Possible Eoarchean records of the geomagnetic field preserved in the Isua Supracrustal Belt, southern west Greenland

We present paleomagnetic field tests that hint that a record of Earth’s 3.7-billion-year (Ga) old magnetic field may be preserved in the banded iron formation from the northeastern Isua Supracrustal belt. Magnetite in the banded iron formation has a Pb-Pb age of 3.7 Ga (Frei et al., 1999). The U-Pb system has a closure temperature between 150-400°C for the magnetite grain size range observed in the banded iron formation, suggesting the rocks have not been significantly heated since magnetization was acquired. We use a `pseudo' baked contact test to assess paleodirections in the banded iron formation that have avoided thermal overprints from subsequent igneous intrusions. We demonstrate that specimens that pass this test also go on to pass fold and reversal tests. We argue that the banded iron formation acquired a chemical remanent magnetization via magnetite growth during a multi-phase Eoarchean metamorphic event. We recover what appears to be the oldest known whole rock record of the geomagnetic field, and oldest known records of reversals and secular variation suggesting that Earth’s magnetic field behaviour in the Eoarchean may have been similar to that observed today.本研究提出的古地磁测试结果表明，伊苏超壳带东北部的条带状铁建造（banded iron formation, BIF）中或保存了地球37亿年（Ga）前的古地磁记录。该条带状铁建造中的磁铁矿的铅-铅年龄为37 Ga（Frei等，1999）。针对该条带状铁建造中观测到的磁铁矿粒径范围，铀-铅体系的封闭温度介于150~400℃之间，这表明岩石自获得磁化以来未遭受显著热扰动。我们采用“伪”烘烤接触测试（pseudo baked contact test）来评估条带状铁建造中的古地磁方向，该方法可规避后续火成侵入活动带来的热叠加效应。研究发现，通过该测试的样品同时可通过褶皱测试与倒转测试。我们认为，该条带状铁建造是在多幕式始太古代变质事件中通过磁铁矿生长获得了化学剩余磁化强度。本研究获取了目前已知最古老的全岩地磁场记录，以及最古老的磁场倒转与长期变化记录，这表明始太古代时期地球的磁场行为或与现代观测结果相近。