JpGU-AGU Joint Meeting 2017

講演情報

[EE] 口頭発表

セッション記号 S (固体地球科学) » S-EM 固体地球電磁気学

[S-EM18] [EE] General Contributions in Geomagnetism, Paleomagnetism, and Rockmagnetism

2017年5月20日(土) 15:30 〜 17:00 A07 (東京ベイ幕張ホール)

コンビーナ:小田 啓邦(産業技術総合研究所地質情報研究部門)、望月 伸竜(熊本大学大学院先導機構)、Joshua M Feinberg(University of Minnesota)、Myriam Annie Claire Kars(Center for Advanced Marine Core Research)、座長:望月 伸竜(熊本大学大学院先導機構)、座長:Lascu Ioan(University of Cambridge)、座長:Kars Myriam(Center for Advanced Marine Core Research)

16:35 〜 16:55

[SEM18-05] Pseudo single domain magnetite as a stable natural remanent magnetization carrier in obsidian

★招待講演

*Ioan Lascu1Matthew Ball1Joshua Einsle1Richard Harrison1 (1.University of Cambridge)

Most natural samples contain so-called “non-ideal” paleomagnetic recorders, which are magnetic particles larger than ideal, single domain recorders, but smaller than proper multi domain grains, which are poor paleomagnetic recorders. The grain size range for these recorders, which for magnetite comprises grains from ~100 nm to a few μm in size, is known as the pseudo single domain (PSD) domain state. Natural samples containing abundant PSD grains have been shown to reliably record thermomagnetic remanent magnetizations that are stable over billions of years. Here we investigate obsidian varieties from Glass Butte, Oregon, USA, which present the opportunity to study the simple case of PSD grains encapsulated in volcanic glass. To do this, we combine paleointensity experiments, rock magnetism, scanning electron microscopy (SEM) nanotomography, and finite-element micromagnetic modelling. Results from the Thellier-IZZI protocol indicate that PSD grains acquire a thermoremanent magnetization efficiently and have high blocking temperatures, similar to stable single domain grains. Using rock magnetism we identify PSD signatures via their diagnostic fingerprint in first-order reversal curve (FORC) diagrams. Tomographic reconstructions obtained by stacking SEM images acquired via sequential milling through sample volumes of a few tens of cubic μm reveal the presence of abundant grains that span the PSD grain size interval. These grains have a variety of shapes, from simple ellipsoidal particles, to more complex morphologies attained via the coalescence of neighbouring grains during crystallization, to intricate “rolling snowball” morphologies that formed during growth in a dynamic environment as the flowing lava cooled. Micromagnetic modelling of the simplest morphologies reveals that these grains are in single vortex states, with the remanence controlled by irregularities in grain morphology. Larger grains contain complex, multi-vortex structures and incipient domain walls, with remanence being controlled by the collection of PSD states from areas with pronounced shape anisotropy. Modelling the properties of these grains as a function of field and temperature allows a better understanding of PSD remanence acquisition in natural samples.