Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived 10Be and for the possible presence of the short-lived nuclide 7Be in the early solar system
Introduction
The discovery in Ca-, Al-rich inclusions (CAIs) from carbonaceous chondrites of isotopic anomalies in boron due to the in situ decay of short-lived (T1/2 = 1.5 Ma) 10Be (MacPherson et al., 2003, McKeegan et al., 2000, Sugiura et al., 2001) has renewed an old idea according to which a fraction of primitive solar system materials underwent irradiation by energetic particles from the young Sun. Independently, X-ray observations of low-mass stars in pre-T-Tauri and T-Tauri phases have suggested that these young stellar objects are a source of an intense flux of accelerated particles (Feigelson and Montmerle, 1999). Recent X-ray observations in the Orion nebular cluster of 43 young stellar objects with ages from <0.3 to ≈10 Ma and masses ranging from 0.7 to 1.4 M⊙ demonstrate that essentially all solar-type stars go through an epoch of greatly enhanced X-ray luminosity (Feigelson et al., 2002). Scaling of X-ray luminosity leads to estimates of enhancement factors of ∼105 for the proton flux relative to that of the present day Sun, and the population distribution suggests that this high activity lasts for periods of up to a few million years (Feigelson et al., 2002). From the astronomical observations, it seems inevitable that the Sun would have passed through such a phase during its formation and early evolution. Whether or not the components of primitive meteorites witnessed this epoch is a question that, in principle, can be addressed by examining the isotopic records of the earliest solar system rocks.
Models developed to describe the magnetic interaction between a forming star and its accretion disk (Shu et al., 1994) predict the existence close to the star of a gas-free region in which dust grains might be irradiated by energetic particles associated with magnetic reconnection events. Because of its proximity to the accreting star (as close as ∼0.06 AU), the dust is severely thermally processed, undergoing a series of evaporation/recondensation events and possibly melting. Surviving distillates or recondensed solids can migrate back to the so-called X region (at the termination of the accretion disk) through fluctuations in the accretion rate and/or magnetic field strength of the accreting star (Gounelle et al., 2001, Lee et al., 1998, Shu et al., 1996, Shu et al., 1997). These irradiated and thermally processed solids can then be launched by a magnetocentrifugal “x-wind” and fall back on the mid plane of the accretion disk at distances of several AU where these refractory solids may agglomerate with thermally unprocessed materials into the parent bodies of primitive meteorites. The presence in CAIs of specific isotopic anomalies due to the irradiation process would be a strong argument in favor of a scenario of the x-wind type for the evolution of early solar system (Lee et al., 1998).
In principle, the isotopic compositions of Li, Be, and B in CAIs can provide significant constraints on the existence of such irradiation processes in the early solar system. Because these elements are destroyed in stellar interiors and are produced primarily by spallation reactions on carbon and oxygen target nuclei, they have relatively low abundances in the solar system (Anders and Grevesse, 1989). Furthermore, the isotopic compositions produced by spallation reactions are energy dependent. For example, the 7Li/6Li ratio produced by high energy (GeV range) galactic cosmic ray (GCR) spallation of O and C nuclei is ≈2 (Meneguzzi et al., 1971), which is much less than the average chondritic ratio of 12.02 (James and Palmer, 2000, McDonough et al., 2003). Calculations indicate that for a range of realistic compositions and energetic spectra the spallation-produced 7Li/6Li ratios are always much lower than 12 (Ramaty et al., 1996), which means that the chondritic ratio must be due to mixing between 7Li produced during the Big-Bang, and 7Li-poor Li produced by spallation reactions at various energies during the evolution of the galaxy (Reeves, 1994). The situation is less clear for B because it is produced during low-energy (MeV range) spallation reactions with various 11B/10B ratios ranging from ∼2.5 to 7 (Cassé et al., 1995, Ramaty et al., 1996) which bracket the chondritic value of 11B/10B ≈ 4.04 (Zhai et al., 1996). In general, the low abundances and the strong isotopic contrasts of various spallation sources of Li and B make the isotopic composition of these elements very sensitive to small additions of a specific irradiation-produced component. Finally, two short-lived radioactive isotopes of Be exist: 10Be β-decays to 10B with a half-life of 1.5 My and 7Be decays to 7Li by electron capture with a half-life of 53 days.
The rather systematic incorporation by CAIs of 10Be during their formation (MacPherson et al., 2003, McKeegan et al., 2000, Sugiura et al., 2001) was originally taken as a decisive argument for the presence of irradiation processes in the early solar system (McKeegan et al., 2000). This view was recently challenged (Desch et al., 2004) based on calculations of the concentrations of 10Be that might be achieved by magnetic trapping of GCRs in the molecular cloud parental to the solar system. Although the calculations do not prove that the 10Be observed in CAIs could not have been produced by irradiation around the early Sun, they nevertheless raise the possibility that a significant fraction of it is of pre-solar origin, which is conceivable because of the relatively long half-life of 10Be. Obviously, a definitive argument in favor of a production of Be by solar system irradiation processes would be the incorporation in CAIs of 7Be since its very short half-life precludes any pre-solar heritage. Previous searches for 7Be in CAIs (Chaussidon et al., 2001, Chaussidon et al., 2002) have shown that large 7Li/6Li isotopic variations are present but no unambiguous “isochron-type” relationship could be demonstrated between the 7Li/6Li and the 9Be/6Li ratios. The Li isotopic variations in one Allende CAI (USNM 3515) were tentatively explained by a diffusive relaxation during cooling of Li isotopic anomalies due to the in situ decay of 7Be, but no petrographic evidence could be found to support (or deny) this interpretation (Paque et al., 2003). Another problem with the study of USNM 3515 was that this inclusion showed anomalously high B concentrations (in the ppm range) precluding the detection of 10Be in it.
In the present study, we report detailed analyses of the Li–Be–B isotopic compositions of another Allende CAI (3529-41), which is known to have contained 10Be (McKeegan et al., 2000) at close to the highest level found so far in any CAI. The petrology of 3529-41 was previously studied by Podosek et al. (1991) who also demonstrated that it contained 26Al at close to the canonical value at the time of its formation (Podosek et al., 1991). Our data show that correlations exist between CAI mineralogy and Li–Be concentrations and isotopic compositions suggesting that, when it last crystallized, CAI 3529-41 incorporated live 7Be into its mineral structures.
Section snippets
Allende CAI 3529-41
Allende 3529-41 was previously studied for its petrology (major and trace elements) as well as for its Mg and Sr isotopic compositions by Podosek et al. (1991). It is a coarse-grained type B1 CAI according to the definitions of MacPherson et al. (1988) and of Wark and Lovering (1982) containing a melilite-rich mantle surrounding a fassaite + spinel-rich core. In the studied section, the CAI appears irregular in shape with a maximum size of ≈6 mm (Fig. 1). The inclusion is surrounded by a
Li–Be–B distributions in Allende CAI 3529-41
The absolute Li–Be–B concentrations ([Li], [Be], and [B], respectively), as well as those of Be/Li and Be/B are highly variable on small spatial scales in 3529-41 (Table 3A, Table 3B, Table 4A, Table 4B). The 9Be/6Li ratios range from 0.6 to 3337 in melilite, from 11.8 to 437.1 in fassaite, and from 6.4 to 942.9 in anorthite. The 9Be/11B ratios range from 0.1 to 197.3 in melilite, from 0.06 to 3.4 in fassaite, and from 0.005 to 17.2 in anorthite. Clearly the largest fractionation of B relative
Processes controlling the Li–Be–B distributions in Allende CAI 3529-41
The major difficulty in searching for traces of the in situ decay of radioactive 7Be and 10Be in CAIs, apart from the generally low Li–Be–B concentrations, is that the “pristine” Li–Be–B distributions and/or Li and B isotopic compositions may have been disturbed locally by secondary processes. That such processes have occurred generally in Allende CAIs and in 3529-41 in particular is demonstrated convincingly by the numerous disturbances of the 26Al/26Mg system, especially in melilite,
Conclusions
Large variations in the concentrations and isotopic compositions of lithium and boron are found in Allende CAI 3529-41. Isotopic compositions, corrected for recent exposure to GCR, range from 7Li/6Li = 9.2 ± 0.22 to 13.44 ± 0.56 and 10B/11B = 0.2468 ± 0.0057 to 0.4189 ± 0.0493. 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. Disturbances in the
Uncited reference
Hinton (1990).
Acknowledgements
We thank Guy Libourel, Matthieu Gounelle, Jean Duprat, and Ingo Leya for stimulating discussions and thoughtful advice, and Shogo Tachibana and two anonymous reviewers for their comments and suggestions. Allende CAI 3529-41 was kindly made available by Glenn MacPherson, curator of meteorites at the Smithsonian Institution. This work was supported by PNP-INSU grants and Région Lorraine grants to Marc Chaussidon; Kevin McKeegan gratefully acknowledges support from the NASA Cosmochemistry Program.
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