- Email: [email protected]
Carbon and helium isotope systematics of North Fiji Basin basalt glasses: carbon geochemical cycle in the subduction zone
Earth and Planetary Science Letters 154 Ž1998. 127–138
Carbon and helium isotope systematics of North Fiji Basin basalt glasses: carbon geochemical cycle in the subduction zone Yoshiro Nishio
a,)
, Sho Sasaki
a,1
, Toshitaka Gamo Yuji Sano d,4
b,2
, Hajime Hiyagon
c,3
,
a
c
Geological Institute, School of Science, UniÕersity of Tokyo, Tokyo 113, Japan b Ocean Research Institute, UniÕersity of Tokyo, Tokyo 164, Japan Department of Earth and Planetary Physics, School of Science, UniÕersity of Tokyo, Tokyo 113, Japan d Department of Earth and Planetary Science, Hiroshima UniÕersity, Higashi-Hiroshima 739, Japan Received 27 May 1997; revised 10 September 1997; accepted 29 September 1997
Abstract We have measured d 13 C values and CO 2r 3 He ratios of vesicle-gas, and chemical compositions of North Fiji Basin basalt glasses, to estimate the contribution of subducted carbon in back-arc basin basalt quantitatively. It has made clear that the CO 2r 3 He ratio increases and the d 13 C value decreases with K 2 O content. Since the variety of K 2 O content of North Fiji Basin basalt is the result of two-component mixing, the variety of the CO 2r 3 He ratio and d 13 C value of the North Fiji Basin basalt should be mainly a result of mixing between the mantle component Žlow-CO 2r 3 He, high-d 13 C and low-K 2 O. and the subducted component Žhigh-CO 2r 3 He, low-d 13 C and high-K 2 O.. From a simple mass-balance calculation, it is derived that the subducted end-member source has 70% carbonate and 30% organic matter in origin. Assuming that complete decomposition of the subducted organic matter has occurred, most Ž; 90%. carbonates, which subducted without accretion to wedge, is free from decomposition in North Fiji back-arc, since the subducting carbonate and organic matter throughout the North Fiji subduction zone are estimated in a proportion of 20:1. This may be one clue that carbonate can be transported into the mantle through the subduction zones. q 1998 Elsevier Science B.V. Keywords: carbon; helium; isotopes; basalts; back-arc basins; Fiji
1. Introduction
)
Corresponding author. Fax: q81-3-3815-9490; E-mail: [email protected] 1 Fax: q81-3-3815-9490; E-mail: [email protected] 2 Fax: q81-3-5351-6452; E-mail: [email protected] 3 Fax: q81-3-3818-3247; E-mail: [email protected]. u-tokyo-.ac.jp 4 Fax: q81-824-24-0735; E-mail: [email protected]. hiroshima-u.ac.jp
In the carbon geochemical cycle, one of the important processes is that sedimentary carbon should be re-injected into the mantle at the subduction zone w1,2x. Mattey et al. w3x noted the contribution of the subducted sedimentary organic carbon to certain back-arc basin basalt ŽBABB., since the d 13 C values of BABBs from the East Scotia Sea and the Mariana Trough are significantly lower than those of mid-ocean ridge basalt ŽMORB.. The low d 13 C values,
0012-821Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 1 2 - 8 2 1 X Ž 9 7 . 0 0 1 8 7 - 8
128
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
however, can be produced not only by mixing of the organic carbon with the mantle carbon, but also by progressive degassing, because of the isotopic fractionation between CO 2 vapor and carbon in the melt w4x. In addition, carbon resulting from the mixing of carbonate Ž d 13 C ; 0‰. with organic matter Ž d 13 C - y20‰. may have a 13 Cr 12 C ratio compatible with that of the mantle Ž d 13 C f y6.5 " 2.5‰.. From this, the contribution of the subducted carbon in BABB cannot be determined by carbon isotopic data only. Cr 3 He ratios of island arc volcanic gases range from 3 = 10 9 to 1.3 = 10 11 , which is higher than those of MORB wŽ2.0 " 0.5. = 10 9 x w5,6x. Since this excess carbon Žrelative to the mantle. can be attributed to the subducted sediment, the Cr 3 He ratio is one of the more powerful tracers to reveal the recycling of the subducted carbon w5–7x. From this point of view, there are several reports on both 13 Cr 12 C and CO 2r 3 He ratios of island arc volcanic gases w7–11x; e.g., Sano and Marty w7x gave quantitative estimates of the contribution of the subducted carbon in island arc volcanic gases using a mass-balance calculation between 13 Cr 12 C and CO 2r 3 He ratios. However, 13 Cr 12 C and CO 2r 3 He ratios of BABBs have not been documented simultaneously. Because of less crustal contamination properties, BABBs should be good sample material for the study of recycling in the subduction zone. This study aims to reveal the detailed carbon and helium isotopic systematics in a back-arc basin ŽNorth Fiji Basin.. Furthermore, this paper discusses the origin of carbon in BABB using 13 Cr 12 C and CO 2r 3 He data. 2. Samples Analyzed samples were collected from the North Fiji Basin Žhereafter NFB. during the French–Japan STARMER project. The NFB ŽFig. 1. is located east of the New Hebrides Trench, where the Indo-Australian Plate is being subducted beneath the Pacific Plate. The NFB is the largest back-arc basin among the presently active basins, and the distance from the major spreading segments to the trench is ) 500 km, which is remarkably long in comparison with smaller back-arc basins such as the Mariana Trough and the Lau Basin. The presently active spreading system in
NFB can be divided into four segments between 158S and 228S, which are north to south N1608, N158, N–S and 1748E segments, based on topography w12x Žsee Fig. 1.. The basalts in the central NFB ŽN158 and N–S segments. are similar to an N-MORB, which is depleted in large-ion lithophile elements ŽLILE., high-field-strength elements ŽHFSE. and light rareearth elements ŽLREE. w13x. In the southern NFB, on the 1748E segment, several basalts show a significant subduction-related contamination with a negative Nb anomaly and a high LOI Žlost of ignition. w13,14x. Since the 1748E segment is the nearest to the trench among the four NFB segments, the basalts from this area may be influenced by the fluid dehydrated from the slab. In the northern NFB w1608 spreading segment including the Triple Junction ŽTJ.x, many basalts result from the mixing of N-MORB and oceanic island basalt ŽOIB. sources with significant enrichment in LILE, HFSE and LREE w13,14x. The detailed petrological and geochemical results on analyzed basalts are reported elsewhere w12–15x. We have analyzed 20 basalt glasses, which were collected from stations 4 ŽTJ., 6 ŽN158., 14 ŽN–S., 15 ŽN–S., 21 Ž1748E., 53 ŽN1608., 55 ŽN1608. and 58 ŽN1608., by the RrV ‘‘Kaiyo’’ and RrV ‘‘Yokosuka’’ cruises. These sample sites, which encompass all four spreading segments and the Triple Junction, are shown in Fig. 1. 3. Analytical methods 3.1. Sample preparation Analyzed glass samples are the chilled margin of the pillow lava. After repeated crushing, fresh glasses without oxidation haloes were collected manually. The glass chips were washed ultrasonically in a diluted HNO 3 solution and then cleaned ultrasonically in distilled water, in ethanol, and then in acetone. After drying at 1008C for 24 h to remove organic solvents, a basalt glass sample Ž; 1.5 g. was loaded into the vacuum vessel for crushing and degassed at 1508C in vacuum for 1 h. Even though the degassing may lead to the loss of He by diffusion, as predicted by the helium diffusion coefficient in basaltic glasses, this effect may not be so serious for vesicle-gas. In order to liberate vesicle-gases, the
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
129
Fig. 1. Major structures and sample sites in North Fiji Basin w13x. Short dashed line s spreading centers; F.Z.s fracture zone; T.J.s Triple Junction; black line with black triangles shows the active New Hebrides subduction zone; dashed line with open triangles shows paleo-subduction zone. Areas with less than 2000 m of water are stippled. In this map, ODP Site 828 reported by Collot et al. w29,30x is shown as I. A sample site along spreading segments gets near to the trench with the latitude.
glass sample was crushed for 5 min in a ball mill. After 5-min crushing, the glass chips were finely powdered. It is estimated that most of the vesicle-gas could be liberated by 5-min crushing, since a longer crushing time could not liberate more vesicle-gas significantly. While the liberated gases were collected at a cold trap, the crushed sample together with the vessel was heated at 1508C in order to degas the adsorbed gases on the surface of the powder. In this study, separate crushings were performed for carbon and helium analyses. Carbon and helium were purified in each vacuum line. 3.2. Carbon The 13 Cr 12 C ratios were measured by a conventional stable-isotope mass spectrometer ŽFinnigan
MAT 250. installed at the Ocean Research Institute, University of Tokyo, after separation of CO 2 from other chemical components using a trap held at liquid N2 temperature and a trap at ethanol–dry ice temperature. In addition, H 2 S was completely removed by a trap with a 1 M CuCl 2 solution which had already been evacuated by freezing, since H 2 S may affect the measured 13 Cr 12 C ratios of CO 2 w16x. Concentrations of the separated CO 2 were determined by a mercury manometer. In this case, the uncertainty of measurement was ; 20% at 1 s , which was estimated by the repeated analysis of a standard sample. Observed 13 Cr 12 C ratios are expressed by the familiar delta notation relative to PDB ŽPeedee formation Belemnite., d 13 C. The treatment of d 13 C error in this paper is as follows: Uncertainties of the d 13 C values whose CO 2 amount is more
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
130
Table 1 Vesicle-CO2r 3 He ratios in reference North Mid-Atlantic Ridge basalt glasses Sample
4 He Ž10y10 molrg.
3 Her 4 He a Ž RrRA .
CO 2 Ž10y6 molrg.
CO 2 r 3 He Ž=10 9 .
Reference
CH31-DR11 CH31-DR11 CH31-DR11
3.39
7.93
8.32
2.2 2.0 Ž6.6.
this work w19x w5x
CH98-DR11 CH98-DR11 CH98-DR11
2.10
8.24
3.90
1.6 1.6 1.8
this work w20x w5x
a
Sample 3 Her 4 He ratio normalized to the atmospheric 3 Her 4 He ratio, RA s 1.4 = 10y6 .
than 1.8 = 10y6 mol are "0.1‰ at 1 s estimated by the repeated measurements of a standard sample. Excluding one sample ŽST6-D4-4., the error margin of d 13 C values listed in this paper is "0.1‰. The extracted CO 2 amount of the ST6-D4-4 sample is 1.4 = 10y6 mol, and the error margin of the d 13 C value of ST6-D4-4 sample is "0.2‰ at 1 s , estimated by the repeated measurements of a standard sample. d 13 C data of samples whose CO 2 amount is - 0.9 = 10y6 mol are not presented in this paper. 3.3. Helium The 3 Her 4 He and 4 Her 20 Ne ratios, and 4 He concentrations were measured by a noble gas mass spectrometer Ž6-60-SGA, Nuclide Co.. installed at the Department of Earth and Planetary Physics, University of Tokyo, after purification and separation of helium and neon using a hot Ti getter and activated charcoal traps held at liquid N2 temperature w17x. Although helium was not separated from neon, which interferes with helium isotopic analysis, neon presence in the measured basalt glasses was too small to generate any significant uncertainty in absolute 3 Her 4 He ratios w18x. As for the helium isotopic ratio, experimental error, which is constrained by the reproducibility of the repeated measurements, was ; 5% at 1 s . The helium concentrations were determined from the peak height in comparison with an air standard of a known volume. In this case, the uncertainty of measurement was ; 20% at 1 s , which is estimated by repeated analysis of atmospheric standard samples. The 3 Her 4 He ratios are expressed relative to the atmospheric ratio Ž RrRA .,
using air helium as the absolute standard Ž R A s 1.4 = 10y6 .. 3.4. CO2 r 3 He ratio In this paper, CO 2r 3 He ratios are calculated by combining observed 3 Her 4 He ratios, and concentrations of CO 2 and He which were extracted separately. Although it is preferable to extract CO 2 and He simultaneously, the CO 2r 3 He ratio by separate extraction agrees with those produced by simultaneous extraction within the experimental error, "20%, which is estimated by the repeated measurements of CO 2 and He concentrations. Table 1 lists CO 2r 3 He ratios of two reference MORB samples, which were reported previously by Marty and Jambon w5x; vesicle-CO 2r 3 He ratios measured by the authors agree with those of other laboratories w5,19,20x within the experimental error, "20%. 3.5. Chemical compositions After carbon and helium analyses, chemical compositions of samples were measured by a conventional X-ray fluorescence apparatus installed at the Geological Institute, University of Tokyo. Experimental details will be given elsewhere w21x. 4. Results Results of vesicle-gases and chemical compositions are summarized in Tables 2 and 3, respectively. According to the SiO 2 content, from 48.0 to 50.0 wt% ŽTable 3., all analyzed samples are classified as basalt. The analyzed basalt glasses are free from air
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
131
Table 2 He and C compositions of vesicle-gas in NFB basalt glasses Latitude Ž8 X S.
Longitude Ž8 X E.
Depth Žm.
4 He Ž10y10 molrg.
3 Her 4 He a Ž RrRA .
CO 2 Ž10y6 molrg.
16 27 15 59 16 18
173 31 173 23 173 34
3464 2962 3599
0.52 0.49 0.13
8.82 8.58 8.58
0.80 n.a. 1.38
17 00 17 00 17 00 17 00 17 00 17 00 16 57 16 57 16 57
173 55 173 55 173 55 173 55 173 55 173 55 173 55 173 55 173 55
2034 2034 2034 2034 2034 2034 1970 1970 1970
4.09 7.22 4.40 6.67 5.29 4.85 4.89 5.02 4.06
8.16 8.53 7.91 8.42 8.16 8.36 8.82 8.88 9.80
2.90 2.46 1.34 1.99 4.73 6.34 4.69 4.96 5.22
18 07
173 29
2727
2.79
9.03
1.29
18 49 18 49 19 16 19 16
173 30 173 30 173 27 173 27
2721 2721 2726 2726
5.71 3.09 3.26 5.50
8.53 8.13 8.69 8.19
ST21-D3-1 21 02 ST21-D3-9 21 02 ST21-D3-Ž19–23. 21 02
174 09 174 09 174 09
2832 2832 2832
1.83 0.94 2.03
8.09 8.82 8.28
Sample
d 13 C Ž‰ vs. PDB.
CO 2 r 3 He Ž=10 8 .
4 He r 20 Ne
n.a. n.a. y7.61
12.5 n.a. 92.4
56 1500 750
y8.47 y7.48 y7.46 y6.12 l.s. y10.45 y6.33 y7.67 y7.32
6.21 2.85 2.75 2.53 7.83 11.2 7.76 7.94 9.39
740 580 1700 750 1500 1300 830 1800 8200
y4.17 b
3.67
230
2.72 2.41 2.50 3.71
y6.67 y7.61 y7.19 y6.92
3.99 6.86 6.30 5.87
130 520 340 1100
2.77 2.05 3.57
y8.21 y8.03 y8.37
N1608 segment: ST53-D9-1-3 ST55-D11-1-6 ST58-DT8r9-7
Triple Junction Area: ST4-DV17-2 ST4-DV17-3 ST4-DV18-2 ST4-DV19-1 ST4-DV21-1 ST4-DV21-5 ST4-D2-1 ST4-D2-2 ST4-D2-4 N158 segment: ST6-D4-4 N–S segment: ST14-DT7-2 ST14-DT7-X ST15-D11-12 ST15-D11-X 1748E segment: 13.4 17.8 15.2
980 450 340
n.a.s not analyzed sample which is too little to analyze; l.s.s lost sample in analysis. a Sample 3 Her 4 He ratio normalized to the atmospheric 3 Her 4 He ratio, RA s 1.4 = 10y6 . b Only this d 13 C error is 0.2‰ Žthe others are 0.1‰..
contamination, since observed 4 Her 20 Ne ratios, from 56 to 8200, which are thought to reflect the degree of air contamination w22,23x, are much higher than the atmospheric ratio, 0.318. Significantly high CO 2 concentrations, from 8.0 = 10y7 to 6.3 = 10y6 molrg, indicate that, for the most part, all analyzed basalt glasses preserve carbon as vesicle-CO 2 ŽTable 2.. Fig. 2a, b and c shows the along-ridge variation of CO 2r 3 He ratios, 3 Her 4 He ratios and d 13 C values, respectively. Vesicle-CO 2r 3 He ratios of NFB
basalt glasses vary significantly from 2.5 = 10 8 to 9.2 = 10 9 ŽFig. 2a., although those of MORB reported by Marty and Jambon w5x range within Ž1.5 " 0.5. = 10 9 uniformly. Among analyzed NFB basalt glasses, the CO 2r 3 He ratio of the ST58-DT8r9-7 basalt from N1608 segment is 9.2 = 10 9 , which is much higher than those of the other NFB basalts Ž2.5 = 10 8 –1.8 = 10 9 . ŽFig. 2a., while its 3 Her 4 He ratio is not different from the other basalts ŽFig. 2b.. The 3 Her 4 He ratios Ž7.9–9.8 RrRA . of the measured NFB basalts are slightly higher than the aver-
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
132
Table 3 Chemical compositions of NFB basalt glasses Sample
SiO 2 Žwt%.
Al 2 O 3 Žwt%.
TiO 2 Žwt%.
n.a. 48.5 48.7
n.a. 16.2 15.5
n.a. 1.55 2.24
49.3 49.8 49.2 49.1 48.6 48.5 50.0 49.8 49.9
14.3 13.7 15.3 15.2 17.6 17.6 14.6 14.5 14.5
ST6-D4-4 N–S segment:
49.6
ST14-DT7-2 ST14-DT7-X ST15-D11-12 ST15-D11-X
Fe 2 O 3 Žwt%.
MnO Žwt%.
MgO Žwt%.
CaO Žwt%.
Na 2 O Žwt%.
K 2O Žwt%.
P2 O5 Žwt%.
Total Žwt%.
n.a. 9.9 9.1
n.a. 0.16 0.15
n.a. 8.09 9.57
n.a. 11.6 9.5
n.a. 2.75 3.16
0.45a 0.29 0.88
n.a. 0.21 0.38
n.a. 99.2 99.2
1.62 1.65 1.35 1.35 1.79 1.78 1.41 1.42 1.42
12.1 12.7 10.9 11.0 8.0 7.9 11.1 11.0 11.0
0.20 0.21 0.18 0.18 0.13 0.13 0.19 0.19 0.19
7.42 7.26 8.26 8.25 8.38 8.37 7.41 7.40 7.39
11.6 11.5 12.1 12.1 10.9 10.9 12.0 12.0 12.0
2.58 2.52 2.46 2.46 2.79 2.76 2.99 3.02 3.01
0.19 0.08 0.06 0.06 0.77 0.76 0.10 0.10 0.10
0.18 0.16 0.13 0.12 0.32 0.32 0.14 0.14 0.14
99.4 99.7 100.0 99.7 99.3 99.1 99.8 99.6 99.6
14.9
1.24
10.9
0.18
8.31
12.1
2.45
0.03
0.11
99.8
49.7 49.6 48.8 48.9
15.4 14.5 15.2 15.2
1.22 1.59 1.68 1.68
10.7 11.9 11.3 11.2
0.18 0.19 0.19 0.19
8.65 7.52 7.96 8.01
12.6 11.7 11.7 11.7
2.37 2.60 2.70 2.70
0.03 0.17 0.12 0.12
0.11 0.17 0.18 0.18
100.9 99.8 99.9 99.9
ST21-D3-1 48.0 ST21-D3-9 48.3 ST21-D3-Ž19 ; 23. 48.1
15.9 15.7 16.0
1.71 1.65 1.71
10.1 10.1 10.1
0.16 0.16 0.16
9.49 9.37 9.50
10.6 10.8 10.6
3.02 2.88 3.01
0.35 0.32 0.36
0.25 0.23 0.25
99.6 99.6 99.7
N1608 segment: ST53-D9-1-3 ST55-D11-1-6 ST58-DT8r9-7
Triple Junction Area: ST4-DV17-2 ST4-DV17-3 ST4-DV18-2 ST4-DV19-1 ST4-DV21-1 ST4-DV21-5 ST4-D2-1 ST4-D2-2 ST4-D2-4 N158 segment:
1748E segment:
n.a.s not analyzed sample which is too little to analyze. a Data from Hirose et al. w15x.
age MORB value, 8.2 RrRA w24x, although within the MORB value, 8.2 " 0.7RrRA w24x, approximately ŽFig. 2b.. In southern NFB ŽN158, N–S and 1748E segments., there are systematic variations in CO 2r 3 He ratios and d 13 C values: CO 2r 3 He ratio increases ŽFig. 2a. and d 13 C value decreases ŽFig. 2c. as the sample site becomes closer to the trench, since a sample site along spreading segments gets nearer to the trench with the latitude Žsee Fig. 1.. As previously reported, NFB basalts are characterized by extreme variations in LILE, HFSE and REE contents and Sr–Nd isotopic ratios, whose data demonstrate that at least two different magma source
components were mixed and trapped w13,14x. One is a depleted mantle source and the other is a LILE, LREE and HFSE enriched OIB-like source w14x. In order to identify the analyzed NFB basalts, this paper uses K 2 O content, which reflects the characteristics of REE pattern and Sr–Nd isotope systematics w13,14x. Fig. 3a and b, the CO 2r 3 He–K 2 O diagram and the d 13 C–K 2 O diagram, respectively, shows that the CO 2r 3 He ratio increases and the d 13 C value decreases with K 2 O content. From this, the wide variation of CO 2r 3 He ratios and d 13 C values of NFB basalts should be mainly a result of the two-component mixing, rather than fractional de-
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
133
refers to the former as a ‘‘depleted’’ source and the latter has an ‘‘enriched’’ source in terms of K 2 O abundance.
5. Discussion 5.1. Depleted source (low-CO2 r 3 He, high-d 13 C and low-K 2 O) Assuming that the CO 2r 3 He ratio and the d 13 C value of the NFB mantle are those of basalt whose K 2 O content is the lowest among NFB basalts, it may be inferred that the CO 2r 3 He ratio and the d 13 C value of NFB mantle source range from 2.5 = 10 8 to 4.0 = 10 8 and from y6.7 to y4.2‰, respectively Žsee Fig. 3a and b.. The inferred NFB mantleCO 2r 3 He ratio, from 2.5 = 10 8 to 4.0 = 10 8 , is lower than that of North Atlantic and East Pacific MORB, Ž1.5 " 0.5. = 10 9 w5,6x. This disagreement, however, is not unique: Rodriguez Triple Junction MORBs in the Indian Ocean have low CO 2r 3 He ratios, from 1.2 = 10 8 to 4.2 = 10 8 w25x, and the
Fig. 2. Along-ridge variation of: Ža. CO 2 r 3 He ratio; Žb. 3 Her 4 He ratio; and Žc. d 13 C value, of vesicle-gases in NFB basalt glasses. The 3 Her 4 He diagram Žb. also show normal MORB range, 8.18"0.73Ž1 s . RA w24x. Circles Ž` and v . show the samples from the northern segment ŽN1608 segment including Triple Junction. and the southern segments ŽN158, N–S and 1748E segments., respectively. Among the southern NFB samples Žv ., there is a correlation that CO 2 r 3 He ratio increases Ža. and d 13 C value decreases Žc. as the sample site becomes closer to the trench. The estimated CO 2 r 3 He ratio Ža. and d 13 C value Žc. of NFB-mantle are stippled.
gassing. The mixing end-member source should be NFB mantle source Žlow-CO 2r 3 He, high-d 13 C and low-K 2 O. and the other end-member source ŽhighCO 2r 3 He, low-d 13 C and high-K 2 O.. This paper
Fig. 3. Correlation diagrams: Ža. between CO 2 r 3 He ratio and K 2 O content; and Žb. between d 13 C value and K 2 O content, of the North Fiji back-arc basin basalt glasses. The estimated CO 2 r 3 He ratio and d 13 C value of NFB-mantle are stippled.
134
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
N-MORB-like basalt from the Mariana Trough ŽBABB. also has low CO 2r 3 He ratio, 7.2 = 10 8 w26x. This difference will be discussed in a separate paper. 5.2. Enriched source (high-CO2 r 3 He, low-d 13 C and high-K 2 O) As seen in Fig. 3, the signature of the enriched source appears in samples from the N1608 segment including the Triple Junction and the 1748E segment which is close to the trench. Among analyzed NFB basalt glasses, the CO 2r 3 He ratio of the ST58DT8r9-7 basalt from the N1608 segment is 9.2 = 10 9 , which is much higher than those of the other NFB basalts wŽ0.3–1.8. = 10 9 x, while its 3 Her 4 He ratio is not different from the other basalts ŽFig. 2a and b.. Although there is a possibility that a high CO 2r 3 He ratio may be contributed by the lower mantle w27x, the helium isotopic data indicate that the excess carbon in NFB enriched source is not of deep origin. Because of this, the excess carbon Žrelative to the mantle. in NFB enriched source can be attributed convincingly to the subducted sedimentary carbon. In order to characterize the subducted sedimentary carbon, we make quantitative estimates of carbon
contributions from the mantle, carbonate and organic matter, in the analyzed NFB basalt, based on a mass balance using CO 2r 3 He ratios and d 13 C values w7x. The distribution of data in the CO 2r 3 He– d 13 C diagram ŽFig. 4. suggests that the carbon in NFB basalts can be explained by a three-component mixing. When carbon in a sample is a mixture of carbon from the mantle, carbonate and organic matter components with relative masses M, C and O, respectively, the following equations are derived:
d 13 C obs s d 13 C man = M q d 13 C car = C q d 13 C org = O 12
1r Ž Cr 3 He . obs 12
12
s Mr Ž Cr 3 He . man q Cr Ž Cr 3 He . car 12
qOr Ž Cr 3 He . org MqCqOs1 where subscripts obs, man, car and org refer to the observed samples, mantle, sedimentary carbonate and sedimentary organic matter, respectively. Taking values of d 13 C man s y6.0‰; d 13 C car s 0‰ w28x; d 13 C org s y26‰ w28x; Ž 12 Cr 3 He. man s 2.5 = 10 8 ; Ž 12 Cr 3 He.car s 1 = 10 13 w7x and Ž 12 Cr 3 He. org s 1
Fig. 4. Correlation diagram between d 13 C value and CO 2r 3 He ratio of NFB basalt glasses. Solid lines indicate the mixing trends between NFB mantle and carbonate, that between NFB mantle and organic matter, and that between carbonate and organic matter. Numbers indicate contribution of the sedimentary carbon Ž%.. r s organicrcarbonate. Dotted lines indicate the mixing trends between NFB mantle and the subducted end-member source whose carbonate and organic matter are in the ratios 1:1, 7:3 and 20:1, respectively.
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
= 10 13 w7x, one can calculate quantitatively a mixing ratio of three components, M, C and O, in the sample w7x. Uncertainty caused by the variation of d 13 C car , d 13 C org , Ž 12 Cr 3 He.car and Ž 12 Cr 3 He. org is recorded by Sano and Marty w7x. In this mass-balance calculation, Cr 3 He ratios of carbonate and organic matter were assumed as those of most crustal CO 2-rich continental gases w7x. Even though the Cr 3 He ratios of carbonate and organic matter may very significantly, the calculation results are not affected w7x. Estimated carbon fractions are plotted in Fig. 5, which shows that the mantle carbon in NFB basalts vary widely from 3% to 99%. Contrary to a wide variation of the mantle carbon, carbonate and organic matter are consistently in a ratio of approximately 7:3, as opposed to the samples which are depleted in the sedimentary carbon Žcarbonateq organic matter. Žsee Fig. 5.. Although the subducted carbon is enriched in samples from the two areas, the N1608 segment Žincluding TJ. and the 1748E segment, both carbonaterorganic matter ratios are almost the same. This result suggests that the NFB subducted endmember source should have 70% carbonate and 30% organic matter in origin, regardless of sample site. Although the inferred CO 2r 3 He ratio and the d 13 C
135
value of the NFB mantle source vary from 2.5 = 10 8 to 4.0 = 10 8 and from y6.7 to y4.2‰, respectively, these variations are not significant for the estimated carbonaterorganic matter ratios. In contrast, it is estimated that the subducting carbonate and organic matter throughout the North Fiji subduction zone are in a proportion of 20:1, judging from the reports of Collot et al. w29,30x: Seismic data ŽSCS Line 83. and core-sample data ŽHole 828. indicate that carbonate-layer ŽUnit II and III., which has ; 10 wt% inorganic carbon Ž80 wt% CaCO 3 . and ; 0.5 wt% organic carbon, is subducting throughout the New Hebrides Trench. Assuming this ratio is the same as that of the subducting sediment, the following equations are described: O 1 s Ž 1. C 20 where C and O are masses of carbonate and organic matter, respectively, in the subducting marine sediment. On the other hand, in North Fiji back-arc basin basalts, the subducted end-member source has 70% carbonate and 30% organic matter in origin. From this, the following equations are expressed as: Od 3 s Ž 2. Cd 7 where C d and Od are masses of carbonate and organic matter, respectively, in the subducted source in the North Fiji back-arc basin basalt. Further assuming that complete decomposition of the subducted organic matter has occurred ŽO s Od ., Eqs. Ž1. and Ž2. can be rewritten as: Cs C
s
C y Cd C
s1y
Cd C
s1y
ž
O C
=
Cd Od
/
f 0.9
Ž 3.
Fig. 5. Estimated carbon contributions from mantle, carbonate and organic matter in the NFB basalt glasses. The line with an arrow shows the mixing line between NFB mantle source and the subducted end-member source. Contrary to a wide variation of the mantle carbon, carbonate and organic matter are consistently in a ratio of approximately 7:3, as opposed to the samples which are depleted in the sedimentary carbon Žcarbonateqorganic matter..
where C s is mass of carbonate which is free from the decomposition in the North Fiji back-arc. From Eq. Ž3., it is derived that ; 90% of the carbonate, which subducted without accretion to wedge, is free from decomposition in the area studied. Assuming that the result of BABB represent the signature of the whole subduction zone, the surviving carbonate may be re-injected into mantle in the area studied. Since the lack of North Fiji arc data such as Vanuatsu island-arc volcanic gases, the authors cannot deny the possibility that the carbonate was decomposed priory at the
136
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
North Fiji arc. However, the evaluation of crustal contamination in arc sample would be difficult. Less crustal contamination is one of the advantages of BABB sample. The possibility that carbonate can be transported into the mantle through the subduction zones has been proposed due to the results of highpressure and -temperature experiments of carbonate decomposition Že.g., w31–33x.. Moreover, the evidence for the burial of carbonate at depths in excess of 100 km was reported by Becker and Altherr w34x. This connection, along with the result of this study may be compelling evidence that carbonate can be transported into the mantle through the North Fiji subduction zones. 5.3. Distribution of the subducted carbon As stated previously, the NFB subducted endmember source should have 70% carbonate and 30% organic matter in origin, regardless of the sample site. In this paper, the subducted end-member source means the carbon-rich source generated from the subducted sediment. The subducted end-member source is enriched in samples from two areas: one is the 1748E segment and the other is the N1608 segment Žincluding TJ.. In the southern segments ŽN158, N–S and 1748E., the CO 2r 3 He ratio increases ŽFig. 2a., and the d 13 C value decreases ŽFig. 2c. as the sample site becomes closer to the trench. In NFB, a significant subduction-related contamination with a negative Nb anomaly and a high LOI are observed in several basalts mainly from the 1748E segment w13,14x. The NFB is a large back-arc basin, and the distance from the northern segments to the trench is ) 500 km, which is remarkably long in comparison with smaller back-arc basins such as the Mariana Trough and the Lau Basin. Since the 1748E segment is relatively nearer to the trench than the other three segments, the subducted end-member source in samples from the 1748E segment would be transported with the water dehydrated from the subducted slab. Although a negative Nb anomaly and a high LOI were not observed in ST21 samples which were the only 1748E samples analyzed in this paper w13x, carbon tracers such as CO 2r 3 He ratio and d 13 C value can detect the subducted carbon. Contrary to this, the subducted end-member source in samples from the N1608 segment, including TJ, would be transported by the mantle plume.
6. Conclusions From the above, the following conclusions can be drawn: Ž1. Although CO 2r 3 He ratios and d 13 C values of NFB basalts vary significantly, there are correlation that CO 2r 3 He ratio increases and d 13 C value decreases with K 2 O content. Since REE and Sr–Nd isotopic data suggest that the variety of K 2 O content of the NFB basalt is the result of two-component mixing w14x, the wide variation of CO 2r 3 He ratios and d 13 C values of NFB basalts should be mainly a result of the two-component mixing, rather than fractional degassing. The mixing end-member source should be NFB mantle source Žlow-CO 2r 3 He, highd 13 C and low-K 2 O. and the other end-member source Žhigh-CO 2r 3 He, low-d 13 C and high-K 2 O.. Ž2. Assuming that the CO 2r 3 He ratio and the 13 d C value of the NFB mantle are those of basalt whose K 2 O content is the lowest among NFB basalts, it may be inferred that the CO 2r 3 He ratio and the d 13 C value of NFB mantle source range from 2.5 = 10 8 to 4.0 = 10 8 and from y6.7 to y4.2‰, respectively. Ž3. In NFB basalt, the 3 Her 4 He ratio does not correlate with CO 2r 3 He ratio and d 13 C value. This suggests that the excess carbon Žrelative to the mantle. of NFB enriched source can be attributed convincingly to the subducted sedimentary carbon, rather than the lower mantle. Ž4. From a simple mass-balance calculation, it is derived that the subducted end-member source has 70% carbonate and 30% organic matter in origin. Assuming that complete decomposition of the subducted organic matter has occurred, most Ž; 90%. carbonates, which subducted without accretion to wedge, is free from decomposition in North Fiji back-arc. This may be one clue that carbonate can be transported into the mantle through the subduction zones.
Acknowledgements The authors are grateful to K. Hirose and T. Urabe for providing the NFB basalt glass samples. The authors would like to thank M. Tsutsumi, H. Yoshida, K. Kiyota, N. Takahata, J. Yamamoto, H.
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
Namba, K. Sugai, M. Joshima and N. Geshi for their help with experiments. Thanks are due to E. Tajika, Y. Nagahara, I. Kaneoka, M. Toriumi, K. Ozawa, T. Sasada and M. Ozima for many helpful comments and suggestions. The authors would like to express their appreciation to Y. Nozaki and N. Sugiura for the use of their laboratories. The authors also thank B. Marty and anonymous reviewers for thoughtful reviews of the manuscript. JSPS Research Fellowship for Young Scientists to YN partly funded this study.
w15x
w16x
w17x
w18x
w19x
References w1x M. Javoy, F. Pineau, C.J. Allegre, Carbon geodynamic cycle, ` Nature ŽLondon. 300 Ž1982. 171–173. w2x D.H. Green, S.M. Eggins, G. Yaxley, The other carbon cycle, Nature ŽLondon. 365 Ž1993. 210–211. w3x D.P. Mattey, R.H. Carr, I.P. Wright, C.T. Pillinger, Carbon isotopes in submarine basalts, Earth Planet. Sci. Lett. 70 Ž1984. 196–206. w4x C. Macpherson, D. Mattey, Carbon isotope variations of CO 2 in Central Lau Basin basalts and ferrobasalts, Earth Planet. Sci. Lett. 121 Ž1994. 263–276. w5x B. Marty, A. Jambon, Cr 3 He in volatile fluxes from the solid Earth: implications for carbon geodynamics, Earth Planet. Sci. Lett. 83 Ž1987. 16–26. w6x R.H. Kingsley, J.-G. Schilling, Carbon in Mid-Atlantic Ridge basalt glasses from 288N to 638N: Evidence for a carbon-enriched Azores mantle plume, Earth Planet. Sci. Lett. 129 Ž1995. 31–53. w7x Y. Sano, B. Marty, Origin of carbon in fumarolic gas from island arc, Chem. Geol. 119 Ž1995. 265–274. w8x B. Marty, A. Jambon, Y. Sano, Helium isotopes and CO 2 in volcanic gases of Japan, Chem. Geol. 76 Ž1989. 25–40. w9x Y. Sano, J. Hirabayashi, T. Oba, T. Gamo, Carbon and helium isotopic ratios at Kusatsu-Shirane volcano, Japan, Appl. Geochem. 9 Ž1994. 371–377. w10x Y. Sano, T. Gamo, K. Notsu, H. Wakita, Secular variations of carbon and helium isotopes at Izu-Oshima Volcano, Japan, J. Volcanol. Geotherm. Res. 64 Ž1995. 83–94. w11x Y. Sano, Y. Nishio, S. Sasaki, T. Gamo, K. Nagao, Helium and carbon isotope systematics at Ontake volcano, Japan, Geochim. Cosmochim. Acta Žsubmitted.. w12x J.-P. Eissen, C. Lefevre, P. Maillet, G. Morvan, M. Nohara, ` Petrology and geochemistry of the central North Fiji Basin spreading centre ŽSouthwest Pacific. between 168S and 228S, Mar. Geol. 98 Ž1991. 201–239. w13x J.-P. Eissen, M. Nohara, J. Cotten, K. Hirose, North Fiji Basin basalts and their magma sources: Part I. Incompatible element constraints, Mar. Geol. 116 Ž1994. 153–178. w14x M. Nohara, K. Hirose, J.-P. Eissen, T. Urabe, M. Joshima, The North Fiji Basin basalts and their magma sources: Part
w20x
w21x
w22x
w23x
w24x
w25x
w26x
w27x
w28x
w29x w30x
137
II. Sr–Nd isotopic and trace element constraints, Mar. Geol. 116 Ž1994. 179–195. K. Hirose, M. Nohara, H. Hiyagon, M. Tanahashi, J.-P. Eissen, Petrology of the central ridge in the North Fiji Basin, Earth Planet. Sci. Lett. Žsubmitted.. Y. Nishio, M. Tsutsumi, T. Gamo, Y. Sano, Hydrogen effect on the d 13 C value of CO 2 measured by mass spectrometry with electron-impact ionization, Anal. Sci. 11 Ž1995. 9–12. Y. Sano, H. Wakita, Geographical distribution of 3 Her 4 He ratios in Japan: Implications for arc tectonics and incipient magmatism, J. Geophys. Res. 90 Ž1985. 8729–8741. Y. Sano, H. Wakita, Precise measurement of helium isotopes in terrestrial gases, Geochim. Cosmochim. Acta 61 Ž1988. 1153–1157. F. Pineau, M. Jaboy, Carbon isotopes and concentrations in mid-oceanic ridge basalts, Earth Planet. Sci. Lett. 62 Ž1983. 239–257. A. Jambon, J.L. Zimmermann, Major volatiles from a North Atlantic MORB glass and calibration to He: a size fraction analysis, Chem. Geol. 62 Ž1987. 177–189. H. Yoshida, N. Takahashi, Chemical behavior of major and trace elements in the Horoman mantle diapir, Hidaka belt, Hokkaido, Japan, J. Mineral. Petrol. Econ. Geol. Žsubmitted.. H. Craig, J.E. Lupton, Y. Horibe, A mantle helium content in circum-Pacific volcanic gases: Hakone, the Marianas, and Mt. Lassen, in: E.C. Alexander, M. Ozima ŽEds.., Terrestrial Rare Gases, Jpn. Sci. Soc. Press, Tokyo, 1978, pp. 3–16. T. Torgersen, W.J. Jenkins, Helium isotopes in geothermal systems: Iceland, the Geysir, Raft River and Steamboat Springs, Geochim. Cosmochim. Acta 46 Ž1982. 739–748. D.R. Hilton, K. Hammerschmidt, G. Loock, H. Friedrichsen, Helium and argon isotope systematics of the central Lau Basin and Valu Fa Ridge: Evidence of crustrmantle interactions in a back-arc basin, Geochim. Cosmochim. Acta 57 Ž1993. 2819–2841. Y. Nishio, S. Sasaki, T. Gamo, T. Ishii, Y. Sano, Carbon and helium isotope systematics of mid-ocean ridge basalt from Rodriguez Triple Junction in the Indian Ocean, Abstr. 1996 Fall Meet. Volcanol. Soc. Jpn., 1996, p.42. Y. Sano, Y. Nishio, T. Gamo, A. Jambon, B. Marty, Noble gas and carbon isotopes in Mariana Trough basalt glasses, Appl. Geochem. Žin press.. T. Trull, S. Nadeau, F. Pineau, M. Polv, M. Javoy, C–He systematics in hotspot xenoliths: Implications for mantle carbon contents and carbon recycling, Earth Planet. Sci. Lett. 118 Ž1993. 43–64. M. Schidlowski, A 3,800-million-year isotopic record of life from carbon in sedimentary rocks, Nature ŽLondon. 333 Ž1988. 313–318. J.-Y. Collot, H.G. Greene, L.B. Stokking, et al., Init. Rep., Proc. Ocean Drill. Proj. 134 Ž1992. 159. J.-Y. Collot, H.G. Greene, M.A. Fisher, E. Geist, Tectonic accretion and deformation of the accretionary wedge in the North d’Entrecasteaux Ridge–New Hebrides Island Arc collision zone: evidence from multichannel seismic reflection profiles and Leg 134 results, Sci. Rep., Proc. Ocean Drill. Proj. 134 Ž1994. 5–18.
138
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
w31x R.C. Newton, W.E. Sharp, Stability of forsteriteqCO2 and its bearing on the role of CO 2 in the mantle, Earth Planet. Sci. Lett. 26 Ž1975. 239–244. w32x P.J. Wyllie, W.L. Huang, Carbonation and melting reactions in the system CaO–MgO–SiO 2 –CO 2 at mantle pressures with geophysical and petrological applications, Contrib. Mineral. Petrol. 54 Ž1976. 79–107.
w33x C. Biellmann, P. Gillet, F. Guyot, J. Peyronneau, B. Reynard, Experimental evidence for carbonate stability in the Earth’s lower mantle, Earth Planet. Sci. Lett. 118 Ž1993. 31–41. w34x H. Becker, R. Altherr, Evidence from ultra-high-pressure marbles for recycling of sediments into the mantle, Nature ŽLondon. 748 Ž1992. 745–748.
Carbon and helium isotope systematics of North Fiji Basin basalt glasses: carbon geochemical cycle in the subduction zone Yoshiro Nishio
a,)
, Sho Sasaki
a,1
, Toshitaka Gamo Yuji Sano d,4
b,2
, Hajime Hiyagon
c,3
,
a
c
Geological Institute, School of Science, UniÕersity of Tokyo, Tokyo 113, Japan b Ocean Research Institute, UniÕersity of Tokyo, Tokyo 164, Japan Department of Earth and Planetary Physics, School of Science, UniÕersity of Tokyo, Tokyo 113, Japan d Department of Earth and Planetary Science, Hiroshima UniÕersity, Higashi-Hiroshima 739, Japan Received 27 May 1997; revised 10 September 1997; accepted 29 September 1997
Abstract We have measured d 13 C values and CO 2r 3 He ratios of vesicle-gas, and chemical compositions of North Fiji Basin basalt glasses, to estimate the contribution of subducted carbon in back-arc basin basalt quantitatively. It has made clear that the CO 2r 3 He ratio increases and the d 13 C value decreases with K 2 O content. Since the variety of K 2 O content of North Fiji Basin basalt is the result of two-component mixing, the variety of the CO 2r 3 He ratio and d 13 C value of the North Fiji Basin basalt should be mainly a result of mixing between the mantle component Žlow-CO 2r 3 He, high-d 13 C and low-K 2 O. and the subducted component Žhigh-CO 2r 3 He, low-d 13 C and high-K 2 O.. From a simple mass-balance calculation, it is derived that the subducted end-member source has 70% carbonate and 30% organic matter in origin. Assuming that complete decomposition of the subducted organic matter has occurred, most Ž; 90%. carbonates, which subducted without accretion to wedge, is free from decomposition in North Fiji back-arc, since the subducting carbonate and organic matter throughout the North Fiji subduction zone are estimated in a proportion of 20:1. This may be one clue that carbonate can be transported into the mantle through the subduction zones. q 1998 Elsevier Science B.V. Keywords: carbon; helium; isotopes; basalts; back-arc basins; Fiji
1. Introduction
)
Corresponding author. Fax: q81-3-3815-9490; E-mail: [email protected] 1 Fax: q81-3-3815-9490; E-mail: [email protected] 2 Fax: q81-3-5351-6452; E-mail: [email protected] 3 Fax: q81-3-3818-3247; E-mail: [email protected]. u-tokyo-.ac.jp 4 Fax: q81-824-24-0735; E-mail: [email protected]. hiroshima-u.ac.jp
In the carbon geochemical cycle, one of the important processes is that sedimentary carbon should be re-injected into the mantle at the subduction zone w1,2x. Mattey et al. w3x noted the contribution of the subducted sedimentary organic carbon to certain back-arc basin basalt ŽBABB., since the d 13 C values of BABBs from the East Scotia Sea and the Mariana Trough are significantly lower than those of mid-ocean ridge basalt ŽMORB.. The low d 13 C values,
0012-821Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 1 2 - 8 2 1 X Ž 9 7 . 0 0 1 8 7 - 8
128
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
however, can be produced not only by mixing of the organic carbon with the mantle carbon, but also by progressive degassing, because of the isotopic fractionation between CO 2 vapor and carbon in the melt w4x. In addition, carbon resulting from the mixing of carbonate Ž d 13 C ; 0‰. with organic matter Ž d 13 C - y20‰. may have a 13 Cr 12 C ratio compatible with that of the mantle Ž d 13 C f y6.5 " 2.5‰.. From this, the contribution of the subducted carbon in BABB cannot be determined by carbon isotopic data only. Cr 3 He ratios of island arc volcanic gases range from 3 = 10 9 to 1.3 = 10 11 , which is higher than those of MORB wŽ2.0 " 0.5. = 10 9 x w5,6x. Since this excess carbon Žrelative to the mantle. can be attributed to the subducted sediment, the Cr 3 He ratio is one of the more powerful tracers to reveal the recycling of the subducted carbon w5–7x. From this point of view, there are several reports on both 13 Cr 12 C and CO 2r 3 He ratios of island arc volcanic gases w7–11x; e.g., Sano and Marty w7x gave quantitative estimates of the contribution of the subducted carbon in island arc volcanic gases using a mass-balance calculation between 13 Cr 12 C and CO 2r 3 He ratios. However, 13 Cr 12 C and CO 2r 3 He ratios of BABBs have not been documented simultaneously. Because of less crustal contamination properties, BABBs should be good sample material for the study of recycling in the subduction zone. This study aims to reveal the detailed carbon and helium isotopic systematics in a back-arc basin ŽNorth Fiji Basin.. Furthermore, this paper discusses the origin of carbon in BABB using 13 Cr 12 C and CO 2r 3 He data. 2. Samples Analyzed samples were collected from the North Fiji Basin Žhereafter NFB. during the French–Japan STARMER project. The NFB ŽFig. 1. is located east of the New Hebrides Trench, where the Indo-Australian Plate is being subducted beneath the Pacific Plate. The NFB is the largest back-arc basin among the presently active basins, and the distance from the major spreading segments to the trench is ) 500 km, which is remarkably long in comparison with smaller back-arc basins such as the Mariana Trough and the Lau Basin. The presently active spreading system in
NFB can be divided into four segments between 158S and 228S, which are north to south N1608, N158, N–S and 1748E segments, based on topography w12x Žsee Fig. 1.. The basalts in the central NFB ŽN158 and N–S segments. are similar to an N-MORB, which is depleted in large-ion lithophile elements ŽLILE., high-field-strength elements ŽHFSE. and light rareearth elements ŽLREE. w13x. In the southern NFB, on the 1748E segment, several basalts show a significant subduction-related contamination with a negative Nb anomaly and a high LOI Žlost of ignition. w13,14x. Since the 1748E segment is the nearest to the trench among the four NFB segments, the basalts from this area may be influenced by the fluid dehydrated from the slab. In the northern NFB w1608 spreading segment including the Triple Junction ŽTJ.x, many basalts result from the mixing of N-MORB and oceanic island basalt ŽOIB. sources with significant enrichment in LILE, HFSE and LREE w13,14x. The detailed petrological and geochemical results on analyzed basalts are reported elsewhere w12–15x. We have analyzed 20 basalt glasses, which were collected from stations 4 ŽTJ., 6 ŽN158., 14 ŽN–S., 15 ŽN–S., 21 Ž1748E., 53 ŽN1608., 55 ŽN1608. and 58 ŽN1608., by the RrV ‘‘Kaiyo’’ and RrV ‘‘Yokosuka’’ cruises. These sample sites, which encompass all four spreading segments and the Triple Junction, are shown in Fig. 1. 3. Analytical methods 3.1. Sample preparation Analyzed glass samples are the chilled margin of the pillow lava. After repeated crushing, fresh glasses without oxidation haloes were collected manually. The glass chips were washed ultrasonically in a diluted HNO 3 solution and then cleaned ultrasonically in distilled water, in ethanol, and then in acetone. After drying at 1008C for 24 h to remove organic solvents, a basalt glass sample Ž; 1.5 g. was loaded into the vacuum vessel for crushing and degassed at 1508C in vacuum for 1 h. Even though the degassing may lead to the loss of He by diffusion, as predicted by the helium diffusion coefficient in basaltic glasses, this effect may not be so serious for vesicle-gas. In order to liberate vesicle-gases, the
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
129
Fig. 1. Major structures and sample sites in North Fiji Basin w13x. Short dashed line s spreading centers; F.Z.s fracture zone; T.J.s Triple Junction; black line with black triangles shows the active New Hebrides subduction zone; dashed line with open triangles shows paleo-subduction zone. Areas with less than 2000 m of water are stippled. In this map, ODP Site 828 reported by Collot et al. w29,30x is shown as I. A sample site along spreading segments gets near to the trench with the latitude.
glass sample was crushed for 5 min in a ball mill. After 5-min crushing, the glass chips were finely powdered. It is estimated that most of the vesicle-gas could be liberated by 5-min crushing, since a longer crushing time could not liberate more vesicle-gas significantly. While the liberated gases were collected at a cold trap, the crushed sample together with the vessel was heated at 1508C in order to degas the adsorbed gases on the surface of the powder. In this study, separate crushings were performed for carbon and helium analyses. Carbon and helium were purified in each vacuum line. 3.2. Carbon The 13 Cr 12 C ratios were measured by a conventional stable-isotope mass spectrometer ŽFinnigan
MAT 250. installed at the Ocean Research Institute, University of Tokyo, after separation of CO 2 from other chemical components using a trap held at liquid N2 temperature and a trap at ethanol–dry ice temperature. In addition, H 2 S was completely removed by a trap with a 1 M CuCl 2 solution which had already been evacuated by freezing, since H 2 S may affect the measured 13 Cr 12 C ratios of CO 2 w16x. Concentrations of the separated CO 2 were determined by a mercury manometer. In this case, the uncertainty of measurement was ; 20% at 1 s , which was estimated by the repeated analysis of a standard sample. Observed 13 Cr 12 C ratios are expressed by the familiar delta notation relative to PDB ŽPeedee formation Belemnite., d 13 C. The treatment of d 13 C error in this paper is as follows: Uncertainties of the d 13 C values whose CO 2 amount is more
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
130
Table 1 Vesicle-CO2r 3 He ratios in reference North Mid-Atlantic Ridge basalt glasses Sample
4 He Ž10y10 molrg.
3 Her 4 He a Ž RrRA .
CO 2 Ž10y6 molrg.
CO 2 r 3 He Ž=10 9 .
Reference
CH31-DR11 CH31-DR11 CH31-DR11
3.39
7.93
8.32
2.2 2.0 Ž6.6.
this work w19x w5x
CH98-DR11 CH98-DR11 CH98-DR11
2.10
8.24
3.90
1.6 1.6 1.8
this work w20x w5x
a
Sample 3 Her 4 He ratio normalized to the atmospheric 3 Her 4 He ratio, RA s 1.4 = 10y6 .
than 1.8 = 10y6 mol are "0.1‰ at 1 s estimated by the repeated measurements of a standard sample. Excluding one sample ŽST6-D4-4., the error margin of d 13 C values listed in this paper is "0.1‰. The extracted CO 2 amount of the ST6-D4-4 sample is 1.4 = 10y6 mol, and the error margin of the d 13 C value of ST6-D4-4 sample is "0.2‰ at 1 s , estimated by the repeated measurements of a standard sample. d 13 C data of samples whose CO 2 amount is - 0.9 = 10y6 mol are not presented in this paper. 3.3. Helium The 3 Her 4 He and 4 Her 20 Ne ratios, and 4 He concentrations were measured by a noble gas mass spectrometer Ž6-60-SGA, Nuclide Co.. installed at the Department of Earth and Planetary Physics, University of Tokyo, after purification and separation of helium and neon using a hot Ti getter and activated charcoal traps held at liquid N2 temperature w17x. Although helium was not separated from neon, which interferes with helium isotopic analysis, neon presence in the measured basalt glasses was too small to generate any significant uncertainty in absolute 3 Her 4 He ratios w18x. As for the helium isotopic ratio, experimental error, which is constrained by the reproducibility of the repeated measurements, was ; 5% at 1 s . The helium concentrations were determined from the peak height in comparison with an air standard of a known volume. In this case, the uncertainty of measurement was ; 20% at 1 s , which is estimated by repeated analysis of atmospheric standard samples. The 3 Her 4 He ratios are expressed relative to the atmospheric ratio Ž RrRA .,
using air helium as the absolute standard Ž R A s 1.4 = 10y6 .. 3.4. CO2 r 3 He ratio In this paper, CO 2r 3 He ratios are calculated by combining observed 3 Her 4 He ratios, and concentrations of CO 2 and He which were extracted separately. Although it is preferable to extract CO 2 and He simultaneously, the CO 2r 3 He ratio by separate extraction agrees with those produced by simultaneous extraction within the experimental error, "20%, which is estimated by the repeated measurements of CO 2 and He concentrations. Table 1 lists CO 2r 3 He ratios of two reference MORB samples, which were reported previously by Marty and Jambon w5x; vesicle-CO 2r 3 He ratios measured by the authors agree with those of other laboratories w5,19,20x within the experimental error, "20%. 3.5. Chemical compositions After carbon and helium analyses, chemical compositions of samples were measured by a conventional X-ray fluorescence apparatus installed at the Geological Institute, University of Tokyo. Experimental details will be given elsewhere w21x. 4. Results Results of vesicle-gases and chemical compositions are summarized in Tables 2 and 3, respectively. According to the SiO 2 content, from 48.0 to 50.0 wt% ŽTable 3., all analyzed samples are classified as basalt. The analyzed basalt glasses are free from air
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
131
Table 2 He and C compositions of vesicle-gas in NFB basalt glasses Latitude Ž8 X S.
Longitude Ž8 X E.
Depth Žm.
4 He Ž10y10 molrg.
3 Her 4 He a Ž RrRA .
CO 2 Ž10y6 molrg.
16 27 15 59 16 18
173 31 173 23 173 34
3464 2962 3599
0.52 0.49 0.13
8.82 8.58 8.58
0.80 n.a. 1.38
17 00 17 00 17 00 17 00 17 00 17 00 16 57 16 57 16 57
173 55 173 55 173 55 173 55 173 55 173 55 173 55 173 55 173 55
2034 2034 2034 2034 2034 2034 1970 1970 1970
4.09 7.22 4.40 6.67 5.29 4.85 4.89 5.02 4.06
8.16 8.53 7.91 8.42 8.16 8.36 8.82 8.88 9.80
2.90 2.46 1.34 1.99 4.73 6.34 4.69 4.96 5.22
18 07
173 29
2727
2.79
9.03
1.29
18 49 18 49 19 16 19 16
173 30 173 30 173 27 173 27
2721 2721 2726 2726
5.71 3.09 3.26 5.50
8.53 8.13 8.69 8.19
ST21-D3-1 21 02 ST21-D3-9 21 02 ST21-D3-Ž19–23. 21 02
174 09 174 09 174 09
2832 2832 2832
1.83 0.94 2.03
8.09 8.82 8.28
Sample
d 13 C Ž‰ vs. PDB.
CO 2 r 3 He Ž=10 8 .
4 He r 20 Ne
n.a. n.a. y7.61
12.5 n.a. 92.4
56 1500 750
y8.47 y7.48 y7.46 y6.12 l.s. y10.45 y6.33 y7.67 y7.32
6.21 2.85 2.75 2.53 7.83 11.2 7.76 7.94 9.39
740 580 1700 750 1500 1300 830 1800 8200
y4.17 b
3.67
230
2.72 2.41 2.50 3.71
y6.67 y7.61 y7.19 y6.92
3.99 6.86 6.30 5.87
130 520 340 1100
2.77 2.05 3.57
y8.21 y8.03 y8.37
N1608 segment: ST53-D9-1-3 ST55-D11-1-6 ST58-DT8r9-7
Triple Junction Area: ST4-DV17-2 ST4-DV17-3 ST4-DV18-2 ST4-DV19-1 ST4-DV21-1 ST4-DV21-5 ST4-D2-1 ST4-D2-2 ST4-D2-4 N158 segment: ST6-D4-4 N–S segment: ST14-DT7-2 ST14-DT7-X ST15-D11-12 ST15-D11-X 1748E segment: 13.4 17.8 15.2
980 450 340
n.a.s not analyzed sample which is too little to analyze; l.s.s lost sample in analysis. a Sample 3 Her 4 He ratio normalized to the atmospheric 3 Her 4 He ratio, RA s 1.4 = 10y6 . b Only this d 13 C error is 0.2‰ Žthe others are 0.1‰..
contamination, since observed 4 Her 20 Ne ratios, from 56 to 8200, which are thought to reflect the degree of air contamination w22,23x, are much higher than the atmospheric ratio, 0.318. Significantly high CO 2 concentrations, from 8.0 = 10y7 to 6.3 = 10y6 molrg, indicate that, for the most part, all analyzed basalt glasses preserve carbon as vesicle-CO 2 ŽTable 2.. Fig. 2a, b and c shows the along-ridge variation of CO 2r 3 He ratios, 3 Her 4 He ratios and d 13 C values, respectively. Vesicle-CO 2r 3 He ratios of NFB
basalt glasses vary significantly from 2.5 = 10 8 to 9.2 = 10 9 ŽFig. 2a., although those of MORB reported by Marty and Jambon w5x range within Ž1.5 " 0.5. = 10 9 uniformly. Among analyzed NFB basalt glasses, the CO 2r 3 He ratio of the ST58-DT8r9-7 basalt from N1608 segment is 9.2 = 10 9 , which is much higher than those of the other NFB basalts Ž2.5 = 10 8 –1.8 = 10 9 . ŽFig. 2a., while its 3 Her 4 He ratio is not different from the other basalts ŽFig. 2b.. The 3 Her 4 He ratios Ž7.9–9.8 RrRA . of the measured NFB basalts are slightly higher than the aver-
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
132
Table 3 Chemical compositions of NFB basalt glasses Sample
SiO 2 Žwt%.
Al 2 O 3 Žwt%.
TiO 2 Žwt%.
n.a. 48.5 48.7
n.a. 16.2 15.5
n.a. 1.55 2.24
49.3 49.8 49.2 49.1 48.6 48.5 50.0 49.8 49.9
14.3 13.7 15.3 15.2 17.6 17.6 14.6 14.5 14.5
ST6-D4-4 N–S segment:
49.6
ST14-DT7-2 ST14-DT7-X ST15-D11-12 ST15-D11-X
Fe 2 O 3 Žwt%.
MnO Žwt%.
MgO Žwt%.
CaO Žwt%.
Na 2 O Žwt%.
K 2O Žwt%.
P2 O5 Žwt%.
Total Žwt%.
n.a. 9.9 9.1
n.a. 0.16 0.15
n.a. 8.09 9.57
n.a. 11.6 9.5
n.a. 2.75 3.16
0.45a 0.29 0.88
n.a. 0.21 0.38
n.a. 99.2 99.2
1.62 1.65 1.35 1.35 1.79 1.78 1.41 1.42 1.42
12.1 12.7 10.9 11.0 8.0 7.9 11.1 11.0 11.0
0.20 0.21 0.18 0.18 0.13 0.13 0.19 0.19 0.19
7.42 7.26 8.26 8.25 8.38 8.37 7.41 7.40 7.39
11.6 11.5 12.1 12.1 10.9 10.9 12.0 12.0 12.0
2.58 2.52 2.46 2.46 2.79 2.76 2.99 3.02 3.01
0.19 0.08 0.06 0.06 0.77 0.76 0.10 0.10 0.10
0.18 0.16 0.13 0.12 0.32 0.32 0.14 0.14 0.14
99.4 99.7 100.0 99.7 99.3 99.1 99.8 99.6 99.6
14.9
1.24
10.9
0.18
8.31
12.1
2.45
0.03
0.11
99.8
49.7 49.6 48.8 48.9
15.4 14.5 15.2 15.2
1.22 1.59 1.68 1.68
10.7 11.9 11.3 11.2
0.18 0.19 0.19 0.19
8.65 7.52 7.96 8.01
12.6 11.7 11.7 11.7
2.37 2.60 2.70 2.70
0.03 0.17 0.12 0.12
0.11 0.17 0.18 0.18
100.9 99.8 99.9 99.9
ST21-D3-1 48.0 ST21-D3-9 48.3 ST21-D3-Ž19 ; 23. 48.1
15.9 15.7 16.0
1.71 1.65 1.71
10.1 10.1 10.1
0.16 0.16 0.16
9.49 9.37 9.50
10.6 10.8 10.6
3.02 2.88 3.01
0.35 0.32 0.36
0.25 0.23 0.25
99.6 99.6 99.7
N1608 segment: ST53-D9-1-3 ST55-D11-1-6 ST58-DT8r9-7
Triple Junction Area: ST4-DV17-2 ST4-DV17-3 ST4-DV18-2 ST4-DV19-1 ST4-DV21-1 ST4-DV21-5 ST4-D2-1 ST4-D2-2 ST4-D2-4 N158 segment:
1748E segment:
n.a.s not analyzed sample which is too little to analyze. a Data from Hirose et al. w15x.
age MORB value, 8.2 RrRA w24x, although within the MORB value, 8.2 " 0.7RrRA w24x, approximately ŽFig. 2b.. In southern NFB ŽN158, N–S and 1748E segments., there are systematic variations in CO 2r 3 He ratios and d 13 C values: CO 2r 3 He ratio increases ŽFig. 2a. and d 13 C value decreases ŽFig. 2c. as the sample site becomes closer to the trench, since a sample site along spreading segments gets nearer to the trench with the latitude Žsee Fig. 1.. As previously reported, NFB basalts are characterized by extreme variations in LILE, HFSE and REE contents and Sr–Nd isotopic ratios, whose data demonstrate that at least two different magma source
components were mixed and trapped w13,14x. One is a depleted mantle source and the other is a LILE, LREE and HFSE enriched OIB-like source w14x. In order to identify the analyzed NFB basalts, this paper uses K 2 O content, which reflects the characteristics of REE pattern and Sr–Nd isotope systematics w13,14x. Fig. 3a and b, the CO 2r 3 He–K 2 O diagram and the d 13 C–K 2 O diagram, respectively, shows that the CO 2r 3 He ratio increases and the d 13 C value decreases with K 2 O content. From this, the wide variation of CO 2r 3 He ratios and d 13 C values of NFB basalts should be mainly a result of the two-component mixing, rather than fractional de-
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
133
refers to the former as a ‘‘depleted’’ source and the latter has an ‘‘enriched’’ source in terms of K 2 O abundance.
5. Discussion 5.1. Depleted source (low-CO2 r 3 He, high-d 13 C and low-K 2 O) Assuming that the CO 2r 3 He ratio and the d 13 C value of the NFB mantle are those of basalt whose K 2 O content is the lowest among NFB basalts, it may be inferred that the CO 2r 3 He ratio and the d 13 C value of NFB mantle source range from 2.5 = 10 8 to 4.0 = 10 8 and from y6.7 to y4.2‰, respectively Žsee Fig. 3a and b.. The inferred NFB mantleCO 2r 3 He ratio, from 2.5 = 10 8 to 4.0 = 10 8 , is lower than that of North Atlantic and East Pacific MORB, Ž1.5 " 0.5. = 10 9 w5,6x. This disagreement, however, is not unique: Rodriguez Triple Junction MORBs in the Indian Ocean have low CO 2r 3 He ratios, from 1.2 = 10 8 to 4.2 = 10 8 w25x, and the
Fig. 2. Along-ridge variation of: Ža. CO 2 r 3 He ratio; Žb. 3 Her 4 He ratio; and Žc. d 13 C value, of vesicle-gases in NFB basalt glasses. The 3 Her 4 He diagram Žb. also show normal MORB range, 8.18"0.73Ž1 s . RA w24x. Circles Ž` and v . show the samples from the northern segment ŽN1608 segment including Triple Junction. and the southern segments ŽN158, N–S and 1748E segments., respectively. Among the southern NFB samples Žv ., there is a correlation that CO 2 r 3 He ratio increases Ža. and d 13 C value decreases Žc. as the sample site becomes closer to the trench. The estimated CO 2 r 3 He ratio Ža. and d 13 C value Žc. of NFB-mantle are stippled.
gassing. The mixing end-member source should be NFB mantle source Žlow-CO 2r 3 He, high-d 13 C and low-K 2 O. and the other end-member source ŽhighCO 2r 3 He, low-d 13 C and high-K 2 O.. This paper
Fig. 3. Correlation diagrams: Ža. between CO 2 r 3 He ratio and K 2 O content; and Žb. between d 13 C value and K 2 O content, of the North Fiji back-arc basin basalt glasses. The estimated CO 2 r 3 He ratio and d 13 C value of NFB-mantle are stippled.
134
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
N-MORB-like basalt from the Mariana Trough ŽBABB. also has low CO 2r 3 He ratio, 7.2 = 10 8 w26x. This difference will be discussed in a separate paper. 5.2. Enriched source (high-CO2 r 3 He, low-d 13 C and high-K 2 O) As seen in Fig. 3, the signature of the enriched source appears in samples from the N1608 segment including the Triple Junction and the 1748E segment which is close to the trench. Among analyzed NFB basalt glasses, the CO 2r 3 He ratio of the ST58DT8r9-7 basalt from the N1608 segment is 9.2 = 10 9 , which is much higher than those of the other NFB basalts wŽ0.3–1.8. = 10 9 x, while its 3 Her 4 He ratio is not different from the other basalts ŽFig. 2a and b.. Although there is a possibility that a high CO 2r 3 He ratio may be contributed by the lower mantle w27x, the helium isotopic data indicate that the excess carbon in NFB enriched source is not of deep origin. Because of this, the excess carbon Žrelative to the mantle. in NFB enriched source can be attributed convincingly to the subducted sedimentary carbon. In order to characterize the subducted sedimentary carbon, we make quantitative estimates of carbon
contributions from the mantle, carbonate and organic matter, in the analyzed NFB basalt, based on a mass balance using CO 2r 3 He ratios and d 13 C values w7x. The distribution of data in the CO 2r 3 He– d 13 C diagram ŽFig. 4. suggests that the carbon in NFB basalts can be explained by a three-component mixing. When carbon in a sample is a mixture of carbon from the mantle, carbonate and organic matter components with relative masses M, C and O, respectively, the following equations are derived:
d 13 C obs s d 13 C man = M q d 13 C car = C q d 13 C org = O 12
1r Ž Cr 3 He . obs 12
12
s Mr Ž Cr 3 He . man q Cr Ž Cr 3 He . car 12
qOr Ž Cr 3 He . org MqCqOs1 where subscripts obs, man, car and org refer to the observed samples, mantle, sedimentary carbonate and sedimentary organic matter, respectively. Taking values of d 13 C man s y6.0‰; d 13 C car s 0‰ w28x; d 13 C org s y26‰ w28x; Ž 12 Cr 3 He. man s 2.5 = 10 8 ; Ž 12 Cr 3 He.car s 1 = 10 13 w7x and Ž 12 Cr 3 He. org s 1
Fig. 4. Correlation diagram between d 13 C value and CO 2r 3 He ratio of NFB basalt glasses. Solid lines indicate the mixing trends between NFB mantle and carbonate, that between NFB mantle and organic matter, and that between carbonate and organic matter. Numbers indicate contribution of the sedimentary carbon Ž%.. r s organicrcarbonate. Dotted lines indicate the mixing trends between NFB mantle and the subducted end-member source whose carbonate and organic matter are in the ratios 1:1, 7:3 and 20:1, respectively.
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
= 10 13 w7x, one can calculate quantitatively a mixing ratio of three components, M, C and O, in the sample w7x. Uncertainty caused by the variation of d 13 C car , d 13 C org , Ž 12 Cr 3 He.car and Ž 12 Cr 3 He. org is recorded by Sano and Marty w7x. In this mass-balance calculation, Cr 3 He ratios of carbonate and organic matter were assumed as those of most crustal CO 2-rich continental gases w7x. Even though the Cr 3 He ratios of carbonate and organic matter may very significantly, the calculation results are not affected w7x. Estimated carbon fractions are plotted in Fig. 5, which shows that the mantle carbon in NFB basalts vary widely from 3% to 99%. Contrary to a wide variation of the mantle carbon, carbonate and organic matter are consistently in a ratio of approximately 7:3, as opposed to the samples which are depleted in the sedimentary carbon Žcarbonateq organic matter. Žsee Fig. 5.. Although the subducted carbon is enriched in samples from the two areas, the N1608 segment Žincluding TJ. and the 1748E segment, both carbonaterorganic matter ratios are almost the same. This result suggests that the NFB subducted endmember source should have 70% carbonate and 30% organic matter in origin, regardless of sample site. Although the inferred CO 2r 3 He ratio and the d 13 C
135
value of the NFB mantle source vary from 2.5 = 10 8 to 4.0 = 10 8 and from y6.7 to y4.2‰, respectively, these variations are not significant for the estimated carbonaterorganic matter ratios. In contrast, it is estimated that the subducting carbonate and organic matter throughout the North Fiji subduction zone are in a proportion of 20:1, judging from the reports of Collot et al. w29,30x: Seismic data ŽSCS Line 83. and core-sample data ŽHole 828. indicate that carbonate-layer ŽUnit II and III., which has ; 10 wt% inorganic carbon Ž80 wt% CaCO 3 . and ; 0.5 wt% organic carbon, is subducting throughout the New Hebrides Trench. Assuming this ratio is the same as that of the subducting sediment, the following equations are described: O 1 s Ž 1. C 20 where C and O are masses of carbonate and organic matter, respectively, in the subducting marine sediment. On the other hand, in North Fiji back-arc basin basalts, the subducted end-member source has 70% carbonate and 30% organic matter in origin. From this, the following equations are expressed as: Od 3 s Ž 2. Cd 7 where C d and Od are masses of carbonate and organic matter, respectively, in the subducted source in the North Fiji back-arc basin basalt. Further assuming that complete decomposition of the subducted organic matter has occurred ŽO s Od ., Eqs. Ž1. and Ž2. can be rewritten as: Cs C
s
C y Cd C
s1y
Cd C
s1y
ž
O C
=
Cd Od
/
f 0.9
Ž 3.
Fig. 5. Estimated carbon contributions from mantle, carbonate and organic matter in the NFB basalt glasses. The line with an arrow shows the mixing line between NFB mantle source and the subducted end-member source. Contrary to a wide variation of the mantle carbon, carbonate and organic matter are consistently in a ratio of approximately 7:3, as opposed to the samples which are depleted in the sedimentary carbon Žcarbonateqorganic matter..
where C s is mass of carbonate which is free from the decomposition in the North Fiji back-arc. From Eq. Ž3., it is derived that ; 90% of the carbonate, which subducted without accretion to wedge, is free from decomposition in the area studied. Assuming that the result of BABB represent the signature of the whole subduction zone, the surviving carbonate may be re-injected into mantle in the area studied. Since the lack of North Fiji arc data such as Vanuatsu island-arc volcanic gases, the authors cannot deny the possibility that the carbonate was decomposed priory at the
136
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
North Fiji arc. However, the evaluation of crustal contamination in arc sample would be difficult. Less crustal contamination is one of the advantages of BABB sample. The possibility that carbonate can be transported into the mantle through the subduction zones has been proposed due to the results of highpressure and -temperature experiments of carbonate decomposition Že.g., w31–33x.. Moreover, the evidence for the burial of carbonate at depths in excess of 100 km was reported by Becker and Altherr w34x. This connection, along with the result of this study may be compelling evidence that carbonate can be transported into the mantle through the North Fiji subduction zones. 5.3. Distribution of the subducted carbon As stated previously, the NFB subducted endmember source should have 70% carbonate and 30% organic matter in origin, regardless of the sample site. In this paper, the subducted end-member source means the carbon-rich source generated from the subducted sediment. The subducted end-member source is enriched in samples from two areas: one is the 1748E segment and the other is the N1608 segment Žincluding TJ.. In the southern segments ŽN158, N–S and 1748E., the CO 2r 3 He ratio increases ŽFig. 2a., and the d 13 C value decreases ŽFig. 2c. as the sample site becomes closer to the trench. In NFB, a significant subduction-related contamination with a negative Nb anomaly and a high LOI are observed in several basalts mainly from the 1748E segment w13,14x. The NFB is a large back-arc basin, and the distance from the northern segments to the trench is ) 500 km, which is remarkably long in comparison with smaller back-arc basins such as the Mariana Trough and the Lau Basin. Since the 1748E segment is relatively nearer to the trench than the other three segments, the subducted end-member source in samples from the 1748E segment would be transported with the water dehydrated from the subducted slab. Although a negative Nb anomaly and a high LOI were not observed in ST21 samples which were the only 1748E samples analyzed in this paper w13x, carbon tracers such as CO 2r 3 He ratio and d 13 C value can detect the subducted carbon. Contrary to this, the subducted end-member source in samples from the N1608 segment, including TJ, would be transported by the mantle plume.
6. Conclusions From the above, the following conclusions can be drawn: Ž1. Although CO 2r 3 He ratios and d 13 C values of NFB basalts vary significantly, there are correlation that CO 2r 3 He ratio increases and d 13 C value decreases with K 2 O content. Since REE and Sr–Nd isotopic data suggest that the variety of K 2 O content of the NFB basalt is the result of two-component mixing w14x, the wide variation of CO 2r 3 He ratios and d 13 C values of NFB basalts should be mainly a result of the two-component mixing, rather than fractional degassing. The mixing end-member source should be NFB mantle source Žlow-CO 2r 3 He, highd 13 C and low-K 2 O. and the other end-member source Žhigh-CO 2r 3 He, low-d 13 C and high-K 2 O.. Ž2. Assuming that the CO 2r 3 He ratio and the 13 d C value of the NFB mantle are those of basalt whose K 2 O content is the lowest among NFB basalts, it may be inferred that the CO 2r 3 He ratio and the d 13 C value of NFB mantle source range from 2.5 = 10 8 to 4.0 = 10 8 and from y6.7 to y4.2‰, respectively. Ž3. In NFB basalt, the 3 Her 4 He ratio does not correlate with CO 2r 3 He ratio and d 13 C value. This suggests that the excess carbon Žrelative to the mantle. of NFB enriched source can be attributed convincingly to the subducted sedimentary carbon, rather than the lower mantle. Ž4. From a simple mass-balance calculation, it is derived that the subducted end-member source has 70% carbonate and 30% organic matter in origin. Assuming that complete decomposition of the subducted organic matter has occurred, most Ž; 90%. carbonates, which subducted without accretion to wedge, is free from decomposition in North Fiji back-arc. This may be one clue that carbonate can be transported into the mantle through the subduction zones.
Acknowledgements The authors are grateful to K. Hirose and T. Urabe for providing the NFB basalt glass samples. The authors would like to thank M. Tsutsumi, H. Yoshida, K. Kiyota, N. Takahata, J. Yamamoto, H.
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
Namba, K. Sugai, M. Joshima and N. Geshi for their help with experiments. Thanks are due to E. Tajika, Y. Nagahara, I. Kaneoka, M. Toriumi, K. Ozawa, T. Sasada and M. Ozima for many helpful comments and suggestions. The authors would like to express their appreciation to Y. Nozaki and N. Sugiura for the use of their laboratories. The authors also thank B. Marty and anonymous reviewers for thoughtful reviews of the manuscript. JSPS Research Fellowship for Young Scientists to YN partly funded this study.
w15x
w16x
w17x
w18x
w19x
References w1x M. Javoy, F. Pineau, C.J. Allegre, Carbon geodynamic cycle, ` Nature ŽLondon. 300 Ž1982. 171–173. w2x D.H. Green, S.M. Eggins, G. Yaxley, The other carbon cycle, Nature ŽLondon. 365 Ž1993. 210–211. w3x D.P. Mattey, R.H. Carr, I.P. Wright, C.T. Pillinger, Carbon isotopes in submarine basalts, Earth Planet. Sci. Lett. 70 Ž1984. 196–206. w4x C. Macpherson, D. Mattey, Carbon isotope variations of CO 2 in Central Lau Basin basalts and ferrobasalts, Earth Planet. Sci. Lett. 121 Ž1994. 263–276. w5x B. Marty, A. Jambon, Cr 3 He in volatile fluxes from the solid Earth: implications for carbon geodynamics, Earth Planet. Sci. Lett. 83 Ž1987. 16–26. w6x R.H. Kingsley, J.-G. Schilling, Carbon in Mid-Atlantic Ridge basalt glasses from 288N to 638N: Evidence for a carbon-enriched Azores mantle plume, Earth Planet. Sci. Lett. 129 Ž1995. 31–53. w7x Y. Sano, B. Marty, Origin of carbon in fumarolic gas from island arc, Chem. Geol. 119 Ž1995. 265–274. w8x B. Marty, A. Jambon, Y. Sano, Helium isotopes and CO 2 in volcanic gases of Japan, Chem. Geol. 76 Ž1989. 25–40. w9x Y. Sano, J. Hirabayashi, T. Oba, T. Gamo, Carbon and helium isotopic ratios at Kusatsu-Shirane volcano, Japan, Appl. Geochem. 9 Ž1994. 371–377. w10x Y. Sano, T. Gamo, K. Notsu, H. Wakita, Secular variations of carbon and helium isotopes at Izu-Oshima Volcano, Japan, J. Volcanol. Geotherm. Res. 64 Ž1995. 83–94. w11x Y. Sano, Y. Nishio, S. Sasaki, T. Gamo, K. Nagao, Helium and carbon isotope systematics at Ontake volcano, Japan, Geochim. Cosmochim. Acta Žsubmitted.. w12x J.-P. Eissen, C. Lefevre, P. Maillet, G. Morvan, M. Nohara, ` Petrology and geochemistry of the central North Fiji Basin spreading centre ŽSouthwest Pacific. between 168S and 228S, Mar. Geol. 98 Ž1991. 201–239. w13x J.-P. Eissen, M. Nohara, J. Cotten, K. Hirose, North Fiji Basin basalts and their magma sources: Part I. Incompatible element constraints, Mar. Geol. 116 Ž1994. 153–178. w14x M. Nohara, K. Hirose, J.-P. Eissen, T. Urabe, M. Joshima, The North Fiji Basin basalts and their magma sources: Part
w20x
w21x
w22x
w23x
w24x
w25x
w26x
w27x
w28x
w29x w30x
137
II. Sr–Nd isotopic and trace element constraints, Mar. Geol. 116 Ž1994. 179–195. K. Hirose, M. Nohara, H. Hiyagon, M. Tanahashi, J.-P. Eissen, Petrology of the central ridge in the North Fiji Basin, Earth Planet. Sci. Lett. Žsubmitted.. Y. Nishio, M. Tsutsumi, T. Gamo, Y. Sano, Hydrogen effect on the d 13 C value of CO 2 measured by mass spectrometry with electron-impact ionization, Anal. Sci. 11 Ž1995. 9–12. Y. Sano, H. Wakita, Geographical distribution of 3 Her 4 He ratios in Japan: Implications for arc tectonics and incipient magmatism, J. Geophys. Res. 90 Ž1985. 8729–8741. Y. Sano, H. Wakita, Precise measurement of helium isotopes in terrestrial gases, Geochim. Cosmochim. Acta 61 Ž1988. 1153–1157. F. Pineau, M. Jaboy, Carbon isotopes and concentrations in mid-oceanic ridge basalts, Earth Planet. Sci. Lett. 62 Ž1983. 239–257. A. Jambon, J.L. Zimmermann, Major volatiles from a North Atlantic MORB glass and calibration to He: a size fraction analysis, Chem. Geol. 62 Ž1987. 177–189. H. Yoshida, N. Takahashi, Chemical behavior of major and trace elements in the Horoman mantle diapir, Hidaka belt, Hokkaido, Japan, J. Mineral. Petrol. Econ. Geol. Žsubmitted.. H. Craig, J.E. Lupton, Y. Horibe, A mantle helium content in circum-Pacific volcanic gases: Hakone, the Marianas, and Mt. Lassen, in: E.C. Alexander, M. Ozima ŽEds.., Terrestrial Rare Gases, Jpn. Sci. Soc. Press, Tokyo, 1978, pp. 3–16. T. Torgersen, W.J. Jenkins, Helium isotopes in geothermal systems: Iceland, the Geysir, Raft River and Steamboat Springs, Geochim. Cosmochim. Acta 46 Ž1982. 739–748. D.R. Hilton, K. Hammerschmidt, G. Loock, H. Friedrichsen, Helium and argon isotope systematics of the central Lau Basin and Valu Fa Ridge: Evidence of crustrmantle interactions in a back-arc basin, Geochim. Cosmochim. Acta 57 Ž1993. 2819–2841. Y. Nishio, S. Sasaki, T. Gamo, T. Ishii, Y. Sano, Carbon and helium isotope systematics of mid-ocean ridge basalt from Rodriguez Triple Junction in the Indian Ocean, Abstr. 1996 Fall Meet. Volcanol. Soc. Jpn., 1996, p.42. Y. Sano, Y. Nishio, T. Gamo, A. Jambon, B. Marty, Noble gas and carbon isotopes in Mariana Trough basalt glasses, Appl. Geochem. Žin press.. T. Trull, S. Nadeau, F. Pineau, M. Polv, M. Javoy, C–He systematics in hotspot xenoliths: Implications for mantle carbon contents and carbon recycling, Earth Planet. Sci. Lett. 118 Ž1993. 43–64. M. Schidlowski, A 3,800-million-year isotopic record of life from carbon in sedimentary rocks, Nature ŽLondon. 333 Ž1988. 313–318. J.-Y. Collot, H.G. Greene, L.B. Stokking, et al., Init. Rep., Proc. Ocean Drill. Proj. 134 Ž1992. 159. J.-Y. Collot, H.G. Greene, M.A. Fisher, E. Geist, Tectonic accretion and deformation of the accretionary wedge in the North d’Entrecasteaux Ridge–New Hebrides Island Arc collision zone: evidence from multichannel seismic reflection profiles and Leg 134 results, Sci. Rep., Proc. Ocean Drill. Proj. 134 Ž1994. 5–18.
138
Y. Nishio et al.r Earth and Planetary Science Letters 154 (1998) 127–138
w31x R.C. Newton, W.E. Sharp, Stability of forsteriteqCO2 and its bearing on the role of CO 2 in the mantle, Earth Planet. Sci. Lett. 26 Ž1975. 239–244. w32x P.J. Wyllie, W.L. Huang, Carbonation and melting reactions in the system CaO–MgO–SiO 2 –CO 2 at mantle pressures with geophysical and petrological applications, Contrib. Mineral. Petrol. 54 Ž1976. 79–107.
w33x C. Biellmann, P. Gillet, F. Guyot, J. Peyronneau, B. Reynard, Experimental evidence for carbonate stability in the Earth’s lower mantle, Earth Planet. Sci. Lett. 118 Ž1993. 31–41. w34x H. Becker, R. Altherr, Evidence from ultra-high-pressure marbles for recycling of sediments into the mantle, Nature ŽLondon. 748 Ž1992. 745–748.