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LETTERS
PUBLISHED ONLINE: 23 JANUARY 2011 | DOI: 10.1038/NGEO1060
Tracing two orogenic cycles in one eclogite sample
by Lu–Hf garnet chronometry
Daniel Herwartz1 *, Thorsten J. Nagel1 , Carsten Münker2 , Erik E. Scherer3 and Nikolaus Froitzheim1
Subduction of rocks into the mantle results in high-pressure
metamorphism and the formation of eclogites from basaltic
precursor rocks. At the Earth’s surface, eclogites often occur
as isolated fragments embedded in crustal rocks that lack
evidence for high-pressure conditions1–3 . The high-pressure
rocks are therefore often viewed as dismembered fragments
that have been assembled and intercalated with rocks devoid of
any high-pressure history at shallow crustal levels4–8 , forming
a tectonic mélange. Such mélange models were supported
by age discrepancies among high-pressure rocks from the
Adula nappe (Central Alps)9–12 , which was thought to represent
a classic example of such a situation4,5 . Here we present
Lu–Hf age data from two populations of the high-pressure
mineral garnet, found within a single eclogite sample taken
from Trescolmen, in the Central Adula nappe. We report a
minimum Variscan age of 332.7 Myr and a maximum Alpine
age of 38 Myr for the two populations. We suggest that
the Trescolmen eclogite was subducted to mantle depth and
subsequently exhumed, becoming part of a continental crust
during the Variscan orogeny. Later, during the Alpine orogeny,
the Adula nappe must have been subducted to—and exhumed
from—mantle depth a second time, as one coherent unit. We
conclude that the Adula nappe is not a mélange, and therefore,
the crustal rocks that envelope the eclogites have also been
subjected to high-pressure conditions through deep subduction
during the Alpine event13,14 .
Subduction to mantle depths of >40 km and exhumation back
to the surface are documented by the occurrence of eclogite-facies
rocks formed at high-pressure in Alpine-type orogens. Such rocks
occur either in units derived from subducted oceanic crust or,
alternatively, in tectonic nappes dominantly made of felsic crust
from a subducted continental margin. Rocks that have reached
ultrahigh-pressure conditions (that is, the stability field of coesite;
>70 km) typically derive from the second type of high-pressure
units and are associated with abundant continental basement1–3 .
The high-pressure mineral assemblages are usually preserved
within isolated blocks of eclogite or garnet peridotite, whereas the
enveloping crustal rocks typically lack evidence of high-pressure
conditions1–3 . Thus, a paramount question in reconstructing the
geologic history of high-pressure domains is whether these nappes
represent (1) units having different metamorphic histories that
were mechanically juxtaposed within a subduction channel4–8,11
or (2) coherent terranes of pre-existing continental basement that
underwent high-pressure metamorphism and exhumation as a
whole2,3,14–16 (Fig. 1).
The Adula nappe in the Lepontine Alps (Switzerland) has served
as a natural laboratory for many studies addressing subductionzone processes4,5,11,13,14,17–20 . It comprises crustal gneisses, metasediments and mica schists, as well as mafic and ultramafic
units including eclogites, amphibolites and peridotites13,14,17,19,20 . So
far, peak-pressure conditions have been recognized in eclogites,
associated garnet mica schists, and scarce garnet peridotites13,14,19,20 .
These rocks show a gradient of southward-increasing pressure and
temperature (Fig. 2), interpreted to reflect burial in a southwarddipping subduction zone during the Tertiary Alpine orogeny19,20 .
After the high-pressure event, the entire unit was subjected to
amphibolite-facies (low-pressure, high-temperature) conditions,
with the intensity of the overprint also increasing to the south.
However, previous geochronology yielded inconsistent results, so
the timing of high-pressure metamorphism in the Adula nappe
remains ambiguous. Eclogites and garnet peridotites from the
southwestern part of the nappe yield ages between 42 and 35 Myr
using Lu–Hf and Sm–Nd dating of garnet9,11 and U–Pb dating of
zircon10 . In marked contrast, zircon grains from eclogites in the
central and northeastern part of the nappe yield only Palaeozoic
ages for the high-pressure stage12 (Fig. 2). These observations
spurred models suggesting that the Adula nappe was not always
a single coherent unit but rather comprises different tectonic
slices that were metamorphosed at different times before final
assembly5,11 (Fig. 1a).
We have investigated an eclogite from the classic Trescolmen
locality in the central Adula nappe (Fig. 2). Pressures and
temperatures associated with the main eclogite-facies assemblages
are about 2.1–2.2 GPa and about 680 ◦ C (ref. 20; Fig. 2). The
sample investigated is composed of garnet, omphacite, minor
amounts of quartz, euhedral amphibole, and rutile (Fig. 3).
There is no sign of amphibolite-facies retrogression and the
sample is feldspar free. Omphacite crystals are equigranular and
chemically homogeneous, whereas garnet porphyroblasts exhibit
strong chemical zonation and a bimodal grain-size distribution.
A few large garnet grains up to 4 mm in diameter comprise
dark cores and pale rims. A second population of smaller (0.1–
0.3 mm), euhedral garnet porphyroblasts is distributed throughout
the sample. High-resolution X-ray maps confirm the presence
of two generations (Fig. 3; Supplementary Fig. S1). Cores of
the large porphyroblasts (grt1) are corroded, have oscillatory
grossular zoning, and irregular, almandine + spessartine-rich and
pyrope-poor patches. These relict cores are cross-cut by jagged
fissures filled with grossular-rich garnet (grt2a), which also
mantles grt1 cores. The outermost rim (grt2b) is again poorer
in grossular component. The small, euhedral garnet population
consists solely of grt2a (in cores) and grt2b (at rims). Both
garnet generations contain rare inclusions of quartz, rutile and
apatite. In addition, grt1 contains inclusions of Al-rich amphibole
and grt2 contains omphacite. We interpret the grt1 population
to have been partially resorbed at high temperature, thereby
producing the fissures. We also attribute the patchy distribution of
almandine, pyrope and spessartine components to this event, with
1 Steinmann-Institut,
Universität Bonn, Bonn 53115, Germany, 2 Institut für Geologie und Mineralogie, Universität zu Köln, Köln 50674, Germany, 3 Institut
für Mineralogie, Universität Münster, Münster 48149, Germany. *e-mail: [email protected].
178
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NATURE GEOSCIENCE DOI: 10.1038/NGEO1060
a
LETTERS
b
0
20
20
40
40
(km)
(km)
0
60
80
100
Unit containing separate
ultra (high) pressure a rocks
before they are welded together
at shallow crustal levels
60
80
Unit comprising a large,
coherent ultra (high) pressure body
100
Figure 1 | Two contrasting models for the exhumation mechanism of geological units comprising (ultra) high-pressure rocks. a, Small parts of the
subducting slab are plucked off at various depths and then exhumed within a ‘subduction channel’ and welded into a megascale mélange at shallow crustal
levels. b, Nappes are subducted and exhumed as coherent bodies. Hence, in model a only small parts of the nappe have reached (ultra) high-pressure
conditions, whereas in model b the entire nappe is subjected to high-pressure conditions.
a
9°
A
Bündnerschiefer
Zürich
Va: Vals
520 °C/ 1.3 GPa
580 °C/ 1.7 GPa
About 330 Myr (U¬Pb)
N
Va
Adula
European margin
Apulian margin
Cf: Confin
570 °C/2.2 GPa
700 °C/1.9 GPa
About 340 Myr (U¬Pb)
Simano
Cf
Trescolmen
A
VL: Val Large
About 370 Myr (U¬Pb)
Le
South Penninic ocean
Tertiary intrusions
A′
A′′
10 km
Simano
VL
ina
nt
ve
Latitude (° N)
100 km
Milano
46° 30′
IL
Bergell
Tr
A′
Cv: Calvaresc
About 370 Myr (U¬Pb)
46° 20′
Tr: Trescolmen
660 °C/2.2 GPa
700 °C/2.1 GPa
About 370 Myr (U¬Pb)
Cv
Ga
Suretta
Tambo
46° 20′
Chiavenna
Ga: Gagnone
740 °C/3 GPa
About 40 Myr (Sm¬Nd)
Ar: Alpe Arami
840 °C/3.2 GPa
1120 °C/5 GPa
1180 °C/5.9 GPa
About 35 Myr (U¬Pb)
About 39 Myr (Sm¬Nd)
About 37 Myr (Lu¬Hf)
46° 10′
Ca: Caurit
720 °C/ 2.4 GPa
Ar
Ca
Go
Du
Go: Gorduno
750 °C/2.3 GPa
About 38 Myr(Lu¬Hf)
Bergell
Du: Duria
830 °C/3 GPa
830 °C/2.8 GPa
About 35 Myr (U¬Pb)
46° 10′
10 km
A′′
Periadriatic (Insubric) line
Southern Alps
9°
9° 30′
Longitude (° E)
Figure 2 | Location of the Adula nappe and the Alp Trescolmen. a, Tectonic sketch map of the Adula nappe. Stars denote prominent eclogite or garnet
peridotite localities. Rectangles illustrate a compilation of peak pressure–temperature conditions20 , as well as known high-pressure ages9–12 . Ambiguous
Ar–Ar (ref. 22), K–Ar and Rb–Sr (ref. 23) data are not shown (Va, Vals; Cf, Confin; VL, Val Large; Tr, Trescolmen; Cv, Calvaresc; Ga, Gagnone; Ar, Alpe
Arami; Go, Gorduno; Ca, Caurit; Du, Duria). Insets show an overview of the Alpine realm (top) and a tectonic cross-section through the Adula nappe20
(bottom; IL, Insubric line).
the almandine+spessartine-rich areas representing the original grt1
composition. Other grt1 domains have apparently been subjected
to diffusive re-equilibration. This view is corroborated by the
observation that the spessartine content within the patches still
reflects original growth zoning; that is, it consistently increases
towards the grain centres as defined by the concentric, oscillatory
grossular zoning. Garnet generations grt2a and grt2b clearly grew
after the resorption event, with grt2a healing the fissures and
forming new nuclei within the rock matrix. The presence of sharp
chemical gradients between grt1 domains and the surrounding
grt2 indicates that the aforementioned diffusive re-equilibration
must have occurred before grt2 growth. Enclosed relics of grt1 in
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NATURE GEOSCIENCE DOI: 10.1038/NGEO1060
LETTERS
a
b
Fe
Mg
omp
A
A
grt
am
B
B
B
B
A
A
qtz
c
d
Ca
Mn
A
A
B
B
B
A
A
B
1 mm
Oscillatory zoning
e
0.7
A′
A
Almandine
Pyrope
f
0.7
Grossular
Spessartine
B′
0.5
X in garnet
X in garnet
0.5
B
0.3
0.1
0.3
0.1
h
g
grt1
relics
grt1
j
i
grt2b
grt2a
Figure 3 | Illustration of the different garnet generations in Trescolmen sample TRC1. a–d, Element maps of Fe (a), Mg (b), Ca (c) and Mn (d). In b, bluish
colours are garnet (grt), green is omphacite (omp), yellow is amphibole (am) and black denotes quartz (qtz). e,f, Compositional cross-section through
large (e) and small (f) garnet crystals. Location of cross-sections is indicated in a–d. g–j, Illustration of garnet generations. grt1 represents corroded garnet
relics from a first (Variscan) orogenic cycle (g,h) that have been partially replaced and overgrown by two successive (Alpine) generations, grt2a (i) and
grt2b (j).
180
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LETTERS
Table 1 | Lu–Hf data obtained from the Trescolmen (Adula nappe) eclogite sample TRC1.
Sample ID
Fraction
Lu (ppm)
Hf (ppm)
176 Lu/177 Hf
176 Hf/177 Hf
DH55
Whole rock 1
(selective tabletop digestion)
Whole rock 2
(PARR bomb digestion)
Omphacite
Variscan garnet
Variscan garnet
Variscan garnet
Variscan garnet
Variscan garnet
Variscan garnet
Variscan garnet
(purest sample)
Variscan garnet
Variscan garnet
Alpine garnets ≤150 µm
(purest sample)
Alpine garnet ≤150 µm
Alpine garnet ≤150 µm
Alpine garnet ≥250 µm
Alpine garnet ≥250 µm
Alpine garnet ≥250 µm
(impure garnet separate)
Residue of whole rock 1
Residue of Variscan garnet
Residue of Variscan garnet
Residue of Variscan garnet
Residue of Variscan garnet
0.662
0.221
0.4240 (13)
0.283987 (21)
1.27
6.04
0.02990 (9)
0.283074 (9)
0.0205
2.47
2.68
3.29
4.38
2.09
1.91
4.37
0.249
0.0496
0.0752
0.112
0.135
0.0392
0.0746
0.0889
0.01167 (11)
7.114 (23)
5.092 (18)
4.187 (16)
4.638 (4)
7.647 (24)
3.637 (11)
7.037 (25)
0.283171 (14)
0.32333 (14)
0.312218 (30)
0.30722 (11)
0.311691 (7)
0.32567 (16)
0.301684 (30)
0.327306 (36)
4.27
3.72
0.945
0.0813
0.0759
0.0259
7.508 (23)
7.013 (23)
5.174 (18)
0.327553 (30)
0.326723 (46)
0.286748 (44)
0.960
1.02
0.965
0.951
0.974
0.0259
0.0266
0.0294
0.0274
0.0558
5.272 (17)
5.442 (3)
4.665 (14)
4.939 (16)
2.478 (8)
0.287067 (54)
0.287197 (26)
0.287089 (15)
0.287077 (56)
0.285292 (63)
–
–
–
–
–
–
–
–
–
–
0.0001875 (7)
0.001832 (6)
0.0009642 (71)
0.001369 (8)
0.002141 (9)
0.283022 (9)
0.283038 (14)
0.282959 (22)
0.283067 (25)
0.283063 (29)
DH78
DH61
DH53
DH62
DH63
DH134
DH156
DH157
DH158
DH159
DH160
DH161
DH64
DH133
DH54
DH65
DH66
DH55-R
DH53-R
DH156-R
DH157-R
DH159-R
Reported uncertainties on the last decimal places (in parentheses) are the estimated 2σ external reproducibility for 176 Lu/177 Hf and 2σm internal run statistics for 176 Hf/177 Hf. For the isochron regressions,
the empirical relationship between external and internal uncertainties was employed, where the 2σ external uncertainties of 176 Hf/177 Hf are about twice the 2σm internal analysis statistics27 . External
reproducibility for 176 Lu/177 Hf of ±0.25% (2σ ) includes average error magnification for over- or under-spiked samples. ‘Residues’ are the undigested fraction left after the selective dissolution of whole
rock and garnet by the tabletop procedure.
grt2-filled fissures are as small as 10 µm and still preserve their
distinct chemical composition.
We separated several garnet fractions by hand picking dark
red fragments of grt1 and light red, euhedral crystals of grt2
in two different grain sizes. Two different digestion methods
were applied, one of which, in steel-jacketed Teflon bombs
(PARR bombs), ensures full sample digestion21 , whereas a tabletop
digestion method efficiently dissolves garnet while leaving behind
refractory Hf-bearing phases such as zircon and rutile21 . The
undigested refractory minerals (residues) from the tabletop
digestion were rinsed several times and then digested in PARR
bombs. The Lu–Hf data from the different garnet separates define
two well-constrained arrays in 176 Hf/177 Hf versus 176 Lu/177 Hf
space, documenting two distinct metamorphic events (Table 1;
Fig. 4). Eight out of nine grt1 fractions yield Carboniferous
ages (336–299 Myr), whereas <150 µm grt2 fractions consistently
define Eocene ages (40–37 Myr). We interpret the Eocene age to
date prograde growth of grt2 as Alpine peak-pressure conditions
were approached, whereas the Carboniferous age may represent
growth or re-equilibration of grt1. As our physical separation of
the two main garnet generations was not perfect, the Variscan
and Alpine isochrons strictly represent minimum and maximum
ages, respectively. There is clear evidence for contamination of
>250 µm grt2 fractions by grt1 inclusions, as the former clearly
plot above the isochron defined by the smaller grain size fractions
of grt2 (<150 µm). Although not visible in thin section, there is
probably a minor grt1 component in the <150 µm grt2 fractions
as well, as evident from a slight scatter in 176 Hf/177 Hf that is
not correlated with 176 Lu/177 Hf. Assuming that the grt2 fraction
having the lowest 176 Hf/177 Hf at a given 176 Lu/177 Hf is the least
contaminated, we obtain a maximum garnet–omphacite isochron
age of 37.10 ± 0.94 Myr for the Alpine event. Alternatively, an
age of 38.5 ± 2.5 Myr is calculated using the residues of grt1
and the purest grt2 fraction. As residual components of grt1
cores preserve unradiogenic Hf that was not available when new
garnet formed during Alpine eclogite-facies metamorphism, the
use of this low Variscan initial 176 Hf/177 Hf is probably unrealistic.
Nevertheless, it constrains a firm upper age limit for Alpine
garnet growth. Analogous to our reasoning for grt2 fractions,
we regard grt1 separates that tend towards lower 176 Hf/177 Hf
at a given 176 Lu/177 Hf as being affected by the younger grt2
domains. A minimum age of 336.0 ± 3.3 Myr is obtained for
the older event by an isochron defined by grt1 separates with
the comparatively highest 176 Hf/177 Hf and the solid residues
that remained after garnet dissolution. The residues consist of
>99% rutile and should have preserved a Variscan initial Hf
isotope composition. Using the fully digested whole rock instead
of the residues for isochron regression yields an identical age
of 337.1 ± 1.3 Myr.
Eclogites from Trescolmen can now be shown to have been
subjected to the high-pressure phases of two orogenic cycles17 , a
history that may also apply to the entire basement of the Adula
nappe, as we will argue below. The Variscan ‘minimum’ age of
332.7 Myr is consistent with more precise U–Pb zircon ages of
about 370 Myr typically found at Trescolmen12 . These 370 Myr ages
probably reflect the true timing of garnet growth because they
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LETTERS
a
0.33
ag
e
0.31
im
um
0.30
Age = 37.1 ± 0.94 Myr
(176Hf/177Hf)i = 0.283163 ± 0.000027
M
in
176Hf/177Hf
0.32
Age = 336.0 ± 3.3 Myr
(176Hf/177Hf)i = 0.283020 ± 0.000034
MSWD = 3.3
0.29
Maximum
0.28
age
b
0
2
4
6
8
10
176Lu/177Hf
b
0.2835
Age = 336.0 ± 3.3 Myr
(176Hf/177Hf)i = 0.283020 ± 0.000034
0.2834 MSWD = 3.3
176Hf/177Hf
0.2833
0.2832
0.2831
Age = 37.1 ± 0.94 Myr
(176Hf/177Hf)i = 0.283163 ± 0.000027
0.2830
0.2829
0
0.01
0.02
0.03
176Lu/177Hf
WR1 (tabletop)
WR2 (PARR bomb)
Omphacite
grt1
grt2 ≤ 150 μm
grt2 ≥ 250 μm
Residue of WR1
Residues of grt1
Figure 4 | Isochron plots illustrating the different ages obtained for two
garnet generations present in eclogite sample TRC1 from Trescolmen.
a, A six-point isochron defined by the purest (with respect to different
garnet generations) grt1 separate, four residues of grt1 fractions and one
whole-rock residue (WR1) yields a Variscan age of 336.0 ± 3.3 Myr. A
two-point isochron defining an Alpine age of 37.10 ± 0.94 Myr is obtained
from the purest grt2 fraction and the omphacite separate. MSWD = mean
square of weighted deviates. b, Close-up showing the low-Lu/Hf analyses.
were interpreted to reflect high-pressure conditions12 . The Eocene
‘maximum’ age of 38 Myr is consistent with high-pressure ages
determined by Lu–Hf (ref. 11), Sm–Nd (ref. 9) and U–Pb (ref. 10)
chronometry in the southwestern Adula nappe (Fig. 2). It also
concurs with Ar–Ar phengite ages (about 40.5 Myr; ref. 22) from
the central Adula nappe as well as a prominent Rb–Sr phengite-age
population (about 38–35 Myr; ref. 23) that has been interpreted to
date the Alpine high-pressure event20,23 . As the intensity of Alpine
metamorphic overprint increases towards the south, evidence of the
Variscan metamorphism is best preserved in the northeastern part
of the nappe. Fortunately, our sample from the central part of the
nappe was only partially overprinted such that two distinct garnet
generations are present.
182
Clearly, vestiges of Variscan metamorphism are preserved only
as relict cores of garnet, whereas the rest of our eclogite has
equilibrated to Alpine high-pressure conditions. Hence, the pressure and temperature estimates for the Trescolmen locality probably reflect Eocene conditions13,14,19,20 . This assumption is even
more likely to apply for the southwestern Adula nappe where
the Alpine overprint was even more pronounced. Although it
cannot be resolved here whether the petrological features in the
northern Adula nappe record Eocene or Palaeozoic conditions, it
is simpler to attribute a coherent pressure–temperature gradient
to a single event rather than to two. Collectively, we infer that
the north-to-south pressure gradient in the Adula nappe is an
entirely Alpine feature and may well reflect the position of the
Adula nappe as a coherent unit in an Alpine subduction zone15,20 .
Subduction as a coherent unit is also supported by our new age data
because (1) Alpine high-pressure metamorphism is consistently
dated as upper Eocene within a variety of rocks throughout the
Adula nappe and (2) our Variscan high-pressure age in sample
TRC1 clearly shows that this sample was part of the continental
basement before the Alpine orogeny. These features would be
expected for basement nappes that are subducted and exhumed as
coherent slabs, whereas they must be viewed as rather coincidental
in a mélange model.
Although the continental host rocks in the Adula nappe generally
lack evidence for deep subduction, ‘coherent unit type’ subduction
models require that they must have been subjected to peak
pressures similar to those of the associated eclogites2 . Regardless
of whether these rocks had actually recrystallized to high-pressure
assemblages, they would have had large buoyancies, contributing
to fast exhumation1–3,24 . The complete absence of any diffusive
re-equilibration between Variscan grt1 and Alpine grt2 populations
during the entire Alpine cycle indicates that exhumation indeed
occurred rapidly. As evident from the available age data, deep
subduction of the Adula nappe did not commence until about
37 Myr ago and the unit reached peak-pressure conditions about
35 Myr ago10 , before it was exhumed to shallow crustal levels
(0.55 GPa) about 33–32 Myr ago10,12 . Hence, it seems unlikely that
the Trescolmen eclogites have resided in a deep subduction channel
for several million years as proposed for other high-pressure rocks
in the Adula nappe5,11 and elsewhere in the Alps6,7 .
Our data support models where the Adula nappe represents a
basement nappe that was subducted and rapidly exhumed as one
coherent unit3,14–16,20 . The methods applied here can now be used
to test for similar scenarios in other continental high-pressure and
ultrahigh-pressure units in the Alps and elsewhere.
Methods
The eclogite sample was crushed and divided into two splits, one of which was
powdered in an agate mill, whereas the other was processed for mineral separation.
Two different digestion procedures were applied. (1) Selective dissolution of garnet,
omphacite, and one whole-rock fraction (TRC1-WR1) efficiently dissolves major
silicate phases, while leaving behind refractory, Hf-bearing accessory phases such
as rutile and zircon21 . (2) One whole rock (TRC1-WR2) and the refractory mineral
inclusions remaining after selective dissolution (‘residues’) were digested in Savillex
vials placed inside steel-jacketed Teflon PARR bombs for 3–5 days at 180 ◦ C,
ensuring full sample digestion21 . A mixed 176 Lu–180 Hf tracer was added before all
digestions. The refractory mineral inclusions remaining from the selective tabletop
digestion were rinsed repeatedly with 3N HCl and deionized water before respiking
and digestion in PARR bombs. Separation of Lu and Hf from the rock matrix was
achieved by Ln-Spec resin column chemistry25 . Lutetium and Hf measurements
were carried out on a Finnigan Neptune multi-collector inductively coupled
plasma mass spectrometer (MC-ICP-MS) at the Steinmann Institut, Bonn21,26 .
Measured 176 Hf/177 Hf values are reported relative to 176 Hf/177 Hf = 0.282160 for
the Münster Ames Hf standard, which is isotopically identical to the JMC-475
standard. For isochron calculations, the external reproducibility was estimated
by the empirical relationship 2σ external reproducibility ≈4σm , where σm is the
standard error of an individual analysis27 . An external reproducibility of ±0.2%
(2σ ) for the 176 Lu/177 Hf is typical for ideally spiked sample solutions if naturally
occurring Yb in the Lu cuts is used for mass bias and interference corrections26 .
Isochron regressions and ages were calculated using ISOPLOT v.2.49 (ref. 28) and
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λ176 Lu = 1.867 × 10−11 yr−1 (refs 29,30). Blanks for Lu and Hf ranged between
3–15 pg and 6–25 pg, respectively. Total amounts of Hf analysed ranged from 1.2 to
584 ng. Sample-to-blank ratios were all >80. The blank uncertainty results in slight
variations of 176 Hf/177 Hf composition, but the effect on isochron ages is negligible.
Electron microprobe analyses were carried out using the JEOL-JXA-8900 at the
Steinmann Institut, Bonn.
Received 3 March 2010; accepted 7 December 2010;
published online 23 January 2011
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Acknowledgements
We thank S. Weber for his help with preparing mineral separates, S. Oppel for
processing thin sections, A. Luguet for technical assistance with the MC-ICP-MS
instrument, J.E. Hoffmann for critical discussion and S. Kramer for proofreading.
We are grateful for the thorough and constructive reviews by J. Kramers, H.K.
Brueckner and C. Beaumont.
Author contributions
D.H. acquired the Lu–Hf isotope data including MC-ICP-MS measurements and data
processing. T.J.N. collected, processed and refined electron microprobe analyses. C.M.,
N.F. and E.E.S. substantially contributed to the study design, data evaluation and
interpretation. E.E.S. developed the selective digestion method and C.M. developed the
column chemistry and the analytical protocol. D.H. and T.J.N. wrote the manuscript and
contributed equally to the study.
Additional information
The authors declare no competing financial interests. Supplementary information
accompanies this paper on www.nature.com/naturegeoscience. Reprints and permissions
information is available online at http://npg.nature.com/reprintsandpermissions.
Correspondence and requests for materials should be addressed to D.H.
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