Earth and Planetary Science Letters, 115 (1993) 245-256 245 Elsevier Science Publishers B.V., Amsterdam

[DT]

The Pb isotopic evolution of the Earth: inferences from river water suspended loads

Yemane Asmerom 1 and Stein B. Jacobsen Harvard Center for Isotope Geochemistry, Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St.,

Cambridge, MA 02138, USA

(Received July 20, 1992; revision accepted December 15, 1992)

ABSTRACT

Pb and U concentrations and Pb isotopic variations in the suspended loads of major rivers draining regions with a range of ages of crustal formation are reported. The 2°6pb/z°4pb, 2°7pb/2°4pb and 2°spb/2°4pb ratios are positively correlated with 87Sr/S6Sr and negatively correlated with end values in the samples. A coherent negative correlation of the Pb isotope ratios with EN~ values allows us to estimate the average Pb isotope composition of the upper continental crust at a°6pb/Z°4pb = 19.32 _+ 0.28, 2°7pb/2°4pb = 15.76 + 0.09 and 2°8pb/2°4pb = 39.33 + 0.39.

The Z°6pb/2°4pb ratios are positively correlated with their Nd model ages, reflecting the higher upper continental crust /x values relative to mantle Ix values. The upper continental crust, as well as the MORB and OIB sources, plot to the right of the geochron. Assuming that the lower crust is the missing low-/x reservoir, we have calculated a range of possible lower crust Pb isotope compositions similar to many granulite terrains.

Overall, the upper continental crust has somewhat higher 2°Tpb/Z°4Pb than MORB or OIB, while 2°6pb/2°4pb overlaps with the MORB values and is less radiogenic than some OIB values. Typical measured /L values for the suspended loads of large rivers are ~ 4, while the ~ values of their crustal source rocks (based on the Pb isotope composition) are ~ 12, allowing for up to 65% of U from continental weathering being carried in the dissolved load to the oceans. The near-overlap of the 2°6pb/2°4pb values of MORB and continental crust is consistent with the suggestion that U is recycled into the mantle through altered oceanic crust.

The exception to these trends are the samples with the oldest Nd model ages (~ 3.5 Ga). The Pb isotope ratios show drastic deviation from the Pb and Sr correlation trend suggesting an inherent low/z value in the mantle source of the early Archean continental crust,

1. Introduction

The best constraints on m a n t l e - c r u s t evolu-

t ion mode ls are der ived f rom S m - N d and R b - S r

systematics in mant le and crustal reservoirs. Both

of these systems show large and relatively well

unde r s tood e l emen ta l f ract ionat ions be tween the

mant le and the crust. Also, because of the very

long m e a n life of these systems, thei r in terpre ta-

t ion does not d e p e n d on assumptions about the

history of the Ea r th pr ior to fo rmat ion of the

oldest p rese rved cont inenta l crustal rocks. These

systems pr imari ly yield in format ion about the

1 Present address: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, USA.

m e a n age of mant le and crustal reservoirs, as well

as their relat ive sizes as a funct ion of t ime [1].

In contrast , the T h - U - P b isotopic system can

potent ia l ly yield in format ion about the m e a n ages,

as well as in format ion about the very early history

of the Ear th , since the half lives in this system

range f rom 0.7 to 14 Ga. T h e main p rob lem with

this system is that U - T h - P b e lementa l f ract iona-

t ion be tween the crust and the mant le is poor ly

unders tood , the isotopic d i f ferences be tween the

crust and the mant le appea r to be relat ively small,

and the in te rp re ta t ion depends heavily on as-

sumpt ions about the age of the Ear th . The largest

U - T h - P b f rac t ionat ion in the silicate por t ion of

the Ea r th appears to occur within the cont inenta l

crust, a l though the magni tude of this f ract iona- t ion and the Pb isotope composi t ion of the conti-

0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

246 Y. ASMEROM AND S.B. JACOBSEN

nental crust is not well characterized. Previous empirical estimates of the upper continental crust were based on Pb ores or deep sea sediments. The Pb ore data reflect source compositions which may not all be upper crust. Deep sea sediments do give a reasonable estimate of the Pb isotope composition; however, some of the provenance and age information is lost by mixing during sedimentation.

To improve on this situation we decided to measure U - T h - P b isotopic systematics in the suspended loads of river water to obtain bet ter estimates of the Pb isotope composition of the Earth 's upper continental crust. These data are used to understand more clearly the U - P b iso- topic evolution in the mant le -crus t system, as well for understanding the behavior of U in the exogenic cycle.

2. Sample description and methodology

Collection of the river water suspended load samples used in this work is described by Gold- stein and Jacobsen [2,3]. The suspended load samples come from rivers draining a variety of tectonic environments: from young arcs (such as Pampanga in the Philippines and Mogami in Japan), to old cratonic terrains (such as small

rivers at Isua, west Greenland, and the upper part of the Mississippi). Samples were selected with the widest possible range in Nd model ages (from 3.5 Ga to 0.5 Ga) in order to examine the Pb isotopic variations in upper continental crust segments as a function of their age of extraction from the mantle. The selection of age distribution was based on Nd model ages of a larger set of samples used by Goldstein and Jacobsen [3,4] for REE, Nd and Sr isotopic studies of suspended loads of river waters.

Approximately 30 mg of sample was dissolved in teflon bombs with a H F - H N O 3 mixture. An aliquot of the sample was spiked with 2°8pb and 235U for determination of Pb and U concentra- tions. Pb was separated in an anion ion-exchange column using HBr. U was separated using H N O 3 elution, following the method of Manhes et al. [5]. Total procedural blanks were about 200 pg for Pb and U. Pb was loaded on a single Re filament using a silica g e l - H 3 P O 4 acid mixture. U was loaded on Re triple filaments using H N O 3. Concentration and isotopic composition measure- ments were obtained using a VG-54 mass spec- trometer. Pb isotope fractionation was estimated by running similar size samples of the NBS-981 Pb standard. The measurements were corrected for blank and fractionation (0.12% and 0.20% per atomic mass unit for Pb and U, respectively).

TABLE 1

The isotopic composition of Pb and concentrations of U, and Pb in river water suspended load samples i

Sample Pb U 206p b 207p b 20Sp b 23Su ENd(0) 2 S7Sr / 86Sr 2

(ppm) (ppm) 2o4 Pb 2o4 Pb 2o4 Pb 2o4 Pb

Isua-B 66,74 7,60 16.939 14.701 37.597 6.92 - 42.6 0.77634 Isua-F 52,71 7,77 17.59 14.755 37.508 9.03 - 42.0 0.80016 Mississippi-upper 27,31 1.55 19.219 15.688 38.848 3.66 - 19.3 0.72008 Bruneau 37,16 5.76 18.838 15.863 39.534 10.08 - 14.2 0.70998 Ohio 44,12 2.53 19.281 15.755 39.006 3.72 - 12.4 0.72491 Bear 23.76 3.30 19.419 15.799 39,470 9.09 - 11.5 0.71594 Mississippi-lower 38.17 2.40 19.182 15.614 38.813 4.06 - 10.0 0.71918 Amazon 8.120 1.26 18.832 15.662 39.087 9.99 - 9.2 - Murray 28.44 1.66 18.472 15.610 38.343 3.70 - 4.7 0.71428 Mogami 61.54 2.60 18.471 15.646 38.362 2.68 + 0.5 0.70695 Pampagna 13.19 0.712 18.073 15.528 37.656 3.37 + 7.1 0.70491 Agno 45.61 0.308 17.897 15.603 37.862 0.422 + 6.5 0.70435

1 The uncertainties in the U and Pb concentrations are < 0,5% and < 0.2%, respectively. The Pb isotopic ratios are corrected for fractionation (0.12% per atomic mass unit), which is the largest source of uncertainty, as the precision of the measurements were much better.

2 Nd and Sr isotope data from Goldstein and Jacobsen [3] and except for the Amazon ENd value of Goldstein et al. [6].

Pb ISOTOPIC E V O L U T I O N O F T H E E A R T H 247

3. Results

The Pb isotopic data, as well as the concentra- tions of Pb and U obtained for the suspended load samples, are given in Table 1. Also given in Table 1 are the 875r/86Sr and eNa(0) values reported for these samples by Goldstein and Ja- cobsen [3].

3.1. Pb isotopic variations

The Pb isotope variations measured in the suspended load samples are shown in Fig. 1 (tri- angles). The 2°6pb/2°4pb ratios of the samples range between 16.9 and 19.2, the 2°7pb/2°apb ratios range from 14.7 to 15.9 and the 2°spb/2°4pb ratios are between 37.5 and 39.5. The two Isua samples show 2°7pb/2°4pb values that are unusu- ally low. The remaining data show rather coher- ent data arrays in both 2°8pb/2°6pb and 2 ° 7 p b / 2 ° 6 p b p l o t s .

The coupled decay of 238U and 235U to 2°6pb and 2°7pb can be used to constrain the time integrated ~ value (-= 238U/2°4pb) in the sources of the suspended loads. As a first approximation we can use a two-stage model. Such an approach has also been used for oceanic mantle [7]. The Pb isotopic composit ion of a given sample can be considered to reflect evolution in the mantle with a first stage, /Zl, value, followed by a second stage, /z 2, value, due to evolution in the continen- tal crust. Given an estimate of the age of extrac-

16 a I I I I I A ~

.j~ ~ 2.5 ' A , - x

14 /

°

39 b l ' ' ' I ~ A

33 s

5 K[ = 3.8

/a I = 8.26 3O

~_.~4.56 Ga I I I I

10 12 14 16 18 20

2°6pb / 2°4 pb

Fig. 1. Pb isotope growth curves with Canyon Diablo troilite Pb composition for initial Pb and using an age of the Earth of 4.56 Ga. A /x 1 value of 8.26 and a K 1 value of 3.8 were used for the calculation. ~, = measured Pb isotope composition of suspended loads; • = initial Pb isotope compositions of sus- pended load samples, calculated using a two-stage model and their Nd model ages as estimates of their crustal residence

ages (see text for detail).

tion of the crustal source rocks of the suspended load samples from the mantle (T) we can then use the measured Pb isotope composit ion (m) of

TABLE 2

Calculated two-stage/x and K values of river water suspended load samples

T N d 4 Sample / ~ /z~ /x 3 Nd Pb K 2 --DM

(ppm) N--d

Isua-B 6.92 6.36 7.86 56.7 1.18 4.41 3.53 Isua-F 9.03 6.05 9.01 52.6 1.00 3.89 3.40 Miss-upper 3.66 8.09 12.00 30.4 0.898 3.71 2.20 Bruneau 10.1 8.58 10.64 58.9 0.631 4.79 1.90 Ohio 3.72 8.30 12.91 39.0 1.13 3.77 1.75 Bear 9.09 8.37 13.46 34.9 0.681 4.04 1.67 Miss-lower 4.06 8.08 13.68 40.8 0.936 3.64 1.59 Amazon 9.99 8.24 12.14 32.6 0.249 4.34 1.54 Murray 3.70 8.22 11.56 35.0 0.813 4.01 1.24 Mogami 2.68 8.30 12.05 31.6 1.95 4.10 1.00 Pampagna 3.37 8.15 12.47 12.3 1.07 3.48 0.55 Agno 0.42 8.30 8.96 20.3 2.24 5.23 0.53

i Measured /x values. 2 Calculated first stage (mantle)/x values. 3 Calculated second stage (crust)/z values. 4 Nd model ages from [3,6].

248 Y. ASMEROM AND S.B. JACOBSEN

a sample to calculate both /,( and 11,2 from the following two equations:

206 Pb 2o6 Pb

( 2 0 - ' 0 ~ ) m ~ (20~pb)BEi

+ ~ l (e A2-~vo - eA~: r)

+/~2(e a:3sr - 1) (1)

2o7 Pb 2o7 Pb

1 + -

q-/.£ 2(e A235T - 1)] (2)

where T O is the age of the Earth; T is the mean age of the crust being sampled and the decay constants are A238 =0.155125 Ga -1 and A23 s = 0.98485 Ga 1. In this case we are using the Nd model age of a sample to estimate T. The sub- script BEi denotes the bulk Earth initial Pb iso- topic ratios, for which we use the troilite Pb isotope composition of the iron meteorite Canyon Diablo [8,9]. The calculated/x~ and /z 2 values are given in Table 2. The calculated 2°7pb/2°4pb and 2°6pb/2°4pb ratios at the end of stage 1 are also shown in Fig. 1 (solid triangles). A single-stage 2°7"2°6pb growth curve for the source of the conti- nental crust (the depleted mantle) is shown in Fig. 1. The curve is determined by the averagd/z~ value (see below) of the samples given in Table 2.

Variations in the 232Th/Z38U ratio ( = K ) of the crustal sources of the suspended loads may be constrained by considering their measured 2°spb/2°4pb ratios. Assuming a common ~c 1 value, we may use /.t values estimated from equations (1) and (2) to calculate the K 2 value for each of the samples using the following equation:

2(18 Pb ( 2os Pb

-~ Kl/R,l(e a232To -- eA232 7")

"4- K2~.L2(e A232T- 1) (3)

where Az32 = 0.049475 Ga -( and we used a chon- dritic ~q value of 3.8 [10]. The calculated K 2 values are given in Table 2.

Figure 2 shows the Pb isotopic variations in the suspended loads compared to the fields for

g=8.26' ' / ' ,," ' ' a ~ 16"0 - ~ ,//

~, ~ "°""- ~ :~7_~7:' ~ .........

15.0 " O"" •

4O ~ ............. "OIB

38

36 ~ l G a

:].56 G a i i

16 17 18 119 210 ;1 2°6pb / 2°4 pb

Fig. 2. Comparison of Pb isotope composition of suspended loads with Pb isotope compositions of MORB, OIB and pelagic sediments. MORB data from Ito et al. [11]; OIB field from Zindler and Hart [12], pelagic sediments from Sun [13]. The 2°6pb/2°4pb ratio of MORB and suspended load sedi- ments are essentially the same, while OIBs have the most radiogenic component. The 2°7pb/2°4pb ratios of suspended loads are distinctly higher than the MORB and OIB data. Other continental crust samples, such as continental arcs, have similarly higher 2°7pb/2°4pb ratios than MORB and

OIB [14].

mid-ocean ridge basalts (MORB), ocean island basalts (OIB) and pelagic sediments. Suspended loads and MORBs show a similar range in the 2°6pb/2°4pb ratio (Fig. 2). The 2°6pb/2°4pb ratio

of some OIBs is considerably more radiogenic than that of suspended loads. The 2°Tpb/2°4pb data for suspended loads is noticeably higher than that of MORB or OIB (Fig. 2). Continental arc rocks show similar higher 2°7pb/2°4pb values compared to oceanic rocks [14]. The data for pelagic sediments fall in the central part of the composition range of the suspended loads. Also shown are some reference, single-stage Pb iso- tope growth curves using a /~ value of 9 and a K value of 3.8. In the 2°7pb-2°6pb diagram we also compare the results if 4.45 Ga and 4.56 Ga are used for the age of the Earth. The samples plot to the right of the 4.56 Ga geochron, while the

P b I S O T O P I C E V O L U T I O N O F T H E E A R T H 249

4.45 Ga geochron cuts right through the middle of the suspended load data points.

The measured Pb isotope compositions reflect the variability in the upper continental crust. The data show that, overall, the 2°6pb/2°apb and 2°spb/2°4pb composition of the upper continen- tal crust is not noticeably different from the de- pleted mantle, however, the 2°Tpb/204 Pb value is slightly higher in the upper continental crust.

The exception to the main suspended load trend are the samples from Isua, which have very low 2°7pb/2°4pb and 2°6pb/2°4pb ratios. They sample drainages dominated by the Isua metased- iments and the Amitsoq gneiss; these are rocks which had experienced both Archean and Pro- terozoic metamorphism. Their unusual composi- tion could either reflect a complex metamorphic history or unusual isotopic systematics in the sources of the oldest preserved continental crust [15,161.

3.2. U-Pb systematics

Pb and Nd are transported primarily in the suspended load fraction of river waters, due to their low solubility in normal river waters. The average P b / N d ratio of 1.06 + 0.32 in suspended loads (Table 2) should, therefore, reflect this ra- tio in the upper continental crust. Using a Nd concentration of 26 ppm for the crust [17] this yields an upper crustal Pb concentration of 27.6 + 8.3 ppm. This is similar to the concentration of 20 ppm for the average upper crust concentration given by Taylor and McLennan [18].

U, in contrast to Pb, is more soluble in natural waters in the U 6+ oxidation state, forming a soluble uranyl-carbonate complex [19]. Sus- pended loads from most river waters have U concentrations lower than the average upper con- tinental crust estimate of 4 ppm [18,20]. Excep- tions to this are rivers draining small drainage basins, such as Isua and Bruneau. The sediment input to such rivers may be dominated by me- chanical erosion products rather than chemical weathering products.

The U - P b system of suspended loads does not show a coherent pattern in a U - P b isochron diagram (Fig. 3). Most of the data plot in a region that indicates a loss of U. The calculated /x~ and iz 2 values and the measured tx values are shown

20 . . . . , . . . . , . . . . , . . . . i . . . . , . . . .

Oo 19

• O0 ~ O •

17

1 6 . . . . l e - , , i , , , i ¢ , , , k . . . .

2 4 6 8 1 0

238U/204pb

Fig. 3. 2 3 8 U - 2 ° 6 p b d i a g r a m showing p r e s e n t d a y e s t i m a t e d

bu lk E a r t h va lue a n d 2.0 G a a n d 4.56 G a i sochrons . N o t e t ha t

mos t o f t he s u s p e n d e d load s a m p l e s p lo t in a r eg ion t ha t

w o u l d i nd i ca t e t ha t they e x p e r i e n c e d U loss.

12

in Fig. 4 and Table 2. Most of the samples have calculated /x 2 values considerably higher than the measured ~ values; on average, about 65% higher. This is consistent with U / P b fractiona- tion during weathering. Exceptions to this trend are samples from river systems with small drainage basins or samples that drain young is- land arc terrains. The measured Iz values of samples from rivers with small drainages are in the range of the tx values expected for the upper continental crust and /x values calculated from the Pb isotopic data. As stated previously, in the small drainage river systems erosion seems to be dominated by mechanical erosion rather than

S 8

II

~ 4

V V

• • •

0 0 0

V V o

0 0 ~ o • v

0 0 0 I ~= / i ~ ] =

L I1 . . . . = 0 0 C ) , i . . . . i . . . . J . . . . i , ,

0.8 1.6 2.4 3.2

Nd model age (Ga)

Fig. 4. M e a s u r e d (©) a n d c a l c u l a t e d e 3 s u / 2 ° 4 p b ra t ios (Ix) o f

r iver w a t e r s u s p e n d e d loads . T h e ~1 ( 0 ) a n d /x 2 (zx) va lues

we re c a l c u l a t e d us ing a two- s t age m o d e l wi th N d m o d e l ages

as the c rus ta l r e s i d e n c e age o f the samples . Overa l l , t h e r e is a b o u t 6 5 % d i f f e r ence b e t w e e n the m e a s u r e d a n d c a l c u l a t e d tx

va lues as a resu l t o f U loss to the d i sso lved f r ac t ion .

250 Y . A S M E R O M A N D S . B . J A C O B S E N

chemical weathering, resulting in high measured Ix values. The extremely low tz value of the Agno sample (0.42) and the low/z values for the other young island arc river samples is most likely to result of U loss and the fact that island arc rocks tend to have low/~ values [21]. The two-stage Pb evolution model does not take into account com- plexities that arise during intra-crustal recycling. However, it seems a reasonable approximation in light of the fact that the calculated ~ values are realistic (mean ~l = 8.26 + 0.09 and mean ~2 = 12.0 _+ 0.9). The Isua samples show complexities that may not follow a simple two-stage history, due to metamorphism at around 2.7 Ga [22]. The average two-stage model /x t and/--(,2 values of 6.21 and 8.43, respectively, are distinctly different from the p~ values of the other samples.

3.3. P b - S r - N d isotopic relations

The Nd and Sr isotope composition of the upper crust inferred from the suspended load data of Goldstein and Jacobsen [3] are distinctly different from the MORB and OIB sources and appear to be the enriched complement to the depleted MORB source. In contrast, the Pb iso- topic composition of the upper crust is not partic- ularly radiogenic, compared to MORBs and some OIBs (Fig. 2). There is a systematic increase in,Pb isotopic ratios of the suspended loads with in- creasing Nd model age (Tables 1 and 2; Fig. 5a). The distinct exceptions are the samples from Isua. They have the least radiogenic Pb of all the samples, although they have the oldest Nd model age.

Figure 5b shows that there is a positive corre- lation between 2°6pb/2°4pb and 87Sr/86Sr, with the exception of the Isua data. Pb isotopic ratios are negatively correlated with Nd isotopic ratios (Fig. 6). The P b -Nd data define a more coherent pattern than the Pb-Sr data. The scatter in the Pb-Sr data is probably the result of both higher Sr solubility in river water, as well as a variable carbonate to silicate component in the sources of the various samples [2]. The observed difference in 87Sr/86Sr between dissolved and suspended loads may also contribute to the scatter in the Pb-Sr data. The general correlation trends ob- served in the suspended load data (positive Pb-Sr

3

2

0

0.78 0,73

0.72

• ISUA •

a

I i i i

ISUA

i [ i

• o o o

0.71 •

17 18 19

2°6 p b / 2°4 p b

Fig. 5. (a) 2°6pb/2°4pb variation in suspended loads with Nd model ages of Goldstein and Jacobsen [3]. The Pb isotope data indicate that the upper crust has a higher ~ value than the depleted mantle. (b) 2°6pb/2°4pb versus 87Sr/S6Sr dia- gram of river water suspended load material. The Sr isotopic data are from Goldstein and Jacobsen [3]. There is broad positive correlation of the two isotopic systems, similar to MORB data. The scatter is more pronounced than that ob- served in the 2°6pb/204pb-ENd correlation diagram (Fig. 6).

and negative P b -N d isotope correlation) has also been observed in MORB and OIB data [12,23,24].

The coherent correlation between Pb and Nd isotopic composition of suspended load samples allows us to infer the average Pb isotopic compo- sition of the upper continental crust. Goldstein and Jacobsen [3] estimated an average eNd value of --16.7 for the present day upper continental crust. Accordingly, based on the N d -P b isotope correlation (Fig. 6), we propose 2°6pb/Z°4pb = 19.32 _+ 0.28, 2°7pb/2°4pb = 15.76 +_ 0.09 and 2°spb/2°4pb = 39.33 _+ 0.39 as average values for the upper continental crust.

4. Discussion

4.1. The Pb isotopic composition of the upper con- tinental crust

The Pb isotopic composition of suspended loads plots to the right of the 4.56 Ga geochron,

Pb ISOTOPIC EVOLUTION OF THE EARTH 251

• -~ 40 . . ~ 39.33±0.39

39

38

16.0 15.76:~0.09

~ 15.8

~ 15.6

15.4 t I I ] ~, '. '. I '. '. '. '. ~ '. '. '. '. : : '. '. '. ~ '. '. '. : t i ~

19.32:k0.28 ~,~ 20 ~

~ 19

~ 18

-20 -10 0 10

Fig. 6. 2 ° 6 ' 2 ° 7 ' 2 ° s p b / 2 ° 4 p b - c o r r e l a t i o n d i a g r a m for r i ve r eNa s u s p e n d e d l o a d m a t e r i a l . N d i s o t o p i c d a t a f r o m G o l d s t e i n a n d

J a c o b s e n [3]. T h e r e is m o d e r a t e l y g o o d c o r r e l a t i o n , e x c e p t for t h e I s u a s a m p l e s . T h e c o r r e l a t i o n is a g o o d i n d i c a t i o n t h a t t h e

Pb i s o t o p i c d a t a f r o m t h e s u s p e n d e d l o a d s r e f l e c t s t h e iso- t o p i c c o m p o s i t i o n o f t h e a v e r a g e c r u s t a l s o u r c e rocks . T h e c o r r e l a t i o n a l l ows an e s t i m a t e o f t h e a v e r a g e Pb i s o t o p i c

c o m p o s i t i o n o f t h e u p p e r c o n t i n e n t a l c rus t .

similar to OIB, and most MORB Pb isotope data (Fig. 2). Our results, consistent with data from other sediment data, such as pelagic sediments [13] and other ocean sediments [25,26], clearly establish that the upper continental crust is more radiogenic than that allowed by single stage growth with an age of the Earth of 4.56 Ga.

As observed in Fig. 2, it is clear that the MORB source mantle is not a depleted compli- mentary reservoir to the continental crust for the U - P b system. The fact that the Pb isotopic data from both the continental crust and depleted mantle sources, such as MORBs, lie to the right of the geoehron has led to what has been referred as the "Pb paradox". Different suggestions have been made to resolve the dilemma: (1) a low Iz lower crustal reservoir of Pb [20,27,28]; (2) the

fractionation of Pb into the core [29,30,31,53]; (3) an age of the Earth of ~ 50-100 Ma younger than the solar system age of 4.56 Ga [22,32,33].

The largest U / P b and U / T h fractionation occurs between the lower and upper continental crust, as a result of U mobility during high-grade metamorphism. Thus, the lower crust is a plausi- ble reservoir for unradiogenic Pb and is likely to have a low/z value compared to the upper crust. However, it is possible that the Pb isotopic data of lower crustal xenoliths may have been affected by upper crustal Pb contamination [34].

The fact that all our continental crust samples plot within the 2°6pb/2°4pb range of MORB (Fig. 2) may suggest that U and Pb have a very similar degree of incompatibility during crust for- mation from the depleted mantle. The result of the two-stage model calculations (Table 2) shows that, in general, the first-stage (depleted mantle) Iz value is ~ 20-30% lower than the second-stage (crustal residence)/x values. Figure 5a shows the variation in 2°6pb/2°4pb with Nd model ages of suspended load samples. Except for the Isua sam- ples, there is a general trend of increasing 2°6pb/z°apb with increasing age. These data clearly show that the upper crust has higher /x value than the mantle. The reverse pattern would be shown if depleted mantle tz values were higher than the upper crust. However, /x values for the total continental crust may be lower than the average/x value for the depleted mantle, if large U / P b and U / T h fractionation between the up- per crust and lower crust is assumed.

Suspended load samples, similar to other con- tinental crust samples have distinctly higher 2°7pb/2°4pb ratios than MORB or OIB (Fig. 2). As a result of the short half-life of 235U compared to 238U, the bulk of the radiogenic 2°7pb was produced during the Archean. Thus, the high 2°7pb/2°4pb ratios of the continental crust sam- ples reflect contribution from an old component of Pb that is not being sampled by MORB or OIB. U, Th and Pb are highly incompatible ele- ments and the continental crust contains ~ 10 times the amount of U, Th and Pb compared to the depleted mantle [12,20]. A small fraction of crustal Pb recycled into the mantle would domi- nate the Pb isotopic signature of the mantle [35]. The distinct 2°Tpb/2°4pb ratio of the upper con- tinental crust rules out a very high rate of crustal

252 Y. ASMEROM AND S.B. JACOBSEN

Pb recycl ing into the mant le dur ing the la te r half of E a r t h history, as p r o p o s e d by A r m s t r o n g [36]. A t presen t , s ed imen t subduc t ion in arcs may be one of the poss ible ways that some amoun t of crust may be recycled. W h i t e and Pa tche t t [37] (based on Nd and Hf da ta ) and Ben O t h m a n et al. [26] (based on Pb isotopic da ta ) rule out a s ignif icant amoun t of s ed imen t subduc t ion in m o d e r n is land arcs. The Pb da ta does not rule out a large amoun t of recycl ing dur ing the ear ly par t of the Ea r th ' s history. As a resul t of s t rong incompat ib i l i ty of U, Th and Pb, the 2°7pb and 2o6 Pb isotopic compos i t ion of the d e p l e t e d man t l e may ref lect the p r e f e r r e d recycl ing of U in al- t e r ed oceanic crust, especia l ly dur ing pe r iods when crust p roduc t ion was low and ocean crust recycl ing was high.

O u r es t imate of the Pb isotopic compos i t ion of the u p p e r crust is in a g r e e m e n t with the p lumbo- tectonics m o d e l es t imate of Z a r t m a n and Ha ines [20]. The except ion to these t r ends are the sam- ples with the o ldes t Nd mode l ages ( ~ 3.5 Ga). The Pb i so tope rat ios show large devia t ions f rom the Pb and Sr co r re la t ion t rend. The init ial Pb i so tope compos i t ion ca lcu la ted using a two stage mode l for the Isua samples is close to the lowest init ial t e r res t r i a l Pb, m e a s u r e d on ga lena [15]. This suggests an inhe ren t low /x va lue in the

mant le source of the ear ly A r c h e a n cont inen ta l crust. This is consis tent with suggest ions m a d e by a n u m b e r of previous workers in an a t t empt to mode l the evolut ion of ear ly Pb [32].

4.2. The Pb isotope mass balance o f the crust and upper mant le system

The isotopic compos i t ion of the u p p e r crust d e t e r m i n e d here provides new const ra in ts on the c r u s t - m a n t l e mass ba lance for Pb isotopes. The p resen t mass ba lance for the U - T h - P b system is given in Table 3. The bulk E a r t h Pb i so tope rat ios were ca lcu la ted using the /x value of Hof- mann [38] and assigning it an uncer ta in ty of + 0.5. T h e / x values of lower and to ta l crust were calcu- la ted by mass ba lance f rom the following equa- t ion for the bulk si l icate E a r t h ~ value:

/A'BE = E [d'jCpbjYMj (4) J

where /.%, Cpb j a n d YMj are the p~ value, the concen t ra t ion of Pb and mass f ract ion of the to ta l si l icate Ear th Pb of reservoir j ( the u p p e r crust, lower crust and d e p l e t e d mant le) . The u p p e r crust and d e p l e t e d mant le ~ values of 12.0 + 0.9 and 8.26 + 0.09, respect ively, a re the average val- ues ca lcu la ted using a two-s tage mode l for our

TABLE 3

Pb concentration and isotope composition estimates for the mantle and continental crust

Mass YM j C Pb t.Z 206 Pb 207 Pb 208 Pb

(10 24 g) (ppm) 204p b ~204 pb 204pb

Upper crust 6.9 0.0048 20.0 12.0 +_ 0.9 19.3 + 0.2 15.76 _+ 0.09 39.8 _+ 0.4 Lower crust 15.7 0.0145 4.0 (5.4) (18.59) (17.47) (36.53)

(3. 9) (17. 03) (16.50) (35. 08) (2.4) (15.48) (15.54) (33.62)

Total crust 22.6 0.0193 (8.0) (9.51) (19.03) (16.40) (38.57) (8.95) (18.45) (16.04) (38.02) (8.38) (17. 86) (15. 68) (37. 47)

Upper mantle 1170 0.9807 (0.0214) 8.26 _+ 0.09 18.40 15.50 37.98 Bulk Earth 4034 - 0.175 9.38 (18. 96) (16.29) (38.50)

8.88 (18.44) (15.98) (38.02) 8.38 (17.93) (15.66) (37.54)

Masses and mass fractions from Jacobsen and Wasserburg [1], Jacobsen [17] and Zartman and Haines [20]. The Pb concentrations for the continental crust are from Taylor and McLennan [18] and the bulk Earth concentration for Pb is from Hofmann [38]. Upper crustal and upper mantle /x values from this work, lower crust similar to estimate of Zartman and Haines [20]. Pb isotope composition for the upper crust from this study. Pb isotope composition of the upper mantle from Ito et al. [11], based on MORB data. Values in parentheses are calculated (see text). The Pb isotope bulk Earth values were calculated using initial values for the Earth from Tatsumoto et al. [8] (2°6pb /2°4pb = 9.307, 2°7pb /2°4pb = 10.294 and 2°8pb/2°4pb = 29.476) and an age of the Earth of 4.56 Ga.

Pb ISOTOPIC EVOLUTION OF THE EARTH 253

samples (Table 2). The first-stage/z value (].Z 1) is assumed to be the depleted mantle value. This is lower than the /~ value of 10.01 of Zar tman and Haines [20] and the depleted mantle /z value of 9.4 of Zindler and Har t [12]. However, we note that the mantle /z of Zar tman and Haines was time dependent and only took on a value of 10.01 at the present. Values as low as 5 have been suggested previously [39]. The largest uncertainty in estimating the U - P b mass balance for the silicate portion of the Earth lies in estimating the lower crust /.t value. We obtain a range of lower crustal tt values from 2.4 to 5.4 for the lower crust, lower than the estimate of 6.49 by Zar tman and Haines [20]. The range obtained is reason- able for the lower crust, in light of the low /.t values exhibited by lower crust xenoliths and granulites terrains. The range in the /.t values used for the bulk Earth (8.4-9.4) covers most of the estimates in the literature. Lower crust xeno- liths provide a very confusing picture because they are associated with mantle-derived basaltic magmatism [34]. Granulite terrains, which are probably the closet analogs to lower crust compo- sition, have very low/.t values and, in many cases, very unradiogenic Pb [40-42], although others show normal Pb compositions [34].

For the Pb isotope composition of the de- pleted M O R B source mantle we use the values of Ito et al. [11]. Using this we calculated the Pb isotope composition of the lower and total crust by mass balance using:

206 Pb ] 206 Pb

and similar equations for 2°7pb/2°4pb and 2°spb/2°4pb. The resulting estimates of the lower crust (LC) and total crust (TC) are shown in Fig. 7. The lower continental crust remains a possible source of unradiogenic Pb. The simple calcula- tion results in an estimate of 2°6pb/a°4pb = 15.48-18.59; 2°7pb/2°4pb = 15.54-17.47 and 2°spb/2°4pb =33.62-36.53 for the Pb isotope composition of the lower crust. The LC1 value shown in Fig. 7 may appear to be the most reasonable value, since it plots close to the man- tle growth curve. The LC2 and LC3 estimates are rather unusual compositions, even for lower crustal values. The mean Pb isotope composition

17.5

17.0 ~ 3

.~ re ~ 16.0

TC LC 1 ~ U C

t 5 . 5 ~ I ~ ~ , ,'/BE ~'DM

tS.0 = . . 8 . 2 3 i LC(R&~) / , , ,

16 17 18 19 20 2°6 pb / 2o4 pb

Fig. 7. Est imates of the Pb isotopic composition for the upper continental (UC) crust from suspended load data compared to previously published estimates of the Pb isotopic compositions for the crust and depleted mantle. Also shown is the depleted mantle (DM) value of Ito et al. [11] and the lower crust (LC, R&G) value of Rudnick and Goldstein [34], a range of possi- ble bulk earth values (BE), and our estimates for the lower continental crust (LC1, LC2 and LC3) and the total conti-

nental crust (TC).

for the lower crust values are similar to the Pb isotope composition of many Archean granulites [34]. There are large, early Proterozoic and Archean cratons that have not been reactivated by Phanerozoic magmatism. The lower crust of these terrains may store the low-p, component of terrestrial Pb. If the composition of the lower crust is similar to this estimate then lower crust is the solution to the "Pb paradox". Better con- straints on the Pb isotopic composition of the lower crust are clearly needed to resolve this issue.

4.3. The U-Pb fractionation during continental weathering

The 65% discrepancy between the measured and calculated /.t values of suspended loads of major rivers may be caused by fractionation of U-rich heavy minerals, such as zircon, into the river bed load. The only river bed sediment sam- ple, the sample from the Amazon, has a mea- sured /.t value slightly lower than the calculated value. If this sample is taken as a representative sample of river bed load sediments, then they do not show significant enrichment to make up for the deficit in the suspended load. At most, they are close to bulk rock composition. Hf, which like

254 Y. A S M E R O M A N D S.B. J A C O B S E N

U is enriched in zircons, has concentration in suspended loads [43] close to that of the upper continental crust: 6 ppm [18]. Goldstein and Ja- cobsen [3], based on REE data, concluded that heavy mineral separation into river bed load does not play a significant role in REE fractionation.

The most important source of discrepancy be- tween the measured and calculated ~ values is the U loss to the dissolved load. The bulk of this U ends up in the ocean [44]. The riverine dis- solved U input into present day oceans includes U from fertilizer usage. A semi-quantitative esti- mate of the pre-agriculture riverine U input into the oceans may be at tempted from the Pb isotope data of suspended loads (Tables 1 and 2). The measured ~ (/x m, Table 2) values of the large river systems is surprisingly near constant at 3.79 + 0.16. The calculated average second stage (crust residence) ~z (/x2, Table 2) is 12.0 + 0.9. The difference between these two values represents the dissolved fraction, which amounts to 65% of the total amount. The annual suspended load input into the oceans is estimated at 7 × 1015 g [45]. The average U concentration of the upper continental crust is estimated at 2.4 ppm [18]. The total annual dissolved riverine input, the average crust U concentration multiplied by the annual suspended load input and dissolved frac- tion (0.65), amounts to 1.1 × 101° g. This is nearly identical to the 1.0 × 10 l° g / y r estimated by vari- ous workers using the concentration of U in the discharge of rivers and estimates of annual dis- charge [46,47]. However, estimates as high as 1.9 × 101° g / y r have been suggested [44].

Ocean crust represents a significant sink for U, removing nearly 62% of the riverine input during low-temperature alteration [44], although figures as low as 25% of the riverine input have been suggested [48]. In addition, the U / T h and U / P b ratios are increased during hydrothermal ridge alteration [49,50]. The return of altered ocean crust into the mantle constitutes a potentially large source of U injection into the mantle. The ultimate fate of U in basalt is, however, compli- cated by subduction zone processes. It is difficult to assess the magnitude of how much U makes it to the mantle and how much is returned to the crust through arc magmatism.

The similarity in 2°6pb/2°4pb composition of MORB and continental crust samples, while

maintaining a distinct difference in 2°7pb/2°4pb ratios, may be explained by introduction of U-en- riched ocean crust into the mantle since the initi- ation of present-style plate tectonics. The dispar- ity in 2°8pb/2°4pb ratios can be explained simi- larly, as alteration of the ocean crust lowers the P b / U more severely (up to 40%) than the T h / U ratio (between 10% and 25%) [50]. OIB contain the most radiogenic Pb component (except for 2°7pb) of all the major reservoirs (Fig. 2). U-en- riched subducted ocean crust represents the best candidate for the radiogenic component in OIB, although we do not claim to reconcile all the geochemical anomalies of OIB based on the Pb isotopic data alone. Similar ideas have been pre- viously suggested by other workers [50-52].

5. Conclusions

From the above, the following conclusions can be drawn:

(1) The coherent negative correlation between Pb isotope composition and end values of sus- pended load samples allows us to arrive at an empirical estimate of the Pb isotope composition of the upper crust as follows: 2°6pb/2°4pb = 19.32 + 0.28, 2°7pb/2°4pb = 15.76 + 0.09 and 2°spb/2°4pb = 39.33 + 0.39. The Pb isotope ra- tios are positively correlated with 87Sr/86Sr, ex- cept for the samples from Isua, which have the highest Nd model ages ( ~ 3.5 Ga). The Pb iso- tope composition of the Isua samples may suggest a low/x value for the sources of continental crust during the early Earth.

(2) The upper part of the continental crust, similar to the MORB and OIB sources, plots to the right of the geochron leading to what is referred to as the "Pb paradox". One possible solution to this paradox is the removal of a low-p~ component to various reservoirs, such as the core or the lower crust. Results from our simple mass balance calculation using our new upper crustal Pb isotope composition, in combination with other data, and parameter estimates allow for the pos- sibility that the lower crust may contain the low-~ component of terrestrial Pb.

(3) The Pb isotope composition and U concen- tration of suspended loads allow us to estimate the magnitude of U / P b fractionation during the weathering of continental crust. It is estimated

Pb I S O T O P I C E V O L U T I O N O F T H E E A R T H 255

t h a t 6 5 % of t h e U is p a r t i t i o n e d i n t o t h e dis-

s o l v e d l o a d o f r i ve r w a t e r s . I n a d d i t i o n , t h e an -

n u a l i n p u t o f U i n t o t h e o c e a n s ( i n c l u d i n g e s t u a r -

ies) is e s t i m a t e d a t 1.1 x 101° g, s i m i l a r to f i g u r e s

b a s e d o n o t h e r a p p r o a c h e s . T h e f r a c t i o n a t i o n

b e t w e e n U a n d P b d u r i n g c o n t i n e n t a l w e a t h e r i n g

a n d u p t a k e o f U b y a l t e r e d o c e a n c r u s t m a y e x p l a i n t h e s i m i l a r i t y in 2 ° 6 p b / 2 ° 4 p b c o m p o s i t i o n

b e t w e e n t h e c o n t i n e n t a l c r u s t a n d M O R B s o u r c e ,

a n d s o m e o f t h e O I B .

Acknowledgements

T h i s w o r k was s u p p o r t e d by N a t i o n a l S c i e n c e

F o u n d a t i o n g r a n t E A R 90-04821 .

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