Paleointensity of the Paleoproterozoic Geomagnetic Field as Recorded by Single Silicate Crystals: Testing the "Proterozoic Dipole Low"

该项目将通过获得古地球磁场强度（古强度）的高质量数据，推进对地球早期演化的理解。今天，地球的磁场是由地球外核的熔融铁对流产生的。这个过程被称为地球发电机，是由密度沉降和结晶内核的聚变热提供动力的。然而，根据大多数模型，内部的固体核心相对年轻，可能只存在于约45亿年地质历史的最后5亿年。同时，已经确定地磁场至少在35亿年前就存在了。因此，在内核形成之前，地球动力机制应该是不同的。特别是，最近的一个假设表明，在24.5亿年前，在基底-地幔岩浆海洋中产生了一个强大的磁场，这是一层熔融硅酸盐地幔，位于核心-地幔边界之上。地幔完全凝固后，在完全液态的地核内产生了一个更弱、更不稳定的磁场（“元古代低偶极子”）。产生强磁场的现代地球发电机始于大约4亿年前固体内核的形成。这项研究的目的是通过获得高质量的古强度值（古代磁场强度的测量）来验证这一假设，该时间段（19.8亿至24.1亿年前）紧接在提出的基幔海洋地球动力学消亡之后。古强度实验将采用一种基于单硅酸盐晶体研究的新技术，在印度、加拿大和澳大利亚的几组快速冷却的古元古代侵入岩上进行。这项研究将为产生地球磁场的机制和地球深层内部（地核和地幔）的演变提供重要的见解。这项研究更广泛的意义包括更好地理解地球磁场演化与生物圈和大气演化之间的联系。该项目将涉及密歇根理工大学的本科生和研究生，从而培养下一代科学家。为了提高公众对地球科学的认识，研究结果将通过一系列科学探索会议进行传播。前寒武纪地球磁场强度（古强度）的长期演化数据对于理解早期地球动力学的性质至关重要。这些数据还可以为了解地球内部的重要过程提供见解，例如固体内核的形成和生长，或者影响地球动力学强迫的地幔对流的长期变化。然而，我们对前寒武纪古强度的了解仍然非常有限。常规的古强度方法在研究前寒武纪岩石时，由于样品的热蚀变，成功率较低。为了解决这个问题，一种新的技术已经被开发出来，它使用单个的形成岩石的硅酸盐晶体来测量古强度。这种晶体通常含有单畴或伪单畴铁磁包裹体，免受硅酸盐宿主的自然蚀变的影响，并且相对于实验蚀变而言是稳定的。本文的研究目的是通过对印度三个基性岩脉群（Dharwar克拉通的~2.37 Ga Bangalore岩脉、~2.18 Ga Mahhubnagar岩脉和Bundelkhand克拉通的~1.98 Ga岩脉）和加拿大Superior克拉通的~2.12 Ga和~2.10 Ga Marathon岩脉的斜长石进行古强度分析，探讨元古代地球动力学的强度。此外，将对~2.41 Ga Widgiemooltha岩脉样品进行单晶古强度测定，获得可靠的块状岩石古强度测定方法。通过岩石磁学和古强度的初步研究，证明了岩脉组合体的结晶适合于古强度实验。该项目将增加前寒武纪高质量古强度测定的数量，为了解地球动力学机制和地球的长期演化提供洞见。重要的是，所选岩脉组的年龄将使我们能够检验从~2.45 Ga以前的基底-地幔岩浆海洋中产生的强场地球发电机到没有固体内核的液体核中产生的弱场地球发电机的过渡假设（“元古代偶极子低”）。从同一岩石单元的块状岩石和单晶样品中比较岩石的磁性和古强度，将有助于我们对岩石和磁性矿物形成和蚀变过程的理解，这可能会影响岩石和晶体作为古强度记录器的保真度。该项目将支持一名研究生（博士）和几名本科生研究助理。

This project will advance understanding of our planet's early evolution by obtaining high-quality data on the strength (paleointensity) of ancient Earth's magnetic field. Today, the Earth's magnetic field is generated by convection of molten iron in the planet's outer core. This process, called the geodynamo, is powered by density settling and heat of fusion of the crystallizing inner core. However, according to the most models, the inner solid core is relatively young and may have existed only for the last 500 million years of the ~4.5 billion years of the geological history. At the same time, it has been established that the geomagnetic field has existed since at least 3.5 billion years ago. Hence, before the inner core formation, the geodynamo mechanisms should have been different. In particular, a recent hypothesis suggests that before ~2.45 billion years ago, a strong field was produced within a basal-mantle magma ocean, a layer of molten silicate mantle above the core-mantle boundary. After the mantle completely solidified, a weaker and less stable magnetic field was produced within the entirely liquid core ("the Proterozoic Dipole Low"). The modern style geodynamo producing a strong magnetic field started with the formation of solid inner core at about 400 million years ago. The goal of this research is to test this hypothesis by obtaining high-quality paleointensity values (measurements of the ancient magnetic field strength) for the time period (between 1.98 and 2.41 billion years ago) immediately following the demise of the proposed basal mantle ocean geodynamo. The paleointensity experiments will be conducted on several suites of quickly-cooled intrusive Paleoproterozoic rocks in India, Canada, and Australia using a novel technique based on investigation of single silicate crystals. The study will provide important insights into the mechanism that generates Earth's magnetic field and the evolution of the Earth's deep interior (the core and the mantle). Broader implications of the study include a better understanding of the link between evolution of Earth's magnetic field and evolution of biosphere and atmosphere. The project will involve Michigan Tech undergraduate and graduate students, thus training the next generation of scientists. In order to increase the general public awareness of Earth science, the results will be disseminated through a series of science exploration sessions.Data on the long-term evolution of Earth's magnetic field strength (paleointensity) in the Precambrian are crucial for understanding the nature of early geodynamo. These data may also provide insights into important processes within the Earth's interior, such as the formation and growth of the solid inner core, or long-term changes in mantle convection affecting the forcing of the geodynamo. However, our knowledge of the Precambrian paleointensity remains very limited. Conventional paleointensity methods often have low success rate when applied to Precambrian rocks due to heating-induced alteration of samples. In order to circumvent this problem, a novel technique has been developed that uses individual rock-forming silicate crystals to measure paleointensity. Such crystals often contain single-domain to pseudosingle-domain ferromagnetic inclusions protected from natural alteration by the silicate host and stable with respect to the experimental alteration. The research objective of this proposal is to investigate the strength of the Proterozoic geodynamo by paleointensity analyses of plagioclase separated from three mafic dike swarms in India (the ~2.37 Ga Bangalore dikes, the ~2.18 Ga Mahhubnagar dikes in the Dharwar craton, and the ~1.98 Ga dikes in the Bundelkhand craton), and from the ~2.12 Ga and ~2.10 Ga Marathon dikes in the Superior Craton (Canada). In addition, single crystal paleointensity determinations will be conducted for a methodological investigation on samples from the ~2.41 Ga Widgiemooltha dikes for which reliable bulk rock paleointensity determinations have been obtained. The suitability of the crystals from the proposed dike suites for paleointensity experiments has been demonstrated by pilot rock magnetic and paleointensity investigations. This project will increase the number of high-quality paleointensity determinations for the Precambrian providing insights into the mechanisms of the geodynamo and the long-term evolution of Earth. Importantly, the age of selected dike suites will allow us to test the hypothesis of a transition from a strong field geodynamo produced within a basal-mantle magma ocean before ~2.45 Ga to a weak field geodynamo produced within the liquid core without the solid inner core ("the Proterozoic Dipole Low"). The proposed comparison of rock magnetic properties and paleointensities from the bulk rock and single crystal samples derived from the same rock unit will advance our understanding of the processes of rock and magnetic mineral formation and alteration, which may affect the fidelity of rocks and crystals as paleointensity recorders. The project will support a graduate (Ph.D.) student and several undergraduate research assistants.