Geochemistry of apollo 15 basalt 15555 and soil 15531

Area of Science:

• Lunar geology and geochemistry
• Petrology of extraterrestrial materials

Background:

• Apollo 15 mission returned diverse lunar samples, including mare basalts.
• Understanding lunar basalt composition provides insights into planetary evolution and differentiation.

Purpose of the Study:

• To determine the major and trace element concentrations of Apollo 15 mare basalt 15555 and associated mineral and soil samples.
• To compare the composition of basalt 15555 with other lunar basalts and understand its differentiation status.

Main Methods:

• Atomic absorption spectrophotometry
• Colorimetry
• Isotope dilution analysis

Main Results:

• Basalt 15555 exhibits the lowest reported concentrations of Li, K, Rb, Ba, rare-earth elements, and Zr in lunar basalts.
• The negative europium anomaly in basalt 15555 is the smallest observed, suggesting minimal plagioclase fractionation.
• Soil 15531 can be modeled as a mixture of basalt 15555, KREEP, and plagioclase.

Conclusions:

• Apollo 15 mare basalt 15555 may represent the least differentiated lunar material analyzed to date.
• The observed europium anomaly can be explained by minor plagioclase crystallization, or a multistage origin is possible if plagioclase is not on the liquidus.
• Mineralogical data suggest plagioclase and pyroxene approached quasi-equilibrium during crystallization.