Normative composition and classification of lunar igneous rocks and glasses, II Lunar glasses

Abstract

Rittman norms of lunar glasses were calculated for 1499 major element analyses from the literature and unpublished data obtained in our laboratory. The norm values, grouped according to increasing clinopyroxene content, were plotted into quartz-plagioclase-orthopyroxene and olibine-plagioclase-orthopyroxene triangles, respectively. The plots indicate that lunar glasses, like lunar rocks, form a compositional continuum which starts from glasses very high in plagioclase and continues, with increasing clinopyroxene, to plagioclase-poorer and orthopyroxene- and olivine-richer glasses. According to apparent clusters in the plots and taking into account the rock groups defined in Part I of this paper, the continuum was divided into eleven glass groups: GI 1 (anorthite), GI 2 (anorthositic), GII 1 (noritic and gabbronoritic), GII 2 (troctolitic), GIIIA (K-rich noritic), GIIIB (noritic), GIV (mafitic tholeiitic), GV (Ti-rich tholeiitic), GVI 1 (Ti-poor olivine gabbronoritic), GVI 2 (olivine gabbronoritic), GVI 3 (Ti-rich olivine gabbronoritic). Chemical and normative correspondence between rock (R) and glass (G) groups could be achieved by a refinement of our rock classification given in Part I of this paper: RI was divided into RI 1 (>95%plag), and RI 2 (90–95% plag); RII into RII 1 (75–90% plag) and RII 2 (50–75% plag); RIIIB into RIIIB 1 (40–55% plag) and RIIIB 2 (55–75% plag). This refined rock classification comprises 10 groups. Reasonable chemical and normative correspondence exists between the following pairs: RI 1/GI 1, RI 2/GI 2, RII 1/GII 1, RIIIA/GIIIA, RIIIB 1/GIIIB, RIV/GIV, and RVB/GV. No equivalent glasses exist for rocks RII 2, RIIIB 2 and RVA. No equivalent rocks are known for glasses GII 2, GVI 1 (Apollo 15 green glasses), GVI 2, and GVI 3 (Apollo 17 orange glasses). Figures are given for the frequencies of rock and glass groups at the Apollo and Luna landing sites. The abundance of chemically corresponding rocks and glasses at each site is similar in most cases, suggesting genetic relationships. It is suggested that GI, GII, GIII, GIV and GV glasses were formed by impact fusion of chemically corresponding rocks. GVI 1, GVI 2 and GVI 3 glasses are probably of volcanic origin. Frequency distributions of major element concentrations in glass and rock groups show that glass groups contain an excess of Na-poor, some also of K-poor samples, in comparison with corresponding rock groups. It is suggested that this difference is due to alkali volatilization caused by impact fusion at high temperatures. Average data are given on colours and refractive indices of the glasses.