The early evolution of small terrestrial bodies: a palaeomagnetic and geochemical study of meteorites

Abstract

Meteorites act as windows into the early solar system, providing a wealth of information on the composition, formation, and evolution of planetary bodies. After their formation, planetary bodies are reworked by several secondary processes which modify their original chemical and physical properties. In this thesis, I study two such processes (aqueous alteration and impacts), using magnetic and geochemical techniques on stony-iron, iron, and chondritic meteorites. The formation of stony-iron meteorites is a topic of intense research and both mesosiderites and pallasites are hypothesised to have formed through impact mixing of metal and silicates. I use palaeomagnetic and microscopy techniques on the mesosiderite Estherville and show that Estherville does not record a high-coercivity natural remanence. However, Estherville has the ability to record strong (>150 μT) fields. The slow cooling rates recorded by mesosiderites (<1 K/Myr) imply a formation close to the core-mantle boundary, suggesting that they would have been exposed to strong fields if a dynamo was active. However, they do not carry a high coercivity remanence, which may indicate that the mesosiderite-forming impact shut off the dynamo. To investigate the formation of pallasites, I use in situ chemical analysis and find that all metal in Seymchan has the same origin. Based on this result and textural observations, I propose a pallasite formation model involving two or more impacts, and provide additional evidence for the previously proposed link between Main Group pallasites and IIIAB meteorites. Finally, I study aqueous alteration using quantum diamond microscope magnetic mapping to spatially resolve the magnetic signals of the CM chondrite MCY 05231. I suggest that magnetite forms via different reaction pathways in this sample, and that magnetite formed via pseudomorphic replacement of metal leads to the inheritance of magnetic remanence from the precursor grain. I use these results to suggest a magnetic history for MCY 05231, and to discuss implications for bulk magnetic studies of chondrites