Disentangling Mg-Suite Petrogenesis via Petrologically Heterogenous Olivine-Hosted Melt Inclusions in Apollo Troctolite 76535

The magnesian (Mg-) suite is a group of lithologies associated with the lunar highlands that generally consist of Mg-rich mafic phases (olivine, pyroxene) and calcic plagioclase. Generally inferred to be magmatic in origin, the Mg-suite represents the earliest stage of secondary crust building on the Moon, although diverse petrogenetic models have been developed to understand seemingly contradictory characteristics of these rocks. A key aspect of many of these models is the composition of KREEP (potassium, rare earth elements, phosphorus) incorporated into these magmas, as well as the timing and mechanism of its incorporation. Determining these factors will help dis-criminate between the various petrogenetic models.

Apollo 17 troctolite 76535 is a hallmark of the lunar Mg-suite, due to its relatively large mass, inferred pristinity, and ancient age. Olivine-hosted melt inclusions (MIs) within 76535 have been documented by previous researchers but not studied in detail. Melt inclusions in igneous rocks are valuable because they represent pockets of melt trapped during crystallization and can provide information on magma composition and history.

In order to better understand the role of KREEP in Mg-suite magmas and their relevance to Mg-suite petrogenesis, we are conducting a systematic study of olivine-hosted MIs in 76535, including their occurrence, size, petrology, geochemistry, and mineralogy. This work investigates 76535,218, a collection of 192 loose olivine grains that were selected based on optical identification of potential inclusions.