Thermochemical remanent magnetisations: How do they affect ancient magnetic field intensities from the Earth and Solar System?

储存在岩石和陨石中的古代磁场记录是回答地球和行星科学中一些最基本问题的关键，这些问题包括地核和地球发电机的演变，以及太阳系的形成。特别是，正是对古代磁场强度的估计，使我们能够解决许多问题，从太阳演化的限制性理论，到将地球发电机的开始与地球上生命的开始联系起来的想法。为了恢复古代磁场强度，我们研究了在冷却过程中记录了热磁化（TRM）的火成岩。TRM是磁性矿物在像地球这样的弱磁场中从居里温度（~600℃）以上冷却下来时所记录的剩余磁化。居里温度是定义材料表现出磁化的最高温度的关键参数。在TRM采集过程中，假定磁性矿物在化学上是稳定的，在冷却过程中不会发生物理或化学变化。这样的trm可以比宇宙的年龄稳定几倍。火成岩中的磁性矿物，特别是玄武岩中的磁性矿物，通常是钛磁铁矿Fe2.4Ti0.6O4。玄武岩在地球上无处不在，例如，大部分海洋地壳的顶部（占地球表面的70%）是玄武岩。几十年来，人们已经知道，当fe2.4 ti0.6 _4冷却时，它会分解（溶解）成磁性磁铁矿相（Fe3O4）和非磁性ulvöspinel相（Fe2TiO4）。自20世纪50年代以来，人们对这种分解进行了广泛的研究，结果表明，这种分解发生在居里温度以上和以下的温度下。分解停止的确切温度取决于许多因素，如冷却速度，较慢的冷却速度更有可能在低温下产生溶出结构。多年来，研究古地磁强度的古地磁学家一直认为，在居里温度以上的温度下，出溶过程停止，岩石获得trm；然而，越来越多的证据表明，在居里温度以下，矿物继续分解，从而化学报警并记录另一种类型的磁剩余磁化，称为热化学剩余磁化（TCRM）。这是一个问题，因为测定古代磁场强度的方法假设岩石携带TRM而不是TCRM。地球科学界维护着一个全球古磁场强度的数据库。对该提案的分析表明，在过去60年收集的数据库中，至少有51%的4293强度估计（场地水平）可能会因磁化是TRM的错误假设而受到损害，而实际上它是TCRM。这可能是在数据库中发现大量分散的原因。迄今为止，很少有人尝试确定TCRM对古地磁强度测定的影响，主要是因为问题的复杂性。近年来，PI, coi，访问研究员和项目合作伙伴开发了新的纳米成像，数值算法（MERRILL）和磁测量协议来研究TRM采集，现在使TCRM问题易于处理。我们的目标是在温度下解混过程中对富钛氧化铁的磁性结构进行纳米成像，以使我们了解解混过程如何影响磁化。我们将把这些信息与纳米化学制图结合起来，使用美林新添加的多相技术建立数值模型。数值模型将使我们能够：(1)根据磁测量结果做出预测，(2)确定TCRM在地质时间尺度上的稳定性，(3)确定TCRM对古代磁场强度测定的贡献。我们将利用结果开发新的古代场强估计协议，并对遗留数据提供更正。

Ancient records of magnetic fields stored in rocks and meteorites hold the key to answering some of the most fundamental questions in Earth and Planetary Sciences including the evolution of the Earth's Core and geodynamo, and the formation of the Solar System. In particular, it is the estimates of ancient field intensities that allows us to solve many of these questions, from constraining theories of Solar evolution, to ideas that link the start of the geodynamo to the beginning of life on Earth.To recover ancient field intensities, we study igneous rocks that have recorded thermoremanent magnetisations (TRM) during cooling. A TRM is the remanent magnetisation recorded by magnetic minerals as they cool from above the Curie temperature (~600 C) in weak magnetic fields like the Earth's. The Curie temperature is a key parameter that defines the maximum temperature at which a material exhibits magnetisation. During TRM acquisition it is assumed that the magnetic minerals are chemically stable, and do not physically or chemically alter during cooling. Such TRMs can be stable for times greater than the age of the Universe.The magnetic mineral in igneous rocks, particularly basalts, is usually titanomagnetite Fe2.4Ti0.6O4. Basalts are ubiquitous on Earth, for example, most of the top of the ocean crust (70% of the Earth's surface) is basalt. It has been known for many decades that as Fe2.4Ti0.6O4 cools it unmixes (exsolves) into a magnetic magnetite phase (Fe3O4) and a non-magnetic ulvöspinel phase (Fe2TiO4). The unmixing has been extensively studied since the 1950s and has been shown to occur at temperatures above and below the Curie temperature. The exact temperature at which unmixing stops depends on many factors like the cooling rate, with slower cooling rates more likely to give rise to exsolution structures at low temperatures.For many years palaeomagnetists who study ancient field intensities have assumed that exsolution processes stop at temperatures above the Curie temperature, and that rocks acquire TRMs; however, there is growing evidence to suggest that the minerals continue to unmix below the Curie temperature, thereby chemically alerting and recording another type of magnetic remanent magnetisation termed a thermochemical remanent magnetisation (TCRM). This is a problem, as methods for ancient magnetic field intensity determination assume that rocks carry a TRM not a TCRM.The Earth Science community maintains a database of global ancient field intensities. Analysis for this proposal indicates at least ~51% of the 4293 intensity estimates (site-level) in the database collected over the last 60 years, could be compromised by the incorrect assumption that the magnetisation is a TRM when it is in fact a TCRM. This maybe the reason for the large scatter found in the database.Hitherto little attempt has been made to determine the effect of TCRM on ancient field intensity determination, primarily because of the complexity of the problem. In recent years the PI, CoIs, Visiting Fellow and Project Partners, have developed new nanometric imaging, numerical algorithms (MERRILL) and magnetic measurement protocols to study TRM acquisition, that now make the TCRM problem tractable. We aim to nanometrically image magnetic structures in Ti-rich iron oxides during unmixing at temperature, to allow us to understand how the magnetisation is affected by the unmixing process. We will combine this information with nanometric chemical mapping to build numerical models, using a new multiphase addition to MERRILL. The numerical model will allow us to: (1) make predictions which we will ground-truth against magnetic measurements, (2) determine the stability of TCRM on geological timescales, and (3) to determine the contribution of TCRM to ancient magnetic field intensity determinations. We will use the results to develop new ancient field intensity estimations protocols and provide corrections to legacy data.