The clast has mineral chemistry characteristics that differentiate it from known lunar lithologies (Figs. 5, 6, 8, and 9). In particular, the olivine and pyroxene crystals have Fe/Mn ratios that are unique compared with previously sampled lunar rocks types. The ratio of Fe/Mn in mafic minerals and bulk samples is indicative of planetary reservoirs and subsequent geological evolution of planetary bodies (i.e., volatile loss, metal segregation during core removal, oxygen fugacity, and melt fractionation: Drake et al. 1989; Papike 1998; Karner et al. 2003, 2006; Papike et al. 2003; Goodrich and Delaney 2000; Gross and Treiman 2010; Gross et al. 2011). The possible planetary sources of the clast are discussed below.
Sampling a Unique Region of Lunar Crust?
Although the olivine and pyroxene mineral compositions are not lunar-like (Figs. 5, 6, 8, and 9), other characteristics may be consistent with the clast being derived from lunar rocks. Its magnesian nature is similar to rocks from the lunar Mg-Suite, although plagioclase is alkali-rich compared with plagioclase in Mg-Suite rocks (Fig. 7). Conversely, although the clast's aluminous and alkali-rich nature is more similar to samples from the High Alkali Suite, olivine and pyroxene in the clast are too magnesian (Fig. 7). Plagioclase trace element concentrations (Fig. 3) are similar to rocks from both the High Alkali Suite and Mg-Suite. If the rock is lunar, then it shares characteristics of both these magmatic suites, although it differs from both. It is plausible that the rock represents a Mg-Suite cumulate that was infiltrated by late-stage–evolved K-rich fluids (akin to High Alkali Suite or KREEP basalt melts) to account for the alkali-rich plagioclase and trapped mesostasis Na-K–rich glass.
If the clast originated from the Moon, then an explanation is required for the nonlunar Fe/Mn ratios, in both the early formed olivine (Figs. 8a and 8b) and the later crystallized pyroxene (Figs. 8c and 8d). The following mechanisms could account for difference in Fe/Mn ratios between the clast and lunar rocks:
- Oxygen fugacity effects. Low Fe/Mn ratio in the clast could result from a source region with higher ƒO2 than typical lunar melts. However, no ferric mineral phases are present in the clast that would support this model. Alternatively, the low ratio could imply that the clast has experienced reduction to remove Fe from olivine and pyroxene that could have decreased both minerals’ Fe/Mg and Fe/Mn ratios, and increased Ni concentrations in the clast compared with lunar rocks. However, no metallic Fe is observed in the clast, so if reduction occurred, the resulting metal products and siderophile elements were effectively removed from the rock before it crystallized.
- Crystallization or fractionation effects. A decrease in Fe/Mn ratio in mafic phases could indicate that olivine was removed (fractionated) from the sample's source region, as Mn is somewhat incompatible in olivine and Fe is compatible (Humayun et al. 2004; Qin and Humayun 2008); this process could decrease the system's bulk Fe and increase the bulk Fe/Mg and decrease the Fe/Mn ratio. However, as the clast has a bulk-rock Mg# of 84–89 (Table 1), precipitating olivine in equilibrium should be Fo94–96 (calculated using equation 3 of Joy et al. 2008 and references therein). As these calculated values are similar to the most primitive olivine composition measured in the clast (Fo93), this indicates that little or no olivine was removed from the parent system and so this is not likely to be the cause of the Fe/Mn variation. Fe/Mn ratios could potentially also be lowered if Fe-Ti or Fe-Cr-Al oxides precipitated as an early phase removing Fe from the melt (Karner et al. 2003; Gross et al. 2011); however, both olivine and pyroxene in the clast are generally Ti-rich, Cr-rich, and Al-rich compared with lunar phases with similar Mg# (Figs. 5 and 6), suggesting that early oxide removal has not been extensive.
- Unique lunar crustal or mantle mineralogy. The lunar mantle and crust is heterogeneous, with regions that contain differing amounts of volatile elements (e.g., Hauri et al. 2011; McCubbin et al. 2011; Tartèse et al. 2013). The clast could, therefore, have been sourced from a region with higher concentrations of volatile elements. Manganese is a moderately volatile element and generally has low concentrations in lunar materials, presumably because it was volatilized and depleted during the Moon's formation by giant impact (e.g., Hartmann and Davis 1975; Papike et al. 2003; O'Neill and Palme 2008). However, in principle, as yet undiscovered relatively volatile-rich regions may exist in the lunar crust or mantle from which this clast might have been derived. An origin in such a region might also explain the relatively alkali-rich nature of the plagioclase grains within the clast.
In summary, although there are possibly mechanisms to account for the clasts's non–lunar-like Fe/Mn ratios in olivine and pyroxene, such models would also have to account for the clast's different mineral chemistry compared with known lunar rock types (Table 3). Indeed, the clast appears sufficiently compositionally unique compared with known lunar rocks that it may not be lunar at all.
Table 3. Summary of compositional and mineralogical similarities and differences between the clast in MAC 88105,159 and other planetary bodies.
|Oxygen isotopes||Clast weighted mean Δ17O indistinguishable (2 sigma) from TFL, distinct from HED samples||Yes||Yes||Yes||No||Unknown||Unknown||Yes|
|Bulk K and Th values||Within range of lunar HAS rocks, higher than HMS rocks. Within range of terrestrial Archean rocks. Higher than terrestrial oceanic crust. Higher than HED and SNC samples. Higher than that recorded Mercury surface regoliths and Venus crust.||Uncommon, but like some HAS samples||Yes for Archean rocks, No for oceanic basaltic crust||No||No||No||No||Maybe|
|Magnesian mafic minerals||More magnesian than HED and SNC samples studied to date. Similar to lunar Mg-Suite and terrestrial basalts/Archean rocks||Yes like lunar Mg-Suite rocks||Yes||No ||No||Unknown||Unknown||Yes|
|Olivine FeO/MnO ratios||Nonlunar olivine FeO/MnO ratios||No||Ol: Overlap but not same trend||Ol: Yes||No||Unknown||Unknown||Yes|
|Pyroxene FeO/MnO ratios||Nonlunar pyroxene FeO/MnO ratios||No||Pyx: Overlap but not same trend||Pyx: No||Pyx: No||Unknown||Unknown||Yes|
|Plagioclase An#81, Eu × ~40cn||Higher than equivalent An# plag. in terrestrial basalts and SNCs, but similar to lunar HAS Suite and HEDs (Karner et al. 2004)||Yes—within range of lunar HAS Suite and phases in late-stage fractionates of some mare basalts||Yes||No, too calcic||Yes||Unknown||Unknown||Maybe|
|Sodic augites||Not-HED affiliated or martian; unusual for the Moon, but some overlap with terrestrial pyroxenes||Unusual||Yes||No||No||Unknown||Unknown||Maybe|
|Olivine low-Ni (<150 ppm)||Suggest nonterrestrial origin (Archean and modern day oceanic basalt olivines typically >600 ppm Ni)||Yes||No||Yes||Yes||Unknown||Unknown||Maybe|
A Nonlunar Origin?
As the Fe/Mn ratios of the olivine and pyroxene are not lunar-like (Figs. 8 and 9), it is plausible that the clast may have been sourced from a different parent body and survived delivery to the Moon as impact debris. Meteoritic debris have previously been identified on the Moon as rare samples found in the lunar regolith (see Joy et al. 2012 for a summary).
Compositional and isotopic constraints for a meteoritic origin, and potential parent bodies, are listed in Table 3. Olivine grains in the clast have nonlunar Fe/Mn ratios that are more similar to trends in martian meteorites, terrestrial samples, and some chondrules in ordinary chondrites (Fig. 8). However, we do not consider that the clast is a chondrule relic as there (1) are no Fe-metal, sulfide, Al-rich spinel, or nepheline grains present, indicative of plagioclase-rich chondrules; (2) the clast bulk MgO/Al2O3 ratio (approximately 0.8) is lower than bulk chondrules (typically >>1.5: McSween 1977), and clast bulk MgO/TiO2 is typically lower (<0.21) than in chondrules (typically >>45: McSween 1977); and (3) plagioclase grains in the clast are a lot blockier than found in plagioclase-rich chondrules (Krot et al. 2002). The clast pyroxenes have Fe/Mn ratios that are distinct from most basaltic achondrite groups, although are within the spread of terrestrial pyroxene data (Fig. 8).
Additional constraints are provided by the oxygen-isotope data. Minerals in the clast have oxygen-isotope ratios (Table 2) that are (1) statistically indistinguishable from the Terrestrial Fractionation Line (i.e., the clast could be a terrestrial or a lunar sample), (2) are statistically (2σ error) different from the bulk HED meteorite trend, and (3) are within 2σ error of the bulk SNC meteorite and angrite meteorite oxygen-isotope trends (Fig. 10).
Although the oxygen isotopic composition is consistent with the TFL, and the plagioclase An# values overlap with terrestrial values (Fig. 9), we provisionally discount a terrestrial origin for the clast. This is because the olivine Ni contents are lower (Clast = <150 ppm Ni: Table S1) than high-Fo (Fo>80) olivine in terrestrial mafic rocks (typically >500 ppm Ni; Karner et al. 2003; PetDB database www.petdb.org/) and Archean samples (>600 ppm Ni; Barnes et al. 1983; Karner et al. 2003; Mondal et al. 2006; Pettigrew and Hattori 2006; Cheng and Kusky 2007). In addition, although the Fe/Mn ratios in the clast's mafic phases overlap with examples from terrestrial samples, they do not exactly follow the terrestrial Fe/Mn ratio trend (Fig. 8).
It is notable that the Fe/Mn ratios in olivine fall very close to the martian olivine trend (Figs. 8a and 8b), and that the oxygen-isotope values do not rule out a martian origin (Fig. 10). However, evidence from other mineral chemistry data appears to discount a martian source, as the olivine and pyroxene mineral compositions are atypically magnesian, and the plagioclase too Ca-rich (anorthitic) compared with known martian meteorites (i.e., Papike et al. 2003, 2009; Karner et al. 2003, 2004, 2006; Sarbadhikari et al. 2009; Fig. 9). Moreover, unlike the olivine, the Fe/Mn ratios in pyroxene do not follow the martian trend (Karner et al. 2003, 2006; see Figs. 8 and 9). However, we have to recognize that our current set of martian meteorites are derived from a very few locations on Mars, and it would be unwise to assume that we have anything approaching a complete picture of the range of the composition of martian igneous rocks that we could use for such a comparison.
As we have no recognizable meteorite samples from Venus or Mercury to compare with, it is difficult to assess if these planets could have been the source of the clast. However, the clast bulk K (7300–8400 ppm; Table 1) and Th (approximately 2.5 ppm) in the clast is notably higher than that recorded in any Mercury surface regoliths by the Messenger mission gamma-ray spectrometer (GRS) instrument (1150 ± 220 ppm K; 0.22 ± 0.06 ppm Th; see Peplowski et al. 2011). It is also higher than that recorded by the GRS instrument on board the Venera landers (3000–4500 ppm K; 0.7–2 ppm Th; see fig. 2 of Peplowski et al. 2011 and references therein). This suggests that the clast is unlike typical rocks in Mercury's or Venus's upper crust, although clearly the full diversity of these crustal rocks is presumably greater than deduced from relatively low spatial resolution orbital remote sensing of Mercury and three in situ measurements made on Venus.
Compared with asteroid material sampled at the present day by near Earth objects (NEOs), mafic mineral compositions in the clast are dissimilar to lithologies sampled by aubrite (Brearley and Jones 1998), winonaite (Benedix et al. 2005), acopulcoite, lodronite (McCoy et al. 1996, 1997), mesosiderite (Nehru et al. 1980), and ureilite (Downes et al. 2008) achondritic meteorites. Plagioclase grains in the clast are not as calcic, and Fe/Mn ratios in olivine and pyroxene are lower than in angrite meteorites (Fig. 9) (Papike et al. 2003). Fe/Mn ratios and the augite-rich Na-bearing nature of pyroxene are also dissimilar to those in HED pyroxene (Figs. 8c and 8d, and Fig. 9; see also McSween et al. 2012; Beck et al. 2012). HED meteorites have already been shown to not fit well with the clast's oxygen-isotope composition (Fig. 10).
The clast could, therefore, have originated from a different, so-far unsampled, achondritic parent body with differentiates that were melted from a primitive reserve (to account for magnesian mafic phases) and included a fractionated residual liquid component (to account for the Na-K–rich mesostasis). Rare granitic igneous fragments and glasses (some magnesian) have been reported in a number of meteorites that are presumed to have originated within differentiated crusts by magmatism or impact processes on small asteroidal parent bodies (e.g., Bonin 2012). It is, therefore, possible that this clast could represent a lithology from a differentiated asteroid parent body that is poorly represented, or not represented, in meteorites being delivered to Earth at the present day.
Whatever its source, if the clast is exogenous to the Moon, the timing of its delivery to the lunar surface could help to shed new light on the sources of projectiles being delivered to the Moon at different points in lunar history. Constraining the age of lunar regoliths is complicated as they contain many different rock types that may have undergone several formation and space-exposure episodes. In regolith samples that have undergone exposure to the space environment, the bulk-rock ratio of “trapped” (parentless) 40Ar to solar wind–implanted 36Ar has been shown to be indicative of the last time the regolith system was closed from surface exposure (i.e., it was turned from a soil into a rock). This isotope ratio can then be calibrated to a temporal antiquity age record using the argon isotope record of Apollo samples of known age (Eugster et al. 2001; Joy et al. 2011a). The trapped 40Ar/36Ar of MAC 88015 was measured by Eugster et al. (1991) to be 5.7. Applying this ratio to the age calibration of Joy et al. (2011a) implies that MAC 88105 was closed from lunar surface exposure at approximately 2.82 Ga. Therefore, any meteorite components in the MAC 88105 parent regolith would have to have been delivered to the lunar surface before this time. Eugster et al. (1991) report that the parent regolith was immature and had a surface residence time of about 650 Ma prior to brecciation: this implies that the clast possibly was delivered between approximately 3.47 Ga and approximately 2.82 Ga during Late Imbrian epoch to early Eratosthenian period.
Highly siderophile-element signatures for impact melts (e.g., Puchtel et al. 2008; Fischer-Gödde and Becker 2012; Galenas et al. 2012), and discoveries of projectiles in ancient breccias (Joy et al. 2012), imply that chondritic asteroids were common sources of impactors during the basin-forming epoch (>3.7 Ga). Delivery of achondritic material to the lunar surface during an interval of approximately 3.47 Ga and approximately 2.82 Ga is consistent with a variety of impactors (chondritic, achondritic, iron) found in younger Apollo 16 regolith breccias and Apollo landing site soils (see Joy et al. 2012 for a summary), and reflects a possible diversification of impactor sources in post–basin-forming epoch (<3.7 Ga) projectile populations.