Magnetic Remanence in the Murchison Meteorite

Meteoritics & Planetary Science 38, Nr 3, 399–405 (2003) Abstract available online at http://meteoritics.org

Magnetic remanence in the Murchison meteorite

Gunther KLETETSCHKA, 1, 2* Tomas KOHOUT, 3 and Peter J. WASILEWSKI 2

1Howard University, Washington D.C. 20059, USA 2Astrochemistry Branch, Code 691, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA 3Faculty of Natural Sciences, Charles University, Prague, Czech Republic, and Institute of Geology, Academy of Sciences, Czech Republic *Corresponding author. E-mail: [email protected]

(Received 29 August 2002; revision accepted 8 January 2003)

Abstract–The Murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules, various inclusions, and matrix with occasional porphyroblasts of olivine and/or pyroxene. It also contains amino acids that may have served as the necessary components for the origin of life. Magnetic analyses of Murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence. The rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids. The level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition. We support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination. When using this approach, 1–2% of the natural remanence left in terrestrial rocks with TRM and/or CRM determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral (with the exception of hematite). The same method is applied to the Murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200–2000 nT during and/or after the formation of the parent body.

INTRODUCTION NEW METHOD

The amount of acquired magnetization in rocks can be By normalizing the experimental mineral TRM characterized by ratio (REM) between the natural remanence acquisitions for SD, PSD (pseudo single domain), and MD (NRM) and saturated isothermal remanence (SIRM) (multi domain) minerals (Dunlop and Waddington 1975; (Cisowski et al. 1983; Cisowski and Fuller 1986; Wasilewski Kletetschka, Wasilewski, and Taylor 2000; Ö zdemir and 1977). Most of the rocks on Earth acquire thermal remanence O’Reilly 1982; Tucker and O’Reilly 1980; Wasilewski 1981) (TRM—the rocks’ temperature is lowered through the we obtain a uniform trend, indicating that magnetic grains of blocking temperature) and chemical remanence (CRM— various mineral domain-states and/or mineral types saturate magnetic minerals chemically precipitated) in a geomagnetic near 20 mT (see Fig. 1). The only exception from this trend is field. Rocks can also acquire Detrital remanent magnetization MD hematite, which saturates near 0.1 mT due to its low (DRM) during the sedimentation processes. However, this spontaneous magnetization and demagnetizing field, allowing DRM is often overprinted by later CRM acquired during the domain walls to nearly saturate in magnetic fields of low cementation processes. Methods used for paleofield intensity (Dunlop and Kletetschka 2001; Kletetschka, determination (Fuller 1974; Stephens and Collinson 1974; Wasilewski, and Taylor 2000). The normalized TRM Thellier and Thellier 1959) require an extensive sample acquisition trends allow quick and rough estimation of a heating. SD (single domain) grains (<40 nm in size) that carry paleofield in rocks that formed in an unknown magnetic the stable component of NRM have large surfaces and, environment. To test this approach, we used NRM and SIRM therefore, a mild heating promotes chemical reactions values from 85 terrestrial samples of various origin (Table 1) destroying the original record. W e offer an alternative (Goddard database). Statistical means of NRM/SIRM ratios approach accessing an approximate value of the primitive are 0.009 ± 0.002, 0.011 ± 0.004, and 0.017 ± 0.003 for paleofield information without heating the sample. metamorphic, sedimentary (with CRM), and igneous rock

399 © Meteoritical Society, 2003. Printed in USA. 400 G. Kletetschka, T. Kohout, and P. J. Wasilewski

Fig. 1. RM/SIRM (RM is a remanent magnetization, e.g. CRM, TRM) is plotted against an acquisition magnetic field for various materials. This plot provides a basis for paleofield estimates. Data are calculated from TRM acquisitions on 40 nm titanomagnetite (Özdemir and O’Reilly 1982), 1900 nm and 2 mm titanomagnetite (Tucker and O’Reilly 1980), Columbia plateau basalt (Dunlop and Waddington 1975), iron-nickel spheres (Wasilewski 1981), and 1 mm hematite and magnetite (Kletetschka, Wasilewski, and Taylor 2000). The acquisition trend of all data except hematite is emphasized by the dashed line. types, respectively. The statistical spread is caused by values observed previously [Banerjee and Hargraves 1972]) multiple remanent components in some of these rocks with drifted as soon as the sample was shielded from the terrestrial contrasting direction recorded throughout the rocks’ history. field. This magnetic sensitivity, caused by fields as low as the When we use these statistical means in Fig. 1, we obtain geomagnetic field, has never been reported in the Murchison acquisition fields between 35,000 and 65,000 nT. This range meteorite despite multiple magnetic analyses (Brecher and of values falls within the actual values of geomagnetic fields. Arrhenius 1974; Banerjee and Hargraves 1972). Similar soft This 1% of TRM remanence left in various rock has been behavior has been observed in chondrules extracted from the noted previously. Cisowski observed that a typical ratio in Bjurbole meteorite (Kletetschka, Wasilewski, and fine-grained magnetic material for TRM acquired in Berdichevsky 2001). The effect of exposing and shielding geomagnetic field will be of an order of 10 -2 (Cisowski and meteorite fragments to and from the geomagnetic field, Fuller 1986). Sedimentary samples where fine-grained respectively, is illustrated in Fig. 2, where the acquired/ secondary magnetite forms by chemical precipitation, giving relaxed component is shown to increase linearly with the rise to CRM, again gives magnetization about 1 part in 100 of logarithm of time. This property is independent of further SIRM (Hart and Fuller 1988). In light of these observations demagnetization and/or acquisition and, thus, must be and the experimental data presented in Fig. 1, we apply an considered when deciphering the extraterrestrial extension of the curve approximated by the data (Fig. 1) for magnetization signature. estimation of pre-existing magnetic fields recorded in the The ultrasoft magnetic decay at 77 K is about 70% of Murchison meteorite. room temperature decay. However, the NRM increased more than twice from 5 E-05 A m 2/kg at 300K to 13 E-05 A m 2/kg MAGNETIC SIGNATURE OF MURCHISON at 77 K. Contrary to intuition, the samples with low coercivity do not contain the ultrasoft component and have relatively NRM measurements of the interior piece of Murchison high NRM. The SIRM/Js (Js = saturation magnetization) ratio revealed a peculiar magnetic instability. The natural remanent (Table 2) of low coercivity samples suggests that magnetization (NRM ~5 E-05 A m 2/kg, consistent with magnetization carriers are multi-domain grains. This is also Magnetic remanence in the Murchison meteorite 401 ) 0 5 3 6 4 2 5 5 4 5 5 6 5 2 2 5 4 4 1 4 4 2 5 5 3 5 6 3 5 5 2 1 6 6 3 2 6 1 5 6 5 4 3 g 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 k ------+ ------

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N e n l t d s o i g n r l l w d o C r c b g C , s a e r e C p , , l l p r g B c y h e i u r s i l o I C o C

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a a 0 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 4 5 6 1 T W 1 2 3 4 5 6 7 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 402 G. Kletetschka, T. Kohout, and P. J. Wasilewski

Fig. 2. Magnetic component gained and relaxed after exposing to and shielding from the geomagnetic field, respectively. This ultrasoft component modifies magnetization measurements (NRM ~5 E-05 A m 2/kg) during the examination and is always parallel to the external (geomagnetic) field. The inset picture shows the size of the actual fragment of the Murchison meteorite (Smithsonian Institution).

Table 2. Hysteresis parameters for representative samples at 300 K (suffix R) and 77 K (suffix N). Js (A m 2/kg) is saturation magnetization, SIRM (A m 2/kg) is saturation remanence, Hc (mT) is coercivity, and Fr (%) is remanence left after 5 minutes in magnetic vacuum. Sample Mass (g) JsR HcR SIRMR/JsR JsN HcN SIRMN/JsN FrR(%) 1 0.0056 3.870 8.97 0.033 3.860 10.95 0.039 99 2 0.0097 2.210 13.72 0.070 2.110 30.76 0.134 93 3 0.0030 1.110 16.10 0.132 1.700 42.47 0.116 68 4 0.0059 0.768 19.30 0.114 0.931 52.80 0.175 86 5 0.0038 0.792 20.30 0.100 0.838 48.80 0.129 59 6 0.0346 0.857 21.50 0.105 0.938 42.06 0.157 60 7 0.0073 0.471 24.30 0.140 0.506 45.10 0.223 61 8 0.0509 0.532 25.80 0.140 0.668 51.30 0.181 70

consistent with high value of Js indicating a high some part of each specimen was severely heated during the concentration of iron (Table 2). The samples with the ultrasoft meteorite landing and acquired a terrestrial TRM component. component have high coercivity and low remanence and point One specimen within Group A clearly has a large level of to a mixture of SD and super-paramagnetic grains finely magnetization (NRM/SIRM ~0.01 in Fig. 3) but is part of the dispersed throughout the sample. This is important because Murchison interior with no evidence of fusion crust. This magnetic remanence carried by SD fraction of these grains is specimen also has the lowest Hc, indicating the magnetically stable against artificial remanence acquisition and, thus, may softest material (See Table 2, Specimen 1 with mass 0.0056 preserve a record of pre-terrestrial magnetic events. g). Closer examination revealed a multi-domain metallic Magnetic results from Murchison fragments (Fig. 3) piece within this sample. Soft MD magnetic properties allow allow division of Murchison material into two distinct groups. geomagnetic field contamination, characterized by a higher Group A contains six fragments with REM just under 1 E-02. value of the measured natural magnetization. Soft Five of these samples come from the part of the specimen magnetization in this sample also has low stability against the (Czech Republic) that contained the fusion crust. Therefore, NRM demagnetization by alternating magnetic field (>90% Magnetic remanence in the Murchison meteorite 403

Fig. 3. Natural remanent magnetization versus saturation remanence for parts of Murchison is shown in comparison with terrestrial samples exposed to the lightning discharge (fulgurites and lodestones).

NRM loss in 20 mT). Group B contains only fragments from magnetization levels in Fig. 3. After subtracting this ultrasoft the Murchison’s interior, and none of these fragments has a component (1.5 E-5 A m 2/kg) from each sample’s NRMs, the magnetization level over 0.001 (Fig 3). AF demagnetization range of magnetization levels from the Murchison interior is of these samples revealed fairly stable NRM (<60% NRM between 1 E-04 and 8 E-04. To use this range in the acquisition loss in 60 mT). diagram (Fig. 1), we need to acknowledge the absence of In summary, Group A has samples with strong terrestrial mineral acquisition data for low field values. Assuming that magnetic components and Group B has a record of fairly the acquisition trend extends linearly into low fields, we weak, stable, and possibly extraterrestrial paleofield. obtain a paleofield of at least 200 and at most 2000 nT. This Therefore, only Group B can be considered for paleofield field was recorded by high coercivity fraction of magnetic estimation. carriers and, therefore, it is likely that the Murchison meteorite RM (Remanent magnetization) values of Group B was exposed to this field during its formation. samples are still subject to the ultrasoft component discussed earlier. According to Fig. 2, the Murchison meteorite is MURCHISON METEORITE AND AMINOACIDS capable of acquiring almost 4 E-05 A m 2/kg in several days. To double this value, the sample would have to be exposed to The Murchison meteorite is well known for its content of the geomagnetic field for more than 2000 years, due to the amino acids (Engel and Macko 1997; Epstein et al. 1987; logarithmic nature of remanence acquisition. Because the Kvenvolden, Lawless, and Ponnamperuna 1971; Oró 1990). sample landed on Earth 32 years ago, the maximum extent of The formation of organic compounds and their accumulation the ultrasoft component can not exceed 7 E-05 A m 2/kg is considered a prerequisite to the appearance of life on according to the linear dependence in Fig. 2. During the course primordial Earth (Oparin 1957). V arious sources of energy, of measurement (1–5 minutes per sample), samples acquire an such as heat from volcanoes, heat and ultraviolet light from ultrasoft component of more than 1.5 E-05 A m 2/kg. This the sun, ionizing radiation from radionuclides, and electric component can significantly contribute to the Murchison discharges may be responsible for massive organo-synthesis meteorite samples whose NRM range is 3–160 E-05 A m 2/kg. from prebiotic compounds. Electric discharge events (Miller Ultrasoft acquisition will cause a slight overestimate of 1957) probably operated during the first stages of the solar 404 G. Kletetschka, T. Kohout, and P. J. Wasilewski

nebula development (Desch and Cuzzi 2000). This saturation. However, subsequent formation of the Murchison mechanism can be responsible for the major synthesis of parent body would essentially randomize the strong magnetic amino acids in carbonaceous meteorites. This is supported by moments, lowering the level of overall magnetization. This a similarity between the products and relative abundance of scenario is consistent with new isotopic and experimental the amino acids produced by electric discharge and the amino evidence that suggest that the synthesis of amino acids (or acids present in the Murchison meteorite (Cronin and Moore their precursors) may have preceded the formation of the 1971; Wolman, Haverland, and Miller 1972). Murchison parent body (Caro et al. 2002). Models of early solar nebula evolution predict the presence of lightning discharges due to turbulent flows Acknowledgments–We thank Tim McCoy (Smithsonian carrying dust particles rich in metal and silica (Desch and Institution, W ashington DC) and Marcela Bukovanska Cuzzi 2000). These authors speculate that discharge formed (National Museum, Prague) from whom we obtained the this way is comparable and/or several times more intense than Murchison meteorite fragments. terrestrial lightning and occupies larger volumes and distances during the stroke. The presence of lightning strokes Editorial Handling—Dr. Beth Ellen Clark-Joseph in dust during the early solar nebula development may have been associated with magnetic field pulses stronger than REFERENCES magnetic fields generated by terrestrial lightnings. The magnetization acquired by primitive matter should closely Banerjee S. K. and Hargraves R. B. 1972. 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