The quest for primary magnetisation in Earth's oldest materials

地球的磁场是由其外核中的液态铁不断搅动产生的。这种“地球发电机”对现代生活至关重要：如果没有它，我们的大气层将逐渐被太阳风剥离，我们将暴露在潜在致命剂量的高能宇宙射线中(这一前景有待前往火星的宇航员)。40亿年前，火星失去了磁场和大部分大气。在创造约40亿年前地球上生命出现所需的条件方面，地球发电机很可能发挥了同样重要的作用。然而，我们对地球发电机在这一关键时期的行为知之甚少。地球发电机的最早证据来自35亿年前的岩石，但由于地球是在45亿年前形成的，目前我们对地球磁史的了解差距超过10亿年。这一提议是从地球上最古老的物质中提取原始磁信号的国际探索的一部分，最终目的是填补古地磁记录中的这一空白。早期地球的磁性数据的缺乏很容易解释：这个年龄的岩石极其罕见，那些存在的岩石在漫长的地质历史中遭受了不同程度的加热和/或化学改造。随着时间的推移，古老岩石中微小的磁性矿物颗粒携带的磁性信号会受到破坏。如果加热过多，主磁信号就会永远被破坏。低温化学蚀变过程中新磁性矿物的生长可以掩盖或取代原生磁性信号。只有最热稳定的磁性颗粒才有可能粘住它们的原始磁化作用。在寻找理想的磁记录器的过程中，“单晶”古地磁方法已经成为一个令人兴奋的前景。不是分析块状岩石(可能主要是次生磁性矿物)，而是对名义上非磁性矿物(例如石英)的单晶进行测量，这些单晶含有原生磁性矿物的亚微米包裹体。这些磁性包裹体不仅受到其寄主硅酸盐的保护，不受化学作用的影响，而且足够小，足以包含稳定的磁性结构，这些结构在除最极端的变质温度之外的所有温度下都能经受住加热。最近，来自有30亿年历史的杰克·希尔斯准砾岩的碎屑锆石单晶引起了人们的强烈关注。这些晶体中的一小部分可以追溯到赫迪亚时代(即超过40亿年)。据称，这些锆石在42亿年前被活跃的地球发电机磁化；将地球发电机的开始时间推迟了7亿年。然而，一份反声明断言，杰克山的样品已经被普遍地重新磁化，不再含有任何初级磁化的痕迹。解决这一争议需要对磁性包裹体进行深入分析。该项目将把单晶古地磁方法建立在可靠的物理基础上。一系列层析成像方法，在多个长度尺度上相互关联，将能够以前所未有的细节重建单晶的内部结构，为确定其磁性包裹体的初级/次级性质提供证据。铁同位素将作为区分高温原生包裹体和低温化学蚀变产物的潜在方法而被研究。原生磁性包裹体的三维分布将用于开发重建磁记录过程的计算机模拟。这些方法将应用于杰克山锆石，并与高分辨率磁成像相结合，以便能够明确地确定Hadean磁场的性质。

The Earth's magnetic field is generated by the constant churning of liquid iron in its outer core. This "geodynamo" is crucial to modern life: without it our atmosphere would be gradually stripped away by the solar wind and we would be exposed to potentially lethal doses of high-energy cosmic rays (a prospect that awaits astronauts journeying to Mars, which lost its magnetic field along with much of its atmosphere 4 billion years ago).The geodynamo is likely to have played an equally important role in creating the conditions necessary for the emergence of life on Earth around 4 billion years ago. However, we know little, if anything, about the behaviour of the geodynamo during this critical period. The earliest evidence for the geodynamo comes from rocks that are 3.5 billion years old, but since the Earth formed over 4.5 billion years ago, there is currently a gap of more than one billion years in our knowledge of Earth's magnetic history. This proposal forms part of an international quest to extract pristine magnetic signals from Earth's oldest materials, with the ultimate aim of plugging this gap in the paleomagnetic record.The lack of magnetic data for the early Earth is easy to explain: rocks of this age are extremely rare, and those that exist have suffered varying degrees of heating and/or chemical alteration during their long geological history. The magnetic signals carried by tiny magnetic mineral grains in ancient rocks become corrupted over time. Too much heating and the primary magnetic signals are destroyed forever. Growth of new magnetic minerals during low-temperature chemical alteration can obscure or replace the primary magnetic signals. Only the most thermally stable magnetic grains, which have been fully protected from chemical alteration, have the potential to cling on to their primary magnetisation.In the search for ideal magnetic recorders, the "single-crystal" paleomagnetism method has emerged as an exciting prospect. Instead of analysing bulk rocks (which are likely to be dominated by secondary magnetic minerals), measurements are made on single crystals of nominally non-magnetic minerals (e.g. quartz) containing sub-micrometre inclusions of primary magnetic minerals. These magnetic inclusions are not only protected from chemical alteration by their host silicate, but are small enough to contain stable magnetic structures that survive heating to all but the most extreme metamorphic temperatures. Intense attention has recently focussed on single crystals of detrital zircon from the 3 billion year old Jack Hills metaconglomerate. A small fraction of these crystals have been dated to the Hadean era (i.e. more than 4 billion years old). It is claimed that these zircons were magnetised by an active geodynamo 4.2 billion years ago; pushing back the start of the geodynamo by 700 million years. A counter claim asserts, however, that the Jack Hills samples have been pervasively remagnetised, and no longer contain any vestige of primary magnetisation. Resolving this controversy requires in-depth analysis of the magnetic inclusions.This project will place the single-crystal paleomagnetism method on a sound physical basis. A range of tomographic methods, correlated across multiple length scales, will enable the internal architecture of single crystals to be reconstructed in unprecedented detail, providing evidence to establish the primary/secondary nature of their magnetic inclusions. Fe isotopes will be investigated as a potential method to distinguish high-temperature primary inclusions from the low-temperature products of chemical alteration. The 3D distribution of primary magnetic inclusions will be used to develop computer simulations that recreate the magnetic recording process. These methods will be applied to the Jack Hills zircons and combined with high-resolution magnetic imaging to enable to properties of the Hadean magnetic field to be determined unambiguously.