# Effect of hot desert weathering on the bulk-rock iron isotope composition of L6 and H5 ordinary chondrites

Gaëlle Saunier, Franck Poitrasson, Bertrand Moine, Michel Grégoire, A.

Seddiki

► To cite this version:

Gaëlle Saunier, Franck Poitrasson, Bertrand Moine, Michel Grégoire, A. Seddiki. Effect of hot desert weathering on the bulk-rock iron isotope composition of L6 and H5 ordinary chondrites. Meteoritics and Planetary Science, 2010, 45 (2), pp.195-209. 10.1111/j.1945-5100.2010.01017.x. hal-00576130

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# Effect of hot desert weathering on the bulk-rock iron isotope composition of L6 and H5 ordinary chondrites

Gaëlle SAUNIER<fnref n="1" />, Franck POITRASSON<fnref n="1" />*, Bertrand MOINE<fnref n="2" />, Michel GREGOIRE<fnref n="3" />, and Abdelmadjid SEDDIKI<fnref n="4" />

**Abstract**—Although iron isotopes are increasingly used for meteorites studies, no attempt has been made to evaluate the effect of terrestrial weathering on this isotopic tracer. We have thus conducted a petrographic, chemical, and iron isotopic study of equilibrated ordinary chondrites (OC) recovered from hot Moroccan and Algerian Saharan deserts environment. As previously noticed, we observe that terrestrial desertic weathering is characterized by the oxidation of Fe-Ni metal (Fe⁰), sulfide and Fe<fnref n="2" />⁺ occurring in olivine and pyroxene. It produces Fe-oxides and oxyhydroxides that partially replace metal, sulfide grains and also fill fractures. The bulk chemical compositions of the ordinary chondrites studied show a strong Sr and Ba enrichment and a S depletion during weathering. Bulk meteoritic iron isotope compositions are well correlated with the degree of weathering and S, Sr, and Ba contents. Most weathered chondrites display the heaviest isotopic composition, by up to 0.1‰, which is of similar magnitude to the isotopic variations resulting from meteorite parent bodies' formation and evolution. This is probably due to the release of isotopically light Fe<fnref n="2" />⁺ to waters on the Earth's surface. Hence, when subtle Fe isotopic effects have to be studied in chondrites, meteorites with weathering grade above W2 should be avoided.

## INTRODUCTION

Meteorites are amongst the few remaining witnesses of the early solar system history and they thus represent a valuable record of planet formation processes (e.g., Taylor 2001). The number of studies on the stable iron isotope composition of meteorites (carbonaceous chondrites, ordinary chondrites, enstatite chondrites, achondrites, pallasites, and iron meteorites) is growing rapidly given the high level of precision now achievable. These works addressed parent bodies processing, planetary formation, accretion, and differentiation (Zhu et al. 2001; Kehm et al. 2003; Poitrasson et al. 2004, 2005; Mullane et al. 2005;

Schoenberg and von Blanckenburg 2006; Hezel et al. 2008; Theis et al. 2008).

Many of the meteorites collected today come from hot (e.g., Sahara) and cold (e.g., Antarctica) deserts. These meteorites may have resided on the Earth's surface for thousands of years, where they have been affected by different weathering agents such as rain water, atmospheric oxygen, salts from aerosols, wind, and temperature variations. All these factors influence the mechanisms and rates of weathering. Weathering is therefore dependent on the residence time of meteorites at the Earth's surface, and the local climate of the place where these meteorites fell. Meteorite permeability also influences the mechanism and rate of weathering. In a

<footnote n="1">¹Laboratoire d'Étude des Mécanismes de Transfert en Géologie, Université de Toulouse, Centre National de la Recherche Scientifique, IRD, 14, avenue Edouard Belin, 31400 Toulouse, France</footnote>

<footnote n="2">²Département de Géologie, Faculté des Sciences, Université Jean Monnet, 23 Avenue du Docteur P. Michelon, 42023 St-Etienne, France</footnote>

<footnote n="3">³Laboratoire Dynamique Terrestre et Planétaire, Centre National de la Recherche Scientifique, Université de Toulouse, CNES, 14, avenue Edouard Belin, 31400 Toulouse, France</footnote>

<footnote n="4">⁴Laboratoire de Magmatisme et Synthèse Géodynamique des Bassins Algériens, Université d'Oran Es-senia, BP. 1524, Oran 3 1000, Algérie</footnote>

<footnote>*Corresponding author. E-mail: Franck.Poitrasson@lmtg OBS-MIP.fr</footnote>

<footnote>*(Received 22 September 2008; revision accepted 06 October 2009)</footnote>
<table><caption>Table 1. Meteorite samples studied and their weathering grade and shock stage.</caption><thead><tr><th></th><th>Class</th><th>Weathering grade</th><th>Shock stage</th><th>Fayalite (% mol)</th><th>Ferrosilite (% mole)</th><th>Place</th><th>Year of find</th></tr></thead><tbody><tr><td>Increasing weathering</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>W0-1</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mocs (fall)<sup>a</sup></td><td>L5-6</td><td>W0-1</td><td>S3-5</td><td>25</td><td>21.0</td><td>Transylvania</td><td>1882</td></tr><tr><td>W2</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NWA<sup>b</sup> 4260</td><td>L6</td><td>W2</td><td>S3</td><td>25.8</td><td>21.7</td><td>Algeria</td><td>2003</td></tr><tr><td>NWA 4278</td><td>L6</td><td>W2</td><td>S2</td><td>25.2</td><td>21.1</td><td>Morocco</td><td>?</td></tr><tr><td>NWA 2258</td><td>L6</td><td>W2</td><td>S3</td><td>25.5</td><td>21.1</td><td>Algeria</td><td>2001</td></tr><tr><td>W3</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NWA 4279</td><td>L6</td><td>W3</td><td>S2</td><td>26.7</td><td>20.9</td><td>Morocco</td><td>?</td></tr><tr><td>W4</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>NWA 2261</td><td>L6</td><td>W4</td><td>S3</td><td>25.9</td><td>21.9</td><td>Algeria</td><td>2002</td></tr><tr><td>NWA 5482</td><td>H5</td><td>W3-4</td><td>S2-3</td><td>18.5</td><td>16.9</td><td>Morocco</td><td>2002</td></tr><tr><td>NWA 5583</td><td>H5</td><td>W5</td><td>S2-3</td><td>19.1</td><td>15.9</td><td>Morocco</td><td>?</td></tr></tbody></table>

previous study, Bland et al. (1998, 2006) showed that the rate of weathering of meteorites under hot desert conditions is characterized by a rapid initial increase and then remains nearly constant. The initial stage (first 20 kyr) is dominated by the alteration of Fe-Ni metal, and the second stage by the alteration of more resistant troilite to form hydrous Fe-rich minerals, referred to in this paper as Fe-oxide/oxyhydroxide. This latter weathering process is accompanied by the release of S and its partial combination with Ba and Sr to form sulfate minerals (Bland et al. 1998). Recently, meteorite weathering experiments have confirmed the abundance of iron oxyhydroxides and sulfates in alteration products (Lee et al. 2006; Gronstal et al. 2009). Iron oxides and Fe-oxyhydroxides occluding primary pores and forming coalesced veins are considered as the result of oxidation of Fe⁰ from Fe-Ni alloy and Fe²⁺ from sulfides. Divalent iron in olivine and pyroxene may also be affected and oxidized by atmospheric oxygen to give Fe³⁺. Hence, trivalent iron is essentially a terrestrial weathering product that is used to determine the meteorite weathering grade (Bland et al. 1996, 1998; Crozaz and Wadhwa 2001; Gillet et al. 2002; Lee and Bland 2004; Al-Kathiri et al. 2005; Bland 2006).

Building on these previous studies, we have investigated the effects of hot desert-related meteorite weathering, with particular emphasis on the isotopic composition of Fe. We have first evaluated the weathering grade of ordinary chondrites (L and H type) using optical microscopy, scanning electron microscopy (SEM) for texture and mineralogy and inductively coupled plasma-mass spectrometer (ICP-MS) for trace element contents. We have then evaluated for the first time the effect of hot desert weathering on the Fe-isotope signatures measured by high-precision multi

collector inductively coupled plasma-mass spectrometer (MC-ICP-MS).

## SAMPLES

Five L6 and two H5 ordinary chondrites collected in deserts of Morocco and Algeria (northwest Africa) have been studied. The meteorite Mocs, an observed fall from Romania (in 1882), is used as a “least weathered” reference in this study. Ordinary chondrites are the most abundant types of meteorites and they account for ~80% of observed falls (Grady 2000). The five L6 and the two H5 investigated have been observed by optical microscopy in order to verify their weathering grade and shock classification (Table 1), following the scheme of Wlotzka (1993) that is based on the mineralogical changes observed in polished sections and that of Stöffler et al. (1991) for the shock stage.

## EXPERIMENTAL

### Mineralogy and Petrography

Polished thick sections were studied using reflected light microscopy (Nikon Eclipse E600 POL), and electron microscopy (JEOL-JSM6360LV). To ascertain meteorite classification, chemical compositions of olivines, pyroxenes, and plagioclases were determined at the Université de Toulouse, Laboratoire des Mécanismes et Transferts en Géologie (LMTG) using a CAMECA SX50 microprobe with SAMx automation. The operating conditions were an accelerating voltage of 15 kV and a beam current of 20 nA. The analyzed surface was approximately of 2 × 2 µm². Natural and synthetic minerals were used as standards.

<footnote><sup>a</sup>Chemical data for the Mocs meteorite are from Miura et al. (1995).</footnote>

<footnote><sup>b</sup>NWA = Northwest Africa.</footnote>
## Trace Elements Analysis

Each meteorite was sawn to remove fusion crusts in order to avoid Fe isotope fractionation (see Poitrasson et al. 2004). To have a representative aliquot of whole-rock, approximately 1 g was powdered using an agate pestle and mortar.

For ICP-MS (and MC-ICP-MS) analysis, approximately 50 mg of powdered sample was weighed and placed in a Teflon vessel with 0.05 mL HNO₃ (~15N). This solution was taken to dryness at 100 °C. In order to eliminate organic matter, ~5 mL of H₂O₂ was progressively added to samples and slowly evaporated. Subsequently, 1 mL of HNO₃ (~15N) was added and taken to dryness and this was repeated with 1 mL of HNO₃ (2M). The residues were placed into high-pressure beakers with 0.2 mL double-distilled HNO₃, 0.2 mL double-distilled HCl, and 2 mL of Merck Suprapure HF, and then placed in an oven at 135 °C for 3 days. After cooling, the samples were taken to dryness (120 °C). Finally, 2 mL HCl (6M) was added and the high pressure beakers were put in the oven for one night at 135 °C. One-fifth of each sample was used for trace element determinations whereas the remaining was kept for Fe isotope analysis. Bulk rocks trace elements were analyzed using an ELAN 6000 ICP-MS at LMTG into a 2 wt% HNO₃ solution yielding a total dilution factor of 3000. Indium and Rhenium were added as internal standards. Uncertainties obtained with this technique are usually better than 5% (2SD) (see Pili et al. 2002 for more details).

## Sulfur Analyses by Pyrohydrolysis

Due to imprecise and inaccurate value obtained by the ICP-MS technique, elemental sulfur was analyzed using pyrohydrolysis at the University of Saint-Etienne (Dreibus et al. 1989; Michel and Villemant 2003). Extractions were performed on 50 mg of bulk-rock powder mixed with 500 mg of vanadium oxide (V₂O₅) in a platinum crucible at 1000 °C in a stream of H₂O vapor. The vapor was condensed and collected in a diluted NaOH solution (25 mM). Sulfur was trapped as SO₄²⁻ in solution and analyzed by ion chromatography on anionic columns (Dionex ICS90 with AS14A column and AMMS III suppressor). Blanks and international rock standards (BE-N, SY-2, BCR-2) were analyzed with our samples. Accuracy and precision are estimated to be within 7% (see Table 2) and detection limit is less than 20 ppm. The sulfur content in the BCR-2 international reference material proposed by Michel and Villemant (2003) differs from our results. However, these authors performed only two analyses of this

<table><caption>Table 2. Sulfur content in three geological reference materials.</caption><thead><tr><th rowspan="3">Samples</th><th colspan="4">Sulfur</th></tr><tr><th colspan="2">Measured value<br>(this work)</th><th colspan="2">Measured value<br>(Michel and<br>Villemant 2003)</th></tr><tr><th>ppm</th><th>SD (%)</th><th>ppm</th><th>SD (%)</th></tr></thead><tbody><tr><td>BCR-2</td><td>285</td><td>3.5</td><td>158–165</td><td>–</td></tr><tr><td>SY-2</td><td>114</td><td>6.1</td><td>115</td><td>22</td></tr><tr><td>BE-N</td><td>312</td><td>2.2</td><td>308</td><td>10</td></tr></tbody></table>

SD: standard deviation in %.

standard whereas we did eight analyses. Our higher value was as precisely determined as the other geostandards and we have no reason to consider it as less accurate than the others. For SY-2 and BE-N our values are indeed in excellent agreement with previously published values (Table 2; Michel and Villemant 2003; Govindaraju 1994).

## Iron Isotopes Chromatography and Analysis by MC-ICP-MS

Fractions of meteorite dissolutions were taken through anion exchange chromatography in an HCl medium in order to separate iron from other elements, following the procedure described in Poitrasson et al. (2004). Isotope measurements were performed with a Thermo Electron Neptune MC-ICP-MS at LMTG in HCl 0.05 M solutions, after the addition of Ni for mass-bias correction (Poitrasson and Freydier 2005). Iron concentrations were determined prior to isotopic measurements. The iron isotope results are quantified in ‰, relative to the IRMM-014 isotopic reference material. The ⁵⁷Fe/⁵⁴Fe and ⁵⁶Fe/⁵⁴Fe ratios are expressed in the conventional delta notation, e.g.,

$$ \delta^{57}\text{Fe}/^{54}\text{Fe} = \left( \frac{^{57}\text{Fe}/^{54}\text{Fe}_{\text{sample}}}{^{57}\text{Fe}/^{54}\text{Fe}_{\text{IRMM-014}}} - 1 \right) \times 1000 \quad (1) $$

⁵⁷Fe/⁵⁴Fe sample is the measured sample ⁵⁷Fe/⁵⁴Fe ratio and ⁵⁷Fe/⁵⁴Fe IRMM-14 is the average of the measured ⁵⁷Fe/⁵⁴Fe ratios of the IRMM-014 standards placed before and after the sample in the analytical sequence. The long-term reproducibility is estimated to be ~0.013‰/amu (Poitrasson and Freydier 2005). Instrumental mass bias was corrected by considering that fractionation factors of iron and nickel are different (Maréchal et al. 1999), which was found to correspond to the best reproducibility. Each sample was measured six times in order to optimize measurement uncertainties.
<figure><img src="image_2.png" /><figcaption>Fig. 1. a) Reflected light microscopic view of weakly weathered meteorite Northwest Africa (NWA) 4278 (W2–L6 type): Fe-Ni alloy and minor amounts of sulfide are the brightest areas. The matrix consists of olivine, pyroxene, and Fe-oxide/oxyhydroxide. Iron metal with incipient weathering can be seen. b) Reflected light microscopic view of a strongly weathered meteorite NWA 2261 (W4–L6): only a few Fe-metal inclusions remain and the majority of sulfides are altered. Cracks filled by iron oxyhydroxides are common.</figcaption></figure>

# RESULTS

## Microscopic Observations

Reflected light micrographs of meteorites polished sections are shown in Fig. 1. Major minerals are olivine, pyroxene, plagioclase, sulfide, and Fe-Ni metal. The weathering products are represented by Fe-oxides/ oxyhydroxides.

Weakly weathered samples (W2) display relatively well preserved and individualized chondrules (notably barred, with iron and sulfide inclusions). Accessory

minerals such as spinel, phosphate, maskelynite (plagioclase transformed into glass by shock-related high pressures) are present in small quantity. A minor amount of iron metal is replaced by Fe-oxides/ oxyhydroxides (Fig. 1a). Sulfides are more abundant than Fe-Ni metal. Veinlets are rare in these meteorites. On the other hand, samples with intermediate weathering level (W3) do show veinlets filled with Fe-oxides/ oxyhydroxides cross-cutting the primary mineralogy. Chondrules progressively disappear and remain as relics. In the most extreme cases of weathering studied, i.e., samples characterized by weathering grade W4/5 (samples NWA 2261 [Fig. 1b] and NWA 5483 not shown here) Fe-Ni alloys and sulfides are largely replaced by Fe-oxides/oxyhydroxides. As illustrated in Fig. 1b, sulfides seem to be more resistant to alteration than Fe-Ni alloys, in agreement with previous studies (Lee and Bland 2004; Al-Kathiri et al. 2005). Opaque mineral alteration aureoles and fractures filled by Fe oxyhydroxides are best studied using the scanning electron microscope (see Fig. 2).

## Mineral Compositions

Compositions of olivines and pyroxenes determined by electron microprobe are reported in Table 1. Figure 3 displays a classification plot for ordinary chondrites. Our measured fayalite and ferrosilite components of olivine and pyroxene, respectively, fall in the range of L and H chondrites.

## Trace Elements

The ICP-MS results obtained on the five L6 and the two H5 chondrites (Table 3) have a precision of 3–10% relative standard deviation (RSD) for most elements. The majority of the trace element concentration measured in this study are similar to literature data reported by Wasson and Kallemeyn (1988) for L meteorites. For H chondrites, concentrations are similar to average H from Wasson and Kallemeyn (1988) except for Cl, Cr, Zn, Th, Ba, and Sr. Barium, Sr, Cl, and Th display positive anomalies, while S, Co, and Ge display negative anomalies (Fig. 4). Interestingly, the most unweathered sample (Mocs) does not show high Sr and Ba enrichment (at 10.7 and 7 ppm, respectively). Weakly weathered meteorites have slightly higher Ba and Sr concentrations (corresponding to 2 to 6 times the L-type mean chondrite) and the most weathered samples contain the higher Ba and Sr contents (corresponding to 20 to 30 times to L- and H-type chondritic values) (see Table 3 and Fig. 4). For rare earth elements (REE) we do not see effect of terrestrial weathering. Only the most weathered H5 (NWA 5483)
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. a) Backscattered electron (BSE) image of NWA 4279 (W3-L6): alteration products are visible and form a thick rim to the Fe-Ni metal grains. In the upper left, cracks filled with alteration products are apparent. Some grains of Fe-Ni metal have a concentrically laminated rim of weathering products (see double arrows). b) BSE image of NWA 5483 (W5-H5): feldspar, olivine, and pyroxene occur as breccias cemented by iron silicates that are cross cut by Fe-oxide/oxyhydroxide veins (NE-SW direction). To the bottom right, dissolved silicates which leave a network of holes (see arrows) can be seen. Olivine and pyroxene appear in gray and are difficult to differentiate due to their close mean atomic number. Plagioclases appear in dark gray.</figcaption></figure>

and L6 (NWA 2261) samples are LREE enriched, but our “least weathered” reference, Mocs, has higher LREE-normalized values as well (Fig. 5).

When normalized to average H- and L-type bulk chondrite compositions, it is apparent that the S content of our studied meteorites decrease with increasing weathering grade (such as NWA 5483, NWA 2261, or NWA 4278 in Fig. 4). The sulfur content decreases

<figure><img src="image_5.png" /><figcaption>Fig. 3. Plot of the fayalite content of olivine against the ferrosilite content of orthopyroxene in the studied ordinary chondrites. The two oxidation groups (dashed circles) fit with the relative proportion of ferrosilite given in the literature for H- and L-type chondrites.</figcaption></figure>

when alteration increases, from 21900 ppm in Mocs (L5-6/W0-1) to 3 600 ppm in NWA 2261 (L6/W4) and 2000 ppm in NWA 5483 (H5/W5). In order to further characterize weathering, sample NWA 4260 was cut into three fragments from border to center, providing a transition from the most to the least weathered (NWA 4260-1, NWA 4260-2, and NWA 4260-3) (Fig. 6a). The decrease in weathering from slice 1 to 3 is also visible in the thick section (see Fig. 6a), where the dark slice represents the most weathered part. Concentrations of Sr and Ba decrease from the weathered rim to the less weathered core (Fig. 6b). In contrast, the more weathered rim is S-depleted whereas the less weathered core is somewhat S-enriched (Fig. 6c and Table 3).

## Iron Isotopes

Our results show that iron isotope compositions are heavier with increasing degrees of weathering (W4-5). A slight tendency is observed, with the less weathered meteorites that tend to have lower isotopic ratios ($\delta^{57}\text{Fe}/^{54}\text{Fe}$: -0.017 ± 0.027‰ to 0.064 ± 0.075‰) whereas very weathered ones tend to show higher values ($\delta^{57}\text{Fe}/^{54}\text{Fe}$: 0.098 ± 0.069 ‰ for NWA 5483 (H5/W5), 0.116 ± 0.046 ‰ for NWA 2261 (L5/W4)) (Table 4). The least weathered meteorites (e.g., Mocs W0-1) have $\delta^{57}\text{Fe}/^{54}\text{Fe}$ values indistinguishable from those previously published for chondrites corresponding to “falls” or “finds” from Antarctica (e.g., Zhu et al. 2001; Kehm et al. 2003; Poitrasson et al. 2005; Dauphas et al. 2009), whereas values from the very weathered ones are distinctly heavier. Furthermore, there are geochemical correlations
<table><caption>Table 3. Trace elements results for L and H chondrites (<d.l., under detection limit; n.a., not analyzed).</caption><thead><tr><th></th><th>Unit</th><th>Mocs*<br>L5-6/<br>W0-1</th><th>NWA<br>4260-1<br>L6/W2</th><th>NWA<br>4260-2<br>L6/W2</th><th>NWA<br>4260-3<br>L6/W2</th><th>NWA<br>4278<br>L6/W2</th><th>NWA<br>2258<br>L6/W2</th></tr><tr><th></th><th></th><th></th><th>2σ<br>(%)</th><th>2σ<br>(%)</th><th>2σ<br>(%)</th><th>2σ<br>(%)</th><th>2σ<br>(%)</th></tr></thead><tbody><tr><td>S</td><td>wt%</td><td>2.19</td><td>1.43</td><td>5.0%</td><td>1.69</td><td>5.0%</td><td>2.00</td></tr><tr><td>Cl</td><td>ppm</td><td>n.a</td><td>633</td><td>—</td><td>203</td><td>—</td><td>138</td></tr><tr><td>Sc</td><td>ppm</td><td>n.a</td><td>10.0</td><td>3.3</td><td>8.4</td><td>3.3</td><td>10.0</td></tr><tr><td>V</td><td>ppm</td><td>n.a</td><td>120</td><td>4.1</td><td>82</td><td>4.2</td><td>113</td></tr><tr><td>Cr</td><td>ppm</td><td>n.a</td><td>5840</td><td>3.9</td><td>4070</td><td>4.1</td><td>5530</td></tr><tr><td>Co</td><td>ppm</td><td>460</td><td>320</td><td>4.2</td><td>378</td><td>4.6</td><td>327</td></tr><tr><td>Ni</td><td>ppm</td><td>13200</td><td>13000</td><td>3.2</td><td>9690</td><td>3.5</td><td>13060</td></tr><tr><td>Cu</td><td>ppm</td><td>n.a</td><td>89.9</td><td>3.4</td><td>58.3</td><td>3.5</td><td>98.7</td></tr><tr><td>Zn</td><td>ppm</td><td>n.a</td><td>88.0</td><td>3.8</td><td>4.0</td><td>4.3</td><td>82.9</td></tr><tr><td>Ga</td><td>ppm</td><td>n.a</td><td>7.10</td><td>3.7</td><td>3.7</td><td>4.70</td><td>7.70</td></tr><tr><td>Ge</td><td>ppm</td><td>n.a</td><td>3.30</td><td>6.3</td><td>4.9</td><td>2.60</td><td>2.00</td></tr><tr><td>As</td><td>ppm</td><td>n.a</td><td>4.50</td><td>4.0</td><td>5.5</td><td>2.80</td><td>4.40</td></tr><tr><td>Se</td><td>ppm</td><td>n.a</td><td>19.8</td><td>8.5</td><td>14</td><td>10.9</td><td>21.1</td></tr><tr><td>Rb</td><td>ppm</td><td>3.20</td><td>3.00</td><td>3.5</td><td>4.0</td><td>2.10</td><td>3.30</td></tr><tr><td>Sr</td><td>ppm</td><td>10.7</td><td>21.1</td><td>3.8</td><td>4.0</td><td>12.6</td><td>111</td></tr><tr><td>Y</td><td>ppm</td><td>1.90</td><td>2.00</td><td>2.9</td><td>3.6</td><td>1.20</td><td>2.00</td></tr><tr><td>Zr</td><td>ppm</td><td>9.80</td><td>6.40</td><td>3.4</td><td>4.4</td><td>4.60</td><td>7.20</td></tr><tr><td>Nb</td><td>ppm</td><td>0.64</td><td>0.55</td><td>5.2</td><td>6.8</td><td>0.31</td><td>0.51</td></tr><tr><td>Sn</td><td>ppm</td><td>n.a</td><td>0.55</td><td>5.3</td><td>0.56</td><td>0.36</td><td>0.56</td></tr><tr><td>Sb</td><td>ppm</td><td>n.a</td><td>0.06</td><td>15</td><td>0.05</td><td>0.05</td><td>6.8</td></tr><tr><td>Cs</td><td>ppm</td><td>0.03</td><td>&lt;d.l</td><td>—</td><td>&lt;d.l</td><td>—</td><td>&lt;d.l</td></tr><tr><td>Ba</td><td>ppm</td><td>7.00</td><td>10.3</td><td>3.4</td><td>4.30</td><td>4.4</td><td>36.8</td></tr><tr><td>La</td><td>ppm</td><td>0.58</td><td>0.44</td><td>5.1</td><td>4.4</td><td>0.20</td><td>4.1</td></tr><tr><td>Ce</td><td>ppm</td><td>1.34</td><td>0.89</td><td>3.1</td><td>3.8</td><td>0.48</td><td>3.6</td></tr><tr><td>Pr</td><td>ppm</td><td>0.17</td><td>0.13</td><td>4.2</td><td>4.9</td><td>0.07</td><td>0.14</td></tr><tr><td>Nd</td><td>ppm</td><td>0.78</td><td>0.66</td><td>6.8</td><td>9.3</td><td>0.38</td><td>0.74</td></tr><tr><td>Sm</td><td>ppm</td><td>0.21</td><td>0.21</td><td>12</td><td>10</td><td>0.11</td><td>0.23</td></tr><tr><td>Eu</td><td>ppm</td><td>0.08</td><td>0.09</td><td>7.4</td><td>9.1</td><td>0.08</td><td>6.7</td></tr><tr><td>Gd</td><td>ppm</td><td>0.30</td><td>0.25</td><td>8.1</td><td>9.1</td><td>0.16</td><td>7.6</td></tr><tr><td>Tb</td><td>ppm</td><td>0.05</td><td>0.05</td><td>8.2</td><td>9.6</td><td>0.03</td><td>3.8</td></tr><tr><td>Dy</td><td>ppm</td><td>0.34</td><td>0.35</td><td>5.7</td><td>7.6</td><td>0.21</td><td>5.8</td></tr><tr><td>Ho</td><td>ppm</td><td>0.07</td><td>0.08</td><td>4.7</td><td>6.6</td><td>0.05</td><td>7.4</td></tr><tr><td>Er</td><td>ppm</td><td>0.21</td><td>0.23</td><td>6.1</td><td>8.9</td><td>0.15</td><td>6.1</td></tr><tr><td>Tm</td><td>ppm</td><td>0.03</td><td>0.03</td><td>7.9</td><td>0.03</td><td>0.02</td><td>7.1</td></tr><tr><td>Yb</td><td>ppm</td><td>0.21</td><td>0.25</td><td>6.7</td><td>6.6</td><td>0.16</td><td>7.1</td></tr><tr><td>Lu</td><td>ppm</td><td>0.03</td><td>0.04</td><td>9.3</td><td>5.0</td><td>0.02</td><td>7.4</td></tr><tr><td>Hf</td><td>ppm</td><td>0.18</td><td>0.17</td><td>11</td><td>11</td><td>0.12</td><td>9.9</td></tr><tr><td>Ta</td><td>ppm</td><td>0.02</td><td>0.02</td><td>20</td><td>&lt;d.l</td><td>&lt;d.l</td><td>0.03</td></tr><tr><td>Pb</td><td>ppm</td><td>0.51</td><td>0.16</td><td>8.9</td><td>12</td><td>0.09</td><td>12</td></tr><tr><td>Th</td><td>ppm</td><td>0.17</td><td>0.06</td><td>6.9</td><td>7.6</td><td>0.02</td><td>5.0</td></tr><tr><td>U</td><td>ppm</td><td>0.04</td><td>0.03</td><td>11</td><td>9.1</td><td>0.004</td><td>9.1</td></tr></tbody></table>

linking δ⁵⁷Fe/⁵⁴Fe varies with an increasing degree of weathering class (Fig. 7) and S, Ba, Sr contents with δ⁵⁷Fe/⁵⁴Fe (Figs. 8 and 9). A positive correlation between Ba-Sr and the Fe-isotopic composition of the bulk chondrites, as well as a negative correlation between Fe-isotopic composition and S are observed (see the Discussion section).

DISCUSSION

Effect of Weathering on Chondrite Textures and Chemistry

Alteration is texturally characterized by the progressive replacement of pre-existing Fe⁰⁻ and Fe²⁺⁻

<footnote>*Ni and Co data are from Miura et al. (1995)</footnote>
<table><thead><tr><th>NWA<br>4279<br>L6/W3</th><th>2σ<br>(%)</th><th>NWA<br>2261<br>L6/W4</th><th>2σ<br>(%)</th><th>NWA<br>5482<br>H5/W3-4</th><th>2σ<br>(%)</th><th>NWA<br>5483<br>H5/W5</th><th>2σ<br>(%)</th><th>Quantification<br>Limit<br>(ppm)</th><th>Wasson and<br>Kallemeyn<br>(1988)<br>Average<br>L<br>H</th><th>Al-Kathiri<br>et al. (2005)<br>15-30 kyr:<br>Average<br>L6 W2-3</th><th>Al-Kathiri<br>et al. (2005)<br>>30 kyr:<br>Average<br>L6 W4</th></tr></thead><tbody><tr><td>1.75</td><td>5.0%</td><td>0.36</td><td>5.0%</td><td>1.53</td><td>5.0%</td><td>0.20</td><td>5.0%</td><td>–</td><td>2.2</td><td>2.0</td><td>0.81</td><td>0.47</td></tr><tr><td>655</td><td>–</td><td>291</td><td>–</td><td>342</td><td>–</td><td>165</td><td>–</td><td>–</td><td>76</td><td>80</td><td>–</td><td>–</td></tr><tr><td>10.5</td><td>3.4</td><td>10.6</td><td>3.3</td><td>10.9</td><td>3.3</td><td>12.3</td><td>3.2</td><td>3.941</td><td>8.6</td><td>7.9</td><td>8.72</td><td>9.36</td></tr><tr><td>103</td><td>3.2</td><td>101</td><td>4.3</td><td>95</td><td>3.9</td><td>121</td><td>3.3</td><td>0.449</td><td>77</td><td>74</td><td>59.2</td><td>59.9</td></tr><tr><td>4930</td><td>3.4</td><td>5003</td><td>4.1</td><td>4530</td><td>4.1</td><td>5620</td><td>3.3</td><td>2.430</td><td>3880</td><td>3660</td><td>3675</td><td>4193</td></tr><tr><td>409</td><td>3.5</td><td>497</td><td>4.5</td><td>838</td><td>4.7</td><td>514</td><td>3.8</td><td>0.079</td><td>590</td><td>810</td><td>495</td><td>517</td></tr><tr><td>14650</td><td>3.4</td><td>7890</td><td>3.5</td><td>16220</td><td>3.2</td><td>16040</td><td>3.6</td><td>2.510</td><td>12000</td><td>16000</td><td>10215</td><td>9828</td></tr><tr><td>109.5</td><td>3.6</td><td>89.6</td><td>3.6</td><td>104.1</td><td>3.3</td><td>133.2</td><td>3.6</td><td>0.342</td><td>90</td><td>82</td><td>77</td><td>80.1</td></tr><tr><td>78.9</td><td>3.9</td><td>74.6</td><td>4.1</td><td>78.1</td><td>4.2</td><td>76.2</td><td>4.2</td><td>2.052</td><td>50</td><td>47</td><td>75.2</td><td>110</td></tr><tr><td>7.30</td><td>3.4</td><td>6.70</td><td>4.1</td><td>6.80</td><td>3.2</td><td>9.60</td><td>4.0</td><td>0.213</td><td>5.7</td><td>6.0</td><td>4.86</td><td>5.13</td></tr><tr><td>3.30</td><td>3.8</td><td>3.80</td><td>4.9</td><td>4.60</td><td>4.2</td><td>12.1</td><td>4.0</td><td>0.497</td><td>10</td><td>13</td><td>–</td><td>–</td></tr><tr><td>4.70</td><td>4.7</td><td>4.60</td><td>5.8</td><td>5.70</td><td>4.8</td><td>8.10</td><td>5.2</td><td>0.287</td><td>1.55</td><td>2.05</td><td>–</td><td>–</td></tr><tr><td>23.3</td><td>12</td><td>12.8</td><td>9.2</td><td>12.4</td><td>7.4</td><td>18.6</td><td>9.7</td><td>1.947</td><td>9.0</td><td>7.7</td><td>7.32</td><td>7.05</td></tr><tr><td>3.00</td><td>3.4</td><td>3.10</td><td>3.6</td><td>2.40</td><td>3.2</td><td>2.80</td><td>3.7</td><td>0.100</td><td>3.1</td><td>2.9</td><td>2.1</td><td>2.54</td></tr><tr><td>31.2</td><td>3.7</td><td>24.9</td><td>3.7</td><td>27.7</td><td>3.0</td><td>44.8</td><td>3.6</td><td>0.191</td><td>11.1</td><td>10.0</td><td>64.9</td><td>343</td></tr><tr><td>2.80</td><td>4.0</td><td>2.40</td><td>3.2</td><td>1.80</td><td>2.8</td><td>2.90</td><td>4.0</td><td>0.017</td><td>2.1</td><td>2.2</td><td>–</td><td>–</td></tr><tr><td>7.30</td><td>4.1</td><td>6.20</td><td>3.8</td><td>6.20</td><td>3.6</td><td>7.90</td><td>4.0</td><td>0.075</td><td>5.9</td><td>6.3</td><td>5.99</td><td>6.23</td></tr><tr><td>0.58</td><td>4.1</td><td>0.52</td><td>35</td><td>0.45</td><td>5.6</td><td>0.62</td><td>4.3</td><td>0.016</td><td>0.39</td><td>0.36</td><td>0.47</td><td>0.44</td></tr><tr><td>0.65</td><td>6.1</td><td>0.46</td><td>14</td><td>0.53</td><td>13</td><td>0.94</td><td>4.4</td><td>0.015</td><td>0.71</td><td>0.86</td><td>–</td><td>–</td></tr><tr><td>0.12</td><td>13</td><td>0.06</td><td>25</td><td>0.07</td><td>10</td><td>0.13</td><td>13</td><td>0.027</td><td>0.068</td><td>0.070</td><td>–</td><td>–</td></tr><tr><td>&lt;d.l</td><td>–</td><td>&lt;d.l</td><td>–</td><td>0.02</td><td>13</td><td>0.03</td><td>11</td><td>0.016</td><td>0.28</td><td>0.12</td><td>–</td><td>–</td></tr><tr><td>9.3</td><td>3.8</td><td>55</td><td>3.5</td><td>6.9</td><td>3.3</td><td>188</td><td>3.5</td><td>0.073</td><td>3.7</td><td>4.2</td><td>13.7</td><td>53.2</td></tr><tr><td>0.51</td><td>4.2</td><td>0.52</td><td>3.4</td><td>0.29</td><td>4.3</td><td>0.72</td><td>4.3</td><td>0.004</td><td>0.310</td><td>0.295</td><td>0.38</td><td>0.46</td></tr><tr><td>1.23</td><td>3.4</td><td>1.15</td><td>3.7</td><td>0.74</td><td>2.7</td><td>1.77</td><td>3.4</td><td>0.004</td><td>0.90</td><td>0.83</td><td>0.91</td><td>1.04</td></tr><tr><td>0.18</td><td>5.4</td><td>0.16</td><td>4.0</td><td>0.11</td><td>5.1</td><td>0.25</td><td>5.2</td><td>0.004</td><td>0.132</td><td>0.123</td><td>0.14</td><td>0.15</td></tr><tr><td>0.88</td><td>7.3</td><td>0.86</td><td>6.5</td><td>0.59</td><td>6.2</td><td>1.16</td><td>5.2</td><td>0.012</td><td>0.682</td><td>0.628</td><td>0.67</td><td>0.72</td></tr><tr><td>0.30</td><td>9.0</td><td>0.27</td><td>8.7</td><td>0.19</td><td>8.0</td><td>0.34</td><td>6.5</td><td>0.008</td><td>0.195</td><td>0.185</td><td>0.22</td><td>0.23</td></tr><tr><td>0.12</td><td>7.2</td><td>0.11</td><td>7.7</td><td>0.09</td><td>9.0</td><td>0.12</td><td>6.9</td><td>0.008</td><td>0.078</td><td>0.073</td><td>0.08</td><td>0.08</td></tr><tr><td>0.44</td><td>8.1</td><td>0.35</td><td>6.6</td><td>0.24</td><td>9.1</td><td>0.41</td><td>7.9</td><td>0.024</td><td>0.310</td><td>0.299</td><td>0.31</td><td>0.31</td></tr><tr><td>0.07</td><td>6.5</td><td>0.06</td><td>6.3</td><td>0.04</td><td>5.8</td><td>0.08</td><td>6.1</td><td>0.004</td><td>0.057</td><td>0.053</td><td>–</td><td>–</td></tr><tr><td>0.49</td><td>6.7</td><td>0.42</td><td>6.0</td><td>0.32</td><td>6.0</td><td>0.52</td><td>4.6</td><td>0.004</td><td>0.366</td><td>0.343</td><td>–</td><td>–</td></tr><tr><td>0.10</td><td>5.4</td><td>0.09</td><td>4.9</td><td>0.07</td><td>6.1</td><td>0.11</td><td>4.7</td><td>0.005</td><td>0.081</td><td>0.073</td><td>–</td><td>–</td></tr><tr><td>0.31</td><td>7.8</td><td>0.28</td><td>7.3</td><td>0.21</td><td>6.5</td><td>0.33</td><td>9.7</td><td>0.007</td><td>0.248</td><td>0.226</td><td>–</td><td>–</td></tr><tr><td>0.04</td><td>7.4</td><td>0.04</td><td>8.1</td><td>0.03</td><td>8.6</td><td>0.05</td><td>8.6</td><td>0.004</td><td>0.039</td><td>0.039</td><td>–</td><td>–</td></tr><tr><td>0.31</td><td>6.4</td><td>0.27</td><td>5.2</td><td>0.22</td><td>7.4</td><td>0.33</td><td>6.7</td><td>0.006</td><td>0.220</td><td>0.205</td><td>–</td><td>–</td></tr><tr><td>0.05</td><td>9.0</td><td>0.04</td><td>4.9</td><td>0.03</td><td>7.9</td><td>0.05</td><td>6.0</td><td>0.003</td><td>0.033</td><td>0.031</td><td>–</td><td>–</td></tr><tr><td>0.22</td><td>9.1</td><td>0.25</td><td>11</td><td>0.33</td><td>9.7</td><td>0.23</td><td>8.9</td><td>0.003</td><td>0.170</td><td>0.180</td><td>0.15</td><td>0.16</td></tr><tr><td>0.04</td><td>10</td><td>0.06</td><td>120</td><td>0.01</td><td>22</td><td>0.03</td><td>8.7</td><td>0.010</td><td>0.023</td><td>0.022</td><td>–</td><td>–</td></tr><tr><td>0.16</td><td>11</td><td>2.53</td><td>3.2</td><td>0.25</td><td>9.7</td><td>0.26</td><td>7.8</td><td>0.007</td><td>0.370</td><td>0.240</td><td>–</td><td>–</td></tr><tr><td>0.06</td><td>7.1</td><td>0.14</td><td>6.1</td><td>1.25</td><td>5.3</td><td>0.08</td><td>6.4</td><td>0.001</td><td>0.043</td><td>0.042</td><td>–</td><td>–</td></tr><tr><td>0.09</td><td>4.9</td><td>0.02</td><td>11</td><td>0.01</td><td>12</td><td>0.04</td><td>6.9</td><td>0.001</td><td>0.013</td><td>0.012</td><td>–</td><td>–</td></tr></tbody></table>

minerals by Fe-oxides/oxyhydroxides and the appearance of Fe-oxides/oxyhydroxides veins cross-cutting primary minerals (Figs. 1 and 2). Compared to Fe-Ni alloys, sulfides are more resistant to weathering (Fig. 1b), possibly because their Fe²⁺ is less remote to atmospheric redox equilibrium conditions than Fe metal. In H-type chondrites, most Fe-Ni metal is

replaced by Fe-oxides/oxyhydroxides during weathering and veins filled Fe-oxides/oxyhydroxides are abundant.

The concentrations of Ba and Sr in Mocs (7.0 and 10.7 ppm), the only investigated fall, are roughly similar to that of average L chondrites (Wasson and Kallemeyn 1988; 3.7 and 11.1 ppm respectively, see Fig. 4 and Table 3). This indicates that high values of Ba and Sr
<figure><img src="image_6.png" /><figcaption>Fig. 4. Logarithmic plots of the bulk chemical composition of the different weathered L and H chondrites normalized to average L and H chondrites from Wasson and Kallemeyn (1988). The three class of weathering <15 kyr (W2), 15–30 kyr (W3), and >30 kyr (W4) are represented with ages estimate following Wlotzka (1993). Unfilled symbols represent H chondrites.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. REE values for the studied L and H chondrites normalized to average L and H values in Wasson and Kallemeyn (1988).</figcaption></figure>

observed in more weathered chondrites are related to chemical alteration processes at the Earth's surface. Significant levels of Sr, Ba (and U) due to hot desert weathering were previously reported for a shergottite by Barrat et al. (2001). The decrease in S is mainly related to the weathering of troilite and subsequent leaching from the meteorite (Fig. 1b) (Al-Kathiri et al. 2005). A relationship exists between Sr-Ba enrichment and

decrease in S and the increasing degrees of weathering for L chondrites (except for NWA 2258 with very high value of Sr and Ba and NWA 4279 with very low Ba concentration for its degree of weathering) and H chondrites (Fig. 8). For S, an increase in concentration with increase in weathering is clear, although NWA 4279 seems to be an outlier (Fig. 9). One meteorite (NWA 2258) also does not follow the trend highlighted for Ba, Sr, and δ<sup>57</sup>Fe/δ<sup>54</sup>Fe versus weathering grade evolution. The relationship between Ba content and increasing weathering grade for L chondrites is not so clear, from W2 to W3 weathering grade due to samples NWA 2258 and NWA 4279. But between two extreme groups of weathering grade, the Ba enrichment is observed for L type compared to average Ba concentration from Wasson and Kallemeyn (1988) (see Figs. 8a and 8b). This is confirmed by the analysis of three fragments sampled progressively from the rim to the less weathered core of a single meteorite (Fig. 6a) that shows similar relationship. Stelzner et al. (1999) observed the same tendency for Ba and Sr concentrations to decrease with sampling depth in H5 meteorites. Scandium, Ti, V, Cr, and Co are concentrated in spinels which are not sensitive to alteration. Therefore these elements display only little concentration variation with increasing weathering grade (Fig. 4). In a study of L6 (W4–5) meteorites from
a)

<figure><img src="image_8.png" /></figure>

b)

<figure><img src="image_9.png" /></figure>

c)

<figure><img src="image_10.png" /><figcaption>Fig. 6. a) Three fragments of NWA 4260 (NWA 4260-1, NWA 4260-2, and NWA 4260-3), by decreasing order of alteration: b) concentrations of Ba, Sr, and c) and S. Enrichment of Ba and Sr from center to border can be seen. On the other hand, sulfur shows only a slight depletion in the more external, and more weathered part.</figcaption></figure>

hot desert, Lee and Bland (2004) found other weathering product such as gypsum, barite, jarosite, sulfates, Ca-carbonate's and silica. Specifically, barite is found in association with veins and has been partially occluded by Fe-oxide/oxyhydroxides. However, in our

study those minerals were not observed during our electron microscopic observations.

Al-Kathiri et al. (2005) also observed similar chemical evolutions from less (W2-3) to more weathered (W4) L chondrites (Table 3) except for Sr concentration in very weathered meteorites. However, a large scatter in concentrations for meteorites from a given weathering grade appears in the Al-Kathiri et al. (2005) dataset, a scatter that is not so strong in our own dataset. This could be related to the fact that Al-Kathiri et al. (2005) have investigated more samples. Analytical problems with these authors' inductively coupled plasma optical emission spectroscopy (ICP-OES) data might be another issue, as suggested by our initial attempt to obtain S concentration with similar technique (see the Experimental section). Moreover, Al-Kathiri et al. (2005) provide neither uncertainties nor concentrations obtained on geostandards to assess the analytical techniques they used.

As reported in a previous study focusing on achondrites (non-cumulate eucrites) by Barrat et al. (2003), REE do not seem strongly affected by terrestrial weathering. Only the most weathered H5 (NWA 5483) and L6 (NWA 2261) samples are LREE enriched (Fig. 5). More data on H5 with various weathering grade are nevertheless required to confirm this result, however, since the investigated fall (Mocs) also shows a LREE enrichment.

## Iron Isotopes

The higher value of δ⁵⁷Fe/⁵⁴Fe in very weathered meteorites is possibly related to the partial leaching and subsequent oxidation under terrestrial oxidizing atmospheric conditions of the mobile Fe⁰ and Fe²⁺ occurring in primary minerals such as metal, troilite, olivine, and pyroxene. Subsequently, less soluble Fe³⁺ precipitates to form Fe-oxide/oxyhydroxide. It has been shown theoretically and experimentally that the partial oxidation of iron leads to isotopic fractionation, the more oxidized species being isotopically heavier (e.g., Polyakov and Mineev 2000; Johnson et al. 2002; Welch et al. 2003; Anbar et al. 2005; Polyakov et al. 2007). In the specific case of chondrites, our mineralogical observations (Figs. 1 and 2) indicate that the more weathered meteorites (W4-5) have a higher content of ferric iron-bearing phases relative to the fresher samples, and would therefore display heavier Fe isotopic compositions.

A correlation is observed between δ⁵⁷Fe/⁵⁴Fe and increasing degree of weathering grade from W0-1 to W5. However, there is some scatter and in a given meteorite class (H or L) it is not always possible to classify a specific sample isotopically in terms of
<table><caption>Table 4. Iron isotopic ratio and iron concentration for studied meteorites. In italics H meteorites type.</caption><thead><tr><th>Type/weathering grade</th><th>Sample</th><th>Fe (wt%)<sup>b</sup></th><th>Sample mass (mg)</th><th>δ<sup>57</sup>Fe/δ<sup>54</sup>Fe (‰)</th><th>Uncertainty (2SE)<sup>a</sup></th><th>δ<sup>56</sup>Fe/δ<sup>54</sup>Fe (‰)</th><th>Uncertainty (2SE)<sup>a</sup></th></tr></thead><tbody><tr><td>L5-6/W0-1</td><td>Mocs</td><td>23.0*</td><td>3.38</td><td>-0.017</td><td>0.027</td><td>-0.019</td><td>0.018</td></tr><tr><td>L6/W2</td><td>NWA 2258</td><td>18.1</td><td>48.58</td><td>-0.055</td><td>0.035</td><td>-0.040</td><td>0.031</td></tr><tr><td>L6/W2</td><td>NWA 4260-1</td><td>16.8</td><td>53.02</td><td>-0.026</td><td>0.050</td><td>-0.027</td><td>0.031</td></tr><tr><td>L6/W2</td><td>NWA 4260-2</td><td>17.0</td><td>48.17</td><td>0.030</td><td>0.093</td><td>0.018</td><td>0.065</td></tr><tr><td>L6/W2</td><td>NWA 4260-3</td><td>18.0</td><td>52.65</td><td>0.008</td><td>0.054</td><td>0.010</td><td>0.039</td></tr><tr><td>L6/W2</td><td>NWA 4278</td><td>16.2</td><td>53.89</td><td>0.064</td><td>0.075</td><td>0.052</td><td>0.053</td></tr><tr><td>L6/W3</td><td>NWA 4279</td><td>19.3</td><td>49.39</td><td>-0.021</td><td>0.039</td><td>-0.020</td><td>0.026</td></tr><tr><td>L6/W4</td><td>NWA 2261</td><td>18.3</td><td>51.51</td><td>0.116</td><td>0.046</td><td>0.071</td><td>0.035</td></tr><tr><td>H5/W3-4</td><td>NWA 5482</td><td>21.3</td><td>33.1</td><td>0.028</td><td>0.067</td><td>0.009</td><td>0.036</td></tr><tr><td>H5/W5</td><td>NWA 5483</td><td>22.4</td><td>47.62</td><td>0.098</td><td>0.069</td><td>0.055</td><td>0.060</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>Fig. 7. Variation of δ<sup>57</sup>Fe/δ<sup>54</sup>Fe with weathering grade for L and H chondrites, with a class differentiation. Average chondrite is δ<sup>57</sup>Fe/δ<sup>54</sup>Fe = -0.043 ± 0.042‰ (Poitrasson 2007).</figcaption></figure>

increasing weathering grade. Therefore, the use of the average of a given weathering grade in the two investigated meteorite classes is more pertinent to illustrate the increasing δ<sup>57</sup>Fe/δ<sup>54</sup>Fe with increasing weathering grade (Fig. 7). From W0-1 to W3, no δ<sup>57</sup>Fe/δ<sup>54</sup>Fe increase is observed, but from W3 δ<sup>57</sup>Fe/δ<sup>54</sup>Fe is heavier due to the effect of weathering.

The relationships between δ<sup>57</sup>Fe/δ<sup>54</sup>Fe and S, Ba, and Sr contents confirm this interpretation (Fig. 10). A negative tendency between δ<sup>57</sup>Fe/δ<sup>54</sup>Fe and S is observed, whereas positive relationships link δ<sup>57</sup>Fe/δ<sup>54</sup>Fe and Sr or Ba contents. This is consistent with a general picture of a leaching of S coupled to Sr and Ba enrichment in increasingly weathered meteorites. These two processes are linked to an increase in δ<sup>57</sup>Fe/δ<sup>54</sup>Fe

related to the preferential leaching of the isotopically light Fe (Fe<sup>2+</sup> enriched in δ<sup>54</sup>Fe; see above). During the weathering process, primary minerals will be progressively dissolved and will release Fe<sup>2+</sup> in aqueous agents. Part of this iron will be subsequently oxidized, and therefore δ<sup>57</sup>Fe-enriched following the reaction: 4Fe<sup>2+</sup> + 3O<sub>2</sub> + 6H<sub>2</sub>O = 4Fe(OH)<sub>3</sub>(s). This is an insoluble form that will preferentially remain within meteorites (Fig. 11). This theory can be assessed in comparing concentration measured in our investigated meteorites with total iron content (both oxidized iron and metal) of H and L chondrites given in the literature. A literature survey reveals that H-type generally have 25–30 wt% and L-type 20–23 wt% of total iron, in good agreement with unweathered H- and L-type meteorites data of Wasson and Kallemeyn (1988), with 27.8 wt% and 21.5 wt%, respectively. Whereas our L- and H-type weathered meteorite show systematic lower total iron concentration (average W2 of 17.2 wt%, W3 of 19.3 wt% and W4 of 18.3 wt% for L-type and W3–4 of 21.3 wt% and W5 of 22.4 wt% for H-type) Mocs has a concentration of 23 wt%, i.e., within the range of L-type chondrite because it is fresh. This produces a loss of iron from the meteorites with weathering and confirms our interpretation of leaching of light iron isotope during terrestrial weathering. Accordingly, Fe<sup>2+</sup> oxidation followed by a quantitative precipitation of Fe-oxide/oxyhydroxide without Fe loss from the meteorite cannot produce isotopic fractionation at the bulk sample scale. This may explain why some meteorites display some mineralogical and geochemical evidence of weathering without any isotopic fractionation (e.g., NWA 4279); in this case, Fe-loss did not occur. This δ<sup>57</sup>Fe/δ<sup>54</sup>Fe versus weathering grade correlation (Fig. 10) is observed among different investigated meteorites but not within variably weathered portions of sample NWA 4260

<footnote><sup>a</sup>The iron isotope composition and two standard error uncertainties quoted are calculated from the number of analyses and using the Student's t-correcting factors (Platzner 1997).</footnote>

<footnote><sup>b</sup>Uncertainties on iron concentration determination are < 5%.</footnote>

<footnote>*Fe concentration from Miura et al. (1995).</footnote>
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 8. a) Strontium and Ba concentration versus weathering grade for the six L chondrites. Dashed lines represent Sr and Ba concentration for average L chondrites from Wasson and Kallemeyn (1988). b) Increasing Sr and Ba concentration versus weathering grade for the two H chondrites (NWA 5482 [W3–4/H5] and NWA 5483 [W5/H5]). Dashed lines represent Sr and Ba concentration for average H chondrites from Wasson and Kallemeyn (1988).</figcaption></figure>

(Fig. 6). This is probably because the weathering gradient observed within this meteorite is too weak to notably affect Fe isotope signatures, or no differential Fe-loss occurred between the three sampled zones (Table 3).

One meteorite (NWA 2258) seems to be an outlier in the above trends (question mark in Fig. 10). In particular, it displays anomalously high Sr and Ba contents given its Fe isotope composition. The reason for this remains unclear. Perhaps this meteorite shows an alteration process that occurred on its parent body highlighted here by high Sr and Ba concentrations for its weathering class, and negative δ⁵⁷Fe/⁵⁴Fe, as suggested by Dauphas et al. (2009). Those authors also observed a very negative δ⁵⁷Fe/⁵⁴Fe = -0.21 ± 0.045‰

<figure><img src="image_14.png" /><figcaption>Fig. 9. Sulfur concentration variation with increasing degree of weathering for all type of studied meteorites (H and L).</figcaption></figure>

for a EL6 meteorite (Blithfield) and proposed an extensive redistribution process of Fe between sulfide, metal, and silicate to explain this value.

Although the effect of weathering leads to a small Fe isotope variation at the bulk sample scale, this is nevertheless significant as a 0.1‰ variation in δ⁵⁷Fe/⁵⁴Fe is the level of difference observed between different planetary reservoirs, or even between different planets (e.g., Zhu et al. 2001; Poitrasson et al. 2004, 2005; Schoenberg and von Blanckenburg 2006). Relatively few bulk iron isotope data for ordinary chondrites are available in the literature. With the currently growing database, if high δ⁵⁷Fe/⁵⁴Fe would be observed in the future for meteorite finds, especially in hot desert, they could be related to weathering.

A more comprehensive iron isotope study of all types of chondrites coming from deserts is necessary to see if the tendency highlighted above is characteristic of all types of weathered meteorites. Nevertheless, from the present study, it is recommended to avoid meteorites from hot deserts for future Fe isotope studies, or at least avoid significantly weathered samples, i.e., above grade W2.

Bulk chondrite iron isotope compositions, with an average of δ⁵⁷Fe/⁵⁴Fe = -0.043 ± 0.042‰ (Poitrasson 2007), seem to be homogeneous. The more comprehensive recent database of Dauphas et al. (2009) confirms this. On the other hand, their inclusions (metal, CAI or chondrules) give highly variable iron isotope compositions, to more than the ‰ level (up to 0.6‰ in δ⁵⁷/⁵⁴Fe for matrix and down to -0.65‰ for chondrules in Chainpur [LL3]) (Zhu et al. 2001; Kehm et al. 2003; Mullane et al. 2005; Theis et al. 2008; Hezel et al. 2008). These variations are significant and provide important constraints on chondrule formation as well as on mechanisms affecting their parent body (Kehm et al. 2003; Mullane
a)

<figure><img src="image_15.png" /></figure>

b)

<figure><img src="image_16.png" /></figure>

c)

<figure><img src="image_17.png" /></figure>

Fig. 10. Relationship between δ<sup>57</sup>Fe/δ<sup>54</sup>Fe with a) Sr, b) Ba, and c) S concentrations. Average chondrite is δ<sup>57</sup>Fe/δ<sup>54</sup>Fe = -0.043 ± 0.042‰ (Poitrasson 2007). Dashed arrows show the effect of increasing weathering. The trend in (a) and (b) were just by eye whereas in (c) it is a data fit with a second order polynomial. Question marks represent data for NWA 2258 which displays anomalously high Sr and Ba given its Fe isotope composition, for unknown reasons, whereas its S concentration fits well with other samples.

et al. 2006). However, the isotopic variations are in that case an order of magnitude higher than those resulting from terrestrial weathering. The weathering grade of those meteorites is therefore less of an issue for studies of inclusions in chondrites. Nevertheless, it remains feasible that weathering may generate larger isotopic variations at the inclusion-scale. This will have to be investigated in the future.

## Meteorites from Cold Desert

Studies of the effect of terrestrial weathering on meteorites from Antarctica have been published

(Crozaz and Wadhwa 2001). Like hot desert-derived meteorites, those meteorites can also be affected by terrestrial weathering, though producing different effects on the meteorite mineralogy and geochemistry. This is because these objects spend most of their long terrestrial residence time encased in ice so their compositions remain essentially unchanged. Antarctic meteorites generally experienced lesser chemical alteration than hot desert-derived meteorites despite longer residence time at the surface of the Earth (~2 Ma) compared to hot desert meteorites (> 10<sup>4</sup> yr) (Crozaz et al. 2003). Therefore, even if hot desert meteorites have spent less time on Earth than cold
<figure><img src="image_18.png" /><figcaption>Fig. 11. Our model for iron mobilization during weathering and oxidation of Fe⁰ and Fe²⁺ contained in metal and primary minerals like olivine and pyroxene to Fe³⁺. The high δ⁵⁷Fe/δ⁴Fe ratio of the remaining, oxidized Fe may be due to the possible leaching of the isotopically lightest and reduced Fe. (M = mobilization and O = oxidation).</figcaption></figure>

desert meteorites, they are more rapidly weathered (Crozaz and Wadhwa 2001). Cold desert meteorites show a lower FeO concentration (~15.4 %wt for L6 chondrites: Jarosewich 1984) than hot desert meteorites (17.22 %wt for W2 and 18.30 %wt for W4 in our investigated L6 chondrite). Another study on Antarctic meteorites has been published by Lee and Bland (2004) who evidence some similarities, but especially differences between hot deserts and Antarctica environments. For example, troilite is weathered more slowly than Fe-Ni metal in hot desert environments (Figs. 1 and 2) whereas there is very little evidence of troilite weathering is observed in Antarctic meteorites. Under Antarctic weathering conditions, sulfates, carbonate and silica precipitate, in addition to Fe-oxide/ oxyhydroxide. Antarctic meteorites are characterized by chlorine-bearing and silicate weathering products, indicating that olivine and pyroxene are also affected. Their alteration products are essentially Fe-rich (Lee and Bland 2004).

Previous iron isotope studies did not reveal any obvious effects at the bulk sample scale on “finds” from Antarctica (Poitrasson et al. 2004) but more work is needed as there are still only few data available for Antarctica ordinary chondrites “finds” from the L and LL groups (ALHA77216 [L3.7/L3.9] and ALHA77278 [LL3.7] no weathered; Kehm et al. 2003).

Hence, Antarctic meteorites seem to be better candidates than hot desert meteorites to study their parent body formation and subsequent evolution using Fe isotopes, because of their low rate of weathering. Obviously, “falls” remain the best choice.

## CONCLUSION

This study shows that weathering not only affects the iron mineralogy in meteorites recovered from hot deserts. Iron isotope signatures can also be affected by more than 0.1‰ in the most extreme cases. This is obviously of importance given the similar magnitude of isotopic effects studied in bulk meteorite finds (Zhu et al. 2002; Poitrasson et al. 2004, 2005; Schoenberg and von Blanckenburg 2006). An increase in the meteorite weathering grade is accompanied by higher Sr, Ba contents and a decrease in S concentrations, with concomitant increase in Fe isotope compositions at the bulk meteorite scale. This is interpreted as reflecting the release of S and isotopically light Fe²⁺, with a relative enrichment in Sr and Ba in the weathered meteorites.

Hence, if subtle Fe isotopic effects have to be studied, meteorites from hot deserts should be considered with caution. Those above weathering grade W2 should be avoided. It is better to work on meteorite “falls” and Antarctic “finds” (e.g., Poitrasson et al. 2004) because their Fe mineralogy and geochemistry have been least affected by weathering than meteorites from hot deserts (Crozaz et al. 2003).

*Acknowledgments*—Bernard Mougenot is acknowledged for providing some of the meteorites studied whereas others were collected within the framework of CNRS-PICS project led by Jean-Yves Cottin. Thanks to people who have contributed directly or indirectly to this project: chemistry team (Carole Boucayrand for fusion method, Maïté Carayon for standards preparation),
Philippe De Parseval for microprobe analyses, Thierry Aigouy for MEB observations, Frédéric Candaudap for ICP-MS analyses, and Rémi Freydier for maintaining the MC-ICP-MS in good working conditions. Martin Lee, Dominik Hezel, Karl Khem, and Associate Editor Nancy Chabot are acknowledged for their thorough help and patience in assisting us to improve this manuscript. This research was funded by the PNP CNRS/ INSUprogram.

*Editorial Handling*—Dr. Nancy Chabot

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