Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan

Abstract

This paper reports on the rock-magnetic properties of single zircon crystals, which are essential for future work establishing the reliable paleointensity method using single zircon crystals. Zircon crystals used in this study were sampled from the Nakagawa River, which crosses the Tanzawa tonalitic pluton in central Japan. Rock-magnetic measurements were conducted on 1037 grains of zircons, but many of these measurements are below the limits of the sensitivity of the magnetometers employed. Isothermal remanent magnetizations (IRMs) of 876 zircon crystal are below the practical resolution of this study; we infer that these crystals contain no or only minute quantities of ferromagnetic minerals. The other zircon crystals contain enough magnetic minerals to be measured in the DC SQUID magnetometer. Am 2 (80 zircons), combining the rock-magnetic parameter, we proposed the sample-selection criteria for future study of paleointensity experiments using single zircon crystals. In the case that the samples had high coercivity (B c ) values (>10 mT) or high M NRM /M IRM values (>~0.1), main remanence carriers are probably pyrrhotite and these samples are inappropriate for the paleointensity study. In the case that the samples had low B c values (<10 mT) and low M NRM /M IRM values (<~0.1), main remanence carriers seem to be nearly pure magnetite with pseudo-single-domain grain sizes, and these samples are expected to appropriate for the paleointensity study. Total thermoremanent magnetization (TRM) acquisition experiments were also carried out for 12 samples satisfying the above criteria. The TRM intensity was 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 for last 5 Myr.

Widespread occurrence of silicate‐hosted magnetic mineral inclusions in marine sediments and their contribution to paleomagnetic recording

“…The importance of magnetic inclusions for paleomagnetic studies has been recognized since the discovery of ultrafine‐grained iron oxides in silicates in the Modipe Gabbro [ Evans et al ., ; Evans and Wayman , ]. More recently, there has been renewed interest in magnetic mineral inclusions due to the development of single‐crystal‐based paleomagnetic analysis and the much increased sensitivity of modern cryogenic magnetometers [e.g., Cottrell and Tarduno , ; Tarduno et al ., , , ; Muxworthy and Evans , ; Sato et al ., ]. Paleomagnetic signals from magnetic mineral inclusions have been investigated extensively to expand knowledge of past magnetic field behavior of the Earth and other bodies within the solar system [e.g., Tarduno et al ., , , ; Feinberg et al ., , ; Lappe et al ., , ; Muxworthy et al ., ; Usui et al ., ].…”

Special issue on “Recent advances in environmental magnetism and paleomagnetism”

“…A frontier letter by Sato et al (2015) presented the rock magnetic properties of 1037 single zircon crystals sampled from the Nakagawa River, which crosses the Tanzawa tonalitic pluton in central Japan, for the purpose of future paleointensity studies. The isothermal remanent magnetization (IRM) intensities of 161 zircon crystals (>~4 × 10 −12 Am), which contained enough magnetic minerals, could be measured in a DC SQUID magnetometer.…”

Scanning SQUID microscope system for geological samples: system integration and initial evaluation

“…Kirschvink et al (2015) reported the best sensitivity for a 2G SRM is ~10 −13 Am 2 using ultraclean quartz glass sample holder. The measurement of a small specimen, such as a magnetic inclusion in a single crystal (e.g., Sato et al 2015;Tarduno et al 2015), can be improved by using a SQUID microscope for measuring the magnetic moment. Recently, Lima and Weiss (2016) …”

“…The practical limit of a normal-type DC SQUID rock magnetometer is 4 × 10 −12 Am 2 (Sato et al 2015), whereas that of an improved magnetometer with a smaller bore of 6.2 mm is an order of magnitude smaller (Tarduno et al 2015). Recently, Fu et al (2016) used a scanning SQUID microscope to measure the magnetic moment of a set of single zircon crystals to estimate paleointensity.…”

“…A single mineral crystal such as zircon with magnetic inclusions has been used to measure natural remanent magnetization (e.g., Sato et al 2015;Tarduno et al 2015), which could be used to determine the magnetic field of the Earth in the distant past (Tarduno et al 2015). The practical limit of a normal-type DC SQUID rock magnetometer is 4 × 10 −12 Am 2 (Sato et al 2015), whereas that of an improved magnetometer with a smaller bore of 6.2 mm is an order of magnitude smaller (Tarduno et al 2015).…”