The natural thermoluminescence of Antarctic meteorites and their terrestrial ages and orbits: A 2010 update

Abstract– We have examined the relationship between natural thermoluminescence (TL) and ²⁶Al in 120 Antarctic meteorites in order to explore the orbital history and terrestrial ages of these meteorites. Our results confirm the observations of Hasan et al. (1987) which were based on 23 meteorites. For most meteorites there was a positive correlation between natural TL and ²⁶Al, reflecting their similarity in decay rate under Antarctic conditions and thus in terrestrial age. For a small group with low TL and high ²⁶Al a small perihelion was proposed. Within this group, natural TL decreases with terrestrial age as determined by ³⁶Cl measurements, although the rate of TL decay is faster (half‐life approximately 10 ka) and the ages that can be determined are smaller (<200 ka) than for most meteorites. The faster decay rate and lower natural TL levels are a reflection of recent exposure to higher radiation doses and higher temperatures, since this history would populate less stable TL traps with smaller electron densities. We sort the 120 meteorites by perihelion and terrestrial age. The normal perihelion group range up to approximately 1000 ka and the small perihelion group range up to approximately 200 ka. An intermediate perihelion group tends to have short terrestrial ages (20–60 ka). There is acceptable agreement between most (34 out of 43) of our present terrestrial age estimates and those determined by isotopic means, the exceptions reflecting complex irradiation histories, long burial times in the Antarctic, or other issues.     

Rapid changes in the nature of the H chondrites falling to Earth

Abstract— We have previously identified a subgroup of Antarctic H chondrites that are significantly different from H chondrites among the modern falls in terms of induced thermoluminescence (TL), metallographic cooling rate, and cosmogenic inert gas contents. Here we examine their terrestrial and thermal history as apparent in their natural TL and radioactive cosmogenic isotope abundances. These meteorites have a tendency towards high ²⁶Al activities and fairly short ¹⁴C and ³⁶Cl terrestrial ages (generally <100 ka). They also sometimes exhibit unusually high natural TL levels, which we have previously interpreted as indicating orbital evolution from perihelia >1.2 AU to ～1 AU within the last <10⁵ years. We suggest that the nature of the meteorites falling to Earth is not independent of time but depends on stochastic events, such as the breakup of parent bodies and recent variations in orbit.     

Thermoluminescence survey of 12 meteorites collected by the European 1988 Antarctic meteorite expedition to Allan Hills and the importance of acid washing for thermoluminescence sensitivity measurements

Abstract— Natural and induced thermoluminescence (TL) data are reported for 12 meteorites recovered from the Allan Hills region of Antarctica by the European field party during the 1988/89 field season. The samples include one with extremely high natural TL, ALH88035, suggestive of exposure to unusually high radiation doses (i.e., low degrees of shielding), and one, ALH88034, whose low natural TL suggests reheating within the last 10⁵ years. The remainder have natural TL values suggestive of terrestrial ages similar to those of other meteorites from Allan Hills. ALH88015 (L6) has induced TL data suggestive of intense shock. TL sensitivities of these meteorites are generally lower than observed falls of their petrologic types, as is also observed for Antarctic meteorites in general. Acid-washing experiments indicate that this is solely the result of terrestrial weathering rather than a nonterrestrial Antarctic—non-Antarctic difference. However, other TL parameters, such as natural TL and induced peak temperature-width, are unchanged by acid washing and are sensitive indicators of a meteorite's metamorphic and recent radiation history.     

Exposure of Allan Hills 84001 and other achondrites on the Antarctic ice

Abstract— The enrichment of F on Antarctic meteorites is the result of their exposure to the atmosphere, and its measurement allows a subdivision of the terrestrial age into a duration of exposure on the ice and the time a meteorite was enclosed by the ice. In many cases, the periods of surface exposure are only small fractions of the terrestrial ages of meteorites collected in Antarctica. The enrichment of F on the surfaces of Antarctic achondrites was investigated by means of nuclear reaction analysis (NRA): scanning proton beams with an energy of 2.7 and 3.4 MeV were used to induce the reactions ¹⁹F(p,αγ)¹⁶O and ¹⁹F(p, p'γ)¹⁹F, respectively. Gamma signals proportional to the F content were measured. The following Antarctic achondrites were investigated: Martian meteorite ALH 84001; diogenite ALHA77256; the eucrites ALHA81011 and ALHA78132; and in addition, the H5 chondrite ALHA79025. For ALH 84001, our data indicate a period of exposure on the ice of <500 years. Thus, this specimen was enclosed in the ice >95% of its terrestrial age of 13 000 years.     

Meteorite infall as a function of mass: Implications for the accumulation of meteorites on Antarctic ice

Abstract— Antarctic meteorites are considerably smaller, on average, than those recovered elsewhere in the world, and seem to represent a different portion of the mass distribution of infalling meteorites. When an infall rate appropriate to the size of Antarctic meteorites is used (1000 meteorites 10 grams or larger/km²/10⁶ years), it is found that direct infall can produce the meteorite accumulations found on eight ice fields in the Allan Hills region in times ranging from a few thousand to nearly 200 000 years, with all but the Allan Hills Main and Near Western ice fields requiring less than 30 000 years. Meteorites incorporated into the ice over time are concentrated on the surface when the ice flows into a local area of rapid ablation. The calculated accumulation times, which can be considered the average age of the exposed ice, agree well with terrestrial ages for the meteorites and measured ages of exposed ice. Since vertical concentration of meteorites through removal of ice by ablation is sufficient to explain the observed meteorite accumulations, there is no need to invoke mechanisms to bring meteorites from large areas to the relatively small blue-ice patches where they are found. Once a meteorite is on a bare ice surface, freeze-thaw cycling and wind break down the meteorite and remove it from the ice. The weathering lifetime of a 100-gram meteorite on Antarctic ice is on the order of 10 000 ± 5000 years.     

Terrestrial ages of ordinary chondrites from the Lewis Cliff stranding area,East Antarctica

Abstract— We determined terrestrial ages of ordinary chondrites from the Lewis Cliff stranding area, East Antarctica, on the basis of the concentrations of cosmogenic ¹⁰Be (t_½; = 1.51 Ma), ²⁶Al (t_½; = 0.705 Ma), and ³⁶Cl (t_½; = 0.301 Ma). After an initial ²⁶Al γ-ray survey of 91 meteorites suggested that many have terrestrial ages >0.1 Ma, we selected 62 meteorites for ¹⁰Be and ²⁶Al measurements by accelerator mass spectrometry (AMS) and measured ³⁶Cl in twelve of those. Low terrestrial ages (<0.1 Ma) were found for ～60% of the meteorites, whereas all others have ages between 0.1 and 0.5 Ma, except for one exceptional age of >2 Ma (Welten et al., 1997). Our major conclusions are: (1) The Lewis Cliff H-chondrites show similar ages to those from the Allan Hills icefields, but the L-chondrites are about a factor of 2 younger than those from Allan Hills, which indicates that Lewis Cliff is a younger stranding area. (2) The terrestrial age distributions at different parts of the Lewis Cliff stranding area generally agree with simple meteorite concentration models, although differences in weathering rate may also play a role. (3) We confirm that meteorites with natural thermoluminescence (TL) levels >80 krad are associated with low terrestrial ages (Benoit et al., 1992) but conclude that natural TL levels <80 krad can not be used to calculate the terrestrial age of a meteorite. Natural TL levels do seem useful to estimate relative terrestrial ages of large groups of meteorites and to determine differences in the surface exposure age of paired meteorite fragments. (4) Of the 62 meteorites measured with AMS, 31 were assigned to 11 different pairing groups, mainly on the basis of their cosmogenic nuclide record. The meteorites are estimated to represent between 42 and 52 distinct falls.     

The natural thermoluminescence of meteorites VI: Carbon-14, thermoluminescence and the terrestrial ages of meteorites

Abstract Research on meteorite finds, especially those from the Antarctic and from desert regions in Australia, Africa, and America, has become increasingly important, notably in studies of possible changes in the nature of the meteorite flux in the past. One important piece of information needed in the study of such meteorites is their terrestrial age which can be determined using a variety of methods, including ¹⁴C, ³⁶Cl, and ⁸¹Kr. Natural thermoluminescence (TL) levels in meteorites can also be used as an indicator of terrestrial age. In this paper, we compare ¹⁴C-determined terrestrial ages with natural TL levels in finds from the Prairie States (central United States), a group of finds from Roosevelt County (New Mexico, USA), and a group from the Sahara Desert. We find that, in general, the natural TL data are compatible with the ¹⁴C-derived terrestrial ages using a 20 °C TL decay curve for the Prairie States and Roosevelt County and a 30 °C decay curve for the Saharan meteorites. We also present TL data for a group of meteorites from the Sahara desert which has not been studied using cosmogenic radionuclides. Within these data there are distinct terrestrial age clusters which probably reflect changes in meteorite preservation efficiency over ～ 15, 000 years in the region.     

The thermal and radiation exposure history of lunar meteorites

Abstract— We have measured the natural and induced thermoluminescence (TL) of seven lunar meteorites in order to examine their crystallization, irradiation, and recent thermal histories. Lunar meteorites have induced TL properties similar to Apollo samples of the same provenance (highland or mare), indicating similar crystallization and metamorphic histories. MacAlpine Hills 88104/5 has experienced the greatest degree of impact/regolith processing among the highland-dominated meteorites. The basaltic breccia QUE 94281 is dominated by mare component but may also contain a significant highland component. For the mare-dominated meteorites, EET 87521 may have a significant highland impact-melt component, while Asuka 881757 and Y-793169 have been heavily shocked. The thermal history of Y-793169 included slow cooling, either during impact processing or during its initial crystallization. Our natural TL data indicate that most lunar meteorites have apparently been irradiated in space a few thousand years, with most <15,000 a. Elephant Moraine 87521 has the lowest irradiation exposure time, being <1,000 a. Either the natural TL of ALHA81005, Asuka 881757 and Y-82192 was only partially reset by lunar ejection or these meteorites were in small perihelia orbits (≤0.7 AU).     

Concentrations of siderophile elements in nonmagnetic fractions of Antarctic H- and L-chondrites: A quantitative approach on weathering effects

Abstract— The nonmagnetic separates of Antarctic H- and L-chondrite finds from the Lewis Cliff stranding area are enriched in Fe and siderophile elements such as Ni, Co, Cu, As, Ir, and Au, relative to those of non-Antarctic falls. The siderophile enrichments are caused by the oxidation of metallic Fe-Ni to nonmagnetic hydrous Fe-Ni oxides. The concentrations of Fe, Ni, and Co in each sample were used to calculate the amount of oxidized metal, which serves as a quantitative measure of the degree of weathering. The amount of oxidized metal is generally between 1–10 wt% and shows a rough correlation with the qualitative A-B-C-weathering classification for increasing rustiness. No clear correlation was found between weathering and terrestrial age, although meteorites older than 0.2 Ma contain on the average ～50% more oxidized metal. Also, no correlation was found between the degree of weathering and the natural thermoluminescence (TL) level, which is dominated by the thermal history in space rather than by the thermal history in or on the Antarctic ice. The only significant parameter for the degree of weathering seems to be the location of find, with the most weathered specimens being found in a part of the stranding area where melt-water ponds are known to occur. This observation confirms that the occasional presence of liquid water plays an important role in the Antarctic weathering process and explains the poor correlation of the degree of weathering of Antarctic meteorites with terrestrial age and surface exposure age.     

X‐ray computed tomography imaging: A not‐so‐nondestructive technique

X‐ray computed tomography has become a popular means for examining the interiors of meteorites and has been advocated for routine curation and for the examination of samples returned by missions. Here, we report the results of a blind test that indicate that CT imaging deposits a considerable radiation dose in a meteorite and seriously compromises its natural radiation record. Ten vials of the Bruderheim L6 chondrite were placed in CT imager and exposed to radiation levels typical for meteorite studies. Half were retained as controls. Their thermoluminescence (TL) properties were then measured in a blind test. Five of the samples had TL data unaltered from their original (~10 cps) while five had very strong signals (~20,000 cps). It was therefore very clear which samples had been in the CT scanner. For comparison, the natural TL signal from Antarctic meteorites is ~5000–50,000 cps. Using the methods developed for Antarctic meteorites, the apparent dose absorbed by the five test samples was calculated to be 83 ± 5 krad, comparable with the highest doses observed in Antarctic meteorites and freshly fallen meteorites. While these results do not preclude the use of CT scanners when scientifically justified, it should be remembered that the record of radiation exposure to ionizing radiations for the sample will be destroyed and that TL, or the related optically stimulated luminescence, are the primary modern techniques for radiation dosimetry. This is particularly important with irreplaceable samples, such as meteorite main masses, returned samples, and samples destined for archive.     

The meteorite collection sites of Antarctica

Abstract— Antarctic meteorites have been and are being well studied but the potential for glaciological and climatological information in the sites where they are found is only beginning to be realized. To date, meteorite stranding surfaces have been identified only in East Antarctica: (1) The MacKay Glacier/David Glacier region contains the Allan Hills and the Reckling Moraine/Elephant Moraine stranding surfaces. Because the Allan Hills Main Icefield has a large proportion of meteorites with long terrestrial ages, these concentrations of meteorites must have had catchment areas extending well inland, in contrast to the present. Where known, bedrock topography is mesa-like in form and influences ice flow directions. Ice levels at the Allan Hills may have been higher by 50–100 m in the past. Reckling Moraine and Elephant Moraine are located on a long patch of ice running westward from Reckling Peak; the ice appears to be pouring over a bedrock escarpment. (2) In North Victoria Land, ice diverges around Frontier Mountain and flows into a site behind the barrier where ablation occurs extensively. It is proposed that meteorites and rocks were dumped by ice flow at the mouth of a valley in the lee of the mountain at the site where a meltwater pond existed, in a depression produced by ablation. Later, the pond migrated headward along the valley to a point where it is today, leaving a morainal deposit with the meteorites at a higher level. (3) Between the Beardmore and Law Glaciers, ice flows sluggishly into the southwestern margin of the Walcott Névé. Northeastern sections of the Walcott are virtually barren of meteorites. The entering Plateau ice is diverted northward to flow along the base of Lewis Cliff. This flow apparently terminates in an ice tongue protruding into a vast moraine, where a very large concentration of meteorites was found on the ice. This final segment of flowing ice is called the Lewis Cliff Ice Tongue. Meteorite Moraine, a subsidiary occurrence 2 km to the northeast, is also found against morainal deposits. The origin of the moraines and the history of meteorite concentration at this site is the subject of some debate. (4) The Transantarctic Mountains are submerged along one segment many hundreds of km in length by ice flowing off the Polar Plateau. The Thiel Mountains, Pecora Escarpment and Patuxent Range are the only surface indications of the underlying mountains along this interval, and meteorite stranding surfaces are found at each of these sites. Little is yet known about ice dynamics at these sites. (5) The immense Yamato Mountains meteorite stranding surface covers an area of about 4000 km². So far, most meteorites have been recovered in the upper reaches of this blue ice field, where ice flow is slowed by outlying subice barriers of the Yamato Mountains. Individual massifs in this range extend northward over 50 km, and the Yamato Meteorite Icefield loses 1100 m in elevation over this distance. (6) The Sør Rondane Mountains form a barrier to ice flow off the Polar Plateau. The major meteorite stranding surface associated with this barrier is the Nansenisen Icefield, a large ablation area about 50 km upstream of the mountains. The existence of a meteorite stranding surface at this site has not been explained so far. Most meteorite stranding surfaces have been functioning for a long time. They are sites where net ablation of the surface is occurring; the ice at these sites is stagnant or flowing only slowly, and the numbers of meteorites with great terrestrial ages decrease exponentially. Concentration mechanisms operating at these sites involve ablation, direct infall, time, low temperatures, moderate weathering and wind ablation. Detrimental to concentration are ice flow out of the area and extreme weathering. In spite of the fact that the Antarctic Ice Sheet is thought to be over 10 Ma old, we do not find stranding surfaces with meteorites having greater terrestrial ages than 1 Ma. This suggests that stranding surfaces are transient features, affected on a continental scale by possible extreme warming during late Pliocene and on a smaller scale by regional changes that produce differential effects between icefields. The latter effect is suggested by differences in the average terrestrial age of meteorites at different stranding surfaces. In either case, these sites seem to appear as a result of thinning near the edges of the ice sheet, and stratigraphic sequences may be exposed in the ice at stranding surfaces. We review five models for the production of meteorite stranding surfaces: (1) simple deflation of the ice sheet, in which ablation removes great thicknesses of overlying ice, exposing the contained meteorites while allowing direct falls to accumulate, (2) simple accumulation of direct falls on a bare ice surface that is not deflating, (3) ablation of ice trapped against a barrier, in which meteorites accumulate by direct infall while inflowing ice contributes meteorites by ablation discovery, (4) deceleration of ice by a subice barrier, which allows ablation discovery of meteorites in incoming ice and accumulation of other meteorites on the surface by direct infall and (5) stagnation of ice by encounter with an ice mass able to produce an opposing flow vector, in which ablation discovery and direct infall accumulation processes operate to build the meteorite concentration.     

Allan Hills 88019: An Antarctic H-chondrite with a very long terrestrial age

Abstract— We have measured the concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al and ³⁶Cl (half-lives 1.51 Ma, 716 ka, and 300 ka, respectively) in two different laboratories by accelerator mass spectrometry (AMS) techniques, as well as concentrations and isotopic compositions of stable He, Ne and Ar in the Antarctic H-chondrite Allan Hills (ALH) 88019. In addition, nuclear track densities were measured. From these results, it is concluded that the meteoroid ALH 88019 had a preatmospheric radius of (20 ± 5) cm and a shielding depth for the analyzed samples of between 4 and 8 cm. Using calculated and experimentally determined production rates of cosmogenic nuclides, an exposure age of ～40 Ma is obtained from cosmogenic ²¹Ne and ³⁸Ar. The extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 ± 0.4 Ma. A similarly long terrestrial age was found so far only for the Antarctic L-chondrite Lewis Cliff (LEW) 86360. Such long ages establish one boundary condition for the history of meteorites in Antarctica.     

The age of the meteorite recovery surfaces of Roosevelt County,New Mexico,USA

Abstract— We have obtained minimum age estimates for the sand units underlying the two largest meteorite deflation surfaces in Roosevelt County, New Mexico, USA, using thermoluminescence dating techniques. The dates obtained ranged from 53.5 (±5.4) to 95.2 (±9.5) ka, and must be considered lower limits for the terrestrial ages of the meteorites found within these specific deflation surfaces. These ages greatly exceed previous measurements from adjacent meteorite-producing deflation basins. We find that Roosevelt County meteorites are probably terrestrial contemporaries of the meteorites found at most accumulation zones in Antarctica. The apparent high meteorite accumulation rate reported for Roosevelt County by Zolensky et al. (1990) is incorrect, as it used an age of 16 ka for all Roosevelt County recovery surfaces. We conclude that the extreme variability of terrestrial ages of the Roosevelt County deflation surfaces effectively precludes their use for calculations of the meteorite accumulation rate at the Earth's surface.     

Carbon-14 terrestrial ages and weathering of 27 meteorites from the southern high plains and adjacent areas (USA)

Abstract We report on a series of 27 ¹⁴C terrestrial ages of meteorites from four states in the central and southwestern USA. These results were compared to the earlier work of Boeckl (1972). Our results showed that the weathering rate for destruction of meteorites is lower than suggested by Boeckl (1972). We estimated a “half-life” for removal of meteorites of about 10 to 15 ka, similar to that derived for Roosevelt County meteorites. We also studied the weathering of these meteorites compared to terrestrial age. Only a weak correlation was observed, and for these meteorites the degree of weathering can only be taken as a weak indicator of terrestrial residence time. We also measured the δ ¹³C and ¹⁴C and amount of weathering-product carbonates which show some interesting variations with the length of time the meteorites have been exposed to weathering.     

Terrestrial microfossils in Antarctic ordinary chondrites

Abstract— Microfossils have been separated and identified in four high metamorphic grade chondrites from Allan Hills and Queen Alexandra Range, Antarctica. Diatoms and opal phytoliths representing both marine and terrestrial flora were recognized among the dust removed from cracks in all meteorites studied. It is likely that contamination of Antarctic meteorites with such biogenic material is ubiquitous. Standard clean room procedures to avoid laboratory introduction of microfossils into the meteorites were followed, and the genera and species identified so far are characteristic of marine, freshwater, and continental environments. The most probable mechanism for introduction of the microfossils into the meteorites is eolian transport to and on the polar ice cap. It is likely that wind-driven systems may sample atmospherically transported material from large portions of the southern hemisphere. Entrainment of terrestrial microfossils is probably a typical interaction of meteorites with the Antarctic environment and must be recognized and accounted for in any attempt to use Antarctic meteorites as sources of extraterrestrial life forms. Organic molecules derived from microfossils are likely to be pervasive throughout any crack network present in a meteorite at all scales from millimeter to submicron. Cracks are a ubiquitous consequence of weathering in and on the Antarctic ice and the probability that crack surfaces contain terrestrial organic materials is high.     

Uranium accumulation during weathering of Cañon Diablo meteoritic iron

Abstract— Cañon Diablo meteoritic iron oxide consists mainly of goethite and maghemite and contains 209 to 630 ppb uranium compared to <0.02 ppb in the unweathered octahedrite. Significant radioactive disequilibria between ²³⁸U, ²³⁴U and ²³⁰Th indicate that uranium was sorbed from soil porewater during terrestrial weathering after meteorite impact 50 ka ago. Depending on the model assumed for U uptake, corrected ²³⁰Th-²³⁴U ages of 24 to 48 ka were obtained. While the data presented may not allow an unambiguous interpretation, the potential of this approach in obtaining minimum terrestrial ages for weathered meteorites is demonstrated.     

Weathering of ordinary chondrites from Oman: Correlation of weathering parameters with 14C terrestrial ages and a refined weathering scale

We have investigated 128 ¹⁴C‐dated ordinary chondrites from Oman for macroscopically visible weathering parameters, for thin section‐based weathering degrees, and for chemical weathering parameters as analyzed with handheld X‐ray fluorescence. These 128 ¹⁴C‐dated meteorites show an abundance maximum of terrestrial age at 19.9 ka, with a mean of 21.0 ka and a pronounced lack of samples between 0 and 10 ka. The weathering degree is evaluated in thin section using a refined weathering scale based on the current W0 to W6 classification of Wlotzka (1993), with five newly included intermediate steps resulting in a total of nine (formerly six) steps. We find significant correlations between terrestrial ages and several macroscopic weathering parameters. The correlation of various chemical parameters including Sr and Ba with terrestrial age is not very pronounced. The microscopic weathering degree of metal and sulfides with newly added intermediate steps shows the best correlation with ¹⁴C terrestrial ages, demonstrating the significance of the newly defined weathering steps. We demonstrate that the observed ¹⁴C terrestrial age distribution can be modeled from the abundance of meteorites with different weathering degrees, allowing the evaluation of an age‐frequency distribution for the whole meteorite population.     

Terrestrial ages of meteorites from the Nullarbor region,Australia, based on 14C and 14C–10Be measurements

Abstract– We have investigated the terrestrial ages, or residence times, of 78 meteorites (representing 73 discrete falls) recovered in Western Australia, and one from South Australia, using both ¹⁴C measurements and also ¹⁴C/¹⁰Be. The samples studied included two ureilites, one CK and one EL chondrite. We have included ¹⁰Be measurements from 30 meteorites, including some meteorites for which the ¹⁴C terrestrial age was previously determined. We find that the ¹⁴C/¹⁰Be terrestrial ages are more precise than ¹⁴C alone, as we can correct for shielding effects. In general, the two different age determinations age by ¹⁴C–¹⁰Be are precise to 0.5–1 ka and ¹⁴C alone within 1–2 ka. However, measurement of the ¹⁴C age alone gives good agreement with the ¹⁴C–¹⁰Be for most samples. The study of the terrestrial ages of meteorites gives us useful information concerning the storage and weathering of meteorites and the study of fall times and terrestrial age. We have compared the terrestrial ages to weathering, degree of oxidation (estimated from Mössbauer studies) and Δ¹⁷O. In this study, we found that weathering is not well correlated with terrestrial age for Nullarbor meteorites. However, there is a good correlation between degree of oxidation and Δ¹⁷O. The implications for the study of terrestrial ages and weathering from other desert environments will be discussed.     

Weathering in Antarctic H and CR chondrites: Quantitative analysis through Mössbauer spectroscopy

Abstract— Mössbauer spectroscopy is a very useful tool for identifying ferric iron weathering products in meteorites because of the capability to quantify the relative amounts of ferric iron in them. Mössbauer measurements were made of 33 Antarctic H chondrites (predominately H5) and two paired Antarctic CR chondrites. The primary goals of this study are to determine if Mössbauer spectroscopy can be used to determine which phases are weathering in Antarctic meteorites and if the relative amounts of ferric iron correlate with terrestrial age. Determining which minerals are weathering in ordinary chondrites appears very difficult due to variations in composition for different ordinary chondrites of the same meteorite class and possible problems in preparing homogeneous samples. The analysis of the two paired CR chondrites appears to indicate that metallic iron is predominately weathering to produce ferric iron for this class of meteorite. No correlation is seen between the relative amounts of ferric iron and terrestrial age for ordinary chondrites. One Antarctic H5 chondrite (ALHA77294) with a short ¹⁴C age of 135 ± 200 years from the dating of interior carbonate weathering products does have a relatively low amount of ferric iron, which is consistent with this meteorite being exposed on the surface for a relatively short time.