AL-26 AS A HEAT-SOURCE FOR EARLY MELTING OF PLANETESIMALS - RESULTS FROM ISOTOPIC STUDIES OF METEORITES

Ion microprobe studies of magnesium isotopic composition in igneous co mponents from several chondritic meteorites have been carried out to l ook for Mg-26 excess that may be attributed to the presence of the now -extinct radionuclide Al-26(tau similar to 1 Ma) at the time of format ion of these objects. A positive evidence for the presence of Al-26 in the analysed objects will strengthen?ts case as the primary heat sour ce for the early thermal metamorphism/melting of meteorite parent bodi es. Based on calculated temperature profiles inside chondritic objects of different sizes and initial Al-26/Al-27 ratios, we have estimated the initial abundances of Al-26 needed to provide the heat necessary f or the wide range of thermal processing seen in various types of meteo rites. The magnesium isotopic data obtained by us do not provide defin itive evidence for the presence of Al-26 at the time of formation of t he analysed igneous phases in different chondritic meteorites. Experim ental evidence for a planetary scale distribution of Al-26 in the earl y solar system to serve as a significant heat source for the thermal m etamorphism and melting of meteorite parent bodies (planetesimals) rem ains elusive.