An early solar system magnetic field recorded in CM chondrites
Introduction
The study of the remanent magnetization (paleomagnetism) of extraterrestrial materials gives clues to the history of the primitive solar system and its evolution. Indeed, paleomagnetic studies of meteorites provide a unique window into understanding early solar magnetic fields within the solar protoplanetary disk itself as well as dynamo magnetic fields generated within the convecting metallic cores of planetesimals (e.g., Weiss et al., 2010). In this study we focus on CM carbonaceous chondrites, a meteorite group whose paleomagnetism has not been comprehensively analyzed.
CM chondrites are of particular interest because even though they represent only 1.5% of meteorites falls (Grady, 2000), CM-like materials represent a significant fraction of the micrometeorite flux (Gounelle et al., 2005a, Gounelle et al., 2005b). In addition, they are regarded as some of the most chemically primitive materials available in our solar system (Mason, 1962, Wood, 1963, Anders, 1971) and may be related to asteroids and/or comets (Aléon et al., 2009, Haack et al., 2011). They could have formed in the outer solar system (Wasson, 1976, Wasson and Wetherill, 1979, Gounelle et al., 2008). As such, they offer the possibility to estimate magnetic field strengths present in the early solar system.
CM chondrites consist of chondrules, mineral and chondrule fragments, and calcium–aluminum inclusions (CAIs) embedded in a fine-grained matrix. These rocky components were accreted together with ice to form the CM parent-body (Young et al., 1999, Grimm and McSween, 1989). Following accretion, the ice melt and aqueous alteration took place on the parent body. This process generated alteration minerals such as phyllosilicates, carbonates, iron sulfides and iron oxides (Rosenberg et al., 2001, Brearley, 2006). A range of intensity of aqueous alteration has been observed in CM chondrites (Browning et al., 1996, Rubin et al., 2007, Howard et al., 2011, Hewins et al., 2014).
Almost all CM chondrites are unshocked (classified as S1) (Scott et al., 1992). However, they are breccias that contain sub-centimeter size clasts whose degree of alteration is sometimes different (Rubin and Wasson, 1986, Alexander et al., 2010, Zanda et al., 2010, Jenniskens et al,, 2012, Hewins et al., 2014). As such, it was proposed that aqueous alteration in CM may have been triggered by low intensity impacts (Rubin, 2012, Lindgren et al., in press). It is noteworthy that although some CM chondrites show evidence of thermal metamorphism likely generated by shock processes (Nakato et al., 2013), those considered here escaped heating above ∼40 °C (Guo and Eiler, 2007, Kimura et al., 2011).
The generally accepted view is that aqueous alteration of CM chondrites was a post-accretionary asteroidal process (e.g., McSween, 1979, Tomeoka et al., 1989; Hanowski and Brearley, 2000, Hanowski and Brearley, 2001). However aqueous alteration in small precursor planetesimals prior to the formation of the CM parent asteroid is also advocated for some CM chondrites (Metzler et al., 1992, Bischoff, 1998, Lauretta et al., 2000). I–Xe dating indicates that magnetite in Murchison formed after CAIs (Pravdivtseva et al., 2013). Carbonates in CM chondrites have also been dated using the Mn/Cr system. Because the solar system initial abundance of 53Mn is poorly known, and because suitable carbonate standards are lacking, these ages are to be taken with caution. The most recent work indicates that carbonates in CM chondrites formed roughly 4 Myr after CAIs (Fujiya et al., 2012). Even if no firm timing constraints arise, it is clear that aqueous alteration occurred dominantly very early in solar system history, through a single event or episodic events over a period encompassing at least 2.4–4 million years (Myr) after CAI formation (Brearley, 2006, De Leuw et al., 2009).
Ejection of most CM meteorites from their parent body took place less than 2 Myr ago (e.g., Eugster et al., 2006).
The magnetic properties of a dozen CM chondrites were studied previously (review in Weiss et al., 2010, Elmaleh et al., 2012). Although all CM chondrites are chemically similar, considerable differences exist in their mineralogy as illustrated by their ferromagnetic phases (Hyman and Rowe, 1983). The magnetic mineralogy of CM chondrites is composed, in various proportions, of metallic iron (<1.5 wt%), magnetite (<3 wt%), and iron–nickel sulfides (especially pyrrhotite, 0.7–3.1 wt%) (Hyman and Rowe, 1986, Burgess et al., 1991, Rochette et al., 2008, Howard et al., 2009). The magnetic susceptibility of CM chondrites varies by nearly two orders of magnitude (Rochette et al., 2008), indicating large variations in the magnetic mineral assemblage, abundance, and possibly grain size variations.
Magnetite in CM chondrites is a secondary mineral formed on the parent asteroid by oxidation of carbides and pyrrhotite (Brearley, 2003, Brearley, 2011). Two populations of pyrrhotite may be distinguished in CM chondrites. A coarse grained (∼10 μm) pyrrhotite of primary origin, located in chondrules (Harries and Langenhorst, 2013, Brearley and Martinez, 2010), is present only in low abundance (<0.2 vol%). Secondary pyrrhotite nanoparticles are observed in the matrix and in the fine grained rims (Chizmadia and Brearley, 2008). This latter was formed on the CM parent body through aqueous alteration process. CM chondrites can also contain metal (Rubin et al., 2007, Hewins et al., 2014) and schreibersite (Nazarov et al., 2009).
The paleomagnetism of CM chondrites has been previously investigated (review in Weiss et al., 2010). These meteorites possess a natural remanent magnetization (NRM) (Banerjee and Hargraves, 1971). Partial alternating field (AF) demagnetization of four meteorites showed that this NRM is stable (Larson et al., 1973). The NRM was interpreted as a thermoremanent magnetization (TRM), thermochemical remanent magnetization (TCRM), or chemical remanent magnetization (CRM) (Banerjee and Hargraves, 1972, Larson et al., 1973). For Murchison, a minimum paleointensity of was estimated with the Thellier–Thellier method (Banerjee and Hargraves, 1972). Much lower values of were determined by non-heating methods (Kletetschka et al., 2003). From these previous studies, no clear picture regarding the nature of the NRM, and the nature and intensity of the magnetizing field has emerged.
Here, we performed a paleomagnetic study of seven CM carbonaceous chondrites: Banten, Cold Bokkeveld, Murchison, Murray, Mighei, Nogoya (falls), and Paris (fresh find). Unlike previous studies, we studied mutually-oriented subsamples, and used both AF and thermal demagnetization. We also evaluated the possibility of viscous remanent magnetization (VRM) and shock remanent magnetization for each meteorite, and discussed the time–temperature stability of the NRM. Our aim was to characterize their magnetic mineralogy, determine the nature and age of NRM, and estimate the intensity and nature of the paleofield.
Section snippets
Samples
Samples of seven CM2 chondrites were supplied by the Muséum national d'Histoire naturelle (Paris, France) (list in Supplemental Table A). They collectively span a wide range of aqueous alteration (Browning et al., 1996, Rubin et al., 2007).
All samples were stored in a magnetically shielded room (field <400 nT) for at least one month to allow partial decay of the VRM acquired in the geomagnetic field since their fall. As the original orientations of the samples on the parent body are unknown,
Low temperature measurements
Low temperature remanence measurements performed on the Paris and Murchison meteorites show the presence of a Verwey transition around 120 K (Supplemental Fig. A), as previously observed by Elmaleh et al. (2012). Monitoring of magnetic susceptibility of Cold Bokkeveld, Mighei, Nogoya, and Paris at low temperature confirm the existence of a Verwey transition in all samples but one (the less aqueously altered lithology of Paris meteorite, Supplemental Fig. B). The Verwey transition indicates the
Paleomagnetic results
AF demagnetization up to 170 mT was performed on at least one subsample of each studied meteorite. Thermal demagnetization was also conducted after removal of the low coercivity (LC) component with AF demagnetization. The directions of the components were computed using principal component analysis (Kirschvink, 1980). The results are listed in Table 3, and displayed in Fig. 5, Fig. 6, Fig. 7.
For Cold Bokkeveld, one sample with fusion crust and four interior mutually-oriented samples were
Nature of the NRM
We identified two stable components of magnetization in all studied CM chondrites. These two components were best isolated by thermal demagnetization. An LC component was usually isolated below ∼15 mT and an HC component between ∼20 and ∼90 mT. A HT component was isolated between ∼120 °C and 260 to 585 °C. The HC and HT directions isolated by AF and thermal demagnetization, respectively, in mutually-oriented samples are similar considering their 95% confidence interval and the sub-samples
Conclusion
We performed magnetic measurements on seven CM chondrites. The magnetic mineralogy of all studied meteorites is composed of variable amounts of pyrrhotite and magnetite formed during aqueous alteration on the parent body, as well as primary pyrrhotite and metallic FeNi. Although magnetite dominates the magnetic susceptibility of all studied CM chondrites, pyrrhotite dominates the remanent properties of all of them with the exception of Murchison (dominated by magnetite). Paris is the sample
Acknowledgments
We acknowledge France Lagroix (IPGP) for MPMS measurements and François Demory (CEREGE) for assistance in the laboratory. Funding from ANR (project ANR-09-BLAN-0042), and Marie Curie People Programme (FP7/2077-2013, REA grant agreement No. 29835) are acknowledged. We thank the anonymous reviewer and the Professor Adrian Brearley for their constructive review.
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