Fine-grained Inclusions Research Articles

Characterization of the mode of occurrence of gold in Jokisivu pilot feed and products

The best processing methods for recovering gold are determined essentially by the mineralogy and particle size distribution of the gold. Therefore detailed and quantitative mineralogical work is essential for process optimization. This paper describes the process mineralogical study carried out for pilot testwork feed and products of Jokisivu gold ore. The occurrence of coarse and liberated gold particles favors gravity separation, the ensuing bulk sulfide flotation recovering the remaining sulfides. The gold lost in the tailings occurs mainly as fine-grained inclusions in silicates and also rarely as coarse grains that by-passed the gravity circuit and did not float.

Composition, Geochemical Alteration, and Alpha-Decay Damage Effects of Natural Brannerite

To investigate the long-term alteration behavior of brannerite, we have undertaken a study of twelve natural samples from a range of geological environments. Our results indicate that seven of the samples exhibit only minor alteration, usually within veinlets or around the rim of the sample. The remaining five samples consist of variable amounts of unaltered and altered brannerite. SEMEDX analyses of unaltered areas indicate that the chemical formulae may deviate from the ideal stoichiometry. The U content ranges from 0.45 to 0.88 atoms per formula unit (pfu). Maximum amounts of the other major cations on the U-site are 0.48 Ca, 0.22 Th, 0.14 Y, and 0.07 Ln (lanthanide = Ce, Nd, Gd, Sm) atoms pfu. The Ti content ranges from 1.86 to 2.10 atoms pfu. Maximum values of other cations on the Ti-site are 0.15 Fe, 0.14 Si, 0.09 Al, 0.06 Nb, 0.04 Mn, and 0.04 Ni atoms pfu. Altered regions of brannerite contain significant amounts of Si and other elements incorporated from the fluid phase, and up to 40-90% of the original amount of U has been lost as a result of alteration. SEM-EDX results also provide evidence for TiO 2 phases, galena, and a thorite-like phase as alteration products. Electron diffraction patterns of all samples typically consist of two broad, diffuse rings that have equivalent d-spacings of 0.31 nm and 0.19 nmi, indicating complete amorphization of the brannerite. Many of the grains also exhibit weak diffraction spots due to fine-grained inclusions of a uranium oxide phase and galena. Using the available age data, these samples have average accumulated alpha-decay doses of 2-170 × 10 16 alphas/mg. Our results indicate that brannerite is subject to amorphization and may lose U under certain P-T-X conditions, but the overall durability of the titanate matrix remains high.

A unique, (almost) unaltered spinel‐rich fine‐grained inclusion in Kainsaz

Abstract We report a unique, spinel‐rich, extremely porous fine‐grained inclusion in the Kainsaz (CO3) meteorite. This inclusion is the least altered fine‐grained inclusion yet discovered, having escaped almost entirely the secondary alterations experienced by Allende fine‐grained inclusions. The inclusion is comprised of loosely packed 5–30 μm spinel grains mantled by thin layers of melilite, anorthite, and diopsidic pyroxene. The inclusion, which has over 30 vol% void space, is one of the most spinel‐rich, most porous fine‐grained inclusions seen to date. The mineralogy of the inclusion matches that which has been predicted for a precursor of the altered mineral assemblages of Allende fine‐grained inclusions, though a lack of interstitial material in the Kainsaz inclusion reduces the likelihood of a direct genetic relationship between the two (Allende fine‐grained inclusions contain abundant interstitial material). Its mineralogical composition confirms that the precursors of other, more altered, fine‐grained inclusions were assemblages of refractory minerals exclusively.

Refractory inclusions with unusual chemical compositions from the Vigarano carbonaceous chondrite

Ten inclusions, nine from Vigarano and one from Leoville, both members of the reduced subgroup of C3V chondrites, were analyzed for major and trace elements by neutron activation. Most have some refractory element characteristics that are common in refractory inclusions from Allende, a member of the oxidized subgroup of C3V chondrites. For instance, three (Leo 3537-2, Vig 1623-8, Vig 1623-11) are Type B inclusions with Group I or modified Group I REE patterns, three (Vig 1623-5, Vig 1623-10, Vig 1623-13) are olivine-rich inclusions, each of which is rather uniformly diluted in most refractory elements compared to Group I inclusions, and two (Vig 1623-14, Vig 1623-16) are fine-grained inclusions with Group II REE patterns. Six of the Vigarano inclusions, however, have refractory element fractionations that are unusual in Allende inclusions. Vig 1623-11 has Cl chondrite-normalized enrichment factors for Th and refractory siderophiles that are lower than those for REE by 60% or more, a positive Mo anomaly ( Mo Ir = 1.92 ± 0.02 relative to Cl chondrites) and an unusually low Zr Hf ratio (0.67 ± 0.06 relative to Cl). Relative to Cl chondrites, the Ir enrichment factor is 1.7 times greater than the mean enrichment factor for Re, Os, and Ru in Vig 1623-10. Vig 1623-13 has one of the lowest Ir Sm ratios (0.00044 ± 0.00007 relative to Cl) ever measured in an olivine chondrule. Vig 1623-16 has a modified Group II REE pattern which reflects removal of refractory REEs from the nebular gas at unusually low temperatures. Vig 477-5 has a REE pattern with a small negative Ce anomaly ( Ce La = 0.79 ± 0.01 relative to Cl), a large negative Yb anomaly ( Yb Lu = 0.57 ± 0.02 relative to Cl) and no Eu anomaly. This inclusion also has a positive Mo anomaly ( Mo Ir = 2.25 ± 0.03 relative to Cl) and enrichment factors for Zr and Th that are lower than those for Lu by about 65%. Vig 477-B has a REE pattern that may be a mixture of five parts of a component with an ultrarefractory REE pattern and ninety-five parts of another with a modified Group II REE pattern. Vigarano apparently sampled a different population of refractory inclusions from Allende, presumably because refractory nebular materials were not well mixed where and when C3V chondrites accreted. The Vigarano and Leoville coarse-grained inclusions studied here, like others analyzed previously, tend to have lower concentrations of Na and Au than Allende coarse-grained inclusions, but the two Vigarano fine-grained inclusions of this study have concentrations of these elements that are comparable to those of Allende fine-grained inclusions. Thus, although coarse-grained inclusions in the reduced subgroup of C3V chondrites were altered under different conditions from those in the oxidized subgroup, some fine-grained inclusions in the reduced subgroup were altered under similar conditions as some in the oxidized subgroup.

Chemical compositions of refractory inclusions from the Vigarano and Leoville carbonaceous chondrites

Major and trace elements were determined by INAA in two hibonite-rich refractory inclusions and a dark clast from Vigarano, three refractory inclusions from Leoville, and a hibonite-rich and a melilite-rich fraction from a composite refractory inclusion from Allende. The refractory inclusions from Vigarano and Leoville are all similar to their counterparts in Allende in that they have groups I, II, and III refractory element fractionation patterns, but a large proportion of these Vigarano and Leoville inclusions also contain refractory element features that are very infrequently encountered in Allende inclusions. For example, Vigarano 1623-2 is a group I inclusion with a large Ce anomaly ( La Ce = 2.34 ± 0.32 relative to Cl); Vigarano 1623-3 is a group III in which Os is strongly fractionated relative to other refractory siderophiles ( Os Re = 2.13 ± 0.08 relative to Cl); and Leoville 3537-1 is a modified group II in which removal of refractory REEs and condensation of the remaining REEs both occurred at unusually low temperatures. All samples from Vigarano and Leoville, including a fine-grained inclusion, have Na, Au, and other volatile element concentrations at or below the low end of the concentration ranges for these elements in Allende coarse-grained inclusions. This is consistent with the much lower amounts of Na-rich, secondary alteration products in the Vigarano and Leoville inclusions than in the ones from Allende. Even the Vigarano dark clast has a lower Na content than Allende dark clasts. Secondary alteration of refractory inclusions in the reduced subgroup of C3 V meteorites occurred in a part of the solar nebula where grain-gas separation processes removed the inclusions from chemical communication with the gas at a higher temperature or after a shorter time than in the case of Allende inclusions. The hibonite- and melilite-rich parts of the Allende inclusion could have formed in separate parts of the same chemical reservoir by separation of early condensing hibonite from melilite. The relatively hibonite-rich Vigarano inclusions may have accumulated originally from condensates preferentially enriched in hibonite by gravitational settling of grains of this early condensate or by their transport in convection cells.

Origin of zoned fine-grained inclusions in the Allende meteorite

Many fine-grained Ca-Al-rich inclusions in the Allende meteorite show a consistent pattern of mineralogic and textural zonation. The center, zone A, consists mostly of spinel, nepheline, Al-diopside and salite, with minor amounts of olivine, sodalite, grossular, anorthite, perovskite, and ilmenite. In most inclusions, these minerals occur as loosely packed, discrete, anhedral to subhedral grains with only minor intergrowth between phases. In the others, zone A is made of 5 to 30 μm-sized, spinel-centered, multi-rimmed objects, and of finer-grained, loosely packed, interstitial material. The intermediate zone, B, is characterized by the presence of hedenbergite and andradite (in addition to the minerals which occur in zone A) partially filling voids. One type of outer zone, C, consists mostly of intergrown olivine and nepheline with small ilmenite grains enclosed. The other type of zone C consists of symplectic texture with spinel rimmed by nepheline or anorthite, rimmed by Al-diopside. The textural and mineralogical similarity of zone A, except for the rimmed spinel objects, to altered areas of coarsegrained melilite-rich CAIs suggests that a similar open system alteration affected the fine-grained inclusions and that zone A's contained melilite before being completely altered to anorthite, grossular, nepheline and sodalite. By comparing the mineralogies before and after the alteration, some Ca must have evaporated from—and Si, Fe, Na and Cl condensed onto—the inclusions. Under conditions of low temperatures (<1000 K) and/or at H O ratios lower than the cosmic abundance, Ca is more volatile than Si due to the formation of gaseous Ca-hydroxides. The thermodynamic stability of minerals in zones A and B also indicates alteration at temperatures less than 1000 K. The mode of occurrence of hedenbergite and andradite in zone B implies that they are condensates deposited on the edge of zone A. It is likely that the Ca migrating out of the center of the inclusions and the Si and Fe migrating inward from the surrounding nebular gas coprecipitated according to the thermodynamic stability of these minerals. Zone C, on the other hand, most likely existed even before alteration, surrounding what is now zone A. However, the mineralogy of zone C appears to have changed by alteration: the zone C consisting of olivine, nepheline and ilmenite was probably a spinel-rich belt mantling the inclusion.

Physicochemical Isotope anomalies

Isotopic composition of refractory elements can be modified, by physical processes such as distillation and sputtering, in unexpected patterns. Distillation enriches the heavy isotopes in the residue and the light isotopes in the vapour. However, current models appear to be inadequate to describe the detailed mass dependence, in particular for large fractionations. Coarse- and fine-grained inclusions from the Allende meteorite exhibit correlated isotope effects in Mg both as mass-dependent fractionation and residual anomalies. This isotope pattern can be duplicated by high temperature distillation in the laboratory. A ubiquitous property of meteoritic inclusions for Mg as well as for most of the other elements, where measurements exist, is mass-dependent fractionation. In contrast, terrestrial materials such as microtektites, tektite buttons as well as lunar orange and green glass spheres have normal Mg isotopic composition. A subset of interplanetary dust particles labelled as chondritic aggregates exhibit excesses in 26Mg and deuterium anomalies. Sputtering is expected to be a dominant mechanism in the destruction of grains within interstellar dust clouds. An active proto-sun as well as the present solar-wind and solar-flare flux are of sufficient intensity to sputter significant amounts of material. Laboratory experiments in Mg show widespread isotope effects including residual 26Mg excesses and mass dependent fractionation. It is possible that the 26Mg excesses in interplanetary dust is related to sputtering by energetic solar-wind particles. The implication of the laboratory distillation and sputtering effects are discussed and contrasted with the anomalies in meteoritic inclusions and other extraterrestrial materials we have access to.

Carrier phases for iodine in the Allende meteorite and their associated 129Xe r/ 127I ratios: A laser microprobe study

A new mass spectrometer of high sensitivity (about 500 atoms of 129Xe can be detected) coupled to a laser microprobe sample extraction system was used to determine the 129Xe r/I ratios of minerals which carry 129Xe r in the Allende meteorite. In sodalite-rich fine-grained inclusions, I correlates with Cl verifying that sodalite carries the majority of I. One sample heated above the decomposition point of sodalite had a small amount of residual I indicating the presence of at least one other carrier. The Cl/l ratio of the matrix was different from that of the fine-grained inclusions and the 129Xe r correlates with trapped Xe. This implies either a different Cl/l ratio for sodalite in the matrix or a different carrier for I in matrix. In melilite from one coarse-grained inclusion, I is variable and correlates with both Xe and, surprisingly, 131Xe produced from Ba during the irradiation. Since this is unlikely for either sodalite or contaminating I, some other I carrier is inferred. In coarse-grained inclusions with Cl-rich alteration products in cracks, most of the 129Xe r is found in those cracks. A residual amount of 129Xe r is present in melilite samples which show no Cl-rich alteration products, and is a trace impurity in the melilite. The 129Xe r/ 127l ratio of sodalite from the fine-grained inclusions, and matrix is similar—between 1.25 and 1.35 × 10 −4. This implies an age for these samples within 2 million years of a Bjurbole chondrule included in the irradiation as a standard. The 129Xe r/ 127l ratio of a sample of a fine-grained inclusion heated to 1100° C is 2.4 × 10 −4 or 16 million years before the Bjurbole chondrule. A coarse-grained inclusion on the other hand showed a 129Xe r/ 127I ratio of 3.15 × 10 −5 or 35 million years after the Bjurbole chondrule.

Identification of cosmogenic argon components in Allende by laser microprobe

New techniques are presented for using a laser microprobe to determine the spallation argon systematics of calcium‐aluminum inclusions. The 38Ars amounts determined for melilite and anorthite in a coarse‐grained inclusion from Allende are 2.9 × 10−8 and 1.3 × 10−8 cc/g, respectively. The ratio of the amounts is consistent with the calcium contents of these two minerals. The 38Ars amount determined for a fine‐grained inclusion from Allende is 1.1 × 10−8 cc/g. Calcium and potassium amounts were determined from irradiated samples of the same inclusions so that production of 38Ar from calcium during the cosmic ray exposure of Allende could be determined for these samples. The production observed was 12.4 ± 2.1 × 10−8 ccSTP 38Ar/g Ca for the coarse‐grained inclusion and 9.9 ± 2.4 ccSTP 38Ar/g Ca for the fine‐grained inclusion. No evidence of unusual exposure was observed in the two inclusions studied.

Evidence for two-stage mixing in magmatic inclusions and rhyolitic lava domes on Niijima Island, Japan

Fine-grained inclusions are found in the Achiyama and the Mukaiyama rhyolitic lava domes on Niijima Island, Japan. The existence of fine-grained rims in the inclusions indicates that these inclusions are a quenched mafic magma incorporated into their rhyolite host. The chemical composition and the phenocryst assemblage of the inclusions in the Achiyama dome are identical with those of the effusive basaltic lava on Niijima Island (Wakago basalt). The inclusions of the Mukaiyama dome are dacitic and have a disequilibrium phenocryst assemblage (olivine, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, quartz and opaque minerals); whole-rock compositions lie on a mixing line between the host rhyolite and Wakago basalt. The dacitic magma is, therefore, a product of mixing of the basaltic and rhyolitic magmas. The occurrence of two different inclusions (basaltic and dacitic) in two different domes can be explained by a two-stage magma mixing model. The dacite is formed in the first stage by mixing in a vertically stratified magma chamber caused by an interfacial instability between rhyolitic and basaltic layers. A small amount of rhyolitic magma is entrained into the lower basaltic magma layer, and two magmas can mix essentially in a liquid state. The mixed dacitic magma is formed in the lower layer. The second stage is mixing caused by forced convection during the ascent through a conduit just before eruption. The stably stratified silicic and mafic magmas in a magma chamber can turn over and efficiently mix because of the formation of an unstable flow in a conduit. Under this condition, the system reaches thermal equilibrium immediately after the mafic magma is disaggregated and dispersed in the silicic host. The mafic magma solidifies when the silicic endmember is predominant in the mixture; hence the equilibrium temperature is below the solidus of the mafic magma. Mafic inclusions are formed at this stage. Whether the inclusions are basaltic or dacitic depends on whether the final ascent through a conduit (eruption) occurred before or after the formation of the layer of a mixed dacitic magma in a magma chamber.

Dating blueschist metamorphism: A combined 40Ar/ 39Ar and electron microprobe approach

40Ar 39Ar and electron microprobe examination of blueschist samples from the Iceberg Lake schist, southern Alaska suggest that phengite inclusions are the source of 40Ar in crossite. Because such finegrained inclusions may be susceptible to argon loss, caution should be exercised in interpreting K-Ar ages from this phase, and possibly other low-K amphiboles from blueschist suites. The estimated blocking temperature for phengite in the matrix (314° to 450°C), however, is close to the estimated peak metamorphic temperatures (325° ± 50° C), suggesting that phengite 40Ar 39Ar plateau dates may coincide closely with the time of blueschist metamorphism.

Isotope anomalies induced in laboratory distillation

We have carried out high-temperature distillation experiments at 5 × 10−5 torr on terrestrial pyroxene crystals and measured24Mg, 25Mg and 26Mg simultaneously using a 61-cm multi-cup mass spectrometer. The raw data were corrected for isotope fractionation using the power law and the Rayleigh law. Regardless of the correction procedure, the sample residues are depleted in 26Mg and the condensates are enriched in 26Mg, relative to non-distilled standards. These effects mimic the 'isotope anomalies' in Mg observed in Allende coarse- and fine-grained inclusions, previously attributed to nucleosynthetic processes involving 26Al.

Titanium isotopic anomalies in meteorites

High-precision analyses of Ti are reported for samples from a variety of meteorite classes. The expanded data base for Allende inclusions still shows Ti isotopic anomalies in every inclusion. All the coarse-grained inclusions give quite similar patterns, but fine-grained inclusions show more variable, and sometimes larger, anomalies. One inclusion, 3675A, was analyzed because others identified it as a possible “FUN” inclusion due to its mass-fractionated Mg. This designation is supported by the significantly more complex Ti isotopic pattern for 3675A compared to all our other Allende inclusions. Available data fail to suggest that any particular Allende mineral phase, including a chromite-carbon fraction from an acid residue, is especially rich in anomalous Ti. We also find anomalous Ti in a bulk sample of a Cl chondrite and in matrix separates from C2 chondrites. The excesses of 50Ti are smaller than for Allende inclusions, and subtle differences in Ti isotopic patterns tentatively suggest that parent materials for C1-C2 matrix and Allende inclusions are not directly related. Analyses of chondrules from unequilibrated ordinary chondrites did not yield clear evidence for anomalous Ti, but some “larger than usual” deficits at 50 46 give encouragement for future work in this direction. Comparing the magnitude of isotopic shifts at 50Ti and 16O for all these meteorite samples indicates that they are not correlated, but the data do not preclude a correlation between concentrations of “exotic” 50Ti and 16O atoms. Whether or not Allende “FUN” inclusions are considered, at least 4 distinct isotopic components of Ti are required to account for the observed isotopic variations. The Ti data cannot be plausibly explained in terms of an early solar-system particle irradiation; instead, neutron-rich hydrostatic burning within a star is probably responsible for the dominant 50Ti anomalies, while s-process mechanisms are viable sources for some of the more subtle Ti variations. We suggest that the Ti anomalies are linked to a diversity of nucleosynthetic sources and the highly refractory behavior of Ti. Therefore, some form of “chemical memory” from the ISM, rather than “late stage supernova injection”, is most likely responsible for the preservation of observed isotopic heterogeneities.

Ca isotopes in refractory inclusions

We report measurements of the absolute isotope abundance of Ca in Ca-Al-rich inclusions from the Allende and Leoville meteorites. Improved high precision measurements are reported also for 46Ca. We find that nonlinear isotope effects in Ca are extremely rare in these inclusions. The absence of nonlinear effects in Ca, except for the effects in FUN inclusions, is in sharp contrast to the endemic effects in Ti. One fine-grained inclusion shows an excess of 46Ca of (7 ± 1)%., which is consistent with addition of only 46Ca or of an exotic (∗) component with 46Ca ∗ ~ 48Ca ∗. FUN inclusion EK-1-4-1 shows a small 46Ca excess of (3.3 ± 1.0)%.; this confirms that the exotic Ca components in EK-1-4-1 were even more deficient in 46Ca relative to 48Ca than is the case for normal Ca. The Ca in the Ca-Al-rich inclusions shows mass dependent isotope fractionation effects (as deduced from the absolute 40Ca 44Ca ) which have a range from −3.8 to +6.7%. per mass unit difference. This range is a factor of 20 wider than the range previously established for bulk meteorites and for terrestrial and lunar samples. Ca and Mg isotope fractionation effects in the Ca-Al-rich inclusions are common and attributed to kinetic isotope effects which imply the production of the inclusions by complex sequences of condensation, vaporization and recondensation. A correlation was found between Ca and Mg isotope fractionation effects and inclusion type. A possible correlation between isotope fractionation and rare earth element abundance patterns is discussed.

Trace element concentrations in the isotopically unique Allende inclusion, EK 1-4-1

Concentrations of rare-earth elements (REE), Sc, Fe, Co, Cr, Na and Ir in the bulk sample and mineral separates of the isotopically unique Allende inclusion, EK 1-4-1, were determined by instrumental neutron activation analysis. REE concentrations were also determined by mass-spectrometric isotope dilution for two density separates. The inclusion showed enrichment of light REE over heavy REE with a positive Yb anomaly, thus showing a tendency to resemble group II fine-grained inclusions in REE abundances, although EK 1-4-1 is a coarse-grained inclusion. High Cr concentrations also indicate group II affinity. However, high Ir (6 ppm) and Sc (105 ppm), and low FeO (1.80%), and Co (13.8 ppm) concentrations in the bulk sample and low Na concentrations in mineral separates show group I affinity. Between melilite and pyroxene fractions, the same samples in which mass-fractionated O isotope ratios were observed (Clayton and Mayeda, 1977), REE, Sc, Co and Fe showed distributions which are substantially different from those observed for “ordinary” Allende type B inclusions. These two minerals do not appear to be in equilibrium with respect to trace element distributions. The results indicate that the isotopically unique EK 1-4-1 is also unusual in its elemental abundances and distributions.

Ubiquitous isotopic anomalies in Ti from normal Allende inclusions

A newly developed technique for high-precision isotopic analyses of titanium was applied to terrestrial rocks and course- and fine-grained Allende inclusions. Repeated analyses of three terrestrial rocks gave excellent agreement (usually less than 2 × 10 −4 deviations) with a Ti metal standard. All seven Allende inclusions studied here were previously determined to contain isotopically normal Nd and/or Sm, indicating that none belongs to a small group of peculiar inclusions, dubbed as FUN inclusions. Yet, every inclusion showed a clearly resolvable excess at 50Ti, with ε(50/46) [deviation from the Ti standard in parts in 10 4] ranging from +7 to +10 for five of the inclusions, while two pink fine-grained inclusions gave larger excesses of +15 and +28. Six inclusions also exhibited greater than 2σ deficits at 47Ti/ 46Ti, with an average value greater than −2 ε-units. Analyses of pyroxene and melilite separates and bulk samples from one inclusion furnished no evidence for Ti isotopic disequilibrium within a single inclusion. However, the possibility that pyroxene contributed the bulk of the Ti in all samples renders this a rather insensitive test. A graphical presentation of Ti isotopic abundances for these normal Allende inclusions, two previously analyzed FUN inclusions, and terrestrial samples demonstrates that at least three distinct components are required. The discovery of widespread isotopic anomalies in Ti from normal Allende inclusions establishes Ti as the first non-noble-gas element studied since oxygen to show such isotopic heterogeneity. A survey of published nucleosynthetic origins of Ti isotopes suggests that the dominant 50Ti excesses in these inclusions are due to the relative enrichment of isotopes synthesized during hydrostatic burning in or near the core of a massive star. Such a source is seemingly consistent with the absence of isotopic anomalies in previously analyzed elements, and can be tested via its prediction of similar excesses for the neutron-rich isotopes of Cr, Fe and Ni and their respective radioactive decay products.

The isotopic composition of uranium and lead in Allende inclusions and meteoritic phosphates

We measured the isotopic compositions of U and Pb in eight Ca-Al-rich inclusions from the Allende chondrite, in whitlockite from the St. Séverin chondrite, and in a whitlockite and a whole rock sample from the Angra dos Reis achondrite (ADOR). The total range in 235U/ 238U in the acid-soluble phases of the Allende inclusions and meteoritic whitlockites is from 1/137.6 to 1/138.3, and to within the error limits (3–20‰) indistinguishable from normal terrestrial U. The 235U/ 238U ratios for whitlockites from St. Séverin and ADOR are the same as that in the bulk meteorites and are normal. The Pb isotopic compositions of the acid-soluble phases of the Allende coarse-grained inclusions (except C-1) give a mean 207Pb/ 206Pb model age of 4.559 ± 0.015AE which is consistent with the observation of normal U isotopic composition. The Pb isotopic compositions of coarse-grained (FUN) inclusion, C-1, and of two fine-grained inclusions from Allende indicate that the U-Pb systems of these samples have evolved differently from the rest of the meteorite. The 207Pb/ 206Pb model ages for the whitlockites and for bulk ADOR are the same as those for St. Séverin whitlockite, 4.551 ± 0.004AE. The Pb results from St. Séverin indirectly constrain the value of the U isotopic abundance to be within 7.4‰ of normal. Both the directly measured 235U/ 238U and the Pb data on St. Séverin are in complete disagreement with claims of large 235U excess reported by other workers on the same meteorite. The Th/U ratios of some acid-soluble phases of Allende inclusions and of meteoritic whitlockite show varying degrees of enrichment of Th relative to U by factors of 2 to 25 relative to the bulk meteorites. Previous studies have shown that 244Pu/ 238U ratios in whitlockite are also fractionated relative to the bulk meteorites. The observation of no isotopic effects in U reported here does not support the claims of either fractionation of 247Cm/ 238U or of a large amount of live 247Cm at the early stage of formation of the solar system. Using the present data we obtain a limit of 247Cm/ 235U≤ 4 × 10 −3 at the time of meteorite formation or differentiation. This implies that the last major “r” process contribution to the material in the protosolar nebula was ∼10 8 years prior to 26Al formation and injection.

Boron concentrations in carbonaceous chondrites

We have analyzed B in carbonaceous chondrites in order to clarify a factor of 100 difference between the solar system B abundance derived from the solar photosphere and that inferred from previous meteorite data. Consistent results were obtained from two instrumental methods for B analysis: (a) counting of the high energy betas from 12B produced by the 11B(d,p) reaction, and (b) measurement of particle track densities from 10B(n,α) 7Li in a plastic track detector affixed to a homogenized meteorite sample. Contamination is a major problem in B analyses, but extensive testing showed that our results were not seriously affected. Our B concentrations are typically 1–2 ppm and are a factor of 2–6 lower than previous carbonaceous chondrite measurements. Our data for the Cl chondrites Ivuna and Orgueil would indicate a solar system B/Si atomic abundance ratio of 58 × 10 −6, but this is still a factor of 2–10 higher than the photospheric estimates. It may be that B is depleted in the sun by thermonuclear processes; however, the similarity of photospheric and meteoritic Be abundances is a problem for this point of view. Alternatively, B may be enhanced in carbonaceous chondrites, but this would make B a cosmochemically unique element. A mm-sized (Fe,Mn,Mg)CO 3 crystal from Orgueil shows no B enrichment. We find 10B ≤ 10 16 atoms/g in two Allende fine-grained inclusions suggesting that B is not a refractory element under solar nebula conditions. This 10B limit, when taken as a limit on 10Be when the inclusion formed, puts constraints on the possibility of a solar system synthesis of 26Al. For a proton spectrum of E −a, a must be ≥ 3 if a solar gas is irradiated or a ≥1.5 if dust of solar composition is irradiated.

Petrographic variations among carbonaceous chondrites of the Vigarano type

The Vigarano subtype is a petrographically complex class of meteorites. Oxidized and reduced groups can be distinguished on the basis of metal vs magnetite abundances and Ni contents of sulfide minerals. These meteorites also differ in the proportions of matrix and chondrules and in polymict character. Slight bulk chemical differences correlate with the recognized petrologic groupings. It is likely that the Vigarano subtype includes several previously unrecognized subgroups. Metamorphism has affected Coolidge, Mulga (West) and, to a lesser extent, Allende, as evidenced by ferromagnesian mineral equilibration, Fe-enrichment of fine-grained inclusions, and loss of some volatile gases. Because of the metamorphic effects in the Allende chondrite (the only meteorite of the group that has been intensively studied) and the petrographie differences among all meteorites of the Vigarano subtype, it is suggested that Allende alone may not adequately reflect the wide spectrum of properties in this important class of meteorites.

Trace element distribution in mineral separates of the Allende inclusions and their genetic implications

Concentrations of the REE, Sc, Co, Fe, Zn, Ir, Na and Cr were determined by instrumental neutron activation and mass spectrometric isotope dilution analysis for mineral separates of the coarseand fine-grained types (group I and II of Martin and Mason's classification) of the Allende inclusions. These data, combined with data on mineral/liquid partition coefficients, oxygen isotope distributions and diffusion calculations, suggest the following: (1) Minerals in the coarse-grained inclusions (group I) crystallized in a closed system with respect to refractory elements. On the other hand, differences in oxygen isotope distributions among minerals preclude a totally molten stage in the history of the inclusion. Group I inclusions were formed by rapid condensation (either to liquid or solid) in a supercooled solar nebula; extrasolar pyroxene and spinel dust were included but not melted in the condensing inclusions, thus preserving their extrasolar oxygen isotope composition. REE were distributed by diffusion during the subsequent heating at subsolidus temperatures; because oxygen diffuses much more slowly at these temperatures, the oxygen isotope anomalies were preserved. (2) The fine-grained (group II) inclusions were also formed by condensation from a super-cooled nebular gas; however, REE-rich clinopyroxene and spinel were formed early and REE-poor sodalite and nepheline were formed later and mechanically mixed with clinopyroxene and spinel to form the inclusions. The REE patterns of the bulk inclusions and the mineral separates are fractionated, indicating that REE abundances in the gaseous phase were already fractionated at the time of condensation of the minerals. (3) Pre-existing Mg isotope anomalies in the coarse-grained inclusions must have been erased during the heating stage thus resetting the 26Al- 26Mg chronometer.