APATITE: A VERSATILE RECORDER OF THE HISTORY OF LUNAR VOLATILES

Lunar apatite has been the main target for numerous recent investigations (as reviewed in Anand et al., 2014, Phil. Trans. Royal Soc. A, in press) because of its widespread occurrence in a variety of lunar rocks and its potential to reveal the volatile inventory of the Moon. Of the three volatiles (F, Cl and OH) present in lunar apatites, it is the abundance of OH and its H isotopic composition that have received most attention.

Previously, only Cl abundances could reliably be measured in lunar apatites using an electron microprobe, while F measurements were associated with large uncertainties and OH abundances were estimated by difference based on stoichiometry (e.g., McCubbin et al., 2011, GCA,75, 5073-5093). Advancements in analytical instrumentation such as the ion probe have facilitated more accurate and precise measurements of not only abundances of OH, F and Cl but also measurements of H and Cl isotopic compositions in lunar apatites. A number of groups worldwide are currently engaged in ion probe measurements of volatiles in apatites from a variety of lunar rocks. Such concerted efforts have yielded a wealth of new data highlighting a number of processes (e.g., degassing, spallation, fractional crystallisation etc.) that need to be taken into consideration before the history of indigenous lunar volatiles can be fully comprehended.

We have measured OH-δD systematics of apatites in lunar rocks from the ancient highlands terrain as well as the younger maria and various impact-related rock types. We have also begun coupled analyses of H and Cl isotopes in individual apatite grains from a variety of lunar samples to get further constraints on the effects of different processes on abundance and isotopic composition of H and Cl.

Some of the major highlights from recent work carried our on lunar apatites has been the revelation of complex partitioning behaviour of F, Cl and OH in apatite during fractional crystallisation from a basaltic melt, significant effects of spallation and degassing processes on the H and Cl isotopic composition, and an almost identical source of water in the Earth-Moon system. The field of lunar apatite research is rapidly evolving with acquisition of high-quality data necessitating paradigm changes and revisions in established hypotheses and models for the origin and evolution of volatiles in the Moon.