Isotopic properties of silicon carbide X grains from the Murchison meteorite in the size range 0.5‐1.5 μm

Abstract— We report isotopic abundances for C, N, Mg‐Al, Si, Ca‐Ti, and Fe in 99 presolar silicon carbide (SiC) grains of type X (84 grains from this work and 15 grains from previous studies) from the Murchison CM2 meteorite, ranging in size from 0.5 to 1.5 μm. Carbon was measured in 41 X grains, n in 37 grains, Mg‐Al in 18 grains, Si in 87 grains, Ca‐Ti in 25 grains, and Fe in 8 grains. These X grains have ¹²C/¹³C ratios between 18 and 6800, ¹⁴N/¹⁵n ratios from 13 to 200, δ²⁹Si/²⁸Si between ?750 and +60%₀, δ³⁰Si/²⁸Si from ?770 to ?10%₀, and ⁵⁴Fe/⁵⁶Fe ratios that are compatible with solar within the analytical uncertainties of several tens of percent. Many X grains carry large amounts of radiogenic ²⁶Mg (from the radioactive decay of ²⁶Al, half‐life ? 7 times 10⁵ years) and radiogenic ⁴⁴Ca (from the radioactive decay of ⁴⁴Ti, half‐life = 60 years). While all X grains but one have radiogenic ²⁶Mg, only ～20% of them have detectable amounts of radiogenic ⁴⁴Ca. Initial ²⁶Al/²⁷Al ratios of up to 0.36 and initial ⁴⁴Ti/⁴⁸Ti ratios of up to 0.56 can be inferred. The isotopic data are compared with those expected from the potential stellar sources of SiC dust. Carbon stars, Wolf‐Rayet stars, and novae are ruled out as stellar sources of the X grains. The isotopic compositions of C and Fe and abundances of extinct ⁴⁴Ti are well explained both by type Ia and type II supernova (SN) models. The same holds for ²⁶Al/²⁷Al ratios, except for the highest ²⁶Al/²⁷Al ratios of >0.2 in some X grains. Silicon agrees qualitatively with SN model predictions, but the observed ²⁹Si/³⁰Si ratios in the X grains are in most cases too high, pointing to deficiencies in the current understanding of the production of Si in SN environments. The measured ¹⁴n/¹⁵n ratios are lower than those expected from SN mixing models. This problem can be overcome in a 15 M_odot; type II SN if rotational mixing, preferential trapping of N, or both from ¹⁵n‐rich regions in the ejecta are considered. The isotopic characteristics of C, N, Si, and initial ²⁶Al/²⁷Al ratios in small X grains are remarkably similar to those of large X grains (2–10 μm). Titanium‐44 concentrations are generally much higher in smaller grains, indicative of the presence of Ti‐bearing subgrains that might have served as condensation nuclei for SiC. The fraction of X grains among presolar SiC is largely independent of grain size. This implies similar grain‐size distributions for SiC from carbon stars (mainstream grains) and supernovae (X grains), a surprising conclusion in view of the different conditions for dust formation in these two types of stellar sources.     

Coordinated EDX and micro‐Raman analysis of presolar silicon carbide: A novel,nondestructive method to identify rare subgroup SiC

We report the development of a novel method to nondestructively identify presolar silicon carbide (SiC) grains with high initial ²⁶Al/²⁷Al ratios (>0.01) and extreme ¹³C‐enrichments (¹²C/¹³C ≤ 10) by backscattered electron‐energy dispersive X‐ray (EDX) and micro‐Raman analyses. Our survey of a large number of presolar SiC demonstrates that (1) ~80% of core‐collapse supernova and putative nova SiC can be identified by quantitative EDX and Raman analyses with >70% confidence; (2) ~90% of presolar SiC are predominantly 3C‐SiC, as indicated by their Raman transverse optical (TO) peak position and width; (3) presolar 3C‐SiC with ¹²C/¹³C ≤ 10 show lower Raman TO phonon frequencies compared to mainstream 3C‐SiC. The downward shifted phonon frequencies of the ¹³C‐enriched SiC with concomitant peak broadening are a natural consequence of isotope substitution. ¹³C‐enriched SiC can therefore be identified by micro‐Raman analysis; (4) larger shifts in the Raman TO peak position and width indicate deviations from the ideal 3C structure, including rare polytypes. Coordinated transmission electron microscopy analysis of one X and one mainstream SiC grain found them to be of 6H and 15R polytypes, respectively; (5) our correlated Raman and NanoSIMS study of mainstream SiC shows that high nitrogen content is a dominant factor in causing mainstream SiC Raman peak broadening without significant peak shifts; and (6) we found that the SiC condensation conditions in different stellar sites are astonishingly similar, except for X grains, which often condensed more rapidly and at higher atmospheric densities and temperatures, resulting in a higher fraction of grains with much downward shifted and broadened Raman TO peaks.     

Isotopic compositions of different presolar silicon carbide size fractions from the Murchison meteorite

Abstract— We report measurements of isotopic ratios of C, N, Mg, Si, Ca, Ti, Cr, and Fe in bulk samples (aggregates of many grains) of up to seven different fractions of silicon carbide (SiC), ranging from 0.38 to 3.0μm in diameter, from the Murchison CM2 carbonaceous chondrite. Ratios of ¹²C/¹³C range from 37 to 42 and ¹⁴N/¹⁵N ratios from 370 to 520, within the range of single‐grain measurements on coarser samples and in agreement with an asymptotic giant branch (AGB) star origin of most of the grains. Variations among size fractions do not show any simple trend and can be explained by varying contamination with isotopically normal material. Silicon isotopic ratios vary only little and, with one exception, lie to the right of the singlegrain mainstream correlation line. This might indicate a higher percentage of the minor populations Y and Z among finer grain‐size fractions. All bulk samples have large ²⁶Mg excesses attributed to the presence of short‐lived ²⁶Al at the time of grain formation. Inferred ²⁶Al/²⁷Al ratios are much larger than those measured in single larger mainstream grains. This is probably because of the presence of SiC grains of type X; we obtain an estimate of 0.4 for their ²⁶Al/²⁷Al ratio. Our Ca‐isotopic measurements, the first made on presolar SiC grains, show excesses in ⁴²Ca and ⁴³Ca, which is in general agreement with theoretical expectations for AGB stars. Calcium‐44 excesses are much larger than expected and are probably because of X grains, which have high⁴⁴Ca excesses because of the decay of short‐lived ⁴⁴Ti produced in supernova explosions. We arrive at an estimate of 0.014 for the initial ⁴⁴Ti/⁴⁸Ti ratio of the X grains, within the range obtained from previous single X grain measurements. The Ti‐isotopic ratios of the bulk samples show a V‐shaped pattern with excesses of all isotopes relative to ⁴⁸Ti. Isotopes ⁴⁶Ti, ⁴⁷Ti, and ⁵⁰Ti show excesses relative to the correlation between Ti and Si ratios for single grains and are in general agreement with theoretical models of s‐process nucleosynthesis in AGB stars. In contrast, ⁴⁹Ti does not show any excess relative to the singlegrain data; it also fails to agree with theory, which predicts much larger excesses than observed. Measured ⁵³Cr/⁵²Cr and ⁵⁷Fe/⁵⁶Fe ratios are normal within errors. The first result is expected even for Cr in AGB star envelopes, but the second result suggests that most of the Fe analyzed originates from contamination. We have found no simple trends in isotopic composition with respect to grain size that can be interpreted in terms of nucleosynthetic origin, unlike the results for Kr, Xe, Ba, and Sr.     

Search for extinct aluminum‐26 and titanium‐44 in nanodiamonds from the Allende CV3 and Murchison CM2 meteorites

Abstract– We report Mg‐Al and Ca‐Ti isotopic data for meteoritic nanodiamonds separated from the Allende CV3 and Murchison CM2 meteorites. The goal of this study was to search for excesses in ²⁶Mg and ⁴⁴Ca, which can be attributed to the in situ decay of radioactive and now extinct ²⁶Al and ⁴⁴Ti, respectively. Previous work on presolar SiC and graphite had shown that ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios in presolar grains can be used to discriminate between different types of stellar sources. Aluminum and Ti concentrations are low in the meteoritic nanodiamonds of this study. Murchison nanodiamonds have higher Al and Ti concentrations than the Allende nanodiamonds. This can be attributed to contamination and the presence of presolar SiC in the Murchison nanodiamond samples. ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca ratios are close to normal in Allende nanodiamonds with upper limits on the initial ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios of approximately 1 × 10^?3. These ratios are factors of 10–1000 and, respectively, 1–1000 lower than those of presolar SiC and graphite grains from supernovae. The ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti data for nanodiamonds are compatible with an asymptotic giant branch star or solar system origin, but not with a supernova origin of a major fraction of meteoritic nanodiamonds. The latter possibility cannot be excluded, though, as the diamond separates may contain significant amounts of contaminating Al and Ti, which would lower the inferred ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios considerably.     

Sulfur in presolar silicon carbide grains from asymptotic giant branch stars

We studied 14 presolar SiC mainstream grains for C‐, Si‐, and S‐isotopic compositions and S elemental abundances. Ten grains have low levels of S contamination and CI chondrite‐normalized S/Si ratios between 2 × 10^?5 and 2 × 10^?4. All grains have S‐isotopic compositions compatible within 2σ of solar values. Their mean S isotope composition deviates from solar by at most a few percent, and is consistent with values observed for the carbon star IRC+10216, believed to be a representative source star of the grains, and the interstellar medium. The isotopic data are also consistent with stellar model predictions of low‐mass asymptotic giant branch (AGB) stars. In a δ³³S versus δ³⁴S plot the data fit along a line with a slope of 1.8 ± 0.7, suggesting imprints from galactic chemical evolution. The observed S abundances are lower than expected from equilibrium condensation of CaS in solid solution with SiC under pressure and temperature conditions inferred from the abundances of more refractory elements in SiC. Calcium to S abundance ratios are generally above unity, contrary to expectations for stoichiometric CaS solution in the grains, possibly due to condensation of CaC₂ into SiC. We observed a correlation between Mg and S abundances suggesting solid solution of MgS in SiC. The low abundances of S in mainstream grains support the view that the significantly higher abundances of excess ³²S found in some Type AB SiC grains are the result of in situ decay of radioactive ³²Si from born‐again AGB stars that condensed into AB grains.     

NanoSIMS studies of Ba isotopic compositions in single presolar silicon carbide grains from AGB stars and supernovae

Abstract— We have studied 74 single presolar silicon carbide grains with sizes between 0.2 and 2.6 μm from the Murchison and Murray meteorites for Ba isotopic compositions using NanoSIMS. We also analyzed 7 SiC particles either consisting of sub‐micron‐size SiC grains or representing a morphologically and isotopically distinct subgroup. Of the 55 (likely) mainstream grains, originating from asymptotic giant branch (AGB) stars, 32 had high enough Ba contents for isotopic analysis. For 26 of them, CsH_x interferences were either negligible or could be corrected with confidence. They exhibit typical s‐process Ba isotopic patterns with slightly higher than solar ¹³⁴Ba/¹³⁶Ba and lower than solar ^135,137,138Ba/¹³⁶Ba ratios. Results are generally well explained in the context of neutron capture nucleosynthesis in low mass (1–3 M_⊙) AGB stars and provide constraints on AGB models, by reducing the needed ¹³C spread from factor of ～20 down to 2. Out of the 19 supernova X grains, three had sufficient concentrations for isotopic analysis. They tend to exhibit higher than solar ¹³⁴Ba/¹³⁶Ba and ¹³⁸Ba/¹³⁶Ba ratios, close to solar ¹³⁷Ba/¹³⁶Ba, and ¹³⁵Ba/¹³⁶Ba lower than solar but higher than in mainstream grains. This signature could indicate a mixture of n‐burst type Ba with either “normal Ba” more s‐process‐rich than solar, or normal Ba plus weak s‐process Ba. In the n‐burst component Cs may have to be separated from Ba at ～10 years after the SN explosion. Depending on predictions for its composition, another possibility is early separation (at ～1 year) coupled with addition of some unfractionated n‐burst matter. Abundances of trace elements (Sr, Zr, Cs, La, and Ce) analyzed along with Ba signify that implantation may have been an important process for their introduction.     

Stardust in Antarctic micrometeorites

Abstract— We report the discovery of presolar silicate, oxide (hibonite), and (possibly) SiC grains in four Antarctic micrometeorites (AMMs). The oxygen isotopic compositions of the eighteen presolar silicate (and one oxide) grains found are similar those observed previously in primitive meteorites and interplanetary dust particles, and indicate origins in oxygen‐rich red giant or asymptotic giant branch stars, or in supernovae. Four grains with anomalous C isotopic compositions were also detected. ¹²C/¹³C as well as Si ratios are similar to those of mainstream SiC grains; the N isotopic composition of one grain is also consistent with a mainstream SiC classification. Presolar silicate grains were found in three of the seven AMMs studied, and are heterogeneously distributed within these micrometeorites. Fourteen of the 18 presolar silicate grains and 3 of the 4 C‐anomalous grains were found within one AMM, T98G8. Presolar silicate‐bearing micrometeorites contain crystalline silicates that give sharp X‐ray diffractions and do not contain magnesiowüstite, which forms mainly through the decomposition of phyllosilicates and carbonates. The occurrence of this mineral in AMMs without presolar silicates suggests that secondary parent body processes probably determine the presence or absence of presolar silicates in Antarctic micrometeorites.     

Origin of hibonite-pyroxene spherules found in carbonaceous chondrites

Abstract— We have studied both of the known glass-free, hibonite-pyroxene spherules: MYSM3, from Murray (CM2), and Y17–6, from Yamato 791717 (CO3). They consist of hibonite plates (～2 wt% TiO^tot₂) enclosed in Al-rich pyroxene that has such high amounts of CaTs (CaAl₂SiO₆) component, up to ～80 mol%, that it must have crystallized metastably. Within the pyroxene, abundances of MgO and SiO₂ are strongly correlated with each other and are anticorrelated with those of Al₂O₃, reflecting an anticorrelation between the diopside and CaTs components of the pyroxene. In contrast with previous results for Type B fassaite, however, we do not observe an anticorrelation between MgO and TiO^tot₂, possibly reflecting different relative distribution coefficients for Ti³⁺ and Ti⁴⁺ in the aluminous pyroxene of the spherules from those found for fassaite in Type B inclusions. Previously described hibonite-silicate spherules have ²⁶Mg deficits but the present samples do not. Furthermore, the pyroxene in Y17-6 has excess ²⁶Mg, while the hibonite it encloses does not, indicating that the two phases either had different initial ²⁶Al/²⁷Al ratios or different initial ²⁶Mg/²⁴Mg ratios. The Ti isotopic compositions of the present samples are highly unusual: δ⁵⁰Ti = 103.4 ± 5.2%o in MYSM3 and -61.4 ± 4.1%0 in Y17-6, which are among the largest ⁵⁰Ti anomalies reported for any refractory inclusion. The textures suggest that hibonite crystallized first; but based on the calculated bulk compositions of both spherules, it is not the liquidus phase in either sample, which suggests that the hibonite in both samples is relict. The presence of ragged hibonite grains in MYSM3 and rounded hibonite grains in Y17-6 and a lack of isotopic equilibrium between pyroxene and hibonite support this conclusion. The spherules crystallized from liquid droplets that probably formed as a result of the melting of solid precursor grains that included hibonite. The heating events were too short and/or not hot enough to melt all the hibonite. The droplets cooled quickly enough that CaTs-rich pyroxene crystallized instead of anorthite. Based on the observed differences in isotopic composition, it is unlikely that the precursors of the present samples formed in the same reservoir as each other or as the previously described hibonite-silicate spherules, providing further evidence of the isotopic heterogeneity of the early solar nebula.     

Trace-element concentrations in single circumstellar silicon carbide grains from the Murchison meteorite

Abstract— Concentrations of the trace elements Mg, Al, Ca, Ti, V, Fe, Sr, Y, Zr, Ba and Ce were determined by ion microprobe mass spectrometry in 60 individual silicon carbide (SiC) grains (in addition, Nb and Nd were determined in 20 of them), from separate KJH (size range 3.4–5.9 μm) of the Murchison carbonaceous meteorite, whose C-, N- and Si-isotopic compositions have been measured before (Hoppe et al., 1994) and provide evidence that these grains are of stellar origin. The selected SiC grains represent all previously recognized subgroups: mainstream (20 < ¹²C/¹³C < 120; 200 < ¹⁴N/¹⁵N; Si isotopes on slope 1.34 line), grains A (¹²C/¹³C < 3.5), grains B (3.5 < ¹²C/¹³C < 10), grains X (¹⁵N excesses, large ²⁸Si excesses) and grains Y (150 < ¹²C/¹³C < 260; Si isotopes on slope 0.35 line). Data on these grains are compared with measurements on fine-grained SiC fractions. Trace-element patterns reflect both the condensation behavior of individual elements and the source composition of the stellar atmospheres. A detailed discussion of the condensation of trace elements in SiC from C-rich stellar atmospheres is given in a companion paper by Lodders and Fegley (1995). Elements such as Mg, Al, Ca, Fe and Sr are depleted because their compounds are more volatile than SiC. Elements whose compounds are believed to be more refractory than SiC can also be depleted due to condensation and removal prior to SiC condensation. Among the refractory elements, however, the heavy elements from Y to Ce (and Nd) are systematically enriched relative to Ti and V, indicating enrichments by up to a factor of 14 of the s-process elements relative to elements lighter than Fe. Such enrichments are expected if N-type carbon stars (thermally pulsing AGB stars) are the main source of circumstellar SiC grains. Large grains are less enriched than small grains, possibly because they are from different AGB stars. The trace-element patterns of subgroups such as groups A and B and grains X can at least qualitatively be understood if grains A and B come from J-type carbon stars (known to be lacking in s-process enhancements shown by N-type carbon stars) or carbon stars that had not experienced much dredge-up of He-shell material and if grains X come from supernovae. However, a remaining puzzle is how stars become carbon stars without much accompanying dredge-up of s-process elements.     

Aluminum‐26 in calcium‐aluminum‐rich inclusions and chondrules from unequilibrated ordinary chondrites

Abstract— In order to investigate the distribution of ²⁶A1 in chondrites, we measured aluminum‐magnesium systematics in four calcium‐aluminum‐rich inclusions (CAIs) and eleven aluminum‐rich chondrules from unequilibrated ordinary chondrites (UOCs). All four CAIs were found to contain radiogenic ²⁶Mg (²⁶Mg*) from the decay of ²⁶A1. The inferred initial ²⁶Al/²⁷Al ratios for these objects ((²⁶Al/²⁷Al)₀ ? 5 × 10^?5) are indistinguishable from the (²⁶Al/²⁷Al)₀ ratios found in most CAIs from carbonaceous chondrites. These observations, together with the similarities in mineralogy and oxygen isotopic compositions of the two sets of CAIs, imply that CAIs in UOCs and carbonaceous chondrites formed by similar processes from similar (or the same) isotopic reservoirs, or perhaps in a single location in the solar system. We also found ²⁶Mg* in two of eleven aluminum‐rich chondrules. The (²⁶Al/²⁷Al)₀ ratio inferred for both of these chondrules is ～1 × 10^?5, clearly distinct from most CAIs but consistent with the values found in chondrules from type 3.0–3.1 UOCs and for aluminum‐rich chondrules from lightly metamorphosed carbonaceous chondrites (～0.5 × 10^?5 to ～2 × 10^?5). The consistency of the (²⁶Al/²⁷Al)₀ ratios for CAIs and chondrules in primitive chondrites, independent of meteorite class, implies broad‐scale nebular homogeneity with respect to ²⁶Al and indicates that the differences in initial ratios can be interpreted in terms of formation time. A timeline based on ²⁶Al indicates that chondrules began to form 1 to 2 Ma after most CAIs formed, that accretion of meteorite parent bodies was essentially complete by 4 Ma after CAIs, and that metamorphism was essentially over in type 4 chondrite parent bodies by 5 to 6 Ma after CAIs formed. Type 6 chondrites apparently did not cool until more than 7 Ma after CAIs formed. This timeline is consistent with ²⁶Al as a principal heat source for melting and metamorphism.     

Magnesium isotopic fractionation in chondrules from the Murchison and Murray CM2 carbonaceous chondrites

We present high‐precision measurements of the Mg isotopic compositions of a suite of types I and II chondrules separated from the Murchison and Murray CM2 carbonaceous chondrites. These chondrules are olivine‐ and pyroxene‐rich and have low ²⁷Al/²⁴Mg ratios (0.012–0.316). The Mg isotopic compositions of Murray chondrules are on average lighter (δ²⁶Mg ranging from ?0.95‰ to ?0.15‰ relative to the DSM‐3 standard) than those of Murchison (δ²⁶Mg ranging from ?1.27‰ to +0.77‰). Taken together, the CM2 chondrules exhibit a narrower range of Mg isotopic compositions than those from CV and CB chondrites studied previously. The least‐altered CM2 chondrules are on average lighter (average δ²⁶Mg = ?0.39 ± 0.30‰, 2SE) than the moderately to heavily altered CM2 chondrules (average δ²⁶Mg = ?0.11 ± 0.21‰, 2SE). The compositions of CM2 chondrules are consistent with isotopic fractionation toward heavy Mg being associated with the formation of secondary silicate phases on the CM2 parent body, but were also probably affected by volatilization and recondensation processes involved in their original formation. The low‐Al CM2 chondrules analyzed here do not exhibit any mass‐independent variations in ²⁶Mg from the decay of ²⁶Al, with the exception of two chondrules that show only small variations just outside of the analytical error. In the case of the chondrule with the highest Al/Mg ratio (a type IAB chondrule from Murchison), the lack of resolvable ²⁶Mg excess suggests that it either formed >1 Ma after calcium‐aluminum‐rich inclusions, or that its Al‐Mg isotope systematics were reset by secondary alteration processes on the CM2 chondrite parent body after the decay of ²⁶Al.     

Al‐Mg systematics of CAIs,POI, and ferromagnesian chondrules from Ningqiang

Abstract— We have made aluminum‐magnesium isotopic measurements on 4 melilite‐bearing calcium‐aluminum‐rich inclusions (CAIs), 1 plagioclase‐olivine inclusion (POI), and 2 ferromagnesian chondrules from the Ningqiang carbonaceous chondrite. All of the CAIs measured contain clear evidence for radiogenic ²⁶Mg* from the decay of ²⁶Al ( = 1.05 Ma). Although the low Al/Mg ratios of the melilites introduce large uncertainties, the inferred initial ²⁶Al/²⁷Al ratios for the CAIs are generally consistent with the value of 5 times 10^?5. There is clear evidence of ²⁶Al* in one POI and two chondrules, but with considerable uncertainties in the value of (²⁶Al/²⁷Al)₀. The (²⁶Al/²⁷Al)₀ ratios for the POI and the chondrules are 0.3–0.6 times 10^?5, roughly an order of magnitude lower than the canonical value. Ningqiang shows very little evidence of metamorphism as a bulk object and the (²⁶Al/²⁷Al)₀ ratios in its refractory inclusions and chondrules are consistent with those found in other unmetamorphosed chondrites of several different classes. Our observations and those of other workers support the view that ²⁶Al was widely and approximately homogeneously distributed throughout the condensed matter of the solar system. The difference in (²⁶Al/²⁷Al)₀ between CAIs and less refractory materials seems reasonably interpreted in terms of a ～2 million year delay between the formation of CAIs and the onset of formation of less refractory objects. The POI shows clear differences in ²⁵Mg/²⁴Mg between its constituent spinels and olivine, which confirms that they are partially reprocessed material from different sources that were rapidly quenched.     

Revisiting 26Al‐26Mg systematics of plagioclase in H4 chondrites

Zinner and Göpel ( 1992 , 2002 ) found clear evidence for the former presence of ²⁶Al in the H4 chondrites Ste. Marguerite and Forest Vale. They assumed that the ²⁶Al‐²⁶Mg systematics of these chondrites date “metamorphic cooling of the H4 parent body.” Plagioclase in these chondrites can have very high Al/Mg ratios and low Mg concentrations, making these ion probe analyses susceptible to ratio bias, which is inversely proportional to the number of counts of the denominator isotope (Ogliore et al. 2011 ). Zinner and Göpel ( 2002 ) used the mean of the ratios to calculate the isotope ratios, which exacerbates this problem. We analyzed the Al/Mg ratios and Mg isotopic compositions of plagioclase grains in thin sections of Ste. Marguerite, Forest Vale, Beaver Creek, and Sena to evaluate the possible influence of ratio bias on the published initial ²⁶Al/²⁷Al ratios for these meteorites. We calculated the isotope ratios using total counts, a less biased method of calculating isotope ratios. The results from our analyses are consistent with those from Zinner and Göpel ( 2002 ), indicating that ratio bias does not significantly affect ²⁶Al‐²⁶Mg results for plagioclase in these chondrites. Ste. Marguerite has a clear isochron with an initial ²⁶Al/²⁷Al ratio indicating that it cooled to below 450 °C 5.2 ± 0.2 Myr after CAIs. The isochrons for Forest Vale and Beaver Creek also show clear evidence that ²⁶Al was alive when they cooled, but the initial ²⁶Al/²⁷Al ratios are not well constrained. Sena does not show evidence that ²⁶Al was alive when it cooled to below the Al‐Mg closure temperature. Given that metallographic cooling rates for Ste. Marguerite, Forest Vale, and Beaver Creek are atypical (>5000 °C/Myr at 500 °C) compared with most H4s, including Sena, which have cooling rates of 10–50 °C/Myr at 500 °C (Scott et al. 2014 ), we conclude that the Al‐Mg systematics for Ste. Marguerite, Forest Vale, and Beaver Creek are the result of impact excavation of these chondrites and cooling at the surface of the parent body, instead of undisturbed cooling at depth in the H chondrite parent body, like many have assumed.     

Presolar grains from novae: their isotopic ratios and radioactivities

Five SiC and two graphite presolar grains exhibit isotopic ratios characteristic of ONe nova nucleosynthesis: low ¹²C/¹³C (4–9), low ¹⁴N/¹⁵N (5–20), high ²⁶Al/²⁷Al, high ³⁰Si/²⁸Si (2×solar) with close-to-normal ²⁹Si/²⁸Si. The upper limit of ²⁰Ne/²²Ne (<0.01) of one graphite grain suggests that the ²²Ne excess is due to the decay of ²²Na. In order to achieve the isotopic ratios of the grains, however, synthesized material during nova explosion had to be mixed with isotopically close-to-solar material, which should consist of more than 95% of the mix.     

The relative formation ages of ferromagnesian chondrules inferred from their initial aluminum‐26/aluminum‐27 ratios

Abstract— We performed a systematic high‐precision secondary ion mass spectrometry ²⁶Al‐²⁶Mg isotopic study for 11 ferromagnesian chondrules from the highly unequilibrated ordinary chondrite Bishunpur (LL3.1). The chondrules are porphyritic and contain various amounts of olivine and pyroxene and interstitial plagioclase and/or glass. The chemical compositions of the chondrules vary from FeO‐poor to FeO‐rich. Eight chondrules show resolvable ²⁶Mg excesses with a maximum δ²⁶Mg of ?1% in two chondrules. The initial ²⁶Al/²⁷Al ratios inferred for these chondrules range between (2.28 ± 0.73) × 10^?5 to (0.45 ± 0.21) × 10^?5. Assuming a homogeneous distribution of Al isotopes in the early solar system, this range corresponds to ages relative to CAIs between 0.7 ± 0.2 Ma and 2.4_+0.7^?0.4 Ma. The inferred total span of the chondrule formation ages is at least 1 Ma, which is too long to form chondrules by the X‐wind. The initial ²⁶Al/27Al ratios of the chondrules are found to correlate with the proportion of olivine to pyroxene suggesting that olivine‐rich chondrules formed earlier than pyroxene‐rich chondrules. Though we do not have a completely satisfactory explanation of this correlation we tentatively interpret it as a result of evaporative loss of Si from earlier generations of chondrules followed by addition of Si to the precursors of later generation chondrules.     

Oxygen isotope and 26Al‐26Mg systematics of aluminum‐rich chondrules from unequilibrated enstatite chondrites

Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for ²⁶Al‐²⁶Mg systematics, only one aluminum‐rich chondrule contains excess ²⁶Mg from the in situ decay of ²⁶Al; the inferred initial ratio (²⁶Al/²⁷Al)_o = (6.8 ± 2.4) × 10^?6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ?0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more ¹⁶O‐rich compositions with a range in δ¹⁸O of about ?12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess ²⁶Mg from the in situ decay of ²⁶Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of ²⁶Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that ²⁶Al was at least approximately homogeneously distributed across the chondrite‐forming region.     

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

Abstract— MacAlpine Hills (MAC) 87300 and 88107 are two unusual carbonaceous chondrites that are intermediate in chemical composition between the CO3 and CM2 meteorite groups. Calcium‐aluminum‐rich inclusions (CAIs) from these two meteorites are mostly spinel‐pyroxene and melilite‐rich (Type A) varieties. Spinel‐pyroxene inclusions have either a banded or nodular texture, with aluminous diopside rimming Fe‐poor spinel. Melilite‐rich inclusions (Åk_4–42) are irregular in shape and contain minor spinel (FeO <1 wt%), perovskite and, more rarely, hibonite. The CAIs in MAC 88107 and 87300 are similar in primary mineralogy to CAIs from low petrologic grade CO3 meteorites but differ in that they commonly contain phyllosilicates. The two meteorites also differ somewhat from each other: melilite is more abundant and slightly more Al‐rich in inclusions from MAC 88107 than in those from MAC 87300, and phyllosilicate is more abundant and Mg‐poor in MAC 87300 CAIs relative to that in MAC 88107. These differences suggest that the two meteorites are not paired. The CAI sizes and the abundance of melilite‐rich CAIs in MAC 88107 and 87300 suggests a genetic relationship to CO3 meteorites, but the CAIs in both have suffered a greater degree of aqueous alteration than is observed in CO meteorites. Aluminum‐rich melilite in CAIs from both meteorites generally contains excess ²⁶Mg, presumably from the in situ decay of ²⁶Al. Although well‐defined isochrons are not observed, the ²⁶Mg excesses are consistent with initial ²⁶Al/²⁷Al ratios of approximately 3–5 times 10^?5. An unusual hibonite‐bearing inclusion is isotopically heterogeneous, with two large and abutting hibonite crystals showing significant differences in their degrees of mass‐dependent fractionation of ²⁵Mg/²⁴Mg. The two crystals also show differences in their inferred initial ²⁶Al/²⁷Al ratios, 1 × 10^?5 vs. ≤3 × 10^?6.     

Presolar silicon carbide grains and their parent stars

Abstract— Carbon stars are an important source of presolar TiC, SiC, and graphite grains found in meteorites. The elemental abundances in the stellar sources of the SiC grains are inferred by using condensation calculations. These elemental abundances, together with C isotopic compositions, are used to identify possible groups of carbon stars that may have contributed SiC grains to the presolar dust cloud. The most likely parent stars of meteoritic SiC mainstream grains are N-type carbon stars and evolved subgiant CH stars. Both have s-process element abundances higher than solar and 10 < ¹²C/¹³C < 100 ratios. The J stars and giant CH stars, with solar and greater than solar abundances of s-process elements, respectively, are good candidate parents for the ‘A’ and ‘B’ SiC grains with low ¹²C/¹³C ratios. A special subgroup of CH giant stars with very large ¹²C/¹³C ratios could have parented the ‘Y’ SiC grains with ¹²C/¹³C ratios > 100. The carbon star population (e.g., N, R, J, CH groups) needed to provide the observed SiC grains is compared to the current population of carbon stars. This comparison suggests that low-metallicity CH stars may have been more abundant in the past (>4.5 Ga ago) than at present. This suggestion is also supported by condensation-chemistry modeling of the trace element patterns in the SiC grains that shows that subsolar Fe abundances may be required in the stellar sources for many SiC grains. The results of this study suggest that presolar SiC grains in meteorites can provide information about carbon stars during galactic evolution.     

Aluminum‐26 in H4 chondrites: Implications for its production and its usefulness as a fine‐scale chronometer for early solar system events

Abstract— In order to investigate whether or not ²⁶Al can be used as a fine‐scale chronometer for early solar system events we measured, with an ion microprobe, Mg isotopes and Al/Mg ratios in separated plagioclase, olivine, and pyroxene crystals from the H4 chondrites Ste Marguerite (SM), Forest Vale (FV), Beaver Creek and Quenggouk and compared the results with the canonical ²⁶Al/²⁷Al ratio for calcium‐aluminum‐rich inclusions (CAIs). For SM and FV, Pb/Pb and Mn‐Cr ages have previously been determined (Göpel et al., 1994; Polnau et al., 2000; Polnau and Lugmair, 2001). Plagioclase grains from these two meteorites show clear excesses of ²⁶Mg. The ²⁶Al/²⁷Al ratios inferred from these excesses and from isotopically normal Mg in pyroxene and olivine are (2.87 ± 0.64) × 10^?7 for SM and (1.52 ± 0.52) × 10^?7 for FV. The differences between these ratios and the ratio of 5 times 10^?5 in CAIs indicate time differences of 5.4 ± 0.1 Ma and 6.1 ± 0.2 Ma for SM and FV, respectively. These differences are in agreement with the absolute Pb/Pb ages for CAIs and SM and FV phosphates but there are large discrepancies between the U‐Pb and Mn‐Cr system for the relative ages for CAIs, SM and FV. For example, Mn‐Cr ages of carbonates from Kaidun are older than the Pb/Pb age of CAIs. However, even if we require that CAIs are older than these carbonates, the time difference between this “adjusted” CAI age and the Mn‐Cr ages of SM and FV require that ²⁶ Al was widely distributed in the early solar system at the time of CAI formation and was not mostly present in CAIs, a feature of the X‐wind model proposed by Shu and collaborators (Gounelle et al., 2001; Shu et al., 2001). From this we conclude that there was enough ²⁶Al to melt small planetary bodies as long as they formed within 2 Ma of CAIs, and that ²⁶Al can serve as a fine‐scale chronometer for early solar system events.     

Mg isotopic heterogeneity,Al‐Mg isochrons,and canonical 26Al/27Al in the early solar system

There is variability in the Mg isotopic composition that is a reflection of the widespread heterogeneity in the isotopic composition of the elements in the solar system at approximately 100 ppm. Measurements on a single calcium‐aluminum‐rich inclusion (CAI) gave a good correlation of ²⁶Mg/²⁴Mg with ²⁷Al/²⁴Mg, yielding an isochron corresponding to an initial (²⁶Al/²⁷Al)_o = (5.27 ± 0.18) × 10^?5 and an initial (²⁶Mg/²⁴Mg)_o = ?0.127 ± 0.032‰ relative to the standard. This isochron is parallel to that obtained by Jacobsen et al. (2008) , but is distinctively offset. This demonstrates that there are different initial Mg isotopic compositions in different samples with the same ²⁶Al/²⁷Al. No inference about uniformity/heterogeneity of ²⁶Al/²⁷Al on a macro scale can be based on the initial (²⁶Mg/²⁴Mg)_o values. Different values of ²⁶Al/²⁷Al for samples representing the same point in time would prove heterogeneity of ²⁶Al/²⁷Al. The important issue is whether the bulk solar inventory of ²⁶Al/²⁷Al was approximately 5 × 10^?5 at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with ²⁶Al/²⁷Al from approximately 5 × 10^?5 to below 0.2 × 10^?5. We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of ¹⁶O‐poor material; and the relationship to earlier formed ¹⁶O‐rich material in the disk. ²⁶Al‐free refractories can be produced at a later time by late infall, if there is an adequate heat source, or from original heterogeneities in the placental molecular cloud from which the solar system formed.