Nominally hydrous magmatism on the Moon

Abstract

For the past 40 years, the Moon has been described as nearly devoid of indigenous water; however, evidence for water both on the lunar surface and within the lunar interior have recently emerged, calling into question this long-standing lunar dogma. In the present study, hydroxyl (as well as fluoride and chloride) was analyzed by secondary ion mass spectrometry in apatite [Ca 5 ðPO 4 Þ 3 ðF,Cl,OHÞ] from three different lunar samples in order to obtain quantitative constraints on the abundance of water in the lunar interior. This work confirms that hundreds to thousands of ppm water (of the structural form hydroxyl) is present in apatite from the Moon. Moreover, two of the studied samples likely had water preserved from magmatic processes, which would qualify the water as being indigenous to the Moon. The presence of hydroxyl in apatite from a number of different types of lunar rocks indicates that water may be ubiquitous within the lunar interior, potentially as early as the time of lunar formation. The water contents analyzed for the lunar apatite indicate minimum water contents of their lunar source region to range from 64 ppb to 5 ppm H 2 O. This lower limit range of water contents is at least two orders of magnitude greater than the previously reported value for the bulk Moon, and the actual source region water contents could be significantly higher. O ne of the scientific discoveries resulting from the Apollo missions was the pervasive waterless nature of the Moon and its rocks. The initial studies of the returned samples were pristine and showed no evidence of aqueous alteration, and water analyses were below detection limits. Moreover, hydrous phases were absent from the lunar rocks aside from a few unconfirmed reports of possible amphibole grains (1), which are now considered by many to represent either misidentifications or terrestrial contamination (as summarized by ref. 2). The subsequent forty years of lunar sample analysis have only supported and strengthened the idea that indigenous water was nearly absent from the Moon's interior. In fact, this conclusion has been incorporated into many petrologic and geophysical models constructed to aid in our understanding of lunar formation and lunar geology (3-11). The bulk water content of the Moon was recently estimated to be less than 1 ppb (11), which would make the Moon at least six orders of magnitude drier than the interiors of Earth (12, 13) and Mars (14). This extremely low water content is in keeping with the pervasive volatile-element depletion signature recorded in all lunar materials, because hydrogen is the most volatile of the elements. The exact cause for this volatile-element depletion is still under question; however, many have argued that it stems from the high temperatures associated with the Moon-forming giant-impact event at ∼4.5 Ga (i.e., refs. 3,8,15,and 16).Facilitated by advancements in analytical detection sensitivities for water, several recent discoveries have indicated that the story of water on the Moon is far from complete. Evi...

Solubility of water in lunar basalt at low pH2O

“…Recent detections of dissolved water in lunar volcanic glasses and melt inclusions (Saal et al 2008;Hauri et al 2011;Saal et al 2013;Chen et al 2015;Wetzel et al 2015), lunar apatites (Boyce et al 2010;McCubbin et al 2010;Greenwood et al 2011;Tartèse et al 2014), and plagioclase from lunar highland anorthosites (Hui et al 2013) have led to a reevaluation of what has appeared for decades to be one of the definitive results of the study of lunar samples: i.e., that the sources of lunar magmas-and, by inference, the entire Moon-are much poorer in water than the Earth; indeed the Moon had been described as "bone dry" (Newsom and Taylor 1989). In particular, direct measurements of water in incompletely degassed, primitive lunar glasses made in several laboratories have shown that water concentrations are similar to those observed in magmas from Earth's depleted upper mantle (Saal et al 2008;Chen et al 2015;Hauri et al 2015;Wetzel et al 2015) and that the isotopic composition of this water is approximately chondritic (Friedman et al 1974;Saal et al 2013;Barnes et al 2014;Füri et al 2014;Tartèse et al 2014).…”

Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon

“…Water contents of apatite grains in mare basalts range from 0 to 6050 ppm H 2 O (Figure 2 and Table S5). These are the highest water contents measured in lunar samples [6][7][8][9] . Both high-Ti (10044 and 75055) and low-Ti (12039 and 14053) mare basalts contain apatite grains with abundant water.…”

Water in alkali feldspar: The effect of rhyolite generation on the lunar hydrogen budget

“…Volatile data from apatites in lunar basalts and alkali-suite clasts from McCubbin et al (2010) show a conflicting trend with lower OH -and higher Cl/F in rocks that have more alkali feldspar (i.e. more K-rich).…”

“…Apatite data are from McCubbin et al (2010). Water estimates from apatite assume 95 % crystallisation prior to apatite crystallisation.…”

“…urKREEP). However, sample-based estimates of water content of KREEP-rich magmas from measurements of OH -, F, and Cl in lunar apatites suggest a low water concentration in the KREEP component with 2 to 140 ppm magmatic water (McCubbin et al, 2010). Using these data Elkins-Tanton and Grove (2011) predict that the bulk water content of the magma ocean would have been <10 ppm.…”