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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.

Hadean (≥4.0 Ga) zircon grains provide the only direct record of the first half billion years of Earth's history. Determining accurate and precise crystallization ages of these ancient zircons is a prerequisite for any interpretation of crustal evolution, surface environment and geodynamics on the early Earth, but this may be compromised by mobilization of radiogenic Pb due to subsequent thermal overprinting. Here we report a detrital zircon from the Jack Hills (Western Australia) with 4486-4425 Ma concordant ion microprobe ages that yield a concordia age of 4463 ± 17 Ma (2σ). These are the oldest zircon ages recorded from Earth. However, scanning ion imaging reveals that these >4.4 Ga apparent ages resulted from incorporation of micrometre-scale patches of unsupported radiogenic Pb with extremely high 207Pb/206Pb ratios corresponding to >4.5 Ga 207Pb/206Pb ages. These patches likely resulted from redistribution of radiogenic Pb into nanometer-scale domains in a ~4.3 Ga zircon during a ~3.8 Ga or older event. This highlights that even a concordia age can be spurious and should be carefully evaluated before being interpreted as the crystallization age of ancient zircon.

subduction beneath Laurentia Enriched Grenvillian lithospheric mantle as a consequence of long-lived Email alerting services articles cite this article to receive free e-mail alerts when new www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science. This file may not be posted to any Web site, but authors may post works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent their employment. Individual scientists are hereby granted permission, without fees or further

-The North Singhbhum Mobile Belt (NSMB) is a 200 km long, curved Proterozoic foldthrust belt that skirts the northern margin of the Archean Singhbhum Craton of NE India. The Singhbhum Shear Zone (SSZ) developed between the Dhanjori and Chaibasa formations near the southern margin of the NSMB and represents an important Cu-U-P metallotect. A SHRIMP U-Pb zircon date of 1861 ± 6 Ma, obtained for the syn-to post-kinematic Arkasani Granophyre that has intruded the SSZ, provides a minimum age for the prolonged tectonic activity and mineralization along the SSZ and for the time of closure of the Chaibasa and Dhanjori sub-basins. The Dalma Volcanic Belt, a submarine rift-related bimodal mafic-felsic volcanic suite, forms the spine of the NSMB. A SHRIMP U-Pb zircon igneous crystallization date of 1631 ± 6 Ma was obtained for an unfoliated felsic volcanic rock from the base of the Dalma volcanic sequence. These new findings suggest that the different sub-basins in the NSMB evolved diachronously under contrasting tectonic environments and were juxtaposed during a later orogenic movement.

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