For both, the olivine and the gibbsite, a larger amount of 84Kr and 132Xe were found in the re-exposed sample compared to the original one (Table 2, Fig. 4). For olivine, the measured values can be compared to each other in a straightforward way, because no mineralogical or weight changes were observed after heating during the first measurement. Olivine therefore adsorbed about twice as much 84Kr as the original sample contained. The increase in 132Xe is a factor of about 5. For gibbsite, which lost about 25% of its weight during the first heating, the case is more complicated because of the structural change and the large increase in active surface area: 3.77 m2 g−1 in the unheated versus 65.33 m2 g−1 in the heated sample. The original increase in 84Kr (146 × 10−12 cm3 STP/g before and 6424 × 10−12 cm3 STP/g after; Table 2) and in 132Xe (16 × 10−12 cm3 STP/g before and 1737 × 10−12 cm3 STP/g after; Table 2) has to be scaled to this increase in surface area. Concentrations per surface area are 38.5 × 10−12 cm3 STP/m2 (before) and 98.3 × 10−12 cm3 STP/m2 (after) for 84Kr and 4.3 × 10−12 cm3 STP/m2 (before) and 26.6 × 10−12 cm3 STP/m2 (after) for our first gibbsite experiment. The values for the second gibbsite experiment are similar. The resulting factors of increase are 2.5 (1.8 for the second experiment) and 6 for 84Kr and 132Xe, respectively. This result matches the increase observed for olivine rather closely. We also observed—especially in gibbsite—loss of significant amounts of other reactive volatiles (removed by exposure to the Ti- and Al-Zr getters) during the first measurement, which was detected by the pressure increase in the system during heating of the sample. In the second set of measurements, i.e., after pre-degassing by the first measurement, the pressure increase during the heating phase was considerably less, suggesting that the pressure in the first run stemmed from water and other volatiles incorporated into the sample during mineral formation. In addition to the structurally bound water in gibbsite, many of the potential adsorption sites (e.g., grain boundaries) must have been occupied by these volatiles; and since they were removed during the first set of measurements, a fresh, clean surface was exposed to air during the adsorption phase. In laboratory air, especially when compared to water in hydrous mineral formation, the competition is less severe, which may lead to an increased adsorption of noble gases on fresh surfaces. Bernatowicz et al. (1984) suggest that this effect may account for the discrepancy between laboratory measured adsorption constants and observed natural noble gas concentrations. The effect may also explain the gain in noble gases upon grinding, because the breakup of grains exposes fresh and dry surfaces to air, just as it happens after the cleaning steps. In fact, this heating and cleaning may have altered the surface in a similar manner as described by Bernatowicz et al. (1982) who observed Henry constants that were higher by an order of magnitude after heating a feldspar sample as compared to crushing it. The authors discuss two possible explanations for the observation: either crushing may have opened up fractures and therefore the new surfaces were not truly pristine; or crushing released volatiles contained in the crystals, which were then competing for adsorption sites. They favor the second option, because a (volatile free) lunar sample did not show any difference between being crushed and being heated. Martian meteorites may have less volatiles stored in them than Bernatowicz’s labradorite, which would minimize “self-contamination” of surfaces. Heating under vacuum, however, will remove any volatiles that are released, e.g., due to breaking open fluid inclusions. We therefore expect both, the crushed Martian meteorites and the heated samples, to present fresh, clean surfaces to the air they are exposed to.
The noble gas elemental composition of the gibbsite sample changes dramatically. The original samples show 36Ar/132Xe and 84Kr/132Xe ratios that scatter considerably and fall into the vicinity of atmospheric noble gases dissolved in water (Fig. 5). The adsorbed component, however, is similar in all samples ranging from approximately 40–48 for 36Ar/132Xe and from 3 to 5 for 84Kr/132Xe. This is much lower than in the original sample and—for Kr and Xe—resembles the adsorption values found upon grinding. Note that, like described in the case of grinding (Niemeyer and Leich 1976; Niedermann and Eugster 1992), the adsorbed component is not predominantly released in the first temperature steps, but a large fraction of it degasses at higher temperature (Table 2). One explanation is that heating created fresh surfaces that are as active (i.e., have an order of magnitude higher Henry constants and higher surface energies; Bernatowicz et al. 1982) as freshly crushed surfaces. Volatiles of any sort will take advantage of that and thus be strong competitors for noble gas adsorption. Fluids, i.e., strong competitors, occur in mineral formation environments to a much higher degree than in dry laboratory air, but may also be released upon heating or grinding of crystals that host fluids in inclusions or on grain boundaries (Bernatowicz et al. 1982).
Ground Martian Meteorites
We will now compare our results on pure adsorption to adsorption under the mechanical stress of grinding. Unfortunately, seen in their entirety, the elemental abundances in the ground samples do not display a clear picture of the effect that grinding may have on Martian meteorites (Fig. 1). Comparing the ground samples with untreated samples (literature values, Table 3) by calculating the excess amounts of 84Kr and 132Xe in the ground samples, spans a range from apparent loss to an excess as large as 650%. A definite “gain,” i.e., a higher Xe abundance in the ground sample than in the reference literature samples, can be observed for Shergotty, Nakhla, Governador Valadares, ALH84001, and Zagami. EETA79001 (Lith. A) spans a wide range from apparent deficits to apparent excesses, whereas Lafayette, ALHA77005 and Chassigny apparently experienced “losses.” The literature database, however, is small, and, moreover, there is considerable scatter among the untreated (to our knowledge) reference samples. Especially in the case of finds—and depending on sampling location—alteration of the meteorite’s surface and the presence of internal cracks may cause air being present in an untreated sample. Although this effect may offer an explanation for the “loss” (i.e., lower Xe in the ground specimen) in the finds Lafayette and ALHA77005, it does not apply to Chassigny, which is a fall. A potential reason for “loss” rather than “gain” in Chassigny may be the fact that it consists almost exclusively of olivine (see below).
Table 3. 36Ar, 84Kr, and 132Xe in Martian meteorites from the literature. 36Ar in 10−9 cm3 STP/g, 84Kr, and 132Xe in 10−12 cm3 STP/g; “bulk chips” refer to cases in which the authors only state that they took small pieces of the sample, but did not specify any further treatment. Our samples of Lafayette (Ott et al. 1988) were obtained from the Field Museum in Chicago and were slightly crushed to reduce grain size but they were not ground.
|ALHA77005||8.96||558||303||29.57||1.84||Bulk chips|| Miura et al. (1995) |
|10.0||558||303||33.00||1.84||Not reported|| Nagao (1987) |
|ALH84001||1.29||171||19.9||64.82||8.59||Not reported, likely untreated pieces|| Bogard and Garrison (1998) |
|1.00||114||15.9||62.89||7.17||Not reported, likely untreated pieces|| Bogard and Garrison (1998) |
|1.38||184||24.6||56.10||7.48||Not reported, likely untreated pieces|| Bogard and Garrison (1998) |
|1.25||157||24.1||51.95||6.51||Bulk chips|| Miura et al. (1995) |
|1.63||175||27.6||59.09||6.34||Bulk chips|| Miura et al. (1995) |
|1.42||185||28.1||50.50||6.59||Bulk chips|| Murty and Mohapatra (1997) |
|8.77||59.0||11.5||762.61||5.13||Bulk chips|| Swindle et al. (1995) |
|1.73||156||28.0||61.91||5.60||Bulk chips|| Mathew and Marti (2001) |
|Chassigny||0.81||53.4||43.2||18.70||1.24||Bulk chips|| Ott (1988) |
|2.24||83.7||47.3||47.41||1.77||Bulk chips|| Mathew and Marti (2001) |
|1.81||81.8||44.7||40.39||1.83||Bulk chips|| Mathew and Marti (2001) |
|0.89||61.7||46.1||19.28||1.34||Bulk chips|| Mathew and Marti (2001) |
|EETA79001||0.40||118||21.0||19.05||5.62||Not reported, likely untreated pieces|| Bogard et al. (1984) |
|1.30||29.0||4.7||276.60||6.17||Not reported, likely untreated pieces|| Bogard et al. (1984) |
|1.89||20.5||31.1||60.87||0.66||Not reported, likely untreated pieces|| Bogard et al. (1984) |
|Governador Valadares||12.0||21.0||6.2||1935.48||3.39||Bulk chips1|| Swindle et al. (1989) |
|Lafayette|| ||98.4||32.4|| ||3.04||Small chips|| Ott et al. (1988) |
| ||56.5||20.4|| ||2.77||Small chips|| Ott et al. (1988) |
|Nakhla||0.90||39.6||11.5||77.83||3.44||Bulk chips|| Ott (1988) |
|0.88||34.9||8.2||107.73||4.28||Bulk chips|| Ott (1988) |
|0.86||40.9||13.3||64.66||3.08||Bulk chips|| Ott (1988) |
|13.7||24.0||9.0||1522.22||2.67||Not reported|| Busemann and Eugster (2002) |
| ||1.05||55.7||12.7||83.25||4.40||Most likely untreated pieces of bulk rock|| Mathew and Marti (2002) |
|Shergotty||0.70||23.7||5.3||132.08||4.47||Not reported, likely untreated pieces|| Bogard and Garrison (1998) |
|1.24||66.6||12.7||97.87||5.24||Bulk chips|| Ott (1988) |
|1.28||53.7||8.8||144.80||6.07||Bulk chips|| Ott (1988) |
|Zagami||5.50||43.0||3.3||1666.67||13.03||Not reported, likely untreated pieces|| Bogard et al. (1984) |
|7.70||179||10.0||770.00||17.99||Small pieces from a larger, untreated sample|| Ott et al. (1988) |
Grinding of our samples was done under laboratory atmosphere. Upon grinding, the active surface area of a sample is increased. Furthermore, grinding increases the temperature of the sample by friction heating, and applies mechanical stress on the individual grains. The combination of these factors causes terrestrial air to be the most likely contaminant introduced by the sample preparation. To estimate the relative and absolute amounts of contaminating air to be expected we will first look at the adsorption process in detail. For this, it seems safe to assume that the application of Henry’s law is possible, because krypton and xenon are rare (Ozima and Podosek 2002). Air contains 0.114 × 10−3 vol. % Kr and 8.7 × 10−6% vol. Xe, which at P = 1.013 atm corresponds to p(Kr) of 1.155 × 10−6 atm, and p(Xe) of 8.81 × 10−8 atm. The influence of temperature on adsorption coefficients in geologic materials within the range of 0–25 °C is less severe than the influence of material properties, but in most geologic material Xe adsorbs to a greater extent than does Kr (Ozima and Podosek 2002). Deciding which adsorption coefficients for Kr and Xe are applicable to the grinding experiment is not straightforward because of the dependence on the material.
Comparing available data for shale, basalt and quartzite (see Ozima and Podosek 2002, their Table 3.2; data from Podosek et al. 1981) reveals that fine-grained clay adsorbs more and fractionates Kr from Xe more severely than do basalt and quartzite (Table 4). Especially for the fine-grained clay, the effect is a shift in the 84Kr/132Xe ratio from terrestrial atmosphere toward lower values. From this, the expected 84Kr/132Xe ratio for analytical grade powder is about 2.1, which is in the range of what is observed in our samples, independent of the original 84Kr/132Xe ratio as reflected by the literature values (Fig. 2) or the gain or loss factor (Fig. 1). In addition, it has been shown that the increase of the surface area is only one aspect of the enhanced adsorption by crushing-generated surfaces: crushing may furthermore cause an order of magnitude increase in the Henry constant (for the case of plagioclase see Bernatowicz et al. 1982) and may produce more energetic surfaces compared to the uncrushed sample (Bernatowicz et al. 1982). Thus, most likely adsorption contamination by terrestrial air has led to the elemental ratios observed in our ground samples and has overprinted the Martian signatures.
Table 4. Henry constants, amounts of Kr and Xe and 84Kr/132Xe ratios of adsorbed air. All numbers are taken from Ozima and Podosek (2002), their table 2.2, p. 39). T in °C, Henry constant in cm3STP g−1 atm−1, and N in cm3STP g−1. VA shale = volcanic ash shale, CR basalt = Columbia River basalt, and U qtz = Uncomphagre quartzite. f is the fractionation factor that describes the enrichment of Xe relative to Kr.
|U qtz||0||3 * 10−4||3.46 * 10−10||5 * 10−5||4.41 * 10−12||78.6||167||0.167|
|CR basalt||0||0.28||3.23 * 10−7||0.7||6.17 * 10−8||5.24||11.1||2.50|
|VA shale||0||0.43||4.97 * 10−7||5.6||4. 94 * 10−7||1.01||2.13||13.0|
|VA shale||25||0.21||2.43 * 10−7||2.5||2.20 * 10−7||1.10||2.33||11.9|
The air contamination becomes evident in the isotopic signatures of the ground samples compared to their untreated counterparts. On a plot of 136Xe/132Xe versus 129Xe/132Xe all ground samples, except Shergotty, plot in a small area close to air, whereas the untreated samples cover a much wider range, including the display of some fissiogenic 136Xe (Fig. 3). Our conclusion is in agreement with observations on lunar samples (Niemeyer and Leich 1976; Niedermann and Eugster 1992) and the Allende chondrite (Srinivasan et al. 1978). However, compared to these cases, it is considerably more and critically important to the study of Martian meteorites because of the unique similarity of the contaminant with the Martian interior component. A particular case is the 129Xe/132Xe isotopic ratio, which is often used as an indicator for the presence of Martian gas components. With gas amounts small, analytical precision of Xe analyses of Martian meteorites is often poor and data do not allow distinguishing between Martian interior (129Xe/132Xe = 1.03 ± 0.02; Ott 1988) and air (0.9832 ± 0.0012; Ozima and Podosek 2002). Fissiogenic 136Xe has been observed previously in other Martian meteorites (Mathew et al. 1998, 2003; Mathew and Marti 2001, 2002). If the contamination problem is not recognized in such a case, this then can lead to misinterpretation of the results.
Looking at one of the common elemental ratio plots in Martian meteorite noble gas research (84Kr/132Xe versus 36Ar/132Xe; Fig. 2) demonstrates the case: The Martian meteorite measurements of untreated samples (literature values) span a wide range and trend toward Martian atmosphere. The ground samples, on the other hand, cluster in a much smaller area (36Ar/132Xe between about 40 and 100, 84Kr/132Xe between 2 and 5) independent of any differences they may have had before the grinding procedure.
The three samples that show deficits of krypton and xenon are Lafayette, ALHA77005, and Chassigny. Lafayette, which belongs to the group of nakhlites, is the one with the highest olivine abundance (about 17%) in this study (Treiman 2005) and contains approximately 10% of iddingsite. As nakhlites do not contain shock melt, iddingsite may appear a likely host phase from which trapped gas could have been lost. This, however, does not fit with the results for Governador Valadares, which contains a similar amount of iddingsite, and thus would be expected to be similar to Lafayette, contrary to observation. A possible reason may be enhanced exposure of Lafayette to air prior to collection, e.g., through cracks or find location conditions and terrestrial residence time. Nakhla is an observed fall, and Governador Valadares may have been collected on the morning after its fall. In contrast, Lafayette is a find with a terrestrial age of about 3000 yr (Treiman  and references therein) and was actually collected as a glacially striated stone in the first place. Whatever the reasons, it is important to keep in mind that among the nakhlites only for Nakhla there are multiple measurements of untreated samples (Ott 1988; Busemann and Eugster  as well as Mathew and Marti  report Kr and Xe-data; while Gilmour et al. [1999, 2001] report Xe only), while for Governador Valadares (Swindle et al. 1989) and Lafayette (Ott et al. 1988) there are two reference measurements only (Table 3). This is a problem, of course, because of sample inhomogeneities, and the small amount of sample actually used for noble gas analysis.
The second meteorite to display “loss” is ALHA77005, an olivine rich (∼60% olivine; Treiman et al. 1994) shergottite, which also contains a considerable amount of shock glass (∼14%), whereas the third one is Chassigny, a dunite (about 92% olivine, Floran et al. 1978). In Chassigny melt inclusions in olivine are described that contain amphiboles with “substantial hydrogen” and some fluorine (Floran et al. 1978). Grinding may have led to loss of those volatiles and of the noble gases that may have accompanied them. It appears questionable, however, whether this can account for the entirety of the loss, because melt inclusions make up only 0.3% of the rock. The case is different for ALHA77005, which contains a considerable amount of shock glass. Shock glass is known as the carrier of Martian atmosphere as found in EETA79001 (Bogard and Johnson 1983; Bogard and Garrison 1998), which may have been lost from this phase in principle.
Loss, however, is not very likely to have occurred under mild crushing (grinding) conditions as employed here. This is indicated by numerous experiments on terrestrial samples, which have been crushed to extract the noble gases in fluid inclusions, and subsequently heated to understand the signatures of the minerals enclosing these (e.g., Kelley et al. 1986; Turner and Songshan 1992; Kendrick et al. 2001, 2006; Kendrick and Phillips 2009; Kamensky and Skiba 2011). It also fits with the observation that the ground samples have retained their 3He and 4He and compare well to literature data (see Schwenzer et al. 2008), and it is in accordance with “true” crushing experiments on Martian meteorites as well. Crushing EETA79001 (Wiens 1988) revealed the special EETV-component that was interpreted to be located in vesicles, while heating of the sample after the crushing experiment showed Martian meteorite noble gas signatures as seen in other heating experiments of this and other Martian meteorites. While hints for a possible redistribution of Martian components during crushing have been seen (Mohapatra et al. 2009), no evidence for losses has been found in the same experiment. Thus, the major concern with mildly crushed samples is not loss of noble gases from lattice sites but rather contamination by introduced air. It is also noteworthy that both Chassigny and ALHA77005 (with apparent “losses”) join in the narrow area of the ground samples in the elemental ratios plot (Fig. 2), which argues for a common noble gas component introduced into them independent of what they may have lost.