Ann. Rev. Earth planet. Sci. 1985. 13: 201-40 Copyright © 1985 by Annual Reviews Inc. All rights reserved THE MAGMA OCEAN CONCEPT AND LUNAR EVOLUTION Paul H. Warren Institute of Geophysics and Planetary Physics, Department of Earth and Space Sciences, University of California, Los Angeles, California 90024 INTRODUCTION Despite its small size overall, the Moon has an igneously differentiated crust roughly 55-75 km thick (Toksoz 1979). The oldest lunar crust is anortho­ sitic, with an average (depending on petrogenetic model) of 75-95% plagioclase (Ca,Na-aluminosilicate). One way that anorthosite may form is if plagioclase, which has a density of 2. 7 g cm - 3, floatsas it crystallizes from denser basaltic magma. There is little doubt that plagioclase is buoyant in anhydrous lunar magmas (e.g. Walker & Hays 1977, Warren & Wasson 1979b). The global distribution and purity of the anorthosites, produced during a time span of < 300 Myr, suggest flotation over a magma "ocean" -a global, near-surface shell of magma, tens or hundreds of kilometers thick. "Magma ocean" is probably a hyperbole. Among other things, "ocean" implies the system is virtually 100% liquid, with a mainly gas/liquid upper by University of Hawaii at Manoa Library on 08/19/11. For personal use only. surface and a waterlike viscosity of the order of 10-2 poise. Warren (1984b) suggests that in most contexts the more general term "magmasphere" Annu. Rev. Earth Planet. Sci. 1985.13:201-240. Downloaded from www.annualreviews.org would be preferable. Nevertheless, massive primordial magmatism in­ fluenced almost all phases of the Moon's evolution. The Moon's bulk composition and origin are constrained largely by data for its crust; much of this crust formed by primordial magmatism. StUdying the primordial lunar differentiation is an indirect means to constrain the origin of the solar system and of the Earth in particular (Ringwood 1979), because primordial heating probably affected the Earth in a similar fashion. It is important from a purely terrestrial point of view to study the nature and extent of the lunar primordial differentiation. 201 0084-6597/85/0515-0201$02.00 202 WARREN Beginnings of Magma Oceanography (1864-1976) The science of magma oceanography is immature but not altogether new. In the nineteenth century, the planets were presumed to have formed from incandescent matter; this assumption implied that the entire Earth was once molten. Lord Kelvin (Thomson 1864), noting that silicate adiabats "most probably" have small dT/dP (T = temperature, P = pressure) relative to melting curves, inferred that crystallization of a magma ocean would "commence at the bottom, or at the center, if there is no solid nucleus to begin with, and would proceed outwards." Most US Apollo (manned) and Russian Luna (unmanned) missions landed within maria-"seas" of comparatively young basalt flows. Representative portions of the ancient highlands were only sampled by Luna 20 (February 1972) and Apollo 16 (April 1972). Nevertheless, a primordial magma ocean was postulated on the basis of a few anorthositic particles of Apollo 11 soil (Wood et al 1970, Smith et al 1970). After the initial round of Moon rock studies (the last US lunar mission, Apollo 17, flew in December 1972), detailed theoretical analyses of magma ocean crystallization began to appear (Binder 1974, Wood 1975, Walker et al 1975, Drake 1976). More refined treatments still tend to favor some sort of magma ocean. But skepticism seems to be increasing (Walker 1983, Longhi & Ashwal 1984), mainly because it is not clear how sufficient heat could have accumulated to form a magma ocean, and because a single magma seems inadequate to account for the diversity of the (growing) data base for ancient pristine rocks. CLASSIFICATION OF LUNAR ROCKS The Pristine Rock Concept The sampled portion of the lunar crust contains few relics of rocks that formed before the waning of intense meteoritic bombardment due to late by University of Hawaii at Manoa Library on 08/19/11. For personal use only. planetary accretion, about 4.0 Gyr ago. Consequently, most rocks from the ancient highlands are polymict impact breccias, produced by mixing of Annu. Rev. Earth Planet. Sci. 1985.13:201-240. Downloaded from www.annualreviews.org older rocks. Any rock fr agment (or large breccia clast) that escaped mixing, and retains an endogenetic igneous or metamorphic composition, is classified as "pristine." Except for mare basalts, the rate of discovery of pristine samples (mostly as clasts within polymict breccias) has been slow, roughly 10 per year. For an understanding of the ancient, endogenetic crust, pristine samples are indispensable. The complex problem of how to distinguish pristine from nonpristine rocks was reviewed by Warren & Wasson (1977) and Norman & Ryder THE LUNAR MAGMA OCEAN CONCEPT 203 (1979) [see also Ryder et al (1980), Shervais et al (1983), and Warren et al (1983b)]. Texture is often crucial, but even among the limited set of ancient samples with pristine compositions, pristine textures are rare. The least ambiguous single means for identifying pristine rocks is by analysis of siderophile elements. Most meteorites have high siderophile contents, and polymict impact breccias almost always contain small but telltale meteoritic components. Of the limited set oflunar rocks that on the basis of other indicators (such as texture) are unambiguously pristine, virtually all have extremely low siderophile contents. Mare Basalts Mare basalts are distinctive dark, ferro an lava flows that are concentrated within great impact basins. They cover about 17% of the lunar surface (Harz 1978). Recently, a 4.2-Gyr-old mare basalt was discovered (L. A. Taylor et aI 1983), but most are between 2.9 and 3.9 Gyr old. Pristine mare basalts are common, and except fo r regolith breccias, polymict impact breccias with large mare components are rare. KREEP Lithologies The key characteristic of KREEP rocks is that they contain high concentrations of incompatible elements (such as K, Rare Earth Elements, and P), always in the same pattern of element : element ratios (e.g. the ratio of the lightest REE, La, to the heaviest REE, Lu, is always about 2.2 times the chondritic value). The few known pristine KREEP rocks are about 3.9 Gyr old (Table 1) and are generally basaltic in major element composition and texture (see reviews by Meyer 1977, Warren & Wasson 1979b). A KREEP component dominates trace element patterns of most highlands polymict breccias. The ages of the pristine KREEP samples may be unrepresentative, however. Pristine KREEP emplaced into the upper crust before 3.9 Gyr ago was vulnerable to destruction by mixture into polymict breccias; and most KREEP rocks are polymict breccias. by University of Hawaii at Manoa Library on 08/19/11. For personal use only. Ancient Lithologies Annu. Rev. Earth Planet. Sci. 1985.13:201-240. Downloaded from www.annualreviews.org Mare basalt probably amounts to less than 1% of the lunar crust (Taylor 1982a), and KREEP is comparably minor. Anorthositic materials cover most of the ancient, heavily cratered highlands and probably dominate the upper half of the crust. Most nonmare (or highlands) rocks are dominated by plagioclase, orthopyroxene, and olivine. Early papers (Keil et al 1972) classified themas a single group, the ANT (Anorthosite-Norite- Troctolite) suite. It later became apparent that the anorthosites are fundamentally unrelated to the norites and troctolites (the A probably formed separately 204 WARREN Table 1 Sm-Nd and Rb-Sr isotopic age data for pristine rocks' Sample Lithology Sm-Nd age Rb-Sr ageb Initial 87Sr/86Sr Range mare basalts (100) 2.9-3.8e 0.6991-0.6997 Range KREEP basalts (8) 3.83-3.93 0.6994-0.7006 14321c granite 4.11±0.20 4.oo±0.11 0.70318 15455c Mg-rich norite 4.48 (1) 4.48±0.12 0.69896±3 67667 Mg-rich lherzolite 4.18±0.07 4.18 (1) 0.69905 (1) 72255c Mg-rich norite no data 4.08±0.05 0.69913±7 72417 Mg-rich dunite no data 4.4510.10 0.699OO±7 73255c Mg-rich norite 4.23±0.05 no data no data 76535 Mg-rich troctolite 4.2610.06 4.51 ±0.07 0.69900 ±3 77215 Mg-rich norite 4.37±0.07 4.33±0.04 0.69901±7 78236 Mg-rich norite 4.34, 4.43 4.29±0.02 0.69901±2 Meand fe rroan anorthosites (5) no data no data 0.698949 ± 11 • Data source: review by Nyquist (1982), except for 14321c (Nyquist et al 1983), basalts, and ferroan anorthosites (Nyquist 1977). Note: Nyquist (1977) lists one of the KREEP basalts (72275,171) as a pigeonite basalt. bThroughout this article, the time constant for decay of 87Rb is assumed to be 0.0142 Gyr-1. Ages reported using 0.0139 Gyr-1 are adjusted here by multiplying times 0.979. C One basalt of mare affinityfrom Apollo 14 has a Rb-Sr age of 4.14 ± 0.05 Gyr (L. A. Taylor et aI1983), but this age is highly exceptional, at least among sampled mare basalts. Least-squares-weighted mean and uncertainty of mean, based on data for d 16 15415, 60015, 60025, 61016c, and 64423,13,1. The range of the data is from 0.69887 ± 7 to 0.69910 ± 12. The listed mean does not include 4 data from Nunes et al (1974), which would drive it down to 0.698925 ± 10. from the N and the T), and unusual rocks like dunite and gabbro had to be classified. In this paper, all nonmare lithologies except KREEP are termed "ancient." FERROAN ANORTHOSITES Roughly 50% of all ancient rocks are ferro an anorthosites. Dowty et al (1974) first noted thedistinctiveness of ferro an anorthosites, so-called because their sparse (generally < 5%) mafic silicates have consistently low Mg/(Mg + Fe), or mg, by ancient lunar standards.