Abstract
Abstract Since ejecta around an impact crater is excavated from a depth, its mineralogy and chemistry will provide us with information on the composition of the pre-impact subsurface. The depth from which crater ejecta were excavated was determined from laboratory experiments, field studies, and a simplified quantitative model (Z-model and the scaling law of ejection velocity). Based on the results of these studies, it is believed that surface material of an ejecta blanket between 1.1 and 1.5 radii from the crater was excavated from a depth of 0.13 to 0.15 radii. The following results were obtained from combining the surface and subsurface basalt distributions with crater-counting ages for the mare basalt, we obtained the following results: (1) The averages of TiO2 and FeO increased with time from the Imbrian to the Eratosthenian periods, which is represented by a continuous trend curve on the TiO2-FeO graph: (2) volcanic activities in Mare Imbrium drastically decreased and basalts changed from a low-Ti to high-Ti content around the transition of the Imbrian to Eratosthenian period: (3) basalts with less than 3 wt% TiO2 erupted in succession mainly in the Imbrian period.
Highlights
Lunar mare basalt deposits cover 17% of the lunar surface and have a total volume estimated at 107 km3, which is less than 1% of the total volume of the crust (Head, 1975)
This study investigated the distribution of geologic units in Mare Imbrium
The compositional data of crater ejecta were used as an indicator to reveal subsurface distributions of the mare basalt
Summary
Lunar mare basalt deposits cover 17% of the lunar surface and have a total volume estimated at 107 km, which is less than 1% of the total volume of the crust (Head, 1975). The ages of Ti-rich basalts (Apollo and 17) are 3.6 to 3.9 Ga while those of the less-Tirich basalts (Apollo and 15) are 3.2 to 3.4 Ga.Based on an assimilation of chemical, mineral, isotopic, and other data collected on these basalts, several models for mare basalt generation have been proposed (Green and Ringwood, 1973; Taylor and Jakes, 1974; O’Hara et al, 1975; Kesson, 1975; Hubbard and Minear, 1975; Ringwood, 1975; Shih and Schonfeld, 1976; Ringwood and Kesson, 1976; Binder, 1982) These models have a basis in the fact that the ages of high-Ti basalt (3.6–3.9 Ga) are older than those of low-Ti basalts (3.2–3.4 Ga). Within the context of the results, wi will discuss the genesis and thermal conditions of mare basalt under Imbrium basin
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