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ISSN 0869-5911, Petrology, 2018, Vol. 26, No. 2, pp. 145–166. © Pleiades Publishing, Ltd., 2018. Original Russian Text © N.L. Mironov, M.V. Portnyagin, 2018, published in Petrologiya, 2018, Vol. 26, No. 2, pp. 140–162.

Coupling of Redox Conditions of Mantle Melting and Copper and Sulfur Contents in Primary Magmas of the Tolbachinsky Dol (Kamchatka) and Juan de Fuca Ridge (Pacific Ocean) N. L. Mironova, * and M. V. Portnyagina, b aVernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia b GEOMAR Helmholtz Center for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany *е-mail: [email protected]

Received October 22, 2016; in final form, January 23, 2017

Abstract⎯The compositions of parental melts of Tolbachinsky Dol (Kamchatka) basalts were estimated from the compositions of olivine-hosted (Fo90.5-83.1) primitive melt inclusions in the rocks of the Northern breakthrough of the Great Tolbachik Fissure Eruption (1975 A.C.) and of the late-Holocene cone “1004”. The parental melts contain 100–150 ppm Cu and 0.16–0.30 wt % S. These concentrations are much higher than those determined for the initial magmas of mid-ocean ridge basalts (MORB), for example of the Juan de Fuca ridge (Cu = 55–105 ppm, S=0.09–0.12 wt %). Modeling of mantle melting under variable redox conditions demonstrated that the high Cu and S contents in the Tolbachinsky Dol melts can be obtained by 6– 12% melting of DMM-like source under oxidized conditions (ΔQFM = +1.2 ± 0.1) and do not require a significant (>30–35% for S) subduction-related influx of these elements to the mantle source. The high contents of Cu and S in the Tolbachinsky Dol melts are largely explained by the increase of sulfide solubility in a silicate melt under oxidized conditions. In contrast, relatively reduced (ΔQFM ~ 0) conditions of MORB generation result in low contents of Cu and S in their initial magmas. The estimated ΔQFM values agree well with the data obtained using the Cr-spinel–olivine oxybarometer. The high oxygen potential of Tolbachinsky Dol primary magmas is inherited by more evolved magmas, thus favouring Cu enrichment up to 270 ppm during magma fractionation, approaching maximum copper contents in the global systematics of island-arc rocks. DOI: 10.1134/S0869591118020030

INTRODUCTION Redox regime usually described by the oxygen fugacity ( fO2 ) is one of the most important factors of the Earth’s evolution (e.g., Kadik, 2006; Ryabchikov and Kogarko, 2010). It determines the composition of mantle magmas (e.g., Green et al., 1987), their differentiation trends (e.g., Ariskin and Barmina, 2004; Portnyagin et al., 2012), and the behavior of many chemical elements, including chalcophile ore elements (for instance, Cu, Ag, and Au), in magmatic processes (Mungall, 2002; Borisov, 2005; Botcharnikov et al., 2011). The more oxidized nature of island-arc magmas and their sources as compared to oceanic magmas for a long time was regarded as reliably established fact (e.g., Ballhaus et al., 1991). However, this paradigm has been challenged in recent decades. Based on trace element (Sc, V, Zn, Сu) systematics and Fe isotopes, some researchers proposed that the oxygen fugacity during formation of island-arc magmas is close to that of the mid-ocean ridge basalts (Lee et al., 2005, 2012)

(MORB, ΔQFM ~ 0). These data also arised some doubts concerning the initial enrichment of the island-arc magmas in chalcophile elements, for instance, in Cu (Lee et al., 2012). Other authors found, however, several lines of additional evidence in support of the more oxidized state of primary islandarc magmas (Kelley and Cottrell, 2009; Evans et al., 2012). Variations in the redox conditions at ΔQFM > 0 during mantle melting can be traced using sulfur content in melts. For instance, according to experimental data for water-saturated basaltic melt at P = 0.2 GPa and T = 1050°С (Jugo et al., 2010), the total solubility of sulfur species (sum of S2– and S6+) sharply increases from 0.13 to 1.3 wt % in response to the increase of S6+ solubility. This effect is documented within a narrow ΔQFM range from 0 to +1.5 in the field of the possible sulfide stability. The S content in melt remains high (1.3–1.2 wt %) under more oxidized conditions (ΔQFM > 1.5) in the sulfate saturation field (Jugo et al., 2010). However, the information on the initial sul-

145

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fur content is inaccessible from the composition of subaerially erupted island-arc rocks (Wallace, 2005), while data on the sulfur content in melt inclusions need to be treated with caution, with allowance, for instance, of possible melt degassing before its trapping as inclusion (e.g., Sadofsky et al., 2008). The contribution of different sources (mantle vs. subduction component) to the sulfur content in magmas is also uncertain (Richards, 2015). All these complications seem to prevent researchers from direct application of data on sulfur content in natural melts to reconstruct the redox conditions of magmas and their sources (Lee et al., 2012). This work reports new data on the Сu and S contents in the parental melts of Tolbachinsky Dol at Kamchatka, which were obtained from study of melt inclusions in high-Mg olivine. Data on quenched MORB glasses from the Juan De Fuca Ridge, Pacific Ocean, were taken for comparison (Gale et al., 2013; Jenner and O’Neill, 2012). These data are used for modeling of peridotite melting under different redox conditions. Our results indicate that the mantle melting beneath Tolbachinsky Dol occured under oxidized conditions (ΔQFM ~ +1.1…+1.3), which were preserved during subsequent magma differentiation. The data serve in support of more oxidized conditions of generation of island-arc magmas as compared to those of MORB, but raise doubts as to significant fluid influx of Cu and S in their source. OBJECTS Volcanism of Tolbachinsky Dol Tolbachinsky Dol (TD) exists as an active volcanic zone during at least Holocene (~10 000 years) and produces fissure areal monogenic basaltic eruptions (The Great Tolbachik …, 1983). Owing to two recent large eruptions (GTFE in 1975–1976 and in 2012– 2013), TD is geologically well-studied region and continues to attract attention of researchers, including petrologists and geochemists (2012–13 Tolbachik eruption, 2015). It was shown (Portnyagin et al., 2015) that the observed rock diversity of TD can be explained by the open-system cyclic fractionation of the primary high-Mg moderate-K magma and mixing of its fractionation products. The primary magmas were likely derived from a mantle-wedge peridotite compositionally similar to the moderately depleted MORB source (Churikova et al., 2001; Portnyagin et al., 2007a), which was melted (up to ~10%) at temperature of 1250–1300°C and pressure ~2 GPa under the influence of slab-derived hydrous melts (Portnyagin et al., 2015). The rocks of TD are ascribed to the tholeiitic series (Portnyagin et al., 2007b, Fig. 3), with the “tholeiitic index” (THI) according to (Zimmer et al., 2010) of 1.04 (THI > 1 for tholeiitic and 8.0 wt %) of the JDF show the same range of Cu contents (52–105 ppm, on average 74 ppm; Fig. 2а, Table 2). Similar concentrations were also found in the melt inclusion glasses in olivine from the Garrett Fracture Zone in the East Pacific Rise (70–100 ppm, on average ~80 ppm; Fig. 2а; Beaudoin et al., 2007). Based on these data, the typical initial Cu contents in MORB are assumed to be close to the average copper content in the primitive JDF basalts (MgO > 8.0 wt %, Cu 74 ± 12 ppm; Fig. 2a, Table 2). It should be noted that these values are also close to the copper content in the primitive rocks of some other volcanoes of Kamchatka, for instance, Klyuchevskoy volcano (Cu 50–100 ppm, MgO > 10 wt %; Portnyagin and Manea, 2008; GEOROC, 2017). Unlike rocks, melt inclusions can retain information on the volatile content, in particular, S content, in primary magmas (Wallace, 2005; Sadofsky et al., 2008). The highest S content up to 0.33 wt % for TD was found in primitive MIs. Matrix glasses and bulk rocks have S contents close to the detection limit of microprobe analysis (~0.01 wt %), which is related to the substantial magma degassing during eruption (Fig. 2b). The primitive TD melts differ from MORB in much higher concentrations of S and Cu (Fig. 2). The high-Mg glasses (MgO > 8 wt %) of the JDF ridge contain from 0.09 to 0.12 wt % S (Jenner and O’Neill, 2012), whereas the primitive melts of TD contain 0.14–0.33 wt % S (Fig. 2b; Kamenetsky et al., 2017). Since sulfur variations in the TD melts can be related to the partial degassing of melts prior to their trapping by host mineral and to the possible presence of S-bearing daughter phases in the inclusions (Kamenetsky et al., 2002), the range of S content for primary TD magmas was estimated from the average and maximum sulfur contents in primitive MIs. The estimated S contents in primary melts of the Northern Breakthrough of the TD (sample NT) are 0.19–0.27 wt % (average-maximum), while those of the Cone “1004” (sample K01-30) are 0.21–0.30 wt % (Figs. 2b, 3). The upper estimate for the Northern Breakthrough was obtained using expanded data set of inclusions, including those not analyzed for copper content. The S content in the primary magmas of the JDF ridge was estimated to be ~0.11 wt % based on data on the most magnesium-rich glasses (MgO > 8 wt %) (Figs. 2b, Table 2). This value is well consistent with sulfur contents (0.08–0.11 wt %) in the most primitive melts trapped by high-Mg olivine phenocrysts (Fo88.6-90.7)

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Rocks and melts of TD

Cu, ppm

200 Kamchatka rocks 150

100

50 MORB (JDF) 0

2

4

6

8

10

12

14

0.35 (b) 0.30

TD melts

S, wt %

0.25 0.20 0.15 MORB (JDF) 0.10 0.05 Primary melts 0

2

4

Tolbachinsky Dol (TD): Primitive inclusions MORB:

6 8 MgO, wt %

10

12

14

Rocks (Portnyagin et al., 2015)

NT Northern Breakthrough (sample NT) Hybrid Cone “1004” (sample K01-30) K01-30 melts Sample NT n.a. for Cu Groundmass (sample NT) JDF (Jenner, O’Neill, 2012) JDF (Gale et al., 2013) Primary melts:

NT

MI in olivine K01-30

FZ Garrett TF Siqueiros MORB (JDF)

Fig. 2. Variations of Cu, S, and MgO contents in melt inclusions, matrix glasses, and rocks of Tolbachinsky Dol (TD) and Pacific MORB basalts of the Juan de Fuca (JDF) Ridge. Compositions of studied melt inclusions in olivine and matrix glasses from TD rocks are shown here. Calculated average compositions of initial, primary melts (recalculated to equilibrium with olivine Fo91) are shown by large symbols (see also their composition in Table 2). Additional set of primitive melt inclusions from NT sample analyzed only for sulfur content (not analyzed, n.a. for Cu) is shown with smaller (yellow) circles. The compositions of oceanic glasses of the JDF ridge basalts (>40° N) (Jenner, O’Neill, 2012; Gale et al., 2013), melt inclusions in olivine from basalts of the Garrett Fracture Zone (FZ), East Pacific Rise (Beaudoin et al., 2007), melt inclusions in olivine from the Siqueiros Transform Fault (TF), East Pacific Rise (Danyushevsky et al., 2003), compositions of the TD rocks (Portnyagin et al., 2015), and volcanic rocks of Kamchatka (GEOROC, 2017) are also shown for comparison. PETROLOGY

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Table 1. Composition of melt inclusions in olivine and groundmass glasses in the studied samples of Tolbachinsky Dol (TD) Sample

NT

NT

NT

NT

NT

NT

NT

NT

NT

NT

Inclusion

1

2

3a

3b

4

5a

6

7

8

10

Experiment, Tquench, °C

no

no

no

no

no

no

no

no

no

no

Inclusion size, а, μm

77

85

79

85

60

189

45

89

117

94

Inclusion size, b

74

113

64

113

94

83

145

57

115

85

Averaged diameter, d

75

99

72

99

77

136

95

73

116

90

Bubble, diameter, μm

19

36

17

26

25

32

19

26

38

26

87.5

88.5

83.1

83.3

87.9

90.1

85.4

87.9

88.9

90.0

2

1

2

2

2

1

2

1

1

1

Host olivine, Fo Melt group (1, 2, 3)

Composition of inclusion glasses, wt % SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 S Cl H2O Total CaO/Al2O3 CaO/K2O Cu, ppm

49.53 47.32 50.10 50.34 49.64 46.90 49.85 47.02 1.26 1.08 1.37 1.35 1.26 1.10 1.30 1.08 16.28 15.87 16.28 16.62 16.30 15.91 16.01 15.48 7.09 7.10 8.32 8.06 7.00 6.41 8.85 8.16 0.08 0.14 0.14 0.10 0.15 0.12 0.10 0.16 5.90 6.60 5.46 5.46 5.75 6.77 5.72 6.53 11.80 14.04 9.52 9.38 11.93 14.60 9.57 13.65 3.06 2.78 3.22 3.41 3.06 2.82 3.15 2.87 1.34 0.98 1.62 1.63 1.32 0.91 1.50 0.93 0.32 0.22 0.39 0.45 0.26 0.21 0.38 0.17 0.176 0.239 0.089 0.088 0.166 0.254 0.081 0.223 0.092 0.129 0.059 0.049 0.075 0.129 0.059 0.125 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 96.91 96.49 96.56 96.93 96.90 96.12 96.57 96.38 0.73 0.88 0.58 0.56 0.73 0.92 0.60 0.88 8.84 14.38 5.88 5.75 9.07 15.97 6.38 14.64 182 143 201 206 168 145 209 163 Inclusion composition calculated to equilibrium with host olivine, wt %

47.96 47.18 1.01 1.09 15.04 16.22 7.20 6.45 0.08 0.14 6.60 5.83 13.85 14.56 2.65 2.76 0.94 0.86 0.16 0.20 0.202 0.280 0.116 0.131 n.a. n.a. 95.80 95.69 0.92 0.90 14.76 16.92 116 126

SiO2 TiO2 Al2O3 FeOtot MnO MgO CaO

49.6 1.15 14.87 9.5 0.07 9.54 10.78

47.84 0.99 14.6 9.5 0.13 10.16 12.92

51.22 1.36 16.11 9.5 0.14 6.89 9.42

51.13 1.32 16.26 9.5 0.1 6.95 9.18

49.56 1.13 14.68 9.5 0.14 9.89 10.75

47.2 0.97 13.99 9.5 0.11 11.76 12.84

50.92 1.27 15.61 9.5 0.1 8.16 9.33

48.01 1.03 14.73 9.5 0.15 9.61 12.99

48.68 0.93 13.8 9.5 0.07 10.7 12.71

47.51 0.94 14.04 9.5 0.12 11.79 12.6

Na2O

2.79

2.56

3.19

3.34

2.76

2.48

3.07

2.73

2.43

2.39

K2O

1.22

0.9

1.6

1.59

1.19

0.8

1.46

0.88

0.86

0.74

P2O5

0.29

0.2

0.39

0.44

0.23

0.18

0.37

0.16

0.15

0.17

Total Tcalc (1 atm, dry), °С

99.8

99.8

1254

1258

99.8 1195

99.8 1200

99.8 1262

99.8 1298

99.8 1230

99.8 1245

99.8 1271

99.8 1296

Corr. coeff.

0.91

0.92

0.99

0.98

0.90

0.88

0.97

0.94

0.92

0.86

Cl

0.084

0.119

0.058

0.047

0.068

0.113

0.057

0.118

0.106

0.113

S

0.160

0.221

0.088

0.086

0.150

0.223

0.079

0.211

0.185

0.241

Cu

166

132

198

201

152

127

204

154

106

108

S and Cu contents in a primary melt (Fo91) S

0.20

Cu

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Table 1. (Contd.) Sample

NT

NT

NT

NT

NT

NT

NT

NT

NT

NT

Inclusion

11

12

13a

13b

14

15

16

17

18

19


no

no

no

no

no

no

no

no

no

no


160

123

43

72

117

104

189

72

117

113

Inclusion size, b

208

75

72

40

49

60

79

40

121

34


184

99

58

56

83

82

134

56

119

74

Bubble diameter, μm

75

28

11

15

21

25

42

13

28

26

88.0

88.3

85.0

84.6

86.9

84.5

87.9

83.1

89.0

88.9

1

1

2

2

1

2

1

2

1

1

47.97

46.87

48.71

46.98

47.37

Host olivine, Fo Melt group (1, 2, 3) SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 S Cl H2O Total CaO/Al2O3 CaO/K2O Cu, ppm

1.08 15.14 7.54 0.16 6.71 13.64 2.84 0.95 0.21 0.145 0.118 n.d. 96.49 0.90 14.40 154

Composition of inclusion glasses, wt % 49.31 49.65 47.25 50.12 47.25

1.08 1.32 1.32 1.05 1.39 1.11 15.92 16.30 16.21 15.71 16.58 15.72 7.47 8.65 8.91 8.19 7.78 7.18 0.11 0.15 0.18 0.14 0.12 0.11 6.49 5.53 5.62 6.57 5.54 6.37 13.65 10.75 9.47 13.08 9.30 13.45 2.75 3.23 3.28 2.87 3.36 2.79 1.03 1.41 1.51 0.95 1.59 0.99 0.24 0.30 0.37 0.18 0.41 0.20 0.238 0.133 0.080 0.232 0.086 0.211 0.139 0.081 0.061 0.123 0.066 0.130 n.d. n.d. n.d. n.d. n.d. n.d. 95.97 97.15 96.64 96.34 96.33 95.49 0.86 0.66 0.58 0.83 0.56 0.86 13.23 7.65 6.29 13.78 5.87 13.61 151 206 209 156 142 154

1.22 1.12 15.81 15.71 9.09 6.91 0.18 0.13 5.86 6.41 10.98 13.77 3.12 2.81 1.27 0.94 0.22 0.19 0.169 0.217 0.097 0.122 n.d. n.d. 96.72 95.30 0.69 0.88 8.65 14.64 145 148

1.05 15.64 7.42 0.13 6.51 13.89 2.89 0.92 0.17 0.246 0.124 n.d. 96.35 0.89 15.01 140

Composition of inclusions calculated to equilibrium with host olivine, wt % SiO2

48.63

47.81

TiO2

1.01

1.01

Al2O3

14.2

14.9

50.1

50.8

48.42

51.02

48.32

50.21

47.93

47.97

1.28

1.3

1.02

1.35

1.04

1.24

1.03

0.96

15.81

15.98

15.26

16.06

14.77

16.05

14.39

14.34

FeOtot

9.5

9.5

9.5

9.5

9.5

9.5

9.5

9.5

9.5

9.5

MnO MgO CaO Na2O

0.15 9.79 12.8

0.1 9.97 12.77

0.15 7.78 10.43

0.18 7.63 9.33

0.14 8.89 12.7

0.12 7.57 9.01

0.1 9.7 12.64

0.18 6.82 11.15

0.12 10.64 12.62

0.12 10.55 12.73

2.66

2.57

3.13

3.23

2.79

3.25

2.62

3.17

2.57

2.65

K2O

0.89

0.96

1.37

1.49

0.92

1.54

0.93

1.29

0.86

0.84

P2O5

0.2

0.22

0.29

0.36

0.17

0.4

0.19

0.22

0.17

0.16

Total Tcalc (1 atm, dry), °С Corr. coeff. Cl S Cu S Cu

99.8 1249 0.94 0.111 0.136 144 0.12 129

99.8 1253

99.9 1212

99.8 1217

99.8 1226

99.8 1218

99.8 1247

0.93 0.98 0.99 0.97 0.97 0.94 0.130 0.079 0.060 0.119 0.064 0.122 0.222 0.130 0.079 0.225 0.084 0.198 140 201 207 151 138 144 S and Cu contents in a primary melt (Fo91) 0.20 126

0.20 132

99.8 1177

99.8 1271

1.02 0.91 0.098 0.111 0.172 0.198 148 135

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Table 1. (Contd.) Sample

NT

NT

NT

NT

NT

Inclusion

20

4gm*

7gm

3gm*

15gm

2

3

4

5

6


no

no

no

no

no

1180

1190

1250

1185

1167

8.40

10.00

14.45

11.15

7.10

Experimental time, min

K01-30 K01-30 K01-30 K01-30 K01-30


47

110

130

135

115

142

Inclusion size, b

94

110

110

130

130

112


71

110

120

133

123

127


26

30

35

50

42

37

Host olivine, Fo

90.3

73.9

1

3

Melt group (1, 2, 3)

78.9

77.8

77.7

88.7

87.6

90.5

87.9

87.1

3

3

3

1

1

1

1

2


47.38 51.83 51.05 52.34 51.45 46.84 46.80 49.78 1.10 1.98 1.39 1.62 1.48 0.87 0.90 0.81 16.55 14.83 16.76 16.04 16.73 13.49 13.23 11.31 5.90 10.78 8.53 9.77 9.01 8.53 9.40 8.12 0.04 0.22 0.11 0.20 0.18 0.15 0.18 0.17 6.46 4.31 4.74 5.04 4.61 9.63 11.06 13.51 13.86 8.95 9.64 9.66 9.57 13.38 11.23 11.78 2.80 3.57 3.34 3.37 3.56 2.34 2.20 1.73 1.04 2.48 1.69 1.88 1.76 0.78 0.64 0.65 0.20 0.59 0.39 0.44 0.39 0.17 0.13 0.13 0.232 0.017 0.011 0.008 0.022 0.261 0.197 0.169 0.121 0.076 0.063 0.056 0.064 0.140 0.127 0.107 n.d. n.d. n.d. n.d. n.d. 1.8 2.8 3.0 95.67 99.64 97.72 100.42 98.82 98.39 98.93 101.26 0.84 0.60 0.58 0.60 0.57 0.99 0.85 1.04 13.30 3.61 5.70 5.14 5.44 17.06 17.61 18.03 153 272 258 219 227 129 129 108 Inclusion compositions calculated to equilibrium with host olivine, wt %

SiO2 TiO2 Al2O3 FeOtot MnO MgO CaO Na2O K2O P2O5

Corr. coeff. Cl S Cu

47.54 51.83 51.05 52.34 51.45 48.3 49.27 50.33 50.32 51.44 0.95 1.98 1.39 1.62 1.48 0.88 0.99 0.81 0.96 1.45 14.23 14.83 16.76 16.04 16.73 13.65 14.54 11.35 13.82 13.51 9.5 10.8 8.53 9.77 9.01 9.54 9.52 9.50 9.51 9.54 0.03 0.22 0.11 0.20 0.18 0.15 0.2 0.17 0.18 0.12 12.18 4.31 4.74 5.04 4.61 10.46 9.7 13.28 10.18 9.32 11.92 8.95 9.64 9.66 9.57 13.54 12.34 11.83 11.67 9.44 2.41 3.57 3.34 3.37 3.56 2.37 2.42 1.74 2.33 3.35 0.89 2.48 1.69 1.88 1.76 0.79 0.70 0.65 0.67 1.30 0.17 0.59 0.39 0.44 0.39 0.17 0.14 0.13 0.18 0.36 99.8 99.5 97.6 100.4 98.7 99.85 99.82 99.79 99.82 99.83 1309 1115 1123 1131 1118 1261 1241 1320 1255 1264 0.85 1.26* 1.10* 1.01 1.10 0.99 1.06 0.96 0.103 0.076 0.063 0.056 0.064 0.141 0.140 0.106 0.127 0.080 0.198 0.017 0.011 0.008 0.022 0.262 0.216 0.168 0.242 0.215 131 342 258 241 227 130 142 107 123 130

S Cu

0.19 125

Total Tcalc (1 atm, dry), °С

48.45 51.79 0.90 1.51 13.01 14.09 9.52 8.20 0.17 0.12 10.72 8.23 10.98 9.84 2.19 3.49 0.63 1.36 0.17 0.38 0.227 0.224 0.119 0.084 2.8 1.7 99.89 101.06 0.84 0.70 17.45 7.24 116 136

S and Cu contents in a primary melt (Fo91)

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Table 1. (Contd.) Sample Inclusion

K01-30 K01-30 K01-30 K01-30 K01-30 K01-30 K01-30 K01-30 K01-30 K01-30 7

8-1

8-2

9

10

11

12

14

18-1

19-1


1235

1192

1192

1178

1161

1185

1166

1230

1207

1186


10.50

13.45

13.45

10.00

8.00

13.15

9.30

9.10

14.30

7.15


170

88

110

140

125

162

100

165

90

88

Inclusion size, b

120

125

70

120

160

132

132

140

55

45


145

107

90

130

143

147

116

153

73

66


48

35

22

40

52

50

44

52

20

20

88.7

88.5

88.5

89.0

88.9

88.6

89.1

88.9

89.3

89.9

1

2

2

1

1

1

1

1

1

1

Host olivine, Fo Melt group (1, 2, 3)


46.58 0.87 11.54 9.31 0.15 13.04 11.61 1.88 0.64 0.13 0.199 0.115 2.6 98.69 1.01 18.11 130

48.78 1.13 12.66 8.18 0.14 9.97 9.79 2.92 1.31 0.38 0.185 0.087 3.0 98.55 0.77 7.50 137

49.11 48.96 1.32 0.86 12.63 12.69 8.24 8.64 0.16 0.16 10.07 10.21 11.00 13.08 2.61 2.04 1.31 0.72 0.61 0.15 0.183 0.190 0.077 0.103 2.5 2.6 99.89 100.35 0.87 1.03 8.39 154

18.20 116

47.33 0.94 13.92 8.25 0.16 9.50 12.17 2.71 0.61 0.12 0.328 0.166 2.6 98.83 0.87 19.84 143

47.45 0.92 12.61 8.99 0.19 11.14 12.29 2.22 0.65 0.13 0.210 0.114 2.6 99.55 0.97 18.83 109

47.60 0.96 13.86 8.37 0.14 9.55 12.69 2.38 0.70 0.19 0.269 0.124 2.1 98.92 0.92 18.15 151

47.78 0.82 11.86 9.00 0.14 12.78 11.78 1.92 0.62 0.18 0.243 0.120 2.6 99.86 0.99 19.06 122

49.24 0.90 12.28 8.43 0.14 10.66 12.98 2.20 0.68 0.12 0.293 0.163 n.d. 98.11 1.06 19.23 126

48.10 0.81 12.67 7.71 0.17 11.18 13.43 1.87 0.63 0.10 0.190 0.130 n.d. 97.01 1.06 21.16 131

Inclusion composition calculated to equilibrium with host olivine, wt % SiO2

49.26

50.69

50.05

49.72

48.85

49.16

48.71

49.64

49.88

TiO2

0.98

1.16

1.32

0.85

0.94

0.97

0.95

0.89

0.89

0.8

Al2O3

13.06

12.96

12.65

12.59

13.94

13.29

13.68

12.87

12.16

12.5

FeOtot MnO MgO CaO Na2O

9.52 0.17 10.69 13.14 2.13

9.53 0.14 10.57 10.02 2.99

9.50 0.16 10.56 11.01 2.61

9.52 0.16 11.11 12.98 2.02

9.53 0.16 10.76 12.19 2.71

9.53 0.2 10.55 12.96 2.34

9.53 0.14 11.04 12.53 2.35

9.53 0.15 11.03 12.78 2.08

9.52 0.14 11.4 12.86 2.18

9.51 0.17 12.05 13.25 1.84

K2O

0.72

1.34

1.31

0.71

0.61

0.69

0.69

0.67

0.67

0.62

P2O5 Total Tcalc (1 atm, dry), °С Corr. coeff. Cl S Cu S

49

0.15 0.39 0.61 0.15 0.12 0.14 0.19 0.2 0.12 0.1 99.82 99.79 99.78 99.81 99.81 99.83 99.81 99.84 99.82 99.84 1263 1287 1276 1273 1274 1262 1276 1271 1282 1292 1.12 1.03 1.00 0.99 0.99 1.06 0.99 1.08 0.99 0.98 0.129 0.089 0.077 0.101 0.165 0.121 0.122 0.130 0.162 0.127 0.223 0.190 0.183 0.187 0.326 0.222 0.266 0.263 0.290 0.185 146 140 153 114 142 116 149 132 125 128 0.20

Content of S and Cu in a primary melt (Fo91) 0.17 0.30 0.20 0.24

0.24

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Table 1. (Contd.) Cu

133

Sample

K01-30 K01-30 K01-30 K01-30 K01-30 K01-30 K01-30 K01-30

Inclusion

105

130

105

137

121

23

27

28

31

32

33

35

39


1180

1166

1213

1167

1183

1155

1206

1215


116

121

NT+K01-30 (average)

7.00

10.30

13.45

7.00

8.20

7.00

11.40

13.30


50

55

113

168

75

65

160

64

Inclusion size, b

75

75

83

100

85

70

50

90


63

65

98

134

80

68

105

77


19

20

31

48

23

20

30

23

89.1

90.4

88.9

88.9

89.4

90.3

88.8

88.6

88.9

85.7

77.1

1

1

1

1

1

1

1

1

1

2

3

48.13

47.91

46.13

47.52

49.74

51.67

Host olivine, Fo Melt group (1, 2, 3) SiO2

Composition of inclusion glasses, wt % 46.73 48.70 43.45 48.56 48.62

TiO2

1.09

1.21

0.81

0.83

1.03

1.15

0.94

0.87

0.98

1.31

1.62

Al2O3

14.81

14.68

12.72

13.18

13.63

15.14

13.09

12.88

14.11

15.48

16.09

FeO MnO MgO CaO Na2O

8.14 0.12 8.99 13.38

6.91 0.13 9.71 12.96

9.02 0.15 11.28 12.91

8.94 0.18 9.97 12.80

7.97 0.14 9.84 13.76

7.25 0.12 8.93 13.04

9.25 0.17 10.58 13.16

8.95 0.14 10.53 13.29

8.02 0.14 9.08 13.09

8.20 0.14 6.59 10.28

9.52 0.18 4.68 9.46

2.47

2.61

2.00

2.12

2.27

2.55

2.04

2.18

2.42

3.16

3.46

K2O

1.02

0.63

0.67

0.73

0.78

0.90

0.67

0.72

0.80

1.43

1.95

P2O5

0.25

0.19

0.16

0.20

0.20

0.21

0.13

0.18

0.17

0.37

0.45

S Cl H2O

0.243 0.129

0.261 0.161

0.214 0.140

0.235 0.133

0.208 0.152

0.189 0.110

0.237 0.128

0.225 0.153

0.228 0.129

0.138 0.074

0.014 0.065

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

2.5

2.4

Total CaO/Al2O3

98.80

97.37

96.82

98.02

93.45

98.17

99.04

96.29

97.5

97.5

0.90

0.88

1.01

0.97

1.01

0.86

1.01

1.03

CaO/K2O

13.16

20.71

19.21

17.53

17.67

14.56

19.67

18.53

Cu, ppm SiO2

169

136

129

119

155

141

135

129

Inclusion composition calculated to equilibrium with host olivine, wt% 48.06 48.02 48.49 49.47 46.28 48.16 49.12 48.08

TiO2

1.03

1.11

0.86

Al2O3

14.02

13.5

13.44

FeOtot

9.52

9.53

9.51

MnO MgO CaO Na2O

0.11 10.86 12.66

0.12 12.48 11.92

2.34

K2O

137 48.56

0.67

0.6

7.3

5.0

175 50.56

244 51.67

1.08

1.03

0.95

0.92

0.96

1.28

1.62

14.26

13.61

13.18

13.59

13.73

15.05

16.09

9.52

9.53

9.52

9.50

9.50

9.52

9.53

9.52

0.16 10.74 13.64

0.18 10.99 12.72

0.15 10.72 14.4

0.11 12.38 11.72

0.17 10.79 13.25

0.15 10.28 14.03

0.14 10.85 12.77

0.13 8.47 9.99

0.18 4.68 9.46

2.4

2.11

2.11

2.38

2.29

2.05

2.3

2.35

3.07

3.46

0.97

0.58

0.71

0.73

0.82

0.81

0.67

0.76

0.77

1.39

1.95

P2O5

0.24

0.17

0.17

0.2

0.21

0.19

0.13

0.19

0.17

0.36

0.45

Total

99.81

99.83

99.83

99.84

99.83

99.82

99.81

99.80

Tcalc (1 atm, dry), °С

1274

1312

1263

0.82

0.93 16.8

99.1

13.1

1271

1268

1310

1264

1254

Corr. coeff.

0.95

0.93

1.06

1.00

1.05

0.90

1.00

1.06

Cl

0.123

0.149

0.148

0.133

0.160

0.100

0.128

0.162

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99.8 1233 0.072

99.1 1122 0.065

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Table 1. (Contd.) S

0.232

0.241

Cu

161

126

S Cu

0.21 148

0.23 121

0.226

0.235

0.218

0.171

0.237

136 119 164 128 135 S and Cu contents in a primary melt (Fo91) 0.21 125

0.21 109

0.20 153

0.16 122

0.22 123

0.238

0.223

136

133

0.22 124

0.204 122

0.135 170

0.014 267

Here shown are compositions of melt inclusions (MI) for two studied TD samples: sample of 1975 Northern breakthrough (NT) and cone “1004” (K01-30). Inclusions from sample NT are represented by naturally quenched MI. Inclusions from sample K01-30 were reheated and quenched, with shown heating time (exposure time from 1064°C to the quenching temperature, T = 1064°C was controlled by the moment of Au melting) in minutes and seconds (for instance, 7.10, i.e. 7 min and 10 s). Melt groups: (1) Primitive melts unaffected or slightly affected by mixing with differentiated subalkaline melts. The group was distinguished based on K2O < 1 wt % (for compositions corrected for equilibrium with host olivine) (Fig. 1a); (2) Hybrid melt group showing signs of mixing; (3) groundmass glass. MI compositions were corrected for equilibrium with host olivine with allowance for iron loss using an “Fe-loss” software (Danyushevsky et al., 2000); FeOinitial = 9.5 wt % (corresponds to the average content in high-Mg rocks of TD), Fe2+/Fe3+ in a melt was given as constant value equal 5.5 (FeO = 8.05, Fe2O3 = 1.63 wt %), estimated from study of syngenetic spinel inclusions in olivine (Table 4). Primary melts were recalculated to equilibrium with olivine Fo91 (only for group of primitive inclusions) using Petrolog3 software (Danyushevsky and Plechov, 2011) by reverse olivine fractionation at P = 10 kbar and fO2 = QFM+1. The Fo values for the groundmass were calculated using a Petrolog3 software at 1 atm and fO2 = NNO. Correction coefficients (Corr. coeff.) were used to correct sulfur and copper contents in melts calculated to equilibrium with their olivine. The coefficients were calculated from ratios: K2O in a melt (value corrected for equilibrium with olivine)/K2O in an inclusion glass (measured value). Correction coefficients for calculation of S and Cu contents in primary melts from equilibrium composition of melt inclusions (not listed in table) varied from 0.87 to 0.97, 0.92 on average. * In name (3gm* and 4gm*) is the correction for copper content in the groundmass glass due to trapping the Pl–Px microlites during laser analysis; uncorrected values for them are shown in Fig. 2a; n.a. for H2O means not analyzed. Oxides, S, and Cl are given in wt %, Fo, in mol %, Cu in ppm.

from the Siqueiros Transform Fault in the East Pacific Rise (Fig. 2b; Danyushevsky et al., 2003). DISCUSSION Model of the Cu and S Behavior during Mantle Melting Variations of Cu and S contents in the primary magmas of Tolbachinsky Dol as compared to MORB can be explained using mantle melting model of Lee et al. (2012). This model made it possible to predict the Cu and S contents in the melts derived by different degrees of peridotite melting at variable oxygen fugacity, Cu and S contents in a source, Cu sulfide–melt partition coefficient, pressure and temperature of melting. The model of Lee et al. (2012) is underlain by (1) model of Mavrogenes and O’Neill (1999) of S2– solubility in a melt depending on Т and Р, and (2) model of Jugo et al. (2010) describing the total sulfur content in melt as a function of oxygen fugacity and accounting for a sharp increase of S6+ solubility and, respectively, the total sulfur content within the range of QFM + 1 – QFM + 1.5. It is unclear, however, whether these models provide adequate description of sulfur solubility depending on P-T conditions and melt composition (for instance, depending on water or nickel content in melt) (Ariskin et al., 2013). The accuracy of the Jugo et al. (2010) model is also an open question. For instance, the dependence of S solubility on ΔQFM implied by Jugo et al. (2010) and, respectively, by Lee et al. (2012) suggests that log S6+/S2– iso-

pleths in the 1/T–log fO2 field are parallel to QFM equilibrium line, which is not likely the case (A.A. Borisov, personal communication). The pressure effect on the sulfur solubility is also uncertain (Jugo et al., 2010; Matjuschkin et al., 2016). In this work, we are based on the model (Lee et al., 2012), because no alternative mutually consistent model has been proposed yet that would take into account discovered shortcomings. Estimated absolute fO2 values are certainly model dependent, but the relative difference in the redox conditions of magma generation for TD and MORB is not. The difference in fO2 may increase if the higher pressure formation of the TD magmas (2 GPa) as compared to MORB magmas (1 GPa) is taken into account (Matjuschkin et al., 2016). The Cu content in the mantle sources of TD and MORB magmas was taken to be 30 ppm, which corresponds to the DMM composition (Salters and Stracke, 2004); the sulfur content—200 ppm (Lee et al., 2012). It should be noted that the estimated sulfur concentrations in the upper mantle varies from 119 ppm (Salters and Stracke, 2004), 146 ± 35 ppm (Saal et al., 2002) and up to 190 ± 90 ppm (Lorand, 1990). The sulfur content of 200 ppm taken in this work corresponds to the upper limit of the inferred contents. Such S content in a source allows us to closely reproduce the Cu and S contents in typical MORB and to compare our modeling results with those in Lee et al. (2012). The pressure of the primary magma generation for TD was estimated by Portnyagin et al. (2015) to PETROLOGY

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Table 2. Average compositions of primitive and primary melts for the studied samples of Tolbachinsky Dol (TD) and Pacific basalts of the Juan de Fuca (JDF) ridge and estimated conditions of crystallization and formation Object (sample) TD (NT) TD (K01-30) MORB (JDF) Melts I st. dev. II I st. dev. II I st. dev. II (I—primitive, II—primary) Equilibrium olivine, Fo 88.6 1.0 91 89.1 0.7 91 85 89.4 Melt composition SiO2, wt % 48.07 0.52 47.34 48.91 0.95 48.2 49.55 0.78 48.77 TiO2 0.99 0.04 0.9 0.94 0.09 0.86 1.33 0.24 0.85 Al2O3 14.45 0.41 13.14 13.31 0.75 12.24 15.99 0.74 18.69 FeOtot 9.52 0.01 9.57 9.52 0.01 9.55 9.45 0.82 7.37 MnO 0.11 0.03 0.1 0.16 0.03 0.15 MgO 10.40 1.00 13.81 11.07 0.86 14.00 8.43 0.34 10.0 CaO 12.68 0.26 11.53 12.82 0.75 11.79 11.90 0.51 11.5 Na2O 2.57 0.12 2.34 2.22 0.22 2.04 2.55 0.27 2.43 K2O 0.87 0.06 0.79 0.71 0.09 0.65 0.13 0.05 0.08 P2O5 0.18 0.02 0.16 0.16 0.04 0.15 0.14 0.04 0.090 Total 99.83 0.01 99.7 99.8 0.01 99.6 99.46 0.40 99.78 CaO/Al2O3 0.88 0.03 0.88 0.97 0.07 0.96 0.75 0.05 0.62 Cl 0.115 0.007 0.105 0.134 0.020 0.123 0.010 0.002 S 0.207 0.026 0.188 0.232 0.039 0.212 0.106 0.010 0.106 (n = 19) S max 0.274 0.298 Cu, ppm 133 15 121 133 15 121 74 12 74 Number of analyses, n 12 20 44 P-T crystallization conditions P, GPa 1 1 0.5 T, °С (dry, 1 atm) 1206 T, °С (dry, 0.5 GPa) 1270 T, °С (dry, 1 GPa) 1314 25 1392 1324 20 1390 1295 1205 1283 1215 1281 T, °С (4 wt % H2O, 1 GPa) P-T-F formation conditions 2.0 1360

P, GPa TDPS,°С (Zhang and Hirschmann, 2016) H2O, wt % dTDPS,°С (Portnyagin et al., 2007a) T, °С (TDPS+dTDPS) Fmelt, % P, GPa (Lee et al., 2009) T, °C (Lee et al., 2009)

2

4 –75 1285 7 2.0 1332

1

2.0 1360

1.0 1240

4 –60 1300 10 1.8 1325

0 60 1300 8 1.2 1308

2

Table shows the average compositions of primitive melts (group 1 in Table 1) of TD and standard deviation from mean. For primitive MORB composition of the JDF ridge, we took the average composition of high-Mg basalts with MgO > 8 wt % according to (Jenner and O’Neill, 2012, except for data on seamount basalts) and (Gale et al., 2013). Primary melts were calculated for TD by reverse fractionation of olivine to equilibrium with Fo91, at 10 kbar, QFM + 1 using a Petrolog3 software (Danyushevsky and Plechov, 2011). For MORB compositions of JDF, the calculations were carried out by reverse fractionation of Ol (10%) and Pl (25%) up to MgO = 10 wt % at 5 kbar and QFM using the same program. Obtained composition is close to the primary melt TOR-2 (Sobolev et al., 1989). Crystallization temperature was estimated with allowance for pressure (+5 С/kbar) and influence of water content (according to the model of Almeev et al., 2007 T decrease for 4 wt % H2O is 110°C)). Temperature of formation (or melting) was calculated using formula TDPS + dTDPS, where TDPS is the temperature of dry peridotite solidus at P = 2 GPa and 1 GPa (Zhang and Hirschmann, 2016, Fig. 10), while dTDPS is the deviation of true melting temperature from TDPS (dTDPS, ~ –75–60°C for TD and +60°С for MORB). The dTDPS values were estimated according to (Portnyagin et al., 2007a, Fig. 7b and H2O–Fmelt relation in Supplementary there), using estimated degrees of melting for TD and JDF and water content in primary magmas of ~4 wt % and 0 wt %, respectively. In all three cases (NT, K01-30, JDF), melting temperatures are close to 1300°С. Melting degree, Fmelt, for TD was estimated using technique (Portnyagin et al., 2007а) on the basis of trace element contents in the average compositions of initial melts. Fmelt for JDF (MORB) was estimated from Fmelt– Na2O relation (Langmuir, 1992, Fig. 42 where Na2O ~ 2.1–2.7 wt % correspond to Fmelt ~ 10–6%, 8%, on average). The pressures and temperatures of the formation of the same primary melts according to model (Lee et al., 2009) are shown for comparison. PETROLOGY

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Table 3. Composition of crystalline inclusions Cpx and Opx in olivine from Tolbachinsky Dol (TD), cone “1004” TD, cone “1004” (sample K01-30) Components 22

23

26

24

25a

25b

25c

27a

27b

Inclusion

Cpx

Cpx

Cpx

Opx

Opx

Opx

Opx

Opx

Opx

SiO2

53.91

53.10

53.48

55.89

56.80

56.77

55.32

57.57

56.45

TiO2

0.15

0.16

0.19

0.08

0.01

0.06

0.03

0.14

0.14

Al2O3

1.02

1.52

1.47

0.65

0.98

0.99

0.80

0.87

0.68

FeOtot MnO MgO CaO Na2O

3.45 0.07 18.82 21.70 0.24

3.60 0.10 17.72 22.04 0.25

3.67 0.08 18.14 22.16 0.25

6.77 0.12 33.30 1.90 0.01

6.43 0.16 33.42 1.79 0.00

6.27 0.18 33.47 1.80 0.00

6.71 0.14 34.81 1.69 0.00

6.66 0.19 33.40 2.03 0.01

6.84 0.18 33.90 1.97 0.03

K2O

0.01

0.01

0.00

0.00

0.01

0.01

0.01

0.01

0.01

0.69 0.07 100.1 90.7 1185 –2.2

0.79 0.01 99.3 89.8 1266 7.7

0.72 0.04 100.2 89.8 1245 5.2

0.29 0.14 99.2 89.8

0.41 0.09 100.1 90.3

0.45 0.05 100.1 90.5

0.43 0.10 100.0 90.2

0.43 0.02 101.3 89.9

0.31 0.05 100.6 89.8

Ol

Ol

Ol

Ol

Ol

Ol

38.70 10.20 0.19 48.63 0.19 0.27 0.04 98.2 89.5

40.09 9.77 0.17 48.94 0.16 0.36 0.05 99.5 89.9

39.98 9.80 0.19 48.92 0.15 0.31 0.04 99.4 89.9

40.08 9.76 0.19 49.16 0.16 0.34 0.04 99.7 90.0

Cr2O3 NiO Total Px, #Mg T, °С (Putirka et al., 2003) P, kbar (Putirka et al., 2003) Host mineral

Ol

Ol

SiO2

39.92 9.92 0.16 49.15 0.20 0.22 0.04 99.6 89.8

39.92 10.79 0.20 48.43 0.19 0.23 0.03 99.8 88.9

FeO MnO MgO CaO NiO Cr2O3 Total Fo

Ol 40.48 11.43 0.20 48.14 0.18 0.22 0.05 100.7 88.2

40.88 10.27 0.18 49.32 0.17 0.27 0.04 101.1 89.5

41.05 10.17 0.20 49.32 0.18 0.27 0.05 101.2 89.6

Pressure and temperature of crystallization were calculated using Cpx–melt model (Putirka et al., 2003); the average primitive composition for sample K01-30 (Table 2) estimated from melt inclusion data in olivine was used as a melt. For unclear reasons, one Cpx inclu+ sion (K01-30-22) yielded negative pressure and lowered temperature. Px, #Mg = Mg/(Mg + Fe2tot ).

be ~2 GPa. The formation pressure of the primary magmas of the JDF ridge was assumed to be ~1 GPa on the basis of previous estimates made for TOR-2 basalts (Dmitriev et al., 2006). Estimated melting temperature of a source of TD rocks and basalts of the JDF ridge was ~1300°C (Table 2). This value is well consistent with the liquidus temperature of crystallization of the initial magmas of the TD (1280–1285°C at 1 GPa and 4 wt % H2O) and of the JDF ridge (1270– 1295°C at 0.5–1 GPa) (Table 2). Similar pressures and temperatures of primary magma generation (2.0–1.8 GPa and 1330–1325°C for TD; ~1.2 GPa and 1310°С for JDF ridge) are also predicted by the model (Lee et al., 2009) (Table 2).

Calculated Cu and S contents in melts are dependent on the pressure and temperature variations owing to their effect on sulfur solubility in a melt. In particular, variations ±50°C or ±0.5 GPa have approximately the same effect as variations ±0.05–0.1 log units ΔQFM in calculations according to model (Lee et al., 2012). A temperature increase by 50°С leads to the increase of S solubility by ~0.03 wt % and is equivalent to a decrease of pressure by 0.5 GPa or increase of ΔQFM by 0.05–0.1 log units. Thus the existing uncertainties of the estimated temperature and pressure of magma formation do not affect the main conclusions of this work. Based on data (Zhang and Hirschmann, 2016), the temperature of sulfide liquidus at a pressure of 2 GPa PETROLOGY

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is 1236°C. This denotes that at temperature of mantle melting of 1300°C, sulfide exists as sulfide liquid (SL). With allowance for this fact, we calculated the Cu partition coefficient between sulfide liquid (SL) and siliSL-SM cate melts (SM) (DCu ) using model (Li and Audetat, 2015). At a given temperature of 1300°C and ΔQFM = +1, FeO = 9.5 wt % in a silicate melt for TD and ΔQFM = 0 and FeO = 7.5–8 wt % for the JDF SL-SM Ridge, DCu were ~600 (TD) and ~700 (JDF), respectively. The calculated values are the least sensitive to variations of given ΔQFM (Li and Audetat, SL-SM 2015). The use of the lower DCu than in the work of SL-SM Lee et al. (2012) ( DCu = 800) results in the increase of the calculated copper contents in primary melts by 10–20 ppm.

Assessment of Conditions of Magma Formation on the Basis of Cu–S Systematics Our modeling makes it possible to compare the observed Cu and S contents in primary magmas with their model contents at the estimated temperature and pressure of magma formation, to quantify independently the degree and redox conditions of mantle melting beneath TD, and compare the estimated parameters with the conditions of MORB formation (Fig. 3). As demonstrated by the numerical modeling, the high observed contents of Cu (100–150 ppm, on average 120 ppm) and S (0.16–0.30 wt %, on average 0.21 wt %) in the initial TD melts correspond to the degree of mantle melting from ~6 to 12% and ΔQFM ranging from +1 to +1.3 (Fig. 3). Obtained degrees of mantle melting are close to those estimated independently from the average MIs trace-element compositions (5– 11%, Table 2) and for TD magmas in general (~6%, Portnyagin et al., 2015). However, it should be noted that independent estimates of degree of melting are required for each of the individual melt inclusions for a more accurate comparison of modeling results. The modeling results can be also used to estimate the possible sulfur influx in a mantle source and sulfur content in a source. Obtained data show that no sulfur influx is required to explain the average sulfur composition of primary magmas of the Northern Breakthrough. The maximum sulfur contents in these melts can be explained by 7% melting of MORB-type mantle source under oxidized conditions (ΔQFM=+1.25) (Fig. 3). The average composition of the initial magmas of Cone “1004” also does not require an additional sulfur influx and can be obtained by 9% melting at ΔQFM from +1.1 to +1.2. The maximum sulfur contents (0.30 wt %) in the primary magmas of Cone “1004” also can be obtained by melting of mantle with up to 200 ppm S, but at sufficiently low degrees of melting (~5–7%), which are lower than estimated for this sample from trace elements (9–11%, Table 2). PETROLOGY

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Alternatively, the observed sulfur and copper contents in MIs from Cone “1004” can be obtained at ΔQFM = +1.28 by higher degrees of melting Fmelt ~10–11% of a more S-rich source (300 ppm) than DMM (Fig. 3), which may imply up to 30–35% sulfur in primary melt derived from the subducted plate. The redox conditions of the crystallization of the studied primitive TD melts were also estimated using oxybarometer based on the dependence of composition of the Ol–Cr-Spl–(Opx) equilibrium assemblage on fO2 (Ballhaus et al., 1991; Nikolaev et al., 2016). An application of this oxybarometer for TD is well justified, because the primitive melts are in equilibrium with this assemblage comprising co-genetic crystalline Cr-spinel as well as orthopyroxene inclusions in highMg olivine phenocrysts (Tables 3, 4). The calculated ΔQFM values for olivine Fo88.2-90.7 according to the model (Ballhaus et al., 1991) (at P = 1 atm) vary within the range of +1.5…+2 (on average +1.8) regardless of olivine composition. There is no difference between two samples studied (Fig. 4, Table 4). The values of ΔQFM obtained using modified model (Nikolaev et al., 2016) are systematically by ~0.4 log units lower (ΔQFM +1.2…+1.6, on average +1.4) (Fig. 4, Table 4). The estimates obtained from modeling of Cu–S systematics (+1.1 < ΔQFM +1) and FeOtot = 9–11 wt % (Portnyagin et al., 2012, Fig. 10). The persistent or increasing oxidation during magma fractionation presumably also provides favorable conditions for postmagmatic abundant copper sulfate and chloride sulfide-free PETROLOGY

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mineralization of exhalations, which are typical of TD (Pekov et al., 2014; I. Pekov, personal communication, 2016). Contribution in the Discussion of Conditions of the Island-Arc Magma Formation The Cu and S contents in primary melts are insensitive to the possible oxidation of magmas after their extraction from a mantle source. Oxidation can only retard the melt saturation in sulfide phase and prior to this moment, Cu and S will behave like elements strongly incompatible in silicate minerals (for instance, Ba, La, Nb). Therefore, our conclusion concerning the similarity of the estimated redox conditions during melting of island-arc mantle (Cu–S systematics) and crystallization of primitive melts (olivine–spinel oxybarometry) has an important petrogenetic significance. Firstly, this indicates that the redox state of magmas can be preserved during ascent of primary magmas from the mantle and their further crystallization at crustal conditions (P ~5–8 kbar, see above, and Table 3). Therefore olivine–spinel oxybarometry can be informative about the conditions of magma generation in the mantle (Fig. 4; Evans et al., 2012). This conclusion casts some doubts on the widely discussed hypothesis about significant oxidation of mantle magmas en route to the surface (Lee et al., 2012) Secondly, obtained data raise questions regarding the main conclusion of Lee et al. (2012) on similarity of the redox conditions during formation of midocean ridge basalts (MORB) and island-arc ( subduction-related) magmas based on the close initial content of only copper (50–100 ppm), without consideration of sulfur content data. In contrast to (Lee et al., 2012), our work involves data on sulfur contents in primitive melts, which in combination with Cu content unambiguously indicate the more oxidized conditions of the formation of Tolbachinsky island-arc magmas as compared to those of MORB. At the same time, the moderate oxidation of the island-arc mantle and formation of primary magmas in the presence of sulfide phase place significant restriction on the possible enrichment of these magmas in copper and other strongly chalcophile elements (Mungall, 2002). Future studies of primitive melt inclusions on regional scale would make it possible to analyze the Cu–S systematics for different island-arc complexes and to provide alternative estimate of redox conditions of the formation of the primary island-arc magmas. Data obtained in this work also demonstrate that the observed Cu and S contents in the initial TD magmas can be generated from MORB-like source without S enrichment, which has been suggested for suprasubduction zones (e.g., Richards, 2015 and references therein). The elevated sulfur content in the TD melts was likely caused by the increase of sulfide solubility at PETROLOGY

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a moderate increase of fO2 (Jugo et al., 2010). The missing or very limited subduction-related sulfur influx to the initial magmas of the Northern Breakthrough of Tolbachinsky Dol cast doubts concerning the key role of sulfate fluids in the oxidation of mantle sources of island-arc magmas (e.g., Jégo and Dasgupta, 2014). In this case, the more probable oxidizing agent in the mantle can be Fe2O3 dissolved in slab-derived melts (Mungall, 2002), as is also suggested by recent data on TD emphasizing a major role of slab-derived hydrous melts in the enrichment of its mantle source (Portnyagin et al., 2015). CONCLUSIONS (1) The study of melt inclusions in olivine (Fo83.1from the rocks of Tolbachinsky Dol (TD) (Kamchatka) showed that the most primitive melts have higher contents of Cu (100–150 ppm) and S (0.16– 0.30 wt %) as compared to typical MORB, for instance, from the Juan de Fuca (JDF) ridge (Pacific Ocean), which are characterized by the moderate contents of Cu (55–105 ppm) and S (0.09–0.12 wt %) in the parental melts. (2) Modeling algorithm of Lee et al. (2012) showed that the primary melts of TD and JDF ridge could be derived from a similar mantle source with S ~200 ppm and Cu ~30 ppm, but at different oxygen fugacity: +1.1