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

行星吸积和岩浆分异是控制地球形成的关键过程。小行星是早期太阳系中的小天体（卵石大小到月球大小），其现今的残余物是小行星、柯伊伯带天体和彗星。当一个天体形成时，它会吸收一定量的放射性物质，这主要取决于它的大小和吸积时间。随着放射性物质的衰变，它会加热身体的内部，这可能导致熔化和分化成核心，地幔和地壳，就像地球上的情况一样。在微行星中，有时情况并非如此，因为它们的尺寸很小，而且吸积的开始也不一样（早期的吸积吸收了更多的放射性物质，这导致了更多的熔化）。因此，星子经历了比行星更广泛的热历史。当行星完全熔化和分化时，星子表现出各种中间熔化状态，并提供了一个窗口，了解行星吸积和分化的早期阶段在地球等天体上的过程。由于大多数可观测到的小行星都有未分化的地壳，因此人们认为分化的星子和小行星是罕见的。这一观点正受到越来越多的古地磁证据的挑战，这些证据表明，几个陨石母体具有内部产生的磁场（发电机场），需要一个熔融的核心。这是目前一些学者的情况（例如Gattacceca等人，2016）、无球粒陨石（韦斯等人，2008）、钯（布赖森等人，2014）和铁陨石（Maurel等人，2018年）。因此，早期的太阳系似乎比以前认为的更具岩浆和磁性。然而，鉴于上述大多数陨石母体预计不会形成核心，目前对星子中的发电机生成还没有很好的了解。在行星的例子中，一个被提出的解释是，行星盖在分化的天体上是常见的，要么：（1）作为微行星的一部分，它分化了它的内部，而不是地壳，要么（2）在行星吸积的后期阶段，在一个完全分化的微行星上吸积（韦斯和埃尔金斯-坦顿，2013）。在这个项目中，我的目标是调查这两种情况：1。通过对几个原始无球粒陨石群（无球粒陨石、钾长花岗岩、钠长花岗岩）的古地磁分析，确定在分化的星子表面是否存在无球粒陨石盖。这些陨石是部分熔融的，所以我的目标是测试它们是否形成于南极（未分化）地壳和分化内部之间的部分分化层（场景1）。分析毁灭性撞击对星子发电机的影响及其在中陨铁陨石中的磁记录，并将这些撞击纳入后期吸积的更广泛的情况，以确定是否可以向已经分化的天体添加南极物质（设想2）。分析来自古老大陆地壳（>3 Ga，格陵兰岛Isua Greenstone Belt）的陆地样品记录的磁信号，以比较和对比星子和行星发电机的早期阶段。

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.