WATER IN LUNAR PLAGIOCLASE AND EVIDENCE FOR A WET EARLY MOON

Since the Apollo era, the standard view has been that the Moon had lost its volatiles during its formation and early history. However, recent discoveries of water on its surface and in lunar samples suggest that indigenous water is present and heterogeneously distributed in the lunar interior, with some parts of lunar mantle containing as much water as Earth’s upper mantle. This water is thought to have been brought in part through solar wind implantation and meteorite impacts after the formation of the primary crust.

Identification of measureable quantities of water in ferroan anorthosite (FAN), a primary product of the Lunar Magma Ocean (LMO), would negate the need for later addition of water to the Moon after the formation of the primary crust. It is generally accepted that plagioclase, after crystallization, floated to the top of the LMO, forming FAN as the original crust. Therefore, any indigenous water preserved in FAN was partitioned from the LMO. Polarized Fourier transform infrared spectroscopy was used to measure water contents in plagioclases of FANs 15415,238 and 60015,787. The olivines and plagioclases in troctolite 76535,164 were also analyzed to assess the water inventory in the lunar highland upper crust. We measured ~6 ppm of water in ferroan anorthosite and up to ~2.7 ppm in plagioclase of troctolite 76535. More work on other FAN is in progress and will be presented.

From these measurements, we estimated the initial water content of the LMO to have been ~320 ppm wt H₂O. Water accumulating in the final 2 vol % LMO residuum could reach amounts of ~1.4 wt %. We therefore show that water has been present in the lunar interior since its earliest history, during LMO formation and crystallization. This primordial water in the LMO may be difficult to reconcile with the favored moon formation scenario. The presence of water would facilitate a more prolonged crystallization of the LMO than a dry scenario and could have affected the genesis of lunar basalts. Furthermore, the amount of water calculated for the LMO is sufficient to explain those detected in mare basalts, which are considered to have been derived from LMO cumulates. Finally, the water detected in the FANs and troctolite 76535 may contribute a significant portion to the trace amount of water/hydroxyl recently detected on the surface of the lunar highland crust from orbit.