Constraining the early evolution of the Solar System and the Earth using meteorite palaeomagnetism

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Planetary accretion and magmatic differentiation are key processes that govern the formation of a terrestrial body. Planetesimals are small bodies (pebble-size to Moon-size) in the early Solar System, whose present-day remnants are asteroids, Kuiper belt objects, and comets. When a terrestrial body forms, it takes in an amount of radiogenic material which mainly depends on its size and timing of accretion. As the radiogenic material decays, it heats up the interior of the body, which can lead to melting and differentiation into a core, mantle and crust, as was the case on Earth. In planetesimals, this is sometimes not the case because of their small sizes and variable onset of accretion (earlier accretion takes in more radiogenic material, which leads to more melting). Therefore, planetesimals experienced a significantly wider range of thermal histories than planets. Whilst planets melted and differentiated completely, planetesimals exhibit a variety of intermediate melting states, and offer a window into the processes operating during the early stages of planetary accretion and differentiation on bodies such as the Earth. Since most observable asteroids have chondritic (undifferentiated) crusts, it has been assumed that differentiated planetesimals and asteroids are rare. This view is being challenged by a growing body of palaeomagnetic evidence suggesting that several meteorite parent bodies had an internallygenerated magnetic field (a dynamo field) that required a molten core. This is currently the case for some chondrites (e.g. Gattacceca et al., 2016), achondrites (Weiss et al., 2008), pallasites (Bryson et al., 2014) and iron meteorites (Maurel et al., 2018). Thus, the early Solar System appears significantly more magmatically and magnetically active than previously thought. However, dynamo generation in planetesimals is not well understood currently, given that most of the meteorite parent bodies mentioned above are not expected to form a core. In the case of chondrites, one proposed explanation is that chondritic lids are common on differentiated bodies, either: (1) as part of a planetesimal which differentiated its interior but not the crust, or (2) accreted on a fully differentiated planetesimal at a later stage during planetary accretion (Weiss & Elkins-Tanton, 2013). In this project, I aim to investigate both of these scenarios by:1. Determining whether chondritic lids can exist on the surface of differentiated planetesimals, through palaeomagnetic analyses of several primitive achondrite groups (acapulcoites, lodranites, ureilites). These meteorites are partially molten, so I aim to test if they formed in a partially differentiated layer between the chondritic (undifferentiated) crust and the differentiated interior (scenario 1).2. Analysing the effect of destructive impacts on a planetesimal dynamo and its magnetic recording in a mesosiderite meteorite, and integrating these impacts into the wider picture of late-stage accretion, to determine whether chondritic material can be added to an already differentiated body (scenario 2).3. Analysing the magnetic signals recorded by terrestrial samples from old continental crust (>3 Ga, Isua Greenstone Belt, Greenland), in order to compare and contrast the early stages of the dynamos of a planetesimal and a planet.