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From Magma Ocean to Crust Formation E Evolution, Lunar: From Magma topography of the lunar surface; new gravity Ocean to Crust Formation data; geochronology; geochemistry, especially isotopic constraints; and the abundances and Juliane Gross1,2,3 and Katherine H. Joy4 source reservoirs of lunar volatiles have brought, 1Department of Earth and Planetary Sciences, and continue to bring, valuable insights to under- Rutgers University, Piscataway, NJ, USA standing the Moon’s geological evolution (e.g., 2American Museum of Natural History, New Shearer et al. 2006; Elkins-Tanton et al. 2011; York, NY, USA Elardo et al. 2011; Zuber et al. 2013; Wieczorek 3Lunar and Planetary Institute, Houston, TX, et al. 2013; Borg et al. 2015). USA 4School of Earth and Environmental Sciences, The Lunar Magma Ocean Paradigm University of Manchester, Manchester, UK The early history and broad-scale petrogenesis of the Moon were glimpsed from the first manned and unmanned missions to the Moon that returned Introduction 382 kg of lunar rocks and soils from the lunar surface (Vaniman et al. 1991). These were col- The lunar crust provides a record of the planetary lected by the Apollo and Luna missions from the formation and early evolutionary processes and central and eastern nearside of the Moon (Fig. 1). contains a wealth of information about the origin Based on numerous studies of these returned and evolution of the Earth-Moon system (e.g., Apollo and Luna samples, hypotheses of the Taylor 1982; NRC 2007; Canup 2008, 2012; Moon’s differentiation and crust formation were Cuk and Stewart 2012; Young et al. 2016). Under- made. The most supported model for the forma- standing these processes is crucial for the recon- tion and evolution of the Moon is that of a giant struction of the early evolutionary stages of the impact between proto-Earth and another large Earth, e.g., the early geological evolution of a planetary body early in the Solar System history terrestrial planet, inner Solar System impact bom- (e.g., Canup 2008, 2012; Canup et al. 2015; Cuk bardment, and the solar and galactic environment and Stewart 2012), chemically mixing the two throughout the last 4.5 billion years (Ga) (e.g., bodies (e.g., Young et al. 2016). This produced a NRC 2007; Crawford et al. 2012). lunar magma ocean (LMO), although the global Our knowledge of the lunar highland crust has extent of this ocean and depth of melting that advanced enormously. Studies of lunar meteor- occurred is debated (Wood et al. 1970; Smith ites; experimental and computational studies; et al. 1970; Solomon 1986; Binder 1986; remote sensing of mineralogy, chemistry, and # Springer International Publishing AG 2016 B. Cudnik (ed.), Encyclopedia of Lunar Science, DOI 10.1007/978-3-319-05546-6_39-1 2 Evolution, Lunar: From Magma Ocean to Crust Formation Evolution, Lunar: From Magma Ocean to Crust For- positive Eu-anomalies (O’Hara and Nui 2015). It is impor- mation, Fig. 1 Clementine mission albedo map of the tant to note that the gamma-ray spectrometer data sample Moon in a cylindrical projection, overlain with locations of the upper 30 cm of the lunar regolith, and mapped regoliths the Apollo sample return missions (as indicated by will include a wide mix of igneous rocks, reworked impact crosses). (a) Map is overlain with the distribution and melt breccias, and impact glass over a scale of 60 km per concentration of regions with >2 ppm Th as mapped by pixel, so we also highlight in Fig. 2b regoliths with the Lunar Prospector mission (2 per pixel data calibration; <0.3 ppm Th, taken as equivalent to significant positive Prettyman et al. 2006) showing surface expression of Eu-anomalies akin to Apollo igneous FAN rocks KREEP-bearing lithologies. Major crustal terranes as (<0.15 ppm Th, Taylor 2009), bearing in mind the average mapped by Jolliff et al. (2000) are denoted as South Pole- uncertainty of the Lunar Prospector dataset (Prettyman Aitken (SPA) basin, feldspathic highland terrane (FHT) et al. 2006). An interesting point to note is that none of and Procellarum KREEP Terrane (PKT). (b) Map is over- the average Apollo landing site soils have positive Eu- lain with the distribution and concentration of regions with anomalies as they all contain soil components rich in <0.7 ppm Th (cyan-colored pixels) and <0.3 ppm Th negative Eu-anomalies-bearing KREEPy mafic impact (dark blue-colored pixels) (Prettyman et al. 2006). The melt breccias and/or mafic mare basalt material threshold <0.7 ppm Th is taken as regolith with no or a Evolution, Lunar: From Magma Ocean to Crust Formation 3 Evolution, Lunar: From Magma Ocean to Crust For- highlands seen on the lunar surface today. The residual mation, Fig. 2 Schematic sketch of the (a) differentiation magma, trapped between the anorthositic crust and the and (b) crystallization of the global lunar magma ocean underlying ultramafic mantle, continued to crystallize and (LMO). (b) The LMO crystallized mafic cumulates of late-stage melt became increasingly enriched in KREE- olivine and pyroxene crystals. These cumulates sank into P. For more details, see text (Modified from the classroom the interior to form the lunar mantle (Shearer et al. 2006 illustration of Jennifer Rapp (2013) from the Lunar and and refs. therein; Elardo et al. 2011). (c) After 75 to 80 % Planetary Institute; Center for Lunar Science and Explora- of LMO crystallization, plagioclase began to crystallize tion http://www.lpi.usra.edu/nlsi/training/illustrations/ (Snyder et al. 1992) and floated to the top of the LMO planetaryInteriors) producing the global anorthositic crust, the light-colored Pritchard and Stevenson 2000; Shearer et al. 2006 suggest that initialization of plagioclase crystalli- and refs. therein; Delano 2009). zation occurred at depths of >75 km in the LMO As this LMO cooled, crystals that precipitated (Nakvasil et al. 2015). Upon formation, this from the melt separated due to density differences low-density plagioclase overcame a density con- (Fig. 2). This has been modeled (e.g., Solomon trast with dense comagmatically crystallizing and Longhi 1977; Dreibus et al. 1977; Longhi mafic phases by buoyantly rising and migrating 1980, 2003; Tonks and Melosh 1990; Snyder toward the lunar surface, forming a thick “onion et al. 1992; Meyer et al. 2010; Elkins-Tanton shell” anorthosite crust (Warren 1990; Elkins- et al. 2011) and experimentally tested (Elardo Tanton et al. 2011) (Fig. 2). This crystal separation et al. 2011; Rapp and Draper 2012, 2013). These also removed the plagiophile compatible element models consider two end-member forms: (1) frac- Eu from the magma ocean melt, providing the tional crystallization from start to finish (the “one- anorthositic crust with a positive chondrite- stage model”) and (2) a “two-stage model” in normalized (cn) Eu-anomalies (i.e., Eu/Eu* >1, which early equilibrium crystallization occurred where Eu/Eu* = (Eucn /(Smcn +Gdcn)^0.5)) and followed by fractional crystallization of the resid- rocks that have low concentrations of incompati- ual magma ocean. In all models, Mg-rich olivine ble trace elements. Analyses of Apollo returned is the liquidus phase, followed by orthopyroxene samples show that this primary feldspathic crust is crystallization. These dense mafic (Mg-rich) min- composed of anorthositic rock with plagioclase erals sank and formed the mantle, enriching the An# (molar Ca/[Ca + Na + K]) of 94–98, with residual magma in iron and more incompatible mafic minerals that have a Mg# (molar Mg/[Mg + elements (e.g., Th, Ti, K). Fe]) ranging from 40 to 70, although varieties After 75–80 % of the LMO had solidified, within that range exist (Warren et al. 1983; James the calc-end-member of plagioclase feldspar et al. 1989; Jolliff and Haskin 1995; Nyquist (anorthite) began to crystallize. Recent studies et al. 2006) (Fig. 3a). These rocks are called the 4 Evolution, Lunar: From Magma Ocean to Crust Formation Evolution, Lunar: From Magma Ocean to Crust For- Anorthosite clasts in lunar feldspathic meteorites mation, Fig. 3 Graph of An (molar Ca/[Ca + Na + K]) in (Modified from Gross et al. (2014). Data from Warner plagioclase versus Mg# (molar Mg/[Mg + Fe]*100) in et al. (1976), Warren and Wasson (1979), Warren (1993), mafic minerals (olivine and pyroxene) in feldspathic rock and Meyer (2010) and from Gross et al. (2014) and refer- fragments in lunar samples. (a) Apollo samples. (b) ences therein) ferroan anorthosite suite (FAN; Fig. 3a). FANs are thorium (Th), potassium (K), rare earth elements common at the Apollo 16 highland landing site (REE), and phosphorus (P) that were not incorpo- and are also present at the mare regions visited by rated into the previously formed cumulates. This the other Apollo missions (Wood et al. 1970). residual melt is called the KREEP (K, REE, and P) The development of a thick “onion shell” anor- geochemical component (Warren and Wasson thosite crust would have likely caused a large 1979; Snyder et al. 1995; also see review by thermal blanketing effect on the lunar interior Shearer et al. 2006 and refs. therein; Taylor and (Shearer et al. 2006). The rate of cooling in the McLennan 2009). remaining molten lunar interior (be it the dregs of By the end of differentiation, the Moon had a a magma ocean, more localized melt environ- small dense core, an inner mantle made of Mg- ments, or Moon-wide partial melts) would have and Fe-rich silicate minerals (olivine and pyrox- rapidly slowed, and the melt itself would have ene), and an outer feldspathic crust, the so-called become increasingly FeO-rich and dense “primary” crust which makes up much of the (Warren 1990) and crystallized to yield abundant white highland areas of the Moon (Fig. 1). Numer- ilmenite (FeTiO3). Very late-stage residual melt ical models predict that upon solidification of the products (after 90–95 % of crystallization) would LMO, a density gradient existed, with denser have ponded and crystallized in the intermediate (cooler and more Fe-rich) cumulates near the sur- regions between the “buoyantly risen” face, under the buoyant anorthosite crust, com- anorthosite-rich crust and the “sunken” mafic pared to less dense cumulates (Mg-rich) at depth.
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