Showing papers in "Meteoritics in 1981"

Relative abundances of chondrule primary textural types in ordinary chondrites and their bearing on conditions of chondrule formation

Abstract: A petrographic survey of > 1600 chondrules in thin-sections of 12 different mildly to highly unequilibrated H-, L-, and LL-chondrites, as well as morphological and textural study of 141 whole chondrules separated from 11 of the same chondrites, was used to determine the relative abundances of definable chondrule primary textural types Percentage abundances of various chondrule types are remarkably similar in all chondrites studied and are ∼ 47–52 porphyritic olivine-pyroxene (POP), 15–27 porphyritic olivine (PO), 9–11 porphyritic pyroxene (PP), 3–4 barred olivine (BO), 7–9 radial pyroxene (RP), 2–5 granular olivine-pyroxene (GOP), 3–5 cryptocrystalline (C), and ≤ 1 metallic (M) Neither chondrule size nor shape is strongly correlated with textural type Compound and cratered chondrules, which are interpreted as products of collisions between plastic chondrules, comprise ∼ 2–28% of nonporphyritic (RP, GOP, C) but only ∼ 2–9% of porphyritic (POP, PO, PP, BO) chondrules, leading to a model-dependent implication that nonporphyritic chondrules evolved at number densities (chondrules per unit volume of space) which were 102 to 104 times greater than those which prevailed during porphyritic chondrule formation (total range of ∼ 1 to ∼ 106 m−3) Distinctive “rims” of fine-grained sulfides and/or silicates occur on both porphyritic and nonporphyritic types and appear to post-date chondrule formation Apparently, either the same process(es) contributed chondrules to all unequilibrated ordinary chondrites or, if genetically different, the various chondrule types were well mixed before incorporation into chondrites Melting of pre-existing materials is the mechanism favored for chondrule formation

The innisfree meteorite fall: a photographic analysis of fragmentation, dynamics and luminosity

Abstract: The Innisfree meteorite was the third fall for which accurate orbital data were secured from a camera network. Nine fragments were found within three months of the fall with a total mass of 4.58 kg. The ellipse of fall is unusually small because of the steep path in the atmosphere. The photograph from the Vegreville station reveals six trails below 26 km and these are correlated with the six main fragments, all with masses in excess of 300 g. A photometric study indicates that Innisfree had a peak absolute magnitude Mpan = −12.1 at a height of 36 km. The recovered meteorites provide known masses for the late stages of the photographic trails which, combined with dynamical data, allow luminous efficiencies to be derived with unusual confidence. Late in the flight where shock wave effects dominate ablation, luminous efficiencies vary from 3 × 10−5 to 5 × 10−2 for velocities between 3 and 10 km s−1 and masses from 0.3 to 2.0 kg. The mean luminous efficiency for the entire flight is estimated between 4 × 10−2 and 8 × 10−2.

Irghizites and zhamanshinites: zhamanshin crater, ussr

Abstract: Two irghizites, three zhamanshinites and one sample each of lechatelierite, vein-quartz, Palaeogene silty clay and Palaeogene quartzite are analyzed using neutron activation analysis. A silicate analysis of the Palaeogene silty clay is carried out, as is an incomplete analysis of the Palaeogene quartzite from the Zhamanshin impact crater. The REE abundances of the irghizites are found to resemble those of sedimentary rocks. It is noted that on the Koehler and Raaz diagram all projection points of the irghizites lie inside the field of tektites and indicate that they were derived from terrestrial sedimentary rocks. The Zhamanshin impact glasses are divided into three types: silica-rich zhamanshinites, zhamanshinites, and silica-poor zhamanshinites. It is pointed out that these are also characterized by varying proportions of alkalis and Al, Mn, and Ca contents. Irghizites and silica-rich zhamanshinites show a depletion of Eu, zhamanshinites, however, do not show this depletion. It is assumed that partial melting is an important process in the origin of zhamanshinites.

Unusual weathering products of oldhamite parentage in the Norton County enstatite achondrite

Abstract: Oldhamite, CaS, is found only in enstatite chondrites and enstatite achondrites. During the course of a comprehensive study of Norton County, oldhamite was found in several polished thin sections of enstatite achondrite. Inspection of thousands of specimens of Norton County showed that occasionally white, pale-yellow and brown specks, generally a few millimeters in size, occur on their surfaces. X-ray powder analysis of these materials shows that they contain portlandite, Ca(OH)2, vaterite (CaCO3, hexagonal), calcite (CaCO3, trigonal), bassanite (CaSO4 1/2 H2O) and one unknown species. It is suggested that these minerals were produced by terrestrial weathering of oldhamite.

Chemical Variations Among L-Group Chondrites, II. Chemical Distinctions Between L3 and LL3 Chondrites

Abstract: New chemical analyses of the Krymka and Manych chondrites and a review of data for other low-iron type 3 chondrites show that the ratio of metallic to total iron varies widely in LL3 chondrites and is an imperfect basis for distinguishing between these meteorites and L3 chondrites. More reliable chemical criteria — total Fe/Mg and Ni/Mg ratios, and Fe-S relationships — indicate that Krymka, Manych, Carraweena and Bishunpur are LL3 chondrites rather than samples of an iron-poor subgroup of the L-group.

Meteorite falls and finds: some statistics

Abstract: The statistics of meteorite falls and finds are presented. Histograms give the distribution of falls as a function of year, month and time of day. The distributions of the retrieved masses of fallen and found meteorites are given, as is also their distribution over the Earth's surface. The data for this analysis have been taken from the British Museum's Catalogue of Meteorites (1966) and Appendix (1977).

Impact and Impact-Like Structures in Algeria. Part II: Multi-Ringed Structures

Abstract: While Part I was devoted to the study of bowl-shaped depressions in Algeria, the present article focuses on multi-ringed structures of possible impact origin on the Algerian Sahara platform. Four structures were selected from orbital, aviation and geologic documents but only three were visited. TIN BIDER (27°36′N; 005°07′E) is a concentric multiple ring structure at least 6 km in diameter. Upper-Cretaceous sedimentary beds outside the structure dip inward a few degrees at the periphery and become extremely folded nearer the center, yet a general circular symmetry is always retained. A clearly exposed contact between upper deformed beds and underlying on-deformed beds is remarkably flat, dipping less than 10° inward. The upper beds display strong centrifugal folding. In the center of the structure Lower-Cretaceous sandstones about 0.5 km above their normal stratigraphic position are exposed. Although no shatter cones, intensive brecciation, or fracturing were observed, there is definite petrographic evidence of shock metamorphism (planar elements) in the quartz grains of the central sandstones. Tin Bider is a probable impact structure. It is the only astrobleme known with such prominent ductile deformations, a characteristic which may be due to the nature of the target materials. Detailed studies are now required to understand the mechanism of deformation of this multi-ringed structure. Its formation may be early Tertiary in age. FOUM TEGUENTOUR (26°14.5′N; 002°25′E) is an 8 km diameter bull's eye ring pattern. Although the high circularity and the morphology are consistent with an impact origin, the prominence of ductile deformation, the nature of the formations (clay-gypsum with sandstone intercalcations), the type of folds, the relationships between the structure and a surrounding plateau, and the lack of any evidence of shock effects better support a diapiric origin. MAZOULA (28°24′N; 007°49′E) is an 800 m diameter multi-ring feature with a 300 m wide anticlinal dome which rises some 30–35 m above the surrounding horizontal strata. The dome is capped by a flat-lying massive carbonate layer dipping on the flanks in accordance to the surface topography. There are no special disturbances, no fractures or breccias. Mazoula is not impact in origin but may be a Cretaceous rudistid reef and/or a salt diapir.

The Major Element Chemistry of Libyan Desert Glass and the Mineralogy of its Precursor

Abstract: Pieces of high-silica, natural glass (Libyan Desert Glass), found on the desert surface of western Egypt, have been treated as an enigma for 50 years although it is virtually certain they are similar to tektites in being impact-derived products. New major element analyses of four Libyan Desert Glass specimens agree extremely well with the only other recent analysis and demonstrate that the original bulk analyses reported by Spencer (1939) are in error. The five modern analyses define a very tight chemical range for SiO2 (97.38–98.25 wt %), Al2O3 (1.16–2.26 wt %), total Fe (0.15–0.60 wt % as Fe2O3) and TiO2 (0.13–0.19 wt %). Measurable MgO (0.04–0.20 wt %) was found in one specimen. No other elements are present in greater than trace amounts. Microprobe analyses show that Al, Fe and Ti are all positively correlated with one another and are almost ubiquitously distributed throughout the glass. They must also have been so distributed in the LDG precursor material as mechanical mixing and elemental diffusion in the short-lived melt were limited. In contrast, Mg is sharply restricted in occurrence and correlates only with Fe, strongly suggesting a precursor Mg-Fe oxide or silicate mineral present as rare, discrete grains. Aside from rare accessory minerals, the parent material was a sand or sandstone composed of quartz grains coated with a mixture of kaolinite, hematite and anatase. This conclusion is based solely on the elemental distribution in the glass but is buttressed by the occurrence of both sand and sandstone, in southwestern Egypt, with the requisite mineralogy. However, mineralogic identity need not, in general, translate to a chemical match and it is entirely possible that the specific sand or sandstone facies involved in the glass formation no longer exists after 28 million years. Consequently, it may well be that evidence other than chemical comparisons will be needed to identify the presently unknown parent crater.

Terrestrial-Type Xenon in Meteoritic Troilite

Abstract: Recently it has been realized that the isotopic composition of Xe in different phases of chondrites is not uniform and that AVCC Xe is a mixture of the different nucleogenetic components trapped in these phases1–4. We show here a similar abundance pattern for the nonradiogenic xenon isotopes in air and in meteoritic troilite (FeS), which suggests that the isotopic composition of atmospheric Xe was not produced by unique events in the history of terrestrial material but represents a particular mix of the different nucleogenetic components of Xe that was dominant in a central Fe- and S-rich region of the protoplanetary nebula5.

Chemical Variations Among L-Group Chondrites, III. Major Element Variation in L6 Chondrites

Abstract: Bulk chemical and mineral analyses of five L6 chondrites of shock facies d to f bring the number of L6 falls analyzed by Jarosewich to 20 and enable us: 1) to examine the chemical effects of shock melting in chondrites of the same petrologic type that presumably sample a limited stratigraphic range in their parent body; and 2) to seek depth-related chemical variations by comparing the compositions of L3 and melt-free L6 chondrites. The mean Fe/Mg, Si/Mg, S/Mg and Ni/Mg ratios of melt-free L6 chondrites (shock facies a to c) are virtually identical to those of L3 chondrites, suggesting that L-group material had the same bulk composition early (L6) and late (L3) in the accretion of the parent body. Wider variations of S/Mg and Ni/Mg in L6 chondrites may signify that L6 material was less well mixed than L3, or that some mobilization of metal and troilite occurred at shock intensities (facies c) too low to melt silicates. L6 chondrites that experienced shock melting of silicates (facies d to f) show wide variations of Fe/Mg, Si/Mg, S/Mg and Ni/Mg. It appears that most of the major element variation in the L-group is tertiary (shock-related) rather than primary (nebular, accretionary) or secondary (metamorphic). There is some evidence that L-group chondrites comprise two subgroups with different Fe/S ratios, but these subgroups are now poorly defined and their significance is unknown.

Tierra Blanca: an Unusual Achondrite from West Texas

Abstract: Tierra Blanca is an achondrite (sensu stricto) composed chiefly of orthopyroxene, olivine, plagioclase and clinopyroxene. The mineralogy, major element chemistry and texture are similar to Winona and also Mt. Morris, Acapulco and Antarctic meteorite ALHA 77081 to some degree. The designation of this group as “Winonaites,” as suggested by Prinz et al. (1980), seems justified and useful.

Thermal History of the Abee Enstatite Chondrite II; Thermal Measurements and Heat Flow Calculations

Abstract: Specimens from the center and the outer surface of Abee exhibit identical microstructures within their metal phase—platelets of an iron carbide. Thus the entire body must have cooled at a nearly uniform rate. The density, heat capacity, and thermal conductivity have been measured and used in a theoretical heat flow analysis. From these calculations and observations it is concluded that Abee cooled from 700 °C to 200 °C in about two hours.

The lahrauli ureilite

Abstract: A meteorite which fell at Lahrauli, district Basti, U.P. (India) in 1955 has been examined for mineralogical, chemical and cosmogenic characteristics. It contains pigeonite Wo(7.7)Fs(18) (with Cr2O3 = 1.2% and Al2O3 = 0.4%), olivine Fo(79) (with CaO = 0.3% and Cr2O3 = 0.7%) and diamond. The basic similarities of this meteorite to Goalpara, Dyalpur, Havero and Novo Urei indicate that it is a ureilite. Cosmic ray tracks have been measured in the olivine and pyroxene grains. Track density of 2.3 × 10 to the 6th/sq cm in olivines and VVH/VH ratio of 0.001 is similar to that observed in other meteorites.

Composition and origin of the unusual Oktibbeha County iron meteorite

Abstract: Oktibbeha County, the most Ni-rich iron meteorite, has been analyzed for Ni, Co, Cu, Ga, Ge, As, Sb, Ir, and Au. Cu and Sb are higher than in any other iron, but other trace elements are within the ranges typically found in iron meteorites. Extrapolation of trace element trends in group IAB indicates that Oktibbeha County is a member of this group. This sheds light on the origin of groups IAB and IIICD, which are thought to be derived from impact melts on parent bodies of chondritic composition. Lafayette (iron), another sample reported in the literature to have a similarly high Ni content, is probably a pseudometeorite.

Cm/U, Th/U, and 235 U/ 238 U in Meteorites

Abstract: The relative abundances of the actinides, ^(247)Cm, ^(244)Pu, ^(235)U, ^(238)U, and ^(232)Th are important for the estimation of the duration of galactic nucleosynthesis and the time interval between the termination of the last contributing "r" process nucleosynthetic event and formation of the solar system. ^(247)Cm is particularly important because of its half life (1.56 x 10^7y). As ^(247)Cm is an extinct nuclide, the basic evidence for its presence must come from observed shifts in the ^(235)U/^(238)U ratio and anomalies in ^(207)Pb/^(206)Pb and U-Pb ages in early solar system samples.

Grier(b), a ""Monomict,"" Brecciated Chondrite

Abstract: The Grier(b), New Mexico meteorite, a single mass of 929.4 grams, was found in 1969. This brecciated chondrite can be classified as an L-group from the bulk chemical analysis, ∼ 8 wt % metal with an estimated total iron content of 25 wt %, and the constant olivine (Fa25.5) and orthopyroxene (Fs23) compositions. The main portion of the meteorite fits the criteria for an L5 (grey to intermediate hypersthene) chondrite. A conspicuous, large (several cm3) dense fragment, texturally an L6–7 chondrite, contains practically no metal or chondrules. However, there is little variation in the bulk silicate and individual phase compositions between the fragment and the matrix. In spite of this, it seems unlikely that the fragment was created in situ because metal and sulfide are not found in the fragment-matrix contact zone; thus the formation of olivines and pyroxenes in both parts, as well as the “draining” of metal from the fragment, occurred prior to accretion with little, if any, subsequent thermal metamorphism.