Paleointensity, morphology, and stability of the Proterozoic geomagnetic field as recorded by mafic dikes in India

印度镁铁质岩脉记录的元古代地磁场的古强度、形态和稳定性

• 批准号: 1112952
• 负责人: Aleksey Smirnov
• 金额: $ 20.44万
• 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: 2011
• 资助国家: 美国
• 起止时间: 2011-09-01 至 2015-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1112952&HistoricalAwards=false
• 关键词: Paleointensity; morphology; stability; Proterozoic; geomagnetic

The Earth's magnetic field is generated by convection of molten iron in the planet's outer core. This process, or the geodynamo, is powered by density settling and heat of fusion of the crystallizing inner core. The Earth's magnetic field can be well approximated by that of a dipole (i.e. bar magnet) positioned at Earth's center and aligned with the rotational axis. This allows us to apply the dipole equations to reconstruct past positions of continents using the direction of ancient magnetic field recorded in rocks (fossil magnetism). It is well established that the field has had this dipolar geometry for at least 500 milion years, and that the field has been strong enough to provide magnetic shielding of the biosphere and atmosphere from solar radiation. However, our knowledge of the field characteristics for earlier times remains limited. Current thinking suggests that the solid inner core formed sometime during the Proterozoic eon (2500 to 542 million years ago). Before the inner core formation, the geodynamo could have produced a weaker and less stable magnetic field with large episodic deviations from the dipolar geometry. An attendant weaker magnetic shielding would allow solar radiation to affect the life evolution and atmospheric chemistry. Our research seeks to obtain high-quality data on the Proterozoic field by investigating the fossil magnetism of several suites of quickly-cooled intrusive Proterozoic rocks in Peninsular India. This study will provide important insight in our understanding the mechanism that generates Earth's magnetic field and would have important implications in how we use the magnetic field records to decipher the geological history of our planet, including the age of the inner core. Broader implications of the study include better understanding of a potential link between the evolution of Earth's magnetic field and the evolution of biosphere and atmosphere. The project will involve undergraduate students at Michigan Tech in the laboratory analyses of the samples thus training the next generation of scientists. This research will also become a part of a Ph.D. thesis. 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 behavior and configuration of the geomagnetic field during the Precambrian are crucial in understanding the nature of Earth's early geodynamo. These data are also important for investigating potential causative links between the evolution of geomagnetic field and other components of the Earth system. For example, a weak or unstable field of an early geodynamo could result in weaker magnetosphere shielding and, hence, a stronger effect on the atmosphere and biosphere from solar and cosmic radiation. In addition, long-term trends in the strength and stability of geomagnetic field may provide insight into the timing of some important transitions in the Earth's interior such as the formation and growth of the solid inner core. In the absence of strict theoretical constraints, paleomagnetic data become a principal source of information about the Precambrian field. However, our knowledge of the field characteristics during the first four billion years of Earth history remains very limited. In particular, the database on the field strength contains only a handful of reliable data points. In our project, we will investigate the strength, morphology, and stability of the Proterozoic geodynamo by detailed paleodirectional and paleointensity analyses of five mafic dike swarms in Peninsular India, which have been reliably dated and shown to contain pristine paleomagnetic records. Our proposed targets are the~2.37 Ga and 2.18 Ga swarms in the Dharwar craton, the ~1.99 Ga swarm in the Bundelkhand craton, the ~1.89 Ga swarm in the southern Bastar craton, and ~0.75 Ga swarm of the Malani Igneous Suite in the Marwar Terrain. In this study, we will use both traditional and novel approaches such as single crystal technique for paleointensity determinations. This study will provide a synoptic view of the Proterozoic geodynamo, including time-averaged paleointensity values, estimates of the relative contributions from dipole and non-dipole field components, and paleolatitude-dependent estimates of paleosecular variation.

地球的磁场是由地球外核的熔融铁对流产生的。这个过程，或称地球发电机，是由密度沉降和结晶内核的熔化热提供动力的。地球的磁场可以很好地近似为位于地球中心并与旋转轴对齐的偶极子（即条形磁铁）。这使我们能够应用偶极子方程，利用岩石中记录的古代磁场方向（化石磁性）来重建大陆过去的位置。已经确定的是，磁场具有这种偶极几何形状至少有5亿年，并且磁场足够强大，可以为生物圈和大气层提供磁屏蔽，使其免受太阳辐射的影响。然而，我们对早期磁场特征的了解仍然有限。目前的想法表明，固体内核形成于元古代（2500至5.42亿年前）的某个时候。在内核形成之前，地球发电机可能会产生一个较弱和较不稳定的磁场，与偶极几何形状有很大的幕式偏差。随之而来的较弱的磁屏蔽将允许太阳辐射影响生命进化和大气化学。我们的研究旨在通过调查印度半岛几套快速冷却的侵入元古代岩石的化石磁性来获得元古代领域的高质量数据。这项研究将为我们理解产生地球磁场的机制提供重要的见解，并将对我们如何使用磁场记录来破译地球的地质历史，包括内核的年龄产生重要影响。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潜在联系。该项目将让密歇根理工大学的本科生参与样本的实验室分析，从而培训下一代科学家。这项研究也将成为博士学位的一部分。论文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研究结果。有关前寒武纪地磁场长期行为和结构的数据对于了解地球早期地球发电机的性质至关重要。这些数据对于研究地磁场演变与地球系统其他组成部分之间的潜在因果关系也很重要。例如，早期地球发电机的弱场或不稳定场可能导致较弱的磁层屏蔽，因此，太阳和宇宙辐射对大气和生物圈的影响更大。此外，地磁场的强度和稳定性的长期趋势可以提供对地球内部一些重要转变的时间的洞察，例如固体内核的形成和增长。在缺乏严格理论约束的情况下，古地磁数据成为有关前寒武纪磁场的主要信息来源。然而，我们对地球历史最初40亿年期间磁场特征的了解仍然非常有限。特别是，关于场强的数据库只包含少数可靠的数据点。在我们的项目中，我们将调查的强度，形态和稳定性的元古代地球发电机的详细古方向和古强度分析的五个基性岩墙群在印度半岛，这已被可靠地定年，并显示包含原始的古地磁记录。我们建议的目标是达尔瓦尔克拉通的~2.37 Ga和~ 2.18 Ga岩群，本德尔坎德克拉通的~1.99 Ga岩群，巴斯塔克拉通南部的~1.89 Ga岩群，以及马尔瓦尔地体的马拉尼火山岩套的~0.75 Ga岩群。在这项研究中，我们将使用传统的和新的方法，如单晶技术的古强度测定。这项研究将提供一个元古宙地球发电机的天气视图，包括时间平均古强度值，估计偶极和非偶极场分量的相对贡献，和古纬度依赖的估计古长期变化。