Physics of the Earth and Planetary Interiors 177 (2009) 12–18 Contents lists available at ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi Ancient stable magnetism of the Richardton H5 chondrite Yongjae Yu a , Seong-Jae Doh b,∗ , Wonnyon Kim b , Kyoungwon Min c a Department of Geology and Earth Environmental Sciences, Chungnam National University, Daejeon 305-764, Republic of Korea b Department of Earth and Environmental Sciences, Korea University, Seoul 136-713, Republic of Korea c Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA article info Article history: Received 2 October 2008 Received in revised form 1 June 2009 Accepted 1 July 2009 Keywords: Richardton Chondrite Meteorite Fe–Ni system Kamacite abstract Investigating mineral magnetic properties of meteorites is essential to understanding the formation and evolution of planetary bodies in the solar system. In order to decipher ancient magnetic records, demag- netization experiments were carried out for the ∼4550 Ma Richardton H5 chondrite. Alternating-field demagnetization revealed a soft fraction as well as a hard fraction. Conventional thermal demagnetiza- tion in air showed severe alterations. But, a few thermal demagnetizations in vacuum were successful in isolating a stable paleomagnetic record. On the basis of microscopic analysis, we found that fine-grained clinopyroxene-hosted kamacite is abundant, responsible for the stable and permanent magnetic record of Richardton. The experimental data imply a thermal or thermochemical origin for the stable paleomag- netic record of Richardton. However, the possibility of pressure (re)magnetization cannot be evaluated because the effect of pressure on magnetization for the Fe–Ni system is unknown. © 2009 Elsevier B.V. All rights reserved. 1. Introduction As the most common type of meteorites, chondrites originate from debris of the solar nebula (e.g., Hutchison, 2004; Zanda, 2004). Due to their unique origin, chondrites are regarded as the sole witness for the formation of the early solar system. Despite such importance in unraveling the history of the early solar system, intensive scientific investigation on meteorites has been limited mainly because of the scarcity of the available samples and because meteorites are easily altered when reheated (e.g., Pellas and Storzer, 1981; Clayton, 1993). For instance, samples are hardly recyclable in other scientific investigation once they were heat-treated. Paleo- magnetic analysis of meteorites is of particular interest because it provides basic information on the evolution of planetary material (e.g., Nagata, 1979, 1980; Wasilewski and Dickinson, 2000; Weiss et al., 2000; Antretter et al., 2003). In particular, deciphering the ori- gin of natural remanent magnetization (NRM) of chondrites bears broad implications for asteroidal differentiation and collision. Per- haps the biggest challenge of meteoritic research is the fact that extraterrestrial materials display unstable magnetic behavior under heating (e.g., Fuller, 1974; Cisowski, 1986; Pearce et al., 1976; Yu and Gee, 2005). To date, the best well-known experimental design that may minimize mineralogical alteration during heating is the double-buffering approach (Taylor, 1979). In addition to thermal instability, the effect of shock demagnetization/remagnetization ∗ Corresponding author. Tel.: +82 2 3290 3173; fax: +82 2 3290 3189. E-mail address: sjdoh@korea.ac.kr (S.-J. Doh). needs to be considered in analyzing extraterrestrial NRM. Accord- ing to recent tests (e.g., Funaki et al., 2000; Gattacceca et al., 2006, 2008a,b; Gilder et al., 2006; Bazaeva et al., 2007a), terrestrial iron- oxides are easily influenced by mild shocks of <2.0 GPa. It is also noteworthy that the shock-induced remanence is about an order less intense than the thermoremanent magnetization (TRM). As an equilibrated ordinary chondrite (H5) (van Schmus and Wood, 1967), the Richardton meteorite fell on Earth on July 21, 1919 (Quirke, 1919). A total of ∼90 kg of stony meteorite material was retrieved at Richardton, North Dakoda, USA (Quirke, 1919). Richard- ton was classified as an S2 meteorite which corresponds to a shock pressure of 5–10 GPa on the Stöffler et al. (1991) scale (Grady, 2000). A Pb–Pb isochron dates the Richardton meteorite at 4550 ± 2.6 Ma using multiple phosphate fractions (Amelin et al., 2005). Compared to finds, falls may represent fresher and less weathered meteorite although both falls and finds are not free from artificial magnetic contamination (Wasilewski and Dickinson, 2000). In the present study, we use NRM to determine the stable paleomagnetic record of the Richardton chondrite. In addition to conventional alternating-field (AF) demagnetization and thermal demagnetization in air, thermal demagnetization on vacuum- sealed samples was also carried out. A number of rock magnetic tests such as low-temperature cycling of isothermal remanent mag- netization (IRM) and demagnetization of saturation isothermal remanent magnetization (SIRM) were also included. 2. Experiments and results Fourteen individual chondrite samples (287–619 mg) of Richardton were used in this study. Stepwise AF demagnetization 0031-9201/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2009.07.003