Hidden in Plain Sight: Spinel-Rich Deposits on the Central Nearside of the Moon

A new and unexpected spinel-rich lithology has been identified on the central lunar nearside by the Moon Mineralogy Mapper (M3: a 0.43 to 3.0 μm imaging spectrometer on India’s Chandrayaan-1 spacecraft). While spinel group minerals are common in lunar samples, they are only known to occur as accessory phases at very low abundances (<10%). For the first time, M3 has identified deposits on the surface of the Moon that have low mafic silicate abundances (<20% olivine and pyroxene) and are spectrally dominated by spinel. Spinel group minerals are characterized by strong absorptions near 2 μm. Pyroxene spectra also have absorptions in the 2 μm region, yet they have even stronger features near 1 μm. Conversely, olivine spectra lack 2 μm absorptions and are instead characterized by a complex feature at 1 μm. These newly identified regions have strong 2 µm absorptions and extremely weak or absent 1 μm absorptions and are thus identified as spinel-rich. It is therefore the combination of high signal-to-noise, spectral range (covering both 1 and 2 µm regions), and spatial resolution of the M3 (140 m/pixel) data that has unveiled this unique spinel-rich deposit, which has been hidden in plain sight. On the nearside, spinel-rich lithologies have thus far only been in identified in a single region centered near (5°N, 11°W), amongst the Sinus Aestuum and Gambart pyroclastic/dark mantle deposits (DMD). This region has undergone a complex geologic history, including modification by Imbrium basin ejecta, pyroclastic eruptions, mare flooding, and modification by Copernicus ejecta. Where exactly these unusual compositions fit in this sequence is not yet clear. The spinel-rich unit is regional in scale (100’s km2) and includes some areas between these two DMDs that have been covered by mare basalts. However, spinel is notably absent in the large Rima Bode DMD located to the northeast. Some of the darkest areas associated with the DMDs are spinel-rich, but not all DMDs in the region correlate with spinel enhancements. Similarly, not all spinel-rich areas appear directly related to the DMDs. Moreover, spinel-rich outcrops are also associated with craters and are exposed, for example, in crater walls. Thus while these surprising spinel-rich deposits are apparently associated with the pyroclastics and are likely volcanic in origin, their specific relationship to pyroclastic volcanism is not yet clear. Nonetheless, these spinel-rich lithologies are certain to provide unique insights into the processes and source regions of early lunar volcanism.