Rb-Sr ages of igneous rocks from the Apollo 14 mission and the age of the Fra Mauro formation

Abstract

Internal Rb-Sr isochrons were determined on four basaltic rocks and on a basaltic clast from a breccia from the Fra Mauro landing site. Rocks 14310, 14073 and 14001,7,3 yield essentially identical ages T = 3.88 ± 0.04 AE and identical high initial ⁸⁷Sr/⁸⁶Sr, I = 0.70035 ± 4. Rock 14053 and the clast from breccia 14321 both yield a higher age T = 3.95 ± 0.04 AE and a distinctly lower I = 0.69945 ± 4. Model ages relative to BABI for these rocks range from 4.3 to 4.6 AE. Model ages were determined for soil samples 14141, 14149, 14163, 14259 yielding T_(BABI) of 4.4, 4.5, 4.5 and 4.7 AE respectively. An internal isochron was determined for 12004 and yielded (3.29 ± 0.07 AE, 0.69948 ± 5) in agreement with our previous results for basaltic rocks from the Apollo 12 site. The crystallization ages for Apollo 14 basalts are only 0.2 to 0.3 AE older than were found for mare basalts from the Sea of Tranquillity. Assuming these leucocratic igneous rocks to be representative of the Fra Mauro site, it follows that there were major igneous processes active in these regions, and presumably throughout the highlands, at times only slightly preceding the periods at which the maria were last flooded. If the breccias represent the Fra Mauro formation and if this is the result of excavation of the Imbrium Basin, we must conclude that a major bombardment of the earth-moon system continued to take place at least as late as 3.9 AE or about 0.7 AE after the formation of the solar system. Such a late major bombardment could remove the need for internal heat sources to explain some mare lava flows. Some of the lunar differentiation processes could possibly represent the result of collisional melting of a thin outer layer. This late bombardment requires that planetary objects ∼ 100 km in size must be stored in unstable orbits with lifetimes of 10⁸ yr or more.