Rb-Sr ages and elemental abundances of K, Rb, Sr, and Ba in samples from the Ocean of Storms

Abstract

Rb-Sr internal isochron ages for three basaltic crystalline rocks from the Ocean of Storms are 300–600 m.y. younger than the ages of basaltic rocks from the Sea of Tranquillity. Rock 12063 has an age of 3.34 ± 0.10AE and [ 87Sr 86Sr ] I of 0.69914 ± 3. Internal isochron ages of two separate samples of rock 12004 yield ages of 3.01 ±0.11 and 3.03 ± 0.09AE indicating that this rock is distinctly younger than rock 12063 by about 300 m.y. A relatively imprecise age of 2.92 ± 0.18AE is obtained for rock 12052. The range in ages obtained by us indicate magmatic activity extending over a 300 m.y. period in the Ocean of Storms. Initial Sr compositions for the three rocks are distinctly higher than BABI value and provide evidence for the extreme alkali depletion in the source areas of these basalts, as also indicated by the trace element data. The differences in the initial Sr compositions and ages suggest that these lavas are produced from different source materials. An internal isochron age of 3.63 ± 0.11AE and a [ 87Sr 86Sr ] I of 0.69903 ± 5 has been obtained on the Apollo 11 Sample 10058. Model ages for the soil samples 12033 and 12070 are 4.3AE and 4.6AE respectively. The different model ages and the abundances of K, Rb, Sr and Ba in the soils indicate the heterogeneous nature and lack of mixing in the soils. The soils are highly radiogenic compared to the rocks. Both the trace element data and the Sr-isotopic data from leaching experiments show that the soil is not simply derived from the mare basalts, but contains a highly radiogenic and K, Rb, Ba rich component similar in composition to rock 12013. We infer that this exotic component is more ubiquitous on the lunar surface than present sampling indicates.