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Irradiation Processes in the Early Solar System 323 Chaussidon and Gounelle: Irradiation Processes in the Early Solar System 323 Irradiation Processes in the Early Solar System Marc Chaussidon Centre de Recherches Pétrographiques et Géochimiques Matthieu Gounelle Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse–Université Paris XI During recent years, a growing body of evidence, both astrophysical observations of young stellar objects and cosmochemical observations made on meteorites, has strengthened the case that a fraction of the dust and gas of the solar accretion disk has been irradiated by cosmic rays emitted by the young forming active Sun. We review this meteoritic evidence, specifically the most convincing data obtained on Ca-Al-rich inclusions (CAIs) of primitive chondrite me- teorites. Boron-isotopic variations in CAIs can be explained by decay products of short-lived 10 7 Be (T1/2 = 1.5 m.y.) while a combination of extinct Be (T1/2 = 53 d) and of spallogenic Li may explain 7Li/6Li variations. We then discuss irradiation models that attempt to account for the origin of short-lived radionuclides and evaluate the predictions of these models in light of experimental data. Several anomalies, such as 21Ne and 36Ar excesses, suggest an exposure of CAIs and of chondrules (or their precusors) to cosmic rays emitted by the young Sun. The possible effects of early solar irradiation processes on the mineralogy, chemistry, and light- element isotopic compositions (e.g., O, H, N) of early solar system solids are also considered and evaluated. 1. INTRODUCTION of interstellar graphite grains, which would be incorporated in the presolar nebula (Dwek, 1978a). It has long been recognized that, because light elements It is important to point out here that, historically, the (D, Li, Be, and B) could not be synthesized by thermonu- situation with short-lived 26Al is quite similar to that of Li- clear reactions taking place at temperatures higher than Be-B. In fact, two possible sources were proposed for 26Al ≈106 K in stellar interiors, other nucleosynthetic processes when it was discovered in CAIs: either an external seed- had to be invoked to explain their presence in the galaxy ing of the solar system by fresh supernova products or a and the solar system (Burbidge et al., 1957). This is espe- local production within the solar system by irradiation pro- cially true for 6Li, B, and Be since 7Li can also be produced cesses around the young active Sun (Lee et al., 1976). The by Big Bang nucleosynthesis and during stellar evolution idea of an internal source by irradiation was nearly aban- either in the red giant phase or in the H-explosive-burning doned for many years, but the original calculations of the phase of nova (e.g., Audouze and Vauclair, 1980). Among fluence required to produce the amount of 26Al observed the different mechanisms studied for the production of Li- in CAIs yielded values (≈1020 protons cm–2) of the same Be-B elements, it was suggested that endothermic spalla- order of magnitude as those calculated in the latest irradia- tion reactions that occur at high energies (typically higher tion models. than a few MeV) can produce these elements in significant This old idea that a fraction of solar system material was amounts. irradiated by solar cosmic rays (SCRs) around the young From the observation that concentrations of Li-Be-B ele- active Sun was recently renewed from two different lines ments were higher by a factor of ≈106 in galactic cosmic of evidence. First, observations of solar-type stars during rays (GCRs) relative to solar system abundances, Reeves et their pre-main-sequence phases (protostars, “classical” al. (1970) found that these elements were synthesized by T Tauri stars with accretion disks, and “weak-lined” T Tauri spallation reactions between high-energy GCR particles stars without accretion disks) show powerful X-ray flares (mostly protons and α particles) and the heavy nuclei 12C that are very likely accompanied by intense fluxes of ac- and 16O. However, before this understanding of the nucleo- celerated particles (Feigelson and Montmerle, 1999; Gudel, synthetic origin of Li-Be-B elements was reached, it was 2004). The X-ray flares are seen in virtually all young stars, originally proposed that these elements were produced in while radio gyrosynchrotron emission from flare MeV elec- the early solar system by irradiation of meter-sized plan- trons are detected in some cases. Recent studies of the pre- etesimals with high-energy solar particles (Fowler et al., main-sequence population in the Orion nebula have shown 1962; Burnett et al., 1965). Other scenarios were envisaged, that analogs of the young Sun exhibit X-ray flare enhance- such as the production of Li-Be-B elements by irradiation ments by factors of 104 and inferred proton fluence en- 323 324 Meteorites and the Early Solar System II hancements by factors of 105 (Feigelson et al., 2002; Wolk bulk carbonaceous chondrites (James and Palmer, 2000; et al., 2005). X-ray absorption and Fe fluorescent emission McDonough et al., 2003). However, the isotopic composi- line measurements give direct evidence that X-rays effi- tions of Li-Be-B elements in components of chondrites ciently irradiate the protoplanetary disks (Tsujimoto et al., (CAIs, refractory grains, chondrules, etc.) are anticipated 2005; Kastner et al., 2005). Second, the discovery of short- to be much more variable for several reasons. These varia- lived 10Be in CAIs from primitive chondrites (McKeegan tions, if present, might be a very rich source of informa- et al., 2000; Sugiura et al., 2001; MacPherson et al., 2003) tion concerning irradiation processes in the presolar or was taken as a strong argument in favor of the presence of protosolar nebula. irradiation products among the precursors of CAIs. Al- First, Li-Be-B elements are efficiently produced by spal- though this view was recently challenged from a calcula- lation reactions during collisions between accelerated pro- tion of the amount of 10Be trapped during stopping of GCRs tons and C and O nuclei (e.g., Reeves, 1994). Because the in the molecular cloud core, parent of the solar system solar system abundance of Li-Be-B elements (Anders and (Desch et al., 2004), the presence of radioactive 7Be in one Grevesse, 1989) is lower by 5 to 7 orders of magnitude Allende CAI (Chaussidon et al., 2004, 2005) would be a relative to that of the target elements C and O (which have decisive argument for an irradiation of a fraction of the solar a different nucleosynthetic origin than Li-Be-B), even a nebula. However, the evidence for 7Be needs to be secured small addition of a spallation component to a solar system from other CAIs in the future. Li-Be-B abundance might be detectable. Second, the spal- In the following sections, we review the few traces of lation Li and B components have a priori isotopic compo- irradiation processes that have been reported so far in me- sitions that will be very different from bulk chondrites. This teorites. This includes either interactions with accelerated comes from the fact that the cross sections for the produc- protons or α particles that result in spallation reactions or tion of the different Li and B isotopes vary as a function of interactions with X-rays or UV radiation. The strongest the energy: Ramaty et al. (1996) calculated 7Li/6Li pro- evidence that we discuss in more detail is the variation in duction ratios ranging from ≈1.5 to ≈8 and 11B/10B ratios the Li- and B-isotopic compositions in CAIs and the in- ranging from ≈2.5 to ≈8 for spallation reactions at energies ferred presence in the early solar system of short-lived 7Be between a few and 100 MeV/nucleon using variable com- and 10Be. In addition, we discuss more briefly (1) the case positions for the cosmic ray and the target at rest (spalla- of 21Ne and 36Ar excesses reported in gas-rich meteorites tion 7Li/6Li and 11B/10B ratios are the lowest at high energies and in chondrules, (2) the recent claims for UV irradiation >100 MeV). The difference between the chondritic and the as a source of non-mass-dependent O-isotopic anomalies spallation isotopic ratio is large for Li because solar sys- and of 15N enrichment in the early solar system, and (3) the tem Li also contains 7Li produced during Big Bang nucleo- chemical and mineralogical traces of electron and ion irra- synthesis. Thus, in any case the spallogenic 7Li/6Li will be diation in irradiated silicates. We also mention the possible significantly lower than the chondritic 7Li/6Li of ≈12. Third, effects of X-rays as a source of ionization that can promote two radioactive isotopes of Be having very different half- ion-molecule reactions responsible for D- and N-isotopic lives exist: 7Be (half-life of 53 d) decays by electron cap- variations. The existing models of irradiation are then de- ture to 7Li and 10Be (half-life of 1.5 m.y.) decays by β decay scribed and their contrasted predictions evaluated concern- to 10B. These two isotopes are produced efficiently by spal- ing the production of the different short-lived radioactivities lation reactions: the cross sections for the production by observed in CAIs (7Be, 10Be, 26Al, 41Ca, and 53Mn). collisions between accelerated protons and O nuclei are, for instance, at 650 MeV of ≈11 mb, ≈3 mb, and ≈1.5 mb for 2. TRACES OF EARLY IRRADIATION 7Be, 9Be, and 10Be, respectively (Read and Viola, 1984; PROCESSES IN METEORITES Michel et al., 1997; Sisterson et al., 1997). 2.1.2. Evidence for the incorporation of short-lived be- 2.1.
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