The Lunar Mare Ring-Moat Dome Structure (RMDS) Age Conundrum : Contemporaneous With Imbrian-Aged Host Lava Flows or Emplaced in the Copernican?

Zhang, F. and Head, J.W. and Wöhler, C. and Basilevsky, A.T. and Wilson, L. and Xie, M. and Bugiolacchi, R. and Wilhelm, T. and Althoff, S. and Zou, Y.L. (2021) The Lunar Mare Ring-Moat Dome Structure (RMDS) Age Conundrum : Contemporaneous With Imbrian-Aged Host Lava Flows or Emplaced in the Copernican? Journal of Geophysical Research: Planets, 126 (8): e2021JE006. ISSN 2169-9100

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Abstract

Ring-moat dome structures (RMDSs) are small circular mounds of diameter typically about 200 m and ∼3–4 m in height, surrounded by narrow, shallow moats. They occur in clusters, are widespread in ancient Imbrian-aged mare basalt host units and show mineralogies comparable to those of their host units. Based on these close associations and similarities, a model has been proposed for the formation of RMDS as the result of late-stage flow inflation, with second boiling releasing quantities of magmatic volatiles that migrate to the top of the flow as magmatic foams and extrude through cracks in the cooled upper part of the flow to produce the small RMDS domes and surrounding moats. In contrast to this model advocating a contemporaneous emplacement of RMDSs and their host lava flows, a range of observations suggests that the RMDS formed significantly after the emplacement and cooling of their host lava flows, perhaps as recently as in the Copernican Period (∼1.1 Ga to the present). These observations include: (a) stratigraphic embayment of domes into post-lava flow emplacement impact craters; (b) young crater degradation age estimates for the underlying embayed craters; (c) regolith development models that predict thicknesses in excess of the observed topography of domes and moats; (d) landform diffusional degradation models that predict very young ages for mounds and moats; (e) suggestions of fewer superposed craters on the mounds than on the adjacent host lava flows, and (f) observations of superposed craters that suggest that the mound substrate does not have the properties predicted by the magmatic foam model. Together, these observations are consistent with the RMDS formation occurring during the period after the extrusion and solidification of the host lava flows, up to and including the geologically recent Late Copernican, that is, the last few hundreds of millions of years of lunar history. We present and discuss each of these contradictory data and interpretations and summarize the requirements for magma ascent and eruption models that might account for young RMDS ages. We conclude with a discussion of the tests and future research and exploration that might help resolve the RMDS age and mode of emplacement conundrum.