THERMAL STABILITY OF THERMOLUMINESCENCE IN CHONDRITES

New results are presented concerning the decay of thermoluminescence in ordinary chondrites. No evidence is found for the existence of two different decay rates as previously reported and a new interpretation of the earlier data is presented. The relevance to determination of terrestrial ages by thermoluminescence is discussed.     

CHEMICAL VARIATIONS AMONG L-CHONDRITES—IV. ANALYSES,WITH PETROGRAPHIC NOTES,OF 13 L-GROUP AND 3 LL-GROUP CHONDRITES

We review our procedures for selecting, preparing and analyzing meteorite samples, present new analyses of 16 ordinary chondrites, and discuss variations of Fe, S and Si in the L-group. A tendency for Fe/Mg, S/Mg and Si/Mg to be low in L chondrites of fades d to f testifies that post-metamorphic shock melting played a significant role in the chemical diversification of the L-group. However, these ratios also vary widely and sympathetically in melt-free chondrites, indicating that much of the L-group's chemical variation arose prior to thermal metamorphism and is in that sense primary. If all L-chondrites come from one parent body, type-correlated chemical trends suggest: 1) that the body had a traditional “onion skin” structure, with metamorphic intensity increasing with depth; and 2) that it formed from material that became more homogeneous, slightly poorer in iron, and significantly richer in sulfur as accretion proceeded.     

METAL OF HIGH Co CONTENT IN LL CHONDRITES

Measurements on the metal of the LL chondrites Appley Bridge, Jelica, Olivenza, and Khanpur have shown that the Co: Ni ratio of the metal is near 1.24 and that Prior's law appears to be followed to some extent within the LL group.     

SOLAR GASES IN METEORITES: THE ORIGIN OF CHONDRITES AND C1 CARBONACEOUS CHONDRITES

Because of their short cosmic ray exposure ages, chondritic meteorites are more likely to have been broken off from parent bodies in Earth-crossing orbits than from parent bodies in the asteroid belt. The radii of the objects now in the vicinity of the Earth (Apollo and Amor objects) are too small to be unfragmented asteroids of the theory for the origin of gas-rich meteorites of Anders. Because of the abundant evidence for very heavy shock and reheating among L- and H-chondrites, I conclude that the asteroidal origin for the ordinary chondrites is still the most likely. A cometary origin for the CI chondrites is examined. Regolith and megaregolith do not necessarily have to be formed by impacts on the cometary nucleus. The short-period comet Encke receives about 1/10 the solar-wind flux of a belt asteroid at 2.5 AU in its present orbit. The thickness of the megaregolith (C1 chondrites) is estimated between 0.1 and 0.3 km. Stirring of the megaregolith without substantial loss of dust from the comet might occur when the comet is transitional between “active” and “dead.” The consolidation of C1- “dust” into rock is somewhat problematic, but if liquid water and water vapor have played a role, then a crust rich in solar gases might form in the outer regions of a comet. A testable alternative explanation is suggested, namely that the solar gases in the C1 chondrites do not come from the Sun.     

VUGS IN ORDINARY CHONDRITES

Large vugs occur in the ordinary chondrite, Farmington. They contain the same phases present in the body of the meteorite, either as vug wall lining or as crystals attached to linings. The morphologies of these phases indicate a history of melting and vapor deposition. These vugs are compared with vugs in the ordinary chondrites, Orvinio and Tadjera, and with published reports of vugs in Rose City and Shaw. It is concluded that although shock events may not cause heating above the liquidus for the body of the meteorite, local pockets of melting and vapor formation do occur due to inhomogenieties in the shock wave pattern. Vugs represent such pockets. Vug formation, as a consequence of shock processing, is widespread among ordinary chondrites and shows no correlation with the average temperature of shock heating, subsequent average cooling rate (as indicated by metallurgical criteria), or blackening of body color.     

FRACTIONATION OF MODERATELY VOLATILE ELEMENTS IN ORDINARY CHONDRITES

Observed differences in the abundance ratios of moderately volatile elements found in ordinary chondrites relative to CI chondrites may have resulted from a continuous loss of nebular gas from the ordinary-chondrite formation region during condensation. If this occurred, the nebular volatility of these elements should be inversely correlated with their abundance ratios. Such a nebular gas loss can occur as a result of momentum exchange between solids and gases, as a result of interactions between the nebular gas and solar photons or particles at the surface of the nebula, or as a result of the settling of previously condensed solids to the median plane of the nebula.     

PETROLOGICAL FEATURES OF SHOCK METAMORPHISM IN CHONDRITES: ALFIANELLO

The Alfianello meteorite was inspected by optical microscopy, both by transmitted and reflected light, in order to look for evidence of shock metamorphism. The stone is a well-recrystallized olivine-hypersthene chondrite that shows unambiguous features of dynamic deformation. Some examples of planar deformation structures, which are notable in olivine and plagioclase feldspar crystals, are reported. The presence of several shock-transformed components is described in some detail. Alfianello appears to belong to a class of chondrites lightly-to-moderately shocked.     

SIZE FREQUENCY DISTRIBUTIONS OF FLUID DROP CHONDRULES IN ORDINARY CHONDRITES

The size frequency distributions of fluid drop chondrules in 11 ordinary chondrites (five H3, one H4, four L3, one LL3) have been determined by optical measurements in petrographic thin sections. The extreme range of median size of the fluid drop chondrules in individual meteorites is only slightly greater than lφ unit, and the grain size frequency distributions are approximately log normal. Chondrule size frequency distributions generally are fine skewed, platykurtic, and indicate moderate sorting. The size frequency distributions of fluid drop chondrules in ordinary chondrites are distinctly coarser than similar chondrules measured previously in CM2 and C03 carbonaceous chondrites.     

VANADIUM AND COPPER IN CHONDRITES

Emission spectrographic analyses for vanadium and copper in sixty-seven chondritic meteorites including members from ordinary, carbonaceous, and enstatite chondrite groups give a total range of 41–84 ppm vanadium and a total range of 77–180 ppm copper. The respective average values are 61 ± 7 ppm and 115 ± 12 ppm. Detailed patterns of vanadium and copper distributions between different chondrite groups are discussed in light of recent mechanisms for chemical fractionations in meteorites     

ON THE DEPLETION OF MODERATELY VOLATILE ELEMENTS IN ORDINARY CHONDRITES

Within the accuracy of present data, elements of intermediate volatility (condensation temperatures between about 1100 and 600° K) do not show a correlation of abundance with volatility in equilibrated ordinary chondrites. There is no need to postulate continuous loss of gas during condensation.     

DISTRIBUTION OF RARE EARTH ELEMENTS AND URANIUM IN VARIOUS COMPONENTS OF ORDINARY CHONDRITES

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 Séverin (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% of the light REE in all chondrites. On the other hand, only 20–40% 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.     

ON TYPE III PLESSITE IN CHONDRITES

Questions are raised concerning the possible sources of heat necessary for converting martensite to coarse Type III plessite in ordinary chondrites. It is suggested that the unusual Type III plessite in the Kingfisher, Oklahoma black chondrite was formed by partial homogenization of preexisting Type III plessite as a result of shock reheating of the metal into the γ field of the Fe-Ni phase diagram, rather than by decomposition of shock reheated prior martensite in the α + γ field, as originally proposed by Taylor and Heymann. Because martensite is sporadically distributed within Kingfisher plessite it is suggested that microstructures of this kind be called Type II-III plessite.     

GAS PERMEABILITY OF SHOCKED CHONDRITES

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 permeability decreased by as much as 50% 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 permeability of heavily-shocked chondrites is less than that of mildly shocked chondrites. Using the measured permeability data we estimated the size of a possible shocked-chondrite precursor body.     

SATURATION MAGNETIZATION MEASUREMENTS OF CARBONACEOUS CHONDRITES

The saturation magnetization of some carbonaceous chondrites was studied using a Faraday balance. The Faraday balance was shown to be an accurate (± 3%), reliable technique for measuring saturation magnetization by comparison with vibrating-sample magnetometer measurements on the same samples. Hyman and Rowe (1983) previously used these saturation magnetization measurements to measure the magnetite content of the five CI chondrites. Here, we present measurements on the magnetite contents of some CM2, CV3 and a CV5 chondrites. The method was also used to measure the content of metallic nickel-iron in Ornans, 3.4 ± 0.3%. Of the CM2 chondrites examined, only Bells, Essebi and Haripura had magnetite contents over about 1% by weight. A number of CV chondrites have magnetite between 2.3 and 13%, with little or no metallic iron. Leoville and Vigarano contain both magnetite and metallic iron, complicating the saturation magnetization results. Arch and Allende have very little metallic iron or magnetite, probably < 1% of either. This technique measures only ferrimagnetic magnetite; superparamagnetic magnetite with particle size < 300Å is ignored.     

CALCIUM-ALUMINIUM-RICH CHONDRULES IN THE UNEQUILIBRATED ORDINARY CHONDRITES

Calcium-aluminium-rich chondrules were found in L-3 (ALH-77015) and LL-3 (ALH-77278) chondrites. They consist of spinel, olivine, fassaite and glassy groundmass enriched in anorthite component. The bulk chemical compositions of the two chondrules are nearly the same and resemble those of the calcium-aluminium-rich inclusions in the carbonaceous chondrites, but the former is more enriched in MgO and SiO₂ and depleted in Al₂O₃ and CaO. The calcium-aluminium-rich chondrules would have been primarily early condensates which existed at the place where the chondrules were formed, and heated up to 1400 °C or higher to have melted and cooled at the rate similar to those for other chondrules, or the products of intensive heating followed by reaction with gas.     

ASTEROIDAL SOURCE OF ORDINARY CHONDRITES*

The orbital evolution of asteroidal fragments with diameters ranging from 10 cm to 20 km, injected into the 3:1 Kirkwood gap at 2.50 A.U., has been investigated using Monte Carlo techniques. It is assumed that this material can become Earth-crossing on a time scale of 10⁶ years, as a result of a chaotic zone discovered by Wisdom, associated with the 3:1 resonance. This phenomenon, as well as close encounter planetary perturbations, the v₆ secular resonance, and the ablative effects of the Earth's atmosphere are included in the determination of the orbital characteristics of meteorites impacting the Earth derived by fragmentation of this asteroidal material. It is found that the predicted meteorite orbits closely match those found for observed ordinary chondrites, and the total flux is in approximate agreement with the observed fall rate of ordinary chondrites. About 10% of the predicted impacting bodies are meteorite-size bodies originating directly from the asteroid belt. The remainder are obtained by subsequent fragmentation of larger (～1 m to 20 km diameter) Earth-crossing asteroidal fragments. The largest of these fragments are observable as Apollo-Amor objects. Thus the apparent paradox between the orbital characteristics of observed ordinary chondrites and those predicted from Apollo object sources is reconciled. Both appear to be complementary aspects of the same phenomena. No other asteroidal resonance is found to be satisfactory as a source of ordinary chondrites. These meteorites are therefore most likely to be derived from S asteroids in this limited region of the asteroidal belt, the largest of which are 11 Parthenope, 17 Thetis, and 29 Amphitrite.     

CHEMICAL VARIATIONS AMONG L-GROUP CHONDRITES,III. MAJOR ELEMENT VARIATION IN L6 CHONDRITES

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.     

VEIN FORMATION IN THE C1 CARBONACEOUS CHONDRITES

Veins in the C1 chondrites Orgueil, Alais, and Ivuna have been deposited during an extended period of impact brecciation and leaching. At least three generations of mineralization, dominated successively by carbonates, calcium sulfate, and magnesium sulfate, can be recognized. Vein minerals are derived locally by closed-system reactions between matrix phyllosilicates and an aqueous fluid, with the result that few, if any, primitive mineral phases still exist in the C1s     

CLASSIFICATION OF EIGHT ORDINARY CHONDRITES FROM TEXAS

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.