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Session 49 - Galactic Radioactivity, 26A1 & Solar System Origin.
Oral session, Wednesday, June 11
North Main Hall F/G,

[49.04] 26Al in the early Solar System: Evidence from Meteorites

G. J. MacPherson (Smithsonian Institution)

Evidence for live ^26Al (t^1/21/2 0.7 million years) in the early Solar System is preserved in two kinds of preplanetary particles contained within chondrite meteorites: chondrules (mostly silicates of magnesium and iron), and Ca-Al-rich Inclusions (CAIs; mostly oxides and silicates of calcium, aluminum, magnesium, and titanium). Both formed 4.6 Gy ago at high temperatures in the presolar nebula, in many cases solidifying from melt droplets, and both record the chemical and isotopic characteristics of their precursor dust. The former presence of ^26Al in any object is indicated by excess abundances of the daughter product, ^26Mg, that correlate with the bulk Al/Mg in individual mineral phases; if the different phases originally co-crystallized in a closed system, such a correlation demonstrates in situ decay of ^26Al. Most CAIs formed with a very consistent inferred initial abundance ratio, ^26Al/^27Al 5 x 10^-5, regardless of which chondrite sub-group (asteroidal parent body) they occur in. Some CAIs have little or no evidence for ^26Al, in some cases because later thermal events reset the isotopic systems as much as 2-3 million years after CAI formation and in a rare few cases because little or no ^26Al was originally present. Only relatively rare chondrules that contain Al-rich, Mg-poor minerals are suitable for ion probe detection of former ^26Al. In such chondrules, the highest initial ratio that has been measured is ^26Al/^27Al 2.5 x 10^-5; others have ratios 1 x 10^-5, and in many there is no detectable excess of ^26Mg. If CAIs and chondrules formed from a single initially-homogeneous isotopic reservoir (controversial!), then their isotopic differences reflect an extended chronology of nebular events: CAIs formed first, chondrules began forming 0.7 million years later, both experienced at least 2-3 million years of nebular history, and a total nebula lifetime of >3-4 million years is indicated.

Program listing for Wednesday