4. On the Early Thermal History of Chondritic Asteroids Derived by 244-Plutonium Fission Track Thermometry

Abstract

Cooling curves were determined for ordinary chondrites within the time- and temperature intervals from ~4.6 to ~4.1 × 109 years and ~1400 K to 300 K respectively. This was done by analyzing the 244-plutonium fission track record in whitlockite and adjacent mineral track detectors. The resulting cooling rates constrain the sizes of the parent asteroids to ~120 - 200 km in radius. These dimensions together with the fact that chondrites had never been molten, suggest lower 26Al/27Al initial ratios for ordinary chondritic than for Allende refractory materials.