Engineering topics
Schultz, L.
Publications and source records attributed to Schultz, L..
Cosmogenic Records in 18 Ordinary Chondrites from the Dar Al Gani Region, Libya: Noble Gases - 1
In the last decade thousands of meteorites have been recovered from hot deserts in the Sahara and Oman. One of the main meteorite concentration surfaces in the Sahara is the Dar al Gani plateau in Libya, which covers a total area of ~8000 km2. More than 1000 meteorites have been reported from this area. The geological setting, meteorite pairings and the meteorite density of the Dar al Gani (DaG) field are described in more detail in [1]. In this work we report concentrations of the noble gas isotopes of He, Ne, Ar as well as 84Kr and 132Xe in 18 DaG meteorites. In a separate paper we will report the cosmogenic radionuclides [2]. We discuss the thermal history and cosmic-ray exposure (CRE) history of these meteorites, and evaluate the effects of the hot desert environment on the noble gas record.
Cosmogenic Records in 18 Ordinary Chondrites from the Dar Al Gani Region, Libya: Radionclides - 2
In the past decade more than 1000 meteorites have been recovered from the Dar al Gani (DaG) plateau in the Libyan part of the Sahara. The geological setting, meteorite pairings and density are described. So far, only a few terrestrial ages are known for DaG meteorites, e.g. 60+/- 20 kyr for the DaG 476 shergottite shower and 80+/- 20 kyr for the lunar meteorite DaG 262. However, from other desert areas, such as Oman, it is known that achondrites may survive much longer than chondritic meteorites, so the ages of these two achondrites may not be representative of the majority of the DaG meteorite collection, of which more than 90% are ordinary chondrites. In this work we report concentrations of the cosmogenic radionuclides, 14C (half-life = 5,730 yr), 41Ca (1.04x10 superscript 5 yr), Cl-36 (3.01x10 superscript 5 yr), Al-26 (7.05x10 superscript 5 yr) and 10Be (1.5x10 superscript 6 yr) to determine the terrestrial ages of DaG meteorites and constrain their pre-atmospheric size and exposure history.
Cosmic-Ray-Exposure Ages of Diogenites and the Collisional History of the HED Parent Body or Bodies
Cosmic-ray-exposure ages of meteorites provide information on the collisional history of their parent bodies and the delivery mechanism of meteorites to Earth. The exposure-age distributions of ordinary chondrites show distinct patterns for H, L, and LL types, consistent with their origin on different parent bodies. The exposure-age distributions of howardites, eucrites. and diogenites (HEDS) show a common pattern with major peaks at 22 Ma and 38 Ma This provides additional evidence for a common origin of the HED meteorites, possibly 4 Vesta, although orbital dynamics calculations showed that the delivery of meteorites from Vesta to Earth is difficult. However, the discovery of several kilometer-sized Vesta-like asteroids in the region between Vesta and the 3:1 resonance suggested that these seem more likely parent bodies of the HEDs than Vesta itself. This implies that the exposure-age clusters may represent samples of several parent bodies. Therefore, the near-absence of diogenites with ages <20 Ma might be of interest for the composition of these kilometer-sized fragments of Vesta. Here we present cosmic-ray-exposure ages of 20 diogenites, including 9 new meteorites. In addition, we calculate the probability for each peak to occur by chance, assuming a constant production rate of HED fragments.
Noble gases in twenty Yamato H-chondrites: Comparison with Allan Hills chondrites and modern falls
Concentration and isotopic composition of noble gases have been measured in 20 H-chrondrites found on the Yamato Mountains ice fields in Antarctica. The distribution of exposure ages as well as of radiogenic He-4 contents is similar to that of H-chrondrites collected at the Allan Hills site. Furthermore, a comparison of the noble gas record of Antarctic H-chrondrites and finds or falls from non-Antarctic areas gives no support to the suggestion that Antarctic H-chrondrites and modern falls derive from differing interplanetary meteorite populations.
Terrestrial Kr-81-Kr ages of Antarctic meteorites
The production rate of Ar-38 in meteorites P(38), has been determined, as a function of the sample's chemical composition, from Kr-81-Kr exposure ages of four eucrite falls. The cosmogenic Kr-78/Kr-83 ratio is used to estimate the shielding dependence of P(38). From the 'true' Ar-38 exposure ages and the apparent Kr-81-Kr exposure ages of nine Antarctic eucrite finds, terrestrial ages are calculated. They range from about 300,000 ages (Pecora Escarpment 82502) to very recent falls (Thiel Mountains 82502). Polymict eucrites from the Allan Hills (A78132, A79017, and A81009) have within the limits of error the same exposure age (15.2 x 10 to the 6th ages) and the same terrestrial age (110,000 ages). This is taken as strong evidence that these meteorites are fragments of the same fall. A similar case is the Elephant Moraine polymict eucrites A79005, A79006, and 82600 with an exposure age of 26 x 10 to the 6th ages and a terrestrial age of 180,000 ages. EETA 79004 may be different from this group because its exposure age and terrestrial age are 21 x 10 to the 6th ages and 250,000 ages, respectively. The distribution of terrestrial ages of Allan Hills meteorites is discussed. Meteorites from this blue ice field have two sources: directly deposited falls and meteorites transported to the Allen Hills inside the moving Antarctic ice sheet. During the surface residence time meteorites decompose due to weathering processes. The weathering 'half-life' is about 160,000 ages. From the different age distributions of Allan Hills and Yamato meteorites, it is concluded that meteorite concentrations of different Antarctic ice fields need different explanations.
International Workshop on Antarctic Meteorites
Topics addressed include: meteorite concentration mechanisms; meteorites and the Antarctic ice sheet; iron meteorites; iodine overabundance in meteorites; entrainment, transport, and concentration of meteorites in polar ice sheets; weathering of stony meteorites; cosmic ray records; radiocarbon dating; element distribution and noble gas isotopic abundances in lunar meteorites; thermoanalytical characterization; trace elements; thermoluminescence; parent sources; and meteorite ablation and fusion spherules in Antarctic ice.
Discovery of meteorites on a blue-ice field near the Frontier Mountains, North Victoria Land, Antarctica
A high concentration of meteorites were discovered on a blue ice field northeast of the Frontier Mountains. As a result of a systematic search, a total of 42 meteorites were recovered. The current glacial situation has evolved through various stages, which are discussed in relationship to the concentration of meteorites. Ice flow patterns are summarized. The chemical composition and terrestrial ages of the meteorites are discussed.
Mysterious iodine-overabundance in Antarctic meteorites
Halogen as well as other trace element concentrations in meteorite finds can be influenced by alteration processes on the Earth's surface. The discovery of Antarctic meteorites offered the opportunity to study meteorites which were kept in one of the most sterile environment of the Earth. Halogen determination in Antartic meteorites was compared with non-Antarctic meteorites. No correlation was found between iodine concentration and the weathering index, or terrestrial age. The halogen measurements indicate a contaminating phase rich in iodine and also containing chlorine. Possible sources for this contamination are discussed.
Terrestrial ages of Antarctic meteorites: Implications for concentration mechanisms
Antarctic meteorites differ from meteorites fallen in other places in their mean terrestrial ages. Boeckl estimated the terrestrial half-life for the disintegration of stone meteorites by weathering under the climatic conditions of the Western United States to be about 3600 years. Antarctic meteorites, however, have terrestrial ages up to 70000 years, indicating larger weathering half-lives. The terrestrial ages of meteorites are determined by their concentration of cosmic-ray-produced radionuclides with suitable half-lives (C-14, Al-26, and Cl-36). These radionuclides have yielded reliable ages for the Antarctic meteorites. The distribution of terrestrial ages of Allan Hills and Yamato meteorites are examined.
The production rate of cosmogenic 21-Ne in chondrites deduced from 81-Kr measurements
Cosmogenic Ne-21 is used widely to calculate exposure ages of stone meteorites. In order to do so, the production rate P(21) must be known. This rate, however, is dependent on the chemical composition of the meteorite as well as the mass of, and position within, the meteoroid during its exposure to the cosmic radiation. Even for a mean shielding the production rates determined from measurments of different radionuclides vary by a factor of two. A method that can be used to determine exposure ages of meteorites that avoids shielding and chemical composition corrections is the -81-Kr-Kr-method. However, for chondrites, in many cases, the direct determination of production rates for the Kr isotopes is prevented by the trapped gases and the neutron effects on bromine. Therefore, this method was applied to four eucrite falls and then their 81-Kr-83-Kr-ages were compared to their cosmogenic Ne-21 and Ar-38 concentrations. The eucrites Bouvante-le-Haut, Juvinas, Sioux County, and Stannern were chosen for these measurements because of their similar chemical composition regarding the major elements.
Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy
Improved exposure ages are derived for 201 H, 203 L, and 38 LL chondrites in an effort to understand the characteristics of the chondrite parent body. The Ne-21 exposure ages were calculated from literature values taking into account shielding differences, a trapped component and radiogenic He. The exposure age distributions show clear peaks at 4.5 and 20 million years for the H chondrites, while the Ls and LLs appear more as a continuous series of intermediate peaks which may be modeled by at least six peaks between 1 and 35 million years in the case of L chondrites. The observations that every petrological type occurs in each large peak and contain solar wind gases suggest that the parent bodies have been fragmented and reassembled into a megabreccia. The H meteorites are proposed to represent the surface layer of a body with a substantial, active regolith as indicated by the relatively high abundances of solar gases. The L chondrites, on the other hand, are attributed to a parent body that was fragmented by collision about 500 million years ago.
He, Ne and Ar in chondritic Ni-Fe as irradiation hardness sensors.
Noble gas analyses of the Ni-Fe of 9 L, 5 H, and 2 LL chondrites quantitatively support previous suggestions of radiogenic He-4 recoil and He-3 deficits. Furthermore, noble gases in the Ni-Fe show evidence for in situ produced radiogenic He-4 and in some cases for recoil loss of Ar-38 and gain of Ne-21. The ratio of spallogenic Ar-38 and Ne-21 in the metal phase is found to correlate strongly with He-3/Ne-21 and Ne-22/Ne-21 in bulk samples of these chondrites. This is proof of the dependence of these ratios on the irradiation hardness experienced by the meteoroid in space. 'Hardness indices' n equals 1.9-2.2 are found, indicating that on the average the stone meteoroids from which the samples came were smaller in mass than iron meteoroids. The spallogenic Ne-21/Ar-38 ratio in metallic Ni-Fe can be used with the semi-empirical production model deduced from the Grant iron meteorite to calibrate spallogenic He-3/Ne-21 and Ne-21/Ar-38 in bulk samples of L, LL and H chondrites for meteoroid size and sample location allowing the estimation of minimal meteoroid masses.