Albert Joseph Fahey

26Al, 244Pu, 50Ti, REE, and trace element abundances in hibonite grains from CM and CV meteorites

The ion microprobe was used to measure Ti and Mg isotopes as well as rare earth and other trace elements in ten hibonites from the CM carbonaceous chondrites Murchison, Murray, and Cold Bokkeveld and in two hibonites and Ti-rich pyroxene from the CV chondrite Allende. In hibonites from Murchison and Murray fission track densities were also measured, as were Th and U concentrations. Eight of the hibonites, from all four meteorites, exhibit large Ti isotopic anomalies, particularly in 50Ti. Two grains from Murray have 50Ti excesses of ~ 10%. At least four nucleosynthetic components are required to account for all the Ti isotopic data. Neutron-rich nuclear statistical equilibrium nucleosynthesis is the most likely process to account for a 50Ti-rich component (with 50Ti49Ti

Correlated study of initial 87Sr86Sr and AlMg isotopic systematics and petrologic properties in a suite of refractory inclusions from the Allende meteorite

20). The ion probe Ti isotopic measurements confirm that the solar nebula was isotopically heterogeneous on a small spatial scale and argue for a chemical memory origin of the Ti isotopic anomalies which were probably carried into the solar system in the form of refractory dust grains. However, there is no experimental evidence that such interstellar grains survived the formation of the hibonites. The REE and trace element patterns of the hibonites are similar to those seen in CAIs and can be interpreted in terms of fractionation effects during condensation from a gas of solar composition, thus arguing for a solar system origin of the hibonites. Additional evidence for such an origin is provided by the PuTh ratios, which are ~ 10−4, and by the Mg isotopic compositions which are normal except for 26Mg∗ due to 26Al. Only three out of ten hibonites exhibit 26Mg∗, consistent with previous studies which demonstrated the paucity of 26Mg∗ in hibonites. Because of the refractory nature of hibonite and the presence of large Ti isotopic effects, we conclude that a heterogeneous distribution of 26Al in the early solar system is the most likely reason. In particular, our observations of δ50Ti = 15%. and of an isochron with (26Al27Al)0 = 5 × 10−8 in the FUN inclusion HAL are evidence against both late formation and Mg redistribution to explain the lack of 26Al in hibonites. There are no obvious correlations between the Ti isotopic compositions, the presence of 26Mg∗, the presence of 244Pu, and the REE and trace element patterns in individual hibonites. This indicates that the anomalous 50Ti, as well as 26A1 and 244Pu, were not co-produced in a single astrophysical source, and/or that these nuclides were introduced into the solar nebula by different carriers before being incorporated into the hibonites.

Extreme ^(26)Mg and ^(17)O enrichments in an Orgueil corundum: Identification of a presolar oxide grain

We have studied a suite of six coarse-grained Ca-Al-rich inclusions (CAIs) from the Allende meteorite, applying in concert detailed petrographic and chemical characterization, ion probe mass spectrometric analyses of the Al-Mg isotopic system to determine the abundance and distribution of 26A1, and thermal emission mass spectrometric analyses of the Rb-Sr system to determine initial 87Sr86Sr ratios. The simplest inclusion is a Type Bl, USNM 3529-Z, that shows evidence of minor alteration and recrystallization since solidification from a melt droplet; e.g., some of the coarse-grained anorthite in the inclusion (especially near the inclusion rim) apparently formed by recrystallization or melting of fine-grained secondary anorthite that had replaced melilite. Four other Type Bs contain evidence of more widespread recrystallization in addition to evidence of local replacement of melilite by coarse anorthite; e.g., melilite zoning is complex and not igneous in origin, and in one inclusion (USNM 3529-21) melilite and pyroxene crystals enclose relicts of an earlier generation of the same phases. The sixth inclusion, USNM 3898, is a Type A whose properties appear to be largely metamorphic in origin. All six inclusions show variations in Mg isotopic compositions indicating that they incorporated 26A1. In 3529-Z the relative abundance of 26A1 (26Al27Al = (4.0 ± 0.1) × 10−5) is close to the “canonical” value (26Al27Al = 5 × 10−5) for CAIs, but both anorthite and melilite show minor departures from a strict isochron relationship between Mg and Al. In particular, the “coarse” anorthite near the inclusion rim has initial 26Al27Al ratios of 1−2 × 10−5, implying formation at least 1.5 million years after the “first-generation” anorthite. The other four Type Bs exhibit larger disturbances of the Al-Mg system that can be understood in terms of local recrystallization and isotopic exchange, primarily between anorthite and melilite. Only 3898 is consistent with a strict Al-Mg isochron relationship, but this CAI lacks primary anorthite, and the small range of Al/Mg among the phases analyzed may preclude resolution of any small isotopic disturbances. The RbSr isotopic system also typically indicates some level of disturbance, some of which must have been relatively recent. Nevertheless, phases characterized by very low Rb/Sr permit precise identification of initial 87Sr86Sr. Our Al-Mg and Sr data are largely consistent with a simple chronological interpretation of both systems; i.e., the 26Al results suggest that all the initial 87Sr86Sr ratios should have the same value and, with one exception, our initial 87Sr86Sr ratios are consistent with a single value; this value is somewhat higher than the primitive value ALL reported by Gray et al. (1973), however. Also with one exception, the very low Rb/Sr CAI data reported by Gray et al. (1973) are consistent with the same value. The exception is the same inclusion in both studies; our analysis of USNM 3898 yields an initial 87Sr86Srhigher than that of the other CAIs, while Gray et al. (1973) obtained an initial 87Sr86Srlower than that of the other CAIs for the same inclusion (labeled as D7, on which ALL is based). Neither result can be readily explained as a chronological aberration, since 3898 has nearly canonical 26Al27Al. The present data are the first which establish a firm association between primitive 26Al27Al and primitive 87Sr86Sr by analysis of both isotopic systems in the same inclusions. The comparison of both isotopic systems, interpreted as simple nebular chronometers, does not reveal any chronological inconsistencies that demand resolution in terms of a grossly heterogeneous distribution of 26Al.

Oxygen isotopes in circumstellar Al203 grains from meteorites and stellar nucleosynthesis

A corundum (Al_2O_3) grain from the Orgueil meteorite is greatly enriched in ^(17)O and ^(26)Mg^*. The measured ^(16)O/^(17)O is 1028 ± 11 compared to ^(16)O/^(17)O)_☉ = 2610. This is the largest ^(17)O excess so far observed in any meteoritic material. The ^(26)Mg excess (^(26)Mg^*) is most plausibly due to in situ decay of ^(26)Al. The inferred ^(26)Al/^(27)Al)_0 ratio of 8.9 x 10^(-4) is ~ 18 times larger than the 5 x 10^(-5) value commonly observed in refractory inclusions formed m the solar system. The large ^(17)O excess and high ^(26)Mg^*/^(27)Al ratio unambiguously identify this corundum as a presolar oxide grain. Enrichments in ^(17)O and ^(26)Al are characteristic of H-burning and point to red giant or AGB stars as likely sources.

Microdistributions of Mg isotopes and REE abundances in a Type A calcium-aluminum-rich inclusion from Efremovka☆

We have found a ~3 µm Al_2O_3 grain (B39) in the Bishunpur LL3.1 chondrite that is enriched in ^(17)O by a factor of ~ 6.8 (^(16)O/^(17)O = 385 ± 9) and depleted in 180 by almost 60% (^(16)O/^(18)O = 853 ± 30) relative to solar system oxygen and has an initial ^(26)Al/^(27)Al = 1.7 ± 0.2 x 10^3, ~34 times greater than the initial solar system value. The isotopic compositions of B39 and two other Al_2O_3 grains previously reported from the Orgueil and Murchison meteorites show that these grains formed directly from the ejecta of low-mass AGB stars with C/0 < 1. A simple theoretical analysis is presented showing that the oxygen systematics of the grains are a natural consequence of main-sequence evolution followed by first dredge-up. ^(26)AI is the result of third dredge-up. Circumstellar Al_2O_3 grains provide very precise isotopic data for stellar ejecta that complement spectroscopic observations of oxygen-rich stars. Isotopic differences indicate that the Al_2O_3 grains come from separate stars of different mass and initial oxygen composition that originated in molecular clouds different from the one in which the solar system formed.

Evidence for distillation in the formation of HAL and related hibonite inclusions

Abstract Ion probe measurements were performed in the core and rim of a compact Type A inclusion, E2, from the CV3 chondrite Efremovka. Rare earth element (REE) and selected trace element (Sc, V, Sr, Y, Zr, Nb, Ba, Hf) abundances as well as Mg, Si, and Ti isotopes were measured. The REE are concentrated in perovskite except for Eu which mainly resides in melilite. REE patterns in E2 perovskite are unfractionated and have a Eu depletion. No systematic differences in REE concentrations or patterns between core and rim perovskites were found. In melilite, the concentrations of Ba, La, Ce, and Dy are independent of the distance from the rim. The Mg isotopic composition of melilite and spinel is uniformly heavy (by ~ 10%./ amu) in the interior and smoothly grades to normal in and near the rim. This indicates that the rim must have formed by condensation of minerals onto a solid core at high temperatures. Excesses of 26 Mg are present and correlate with the 27 Al 24 Mg ratio, both in the core and rim regions of the inclusion. The slope of the apparent isochron in the rim region is higher than the slope measured in the inner core, providing evidence that 26 Al was inhomogeneously distributed in the early solar nebula. In addition, the intercepts differ, pointing to a heterogeneous 26 Mg 24 Mg ratio. No Si isotopic mass fractionation was found. The Ti isotopic composition of E2 perovskite is normal.

Hibonite-bearing microspherules - A new type of refractory inclusions with large isotopic anomalies

Abstract Four hibonite-bearing refractory inclusions, HAL from Allende, DH-H1 from the Dhajala H3 chondrite, 7–404 and 7–971 from the Murchison CM2 chondrite, have related chemical and isotopic systematics: they exhibit large Ce and V depletions and very low Mg and Ti concentrations compared to other meteoritic hibonites and have mass-fractionated Ca- and Ti-isotopic compositions with enrichments in the heavy isotopes by up to +13%./amu for Ca and up to +19%./amu for Ti. There is no correlation between Ca- and Ti-isotopic mass fractionations, but the Ti-isotopic mass fractionation is inversely correlated with the Ti concentration as expected for Rayleigh-law mass fractionation. The inclusions also have nonlinear 48Ca anomalies within error of −5%. while nonlinear 50Ti anomalies in HAL and 7–971 are +15.0 ± 3.6%. (2σ) and −4.4 ± 6.5%., respectively. Oxygen-isotopic compositions were measured in HAL, 7–404, and 7–971, as well as a range of morphologically different refractory inclusions from Murchison. The Murchison refractory inclusions all show enrichments in 16O relative to terrestrial with a mean value of +46.8 ± 1.6%. The three HAL-type inclusions are also enriched in 16O but are mass fractionated in their O isotopes in favor of the heavy isotopes with variable degrees of mixing with isotopically normal O. The scatter of the data from 7–404 and 7–971 in terms of mass fractionation exceeds the reproducibility predicted from measurements of the Burma-spinel standard and indicates the presence of intrinsic O-isotopic heterogeneities in these hibonites. These chemical and isotopic characteristics are consistent with the formation of HAL-type inclusions as distillation residues. A distillation origin is supported by chemical and isotopic measurements of a hibonite-bearing distillation residue produced in the laboratory by evaporating terrestrial kaersutite. The residue from this experiment has large Ce and V depletions, no detectable Mg, and mass fractionated Ca- and Ti-isotopic compositions. However, while the HAL-type refactory inclusions owe their chemical and isotopic characteristics to a distillation origin, subsequent thermal events are required to explain certain petrographic and chemical characteristics.

Trace-element abundances in hibonites from the Murchison carbonaceous chondrite: Constraints on high-temperature processes in the solar nebula

Reported are petrographic descriptions, major and trace element chemistry and, Mg, Ca, and Ti isotopic compositions of a new class of refractory inclusions that consist of spherules composed of hibonite and a silicate glass. The distinctive features of these inclusions are excesses in 48Ca and 50Ti in both glass and hibonite, and 26Mg depletions relative to terrestrial isotopic compositions. Three spherules have been examined and analyzed, one from the Lance CO3 meteorite and two from the Murchison CM2 meteorite. Lance 3413-1/31 (LA3413-1/31) and Murchison 7-228 (MUR7-228) have euhedral to subhedral hibonite crystals enclosed within glass. Murchison 7-753 (MUR7-753) has a rounded hibonite core with several small inclusions of perovskite. A small fragment of glass is attached to the hibonite and an Fe-silicate rim is imperfectly preserved around the grain. LA3413-1/31 has a Group II REE pattern; MUR7-228 a refractory pattern with depletions in the relatively volatile elements Sr, Ba, Nb, V and Eu; and MUR7-753 a pattern characterized by the prior removal of an ultrarefractory component and overall fractionation of all REEs. The partitioning of the LREEs between hibonite and glass in MUR7-228 is consistent with equilibrium hibonite-liquid partition coefficients previously determined; LA3413-1/31 shows much less partitioning, while MUR7-753 shows no evidence for partitioning and preserves an unequilibrated refractory component highly enriched in Gd. All spherules have initial magnesium depleted in 26Mg by around 3‰ relative to terrestrial Mg, but only MUR7-228 shows evidence for in situ decay of 26Al, with an initial 26Al27Al of (1.7 ± 0.7) × 10−5. Both hibonite and glass in all three spherules show excesses of 48Ca and 50Ti, ranging up to +40 and +20‰, respectively, relative to terrestrial Ca and Ti. Spherules such as these are rare and the only other occurrence is in the unique chondrite ALH85085. The hibonite in the spherules shows similarities to isotopically anomalous hibonite crystal fragments (PLACs), but it is unlikely that the spherules formed by remelting of PLACs. The precursors include isotopically anomalous Ca-Ti carriers but also isotopically normal refractory components that probably formed as condensates. The spherules formed by melting of these precursors under disequilibrium conditions and rapid cooling after hibonite crystallization. These inclusions must have formed early, prior to the dilution of isotopic anomalies by mixing processes and in an area characterized by excesses of 48Ca and 50Ti, depletions of 26Mg and lack of 26Al.

O-16 excesses in Murchison and Murray hibonites - a case against a late supernova injection origin of isotopic anomalies in O, Mg, Ca, and Ti

Abundances of trace elements, including 14 rare earth elements (REE) and 8 other refractory trace elements (RTE) have been measured in 33 hibonite- and 3 perovskite-bearing grains from the Murchison carbonaceous chondrite. The normalized abundance patterns show a good correlation with three morphological and chemical groups of hibonites. Sixteen platy hibonite crystals (PLACs) typically have patterns which are depleted in the relatively volatile trace elements V, Nb, Ba, Eu, and Yb (Group III). Three PLACs also show large depletions in Ce; one PLAC is enriched in the ultrarefractory REE, while two other PLACs have fractionated patterns enriched in the light REE, with positive Tm anomalies. The PLACs commonly have a roll-off in the abundances of the HREE, which is consistent with solid-liquid partitioning. Three blue aggregates (BAGs) have patterns with low overall abundances and depletions in the ultrarefractory REE except Tm (Group II). Spinel-hibonite grains (SHIBs) show a variety of patterns mainly of Group II, but 4 SHIBs have relatively smooth patterns with anomalies in the volatile REE Ce, Eu, and Yb. The perovskites have patterns similar to Group II but with excesses in Eu, and Yb. The HREE roll-off in the PLACs and the igneous morphologies of the SHIBs indicate that these grains formed by the crystallization of melts. Distillation may be important for the BAGs and two of the PLACs which have large isotopic mass-fractionation effects. PLACs have the simplest trace element patterns and the largest Ti isotopic anomalies and probably were the first hibonites to form in the early solar system. The SHIBs have trace-element signatures that require multiple processing episodes. They are somewhat complementary to the PLACs in that they are enriched in the volatile elements which are depleted in the PLACs. On average, the SHIBs contain isotopically less-anomalous Ti than the PLACs. However, SHIBs contain 26Mg excesses that are consistent with the in situ decay of 26Al, whereas PLACs generally have only small, or no excesses of 26Mg despite high 27Al24Mg. While an overall relationship exists between the morphology, chemistry, and Ti and Mg isotopic systematics of meteoritic hibonites, they do not, as yet, fit into a simple scenario for the formation of the solar system.

Large Ca-48 anomalies are associated with Ti-50 anomalies in Murchison and Murray hibonites

Ion probe measurements of the oxygen isotopic composition of seven hibonite samples from the CM chondrites Murchison and Murray are reported. All samples show large O-16 excesses relative to terrestrial oxygen. The data for all samples plot along the carbonaceous chondrite O-16-rich mixing line and show no evidence for isotopic mass fractionation effects characteristic of FUN inclusions. These hibonites have the largest Ca-48 and Ti-50 isotopic anomalies found to date; thus there is no intrinsic relationship between anomalies of a nucleosynthetic origin and isotopic mass fractionation effects. The large O-16 excess seen in the sample with the largest measured Ca-48 and Ti-50 depletions argues against a late injection of exotic material from a nearby supernova as a source for the isotopic anomalies. 38 references.