LUNAR SAMPLE NEWSLETTER NUMBER 44 JULY 19, 1985 Douglas P. Blanchard . Lunar Sample Curator John W. Dietrich. Newsletter Editor Planetary Materials Branch. SN2. NASA/ JSC Houston . Texas 77058. 713-483-3274 CONTENTS Page WORKSHOP ON THE GEOLOGY AND PETROLOGY OF THE APOLLO 15 LANDING SITE 2 PRELIMINARY AGENDA--WORKSHOP ON THE GEOLOGY AND PETROLOGY OF THE APOLLO 15 LANDING SITE 6 LPSC XVI LUNAR SAMPLE FIELD TRIPS A BIG SUCCESS 8 NEXT LAPST MEETING WILL BE NOVEMBER 16-18, 1985 9 STUDY OF NEW BRECCIA SURFACES CONTINUES 9 UPCOMING DATES OF INTEREST 10 APPENDIX I: MAPPING OF BRECCIA 14303 11 1 , ., WORKSHOP ON THE GEOLOGY AND PETROLOGY OF THE APOLLO 15 LANDING SITE November 13-15, 1985 at LPI, Houston (Co-convenors: P. Spudis (U.S.G.S.) and G. Ryder (LPI» The geology of the Apollo 15 landing site remains poorly understood, in contrast with that of the geology and samples of the Apollo 16 and 17 landing sites. The Apollo 15 site is on the rim of the Imbrium basin, the remains of a paramount event in lunar geologic history. It encompasses a remarkably complete stratigraphic section ranging from pre-Imbrian to Copernican, unique among Apollo sites. Within the Apollo 15 samples, site photographs, surface experiments, and crew reports is recorded a variety of lunar processes and historical events, many of which are at present only dimly perceived. The petrology and stratigraphy of site materials are relevant to lunar crustal composition, formation, and origin, the mechanics and ejecta depositional processes of craters ranging from large basins to secondary clusters; and a whole gamut of volcanic processes. However, the Apollo 15 mission has often been felt to have received short shrift and to have been overshadowed by the succeeding Apollo 16 landing. It has never had a "conference of its own" at which multidisciplinary approaches could focus on its scientific opportunities. There is a perception that we have glaring deficiencies in our understanding which would be remedied by a multidisciplinary examination of the Apollo 15 landing site, especially in the light of the results from other missions. To examine a.nd synthesize our present knowledge, to discuss and dispute varied pertinent concepts, and to formulate research directions likely to advance our understanding of the landing site and of the moon, the Lunar and Planetary Institute, at the instigation of LAPST, is sponsoring a "Workshop on the Geology and Petrology of the Apollo 15 Landing Site." Keynote talks, contributed papers, and discussions are being arranged around eight main topics, following keynote presentations providing background (see preliminary agenda, below). Following the meeting, the contributed and invited abstracts and summaries of the discussion sessions will be published as an LIP Tech- nical Report. Members of the prime, backup, and support crews have expressed great interest in participating in the Workshop. In the following paragraphs we summarize, in order of the Workshop topics, some of the rationale and questions which future studies might at least partly answer. Sampling of the Apennine Front was the prime target of the Apollo 15 mission, yet its petrology remains one of the major outstanding problems . The talus deposit on the lower slopes of Hadley Delta is dominated by mare debris and mare-rich breccias; highlands materials is generally cryptic or at least small. Small samples, including coarse-fines from the regoliths, were scantily regarded in the Apollo mission days, partly because of time constraints. These small samples are now a target for study. Why was so little highlands materials found? Is the Front dominantly very friable material? A rough average composition appears to be some form of low-K Fra Mauro (LKFM, a low-KREEP basaltic composition), and there are some impact melts of this broad composition: these might represent Imbrium basin impact melt. We do not know the range of compositions in the highlands, although igneous ferroan anorthites, norites, and troctolites have been found. These cannot mix to produce the average; the LKFM composition has so far been 2 found as non-igneous rocks and its origin is a recurring question which investigation of the Front samples might solve. The regolith throughout the site contains highlands components, mostly in a cryptic form. Up to the present, petrographic studies of particle populations and synthesis of chemistry (especially mixing models) have not been particularly directed at defining the highlands materials. Not until the terra components are identified can the events and processes 'which formed them be deciphered. The common pre-mission interpretation of massif materiais forming the Front is of an Imbrium and Serenitatis basin origin. The sample suite is at present too poorly understood to adequately assess this interpretation, or whether other sources also provided Front material. Can material identified at the Apollo 17 site, e.g., the Serenitatis melt sheet, be identified among the Apollo 15 samples? Ejecta comprises older material: there are some deeply-derived, lower crustal (?) samples in the collection but their significance has not been adequately discussed. Basin-related rocks and ejecta can provide much information about multi-ring basin formation. Volcanic KREEP basalts were an unexpected discovery among the Apollo 15 samples. They are ubiquitous and numerous but small: only two are individually numbered rocks and the largest is 7.5 g. Their investigation is essential in shedding light on the development of KREEP on the moon. They have crystallization ages of ~ 3.85 b.y. and according to Sr-isotopic studies at least two distinct extrusions have been sampled. Their age cannot yet be distinguished from that of the Imbrium impact, but there is evidence that they are derived from the Apennine Bench Formation, hence are post-Imbrium. Was pressure-release significant in their genesis? The number of flows, their fractionation, and their origin is not yet known. How did they get distributed around the site as tiny fragments? - from beneath the local mare units or delivered laterally by rays? Why are their rare earth abundances so much lower than the Apollo 14 (brecciated) KREEP? How does the much older zircon age of the quartz-monzodiorite clasts in 15405 fit in with KREEP petrogenesis? A few workers remain un convinced of the origin of Apollo 15 KREEP as volcanic flows, suggesting instead that they are impact melts, perhaps from Imbrium itself. The Apollo 15 highlands, once its composition and stratigraphy have been established, offers a perspective on lunar basins, especially with inte- gration of information from other landing sites. One important potentially solvable question is the age of the Imbrium basin. Can we identify Imbrium basin ejecta at Apollos 14 and 16 and compare it with that of Apollo 15? What can cratering mechanics and . remote sensing tell us about the target stratigraphy? Understanding the relationship between Apollo 15 KREEP basalts and the Imbrium basin is of fundamental importance in establishing crustal responses to large impacts and the thermal state of the moon's crust at ~ 3.9 b.y. ago. How did the pronounced layering at Silver Spur form? Cratering mechanics and the lateral and vertical redistribution of crustal materials are intimately related. Understanding of Imbrium ejecta and its distribution is a profitable approach towards understanding these problems. Hadley Rille was an important target and has been well-described, but its origin as a lava channel or tube is not undisputed. But even if it is a lava . tube, the mechanism of its formation is unclear. It has not been clearly related to any of the sampled lavas. Can the features seen in its walls be 3 adequately correlated with the known characteristics of the rocks, for ' instance the thickness of flows as determined from samples? The rille might expose unsampled lava types. If so, then we need to explain how Apollo 15 ·!CREEP volcanics and the yellow volcanic glasses were distributed around the landing site without exposing them. Perhaps distinct mare volcanics do exist as small samples (e.g., coarse fines) but have not been recognized. There is a dark feature around the base of the _massifs which has been disputably interpreted as a "high lava" mark. Is there an episode of lava ponding recorded within the mare basalt samples? The landing site lies upon a topographic ridge, and to the north is the raised mound of the North Complex, a planned sampling location not eventually visited. Are these features mare-related or older (e.g., Apollo 15 !CREEP)? The inventory of basalt types has not necessarily been completed, because some small samples have not been adequately characterized and yet seem to be distinct. The olivine-normative and quartz-normative mare basalts are distinct in major element chemisty, yet have indistinguishable ages and isotopic systematics, and almost identical trace element patterns. The proper interpretation of this puzzling feature has never been addressed, yet surely is of deep significance for the petrogenesis of mare basalts in general. Several geochemists have suggested on the basis of small differences in trace element ratios that the two main mare basalts have sub-groups. If the existence of these sub-groups is verified, they have import for mantle processes or assimilations. Hadley Rille formation might include assimilation if it incorporated downcutting. Can recent suggestions that terrestrial komatrites assimilated older flows guide us in interpretations of Hadley Rille and the chemistry of the mare basalts? Where are the source vents for the lavas and what are they like? If the vents are some distance away, then it is quite likely that surface fractionati on has occurred and that the magmas as erupted have not been sampled. If the flows came any great distance, one might not expect volatiles in sufficient abundance to have created the 30 to 40% vesicularity of many of the olivine-normative mare basalts.