Chlorine distribution and its isotopic composition in “rusty rock” 66095. Implications for volatile element enrichments of “rusty rock” and lunar soils, origin of “rusty” alteration, and volatile element behavior on the Moon

Abstract

An interesting characteristic of the pyroclastic glass bead deposits, select impact produced lithologies such as the “rusty rock” 66095, and unique lunar soils from the Apollo 16 landing site, is their unusual enrichments in 204Pb, Cd, Bi, Br, I, Ge, Sb, Tl, Zn, and Cl which indicates that portions of these sample contain a substantial volatile component. Sample 66095, a fine-grained, subophitic to ophitic polymict melt breccia, also hosts a pervasive low-temperature, volatile-rich, oxyhydrated mineral assemblage. The volatile element enrichments in these assorted lunar lithologies have been attributed to a variety of extra-lunar and lunar processes, whereas the oxyhydration in 66095 has long been thought to represent either terrestrial alteration of lunar chlorides and Fe–Ni metal to βFeO(OH,Cl) or indigenous lunar processes. In 66095, Cl is accommodated in FeO(OH,Cl), phosphates, and chlorides and is heterogeneously distributed. The low-temperature alteration occurs as rims around Fe–Ni metal and sulfide grains, and as dispersed grains in the adjacent matrix. Micro-Raman and transmission electron microscope (TEM) imaging indicate that akaganéite (βFeO(OH,Cl)) is the dominant FeO(OH) polymorph and is intergrown with goethite (αFeO(OH)) and hematite (αFe2O3). TEM observations indicate a well-defined “nanometer-scale” stratigraphy” to the alteration. For example, kamacite (body centered cubic)→face-centered cubic (fcc) Fe–Ni alloy→lawrencite (FeCl2)→akaganéite. The lunar lawrencite (Fe,Ni)Cl2 in 66095 does not react directly to akaganéite on Earth. Rather, lawrencite exposed to terrestrial conditions reacts to form an amorphous Fe- and Cl-bearing phase, nano-crystalline goethite, and hematite. The morphology of these terrestrial alteration products is significantly different than that of the akaganéite occurring in 66095. The chlorine isotopic compositions of these volatile-rich samples are enriched in heavy Cl. For 66095, the δ37Cl ranges from +14.0‰ to +15.6‰, whereas the δ37Cl for the volatile-rich A16 soils ranges from +5.6‰ to +15.7‰. Based on these data it appears likely that the volatile element enrichments and the Cl isotopic fractionation observed in 66095 and the Apollo 16 soils did not result from extra-lunar additions, but are most likely indigenous to the Moon. Lawrencite was deposited on mineral surfaces at approximately 650°C to 570°C from a metal-chloride-bearing, H-poor gas phase. This gas phase was also responsible for the transport of other metals (e.g. Zn, Cu, Pb, Fe). The fractionation of Cl isotopes in the rusty rock can be attributed to fumarole processes in a low-H system. The origin and formation of the akaganéite is more enigmatic. The Cl-isotopes are consistent with it replacing lawrencite. However, numerous nanometer-scale observations are not consistent with a terrestrial origin and indicate multiple episodes of oxyhydration.