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In Situ Identification, Pairing, and Classification of Meteorites from Antarctica Through Magnetic Susceptibility Measurements

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In Situ Identification, Pairing, and Classification of Meteorites from Antarctica Through Magnetic Susceptibility Measurements Meteoritics & Planetary Science 41, Nr 3, 343–353 (2006) Abstract available online at http://meteoritics.org In situ identification, pairing, and classification of meteorites from Antarctica through magnetic susceptibility measurements Luigi FOLCO1, 2*, Pierre ROCHETTE1, Jérôme GATTACCECA1, and Natale PERCHIAZZI3 1CEREGE, Universitè Aix-Marseille 3, Aix en Provence, France 2Museo Nazionale dell’Antartide, Siena, Italy 3Dipartimento di Scienze della Terra, Università di Pisa, Italy *Corresponding author. E-mail: [email protected] (Received 28 April 2005; revision accepted 24 October 2005) Abstract–We report on the effectiveness of using magnetic measurements in the search for meteorites on the Antarctic ice sheet, which is thus far the Earth’s most productive terrain. Magnetic susceptibility measurements carried out with a pocket meter (SM30) during the 2003/04 PNRA meteorite collection expedition to northern Victoria Land (Antarctica) proved to be a rapid, sensitive, non-destructive means for the in situ identification, pairing, and classification of meteorites. In blue ice fields characterized by the presence of moraines and glacial drifts (e.g., Miller Butte, Roberts Butte, and Frontier Mountain), magnetic susceptibility measurements allowed discrimination of meteorites from abundant terrestrial stones that look like meteorites thanks to the relatively high magnetic susceptibility of the former with respect to terrestrial rocks. Comparative measurements helped identify 16 paired fragments found at Johannessen Nunataks, thereby reducing unnecessary duplication of laboratory analyses and statistical bias. Following classifications schemes developed by us in this and previous works, magnetic susceptibility measurements also helped classify stony meteorites directly in the field, thereby providing a means for selecting samples with higher research priority. A magnetic gradiometer capable of detecting perturbations in the Earth’s magnetic field induced by the presence of meteorites was an efficient tool for locating meteorites buried in snow along the downwind margin of the Frontier Mountain blue ice field. Based on these results, we believe that magnetic sensors should constitute an additional payload for robotic search for meteorites on the Antarctic ice sheet and, by extension, on the surface of Mars where meteorite accumulations are predicted by theoretical works. Lastly, magnetic susceptibility data was successfully used to cross- check the later petrographic classification of the 123 recovered meteorites, allowing the detection of misclassified or peculiar specimens. INTRODUCTION Abundant dark terrestrial stones disseminated on the blue ice surface can be rather common (e.g., Lewis Cliff, Reckling Antarctica is one of the most fertile terrains for the search Moraine, etc.) (Cassidy et al. 1992), making discrimination of for meteorites on Earth. Each year various research programs meteorites extremely difficult even for well-trained searchers. retrieve hundreds to thousands of specimens from blue ice Furthermore, the seasonal snow cover present in most blue ice fields at the edge of the Antarctic plateau. The high number of fields can transiently hide meteorite accumulations (e.g., finds is favored by concentration mechanisms produced by Frontier Mountain) (Folco et al. 2002). The high number of the glacial dynamics of the Antarctic ice sheet and by the low finds from Antarctica generates an enormous amount of weathering rates determined by polar climatic conditions curatorial work, in terms of classification and allocation of (Cassidy et al. 1992; Harvey 2003). As searches are usually subsamples to the scientific community. Methods that can carried out visually, the ease with which meteorites can be help find meteorites in the field, reduce curatorial costs, and spotted on bare ice is another important factor contributing to contribute to the rapid and accurate classification of this the extraordinary number of finds. However, the latter unique sampling of the solar system materials are highly condition is not always met, and there are several blue ice desirable. fields where the search for meteorites can be problematic. Magnetic susceptibility measures the ability of a material 343 © The Meteoritical Society, 2006. Printed in USA. 344 L. Folco et al. Victoria Land, Antarctica, during which we visited the Miller Butte (72°42′S, 160°15′E), Roberts Butte (72°39′S, 160°12′E), Johannessen Nunataks (72°51′S, 159°50′E), Mount Walton (72°27′S, 160°18′E), Frontier Mountain (72°59′S, 160°20′E) and Mount DeWitt (77°12′S, 159°50′E) blue ice fields and recovered 123 meteorite specimens. We also discuss the application of these techniques in relation to robotic search for meteorites on the Antarctic ice sheet and other productive terrains in the solar system such as the surface of Mars. THE MAGNETIC CLASSIFICATION OF METEORITES Magnetic susceptibility reported here is the mass normalized one, χ, in m3/kg, as mass is an easier parameter to measure than volume. Following Rochette et al. (2003), it will be expressed as the decimal logarithm of χ in 10−9 m3/kg in order to account for the 5 order of magnitude variation in rocks. For strongly magnetic material (log χ > 3), χ is proportional to the amount of Fe,Ni metal, magnetite, maghemite, cohenite, and schreibersite, i.e., minerals with Fig. 1. The decimal logarithm of magnetic susceptibility χ in 10−9 m3 kg −1 for different classes of meteorites (mean value and standard practically equal specific χ. The contribution of less magnetic deviation), as well as for plutonic, volcanic, metamorphic, and iron-bearing minerals, such as other iron oxides and sedimentary rocks from northern Victoria Land, Antarctica, hydroxides (including metal oxidation products), pyrrhotite, belonging to the McMurdo Volcanics, Ferrar Supergroup and Beacon and paramagnetic iron-bearing silicates, is no longer Supergroup, Admiralty Intrusive, Granite Harbour Intrusive, Wilson negligible at log χ ≤ 3. Laboratory instruments used to Metamorphic Complex, and Bowers Metamorphic Complex. Weathered metal bearing meteorites show lower values than the measure χ reported in this study are either the Kappabridge or means above (derived from falls). the Bartington bridge (see Sagnotti et al. 2003 for their calibration). Instrumental precision is 1% or better, but as to acquire a magnetization when placed in an external discussed in Rochette et al. (2003), irregular sample shape magnetic field. Rochette et al. (2003, 2004) have and anisotropy may lead to dispersion of duplicate demonstrated that a given meteorite has a well-defined measurements of up to 20% (i.e., 0.08 in log unit). A database magnetic susceptibility and that the various meteorite classes of χ measurements on more than 1500 different meteorites have characteristic ranges of magnetic susceptibility, (each represented by at least 3 g of material and one reflecting different contents of magnetic minerals (mainly specimen) from various collections worldwide has been metal and magnetite). We will show that the magnetic compiled using different laboratory instruments. Multiple susceptibility of meteorites tends to be higher than that of the specimen measurements for a single meteorite usually yield a terrestrial rocks available at the Earth’s surface. In order to standard deviation of less than 0.1 (Rochette et al. 2003). provide a simple, sensitive, accurate, and non-destructive Figure 1, created by averaging log χ values per meteorite means for in situ magnetic susceptibility measurements of the class, shows means and standard deviations in the 0.1–0.5 of whole stones of any size and shape, Gattacceca et al. (2004) range. Although irons and stony-irons were not included in calibrated a pocket contact probe (the SM30, by ZH this database due to specific measuring problems, they would Instruments). As such, magnetic susceptibility measurements obviously plot in the >5 range (Terho et al. 1993). One can can be used during a meteorite search campaign as a distinguish three ranges of log χ: strongly magnetic (≥4.5), diagnostic parameter to discriminate meteorites from which includes 88% of the total world collection of terrestrial rocks, to pair fragments belonging to the same fall, meteorites; weakly magnetic (≤3.5), which is comprised of R and to gain significant insight into their classification directly chondrites, lunar, Martian, angrite, HED achondrites; and in the field. Moreover, the very high magnetic susceptibility intermediate classes comprised of LL, CM, oxydized CV of most meteorites can also be used to help locate them when chondrites, and aubrites. As described in details for ordinary they are buried in snow (or glacial or aeolian deposits) using chondrite falls in Rochette et al. (2003), log χ is potentially a a magnetic gradiometer. very efficient tool for classifying meteorites, although non- We report henceforth on a test of these techniques carried unique assignments are obvious, especially around log χ = 3 out in the 2003/04 PNRA meteorite collection expedition to or 5. Moreover, weathering induces a significant decrease in In situ identification, pairing, and classification of meteorites from Antarctica 345 log χ for metal bearing meteorites, i.e., about 0.1–0.2 per weathering grade. For example, a log χ of 4.9, which is typical of a fresh L, can also be found in a weathered H or E chondrite. A database of log χ values for terrestrial rocks was generated using samples of magmatic, metamorphic, and sedimentary rocks cropping out in northern Victoria
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