New research has deciphered an important process in the formation of a unique rock on the moon. The discovery explains its characteristic composition and its presence on the lunar surface, unraveling a mystery that has long baffled scientists. The study, published January 15 in the journal Nature Geoscience, reveals a key step in the formation of these unique magmas.

The picture shows astronaut and geologist Harrison H. Schmitt standing next to a huge lunar boulder during NASA's Apollo 17 mission in 1972. Scientists in this study used rock samples from this Apollo mission. Source: NASA/GeneCernan

High-temperature laboratory experiments using lava combined with sophisticated isotope analysis of lunar samples identified key reactions that control their composition.

The image shows a sample of lunar rock (known as high-titanium basalt) from the Apollo 17 mission, similar to the sample analyzed in this study. Source: NASA

core reaction

This reaction, which occurred deep inside the moon about 3.5 billion years ago, involves the exchange of the element iron (Fe) in the magma with the element magnesium (Mg) in the surrounding rock, changing the chemical and physical properties of the melt.

Co-first author Tim Elliott, Professor of Earth Sciences at the University of Bristol, said: "The origin of volcanic moon rocks is a fascinating story, involving an 'avalanche' of unstable planet-scale crystal piles created as the primordial magma ocean cooled.

"At the heart of this epic history is the existence of a type of magma unique to the Moon, but explaining how this magma got to the lunar surface and was sampled by space missions has been a tricky problem. It's great to be able to solve this puzzle."

Image showing titanium abundance on the lunar surface obtained by NASA's Clementine spacecraft. The red parts represent extremely high concentrations compared to terrestrial rocks. Image source: Lunar and Planetary Institute

Learn about high titanium basalt

Back in the 1960s and 1970s, NASA's Apollo missions successfully retrieved samples of solidified ancient lava from the lunar crust, and it has been known ever since that parts of the lunar surface have surprisingly high concentrations of the element titanium (Ti). Recent orbiting satellite mapping shows that these magmas, known as "high titanium basalts," are widespread on the moon.

"So far, models have been unable to reproduce magma compositions consistent with the fundamental chemical and physical characteristics of high-titanium basalts.Co-first author Dr. Martijn Klaver, researcher at the Institute of Mineralogy at the University of Münster, added: "It has proven particularly difficult to explain their low density, which enabled them to erupt some 3.5 billion years ago.""

Electron microscopy images of this research experiment. The melt (brown) reacts with the surrounding crystals (green), resulting in a melt with a lower iron content. Source: University of Bristol/University of Münster

An international team of scientists led by the University of Bristol in the UK and the University of Münster in Germany successfully simulated the formation process of high-titanium basalt in the laboratory using high-temperature experiments. Measurements of high-titanium basalt also revealed a unique isotopic composition that provides a fingerprint for the reaction reproduced by the experiment.

Both results clearly demonstrate that melt-solidification reactions are integral to understanding the formation of these unique magmas.