Deciphering trace element behavior during basaltic magmatism on the moon through ion microprobe analyses of ancient lunar basalts, lunar volcanic glasses, and a terrestrial analogue.

摘要

Ion microprobe analyses of individual phases in basaltic samples from the Earth and Moon were used to provide information about the behavior and distribution of trace elements during basaltic magmatism. This information in turn has provided clues about the origin of lunar basalts, the composition of the lunar interior, and the formation of the Moon itself.; Detailed analyses of basaltic samples from the Makaopuhi Lava Lake, Hawaii were used to determine what factors affect the trace element record of a basaltic system. Geologic mechanisms such as gravitational settling and convective transport of minerals have significant effects on the trace element record. However, it is also shown that micro-scale factors such as mineral-melt interface kinetics can also affect the trace element record. The insights gained from these results have been extended to make inferences for basaltic systems where geological constraints are limited (i.e., the Moon).; Analyses of basaltic samples from the Moon provide a glimpse into the composition and structure of the lunar interior. For example, thorium and samarium data from lunar volcanic glasses show that the lunar mantle is heterogeneous on a large-scale. In addition, these data show that compositional variations in the glasses are a function of phase heterogeneity in the cumulate source region. This compositional information is important because it allows one to calculate the bulk composition of the mantle, which in turn is critical for understanding and testing different models of lunar evolution, such as the formation of a lunar magma ocean, generation of mare basalts, the origin of compositional asymmetry, and the thermal evolution of the Moon.; Trace element analyses of individual minerals in the Apollo 14 high-Al basalts provide evidence for compositional complexity in the lunar mantle. The trace element data show that these basalts were produced by fractional crystallization of basaltic melts that were produced by melting multiple trace element-rich sources in the deep lunar mantle. These results require efficient processing and convective overturn within the lunar mantle and imply that overturn and mixing of the mantle had to occur within 260 million years after the formation of the Moon.