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Water‐Soluble Salts and Temperature Variation in Ordinary Chondrites And

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Water‐Soluble Salts and Temperature Variation in Ordinary Chondrites And Meteoritics & Planetary Science 48, Nr 10, 1958–1980 (2013) doi: 10.1111/maps.12211 “Sweating meteorites”—Water-soluble salts and temperature variation in ordinary chondrites and soil from the hot desert of Oman Florian J. ZURFLUH1*, Beda A. HOFMANN2, Edwin GNOS3, and Urs EGGENBERGER1 1Institut fur€ Geologie, Universitat€ Bern, Baltzerstrasse 1 + 3, Bern CH-3012, Switzerland 2Naturhistorisches Museum der Burgergemeinde Bern, Bernastrasse 15, Bern CH-3005, Switzerland 3Museum d’histoire naturelle de la Ville de Geneve, 1 Route de Malagnou, CP 6434 CH-1211 Geneve 6, Switzerland *Corresponding author. E-mail: zur0fl[email protected] (Received 20 June 2012; revision accepted 05 September 2013) Abstract–The common appearance of hygroscopic brine (“sweating”) on ordinary chondrites (OCs) from Oman during storage under room conditions initiated a study on the role of water-soluble salts on the weathering of OCs. Analyses of leachates from OCs and soils, combined with petrography of alteration features and a 11-month record of in situ meteorite and soil temperatures, are used to evaluate the role of salts in OC weathering. 2+ 2À À + À Main soluble ions in soils are Ca ,SO4 ,HCO3 ,Na , and Cl , while OC leachates are 2+ 2+ À 2À dominated by Mg (from meteoritic olivine), Ca (from soil), Cl (from soil), SO4 (from meteoritic troilite and soil), and iron (meteoritic). “Sweating meteorites” mainly contain Mg2+ and ClÀ. The median Na/Cl mass ratio of leachates changes from 0.65 in soils to 0.07 in meteorites, indicating the precipitation of a Na-rich phase or loss of an efflorescent Na-salt. The total concentrations of water-soluble ions in bulk OCs ranges from 600 to 9000 lggÀ1 (median 2500 lggÀ1) as compared to 187–14140 lggÀ1 in soils (median 1148 lggÀ1). Soil salts dissolved by rain water are soaked up by meteorites by capillary forces. Daily heating (up to 66.3 °C) and cooling of the meteorites cause a pumping effect, resulting in a strong concentration of soluble ions in meteorites over time. The concentrations of water-soluble ions in meteorites, which are complex mixtures of ions from the soil and from oxidation and hydrolysis of meteoritic material, depend on the degree of weathering and are highest at W3. Input of soil contaminants generally dominates over the ions mobilized from meteorites. Silicate hydrolysis preferentially affects olivine and is enhanced by sulfide oxidation, producing local acidic conditions as evidenced by jarosite. Plagioclase weathering is negligible. After completion of troilite oxidation, the rate of chemical weathering slows down with continuing Ca-sulfate contamination. INTRODUCTION Langenauer and Krahenb€ uhl€ 1993) and the efflorescence of salts on meteorite surfaces (e.g., Jull et al. 1988; Water-soluble salts were found in the ordinary Velbel 1988; Velbel et al. 1991; Losiak and Velbel chondrites (OCs) Monahans and Zag that both fell in 2011). In hot deserts, daily temperature variations are 1998 (Grossman 1998, 1999). The halite and sylvite much larger and liquid water and water-soluble salts are grains found in these meteorites were identified to be of much more abundant and play an important role during extraterrestrial origin (e.g., Rubin et al. 2002). So far, weathering of meteorites (e.g., Al-Kathiri et al. 2005; these are the only OCs from which extraterrestrial salts Bland et al. 2006). However, the only investigation were described. However, meteorites are subject to where salt contamination in hot desert meteorites was terrestrial alteration and contamination from the first studied focused on the halogen contamination of moment they reach the Earth. eucrites and OCs from Western Australia (Krahenb€ uhl€ A few studies focused on the halogen and Langenauer 1994, 1995). So far, a systematic survey contamination of meteorites from Antarctica (e.g., of salts within hot desert meteorites is missing. © The Meteoritical Society, 2013. 1958 Water-soluble salts in chondrites from Oman 1959 In this study, we have analyzed the water-soluble salt concentration of 15 samples from the JaH 091 meteorite shower (Gnos et al. 2006) and of 16 individual OC samples collected in the hot desert of Oman during the Omani-Swiss meteorite search project. This study was initiated after the common observation of a hygroscopic behavior (“sweating”) of freshly sawn surfaces of Oman meteorites (Fig. 1). We selected samples from various geographic locations and different macroscopic appearances to be able to interpret the variability in salt concentrations. We recognized several types of salt contamination in meteorites (Fig. 2) (1) hygroscopic (“sweating”) meteorites as shown in Figs. 1, 2a, and 2b; (2) “dark green and dry” meteorites, which appear macroscopically relatively unaltered, but generally belonging to W ≥ 3 weathering grade (Wlotzka 1993) showing only minor efflorescence Fig. 1. Cut surface of meteorite JaH 091 (individual 0703-703) of salts (Fig. 2c); (3) “dry rusty meteorites,” typically of with brine droplets “sweating” caused by the presence of weathering degree W4 (Wlotzka 1993) often show hygroscopic salts. Even clear droplets (indicated by arrows) macroscopically visible white pore space fillings are Mg2+ and ClÀ rich. Some ferrous iron is mobilized from (Fig. 2d). the meteorite interior and oxidized to iron hydroxides on the Special attention was paid to solve the enigma of surface. Note only minor hygroscopic behavior of the dark, slightly leached outermost rim of the meteorite. the “sweating meteorites,” i.e., the hygroscopic behavior of freshly cut meteorite surfaces (Fig. 1). In a relatively humid environment (room condition and 50–60% which is the lowest out of 10 recording stations in relative humidity), salts in meteorites attract water and Oman (Fisher 1994). Additionally, fog may serve as a droplets of brine appear on the surface. This effect is source of moisture in this area. During our fieldwork, described by Nininger (1929) for iron meteorites, but we often observed fog in mornings, causing wetting of otherwise apparently unobserved. The “sweating” of the desert soil surface, especially with wind from iron meteorites is linked to the presence of the mythic southerly directions, i.e., the Arabian Sea. mineral lawrencite (FeCl2) that was claimed to be Daily temperature fluctuations play an important nonexistent (Buchwald and Clarke 1988). Extreme role in the development of the observed salt hygroscopic behavior as seen in Fig. 1 is commonly contamination. In this study, we present data for water- observed on OCs recovered from Oman, but has not soluble salts and temperature variation in an OC from been reported from other deserts like the Sahara, Oman and discuss their role in the weathering of Omani Roosevelt County, or Australia (personal meteorites. communication with persons investigating meteorites from these provenances, e.g., Rainer Bartoschewitz, SAMPLES AND ANALYTICAL TECHNIQUES Alex Bevan, Addi Bischoff, Anne Black, Luici Folco, Marc Jost, Martin Lee, Peter Marmet, Alex Ruzicka, Sample Selection and Leaching Experiments Jochen Schluter,€ and anonymous collectors). Liquid water is a prerequisite to mobilize salts in As base for the study, we used samples collected meteorites and initiate weathering. Precipitation in the during fieldwork in 2007, 2009, and 2010 of the joint Oman inland desert is sparse, but occurs nearly Oman-Swiss meteorite research team. The annually. Maps displaying the distribution of concentrations of water-soluble ions were determined by precipitation show a gradual decrease in mean annual leaching experiments using soil and meteorite samples. precipitation from the coastal regions of Oman toward In addition, six water samples from Oman were inland where typical meteorite recovery surfaces receive analyzed for comparison. In Table 1, an overview of the less than 70 mm per year of precipitation (Edgell 2006). performed experiments is given. Mostly, the rains occur as episodic events during February and March. Meteorological data from Meteorite Samples Yalooni station (19°56′ N, 57°06′ E), the only station All Oman meteorites used here were collected with located within the meteorite recovery area, yielded a tweezers, packed into polypropylene bags, and mean annual precipitation of 38.9 mm for 1979–1990, unpacked in the lab wearing gloves to avoid further 1960 F. J. Zurfluh et al. Fig. 2. Cut slabs of JaH 091 samples showing the distribution of salts. Images (a) and (b) are from sample 0703-331. a) The major part shows presence of MgCl-rich brine with only minor iron hydroxide. Some spots are free of brine, as indicated by À “dry.” b) The same cut surface as (a), but treated with AgNO3 solution. White precipitation of AgCl shows distribution of Cl . c) Sample 0603-239 (JaH 091) has a zoned occurrence of different types of salts. In the middle of the sample, a hygroscopic MgFeCl-rich brine is present. A greenish area surrounds it with efflorescence of whitish, hairy MgSO4, which is not hygroscopic. The outermost part shows no salt efflorescence or hygroscopic behavior and displays a rusty color. d) Meteorite 0702-241 (JaH À 091) is a typical W4 sample with white minerals, most likely Ca-sulfates, in pores. After treatment with AgNO3,noCl rich spots are observed. contamination. Samples were cleaned from attached soil produced from meteorite interiors at least one material with pressured air and stored under controlled centimeter below natural surfaces. Parts of the conditions (20 °C, approximately 40% rel. humidity) at meteorites free of calcite veins or cracks were the Natural History Museum Bern. The samples were selected. Typical sample size was about cut using isopropanol as cooling agent to reduce 2 9 1 9 0.8 cm (2.02–7.17 g, mean 4.60 g). After leaching of the soluble pore minerals. weighing, samples were transferred to small Teflon Three experiments consisting of leaching series were flasks and filled with deionized Millipore-Q water performed to determine the water-soluble salts in (meteorite/water ratio approximate 1–4 by mass). meteorites (Table 1) The meteorites were leached over a period of 1.
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