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陆地天体形成的关键过程。行星体是早期太阳系中的小天体(卵石大小到月亮大小)，其今天的残余物是小行星、柯伊伯带天体和彗星。当一个陆地天体形成时，它吸收了大量的放射性物质，这主要取决于它的大小和吸积的时间。随着放射性物质的衰变，它会加热人体内部，这可能会导致熔化和分化为核心、地幔和地壳，就像地球上的情况一样。在行星体中，有时情况并非如此，因为它们的体积很小，而且吸积的开始时间可变(较早的吸积吸收了更多的放射性物质，这会导致更多的熔化)。因此，行星体经历了比行星更广泛的热历史。虽然行星完全融化和分化，但行星体表现出各种中间融化状态，并提供了一个窗口，可以了解行星在地球等天体上吸积和分化的早期阶段的操作过程。由于大多数可观测到的小行星都有软骨化(未分化)的壳，因此人们认为分化的小行星和小行星很少见。越来越多的古地磁证据表明，几个陨石母体具有内部产生的磁场(发电机磁场)，需要熔融的核心，这一观点受到了挑战。目前，一些球粒陨石(如Gattacceca等人，2016年)、非球粒陨石(Weiss等人，2008年)、球粒陨石(Bryson等人，2014年)和铁陨石(Maurel等人，2018年)就是这种情况。因此，早期太阳系看起来比之前认为的更具魔力和磁性。然而，考虑到上面提到的大多数陨石母体预计不会形成核心，目前对行星体中的发电机产生还没有很好的了解。对于球粒陨石，一种提出的解释是球粒陨石盖在分化的天体上很常见，或者(1)作为小行星的一部分，分化其内部而不是地壳，或(2)在行星吸积过程的后期阶段，在完全分化的小行星上聚积(Weiss&Elkins-Tanton，2013)。在这个项目中，我的目标是通过以下两种方式来研究这两种情况：1.通过对几个原始的无球粒陨石群(无球粒陨石、球粒陨石、球粒陨石)进行古地磁分析，确定在分化的行星体表面是否存在球粒陨石盖。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球粒状(未分化的)地壳和分化的内部之间的部分分化层(场景1)。分析破坏性撞击对小行星发电机的影响及其在中铁陨石陨石中的磁记录，并将这些撞击结合到后期吸积的更广泛图景中，以确定是否可以将软骨状物质添加到已经分化的天体中(情景2)。分析旧大陆地壳(&gt；3Ga，格陵兰伊苏阿绿岩带)的陆地样品记录的磁信号，以比较和对比小行星和行星的早期发电机阶段。

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.