Paleomagnetic studies on single crystals separated from the middle Cretaceous Iritono granite

Creators: Kato, Chie; Sato, Masahiko; Yamamoto, Yuhji; Tsunakawa, Hideo; Kirschvink, Joseph L.

Abstract

Investigations of superchrons are the key to understanding long-term changes of the geodynamo and the mantle's controlling role. Granitic rocks could be good recorders of deep-time geomagnetic field behavior, but paleomagnetic measurements on whole-rock granitic samples are often disturbed by alterations like weathering, and the presence of multi-domain magnetite. To avoid such difficulties and test the usefulness of single silicate crystal paleomagnetism, here we report rock-magnetic and paleomagnetic properties of single crystals and compare those to the host granitic rock. We studied individual zircon, quartz and plagioclase crystals separated from the middle Cretaceous Iritono granite, for which past studies have provided tight constraints on the paleomagnetism and paleointensity. The occurrence of magnetite was very low in zircon and quartz. On the other hand, the plagioclase crystals contained substantial amounts of fine-grained single-domain to pseudo-single-domain magnetite. Microscopic features and distinctive magnetic behavior of plagioclase crystals indicate that the magnetite inclusions were generated by exsolution. We therefore performed paleointensity experiments by the Tsunakawa–Shaw method on 17 plagioclase crystals. Nine samples passed the standard selection criteria for reliable paleointensity determinations, and the mean value obtained was consistent with the previously reported whole-rock paleointensity value. The virtual dipole moment was estimated to be higher than 8.9 ± 1.8 × 1022 Am2, suggesting that the time-averaged field strength during middle of the Cretaceous normal superchron was several times as large as compared to that of non-superchron periods. Single plagioclase crystals which have exsolved magnetite inclusions can be more suitable for identification of magnetic signals and interpretation of paleomagnetic records than the conventional whole-rock samples or other silicate grains.

Additional Information

© 2018 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Received: 3 July 2018; Accepted: 25 October 2018; Published: 13 November 2018. We thank Shinji Yamamoto for petrological discussions. The microscopic photograph of plagioclase sample (Fig. 7) was taken by Yujiro Tamura. We thank lead guest editor John Tarduno and two anonymous reviewers for their constructive comments. Rock- and paleomagnetic measurements were taken under the cooperative research program of Center for Advanced Marine Core Research (CMCR), Kochi University (Accept Nos. 16A009, 16B009, 17A028 and 17B028). This work was supported by the Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists (DC1) No. 15J11812. Authors' contributions: YY and HT collected the samples. CK conducted the magnetic measurements. All contributed to discussion and writing the manuscript. All authors read and approved the final manuscript. The authors declare that they have no competing interests. Availability of data and materials: The data and materials used in this study are available on request to the corresponding author, Chie Kato (chiekato15@gmail.com). Ethics approval and consent to participate: Not applicable. This work was supported by the Japan Society for the Promotion of Science (JSPS) Research Fellowship for Young Scientists (DC1) No. 15J11812.

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