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 (Δ¹⁷O ∼ −24 ± 2‰) nebular gas. The ²⁶Al-poor [(²⁶Al/²⁷Al)₀ < 1 × 10⁻⁵], ¹⁶O-rich (Δ¹⁷O ∼ −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 (Δ¹⁷O > −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 (Δ¹⁷O > −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.     

Implications of the carbonaceous chondrite Mn-Cr isochron for the formation of early refractory planetesimals and chondrules

An excellent ⁵³Mn-⁵³Cr isochron for bulk CI, CM, CO, CV, CB, and ungrouped C3 chondrites seems to suggest that each carbonaceous chondrite group acquired its Mn/Cr ratio 4568 ± 1 Myr ago. This age is indistinguishable from the age of Ca-Al-rich inclusions (CAIs), which is considered to be the start of the solar system (t₀). However, carbonaceous chondrites were not assembled until at least 1.5-5 Myr after t₀, to judge by the ²⁰⁷Pb-²⁰⁶Pb and ²⁶Al-²⁶Mg ages of the chondrules within them, and by the fact that they were not melted by heat from the decay of ²⁶Al. Presumably, therefore, these meteorites inherited their bulk Mn-Cr isochron from precursor materials which experienced Mn-Cr fractionation at t₀. As a possible physical mechanism for how the isochron was established initially, and later inherited by the carbonaceous chondrites, we propose the rapid formation at t₀ of planetesimals that were variably depleted in moderately volatile elements, and hence had variably low Mn/Cr. The planetesimals and the undepleted (high Mn/Cr) primitive dust from which they were made shared the same initial ε⁵³Cr, and therefore evolved on an isochron. We suggest that later impact-disruption of the planetesimals produced dusty debris, which became mixed, in various proportions, with unprocessed (high Mn/Cr) dust before accreting to the carbonaceous chondrite parent bodies. With mixing in a closed system, the isochron was unchanged. We infer that some debris-rich material was converted to chondrules prior to accretion. The chondrules could have been formed by flash melting of the mixed dust, or could instead have been made directly by the impact splashing of molten planetesimals, or by condensation from impact-generated vapor plumes.     

Al-Mg dating of asteroidal magmatism in the young Solar System

We present high-precision Mg isotope data for most classes of basaltic meteorites including eucrites, mesosiderite silicate clasts, angrites and the ungrouped Northwest Africa (NWA) 2976 measured by pseudo-high-resolution multiple-collector inductively coupled plasma mass spectrometry and utilising improved techniques for chemical purification of Mg. With the exception of the angrites Angra dos Reis, Lewis Cliff (LEW) 86010, NWA 1296 and NWA 2999 and the diogenite Bilanga, which have either been shown to have young ages by other dating techniques or have low Al/Mg ratios, all bulk samples of basaltic meteorites have ²⁶Mg excesses (δ²⁶Mg^∗=+0.0135 to +0.0392‰). The ²⁶Mg excesses cannot be explained by analytical artefacts, cosmogenic effects or heterogeneity of initial ²⁶Al/²⁷Al, Al/Mg ratios or Mg isotopes in asteroidal parent bodies as compared to Earth or chondrites. The ²⁶Mg excesses record asteroidal melting and formation of basaltic magmas with super-chondritic Al/Mg and confirm that radioactive decay of short-lived ²⁶Al was the primary heat source that melted planetesimals. Model ²⁶Al-²⁶Mg ages for magmatism on the eucrite/mesosiderite, angrite and NWA 2976 parent bodies are 2.6-3.2, 3.9-4.1 and 3.5 Myr, respectively, after formation of calcium-aluminium-rich inclusions (CAIs). However, the validity of these model ages depends on whether the elevated Al/Mg ratios of basaltic meteorites result from magma ocean evolution on asteroids through fractional crystallisation or directly during partial melting. Mineral isochrons for the angrites Sahara (Sah) 99555 and D’Orbigny, and NWA 2976, yield ages of and , respectively, after CAI formation. Both isochrons have elevated initial δ²⁶Mg^∗ values. Given the brecciated and equilibrated texture of NWA 2976 it is probable that its isochron age and elevated initial δ²⁶Mg^∗(+0.0175±0.0034‰) reflects thermal resetting during an impact event and slow cooling on its parent body. However, in the case of the angrites the marginally elevated initial δ²⁶Mg^∗(+0.0068±0.0058‰) may reflect either δ²⁶Mg^∗ ingrowth in a magma ocean prior to eruption and crystallisation or in an older igneous protolith with super-chondritic Al/Mg prior to impact melting and crystallisation of these angrites, or partial internal re-equilibration of Mg isotopes after crystallisation. ²⁶Al-²⁶Mg model ages and an olivine + pyroxene + whole rock isochron for the angrites Sah 99555 and D’Orbigny are in good agreement with age constraints from ⁵³Mn-⁵³Cr and ¹⁸²Hf-¹⁸²W short-lived chronometers, suggesting that the ²⁶Al-²⁶Mg feldspar-controlled isochron ages for these angrites may be compromised by the partial resetting of feldspar Mg isotope systematics. Even when age constraints from the ²⁶Al-²⁶Mg angrite model ages or the mafic mineral + whole rock isochron are considered, the relative time difference between Sah 99555/D’Orbigny crystallisation and CAI formation cannot be reconciled with Pb-Pb ages for Sah 99555/D’Orbigny and CAIs, which are ca. 1.0 Myr too old (angrites) or too young (CAIs) for reasons that are not clear. This discrepancy might indicate that ²⁶Al was markedly lower (ca. 40%) in the planetesimal- and planet-forming regions of the proto-planetary disc as compared to CAIs, or that CAI Pb-Pb ages may not accurately date CAI formation, which might be better dated by the ¹⁸²Hf-¹⁸²W and ²⁶Al-²⁶Mg chronometers as 4568.3±0.7 (Burkhardt et al., 2008) and (herein), respectively, when mapped onto an absolute timescale using Pb-Pb ages for angrites.     

Al-Mg systematics of chondrules in a primitive CO chondrite

We have conducted systematic investigations of formation age, chemical compositions, and mineralogical characteristics of ferromagnesian chondrules in Yamato-81020 (CO3.05), one of the most primitive carbonaceous chondrites, to get better understanding of the origin of chemical groups of chondrites. The ²⁶Al-²⁶Mg isotopic system were measured in fourteen FeO-poor (Type I), six FeO-rich (Type II) and two aluminum-rich (Al-rich) chondrules using a secondary ion mass spectrometer. Excesses of ²⁶Mg in plagioclase (1.0-13.5‰) are resolved with sufficient precision (mostly 0.4-6.6‰ at 2σ level) in all the chondrules studied except one. Chemical zoning of Mg and Na in plagioclase were investigated in detail in order to evaluate the applicability of ²⁶Al-²⁶Mg chronometer. We conclude that the Al-Mg isotope system of the chondrules in Y-81020 have not been disturbed by parent-body metamorphism and can be used as chronometer assuming homogeneous distribution of ²⁶Al. Assuming an initial ²⁶Al/²⁷Al ratio of 5 × 10⁻⁵ in the early solar system, ²⁶Al-²⁶Mg ages were found to be 1.7-2.5 Ma after CAI formation for Type I, 2.0-3.0 Ma for Type II and 1.9 and 2.6 Ma for Al-rich chondrules.The formation ages of ferromagnesian chondrules in Y-81020 are in good agreement with those of L and LL (type 3.0-3.1) chondrites in the literature, which indicates that common chondrules in the CO chondrite were formed contemporaneously with those in L and LL chondrites. The concurrent formation of chondrules of CO and L/LL chondrites suggests that the chemical differences between CO and L/LL chondrites might be caused by spatial separation of chondrule formation environments in the protoplanetary disk.     

Hf-W chronology of the accretion and early evolution of asteroids and terrestrial planets

The ¹⁸²Hf-¹⁸²W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accretion and earliest evolution of asteroids and terrestrial planets and lead to the following succession and duration of events in the earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3 ± 0.7 Ma was followed by the accretion and differentiation of the parent bodies of some magmatic iron meteorites within less than ∼1 Myr. Chondrules from H chondrites formed 1.7 ± 0.7 Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their ²⁶Al-²⁶Mg ages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon as ∼3 Myr after CAI formation and may have continued until ∼10 Myr. A similar timescale is obtained for the high-temperature metamorphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that the different thermal histories of meteorite parent bodies primarily reflect their initial abundance of ²⁶Al, which is determined by their accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear to have induced large-scale melting. For instance, Hf-W ages for eucrite metals postdate CAI formation by ∼20 Myr and may reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf-W systematics of some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remains poorly constrained.The strong fractionation of lithophile Hf from siderophile W during core formation makes the Hf-W system an ideal chronometer for this major differentiation event. However, for larger planets such as the terrestrial planets the calculated Hf-W ages are particularly sensitive to the occurrence of large impacts, the degree to which impactor cores re-equilibrated with the target mantle during large collisions, and changes in the metal-silicate partition coefficients of W due to changing fO₂ in differentiating planetary bodies. Calculated core formation ages for Mars range from 0 to 20 Myr after CAI formation and currently cannot distinguish between scenarios where Mars formed by runaway growth and where its formation was more protracted. Tungsten model ages for core formation in Earth range from ∼30 Myr to >100 Myr after CAIs and hence do not provide a unique age for the formation of Earth. However, the identical ¹⁸²W/¹⁸⁴W ratios of the lunar and terrestrial mantles provide powerful evidence that the Moon-forming giant impact and the final stage of Earth’s core formation occurred after extinction of ¹⁸²Hf (i.e., more than ∼50 Myr after CAIs), unless the Hf/W ratios of the bulk silicate Moon and Earth are identical to within less than ∼10%. Furthermore, the identical ¹⁸²W/¹⁸⁴W of the lunar and terrestrial mantles is difficult to explain unless either the Moon consists predominantly of terrestrial material or the W in the proto-lunar magma disk isotopically equilibrated with the Earth’s mantle.Hafnium-tungsten chronometry also provides constraints on the duration of magma ocean solidification in terrestrial planets. Variations in the ¹⁸²W/¹⁸⁴W ratios of martian meteorites reflect an early differentiation of the martian mantle during the effective lifetime of ¹⁸²Hf. In contrast, no ¹⁸²W variations exist in the lunar mantle, demonstrating magma ocean solidification later than ∼60 Myr, in agreement with ¹⁴⁷Sm-¹⁴³Nd ages for ferroan anorthosites. The Moon-forming giant impact most likely erased any evidence of a prior differentiation of Earth’s mantle, consistent with a ¹⁴⁶Sm-¹⁴²Nd age of 50-200 Myr for the earliest differentiation of Earth’s mantle. However, the Hf-W chronology of the formation of Earth’s core and the Moon-forming impact is difficult to reconcile with the preservation of ¹⁴⁶Sm-¹⁴²Nd evidence for an early (<30 Myr after CAIs) differentiation of a chondritic Earth’s mantle. Instead, the combined ¹⁸²W-¹⁴²Nd evidence suggests that bulk Earth may have superchondritic Sm/Nd and Hf/W ratios, in which case formation of its core must have terminated more than ∼42 Myr after formation of CAIs, consistent with the Hf-W age for the formation of the Moon.     

Cosmic-ray constancy and cosmogenic production rates in short-lived chondrites

Five chondrites with short cosmic ray exposure ages (< 3Myr) have been analyzed for ⁵³Mn (t_1.2 = 3.7Myr), ^26Al(t_1.2 = 0.72Myr) and the stable isotopes of He, Ar, and in particular, Ne. In addition, rare gases were determined for five other short-lived chondrites from the same aliquots which had previously been used for ²⁶Al and ⁵³Mn determinations by the Cologne Laboratory. Improved curves of growth were constructed for ²⁶Al and ⁵³Mn to deduce the respective saturation values ²⁶Al₀ and ⁵³Mn₀ and to obtain ²¹Ne production rates (²¹P).²⁶Al and ⁵³Mn saturation values deduced from short-lived chondrites alone agree within 20%, with those deduced from all chondrites. Values of ²¹p based on ⁵³Mn and ²⁶Al are 0.30 ± 0.03 and 0.46 ± 0.05, respectively, in H chondrites. Possible causes for the difference include half-life errors, data selection and a recent (?2.5Myr) increase in the cosmic ray intensity.     

Al in plagioclase-rich chondrules in carbonaceous chondrites: Evidence for an extended duration of chondrule formation

The ²⁶Al-²⁶Mg isotope systematics in 33 petrographically and mineralogically characterized plagioclase-rich chondrules (PRCs) from 13 carbonaceous chondrites (CCs) - one ungrouped (Acfer 094), six CR, five CV, and one CO - reveal large variations in the initial ²⁶Al/²⁷Al ratio, (²⁶Al/²⁷Al)₀. Well-resolved ²⁶Mg excesses (δ²⁶Mg) from the in situ decay of the short-lived nuclide ²⁶Al (t_1/2 ∼ 0.72 Ma) were found in nine chondrules, two from Acfer 094, five from the CV chondrites, Allende and Efremovka, and one each from the paired CR chondrites, EET 92147 and EET 92042, with (²⁶Al/²⁷Al)₀ values ranging from ∼3 × 10⁻⁶ to ∼1.5 × 10⁻⁵. Data for seven additional chondrules from three CV and two CR chondrites show evidence suggestive of the presence of ²⁶Al but do not yield well defined values for (²⁶Al/²⁷Al)₀, while the remaining chondrules do not contain excess radiogenic ²⁶Mg and yield corresponding upper limits of (11-2) × 10⁻⁶ for (²⁶Al/²⁷Al)₀. The observed range of (²⁶Al/²⁷Al)₀ in PRCs from CCs is similar to the range seen in chondrules from unequilibrated ordinary chondrites (UOCs) of low metamorphic grade (3.0-3.4). However, unlike the UOC chondrules, there is no clear trend between the (²⁶Al/²⁷Al)₀ values in PRCs from CCs and the degree of thermal metamorphism experienced by the host meteorites. High and low values of (²⁶Al/²⁷Al)₀ are found equally in PRCs from both CCs lacking evidence for thermal metamorphism (e.g., CRs) and CCs where such evidence is abundant (e.g., CVs). The lower (²⁶Al/²⁷Al)₀ values in PRCs from CCs, relative to most CAIs, are consistent with a model in which ²⁶Al was distributed uniformly in the nebula when chondrule formation began, approximately a million years after the formation of the majority of CAIs. The observed range of (²⁶Al/²⁷Al)₀ values in PRCs from CCs is most plausibly explained in terms of an extended duration of ∼2-3 Ma for the formation of CC chondrules. This interval is in sharp contrast to most CAIs from CCs, whose formation appears to be restricted to a narrow time interval of less than 10⁵ years. The active solar nebula appears to have persisted for a period approaching 4 Ma, encompassing the formation of both CAIs and chondrules present in CCs, and raising important issues related to the storage, assimilation and mixing of chondrules and CAIs in the early solar system.     

Lack of relationship between Al ages of chondrules and their mineralogical and chemical compositions

Previous studies have suggested the existence of a correlation between ²⁶Al relative crystallisation ages and mineralogical and bulk chemical compositions of ferromagnesian chondrules from the Bishunpur and Semarkona unequilibrated ordinary (0120 and 0150). However, because the precision in ²⁶Al ages was moderate, these correlations are questionable. Here, we report mineralogical and chemical compositions of 14 ferromagnesian chondrules from Semarkona for which precise ²⁶Al ages were previously obtained (Villeneuve et al., 2009). We find global correlation of ²⁶Al ages neither with bulk chemical composition of chondrules, nor with the different types of ferromagnesian chondrules, i.e. PO, POP and PP. This indicates that if some kind of correlations between chemical compositions of chondrules and their ages of formation exists, they do not exist at timescales that can be measured with the ²⁶Al-²⁶Mg systematics but presumably at much shorter timescales.     

Al-Mg systematics in D’Orbigny and Sahara 99555 angrites: Implications for high-resolution chronology using extinct chronometers

We report on an investigation of the ²⁶Al-²⁶Mg isotope systematics in the D’Orbigny and Sahara 99555 angrites. High precision Mg isotope compositions and Al/Mg ratios were measured in mineral separates and whole rock samples from D’Orbigny and Sahara 99555 using multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Plagioclase separates from both angrites have resolvable excesses in ²⁶Mg (Δ²⁶Mg) that correlate with their respective Al/Mg ratios. ²⁶Al-²⁶Mg systematics in the mineral separates and whole rocks define precise isochrons that correspond to ²⁶Al/²⁷Al ratios of (5.06 ± 0.92) × 10⁻⁷ and (5.13 ± 1.90) × 10⁻⁷ and initial Δ²⁶Mg values of −0.006 ± 0.040‰ and −0.016 ± 0.047‰ for D’Orbigny and Sahara 99555, respectively. The slopes and initial Δ²⁶Mg values are identical for these two meteorites within errors and the data for both angrites considered together define an isochron corresponding to a ²⁶Al/²⁷Al ratio of (5.10 ± 0.55) × 10⁻⁷ and initial Δ²⁶Mg value of −0.012 ± 0.019. Relative to the Efremovka E60 CAI, the ²⁶Al/²⁷Al values reported here for these angrites imply ²⁶Al-²⁶Mg ages of 4562.42 ± 0.29 Ma and 4562.43 ± 0.53 Ma for D’Orbigny and Sahara 99555, respectively. These ²⁶Al-²⁶Mg ages are concordant with model ages determined using other extinct radionuclide chronometers (e.g., ⁵³Mn-⁵³Cr and ¹⁸²Hf-¹⁸²W), but are ∼2 Myr younger than the absolute ²⁰⁷Pb-²⁰⁶Pb ages that have been reported recently for these angrites. The reason for this discrepancy is not presently known, but may imply disturbance of one or more of the isotope systems under consideration or a possible bias in the ²⁰⁷Pb-²⁰⁶Pb ages of the angrites resulting from natural or analytical causes.     

Ancient relative and absolute ages for a basaltic meteorite: Implications for timescales of planetesimal accretion and differentiation

Asuka 881394 is a unique basaltic meteorite that originated in the crust of a differentiated planetesimal in the early Solar System. We present high precision Pb, Mg, and Cr isotopic compositions of bulk samples and mineral separates from this achondrite. A ²⁰⁷Pb-²⁰⁶Pb internal isochron obtained from the radiogenic pyroxene and whole-rock fractions of Asuka 881394 yields an absolute age of 4566.5 ± 0.2 Ma, which we consider to be the best estimate for the crystallization age of this basaltic achondrite. The ²⁶Al-²⁶Mg systematics show some evidence of disturbance, but 5 of the 6 analyzed whole-rock and mineral fractions define an isochron corresponding to a ²⁷Al/²⁶Al ratio of (1.28 ± 0.07) × 10⁻⁶. Comparison with the ²⁶Al-²⁶Mg and Pb-Pb systematics in the D’Orbigny achondrite translates to a ²⁶Al-²⁶Mg age of 4565.4 ± 0.2 Ma for Asuka 881394. The ⁵³Mn-⁵³Cr systematics in whole-rock, silicate and chromite fractions correspond to a ⁵³Mn/⁵⁵Mn ratio of (3.85 ± 0.23) × 10⁻⁶. Compared to the most precise ⁵³Mn-⁵³Cr and Pb-Pb systematics available for the D’Orbigny angrite, this translates to a ⁵³Mn-⁵³Cr age of 4565.3 ± 0.4 Ma; similarly, a comparison with the NWA 4801 angrite yields a ⁵³Mn-⁵³Cr age of 4565.5 ± 0.4 Ma, in agreement with the age obtained relative to D’Orbigny. While the ²⁶Al-²⁶Mg and ⁵³Mn-⁵³Cr ages appear to be concordant in Asuka 881394, these ages are ∼1 Ma younger than its ²⁰⁷Pb-²⁰⁶Pb age. This discordance might have been caused by one or more of several reasons, including differences in the closure temperatures for Pb versus Cr and Mg diffusion in their host minerals combined with slow cooling of the parent body as well as differential resetting of isotopic systems by a process other than volume diffusion, e.g., shock metamorphism. The ancient age of Asuka 881394 suggests that basaltic volcanism on its parent planetesimal occurred within ∼3 Ma of the formation of earliest solids in the Solar System, essentially contemporaneously with chondrule formation. This requires that the Asuka 881394 parent body was fully accreted within ∼500,000 yrs of Solar System formation.     

The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization

We evaluate initial (²⁶Al/²⁷Al)_I, (⁵³Mn/⁵⁵Mn)_I, and (¹⁸²Hf/¹⁸⁰Hf)_I ratios, together with ²⁰⁷Pb/²⁰⁶Pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common, closed isotopic system, i.e., the early solar nebula. The relative initial isotopic abundances of ²⁶Al, ⁵³Mn, and ¹⁸²Hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values, here denoted by I(Al)_SS, I(Mn)_SS, and I(Hf)_SS, respectively. I(Mn)_SS and I(Hf)_SS = 9.1 ± 1.7 × 10⁻⁶ and 1.07 ± 0.08 × 10⁻⁴, respectively, correspond to “canonical” I(Al)_SS = 5.1 × 10⁻⁵. I(Hf)_SS so determined is consistent with I(Hf)_SS = 9.72 ± 0.44 × 10⁻⁵ directly determined from an internal Hf-W isochron for CAI minerals. I(Mn)_SS is within error of the lowest value directly measured for CAIs. We suggest that erratically higher values measured for CAIs in carbonaceous chondrites may reflect proton irradiation of unaccreted CAIs by the early Sun after other asteroids destined for melting by ²⁶Al decay had already accreted. The ⁵³Mn incorporated within such asteroids would have been shielded from further “local” spallogenic contributions from within the solar system. The relative initial isotopic abundances of the short-lived nuclides are less consistent with the ²⁰⁷Pb/²⁰⁶Pb ages of the corresponding materials than with one another. The best consistency of short- and long-lived chronometers is obtained for (¹⁸²Hf/¹⁸⁰Hf)_I and the ²⁰⁷Pb/²⁰⁶Pb ages of angrites. (¹⁸²Hf/¹⁸⁰Hf)_I decreases with decreasing ²⁰⁷Pb/²⁰⁶Pb ages at the rate expected from the 8.90 ± 0.09 Ma half-life of ¹⁸²Hf. The model solar system age thus determined is T_SS,Hf-W = 4568.3 ± 0.7 Ma. (²⁶Al/²⁷Al)_I and (⁵³Mn/⁵⁵Mn)_I are less consistent with ²⁰⁷Pb/²⁰⁶Pb ages of the corresponding meteorites, but yield T_SS,Mn-Cr = 4568.2 ± 0.5 Ma relative to I(Al)_SS = 5.1 × 10⁻⁵ and a ²⁰⁷Pb/²⁰⁶Pb age of 4558.55 ± 0.15 Ma for the LEW86010 angrite. The Mn-Cr method with I(Mn)_SS = 9.1 ± 1.7 × 10⁻⁶ is useful for dating accretion (if identified with chondrule formation), primary igneous events, and secondary mineralization on asteroid parent bodies. All of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies. For I(Mn)_SS = 9.1 ± 1.7 × 10⁻⁶, parent body differentiation is found to extend at least to ∼5 Ma post-T_SS, i.e., until differentiation of the angrite parent body ∼4563.5 Ma ago, or ∼4564.5 Ma ago using the directly measured ²⁰⁷Pb/²⁰⁶Pb ages of the D’Orbigny-clan angrites. The ∼1 Ma difference is characteristic of a remaining inconsistency for the D’Orbigny-clan between the Al-Mg and Mn-Cr chronometers on one hand, and the ²⁰⁷Pb/²⁰⁶Pb chronometer on the other. Differentiation of the IIIAB iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body, and at nearly the same time as one another as shown by the Mn-Cr ages of IIIAB irons and ureilites, respectively. The latest recorded episodes of secondary mineralization are for carbonates on the CI carbonaceous chondrite parent body and fayalites on the CV carbonaceous chondrite parent body, both extending to ∼10 Ma post-T_SS.     

Extinct Be in Type A calcium-aluminum-rich inclusions from CV chondrites

We have found clear evidence of live ¹⁰Be in five normal Type A Calcium-aluminum-rich inclusions (CAIs), one normal Type B CAI, and one FUN Type A CAI, all from CV3 chondrites. The (¹⁰Be/⁹Be)₀ ratios range from ∼0.36 × 10^-3 to ∼0.77 × 10^-3 and are similar to those found by previous workers. The (¹⁰Be/⁹Be)₀ ratios do not correlate in a temporal fashion with (²⁶Al/²⁷Al)₀, suggesting that ¹⁰Be and ²⁶Al were produced by different mechanisms. An examination of possible sources for the short-lived radionuclides indicates that production of ¹⁰Be was almost certainly by particle irradiation, possibly within the solar system, and was probably accompanied by significant production of ⁴¹Ca and ⁵³Mn. In contrast, all of the ⁶⁰Fe, most of the ²⁶Al, and some of the ⁵³Mn were probably produced in stars and were imported into the solar system within presolar dust grains.     

Early core formation in asteroids and late accretion of chondrite parent bodies: Evidence from Hf-W in CAIs, metal-rich chondrites, and iron meteorites

The ¹⁸²Hf-¹⁸²W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the ¹⁸²Hf/¹⁸⁰Hf and ε_W at the time of CAI formation to (1.07 ± 0.10) × 10⁻⁴ and −3.47 ± 0.20, respectively. All magmatic iron meteorites examined here have initial ε_W values that are similar to or slightly lower than the initial value of CAIs. These low ε_W values may in part reflect ¹⁸²W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ∼0.3 ε units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ∼1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.Chondrule formation ∼2-5 Myr after CAIs, as constrained by published Pb-Pb and Al-Mg ages, postdates core formation in planetesimals, and indicates that chondrites do not represent the precursor material from which asteroids accreted and then differentiated. Chondrites instead derive from asteroids that accreted late, either farther from the Sun than the parent bodies of magmatic iron meteorites or by reaccretion of debris produced during collisional disruption of older asteroids. Alternatively, chondrites may represent material from the outermost layers of differentiated asteroids. The early thermal and chemical evolution of asteroids appears to be controlled by the decay of ²⁶Al, which was sufficiently abundant (initial ²⁶Al/²⁷Al >1.4 × 10⁻⁵) to rapidly melt early-formed planetesimals but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. The preservation of the primitive appearance of chondrites thus at least partially reflects their late formation rather than their early and primitive origin.     

Formation of chondritic refractory inclusions: The astrophysical setting

This study attempts to identify the astrophysical setting in which properties of the Ca,Al-rich inclusions (CAIs) found in chondritic meteorites are best understood. Importance is attached to the short time period in which most or all of the CAIs were formed (<∼0.5 Myr, corresponding to the observed dispersion of values of initial ²⁶Al/²⁷Al about the canonical value of ∼5 × 10⁻⁵), a constraint that has been overlooked. This period is dissimilar to the time scale of evolution of T Tauri stars, ∼10 Myr; it corresponds instead to the time scale of Class 0 and Class I young stellar objects, protostars as they exist during the massive infall of interstellar material that creates stars. The innermost portion of the sun’s rapidly accreting nebular disk, kept hot during that period by viscous dissipation, is the most plausible site for CAI formation. Once condensed, CAIs must be taken out of that hot zone rather promptly in order to preserve their specialized mineralogical compositions, and they must be transported to the radial distance of the asteroid belt to be available for accretion into the chondrites that contain them today. Though this paper is critical of some aspects of the x-wind model of CAI formation, something akin to the x-wind may be the best way of understanding this extraction and transport of CAIs.     

Oxygen isotope compositions of chondrules in CR chondrites

We report in situ ion microprobe analyses of oxygen isotopic compositions of olivine, low-Ca pyroxene, high-Ca pyroxene, anorthitic plagioclase, glassy mesostasis, and spinel in five aluminum-rich chondrules and nine ferromagnesian chondrules from the CR carbonaceous chondrites EET92042, GRA95229, and MAC87320. Ferromagnesian chondrules are isotopically homogeneous within ±2‰ in Δ¹⁷O; the interchondrule variations in Δ¹⁷O range from 0 to −5‰. Small oxygen isotopic heterogeneities found in two ferromagnesian chondrules are due to the presence of relict olivine grains. In contrast, two out of five aluminum-rich chondrules are isotopically heterogeneous with Δ¹⁷O values ranging from −6 to −15‰ and from −2 to −11‰, respectively. This isotopic heterogeneity is due to the presence of ¹⁶O-enriched spinel and anorthite (Δ¹⁷O = −10 to −15‰), which are relict phases of Ca,Al-rich inclusions (CAIs) incorporated into chondrule precursors and incompletely melted during chondrule formation. These observations and the high abundance of relict CAIs in the aluminum-rich chondrules suggest a close genetic relationship between these objects: aluminum-rich chondrules formed by melting of spinel-anorthite-pyroxene CAIs mixed with ferromagnesian precursors compositionally similar to magnesium-rich (Type I) chondrules. The aluminum-rich chondrules without relict CAIs have oxygen isotopic compositions (Δ¹⁷O = −2 to −8‰) similar to those of ferromagnesian chondrules. In contrast to the aluminum-rich chondrules from ordinary chondrites, those from CRs plot on a three-oxygen isotope diagram along the carbonaceous chondrite anhydrous mineral line and form a continuum with amoeboid olivine aggregates and CAIs from CRs. We conclude that oxygen isotope compositions of chondrules resulted from two processes: homogenization of isotopically heterogeneous materials during chondrule melting and oxygen isotopic exchange between chondrule melt and ¹⁶O-poor nebular gas.     

Hf-W mineral isochron for Ca,Al-rich inclusions: Age of the solar system and the timing of core formation in planetesimals

Application of ¹⁸²Hf-¹⁸²W chronometry to constrain the duration of early solar system processes requires the precise knowledge of the initial Hf and W isotope compositions of the solar system. To determine these values, we investigated the Hf-W isotopic systematics of bulk samples and mineral separates from several Ca,Al-rich inclusions (CAIs) from the CV3 chondrites Allende and NWA 2364. Most of the investigated CAIs have relative proportions of ¹⁸³W, ¹⁸⁴W, and ¹⁸⁶W that are indistinguishable from those of bulk chondrites and the terrestrial standard. In contrast, one of the investigated Allende CAIs has a lower ¹⁸⁴W/¹⁸³W ratio, most likely reflecting an overabundance of r-process relative to s-process isotopes of W. All other bulk CAIs have similar ¹⁸⁰Hf/¹⁸⁴W and ¹⁸²W/¹⁸⁴W ratios that are elevated relative to average carbonaceous chondrites, probably reflecting Hf-W fractionation in the solar nebula within the first ∼3 Myr. The limited spread in ¹⁸⁰Hf/¹⁸⁴W ratios among the bulk CAIs precludes determination of a CAI whole-rock isochron but the fassaites have high ¹⁸⁰Hf/¹⁸⁴W and radiogenic ¹⁸²W/¹⁸⁴W ratios up to ∼14 ε units higher than the bulk rock. This makes it possible to obtain precise internal Hf-W isochrons for CAIs. There is evidence of disturbed Hf-W systematics in one of the CAIs but all other investigated CAIs show no detectable effects of parent body processes such as alteration and thermal metamorphism. Except for two fractions from one Allende CAI, all fractions from the investigated CAIs plot on a single well-defined isochron, which defines the initial ε¹⁸²W = −3.28 ± 0.12 and ¹⁸²Hf/¹⁸⁰Hf = (9.72 ± 0.44) × 10⁻⁵ at the time of CAI formation. The initial ¹⁸²Hf/¹⁸⁰Hf and ²⁶Al/²⁷Al ratios of the angrites D’Orbigny and Sahara 99555 are consistent with the decay from initial abundances of ¹⁸²Hf and ²⁶Al as measured in CAIs, suggesting that these two nuclides were homogeneously distributed throughout the solar system. However, the uncertainties on the initial ¹⁸²Hf/¹⁸⁰Hf and ²⁶Al/²⁷Al ratios are too large to exclude that some ²⁶Al in CAIs was produced locally by particle irradiation close to an early active Sun. The initial ¹⁸²Hf/¹⁸⁰Hf of CAIs corresponds to an absolute age of 4568.3 ± 0.7 Ma, which may be defined as the age of the solar system. This age is 0.5-2 Myr older than the most precise ²⁰⁷Pb-²⁰⁶Pb age of Efremovka CAI 60, which does not seem to date CAI formation. Tungsten model ages for magmatic iron meteorites, calculated relative to the newly and more precisely defined initial ε¹⁸²W of CAIs, indicate that core formation in their parent bodies occurred in less than ∼1 Myr after CAI formation. This confirms earlier conclusions that the accretion of the parent bodies of magmatic iron meteorites predated chondrule formation and that their differentiation was triggered by heating from decay of abundant ²⁶Al. A more precise dating of core formation in iron meteorite parent bodies requires precise quantification of cosmic-ray effects on W isotopes but this has not been established yet.     

Extremely Na- and Cl-rich chondrule from the CV3 carbonaceous chondrite Allende

We report on a study of Al3509, a large Na- and Cl-rich, radially-zoned object from the oxidized CV carbonaceous chondrite Allende. Al3509 consists of fine-grained ferroan olivine, ferroan Al-diopside, nepheline, sodalite, and andradite, and is crosscut by numerous veins of nepheline, sodalite, and ferroan Al-diopside. Some poorly-characterized phases of fine-grained material are also present; these phases contain no significant H₂O. The minerals listed above are commonly found in Allende CAIs and chondrules and are attributed to late-stage iron-alkali-halogen metasomatic alteration of primary high-temperature minerals. Textural observations indicate that Al3509 is an igneous object. However, no residual crystals that might be relicts of pre-existing CAI or chondrule minerals were identified. To establish the levels of ²⁶Al and ³⁶Cl originally present, ²⁶Al-²⁶Mg and ³⁶Cl-³⁶S isotopic systematics in sodalite were investigated. Al3509 shows no evidence of radiogenic ²⁶Mg^∗, establishing an upper limit of the initial ²⁶Al/²⁷Al ratio of 3 × 10⁻⁶. All sodalite grains measured show large but variable excesses of ³⁶S, which, however, do not correlate with ³⁵Cl/³⁴S ratio. If these excesses are due to decay of ³⁶Cl, local redistribution of radiogenic ³⁶S^∗ after ³⁶Cl had decayed is required. The oxygen-isotope pattern in Al3509 is the same as found in secondary minerals resulting from iron-alkali-halogen metasomatic alteration of Allende CAIs and chondrules and in melilite and anorthite of most CAIs in Allende. The oxygen-isotope data suggest that the secondary minerals precipitated from or equilibrated with a fluid of similar oxygen-isotope composition. These observations suggest that the formation of Al3509 and alteration products in CAIs and chondrules in Allende requires a very similar fluid phase, greatly enriched in volatiles (e.g., Na and Cl) and with Δ¹⁷O ∼ −3‰. We infer that internal heating of planetesimals by ²⁶Al would efficiently transfer volatiles to their outer portions and enhance the formation of volatile-enriched minerals there. We conclude that the site for the production of Na- and Cl-rich fluids responsible for the formation of Al3509 and the alteration of the Allende CAIs and chondrules must have been on a protoplanetary body prior to incorporation into the Allende meteorite. Galactic cosmic rays cannot be the source of the inferred initial ³⁶Cl in Allende. The problem of ³⁶Cl production by solar energetic particle (SEP) bombardment and the possibility that ³⁶Cl and ⁴¹Ca might be the product of neutron capture resulting from SEP bombardment of protoplanetary surfaces are discussed. This hypothesis can be tested comparing inferred “initial” ³⁶Cl with neutron fluencies measured on the same samples and on phases showing ³⁶S^∗ by Sm and Gd isotopic measurements.     

Ca–Al-rich inclusions in carbonaceous chondrites: the oldest solar system objects

This paper presents a review of recent available data on the first solid condensates of the Solar System, which include refractory CAIs (Ca–Al-rich Inclusions) mostly composed of Ca, Al, Mg, and Ti minerals. A theoretical condensation sequence calculated from thermodynamic data confirmed that CAIs formed as fine-grained aggregates in the protoplanetary disk from an ¹⁶О-rich gas of solar composition at temperatures >1300° K and pressures <10^–4 bar. Based on the diversity of CAI types, their mineralogical, bulk chemical, and isotopic compositions, it can be concluded that CAIs experienced melting and evaporation, possibly by shock waves, which may have occurred in the protoplanetary disk within a brief time interval. Some CAIs may have experienced multiple events such as melting, evaporation, and recycling back to the disk by means of a bipolar outflow. The CAIs having an absolute age of 4567.30 ± 0.16 Myr are the oldest objects in the Solar System. The study of CAIs revealed two distinct oxygen isotope reservoirs (¹⁶О-rich and ¹⁶О-poor) and established a chronology of the sequence of processes forming individual CAI components using Mg–Al, Cr–Mn and Pb–Pb isotopic systematics.     

Oxygen- and magnesium-isotope compositions of calcium-aluminum-rich inclusions from CR2 carbonaceous chondrites

We report both oxygen- and magnesium-isotope compositions measured in situ using a Cameca ims-1280 ion microprobe in 20 of 166 CAIs identified in 47 polished sections of 15 CR2 (Renazzo-type) carbonaceous chondrites. Two additional CAIs were measured for oxygen isotopes only. Most CR2 CAIs are mineralogically pristine; only few contain secondary phyllosilicates, sodalite, and carbonates - most likely products of aqueous alteration on the CR2 chondrite parent asteroid. Spinel, hibonite, grossite, anorthite, and melilite in 18 CAIs have ¹⁶O-rich (Δ¹⁷O = −23.3 ± 1.9‰, 2σ error) compositions and show no evidence for postcrystallization isotopic exchange commonly observed in CAIs from metamorphosed CV carbonaceous chondrites. The inferred initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, in 15 of 16 ¹⁶O-rich CAIs measured are consistent with the canonical value of (4.5-5) × 10⁻⁵ and a short duration (<0.5 My) of CAI formation. These data do not support the “supra-canonical” values of (²⁶Al/²⁷Al)₀ [(5.85-7) × 10⁻⁵] inferred from whole-rock and mineral isochrons of the CV CAIs. A hibonite-grossite-rich CAI El Djouf 001 MK #5 has uniformly ¹⁶O-rich (Δ¹⁷O = −23.0 ± 1.7‰) composition, but shows a deficit of ²⁶Mg and no evidence for ²⁶Al. Because this inclusion is ¹⁶O-rich, like CAIs with the canonical (²⁶Al/²⁷Al)₀, we infer that it probably formed early, like typical CAIs, but from precursors with slightly nonsolar magnesium and lower-than-canonical ²⁶Al abundance. Another ¹⁶O-enriched (Δ¹⁷O = −20.3 ± 1.2‰) inclusion, a spinel-melilite CAI fragment Gao-Guenie (b) #3, has highly-fractionated oxygen- and magnesium-isotope compositions (∼11 and 23‰/amu, respectively), a deficit of ²⁶Mg, and a relatively low (²⁶Al/²⁷Al)₀ = (2.0 ± 1.7) × 10⁻⁵. This could be the first FUN (Fractionation and Unidentified Nuclear effects) CAI found in CR2 chondrites. Because this inclusion is slightly ¹⁶O-depleted compared to most CR2 CAIs and has lower than the canonical (²⁶Al/²⁷Al)₀, it may have experienced multistage formation from precursors with nonsolar magnesium-isotope composition and recorded evolution of oxygen-isotope composition in the early solar nebula over  My. Eight of the 166 CR2 CAIs identified are associated with chondrule materials, indicating that they experienced late-stage, incomplete melting during chondrule formation. Three of these CAIs show large variations in oxygen-isotope compositions (Δ¹⁷O ranges from −23.5‰ to −1.7‰), suggesting dilution by ¹⁶O-depleted chondrule material and possibly exchange with an ¹⁶O-poor (Δ¹⁷O > −5‰) nebular gas. The low inferred (²⁶Al/²⁷Al)₀ ratios of these CAIs (<0.7 × 10⁻⁵) indicate melting >2 My after crystallization of CAIs with the canonical (²⁶Al/²⁷Al)₀ and suggest evolution of the oxygen-isotope composition of the inner solar nebula on a similar or a shorter timescale. Because CAIs in CR2 and CV chondrites appear to have originated in a similarly ¹⁶O-rich reservoir and only a small number of CR2 and CV CAIs were affected by chondrule melting events in an ¹⁶O-poor gaseous reservoir, the commonly observed oxygen-isotope heterogeneity in CAIs from metamorphosed CV chondrites is most likely due to fluid-solid isotope exchange on the CV asteroidal body rather than gas-melt exchange. This conclusion does not preclude that some CV CAIs experienced oxygen-isotope exchange during remelting, instead it implies that such remelting is unlikely to be the dominant process responsible for oxygen-isotope heterogeneity in CV CAIs. The mineralogy, oxygen and magnesium-isotope compositions of CAIs in CR2 chondrites are different from those in the metal-rich, CH and CB carbonaceous chondrites, providing no justification for grouping CR, CH and CB chondrites into the CR clan.     

Timing of metal–silicate differentiation in the Eagle Station pallasite parent body

The time of the metal–silicate differentiation of the Eagle Station pallasite (ESP) parent body was investigated using the ²⁶Al–²⁶Mg short-lived chronometer (half-life of 0.72 Myr). The Mg isotope ratios were measured in ESP olivines by both MC–SIMS and HR-MC–ICPMS, allowing us to check the consistency between the results given by two different analytical protocols and data reduction processes. Results show that the two datasets are consistent, with a (δ²⁶Mg*)_av. value of –0.003 (± 0.005)‰ (2 s.e., n = 89). Such a value, associated with data from the ¹⁸²Hf–¹⁸²W short-lived systematics (half-life of 8.9 Myr), indicates an ESP parent body metal–silicate differentiation occurring most likely at least at ∼ 2 Ma, but possibly 4 Ma, after CAI formation. From the ²⁷Al/²⁴Mg ratios measured in ESP olivines using MC–SIMS, the duration of the olivine crystallization process was inferred to have lasted over ∼ 275 kyr if the core has differentiated as early as 2 Ma after CAIs, while in the case of a core differentiation occurring 4 Ma after CAIs, the silicate–silicate differentiation should have lasted for another 4 Myr.