Constraints on earth evolution from antimony in mantle-derived rocks

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ISOTOPE GEOSCIENCE Chemical Geology 139 (1997) 39-49

Constraints on earth evolution from antimony in mantle-derived rocks K.P. Jochum *, A.W. Hofmann Max-Planck-lnstitut fiir Chemie, Pos~ach 3060, D-55020, Mainz, Germany Received 15 December 1996; accepted 11 March 1997

Abstract We have analyzed Sb in a variety of mantle-derived volcanic rocks, peridotites, and in the CI chondrite Orgueil by spark source mass spectrometry. Concentrations vary from 0.02 to 0.8 ppm in oceanic basalts (mid-ocean ridge basalts, MORB; oceanic island basalts, OIB). Antimony is a moderately siderophile element which behaves like the incompatible lithophile element Pr during igneous processes in the mantle. Both MORB and OIB samples have similar Sb/Pr ratios of about 0.02, which are different from those in continentalcrustal rocks. Antimony resembles Pb in that it behaves like a highly incompatible element during formation of continental crust, whereas it behaves only moderately incompatible during formation of oceanic basalts (MORB or OIB). Consequently, Sb/Pb ratios of oceanic basalts agree within error limits with those of the continental crust and with the CI chondritic value, indicating that Sb/Pb is not strongly fractionated during crust-mantle-core differentiation. From the Sb/Pb ratios we estimate a Sb concentration of 11 + 5 ppb for the primitive mantle. An alternative estimate for the primitive-mantle abundance is obtained by assuming the Sb/Pr ratio of the primitive mantle to be intermediate between the MORB-OIB value (0.02) and that of the continental crust (0.05). This approach yields Sb = 8 + 4 ppb for the primitive mantle. Antimony is depleted in the bulk silicate Earth by a factor of 45. The volatility-corrected depletion factor of 7 is similar to other moderately siderophile elements. © 1997 Elsevier Science B.V. Keywords: antimony; moderately siderophile elements; primitive mantle; oceanic basalts; CI chondrite; mantle sources

1. Introduction Chemical elements such as Pb, Sn, Sb, W, Mo, T1, which are moderately siderophile a n d / o r chalcophile as well as being magmaphile (i.e. 'incompatible' in the conventional sense), are of particular geochemical interest because they have participated

" Corresponding author. FAX: 49-6131-371051; E-mail: kpj @geobar.mpch-mainz.mpg.de.

in both of the major differentiation processes of the Earth, the segregation of the core, and the segregation of the crust. During these processes, their 'doubly incompatible' geochemical properties cause them to be partitioned into metal/sulphide as well as into silicate liquids. Consequently, they are relatively enriched in the Earth's core as well as in the crust. Some of these elements, including Sb, are also moderately volatile in the cosmochemical sense, so that their abundances in the bulk Earth are afflicted by an additional uncertainty.

0009-2541/97/$17.00 ~) 1997 Elsevier Science B.V. All fights reserved. Pll S 0 0 0 9 - 2 5 4 1 ( 9 7 ) 0 0 0 3 2 - 6

40

K.P. Jochum, A. W. Hofmann / Chemical Geology 139 (1997) 39-49

The behaviour of W, Mo, Pb, Sn has previously been studied by Newsom and Palme (1984), Newsom et al. (1986, 1996), Sims et al. (1990), Jochum et al. (1993), Yi et al. 0995). However, comparatively little is known about Sb, mainly because of analytical difficulties. Therefore, the primitive-mantle abundance of Sb is not well-known. Some oceanic basalts have been investigated by Hertogen et al. (1980) and Sims et al. (1990). Recently, Noll et al. (1996) analyzed Sb together with other siderophile and chalcophile elements in lavas from subduction zones. These studies indicate that the 'behaviour' of Sb during subduction processes and, ultimately, crust formation is similar to that of Pb. The purpose of this paper is to investigate Sb together with other trace elements in a variety of oceanic basalts, mainly MORB and OIB, Precambrian and Tertiary basalts and komatiites, 'primitive' mantle xenoliths, and in the CI carbonaceous chondrite Orgueil, using spark source mass spectrometry (SSMS). The results are used to determine the relative level of incompatibility of Sb, its primitive-mantle abundance and its depletion in the bulk silicate Earth, as well as to constrain crust-mantle differentiation. Preliminary results of this study have been previously published by Jochum and Hofmann (1994).

Table 1 Sb concentrations (ppm) in international reference materials determined by SSMS (the data are compared with working values of Govindaraju, 1994) Reference material

SSMS

Working value

BCR- 1 W-1 AGV-1 W-2 BIR-1 BHVO-1 BE-N SDC- 1 QLO- 1 DTS-1 PCC-1 RGM-1 JG-3 a JB-2 a JA-3 a JF-1 a JF-2 a JA-2 a JG-2 a JR-1 a JA-1 a JR-2 a JB-la a JB-3 a JGb-1 a JG-la a

0.67 1.15 4.27 1.3 0.37 0.2 0.29 0.55 1.68 1 1 1.13 0.087 0.28 0.32 0.069 0.028 0.18 0.064 1.3 0.24 1.2 0.3 0.16 0.09 0.085

0.62 1.04 4.3 0.79 0.58 0.16 0.26 0.54 2.1 0.5 1.28 1.26 0.07 0.27 0.34 0.06 0.04 0.13 0.06 1.48 0.26 1.83 0.28 0.15 0.1t 0.06

Analyses of the GSJ geostandards have been previously published by Jochum and Jenner (1994).

a

2. Analytical technique Spark source mass spectrometry (SSMS) was used for simultaneous determination of Sb and other trace elements (Jochum et al., 1988). About 50 mg of sample were mixed with a spiked graphite containing 12 spike isotopes (e.g., 143Nd). Antimony concentrations were determined by measuring S b / N d ratios using the isotopes 121Sb, 123Sb and 145Nd, 146Nd, calibrating with a sensitivity factor of Sb relative to Nd of 0.85 and using Nd as internal standard element which has been determined by isotope dilution. Replicate analyses show that precision is about 7 10%. The detection limit for Sb is about 0.001 ppm, allowing its determination in a wide variety of rock types. Accuracy is similar to the precision. This i s demonstrated in Table 1 where the results for international reference materials are compared with the working values by Govindaraju (1994). The agree-

ment of our data with the reference values is better than 15% for most of the samples with no systematic differences apparent.

3. Samples Many samples of differing provenance and tectonic setting were analyzed. Sample locations and references for major element, trace element and isotopic data of most samples are published in Jochum et al, (1993). Brief descriptions are given below. Most of the mid-ocean ridge basalt(MORB) samples come from the basalt glass collection of the Smithsonian Institution. They include glasses from different locations of the Atlantic, the Pacific and the Indian Ocean (Melson et al., 1977; Puchelt and

41

K.P. Jochum, A.W. Hofmann/ Chemical Geology 139 (1997) 39-49

Emmermann, 1983). The other M O R B samples are from the region between the Kane and Hayes Fracture Zones (obtained from C. Langrnuir). The ocean island basalts (OIB) come from the Hawaiian Islands, Samoa, St. Helena, Tubuai, Azores, Society Islands, Tristan da Cunha, Gough Island, Comores, Reunion, and Galapagos. Most OIB are tholeiites and alkali basalts. They are fresh and taken from historic eruptions where possible. Archaean to Tertiary komatiites and basalts are 3.4 Ga old rocks from Barberton (S. Africa) and Pilbara (Australia), 2.7 Ga old rocks from Kambalda (Australia), Abitibi (Canada) and B e l i n g w e (Zimbabwe), and 1.9 Ga old rocks from the Ottawa Islands (Canada). The youngest samples are Tertiary rocks from Gorgona Island (Colombia). Locations of the analyzed spinel peridotite xenoliths are San Carlos (Arizona, USA), Kilboume Hole and Potrillo (New Mexico, USA), Landoz (Massif Central, France), Dreiser Weiher (Eifel, Germany) and southeastern Australia. To determine the depletion of Sb in the primitive mantle, the CI carbonaceous chondrite Orgueil was also analyzed.

Antimony concentrations in the mantle-derived rocks and in the CI chondrite Orgueil are listed in Tables 2 - 5 . Because Sb behaves like moderately incompatible and lithophile elements (such as the light rare earth elements, LREE) during magmatic processes, we have also included La, Ce, Pr, Nd abundances and S b / P r ratios in the tables. LREE concentrations were determined by SSMS together with Sb. MORB samples have Sb concentrations from 0.02 to 0.05 ppm (Table 2) and S b / P r ratios of about 0.02 (see next section and Fig. 2). Jochum and Verma (1996) have recently shown that seawater may enrich Sb abundances and S b / P r ratios extremely. Therefore, only clean glass fragments from the samples, carefully hand-picked and with no traces of palagonite, were selected for this investigation. Our Sb values are higher than the data of four MORB samples (0.005-0.021 ppm) obtained by Sims et al. (1990). We are not certain whether this difference is caused by sampling bias or analytical discrepancies between SSMS and the radiochemical

Table 2 Analytical results of MORB Sample Sb (ppm)

La (ppm)

Ce (ppm)

Pr (ppm)

Nd (ppm)

Sb/Pr

Pb (ppm) a

VG744 VG937 P6909-28B GS7309-94 GS7309-75 VG198 R3-3-D10 K62A-D143G b VG1583 VG3095 b OC 180 26-3 OC180 24-2 OC 180 25-1 OC180 12-2 OC180 10-1 OC180 14-2 OC180 34-1 OC 180 9-2

3.87 3.86 4.91 3.74 5.65 4.24 6.29 4.5 3.41 2.79 4.73 2.78 2.3 2.75 1.4 2.78 2.56 2.74

12.2 11.4 14.2 11 14.7 15.5 17.4 12.8 12.6 9.13 17.4 10.4 8.29 10.4 6.48 10.2 10.2 8.89

2.23 2.03 2.27 1.83 2.33 2.58 2.7 2.06 2.33 1.68 2.94 1.78 1.42 1.64 1.02 1.63 1.52 1.73

12.2 10.8 12.3 10.1 11.6 13.5 14.1 10.7 12.9 9.1 14.9 9.25 7.23 8.29 5.19 8.4 7.66 9.12

0.019 0.023 0.017 0.026 0.019 0.019 0.019 0.021 0.015 0.024 0.013 0.015 0.016 0.018 0.030 0.017 0.028 0.028

0.454 0.556 0.589 0.495 0.52 0.495 0.709 0.426 0.645 0.505 0.502 0.508 0.375 1.11 0.772 0.564 0.412 0.6

0.043 0.046 0.038 0.047 0.045 0.05 0.052 0.043 0.034 0.04 0.039 0.027 0.023 0.029 0.031 0.027 0.042 0.049

a Hofmann et al., 1986. b REE concentrations from Newsom et al., 1986.

4. Results

42

K.P. Jochum, A. W. Hofmann / Chemical Geology 139 (1997) 39-49

epithermal neutron activation analysis procedure used by Sims et al. O I B samples h a v e Sb concentrations ranging f r o m about 0.05 to 0.8 p p m (Tables 3 and 4). The large variation is caused by source mantle heterogeneity and different degrees o f partial m e l t i n g and fractional crystallization. The l o w e s t Sb concentrations are found in H a w a i i a n tholeiites, whereas a differentiated H a w a i i a n trachyte and samples f r o m Tubual and the Society Islands h a v e m u c h higher concentrations. Similar variations have b e e n found for other m o d e r a t e l y siderophile elements, e.g. Sn ( J o c h u m et al., 1993). A l t h o u g h Sb concentrations in OIB are

m u c h h i g h e r than in M O R B , S b / P r ratios o f about 0.02 are similar. Precambrian komatiites and basalts s h o w a large variation in Sb abundances (Tables 5 and 6). The unusually high S b / R E E ratios in m o s t samples, especially those f r o m Kambalda, are p r e s u m a b l y caused by c o n t a m i n a t i o n by crustal rocks ( A m d t and Jenner, 1986). H o w e v e r , m o s t Tertiary G o r g o n a komatiites and basalts h a v e l o w Sb concentrations similar to M O R B but S b / R E E ratios that are about a factor o f 2 h i g h e r than M O R B . N o n - M O R B ratios o f G o r g o n a and other komatiites are also found for other e l e m e n t pairs, such as N b / T h ( J o c h u m et al.,

Table 3 Analytical results of OIB Sample

Sb (ppm)

La (ppm)

Ce (ppm)

Pr (ppm)

Nd (ppm)

Pb (ppm) a

Sb/Pr

Azores F-33 b Azores SM-12 Azores SM-6 Azores P-21 Galapagos E-20 St. Helena 2882 St. Helena 2926 b St. Helena 102 b St. Helena SC74 St. Helena SC68 Samoan Isl. Vpo 1 Samoan Isl. VP-2 Samoan Isl. Ut~-7 b Samoan Isl. JKU-1 Samoan Isl. US-1 Tristan da Cunha TR-1 b Tristan da Cunha TR-4 b Tristan da Cunha TR-5 Tristan da Cunha TR-6 Gough G-8 Reunion RE24-1 b Society Isl. Hanh 769 Society Isl. Tahi 343 Society Isl. Tahaa 73-185 Comoros AJ 21-9 Tubuai 5433 Tubuai TU9 Tubuai 105 Z Tubuai K109 Tubuai 5436

0.258 0.367 0.214 0.132 0.122 0.11 0.24 0.367 0.354 0.164 0.167 0.161 0.228 0.475 0.35 0.312 0.184 0.407 0.475 0.217 0.178 0.44 0.152 0.512 0.256 0.27 0.56 0.51 0.16 0.16

36.5 52.6 45.7 22.3 12 28.6 49.7 81.6 42.6 26.5 33 23.4 82.3 62.1 42.4 110 60.7 57.3 78.1 40.7 21.9 80.7 28.1 84.2 36.5 68.5 169 182 59.4 47.1

74.9 110 99.3 48.6 29 60.7 103 160 83.7 56.3 70.8 51.7 169 125 88.1 209 122 116 167 80.8 49 161 67.5 186 68.4 134 330 324 112 93

8.9 13.6 12.6 6.44 4.34 7.8 12.2 19.3 11.2 6.51 8.67 6.64 19.7 15.7 11.2 24.8 16.4 16 20 10.8 6.47 20 8.74 25.1 9.11 16.4 37.8 31.9 12.9 12.1

35.6 54 50.2 27.1 20.4 31.2 49.5 67.8 43.7 25.8 33.6 27.8 75.9 62.4 45.6 90.6 59.7 63.8 75.9 44.2 27.4 72.7 38.4 94.9 38.1 57.6 127 102 46.6 48.6

3.02

0.029 0.027 0.017 0.020 0.028 0.014 0.020 0.019 0.032 0.025 0.019 0.024 0.012 0.030 0.031 0.013 0.011 0.025 0.024 0.020 0.028 0.022 0.017 0.020 0.028 0.016 0.015 0.016 0.012 0.013

a Pb data from Hofmann et al. (1986) and Chauvel et al. (1992). b REE concentrations from Newsom et al., 1986.

2.83 4.74

5.02

9.28 4.7

2.06

4.24 4.2

3.2 3.1

K.P. Jochum, A. W. Hofraann/ Chemical Geology 139 (1997) 39-49

43

Table 4 Analytical results of samples from the Hawaiian Islands Sample

Rock type

Sb (ppm)

La (ppm)

Ce (ppm)

Pr (ppm)

Nd (ppm)

Pb (ppm)

Sb/Pr

Kauai Kau-1 C 46 OA 2 OA 7 OA 8 OA 9 OA 11 OA 1

tholeiite tholeiite tholeiite tholeiite tholeiite tholeiite tholeiite mell. neph.

0.075 0.069 0.074 0.108 0.061 0.076 0.057 0.34

6.42 14 13.3 13.6 14.4 8.15 13 75.6

17.1 31.8 29.9 35.5 32.5 18.8 30 145

2.37 4.82 4.58 4.95 5.37 3.3 4.65 18.1

10.6 20.6 20.7 23 24.7 16 21.8 68.6

0.743 1.41 1.47 1.65 1.69 1.33 1.45 4.11

0.032 0.014 0.016 0.022 0.011 0.023 0.012 0.019

Moiokai C 44 71-WAIK 8F

alk. basalt tholeiite

0.285 0.15

38. t 10.8

84.1 26.9

11.8 4.13

49.6 18.3

2.41 0.949

0.024 0.036

Maui C 149 C 116 HMT 79-2b C 127

mugeatite trachyte alk. basalt hawaiite

0.213 0.85 0.229 0.163

62.9 86.1 17.2 37.5

119 175 38.9 81.4

17.9 22.3 6.04 10.6

72.1 78.5 26.5 43.9

5.4 5.83 2.04 2.55

0.012 0.038 0.038 0.015

Kahoolawe KW-24 KW-25 KW- 1 KW-2 KW-5 KW-7 KW-6 KW-23 KW- 19 KW-18

tholeiite tholeiite tholeiite tholeiite tholeiite tholeiite tholeiite tholeiite tholeiite tholeiite

0.079 0.078 0.072 0.097 0.08 0.073 0.074 0.051 0.06 0.078

4.7 5.88 9.32 10.6 11.3 15.2 7.79 10.7 13.2 10.1

12.4 14.7 23.4 26.2 29.8 35.5 19.1 28.4 33.8 26.1

1.61 2.24 3.31 3.69 3.7 4.78 2.76 3.79 4.9 3.23

7.08 10.5 " 15.2 16.3 16 20.4 13.5 17.4 21.4 14.9

Hawaii 1801 KL-2 a

alk. basalt tholeiite

0.083 0.146

21.3 13.7

43.8 33.7

5.7 4.71

24.2 22.3

Loihi Dredge 2

tholeiite

0.16

12.4

30.5

3.98

17.2

0.049 0.035 0.022 0.026 0.022 0.015 0.027 0.013 0.012 0.024

1.02

0.015 0.031 0.040

a REE data from Newsom et al., 1986.

1991), suggesting that the origin o f komatiites was m a r k e d l y different :from that o f normal oceanic basalts. In peridotite xenoliths, Sb abundances are v e r y l o w (Table 7); the m e a n concentration is 0.007 ppm. T h e nodule SC-1, w h i c h is ' p r i m i t i v e ' with respect to its m a j o r e l e m e n t and c o m p a t i b l e t r a c e - e l e m e n t chemistry (Jagoutz et al., 1979) has a Sb concentration o f about 0.006 p p m and a S b / P r ratio o f 0.02. C I c a r b o n a c e o u s chondrite abundances a r e usually used as reference o f unfractionated solar-system ma-

terial. S S M S analyses o f the C I chondrite Orgueil by RochoU and J o c h u m (1993) g i v e a Sb concentration o f 0.16 ppm. Our n e w v a l u e (Table 7) is s o m e w h a t lower, in g o o d a g r e e m e n t with the c o m p i l a t i o n value o f A n d e r s and G r e v e s s e (1989).

5. Incompatibility of Sb W h e n no metal or sulphide phases are present, Sb b e h a v e s like an ordinary (lithophile) i n c o m p a t i b l e

44

K.P. Jochum, A. W. Hofmann/ Chemical Geology 139 (1997) 39-49

Table 5 Analyticalresults of Precambrianand Recent komatiites. Sample

Sb (ppm)

Pr (ppm) a

Sb/Pr

Onverwacht 8241 Onverwacht 8243 Onverwacht 5031 Onverwacht 5019 KambaldaA1140 Kambalda476.3 Kambalda477.4 BelingweZ2 Alexo M664 Simbawe ZA1 Gorgona GOR159 GorgonaGOR160

0.049 0.39 0.18 0.23 0.27 0.23 0.2 0.055 0.02 0.038 0.026 0.03

0.67 0.27 0.93 0.54 0.31 0.39 0.42 0.44 0.28 0.39 0.43 0.46

0.073 1.444 0.194 0.426 0.871 0.590 0.476 0.125 0.071 0.097 0.060 0.065

a

Jochum et al., 1991.

element during igneous processes. Following the approach previously employed by Jochum et al. (1993), we have determined the compatibility of Sb relative to those of the REE by systematically testing global correlations of Sb with the REE using the data on oceanic basalts (Tables 2 - 4 ) . Fig. 1 shows the slopes of the log Sb versus log (La, Ce, Pr, Nd) regression lines, calculated using the two-error regression of Table 6 Analyticalresults of Precambrianand Recent basalts Sample

Sb (ppm)

Pr (ppm) a

Sb/Pr

Onverwacht 5038 Onverwacht 5080 Onverwacht5077 Pilbara 92 Pilbara 56A Pilbara 26A KambaldaKA1 KambaldaC592 Newton NEW 91 Newton NEW 20 Munro C 1 Munro C 126 Munro C31 Munro C6 Ottawa Isl. G6 Ottawa Isl. G37B Ottawa G12 Wawa 59 Gorgona 117 Gorgona 167 Gorgona 54

0.054 0.12 0.16 0.27 0.42 0.18 0.31 0.14 0.486 0.038 0.071 0.16 0.035 0.067 0.062 0.04 0.061 0.57 0.03 0.033 0.017

6.27 1.8 1.56 1.15 3 1.95 1.18 3.36 1.28 0.79 3.01 0.7 1.63 0.94 1.36 0.87 1.12 1.07 0.6 0.48 2.42

0.009 0.067 0.103 0.235 0.140 0.092 0.263 0.042 0.380 0.048 0.024 0.229 0.021 0.071 0.046 0.046 0.054 0.533 0.050 0.069 0.007

a Jochum et al., 1991.

Table 7 Analytical results of spinel peridotite xenoliths and the CI carbonaceous chondrite Orgueil Sample

Sb (ppm)

Pr (ppm)

Sb/Pr

San Carlos SC-1 a Potrillo Po-1 a Landoz Fr-1 a Dreiser WeiherD-1 a SE Australia84--402 SE Australia84-413 SE Australia85-168

0,0064 0,0043 0.0091 0.0068 0.0051 0.015 0.01

0,268 0.103 0.176 0.085 0.095 1.43 0.204

0.024 0.042 0.052 0.080 0.054 0.010 0.049

Orgueil

0.14

0.097

1.443

a Pr data from Jochum et al., 1989.

York (1969). (A log plot of the inverse concentrations 1 / C 1 and 1 / C 2 , as used by Minster and All~gre (1978), yields entirely equivalent results.) The slopes in Fig. 1 increase monotonically from 0.75 to 1.17 in the progression of Sb vs. La, Sb vs. Ce, Sb vs. Pr to Sb vs. Nd. The correlation between log Sb and log Pr, with a slope equalling unity, is shown on Fig. 2. As previously shown by Jochum et al. (1993), this is equivalent to having a slope of zero on a C 1 / C 2 vs. C l plot used by us in previous publications (e.g. Jochum et al., 1983; Hofmann et al., 1986). Fig. 2b shows such a S b / P r plot, which has a zero slope within the scatter of data. We recommend the evaluation of compatibility according to Figs. 1 and 2a, because the representation

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between Sb and La, Ce, Pr, and Nd abundancesin MORB and OIB, with the two-error regression of York (1969). The slope of the Sb-Pr correlation(see also Fig. 2) equals unity within error (0.98 +0.05, 1 or), indicating that Sb and Pr are approximately equally incompatible.

K.P. Jochum, A. W. Hofmann/ Chemical Geology 139 (1997) 39-49

given in Fig. 2b does not lend itself to convenient statistical testing. At zero slope, a completely uncorrelated array of data and a highly correlated array on Fig. 2a would both have zero correlation coefficients on Fig. 2b so that the significance of the zero slope in Fig. 2b is not determined. One may question whether MORB and OIB should be treated as a single population. Inspection of Fig. 2 shows that analyses of the MORB data alone do not yield a well defined correlation, because the variation of the concentrations is similar to the scatter of the ratios. However, the OIB data, taken alone, do yield a very well defined correlation and slope in Fig. 2a, and the MORB population clearly lies on the same correlation line. We conclude from Figs. 1 and 2 that the S b / P r ratios are nearly uniform and that Sb behaves like the moderately incompatible element Pr in oceanic basalts, and consequently the two elements are about

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45

equally incompatible. The degree of incompatibility of a moderately siderophile element can also be estimated by ionic properties as demonstrated by Jochum et al. (1993). Bulk partitioning depends on ionic radius and also quite strongly on ionic charge. Siderophile/chalcophile-lithophile element groups which have constant ratios form subparallel arrays in an ionic radius-ionic charge plot. Fig. 3 shows this relationship for W - T h - B a , Pb-Ce, Mo-Pr, and SnSm. Pentavalent Sb has an ionic radius of 0.69 A, very close to Mo (0.71 ,~). If the previously established contours shown in Fig. 3 were used to predict the best REE compatibility analog for Sb by drawing parallel lines, the 'best' REE analog would have an ionic radius of 1.10 + 0.05 ,~, which is nearly that of Pr (1.08 ,~), the same REE as determined by the relationship shown in Figs. 1 and 2. Sims et al. (1990) and Noll et al. (1996) determined Ce as best REE analog for Sb, an element with a similar ionic radius (1.09 A) as Pr. o

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Sb (ppm) Fig. 2. Antimony is best correlated with Pr for oceanic basalts. The log Sb-log Pr diagram (a) shows a slope of about 1 and S b / P r ratios provide the best concentration-independent fit and are approximately constant (b).

6. Global differentiation

Fig. 4 shows the Sb-Pr data for recent MORB and OIB in comparison with older (with exception of

46

K.P. Jochura~ A. W. Hofmann / Chemical Geology 139 (1997) 39"49

of about 0.02. This value differs from estimates of the bulk continental crust of about 0.05 (Taylor and McLennan, 1985; Wedepohl, 1995). This is analogous to the crust-mantle relationships previously described for Pb (Hofmann et al., 1986; Newsom et al., 1986). Our results therefore extend, confirm and refine those of Sims et al. (1990) who found (on the basis of a much smaller number of oceanic basalt samples including 4 MORB and 11 OIB) that crustal-derived materials are enriched in S b / C e (Ce being only very slightly more incompatible than Pr) compared to oceanic basalts. To illustrate the difference in S b / R E E relationships between the OIB-MORB environment and island arc volcanic rocks, we have plotted Sb versus Ce from the oceanic basalts together with the data for arc volcanics taken from Noll et al. (1996) (Fig. 5). (We use Ce for this comparison because Noll et al. did not give Pr data.) The strongly correlated array for MORB-OIB contrasts sharply with the roughly triangular array for the arc rocks, which shows little or no correlation. Clearly, Sb behaves essentially independently of Ce (or Pr) in the subduction environment, and Noll et al., using S b / C e versus B / L a correlations, have convincingly argued that this apparent incompatibility is caused by nonigneous processes, which almost certainly involve Sb and B transport by a fluid phase. We note here in passing that the S b / C e versus Ce plot used by Noll et al. shows a negative correlation in contrast to the virtual absence of any correlation seen in Fig. 5 (which plots Sb versus Ce). This

the Gorgona samples, Archaean) basalts and komatiites, 'primitive' mantle xenoliths, estimates of average continental crust and the abundances in carbonaceous chondrites. As expected, the results for the mantle xenoliths are similar to the recent basalt data, confirming the inference made by Hofmann et al. (1986) that when an incompatible trace element ratio is independent of the absolute concentration in a suite of basaltic rocks, then this also represents the ratio of the mantle sources of these rocks. In contrast, almost all the older volcanic rocks have higher S b / P r ratios than the recent basalts. However, we do not think that this represents any kind of secular trend in mantle composition; instead this effect is most likely caused by the very high mobility of antimony during alteration as demonstrated by Jochum and Verma (1996), which makes all Precambrian komatiites and basalts highly susceptible to secondary uptake of Sb. Both published estimates of average continental crust show higher S b / P r ratios than any of our recent oceanic basalts. The highest S b / P r ratio is found in CI meteorites, consistent with the expectation that all accessible terrestrial silicate reservoirs are depleted in Sb due to the combined effects of volatility and loss to the Earth's core. The Sb data are consistent with earlier constraints on crust-mantle evolution (and core formation) using other moderately siderophile elements (Hofmann et al., 1986; Newsom et al., 1986; Jochum et al., 1993; Noll et al., 1996). Fig. 4 shows that both MORB and OIB samples have similar S b / P r ratios 10

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K.P. Jochum, A. W. Hofmann/ Chemical Geology 139 (1997) 39-49

illustrates a problem with using ratios of elements that are not clearly correlated with each other. Thus, whereas the logarithmic plot of S b / C e versus Ce (of the data of Noll et al.) shows a strong negative correlation, a plot of S b / C e versus Sb shows a similarly strong positive correlation. This kind of ambiguity can result easily from arrays of data when one element is much more variable than the other, and it is not clear in such cases what the meaning of an element ratio is. The C ] / C 2 versus C 1 plot has a theoretical justification when the effects of partial melting are being assessed (e.g. Minster and All~gre, 1978; Hofmann et al., 1986). The same cannot be said for C ] / C 2 versus C 2 plots, which have also been used by other authors in recent years, and are not well grounded in partial melting theory. Both plots should probably be avoided when the intrinsic element-element correlations are poor or absent, and when totally different (non-igneous) processes are involved. The most convincing diagram used by Noll et al., namely S b / C e versus B / L a , could be replaced by a Sb versus B plot, which shows a rather similar correlation and demonstrates the same point. There is an additional danger in ratio-ratio plots when the variables used in the denominator are themselves highly correlated, as is true for La and Ce. In such a case one can obtain strong and possibly spurious correlations when the two numerators are completely uncorrelated and the denominators are highly variable in absolute magnitude. These remarks are intended as a cautioning note and should , ,,,i,,, I

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47

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Pb (ppm) Fig, 6. Sb vs. Pb plot. Sb and Pb are correlated for oceanic basalts and arc rocks (Noll et al., 1996), having somewhat different slopes of correlation lines however.

in no way detract from the validity of the interpretation given by Noll et al., which we consider to be very well substantiated, particularly in view o f the fact that Sb and B, two highly mobile elements (in the presence of aqueous fluids), correlate very well with each other but not with the REE. As noted above, the behaviour of Sb is rather similar to that of Pb, both in oceanic basalts (using present data and those of Hofmann et al., 1986, and Newsom et al., 1986) and in subduction zone magmas (Noll et al., 1996). This is illustrated in Fig. 6, a plot of Sb versus Pb. The slope of the correlation of Sb versus Pb is close to unity in MORB and OIB, but is somewhat steeper for the island arc magmas, and this is consistent with the conclusion of Noll et al. that Sb is more mobile than Pb in the subduction environment and should therefore be transferred to the continental crust even more efficiently than Pb. Thus, one might expect to the S b / P b ratio of the average continental crust to be slightly greater than that of the oceanic crust. However, the amount and quality of data for crustal rocks and the high mobility of Sb conspire to make the estimates of the continental crust highly uncertain. In summary, it is clear that both element ratios, P b / C e and Sb/Pr, are significantly higher in the continental crust than in oceanic magmas and in the present-day upper mantle, but that S b / P b ratios are much more similar in the two reservoirs. Both Sb and Pb behave like the moderately incompatible elements Pr and Ce in the MORB-OIB environment. Their apparently much more incompatible behaviour

48

K.P. Jochum, A. W. Hofmann / Chemical Geology 139 (1997) 39-49

in the subduction environment and the resulting anomalous enrichment in the continental crust is almost certainly caused by non-magmatic mobilization through aqueous fluids as long suspected by Hofmann et al. (1986), Newsom et al. (1986), Hofmann (1988), Peucker-Ehrenbrink et al. (1994), Miller et al. (1994), Chauvel et al. (1995), and confirmed by Noll et al. (1996).

7. Primitive-mantle abundance The abundance of Sb in the primitive mantle (PRIMA = bulk silicate Earth) can be estimated from Fig. 6 and from previously made estimates of the lead abundance in the primitive mantle. As noted above, and as is true for many estimates of absolute abundances of volatile or siderophile elements, such estimates are somewhat model-dependent and will also be affected by uncertainties in the composition of the continental crust. A primitive-mantle abundance of Pb = 175 ppb has been estimated by Hofmann (1988) using the abundance estimate of the refractory lithophile element uranium (20,4 ppb) and a U / P b ratio estimated from lead isotopes in mantle and crust (expressed in terms of /x = (238U/2°4pb) o = 8.88). The value given by Hofmann (1988) is slightly erroneous in that it is not consistent with the stated values for U and /x. The correct value is 146 ppb. The mean S b / P b ratio of 0.074 + 0.031 then yields an estimated Sb = 11 ± 5 ppb for present-day OIB-MORB mantle and, because of the similarity of subduction magmas, the continental crust and CI chondrites ( S b / P b = 0.057; Anders and Grevesse, 1989), approximately for the primitive mantle as well. An alternative way to estimate the primitive-mantle abundance of antimony is to start with the S b / P r ratio of 0.02 for MORB and OIB. Estimates of the bulk continental crust S b / P r ratio are higher (about 0.05). The primitive-mantle value is likely to lie between the average values for MORB-OIB and for the continental crust. Thus we~obtain a rough estimate for PRIMA S b / P r = 0.035 + 0.015. Assuming Pr = 0.24 ppm for PRIMA (Hofmann, 1988), we obtain Sb = 8.4 _ 3.6 ppb for the bulk silicate Earth. A PRIMA composition can also be estimated from the chemistry of fertile spinel peridotite xenoliths.

Our analyses of spinel peridotite xenoliths (Table 7) give a mean Sb concentration of 7 ppb which is identical with the estimate derived from oceanic basalts. This value is slightly higher than the PRIMA estimate of Sb = 5.7 ppb by WSnke et al. (1984). Antimony is depleted in the PRIMA relative to CI chondritic abundances by a factor of about 45 (Fig. 4). Because Sb is a volatile element (having a 50% condensation temperature of 912 K at a pressure of 10 - 4 atm; Wasson, 1985), the depletion is due to both volatility and siderophilicity. Volatility can be corrected using the procedure of Newsom (1990) by subtracting out the depletion of volatile lithophile elements of similar volatility. Using this correction we obtain a depletion factor of 7 for Sb. This factor is similar to other moderately siderophile elements (5-20; Newsom, 1990) and in agreement with the heterogeneous accretion model of W~inke (1981).

Acknowledgements The samples used in this study have been accumulated over the course of many years. They have been contributed by many individuals, especially C. Langmuir, E. Ito, W.M. White, N.T. Amdt, L.M. Echeverria, W.F. McDonough, E. Jagoutz, M. Tatsumoto, W.P. Leeman, R. Cliff and H. Palme. We thank S. Midinet-Best for technical assistance. We are grateful to M. Drake and H. Newsom for helpful reviews.

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