Hibonite Crystals Research Articles

Titanium and chromium isotopic compositions of calcium-aluminum-rich inclusions: Implications for the sources of isotopic anomalies and the formation of distinct isotopic reservoirs in the early Solar System

As the earliest-dated solids in our Solar System, calcium-aluminum-rich inclusions (CAIs) provide a record of their formation environment near the young Sun and hold clues to the formation of planetary-scale isotopic reservoirs in the solar protoplanetary disk. Although CAIs from several CV, CK, CM, CO, and ordinary chondrites have been analyzed previously for their Ti isotopic compositions, CAIs from just three CV chondrites have been analyzed for their Cr isotopic compositions, and only a handful of CAIs have been measured for both their Ti and Cr isotopic compositions. We report mass-independent Ti and Cr isotopic anomalies in several CAIs from CV and CK chondrites; this is the first report of the Cr isotopic composition of a CAI from a CK chondrite. With these additional data, we aim to better constrain the compositional range of CAIs in ε50Ti versus ε54Cr space, thereby facilitating the isotopic characterization of the material inherited by the solar protoplanetary disk and the role of CAIs in the formation of distinct planetary-scale isotopic reservoirs in our early Solar System. The narrow range in isotopic anomalies in CAIs when compared to other early-formed refractory inclusions such as platy hibonite crystals (PLACs) and spinel-hibonite inclusions (SHIBs) suggests that CAIs record the mixing of these precursor materials and the averaging of their larger isotopic anomalies. The isotopic composition of CAIs is therefore likely the result of a combination of factors, including mixing of material inherited from their formation region, heterogeneous carrier phase distribution, and thermal processing in the disk. The ε50Ti and ε54Cr isotopic compositions of CAIs are not correlated, further demonstrating that these isotopic anomalies have different carrier phases. The Ti and Cr isotopic compositions of CAIs additionally show that CAIs alone cannot be responsible for the compositional difference between the non-carbonaceous chondritic (NC) and carbonaceous chondritic (CC) isotopic reservoirs but nevertheless do play a role in the formation of these large-scale isotopic reservoirs in the early Solar System.

Calcium and titanium isotopes in refractory inclusions from CM, CO, and CR chondrites

Previous studies have shown that CV and CM chondrites incorporated Ca, Al-rich inclusions (CAIs) with different isotopic characteristics, which may represent different snapshots in the isotopic evolution of the early Solar System. To better understand how the isotopic characteristics of CAIs vary between different chondrite groups, we have studied calcium and titanium isotopes in CAIs from CM, CO, and CR chondrites. We show that all three chondrite groups contain CAIs with large anomalies in 48Ca and/or 50Ti (10s of ‰ or 100s of ε-units) as well as CAIs with no anomalies resolved beyond measurement uncertainties.Isotopically, the anomalous CO and CR chondrite CAIs resemble the platy hibonite crystals (PLACs) from CM chondrites, but they are more mineralogically complex. The new data are consistent with the well-established mutual exclusivity relationship between incorporation of 26Al and the presence of large anomalies in 48Ca and 50Ti. The two highly anomalous CO chondrite CAIs have correlated anomalies in 46Ti and 50Ti, while most other highly anomalous CAIs do not. This result could indicate that the reservoir with coupled 46Ti and 50Ti that was sampled by bulk meteorites and CV chondrite CAIs already existed before arrival and/or homogeneous distribution of 26Al in the protoplanetary disk.Among the studied CM chondrite CAIs are ten spinel-hibonite inclusions (SHIBs) with known oxygen isotopic compositions. Our results show that these objects sampled a reservoir that was well-mixed in oxygen, calcium, and titanium isotopes. We further show that SHIBs tend to be slightly enriched in the heavy calcium isotopes, suggesting that their formation history was different from CV chondrite CAIs.

A link between oxygen, calcium and titanium isotopes in 26Al-poor hibonite-rich CAIs from Murchison and implications for the heterogeneity of dust reservoirs in the solar nebula

PLACs (platy hibonite crystals) and related hibonite-rich calcium-, aluminum-rich inclusions (CAIs; hereafter collectively referred to as PLAC-like CAIs) have the largest nucleosynthetic isotope anomalies of all materials believed to have formed in the solar system. Most PLAC-like CAIs have low inferred initial 26Al/27Al ratios and could have formed prior to injection or widespread distribution of 26Al in the solar nebula. In this study, we report 26Al–26Mg systematics combined with oxygen, calcium, and titanium isotopic compositions for a large number of newly separated PLAC-like CAIs from the Murchison CM2 chondrite (32 CAIs studied for oxygen, 26 of these also for 26Al–26Mg, calcium and titanium). Our results confirm (1) the large range of nucleosynthetic anomalies in 50Ti and 48Ca (our data range from −70‰ to +170‰ and −60‰ to +80‰, respectively), (2) the substantial range of Δ17O values (−28‰ to −17‰, with Δ17O=δ17O−0.52×δ18O), and (3) general 26Al-depletion in PLAC-like CAIs.The multielement approach reveals a relationship between Δ17O and the degree of variability in 50Ti and 48Ca: PLAC-like CAIs with the highest Δ17O (∼−17‰) show large positive and negative 50Ti and 48Ca anomalies, while those with the lowest Δ17O (∼−28‰) have small to no anomalies in 50Ti and 48Ca. These observations could suggest a physical link between anomalous 48Ca and 50Ti carriers and an 16O-poor reservoir. We suggest that the solar nebula was isotopically heterogeneous shortly after collapse of the protosolar molecular cloud, and that the primordial dust reservoir, in which anomalous carrier phases were heterogeneously distributed, was 16O-poor (Δ17O⩾−17‰) relative to the primordial gaseous (CO+H2O) reservoir (Δ17O<−35‰). However, other models such as CO self-shielding in the protoplanetary disk are also considered to explain the link between oxygen and calcium and titanium isotopes in PLAC-like CAIs.

A heterogeneous solar nebula as sampled by CM hibonite grains

The magnesium isotopic compositions of 43 hibonite-bearing inclusions extracted from CM2 meteorites Murchion and Paris were measured with a Cameca 1280 HR2 ion microprobe. The results are broadly consistent with the literature data: most Spinel–HIBonite spherules (SHIBs) bear evidence for in situ decay of 26Al at the canonical level, whereas PLAty–hibonite Crystals (PLACs) either lack resolvable 26Mg excesses or are characterized by apparent 26Mg deficits by up to ~5‰ relative to the terrestrial composition. With higher precisions achieved in this study, the 26Al/27Al scatter ranging from ~6×10−6 to ~6×10−5 previously seen in lower precision SHIB data can be clearly resolved. A statistical treatment to all the data reveals three distinctive peaks at 26Al/27Al=4.9×10−5, 3.5×10−5 and 6.5×10−6, and two minor (barely resolved) components at 26Al/27Al=6×10−5 and 2.5×10−5. Possible explanations for the observed 26Al/27Al scatter in SHIBs include (1) isotopic disturbance, (2) protracted SHIB formation, (3) early formation before complete homogenization of 26Al in the solar nebula, and (4) initial Mg isotopic heterogeneity during SHIB formation. We argue that the early formation of SHIBs was the most plausible scenario. The PLACs could have formed in a reservoir characterized by 26Al/27Al similar to the average galactic background value, or in an 26Al-free environment. In either case, it is almost certain that the Mg isotope distribution in the PLAC-forming region was heterogeneous. Combining the hydrodynamic timescales of isotope mixing in the solar nebula, one could infer that the duration for PLAC and SHIB formation should only be around tens of thousands of years.

HETEROGENEOUS DISTRIBUTION OF 26 Al AT THE BIRTH OF THE SOLAR SYSTEM

It is believed that Al-26, a short-lived (t1/2 = 0.73 Ma) and now extinct radionuclide, was uniformly distributed in the nascent Solar System with the initial Al-26/Al-27 ratio of ~5.2\times10-5, suggesting its external stellar origin. However, the stellar source of Al-26 and the manner in which it was injected into the solar system remain controversial: the Al-26 could have been produced by an asymptotic giant branch star, a supernova, or a Wolf-Rayet star and injected either into the protosolar molecular cloud or protoplanetary disk. Corundum (Al2O3) is thermodynamically predicted to be the first condensate from a cooling gas of solar composition. Here we show that micron-sized corundum condensates from O-16-rich gas (Big Delta O-17 ~ -25%) of solar composition recorded heterogeneous distribution of Al-26 at the birth of the solar system: the inferred initial Al-26/Al-27 ratio ranges from ~6.5x10-5 to <2x10-6; ~50% of the corundum grains measured are Al-26-poor. Other Al-26-poor, O-16-rich refractory objects include grossite (CaAl4O7)- and hibonite(CaAl12O19)-rich calcium-aluminum-rich inclusions (CAIs) in CH chondrites, platy hibonite crystals in CM chondrites, and FUN (fractionation and unidentified nuclear isotopic anomalies) CAIs in CV, CO, and CR chondrites. Considering the apparently early and short duration (<0.3 Ma) of condensation of refractory O-16-rich solids in the solar system, we infer that Al-26 was injected into the collapsing protosolar molecular cloud and later homogenized in the protoplanetary disk. The apparent lack of correlation between Al-26 abundance and O-isotope compositions of corundum grains put important constraints on the stellar source of Al-26 in the solar system.

LITHIUM–BERYLLIUM–BORON ISOTOPIC COMPOSITIONS IN METEORITIC HIBONITE: IMPLICATIONS FOR ORIGIN OF 10 Be AND EARLY SOLAR SYSTEM IRRADIATION

NanoSIMS isotopic measurements of Li, Be, and B in individual hibonite grains extracted from the Murchison meteorite revealed that 10B excesses correlate with the 9Be/11B ratios in 26Al-free PLAty hibonite Crystals. From these data, an initial 10Be/9Be = (5.5 ± 1.6) × 10–4 (2σ) and 10B/11B = 0.2508 ± 0.0015 can be inferred. On the other hand, chondritic boron isotopic compositions were found in 26Al-bearing Spinel-HIBonite spherules, most likely due to contamination with normal boron. No 7Li excesses due to 7Be decay were observed. When combined with previously reported data, the new data yield the best defined 10Be/9Be = (5.3 ± 1.0) × 10–4 (2σ) and 10B/11B = 0.2513 ± 0.0012 for PLACs. A comparison of this value and the best constrained 10Be/9Be = (8.8 ± 0.6) × 10–4 in CV Ca-Al-rich inclusions supports a heterogeneous distribution of 10Be and its protosolar irradiation origin. We consider two possible irradiation scenarios that could potentially lead to the observed Li-Be-B isotopic compositions in PLACs. Although in situ irradiation of solids with hibonite chemistry seems to provide the simplest explanation, more high quality data will be needed for quantitatively constraining the irradiation history.

Origin and chronology of chondritic components: A review

Mineralogical observations, chemical and oxygen–isotope compositions, absolute 207Pb– 206Pb ages and short-lived isotope systematics ( 7Be– 7Li, 10Be– 10B, 26Al– 26Mg, 36Cl– 36S, 41Ca– 41K, 53Mn– 53Cr, 60Fe– 60Ni, 182Hf– 182W) of refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], chondrules and matrices from primitive (unmetamorphosed) chondrites are reviewed in an attempt to test (i) the x-wind model vs. the shock-wave model of the origin of chondritic components and (ii) irradiation vs. stellar origin of short-lived radionuclides. The data reviewed are consistent with an external, stellar origin for most short-lived radionuclides ( 7Be, 10Be, and 36Cl are important exceptions) and a shock-wave model for chondrule formation, and provide a sound basis for early Solar System chronology. They are inconsistent with the x-wind model for the origin of chondritic components and a local, irradiation origin of 26Al, 41Ca, and 53Mn. 10Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology. 41Ca– 41K, and 60Fe– 60Ni systematics are important for understanding the astrophysical setting of Solar System formation and origin of short-lived radionuclides, but so far have limited implications for the chronology of chondritic components. The chronological significance of oxygen–isotope compositions of chondritic components is limited. The following general picture of formation of chondritic components is inferred. CAIs and AOAs were the first solids formed in the solar nebula ∼4567–4568 Myr ago, possibly within a period of <0.1 Myr, when the Sun was an infalling (class 0) and evolved (class I) protostar. They formed during multiple transient heating events in nebular region(s) with high ambient temperature (at or above condensation temperature of forsterite), either throughout the inner protoplanetary disk (1–4 AU) or in a localized region near the proto-Sun (<0.1 AU), and were subsequently dispersed throughout the disk. Most CAIs and AOAs formed in the presence of an 16O-rich (Δ 17O ∼ −24 ± 2‰) nebular gas. The 26Al-poor [( 26Al/ 27Al) 0 < 1 × 10 −5], 16O-rich (Δ 17O ∼ −24 ± 2‰) CAIs – FUN (fractionation and unidentified nuclear effects) CAIs in CV chondrites, platy hibonite crystals (PLACs) in CM chondrites, pyroxene–hibonite spherules in CM and CO chondrites, and the majority of grossite- and hibonite-rich CAIs in CH chondrites—may have formed prior to injection and/or homogenization of 26Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are 16O-depleted (Δ 17O > −10‰) and have ( 26Al/ 27Al) 0 similar to those in chondrules (<1 × 10 −5). These CAIs probably experienced melting during chondrule formation. Chondrules and most of the fine-grained matrix materials in primitive chondrites formed 1–4 Myr after CAIs, when the Sun was a classical (class II) and weak-lined T Tauri star (class III). These chondritic components formed during multiple transient heating events in regions with low ambient temperature (<1000 K) throughout the inner protoplanetary disk in the presence of 16O-poor (Δ 17O > −5‰) nebular gas. The majority of chondrules within a chondrite group may have formed over a much shorter period of time (<0.5–1 Myr). Mineralogical and isotopic observations indicate that CAIs were present in the regions where chondrules formed and accreted (1–4 AU), indicating that CAIs were present in the disk as free-floating objects for at least 4 Myr. Many CAIs, however, were largely unaffected by chondrule melting, suggesting that chondrule-forming events experienced by a nebular region could have been small in scale and limited in number. Chondrules and metal grains in CB chondrites formed during a single-stage, highly-energetic event ∼4563 Myr ago, possibly from a gas-melt plume produced by collision between planetary embryos.

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.

Refractory inclusions from the ungrouped carbonaceous chondrites MacAlpine Hills 87300 and 88107

Abstract— MacAlpine Hills (MAC) 87300 and 88107 are two unusual carbonaceous chondrites that are intermediate in chemical composition between the CO3 and CM2 meteorite groups. Calcium‐aluminum‐rich inclusions (CAIs) from these two meteorites are mostly spinel‐pyroxene and melilite‐rich (Type A) varieties. Spinel‐pyroxene inclusions have either a banded or nodular texture, with aluminous diopside rimming Fe‐poor spinel. Melilite‐rich inclusions (Åk4–42) are irregular in shape and contain minor spinel (FeO &lt;1 wt%), perovskite and, more rarely, hibonite. The CAIs in MAC 88107 and 87300 are similar in primary mineralogy to CAIs from low petrologic grade CO3 meteorites but differ in that they commonly contain phyllosilicates. The two meteorites also differ somewhat from each other: melilite is more abundant and slightly more Al‐rich in inclusions from MAC 88107 than in those from MAC 87300, and phyllosilicate is more abundant and Mg‐poor in MAC 87300 CAIs relative to that in MAC 88107. These differences suggest that the two meteorites are not paired. The CAI sizes and the abundance of melilite‐rich CAIs in MAC 88107 and 87300 suggests a genetic relationship to CO3 meteorites, but the CAIs in both have suffered a greater degree of aqueous alteration than is observed in CO meteorites. Aluminum‐rich melilite in CAIs from both meteorites generally contains excess 26Mg, presumably from the in situ decay of 26Al. Although well‐defined isochrons are not observed, the 26Mg excesses are consistent with initial 26Al/27Al ratios of approximately 3–5 times 10−5. An unusual hibonite‐bearing inclusion is isotopically heterogeneous, with two large and abutting hibonite crystals showing significant differences in their degrees of mass‐dependent fractionation of 25Mg/24Mg. The two crystals also show differences in their inferred initial 26Al/27Al ratios, 1 × 10−5 vs. ≤3 × 10−6.

Hibonite-hercynite inclusion HH-1 from the Lancé (CO3) meteorite: The history of an ultrarefractory CAI

We studied the petrography, major and trace element chemistry, and magnesium, calcium, titanium and iron isotopic compositions of a hibonite-rich refractory inclusion from the Lancé CO3 meteorite. The inclusion, 290 × 230 μm in size, consists of a core of hibonite crystals containing small perovskite grains, surrounded by a hercynite mantle and a thin diopside rim. The hercynite contains many small perovskite, ilmenite and sulfide grains. Trace element abundances show an ultrarefractory signature in all measured phases with an increasing depletion of the REEs heavier than Tb with increasing mass in hibonite, perovskite and ilmenite, but not hercynite. Calcium and titanium are isotopically anomalous with large deficits in 48Ca and 50Ti. Deficits are 60%. in 50Ti and 11.5%. in 49Ti in hibonite, hercynite, perovskite and ilmenite. In addition, the titanium isotopes show a mass-dependent isotopic fractionation of 8.5%./amu in favor of the heavy isotopes. Calcium isotope deficits in hibonite are 6.9%. in 42Ca, 5.0%. in 43Ca and 32.5%. in 48Ca relative to 40Ca- 44Ca. The deficits are smaller in the rim diopside ( δ 48 Ca = − 21%.) and a neighboring pyroxene grain( δ 48 Ca = − 8%.). Magnesium shows excesses in 26Mg, but the lack of an isochron relationship between 26Mg 24Mg and 27Al 24Mg indicates redistribution of radiogenic 26Mg. Magnesium in hibonite is isotopically light ( F Mg = − 5.3%./amu). The iron isotopes in hercynite are normal. The data show that inclusion HH-1 experienced a complex metasomatic history including Fe-metasomatism, sulfurization and formation of nepheline and sodalite. The isotopic data provide not only a record of nucleosynthetic processes and nebular inhomogeneity but also a tracer of the relative movement of elements which may have occurred during alteration.

The occurrence of grossite (CaAl 4O 7) in chondrites

The mineral grossite (CaAl 4O 7; Ca-dialuminate, also termed CA2) is a rare phase in terrestrial and extraterrestrial rocks. In meteorites, grossite was found in Ca,Al-rich inclusions (CAIs) from Allende (CV3), Vigarano (CV3), Leoville (CV3), Efremovka (CV3), Colony (CO3), Murchison (CM2), ALH 85085 (CH), Acfer 182 (CH), Acfer 059-E1 Djouf 001 (CR2), and the anomalous C-chondrite Adelaide. In Ca,Al-rich inclusions from most of these meteorites, grossite is an accessory phase; however, in many CAIs from ALH 85085, Acfer 182, and Acfer 059-E1 Djouf 001 it is a major phase. In this paper, we summarize the results of literature reports on grossite-bearing inclusions and focus, in detail, on the occurrence, mineralogy, and chemistry of CaAl 4O 7-containing Ca,Al-rich inclusions in Acfer 182 and Acfer 059-E1 Djouf 001. In these carbonaceous chondrites, abundant CAIs with high modal abundances of grossite (> 30 vol%, often >60 vol%) were discovered. To classify Ca,Al-rich inclusions with such high modal abundances of CaAl 4O 7 we suggest to introduce a new type of Ca,Al-rich inclusion: grossite-rich inclusions. In Acfer 059-E1 Djouf 001 and Acfer 182, grossite-containing inclusions occur with different petrographic characteristics. The most common type consists of a core of grossite rimmed by melilite, Ca-pyroxene ± spinel ± hibonite. In an other type, anhedral, polycrystalline grossite complexes of irregular shape are enclosed in melilite. In some spherical CA2-inclusions abundant euhedral to subhedral grossite and/or hibonite crystals are embedded in melilite. In two inclusions, a core of hibonite is mantled by grossite. Additionally, some grossite-bearing inclusions with unique texture and/or mineralogy were found. One of these inclusions is rich in Zr,Y-rich perovskite and contains phases rich in the ultrarefractory elements Sc and/or Zr. Almost all of the grossite-containing inclusions are rich in Al 2O 3 (>50 wt%), even if grossite is only a minor phase. Based on textural appearance and/or on REE contents in some inclusions (core minerals with Group II patterns) a condensation history for most inclusions is suggested. A single inclusion rich in perovskite and other phases containing high concentrations of ultrarefractory elements appears to have formed as an early condensate from the solar nebula.

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

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 Lancé CO3 meteorite and two from the Murchison CM2 meteorite. Lancé 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 26Al 27Al 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.

Presolar isotopic and chemical signatures in hibonite-bearing refractory inclusions from the Murchison carbonaceous chondrite

Magnesium-, calcium-, and titanium-isotopic compositions, as well as trace-element abundances, have been determined by ion microprobe mass spectrometry for 37 hibonite-bearing inclusions from the Murchison carbonaceous chondrite. These and previously analyzed hibonite-bearing inclusions can be divided into three main groups based on morphological and mineralogical criteria. PLAty Crystal fragments (PLACs) generally have depletions in the relatively volatile refractory trace elements (Eu, Yb, Mb, V, Ba), little or no evidence of 26Al, I.E., little or no radiogenic 26Mg ( 26Mg∗), and widely variable Ca- and Ti-isotopic compositions. One PLAC, 13-13, has excess 48Ca of 104%. and 50Ti of 273%., the largest effects yet measured in these elements. Four Blue AGgregates (BAGs) are distinctive in having low overall trace-element abundances with large relative depletions in the ultrarefractory elements Gd-Er, Lu, Zr, Hf, Y, Sc, and mass-fractionated Mg-isotopic compositions enriched in the heavy isotopes. Large 50Ti deficits have been measured in three of the four BAGs; the only BAG analyzed for its Ca-isotopic composition also shows a deficit in 48Ca. Spinel HIBonite inclusions (SHIBs) show a wide variety of morphologies ranging from bladed hibonite crystals within spinel, to spherules of hibonite and spinel, to large uniform hibonite crystals within spinel. SHIBs consisting of bladed hibonite in spinel or hibonite-spinel spherules typically have ultrarefractory-depleted trace-element patterns, although uncommon ultrarefractory-enriched patterns were also found, 26Mg∗ at a level consistent with 5 × 10 −5 × 27Al , and generally small (<10%.) Ca- and Ti-isotopic anomalies. One of the SHIB spherules, 7–170, has highly unusual systematics with a large 50Ti deficit, only a small 48Ca deficit, and no 26Mg∗. The large hibonite crystals within spinel have variable trace-element patterns, commonly with anomalies in the relatively volatile elements Ce, Eu, and Yb. 26Mg∗ is present in some of these inclusions but none of this type of SHIB has large Ca- or Ti-isotopic anomalies. While at least four distinct components are required to model the Ti-isotopic compositions, three components are sufficient to describe the variations in Ca-isotopic compositions measured thus far. The systematics of PLAC 13-13 with large excesses of 48Ca and 50Ti match the predictions of the multiple-zone-mixing model for nucleosynthesis in supernova ejecta to a maximum neutron excess of 0.159. Compositions depleted in 48Ca and 50Ti are unlikely to represent discrete nucleosynthetic components but represent material with a shortfall of neutron-rich material relative to the solar system abundances. Additional components must be present but are not easily resolved. Within the morphological groups of hibonite-bearing inclusions, a few have exceptional chemical and isotopic characteristics such as SHIB spherule 7–170. However, independent of the morphological groups, there are correlations in hibonite isotopic systematics and chemistry that hold for all inclusions studied: (1) no inclusion has both a large 50Ti anomaly and 26Mg∗; and (2) inclusions depleted in the ultrarefractory elements have 26Mg∗, with the exception of those which have a large 50Ti anomaly. These features clearly establish a relationship between chemical and isotopic characteristics of the inclusions which must predate their formation. Several components were combined, probably as mixtures of dust grains or crystals, and were melted to form refractory inclusions. The nucleosynthetic anomalies must be inherited from stellar environments; it is possible that the depletions in ultrarefractory elements are also a memory of stellar condensation and are not necessarily related to a condensation event in the solar nebula as has generally been assumed.

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

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 27Al 24Mg . 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.

Refractory inclusions in the Murchison meteorite

Mineralogical and petrographic studies of a wide variety of refractory objects from the Murchison C2 chondrite have revealed for the first time melilite-rich and feldspathoid-bearing inclusions in this meteorite, but none of these is identical to any inclusion yet found in Allende. Blue spinel-hibonite spherules have textures indicating that they were once molten, and thus their SiO 2-poor bulk composition requires that they were exposed to higher temperatures (>1550°C) than those deduced so far from any Allende inclusion. Melilite-rich inclusions are similar to Allende compact Type A's, but are more Al-, Ti-rich. One inclusion (MUCH-1) consists of a delicate radial aggregate of hibonite crystals surrounded by alteration products, and probably originated by direct condensation of hibonite from the solar nebular vapor. The sinuous, nodular and layered structures of another group of inclusions, spinel-pyroxene aggregates, suggest that these also originated by direct condensation from the solar nebular gas. Each type of inclusion is characterized by a different suite of alteration products and/or rim layers from all the other types, indicating modification of the inclusions in a wide range of different physico-chemical environments after their primary crystallization. All of these inclusions contain some iron-free rim phases. These could not have formed by reaction of the inclusions with fluids in the Murchison parent body because the latter would presumably have been very rich in oxidized iron. Other rim phases and alteration products could have formed at relatively low temperatures in the parent body, but some inclusions were not in the locations in which they were discovered when this took place. Some of these inclusions are too fragile to have been transported from one region to another in the parent body, indicating that low temperature alteration of these may have occurred in the solar nebula.

Mineralogy and petrography of HAL, an isotopically-unusual Allende inclusion

A detailed mineralogical study of HAL was initiated to elucidate the origin of this inclusion because Lee et al. (1979) had found large mass fractionation effects and small nuclear effects in its Ca isotopic composition, but no 26Mg excesses in samples of it with very high 27Al 24Mg ratios. HAL's 1.0 mm radius interior consists almost entirely of three hibonite crystals and is surrounded by a 2.0 mm thick, multi-layered rim. The first layer, called the black rim, is black and compact, resembles a devitrified glass and contains an anisotropic Al-Fe-oxide similar to hercynite in composition. This is followed by a friable rim sequence, layer I of which is predominantly feldspathoids with minor anorthite, Ti-Fe-oxide and Al-Fe-oxide. Layer II contains abundant perovskite, plus grossular, andradite and pyroxene in addition to the minerals of layer I. Layer III is mostly Ca-phosphate, possibly hydroxy-apatite and perovskite. Layer IV is rich in hibonite, Al-Fe-oxide, perovskite, nepheline and the two garnets, lacks Ca-phosphate but contains traces of a Ti-Sc-Zr-oxide. Layer V is rich in Al-Fe-oxide, pyroxene, nepheline, the two garnets and olivine whose crystals display peculiar rectangular cross-sections. The black rim does not completely surround the hibonite core. Sectors of the friable rim exist where layer I is missing and where the mineralogy of adjacent layers is no different from that of the same layers in other sectors. Pentlandite, nickel-iron and barrel-shaped olivine crystals, minerals typical of the matrix of Allende and found nowhere else in HAL, are found in layer V and increase in abundance toward its exterior, as if grains of these phases accreted together with the other minerals of layer V. This layer also contains alternating olivine-rich and garnet-, pyroxene-rich bands, resembling rhythmic layering. For these reasons, we conclude that each of the layers of the friable rim formed by the accretion of an assemblage of condensate grains rather than by complete reaction of a precursor to HAL with a nebular gas. Thus, the unusual isotopic characteristics of HAL are thought to have been inherited from a nebular reservoir which was isotopically distinct from that which gave rise to the bulk of the material in Allende. HAL's mineralogical peculiarities indicate that its formation reservoir was also chemically distinct from the latter one.

Magnesium isotopic variations in hibonite from the Murchison Meteorite: An ion microprobe study

Ion microprobe measurements show that some hibonite crystals from refractory‐rich inclusions in Murchison contain isotopically anomalous Mg. The observed effects include (1) the largest mass fractionation ever observed in nature for this element, ∼10% per mass unit; (2) small 26Mg excesses; (3) isotopic variability on a scale of microns or less in individual grains. These observations are corroborative evidence for the primitiveness of the Murchison inclusions. More importantly, they indicate that the types of Mg isotopic anomalies previously observed in Allende and Leoville inclusions are not confined to CV meteorites.