Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...1..559w&link_type=abstract
Treatise on Geochemistry, Volume 1. Editor: Andrew M. Davis. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 71
Mathematics
Logic
7
Scientific paper
Oxygen isotopic data suggest that there is a genetic relationship between the constituent matter of the Moon and Earth (Wiechert et al., 2001). Yet lunar materials are obviously different from those of the Earth. The Moon has no hydrosphere, virtually no atmosphere, and compared to the Earth, lunar materials uniformly show strong depletions of even mildly volatile constituents such as potassium, in addition to N2, O2, and H2O (e.g., Wolf and Anders, 1980). Oxygen fugacity is uniformly very low ( BVSP, 1981) and even the earliest lunar magmas seem to have been virtually anhydrous. These features have direct and far-reaching implications for mineralogical and geochemical processes. Basically, they imply that mineralogical diversity and thus variety of geochemical processes are subdued; a factor that to some extent offsets the comparative dearth of available data for lunar geochemistry.The Moon's gross physical characteristics play an important role in the more limited range of selenochemical compared to terrestrial geochemical processes. Although exceptionally large (radius=1,738 km) in relation to its parent planet, the Moon is only 0.012 times as massive as Earth. By terrestrial standards, pressures inside the Moon are feeble: the upper mantle gradient is 0.005 GPa km -1 (versus 0.033 GPa km -1 in Earth) and the central pressure is slightly less than 5 GPa. However, lunar interior pressures are sufficient to profoundly influence igneous processes (e.g., Warren and Wasson, 1979b; Longhi, 1992, 2002), and in this sense the Moon more resembles a planet than an asteroid.Another direct consequence of the Moon's comparatively small size was early, rapid decay of its internal heat engine. But the Moon's thermal disadvantage has resulted in one great advantage for planetology. Lunar surface terrains, and many of the rock samples acquired from them, retain for the most part characteristics acquired during the first few hundred million years of solar system existence. The Moon can thus provide crucial insight into the early development of the Earth, where the direct record of early evolution was effectively destroyed by billions of years of geological activity. Lunar samples show that the vast majority of the craters that pervade the Moon's surface are at least 3.9 Gyr old (Dalrymple and Ryder, 1996). Impact cratering has been a key influence on the geochemical evolution of the Moon, and especially the shallow Moon.The uppermost few meters of the lunar crust, from which all lunar samples are derived, is a layer of loose, highly porous, fine impact-generated debris - regolith or lunar "soil." Processes peculiar to the surface of an atmosphereless body, i.e., effects of exposure to solar wind, cosmic rays, and micrometeorite bombardment, plus spheroidal glasses formed by in-flight quenching of pyroclastic or impact-generated melt splashes, all are evident in any reasonably large sample of lunar soil (Lindsay, 1992; Keller and McKay, 1997; Eugster et al., 2000). The lunar regolith is conventionally envisaged as having a well-defined lower boundary, typically 5-10 m below the surface ( McKay et al., 1991); below the regolith is either (basically) intact rock, or else a somewhat vaguely defined "megaregolith" of loose but not so finely ground material. Ancient highland terrains tend to have a regolith roughly 2-3 times than that of the maria ( Taylor, 1982). However, in much of the highlands the regolith/megaregolith "boundary" may be gradational. The growth of a regolith can approach a steady-state thickness by shielding its substrate against further impacts ( Quaide and Oberbeck, 1975), but there is no reason to believe that the size-frequency spectrum of impactors bombarding the Moon ( Melosh, 1989; Neukum et al., 2001) features a discontinuity at whatever size (of order 1-10 m) would be necessary to limit disintegration to ˜10 m.All lunar samples are from the regolith, so the detailed provenance of any individual lunar sample is rarely obvious; and for ancient highland samples, never obvious. The closest approach toin situ sampling of bedrock came on the Apollo 15 mission. The regolith is very thin near the edge of the Hadley Rille, and many samples of clearly comagmatic basalts were acquired within meters of their 3.3 Ga "young," nearly intact, lava flow, so that their collective provenance is certain (Ryder and Cox, 1996). Even the regional provenance of any individual lunar sample is potentially allocthonous. However, most lunar rocks, even ancient highland rocks, are found within a few hundred kilometers of their original locations. This conclusion stems from theoretical modeling of cratered landscapes ( Shoemaker et al., 1970; Melosh, 1989), plus observational evidence such as the sharpness of geochemical boundaries between lava-flooded maria and adjacent highlands (e.g., Li and Mustard, 2000).Besides breaking up rock into loose debris, impacts create melt. Traces of melt along grain boundaries may suffice to produce new rock out of formerly loose debris; the resultant rock would be classified as either regolith breccia or fragmental breccia, depending upon whether surface fines were important, or not, respectively, in the precursor matter (Stöffler et al., 1980). Features diagnostic of a surface component include the presence of glass spherules (typically a mix of endogenous mare-pyroclastic glasses and impact-splash glasses) or abundant solar-wind-implanted noble gases (e.g., Eugster et al., 2000).Elsewhere, especially in the largest events in which a planet's gravitational strength limits displacement and the kinetic energy of impact is mainly partitioned into heat (Melosh, 1989), impact melt may constitute a major fraction of the volume of the material that becomes new rock. Rocks formed in this manner are classified as impact-melt breccias and subclassified based on whether they are clast-poor or clast-rich, and whether their matrix is crystalline or glassy ( Stöffler et al., 1980). Obvious lithic and mineral clasts are very common in impact-melt breccias, although the full initial proportion of clasts may not be evident in the final breccia. Some of the clasts may be so pulverized, especially in large impact events ( Schultz and Mendell, 1978), that they are "lost" by digestion into comingled superheated impact melt ( Simonds et al., 1976). By some definitions, the term impact-melt breccia may be applied to products of melt plus clast mixtures with initial melt proportion as low as 10 wt.% ( Simonds et al., 1976; Papike et al., 1998).A few impactites feature a recrystallized texture, i.e., they consist dominantly of a mosaic of grains meeting at ˜120° triple junctions. These metamorphic rocks, termed granulitic breccias, may form from various precursor igneous or impactite rocks, and the heat source may be regional (burial) or local, such as a nearby impact melt (Stöffler et al., 1980). But lunar granulitic breccias are almost invariably fine grained, and they tend to be "contaminated" with meteoritic siderophile elements (e.g., M. M. Lindstrom and D. J. Lindstrom, 1986; Warren et al., 1991; Cushing et al., 1999), implying that the precursor rocks were probably mostly shallow impact breccias (brecciation and siderophile-element contamination being concentrated near the surface), and the heat source was probably most often a proximal mass of impact melt.Besides impactites, which are predominant near the bombarded surface, virtually all other lunar crustal rocks are igneous or annealed-igneous. The super-arid Moon has never produced (by any conventional definition) sedimentary rock, and most assuredly has never hosted life. Even metamorphism is of reduced scope, with scant potential for fluid-driven metasomatism. Evidence for metamorphism among returned lunar samples is mostly confined to impact shock and thermal effects. Although regional burial metamorphism may occur (Stewart, 1975), deeply buried materials seldom find their way into the surface regolith, whence all samples come. Annealing of lunar rocks is more likely a product of simple postigneous slow cooling (at significant original
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