9:00 AM - 9:15 AM
[MIS25-01] Geological verification of linear gravity anomalies on the Moon
Keywords:Lunar Volcanism, Linear Gravity Anomaly, Intrusive Materials, Lunar Explorer Kaguya, Crater
Radiometric ages of lunar meteorites and stratigraphic relationship between the cryptomaria and impact basins suggest that magmatic activity on the Moon have started before the youngest impact basin formation (3.8 billion years ago), but the flux of magmatic production and its temporal variation in the early Moon are not well understood. Gravity measurements made by NASA's Gravity Recovery and Interior Laboratory (GRAIL) have revealed linearly extending positive gravity anomalies, the Linear Gravity Anomalies (LGAs), in the Bouguer Gravity Field (Andrews-Hanna et al., 2013). LGAs are stratigraphically inferred to have formed in the early phase of the Moon (~3.8 billion years ago) (Andrews-Hanna et al., 2013, Sawada et al., 2016) and are expected to reflect a history of thermal evolution of the early Moon. A formation hypothesis is that the lithosphere was extended by the increase of the lunar radius, and magmas ascending from the mantle intruded into the extension field.
Crater formation by meteorite impacts excavates subsurface materials and deposited those around the craters. In this study, we used the reflectance map of the Multi-band Imager (MI) onboard the lunar orbiter KAGUYA (SELENE) to investigate whether the intrusive materials are exposed around large craters superposed on LGAs. We estimated iron and titanium abundances, and spectral absorption depths at 950 nm, 1050 nm, and 1250 nm, which indicates the relative amounts of pyroxenes, olivine, and plagioclase, respectively.
We investigated two LGAs on the lunar farside, LGA 1 and LGA 20 (Andrews-Hanna et al., 2013) because massive magma eruptions have not been occurred after the LGA formation and the contamination from post-LGA basalts is expected to be limited. The results indicate that there is no region with high iron and titanium contents around LGA1, suggesting no exposure of intrusive materials. On the rim of the Lomonosov crater around LGA 20, we found regions with high Fe content (12-14 wt%) and mineral abundance ratio close to the basaltic composition compared to the lower crustal material (here considered to be norite consisting mainly of orthopyroxene and plagioclase). The presence of these materials on the Lomonosov rim indicates that they existed in the subsurface before the Lomonosov-forming impact. Their elemental and mineral compositions suggest that they are mixtures of basaltic and noritic materials. Therefore, it is highly likely that the high Fe-rich material is originated from intrusive materials that constitute the LGA. In this case, the lower limit of the top depth of magma intrusion is estimated to be about 7 km based on an estimate of the Lomonosov-excavating depth by iSALE simulation.
Crater formation by meteorite impacts excavates subsurface materials and deposited those around the craters. In this study, we used the reflectance map of the Multi-band Imager (MI) onboard the lunar orbiter KAGUYA (SELENE) to investigate whether the intrusive materials are exposed around large craters superposed on LGAs. We estimated iron and titanium abundances, and spectral absorption depths at 950 nm, 1050 nm, and 1250 nm, which indicates the relative amounts of pyroxenes, olivine, and plagioclase, respectively.
We investigated two LGAs on the lunar farside, LGA 1 and LGA 20 (Andrews-Hanna et al., 2013) because massive magma eruptions have not been occurred after the LGA formation and the contamination from post-LGA basalts is expected to be limited. The results indicate that there is no region with high iron and titanium contents around LGA1, suggesting no exposure of intrusive materials. On the rim of the Lomonosov crater around LGA 20, we found regions with high Fe content (12-14 wt%) and mineral abundance ratio close to the basaltic composition compared to the lower crustal material (here considered to be norite consisting mainly of orthopyroxene and plagioclase). The presence of these materials on the Lomonosov rim indicates that they existed in the subsurface before the Lomonosov-forming impact. Their elemental and mineral compositions suggest that they are mixtures of basaltic and noritic materials. Therefore, it is highly likely that the high Fe-rich material is originated from intrusive materials that constitute the LGA. In this case, the lower limit of the top depth of magma intrusion is estimated to be about 7 km based on an estimate of the Lomonosov-excavating depth by iSALE simulation.