# Early Silicate Earth Differentiation

Guillaume Caro

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Guillaume Caro. Early Silicate Earth Differentiation. Annual Review of Earth and Planetary Sciences, 2011, 39 (1), pp.31-58. 10.1146/annurev-earth-040610-133400. hal-02329920

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Early Silicate Earth
Differentiation

Guillaume Caro

Centre de Recherches Pétrographiques et Géochimiques, Centre National de la Recherche Scientifique, Université de Lorraine, 54501 Vandoeuvre-les-Nancy, France;
email: caro@crpg.cnrs-nancy.fr

Annu. Rev. Earth Planet. Sci. 2011. 39:31–58

First published online as a Review in Advance on
January 3, 2011

The *Annual Review of Earth and Planetary Sciences* is
online at earth.annualreviews.org

This article's doi:
10.1146/annurev-earth-040610-133400

Copyright © 2011 by Annual Reviews.
All rights reserved

0084-6597/11/0530-0031$20.00

Keywords

extinct radioactivity, <sup>142</sup>Nd, Hadean, magma ocean, hidden reservoir, depleted mantle

Abstract

The discovery of small <sup>142</sup>Nd anomalies in early Archean rocks has brought about a revolution in our understanding of early planetary differentiation processes. <sup>142</sup>Nd is a radiogenic isotope produced by the decay of now-extinct <sup>146</sup>Sm in crustal and mantle reservoirs. Given that <sup>142</sup>Nd heterogeneities can be produced only prior to 4.2 Gya, this short-lived chronometer provides selective information on the very early evolution of primordial silicate reservoirs. This information is particularly crucial for Earth, where the fingerprints of the earliest crustal formation processes have been almost entirely erased from the geological record. This article reviews the history of the field, from the pioneering applications of the <sup>147</sup>Sm-<sup>143</sup>Nd and <sup>146</sup>Sm-<sup>142</sup>Nd systems to ancient crustal rocks, to the more recent insights gained from application of <sup>146</sup>Sm-<sup>142</sup>Nd to meteorites and lunar samples.
# 1. INTRODUCTION

The first hundred million years of planetary evolution were punctuated by highly energetic collisions between Moon-to-Mars-sized embryos (Chambers 2001, Chambers 2004). These giant impacts likely created global magma oceans on the accreting planets (Benz & Cameron 1990, Melosh 1990) and thus provided the energy required for differentiating planetary mantles. In small bodies such as the Moon and Mars, these early events were recorded by radiogenic isotope systems, which point to very early (4.4–4.5 Gya) formation of mantle and crustal reservoirs (e.g., Blichert-Toft et al. 2002, Borg & Draper 2003, Debaille et al. 2008, Elkins-Tanton et al. 2003, Snyder et al. 2000, Taylor et al. 2009). On Earth, however, continuous exchanges between the mantle and crust have led to apparently complete rejuvenation of the surface over the past 4.5 Ga (Condie 2000). As a result, the isotopic and chemical heterogeneities observed in modern mantle-derived rocks reflect later production and recycling of continental and oceanic crust through geological time (Allegre et al. 1980, Armstrong 1968, Hofmann 1988) and bear no relation to primordial mantle differentiation processes.

Despite the homogenizing effect of crustal recycling and mantle convection, isotopic heterogeneities resulting from primordial differentiation of Earth's mantle were not completely erased from the geological record. Small isotopic anomalies, created by radioactive decay in Hadean (4–4.5 Gya) silicate reservoirs, have been sampled by Archean magmatism and preserved in these ancient rocks long after the early terrestrial crust had disappeared. These isotopic signatures can be used to date the formation of these now vanished reservoirs and to quantify their interactions through geological time. This chronological information provides key constraints on early crustal formation processes that would otherwise remain inaccessible owing to the lack of a preserved Hadean rock record.

The most commonly employed isotope systems for studying early silicate Earth differentiation are the <sup>147</sup>Sm-<sup>143</sup>Nd and <sup>146</sup>Sm-<sup>142</sup>Nd chronometers. These systems regularly exhibit sufficient parent-daughter fractionation during partial melting so as to allow isotopic differences to develop in planetary reservoirs. In addition, they have quite different half-lives, which make them sensitive to processes that took place at different periods of Earth's history. The <sup>147</sup>Sm-<sup>143</sup>Nd chronometer, with a half-life of 103 Ga, provides an integrated record of mantle-crust evolution over the past 4.5 Ga, whereas the short-lived <sup>146</sup>Sm-<sup>142</sup>Nd chronometer provides selective information on very early events (>4.2 Gya). Applied together, the <sup>147</sup>Sm-<sup>143</sup>Nd and <sup>146</sup>Sm-<sup>142</sup>Nd systems represent a powerful tool for unraveling the chronology and nature of early mantle-crust differentiation processes.

This review begins with an examination of commonly accepted <sup>147</sup>Sm-<sup>143</sup>Nd constraints on the evolution of the mantle-crust system over the past 4.5 Ga. The fundamental aspects of <sup>147</sup>Sm-<sup>143</sup>Nd geochemistry are reviewed, with emphasis on unresolved issues raised by the Archean <sup>143</sup>Nd record. These questions are addressed in light of recent advances in the application of the <sup>146</sup>Sm-<sup>142</sup>Nd system, which provide a new way of estimating the age of differentiation of primordial silicate reservoirs and shed light on the dynamics of the mantle-crust system during the first billion years of Earth history. The last part of this review covers the <sup>142</sup>Nd record of meteorites and lunar samples, and discusses the implications of very early differentiation processes with regard to the structure and bulk composition of the silicate Earth.

# 2. Nd ISOTOPE ESSENTIALS

## 2.1. Chronometric Equations

Sm is an intermediate rare earth element (REE) with seven isotopes, four of which are radioactive: The long-lived <sup>147</sup>Sm α-decays to <sup>143</sup>Nd with a half-life of 103 Ga, and now-extinct <sup>146</sup>Sm, with
a shorter half-life of 106 Ma, decayed into ¹⁴²Nd (Audi et al. 1997). ¹⁴⁸Sm and ¹⁴⁹Sm are also radioactive, decaying to ¹⁴⁴Nd and ¹⁴⁵Nd, respectively, but their half-lives are too long to generate detectable radiogenic effects in planetary materials. The chronometric equation describing the closed-system evolution of the ¹⁴³Nd/¹⁴⁴Nd ratio in reservoir *i* can be obtained from the following mass balance relationship:

$$^{147}\text{Sm}_{t_p} + ^{143}\text{Nd}_{t_p} = ^{147}\text{Sm}_t + ^{143}\text{Nd}_t, \qquad (1)$$

where *t* is time running forward from the origin of the solar system (*t*₀ = 0) to the present day [*t*ₚ = 4.567 Ga (Bouvier et al. 2007)]. Substituting ¹⁴⁷Smₜ = ¹⁴⁷Smₜ *e*<sup>λ₁₄₇(*t*ₚ-*t*)</sup> into Equation 1 yields the following expression:

$$\left(\frac{^{143}\text{Nd}}{^{144}\text{Nd}}\right)_t^i = \left(\frac{^{143}\text{Nd}}{^{144}\text{Nd}}\right)_{t_p}^i + \left(\frac{^{147}\text{Sm}}{^{144}\text{Nd}}\right)_{t_p}^i (1 - e^{-\lambda_{147}(t_p - t)}), \qquad (2)$$

where λ₁₄₇ = 6.53 × 10⁻³ Ga⁻¹ is the decay constant of ¹⁴⁷Sm. This standard chronometric equation can be used for any radiogenic system in which the parent nuclide is extant (e.g., ⁸⁷Rb-⁸⁷Sr, ¹⁷⁶Lu-¹⁷⁶Hf). For a short-lived radioactivity such as ¹⁴⁶Sm (λ₁₄₆ = 6.74 Ga⁻¹), the present-day parent/daughter isotopic ratio (i.e., ¹⁴⁶Sm/¹⁴⁴Nd) is almost zero, and thus isotope evolution needs to be expressed relative to initial solar system parameters,

$$\left(\frac{^{142}\text{Nd}}{^{144}\text{Nd}}\right)_t^i = \left(\frac{^{142}\text{Nd}}{^{144}\text{Nd}}\right)_{t_0}^i + \left(\frac{^{146}\text{Sm}}{^{144}\text{Nd}}\right)_{t_0}^i (1 - e^{-\lambda_{146}t}). \qquad (3)$$

The general chronological equation for the ¹⁴⁶Sm-¹⁴²Nd system is then obtained by decomposing (¹⁴⁶Sm/¹⁴⁴Nd)ᵢ₀ into readily measurable parameters,

$$\left(\frac{^{142}\text{Nd}}{^{144}\text{Nd}}\right)_t^i = \left(\frac{^{142}\text{Nd}}{^{144}\text{Nd}}\right)_{t_0}^i + \left(\frac{^{146}\text{Sm}}{^{144}\text{Sm}}\right)_{t_0}^i \left(\frac{^{144}\text{Sm}}{^{147}\text{Sm}}\right)_{t_p}^i \left(\frac{^{147}\text{Sm}}{^{144}\text{Nd}}\right)_{t_p}^i (1 - e^{-\lambda_{146}t}), \qquad (4)$$

where (¹⁴⁶Sm/¹⁴⁴Sm)ᵢ₀ = 0.0085 ± 0.0007 is the initial abundance of ¹⁴⁶Sm in the early solar system (Boyet et al. 2010), and (¹⁴⁴Sm/¹⁴⁷Sm)ᵢₚ = 0.20454. The present-day parent/daughter ratio in reservoir *i* is usually expressed as ¹⁴⁷Sm/¹⁴⁴Nd (instead of ¹⁴⁴Sm/¹⁴⁴Nd) to maintain consistency with the ¹⁴⁷Sm-¹⁴³Nd chronometric equation (Equation 2).

## 2.2. Geochemical Properties

The Sm-Nd isotopic schemes present ideal properties for tracing the evolution of differentiated silicate reservoirs (e.g., DePaolo & Wasserburg 1976, Jacobsen & Wasserburg 1979). Both elements are refractory and lithophilic; as a result, the Sm/Nd ratio is not affected by metal-silicate segregation, and volatility-controlled fractionation is limited to high-temperature processes (Boynton 1975, Davis & Grossman 1979). On planetary scales, Sm/Nd fractionation occurs during partial melting and fractional crystallization related to crustal formation and differentiation. As Sm and Nd are both incompatible, they are enriched in partial melts and depleted in solid residues, so planetary crusts concentrate a large budget of these elements (Hofmann 1988, Taylor & McLennan 1985). Nd has a larger ionic radius and is therefore more incompatible than Sm. Consequently, crustal reservoirs have lower Sm/Nd ratios than their complementary depleted mantles (DMs). Closed-system evolution of differentiated silicate reservoirs will, therefore, result in the development of distinct radiogenic ¹⁴³Nd/¹⁴⁴Nd signatures. If differentiation took place prior to extinction of ¹⁴⁶Sm (that is, >4.2 Gya), ¹⁴³Nd/¹⁴⁴Nd heterogeneities will be associated with variations in the ¹⁴²Nd/¹⁴⁴Nd ratio, which are often referred to as ¹⁴²Nd anomalies.
By convention, variations in ¹⁴²Nd/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd ratios in planetary reservoirs are expressed in parts per 10,000 relative to a reference isotopic ratio, using the epsilon notation,

$$ \epsilon^{142.143} \text{Nd} = \left[ \frac{(^{142.143} \text{Nd}/^{144} \text{Nd})_i}{(^{142.143} \text{Nd}/^{144} \text{Nd})_{\text{Ref}}} - 1 \right] \times 10^4. \quad (5) $$

For the ¹⁴⁷Sm-¹⁴³Nd system, the reference ratio for normalization is the chondritic composition [labeled CHUR for chondritic uniform reservoir (DePaolo & Wasserburg 1976)]. For the ¹⁴⁶Sm-¹⁴²Nd system, the convention is to use a terrestrial standard for normalization so that the present-day mantle and crust have, by definition, $\epsilon^{142} \text{Nd} = 0$.

# 3. THE ¹⁴⁷Sm-¹⁴³Nd SYSTEM AS A TRACER OF MANTLE-CRUST EVOLUTION

## 3.1. Early Developments

The ¹⁴⁷Sm-¹⁴³Nd isotope system was first developed as a chronometer for dating basaltic achondrites (Lugmair & Scheinin 1975). In comparison with the Rb-Sr system, which was already successfully applied as a dating technique, application of the Sm-Nd scheme was viewed as more challenging, owing to the limited range of Sm/Nd fractionation in planetary material. Two important properties of the Sm-Nd chronometer, however, motivated its application to problems linked to mantle-crust differentiation. First, in contrast to more volatile elements such as Rb, Sm and Nd were not significantly affected by cosmochemical fractionation in the solar nebula (Larimers & Anders 1967, Wasson & Chou 1974). Their relative abundance in chondrites is constant, averaging ¹⁴⁷Sm/¹⁴⁴Nd = 0.1966 ± 0.004 (Bouvier et al. 2008, Jacobsen & Wasserburg 1984, Patchett et al. 2004), and this chondritic value is indistinguishable from the solar composition within the ±7% uncertainty of photosphere estimates (Asplund et al. 2006). It was thus conservatively assumed that the bulk silicate Earth (BSE) and other terrestrial planets had perfectly chondritic Sm/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios. As discussed in Section 5, this assumption is now being reconsidered in light of recent evidence from the ¹⁴⁶Sm-¹⁴²Nd system. However, for the past 30 years, the CHUR parameters have provided an important reference for the interpretation of isotopic signatures in planetary reservoirs, in terms of enrichment or depletion relative to a primitive mantle (PM) (e.g., DePaolo 1980; DePaolo & Wasserburg 1976, 1979; Jacobsen 1988; Jacobsen & Wasserburg 1979; Richard et al. 1976).

A second advantage of the Sm-Nd system lies in its relative resistance to element redistribution during metamorphism, which allows the precise determination of the initial isotopic composition of ancient rocks at the time of their formation (e.g., Basu et al. 1984, Blichtert-Toft et al. 1999, Moorbath et al. 1997, Shirey & Hanson 1986, Wilson & Carlson 1989). The initial ¹⁴³Nd/¹⁴⁴Nd composition (henceforth noted $\epsilon^{143} \text{Nd}_{\text{initial}}$) can be obtained from the y-intercept of a whole-rock isochron or from back-calculating in situ ¹⁴⁷Sm decay at the supposed age of crystallization. This initial isotopic signature reflects both the Sm/Nd ratio and the Nd residence time in the mantle or crustal reservoir from which the parental magmas are derived. Juvenile rocks derived from a DM will have positive $\epsilon^{143} \text{Nd}_{\text{initial}}$, whereas a negative $\epsilon^{143} \text{Nd}_{\text{initial}}$ reflects melting or assimilation of a preexisting crustal component.

The pioneering results obtained by DePaolo & Wasserburg (1976) suggested that $\epsilon^{143} \text{Nd}_{\text{initial}}$ values in Archean and Proterozoic rocks were close to that expected for a PM with chondritic Sm/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios. These authors concluded that either Earth's mantle had not experienced very early differentiation, or ancient crustal and mantle reservoirs were remixed prior to
<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>

$\varepsilon^{143}\text{Nd}_{\text{initial}}$ in juvenile mafics and ultramafic rocks (orange circles), from 3.8 Gya to present [data from Basu et al. (1984), Lahaye et al. (1995), Polat & Munker (2004), Shirey & Hanson (1986), Vervoort & Blichtert-Toft (1999), Vervoort et al. (1994), Wilson & Carlson (1989)]. The trend toward increasingly radiogenic signatures indicates derivation from a mantle reservoir with a Sm/Nd ratio 5–10% higher than the chondritic uniform reservoir (CHUR). (a) The standard interpretation is that the observed trend reflects continuous crustal formation from a chondritic primitive mantle. This model, however, provides a good fit to $^{143}\text{Nd}$ data only if the depleted mantle (DM) represents less than 25% of the bulk silicate Earth (BSE). Extraction of the present-day continental mass from the entire mantle would generate much smaller radiogenic effects in the depleted reservoir. This model also fails to account for the radiogenic $\varepsilon^{143}\text{Nd}_{\text{initial}}$ in the oldest rocks (Caro & Bourdon 2010). Armstrong (1991) interpreted this as reflecting rapid growth of the continental crust prior to 4 Gya and subsequent evolution in steady state until the present day. Similar to the standard growth model, Armstrong’s Constant Crustal Mass (CCM) model can generate the observed isotopic evolution only if the upper mantle has remained separated from the lower mantle for >4 Ga. (b) $\varepsilon^{143}\text{Nd}$ evolution of the primitive mantle and DM assuming a superchondritic composition for the BSE ($^{147}\text{Sm}/^{144}\text{Nd} = 0.2082$; $\varepsilon^{143}\text{Nd} = +6.9$) resulting from the loss of an early enriched reservoir (EER) at an early stage of Earth’s formation (see discussion in Section 5). The predicted evolution matches that defined by juvenile rocks until 2 Gya, after which the DM deviates toward increasingly radiogenic $\varepsilon^{143}\text{Nd}$. In this nonchondritic Earth model, continuous extraction of the continental mass from the whole mantle can account for this deviation, without the need to call upon massive early crustal growth (Caro & Bourdon 2010). Abbreviation: MORB, mid-ocean ridge basalt.

the formation of the oldest cratons. In retrospect, the alignment of $\varepsilon^{143}\text{Nd}_{\text{initial}}$ along the CHUR evolution line was fortuitous, as it is now well established that mafic rocks as old as 3.8 Ga have positive $\varepsilon^{143}\text{Nd}_{\text{initial}}$ values (e.g., Blichtert-Toft et al. 1999, Moorbath et al. 1997). As shown in **Figure 1a**, juvenile rocks display increasingly radiogenic $\varepsilon^{143}\text{Nd}_{\text{initial}}$ values through time, indicating their extraction from a depleted reservoir with an Sm/Nd ratio 5–10% higher than chondritic. The roughly chondritic $\varepsilon^{143}\text{Nd}_{\text{initial}}$ reported in early studies reflected mixtures between mantle-derived magmas and the unradiogenic crustal material, and had no significance in terms of bulk terrestrial composition.

Despite the early recognition that no terrestrial reservoir had evolved along the isotopic evolution line predicted for CHUR, the chondritic BSE hypothesis remained a cornerstone of models of crustal evolution and chemical geodynamics (e.g., Albarede & Rouxel 1987, Allegre 1982, Jacobsen 1988, Zindler & Hart 1986). An important result of early Sm-Nd investigations was
**MORB:** mid-ocean
ridge basalt

**CMB:** core-mantle
boundary

that mid-ocean ridge basalts (MORBs) have positive ε¹⁴³Nd values between +7 and +12, indicating extraction from a mantle reservoir with a time-integrated depletion history (DePaolo & Wasserburg 1976, Richard et al. 1976, Salters & Stracke 2004). This, in turn, requires that a significant budget of trace elements was extracted from the mantle and isolated in a long-lived reservoir with low Sm/Nd and subchondritic ε¹⁴³Nd. The continental crust, with a mean age of 2 Ga and ε¹⁴³Nd of -15, satisfies all these constraints (Jacobsen & Wasserburg 1979, McCulloch & Wasserburg 1978, Taylor et al. 1983). Thus, for the past 30 years, continuous crustal growth has been widely considered to be the major cause of the chemical and isotopic evolution of the mantle (Hofmann 1988, Jacobsen 1988, Jacobsen & Wasserburg 1979).

## 3.2. Unresolved Issues

Two observations sharply contradict the standard mantle-crust evolution model. First, the ubiquity of positive ε¹⁴³Ndinitial in the most ancient crustal rocks indicates that the sampled upper mantle had experienced depletion in incompatible elements several hundred million years prior to the stabilization of the oldest cratons (e.g., Bennett et al. 1993, Moorbath et al. 1997). Because the present-day continental mass contains only a negligible amount of pre-3-Ga-old terranes (Condie 2000), the formation of the continents as we observe them today cannot account for large-scale mantle depletion in the early Archean. This implies either that the volume of the early Archean crust was much larger in the past or that an early (>4 Gya) event depleted Earth's mantle prior to the onset of continental growth.

The second challenge to the standard model is that the present-day crust does not contain a large enough budget of REEs to balance the radiogenic ε¹⁴³Nd measured in MORBs, unless the MORB source represents less than 25% of the total mass of the mantle (Caro & Bourdon 2010, DePaolo 1980, Jacobsen & Wasserburg 1979). Extraction of the continental crust from the whole mantle would generate only a small radiogenic effect of +2 in the DM (Figure 1a), far below the observed MORB value of +9 (Salters & Stracke 2004). This observation was first interpreted as evidence for layered mantle convection with strict separation at the 660-km discontinuity (Allegre 1982). The requirement for a less degassed reservoir to account for the low flux of ⁴He from the mantle (O'Nions & Oxburgh 1983) and to satisfy argon mass balance constraints (Allegre 1982, Allegre et al. 1996) also supported the idea of a primitive lower mantle. However, this model fell into disfavor as tomographic imaging of the deep mantle showed evidence for vertical exchanges through the 660-km discontinuity (e.g., Grand 2002, Montelli et al. 2004), lending support to whole-mantle convection models. Although these observations do not preclude the existence of a deep primitive reservoir, they demonstrate that the lower mantle is not completely isolated from the upper mantle. The striking absence of primitive trace element or isotopic ratios in oceanic island basalts (Hofmann et al. 1986, Zindler & Hart 1986), including those originating from deep mantle plumes (Montelli et al. 2004), precludes a simple model of layered convection with a chondritic lower mantle.

The requirement for a DM reservoir in the early Archean quickly became a cornerstone of the Big Bang model of crustal growth (Armstrong 1981), also known as the Constant Crustal Mass model. Armstrong (1991) showed that positive ε¹⁴³Nd values in the Archean mantle can be explained by this end-member scenario, whereby a continental mass similar to that of the present day already was formed by 4 Gya and has since remained in a steady state under balanced production and recycling fluxes (see Harrison 2009 for a review). Alternative models involving permanent or transient storage of a Hadean mafic crust at the core-mantle boundary (CMB) were also shown to provide a satisfying fit to ¹⁴³Nd data (Chase & Patchett 1988, Galer & Goldstein 1991, Tolstikhin et al. 2006). Although these models involve very different processes, they have
one important feature in common: They call for the creation of long-lived enriched reservoirs at a very early stage of Earth's evolution. This leads to a paradoxical observation: Although the isotopic and chemical heterogeneities in oceanic basalts reflect mantle-crust exchanges in relatively recent times (Allegre et al. 1980, Christensen & Hofmann 1994, Sun 1980, Tatsumoto 1978), the main features of ¹⁴⁷Sm-¹⁴³Nd systematics in the silicate Earth appear to result from a much earlier event (>4 Gya). This apparent paradox is most evident when considering hidden reservoir models. For example, Tolstikhin et al. (2006) suggested that the D'' layer formed through recycling of a dense basaltic proto-crust immediately following Earth's accretion. These authors estimated that the recycling of 25 × 10²⁷ g of komatiitic crust (the approximate mass of D'') would satisfy Nd mass balance constraints and account for the isotopic evolution of the DM, without the need to call upon layered mantle convection. This reservoir, therefore, would represent 10 times the mass of the continental crust and concentrate 25% of the total REE budget of the silicate Earth. An outstanding question thus arises as to why heterogeneities found in oceanic island basalts are dominated by the recycling of continental and oceanic crust, whereas the geochemical signature of the much larger hidden reservoir is so strikingly absent from mantle plumes thought to originate at the CMB.

EDM: early depleted mantle

# 4. THE TERRESTRIAL ¹⁴⁶Sm-¹⁴²Nd RECORD: ZOOMING IN ON EARLY EVENTS

## 4.1. The Coupled ¹⁴⁶,¹⁴⁷Sm-¹⁴²,¹⁴³Nd Chronometer

The main difficulty in testing models of early mantle-crust evolution lies in the poor chronological resolution of the ¹⁴⁷Sm-¹⁴³Nd system. This limitation can be illustrated in the context of a two-stage model of mantle-crust evolution (Figure 2a). In this simple model, a PM experiences instantaneous differentiation at $t = t_d$, leading to the formation of an enriched crust and one or several depleted reservoirs. During the second stage ($t > t_d$), differentiated reservoirs evolve as closed systems (i.e., there is no further recycling or crustal growth after $t_d$). The ¹⁴³Nd/¹⁴⁴Nd evolution of an early depleted mantle (EDM) can then be derived from Equation 2:

$$ \left( \frac{^{143}_{144}\text{Nd}}{^{144}_{144}\text{Nd}} \right)_t^{\text{EDM}} = \left( \frac{^{143}_{144}\text{Nd}}{^{144}_{144}\text{Nd}} \right)_{\text{fp}}^{\text{PM}} + \left( \frac{^{147}_{144}\text{Sm}}{^{144}_{144}\text{Nd}} \right)_{\text{fp}}^{\text{PM}} [1 - e^{\lambda_{147}(t_p - t_d)}] \\ + \left( \frac{^{147}_{144}\text{Sm}}{^{144}_{144}\text{Nd}} \right)_{\text{fp}}^{\text{EDM}} [e^{\lambda_{147}(t_p - t_d)} - e^{\lambda_{147}(t_p - t)}]. \quad (6) $$

From Equation 6, it is apparent that the isotopic composition of the EDM at a given time is not only dependent on the age of differentiation of the mantle ($t_d$) but also on the extent of Sm/Nd fractionation during differentiation [i.e., (¹⁴⁷Sm/¹⁴⁴Nd)EDM]. The latter cannot be measured directly because mantle-crust exchanges for the past 4.5 Ga have long erased the geochemical fingerprint of the earliest crust. Therefore, the ¹⁴⁷Sm-¹⁴³Nd chronometric equation of the EDM is underconstrained, preventing an accurate determination of $t_d$.

This limitation prompted the development, in the mid-1990s, of the short-lived ¹⁴⁶Sm-¹⁴²Nd chronometer (Harper & Jacobsen 1992). With a half-life of 106 Ma, ¹⁴⁶Sm can be considered extinct after only 500 Ma. This isotope system thus provides a selective record of mantle-crust differentiation processes that took place prior to 4.2 Gya. Lugmair et al. (1983) demonstrated the presence of ¹⁴⁶Sm in the early solar system, reporting large ¹⁴²Nd excesses from α-recoil in carbon-chromite fractions of the Allende meteorite. The initial ¹⁴⁶Sm/¹⁴⁴Sm ratio was then estimated more precisely using mineral isochrons in chondrites and achondrites, yielding a value of 0.0085 ± 0.0007 (Boyet et al. 2010, Lugmair & Galer 1992, Prinzhöfer et al. 1992).
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>

Illustration of the two-stage model of isotopic evolution for the coupled ¹⁴⁶.¹⁴⁷Sm-¹⁴².¹⁴³Nd chronometer. The model assumes instantaneous differentiation 4.5 Gya and 4.2 Gya, corresponding to $t_d = 0.06$ Ga and 0.36 Ga after solar system formation, respectively, for two early depleted mantle (EDM) reservoirs (labeled EDM-Hi and EDM-Lo) and a complementary early enriched reservoir (EER). A case illustrating the effect of late (<4.2 Gya) disturbance of the Sm-Nd systems is also shown. (a) Because of its long half-life, the ¹⁴⁷Sm-¹⁴³Nd system does not provide precise chronological information. Positive ε¹⁴³Ndₗinitial in the early Archean mantle could reflect formation of a highly depleted mantle during a late (4.2 Gya) event or more moderate mantle depletion at an earlier (4.5 Gya) stage. (b) The ¹⁴⁶Sm-¹⁴²Nd chronometer, with a shorter half-life, is more sensitive to small time differences, which allows the two aforementioned scenarios to be distinguished. In addition, the ¹⁴²Nd composition of crustal rocks should be less sensitive to late (<4.2 Gya) metamorphic disturbance compared with their ¹⁴³Nd signature. (c) By coupling both isotope systems, a model age of differentiation $T_d$ can be estimated ($T$ is time running backward from the present day to the origin of the solar system, so that $T_d = 4.567 - t_d$). This also provides an estimate of the Sm/Nd ratio in the parent reservoir. (d) ¹⁴²Nd data are often represented in planetary isochron diagrams. Here the ε¹⁴²Nd values of the rocks are plotted against the ¹⁴⁷Sm/¹⁴⁴Nd ratio of their mantle (or crustal) source reservoir, which is estimated directly using the initial ¹⁴³Nd/¹⁴⁴Nd composition of the rock. Planetary isochrons resulting from crystallization of global magma oceans should intersect the solar system isochron at a composition corresponding to that of the primitive mantle (PM). This method can be used to test compositional models for the terrestrial planets.
Because the ¹⁴⁶Sm-¹⁴²Nd system shares the same properties as the ¹⁴⁷Sm-¹⁴³Nd couple (Figure 2b), a chronometric equation similar to that derived above can be obtained for the ¹⁴²Nd/¹⁴⁴Nd evolution of the EDM:

$$ \left( \frac{^{142}_{144}\text{Nd}}{^{144}_{144}\text{Nd}} \right)_t^{\text{EDM}} = \left( \frac{^{142}_{144}\text{Nd}}{^{144}_{144}\text{Nd}} \right)_{t_p}^{\text{PM}} + \frac{\left( \frac{^{146}_{144}\text{Sm}^{144}\text{Sm}}{^{147}_{144}\text{Sm}} \right)_{t_0}}{\left( \frac{^{146}_{144}\text{Sm}^{144}\text{Sm}}{^{147}_{144}\text{Sm}} \right)_{t_p}} \left[ \left( \frac{^{147}_{144}\text{Sm}}{^{144}_{144}\text{Nd}} \right)_{t_p}^{\text{PM}} \times e^{-\lambda_{146}t_d} + \left( \frac{^{147}_{144}\text{Sm}}{^{144}_{144}\text{Nd}} \right)_{t_p}^{\text{EDM}} \times [e^{-\lambda_{146}t_d} - e^{-\lambda_{146}t}] \right] . \quad (7) $$

By combining both chronometers, one obtains a system of two equations (Equations 6 and 7) from which both the age of differentiation ($t_d$) and the ¹⁴⁷Sm/¹⁴⁴Nd ratio of the EDM can be estimated without assumptions regarding the composition of the early crust (Figure 2c,d). It was therefore anticipated that this coupled ¹⁴⁶,¹⁴⁷Sm-¹⁴²,¹⁴³Nd chronometer would provide a means of distinguishing differentiation resulting from magma ocean crystallization from that produced by later crustal growth.

## 4.2. The Archean ¹⁴⁶Sm-¹⁴²Nd Record

Harper & Jacobsen (1992) presented the first attempt to measure the ¹⁴²Nd composition of terrestrial rocks. These authors reported a positive anomaly of 33 ppm in a volcaniclastic metasediment from the Isua Greenstone Belt (3.8 Gya, West Greenland). Using the coupled ¹⁴⁶,¹⁴⁷Sm-¹⁴²,¹⁴³Nd chronometer, Harper & Jacobsen concluded that Earth's mantle had experienced global differentiation immediately following accretion. However, these results were met with skepticism after several groups reported normal ¹⁴²Nd compositions for early Archean rocks at a ±20 ppm level of precision (Goldstein & Galer 1992, McCulloch & Bennett 1993, Regelous & Collerson 1996). Subsequently, Sharma et al. (1996) showed that a ¹⁴²Nd excess in Harper & Jacobsen's sample could not be ruled out but that the uncertainty quoted by the authors was probably underestimated.

When a more precise thermal ionization mass spectrometer was developed in the early 2000s, it became possible to break the 20 ppm precision barrier of the previous generation of instruments and resolve effects of >5 ppm on the ¹⁴²Nd/¹⁴⁴Nd ratio (Caro et al. 2003). The first high-precision ¹⁴²Nd study was conducted on early Archean metasediments from the Isua Greenstone Belt characterized by a positive $\epsilon^{143}\text{Nd}_{\text{initial}}$ of +1.9 ± 0.6 (Caro et al. 2003, 2006). The results showed a ubiquitous ¹⁴²Nd anomaly of 15 ± 6 ppm compared with modern oceanic basalts. Further analyses revealed that the lithologies from the Isua Greenstone Belt and West Greenland craton exhibit slightly different ¹⁴²Nd signatures ranging from +7 ppm for 3.65-Ga-old tonalites to +20 ppm for >3.85-Ga-old amphibolites (Bennett et al. 2007, Boyet & Carlson 2006, Boyet et al. 2004, Caro et al. 2006). Larger effects (>20 ppm) were not confirmed, as high-precision analyses of Harper & Jacobsen's (1992) sample showed no detectable excess at the ±10 ppm level of precision (Papanastassiou et al. 2003).

Two recent studies also reported negative ¹⁴²Nd anomalies, indicating the preservation of Hadean crustal components in early Archean terranes. O'Neil et al. (2008) first measured $\epsilon^{142}\text{Nd}$ values down to -15 ppm in mafic amphibolites and felsic gneisses from the Nuvvuagittuq Greenstone Belt (NGB, Quebec). In contrast to West Greenland rocks, ¹⁴²Nd effects in NGB rocks display a well-defined correlation with their Sm/Nd ratio (Figure 3). The debate has centered on the significance of this correlation. If interpreted as an isochron, a crystallization age of 4,280±⁵³ Ma can be estimated for the protolith of the faux amphibolite, making the NGB the oldest (oceanic) crustal sequence preserved on Earth. This age, however, is significantly older than the ¹⁴⁷Sm-¹⁴³Nd age of 3,819 ± 270 Ma defined by the amphibolite (O'Neil et al. 2008). Similarly,
<figure><img src="image_5.png" /><figcaption>Figure 3</figcaption></figure>

Coupled <sup>146,147</sup>Sm-<sup>142,143</sup>Nd systematics in early Archean rocks from the West Greenland and Yilgarn cratons (Bennett et al. 2007, Caro et al. 2006), Barberton komatiites (Caro et al. 2006), and the Nuvvuagittuq Greenstone Belt (NGB; O'Neil et al. 2008). ε<sup>142</sup>Nd values in NGB samples are plotted against their respective <sup>147</sup>Sm/<sup>144</sup>Nd ratios. If interpreted as an isochron, the trend defined by the faux amphibolites and gabbros defines an age of 4.28 Ga. For early Archean rocks from West Greenland and the Yilgarn Craton, positive anomalies are inherited from a depleted mantle source. Therefore, data for individual rock samples are plotted against the <sup>147</sup>Sm/<sup>144</sup>Nd ratio of their source reservoir, which was estimated from their ε<sup>143</sup>Nd<sub>initial</sub> values using the two-stage model equations presented in Section 4.1. The depleted ε<sup>142</sup>Nd signatures of the West Greenland and Yilgarn TTGs (tonalites, trondhjemites, granodiorites) define a mixing relationship between an early depleted mantle (ε<sup>142</sup>Nd = 15–20 ppm, <sup>147</sup>Sm/<sup>144</sup>Nd = 0.23–0.24) and an undifferentiated reservoir with ε<sup>142</sup>Nd = 0 and <sup>147</sup>Sm/<sup>144</sup>Nd = 0.208. The NGB isochron and the early Archean mantle array converge toward this common end member, which likely defines the composition of Earth's primitive mantle. The Barberton komatiite appears to be derived from this primitive reservoir, whereas intermediate ε<sup>142</sup>Nd values found in tonalites of the Yilgarn craton reflect a mixed source. Late disturbance of the <sup>147</sup>Sm-<sup>143</sup>Nd system is evident in several samples of the Isua Greenstone Belt and the Itsaq gneiss complex, as previously documented (Polat et al. 2003). Samples represented by smaller squares are likely to have experienced such disturbance as these samples yield model ages in excess of the age of the solar system. The depleted and enriched components represented here cannot be cogenetic. Formation of the NGB postdates by several hundred million years the event that led to differentiation of the Isua mantle source. The solar system isochron, which passes through the composition of ordinary chondrites (OCs), is shown for comparison. Abbreviation: BSE, bulk silicate Earth.

U-Pb zircon dates from orthogneisses crosscutting supracrustal units provide only a minimum age of >3,750 Ma for the NGB (Cates & Mojszis 2007). O'Neil et al. (2008) speculated that the observed ε¹⁴²Nd versus Sm/Nd array might be a mixing line, rather than a true isochron, reflecting the assimilation of a >4.28-Ga-old, light REE-rich component in the 3.8-Ga-old parental melt of the amphibolite.
Upadhyay et al. (2009) recently reported a second case. These authors found negative $\varepsilon^{142}$Nd in a 1.48-Ga-old syenite intruding the early Archean Kharyar craton (India). Four samples have distinctly lower $^{142}$Nd signatures of −8 to −13 ppm compared with a terrestrial standard, whereas three others yield normal $^{142}$Nd composition within errors. As the rocks are too young to have developed in situ $^{142}$Nd effects, the authors interpreted these results as reflecting a variable degree of assimilation of a >4.2-Ga-old mafic component at lower crustal levels during emplacement of the syenitic magma. A puzzling aspect of the results is that both the anomalous and the normal group include samples with identical $\varepsilon^{143}$Nd$_{initial}$ values. This observation is seemingly inconsistent with mixing models, as such processes would necessarily generate coupled variations in $\varepsilon^{143}$Nd$_{initial}$ and $\varepsilon^{142}$Nd. If these results are confirmed, however, they would suggest that Hadean mafic or ultramafic components were incorporated and preserved within the roots of much younger Archean cratons (Upadhyay et al. 2009).

OC: ordinary chondrite

### 4.3. The Age of the Early Terrestrial Crust

Although $^{142}$Nd investigations of Archean terranes are still in their infancy, a new picture of early mantle-crust evolution is starting to emerge from the data presented above. As shown in **Figure 1a**, juvenile rocks older than 2.5 Ga have fairly homogeneous $\varepsilon^{143}$Nd$_{initial}$ signatures. Thus, from the perspective of this long-lived isotope system, the Archean mantle appears to be uniformly depleted. However, when the coupled $^{142,143}$Nd systematics is considered, it is apparent that at least two distinct reservoirs are needed to account for mantle heterogeneity in the early Archean (**Figure 3**). On the one hand, rocks from the West Greenland craton and Isua Greenstone Belt are derived from a depleted mantle, with $^{147}$Sm/$^{144}$Nd = 0.23–0.24 and $\varepsilon^{142}$Nd = +15–20 ppm. On the other hand, the Barberton komatiite and gabbros from the Nuvvuagittuq Greenstone Belt were extracted from a primitive reservoir, with $\varepsilon^{142}$Nd = 0 and $^{147}$Sm/$^{144}$Nd = 0.208. (This primitive end-member has a superchondritic Sm/Nd composition; this aspect will be discussed further in Section 5.) The intermediate values measured in Yilgarn tonalites appear to reflect mixing between these two end-members.

By combining $^{142,143}$Nd data for all Archean rocks, a well-defined mantle isochron is obtained that is similar to that defined by martian shergottites and lunar basalts (Brandon et al. 2009, Caro et al. 2008b, Debaille et al. 2007) (**Figure 3**). This isochron establishes the age of differentiation of Earth's mantle at 150–180 Ma after formation of the solar system (i.e., 4.39–4.42 Gya). This age is significantly younger than the 30–70 Ma age range calculated by Bennett et al. (2007) because these authors estimated a model age based on the assumption of a chondritic BSE (CHUR). Indeed, forcing the Archean mantle isochron to pass through the chondritic composition [ordinary chondrite (OC) in **Figure 3**] would result in a much steeper slope and thus an older age. However, as shown in Section 5, the assumption of a perfectly chondritic bulk Earth is no longer justified, calling into question the validity of CHUR model ages.

### 4.4. Differentiation and Composition of Hadean Silicate Reservoirs

A plausible but speculative interpretation of $^{146,147}$Sm-$^{142,143}$Nd chronological constraints is that early mantle differentiation resulted from crystallization of a deep magma ocean following the Moon-forming giant impact (Bennett et al. 2007; Caro et al. 2003, 2005). According to $^{182}$Hf-$^{182}$W chronometry, this event probably took place >50 Ma after formation of the solar system (Touboul et al. 2007) and led to global melting of Earth's interior (Canup 2004). During the initial stage of solidification, crystallization of Mg-perovskite in the lower mantle, followed by majorite and olivine in the upper mantle, would have concentrated incompatible elements in an enriched
residual liquid (Abe 1997, Agee 1990, Solomatov & Stevenson 1993a). Although this initial stage was probably rapid ($10^3-10^6$ years; Abe 1997), fractionation of the REE and other incompatible species likely would have taken place at a later stage, when this residual liquid segregated according to its density contrast with crystal cumulates (Solomatov & Stevenson 1993a,b). Depending on the blanketing effect of the early atmosphere, model calculations suggest that final solidification of the terrestrial magma ocean would have taken place $10^7-10^8$ years after the giant impact (Abe 1997, Matsui & Abe 1986).

The consequences of magma ocean crystallization for the structure and chemical composition of the mantle remain the most speculative aspect of early silicate Earth evolution (Caro et al. 2005, Labrosse et al. 2007). Because trace elements are highly concentrated in residual liquids, extensive crystal/melt separation is needed to achieve significant fractionation. Trapping of a minor amount of liquid in an olivine or perovskite cumulate would buffer any trace element fractionation created by crystallization of these mineral phases. Thus, REE fractionation in the magma ocean is controlled not only by the nature of the crystallizing mineral assemblage but also by the differential rates of melt migration and progression of the solidification front (Solomatov & Stevenson 1993a). At the pressures of the upper mantle, melt percolation is expected to be faster than the rate of solidification (Solomatov & Stevenson 1993a). This would have favored the migration of this residual liquid toward the surface and the formation of the first terrestrial proto-crust. However, in the rapidly cooling lower mantle, the magnitude of Sm/Nd fractionation would be limited by the lower efficiency of melt percolation and by the buffering effect of Ca-perovskite (Caro et al. 2005, Corgne et al. 2005). In this magma ocean scenario, the depleted signature of the Archean mantle would more likely reflect differentiation of the uppermost mantle. The Archean mantle array (Figure 3) may then be seen as reflecting rehomogenization of a deep PM with shallow reservoirs that had been depleted by extraction of a residual melt during the final stage of magma ocean solidification.

An alternative interpretation, which does not require global melting of Earth's interior, is that early mantle depletion was caused by massive extraction of continental crust starting 4.5 Gya (Figure 4). Initially proposed by Armstrong (1981), this highly speculative model has found support in studies of Hadean detrital zircons from the Jack Hills metaconglomerate (<4.4 Gya, Western Australia) (e.g., Amelin et al. 1999, Caro et al. 2008a, Harrison 2009). Jack Hills zircons are interpreted as having crystallized from hydrous meta- and peraluminous magmas in an environment characterized by a relatively low heat flow (Hopkins et al. 2008). $^{176}$Lu-$^{176}$Hf studies of Jack Hills reported highly unradiogenic $\varepsilon^{176}\text{Hf}_{\text{initial}}$ as early as 4.3 Gya, and the evolution of $\varepsilon^{176}\text{Hf}_{\text{initial}}$ versus time requires sequestration of the zircons in a low Lu/Hf crustal reservoir extracted from the mantle ~4.4 to 4.5 Gya (Figure 4) (Harrison et al. 2005, 2008; Kemp et al. 2010). On the basis of these results, Harrison et al. (2005, 2008) suggested that sites of granite generation similar to modern subduction zones were in place and that they led to the development of continental masses similar to present-day continents as early as 4.5 Gya, whereas Kemp et al. (2010) favored a model involving a predominantly mafic crust formed $4.46 \pm 0.12$ Gya. Independently of these different interpretations, the general agreement between the model age of mantle depletion recorded by the $^{146}\text{Sm}-^{142}\text{Nd}$ system and the ages obtained from Hf isotope studies suggests that part or all of the proto-crust quickly evolved into a highly differentiated reservoir that contained both felsic and mafic components. Although they may have been present in different proportions, these Hadean lithologies appear compositionally similar to their Archean and Proterozoic counterparts (Harrison 2009, Kemp et al. 2010, O'Neil et al. 2008). This leaves open the possibility that early mantle differentiation resulted from the onset of continental growth rather than from magma ocean crystallization.
<figure><img src="image_6.png" /><figcaption>**Figure 4**</figcaption></figure>

In situ Hf-Pb isotopic data for Hadean and early Archean zircons from the Jack Hills metaconglomerate (Harrison et al. 2008, Kemp et al. 2010). Most grains have ε¹⁷⁶Hfₗₘ₀ial ranging between the predicted evolution of a mafic crust with ¹⁷⁶Lu/¹⁷⁷Hf = 0.02 and that of a felsic reservoir with ¹⁷⁶Lu/¹⁷⁷Hf = 0.01 (Harrison et al. 2008, Kemp et al. 2010). Although the geodynamic implications of these results have remained a matter of debate (Kemp et al. 2010), the results clearly indicate the presence of differentiated crustal terranes 4.4–4.5 Gya. This crustal reservoir formed immediately following terrestrial accretion and remained preserved from recycling in the mantle for 1–2 Ga, in agreement with ¹⁴⁶Sm-¹⁴²Nd constraints on early mantle-crust evolution (Figure 5). Abbreviation: EDM, early depleted mantle (¹⁷⁶Lu/¹⁷⁷Hf = 0.0375; Caro & Bourdon 2010).

**4.5. Longevity of the Hadean Crust**

The longevity of the Hadean crust is perhaps one of the most speculative aspects of early crustal evolution. Models range from very short-lived, unstable crusts (e.g., van Thienen et al. 2004) to long-lived reservoirs with a lifetime similar to that of present-day continents (Armstrong 1981, Galer & Goldstein 1991). As shown in **Figure 5**, the preservation of the anomalous ¹⁴²Nd signature in the Archean mantle provides a first-order constraint on the lifetime of the now-vanished proto-crust. In essence, a stable Hadean reservoir is needed to allow the preservation of ¹⁴²Nd heterogeneity in the mantle until 3.6 Gya and possibly later (Bennett et al. 2010), but this early crust must have been largely remixed in the mantle by the end of the Archean to account for the perfectly homogeneous ¹⁴²Nd signature of the present-day mantle and crust (Andreasen et al. 2007, Boyet & Carlson 2006, Caro et al. 2006, Murphy et al. 2010). As shown in **Figure 5**, these observations are consistent with a lifetime on the order of 1–2 Ga for the Hadean crust (Caro et al. 2006, Jacobsen & Harper 1996).

The requirement for a long-lived proto-crust could provide important, albeit indirect, information on the composition of Hadean silicate reservoirs. As shown in **Figure 5**, a felsic crust with a lifetime of 1–2 Ga would evolve toward negative ¹⁴²Nd anomalies of –20 to –30 ppm during the Archean. This is due to the low Sm/Nd ratios in felsic lithologies favoring the development of negative ε¹⁴²Nd effects within relatively short periods of time. Reworking of Hadean crustal terranes within the growing Archean continental mass should have left a detectable fingerprint
<figure><img src="image_7.png" /><figcaption>Figure 5</figcaption></figure>

Isotopic evolution of differentiated crustal and mantle reservoirs calculated using a two-box model with recycling (Caro et al. 2006). In this model, crustal and mantle reservoirs differentiated 4.5 Gya and subsequently evolved at steady state. In contrast to the simple two-stage model described in Section 4.1, differentiated reservoirs do not evolve as perfectly closed systems but instead exchange material at a constant rate. In essence, recycled crustal material is balanced by new additions from the complementary depleted mantle. The average $ε^{142}$Nd evolution of the crust is then a function of its bulk composition and the residence time of Nd in this reservoir [i.e., the lifetime of the crust ($τ_c$)]. The preservation of $^{142}$Nd anomalies in the early Archean mantle requires extraction of a primordial crust with a lifetime of approximately 1–2 Ga. The blue curve represents the isotopic evolution of a continental reservoir with a composition similar to modern crust (i.e., $^{147}$Sm/$^{144}$Nd = 0.12). The predicted evolution for the complementary depleted mantle is shown in green. The purple curve illustrates the evolution of a basaltic crust, with $^{147}$Sm/$^{144}$Nd = 0.15 and $τ_c$ = 1–2 Ga. The preservation of negative $^{142}$Nd anomalies in the Archean is dependent upon the chemical composition of the crustal reservoir. Nuvvuagittuq Greenstone Belt data come from O’Neil et al. (2008). West Greenland data come from Bennett et al. (2007) and Caro et al. (2006). Komatiite data come from Caro et al. (2006) and Boyet and Carlson (2006). Abbreviation: BSE, bulk silicate Earth.

in the form of negative $^{142}$Nd anomalies in early Archean metasediments and granitoids. So far, most studies of Archean rocks yielded either depleted ($ε^{142}$Nd > 0) or primitive ($ε^{142}$Nd = 0) signatures (Bennett et al. 2007; Boyet & Carlson 2005, 2006; Caro et al. 2006, 2008a), and the only confirmed occurrence of negative $^{142}$Nd anomalies in the terrestrial rock record is derived from mafic lithologies (O’Neil et al. 2008). If these results were confirmed on a broader scale, they would favor models involving a predominantly mafic or ultramafic crust over those calling for rapid continental growth in the Hadean (Armstrong 1981, 1991). Indeed, a basaltic crust with a lifetime of 1–2 Ga would probably develop a smaller negative $^{142}$Nd anomaly of –5 to –10 ppm (Figure 5), so its involvement in Archean crustal formation processes would be far more difficult to detect than the contribution of a Hadean felsic component.
## 5. THE CHONDRITIC <sup>146</sup>Sm-<sup>142</sup>Nd RECORD AND THE CONUNDRUM OF THE MISSING RESERVOIR

In the previous section, the Archean mantle array was proposed to reflect ongoing homogenization of a deep primitive reservoir with shallow mantle domains depleted by melt extraction from the solidifying upper mantle. These heterogeneities have since been remixed by convection, yielding a homogeneous $\varepsilon^{142}$Nd signature in the present-day mantle. A puzzling aspect of the Archean $^{142,143}$Nd results, however, is that mantle-derived rocks with positive $\varepsilon^{143}$Nd$_{initial}$ are not always associated with positive $\varepsilon^{142}$Nd. For example, the 3.5-Ga-old Barberton komatiite, with an $\varepsilon^{143}$Nd$_{initial}$ of +1.4 [corresponding to ($^{147}$Sm/$^{144}$Nd)$_{source}$ = 0.208], has a normal $\varepsilon^{142}$Nd composition (Caro et al. 2006). A question thus arises as to which process is responsible for creating depleted $\varepsilon^{143}$Nd signatures that are not associated with any $^{142}$Nd effects.

The Barberton $^{142,143}$Nd signature was initially interpreted as reflecting crustal extraction processes that depleted the upper mantle once $^{146}$Sm was extinct (that is, <4.2 Gya) (Caro et al. 2006). However, when the Archean $^{142,143}$Nd record is considered together with that of chondrites, a far less intuitive scenario emerges: The lack of a positive $^{142}$Nd anomaly in the depleted Barberton mantle source is not related to late crustal extraction; rather, it is a consequence of the formation, prior to crystallization of the terrestrial magma ocean, of an early enriched reservoir (EER) that was never remixed into the mantle. In essence, the debate centers on one fundamental question: Is this missing reservoir hidden in the deep mantle, or was it lost during accretion of the proto-Earth?

EER: early enriched reservoir

CC: carbonaceous chondrite

### 5.1. The Chondritic <sup>142</sup>Nd Signature

Since the pioneering studies of the late 1970s, the interpretation of Nd isotopic data has relied on the fundamental assumption that the bulk Earth had a perfectly chondritic Sm/Nd ratio (Allegre et al. 1979, DePaolo & Wasserburg 1979, Jacobsen 1988, Jacobsen & Wasserburg 1979). Jacobsen & Wasserburg (1984) and subsequent studies (Bouvier et al. 2008, Patchett et al. 2004) showed that Sm/Nd does not vary by more than 4% among the different types of chondrite. A 4% heterogeneity in Sm/Nd should generate a dispersion of $\pm 6$ ppm in the $^{142}$Nd/$^{144}$Nd ratio after 4.56 Ga. Therefore, if the silicate Earth had a chondritic Sm/Nd, its $^{142}$Nd signature and that of chondrites should be indistinguishable at the level of precision achieved by modern mass spectrometers ($\pm 2\sim 5$ ppm).

High-precision studies of meteorites, however, yielded strikingly different results. Boyet & Carlson (2005) reported $^{142}$Nd data from a set of meteorites including OCs and carbonaceous chondrites (CCs) as well as basaltic achondrites. Their results showed that all chondrites, regardless of their classification, are characterized by $\varepsilon^{142}$Nd values shifted downward by approximately 20 ppm compared with terrestrial rocks (Figure 6a). Basaltic eucrites, which have near-chondritic Sm/Nd ratios, also exhibited the same negative anomaly, suggesting that their parent asteroid (4-Vesta) accreted from chondritic material.

When examined in detail, it is apparent that the range of $^{142}$Nd anomalies exhibited by CCs is much larger than that expected on the basis of their Sm/Nd heterogeneity. These meteorites have low $\varepsilon^{142}$Nd, ranging between -45 and -20 ppm, and this dispersion exceeds analytical uncertainty (Figure 6a). Furthermore, $\varepsilon^{142}$Nd values in CCs show no correlation with their Sm/Nd ratios, indicating that $^{142}$Nd heterogeneities do not result from the decay of $^{146}$Sm (Carlson et al. 2007). OCs, however, define a distinct population with a homogeneous and less negative $\varepsilon^{142}$Nd signature of $\pm 18 \pm 5$ ppm (Figure 6a).

The difference in composition between CCs and OCs has been interpreted as the result of incomplete mixing of isotopically diverse nucleosynthetic material in the early solar system
<figure><img src="image_8.png" /><figcaption>Figure 6</figcaption></figure>

(a) Compilation of ¹⁴²Nd measurements in ordinary chondrites (OCs) and carbonaceous chondrites (CCs) compared with the modern terrestrial composition (Andreasen & Sharma 2006, Boyet & Carlson 2005, Carlson et al. 2007). OCs define a homogeneous population with an average ε¹⁴²Nd value of –18 ppm, whereas CCs exhibit more heterogeneous signatures. (b) The more negative ε¹⁴²Nd values observed in CCs display an inverse correlation with the abundance of the r-process nuclide ¹⁴⁸Nd (Carlson et al. 2007). This reflects a mixture between normal Nd and material carrying nucleosynthetic anomalies such as FUN (fractionation and unknown nuclear effects) inclusions or presolar grains (Carlson et al. 2007). OCs, however, have normal ¹⁴⁸Nd abundance within errors.

(Andreasen & Sharma 2006, 2007; Carlson et al. 2007). ¹⁴²Nd is produced mainly by s-process (slow neutron capture) in the interior of red giants, whereas heavier Nd isotopes are also produced by r-process (rapid neutron capture) in supernovae (Wallerstein et al. 1997). Accordingly, primordial heterogeneities arising from imperfect homogenization of nucleosynthetic products are expected to result in large ¹⁴²Nd anomalies. As shown in **Figure 6b**, ¹⁴²Nd deficits in CCs are correlated with excesses of the r-only nuclide ¹⁴⁸Nd (Carlson et al. 2007). These effects appear to be inherited from material such as FUN (fractionation and unknown nuclear effects) inclusions or presolar grains, which long have been known to carry large nucleosynthetic anomalies (see Birck 2004 for a review). This interpretation is reinforced by the observation of similar deficits in s-process and p-process nuclides from other heavy elements such as barium (Andreasen & Sharma 2007, Carlson et al. 2007) and samarium (Andreasen & Sharma 2006). Overall, these observations demonstrate that the negative and heterogeneous ¹⁴²Nd signatures characterizing CCs partly reflect the incorporation of presolar material with a distinctively low abundance of s-process nuclides. In contrast, OCs, which exhibit normal isotopic composition for heavy elements, appear to be representative of planet-building material (Andreasen & Sharma 2006, 2007; Carlson et al. 2007). The ¹⁴²Nd excess of 18 ± 5 ppm in the accessible Earth compared with OCs has, therefore, been interpreted as resulting from ¹⁴⁶Sm decay, rather than from isotopic heterogeneity in the early solar system (Andreasen & Sharma 2006, Carlson et al. 2007).

## 5.2. Hidden Reservoir Models

The chondritic ¹⁴²Nd signature has potentially important consequences for our understanding of planetary formation. Yet, the interpretation of the ¹⁴²Nd excess observed in terrestrial rocks remains a contentious issue. If, as argued above, the negative anomaly in OCs is radiogenic, then it follows that the accessible mantle evolved for more than 4.5 Ga with a superchondritic Sm/Nd
ratio. Boyet & Carlson (2005), following ¹⁴²Nd evidence for early mantle depletion, interpreted this observation in terms of internal differentiation processes. The rationale of their model is that if the BSE has a perfectly chondritic Sm/Nd ratio, then the more radiogenic ¹⁴²Nd/¹⁴⁴Nd composition of the accessible silicate reservoirs (i.e., the crust and the source reservoirs of oceanic basalts) must be balanced by a hidden reservoir with a low Sm/Nd ratio and subchondritic ε¹⁴²Nd (Boyet & Carlson 2005, 2006). Because no negative anomaly has been found in mantle-derived rocks, this hidden reservoir must have remained perfectly isolated from mantle convection throughout the entire Earth's history (Andreasen et al. 2007, Boyet & Carlson 2006, Caro et al. 2006, Murphy et al. 2010). Boyet & Carlson (2005) first suggested that a Hadean crust with a KREEP-like composition [i.e., having elevated potassium (K), rare earth elements (REEs), and phosphorus (P) contents] could have been recycled in the deep mantle and remained isolated near the CMB. As pointed out by Davaille et al. (2003), the density contrast between basaltic and peridotitic compositions may have been sufficient to prevent remixing of this early crust in the convective mantle, although it is doubtful that such a reservoir could have remained unsampled for the past 4.5 Ga. Alternatively, Labrosse et al. (2007) proposed that the superchondritic Sm/Nd ratio of the accessible mantle may reflect concentration of incompatible elements in a basal magma ocean. This reservoir would have formed owing to downward migration of intrinsically dense melts toward the CMB during crystallization of the lowermost mantle. Such a mechanism provides means to create a reservoir enriched in incompatible elements at the base of the mantle without the need to call upon crustal recycling. However, modeling of trace element partitioning at high pressure does not provide a satisfying fit to ¹⁴²Nd data, as separation and crystallization of the basal magma ocean would generate only negligible ¹⁴²Nd excesses (<2 ppm) in the DM. Furthermore, this model predicts continuous injection of isotopic heterogeneity in the convective mantle, as the less dense crystallized fraction is progressively reintrained in the mantle. This seems inconsistent with the lack of negative ε¹⁴²Nd signatures in oceanic basalts, which suggests that the hypothetical hidden reservoir does not contribute to the chemical and isotopic heterogeneity of the modern mantle (Andreasen et al. 2007, Boyet & Carlson 2006, Caro et al. 2006, Murphy et al. 2010).

### 5.3. The Age of the Missing Reservoir

An important constraint on the aforementioned models lies in the age of formation of the EER relative to the chronology of lunar and core formation. The minimum age of differentiation of this reservoir can be constrained by coupled ¹⁴⁶,¹⁴⁷Sm-¹⁴²,¹⁴³Nd chronometry using the two-stage model equations presented in Section 4.1 (Bourdon et al. 2008, Korenaga 2009). To apply this chronometer, we must first estimate the ε¹⁴³Nd and ε¹⁴²Nd values for the EDM complementary to the EER. Because the continental crust was extracted from the EDM, the present-day DM is chemically more depleted, and has a more radiogenic ε¹⁴³Nd, than the EDM. To estimate ε¹⁴³Nd_EDM, it is therefore necessary to “remix” the continental crust into the DM. In the most conservative scenario, whereby the continental crust was extracted from the entire mantle, mass balance constraints require that $R_{EDM} = R_{CC}\varphi_{CC} + R_{DM}\varphi_{DM}$, where $R_i = (^{143}\text{Nd}/^{144}\text{Nd})$, $\varphi_i = (\text{Nd})_i/(\text{Nd})_{EDM}$, and $(\text{Nd})_i$ is the total number of moles of Nd in reservoir $i$. In addition, the Nd budget of the EDM is such that $(\text{Nd})_{EDM} = (\text{Nd})_{BSE} - (\text{Nd})_{EER}$ and that of the modern DM is $(\text{Nd})_{DM} < (\text{Nd})_{EDM} - (\text{Nd})_{CC}$. Using a BSE concentration of 1.25 ppm (McDonough & Sun 1995) and a crustal concentration of 20 ppm (Rudnick & Fountain 1995), one obtains an upper bound for [Nd]$_{DM}$ of <1.14 ppm, where [Nd]$_i$ is the Nd concentration in reservoir $i$. As $R_{CC} = 0.5120$ and $R_{DM} = 0.5131$ (Salters & Stracke 2004), $R_{EDM}$ is constrained to be <0.5130, which corresponds to $\epsilon^{143}\text{Nd}_{\text{EDM}} < +7$. This value represents a strict upper limit because it does not take into account the partitioning of Nd in the EER. If, for example, one considers that this
reservoir contains 25% of the total Nd budget of the BSE, as suggested by Tolstikhin et al. (2006), then $[Nd]_{DM} < 0.83$ ppm, yielding $\epsilon^{143}Nd_{EDM} < +6$.

The data compiled in **Figure 6a** suggest that the radiogenic $^{142}$Nd excess in accessible terrestrial reservoirs is probably no less than 15 ppm. Application of the coupled $^{142,143}$Nd chronometer using a chondrite-normalized $\epsilon^{142}Nd_{EDM}$ value of >15 ppm and $\epsilon^{143}Nd_{EDM} < +7$ shows that differentiation of the EER must have taken place less than 20 Ma after formation of the solar system (**Figure 7a**, Model 1). If one considers a more realistic value of $\epsilon^{143}Nd_{EDM} < +6$ (Model 2), then the age of the EER becomes identical within errors to that of the solar system (i.e., 4.567 Gya; Bouvier et al. 2007), which is seemingly incompatible with an internal differentiation process. This simple calculation also shows that the $^{142}$Nd excess in the EDM cannot exceed 18 ppm, as this would yield model ages older than the age of the solar system (Model 3).

These chronological constraints outline a major challenge to all hidden reservoir models (Bourdon et al. 2008, Korenaga 2009). It has been estimated, on the basis of $^{182}$Hf-$^{182}$W chronometry, that the earliest possible age of completion of core formation is likely >30 Ma (Kleine et al. 2004, Yin et al. 2002) and that the putative Moon-forming impact must have taken place >50 Ma

<figure><img src="image_9.png" /><figcaption>**Figure 7**</figcaption></figure>

(a) Model age estimates for the hidden reservoir. Model 1 assumes that the early depleted mantle (EDM) has a $^{142}$Nd excess of >15 ppm compared with ordinary chondrites (OCs) and a maximum $\epsilon^{143}Nd_{EDM}$ of +7. This conservative model yields an age of 20 Ma after formation of the solar system (i.e., 4.546 Gya). As Model 1 does not take into account the budget of Nd stored in the early enriched reservoir (EER), this should be considered the youngest possible age. In Model 2, it is assumed that the hidden reservoir contains 25% of the Nd budget of the bulk silicate Earth (BSE) (Tolstikhin et al. 2006). The maximum $\epsilon^{143}Nd_{EDM}$ is +6, and coupled $^{146,147}$Sm chronometry yields a model age of 4.56 Ga ($t_d$ = 0) for the hidden reservoir. A similar calculation assuming an $\epsilon^{142}Nd_{EDM}$ excess of +18 ppm also yields a model age identical within errors to that of the solar system (Model 3). Conversely, if the Pb-Pb age of Earth (65–85 Ma) is considered as the earliest possible age for the hidden reservoir, then a maximum $\epsilon^{142}Nd_{EDM}$ of +8 to +11 ppm is obtained (Model 4). $\epsilon^{142}Nd$ values are normalized to OCs instead of a terrestrial standard. (b) Ages of differentiation of the major terrestrial reservoirs compared with that estimated for the EER. The minimum age of completion of core formation is based on $^{182}$Hf-$^{182}$W chronometry (Kleine et al. 2004, Yin et al. 2002). The Pb-Pb age of Earth, which likely reflects late-stage core formation (Wood & Halliday 2010), is from Wood & Halliday (2005). The $^{129}$I-$^{129}$Xe closure age of the atmosphere is from Kunz et al. (1998) and Ozima & Podosek (1999).
after solar system formation (Bourdon et al. 2008, Touboul et al. 2007). The Pb-Pb age of Earth, which is thought to reflect late-stage core formation (e.g., Gancarz & Wasserburg 1977, Pidgeon 1978), is estimated at 65–85 Ma (Wood & Halliday 2005, 2010), and the closure of the atmosphere recorded by xenon isotopes probably took place >50 Ma after formation of the solar system (Caffee et al. 1999, Kunz et al. 1998, Ozima & Podosek 1999, Pepin & Porcelli 2006). Thus, all four chronometers, although sensitive to different differentiation processes, consistently record ages younger than the age of the EER (Figure 7b). It is difficult to envision how the separation of a long-lived silicate reservoir in the deep mantle could have taken place prior to completion of core formation. However, if one takes the Pb-Pb age of Earth as the earliest possible time for magma ocean crystallization, then the maximum ¹⁴²Nd excess in the EDM, for the two cases outlined above, is +8 to +11 ppm (Model 4), only half the value needed to account for the observations (Figure 7a).

## 5.4. Hidden Reservoir Versus Lost Reservoir: A Lunar Perspective

A second test of hidden reservoir models comes from examination of ¹⁴²Nd systematics in lunar samples. It is well established that the Moon and the silicate Earth have identical oxygen and tungsten isotopic compositions (Touboul et al. 2007, Wiechert et al. 2001). This suggests that the Moon accreted from material ejected from Earth's mantle or from impactor material that had experienced extensive equilibration with the terrestrial magma ocean after the giant impact (Pahlevan & Stevenson 2007). In the aftermath of such an event, Earth's mantle would have been completely molten (Canup 2004) and partially vaporized, leading to the formation of a global magma ocean surrounded by a thick atmosphere of silicate dust and vapor (Pahlevan & Stevenson 2007). Average mantle temperatures after the giant impact were probably in excess of 10,000 K (Canup 2004), and turbulent convection in this hot magma ocean must have quickly rehomogenized any preexisting stratification. Therefore, it was reasoned that a Moon with chondritic ε¹⁴²Nd would imply that Earth was also chondritic at the time of the giant impact. Later internal differentiation would then be needed to account for the nonchondritic ¹⁴²Nd composition of the accessible silicate Earth.

Nyquist et al. (1995) were the first to apply the ¹⁴⁶Sm-¹⁴²Nd method to lunar rocks. These authors found that mare basalts and KREEPs plot along a linear array, which, if interpreted as an isochron, defines a late age of crystallization of 4.3 Ga. Rankenburg et al. (2006), using higher precision mass spectrometry, obtained results suggesting that ε¹⁴²Nd values reported by Nyquist et al. (1995) were overestimated. However, their measurements were affected by an analytical bias, and the subsequent revision by Brandon et al. (2009) yielded ε¹⁴²Nd compositions within errors of previous estimates (Boyet & Carlson 2007, Nyquist et al. 1995).

A compilation of lunar ¹⁴²Nd data (Boyet & Carlson 2007, Brandon et al. 2009, Nyquist et al. 1995), excluding those requiring large corrections for neutron capture effects, is shown in **Figure 8**. As can be seen, the lunar array does not intersect the composition of OCs, as would be expected if the source reservoirs of lunar basalts had differentiated from a chondritic PM. In fact, the bulk ¹⁴²Nd composition of the Moon (corresponding to the intercept of the lunar array with the solar system isochron) is indistinguishable from that of the modern silicate Earth and lies within the errors of the primitive end-member defined by early Archean rocks (Figure 3). Caro et al. (2008b) interpreted this as reflecting differentiation of lunar and terrestrial primordial reservoirs from a primitive source with identical, but nonchondritic, Sm/Nd and ¹⁴²Nd/¹⁴⁴Nd ratios. This implies that the ¹⁴²Nd signature found in all modern terrestrial reservoirs is not restricted to the uppermost mantle and crust; instead, this radiogenic excess is a feature characterizing the entire Earth-Moon system. This precludes the possibility that the EER resulted from the crystallization
<figure><img src="image_10.png" /><figcaption>Figure 8</figcaption></figure>

Coupled <sup>146,147</sup>Sm-<sup>142,143</sup>Nd systematics of lunar rocks. The data are compiled from Nyquist et al. (1995), Boyet & Carlson (2007), and Brandon et al. (2009). A regression through all data yields an age of mantle differentiation of 4,325 x 22 Ma [mean standard weighted deviation (MSWD) = 5.2]. This late age may reflect either the final stage of solidification of the lunar mantle or subsequent overturn of magma ocean cumulates (Bourdon et al. 2008). The lunar array, the solar system isochron, and the Archean mantle isochron define a triple intersect at ε<sup>142</sup>Nd ≈ 0, and <sup>147</sup>Sm/<sup>144</sup>Nd ≈ 0.21, which likely defines the bulk composition of the Earth-Moon system (Caro et al. 2008b). Abbreviations: KREEPs, having potassium (K), rare earth elements (REEs), and phosphorus (P) contents; OC, ordinary chondrite.

of a magma ocean after the Moon-forming giant impact and eliminates the possibility that the Moon itself is the missing reservoir. Considered together with the chronological constraints on EER formation (see Section 5.3), the nonchondritic composition of the Moon makes it more difficult to explain the terrestrial <sup>142</sup>Nd signature in terms of an internal differentiation process (Boyet & Carlson 2007). Although a deep hidden reservoir might have formed prior to the giant impact and escaped rehomogenization in a largely molten mantle, this scenario seems unlikely. Rather, these constraints suggest that Earth accreted from material that had already experienced significant losses of enriched, low Sm/Nd material (Bourdon et al. 2008, Caro et al. 2008b). The Moon may have then inherited this nonchondritic signature from the proto-Earth.

**5.5. Collisional Erosion Models**

The idea that the bulk Earth and Moon may not have perfectly chondritic Sm/Nd ratios stands in contradiction with the standard paradigm that refractory elements did not experience fractionation during planetary formation. Such fractionation, however, may be an inevitable outcome of the accretion process. Simulations of planetary formation show that large impacts do not always result in the accretion of the impactor but also induce mass loss from the targets (Agnor & Asphaug
2004, Agnor et al. 1999). The net accretion of impactors occurs only in the case of low-velocity or head-on collisions, whereas high-velocity and tangential impacts tend to result in fragmentation of the impactor, ejection of mantle and crustal material from the target, and stripping of planetary cores from silicate mantles. In the simulations of Agnor & Asphaug (2004), hit and run collisions represent one-third to one-half of the large impacts on the proto-planets, suggesting that Earth and/or its precursors experienced significant loss of silicate components during one or several collisions.

O'Neill & Palme (2008) laid out the case for collisional erosion during Earth's accretion. These authors pointed out that the bulk Fe/Mg ratio of Earth (2.2) is significantly higher than the solar value of 1.9, suggesting 10% loss of silicate material relative to metal. O'Neill & Palme (2008) and Korenaga (2009) also suggested that erosional impacts on the proto-Earth induced preferential loss of crustal material, which depleted the mantle in incompatible elements and generated chemical fractionations similar to those commonly attributed to internal mantle-crust differentiation processes.

Empirically, this model is supported by numerous observations of planetary objects of the solar system, the most obvious example being the high density of Mercury, which likely results from collisional stripping of its silicate reservoirs (Benz et al. 1988). Differences in the density of smaller planetary bodies from the asteroid belt may have a similar cause (Asphaug 2009), which indicates that planets accreted from highly differentiated planetesimals that had lost variable amounts of silicate material. Lastly, the presence of a large impact crater on the southern side of Vesta provides evidence for erosional loss of planetary crusts. According to Thomas et al. (1997), Vesta's crater was caused by a collision with a 30 km-diameter impactor that resulted in the loss of 1% of the total mass of the planet. If the ejected material was predominantly crustal, with a composition similar to basaltic eucrites, then Vesta would have lost roughly a third of its REE budget in a single impact (Caro & Bourdon 2010). This shows that the trace element budgets of the terrestrial planets, which are highly concentrated in shallow crustal reservoirs, are likely to be severely affected by erosional impacts.

## 5.6. Mantle-Crust Evolution in a Nonchondritic Earth

A consequence of collisional erosion is that lithophilic elements are expected to be fractionated not only as a function of volatility but also according to their degree of compatibility in silicate melts. By removing crustal material from differentiated planetesimals, this process would have created in Earth and other terrestrial planets a REE signature resembling the pattern of mantle depletion produced by crustal growth (O'Neill & Palme 2008, Korenaga 2009). The associated Sm/Nd fractionation would be small (<10%) compared with cosmochemical fractionations experienced by more volatile elements in the solar nebula. However, even a slightly nonchondritic Sm/Nd for the BSE would contribute an entirely new perspective on the evolution of the mantle-crust system (Caro & Bourdon 2010, O'Neill & Palme 2008, Korenaga 2009). Specifically, if one assumes that the ¹⁴²Nd excess of 18 ppm characterizing the silicate Earth is due to the loss of a low Sm/Nd component 4.56 Gya (O'Neill & Palme 2008, Warren 2008), then the ¹⁴⁷Sm/¹⁴⁴Nd ratio and ε¹⁴³Nd composition of the BSE can be estimated to be 0.2082 and +6.9, respectively (Caro & Bourdon 2010). In this nonchondritic model, the DM (MORB source) would display a radiogenic excess of 2.1 ε-units compared with the BSE, instead of 9 ε-units in the CHUR model (Figure 1b). This, in turn, modifies the mass balance relationships in the mantle-crust system. Assuming that this radiogenic excess is due to continental formation, the mass fraction of the DM can be estimated to be 90% of that of the silicate Earth, instead of the 25% estimated using CHUR parameters. It follows that separation of the upper and lower mantle would no longer be necessary
to account for the radiogenic $\varepsilon^{143}$Nd signature characterizing the MORB source, a constraint more consistent with geophysical evidence for whole-mantle convection. These revised BSE parameters also provide an excellent fit to the observed $\varepsilon^{143}$Nd evolution of the Archean mantle. As illustrated in **Figure 1b**, continuous extraction of the continental crust from the whole mantle can account for the isotopic evolution recorded in juvenile rocks from 3.8 Gya to present, obviating the need for early continental growth or a hidden reservoir at the CMB.

Collisional erosion of low Sm/Nd crustal components would thus provide a simple answer to the most puzzling questions raised by the $^{143}$Nd systematics of the mantle (see Section 3.2). In particular, this process would explain why the EER does not appear to contribute isotopic or chemical heterogeneity to the modern mantle. It would also solve the apparent paradox of a seemingly depleted Archean mantle despite the scarcity of ancient (>4 Gya) continental crust. Lastly, a nonchondritic Earth model would make it easier to interpret isotope systematics in oceanic basalts, by eliminating Nd mass balance constraints for layered mantle convection and by associating primitive $^3$He/$^4$He with primitive $\varepsilon^{143}$Nd signatures (Caro & Bourdon 2010). If correct, this interpretation would imply that early REE fractionation not only was produced by internal differentiation but also was generated by the accretion process at an earlier stage of planetary evolution. Distinguishing the effects of crustal differentiation and recycling from those arising from earlier collisional loss represents a major challenge in future studies of early mantle-crust differentiation.

## SUMMARY POINTS

1. The presence of positive $^{142}$Nd anomalies in rocks from the West Greenland and Yilgarn cratons is unambiguous evidence for early formation (4.39–4.42 Gya) of Earth’s primordial crust. This Hadean reservoir was preserved from remixing in the mantle for at least 1 Ga after its formation. The modern mantle, however, has a homogeneous $^{142}$Nd signature, indicating extensive rehomogenization of chemical and isotopic heterogeneities arising from early (>4.2 Gya) crustal formation processes.

2. Several lines of evidence suggest that this primordial differentiation was related to the crystallization of a global magma ocean following the giant impact. The lack of positive anomalies in several early Archean terranes—including the Barberton komatiite and gabbros from the Nuvvuagittuq Greenstone belt—requires that a substantial fraction of the mantle was not affected by this process. The preservation of pristine mantle domains until the early Archean may reflect nonfractional crystallization of the lower mantle due to rapid solidification rates compared with rates of melt percolation.

3. All classes of chondrites are characterized by negative $\varepsilon^{142}$Nd compositions compared with modern terrestrial samples. The heterogeneous $\varepsilon^{142}$Nd signatures observed in CCs reflect imperfect homogenization of presolar material carrying deficits in s-process nuclides ($^{142}$Nd) and p-process nuclides ($^{146}$Sm). However, the homogeneous anomaly found in OCs most likely results from $^{146}$Sm decay in planetary reservoirs with fractionated REE abundances.

4. The superchondritic $^{142}$Nd composition of the accessible Earth implies the formation of an EER that was never remixed into the mantle. Chronological constraints from coupled $^{146,147}$Sm-$^{142,143}$Nd studies, as well as compositional estimates for the bulk Moon, indicate that this event took place prior to the giant impact and probably prior to completion of core formation, and thus predates the event that led to depletion of the Isua mantle source.
5. It has been speculated that collisional erosion, induced by hit-and-run collisions at an early stage of planetary accretion, preferentially removed crustal material from planetary embryos. This would have progressively depleted the accreting planets in incompatible elements and generated chemical fractionations similar to those associated with internal mantle-crust differentiation processes. Because of this similarity, the consequences of erosional impacts for the bulk composition of the terrestrial planets remain highly uncertain and represent a new, largely unexplored field of investigation.

## DISCLOSURE STATEMENT

The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

## ACKNOWLEDGMENTS

Thanks are due to Bernard Bourdon, with whom many ideas expressed in this paper were developed. This work also benefited from discussions with many colleagues, including Vickie Bennett, Stéphane Labrosse, Thorsten Kleine, Vincianne Debaille, Jonathan O'Neil, Emilie Thomassot, Mathieu Touboul, and Steve Mojzsis. T. Mark Harrison is thanked for his review of the manuscript.

## LITERATURE CITED

* Abe Y. 1997. Thermal and chemical evolution of the terrestrial magma ocean. *Phys. Earth Planet. Int.* 100:27–39

* Agee CB. 1990. A new look at differentiation of the Earth from melting experiments on the Allende meteorite. *Nature* 346:834–37

* Agnor C, Asphaug E. 2004. Accretion efficiency during planetary collisions. *Astrophys. J.* 613:157–60

* Agnor C, Canup RM, Levison HF. 1999. On the character and consequences of large impacts in the late stage of terrestrial planet formation. *Icarus* 142:219–37

* Albarede F, Rouxel M. 1987. The Sm/Nd secular evolution of the continental crust and the depleted mantle. *Earth Planet. Sci. Lett.* 82:25–35

* Allegre CJ. 1982. Chemical geodynamics. *Tectonophysics* 81:109–32

* Allegre CJ, Brévart O, Dupré B, Minster JF. 1980. Isotopic and chemical effects produced in a continuously differentiating convecting Earth mantle. *Philos. Trans. R. Soc. Ser. A* 297:447–77

* Allegre CJ, Hofmann AW, O'Nions K. 1996. The argon constraints on mantle structure. *Geophys. Res. Lett.* 23:3555–57

* Allegre CJ, Othman DB, Polve M, Richard D. 1979. Nd-Sr isotopic correlation in mantle materials and geodynamic consequences. *Phys. Earth Planet. Int.* 19:293–306

* Amelin Y, Lee D-C, Halliday AN, Pidgeon RT. 1999. Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons. *Nature* 399:252–55

* Andreasen R, Sharma M. 2006. Solar nebula heterogeneity in p-process samarium and neodymium isotopes. *Science* 314:806–9

* Andreasen R, Sharma M. 2007. Mixing and homogenization in the early solar system. *Astrophys. J.* 665:874–83

* Andreasen R, Sharma M, Subbarao KV, Viladkar SG. 2007. Where on Earth is the enriched Hadean reservoir? *Earth Planet. Sci. Lett.* 266:14–28

* Armstrong RL. 1968. A model for the evolution of strontium and lead isotopes in a dynamic Earth. *Rev. Geophys.* 6:175–99

* Armstrong RL. 1981. The case for crustal recycling on a near-steady-state no-continental-growth Earth. *Philos. Trans. R. Soc. Ser. A* 301:443–72
Armstrong RL. 1991. The persistent myth of crustal growth. *Aust. J. Earth Sci.* 38:613-30

Asphaug E. 2009. Growth and evolution of asteroids. *Annu. Rev. Earth Planet. Sci.* 37:413-48

Asplund M, Grevesse N, Sauval AJ. 2006. The solar chemical composition. *Nuclear Phys. A* 777:1-4

Audi G, Bersillon O, Blachot J, Wapstra AH. 1997. The NUBASE evaluation of nuclear and decay properties. *Nuclear Phys. A* 624:1-124

Basu AR, Goodwin AM, Tatsumoto M. 1984. Sm-Nd study of Archean alkalic rocks from the superior province of the Canadian Shield. *Earth Planet. Sci. Lett.* 70:40-46

Bennett VC, Brandon AD, Jenner FE, Nutman AP. 2010. Hadean isotopic signatures in Mesoarchean pillow basalts, southern West Greenland. *VM Goldschmidt Conf., 10th*

Bennett VC, Brandon AD, Nutman AP. 2007. Coupled <sup>142</sup>Nd–<sup>143</sup>Nd isotopic evidence for Hadean mantle dynamics. *Science* 318:1907–10

Bennett VC, Nutman AP, McCulloch MT. 1993. Nd isotopic evidence for transient, highly depleted mantle reservoirs in the early history of the Earth. *Earth Planet. Sci. Lett.* 119:299-317

Benz W, Cameron AGW. 1990. Terrestrial effects of the giant impact. In *Origin of the Earth*, ed. HE Newsom, JH Jones, pp. 61–68. Oxford: Oxford Univ. Press

Benz W, Slattery WL, Cameron AGW. 1988. Collisional stripping of Mercury's mantle. *Icarus* 74:516-28

Birck J-L. 2004. An overview of isotopic anomalies in extraterrestrial materials and their nucleosynthetic heritage. In *Geochemistry of Nontraditional Stable Isotopes*, ed. CM Johnson, BL Beard, F Albarède, pp. 25–64. Washington, DC: Mineral. Soc. Am./Geochem. Soc.

Blichtert-Toft J, Albarède F. 2008. Hafnium isotopes in Jack Hills zircons and the formation of the Hadean crust. *Earth Planet. Sci. Lett.* 265:686-702

Blichtert-Toft J, Albarède F, Rosing M, Frei R, Bridgwater D. 1999. The Nd and Hf isotopic evolution of the mantle through the Archean. Results from the Isua supracrustals, West Greenland, and from the Birimian terranes of West Africa. *Geochim. Cosmochim. Acta* 63:3901–14

Blichtert-Toft J, Boyet M, Telouk P, Albarède F. 2002. <sup>147</sup>Sm-<sup>143</sup>Nd and <sup>176</sup>Lu-<sup>176</sup>Hf in eucrites and the differentiation of the HED parent body. *Earth Planet. Sci. Lett.* 204:167-81

Borg LE, Draper DS. 2003. A petrogenetic model for the origin and compositional variation of the Martian basaltic meteorites. *Meteorit. Planet. Sci.* 38:1713-31

Bourdon B, Caro G. 2007. The early terrestrial crust. *C. R. Geosci.* 339:928-36

Bourdon B, Touboul M, Caro G, Kleine T. 2008. Early differentiation of the Earth and the Moon. *Philos. Trans. R. Soc. Ser. A* 366:4105-28

Bouvier A, Blichtert-Toft J, Moynier F, Vervoort JD, Albarede F. 2007. Pb-Pb dating constraints on the accretion and cooling history of chondrites. *Geochim. Cosmochim. Acta* 71:1583-604

Bouvier A, Vervoort JD, Patchett PJ. 2008. The Lu-Hf and Sm-Nd isotopic composition of CHUR: constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. *Earth Planet. Sci. Lett.* 272:48-57

Boyet M, Blichtert-Toft J, Rosing M, Storey CD, Télouk M, Albarede F. 2004. <sup>142</sup>Nd evidence for early Earth differentiation. *Earth Planet. Sci. Lett.* 214:427-42

Boyet M, Carlson RW. 2005. <sup>142</sup>Nd evidence for early (> 4.53 Ga) global differentiation of the silicate Earth. *Science* 214:427-42

Boyet M, Carlson RW. 2006. A new geochemical model for the Earth's mantle inferred from <sup>146</sup>Sm-<sup>142</sup>Nd systematics. *Earth Planet. Sci. Lett.* 250:254-68

Boyet M, Carlson RW. 2007. A highly depleted moon or a nonmagma ocean origin for the lunar crust? *Earth Planet. Sci. Lett.* 262:505-16

Boyet M, Carlson RW, Horan M. 2010. Old Sm-Nd ages for cumulate eucrites and redetermination of the solar system initial <sup>146</sup>Sm/<sup>144</sup>Sm ratio. *Earth Planet. Sci. Lett.* 291:172-81

Boynton WV. 1975. Fractionation in the solar nebula: condensation of yttrium and the rare earth elements. *Geochim. Cosmochim. Acta* 39:569-84

Brandon AD, Lapen TJ, Debaille V, Beard BL, Rankenburg K, Neal C. 2009. Re-evaluating <sup>142</sup>Nd/<sup>144</sup>Nd in lunar mare basalts with implications for the early evolution and bulk Sm/Nd of the Moon. *Geochim. Cosmochim. Acta* 73:6421-45

Caffee MW, Hudson GB, Velsko C, Huss GR, Alexander EC Jr, Chivas AR. 1999. Primordial noble gases from Earth's mantle: identification of a primitive volatile component. *Science* 285:2115-18
Canup RM. 2004. Simulations of a late lunar-forming impact. *Icarus* 168:433–56

Carlson RW, Boyet M, Horan M. 2007. Chondrite barium, neodymium and samarium isotopic heterogeneity and early earth differentiation. *Science* 316:1175–78

Caro G, Bennett VC, Bourdon B, Harrison TM, von Quadt A, et al. 2008a. Application of precise <sup>142</sup>Nd/<sup>144</sup>Nd analysis of small samples to inclusions in diamonds (Finsch, South Africa) and Hadean Zircons (Jack Hill, Western Australia). *Chem. Geol.* 247:253–65

Caro G, Bourdon B. 2010. Non-chondritic Sm/Nd ratio in the terrestrial planets: consequences for the geochemical evolution of the mantle-crust system. *Geochim. Cosmochim. Acta* 74:3333–49

Caro G, Bourdon B, Birck J-L, Moorbath S. 2003. <sup>146</sup>Sm-<sup>142</sup>Nd evidence for early differentiation of the Earth's mantle. *Nature* 423:428–32

Caro G, Bourdon B, Birck J-L, Moorbath S. 2006. High-precision <sup>142</sup>Nd/<sup>144</sup>Nd measurements in terrestrial rocks: constraints on the early differentiation of the Earth's mantle. *Geochim. Cosmochim. Acta* 70:164–91

Caro G, Bourdon B, Halliday AN, Quitté G. 2008b. Super-chondritic Sm/Nd ratios in Mars, the Earth and the Moon. *Nature* 452:336–39

Caro G, Bourdon B, Wood BJ, Corgne A. 2005. Trace element fractionation generated by melt segregation from a magma ocean. *Nature* 436:246–49

Cates NL, Mojzsis SJ. 2007. Pre-3750 Ma supracrustal rocks from the Nuvvuagittuq supracrustal belt, northern Québec. *Earth Planet. Sci. Lett.* 255:9–21

Chambers JE. 2001. Making more terrestrial planets. *Icarus* 152:205–24

Chambers JE. 2004. Planetary accretion in the inner Solar System. *Earth Planet. Sci. Lett.* 223:241–52

Chase CG, Patchett PJ. 1988. Stored mafic/ultramafic crust and early Archean mantle depletion. *Earth Planet. Sci. Lett.* 91:66–72

Christensen UR, Hofmann AW. 1994. Segregation of subducted oceanic crust in the convecting mantle. *J. Geophys. Res.* 99:19867–84

Condie KC. 2000. Episodic growth models: afterthoughts and extensions. *Tectonophysics* 322:153–62

Corgne A, Liebske C, Wood BJ, Rubie DC, Frost DJ. 2005. Silicate perovskite-melt partitioning of trace elements and geochemical signature of a deep perovskitic reservoir. *Geochim. Cosmochim. Acta* 69:485–96

Davaille A, Le Bars M, Carbonne C. 2003. Thermal convection in a heterogeneous mantle. *C. R. Geosci.* 335:141–56

Davis AM, Grossman L. 1979. Condensation and fractionation of rare earths in the solar nebula. *Geochim. Cosmochim. Acta* 43:1611–32

Debaille V, Brandon AD, Yin QZ, Jacobsen B. 2007. Coupled <sup>142</sup>Nd-<sup>143</sup>Nd evidence for a protracted magma ocean in Mars. *Nature* 450:525–28

Debaille V, Yin QZ, Brandon AD, Jacobsen B. 2008. Martian mantle mineralogy investigated by the <sup>176</sup>Lu-<sup>176</sup>Hf and <sup>147</sup>Sm-<sup>143</sup>Nd systematics of shergottites. *Earth Planet. Sci. Lett.* 269:186–99

DePaolo DJ. 1980. Crustal growth and mantle evolution: inferences from models of element transport and Nd and Sr isotopes. *Earth Planet. Sci. Lett.* 44:1185–96

DePaolo DJ, Wasserburg GJ. 1976. Inferences about magma sources and mantle structure from variations of <sup>143</sup>Nd/<sup>144</sup>Nd. *Geophys. Res. Lett.* 3:743–46

DePaolo DJ, Wasserburg GJ. 1979. Sm-Nd age of the Stillwater complex and the mantle evolution curve for neodymium. *Geochim. Cosmochim. Acta* 43:999–1008

Elkins-Tanton LT, Parmentier EM, Hess PC. 2003. Magma ocean fractional crystallization and cumulate overturn in terrestrial planets: implications for Mars. *Meteorit. Planet. Sci.* 38:1753–71

Galer SJG, Goldstein SL. 1991. Early mantle differentiation and its thermal consequences. *Geochim. Cosmochim. Acta* 55:227–39

Gancarz AJ, Wasserburg GJ. 1977. Initial Pb of the Amitsoq gneiss, West Greenland, and implications for the age of the Earth. *Geochim. Cosmochim. Acta* 41:1283–301

Goldstein SL, Galer SJG. 1992. On the trail of early mantle differentiation: <sup>142</sup>Nd/<sup>144</sup>Nd ratios of early Archean rocks. *Eos Trans. AGU* 73:S323

Grand SP. 2002. Mantle shear-wave tomography and the fate of subducted slabs. *Philos. Trans. R. Soc. Ser. A* 360:2475–91

Harper CL, Jacobsen SB. 1992. Evidence from coupled <sup>147</sup>Sm-<sup>143</sup>Nd and <sup>146</sup>Sm-<sup>142</sup>Nd systematics for very early (4.5 Gyr) differentiation of the Earth's mantle. *Nature* 360:728–32
Harrison TM. 2009. The Hadean crust: evidence from >4 Ga zircons. *Annu. Rev. Earth Planet. Sci.* 37:479–505

Harrison TM, Blichtert-Toft J, Muller W, Albarede F, Holden P, Mojzsis SJ. 2005. Heterogeneous Hadean hafnium: evidence for continental crust by 4.4–4.5 Ga. *Science* 310:1947–50

Harrison TM, Schmitt AK, McCulloch MT, Lovera OM. 2008. Early (≥4.5 Ga) formation of terrestrial crust: Lu-Hf, δ¹⁸O, and Ti thermometry results for Hadean zircons. *Earth Planet. Sci. Lett.* 268:476–86

Hofmann AW. 1988. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. *Earth Planet. Sci. Lett.* 90:297–314

Hofmann AW, Jochum KP, Seufert M, White WM. 1986. Nb and Pb in oceanic basalts: new constraints on mantle evolution. *Earth Planet. Sci. Lett.* 79:33–45

Hopkins M, Harrison TM, Manning CE. 2008. Low heat flow inferred from >4 Ga zircons suggests Hadean plate boundary interactions. *Nature* 456:493–96

Jacobsen SB. 1988. Isotopic constraints on crustal growth and recycling. *Earth Planet. Sci. Lett.* 90:315–29

Jacobsen SB, Harper CL Jr. 1996. Accretion and early differentiation history of the Earth based on extinct radionuclides. In *Earth Process: Reading the Isotopic Code*, ed. A Basu, S Hart, pp. 47–74. Geophys. Monogr. 95. Washington, DC: AGU

Jacobsen SB, Wasserburg GJ. 1979. The mean age of mantle and crustal reservoirs. *J. Geophys. Res.* 84:7411–24

Jacobsen SB, Wasserburg GJ. 1984. Sm-Nd evolution of chondrites and achondrites, II. *Earth Planet. Sci. Lett.* 67:137–50

Kemp AIS, Wilde SA, Hawkesworth CJ, Coath CD, Nemchin A, et al. 2010. Hadean crustal evolution revisited: new constraints from Pb-Hf isotope systematics of the Jack Hills zircons. *Earth Planet. Sci. Lett.* 296:45–56

Kleine T, Mezger K, Munker C, Palme H, Bischoff A. 2004. ¹⁸²Hf-¹⁸²W isotope systematics of chondrites, eucrites, and martian meteorites: chronology of core formation and early differentiation in Vesta and Mars. *Geochim. Cosmochim. Acta* 68:2935–46

Korenaga J. 2009. A method to estimate the composition of the bulk silicate Earth in the presence of a hidden geochemical reservoir. *Geochim. Cosmochim. Acta* 73:6952–64

Kunz J, Staudacher T, Allegre CJ. 1998. Plutonium-fission xenon found in Earth's mantle. *Science* 280:877–80

Labrosse S, Herlund JW, Coltice N. 2007. A crystallizing dense magma ocean at the base of the Earth's mantle. *Nature* 450:866–69

Lahaye Y, Arndt N, Byerly G, Chauvel C, Fourcade S, Gruau S. 1995. The influence of alteration on the trace-element and Nd isotopic compositions of komatiites. *Chem. Geol.* 126:43–64

Larimers JW, Anders E. 1967. Chemical fractionation in meteorites, II. Abundance patterns and their interpretation. *Geochim. Cosmochim. Acta* 31:1215–38

Lugmair GW, Galer SJG. 1992. Age and isotopic relationships among the angrite Lewis Cliff 86010 and Angra Dos Reis. *Geochim. Cosmochim. Acta* 56:1673–94

Lugmair GW, Scheinin NB. 1975. Sm-Nd systematics of the Stannern meteorite. *Meteoritics* 10:447–48

Lugmair GW, Scheinin NB, Marti K. 1983. Samarium-146 in the early solar system: evidence from neodymium in the Allende meteorite. *Science* 222:1015–18

Matsui T, Abe Y. 1986. Evolution of an impact induced atmosphere and magma ocean on the accreting Earth. *Nature* 319:303–5

McCulloch MT, Wasserburg GJ. 1978. Sm-Nd and Rb-Sr chronology of continental crust formation. *Science* 200:1003–11

McCulloch MT, Bennett VC. 1993. Evolution of the early Earth: constraints from ¹⁴³Nd-¹⁴²Nd isotopic systematics. *Lithos* 30:237–55

McDonough WF, Sun S-S. 1995. The composition of the Earth. *Chem. Geol.* 120:223–53

Melosh HJ. 1990. Giant impacts and the thermal state of the early Earth. In *Origin of the Earth*, ed. HE Newson, JH Jones, pp. 69–84. New York: Oxford Univ. Press

Montelli R, Nolet G, Dahlen FA, Masters G, Engdahl ER, Hung S-H. 2004. Finite-frequency tomography reveals a variety of plumes in the mantle. *Science* 303:338–43

Moorbath S, Whitehouse MJ, Kamber BS. 1997. Extreme Nd-isotope heterogeneity in the early Archaean—fact or fiction? Case histories from northern Canada and West Greenland. *Chem. Geol.* 135:213–31

Murphy DT, Brandon AD, Debaille V, Burgess R, Ballentine C. 2010. In search of a hidden long-term isolated subchondritic ¹⁴²Nd/¹⁴⁴Nd reservoir in the deep mantle: implications for the Nd isotope systematics of the Earth. *Geochim. Cosmochim. Acta* 74:738–50
Nyquist LE, Wiesmann H, Bansal B, Shih C-Y, Keith JE, Harper CL. 1995. <sup>146</sup>Sm-<sup>142</sup>Nd formation interval for the lunar mantle. *Geochim. Cosmochim. Acta* 13:2817–37

O'Neil J, Carlson RW, Francis D, Stevenson RK. 2008. Neodymium-142 evidence for Hadean mafic crust. *Science* 321:1821–31

O'Neill HSC, Palme H. 2008. Collisional erosion and the nonchondritic composition of the terrestrial planets. *Philos. Trans. R. Soc. Ser. A* 366:4205–38

O'Nions K, Oxburgh ER. 1983. Heat and helium in the Earth. *Nature* 306:429–31

Ozima M, Podosek FA. 1999. Formation age of the Earth from <sup>129</sup>I/<sup>127</sup>I and <sup>244</sup>Pu/<sup>238</sup>U systematics and the missing Xe. *J. Geophys. Res.* 104:25493–99

Pahlevan K, Stevenson DJ. 2007. Equilibration in the aftermath of the lunar-forming giant impact. *Earth Planet. Sci. Lett.* 262:438–49

Papanastassiou DA, Sharma M, Ngo HH, Wasserburg GJ, Dymek RF. 2003. *No* <sup>142</sup>Nd excess in the early Archean Isua gneiss IE 715-28. Presented at Lunar Planet. Sci. Conf., 34th, League City, Tex.

Patchett PJ, Vervoort JD, Söderlund U, Salters VJM. 2004. Lu-Hf and Sm-Nd isotopic systematics in chondrites and their constraints on the Lu-Hf properties of the Earth. *Earth Planet. Sci. Lett.* 202:345–60

Pepin RO, Porcelli D. 2006. Xenon isotope systematics, giant impacts, and mantle degassing on the early Earth. *Earth Planet. Sci. Lett.* 250:470–85

Pidgeon RT. 1978. Big Stubby and the early history of the Earth. In *Short Papers of the 4th ICOG*, ed. RE Zartman, pp. 334–35. *U.S. Geol. Surv. Open File Rep.* 78-701

Polat A, Hofmann AW, Münker C, Regelous M, Appel PWU. 2003. Contrasting geochemical patterns in the 3.7–3.8 Ga pillow basalt cores and rims, Isua greenstone belt, Southwest Greenland: implications for postmagmatic alteration processes. *Geochim. Cosmochim. Acta* 67:447–57

Polat A, Munker C. 2004. Hf-Nd isotope evidence for contemporaneous subduction processes in the source of late Archean arc lavas from the Superior Province, Canada. *Chem. Geol.* 213:403–29

Prinzhofer DA, Papanastassiou DA, Wasserburg GJ. 1992. Samarium-neodymium evolution of meteorites. *Geochim. Cosmochim. Acta* 56:797–815

Rankenburg K, Brandon AD, Neal CR. 2006. Neodymium isotope evidence for a chondritic composition of the Moon. *Science* 312:1369–72

Regelous M, Collerson KD. 1996. <sup>147</sup>Sm-<sup>143</sup>Nd, <sup>146</sup>Sm-<sup>142</sup>Nd systematics of early Archean rocks and implications for crust-mantle evolution. *Geochim. Cosmochim. Acta* 60:3513–20

Richard D, Shimizu N, Allegre CJ. 1976. <sup>143</sup>Nd/<sup>146</sup>Nd, a natural tracer: an application to oceanic basalts. *Earth Planet. Sci. Lett.* 31:269–78

Rudnick RL, Fountain DM. 1995. Nature and composition of the continental crust: a lower crustal perspective. *Rev. Geophys.* 33:267–309

Salters VJM, Stracke A. 2004. Composition of the depleted mantle. *Geochem. Geophys. Geosys.* 5:Q05B07

Sharma M, Papanastassiou DA, Wasserburg GJ, Dymek RF. 1996. The issue of the terrestrial record of <sup>146</sup>Sm. *Geochim. Cosmochim. Acta* 60:2037–47

Shirey SB, Hanson GN. 1986. Mantle heterogeneity and crustal recycling in Archean granite-greenstone belts: Evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada. *Geochim. Cosmochim. Acta* 50:2631–51

Snyder GA, Borg LE, Nyquist LE, Taylor LA. 2000. Chronology and isotopic constraints on lunar evolution. In *Origin of the Earth and Moon*, ed. RM Canup, K Righter, pp. 361–95. Tucson: Univ. Ariz. Press

Solomatov VS, Stevenson DJ. 1993a. Nonfractional crystallization of a terrestrial magma ocean. *J. Geophys. Res.* 98:5391–406

Solomatov VS, Stevenson DJ. 1993b. Suspension in convective layers and style of differentiation of a terrestrial magma ocean. *J. Geophys. Res.* 98:5375–90

Sun S-S. 1980. Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs. *Philos. Trans. R. Soc. Ser. A* 297:409–45

Tatsumoto M. 1978. Isotopic composition of lead in oceanic basalt and its implication to mantle evolution. *Earth Planet. Sci. Lett.* 38:63–87

Taylor SR, McLennan SM. 1985. *The Continental Crust: Its Composition and Evolution*. Oxford: Blackwell Sci.

Taylor DJ, McKeegan KD, Harrison TM. 2009. Lu-Hf zircon evidence for rapid lunar differentiation. *Earth Planet. Sci. Lett.* 279:157–64
Taylor SR, McLennan SM, McCulloch MT. 1983. Geochemistry of Loess, continental crustal composition and crustal model ages. *Geochim. Cosmochim. Acta* 47:1897–905

Thomas PC, Binzel RP, Gaffey MJ, Storrs AD, Wells EN, Zellner BH. 1997. Impact excavation on asteroid 4 Vesta: Hubble Space Telescope results. *Science* 277:1492–95

Tolstikhin IN, Kramers JD, Hofmann AW. 2006. A chemical Earth model with whole mantle convection: the importance of a core-mantle boundary layer (D') and its early formation. *Chem. Geol.* 226:79–99

Touboul M, Kleine T, Bourdon B, Palme H, Wieler R. 2007. Late formation and prolonged differentiation of the Moon inferred from W isotopes in lunar metals. *Nature* 450:1206–9

Upadhyay D, Scherer EE, Mezger K. 2009. <sup>142</sup>Nd evidence for an enriched Hadean reservoir in cratonic roots. *Nature* 459:1118–21

van Thienen P, van den Berg AP, Vlaar NJ. 2004. Production and recycling of oceanic crust in the early Earth. *Tectonophysics* 386:41–65

Vervoort JD, Blichert-Toft J. 1999. Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time. *Geochim. Cosmochim. Acta* 63:533–56

Vervoort JD, White WM, Thorpe RI. 1994. Nd and Pb isotope ratios of the Abitibi greenstone belt: new evidence for very early differentiation of the Earth. *Earth Planet. Sci. Lett.* 128:215–29

Wallerstein G, Iben I, Parker P, Boesgaard AM, Hale GM, et al. 1997. Synthesis of the elements in stars: forty years of progress. *Rev. Mod. Phys.* 69:995–1084

Warren PH. 2008. A depleted, not ideally chondritic bulk Earth: the explosive-volcanic basalt loss hypothesis. *Geochim. Cosmochim. Acta* 72:2217–35

Wasson JT, Chou CL. 1974. Fractionation of moderately volatile elements in meteorites. *Meteoritics* 9:69–84

Wiechert U, Halliday AN, Lee D-C, Snyder GA, Taylor LA, Rumble D. 2001. Oxygen isotopes and the moon-forming giant impact. *Science* 294:345–48

Wilson AH, Carlson RW. 1989. A Sm-Nd and Pb isotope study of Archean greenstone belts in the southern Kaapvaal craton. *Earth Planet. Sci. Lett.* 96:89–105

Wood BJ, Halliday AN. 2005. Cooling of the Earth and core formation after the giant impact. *Nature* 437:1346–48

Wood BJ, Halliday AN. 2010. The lead isotopic age of the Earth can be explained by core formation alone. *Nature* 465:767–71

Yin QZ, Jacobsen SB, Yamashita K, Blichert-Toft J, Telouk P, Albarede F. 2002. A short timescale for terrestrial planet formation from Hf-W chronometry of meteorites. *Nature* 418:949–52

Zindler A, Hart S. 1986. Chemical geodynamics. *Annu. Rev. Earth Planet. Sci.* 14:493–571