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

单硅酸盐晶体记录的古元古代地磁场的古强度：测试“元古代偶极低”

• 批准号: 1519967
• 负责人: Aleksey Smirnov
• 金额: $ 25万
• 依托单位: Michigan Technological University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-08-15 至 2019-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1519967&HistoricalAwards=false
• 关键词: Paleointensity; Paleoproterozoic; Geomagnetic; Field; Recorded

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.

该项目将通过获得关于古代地球磁场强度（古强度）的高质量数据，促进对地球早期演变的了解。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，由密度沉降和结晶内核的熔化热提供动力。然而，根据大多数模型，内部固体核心相对年轻，可能只存在于地质历史的最后5亿年。与此同时，已经确定地磁场至少在35亿年前就已经存在。因此，在内核形成之前，地球发电机机制应该是不同的。特别是，最近的一个假说表明，在24.5亿年前，一个强大的磁场产生于基底-地幔岩浆海洋中，一层熔融的硅酸盐地幔在核幔边界之上。在地幔完全凝固后，在完全液态的地核内产生了一个较弱和较不稳定的磁场（“元古代偶极低压”）。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（古磁场强度的测量）的时间段（19.8和24.1亿年前）紧接着消亡的建议的基底地幔海洋地球发电机来测试这一假设。古强度实验将在印度、加拿大和澳大利亚的几套快速冷却的侵入性古元古代岩石上进行，使用基于硅酸盐单晶研究的新技术。这项研究将为了解地球磁场的产生机制和地球深部（地核和地幔）的演变提供重要的见解。这项研究的更广泛意义包括更好地理解地球磁场演变与生物圈和大气演变之间的联系。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，将通过一系列科学探索会议传播成果，关于前寒武纪地球磁场强度（古强度）长期演变的数据对于了解早期地球发电机的性质至关重要。这些数据还可以提供对地球内部重要过程的深入了解，例如固体内核的形成和增长，或影响地球发电机作用力的地幔对流的长期变化。然而，我们对前寒武纪古地震烈度的认识仍然非常有限。由于样品的热致蚀变，传统的古强度方法应用于前寒武纪岩石时往往成功率较低。为了解决这个问题，一种新的技术已经开发出来，使用单个的岩石形成硅酸盐晶体来测量古强度。这种晶体通常含有单畴至伪单畴铁磁内含物，这些内含物受到硅酸盐主体的保护而不受自然蚀变的影响，并且相对于实验蚀变是稳定的。本项目的研究目标是通过对印度三个基性岩墙群中分离出的斜长石进行古强度分析，研究元古代地球发电机的强度（~2.37 Ga班加罗尔岩脉、~2.18 Ga Dharwar克拉通Mahhubnagar岩脉和~1.98 Ga Bundelkhand克拉通岩脉），以及加拿大苏必利尔上级基隆的~2.12 Ga和~2.10 Ga马拉松岩脉。此外，单晶古强度测定将进行方法学研究的样品从~2.41 Ga Widgiemooltha脉已获得可靠的块体岩石古强度测定。从拟议的岩脉套房的晶体paleointensity实验的适用性已被证明由试点岩石磁性和paleointensity调查。 该项目将增加前寒武纪高质量古强度测定的数量，为地球发电机的机制和地球的长期演化提供见解。重要的是，年龄选定的岩脉套件将使我们能够测试的假设，从一个强场geodynamo内产生的基底-地幔岩浆海洋前2.45 Ga到一个弱场geodynamo内产生的液体核心没有固体内核（“元古代偶极子低”）的过渡。来自同一岩石单元的大块岩石和单晶样品的岩石磁性和古强度的拟议比较将促进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。该项目将支持一个研究生（博士）学生和几名本科生研究助理。