Ca,Al-rich Inclusions Formation Research Articles

A temporal shift of chondrule generation from the inner to outer Solar System inferred from oxygen isotopes and Al-Mg chronology of chondrules from primitive CM and CO chondrites

Deciphering the spatial and temporal evolution of chondrules allows for a better understanding of how asteroidal seeds formed, migrated, and eventually accreted into parent asteroids. Here we report high precision Al-Mg ages and oxygen three-isotope ratios of fifteen FeO-poor chondrules from the least metamorphosed Mighei-like (CM) and Ornans-like (CO) carbonaceous chondrites, Asuka 12236 (CM2.9), Dominion Range 08006 (CO3.01), and Yamato 81020 (CO3.05). This is the first report of Al-Mg ages of chondrules from the CM chondrite group. All but one of the fifteen chondrules exhibit a restricted range of inferred initial 26Al/27Al ratios, and all ratios are ≤ 6.0 × 10−6, which is systematically lower than those of the majority of ordinary chondrite (OC) chondrules. These observations indicate that the majority of chondrules in the outer Solar System were produced ≥2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), which postdates OC chondrule formation in the inner Solar System (≤2.2 Ma after CAI formation). We propose that the discrete chondrule-forming events in different disk regions reflect a time difference in growth and orbital evolution of planetesimals within the first 4 Ma of the Solar System.One chondrule from Asuka 12236 has an age of 1.9 Ma after CAI formation and is therefore significantly older than the other fourteen chondrules, meaning this chondrule formed contemporaneously with the majority of OC chondrules. This old chondrule also exhibits 16O-depleted oxygen isotope characteristics compared to the other chondrules, suggesting a distinct formation region, probably inside the disk region relative to where the majority of CM and CO chondrules formed. Our results indicate that this old chondrule has migrated from the inner to the outer part of the protoplanetary disk within ∼1 Ma and then accreted into the CM parent asteroid >3 Ma after CAI formation, although its formation exterior to the accretion region of the CM parent asteroid and subsequent inward migration cannot be ruled out completely.

Fingerprints of the protosolar cloud collapse in the Solar System: Refractory inclusions distribution and isotopic anomalies in meteorites

AbstractIncreasing evidences suggest that the building blocks of Ca-Al-rich inclusions (CAIs) could have formed with the Sun, during the collapse of the parent cloud. However, determination of the relative age of CAIs relies on the homogeneous distribution of their short-lived radionuclide 26Al that is used as a chronometer. Some CAIs show evidence of 26Al/27 Al variation that is independent of decay.We investigate the dynamical and chemical evolution of refractories from the collapsing cloud to their transport in the protoplanetary disk focusing to the predicted isotopic anomalies resulting from 26Al heterogeneities.The interplay between the thermal properties of the dust, the isotopic zoning in the cloud and disk dynamics produce aggregates that resemble chondrites. An abrupt raise of 26Al close the center of the cloud followed by a plateau throughout the cloud best matches the observations. As a consequence, the 26Al -chronometer retains validity from the formation of canonical CAIs onward.

Ruthenium Isotope Composition of Allende Refractory Metal Nuggets

Abstract Refractory metal nuggets (RMNs) are among the first solids formed in the nascent solar system. They contain high abundances of refractory metals like Re, Os, W, Ir, Ru, and Pt. The isotopic compositions of these elements bear testimony to the stellar sources that contributed to the nucleosynthetic makeup of our solar system. We report the first high-precision Ru isotope data for a bulk RMN sample prepared from the Allende meteorite. The RMNs display well-resolved mass-independent anomalies with positive anomalies for 96Ru, 98Ru, 100Ru, 102Ru, and 104Ru. These are best explained by a deficit in r-process combined with a slight deficit in p-process nuclides. This finding stands in stark contrast to the s-process deficit isotopic patterns observed for Allende Ca–Al-rich inclusions (CAIs), bulk Allende, and other bulk meteorites. The contrasting r-, p-deficit versus s-deficit Ru isotopic signatures observed between RMNs and CAIs is surprising, given that CAIs are assumed to be a major host phase of RMNs. One way to explain the s-deficit patterns observed for CAIs and bulk meteorites is that r- and p-process Ru nuclides were added to the solar nebula after RMN formation and prior to the formation of CAIs and the accretion of meteorite parent bodies. A possible source may have been a nearby core-collapse supernova that injected freshly synthesized r- and p-process nuclides into the nascent solar system. The injection of such r- and p-enriched matter represents an alternative mechanism to account for the s-process variability presented by CAIs and bulk carbonaceous meteorites.

Hf-W chronology of CR chondrites: Implications for the timescales of chondrule formation and the distribution of 26Al in the solar nebula

Renazzo-type carbonaceous (CR) chondrites are distinct from most other chondrites in having younger chondrule 26Al-26Mg ages, but the significance of these ages and whether they reflect true formation times or spatial variations of the 26Al/27Al ratio within the solar protoplanetary disk are a matter of debate. To address these issues and to determine the timescales of metal-silicate fractionation and chondrule formation in CR chondrites, we applied the short-lived 182Hf-182W chronometer to metal, silicate, and chondrule separates from four CR chondrites. We also obtained Mo isotope data for the same samples to assess potential genetic links among the components of CR chondrites, and between these components and bulk chondrites.All investigated samples plot on a single Hf-W isochron and constrain the time of metal-silicate fractionation in CR chondrites to 3.6 ± 0.6 million years (Ma) after the formation of Ca-Al-rich inclusions (CAIs). This age is indistinguishable from a ∼3.7 Ma Al-Mg age for CR chondrules, suggesting not only that metal-silicate fractionation and chondrule formation were coeval, but also that these two processes were linked to each other. The good agreement of the Hf-W and Al-Mg ages, combined with concordant Hf-W and Al-Mg ages for angrites and CV chondrules, provides strong evidence for a disk-wide, homogeneous distribution of 26Al in the early solar system. As such, the young Al-Mg ages for CR chondrules do not reflect spatial 26Al/27Al heterogeneities but indicate that CR chondrules formed ∼1–2 Ma later than chondrules from most other chondrite groups.Metal and silicate in CR chondrites exhibit distinct nucleosynthetic Mo and W isotope anomalies, which are caused by the heterogeneous distribution of the same presolar s-process carrier. These data suggest that the major components of CR chondrites are genetically linked and therefore formed from a single reservoir of nebular dust, most likely by localized melting events within the solar protoplanetary disk. Taken together, the chemical, isotopic, and chronological data for components of CR chondrites imply a close temporal link between chondrule formation and chondrite accretion, indicating that the CR chondrite parent body is one of the youngest meteorite parent bodies. The relatively late accretion of the CR parent body is consistent with its isotopic composition (for instance the elevated 15N/14N) that suggests a formation at a larger heliocentric distance, probably beyond the orbit of Jupiter. As such, the accretion age of the CR chondrite parent body of ∼3.6 Ma after CAI formation provides the earliest possible time at which Jupiter's growth could have led to scattering of carbonaceous meteorite parent bodies from beyond its orbit into the inner solar system.

Distribution of p-process 174Hf in early solar system materials and the origin of nucleosynthetic Hf and W isotope anomalies in Ca–Al rich inclusions

Some nuclides that were produced in supernovae are heterogeneously distributed between different meteoritic materials. In some cases these heterogeneities have been interpreted as the result of interaction between ejecta from a nearby supernova and the nascent solar system. Particularly in the case of the oldest objects that formed in the solar system – Ca–Al rich inclusions (CAIs) – this view is confirm the hypothesis that a nearby supernova event facilitated or even triggered solar system formation. We present Hf isotope data for bulk meteorites, terrestrial materials and CAIs, for the first time including the low-abundance isotope 174Hf (∼0.16%). This rare isotope was likely produced during explosive O/Ne shell burning in massive stars (i.e., the classical “p-process”), and therefore its abundance potentially provides a sensitive tracer for putative heterogeneities within the solar system that were introduced by supernova ejecta. For CAIs and one LL chondrite, also complementary W isotope data are reported for the same sample cuts. Once corrected for small neutron capture effects, different chondrite groups, eucrites, a silicate inclusion of a IAB iron meteorite, and terrestrial materials display homogeneous Hf isotope compositions including 174Hf. Hafnium-174 was thus uniformly distributed in the inner solar system when planetesimals formed at the <50 ppm level. This finding is in good agreement with the evidently homogeneous distributions of p-process isotopes 180W, 184Os and possibly 190Pt between different iron meteorite groups. In contrast to bulk meteorite samples, CAIs show variable depletions in p-process 174Hf with respect to the inner solar system composition, and also variable r-process (or s-process) Hf and W contributions. Based on combined Hf and W isotope compositions, we show that CAIs sampled at least one component in which the proportion of r- and s-process derived Hf and W deviates from that of supernova ejecta. The Hf and W isotope anomalies in CAIs are therefore best explained by selective processing of presolar carrier phases prior to CAI formation, and not by a late injection of supernova materials. Likewise, other isotope anomalies in additional elements in CAIs relative to the bulk solar system may reflect the same process. The isotopic heterogeneities between the first refractory condensates may have been eradicated partially during CAI formation, because W isotope anomalies in CAIs appear to decrease with increasing W concentrations as inferred from time-integrated 182W/184W. Importantly, the 176Lu–176Hf and 182Hf–182W chronometers are not significantly affected by nucleosynthetic heterogeneity of Hf isotopes in bulk meteorites, but may be affected in CAIs.

26Al–26Mg chronology and oxygen isotope distributions of multiple melting for a Type C CAI from Allende

Disequilibrium oxygen isotopic distributions of Ca–Al-rich inclusions (CAIs) correspond to multiple melting events in the solar nebula. 26Al–26Mg systematics may be applicable for age differences among such melting events. We have carried out a coordinated study of detailed petrographic observations and in-situ oxygen and magnesium isotope measurements for a Type C CAI, EK1-04-2, from the Allende CV3 meteorite to determine the melting events and their ages. The CAI consists mainly of spinel, anorthite, olivine, and pyroxene, and has a core and mantle structure. Petrography of the core suggests that the crystallization sequence of the core minerals is from spinel, anorthite, olivine, and to pyroxene. The mantle has the same mineral assemblage as the core, and shows incomplete melting and solidification textures. Oxygen isotopic compositions of the minerals are distributed along the carbonaceous chondrite anhydrous mineral (CCAM) line (δ18O=−44‰ to +9‰), which indicates to preserve a chemical disequilibrium status in the CAI. Spinel shows a 16O-rich signature (δ18O∼−43‰), while anorthite is 16O-poor (δ18O ∼+8‰). Olivine and pyroxene in the core have the same oxygen isotopic composition (δ18O∼−15‰), which indicates their equilibrium. Olivine and pyroxene in the mantle have variable oxygen isotopic compositions and are slightly depleted in 16O (δ18O=−13‰ to −4‰) compared with the same minerals in the core. The 26Al–26Mg systematics is consistent with the disequilibrium status observed according to the petrography and oxygen isotopes. Spinel is plotted on a line of (26Al/27Al)0=(3.5±0.2)×10−5, anorthite is plotted on a line of (−1±5)×10−7, and olivine and pyroxene in the core are plotted on a line of (−1±7)×10−6. Plots of olivine and pyroxene in the mantle are scattered below the isochron of these minerals in the core. This study indicates that the EK1-04-2 Type C CAI underwent multiple heating events after the formation of its CAI precursor. The precursor CAI was formed ∼0.4Myr after the formation of the Solar System defined by canonical CAI formation. At least 1.6Myr after the precursor CAI formation, the CAI was partially melted and the melt exchanged oxygen isotopes with surrounding 16O-poor nebular gas. 16O-poor olivine and pyroxene in the core crystallized from the melt. Subsequently, Al-rich chondrules accreted onto the CAI, and the CAI experienced partial melting again and recrystallized to form the mantle. The oxygen and magnesium isotopes in anorthite were redistributed during thermal metamorphism in the Allende parent body. Our study reveals that the CAI had been retained in the solar nebula for at least 1.6Myr and underwent multiple melting events in the nebula, and oxygen and 26Al–26Mg systematics has been partially disturbed depending on crystal sizes by metamorphism on the parent body.

Rb–Sr chronology of volatile depletion in differentiated protoplanets: BABI, ADOR and ALL revisited

A strong depletion in moderately volatile elements is a characteristic feature of many planetary bodies in the inner solar system and either reflects the rapid accretion of planetesimals from an incompletely condensed solar nebula, or is the result of energetic collisions during planetary accretion. To better constrain the origin and timescales of this volatile depletion, we have precisely measured Sr isotope compositions in angrites, eucrites and Ca–Al-rich inclusions (CAI). Angrites have an initial (87Sr/86Sr)ADOR=0.698978±0.000004, which is indistinguishable from (87Sr/86Sr)BABI=0.698970±0.000028 obtained for eucrites. In agreement with earlier studies we find that angrites and eucrites have higher initial 87Sr/86Sr ratios than CAI, at face value corresponding to model timescales for volatile loss of several millions of years. However, all the investigated CAI are characterized by elevated 84Sr/86Sr ratios compared to angrites and eucrites, which we interpret to reflect an excess of r-process Sr in the CAI. If this is correct, then the nucleosynthetic Sr isotope anomalies in the CAI require an upward correction of their measured 87Sr/86Sr. After this correction CAI have an initial (87Sr/86Sr)CAI=0.698975±0.000008, which is indistinguishable from ADOR and BABI. This implies volatile loss from the angrite and eucrite parent bodies within less than ~1Ma after formation of CAI. The volatile-depleted nature of these differentiated protoplanets thus most likely reflects their rapid accretion from volatile-poor dust in an incompletely condensed solar nebula.

A LOWER INITIAL ABUNDANCE OF SHORT-LIVED41Ca IN THE EARLY SOLAR SYSTEM AND ITS IMPLICATIONS FOR SOLAR SYSTEM FORMATION

The short-lived radionuclide Ca-41 plays an important role in constraining the immediate astrophysical environment and the formation timescale of the nascent solar system due to its extremely short half-life (0.1 Myr). Nearly 20 years ago, the initial ratio of Ca-41/Ca-40 in the solar system was determined to be (1.41 +/- 0.14) x 10(-8), primarily based on two Ca-Al-rich Inclusions (CAIs) from the CV chondrite Efremovka. With an advanced analytical technique for isotopic measurements, we reanalyzed the potassium isotopic compositions of the two Efremovka CAIs and inferred the initial ratios of Ca-41/Ca-40 to be (2.6 +/- 0.9) x 10(-9) and (1.4 +/- 0.6) x 10(-9) (2 sigma), a factor of 7-10 lower than the previously inferred value. Considering possible thermal processing that led to lower Al-26/Al-27 ratios in the two CAIs, we propose that the true solar system initial value of Ca-41/Ca-40 should have been similar to 4.2 x 10(-9). Synchronicity could have existed between Al-26 and Ca-41, indicating a uniform distribution of the two radionuclides at the time of CAI formation. The new initial Ca-41 abundance is 4-16 times lower than the calculated value for steady-state galactic nucleosynthesis. Therefore, Ca-41 could have originated as part of molecular cloud materials with a free decay time of 0.2-0.4 Myr. Alternative possibilities, such as a last-minute input from a stellar source and early solar system irradiation, could not be definitively ruled out. This underscores the need for more data from diverse CAIs to determine the true astrophysical origin of Ca-41.

Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca‐Al‐rich inclusion of Efremovka CV3 chondrite

Abstract– Different oxygen isotopic reservoirs have been recognized in the early solar system. Fluffy type A Ca‐Al‐rich inclusions (CAIs) are believed to be direct condensates from a solar nebular gas, and therefore, have acquired oxygen from the solar nebula. Oxygen isotopic and chemical compositions of melilite crystals in a type A CAI from Efremovka CV3 chondrite were measured to reveal the temporal variation in oxygen isotopic composition of surrounding nebular gas during CAI formation. The CAI is constructed of two domains, each of which has a core‐mantle structure. Reversely zoned melilite crystals were observed in both domains. Melilite crystals in one domain have a homogeneous 16O‐poor composition on the carbonaceous chondrite anhydrous mineral (CCAM) line of δ18O = 5–10‰, which suggests that the domain was formed in a 16O‐poor oxygen isotope reservoir of the solar nebula. In contrast, melilite crystals in the other domain have continuous variations in oxygen isotopic composition from 16O‐rich (δ18O = −40‰) to 16O‐poor (δ18O = 0‰) along the CCAM line. The oxygen isotopic composition tends to be more 16O‐rich toward the domain rim, which suggests that the domain was formed in a variable oxygen isotope reservoir of the solar nebula. Each domain of the type A CAI has grown in distinct oxygen isotope reservoir of the solar nebula. After the domain formation, domains were accumulated together in the solar nebula to form a type A CAI.

Neutron capture on Pt isotopes in iron meteorites and the Hf–W chronology of core formation in planetesimals

The short-lived 182Hf–182W isotope system can provide powerful constraints on the timescales of planetary core formation, but its application to iron meteorites is hampered by neutron capture reactions on W isotopes resulting from exposure to galactic cosmic rays. Here we show that Pt isotopes in magmatic iron meteorites are also affected by capture of (epi)thermal neutrons and that the Pt isotope variations are correlated with variations in 182W/184W. This makes Pt isotopes a sensitive neutron dosimeter for correcting cosmic ray-induced W isotope shifts. The pre-exposure 182W/184W derived from the Pt–W isotope correlations of the IID, IVA and IVB iron meteorites are higher than most previous estimates and are more radiogenic than the initial 182W/184W of Ca–Al-rich inclusions (CAI). The Hf–W model ages for core formation range from +1.6±1.0 million years (Ma; for the IVA irons) to +2.7±1.3Ma after CAI formation (for the IID irons), indicating that there was a time gap of at least ∼1Ma between CAI formation and metal segregation in the parent bodies of some iron meteorites. From the Hf–W ages a time limit of <1.5–2Ma after CAI formation can be inferred for the accretion of the IID, IVA and IVB iron meteorite parent bodies, consistent with earlier conclusions that the accretion of differentiated planetesimals predated that of most chondrite parent bodies.

Oxygen isotopic variations in a type A Ca–Al‐rich inclusion revealed by high‐precision secondary ion mass spectrometry analysis with micrometer resolution

Ca–Al‐rich inclusions (CAIs) are thought to be the oldest rocks in the solar system. CAIs have oxygen isotope heterogeneities on inter‐mineral and intra‐mineral scales that provide information about environments of the early solar system. We studied a type A CAI from the Allende meteorite, showing a core–mantle structure. Reversely zoned melilite found in the mantle was identified by a combination of energy dispersive X‐ray spectrometer and electron back‐scattered diffraction techniques. For investigation of oxygen isotopic variations of reversely zoned melilite, we developed a high‐precision secondary ion mass spectrometry method that achieves to ~1‰ (1σ) under the spatial resolution of ~3 µm in diameter. Oxygen isotopic composition of core melilite is uniformly 16O‐poor (δ17, 18O ≈ −5 to 0‰), whereas mantle melilite varies widely from 16O‐poor (δ17, 18O ≈ −5‰) to rich (δ17, 18O ≈ −45‰) at the micrometer scale. The variation possibly suggests that the oxygen isotopic composition of solar nebular gas was changed from 16O‐poor to 16O‐rich at the last stage of the CAI formation. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Bulk chemical compositions of Al-rich objects from Rumuruti (R) chondrites: Implications for their origin

R chondrites are a distinct group of chondritic meteorites with unique mineralogical and chemical compositions. They contain various types of Al-rich objects [Ca,Al-rich inclusions (CAIs), Al-rich chondrules and fragments], whose mineralogical compositions and classifications were previously determined by Rout and Bischoff (2008). Here, we report on the bulk compositions of 126 such Al-rich objects determined by broad-beam electron probe microanalysis. Most of the CAIs, except a few, are significantly altered by complex nebular and/or parent body processes and the determination of their pristine composition is difficult. We found that the simple concentric spinel-rich inclusions have high Al 2O 3 (21–72 wt%) correlated with their high modal spinel. The subgroup of simple concentric spinel-rich CAIs have a high Al 2O 3 (21–57 wt%) and also sometimes high FeO (up to 36 wt%), due to a high hercynitic component in the spinel. One simple concentric spinel- and hibonite-rich CAI H030/L, identified as a HAL-type CAI by isotopic studies reported elsewhere, has a highly refractory composition (Al 2O 3∼72 wt%). Most of the simple concentric spinel- and fassaite-rich CAIs have consistently high CaO (∼2.5–17 wt%) compared to other simple concentric spinel-rich inclusions group, where only some inclusions have high CaO (up to 15 wt%). Simple concentric spinel- and Na,Al-alteration product-rich CAIs are heavily altered and have high Na 2O (up to ∼12.5 wt%). The three analyzed fassaite-rich spherules have high CaO and Al 2O 3, and complex spinel- and fassaite-rich CAIs have high CaO (up to 23 wt%) and SiO 2 (up to 41 wt%). Most of the complex spinel- and plagioclase-rich CAIs are altered and contain high amounts of secondary oligoclase. However, a few are less affected by secondary alteration and these are characterized by relatively high CaO (up to 24 wt%) and Al 2O 3 (18–33 wt%); complex spinel and Na,Al-alteration product-rich CAIs are similar to the concentric spinel- and Na,Al-alteration product-rich CAIs. Due to Fe- and alkali-metasomatism, the vast majority of the inclusions in this subgroup were heavily altered, either in a nebular or parent body environment. As a result of this alteration, they contain high FeO and Na 2O+K 2O+Cl. Almost all inclusions have a Ca/Al-ratio below the solar ratio. This suggests that significant Ca/Al fractionation occurred during the formation of most CAIs, most probably due to disequilibrium condensation of spinel prior to melilite. However, a distillation process cannot be ruled out for some CAIs in producing the spinel enrichment. Some porous and fine grained CAIs may have been produced by agglomeration of refractory dust rich in spinel and fassaite. The HAL-type CAI, H030/L, most likely formed by distillation, similar to most of the HAL-type inclusions. Phase equilibrium analysis, in the CMAS system, shows that the fassaite-bearing spherules most likely formed by metastable crystallization and disequilibrium processes. Al-rich chondrules are characterized by >10 wt% Al 2O 3, and most of these also have high FeO and Na 2O. Considering their bulk compositions, their precursors seem to have been a mixture of a ferromagnesian chondrule component rich in olivine and an anorthite–spinel–pyroxene–nepheline-rich CAI component. The mineral assemblages of some of the less altered Al-rich chondrules conform to those predicted by phase equilibrium studies.

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 fO 2 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.

Hf–W chronometry of the IAB iron meteorite parent body

The IAB iron meteorites are unusual in having abundant silicate inclusions, but the relationship of metal and silicates is not well understood. In order to study the sequence and the timing of events that led to the observed structure of IAB iron meteorites, the Hf–W isotope system was applied to metal and silicate separates from seven IAB iron meteorites. A major metal segregation event on the IAB parent body is dated by the average 182W/ 184W ratio of the IAB metal phases (− 3.1 ± 0.2 ε 182W-units), corresponding to a time of 1.8 + 2.3 / − 2.0 Ma after the formation of Ca,Al-rich inclusions (CAIs). The 182Hf– 182W data for three bulk inclusions from Landes, Copiapo and El Taco define a linear trend interpreted as an isochron, corresponding to an age of 2.5 + 2.3 / − 2.0 Ma after CAI formation. This age most likely dates silicate differentiation on the IAB parent body and is, within analytical uncertainty, indistinguishable from the major metal segregation event. Several silicate inclusions are displaced from the IAB silicate isochron towards less radiogenic W compositions, most likely reflecting thermal events on the parent body. Metal segregation and silicate differentiation occurred early enough, so that these processes could have at least partially been triggered by an internal heat-source (e.g., 26Al). Basaltic inclusions of the Caddo County specimen define an age of 4.6 + 0.7 / − 0.6 Ma, most likely dating a partial melting event on the IAB parent body. Three silicate-separates from another El Taco inclusion yield an age that postdates CAI formation by 10.8 + 2.4 / − 2.0 Ma and the IAB silicate isochron by ~ 8.3 Ma. This prolonged interval of local thermal events on the IAB parent body are best explained by collisional heat sources (i.e. impacts). The Hf–W ages for thermal events on the IAB parent body are within the range of previously reported 129I– 129Xe ages but are significantly older than published 87Rb– 87Sr, 147Sm– 143Nd and most 40 K– 40Ar ages. Most likely, these chronometers were reset during later thermal events that had only a minor effect on the 182Hf– 182W and 129I– 129Xe systems.

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 ε 182W = −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 ε 182W 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.

Multiple Generations of Refractory Inclusions in the Metal‐Rich Carbonaceous Chondrites Acfer 182/214 and Isheyevo

Ca,Al-rich inclusions (CAIs) are believed to have formed by evaporation, condensation, and melting of the pre-existing solids during the earliest stages of the solar system evolution. Most CAIs in unmetamorphosed chondrites contain detectable excesses of 26Mg(26Mg*), a decay product of the short-lived radionuclide 26Al ( -->T1/2 ~ 730,000 yr), that correspond to an initial 26Al/27Al ratio of ~ -->(4–7) × 10−5. It is suggested that 26Al was injected into the protosolar molecular cloud or protoplanetary disk by a nearby core-collapse supernova (SN Type II) and uniformly distributed in the solar system; CAI formation started shortly after injection of 26Al and lasted less than 20,000 yr . Here we show that CAIs from the metal-rich carbonaceous chondrites Acfer 214 (CH) and Isheyevo (CH/CB-like) have a bimodal distribution of 26Mg*. Most CAIs composed of grossite (CaAl4O7), hibonite (CaAl12O19), Al-rich pyroxene, perovskite (CaTiO3), and gehlenitic melilite (Ca2Al2SiO7-Ca2MgSi2O7) show either unresolvable or small 26Mg* corresponding to an initial 26Al/27Al ratio of ~ -->4 × 10−7. Some of the grossite-rich CAIs and the less refractory inclusions composed of melilite, spinel (MgAl2O4), Al,Ti-pyroxene, and anorthite (CaAl2Si2O8) have large 26Mg* corresponding to the initial 26Al/27Al ratio of ~ -->5 × 10−5. The 26Al-poor and 26Al-rich CAIs are characterized by 16O-rich (Δ -->17O < − 20‰) compositions typical of CAIs. We suggest that the 26Al-poor and 26Al-rich CAIs represent samples of at least two generations of CAIs formed before and after injection of 26Al into the solar system, respectively. Model yields of 16O,17O, and 18O for SN wind prior to explosion, during explosion, and in total, combined with the observations that both 26Al-poor and 26Al-rich CAIs plot on a three-isotope oxygen diagram (δ17O vs. δ18O) along a single line with a slope of ~1 are consistent with injection of 26Al with the SN wind into the protosolar molecular cloud rather with the SN explosion into the disk.

Al‐Mg isotopic evidence for episodic alteration of Ca‐Al‐rich inclusions from Allende

Abstract— Textures, mineral assemblages, and Al‐Mg isotope systematics indicate a protracted, episodic secondary mineralization history for Allende Ca‐Al‐rich inclusions (CAIs). Detailed observations from one type B1 CAI, one B2, one compact type A (CTA), and one fluffy type A (FTA) indicate that these diverse types of CAIs are characterized by two distinct textural and mineralogic types of secondary mineralization: (1) grossular‐rich domains, concentrated along melilite grain boundaries in CAI interiors, and (2) feldspathoid‐bearing domains, confined mostly to CAI margins just interior to the Wark‐Lovering rim sequence.The Al‐Mg isotopic compositions of most secondary minerals in the type B1 CAI, and some secondary minerals in the other CAIs, show no resolvable excesses of 26Mg, whereas the primary CAI phases mostly yield correlated excesses of 26Mg with increasing Al/Mg corresponding to “canonical” initial 26Al/27Al ∼ 4.5–5 × 10−5. These secondary minerals formed at least 3 Ma after the primary CAI minerals. All but two analyses of secondary minerals from the fluffy type‐A CAI define a correlated increase in 26Mg/24Mg with increasing Al/Mg, yielding (26Al/27Al)0 = (4.9 ± 2.8) × 10−6. The secondary minerals in this CAI formed 1.8–3.2 Ma after the primary CAI minerals. In both cases, the timing of secondary alteration is consistent with, but does not necessarily require, alteration in an asteroidal setting. One grossular from the type B2 CAI, and several grossular and secondary feldspar analyses from the compact type A CAI, have excesses of 26Mg consistent with initial 26Al/27Al ∼ 4.5 × 10−5. Especially in the compact type A CAI, where 26Mg/24Mg in grossular correlates with increasing Al/Mg, these 26Mg excesses are almost certainly due to in situ decay of 26Al. They indicate a nebular setting for formation of the grossular.The preservation of these diverse isotopic patterns indicates that heating on the Allende parent body was not pervasive enough to reset isotopic systematics of fine‐grained secondary minerals. Secondary mineralization clearly was not restricted to a short time interval, and at least some alteration occurred coincident with CAI formation and melting events (chondrule formation) in the nebula. This observation supports the possibility that alteration followed by melting affected the compositional evolution of CAIs.

Remelting of refractory inclusions in the chondrule‐forming regions: Evidence from chondrule‐bearing type C calcium‐aluminum‐rich inclusions from Allende

Abstract— We describe the mineralogy, petrology, oxygen, and magnesium isotope compositions of three coarse‐grained, igneous, anorthite‐rich (type C) Ca‐Al‐rich inclusions (CAIs) (ABC, TS26, and 93) that are associated with ferromagnesian chondrule‐like silicate materials from the CV carbonaceous chondrite Allende. The CAIs consist of lath‐shaped anorthite (An99), Cr‐bearing Al‐Ti‐diopside (Al and Ti contents are highly variable), spinel, and highly åkermanitic and Na‐rich melilite (Åk63–74, 0.4–0.6 wt% Na2O). TS26 and 93 lack Wark‐Lovering rim layers; ABC is a CAI fragment missing the outermost part. The peripheral portions of TS26 and ABC are enriched in SiO2 and depleted in TiO2 and Al2O3 compared to their cores and contain relict ferromagnesian chondrule fragments composed of forsteritic olivine (Fa6–8) and low‐Ca pyroxene/pigeonite (Fs1Wo1–9). The relict grains are corroded by Al‐Ti‐diopside of the host CAIs and surrounded by haloes of augite (Fs0.5Wo30–42). The outer portion of CAI 93 enriched in spinel is overgrown by coarse‐grained pigeonite (Fs0.5–2Wo5–17), augite (Fs0.5Wo38–42), and anorthitic plagioclase (An84). Relict olivine and low‐Ca pyroxene/pigeonite in ABC and TS26, and the pigeonite‐augite rim around 93 are 16O‐poor (Δ17O ∼ −1‰ to −8‰). Spinel and Al‐Ti‐diopside in cores of CAIs ABC, TS26, and 93 are 16O‐enriched (Δ17O down to −20‰), whereas Al‐Ti‐diopside in the outer zones, as well as melilite and anorthite, are 16O‐depleted to various degrees (Δ17O = −11‰ to 2‰). In contrast to typical Allende CAIs that have the canonical initial 26Al/27Al ratio of ∼5 × 10−5 ABC, 93, and TS26 are 26Al‐poor with (26Al/27Al)0 ratios of (4.7 ± 1.4) × 10−6 (1.5 ± 1.8) × 10−6 &lt;1.2 × 10−6 respectively. We conclude that ABC, TS26, and 93 experienced remelting with addition of ferromagnesian chondrule silicates and incomplete oxygen isotopic exchange in an 16O‐poor gaseous reservoir, probably in the chondrule‐forming region. This melting episode could have reset the 26Al‐26Mg systematics of the host CAIs, suggesting it occurred ∼2 Myr after formation of most CAIs. These observations and the common presence of relict CAIs inside chondrules suggest that CAIs predated formation of chondrules.

Pb isotopic age of the Allende chondrules

Abstract— We have studied Pb‐isotope systematics of chondrules from the oxidized CV3 carbonaceous chondrite Allende. The chondrules contain variably radiogenic Pb with a 206Pb/204Pb ratio between 19.5–268. Pb‐Pb isochron regression for eight most radiogenic analyses yielded the date of 4566.2 ± 2.5 Ma. Internal residue‐leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566.6 ± 1.0 Ma, our best estimate for an average age of Allende chondrule formation. This Pb‐Pb age is consistent with the range of model 26Al‐26Mg ages of bulk Allende chondrules reported by Bizzarro et al. (2004) and is indistinguishable from Pb‐Pb ages of Ca‐Al‐rich inclusions (CAIs) from CV chondrites (4567.2 ± 0.6 Ma) (Amelin et al. 2002) and the oldest basaltic meteorites. We infer that chondrule formation started contemporaneously with or shortly after formation of CV CAIs and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids. The entire period of chondrule formation lasted from 4566.6 ± 1.0 Ma (Allende) to 4564.7 ± 0.6 Ma (CR chondrite Acfer 059) to 4562.7 ± 0.5 Ma (CB chondrite Gujba) and was either continuous or consisted of at least three discrete episodes. Since chondrules in CB chondrites appear to have formed from a vapor‐melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated (Krot et al. 2005), there were possibly a variety of processes in the early solar system occurring over at least 4–5 Myr that we now combine under the umbrella name of “chondrule formation.”

Stellar nucleosynthetic contribution of extinct short-lived nuclei in the early solar system and the associated isotopic effects

Abstract— A wide range of stellar nucleosynthetic sources has been analyzed to derive their contributions of short-lived and stable nuclei to the presolar cloud. This detailed study is required to infer the most plausible source(s) of short-lived nuclei through a critical comparison among the various stellar sources that include AGB stars, novae, supernovae II, Ia, and Wolf-Rayet stars that evolved to supernovae Ib/c. In order to produce the canonical value of 26Al/27Al in the early solar system, almost all stellar sources except low-mass AGB stars imply large isotopic anomalies in Ca-Al-rich inclusions (CAIs). This is contrary to the observed isotopic compositions of CAIs. The discrepancy could impose stringent constraints on the formation and thermal evolution of CAIs from different chondrites. Among the various stellar scenarios, the injection of short-lived nuclei into the previously formed solar protoplanetary disc by a massive star of an ad hoc chosen high-injection mass cut is a possible scenario. There is a possibility of the contribution of short-lived nuclides by a 1.5–3 M⊙ AGB star as it implies the smallest shift in stable isotopes. A low-mass AGB star of relatively low metallicity would be even a better source of short-lived nuclei. However, this scenario would require independent gravitational collapse of the presolar cloud coupled with ambipolar diffusion of magnetic flux. Alternatively, numerous scenarios can be postulated that involve distant (≥10 pc) massive stars can contribute 60Fe to the presolar cloud and can trigger its gravitational collapse. These scenarios would require production of 26Al and 41Ca by irradiation in the early solar system. Significant production of 26Al and 60Fe can be explained if massive, rotating Wolf-Rayet stars that evolved to supernovae Ib/c were involved.