FUN Inclusions Research Articles

HIDALGO: A FUN object from the earliest epoch of the solar system’s history

Chemical and isotopic measurements of HIDALGO, a stoichiometrically pure hibonite inclusion found in the matrix of the Dar al Gani 027 meteorite, were conducted by secondary ion mass spectrometry to investigate its origin and evolution. HIDALGO is characterized by large mass-dependent isotope fractionations in O, Ca, and Ti, as well as large negative anomalies in neutron-rich 48Ca and 50Ti, making it the newest member of the HAL-type FUN inclusions. The highly fractionated Ca and Ti isotopes but unfractionated Mg isotopes are consistent with HIDALGO being a residue from an extensive evaporation event, during which large fractions of initial Ca and Ti, and essentially all the initial Mg, in the precursor material were lost. HIDALGO appears to have incorporated live 26Al at a higher level than other HAL-type inclusions, but still at a lower amount compared to the Solar System’s initial 26Al abundance typically found in non-FUN CAIs. Interestingly, the inferred 10Be abundance in HIDALGO is comparable to the values observed in the majority of CV3 CAIs but ∼ 2.5 times higher than those in HAL-type samples. HIDALGO’s unusual 26Al/27Al and 10Be/9Be ratios, together with the 48Ca-50Ti anomalies, can be best explained by the formation of its precursor material in the isotopically heterogeneous solar nebula. Finally, large 7Li excesses correlating with Be/Li were found in HIDALGO, a behavior that can be interpreted as due to in-situ decay of live 7Be. Charged particle spallation of initially Li-free HIDALGO can simultaneously account for the inferred 7Be abundance and the measured Li elemental concentration. The consistency between the measurement and spallation calculation results provides support for the prior existence of 7Be in HIDALGO, possibly produced by irradiation close to the Sun

In situ oxygen, magnesium, and silicon isotopic compositions of the FUN inclusion Vigarano 1623‐5

AbstractOxygen, magnesium, and silicon isotopic abundances in Vigarano 1623‐5 were studied using secondary ion mass spectrometry to investigate correlations between isotopic and petrologic properties of this unique forsterite‐bearing FUN inclusion. Vigarano 1623‐5 displays large, correlated mass‐dependent fractionation effects, tightly linked to mineralogy within distinct petrologic units of the inclusion. The pyroxene‐rich and melilite‐rich interior parts of the inclusion display highly mass‐fractionated isotopic compositions of oxygen, magnesium, and silicon, consistent with Rayleigh distillation during evaporation of a melt with initial oxygen composition close to a solar composition. However, the chemical composition, enriched in magnesium and silicon, suggests a precursor already fractionated by prior melt evaporation. A discontinuous igneous rim was produced by a flash‐melting event followed by isotopic exchange in the rim melilite with planetary‐like oxygen, mechanical fragmentation, and reassembly with an accretionary rim of heterogeneous materials. Al‐rich minerals in 1623‐5 show evidence for having crystallized with live 26Al but at less than the “canonical” level of most CV calcium‐aluminum‐rich inclusions. However, well‐defined 26Al‐26Mg isochrons are not found and temporal implications are ambiguous.

Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology, and oxygen isotopic compositions

We present a detailed characterization of the mineralogy, petrology, and oxygen isotopic compositions of twelve FUN CAIs, including C1 and EK1-4-1 from Allende (CV), that were previously shown to have large isotopic fractionation patterns for magnesium and oxygen, and large isotopic anomalies of several elements. The other samples show more modest patterns of isotopic fractionation and have smaller but significant isotopic anomalies. All FUN CAIs studied are coarse-grained igneous inclusions: Type B, forsterite-bearing Type B, compact Type A, and hibonite-rich. Some inclusions consist of two mineralogically distinct lithologies, forsterite-rich and forsterite-free/poor. All the CV FUN CAIs experienced postcrystallization open-system iron-alkali-halogen metasomatic alteration resulting in the formation of secondary minerals commonly observed in non-FUN CAIs from CV chondrites. The CR FUN CAI GG#3 shows no evidence for alteration. In all samples, clear evidence of oxygen isotopic fractionation was found. Most samples were initially 16O-rich. On a three-oxygen isotope diagram, various minerals in each FUN CAI (spinel, forsterite, hibonite, dmisteinbergite, most fassaite grains, and melilite (only in GG#3)), define mass-dependent fractionation lines with a similar slope of ∼0.5. The different inclusions have different Δ17O values ranging from ∼−25‰ to ∼−16‰. Melilite and plagioclase in the CV FUN CAIs have 16O-poor compositions (Δ17O ∼−3‰) and plot near the intercept of the Allende CAI line and the terrestrial fractionation line. We infer that mass-dependent fractionation effects of oxygen isotopes in FUN CAI minerals are due to evaporation during melt crystallization. Differences in Δ17O values of mass-dependent fractionation lines defined by minerals in individual FUN CAIs are inferred to reflect differences in Δ17O values of their precursors. Differences in δ18O values of minerals defining the mass-dependent fractionation lines in several FUN CAIs are consistent with their inferred crystallization sequence, suggesting these minerals crystallized during melt evaporation. In other FUN CAIs, no clear correlation between δ18O values of individual minerals and their inferred crystallization sequence is observed, possibly indicating gas-melt back reaction and oxygen-isotope exchange in a 16O-rich gaseous reservoir. After oxygen-isotope fractionation, some FUN CAIs could have experienced partial melting and gas-melt oxygen-isotope exchange in a 16O-poor gaseous reservoir that resulted in crystallization of 16O-depleted fassaite, melilite and plagioclase. The final oxygen isotopic compositions of melilite and plagioclase in the CV FUN CAIs may have been established on the CV parent asteroid as a result of isotope exchange with a 16O-poor fluid during hydrothermal alteration.We conclude that FUN CAIs are part of a general family of refractory inclusions showing various degrees of fractionation effects due to evaporative processes superimposed on sampling of isotopically heterogeneous material. These processes have been experienced both by FUN and non-FUN igneous CAIs. Generally, the inclusions identified as FUN show larger isotope fractionation effects than non-FUN CAIs. There is a wide spread in UN isotopic anomalies in a large number of CAIs not exhibiting large fractionation effects in oxygen, magnesium, and silicon. The question of why some FUN CAIs show more extreme UN isotopic effects is attributed by us to limited sampling and not a special source of isotopically anomalous material. We consider the majority of igneous CAIs to be the result of several stages of thermal processing (evaporation, condensation, and melting) of aggregates of solid precursors composed of incompletely isotopically homogenized materials. The unknown nuclear effects in CAIs are common to both FUN and non-FUN CAIs, and are not a special characteristic of FUN inclusions but represent the spectrum of results from sampling a very heterogeneous medium in the accreting Solar System.

Isotopic records in CM hibonites: Implications for timescales of mixing of isotope reservoirs in the solar nebula

The magnesium isotopic compositions of 26 hibonite-bearing inclusions from the CM chondrite Murchison, as well as isotopic measurements on a subset of these samples for oxygen, titanium, and lithium–beryllium–boron are reported along with oxygen isotopic data for an additional 13 hibonites that were previously investigated for other isotope systems (magnesium, potassium, calcium, and titanium) and rare earth element concentrations. Magnesium isotopic compositions divide CM hibonites into two distinct populations which correlate perfectly with their mineralogy and morphology, as previously discovered by Ireland [Ireland T. R. (1988) Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta 52, 2827–2839]: Spinel-HIBonite spherules (SHIBs) bear evidence of in situ 26Al decay, whereas PLAty-Crystals (PLACs) and Blue AGgregates (BAGs) either lack resolvable 26Mg-excesses or exhibit 26Mg deficits by up to ∼4‰. High precision, multiple collector SIMS analyses show that 6 of 7 SHIBs investigated fall on a single correlation line implying 26Al/ 27Al = (4.5 ± 0.2) × 10 −5 at the time of isotopic closure, consistent with the “canonical” 26Al abundance characteristic of internal isochrons in many calcium–aluminum-rich inclusions (CAIs). One SHIB sample exhibits Δ 26Mg ∗ consistent with a “supracanonical” 26Al/ 27Al ratio of (6.4 ± 0.5) × 10 −5. The PLAC hibonites contain highly anomalous titanium isotopic compositions, with δ 50Ti values ranging from −80‰ to almost +200‰, whereas SHIBs generally lack large Ti isotopic anomalies. Eight out of 11 26Al-free PLAC hibonite grains record 10B/ 11B excesses that correlate with Be/B; the inferred initial 10Be/ 9Be ratio of (5.1 ± 1.4) × 10 −4 is lower than the best-constrained 10Be/ 9Be of (8.8 ± 0.6) × 10 −4 in a CV CAI. The data demonstrate that 10Be cannot be used as a relative chronometer for these objects and that most of the 10Be observed in CAIs must be produced by irradiation of precursor solids in the early solar system. The lack of 26Al in PLAC hibonites indicates that significant amounts of 26Al were not formed in the same spallogenic processes that made 10Be in PLAC precursors. This is most easily understood as indicating very early formation of the PLAC hibonites, prior to the incorporation and mixing of 26Al into the solar nebula, although an alternative scenario, which invokes irradiation under different solar flare conditions, cannot be ruled out. Lithium isotopes are normal within uncertainties, probably reflecting contamination and/or postcrystallization exchange. The oxygen isotopic compositions of SHIBs and PLACs are all highly 16O-enriched, but are not derived from a homogeneous reservoir: Δ 17O values span a range of ∼−28‰ to −15‰. The ranges of 16O-enrichment in SHIBs and PLACs overlap and are less “anomalous” than the most 16O-enriched compositions found in meteorites [Kobayashi S., Imai H. and Yurimoto H. (2003) New extreme 16O-rich chondrule in the early solar system. Geochem. J. 37, 663–669]. Both PLACs and SHIBs formed in 16O-enriched reservoirs characterized by small-scale heterogeneities in the gas phase. If such heterogeneities were generated by an admixture of relatively 16O-poor gas created by self-shielding during CO photolysis and transported to the hot inner regions of the accretion disk, then this process must have been initiated very early on, prior to the arrival of fresh radioactivity into the inner solar system. Oxygen isotope heterogeneities persisted throughout the formation interval of PLACs, CAI precursors, and SHIBs which could be as long as 3 × 10 5 years based on 26Al records. One SHIB and one BAG exhibit mass fractionated oxygen isotopic compositions similar to those seen in FUN inclusions and in several platy hibonite crystals [Lee T., Mayeda T. K. and Clayton R. N. (1980) Oxygen isotopic anomalies in Allende inclusion HAL. Geophys. Res. Lett. 7, 493–496; Ireland T. R., Zinner E. K., Fahey A. J. and Esat T. M. (1992) Evidence for distillation in the formation of HAL and related hibonite inclusions. Geochim. Cosmochim. Acta 56, 2503–2520; Ushikubo T., Hiyagon H. and Sugiura N. (2007) A FUN-like hibonite inclusion with a large 26Mg-excess. Earth Planet. Sci. Lett. 254, 115–126]. The suite of mass-fractionated hibonites exhibit a range of isotopic properties, including 26Al/ 27Al ratios from below detection to a “canonical” level and oxygen and titanium anomalies that are not exceptional by PLAC standards. This suggests that F (fractionation) processes—evaporation under (oxidizing) conditions—are not necessarily associated with sampling a special isotopic reservoir.

Discovery of a New FUN CAI from a CV Carbonaceous Chondrite: Evidence for Multistage Thermal Processing in the Protoplanetary Disk

We report the mineralogy, petrography, as well as oxygen and magnesium isotope data of a newly identified FUN inclusion from the CV carbonaceous chondrite NWA 779. Variability in the texture of the mineral phases coupled with oxygen isotope data provides evidence for multistage evolution of this inclusion under distinct thermal regimes: slow crystallization of 16O-rich melt accompanied by evaporation, and subsequent remelting in an 16O-poor reservoir during transient heating events, possibly associated with the formation of CV chondrules. The inferred oxygen isotope composition of the precursor material of this inclusion (δ17,18O = − 48.4+ 2.5−3.0‰) is consistent with that observed for CAIs and amoeboid olivine aggregates from least metamorphosed carbonaceous chondrites, suggesting that both FUN and normal CAIs formed in an16O-rich reservoir with oxygen isotope composition similar to that inferred for the Sun. However, in contrast to normal CAIs, most FUN inclusions show no evidence for live 26Al at the time of their formation. Based on these observations, we propose that the protosolar molecular cloud was polluted with stellar-derived 26Al prior to its collapse. Thus, FUN CAIs formed at a time when dust inherited from the molecular cloud—including the carrier of 26Al—was still poorly homogenized in the protoplanetary disk.

The White Angel: A unique wollastonite‐bearing, mass‐fractionated refractory inclusion from the Leoville CV3 carbonaceous chondrite

Abstract— We report the study of an unusual compact type A refractory inclusion, named the White Angel, from the Leoville CV3 meteorite. The petrologic, mineral chemical, isotopic, and trace‐element signatures of this once‐molten Ca‐Al‐rich inclusion (CAI), which contains large, equant wollastonite crystals, indicate a short multistage history that occurred very early, before substantial decay of 26Al. Magnesium in the inclusion is isotopically heavy, with FMg reaching 18‰/amu, in the range of fractionated and with unidentified nuclear effects (FUN) inclusions. However, the absence of any nuclear anomalies in Ca and Ti and an inferred 26Al/27Al ratio of (5.5 ± 0.9) × 10−5 indicate that the White Angel belongs to the F inclusions. Silicon and oxygen are also mass fractionated in favor of the heavy isotopes, but to a lesser extent. The O isotopes show a range in 16O excesses. On an O three‐isotope plot, data points lie on a line parallel and to the right of the carbonaceous chondrite anhydrous mineral mixing line, with wollastonite being the most 16O‐rich phase. The chondrite‐normalized rare earth and trace‐element pattern of the whole inclusion is the complement of an ultrarefractory pattern indicating that precursor phases of the CAI must have condensed in an Al‐, heavy rare earth element (HREE)‐depleted reservoir. Melting of those precursor phases in an 16O‐rich environment and evaporation led to mass‐dependent isotopic fractionation of Mg, Si, and O. Partial isotopic exchange with a reservoir containing unfractionated Mg took place at a later stage but before any measurable decay of 26Al. Some minerals (melilite and perovskite) in the White Angel equilibrated oxygen isotopes with a relatively 16O‐poor reservoir that was also mass‐fractionated toward the heavy isotopes, different from that with which the normal or FUN inclusions interacted.

A FUN-like hibonite inclusion with a large 26Mg-excess

We found a unique hibonite (CaAl12O19) inclusion Kz1–2 from the Kainsaz (CO3) chondrite, which exhibits evidence for a strong link between “normal” CAIs and FUN inclusions. Kz1–2 shows strongly fractionated isotopic compositions of O, Ca, and Ti preferring heavy isotopes and a characteristic REE abundance pattern similar to those of FUN (or HAL-type hibonite) inclusions, suggesting an intensive distillation process in the formation of this inclusion. Unlike FUN inclusions, however, Kz1–2 shows large excesses in 26Mg and possibly in 41K most likely from the decays of now-extinct nuclides 26Al and 41Ca with the inferred initial ratios of (26Al/27Al)0∼5.3×10−5 and (41Ca/40Ca)0∼1.1×10−8, respectively, and no detectable mass-independent isotopic anomalies in Ca and Ti, both of which are characteristic of “normal” CAIs. These observations suggest that the distillation process which produced isotopic fractionation in Kz1–2 occurred as early as the formation of “normal” CAIs. It is also inferred that the formation process of FUN inclusions is closely related to that of “normal” CAIs.

Nuclear field vs. nucleosynthetic effects as cause of isotopic anomalies in the early Solar System

Even in the absence of nucleosynthesis, mass-independent isotope effect due to nuclear field can lead to a number of isotope anomalies found in refractory inclusions of chondrites. Isotope anomalies observed for Ba, Ca, Sr, Ti, and Cr in FUN inclusions may simply be explained by the nuclear field effects of these elements. A whole class of isotopic heterogeneities may therefore reflect evaporation/condensation processes in the solar nebula rather than nucleosynthetic effects. We speculate that the current estimates of the r-process component of the normal solar material may be affected by non-mass dependent fractionation effects.

Chemical equilibrium and kinetic constraints for chondrule and CAI formation conditions

While many uncertainties remain, a kinetic evaporation-condensation model is used to show that type A chondrules, and compact Type A and B calcium-aluminum-rich inclusions (CAIs) could have formed from CI-like precursors under conditions that are consistent with predictions for 2–3 AU in a canonical solar nebula. Type B and Al-rich chondrules, and Type C CAIs, on the other hand, may have formed from fractionated precursors. Based primarily on chondrule and CAI isotopic compositions, previous studies have reached different conclusions because they did not take into account the effects of gas-melt exchange. Assuming CI-like precursor compositions, equilibrium silicate melts with elemental compositions like those of type A chondrules could have formed over a wide range of conditions (T, P tot, solid/gas/solar). Metal is not predicted to be stable when T ≥ 1600°C. When T < 1600°C, the abundances and compositions of metal in chondrules appear to be less successfully reproduced than the silicates, e.g., at a given temperature more metal is predicted in type II chondrules than is generally observed, and under some conditions type IIs are predicted to be more metal-rich than type Is. These differences could be overcome if type Is formed from precursors that were more reduced than CI, and if type IIs formed after significant metal-silicate fractionation. The formation conditions of molten CAIs are much more restricted than for chondrules, perhaps in part explaining their lower abundances. The Mg, Si and O isotopic mass fractionations in non-FUN CAIs can be reproduced if they formed between ∼1400 to 1500°C in regions where CAI-like equilibrium melts were stable, but they did not quite reach equilibrium with the gas. CAI formation times at P tot = 10 −4-10 −3 bars are consistent with estimates of Type B CAI cooling times, but pressures much below this require formation times that are too long. The isotopic mass fractionations in FUN CAIs can be explained if they formed at or below the ranges of solid/gas/solar ratios where CAI-like equilibrium compositions are stable. Under these conditions, FUN inclusions undergo less gas-melt exchange than non-FUN CAIs. The FUN CAI formation temperatures are consistent with formation at 1400 to 1500°C, but may have been higher. Two general explanations for the distribution of O mass independent fractionations (MIF) in chondrules/CAIs have been explored: creation of the MIF before chondrule/CAI formation, and creation of the MIF during chondrule/CAI formation. If the MIF was established before chondrule/CAI formation, the most promising explanation is that H 2O (presumably as ice) and silicate dust with MIFs of opposite sign are fractionated together from the remaining gas. On heating, the H 2O now in the gas exchanges with the melt. If the MIF was generated during chondrule/CAI formation, it must be generated in the H 2O, because it exchanges most rapidly with the melt, and mass balance requires creation of MIF of opposite sign in CO. Self-shielding from UV radiation is one possibility, but the effect may be quenched at high temperatures. Non-RRKM intramolecular kinetic isotope effects are another possibility, but a continuous source of radiation may be needed to prevent gas phase reactions from approaching equilibrium.

A petrographic, chemical, and isotopic study of calcium‐aluminum‐rich inclusions and aluminum‐rich chondrules from the Axtell (CV3) chondrite

Abstract— Petrographic, compositional, and isotopic characteristics were studied for three calcium‐aluminum‐rich inclusions (CAIs) and four plagioclase‐bearing chondrules (three of them Al‐rich) from the Axtell (CV3) chondrite. All seven objects have analogues in Allende (CV3) and other primitive chondrites, yet Axtell, like most other chondrites, contains a distinctive suite of CAIs and chondrules. In common with Allende CAIs, CAIs in Axtell exhibit initial 26Al/27Al ratios ((26Al/27Al)0) ranging from ∼5 × 10−5 to &lt;1.1 × 10−5, and plagioclase‐bearing chondrules have (26Al/27Al)0 ratios of ∼3 × 10−6 and lower. One type‐A CAI has the characteristics of a FUN inclusion. The Al‐Mg data imply that the plagioclase‐bearing chondrules began to form &gt;2 Ma after the first CAIs. As in other CV3 chondrites, some objects in Axtell show evidence of isotopic disturbance. Axtell has experienced only mild thermal metamorphism (&lt;600 °C), probably not enough to disturb the Al‐Mg systematics. Its CAIs and chondrules have suffered extensive metasomatism, probably prior to final accretion. These data indicate that CAIs and chondrules in Axtell (and other meteorites) had an extended history of several million years before their incorporation into the Axtell parent body. These long time periods appear to require a mechanism in the early solar system to prevent CAIs and chondrules from falling into the Sun via gas drag for several million years before final accretion.We also examined the compositional relationships among the four plagioclase‐bearing chondrules (two with large anorthite laths and two barred‐olivine chondrules) and between the chondrules and CAIs. Three processes were examined: (1) igneous differentiation, (2) assimilation of a CAI by average nebular material, and (3) evaporation of volatile elements from average nebular material. We find no evidence that igneous differentiation played a role in producing the chondrule compositions, although the barred olivine compositions can be related by addition or subtraction of olivine. Methods (2) and (3) could have produced the composition of one chondrule, AXCH‐1471, but neither process explains the other compositions. Our study indicates that plagioclase‐bearing objects originated through a variety of processes.

An Isotopic and Petrologic Study of Calcium-Aluminum-Rich Inclusions from CO3 Meteorites

We have studied the mineralogy and petrology of 229 calcium-aluminum-rich inclusions (CAIs) from ten CO3 meteorites of petrologic types 3.0–3.7. Subsets of these inclusions were measured by ion probe for magnesium, calcium, and titanium isotopes and REE abundances. Most CAIs from CO3 meteorites fall into three major types: (1) melilite-rich inclusions, which also contain spinel/hercynite, perovskite, and occasionally hibonite; (2) spinel-pyroxene inclusions; and (3) hibonite-hercynite inclusions. In addition, several isolated hibonite grains, two grossite (CaAl4O7) bearing CAIs, two hibonite-fassaite microspherules, and one anorthite-spinel-pyroxene inclusion were found. CAIs from CO3 meteorites exhibit all of the REE patterns commonly seen in inclusions from CV3 and CM2 chondrites. Most exhibit evidence of 26Al, and many have inferred (26Al/27Al)o ≈ 5 × 10−5. The relative abundances of different types of CAIs in CO3 chondrites differ from those in CV3 and CM2 chondrites.CAIs in CO3 chondrites have experienced considerable secondary alteration, both before and after accretion. Signatures of nebular alteration include Wark-Lovering rims and the high Fe contents in spinels from all hibonite/hercynite inclusions. Occasionally, melilite and anorthite show evidence of nebular alteration to feldspathoids and pyroxene. The magnesium-aluminum systematics of some melilite-rich inclusions were apparently disturbed prior to final accretion of the parent body. Parent body alteration is indicated by correlations between CAI characteristics and the petrologic type of the host meteorite. Spinel in melilite-rich and coarse-grained spinel-pyroxene inclusions becomes more Fe rich, with the development of relatively homogeneous hercynitic spinel (∼50–60 mol%) in CAIs from metamorphic grades >3.4. Perovskite has been converted to ilmenite in types >3.4. Melilite-rich inclusions are abundant in CO3.0–3.3 meteorites, rare in 3.4 meteorites, and absent meteorites of types 3.5–3.7; melilite-rich CAIs are probably replaced by inclusions rich in feldspathoids, pyroxene, and Fe-rich spinel. Isotopic disturbance of the magnesium-aluminum systematics may be more severe in higher petrologic types. Hibonite seems to be unaffected by this level of metamorphism.Three isotopically unusual inclusions were found. One single-crystal hibonite, Isna SP16, has a REE pattern strongly depleted in Ce and Y, (26Al/27Al)o = (2.4 ± 0.3) × 10−5, and mass fractionated calcium (FCa = +12 ± 2‰/amu), but no resolvable nuclear anomalies in neutron-rich calcium isotopes. The REE pattern, which is thought to reflect nebular conditions, and mass-fractionated calcium, indicative of evaporation, are similar to those of the FUN inclusion, HAL, and related hibonites, indicating similar formation conditions. The absence in Isna SP16 of the nuclear anomalies observed in HAL and the difference in (26Al/27Al)o between HAL and Isna SP16 indicate that the processes that produced HAL-type hibonites operated on diverse materials. Two hibonite-bearing microspherules, Colony SP1 and ALH82101 SP15, exhibit nearly flat REE patterns with negative europium anomalies and slightly negative δ26Mg. ALH82101 SP15 has resolved excesses of 48Ca and 50Ti. These characteristics are similar to those of previously described microspherules from Murchison and Lance, implying that the microspherules formed via a single process from related, but not identical source materials.

Isotopically distinct reservoirs in the solar nebula: Isotope anomalies in Vigarano meteorite inclusions

The isotopic compositions of Mg, Ca, Ti, Cr, Zn, Sr, Ba, Nd, and Sm were measured in four relatively unaltered refractory inclusions from the Vigarano carbonaceous chondrite meteorite. Three of the inclusions (USNM 1623-2, 1623-3, and 1623-8) show similar Mg, Ca, Ti, and Cr isotopic compositions to those found in most inclusions in the Allende carbonaceous chondrite. This indicates that these Vigarano inclusions sampled the same isotopic reservoirs as the majority of the Allende inclusions that isotope signatures in the latter were not significantly modified by the secondary alteration that permeates most Allende inclusions. In contrast, inclusion 1623-5 has large deficits in Mg-26, Ca-48, and Ti-50 and small but distinct Cr-54, Zn-66, Sr-84, Ba-135, Ba-137, and Sm-144 anomalies. The magnitudes of these unusual anomalies in the refractory elements are within analytical uncertainty of those found in the Allende 'FUN inclusion C1, yet 1623-5 has a very different bulk chemical composition from C1. The fact that 1623-5 and C1 have identical isotopic anomalies yet have significantly distinct major and trace element contents provide convincing evidence for the presence of isotopically distinct reservoirs in the early solar system.

An ion microprobe study of corundum in the Murchison meteorite: Implications for 26A1 and 16O in the early solar system

Twenty-six A1 2O 3 grains from the Murchison CM2 chondrite have been analyzed by ion microprobe mass spectrometry for the isotopes of O, Mg, and Ti and the abundances of Mg, Ca, Sc, Ti, V, Sr, Y, Zr, La, and Ce. Being the most refractory major phase in solar matter, A1 2O 3 retains a particularly durable record of the early solar system. 26Mg 24Mg ranges up to 56× the solar-system ratio, owing to decay of extinct 26A1, but the initial 26Al 24Al ratios do not exceed the canonical maximum of 5 × 10 −5 established in earlier work. There is no evidence for fossil radiogenic 26Mg surviving from presolar times. Oxygen isotope compositions cluster mainly near δ 18 O = −50%. (lighter than bulk spinel), but range from −94 to −11%.. The grains divide into three groups on the basis of 26A1, 16O, Ti, and V content, and 26Al and O show distinctive correlations (in contrast to all previous studies), suggesting an origin from the following components. Group 1 (high 26A1, Ti, V): mixture of material with 26Al 24Al = 5 × 10 −5 and δ 18 O = −45%. with dead Al of δ 18 O ≈ −100%.. Group 2 (low 26A1, Ti, V): mixture of material with 26Al 27Al = 5 × 10 −6 with dead Al, with complex fractionation and exchange of O resembling that of FUN inclusions. Group 3 (no 26A1; high Ti, V): dead Al from various sources. In terms of this model, the corundum formed from two components with live 26Al and a mass fraction of 43% dead Al, but we do not know whether this figure is typical of carbonaceous chondrites in toto, let alone the entire solar nebula. Trace element abundances in corundum are generally at less than Cl levels relative to Al, and decline with increasing volatility, from Zr to Ca.

Melt solidification and late-stage evaporation in the evolution of a FUN inclusion from the Vigarano C3V chondrite

Vigarano 1623-5 is a forsterite-bearing refractory inclusion in which oxygen, magnesium, and silicon show large degrees of mass-dependent isotopic fractionation. The core of 1623-5 consists of forsteritic olivine, fassaite, melilite (Åk 89), and spinel, all of which show isotopic mass fractionation, averaging ~30.6%./amu for magnesium and ~10.0%./amu for silicon; it is enclosed in a mantle consisting of aluminous melilite (Åk 16–60), spinel, perovskite, and hibonite. Hibonite and spinel in the mantle are enriched in heavy isotopes of magnesium by 8–15%./amu relative to the core. 1623-5 originally formed by solidification of a magnesium-rich melt. The endemic isotopic mass fractionation may have been caused by evaporation of that melt or may reflect an earlier evaporation event affecting the reservoir from which the interior of 1623-5 formed. Later flash remelting of the outer part of the core, and volatilization of SiO 2 and MgO from the melt so produced, caused formation of a mantle that is in isotopic and petrologic disequilibrium with the core.

The isotopic composition of nitrogen in ureilitic diamonds

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$̆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.

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

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.

Isotopic studies of Mg, Fe, Mo, Ru and W in Fremdlinge from Allende refractory inclusions

The isotopic abundances of Mg, Fe, Mo, Ru and W in Fremdlinge and a refractory metal nugget in coarse-grained, Ca-Al-rich inclusions from the Allende meteorite have been determined using the PANURGE ion microprobe. These data represent the first measurement of the isotopic composition of Fremdlinge. No evidence for large, non-mass-dependent deviations (≳1%) from normal solar isotopic abundances was found for any of the elements measured. The only hint, of a nonlinear isotopic anomaly is for Mo in Fremdling 1 of Allende inclusion 5171; the measured 100 Mo 98 Mo ratio lies slightly more than two standard deviations above the value measured in standards. Isotopic variations due to mass-dependent fractionation are suggested for Mo in two Fremdlinge, Zelda of Egg 6 and Fremdling 1 of 5171, and for Ru in a refractory metal nugget in the FUN inclusion Cl. Mo is enriched in the lighter isotopes by ∼4‰/ amu and ∼9‰/ amu , respectively, while Ru is enriched in the heavier isotopes by ∼4‰/ amu . Despite their unusual mineralogy and extreme enrichment in refractory siderophile elements, Fremdlinge do not contain an enhanced component from a single nucleosynthetic source, but appear to contain the normal solar system mixture of e-, s-, and r-process material. While the possibility that some Fremdlinge may have formed under presolar conditions cannot be excluded, we believe that the chemical, petrologic and isotopic data are most plausibly interpreted in terms of the origin of Fremdlinge within the solar nebula

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 50Ti 49Ti $ ̆ 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 Pu Th 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 δ 50 Ti = 15%. and of an isochron with ( 26Al 27Al ) 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.

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

Ion microprobe measurements of the Ca isotopic compositions of eight individual hibonite grains from the Ca chondrites Murray and Murchison are reported. Large Ca-48 anomalies two orders of magnitude larger than those observed in normal inclusions and at least a factor of four larger than in the FUN inclusions are found. These anomalies are qualitatively correlated with the Ti-50 anomalies in the sense that the delta Ca-48 and delta Ti-50 values have the same sign. These results confirm previous conclusions concerning the presence of a nucleosynthetic component produced by neutron-rich nuclear statistical equilibrium processes. They suggest the preservation of the isotopic anomalies with interstellar dust grains as carriers of the n-rich Ca and Ti components. The large variation in the delta Ca-48/delta Ti-50 ratios indicates multiple nucleosynthetic components and/or chemical fractionation between Ca and Ti prior to hibonite formation.

Absolute isotopic abundances of Ti in meteorites

The absolute isotope abundance of Ti has been determined in Ca-Al-rich inclusions from the Allende and Leoville meteorites and in samples of whole meteorites. The absolute Ti isotope abundances differ by a significant mass dependent isotope fractionation transformation from the previously reported abundances, which were normalized for fractionation using 46Ti 48Ti . Therefore, the absolute compositions define distinct nucleosynthetic components from those previously identified or reflect the existence of significant mass dependent isotope fractionation in nature. We provide a general formalism for determining the possible isotope compositions of the exotic Ti from the measured composition, for different values of isotope fractionation in nature and for different mixing ratios of the exotic and normal components. The absolute Ti and Ca isotopic compositions still support the correlation of 50Ti and 48Ca effects in the FUN inclusions and imply contributions from neutron-rich equilibrium or quasi-equilibrium nucleosynthesis. The present identification of endemic effects at 46Ti, for the absolute composition, implies a shortfall of an explosive-oxygen component or reflects significant isotope fractionation. Additional nucleosynthetic components are required by 47Ti and 49Ti effects. Components are also defined in which 48Ti is enhanced. Bulk samples of carbonaceous meteorites (C2 and C3 types) show distinct excesses at 50Ti but no nonlinear effects at the other Ti isotopes. Other chondrites, including Orgueil (Cl), show no nonlinear effects. Relative to terrestrial Ti, a small isotope fractionation is found for only an enstatite chondrite. The Ti absolute compositions in Ca-Al-rich inclusions show significant isotope fractionation effects corresponding to an enhancement in the heavier isotopes relative to the lighter isotopes as compared to Ti in a TiO 2 standard and in chondrites. The absence of a correlation of Ti isotope fractionation effects with those for Ca and Mg is indicative of multiple processes of condensation, volatilization and recondensation; however, the mechanisms causing the isotope fractionation are not well understood.