THE SYSTEMATICS OF LIGHT LITHOPHILE ELEMENTS (LI, BE, AND B) IN LUNARPICRITIC GLASSES - IMPLICATIONS FOR BASALTIC MAGMATISM ON THE MOON AND THE ORIGIN OF THE MOON

Lunar picrites, represented by high-Mg volcanic glasses, are thought t o be products of either partial melting of the deep lunar mantle follo wed by rapid ascent or polybaric partial melting initiated in the deep lunar mantle. The near primary compositions of these volcanic glasses provide us with a unique perspective for evaluating basaltic magmatis m, the characteristics and evolution of the lunar mantle, and the orig in of the Moon. The light lithophile elements (LLE = Li, Be, B) in pla netary materials have been used to estimate planetary compositions and evaluate magmatic processes. Ion microprobe analyses of these glasses for LLEs were conducted using a Cameca 4f ion microprobe. This suite of glass beads ranged in TiO2 from 0.3 to 17 wt%. Seventy-one individu al glass beads were analyzed for the LLEs. In addition, core-rim analy ses of individual glass beads were made. The LLEs show a wide range of variability with Li ranging from 1.2 to 23.8 ppm, Be ranging from 0.0 6 to 3.09 ppm, and B ranging from 0.11 to 3.87 ppm. B/Be ranges from 0 .40 to 4.6. Li/Be ranges from 2.7 to 41.7, although 90% of the Li/Be v alues range from 14 to 30. Both B/Be and Li/Be values for the picritic glasses are less than chondrite. Be/Nd for the glasses ranges from .0 4 to .06 and are similar to chondrite (.058). Traverses across individ ual beads indicate that they are generally homogeneous with regards to LLEs regardless of TiO2 content. The individual glass groups show lim ited variations in LLE characteristics. The exceptions to this observa tion are the A17 VLT and the A15 yellow glasses. At individual Apollo sampling sites, and LLE content is generally correlated to TiO2. The h igh-Ti glasses are displaced toward higher Li at similar B and Be rela tive to the very low-Ti glasses. LLE concentrations also parallel the enrichments of other lithophile elements such as Ba, Zr, Sr, and REEs. As noted for other trace element characteristics, glasses from each s ampling site have similar LLE signatures. For example, the Apollo 14 g lasses generally have higher LLE concentrations relative to glasses of similar TiO2 content from other sites. The LLE data support mantle in homogeneity and Lunar Magma Ocean (LMO) cumulate overturn models sugge sted by previous studies. A KREEP component had been incorporated into some of these picritic glasses. This is consistent with other trace e lements and probably reflects the recycling of KREEP and/or other late stage LMO cumulates into the deep lunar mantle. The picritic glasses are compositionally distinct from the crystalline mare basalts in LLEs . They are not related by either fractional crystallization or partial melting processes. This suggests that they were derived from distinct ively different mantle sources. Estimates of the bulk compositions of the Earth and the Moon have previously been made based on the assumpti on that the ratio of Li to Be is a direct measure of the ratio of the high temperature condensates (HTC, refractory components) to the Mg-si licates (less refractory components) in a planet. We assert that if Li /Be is to be used to estimate bulk Moon composition, the picritic glas ses provide fewer pitfalls and a better estimate than the crystalline mare basalts. Differences in partition coefficients (D) for Li and Be indicate that fractional crystallization and partial melting will modi fy the Li/Be ratio. Estimates based on the picritic glasses infer a hi gher Li/Be for the bulk Moon than estimated from the mare basalts. Thi s would indicate that the bulk Moon is less refractory than previously calculated by Li/Be and approaches the bulk composition of the Earth.