Experimental investigation of Apollo 16 “Rusty Rock” alteration by a
lunar fumarolic gas
Abstract
The Apollo 16 sample 66095, named „Rusty Rock”, is enriched in volatile
and moderately volatile elements. The impact melt breccia is
characterized by the abundant occurrence of 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 1005±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±5
°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”.