PerspectivePlanetary Science

Analyzing Moon Rocks

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Science  25 Apr 2014:
Vol. 344, Issue 6182, pp. 365-366
DOI: 10.1126/science.1253266

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The paradigm of a “dry Moon” was recently challenged on the basis of reexamination of lunar samples collected during the Apollo missions, raising the possibility of a volatile-rich lunar interior (16). Several of these studies measured appreciable quantities of water (reported as equivalent H, OH, or H2O) and other volatiles (e.g., Cl, F) in the mineral apatite (46), which is ubiquitous in lunar basalts (mare basalts) (see the figure). However, an accurate estimation of the water content of the magmatic liquid from which apatite formed, and ultimately of the mantle source regions of mare basalts, depends on a number of parameters. In cases where apatite in a mare basalt formed through the process of fractional crystallization (when newly formed crystals in a cooling magma are physically separated, preventing any further interaction with the remaining melt), it may not be possible to obtain any reliable estimates of the water contents of the parental magma (and its source region). On page 400 of this issue, Boyce et al. (7) present an elegant numerical model, applicable to mare basalt apatite formed through fractional crystallization, to demonstrate that some of the highest water contents reported for lunar apatite can be reconciled with an original melt containing not much water at all. These new results cast doubt on the utility of apatite volatile abundances in reliably estimating the water content of mare basalt source regions.