9 Lu–Hf and other lithophile
isotope systems
Lutetium lies at the end of the lanthanide
series as the ‘heaviest’ of the rare earth elements (REE). It has two isotopes 175Lu
and 176Lu whose respective abundances are 97.4 and 2.6%. 176Lu
displays a branched isobaric decay, by $! emission to 176Hf and by
electron capture to 176Yb. However the latter makes up only a few
per cent at most of the total activity and can be more or less ignored (Dixon et al., 1954). 176Hf is left
in an excited state after $ emission, and decays to the ground state by ( emission. It is one of six isotopes
and makes up 5.2% of total hafnium, an element which is not a rare earth but
resembles Zr very closely in its crystal chemical
behaviour.
9.1 Lu)Hf geochronology
The decay scheme:
17671Lu 6 17672Hf + $!
+ <
+ Q
yields a decay equation:
176Hf = 176HfI + 176Lu
(e8t ! 1) [9.1]
This is conveniently divided through by 177Hf:
176Hf (176Hf) 176Lu
))) = ())))) + ))) (e8t ! 1) [9.2]
177Hf (177Hf)I 177Hf
The
first Lu)Hf geochronological measurement was made by Herr et al. (1958), who attempted to
determine the half-life of 176Lu by analysing the isotopic
composition of Hf in the heavy-REE-rich mineral
gadolinite (containing several thousand ppm Lu).
However, routine Hf isotope analysis was prevented
until 1980 by the difficulties of low blank chemical separation and by the poor
ionisation efficiency of Hf during Thermal Ionisation
Mass Spectrometry (TIMS). These problems were finally overcome by Patchett and Tatsumoto (1980a),
using a modified form of triple filament analysis with a very hot centre
filament (section 2.2.1). After this breakthrough, TIMS
analysis of Hf continued to be limited by poor
ionisation efficiency, although improvements were achieved by various methods.
For example, one technique involved bombarding the heated sample with an ion
beam during TIMS analysis, hence ‘hot SIMS’ (Salters and Zindler, 1995). However,
TIMS analysis of Hf has now
been superseded by MC-ICP-MS (section 2.5.2).
9.1.1 The Lu decay constant and the CHUR
composition
Patchett and Tatsumoto
(1980b) presented the first Lu)Hf isochron,
based on a suite of eucrite meteorites (achondrites). These have an estimated age of ca. 4.55 Byr, from which Patchett and Tatsumoto were able to make a geological determination of
the 176Lu decay constant. Their original ten-point whole-rock isochron was improved by the addition of three extra points
(Tatsumoto et
al., 1981) to yield a half-life of 35.7 " 1.2 Byr
(equivalent to a decay constant of 1.94 H 10!11 yr!1) and an initial 176Hf/177Hf
ratio of 0.27978 " 9 (2F), Fig. 9.1. This half-life compared very well with the value of 35.4 " 1.1 Byr
which Patchett and Tatsumoto
(1980b) calculated as a weighted mean of five physical half-life determinations
made since 1960 (Faure, 1977). However, it should be
noted that the good isochron fit obtained from achondrites was achieved by rejecting one sample, the Antarctic
meteorite ALHA.
Fig. 9.1. Lu)Hf isochron
for eucrite meteorites.
More
recent physical determinations yield significantly higher values of the
half-life, averaging 37.3 " 0.1 Byr (Nir-El and
Lavi, 1998), equivalent to a decay constant of 1.86 " 0.005 H 10!11 yr!1. This value has been supported by geological
half-life determinations based on four Proterozoic
rock samples (Scherer et al., 2001).
Three of these samples comprised pegmatites, while
the fourth was a monazite–xenotime
gneiss from Grenville-age rocks of the Hudson
Highlands,
Pettingill and Patchett (1981)
attempted to test their Lu decay constant in a dating study on the Amitsoq gneisses of west
Fig. 9.2. Lu–Hf errorchron for a suite of whole-rock Amitsoq
gneisses and separated zircons. Open symbols were omitted from the regression.
After Pettingill and Patchett
(1981).
Similarly
low Lu–Hf isochron ages
were also obtained in a more recent dating study on the Isua
supracrustals of western Greenland, and on Early Proterozoic (Birimian) metamorphic
rocks of western Africa (Blichert-Toft et al., 1999). In this case the
relatively large discrepancy between Lu–Hf and Sm–Nd ages was attributed to open
system behaviour of the Lu–Hf system. However, the
Lu–Hf and Sm–Nd ages in this study can be reconciled using the new decay
constant (Villa et al., 2001).
The
low Hf abundances in chondrites,
coupled with the poor efficiency of Hf analysis by
TIMS, prevented a direct determination of their Hf
isotope composition until recently. Therefore, Patchett
and Tatsumoto (1981) determined the composition of
the chondritic uniform reservoir (CHUR) from the
intersection of the eucrite meteorite isochron and the 176Lu/177Hf ratio of
0.0334 derived from the carbonaceous chondrites
Murchison and Allende. This gave a present-day chondritic 176Hf/177Hf ratio of
0.28286.
Direct
analysis of chondritic Hf
was finally made possible by MC–ICP–MS (Blichert-Toft
and Albarede, 1997). This work yielded a cluster of
data points very close to the eucrite isochron of Patchett and Tatsumoto, but the average present-day CHUR value was
lowered slightly to 0.28277 " 3. Both values compare quite well
with the Bulk Earth Hf isotope ratio determined from
the intersection of chondritic 143Nd/144Nd
with the Nd)Hf isotope array defined by ocean island basalts.
However, the new value does not fit quite as well as the old one.
Hf isotope ratios can be expressed using the , notation developed for Nd (parts per 10 000 deviation from the chondritic
evolution line). Unfortunately, changes in the Lu decay constant also have a
major impact on the calculation of , Hf values because
they change the slope of the chondritic growth line.
As a result, the new decay constant described above gives rise to a CHUR
evolution line which is approximately 3 epsilon units higher than the old
value. This has major ramifications for the use of Hf
initial ratios as a tracer of crust and mantle evolution, as discussed in more
detail in section 9.2.
Following
the studies described above, additional work was done on the Lu)Hf
systematics of meteorites, with somewhat perplexing
results. The first of these studies, by Blichert-Toft
et al. (2002), re-examined the suite
of eucrites analysed by Patchett
and Tatsumoto. The second study focussed on ordinary chondrites and carbonaceous chondrites,
with a few eucrite analyses for comparison (Bizzarro et al.,
2003).
Blichert-Toft et al.
analysed a suite of 21 whole-rock eucrites, including
most of the samples in Fig. 9.1. This suite actually comprises two different petrological types, the cumulate eucrites
(with Lu/Hf ratios greater than chondrites)
and the basaltic eucrites, also called basaltic achondrites (with lower Lu/Hf
ratios similar to those of chondrites). Basaltic achondrites have been dated successfully by Sm)Nd mineral isochrons
(section 4.1.1), but Sm)Nd analysis of the cumulate eucrites shows evidence of major disturbance, especially
for Moama, the high Lu/Hf
sample that controls the eucrite Lu)Hf isochron
in Fig. 9.1. On the other hand, if the Lu)Hf data set is restricted to basaltic eucrites (achondrites), the
spread in Lu/Hf ratios is not sufficient to obtain a
precise regression.
After
rejecting Moama and two other samples from a
composite Sm)Nd data set of cumulus and basaltic eucrites, Blichert-Toft et al. obtained an age of 4464 " 75 Myr.
Using the old 176Lu decay constant of 1.94 H 10!11 yr!1 then gave a similar age of 4470 " 22 Myr
for the three most radiogenic eucrites in Fig. 9.1 (Moama, Moore County and Serra de
Mage). However, this agreement was only obtained by selecting different samples
for the two isochrons, and is therefore not very
meaningful.
Alternatively,
the basaltic eucrite suite alone yields a Lu)Hf ‘errorchron
age’ of 4604 " 39 Myr (MSWD = 4.52) using the old decay constant, which is within
error of the latest estimate of 4565 Myr for the time
of differentiation of the eucrite parent body
(section 15.5.2). On this basis, Blichert-Toft et al. argued that their data supported
the old decay constant. However, if analytical errors on the basaltic eucrite errorchron are expanded
to equal the geological scatter (MSWD = 1), the errocrchron then provides a relatively weak
constraint on the decay constant. For example, an age of ca. 4.75 " 0.18 Byr
would be obtained using the new 176Lu decay constant, which is just
within error of the earliest solar system ages of 4.57 Byr.
On this basis, the eucrite data cannot definitely
exclude the new decay constant.
Apparently
stronger evidence against the new decay constant was obtained from the study of
Bizzarro et al.
(2003) on chondrites. For example, a composite suite
of ordinary chondrites, carbonaceous chondrites and a few eucrites
gave an isochron fit with a reported MSWD of 1.04.
Assuming an age of 4.56 Byr for the chondrite-forming event, the isochron
implied a high 176Lu decay constant of 1.98 H 10!11 yr!1, equivalent to a half-life of 35.0 Byr. In addition, the steeper slope on the isochron gave a less radiogenic chondritic
initial ratio of 0.27963 " 2. Combining this value with the high decay constant leads to a CHUR
evolution line about 3 epsilon units lower
than Patchett’s original value (in contrast to the
new ‘terrestrial’ value that is 3 epsilon units higher).
The
discrepancy between the new ‘terrestrial’ and ‘meteorite’ decay constants
obviously introduces a lot of uncertainty into the interpretation of hafnium
isotope data, both in terms of dating and in terms of crust)mantle evolution models. Faced with
this dilemma, the present author believes that the terrestrial value is to be
preferred, because it causes fewer problems for several dating studies on
terrestrial rock suites. It is also supported by the new study of Soderlund et al.
(2004). However, until the meteorite initial ratio is clarified, the
interpretation of Hf isotope data must be regarded as
provisional. Hence the CHUR line used in the rest of this chapter will largely
follow the old value of Patchett and Tatsumoto (1981).
9.1.2 Dating metamorphism
The high Lu/Hf ratios
found in garnets make these minerals useful for Lu–Hf
dating of metamorphic events in a manner analogous to Sm–Nd. In particular, the spread of Lu/Hf
ratios measured in metamorphic garnets is typically greater than for Sm/Nd ratios. When coupled with the lower half-life of Lu, this
allows more precise dating of young (Cenozoic)
metamorphic events. Applications have included the dating of garnet granulite lower crustal xenoliths
(Scherer et al., 1997) and the dating
of high-pressure metamorphism in the Alps (Duchene et al., 1997).
Scherer
et al. (2000) made a detailed study of
the application of Lu–Hf garnet geochronology,
including the effects of Hf-rich accessory minerals
and an estimation of the
blocking temperature of the Lu–Hf
system in garnet. Of the major accessory minerals, apatite, monazite and
zircon, only the latter contains high levels of Hf,
whereas monazite contains high levels of Nd. This
means that comparison of Lu–Hf and Sm–Nd ages can be used to test
for the perturbation of garnet–whole-rock isochrons
by these minerals. This is important because zircon may contain as much as 95%
of the total Hf inventory of a rock, so any
discordance between zircon and garnet will have a large effect on the Lu–Hf age. Such discordance is quite likely when we are trying
to date the growth of metamorphic garnet, because the rock will probably
contain pre-metamorphic zircon grains which do not equilibrate with garnet
under peak metamorphic conditions.
Two
alternative scenarios involving discordant zircon are shown in Fig. 9.3. The
first rock (Fig. 9.3a) has zircon grains in the groundmass, but not in the
garnet. Comparison between zircon and whole-rock compositions shows that the
zircon grains bias the whole-rock point to give an apparent isochron
age which is slightly too old. In the other case (Fig. 9.3b) there are zircon
inclusions in the garnet as well as zircon grains in the groundmass. In this
case, comparison of leached and unleached garnets
shows that the apparent age will be too young. To avoid these problems, Scherer
et al. recommended that
inclusion-free garnets should be selected by hand picking. In addition, the
analysis of other zircon-free minerals such as hornblende or clinopyroxene can be used (instead of the whole-rock point)
to avoid the effects of groundmass zircon.
Fig. 9.3. The effects of zircon on Lu–Hf ages of metamorphic garnet growth. a) involving
groundmass zircons only; b) involving zircons inclusions in garnet grains. ( ! ) = garnet; ( " ) = leached garnet; ( Q
) = whole-rock; ( x ) = zircon. After Scherer et al. (2000).
Having
applied such corrections, Scherer compared the resulting Lu–Hf
ages with Sm–Nd ages on the
same samples. Their published ages should also be corrected for the new decay
constant. The result is that the Lu–Hf ages are
either within error of or older than the Sm–Nd ages, so the blocking temperature of the Lu–Hf system in garnet appears to be greater or equal to that
of the Sm–Nd system.
However, because metamorphic garnets have large variations in texture and
chemistry, both isotope systems will have a fairly wide range of blocking
temperatures.
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