Case study of magmatic differentiation trends on the Moon based on lunar meteorite Northwest Africa 773 and comparison with Apollo 15 quartz monzodiorite
Introduction
Apollo era studies of samples returned from the Moon showed that fractional crystallization was an important process for compositional evolution of mare basalts (Papike et al., 1976, Papike et al., 1998, Shearer et al., 2006). Mare basalts occur as mostly crystalline rocks that formed in lava flows and as pyroclastic glass beads, and show wide ranges in TiO2 concentrations (TiO2 < 1 to 14 wt%), particularly in comparison to terrestrial basalts (Taylor et al., 1991, Neal and Taylor, 1992, Grove and Krawczynski, 2009). Several suites of low-Ti mare basalts show variations in major element abundances that can be explained in part by fractionation of olivine or pyroxene, which drove residual liquids to more ferroan (higher FeO/[FeO + MgO]), more Ti-rich compositions (e.g., Apollo 15 mare basalts, Fig. 1a; also see Ryder and Schuraytz, 2001, Schnare et al., 2008). Whole rock major element concentrations suggest that fractionation of olivine also occurred in high-Ti mare basalts, but concurrently with a high-Ti mineral such as ilmenite, armalcolite or ulvospinel, resulting in decreasing TiO2 concentrations of residual liquids during fractional crystallization (Fig. 1b; see Papike et al., 1976). Early fractionation of a mafic phase is supported by textures and mineral compositions of the crystalline mare basalts, which (1) suggest that olivine and/or pyroxene preceded feldspar during crystallization and (2) tend to have normally-zoned pyroxene, indicative of progressively more ferroan liquids during crystallization (Papike et al., 1976, Papike et al., 1998, Taylor et al., 1991, Shearer et al., 2006). Experiments have verified the early crystallization of olivine or pyroxene with a Ti-rich mineral in high-Ti mare liquids and without a Ti-rich phase in low-Ti liquids (e.g., Kesson, 1975, Longhi, 1992). Late stages of fractional crystallization drove residual liquids to highly ferroan compositions where immiscible FeO-rich and alkali-silica-rich silicate liquids formed, as identified in experiments (Fig. 1c; see Hess et al., 1975, Rutherford et al., 1976, Longhi, 1990) and in melt inclusions in natural samples (e.g., Roedder and Weiblin, 1971, Shearer et al., 2001). The presence of silica-rich melt inclusions in mare basalts is an indicator of fractionation over a wide compositional range—at least on a local scale—in order for liquids to reach the ferroan compositions where immiscibility could occur.
The lunar KREEP basalts contrast with mare basalts in several aspects of their composition, mineralogy and evolution. The KREEP geochemical signature includes high concentrations of K, rare earth elements and P; a negative Eu anomaly; enrichment in light vs. heavy rare earth elements (Hubbard et al., 1971) and is associated with enrichment of incompatible elements in late-stage liquids of the lunar magma ocean after fractional removal of Eu into the anorthite-rich flotation crust (Warren and Wasson, 1979, Shearer et al., 2006). The KREEP signature occurs in breccias, plutonic rocks (notably in the magnesian suite, also known as Mg-suite, see James, 1980, Elardo et al., 2011), some basalts returned from the Moon, and in some basaltic lunar meteorites (e.g., Fagan et al., 2002, Fagan et al., 2003, Jolliff et al., 2003, Borg et al., 2009, Joy et al., 2011). Because of its occurrence in a variety of rock-types from several locations, KREEP as a geochemical signature appears to be widespread in the lunar crust (e.g., Lucey et al., 2006). In contrast, rocks referred to as KREEP basalts have been identified in abundance only at the Apollo 15 site (Papike et al., 1998, Shearer et al., 2006, Taylor et al., 2012). The KREEP basalts are distinct from mare basalts in having early-crystallized orthopyroxene and plagioclase feldspar (vs. olivine or pigeonite with or without Ti-rich oxide), and consistently high Al2O3 concentrations (near 15 wt%) in contrast to the widely ranging, but generally lower Al2O3 concentrations of mare basalts (Neal and Taylor, 1992, Papike et al., 1998, Shearer et al., 2006). In spite of these differences, both sets of lunar basalts experienced magmatic evolution to more ferroan compositions during crystallization (Irving, 1977, Taylor et al., 2012). Granite and quartz monzodiorite (QMD) clasts from Apollo 15 breccias are more ferroan and more enriched in incompatible elements than the KREEP basalts, and it has been suggested that they are late-stage products of fractional crystallization, and perhaps immiscibility, of KREEP-rich liquids (Fig. 1c; see Rutherford et al., 1976, Ryder, 1976, Taylor et al., 1980, Taylor et al., 2012, Warren, 1988, Neal and Taylor, 1989a).
A progression to more ferroan compositions is also characteristic of the terrestrial tholeiitic trend. In the tholeiitic (or FeO-enrichment or “Fenner”) trend, magmatic systems evolve from magnesian to ferroan compositions and may generate alkali-silica-rich rocks, but only after extensive FeO-enrichment (Fig. 1c, d; see Fenner, 1929, Wager, 1960, Carmichael, 1964). In contrast, the calc-alkaline (or SiO2-enrichment, or “Bowen”) trend is a set of compositions showing a lower extent of FeO-enrichment and wider ranges of SiO2- and alkali-enrichment (Fig. 1c, d; see Bowen, 1928, Martin and Piwinskii, 1972, Miyashiro, 1974 Grove and Baker, 1984, Zimmer et al., 2010). Oxygen fugacity is one of the many factors that affect magmatic evolution trends on Earth. At low oxygen fugacities in the Earth’s crust and mantle, most Fe occurs as FeO, FeO substitutes for MgO in mafic silicates as crystallization temperatures decrease, and residual liquids and the rocks that form from them evolve along the tholeiitic trend toward more ferroan compositions with relatively little increase in silica until late stages (Osborn, 1962, Kuno, 1965, Miyashiro, 1974). At higher oxygen fugacities, Fe2O3 becomes more abundant and may result in the stabilization of Fe,Ti-oxides; in this case, fractional crystallization results in silica-enrichment of residual liquid with lower proportional increases in FeO (Osborn, 1959, Gill, 1981, Burns, 1985, Yagi and Takeshita, 1987, Lachize et al., 1996, Berndt et al., 2005). Of course, other processes and controls affect terrestrial magmatic differentiation. For example, early crystallization of plagioclase feldspar tends to enrich residual liquids in FeO, leading to the tholeiitic trend (Grove and Baker, 1984). In settings where high pressures or water abundances suppress feldspar or enhance the stability of magnetite relative to mafic silicates, FeO-enrichment may be limited, leading to calc-alkaline differentiation (Sisson and Grove, 1993, Zimmer et al., 2010, Blatter et al., 2013). Magma mixing, assimilation of country rocks and silicate liquid immiscibility are among the other processes that affect the evolution along tholeiitic and calc-alkaline trends (e.g., Grove et al., 1982, Kay and Kay, 1985, Tatsumi and Suzuki, 2009, Charlier et al., 2011, Jakobsen et al., 2011).
In this study, the broad magmatic trends and controls summarized above are used as context for: (1) evaluating whether clasts in the lunar mafic breccia Northwest Africa 773 (NWA 773) can be linked by magmatic differentiation (Fagan et al., 2003, Jolliff et al., 2003); (2) comparing NWA 773 clasts (very low Ti mare heritage, see Jolliff et al., 2003) with Apollo 15 QMD (KREEP heritage, see Ryder, 1976, Nyquist et al., 1977) as rocks from distinct igneous differentiation trends on the Moon; (3) examining controls, particularly the effect of variable TiO2, on lunar magmatic differentiation; and (4) comparing some of the controls and differentiation processes of lunar and terrestrial magmatic systems. We focus on pyroxene compositions and zoning in distinct textural settings to characterize differentiation trends within clasts of NWA 773, to determine whether specific individual clasts in NWA 773 are plausibly co-magmatic, and to compare NWA 773 with Apollo 15 QMD. Results from NWA 773 and Apollo 15405 QMD are compared with experiments conducted by others (e.g., Rutherford et al., 1976) and with MELTS models (Fernandes et al., 2003, Asimow and Ghiorso, 1998) conducted as a part of this study to understand some of the controls on lunar magmatic differentiation.
Section snippets
Samples: Northwest Africa 773 (NWA 773) and Apollo 15405 quartz monzodiorite (QMD)
Mineral textures and compositions were determined from one polished thin section of Northwest Africa 773 (NWA 773, on loan from M. Killgore) and two polished thin sections of Apollo sample 15405 quartz monzodiorite (QMD, subsamples 56 and 145 on loan from NASA Johnson Space Center). NWA 773 is a mafic lunar regolith breccia that was found in 2000 (Grossman and Zipfel, 2001). The original sample consists of three stones weighing a total of 633 g. The sample consists of two main lithologies: a
Breccia and main olivine cumulate (OC) clast in NWA 773
The breccia and main olivine cumulate (OC) clast of NWA 773 have been described in two detailed papers (Fagan et al., 2003, Jolliff et al., 2003) and lithologies of paired meteorites of the NWA 773 clan have also been described (e.g., Bunch et al., 2006, Jolliff et al., 2007, Zhang et al., 2010, Nagaoka et al., 2011). General features of clasts from NWA 773 are described below, with a focus on varieties of pyroxene, which have compositions and zoning patterns that are useful for classifying
Ferroan–titanian trend in NWA 773 breccia and main olivine cumulate (OC) clast
Pyroxene compositions from OC-mg, OC-main, OC-breccia clasts form a fairly continuous array in Wo–En–Fs components, Fe#, Ti# and Ti/Al from primitive (low values of Fs, Fe#, Ti#, Ti/Al) to more evolved compositions (Fig. 4). Pyroxene crystals from the OC clasts show little variation in Fe#, whereas wide ranges in Fe# correlate with Ti# in pyroxene gabbro clasts. These wide variations in both Fe# and Ti# are well-exposed in one clast, where both values increase with proximity to late-stage
Model crystallization of mare basaltic liquids with variable TiO2
One factor for silica-enrichment in the terrestrial calc-alkaline trend is a high oxygen fugacity, which leads to stabilization of the Fe-oxide magnetite (Osborn, 1959, Berndt et al., 2005). Subsequent fractionation of magnetite removes Fe from residual liquid, limiting Fe-enrichment, and at the same time drives liquids to more silica-rich compositions. Lunar basaltic rocks formed under relatively low oxygen fugacities (Longhi, 1992), which would prohibit the role of oxidized iron as described
Conclusions
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Olivine cumulate (OC), zoned pyroxene gabbro, symplectite and alkaline-phase-ferroan clasts from the NWA 773 breccia are linked by origin from a common magmatic system on the Moon. Early differentiation was dominated by fractionation of magnesian olivine with some pyroxene. Lack of zoning in individual crystals indicates that crystals grew under near-equilibrium conditions at this stage, but variations in Fe# of olivines and pyroxenes from different OC clasts suggest that the main residual
Acknowledgments
Financial support for this work was provided by Monbukagakusho Grant-in-Aid of Scientific Research No. 18540483 and Waseda University research funds. We thank A. Yonemochi and H. Miura for assistance with electron microprobe analyses. We are grateful for loans of thin sections from M. Killgore for NWA 773 and from NASA/JSC (G.E. Lofgren and R.A. Zeigler, curators) for Apollo 15405 QMD. Waseda University undergraduate research projects conducted by A. Sasamoto, Y. Kataoka, S. Kodama and S.
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