Single Chondrules Research Articles

Matrix material in type 3 chondrites—occurrence, heterogeneity and relationship with chondrules

Matrix material in type 3 chondrites forms rims on chondrules, metal-sulfide aggregates, Ca,Al-rich inclusions and chondritic clasts; it also forms lumps up to a millimeter in size, which may contain coarser silicates. Chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized. Mean concentrations of Mg, Na, Al and Ca in matrix occurrences show up to fivefold variations in a single chondrite. Variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between H, L and LL matrices. Such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates. Compositions of chondrules and their matrix rims are normally unrelated, although rim compositions are correlated with those of matrix lumps inside chondrules. A single chondrule was found with a composition nearly identical to that of its internal matrix lump, suggesting that some chondrules may have formed from matrix material. Matrix lumps are as heterogeneous as chondrules, but mean chondrule and matrix compositions differ, even allowing for possible loss of metallic Fe,Ni during chondrule formation. Since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for Fe-Mg exchange, our matrix samples cannot represent typical chondrule precursor materials.

Petrography, mineral chemistry and origin of Type I enstatite chondrites

Optical and cathodoluminescence petrography were coupled with electron microprobe analysis to relate the textures and chemical compositions of minerals in the chondrules and matrix of the Indarch, Kota-Kota, Adhi-Kot and Abee Type I enstatite chondrites. Clinoenstatites fall into two distinct chemical groups with characteristic red or blue luminescence; red crystals are higher in Ti, Al, Cr, Mn and Ca, and lower in Na, than blue ones. Rare forsterites in Indarch and Kota-Kota show distinct compositions associated with orange or blue luminescence. The chemical ranges are indistinguishable for each color type in chondrules of all textural types, and the presence of both color types in a single chondrule or a metal fragment requires mechanical aggregation of both crystals and liquids of both color types. Porphyritic chondrules are ascribed mainly to aggregation of existing crystals because both types of pyroxene and olivine occur in the same chondrule. Large crystals of one color type are surrounded by fine-grained crystals of another type in some barred and radiating chondrules. All types of chondrules are surrounded by fine-grained rims rich in sulfide. The matrix contains many broken chondrules and individual silicate grains but is rich in sulfide and metal. Analyses are given of albite (minor elements and luminescence color vary between chondrites), kamacite, schreibersite, oldhamite and niningerite. Although the mineral assemblages do not fit theoretical condensation sequences in detail, the red pyroxene and orange olivine might result ultimately from near-equilibrium crystallization in which early reduced condensates reacted with a gas, while the blue crystals might result from fractional condensation in which early condensates were removed mechanically from a gas. Subsequent episodes involving mixing, melting, crystallization, condensation, fracturing, and mechanical aggregation would be needed to produce the complex textures.

Sr isotopic fractionation in Allende chondrules: A reflection of solar nebular processes

True relative Sr isotopic compositions, determined by the double-spike technique, are reported for 8 olivine chondrules from Allende and a single chondrule from Richardton. The Richardton chondrule has an Sr composition identical with the whole meteorite, but the Allende chondrules are up to 1.4‰ per mass unit light-isotope enriched, closely similar to Ca-Al inclusions (CAI) from the same individual stone. The correspondence of the patterns for chondrules and CAI suggests that both groups of objects derived their fractionated Sr in similar ways. The lack of any detectable non-linear Sr isotopic anomaly in the objects suggests that their Sr compositions did not have some exotic or extrasolar origin, but were derived from normal solar system Sr by mass fractionation. The consistent light-Sr enrichment of Allende objects may be explained by several schemes, and all are heavily model-dependent. Most plausible to the author is that the CAI and chondrules derived their fractionated Sr from a region of the nebula made isotopically light by partial kinetic mass separation of elements in the vapour phase. Later, the solid objects may have moved to an isotopically more normal region, where the Allende matrix accreted.

Search for 129Xe in Mineral Grains from Allende Inclusions: An Exercise in Miniaturized Rare Gas Analysis

Abstract A static mass spectrometer, modified by installation of a Baur-Signer (Zürich) ion source and an ion-counting system, was used to detect xenon from single mineral grains from inclusions in the C3 meteorite Allende. The grains were melted in small conical heaters wound from tungsten wire. In almost all cases excess 129Xe from 129I decay was detected, in concentrations varying from < 1000 to 600,000 atoms per microgram. Inferred iodine concentrations increased on the average from pentlandite (0.6 ppb), to hedenbergite (21 ppb), to olivine (92 ppb), to enstatite (203 ppb) to melilite (422 ppb), but vary widely from one grain to another of the same mineral, indicating that the iodine resides in some minor phase which is included “spottily” in the bulk phases over which we had good mineralogical control. About 23,000 atoms of fissiogenic 132Xe, presumably from 244Pu decay, was detected in the largest melilite sample analyzed, but we cannot determine from this study whether the 244Pu is “spotty” like 129I or is uniformly distributed in the melilite. One can foresee that if the samples could be loaded into previously outgassed heater cones, a system such as the one described in this paper would have 132Xe blanks of ~ 50,000 atoms. It is likely that in such a system the detectability for excess Xe achieved in this paper (~ 20,000 atoms) could be substantially improved. We also ran St. Severin troilite, which contains less than ~ 33 atoms of excess 129Xe per microgram, and a single chondrule from Alllegan, wich contains higher concentrations of radiogenic xenon than were previously reported by Podosek from runs on a pile-irradiated sample.

Determination of an internal 87Rb 87Sr isochron for the Olivenza chondrite

RbSr isotopic determinations were made on five single chondrules, ranging from 4 to 27 milligrams, extracted from Olivenza, an olivine-hypersthene chondrite. Several density fractions with a particle size smaller than 37μ were separated and RbSr analyses made. These data form a well defined linear array on the SrRb evolution diagram and yield an age of 4.63 × 10 9 years and an initial 87Sr 86Sr = 0.6994 . The typical strontium and rubidium blanks were 8 × 10 −10 and 4 × 10 −11 g, respectively. An acid wash experiment shows that preferential leaching takes place and that the resulting data from such a procedure are of doubtful reliability. Constructing an error envelope of the data points, a graphical representation of the initial 87Sr 86Sr versus the age is presented which demonstrates the constraints on these two anticorrelated parameters. This is a useful means of testing whether two data arrays may have the same age and ( 87Sr 86Sr ) I .

Xenon and krypton in the Bruderheim meteorite

In a series of heating experiments, xenon, radiogenic Xe129R, and krypton contents, and the xenon and krypton isotopic composition of the Bruderheim meteorite were studied for the separated minerals feldspar, pyroxene, olivine, and troilite and for numerous chondrule fractions. Important differences among the individual minerals and between minerals and chondrules were observed, and the following conclusions were reached: 1. There are wide variations in the Xe129/Xe132 ratios in xenon from the minerals and from chondrules, and variations of this quantity are observed as a function of heating temperature. These variations, which are not caused by atmospheric contamination, are further evidence that Xe129 anomalies in Bruderheim phases and chondrules result from in situ decay of I129. 2. Relative to the total meteorite, chondrules are depleted in primordial xenon and frequently enriched in Xe129R. The matrix material, in contrast, shows correlated xenon and Xe129R contents, the content of Xe129R varying nearly quadratically with the content of primordial xenon. 3. Because both xenon and Xe129R contents are highest in minerals of highest diffusion constants and lowest activation energies, there appears not to have been gross xenon diffusion since the decay of I129. The observed differences in xenon between chondrules and the matrix minerals argue against a simple mechanism of formation of chondrules directly from the matrix minerals, or the reverse. 4. Fractionation in the release of the four heaviest xenon isotopes from most minerals and chondrules occurs as a function of temperature, and correlations among the anomalies are consistent with the identification of this effect as resulting from the release of a fission-xenon component. If now-extinct Pu244 is the source by spontaneous fission, Pu244-Xe136 formation intervals from zero to 144 m.y. are deduced for chondrules; there is, however, 3 times too much of this suspected fission component in the bulk meteorite to be caused by Pu244 or U238 spontaneous fission after isolation of the solar nebula from the interstellar reservoir, based on elemental abundances calculated by Cameron. 5. Comparison of the release, from neutron-irradiated samples, of Xe129R versus pile-produced Xe128* from iodine or Xe131* from tellurium and barium favors an iodine-Xe129R correlation, but a tellurium-Xe129R correlation cannot be ruled out unequivocally. The galactic synthesis model for I129 yields an I-Xe129 formation interval of 58 m.y. for a single chondrule versus a questionable value of 34 m.y. for bulk Bruderheim. This discrepancy conflicts with the other evidence for an early origin for chondrules and raises some questions about the validity of the I-Xe129 dating method. 6. The unusually high spallation-type anomalies in krypton and xenon from chondrules in Bruderheim are probably due in part to recent cosmic rays. 7. The observed differences between chondrules and matrix material cannot be the result of processes acting after the meteorites formed. They reflect differences extant at the time of formation and argue for the presence of two or more types of primordial xenon. Possibly, a high-temperature component was associated with chondrules, and a lower-temperature component with the matrix minerals. If this identification is correct, it provides support for the hypothesis that chondrules represent a vapor-liquid precipitation from the primitive solar nebula and that matrix minerals represent a vapor-solid condensate. A possible explanation for differences between chondrules and matrix, consistent with the above hypothesis and permitting all solids to be equally irradiated during or after formation, is that during subsequent cooling chondrules were effectively isolated from the gas phase in the nebula by virtue of their larger size, whereas the fine-grained solids remained in equilibrium with the gases and I129 down to a relatively low temperature.