Mare basalts: Crystal chemistry, mineralogy, and petrology

Abstract

Lunar crustal rocks can be divided into two groups: the terra, or highland, types and the mare basalts. Interpretation of the highland samples is complicated by their derivative nature, which resulted from a series of crystallization, shock, and brecciation events. In contrast, mare basalts appear to be much less complicated and to have been rather uncompromised since their arrival at the lunar surface; thus a synthesis appears possible at this time. Although the mare basalts comprise less than 1% of the lunar crust, they contain much information about the thermal history of the moon and the nature of the lunar interior. It is now known that a complete suite of basalts, sampling all of the chemically and temporally distinct units, was not sampled by the Apollo and Luna missions. The mare basalts that have been studied have ages between 3.15 and 3.96 Gy. However, photogeologic evidence (crater counts and crater degradation studies) indicates that basalts as young as 2.5 Gy exist on the moon and were not sampled. The returned samples can be divided into two broad groups: the older, high‐titanium group (ages, ∼3.55–3.85 Gy; TiO2, 9–14 wt %) and the younger, low‐titanium group (ages, 3.15–3.45 Gy; TiO2, 1–5 wt %). Basalts from Apollo 11 and 17 fall into the older, high‐titanium group; basalts from Apollo 12 and 15 and Luna 16 fall into the younger, low‐titanium group. The two major groups of basalts can be further subdivided on the basis of major‐ and minor‐element chemistry. Within each of these subgroups a variety of grain sizes and textures, which result from different cooling histories, are present. Near‐surface fractionation of these basalts involved mainly olivine in the low‐titanium basalts and olivine plus iron‐titanium oxides in the high‐titanium basalts. The alkali‐depleted mare basalts evolved by rapid cooling at the lunar surface under extremely reducing conditions (∼10−13 atm at 1150°C). This low oxygen fugacity resulted in reduced valence states for Ti (Ti4+ → Ti3+) and Cr (Cr3+ → Cr2+), which in turn affected both the chemistry and the stability of the mare basalt minerals. The most important mineralogical species in these rocks are the silicates (pyroxene, feldspar, and olivine) and the Fe‐Ti oxides (ilmenite, spinel, and armalcolite). Models for the source regions of the mare basalts remain controversial. Three basic models for mare basalt source regions have been advanced. These include the cumulate source model (remelting of cumulates resulting from early lunar differentiation), the primitive source model (melting of deep undifferentiated mantle), and the assimilation model (primary melts are contaminated by assimilation). All of these models have problems. If one assumes that at least some of the lunar basalt samples arrived at the surface with unaltered chemistry, the high‐pressure experimental phase equilibria approach can provide constraints on the nature of the source regions for these rocks. Results of these studies indicate that the low‐ and high‐Ti mare basalt groups were derived from mineralogically distinct source regions. The low‐Ti basalts could have been derived from an olivine‐pyroxene source rock at depths ranging from 200 to 500 km, while the high‐Ti basalts could have been derived from olivine‐pyroxene‐ilmenite cumulates in the outer 150 km of the moon.