Significant abundances of trapped argon, krypton, and xenon have been measured in shock-altered phases of the achondritic meteorite Elephant Moraine 79001 from Antarctica. The relative elemental abundances, the high ratios of argon-40 to argon-36 (>/= 2000), and the high ratios of xenon-129 to xenon-132 (>/= 2.0) of the trapped gas more closely resemble Viking data for the martian atmosphere than data for noble gas components typically found in meteorites. These findings support earlier suggestions, made on the basis of geochemical evidence, that shergottites and related rare meteorites may have originated from the planet Mars.

All of the Apollo 16 regolith breccias (18 specimens) have been characterized in terms of their petrography, grain‐size distribution, porosity, major and trace element composition, noble gas contents, and ferromagnetic resonance properties. These breccias vary significantly with respect to their density and porosity, with the more dense breccias showing significant shock damage. The regolith breccias resemble the soils in grain‐size distribution and in the relative proportions of major petrological components, except agglutinates. Many of the breccias are compositionally different from the Apollo 16 soils in that they lack an important mafic component present in the soils. Although some groupings occur, the petrologic and chemical compositions of the regolith breccias do not correlate with the station location of the samples. All but one of the breccias show some evidence of irradiation at the lunar surface (solar gases, measurable FMR, agglutinates), and analyses made on grain‐size separates from two disaggregated breccias indicate that this irradiation occurred before compaction when the breccia material was finely disseminated on the surface. However, the concentrations of surface irradiation parameters (solar gases, FMR, agglutinates) for most breccias are far less than seen in any lunar soils or in regolith breccias from other Apollo missions. Several breccias also contain unusually high trapped 40Ar/36Ar ratios of ∼8–12 and a significant fission Xe component in excess of that expected from in situ production. These observations suggest that the surface irradiation of these breccias occurred as early as 4×109 years ago. We conclude that most of the Apollo 16 regolith breccias were not formed from any known Apollo 16 soil. They appear to be well‐comminuted material that contains ancient regolith developed during the late stage heavy bombardment of the moon when large impacts were much more common relative to small impacts so that regoliths did not have time to significantly mature before being diluted by fresh ejecta and buried. This ancient megaregolith is significantly different from more recent lunar regolith but may be similar to asteroidal regoliths from which some brecciated meteorites have formed.

No abstract

The existence of trapped gases in the EETA 79001 shergottite meteorite that were apparently shock‐implanted from the Martian atmosphere raises important questions as to the mechanism of gas implantation and whether the implanted gas has been mass fractionated. To study the phenomenon of shock‐implantation of gases, we artificially shocked whole‐rock and powder samples of a terrestrial basalt to pressures of 2–40 GPa in the presence of controlled gas mixtures ranging from 10−4 to 3 atmospheres. Argon, Kr, and Xe, and to a lesser extent Ne, were readily implanted into the silicate under a wide range of experimental conditions. As exemplified by Ar, the amount of implanted gas is linearly proportional to its partial pressure over a partial pressure range of 0.0001 to 0.1 atmosphere, the former value being similar to the partial pressure of Ar on Mars. The implanted gas showed no evidence of isotopic fractionation, and for Kr and Xe fractionation limits of 0.1% per mass unit could be set. The elemental composition of shock‐implanted gas also closely resembles the ambient gas phase, except that Ne is only lightly retained and readily leaks from the samples after the shock. Stepwise temperature releases of gas implanted at 2, 5, 20, and 35 GPa indicate two or more lattice sites for the implanted gas that are possibly related to microcracks and other lattice defects. Activation energies, Q, for diffusion of shock‐implanted gas from the samples increase dramatically with increasing shock; values of Q for Ar diffusion for the four shock levels 2, 5, 20, and 35 GPa are 7, 9, 14, and ∼25 kcal/mol, compared to 30–45 kcal/mol for typical radiogenic Ar. Diffusion of Kr and Xe is considerably slower than Ar. The amounts of gas that would have been implanted with 100% efficiency were calculated from the measured porosities of the powder samples and were compared to observed abundances. The implantation efficiencies were approximately 0.5% at 2 GPa, 7% at 5 GPa, and greater than 50% at both 20 and 35 GPa. These high gas implantation efficiencies occur for shock pressures where modest amounts of sample melting at grain boundaries begins, and they suggest that higher shock pressures generating substantial melting could not achieve appreciably greater efficiencies. These experimental data are consistent with shock‐implantation of Martian gases without mass fractionation into shock‐melted phases of EETA 79001; the progenitor of these melts may have been porous, fragmental material.

Several experimentally and naturally shocked silicate samples were analyzed for noble gas contents to further characterize the phenomenon by which ambient gases can be strongly implanted into silicates by shock and to evaluate the possible importance of this process in capturing planetary atmospheres in naturally shocked samples. Gas implantation efficiency is apparently mineral independent, as mono-mineralic powders of oligoclase, labradorite, and diopside and a powdered basalt shocked to 20 GPa show similar efficiencies. The retentivity of shock-implanted gas during stepwise heating in the laboratory is defined in terms of two parameters: activation energy for diffusion as determined from Arrhenius plots, and the extraction temperature at which 50% of the gas is released, both of which correlate with shock pressure. These gas diffusion parameters are essentially identical for radiogenic 4°Ar and shock-implanted 4°Ar in oligoclase and labradorite shocked to 20 GPa, suggesting that the two 4°Ar components occupy analogous lattice sites. Our experiments indicate that gas implantation occurs through an increasing production of microcracks/defects in the lattice with increasing shock pressure. The ease of diffusive loss of implanted gas is controlled by the degree of annealing of these microcracks/defects.Identification of a shock-implanted component requires relatively large concentrations of implanted gas which is strongly retained ti.e., moderate activation energy) in order to separate implanted gas from surface adsorbed gases. Literature data on shocked terrestrial samples indicate only weak evidence for shock-implanted gases, with an upper limit for 40Ar of -10-6 cm 3STP/g. New analyses of shocked samples from the Wabar Crater indicate the presence of shock-implanted Ar, having concentrations (-10-4 cm 3STP/g) and activation energies for diffusive loss which are essentially that expected from experimental studies. Lack of sufficient target porosity or the presence of ground water may explain the sparse evidence for shock-implanted gas at other terrestrial craters. Although Wabar Crater may represent an unusually favorable environment on Earth for shock-implanting gases, surfaces of other planetary bodies, such as Mars, may frequently provide such environments. Analyses of returned samples from old Martian terraines may document temporal changes in earlier atmospheric composition.