NEEP602 Course Notes (Fall 1996)
Resources from Space
American Mineralogist, Volume 76, pages 773-784, 1991
Evolution of the Moon: Apollo model (continued)
HARRISON H. SCHMITT
P.O. Box 14338, Albuquerque, New Mexico 87191, U.S.A
Mare basalt: 3.83.0(?) eons
Surface eruption and subsurface intrusion of mare basalt magmas (see reviews in Basaltic Volcanism Study Project, 1981; Wilhelms, 1987, chapter 5) became a significant crustal phenomena about 3.8 eons (Wilhelms, 1987, chapter 10). Surface eruptions probably appeared first at a selenopotential low in the vicinity of northern Tranquillitatis and southern Serenitatis with subsequent eruptions appearing in roughly concentric zones (Schmitt, 1974, 1975a, p. 266) around this low. The offset of the center of mass of the Moon from its center of figure toward the Earth (Kaula et al., 1974), caused by asymmetrical thinning of the crust during the large basin stages, would have created this selenopotential low and the resulting asymmetrical distribution of surface mare basalts (Taylor, 1982, p. 344345). Progressively younger upward migrations of mare basalt magmas from the mantle would encounter a largely sealed surface near preceding eruptive regions and be forced concentrically away from the selenopotential low. Basin and highland distributions would produce the observed perturbations to idealized concentrlc zomng.
Exact upper and lower age limits for the mare basalt stage will remain uncertain (see Basaltic Volcanism Study Project, 1981; Taylor et al., 1983; Schultz and Spudis, 1983; Wilhelms, 1987) until field studies take place in the major unvisited mare regions of the Moon, including Mare Procellarum, Mare Orientale, and the mare and highlands of the lunar far side. The Apollo 14 breccia clast with mare-related chemistry, a cumulate texture, and an age of 4.2 eons (Taylor, 1982) deserves particular emphasis in this regard. Clearly, however, the span of 3.8 to 3.0 eons encompasses the major identified portion of eruptive as well as intrusive activity involving mare basalt magma (see Taylor, 1982, p. 319; Wilhelms, 1987, chapters 10-13).
Early precursors of the mare basalt stage also may have contributed to light plains deposits. The surface character of light plains in some farside basins observed by the author (unpublished data, 1972) suggests pyroclastic eruptions dominated by crustal debris. Relevant surface characteristics include irregular rimless depressions suggestive of terrestrial pyroclastic vents and collapse features.
Although volatile depletion of the magma ocean appears to have been extensive, some residual volatiles remained as indicated by the vesicular nature of many mare basalts. Evolved with low melting point components before the first true mare basalt magmas in each mantle source zone, such residual volatiles would be expected to entrain crustal debris as they move upward to erupt at various times prior to basaltic magma extrusion or intrusion into any given province. With this scenario, the earliest coherent magmas also would have been charged with volatiles, as found for certain highly fractionated kimberliteminette eruptive sequences on Earth (Schmitt and Swann, 1968). Upon assimilating variable quantities of anorthositic crustal debris, possibly including some KREEP components, early precursor magmas may account for some of the preImbrium (older than 3.8 eons) basalts that are highly fractionated and Al rich.
Late-stage debris eruptions producing similar morphological characteristics, as seen on light plains deposits, also may have occurred in various provinces as subsurface mare basalt intrusions cooled and released volatiles, particularly beneath the lunar highlands. Vesicle assemblages in many mare basalts (Schmitt et al., 1970) further suggest a largely unreactive volatile phase that might drive late eruptions from cooling intrusives. Such eruptions may help explain some of the age relationships of dark halo and dark mantling deposits not associated directly with extrusive mare basalt (Schmitt et al., 1967; Head and McCord, 1978; Head and Wilson, 1979; Wilhelms, 1987, p. 89).
The lowest melting components of the lunar mantle probably were approaching their melting points by 3.8 eons because of the accumulation of radiogenic heat trapped by the highly insulating crust. Unloading of the mantle by the formation of young large basins, however, possibly controlled the specific timing of some early mare basalt eruptions (Schmitt, 1975a). As discussed by Taylor (1975), large scale impact melting does not appear to be a viable source for the mare basalts.
Strong evidence exists that individual mare basalt flows underwent considerable differentiation in place (Schmitt et al., 1970; Schmitt and Sutton, 1971; Basaltic Volcanism Study Project,1981). Both the settling of dense crystals and ironsulfide liquid and the flotation of plagioclase and vesicle assemblages constitute demonstrated processes for significant differentiation in mare basalt magma, the chemical evidence for which has been well documented (James and Wright, 1972; Rhodes et al., 1977). As suggested by Schmitt (1975a), during periods of rapid mare basalt eruption, the thickness of cooling units may be as much as hundreds of meters or more (see Wilhelms, 1987, p. 99101), leading to the possibility of layered complexes comparable to large stratified and resourcerich igneous bodies on Earth.
Schmitt's (1975a, p. 267) analysis ofthe significance of REE distributions among mare basalts of various ages suggests that the remelting of the lunar mantle proceeded inward from zones near the base of the insulating crust. Such a process also would be suggested by pressure considerations and the probable locations of concentrations of radiogenic heat after differentiation of the melted shell. A definitive determination of the sequence of melting depths for the mare basalts, however, has not been accomplished (see Wilhelms, 1987, p. 102).
The discovery of the orange and black pyroclastic materials in the rim of Shorty Crater during the Apollo 17 mission to the Valley of Taurus-Littrow (Schmitt and Cernan,1972; Schmitt,1973) and subsequent recognition of orange materials as a major component of dark mantling deposits at the southwestern edge of Mare Serenitatis (Schmitt and Evans, 1972; Lucchitta and Schmitt, 1974) added excitement, puzzles, and critical new information to our studies of the Moon. The excitement came from the discovery of what, at the time, appeared to be volcanic material the Apollo 17 science team had speculated might be found at a dark halo crater named Shorty. The puzzles emerged when, after the socalled orange soil had been examined carefully on Earth, the conclusion had to be that orange glass beads of pyroclastic origin, 3.6 eons old and underlain by their black devitrified equivalent, somehow had been emplaced in the rim and ejecta blanket of an impact crater 80 m in diameter with only minor contamination by other material (Wilhelms, 1987, Table 11.3; Apollo Field Geology Investigation Team, 1973; Heiken et al., 1974). The critical new information was that the source region for the magma that formed these materials, and others like them at other sites, must be the deep interior of the Moon, possibly below the region included in the original melted shell (Schmitt, 1975a, p. 267; Delano, 1980).
The initial observation of the orange soil came as the result of a serendipitous interaction of mission planning, field observation, and the power of suggestion. The dark halo crater, Shorty, had been thought to be either an impact crater or a volcanic crater that penetrated the lightcolored materials of a probable avalanche. If Shorty were a volcanic vent, local effluents might have altered surrounding material in some recognizable way. Even though visual inspection of Shorty disclosed that it was clearly an impact crater that had penetrated the avalanche and ejected underlying dark material as well as mare basalt fragments, thought processes that tested hypotheses on the origin of Shorty Crater played a key role in calling attention to the very light orange coloration in the thin regolith covering the main orange glass deposit.
A geologic model for the emplacement of the orange and black beads (Schmitt, 1989) comes from personal observations of the effects of the explosion of 500 tons of ammonium nitrate (1971 Dial Pack event) near Medicine Hat, Alberta. This explosion caused the pressurization of underlying H2O-saturated sand and the eruption of the H2Osand mixture along conduits in radial and circumferential fractures cutting the rim and ejecta blanket of the explosion crater. Table 7 summarizes the sequence of analogous events that may have taken place in the vicinity of Shorty Crater.
Experimental data related to lunar pyroclastic glasses (see Delano, 1980) suggest that their magma source region lies 400-500 km below the surface. Further, the geochemistry of the glasses suggests that their magmas came from differentiated material at these depths (see Hughes et al., 1989). On the other hand, the noncrustal and primitive nature of the volatiles adsorbed on the surfaces of the orange and black beads (Meyer et al., 1975; Tera and Wasserburg, 1976; Tatsumoto et al,, 1976) indicate a source region for these volatiles below the zones affected by the loss of volatiles during the magma ocean stage. These seemingly conflicting constraints can be reconciled by postulating the upward migration of volatiles from the primitive lower mantle into the magma source regions and possibly assisting both the partial melting and the upward movement of the integrated magma. Alternatively, Delano (1980) has suggested that the source region may straddle the boundary between differentiated and primordial materials.
The presence of volatiles below 400-500 km also would suggest that their deep source materials remained relatively cool during the processes that formed the Moon and had not been subject to significant earlier differentiation. The possible existence of a density reversal below 400-480 km, a reasonable depth of differentiated crust and mantle, supports this possibility. These potential constraints on lunar composition and structure limit possible mechanisms for the formation of the Moon. In fact, they suggest formation of the moon as an initially cool contemporaneous partner of the Earth rather than derivation from the already differentiated Earth as suggested by others (Hartmann, 1986).
Structural adjustments related to the large basins continued well into the mare basalt stage. For example, in the Valley of Taurus-Littrow, the original southeastnorthwest bounding faults between the valley and the massifs are high angle normal faults formed during dilation of the crust immediately after impact (Apollo Field Geology Investigation Team, 1973; Wolfe et al., 1981); that is, the valley began as graben roughly radial to the center of Serenitatis. A lack of talus and the presence of moatlike depressions at the base of the South Massif (Scott et al., 1972; Schmitt, 1973) may indicate postmare isostatic adjustment by reversal of movement with the massif moving relatively downward and separating from the valley's mare basalt fill. This reversal also can be viewed as progressive southwestnortheast extension along the South Massif's bounding fault (Schmitt, 1975a, p. 276) after the major episode of basin filling by mare basalt. Extension with this orientation also would be consistent with eastward thrusting having produced the northsouth trending LeeLincoln scarp.
Late graben structures spatially associated with pyroclastic deposits near the southwestern rim of Serenitatis in the Sulpicius Gallus region (Carr, 1966) appear to be a more definitive indication of extensional adjustment. ArkaniHamed (1989) has suggested that extensional faults related to these basinedge stresses could propagate to great depth. This particularly might be true in a post-mare basalt upper mantle made increasingly brittle by cooling and magma withdrawal. If so, such faults could act as conduits for pyroclastic glasses from deep source regions.
The mare basalt stage completed the major changes during lunar evolution. Table 8 summarizes the additional changes that had occurred at its conclusion.
Mature surface: 3.0(?) eons to present
The end of major eruptions of mare basalt in Eratosthenian time at about 3.0 eons (Wilhelms, 1987, p. 258-262) ushered in a stage of lunar evolution when only minor changes to the surface occurred (see Wilhelms, 1987, Plates 1011) and completed the major evolutionary sequence on the Moon. Although impact events continued and declined to approximately present frequency levels early in this stage (Wilhelms, 1987, Fig. 7.16), they did little to change the face of the Moon as seen for the last 3 b.y. except for the excavation of the relatively young rayed craters and the deepening, mixing, and maturation of surface regolith. The type example for the few major events of this stage has been that which created the crater Copernicus (see Shoemaker, 1962; Schmitt et al., 1967) about 0.85 eon ago (Silver, 1971).
The bright swirls that appear to have altered all older materials (Schmitt and Evans,1972; Wilhelms,1987, Figs. 4.7, 10.32, and 12.10) and to correlate with high amplitude magnetic anomalies (Hood et al., 1981) constitute one significant puzzle remaining to be solved that may relate to the mature stage of lunar evolution. Some bright swirls appear also to correlate with potential stress induced at the antipodes of large impacts, such as Imbrium and Orientale. Bright swirls in other regions, particularly on Mare Procellarum and throughout the highlands east of Smythii, appear to have no obvious correlations.
Many bright swirls in the highlands east of Smythii have interior zones with darker albedos than the surrounding highlands (Schmitt, unpublished mission notes). These and other bright swirls may be evidence of volatiles released as the lower mantle of the Moon reached its maximum general temperature as a result of radiogenic heat accumulation in relatively undifferentiated material. This same thermal event may in some way correlate with the disappearance of the lunar magnetic field (see Taylor, 1982, p. 368370) if that field originally related to some manifestation of a permanent magnet.
Perhaps the most significant processes of the mature surface stage are related to the local effects of continued primary and secondary impact cratering (see Wilhelms, 1987, Chapters 12 and 13). For example, in addition to many sizes and types of impact craters related to this stage, Apollo 17 (Schmitt and Cernan,1972) investigated the effects of an avalanche offthe side of the South Massif, probably induced by the impact of secondary material from Tycho (Apollo Field Geology Investigation Team, 1973; Wolfe et al., 1981).
Informal premission photographic and analytical interpretation (H. H. Schmitt and R. Shreve, unpublished discussions, 1972) of the plumelike deposit of light colored matenal extending 6 km away from the base of the South Massif produced a plausible working hypothesis for this "light mantle's" origin. Formation as a gaslubricated and suspended avalanche of debris off the side of the 2200m high mountain appeared most likely. Energy considerations led Shreve to conclude that simple landslide mechanics could not have moved debris as far as observed. Thus, it seemed likely that solar wind gases, particularly H, had been released by particle abrasion as the flow of talus debris began to move down the side of the South Massif.
This hypothesis in turn led to a search for supporting evidence while exploring its surface (Schmitt and Cernan, 1972). In situ size screening of both talus and light mantle surface debris showed a distinctly lower frequency of rock fragments larger than about 2 cm in diameter on the light mantle. Consistent with this, visual observations from the Lunar Rover disclosed that the size of boulders excavated by impacts into the avalanche material correlated roughly to the diameter, and therefore the depth of penetration, of the impact crater. These relationships suggest that the deposit was sorted vertically by size as would be expected in debris suspended by gas during transport.
Table 9 summarizes the changes by which the mature surface stage completes this model of lunar evolution.
CONCLUSION
The Apollo era began the modern process of understanding the evolution of the Moon. Through that remarkable effort on the part of national leaders, managers, engineers, workers, industry, a worldwide community of scientists, and the American people, we also have looked with new insight at our own planet and the other terrestrial planets. The Apollo exploration of the Moon stimulated investigations that now reach toward real understanding of the early differentiation of the planets, the nature of their internal structure, the environmental dynamics at the origin of life more than 3.5 eons ago, the geochemical and biological influence of very large impact events, and the effects of early partial melting in protomantles. Further, the Apollo explorations wcre of incalculable value in adding the reality of known materials and processes to the interpretation of subsequent automated exploration of the solar SyStem. It has heen rccently noted (Wittenberg Ct al., 1986; Kulcinski and Schmitt, 1987) that, early in the third millennium, Apollo's discovery of concentrations of 3He and other solar wind gases in the Moon's regolith may lead to vast and environmentally benign energy resources required by Earth and to consumables required for Martian settlement.
ACKNOWLEDGMENTS
The author's own involvement in planetary science began under the irreplaceable influence of friends and teachers such as H.A. Schmitt, R.H. Jahns, lan Campbell, L.T. Silver, R.P. Sham, T.N. Irvine, T.F.W. Barth, H.E. Mckinstry, and R.M. Garrels. Later, interactions continued with many of these same people as well as G.J. Wasserburg, H.P. Taylor, E.M. Shoemaker, G.A. Swann, W.R. Muelhberger, Harold Masursky, D.E. Wilhelms, P.W. Gast, W.C. Phinney, Farouk ElBaz, J.W. Head, and many others who collectively illustrate the universal and synergistic nature of modem science. He most currently is indebted to J.W. Delano and P.H. Warren for their diligent and immeasurably helpful review of the manuscnpt of this paper. Standing at the forefront of these giants in our science, however, one finds the man to whom this issue of American Mineraloglst is dedicated. To Jim Thompson, we all say thank you for giving so much of yourself and your insights to so many (see, e.g., Schmitt, 1963). Best wishes for the future.
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MANUSCRIPT RECEIVED FEBRUARY 1, 1990
MANUSCRIPT ACCEPTED FEBRUARY 25, 1991
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