In the past 30 years tens of thousands of meteorites have been found in Antarctica, the Sahara and Oman. The terrestrial ages of these meteorites are a crucial piece of information to determine the flux of meteorites to Earth and the effects of terrestrial weathering. Terrestrial ages of Antarctic meteorites are mostly determined by measuring cosmogenic 14C in bulk samples or 36Cl in the metal fraction. However, both methods have limitations, i.e. 14C can only be used for ages up to 35-40 kyr, while the 36Cl method has an uncertainty of 50-60 kyr. With a half-life of 1.04x105 yr, 41Ca fills the gap between 14C and 36Cl. In this work we present concentrations of 41Ca in Antarctic diogenites and discuss their terrestrial ages and/or pre-atmospheric size. After dissolution of 100-300 mg of sample in HF/HNO3, Ca is separated as CaF2 and the 41Ca/Ca ratio is measured at LLNL. Measured 41Ca concentrations in diogenites range from 1.5 dpm/kg for A-881838 to 28 dpm/kg for PCA 02008. Although the main target element for the production of 41Ca in diogenites is Fe, accounting for 80-90% of the total 41Ca, small contributions are expected from Ti (~150 dpm/kg•Ti), Cr (~35 dpm/kg•Cr) and Mn (~30 dpm/kg•Mn). In addition, in objects larger than 30 cm in radius, 41Ca is also produced by neutron-capture on 40Ca. We normalized the measured 41Ca concentrations to the “effective” Fe concentration (Fe*) of each sample, using the following equation: Fe*=Fe+6xTi+1.5xCr+1.2xMn. This procedure yields 41Ca concentrations of 23-27 dpm/kg(Fe*) for four small diogenite falls, consistent with the average of ~25 dpm/kg in small iron meteorite falls. Several diogenites, including PCA 02008, show significantly higher 41Ca values of 30-180 dpm/kg(Fe*), which are due to variable contributions of neutron-capture 41Ca (100-1500 dpm/kg•Ca) in objects with pre-atmospheric radii larger than 30 cm. These large pre-atmospheric sizes are consistent with the relatively high concentrations of 10Be (>22 dpm/kg) and 26Al (>70 dpm/kg) in these samples. The remaining Antarctic diogenites show 41Ca concentrations ranging from 10 to 30 dpm/kg(Fe*). These concentrations correspond to terrestrial ages of 0-150 kyr, which is in the same range as most Antarctic chondrites. This suggests that unlike in hot deserts, where achondrites seem to survive longer than chondrites, this is not the case for Antarctica. We will discuss the implications of this observation in terms of the different roles of terrestrial weathering of meteorites in hot and cold deserts.
See more of Astrophysics and Cosmochemistry
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)