Zircon crystals as recorder of the Earth’s magnetic field

(Guest post by Masahiko Sato)

Geomagnetic field paleointensity data provide critical information about the thermal evolution of the Earth, and the state of the geomagnetic field has been closely related to the surface environment. While it is pivotal to understand the variations in geomagnetic field intensity through the history of the Earth, data are still too scarce to resolve billion-year-scale geomagnetic field variation. To overcome this problem, much research in recent years has focused on paleointensity experiments using single silicate crystals, which often accompany magnetic mineral inclusions, such as plagioclase, quartz phenocryst, pyroxene, and olivine. Sato et al. (2015) focus on zircon crystals, which are highly resistant to weathering and may provide our only means to examine the oldest geomagnetic field, as a robust paleomagnetic recorder. They corrected zircon crystals from the Tanzawa tonalitic pluton, which have young crystallization ages and the clear thermal history, and reported the results for systematic rock-magnetic and paleomagnetic measurements.

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Figure 1. Photo images of zircon crystals. The images are composite image of several photos which were taken at different depth of focus. Scale bars are 100 μm.

As stability of the paleomagnetic record changes with the grain size and chemical composition of magnetic mineral, to measure primary remanent magnetization recorded by fine-grained magnetites is essential for reliable paleomagnetic data. However, so far there are few studies conducting magnetic measurement using a zircon single crystal. Sato et al. (2015) conducts systematic rock-magnetic and paleomagnetic measurements for a huge number of single zircon crystals. They measured 1037 grains of naturally recorded remanence (natural remanent magnetization, NRM) using superconducting quantum interference device (SQUID) magnetometer and revealed that 85 grains contain enough magnetic minerals to be measured in the SQUID magnetometer. They applied pulse magnetic field to these zircon grains and artificially imparted remanent magnetization (isothermal remanent magnetization, IRM) in the laboratory, and found that a low NRM/IRM ratio can be used as indicator of fine-grained magnetite and suitable paleomagnetic recorder. They also imparted thermoremanent magnetization (TRM) by heating and cooling in the magnetic field, which simulates acquisition process of NRM, for 12 samples satisfying the above criteria. As the results, the TRM intensity is comparable with that of NRM, and a rough estimation of the paleointensity using NRM/TRM ratios shows field intensities consistent with the average geomagnetic field intensity at the Tanzawa tonalitic pluton, indicating availability of the proposed criteria.

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Figure 2. The NRM intensity is plotted as a function of the IRM intensity. Diagonal lines indicate IRM/NRM ratios of 1, 0.1, 0.01, and 0.001, respectively. Vertical and horizontal grey lines indicate 4 × 10−12 Am2 intensity lines (detection limit). The single domain- and pseudo-single-domain-like samples are plotted as red and blue symbols, respectively.

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