Complex Explosive Volcanic Activity on the Moon in Oppenheimer Crater

Abstract:

Oppenheimer is a floor-fractured crater located within the South Pole-Aitken basin on the Moon, and exhibits more than a dozen localized pyroclastic deposits associated with the fractures. Localized pyroclastic deposits on the Moon are thought to form as a result of intermittently explosive Vulcanian eruptions under low effusion rates, in contrast to the higher-effusion rates and Hawaiian-style fire fountaining inferred to form larger regional deposits. However, using new methods to derive iron mineralogy from Chandrayaan-1 Moon Mineralogy Mapper near-infrared spectra, we find that the mineralogy of the Oppenheimer pyroclastics is not consistent with a simple Vulcanian eruption mechanism.

The Oppenheimer pyroclastic deposits are mixtures of pyroxene sourced from the crater floor, juvenile clinopyroxene (CPX), and juvenile iron-rich glass. A Vulcanian (plugged conduit) eruption should cause significant country rock to be incorporated into the pyroclastic deposit. However, large areas within many of the deposits exhibit spectra consistent with high abundances of juvenile phases (glass or CPX mixed with glass) and very little floor material. Thus, we propose that at least some portion of these deposits must have erupted via a Strombolian or more continuous fire fountaining eruption at higher effusion rates. Significant along-fracture mineralogical variations within many of the deposits suggest multiple eruptions and that eruption styles may have been variable in time and space. Diviner mid-infrared spectra also indicate that these local deposits may be much more iron-rich than regional pyroclastic deposits, and thus are valuable resource targets. These results suggest that local lunar pyroclastic deposits may have a more complex origin and mode of emplacement than previously thought.