Evaporite formation during weathering of Antarctic meteorites A weathering census analysis based on the ANSMET database

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1 Meteoritics & Planetary Science 46, Nr 3, (2011) doi: /j x Evaporite formation during weathering of Antarctic meteorites A weathering census analysis based on the ANSMET database Anna LOSIAK 1* and Michael A. VELBEL 2 1 Department of Lithospheric Research, Center for Earth Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria 2 Department of Geological Sciences, Michigan State University, 206 Natural Science Building, East Lansing, Michigan , USA * Corresponding author. anna.losiak@univie.ac.at (Received 21 April 2010; revision accepted 30 November 2010) Abstract Weathering of meteorites at the scale of the entire Antarctic Search for Meteorites program population is studied by analyzing the recent version of the online Antarctic meteorite classification database that includes information about 15,263 meteorites. This paper updates, supplements, and expands on the last Antarctic meteorite weathering census by Velbel (1988, Meteoritics 23: ). On average 5% of all Antarctic meteorites are indicated as evaporite bearing in the Antarctic Meteorite Database. Evaporite formation depends on compositional group. Evaporites are much more common on C chondrites than on ordinary chondrites, supporting previous findings. Ordinary chondrites of petrologic type 3 more often have evaporites on their surface than meteorites of other petrologic types. Contrary to previous findings, there is no apparent relation between evaporite formation and meteorite rustiness. Some meteorite-bearing fields influence the frequency of evaporitemineral formation on meteorites. The influence of location is apparently related to differences in environmental conditions, most probably microclimate or and hydrologic conditions. There is no relation between abundance of evaporite-bearing meteorites and distance from the sea. Evaporite formation varies with year of collection; however, it was not possible to distinguish whether this is related to annual changes in environment or an artifact of sample categorization or curation. INTRODUCTION Two main types of weathering products on Antarctic meteorites are visible at the hand-sample scale: rust and evaporites. Although many scientific papers discussed problems related to the weathering of Antarctic meteorites (e.g., Gooding 1981, 1986; Velbel and Gooding 1990; Harvey and Score 1991; Mittlefehldt and Lindstrom 1991; Benoit and Sears 1999; Crozaz et al. 2003; Lee and Bland 2004; Tyra et al. 2007), only a few articles have been published about evaporitic products of Antarctic weathering (e.g., Marvin 1980; Jull et al. 1988; Velbel 1988; Velbel et al. 1991; Gounelle and Zolensky 2001). Evaporites are highly water soluble minerals, formed as a result of the evaporation or freezing of bodies of water. Annual average air temperatures in Antarctic meteorite-bearing ice fields are below 0 C (Comiso 2000). However, insolation heating of the generally dark colored and usually fusion-crusted meteorites during wind-free summer days produces conditions beneath the exposed surfaces under which liquid water can exist (Schultz 1986, 1990; Harvey 2003). Capillary water or thin films of unfrozen water can also occur (e.g., Gooding 1981, 1986; Campbell and Claridge 1987). Sometimes the amount of water can be significant; for example Cassidy (2003) reported a large meltwater pond in the area of the Lewis Cliff (LEW) meteorite-bearing field (although it was frozen during the time of this expedition). In addition, some meteorites are found in shallow depressions filled with refrozen ice (Gooding 1986; Harvey 2003). Based on the 443 Ó The Meteoritical Society, 2011.

2 444 A. Losiak and M. A. Velbel carbon isotopic composition of carbonate, evaporites were formed at temperatures around )2 ± 4 C (Grady et al. 1989). Evaporitic materials on Antarctic meteorites consist of mainly Mg- and Ca-carbonates and sulfates (e.g., nesquehonite Mg(HCO 3 )(OH)Æ2H 2 O, hydromagnesite Mg 5 (CO 3 ) 4 (OH) 2 Æ4H 2 O, epsomite MgSO 4 Æ7H 2 O, starkeyite MgSO 4 Æ4H 2 O, gypsum CaSO 4 Æ2H 2 O, amorphous Mgcarbonate, as well as various unidentified K, Fe, and Mg sulfates (Velbel 1988; Velbel et al. 1991; and references therein). Cations, especially Mg, present in evaporites probably come from the weathering of primary minerals (especially olivine) present in meteorites (Velbel et al. 1991). At least some carbon dioxide incorporated into carbonates formed during terrestrial weathering originates from the Earth s atmosphere (e.g., Jull et al. 1988), although in meteorites with abundant indigenous meteoritic carbon some of the carbon in terrestrial carbonates may be terrestrially redistributed extraterrestrial carbon (Tyra et al. 2007). Studies of Antarctic dry-valley soils suggest that sulfates can come from wind-blown sea salt, weathering, volcanic sources, and atmospheric oxidation of reduced gaseous sulfur compounds (Bao et al. 2000; Bao and Marchant 2006). The Antarctic Search for Meteorites (ANSMET) program of the United States of America began in 1976 (Harvey 2003). The recent version of ANSMET database used in this study includes information on meteorites recovered up to 2006 (although sample processing and data entry from 2005 and 2006 were not complete at the time of data acquisition). During the 32 field seasons (one in 1989 was canceled due to weather conditions and logistical problems) meteorites were found in 48 different locations, some of which were visited in multiple field seasons (Harvey 2003). Usually up to two or three meteorite-bearing fields were visited during most years. Only one previous article (Velbel 1988) discussed weathering based on data from the entire available population of Antarctic meteorites. It was written more than 20 years ago and at that time, data for only about 1367 meteorites were available, of which 74 had evaporites. At the time of the present work, the ANSMET database includes information on 15,263 meteorites, of which 757 meteorites have evaporites. This more than 10-fold increase in the number of available meteorites since the last such population study allows for a more complete population-scale analysis. In addition, the large number of meteorites with evaporites allows more detailed analysis of evaporite formation with respect to a larger variety of factors than previously possible. The aim of this paper was to update, supplement, and expand on the last Antarctic meteorite weathering census by Velbel (1988), by examining five possible influences of evaporite formation by weathering on the distribution of evaporite minerals in the population of ANSMET Antarctic meteorites (1) meteorite compositional class, (2) meteorite petrologic type, (3) the weathering classification, (4) the geographic locations of meteorite-bearing ice fields, and (5) the year of collection. The results of this study may be useful as background information in a variety of studies using samples from Antarctic meteorites. It is known that the weathering (including processes of evaporite formation) can modify meteorite characteristics (e.g., trace element abundances; e.g., Velbel 1988), redistribute meteoritic cations (Velbel et al. 1991), and incorporate terrestrial anions (Jull et al. 1988; Velbel et al. 1991). Such compositional modifications can interfere with the retrieval of information about preterrestrial solar system processes from Antarctic meteorites (e.g., Gooding 1981; Bland et al. 2006). This problem can be especially significant when dealing with rare types of meteorites; Antarctic samples may constitute a major fraction of all available samples of some rare groups, and some such groups are especially prone to evaporite formation (e.g., Karoonda-type carbonaceous chondrites). However, much that is known about these phenomena is known from case studies of specific meteorites. Population-scale study is required to establish how such phenomena are distributed. Statistical study of meteorite alteration phenomena using the ANSMET database can also provide information about the environment of Antarctica (e.g., Bland et al. 2000; Harvey 2003), potentially supplementing the limited data available about the interior of this continent. METHODS Data on the distribution of evaporitic materials on Antarctic meteorites recovered by the ANSMET program were retrieved from the online Antarctic meteorite classification database (downloaded on October 12, 2008) on the NASA Astromaterials Curation Web page ( This database yielded information about the total number of meteorites, the number of meteorites with evaporites, the year of collection, the weathering category, and the petrologic type. Within each compositional group and petrologic type, the total number of meteorites, and the number and percentage of evaporite-bearing meteorites were tabulated. All data for evaporite-bearing and evaporite-free meteorites were analyzed with respect to four variables (meteorite compositional class, meteorite weathering classification, meteorite-bearing field, and the year of collection). All meteorites were treated as

3 Evaporites on Antarctic meteorites 445 separate specimens; pairing was not taken into account, although some results regarding pairing were tabulated (Losiak 2009). Results relating to all parameters were recorded, but for further analysis only subgroups with 20 (or more if indicated in footnotes) specimens were used. The number of 20 meteorites was arbitrarily assumed to be sufficient to allow for the analysis. Organizing the data with respect to factors that are known to influence evaporite formation on Antarctic meteorites allowed identifying the influence of individual weathering-controlling factors. Velbel (1988) showed that the probability of evaporite occurrence is related to the compositional classification. Consequently, the present study reports all other factors in relation to meteorite compositional class. Some of the tables were also organized relative to other factors that were postulated to influence evaporite occurrence (location and year of collection), to examine the influence of such individual factors. After implementing these procedures, in most cases only ordinary chondrites were present in sufficient numbers to allow for analysis. Confidence levels were in all cases calculated for 95% probability using Minitab statistics package. The ANSMET database is assumed to be correct, although it may contain some inaccuracies or mistakes. In addition to possible human errors in entering data, some inaccuracies are possible due to the fact that not all meteorites are studied in equal detail; some specimens (especially ordinary chondrites) are classified based mainly on macroscopic examination. Pairing relationships have been established for only a small fraction of the large number of recovered meteorites. In addition, the most important parameter for this study, the presence of evaporites, is assigned on the basis of visual inspection of the specimen by the laboratory technician. It is likely that very small amounts of evaporites elude detection. Different laboratory technicians may have different threshold levels of evaporite amount that is recorded in the database (although this hypothesis was tested in Losiak 2009, and it was not supported). Bearing such caveats in mind, in the rest of the article we rely on the presence or absence of evaporitic material as recorded in the database. The tables illustrating the results are in most cases too large to fit the journal format. Full-sized color versions of the tables with results are available as Supporting Information. RESULTS The analyzed version of ANSMET database includes information about 15,263 meteorites: 14,374 (94.2%) chondrites, 411 (2.7%) achondrites, and 163 (1.1%) irons and stony-irons. The database does not contain information about the classification of 315 meteorites (2.1%). Approximately 5.0% (757) of all U.S. Antarctic meteorites have evaporites. This is consistent with previous findings (5.4% in Velbel 1988). Influence of Meteorite Composition (Class) Characteristics of meteorites such as chemical composition and porosity in large part are related on their classification (e.g., Corrigan et al. 1997; Consolmagno et al. 1998; Flynn et al. 1999; Britt and Consolmagno 2003; Wilkison et al. 2003; Hutchinson 2004). Similarly, the proportion of meteorites with evaporites varies with composition (classification), consistent with Velbel s (1988) findings (Table 1) that attributed those differences to the differences in chemical composition. The carbonaceous chondrite population has the highest percentage of evaporitebearing meteorites (29.2%); achondrites have intermediate abundance (6.6%), and ordinary chondrites the lowest (4.0%) (Fig. 2). Calculated confidence intervals show that with 95% probability the percent of evaporite-bearing meteorites differs between those groups. Thanks to the larger number of available specimens, it was possible to differentiate between the percentages of evaporite-bearing meteorites among different meteorite groups, in contrast to the previous study (Velbel 1988) in which (with exception for H, L, and LL ordinary chondrites) only classes could be compared for most meteorite groups. For example, there is a significant variability among different carbonaceous chondrite classes (Table 1); 54.2% of Karoonda-type carbonaceous chondrites have evaporites, whereas for Renazzo-type carbonaceous chondrites the evaporitebearing fraction is only 10.9%. Within the ordinary chondrite group, there are some important variations in the percent of evaporite-bearing meteorites. Evaporites occur on 5.3% of H chondrites compared with only 1.7% of LL chondrites. Specific percentages of evaporite-bearing meteorites differ between Velbel (1988) and the present study. However, the general relations between different classes and groups remain the same, and differences are within confidence intervals. Paired samples are different fragments of the same meteoroid that disintegrated either as a shower fall or after at Earth s surface (Harvey 2003). The chemical composition and terrestrial age of meteorites within the same pairing group are the same, but their terrestrial history could be different. If formation of evaporites on Antarctic meteorites was fully dependent on

4 446 A. Losiak and M. A. Velbel Table 1. Evaporitic statistics for chondrite classes and weathering categories. Full-sized table available as supporting information (Table S1).

5 Evaporites on Antarctic meteorites 447 composition, all meteorites in the group should have 100% or 0% evaporites, which is not the case. For example, within the Elephant Moraine (EET) (L6 chondrite) pairing group 2.1% of 678 paired meteorites had evaporites visible on their surface. This falls slightly below the population-average values for L chondrites. Unfortunately, because of low sample number in most of the pairing groups, a statistically meaningful comparison is impossible. The fact that not all paired samples (characterized by identical chemical composition and class) develop evaporites suggests that other variables besides or in addition to composition play an important role in the process of evaporite formation. In addition, at least in some cases, this can be due to inhomogeneity of brecciated meteorites, different fragments of which can have very different properties (M.E. Zolensky, personal communication). Influence of Petrologic Type Table 2 presents the percentage of evaporite-bearing meteorites in different petrologic types. Petrologic type refers to the relative degree of static metamorphism and homogenization of mineral compositions that the meteorite underwent in its history on the parent body (three being the most pristine and six the most modified by metamorphic processes; Hutchinson 2004). For all analyzed compositional groups (results for other meteorite groups with small populations are not shown here) petrologic type 3 has the highest percent of evaporite-bearing meteorites. Estimated errors for those two populations do not overlap (with 95% probability). Table 2. Percent of evaporite-bearing meteorites with respect to petrologic types and weathering category of LL, L, and H chondrites. Petrologic type Total Ev. % me) me+ LL LL LL LL LL L L L L L H H H H H For the total population of LL chondrites, 1.7% are evaporite bearing, whereas 16.1% of LL3s are evaporite bearing. This is a statistically significant difference. The relatively small number of available samples in this group makes it highly susceptible to the influence of other factors discussed in this paper. For example, three of five meteorites that had evaporites on them were found in the LEW field. This location seems to favor evaporite formation (Table 3). In addition, three evaporite-bearing LL3 meteorites were found in 1988, a year that also seems to be characterized by overabundance of evaporite-bearing meteorites (Table 3). These three meteorites have not been paired yet although they have relatively similar properties and the possibility that they are paired cannot be excluded. However, even if we treat those three specimens as one, so that the total number of evaporitebearing LL3 chondrites is reduced to three individuals not related by pairing, these paired evaporite-bearing LL3s still consist of 9.6% of total number of LL3s. Similarly, in the H group four of seven evaporitebearing meteorites of petrologic type 3 were also collected in 1988 at LEW ice field. When corrected for possible pairing (counting all these meteorites as one), the percentage of evaporite-bearing meteorites is 7.1%, which is still higher than for H chondrites of any other petrologic type. L chondrites are the most numerous. Of total number of 235 L3 chondrites, 21 have evaporites (8.9%) compared with the average (4.2%) for the entire population of L chondrites. Influence of Weathering (Rust) Index Different individuals from paired groups of meteorites usually do not fall into the same weathering categories (Losiak 2009). Usually weathering categories are closely related (e.g., A B and B, or B, B C, and C for one paired group). However there are groups (e.g., Graves Nunataks or EET 96135) that include samples assigned to four of the five different weathering categories. The influence of weathering category on the frequency of evaporite formation in the ANSMET population of Antarctic meteorites is not apparent (Fig. 1). There is no consistent trend of increase or decrease in the percentage of evaporite-bearing meteorites with weathering category common to all compositional groups. The percent of evaporite-bearing samples varies widely within compositional-weathering groups and confidence levels for different weathering groups overlap. Table 1 presents results aggregated in a manner consistent with table 4 of Velbel (1988). Comparison reveals a large discrepancy in percentages of evaporitebearing meteorites between this and the previous study.

6 448 A. Losiak and M. A. Velbel Fig. 1. The relation between weathering index and percent of meteorites with evaporites. Table 3. The number of meteorites found along with number of meteorites with evaporite deposits in ice fields with 100 or more total cataloged meteorites. Full-color version and map version of the table are available as supporting information (Table S2).

7 Evaporites on Antarctic meteorites 449 Fig. 2. Percent of meteorites with evaporites as a function of compositional group. The dashed line represents average percentage of evaporite-bearing meteorites for entire population of Antarctic meteorites (5%). Numbers refer to the total number of meteorites from a given group found in Antarctica. Velbel (1988) reported 85.7% of C chondrites of weathering category A had evaporites on them, whereas in this study the value is only 32%. This difference is probably at least partially related to the much smaller number of C chondrites samples available at the time of the previous study. Velbel (1988) observed that for the carbonaceous chondrites significant evaporite formation is correlated with the earliest stages of rusting, but the results of the present study do not support this. Influence of Geography Table 3 shows the percentage of evaporite-bearing meteorites distributed according to the field areas from which they were collected. Only ice fields from which a total of more than 100 meteorites have been recovered and classified are shown. Values were calculated for the entire population as well as for the most abundant meteorite classes (ordinary chondrites, to control for the influence of composition). Some meteorite-bearing ice fields show consistently high (GRO, MIL) or low (DOM, LAP, MAC, MET, RBT) proportions of evaporite-bearing meteorites (relative to the entirepopulation average), for all analyzed compositional groups (Losiak and Velbel 2009). However, because of a small sample size the estimated errors are quite large for all compositionally controlled groups. On the other hand, the percentages of evaporite-bearing meteorites differ significantly among collecting areas relative to the total population, even considering the estimated error values. Furthermore, fields characterized by over- and under-abundance of evaporite-bearing meteorites can be located in close proximity to each other. For example, the DOM ice field characterized by the lowest average percentage of evaporites (0.7%) is neighbor to the GRO field that has the highest average percent of evaporitebearing meteorites (13.5%). Other meteorite-bearing ice fields do not show such uniform over- or underabundance of evaporite-bearing meteorites. Influence of Year of Collection Year of collection influences the frequency of evaporite formation in the ANSMET population of Antarctic meteorites. Table 4 presents percentages of evaporite-bearing classes of ordinary chondrites with respect to year in which they were found. The percentage of meteorites with evaporites in different

8 450 A. Losiak and M. A. Velbel Table 4. Number of meteorites found and the number of evaporite-bearing meteorites as a function of a year of collection for years Full-sized table available as supporting information (Table S3). years varies widely from as low as 0.6% in 2004 to as much as 11.9% in Some years (1978, 1981, 1983, 1997, 1999, 2000, 2002, and 2004) show, for all compositional groups as well as for total population, lower than average percentages of evaporite-bearing meteorites. Other years (1988, 1993, 1994, 1995, 2005) have greater than average proportion of meteorites with evaporites. However, there are years in which some groups have higher and others lower than average percentage of evaporite-bearing meteorites (e.g., 2001), but at least in some cases this may be due to an artifact of the low number of collected specimens. Each year meteorites are collected from only a few meteorite-bearing fields (in most years meteorites collected are from one or two field areas). Because of this, the influence of collection year on percent of evaporite-bearing meteorites can be strongly influenced by geographical location (see previous section). Similarly, the percentage of evaporite-bearing meteorites recorded for each location can be a function of the collection year. To exclude the possible influence of location, data for L (the most numerous meteorite compositional groups) (as well as H chondrites, for which data are available as a Supporting Information) were arranged according to year and field of collection (Table 6). This permits comparison of the influence of both variables. Results from Table 6 show that year of collection influences evaporite occurrence. Percentages of evaporite-bearing meteorites are much more similar to each other within yearly groups than within location groups. For example, the evaporite-bearing fractions of L chondrites at the two collecting areas for 1987 were 3.6% and 3.0%, for % and 3.8%, and for 2004 no meteorites with evaporites were found in any of the collected field areas. Year 2003 is unique because 1.2% meteorites from LAP and 19.8% meteorites from GRO fields had evaporites. However, both of those values are higher than the long-term field averages. Within L chondrites, variation in the percentage of evaporitebearing meteorites within one field (but different years) is for EET 2.6% and 4.1%, LEW 3.0% and 7.8%, and Queen Alexandra Range (QUE) 0.9% and 6.5%. Within-year variations between different collecting areas are smaller than between-year variations within individual collecting areas. However, more advanced statistical tests (e.g., chi-square test) should be applied in the future in order to test this hypothesis properly. Changes in the curatorial personnel performing the initial characterization and classification of the meteorites collected by the ANSMET program (Antarctic Meteorites Newsletter ) were analyzed (Losiak 2009). Staff analyzing meteorites did not change abruptly, although during many experienced people retired or otherwise left the curatorial teams at NASA JSC and the Smithsonian Institution. Generally, at least one very experienced lab technician was always present, and new personnel did not process specimens independently for at least a year.

9 Evaporites on Antarctic meteorites 451 DISCUSSION Influence of Composition (Class) Meteorite compositional group influences the frequency of evaporite formation in the ANSMET population of Antarctic meteorites. Different compositional groups are characterized by varying percentages of evaporite-bearing meteorites (Table 1). In addition, if a sample size is larger than 20 specimens, it is clear that percentage of evaporite-bearing meteorites is more similar within individual compositional classes, than between compositional classes. For example, percentages of evaporite-bearing meteorites among different ordinary chondrite groups vary between 1.7 and 5.3% with an average of 4.0%, whereas within carbonaceous chondrite proportions vary among different classes between 10.0 and 54.2% with an average of 29.2% (Fig. 2). Groups with similar composition (i.e., from the same class) have similar frequencies of evaporite occurrence, demonstrating that the differences are not random. Differences in percentages of evaporite-bearing meteorites are related to the differences in chemical and physical characteristics of meteorite groups. Meteorite classification is based on chemical and mineralogical characteristics (Hutchinson 2004); it is thus reasonable to assume that differences in evaporite percentages between compositional groups are due to differences in their chemical properties (as it was previously proposed by Velbel 1988). However, a simple comparison between the averages of the bulk weight percent abundances of selected evaporite-forming elements and percentage of evaporite-bearing meteorites does not show any apparent relationship. Many evaporites found on Antarctic meteorites are sulfates; however, bulk sulfur abundances are relatively uniform in all meteorite groups and do not correlate with percent of evaporite-bearing meteorites. Similarly, there seems to be no relationship with bulk Mg and Ca content. Carbon content and percentage of evaporite-bearing meteorites are on average much lower in ordinary chondrites than in carbonaceous chondrites; however, within the C chondrite group, higher weight percent C does not correlate with increased susceptibility to evaporite formation, possibly because at least part of C is hosted in insoluble carbonates (M. E. Zolensky, personal communication). However, it has been shown that the carbon in the evaporitic minerals is terrestrial (Gooding et al. 1988; Jull et al. 1988; Grady et al. 1989). The weathering rate of primary meteorite minerals, and their contribution to solutes for incorporation into terrestrial weathering products, can depend more on the mineral assemblage than on the bulk chemistry. For Fig. 3. Porosity measurements of individual meteorite samples versus average percentage of meteorites with evaporites within a compositional group. Trend is not conclusive. Data from: Corrigan et al. 1997; Consolmagno et al. 1998; Flynn et al. 1999; Britt and Consolmagno 2003; Wilkison et al example, the Fe-rich variety of olivine weathers much faster than Mg-rich olivine (e.g., Velbel 1999). Meteorites of relatively similar bulk chemistry can be composed of different mineral assemblages and this can be a source of differences in percentages of evaporitebearing meteorites among compositional groups (Hutchinson 2004), although at present this relationship is not clear. It is known that physical properties (especially permeability and porosity) of meteorites can influence the weathering rate of meteorites due to associated differences in ability of water to penetrate inside the rock (Bland et al. 2000). However, porosity measurements vary significantly not only within a compositional group, but even within different portions of a single specimen (Fig. 3) (Corrigan et al. 1997; Consolmagno et al. 1998; Flynn et al. 1999; Britt and Consolmagno 2003; Wilkison et al. 2003). Consequently, a trend between porosity of a meteorite group and percentage of evaporites on meteorites of that group are not conclusive. Fewer permeability than porosity measurements are available for analysis (Fig. 4) (Sugiura et al. 1984; Corrigan et al. 1997). An apparent positive trend is strongly influenced by the single measurement of CK4 Maralinga meteorite. As with the porosity data, permeability measurements of meteorites within the same compositional group can vary widely. For example, the permeability of the CO3 chondrites Lance and Felix are respectively and 1.26 mdarcy, a difference of more than two orders of magnitude

10 452 A. Losiak and M. A. Velbel minerals. This study shows that even minor metamorphic reprocessing (associated with the change from petrologic type 3 to type 4) is sufficient to reprocess and render less reactive most of the primary meteorite phases that are highly susceptible to terrestrial weathering. Influence of Weathering Classification Fig. 4. Permeability of single specimens versus average percentage of meteorites with evaporites within a compositional group (Corrigan et al. 1997). Trend of percentage of evaporites with increasing permeability can be observed, although it is strongly influenced by a single measurement of CK4 Maralinga meteorite. (Corrigan et al. 1997). Finally, none of the meteorites for which permeability measurements are available come from Antarctica. In summary, although it is possible that higher permeability enhances development of evaporites, this relationship cannot be supported using very limited available data. Influence of Petrologic Type Meteorites of petrologic type 3 more commonly bear evaporites than meteorites of petrologic types 4, 5, and 6 (Table 2). This is consistent with findings of Velbel (1988) who suggested that meteorites of lower petrologic types have more evaporites. However, because of a small number of samples available at the time, he was able to observe only aggregated data (types 3 and 4 compared with petrologic types 5 and 6). The new data show that the above-average abundance of evaporite-bearing meteorites is related only to petrologic type 3. Low petrologic-type meteorites include greater abundances of amorphous and poorly crystalline components than higher petrologic types (Hutchinson 2004). Velbel (1988) hypothesized that these components, being highly susceptible to weathering, can release their constituent atoms more readily, and that the rapidly released elements become part of evaporite No relationship between weathering classification and the frequency of evaporite formation in the ANSMET population of Antarctic meteorites can be discerned based on available data (Table 1). For most of the compositional-weathering groups the sample size is very small, which makes it very vulnerable to the influence of other factors. Although the percentages of evaporite-bearing meteorites vary among weathering categories, there is no consistent trend of higher proportion of evaporite-bearing meteorites in the initial stages of weathering as suggested (for C chondrites) by Velbel (1988). Similarly, no other consistent trend is shared by all compositional groups. Although it is possible that all groups have their individual characteristic trends, this is unlikely because compositionally similar groups should behave similarly. It is also possible that multiple generations of evaporites develop on meteorites during their Antarctic exposure; some of the groups (L, LL, CM) show a suggestion of a trend of abundant evaporite-bearing meteorites in the initial and late stages of rusting; however, the relationship is weak. Finally, a strong influence of other parameters may obscure systematic but weak evaporiterust relationships. Influence of Geography The geographic location of meteorite-bearing ice fields influences the frequency of evaporite formation on the ANSMET population of Antarctic meteorites (Table 3). If evaporite formation was fully controlled by composition, there should be no large differences in evaporite formation among different geographic ice fields for compositionally uniform populations. For example, for H chondrites the average number of meteorites with evaporites is 5.3% (of a population of 4908), and the proportion of evaporite-bearing H chondrites varies among geographically different collecting fields from less than 3% (RBT, ALH, DOM) to more than 9% (GRO, MIL, QUE). Similarly for L and LL chondrites, the percentage of meteorites with evaporites varies from less than 1% to 15.9% and from less than 1% to 8.3%, respectively. The fact that some ice fields show consistently above-average or belowaverage proportions of evaporite-bearing meteorites for

11 Evaporites on Antarctic meteorites 453 Table 5. Percent of evaporite-bearing meteorites as a function of location within the LEW ice field for H and L chondrites only sites consisting of more than 100 meteorites in either H or L population were included. LEW total Upper Ice Tongue Lower Ice Tongue No. Ev. % me) me+ No. Ev. % me) me+ No. Ev. % me) me+ H L Meteorite Moraine South Lewis Cliff No. Ev. % me) me+ No. Ev. % me) me+ H L all analyzed compositional classes of meteorites suggests that environmental characteristics at these sites favor the formation of evaporites. If the source of evaporites were terrestrial contamination such as sea salt, one might hypothesize that the number or proportion of evaporite-bearing meteorites decreases with increasing distance from the sea (Wentworth et al. 2005). However, results do not support this hypothesis (refer to the maps provided as Supporting Information and Losiak 2009). Fields with above- and below-average abundances of evaporitebearing meteorites seem to be distributed relatively randomly with distance from marine coasts. This is consistent with previous results, indicating that evaporites on Antarctic meteorites are not of terrestrial marine (e.g., sea salt) origin (Velbel et al. 1991; Wentworth et al. 2005). Weathering rates of meteorites as well as the specific weathering processes depend mostly on climate (e.g., Bland et al. 2000). It is thus reasonable to assume that the number of evaporite-bearing meteorites will depend on variation in climatic conditions. Meteorites collected from field areas that have higher than average temperature (especially during the summer, when liquid water can exist) should have more evaporite-bearing meteorites. Meteorite ice fields searched by ANSMET are located along Transantarctic Mountains (Harvey 2003). Unfortunately, data concerning climatic conditions of the meteorite bearing ice fields are very limited. Comiso (2000) produced maps of Antarctic average monthly temperature for years by modeling based on data from ground stations and infrared satellites. The maps show that (at the available scale and accuracy) climatic (temperature) conditions are similar for all meteorite fields. In addition, some ice fields with higher-than-average abundances of evaporitebearing meteorites (for all compositional groups) are immediate neighbors of those with lower-than-average proportions of evaporite-bearing meteorites (e.g., RBT, GRO, MAC; Table 3). Both observations preclude a macroclimatic influence. An alternative explanation is that environmental conditions at the microscale (e.g., microclimatic) are more important in evaporite formation. However, currently available data are not sufficient to test this hypothesis. The LEW meteorite-bearing field is one of the locations that are characterized by above-average proportion of evaporite-bearing materials (Table 3). The LEW ice field consists of multiple collection sites (Cassidy 2003, p. 297). Evaporites are present in this field not only on meteorites, but also in cracks in ice, in morainal sediments, and even around the meltwater ponds (Cassidy 2003). Fitzpatrick (1990) found nahcolite, trona, borax, and other associated minerals in samples from ice and moraine at the LEW ice tongue. Trona formation is a result of the direct evaporation from standing water in lateral kettle ponds existing at this locality during the summer. Nahcolite has precipitated from evaporation of waters rising from a point source (spring-like) beneath the ice. According to Harvey et al. (2006), the evaporites at the LEW are a result of circulation of brine created by melting of snow or ice when in contact with dark colored rocks of moraine followed by leaching of soluble components from the regolith. Further work by Socki et al. (2008) showed that Na-sulfates in this location were created as a result of oxidation of local rocks and moved by water originating from melted ice. If circulation of brine within the ice influences evaporite formation on meteorites, it is reasonable to expect that depending on the specific location within the meteorite field, the percent of evaporite-bearing meteorites will vary geographically. Table 5 presents the percentage of evaporite-bearing meteorites as a function of the specific location within the LEW ice field. In a population of H chondrites, the number of evaporite-bearing meteorites coming from Meteorite Moraine is higher than for both Upper and Lower Ice Tongue. However the same trend is not present for L chondrites probably at least partially because of very low number (17) of available meteorites. Unfortunately, some meteorite-bearing ice fields are represented by samples collected during only one or two field seasons. Because of this, it is possible that the over-

12 454 A. Losiak and M. A. Velbel Table 6. Number of evaporite-bearing L as a function of year and field of collection. Full-sized table available as supporting information (Table S4). EET LEW QUE LAP MAC GRO Total Ev. % me) me+ Total Ev. % me) me+ Total Ev. % me) me+ Total Ev. % me) me+ Total Ev. % me) me+ Total Ev. % me) me The average for all L chondrites is 4.2%. Only years that have more than 125 meteorites of a given type were taken into account; only fields that in a given year had more than 50 meteorites were taken into account. The number below the symbol of meteorite-bearing ice field refers to the average percentage of evaporite-bearing meteorites from all years of given field (including data for years not shown in the table).

13 Evaporites on Antarctic meteorites 455 Fig. 5. Percent of meteorite with evaporites for total population collected in given year. Years marked with gray have a total number of meteorites lower than 200. or under-abundance of evaporites in the specific field is not a result of influence of a field, but a consequence of collecting meteorites during a limited number of years (that happened to be unusually cold warm etc., which produced under- or over-abundance of evaporites). However, Table 6 shows that some fields (LAP, GRO) have a consistent trend for multiple years. Influence of Collection Year The year of collection influences the frequency of evaporite occurrence in the ANSMET population of Antarctic meteorites. Some years show a consistent trend of higher or lower than usual percentage of evaporite-bearing meteorites for all analyzed compositional groups as well as for the entire population collected in a given year (Table 4; Fig. 5). The most obvious variable that changes among different years is weather. It could be expected that the higher temperature during the summer months, the higher percentage of evaporite-bearing meteorites is observed. As was already mentioned in the previous section, the climatological data concerning meteorite-bearing ice fields are very limited, and the available information comes mostly from the analysis of the satellite data (Comiso 2000) as well as few automatic and normal weather stations deployed (e.g., Carrasco et al. 1997). However, during the season of , the first automatic weather station was deployed at the meteorite-bearing ice field at the Miller Range, so more detailed data to support further investigation of climatological influences may be available in the future. The weather in Antarctica varies among years. However, there is no obvious trend between temperature (at the available spatial scale) and abundances of evaporite-bearing meteorites. It is possible that evaporite development depends on maximal, not average, temperatures, or that only a few days of warm weather are sufficient to produce evaporite minerals. Other possible hypotheses include meteorite temperature being influenced by the strength of catabatic winds; when the wind dies out, ice and snow melt and leach meteorites allowing development of evaporites (M. E. Zolensky, personal communication). Multiple generations of evaporites can develop during cycles of wind-free periods and subsequent removal by wind erosion during normal, windy conditions. Once again, the available microclimatic data available are insufficient to test such hypotheses unequivocally. Other factors that vary between years can also influence the formation of evaporite minerals on meteorites. For example, meteorites collected during the 2003 season were stored in a freezer that experienced a power loss (Antarctic Meteorite Newsletter ). One specific consequence noted by the curatorial staff was the appearance of evaporites. Thus, in this particular case, the higher-than-field-average evaporite abundances for 2003 acquisitions are almost certainly due to the laboratory environmental-control failure and not to unique field weather conditions in collecting year On the other hand, this example shows that even brief increases in temperature of meteorites can result in significant modifications of their characteristics. It is also known that even the controlled environment of the ANSMET curatorial facility is not a guarantee of retaining original properties of a meteorite; at least one meteorite (H5 chondrite LEW 85320) developed significant evaporites after transporting it to Houston (e.g., Jull et al. 1988; Velbel et al. 1991; Socki et al. 1993). Another possible explanation of the differences in the percent of evaporites present is that they are an artifact of different curatorial personnel processing the

14 456 A. Losiak and M. A. Velbel samples. Some of the parameters analyzed in the study (especially weathering class) can be assigned slightly different values depending on decision of the curatorial worker assigning the weathering category. It is also not clear if two different people would assign the same meteorite to evaporite or nonevaporite group, especially if the amount of visible evaporites is low. However, comparison of the annual changes in abundance of evaporite-bearing meteorites with changes in ANSMET staff does not show an obvious relationship (Losiak 2009). In addition, the process of training new staff members should preclude major differences in way meteorites are categorized. On the other hand, the NASA JSC and Smithsonian ANSMET laboratories experienced a major staff change between 1994 and 1996, and over the same time there was a decrease in the average percentage of evaporite-bearing meteorites (Fig. 5). This decrease can be either caused by the difference in environmental conditions (e.g., a climatic trend) or curatorial processing. However, it is not possible to test which of the hypotheses is supported with the available data. As with the influence of the geographic location of the meteorite-bearing ice field, the influence of the year of collection can be influenced by the fact that in each year usually only one or two fields are visited. Consequently, high abundances of meteorites bearing evaporites can be a result of the location rather than the year of collection. Relative Importance of Parameters At least three parameters influence the probability of evaporite occurrence on the Antarctic meteorites: composition of meteorite (including its petrologic type), location of meteorite recovery, and year of collection. It appears that the relative importance of those three factors varies from year to year and from location to location. Apparently, in some locations (e.g., DOM, LAP, MET, GRO, MIL) the influence of the local environment is so strong that all other factors have only secondary importance. In other meteorite-bearing ice fields, local geographical factors do not have such a strong impact, which allows other factors (e.g., weather) to dominate. Similarly, the influence of collection year varies not only in direction but also in strength; some locations (e.g., MET) are associated with a systematic but slight decrease in the proportion of evaporite-bearing meteorites, others (e.g., DOM) with a pronounced decrease in percent of evaporitic meteorites (Table 3). Controlling factors for all three parameters (Table 6) reveals that in most cases, percentages are more similar within a given collection year than within the same location at different times. However, there are some exceptions; for example, in 2003, two fields were sampled that have especially strong influence, resulting in high over- (GRO average percent of evaporitebearing meteorites of 15.3%) and under-abundance (LAP 0.7%) of evaporite-bearing meteorites. In 2003, exposure of meteorites to unusually high temperatures due to equipment failure caused increase in evaporite formation. Even though the entire collection was subjected to the freezer failure, only 1.2% of meteorites found in LAP field had evaporites, whereas in GRO field the same is true for 19.8%. On the other hand, some years have such a strong influence that the influence of geographical location (especially if fields visited do not have a very distinctive positive or negative influence) is obscured. For example, in % H chondrites had evaporites on them even though the average for the entire population is 5.3%. CONCLUSIONS 1. Evaporite formation depends on compositional group, supporting the findings of Velbel (1988). a. C chondrites are a class characterized by the highest percentage (approximately 30%) of evaporite-bearing meteorites. b. Groups within the same class can have significantly different percent of evaporitebearing meteorites, e.g., in L chondrites group 4.2% of meteorites have evaporites, whereas the same is true for only 1.7% of LL chondrites. 2. More evaporites are forming on meteorites of petrologic type 3 than on meteorites of higher petrologic types. This finding supports and refines the findings of Velbel (1988). 3. There is no apparent relation between evaporite formation and meteorite rustiness, which contradicts the findings of Velbel (1988). 4. Some meteorite-bearing fields influence the frequency of evaporite-mineral formation on meteorites. The influence of location is apparently related to differences in environmental conditions, most probably microclimate or and hydrologic conditions. There is no relation between abundance of evaporite-bearing meteorites and distance from the sea. This provides further support for the nonmarine source of evaporitic products of Antarctic meteorite weathering (e.g., Velbel et al. 1991). 5. Evaporite formation varies with year of collection. This may be related to annual changes in weather, annual variation in any other factor (e.g., hydrologic conditions), or an artifact of sample categorization or curation.

15 Evaporites on Antarctic meteorites The relative importance of different factors is difficult to estimate. The importance of factors influencing evaporite formation varies between years and locations. However, the influence of compositional group appears to be uniformly relatively strong. Acknowledgments We thank Prof. Harvey for all his comments. We are also grateful to the staff of the Astromaterials Acquisition and Curation Office at the Johnson Space Center, especially to Carlton Allen, Kevin Righter, and Cecilia Satterwhite. Reviews by Rick Socki, Mike Zolensky, and Allan Treiman helped shape the final manuscript and are greatly appreciated. Editorial Handling Dr. Allan Treiman REFERENCES Antarctic Meteorite Newsletter jsc.nasa.gov/antmet/amn/amnfeb06/amnfeb06.pdf Bao H. and Marchant D. R Quantifying sulfate components and their variations in soils of the McMurdo Dry Valleys, Antarctica. Journal of Geophysical Research 111:D doi: /2005jd Bao H., Campbell D. A., Bockheim J. 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