Article
Volume 13, Number 3
3 March 2012
Q03002, doi:10.1029/2011GC004003
ISSN: 1525-2027
Excess hafnium-176 in meteorites and the early Earth
zircon record
Martin Bizzarro and James N. Connelly
Centre for Star and Planet Formation, Natural History Museum of Denmark, University of
Copenhagen, DK-1350 Copenhagen, Denmark ([email protected])
Kristine Thrane
Geological Survey of Denmark and Greenland, DK-1350 Copenhagen, Denmark
Lars E. Borg
Lawrence Livermore National Laboratory, Livermore, California 94550, USA
Lu-to-176Hf decay system is a powerful tool to understand ancient chemical fractionation events associated with planetary differentiation. Detrital Hadean zircons (>3.8 Gyr) from the Jack Hills
metasedimentary belt of Western Australia record extremely enriched Hf-isotope signals suggesting early
extraction of a continental crust (>4.5 Gyr) but fail to identify a prevalent complementary depleted mantle
reservoir, suggesting that crust formation processes in the early Earth were fundamentally distinct from
today. However, this conclusion assumes that the Hf-isotope composition of bulk chondrite meteorites can
be used to estimate the composition of Earth prior to its differentiation into major silicate reservoirs, namely
the bulk silicate Earth (BSE). We report a 176Lu-176Hf internal mineral isochron age of 4869 ! 34 Myr for
the pristine SAH99555 angrite meteorite. This age is "300 Myr older than the age of the Solar System, confirming the existence of an energetic process yielding excess 176Hf in affected early formed Solar System
objects through the production of the 176Lu isomer (t1/2 "3.9 hours). Thus, chondrite meteorites contain
excess 176Hf and their present-day composition cannot be used to infer the Lu-Hf parameters of BSE. Using
a revised BSE estimate based on the SAH99555 isochron, we show that Earth’s oldest zircons preserve a
record of coexisting enriched and depleted hafnium reservoirs as early as "4.3 Gyr in Earth’s history, with
little evidence for the existence of continental crust prior to "4.4 Gyr. This new view suggests continuous
juvenile crustal growth and recycling throughout the Hadean and Archean eras, perhaps analogous to modern
plate tectonics.
[1] The long-lived
176
Components: 5100 words, 2 figures, 4 tables.
Keywords: Bulk Silicate Earth; Lu-Hf; angrites; meteorites.
Index Terms: 1040 Geochemistry: Radiogenic isotope geochemistry.
Received 14 December 2011; Revised 19 January 2012; Accepted 23 January 2012; Published 3 March 2012.
Bizzarro, M., J. N. Connelly, K. Thrane, and L. E. Borg (2012), Excess hafnium-176 in meteorites and the early Earth zircon
record, Geochem. Geophys. Geosyst., 13, Q03002, doi:10.1029/2011GC004003.
Copyright 2012 by the American Geophysical Union
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1. Introduction
[2] Zircon is a ubiquitous accessory mineral of
Earth’s continental crust and, because of its durability and antiquity, is unique in preserving a record
of the evolution of the terrestrial crust-mantle system in deep time. In addition to its standard utility
for U-Pb geochronology, zircon is also ideally
suited for the application of the 176Lu-to-176Hf
decay system (half-life of "37 Gyr), a powerful
isotopic clock and tracer that allows identification of ancient chemical fractionation associated
with planetary differentiation and crust formation
[Patchett et al., 1981; Stevenson and Patchett, 1990;
Amelin et al., 1999, 2000; Harrison et al., 2005;
Kemp et al., 2006, 2009; Blichert-Toft and
Albarède, 2008; Harrison et al., 2008; Pietranic
et al., 2008; Kemp et al., 2010; Amelin et al.,
2011]. Using this approach, concurrent U-Pb and
Lu-Hf studies in Archean and Hadean zircons–
including the oldest remnants of Earth’s earliest
crust–have identified unradiogenic Hf isotope signals [Harrison et al., 2008; Kemp et al., 2010] with
little evidence for a complementary depleted reservoir. An implication of these data is that Earth’s first
crust was extracted from a primitive mantle as early
as "4.5 Gyr, and extensively reprocessed throughout the Hadean era without additional generation
of juvenile crust [Kemp et al., 2010]. This observation is difficult to reconcile with a modern-style
plate tectonic regime, implying that crust formation
in the Archean and Hadean epochs occurred by
fundamentally distinct processes. However, these
conclusions, based on the 176Lu-176Hf system, presume accurate knowledge of the initial Hf isotope
composition of the Solar System, which is used to
infer the Hf isotope composition of Earth prior to its
differentiation into major silicate reservoirs, that is,
the Bulk Silicate Earth (BSE). Previous attempts
to directly define the initial 176Hf/177Hf ratio of the
Solar System were based on whole-rock measurements of sample suites with complex chemical and/
or thermal histories such as chondrite and eucrite
meteorites [Patchett and Tatsumoto, 1980; BlichertToft and Albarède, 1997; Bizzarro et al., 2003;
Patchett et al., 2004; Bouvier et al., 2008; BlichertToft et al., 2002], which fail to define isochronous
relationships [Thrane et al., 2010]. Thus, estimates
of the initial 176Hf/177Hf ratio of the Solar System
are now based on the present-day Hf isotope composition of least metamorphosed chondrite meteorites [Bouvier et al., 2008], but this necessitates
knowledge of the 176Lu decay rate (l176Lu) as well
as 176Lu/177Hf ratio of the chondritic reservoir to
10.1029/2011GC004003
back-calculate the 176Hf/177Hf value to the initial
value of BSE. Although recent calibrations of the
l176Lu by geological comparison based on both
terrestrial and extraterrestrial materials are now in
excellent agreement [Scherer et al., 2001; Amelin,
2005; Söderlund et al., 2004], the 176Hf/177Hf and
176
Lu/177Hf ratios of chondrite meteorites are
sufficiently variable to make the choice for the correct value for BSE difficult. Additionally, the
176
Hf/177Hf ratios of chondrite and eucrite meteorites were apparently affected by irradiation processes
in the early Solar System resulting in accelerated
176
Lu decay through the formation of the 176Lu isomer (176mLu) at 123 keV excitation energy above the
ground state, which decays to 176Hf in 3.7 hours
[Albarède et al., 2006]. If correct, this model seriously undermines the assumption of a Lu-Hf chondritic Earth, because it requires that the material that
accreted to form the terrestrial planets and chondrite
meteorites were identically irradiated.
[3] In light of these uncertainties, we have investigated the 176Lu-176Hf systematics of the SAH99555
angrite to provide the first determination of the
initial 176Hf/177Hf ratio of the Solar System by the
internal isochron approach. Angrites are rapidly
cooled and un-metamorphosed igneous meteorites
of basaltic composition believed to have erupted
at the surface of the angrite parent body in the
first 10 Myr of the Solar System [Amelin, 2008].
They are characterized by an unusual mineralogy
and chemistry mainly composed of Ca-rich olivine,
fassaitic clinopyroxene, and anorthite [Mittlefehldt
et al., 2002; Floss et al., 2003]. Some angrites display evident quenching textures due to rapid cooling
from magmas. In these quenched angrites, ulvöspinel,
troilite, and silico-phosphates are minor, but common late-stage crystallization products [Mikouchi
and Barrat, 2009]. With an absolute Pb-Pb age of
4564.58 ! 0.14 Myr [Connelly et al., 2008] and
evidence for live 26Al at the time of its crystallization
[Spivak-Birndorf et al., 2009; Schiller et al., 2010],
the SAH99555 quenched angrite is one of the oldest
and most pristine basaltic meteorites. Moreover,
trace element variations in SAH99555 indicate rapid
crystallization under near closed system conditions,
consistent with its mineralogical and textural features
[Floss et al., 2003]. Thus, the SAH99555 angrite
is ideally suited for high-resolution 176Lu-176Hf
systematics.
2. Methodology and Results
[4] A three gram piece of SAH99555 acquired from
Labenne Meteorites was lightly crushed with an
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Table 1.
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BIZZARRO ET AL.: EXCESS HAFNIUM-176 IN METEORITES
10.1029/2011GC004003
Lu-176Hf Data for the SAH99555 Angritea
Fraction
Weight (mg)
Lu (ppm)
Hf (ppm)
Bulk-rock (1)
Bulk-rock (2)
Pyroxene (1)
Pyroxene (2)
Olivine (1)
Olivine (2)
51.4
45.0
20.2
20.0
42.2
38.5
0.291
0.300
0.493
0.497
0.0472
0.0531
1.62
1.66
4.38
4.44
0.137
0.129
176
Lu/177Hf
0.02573!12
0.02589!12
0.01601!08
0.01589!07
0.04886!23
0.05836!27
176
Hf/177Hf
0.282120!09
0.282158!10
0.281208!09
0.281200!09
0.284319!21
0.285257!26
a
Analytical uncertainties for the 176Lu/177Hf values refer to the last digits and represent the external reproducibility (0.47%, 2sd) estimated from
the repeated analyses of the BHVO-1 standard. Analytical uncertainties for the 176Hf/177Hf values refer to the last digits and represent the internal
precision (2se) of individual measurements. The external reproducibility of the 176Hf/177Hf values is 55 ppm (2sd) and was estimated by repeated
analyses of the BHVO-1 standard.
agate mortar and pestle and sieved to approximately
100 mm grain size. Following gentle cleaning in
Milli-Q water and acetone, this material was passed
through a Frantz magnetic separater to create concentrates of olivine and pyroxene that were subsequently purified by hand picking using a binocular
microscope. An uncrushed piece of SAH99555 was
broken into 2 pieces of approximately 100 mg each
for duplicate whole rock analyses. The whole rock
pieces were not cleaned or leached prior to dissolution to avoid dissolving soluble accessory phases
such as phosphate minerals. Mineral fractions were
pre-cleaned in the clean laboratory in several short
cycles of warm distilled water and 0.1M HNO3
before they were spiked with 176Lu and 180Hf tracer
solutions and dissolved using a 5:1 mixture of 28M
HF and 14M HNO3 on a 130# C hotplate for 5 days.
Whole rock fragments were spiked and digested in
the same fashion as the mineral fractions, but without any pre-cleaning. The samples were then dried
down and fluxed in 14M HNO3 in as many cycles as
was necessary to fully dissolve the sample. Purification of Lu and Hf utilized a first stage cation
column and a second stage TODGA (Eichrom
Industries) following the methods outlined by
Connelly et al. [2006]. Lu and Hf were analyzed by
MC-ICPMS following methods outlined by
Bizzarro et al. [2003]. The concentration and isotopic data are presented in Table 1. Following Lu
and Hf recovery, the remaining REE were purified
using RE-spec resin in 0.05N and 1N HNO3 prior to
Table 2.
147
loading on pressurized 2-hydroxyisobutyric acid
columns. The 2-hydroxyisobutyric acid was separated from the Sm and Nd using 2 mL cation cleanup columns. Total procedural blanks (N = 5) include
contributions associated with the tracers and were:
Sm = 6 ! 3 picograms (pg) and Nd = 7 ! 2 pg
(1 sd). Isotopic measurements were done in the
Radiogenic Isotope Laboratory at the University of
New Mexico on a Micromass (VG) Sector 54 mass
spectrometer. Neodymium and Sm were run as
oxides on single Re filaments using an oxygen bleed
system at 5 $ 10%6 torr. Mass fractionation of Nd
was corrected using 146Nd/144Nd = 0.7219, whereas
Sm interference was corrected using 147Sm/152Sm =
0.560828. Measured 143Nd/144Nd ratios were
normalized to the LaJolla Nd standard value of
0.511850. The Sm-Nd results are compiled in
Table 2.
[5] Two pyroxene, two olivine and two total rock
fractions define an isochron in 176Lu/177Hf and
176
Hf/177Hf space with slope and intercept of
0.09516 ! 0.00069 and 0.279685 ! 0.000019,
respectively (Figure 1). Using an average l176Lu of
1.867 ! 0.008 $ 10%11yr%1 determined by recent
geological calibrations [Söderlund et al., 2004], we
calculate a 176Lu-176Hf age of 4869 ! 34 Myr.
This age is approximately 300 Myr older than the
currently accepted age of the Solar System defined
by condensation of calcium-aluminum-rich inclusions at 4567.18 ! 0.50 Myr [Amelin et al., 2010]
Sm-143Nd Fata for the SAH99555 Angritea
Fraction
Weight (mg)
Sm (ppm)
Nd (ppm)
Bulk-rock (1)
Bulk-rock (2)
La Jolla Nd (N = 11)
51.4
45.0
2.00
2.00
6.28
6.21
147
Sm/144Nd
0.19339!19
0.19441!19
143
Nd/144Nd
0.512556!08
0.512582!07
0.511863!15
a
All samples and standards run as NdO+. Error limits apply to last digits and include a minimum uncertainty of 0.5% plus 50% of the blank
correction for Sm and Nd added quadratically. Normalized to 146Nd/144Nd = 0.7219. Uncertainties refer to last digits and are 2sm calculated
from the measured isotopic ratios. 2sm = [S(mi % m)2/(n(n % 1))]0.5 for n ratio measurements mi with mean value m. Error limits refer to last
digits and are 2sp. 2sp = [S(Mi % p)2/(N % 1)]0.5 for N measurements Mi with mean value p.
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Figure 1. Lu-Hf isochron diagram for the SAH9995 angrite. We interpret the intercept of the regression
[(176Hf/177Hf)0] to represent the initial 176Hf/177Hf ratio of the Solar System. MSWD, mean square of weighted deviations. Errors on the 176Lu/177Hf and 176Hf/177Hf are smaller than symbols. The external reproducibility of the
176
Hf/177Hf and 176Lu/177Hf values, determined by repeated analysis of 36 sample digestions of the BHVO-1 basalt,
are 0.47% and 0.0055%, respectively. For the isochron calculations, we used the external reproducibility or the internal
precision of the individual measurements, whichever is larger.
and, thus, cannot represent the true crystallization
age of SAH99555.
3. Discussion
[6] The accuracy of the analytical protocol
employed here for the Lu-Hf measurements has
been evaluated by analysis of matrix-matched terrestrial rock standards processed though our chemical purification scheme in an identical manner
as the extraterrestrial samples. The reproducibility
and accuracy of the 176Lu/177Hf and 176Hf/177Hf
measurements was estimated by repeated analysis
of a reference rock standard. During the course of this
study, 36 sample digestions of the Hawaiian basalt
BHVO-1 were processed through the chemical
purification procedure and analyzed by MC-ICPMS,
yielding average values of 0.00876 ! 0.00004 and
0.283110 ! 0.000016 for the 176Lu/177Hf and
176
Hf/177Hf ratios, respectively. These values are
identical, within analytical uncertainties, to that
obtained in previous studies [Blichert-Toft, 2001;
Münker et al., 2001; Bizzarro et al., 2003; Vervoort
et al., 2004; Connelly et al., 2006]. Isotope anomalies of nucleosynthetic origin affecting the hafnium
mass array could, in principle, compromise the
accuracy of the 176Hf/177Hf and 176Lu/177Hf isotope
ratios presented here for SAH99555. However, the
analysis of an un-spiked total rock aliquot indicates
that the Hf stable isotopes ratios of SAH99555
are identical, within analytical uncertainty, to the
terrestrial values (Table 3). This result is consistent with a recent study suggesting the absence of
nucleosynthetic Hf-isotope heterogeneity in chondrite meteorites [Sprung et al., 2010]. Quenched
angrites such as SAH99555 typically contain minor
Table 3. Stable Hf-Isotope Composition of an Unspiked
Aliquot of the SAH999555 Angritea
Fraction
Bulk-rock (1)
Accepted values
178
Hf/177Hf
1.46716!1
1.46717
179
Hf/177Hf
0.732507!7
0.7325
180
Hf/177Hf
1.88663!2
1.8867
a
Analytical uncertainties for all isotope ratios refer to the last digits
and represent the internal precision (2se) of individual measurements.
The accepted Hf stable isotope ratios are based on Stevenson and
Patchett [1990] and Blichert-Toft et al. [1997].
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accessory phases such as ulvöspinel and silicophosphates, which can potentially strongly fractionate rare earth and high-field strength elements.
Although we have not analyzed the 176Lu-176Hf
systematics of these phases separately, we emphasize that the two whole-rock fractions lie on the
mineral isochron defined by the olivine and pyroxene fractions. This observation is consistent with a
closed system behavior of the 176Lu-176Hf systematics in SAH99555, suggesting that the Lu-Hf
regression line presented in Figure 1 represents a
meaningful isochron. Finally, given that the U-Pb
system and short-lived radioisotope chronometers
(26Al-26Mg, 53Mn-53Cr and 182Hf-182W) define
crystallizations ages for SAH99555 that are within
5 Myr of Solar System formation [Spivak-Birndorf
et al., 2006, 2009; Markowski et al., 2007; Connelly
et al., 2008; Schiller et al., 2010], it is unlikely
that the apparent old 176Lu-176Hf crystallization
age for this meteorite results from a complex postcrystallization history. Thus, we conclude that the
Hf isotope data and Lu/Hf ratios presented here
for SAH99555 are accurate within the stated uncertainties, free of nucleosynthetic effects within the
resolution of our analyses, and not affected by postcrystallization disturbance(s).
Lu-176Hf isochron age
defined by minerals of the SAH99555 angrite is
similar to that obtained for bulk chondrite and
eucrite meteorites [Patchett and Tatsumoto, 1980;
Blichert-Toft and Albarède, 1997; Bizzarro et al.,
2003; Patchett et al., 2004; Bouvier et al., 2008;
Blichert-Toft et al., 2002], suggesting the presence
of variable excess 176Hf correlated to the 176Lu/177Hf
ratios in all these extraterrestrial samples. Thus, our
results confirm the existence of an energetic process
yielding excess 176Hf in early formed Solar System
solids and planetesimals through the production of
176m
Lu, thereby resulting in 176Lu-176Hf isochron
ages that are consistently too old [Albarède et al.,
2006]. This process has been attributed to the influx
of solar or supernova(e)-generated g-radiation of
early Solar System solids, an energy source with a
limited penetration of a few centimeters into silicate and metal materials [Albarède et al., 2006].
Petrographic features of quenched angrites such as
SAH99555 suggest cooling rates of 10–50# C/hour
[Mikouchi et al., 2001], indicating that these basaltic
meteorites crystallized at depths "0.5%2 m (calculated with a thermal diffusivity of 0.004 cm2/s, a
typical value for solid rock [Myamoto et al., 1986]).
Thus, solar or supernova(e)- generated g-radiation
cannot account for the excess 176Hf in SAH99555.
While our results do not necessarily rule-out the
g-radiation model, they suggest the existence of an
[7] The apparent old
176
10.1029/2011GC004003
additional more penetrative energy source(s) affecting early Solar System objects.
[8] Fully penetrative supernova-derived neutrinos
could, in principle, provide sufficient energy for the
production of the 176Lu isomer and associated
excesses 176Hf. But the required neutrino flux
for adequate production of 176mLu would place
the supernova within one astronomical unit of the
angrite parent body making this scenario implausible [Thrane et al., 2010]. Alternatively, excitation
of 176Lu to its isomeric state can occur from the
energy released through the impact of cosmic rays
penetrating the surface of accreted planetesimals,
via Coulomb excitation and Bremsstrahlung radiation. Cosmic rays are high-energy particles pervading the galaxy commonly believed to be accelerated
by supernova shock waves [Uchiyama et al., 2007].
Thus, Solar System formation in association with
massive stars [Hester et al., 2004] would have
exposed early formed solids and planetesimals to
a high cosmic ray flux. Recent numerical models
[Thrane et al., 2010] indicate that the energy
released from one proximal supernova is more than
adequate to enhance the production of 176mLu
within accreted planetesimals to a maximum depth
of "200 m and, hence, account for the presence of
176
Hf excesses in meteorites. If correct, this implies
that Solar System solids were exposed to a significant flux of cosmic rays after the crystallization of
SAH99555 at 4564.58 ! 0.14 Myr but before formation of the 4557 Myr old phosphates from the
Acapulco meteorite [Amelin, 2005], if these represent samples that existed within the upper 200 m of
a planetesimal. Despite uncertainties regarding the
relative contributions of the g-radiation and cosmic
ray models to the irradiation history of early Solar
System solids, the overriding result is the limited
penetration depth (<200 m) of these energy sources
within accreted planetesimals.
Hf/177Hf ratio
determined here by the internal isochron approach is
lower by "4 to 5 !-units compared to that backcalculated from the present-day Lu-Hf parameters
of bulk chondrite meteorites based on Blichert-Toft
and Albarède [1997] and Bouvier et al. [2008],
respectively, and the l176Lu value of 1.867 $
10%11yr%1 [Söderlund et al., 2004]. However,
it is identical within analytical uncertainty to less
precise estimates based on linear arrays (errorchrons) defined by bulk samples of basaltic eucrites
and chondrites [Patchett and Tatsumoto, 1980;
Blichert-Toft and Albarède, 1997; Bizzarro et al.
2003; Patchett et al., 2004; Bouvier et al., 2008;
[9] The Solar System’s initial
176
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Table 4. Lu-Hf BSE Parameters Based on the
SAH99555 Angritea
176
Hf/177Hf
0.282685!21
(176Hf/177Hf )T =
0.279685!19
0
176
Lu/177Hf
0.0337!1
a
The estimate of the initial Hf-isotope composition of the
Solar System [(176Hf/177Hf )T = 0] is based on the intercept of the
176
Lu-176Hf isochron of the SAH99555 angrite and is believed to
representan irradiation-free composition (176Lu/177Hf = 0). The
present-day Hf-isotope composition of the BSE was calculated by
forward projection of the Solar System initial 176Hf/177Hf using the
l176Lu of Söderlund et al. [2004] and a 176Lu/177Hf of 0.0337, which
represents an estimate based on chondrite meteorites [Bouvier et al.,
2008] and adjusted for a 0.3% 176 Lu burnout inferred from the excess
176
Hf in SAH99555.
Blichert-Toft et al., 2002]. Blichert-Toft et al. [2002]
reported a Lu-Hf errorchron for bulk basaltic
eucrites with the following regression parameters:
slope = 0.09294 ! 0.00080 and intercept =
0.279660 ! 0.000020 (MSWD = 4.5). The precision of these regression parameters appears similar
to that presented here for the SAH99555 internal
isochron. However, we were not able to reproduce
the uncertainties quoted by these authors using
the widely available Isoplot regression software
[Ludwig, 1991], which is the standard in the
geoscience community. Indeed, when using the
exact same dataset as Blichert-Toft et al. [2002],
we obtained the following regression parameters:
slope = 0.0927 ! 0.0037 and intercept = 0.279670 !
0.000110 (MSWD = 4.5). The uncertainty on the
initial 176Hf/177Hf defined by the basaltic eucrite
isochron corresponds to nearly 4 !-units and, therefore, cannot be used to define the Lu-Hf parameters of the bulk silicate Earth.
[10] The two whole rock fractions of SAH99555
that, together with pyroxene and olivine mineral
separates, define the 176Lu-176Hf isochron in Figure 1
were also studied for 147Sm-143Nd systematics
(Table 2). Using the Pb-Pbage of SAH99555 and
a l147Sm values of 6.539 $ 10%12y%1 [Begemann
et al., 2001], we calculate initial 143Nd/144Nd ratios
of 0.506694 ! 8 and 0.506687 ! 7 at the time of
solidification of SAH99555 for these two whole
rock aliquots, consistent with previous Solar System
initial 143Nd/144Nd estimates based on differentiated and chondrite meteorites [Bouvier et al., 2008;
Jacobsen and Wasserburg, 1984]. These results
support our interpretation that the initial 176Hf/177Hf
value defined by the SAH99555 angrite internal
isochron is primary and that its initial ratio represents the best estimate of the irradiation-free initial
Hf-isotope composition of the Solar System.
10.1029/2011GC004003
Hf/177Hf value defines
BSE depends on the irradiation history of the
material that accreted to form the Earth. Although
the mechanisms responsible for the initial growth
of dust-like particles into km-size objects are still
poorly understood, this process must have occurred
over timescales of "104 to "105 years after formation of the solar protoplanetary disk – well before
the irradiation event that produced excess 176Hf in
SAH99555 – to avoid Earth’s precursor material
spiraling into the Sun [Youdin and Shu, 2002;
Chambers, 2004). Indeed, Mg- and W-isotope data
of differentiated meteorites indicate that accretion
and differentiation of planetesimals >100 km in
diameter also occurred within the first few million
years of the Solar System [Scherstén et al., 2006;
Schiller et al., 2011]. As such, the bulk of Earth’s
precursor material most likely consisted of planetesimals >10 km that completed their accretions over
timescales comparable to that of the angrite parent
body and, hence, were mostly shielded from galactic cosmic rays and/or g-radiation given the limited
penetration depth (<200 m) of these energy sources.
We calculate that the effect of the accelerated decay
of 176Lu on the 176Hf/177Hf of BSE are negligible
(<30 ppm) if Earth formed from planetesimals at
least 10 km in diameter. Therefore, we conclude that
Earth’s precursor material consisted predominantly
of un-irradiated material such that the present-day
Hf isotopic composition of the irradiated chondrite
meteorites cannot be used to estimate the Hf isotopic composition of BSE.
[11] Whether this initial
176
[12] Using
our newly defined BSE Lu-Hf
parameters based on the internal isochron of the
SAH99555 angrite (Table 4), we re-explore the
depletion of Earth’s mantle at the time of formation of the Jack Hills zircons (Figure 2). We note
that Harrison et al. [2005] and Blichert-Toft and
Albarède [2008] reported heterogeneous !Hf
values for >4000 Myr zircons from the Jack Hills
metasedimentary belt of Western Australia, including highly radiogenic signals suggesting the existence of short-lived, ultra-depleted Hadean mantle
domains. The Hf isotope compositions in these
previous studies were obtained either by laser ablation using large beam sizes (62–81 mm [Harrison
et al., 2005]), or by whole grain dissolution
[Harrison et al., 2005; Blichert-Toft and Albarède,
2008]. However, the highly radiogenic signatures
reported by Harrison et al. [2005] and Blichert-Toft
and Albarède [2008] were not confirmed by later
studies of Jack Hills zircons, where the Hf and
Pb isotope data were obtained concurrently on the
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Figure 2. Hf isotope evolution diagrams for Archaean and Hadean igneous and detrital zircons. Data taken from
Amelin et al. [2000, 2001], Harrison et al. [2008], Kemp et al. [2009], Pietranic et al. [2008], and Kemp et al.
[2010]. (a) Calculated with the old BSE parameters based on chondrite meteorites [Blichert-Toft and Albarède,
1997; Bouvier et al., 2008]. (b) Calculated using the BSE parameters based on the initial Hf-isotope composition of
the Solar System defined by the SAH99555 mineral isochron (Table 4). The l176Lu of Söderlund et al. [2004] was used
in all calculations. The depleted mantle growth line depicts the evolution of a reservoir extracted from a primitive
mantle at 4530 Myr [Boyet and Carlson, 2005] to a modern mid-oceanic ridge basalt (MORB) 176Hf/177Hf value of
0.2833 [Chauvel and Blichert-Toft, 2001]. The evolution of the putative felsic crust reservoir was calculated with a
176
Lu/177Hf of "0.01. Harrison et al. [2005] and Blichert-Toft and Albarède [2008] reported heterogeneous !Hf values
for >4000 Myr Jack Hills zircons, including highly radiogenic signals. However, these highly radiogenic signals were
not confirmed by recent studies where Hf and Pb data were obtained by concurrent analysis of carefully characterized
zircons, thereby affirming concerns [Harrison et al., 2008; Kemp et al., 2010] that the earlier results may be compromised by sampling domains with disparate age and 176Hf/177Hf values. Therefore, we have omitted these data from our
compilation. Using the BSE parameters based on chondrite meteorites (Figure 2a), some of the most unradiogenic Jack
Hills zircons plot in an impossible region of the Hf isotope diagram, which requires their formation prior to the accretion
of the Earth if these minerals were derived from a source lithology with a Lu/Hf "0.01. However, using the revised BSE
parameters determined in our study (Figure 2b), we note that all data can be explained if crustal reservoirs of felsic
composition existed on Earth from "4400 Myr.
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same spatial domain of well-characterized zircons
[Harrison et al., 2008; Kemp et al., 2010]. For
example, Harrison et al. [2008] reported concurrent
Hf and U-Pb data for 68 Jack Hills zircons with ages
greater than "3.9 Gyr, yielding !HfT values ranging
from "%11 to %1. Similarly, Kemp et al. [2010]
recently reported data for 96 Jack Hills zircons for
which the Hf and Pb isotope ratios were measured
concurrently, and obtained !HfT values ranging
from "%11 to %1. We concur with Harrison et al.
[2008] and Kemp et al. [2010], who concluded
that the highly radiogenic Hf signals reported
by Harrison et al. [2005] and Blichert-Toft and
Albarède [2008] are spurious, caused by the analysis of complexly zoned zircons resulting in
mixed sampling of domains with disparate age and
176
Hf/177Hf ratios. This feature has been clearly
demonstrated by Kemp et al. [2010, Figure 2a],
where a range of up to 15 !-units is observed within
a single complexly zoned grain, with the most
radiogenic !HfT values associated with the younger
domains of the zircon. Similarly, Bizzarro et al.
[2002] showed that whole grain dissolution of
"3 Gyr cogenetic zircons and baddeleyites from
a single carbonatite analyzed by solution mode
MC-ICPMS (the same technique used by BlichertToft and Albarède [2008]) have a range of up
to 6 !-units, although these grains have similar
207
Pb/206Pb ages. However, the !HfT values are
correlated to the 206Pb/238U ratios, with the most
discordant grains showing the more radiogenic !HfT
values [see Bizzarro et al., 2002, inset in Figure 1].
Given these concerns, we have not included the
earlier data of Harrison et al. [2005] and BlichertToft and Albarède [2008] in our compilation of
Figure 2.
[13] The most important result emerging from our
study is that, contrary to current thinking, the terrestrial Hadean zircon record supports coexistence
of both enriched and depleted hafnium reservoirs as
early as "4.3 Gyr in Earth’s history. Approximately
50% of the zircons older than 4 Gyr have !Hf >0,
indicating that these grains were derived from a
source reservoir with a time-integrated record of
depletion. Equally conspicuous is that a significant
portion of the younger zircons (<3.8 Gyr) from
Greenland, Canada, Australia and South Africa
were extracted from depleted source regions compatible with the extent of depletion recorded by the
>4-Gyr grains. Moreover, the extent of depletion
defined by the most radiogenic Archean and Hadean
zircons is broadly consistent with a source reservoir
that evolves to the modern depleted mantle. This
indicates that the crustal rocks from which these
10.1029/2011GC004003
zircons crystallized were ultimately derived from
a mantle source region with a 176Lu/177Hf ratio
similar to that of the present-day mantle as sampled
by mid-ocean ridge basalts.
[14] The genesis of silicic magmas required to sta-
bilize zircon crystallization did not occur via direct
melting of a mantle source, but involved further
differentiation processes and/or re-melting of older
likely more mafic juvenile lithologies. In previous
studies [Kemp et al., 2006; Pietranic et al., 2008;
Kemp et al., 2010], the lack of zircons with Hfisotope signatures matching that of the depleted
mantle evolution curve was interpreted to reflect a
temporal decoupling between juvenile crust generation and zircon crystallization, suggesting that the
genesis of silicic magmas may have been delayed by
100 to 400 million years following crust extraction.
However, the new view presented here establishes
that, compatible with modern plate tectonics
[Tamura and Tasumi, 2002; Vogel et al., 2004],
juvenile crust generation and subsequent differentiation in the Hadean epoch were nearly contemporaneous in some cases and, therefore, intrinsically
linked. The smear of zircon Hf-isotope compositions along and below the depleted mantle curve
is indicative of continuous juvenile crust formation and reprocessing throughout the Hadean and
Archean eras. Taking the Hf-isotope signal of the
>4.1 Gyr zircons at face value, a significant result
of our new BSE reference framework is the lack of
evidence for the existence of continental crust prior
to "4.4 Gyr, a time that may coincide with the
establishment of a plate tectonic regime similar to
that operating today.
[15] The present-day Hf-Nd BSE isotopic value,
based on chondrite meteorites, is slightly displaced
toward lower 176Hf/177Hf ratios ("3 !-unit) compared to the modern mantle array, which apparently
requires the presence of an unidentified reservoir in
Earth’s mantle [Blichert-Toft and Albarède, 1997].
Our revised Lu-Hf parameters for the BSE lowers
the estimated Hf isotopic composition of the BSE
through time by "5 !-unit, which exacerbates the
observed offset between BSE and the modern
mantle array and, therefore, increases the need
for a missing reservoir enriched in incompatible
elements. The striking consistency in the timeintegrated Hf-isotope composition of the presentday, Archean and Hadean depleted mantle source
reservoirs is consistent with the idea that depletion
of the terrestrial mantle resulted from the extraction
of this early formed complementary enriched reservoir, perhaps related to a global differentiation
event that occurred by 4.53 Gyr [Boyet and Carlson,
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2005]. Apart from an ambiguous Hf-isotope signal
in deep-seated magmas such as carbonatites and
kimberlites [Bizzarro et al., 2002], there is little
evidence that the unidentified enriched reservoir
has contributed to surface magmatism, requiring its
isolation in the lower mantle. If correct, this model
implies that continuous extraction and recycling
of crust throughout Earth’s history has had limited
impact on the chemical budget of the mantle sampled by mid-ocean ridge basalts through geologic
time.
Acknowledgments
[16] The Centre for Star and Planet Formation is financed by
the Danish National Research Foundation and the University
of Copenhagen’s Programme of Excellence. We thank Yuri
Amelin for comments on an earlier version of this paper.
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