondrites when they formed. Here we report additional data on Fe-60 from sulfides in enstatite chondrites and on the potential relationship between the Fe-60-Ni-60 and Mn-53-Cr-53 systems.

Guan, Y.; Huss, G. R.; Leshin, L. A.

2004-01-01

259

Chondrules in H3 chondrites - Textures, compositions and origins  

NASA Technical Reports Server (NTRS)

Compositional and textural variations among chondrules in unequilibrated (type 3) H-group chondrites are examined in order to determine possible relations between chondrule compositions and textures. Bulk compositions of polished thin sections of 90 individual chondrules and 16 compound chondrule sets from the Sharps, Tieschitz and Bremervorde chondrites were measured by broad-beam electron probe analyses; the chondrules were also classified petrographically as barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene or fine-grained. The mean compositions of each type are found to be distinct as verified by discriminant analysis, despite a large scatter, with the olivine-rich chondrules characterized by low SiO2 and high FeO and MgO contents, greater concentrations of TiO2, Al2O3, Na2O and K2O, and lower Cr2O3 and MnO relative to pyroxene-rich chondrules. Data suggest that composition, together with cooling rate, has played a conspicuous role in producing observed chrondrule textures, and are consistent with chondrule formation from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space.

Lux, G.; Keil, K.; Taylor, G. J.

1981-01-01

260

Chondrules in H3 chondrites - Textures, compositions and origins  

NASA Astrophysics Data System (ADS)

Compositional and textural variations among chondrules in unequilibrated (type 3) H-group chondrites are examined in order to determine possible relations between chondrule compositions and textures. Bulk compositions of polished thin sections of 90 individual chondrules and 16 compound chondrule sets from the Sharps, Tieschitz and Bremervorde chondrites were measured by broad-beam electron probe analyses; the chondrules were also classified petrographically as barred olivine, porphyritic olivine, porphyritic pyroxene, barred pyroxene, radiating pyroxene or fine-grained. The mean compositions of each type are found to be distinct as verified by discriminant analysis, despite a large scatter, with the olivine-rich chondrules characterized by low SiO2 and high FeO and MgO contents, greater concentrations of TiO2, Al2O3, Na2O and K2O, and lower Cr2O3 and MnO relative to pyroxene-rich chondrules. Data suggest that composition, together with cooling rate, has played a conspicuous role in producing observed chrondrule textures, and are consistent with chondrule formation from mixtures of differing fractions of high-, intermediate- and low-temperature nebular condensates that underwent melting in space.

Lux, G.; Keil, K.; Taylor, G. J.

1981-05-01

261

Experimental Impacts into Chondritic Targets. Part 1; Disruption of an L6 Chondrite by Multiple Impacts  

NASA Technical Reports Server (NTRS)

A fragment of an L6 chondrite (ALH 85017,13) with an initial mass (M(sub 0)) of 464.1 g was the target in a series of experimental impacts in which the largest remaining fragment (M(sub R)) after each shot was impacted by a 3.18-mm ceramic sphere at a nominal speed of 2 km/s. This continued until the mass of the largest remaining piece was less than half the mass of the target presented to that shot (M(sub S)). Two chunks of Bushveldt gabbro with similar initial masses were also impacted under the same conditions until M(sub R) was less than half M(sub 0). The two gabbro targets required a total of 1.51x10(exp 7) and 1.75x10(exp 7) erg/g to attain 0.27 and 0.33 M(sub R)/M(sub 0), respectively; the chondrite, however, was considerably tougher, reaching 0.40 and 0.21 M(sub R)/M(sub 0) only after receiving 2.37x10(exp 7) and 3.10x10(exp 7) erg g-1, respectively. The combined ejecta and spallation products from the gabbro impacts were coarser than those from the chondrite and in sufficient quantities that the new surface areas exceeded those from the meteorite until the fifth shot in the chondrite series, which was the number of impacts required to disrupt each gabbro target (i.e., MR/M0 = 0.5). Unlike the behavior shown in previous regolith-evolution series, neither gabbro target produced an enhancement in the size fraction reflecting the mean size of the crystals composing the rock (about 3 mm), an effect possibly related to the width of the shock pulse. The original chondrite was so fine-grained and fractured, and the variance in its grain-size distribution so large, that effects related to grain-size were relegated to the <63- m fraction. Impacts into ALH 85017 produced abundant, fine-grained debris, but otherwise the slopes of its size distributions were comparable to those from other experiments involving natural and fabricated terrestrial targets. The characteristic slopes of the chondrite's size distributions, however, were notably more constant over the entire nine-impact series than those from any of the terrestrial targets, a testament to the control over comminution apparently exerted by pre-existing fractures and other, microscopic damage in the meteorite. The enhancement in the finer fraction of debris from ALH 85017 indicates that ordinary chondrites in solar orbit would be very efficient contributors to the cosmic-dust complex. At the same time, the greater resistance to disruption displayed by ordinary chondrites relative to that exhibited by igneous rocks indicates that a selection effect could be operative between the annealed, ordinary-chondritic breccias and relatively weaker, differentiated meteorites. Preferential survival from their time in the regoliths of their parent bodies through their transit to Earth and passage through the atmosphere suggests that meteorite collections could be biased in favor of the ordinary chondrites.

Cintala, Mark J.; Horz, Friedrich

2007-01-01

262

Comparative 187Re- 187Os systematics of chondrites . Implications regarding early solar system processes  

NASA Astrophysics Data System (ADS)

A suite of 47 carbonaceous, enstatite, and ordinary chondrites are examined for Re-Os isotopic systematics. There are significant differences in the 187Re/ 188Os and 187Os/ 188Os ratios of carbonaceous chondrites compared with ordinary and enstatite chondrites. The average 187Re/ 188Os for carbonaceous chondrites is 0.392 ± 0.015 (excluding the CK chondrite, Karoonda), compared with 0.422 ± 0.025 and 0.421 ± 0.013 for ordinary and enstatite chondrites (1? standard deviations). These ratios, recast into elemental Re/Os ratios, are as follows: 0.0814 ± 0.0031, 0.0876 ± 0.0052 and 0.0874 ± 0.0027, respectively. Correspondingly, the 187Os/ 188Os ratios of carbonaceous chondrites average 0.1262 ± 0.0006 (excluding Karoonda), and ordinary and enstatite chondrites average 0.1283 ± 0.0017 and 0.1281 ± 0.0004, respectively (1? standard deviations). The new results indicate that the Re/Os ratios of meteorites within each group are, in general, quite uniform. The minimal overlap between the isotopic compositions of ordinary and enstatite chondrites vs. carbonaceous chondrites indicates long-term differences in Re/Os for these materials, most likely reflecting chemical fractionation early in solar system history. A majority of the chondrites do not plot within analytical uncertainties of a 4.56-Ga reference isochron. Most of the deviations from the isochron are consistent with minor, relatively recent redistribution of Re and/or Os on a scale of millimeters to centimeters. Some instances of the redistribution may be attributed to terrestrial weathering; others are most likely the result of aqueous alteration or shock events on the parent body within the past 2 Ga. The 187Os/ 188Os ratio of Earth's primitive upper mantle has been estimated to be 0.1296 ± 8. If this composition was set via addition of a late veneer of planetesimals after core formation, the composition suggests the veneer was dominated by materials that had Re/Os ratios most similar to ordinary and enstatite chondrites.

Walker, R. J.; Horan, M. F.; Morgan, J. W.; Becker, H.; Grossman, J. N.; Rubin, A. E.

2002-12-01

263

Petrology and oxygen isotopes of NWA 5492, a new metal-rich chondrite  

NASA Astrophysics Data System (ADS)

Northwest Africa 5492 is a new metal-rich chondrite breccia that may represent a new oxygen reservoir and new chondrite parent body. It has some textural similarities to CB and CH chondrites, but silicates are more reduced, sulfides are more common and not associated with metal, and metal compositions differ from CB and CH chondrites. Oxygen isotope ratios indicate that Northwest Africa (NWA) 5492 components (chondrules and lithic fragments) formed in at least two different oxygen reservoirs. The more common, and presumably host, component plots in a region above the terrestrial fractionation line, below ordinary chondrite compositions, and just above enstatite chondrites in 3-oxygen space. The only other chondritic materials that plot in this region are chondrules from the Grosvenor Mountains (GRO) 95551 ungrouped metal-rich chondrite. The other rare component plots near the CR, CB, and CH chondrites. Based on petrologic characteristics and oxygen isotopic compositions, NWA 5492 appears to be related to the ungrouped metal-rich GRO 95551 chondrite.

Weisberg, Michael K.; Bunch, Ted E.; Wittke, James H.; Rumble, Douglas; Ebel, Denton S.

2012-03-01

264

Chemical and physical studies of type 3 chondrites 12: The metamorphic history of CV chondrites and their components  

NASA Technical Reports Server (NTRS)

The induced thermoluminescence (TL) properties of 16 CV and CV-related chondrites, four CK chondrites and Renazzo (CR2) have been measured in order to investigate their metamorphic history. The petrographic, mineralogical and bulk compositional differences among the CV chondrites indicate that the TL sensitivity of the approximately 130 C TL peak is reflecting the abundance of ordered feldspar, especially in chondrule mesostasis, which in turn reflects parent-body metamorphism. The TL properties of 18 samples of homogenized Allende powder heated at a variety of times and temperatures, and cathodoluminescence mosaics of Axtell and Coolidge, showed results consistent with this conclusion. Five refractory inclusions from Allende, and separates from those inclusions, were also examined and yielded trends reflecting variations in mineralogy indicative of high peak temperatures (either metamorphic or igneous) and fairly rapid cooling. The CK chondrites are unique among metamorphosed chondrites in showing no detectable induced TL, which is consistent with literature data that suggests very unusual feldspar in these meteorites. Using TL sensitivity and several mineral systems and allowing for the differences in the oxidized and reduced subgroups, the CV and CV-related meteorites can be divided into petrologic types analogous to those of the ordinary and CO type 3 chondrites. Axtell, Kaba, Leoville, Bali, Arch and ALHA81003 are type 3.0-3.1, while ALH84018, Efremovka, Grosnaja, Allende and Vigarano are type 3.2-3.3 and Coolidge and Loongana 001 are type 3.8. Mokoia is probably a breccia with regions ranging in petrologic type from 3.0 to 3.2. Renazzo often plots at the end of the reduced and oxidized CV chondrite trends, even when those trends diverge, suggesting that in many respects it resembles the unmetamorphosed precursors of the CV chondrites. The low-petrographic types and low-TL peak temperatures of all samples, including the CV3.8 chondrites, indicates metamorphism in the stability field of low feldspar (i.e., less than 800 C) and a metamorphic history similar to that of the CO chondrites but unlike that of the ordinary chondrites.

Guimon, R. Kyle; Symes, Steven J. K.; Sears, Derek W. G.

1995-01-01

265

Isotopic Composition of Silicon Carbide in the CO3 Chondrite Colony  

NASA Technical Reports Server (NTRS)

Presolar grains have been identified in primitive members of all chondrite classes. The isotopic compositions of presolar grains provide probes of galactic evolution and nucleosynthesis in stars, while the abundances and characteristics of presolar grains contain a record of thermal processing in the solar system. Most of the detailed isotopic work has been done on SiC from Murchison and Orgueil, supplemented by a few studies of ordinary and enstatite chondrites. This work investigates SiC in the Colony CO3.O meteorite. SiC is present in Colony at a matrix-normalized abundance of approximately 3.7 ppm, much less than in CI chondrites and the matrixes of CM and primitive ordinary and enstatite chondrites. The abundances of SiC and other presolar grains in Colony seem to correlate with the chemical processing that produced CO3 chondrites. This implies that the known presolar grains experienced the same processing as the bulk CO3 material and assumes that the parent material was the same as that for other chondrites, including CI. That parent material is most plausibly the average material in the sun s parent molecular cloud. One test of this idea is to look for primary differences between SiC in Colony (and other meteorites) and that in CI and CM chondrites. True differences not related to thermal processing would falsify the assumption that all chondrite classes originated from the same reservoir of presolar dust.

Smith, J. B.; Huss, G. R.

2003-01-01

266

Primitive material surviving in chondrites - Refractory inclusions  

NASA Technical Reports Server (NTRS)

The refractory inclusions embedded in the matrix of many carbonaceous and some ordinary or enstatite chondrites exhibit mineralogical and bulk composition characteristics which correlate with the first, or highest-temperature, condensation products that have been predicted to form from a hot, but cooling, solar nebula. These inclusions' isotopic compositions indicate presolar nucleosynthetic processes that have contributed to the production of the elements from which the solar system was formed; subsequently, the inclusions experienced melting, recrystallization, and alteration histories that have obscured the formation processes.

Macpherson, Glenn J.; Wark, D. A.; Armstrong, John T.

1988-01-01

267

Chondrite thermal histories constrained by experimental annealing of Quenggouk orthopyroxene  

NASA Astrophysics Data System (ADS)

Attention is given to the series of ordinary chondrite petrological types, numbered 3-6, which exhibit increasingly homogeneous minerals and annealed textures. While the annealing is usually interpreted as due to reheating after cold accretion of the parent bodies, evidence has been adduced for texture and mineral composition origins during primary cooling. The first constraint on chondrite 'matamorphic' history is presently derived from experimental annealing of a silicate mineral in natural meteoric specimens. Striated orthopyroxene was almost entirely converted to its normal form in 1 week at about 800 C, without detectable change of composition. These results support the primary cooling hypothesis.

Ashworth, J. R.; Mallinson, L. G.; Hutchison, R.; Biggar, G. M.

1984-03-01

268

Rhenium-osmium isotope systematics of carbonaceous chondrites  

USGS Publications Warehouse

Rhenium and osmium concentrations and Os isotopic compositions of eight carbonaceous chondrites, one LL3 ordinary chondrite, and two iron meteorites were determined by resonance ionization mass spectrometry. Iron meteorite 187Re/186Os and 187OS/186Os ratios plot on the previously determined iron meteorite isochron, but most chondrite data plot 1 to 2 percent above this meteorite isochron. This suggests either that irons have significantly younger Re-Os closure ages than chondrites or that chondrites were formed from precursor materials with different chemical histories from the precursors of irons. Some samples of Semarkona (LL3) and Murray (C2M) meteorites plot 4 to 6 percent above the iron meteorite isochron, well above the field delineated by other chondrites. Murray may have lost Re by aqueous leaching during its preterrestrial history. Semarkona could have experienced a similar loss of Re, but only slight aqueous alteration is evident in the meteorite. Therefore, the isotopic composition of Semarkona could reflect assembly of isotopically heterogeneous components subsequent to 4.55 billion years ago or Os isotopic heterogeneities in the primordial solar nebula.

Walker, R. J.; Morgan, J. W.

1989-01-01

269

Rhenium-osmium isotope systematics of carbonaceous chondrites.  

PubMed

Rhenium and osmium concentrations and Os isotopic compositions of eight carbonaceous chondrites, one LL3 ordinary chondrite, and two iron meteorites were determined by resonance ionization mass spectrometry. Iron meteorite (187)Re/(186)Os and (l87)Os/(l86)Os ratios plot on the previously determined iron meteorite isochron, but most chondrite data plot 1 to 2 percent above this meteorite isochron. This suggests either that irons have significantly younger Re-Os closure ages than chondrites or that chondrites were formed from precursor materials with different chemical histories from the precursors of irons. Some samples of Semarkona (LL3) and Murray (C2M) meteorites plot 4 to 6 percent above the iron meteorite isochron, well above the field delineated by other chondrites. Murray may have lost Re by aqueous leaching during its preterrestrial history. Semarkona could have experienced a similar loss of Re, but only slight aqueous alteration is evident in the meteorite. Therefore, the isotopic composition of Semarkona could reflect assembly of isotopically heterogeneous components subsequent to 4.55 billion years ago or Os isotopic heterogeneities in the primordial solar nebula. PMID:17799187

Walker, R J; Morgan, J W

1989-01-27

270

In-Situ Oxygen Isotopic Composition of Tagish Lake: An Ungrouped Type 2 Carbonaceous Chondrite  

NASA Technical Reports Server (NTRS)

We have measured the oxygen isotopic composition of several components of Tagish Lake by ion microprobe. This meteorite constitutes the best preserved sample of C2 matter presently available for study. It presents two different lithologies (carbonate-poor and -rich) which have fairly comparable oxygen isotopic composition, with regard to both the primary or secondary minerals. For the olivine and pyroxene grains, their delta O-18 values range from - 10.5% to + 7.4% in the carbonate-poor lithology, with a mean Delta O-17 value of - 3.7 2.4%. In the carbonate-rich lithology, delta O-18 varies from - 7.9% to + 3.3%, and the mean Delta O-17 value is - 4.7 +/- 1.4%. Olivine inclusions (Fo(sub >99)) with extreme O-16-enrichment were found in both lithologies: delta O-18 = - 46.1 %, delta O-187= - 48.3% and delta O-18 = - 40.6%, delta O-17 = - 41.2% in the carbonate-rich lithology; delta O-18 = - 41.5%, delta O-17 = -43.4%0 in the carbonate-poor lithology. Anhydrous minerals in the carbonate-poor lithology are slightly more O-16-rich than in the carbonate-rich one. Four low-iron manganese-rich (LIME) olivine grains do not have an oxygen isotopic composition distinct from the other "normal" olivines. The phyllosilicate matrix presents the same range of oxygen isotopic compositions in both lithologies: delta O-18 from approximately 11 % to approximately 6%, with an average Delta. O-17 approximately 0%. Because the bulk Tagish Lake oxygen isotopic composition given by Brown et al. is on the high end of our matrix analyses, we assume that this "bulk Tagish Lake" composition probably only represents that of the carbonate-rich lithology. Calcium carbonates have delta O-18 values up to 35%, with Delta O-17 approximately 0.5%0. Magnetite grains present very high Delta O-17 values approximately + 3.4%0 +/- 1.2%. Given our analytical uncertainties and our limited carbonate data, the matrix and the carbonate seem to have formed in isotopic equilibrium. In that case, their large isotopic fractionation would argue for a low temperature (CM-like, T approximately 0 deg) formation. Magnetite probably formed during a separate event. Tagish Lake magnetite data is surprisingly compatible with that of R-chondrites and unequilibrated ordinary (LL3) chondrites. Our oxygen isotope data strongly supports the hypothesis of a single precursor for both lithologies. Drastic mineralogical changes between the two lithologies not being accompanied with isotopic fractionation seem compatible with the alteration model presented by Young et aI. Tagish Lake probably represents the first well preserved large sample of the C2 matter that dominates interplanetary matter since the formation of the solar system.

Zolensky, Michael E.; Engrand, Cecile; Gounelle, Matthieu; Zolensky, Mike E.

2001-01-01

271

Crustal structure and igneous processes in a chondritic Io  

NASA Technical Reports Server (NTRS)

Liquid sulfur can form when metal-free C1 or C2 chondrites are heated. It may be obtained either by direct melting of native sulfur in disequilibrated C1 or C2 chondrites or by incongruent melting of pyrite and other sulfides in thermodynamically equilibrated rocks of the same composition. Hence, Lewis considered C2 chondrites to be the best meteoritic analog for Io's bulk composition. Metal-bearing C3 and ordinary chondrites are too chemically reduced to yield liquid sulfur and are not thought to represent plausible analogs of Io's bulk composition. An important aspect of Lewis' work is that CaSO4 and MgSO4 are predicted to be important in Io. Real C1 and C2 chondrites contain averages of, respectively, 11 percent and 3 percent by mass of salts (plus water of hydration). The most abundant chondritic salts are magnesium and calcium sulfates, but other important components include sulfates of sodium, potassium, and nickel and carbonates of magnesium, calcium, and iron. It is widely accepted that chondritic salts are formed by low-temperature aqueous alteration. Even if Io originally did not contain salts, it is likely that aqueous alteration would have yielded several percent sulfates and carbonates. In any event, Io probably contains sulfates and carbonates. This report presents the results of a model of differentiation of a simplified C2 chondrite-like composition that includes 1.92 percent MgSO4, 0.56 percent CaSO4, 0.53 percent CaCO3, and 0.094 percent elemental sulfur. The temperature of the model is gradually increased; ensuing fractional melting results in these components extruding or intruding at gravitationally stable levels in Io's crust. Relevant phase equilibria were reviewed. A deficiency of high-pressure phase equilibria renders the present model qualitative.

Kargel, J. S.

1993-01-01

272

Crustal structure and igneous processes in a chondritic Io  

NASA Astrophysics Data System (ADS)

Liquid sulfur can form when metal-free C1 or C2 chondrites are heated. It may be obtained either by direct melting of native sulfur in disequilibrated C1 or C2 chondrites or by incongruent melting of pyrite and other sulfides in thermodynamically equilibrated rocks of the same composition. Hence, Lewis considered C2 chondrites to be the best meteoritic analog for Io's bulk composition. Metal-bearing C3 and ordinary chondrites are too chemically reduced to yield liquid sulfur and are not thought to represent plausible analogs of Io's bulk composition. An important aspect of Lewis' work is that CaSO4 and MgSO4 are predicted to be important in Io. Real C1 and C2 chondrites contain averages of, respectively, 11 percent and 3 percent by mass of salts (plus water of hydration). The most abundant chondritic salts are magnesium and calcium sulfates, but other important components include sulfates of sodium, potassium, and nickel and carbonates of magnesium, calcium, and iron. It is widely accepted that chondritic salts are formed by low-temperature aqueous alteration. Even if Io originally did not contain salts, it is likely that aqueous alteration would have yielded several percent sulfates and carbonates. In any event, Io probably contains sulfates and carbonates. This report presents the results of a model of differentiation of a simplified C2 chondrite-like composition that includes 1.92 percent MgSO4, 0.56 percent CaSO4, 0.53 percent CaCO3, and 0.094 percent elemental sulfur. The temperature of the model is gradually increased; ensuing fractional melting results in these components extruding or intruding at gravitationally stable levels in Io's crust. Relevant phase equilibria were reviewed. A deficiency of high-pressure phase equilibria renders the present model qualitative.

Kargel, J. S.

1993-03-01

273

Origin of the differences in refractory-lithophile-element abundances among chondrite groups  

NASA Astrophysics Data System (ADS)

Chondrite groups can be distinguished on the basis of their abundances of refractory lithophile elements (RLE). These abundances are, in part, functions of the mass fraction of Ca-Al-rich inclusions (CAIs) within the chondrites. Carbonaceous chondrites contain the most CAIs and the highest RLE abundances; they also contain modally abundant fine-grained matrix material that consists largely of modified nebular dust. The amount of dust varied throughout the solar nebula: enstatite and ordinary chondrites formed in low-dust regions in the inner part of the nebula, R chondrites formed in higher-dust zones at somewhat greater heliocentric distances, and carbonaceous chondrites formed in even dustier regions farther from the Sun. The amount of ambient dust peaked in the region where CV and CK chondrites accreted; these chondrites have abundant matrix, the highest modal abundances of CAIs, and the highest bulk RLE contents. Substantial amounts of nebular dust occurred in highly porous multi-millimeter-to-centimeter-size dustballs that were on the order of 100 times more massive than CAIs. Radial drift processes in the nebula affected these dustballs to approximately the same extent as the CAIs; both types of objects were aerodynamically concentrated in the same nebular regions. These regions maintained approximately the same relative amounts of dust through the periods of chondrule formation and chondrite accretion.

Rubin, Alan E.

2011-06-01

274

Primordial noble gases in chondrites: the abundance pattern was established in the solar nebula.  

PubMed

Ordinary chondrites, like carbonaceous chondrites, contain primordial noble gases mainly in a minor phase comprising

Alaerts, L; Lewis, R S; Anders, E

1977-12-01

275

Chondrites: initial strontium-87/strontium-86 ratios and the early history of the solar system.  

PubMed

A sodium-poor, calcium-rich inclusion in the carbonaceous chondrite Allende had a (87)Sr/(86)Sr ratio at the time of its formation of 0.69880, as low a value as that found in any other meteorite. The higher (87)Sr/(86)Sr ratios found in ordinary chondrites indicate that their formation or isotopic equilibration occurred tens of millions of years later. PMID:17749309

Wetherill, G W; Mark, R; Lee-Hu, C

1973-10-19

276

Heterogeneities in the solar nebula. [oxygen isotopic composition in carbonaceous chondrites  

NASA Technical Reports Server (NTRS)

Oxygen isotopic compositions of the high-temperatue phases in carbonaceous chondrites define a mixing line with an O-16 rich component and show little superimposed chemical isotope fractionation. Within a single inclusion in Allende, variations of delta O-18 and delta O-17 of 39% are found. The ordinary chondrites are slightly displaced from the terrestrial fractionation trend, implying that at least 0.2% of the oxygen in terrestrial rocks was derived from the O-16 rich component.

Clayton, R.; Grossman, L.; Mayeda, T. K.; Onuma, N.

1974-01-01

277

Re-Os Systematics and HSE Distribution in Metal from Ochansk (H4) Chondrite  

NASA Technical Reports Server (NTRS)

Previous studies of the Re-Os systematics of chondrites have documented considerable variation in the Re/Os ratios of whole rock samples. For some whole rock chondrites, Re-Os systematics display large deviations from the primitive isochron that are considerably larger than deviations in other isotope systems. Possible interpretation of these facts is that the Re-Os system in chondrites is particularly sensitive to post-formation alteration processes, thus providing a useful tool to examine such processes. Significant variations that have been detected in highly siderophile element (HSE) patterns for ordinary chondrites support this conclusion. We report Re-Os isotope data for metal separates from the Ochansk H4 chondrite coupled with abundance data for Ru, Pd, Ir, and Pt, determined in the same samples by isotope dilution. We chose this meteorite mainly because it is an observed fall with minimal signs of weathering, and its low metamorphic grade (H4) and shock stage (S3).

Smoliar, M. I.; Horan, M. F.; Alexander, C. M. OD.; Walker, R. J.

2003-01-01

278

Plagioclase-rich inclusions in carbonaceous chondrite meteorites - Liquid condensates?  

NASA Astrophysics Data System (ADS)

The characteristics and formation of coarse-grained, plagioclase-rich inclusions are investigated. The textures, mineralogical compositions, and initial Al-26/Al-27 ratios for the plagioclase-rich inclusions are described. It is observed that plagioclase-rich inclusions in carbonaceous chondrites are either Ca-Al-rich inclusions (CAIs) composed of 30-60 vol pct anorthite, and less than 35 vol pct Ti-Al-pyroxene and melilite, or CA chondrites composed of plagioclase, pyroxene, olivine, spinel, and melilite. It is observed that CA chondrules are chemically and mineralogically the most similar components shared by carbonaceous and ordinary chondrites. The textural changes observed in the inclusions are examined. The data reveal that the CAIs have three textural groups: coarse anorthite laths, equigranular anorthite and Ti-Al-pyroxene, and lacy Ti-Al-pyroxene and fine-grained anorthite.

Wark, D. A.

1987-02-01

279

Magnetic Record in Chondrite Meteorites.  

National Technical Information Service (NTIS)

What we know about the magnetic record in chondrite meteorites based on new data and previously published results is summarized. Strips from thin slabs of chondrite meteorites were cut into near cubical subsamples (several mm on edge) numbering approximat...

P. J. Wasilewski J. L. Faris M. V. Obryan

1993-01-01

280

Solving Ordinary Differential Equations  

NASA Technical Reports Server (NTRS)

Initial-value ordinary differential equation solution via variable order Adams method (SIVA/DIVA) package is collection of subroutines for solution of nonstiff ordinary differential equations. There are versions for single-precision and double-precision arithmetic. Requires fewer evaluations of derivatives than other variable-order Adams predictor/ corrector methods. Option for direct integration of second-order equations makes integration of trajectory problems significantly more efficient. Written in FORTRAN 77.

Krogh, F. T.

1987-01-01

281

The L3-6 chondritic regolith breccia Northwest Africa (NWA) 869: (I) Petrology, chemistry, oxygen isotopes, and Ar-Ar age determinations  

NASA Astrophysics Data System (ADS)

Northwest Africa (NWA) 869 consists of thousands of individual stones with an estimated total weight of about 7 metric tons. It is an L3-6 chondrite and probably represents the largest sample of the rare regolith breccias from the L-chondrite asteroid. It contains unequilibrated and equilibrated chondrite clasts, some of which display shock-darkening. Impact melt rocks (IMRs), both clast-free and clast-poor, are strongly depleted in Fe,Ni metal, and sulfides. An unequilibrated microbreccia, two different light inclusions and two different SiO2-bearing objects were found. Although the matrix of this breccia appears partly clastic, it is not a simple mixture of fine-grained debris formed from the above lithologies, but mainly represents an additional specific lithology of low petrologic type. We speculate that this material stems from a region of the parent body that was only weakly consolidated. One IMR clast and one SiO2-bearing object show ?17O values similar to bulk NWA 869, suggesting that both are related to the host rock. In contrast, one light inclusion and one IMR clast appear to be unrelated to NWA 869, suggesting that the IMR clast is contaminated with impactor material. 40Ar-39Ar analyses of a type 4 chondrite clast yield a plateau age of 4402 ± 7 Ma, which is interpreted to be the result of impact heating. Other impact events are recorded by an IMR clast at 1790 ± 36 Ma and a shock-darkened clast at 2216 ± 40 Ma, demonstrating that NWA 869 escaped major reset in the course of the event at approximately 470 Ma that affected many L-chondrites.

Metzler, Knut; Bischoff, Addi; Greenwood, Richard C.; Palme, Herbert; Gellissen, Marko; Hopp, Jens; Franchi, Ian A.; Trieloff, Mario

2011-05-01

282

Paleomagnetism of enstatite chondrites  

NASA Astrophysics Data System (ADS)

Chondritic meteorites are widely thought to have originated on unmelted parent bodies. However, recent studies of CV carbonaceous chondrites have observed stable remanent magnetization acquired after accretion that may have been imparted by a core dynamo on the parent body. This suggests that CV chondrites may have originated from an internally melted, partially differentiated parent body with a relic chondritic crust. Although diverging from the predominant view that chondrites are samples of unmelted bodies, this idea has deep roots in the history of meteoritics. In particular, a common parent body has often been invoked for enstatite chondrites and enstatite achondrites (aubrites), which share many compositional, mineralogical, and isotopic similarities. Therefore, enstatite chondrites are a natural target for further testing the partial differentiation hypothesis. However, there are very few previous paleomagnetic studies of these meteorites. To address this, we studied the magnetic properties and paleomagnetism of three enstatite chondrites (Pillistfer EL6, Eagle EL6, and Sahara 97158 EH3) to examine the feasibility of dynamo generation on the enstatite chondrite parent body. In Pillistfer, our alternating field (AF) demagnetization of mutually oriented interior and fusion-crusted subsamples revealed three low coercivity components blocked from ~1.5-2.5 mT (component A1), ~2.5-7 mT (component A2), and ~7-9 mT (component A3). The A2 and A3 components are poorly defined, likely due to spurious anhysteretic remanent magnetization (ARM) acquired during AF demagnetization. Thermal demagnetization revealed low temperature (T1) and medium temperature (T2) components, ranging from 50-600°C and 600-700°C, respectively. The A1 and T1 components coincided, while the A2 and T2 components were more scattered (although nonrandomly distributed). Components A1 and A2 of fusion-crusted samples were similarly oriented to those of interior samples. The ratio of natural remanent magnetization (NRM) to isothermal remanent magnetization (IRM) over the A1 coercivity range is 10%. Combined with the failed fusion crust test, this high value of NRM/IRM suggests the sample has been partially, but not completely, remagnetized since arriving on Earth (perhaps by a hand magnet). Furthermore, we found that using AF paleointensity methods, we are only capable of recovering paleointensities from thermoremanence in Pillistfer down to 200 ?T, indicating it has poor magnetic recording properties and preventing us from obtaining paleointensities for the A2 and A3 components. Electron microscopy analyses of Pillistfer show the prevalence of FeNi metal with varying amounts of nickel (ranging in composition from kamacite into the taenite equilibrium fields and schriebersite as the most abundant ferromagnetic phases). Preliminary results on Eagle and Sahara 97158 enstatite chondrites indicate that, like in Pillistfer, the remanence is dominated by soft multidomain kamacite, making them poor paleomagnetic recorders. It is currently unclear whether enstatite chondrites may be suitable for retaining paleomagnetic fields imparted from a parent body-generated dynamo.

Feng, H.; Weiss, B. P.; Tikoo, S. M.; Gattacceca, J.; Suavet, C. R.; Andrade Lima, E.

2013-12-01

283

Magnetic anisotropy and porosity of chondrites  

NASA Astrophysics Data System (ADS)

The measured magnetic anisotropy and porosity of various chondrites are found to be inversely correlated when they are considered by class of chondrites (E, H and L + LL). For chondrites with similar porosities, the anisotropies for H and E chondrites are smaller than for L and LL chondrites, which have lower metal contents. The anisotropy and porosity are not dependent on the metamorphic grade of chondrites. K-Ar ages of strongly anisotropic and/or less porous chondrites are younger than those of less anisotropic and/or more porous chondrites. These observations suggest that impacts which reset the K-Ar ages produced the anisotropy and reduced the porosity of chondrites.

Sugiura, N.; Strangway, D. W.

1983-01-01

284

Halogens in CM Chondrites  

NASA Astrophysics Data System (ADS)

We set up an extraction line of halogens (fluorine, chlorine) by pyrohydrolysis with 50 mg of rock. We analyzed 7 CM2 chondrites found in Antarctica and found that the Cl content of meteorites with an intact fusion crust is higher than those without.

Menard, J. M.; Caron, B.; Jambon, A.; Michel, A.; Villemant, B.

2013-09-01

285

Magnetite in carbonaceous chondrites  

Microsoft Academic Search

Magnetite (Fe3O4) occurs in many carbonaceous chondrites (CCs) and has long attracted the attention of meteorite researchers. However, its origin is unresolved; suggestions range from condensates from the solar nebular to products from aqueous alteration on the meteorite parent body. The report of possible evidence of former life in ancient Martian meteorite ALH84001 suggested an additional and highly intriguing origin

Xin Hua; Peter R. Buseck

1997-01-01

286

39Ar -40Ar chronology of R chondrites  

NASA Astrophysics Data System (ADS)

This study presents the first determinations of 39Ar-40Ar ages of R chondrites for the purpose of understanding the thermal history of the R chondrite parent body. The 39Ar-40Ar ages were determined on whole-rock samples of four R chondrites: Carlisle Lakes, Rumuruti, Acfer 217, and Pecora Escarpment #91002 (PCA 91002). All samples are breccias except for Carlisle Lakes. The age spectra are complicated by recoil and diffusive loss to various extents. The peak 39Ar-40Ar ages of the four chondrites are 4.35, ~4.47 ± 0.02, 4.30 ± 0.07 Ga, and 4.37 Ga, respectively. These ages are similar to Ar-Ar ages of relatively unshocked ordinary chondrites (4.52-4.38 Ga) and are older than Ar-Ar ages of most shocked ordinary chondrites (<<4.2 Ga). Because the meteorites with the oldest (Rumuruti, ~4.47 Ga) and the youngest (Acfer 217, ~4.30 Ga) ages are both breccias, these ages probably do not record slow cooling within an undisrupted asteroidal parent body. Instead, the process of breccia formation may have differentially reset the ages of the constituent material, or the differences in their age spectra may arise from mixtures of material that had different ages. Two end-member type situations may be envisioned to explain the age range observed in the R chondrites. The first is if the impact(s) that reset the ages of Acfer 217 and Rumuruti was very early. In this case, the ~170 Ma maximum age difference between these meteorites may have been produced by much deeper burial of Acfer 217 than Rumuruti within an impact-induced thick regolith layer, or within a rubble pile type parent body following parent body re-assembly. The second, preferred scenario is if the impact that reset the age of Acfer 217 was much later than that which reset Rumuruti, then Acfer 217 may have cooled more rapidly within a much thinner regolith layer. In either scenario, the oldest age obtained here, from Rumuruti, provides evidence for relatively early (~4.47 Ga) impact events and breccia formation on the R chondrite parent body.

Dixon, E. T.; Bogard, D. D.; Garrison, D. H.

2003-03-01

287

Noble Gases in Several New Chondrite Falls and Finds  

NASA Astrophysics Data System (ADS)

Noble gases in chondrites are a mixture of different components that contain information on past "geological" events of the meteoritic material on their parent bodies and also on the irradiation and thermal history of the meteoroid. Furthermore, together with the mineralogical-petrographical record, the noble gases yield hints on the question of possible pairing of specimens. Seventeen different ordinary chondrites have been investigated. Results are given in Table 1. Twelve of these chondrites--marked with an asterisk--fell or have been recognized as meteorites during the last seven years. As shown by the presence of solar gases, two chondrites are regolithic breccias (Noblesville and Chiang Khan). Our analyses of Chiang Khan are different from that given by Eugster et al. (1990). It is possible that two different meteorites are known under this name. Glanerbrug, although a brecciated chondrite, does not contain solar gases. The Dunedin is a "museum-find." In 1987 it was proposed as a new meteorite (Sipiera et al., 1987). The noble gas record of Dunedin is different from that of all other yet measured LL3 and LL4 chondrites (Schultz and Kruse, 1989). Thus, these measurements do not contradict the suggestion of a distinct meteorite. A ^3He/^21Ne <1 is observed in Gilzem indicating a deficit of ^3He. This meteorite is not paired with the H5-shower of Simmern that fell in 1920 at a distance of about 40 km from Gilzem. Also the L-chondrites Pampa (a), (b), and (c) (R. Wieler, priv. communication) do not belong to the same fall. The ^3He/^21Ne-- and ^22Ne/^21Ne--ratios of individual fragments of the Kokubunji shower follow the trend-lines of the "Bern-plot." References: Eugster O., Michel Th., and Wang D. (1990) Lunar Planet. Sci. XXI, 339. Sipiera P.P., Brooks R.R. Johnston J.H., Holzbecher J., and Ryan D.E. (1987) Chem. Geol. 64, 351. Schultz L. and Kruse H. (1989) Meteoritics 24, 155. Table 1, which in the hard copy appears here, shows noble gas concetrations of the 17 ordinary chondrites under investigation.

Loeken, T.; Scherer, P.; Weber, H. W.; Schultz, L.

1992-07-01

288

Solving Ordinary Differential Equations  

NSDL National Science Digital Library

EJS and OSP examples to accompany the Solving Ordinary Differential Equations chapter in the Handbook of Dynamical Systems edited by Paul Fishwick. Dynamic models are described in the chapter with a âcomputer science slantâ toward the problems of model design, representation, and analysis. EJS and OSP implementations are distributed in two ready-to-run Launcher packages.

Christian, Wolfgang; Franciscouembre

2009-08-07

289

Celebrating the Ordinary  

ERIC Educational Resources Information Center

Memento. Memoir. Memorable. Memory. Memorial. Commemorate. In Memoriam. These words may remind a person of stone monuments, or larger-than-life heroes and loved ones far distanced by space and time. The act of remembering, though, also belongs in the world of the everyday and the ordinary, and has a valuable place in an art classroom. In this…

Horst, Carol

2010-01-01

290

Sculptures of Ordinary People.  

ERIC Educational Resources Information Center

Discusses the presence of ordinary people in art. Features four sculptors and examples of their work: (1) "Janitor" by Duane Hanson; (2) "The Red Light" by George Segal; (3) "The Sodbuster" by Luis A. Jimenez; and (4) "The Driller" by Mahonri Young. (CMK)

Hubbard, Guy

2000-01-01

291

A search for spectral alteration effects in chondritic gas-rich breccias  

NASA Technical Reports Server (NTRS)

Several samples of gas-rich breccias were selected, including slabs of the Kapoeta howardite, the ordinary chondrites Dubrovnik, Cangas de Onis, and Dimmit. Numerous 0.8 to 2.5 micron reflection spectra of selected areas on sawed or broken surfaces were measured with the Planetary Geosciences Division spectrogoniometer. While these spectra are not directly comparable to those of powered samples, comparisons within the data set should reveal any spectral differences due to weathering. These results indicate that unknown regolith processes do not confer the ordinary-chondrite parent bodies with an altered layer exhibiting S-class spectral properties. This is consistent with recent interpretations of the new Q-class of asteroids as the ordinary-chondrite parent bodies. However, significant spectral effects do occur in asteroid regoliths: darkening and suppression of absorption bands in highly shocked material, as seen previously in the so-called black chondrites; and segregation of metal in large impact melt pools on chondritic asteroids, which may have achondritic spectra. Neither of these effects is likely to be significant in interpreting current integral-disk spectra, but should be searched for in spectral maps returned by future spacecraft.

Bell, J. F.; Keil, Klaus

1987-01-01

292

CARBONACEOUS MATTER PRECURSORS AND METAMORPHIC CONDITIONS IN THERMALLY PROCESSED CHONDRITES  

NASA Astrophysics Data System (ADS)

Unravelling the origin of carbonaceous matter in pristine chondrites requires the understanding of the effect of post-accretion processes. In chondrites of petrologic type 3, thermal metamorphism modified to various extents the composition and structure of carbonaceous matter. Interestingly, this process controls the degree of structural order of carbonaceous matter, and clues on the thermal history of the parent body may be recovered from the physico-chemical study of carbonaceous matter. Following this framework, geothermometers based on Raman spectrometry of carbonaceous matter and covering a wide range of temperatures (100-650 °C) have been developed over recent years, both on terrestrial rocks and chondrites. While Raman data have been largely interpreted in terms of temperature, they are also the fingerprint of certain metamorphic conditions, especially in the low temperature range relevant to poorly ordered carbonaceous matter. This study investigates the Raman spectra of two series of chondritic carbonaceous matter and coal samples formed from different precursors and under different metamorphic conditions. The Raman spectra of Polyaromatic Carbonaceous Matter (PCM) from 42 chondrites and 27 coal samples, measured with visible (514 nm) and ultra-violet (244 nm) excitation wavelengths, are analyzed. The Raman spectra of low rank coals and chondrites of petrologic types 1 and 2, which contain the more disordered PCM, reflect the distinct carbon structures of their precursors. The 514 nm Raman spectra of high rank coals and chondrites of petrologic type 3 exhibit continuous and systematic spectral differences reflecting different carbon structures present during the metamorphism event. They result from differences in the chemical structures of the precursors concerning for instance the reticulation of polyaromatic units or an abundance of ether functional groups, or possibly from a lack of carbonization processes to efficiently expel oxygen heteroatoms, due to weak lithostatic pressure and confinement. These results suggest that the use of lowtemperature carbon thermometers should be restricted to a given geological context. At the same time, the sensitivity of Raman spectra to precursors and certain metamorphic conditions could be used to obtain information other than temperature. The analysis also provides evidence of the accretion of relatively homogeneous PCM precursors among ordinary CO and CV carbonaceous chondrite parent bodies, given that the 514nm Raman spectra of PCM efficiently trace the metamorphism grades. Looking closer, however, the 514 nm Raman data are more scattered in chondrites than in coals and the maturity tracers are less sensitive and miscorrelate with the atomic H/C ratio, suggesting slight compositional and structural differences among the PCM precursors accreted.

Quirico, E.; Montagnac, G.; Rouzaud, J.; Bonal, L.; Bourot-Denise, M.; Duber, S.; Reynard, B.

2009-12-01

293

Temperature and oxygen fugacity constraints on CK and R chondrites and implications for water and oxidation in the early solar system  

NASA Astrophysics Data System (ADS)

Recent chondritic meteorite finds in Antarctica have included CB, CH, CK and R chondrites, the latter two of which are among the most oxidized materials found in meteorite collections. In this study we present petrographic and mineralogic data for a suite of CK and R chondrites, and compare to previous CK and R chondrites, as well as some CV. In particular we focus on the opaque minerals - magnetite, chromite, sulfides, and metal - as well as unusual silicates - hornblende, biotite, and plagioclase. Several mineral thermometers and oxy-barometers are utilized to calculate temperatures and oxygen fugacities for these unusual meteorites compared to other more common chondrite groups. R and CK chondrites show lower equilibrium temperatures than ordinary chondrites, even though they are at similar petrologic grades (e.g., thermal type 6). Oxygen fugacity calculated for CV and R chondrites ranges from values near the iron-wustite (IW) oxygen buffer to near the fayalite-magnetite-quartz (FMQ) buffer. In comparison, the fO 2 recorded by ilmenite-magnetite pairs from CK chondrites are much higher, from FMQ + 2 to FMQ + 4.5. The latter values are the highest recorded for materials in meteorites, and place some constraints on the formation conditions of these magnetite-bearing chondrites. Differences between mineralogic and O isotopic compositions of CK and R chondrites suggest two different oxidation mechanisms, which may be due to high and low water:rock ratios during metamorphism, or to different fluid compositions, or both.

Righter, K.; Neff, K. E.

294

Temperature and Oxygen Fugacity Constraints on CK and R Chondrites and Implications for Water and Oxidation in the Early Solar System  

NASA Technical Reports Server (NTRS)

Recent chondritic meteorite finds in Antarctica have included CB, CH, CK and R chondrites, the latter two of which are among the most oxidized materials found in meteorite collections. In this study we present petrographic and mineralogic data for a suite of CK and R chondrites, and compare to previous studies of CK and R, as well as some CV chondrites. In particular we focus on the opaque minerals magnetite, chromite, sulfides, and metal as well as unusual silicates hornblende, biotite, and plagioclase. Several mineral thermometers and oxy-barometers are utilized to calculate temperatures and oxygen fugacities for these unusual meteorites compared to other more common chondrite groups. R and CK chondrites show lower equilibrium temperatures than ordinary chondrites, even though they are at similar petrologic grades (e.g., thermal type 6). Oxygen fugacity calculated for CV and R chondrites ranges from values near the iron-wustite (IW) oxygen buffer to near the fayalite-magnetite-quartz (FMQ) buffer. In comparison, the fO2 recorded by ilmenite-magnetite pairs from CK chondrites are much higher, from FMQ+3.1 to FMQ+5.2. The latter values are the highest recorded for materials in meteorites, and place some constraints on the formation conditions of these magnetite-bearing chondrites. Differences between mineralogic and O isotopic compositions of CK and R chondrites suggest two different oxidation mechanisms, which may be due to high and low water: rock ratios during metamorphism, or to different fluid compositions, or both.

Righter, K.; Neff, K. E.

2007-01-01

295

Chondrite thermal histories from Low-CA pyroxene microstructures: Autometamorphism versus prograde metamorphism revisited  

NASA Technical Reports Server (NTRS)

In order to constrain the thermal histories of chondritic meteorites, a detailed study of the microstructures of low-Ca pyroxenes produced experimentally and in types 4 and 5 ordinary chondrites was carried out. Cooling experiments on synthetic MgSiO3 at cooling rates between 2 and 10000 C/hr from the protopyroxene stability field into that of orthopyroxene (OPX) were performed and the products of these experiments were annealed for a variety of annealing times. There are clear microstructural differences between samples which were cooled and those which were subsequently annealed. A comparison of the microstructures observed in the experimental samples with those in H4-5 ordinary chondrites shows that they cannot have experienced a single stage cooling history, as proposed for the autometamorphism model.

Brearley, Adrian J.; Jones, Rhian H.; Papike, J. J.

1993-01-01

296

The origin of chondrites: Metal-silicate separation experiments under microgravity conditions - II  

Microsoft Academic Search

In order to understand mechanical processes likely to be occurring on the surfaces of asteroids, we have performed experiments with mineral mixtures on NASA's KC-135 microgravity facility. The behavior of sand and iron filing mixtures, the sand and metal being in the proportions and with grain sizes of ordinary chondrite meteorites, was observed with digital cameras as gas was flowed

S. R. Moore; M. Franzen; P. H. Benoit; D. W. G. Sears; A. Holley; M. Meyer; R. Godsey; J. Czlapinski

2003-01-01

297

Photographic observations of Neuschwanstein, a second meteorite from the orbit of the Príbram chondrite  

Microsoft Academic Search

Photographic observations of meteoroids passing through the atmosphere provide information about the population of interplanetary bodies in the Earth's vicinity in the size range from 0.1m to several metres. It is extremely rare that any of these meteoroids survives atmospheric entry to be recovered as a meteorite on the ground. Príbram was the first meteorite (an ordinary chondrite) with a

Pavel Spurný; Jürgen Oberst; Dieter Heinlein

2003-01-01

298

Ordinary electromagnetic mode instability  

NASA Technical Reports Server (NTRS)

The instability of the ordinary electromagnetic mode propagating perpendicular to an external magnetic field is studied for a single-species plasma with ring velocity distribution. The marginal instability boundaries for both the purely growing mode and the propagating growing modes are calculated from the instability criteria. The dispersion characteristics for various sets of plasma parameters are also given. The typical growth rates are of the order of the cyclotron frequency.

Cheng, C. Z.

1974-01-01

299

Silver isotope variations in chondrites: Volatile depletion and the initial 107Pd abundance of the solar system  

NASA Astrophysics Data System (ADS)

The extinct radionuclide 107Pd decays to 107Ag (half-life of 6.5 Ma) and is an early solar system chronometer with outstanding potential to study volatile depletion in the early solar system. Here, a comprehensive Ag isotope study of carbonaceous and ordinary chondrites is presented. Carbonaceous chondrites show limited variations ( ?107Ag = -2.1 to +0.8) in Ag isotopic composition that correlate with the Pd/Ag ratios. Assuming a strictly radiogenic origin of these variations, a new initial 107Pd/ 108Pd of 5.9 (±2.2) × 10 -5 for the solar system can be deduced. Comparing the Pd-Ag and Mn-Cr data for carbonaceous chondrites suggests that Mn-Cr and Pd-Ag fractionation took place close to the time of calcium-aluminium-rich inclusion (CAI) and chondrule formation ˜4568 Ma ago. Using the new value for the initial 107Pd abundance, the revised ages for the iron-rich meteorites Gibeon (IVA, 8.5 +3.2/-4.6 Ma), Grant (IIIAB, 13.0 +3.5/-4.9 Ma) and Canyon Diablo (IA, 19.5 +24.1/-10.4 Ma) are consistent with cooling rates and the closure temperature of the Pd-Ag system. In contrast to carbonaceous chondrites, ordinary chondrites show large stable isotope fractionation of order of 1 permil for 107Ag/ 109Ag. This indicates that different mechanisms of volatile depletion were active in carbonaceous and ordinary chondrites. Nebular processes and accretion, as experienced by carbonaceous chondrites, did not led to significant Ag isotope fractionation, while the significant Ag isotope variations in ordinary chondrites are most likely inflicted by open system parent body metamorphism.

Schönbächler, M.; Carlson, R. W.; Horan, M. F.; Mock, T. D.; Hauri, E. H.

2008-11-01

300

Ar-40/Ar-39 dating of collisional events in chondrite parent bodies  

NASA Technical Reports Server (NTRS)

Ar-40/Ar-39 age dating of a number of shocked ordinary chondrites is interpreted in terms of collisional degassing events of meteorite parent bodies, probably in the asteroid belt. Examples of L, H, and at least one LL chondrite show episodic degassing. Degassing ages suggest several distinct events ranging from about 0.03 aeon to 0.7 aeon and probably higher. All specimens of either the H or L chondrites are not consistent with a single age event. A direct correlation exists between the degree of shock heating and the fraction of argon lost during degassing. However, no chondrite yet analyzed shows complete degassing of its high-temperature phase. Consequently, whole rock K-Ar ages are not accurate monitors of the time of the shock event.

Bogard, D. D.; Wright, R. J.; Husain, L.

1976-01-01

301

On the Relationship between Cosmic Ray Exposure Ages and Petrography of CM Chondrites  

NASA Technical Reports Server (NTRS)

Carbonaceous (C) chondrites are potentially the most primitive among chondrites because they mostly escaped thermal metamorphism that affected the other chondrite groups. C chondrites are chemically distinguished from other chondrites by their high Mg/Si ratios and refractory elements, and have experienced various degrees of aqueous alteration. They are subdivided into eight subgroups (CI, CM, CO, CV, CK, CR, CB and CH) based on major element and oxygen isotopic ratios. Their elemental ratios vary over a wide range, in contrast to those of ordinary and enstatite chondrites which are relatively uniform. It is critical to know how many separate bodies are represented by the C chondrites. In this study we defined 4 distinct cosmic-ray exposure (CRE) age groups of CMs and systematically characterized the petrography in each of the 4 CRE age groups to determine whether the groups have significant petrographic differences with such differences probably reflecting different parent body (asteroid) geological processing, or multiple original bodies. We have reported the results of a preliminary grouping at the NIPR Symp. in 2013 [3], however, we revised the grouping and here report our new results.

Takenouchi, A.; Zolensky, M. E.; Nishiizumi, K.; Caffee, M.; Velbel, M. A.; Ross, K.; Zolensky, A.; Lee, L.; Imae, N.; Yamaguchi, A.; Mikouchi, T.

2014-01-01

302

To Be an Ordinary Department  

ERIC Educational Resources Information Center

In this article, the author shares his experience being the "chair" of the Department of Latin American and Puerto Rican Studies at Lehman College of the City University of New York. He also shares how this department proved to be an "ordinary" department at an "ordinary" liberal arts college. However, to conclude that they are an ordinary

Colburn, Forrest D.

2003-01-01

303

Fluid Inclusions in Carbonaceous Chondrites  

NASA Technical Reports Server (NTRS)

Fluid inclusions are present in carbonaceous chondrites. Of the chondrites studied (CI1, CM1 and 2, CV3) fluid inclusions were found only in CM2s and CI1s, and by extrapolation are most likely to be found there in the future. Additional information is contained in the original extended abstract.

Saylor, J.; Zolensky, M. E.; Bodnar, R. J.; Le L.; Schwandt, C.

2001-01-01

304

Chondrite thermal histories - Clues from electron microscopy of orthopyroxene  

NASA Astrophysics Data System (ADS)

Optically 'striated' orthopyroxenes in two ordinary chondrites, Allegan (H5) and Quenggouk (H4), are compared with shock-affected orthopyroxenes in Saint-Severin (LL6) and Ambapur Nagla (H5) by high-voltage transmission electron microscopy. The striated orthopyroxenes have very many, thin, evenly distributed lamellae of clinopyroxene. They are undeformed and also lack evidence of partial inversion from clinopyroxene to orthopyroxene. Striated orthopyroxene does not seem to be a reliable indicator of prograde metamorphism. Instead, it is interpreted as inverted protopyroxene, produced during the cooling of chondrules at slower rates than the rapid quenching of Type 3 chondrules. The conclusions are consistent with retrograde models for the evolution of H-group chondrites, in which the higher petrologic types are attributed to retarded cooling due to accretionary processes leading to the growth of the parent body.

Ashworth, J. R.

1980-01-01

305

The Galatia, Kansas, chondrite  

NASA Technical Reports Server (NTRS)

The paper describes the Galatia meteorite found August 1971 approximately 7 km ENE of Galatia, Barton County, Kansas (98 deg 53 min W, 38 deg 39.5 min N). The single stone weighed 23.9 kg and is partially weathered. Olivine (Fa 24.9) and pyroxene (Fs 20.9) compositions indicate L-group classification, and textural observations indicate that the stone is of petrologic type 6. While Galatia is similar in many respects to the Otis L6 chondrite found 20 miles to the west, Galatia does not have the brecciated structure of Otis and is therefore not part of the same fall.

Van Schmus, W. R.; Keil, K.; Lange, D. E.; Conrad, G. H.

1978-01-01

306

Asteroid 2008 TC3—Almahata Sitta: A spectacular breccia containing many different ureilitic and chondritic lithologies  

NASA Astrophysics Data System (ADS)

Asteroid 2008 TC3 impacted Earth in northern Sudan on October 7, 2008. The meteorite named Almahata Sitta was classified as a polymict ureilite. In this study, 40 small pieces from different fragments collected in the Almahata Sitta strewn field were investigated and a large number of different lithologies were found. Some of these fragments are ureilitic in origin, whereas others are clearly chondritic. As all are relatively fresh (W0-W0/1) and as short-lived cosmogenic radioisotopes were detected within two of the chondritic fragments, there is strong evidence that most, if not all belong to the Almahata Sitta meteorite fall. The fragments can roughly be subdivided into achondritic (ureilitic; 23 samples) and chondritic lithologies (17 samples). Among the ureilitic rocks are at least 10 different lithologies. A similar number of different chondritic lithologies also exist. Most chondritic fragments belong to at least seven different E-chondrite rock types (EH3, EL3/4, EL6, EL breccias, several different types of EL and EH impact melt rocks and impact melt breccias; some of the latter are shock-darkened). In addition, two H-group ordinary chondrite lithologies were identified, and one sample of a chondrite type that is so far unique. The latter has some affinities to R chondrites. Oxygen isotope compositions of 14 fragments provide further fundamental information on the lithological heterogeneity of the Almahata Sitta meteorite. Based on the findings presented in this study, the reflectance spectrum of asteroid 2008 TC3 has to be evaluated in a new light.

Bischoff, Addi; Horstmann, Marian; Pack, Andreas; Laubenstein, Matthias; Haberer, Siegfried

2010-10-01

307

26Mg-excess in hibonites of the Rumuruti chondrite Hughes 030  

NASA Astrophysics Data System (ADS)

The Rumuruti chondrites (R chondrites) constitute a new, well-established, chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Most samples of this group are gas-rich regolith breccias showing the typical light/dark structure and consist of abundant fragments of various parent body lithologies embedded in a fine-grained, olivine-rich matrix. Most R chondrites contain the typical components of primitive chondrites including chondrules, chondrule and mineral fragments, sulfides, and rare calcium-aluminum-rich inclusions (CAIs). In Hughes 030, an interesting CAI consisting of abundant hibonite and spinel was found. Mg isotopic analyses revealed excess 26Mg in components of R chondrites for the first time. The hibonite grains with high Al/Mg values (~1500 to 2600) show resolved 26Mg excess. The slope of the correlation line yields an initial 26Al/ 27Al = (1.4 ± 0.3) x 10-6, which is ~40 times lower than the initial value measured in CAIs from primitive meteorites. The inferred difference in 26Al abundance implies a time difference of ~4 million years for the closure of the Al-Mg system between CAIs from primitive chondrites and the Hughes 030 CAI. Based on mineralogy and the petrographic setting of the hibonite-rich CAI, it is suggested that 4 million years reflect the time interval between the formation of the CAI and the end of its secondary alteration. It is also suggested that most of this alteration may have occurred in the nebula (e.g. Zn- and Fe-incorporation in spinels). However, the CAI could not have survived in the nebula as a free floating object for a long period of time. Therefore, the possibility of storage in a precursor planetesimal for a few million years, resetting the magnesium-aluminum isotopic system, prior to impact brecciation, excavation, and accretion of the final R chondrite parent body cannot be ruled out.

Bischoff, A.; Srinivasan, G.

2003-01-01

308

Effect of metamorphism on isolated olivine grains in CO3 chondrites  

NASA Technical Reports Server (NTRS)

The presence of a metamorphic sequence in the CO3 chondrite group has been shown previously to result in changes in properties of chondrule silicates. However, the role of isolated olivine grains during metamorphism of these chondrites has not been addressed. Isolated olivine grains in two metamorphosed CO3 chondrites, Lance and Isna, have been investigated in this study in order to assess the compositional properties of isolated olivine grains that may be attributable to metamorphism. Compositional changes in isolated olivines with increasing petrologic subtype are very similar to changes in chondrule olivines in the same chondrites. Olivine compositions from all occurrences (chondrules, isolated grains, and matrix) converge with increasing petrologic subtype. The degree of equilibration of minor elements is qualitatively related to the diffusion rate of each element in olivine, suggesting that diffusion-controlled processes are the most important processes responsible for compositional changes within the metamorphic sequence. The data are consistent with metamorphism taking place in a closed system on the CO3 chondrite parent body. Fe-poor olivine grains in metamorphosed chondrites are characterized by an Fe-rich rim, which is the result of diffusion of Fe into the grains from Fe-rich matrix. In some instances, 'complex', Fe-rich rims have been identified, which appear to have originated as igneous overgrowths and subsequently to have been overprinted by diffusion processes during metamorphism. Processes experienced by CO3 chondrites are more similar to those experienced by the ordinary chondrites than to those encountered by other carbonaceous chondrites, such as the CV3 group.

Jones, Rhian H.

1993-01-01

309

Petrology and oxygen isotope compositions of chondrules in E3 chondrites  

NASA Astrophysics Data System (ADS)

Chondrules in E3 chondrites differ from those in other chondrite groups. Many contain near-pure endmember enstatite (Fs <1). Some contain Si-bearing FeNi metal, Cr-bearing troilite, and, in some cases Mg, Mn- and Ca-sulfides. Olivine and more FeO-rich pyroxene grains are present but much less common than in ordinary or carbonaceous chondrite chondrules. In some cases, the FeO-rich grains contain dusty inclusions of metal. The oxygen three-isotope ratios (? 18O, ? 17O) of olivine and pyroxene in chondrules from E3 chondrites, which are measured using a multi-collection SIMS, show a wide range of values. Most enstatite data plots on the terrestrial fractionation (TF) line near whole rock values and some plot near the ordinary chondrite region on the 3-isotope diagram. Pyroxene with higher FeO contents (˜2-10 wt.% FeO) generally plots on the TF line similar to enstatite, suggesting it formed locally in the EC (enstatite chondrite) region and that oxidation/reduction conditions varied within the E3 chondrite chondrule-forming region. Olivine shows a wide range of correlated ? 18O and ? 17O values and data from two olivine-bearing chondrules form a slope ˜1 mixing line, which is approximately parallel to but distinct from the CCAM (carbonaceous chondrite anhydrous mixing) line. We refer to this as the ECM (enstatite chondrite mixing) line but it also may coincide with a line defined by chondrules from Acfer 094 referred to as the PCM (Primitive Chondrite Mineral) line ( Ushikubo et al., 2011). The range of O isotope compositions and mixing behavior in E3 chondrules is similar to that in O and C chondrite groups, indicating similar chondrule-forming processes, solid-gas mixing and possibly similar 16O-rich precursors solids. However, E3 chondrules formed in a distinct oxygen reservoir. Internal oxygen isotope heterogeneity was found among minerals from some of the chondrules in E3 chondrites suggesting incomplete melting of the chondrules, survival of minerals from previous generations of chondrules, and chondrule recycling. Olivine, possibly a relict grain, in one chondrule has an R chondrite-like oxygen isotope composition and may indicate limited mixing of materials from other reservoirs. Calcium-aluminum-rich inclusions (CAIs) in E3 chondrites have petrologic characteristics and oxygen isotope ratios similar to those in other chondrite groups. However, chondrules from E3 chondrites differ markedly from those in other chondrite groups. From this we conclude that chondrule formation was a local event but CAIs may have all formed in one distinct place and time and were later redistributed to the various chondrule-forming and parent body accretion regions. This also implies that transport mechanisms were less active at the time of and following chondrule formation.

Weisberg, Michael K.; Ebel, Denton S.; Connolly, Harold C.; Kita, Noriko T.; Ushikubo, Takayuki

2011-11-01

310

Birth in an Ordinary Instant  

PubMed Central

Our daily lives are a series of ordinary moments and unnoticed thresholds—times that define us in ways we often do not give much attention. While we consider childbirth to be one of life's extra-ordinary events, the hours of labor and birth need not be dramatic (or traumatic) ones. I describe a quiet, well-supported birth in the Netherlands that is cause for celebration of the beauty of an ordinary instant that can define and enrich the human experience.

De Vries, Charlotte

2010-01-01

311

Deuterium-depleted hydrogen in acid-insoluble residues of the Efremovka chondrite  

NASA Astrophysics Data System (ADS)

The paper presents results of an analysis of the isotope composition of hydrogen in three acid-insoluble residues of the Efremovka carbonaceous chondrite. The magnitude of deuterium depletion in two of these residue samples is similar to that observed in ordinary and enstatitic chondrites, suggesting that a single reservoir of hydrogen with D/H = 0.0001 existed in the early solar system. This reservoir might have been formed as the result of mixing of interstellar hydrogen with hydrogen released during the vaporization of deuterium-enriched interstellar dust.

Fisenko, A. V.; Devirts, A. L.; Lagutina, E. P.; Semenova, L. F.; Lavrukhina, A. K.

312

Formation and Processing of Amorphous Silicates in Primitive Carbonaceous Chondrites and Cometary Dust  

NASA Technical Reports Server (NTRS)

Chondritic-porous interplanetary dust particles (CP IDPs) exhibit strongly heterogeneous and unequilibrated mineralogy at sub-micron scales, are enriched in carbon, nitrogen and volatile trace elements, and contain abundant presolar materials [1-4]. These observations suggest that CP IDPs have largely escaped the thermal processing and water-rock interactions that have severely modified or destroyed the original mineralogy of primitive meteorites. CP IDPs are believed to represent direct samples of the building blocks of the Solar System - a complex mixture of nebular and presolar materials largely unperturbed by secondary processing. The chemical and isotopic properties of CP IDPs and their atmospheric entry velocities are also consistent with cometary origins. GEMS (glass with embedded metal and sulfides) grains are a major silicate component of CP IDPs. GEMS grains are < 0.5 microns in diameter objects that consist of numerous 10 to 50 nm-sized Fe-Ni metal and Fe-Ni sulfide grains dispersed in a Mg-Si-Al-Fe amorphous silicate matrix [2, 5]. Based on their chemistry and isotopic compositions, most GEMS appear to be non-equilibrium condensates from the early solar nebula [2]. If GEMS grains are a common nebular product, then they should also be abundant in the matrices of the most physically primitive chondritic meteorites. Although amorphous silicates are common in the most primitive meteorites [6-9], their relationship to GEMS grains and the extent to which their compositions and microstructure have been affected by parent body processing (oxidation and aqueous alteration) is poorly constrained. Here we compare and contrast the chemical, microstructural and isotopic properties of amorphous silicates in primitive carbonaceous chondrites to GEMS grains in IDPs.

Keller, Lindsay P.; Messenger, S.

2012-01-01

313

Partial asteroid differentiation revealed by paleomagnetism of R-chondrite meteorites  

NASA Astrophysics Data System (ADS)

The study of the paleomagnetism of extraterrestrial material allows constraining magnetic fields in the early solar system. This can help us to understand primordial aspects of the history of the early solar system. Indeed, nebular or solar magnetic fields could have played a major role in the accretion process that generated the primary components of our solar system. Internal fields (i.e. generated by a dynamo within a solid body) are also of substantial interest since they provide information on parent body evolution, especially on parent body differentiation. In this study we focused on Rumuruti chondrites (R chondrites) [1]. This meteorite group is of particular interest because R chondrites parent body is believed to have formed at a heliocentric distance greater than ordinary chondrites and less than carbonaceous chondrites [2]. As such, more than a simple new chondrite group, R chondrites offer the possibility to estimate the magnetic fields strength present in a yet unstudied part of the early solar system. Only preliminary paleomagnetic data are available for these meteorites [3]. We performed a detailed magnetic and paleomagnetic study of two R chondrites, PCA91002 and LAP03639. Our aim was to establish the nature and the origin of the magnetic field recorded in these meteorites. Our results show that these two meteorites contain sulfide (pyrrhotite). Magnetite was also found in PCA91002. Paleomagnetic analyses using thermal and alternating field demagnetization evidenced a stable and homogenous magnetization in both R chondrites. Because magnetic carriers in these meteorites are secondary phases formed during a metamorphic event several Myr after the parent body formation (I-Xe dating on magnetite, [4]), the magnetization was acquired after the possible existence of solar and nebular magnetic fields. Therefore the magnetizing field was most probably of internal origin. Using alternating field normalizing methods we estimate that the magnetization was acquired in a magnetic field of about 5 µT. In view of the intensity and the likely internal (dynamo) origin of the magnetizing field, partial differentiation of the R chondrite parent body seems necessary. This idea, with a chondritic crust overlying a differentiated inner body has recently been proposed for CV chondrites parent body [5,6]. [1] Schulze et al., 1994. Meteoritics 29, 275-286. [2] Khan et al., 2013. 44th Lunar and Planet. Sci. Conf., abstract 2059. [3] Gattacceca and Rochette 2004. Earth Planet . Sci. Lett., 277, 377-393. [4] Claydon et al., 2013. 44th Lunar and Planet. Sci. Conf., abstract 2211. [5] Carporzen et al., 2011. Proc. National Acad. Sci., 108, 6386-6389. [6] Elkins-Tanton et al., 2011. Earth Planet . Sci. Lett., 305, 1-10.

Cournède, Cécile; Gattacceca, Jérôme; Rochette, Pierre

2014-05-01

314

Magnetite-sulfide chondrules and nodules in CK carbonaceous chondrites - Implications for the timing of CK oxidation  

NASA Technical Reports Server (NTRS)

CK carbonaceous chondrites contain rare (about 0.1 vol pct) magnetite-sulfide chondrules that range from about 240 to 500 microns in apparent diameter and have ellipsoidal to spheroidal morphologies, granular textures, and concentric layering. They resemble the magnetite-sulfide nodules occurring inside mafic silicate chondrules in CK chondrites. It seems likely that the magnetite-sulfide chondrules constitute the subset of magnetite-sulfide nodules that escaped as immiscible droplets from their molten silicate chondrule hosts during chondrule formation. The intactness of the magnetite-sulfide chondrules and nodules implies that oxidation of CK metal occurred before agglomeration. Hence, the pervasive silicate darkening of CK chondrites was caused by the shock mobilization of magnetite and sulfide, not metallic Fe-Ni and sulfide as in shock-darkened ordinary chondrites.

Rubin, Alan E.

1993-01-01

315

Magnetite-sulfide chondrules and nodules in CK carbonaceous chondrites - Implications for the timing of CK oxidation  

NASA Astrophysics Data System (ADS)

CK carbonaceous chondrites contain rare (about 0.1 vol pct) magnetite-sulfide chondrules that range from about 240 to 500 microns in apparent diameter and have ellipsoidal to spheroidal morphologies, granular textures, and concentric layering. They resemble the magnetite-sulfide nodules occurring inside mafic silicate chondrules in CK chondrites. It seems likely that the magnetite-sulfide chondrules constitute the subset of magnetite-sulfide nodules that escaped as immiscible droplets from their molten silicate chondrule hosts during chondrule formation. The intactness of the magnetite-sulfide chondrules and nodules implies that oxidation of CK metal occurred before agglomeration. Hence, the pervasive silicate darkening of CK chondrites was caused by the shock mobilization of magnetite and sulfide, not metallic Fe-Ni and sulfide as in shock-darkened ordinary chondrites.

Rubin, A. E.

1993-03-01

316

The Carlisle Lakes-type chondrites: A new grouplet with high. Delta. sup 17 O and evidence for nebular oxidation  

SciTech Connect

Carlisle Lakes, ALH85151, and Y75302 are similar ungrouped chondrites which have petrologic and bulk compositional similarities to the ordinary chondrites, but are more oxidized; and their oxygen isotopic compositions differ. They represent a new grouplet which the authors call the Carlisle Lakes-type chondrites. They have the highest {Delta}{sup 17}O values (up to 2.91) measured to date. The whole chondrites and most of their chondrules plot on the same mass fractionation line on an oxygen 3-isotope diagram. They are olivine rich (>70 vol%), essentially metal free, and most olivine is FeO rich, equilibrated at Fa{sub 38}. Rare olivine and pyroxene grains in chondrules and fragments are zoned, and these are important in discerning the history of these chondrites. The zoning does not appear to have formed during crystallization from a melt droplet chondrule, but post-dated chondrule formation. Two hypotheses are postulated to explain the zoning: (1) parent-body thermal metamorphism and (2) nebular gas-solid exchange reactions accompanied by condensation of new FeO-rich olivine, utilizing existing olivine surfaces as nucleation sites. The occurrence of steep Fe-Mg compositional gradients of core-to-rim profiles, oscillatory zoning in olivine, fayalitic rims of Fa{sub 45} that exceed instead of approach the equilibrium composition of the matrix (Fa{sub 38}), and olivine-filled veins in zoned pyroxenes are more compatible with the nebular hypothesis. The Carlisle Lakes-type chondrites may have originally been derived from an ordinary chondrite-like precursor which was later oxidized, prior to its final lithification. However, the oxygen isotopic compositions of the whole chondrites and most of their chondrules suggest that the precursor probably formed in an oxygen isotopically distinct environment.

Weisberg, M.K. (American Museum of Natural History, New York, NY (United States) Brooklyn Coll., NY (United States)); Prinz, M. (American Museum of Natural History, New York, NY (United States)); Kojima, Hideyasu; Yanai, Keizo (National Inst. of Polar Research, Tokyo (Japan)); Clayton, R.N.; Mayeda, T.K. (Univ. of Chicago, IL (United States))

1991-09-01

317

Accretion and differentiation of the Earth in an E-chondrite framework  

NASA Astrophysics Data System (ADS)

The isotopic similarities between Enstatite chondrites and the Earth can be taken as the indication of a strong parenthood between the two materials. We combine the composition of E-chondrites with terrestrial constraints on the Primitive Upper Mantle (PUM) and with geophysical constraints on the light elements contained in the Core to assess the composition of the Bulk Earth. This "E-chondrite framework" predicts a Primitive Lower Mantle (PLoM) enriched in Si and Fe, and depleted in Mg, Ca and Al, relative to the PUM. Using experimentally determined metal/silicate partition coefficients, we show that the difference of chemical composition between PUM and PLoM can be fully accounted for by a change of P,T conditions prevailing during the formation of the Core. The PLoM's Si, Fe content reflect relatively low P,T conditions of melt extraction from the earth building material during a first stage of Core formation, whereas the pyrolitic composition of PUM reflect metal extraction under high P,T conditions. The determination of these latter conditions correspond those inferred for a magma ocean produced by the giant impact, extending down to about 1350 km, or 55 GPa. The PUM/PLoM chemical heterogeneity did not require any additional crystal fractionation between the two mantles, or any ad hoc evolution of the chemical composition of the accreting material. The composition inferred for the Earth Building Material (EBM), notably its Si/Mg, Fe/Al and Mg/Ca ratios, points out to a closer parenthood with EH3 chondrites, whereas ad hoc mixtures between ordinary and carbonaceous chondrites are ruled out by isotopic as well as chemical arguments. The only differences between EH3 and Earth Building Material are the higher Sulfur content of EH3 chondrites, as well as their slight ?30Si depletion relative to the Earth's mantle. We show that these differences are likely to result from Si isotopic equilibration and sulfurization of residual EBM during its outward migration from 1 UA toward the asteroid belt, due to an associated decrease of temperature of 300-400 K.hemical (RedOx) constraints on the Earth Building Material (EH, EL = enstatite chondrites; CI, CM, CV, CO = carbonaceous chondrites; L, H, LL = ordinary chondrites). Because LL iron concentration is low and largely oxidized, and CI iron totally oxidized, a mixture of them (FBM) plots far away from the Earth.

Kaminski, E.; Javoy, M.

2012-12-01

318

Chondritic meteorites and the solar nebula  

NASA Astrophysics Data System (ADS)

Present understanding of the formation of chondritic meteorites in the solar nebula is reviewed, focusing on the nebular history of chondritic material. The stages in the history of chondritic material are summarized. The processing of chondrules and refractory inclusions is considered, and the relatedness of chondrules and refractory inclusions is discussed.

Wood, J. A.

319

Porosity and Permeability Determinations of Chondritic Materials  

NASA Astrophysics Data System (ADS)

We describe results of porosity and permeability determinations of chondritic meteorites of low petrologic grade (1-3). The porosity results are comparable to those for chondritic interplanetary dust particles. Permeabilities of CM2 chondrites are slightly higher than those for CV3s.

Corrigan, C. M.; Zolensky, M. E.; Long, M.; Sapp, C. A.; Dahl, J. M.

1996-03-01

320

Origin and chronology of chondritic components: A review  

NASA Astrophysics Data System (ADS)

Mineralogical observations, chemical and oxygen-isotope compositions, absolute 207Pb- 206Pb ages and short-lived isotope systematics ( 7Be- 7Li, 10Be- 10B, 26Al- 26Mg, 36Cl- 36S, 41Ca- 41K, 53Mn- 53Cr, 60Fe- 60Ni, 182Hf- 182W) of refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], chondrules and matrices from primitive (unmetamorphosed) chondrites are reviewed in an attempt to test (i) the x-wind model vs. the shock-wave model of the origin of chondritic components and (ii) irradiation vs. stellar origin of short-lived radionuclides. The data reviewed are consistent with an external, stellar origin for most short-lived radionuclides ( 7Be, 10Be, and 36Cl are important exceptions) and a shock-wave model for chondrule formation, and provide a sound basis for early Solar System chronology. They are inconsistent with the x-wind model for the origin of chondritic components and a local, irradiation origin of 26Al, 41Ca, and 53Mn. 10Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology. 41Ca- 41K, and 60Fe- 60Ni systematics are important for understanding the astrophysical setting of Solar System formation and origin of short-lived radionuclides, but so far have limited implications for the chronology of chondritic components. The chronological significance of oxygen-isotope compositions of chondritic components is limited. The following general picture of formation of chondritic components is inferred. CAIs and AOAs were the first solids formed in the solar nebula ˜4567-4568 Myr ago, possibly within a period of <0.1 Myr, when the Sun was an infalling (class 0) and evolved (class I) protostar. They formed during multiple transient heating events in nebular region(s) with high ambient temperature (at or above condensation temperature of forsterite), either throughout the inner protoplanetary disk (1-4 AU) or in a localized region near the proto-Sun (<0.1 AU), and were subsequently dispersed throughout the disk. Most CAIs and AOAs formed in the presence of an 16O-rich (? 17O ˜ -24 ± 2‰) nebular gas. The 26Al-poor [( 26Al/ 27Al) 0 < 1 × 10 -5], 16O-rich (? 17O ˜ -24 ± 2‰) CAIs - FUN (fractionation and unidentified nuclear effects) CAIs in CV chondrites, platy hibonite crystals (PLACs) in CM chondrites, pyroxene-hibonite spherules in CM and CO chondrites, and the majority of grossite- and hibonite-rich CAIs in CH chondrites—may have formed prior to injection and/or homogenization of 26Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are 16O-depleted (? 17O > -10‰) and have ( 26Al/ 27Al) 0 similar to those in chondrules (<1 × 10 -5). These CAIs probably experienced melting during chondrule formation. Chondrules and most of the fine-grained matrix materials in primitive chondrites formed 1-4 Myr after CAIs, when the Sun was a classical (class II) and weak-lined T Tauri star (class III). These chondritic components formed during multiple transient heating events in regions with low ambient temperature (<1000 K) throughout the inner protoplanetary disk in the presence of 16O-poor (? 17O > -5‰) nebular gas. The majority of chondrules within a chondrite group may have formed over a much shorter period of time (<0.5-1 Myr). Mineralogical and isotopic observations indicate that CAIs were present in the regions where chondrules formed and accreted (1-4 AU), indicating that CAIs were present in the disk as free-floating objects for at least 4 Myr. Many CAIs, however, were largely unaffected by chondrule melting, suggesting that chondrule-forming events experienced by a nebular region could have been small in scale and limited in number. Chondrules and metal grains in CB chondrites formed during a single-stage, highly-energetic event ˜4563 Myr ago, possibly from a gas-melt plume produced by collision between planetary embryos.

Krot, A. N.; Amelin, Y.; Bland, P.; Ciesla, F. J.; Connelly, J.; Davis, A. M.; Huss, G. R.; Hutcheon, I. D.; Makide, K.; Nagashima, K.; Nyquist, L. E.; Russell, S. S.; Scott, E. R. D.; Thrane, K.; Yurimoto, H.; Yin, Q.-Z.

2009-09-01

321

Serpentine Nanotubes in CM Chondrites  

NASA Technical Reports Server (NTRS)

The CM chondrites are primitive meteorites that formed during the early solar system. Although they retain much of their original physical character, their matrices and fine-grained rims (FGRs) sustained aqueous alteration early in their histories [1- 3]. Serpentine-group minerals are abundant products of such alteration, and information regarding their structures, compositions, and spatial relationships is important for determining the reactions that produced them and the conditions under which they formed. Our recent work on FGRs and matrices of the CM chondrites has revealed new information on the structures and compositions of serpentine-group minerals [4,5] and has provided insights into the evolution of these primitive meteorites. Here we report on serpentine nanotubes from the Mighei and Murchison CM chondrites [6].

Zega, Thomas J.; Garvie, Laurence A. J.; Dodony, Istvan; Buseck, Peter R.

2004-01-01

322

Porosity of Chondrites: Some Regularities  

NASA Astrophysics Data System (ADS)

Porosity depends on processes of formation and evolutionary history of composite materials of chondrites. The same processes may stipulate, in particular, the content of radiogenic 4He and 40Ar in the meteorites. We have analysed the distribution of porosity of H and L chondrites depending on ratio of T(sub)4/T(sub)40, (where T(sub)4 and T(sub)40 are U, Th-He and K-Ar gas-retention ages, respectively) for determination of possible interrelation between these features of meteorites.

Alexeev, V. A.; Kuyunko, N. S.

1996-03-01

323

Magnetite-Pentlandite Chondrules in CK Chondrites  

NASA Astrophysics Data System (ADS)

Opaque-mineral-rich chondrules are among the least common chondrule types and have received scant attention since their discovery by Gustav Rose in 1864. This category includes chondrules comprised principally of metallic Fe-Ni (Tschermak, 1885; Gooding and Keil, 1981) or chrome-spinel (Ramdohr, 1967; Krot et al., 1992). Here I report the occurrence of seven magnetite-pentlandite chondrules in Karoonda (CK4), PCA82500 (CK4/5) and EET90007 (CK5). The chondrules range in size from 225x255 micrometers to 440x570 micrometers and have ellipsoidal or spheroidal morphologies. All are concentrically layered: five of the chondrules have 20-60-micrometer-thick magnetite rims surrounding pentlandite-rich cores or mantles; two of the chondrules have thin pentlandite rims surrounding magnetite-rich cores and mantles. One chondrule from Karoonda has four distinct alternating layers of magnetite and pentlandite. Accessory phases, which occur in one or more of the chondrules, include pyrrhotite, chlorapatite, ilmenite, and chalcopyrite. The chondrules have finely to coarsely granular textures; in the center of one chondrule from PCA82500 there is a 25x100 micrometers subhedral crystal of pentlandite. All of the chondrules appear to be recrystallized, presumably due to metamorphism of their host rocks. The magnetite-pentlandite chondrules are very similar to the magnetite-pentlandite-rich nodules within mafic silicate chondrules in CK chondrites. I examined four nodules that range in size from 58x64 micrometers to 400x670 micrometers and have ellipsoidal or spheroidal morphologies. All but one are concentrically layered; one nodule from a Karoonda chondrule has four concentric layers of magnetite+-pentlandite. The nodules probably formed from immiscible sulfide-oxide droplets within their molten silicate chondrule hosts during chondrule formation. Upon cooling, magnetite and monosulfide solid solution (Mss) precipitated during cotectic crystallization; the Mss transformed into pentlandite after cooling below 610 degrees C. Dense immiscible liquid droplets tend to get expelled from the equators of their spinning molten spheroidal hosts if they do not happen to be located in the molten spheroids' centers where the centrifugal forces are minimal. This phenomenon also affected the four magnetite-pentlandite nodules: one nodule is near the margin and three are at the centers of their host silicate chondrules. The similarities in size, shape, mineralogy, and texture between the magnetite-pentlandite chondrules and nodules indicate that these chondrules constitute the set of immiscible nodule droplets that were lost to their mafic silicate chondrule hosts after melting. The occurrence of magnetite-pentlandite chondrules and nodules has important implications for the timing of CK chondrite oxidation. If oxidation had occurred after agglomeration and transformed metallic Fe-Ni into magnetite, the large (factor of 2) increase in molar volume would have disrupted the nodules and chondrules and destroyed the evidence for rhythmic layering. The intactness of the chondrules and nodules implies that the oxidation of fine-grained metallic Fe-Ni into magnetite probably occurred before agglomeration, either during chondrule formation in a region of high fo(sub)2 or within porous chondrule-precursor dust clumps after nebular temperatures cooled below ~130 degrees C. Hence, the pervasive silicate darkening of CK chondrites (Kallemeyn et al., 1991; Rubin, 1992) was caused by the shock mobilization of magnetite and pentlandite, not metallic Fe-Ni and troilite as in shock-darkened ordinary chondrites. References: Gooding J.L. and Keil K. (1981) Meteoritics 16, 17- 43; Kallemeyn G.W., Rubin A.E. and Wasson J.T. (1991) Geochim. Cosmochim. Acta 55, 881-892; Krot A., Ivanova M.A. and Wasson J.T. (1992) Earth Planet. Sci. Lett., submitted; Ramdohr P. (1967) Geochim. Cosmochim. Acta 31, 1961-1967; Rubin A.E. (1992) Geochim. Cosmochim. Acta 56, 1705-1714; Tschermak G. (1885) Die Mikroskopische Beschaffenheit der Meteoriten. Schweizerbart'sche Verlagshandlung, Stuttga

Rubin, A. E.

1992-07-01

324

Evidence for fractional condensation and reprocessing at high temperatures in CH chondrites  

NASA Astrophysics Data System (ADS)

We performed a detailed study of silica-rich components (SRC) in the paired CH chondrites Acfer 182 and 207. These SRCs appear either as chondrules or fragments, and they contribute <0.1 vol% to the bulk meteorite. They usually contain a silica and a silicate portion. Both portions are, in most cases, cryptocrystalline and have bulk SiO2-concentrations between 65 and 85 wt%. The silicate generally has a pyroxene normative composition. The silica often appears as blebs within the silicate matrix or vice versa. If there are no blebs, silica and silicate still form rounded interfaces. The SRCs are depleted in refractory elements like Ca, Al, and Ti relative to CI. A few SRC- like objects are extremely rich in Mn and show no depletion in refractory elements. We conducted micro-Raman studies on the silica portions of the SRCs to determine their structure, and we identified several silica phases: -quartz, cristobalite, glass, and a yet unidentified polymorph. The silicate portion is glass when the silica is glass and crystalline when the silica is crystalline. The low contents of Al and Ca make an igneous origin of the SRCs very unlikely, and the absence of metal excludes the formation by reduction of pyroxene. We suggest, instead, a fractional condensation origin of the SRCs from a Si-enriched gas after removal of gaseous Mg by forsterite condensation. Additional evidence for fractional condensation is provided by a unique layered object with olivine in the core, pyroxene and metal at the rim, and silica at the outermost border; these layers record the condensation sequence. Two chondrules were found with several percent of Mn and high Cr, Na, and K contents, providing further evidence for condensation from a fractionated gas. The texture of the SRCs and the occurrence of cristobalite and silica glass, however, require formation by liquid immiscibility at high temperatures, above 1968 K, and subsequent fast cooling. Therefore, we propose a 2-stage model for the formation of SRCs in CH chondrites: 1) fractional condensation of forsterite, enstatite, and SiO2-rich phases; and 2) reheating of SiO2-rich components to temperatures above 1968 K followed by rapid cooling. All other phases identified in CH chondrites can be understood within the framework of this model. Thus, the extremely unequilibrated CH chondrites provide a wealth of evidence for fractional condensation processes in the early solar nebula, in metals (Meibom et al. 1999), and in silicates.

Hezel, D. C.; Palme, H.; Brenker, F. E.; Nasdala, L.

2003-08-01

325

Amino acids in carbonaceous chondrites.  

PubMed

For almost 20 years laboratory experiments have advanced the concepts of chemical evolution, particularly with regard to formation of the amino acids. What has been generally lacking is concrete natural evidence for this chemical evolution hypothesis. The recent development of sophisticated analytical techniques and availability of carbonaceous chondrites with a minimum of terrestrial contamination has resulted in the identification of amino acids which provide strong evidence for a natural extraterrestrial chemical synthesis. Since the initial find in the Murchison meteorite (a type II carbonaceous chondrite) of both protein and nonprotein amino acids with nearly equal abundances of D and L isomers, further studies have been carried out. These studies have revealed the presence of at least 35 amino acids; the population consists of a wide variety of linear, cyclic and polyfunctional amino acids which shows a trend of decreasing concentration with increasing carbon number. Investigations of the Murray meteorite (a type II carbonaceous chondrite) has produced similar results, but studies of the Orgueil meteorite (a type I carbonaceous chondrite) show only a limited suite of amino acids, some of which appear to be indigenous while others appear to be terrestrial contaminanats. A sample of the Murchison meteorite was extracted with D2O and in addition of 'free' amino acids, showing no deuterium incorporation, some amino acids showed the presence of deuterium suggesting either a 'precursor(s)' or hydrogen-deuterium exchange which require(s) formation of carbon-hydrogen bonds. PMID:1153189

Lawless, J G; Peterson, E

1975-01-01

326

Magnetic study of CM chondrites  

NASA Astrophysics Data System (ADS)

The study of the paleomagnetism of carbonaceous chondrites can lead to an estimate of the magnetic fields present in the early solar system. CM chondrites contain abundant magnetite formed during aqueous alteration on their parent body, and have not been heated after that, making them interesting targets for paleomagnetism. We performed a detailed and comparative magnetic study (paleomagnetism and rock magnetism) of three CM chondrites: Paris, Cold Bokkeveled and Murchison. These three meteorites cover a wide range of aqueous alteration, with increasing alteration from Paris [1] to Murchison to Cold Bokkeveld [2]. Paris is a unique CM chondrite significantly less aqueously altered than other CM chondrites. Our magnetic data show that in contrast with other CM, Paris meteorite contains abundant FeNi metal (of nebular origin) together with magnetite and pyrrhotite (of asteroidal origin). Paleomagnetic results of Paris show that unfortunately the meteorite has been exposed to a strong artificial magnetic field (magnet), precluding the study of the natural magnetization (of possible nebular origin) carried by FeNi. However, a high-coercivity magnetization carried by pyrrhotite is still preserved in the meteorite. It is homogeneous in direction and intensity at the scale of the meteorite. We interpret this high-coercivity magnetization as a pre-terrestrial chemical remanent magnetization acquired on the parent body in a field of a few µT. Our preliminary results on Murchison also evidenced an stable and homogeneous magnetization in the meteorite. Therefore a long-lasting stable magnetizing field seems necessary to account for the paleomagnetism of both meteorites. Because crystallization of pyrrhotite and magnetite occurred several Myr after the formation of the parent body [3] (i.e. after possible existence of strong solar and nebular magnetic field), the magnetizing field was most probably created on the parent body. In view of its intensity, the most plausible origin for the magnetizing field is an internally generated dynamo field. This would imply that the parent body of CM chondrites was partially differentiated with a convecting metallic core. Such process has recently been proposed for the parent body of CV chondrites [4, 5]. [1] Zanda et al., 2010. Meteoritics Planetary Sci., 45, 222-222. [2] Rubin et al., 2007. Geo. et Cosmo. Acta, 71, 2361-2382 [3] Krot et al., 2005. UCRL-BOOK-217207 [4] Carporzen et al., 2011. Proc. National Acad. Sci., 108, 6386-6389. [5] Elkins-Tanton et al., 2011. Earth Planet . Sci. Lett., 305, 1-10.

Cournède, C.; Gattacceca, J.; Zanda, B.; Rochette, P.

2012-04-01

327

Ordinary People, Ordinary Places: The Civil Rights Movement. [Lesson Plan].  

ERIC Educational Resources Information Center

Martin Luther King, Jr. framed the goals and strategies of the Civil Rights Movement for a national audience, and with his message of nonviolent protest, he inspired ordinary African Americans to demand equal rights as American citizens. This lesson focuses on the individual men and women who embraced King's message and advanced the Civil Rights…

2002

328

New component of the Mezo-Madaras breccia - a microchondrule- and carbon-bearing L-related chondrite  

SciTech Connect

Microchondrules with apparent diameters 2-150 microns are found in a black carbon-bearing inclusion in Mazo-Madaras. Some are homogeneous (glassy or microcrystalline); others show two phases (mainly silica and pyroxene-rich glass). The bulk chemical composition of the inclusion is related to the host chondrite, in which silica-pyroxene chondrules are ubiquitous. Small black lumps of the same kind are dispersed in bulk Mezo-madaras. This L-related carbon-bearing material may represent a new specimen of C-rich ordinary chondrite. 13 references.

Michel-Levy, M.C.

1988-03-01

329

A refractory inclusion in the Kaba CV3 chondrite - Some implications for the origin of spinel-rich objects in chondrites  

NASA Astrophysics Data System (ADS)

The first detailed petrographic and mineralogical study of a Ca, Al-rich inclusion (CAI) from the Kaba CV3 chondrite is reported. This 'fine-grained' CAI contains abundant small, rounded, rimmed, spinel-rich objects which have important features in common with the spinel-rich objects in other carbonaceous and ordinary chondrites. These nodules are interpreted as fractionated distillation residues of primitive dust. However, the available data do not unambiguously rule out a condensation origin for at least some of these objects. Finally, the preservation of distinct diopside-hedenbergite rims on the spinel-rich bodies and the small grain size of many minerals in the CAI matrix material both suggest that the CAI accreted cool and had a relatively cool thermal history in the Kaba parent body.

Fegley, B.; Post, J. E.

1985-11-01

330

Magnetic record in chondrite meteorites  

NASA Technical Reports Server (NTRS)

What we know about the magnetic record in chondrite meteorites based on new data and previously published results is summarized. Strips from thin slabs of chondrite meteorites were cut into near cubical subsamples (several mm on edge) numbering approximately 60 to approximately 120 per meteorite. A common orientation was assigned to each subsample from a given meteorite in order to ensure that we could discover the vector makeup of the bulk meteorite. The new data set includes: Shaw (L7), Roy (L5/6), Claytonville (L5), Plainview (H5), Etter (H5), Leoville (C3V), and Allende (C3V). In addition to these new results, literature data of sufficient detail, e.g. Bjurbole (L4), ALH769 (L6), Abee (E4), Allende (C3V), and Olivenza (L5), is considered.

Wasilewski, P. J.; Faris, J. L.; Obryan, M. V.

1993-01-01

331

The extent of aqueous alteration in C-class asteroids, and the survival of presolar isotopic signatures in chondrites  

NASA Astrophysics Data System (ADS)

Several sample return missions are being planned by different space agencies for in situ sampling of undifferentiated bodies. Such missions wish to bring back to Earth pristine samples from C-class asteroids and comets to obtain clues on solar system formation conditions. A careful selection of targeted areas is required as many C-class asteroids and periodic comets have been subjected to collisional and space weathering processing since their formation. Their surfaces have been reworked by impacts as pointed out by the brecciated nature of many chondrites arrived to Earth, exhibiting different levels of thermal and aqueous alteration. It is not surprising that pristine chondrites can be considered quite rare in meteorite collections because they were naturally sampled in collisions, but several groups of carbonaceous chondrites contain a few members with promising unaltered properties. The CI and CM groups suffered extensive aqueous alteration [1], but for the most part escaped thermal metamorphism (only a few CMs evidence heating temperature over several hundred K). Both chondrite groups are water-rich, containing secondary minerals as consequence of the pervasive alteration of their primary mineral phases [2]. CO, CV, and CR chondrite groups suffered much less severe aqueous alteration, but some CRs are moderately aqueously altered. All five groups are good candidates to find unequilibrated materials between samples unaffected by aqueous alteration or metamorphism. The water was incorporated during accretion, and was released as consequence of shock after impact compaction, and/or by mild radiogenic heating. Primary minerals were transformed by water into secondary ones. Water soaking the bodies participated in chemical homogenization of the different components [1]. Hydrothermal alteration and collisional metamorphism changed the abundances of isotopically distinguishable presolar silicates [3]. Additional instruments in the landers to identify aqueous alteration signatures could help to get samples unbiased by parent body processes. Future work in this regard could be essential to successfully getting back to Earth samples to unveil the conditions in which the solar system formed. REF: [1] Trigo-Rodriguez J.M. & Blum J. 2009. Plan. Space Sci.57,243; [2] Rubin et al. (2007) GCA 71,2361; [3] Trigo-Rodriguez J.M. & Blum J. (2009). Pub.Ast.Soc.Aust.26,289

Trigo-Rodriguez, J. M.

2011-05-01

332

Fluorescent organic matter in carbonaceous chondrites.  

PubMed

Fluorescent organic matter in carbonaceous chondrites was investigated using a microscope equipped with a fluorescence spectrophotometer. Fluorescent particles were observed in powdered CM2 carbonaceous chondrites (Y-74662, Y-7791198, and Murchison) without carbon enrichment by acid treatments. Although it was difficult to find fluorescent particles in powdered sample of C3 chondrites (ALH-77307, Y-791717, and Allende) without acid treatments, many fluorescent particles were observed after carbon enrichment by acid treatments. Fluorescence of coronene and shock-altered graphite were observed using the same microscope and the same conditions as those for carbonaceous chondrites. PMID:11543334

Murae, T

1999-01-01

333

Oxygen Isotope Systematics of Chondrules from the Least Equilibrated H Chondrite  

NASA Technical Reports Server (NTRS)

Oxygen isotope compositions of bulk chondrules and their mineral separates in type 3 ordinary chondrites (UOC) show several % variability in the oxygen three isotope diagram with slope of approx.0.7 [1]. In contrast, ion microprobe analyses of olivine and pyroxene phenocrysts in ferromagnesian chondrules from LL 3.0-3.1 chondrites show mass dependent isotopic fractionation as large as 5% among type I (FeO-poor) chondrules, while type II (FeO-rich) chondrules show a narrow range (less than or equal to 1%) of compositions [2]. The .Delta(exp 17)O (=delta(exp 17)O-0.52xdelta(exp 18)O) values of olivine and pyroxene in these chondrules show a peak at approx.0.7% that are systematically lower than those of bulk chondrule analyses as well as the bulk LL chondrites [2]. Further analyses of glass in Semarkona chondrules show .17O values as high as +5% with highly fractionated d18O (max +18%), implying O-16-poor glass in chondrules were altered as a result of hydration in the parent body at low temperature [3]. Thus, chondrules in LL3.0-3.1 chondrites do not provide any direct evidence of oxygen isotope exchange between solid precursor and O-16-depleted gas during chondrule melting events. To compare the difference and/or similarity between chondrules from LL and H chondrites, we initiated systematic investigations of oxygen isotopes in chondrules from Yamato 793408 (H3.2), one of the least equilibrated H chondrite [4]. In our preliminary study of 4 chondrules, we reported distinct oxygen isotope ratios from dusty olivine and refractory forsterite (RF) grains compared to their host chondrules and confirmed their relict origins [5].

Kita, N. T.; Kimura, M.; Ushikubo, T.; Valley, J. W.; Nyquist, L. E.

2008-01-01

334

Micro-Scale Distributions of Major and Trace Elements in Chondrites  

NASA Technical Reports Server (NTRS)

The Hayabusa spacecraft has successfully returned to Earth after two touchdowns on the surface of Asteroid 25143 Itokawa. This asteroid is classified as an S-type and inferred to consist of materials similar to ordinary chondrites or primitive achondrites [1]. More than 1500 particles have been identified consisting of olivine, pyroxene, plagioclase, Fe sulfide and Fe metal, with compositions consistent with being of LL origin. While the chondritic components are familiar to us, the level of detail to which the Itokawa samples will be exposed to will be unprecedented given that the samples are reasonably large and accessible to a wide variety of techniques. In many ways, we expect that our knowledge base of the comparator chondrites will be found to be wanting. Chondrites are the building blocks of the solar system. However, these rocks are essentially breccias and they are quite variable in bulk element compositions as well as compositions of the individual components. We have initiated a program of analysis for chondrites focusing on major and trace element distributions between the mineral components and the matrix. The issues to be addressed include the homogeneity of matrix and chondrule components and the representivity of any given sample to the bulk meteorite. This may be particularly important given the limited numbers of Itokawa grains that may be available for a specific analysis. As an initial study, we have taken thin sections of carbonaceous chondrites to study the representivity of the matrix compositions. Spot locations were constrained to limited regions of the sections so as to assess the variability of a local scale. Further work will be required to assess variability over a centimeter scale.

Ireland, T. R.; Zolensky, M.

2011-01-01

335

Evidence for multiple sources of diamond from primitive chondrites.  

PubMed

Fine-grained diamonds, the most abundant form of circumstellar dust isolated from primitive meteorites, have elemental and isotopic characteristics that are dependent on the host meteorite type. Carbon isotopic compositions vary from -32 to -38 per mil, and nitrogen associated with the diamond changes in overall abundance by over a factor of four from 0.2 to 0.9 weight percent, between ordinary and CM2-type chondrites. Although the ratio of carbon to nitrogen evolves in a distinctive way during combustion of diamond separates, metamorphic degassing of nitrogen is not the main cause of the differences in nitrogen content. The data suggest that intrinsic differences must have been inherited by the diamonds at the time of their formation and that the diamonds were distributed heterogeneously in the solar nebula during condensation. However, the hypothesis that a distinct nitrogen carrier remains hidden within the diamond cannot be ruled out. PMID:17776409

Russell, S S; Arden, J W; Pillinger, C T

1991-11-22

336

Photometric and polarimetric properties of the Bruderheim chondritic meteorite  

NASA Technical Reports Server (NTRS)

Photometric and polarimetric laboratory measurements were made as a function of phase angle in the U(0.36 microns), G(0.54 microns) and R(0.67 microns) bands for 0, 30 and 60 deg incident illumination on four particle size ranges of Bruderheim, an L6 olivine-hypersthene chondritic meteorite. The four particle size ranges were: 0.25-4.76 mm coated with less than 74 microns powder, 74-250 microns, and less than 37 microns. In addition, normal reflectance measurements were made in the spectral range from 0.31 to 1.1 microns. Comparison with astronomical data reveals that none of the asteroids in the main belt for which adequate observations exist can be matched with Bruderheim, which is representative of the most common meteoritic material encountered by the Earth. However, it appears from the polarization and photometry data that the surface of the Apollo asteroid Icarus is consistent with an ordinary chondrite composition. This suggests the possibility that this material, although common in Earth-crossing orbits, is rare as a surface constituent in the main asteroid belt.

Egan, W. G.; Veverka, J.; Noland, M.; Hilgeman, T.

1973-01-01

337

Volatile elements in chondrites - Metamorphism or nebular fractionation  

NASA Technical Reports Server (NTRS)

Three of the most highly metamorphosed meteorites of their respective classes, Shaw (LL7), Karoonda (C5), and Coolidge (C4), were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Te, Tl, U, and Zn. Comparison with data by Lipschutz and coworkers (1977) on artificially heated primitive meteorites shows that the natural metamorphism of meteorites cannot have taken place in a system open to volatiles. Shaw, metamorphosed at 1300 C for more than 1 million yr, is less depleted in In, Bi, Ag, Te, Zn, and Tl than Krymka heated at 1000 C for 1 week. Karoonda, metamorphosed at 600 C for many millennia, is less depleted in Bi and Tl than Allende heated at 600 C for 1 week. Data on primordial noble gases also show that the volatile-element patterns of ordinary and carbonaceous chondrites were established by nebular condensation and changed little, if at all, during metamorphism. For enstatite chondrites, the evidence is still incomplete but seems to favor a nebular origin of the volatile pattern.

Takahashi, H.; Gros, J.; Higuchi, H.; Morgan, J. W.; Anders, E.

1978-01-01

338

Origin and history of chondrite regolith, fragmental and impact-melt breccias from Spain  

NASA Astrophysics Data System (ADS)

Six ordinary chondrite breccias from the Museo Nacional de Ciencias Naturales, Madrid (Spain), are described and classified as follows: the solar gas-rich regolith breccia Oviedo (H5); the premetamorphic fragmental breccias Cabezo de Mayo (type 6, L-LL), and Sevilla (LL4); the fragmental breccias Canellas (H4) and Gerona (H5); and the impact melt breccia, Madrid (L6). It is confirmed that chondrites with typical light-dark structures and petrographic properties typical of regolith breccias may (Oviedo) or may not (Canellas) be solar gas-rich. Cabezo de Mayo and Sevilla show convincing evidence that they were assembled prior to peak metamorphism and were equilibrated during subsequent reheating. Compositions of olivine and low-Ca pyroxene in host chondrite and breccia clasts in Cabezo de Mayo are transitional between groups L and LL. It is suggested, based on mineralogic and oxygen isotopic compositions of host and clasts, that the rock formed on the L parent body by mixing, prior to peak metamorphism. This was followed by partial equilibrium of two different materials: the indigenous L chondrite host and exotic LL melt rock clasts.

Casanova, I.; Keil, K.; Wieler, R.; San Miguel, A.; King, E. A.

1990-06-01

339

Origin and history of chondrite regolith, fragmental and impact-melt breccias from Spain  

NASA Technical Reports Server (NTRS)

Six ordinary chondrite breccias from the Museo Nacional de Ciencias Naturales, Madrid (Spain), are described and classified as follows: the solar gas-rich regolith breccia Oviedo (H5); the premetamorphic fragmental breccias Cabezo de Mayo (type 6, L-LL), and Sevilla (LL4); the fragmental breccias Canellas (H4) and Gerona (H5); and the impact melt breccia, Madrid (L6). It is confirmed that chondrites with typical light-dark structures and petrographic properties typical of regolith breccias may (Oviedo) or may not (Canellas) be solar gas-rich. Cabezo de Mayo and Sevilla show convincing evidence that they were assembled prior to peak metamorphism and were equilibrated during subsequent reheating. Compositions of olivine and low-Ca pyroxene in host chondrite and breccia clasts in Cabezo de Mayo are transitional between groups L and LL. It is suggested, based on mineralogic and oxygen isotopic compositions of host and clasts, that the rock formed on the L parent body by mixing, prior to peak metamorphism. This was followed by partial equilibrium of two different materials: the indigenous L chondrite host and exotic LL melt rock clasts.

Casanova, I.; Keil, K.; Wieler, R.; San Miguel, A.; King, E. A.

1990-01-01

340

H/L chondrite LaPaz Icefield 031047 ? A feather of Icarus?  

SciTech Connect

Antarctic meteorite LAP 031047 is an ordinary chondrite composed of loosely consolidated chondritic fragments. Its petrography, oxygen isotopic composition and geochemical inventory are ambiguous and indicate an intermediate character between H and L chondrites. Petrographic indicators suggest LAP 031047 suffered a shock metamorphic overprint below {approx}10 GPa, which did not destroy its unusually high porosity of {approx}27 vol%. Metallographic textures in LAP 031047 indicate heating above {approx}700 C and subsequent cooling, which caused massive transformation of taenite to kamacite. The depletion of thermally labile trace elements, the crystallization of chondritic glass to microcrystalline plagioclase of unusual composition, and the occurrence of coarsely crystallized chondrule fragments is further evidence for post-metamorphic heating to {approx}700-750 C. However, this heating event had a transient character because olivine and low-Ca pyroxene did not equilibrate. Nearly complete degassing up to very high temperatures is indicated by the thorough resetting of LAP 031047's Ar-Ar reservoir {approx}100 {+-} 55 Ma ago. A noble gas cosmic-ray exposure age indicates it was reduced to a meter-size fragment at < 0.5 Ma. In light of the fact that shock heating cannot account for the thermal history of LAP 031047 in its entirety, we test the hypothesis that this meteorite belonged to the near-surface of an Aten or Apollo asteroid that underwent heating during orbital passages close to the Sun.

Wittmann, Axel; Friedrich, Jon M.; Troiano, Julianne; Macke, Robert J.; Britt, Daniel T.; Swidle, Timothy D.; Weirich, John R.; Rumble III, Douglas; Lasue, Jeremie; King, David A. (Central Florida); (Lunar and Planetary Institute); (CIW); (Ariz); (Fordham); (LANL)

2011-10-28

341

Thermally Mobile Trace Elements in Carbonaceous Chondrites from Cold and Hot Deserts  

NASA Technical Reports Server (NTRS)

Some decades ago, Anders and co-workers used RNAA to classify a number of trace elements as being volatile during nebular condensation and accretion into primitive objects based upon their strong depletion in (equilibrated) ordinary chondrites relative to C1 chondrites. Such elements, e.g. Ag, Bi, Cd, Cs, In, Se, Te, Tl, Zn and others, exhibit nearly constant, C1-normalized atomic abundances in C2 (CM2) and in C3 chondrites. They interpreted the near-constancy of these abundances according to a 2-component model in which volatiles were introduced into carbonaceous (and other) chondrites as Cl material which was diluted with differing proportions of high-temperature (i.e. volatile-free) components. In this view, mean volatile element abundances of 0.48 in C2 and 0.24-0.29 x C1 in C3 chondrites indicated that C2 and C3 chondrites are, respectively, about 1:1 and 1:2-3 mixtures of Cl-like and high temperature materials. More recently, C1 normalized abundances of such volatile elements are nearly constant in most C2-6 chondrites (i.e. 25 non-Antarctic meteorites, nearly all falls, and 36 Antarctic finds) consistent with a 2- component mixing model. However, rather than being quantized, mean volatile element contents in each chondrite define a continuum from 0.92-0.14 x Cl for these 61 chondrites. A few carbonaceous chondrites - the first having been the NIPR consortium samples B-7904, Y-82162 and Y-86720 - show an altered pattern: many of the volatile elements in each exhibit the usual constancy of C1-normalized atomic abundances, but modified by further depletion of Cd and other elements like Tl and Bi. These are the most mobile trace elements, i.e. those most readily vaporized and lost from primitive meteorites during week-long heating at greater than or equal to 400 C under low ambient pressures (initially 10 (exp -5) atm H2), simulating metamorphic conditions in a primitive parent body. Similarities between mobile element data for B-7904, Y-82162 and Y-86720 with those for Murchison heated at 500-700 C suggest that these Antarctic C1 and C2-like chondrites were metamorphosed at temperatures like these in the interiors of their parent bodies. Metamorphic temperatures inferred from RNAA data and textural/mineralogic alterations are internally consistent, agreeing with those evident in heated Murchison samples. These 3 chondrites were also heated late in their histories since all have lost cosmogenic 3-He, presumably during close solar approach, and B-7904 and Y-86720 seem also to have lost substantial proportions of radiogenic He-4 and Ar-40, cf. data. Similarities in spectral reflectance data for C-, G-, B- and F- asteroids, for these meteorites and for heated Murchison samples suggest that thermally metamorphosed interior materials in these asteroids were excavated by impacts and re-deposited on them, forming their present surfaces. Establishment of the thermal metamorphic histories of carbonaceouis chondrites, then, is essential to establishing the evolution and present-day nature of C-type and related asteroids.

Lipschutz, M. E.

1999-01-01

342

Thermally Mobile Trace Elements in Carbonaceous Chondrites From Cold and Hot Deserts  

NASA Technical Reports Server (NTRS)

Some decades ago, Anders and co-workers used RNAA to classify a number of trace elements as being volatile during nebular condensation and accretion into primitive objects based upon their strong depletion in (equilibrated) ordinary chondrites relative to C1 chondrites. Such elements, e.g. Ag, Bi, Cd, Cs, In, Se, Te, Tl, Zn and others, exhibit nearly constant, C1-normalized atomic abundances in C2 (CM2) and in C3 chondrites. They interpreted the near-constancy of these abundances according to a 2-component model in which volatiles were introduced into carbonaceous (and other) chondrites as C1 material which was diluted with differing proportions of high-temperature (i.e. volatile-free) components. In this view, mean volatile element abundances of 0.48 in C2 and 0.24-0.29 x C1 in C3 chondrites indicated that C2 and C3 chondrites are, respectively, about 1:1 and 1:2-3 mixtures of C1-like and high temperature materials. More recently, Xiao and Lipschutz found that C-normalized abundances of such volatile elements are nearly constant in most C2-6 chondrites (i.e. 25 non-Antarctic meteorites, nearly all falls, and 36 Antarctic finds) consistent with a 2-component mixing model. However, rather than being quantized, mean volatile element contents in each chondrite define a continuum from 0.92-0.14 x C1 for these 61 chondrites. A few carbonaceous chondrites, the first having been the NIPR consortium samples B-7904, Y-82162 and Y-86720, show an altered pattern: many of the volatile elements in each exhibit the usual constancy of C1-normalized atomic abundances, but modified by further depletion of Cd and other elements like Tl and Bi. These are the most mobile trace elements, i.e. those most readily vaporized and lost from primitive meteorites during week-long heating at greater than or equal to 400 C under low ambient pressures (initially 10(exp -5) atm H2), simulating metamorphic conditions in a primitive parent body. Similarities between mobile element data for B-7904, Y-82162 and Y-86720 with those for Murchison heated at 500-700 C, suggest that these Antarctic C1 - and C2-like chondrites were metamorphosed at temperatures like these in the interiors of their parent bodies. Metamorphic temperatures inferred from RNAA data and textural/mineralogic alterations are internally consistent, agreeing., with those evident in heated Murchison samples. These 3 chondrites were also heated late in their histories since all have lost cosmogenic He-3, presumably during close solar approach, and B-7904 and Y-86720 seem also to have lost substantial proportions of radiogenic He-4 and Ar-40 cf. data in. Similarities in spectral reflectance data for C-, G-, B- and F- asteroids, for these meteorites and for heated Murchison samples suggest that thermally metamorphosed interior materials in these asteroids were excavated by impacts and re-deposited on them, forming their present surfaces. Establishment of the thermal metamorphic histories of carbonaceous chondrites, then, is essential to establishing the evolution and present-day nature of C-type and related asteroids.

Lipschutz, M. E.

2000-01-01

343

What Are Space Exposure Histories Telling Us about CM Carbonaceous Chondrites?  

NASA Technical Reports Server (NTRS)

Chondrites are chemically primitive and carbonaceous (C) chondrites are potentially the most primitive among them because they mostly escaped thermal metamor-phism that affected the other chondrite groups and ratios of their major, non-volatile and most of the volatile elements are similar to those of the Sun. Therefore, C chondrites are ex-pected to retain a good record of the origin and early history of the solar system. Carbonaceous chondrites are chemically differentiated from other chondrites by their high Mg/Si ratios and refractory elements, and have experienced various degrees of aqueous alteration. They are subdivided into eight subgroups (CI, CM, CO, CV, CK, CR, CB and CH) based on major element and oxygen isotopic ratios. Their elemental ratios spread over a wide range though those of ordinary and enstatite chondrites are relatively uniform. It is critical to know how many sepa-rate bodies are represented by the C chondrites. In this study, CM chondrites, the most abundant carbona-ceous chondrites, are examined. They are water-rich, chon-drule- and CAI-bearing meteorites and most of them are brec-cias. High-temperature components such as chondrules, iso-lated olivine and CAIs in CMs are frequently altered and some of them are replaced by clay minerals and surrounded by sul-fides whose Fe was derived from mafic silicates. On the basis of degrees of aqueous alteration, CMs have been classified into subtypes from 1 to 2, although Rubin et al. [1] assigned subtype 1 to subtype 2 and subtype 2 to subtype 2.6 using various petrologic properties. The classification is based on petrographic and mineralogic properties. For example, though tochilinite (2[(Fe, Mg, Cu, Ni[])S] 1.57-1.85 [(Mg, Fe, Ni, Al, Ca)(HH)2]) clumps are produced during aqueous alteration, they disappear and sulfide appears with increasing degrees of aqueous alteration. Cosmic-ray exposure (CRE) age measurements of CM chondrites reveal an unusual feature. Though CRE ages of other chondrite groups range from several Myr to tens of Myr, CMs exposure ages are not longer than 7 Myr with one-third of the CM having less than 1 Myr CRE age. For those CM chondrites that have CRE ages <1 Myr, there are two discern-able CRE peaks. Because a CRE age reflects how long a me-teorite is present as a separate body in space, the peaks pre-sumably represent collisional events on the parent body (ies) [2]. In this study we defined 4 distinct CRE age groups of CMs and systematically characterized the petrography in each of the 4 CRE age groups to determine whether the groups have significant petrographic differences, with such differences probably reflecting different parent body (asteroid) geological processing, or multiple original bodies.

Takenouchi, A.; Zolensky, Michael E.; Nishiizumi, K.; Caffee, M.; Velbel, M. A.; Ross, K.; Zolensky, P.; Le, L.; Imae, N.; Yamaguchi, A.; Mikouchi, T.

2013-01-01

344

Chondrites and the Protoplanetary Disk, Part 4  

NASA Technical Reports Server (NTRS)

Topics discussed include: Cosmochemical fractionations, Chondritic meteorites and their components, Jet flows: Formation and thermal processing of solids in protoplanetary disks, A Search for Solar-System processing signatures in presolar grains, Experimental study of iron metal condensation, The chondrite types and their origins, Spinel-rich spherules from murchison, etc.

2004-01-01

345

The composition of carbonaceous chondrite matrix  

NASA Technical Reports Server (NTRS)

Matrix compositions of 32 of the approximately 40 known carbonaceous chondrites were analyzed using an electron probe defocussed-beam technique. Except in those chondrites that show evidence of metamorphism, matrices are compositionally similar and have correlation coefficients of +0.96 or greater. The implications of these analyses for the chemistry of individual matrix phases and for primitive solar system material are discussed.

Mcsween, H. Y., Jr.; Richardson, S. M.

1977-01-01

346

Re-Os systematics in chondrites and the fractionation of the platinum group elements in the early solar system  

Microsoft Academic Search

We have investigated the Re-Os system for samples of whole rock, metal, and sulfide from ordinary chondrites. Using closed-system analytical techniques, we found complete exchange between sample and tracer isotopes for silicate-containing samples and obtained precise and reliable Re-Os concentration measurements. Results on two Group IVA iron meteorites and on a silicate-rich iron (Steinbach, IVA-AN) are consistent with the IVA-IVB

J. H Chen; D. A Papanastassiou; G. J Wasserburg

1998-01-01

347

Composition and Origin of SiO2-rich Objects in Carbonaceous and Ordinary Chondrites  

NASA Astrophysics Data System (ADS)

SiO_2-rich objects provide information about processes prior to chondrule formation. REE and major element data of these objects will be presented in order to decide, how SiO_2 was enriched. Fractional condensation is the most plausible mechanism.

Hezel, D. C.; Palme, H.; Brenker, F. E.

2004-03-01

348

Fusion crust and the measurement of surface ages of Antarctic ordinary chondrites  

NASA Astrophysics Data System (ADS)

Miono et al. (1995) generated a reasonable agreement between terrestrial ages calculated using the thermoluminescence (TL) of fusion crust and those calculated from the abundance of cosmogenic nuclides. In Cunningham et al. (1996) it was shown that this relationship could be improved if the ages were calculated using an equation that accounted for the decay of natural TL as well as build up. With these corrections Miono et al.'s data seem to suggest that the natural TL of the fusion crust would be useful in determining the terrestrial ages of meteorites between 40 and 200 ka. Their data were also consistent with meteorite temperatures on the order of 0 C. The equivalent doses exhibited by a suite of Antarctic meteorites, studied in our laboratory, seem to suggest that the surface of the meteorites reached temperatures on the order of 10-15 C. This temperature range limits the use of fusion crust to those meteorites with surface ages less than 20 ka.

Akridge, Janette M. C.; Benoit, Paul H.; Sears, Derek W. G.

1997-03-01

349

Fusion Crust and the Measurement of Surface Ages of Antarctic Ordinary Chondrites  

NASA Technical Reports Server (NTRS)

Natural thermoluminescence (TL) reflects radiation exposure and storage temperature. Meteorites generally exhibit thermoluminescence acquired during their long exposure to galactic cosmic rays in space. During atmospheric passage, temperatures are high enough to completely drain the TL, in the first mm of material under the fusion crust. We therefore refer to this surface layer as "fusion crust" although it does include some unmelted material just below the crust. When the meteorite lands on earth this drained layer will begin to build up natural TL once again due to radiation from cosmic rays and internal radionuclides. Cosmic ray annual dose is estimated to be between 0.04 and 0.06 rad/yr on the earth's surface in Antarctica while the internal radionuclides contribute only about 0.01 rad/yr. Therefore the total annual dose received by the meteorite while it is on the surface is between 0.05 and 0.07 rad/yr. If the meteorite is buried deeply in the ice it is effectively shielded from most cosmic rays and thus only internal radioactivity contributes to the annual dose.

Akridge, Jannette M. C.; Benoit, Paul H.; Sears, Derek W. G.

1997-01-01

350

Fusion Crust and the Measurement of Surface Ages of Antarctic Ordinary Chondrites  

Microsoft Academic Search

Natural thermoluminescence (TL) reflects radiation exposure and storage temperature. Meteorites generally exhibit thermoluminescence acquired during their long exposure to galactic cosmic rays in space. During atmospheric passage, temperatures are high enough to completely drain the TL, in the first mm of material under the fusion crust. We therefore refer to this surface layer as \\

Jannette M. C. Akridge; Paul H. Benoit; Derek W. G. Sears

1997-01-01

351

Presolar corundum and spinel in ordinary chondrites: origins from AGB stars and a supernova.  

PubMed

On the basis of anomalous isotopic compositions of oxygen and magnesium, 14 oxide grains from two primitive meteorites (Bishunpur and Semarkona) have been identified as circumstellar condensates. One corundum grain has a high 18O/16O ratio and isotopic compositions of magnesium, calcium, and titanium that are compatible with a formation in ejecta of a type II supernova that was about 15 times the mass of the sun. The other grains have oxygen, magnesium, and titanium compositions that are consistent with a formation around asymptotic giant branch (AGB) stars with a range of mass and initial composition. The large range of aluminum/magnesium in circumstellar corundum and spinel is considered to reflect various stages of back-reaction between condensed corundum and gaseous magnesium in cooling stellar ejecta. PMID:9812886

Choi, B G; Huss, G R; Wasserburg, G J; Gallino, R

1998-11-13

352

Partial Melting of Ordinary Chondrite: Implications for Siderophile Behavior During Early Differentiation  

Microsoft Academic Search

Core formation scenarios in growing planetesimals include a variety of possible physical mechanisms such as segregation in a solid or partially molten silicate body or whether or not the body was actively deforming. The resulting geochemical composition of the metal and silicate phases will also be a function of bulk composition, percent of metal melted and oxygen fugacity. To explore

T. Rushmer

2002-01-01

353

Evidence for primitive nebular components in chondrules from the Chainpur chondrite  

NASA Astrophysics Data System (ADS)

In view of the fact that the least equilibrated ordinary chondrites contain chondrules that have changed little since the time of their formation in the early solar system, and are therefore excellent indicators of the physical and chemical nature of the solar nebula, 36 chondrules were separated from the Chainpur chondrite and analyzed for 20 elements and petrographic properties. The dominant nebular components found are: (1) a mixture of metal and sulfide whose composition is similar to whole rock metal and sulfide, (2) Ir-rich metal, (3) olivine-rich silicates, (4) pyroxene-rich silicates, and possibly (5) a component containing the more volatile lithophiles. Although etching experiments confirm that chondrule rims are enriched in metal, troilite and moderately volatile elements relative to the bulk chondrules, a large fraction of the volatiles remains in the unetched interior.

Grossman, J. N.; Wasson, J. T.

1982-06-01

354

Origin and metamorphic redistribution of silicon, chromium, and phosphorus in the metal of chondrites.  

PubMed

Chromium, silicon, and phosphorus concentrations of 0.1 to 1 percent by weight are common in metal grains in the least metamorphosed ordinary and carbonaceous chondrites. These concentrations are fairly uniform within single chondrules (but different from chondrule to chondrule) and are inversely correlated with the fayalite concentrations of the chondrule olivines. This relation shows that these chromium, silicon, and phosphorus concentrations could not have been established by condensation or equilibration in the solar nebula but are the result of metal-silicate equilibration within chondrules. Two generations of inclusions made by the exsolution of those elements have been identified: One formed during chondrule cooling and the other formed during metamorphism. The distribution and composition of the latter in type 3 to type 5 chondrites are consistent with increasing metamorphism relative to type 2 and type 3.0 material. PMID:17797224

Zanda, B; Bourot-Denise, M; Perron, C; Hewins, R H

1994-09-23

355

The role of redox processes in the evolution of chondrite matter (NGR studies)  

NASA Astrophysics Data System (ADS)

An NGR study was carried out on natural and heat-treated carbonaceous chondrites. Results obtained at elevated temperatures (400-1400 C) show that iron-bearing phases transformed rapidly (in the course of 1-5 min). At 700-1300 C, the CM2(1) phase was found to transform into the CO3(CV3) phase, and, above 1300 C, the latter phase transforms into the EH phase. A phase transition found at 1050 is considered to be a temperature boundary separating the two principal components of ordinary chondrites: the matrix at a temperature of or below 1050 C, and the chondrules at a temperature of or above 1050 C.

Malysheva, T. V.

1992-09-01

356

Mineralogy and possible origin of an unusual Cr-rich inclusion in the Los Martinez (L6) chondrite  

NASA Technical Reports Server (NTRS)

During a petrological study of the previously unclassified ordinary chondrite Los Martinez a highly unusual Cr-rich inclusion is found which is unique in both extraterrestrial and terrestrial mineralogy. Detailed SEM and TEM studies show that the inclusion consists of a highly zoned single crystal of plagioclase intergrown with chromium-rich spinel which indicates that it is the product of exsolution. The Cr-rich precursors of the inclusion probably have close affinities to the chronite-plagioclase chrondrules observed by Ramdohr (1967) in several ordinary chondrites. Based on the zoning in the inclusion it is suggested that it is the product of fractional crystallization from a melt, which may have formed as a liquid condensate, or by melting of solid condensates, in the solar nebula. Subsequent cooling of this melt condensate resulted in crystallization of the unidentified phase. After crystallization, the inclusion was probably incorporated into a parent body where it underwent metamorphism and was probably shocked to some degree. During this period of parent body metamorphism, exsolution and decomposition of the unknown precursor occurred to produce the observed intergrowth of plagioclase and chromite. Los Martinez is classified as an L6 ordinary chondrite breccia.

Brearley, Adrian J.; Casanova, Ignacio; Miller, Mark L.; Keil, Klaus

1991-01-01

357

Hotter, Faster: A Thermal Model for the H-Chondrite Parent Body Consistent with Chronology and Cooling Rates  

NASA Astrophysics Data System (ADS)

HOTTER, FASTER: A THERMAL MODEL FOR THE H-CHONDRITE PARENT BODY CONSISTENT WITH CHRONOLOGY AND COOLING RATES. H. Y. McSween, Jr. and M. E. Bennett, III, Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996, USA. Because of the abundant sampling and relatively low shock levels of H chondrites, their thermal histories are more tightly constrained than for other ordinary chondrites; consequently, rigorous models for the thermal evolution of their parent asteroid can be formulated that are not possible for other chondrite groups. A revised thermal model for the H-chondrite parent asteroid [Bennett and McSween], based on heating by decay of 26Al, follows the formulation of [Miyamoto and Fujii] except: the unfounded constraint that the relative volumes of different petrologic types must mimic meteorite fall statistics is removed, a shortened thermal history of 60 Ma [Gopel et al] rather than 100 Ma is adopted, and improved geothermometry constraints and measurements of thermal properties [Yomogida and Matsui] are used. Our new model predicts a parent body of approximately 88 kilometers radius, containing a much larger volumetric proportion (71%) of H6 material than in the previous model, with a high thermal gradient and correspondingly small proportions of H5 and H4 material (together comprising 10%) near the surface. Constraints on chronology and cooling rates from H chondrites are used as independent tests of the model. 26Al heating requires that the body accreted 1.5-3.1 Ma after formation of CAIs to reach the measured peak temperature for H6 chondrites, consistent with the 3.0 + 2.6 Ma estimate from Pb/Pb chronology [Gopel et al]. Times of Pb isotopic closure, relative to CAIs, in H-chondrite phosphates (3-5 Ma for H4, 10-16 Ma for H5, 42-62 Ma for H6, from [Gopel et al]) precisely overlap the thermal model estimates. In particular, the markedly shorter duration of heating for H4-5 chondrites agrees with model predictions. The model also predicts post-metamorphic cooling rates through the temperature interval 800-500K of 26 K/Ma for H4, 22 K/Ma for H5, and 16 K/Ma for H6. These values approximately coincide, in both trend and magnitude, with metallographic and fission track cooling rate data for unshocked H chondrites [Lipschutz et al]. _

McSween, H. Y., Jr.; Bennett, M. E., III

1995-09-01

358

Electrical conductivity of chondritic meteorites  

NASA Technical Reports Server (NTRS)

The electrical conductivity of samples of the Murchison and Allende carbonaceous chondrites is 4 to 6 orders of magnitude greater than rock forming minerals such as olivine for temperatures up to 700 C. The remarkably high electrical conductivity of these meteorites is attributed to carbon at the grain boundaries. Much of this carbon is produced by pyrolyzation of hydrocarbons at temperatures in excess of 150 C. As the temperature increases, light hydrocarbons are driven off and a carbon-rich residue or char migrates to the grain boundaries enhancing electrical conductivity. Assuming that carbon was present at the grain boundaries in the material which comprised the meteorite parent bodies, the electrical heating of such bodies was calculated as a function of body size and solar distance during a hypothetical T-Tauri phase of the sun. Input conductivity data for the meteorite parent body were the present carbonaceous chondrite values for temperatures up to 840 C and the electrical conductivity values for olivine above 840 C.

Duba, AL; Didwall, E. M.; Burke, G. J.; Sonett, C. P.

1987-01-01

359

Organic constituents of carbonaceous chondrites.  

PubMed

From a brief discussion of forms of meteorite carbon it is concluded that almost all the carbon in the carbonaceous chondrites is present as organic matter. Attempts to extract and identify this organic matter are then reviewed. It is shown that only 25 per cent has been extracted and only about 5 per cent chemically characterized. Of this 5 per cent, most is a complex mixture of hydroxylated aromatic acids together with various hydrocarbons of the paraffin, naphthene and aromatic series. Small amounts of amino acids, sugars and fatty acids also are present. The possible chemical nature of the major fraction is discussed. It is suggested to be a mixture of high-molecular weight aromatic and hydrocarbon polymers. Possible sources of contamination of the meteorites are described and evidence indicating a general lack of organic contaminants is presented. It is concluded, that most of the organic constituents are indigenous to the meteorites and are extra terrestrial in origin. Synthetic processes for the compounds are mentioned and it is concluded that the organic material is probably of abiogenic origin. A brief review on studies of "organized elements" contained within the meteorites is presented. Difficulties of identification are discussed and photographs of some microstructures of several carbonaceous chondrites are presented. No final conclusion about the nature of these objects is possible, but some appear to be various indigenous organic and mineral structures, while others are terrestrial contaminants. PMID:11881656

Briggs, M H; Mamikunian, G

1964-01-01

360

Variations of Li and Mg isotope ratios in bulk chondrites and mantle xenoliths  

NASA Astrophysics Data System (ADS)

We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (? 25Mg = -0.14 ± 0.06; ? 26Mg = -0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter ? 7Li (by up to ˜3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of ? 7Li = 3.5 ± 0.5‰, ? 25Mg = -0.10 ± 0.03‰ and ? 26Mg = -0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk ? 7Li and ? 26Mg, which vary from -3.7‰ to +14.5‰, and -0.36‰ to + 0.06‰, respectively. Values of ? 7Li and ? 26Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ˜5-10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High ? 7Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy ? 7Li seen in some oceanic basalts.

Pogge von Strandmann, Philip A. E.; Elliott, Tim; Marschall, Horst R.; Coath, Chris; Lai, Yi-Jen; Jeffcoate, Alistair B.; Ionov, Dmitri A.

2011-09-01

361

Porosity and Permeability of Chondritic Materials  

NASA Technical Reports Server (NTRS)

We have investigated the porosity of a large number of chondritic interplanetary dust particles and meteorites by three techniques: standard liquid/gas flow techniques, a new, non-invasive ultrasonic technique, and image processing of backscattered images . The latter technique is obviously best suited to sub-kg sized samples. We have also measured the gas and liquid permeabilities of some chondrites by two techniques: standard liquid/gas flow techniques, and a new, non-destructive pressure release technique. We find that chondritic IDP's have a somewhat bimodal porosity distribution. Peaks are present at 0 and 4% porosity; a tail then extends to 53%. These values suggest IDP bulk densities of 1.1 to 3.3 g/cc. Type 1-3 chondrite matrix porosities range up to 30%, with a peak at 2%. The bulk porosities for type 1-3 chondrites have the same approximate range as exhibited by matrix, indicating that other components of the bulk meteorites (including chondrules and aggregates) have the same average porosity as matrix. These results reveal that the porosity of primitive materials at scales ranging from nanogram to kilogram are similar, implying similar accretion dynamics operated through 12 orders of size magnitude. Permeabilities of the investigated chondrites vary by several orders of magnitude, and there appears to be no simple dependence of permeability with degree of aqueous alteration, or chondrite type.

Zolensky, Michael E.; Corrigan, Catherine M.; Dahl, Jason; Long, Michael

1996-01-01

362

Spectral reflectance properties of carbonaceous chondrites: 3. CR chondrites  

NASA Astrophysics Data System (ADS)

Powdered samples of a suite of 14 CR and CR-like chondrites, ranging from petrologic grade 1 to 3, were spectrally characterized over the 0.3-2.5 ?m interval as part of a larger study of carbonaceous chondrite reflectance spectra. Spectral analysis was complicated by absorption bands due to Fe oxyhydroxides near 0.9 ?m, resulting from terrestrial weathering. This absorption feature masks expected absorption bands due to constituent silicates in this region. In spite of this interference, most of the CR spectra exhibit absorption bands attributable to silicates, in particular an absorption feature due to Fe 2+-bearing phyllosilicates near 1.1 ?m. Mafic silicate absorption bands are weak to nonexistent due to a number of factors, including low Fe content, low degree of silicate crystallinity in some cases, and presence of fine-grained, finely dispersed opaques. With increasing aqueous alteration, phyllosilicate: mafic silicate ratios increase, resulting in more resolvable phyllosilicate absorption bands in the 1.1 ?m region. In the most phyllosilicate-rich CR chondrite, GRO 95577 (CR1), an additional possible phyllosilicate absorption band is seen at 2.38 ?m. In contrast to CM spectra, CR spectra generally do not exhibit an absorption band in the 0.65-0.7 ?m region, which is attributable to Fe 3+-Fe 2+ charge transfers, suggesting that CR phyllosilicates are not as Fe 3+-rich as CM phyllosilicates. CR2 and CR3 spectra are uniformly red-sloped, likely due to the presence of abundant Fe-Ni metal. Absolute reflectance seems to decrease with increasing degree of aqueous alteration, perhaps due to the formation of fine-grained opaques from pre-existing metal. Overall, CR spectra are characterized by widely varying reflectance (4-21% maximum reflectance), weak silicate absorption bands in the 0.9-1.3 ?m region, overall red slopes, and the lack of an Fe 3+-Fe 2+ charge transfer absorption band in the 0.65-0.7 ?m region.

Cloutis, E. A.; Hudon, P.; Hiroi, T.; Gaffey, M. J.

2012-01-01

363