THE USE OF APATITE AS AN ARCHIVE OF PROCESSES IN MAN AND THE MOON

Apatite minerals are compositionally diverse and ubiquitous in terrestrial and lunar systems. The goal of this study was to determine apatite geochemistry from far disparate origins - a biogenic and an abiogenic source — human bone and lunar basalts — to highlight the utility of apatite in its variable forms as an archive of geochemical processes. On Earth, apatite, as hydroxyapatite in bone, is the main constituent of the vertebrate skeletal system. Bone’s major- and trace-element chemistry and structure reflect physical and chemical interactions with an organism and its environment. Within the human bone trabeculae, concentrations of phosphorus, calcium, and sodium were variable: zones of depletion surrounding osteons suggest biotic mobilization of these elements for vital processes. An absence of chemical zonation in the cortex suggests relative stasis in the diet or health state of the organism, or remodeling of prior perturbations.

In apatite on the Moon, the concentrations of F, Cl, and OH ions provide insights into the chemical nature and origin of the magmas that came from the lunar interior, the mantle, which erupted to form the basaltic rocks of the Maria. The water and other volatile contents provide a major control on the crystallization and chemistry of the lunar minerals forming the various rocks of the Moon. Hydrogen isotopes in lunar apatites, D/H values, provide clues as to the origin of the volatiles in the Moon, possibly inherited from comet-bombardment or the Mars-sized bolide (Theia) that caused the Giant Impact origin of the Moon or even the Earth during this early torrential collision. Thus, the OH in lunar magmatic apatites may contain evidence for the origin of the water in the Moon, as well as help explain the consequences of this water on the chemical and physical evolution of the Moon, a paradigm for all the terrestrial planets. Despite the variable compositions and mechanisms of formation, apatite can provide critical information on the formational history of the host (vertebrate or rock) and changes after crystallization (biogenic remodeling or chemical alteration).