E L S E V I E R Chemical Geology 126 (1995) 65-75

CHEMICAL GEOLOGY

tNCLUOING

ISOTOPE GEOSCIENCE

Hydration and dehydration of oceanic crust controls Pb evolution in the mantle

C. Chauvel a ,b ,* , S.L. Goldstein a, A.W. Hofmann a

a Max-Planck-lnstitutfiir Chemie, PosOCach 3060, D-55020 Mainz, Germany Ggosciences Rennes, CNRS UPR 4661, Universitd de Rennes 1, F-35042 Rennes Cedex, France

Accepted 12 January 1995; accepted 31 July 1995 after revision

Abstract

The position of the Pb isotopic compositions of mid-ocean ridge basalts (MORB) to the right of the geochron has long puzzled geochemists. Uranium is more incompatible than Pb during mantle melting, and the mantle source of MORB, being depleted in incompatible elements, should have low Pb isotope ratios and plot to the left of the geochron. This has been called the "lead paradox". MORB have a second peculiar characteristic: Pb concentrations are drastically depleted relative to other elements of similar compatibility such as Ce or Nd. We suggest that both characteristics can be explained by preferential mobility of Pb during hydrothermal alteration of the oceanic crust associated with sea-floor spreading, and subsequent dehydration during subduction. A portion of this Pb migrates into the mantle wedge and is then added to the continental crust via arc magmas. Parts of the Pb-depleted recycled oceanic crust are stored at some deep level in the mantle and eventually become the source of ocean island basalts, the rest mixes into the mantle to become the MORB source. This model is evaluated quantitatively and special attention is given to the evolution of the Ce/Pb ratio in the depleted mantle from the beginning of Earth history to the present.

1. Introduct ion

The partitioning of most elements between the man- tle and the continental crust over Earth history closely parallels their relative incompatibility during mantle melting. This is reflected in long-lived radioactive decay systems such as Rb-Sr, Sm-Nd and Lu-Hf. Rb, Nd and Hf are more incompatible than Sr, Sm and Lu, respectively, and mantle-derived rocks generally have low 878r/86Sr, and high 143Nd/ln4Nd and 176Hf/177Hf

compared to the continents, providing evidence that the depletion process has operated for much of Earth his- tory. This is not the case for the U-Pb system. During mantle melting, U acts as a highly incompatible ele- ment like Th, Ba and Rb, while Pb acts as a moderately

* Corresponding author.

0009-2541/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved S S D 1 0 0 0 9 - 2 5 4 1 ( 9 5 ) 0 0 103-4

incompatible element like Ce or Nd (Hofmann et al., 1986; Newsom et al., 1986; Hofmann, 1988). There- fore, Pb isotope ratios of mantle-derived rocks would be expected to reflect a time-integrated reduction of U/ Pb. Fig. I, a compilation of 2°6pb/2°4Pb and 2°7pb/ Z°4Pb ratios in mid-ocean ridge basalts (MORB), shows that this is not the case. The two geochrons correspond to present-day Pb isotope ratios that evolved from primordial Pb values in a closed system over 4.55 or 4.50 Ga, corresponding to the Pb age of the Earth. Samples plotting on either side of these lines had a more complex history involving stages with low or high/z-values (the 238U / 2°4pb ratio). If U-Pb decay simply reflected the same processes as the other isotope systems, MORB should lie to the left of the geochron appropriate for the age of the Earth. The position of MORB near or to the right of the geochron requires

66 C. Chauvel et al. / Chemical Geology 126 (1995) 65-75

15.8

I5.7

, '~ 15.6

15.5

~ 15.4 ¢'-I

antic

VIORB

15.3

17 17.5

I I 18 18.5 19 19.5 20 20.5

206Pb/204Pb

Fig. 1.2°6pb/2°4pb vs. 2°7Pb/Z°apb ratios in mid-ocean ridge basalts showing that most samples plot on or to the right of 4.50- and 4.55- Ga geochrons.

that their source had a U/Pb ratio that was similar to or higher than the bulk Earth value for much of their evolution. This has been termed the "Pb paradox" (All~gre, 1969).

The special behavior of the U-Pb system can also be seen in their elemental abundances. For most element pairs with similar silicate solid/melt partitioning behavior, the ratio is the same in the bulk Earth, the depleted mantle, and the continental crust (Jochum et al., 1983; Jochum et al., 1986). Oceanic basalts have been shown to have Ce/Pb ratios much higher than the bulk Earth ( Hofmann et al., 1986 ) while the continental crust has lower Ce/Pb than bulk Earth, indicating that Pb has migrated out of the mantle and into the crust compared to Ce, despite similar melting behavior. In this paper, we propose a model first presented by Chau- vel et al. (1991), which accounts for the relatively radiogenic Pb isotopic compositions of MORB as well as their high Ce/Pb ratios. Other aspects of this problem have been treated recently by Miller et al. (1994), who evaluated the Pb budget during subduction and the Ce/ Pb mass balance between the continent and the mantle based on Ce-Pb data and Pb isotope ratios from the Aleutians, and by Peucker-Ehrenbrink et al. (1994), who estimated the global transfer of hydrothermal Pb from oceanic crust into metalliferous sediments using Pb isotope and concentration data from deep-sea cores.

2. Previous models

Of the models that have been proposed to account for the Pb isotopic compositions of depleted mantle,

"Pb pumping" - - the migration of Pb into the core - - is the best known. Vollmer (1977), Vidal and Dosso (1978) and All~gre et al. (1982) suggested that the core continued to grow over an extended period during the history of the Earth, and that loss of Pb to the core led to a progressive increase of the U/Pb ratio in the residual mantle. This model has been questioned by Newsom et al. (1986) who used chalcophile and sid- erophile element ratios in oceanic basalts to suggest that the growth of the Earth's core had no apparent influence on the compositions of young rocks and was not responsible for the high U/Pb ratios that generated the high Pb isotopic compositions of oceanic basalts. Anderson (1982) and Devey et al. (1990) presented the alternative argument that the MORB source is always contaminated to some extent by the ocean island basalt (OIB) source such that ocean ridge basalts reflect neither the Pb isotopic composition (Anderson, 1982) nor the Ce/Pb ratio (Devey et al., 1990) of the pristine MORB source. These attempts to explain the problem miss an essential point relating to the overall mass balance: even if the MORB isotope and trace- element ratios are biased by the ubiquitous presence of enriched mantle material, it is the average composition of the depleted mantle and not that of a more-or-less hypothetical end-member composition that is crucial for the purpose of mass balance. We therefore must find another process to account for the behavior of the U-Pb and Pb isotopic systems, and to explain the high Ce/Pb ratios in the mantle.

3. Exogenous partition of Pb between mantle and continental crust

In this paper we develop and quantify early sugges- tions by Newsom et al. (1986) than the peculiar behav- ior of the U-Pb system results from processes active during the generation, alteration, subduction and recy- cling of oceanic crust. The essence of the model is that Pb migrates more efficiently that Ce from the interior of the oceanic crust to the surface during hydrothermal alteration, where it concentrates in sulfides and oxides and is thereby separated from other trace elements both spatially and mineralogically. During subduction, the sulfides and oxides break down, and the Pb dissolves in the fluid phase which then migrates into the overlying mantle wedge. Additional oceanic Pb might also be lost

C. Chauvel et al. / Chemical Geology 126 (1995) 65-75 67

from the oceanic crust through dehydration during sub- duction. Mantle Pb is thus removed from the recycled oceanic crust and enriched in the mantle region that melts to form island arc magmas and eventually con- tinental crust. We suggest, therefore, that the migration of Pb from mantle to continental crust is governed by secondary, non-magmatic processes, and that the Pb enrichment in the continental crust is due to enrichment in the mantle source and not to a peculiarity in the geochemical behavior of Pb during partial melting in the continental environment.

3.1. Hydrothermal alteration of mid-ocean ridge basalts

207pb 204Pb

15.6

15.5

15.4

15.3 17.5

• i : "

l I" I I I

18 18.5 19 19.5 206Pb/204pb

The volume of oceanic crust affected by hydrother- mal alteration at ocean ridges has been estimated through stable isotope studies, particularly in ophiolitic sequences. These studies show that seawater penetrates downwards into the upper portion of the oceanic crust to reach the bottom part of the sheeted dikes, where it is heated to ~400°C (Gregory and Taylor, 1981; Cocker et al., 1982). The fluids then ascend adiabati- cally through the crust to erupt on the sea floor at tem- peratures of 300-400°C. Cooling and mixing with seawater results in the deposition of sulfides and oxides. The high-temperature alteration occurs only close to the ridge and affects about half of the total thickness of the crust. When the plate migrates to a few tens of kilometers from the ridge axis, low-temperature fluids ( ~ 10°C) alter only the surface of the oceanic crust (Nicolas, 1989).

The hydrothermal activity at oceanic ridges is responsible for the leaching of some elements from basaltic crust and the addition of other elements into the altered basalt. Michard et al. (1983, 1985) have shown that U in seawater is added to the basaltic crust during hydrothermal circulation. Other elements such as Na, Rb and K are leached from the basalt and enter seawater (Edmond et al., 1979; Edmond et al., 1982; Michard et al., 1984), but during low-temperature alteration, they are added back to the oceanic crust (Hart and Staudigel, 1982). Although very few studies have addressed directly the effect of hydrothermal alteration on the Pb concentrations of basalts, Chen et al. (1983) and Michard and Albar~de (1985) have shown that Pb concentrations in hydrothermal fluids from vents on the ocean floor are extremely high.

Fig. 2. Pb isotopic compositions of hydrothermal deposits next to ocean ridges. Data from O'Nions et al. (1978), Br~vart et al. ( 1981 ) and Vidal and Clauer ( 1981 ) for the large symbols and from sources too numerous to he all cited for Pacific MORB.

Exhalative sulfides deposited from such fluids, both in modern examples on the ocean floor, and fossil exam- ples in the form of ore deposits, also contain high con- centrations of Pb. Br6vart et al. (1981) have shown that sulfide deposits found next to the East Pacific Rise at 21°N have Pb contents between 700 and 1800 ppm, with Pb isotopic compositions indistinguishable from underlying MORB (Fig. 2). In contrast, sediments and Mn nodules collected in the same area have Pb isotopic compositions significantly different from those of the basalts, intermediate between values in MORB and pelagic sediment (Fig. 2) (Br6vart et al., 1981 ). These data clearly demonstrate that the Pb in the hydrothermal sulfides originates from the underlying MORB crust. Similar conclusions were reached by other studies of sulfides and Mn-oxides from other oceanic areas (Fig. 2) (Dasch et al., 1971; Unruh and Tatsumoto, 1976; O'Nions et al., 1978; Vidal and Clauer, 1981). It seems, therefore, that during hydrothermal alteration, Pb is leached from the oceanic crust and re-deposited within sulfides and oxides that are either concentrated on the ocean floor or disseminated throughout the altered basalt (Styrt et al., 1981). In either case, Pb migrates upward in the oceanic crust and concentrates in phases distinct from those that contain elements such as U, Th, Rb, Sr, and the rare-earth elements (REE).

68 C. Chauvel et al. / Chemical Geology 126 (1995) 65-75

3.2. Selective dissolution of sulfides in subduction zones

When oceanic crust is subducted it heats up, and the phases produced earlier by hydrothermal processes at the ocean ridges break down. Most of the changes involve dehydration: serpentine and chlorite break down at ~ 600°C, amphibole around 700°C and phlog- opite around 1100°C (Tatsumi, 1989). Experimental studies of ore minerals have shown that most sulfides become unstable at T > 500°C (Barnes, 1979) and that the solubility of Pb in aqueous fluids increases with rising temperatures (Wood et al., 1987). The temper- ature in the upper part of subducted oceanic crust increases at a rate of 5-10°C per km of subduction (Hasebe et al., 1970; Honda and Uyeda, 1983; van de Beukel and Wortel , 1987) and reaches a temperature of ~ 500°C as the crust reaches a depth of 50-100 km (Minear and Toks6z, 1970; van de Beukel and Wortel, 1987). As the oceanic plate is subducted, temperatures in the upper part, where Pb is concentrated, increase to the level at which the sulfides break down and the secondary silicates dehydrate. Serpentine and chlorite contain 12-13% H20; amphibole 1-3% H20 and altered oceanic crust 1-3%. The latter figure corre- sponds to ~ 6-18 vol%. Release of this fluid creates a tremendous flux of Pb from subducted crust to over- lying mantle. Thus the liberated Pb is transported in the fluid from oceanic crust into the overlying mantle wedge.

Pb from the subducted oceanic crust is thereby added to the region of the mantle that melts to form arc mag- mas. We suggest that the high Pb contents reported for such rocks are due to source enrichments and not to a difference in chemical behavior of Pb during oceanic magmatism and continent-related magmatism. Similar conclusions have been reached by Mil ler et al. (1994) who demonstrated that in the Aleutians, Pb is enriched relative to Ce by a factor of 5 -10 compared with oce- anic basalts, and that 40-60% of the Pb is mantle derived. They also showed that the compatibil i ty of Pb is similar to that of Ce in the magmas and lower than those of highly incompatible elements like Rb and U; therefore, a non-magmatic process must transport man- tle Pb to the arc magma source prior to melting. This preferential enrichment in Pb relative to Ce is seen in low C e / P b ratios of island arc magmas, as shown in Fig. 3.

30

20

MORB + OIB

I s l a n d a r c v o l c a n i c s

0 0 50 100

Ce (ppm)

i

150 200

Fig. 3. Ce/Pb in island arcs vs. Ce concentrations in the lavas show- ing the constancy of the ratio independent of the absolute concentra- tion of the trace element in the rocks. Note the low value of Ce/Pb in arc magmas relative to that of MORB and OIB and that of primitive mantle (~ 11 ). Data from Oversby (1972), Hawkesworth et al. (1977), McCulloch and Perfit ( 1981 ), Cohen and O' Nions (1982), Barreiro (1983), Thirlwall and Graham (1984), White and Patchett (1984), White and Dupr6 (1986), Stern et al. ( 1991 ) and Miller et al. (1994).

The products of arc volcanism are later reprocessed into continental crust, which ends up having an abun- dance of Pb greater than those of other elements with similar compatibil i ty during mantle melting. The high concentration of Pb of the continental crust (Taylor and McLennan, 1985; Hofmann et al., 1986), relative to Ce and Nd, results from source enrichment in the mantle wedge at convergent plate margins (Mil ler et al., 1994), and not because the behavior of Pb during crust-formation events is different from that during magma genesis at ridges and in oceanic islands (Hof- mann et al., 1986; Hofmann, 1988).

The dehydrated, Pb-depleted oceanic crust is sub- ducted deeper into the mantle. Portions may be stored for a period in a boundary layer, to be reactivated later as the source of oceanic island magmas (Hofmann and White, 1982; Weaver et al., 1987; Weaver, 1991 ; Chau- vel et al., 1992), but most mixes into the surrounding depleted mantle and becomes a source of MORB.

4. Evolution of Ce/Pb ratios in the mantle

The explanation presented above has important implications for the evolution of the depleted mantle and continental crust. Preferential loss of Pb from sub- ducted oceanic crust, migration into the mantle wedge,

C. Chauvel et aL / Chemical Geology 126 (1995) 65-75 69

and ultimately to the continental crust, are unidirec- tional processes that change continuously the relative abundances of Pb and other trace elements (e.g., Ce and Nd). Whereas the partitioning of Ce between con- tinental crust and mantle is probably dominated by melting processes, the partitioning of Pb is dominated by the secondary processes. Recycled oceanic crust loses more Pb than Ce, and the residue, which has high Ce/Pb, mixes back into the mantle. The continuous input of such material should result in a gradual increase in Ce/Pb ratios in the mantle.

Few high-quality measurements of Ce/Pb ratios are available for island arc volcanics and continental mate- rials but available data show that they have low Ce/Pb ratios (Fig. 3). The low Ce/Pb ratios cannot be derived from subducted continental sediment alone, but a large portion of the extra Pb which has mantle isotopic ratios, may be derived from subducted oceanic crust (Miller et al., 1994). These findings support strongly our model by showing that the magma source beneath island arcs is enriched in Pb relative to Ce.

4.1. Quantitative model of Ce/Pb ratio of the mantle and Pb concentration of the continental crust

We present here the results of a quantitative model describing the effect that the migration of Pb from subducting oceanic crust to mantle wedge would have on the evolution of depleted mantle and continental crust.

Using estimates for the Ce concentration and Ce/Pb ratio in the continental crust, the total amount of Pb present in the continental crust can be estimated (Hof- mann et al., 1986; Miller et al., 1994). Because Ce and Pb have similar incompatibility levels during mantle melting, the amount of extra Pb in the continents rela- tive to Ce can be evaluated. If this excess is assumed to be the amount of Pb extracted from the oceanic crust through the alteration/subduction processes, it can be used to estimate the amount of oceanic crust that must have been altered and subducted through Earth history to account for the enrichment seen in the continental crust. Miller et al. (1994) made an estimate based on present-day element fluxes to two adjacent volcanoes in the Aleutians. Here we focus on the effects that mobilize Pb at mid-ocean ridges.

REE contents in the continental crust are relatively well constrained and for Ce, concentration estimates

range between 33 and 55 ppm (Taylor and McLennan, 1985; Galer et al., 1989). Ce/Pb ratios in crustal mate- rials are not often measured with high precision but available data scatter close to 5. This provides an esti- mated abundance of 6.6-I 1 ppm for Pb in the conti- nental crust. The amount of excess Pb is therefore 3.6- 6 ppm, based on a bulk Earth Ce/Pb ratio of 11. When applied to the total mass of continental crust (2.15. 1025g) (Galer et al., 1989), the total amount of extra Pb can be calculated as (8-13) . 1019 g.

We can now quantify our model using this value for the amount of Pb lost from the subducted oceanic crust. For the initial calculation we make simplifying assump- tions, and then we discuss the effects of more complex scenarios:

(a) The Ce concentration of the oceanic crust is assumed to be 10 times higher than that of the mantle source region, an estimate in agreement with the con- centrations in mantle and MORB given by Hofmann (1988). The Ce concentration of the mantle source of oceanic crust through time is taken as 1.6 ppm (the primitive mantle concentration) at the beginning of Earth history and decreases linearly through time to reach a present-day value of 1 ppm. This evolution was chosen to reflect the depletion process that affected the mantle due to the formation of the continental crust.

(b) The Ce/Pb ratio is not fractionated during man- tle melting to form oceanic basalts. This means that the Ce/Pb ratio of basaltic melts from the undifferentiated mantle is the same as the bulk Earth ( ~ 11). Pb loss during subduction increases the /x-value in the sub- ducted oceanic crust and the Ce/Pb ratio by a factor of

2. These ratios were chosen because they correspond to the values necessary to model the source of HIMU (high-2°6pb/2°4pb mantle end-member) volcanics (Chauvel et al., 1992). As discussed in Goldstein and Chauvel (1990), the process that creates the HIMU source may also control the Pb isotopic composition of the MORB source.

(c) The effects are calculated for three conditions: (1) the total volume, and the rate of formation and subduction of oceanic crust, have been constant since the beginning of Earth history; (2) the rate has decreased linearly through geological time; and (3) the rate has decreased in a non-linear manner.

(d) The proportion of oceanic crust affected by hydrothermal alteration, on the basis of oxygen isotopic studies (Bowers and Taylor, 1985), is ~ 50%, and we

70 C. Chauvel et al. / Chemical Geology 126 (1995) 65-75

1

0.8

0.6

0.4

"~ O.2

10 20 30 40 50 60 70 Ce (ppm) in cont crust

Fig. 4. Calculated proportion of mantle that must be depleted to account for the Pb present in the continental crust, as a function of Ce concentration in the continental crust. For a Ce content of 33 ppm (as suggested by Taylor and McLennan, 1985) and a Ce/Pb ratio of

5, the proportion of depleted mantle corresponds roughly to the

relative mass of the upper mantle.

assume that Pb is extracted from the same proportion of crust during hydrothermal alteration at ridges. Using this figure and the present-day volume of oceanic crust created per year (20 km 3), the volume of oceanic crust affected by the alteration process would be 109 km 3 Ga - ~ and the total proportion of Pb extracted from the oceanic crust would therefore be ~ 25%. This value is similar to the 29% estimate of Miller et al. (1994) and can be compared with constraints from metalliferous sediments discussed by Peucker-Ehrenbrink et al. (1994). These authors concluded that only ~ 12% ( ___ 8%) of Pb in oceanic crust is leached out of the basalts and redeposited in marine sediments. The remaining Pb might be hosted in sulfides disseminated in the altered basalts.

(e) The proportion of the mantle affected by the process is assumed to be equal to that of the upper mantle. This situation corresponds to two-layer mantle convection. It satisfies a straightforward mass-balance calculation and is consistent with estimates of the Ce/ Pb ratio and Ce abundance of the continental crust. The relationship between Ce /Pb ratio and Ce concentration in the continental crust and the size of the affected mantle is shown in Fig. 4.

The results are listed in Table 1 ; details of the cal- culation are given in the Appendix. It can be seen that if oceanic crust has been generated and recycled to the mantle since the start of Earth history, and Pb has been extracted in subduction zones during the entire period, the upper mantle would currently have a Ce /Pb of 22 (Fig. 5a). This value is lower than the average ratio of

~ 2 5 measured in MORB and OIB (Hofmann et al., 1986; Newsom et al., 1986). The amount of Pb extracted from the subducted material is also too low. If oceanic crust started to be recycled only at the end of the Archean ( ~ 2.5 Ga ago), this would lead to still lower Ce/Pb ratios for the upper mantle.

One way to obtain a Ce /Pb ratio of 25 for the upper mantle (Fig. 5a) is to have higher temperatures and faster crustal production rates during the Archean. To model this, we used recycling rates starting at high values at the beginning of Earth history and decreasing through time to reach present-day production rate. The decrease in oceanic crust production rate has been mod- eled in two ways: (a) a linear model starting at 1.5 times the present-day rate, and (b) a model of rapid and non-linear decrease starting at 5.5 times the pres- ent-day rate (see Eq. A-5 in the Appendix). Both mod- els produce reasonable Ce/Pb ratios for the depleted

Table 1 Results of the mass-balance calculation

T Ce/Pb

(Ga) constant recycling rate high recycling rate in the and Archean

50% 60% ~ 8% linear rapid early altered altered melting decrease decrease

4.50 11.00 I 1.00 11.00 11.00 11.00 4.25 11.38 11.46 11.47 11.54 12.13 4.00 11.83 1 2 . 0 1 12.02 12.17 13.15 3.75 12.30 12.59 12.59 12.82 13.92

3.50 12.79 13.19 13.20 13.50 14.53 3.25 13.30 13.82 13.84 14.20 15.11 3.00 13.82 14.49 14.50 14.93 15.71 2.75 14.37 15.18 15.20 15.68 16.33 2.50 14.94 1 5 . 9 1 15.93 16.45 16.98 2.25 15.53 16.67 16.69 17.24 17.65 2.00 16.15 17.47 17.50 18.06 18.35 1.75 16.79 1 8 . 3 1 18.34 18.90 19.08 1.50 17.45 19.18 19.22 19.76 19.83 1.25 18.14 20.10 20.15 20.64 20.62 1.00 18.86 21.07 21.11 21.53 21.44 0.75 19.61 22.08 22.13 22.45 22.29 0.50 20.39 23.13 23.19 23.38 23.17 0.25 21.20 24.24 24.31 24.33 24.09 0.00 22.04 25.40 25.48 25.30 25.04

Pb" (ppm) 2.59 2.95 3.07 3.10 3.40

Details of the calculations are given in the Appendix. aPb added to the continental crust by the process.

C. Chauvel et al. / Chemical Geology 126 (1995) 65-75 71

L)

30

25

20

15

tO

5

0

(a)

~ ' a ' i ~ ' " . . ~ . 4 + >

. . . . . . . . . . . . p resent rate

- - 60% altered o r 8 + m e l t

l inear deen~ase . . . . rap id early decrease

I I I I

l 2 3 4

T (Ga)

,-~ 0.8

0 . 6 0

0.4

~" o.2

0 0

~.. _ ......... ....~ (b)

....... - \ \ \ - - 8% me l t "%x'~N

- - l inear decrease ~

l 2 3 4

T (Ga)

Fig. 5. a. Evolution of the Ce/Pb ratio of the mantle since the begin- ning of Earth history assuming that only the upper mantle is depleted. The four lines correspond to: (1) constant rate of production of oceanic crust since the beginning of Earth history; (2) alteration of 60% of the oceanic crust (or ~ 8% melt) as well as constant present- day production rate of oceanic crust; (3) linear decrease of the production rate of oceanic crust (starting at 1.5 times present rate); (4) rapid decrease in the volume of oceanic crust (starting at 5.5 times present rate). b. Amount of Pb extracted from the oceanic crust through time. If it is assumed that the concentration of Pb in continental crust material has been constant through time, this value can be directly compared to a continental growth curve.

mantle (Fig. 5a) and lead to similar amounts of Pb extracted from the subducted material (3.1 and 3.4 ppm, respectively, when normalized to the mass of present-day continental crust, Table 1 ). However, the rates of extraction from the mantle differ: Fig. 5a and b shows the evolution of the C e / P b ratio and of the fraction of total Pb extracted through time. A rapid initial decrease in oceanic crust production rate pro- duces a rapid increase in C e / P b and of the amount of

Pb transferred to the continental crust early on, fol- lowed by a slower increase. The linear decrease of oceanic crust production rate leads to a slower increase of the C e / P b and of the extraction of Pb, and corre- sponds to a more linear rate of transport (Fig. 5a and b).

Another way to obtain a C e / P b = 25 for the depleted upper mantle, without rapid recycling in the Archean, is to increase the portion of the altered oceanic crust or to decrease the degree of melting that produces the oceanic crust. Leaching Pb out of 60% of the oceanic crust produces a C e / P b ratio of 25.4 and leads to an acceptable amount of Pb extracted to the continents (6.3.1019 g) (Fig. 5a and b) ; similarly, acceptable values (25.5 and 6.6.1019 g) are obtained if the oceanic crust was produced by lower degree melting ( ~ 8%, which corresponds to an enrichment factor of 13 rela- tive to the source) (Fig. 5a and b) . In both cases, deple- tion of the upper mantle occurs later in the Earth history (Fig. 5a) and the Pb extraction curves mimic an almost linear transport rate to the continental crust (Fig. 5b) .

Clearly, the present-day M O R B - O I B value of Ce / Pb = 25 can be reproduced by the model. If the produc- tion rates of oceanic crust were higher in the early Earth history, the model produces Pb extraction curves that are comparable to early growth of the continental crust. Similarly, early rapid production of oceanic crust leads to a relatively rapid increase of the C e / P b ratio in the depleted mantle. Unfortunately, the difference between the various models cannot be evaluated using existing data because constraints on the C e / P b ratio of the Archean mantle are essentially nonexistent. The few attempts to measure Ce and Pb concentrations on Archean komatiites and basalts have shown that Pb acted as a mobile element since rock emplacement, and that measured C e / P b ratios could not be interpreted as original values (Jochum et al., 1991; Lahaye and Arndt, 1995).

Our model does not include the effects of recycling of continental material into the mantle but we can pre- dict that its influence would require even higher recy- cling rates of oceanic crust, to balance the decrease in C e / P b caused by mixing of low-Ce/Pb crustal material into the h igh-Ce/Pb mantle. Similarly, if the depleted mantle is larger than the upper mantle (Fig. 4) , then the Pb must be extracted from a larger volume of mantle and high sea-floor spreading rates are necessary to bring the C e / P b ratio to 25.

72 C. Chauvel et al. / Chemical Geology 126 (1995) 65-75

15.8

15.7

e~ 15.6

15.5

~ 15.4 e,i

15.3

15.2 15.8

15.7

.~ 15.6 eL

15.5

~ 15.4

15.3

15.2 t 16.5 17

MR

17.5 18 18.5 19 19.5 20

206 p b / 2 0 4 p b

Fig. 6. a°6Pb/2°4Pb vs. 2°7pb/2°4pb diagram showing: (a) loci of two-stage evolution over Earth history of altered recycled oceanic crust, with/z2-values shown in boxes and the age of the fractionation event shown in italics; and (b) mixing arrays between the high-p` components (recycled oceanic crust) and primitive or low-p, sur- rounding mantle. The MORB array can be reproduced by continuous mixing of Pb-depleted recycled oceanic crust (with various ages and/or p`-values) and surrounding depleted mantle.

5. Pb isotopic compositions of MORB

For simplicity, we assume that the depleted mantle would plot on the geochron if the Pb depletion process described here had not operated. The present-day com- position of subducted oceanic crust with various ages can be calculated using a range of/z-values and ages. Results of the calculation are shown in Fig. 6a. They will be explained in more detail in a companion paper being prepared by Goldstein and Chauvel (1995). Var- iations in Pb isotopic compositions due to different/z- values for a given age of the recycled oceanic crust are also shown. Fig. 6a shows that the evolution path of recycled MORB crust coincides with the field of pres- ent-day MORB. Similarly, recycling of oceanic crust depleted in Pb relative to Th will produce Z°8pb/2°4pb

isotopic compositions consistent with those measured in MORB.

Because any recycled oceanic crust will be mixed by convection into the upper mantle, the source of any present-day MORB cannot be pure recycled oceanic crust but must be a mixture of old recycled oceanic crust and peridotite. Mixing trends between peridotite and older recycled crust are represented in Fig. 6b by arrows: they show that any reasonable mixing propor- tion between peridotite and old oceanic crust leads to Pb isotopic compositions comparable to those of pres- ent-day MORB.

6. Conclusions

The high 2°6pb/2°4pb ratios of MORB are due to Pb loss resulting from hydrothermal alteration of the oce- anic crust at mid-ocean ridges, and dehydration of oce- anic crust during subduction, followed by remixing of the Pb-depleted residue back into the mantle. Hydro- thermal alteration of the oceanic crust plays a key role. Pb migrates from basalt to sulfides and oxides at the surface of the oceanic crust during hydrothermal cir- culation near the ridge crest, and then into the overlying mantle wedge during dehydration in the subduction zone. The enrichment of Pb in arc magmas leads even- tually to high Pb concentrations in the continental crust. The Pb-depleted oceanic crust mixes into the depleted mantle, increasing the/z and Ce /Pb ratios in the source of MORB.

Quantitative modeling shows that these processes can account for the contrasting Ce/Pb ratios of depleted mantle and continental crust and for the high average Pb content of continental crust, if the recycling rate was higher during the early history of the Earth.

Acknowledgements

We appreciated discussions with colleagues at MPI and G6osciences Rennes, over the period of 5 years since these ideas were first formulated. We would par- ticularly like to thank Nick Arndt, Steve Galer, Charlie Langmuir and Dan Miller. Comments provided by the reviewers, Marc Chaussidon, CI6ment Gari6py and Terry Plank, helped improving the final version of the

C. Chauvel et al. / Chemical Geology 126 (1995) 65-75 73

manuscript. This work receivea financial support from the French DBT I programme.

Appendix

Results presented in Table 1 are calculated using the following parameters and equations:

The masses of upper mantle (UM) and continental crust (CC) are taken from Galer et al. (1989) :

masscc = 2.149.1025 g (A-I)

massuM = 1.097.10 2 7 - - masscc g (A-2)

The volume of subducted oceanic crust is taken as 2.10 25 c m 3 Ga- ~ assuming present-day parameters for the rate of production of oceanic crust. The evolution of Ce concentration in the depleted mantle is assumed to be linear and changes from 1.6 ppm at the beginning of Earth history to 1 ppm for the present-day concen- tration.

The evolution of the Ce/Pb ratio in the depleted mantle was calculated using 250-Ma increments and the following equation:

(~b)j=ICeA, X%OC,-i+Cec~X(I-%OC,-j)I/

( CeB, x Vo,. CeA, x VO1A ] \(Ce/Pb)B~ ( V o I B + V o 1 A ) + ~ (Vo1B+VOlA)]

Cec × %oc,_j + ~ × ( 1 - ~oc_j) ] (A-3)

where Ce = concentration of Ce in ppm; Pb = concentration of Pb in ppm; Vol = volume of crust created per Ga; and %OCi_i: relative proportion of recy- cled oceanic crust compared to depleted mantle over the time interval i - j , with the following indexes: A=oceanic crust; B=altered oceanic crust; C = depleted mantle; i = initial composition in the inter- val i - j ; and j = final composition in the interval i - j .

Pb extracted from the mantle independently from Ce is calculated using the following equation:

CeAi Pbi_j = (.(Ce-7-~) cl) x VOIBi_j × d (A-4)

where d = average density of oceanic crust (taken here as 2.8 g cm-3).

The Pb extracted from the mantle independently from Ce and over the Earth history results from the sum

of all incremental quantities of Pb and is expressed in terms of concentrations relative to the present-day mass of continental crust.

Ce/Pb ratios of the depleted mantle and excess Pb extracted from the mantle have been calculated using five different models:

(1) Assuming a constant production rate of oceanic crust equal to the present-day production (20 km 3 yr - ~), and an enrichment factor of 10 for the Ce con- centration of oceanic crust relative to that of the depleted mantle. Results are reported in column 1 of Table 1.

(2) Assuming a constant production rate of oceanic crust, an enrichment factor of 10 for Ce between oce- anic crust and depleted mantle, and alteration of 60% of the crust instead of 50%. These results are in column 2 of Table 1.

(3) Assuming a constant production rate of oceanic crust, alteration of 50% of the crust, and a higher enrich- ment factor ( ~ 13) for Ce between oceanic crust and depleted mantle. Results are in column 3 of Table 1.

(4) Assuming an enrichment factor of 10 for Ce between oceanic crust and depleted mantle, alteration of 50% of the crust, and higher production rate of oce- anic crust in the early Earth history. In this model, crust production at 4.5 Ga was 1.5 times the present-day rate and decreased linearly to reach the present-day rate for T= 0 Ga. Results are reported in column 4.

(5) Assuming an enrichment factor of 10 for Ce between oceanic crust and depleted mantle, alteration of 50% of the crust, and higher production rate of oce- anic crust in the early Earth history. In this model, crust production at 4.5 Ga was 5.5 times the present-day rate and decreased using the following relation:

x i = xi - log ( x i ) ( A - 5 )

to reach the present-day rate for T= 0 Ga (where xi =rate at the beginning of the time interval and xj=rate at the end of the time interval). Results are reported in column 5.

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