Experimental Investigation of Apollo 16 “Rusty Rock” Alteration by a Lunar Fumarolic Gas
Creators: name: Renggli, C. J.
name: Klemme, S.
Publisher: name: FID GEO
Year: 2021

description: The Apollo 16 sample 66095, named “Rusty Rock”, is enriched in volatile and moderately volatile elements. The impact melt breccia is characterized by abundant Fe‐rich sulfide and chloride alteration phases, including FeS, ZnS, and FeCl2. These phases have previously been interpreted to be the result of fumarolic alteration of the breccia. Here we present the results of two different experimental approaches, which aim to constrain the temperature conditions and the process under which the “Rusty Rock” alteration formed. The first experimental set‐up assumes that the metals Zn, Cu, and Fe were introduced into the rock by a C‐O‐S‐Cl gas phase, and that the Fe‐rich sulfides and chlorides were deposited from this gas phase. This “gas deposition” experiment suggests that the alteration assemblage formed over the temperature range of 538–638 ± 5°C. The second experimental set‐up simulates a scenario, where Fe metal particles in the lunar rock react with a Zn‐C‐O‐S‐Cl gas phase at six different temperatures between 396 ± 5°C and 1,005 ± 5°C. This latter “metal reaction” experiment resulted in the formation of sulfide and chloride coatings on the Fe metal chips. The “Rusty Rock” alteration phases FeCl2 and (Zn,Fe)S were abundantly present in the coating of the Fe metal chip reacted at 580 ± 10°C. Both experiments lead to results which are in agreement, providing a temperature of 580 ± 50°C for the fumarolic alteration on the Moon, as observed in the Apollo 16 “Rusty Rock”.
description: Plain Language Summary: The Apollo 16 sample 66095, colloquially named “Rusty Rock”, is an unusual lunar rock which is enriched in volatile elements such as sulfur and chlorine. We investigate two processes by which sulfides and chlorides may form in a lunar fumarolic system, by conducting experiments in evacuated silica glass tubes at reducing conditions. First, we assume that metals and volatiles (Zn, Cu, Fe, S, and Cl) are all deposited from a gas phase (gas deposition experiments), and second, we assume that Fe metal is already present in the rock and that the Fe altered by the introduction of a Zn‐S‐Cl‐bearing gas phase (metal reaction experiment). In both experimental setups we observe the formation of “Rusty Rock” alteration phases FeCl2 and (Zn, Fe)S at 580 ± 50°C, constraining the temperature of fumarolic alteration recorded in the Apollo 16 sample 66095. Hence, our experiments confirm that the characteristic S‐ and Cl‐rich minerals found in the lunar “Rusty Rock” were formed by a lunar fumarole. More broadly, lunar metal deposits may be associated with ancient fumarolic processes.
description: Key points: Experiments constrain the temperature of fumarolic Apollo 16 “Rusty Rock” alteration to 580 ± 50°C. The phase assemblage of FeCl2 and (Zn,Fe)S constrains the composition of the fumarolic gas at 600°C. Gas deposition and gas‐solid metal reaction experiments reproduce the “Rusty Rock” alteration phases FeCl2 and (Zn, Fe)S.
description: Swiss National Science Foundation
description: Deutsche Forschungsgemeinschaft