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Warren and Taylor-2014-In Tog-The Moon-'Author's Personal Copy'.Pdf This article was originally published in Treatise on Geochemistry, Second Edition published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non- commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Warren P.H., and Taylor G.J. (2014) The Moon. In: Holland H.D. and Turekian K.K. (eds.) Treatise on Geochemistry, Second Edition, vol. 2, pp. 213-250. Oxford: Elsevier. © 2014 Elsevier Ltd. All rights reserved. Author's personal copy 2.9 The Moon PH Warren, University of California, Los Angeles, CA, USA GJ Taylor, University of Hawai‘i, Honolulu, HI, USA ã 2014 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by P. H. Warren, volume 1, pp. 559–599, © 2003, Elsevier Ltd. 2.9.1 Introduction: The Lunar Context 213 2.9.2 The Lunar Geochemical Database 214 2.9.2.1 Artificially Acquired Samples 214 2.9.2.2 Lunar Meteorites 214 2.9.2.3 Remote-Sensing Data 215 2.9.3 Mare Volcanism 216 2.9.3.1 Classification of Mare Rocks 216 2.9.3.2 Chronology and Styles of Mare Volcanism 218 2.9.3.3 Mare Basalt Trace Element and Isotopic Trends 224 2.9.4 The Highland Crust: Impact Bombardment and Early Differentiation 227 2.9.4.1 Polymict Breccias and the KREEP Component 227 2.9.4.2 Bombardment History of the Moon 229 2.9.4.3 Impactor Residues: Siderophile and Fragmental 230 2.9.4.4 Pristine Highland Rocks: Distinctiveness of the Ferroan Anorthositic Suite 231 2.9.4.5 The Magma Ocean Hypothesis 235 2.9.4.6 Alternative Models 237 2.9.5 Water in the Moon 238 2.9.5.1 Traditional View of a Dry Moon 238 2.9.5.2 Water in Pyroclastic Glasses 239 2.9.5.3 Water in Apatite in Mare Basalts and KREEP-Related Samples 239 2.9.5.4 Water in the Lunar Mantle 240 2.9.5.5 Implications of Water in the Lunar Interior 240 2.9.6 The Bulk Composition and Origin of the Moon 241 Acknowledgments 242 References 242 2.9.1 Introduction: The Lunar Context Longhi, 1992, 2003; Warren and Wasson, 1979b), and in this sense, the Moon more resembles a planet than an asteroid. Stable isotopic data suggest a remarkably similar pedigree for Another direct consequence of the Moon’s comparatively the constituent matter of the Moon and Earth, given the great small size was early, rapid decay of its internal heat engine. But isotopic diversity among sampled components of the solar the Moon’s thermal disadvantage has resulted in one great system (Warren, 2011; Zhang et al, 2012). Yet, lunar materials advantage for planetology. Lunar surface terrains, and many are obviously different. The Moon has no hydrosphere and of the rock samples acquired from them, retain for the most virtually no atmosphere, and lunar materials show strong part characteristics acquired during the first few hundred Ma of depletions of even mildly volatile constituents, not just N2, solar system existence. The Moon can thus provide crucial O , and H O (e.g., Wolf and Anders, 1980). Oxygen fugacity insight into the early development of the Earth, whereas the 2 2 is uniformly very low (BVSP, 1981; Nicholis and Rutherford, terrestrial record of early evolution was largely destroyed by 2009). These idiosyncrasies have direct and far-reaching impli- billions of years of geological activity and the first 500 Ma of cations for mineralogy and geochemical processes. Basically, Earth history are missing altogether. Lunar samples show that they imply that mineralogical (and thus process) diversity is the vast majority of the craters that pervade the Moon’s surface subdued, a factor that to some extent offsets the comparative are at least 3.9 Ga old (Norman et al., 2006). Impact cratering dearth of available data for lunar geochemistry. has been a key influence on the geochemical evolution of the The Moon’s gross physical characteristics play an important Moon, and especially the shallow Moon. The popular giant role. Although exceptionally large (radius¼1738 km) in rela- impact model holds that the Moon originated as a form of tion to its parent planet, the Moon is only 0.012 times as impact spall after a collision between the proto-Earth and a massive as the Earth. By terrestrial standards, pressures inside the doomed Mars-sized (or larger) body (e.g., Cameron and Ward, À Moon are feeble: The upper mantle gradient is 0.005 GPa km 1 1976; Canup, 2004). À (vs. 0.033 GPa km 1 in Earth), and the central pressure is The uppermost few meters of the lunar crust, from which slightly less than 5 GPa. However, lunar interior pressures are all lunar samples derive, are a layer of loose, highly porous, sufficient to significantly influence igneous processes (e.g., fine impact-generated debris – regolith or lunar ‘soil.’ Processes Treatise on Geochemistry 2nd Edition http://dx.doi.org/10.1016/B978-0-08-095975-7.00124-8 213 Treatise on Geochemistry, Second Edition, (2014), vol. 2, pp. 213-250 Author's personal copy 214 The Moon peculiar to the surface of an atmosphereless body, that is, effects such as a nearby impact melt (Cushing et al., 1999). But lunar of exposure to solar wind, cosmic rays, and micrometeorite granulitic breccias are almost invariably fine grained, and they bombardment, plus spheroidal glasses formed by in-flight tend to be ‘contaminated’ with meteoritic siderophile elements quenching of pyroclastic or impact-generated melt splashes, all (e.g., Cushing et al., 1999; Lindstrom and Lindstrom, 1986; are evident in any reasonably large sample of lunar soil (Eugster Warren et al., 1991b; however, cf. Treiman et al., 2010), so the et al., 2000; Keller and McKay, 1997). The lunar regolith is precursor rocks were probably mostly shallow impact breccias conventionally envisaged as having a well-defined lower bound- (brecciation and siderophile contamination being concen- ary, typically about 5–10 m below the surface (McKay et al., trated near the surface), and the heat source was probably 1991a); below the regolith is either (basically) intact rock or most often a proximal mass of impact melt. else a somewhat vaguely defined ‘megaregolith’ of loose but not Besides impactites, which are predominant near the bom- so finely ground-up material. Ancient highland terrains tend to barded surface, virtually all other lunar crustal rocks are igne- have roughly two to three times thicker developments of rego- ous or annealed-igneous. The superarid Moon has never lith than maria (Taylor, 1982). produced (by any conventional definition) sedimentary rock All lunar samples are derived through the regolith, so the and most assuredly has never hosted life. Even metamorphism detailed provenance of any individual lunar sample is rarely is of reduced scope, with scant potential for fluid-driven meta- obvious, and for ancient highland samples, never obvious. The somatism. Sampled lunar metamorphism is virtually confined closest approach to in-place sampling of bedrock came on the to impact-shock and thermal effects. Although regional burial Apollo 15 mission, when many tens of clearly comagmatic metamorphism may occur (Stewart, 1975), deeply buried basalts were acquired within meters of their 3.3 Ga ‘young’ materials seldom find their way into the surface regolith, (and thus nearly intact) lava flow, so that their collective prov- whence all samples come. Annealing, among lunar samples, is enance is certain (Ryder and Cox, 1996). Lunar meteorites show more likely a product of simple postigneous slow cooling that impacts occasionally eject rocks clear off the Moon. How- (at significant original depth), or dry baking in proximity to ever, in a statistical way, most lunar rocks, even ancient highland an intrusion, or baking within a zone of impact heating. rocks, are found within a few hundred kilometers of their orig- The Moon’s repertoire of geochemical processes may seem inal locations. This conclusion stems from theoretical modeling limited and weird, but the Moon represents a key link between of cratered landscapes (Melosh, 1989; Shoemaker et al., 1970), the sampled asteroids and the terrestrial planets. Four billion plus observational evidence, such as the sharpness of geochem- years ago, at a time when all but microscopic bits of the ical boundaries between lava-flooded maria and adjacent Earth’s dynamic crust were fated for destruction, most of the highlands (e.g., Li and Mustard, 2000). Moon’s crust had already achieved its final configuration. Besides breaking up rock into loose debris, impacts create The Moon thus represents a unique window into the early important proportions of melt. A trace of melt along grain thermal and geochemical state of a moderately large object boundaries may suffice to produce new rock out of formerly in the inner solar system and into the cratering history of loose debris; the resultant rock would be classified as either near-Earth space. regolith breccia or fragmental breccia, depending upon whether surface fines were important in the precursor matter 2.9.2 The Lunar Geochemical Database (Sto¨ffler et al., 1980). Features diagnostic of a surface compo- nent include a smattering of glass spherules (of order 0.1 mm 2.9.2.1 Artificially Acquired Samples in diameter; typically a mix of endogenous mare-pyroclastic Six Apollo missions acquired a total of 382 kg of rocks and soil.
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