COORDINATED MICROANALYSIS OF THE CHEMICAL COMPOSITION, MICROSTRUCTURE, AND VOLATILE INVENTORY OF APATITE IN HIGH-TI MARE BASALTS FROM THE MOON

Apatite, Ca₅(PO₄)₃(F,Cl,OH), is the main volatile-bearing mineral in lunar rocks. The presence of volatiles in lunar apatite has recently been exploited with the goal of better understanding the origin of volatiles in the Moon and the roles they played during lunar magmatic processes.

Backscattered electron images of apatite in high-Ti mare basalts indicate chemical zoning. Qualitative energy dispersive spectroscopic (EDS) analysis indicates that the zonation is represented by core-to-rim variations in SiO₂ and rare earth element abundances, which make up a coupled substitution in apatite for P₂O₅ and Ca, respectively. We also note that in high-K, high-Ti basalts apatite commonly contains thin, vein-like features that could represent an exsolved component, although the veins were too fine-scale (< 1µm) to be chemically characterized by scanning electron microscopy (SEM). Furthermore, nano-scale secondary ion mass spectrometry (NanoSIMS) analyses of apatite crystals in high-K, high-Ti basalt 10024 display fractionated δ³⁷Cl values (~+8 to +16 ‰, relative to SMOC) and Cl contents up to ~5000 ppm. Overall, the isotopic compositions obtained agree with our analyses of other high- and low-K, high-Ti lunar basalts, which deviate from the overall δ³⁷Cl_Apatite-bulk rock trace element trend defined by low-Ti basalts, including KREEP basalts, from the Moon.

Here we present the results of our analyses of apatite crystals from two high-Ti mare basalts by coordinated SEM, focused ion beam (FIB), transmission electron microscopy (TEM), and NanoSIMS. The goal of this work is to fully document the chemical composition, crystal structure, and volatile abundances (F, Cl, OH, including Cl isotopes) of apatite in order to understand their history of formation and origin of volatiles. A zoned apatite from 10029 (low-K, high-Ti) and an apatite with exsolution features from 10024 were selected for coordinated analysis. To date, we have performed preliminary documentation by SEM and have taken FIB sections for analysis in the TEM. We will present results quantifying the chemical composition and crystal structure of zoned lunar apatite and exsolution features at the meeting.