Distribution Of 26Al Research Articles

Al-Mg and U-Pb chronological records of Erg Chech 002 ungrouped achondrite meteorite

Achondrite meteorites are remnants of the earliest planetary differentiation processes in the Solar System. They have been used to anchor short-lived radiochronometers to absolute ages determined from long-lived radiochronometers. More specifically, when comparing the isotopic systematics of the short-lived 26Al-26Mg chronometer anchored to absolute U-corrected Pb-Pb ages, inferences about the distribution of 26Al (half-life of ∼717 000 yr) in the protoplanetary disk can be evaluated. The ungrouped achondrite Erg Chech (EC) 002 has a distinct mineralogy and more evolved elemental composition compared to basaltic achondrites. In situ and solution 26Al-26Mg chronometry and 53Mn-53Cr chronometry suggest that EC 002 formed within ∼0.7 to 2.2 Ma after the formation of Ca-Al-rich inclusions (CAIs), making it the oldest known sample of igneous crust in the Solar System (Barrat et al., 2021; Anand et al., 2022; Zhu et al., 2022; Fang et al., 2022). Here we present the U-corrected Pb-Pb age and 26Al-26Mg age obtained by MC-ICPMS solution analysis of the same mineral separate samples of EC 002. In addition, six merrillite grains were analyzed by in-situ SIMS to determine their Pb-Pb individual ages.The U isotope composition of EC 002 exhibits internal heterogeneities between leached pyroxene (238U/235U = 137.766 ± 0.027) and the bulk rock (238U/235U = 137.8190 ± 0.0074). The Pb isotope composition of progressively leached pyroxenes are characterized by radiogenic 206Pb/204Pb ratios (ranging from 41 to 23487). Using the U isotope composition of the leached pyroxenes, the resulting age of the 207Pb/206Pb-204Pb/206Pb isochron is 4565.87 ± 0.30 Ma (2σ). The weighted mean of the Pb-Pb ages of seven SIMS analyses of merrillites are 4564.3 ± 5.2 Ma (2σ). These similar ages (within uncertainty) indicate rapid cooling and the absence of significant thermal events after ∼4559 Ma on the parent body of EC 002. The 26Al-26Mg isochron through a bulk rock, pyroxene, fine-grained and four plagioclase fractions defines an initial 26Al/27Al ratio of [8.89 ± 0.79] × 10−6 corresponding to a formation age of 1.83 ± 0.12 Ma after CAIs ([5.23 ± 0.13] × 10−5; Jacobsen et al., 2008). The initial 26Al abundance is consistent with previous MC-ICP-MS 26Al-26Mg reported systematics for EC 002 (Fang et al., 2022), but 0.46 ± 0.13 Myr older than the in situ SIMS 26Al-26Mg age previously reported by Barrat et al. (2021).When anchored to the absolute Pb-Pb age of CV3 CAIs (4567.30 ± 0.16 Ma; Connelly et al., 2012), the Al-Mg model age of EC 002 is 4565.47 ± 0.20 Ma, slightly younger than its U-corrected Pb-Pb age.The concordance of the Pb-Pb and 26Al-26Mg ages of ungrouped CC achondrites when anchored to EC 002 suggest that 26Al was homogeneously distributed between the NC and CC reservoirs at the time of their parent body accretion. Furthermore, the presence of internal U isotope heterogeneities found between mineral and whole-rock samples of EC 002 supports the need of U isotope analysis of meteoritic samples dated using the Pb-Pb chronometer.

Distribution of 26Al in the CR chondrite chondrule-forming region of the protoplanetary disk

We report on the mineralogy, petrography, and in situ measured oxygen- and magnesium-isotope compositions of eight porphyritic chondrules (seven FeO-poor and one FeO-rich) from the Renazzo-like carbonaceous (CR) chondrites Graves Nunataks 95229, Grosvenor Mountains 03116, Pecora Escarpment 91082, and Queen Alexandra Range 99177, which experienced minor aqueous alteration and very mild thermal metamorphism. We find no evidence that these processes modified the oxygen- or Al–Mg isotope systematics of chondrules in these meteorites. Olivine, low-Ca pyroxene, and plagioclase within an individual chondrule have similar O-isotope compositions, suggesting crystallization from isotopically uniform melts. The only exceptions are relict grains in two of the chondrules; these grains are 16O-enriched relative to phenocrysts of the host chondrules. Only the FeO-rich chondrule shows a resolvable excesses of 26Mg, corresponding to an inferred initial 26Al/27Al ratio [(26Al/27Al)0] of (2.5±1.6)×10−6 (±2SE). Combining these results with the previously reported Al–Mg isotope systematics of CR chondrules (Nagashima et al., 2014, Geochem. J. 48, 561), 7 of 22 chondrules (32%) measured show resolvable excesses of 26Mg; the presence of excess 26Mg does not correlate with the FeO content of chondrule silicates. In contrast, virtually all chondrules in weakly metamorphosed (petrologic type 3.0–3.1) unequilibrated ordinary chondrites (UOCs), Ornans-like carbonaceous (CO) chondrites, and the ungrouped carbonaceous chondrite Acfer 094 show resolvable excesses of 26Mg. The inferred (26Al/27Al)0 in CR chondrules with resolvable excesses of 26Mg range from (1.0±0.4)×10−6 to (6.3±0.9)×10−6, which is typically lower than (26Al/27Al)0 in the majority of chondrules from UOCs, COs, and Acfer 094. Based on the inferred (26Al/27Al)0, three populations of CR chondrules are recognized; the population characterized by low (26Al/27Al)0 (<3×10−6) is dominant. There are no noticeable trends with major and minor element or O-isotope compositions between these populations. The weighted mean (26Al/27Al)0 of 22 CR chondrules measured is (1.8±0.3)×10−6. An apparent agreement between the 26Al–26Mg ages (using weighted mean value) and the revised (using 238U/235U ratio for bulk CR chondrites of 137.7789±0.0085) 207Pb–206Pb age of a set of chondrules from CR chondrites (Amelin et al., 2002, Science297, 1678) is consistent with the initial 26Al/27Al ratio in the CR chondrite chondrule-forming region at the canonical level (∼5.2×10−5), allowing the use of 26Al–26Mg systematics as a chronometer for CR chondrules. To prove chronological significance of 26Al for CR chondrules, measurements of Al–Mg and U–Pb isotope systematics on individual chondrules are required. The presence of several generations among CR chondrules indicates some chondrules that accreted into the CR chondrite parent asteroid avoided melting by later chondrule-forming events, suggesting chondrule-forming processes may have occurred on relatively limited spatial scales. Accretion of the CR chondrite parent body occurred at >4.0-0.3+0.5Ma after the formation of CAIs with the canonical 26Al/27Al ratio, although rapid accretion after formation of the major population of CR chondrules is not required by our data.

Gamma-ray telescopes

The last half-century has seen dramatic developments in γ−ray telescopes, from their initial conception and development through to their blossoming into full maturity as a potent research tool in astronomy. Gamma-ray telescopes are leading research in diverse areas such as γ−ray bursts, blazars, Galactic transients, and the Galactic distribution of 26Al.

Potassium, stardust, and the last supernova

No isotopic anomalies have yet been reported for K, but the relevant published literature is sparse and error limits for the scarce (0.01% abundance) isotope 40K are relatively large, 0.5 to 1%. We have developed thermal ionization mass spectrometric procedures by which error limits on 40K are an order of magnitude lower and applied them to analysis of a series of sequential dissolution fractions of the carbonaceous chondrites Orgueil (CI) and Murchison (CM), a sampling procedure known to reveal pervasive isotopic anomalies in Cr. Most of the fractions analyzed have 40K abundances that are normal (i.e., consistent with terrestrial composition) within analytical error limits. Whole–rock 40K abundances of Orgueil and Murchison are normal within about 1 permil or less. However, some dissolution fractions do exhibit evident isotopic anomalies, excesses of 40K up to about 35 ϵ. For K, as for Cr, the most plausible interpretation is that the anomalies reflect the presence of presolar grains that have not been thoroughly mixed with other solar system materials. In detail, the K anomalies do not correlate with the Cr anomalies, and thus probably represent different mineral carriers. Neither carrier phase is yet identified, but they differ from known and well-studied forms of presolar grains in that they are not acid-resistant. The isotopes of K are likely co-synthesized with some short-lived radionuclides, notably 26Al, the presence of which in the early solar system demands a “late” nucleosynthetic injection into the interstellar molecular cloud from which the solar system formed, no more than about 1 Ma before its collapse. It has been suggested that the distribution of 26Al (and other short-lived radionuclides) in early solar system materials was radically heterogeneous, perhaps because of the late injection. Because of its relatively short half-life (1.25 Ga), not in the “extinct radionuclides” range but still short compared to the age of the galaxy, 40K provides a usefully sensitive measure of the distribution of late nucleosynthetic additions to the solar system or its antecedent cloud. As a specific quantitative illustration, if a model 25-solar-mass supernova is invoked to account for observed levels of 26Al it will also provide about 1% of nebular 39K and about 3% of nebular 40K; the difference in the proportions of the K isotopes simply reflects the circumstance that by the time of solar system formation most of the 40K ever added to the sun’s precursor materials over the history of the galaxy had already decayed. There would be a 24‰ 40K anomaly between materials that did or did not incorporate such a contribution. The absence of so large a difference between the earth and the carbonaceous chondrites implies that they incorporate nearly the same amounts of any freshly synthesized K component of this magnitude. Unless some efficient mechanism for nebular scale chemical separation is postulated, the same should be true for co-synthesized nuclides such as 26Al.

Injection of Radioactive Nuclides from the Stellar Source that Triggered the Collapse of the Presolar Nebula

We examine the gravitational capture of supersonic gas and dust as it impacts and triggers the collapse of a molecular cloud core. We use two techniques to track the triggering material in two dimensions, a set of tracer particles and a color field, much like a dye, computed in the same manner as the hydrodynamic density variable. The two tracking techniques produce very similar results. We find that about 10% to 20% of the supersonic material with an initial impact parameter less than the molecular cloud core's initial radius is captured by the collapsing cloud. This fraction is less than the 100% capture estimate often used to constrain the distance to possible stellar sources of radioactive isotopes, and hence may require these stars to be closer than would otherwise be the case. Rayleigh-Taylor instabilities occur and aid in the mixing of the shock material with the target cloud. The impacting material is injected into the outer layers of the collapsing protostar roughly one free-fall time (2 × 105 yr) after the first contact of the triggering material with the cloud, and injection continues for approximately two more free-fall times. These time intervals are substantially less than the mean life of one of the radioactive nuclides of interest, 1.1 × 106 yr for 26Al, and are comparable to the mean life (1.5 × 105 yr) of another short-lived nuclide,41Ca. Evidence for live 26Al and 41Ca in the early solar system is thus consistent with a scenario involving supersonic triggering and injection of freshly synthesized radioactive nuclides into the presolar cloud. Because injection proceeds at a steady pace, it does not appear to be a significant source of temporal heterogeneity in the distribution of 26Al, though the outer layers of the presolar cloud are preferentially enriched in the injected material.

AMS measurements to study uptake and distribution of 26Al in mice and the role of the transferrin receptor in aluminium absorption mechanisms

We report the use of a 20 MV tandem Van de Graaff accelerator and accelerator mass spectrometry (AMS) to measure the distribution and uptake mechanisms of 26Al in vivo in mice. The influence of antibodies against the transferrin receptor and of hypotransferrinaemia (mice with virtual absence of circulating transferrin) were studied with regard to the uptake of 26Al from infusion with aluminium citrate. In both hypotransferrinaemic and antibody-treated mice, uptake of 26Al was similar to control values in all tissues except spleen and muscle for the antibody-treated mice and bone for the hypotransferrinaemic mice. It appears that non-transferrin mediated transport of aluminium can occur into tissues and that transport of the metal bound to citrate and other low molecular weight complexes may be important. Generally, the uptake of aluminium into control tissue follows the order bone (femur) > renal cortex > skeletal muscle ≈ liver > spleen > brain. Comparisons with the uptake and distribution of the radiotracer analogue of aluminium, (Ga-67) were made.

26Al as a heat source for early melting of planetesimals: Results from isotopic studies of meteorites

Ion microprobe studies of magnesium isotopic composition in igneous components from several chondritic meteorites have been carried out to look for26Mg excess that may be attributed to the presence of the now-extinct radionuclide26Al(τ ∼ 1 Ma) at the time of formation of these objects. A positive evidence for the presence of26Al in the analysed objects will strengthen its case as the primary heat source for the early thermal metamorphism/melting of meteorite parent bodies. Based on calculated temperature profiles inside chondritic objects of different sizes and initial26Al/27Al ratios, we have estimated the initial abundances of26Al needed to provide the heat necessary for the wide range of thermal processing seen in various types of meteorites. The magnesium isotopic data obtained by us do not provide definitive evidence for the presence of26Al at the time of formation of the analysed igneous phases in different chondritic meteorites. Experimental evidence for a planetary scale distribution of26Al in the early solar system to serve as a significant heat source for the thermal metamorphism and melting of meteorite parent bodies (planetesimals) remains elusive.

Isotopic analysis of early solar system objects by an ion microprobe: Parametric studies and initial results

Magnesium, potassium and calcium isotope compositions in terrestrial samples and refractory phases from primitive meteorites are determined using an ion microprobe. A thorough investigation of the different instrument parameters is carried out to ensure that conditions necessary for high mass resolution and high precision isotopic studies are adequately satisfied. The instrument can be tuned to achieve mass resolution (M/ΔM) of up to 10,000 (M≤60); it has a very good dynamic stability (ΔB/B≤10 ppm over durations of ≤40 minutes) and the counting system has an effective dead-time of ≤25 nsec and a dynamic background of ≤0·01 c/s. Reproducibility and precision of isotopic measurements are checked by analyzing magnesium and titanium isotopic compositions in terrestrial standards and isotopically doped silicate glasses. A precision of 2‰ (2σ m ) was achieved during magnesium isotopic analysis in samples with low Mg content (200 ppm). Results from studies of magnesium and potassium isotopic compositions in several Ca−Al-rich refractory inclusions (CAIs) from the primitive meteorites Efremovka and Grosnaja, representing some of the early solar system objects, are presented. The well-behaved Mg−Al isotopic systematics confirm the pristine nature of the Efremovka CAIs inferred earlier from petrographic and trace element studies. The Grosnaja CAIs that have experienced secondary alterations show disturbed magnesium isotopic systematics. Observation of excess26Mg in several of the analyzed CAIs confirms the presence of the now extinct26Al (t 1/2=7×105 years) in the solar nebula at the time of CAI formation. Our data also suggest a relatively uniform distribution of26Al in the solar nebula. Several Efremovka CAIs with excess26Mg also have excess41K resulting from the decay of41Ca (t 1/2≃105 years). This observation constrains the time interval between cessation of nucleosynthetic input to the solar nebula and the formation of some of the first solar system solids (CAIs) to less than a million years.