Signals from the ancient geodynamo: A paleomagnetic field test on the Jack Hills metaconglomerate
Signals from the ancient geodynamo: A paleomagnetic field test on the Jack Hills metaconglomerate
复制标题
来自古代地球发电机的信号：对杰克山超砾岩的古磁场测试
DOI：
10.1016/j.epsl.2013.02.008
复制
发表时间：
2013
影响因子：
5.3
通讯作者：
R. Cottrell
中科院分区：
文献类型：
--
作者：
J. Tarduno;R. Cottrell
地球磁场最古老的历史不能直接从现存的大块岩石中读出，因为随后的变质作用温度接近或超过了普通磁性矿物的居里温度。西澳大利亚的杰克山变质沉积层的峰值变质温度较低，其中含有距今44亿年的锆石。为了评估这些沉积物是否保留了地球动力的古代信号，我们在杰克山的一个含鹅卵石的单元上进行了古磁砾岩测试。热退磁显示出明显的磁性成分，其解封温度在~ 550 ~ 580°C之间，通过砾岩测试，表明在砾岩沉积之前磁化。这一结果与岩石磁性数据一起表明，高解封温度成分是由磁铁矿携带的，磁铁矿记录了周围磁场的磁化强度，最简单的解释是发电机的存在。现有的地质年代学数据表明，这些碎屑中可能含有矿物混合物，其年龄仅比最大沉积年龄（30.5亿年前）稍早。然而，这里报告的阳性砾岩测试表明，杰克山的变质沉积层有可能在碎屑或亚碎屑矿物尺度上记录古奥古世到冥古宙的磁场。
The oldest history of Earth's magnetic field cannot be directly read from extant bulk rocks because of subsequent metamorphism at temperatures close to or exceeding the Curie temperature of common magnetic minerals. The Jack Hills metasediments of Western Australia, which have seen lower peak metamorphic temperatures, contain zircons as old as ∼4.4 billion-yr-old. To assess whether these sediments can retain an ancient signal of the geodynamo, we present a paleomagnetic conglomerate test on a cobble-bearing Jack Hills unit. Thermal demagnetization reveals a distinct magnetic component with high unblocking temperatures between ∼550 and 580°C that passes the conglomerate test, indicating magnetization prior to deposition of the conglomerate. This result, together with rock magnetic data, indicates that the high unblocking temperature component is carried by magnetite which records magnetization in an ambient field, and the simplest explanation is that a dynamo was present. Existing geochronological data imply that the clasts could contain mixtures of minerals extending to ages only slightly older than the maximum depositional age at 3.05 billion-yr-ago. However, the positive conglomerate test reported here indicates that the Jack Hills metasediments have the potential to record Paleorchean to Hadean magnetic fields, on a clast or sub-clast mineral scale.