6.3 Pb isotope
geochemistry
Pb isotopes are a powerful tool in studies of
mantle and crustal evolution because the three different radiogenic isotopes are
generated from parents with a wide span of half-lives, two of which are
isotopes of the same element. By using the different isotopes in conjunction,
it is not only possible to identify the nature of differentiation events, but
also to place constraints on their timing.
Early
inferences about the Pb isotopic evolution of the mantle were based on the
analysis of galenas. These continue to give important information about Archean
Pb isotope evolution. However, younger galena ores are plagued by the complex
evolutionary history implied by the formation of ore deposits, involving
both mantle and crustal residence times. As analytical methods improved, it became
possible to analyse mantle-derived samples such as basic magmas. These have
much lower lead contents, but usually a much simpler history, allowing
inferences about the mantle source to be made with greater confidence.
6.3.1 Pb)Pb isochrons and the lead paradox
Ocean Island Basalt (OIB) leads were found by
several workers (e.g. Gast et al.,
1964; Tatsumoto, 1966; 1978; Sun and co-workers, 1975) to define a series of
arrays to the right of the geochron on the Pb)Pb ‘isochron’ diagram (Fig. 6.21). The slopes of
these OIB arrays correspond to apparent ages of between 1 and 2.5 Byr, and can
be interpreted in three principal ways: as the products of two component mixing processes; as resulting from discrete
mantle differentiation events; or as resulting from continuous evolution of
reservoirs with changing : values. Each of these models may be applicable to different magmatic
suites.
Fig. 6.21. Pb)Pb ‘isochron’ diagram showing linear arrays of
data defined by ocean island basalts. After Sun (1980).
The
mixing model was championed by Sun et al.
(1975), who showed that the array of Pb isotope compositions in Reykjanes Ridge
basalts was best explained by two component mixing of ‘plume’ and
‘low-velocity-zone’ (upper mantle) components under Iceland. They suggested
that the linear Pb isotope arrays generated by several other ocean islands
might be explained by the same mechanism. However, since these arrays have
different slopes, a mixing model can only work if each array is attributed to
mixing of the MORB reservoir with a different enriched source (Sun, 1980).
Therefore, the problem of explaining the origin of these radiogenic sources
still remains.
The
differentiation model was studied by Chase (1981), who evaluated OIB data in
terms of a two-stage Pb isotope evolution model. This allowed 238U/204Pb
values to be calculated for an ‘original’ mantle reservoir (:1) and for the secondary sources (:2) which yield OIB Pb)Pb arrays. Chase found that values for :2 are variable within each island group and
between groups, but the calculated :1 value was remarkably constant (7.84)7.96) for all of the data (Fig.
6.22). He therefore concluded that ocean islands were derived from separate OIB
sources of variable age, but that these in turn were derived from a single
long-lived primary reservoir.
Fig. 6.22. Range of : values required to explain OIB
sources using a two-stage Pb evolution model. Parental mantle (:1) undergoes differentiation events at different
times to yield discrete OIB source domains (:2). After Chase (1981).
The
model of continuous mantle evolution with a changing : value was adopted by Dupre and
Allegre (1980) to explain the Pb isotope composition of leached basalt samples
dredged from the Mid Atlantic Ridge. The data define a linear array to the
right of the geochron, whose slope yields an apparent Pb)Pb isochron of 1.7 Byr age. However,
this result was interpreted, not as a world-wide mantle differentiation event,
but as an average age for continuous differentiation from ca. 3.8 Byr ago until
the present. This could have occurred by the mixing of enriched components with
depleted mantle in numerous small events.
The
distribution of OIB Pb)Pb arrays to the right of the geochron presents a problem in
understanding Pb evolution in the Earth as a whole, since it implies that the
depleted mantle has an average composition more radiogenic than the Geochron
(Bulk Earth). This is the opposite of the expected behaviour, since
experimental evidence (e.g. Tatsumoto, 1988) suggests that U is more
incompatible than Pb, and should generate low U/Pb ratios in the depleted
mantle. This problem has been termed the ‘lead paradox’ (Allegre et al., 1980). A complementary reservoir
with unradiogenic Pb must exist to balance the radiogenic depleted mantle, but
this other reservoir has proved hard to locate.
In
the Plumbotectonics model (section 5.4.3) unradiogenic Pb was located in the
lower crust. However, this model was not actually intended to solve the Pb
paradox, but simply to explain the distribution of U and Pb in various crustal
and mantle reservoirs. In fact the earlier versions of the model did not deal
at all with the Pb paradox, while the later versions assumed a young age for
the Earth to solve the paradox.
Notwithstanding
these historical caveats, the Plumbotectonics model can go some way towards
solving the Pb paradox. By retaining unradiogenic Pb in the lower crust, recycling
of upper crustal Pb can partially explain the distribution of oceanic volcanic
Pb to the right of the geochron. However, the U/Pb ratios of typical ocean
floor sediment (White et al., 1985)
are not great enough to explain the radiogenic Pb signatures of most OIB
sources.
An
alternative model proposed by Vidal and Dosso (1978) and Allegre (1982)
suggested that Pb fractionation from a lower mantle OIB reservoir into the
Earth’s core could increase the : value of OIB sources so as to generate the Pb)Pb arrays to the right of the
geochron. These authors proposed that while core segregation progressed very
rapidly after the Earth’s accretion, and was probably almost complete after 100
Myr, it nevertheless continued at a slow rate up to ca. 1.5 Byr ago, preferentially
incorporating Pb.
Newsome
et al. (1986) argued that this model
could be tested by examining the distribution of other elements such as Mo and
W that have much higher distribution coefficients from a lithophile (mantle) to
siderophile (core) phase than Pb. Hence, if late Pb partitioning into the core
is invoked to explain radiogenic OIB sources (e.g.
The
model was tested by comparing Pb isotope data in OIB sources with Mo elemental
data. However, allowance must first be made for the behaviour of Mo during
solid)liquid
partitioning in OIB magma genesis. This is done by comparing Mo with another
element with similar bulk partition coefficients for an upper
mantle mineralogy. Experimental evidence suggests that light REEs such as Pr
have this behaviour. After removing upper mantle effects by normalising against
Pr, Mo abundances exhibit no correlation with radiogenic Pb isotope ratios in
OIB (Fig. 6.23). This suggested that the core-fractionation model for OIB leads
should be
rejected. More recently, the core fractionation model has been reinstated in
order to explain the general
distribution of terrestrial Pb isotope reservoirs to the right of the Geochron
(section 5.4.3). However, it now appears that even this effect is weaker than
first thought.
Fig. 6.23. Pb isotope data for OIB, expressed
in terms of the :2 value of
a two-stage evolution model. These show no correlation with a trace element
index which measures possible fractionation of siderophile elements into the
Earth’s core. After Newsome et al. (1986).
Since
core fractionation has been reduced in importance as a solution for the Pb
paradox, we are forced to return to explanations involving Pb partition between
the mantle and crust. Following Hofmann and White (1980) and Chase (1981), it
has been widely proposed that subducted oceanic crust can solve this paradox.
U/Pb fractionation during the generation of MORB allows the creation of OIB
sources with relatively high : values, which can generate radiogenic Pb signatures after storage for 1 )2 Byr.
However, the U/Pb ratios of normal MORB are not high enough to explain the
composition of strongly radiogenic OIB sources (termed HIMU by Zindler and
Hart, 1986). Therefore, several mechanisms have been proposed to elevate the
U/Pb ratios of subducted oceanic crust.
6.3.2 The development of HIMU
Seawater alteration has been invoked as one
possible mechanism to elevate U/Pb ratios in oceanic crust (Michard and
Albarede, 1985), but this might also elevate Rb/Sr ratios, generating more
radiogenic strontium than is seen in the HIMU component. A better model
(Weaver, 1991) is to invoke preferential extraction of Pb, relative to U, from
the down-going slab in subduction zones. Weaver argued that the characteristic
trace element signature necessary to generate the HIMU source could indeed be
produced in the dehydration residue of subducted ocean crust, if fluids are
enriched in Pb but depleted in U. This requires that uranium be held in a U4+
state, limiting the formation of soluble U6+ complexes. The model is
supported by U/Pb ratios nearly an order of magnitude lower in island arc
tholeiites than in MORB (Sun, 1980). Mobilisation of Pb from the slab in a
fluid phase could also explain the surprising degree of Pb isotope homogeneity
in arc-related ‘conformable’ galena deposits (section 5.4.2).
This
model was further developed by Chauvel et
al. (1995), who suggested that Pb is removed from subducting oceanic crust
by metasomatism and deposited in the overlying mantle wedge. It would then be
incorporated into the continental crust via arc magmatism. This non-magmatic
movement of Pb can account for the Pb paradox, but it can also explain the
general decoupling of Pb from other isotope systems (section 6.4.1).
In
order to create the most radiogenic Pb signatures with U/Pb ratios that are not
unreasonably high, it is necessary for HIMU reservoirs to be isolated for 1 ) 2.5
Byr. This is a problem, because the process of mantle convection naturally
tends to streak out any heterogeneities into narrow schlieren (Olsen, 1984),
which would then be too small to source large volumes of enriched OIB magmas.
One
way to avoid this problem is to involve the lithosphere. By definition this
material is solid and non-convecting, and Nd model age dating (section 4.2.1)
points to its potentially long life-time. Since the lithosphere is thought to
be generally depleted, it would need to undergo secondary enrichment by the
emplacement of LIL-rich metasomatic fluids in order to become a HIMU source.
McKenzie and O’Nions (1983) suggested that sub-continental lithosphere has a
greater density than the underlying Fe-depleted asthenosphere, so that
over-thickening during continental collision might cause some of the
lithosphere to constrict off and fall into the upper mantle convection system.
If this material was sampled within a few hundred million years then it might
yield OIB magmas, before being homogenised into the MORB source by convection.
The main objection to this model is simply that the magnitude of its effect is inadequate
to create a major deep-mantle reservoir.
This
has led most workers to return to the model of Hofmann and White (1980, 1982)
and Chase (1981), who proposed that the radiogenic OIB reservoirs could be
generated by subduction of U-enriched oceanic crust. The attraction of this
model is the certain fact that vast amounts of this material are subducted back into the mantle, together
with the fact that the oceanic crust is necessarily enriched in many
incompatible elements relative to its depleted mantle source. The main problem
with this model is the possibility outlined above, that such heterogeneities
would be re-homogenised into the mantle before the 1–2 Byr period necessary for
high U/Pb ratios to generate radiogenic Pb isotope signatures.
Ringwood
(1982) attempted to solve this problem by postulating that subducted oceanic
crust and continental sediments collect in large blobs or ‘megaliths’ at the
670 km seismic discontinuity, which was also proposed by many workers as a
boundary layer between upper- and lower-mantle convection. Alternatively, with
the general acceptance of some form of whole mantle convection, recent
suggestions are that subducted oceanic lithosphere could form high-viscosity
reservoirs resistant to convective homogenisation near the core–mantle boundary
(e.g. Davies, 2002).
6.3.3 The terrestrial Th/U ratio
Over many years, the prime focus of Pb isotope
analysis has been on the U)Pb system. However, the combination of 208Pb
and 206Pb isotopes also allows constraints to be placed on the Th/U
ratio or ‘6’ value of Earth reservoirs. This places new limits on models of
terrestrial Pb isotope evolution that can help to constrain MORB and OIB
sources. It should be noted that in all of the following discussion, ancient
Th/U values are normalised for subsequent radioactive decay and presented in
terms of their ‘present-day equivalent’ 6 value.
In
order to use Pb isotope ratios to determine the Th/U ratio of a reservoir, it
is necessary to know the age of the reservoir and its Pb isotope composition at
the start and end of its evolution. For the Bulk Earth system the age is
defined by the Geochron, and the initial ratio at time T (the age of the Earth) is given by the Canyon Diablo composition
(Tatsumoto et al., 1973). The Pb
isotope ratio of a mantle reservoir at time t
(end of the period of mantle evolution considered) is determined from the
initial Pb isotope composition of a mantle-derived magma at that time. Hence
(following Allegre et al., 1986), we
can define the radiogenic 208Pb/206Pb ratio of a mantle
reservoir as:
(208Pb) (208Pb)
())))
! ())))
208Pb* (204Pb)t (204Pb)T
)))
= ))))))))))))))) [6.1]
206Pb* (206Pb) (206Pb)
())))
! ())))
(204Pb)t (204Pb)T
Given a closed system from time T to t,
the Th/U ratio (6 value) of the reservoir can be calculated from 208Pb*/206Pb*
by solving U)Th and U)Pb decay equations for values of T
and t (chapter 5). However, we can
also calculate the average or ‘time-integrated’ Th/U ratio of an open system
from time T to t.
In
the conformable Pb model (section 5.4.2), the closed-system assumption for the
mantle implied a constant 6 value against time, equal to the meteorite value of 3.9 " 0.1 (Tatsumoto et al., 1973). In contrast, the model of Cumming and Richards
(1975) proposed a decrease in terrestrial Th/U ratio from an initial value of
4.13 to a present-day value of 3.84. This model was largely overlooked in
constraining mantle Th/U evolution because of the near coincidence of 6 values in Phanerozoic galenas with
the meteorite value.
In
a major new examination of this problem, Allegre et al. (1986) used initial Pb isotope ratios for Phanerozoic
ophiolite complexes and Archean komatiites to calculate time-integrated Th/U
ratios for the upper mantle from T
(4.57 Byr ago) until the age of eruption (Fig. 6.24). They concluded that the
upper mantle had higher Th/U ratios in the Archean than at the present, and
that this reservoir was progressively depleted in Th/U over time, in a similar
way to its depletion in Rb/Sr and Nd relative to Sm (section 4.2.2). It follows
that the 6 value for the oldest rocks (ca. 4.3) might be expected to approximate
the Bulk Silicate Earth value (i.e. before significant depletion of the upper
mantle reservoir). Similar 6 values of 4.25 and 4.37 may be
calculated from the Isua galenas analysed by Appel et al. (1978) and Frei and Rosing (2001). These data represent the
oldest terrestrial galena analyses and provide strong support for the evolution
curve of time-integrated 6 values proposed by Allegre et al.
(1986).
Fig. 6.24. Pb isotope
evidence for time-integrated mantle Th/U ratio. a) Plot of radiogenic 208Pb/206Pb
ratios for mantle-derived Pb, showing best-fit open-system curve relative to
closed-system evolution lines. b) Calculated variations of time-integrated
mantle Th/U ratio (6t) over
Earth history. After Allegre et al. (1986).
The
Bulk Silicate Earth 6 value derived above was tested by an independent determination from
recent oceanic volcanics (Allegre et al.,
1986). This is analogous to the determination of the Bulk Earth Sr isotope
ratio from the Sr)Nd isotope ‘mantle array’. Radiogenic 208Pb*/206Pb*
ratios in oceanic volcanics define fairly good linear arrays when plotted
against 143Nd/144Nd and 87Sr/86Sr
(Fig. 6.25). The intersection of Bulk Earth Sr and Nd compositions with these
correlation lines yields Bulk Silicate Earth 208Pb*/206Pb*,
and hence time-integrated Bulk Earth 6 values of 4.1 ) 4.2. Thus, the two
different approaches (old and modern leads) yield Bulk Earth 6 values in good agreement, averaging
4.2 . Allegre et
al. argued that this value would also be consistent with the lower Th/U
value of meteorites if early U partition into the core is taken into account.
The similarity between the time-integrated 6 values of MORB and chondrites
therefore appears to be a coincidence.
Fig. 6.25. Plot of
radiogenic 208Pb/206Pb against Nd and Sr isotope ratio
for modern oceanic volcanics, allowing a calculation of the time-integrated
Th/U ratio of Bulk Earth. ( ! ) = MORB; ( " ) = OIB. Box
indicates the uncertainty of the Bulk Earth composition. After
Allegre et al. (1986).
In
addition to determining the time-integrated Th/U ratio of the upper mantle from
Pb isotope data, we can also determine an ‘instantaneous’ present-day Th/U
ratio for the mantle from oceanic volcanics. Tatsumoto (1978) estimated this
value as about 2.5 in the MORB source, based on elemental Th/U ratios in lavas.
This value has been confirmed more recently by 232Th/230Th
activity ratios in oceanic volcanics, which can also be used to calculate the instantaneous 6 value for the mantle source
(section 13.3). Compared with a Bulk Earth 6 value near 4, these data indicate
strong mantle depletion, which can be attributed to crustal extraction.
However, this presents a problem, since the time-integrated 6 value of ca. 3.75 in MORB is much
higher than the instantaneous value, and only slightly less than the Bulk Earth
value.
Galer
and O’Nions (1985) solved this problem by proposing that the MORB reservoir was
buffered over geological time by a less depleted reservoir. In other words, Pb
recently extracted from the MORB source only had a brief residence time in the
depleted reservoir, and spent most of Earth history in a reservoir with a 6 value near that of Bulk Earth. They
calculated that a residence time of 600 Myr in a Th-depleted MORB reservoir
with 6 = 2.5 and residence
for 4 Byr in a reservoir with 6 = 3.9 would give the time-integrated 6 value of 3.75 needed to explain
MORB lead isotope compositions (Fig. 6.26). Galer and O’Nions examined three
possible locations for their proposed Bulk Earth 6 lead source: upper continental
crust, sub-continental lithosphere and lower mantle. However, the short upper
mantle residence time for Pb calculated using their model was a severe test of
the ability of all of these reservoirs to account for MORB Pb.
Fig. 6.26. Evolution of time-integrated 6 values as a function of residence
time in the MORB reservoir, starting at a value of 3.9. Histogram at right
indicates present-day composition of Pacific (hatched) and Atlantic MORB. After Galer and O’Nions (1985).
The
proposal to buffer MORB by upper crustal Pb causes problems because the
continental crust is characterized by higher 207Pb/204Pb
than MORB, and due to the almost complete extinction of the 235U
parent of 207Pb at the present day, these differences must be
long-lived. On the other hand, the low Pb content and relatively small volume of sub-continental lithospheric mantle require an
unreasonably rapid rate of exchange (complete exchange within 1 Byr) to buffer
upper mantle Pb. This is precluded by the old Sm)Nd model ages of inclusions in diamonds (section
4.2.1). Finally, buffering of MORB Pb by the lower mantle might be possible if
the depleted upper mantle constituted only the upper 670 km, but more recent
evidence for a larger volume of upper mantle make it impossible to adequately
buffer this reservoir with a Pb residence time of only 600 Myr.
These
problems could be overcome by using a higher Bulk Earth 6 value of 4.2, leading to an upper
mantle Pb residence time of 1.8 Byr (Dickin, 1995).
An alternative approach to solving
this ‘kappa conundrum’ (e.g. Kramers and Tolstikhin, 1997; Elliott et al., 1999) is to argue that the
instantaneous 6 value (Th/U ratio) of the MORB source upper mantle has itself evolved
downwards over geological time, and that this has been the prime control on the
time-integrated 6 value of upper mantle Pb. Models of this second type are compared with
the model of Galer and O’Nions (1985) in Fig. 6.24. In this plot, instantaneous
6 values of
sources generating radiogenic Pb are plotted as a function of time, whereas the
time-integrated 6 value of Pb at the present day is determined by the area under the
graph (Elliott et al., 1999).
Of the four models shown in Fig.
6.27, (a) corresponds to that of Galer and O’Nions, in which Pb is held in a
high-6 source until
it enters a steady-state low-6 MORB reservoir only 600 Myr ago. Figure 6.27b shows a model in which
the MORB source is not buffered by any other source, but undergoes progressive
depletion though geological time due to crustal extraction. This causes a
steady decrease in the upper mantle Th/U value, whose average value determines
the time integrated kappa value recorded by Pb isotope analysis. Figure 6.27c
shows a model proposed by Elliott et al.
(1999) in which the Th/U ratio of the upper mantle remains at a value of 4
until ca. 1.6 Byr ago, after which it decreases rapidly to the present day
value. This model is similar to a model proposed by Kramers and Tolstikhin
(1997), and is based on the argument that crustal uranium would be locked up in
an insoluble form in the sedimentary environment, until oxygenation of the
atmosphere released this uranium into the sea, from where it could be recycled
into the upper mantle. Recycling can occur by uranium enrichment of basaltic
crust on the ocean floor, and subsequent release back to the mantle in
subduction-related fluids (section 13.4). Finally, Fig. 6.27d shows the results
of empirical analysis of elemental Th/U ratios in mafic/ultramafic rocks of
various ages, averaged to reduce the scatter of individual data points. The
curve is a polynomial curve fitted to the analytical data, and could be
explained by a combination of Th/U fractionation during melting and more recent
recycling of dissolved crustal uranium.
Fig. 6.27. Alternative evolution lines for the
instantaneous kappa value of the MORB source that can generate the observed Pb
isotope distribution of MORB. (a) – (c) = model
curves; (d) = average measured Th/U values. The area under each curve yields
time-integrated kappa (6t). For
discussion, see text. After Elliott et al. (1999) and Collerson and Kamber (1999).
The
first three models in Fig. 6.27 all successfully yield a time integrated kappa
value in the MORB source of around 3.75, but they have some problems. Model (a)
proposes an upper mantle residence for Pb which seems impossibly low in
comparison with the ca.1 Byr residence of helium, the most incompatible element
(section 11.1). In addition, this model does not explain how the mantle arrived
at its present day Th/U ratio. Model (b) addresses the latter point, but can
only satisfy the time-integrated upper mantle 6 value by starting from a very high
Th/U ratio of nearly 5. Model (c) addresses both of these
problems by keeping a chondritic Th/U ratio in the MORB source until about 1.6
Byr ago, then causing a rapid decline. However, it ignores the
possibility of exchange of Pb with primordial or recycled sources in the lower
mantle, which could buffer upper mantle Pb. Finally, the best fit curve to the
empirical data in Fig. 6.27d has resemblances to models (b) and (c) but the
time integrated kappa value (area under the curve) is too low to support the Pb
isotope composition of the MORB source (Collerson and Kamber, 1999). However,
this is a good fault because it leaves room for some buffering of MORB source
Pb by input from the lower mantle.
A
Pb isotope model dominated by binary mixing between the MORB source and a lower
mantle Bulk Earth reservoir was proposed by Kamber and Collerson (1999).
However, these authors excluded the effect of any crustal recycling into the
OIB reservoir, and only permitted enrichment of the OIB source in the direction
of increasing : value. In view of the clear evidence for OIB sources with Sr–Nd isotope
signatures enriched relative to Bulk Earth (section 6.2.2), this model cannot
be considered realistic. Nevertheless, this model can be valid if the concept
of mixing between upper and lower mantle sources is broadened to include
enriched lower mantle sources resulting from crustal recycling. These enriched
reservoirs (together with a primordial reservoir) can then buffer the 6 value of the MORB source via the
OIB source, allowing a Pb residence time of up to 1.8 Byr (Dickin, 1995). This
possibility is demonstrated on a plot of instantaneous 6 value against time-integrated 6 value for oceanic volcanics (Fig.
6.28). The OIB samples define an array (interpreted as a mixing line) linking
MORB compositions (solid symbols) and a Bulk Earth point with a 6 value of about 4.2 (Allegre et al., 1986).
Fig. 6.28. Plot of
time-integrated kappa (6t)
against instantaneous kappa (6p)
values for oceanic volcanics. ( ! ) = MORB; ( " ) = OIB. After Allegre et al.
(1986).
6.3.4 The upper mantle : value re-examined
There has been much discussion about the
time-integrated : value of the mantle, as revealed by Pb isotope analysis. It is now
generally agreed that the apparent
increase in : value of the upper mantle over time may be an illusion caused by
recycling of radiogenic Pb from the oceanic and continental lithosphere
(sections 5.4.3 and 6.3.1). However, in order to properly quantify this
process, it is necessary to determine the instantaneous (present day) : value of the upper mantle.
Unfortunately, this cannot be determined directly from the U/Pb ratios of MORB
glasses, since U/Pb fractionation during partial melting is poorly constrained.
Neither is there any isotopic route to this quantity, as was possible for the 6 value. However, White (1993) developed
an indirect approach to the determination of upper mantle : from the relationship between : and uranium content in MORB
glasses.
Analysis
of U and Pb in 82 glasses from the Atlantic, Pacific and Indian oceans revealed
a strong positive correlation between 238U/204Pb ratio and
U content (Fig. 6.29). White (1993) attributed this correlation to
fractionation (of uranium) during partial melting, and argued that it could be
used to estimate the depleted mantle : value. Since U is incompatible, the U content
of the Bulk Silicate Earth estimated from chondrites (0.018 ppm) must be an
upper limit for U in the depleted mantle (MORB source). Applying this value to
the : versus U correlation line leads to a
maximum instantaneous : value of 4.5 in the MORB source (compared with a time-integrated : value of ca. 8.5).
Fig. 6.29. Plot of 238U/204Pb
(:) in MORB
glasses against uranium content, showing a positive correlation, from which a
maximum upper-mantle : value can be estimated. After White (1993).
This
discrepancy is exactly analogous to the kappa conundrum, and again, is due to
the relatively short residence time of Pb in the upper mantle. White proposed
that upper mantle Pb is buffered by the entrainment of radiogenic Pb from
plumes. Therefore, the Pb isotope composition of the upper mantle reflects
dynamic equilibrium between Pb fluxes into and out of this reservoir. In
contrast, radiogenic Sr and Nd in the upper mantle are largely generated by in situ decay of Rb and Sm.
It
is concluded that the U–Th–Pb isotope systematics of the upper mantle can be
reasonably explained by recycling of various enriched sources (and a primordial
source) via lower mantle plumes. However, until such processes are incorporated
into an improved ‘Plumbotectonics’ model, a complete quantitative understanding
of terrestrial U–Th–Pb systematics cannot be claimed.
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