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

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

299

[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. White 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 M o / P r rauo to segregataon of Fe-metal or S-rich metalhc hqmd (sulfide) and the observed constant M o / P r 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 P b / C e ratio of the mantle is essentxally constant, but the present P b / C e rauo m the mantle (=- 0 036) is too low to represent the pnnutlve value (----0 1) derived from Pb isotope systematacs Hagher P b / C e rahos m the crust balance the low P b / C e of the mantle, and crust and mantle appear to sum to a reasonable terrestrial P b / C e ratio The constancy of the P b / C e 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 P b / C e 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 Most oceanic basalts have 143Nd//144Nd and 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 i n c h o n d n U c meteorites [1-3]. Presuming the Earth has chondntlC Sm/Nd and Lu/Hf ratios, we refer that the mantle has been depleted in incompatible 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 v e t o 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 0012-821X/86/$03 50

© 1986 Elsewer Science Pubhshers B V

Lu. Sr Isotopic composmons of mantle-derived rocks also reflect this depletion of incompatible 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 t o r e s u l t from extraction of a parUal melt. Because of the volatlhty of Pb, the U/Pb ratio of the Earth cannot be assumed to be chondritlc. 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 initial Pb isotope ratios equal to that of primordml m e t e o n t l c l e a d [4], t h e 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 fall 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 ~sotop~c composition. Most mantle-derived volcamc rocks plot to the tugh-2°6pb/2°4pb side o f t h i s line, 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&cate the U/Pb ratio of the mantle has in-

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 formation 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 extraction 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. Additional sampling crltena were: a large range in 87Sr/86Sr and 143Nd/144Nd ratios, a wide geograpinc 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 metalsilicate 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 uncertainty 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 ionization 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 uncertaanty 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 described 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 observed 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 incompatible elements so that concentrations of these elements will be affected by magmatIc processes such as partial melting and fractional crystalhzatlon Thus, absolute concentrations of

301 (Q)

159

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....

DEPLETED~ENRICHED

I''"l

....

500

I"

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158 t~ o_ ,do 157 cN 43 o.. r,,, 156 o c~

gQ'x%.l~l~x !

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#/

o

HOWOll MORB St Helena Tr,ston

• Samoa

100

J 0") w

155

50

123

.... , .... ,,, 15¢ , 17 0 17 5 18 0 18 5 19 0 19 5 20 0 20 5 21 0 206pb/204pb

z o n(.2

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I ....

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10

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051281StF Helena i _ Azorea/-t'~..-~ ~. _ __ ~'311 f

. . . . :1

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1

I I I I I I I I I I I I I I I LaCe PrNd

,

0 702

,,,,

, ....

, ....

0.703 0.704 0.705 Q706 87Sr/86Sr

0707

0708

Fig 1 ( a ) L e a d isotope diagram for the U-Pb system, illustrating 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 N d and S r 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-averaged evolution in a depleted reservmr

these elements d o not provide unambiguous information 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 m a y be considered equally incompatible, that is, to

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 coefficients. If the ratio A / B increases as the concentration of A and B increase, then A is more incompatible than B and wee versa. T w o examples of constant trace d e m e n t ratios demonstrated by H o f m a n n and White [17] are the R b / C s and B a / R b ratios. Their data was highly biased toward M O R B , including only a few ocean island representatives, namely from the Galapagas, 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 H o f m a n n 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 ( l o ) , which agrees with the average K / U = 12,700 of Jochum et al. [18] for

302 TABLE 1 Analytical results (concentrations m ppm)

Location

K10A-D33A MORB Pacific 20°36'S, 114o2'W

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

K Rb Cs Sr Ba Hf Zr Nb U Pb Y

1160 1 33 0 015 92 6 12 3 7 15 227 5 93 0 106 0 684 69 1

1220 3 28 0 036 82 2 32 0

1160 3 07 0 028 68 3 23 5

155 0 114 0 001 60 7 1 41

641 0 700 0 006 99 4 8 82

641 0 690 0 007 99 4 8 73

184 6 83 0 144 0 561 63 9

42 0

86 6

96 7

6 48 0 115 0 426 40 3

0 008 0 168 25 9

0 034 0 514 32 6

0 038 0 505 34 5

La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu W Mo

6 93 23 1 4 31 22 8 7 89 2 50 10 9 13 0 2 89 8 46 7 94 1 19 0 022 0 689

4 50 12 8 2 06 10 7 3 59 1 24 5 06 6 25 1 42 4 07 3 96 0 608 0 065 0 578

6 56 20 7 3 39 19 1 6 67 2 11 9 50 11 6 2 61 7 55 7 28 1 10 0 062 0 627

0 972 3 75 0 816 4 81 2 01 0 822 3 17 4 06 0 911 2 63 2 53 0 389 0 002 0 181

2 80 9 12 1 70 9 08 3 28 1 23 4 72 5 70 1 26 3 57 3 33

2 79 9 13 1 68 0 10 3 29 1 22 4 69 5 67 1 26 3 59 3 35 0 506 0 007 0 480

87Sr//86Sr 143Nd/t44Nd 2°6pb/2°4pb 2°7pb/2°4pb 2°spb/2°4pb

0 702463 0 513161 18 321 15 484 37 798

0 702825 0 513041 18 744 15 562 38 566

0 702543 0 513101 18 574 15 515 38 132

Sample

0 702468 0 513198 18 287 15 481 37 816

MORB but displays considerably greater scatter. D e v l a U o n s f r o m t h e J o c h u m e t al. v a l u e a p p e a r t o 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 Mo/Nd ratios m MORB and continental basalts were umform. Our results mdlcate a shghtly better match (more similar ratios) of W to Ba and of Mo t o t h e h g h t r a r e e a r t h Pr. T h e W / B a r a U o s ( a v e r a g e o f 1.61 × 10 - 3 ) a r e 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 ratios (average of 0.227) are within a factor of 4 over a 10 2 c o n c e n t r a t i o n r a n g e . S l i g h t p o s i t i v e s l o p e s o n p l o t s o f W / B a v s B a ( n o t s h o w n ) , a n d M o / P r vs. P r (Fig. 3 a , b ) , s u g g e s t t h a t W m a y b e s l i g h t l y

0 010 0 176 0 703035 0 513083 17 325 15 456 37 287

0 703031 0 513072 17 315 15 443 37 251

19600 54 8 770 584

130 2 51 4 74 81 6 160 19 3 67 8 12 1 3 60 9 95 7 68 1 48 3 83 3 29 0 493 1 25 5 62 0 702960 0 512842 20 816 15 778 40 072

11800 27 2 0 263 665 369 7 56 298 73 2 1 45 2 83 33 2 49 7 103 12 2 49 5 10 2 3 28 9 09 7 20 1 26 3 40 2 73 0 398 0 492 3 25 0 702850 0 512871 20 820 15 801 40 133

more lncompaUble than Ba and Mo shghtly more 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 l o p e , i n d i c a t i n g M o is n o t a s i n c o m p a t i b l e as Ce. I n a d d i t i o n t o a n a l y t i c a l error, the scatter in Mo/Pr and W/Ba rmght reflect minor differences m bulk partition coefficients which in turn aught be due to very minor sulfide fractlonatlon Sulfide fractlonatlon on the o r d e r o f 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 tion m the Mo/Pr raUo. Our data also provide evidence against widespread fractionatlon of sulfide from basaltic 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 tion of lmrmscible sulfide hqmds m the source

303

Tr-1 NMNHll0014 Tristan 1961

Tr-4 NMNHl10017 Tristan

KL-2

ML-3B

UPO-7

82-MT-15

RE 24-1

F-33

Hawml Kalauea East Raft 1983

Hawau Mauna Loa 1975 flow

Samoa Upolu Puapua

Samoa Manua Ta U

Reumon P Fournatse 1984 flow

Azores Fatal 1958

40000 105 1 57 1490 1360 9 88 385 148 3 40 9 28 36 1

20500 65 0

4110 9 36 0 083 363 124 4 35 163 17 0 0 330 1 02 24 4

3250 5 80 0 058 319 81 4 2 43 134 10 0 0 177 0 856 25 8

14800 45 5 0 535 1030 533 8 21 316 76 1 1 62 5 02 35 1

7320 18 1 0 203 295 191 6 53 231 36 4 0 740 2 43 32 0

6590 20 6 0 291 361 200 201 26 5 0 629 2 06 28 9

14300 35 5 0 290 606 421 6 06 213 46 2 1 38 3 02 25 0

13 7 33 7 4 71 22 3 5 78 1 98 6 16 5 44 0 974 2 65 2 12 0 304 0 159 0 980

9 17 23 6 3 53 17 2 4 86 1 69 5 46 5 02 0 947 2 55 2 08 0 305 0 110 0 743

82 3 169 19 7 75 9 142 4 18 11 9 7 84 1 20 2 90 1 89 0 266 0 938 3 52

29 8 67 7 9 06 38 2 882 2 84 8 52 6 82 1 15 3 06 2 31 0 330 0 429 1 89

21 9 49 0 6 47 27 4 640 2 16 6 43 5 78 1 12 2 88 2 32 0 335 0 370 1 30

36 5 74 9 8 90 35 6 707 2 28 6 35 5 11 0 943 2 46 2 01 0 299 0 694 3 39

0 703497 0 512985 18 462 15 477 38 099

0 0 18 15 37

0 0 19 15 39

0 0 18 15 38

0 703930 0 512843 19 312 15 634 39 151

110 209 24 8 90 6 14 0 3 92 9 98 7 20 1 27 3 34 2 72 0 381 1 71 6 34 0 705050 0 512534 18 534 15 546 39 049

1170 747 7 64 279 74 3 1 79 4 70 27 9 60 7 122 16 4 59 7 10 7 3 22 8 29 5 68 1 03 2 40 1 76 0 250 0 786 3 11 0 0 18 15 38

705090 512526 516 526 988

703817 512889 076 451 818

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.

0 705754 0 512729 18 881 15 602 39 073

704632 512843 164 591 305

704197 512844 799 595 907

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 elements 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 coordination (1.26 ~,). The essential umforlmty of ratios such as

304

(a)

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Samba I T;letan St Helena

ppm

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001 0005

Tristan

1

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,

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,,,I 10

Pr

I 10

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. H o f m a n n and White [17] argued that such ratios are xdentxcal to those of the undlfferen tiated sihcate portmn 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

I 100 ppm

....... 1000

Fig 4 (a) The ratm of P b / C e versus the concentraaon of Ce The shaded band labeled "sdlcate Earth" is the calculated P b / C e ratio, assunung a 238U/2°4pb ratio for the p n n u t w e Earth m the range of 6-10 [4,25,26], and a chondntlc U / C e rauo (b) The ratio of P b / C e 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 P b / C e 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 p p m and a 2°4pb concentrataon of 3.55 × 10 -6 # m o 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 conclude that the " m a n t l e " P b / C e raUo of 0.036

305 TABLE 2 Depletion factors for Mo, W, and Pb m the pnrmtlve mantle

Newsom and Palme [10] Tlus work

Mo

W

45 __.5 42 + 5

27 + 15 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 indicated by the upper crustal average of Taylor et al. [28] (Fig 4b). Indeed, it would qualitatively appear that crust and mantle may balance to produce a P b / C e ratio of about 0 1, the value we calculate assuming a slhcate Earth # of 8. Apparently, crust-mantle differentlatmn extracted 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), implying a large range m partial melting. Furthermore, several samples, such as those from Tristan de Cunha and some MORB, have experienced extenswe fractional crystalhzatmn. Yet P b / C e raUos vary by only 30% from the mean. Normal magmauc 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 r o d u o n g oceanic basalt magmas. The surprising result that the constant P b / C e ratio of the mantle is different from that of the

silicate Earth implies that other constant mantle ratios, such as C s / R b , B a / R b , 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 essentially identical to the ratio for mantle-derived samples ( 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 mantle 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 reference 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 conclude that the M o / P r ratio from the mantle derived samples is also representative of the pnnutwe Earth's mantle.

TABLE 3 Partlhon coefficients [38,52] Pb

Ce

Mo

Pr

W

Ba

67

0

1250

0

1

0

0

0

2500

0

36

0

0 03

0 03

S-rich metalhc h q m d (sulfide) silicate h q m d Fe-metal slhcate h q m d bulk mineral slhcate h q m d

0 04

0 04

0 O1

0 O1

306 1

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

i i i ill [

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Pb Isotope ratios in all the ocean Island basalts and most of the MORB samples plot to the righthand 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 significantly 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 H f 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 relationships 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 concentration (Fig 5), which shows a strong positive correlation, indicating that U is more incompatible than Pb (see [18]). This is entirely consistent with the observation that U has the same compatlblhty as K, and with the experimental observations 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

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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] assumes 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 1sotopic 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 island 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 abundances of slderophlle and chalcophtle elements should be detectable in samples of different ages from a single reservoir, or m young samples from

307 reservoirs with different sulfide segregation histories. Our data can be used to investigate the second possibility 4 1. Depletion calculations

The depleUon of chalcophlle or slderophile elements can be calculated for a given metal/sihcate 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 eqmhbrmm Is. X=

a-1 D M/s + a - 1

where D M/s is the bulk metal/total-silicate partition 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 portion 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 coefficient of an incompatible ( = magmaphxle) sideroptule element (e.g. W, P, and Mo) will decrease as the silicate melt fraction increases. This effect is described by the relatlonstup: DM/S

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

where D represents the various partition coefficaents, 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-chalcoplule 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

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 getchron. 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 inferred from the Pb isotopic data, provaded /x1 is known. Chase [38] has shown that the hnear arrays of the 2°Tpb/2°4pb versus 2°6pb/2°4pb correlations of many OIB intersect the geochron at a value of tt = 7.9. This provides a reasonable estimate of #1 for two-stage histories of OIB. Using tlus value, we have calculated bt2 values (presentday, 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 C e / P b or P r / M o ratios would indicate that the lead isotopes of these rocks could be explained by Pb and Mo extraction w~thout simultaneous extraction of Ce and Pr (and other Incompatible hthoptule elements). 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 C e / 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 differences 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

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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 c o m m o n 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 chalcophile than W. For example, the addmonal depletion 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 coefficient 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 neghglble. Regarding the tirmng of the possible segregation events, our data suggest that there was no sulfide segregation since the mantle was last homogemzed, 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 slgmficant 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 accretion 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 concentratlons 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]

309

We conclude that there is no indication of a systematic variation m the depletion of sIderophile or chalcophlle elements as a function of Pb 1sotope 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 systematlcs [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. Nevertheless, 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 essentmlly 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 Zartman [42] and O'Nions et al. [43] have suggested that the unradiogenlc Pb is

stored in the lower crust, and there is a growing body of evidence to support tins suggestion both from direct measurement of granuhte facies rocks (e.g. [44]), and measurements of ISOtOpic ratios of m a g m a generated in or contanunated by lower crust (e.g. [44,45]). Doe and Zartman [42] present a Pb evolution model which involves three reservoirs, upper crust, lower crust, and upper mantle The present isotopic compositions of their reservoirs do not balance to produce a " b u l k 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 (Table 4), however, that uses reasonable isotopic composmons 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 from P b / C e ratios and taken averages of M O R B and deep-sea sediments as representatwe of Pb isotope compositions of the upper mantle and upper crust, respectively. F r o m this, we have computed isotope ratios of the lower crust winch, along with the other isotope ratios, sum to produce 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

U p p e r mantle U p p e r crust Lower crust Whole crust System

Mass (10 24 g)

Pb (ppm)

Total 2°4pb (1015 mol)

Ce (ppm)

Pb/Ce

2°6pb/2°4pb

2°Tpb/2°4pb

2000 8 18 26 2028

0 04 15 75 10 0 163

5 7 9 16 21

12 64 25 38 1 61

0 036 0 23 0 30 0 26 0 101

18 40 18 76 15 96 17 23 17 51

15 49 15 66 15 03 15 32 15 36

31 54 11 65 96

" 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 Z a r t m a n [42] "System" ( = " 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 Concentrations of Pb and Ce m the upper and lower crust are from Taylor and M c L e n n a n [46], Ce concentration m the upper mantle assumes 25% depletion relattve to pnmlUve mantle, consistent wtth the esUmate of Taylor and M c L e n n a n [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 M O R B comptled from the hterature, by Ito et al [53] U p p e r 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 Zartman [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 extracted 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 slgmficantly 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 substantially 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] p o m t 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, therefore, 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 mtdocean ridges. Pb isotopic composltmns of metalhf-

erous sediments [47 49] and Pb concentraUons and isotopic ratios in hydrothermal effluents [50], indicate a substantial transport of Pb from the basalt of the oceamc crust to the marine se&mentseawater system. If most of this sediment is scraped off dunng subductIon of the oceanic plate and ultimately is reincorporated in the continental crust, the process constitutes precisely the nonmagmatlc transfer of Pb from mantle to crust that is required. Transfer of U from seawater to oceanic 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 produce 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 p p m 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 observed. Tins amount of sediment is sufficient to produce significant shifts in Pb isotopic compositions. 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. However, 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

311 the upper mantle has been rmnimal over the past 2 b.y. Finally, the constancy of P b / C e ratms m oceanic basalts and the difference between the observed ratio and any reasonable " p n n u t w e " 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 substantmlly 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 P b / C e , M o / P r and W / B a , ratios of siderophde or chalcophile elements to hthophile elements, provide strong arguments against contmued 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 C s / R b , 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 hydrothermal alteration of oceamc crust and subductmn of this crust may be the process which extracts Pb from the mantle and fractionates P b / C e . (6) The reservoir of unra&ogemc Pb needed to

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 1sotope 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 P b / C e .

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 InstitiJt 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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