USING MODERATELY VOLATILE ELEMENTS TO EXPLORE THE LUNAR AND MARTIAN MANTLES

Moderately volatile elements (MVEs) are valuable geologic tracers of high temperature events that occur during planetary formation but are robust enough to not be reduced to below detection limit abundances in most geologic samples. The recent discovery of volatile compounds (H₂O, S, Cl, & F) in lunar samples has brought a re-evaluation of the Moon forming “Giant Impact”, as well as subsequent lunar differentiation and evolution. The MVEs (Zn, Se, Rb, Ag, Cd, In, Sb, Tl, Pb, & Bi) are incompatible in lunar magma ocean (LMO) phases and are present in low abundances. A custom solution-mode ICP-MS method was used to quantify and remove any major interferences that artificially inflate the measured signal on the element of interest. Thus, a whole rock digestion can be analyzed with no need for isolation from matrix elements, and the entire MVE suite is measured from a single sample. Samples analyzed include 67 lunar basalts (including KREEP and an impact melt), 2 volcanic glass bead types, 4 lunar meteorites, and 9 Martian meteorites. LMO differentiation models state that early forming Mg-rich phases sank. As incompatible elements enriched in the residual melt ilmenite eventually crystallized and subsequently sank to form high-Ti cumulates. Early LMO cumulates are the source region for low-Ti basalts while late stage for the high-Ti basalts. The final dregs of the LMO were highly enriched in incompatible elements and became the source for the “KREEP” signature. The incompatible nature of the MVEs would mean that they are becoming enriched in the melt throughout LMO differentiation. The data shows that the MVEs have relatively equal compositions between the low and high-Ti basalts with KREEP and the impact melt generally being the most enriched. This supports what has been suggested through Cl-isotopic evidence that a crust-breaching impact during LMO crystallization allowed degassing, while subsequent annealing allowed for the renewed enrichment of in incompatible elements in the late stage KREEP reservoir.