Nd and Sr isotopic systematics of river water suspended material: implications for crustal evolution

Earth and Planetary Science Letters, 87 (1988) 249-265 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

249

[6]

Nd and Sr isotopic systematics of river water suspended material: implications for crustal evolution S t e v e n J. G o l d s t e i n * a n d S t e i n B. J a c o b s e n Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138 ( U S.A.)

Received March 25, 1987; revised version received November 23, 1987 Sm-Nd and Rb-Sr isotopic data are presented for the suspended loads of 31 rivers from North America, West Greenland, Australia, Japan, and the Philippines. The Nd model ages for river suspended loads and source rocks are approximately equal. For rivers draining mostly igneous rocks, values of 87Sr/86Sr in the bulk load lie within the range o f 87Sr/86Sr in the source rocks. In general, Sr model ages are less than Nd model ages for suspended loads reflecting Sr loss to solution during chemical weathering of source rocks. An inverse relationship between ~Na and 87Sr/S6Sr in the bulk load of rivers directly reflects the relationship of these parameters in the upper crust exposed to weathering. Sm/Nd decreases and the time-averaged Rb/Sr increases as a function of the depleted mantle Nd model age, ToY~,in the sources of these samples. Such changes may be due to a combination of factors including sampling and preservation bias and progressive depletion of incompatible elements in the mantle through time. The present data base on river water dissolved and suspended material is used to estimate the average Sm-Nd and Rb-Sr concentrations and isotopic parameters of upper crust exposed to weathering: Sm = 5.7 ppm, Nd = 30 ppm, Rb = 95 ppm, Sr = 337 p p m , 1 4 7 S m / 1 4 4 N d = 0.114 a n d 8 7 R b / / 8 6 S r = 0.81, e n d = -- 17, 8 7 S r / 8 6 S r = 0.716. The average T ~ of the suspended load flux from North America to the oceans is estimated to be 1.80 Ga. This is significantly less than the average T ~ of North America of 2.36 Ga and reflects a large contribution of suspended material from young orogenic areas. These data permit a quantitative estimate of the erosion coefficient for North America today. The area weighted Nd model age distribution for North American suspended loads give clear evidence for episodicity in crustal evolution.

1. Introduction Studies of the N d a n d Sr isotopic systematics of a variety of rock types have p l a c e d i m p o r t a n t constraints on the e v o l u t i o n of the crust a n d m a n tle. Because of the limited m o b i l i t y of the rare earth elements d u r i n g w e a t h e r i n g [1-3], the S m - N d isotopic systematics of detrital s e d i m e n ts p r o v i d e a useful average of the S m - N d isotopic systematics of the u p p e r crust e x p o s e d to weathering. Thus, m a j o r river sediments [4] a n d loess [1,5] have b e e n used to d e t e r m i n e the m e a n S m - N d m o d e l age of various parts of the p r e s e n t - d a y c o n t i n e n t a l surface. Shales [1,7,9] h a v e been used to c o n s t r a i n the m e a n S m - N d m o d e l age of the c o n t i n e n t a l surface as a f u n c t i o n of time. This a p p r o a c h is useful for d e s c r i b i n g the age d i s t r i b u t i o n of the * Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A. 0012-821x/88/$03.50

© 1988 Elsevier Science Publishers B.V.

c o n t i n e n t a l surface an d its evolution, however, c o m p l i c a t i o n s arise in relating s e d i m e n t S m - N d m o d e l ages to the m e a n S m - N d m o d e l age of c o n t i n e n t a l crust as a w h o l e [7]. Because sedim e n t s are d er i v ed p r e f e r e n t i a l l y f r o m y o u n g e r o r o g e n i c regions relative to o l d er stable cratons, s e d i m e n t S m - N d m o d e l ages are generally y o u n g e r than the m e a n age of the crust. A d d i t i o n a l d a t a on the S m - N d isotopic systematics of river s u s p e n d e d loads should p e r m i t a q u a n t i t a t i v e e s t i m a t e of this difference w h i ch is n e e d e d to i n t e r p r e t the record of N d isotopes in clastic sediments. In contrast, Sr is q u i t e m o b i l e d u r i n g c h e m i c a l w e a t h e r i n g an d so b o t h dissolved an d su sp en d ed load Sr m u s t be c o n s i d e r e d in d e t e r m i n i n g the average R b - S r isotopic systematics of the continental surface. A l t h o u g h there are d a t a available for the 87Sr/86Sr of the dissolved load of m a n y rivers [3, an d references therein], p u b l i sh ed data on the R b - S r i s o t o p i c systematics of su sp en d ed

250

material in rivers are quite limited [8,29]. Additional data are needed to better estimate the average 87Sr/S6Sr of the continental surface. Combined Sm-Nd and Rb-Sr isotopic data for river dissolved and suspended loads can also be used to explore relationships between the Sm-Nd and RbSr isotopic systematics of the upper crust exposed to weathering. These relationships may yield additional information pertaining to crustal evolution. The purpose of this paper is to present Sm-Nd and Rb-Sr isotopic data on suspended material from rivers draining a variety of rock types and ages in order to (1) compare the bulk load (total dissolved solids + suspended solids) of rivers and their source rocks in terms of their isotopic characteristics and model ages, (2) obtain the Nd and Sr isotope systematics of the upper crust exposed to weathering, (3) obtain the Nd model age distribution of the North American continental surface, and (4) refine the average Sm-Nd and Rb-Sr isotopic parameters for the upper continental crust today. 2. Samples and methods Samples in this study were collected from 30 rivers in North America, West Greenland. Australia, Japan and the Philippines. Sample location maps, basic drainage basin information, and data on the Sm-Nd and Rb-Sr isotopic systematics for the dissolved load of many of these rivers are presented elsewhere [3]. Several samples were selected from rivers with large drainage areas or discharges, including the Mississippi, Missouri, St. Lawrence, Columbia, Colorado, Ohio, Snake, Ottawa, Tennessee, and Murray-Darling. Additional samples were collected to establish a large range of variability of isotopic composition. These include samples from Archean and Proterozoic terrains in West Greenland, West Australia, and the Canadian Shield, as well as samples from relatively young crust in the Philippines, Japan, eastern Australia, and the western United States. Samples were collected with a PVC Van Dorn water sampler. The suspended load was isolated in the field by filtration through 0.2 /~m cellulose filters. The suspended material was then isolated from the cellulose filters via ignition in a platinum

crucible at 8 5 0 ° C for 1 hour. Approximately 50 mg of suspended material was transferred into a teflon container and subjected to standard dissolution procedures in HC1, H N O 3, HF, and HC10 4. These samples were then spiked with a mixed 147Sm-15°Nd tracer and 87Rb and S4Sr tracers. Rb, Sr, Sm and Nd concentrations and Nd and Sr isotopic compositions were determined by mass spectrometry after chemical separation of these elements following the standard methods at Harvard [10]. Filter blanks were 25 ng for Sr and 0.5 ng for Nd and were generally less than 1% of the sample, whereas chemistry blanks were less than 1 ng for Sr and 100 pg for Nd and were negligible. 3. Results Rb, St, Sm and Nd concentrations, isotopic data and model ages are presented in Table 1. Model ages are calculated with respect to the evolution of the depleted mantle, assuming a linear evolution of the depleted mantle (DM) in 143Nd/144Nd and 87Sr/86Sr from the origin of the earth to the present. The depleted mantle Nd model age is calculated from the present values of 143Nd/144Nd and 147Sm/144Nd in the sample using the following expression: Nd

1

TDM = ASm

ln[1 + [

('43Nd/'44Nd)Tampl e -- (143Nd/144Nd)'i')M ] [

am/

ING) sample

(147Sm/,44Nd)°M

]

(1) where X S m = 6 . 5 4 X 1 0 - ~ 2 a ~ is the decay constant for 147Sm, (143Nd/144Nd)°DM and (147Sm/ 144Nd)°)M refer to the ratios in the present-day Sr are calculated in depleted mantle. Values of TDM a similar manner. If the sample is derived from the depleted mantle and no intracrustal p a r e n t / daughter fractionation occurs thereafter, the TDM age is equal to the age of chemical fractionation of the sample from the depleted mantle source. This condition is probably satisfied for the Sm-Nd system; thus, we interpret the T~ d age as a measure of the time of crustal residence of the Nd in the sample. For the Rb-Sr system, the condition of

251 TABLE 1 Analytical results for river water suspended material a Sample

Rb

Sr

87Rb

87Sr b

(ppm)

(ppm)

S6Sr

86Sr

L North America Mississippi-1 156 Missouri 140 St. Lawrence 85 Columbia 84 Colorado 27 Ohio 133 Snake 91 Ottawa 136 Tennessee 124 Bear 148 Mississippi-2 78 St. Louis 94 Bruneau 158 Merced 135 North Potomac 147 Great Whale 124

Csr(0) c

Sm

Nd

147Sm

143Nd d

(ppm)

(ppm)

144N d

144N d

eNd(0) ¢

TI~MSr TI~MNd (Ga) t

(Ga) g

128 151 403 227 2350 120 234 400 151 323 232 259 184 365

3.52 2.68 0.61 1.07 0.033 3.21 1.12 0.98 2.37 1.33 0.97 1.05 2.48 1.07

0.71918_+ 5 0.71691 _+ 5 0.71265_+ 4 0.71161 _+12 0.71077_+ 4 0.72491 _+ 6 0.71173 _+ 6 0.71918_+ 8 0.71666_+ 5 0.71594_+ 9 0.72008 _+ 10 0.72220_+ 6 0.70998 _+ 5 0.70845 _+16

208 176 116 101 89 290 103 208 173 162 221 251 78 56

7.76 7.75 7.48 6.36 4.51 7.28 7.12 9.14 7.66 6.47 5.48 6.31 11.0 8.02

40.8 40.3 40.3 31.6 26.1 39.0 35.4 53.9 40.2 34.9 30.4 35.7 58.9 47.5

0.1151 0.1164 0.1124 0.1219 0.1049 0.1132 0.1218 0.1028 0.1156 0.1124 0.1092 0.1072 0.1136 0.1025

0.511337_+ 19 0.511317+17 0.511152_+21 0.511617_+19 0.511382_+21 0.511212_+20 0.511597-+21 0.510840-+19 0.511256-+13 0.511258_+17 0.510857_+17 0.510782_+17 0.511120_+22 0.511560_+21

- 10.0 -10.4 -13.6 -4.5 -9.1 -12.4 -4.9 -19.7 -11.5 -11.5 -19.3 -20.8 -14.2 -5.6

0.34 0.38 1.28 0.62 0.50 0.60 1.25 0.42 0.74 1.33 1.38 0.21 0.40

1.59 1.64 1.83 1.24 1.38 1.75 1.27 2.10 1.72 1.67 2.20 2.27 1.90 1.10

206 367

2.06 0.98

0.72132_+ 4 0.73128_+12

239 380

8.03 6.19

43.1 40.4

0.1129 0.0928

0.511187_+18 0.510061_+16

12.9 - 34.9

0.65 2.19

1.78 2.91

I1. West Greenland Isua-B 209 Isua-F 270 Nigsik 244

212 168 258

2.86 4.64 2.74

0.77634_+ 7 0.80016_+ 4 0.76861 _+ 4

1020 1358 910

9.05 8.12 9.03

56.7 52.6 56.9

0.0968 0.0937 0.0963

0.509669+38 0.509697+ 21 0.509802+ 17

-42.6 - 42.0 - 40.0

1.85 1.50 1.73

3.53 3.40 3.34

IH. Australia Murray Manning Avon Murchison

134 74 2.4 81

147 174 707 198

2.63 1.22 0.010 1.19

0.71428_+ 0.70902 _+ 0.73322_+ 0.73393 -+

4 6 5 7

139 64 408 418

6.96 6.47 8.74 5.50

35.0 33.9 49.8 30.9

0.1207 0.1300 0.1066 0.1094

0.511605 _+20 0.511831_+13 0.510202_+ 17 0.510709±42

- 4.7 -0.3 -32.1 -22.2

0.32 0.39 1.95

1.24 0.97 3.09 2.40

lid Japan Kitikami Mogami Shinano Tone

59 62 98 72

139 106

1.22 1.68

0.70721 -+ 3 0.70695_+ 2

38 35

146

1.42

0.70839 _+ 4

55

5.84 7.22 6.04 5.52

26.4 31.6 28.4 25.1

0.1340 0.1388 0.1292 0.1335

0.511836_+15 0.511871 _+15 0.511702 + 18 0.511676 _+14

-0.2 + 0.5 - 2.8 - 3.3

0.28 0.19 0.30

1.00 1.00 1.19 1.31

id Philippines Abra Agno Cagayan Pampanga

16 35 28 24

239 208 302 147

0.20 0.48 0.27 0.47

0.70502 _+ 0.70435_+ 0.70524_+ 0.70491 _+

7 -2 11 6

4.41 5.44 4.03 3.49

16.0 20.3 15.1 12.3

0.1669 0.1625 0.1618 0.1718

0.512175 ± 19 0.512181 ± 14 0.512201+_11 0.512208 _+14

+ 6.4 + 6.5 + 6.9 + 7.1

1.19 0.29 0.87 0.39

0.60 0.53 0.47 0.55

3 5 4 5

Uncertainties in Rb concentration < 1%; Sr, Sm, and Nd concentrations < 0.1%. Reported errors for isotopic ratios are 2o of the mean. h Data normalized to 86Sr/SSSr = 0.1194. The measured 87Sr/S6Sr of standard NBS-987 is 0.71025 _+2, and the measured S7Sr/86Sr of Atlantic seawater is 0.70917 + 2. c Deviations in parts in 10 4 from the present-day bulk Earth value of 87Sr/S6Sr = 0.7045 [11]. d Data normalized to 146Nd/142Nd = 0.636151. Deviations in parts in 10 4 from the present-day bulk Earth ( C H U R ) value of 143Ndflla4Nd = 0.511847 [38,39]. The nNdfl standard [12] yielded 143 N d / 144 Nd = 0.511130+10, and the USGS standard BCR-1 yielded 0.511857+15. f Calculated using present-day depleted mantle values of SVSr/86Sr = 0.7026 [33] and SVRb/a6Sr = 0.0541. This corresponds to a linear evolution of depleted mantle from the origin of the earth to the present. g Calculated using present-day depleted mantle values of 143Nd/144Nd = 0.512359 and 147Sm/144Nd = 0.2136. This corresponds to a linear evolution of depleted mantle from eN0(4.55) = 0 to ~Nd(0) = + 10.

252 no intracrustal chemical fractionation is rarely satisfied, thus, T~M Sr ages are generally different from Tff~ ages and have no simple interpretation. Present-day Sm-Nd and Rb-Sr model parameters for the depleted mantle are listed in the footnote to Table 1.

Rivers draining Archean terrains from West Greenland, West Australia, and the Canadian Shield have the lowest eNd and highest Tiff, with ranges of - 1 9 to - 4 3 and 2.1 to 3.5 Ga, respectively. Rivers draining primarily Phanerozoic sediments from North America have intermediate end and Tffd, with ranges of - 9 to - 1 3 and 1.4-1.8 Ga. Fig. 1 also shows that there is an overall trend in the data in which the ]47Sm/]44Nd ratio of river suspended material increases as CNd increases and TDyd decreases. Although there is some scatter in this relationship throughout the data, two rivers in particular, the Merced and the Colorado, fall off the main trend. For the Merced River, this is apparently due to the presence of rocks of anomalously low 147Sm/144Nd in its drainage basin. Cathedral Peak and E1 Capitan, two representative granitic rocks of the drainage area, have 1478m/144Nd of 0.090 and 0.095, respectively [16]. These values are lower than the typical 147Sm/144Nd of = 0.110 for most rocks in the Sierra Nevada Batholith [16]. Sm-Nd isotopic data for other modern clastic sediments, including deep-sea sediments [28], loess deposits [5] and sediments from major rivers [4] are shown for comparison in Fig. 1. Deep-sea sediments from near the Lesser Antilles in the western Atlantic

3.1. Nd and Sr isotope systematics The suspended material in the rivers of this study yielded a very wide range of Sm-Nd system parameters: ENd = --43 to +7, 147Sm/144Nd= 0.094 to 0.172, and T ~ = 0.5-3.5 Ga. These ranges are much greater than previously observed for larger rivers [4] and reflect the greater lithologic and age diversity of the drainage basins for some of the smaller rivers in our data base. The larger rivers in our study (drainage area > 100,000 km 2) have a more restricted range of e Yd = - - 4 to - 2 0 , 147Sm/144Nd=0.103-0.122, and TDy d = 1.2-2.1 Ga. Fig. 1 shows the relationship between the lENd and ]4VSm/]44Nd of river water particulate material. Lines of constant T~d are displayed for reference. Rivers draining young island or continental arc material from the Philippines, Japan, eastern Australia, and the western United States have the highest IENd and lowest T ~ , with ranges of - 5 to + 7 and 0.4 to 1.3 Ga, respectively.

10 /~f 0

--10

Loess

~/

~

1.OGa

..--.

-a

Z

/

-20

O[ ~ . ~ / ' / .

1"oO• o

/./"

/

/

/ --50

j .'~

40 --50

0.05

,

~ / / / - / ~ 5.0 Oa / / . . . . . z i

0.10

//

/

/

/

/ /

0 Norfh America IIW. G r e e n l a n d • Ausfralia • Japan [] P h i l i p p i n e s

/

o/ /

//

/

//

/ /

/

/ / /~

/

/ /

Atl .8"eds /

/

/ " Oa 2.0

j -'~/~/~/ //

//'//

Major rivers ~ - . . ~ ' / - - W .

LtJ

j

-n~

/ 4.0 Ca

. . . .

L

. . . .

0.15

i

0.20

,

,

,

,

0.25

147Sm/144Nd

Fig. 1. ENd(0 ) versus 147Sm/14~Nd for river water suspended material. Fields of major river water sediments [4], western Atlantic sediments [28] and loess [5] are illustrated. Lines of constant depleted mantle Nd model ages (T~d) are displayed for reference. The Nd model age is a measure of the time of crustal residence of Nd in the sample. TDM

253

--

v

. . . .

0.800

ONorth

0.780

• W. G r e e n l a n d • Australia "Japan D Philippines

u~

I

,

America

/

i

,

'

7

i

2.0

Ga

. . . .

/ ~

/

/

•

/-/

/.

/

//

/ /o •

//

/- /

f

oo

./ f / /

1 . 1 . 1 - - - - 1 . 0 Ga 0.740

co

,

Ga

/

/ /

/ /

0.760

, -, / /---- 3.0

1/

0.720

o. oo1 ,o: 0.680 0.0

o

Oo ~

' . . . . . . . . . 1.0 2.0

J o

,

,..5.0

. . . .

,

4.0

. . . . 5.0

87Rb / 86Sr Fig. 2. 87Sr/8asr versus 87Rb/86Sr for river water suspended material. Field for silicate fraction of Atlantic deep-sea sediments [29,31] is illustrated. Lines of constant depleted mantle Sr model ages (TS~) are displayed for reference.

Ocean [28] form a small " b l o b " which overlaps the river and loess data. In general these data agree with the trend shown by our data but show a much smaller range. It is apparent from our new data that there is an overall correlation between end and 1475m/144Ndin suspended loads. The Rb-Sr isotopic data for the suspended material of the rivers in this study are presented in Fig. 2. The suspended material exhibits a wide range of 87Sr/86Sr (0.704-0.800), 87Rb/86Sr (0.01-4.6) and Tt~ srM (0.2-2.0 Ga). The larger rivers in our study (drainage area > 100,000 km 2) have a more restricted range of 87Sr/86Sr = 0.711-0.725, 87Rb/86Sr = 0.03-3.5 and T~M Sr = 0.32--1.3 Ga. Rivers draining young island or continental arc material from the Philippines, Japan, East Australia, and the western United States have the lowest 87Sr/86Sr, with values ranging from 0.7043 to 0.7084. Rivers draining Archean terrains from West Greenland, West Australia, and the Canadian Shield have the highest 87Sr/86Sr, with values ranging from 0.719 to 0.800. Rivers draining primarily Phanerozoic sediments from North America have intermediate 87Sr/86Sr of 0.711-0.725. Rb-Sr model ages are in general substantially lower than Sm-Nd model ages for most rivers, and rivers with the highest R b / S r in their suspended loads generally have the largest difference between ToY~ and Sr T~M.

Previous work on the Rb-Sr systematics of river water suspended material is limited to a few measurements from major rivers. Biscaye and Dasch [29] measured the 87Sr/86Sr of suspended material from the Paran& and Uruguay Rivers and Plata estuary in Argentina and found a range of 87Sr/86Sr = 0.710-0.730 and 87Rb/86Sr = 1.4-4.4. Sediments from Lake Superior [8], with 87Sr/86Sr = 0.738-0.742 and 87Rb/86Sr = 2.0-2.8, plot outside the main "field" shown in Fig. 2. Values of 87Rb/86Sr obtained for the Amazon, Congo, Mekong, and Paran& Rivers range from 1.3 to 6.0 [30] and are broadly consistent with our data. In addition, the Rb-Sr isotopic systematics of other m o d e m erosion products, such as deep-sea clay material and loess deposits, are similar to the data in this study. As illustrated in Fig. 2, the 878r/86Sr and 87Rb/86Sr of the non-carbonate fraction of core-top deep-sea sediments ranges from 0.704 to 0.743 and from near 0 to 5.0, respectively [29,31]. Samples of loess are characterized by 87Sr/86Sr = 0.710-0.719, and 87Rb/86Sr = 0.36-3.0 [5]. Fig. 3a plots end versus 87Sr/86Sr for the suspended load of the rivers in the study. Rivers draining older crust have relatively more radiogenic Sr and non-radiogenic Nd than those draining younger crust and there is thus a general inverse relationship between these parameters. This relationship deviates strongly from linearity to-

254

10 0

--10 G"

O North America

~(~

O ~C)O O

OW. Greenland

•

CO

-2o

(~

Australla A Japan [] Philippines

•

suspended load fluxes are significant for Sr. If cS~ and Cs~ denotes Sr concentrations in dissolved and suspended loads respectively, and ZSL is the mean mass fraction of suspended material in the river, then the 87Sr/86Sr of the bulk load is given by:

t~~" -50 O -40

( UVSr )

c S r ( ~7Sr ) ~ ( ~ r ( 878r ) D L ~ 86S~ DL -}- / ' SL"~SLI S6Sr sL

-$0 0

m

.

.

.

.

.

.

.

(2)

.

-10

@

-20

~

-30

o%

m ©

-40

, , . , .... -50 0.700 0.720

, ......... 0.740

, .....

0.760 0.780 87Sr / 86Sr

, 0.800

Fig. 3. (a) ENd(0 ) versus SVSr/SrSr for river water suspended material. (b) ENd(0) versus SVSr/S6Sr for the bulk load of the rivers in this study using river water dissolved material data of the same samples from Goldstein and Jacobsen [3]. These data reflect the inverse relationship between these parameters in the upper crust exposed to weathering. The relationship appears to have curvature and is characterized by a shallow negative slope for Archean terrains and a steeper negative slope for younger terrains. This curvature reflects variations in the S m / N d and R b / S r of the source rocks with age, as indicated in Fig. 6.

ward high 87Sr/86Sr for rivers of low end. A similar non-linear trend has been observed for the dissolved loads of these rivers [3]. These trends are relatively similar except for rivers draining large regions of marine sediments with low 87Sr/86Sr in the dissolved load and high 87Sr/86Sr in the suspended load. In these rivers the differences in STSr/86Sr reflect the importance of marine precipitates of low R b / S r as a source of dissolved load Sr and shales with high R b / S r as a source of suspended load Sr. The sum of the total dissolved solids and the suspended loads, which we call the bulk load (BL), should most accurately reflect the composition of the source rocks in the drainage basin. In general, the dissolved load (DL) flux of Nd is less than 1% of the suspended load (SL) flux (see Table 4), and BL ~ ENd' SL However, both the dissolved and SO lENd

since ( 1 - Z s c ) = 1. The calculated values of SVSr/86Sr in the bulk loads of the rivers of this study are given in Tables 2 and 3, using YSL values from these tables, dissolved load data from [3] and suspended load data from Table 1. The relationship between CNdBLand (87Sr/86Sr)BL of the rivers in this study is plotted in Fig. 3b, and we note a substantial decrease in the scatter compared to Fig. 3a. 3.2. Relationship to isotopic characteristics of source rocks In general, the chemical and isotopic composition of river dissolved and suspended material can be influenced by a number of sources, including (1) weathering of rocks within the drainage basin, (2) anthropogenic activities, and (3) aeolian or glacial transport of materials from outside the drainage basin. In the absence of any external sources, the bulk load of a river should reflect some weighted average composition of the source rocks within its drainage basin. There are some complexities in comparing the isotopic characteristics of the bulk load with the source rocks. For example, one must know the proportions of the different source rocks within a drainage basin and the isotopic characteristics of each type. In drainage basins which consist of more than one major rock type, it is not possible to precisely determine the relative contribution of the different source rocks to the suspended load, since different rocks erode at different rates. The isotopic characteristics of the source rocks and bulk load of rivers draining young continental and island arcs, Archean terrains, and mostly Phanerozoic sediments from North America are compared in Tables 2 and 3 and Fig. 5

255 TABLE 2 Comparison of Nd and Sr isotope systematics for rivers and source rocks River

N a (Ga) TI~M

suspended load

A (Ga) b source rocks

a

•SL c

87Sr/S6Sr

(ppm)

bulk load e

source rocks

References

I. Continental~island arcs Merced 1.10 Manning 0.97 Murray 1.24 Kitikami 1.00 Mogami 1.00 Shinano 1.19 Tone 1.31 Abra 0.60 Agno 0.53 Cagayan 0.47 Pampanga 0.55

1.0 0.9 1.1 1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.5

- 0.10 - 0.07 -0.14 0.0 0.0 - 0.19 -0.31 - 0.10 - 0.03 0.03 0.05

309 56 1360 1110 545 500 1130 500 500 500 500

0.7084 0.7066 0.7125 0.7070 0.7070 0.7084 0.7056 0.7044 0.7058 0.7052

0.7061-0.7074 0.7035-0.7080 0.7035-0.7080 0.7035-0.7080 0.7035-0.7080 0.7035-0.7040 0.7035-0.7040 0.7035-0.7040 0.7035-0.7040

[16,27] [37] [36] [23,24,26] [23,24,26] [23,24,26] [23,24,26] [25] d [25] d [25] d [25] d

II. Archean terrains Isua-B 3.53 Isua-F 3.40 Nigsik 3.34 Avon 3.09 Murchison 2.40 Great Whale 2.91

3.5 3.5 3.3 3.2 2.5 2.9

-0.03 0.10 - 0.04 0.11 0.10 0.01

321 321 321 321 321 6

0.7802 0.8081 0.7715 0.7326 0.7283 0.7371

0.7163-0.8240 0.7163-0.8240 0.7163-0.8240 0.7144-0.9340 0.7120-0.8600 0.7277-0.7288

[10,21,22] [10,21,22] [18,21,22] [17,50] [13,35] [1]

a Average Nd model age for source rocks in the drainage basin. b A = difference in Nd model age of suspended load and source rock. ° Calculated from the sediment yield data of [41] and water discharges and drainage areas listed in [3]. For rivers of West Greenland and West Australia no suspended load concentrations were available so we used the global average value of 321 ppm (Table 4). d Average Nd model age of source rocks for the Philippines calculated assuming typical island arc values of eNd ----+ 7 [48,49], and assuming 147Sm/144Nd = 0.165 (average of 4 rivers from the Philippines). e Calculated (see text).

Young Bland or continental arcs. Of the rivers draining young island or continental arcs, the isotopic characteristics of the source rocks of the Merced River are best determined. The Merced River drains a portion of the Sierra Nevada Batholith which is characterized by Tff~ = 1.0 G a and 87sr/g6sr = 0.7061-0.7074 [16,27], whereas the bulk load has T ~ = 1.1 G a a n d 8 7 8 r / 8 6 S r = 0.7084. Thus, there is only a small difference between the source rocks and the bulk load in their TDY~ or 87Sr/g6Sr. Similarly, the 8VSr/86Sr of typical volcanic rocks from Japan [23,26] and the Philippines [25] and the bulk loads of the rivers from these areas differ by less than 0.002. N d isotopic data for drainage basins of the rivers in Japan and the Philippines are somewhat less comprehensive, however, the available data for J a p a n [23,24] suggest general agreement between the SmN d model ages of suspended material and source rocks. No Nd isotopic data are available for the volcanic rocks of the Philippines, however, the

suspended load end values of + 6 to + 7 are consistent with typical island arc values of + 5 to + 10 [48,49]. We therefore conclude that in rivers draining young island and continental arcs there are at most small differences between the bulk load and the source rocks in terms of T ~ or 8 7 8 r / 8 6 S r . T h e s e differences in TIN~ ( = 0.1 Ga) and SVSr/86Sr (-- 0.002) are probably less than the uncertainties associated with estimating average values of T ~ and 87Sr/86Sr for the source rocks in the drainage basin. Archean terrains. The source rocks of the rivers in the Isua region of West Greenland have been the subject of intensive geochronological study. The samples Isua-F and Isua-B taken from the lake and a small stream at Isua, drain primarily Amitsoq gneiss and Isua metasediments. These rocks are characterized by ENd ~ - - - 4 0 to - 4 4 , and Tff~ = 3.6-3.7 G a [10,18,20,22], whereas the bulk load of these rivers has ey a = - 4 2 and Tff~ = 3.4-3.5 Ga. There is excellent agreement between

256 TABLE 3 Nd and Sr isotope systematics of North American rivers River

Drainage

Sediment

Nd (Ga) TDM

area (103 km 2)

discharge a (106 tons/yr)

suspended load

basement rocks

(ppm)

2.91 1.70 1.64 1.92 1.83 1.24 1.38 1.75 1.27 2.10 1.72 1.22 1.83 1.67 2.20 2.27 1.90 1.10 1.78

2.9 2.1 2.3

-

Canadian Shield c Mississippi Missouri Mackenzie St. Lawrence Columbia Colorado Ohio Snake Ottawa Tennessee SF Bay Hudson Bear Mississippi-2 St. Louis Bruneau Merced North Potomac Total

5730 3220

46 210

1800 764 654 434 158 98 20 17 13 9 7 0.8 0.2 ~

100 3.3 36 135 0.7 19 1 -

North America

24200

~SL b

( 87Sr )

References

S~Sr BL

2.2 1.3 2.0 1.8 1.5 2.5 1.8

710 1140 14 46 7 136 65 13 108

0.7120 0.7123 0.7097 0.7121 0.7108 0.7121 0.7101 0.7120 0.7125

2.3 2.8 2.8 2.3 1.0 1.6

43 384 266 88 309 17

0.7141 0.7179 0.7193 0.7095 0.7084 0.7130

[1] [4,32] [32] [4] [1,32] [32] [32] [32] [32] [1,32] [32] [4] [3] [32] [1] [1] [32] [16,27] [32]

1183

a Sediment discharges from [41]; Columbia and Colorado river discharges are those prior to damming. b Total suspended load concentrations calculated from sediment yield data of [41] and water discharges and drainage areas listed in

[31. Canadian Shield defined as region of surface exposure of basement rocks in Canada, Baffin Island, and Greenland, excluding St. Lawrence and Ottawa River drainage areas. Nd model age for Canadian Shield river suspended material assumed to be equivalent to Great Whale River.

the bulk load and source rocks with respect to their Sm-Nd isotopic systematics. 87Sr/868r is more difficult to compare since it is quite variable in the source rocks. However, the observed range of 878r//86Sr in the source rocks [10,21] does bracket the range i n 87Sr/86Sr of the bulk load of these rivers. Comparison of the bulk load of the Great Whale River in Northwest Quebec with igneous rock composites from this region [1] show that the values of TDNM a for both are approximately equal, whereas the 8 7 S r / 8 6 S r o f the bulk load is approximately 0.010 greater than the composites. The bulk load of the Avon River in West Australia differs from the source rocks [17] by only 0.1 G a in TDyd, and the bulk load 87Sr/86Sr is bracketed by the observed range in 87sr/g6sr of the source rocks [50] in the drainage area. We therefore conclude that in rivers draining Archean terrains the T ffd of the suspended load and the weighted

average TDNd of the drainage basin are approximately equal, whereas the 87Sr/86Sr of the bulk load generally lies within the observed range in SVSr/86Sr of the source rocks. North American Phanerozoic sediments. The geographic distribution of T ~ d for the North American rivers (excluding the Great Whale) is presented in Fig. 4. It is difficult to estimate the isotopic characteristics of the source rocks of many of the large North American rivers, since they drain mostly Phanerozoic sedimentary rocks for which little or no isotopic data are available. However, there are Sm-Nd isotopic data available for igneous basement rocks. There is general correspondence between the geographic distribution of our river water suspended material N d model ages and Nelson and DePaolo's [32] basement rock Nd model ages for the United States. In both cases, model ages show gross regional trends that are

257 2.3

4.0 =Source rock age oBasement rock age

g {7

/

i ~

3.0

0

E 2.0 "o "o (D "1o

Xxx i

Fig. 4. Geographic distribution of depleted mantle Nd model ages (T~d) in 109 years for the suspended loads of North American rivers (excluding the Great Whale). Data are from this work and [4]. Solid boundaries in this figure correspond to the drainage area of a given river. For most rivers the TDN~ values shown in the figure correspond to the average isotopic characteristics of the drainage area enclosed. Some of the TDN~ values correspond to more than one enclosed area (i.e., the Columbia drainage includes the Snake). Gross regional variations in ToNd (higher T~d for the North Central U.S., lower TN~ for the western and mid western U.S.) correlate with variations in basement rock Nd model ages [32]. consistent with b a s e m e n t rock ages. F o r example, high N d m o d e l ages are o b s e r v e d for the N o r t h C e n t r a l U.S. a n d low m o d e l ages are o b s e r v e d for the central a n d western U.S. T h e r e l a t i o n s h i p of ToNd of river w a t e r susp e n d e d m a t e r i a l to T Nd of the source rocks is shown in Fig. 5. Rivers d r a i n i n g y o u n g i s l a n d or c o n t i n e n t a l arcs a n d A r c h e a n terrains have TDNd ages that are a p p r o x i m a t e l y equal to their igneous source rocks. F o r rivers d r a i n i n g p r e d o m i n a n t l y P h a n e r o z o i c s e d i m e n t a r y rocks we c o m p a r e o u r s u s p e n d e d l o a d m o d e l ages to the N d m o d e l ages of the b a s e m e n t rocks to the s e d i m e n t a r y cover; the m o d e l ages o f s u s p e n d e d m a t e r i a l a n d igneous b a s e m e n t rocks differ b y up to 0.6 G a , b u t are still correlated. T h i r t e e n of these fourteen rivers have y o u n g e r m o d e l ages in the s u s p e n d e d l o a d t h a n in b a s e m e n t rocks a n d the average difference in m o d e l age is 0.35 G a . W e c o n c l u d e f r o m the excellent a g r e e m e n t of N d isotopic p a r a m e t e r s for river s u s p e n d e d l o a d s a n d igneous source rocks that the s u s p e n d e d l o a d s directly reflect the Er~d a n d T Nd values of their source rocks. T h e 87Sr/86Sr of b u l k l o a d s of rivers d r a i n i n g igneous terrains also reflects their values in the source rocks. However, the 87Rb/86Sr of

1.0

=:

Q_

0.0 1.0 2.0 .5.0 4.0 Source or basement rock Nd model age (Ga)

Fig. 5. Relation between TDN~ for river water suspended material and the area-weighted average value of TDN~ for source rocks or basement rocks in the drainage basin. Error bars represent the total range in TONM d ages for basement or source rocks in a given drainage basin. In general, (TDN~)~usp= (TDNMd)so. . . . for rivers draining primarily igneous source rocks. For rivers draining mostly sedimentary rocks, the TDN~ values of their source rocks are not known. Thus, the ToN~ values of rivers draining mostly Phanerozoic sedimentary rocks are compared to basement rock Nd model ages of Nelson and DePaolo [32]. We note that (TDNd)su~p < (TNd)b. . . . . . although these parameters are still correlated. Data for source rocks or basement rocks are from [1,10,14-18,20,22-24,32,35-37].

s u s p e n d e d m a t e r i a l is clearly f r a c t i o n a t e d d u e to Sr loss to s o l u t i o n d u r i n g w e a t h e r i n g (see T a b l e 4) a n d thus T~M Sr m o d e l ages of s u s p e n d e d l o a d s d o n o t reflect source r o c k values a n d are in general m u c h lower t h a n ToNO m o d e l ages.

4. Discussion 4.1. N d and Sr isotopic systematics in the upper crust T h e b u l k l o a d s o f rivers d r a i n i n g p r e d o m i n a n t l y igneous terrains have ENd a n d 87Sr/86Sr values that are similar to the w e i g h t e d average values for source rocks within a d r a i n a g e basin. T h u s in these rivers, the ENd-STSr/86Sr relationship of the b u l k l o a d (Fig. 3b) directly reflects the r e l a t i o n s h i p of these p a r a m e t e r s in u p p e r crustal source rocks d e p e n d i n g o n their m e a n age. F o r rivers d r a i n i n g m o s t l y s e d i m e n t a r y rocks, the ENdS7Sr/S6Sr r e l a t i o n s h i p m a y also b e affected b y m i x i n g of reservoirs of different ENd , 87Sr/86Sr, a n d N d / S r to f o r m the s e d i m e n t a r y source rocks. A t w o - c o m p o n e n t m i x t u r e of: (1) y o u n g island arc

258 0,25

.

.

.

i

.

.

.

.

.

i

.

.

.

.

i

.

.

.

0 North America • W. Greenland • Australia

.

a

• Japan

0.20

C1Philippines

~" 0.15 E

AA 0

0.10

~

:

:

:

~

ooom

©

l

i

I

l

l

i

i

l

•

l

l

l

l

l

l

b

-o o E

2.0

~5

g 1.0

o

g

• 0.5

%

0.0

. . . . 0.0

;0

,°I . . . . 1.0

OC~

~'o ! . . . . 2.0

I

3.0

. . . . 4.0

i Nd

DM

Fig. 6. 14VSm/144Nd and (SVRb/S6Sr)mod~l versus TD~M dd for river water suspended material. Sm/Nd decreases and the Nd time-averaged Rb/Sr increases as a function of TDM in the sources of these samples. These age trends could reflect a progressive depletion in incompatible elements in the upper mantle and new crustal segments through time.

material of ENd= +7, aVSr/S6Sr=0.7045, and N d / S r = 0.06, and (2) Archean material of e yd = - 4 2 , 87Sr/86Sr = 0.80, and N d / S r = 0.32 yields a curved relationship between ENd and SVSr/S6Sr similar to that observed in the data. However, few sedimentary terrains are simple two-component mixtures of Archean and young island arc material. Most sedimentary rocks are in fact multi-component mixtures of crustal material of variable age which will yield a similar trend. As a consequence, the ENd-SvSr/86Sr relationship in the upper crust must be similar in general form to the relationship in the bulk load of rivers shown in Fig. 3b. The progressive change in Fig. 3b from a relatively shallow negative slope for the Archean to a more steeply sloping relationship for more recent crustal additions suggests that additions to the crust have had increasing S m / N d a n d / o r a decreasing R b / S r through time. This suggestion is supported by Fig. 6a which shows a trend toward higher 1478m/144Nd ratios for rivers draining

young crust. These trends may be due to an increase in the extent of mantle depletion through time. In most rivers, the high R b / S r of the suspended loads primarily reflects Sr loss to solution (see Table 4), while almost all of the Rb is transported in the suspended load. Thus, the STSr/86Sr of these samples reflects the age and R b / S r of the source rocks in their drainage basins prior to weathering. Assuming that the age of these materials is equal to T ~ , the R b / S r of these materials prior to weathering can be calculated. This calculation assumes a single-stage evolution of the source rocks of the sample, starting at the isotopic composition of Sr in the depleted mantle at time T = T ~ d and evolving to the measured 8VSr/86Sr in the sample today. The calculated value, denoted (87 Rb/86Sr) model, is the time-averaged 8vRb/86Sr for each sample since its age of formation, T ~ , until present-day weathering. The values of (SVRb/86Sr)moa~l for the river water suspended load samples are plotted versus T~ in Fig. 6b. (87Rb/86Sr)model ranges from = 0.29 to 2.1, and these values are roughly correlated to Tffd. Most samples, including the major rivers, have a limited range of (87Rb/86Sr)model of 0.63-0.96. The average (aTRb/S6Sr)mod~l of the entire data set is 0.69 +_ 0.16 (2o). This value is lower than 87Rb/86Sr estimates of the upper crust based on direct measurement of igneous rocks ( = 1.0 [47]), but is a factor of 2 - 3 greater than estimates for the total crust based on isotopic models [19,33] or compositional models [34].

4.2. Mass distribution of T ~ for North America The cumulative mass distribution of Tffd for North America for basement rocks and river water suspended material is presented in Fig. 7. The mass distributions have been calculated assuming two limits of depleted mantle composition to illustrate the sensitivity of these distributions to the choice of mantle evolution parameters. One limit assumes a linear evolution of ENd in the depleted mantle from ENd(4.55)=0 t o E N d ( 0 ) = awl0, the ENd of mid-ocean ridge basalts. The other assumes a linear evolution to ENd(0 ) = + 6, the lower limit of ENd in primitive island arcs [48,49], and is offset by about 0.2 G a t o lower T r ~ ages. The distribution for basement rocks is recalculated from Nelson and DePaolo [32] using the depleted man-

259

0.8

'North American Basement Rocks / / / ~ /

0.6

[

0.4

JJ /

i/

0,2 0

.+-(.J E)

,.+_ tO D

E > °_ .-I.-

O.B

by a r e a .

? :r

0,6 0.4 0,2

E (.)

North American Surface from / i/ River Particulates, / /

0,8

North American Suspended Load

fo Oceans

0.6

c

Flux /

0,4 0.2

4.0

3.0

2.0

T Nd

DM

1.0

0.0

(Go)

Fig. 7. Cumulative mass distribution of T ~ for (a) basement rocks of North American continent, recalculated from Nelson and DePaolo [32]; (b) the North American continental surface based on the areal distribution of river water suspended material; and (c) suspended load flux from North America to the oceans. The cumulative curves have been calculated assuming two extremes of depleted mantle composition (see text). Assuming a depleted mantle of ~ya(0) = + 10, the mean age of the distributions in (a), (b) and (c) are 2.36 Ga, 2.31 Ga, and 1.80 Ga, respectively.

tle evolution described above. Two distributions are presented for river water suspended material: one weighted by drainage area and one weighted by sediment discharge. The basic data used to calculate these distributions is presented in Table 3. The area and flux weighted averages of Cyd for river water suspended loads from North America are - 2 1 . 9 and - 1 2 . 5 , respectively. We note that the coverage of river waters for North America is 53% in terms of area and 47% in terms of sediment discharge.

The cumulative mass distributions of basement rocks and river water particulate material weighted by area are quite similar in general form. Assuming a depleted mantle of CNa(0) = + 10, the mean age for basement rocks is 2.36 Ga, while the mean age for river water suspended material weighted by area is 2.31 Ga. In contrast, the mass distribution for river water suspended material weighted by sediment discharge is offset to significantly younger ages, due to rapid weathering of young orogenic areas relative to old stable cratonic regions. The mean age for suspended material weighted by discharge is only 1.80 Ga. In addition, the episodicity of crustal growth apparent from the curves for basement rocks and suspended material weighted by area is not as prominent in the curve for suspended material weighted by discharge. This reflects the fact that most of the sediment discharge for North America occurs in major rivers which have similar Sm-Nd isotopic systematics. However, extending this approach to the entire earth's continental crust appears to be the most effective way of obtaining the present-day cumulative age distribution of the entire continental crustal surface.

4.3. Variations of Sm / Nd and Rb / Sr of the upper crust through time Suspended load data show a decrease in S m / N d and an increase in (87Rb/86Sr)vaodel with respect to Tff~ (Fig. 6). The relationship between S m / N d and T ~ for the crust inferred from these data is opposite that proposed by Taylor and McLennan [34], who argued that Proterozoic shales are more light R E E enriched than Archean shales. The age trends in S m / N d and R b / S r shown in our data could be due to one or a combination of factors, including: (1) a systematic bias toward material of more silicic composition for older segments of crust and more marie composition for younger sediments, (2) a secular change in the S m / N d and R b / S r of new additions to the continental crust, or (3) for R b / S r , increased effects of weathering and metamorphism on older segments of upper crust relative to younger segments. The S m / N d and R b / S r of crustal materials depends to some extent on bulk composition, as S m / N d ratios increase and R b / S r ratios decrease with increasingly marie bulk compositions. If the suspended materials from older segments of crust

260 were biased toward more silicic bulk compositions than average crust of similar age, and likewise, materials from younger segments of crust were biased toward more mafic bulk compositions than average crust of equivalent age, the overall relationships between S m / N d and R b / S r and T ~ d might be explained. This bias could represent a sampling bias, or it could reflect some type of preservation bias in which more mafic sections of older crust are preferentially reworked and destroyed through time relative to granitic crust. Alternatively, the relationship could reflect a secular change in the S m / N d and R b / S r of new additions to the continental crust. Such changes could reflect the progressive depletion of the upper mantle and progressive depletion of new segments of the continental crust in incompatible elements through time. Although there is ample Nd isotopic evidence supporting the formation of a depleted mantle relatively early in earth's history [10,20,38,39], its evolution since then is not well constrained by the available depleted mantle data. The maximum change in S m / N d and R b / S r for new additions to the continental crust can be evaluated by mass balance considerations of the depleted mantle + continental crust system. The chemical enrichment factor for the a47Sm/144Nd ratio relative to the bulk earth value (or C H U R ) is defined by fSm/Nd = [(147Sm/144Nd) ....... it/ (147Sm/144Nd)cHUR]- 1. The chemical enrichment factor for new additions to the continental crust from the depleted mantle is given by" fSm/Nd =

~Nd

fSm/Nd +

- 1

(3)

where d i is the effective enrichment of element i in new crust relative to the mantle source [33,40] and fsDm~Nd is the S m / N d enrichment factor for the depleted mantle. An analogous expression holds for Rb-Sr. Using reasonable estimates of the d-values [40] and a knowledge of the S m / N d and R b / S r of undepleted mantle and present-day depleted mantle, the maximum change in the S m / N d and R b / S r of new additions to the crust during the history of the earth can be found. The presentday mass ratio of continental crust to depleted mantle is probably no greater than 0.02 [19,33,40]. The 147Sm/a44Nd of depleted mantle has changed from 0.1967 at the formation of the earth to approximately 0.252 for the present day [40]. Using

dsm = 12 and d N d = 27, the 147Sm/144Nd of new additions to the crust changes from 0.0874 to 0.1120. The magnitude of this change is not very sensitive to changes in the rate of crustal recycling. For dRb = 175 and dsr = 30, the 87Rb/86Sr of new additions to the crust changes from 0.48 initially to 0.04 today; however, this change is sensitive to the recycling of crust back into the mantle as well as the value chosen for dRb. Recycling of crust tends to raise the R b / S r of new additions to the crust today without affecting early evolution of the R b / S r of the crust or mantle. If we choose dRb = 400, the 87Rb/86Sr of new additions to the crust changes from 1.1 to 0.11 through time. In all cases, Rb depletion from the mantle is quite rapid and therefore the R b / S r of new additions to the crust decreases rapidly during early stages of crustal growth and levels off during later stages. In contrast, Sm and N d are more compatible species in the mantle, therefore the increase in S m / N d of new additions to the crust is much more uniform throughout crustal growth. These considerations suggest that the variations in S m / N d and R b / S r observed for river water suspended loads are not solely due to secular variations of the S m / N d and R b / S r of new additions to the crust. Some crustal materials may not be fractionated from the depleted mantle but may contain a volcanic component with a depleted mantle signature. This is particularly true for materials in young island arcs, and to some extent materials in continental margin arcs. If the data for the suspended loads of rivers from Japan and the Philippines that contain a large depleted mantle signature are omitted, the observed change in 147Sm/144Nd from 0.09 to 0.12 is more compatible with possible changes in the S m / N d of new crustal materials that have been fractionated with respect to the depleted mantle [33,40]. For R b / S r , the situation is complicated by the likelihood of increases in the R b / S r of the upper crust due to intracrustal fractionation processes such as weathering and metamorphism. This was ignored in the calculations in section 4.1; however, the decrease in (87Rb/86Sr)model is so large that we do not expect that a proper consideration of these processes would invalidate our conclusion that the R b / S r ratio in new crustal additions decreased markedly with time.

261

We conclude that the relationships of S m / N d and (87Rb/865r) model tO TDTM reflect a progressive depletion in incompatible elements in the upper mantle through time, as suggested by the ~Nd" 87Sr/86Sr plot (Fig. 3). However, this effect has probably been enhanced by a systematic bias in rock composition due to either incomplete sampiing or crustal reworking.

TABLE 4

4.4. S m - N d and Rb-Sr mass balance of the upper continental crust exposed to weathering Rivers integrate over a variety of heterogeneities in the upper crust, and should provide a good way of establishing the typical Sm-Nd and Rb-Sr isotopic systematics of the crust exposed to weathering. Estimates of average river water dissolved and suspended material values for concentrations, fluxes, isotopic ratios, and model ages are given in Table 4. Suspended load concentrations for Rb, Sr, Sm, and Nd are obtained from averages of the data in Table 1. The average ~Nd of river water suspended material based on the average ENd of the 20 largest rivers of this study and [4], is -10.6. The average 87Sr/S6Sr of river water suspended material is -- 0.716 based on the 8 largest rivers in this study. If 52SL is the average concentration of total suspended solids of rivers, and ~DL is the average concentration of total dissolved solids derived from rock weathering, then the bulk load ~BL = Y~SL"~ F~DLrepresent the total concentration derived from rock weathering. The average concentrations in the upper crustal source rocks may be estimated directly from the bulk load concentrations which are given by:

11. Fluxes (1012 g / y ) d JRb 0.07 Jsr 2.52 Jsm 0.0002 JNd 0.001

(4)

CBL = 0¢sLCsL -4" OLDLCDL where

aSK = E S L / ( ~ S L q- E D L )

and

OtDL = (1 --

ESL)/(ESL + ~]DL) ~-- 1/(ESL + EDL). The bulk load concentrations calculated using equation (4) are given in Table 4. The dissolved load and suspended load fluxes calculated from the concentration data show that only insignificantly small amounts of Rb, Sm, and Nd are carried in the dissolved load relative to the suspended load. The concentration of each of these elements in the bulk load is essentially the suspended load concentration multiplied by aSL 0.83. In contrast, the dissolved load flux of Sr is similar to the suspended load Sr flux. The bulk

Rb-Sr and Sm-Nd parameters for average river water Parameter

Dissolved a load

L Concentrations (ppm) Rb 0.0015 Sr 0.060 Sm 0.000008 Nd 0.00004

Suspended b load 109 221 6.86 36.4

Bulk c load 94.5 337 5.69 30.3

1.47 2.98 0.0926 0.491

1.54 5.50 0.0928 0.492

11-1. Isotopic ratios and model ages (Ga) 87Rb/86Sr 0.072 1.42 147Sm/14aNd 0.125 0.114 87Sr/86Sr 0.7101 0.7160 ENd(0 ) -- 8.4 - 10.6 TDS~a 29 0.69 T~ 1.63 1.63

0.806 0.114 0.7133 - 10.6 1.0 1.63

a Data from [3]; average global total dissolved solids concentration is 3ZDL= 67 p p m [30,44] and represents the total amount of dissolved solids derived from rock weathering (i.e. corrected for pollution and the part of H C O ; derived from atmospheric COz). b Data from this study (excluding Colorado and Avon which have atypical values) and [4]; average global suspended load concentration is F~SL= 321 p p m [41] using a total fiver runoff of 4.2 × 1019 g / y r [43]. c Calculated; see text. d Calculated using a total river runoff of 4.2 X 1019 g / y r [43].

load has Rb = 95 ppm, Sr = 337 ppm, S m = 5.7 ppm, N d = 3 0 . 3 ppm, 8VRb/86Sr=0.81, and 147Sm/144Nd = 0.114 (Table 4). These results can be compared to estimates of the abundances of these elements in the upper crust based on a mass balance of the sedimentary reservoir and on direct measurements of igneous rocks. The abundances of these four elements in the sedimentary mass should roughly reflect their abundances in primary crustal materials exposed to weathering. Inputs of Sm and Nd to the sedimentary mass from submarine weathering of basalt are probably minor. For Sr, submarine weathering of basalt affects the 87Sr/S6Sr of the sedimentary reservoir but does not change the abundance of Sr [3]. For Rb, submarine weathering of basalt acts as a sink for approximately 3% of the bulk river flux [6] but is similar to the dissolved load flux of Rb. The proportions of the major rock types in

262 TABLE 5 Upper crustal averages for Rb-Sr and Sm-Nd from various methods Rb (ppm)

Sr (ppm)

Sm (ppm)

Nd (ppm)

S7Rb 86Sr

147Sm i~Nd

Sedimentary reservoir mass balance a

Carbonate Sandstone Shale

3 60 140

610 20 300

0.9 3.1 6.2

4.8 16.4 35.0

0.014 8.64 1.34

0.113 0.114 0.107

Average sediment b

111

294

5.1

28.4

1.09

0.109

110

316

4.5

26.0

1.00

0.105

95

337

5.7

30.3

0.81

0.114

Igneous rock composites

Canadian Shield ~ Upper crust estimated from river water

a Rb and Sr data for sedimentary reservoirs from [42]. Sm and Nd data from [34]. b Composition of total sedimentary mass (15% carbonate + 11% sandstone + 74% shale) from [45,46]. Data from [47].

the s e d i m e n t a r y mass are --15% c a r b o n a t e s , 11% sandstone, a n d ~ 74% shale [45,46]. Estimates of the average c o n c e n t r a t i o n of Rb, St, Sm, and N d in each of these s e d i m e n t a r y rock types are presented in T a b l e 5 and the resulting average s e d i m e n t c o n c e n t r a t i o n s are also given in this table. These values agree to _+ 25% with the river water b a s e d estimates p r e s e n t e d above. These values are also very similar to direct m e a s u r e m e n t of Rb, Sr, Sin, a n d N d c o n c e n t r a t i o n s on igneous rock composites from the C a n a d i a n Shield [47] shown in T a b l e 5. The T ~ d model age of the C a n a d i a n Shield c o m p o s i t e s ~ 2.9 G a [1] which is greater t h a n average u p p e r crust; thus the 1478m/144Nd a n d 87Rb/86Sr of these c o m p o s i t e s are lower a n d higher, respectively, than average u p p e r crust (Fig. 6). T h e average end and T ~ of m a j o r river water s u s p e n d e d m a t e r i a l is a p p r o x i m a t e l y - 1 0 . 6 a n d a b o u t 1.6 Ga. These averages reflect the p o r t i o n of the crust exposed to weathering. Because of the preferential erosion of y o u n g orogenic belts relative to old stable c r a t o n i c regions, w e a t h e r e d crust is generally y o u n g e r than average u p p e r crust for most areas. This is d e m o n s t r a t e d in Fig. 7, where the m e a s u r e d Tffd of large rivers is --- 76% of the value in average b a s e m e n t material. U s i n g this difference a n d the river water s u s p e n d e d m a t e r i a l average TE~d of 1.6 Ga, we o b t a i n 1 ~ d = 2.1 G a for the u p p e r crust. The 147Sm/t44Nd of river w a t e r s u s p e n d e d m a t e r i a l is e q u a l to the

147Sm/144Nd of the u p p e r crust since no significant f r a c t i o n a t i o n of Sm relative to N d takes place d u r i n g weathering. U s i n g ~47Sm/144Nd---0.1140, the average eyd of u p p e r crustal m a t e r i a l is -- 16.7. This average value t o g e t h e r with the relationship b e t w e e n c y d a n d 87Sr/86Sr in the u p p e r crust shown in Fig. 3b i m p l y an SVSr/86Sr value of 0.716 for the u p p e r crust. Values of S m - N d a n d R b - S r isotopic p a r a m e ters in the total crust have been o b t a i n e d from mass b a l a n c e c o n s i d e r a t i o n s for the crust a n d d e p l e t e d m a n t l e system (e.g. [19,33,40]). The c~a of u p p e r crust e s t i m a t e d from river waters ( - 16.7) is similar to the value of eyd = - 1 4 . 2 [40] for the total crust o b t a i n e d from mass b a l a n c e calculations. This suggests that the S m - N d systematics of upper, lower, a n d total crust are a p p r o x i m a t e l y equivalent. However, the e s t i m a t e d values of 87Sr/86Sr a n d 87Rb/86Sr in the u p p e r crust are significantly greater than the values of 87Sr/86Sr = 0.7082 a n d 8VRb/86Sr = 0.23 in the total crust [40] o b t a i n e d from mass balance considerations. This is in a g r e e m e n t with the well established e n r i c h m e n t of R b in the u p p e r crust relative to lower crust. 4.5. Implications for the use of sedimentary rocks as a tracer o f crust-mantle evolution

T h e results p r e s e n t e d a b o v e a n d in Fig. 5 indicate that the S m - N d systematics of river water p a r t i c u l a t e m a t e r i a l do i n d e e d reflect the sys-

263

tematics of the source rocks in a given drainage basin. However, the results (Fig. 7) also suggest that typical sediments will have lower T~M Na ages than average crust. It is clear that an erosion law is needed to relate the model ages of sediments to those of the average crust. Following All~gre and Rousseau [7], we consider erosion products as a mixture of two components, one contributed from young orogenic areas of mass fraction x and age = t, and one contributed from older crustal segments of age To . Assuming constant concentrations of Sm and N d in crustal materials:

= xt + (1 - x ) r 0

(5)

where Tsk is the Nd model age of river water suspended material. Similarly, the entire continent can be considered a two-component mixture of older (age = To) and younger (age = t) materials: rcont =yt + (1 - y ) r o

(6)

The erosion coefficient K is the key parameter which relates TsL to T~ont in this model and is given by:

K

x/(1-x) y/(1 -y)

(7)

F o r N o r t h A m e r i c a today, TsL/Tcon, = 1.80/2.36 = 0.76. Thus 24% of suspended material from North America is derived from young orogenic areas today if t = 0. Almost all continental rocks have a positive depleted mantle model age, including recent volcanics from island arcs (e.g., Japan and the Philippines). If a more reasonable value for t of 0.5 Ga is assumed, then 30% of suspended material is derived from young orogenic areas. To calculate the erosion coefficient K, an estimate of y is needed. The exact value is not well known; however, a reasonable estimate of the proportion of the North American continent composed of young orogenic areas is 0.16 [32]. This corresponds to K = 2.3 for North America today. All6gre and Rousseau [7] modeled the evolution of the crust from shale Sm-Nd data using K = 2, 4, and 6. For K = 2, they obtained a mean age for the G o n d w a n a continent of 1.93 Ga. This is within error of the mean age of total crust of ~ 2.0 G a calculated from Sm-Nd data on the depleted mantle [40], and suggests that a value of K = 2 - 3 may have been applicable through much of earth's history.

In the above discussion we have assumed that the concentration of Nd is constant in crustal material. In reality, N d concentrations are partially dependent on age, with high Nd concentrations (40-50 ppm) in the suspended material of rivers draining Archean rocks, and low N d concentrations (15-30 ppm) in rivers draining young orogenic areas. This factor would tend to bias model ages of mixtures of young and old materials toward greater T~M. Nd In addition, the systematic variation of S m / N d in river water suspended loads results in a non-equivalence of mean age and Nd model age. Nd model ages of two-component mixtures are biased away from the mean .age toward the component that is most greatly fractionated in S m / N d relative to depleted mantle, or toward the older component. However, these effects should generally be small relative to the bias toward lower sediment model ages caused by preferential erosion of younger rocks relative to older rocks.

Acknowledgments We wish to thank Jerry Wasserburg and Stan Hart for their detailed reviews that led to major improvements in this paper. We also thank Jean Titilah for editing the manuscript, and Steve Dobos for collecting the samples from Australia. This work was supported by N S F grants E A R 82-06954 and E A R 85-11912, and NASA grant NAG-9-90.

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