Geochemical Constraints on the Origin of the Moon.

Stuart Ross Taylor Department of Nuclear Physics, Australian National University, Canberra

Lunar and Planetary Institute, Houston, Texas

Tezer M. Esat

Research School of Earth Sciences, Australian National University, Canberra

The Moon has a density indicative of a low content of metallic iron, is bone-dry and is depleted in the very volatile elements (e.g. Bi,T1) relative to abundances in the solar nebula. The alkali elements are depleted relative both to the Earth and the solar nebula. The FeO content of 13% in the bulk Moon is intermediate between that of Mars with 18% and the

terrestrial mantle with 8% FeO. Trace siderophile elements are depleted in order of their metal- silicate partition coefficients, consistent with their removal into a small metallic lunar core. Relative to the Earth, there is an enrichment of refractory elements (eg. Ca, A t, Ti, REE, U, Th) in the Moon. These geochemical properties are examined in the light of the giant impact hypothesis for lunar origin. The high bulk FeO content of the Moon rules out the derivation of the proto-lunar material from any but a small fraction of the terrestrial mantle. Most of the material now in the Moon must have come from the silicate mantle of the impactor. Those elements with condensation temperatures above 1100-1200K are present in the Moon in their solar nebular ratios, so that material subjected to extreme temperatures during the impact event apparently was not incorporated in the Moon. The low value of LUNI (= BABI) argues for very early loss of volatile Rb relative to refractory Sr and is consistent with an early depletion of volatile elements in the solar nebula. The lack of fractionation effects in the potassium isotopes rule out evaporative loss of potassium by Raleigh-type distillation processes during the giant collision. Possible compositions for the impactor include a body with an iron-rich mantle, intermediate between that of Earth and Mars, but with a low volatile content and low

Rb/Sr ratio, established close to To

INTRODUCTION

Collisions with large bodies occurred during planetary formation. Very large impacts are generally held to have caused the obliquities of the inner planets. The evidence for collisions with smaller bodies is revealed by the presence of large multiting basins formed by such impacts on Mercury, Mars and the Moon. Cameron and Ward [1976] proposed to account for the origin of the Moon by the prograde impact of a Mars (or larger) sized impactor on

Earth Processes: Reading the Isotopic Code Geophysical Monograph 95 Copyright 1996 by the American Geophysical Union

the Earth. Such a process can account for the high angular

momentum (3.45 x 10 41 rad. g. cm2/sec) of the Earth- Moon system and the non-equatorial lunar orbit as well as providing extreme temperature conditions which might produce an initially molten Moon and the bone-dry features of lunar geochemistry. The model provides a highly energetic mode of formation. This is a requirement for lunar formation models since the geochemical evidence indicates that much of the Moon was molten at the time

of accretion. The giant single impact model for the origin of the Moon has now become virtually a consensus, since it is the only hypothesis that accounts for the high angular momentum and the unique nature of the Earth- Moon system.

Here we explore the geochemical consequences of a

34 LUNAR ORIGIN

lunar origin by a giant collision, the nature of the poto- lunar material, and the composition of the impactor. Among the various aspects that can be addressed are the relationship of the composition of the Moon to that of the original solar nebula, the comparision between the composition of the terrestrial mantle and that of the bulk Moon, and the composition of the impactor.

Computer simulations of the giant impact hypothesis under conditions that form a lunar mass depleted in metallic iron in terrestrial orbit clearly indicate that it is mostly the material from the silicate mantle of the impactor that finishes up in the Moon [e.g., Cameron and Benz, 1991]. Thus direct comparisions between the composition of the Moon and the silicate mantle of the Earth have little immediate relevance to the problem. This conclusion is reinforced by the geochemical problem of the failure to match Earth mantle and lunar compositions for a nmnber of crucial elements [e.g.,Taylor, 1986a,b; Newsom and Taylor, 1989]. The mass of the impactor is commonly given as 15% of the mass of the Earth. If the Moon (1.2% Earth mass) is derived from that body, less than 10% of the impactor finishes up in the Moon. A basic problem in addressing the origin of the Moon is to try to assess the effect of the high temperatures engendered by the giant impact on the material making up the mantle of the impactor. There are at present many free parameters in the stochastic process that formed the Moon and perhaps only loose constraints can be placed upon the details of the process.

THE COMPOSITION OF THE MOON

The first lunar basaltic samples from Mare Tranquillitatis were, in comparison with terrestrial analogues, depleted in volatile and siderophile elements and enriched in refractory elements; these signatures have proven characteristic of lunar samples in general.

The Highly Volatile Elements

The highly volatile elements (e.g., Bi, TI, In, Cd) are depleted as a class by factors of about 50 relative to the terrestrial mantle, and by factors of about 200 relative to CI cosmic abundances [Wolf and Anders, 1980]. The extreme depletion of the Moon in volatiles is also shown by the total absence of H20 at parts per billion levels.

The Moderately Volatile Elements

The moderately volatile ele•nents, (e.g., K, Rb) are depleted by a factor of about 10 relative to CI cosmic

abundances or about two relative to terrestrial abundances.

The Rb/Cs ratio is well known in the Moon since there is

no aqueous phase present to disturb the system [Norman et al, 1993]. The bulk Moon ratio is probably 20 + 5. In contrast the Earth mantle ratio is about 42 [Jones and Drake, 1993]. The Rb/Cs solar nebula (CI) ratio is 12. The higher Rb/Cs ratio in the Earth, relative to CI is consistent with depletion of more volatile Cs during early solar nebular processes. The Rb/Cs ratios in the Earth and Moon seem to be the reverse of what would be expected, if the Earth's mantle were the source of the proto-lunar material. Cs is more volatile than Rb and during the giant impact, it might be expected that the Rb/Cs ratio of the material that finishes up in the Moon would increase.

Rb/Sr and 87[Sr86Sr Ratios

The low lunar initial 87Sr/86Sr [LUNI = 0.69895 + 3; Nyquist, 1977] is essentially equivalent to the solar system initial ratio of 0.69897 + 1.5 [Lugmair et al.,

1989] based on the very low initial 87Sr/86Sr ratios (BABI = 0.69899 + 4) observed in the achondritic meteorites. The terrestrial Rb/Sr ratio of 0.03 is nearly a factor of 10 lower than the CI ratio, also implying a massive pre-terrestrial depletion of volatile elements, but the terrestrial initial 87Sr/86Sr ratio is not known. The basaltic achondrites, the source of BABI, record ages of about 4550 m.y. and indicate a major separation of Rb from Sr at this early stage of solar system formation. Thus the impactor composition seems to have shared in the depletion in volatile elements which appears to be characteristic of the inner planets.

Iron

Values for the lunar iron content, expressed as FeO, range between 6 and 18% FeO [Warren, 1986]. A principal difficulty with estimates based on petrological grounds is that all samples come from fractionated sources, as shown by their depletion in Eu and Sr and the Lu-Hf systematics [Fujimaki and Tatsurnoto, 1984] and so provide only indirect evidence about the bulk lunar composition. This seems particularly true of the highlands samples (especially the Mg-suite) used by Warren [1986; see discussion by Drake, 1986]. The •nost common estimate, based on density, magnetics, seismology and petrology is about 13+1%, [e.g., Jones and Delano, 1989] which is one-third of the CI volatile-free value

(35.7%FEO). Some of the iron (2-3%) is probably present as metallic Fe in a core. In contrast, the terrestrial

mantle has an FeO content of 8+1%. Controversy exists about the FeO content of the lower terrestrial mantle.

Core formation is likely to be coeval with accretion, so that the silicate mantle composition at 4.5 b.y. was probably not very different from present values. However, all comparisions between the Moon and the terrestrial mantle are bedevilled by the increasing evidence for its heterogeneity and complex history, including possible modification by the arrival of the metallic core of the impactor shortly after the initial impact (Fig. 1).

Trace Siderophile Elements

The trace siderophile elements are depleted in mare basalt samples according to their metal-silicate partition coefficients [Newsotn, 1986; Newsom and Palme, 1984; Drake, 1983]. This depletion is consistent with the removal of these elements into a small lunar core. The

origin of the chondritic-like patterns in the upper mantle of the Earth for Pd, Re, Os, It, Pt and Au has frequently been attributed to a late-stage veneer [e.g. Chou, 1978; Morgan et al, 1981]. The problem with the model is that the Moon does not show evidence of such a late veneer, as pointed out by several workers.

An additional factor is that during terrestrial impact events, the siderophile element abundances found in impact ejecta do not necessarily preserve their meteoritic ratios but may exhibit considerable fractionation [e.g. Attrep et al., 1991]. Whether this effect operates at the scale of the large collision and whether it affects the bulk lunar abundances are subjects for future investigation, but add yet another question to the problem.

Even the lunar value for Ni is not agreed upon. Delano [1986] estimates a value of 520 ppm for the nickel content of the lunar mantle. This compares with 2000 ppm in the terrestrial upper mantle. The lunar value is based on the observation that the Ni-MgO ratio for 1ow-Ti lunar glasses is a factor of four lower than for terrestrial basalts. However, the 1ow-Ti lunar glasses, in contrast to basalts from the terrestrial upper mantle, come from a cumulate region, where Ni is expected to be enriched. All lunar samples to which we have access come from that portion of the Moon which was previously molten. Some Ni will enter the metallic core. A considerable portion will enter olivine and orthopyroxene, crystallising early from the magma ocean. Accordingly, the Delano [1986] value of Ni for the silicate portion of the Moon is likely to be overestimated by at least a factor of two. Thus the nickel content of the Moon is likely to be about an order of magnitude lower than that of the terrestrial mantle.

In summary, although a vast literature has arisen on

TAYLOR AND ESAT 35

1 minute after contact

Impactor '• '"•• core •.'-•i•mantle

mantle

10 rmnutes

I hour

: ....

:'•':•.. :'•:i"; ?._./ ',•.,,' 5 : • '

ß [""'"•:'• ..., .> ß "••-"r•"•Impactor mantle " ' Impactor core

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.....

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4 hour

•iii•i•i '•.•i5 Impactor :::•-:"•,:"2•.,:i•"•<-- core

24 hour

Earth ..".<.(4s

Proto-Moon -->.•

Fig. 1. A cartoon showng several stages of one computer simulation of the formation of the Moon during a giant impact. This depicts the events following the oblique collision with the Earth of an object of 0.14 Earth masses

at a velocity of 5 km s-1. Both the Earth and the impactor have sep,'u'ated into a metallic core and a silicate mantle. The time elapsing from the collision is shown in the upper right corner of each box. Following the collision (a.,b) the impactor speads out (c). Most of the debris from the Earth is in ballistic trajectories and falls back to the Earth. The iron core of the impactor separates from the silicate mantle (d) ,m•d falls into the Earth (e) about four hours after the impact. Twenty-four hours after the impact, a silicate lump of lunar •nass (0.012 Earth mass), mostly derived from the impactor mantle has clumped together by gravitational attraction and is in orbit (f). (Courtesy A. G. W. Cameron)

the alleged similarity of lunar and terrestrial siderophile elements, the situation can be summarised briefly. Core formation occurred in both Earth and impactor.

36 LUNAR ORIGIN

the collision the impactor core was accreted to the Earth. Melting of the Moon during accretion resulted in the formation of a small core which sequested the small remaining lunar budget of siderophile elements in order of their metal-silicate partition coefficients. In contrast, the terrestrial mantle contains an excess of siderophile ele•nents, in chondritic ratios, and is not in equilibrium with the core. Whether the source of these elements was

the impactor core, or due to a late veneer (missing on the Moon!) is not clear. The upshot is that little of genetic significance with respect to the Earth-Moon connection can be gathered frown the siderophile element abundances.

Refractory Lithophile Elements

The abundances of these elements in the Moon have

been the subject of controversy [see discussions by Drake, 1986, and Taylor,1982,1986a]. Discussions of refractory lithophile abundances in the bulk Moon have concentrated typically on the AI and U concentrations, since the abundances of both elements have some independent constraints. The amount of A1 in the bulk Moon must be

adequate to account for the thick feldspathic crust. The amount of U and Th must be sufficient to account for the

heat-flow constraints and for the high near-surface concentrations of these elements. The appearance of plagioclase relatively early during magma ocean crystallisation is needed to produce the depletion in Eu in the mare basalt source regions and requires a higher content of the refractory elements Ca and A1 than in the terrestrial mantle. Observations froin the Galileo Mission

of the lunar farside [Belton et al., 1992] support the concept of a crust dominated by anorthosite. The Clementine Lunar Mission has likewise indicated that the

farside thick lunar highland crust is nearly pure anorthosite, supporting the concept of a high alumina content for the bulk Moon [Lucey et al., 1995].

The essential point about the bulk lunar composition is that it (a) must contain concentrations of Ca and AI that are sufficiently high to account for the thick feldspathic crust (11% of lunar volu•ne) and (b) must allow plagioclase to have precipitated early enough to extrac• Sr and Eu from the •nare basalt source region and for the crystals to be transported freely to the surface. There they float as a thick iceberg-like crust on the liquid magma ocean to produce the lunar highland crust. The low density (hence anorthositic) crust required by the lunar moment of inertia and other geophysical evidence demands that between 40 and 70% of the total A1 budget of the Moon has been segregated into the highland crust.

Mueller and Phillips [1986] made one of the few

combined geophysical and petrological evaluations of the A1 content of the bulk Moon. They concluded that the lunar mantle seis•nic velocity data favor high-alumina bulk Moon compositions (> 5% A1203), compared to models with 3.5% A1203 that match the terrestrial mantle

abundance [Mueller et al., 1988, Hood and Jones, 1986]. The U abundance in the bulk Moon appears to be at

least 30 ppb, while the terrestrial mantle abundance is less than 20ppb [see discussions by Drake, 1986 and Taylor, 1986a]. Since uranium is a refractory element, its abundance will correlate with those of A1, Ca and Ti. A high lunar A1 content •neans a high lunar uranium abundance; refractory elements are not observed to separate under nebular conditions except under the extreme conditions that produced the CAI's. Although lunar uraniu•n values as low as 19ppb [Rasmussen and Warren, 1985] have been proposed, these fail to provide enough uranium (or other refactory elements) for geochemically reasonable lunar models [see detailed discussion by Taylor, 1986a]. The terrestrial U abundance of about 20 ppb is tightly constrained by an interlocking set of isotopic and chemical abundance ratios (K/U, K/Rb, Rb/Sr, Sm/Nd).

In •nagma ocean models [Taylor and Jakes, 1974], KREEP forms the residual melt layer sandwiched between the top of the cumulate pile and the bottom of the anorthositic crust. KREEP, with its very high abundances of incompatible eleinents, including the REE, Zr, Hf, Ba, U, Th and other refactory lithophile elements, appears to be concentrated near the surface only on the near-side, where it has perhaps been excavated from beneath the crust by the Procellaru•n basin collision. Thus its apparent absence on the lunar farside may have more to do with the thickness of the highland crust in that area, and the scarcity of deep basins than any regional difference in the incompatible element abundances. Attempts to limit the abundances of the elements such as U and Th in the

highland crust, and in the bulk Moon by their apparently low surficial concentrations on the lunar farside may merely indicate that the KREEP layer has not been excavated.

V, Cr and Mn

V, Cr and Mn are depleted relative to CI abundances both in the terrestrial upper mantle and the lunar smnples [Wanke et al, 1984, Wanke and Dreibus, 1986]. The question is complicated by the uncertainty in the terrestrial lower •nantle abundances, the possible presence of V in perovskite structures, or in the core. In the Moon, the question is even •nore cmnplex because of the differing oxidation states in the reducing lunar environment.

Schreiber [1977] estimates that 5% exists as V 5+, 25% as V 4+ and 70% as V 3+ with perhaps a trace of V 2+. This will lead to a distribution of V during crystallisation of the maglna ocean unlike that of the terrestrial mantle making comparisions difficult.

Manganese is depleted both in the Earth's mantle and the Moon to about 30-40% of its CI value. Whether this

depletion is due to volatility, entry into a metallic core, or, in the Moon, entry into olivine or orthopyroxene phases buried deeply within the Moon, is uncertain. Thus the apparent si•nilarities in Mn abundances between the terrestrial •nantle and the Moon •nay be due to several causes. These include the very complex core-mantle relationships within the Earth, the unknown composition of the lower mantle (which •nakes norlnalisation to S i in these comparisons of doubtful significance), differing redox states for vanadium and the possible late addition of CI material to the Earth, but not the Moon.

Moreover, V, Cr and Mn show similar depletion patterns to those in the Earth and Moon in the CO, CM and CV chondrites, where they are presumably due to volatile depletion. A principal difficulty with all such che•nical colnparisons is that of non-uniqueness. Similar processes may produce analogous values in different bodies, but such coincidences may not have genetic significance, in agree•nent with the assessment of Humayun and Clayton [1994].

A Unique Composition?

Relative both to the Earth and the primitive solar nebula, the Moon is heavily depleted in the •nost volatile elements as a class [Wolf and Anders, 1980]. It is depleted in the moderately volatile elements by smaller factors, and is enriched in the refractory ele•nents. No other satellite, even Io which has a substantial sulfur content, appears to be so depleted in volatiles as the Moon. Mars has about twice the terrestrial inventory of volatile ele•nents. The satellites of the outer planets are characterised by large ice/rock ratios.

However the compositions of Vesta and Mercury may be analogous. 4 Vesta was identified as the eucrite parent body (EPB) by Consohnagno and Drake [1977], a suggestion recently strengthened by the observations of Binzel and Xu [1993]. The EPB is depleted in K relative to U and in other volatile (e.g. TI) and siderophile elements (Fig. 2) to an extent silnilar to that of the Moon [Morgan et al., 1978]. Although 4 Vesta appears to be unique among the large asteroids, the main significance of

_

the eucrites is that processes producing lunar-like compositions depleted in volatile ,and siderophile elements

TAYLOR AND ESAT 37

were operating close to To.

Mercury may also be depleted in volatiles, but this is uncertain. High te•nperature evaporation has frequently been proposed [e.g. Fegley and Cameron,1987] to account for the high iron/silicate ratio. Their models require 80% evaporation, with the alkalies being severely depleted. The presence of a sodiu•n cloud around Mercury [Hunten et al., 1988] possibly derived frown a lunar-like plagioclase crust is not consistent with such high degrees of vaporisation. The alternative explanation for the high density of Mercury, that the high iron/silicate ratio is due to removal of mantle silicates during a •nassive collision [Wetherill, 1988], does not necessarily require a •najor volatile depletion. If Mercury has a plagioclase-rich crust analogous to the lunar highlands, then it is likely to be depleted in the more volatile elements, since flotation of such a crust in a •nagma ocean requires a water content less than 0.1% [Walker and Hays,1977]. Attempts to secure a K/U ratio for Mercury, which would shed some light on these interesting proble•ns, should be accorded a high priority.

FRACTIONATION EFFECTS OF THE GIANT

COLLISION?

In this section, the composition of the Moon is examined in relation to possible coinpositions produced during the impact of a Mars or larger-sized projectile on the Earth. Can limits be placed on the temperatures to which the proto-lunar Inaterial has been subjected? A caveat worth noting is the state of the Earth at the time of the impact. Although it was certainly melted by the lunar- forming impact, perhaps it was molten already from previous large collisions. We add this to the already formidable list of unknowns. As discussed earlier, the bulk lunar co•nposition can be separated into groups of elements on the basis of relative volatility.

Super-refracto(y Elements

The super-refractory elelnents are Zr, Hf, Y, and Sc. In contrast to their behaviour in some meteorite minerals

(e.g., hibonite) and in CAI refractory inclusions where they are separated from the lesser refractory elements such as the REE, they do not appear to be fractionated in the Moon relative to the other refractory elements.

Refractory Elements

The refractory ele•nents which include AI, Ca and Ti renorig the •najor elements and REE, Th, U, Ba and

38 LUNAR ORIGIN

among the trace elements, have important inferences for lunar origin, for there is no relative fractionation between the various refractory lithophile elements. This contrasts with the evidence for volatility-controlled fractionation in refractory inclusions in meteorites.

When we consider the REE patterns of the refractory inclusions (CAI) in Allende, there is clear evidence of dependence on relative volatility in Groups II, III, VI and the "ultrarefractory" group. In Groups III and VI, Eu and Yb show depletions or enrichments relative to their more refractory neighbours. The extreme patterns shown by Group II and the "ultrarefractory" group reflect more colnplicated scenarios [MacPherson et al., 1988]. Thus the Group II inclusions are depleted both in the most volatile and the most refractory REE. In contrast, the "ultrarefractory" group ,are enriched in the most refractory REE and depleted in the volatile and Inoderately refractory REE. All these effects are attributable to evaporation and condensation involving gas-solid reactions.

However the REE pattens in the Moon show no sign of loss of the more volatile members of the group. They span a considerable range in volatility from moderately refactory (Eu, 1290 K) to very refractory elements (e.g. Er, 1676 K; the telnperatures listed are for 50%

condensation at 10 -3 arm frown Fegley, 1986 and Larimer, 1988). The lunar REE patterns are Inostly sub-parallel to the CI abundances, with minor excursions attributable to crystal-liquid fractionation in igneous melts within the Moon. The most striking deviations are the enrichments and depletions of Eu, due to its separation from the other trivalent REE. This effect occurs in the highly reducing lunar environment, where the smaller Eu 2+ ion mimics the geochemical behaviour of Sr 2+. We can thus attribute the behaviour of relatively volatile Eu in the Moon, to crystal-liquid equilibria since Yb, of about the stone volatility, shows no difference froln its neighbours on chondrite normalized plots. Although the source regions of some In,are basalts Inay be depleted in heavy REE (Gd- Lu) [Nyquist et al., 1977], this does not imply a whole Moon depletion of HREE relative to LREE. This depletion in HREE is consistent with a magma ocean model in which HREE are selectively re•noved in early crystallising olivine and orthopyroxene, prior to the crystallisation of the source regions froln which the mare basalts are derived.

In summary, the material now in the Moon has REE abundances parallel to those in CI, although enriched to a degree depending on one's views of bulk lunar composition, as deduced from the abundances of other refractory elelnents such as Ca, AI, U, and Th.

Sr (1275K) and Ba (1227K; the telnpemtures listed are

for 50% condensation at 10 -3 atln from Fegley, 1986 and Larimer, 1988) are the most volatile of the lithophile refractory trace elements, but they are not depleted relative to the other refractory elements. It thus appears that all elements with condensation temperatures above 1100-

1200 K (at 10 '3 atm.) a_re present in their cosmic abundance proportions. Thus the enrichment of the refractory elements relative to CI abundances is about a factor of 2.5. The terrestrial mantle, or Mars as a proxy for the impactor, do not appear to have the requisite enrichment in refractories to fit the lunar composition. We have argued for only minor modification to the abundance of volatile elements due to the high temperatures generated by the giant impact. Where, then, did the Moon get its refractories? Was the impactor refractory rich even though planets in the terrestrial neighbourhood do not appear to be especially refractory?

Siderophile Elements

The siderophile elements are depleted in the lunar mantle. Their behaviour during the collisional process is difficult to assess. Firsfly they will have undergone extraction into the core of the impactor and the Earth. Secondly, those which were accreted to the Moon have been sequestered into a lunar core. The depletion pattem of siderophile elements in the Moon is correlated with increasing •netal-silicate partition coefficents (Co-W-Ni- Mo-Au-Pd-Re-Ir), not with increasing volatility (Re-W-Ir- Mo-Pd-Au-Co-Ni).

FeO is •nore volatile than the other major oxides during distillation experi•nents [Hashimoto, 1983]. Thus the lunar abundance of 13% FeO presumably represents a minimum for the FeO content of the impactor mantle. Hashimoto [1983] used material of CI composition as his starting material in vacuum distillation experiments. The

residue resulting from 38% evaporation at 1800øC has 12% FeO which closely matches that of the bulk Moon. The refractory elements, A1 and Ca are enriched relative to the starting material, ,as ,are S i and Mg to a lesser degree. Thus it is possible to produce a "lunar-like" composition, for the major elements, by evaporation of material of CI composition.

Could the FeO content of the Moon result from such

a high temperature process by evaporation from a primitive solar nebula composition? Firstly the process is probably not very efficient on the short time scales involved in collisional processes. Secondly, many of the trace siderophile elements (eg. Ir) which are also depleted in the Moon are notably refractory, so that their depletion in the Moon cannot be due to volatilization.

TAYLOR AND ESAT 39

TABLE 1. Comparision between the composition of the Earth's mantle and the bulk Moon shows no correlation with

volatility.

Element Wt Earth Bulk Moon c l•si ticart on % Man tie Moon Earth

Refractory TiO2 0.16 0.3 1.9

Refractory A1203 3.6 6.0 1.6

Refractory CaO 2.9 4.5 1.6 Moderately refractory MgO 35 32 0.91 Moderately refractory SiO2 50 44 0.86

Moderately volatile FeO 8.0 13.0 1.63 Volatile K20 0.02 0.01 0.5

Volatile Na20 0.34 0.09 0.26

when the composition of the terrestrial mantle is compared with that of the bulk Moon, there is no correlation with volatility (Table 1). This result seems robust even given the uncertainties in the two estimates.

In accepting the predictions for the genesis of the Moon from a giant Earth impact, the consequences of one of the more interesting predictions of the models have not been fully explored. Specifically, the models predict at least a 4 hour time difference between the initial impact and the capture of the impactor core. The core then plunges into the already molten Earth magma ocean (Fig. 1). The passage of a large molten mass of iron-nickel through part of the molten Earth crust is akin to a second core for, nation event. The momentum of the impact and the tidal forces generated are likely to result in some mixing between the silicate mantle and the iron-nickel mass of the impactor core. Simply stated the final composition of the Earth mantle is not expected to be the same as it was prior to or after the giant impact but modified by the core capture event. This produces additional complications in comparisons between the Moon and tile present Earth mantle composition.

Moderately Volatile Elements

The moderately volatile elements in tile Moon are depleted by small factors relative to the Earth. Thus although K (1000K) and Rb (1080K; the temperatures are

for 50% condensation at 10 -3 arm from Fegley, 1986 and Larimer, 1988) are depleted by over ml order of magnitude relative to CI abundances, they are depleted only by a

factor of two relative to terrestrial abundances. The

moderately volatile element Mn shows only a minor difference in abundance between Moon and Earth, but is depleted in both bodies to about 40% of its CI abundance. These elements were apparently less depleted by the collisional process than by the initial separation in the nebula at To. This conclusion is supported by the very

low value of LUNI, the lunar initial 87Sr/86Sr ratio, noted earlier. Cs/Rb ratios are higher in the Moon than in the Earth, contrary to expectations based on relative volatility [Kreutzberger et al., 1986; Jones and Drake, 1993, Norman et al., 1993]. Was Cs enriched on the Moon by a late addition of CI material as suggested by Ringwood [1986]? Many other volatile elements would have been added by such an event (e.g., Pb, Bi, H20,

etc.; Jones and Drake, 1986). All these elements are however extremely depleted on the Moon, making a late addition of CI material unlikely.

A significant contribution by Humayun [1994] and Humayun and Clayton [1995] has found that there is no fractionation of the potassium isotopes except for minor effects due to sputtering of lunar surface materials) in the Earth, Moon, and meteorites. This rules out evaporation mechanisms for depleting potassium in the inner solar system. Specifically, it excludes accounting for the low potassium content of the Moon relative to the Earth by boiling off potassium from terrestrial mantle material. This finding places the major depletion of the volatile elements back into the reahn of nebular processes close

to To, a result consistent with the low initial

40 LUNAR ORIGIN

TABLE 2. The loss of volatile elements during tektite formation is shown by this comparison between impact glasses formed at lower temperatures and tektites formed at higher temperatures from similar source rocks. Zhamanshinites and irghizites were formed at the Zhamanshin crater during the same event. The Henbury sediment was the parent material for the Henbury impactites. The composition of the australites are closely similar to that of the Henbury sediments. All samples have similar compositions for major elements and non-volatile trace elements. [Taylor and McLennan, 1979].

Ele•nent Zha•nanshinite Irghizite Henbury Henbury Australite impactite tektite sediment impactite tektite

Cs 7.8 2.3 7.5 8.3 5.2 TI 0.45 <0.02 0.66 0.55 <0.02

Pb 26 2.5 16 20 5.7 Bi 0.12 0.03 0.28 0.22 <0.02

ratios of BABI and LUNI. THE TEKTITE ANALOGUE

The Highly Volatile Elements

These elements which include B i, TI, Cd, Br, Se, Te

and In condense below about 800 K (at 10 -3 atto.). They are not depleted selectively in order of their condensation temperatures but are depleted in the Moon by factors of about 200 relative to CI abundnces and by factors of about 50 relative to their abundances in the terrestrial mantle.

These elements •nay have been •nore strongly depleted in the impactor than in the Earth, as suggested by the low value of LUNI. The uniform depletion of the highly volatile elements ,argues for a condensation mechanism, as noted long ago by Anders and coworkers [e.g., Wolf and Anders, 1980]. Evaporation or boiling off of volatiles will deplete them in order of volatility, which is not observed, as emphasised by Anders m•d coworkers [e.g., Wolf and Arttiers, 1980] and would produce isotopic fractionation, again which is not observed for the volatile element, potassium [Huyamun, 1994; Humayun and Clayton, 1995].

Water is apparently entirely absent on the Moon, except perhaps for some trapped ice in deep polar craters from a late cometary addition.

Those noble gases which ,are present in the lunar stunpies either originate from less volatile radiogenic parents or are derived from the solar wind [Ozima and Podosek, 1983, Swindle et al., 1986]. The absence of any indigenous noble gases, like that of H20, in the Moon is

a first-order observation and •nay perhaps be due to the giant collision, from m•alogy with tektite-fonning events, to which we now turn.

The formation of impact glasses and tektites during hypervelocity i•npacts on terrestrial sedi•nentary rocks [e.g., Taylor, 1973, Koeberl, 1994, Taylor and Koeberl, 1994] provides a s•nall scale analogue for the events discussed here. The impact of km-size bodies with the Earth in these events is in strong contrast with the giant impact hypothesis, which requires a glancing collision of a body of 15% Earth mass to obtain both the appropriate angular momentum and to place material out in lunar orbit. Nevertheless,there appear to be so•ne parallels even though the events are on different scales.

Analyses of tektites reveal that they are composed almost entirely of melted t,'u'get material, with minimal signatures of the impactor. In tektite-forming events, only the most volatile elements are lost from the target material. Two examples are instructive. During the formation of the 10 km diameter Zhamanshin crater in

Russia, both impactites and small tektites were formed. The only significant difference was a depletion in the very volatile ele•nents (e.g., Bi, Pb, TI) in the tektites, formed at higher temperatures, compared to the impact glasses [Taylor and McLennan, 1979]. A si•nilar situation exists with respect to impact glasses formed at the Henbury craters, in central Australia, which closely resemble the country rock composition. By a fortunate happenstance, their composition for most elements is very close to that of the Australasian tektites [Taylor, 1973], although the latter are derived frown different source rocks. Thus it is

possible to use the subgreywackes at Henbury as an analogue for the source rocks of the Australasian tektites.

The comparison shown in Table 2 demonstrates

during the formation of tektites from country rocks, elements more volatile than cesium ,are depleted. The stone conclusion holds for F and CI, more strongly depleted in tektites than impactires, and H20, present at about 80

ppm in tektites [King and Arndt, 1977], although the parent sediments might have contained orders of magnitude more water. However, Humayun and Clayton [1995] record no measurable fractionation of the potassium isotopes in tektites, so that moderately volatile potassium was not subjected to high temperature distillation.

The significant point in the present context is that elements more volatile than Cs, and including water, are depleted during these high-energy tektite-forming events, without affecting the concentrations or isotope ratios of the more refractory elements. Thus it seems possible to attribute the depletion of the Moon in water and very volatile elements to the giant collision, but that elements as volatile as potassium were little affected, as shown by the lack of isotopic fractionation [Hurnayun and Clayton, 1995]. Potassium (and Rb as shown by the low value of LUNI) was already probably depleted in the impactor as a consequence of the early nebula-wide volatile element depletion at To.

Lunar and Martial1 Meteorites

Lunar and martian meteorites can also be included in

the category of impact derived materials. In comparing these events with the single massive Earth impact, where the impactor mass is as large as 15% of the target, the question of scale is relevant. Presumably, more energetic impacts are required in these cases than those responsible for tektites. It is not possible to determine whether samples of the associated projectile material exist in meteorite collections, as these would be indistinguishable from other meteorite types. However, large amounts of target material must have been ejected into lunar and martian orbits and beyond, to have a chance of hitting the Earth, to survive atmospheric entry, and to be recognised and collected. Clearly, material has survived through all of these energetic events, essentially unmelted and intact. The significant point in the present context is that even following a massive impact, capable of ejecting material from one planet to another, unmelted samples of target material can survive the destructive energies and temperatures.

Temperatures as high as 8000 to 14000 K can be generated during the giant impact with Earth [Cameron and Benz, 1991]. The immediate consequence has been assumed to be complete melting and vaporisation of even

TAYLOR AND ESAT 41

the most refractory elements. The material evidence, preservation of all but the most volatile elements and absence of K and Mg isotope fractionation in tektites as well as in lunar materials is contrary to this assumption. In particular, the absence of isotope fractionation in K in the Moon places stringent limits on the degree of Rayleigh type high temperature distillation to less than a few per cent; for Mg the limit is <20%. The contradiction can be resolved by assuming bulk transfer of material into orbit, rather than vaporised material. In any event, there is no time for establishing a convecting well mixed reservoir for Rayleigh type distillation to change the isotope ratios.

The absence of volatile loss, except for the very volatile elements and water, and the absence of isotope fractionation due to volatilisation can be explained by bulk transport of ejecta. Mass loss at the outer envelope is able keep temperatures significantly lower than expected on the basis of energetics alone. Thus it is possible to attribute the depletion of the Moon in water and very volatile elements to the giant collision.

THE SOLAR NEBULA AND POSSIBLE

COMPOSITIONS FOR THE IMPACTOR

We have an excellent understanding of the composition of the now-vanished solar nebula because of the essential

identity between CI and the solar photospheric abundances. In particular, recent work has resolved the outstanding anomaly of distinctly different iron abundances: the new solar values now match the CI iron

abundances [Holweger et al., 1990]. As far as we understand the composition of Venus and

Mars, they seein to share with the Earth a marked depletion in volatile elements. This is shown clearly by the similar K/U ratios for Venus and the Earth. The SNC

meteorites, probably frown Mars, have higher K/U ratios, indicative of a volatile element budget about twice that of the Earth, but still strongly depleted in comparision with the primordial solar nebula values as given by the CI ratios [see review by Taylor, 1992]. The moderately volatile elements (400-1100 K) are depleted in the terrestrial planets and in several classes of •neteorites.

There seems to have been a general depletion of volatile elements throughout the inner nebula that occurred early in solar system history, since very low initial 87Sr/86Sr ratios are observed in the achondritic meteorites. The Pb-Pb and Rb-Sr meteorite isotopic data record a major separation of Rb from Sr and Pb from U and Th before about 4560 m.y. [Tilton, 1988]. The terrestrial Rb/Sr ratio is a factor of 10 lower than the CI

ratio, implying that the Earth accreted from

42 LUNAR ORIGIN

depleted planetesimals [Taylor and Norman, 1990]. What was the cause of this depletion? It could not be

selective condensation, the reverse of evaporation, since isotopic variations would be observed. The isotopic composition of potassium in CI is assumed to be the initial solar nebula ratio and the marked decrease in the

abundance of potassium in the inner nebula did not result in isotopic fractionation.The cause of this depletion of the inner regions of the nebula is most probably connected with early intense sol,'u' activity, flare outbursts and with strong stellar winds, similar to those observed with the young T Tauri and FU Orionis stars. The depletion is not selective and does not correlate with the condensation

temperatures of the individual elements, but rather the group as a whole ,are depleted uniformly.

The depletion of volatile elements must have occurred through physical processes (e.g. sweeping out of fine material by early solar winds) at relatively low temperatures, so that the nebula was cool at that stage. Several other features of the solar system are explained if there was an early physical clearing of water and other volatiles (froIn the inner sol,-u' nebula) that had not yet been accreted into bodies large enough to resist being swept out of the inner sol,'u' system by strong early solar winds. A pile-up of this material at a "snow line" at 5 AU promotes a runaway growth of a 15-20 Earth-mass embryo at that location. Gravitational collapse of the remaing gas in the nebula produces Jupiter before the gaseous nebula is dissipated. Early formation of Jupiter depletes the region of the asteroid belt, which contains

only 10 '3 Earth !nasses, and stunts the subsequent growth of Mars, which is 3000 times less massive than its giant neighbour.

This implies that there was ,an initial complete, rather than p,'u'tial, condensation of potasssium in the nebula and that the depletion of potassium and other volatile elements in the inner nebula wes due to physical processes acting on already condensed !naterial (i.e., the nebula was cool at that stage).

The simih'u'ity in oxygen isotopes between the Earth and Moon indicate derivation of both the Earth and the

impactor from the same region of the nebula, thus excluding models that derive the impactor from the outer

reaches of the sol,'u' syste•n. The si•nilm'ity in 53Cr/52Cr ratios (53Cr is derived in p,'u't froin short-lived 53Mn) between the Earth ,'red the Moon and their contrast with

higher !neteoritic values [Lugmair and Maclsaac, 1995] carries the same implication of derivation of lunar material froin around 1 A.U. A third constraint is the relatively low collision velocity [Benz et al., 1989; Cameron and Benz, 1991] required to produce a Moon-sized body, which

ag,'fin restricts the impactor to be a nearby object. The impactor composition is somewhat of a free

parameter, within the constraint that it must have formed in the same region of the solar nebula as the Earth to account for the similarity in oxygen and chromium isotopes. If the material in the Moon is derived from the impactor, then that body had a lower Rb/Cs ratio than the

Earth. The primitive lunar initial 87Sr/86Sr ratios indicate that the impactor must have been depleted in Rb relative to Sr very close to To.

Current models assume that core mantle separation occurred before impact, to account for the lunar siderophile element abundances and the lunar depletion in iron (13% FeO) relative to primordial solar nebula volatile-free abundance levels (as shown by the CI meteorites) of 36%. The abundance of FeO in the m,'mtle of the impactor must however have been greater than that of the terrestrial mantle (8% FeO), since the bulk Moon contains a much higher abundance (13%).

Such values are intermediate between the FeO

abundance in the m,'u'tian mantle (in which the most recent estimate based on the SNC meteorites is 18%, Longhi et al., 1992) and that of the terrestrial mantle. Since Mars with a high mantle FeO content exists in the inner solar system, the postulate of an impactor with a high mantle FeO content is not unreasonable.

The probable enrichment of refractory elements in the Moon might be adequately accounted for by the vaporisation and recondensation predicted during the impact itself. In summary the impactor m,'mtle was higher in iron, and more depleted in Rb relative to Sr than the terrestrial mantle. However, it was enriched in Cs relative to Rb compared to the Earth [Kreutzberger et al., 1986; Jones and Drake, 1993].

O'Neill [1991, p. 1169]. proposed that the impactor was an "exotic interloper ....... frown further out in the solar system" of CI coinposition, fully oxidised and volatile rich. However, tlu'ee factors constrain the impactor to have originated in the neighbourhood of the Earth. The first two are the similarity of the oxygen and chromium isotopic compositions between E,-u'th and Moon [ Clayton 1981; Lugmair and Maclsaac, 1995]. These are distinct froin those for Mars (SNC •neteorites) and for most of the meteorites. These variations are generally accepted as implying major heterogeneities in the distribution of the O isotopes, and probably Cr isotopes, in the solar system. The third constraint is the relatively low collision velocity noted above. The very primitive value for LUNI indicates that the impactor shared in the early general depletion of volatile elements in the inner nebula, so that it was not of primordial CI

03 10 UJ

Z o

o 1

o

• 10'1

o z

z 10-2

z

10-3

>1660OK 1660OK.14S0OK 14•0-1300 1300.700 700-4S0 %•.•1•0

ß ß ß ß

ß

-I - Ave Løw'TI :I • •• Lunar basalts ß • -- •, Juvlnas --

ß Moore County ß

Ii Serra de Mag• yb U Eu Cr Rb Cs Br BI ß ,

TI

Fig. 2. The close correspondence between the abundances of the lithophile elements in eucrites and 1ow-Ti mare basalts [after Wolf and Allder's, 1980].

In addition, the rare gas and O isotopic evidence make CI chondrites an unsatisfactory component for building the terrestrial planets, so that there is no reason to suppose that planetesimals of CI composition were abundant in the inner nebula. The great distinction between the region now occupied by the terrestrial planets and that from which the CI chondrites are derived lies in

the striking depletion of volatile elements, that, as shown by low K/U ratios, was common to Venus, Earth and Mars, and which was thus probably due to a major event affecting the inner solar nebula.

It is unlikely that a mars-sized body would remain undifferentiated, during the time that it took the Earth to form core and mantle. During the accretion of a mars-sized body, it is likely, at least froin current thinking about the planetesimal hypothesis, to have encountered several collisions with massive objects, inducing melting and subsequent differentiation. The asteroids in tile inner belt, at about 2 AU are nearly all differentiated [Gaffey, 1990] and it is a reasonable supposition that all objects sunward were similarly heated and differentiated before their accretion into the terrestrial planets [ Taylor and Norman, 1990].

Finally, it is worth noting that the eucrite parent body (4 Vesta) produced basaltic material with similar element patterns to low-Ti lunar basalts, including strong depletions in volatile elements (Fig. 2) at about 4550 m. y. ago, c!o:•e to T o. This strengthens the case for an

TAYLOR AND ESAT 43

impactor with a coinposition analogous to the Eucrite Parent Body.

SUMMARY

It will be apparent that this review raises many more questions than it answers, and places a new set of constraints on the origin of the Moon.

The high bulk FeO content of the Moon, amongst other parameters, rules out the derivation of the proto- lunar material from any but a small fraction of the terrestrial mantle. Most of the material now in the Moon

was probably derived froin the silicate mantle of the impactor, although less than 10% of the impactor (15% Earth mass) is required to forin the Moon.

Those elements with condensation temperatures above 1100-1200K are present in the Moon in their solar nebular ratios, so that material subjected to extreme temperatures during the tinpact event apparently was not incorporated in the Moon. Per!laps much of the proto-lunar material was little affected by the impact, by analogy with the relatively pristine nature of the Martian meteorites.

The lack of fractionalion effects in the potassium isotopes rule out evaporative loss of' potassium by Raleigh-type distillation processes during the giant collision. Coupled with the very low value for the initial

87Sr/86Sr ratio (LUNI = BABI), the depletion of the Moon in many volatile elements can be ascribed to a volatile depleted impactor. The formation of tektites during much smaller scale collisional events provides some analogues. Relative to their source material, tektites have lost H20, ,and elements more volatile than Cs (e.g.,

Pb, TI, Bi) but display no potassium isotope fractionalion. The depletion of tile bone-dry Moon in these very volatile elements thus may have occurred as a result of the giant collision.

The composition of the impactor appears to have had an iron-rich m,'mtle, in{ermediate between that of Earth and Mars, but with a low Rb/Sr ratio, established close to T O

and a low volatile content, analogous to the Eucrite Parent Body, in which the depletions in the siderophile elements, alkali elements and more volatile elements are similar to

those in the Moon.

Acknowledgements: We are grateful to R. E. Hannigan and an anonymous, if tardy referee for useful reviews. This is LPI Contribution no 856. The Lunar and Planetary Institute is operated by the Universities Space Research Association under NASA grant no. NASW-3389. This paper is dedicated to Mitsunobo Tatsumoto and George Tilton in honor of their major contributions to geochemistry and

44 LUNAR ORIGIN

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