GP23B-1308
Do Jack Hills Detrital Zircons Contain Records of the Early Geodynamo?

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Benjamin P Weiss1, Adam C Maloof2, Nicholas D Tailby3, Jahandar Ramezani4, Roger R Fu5, David R. Glenn6, Pauli Kehayias6, Ronald L Walsworth6, Veronica Hanus5, Dustin Trail7, E Bruce Watson3, T. Mark Harrison8, Samuel A Bowring9, Joseph L Kirschvink10, Nicholas Swanson-Hysell11, Robert S Coe12, Joshua Franz Einsle13 and Richard J Harrison13, (1)MIT, Earth, Atmospheric and Planetary Sciences, Cambridge, MA, United States, (2)Princeton University, Princeton, NJ, United States, (3)Rensselaer Polytechnic Institute, Troy, NY, United States, (4)MIT-EAPS, Cambridge, MA, United States, (5)Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (6)Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, United States, (7)University of Rochester, Rochester, NY, United States, (8)University of California Los Angeles, Los Angeles, CA, United States, (9)Massachusetts Institute of Technology, Cambridge, MA, United States, (10)California Institute of Technology, Pasadena, CA, United States, (11)University of California Berkeley, Earth and Planetary Science, Berkeley, CA, United States, (12)University of California Santa Cruz, Santa Cruz, CA, United States, (13)University of Cambridge, Cambridge, United Kingdom
Abstract:
It is unknown when Earth’s dynamo magnetic field originated. With crystallization ages ranging from 3.0-4.38 Ga, detrital zircon crystals found in the Jack Hills of Western Australia might preserve a record of the missing first billion years of Earth’s magnetic field history. Recently, Tarduno et al. (2015) argued that magnetization in Jack Hills zircons provides evidence for a substantial geomagnetic field dating back to their U/Pb formation ages (3.3 and 4.2 Ga). However, the identification of such ancient field records requires establishing that the zircons have avoided remagnetization since their formation. At a minimum, it should be demonstrated that they have not been remagnetized since being deposited at ~3.0 Ga. To establish the timing and intensity of the metamorphic and alteration events experienced by the zircon, we conducted 12 paleomagnetic field tests in combination with U-Pb geochronology on their host rocks (see Weiss et al. 2015, EPSL). Our data show that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely from emplacement of the Warakurna large igneous province at 1.1 Ga (see Figure).

Even if some Jack Hills zircons do record a pre-depositional magnetization, they still could have been remagnetized sometime during the 1.4 Gy between their crystallization and their deposition. First, the temperatures capable of remagnetizing magnetite inclusions are well below those that could reset a U-Pb date or result in significant discordance. Therefore, thermal events capable of completely remagnetizing Jack Hills zircons could be undetected by the techniques reported by Tarduno at al. (2015).

Second, the zircons’ magnetization might be dominated by secondary ferromagnetic inclusions or contamination. To address the latter possibility, we are conducting electron microscopy, x-ray tomography, and magnetic field mapping on the zircons. Our initial quantum diamond magnetometry high-resolution (<20 um) imaging has found that the magnetization of many zircons resides in grain exteriors, suggesting a dominantly secondary origin. We conclude that there is currently no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons. We are continuing to search for such magnetic records.

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