American geochemists analyzed for the first time the isotopic composition of sulfur in samples collected during the Apollo 15 and Apollo 17 lunar missions. The results showed that the geological history of the Moon was quite turbulent. The paper was published in the journal Science Advances.
Variations of sulfur isotopes in lavas of mantle origin can tell a lot about the stages of evolution of planetary bodies. If these isotope ratios are well known for terrestrial igneous rocks, so far they have not been studied in detail in the lunar rocks.
Alberto Saal (Alberto Saal) from Brown University in Providence and Eric Hauri (Erik Hauri) from the Carnegie Institution of Science in Washington conducted the first measurements of sulfur isotope ratios 34S/32S in volcanic glasses and olivine melt inclusions. All previous researchers have determined sulfur isotope ratios only in bulk rock samples, from which it is very difficult to draw any conclusions.
The authors found isotopic signatures indicating several important events in the geological history of the Moon: first, the lunar core was separated and an ocean of lunar magma was formed, then this ocean froze, and heterogeneous magma sources were laid down at depth. As they activated, volcanic eruptions – lava outpourings, ejections of pyroclastic and rubble material, and volcanic gas emissions – began. This is where the sulfur isotope fractionation process ended.
Although it does not answer the main question needed to understand the moon’s origin story – whether the Earth’s and the moon’s interior have the same sulfur isotope signature – the results of the new study are an important step toward solving that mystery, scientists say. It is now at least clear that the Moon, like the Earth, went through phases of magmatic differentiation and degassing.
To better understand the processes controlling changes in sulfur isotope ratios, the researchers used the latest method of nanoscale secondary ion mass spectrometry (NanoSIMS), which allows measurements in individual balls of volcanic glass and the smallest inclusions.
The samples studied are represented by two main types of lunar volcanic rocks: so-called marine basalts, which fill lunar seas, and pyroclastic sediments rich in volcanic glass. Both show wide variations in titanium content.
Scientists have found a clear correlation between sulfur isotope values and titanium content in magmatic fluid droplets enclosed in olivine crystals, as well as a positive relationship between sulfur content in the samples and its isotopic values.
The authors suggest that these correlations arose during the emission of magmatic gases during volcanic activity, which produced marine basalts formed already after the formation of the primary lunar crust.