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

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

245

[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 T h e best constraints on m a n t l e - c r u s t evolution m o d e l s are d e r i v e d f r o m S m - N d and R b - S r systematics in m a n t l e and crustal reservoirs. B o t h o f t h e s e systems show large and relatively well u n d e r s t o o d e l e m e n t a l f r a c t i o n a t i o n s b e t w e e n th e m a n t l e an d t h e crust. Also, b e c a u s e o f th e very long m e a n life o f t h e s e systems, t h e i r i n t e r p r e t a tion d o e s n o t d e p e n d on a s s u m p t i o n s a b o u t the history o f t h e E a r t h p r io r to f o r m a t i o n o f th e oldest p r e s e r v e d c o n t i n e n t a l crustal rocks. T h e s e systems p ri m ar i l y yield i n f o r m a t i o n a b o u t the

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

m e a n age o f m a n t l e and crustal reservoirs, as well as t h ei r relative sizes as a f u n c t i o n o f t i m e [1]. In contrast, t h e T h - U - P b isotopic system can p o t e n t i a l l y yield i n f o r m a t i o n a b o u t t h e m e a n ages, as well as i n f o r m a t i o n a b o u t t h e very early history o f the E a r t h , since t h e half lives in this system r a n g e f r o m 0.7 to 14 Ga. T h e m a i n p r o b l e m with this system is that U - T h - P b e l e m e n t a l fractionation b e t w e e n t h e crust an d t h e m a n t l e is p o o r l y u n d e r s t o o d , t h e isotopic d i f f e r e n c e s b e t w e e n the crust and t h e m a n t l e a p p e a r to be relatively small, and the i n t e r p r e t a t i o n d e p e n d s heavily on ass u m p t i o n s a b o u t t h e age o f t h e E a r t h . T h e largest U - T h - P b f r a c t i o n a t i o n in t h e silicate p o r t i o n of t h e E a r t h a p p e a r s to o ccu r within t h e c o n t i n e n t a l crust, a l t h o u g h t h e m a g n i t u d e o f this fractionation and the Pb i so t o p e c o m p o s i t i o n o f 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. D e e p 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 better 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 isotopic evolution in the m a n t l e - c r u s t system, as well for understanding the behavior of U in the exogenic cycle.

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 concentrations. 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 measurements were obtained using a VG-54 mass spectrometer. 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).

2. Sample description and methodology Collection of the river water suspended load samples used in this work is described by Goldstein 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

TABLE 1 The isotopic composition of Pb and concentrations of U, and Pb in river water suspended load samples i Sample

Isua-B Isua-F Mississippi-upper Bruneau Ohio Bear Mississippi-lower Amazon Murray Mogami Pampagna Agno

Pb

U

206pb

207pb

20Spb

23Su

(ppm)

(ppm)

2o4Pb

2o4Pb

2o4Pb

2o4Pb

66,74 52,71 27,31 37,16 44,12 23.76 38.17 8.120 28.44 61.54 13.19 45.61

7,60 7,77 1.55 5.76 2.53 3.30 2.40 1.26 1.66 2.60 0.712 0.308

16.939 17.59 19.219 18.838 19.281 19.419 19.182 18.832 18.472 18.471 18.073 17.897

14.701 14.755 15.688 15.863 15.755 15.799 15.614 15.662 15.610 15.646 15.528 15.603

37.597 37.508 38.848 39.534 39.006 39,470 38.813 39.087 38.343 38.362 37.656 37.862

6.92 9.03 3.66 10.08 3.72 9.09 4.06 9.99 3.70 2.68 3.37 0.422

ENd(0) 2

S7Sr/ 86Sr 2

-

0.77634 0.80016 0.72008 0.70998 0.72491 0.71594 0.71918 0.71428 0.70695 0.70491 0.70435

42.6 42.0 19.3 14.2 12.4 11.5 10.0 - 9.2 - 4.7 + 0.5 + 7.1 + 6.5

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].

247

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

16

3. Results

I

.j~

The Pb isotopic data, as well as the concentrations 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 Jacobsen [3].

I

I

~

I

2.5

/

14

'

I

A ~

A,-x

°

39

3.1. Pb isotopic variations

bl

'

'

'

I ~ A

The Pb isotope variations measured in the suspended load samples are shown in Fig. 1 (triangles). 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 unusually low. The remaining data show rather coherent data arrays in both 2°8pb/2°6pb and 2°7pb/2°6pb

a

plots.

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 composition 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 continental crust. Given an estimate of the age of extrac-

33

s 5

3O

K[ = 3.8

/aI = 8.26 ~_.~4.56 Ga I

10

12

I

I

I

14

16

18

20

2°6pb / 2°4pb 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 K1 value of 3.8 were used for the calculation. ~, = measured Pb isotope composition of suspended loads; • = initial Pb isotope compositions of suspended 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 composition (m) of

TABLE 2 Calculated two-stage/x and K values of river water suspended load samples Sample Isua-B Isua-F Miss-upper Bruneau Ohio Bear Miss-lower Amazon Murray Mogami Pampagna Agno

/~ 6.92 9.03 3.66 10.1 3.72 9.09 4.06 9.99 3.70 2.68 3.37 0.42

/z~ 6.36 6.05 8.09 8.58 8.30 8.37 8.08 8.24 8.22 8.30 8.15 8.30

/x3 7.86 9.01 12.00 10.64 12.91 13.46 13.68 12.14 11.56 12.05 12.47 8.96

Nd

Pb

(ppm)

N--d

56.7 52.6 30.4 58.9 39.0 34.9 40.8 32.6 35.0 31.6 12.3 20.3

1.18 1.00 0.898 0.631 1.13 0.681 0.936 0.249 0.813 1.95 1.07 2.24

K2

TNd 4 --DM

4.41 3.89 3.71 4.79 3.77 4.04 3.64 4.34 4.01 4.10 3.48 5.23

3.53 3.40 2.20 1.90 1.75 1.67 1.59 1.54 1.24 1.00 0.55 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].

Y. ASMEROMAND S.B.JACOBSEN

248

g=8.26' 16"0 ~

a sample to calculate both /,( and 11,2 from the following two equations: 206Pb

~ ~,

2o6Pb

'

"°""-

/'

,//

,,"

~

'

'a~

..... :~7_~7:' ~

( 2 0 - ' 0 ~ ) m ~ (20~pb)BEi + ~ l ( e A2-~vo- eA~:r)

15.0

+/~2(e a:3sr - 1) 2o7Pb

"

O"" •

(1)

2o7Pb

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

~ 1 +

-

38

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 A23s = 0.98485 Ga 1. In this case we are using the Nd model age of a sample to estimate T. The subscript BEi denotes the bulk Earth initial Pb isotopic 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 continental 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 m e a s u r e d 2°spb/2°4pb ratios. Assuming a common ~c1 value, we may use /.t values estimated from equations (1) and (2) to calculate the K2 value for each of the samples using the following equation: 2(18Pb

( 2osPb

-~ Kl/R,l(e a232To-- eA2327") "4- K2~.L2(e A232T- 1)

(3)

where Az32 = 0.049475 Ga -( and we used a chondritic ~q value of 3.8 [10]. The calculated K2 values are given in Table 2. Figure 2 shows the Pb isotopic variations in the suspended loads compared to the fields for

36 ~ l G a :].56 G a

16

i

17

i

18

119

210

;1

2°6pb / 2°4pb 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 sediments 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 M O R B s 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 M O R B 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 isotope 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

Pb

ISOTOPIC

EVOLUTION

OF

THE

249

EARTH

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 continental crust is not noticeably different from the depleted 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 metasediments and the Amitsoq gneiss; these are rocks which had experienced both Archean and Proterozoic metamorphism. Their unusual composition 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 ratio 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]. Suspended loads from most river waters have U concentrations lower than the average upper continental crust estimate of 4 ppm [18,20]. Exceptions to this are rivers draining small drainage basins, such as Isua and Bruneau. The sediment input to such rivers may be dominated by mechanical 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

16

. . . .

le-

,

,

2

i

,

,

,

4

i

¢

,

,

6

8

,

k

. . . .

12

10

238U/204pb Fig. 3. 2 3 8 U - 2 ° 6 p b d i a g r a m s h o w i n g p r e s e n t d a y e s t i m a t e d b u l k E a r t h v a l u e a n d 2.0 G a a n d 4.56 G a i s o c h r o n s . N o t e t h a t m o s t o f t h e s u s p e n d e d l o a d s a m p l e s p l o t in a r e g i o n t h a t w o u l d i n d i c a t e t h a t t h e y e x p e r i e n c e d U loss.

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 fractionation during weathering. Exceptions to this trend are samples from river systems with small drainage basins or samples that drain young island 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

VV V

V

o

S

8

•

•

•

~ 0o

0

0 0

0

•

v

II

~

4

0 0

0

I ~= /i

~]

=

L I1. . . . 0

C),

i

0.8

. . . .

i

1.6

. . . .

J

. . . .

2.4

0

=

i

,

,

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 ratios (Ix) o f r i v e r 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) v a l u e s were calculated using a two-stage model with Nd model ages as t h e c r u s t a l r e s i d e n c e a g e o f t h e s a m p l e s . O v e r a l l , t h e r e is a b o u t 6 5 % d i f f e r e n c e b e t w e e n t h e m e a s u r e d a n d c a l c u l a t e d tx v a l u e s as a r e s u l t o f U loss to the d i s s o l v e d f r a c t i o n .

250

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 complexities 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.

Y. ASMEROM

S.B. JACOBSEN

a

3

• ISUA •

2

•

0

I

i

i

i

i

[

ooo

i

ISUA

0.78 0,73

0.72

0.71

•

17

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 M O R B and OIB sources and appear to be the enriched complement to the depleted MORB source. In contrast, the Pb isotopic composition of the upper crust is not particularly radiogenic, compared to MORBs and some OIBs (Fig. 2). There is a systematic increase in,Pb isotopic ratios of the suspended loads with increasing 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 correlation 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 - N d data define a more coherent pattern than the P b - S r data. The scatter in the P b - S r 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 P b - S r data. The general correlation trends observed in the suspended load data (positive P b - S r

AND

18

19

2°6p b / 2°4p 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 diagram 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 M O R B data. The scatter is more pronounced than that observed 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 composition 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 continental crust The Pb isotopic composition of suspended loads plots to the right of the 4.56 Ga geochron,

Pb ISOTOPICEVOLUTIONOF THE EARTH

•-~ 40 .

~

251

39.3. 3±0.39

39 38

16.0 ~

15.8

~

15.6 15.4

15.76:~0.09

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 - eNa c o r r e l a t i o n d i a g r a m for r i v e r suspended load material. Nd isotopic data from Goldstein and 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 P b 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 isot 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 o w s a n 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 crust.

similar to OIB, and most M O R B 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 M O R B source mantle is not a depleted complimentary 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 plausible 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 M O R B (Fig. 2) may suggest that U and Pb have a very similar degree of incompatibility during crust formation 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 samples, 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 upper crust and lower crust is assumed. Suspended load samples, similar to other continental crust samples have distinctly higher 2°7pb/2°4pb ratios than M O R B 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 samples reflect contribution from an old component of Pb that is not being sampled by M O R B or OIB. U, Th and Pb are highly incompatible elements 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 dominate the Pb isotopic signature of the mantle [35]. The distinct 2°Tpb/2°4pb ratio of the upper continental crust rules out a very high rate of crustal

252

Y. ASMEROM AND S.B. JACOBSEN

Pb recycling into t h e m a n t l e d u r i n g the l a t e 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 p r e s e n t , s e d i m e n t s u b d u c t i o n in arcs m a y b e o n e of the possible ways that s o m e a m o u n t of crust m a y be recycled. W h i t e a n d P a t c h e t t [37] ( b a s e d o n N d a n d H f d a t a ) a n d B e n O t h m a n et al. [26] ( b a s e d on Pb isotopic d a t a ) rule o u t a significant a m o u n t o f s e d i m e n t s u b d u c t i o n in m o d e r n island arcs. T h e Pb d a t a d o e s not rule out a large a m o u n t of recycling d u r i n g t h e early p a r t of t h e E a r t h ' s history. A s a result o f strong i n c o m p a t i b i l i t y o f U, Th a n d Pb, the 2°7pb a n d 2o6Pb isotopic c o m p o s i t i o n o f the d e p l e t e d m a n t l e m a y reflect the p r e f e r r e d recycling o f U in alt e r e d o c e a n i c crust, especially d u r i n g p e r i o d s w h e n crust p r o d u c t i o n was low a n d o c e a n crust recycling was high. O u r e s t i m a t e of t h e Pb isotopic c o m p o s i t i o n of the u p p e r crust is in a g r e e m e n t with t h e p l u m b o tectonics m o d e l e s t i m a t e o f Z a r t m a n a n d H a i n e s [20]. T h e e x c e p t i o n to t h e s e t r e n d s are the samples with t h e o l d e s t N d m o d e l ages ( ~ 3.5 Ga). T h e Pb i s o t o p e ratios show large d e v i a t i o n s f r o m the Pb a n d Sr c o r r e l a t i o n trend. T h e initial Pb i s o t o p e c o m p o s i t i o n c a l c u l a t e d using a two stage m o d e l for the I s u a s a m p l e s is close to t h e lowest initial t e r r e s t r i a l Pb, m e a s u r e d on g a l e n a [15]. This suggests an i n h e r e n t low /x v a l u e in the

m a n t l e source of t h e early A r c h e a n c o n t i n e n t a l crust. This is consistent with suggestions m a d e by a n u m b e r of previous w o r k e r s in an a t t e m p t to m o d e l t h e evolution of early Pb [32].

4.2. The Pb isotope mass balance o f the crust a n d upper mantle system T h e isotopic c o m p o s i t i o n of the u p p e r crust d e t e r m i n e d h e r e p r o v i d e s n e w constraints on t h e c r u s t - m a n t l e mass b a l a n c e for Pb isotopes. T h e p r e s e n t mass b a l a n c e for the U - T h - P b system is given in T a b l e 3. T h e b u l k E a r t h Pb i s o t o p e ratios w e r e c a l c u l a t e d using the /x value of Hofm a n n [38] a n d assigning it an u n c e r t a i n t y of + 0.5. T h e / x values of lower a n d total crust were calcul a t e d by mass b a l a n c e f r o m the following e q u a tion for the b u l k silicate E a r t h ~ value: /A'BE = E J

[d'jCpbjYMj

(4)

w h e r e /.%, Cpbj a n d YMj a r e the p~ value, the c o n c e n t r a t i o n of Pb and mass fraction o f t h e total silicate E a r t h Pb of reservoir j (the u p p e r crust, lower crust a n d d e p l e t e d mantle). T h e u p p e r crust and d e p l e t e d m a n t l e ~ values of 12.0 + 0.9 and 8.26 + 0.09, respectively, a r e the a v e r a g e values c a l c u l a t e d using a two-stage m o d e l for o u r

TABLE 3 Pb concentration and isotope composition estimates for the mantle and continental crust Mass

YMj

(10 24 g)

C Pb

Upper crust Lower crust

6.9 15.7

0.0048 0.0145

20.0 4.0

Total crust

22.6

0.0193

(8.0)

0.9807 -

(0.0214) 0.175

Upper mantle Bulk Earth

1170 4034

t.Z

(ppm)

12.0 +_0.9 (5.4) (3. 9) (2.4) (9.51) (8.95) (8.38) 8.26 _+0.09 9.38 8.88 8.38

206 Pb

207Pb

208Pb

204p b

~204p b

204pb

19.3 + 0.2 (18.59) (17. 03) (15.48) (19.03) (18.45) (17.86) 18.40 (18. 96) (18.44) (17.93)

15.76 _+0.09 (17.47) (16.50) (15.54) (16.40) (16.04) (15.68) 15.50 (16.29) (15.98) (15.66)

39.8 _+0.4 (36.53) (35.08) (33.62) (38.57) (38.02) (37. 47) 37.98 (38.50) (38.02) (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 ° 6 p b / 2 ° 4 p b = 9.307, 2 ° 7 p b / 2 ° 4 p b = 10.294 and 2 ° 8 p b / 2 ° 4 p b = 29.476) and an age of the Earth of 4.56 Ga.

Pb ISOTOPICEVOLUTIONOF THE EARTH samples (Table 2). The first-stage/z value (].Z1) is assumed to be the depleted mantle value. This is lower than the /~ value of 10.01 of Z a r t m a n and Haines [20] and the depleted mantle /z value of 9.4 of Zindler and H a r t [12]. However, we note that the mantle /z of Z a r t m a n 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 Z a r t m a n and Haines [20]. The range obtained is reasonable 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 xenoliths 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 composition, 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 depleted 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: 206Pb ]

206Pb

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 calculation 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 mantle growth curve. The LC2 and LC3 estimates are rather unusual compositions, even for lower crustal values. The m e a n Pb isotope composition

253

17.5 17.0

~3

.~

~

re

16.0 TC LC 1

~

t 5 . 5 ~ tS.0 =..8.23

I~~,

i LC(R&~)/

16

,

17

UC

,'/BE ~D 'M 18

,

,

19

20

2°6pb / 2o4pb Fig. 7. Estimates 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 possible bulk earth values (BE), and our estimates for the lower continental crust (LC1, LC2 and LC3) and the total continental 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 " P b paradox". Better constraints 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 sample, the sample from the Amazon, has a measured /.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

U is enriched in zircons, has concentration in suspended loads [43] close to that of the upper continental crust: 6 p p m [18]. Goldstein and Jacobsen [3], based on R E E data, concluded that heavy mineral separation into river bed load does not play a significant role in R E E fractionation. The most important source of discrepancy between 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 dissolved U input into present day oceans includes U from fertilizer usage. A semi-quantitative estimate of the pre-agriculture riverine U input into the oceans may be attempted 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 p p m [18]. The total annual dissolved riverine input, the average crust U concentration multiplied by the annual suspended load input and dissolved fraction (0.65), amounts to 1.1 × 101° g. This is nearly identical to the 1.0 × 10 l° g / y r estimated by various workers using the concentration of U in the discharge of rivers and estimates of annual discharge [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, complicated 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 M O R B and continental crust samples, while

Y. A S M E R O M

A N D S.B. J A C O B S E N

maintaining a distinct difference in 2°7pb/2°4pb ratios, may be explained by introduction of U-enriched ocean crust into the mantle since the initiation of present-style plate tectonics. The disparity in 2°8pb/2°4pb ratios can be explained similarly, 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-enriched 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 previously 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 suspended 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 ratios are positively correlated with 87Sr/86Sr, except for the samples from Isua, which have the highest Nd model ages ( ~ 3.5 Ga). The Pb isotope 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 M O R B and OIB sources, plots to the right of the geochron leading to what is referred to as the " P b 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 p a r a m e t e r estimates allow for the possibility that the lower crust may contain the low-~ component of terrestrial Pb. (3) The Pb isotope composition and U concentration 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

255

OF THE EARTH

t h a t 6 5 % o f t h e U is p a r t i t i o n e d i n t o t h e dissolved load of river waters. In addition, the annual input of U into the oceans (including estuaries) is e s t i m a t e d a t 1.1 x 101° g, s i m i l a r t o f i g u r e s based on other approaches. The fractionation between U and Pb during continental weathering and uptake of U by altered ocean crust may 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 between the continental crust and MORB and some of the OIB.

source,

Acknowledgements This work was supported by National Science Foundation grant EAR 90-04821.

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