Correlation between rock type and irradiation history of Apollo 11 igneous rocks

Abstract

High-K and low-K Apollo 11 lunar rocks show different distributions of exposure ages T e. The low-K rocks group around 100 × 10 6y(65 × 10 6 ≤T e≤ 130 × 10 6y; except one rock out of9) whereas the high-K rocks have either ages between 30 and 55 × 10 6 y or between 240 and 450 × 10 6 y. From the ( 78K/ 83Kr) sp versus ( 131Xe/ 126Xe) sp correlation diagram it is concluded that the low-K rocks were systematically exposed to a harder irradiation than the high-K rocks. The observed grouping of exposure ages is essentially in agreement with only three impacts originally ejecting the Apollo 11 rocks analyzed so far. Three models explaining the systematically higher shielding of the high-K rocks are discussed. Model A suggests that high-K rocks were initially one (or several) large boulder(s), reduced to present size by space erosion and break-up on the lunar surface. Low-K rocks were directly ejected as small rocks. Model B requires a stratigraphy of the lunar bedrock. High-K material must be closer to the surface than low-K material and has received a substantial preirradiation prior to the ejection. Model C requires that the high-K rocks came from small local craters, whereas the low-K rocks were ejected from a larger, more distant crater. Independent of any model, the exposure age group found for the low-K rocks corresponds to the time of formation of the crater or craters which produced these rocks.