WORKSHOP ON THE GEOLOGY AND PETROLOGY OF THE APOLLO 15 LANDING SITE $ a ~ OLPI Technical Report Number 86-03 ~ a S LUNAR AND PLANETARY INSTITUTE 3303 NASA ROAD 1 HOUSTON, TEXAS 77058-4399 WORKSHOP ON THE GEOLOGY AND PETROLOGY OF THE APOLLO 15 LANDING SITE Edited by Paw D. Spudis and Graham Ryder Sponsored by The Lunar and Planetary Institute A Lunar and Planetary Institute Workshop November 13-15, 1985 LUNAR AND PLANETARY INSTITUTE 3303 NASA ROAD 1 HOUSTON, TEXAS 77058-4399 LPI Technical Report 86-03 Compiled in 1986 by the LUNAR AND PLANETARY INSTITUTE The Institute is operated by Universities Space Research Association under Contract NASW-4066 with the National Aeronautics and Space Administration. Material in this document may be copied without restraint for library, abstract service, educational or personal research purposes; however, republication of any portion requires the written permission of the authors as well as appropriate acknowledgment of this publication. This report may be cited as: Spudis, Paul D. and Ryder, Graham (1986) Workshop on the Geology and Petrology of the Apollo 15 lAnding Site. LPI Tech. Rpt. 86-03. Lunar and Planetary Institute, Houston. 126 pp. Papers in this report may be cited as: Author A. A. (1986) Title of paper. In Workshop on the Geology and Petrology of the Apollo 15 lAnding Site (Spudis, Paul D. and Ryder, Graham eds.), pp. xx-yy. LPI Tech. Rpt. 86-03. Lunar and Planetary Institute, Houston. This report is distributed by: LIBRARY /INFORMATION CENTER Lunar and Planetary Institute 3303 NASA Road 1 Houston, TX 770584399 Mail order requestors will be invoiced for the cost ofpostage and handling. Contents Preface 1 Workshop Rationale and Format 2 Program 5 Summary of Questions Formulated by the Program Committee 9 Discussion Summaries 11 Abstracts 27 Regolith erosion and regolith mixing at the Apollo 15 site on the Moon A. Basu 29 Apollo 15 mare units and their petrogenesis A. B. Binder 32 Comparison of petrology, grain sizes, and sur/ace maturity parameters for Apollo 15 regolith breccias and soils D. D. Bogard, D. S. McKay, R. V. Morris, P. Johnson and S. J. Wentworth 35 Apollo 15 lunar base site: Steep slopes as an energy resource J. D. Burke 38 Extraction of information from major element chemical analyses of lunar basalts J. C. Butler 44 Apollo 15 mare volcanism: Constraints and problems J. W. Delano 47 Characterization of the Apollo 15 feldspathic basalt suite R. F. Dymek 52 Hadley Rille, lava tubes, and mare volcanism at the Apollo 15 site R. G. Greeley and P. D. Spudis 58 The geologic history of quartz-normative and olivine-normative basalts in the vicinity of Hadley Rille (Apollo 15) T. L. Grove 62 Remote sensing of the Hadley-Apennine region B. R. Hawke 65 Petrology and geochemistry of highlands samples /rom the Apennine Front M. M. Lindstrom 70 Chemical components of the Apollo 15 regolith R. L. Korotev 75 Ultramafic parent magmas for mare basalts? J. Longhi 80 Spectral reflectance study of the Hadley-Apennine (Apollo 15) region P. G . Lucey and B. R. Hawke 83 Samples at the Apollo 15 landing site: Types and distribution G. Ryder 86 Apollo 15 mare basalts: A diverse suite or two distinct groups? P. A. Salpas and L. A. Taylor 91 Exotic components at Apollo 15: A relook at secondary cratering P. H. Schultz 94 Apollo 15 regolith breccias and soils: Comparative petrology and chemistry S. B. Simon, J. J. Papike and J. C. Laul 97 The materials and formation of the Imbrium Basin P. D. Spudis 100 The Apennine Bench Formation revisited P.O. Spudis and B. R. Hawke 105 Some observations on the geology of the Apollo 15 landing site G. A. Swann 108 The origin of pristine KREEP: Effects of mixing between urKREEP and the magmas parental to the Mg-rich cumulates P. H. Warren 113 Selection of the Apollo 15 landing site D. E. Wilhelms 116 Geologic setting of the Apollo 15 landing site D. E. Wilhelms 119 Participants 125 1 Preface This report documents the "Workshop on the Geology and . Petrology of the Apollo 15 Landing Site," held at the Lunar and Planetary Institute on November 13-15, 1985. This workshop was one of a series of workshops and topical conferences instigated by the Lunar and Planetary Sample Team (LAPST) to focus community attention on important and interesting topical problems in lunar science. Prior to the workshop, the conveners published a review paper on Apollo 15 site geology as it was then understood (P. D. Spudis and G. Ryder, EOS, Trans. AGU, v. 66, no. 43, pp. 721-726, 1985) and one of the conveners had produced a new, comprehensively annotated catalog of the Apollo 15 rock samples (G. Ryder, Catalog of Apollo 15 Rocks, Curatorial Branch Publication 72, NASA-JSC 10787, 1295 pp., 1985). Both of these documents set the stage for a lively and productive workshop that attempted to define and tackle some major lunar geologic problems and processes from the perspective of one of the most beautiful and fascinating lunar landing sites: the Hadley-Apennine region. The presence of both Dave Scott and Jim Irwin, the astronauts who explored the landing site, was an unprecedented and stimulating factor for such a workshop. 2 Workshop Rationale and Format P. D. Spudis and G. Ryder The geology of the Apollo 15 landing site has remained 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 was often felt to have received short shrift and to have been overshadowed by the succeeding Apollo 16 landing. It had never had a "conference of its own" at which multidisciplinary approaches could focus on its scientific opportunities. There was a perception that there were glaring deficiencies in our understanding that would be remedied by a multidisciplinary examination of the Apollo 15 landing site, especially in the light of the results from other missions. In the following paragraphs we summarize, in order of the workshop topics, some of the rationale and questions, perceived before the workshop, which it and subsequent studies might at least partly answer. Sampling of the Apennine Front was the prime target of the Apollo 15 mission, yet its petrology has remained 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 anorthosites, norites, and troctolites have been found. These cannot mix to produce the average; the LKFM composition has so far been found as non·igneous rocks, and its origin is a recurring question that investigation of the Front samples might solve. The regolith throughout the site contains highlands components, mostly in 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 that formed them be deciphered. The common pre· mission interpretation of massif materials 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 ~.8S 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 unconvinced of the origin of ApoUo 15 KREEP as volcanic flows, suggesting instead that they are impact melts, perhaps from Imbrium itself.