MAGMATIC DEGASSING, ORIGIN AND BUDGET OF HIGHLY VOLATILE ELEMENTS OF THE MOON

Establishing the origin of water and other volatiles, and how and when the budgets of these elements were set in planetary bodies during the earliest stages of Solar System evolution has important implications for understanding the genesis and evolution of the terrestrial planets.

The Moon provides a frozen record of the first few hundred million years of Solar System evolution and, thus, offers a unique opportunity to establish the origin, budget and timing of the water, and other volatiles, delivery to the terrestrial planets.

Presently, there are two distinct views: one, suggesting that the Moon is “bone-dry”, 100 to 1000 times more depleted in highly volatile elements (e.g., water) than Earth. The second view suggests that the Moon is only 5 to 10 times more depleted in highly volatile elements than Earth. Therefore, the estimated extent of the Moon’s volatile depletion differs between the two views by a factor of 20 to 200.

We show that the 20 to 200 factor in the estimated Moon’s depletion of highly volatile elements might originate from the fact that the degassing of volatile elements during lunar magmatic processes has not been carefully considered. Furthermore, the isotopic composition of hydrogen dissolved in lunar primitive volcanic glasses and olivine-hosted melt inclusions, after consideration of magmatic degassing processes, demonstrate that lunar magmatic water has an isotopic composition that is indistinguishable from that of the bulk water in carbonaceous chondrites and similar to that of Earth.

The simplest explanation is that Earth had volatiles from its birth (consistent with dynamic models), and either during the giant impact that formed the Moon they were not completely lost or more volatiles were added during the late veneer.

We have to account for the effect of magmatic degassing before we can estimate the extent of depletion and the isotopic composition of highly volatile elements of the Moon.