Magnetic Brief Review
Beverly Zielonko
Single silicate crystals hosting tiny magnetic inclusions are remarkable targets to study the paleointensities of the Earth and extraterrestrial samples. Since the pioneering work done in late 1990s, paleointensity studies using various silicate minerals such as feldspar, quartz, zircon, pyroxene, and olivine with magnetic inclusions trapped during grain growth or exsolved from the host phase have been reported. It has been shown that some single crystals have the ability to record paleomagnetic information as reliable or more reliable than the whole rock, by direct comparison of the obtained paleointensity estimate from single crystal and the whole-rock sample or the magnetic observatory data. Various rock-magnetic studies also support the fidelity of these crystals. Here, we provide a brief review of the rock-magnetic characteristics of the single crystals, the mineralogical background of the hosting silicates, and experimental procedures developed to obtain reliable data from magnetically weak samples with distinctive rock-magnetic features. We also overview the studies on paleointensity and related topics on various terrestrial and extraterrestrial samples published mainly after the comprehensive reviews in late 2000s. The present review covers the advantages as well as the limitations and caveats of paleointensity studies using single crystal samples and will help readers who wish to utilize this technique in their research.
The strength of the geomagnetic field is an important characteristic throughout the history of the Earth, which is related to the activity of the liquid core (e.g., Olson and Christensen 2006) and affects the surface environment via the interaction with the solar wind (e.g., Tarduno et al. 2014). Paleointensity can be estimated by comparing the strength of the natural remanent magnetization (NRM) of geologic samples to those acquired in a known field in the laboratory. However, such attempts are often hampered by various matters, and obtaining reliable data is challenging.
The alteration of magnetic minerals that has occurred in nature and may occur during laboratory heating is a critical issue in paleointensity experiments. Several experimental protocols have been designed to detect and to correct for the thermal alteration during heating and other non-ideal behavior of the magnetic particles (e.g., Coe 1967; Tsunakawa and Shaw 1994; Yamamoto et al. 2003; Tauxe and Staudigel 2004). However, the older the sample, weathering and other alteration in nature tends to become more severe and samples that are suitable for paleointensity estimate get rarer. Another issue is that the rock-magnetic properties of the samples are not always appropriate for recording paleomagnetic information over geological times. Intrusive rocks are important targets especially for studying older ages, but often contain coarse-grained magnetite that are magnetically unstable and show an undesirable behavior during the paleointensity measurements.
One approach to extend the range of samples for paleointensity study is to extract a portion that contains magnetically and chemically stable magnetic particles exclusively. Single silicate crystal paleointensity (SCP) is a procedure which separates single crystals of rock-forming silicate minerals containing tiny magnetic inclusions and use them for paleointensity experiments (Cottrell and Tarduno 1999; Tarduno et al. 2006; Tarduno 2009). By selecting appropriate crystals, one can obtain samples containing only single domain (SD) and/or pseudo single domain (PSD) magnetite, avoiding multi domain (MD) particles. They are less susceptible to thermal alternation in nature and during laboratory heating as the magnetic particles are encapsulated in the host mineral. Another significant advantage of this method is that the most suitable specimens can be selected from a large number of candidates even with different mineralogy, from a piece of a rock of the same size as a conventional whole-rock sample.
The SCP was developed by Cottrell and Tarduno (1999) and has been actively carried out by their group. The achievements in the first decade and outlook for the future are documented in two review articles by Tarduno et al. (2006) and Tarduno (2009). Tarduno et al. (2006) provided a comprehensive review of early studies on mineralogy, techniques, obtained paleointensity data, and the significance in Earth and planetary sciences. They also presented a vision for study areas in the future, including application to Archean and older rocks and detrital zircons to reveal the early history of the geodynamo, potential and caveats of intrusive rocks and exsolved magnetic minerals, and studies of extraterrestrial samples. The subsequent review by Tarduno (2009) described updates including application to Precambrian Earth and use of oriented samples.
Now, SCP and related studies have been conducted by several research groups, and there have been many advances in the mineralogical background and improvements in measurement techniques, as well as new paleointensity data since the publication of the above reviews. This article aims to provide a guide on mineralogy of the studied samples and experimental techniques for readers interested in this field. We also overview the geophysical achievements of SCP studies on both terrestrial and extraterrestrial samples. The topics presented here are mainly from studies published after Tarduno et al. (2006) and Tarduno (2009), although there are some overlap due to the nature of a review.
Paleointensity estimate requires information on the origin of the magnetic record of the sample (i.e., type of remanence, (un)blocking temperature, and cooling rate) and how it is recorded (e.g., remanence anisotropy of the sample) for selecting appropriate samples to study and properly interpreting the results obtained. These aspects are intimately linked to the origin of the magnetic minerals that carry the remanence of the sample.
There are at least two contrasting origins of the magnetic particles that carry the magnetization of single silicate grains; one is the incorporation into the host mineral during its crystallization and the other is the exsolution from the host phase (Tarduno et al. 2006). The former can take place for any mineral kind in principle, while the latter occurs in feldspars, pyroxenes, amphibole, olivine, and perhaps other minerals. If the magnetic inclusions are xenocrysts trapped during magmatic crystallization of the host phase, they were most likely formed at temperatures well above its Curie point, and magnetic anisotropy is generally not a severe concern. Therefore, the process of remanence acquisition and interpretation of the results of paleomagnetic measurements are straightforward. On the other hand, a large number of samples need to be screened to select those with suitable magnetic properties for paleointensity experiments. One should exclude those with insufficient magnetization or those containing coarse-grained, magnetically unstable particles (Tarduno et al. 2006). If the magnetic inclusions are exsolved magnetite, the situation might be more complex. Exsolution of magnetite sometimes (but not always) results in preferred orientation of long needles causing strong magnetic anisotropy, and requires specific treatment considering it upon paleomagnetic investigations. In addition, if crystallization/crystal growth of the exsolved magnetite occurred at temperatures lower than the Curie point, the sample have acquired thermochemical remanent magnetization (TCRM) and it is difficult to estimate an accurate paleointensity (e.g., Dunlop and zdemir 1997; Smirnov and Tarduno 2005). On the other hand, fine and acicular magnetite dispersed in the host silicate mineral result in high coercivity (Tarduno et al. 2006); this nature makes it a suitable paleomagnetic recorder.
Plagioclase feldspar in basalt lava was the first mineral employed in SCP studies (Cottrell and Tarduno 1999). Large plagioclase phenocrysts up to few millimeters in diameter grown in the magma chamber containing SD to PSD like magnetite inclusions are suited for isolating single crystals and paleomagnetic measurements. Feldspars are contained in a wide range of igneous rocks, and thus, most frequently used in SCP studies (e.g., Cottrell and Tarduno 1999; 2000; Tarduno et al. 2001; 2002; 2007; 2021; Tarduno and Cottrell 2005; Smirnov et al. 2003; Cottrell et al. 2008; Usui and Nakamura 2009; Kato et al. 2018; Bono et al. 2019a; Zhou et al. 2022). Note that some of these studies were on feldspar grains with xenocrystic magnetite, and the others were on grains with exsolved magnetite as a dominant magnetic carrier. Feldspars are relatively easy to be weathered and transformed into altered minerals such as clay minerals. Nonetheless, they are suitable in studies on relatively young volcanic rocks and well-preserved intrusive rocks.
Quartz is also a candidate for SCP studies especially for old acidic rocks since it is a major constituent of crust-forming rocks and is resistant against weathering. To avoid difficulties caused by MD magnetic particles, crystals that do not show visible inclusions under the microscope should be selected (Tarduno et al. 2007). The magnetic carrier of such clean quartz crystals is confirmed to be magnetite in SD to PSD state, based on magnetic hysteresis and low-temperature magnetometry (Tarduno et al. 2007; 2010; Kato et al. 2018).
For terrestrial samples, olivine has not been used for SCP studies due to the low tolerance for alteration which produces secondary magnetite. On the other hand, olivine crystals in extraterrestrial samples have been used for the SCP studies (Tarduno et al. 2012; Fu et al. 2014; Borlina et al. 2021). The olivine crystals in the pallasite meteorites and chondrules do not suffer from severe alteration and these crystals contain pristine fine-grained metals, which are suitable paleomagnetic recorders (see 4.2. Extraterrestrial samples).
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