The volatile (CO2, H2O, F, S, Cl) content of lunar magmas and the origin of lunar magmatism
The composition and origin of the lunar volatiles has been the subject of considerable interest and one of the most important unsolved questions regarding the formation of the Moon. After 40 years of research on the samples returned from the Moon by the Apollo missions, the general consensus today is that the Moon formed and evolved through a single or series of catastrophic heating events in which most of the highly volatile elements were either stripped or evaporated away. Hydrogen being the lightest element is thought to have been completely lost during this period. Our recent results on high-spatial-resolution mass spectrometry measurements on lunar volcanic glasses, represent the first indication of the presence of H2O indigenous to the interior of the Moon. The results suggest that either there was originally water on Earth before the giant-impact event ~4.5 Ga ago that led to the formation of the Moon, and not all the water was lost during that collision, or there was a significant accretion of volatile-rich meteoritic material to the Earth and Moon within the first 100 Ma after the giant-impact and the Moon’s formation. The outcome of our work suggests that, contrary to the prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, and the presence of volatiles, especially water, must be included in models constraining the Moon’s formation and its thermal and chemical evolution.
The new question is not if there is water in the Moon's interior, but how much there is. The volatile budget of the lunar mantle can only be reconstructed from the record preserved in the mare basalts and lunar volcanic glasses. Reconstructing the volatile content of the lunar mantle from basaltic melts is compromised by volatile degassing at the time the lava erupted and subsequent contamination from external sources such as low-pressure condensation following impact, solar wind implantation, and assimilation or sublimation of cometary or meteoritic material. Our measurement of the volatile content of the lunar volcanic glasses represent values after the melt underwent significant degassing, which makes it difficult to establish the primitive volatile content of the lunar basalts. To overcome these obstacles and to put limits on the possible primary abundance of water and other volatiles in lunar magmas, we propose to complement our data on the lunar volcanic glasses with a detailed study of the volatile contents in 1) pristine melt inclusions trapped in phenocrysts (e.g. olivine, plagioclase), and in 2) pyroxenes from lunar mare basalts. The study of volatiles in melt inclusions and pyroxenes from lunar basalts that cooled relatively fast will provide a way to avoid, as much as possible, degassing processes and the addition of volatiles from external sources during a long history of exposure to the extra-lunar environment at the Moon's surface.
Saal, A.E., Hauri, E.H., Lo Cascio, M., Van Orman, J.A., Rutherford, M.J. and Cooper, R.F. - (2008) - The Volatile Content of the Lunar Volcanic Glasses: Evidence for the Presence of Water in the Moon’s Interior. Nature 454, 192-195.
Brown faculty collaborators:
Other project collaborators:
E. Hauri (Carnegie Institution of Washington); J. van Orman (Case Western Reserve University)