~(26)Al as a heat source for early melting of planetesimals: Results from isotopic studies of meteorites

摘要

Ion microprobe studies of magnesium isotopic composition in igneous components from several chondritic meteorites have been carried out to look for ~(26)Mg excess that may be attributed to the presence of the now-extinct radionuclide ~(26)Al(τ ～1 Ma)at the time of formation of these objects. A positive evidence for the presence of ~(26)Al in the analysed objects will strengthen its case as the primary heat source for the early thermal metamorphism/melting of meteorite parent bodies. Based on calculated temperature profiles inside chondritic objects of different sizes and initial ~(26)Al/~(27)Al ratios, we have estimated the initial abundances of ~(26)Al needed to provide the heat necessary for the wide range of thermal processing seen in various types of meteorites. The magnesium isotopic data obtained by us do not provide definitive evidence for the presence of ~(26)Al at the time of formation of the analysed igneous phases in different chondritic meteorites. Experimental evidence for a planetary scale distribution of ~(26)Al in the early solar system to serve as a significant heat source for the thermal metamor-phism and melting of meteorite parent bodies (planetesimals) remains elusive.