Earth and Planetary Sctence Letters, 80 (1986) 299-313 299 Elsewer Soence Pubhshers B V, Amsterdam - Pnnted m The Netherlands

[21

Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core

H.E. N e w s o m 1,2,,, W.M. Whi te L3,,, K.P. J o c h u m 1 and A.W. H o f m a n n 1

i Max-Planck-lnstttutfur Chemte, Saarstr 23, 6500 Mamz (FR G) e lnstttute of Meteortttcs and Department of Geology, Unwerstty of New Mexmo, Albuquerque, NM 87131 (U S A )

3 College of Oceanography, Oregon State Umverszty, Coroalhs, OR 97331 (U S A )

Received October 7, 1985, rewsed version received July 29, 1986

We have investigated the hypothesxs that mantle Pb isotope ratxos reflect continued extraction of Pb into the Earth's core over geologic ttme The Pb, Sr and Nd isotopic composmons, and the abundance of slderophde and chalcoplule elements (W, Mo and Pb) and incomparable hthoplule elements have been determined for a state of ocean island and rind-ocean ndge basalt samples Over the observed range in Pb isotopic composlttons for oceamc rocks, we found no systemalac variation of slderophale or chalcoplule element abundances relative to abundances of slmalarly incompatible, but hthoplule, elements The bagh sensmwty of the Mo/Pr rauo to segregataon of Fe-metal or S-rich metalhc hqmd (sulfide) and the observed constant Mo/Pr ratio rules out the core formatxon model as an explanataon for the Pb paradox The mantle and crust have the same M o / P r and the same W / B a ratios, suggesting that these ratios reflect the ratio m the Earth's pnrmlave mantle

Our data also m&cate that the Pb/Ce ratio of the mantle is essentxally constant, but the present Pb /Ce rauo m the mantle (=- 0 036) is too low to represent the pnnutlve value (---- 0 1) derived from Pb isotope systematacs Hagher Pb /Ce rahos m the crust balance the low Pb/Ce of the mantle, and crust and mantle appear to sum to a reasonable terrestrial Pb/Ce ratio The constancy of the Pb /Ce ratio m a wade variety of oceamc magma types from chverse mantle reservoirs m&cates tlus ratao is not fracttonated by magmatac processes Tlus suggests crust formation must have revolved non-magmatlc as well as magmatlc processes Hydrothermal activity at rind-ocean ndges may result m s~gmficant non-magmatlc transport of Pb from mantle to crust and of U from crust to mantle, producing a lug, her U / P b ratio m the mantle than m the total crust We suggest that the lower crust is baghly depleted m U and has unra&ogemc Pb isotope ratios whach balance the ra&ogemc Pb of upper crust and upper mantle The differences between the Pb/Ce ratio m se&ments, ttus ratio m pnnuttve mantle, and the observed ratio m oceamc basalts preclude both sediment recycling and nuxang of pnnuuve and depleted reservoirs from being important sources of chermcal heterogeneltles m the mantle

1. Introduction

M o s t o c e a n i c b a s a l t s h a v e 143Nd/ /144Nd a n d

176Hf//177 H f r a t i o s t u g h e r t h a n t h o s e in c h o n d n U c

m e t e o r i t e s [ 1 - 3 ] . P r e s u m i n g t h e E a r t h h a s

c h o n d n t l C S m / N d a n d L u / H f ra t ios , we r e f e r

t h a t t h e m a n t l e h a s b e e n d e p l e t e d i n i n c o m p a t i b l e

e l e m e n t s s u c h as N d a n d H f r e l a t i ve to S m a n d

* Present addresses H E Newsom, Institute of Meteonlacs and Department of

Geology, Umverslty of New Mexaco, Albuquerque, NM 87131, U S A

W M Wlute College of Oceanography, Oregon State Umverslty, Corvalhs, OR 97331, U S A

Lu. Sr I s o t o p i c c o m p o s m o n s o f m a n t l e - d e r i v e d

r o c k s a l so r e f l ec t t h i s d e p l e t i o n o f i n c o m p a t i b l e

e l e m e n t s m t h e m a n t l e , w h i c h is t h o u g h t to r e s u l t

f r o m e x t r a c t i o n o f a p a r U a l me l t .

B e c a u s e o f t he v o l a t l h t y of Pb , t h e U / P b r a t i o

o f t h e E a r t h c a n n o t b e a s s u m e d to b e c h o n d r i t l c .

H o w e v e r , i f t h e E a r t h is 4.55 b y. o l d a n d h a d

i n i t i a l P b i s o t o p e r a t i o s e q u a l to t h a t o f p r i m o r d m l m e t e o n t l c l e a d [4], t he 2 ° 7 p b / 2 ° 4 p b a n d 2 ° 6 P b /

2°4pb r a t i o s o f t h e b u l k E a r t h m u s t fa l l o n a 4 .55

b .y . l s o c h r o n p a s s i n g t h r o u g h t h e p r i m o r d i a l P b

~so top~c c o m p o s i t i o n . M o s t m a n t l e - d e r i v e d

v o l c a m c r o c k s p l o t to t h e t u g h - 2 ° 6 p b / 2 ° 4 p b s ide

o f t h i s l ine , k n o w n as t h e g e o c h r o n , a n d h e n c e

m & c a t e t he U / P b r a t i o o f t h e m a n t l e h a s in -

0012-821X/86/$03 50 © 1986 Elsewer Science Pubhshers B V

300

creased at some point in the past. Since U is believed to be more incompatible than Pb, the opposite would be predicted from Nd, Hf and Sr isotope systematlcs.

To explain this so-called "lead paradox" a number of authors have suggested that the Earth's core has grown through time [5-8]. Because of the chalcophile character (that is, having an affinity for sulfide phases) of Pb, extraction of a sulfide phase from the mantle into the core would deplete the mantle m Pb, and increase the U / P b ratio An early study of the depletion of Pb by core forma- tion was made by Oversby and Rlngwood [9]. We have tested this hypothesis of core growth by examamng the depletion of chalcophde elements and siderophde (having an afflmty for Fe-metal) elements m the mantle. The abundance of slderophlle and chalcophile elements provides a record of core formation and accretion for the Earth [10,11]. Because Mo is more chalcopinle than Pb, if mantle Pb isotopic variations reflect extraction of vanable amounts of a sulfide phase into the core, Mo abundances should correlate with Pb ISOtOpic compositions. Mo and W are also more slderophale than Pb so that variable extrac- tion of metalhc Fe into the core should produce a correlation of Mo and W abundances wath Pb isotopic compositions.

2. Sampling, analytical methods, and results

Samples were selected primarily to cover the widest range m Pb isotopic compositions. Ad- ditional sampling crltena were: a large range in 87Sr/86Sr and 143Nd/144Nd ratios, a wide geo-

grapinc distribution, and sample freshness All mad-ocean ridge basalt (MORB) samples were carefully hand-picked and cleaned glasses and five of the ocean island basalt (OIB) samples are from eruptions within the last 38 years (F-33, TR-1, RE24-1, ML3B and KL-2). The remainder are fresh, young lavas, with the exception of the St Helena samples, which are older and somewhat weathered.

Mo and W in the ocean island basalts (sample size 0.1-0.15 g) were deternuned using a metal- silicate extraction technique together with neutron activation analysis [12] A correction was made to Mo for the 99Mo produced by induced fission of 235U using U concentrations deternuned by iso-

tope dilution For the new Mo determinations on OIB's (Table 1) less than 35% of the activity came from fission induced Mo. Analytical uncertainty for Mo is less than 10% for all but sample F-33 winch has a large error of 30% Analytical uncer- tainty for W is less than 10% for all of the ocean island samples, while two of the MORB samples have errors between 10 and 20% (K10a and K73a)

The remalmng elements were analyzed by a combination of thermal ionization and spark source mass spectrometric isotope dilution. K, Rb, Cs, Sr, Ba, U, Pb and rare earth elements (REE) except Pr and Ho were analyzed by thermal ioni- zation isotope dilution [13,14] Hf and Zr were determined using a spark source isotope dilution method [15,16] Many of the elements measured by thermal lomzation were also measured by spark source, with excellent agreement between the two methods Mono-lsotoplc elements Y, Nb, Pr and Ho were determined by conventional spark source mass spectrometry using the elements determined by isotope dilution as internal standards and calibration with geological samples Analytical un- certaanty ranges from 0.5% to 3% for all elements except Y and Nb, for winch it is better than 5%. Isotopic ratios were deterrmned using Flnlgan MAT 261 mass spectrometers and methods de- scribed by White and Patchett [13] and White and Dupr6 [14]

Analytical results are listed in Table 1 The extremely wide range in Pb isotopic compositions (Fig. 1) represents virtually the entire range ob- served in oceanic basalts. Sr and Nd isotope ratios also show an extreme range Tins range m isotopic compositions Indicates the mantle sources of these basalts have experienced widely differing hlstones.

Rare earth patterns are shown in Fig. 2 Again an extreme range in patterns is observed, in part reflecting the varying source compositions, but also reflecting different degrees of partial melting and extents of fractional crystaUlzation.

3. Constant trace element ratios in oceanic basalts

While Pb, Mo and W are chalcophile a n d / o r siderophlle, in silicate systems they behave as in- compatible elements so that concentrations of these elements will be affected by magmatIc processes such as partial melting and fractional crystalhzatlon Thus, absolute concentrations of

(Q) 159

158 t~ o_

,d- o 157 cN 43 o.. r,,, 156

o c~

155

15¢ 17

' " ' l ' ' " l ' " ? " " I . . . . I ' ' " l . . . . I "

DEPLETED~ENRICHED , / ~ N s t Helena\

! So r n o : ~ ° r e ~

# / J

, . . . . , . . . . , , ,

0 17 5 18 0 18 5 19 0 19 5 20 0 20 5 21 0 2 0 6 p b / 2 0 4 p b

(b) 05134[_~.,q ' I . . . . I . . . . I ' ' ' I . . . . I ' ' " ~ . J

d ~ " ~ DEPLETED "I z os,3o r

051281 StF Helena ~ i . _ Azorea/-t'~..-~ _ __ ~ ' 3 1 1 . . . . : 1

f 05126F -

, , , , , , . . . . , . . . .

0 702 0.703 0.704 0.705 Q706 0707 0708 87Sr /86Sr

Fig 1 (a ) L e a d isotope diagram for the U-Pb system, illustrat- ing the evoluuon of the Pb isotopes for the oceamc rocks m a

reservoir that became ennched m the U / P b ratio at some Ume after the formatmn of the Earth The contrasting evolution of P b m an ennched reservoir versus the evolution of Nd and Sr

m a depleted reservoir (Fig lb) is referred to as the lead paradox (b) N d a n d S r ISOtOpic data for mid-ocean ridge basahs (MORB) and ocean island rocks (Table 1) Most of the data suggest that the Nd and Sr ]sotopes reflect a time-aver- aged evolution in a depleted reservmr

these elements do not provide unambiguous infor- mation about metal or sulfide segregation. We therefore need to take account of the effects of magmatlc fractlonatlon.

Relative mcompatlbihty of elements can be at least qualitatively detertmned. Our approach is to seek elements which have constant ratios to each other [10,17,18]. If for two incompatible elements A and B, the ratio A / B is constant over a wide range of concentrations then these elements may be considered equally incompatible, that is, to

301

500

100

0") w 5 0

123 z o n- (.2

~< 10

[] HOWOll • MORB

St Helena gQ'x %.l~l~x o Tr,ston

• Samoa

1 I I I I I I I I I I I I I I I LaCe PrNd SmEu Gd Tb Dy Ha ErTmYb ku

Fig 2 Rare earth element patterns for the samples m this study (Table 1) NoUce the large range m fractlonatlon for the samples

have similar s o l l d / l i q m d bulk parutxon coeffi- cients. If the ratio A / B increases as the concentra- tion of A and B increase, then A is more incom- patible than B and wee versa.

Two examples of constant trace d e m e n t ratios demonstrated by Hofmann and White [17] are the R b / C s and B a / R b ratios. Their data was highly biased toward MORB, including only a few ocean island representatives, namely from the Galapa- gas, the Azores and Hawaii. Our new results yield average values of B a / R b = 1 1 . 5 _ 1.7 ( l o ) and C s / R b -- 10.6 × 10 -3 _+ 2.0 in excellent agreement with the results of Hofmann and White [17] ( B a / R b = 11.55 _+ 0.17, C s / R b = 12.22 × 10 -3 + 0 23). This shows that these constant ratios extend even to islands with "ennched" sources such as Tristan de Cunha and Samoa.

Another example of a constant trace element ratio is K / U . For our data, we fred an average K / U = 12,200 _+ 4000 ( lo ) , which agrees with the average K / U = 12,700 of Jochum et al. [18] for

302

TABLE 1

Analytical results (concentrations m ppm)

Sample

Location

K10A-D33A K62A-D143G K71A-D130H K73A-D123H AII93-11-103 3095 StH 102 StH 2926 MORB NMNHl13716 NMNH109984 NMNH99653 Pacific Pacific Pacific Pacxfic Indaan Indmn MORB St Helena St Helena 20°36'S, 2°37'N, 0°44'N, 1°45'N, 24°59'S, 114o2'W 95o17'W 85o35'W 85°10'W 79°1'E

K 1160 1220 1160 155 641 641 19600 11800 Rb 1 33 3 28 3 07 0 114 0 700 0 690 54 8 27 2 Cs 0 015 0 036 0 028 0 001 0 006 0 007 0 263 Sr 92 6 82 2 68 3 60 7 99 4 99 4 770 665 Ba 12 3 32 0 23 5 1 41 8 82 8 73 584 369 Hf 7 15 7 56 Zr 227 184 42 0 86 6 96 7 298 Nb 5 93 6 48 6 83 130 73 2 U 0 106 0 115 0 144 0 008 0 034 0 038 2 51 1 45 Pb 0 684 0 426 0 561 0 168 0 514 0 505 4 74 2 83 Y 69 1 40 3 63 9 25 9 32 6 34 5 33 2

La 6 93 4 50 6 56 0 972 2 80 2 79 81 6 49 7 Ce 23 1 12 8 20 7 3 75 9 12 9 13 160 103 Pr 4 31 2 06 3 39 0 816 1 70 1 68 19 3 12 2 Nd 22 8 10 7 19 1 4 81 9 08 0 10 67 8 49 5 Sm 7 89 3 59 6 67 2 01 3 28 3 29 12 1 10 2 Eu 2 50 1 24 2 11 0 822 1 23 1 22 3 60 3 28 Gd 10 9 5 06 9 50 3 17 4 72 4 69 9 95 9 09 Dy 13 0 6 25 11 6 4 06 5 70 5 67 7 68 7 20 Ho 2 89 1 42 2 61 0 911 1 26 1 26 1 48 1 26 Er 8 46 4 07 7 55 2 63 3 57 3 59 3 83 3 40 Yb 7 94 3 96 7 28 2 53 3 33 3 35 3 29 2 73 Lu 1 19 0 608 1 10 0 389 0 506 0 493 0 398 W 0 022 0 065 0 062 0 002 0 010 0 007 1 25 0 492 Mo 0 689 0 578 0 627 0 181 0 176 0 480 5 62 3 25

87Sr//86Sr 0 702463 0 702825 0 702543 0 702468 0 703035 0 703031 0 702960 0 702850 143Nd/t44Nd 0 513161 0 513041 0 513101 0 513198 0 513083 0 513072 0 512842 0 512871 2°6pb/2°4pb 18 321 18 744 18 574 18 287 17 325 17 315 20 816 20 820 2°7pb/2°4pb 15 484 15 562 15 515 15 481 15 456 15 443 15 778 15 801 2°spb/2°4pb 37 798 38 566 38 132 37 816 37 287 37 251 40 072 40 133

M O R B b u t d i s p l a y s c o n s i d e r a b l y g r e a t e r sca t t e r .

D e v l a U o n s f r o m t h e J o c h u m et al. v a l u e a p p e a r to

b e c o n s i s t e n t w i t h ~sotope g e o c h e m i s t r y .

N e w s o m a n d P a l m e [10] f o u n d t h a t W / U a n d

M o / N d r a t i o s m M O R B a n d c o n t i n e n t a l b a s a l t s

w e r e u m f o r m . O u r r e s u l t s m d l c a t e a s h g h t l y b e t t e r

m a t c h ( m o r e s i m i l a r r a t i o s ) o f W to B a a n d o f M o

to t h e h g h t r a r e e a r t h Pr. T h e W / B a r aUos ( ave r -

age o f 1.61 × 10 - 3 ) a re w i t h i n a f a c t o r o f 4 o v e r a

10 3 c o n c e n t r a U o n r a n g e a n d t h e M o / P r r a t i o s

( a v e r a g e o f 0 .227) a re w i t h i n a f a c t o r o f 4 o v e r a

10 2 c o n c e n t r a t i o n r a n g e . S l igh t p o s i t i v e s lopes o n

p l o t s o f W / B a vs B a ( n o t s h o w n ) , a n d M o / P r vs.

P r (Fig . 3a ,b) , sugges t t h a t W m a y b e s l i gh t ly

m o r e l n c o m p a U b l e t h a n B a a n d M o s h g h t l y m o r e

i n c o m p a t i b l e t h a n Pr. H o w e v e r , a p l o t o f M o / C e

vs. C e h a s a n e g a t i v e s lope , i n d i c a t i n g M o is n o t

as i n c o m p a t i b l e as Ce. I n a d d i t i o n to a n a l y t i c a l

e r ro r , t h e s c a t t e r i n M o / P r a n d W / B a r m g h t

r e f l ec t m i n o r d i f f e r e n c e s m b u l k p a r t i t i o n coe f f i -

c i e n t s w h i c h in t u r n a u g h t b e d u e to v e r y m i n o r

su l f i de f r a c t l o n a t l o n Su l f i de f r a c t l o n a t l o n o n t h e

o r d e r of 0.1 wt .% c o u l d e x p l a i n t h e f o u r f o l d v a r i a -

t i o n m t h e M o / P r raUo. O u r d a t a a l s o p r o v i d e e v i d e n c e a g a i n s t

w i d e s p r e a d f r a c t i o n a t l o n of su l f ide f r o m b a s a l t i c

m e l t s as p r o p o s e d b y H a m l y n et al. [19]. R e t e n -

t i o n o f l m r m s c i b l e su l f i de h q m d s m the s o u r c e

303

Tr-1 Tr-4 KL-2 ML-3B NMNHl l0014 NMNHl10017 Tristan Tristan Hawml Hawau 1961 Kalauea Mauna Loa

East Raft 1983 1975 flow

UPO-7 82-MT-15 RE 24-1 F-33

Samoa Samoa Reumon Azores Upolu Manua P Fournatse Fatal Puapua Ta U 1984 flow 1958

40000 20500 4110 3250 105 65 0 9 36 5 80

1 57 0 083 0 058 1490 1170 363 319 1360 747 124 81 4

9 88 7 64 4 35 2 43 385 279 163 134 148 74 3 17 0 10 0

3 40 1 79 0 330 0 177 9 28 4 70 1 02 0 856

36 1 27 9 24 4 25 8

110 60 7 13 7 9 17 209 122 33 7 23 6

24 8 16 4 4 71 3 53 90 6 59 7 22 3 17 2 14 0 10 7 5 78 4 86

3 92 3 22 1 98 1 69 9 98 8 29 6 16 5 46 7 20 5 68 5 44 5 02 1 27 1 03 0 974 0 947 3 34 2 40 2 65 2 55 2 72 1 76 2 12 2 08 0 381 0 250 0 304 0 305 1 71 0 786 0 159 0 110 6 34 3 11 0 980 0 743

0 705050 0 705090 0 703497 0 703817 0 512534 0 512526 0 512985 0 512889

18 534 18 516 18 462 18 076 15 546 15 526 15 477 15 451 39 049 38 988 38 099 37 818

14800 7320 6590 14300 45 5 18 1 20 6 35 5

0 535 0 203 0 291 0 290 1030 295 361 606

533 191 200 421 8 21 6 53 6 06

316 231 201 213 76 1 36 4 26 5 46 2

1 62 0 740 0 629 1 38 5 02 2 43 2 06 3 02

35 1 32 0 28 9 25 0

82 3 29 8 21 9 36 5 169 67 7 49 0 74 9

19 7 9 06 6 47 8 90 75 9 38 2 27 4 35 6 142 882 640 707 4 18 2 84 2 16 2 28

11 9 8 52 6 43 6 35 7 84 6 82 5 78 5 11 1 20 1 15 1 12 0 943 2 90 3 06 2 88 2 46 1 89 2 31 2 32 2 01 0 266 0 330 0 335 0 299 0 938 0 429 0 370 0 694 3 52 1 89 1 30 3 39

0 705754 0 704632 0 704197 0 703930 0 512729 0 512843 0 512844 0 512843

18 881 19 164 18 799 19 312 15 602 15 591 15 595 15 634 39 073 39 305 38 907 39 151

regions for our samples should have resulted in large random disturbances of both the M o / P r ratios and the P b / C e ratios as well as disturbing the M o / W ratios. The relatively constant ratios suggest that during magma generation sulfide Is undersaturated or barely saturated.

The P b / C e ratio is constant wlthm a factor of two over the concentration range in oceanic basalts (Fig. 4a). The shght negative slope indicates that Pb is shghtly more compatible than Ce, but Pb is not as compatible as Pr, as a plot of P b / P r vs. Pr (not shown) has a distinct positive slope. The data for oceanic basalts summarized by Sun [20] also show that Pb is fractionated by the same amount as Ce.

The geochemical behavior of these elements is consistent with their respective ionic charges and radii [21]. Tetravalent Mo [22] has a radius of 0.73 .~, assuming octahedral coordination [23] close to that of the moderately incompatible elements Ti and Zr (0.69 ,~ and 0.89 A respectively) The highly Incompatible nature of W is consistent with a charge-radius trend for highly incompatible ele- ments from Ba ( + 2, 1.44 ,~) to Th ( + 4, 1.08 .~) to W (+6 , 0.68 ,~) The covalaance of Pb with Ce ( + 3, 1.09 A) is consistent with the charge ( + 2 [24]) and iomc radius of Pb for octahedral coordi- nation (1.26 ~,).

The essential umforlmty of ratios such as

304

(a)

0

5 0 . . . . I . . . . . . . . I

~ - O R G U E I L 10

5

1 s t _ _

I," o.1 0 .05 Tr i s tan

0 . 0 1 , , , I . . . . . . . . I ,

1 1 0

P r ppm

(b) 50

1 0

5

o 0.5

0.1

0 .05

. . . . I . . . . . . . . I

: CRUSTAL R O C K S ] /

OCEANIC ROCKS / o

g o

O

• o • - _ _ t ~ O • A

ep, O - w w O 0 o

O O

0 . 0 1 1 . . . . I , . . . . , , , I

1 0

P r ppm

Ftg 3 (a) The raao M o / P r versus the concentratzon of Pr The M o / P r ratm for the CI carbonaceous chondnte Orguell is also shown [27] The eovanance of Mo and Pr over an abundance range of a factor of 30 is shown The correlatnon hne is a least squares fit to the data The M o / P r ratms for the St Helen samples, those w~th the most extreme lead isotopes (Fig 1), are the same as for the rest of the samples (b) The M o / P r ratm versus the concentratton of Pr The oceamc basalt data are as m Fig 5 The hnuted amount of crustal data for Mo [54-56], shows httle evidence for a large difference m M o / P r ratms m the ,',-.,~t and mantle

M o / P r and P b / C e m tholenUc to nephehnmc oceamc basalts derived from a variety of mantle reservoirs lmphes that the two elements are not fracUonated from one another during magmatlc processes. Hofmann and White [17] argued that such ratios are xdentxcal to those of the undlfferen tiated sihcate por tmn of the Earth. However, ff we extend tlus line of reasonmg to Pb and Ce, we obtain a P b / C e rauo for the silicate Earth of 0.036 (2°4Pb/Ce = 0.45 × 10-3), and assunung a

(a) 10 5

Q 5

0O5

0 0 1 0 0 0 5

0 0 0 1

. . . . . . . . O'RGUE'IL" '

~,~ SILICATE E A R T H

Hawo. Samba I T;letan St Helena

. . . . . . . . I . . . . . . . . I , , . . . . . . i

lO lOO lOOO

Ce ppm

(b) 10

5

1 Q5

g, g_ Ol

(101 0.005

. . . . . . . . I . . . . . . . . I

s UPPER CRUST o LOESS • OCEANIC ROCKS

fSILICATE EARTH ~ . . . . . . ' ~<:~;~

• o .

Q O 0 1 . . . . . . . . I . . . . . . . . I . . . . . . .

1 10 1 0 0 1 0 0 0 Ce ppm

Fig 4 (a) The ratm of Pb /Ce versus the concentraaon of Ce The shaded band labeled "sdlcate Earth" is the calculated Pb /Ce ratio, assunung a 238U/2°4pb ratio for the pnnutwe

Earth m the range of 6-10 [4,25,26], and a chondntlc U / C e rauo (b) The ratio of Pb /Ce versus the concentration of Ce in ppm The MORB and ocean island data are the same as m Fig 7 Addmonal data for the continental crust from [28] The crust has a much lugher Pb /Ce ratio than the mantle derived samples

Ce concentration of 2.6 tunes that of CI chondntes, we calculate a slhcate Earth Pb concentration of 0.058 ppm and a 2°4pb concentrataon of 3.55 × 10 -6 #mo l /g . The abundance of Pb in the silicate Earth is, however, constrained somewhat by Pb ~sotoplc abundances. All prevaous estimates of the slhcate Earth 238U/2°4pb ratio (/L) he between 6 and 10 (e.g. [4,25,26]). Assuming a terrestrial U abundance of 2.6 tunes CI chondntes (0 0081 p p m for Orguell [27]), we infer from the above Pb concentrauon that # would have to be about 25. Tins value is totally inconsistent with terrestrial Pb isotope systematics and we are forced to con- clude that the "mant le" P b / C e raUo of 0.036

TABLE 2

Depletion factors for Mo, W, and Pb m the pnrmtlve mantle

Mo W

Newsom and Palme [10] 45 __. 5 27 + 15 Tlus work 42 + 5 26 + 9

cannot be the slhcate Earth value Our conclusion is strengthened by the observation that most crustal rocks have higher P b / C e rauos as indi- cated by the upper crustal average of Taylor et al. [28] (Fig 4b). Indeed, it would qualitatively ap- pear that crust and mantle may balance to pro- duce a P b / C e ratio of about 0 1, the value we calculate assuming a slhcate Earth # of 8.

Apparently, crust-mantle differentlatmn ex- tracted Pb from the mantle more efficiently than Ce, yet resulted m a nearly constant P b / C e ratm m the mantle Our samples range from highly trace element depleted tholeutes (e.g. K73A) to highly enriched nephelimtes (e.g. UPO-7), imply- ing a large range m partial melting. Furthermore, several samples, such as those from Tristan de Cunha and some MORB, have experienced exten- swe fractional crystalhzatmn. Yet P b / C e raUos vary by only 30% from the mean. Normal mag- mauc processes would therefore appear incapable of significantly fractlonatmg Pb from Ce Thus crust-mantle dffferentmtion would appear to have involved non-magmatlc processes or at least processes different from those p roduong oceanic basalt magmas.

The surprising result that the constant P b / C e ratio of the mantle is different from that of the

305

silicate Earth implies that other constant mantle ratios, such as Cs /Rb , Ba /Rb , W / B a and M o / P r may not be representative of the slhcate Earth. In notable contrast to Pb, however, the W / B a ratio of the crust (2.14 × 10 -3 [28]) seems to be essen- tially identical to the ratio for mantle-derived sam- ples ( W / B a = 1.61 × 10-3), suggesting that the depletion of W in the mantle relative to chondrites (factor of 26, Table 2) is representative of the pnrmtlve Earth's mantle. The W depletion factor of 26 deduced here from W / B a ratms (Table 3, assuming W / B a = 0.0423 in CI chondrltes, H. Palme, personal commumcatlon, 1984) is identical to that deduced by Newsom and Palme [10] from the W / U ratio, but with a smaller uncertainty of about 30-40%.

The depletion of Mo is deduced from the man- tle M o / P r ratio of 0.227 assuming M o / P r = 2.39 in CI chondntes [29]. The depletion factor of 42 is essentially the same as the value obtained by Newsom and Palme [10] from the M o / N d ratio. Unfortunately the data on crustal materials for Mo are very scarce (Fig 3b). The available data from granitic and sedimentary geochemical refer- ence samples, however, do not indicate a large fractlonatlon of Mo relative to Pr in the crust. In adchtion, the data from the New Bntam island arc [30] show no great difference in M o / P r ratios from the MORB or ocean island data, although this is not the case for P b / C e [14,30]. Therefore, although more information about Mo abundances m crustal materials Is needed, we tentatively con- clude that the M o / P r ratio from the mantle de- rived samples is also representative of the pnnu- twe Earth's mantle.

TABLE 3

Part lhon coefficients [38,52]

Pb Ce Mo Pr W Ba

S-rich metalhc hqmd (sulfide) 6 7 0 1250 0 1 0

silicate hqmd

Fe-metal

slhcate hqmd

bulk mineral

slhcate hqmd

0 0 2500 0 36 0

0 03 0 03 0 04 0 04 0 O1 0 O1

306

4. The lead isotope paradox and the growth of the Earth's core

Pb Isotope ratios in all the ocean Island basalts and most of the MORB samples plot to the right- hand side of the geochron shown in Fig. l a Tins means that the U / P b ratio of their mantle-source rocks must have increased at some time signifi- cantly later than the accretion of the Earth but earher than the present If It is true (as is wadely assumed) that U is more lughly mcompauble than Pb during igneous processes in the mantle, then this result is inconsistent with the incompatible element depletion of the mantle inferred from the ISOtOpic compositions of Sr, Nd and Hf in MORB. In all three isotopic systems, the more highly incompatible element of the mother-daughter pmr is the more depleted one.

Is U really more highly incompatible than pb9 Several hnes of evidence indicate that it is. The first is the admittedly scarce experimental data on U and Pb partitioning [31,32]. The second reason is derived from the relative charge-radius relation- ships of Pb and U. The iomc radius of Pb(2 + ) (1.26 .A) is only shghtly greater than that of the only moderately mcomopatlble Sr(2 + ) (1.21 .~), whereas U(4 + ) (1.08 A) is much larger than the moderately incompatible tetravalent ions such as Zr (0.80 ,~). The third line of evidence IS the observation that all samples except for three of the MORB's have measured U / P b ratios greatly in excess of the prinutwe-mantle value of U / P b , which gives a rough clue that the ratios are higher in the melt than in the source. The fourth evidence is derived from a plot of U / P b versus U con- centration (Fig 5), which shows a strong positive correlation, indicating that U is more incompati- ble than Pb (see [18]). This is entirely consistent with the observation that U has the same compatl- blhty as K, and with the experimental observa- tions on relative partition coefficients of K and Ce summarized by Irving [33]. We conclude that there is little doubt that in partially molten mantle rocks, U is indeed more highly incompatible than Pb, and tins confirms that the "lead paradox" originally discovered by All6gre [34] really does exist.

Several authors [5-7] have proposed that Pb may have been transferred to the Earth's core by conunued core growth (perhaps via a sulfide

. .Q Q_

01

1 i i i i l l [

0 5

0 0 5 •

I I l l l ]

0 01

I I I I I I I I I I I I ' I I ~ I i I I I I l l l J .

O" • •

o BSE • ?

I I I I l l t t J I t I l i i l l [ I I I I l l

0 1 1 10 U

Fig 5 The ratio U / P b versus the concentration of U m oceamc basalts The open circle labeled "BSE" represents the estimated bulk sthcate Earth composltaon The strong posmve correlation shown m this dmgram confirms the assumption that U is more highly incompatible than Pb (see [18])

phase), a long time after accrelaon This "core pumping" would cause the late increase of U / P b (and T h / P b ) required by the Pb isotopic data, and it would thus resolve the lead paradox.

The preferred model of All6gre et al. [7] as- sumes that the Earth was imtxally homogeneous from accretion about 4.55 to 4.35 b.y. when the mantle spht into upper and lower portions. From 4.35 b.y to the present, different mantle reservoirs experienced continued segregation of S-rich metallic liquid (sulfide) to the core at varying rates. The endmember evolution is represented by the upper mantle MORB source that has Pb 1so- topic compositions near the geochron, indicating evolution in a reservoir with a relauvely constant U / P b ratio At the other extreme are the deeper mantle sources (sampled by mantle plumes) of oceanic island basalts, such as St Helena, that evolved with U / P b significantly increasing over time. In this model, the Pb isotopic difference between the two reservoirs requires the addmonal segregation of enough sulfide from the ocean is- land reservoir to increase the size of the core by 15% over geologm time, requiring segregation of at least 7 wt.% sulfide from the mantle [7]

If flus model of continuing core formation over geologic time is correct, variations in the abun- dances of slderophlle and chalcophtle elements should be detectable in samples of different ages from a single reservoir, or m young samples from

reservoirs with different sulfide segregation histo- ries. Our data can be used to investigate the second possibility

4 1. Depletion calculations The depleUon of chalcophlle or slderophile ele-

ments can be calculated for a given metal /s ihcate partition coefficient and metal content. The basic equation [35] for the weight fraction of metal (X) reqmred to achieve a certain depletion factor (a ) in a single partial melting event, assuming eqm- hbrmm Is.

a - 1 X =

D M/s + a - 1

where D M/s is the bulk metal/total-silicate parti- tion coefficient. The depletion factor (a ) is the chondntic abundance of the element normahzed to the refractory content of the Earth &vlded by the abundance of the element in the slhcate por- tion of the Earth.

If the silicate portion of the Earth is partially molten during the extraction of the hqmd metal phase, the bulk-metal/bulk-sihcate partition coef- ficient of an incompatible ( = magmaphxle) sideroptule element (e.g. W, P, and Mo) will de- crease as the silicate melt fraction increases. This effect is described by the relatlonstup:

DM/SL DM/S

F sL + D ss/SL X (1 - F sL)

where D represents the various partition coeffi- caents, F ~s the melt fraction (relative to the total amount of slhcate), and the superscripts M, S, SL, and SS represent bulk metal, bulk silicate, s~hcate liquid, and silicate solid, respectively.

4. 2. Ewdence agamst core formation through ttme Pb is a less chalcophile element than Mo, with

little or no siderophile tendency (Table 3). The large depletion of Pb m the Earth's mantle by a factor of 45 [36] is probably due in large part to volatahty since Pb is no more depleted in the mantle than other smulady volatile but non-chal- coplule elements such as C1, Br, and I [11,36,37]. Also, there is no evidence that the chalcophlle elements in general are strongly partmoned into the core [37]. As discussed above, the P b / C e ratio in the oceanic rocks in F~g. 4a has a maxamum variation in the P b / C e ratio of a factor of two. If

307

the loo of MORB and especially of OIB to the right of the geochron (Fig. la) were caused by late Pb loss to the core, we would expect the C e / P b ratio of the OIB data to correlate positively with the distance of the &splacement from the get- chron. The situation is, however, complicated by the fact that the displacement also depends on the specific age of the hypothetical "core pumping" event A better measure of the lead loss would be the 238U//2°apb ratio m the source rock after the core pumping event. This raUo, P2, may be in- ferred from the Pb isotopic data, provaded /x 1 is known. Chase [38] has shown that the hnear arrays of the 2°Tpb/2°4pb versus 2°6pb/2°4pb correla- tions of many OIB intersect the geochron at a value of tt = 7.9. This provides a reasonable esti- mate of #1 for two-stage histories of OIB. Using tlus value, we have calculated bt2 values (present- day, mantle-source values of 238U/2°4pb) for all the OIB points and those MORB points that lie to the right of the geochron. A positive correlation of these #2 values with the respective Ce /P b or P r / M o ratios would indicate that the lead iso- topes of these rocks could be explained by Pb and Mo extraction w~thout simultaneous extraction of Ce and Pr (and other Incompatible hthoptule ele- ments). Fig. 6 shows that the small variation of P r / M o that does exist does not correlate with P2, and the weak correlation of Ce /P b with #2 can be ascribed entirely to the two samples from St. Helena. The correlation with P r / M o is the more sensitive, though perhaps less direct test, because the partition coefficient for Mo between metallic liquid (25 wt.% S) and silicate liquid is 1250 compared with 6.7 for Pb [39].

Segregation of 7% of a sulfur-rich metalhc liquid, required for the most fractlonated reservoir [7], produces a depletion of Mo by a factor of 95, even if the mantle were totally molten For a sohd-sxhcate or partially molten mantle, the effect would be even larger. For example at 15% partial melting, the depletion factor for Mo would be 500. For segregation of a sulfur-free metallic liquid, the effect would be still larger because of the larger partition coefficient. The actually observed dif- ferences in Mo depletion (Fig. 3a) amount to a maximum of a factor of four, with no sign of a systematic variation as a function of concentration or lead isotopic data (Fig. 6a).

W is not chalcophlle (Table 2), but is a

308

N

20

15

10

I ' I ' I

• o •o oOo •

5 I , I , I

0 2 4 6 8

P r / M o

20

15

txl • •

t o e • • • • lO

5 ) ' , I , , , , I , , , , l , , , , I , , , ,

15 20 25 30 35 40

C e / P b

Fig 6 The ratios (a) P r / M o and (b) C e / P b versus ~2 (the second-stage 238U//2°4pb ratio m the source rock of the basalts) Thas has been calculated from the Pb lsotop,c composxtmns gwen m Table 1, assuming a two-stage evoluUon model and a common Pl value of 7 91 following the method of Chase [38] Tins is one possible approach to est, mate the present-day U / P b ratios of the basalt sources The lack of strong poslUve correlatmns, especially of ~2 versus P r / M o , argues against extraction of Mo and Pb from the mantle independent of the incompatible rare earth elements Pr and Ce

slderophile element sensltwe to metal segregation. The metal/sihcate-hquld partmon coefficient for W is approximately 30. SegregaUon of 7% metal would cause a depleUon of a factor of 15 assuming 15% partml melting. The W / B a raUo vanes by less than a factor of 3, arguing against any metalhc Fe segregation effect.

The M o / W raUo provides another ~mportant constraint. As pointed out by Newsom and Palme [10], the raUo of M o / W m the mantle (6.1, this work) ~s only shghtly less than the chondntlC ratio

of 10 (H. Palme, personal commumcatlon, 1984). Mo, however, is far more slderophde and chal- cophile than W. For example, the addmonal de- pletion of Mo relatwe to W by a factor of 1.6 can be explained by segregaUon of less than 0.1% S-rich metalhc hqmd. Because the partition coeffi- cient for Pb is almost a factor of 200 less than for Mo, the effect of such a small amount of sulfide segregation on the Pb abundance would be negh- glble.

Regarding the tirmng of the possible segrega- tion events, our data suggest that there was no sulfide segregation since the mantle was last ho- mogemzed, wtuch was at least 2 b.y ago judging from Pb isotopic systematxcs. Delano and Stone [40] examined N I / M g and C o / M g ratxos m komatntes of &fferent ages and found no slgmfi- cant variations with age.

The similarity of ratios of many slderophtle elements to the chondrltlC ratios indicates that most of the segregauon of metal or sulfide into the core probably occurred before the end of accre- tion 4 5 b.y ago [10,11,36]. The critical evadence [11] is the depletion of the moderately s,deropbale elements (W, Co, N1, M o ) t o levels of 0.1 to 0.2 times chondrltlC, and the strong depletion of the highly s,derophlle elements (Ir, Os, Re, Au, etc ) to a level approximately 0.002 times chondritlc Virtually all of the metal now present in the Earth's core accreted and segregated into the core by the time the Earth had accreted 85-95% of its mass. At this stage all of the slderophJle elements were depleted in the mantle below their present abundances [10]. The next stage of accretion brought m Mo and W as well as Co and N1 m chondrltlC relative abundances building their con- centratlons to essentially their present levels At this stage, a shght amount of metal or sulfide segregation, less than 0.1%, could explain the shght depleUon of Mo relative to W, and would keep the highly slderophile elements at low concentraUons m the mantle. The final stage of accretion, amounting to less than 1%, ts known as the "late veneer" [41], estabhshing the chondntxc relative abundances of the baghly slderoptule elements, such as Os and Ir, at a level lower than for the moderately slderophile elements. The chondntlc relative abundances of these highly siderophile elements is another piece of ewdence against late core formation [36]

We conclude that there is no indication of a systematic variation m the depletion of sIderophile or chalcophlle elements as a function of Pb 1so- tope composition. Because of the Ingh sensltivaty of Mo to segregation of sulfide and the large range of Pb isotopes represented by our data, we beheve that growth of the Earth's core through geologic time cannot explam the lead paradox

5. Mantle Pb isotopic evolution

Our results indicate the present-day mantle is anomalously depleted in Pb; the P b / C e ratio of the mantle is about a factor of 2.8 lower than the slhcate Earth value deduced from Pb ISOtOpic sys- tematlcs [4,25,26], assuming a chondrltlC C e / U ratio We have argued that tins depletion did not result from extraction of Pb into the core. Never- theless, a reservoir of unradiogemc Pb must exast somewhere m the Earth such that it, the upper continental crust, and the upper mantle sum to produce a Pb isotopic composition which lies on the geochron. Possiblhtles include the deep mantle or lower continental crust

The hypothesis that the unradiogenic Pb is stored in some unsampled mantle reservoir is es- sentmlly untestable and we prefer not to take recourse to it. On the other hand, the quahtative balance of P b / C e ratios between crust and mantle suggests that the reservoir of unradiogemc Pb is m the crust. Doe and Zar tman [42] and O'Nions et al. [43] have suggested that the unradiogenlc Pb is

309

stored in the lower crust, and there is a growing body of evidence to support tins suggestion both f rom direct measurement of granuhte facies rocks (e.g. [44]), and measurements of ISOtOpic ratios of magma generated in or contanunated by lower crust (e.g. [44,45]). Doe and Zar tman [42] present a Pb evolution model which involves three res- ervoirs, upper crust, lower crust, and upper man- tle The present isotopic compositions of their reservoirs do not balance to produce a "bu lk Earth" lying on the geochron, but this is to some degree an artifact of their starting conditions which begin at 4.0 b.y with Pb already evolved to the right of the 4.0 b.y. geochron

A three-reservoir model can be produced (Ta- ble 4), however, that uses reasonable isotopic com- posmons and concentrations and which is at least internally self-consistent in that the reservoirs sum to a reasonable bulk-Earth composition. We have taken crustal Pb concentrations from Taylor and McLennan [46], computed Pb concentrations for the upper (depleted) mantle and primitive mantle f rom P b / C e ratios and taken averages of MORB and deep-sea sediments as representatwe of Pb isotope compositions of the upper mantle and upper crust, respectively. From this, we have com- puted isotope ratios of the lower crust winch, along with the other isotope ratios, sum to pro- duce isotope ratios for the system as a whole that fall on the geochron. This model is, of course, not umque, but it is based on reasonable assumptions about the crust and upper mantle and yields rea-

TABLE 4

Pb isotopic mass balance for the Earth

Mass Pb Total 2°4pb Ce P b / C e 2°6pb/2°4pb 2°Tpb/2°4pb (10 24 g) (ppm) (1015 mol) (ppm)

Upper mantle 2000 0 04 5 31 1 2 0 036 18 40 15 49 Upper crust 8 15 7 54 64 0 23 18 76 15 66 Lower crust 18 7 5 9 11 25 0 30 15 96 15 03 Whole crust 26 10 16 65 38 0 26 17 23 15 32 System 2028 0 163 21 96 1 61 0 101 17 51 15 36

" U p p e r mantle" is assumed to be 50% of the mass of the mantle, masses for upper and lower crust are from Doe and Zar tman [42] "Sys tem" ( = " p n n u t w e mantle") assumes Ce and U 2 6 times the concentrattons m CI chondntes, a 23Su/2°4pb = 8 0 and, therefore, a P b / C e ratio of 0 101 Concentrat ions of Pb and Ce m the upper and lower crust are from Taylor and McLennan [46], Ce concentration m the upper mantle assumes 25% depletion relattve to pnmlUve mantle, consistent wtth the esUmate of Taylor and McLennan [46], that 12 6% of terrestrial Ce is m the crust Pb m the upper mantle is computed from tins Ce concentration and the observed P b / C e rauo Isotopic ratios for depleted mantle are the average of 200 analyses of MORB comptled from the hterature, by Ito et al [53] Upper crust isotopic raUos are the average of 21 analyses of deep-sea sediments (W M Wlute, unpubhshed) Lower crust isotopic ratios are computed by mass balance from the other parameters

310

sonable isotope ratios for the lower crust. The ratios of the lower crust we calculate here are lower than those of Doe and Zar tman [42], but tins is consistent with the lower U / P b for the lower crust of [46].

We have assumed that the crust has been ex- tracted from only 50% of the mantle with the assumption that the remainder of the mantle is "primitive". Mass balance revolving the entire mantle and the ratios and concentratmns in Table 4 would require very low Pb isotope rauos in the lower crust, 2°6pb/2°4pb less than 15 (or slgmfi- cantly higher Pb concentratmn) We beheve such ratios to be unreasonably low, but so httle is known of the lower crust that we cannot entirely rule out such a scenario. On the other hand, the values m Table 4 imply the proportion of mantle depleted to form the crust could not be substan- tially less than 50% With 50% of the mantle involved, 75% depletmn in Pb is reqmred Tins approaches 100% when the proportion of depleted mantle is reduced to 40%. But again, tins depends on the assumed concentratmn of Pb in the crust and tins ~s not well known.

As All6gre et al. [7] pomt out, an Earth with Pb lSOtOplC composmons such as those in Table 4 requires # for the whole crust be lower than that for the mantle, winch xn turn requires Pb be transported from the mantle to the crust more effioently than U. Tins transport cannot, there- fore, be a purely magmatxc one if the arguments given in section 3, also apply to melts destined to form continental crust Thus, the values m Table 4 suggest that the continental crust could not have formed solely by partial melting of the mantle, other, essentmlly non-magmatlc, processes must also have been mvolved Tins should not be surprising as the crust does not appear to have an appropriate composition to be a simple partial melt of the mantle.

We can only speculate on what these other, non-magmatlc, processes might be Since deep-sea sediments are enriched in Pb relative to U [47], weathering and recycling of marine sediment into the mantle would have the effect of lowering the mantle # and increasing the crustal one, and thus producing an effect opposite of the required one

An alternative mechanism hawng the desired effect, involves hydrothermal processes at mtd- ocean ridges. Pb isotopic composltmns of metalhf-

erous sediments [47 49] and Pb isotopic ratios in hydrothermal cate a substantial transport of of the oceamc crust to the seawater system. If most of this

concentraUons and effluents [50], indi- Pb from the basalt marine se&ment-

sediment is scraped off dunng subductIon of the oceanic plate and ultimately is reincorporated in the continental crust, the process constitutes precisely the non- magmatlc transfer of Pb from mantle to crust that is required. Transfer of U from seawater to oce- anic crust during hydrothermal processes [51], will also tend to lower the # of the mantle upon subductlon of the oceamc crust [52]. Detailed studies of hydrothermally altered oceanic crust will be required to determine whether these processes produce sufficiently large changes to actually affect mantle-crust Pb isotopic evolution. We do not argue that tins process accounts for the composition of specific mantle reservoirs such as the St Helena type source We suggest only that ItS operation over much of Earth Instory could pro- duce the excessively radiogenic nature of upper mantle Pb.

The near constancy of the P b / C e ratio In the mantle places strong constraints on the amount of sediment that could have been recycled into the mantle and the degree to which this process could account for isotopic ratio variations in the mantle. Assuming sediment has a P b / C e ratio of 0.58 and 25 ppm Pb (the mean of 20 unpubhshed analyses of D. Ben Othman, W.M. Winte and P.J. Patchett), no more than 0.2% sediment could be added to a mantle reservoir with a P b / C e ratio of the order of 0.036 without shifting the resultant P b / C e ratio of the mixture to values Ingher than those ob- served. Tins amount of sediment is sufficient to produce significant shifts in Pb isotopic composi- tions. The Pb isotopic composition of Tristan de Cunha, for example, could be accounted for by the addition of 0.1% sediment to a mantle with Pb isotope ratios of those of average MORB. How- ever, this amount of sediment addition would result in only small changes in Sr and Nd isotopic ratios.

We conclude that sedimentary recycling has not been a dominant process in producing "the isotopic variations observed in the mantle Indeed, the constancy of the P b / C e ratio appears to indicate that transport of material from crust to the upper mantle or from a possible prlrmtlve reservoir to

the upper mantle has been rmnimal over the past 2 b .y.

Finally, the constancy of P b / C e ratms m oce- anic basalts and the difference between the ob- served ratio and any reasonable "pnnu twe" value preclude primitwe mantle from being a pnnclpal component of any oceanic basalt source, including the Hawanan one. Smaller contnbutmns (up to 10% or so) of prinutwe mantle, which could sub- stantmlly influence He isotopic composltmns, are not precluded.

6. Conclusions

(1) Rauos of Pb//Ce, M o / P r and W / B a in oceamc basalts show no systematic variation with incompatible-element depletmn, enrichment, or Pb isotopic composition, and appear to be nearly constant in the mantle.

(2) Lack of variatmn of Pb isotope ratios with the ratios of Pb/Ce, M o / P r and W/Ba , ratios of siderophde or chalcophile elements to hthophile elements, provide strong arguments against con- tmued core growth and extraction of Pb from the mantle to core through geologic time.

(3) The P b / C e ratm of the mantle, 0.036, is too low by a factor of three to be the primitive mantle ( = bulk sdicate Earth) ratio, assurmng a terrestrial /~ of 6-10 and a chondritlc U / C e ratio for the Earth. Further, the crustal P b / C e ratio appears to be much higher than the mantle ratio. Other ratms, such as Cs /Rb , that are constant m the mantle [17] may also not be the terrestrial ratios.

(4) The mantle M o / P r and W / B a ratios do appear to be prirmtwe mantle ratios because these ratms are ~denUcal in mantle and crust. Thus the conclusions of Newsom and Palme [10] about depletmn of W and Mo in the pnnutwe mantle are confirmed.

(5) because P b / C e is constant m all oceanic basalts, we conclude that magmaUc processes do not affect flus ratm or the W / B a and M o / P r ratios. But because the P b / C e rauo is different in the continental crust and mantle, chfferentmtlon of the Earth into a crust and a mantle must have revolved non-magmauc processes. We suggest hy- drothermal alteration of oceamc crust and subduc- tmn of this crust may be the process which ex- tracts Pb from the mantle and fractionates Pb/Ce.

(6) The reservoir of unra&ogemc Pb needed to

311

balance the radiogemc Pb in the upper mantle and upper crust may be the lower crust. Mass balance can be achieved if the Earth has/x of 8, the lower crust has 2°6pb/2°4pb of approximately 16 and 2°7pb/2°4pb approxtmately 15.0 and ff roughly half the mantle is undepleted.

(7) The constancy of P b / C e ratxos in oceanic basalts which exhibit large variations in Pb 1so- tope ratios, and the differences between the P b / C e ratios of the crust, mantle, and the bulk slhcate Earth tightly constrain the evolution of chemical heterogeneity in the mantle. Apparently, evolution of this heterogeneity has been largely internal and has revolved only processes which fractlonate U / P b and not Pb/Ce.

Acknowledgements

We wish to thank H. Palme, B. Spettel and W. Rammensee for adwce on neutron actwatlon, and S. Kaelinczuk, H. Feldmann, H.M. Seufert, and S. Midinet-Best for technical assistance. Important samples were landly provaded by H. Puchelt, J. Natland, F. Albarrde, and B. Melson. Samples were activated at the research reactor of the In- stitiJt fiir Anorganische Chemic and Kernchenue der Umverslt~it Mamz. H.N. wishes to thank Prof. H. W~inke for his interest and support. Ad&tional support for H.N. was provaded by NASA grant NAG-9-30 (Klaus Keil, pnncipal investigator). Comments by A.E. Rangwood, E.R.D. Scott and the reviewers were helpful.

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