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 meteorites^1,2, 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.