Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo
Secondary magnetite in ancient zircon precludes analysis of a Hadean geodynamo
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DOI：
10.1073/pnas.1811074116
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发表时间：
2018-12
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通讯作者：
Fengzai Tang;Richard J. M. Taylor;J. Einsle;C. Borlina;R. Fu;Benjamin P. Weiss;Helen M. Williams;W. Williams;L. Nagy;Paul A. Midgley;E. Lima;Elizabeth A. Bell;T. M. Harrison;Ellen W. Alexander;R. Harrison
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作者：
Fengzai Tang;Richard J. M. Taylor;J. Einsle;C. Borlina;R. Fu;Benjamin P. Weiss;Helen M. Williams;W. Williams;L. Nagy;Paul A. Midgley;E. Lima;Elizabeth A. Bell;T. M. Harrison;Ellen W. Alexander;R. Harrison
地球的地球发电机对保护我们的大气层至关重要，因此在我们星球的可居住性方面发挥着重要作用。因此，地球的磁场可能在大约40亿年前的Hadean-Archean Eons期间的生命出现中发挥了关键作用。然而，我们对地球发电机在这一关键时期的行为知之甚少。最近的努力集中在杰克山锆石晶体（最古老的地球物质）所蕴含的磁信号上。在这里，我们展示了这种颗粒中的磁性载体。我们的研究结果表明，尽管古老的锆石颗粒可能含有理想的磁性记录器，但它们并没有记录锆石生长时的磁场强度。来自西澳大利亚杰克山的锆石晶体是地球前5亿年为数不多的幸存矿物学记录之一，并被认为包含了冥古宙地球发电机的古地磁记录。保存冥古宙磁化的先决条件是原始火成岩锆石中存在原生磁性包裹体。到目前为止，还没有在古代锆石中的磁记录器的图像。在这里，我们使用高分辨率透射电子显微镜来证明，所有观察到的夹杂物是通过两种不同的机制形成的次要功能。磁铁矿是通过管道扩散机制产生的，由此铁沿着位错的核沿着扩散到辐射损伤的锆石中，并在纳米孔内沉淀，并且还在存在水性流体的情况下在辐射损伤的锆石的低温重结晶期间沉淀。虽然这些磁铁矿可以识别为次生使用透射电子显微镜，否则它们发生在地区，是无法区分的原始火成岩锆石和携带reversible磁化，晚于结晶年龄至少几亿年。没有显微镜证据排除次生磁铁矿，古地磁的情况下，一个Hadean-Eoarchean地球发电机还不能作出。
Significance The Earth’s geodynamo is critical in protecting our atmosphere, and thus plays an important role in the habitability of our planet. As such, the Earth’s magnetic field has likely played a crucial role in the emergence of life around 4 billion years ago during the Hadean–Archean Eons. However, we know little about the behavior of the geodynamo during this critical period. Recent efforts have focused on the magnetic signals harbored by Jack Hills zircon crystals, the oldest terrestrial material. Here we show the magnetic carriers in such grains. Our results demonstrate that although ancient zircon grains may contain ideal magnetic recorders, they do not record the magnetic field strength at the time of zircon growth. Zircon crystals from the Jack Hills, Western Australia, are one of the few surviving mineralogical records of Earth’s first 500 million years and have been proposed to contain a paleomagnetic record of the Hadean geodynamo. A prerequisite for the preservation of Hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon. To date no images of the magnetic recorders within ancient zircon have been presented. Here we use high-resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms. Magnetite is produced via a pipe-diffusion mechanism whereby iron diffuses into radiation-damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low-temperature recrystallization of radiation-damaged zircon in the presence of an aqueous fluid. Although these magnetites can be recognized as secondary using transmission electron microscopy, they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years. Without microscopic evidence ruling out secondary magnetite, the paleomagnetic case for a Hadean–Eoarchean geodynamo cannot yet been made.