A New Interpretation of [TSUP]26[/TSUP]Al in Meteoritic Inclusions

Abstract

We suggest that the large 26 Al/ 27 Al ϭ 5 ϫ 10 Ϫ5 abundance ratio found in calcium-aluminum-rich inclusions (CAIs) in meteorites is produced by energetic particle irradiation in the early solar system but only in a thin (0.2 g cm Ϫ2 ) skin of the solar preplanetary disk that stops the energetic particles. Buildup of that 26 Al concentration happens only during the quiescent, or passive, phase of the solar disk, after accretion and associated turbulence has ceased. We propose that CAIs also originate in the form of fine Al-rich dust within a coronal-type environment atop the disk. In this model only the CAIs among planetary materials would contain this high 26 Al/ 27 Al ratio. Chronological ordering based on 26 Al content within planetary materials would be invalid in this model, which would allow chondrules of ordinary chondrites to be as old as CAIs despite their lack of 26 Al. This temporal reordering could resolve a growing crisis in planetary disk physics. We also outline other problems in planetary history that will be alleviated or reinterpreted if our model is correct. We describe four sources for the energetic particles, noting that for each the power requirements are reduced to credible values by the smallness of the irradiated mass, about 10 Ϫ5 M J of disk matter.

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

“…A more interesting class of heterogeneous-nebula models involves local production of 26A1 through local irradiation of solar system materials (e.g., Heymann and Dziczkaniec, 1976;Clayton and Jin, 1995;Lee et al, 1998). The biggest difficulty faced by these models is the fact that irradiation of average solar system matter by cosmic rays of typical energy and composition sufficient to produce CAI-levels of (26Al/27Al)o also produces other nuclides besides 26AI (e.g., Clayton et al, 1977), but not in their proper relative abundances to 26A1.…”

“…The biggest difficulty faced by these models is the fact that irradiation of average solar system matter by cosmic rays of typical energy and composition sufficient to produce CAI-levels of (26Al/27Al)o also produces other nuclides besides 26AI (e.g., Clayton et al, 1977), but not in their proper relative abundances to 26A1. To overcome this difficulty, modelers have proposed irradiation of a limited region of the nebula, irradiation of chemically fractionated material, and/or irradiation with cosmic rays of restricted energy distribution and composition (e.g., Clayton and Jin, 1995;Lee et al, 1998;Gounelle et al, 2001). The most recent local-production models couple irradiation near the Sun to produce 26A1 with an X-wind model (e.g., Shu et al, 1996) to transport CAIs and other 26Al-bearing objects out to the meteorite parent-body accretion regions (e.g., Lee et al, 1998;McKeegan et al, 1998).…”

Manganese‐chromium formation intervals for chondrules from the Bishunpur and Chainpur meteorites

“…But, "...lacking definitive constraints on the formation locales of CAIs and chondrules or, more fundamentally, a demonstration that 26APAl = 4.5 x 10-5 ever existed in those regions where chondrules formed, it will be difficult to utilize relative 26A1 abundances for chronological purposes." In a similar vein, Shu et al (1996) cautioned that 26A1 may not be the perfect carrier of time information because rather than providing clocks of external seeding, 26A1 may be produced by cosmic-ray reactions inside the early solar system (cf., Clayton and Jin, 1995). In such a case, the zero of time for the 26A1 clock may differ for CAIs and ferromagnesian chondrules depending on when 26A1 was isolated from further production.…”

A stellar origin for the short-lived nuclides in the early Solar System

“…The nuclides could have originated from a single stellar object 2-6 , such as a nearby red-giant or a supernova. But observations of enhanced ion fluxes in a molecular cloud 7 have led to other models [8][9][10] in which these nuclides are formed by energetic particle irradiation of gas and dust in the protosolar molecular cloud; alternatively, irradiation by energetic particles from the active early Sun may have occurred within the solar nebula itself [11][12][13][14][15][16][17][18] . Here we show that there is a correlation between the initial abundances of 41 Ca and 26 Al in samples of primitive meteorite (as inferred from their respective decay products, 41 K and 26 Mg), implying a common origin for the short-lived nuclides.…”