Stellar sources of the short-lived radionuclides in the early solar system

We discuss the possible stellar sources of short-lived radionuclides (SLRs) known to have been present in the early solar system (²⁶Al, ³⁶Cl, ⁴¹Ca, ⁵³Mn, ⁶⁰Fe, ¹⁰⁷Pd, ¹²⁹I, ¹⁸²Hf, ²⁴⁴Pu). SLRs produced primarily by irradiation (⁷Be, ¹⁰Be) are not discussed in this paper. We evaluate the role of the galactic background in explaining the inventory of SLRs in the early solar system. We review the nucleosynthetic processes that produce the different SLRs and place the processes in the context of stellar evolution of stars from 1 to 120 M_⊙. The ejection of newly synthesized SLRs from these stars is also discussed. We then examine the extent to which each stellar source can, by itself, explain the relative abundances of the different SLRs in the early solar system, and the probability that each source would have been in the right place at the right time to provide the SLRs. We conclude that intermediate-mass AGB stars and massive stars in the range from ∼20 to ∼60 M_⊙ are the most plausible sources. Low-mass AGB stars fail to produce enough ⁶⁰Fe. Core-collapse Type II supernovae from stars with initial masses of <20 M_⊙ produce too much ⁶⁰Fe and ⁵³Mn. Sources such as novae, Type Ia supernovae, and core-collapse supernovae of O-Ne-Mg white dwarfs do not appear to provide the SLRs in the correct proportions. However, intermediate-mass AGB stars cannot provide ⁵³Mn or the r-process elements, so if an AGB star provided the ⁴¹Ca, ³⁶Cl, ²⁶Al, ⁶⁰Fe, and ¹⁰⁷Pd, and if a late stellar source is required for ⁵³Mn and the r-process elements, then two types of sources would be required. A separate discussion of the production of r-process elements highlights the difficulties in modeling their production. There appear to be two sources of r-process elements, one that produces the heavy r-process elements, including the actinides, and one that produces the elements from N to Ge and the elements ∼110 < A < ∼130. These can be assigned to SNII explosions of stars of ?11 M_⊙ and stars of 12-25 M_⊙, respectively. More-massive stars, which leave black holes as supernova remnants, apparently do not produce r-process elements.     

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.     

Fe-Ni and Mn-Cr isotopic systems in sulfides from unequilibrated enstatite chondrites

In situ measurements of ⁶⁰Fe-⁶⁰Ni and ⁵³Mn-⁵³Cr isotopic systems with an ion microprobe have been carried out for sulfide assemblages from unequilibrated enstatite chondrites (UECs). Evidence for the initial presence of ⁶⁰Fe has been observed in nine sulfide inclusions from three UECs: ALHA77295, MAC88136, and Qingzhen. The inferred initial (⁶⁰Fe/⁵⁶Fe) [(⁶⁰Fe/⁵⁶Fe)₀] ratios show a large variation range, from ∼2 × 10⁻⁷ to ∼2 × 10⁻⁶. The sulfide inclusions with high Fe/Ni ratios yield (⁶⁰Fe/⁵⁶Fe)₀ ratios of ∼(2-7) × 10⁻⁷, similar to most of the (⁶⁰Fe/⁵⁶Fe)₀ values of troilite and pyroxene observed in unequilibrated ordinary chondrites (UOCs). Inclusions with high inferred (⁶⁰Fe/⁵⁶Fe)₀ ratios (∼1-2 × 10⁻⁶) have low Fe/Ni ratios and the magnitude of the ⁶⁰Ni excesses is similar in two MAC88136 assemblages in spite of a difference of a factor of two in their Fe/Ni ratios. The inferred high (⁶⁰Fe/⁵⁶Fe)₀ ratios were probably the result of Fe-Ni re-distribution in the sulfides during later alteration processes.The ⁵³Mn-⁵³Cr system was measured in five of the sulfide assemblages that were examined for their ⁶⁰Fe-⁶⁰Ni systematics. The ⁵³Mn-⁵³Cr isochrons yielded variable initial (⁵³Mn/⁵⁵Mn) [(⁵³Mn/⁵⁵Mn)₀] ratios from ∼(2-7) × 10⁻⁷. There is no obvious correlation between the (⁶⁰Fe/⁵⁶Fe)₀ and (⁵³Mn/⁵⁵Mn)₀ ratios. The variable ⁵³Mn-⁵³Cr isochrons probably also indicate later disturbance to the isotopic systems in these sulfides. Even though no chronological information can be extracted from the ⁶⁰Fe-⁶⁰Ni and ⁵³Mn-⁵³Cr systems in these UEC sulfides, our results indicate that ⁶⁰Fe was present in the enstatite chondrite formation region of the early Solar System.     

Solar cosmic ray records in lunar rock 64455

Cosmic ray produced ¹⁰Be (half-life = 1.36 × 10⁶ yr), ²⁶Al (7.05 × 10⁵ yr), and ³⁶Cl (3.01 × 10⁵ yr) were measured in a depth profile of 19 carefully-ground samples from the glass-coated lunar surface rock 64455. The solar cosmic ray (SCR) produced ²⁶Al and ³⁶Cl in this rock are present in high concentrations, which in combination with the low observed erosion rate, <0.5 mm/Myr, provide well defined depth profiles characterizing the SCR component of the cosmic rays. In conjunction with new experimentally determined excitation functions, the ³⁶Cl concentrations suggest a softer solar-proton spectral shape than that derived from most previous measurements. The fact that no SCR-produced ¹⁰Be activity could be detected in 64455 is in good agreement with observations in 68815 and also indicates a softer SCR spectrum. Comparison of observed SCR profiles in 64455 with theoretical calculations indicates that the average solar-proton spectrum over the past 2 Myr (based on ²⁶Al) has an exponential rigidity parameter (R₀) of about 90 MV with a proton flux (J) of 73 protons/cm²/s·4π above 10 MeV. Over the last ∼0.5 Myr (based on ³⁶Cl) R₀ is about 70 MV with a flux of ∼196 protons/cm²/s·4π above 10 MeV. These SCR fluxes are consistent with most previous work.     

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.     

On early Solar System chronology: Implications of an heterogeneous spatial distribution of Al and Mn

Early Solar System chronology is usually built with the assumption that the distribution of short-lived radionuclides was homogeneous through the solar accretion disk. At present, there is no unambiguous evidence for a homogeneous distribution of short-lived radionuclides in the solar accretion disk, while some data point to a heterogeneous distribution of short-lived radionuclides. In this paper, we explore a possible chronology based on a heterogeneous distribution of ²⁶Al and ⁵³Mn in the accretion disk. Our basic assumption is that the different abundances of extinct short-lived radionuclides in calcium-aluminium-rich inclusions (CAIs) and chondrules are due to spatial rather than temporal differences. We develop a simple model where CAIs and chondrules form contemporaneously, in different spatial locations, and are characterised by distinct initial ²⁶Al and ⁵³Mn abundances. In this model, all evolved bodies are supposed to be originally chondritic, i.e., to be made of a mixture of CAIs, chondrules, and matrix. This mixture determines the initial content in ²⁶Al and ⁵³Mn of a chondritic parent-body as a function of its CAI and chondrule abundance fraction. This approach enables us to calculate coherent ²⁶Al and ⁵³Mn ages from the agglomeration of the parent-body precursors (CAIs and chondrules) until the isotopic closure of ²⁶Al and ⁵³Mn, thereafter called ²⁶Al-⁵³Mn age. We calculate such ²⁶Al-⁵³Mn ages for a diversity of evolved objects, with the constraint that they should be found for realistic chondritic parent-body precursors, i.e., objects having similar or identical petrograpy to the existing chondrite groups. The so defined age of the d’Orbigny angrite is 4.3 ± 1.1 Myr, for the Asuka-881394 eucrite 2.8 ± 1.0 Myr, for the H4 chondrite Sainte Marguerite ∼3 Myr, and for H4 Forest Vale ∼5 Myr. The calculated ²⁶Al-⁵³Mn ages give timescales for the evolution of the respective parent-bodies/meteorites that can be investigated in the light of further petrographic studies. We anchor the calculated relative chronology to an absolute chronology using absolute Pb-Pb ages and relative Hf-W ages of the objects under scrutiny. The precursors of Sainte Marguerite and Forest Vale agglomerated at the same time (∼4565.8 ± 1.2 Ma ago). The precursors of eucrites (Asuka-881394) agglomerated 4564.8 ± 1.2 Ma ago. The precursors of angrites agglomerated late (4561.5 ± 1.8 Ma ago). Our model provides a fully compatible Al-Mg/Mn-Cr/Pb-Pb chronology, and is shown to be robust to reasonable changes in the input parameters. The calculated initial ²⁶Al/²⁷Al ratios are high enough to have ²⁶Al as a possible heat source for differentiation.     

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

The magnesium isotopic compositions of 26 hibonite-bearing inclusions from the CM chondrite Murchison, as well as isotopic measurements on a subset of these samples for oxygen, titanium, and lithium-beryllium-boron are reported along with oxygen isotopic data for an additional 13 hibonites that were previously investigated for other isotope systems (magnesium, potassium, calcium, and titanium) and rare earth element concentrations. Magnesium isotopic compositions divide CM hibonites into two distinct populations which correlate perfectly with their mineralogy and morphology, as previously discovered by Ireland [Ireland T. R. (1988) Correlated morphological, chemical, and isotopic characteristics of hibonites from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta52, 2827-2839]: Spinel-HIBonite spherules (SHIBs) bear evidence of in situ²⁶Al decay, whereas PLAty-Crystals (PLACs) and Blue AGgregates (BAGs) either lack resolvable ²⁶Mg-excesses or exhibit ²⁶Mg deficits by up to ∼4‰. High precision, multiple collector SIMS analyses show that 6 of 7 SHIBs investigated fall on a single correlation line implying ²⁶Al/²⁷Al = (4.5 ± 0.2) × 10⁻⁵ at the time of isotopic closure, consistent with the “canonical” ²⁶Al abundance characteristic of internal isochrons in many calcium-aluminum-rich inclusions (CAIs). One SHIB sample exhibits Δ²⁶Mg^∗ consistent with a “supracanonical” ²⁶Al/²⁷Al ratio of (6.4 ± 0.5) × 10⁻⁵. The PLAC hibonites contain highly anomalous titanium isotopic compositions, with δ⁵⁰Ti values ranging from −80‰ to almost +200‰, whereas SHIBs generally lack large Ti isotopic anomalies.Eight out of 11 ²⁶Al-free PLAC hibonite grains record ¹⁰B/¹¹B excesses that correlate with Be/B; the inferred initial ¹⁰Be/⁹Be ratio of (5.1 ± 1.4) × 10⁻⁴ is lower than the best-constrained ¹⁰Be/⁹Be of (8.8 ± 0.6) × 10⁻⁴ in a CV CAI. The data demonstrate that ¹⁰Be cannot be used as a relative chronometer for these objects and that most of the ¹⁰Be observed in CAIs must be produced by irradiation of precursor solids in the early solar system. The lack of ²⁶Al in PLAC hibonites indicates that significant amounts of ²⁶Al were not formed in the same spallogenic processes that made ¹⁰Be in PLAC precursors. This is most easily understood as indicating very early formation of the PLAC hibonites, prior to the incorporation and mixing of ²⁶Al into the solar nebula, although an alternative scenario, which invokes irradiation under different solar flare conditions, cannot be ruled out. Lithium isotopes are normal within uncertainties, probably reflecting contamination and/or postcrystallization exchange.The oxygen isotopic compositions of SHIBs and PLACs are all highly ¹⁶O-enriched, but are not derived from a homogeneous reservoir: Δ¹⁷O values span a range of ∼−28‰ to −15‰. The ranges of ¹⁶O-enrichment in SHIBs and PLACs overlap and are less “anomalous” than the most ¹⁶O-enriched compositions found in meteorites [Kobayashi S., Imai H. and Yurimoto H. (2003) New extreme ¹⁶O-rich chondrule in the early solar system. Geochem. J.37, 663-669]. Both PLACs and SHIBs formed in ¹⁶O-enriched reservoirs characterized by small-scale heterogeneities in the gas phase. If such heterogeneities were generated by an admixture of relatively ¹⁶O-poor gas created by self-shielding during CO photolysis and transported to the hot inner regions of the accretion disk, then this process must have been initiated very early on, prior to the arrival of fresh radioactivity into the inner solar system. Oxygen isotope heterogeneities persisted throughout the formation interval of PLACs, CAI precursors, and SHIBs which could be as long as 3 × 10⁵ years based on ²⁶Al records.One SHIB and one BAG exhibit mass fractionated oxygen isotopic compositions similar to those seen in FUN inclusions and in several platy hibonite crystals [Lee T., Mayeda T. K. and Clayton R. N. (1980) Oxygen isotopic anomalies in Allende inclusion HAL. Geophys. Res. Lett.7, 493-496; Ireland T. R., Zinner E. K., Fahey A. J. and Esat T. M. (1992) Evidence for distillation in the formation of HAL and related hibonite inclusions. Geochim. Cosmochim. Acta56, 2503-2520; Ushikubo T., Hiyagon H. and Sugiura N. (2007) A FUN-like hibonite inclusion with a large ²⁶Mg-excess. Earth Planet. Sci. Lett.254, 115-126]. The suite of mass-fractionated hibonites exhibit a range of isotopic properties, including ²⁶Al/²⁷Al ratios from below detection to a “canonical” level and oxygen and titanium anomalies that are not exceptional by PLAC standards. This suggests that F (fractionation) processes—evaporation under (oxidizing) conditions—are not necessarily associated with sampling a special isotopic reservoir.     

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.     

Search for extinct Cl: Vigarano CAIs, the Pink Angel from Allende, and a Ningqiang chondrule

We have searched for excesses of ³⁶S derived from the decay of extinct ³⁶Cl in sodalite, a secondary Cl-rich mineral, in Ca-Al-rich inclusions (CAIs) from the Vigarano and Allende CV3 chondrites and in a chondrule from the Ningqiang carbonaceous chondrite. The presence of sodalite in two CAIs from Vigarano and its absence from surrounding CAI fragments suggests sodalite formation after CAI fragmentation. As for sodalite in the Allende Pink Angel CAI, oxygen isotopic compositions have been interpreted as indicative of high temperature interactions, thus suggesting formation prior to accretion to the parent body, probably in a nebular setting. Sodalite in the Ningqiang chondrule is considered to have formed via alkali-Ca exchange, which is believed to have occurred before accretion to the parent body.Sodalites in the Vigarano CAIs and in the Ningqiang chondrule show no clear evidence for the presence of radiogenic ³⁶S. The inferred 2σ upper limits for ³⁶Cl/³⁵Cl at the time of sodalite formation are 1.6 × 10⁻⁶ (Vigarano CAIs) and 3.3 × 10⁻⁶ (Ningqiang chondrule), respectively. In the Pink Angel CAI sodalite exhibits small ³⁶S excesses which weakly correlate with ³⁵Cl/³⁴S ratios. The inferred ³⁶Cl/³⁵Cl ratio of (1.8 ± 2.2) × 10⁻⁶ (2σ error) is lower than that found by Hsu et al. [Hsu, W., Guan, Y., Leshin, L. A., Ushikubo, T. and Wasserburg, G. J. (2006) A late episode of irradiation in the early solar system: Evidence from extinct ³⁶Cl and ²⁶Al in meteorites. Astrophys. J. 640, 525-529], thus indicative of heterogeneous distribution of ³⁶Cl in this CAI. Spallation reactions induced by energetic particles from the young Sun are suggested for the origin of ³⁶Cl, similar to the case of ¹⁰Be. While ¹⁰Be appears to be present in roughly equal abundance in all studied CAIs, our study indicates the level of ³⁶Cl abundances to be variable so that there seems to be no simple relationship between ¹⁰Be and ³⁶Cl. This would be expected if trapped cosmic rays rather than Early Solar System spallation were the dominant source of ¹⁰Be in the Early Solar System, since their contribution to ³⁶Cl would have been tiny.If the variability of ³⁶Cl abundances is caused by temporal differences in the alteration that formed sodalite, sodalite in the Vigarano CAIs and in the Ningqiang chondrule may have formed ?0.5 and ?0.2 Ma after formation of the sodalite in the Ningqiang CAI analyzed by Lin et al. [Lin, Y., Guan, Y., Leshin, L. A., Ouyang, Z. and Wang, D. (2005) Short-lived chlorine-36 in a Ca- and Al-rich inclusion from the Ningqiang carbonaceous chondrite. Proc. Natl. Acad. Sci. USA 102, 1306-1311]. The Pink Angel sodalite that we analyzed may have formed ∼0.3 Ma after formation of the sodalite in the Pink Angel analyzed by Hsu et al. [Hsu, W., Guan, Y., Leshin, L. A., Ushikubo, T. and Wasserburg G. J. (2006) A late episode of irradiation in the early solar system: Evidence from extinct ³⁶Cl and ²⁶Al in meteorites. Astrophys. J. 640, 525-529]. The estimated temporal differences suggest that alteration extended over at least 0.5 Ma. If previous works showing very low initial ³⁶Cl/³⁵Cl in the Allende CAIs and a H3 chondrite CAI are also considered, then alteration processes may have been more than 1.7 Ma.     

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.     

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.     

Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived Be and for the possible presence of the short-lived nuclide Be in the early solar system

The concentrations and isotopic compositions of lithium, beryllium, and boron, analyzed in situ by ion microprobe in 66 spots of a type B1 Ca-Al-rich inclusion (CAI 3529-41) from the Allende meteorite, are reported. Large variations are observed for both the Li and the B isotopic ratios with ⁷Li/⁶Li ranging from 9.2 ± 0.22 to 12.22 ± 0.43 (a ≈250‰ range in δ⁷Li values) and ¹⁰B/¹¹B ranging from 0.2468 ± 0.0057 to 0.4189 ± 0.0493 (a 410‰ range in δ¹¹B values). The very low Li concentrations (<1 ppb) observed in several anorthite and fassaite grains require that a correction for the contribution of spallogenic Li produced during irradiation of the Allende meteoroid by galactic cosmic rays (GCR) be made (after this correction ⁷Li/⁶Li ranges from 9.2 ± 0.22 to 13.44 ± 0.56, i.e., a ≈350‰ range in δ⁷Li values). In 3529-41, the ¹⁰B/¹¹B ratios are positively correlated with ⁹Be/¹¹B in a manner indicating the in situ decay of short-lived ¹⁰Be (half-life = 1.5 Ma) with a ¹⁰Be/⁹Be ratio at the time of formation of the CAI of 8.8 ± 0.6 × 10⁻⁴, which is in agreement with previous findings [McKeegan, K.D., Chaussidon, M., Robert, F., 2000. Incorporation of short-lived ¹⁰Be in a calcium-aluminum-rich inclusion from the Allende meteorite. Science289, 1334-1337]. The present detailed investigation demonstrates that only minor perturbations of the ¹⁰Be-¹⁰B system are present in 3529-41, contrary to the ²⁶Al/²⁶Mg system for which numerous examples of isotopic redistribution following crystallization were observed [Podosek, F.A., Zinner, E.K., MacPherson, G.J., Lundberg, L.L., Brannon, J.C., Fahey, A.J., 1991. Correlated study of initial ⁸⁷Sr/⁸⁶Sr and Al-Mg systematics and petrologic properties in a suite of refractory inclusions from the Allende meteorite. Geochim. Cosmochim. Acta55, 1083-1110]. Petrographically based criteria were developed to identify within the 66 analyzed spots in 3529-41, those where post-magmatic perturbation of the Li and Be distributions occurred. Li and Be concentrations measured in different analytical spots are compared with those predicted by using experimentally determined partition coefficients according to a model of closed-system crystallization of the CAI melt. These criteria show that 56% of the spots in melilite, 38% in anorthite, and 8% in fassaite suffered post-crystallization perturbations of Li and/or Be distributions. In the remaining spots, which do not show obvious indication of redistribution of Li or Be, the ⁷Li/⁶Li isotopic variations (corrected for GCR exposure) are positively correlated with⁹Be/⁶Li suggesting the in situ decay of now-extinct ⁷Be. The derived isochron implies that at the time of its formation, the CAI melt had a ⁷Be/⁹Be ratio of 0.0061 ± 0.0013 and a ⁷Li/⁶Li ratio of 11.49 ± 0.13. In contrast, all the spots in 3529-41, which do show evidence for post-magmatic redistribution of Li and Be, have relatively constant ⁷Li/⁶Li, averaging 11.72 ± 0.56, which is consistent with mass balance calculations for Li isotopic homogenization in the CAI after the decay of ⁷Be. The incorporation of live ⁷Be in 3529-41 requires, because of the very short half-life of this nuclide (53 days), that it be produced essentially contemporaneously with the formation of the CAI. Therefore, the irradiation processes responsible for production of ⁷Be must have occurred within the solar accretion disk. Calculations developed in the framework of the x-wind model [Gounelle, M., Shu, F.H., Shang, H., Glassgold, A.E., Rehm, E.K., Lee, T., 2004. The origin of short-lived radionuclides and early Solar System irradiation (abstract). Lunar Planet. Sci.35, 1829] reproduce the ⁷Be and ¹⁰Be abundances observed in 3529-41. The correlated presence of ⁷Be and ¹⁰Be in 3529-41 is thus a strong argument that ¹⁰Be, which is observed rather ubiquitously in CAIs, is also a product of irradiation in the early solar system, as might be a significant fraction of other short-lived radionuclides observed in early solar system materials.     

河流阶地样品中石英纯化和宇宙核素~(10)Be和~(26)Al分离方法研究

经历反复埋藏暴露演化过程的河流阶地样品,难以用常规方法将原生宇宙核素10Be、26Al有效分离。本文在前人实验方法基础上,使用人工挑选、磁选仪分选及酸洗方法,分离样品中碳酸盐、含铁矿物及大气生成的10Be,进一步优化了石英提纯实验流程。结果表明:长度为9 cm、内径为1 cm的阴离子交换树脂装置匹配4 mol/L氢氟酸淋滤液,可将B、Mg、Ca、Cr、Fe、Mn、Ni、Ti和Be、Al有效分离,Be、Al回收率分别可达95.7%、85.7%。阳离子交换树脂能有效分离Be、Al,两元素回收率均达到85%。获得10Be/9Be和26Al/27Al流程空白分别为2.19×10-15和1.63×10-15。优化后的实验方法显著提高了河流阶地样品中原生宇宙核素10Be、26Al的纯化效率,且10Be/9Be和26Al/27Al流程空白数值与国内外实验室具有可比性。采用本方法获得了成都平原冲积物10Be、26Al暴露年龄分别是76.36±9.51 ka和69.44±14.13 ka,为评价龙门山前缘隐伏断裂构造特征和活动性提供了年代学依据。     

Mn-Cr systematics of carbonates in CM chondrites: Implications for the timing and duration of aqueous alteration

CM chondrites contain carbonates and other secondary minerals such as phyllosilicates, sulfides, sulfates, oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid. We report in situ Mn-Cr isotope measurements in the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100 using secondary ion mass spectrometry (Cameca ims-1270 ion microprobe). The ⁵³Cr excesses are correlated with the ⁵³Mn/⁵⁵Mn ratio and result from the in situ decay of ⁵³Mn, a short-lived radioisotope with a half-life of 3.7 Ma. If we assume that carbonate grains in samples QUE 93005 and ALH 83100 are cogenetic, then the excesses define initial ⁵³Mn/⁵⁵Mn ratios ((⁵³Mn/⁵⁵Mn)₀) of (4.1 ± 1.2) × 10⁻⁶ and (5.1 ± 1.7) × 10⁻⁶, respectively. These values are comparable to those in carbonates from other CM chondrites as reported in the literature. Initial ⁵³Mn/⁵⁵Mn ratios for calculated model isochrones for individual carbonate grains range from (3.8 ± 1.4) × 10⁻⁶ to (4.8 ± 2.1) × 10⁻⁶ for QUE 93005 and from (3.1 ± 1.6) × 10⁻⁶ to (1.3 ± 0.5) × 10⁻⁵ for ALH 83100. A possible interpretation for the ranges in (⁵³Mn/⁵⁵Mn)₀ could be that alteration in individual CM chondrites was episodic and occurred over an extended period of time. However, isochrones based on the entire set of carbonate grains in each of the CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration on the CM parent asteroid started contemporaneously with or shortly after CAI formation and lasted at least 4 Ma.     

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.     

Paleo-ocean chemistry records in marine opal: Implications for fluxes of trace elements, cosmogenic nuclides (Be and Al), and biological productivity

Here, we provide evidence suggesting that marine (diatom) opal contains not only a high fidelity record of dissolved oceanic concentrations of cosmic ray-produced radionuclides, ¹⁰Be and ²⁶Al, but also a record of temporal variations in a large number of trace elements such as Ti, Fe, Zn and Mn. This finding is derived from measurements in purified biogenic opal that can be separated from detrital materials using a newly developed technique based on surface charge characteristics. Initial results from a sediment core taken near the present-day position of the Antarctic Polar Front (ODP Site 1093) show dramatic changes in the intrinsic concentrations of, Be, Al, Ti, Fe, Mn and Zn in the opal assemblages during the past ∼140 kyr BP. The results imply appreciable climatically controlled fluctuations in the level of bioreactive trace elements. The time series of total Be, Al, Ti, Fe and ¹⁰Be in the sediment core are all well correlated with each other and with dust records in the polar ice cores. The observations suggest that a significant flux of these trace metals to oceans is contributed by the aeolian dust, in this case, presumably from the Patagonia. This observation also allows determination of fluxes of dust-contributed ¹⁰Be to the Antarctica ice sheets. However, our data show that the relationships among the various metals are not perfectly linear. During periods of higher dissolved concentrations of trace elements (indicated by Fe and Ti) the relative concentrations of bioreactive elements, Be, Al, Mn and Zn are decreased. By contrast, the Fe/Zn and Fe/Mn ratios decrease significantly during each transition from cold to warm periods. The relative behavior could be consistent with any of the following processes: (i) enhanced biological productivity due to greater supply of the bioreactive elements (e.g. Zn) during cold periods (ii) increased biological and inorganic scavenging of particle active elements (e.g. Be and Al) during early interglacial periods (iii) differential uptake/removal of the metals by the various diatom taxa whose relative productivity or growth rate changes with large scale climate. In any case, with one sedimentary phase and in single sedimentary sections, we now have the potential to compare directly a proxy for aeolian input of micronutrients (e.g. Fe or Ti), with a proxy for production (e.g. ²⁶Al/Al ratios). We expect that studies of the temporal records of trace elements and cosmogenic nuclides in contrasting regions of upwelling and productivity, which exhibit different sensitivities to global climate fluctuations and micronutrient inputs, would lead to a direct and comprehensive test of ideas such as the hypothesis of iron control of atmospheric carbon dioxide [Martin, J.H., 1990. Glacial-interglacial CO₂ change: the iron hypothesis. Paleoceanography5, 1-13]. Our present data from a single site do not show that increases in dissolved Fe concentrations, per se, were responsible for increased biological productivity. However, a much clearer picture of the effect of increased dust fluxes should emerge when we have data for trace elements and the cosmogenic nuclides, ¹⁰Be and ²⁶Al from various oceanic provinces.     

Volcanic and solar activity, and atmospheric circulation influences on cosmogenic Be fallout at Vostok and Concordia (Antarctica) over the last 60 years

The cosmogenic nuclide beryllium-10 (¹⁰Be), recovered from ice cores, is often used to study solar activity on long timescales. However, the ¹⁰Be signal is also influenced by factors other than the Sun. To identify and quantify various contributions to the ¹⁰Be signal, two Antarctic snow records from the Vostok and Concordia sites spanning the last 60 years were studied at a sub-annual resolution. Three factors that contribute to the ¹⁰Be signal were identified. First, a significant period of approximately 11 yr that can be associated with the modulation of ¹⁰Be production by solar activity was detected in both records. The solar imprint constitutes 20-35% of the variance within the total signal. The 11-yr ¹⁰Be snow component was attenuated by a factor of ∼0.5 and was delayed by ∼1.4 yr compared to the ¹⁰Be production expected within the polar atmosphere. The result could be interpreted as the composite response of a stratospheric ¹⁰Be reservoir with an 11-yr modulation that was attenuated and delayed (with respect to ¹⁰Be polar production) and to a tropospheric ¹⁰Be reservoir with an 11-yr modulation that was not attenuated or delayed. Then, peaks in ¹⁰Be concentrations that were ∼66% and ∼35% higher than the average concentration were observed during the stratospheric volcanic eruptions of Agung (in 1963) and Pinatubo (in 1991), respectively. In light of these new results, published ¹⁰Be ice core records could be reinterpreted because spikes in ¹⁰Be concentration appear at the time of several stratospheric events. The data indicate that stratospheric volcanic eruptions can impact ¹⁰Be transport and deposition as a result of the roles played by the sedimentation of sulfate aerosols and the formation and rapid settling of polar stratospheric clouds (PSC). Also, an interannual variability of ∼4 yr was determined in both ¹⁰Be records, corresponding to ∼26% of the variance within the signal at Vostok. As with species of marine origin (sodium), this 4-yr variability is interpreted as a tropospheric modulation. The 4-yr variability could be associated with atmospheric circulation associated with the coupled Southern Ocean ocean-atmosphere system. The results presented here, from sites within the high Antarctic plateau, open new possibilities for ice core dating over the last few centuries and for the reconstruction of past solar activity in relation to climate.     

Thermal evolution and physics of melt extraction on the ureilite parent body

We develop a physical model of the thermal history of the ureilite parent body (UPB) that numerically tracks the history of its heating, hydration, dehydration, partial melting and smelting as a function of its formation time and the initial values of its composition, formation temperature and water ice content. Petrologic and chemical data from the main group (non-polymict) ureilite meteorites, which sample the interior of the UPB between depths corresponding to pressures in the range 3-10 MPa, are used to constrain the model. We find that to achieve the ∼30% melting inferred for ureilites from all sampled depths, the UPB must have had a radius between ∼80 and ∼130 km and must have accreted about 0.55 Ma after CAI formation. Melting began in the body at ∼1 Ma after CAI, and the time at which 30% melting was reached varied with depth in the asteroid but was always between ∼4.5 and ∼5.8 Ma after CAI. The total rate at which melt was produced in the UPB varied from more than 100 m³ s⁻¹ in the very early stages of melting at ∼1 Ma after CAI to ∼5 m³ s⁻¹ between 2 and 3 Ma after CAI, decreasing to extremely small values as the end of melting was approached beyond ∼5 Ma. Although the initial period of high melt production occupied only a short time around 1 Ma after CAI, it corresponded to ∼half (16%) of total silicate melting, and all strictly basaltic (i.e. plagioclase-saturated) melts must have been produced during this period.A very efficient melt transport network, consisting of a hierarchy of veins and larger pathways (dikes), developed quickly at the start of melting, ensuring rapid (timescales of months) transport of any single parcel of melt to shallow levels, thus ensuring that chemical interaction between melts and the rocks through which they subsequently passed was negligible. Volatile (mainly carbon monoxide) production due to smelting began at the start of silicate melting in the shallowest parts of the UPB and at later times at greater depths. Except at the very start and very end of melting, the volatile content of the melts produced was always high - generally between 15 and 35 mass % - and most of the melt produced was erupted at the surface of the UPB with speeds well in excess of the escape velocity and was lost into space. However, we show that 30% melting at the 3 MPa pressure level was only possible if ∼15% of the total melt produced in the asteroid was retained as a small number (∼5) of very extensive, sill-like intrusions centered at a depth of ∼7 km below the surface, near the base of the ∼8 km thick outer crust of the asteroid that was maintained at temperatures below the basalt solidus by conductive heat loss to the surface. The horizontal extents of these sills occupied about 75% of the surface area of the UPB, and the sills acted as buffers between the steady supply of melt from depth and the intermittent explosive eruption of the melt into space. We infer that samples from these intrusions are preserved as the rare feldspathic (loosely basaltic) clasts in polymict ureilites, and show that the cooling histories of the sills are consistent with these clasts reaching isotopic closure at ∼5 Ma after CAI, as given by ²⁶Al-²⁶Mg, ⁵³Mn-⁵³Cr and Pb-Pb age dates.     

Mn-Cr systematics of the early Solar System revisited

We have reinvestigated the Mn-Cr systematics in a number of primitive meteorites, differentiated planetesimals and terrestrial planets in order to address the chronology of the early stages of protoplanetary disk evolution and planetary formation. Our analytical procedure is based on the assumption of terrestrial abundances for ⁵⁰Cr and ⁵²Cr only; recognizing that a data reduction scheme based on Earth-like ⁵⁴Cr/⁵²Cr abundances in all meteorites is not tenable. Here we show that initial ε_⁵³Cr compositions of ⁵⁴Cr-rich and ⁵⁴Cr-poor acid leach fractions in the primitive carbonaceous chondrite Orgueil differ by 0.9ε, reflecting primordial mineral-scale heterogeneity. However, asteroidal processing effectively homogenized any ε_⁵³Cr variations on the planetesimal scale, providing a uniform present-day solar ε_⁵³Cr=0.20±0.10. Thus, our ⁵³Mn-⁵³Cr data argue against the previously suggested ⁵³Mn heliocentric gradient. Instead, we suggest that inner Solar System objects possessed an initially homogeneous ⁵³Mn/⁵⁵Mn composition, which determined by two independent means is estimated at (6.28 ± 0.66) × 10⁻⁶. Our revised Mn-Cr age for Ste. Marguerite (SM) metamorphism of 4562.9 ± 1.0 Ma is identical to the Pb-Pb age of SM phosphates. Using this age, we confirm that mantle differentiation of the eucrite parent body occurred 4564.9 ± 1.1 Ma ago, and revise the time interval between this event and CAI formation to 2.2 ± 1.1 Ma. We also constrain metamorphism in carbonaceous chondrites of type 2 and 3 to have occurred between 1 and 6 Ma after CAI formation. The ⁵³Mn-⁵³Cr correlation among chondrites, planetesimals and terrestrial planets (the eucrite parent body, Mars and Earth) provides evidence for Mn/Cr fractionation within the protoplanetary disk recorded by all precursor materials of the terrestrial planets and primitive asteroids. This fractionation appears to have occurred within 2 Ma of CAI formation.     

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.