Moon composition important similarities to that of the Earth

The Moon is almost completely covered by regolith material. A reconstruction of the composition of the lunar interior is complicated because all lunar rock samples came from this regolith and thus can never fully represent the outcrop material belonging to a given intrusive body, magmatic province etc. Therefore lunologists must collect evidence on the lunar compositions by grains, in the literal meaning of this word. Because of this fundamental feature of lunar samples, isotopic records, which allow the chemistry of inaccessible reservoirs to be reconstructed, appear to be the most reliable.

In this regard oxygen, the most abundant element of rocky planets, but with isotopic compositions that vary substantially between the different solar system bodies, is of prime importance. The compositions of the Moon's and the Earth's oxygen lie exactly on the same terrestrial fractionation line (Fig. 10.8(b), Wiechert et al., 2001; Spicuzza et al., 2007). Samples from the Moon and the Earth also have indistinguishable 53Cr/52Cr ratios, suggesting that their source materials had similar Mn/Cr and initial 53Mn/55Mn ratios at the time they formed as closed systems for Mn and Cr. In contrast, Mars and the chondrites show different initial 53Cr/52Cr ratios, indicating that the Earth, Mars and the chondrite parent bodies had separate feeding zones, isolated from each other (Lugmair and Shukolyukov, 1998; Carlson and Lugmair, 2000). Also, Mn is a moderately volatile lithophile element (Table 3.1) and the FeO/MnO ratio inferred for the bulk Moon composition is very similar to the terrestrial value, in agreement with the above isotopic constraints (Warren, 2005).

From these isotopic and chemical criteria, the proto-Earth and proto-lunar matter could have originated in a close neighbourhood; the high degree of similarity (especially indicated by O-isotope compositions) favours this conclusion even though it is not a unique one as discussed by Taylor et al. (2006).

The 182Hf-182W chronometer gives ~ 30 Myr as a minimum time interval between solar system formation and the giant Moon-forming impact (Section 18.3). This is consistent with the most reliable estimate of the ancient lunar crust age, 4.456 ± 0.040 Gyr (Norman et al., 2003). Thus no contradiction to the giant-impact hypothesis arises from the available time constraints, and the proto-Moon and proto-Earth could indeed share an overall similar early accretion history.

Moon and Earth compositions: differences?

There are important differences between the bulk compositions of the Earth and Moon. The (uncompressed) density of the Moon (3.34 g cm-3) is much less than the Earth's density (4.45 g cm-3), indicating a heavy-element deficit; only Fe is heavy and abundant enough to make this difference. Geophysical investigations of the Moon can be reconciled with a lunar model having a small metallic core, ~ 1%-3% of the lunar mass, at least an order of magnitude less than that of the Earth (Hood and Zuber, 2000). Even though this interpretation is not unique (Jolliff et al., 2006), the uncompressed density contrast between the Earth and Moon is indeed unarguable.

The abundances of the involatile moderately siderophile elements (e.g. Co and W) in lunar rocks are broadly similar to the terrestrial values; however, the highly siderophile elements in the lunar mantle are depleted by at least a factor 10 compared with the Earth's mantle, and they do not show a chondritic relative abundance pattern. These features have been used to argue that the mantles of the Moon and Earth are too different to have a common source (Wolf and Anders, 1980; Newsom and Taylor, 1989). This could, however, have originated from highly-siderophile-element (HSE) partitioning into the lunar core in the course of its segregation (Kramers, 1998; Righter, 2002) and therefore may shed no light on compositional similarities or dissimilarities to the Earth.

Regarding major lithophile-element abundances, two different models of the bulk silicate Moon are at present still being considered: one is quite similar to the BSE model (Warren, 2005) and the other postulates enhanced abundances, by a factor ~ 1.5, of the most refractory elements, such as Al, Ca, Ti and including heat-producing U and Th (Taylor et al., 2006).

An important difference between the Moon and Earth is the greater depletion of lunar rocks in some volatile elements, first of all the alkali metals, compared with terrestrial rocks. For example, the K/U and Rb/Sr ratios are a factor ~ 4 below the terrestrial values (see Fig. 14.4). Isotopic data corroborate this more severe depletion of the Moon in volatile elements. The U-Th-Pb isotope systematics indicate two major source regions for lunar rocks, both with time-integrated ^ values exceeding the bulk silicate Earth ^ ~ 8. Many lunar crustal rocks were derived from an extremely high-^ source, with ^ > 500. Lead-isotope ratios in lunar meteorites point to a less extreme source with ^ between 10 and 50 but still generally above the terrestrial value (Premo et al., 1999; Snyder et al., 2000).

In the 87Rb-87Sr isotope systematics, the very low initial 87Sr/86Sr ratios of lunar rocks indicate that the loss of volatile Rb occurred from proto-lunar materials before the giant impact, within ~ 15 Myr after the formation of the solar system. Such an early depletion in volatile elements is, however, not a unique feature of lunar rocks, which are almost indistinguishable from a highly differentiated meteorite species, the eucrites. Another type of differentiated meteorites, the angrites, shows an even more extreme depletion (see Fig. 14.4).

The 142Nd/144Nd ratios measured in lunar basalts by Rankenburg et al. (2006) appear to be indistinguishable from chondritic matter, in contrast with those in terrestrial mantle and crustal rocks (Fig. 27.16(b)). This apparent discrepancy between the Moon and Earth may in fact underline the initial similarity of the Sm- and Nd-isotope abundances in the Earth, Moon and chondrites; whereby in the Earth a somewhat enriched reservoir, the D" layer, was isolated at some time after the giant impact (as proposed in Chapter 19).

It should be noted, however, that 142Nd was produced in an s-process whereas 144Nd was yielded by both s- and r- processes; therefore the mixing of the two could have been incomplete, and more detailed studies of the Nd-isotope composition in chondritic meteorites are needed. Andreasen and Sharma (2006) showed that the excess observed in terrestrial rocks is not the effect of any nucleosynthesis presolar anomaly. The lunar 142Nd/144Nd ratios strengthen the case made by these authors that the chondritic 146Sm-142Nd systematics indeed provide a reference for the planets.

The giant-impact hypothesis and peculiarities of lunar composition

The giant-impact hypothesis (Chapter 16) allows some of the compositional differences between the Earth and the Moon to be understood. The Mars-sized impactor had already been differentiated into metal core and silicate mantle. High-resolution simulations of the giant impact (Canup, 2004) show that the impactor's iron core was eventually in the main combined with the proto-Earth core, whereas material derived from the far-side mantle of the impactor was blown away, together with a small portion of its metal, beyond the Roche limit. From this already metal-poor material the Moon was formed ~ 10 years after the impact (Pritchard and Stevenson, 2000). Thus, an impact-related mechanism of Moon formation can readily explain the depletion of the lunar material in metal, even if the impactor itself had an Earth-like bulk-metal/silicate ratio.

Regarding the lunar siderophile-element abundances, several authors have suggested that the Moon inherited siderophile elements from the mantle of the impactor, and afterwards further depletion occurred during equilibrium core formation (New-som and Taylor, 1989; Jones and Hood, 1990; Righter and Drake, 1996). Because of the one-stage segregation process on the Moon and the small fraction of metal available, only highly siderophile elements were partitioned into the lunar core, thus leaving behind a lunar mantle further depleted in accordance with the respective partition coefficients (Kramers, 1998).

The lunar material could have lost volatile elements at the nebula stage, in the course of formation of the Mars-sized impacting body (Ruzicka et al., 2001), and/or during the post-giant-impact vaporization of volatiles (Pritchard and Stevenson, 2000). The Sr-isotope evidence cited above favours the first two scenarios.

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