Investigations of the Sinus Aestuum DMD: An Anomalously Large and Compositionally Distinct Lunar Pyroclastic Deposit

Abstract The Sinus Aestuum (SA) dark mantle deposit (DMD) is a regional pyroclastic deposit on the central lunar near‐side. Spectra from the SA DMD are anomalous when compared to other DMDs and have been interpreted to represent a high abundance of the mineral spinel. The SA spinels are compositionally distinct compared to other lunar spinels and their pyroclastic association suggests that a unique volcanic process has occurred. Spectral mixture analysis is used to map the distribution of the spinel‐bearing pyroclastics (SBPs) across the region. The extent of SBPs reveals that the SA DMD is much larger than previously thought (∼221,000 km 2 ) and is distributed over an approximately circular region, consistent with a large eruption from a single, central source. However, variable abundances of SBPs are observed on smaller scales, suggesting a more complicated history. The distribution of these SBPs, including a portion of non‐spinel bearing DMD discovered by this study within the SA region, is analyzed across a variety of spatial scales to infer what processes may have produced the observed variability. There is evidence of small‐scale volcanism in addition to the larger eruption forming the broader DMD. An ultraviolet absorption, which is absent in other regional DMDs, is discovered in the spectra of SA SBPs, which may indicate the presence of Ti‐bearing oxides. It is unclear whether the spinel itself has caused this feature, or if it is the result of a co‐occurrence of ilmenite within the SBPs.