Department of Earth Sciences, University of Cambridge, CB2 3EQ Cambridge, United Kingdom.
Department of Earth Sciences, University of Cambridge, CB2 3EQ Cambridge, United Kingdom;
Proc Natl Acad Sci U S A. 2019 Jan 8;116(2):407-412. doi: 10.1073/pnas.1811074116. Epub 2018 Dec 31.
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.
来自西澳大利亚州杰克山的锆石晶体是地球最初 5 亿年中少数幸存下来的矿物记录之一,它们被认为包含了冥古宙发电机的古地磁记录。保存冥古宙磁化强度的一个前提是原始火成锆石中存在原生磁性包裹体。迄今为止,还没有展示过古代锆石中磁记录器的图像。在这里,我们使用高分辨率透射电子显微镜证明,所有观察到的包裹体都是通过两种不同机制形成的次生特征。磁铁矿是通过管道扩散机制形成的,铁沿着位错的核心扩散到辐射损伤的锆石中,并在纳米孔中沉淀,也在存在水溶液的情况下,在辐射损伤的锆石低温重结晶期间形成。尽管这些磁铁矿可以通过透射电子显微镜识别为次生矿物,但它们出现在与原始火成锆石无法区分的区域,并携带至少数千万年晚于结晶年龄的剩余磁化强度。如果没有排除次生磁铁矿的微观证据,那么冥古宙-太古宙发电机的古地磁案例就还不能成立。