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Published November 15, 2015 | Supplemental Material
Journal Article Open

Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early geodynamo


It currently is unknown when Earth's dynamo magnetic field originated. Paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3.5 billion years ago (Ga). Detrital zircon crystals found in the Jack Hills of Western Australia are some of the very few samples known to substantially predate this time. With crystallization ages ranging from 3.0–4.38 Ga, these zircons might preserve a record of the missing first billion years of Earth's magnetic field history. However, a key unknown is the age and origin of magnetization in the Jack Hills zircons. The identification of >3.9 Ga (i.e., Hadean) field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz-rich conglomerates at 2.65–3.05 Ga. To address this issue, we have conducted paleomagnetic conglomerate, baked contact, and fold tests in combination with U–Pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks. These tests include the first conglomerate test directly on the Hadean-zircon bearing conglomerate at Erawandoo Hill. Although we observed little evidence for remagnetization by recent lightning strikes, we found that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely due to thermal and/or aqueous effects from the emplacement of the Warakurna large igneous province at ∼1070 million years ago (Ma). Although localized regions of the Jack Hills might have escaped complete remagnetization, there currently is no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons.

Additional Information

© 2015 Elsevier B.V. Received 11 May 2015; Received in revised form 29 July 2015; Accepted 30 July 2015; Available online 28 August 2015. We thank S. Mojzsis for introducing us to the Jack Hills in 2001, the Geological Survey of Western Australia for access to the Jack Hills Geoheritage Reserve, the residents and goat of Beringarra Station for logistical help, Crosslands Resources for showing us their iron ore mine, M. Wingate and R. Pidgeon for helpful discussions and logistical assistance, and D.A.D. Evans for helpful discussions on polar wander. We also thank M. Jackson from the Institute for Rock Magnetism and S. Ceballos, S. Bouma, and F. Hellman from U. C. Berkeley for assistance with magnetometry measurements, B. Carbone for administrative help, R. Bowens–Rubin for acquiring the EHJH5 measurements, and the NSF Early Concept Grants for Exploratory Research program grant EAR-1226293 for support. The UCLA ion microprobe facility is in part supported through a grant from NSF/EAR Instrument and Facilities Program. Partial support was also provided by the NASA Astrobiology Institute (grant NNA09DA80A to The New York Center for Astrobiology). We thank B. Buffett for editorial work and D.A.D. Evans and an anonymous reviewer for helpful reviews of the manuscript.

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