Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo
Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo
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DOI：
10.1073/pnas.1916553117
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发表时间：
2020-01
影响因子：
11.1
通讯作者：
J. Tarduno;R. Cottrell;R. Bono;H. Oda;W. Davis;M. Fayek;O. V. Erve;F. Nimmo;Wentao Huang;E. Thern;S. Fearn;G. Mitra;A. V. Smirnov;E. Blackman
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文献类型：
--
作者：
J. Tarduno;R. Cottrell;R. Bono;H. Oda;W. Davis;M. Fayek;O. V. Erve;F. Nimmo;Wentao Huang;E. Thern;S. Fearn;G. Mitra;A. V. Smirnov;E. Blackman
地球地磁场的年龄和早期历史可以提供对地核和大气演化的洞察。但是测量冥古宙锆石--已知最古老的地球物质--以及确定它们磁化的古老性是古地磁学中最具挑战性的工作之一。新的古地磁、电子显微镜、地球化学和古强度数据表明，在选定的锆石中存在原生磁铁矿包裹体。这些数据支持了地球磁场的存在，以及相关的大气层对太阳风的屏蔽，大约142亿年前。这些锆石在140亿年前记录到的相对较强的磁场可能是地核中化学沉淀为地球发电机提供动力的信号。确定地磁场的年龄对于了解地球的演变至关重要，因为地磁场可以保护大气层免受太阳风的侵蚀。地磁场的存在或不存在也为早期核心条件提供了独特的衡量标准。对杰克山（西澳大利亚州）锆石的古地磁分析表明，在月球形成的巨大撞击之后的几亿年，地磁场的年龄为42亿年（戈伊）。在此，我们提供了新的古地磁和电子显微镜分析，证明了这些锆石中的磁铁矿所携带的主剩磁的存在和新的地球化学数据表明，选择冥古宙锆石逃脱磁复位，因为它们的形成。新的paleointensity和Pb-Pb放射性年龄数据从额外的锆石满足强大的选择标准提供了进一步的证据，为保真度的磁记录，并建议在4.1至4.0十亿年前（Ga）的高地磁场强度的时期，可能代表有效的对流有关的化学沉淀在地球的Hadean液态铁的核心。
Significance The age and early history of Earth’s geomagnetic field can provide insight into the evolution of the core and atmosphere. But measurement of Hadean zircons—the oldest known terrestrial materials—and the determination of the antiquity of their magnetizations are amongst the most challenging endeavors in paleomagnetism. New paleomagnetic, electron microscope, geochemical, and paleointensity data indicate the presence of primary magnetite inclusions in select zircons. These data support the presence of the geomagnetic field, and associated shielding of the atmosphere from the solar wind, ∼4.2 billion years ago. A relatively strong field recorded by these zircons at ∼4 billion years ago may be a signal that chemical precipitation in the core was powering the geodynamo. Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth’s Hadean liquid iron core.