Luna 16 microbreccia lithic fragments: Samples of an early basalt filling of mare fecunditatis

Abstract

Thirty five microbreccia fragments from Mare Fecunditatis recovered by the Luna 16 unmanned mission were studied inPTS by optical microscopy. Bulk compositions were determined by scanning electron beam analysis, and microbreccia constituents (pyroxene, olivine, plagioclase, spinel-group minerals, glasses) were analyzed with the electron microprobe. Results: Microbreccia fragments have bulk and mineral compositions distinctly different from those of aluminous basalt fragments from the same site: FeO, alkalis and FeO/MgO are lower and Cr2O3 is higher in the microbreccia fragments. Furthermore, microbreccia fragments are predominantly olivine-normative, whereas the basalt fragments are predominantly quartz-normative. In addition, in the microbreccia fragments, pyroxene compositions cover the range typical of pyroxenes from Luna 16 basalts but extend to more Mg-rich and Ca-poor compositions. Pyroxferroites found in aluminous basalt fragments were not observed in the microbreccia fragments. Plagioclase populations cluster at ⊂) An95 in the microbreccia fragments as compared to ⊂) An90 in the basalt fragments. Compositions of basaltic glasses from within the microbreccia fragments are similar to those from the soil. Conclusions: We found evidence that at the Luna 16 site, two major basalt types are present. One is represented by the 3.42 b.y. old aluminous basalt fragments, the other by the dominant component in the microbreccia fragments and basaltic glasses. The microbreccia fragments and basaltic glasses represent a here tofore unknown lunar basalt type for which we calculate a flatREE pattern with abundances approximately 10× chondritic, thus making it one of the most primitive basalt types known from the Moon. We suggest that this basalt type formed by a relatively high degree of partial melting of a plagioclase bearing source rock at about 60–100 km depth. We further suggest that this basalt type represents older mare fillings now largely covered by younger flows. These younger flows are represented by the aluminous basalt fragments, a rock type that may have formed from a similar source rock by a relatively low degree of partial melting.