Using apatite to unravel the origin of water in ancient Moon rocks

There have been a limited number of studies investigating the hydrogen isotopic composition of water in samples representing the lunar highlands. This is surprising considering highlands lithologies comprise a large proportion of the returned Apollo samples and are some of the oldest and most pristine samples of the Moon. As such they potentially hold the geological record of the earliest water in the Moon and may ultimately help us decipher the origin of lunar water. We have investigated the δD-H2O systematics of apatite in an Apollo 14 and four Apollo 17 rocks using a Cameca NanoSIMS 50L ion probe. The data were corrected for the contribution of background H2O, its associated D/H ratio, and for spallation effects. Our results indicate that apatites in Apollo 17 troctolite (76535) and granulite (79215) do not preserve magmatic δD-H2O characteristics. Instead, they seem to have recorded the volatile compositions of various metasomatic alteration agents. In the case of the troctolite, metasomatism has likely altered apatite to merrillite, whereas, in the case of the granulite, merrillite has been altered to produce secondary apatite. Consequently, apatite in these samples is not a useful tracer of the original source of lunar interior water. Granite 14303 and two norites (77215 and 78235) collectively display a range in apatite H2O content from 700 to ~ 2000 ppm, and a weighted average δD of -160 ± 74 ‰. After careful consideration of the potential secondary processes that may have altered the indigenous δD signature of these apatites, we conclude that these apatites do indeed preserve their magmatic H-isotopic compositions. By extension, they also record the δD signature of their Mg-rich source regions in the lunar interior. This δD signature is in good agreement with a recent estimate for the δD of the source region of the Ti-rich pyroclastic glasses (Füri et al., 2014. Icarus 229), and is comparable to estimates of the H-isotopic composition of the Earth’s mantle (Lécuyer et al., 1998. Chem. Geol. 145) and the δD of bulk CI-chondrites (Alexander et al., 2012. Science 337). This dataset supports the hypothesis for a common-origin for water in the Earth-Moon system (Füri et al., 2014. Icarus 229; Saal et al., 2013. Science 340)