Evidence from the Semarkona ordinary chondrite for 26A1 heating of small planets

Abstract

The radioactive decay of 26Al, to form 26Mg, may have constituted an important heat source for melting of small planetary bodies in the early Solar System. Although large excesses of radiogenic 26Mg, arising from in situ decay of 26Al (half life ~7.2 x 105 yr), have been found in ten carbonaceous and ordinary chondritic meteorites1,2, the distribution of 26Al in the solar nebula and its importance as a heat source remain to be determined. All previous observations of excess 26Mg are confined to Al-rich minerals in refractory, Ca,Al-rich inclusions (CAIs). An assessment of the distribution of 26Al in the Solar System from these data is difficult because CAIs are only a minor constituent of chondritic meteorites associated with high-temperature nebular processes, and because the initial 26Al/27Al ratio inferred from the CAI data is not constant3 but varies from the commonly measured value of ∼ 5 x 10−5 to values <1 x 10−7. Here we report the first observation of radiogenic 26Mg in non-refractory meteoritic material, a plagio-clase-bearing, olivine-pyroxene clast chondrule in the Semarkona ordinary chondrite. The inferred initial abundance of 26Al (26Al/27Al = (7.7±2.1)xl0−6) is sufficient to produce incipient melting in well insulated bodies of chondritic composition. We conclude that planetary accretion and differentiation must have begun on a timescale comparable to the half life of 26Al and that, even if widespread melting did not occur, 26Al heating played a significant role in thermal metamorphism on small planets.