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

Abstract

Abstract-We determined terrestrial ages of ordinary chondrites from the Lewis Cliff stranding area, East Antarctica, on the basis of the concentrations of cosmogenic loge (ts = 1.5 1 Ma), 26A1 (ts = 0.705 Ma), and 36Cl (t% = 0.301 Ma). After an initial 26A1 pray survey of 91 meteorites suggested that many have terrestrial ages >O. 1 Ma, we selected 62 meteorites for 1OBe and 26A1 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 >SO h a d are associated with low terrestrial ages (Benoit et ul., 1992) but conclude that natural TL levels <80 h a d 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, 3 1 were assigned to 1 1 different pairing groups, mainly on the basis of their cosmogenic nuclide record. The meteorites are estimated to represent between 42 and 52 distinct falls.

Maskelynite in asteroidal, lunar and planetary basaltic meteorites: An indicator of shock pressure during impact ejection from their parent bodies

“…Fritz et al (2005) found a correlation among martian meteorites between high shock pressure induced during launch and short transfer time to Earth. Because the nakhlites and Chassigny appear to have been ejected from Mars in the same impact event before $11 Ma ago and because very few meteorites, even those that fell in Antarctica, are preserved for more than $1 Ma (Welten et al, 1997(Welten et al, , 1999, it is plausible that there were highly shocked nakhlites and chassignites that reached Earth millions of years ago that were lost to the geological record (Fritz et al, 2005). If this selection effect is valid, it would imply that those martian samples that reached Earth more recently tend to be among the less shocked.…”

Meteorites: Messengers from the Early Solar System

“…In Antarctica, weathering is very slow with some meteorites surviving more than 2 Ma. [31] In hot desert environments, most meteorites have terrestrial ages of a few 10'000 years. [32] Metal is completely weathered within less than 20'000 years while troilite is more resistant but generally completely weathered in the oldest meteorites with terrestrial ages >30'000 years.…”

Ejection of Martian meteorites

“…See text for details. meteorites are less than 1 Ma (Welten et al 1997;Welten et al 1999). Given the limited preservation of meteorites on Earth, the lack of highly shocked nakhlites may be due to the fact that they were transferred to Earth substantially faster than the weakly shocked ones, and, therefore, are less likely to be preserved over a given time.…”

“…Assuming that the survival time of meteorites on the surface of Earth is limited to about 1 Ma (Welten et al 1997(Welten et al , 1999, Armstrong et al (2002) evaluated the likelihood for preservation of ejected fragments from the terrestrial planets on the surface of the Moon. They concluded that on the lunar surface samples from terrestrial planets originating from the time of the early heavy bombardment 3.9 Ga ago may be found.…”