Investigation of Lunar Spinels at Sinus Aestuum

Sinus Aestuum is the only known location on the Moon where orbital data have detected Fe‐and/or Cr‐spinel. We analyzed Moon Mineralogy Mapper (M 3 ) visible to near‐infrared data of the largest and strongest spinel signatures and determined that these locations always correspond to impact craters. M 3 spectra show that at least three types of spinels may be present, all of which exhibit a strong and broad absorption at ~2100 nm, and also one of the following: (1) a narrow 700–750 nm absorption, (2) a broad 600–900 nm absorption, or (3) both a weaker 700 nm and stronger 1000 nm absorption. All the spinel detections occur on either larger highland massifs that make up Sinus Aestuum east and west or smaller highland kīpukas and buried highlands within the mare. Almost all of the spinel signatures occur within the mapped pyroclastic dark mantle deposit (DMD). The strong correlation between spinel and DMD distribution on the highlands at Sinus Aestuum is best explained if the spinels were emplaced during the same explosive eruption(s) that deposited the pyroclastics in the Sinus Aestuum DMD. Our observations are most consistent with models of melt‐rock reactions in the anorthositic lunar crust that produce contaminated (high‐Al) regions within a volcanic dike or magmatic reservoir that was capable of erupting pyroclastic glass beads containing pleonaste spinel [Mg,Fe]Al 2 O 4 . Over billions of years, this surface layer of spinels and pyroclastics became heterogeneously mixed into and partially buried within the highland regolith where younger impact craters may sometimes expose it.