Core Formation of Planetary Bodies


All terrestrial planets and many asteroids in our solar system underwent differentiation to segregate a metallic core. The only samples of such cores that are available for laboratory studies are iron meteorites, which are believed to be fragments of cores from asteroids. Chronological studies of such meteorites can provide the timescales of asteroid accretion, core formation, and the time that was necessary for the originally molten metal to cool and crystallise. Due to the unique composition of iron meteorites (mainly Fe-Ni metal) only a few radiogenic systems are able to provide information on the timing of such processes. Particularly precise temporal information for early processes can be obtained from studies of extinct radionuclides. These nuclides have short half-lives such that they were “alive” only in the nascent solar system. Recent results from the extinct radionuclide system 182Hf-182W (t1/2= 9 Myr) have shown that some iron meteorites differentiated within less than 1 Myr after the first solids formed (1, 2). However, the timescale on which iron meteorites cooled and crystallised is not well constrained. Our current research in SEAES is exploiting the unique dating capability of the 107Pd-107Ag chronometer (t1/2=6.5 Myr) to obtain insights into how planetary cores formed and evolved.

The 107Pd-107Ag decay system is ideally suited for dating metal crystallisation and core solidification, because of its short half-life and the strong Pd-Ag fractionation in iron meteorites during these processes (3). It has already been applied to iron meteorites in previous studies (3), however, recent advancements by Schönbächler et al. (4) improved the precision of the analytically challenging Ag isotope measurements by a factor of 4. This opens up the 107Pd-107Ag chronometer for studying a much wider range of iron meteorites and primitive meteorites, where the effects of the radiogenic decay are small. We employ the newly developed analytical technique to obtain high precision Ag isotope analyses using the and the state of the art clean room facility at SEAES.

To date we have analysed iron meteorites (IAB’s (5), IIAB’s and IIIAB’s), ordinary chondrites and an acapulcoite using this technique (4) on the Nu Plasma MC-ICP-MS  here at Manchester. As stated above, most iron meteorites are remnants of asteroid cores and display complete fractional crystallisation, however, this is not the case for IAB iron meteorites. This group of meteorites show evidence of a more complicated formation history as they exhibit only partial elemental fractionation trends and contain abundant silicate inclusions indicating that complete parent body differentiation has not occurred. 107Pd/107Ag analysis revealed that the six meteorite sample set could be divided into two sub groups based on their inclusion petrology and that each sub group defined an isochron that gave relative ages of crystallisation of 18.7 and 14.9 Ma after CAI formation (5). The IAB parent body suffered a catastrophic break up and re-assembly (6) so this data suggests that following the re-assembly the two sub groups were re-buried at different depths and so have different cooling rates.    

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