GP43A-1236
Evaluating the Paleomagnetic Potential of Zircons

Thursday, 17 December 2015
Poster Hall (Moscone South)
Eduardo A. Lima1, Roger R Fu2, Benjamin P Weiss3, David R. Glenn4, Pauli Kehayias4 and Ronald L Walsworth4, (1)Massachusetts Institute of Technology, Cambridge, MA, United States, (2)Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (3)MIT, Earth, Atmospheric and Planetary Sciences, Cambridge, MA, United States, (4)Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, United States
Abstract:
Because zircon crystals commonly display high natural U/Pb ratios and excellent resistance to weathering, paleomagnetic data collected from zircons potentially enjoy the benefits of excellent age controls and minimal remagnetization from infiltrating fluids. We present rock magnetic and paleomagnetic experiments on two sets of zircons with contrasting geologic histories to determine the viability of zircons as paleomagnetic recorders.

First, we characterize primary zircons from the Bishop Tuff, a pyroclastic deposit formed at 767±1 ka in a magnetic field of 43±3 µT. Magnetic field maps with ~10 µm resolution obtained with the nitrogen vacancy (NV) diamond magnetometer indicate that most ferromagnetic sources are situated within zircon interiors, suggesting a primary origin (Fig. 1A). Stepwise thermal demagnetization reveals well-defined components of magnetization blocked in most samples up to 580˚C, indicating the dominance of magnetite, which is the expected primary phase. The intensity of natural remanent magnetization (NRM) is typically 10-12 Am2. Ongoing Thellier-Thellier dual heating experiments will evaluate the accuracy of recovered paleointensities.

Second, we study Hadean and Archean detrital zircons from the Jack Hills. In contrast to the Bishop Tuff samples, magnetic microscopy and stepwise thermal demagnetization demonstrate that the remanent magnetization of >80% of Jack Hills zircon are carried exclusively by secondary hematite situated on grain surfaces (Fig. 1B). NRM intensities range between 10-15 and 10-12 Am2 and decrease by a factor of several upon chemical removal of secondary hematite.

Our analyses reveal a diversity of ferromagnetic mineralogies and distribution in natural zircons. While some zircon populations carry reliable paleomagnetic information, others are dominated by secondary ferromagnetic phases. Without the application of high-resolution magnetic microscopy techniques to identify the main ferromagnetic carrier, it is difficult to establish that Jack Hills zircons retain a credible primary record of the Hadean and Archean geomagnetic field.

<< Previous Abstract | Next Abstract >>