Distinguishing between Apollo 14 impact melt and pristine mare basalt samples by geochemical and textural analyses of olivine

Apollo 14 (A-14) impact melt olivine vitrophyres (OVs), composed of olivine phenocrysts in an opaque glass matrix with minor amounts of pyroxene and plagioclase, are petrographically similar to pristine quenched A-14 high-alumina (high-Al) and Apollo 12 (A-12) basalts. Textural and chemical analyses have been conducted on olivines within impact melt and pristine mare basalt clasts from A-14 breccia 14321 as well as various olivine-phyric Apollo 12 and 17 basalts to be able to distinguish between the petrographically similar samples. The A-14 high-Al basalts examined here represent samples from each of the three chemical groups (A, B, C). Examples from the threeA-12 basalt suites (Ilmenite, Olivine, Pigeonite) and olivine-richApollo 17 (A-17) TypeC basalt, 74275, have also been analyzed. As a complement to the chemical analyses, crystal size distributions (CSDs) were determined for approximately half of the samples investigated here (due to a small number of olivine crystals that did not produce statistically meaningful CSDs on half of the samples), and confirm that although similar, the basalts and impactmelts are texturally distinct. Olivines fromA-14 impact melts have the highest average forsterite contents (average Fo 0.82) and generally the lowest average Co (37 ppm) and Mn (1544 ppm) abundances, thus chemically distinguishing them from the pristine basalts. In addition, chemical compositions obtained for olivine from 14321,1486 aremost likely representative of an impactmelt and thus contradict its previous classification as a high-Al basalt (Neal et al., 1988); olivine from,1486have similar average forsterite (averageFo 0.82) andTi/V-ratio to theOVs.Furthermore, on the basis of their chemical compositions, olivines have been identifiedwithin the impactmelts that are likely inherited from basalts; these olivines are chemically distinct from the rest of the impact melt olivine populationwith lower forsterite (0.67–0.80) and lowTi/V ( 10), which are similar to olivine compositions from the A-14 high-Al basalts. In addition to distinguishing between sample types fromA-14, olivine compositions can also be used to distinguish between pristine mare basalts from various missions. Olivine within the selected samples from the individual A-12 basalt suites can also be distinguished chemically from one another using the Ti/V ratio, where Olivine suite basalts have Ti/V <3 and Ilmenite suite basalts have Ti/V >3.5. The results indicate that combined use of CSDs and chemical analyses of olivine can be used to reliably distinguish between (1) impact melts and pristine Apollo 14 basalts, (2) basalts from different landing sites, and (3) individual basalt suites from a single site. 2012 Elsevier Ltd. All rights reserved.