Thomas Hopkinson
Is there a hidden early enriched reservoir within the Earth?
The discovery in 2005 that the Earth’s mantle’s
142
Nd/144Nd ratio varies from that of
chondritic meteorites, which it was thought represented the bulk composition of the
Earth, has prompted debate over the reason for this deviation. Initial beliefs that there
is an early enriched reservoir deep in the mantle which formed after a differentiation
event in the mantle have, more recently, been replaced with ideas that perhaps the
Earth was not formed with a chondritic composition.
For centuries geologists have attempted to decipher the sequence of events at the very
start of our planet’s existence. It is a challenging subject - as one looks further back
through time, the available evidence (that is to say, rocks of that age) becomes
increasingly rare. Indeed there are no terrestrial rocks older than roughly 3.8 billion
years (Ga) old on the surface of Earth. It is interesting to note that despite it only
being visited six times, the Moon has provided rocks 600 million years (Ma) older
than the most ancient formations on Earth. Despite these problems, a convincing
story has been told of the Earth’s early history. Nearly all of terrestrial accretion
happened within the first 50Ma or so of the solar system’s life, and during this time
the planet differentiated into the iron-rich core and the silicate-rich mantle. It is the
core that is the major repository of the siderophile (‘iron loving’) elements, such as
platinum and gold, whereas the lithophile (‘stone-loving’) elements such as silicon
partitioned into the bulk silicate Earth (BSE). It is in events after this that debate lies
– whether or not there was further differentiation of the BSE.
The first solids to appear in the solar system were calcium-aluminium rich inclusions,
or CAIs, which have been dated to 4567Ma1. Larger solids followed soon after;
chondrules, the major constituent of chondrite meteorites, had formed within 2.5Ma1.
Eventually kilometre-sized planetesimals were produced, which collided with each
other to produce larger and larger bodies2. The heat produced by both the high energy
collisions and radioactive decay of nuclides within planets caused widespread melting
to occur, which facilitated the differentiation between the core and mantle, and also
any possible differentiation within the mantle itself.
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Thomas Hopkinson
In order to understand the chemical evolution of our planet radiogenic isotopes are
investigated. One system that has been utilised is the samarium-neodymium system.
Sm decays to Nd in two ways: 146Sm  142Nd with a half life of 103Ma, and 147Sm 
143
Nd with a half life of 106Ga3. The half life of the shorter lived isotope means it is
useful for investigating processes in the first 500Ma of the solar systems history (five
half lives). Both elements are refractory and lithophile, so their relative abundances
in the Earth should not have been affected by core formation or volatile element loss
from the early solar system. Mid ocean ridge basalts (MORBs) have been shown to
have a higher
143
Nd/144Nd ratio than chondrites, which reflects the depletion of
incompatible elements (those that partition more into melts) in the rocks10. In this
case, Nd is more incompatible than Sm. However, in 2005 Boyet and Carlson
produced high precision data showing that the
142
Nd/144Nd ratio of chondritic
meteorites was 0.2 parts in 10,000 (0.2) less than in terrestrial rocks from various
sources. The 142Nd notation is commonly used for isotopic ratios, and refers to the
difference in
142
Nd/144Nd ratios in parts per ten-thousand, from a modern terrestrial
standard, or 142Nd = ((142Nd/144Nd)sample /(142Nd/144Nd)standard – 1) x 104. The low
Figure 1: Earth’s accretion, possible silicate differentiation and the late veneer
a. Core differentiates as Earth accretes. b. Moon-forming impact occurs as magma ocean is
solidifying; possible lowermost mantle melt reservoir formed5. c. Magma ocean solidifies,
possibly with basal partial melt zone5. d. Late veneer, with chondritic impactors. e. End of late
veneer, Isua mantle source unaffected. Lowermost mantle reservoir, if present, is also
unaffected. f. Today, upper mantle homogenised by mantle convection, potentially still some
remnants of the partial melt zone at the core-mantle boundary.
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Thomas Hopkinson
142Nd in chondrites was an unexpected finding, and meant that either the BSE
composition was superchondritic, or that sometime in the first 300Ma of the Earth’s
life, while the 146Sm isotope was alive, the mantle differentiated into an early enriched
reservoir (EER) of incompatibles and an early depleted reservoir (EDR)3. As all the
samples measured on the Earth’s surface had homogenously higher 142Nd/144Nd ratios
than chondrites; the EER, if it exists, cannot be exposed on the surface, and must be
within the mantle somewhere. The only exception is one sample of 3.8Ga rocks from
Isua, Greenland, which has an even higher ratio than chondrites.
So where could the EER be hidden? Since
there are no subchondritic values of
142
Nd/144Nd at the surface the EER cannot
have ever partaken in volcanism and crust
formation, and therefore must be deep
within the mantle. One problem is that
neither the size nor composition of the
EER, if it exists, is known. One of the
best candidates for where this reservoir is
hidden is the D’’ zone – a 200km thick
zone at the lowermost mantle just above
Figure 2: BSE-normalised element
abundances in MORBs, the continental
crust (CC) with varying concentrations in
average EER and EDR compositions
depending on thickness of proposed EER
(from Boyet and Carlson 2005)
the core-mantle boundary4. Alternatively, the EER could be contained in the whole
lower mantle, although this is unlikely since seismic images suggest that at least the
top three quarters of the mantle is involved in convection, and at least some of it have
been remixed into the mantle3. Figure 2 shows possible compositions of the EER
depending on its size and location, using the simple mass balance equation BSE =
EDR + EER. The
142
Nd/144Nd ratios also allow for timing to be estimated of when
differentiation took place, of which Boyet and Carlson estimated 30Ma into Earth’s
existence; meaning that the mantle differentiation must have been occurring
simultaneously with core formation3. Looking further afield, the supposed EER was
then compared to the KREEP basalts found on the moon, which are rocks rich in
potassium, rare Earth elements and phosphorus: all incompatible elements.
It is
thought that these KREEP basalts were formed from the very last bits of melt in the
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Thomas Hopkinson
Moon – which could potentially be a way that any EER in Earth formed. Labrosse et
al. (2007) examined the possibility that the initial magma ocean could have
crystallized from the bottom up, leaving a small envelope of basal partial melt
surrounding the core that may still exist with a thickness of around 10km today5. As
the layer slowly crystallized over Earth’s history, it would have become more
enriched in incompatible elements – therefore producing the proposed EER.
Further work using the same isotope system was then done on meteorites from Mars,
and it was discovered that Martian rocks also display elevated
142
Nd/144Nd ratios,
meaning that either the Martian mantle also underwent silicate differentiation or that
both planets have a superchondritic composition. However, unlike terrestrial samples,
which have a homogenous 142Nd across the globe, rocks from Mars, the Moon and
Vesta (considered the source of eucrite meteorites) are far more heterogeneous,
remnant from when their mantles solidified and differentiated. Caro et al. in 2008
used data from a range of shergottites, nakhlites and chassignites (together the SNC
meteorites, of Martian origin) to plot an isochron for Mars. It was found that the
Martian isochron intersected a lunar isochron and terrestrial 142Nd data at a single
point that differed from the chondrite composition (see fig.3)6. This also differs from
Vesta’s isochron, which passes through the chondrite composition field. With their
data, they calculated at what age the EER would have formed, and found that any
reservoir would have had to form at the very start of the solar system, rendering the
hypothesis unlikely. Instead, they gave two different explanations. Firstly, there
Figure 3: a. Isochrons of the Moon, Mars and Vesta compared with terrestrial 142Nd and
chondritic composition. b. Bulk compositions of terrestrial planets compared with the
chondritic reservoir and Vesta composition. (Caro et al. 2008)
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Thomas Hopkinson
could have been fractionation and differentiation within the protoplanetary disc,
which would give the region around Earth and Mars higher Sm/Nd but left the
asteroid belt with a chondritic composition. Or, on large bodies such as Mars and
Earth, proto-crusts were continuously being eroded away by large collisions in the
late stages of accretion6,11. These would remove low Sm/Nd material leaving the
remainder of the mantle with a superchondritic Sm/Nd ratio. In 2010 work on early
Archaean rocks in Canada and Greenland, which includes the Isua rocks with their
anomalous 142Nd, revealed no obvious tungsten isotope anomaly. Hafnium-182
decays to tungsten-182 with a half-life of 8.9 million years, so is a good system for
detecting events in the first 50Ma or so of the Earth’s history. As the Earth’s mantle
has a higher
182
W/184W ratio than chondritic meteorites, it has been speculated that
most of Earth’s core formation occurred when
182
Hf was still alive. As the mantle is
enriched in lithophile Hf relative to W, this would produce the higher 182W/184W ratio
observed in mantle rocks7,8. For average terrestrial rocks, 182W = 0, and for average
chondrites, 182W = -2. However Iizuka et al. analysed these Eoarchaean rocks and
discovered no variations in 182W relative to the rest of the terrestrial rocks analysed.
This, they argued, means that if there is an EER, it could not have interacted with the
mantle at all in the 3.8 billion years since these rocks were laid down. Models they
produced also suggested that any EER would have to have a negative 182W, of
sufficient magnitude to lower the whole 182W of the mantle, and therefore increasing
the age of core formation to >30Ma. However the EER would also, by 142Nd
constraints, have to be complete before 30Ma (as suggested by Boyet and Carlson in
2005). For silicate differentiation to be nearly completed before core formation seems
unlikely, and it was deemed more likely that the Earth’s composition was
superchondritic, as suggested by Caro et al. (2008).
These Isua rocks came under scrutiny again a year later, by Willbold et al., who
found, using new high precision isotopic analysis, that the 182W was 0.15 higher than
in the rest of the mantle, similar to how its 142Nd was elevated9. This was explained
using the late veneer hypothesis. The Earth’s mantle is surprisingly abundant in
highly siderophile elements (HSEs), which models suggest should have abundances
close to zero. This is because they should nearly entirely be entrained in the metal
phase during core formation, due to their siderophile nature. It is hypothesised that a
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Thomas Hopkinson
‘late veneer’ of chondritic material was added to the Earth during a terminal
bombardment of meteorites, after core formation had taken place, adding siderophile
elements back to the mantle. This would also have caused isotopic ratios, such as
182W, to move towards a chondritic value. Willbold et al. proposed that the Isua
rocks were sourced from a region of mantle that was unaffected by the late veneer,
and therefore still retained isotopic signatures from the mantle immediately after core
formation terminated9. This would also go some way to explaining the anomalously
high 142Nd in the Isua rocks, too. Also, for the HSE concentrations in the mantle to
now be homogenous, the late veneer material must have been mixed into the mantle.
It is possible that the late veneer initiated mantle mixing, which would have destroyed
much of any mantle layering following its crystallization, and perhaps mixing at least
some of the EER back into the EDR.
So the question as to whether a hidden reservoir exists has clearly not yet been put to
bed.
The example from Isua perhaps shows that we are still far away from fully
answering the questions about our planet’s early existence. That two geochemical
data sets of the same rock, taken only one year apart, can give such different answers
to a problem proves that there is still a lot of work that needs to be done. Ultimately,
it may be that a combination of arguments may be closer to the truth. It is certainly
possible that our Earth has both a non-chondritic composition, either through impact
erosion of a primitive crust or through heterogeneity in the protoplanetary disc; and
also that at some point in the planet’s history silicate differentiation occurred.
References:
1. Amelin, Y. 2002. Lead Isotopic Ages of Chondrules and Calcium-AluminiumRich Inclusions. Science 297, 1678-1683
2. Chambers, J.E. 2004. Planetary Accretion in the inner Solar System. Earth and
Planetary Science Letters 223, 241-252
3. Boyet, M. and Carlson, R.W. 2005. 142Nd Evidence for Early (>4.53 Ga) Global
Differentiation of the Silicate Earth. Science 309, 576-581
4. Peltier, W.R., 2007. Mantle dynamics and the D’’ layer: Impacts on the post
perovskite phase. Geophys. Monograph Series 174, 217-227
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Thomas Hopkinson
5. Labrosse, S. et al. 2007. A crystallizing dense magma ocean at the base of the
Earth’s mantle. Nature 450, 866-869
6. Caro. G. et al. 2008. Super-chondritic Sm/Nd ratios in Mars, the Earth and the
Moon. Nature 452, 336-339
7. Yin, Q. et al. 2002. A short timescale for terrestrial planet formation from Hf-W
chronometry of meteorites. Nature 418, 949-952
8. Kleine, T. et al. 2002. Rapid accretion and early core formation on asteroids and the
terrestrial planets from Hf-W chronometry. Nature 418, 952-955
9. Willbold, M. et al. 2011. The tungsten isotopic composition of the Earth’s mantle
before the terminal bombardment. Nature 477, 195-198
10. Jacobsen, S.B. and Wasserburg, G.J. 1979. The Mean Age of Mantle and Crustal
Reservoirs. Journal of Geophysical Research 84, 7411-7427
11. Agnor, C., and Asphaug, E., 2004. Accretion efficiency during planetary
collisions. Astrophysical Journal 613, 157-160.
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