Geochronology of an Apollo 16 Clast Provides Evidence for a Basin-Forming Impact 4.3 Billion Years Ago

We examined lithic breccias from the Apollo sample collection in order to identify ferroan anorthosite samples suitable for geochronology, and better define the age relationships between rocks of the lunar highlands. Clast 3A is a previously unstudied noritic anorthosite from Apollo 16 lithic breccia 60016 with textural evidence of slow subsolidus recrystallization. We estimate a cooling rate of similar to 10 degrees C/Myr and calculate a pyroxene solvus temperature of 1,100-1,000 degrees C. Pyroxene exsolution lamellae (1-3 mu m) indicate that the last stage of cooling was rapid at similar to 0.2 degrees C/year, typical of rates observed in thick ejecta blankets. We calculate concordant ages from the Sm-147-Nd-143, Sm-146-Nd-142, Rb-Sr, and Ar-Ar isotopic systems of 4,302 +/- 28, 4,296 + 39/-53, 4,275 +/- 38, and 4,311 +/- 31 Ma, respectively, with a weighted average of 4,304 +/- 12 Ma. The closure temperature of the Sm-Nd system is similar to 855 14 degrees C, whereas the closure temperature of the Ar-Ar system is 275 +/- 25 degrees C. Cooling from 855 to 275 degrees C at 10 degrees C/Myr should result in an age difference between the two isotopic systems of similar to 60 Myr. The concordant Sm-Nd, Rb-Sr, and Ar-Ar ages imply that they record the time the rock was excavated by a large impact from the midcrust. The ages clearly predate various late accretion scenarios in which an uptick in impacts at 3.8 Ga is preceded by a period of relative quiescence between 4.4 and similar to 4.1 Ga, and instead are consistent with decreasing accretion rates following the formation of the Moon. Plain Language Summary We investigated a previously unexamined Apollo 16 ferroan anorthosite rock to study the timing of the formation of the lunar crust. Ferroan anorthosites are important because they are thought to be the oldest formed part of the lunar crust. Using three different isotopic geochronology methods, we determined that this rock (Clast 3A) is approximately 4.3 billion years old. The age is slightly younger than all but one recently dated ferroan anorthosite rock. The geology and textures of Clast 3A indicate that it formed deep within the lunar crust. The different dating methods record the age of the rock different temperatures. The Sm-Nd clock records the time when the rock cooled below about 855 degrees C, and the Ar-Ar clock records a temperature of about 275 degrees C. Identical ages determined from the two isotopic systems indicate that the rock experienced rapid cooling from above 855 degrees C to below about 275 degrees C. It appears that Clast 3A formed deep within the crust, but was then excavated 4.3 billion years ago by a massive cratering event. This implies that large basin forming impact events occurred on the Moon early in its history and not just around 3.8 billion years ago during the Late Heavy Bombardment.