Apollo 15 Samples Reveal Multiple Volatile Reservoirs and Processes

In this photo taken during the second moonwalk, James B. Irwin is holding the rover steady at Station 6a behind a greenish boulder sampled by the crew. This station is where the Apollo samples 15400 and 15405 were collected. Credit: NASA.

Our understanding of lunar volatiles (e.g., H, Cl, F), their abundances, isotopic signatures, and distribution on the surface of the Moon is constantly evolving. Over the last 15 years, advances in analytical procedures have completely changed the concept of a “bone dry” Moon by detecting trace amounts of indigenous lunar water in a range of mineral phases. Among these minerals, the calcium phosphate apatite is widely studied because of the apatite crystal structure’s ability to accommodate substantial amounts of volatiles. By studying the isotopic compositions of volatile species within apatite, scientists can “fingerprint” the various sources and evolution (i.e., the processes this reservoir may have experienced) of volatiles within a sample and make inferences about the volatile content of the larger parent body.

In recent work, Thomas Barrett at the Lunar and Planetary Institute and colleagues analyzed apatite within samples known as quartz monzodiorites (QMDs) returned by the Apollo 15 mission. They provide the first Cl isotope data and expand the limited H isotope data for this particular lunar rock type. QMDs are ancient samples (approximately 4.3 billion years old) and very rich in KREEP (potassium [K], rare earth elements [REEs], and phosphorus [P]). This component is hypothesized to be the highly evolved last dregs of the lunar magma ocean. Results from this study show that while Cl isotopes have a similar composition to other KREEP-rich samples, the hydrogen isotopic signature is distinct and extremely light (i.e., rich in the lighter hydrogen isotope compared to the heavier deuterium isotope). The authors hypothesize that this light hydrogen signature could have been inherited from Theia, the body that impacted Earth and formed the Moon. These results indicate a growing diversity of volatile reservoirs and sources early in the Moon’s history. READ MORE