Evidence for Short-Lived Magmatism Early in the Moonโ€™s History

An ancient rock fragment from rake sample 67667 collected at North Ray crater during Apollo 16. Credit: NASA/JSC.

Knowing the age of a rock or suite of rocks is very helpful in understanding geologic history. Geochronology, the science of using the isotopic signatures in a rock to determine its age, has been used for more than 50 years to decipher the sequence of lunar rock formation and the evolution of the Moon. A number of different isotopic chronometers have been used, and over the years, new technologies, new samples, and new ways of thinking have necessitated re-evaluation of data and interpretations made about the geologic history of the Moon.

There are three major suites of plutonic rocks on the Moon: the ferroan anorthosite suite (FAS), the magnesium- (Mg-) suite, and the alkali-suite, based on their very different geochemistry. As the lunar magma ocean cooled after its formation, these rock suites crystallized to form the lunar crust. One model suggests that the FAS represents the upper part of the lunar magma ocean, while the Mg- and alkali-suites could be plutonic crustal rocks that formed later. However, the ages of lunar rocks, based on geochronology, derived from the last 50+ years of study suggest a more complex model where all three suites formed around the same time between 4.3 to 4.5 billion years ago. Two apparent outliers are Mg-suite rocks that were previously measured to have younger ages, suggesting that Mg-suite magmatism could have continued for another 100 million years after FAS magmatism ended. A group of scientists at Lawrence Livermore National Laboratory, led by Lars Borg, reanalyzed one of these rocks (part of Apollo 16 sample 67667) using more modern techniques to verify the previously derived age. The outcome of the new analysis was an isotopic age much older than previously determined and instead consistent with many other Mg-suite samples. This newly determined age strongly suggests Mg-suite magmatism was confined to a relatively short interval of about 20 million years around 4.3 billion years ago. Furthermore, it suggests nearly all of the crustal magmatism that occurred early in the Moonโ€™s history was contemporaneous, including FAS and alkali-suite rocks despite the diverse geochemistry of these rocks requiring different sources. The Mg-suite rocks did not form by the same method as the other crustal rocks and might have formed during a density-driven overturn event during or shortly after solidification of the lunar magma ocean. These findings argue for a geologically complex Moon where lunar crustal formation occurred via different magmatic sources and methods over a short period of time. READ MORE