Abundances of Ag and Cu in mantle peridotites and the implications for the behavior of chalcophile elements in the mantle

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Abundances of Ag and Cu in mantle peridotites and the implications for the behavior of chalcophile elements in the mantle Zaicong Wang ⇑, Harry Becker Freie Universita¨t Berlin, Institut fu¨r Geologische Wissenschaften, Malteserstrasse 74-100, 12249 Berlin, Germany Received 6 November 2014; accepted in revised form 2 April 2015; Available online 10 April 2015

Abstract Silver abundances in mantle peridotites and the behavior of Ag during high temperature mantle processes have received little attention and, as a consequence, the abundance of Ag in the bulk silicate Earth (BSE) has been poorly constrained. In order to better understand the processes that fractionate Ag and other chalcophile elements in the mantle, abundances of Ag and Cu in mantle peridotites from different geological settings (n = 68) have been obtained by isotope dilution ICP-MS methods. In peridotite tectonites and in a few suites of peridotite xenoliths which display evidence for variable extents of melt depletion and refertilization by silicate melts, Ag and Cu abundances show positive correlations with moderately incompatible elements such as S, Se, Te and Au. The mean Cu/Ag in fertile peridotites (3500 ± 1200, 1s, n = 38) is indistinguishable from the mean Cu/Ag of mid ocean ridge basalts (MORB, 3600 ± 400, 1s, n = 338) and MORB sulfide droplets. The constant mean Cu/Ag ratios indicate similar behavior of Ag and Cu during partial melting of the mantle, refertilization and magmatic fractionation, and thus should be representative of the Earth’s upper mantle. The systematic fractionation of Cu, Ag, Au, S, Se and Te in peridotites and basalts is consistent with sulfide melt–silicate melt partitioning with apparent partition coefficients of platinum group elements (PGE) > Au P Te > Cu  Ag > Se P S. Because of the effects of secondary processes, the abundances of chalcophile elements, notably S, Se, but also Cu and the PGE in many peridotite xenoliths are variable and lower than in peridotite massifs. Refertilization of peridotite may change abundances of chalcophile and lithophile elements in peridotite massifs, however, this seems to mostly occur in a systematic way. Correlations with lithophile and chalcophile elements and the overlapping mean Cu/Ag ratios of peridotites and ocean ridge basalts are used to constrain abundances of Ag and Cu in the BSE at 9 ± 3 (1s) ng/g and 30 ± 6 lg/g (1s), respectively. The very different extent of depletion of Ag and Cu in the BSE cannot be explained by low pressure–temperature core formation if currently available metal–silicate partitioning data are applied. Ó 2015 Elsevier Ltd. All rights reserved.

1. INTRODUCTION The abundances and ratios of chalcophile elements in mantle rocks and basaltic magmas yield valuable

⇑ Corresponding author.

E-mail addresses: [email protected] (Z. Wang), [email protected] (H. Becker). http://dx.doi.org/10.1016/j.gca.2015.04.006 0016-7037/Ó 2015 Elsevier Ltd. All rights reserved.

information about their partitioning behavior during mantle processes and magmatic fractionation (e.g., Jenner et al., 2010; Li and Audetat, 2012; Kiseeva and Wood, 2013; Patten et al., 2013; Mungall and Brenan, 2014). Such data also provide important constraints on the composition of chalcophile elements in the bulk silicate Earth (BSE, e.g., McDonough and Sun, 1995; Palme and O’Neill, 2003; Becker et al., 2006; Wang and Becker, 2013), mantle–crust fractionation (Lee et al., 2012) and the conditions required

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for the formation of magmatic ore deposits (e.g., Keays, 1995; Arndt et al., 2005; Richards, 2011). The abundances of the chalcophile elements S, Se, Te, Cu, Au, Re and the platinum group elements (PGE) in mantle peridotites and in basalts have been intensely studied (e.g., Hertogen et al., 1980; BVSP, 1981; Morgan, 1986; Lorand, 1989; Rehka¨mper et al., 1999; Yi et al., 2000; Pearson et al., 2004; Becker et al., 2006; FischerGo¨dde et al., 2011; Jenner and O’Neill, 2012; Lorand et al., 2013; Wang and Becker, 2013). However, the abundances of silver in these rocks have received comparatively little attention. Published Ag data in basalts indicate its chalcophile behavior in terrestrial magmatic processes, similar to Cu (Hertogen et al., 1980; Jenner et al., 2010, 2012; Scho¨nba¨chler et al., 2010; Jenner and O’Neill, 2012; Patten et al., 2013). Very little Ag data have been reported for mantle peridotites, mainly on xenoliths from Kilbourne Hole and massif peridotites from Balmuccia and Baldissero (BVSP, 1981; Garuti et al., 1984; Scho¨nba¨chler et al., 2010). Published Ag data from the Balmuccia and Baldissero peridotite bodies are high (20–100 ng/g) compared to the xenolith data from Kilbourne Hole (mainly 3–10 ng/g) and data on basalts and picrites (15–50 ng/g). Because Ag is believed to behave incompatibly during igneous fractionation (Hertogen et al., 1980; BVSP, 1981; Scho¨nba¨chler et al., 2010; Jenner and O’Neill, 2012), these data are difficult to reconcile. Because sulfides and concentrations of chalcophile elements in mantle xenoliths may have been affected by interaction with magmatic fluids or melts and by oxidation processes related to weathering (e.g., Lorand, 1990; Handler et al., 1999; Lorand et al., 2003, 2013; Ackerman et al., 2009), the significance of the limited available Ag data on mantle rocks is difficult to evaluate. Consequently, the lack of understanding the behavior of Ag during igneous fractionation processes in the mantle results in poorly constrained abundance of Ag in the BSE (McDonough and Sun, 1995; Palme and O’Neill, 2003). Understanding the behavior of Ag in peridotites may also help to constrain the fractionation of chalcophile elements in high temperature mantle processes. At present, it is debated which process controls the distribution of chalcophile elements between peridotites and basic magma during partial melting of the mantle. Many previous studies have been advocating sulfide melt–silicate melt partitioning in order to explain the fractionation of chalcophile elements in these high temperature processes (e.g., Peach et al., 1990; Yi et al., 2000; Patten et al., 2013; Wang and Becker, 2013, 2015; Mungall and Brenan, 2014). Some workers, however, have suggested that monosulfide solid solution (MSS)–sulfide melt partitioning controls the abundances of the PGE (Bockrath et al., 2004; Ballhaus et al., 2006), S, Se and Te (Ko¨nig et al., 2014) during mantle melting. Silver and Cu behave remarkably different in MSS–sulfide melt and sulfide melt–silicate melt partitioning (Li and Audetat, 2012). Because the partitioning processes concurrently affect all chalcophile elements, the Ag and Cu contents in peridotites in combination with the PGE, S, Se and Te will provide comprehensive constraints on the partitioning behavior of chalcophile elements.

In this study we present new Ag and Cu concentration data obtained by isotope dilution and ICP-MS for massif and xenolith peridotites from different geological settings. Also included are new data on peridotites from the Balmuccia and Baldissero bodies, in order to compare with the previously high Ag contents from the same bodies (Garuti et al., 1984). Most samples analyzed in this study have been well studied and particularly, abundances of chalcophile elements such as the PGE, Au, Re, S, Se and Te have been obtained in previous work (Becker et al., 2006; Fischer-Go¨dde et al., 2011; Wang and Becker, 2013; Wang et al., 2013). We will discuss the origin of correlations and the predominant partitioning processes of Ag, Cu and other chalcophile elements during high temperature mantle processes and magma evolution, the commonly low and more scattered abundances of chalcophile elements in many peridotite xenoliths compared to massif peridotites, and new estimates of Ag and Cu abundances in the BSE and their implications for core formation. 2. SAMPLES Samples analyzed in this study are post-Archean mantle peridotites from different geological settings, including massif peridotites (Baldissero and Balmuccia in the Ivrea-Verbano zone in N Italy, Lanzo in NW Italy, Beni Bousera in Morocco, Turon de Te´coue`re in the western Pyrenees), the External and Internal Ligurian ophiolites, and alkali basalt hosted xenoliths from Eifel (Germany) and Hannuoba (China) (Table 1). Most mantle peridotites of this study have been well characterized and abundances of major elements, PGE, Au, Re, S, Se and Te, and 187 Os/188Os data have been reported elsewhere (Lorand et al., 1993, 1999; Fabries et al., 1998; Gao et al., 2002; Becker et al., 2006; Fischer-Go¨dde et al., 2011; Wang and Becker, 2013; Wang et al., 2013; Ko¨nig et al., 2014). Copper abundances have been reported for some samples from Lanzo (Lorand et al., 1993) and Turon de Te´coue`re (Fabries et al., 1998). Additional peridotite samples from the Lanzo peridotite body with protogranular texture that were newly collected away from dikes (>20 cm) are also included. For more details on petrographic descriptions and geochemical data of these samples we refer to previous studies (Bodinier, 1988; Bodinier et al., 1988, 1991; Lorand et al., 1993, 1999; Fabries et al., 1998; Gao et al., 2002; Becker et al., 2006; Fischer-Go¨dde et al., 2011; Wang et al., 2013). These studies have indicated that the peridotites underwent different extents of partial melting and subsequent refertilization with infiltrating basic melts. The melt infiltration processes resulted in variable but often incomplete chemical and Os isotopic equilibration, which is especially indicated by the abundance patterns of the PGE in comparison to their sulfide melt–silicate melt partition coefficients and Os model age distributions. 3. ANALYTICAL METHODS It is widely thought that Cu and Ag are chalcophile and mainly controlled by sulfides, but available peridotite and mid ocean ridge basalts (MORB) data and experimental

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Table 1 Bulk rock concentrations of Ag, Cu, S, Se and Te in mantle peridotites. Comments

Al2O3(*) (wt.%)

Ag (ng/g)

Cu (lg/g)

Baldissero, Italy BD-13 Bomb BD-16 Bomb BD-17 Bomb BD90-3 Bomb BD90-9 Bomb BD90-4 Bomb BD90-4 HPA-S BD90-10 Bomb BD90-10 HPA-S BD11-07 Bomb BD11-07 HPA-S BD11-12 Bomb BD11-12 HPA-S BD11-13 Bomb BD11-13 HPA-S BD11-01 HPA-S BD11-05 HPA-S BD92-2 Bomb BD11-08 Bomb

L L H L L L Replicate L Replicate L Replicate L Replicate L Replicate L L H L

3.1 2.4 1.1 2.9 3.0 2.4

7.19 9.30 24.0 5.46 7.49 19.6 16.6 14.9 11.1 7.33 7.95 2.60 2.22 11.5 11.1 5.52 7.41 4.32 9.79

25.4 24.8 2.59 24.1 22.9 32.9 36.5 24.2 24.3 20.1 22.1 4.92 3.85 4.23 3.98 23.9 29.6 15.1 30.3

Balmuccia, Italy BM90-5 BM90-5 BM90-25 BM90-41 BM90-15 BM11-11 BM11-11 BM11-11 BM-09 BM11-08 BM11-09 BM11-03B BM11-03B BM11-04 BM11-04 BM11-18 BM11-10 BM11-02A BM11-02A BM11-03A BM11-05 BM11–07A BM11-24A

L Replicate L L L L Replicate Replicate L L L H Replicate L Replicate L L L Replicate Dunite Dunite Dunite Dunite

0.3 0.4 0.3 0.5

13.1 18.6 15.2 12.6 8.13 8.54 8.86 8.58 7.64 9.83 6.75 6.33 6.46 14.4 13.9 4.94 7.35 14.1 12.3 2.69 3.45 6.35 1.34

28.6 30.3 30.8 23.4 25.0 8.02 7.44 7.95 37.0 34.3 26.3 24.1 22.7 36.3 35.7 35.6 31.1 55.7 54.8 0.32 0.55 25.2 4.61

Turon de Te´coue`re of Western Pyrenees TUR 7 Bomb TUR 14 Bomb TUR 16 Bomb TUR 23 Bomb TUR11 Bomb TUR21 Bomb

L L L L L L

4.2 4.0 2.2 3.7 3.4 2.9

140 91.3 5.90 65.6 4.72 23.6

32.3 30.0 24.6 28.7 34.2 59.5

Damaping, Hannuoba xenoliths, China DM59 Bomb DM60 Bomb DMP04 Bomb DMP19 Bomb DMP51 Bomb DMP58 Bomb DMP59 Bomb

L L L H L L L

2.6 3.7 2.3 1.9 2.0 3.2 2.6

7.22 10.9 5.18 42.2 19.8 9.12 6.92

24.0 26.6 20.0 10.9 18.6 26.6 23.9

Samples

Digestion methods

Bomb Bomb Bomb Bomb Bomb Bomb Bomb HPA-S Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb

2.7 2.8 2.3 2.5 2.7 3.2 1.8 3.2 2.0 2.5 2.6 2.8 2.1

3.4 3.2 2.7 1.1 2.5 2.8 2.8 3.4

Cu (*) (lg/g)

S(*) (lg/g)

Se(*) (ng/g)

Te(*) (ng/g)

79.0 90.5 6.8 68.4 54.8 85.3

11.2 10.5 1.53 9.72 6.52 13.5

74.8

8.20

171

65.4

10.0

122

36.4

4.69

141

55.0

5.68

162 178 79.9 180

74.2 89.0 37.9 76.1

11.8 11.6 5.77 11.5

76.0

13.3

88.1

91.0 74.5 110 40.9

11.8 9.41 20.5 8.48

182 170 127 75.7

114 80.4 78.2 39.0

13.1 10.1 11.8 7.05

153

80.4

12.1

168 134 349

76.1 89.9 143

10.1 12.2 11.4

28.7 18.9 73.4 27.9

9.63 8.69 35.5 12.3

1.14 1.46 3.53 0.89

284 234 143 227 192 254

93.0 99.2 42.8 64.2 81.4 93.6

12.7 13.4 7.80 9.65 10.2 11.0

118 186 73 91 130 230 118

64.3 68.6

8.82 7.77

64.3

8.82

35.8

36 35 26 36 35 27

(continued on next page)

Line missing

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Table 1 (continued) Samples

Digestion methods

Comments

Al2O3(*) (wt.%)

Ag (ng/g)

Cu (lg/g)

L L L

3.0 3.4 3.7

7.34 6.94 6.79

External and Internal Ligurian ophiolites IL1 Bomb H IL2 Bomb H IL3 Bomb L EL1 Bomb L

1.9 1.7 2.1 3.2

Lanzo, Italy LERS 14 Lanz16 Lanz30 Lanz45 Lanz47N Lanz72A L52 L213 L215 L216 L217

H L L L L L H L L L L H H H H H H

Beni Bousera, Morocco BB3A Bomb BB2G Bomb BB2C Bomb

Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb Bomb

Eifel xenoliths, Germany LeyX1 Bomb LeyX2 Bomb LeyX3 Bomb LeyX4 Bomb Ley1-06 Bomb Ley1-08 Bomb

Cu (*) (lg/g)

S(*) (lg/g)

Se(*) (ng/g)

Te(*) (ng/g)

21.4 26.1 24.3

144 161 306

69.8 76.3 73.4

6.56 10.2 9.59

4.97 4.88 6.06 6.02

20.3 19.2 25.1 23.1

97.6 119 173 241

59.8 56.4 65.2 59.5

7.19 6.66 10.4 8.25

1.2 3.8 3.2 3.0 3.2 4.3 1.4 3.3 2.7 2.3 3.8

1.23 1.90 7.05 6.21 6.54 8.81 2.22 28.5 170 254 56.6

1.45 8.99 28.0 24.1 30.0 33.2 9.41 26.3 17.2 7.32 29.1

54.6 164 156 84.9 173

25.0 66.6 49.1 47.5 78.1

4.29 7.26 6.07 5.01 8.89

1.4 1.7 1.6 1.2 1.6 1.0

0.38 3.36 0.84 0.19 0.79 0.45

1.81 5.01 1.86 1.97 1.16 1.95

3.5 18.9 5.1 3.2 8.6 4.0

7.3 14.2 5.7 6.1 9.9 3.9

0.18 3.28 0.56 0.05 0.05 0.28

13 36 23 10 37

Note: Al2O3, S, Se and Te contents have been published previously (Fischer-Go¨dde et al., 2011; Wang et al., 2013; Wang and Becker, 2013). The symbol (*) indicates the literature data. Additional xenoliths from Damaping, Hannuoba, China (Gao et al., 2002) and some recently collected peridotites from Lanzo, Italy (italicized and underlined samples) were included. The Cu, Ag, S, Se and Te data were obtained by isotope dilution ICP-MS methods and have typically 1–3% combined measurement uncertainty (2s). Digestions methods are Parr bomb or high pressure asher (HPA-S). Copper data on some samples from Lanzo (Lorand et al., 1993) and Turon de Te´coue`re (Fabries et al., 1998) have been reported before. The underlined Ag data (fertile samples with Al2O3 P 3 wt.%) were used to obtain a mean value of 8 ± 2 ng/g (n = 17, 1s). In comments, L represents lherzolite, and H, harzburgite.

partitioning results indicate that silicate phases may also contain a small fraction of Cu and Ag (e.g., Hertogen et al., 1980; Fellows and Canil, 2012; Jenner and O’Neill, 2012; Lee et al., 2012; Li and Audetat, 2012; Patten et al., 2013; Zajacz et al., 2013). Therefore, complete digestion of whole rock samples and spike-sample equilibration are important for the application of isotope dilution to obtain accurate bulk rock Ag and Cu concentration data. We have developed efficient digestion methods using HF–HNO3 digestion in PFA beakers in Parr bombs and glassy carbon vessels in a high pressure asher (HPA-S) for concentration measurements of chalcophile and siderophile elements by isotope dilution and sector-field ICP-MS (Wang et al., 2014). Chromatographic separation methods in combination with an Aridus desolvator have been employed to minimize the effects of isobaric and oxide interferences. These methods have yielded precise data for NIST SRM612 glass, geological reference materials and carbonaceous chondrites (Wang et al., 2014). In the present study we have employed Parr bomb digestion for most samples. Some replicates of peridotites from Balmuccia and

Baldissero also were digested in glassy carbon vessels by HPA-S for comparison. Because of previous detailed descriptions of the method (Wang et al., 2014), we only briefly describe the procedure and minor changes when applied to peridotites. Suitable amounts of a mixed spike solution containing 109 Ag and 65Cu, and about 0.4 g of sample powder were weighted into 15 ml Savillex PFA beakers or 20 ml glassy carbon vessels, followed by the addition of 1 ml 14 mol l 1 HNO3 and 4 ml 24 mol l 1 HF. Samples were digested in Parr bombs at 190 °C for 3 days or, alternatively, in the HPA-S at 220 °C and 100 bar for 16 h. Both methods dissolved all phases, with the exception of small amounts of spinel in some samples. Sample solutions were dried down on a hotplate at 80–90 °C, converted into chloride form in several steps using 2 ml 14 mol l 1 HNO3, 2 ml 9 mol l 1 HCl twice, respectively, and finally were dissolved in 3 ml 4.5 mol l 1 HCl. Minor amounts of fluoride precipitates were occasionally observed, but did not affect the results because of the application of the isotope dilution method and addition of spike solution before high-temperature

Z. Wang, H. Becker / Geochimica et Cosmochimica Acta 160 (2015) 209–226

HF–HNO3 digestion. After centrifuging the sample solution for 15 min, about 1.5 ml of supernatant in 4.5 mol l 1 HCl was taken for chemical separation, and loaded on 2 ml pre-cleaned 100–200 mesh Eichrom 1-X8 anion resin. The details of the separation procedure are given in Wang et al. (2014). Niobium may form oxide interferences on Ag isotopes, however, peridotite commonly has very low Nb abundances. Thus, it is not necessary to separate Ag from Nb. Silver was collected with Cd, Tl and Bi using nitric acid (Wang et al., 2014). After chemical separation, the Cu fraction was collected with Fe in 8 ml 0.4 mol l 1 HCl and the Ag fraction in 28 ml of a mixture of 0.7 mol l 1 HNO3-1 ml/100 ml H2O2. The Ag fraction was dried down to <0.5 ml and dissolved in 4 ml 0.28 mol l 1 HNO3 for analysis. Further purification of the Cu fraction was performed to remove Fe. The Cu fraction was dried down at 80–85 °C and dissolved in a mixture of 0.4 ml 9 mol l 1 HCl–0.4 ml of 0.15 g/ml ascorbic acid solution, which reduces Fe3+ to Fe2+. After reduction, the Cu fraction was further purified by removal of Fe on 1 ml 100–200 mesh anion resin. The Cu fraction was collected in 4.5 ml 0.4 mol l 1 HCl. The solution was dried down with 0.1 ml of 30 ml/100 ml H2O2 to remove remaining ascorbic acid and then dissolved in 4 ml 0.28 mol l 1 HNO3 for ICP-MS analysis. Both Ag and Cu fractions were analyzed by Element XR sector field ICP-MS (Thermo Scientific) with an Aridus-I desolvator to limit oxide formation (CeO+/Ce+ < 0.003) using methods described before (Wang et al., 2014). The medium mass resolution mode (M/DM = 4000) was applied to Cu and low resolution (M/DM = 300) to Ag. The Cu and Ag concentrations were calculated from 65Cu/63Cu and 109 Ag/107Ag. Potential interferences are 47TiO, 49TiO, 91 ZrO and 93NbO, but chemical separation has removed most Ti, Zr and Nb, and monitored intensities of interferences were low, e.g., 47Ti/63Cu < 0.0001, 91Zr/107Ag < 0.1 and 93Nb/109Ag < 0.01, respectively, indicating negligible effects of interferences. During the period of sample measurements, the total procedural blanks (n = 15) were 30 ± 24 pg (1s) for Ag and 11 ± 10 ng (1s) for Cu. A blank correction was always applied and insignificant (<1%) in most samples. The combined measurement uncertainties of the bulk rock Ag and Cu data by the isotope dilution are typically 1–3% (2s). But the blank correction was higher in depleted harzburgites and in dunites from Balmuccia (10–30% for Ag and a few percent for Cu). In these cases, because of the low Ag and Cu contents, the uncertainty of the blank correction dominates the combined measurement uncertainty of the results. 4. RESULTS 4.1. Data quality The precision and accuracy of the methods have been tested by NIST SRM612 glass, UB-N (peridotite), and other geological reference materials, and the results have yielded Ag and Cu data of good precision (mostly < 5%, 2s, Wang et al., 2014). Some peridotite replicates from Baldissero and Balmuccia bodies were analyzed using

213

Parr bomb and HPA-S digestions. The Ag and Cu results by these two methods mostly show a few percent variations, but also up to 20% for some replicates, e.g., BD90-4 and BD11-12 (Table 1). Some replicates digested solely by Parr bomb also show larger differences between replicates, e.g., BM90-5 and BM11-02A. In contrast, BM11-11 was digested three times by different methods with good repeatability (Table 1). Given negligible effect of interferences, low procedural blanks, the repeatable data on BM11-11 and the previous evaluation on the analytical methods by replicates of different reference materials (Wang et al., 2014), the occasional larger variation of Ag and Cu data in replicates should not result from incomplete digestion or other analytical problems. Previous studies have shown that some chalcophile elements such as the PGE, can be strongly affected by the heterogeneous distribution of these elements and the host phases (nugget effect). Repeatable concentration data on such elements requires that the digestion portion size should be large enough (e.g., >1 g, Meisel and Moser, 2004; Wang and Becker, 2014). Because of the application of HF–HNO3 digestion and the potential problems with fluoride precipitates, sample weights of the present work were about 0.4 g. The poor repeatability (about 20%) of Ag and Cu in some samples may reflect heterogeneity of the host phases in sample powders at relatively low sample weights. Copper contents of some samples from Turon de Te´coue`re (Pyrenees) and Lanzo have been obtained before (Bodinier, 1988; Lorand et al., 1993; Fabries et al., 1998). Except one sample TUR21 with very high Cu contents, our results are 5–20% lower than the values of aliquots of these samples from Turon de Te´coue`re and systematically about 30% lower than those from Lanzo (Table 1). Such differences may to some extent reflect sample heterogeneity but also different precision of the applied analytical methods. Incomplete digestion of spinel in most peridotites should affect the results at a limited level. Data from experiments and natural samples indicate spinel-silicate melt partition coefficients of 0.2–0.7 for Cu (Lee et al., 2012; Liu et al., 2014). The volume proportion of spinel in peridotites is small and the maximum budget of Cu hosed by spinel may only account for a few percent or less of the whole rock data. Because of high temperature leaching and partial dissolution of spinel during acid digestion, the effect of remaining spinel should be even less. The effect is unclear for Ag, but presumably similar to Cu due to their similar behavior in magmatic processes, as will be discussed later. HF–HNO3 digestion at different temperatures and pressures by different methods may have dissolved different proportion spinel and yet most samples have yielded repeatable results. Consequently, the Ag and Cu data of the present study should reflect bulk rock concentrations with a few percent uncertainty or slightly more (<20%) if heterogeneity of some sample powders is taken into account. 4.2. Ag and Cu contents in peridotites Most fertile lherzolites from different localities display a relatively narrow range of Ag and Cu contents, 5–15 ng/g

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Z. Wang, H. Becker / Geochimica et Cosmochimica Acta 160 (2015) 209–226 60

60

a

50

40

Cu (µg/g)

40

Cu (µg/g)

b

50

30 20 10

30 20 10

0

0 0

1

2

3

4

5

0

50

100

Al2O3 (wt. %)

200

250

300

350

S (µg/g) 60

60

c

50

d

50 40

Cu (µg/g)

40

Cu (µg/g)

150

30 20

30 20 10

10

0

0 0

50

100

0

150

5

10

15

20

Te (ng/g)

Se (ng/g) Turon de Técouère

Lanzo

Beni Bousera

Baldissero

Balmuccia

Ligurian ophiolites

Balmuccia dunites

Eifel xenoliths

Hannuoba xenoliths

Lherz (Lorand et al 2010)

BSE (this study)

Sulfide melt-silicate melt partitioning

MSS-sulfide melt partitioning

Fig. 1. Positive correlations of Cu with Al2O3, S, Se and Te contents in mantle peridotites from different geological settings (a–d). Solid straight lines are linear regression lines and the correlations are used to estimate Cu abundances in the BSE. Copper abundance variations resulting from partial melting are modeled by MSS–sulfide melt (solid lines) and by sulfide melt–silicate melt (dashed lines) partitioning using partition coefficients and initial contents in Table 2 and assuming 1000 lg/g S solubility in melts (Lorand et al., 1999; Lee et al., 2012). The variation of Al2O3 content with melting is from Niu (1997). Sulfur becomes completely exhausted at 20% melting and the marks on the lines reflect 1% increments of melting degree. The fractionation behavior of Cu and Te is remarkably different in these two partitioning processes. Lherz data are from Lorand and Alard (2010). Previously published abundances of S, Se and Te (Wang and Becker, 2013; Wang et al., 2013) and Al2O3 in the BSE (Lyubetskaya and Korenaga, 2007) are used in the figures.

and 20–35 lg/g, respectively. These concentrations are lower than in MORB with 15–40 ng/g Ag and 60–120 lg/ g Cu (Jenner and O’Neill, 2012). Two samples contains about 50–60 lg/g Cu, and some samples, mainly from Lanzo and Turon de Te´coue`re have high Ag contents >20 ng/g and even up to 250 ng/g (Table 1). These high Ag contents are beyond the range of most peridotites and even higher than in basalts. We note that the newly collected fresh samples from Lanzo have no such anomalously high Ag contents and show results similar to other fertile samples. The depleted harzburgite xenoliths from Eifel (n = 6) contain rather little Ag and Cu, consistent with low contents of the PGE, S, Se and Te (Fischer-Go¨dde et al., 2011; Wang and Becker, 2013). Most replacive dunites from Balmuccia also show low abundances of Ag, Cu and other chalcophile elements; but one of the dunites (BM11-07A) has higher Ag, Cu, S, Se and Te contents (Figs. 1 and 2). Overall, both Cu and Ag contents in peridotites tend to decrease with decreasing Al2O3 content, an indicator of fertility of peridotites (Fig. 1 and Supplementary Fig. 1). This

suggests a moderately incompatible behavior of Cu and Ag. Copper and Ag display positive correlations with S, Se and Te (Figs. 1 and 2 and Supplementary Fig. 1). Copper, Au and most Ag data (except those with >15 ng/g Ag) also display similar positive correlations (Fig. 3). Results of the present study (Fig. 3c) indicate that the Au concentration of the BSE is closer to 1.0 ng/g (McDonough and Sun, 1995), which is lower than 1.7 ± 0.5 ng/g suggested in previous work (Fischer-Go¨dde et al., 2011). Peridotite xenoliths from Damaping (Hannuoba, China) have Cu and Ag contents similar to other massif peridotites and also follow the correlations of Al2O3, Cu, Ag, S, Se and Te for massif peridotites (Figs. 1–3). Samples with Ag > 15 ng/g display strong deviation from the correlations with Al2O3, S, Se, Te and Cu. Ratios of Cu/Ag in most fertile peridotites from different localities range from about 1500 to 4500, and this range is also found in samples from the same locality, e.g., Balmuccia and Baldissero (Fig. 3a). Notably, Cu/Ag displays no systematic variation with Cu or Al2O3 contents. The mean Cu/Ag ratio of fertile peridotites (mean Cu/Ag

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of 3500 ± 1200, 1s, n = 38 for peridotites with Al2O3 P 2 wt.% and excluding a few high Ag samples) is indistinguishable from the mean Cu/Ag of MORB glasses (Cu/Ag  2500–4500, mean 3600 ± 400, 1s, n = 338, Jenner and O’Neill, 2012) and also overlaps the available data on MORB sulfide droplets (Cu/Ag = 3000 ± 300, 1s, n = 7, Patten et al., 2013) (Fig. 3a). Overall, ratios of S/Cu, S/Ag, Se/Cu and Se/Ag in fertile peridotites also change within a limited range (a factor of 1–3) and are slightly lower or similar to those of the least fractionated MORB (Fig. 4 and Supplementary Fig. 2). In contrast, Te/Cu ratios of most peridotites are significantly higher than those of MORB glasses (Fig. 4c, note the log scale for Te/Cu). 5. DISCUSSION 5.1. Comparison with Ag and Cu concentration data from the literature Copper concentrations obtained in the present study are similar to the range of concentrations in previous work on

Fig. 3. Correlations of Cu, Ag and Au contents in mantle peridotites. (a) Ratios of Cu/Ag in fertile peridotites are constant with changing Cu contents, and indistinguishable from the mean Cu/Ag in MORB glasses (Jenner and O’Neill, 2012) and in MORB sulfide droplets (the gray band, Patten et al., 2013). This observation is remarkable because Cu and Ag contents vary significantly between peridotites, basalts and sulfide droplets. (b, c) Copper, Ag and Au concentrations in most peridotites display broadly positive correlations. The Au contents in peridotites are from Fischer-Go¨dde et al. (2011) and Wang et al. (2013), and the value of Au in the BSE used in the figure is 1.0 ng/g from McDonough and Sun (1995). Symbols and modeling for Cu and Ag are the same as in Fig. 1.

peridotites (see review in Lorand et al., 2013), including data on different samples from the Balmuccia and Baldissero peridotite bodies (Garuti et al., 1984). However, the previous Ag concentration results for peridotites from Balmuccia and Baldissero bodies apparently are anomalously high (>20–100 ng/g, Garuti et al., 1984), because the new data from these localities show significantly lower Ag contents (mostly <20 ng/g) for rocks of

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Fig. 4. Variations of abundances and ratios of Cu, Ag, S and Te in peridotites, MORB glasses and MORB sulfide droplets. The ratios of S/Cu (a) and S/Ag (b) in most peridotites vary within a limited range (mostly a factor of 1–2) and are slightly lower than or similar to those of the least fractionated MORB (Yi et al., 2000; Jenner and O’Neill, 2012; Lissner et al., 2014). Similar features also exist for Se/Cu and Se/Ag (see Supplementary Fig. 2). In contrast, Te/ Cu (c) ratios of most peridotites are higher than those of MORB glass and sulfide droplets. The differences in element ratios between peridotites, fractionated MORB glasses and MORB sulfide droplets indicate a relative bulk compatibility in the order of DTe > DCu  DAg > DSe P DS during melting and MORB evolution, consistent with sulfide melt-silicate melt partitioning and experimental results (Li and Audetat, 2012; Brenan, 2015). MORB sulfide droplet data are from Patten et al. (2013) and shown as the gray bands. Note the log scale of Te/Cu in (c).

comparable major element composition. The concentration range of Ag in peridotites from Balmuccia and Baldissero is similar to most fertile peridotites from other localities. Because of the lack of concurrent enrichment of Cu and S, the previous anomalously high values are probably not the result of metasomatism by sulfide melt. We suspect that

the high Ag concentrations may be inaccurate and probably reflect the analytical limits of the applied methods at that time because the study reported an analytical uncertainty of 30% and a detection limit of 20 ng/g (Garuti et al., 1984). Irrespectively of these systematic differences compared to early Ag data, the new results also show that a minor fraction of peridotites (Baldissero, Hannuoba, Lanzo and Turon de Te´coue`re) have Ag contents higher than 20 ng/g (up to 250 ng/g). Silver concentration data from other peridotites reported recently also occasionally show anomalously high Ag contents (up to 104 ng/g, Horan et al., 2014). Such high Ag contents induce substantial scatter in variation diagrams of Ag with Cu, S, Se and Te, and lead to very low Cu/Ag ratios in some samples (Fig. 3a). These values cannot result from melt depletion nor from sulfide melt metasomatism, because removal or addition of sulfide melts should affect Cu, S, Se and Te concurrently. In discordant replacive dunites of the Balmuccia body (Wang et al., 2013), dissolution of sulfides into S undersaturated silicate melt has simultaneously lowered the S, Se, Te, Cu and Ag contents in the dunites compared to lherzolite protoliths (Figs. 1 and 2). Occasionally high Ag contents of 50–250 ng/g in peridotite samples exceed abundances in most MORB and picrites (Hertogen et al., 1980; Scho¨nba¨chler et al., 2010; Jenner and O’Neill, 2012), implying that either spurious contamination or additional processes, such as hydrothermal alteration, may have affected the Ag contents in these peridotites. In this study, the unexplained Ag enrichment (>15 ng/g) mainly occurs in some peridotites from the Lanzo and Turon de Te´coue`re bodies. The samples from Lanzo with high Ag contents (L213, L215, L216 and L217) display variable degrees of serpentinization (6–60%, Lorand et al., 1993). In contrast, the newly collected samples from Lanzo included here show no noticeable alteration in thin sections and low loss on ignition of <2%. The latter samples also display the typical Ag concentration range as in most other fertile peridotites. Although samples from Turon de Te´coue`re are characterized by less than 10% serpentinization, many samples in this area were strongly affected by mylonitization (Fabries et al., 1998). These observations imply the possibility of Ag enrichment by fluid-related metasomatism or supergene alteration. We note that the replicates of a weathered Hawaiian picrites have dramatically enhanced Ag contents (160 ng/g versus 880–1323 ng/ g) and resulted in variable Ag isotopic compositions (Theis et al., 2013). Even the low value of 160 ng/g is still far higher than concentrations in most basalts and other Hawaiian picrites (Scho¨nba¨chler et al., 2010). Although they are only circumstantial observations and the geological environments and rocks are not directly comparable, it indicates that alteration processes, including serpentinization, may lead to a redistribution of Ag. Such features imply that alteration related to fluid infiltration at subsolidus temperatures is likely to be the main reason for occasionally high Ag contents. The details of the redistribution of Ag are unclear at present and require further work. In the following discussion, we will exclude samples from Lanzo and Turon de Te´coue`re that have high Ag contents. The comparison with data on basalts shows that

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these samples likely have little relevance for the fractionation of Ag in magmatic processes in the mantle. 5.2. The behavior of Ag and Cu in mantle processes The chemical behavior of Ag and Cu and their chalcophile nature have been constrained by the evolution of their concentrations in basic rocks and by element partitioning experiments (e.g., Jenner et al., 2010; Li and Audetat, 2012; Kiseeva and Wood, 2013). The constant Cu/Ag of MORB with decreasing Mg number and similar Cu/Ag ratios in MORB glasses and sulfide droplets indicate the importance of sulfide melt–silicate melt partitioning and similar partition coefficients of Cu and Ag during the evolution of MORB magmas (Jenner et al., 2010, 2012; Jenner and O’Neill, 2012; Patten et al., 2013). In contrast, island arc magmas show a marked decrease of Cu/Ag in the magma with sulfide saturation, indicating fractionation of Cu from Ag and more compatible behavior of Cu relative to Ag in sulfides precipitating in the physicochemical environment of arc magmas (Jenner et al., 2010). Although sulfide melt–silicate melt fractionation during the late evolution of basic magma is now a well constrained process, the early magmatic history of basic magma in the mantle remains debated. At issue is, whether the fractionation of Ag, Cu and other chalcophile elements during melt extraction and refertilization in upper mantle peridotites is predominantly controlled by MSS–sulfide melt or by sulfide melt–silicate melt partitioning and how these elements fractionate from each other (e.g., Bockrath et al., 2004; Ballhaus et al., 2006; Li and Audetat, 2012; Wang et al., 2013; Ko¨nig et al., 2014; Mungall and Brenan, 2014; Wang and Becker, 2015). 5.2.1. Fractionation of Ag and Cu in mantle peridotites Peridotites of the present study have undergone variable extents of melt depletion and refertilization, but overall they display well-defined positive correlations for concentrations of Cu, Ag, S, Se and Te (Figs. 1–3). Limited scatter of Cu/Ag ratios and the absence of variations with changing concentrations of Cu and Al2O3 in fertile peridotites also occur (Fig. 3a). These features indicate similar chemical behavior of S, Se, Te, Cu and Ag and limited fractionation of these elements during melting and refertilization. The limited fractionation of Ag and Cu in peridotites is consistent with data from MORBs (Fig. 3a). Although Cu contents in peridotites, basalts and sulfide droplets, vary dramatically, the mean Cu/Ag ratio of peridotites are very similar to Cu/Ag in basaltic glasses (Jenner and O’Neill, 2012) and in MORB sulfide droplets (Patten et al., 2013). As mentioned above, MORBs that have undergone variable degrees of fractional crystallization also display relatively constant Cu/Ag ratios (Jenner et al., 2010, 2012). Collectively, the similar Cu/Ag ratios of peridotites, MORB glasses and MORB sulfide droplets, although with some scatter, clearly indicate the similar behavior of Cu and Ag and very limited systematic fractionation of Cu from Ag during melt extraction, refertilization and fractionation of basic melts under upper mantle and lower oceanic crust conditions.

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Because of the distinctive fractionation of Cu and Ag in sulfide melt–silicate melt and MSS–sulfide melt partitioning (Li and Audetat, 2012), the limited fractionation of Cu and Ag indicates that sulfide melt–silicate melt partitioning is the dominant fractionation process in these environments. This is consistent with experimental results which show the similarity of partition coefficients of Cu and Ag during sulfide melt–silicate melt partitioning (Li and Audetat, 2012; Kiseeva and Wood, 2013). In contrast, Ag is much more incompatible than Cu in melt melt MSS (DMSS–sulfide of 0.3 and DMSS–sulfide of 0.05, Cu Ag calculated from Li and Audetat, 2012), MSS–sulfide melt partitioning would lead to strong enrichment of Ag in melts compared to Cu if MSS is in the residue. But a strong depletion of Ag relative to Cu is not observed in peridotites and an enrichment of Ag (lower Cu/Ag) in basalts does not occur (Fig. 3). This also can be illustrated with the behavior of the peridotite data in Ag–S and Cu–Te diagrams (see modeling in Figs. 1 and 2). For example, Ag is much more incompatible than S in MSS as both DMSS–sulfide melt and DMSS–silicate melt for Ag are lower than for S (Helmy et al., 2010; Li and Audetat, 2012; Brenan, 2015). If MSS is a residual phase, Ag would become depleted much stronger in peridotites than S in peridotites (Fig. 2). Conversely, Ag should always be enriched in the melts relative to S (that is, melts would have much lower S/Ag than peridotites). The data on peridotites and basalts show a different situation in that depleted peridotites display lower S/Ag than most basalts, indicating that S is more incompatible than Ag (Fig. 4b). The behavior of Ag and S is consistent with sulfide melt– silicate melt partitioning. Previous studies on peridotites, mantle pyroxenites and MORBs suggest that sulfide melt–silicate melt partitioning rather than MSS–sulfide melt predominantly controls the fractionation of the PGE, Au, Re, S, Se and Te (e.g., Peach et al., 1990; Fischer-Go¨dde et al., 2011; Patten et al., 2013; Wang et al., 2013; Wang and Becker, 2015). Experimental results (Li and Audetat, 2012; Kiseeva and Wood, 2013; Mungall and Brenan, 2014; Brenan, 2015) and MORB data (Peach et al., 1990; Yi et al., 2000; Patten et al., 2013) have indicated sulfide melt–silicate melt partition coefficients of DPGE (>105) > DAu (3400–10,000) > DTe (1000–6000) > DCu  DAg (500–1800) > DSe P DS (200– 400) at upper mantle conditions. The relative sequence obtained from experimental sulfide melt–silicate melt partition coefficients is consistent with that inferred during fractional crystallization of S saturated MORB that leads to lower S/Cu, Se/Cu, but higher Te/Cu ratios in MORB sulfide droplets than in MORB glass (Fig. 4 and Patten et al., 2013). Furthermore, these ratios, notably S/Cu and S/Ag in fertile peridotites are similar to or slightly lower than ratios in the least fractionated MORBs, whereas Te/Cu ratios in peridotites are higher than in MORB glasses and sulfide droplets (Fig. 4, also see the comparable behavior of Se/Cu and Se/Ag in Supplementary Fig. 2). The comparison of peridotites, MORB glasses and sulfide droplets indicate a consistent relative bulk compatibility in decreasing order of PGE > Te > Cu  Ag > Se P S during common melting and the evolution of MORB magma fractional

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crystallization, consistent with the trends predicted by experimental data on sulfide melt–silicate melt partitioning. 5.2.2. The influence of disequilibrium on chalcophile element fractionation melt–silicate melt The PGE are characterized by Dsulfide > 105 PGE (Mungall and Brenan, 2014), much higher than other less chalcophile elements (e.g., DTe > DCu  DAg > DSe P DS, on the order of hundreds to thousands). The large differences in sulfide melt–silicate melt partition coefficients of different chalcophile elements and the quantitative application of the data imply some fractionation of Cu and Ag from Te, Au and the PGE because the latter elements are much more compatible in sulfides than Cu and Ag. However, broad linear correlations of S, Se, Te, Cu, Ag and Au in peridotites (this work; Wang and Becker, 2013) and positive correlations between S, Re, Au and Pd (e.g., Becker et al., 2006; Fischer-Go¨dde et al., 2011) indicate similar (although not identical) bulk incompatibility. The observation that ‘apparent’ partition coefficients in natural processes are lower than experimentally determined values probably reflects incomplete sulfide melt–silicate melt equilibration, that is, the mass ratio of silicate melts to equilibrated sulfide melts is smaller than the experimental partition coefficient values (called the ‘R factor’, Campbell and Naldrett, 1979; Mungall, 2002). Many studies have shown that the apparent partition coefficients to explain Pd contents in depleted peridotites is in the order of thousands, much lower than data from experimental studies (e.g., Lorand et al., 1999; Luguet et al., 2007; Liu et al., 2009; Wang et al., 2013). The data from MORBs (Patten et al., 2013) and mantle pyroxenites (Wang and Becker, 2015) also indicate variable and significantly lower apparent Dsulfide melt–silicate melt PGE compared to experimental data. Chalcophile elements hosted in sulfide melts may be not completely equilibrated with the coexisting silicate magma due to kinetic effects, e.g., the diffusivities of chalcophile elements in the silicate melt, the size of the sulfide droplets, the strain rate in the enclosing silicate melt, and the duration of the exchange (see details in Mungall, 2002). The apparent lower Dsulfide melt–silicate melt PGE and the smaller observed differences in the fractionation of Cu and Ag from Te, Au and the PGE than expected from their experimental partition coefficients might be explained by the kinetic fractionation of chalcophile elements into the sulfide melts and incomplete sulfide melt-silicate melt equilibration in natural processes (Mungall, 2002). We should keep in mind that most peridotites in this study are not solely affected by partial melting but also refertilized by melt infiltration, which affects the abundances of chalcophile elements via precipitation of sulfidepyroxene-Al phase assemblages (e.g., Le Roux et al., 2007; Lorand and Alard, 2010). In the lherzolite stability field, both processes, melting and refertilization, likely occur nearly simultaneously. Precipitation of variable amount of sulfides during refertilization will replenish the contents of S, Se, Te, Cu, Ag and other chalcophile elements. Correlations of S, Se, Te, Cu, Ag, and Au probably reflect the variable mixing of different generations of sulfides in bulk peridotites (e.g., Le Roux et al., 2007;

Lorand et al., 2010; Ko¨nig et al., 2014), as revealed by Os and Pb isotopes (e.g., Burton et al., 1999, 2012; Alard et al., 2002; Harvey et al., 2011; Warren and Shirey, 2012). Because of the predominant control by sulfide melts and similar sulfide melt–silicate melt partitioning behavior of S, Se, Cu, Ag, Te and Au, the transport of these elements via sulfide dissolution and precipitation during open-system melting and refertilization has limited effects on the ratios of these elements. As sulfide melt–silicate partition coefficients of Cu and Ag are similar and Cu/Ag in MORBs and their sulfide droplets (Patten et al., 2013) are indistinguishable from those in peridotites, it is easy to understand why the precipitation of sulfide melts during refertilization does not lead to significant change of Cu/Ag ratios in fertile peridotites. 5.2.3. Controls on Cu and Ag abundances in silicates and sulfides Experimentally determined partition coefficients of Cu between silicate minerals and melts are variable, and have led to different interpretations of the significance of sulfides and silicates in controlling Cu partitioning during partial melting of peridotites (Fellows and Canil, 2012; Liu et al., 2014, both S-free experiments). Silicate mineral/melt partition coefficients of DCu from Liu et al. (2014) and results estimated from in situ laser ablation ICP-MS analysis of natural samples (Lee et al., 2012) are similar. Comparison with data from Fellows and Canil (2012), Dolivine-melt (about 0.05) are similar and only moderate differences in Dcpx-melt (e.g., 0.06 versus 0.23; the latter from Fellows and Canil, 2012) and Dopx-melt (0.05 versus 0.15) exist between the experimental studies. These results indicate only weak lithophile affinity of Cu in mantle peridotites. Under oxidizing conditions (fO2 > FMQ + 1.2, Liu et al., 2014), more Cu may partition into silicate phases, however, such fO2 are higher than conditions for typical mantle and MORB genesis. Although a small fraction of Cu is hosted in silicate phases, several lines of evidence indicate that sulfides predominantly control abundances and partitioning of Cu in mantle peridotites. Petrological studies of sulfides and in situ analyses of sulfides in mantle peridotites clearly indicate that sulfides contain much Cu (e.g., Lorand, 1991; Lorand et al., 1993; Luguet et al., 2004; Wang et al., 2009). Different experiments also have shown consistent results for sulfide melt–silicate melt partition coefficients of 500–1800 at upper mantle conditions (e.g., Ripley et al., 2002; Li and Audetat, 2012; Kiseeva and Wood, 2013; Mungall and Brenan, 2014), consistent with data from MORB sulfide droplets and glasses (Peach et al., 1990; Patten et al., 2013). Even if we use the higher Dsilicate mineral-melt values (Fellows and Canil, 2012), the mass balance of sulfides and silicate phases and corresponding partition coefficients indicate that the contribution of sulfides accounts for >70% of the bulk rock partition coefficient of Cu during low to moderate degrees of melting (see Supplementary Table 1). The sulfide contribution could be even higher when the lower values of Dsilicate mineral-melt (Lee et al., 2012; Liu et al., 2014) are used for calculations. As shown in Figs. 1 and 2 (also Supplementary Fig. 1), the

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positive correlations of Cu and Ag with S, Se and Te are much better than the correlations with Al2O3, which is mainly hosted in pyroxene and spinel. This also reflects the predominant chalcophile nature of Cu and Ag. Removal and precipitation of variable proportion of sulfides, pyroxene and Al rich phases during melting and refertilization can explain the larger scatter in correlations of Cu and Ag with Al2O3 and other lithophile elements compared to the correlations of S, Se, Te, Cu and Ag. For the much less studied Ag, it was difficult to evaluate how much of it is hosted by silicate or sulfides phases in peridotites. Experimental results show that the solubility of Ag in basaltic melts can be at a few lg/g level, far higher than typical abundances in MORB and in picrite (Zajacz et al., 2013). As discussed above, the partitioning behavior of Cu, S, Se and Te have been well constrained, thus abundances of Cu and Ag in peridotites and MORB and the systematic fractionation of Cu, Ag, S, Se and Te provide constraints on Ag storage in mantle peridotites. Because Ag and Cu have similar sulfide melt–silicate melt partition coefficients at upper mantle conditions (Li and Audetat, 2012; Kiseeva and Wood, 2013), as for Cu, sulfides should be the important phases that control Ag abundances in mantle rocks. Because Cu/Ag ratios remain similar in fertile peridotites, basalts and MORB sulfides, similar behavior of Cu and Ag indicate that the same phases control their behavior during melting, basalt fractionation and sulfide segregation. In spite of the lack of Ag concentration data in silicate phases and experimental partitioning data between silicate minerals and melts for Ag, the similar enrichment factor of Cu and Ag in basalts relative to fertile peridotites and constant Cu/Ag ratios predict that Ag should be as incompatible as Cu in silicate minerals of peridotites. Consequently, Cu and Ag predominantly retain in sulfides of peridotites, and silicate phases play a limited role. The behavior of Ag may change as it becomes more lithophile at lower temperatures or in more evolved magmas (Zajacz et al., 2013). 5.3. Comparison of chalcophile element abundances in massif and xenolith peridotites Previous studies have shown that most peridotite xenoliths are characterized by much lower sulfur concentrations compared to massif peridotites of similar major element compositions (e.g., Lorand, 1990, 1993; Ionov et al., 1992, 1993; Lorand et al., 2003, 2013; Wang and Becker, 2013). Similar observations also have been made for the PGE contents (see review in Lorand et al., 2013). These compositional differences between xenoliths and peridotite massifs have been ascribed to complex secondary processes such as melt/fluid metasomatism, volatilization and supergene weathering for xenoliths (e.g., Lorand, 1990; Rehka¨mper et al., 1997; Handler et al., 1999; Lorand et al., 2003, 2013; Reisberg et al., 2005; Ackerman et al., 2009; Alard et al., 2011). Only few suites of peridotite xenoliths, notably Damaping (Hannuoba, China, Gao et al., 2002; Becker et al., 2006; Liu et al., 2010) and some samples from Kilbourne Hole (SW, USA, BVSP, 1981; Morgan, 1986; Becker et al., 2006) appear to have escaped noticeable

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modification of their chalcophile element inventory. In order to better estimate Cu and Ag abundances in the BSE, it is prudent to compare the data from massif and xenolith peridotites and to evaluate the influence of the secondary processes that have affected many xenoliths on the abundances of Cu and other chalcophile elements. Abundances of Cu, Ag, S, Se and Te in xenoliths from Damaping, Hannuoba (this study, Wang and Becker, 2013) show that the concentration ranges and correlations of Cu–Ag–S–Se–Te–Al2O3 contents are consistent with data on massif peridotites (Figs. 1, 2 and 5). These data are also consistent with the PGE contents of these xenoliths reported before (Gao et al., 2002; Becker et al., 2006; Liu et al., 2010). Some xenoliths from Kilbourne Hole reported earlier (BVSP, 1981; Morgan, 1986; Becker et al., 2006) also display chalcophile element compositions similar to massif peridotites (Fig. 2). But other xenoliths from Kilbourne Hole that were interpreted to be affected by melt metasomatism show very different systematics of trace elements, PGE and Os isotopic compositions (Harvey et al., 2011, 2012). Sulfur abundances in xenoliths are highly variable and typically much lower than 100 lg/g and lower than in massif peridotites at comparable Al2O3 contents (Lorand, 1990, 1993; Ionov et al., 1992, 1993; Lorand et al., 2003, 2013). Some peridotite xenoliths also show high S contents of >250 lg/g (Lorand et al., 2003; Chu et al., 2009; Alard et al., 2011; Fig. 5b). The low or high S abundances in xenoliths compared to massif peridotites with similar fertility or Cu content indicate strong mobilization of S in these samples (e.g., Lorand, 1990; Lorand et al., 2003; Alard et al., 2011). Copper and S in these peridotite xenoliths do not correlate (Fig. 5b). Lower Cu abundances at given Al2O3 contents in some fertile xenoliths (Fig. 5a) may hint at the mobilization of some Cu in these samples as well, but clearly less than for sulfur. Copper and Se are often thought to be much less mobile than S, and the ratios of S to Cu (Handler et al., 1999) or to Se (Lorand et al., 2003) were used to estimate the extent of oxidative sulfide breakdown. Xenoliths from Montferrier in the Massif Central (France) have high Cu and low Se and S contents (Lorand et al., 2003; Alard et al., 2011), indicating decoupling of Cu, Se and S from the correlation of massif peridotites (Fig. 5c). Copper contents in these fertile peridotite xenoliths are similar to those of massif peridotites. This suggests that Se is more mobile during these alteration processes than previously thought and that Cu is much less mobile than S and Se. Many fertile xenoliths display no noticeable alteration and are also depleted both in S and Se (Lorand et al., 2003; Liu et al., 2010), indicating that other processes also may cause concurrent loss of S and Se. Besides, it is noteworthy that the PGE, including compatible Ir and Os commonly show lower concentrations in many xenoliths as well (Fig. 5d, also see review in Lorand et al., 2013). Abundances of Ir in many fertile peridotite xenoliths are even lower than in depleted abyssal harzburgites (Marchesi et al., 2013) and in the serpentinized harzburgites from the Oman ophiolite (Hanghøj et al., 2010) (Fig. 5d). These observations indicate that the secondary processes that have affected mantle xenoliths not only deplete the

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60

a

50 40

40

Cu (μg/g)

Cu (μg/g)

b

50

30 20

30 20 10

10

0

0 0

1

2

3

4

0

5

100

200

60

400

500

600

6

c

50

d

5

40

4

Ir (ng/g)

Cu (μg/g)

300

S (μg/g)

Al2O3 (wt. %)

30

3

20

2

10

1

0

0 0

50

100

150

0

10

20

Se (ng/g)

30

40

50

60

Cu (μg/g)

Massif peridotites

Peridotite xenoliths (literature)

Serpentinized Oman harzburgites

Abyssal harzburgites

Hannuoba peridotite xenoliths

Eifel harzburgite xenoliths

Kaapvaal cratonic harzburgite xenoliths

Bulk silicate Earth

Fig. 5. Comparison of Cu, S, Se and Ir contents in massif and xenolith peridotites, showing the effects of secondary processes on the contents of these elements in xenoliths. (a) Most massif and xenolith peridotites show a positive correlation of Cu and Al2O3. Note that some fertile xenoliths have lower Cu contents at given Al2O3 content compared to peridotite massifs. (b) Most xenoliths display decoupled behavior of Cu and S and differ from massif peridotites because of loss or addition of S (but much less for Cu as indicated in (a). (c) Many fertile xenoliths also display decoupling of Cu and Se and show systematically lower Se contents, implying a depletion of Se relative to Cu in peridotite xenoliths. (d) The contents of the compatible element Ir in many xenoliths are variable and lower than in massif peridotites at given Cu content (similar as at given Al2O3 content because of the broad correlation in (a). Remarkably, the contents of Ir in many fertile xenoliths (those with high Cu contents) are even lower than in depleted abyssal harzburgites. Note that xenoliths from Hannuoba (China) show similar contents and correlations of chalcophile elements as massif peridotites. The gray circles are data of massif peridotites in Fig. 1 and the crosses are xenolith data from the literature (Lorand and Alard, 2001; Lorand et al., 2003; Reisberg et al., 2005; Peltonen and Bru¨gmann, 2006; Ackerman et al., 2009; Chu et al., 2009; Alard et al., 2011). Straight lines in (b) and (c) are the linear regression lines based on the massif peridotites and same as in Fig. 1. Data on highly depleted peridotites are harzburgite xenoliths from the Kaapvaal craton (Griffin et al., 2004), serpentinized harzburgites from the Oman ophiolite (Hanghøj et al., 2010) and serpentinized abyssal harzburgites from ODP Hole1274A (Marchesi et al., 2013).

mobile S and Se, but also to some extent Cu and the PGE which appear to be less mobile in these processes (e.g., Handler et al., 1999; Liu et al., 2009). Most xenoliths display different extents of LREE enrichment, which is often explained to reflect metasomatism by melts or fluids (e.g., Reisberg et al., 2005; Chu et al., 2009; Alard et al., 2011; Liu et al., 2011; Harvey et al., 2012). Sulfide dissolution or breakdown during these metasomatic processes in the lithosphere, or during interaction with fluids from the host magma are the most likely processes leading to the low abundances of the PGE and other chalcophile elements in peridotite xenoliths (e.g., Handler et al., 1999; Bu¨chl et al., 2002; Lorand et al., 2003; Reisberg et al., 2005;

Ackerman et al., 2009; Liu et al., 2010; Alard et al., 2011; Zhang et al., 2012). In summary, complex secondary processes strongly affect sulfides and the budgets of chalcophile elements in mantle xenoliths to variable extents. Following this observation, it is likely that Ag in mantle xenoliths also could be affected. On the other hand, refertilization also changes abundances of chalcophile elements in massif peridotites, however, in a systematic way (see Section 5.2). These features indicate that massif peridotites and those xenoliths which have escaped noticeable modification by secondary processes are better samples for estimation of Cu and Ag contents in the BSE.

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Table 3) after excluding two anomalously values (BM1102A and Lanz16 in Table 1). The calculated value should be regarded as a minimum value due to the incompatible behavior of Cu. Copper displays positive correlations with S, Se and Te contents. Such correlations also can be used to estimate Cu abundances in the BSE if abundances of the other elements have been constrained. According to the correlations with Al2O3, minimum abundances of S, Se and Te in the BSE at 3.52 ± 0.60 wt.% of Al2O3 (Lyubetskaya and Korenaga, 2007) are 210 ± 40 lg/g, 80 ± 17 ng/g and 11.0 ± 1.7 ng/g, respectively (Wang and Becker, 2013). Consistent Cu contents in the BSE of 30 lg/g (with a relative uncertainty of 20%) were obtained from the correlations with S, Se and Te using the minimum estimates of Al2O3, S, Se and Te contents of the BSE (Table 3). These values for Cu are indistinguishable from 30 ± 5 lg/g in McDonough and Sun (1995). In fact, many fertile peridotite xenoliths also have Cu contents around 30 lg/g (Fig. 5a). The previous estimate of 20 ± 10 lg/g in the BSE from peridotite xenoliths (Palme and O’Neill, 2003) is relatively low and probably reflects the effects of secondary processes on Cu abundances in some xenoliths. Some peridotites (mainly from Lanzo and Turon de Te´coue`re) have anomalously high Ag contents and they differ from most fertile samples. These data lead to increased scatter in correlations of Ag with Cu, S, Se and Te and, because of the possible origin by late stage alteration or contamination, should be excluded. Most fertile peridotites have less than 15 ng/g Ag and they yield a mean value of 8 ± 2 ng/g for Ag (n = 17, 1s, Al2O3 P 3 wt.%, Table 1). We have already mentioned that the mean Cu/Ag ratios in fertile peridotites (3500 ± 1200, 1s, n = 38), MORB glasses (3600 ± 400, 1s, n = 338) and sulfide droplets in MORB (3000 ± 300, 1s, n = 7) are overlapping. The

Table 2 Parameters for modeling in Figs. 1–3.

Initial contents Dsulfide melt–silicate DMSS–sulfide melt

melt

S

Cu

Ag

Te

210 lg/g 400 1.2

30 lg/g 1000 0.3

9 ng/g 1000 0.05

11 ng/g 2500 0.02

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Note: Initial contents of S and Te from Wang and Becker (2013), Cu and Ag from this study. Dsulfide melt–silicate melt for Cu and Ag (Li and Audetat, 2012; Kiseeva and Wood, 2013) and S and Te are consistent with the decreasing sequence of relative sulfide meltsilicate melt partitioning of Te > Cu  Ag > Se P S (Mungall and Brenan, 2014; Brenan, 2015). Experimental DMSS–sulfide melt are chosen for S, Te (Helmy et al., 2010) and for Cu and Ag (calculated from Li and Audetat, 2012).

5.4. Abundances of Ag and Cu in the BSE and their implications As a result of the limited available Ag data in peridotites prior to the present study, previous estimates of the Ag concentration in the BSE were 4–8 ng/g with large uncertainty (McDonough and Sun, 1995; Palme and O’Neill, 2003). Although Cu has been well studied, previous estimates also show differences, ranging from 20 ± 10 lg/g (Palme and O’Neill, 2003) to 30 ± 5 lg/g (McDonough and Sun, 1995) based on fertile peridotite xenoliths and massif peridotites, respectively. The systematic behavior of Ag, Cu and other chalcophile elements, the effects of refertilization and the differences between xenolith and massif peridotites provide a better foundation to constrain their abundances in the BSE. We will mainly rely on data from massif peridotites obtained in this study to discuss the Ag and Cu abundances in the BSE model. Fertile samples with Al2O3 P 3 wt.% were used to calculate a mean concentration of 29 ± 4 lg/g for Cu (n = 22, 1s, Table 3 Estimates of the minimum abundances of Cu and Ag in the BSE. Elements

Methods

Results

Comments

Cu

Mean values of fertile peridotites with Al2O3 P 3.0 wt.% Cu–S Cu–Se Cu–Te Updated estimate

Cu = 29 ± 4 lg/g (n = 22, 1s)

Excluding two anomalously values

Cu = 32 ± 7 lg/g Cu = 29 ± 5 lg/g Cu = 30 ± 6 lg/g Cu = 30 ± 6 lg/g

y = 0.15x, R2 = 0.69 y = 360x, R2 = 0.77 y = 2700x, R2 = 0.75

Literature

Peridotite xenoliths Massif peridotites

Cu = 20 ± 10 lg/g (1s) Cu = 30 ± 5 lg/g (1s)

Palme and O’Neill (2003) McDonough and Sun (1995)

Ag

Mean values of fertile peridotites with Al2O3 P 3.0 wt.% Cu/Ag ratios of fertile peridotites and basalts (3500 ± 1000, 1s)

Ag = 8 ± 2 lg/g (n = 17, 1s)

Excluding some anomalously values

Ag = 9 ± 3 lg/g (1s) @ Cu = 30 lg/g

Constant ratios in fertile peridotites (3500 ± 1200, 1s, n = 38), MORB (3600 ± 400, 1s, n = 338) and MORB sulfide droplets (3000 ± 300, 1s, n = 7)

Updated estimate

Ag = 9 ± 3 lg/g (1s)

Ag/Na ratios of peridotites and basalts Derivation from peridotites and basalts

4 ng/g, large uncertainty

Palme and O’Neill (2003)

8 ng/g, large uncertainty

McDonough and Sun (1995)

Literature

(1s) @ S = 210 lg/g (1s) @ Se = 80 ng/g (1s) @ Te = 11 ng/g (1s)

Concentration normalized to CI chondrite and Mg

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a

Lit

Mg Si

1.00

ho de phile ple vo tio lat n i ile n t el Li he em BS en E t K Na

Cu, T50%=1037K

Ga

Ag, T50%= 996K 0.10

B

F Rb

Cu Zn

Refractory lithophile Lithophile volatile Siderophile volatile

Ag

0.01 1600

1400

1200

1000

800

600

50% condensation temperature (K)

Metal-silicate partition coefficients

1000

b Cu

100

Ag

10 Ag: Wood et al 2014 @1.5 GPa and 1460-1650°C Cu: Wood et al 2014 @1.5 GPa and 1460-1650°C Cu: Corgne et al 2008 @ 3.6-7.7 GPa and 1850-2200°C Cu: Righter and Drake 2000 @ 1-9 GPa and 1250-1900°C

1 0%

5%

10%

15%

20%

25%

30%

S content of the metal (wt.%)

Fig. 6. (a) Depletion of CI chondrite and Mg normalized abundances of Cu and Ag in the BSE. (b) Metal–silicate partition coefficients from the literature. Partition coefficients at low P–T conditions (1.5 GPa and 1460–1650 °C) increase with increasing S contents of the metal (Wood et al., 2014). These values are too high to explain the observed depletion of Cu and Ag in the BSE by equilibrium core formation. The different relative depletion of Ag and Cu (a factor of 5–6 more for Ag) is also difficult to be explained by the less siderophile behavior of Ag at S content of the metal <20 wt.%. Additional metal–silicate partition coefficient data for Cu are from Righter and Drake (2000) and Corgne et al. (2008) for comparison. With exception of Cu and Ag abundances from this study, the data in (a) are from Lodders (2003) and Palme and O’Neill (2003).

constant Cu/Ag ratios in peridotites and basalts (3000–4000) thus should be representative of the ratio of Earth’s upper mantle and can be used to better constrain the Ag content in the BSE. Using the Cu/Ag ratio of 3500 ± 1000 and Cu abundance of 30 ± 6 lg/g (1s) in the BSE, the Ag abundance of the BSE is estimated to 9 ± 3 ng/g (1s). The Cu and Ag abundances of 30 ± 6 lg/ g (1s) and 9 ± 3 ng/g (1s), respectively, would be about 20–30% higher if the BSE has Al2O3 content of 4.5 wt.% as proposed by some workers (McDonough and Sun, 1995; Palme and O’Neill, 2003), however, the higher estimate of Al2O3 is not supported by statistical analysis (Lyubetskaya and Korenaga, 2007).

On the basis of the PGE and Re abundances in the BSE, it was estimated that the ‘late veneer’ delivered about 0.5 ± 0.2 wt.% of Earth’s mass after completion of Earth’s core formation (e.g., Walker, 2009). If the late veneer delivered materials with volatile element abundances similar (but not necessarily identical) to CM carbonaceous chondrites (Wang and Becker, 2013), it must also have delivered some Cu and Ag. Mass balance (e.g., using data from the Murchison CM2 chondrite from Wang et al., 2014) indicates that the maximum contribution from the late veneer was <0.8 ng/g Ag and <1 lg/g Cu, that is, less than 10% of the Cu and Ag budgets of the BSE, and within the uncertainty of the estimated values. This suggests that the abundances and isotopic compositions of Ag and Cu in the BSE should predominantly reflect the compositions of the main building materials of the Earth and the effects of core formation (also see Scho¨nba¨chler et al., 2010). The abundances of Cu and Ag in the BSE, normalized to CI chondrite and Mg abundances (0.10 ± 0.02 and 0.018 ± 0.006, respectively, Palme and O’Neill, 2003) are depleted relative to refractory and volatile lithophile elements (Fig. 6). Silver is more depleted than Cu by a factor of 5–6. Silver and Cu are moderately volatile elements in cosmochemical processes with similar 50% condensation temperature (996 K and 1037 K, respectively, Lodders, 2003). If Ag and Cu contents in the Earth’s building materials follow the volatility trend as defined by lithophile volatile elements (Alle`gre et al., 2001; McDonough, 2003), the different relative depletion of Ag and Cu should reflect core formation. Recently determined metal–silicate partition coefficients of Ag and Cu at low P–T conditions (1.5 GPa and 1460–1650 °C) are in the order of several hundreds and higher if sulfur abundances in the metal are high (Wood et al., 2014). These values would require a much stronger depletion of Ag and Cu relative to the volatility trend defined by lithophile volatile elements than actually observed. Published metal–silicate partitioning data for Cu and Ag are inconsistent with equilibrium metal–silicate segregation at low P–T conditions, because the depletion of Ag in the BSE is 5 times stronger than the depletion of Cu, however, experiments indicate that Dmetal–silicate is Ag at S contents in the metal always lower than Dmetal–silicate Cu <20 wt.% (Wood et al., 2014, Fig. 6). Even in the case of disequilibrium metal–silicate segregation, which may lead to lower apparent metal–silicate partition coefficient values, the stronger depletion of Ag compared to Cu would be difficult to explain by core formation alone. Recent work by Righter (2011) has suggested that extrapolation of metal– silicate partitioning data to 27–33 GPa, 3300–3600 K, DIW = 1, and assuming S- and C-bearing metallic liquid (molar fraction Xs = 0.04, Xc = 0.11) can explain abundances of Cu and many other siderophile elements in the BSE by core formation. At present, it is unclear if the depletion of Ag is consistent with these conditions. 6. CONCLUSIONS Abundances of Ag and Cu in mantle peridotites from different geological settings (n = 68), with variable extent of melt depletion and refertilization, have been determined

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by isotope dilution methods to constrain the behavior of Ag, Cu and other chalcophile elements during high temperature mantle processes and to better constrain Ag and Cu abundances in the bulk silicate Earth. Silver and Cu are moderately incompatible elements and are enriched in basic magmas compared to most mantle peridotites. Sulfides are the predominant host phases of Ag and Cu, however, a small fraction of these elements may be hosted in silicate phases of mantle peridotites. Similar ranges of Cu/Ag ratios (3000–4000) of peridotites, MORB glasses and MORB sulfide droplets indicate the similar behavior of Cu and Ag and limited fractionation of these elements during melt extraction, refertilization and fractional crystallization of basic magma. The mean Cu/Ag ratios of peridotites and basalts are representative of the composition of Earth’s upper mantle. Abundances of Cu, Ag, S, Se, Te and Au in most massif peridotites and in a few xenolith suites show positive correlations and indicate the limited fractionation of these elements during melting and refertilization. The limited fractionation of Cu and Ag and the abundance variations of the PGE, Au, S, Se, Te, Cu and Ag in peridotites and basalts are consistent with sulfide melt–silicate melt partitioning with relative bulk partition coefficients in decreasing order of PGE > Au P Te > Cu  Ag > Se P S. The kinetically-controlled fractionation of chalcophile elements into sulfide melts and incomplete sulfide melt–silicate melt equilibrium in natural processes (Mungall, 2002), as well as the effects of reactive melt infiltration and multiple generations of sulfides in peridotites, may lead to the incorporation of larger fractions of chalcophile elements into magmas than expected from experimentally determined partition coefficients. Only few peridotite xenolith suites reported so far (e.g., Hannuoba and some from Kilbourne Hole) show similar abundance ranges and correlations of chalcophile elements as massif peridotites. Complex secondary processes related to melt/fluid metasomatism and alteration lead to the commonly lower and more variable abundances of chalcophile elements in peridotite xenoliths compared to massif peridotites. Refertilization of depleted peridotites may change abundances of chalcophile elements in peridotites and establishes the correlations between S, Se, Te, Cu and Ag. These correlations and constant Cu/Ag ratios of peridotites and MORBs constrain the minimum abundances (at 3.5 wt.% Al2O3) of Ag and Cu in the BSE at 9 ± 3 (1s) ng/g and 30 ± 6 lg/g (1s), respectively. The ‘late veneer’ contributed a small fraction of Ag and Cu (<10%) to the BSE, hence the abundances and isotopic compositions of Ag and Cu in the BSE predominantly reflect the composition of accreted material and the effect of core formation during the main stages of accretion. Available low pressure–temperature metal–silicate partitioning data cannot explain the different extent of depletion of Cu and Ag in the BSE. ACKNOWLEDGEMENTS We thank M. Feth and P. Gleissner for support in the lab. J.P. Lorand, S. Gao, T. Gawronski are thanked for providing some

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samples. This work was supported by funds of Freie Universita¨t Berlin, a China Scholarship Council fellowship to Z. Wang and DFG funding (Be1820/7-1, Be1820/12-1) to H. Becker. We thank M. Horan, T.J. Ireland and an anonymous reviewer for their helpful comments on the manuscript and R.J. Walker for editorial handling.

APPENDIX A. SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.gca.2015.04.006. REFERENCES Ackerman L., Walker R. J., Puchtel I. S., Pitcher L., Jelinek E. and Strnad L. (2009) Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: a study of the upper mantle profile beneath Central Europe. Geochim. Cosmochim. Acta 73, 2400–2414. Alard O., Griffin W. L., Pearson N. J., Lorand J. P. and O’Reilly S. Y. (2002) New insights into the Re–Os systematics of subcontinental lithospheric mantle from in situ analysis of sulphides. Earth Planet. Sci. Lett. 203, 651–663. Alard O., Lorand J.-P., Reisberg L., Bodinier J.-L., Dautria J.-M. and O’Reilly S. Y. (2011) Volatile-rich metasomatism in Montferrier xenoliths (Southern France): implications for the abundances of chalcophile and highly siderophile elements in the subcontinental mantle. J. Petrol. 52, 2009–2045. ´ . (2001) Chemical composition Alle`gre C., Manhe`s G. and Lewin E of the Earth and the volatility control on planetary genetics. Earth Planet. Sci. Lett. 185, 49–69. Arndt N., Lesher C. and Czamanske G. (2005) Mantle-derived magmas and magmatic Ni–Cu-(PGE) deposits. Economic Geology 100th Anniversary volume, 5–24. Bu¨chl A., Brugmann G., Batanova V. G., Munker C. and Hofmann A. W. (2002) Melt percolation monitored by Os isotopes and HSE abundances: a case study from the mantle section of the Troodos Ophiolite. Earth Planet. Sci. Lett. 204, 385–402. Ballhaus C., Bockrath C., Wohlgemuth-Ueberwasser C., Laurenz V. and Berndt J. (2006) Fractionation of the noble metals by physical processes. Contrib. Mineral. Petrol. 152, 667–684. Becker H., Horan M. F., Walker R. J., Gao S., Lorand J.-P. and Rudnick R. L. (2006) Highly siderophile element composition of the Earth’s primitive upper mantle: constraints from new data on peridotite massifs and xenoliths. Geochim. Cosmochim. Acta 70, 4528–4550. Bockrath C., Ballhaus C. and Holzheid A. (2004) Fractionation of the platinum-group elements during mantle melting. Science 305, 1951–1953. Bodinier J. L. (1988) Geochemistry and petrogenesis of the Lanzo peridotite body, western Alps. Tectonophysics 149, 67–88. Bodinier J. L., Dupuy C. and Dostal J. (1988) Geochemistry and petrogenesis of eastern pyrenean peridotites. Geochim. Cosmochim. Acta 52, 2893–2907. Bodinier J. L., Menzies M. A. and Thirlwall M. F. (1991) Continental to oceanic mantle transition-REE and Sr–Nd isotopic geochemistry of the Lanzo Lherzolite Massif. J. Petrol., 191–210. Brenan J. M. (2015) Se–Te fractionation by sulfide–silicate melt partitioning: implications for the composition of mantlederived magmas and their melting residues. Earth Planet. Sci. Lett. (in press).

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