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

利用陨石古地磁学约束太阳系和地球的早期演化

• 批准号: 2102456
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2018
• 资助国家: 英国
• 起止时间: 2018 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2102456
• 关键词: Constraining; early; evolution; Solar; System

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.

行星积聚和岩浆分化是控制陆生体形成的关键过程。行星是在早期太阳系中的小物体（卵石大小到月球尺寸），其当今的残余物是小行星，kuiper带对象和彗星。当形成陆地时，它采用了一定数量的放射原料，主要取决于其大小和积聚的时间。随着放射原料的衰减，它会加热人体的内部，这可能会导致融化并分化成核心，地幔和地壳，就像地球上一样。在行星上，有时并非如此，因为它们的尺寸很小，并且会变化变化（早期的积聚吸收了更多的放射原料，从而导致更多的熔化）。因此，行星比的热历史范围要比行星更宽。尽管行星完全融化并完全区分了，但行星模具表现出各种中间熔化状态，并为行星积聚的早期阶段运行的过程提供了一个窗口，并在地球等物体上分化了。由于大多数可观察到的小行星具有软骨（未分化）地壳，因此已经假定分化的行星和小行星很少见。越来越多的古磁性证据表明，几个陨石母体具有内部生成的磁场（一个发电机场），这一观点正受到挑战。目前，某些软管（例如Gattacceca等，2016），Achondrites（Weiss等，2008），Pallasites（Bryson等，2014）和铁陨石（Maurel等，2018）。因此，早期的太阳系在岩浆和磁性上似乎比以前想象的要高得多。但是，鉴于上面提到的大多数陨石母体的物体不会形成核心，因此目前尚不清楚地球上的发电机生成。对于软骨，一个提出的解释是，在分化的身体上，软骨盖很常见，要么：（1）作为行星的一部分，它是行星内部的，但没有区分地壳，或（2）在完全区分的行星上积聚后来的行星积聚期间（Weiss＆Elkins-Tanton，2013年）。在这个项目中，我旨在通过以下方式调查这两种情况。通过对几个原始的量红色基团（acapulcoites，lodranites，ureilites）的古磁分析，确定在分化的行星表面上是否存在软骨盖。这些陨石是部分熔融的，因此我旨在测试它们是否在软骨（未分化的）外壳和分化的内部（场景1）之间的部分分化层中形成（场景1）。2。分析破坏性影响对速度力陨石中的行星发电机及其磁记录的影响，并将这些影响整合到后期积聚的更广泛的情况下，以确定是否可以将软骨材料添加到已经区分的身体中（方案2） .3。分析来自旧大陆壳（> 3 GA，ISUA绿岩带，格陵兰岛）的地面样品记录的磁信号，以比较和对比行星和行星的发电机的早期阶段。