Rhönite in Luna 24 Pyroxenes: First Find from the Moon, and Implications for Water in Planetary

Compared to the Earth, its Moon is strongly depleted in volatiles, and its interior is generally treated as anhydrous. The internal volatile content of the Moon remains a critical control on its petrogenesis, and can be re-assessed in light of current interest in the Moon. One tool in this assessment is chemical (and isotopic) analysis of minerals that could contain volatile elements. One volatile-bearing mineral is amphibole, which can contain H2O (as OH), F, and Cl. Amphiboles were reported in several lunar rocks [1,2], but these finds have apparently not been verified. A report of amphibole in Luna 24 regolith olivine grains [3], detailed and seemingly reasonable, was the impetus for this study. I have not found amphibole (so far), but have found the first lunar occurrence of the anhydrous mineral rhönite, which has a composition similar to that of kaersutite amphibole. The rhönite is found in magmatic inclusions in pyroxene – the same setting as for kaersutite amphibole in the shergottite Martian meteorites [4,5]. Methods: Grain mount thin sections of Luna 24 regolith were examined at the ARES library, Bldg 31, Johnson Space Center. Sections were examined with petrographic microscopes, and with the Horiba HRLabRam Raman microscope at ARES using red He-Ne laser light. Chemical analyses are by electron microprobe, using the Cameca SX-100 at ARES, 15 kV accelerating potential, beam current 10 na, beam diameter 1 m. Most elements were standardized against synthetic oxides. Other standards were: orthoclase for K; oligoclase for Na, fluorite for F, and tugtupite for Cl. Data were reduced in the Cameca PAP algorithm. Results: The L24 regolith includes mineral fragments, rock fragments, and glasses [6]. Most mineral fragments are pyroxene (pigeonite and augite), and most of them contain glassy inclusions with or without crystalline phases. These inclusions are interpreted as magmatic inclusions [7] – remnant of magma trapped in growing crystals. Magmatic inclusion in several pyroxenes contain tabular brown crystals which are optically anisotropic, pleochroic from pale greenish brown to reddish brown, and with inclined extinction, Figure 1. These optical properties are not consistent with previously reported lunar minerals, but are consistent with kaersutite amphibole as is found in similar magmatic inclusions in Martian meteorite pyroxenes [4,5]. The adjacent opaque grain is ulvöspinel (Table 1). Figure 1. The largest L24 rhönite grain found to date. Top images are plane light, oriented 90° to each other to show pleochroism. Bottom image is crossed polars with host pyroxene near extinction, showing that the grain is anisotropic.