A recent meteorite shower in Antarctica with an unusual orbital history

The Antarctic meteorite collection has proved to be a source of many important discoveries, including a number of previously unknown or very rare meteorite types. A thermoluminescence (TL) survey of meteorite samples recovered by the 1988/89 European expedition and pre-1988 American expeditions to the Allan Hills Main blue ice field resulted in the discovery of 15 meteorites with very high TL levels ( > 100 krad at 250°C in the glow curvc). It is likely that these samples are fragments of a single meteoroid body which: (1) fell very recently and (2) experienced a decrease in orbital perihelia from 1.1 AU to 1 AU within the last 10⁵ yr. Carbon-14 data for two of the samples confirm their young terrestrial age compared to most Antarctic meteorites. Studies of the cosmogenic isotopes in at least one non-Antarctic meteorite which also has very high natural TL, Jilin, indicate that the meteorite experienced a multi-stage irradiation history, the most recent stage being 0.4 Ma in duration following a major break-up of the object. These meteorites, and the few equivalent modern falls, are the only documented samples from bodies which were recently in Earth-approaching (Amor) orbits (i.e., with perihelion > 1.0 AU), as opposed to the Earth-crossing (Apollo) orbits which are the source of most other meteorites. Their rarity indicates that such rapid orbit changes are unusual for meteoroid bodies and may be the result of isolated, large break-up events.     

Al survey of Antarctic meteorites

An²⁶Al survey by non-destructive gamma ray spectrometry was conducted as part of the preliminary examination effort for Antarctic meteorites. A total of 220 samples were studied. The majority of the samples were from the Allan Hills area; however, samples from several other sites, including the Yamato Mountains, were studied. Compared to worldwide falls and finds, the Antarctic group shows a clear trend toward lower²⁶Al levels. At leasr 10% of the samples studied were clearly undersaturated in²⁶Al. The frequency of undersaturated samples suggests that terrestrial ages of several hundred thousand years are common among the Antarctic meteorite collection. The absence of samples with extremely low²⁶Al implies that the upper limit for terrestrial ages in the Antarctic is on the order of 1 million years.     

Dating of meteorite falls using thermoluminescence: application to Antarctic meteorites

The date of fall of a meteorite may be estimated from its thermoluminescence (TL) and in this paper the principle of a method of utilising TL to determine the terrestrial ages of eight Antarctic meteorites (Allan Hills-77) is described. The TL in a meteorite is primarily induced by cosmic ray irradiation in space and once the meteorite is on the Earth's surface, it is shielded from further cosmic ray irradiation. Under these conditions the TL will decay at a rate governed by the thermal stability of the TL and by the environmental temperature.An estimate of the decay rate may be arrived at by using recently calculated data for the trapping parameters associated with meteorite TL. A major problem is the environmental storage temperature. The “effective” storage temperature of the Antarctic meteorites is unknown, but appears to be greater than the mean annual temperature of the region of the meteorite find.Only upper limits to the terrestrial ages can be calculated because the TL at the time of the fall to Earth is highly variable from sample to sample.     

14C content of ten meteorites measured by tandem accelerator mass spectrometry

¹⁴C has been measured in three North American and seven Antarctic meteorites with the Chalk River MP tandem accelerator. In most cases cosmogenic¹⁴C, which is tightly bound, was separated from absorbed atmospheric radiocarbon by stepwise heating extractions. Terrestrial ages obtained by comparing cosmogenic¹⁴C in the meteorite to that in Bruderheim are (7.2 ± 0.6) × 10³ years for Yamato 7304, (11.6 ± 0.4) × 10³ years for Estacado, and range from (32.7 ± 0.5) × 10³ to (41.0 ± 0.8) × 10³ years for six meteorites recovered at Allan Hills and its vicinity. The present upper limit to age determination by the accelerator method varies from 50 × 10³ to 70 × 10³ years depending upon mass and carbon content of the sample. The natural limit caused by cosmic ray production of¹⁴C in silicate rocks at 2000 m elevation is estimated to be (55 ± 5) × 10³ years. “Weathering ages” were estimated for the Antarctic meteorites from the specific activity of loosely-bound CO₂ considered to be absorbed from the terrestrial atmosphere on weathering. The accelerator measurements are in accordance with previous low-level counting measurements but have higher precision and sensitivity.     

14C-39ArMe correlations in chondrites and their pre-atmospheric size

Cosmogenic¹⁴C has been measured in 12 chondrites and the stone phase of the mesosiderite Bondoc. For the chondrites analysed the activities vary between 44 and 72 dpm/kg; the low value of (4.5 ± 0.9) dpm/kg for Bondoc is essentially due to its large pre-atmospheric size and not to a terrestrial age of several half-lives of¹⁴C.In eight cases³⁹Ar in the metal phase from the same meteorite specimens had been measured previously. The results are combined to derive the pre-atmospheric radiiR₀ of the meteoroids and depth of burial of the samples investigated. Values ofR₀ between 35 and 82 cm are obtained; of 14 samples ten came from a depth of 10 cm or less. The preponderance of samples from shallow depths is ascribed to asymmetrical ablation losses of the meteoroids during their passage through the atmosphere.A compilation of all published¹⁴C concentrations in chondrites shows that the variations between different specimens from thesame meteorites are almost as large as those for samples fromdifferent meteorites. Thus, there is no need to invoke different orbits of the meteoroids and a strong spatial gradient in the primary cosmic-ray intensity to explain variations of low-energy-produced cosmogenic nuclides in different meteorites.     

Al andBe production in iron meteorites

We have measured by accelerator mass spectrometry the²⁶Al contents of 20 and the¹⁰Be contents of 14 iron meteorites. The²⁶Al contents are typically 30% or more lower than values obtained by counting techniques; the¹⁰Be contents are 10–15% lower. The production rates (P) of these nuclides decrease by more than a factor of two as the⁴He/²¹Ne ratio increases with increasing shielding from 200 to 400. For the lighter shielding conditions expected in stony meteorites we estimateP₂₆(Fe) as 3–4 dpm/kg andP₁₀(Fe) as 4–5 dpm/kg. The average P/₁₀P₂₆ activity ratio is close to 1.5. Exposure ages calculated from²¹Ne/²⁶Al ratios cannot be calibrated so as to agree with both⁴⁰KK/ ages and ages based on the shorter-lived nuclides³⁹Ar and³⁶Cl. If agreement with the latter is forced, then the disagreement with⁴⁰KK/ ages may signal a 35% increase in the cosmic-ray intensity during the last 10⁷ a.     

Thermal release characteristics of spallogenic He,Ne, and Ar from the Carbo iron meteorite

The thermal release patterns of He, Ne and Ar from samples of the Carbo iron meteorite show that virtually no fractionation of³He,⁴He,²¹Ne and³⁸Ar occurs. Thus, conclusions about iron meteorites based on measured noble gas ratios will be unaffected by gas loss, and measurement of these ratios cannot yield information about possible loss. Further, noble gas loss cannot explain the abnormal elemental and isotopic patterns observed in some iron meteorites, notably hexahedrites.     

Meteoritic and bedrock constraints on the glacial history of Frontier Mountain in northern Victoria Land, Antarctica

In 2001, a small H4 chondrite, Frontier Mountain (FRO) 01149, was found on a glacially eroded surface near the top of Frontier Mountain, Antarctica, about 600 m above the present ice level. The metal and sulphides are almost completely oxidized due to terrestrial weathering. We used a chemical leaching procedure to remove weathering products, which contained atmospheric ¹⁰Be and ³⁶Cl in a ratio similar to that found in Antarctic ice. The FRO 01149 meteorite has a terrestrial age of 3.0 ± 0.3 Myr based on the concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al and ³⁶Cl. This age implies that FRO 01149 is the oldest stony meteorite (fossil meteorites excluded) discovered on Earth. The noble gas cosmic ray exposure age of FRO 01149 is ~ 30 Myr. The meteorite thus belongs to the 33 Myr exposure age peak of H-chondrites.The bedrock surface on which FRO 01149 was found has wet-based glacial erosional features recording a former high-stand of the East Antarctic ice sheet. This ice sheet evidently overrode the highest peaks (> 2800 m a.s.l.) of the inland sector of the Transantarctic Mountains in northern Victoria Land. We argue that FRO 01149 was a local fall and that its survival on a glacially eroded bedrock surface constrains the age of the last overriding event to be older than ~ 3 Myr. The concentrations of in-situ produced cosmogenic ¹⁰Be, ²⁶Al and ²¹Ne in a glacially eroded bedrock sample taken from near the summit of Frontier Mountain yield a surface exposure age of 4.4 Myr and indicate that the bedrock was covered by several meters of snow. The exposure age is also consistent with bedrock exposure ages of other summit plateaus in northern Victoria Land.     

Uranium-lead abundances and isotopic studies in the chondrites Richardton and Farmington

²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb whole meteorite isochrons for Richardton (H5) and Farmington (L5) are presented and give Pb-Pb ages of 4.545 ± 0.010 and 4.620 ± 0.010 Ga respectively (errors ± 2σ). The Pb-Pb isochron for Farmington passes below the Can?on Diablo troilite composition, which may therefore not be the initial Pb composition for this meteorite.All samples show an apparent excess radiogenic lead for single-stage (closed-system) evolution when Can?on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess Pb cannot be explained by terrestrial contamination. There is no unique isotopic composition for initial lead that yields concordant ages at 4.55 Ga for all samples for either meteorite. The data likewise cannot be reconciled to Can?on Diablo initial lead through any of the conventional two- and three-stage evolution models.The apparent excess Pb, with respect to a Can?on Diablo troilite composition and a single-stage closed-system history, and the apparent inhomogeneous initial Pb isotopic compositions, appear to be real. This may be an indication that the U-Pb systems in these meteorites are disturbed, but this disturbance cannot be described consistently by any of the conventional episodic evolution models.     

36Cl and53Mn in Antarctic meteorites and10Be-36Cl dating of Antarctic ice

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) has been measured in twenty Antarctic meteorites by neutron activation analysis.³⁶Cl (t_1/2 = 3.0 × 10⁵years) has been measured in fourteen of these objects by tandem accelerator mass spectrometry. Cosmic ray exposure ages and terrestrial ages of the meteorites are calculated from these results and from rare gases.¹⁴C (t_1/2 = 5740years) and²⁶Al (t_1/2 = 7.2 × 10⁵years) data. The terrestrial ages range from 3 × 10⁴ to 5 × 10⁵ years. Many of the L3 Allan Hills chrondrites seem to be a single fall based on these results. In addition,¹⁰Be (t_1/2 = 1.6 × 10⁶years) and³⁶Cl have been measured in six Antarctic ice samples. The first measurements of¹⁰Be/³⁶Cl ratios in the ice core samples demonstrate a new dating method for ice.     

Source of lead in Central American and Caribbean mineralization

The Zerga meteorite, an LL6 ordinary chondrite found at Aouelloul crater in 1973, is a small fragment of a larger mass whose pre-atmospheric radius was most likely between 20 and 125 cm. A typical amphoterite, it is a monomict breccia that has undergone at least one recrystallization episode.³He and²¹Ne contents define a shielding-corrected, cosmic ray exposure age of 21–24 × 10⁶ years and the²⁶Al content is consistent with a terrestrial age ?500,000 years (2σ limit). The K—Ar gas-retention age is 3.1 × 10⁹ years. The meteorite's areal association with the impact crater is merely coincidental. A new K-Ar age of the glassy impactite found at Aouelloul dates the crater at 3.1 ± 0.3 × 10⁶ years, sensibly indistinguishable from the recently determined age of nearby Tenoumer crater (2.5 ± 0.5 × 10⁶years). The similar ages of these two impact craters, and their almost perfect linear alignment with a third, morphologically similar crater (Temimichat Ghallaman) over a distance of 600 km, suggests a simultaneous triple impact occasioned by the disruption of a large meteorite moving on a very shallow atmospheric trajectory. If so, the concomitant low impact angles may be responsible for the unusually shallow original depths inferred for two of the craters from gravity data.     

Isotopic lead ages of chondritic meteorites

The isotopic composition of lead was determined for two carbonaceous, two H, and two L chondrites. All are falls. The²⁰⁶Pb/²⁰⁴Pb ratios cover a range from 9.45 to 37.33; the²⁰⁷Pb/²⁰⁴Pb ratios range from 10.39 to 26.10. The isotopic data define a²⁰⁷Pb/²⁰⁶Pb age of 4.635 AE. Uranium and lead concentration data indicate that the isotopic lead ages for the Bruderheim chondrite are concordant within approximately 20%. This contrasts with lead data in the literature for chondrites, which consistently indicate discordant isotopic lead ages due to large excesses of radiogenic lead by factors of two or more. The isotopic lead ages for Pultusk may be concordant; those for a sample of Richardton are not. The lack of concordance for Richardton is not due to the analytical procedures, rather it is likely a result of the handling history of the chondrite prior to analysis.The L-3 chondrite, Mezo-Madaras contains such a high concentration of lead - 5.27 PPM - that accurate ratios for primordial lead can be obtained. These are ²⁰⁶Pb/²⁰⁴Pb= 9.310;²⁰⁷Pb/²⁰⁴Pb= 10.296, values which are in close agreement with a recently reported measurement on lead in troilite from the Canyon Diablo iron meteorite.     

Thermoluminescence studies and the preatmospheric shape and mass of the Estacado meteorite

The thermoluminescence (TL) intensity of samples taken systematically across a large section of the Estacado meteorite has been measured. A significant proportion of the TL in meteorites is probably caused by cosmic-ray bombardment and variations in the natural TL throughout the slice reflect the preatmospheric shape of the meteorite in that plane. Before it entered the atmosphere Estacado was an elongated shape, approximating to an ellipse of eccentricity 0.8, with a preatmospheric mass of at least 8 tons. The results are consistent with laboratory experiments which indicate that secondary radiation plays an extremely important part in the production of TL.     

Thermoluminescence of meteorites and their orbits

The thermoluminescence (TL) levels of 45 ordinary chondrites were measured to obtain information about the meteorite orbits. The low-temperature TL reaches equilibrium while the meteorite is in space and reflects the temperature of the meteorite at perihelion. Samples of Pribram, Lost City, and Innisfree, whose orbits are accurately known, were used as control samples. The TL levels in 40 out ot 42 meteorites are similar to the three control samples, indicating that the vast majority of ordinary chondrites that survive atmospheric entry have perihelia similar to three known orbits, i.e., in the range 0.8–1 AU. The effects of albedo and rotation are also considered. A simple model indicates that temperature gradients of 1–2°K/cm are possible in slowly rotating meteoroids and such a temperature gradient is consistent with the unusually large TL gradient measured in the Farmville meteorite. Since slow rotation rates are improbable, other possibilities are examined but no satisfactory explanation has been found. The TL level measured in the Malakal meteorite is two orders of magnitude lower than control samples and is best explained by thermal draining due to solar heating in an orbit with a small perihelion distance. The perihelion is estimated to be～0.5–0.6AU.     

Chemical and physical studies of type 3 chondrites, VII. Annealing studies of the Dhajala H3.8 chondrite and the thermal history of chondrules and chondrites

Samples of the Dhajala H3.8 chondrite have been annealed for 10 hours at 600, 700, 800, 900 and 1000°C and at 1000°C for 1, 2, 20 and 100 h and their thermoluminescence (TL) properties measured. The TL sensitivity decreased by a factor of 2 after annealing at < 900°, but at higher temperatures fell by an order of magnitude. An abrupt increase in the temperature of the TL peak from 172 ± 9°Cto231 ± 8°C and a steady increase in the width of the peak from 169 ± 7°Cto212 ± 5°C were caused by the annealing treatment. The TL phosphor in Dhajala is thought to be feldspar predominantly in the high-temperature (disordered) form, but the present data indicate that a contribution from the low-temperature form is also present and that this low-temperature component is converted to the high form by the annealing treatment. The low-temperature feldspar is located in a few of the chondrules ( 20% of those separated from the meteorite) which are also noteworthy for having high TL sensitivities. These chondrules must have suffered greater crystallization of their mesostasis than the other chondrules, and equilibrated to lower temperatures. It is argued that, for compositional reasons, their mesostasis constituted less of a barrier to diffusion and therefore equilibration. Presumably the post-metamorphic cooling rate of the meteorite through the stability field of the low form was slow enough to permit some crystallization, and the width and temperature of the TL peaks for petrologic types 3.5–3.9 are somehow related to cooling rate. Based on TL, there is no indication of a correlation between petrologic type and cooling rate for types 3.5–3.9; this is not consistent with a simple, single internally heated meteorite parent body.     

A classification of meteorites based on oxygen isotopes

On the basis of¹⁸O/¹⁶O and¹⁷O/¹⁶O ratios, meteorites and planets can be grouped into at least six categories, as follows: (1) the terrestrial group, consisting of the earth, moon, differentiated meteorites and enstatite chondrites; (2) types L and LL ordinary chondrites; (3) type H ordinary chondrites; (4) anhydrous minerals of C2, C3, C4 carbonaceous chondrites; (5) hydrous matrix minerals of C2 carbonaceous chondrites; (6) the ureilites. Objects of one category cannot be derived by fractionation or differentiation from the source materials of any other category.     

Thermoluminescence studies of the Allende meteorite

The Allende meteorite has been examined with a view to applying thermoluminescence (TL) to the study of a meteorite's passage through the atmosphere. At least three kinds of TL-bearing minerals are present. A strong peak at 140°C is due to forsterite, and one at 200°C is probably caused by cordierite. By far the most intense TL comes from an alteration product associated with gehlenite.In the 4-cm diameter meteorite examined the 200°C TL varied in intensity across the stone, showing it to be produced by fragmentation. Temperature gradients induced by atmospheric heating can also be derived, and indicate the orientation of the meteorite. Together with fusion crust measurements these results enable the final phase of the meteorite's passage through the atmosphere to be delineated.     

Elemental and stable isotope variations of organic matter from a terrestrial sequence containing the Cretaceous/Tertiary boundary at York Canyon,New Mexico

Elemental and isotopic composition of organic matter from a terrestrial sequence including the palynological Cretaceous/Tertiary (K-T) boundary together with an Ir anomaly at York Canyon, New Mexico, record information about paleoclimatological and environmental conditions. Six layers of coal, carbonaceous shale and mudstone with high contents of organic material were selected for analysis. A Late Cretaceous coal bed 10 m below the K-T boundary and an Early Tertiary coal bed containing the K-T iridium anomaly at its base were sampled intensively. In the lower bed, the isotopic ratios¹³C/¹²C,¹⁵N/¹⁴N,andD/H and the elemental ratiosC/N andN/H, all varied sympathetically with one another over depth. In contrast, in the upper coal layer, only theD/H,C/N,andN/H ratios showed some coupling. Immediately above the K-T boundary, theδ¹³C values displayed a long-term shift of 1.8‰ to more negative values, while the hydrogen isotope ratios in these samples did not change significantly. We interpret the covariations in both coal layers as sympathetic responses of the isotopic and elemental ratios to climatic and environmental changes, as have been observed for younger sedimentary organic matter. The long-termδ¹³C shift during the early Tertiary is similar to the trend observed forδ¹³C values of marine carbonates. Our data thus support the proposal that the carbon cycle was perturbed globally by the effects of a drastically decreased marine bioproductivity along the K-T transition. The uncoupling of variations in the climatically sensitive isotopic and elemental ratios seen in the Early Tertiary coal bed provides evidence for major geochemical and environmental changes in the York Canyon area at the end of the Mesozoic. On the other hand, the constancy of δD values in the organic matter deposited at and above the K-T boundary gives no indication of significant changes in the hydrologic/climatic regimes as recorded in theD/H ratios at the site for several thousand years following the event which produced the high Ir concentrations. Our results provide constraints on models that have been advanced to explain that event and its consequences.     

Investigations on cosmic-ray-produced nuclides in iron meteorites, 2. New results on41K/40K-4He/21Ne exposure ages and the interpretation of age distributions

The cosmic ray exposure ages of 16 iron meteorites were determined by the⁴¹K/⁴⁰K-⁴He/²¹Ne method. The ages measured in the present and in previous experiments are summarized and presented in form of various histograms characterizing the age distributions of the different chemical groups separately. Age clustering at 650 Ma (mega years) is typical for the group IIIAB. Age clustering at 400 Ma is observed for the IVA irons. Quasi-continuous age distributions are found for the groups IA, IIA, IIB, IVB and for the anomalous irons. The following interpretation is offered. The IIIA and IIIB irons have initially been core material of the same parent asteroid and were ejected in consequence of a single impact event about 650 Ma ago. The IVA irons represent core material of another asteroid which was hit and partially disrupted in consequence of an impact event about 400 Ma ago. The group IA exhibits meteorites with ages between 200 and 1200 Ma. The quasi-continuous character of this age distribution and cosmochemical evidence indicate for these irons a raisin-bread-like character of their initial distribution within the silicate mantle of their parent asteroid. In consequence of several or, perhaps, of many crater-forming impact events the mantle material was gradually destructed and ejected. In the age distribution of the IIA hexahedrites, ages <300 Ma predominate and ages >600 Ma seem to be missing. In attempting to understand this, the possibility must be taken into consideration that the mean life-time of hexahedrites in the interplanetary space might be shorter than that of other irons. The cause might be that the hexahedrite single crystals are perhaps easier cleavable in the space environment. A similar kind of selective mass wastage appears also to be the cause for the absence of stone meteorites with high exposure ages.     

Cosmic-ray-produced53Mn in thirty-one meteorites

Cosmic-ray-produced⁵³Mn (t_½ = 3.7 × 10⁶years) has been determined by neutron activation analysis in twenty-two chondrites including three Antarctic meteorites: Yamato-7301 (j), -7305 (k) and -7304 (m).⁵³Mn was also measured in four mesosiderites, three iron meteorites, Bencubbin (unique) and Udei Station (iron with silicate inclusions). In addition, preliminary results for¹⁰Be (1.6 × 10⁶ years) were obtained in the Yamato meteorites using a low-background needle GM counter. Based on published values of rare gas ages, corrections were made for undersaturation; the average specific saturation activities of⁵³Mn were found in the range 450 ± 90dpm⁵³Mn/kg Fe in most of the chondrites and 490 ± 75dpm⁵³Mn/kg Fe in the mesosiderites. Two meteorites had extremely low contents of⁵³Mn: 102 ± 6 in Yamato-7301 and 48 ± 3dpm⁵³Mn/kg Fe in Bondoc. The Bondoc mesosiderite was already known to have a low concentration of cosmogenic radionuclides due to its large pre-atmospheric size. Several possible mechanisms are discussed to explain the low⁵³Mn activity in Yamato-7301: (1) long terrestrial age of about 7 m.y.; (2) low production rate of⁵³Mn due to heavy pre-atmospheric shielding (>70cm); (3) multi-stage irradiation history resulting in an undersaturation of⁵³Mn; and (4) a mechanism in which two or three of the above factors are combined. The ratio of⁵³Mn production rate in Ni to that in Fe has been estimated to be 1/3, based on the measurements of⁵³Mn in the metallic and silicate phases of St. Séverin meteorite, as well as on published results of some high-energy bombardment experiments.