Determination of Major and Minor Chemical Elements

Taylor, B., Fujioka, K., et al, 1992Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 126

32. PELAGIC AND HEMIPELAGIC SEDIMENTS OF THE IZU-BONIN REGION, LEG 126:GEOCHEMICAL AND COMPOSITIONAL FEATURES1

Akira Nishimura,2 Naoki Mita,3 and Masato Nohara2

ABSTRACT

Chemical analyses were performed on major, minor, and rare-earth elements of pelagic and hemipelagic sediments of theforearc, arc, and backarc sites of the Izu-Bonin Arc, Ocean Drilling Program Leg 126. Analyses of the hemipelagic and pelagicsediments of this area indicate that the chemical composition of this arc is highly affected by the chemical composition of rocksof this arc as a source of sediments. The Oligocene sediments, which are characterized by high MgO contents, reflect the chemicalcomposition of the Paleogene volcanic rocks of the immature arc. Moreover, the late Miocene to Quaternary sediments with lowMgO contents are attributed to the composition of the present arc. We also suggest that the sedimentation rates and topographyof the sedimentary basin affect the MnO and SiO2 contents of pelagic and hemipelagic sediments.

INTRODUCTION

The Izu-Bonin Arc is an intraoceanic arc between the northwesternPacific and the Philippine Sea that was formed from volcanic activi-ties, backarc rifting, and spreading processes since Eocene time(Honza and Tamaki, 1985). Ocean Drilling Program (ODP) Leg 126is the first deep-sea project to drill in the Izu-Bonin Arc area thattransects the entire arc (Fig. 1).

The geochemistry of ocean-floor pelagic sediments has beenstudied to solve various problems, that is, the major source of sedi-ments in the area, the geologic history of hydrothermal activity, theorigin of manganese nodules, and so on (Leinen, 1987; Sugisaki,1980b; Nohara and Kato, 1985). However, the same kinds of studieson near-land areas are very few. The chemical compositions ofargillaceous sediments near the arc were studied on sediments fromthe Japan Trench area (Sugisaki, 1980a; Nohara, 1980). The chemicalcompositions of argillaceous sediments from the Izu-Bonin Arc werereported on only the surface sediments obtained by a piston and agravity cores (Sugisaki and Kinoshita, 1981). The objectives of thispaper are to determine the geologic changes in the chemical featuresof the sediments and to clarify the origins of finer grained sedimentsof the forearc and backarc of the Izu-Bonin Arc, which are related tothe geologic history of the arc. Furthermore, attention was concen-trated on the affect of hydrothermal activity on the composition ofsediments in the Sumisu Rift, which is an active rift that is expectedto contain hydrothermal circulation (Yamazaki, 1988; Nakao et al.,1990). To accomplish these objectives, the bulk chemical compositionof 103 samples and the rare-earth elements of 13 samples from Leg 126were determined and examined.

ANALYTICAL METHODS

The geochemical data of pelagic and hemipelagic sediments arecompared with the lithologic data. The lithologic data are mainlybased on core descriptions from the Proceedings of the Ocean Drill-ing Program, Initial Reports for Leg 126 (Taylor, Fujioka, et al.,1990). Authigenic components in the sediments are sometimes related

1 Taylor, B., Fujioka, K., et al., 1992. Proc. ODP, Sci. Results, 126: College Station,TX (Ocean Drilling Program).

2Marine Geology Department, Geological Survey of Japan, Higashi 1-1-3, Tsukuba,Ibaraki 305, Japan.

3 Geochemistry Department, Geological Survey of Japan, Higashi 1-1-3, Tsukuba,Ibaraki 305, Japan.

to the sedimentation rate. Sedimentation rates were recalculated formuddy sediments, excluding such instantaneous deposits as volcanicashes, turbidites, and debris-flow deposits, based on the originalon-board visual core descriptions and formation microscanner (FMS)columns (Hiscott et al., this volume).

Determination of Major and Minor Chemical Elements

Sediment samples including seawater were dried at room tempera-ture and ground well. The moisture contents (H2O) of samples weredetermined after drying for 3 hr at 110°C.

Total carbon and sulfur contents were measured by infrared spec-trometric determination and high-frequency furnace. Calcium car-bonate contents were converted from nonorganic carbon contents andwere measured by the same methods above after precombustion at450° C for 12 hr. Organic carbon contents were calculated by sub-tracting the nonorganic carbon contents from the total for carboncontents.

Dissolution of sediment samples was achieved to use HC1 (35%)and HF (49%). SiO2, A12O3, and TiO2 were measured by atomicabsorption spectrometry with N2O-C2H2 flame. Other elements weremeasured by atomic absorption spectrometry with air-C2H2 flame. Allanalytical values were converted to 110° C dried basis, excluding themoisture contents. Total Fe contents were calculated as Fe2O3, andCaO contents were calculated by subtracting the CaO contents ascalcium carbonate from the total CaO.

Determination of Strontium and Rare-earth Elements

For Sr analysis, the sediment samples were washed with deionizedwater to remove the effect of seawater. Afterward, the samples wererepeatedly washed with super-grade alcohol. Because the analyzedsediment samples are mainly composed of calcium carbonate, thedried sample was dissolved in 2N HC1 and immediately transfered tothe ion-exchange column to minimize the effect of silica minerals.The Sr isotopic ratio was measured by MAT 262 with multicollectorsystem. Isotopic data are fractionation corrected to 87Sr/86Sr=0.1194.Mean measured standard values are 87Sr/86Sr = 0.710223 for NBS987with a total range of ±0.000010.

For rare-earth-element (REE) analyses, the sediment sampleswere washed with deionized water and were dissolved in mixtures ofultra-pure HC1O4, HC1, and HF. The REEs were determined byICP-MS, after removing major elements by ion-exchange resin. Theprecision of the replicate analyses is better than 5%.

487

A. NISHIMURA, N. MTTA, M. NOHARA

33°N

32C

31'

L-V~Σ v - r

/ Sumisu Jima

140°E Tori Shima 141' 142C

Figure 1. Bathymetric map showing sites drilled on Leg 126 and volcanoes in the vicinity. Bathymetric map of Izu-Bonin is based on Taylor et al.interval in 500-m intervals. A=Higashi-Aoga Shima Caldera, B = Kita-Beyonesu Caldera, C = Myojin-sho Caldera, D = Sumisu Jima Caldera, E =Jima Caldera, and F = Tori Shima Caldera.

(1990). Contour

Minami-Sumisu

GEOLOGIC SETTING AND SAMPLES

The Izu-Bonin Arc is an active intraoceanic arc between thenorthwestern Pacific and the Philippine Sea that has been formingsince the Eocene by intense volcanisms (Honza and Tamaki, 1985).There are submarine calderas in the northern part of the Izu-BoninArc, where Leg 126 was drilled (Fig. 1). Backarc rifts are discontinu-ously present along the volcanic front. The Sumisu Rift, which wasalso drilled on Leg 126, is a backarc rift just behind the volcanic frontbetween Sumisu Jima and Tori Shima islands (Fig. 1) (Honza andTamaki, 1985; Brown and Taylor, 1988; Murakami, 1988; Taylor etal., 1990). The forearc basin, which was drilled on Leg 126, wasformed in the early Oligocene between the frontal- and outer-arc highsof the Paleogene volcanic basement (Taylor, Fujioka, et al., 1990).

Forearc Sites

Three sites (787, 792, and 793) drilled on Leg 126 are composedof geologic sections from the early Oligocene to the Quaternary. Theage-depth curves of the three sites are similar to each other (Shipboard

Scientific Party, 1990a), which suggests that a similar geologic historydominated throughout the forearc basin. The lower to middle parts ofthe Oligocene sequences consist of sediment gravity flow depositswith a high sedimentation rate, such as turbidites and debris-flowdeposits. The latest Oligocene and lower Miocene sequences aredominantly of pelagic mudstone with a very low sedimentation rate.The upper Miocene to Quaternary sequence consists of hemipelagitewith a high content of volcanogenic materials.

Arc Site

The Site 788 sequence drilled on the eastern flank top of theSumisu Rift is mainly composed of pumice deposits. Pelagic claystones/siltstones were found in the lower part of this sequence (Subunits IIAand HB).

Backarc Sites

The sedimentary sequences in the backarc sites are Quaternaryaged (0-1.1 Ma) and are composed of two units. The two units contrast

488

PELAGIC AND HEMIPELAGIC SEDIMENTARY FEATURES

highly in the contents of their ash and tephra layers, but hemipelagitesin the two units possess a similar lithology in their high calcareousnannofossils contents. Two sites that are 2.4 km apart show the samelithologic units with different thicknesses (Shipboard Scientific Party,1990b).

Hemipelagic and pelagic sediments were selected for chemical analy-ses from almost all the units of all Leg 126 sites on the Izu-Bonin Arc(Fig. 1). The chemical analyses of the major and minor elements wereperformed on 103 samples. The samples were checked visually andselected as hemipelagic and pelagic sediments. The analyses of REEswere performed on 13 selected samples of backarc Site 791 and forearcSite 792, which are representative of almost all the lithologic units of Leg126. Chemical analyses were performed on almost all the units of thesedimentary sequences of all sites; however, for Site 788 only one sampleof Subunit IIA was available for analysis because most of the sequencesin this site are composed of volcanogenic sediments.

GEOCHEMICAL FEATURES OF MAJOR ANDMINOR ELEMENTS

The results of the chemical analyses are listed in Table 1. All majorcomponents are given in weight percent of oxides, and minor ones inweight percent of atoms. Furthermore, to determine the origin ofsediments excluding biogenic effects (discussed later), the contents ofthe major elements were recalculated on the basis of being carbonatefree (Tables 2 and 3). The vertical changes in chemical compositionare shown in Figures 2 through 6. Moreover, the Siθ2-Al2O3-10Tiθ2and Fe2O3-MgO-TiO2 triangles (Sugisaki, 1978) show the features ofthe sediments (Figs. 7 and 8). The elements used in these triangles areresistants for chemical weathering.

Throughout the core sequences of all the sites, the calcium carbonatecontent ranges from 1% to 46%, and the carbonate-free contents of themajor elements range as follows: SiO2, 44%-68%; A12O3, 7%-21%;F e A , 3%-ll%; TiO2, 0.1%-0.9%; CaO, l%-26%; MgO, 0%-8%;Na2O, l%-5%; K20,0.4%-4.1%; and MnO, 0.1%-1.4%. The contentsof rare metals range as follows: Co, 6-160 ppm; Ni, 0-50 ppm; Cu,0-200 ppm; Zn, 40-2370 ppm; Pb, 0-250 ppm; Li, 0-50 ppm; and Cr,4-110 ppm. The contents of Cu and Ni in the sediments are very lowcompared with Pacific deep-sea sediments and Philippine Sea sediments(Mitaetal., 1982; Sugisaki, 1980b).

Forearc Sites

The sequences of forearc Sites 787,792, and 793 show the verticalchanges of chemical composition of the sediments since the Oligo-cene age (Tables 1-2 and Figs. 2-4). The sequences of early to lateOligocene age show high Fe2O3 and MgO contents. The sequencesof the Pliocene to the Quaternary show lower SiO2 and higher Fe2O3

contents compared with the backarc Quaternary sequences. The CaO

content of Site 792 is lowest in the upper part of Unit IV (lateOligocene) and that of Site 793 is lowest in Unit IV (early Miocene).

Arc Sites

One sample from Subunit HA has a similar chemical compositionto that of Unit I of forearc Site 792. However, its contents of MgO,CaO, and MnO are slightly higher than those of a later unit (Table 2).

Backarc Sites

The sequences of the two backarc Sites 790 and 791 consist of twolithologic units. Their chemical compositions are similar to eachother, and they show higher mean contents of SiO2 compared withthose of the forearc sites. The CaO and MgO contents of Unit II arehigher than those of Unit I at backarc Sites 790 and 791 (Tables 2-3and Figs. 5-6).

GEOCHEMICAL CHARACTERISTICSOF REEs AND STRONTIUM

The results obtained from the REE analyses for Leg 126 are givenin Table 4, and the corresponding chondrite-normalized REE patternsare shown in Figures 9 and 10. The REE abundances of the sedimentsfrom Sites 791 and 792 are generally higher than those of oceanicbasalts. Their general shapes show that all sediments are enriched inlight REEs and have a marked discontinuity in heavy REEs. Enrich-ment in light REEs is more marked at Site 791 than at Site 792 (Figs. 9and 10).

The Sr isotopic ratios of marine carbonate minerals are as-sumed to be identical to those of seawater at the time of deposi-tion, provided that they have not been altered during diagenesis,dolomitization, or regional metamorphism. The 87Sr/86Sr ratiosin the ocean have changed systematically during Phanerozoictime (Peterman et al., 1970). Burke et al. (1982) obtained theaverage Sr isotopic ratio, 0.70910 ± 0.00004, by analyzing 42samples of modern marine carbonate from all over the world. TheSr isotopic ratios of carbonates from the pelagic carbonate sedi-ments of Unit m of Site 792 are 0.708129 ±0.000016 (Sample 126-792E-28R-1, 57-59 cm) and 0.708237 ± 0.000012 (Sample 126-792E-31R-1, 31-32 cm), which are distinctly different from thoseof modern marine carbonates.

MAJOR SOURCE OF PELAGIC AND HEMIPELAGICSEDIMENTS

Biogenic Components in Chemical Composition

Smear slide observations of the hemipelagic and pelagic sedi-ments suggest that the major biogenic components of this area are

O T -

SiO2 AI2O3 Fe2O3 TiO2 CaO MgO MnO CaCO3

Unit 40 60 0 20 0 10 0 1 0 20 0 10 0 1 0 40 %

Q.CD

Q

300-

IVA

IVB

I

•5•Figure 2. Vertical change of carbonate-free chemical compositions and carbonate content, Site 787.

489

§ Table 1. Chemical compositions of pelagic and hemipelagic sediments of Leg 126, Izu-Bonin Arc.

Core, Section,

126-787B-

126-788D-

126-790A-

126-790B-

126-790C-

126-791A-

Interval

4R-1,6R-2,8R-1 ,9R-2,

15R-4,18R-2,19R-6,20R-2,21R-4,24R-7,26R-6,27R-4,28R-2,29R-4,30R-2,32R-2,33R-2,

5R-1 ,

2H-1,2H-1,

7H-3,

6H-5,10X-2,12X-1,16X-4,20X-4,

4H-4,16H-2,18H-1,19H-1,

(cm)

1 4 -0 -

3 7 -1 4 -6 4 -3 2 -8 9 -7 0 -5 2 -3 6 -

7 -2 6 -2 5 -

121 -3 0 -9 0 -

107-

138-

5 6 -112-

5 0 -

120-4 5 -9 0 -

129-2 2 -

130-9 2 -7 0 -6 0 -

192039156736917 45540102828

1253695

110

140

58114

60

140479 2

1492 4

150947262

Depth(mbsf)

21.5441.5559.6770.64

132.29157.42174.19177.70190.12223.46240.97247.03254.45268.11273.80293.70302.55

259.58

9.369.92

55.50

140.40166.46185.10228.59266.12

29.30

141.62159.20168.70

SJO2

(%)

46.7650.3936.9339.9854.7845.6343.1549.3155.0461.8541.6860.2236.1337.4430.1753.3254.51

44.07

57.1246.32

54.86

46.2950.9248.6650.4543.32

45.6544.1555.7859.50

AI2O3

(%)

10.379.037.62

11.8313.8812.8110.7114.1212.6711.47

9.9211.62

9.029.986.95

10.1313.06

9.11

10.078.78

9.69

7.989.98

10.469.629.72

8.568.339.409.76

Fθ2θ 3

(%)

6.654.914.506.28

10.208.297.118.974.944.716.467.265.285.573.816.88

10.99

9.31

5.304.62

5.24

4.274.696.744.985.84

4.334.184.684.72

TiO2

(%>

0.570.300.280.260.740.480.480.630.370.240.410.450.320.280.220.580.45

0.74

0.500.34

0.52

0.310.420.500.410.43

0.390.360.440.50

CaO

(%)

3.981.689.566.841.862.014.305.801.293.443.564.967.418.158.913.995.86

3.39

3.671.68

3.08

3.393.856.604.896.28

4.0710.89

1.872.34

MgO

(%)

2.131.521.822.625.684.093.08

3.832.132.162.582.962.262.121.663.337.46

3.85

1.571.76

1.95

1.392.102.481.902.27

1.811.741.541.50

N a 2 o(%)

1.921.971.421.402.693.721.782.033.302.511.391.991.551.451.232.011.93

1.61

2.892.06

2.18

1.951.831.901.981.79

1.891.912.563.06

K2O

(%)

0.981.681.440.852.871.991.410.992.621.330.690.800.670.820.730.760.84

0.95

1.191.69

1.33

1.101.981.021.601.06

1.591.591.141.01

MnO

(%)

0.130.210.420.250.180.200.200.220.130.160.200.210.250.190.130.230.22

0.24

0.150.12

0.14

0.160.140.190.160.16

0.110.110.140.15

Org-C

(%)

0.580.302.110.410.230.510.420.360.050.090.070.431.291.300.890.350.08

0.34

1.071.40

1.04

0.761.451.131.230.67

1.750.690.580.20

T-S

(%)

0.440.270.370.160.360.160.180.190.170.140.150.180.210.190.190.160.15

0.22

0.450.49

0.32

0.320.510.490.380.19

0.360.510.340.28

Co(ppm)

5 85 418

1218 0

131109147

4 98 59 94 96 8557 97 5

159

87

3 451

8 5

486 6

1146 28 7

1087 64 77 5

Ni Cu(ppm) (ppm)

000

17000000000000

19

0

010

0

04000

4100

0000000

34000000000

0

00

0

00000

00

00

Zn(ppm)

8 8815668

100937797585 27 59 050574 486

103

78

24683

9 0

71100111

8880

919 081

123

Pb(ppm)

4134 2

64

182 413

4

6252 43 6254 2

00

19

2 4 917

1 1

312 4

2 88

2 8

2 513252 2

U Cr(ppm) (ppm)

14182 9172 513

99

858

141013

91 1

8

9

163 2

2 4

194 9192 718

3 53 21814

37

30383040183441341631303746271727

2 2

3232

35

213 4194727

2 9282 225

CaCθ3

(%)

17.4216.0831.9216.58

2.429.42

20.428.674.582.42

21.7510.5031.5030.2541.83

3.920.50

17.25

11.4227.83

17.58

23.1716.1715.6714.3327.00

19.006.259.004.17

Total

(%)

91.9388.3798.4087.4995.9289.3593.2695.1487.3190.5188.89

101.5995.9097.7796.7585.6796.08

91.10

95.4697.10

97.95

91.1294.0795.8791.9598.75

89.5680.7387.4987.19

Table 1 (continued).

22H-6, 45- 65 204.85 55.21 9.97 4.38 0.43 1.96 1.62 2.59 1.24 0.18 0.42 0.22 70 0 0 88 14 24 4 10.33 88.5639X-1 , 23- 25 352.53 53.74 9.28 4.33 0.50 3.35 1.35 2.44 1.99 0.15 0.89 0.37 56 0 0 76 0 28 26 7.17 85.57

126-791B-13R-1, 97- 99 503.07 45.58 8.51 3.92 0.37 3.98 1.59 1.71 1.69 0.17 1.32 0.46 80 10 0 89 44 36 39 17.58 86.9222R-1, 21- 22 589.21 47.48 7.86 3.34 0.31 4.90 1.71 1.60 1.68 0.15 1.93 0.66 6 0 0 79 29 38 40 17.00 88.6523R-1, 125-126 599.95 39.68 7.05 3.08 0.27 4.88 1.19 1.46 1.53 0.09 2.00 0.47 36 0 0 74 1 1 25 30 23.92 85.6423R-4, 6- 7 603.26 50.03 5.76 2.42 0.23 5.13 1.22 1.41 1.35 0.13 2.20 0.89 17 0 0 64 35 26 24 16.33 87.1324R-1, 62- 64 609.02 51.79 10.21 5.58 0.45 3.74 2.27 2.04 1.25 0.16 0.59 0.32 112 0 0 85 35 20 22 7.75 86.1725R-2, 51- 52 620.11 49.30 11.01 7.49 0.58 5.83 2.89 1.99 0.76 0.19 0.63 0.48 103 0 0 104 15 14 19 8.83 90.0125R-2, 64- 66 620.24 40.20 8.47 4.57 0.38 2.86 1.79 1.69 1.16 0.18 0.66 0.23 114 0 0 89 35 24 36 26.17 88.3827R-1, 30- 31 637.30 48.34 8.01 3.30 0.34 0.72 0.94 2.21 1.12 0.16 0.40 0.29 61 0 0 77 6 17 17 20.00 85.8331R-3, 144-146 680.04 31.20 5.57 2.54 0.17 5.71 1.16 1.38 1.29 0.14 0.92 0.32 6 0 0 71 48 26 34 45.17 95.5934R-2, 139-141 707.49 44.51 6.77 2.53 0.19 2.13 1.03 1.83 1.48 0.13 0.78 0.26 52 0 0 78 20 20 27 30.00 91.6638R-3, 110-112 745.35 50.15 9.34 6.43 0.51 4.71 2.38 1.96 1.37 0.19 0.97 0.21 134 0 0 2370 35 26 33 13.08 91.5539R-2, 45- 46 754.35 43.22 9.31 6.04 0.50 8.43 2.87 1.72 1.18 0.14 1.14 1.81 115 51 0 79 25 21 61 13.08 89.4840R-3, 10- 11 765.20 31.37 6.11 3.37 0.29 7.84 1.53 1.49 1.03 0.18 0.72 0.31 109 0 0 215 22 15 43 39.67 93.9541R-3, 77- 79 775.47 28.03 5.53 3.38 0.20 14.10 1.39 1.25 1.02 0.13 0.74 0.38 69 0 0 61 39 17 33 46.08 102.2642R-2, 72- 73 783.52 40.85 8.15 4.27 0.38 4.74 1.95 1.61 1.46 0.12 0.65 0.49 91 7 0 74 19 27 46 26.50 91.1843R-1, 31 - 32 791.31 48.05 6.05 3.76 0.29 0.78 0.00 1.28 2.11 0.10 0.70 3.63 32 0 0 69 28 16 24 19.17 85.9243R-1, 136-137 792.36 36.47 6.96 3.88 0.29 2.88 1.83 1.46 1.17 0.26 0.21 0.33 54 0 0 61 24 19 44 37.42 93.1746R-2, 34- 36 821.84 36.53 6.56 3.30 0.20 3.31 1.30 1.59 1.06 0.14 0.26 0.60 79 0 0 55 33 14 28 38.25 93.12

126-792A-2H-3, 140-142 14.10 54.52 15.71 7.85 0.64 5.15 2.44 2.42 0.81 0.18 0.24 0.22 124 0 0 104 8 11 21 3.25 93.473H-6, 72- 92 27.32 45.77 14.63 7.55 0.50 6.61 2.88 1.89 0.85 0.15 0.60 0.32 106 0 0 77 13 13 25 10.92 92.694H-6, 100- 102 36.90 43.95 11.71 6.22 0.46 4.69 1.95 1.94 0.93 0.13 0.66 0.26 121 0 0 79 22 17 26 15.00 87.939H-2, 19- 21 77.01 39.78 8.37 4.98 0.36 5.40 1.57 1.97 0.93 0.12 0.68 0.21 70 0 0 70 32 14 2 26.42 90.80

126-792B-8X-1, 112-114 109.02 49.86 14.11 8.79 0.65 6.43 2.55 2.16 0.66 0.19 0.41 0.29 154 0 0 116 20 10 40 3.92 90.059X-2, 13- 15 119.13 51.28 10.73 4.17 0.29 3.74 1.23 2.41 1.18 0.13 0.60 0.29 69 0 0 74 13 12 22 9.25 85.32

126-792E-1R-1, 17- 19 135.77 45.88 12.05 8.11 0.58 5.75 2.61 2.01 0.71 0.17 0.50 0.23 145 0 10 103 15 13 10 9.17 87.802R-1, 7- 9 145.27 47.42 13.30 7.81 0.54 6.42 2.46 2.07 0.74 0.18 0.41 0.32 152 0 0 88 8 1 1 24 8.17 89.87

18R-1, 70- 72 300.20 47.14 12.06 7.66 0.48 4.78 2.55 2.17 0.95 0.18 0.22 0.16 108 0 0 86 2 13 31 7.75 86.1219R-2, 120-122 311.70 42.92 10.67 6.93 0.40 3.98 2.25 1.82 0.74 0.25 0.08 0.20 118 0 0 78 15 9 11 16.58 86.8321R-1, 34- 35 328.64 55.02 14.55 6.75 0.41 4.83 2.01 2.11 0.84 0.21 0.12 0.18 135 0 0 81 0 6 18 5.17 92.1927R-3, 100-102 389.42 30.36 6.23 2.90 0.06 2.75 1.47 1.26 1.06 0.22 0.59 0.28 8 0 0 59 40 22 35 46.50 93.7130R-4, 30- 50 420.00 30.44 7.48 4.37 0.25 3.29 2.17 1.11 2.53 0.41 0.64 0.26 45 0 0 49 46 17 32 38.00 90.9731R-1, 57- 59 425.47 27.79 7.09 3.97 0.14 2.36 2.11 0.96 1.76 0.26 0.52 0.25 76 0 0 61 36 21 30 41.67 88.8936R-4, 2 - 5 477.72 39.05 10.87 7.59 0.36 1.19 5.56 1.95 2.60 0.30 0.40 0.34 89 0 0 76 5 14 33 12.75 82.9937R-2, 62- 63 484.92 43.23 8.68 9.62 0.46 1.62 6.19 1.89 1.66 0.28 0.36 0.24 35 0 10 96 18 14 22 6.67 80.9238R-2, 84- 87 494.84 30.98 10.56 4.72 0.18 0.54 4.17 3.06 0.90 0.70 0.19 0.17 95 0 6 52 20 8 27 29.00 85.1739R-3, 1- 3 505.21 41.35 13.38 6.69 0.51 1.11 5.14 3.83 0.77 0.24 0.10 0.19 133 0 21 69 11 11 29 7.92 81.2540R-5, 85- 87 518.75 35.49 7.66 7.97 0.32 0.93 5.75 1.74 0.31 0.46 0.07 0.22 110 0 0 85 13 12 26 22.67 83.62


Core, Section,Interval

49R-3,51R-1,52R-1,55R-1,57R-1,66R-3,

126-793A-3H-2,5H-7,6H-3,

11H-2,126-793B-

2R-1,3R-2,4R-1,5R-2,6R-1,

13R-2,15R-4,16R-5,17R-2,18R-4,19R-3,24R-6,26R-2,28R-1,35R-1,35R-6,41R-4,46R-2,50R-2,54R-1,57R-2,58R-5,59R-3,71R-2,

(cm)

2 3 -

8 7 -

9 4 -

9 0 -

6 2 -

105-

1 4 0 -1 0 9 -

6 4 -

3 3 -

2 8 -

8 8 -

1 1 2 -1 4 -

125-1 4 -

7 0 -

1 3 3 -6 0 -

2 0 -

3 7 -

1 5 -

5 5 -

21 -7 2 -9 6 -

9 6 -

2 5 -

7 5 -

8 -

1 2 -

106-4 4 -

131 -

2 5

8 9

9 6

94

64

107

142

111

6 6

3 5

3 0

9 0

114

16

127

18

7 2

135

6 2

2 2

3 9

18

57

2 3

74

9 8

9 8

277 7

10

14

108

4 6

133

Depth(mbsf)

601.03617.97627.74656.60675.72765.55

16.4042.2045.6481.03

594.98606.48615.12625.14634.55702.38724.89736.94740.55753.50761.46814.27827.61845.31913.32920.72975.69

1019.391059.251095.781126.321140.421146.781262.00

SK>2

(%)

48.7944.7635.8243.6245.1639.47

33.4441.6129.9649.94

43.5943.6851.2958.7353.4733.4636.9535.5938.0845.8251.1145.5245.9543.1447.7251.7551.2654.2446.8650.0050.0047.7947.6054.24

AI2O3

(%)

11.339.627.32

11.0010.0011.71

6.358.495.698.34

9.9511.6813.4312.4410.30

7.9310.06

9.5810.3513.6718.3513.7713.1016.9015.0812.2120.0117.2912.7812.7714.0514.2515.3219.79

F θ 2 θ 3

(%)

8.469.383.805.815.025.65

3.044.173.123.29

5.787.847.696.034.443.704.764.264.796.897.079.779.595.876.686.886.696.097.749.017.978.057.552.71

TiO 2

(%)

0.710.400.230.410.310.31

0.160.190.150.19

0.350.520.540.370.310.170.240.180.230.650.370.580.600.370.330.470.420.350.330.330.340.390.350.35

CaO

(%)

3.835.024.041.904.360.80

2.733.068.590.93

2.864.494.462.412.856.372.371.140.901.631.434.673.352.366.564.424.154.855.315.897.152.644.324.43

MgO

(%)

5.017.392.585.933.035.71

1.111.711.050.75

2.823.502.972.211.181.902.031.461.453.103.824.954.162.252.622.121.961.586.526.545.546.745.110.40

N a 2 o(%)

1.461.281.121.251.211.05

1.581.731.652.37

1.681.682.002.212.131.251.120.931.031.833.872.902.762.022.001.511.441.351.130.982.644.165.011.62

K2O

(%)

0.430.500.701.610.522.58

1.351.570.961.02

1.191.031.141.611.381.011.151.601.402.222.470.591.392.760.591.191.231.420.940.880.451.700.670.66

MnO

(%)

0.430.370.350.370.310.30

0.110.110.120.18

0.240.190.180.120.140.220.290.630.960.470.380.260.250.210.170.230.230.150.310.210.180.230.180.10

Org-C

(%)

0.150.280.280.230.150.24

0.750.840.370.21

0.500.460.420.060.370.770.300.080.130.040.070.210.190.150.040.130.080.040.020.200.050.040.020.08

T-S

(%)

0.480.240.140.190.140.14

0.510.280.260.27

0.260.200.240.220.270.300.250.210.290.150.200.190.110.160.150.120.130.120.090.080.140.090.170.15

Co(ppm)

104

111

7 3

1 1 0

7 3

8 1

3 3

7 4

6 4

6 2

9 6

116

1 2 2

1 2 9

8 4

7 8

115

7 1

1 2 0

147

1 2 0

1 5 3

131

132

156

124

105

8 4

1 3 8

1 1 8

1 3 2

148

151

6 5

Ni(ppm)

0

1 1

0

3

0

0

4

0

0

0

0

0

0

0

0

0

15

3 3

4 1

0

0

0

0

6

0

0

0

0

19

16

0

2

0

0

Cu Zn(ppm) (ppm)

2 0

0

0

12

0

0

0

0

0

0

0

201

5

0

0

0

0

3 6

19

0

0

0

0

0

0

0

0

0

7 5

13

0

54

9 2

0

8 7

114

41

4 5

52

5 0

6 6

7 3

51

104

8 3

107

105

7 0

6 2

55

84

94

106

8 3

8 3

94

111

81

6 0

8 7

8 7

7 2

6 8

8 3

5 9

6 6

5 9

6 3

Pb(ppm)

713

3 6

0

2 4

2 3

3 3

3 3

3 6

9

12

18

13

0

17

162 7

4 8

2 3

17

2 0

2 2

4

1713

2

12

2 7

2 2

0

3

2 0

70

U Cr(ppm) (ppm)

16

13

8

15

0

2 1

2 5

24

1 2

15

31

3 5

2 2

2 3

14

3 7

3 4

3 3

3 3

4 4

4 2

16

13

11

4

1 2

12

14

15

15

5

8

4

0

3 1

15

15

61

3 2

2 7

2 6

3 2

19

9

3 0

10

2 5

2 3

11

3 6

3 7

3 3

3 0

31

19

2 7

51

3 2

19

12

2 2

2 2

6 6

7 2

1 0 8

5 8

5 8

8

CaCθ3

(%)

7.006.33

34.7513.4221.0018.25

39.9225.5045.5819.33

20.5010.25

5.750.426.75

32.7527.0833.5829.58

7.420.584.337.50

15.420.583.175.334.006.671.420.420.670.583.17

Total

(%)

88.1185.5991.1485.7691.2386.23

91.0689.3097.5186.84

89.7685.5890.1386.8683.6189.8586.6389.2989.2383.9389.7287.7788.9991.6482.5684.2292.9691.5188.7588.3488.9686.7786.9487.70

Note: Major elements in weight percent, and trace elements in parts per million (ppm).



Unit 40 60 0 200 100 10 200 100 1 0 40 %

Q.ΦQ

500--

800-1 LV

IV

\•1

\

•>

•.

•;•

•*#i

>

•

s

*

• *•

<••

•••

•

#

:;>

>•s

•

\

.*

i

#

.*•

•*•

• .#t

•

•

Figure 3. Vertical change of carbonate-free chemical compositions and carbonate content, Site 792.

0-r

100J

600


U n i t 40 60 0 20 0 10 0 1 0 20 0 10 0 1 0 4 0 %

IA

IB

Q.CDD

1000- -

1300-

IV

•

• >

•

*

>\

t

•

\•

/

•t

•

•

i

••*

.•é

• .•\%

*

\i

i


493

A. NISHIMURA, N. MIT A, M. NOHARA

Table 2. Carbonate-free chemical compositions of pelagic and hemipelagic sediments of Leg 126, Izu-Bonin Arc.

Core, Section,

126-787B-

126-788D-

126-790A-

126-790B-

126-790C-

126-791A-

126-791B-

126-792A-

Interval

4R-1,6R-2,8R-1,9R-2,

15R-4,18R-2,19R-6,20R-2,21R-4,24R-7,26R-6,27R-4,28R-2,29R-4,30R-2,32R-2,33R-2,

5R-1,

2H-1,2H-1 ,

7H-3,

6H-5,10X-2,12X-1,16X-4,20X-4,

4H-4,16H-2,18H-1,19H-1,22H-6,39X-1,

13R-1,22R-1,23R-1 ,23R-4,24R-1,25R-2,25R-2,27R-1,31R-3,34R-2,38R-3,39R-2,40R-3,41R-3,42R-2,43R-1,43R-1,46R-2,

2H-3,3H-6,

(cm)

14- 190- 20

37- 3914- 1564- 6732- 3689- 9170- 7452- 5536- 40

7- 1026- 2825- 28

121 - 12530- 3690- 95

107- 110

138- 140

56- 58112- 114

50- 60

120- 14045- 4790- 92

129- 14922- 24

130- 15092- 9470- 7260- 6245- 6523- 25

97- 9921 - 22

125- 1266 - 7

62- 6451 - 5264- 6630- 31

144- 146139-141110- 11245- 4610- 1177- 7972- 7331 - 32

136- 13734- 36

140- 14272- 92

Depth(mbsf)

21.5441.5559.6770.64

132.29157.42174.19177.70190.12223.46240.97247.03254.45268.11273.80293.70302.55

259.58

9.369.92

55.50

140.40166.46185.10228.59266.12

29.30141.62159.20168.70204.85352.53

503.07589.21599.95603.26609.02620.11620.24637.30680.04707.49745.35754.35765.20775.47783.52791.31792.36821.84

14.1027.32

S•O2

(%)

56.6260.0554.2447.9356.1450.3754.2253.9957.6863.3853.2667.2852.7453.6851.8755.4954.78

53.26

64.4864.18

66.56

60.2560.7457.7058.8959.34

56.3647.0961.3062.0961.5757.89

55.3057.2052.1559.8056.1454.0854.4560.4256.9063.5857.7049.7351.9951.9955.5859.4458.2759.16

56.3551.38

AI2O3(%)

12.5610.7611.1914.1814.2214.1413.4615.4613.2811.7512.6812.9813.1714.3111.9510.5413.13

11.01

11.3712.17

11.76

10.3911.9012.4011.2313.31

10.578.89

10.3310.1811.1210.00

10.339.479.276.88

11.0712.0811.4710.0110.169.67

10.7510.7110.1310.2611.097.48

11.1210.62

16.24

16.42

FO2O3

(%)

8.05

5.85

6.61

7.53

10.45

9.15

8.93

9.82

5.18

4.83

8.26

8.11

7.717.996.557.16

11.05

11.25

5.986.40

6.36

5.565.597.995.818.00

5.354.465.144.934.884.66

4.764.024.052.896.058.226.194.124.633.617.406.955.596.275.814.656.205.34

8.118.48

TiO2

(%)

0.690.360.410.310.760.530.600.690.390.250.520.500.470.400.380.600.45

0.89

0.560.47

0.63

0.400.500.590.480.59

0.480.380.480.520.480.54

0.450.370.350.270.490.640.510.420.310.270.590.580.480.370.520.360.460.32

0.660.56

CaO

(%)

4.822.00

14.048.201.902.225.406.361.353.524.555.54

10.8211.6915.324.155.89

4.09

4.152 32

3.74

4.414.607.835.718.61

5.0311.612.052.442.193.61

4.835.906.416.134.056.403.870.91

10.423.055.429.70

12.9926.15

6.450.974.605.36

5.327.42

MgO

(%)

2.581.812.673.145.824.523.874.192.232.213.303.313.303.042.853.477.50

4.65

1.772.44

2.37

1.812.502.942.223.11

2.231.861.691.571.811.45

1.932.061.561.462.463.172.421.172.121.472.743.302.542.582.650.002.922.11

2.523.23

NarO(%)

2.322.352.091.682.764.112.242.223.462.571.782.222.262.082.112.091.94

1.95

3.262.85

2.65

2.542.182.252.312.45

2.332.042.813.192.892.63

2.071.931.921.692.212.182.292.762.522.612.261.982.472.322.191.582.332.57

2.502.12

K2O

(%)

1.192.002.121.022.942.201.771.082.751.360.880.890.981.181.250.790.84

1.15

1.342.34

1.61

1.432.361.211.871.45

1.961.701.251.051.382.14

2.052.022.011.611.360.831.571.402.352.111.581.361.711.891.992.611.871.72

0.840.95

MnO

<%)

0.160.250.620.300.180.220.250.240.140.160.260.230.360.270.220.240.22

0.29

0.170.17

0.17

0.210.170.230.190.22

0.140.120.150.160.200.16

0.210.180.120.160.170.210.240.200.260.190.220.160.300.240.160.120.420.23

0.190.17

Total

(%)

90.2386.1497.6685.0195.8288.2491.5394.6886.7090.2885.81

101.7794.0296.8094.4285.0996.06

89.24

94.8895.98

97.51

88.4492.9295.1090.6198.29

87.1279.4486.2586.6487.2584.46

84.1386.3281.1284.6285.0189.0584.2682.2991.9688.0990.2887.9089.98

104.2088.0082.5889.0988.86

93.2591.79

494



4H-6,9H-2,

126-792B-8X-1.9X-2,

126-792E-1R-1,2R-1,

18R-1,19R-2,21R-1,27R-3,30R-4,31R-1,36R-4,37R-2,38R-2,39R-3,40R-5,49R-3,51R-1,52R-1,55R-1 ,57R-1,66R-3,

126-793A-3H-2,5H-7,6H-3,

11H-2,126-793B-

2R-1,3R-2,4R-1,5R-2,6R-1,

13R-2,15R-4,16R-5,17R-2,18R-4,19R-3,24R-6,26R-2,28R-1 ,35R-1,35R-6,41R-4,46R-2,50R-2,54R-1,57R-2,58R-5,59R-3,71R-2,

100-10219- 21

112- 11413- 15

17- 197- 9

70- 72120- 122

34- 35100-10230- 5057- 59

2 - 562- 6384- 87

1 - 385- 8723- 2587- 8994- 9690- 9462- 64

105- 107

140- 142109- 11164- 6633- 35

28- 3088- 90

112- 11414- 16

125- 12714- 1870- 72

133- 13560- 6220- 2237- 3915- 1855- 5721 - 2372- 7496- 9896- 9825- 2775- 778 - 10

12- 14106- 108

44- 46131 - 133

36.9077.01

109.02119.13

135.77145.27300.20311.70328.64389.42420.00425.47477.72484.92494.84505.21518.75601.03617.97627.74656.60675.72765.55

16.4042.2045.6481.03

594.98606.48615.12625.14634.55702.38724.89736.94740.55753.50761.46814.27827.61845.31913.32920.72975.69

1019.391059.251095.781126.321140.421146.781262.00

51.7154.06

51.8956.51

50.5151.6451.1051.4558.0256.7549.1047.6444.7646.3243.6344.9045.8952.4647.7954.9050.3857.1648.28

55.6555.8555.0561.91

54.8348.6754.4258.9857.3449.7550.6753.5854.0849.4951.4147.5849.6851.0048.0053.4454.1556.5050.2150.7250.2148.1147.8856.01

13.7811.37

14.6911.82

13.2714.4813.0712.7915.3411.6412.0612.1512.469.30

14.8714.539.91

12.1810.2711.2212.7012.6614.32

10.5711.4010.4610.34

12.5213.0114.2512.4911.0511.7913.8014.4214.7014.7718.4614.3914.1619.9815.1712.6121.1418.0113.6912.9514.1114.3515.4120.44

7.326.77

9.154.60

8.938.508.308.317.125.427.056.818.70

10.316.657.27

10.319.10

10.015.826.716.356.91

5.065.605.734.08

7.278.748.166.064.765.506.536.416.807.447.11

10.2110.376.946.727.107.076.348.299.148.008.107.592.80

0.540.49

0.680.32

0.640.590.520.480.430.110.400.240.410.490.250.550.410.760.430.350.470.390.38

0.270.260.280.24

0.440.580.570.370.330.250.330.270.330.700.370.610.650.440.330.490.440.360.350.330.340.390.350.36

5.517.34

6.694.12

6.337.005.184.775.095.135.314.041.361.740.761.211.204.125.366.192.205.520.98

4.544.11

15.791.15

3.605.004.732.423.059.473.241.721.271.771.444.883.622.796.604.564.395.055.695.977.182.654.354.57

2.292.13

2.651.36

2.872.682.762.702.122.753.503.626.376.635.875.587.445.397.893.956.853.846.98

1.852.301.930.93

3.553.903.152.221.272.832.782.202.063.353.845.174.502.662.642.192.071.656.996.635.566.795.140.41

2.282.68

2.252.66

2.212.252.352.182.222.361.791.652.232.024.314.162.251.571.371.721.441.531.28

2.632.323.032.94

2.111.872.122 222.281.861.541.401.461.983.893.032.982.392.011.561.521.411.210.992.654.195.041.67

1.091.26

0.691.30

0.780.811.030.890.891.984.083.022.981.781.270.840.400.460.531.071.860.663.16

2.252.111.761.26

1.501.151.211.621.481.501.582.411.992.402.480.621.503.260.591.231.301.481.010.890.451.710.670.68

0.150.16

0.200.14

0.190.200.200.300.220.410.660.450.340.300.990.260.590.460.400.540.430.390.37

0.180.150.220.22

0.300.210.190.120.150.330.400.951.360.510.380.270.270.250.170.240.240.160.330.210.180.230.180.10

85.8087.49

89.6583.82

86.5788.9784.9584.2191.7788.2485.4380.9680.5079.5679.1279.6478.8287.2184.6286.4283.5588.9083.16

85.1285.6495.4383.69

87.1283.9389.5386.8182.4284.9181.6683.8784.7082.6489.6687.2188.1090.1282.4583.7192.5691.1687.9588.1888.9286.6886.8687.30

Note: Major elements in weight percent.

calcareous nannofossils throughout the sequences. Biogenic siliceouscomponents, such as diatoms, radiolarians, and sponge spicules, arefew except in several horizons that contain up to 10% of them.Therefore, the biogenic components of hemipelagic and pelagic sedi-ments are thought to contribute mainly to the calcium carbonatecontent of sediments in the Izu-Bonin area. Unit V of Site 793 isthought to have been deposited below the calcium carbonate compen-sation depth (CCD), and other sedimentary sequences show deposi-tion above the CCD based on the benthic foraminifer assemblages

(Kaiho, this volume). The calcium carbonate contents of Unit V ofSite 793 are <7%, except for one sample that reaches 15%. On thecontrary, the calcium carbonate contents of the sediments of other unitsare variable and attain values as high and higher than 45% (Table 1).

The Sr isotopic ratios of carbonates from the sediments of Unit IIIof Site 792 are lower in value than those of modern marine carbonates,as already mentioned. These values yield ages of 23.5 Ma for Sample126-792E-28R-1, 57-59 cm, and 28.4 Ma for Sample 126-792E-31R-1, 31-32 cm, based on the data of Hess et al. (1986). The

495


Table 3. Averages and standard deviations of carbonate-free chemical compositions of each unit of the sites drilled on Leg 126, Izu-Bonin Arc.

Site

Unit

Samples (n)

SiO2

Al2θ 3

Fe2O3

TiO2

CaO

MgONa2OK2O

MnO

Site 787Unit II

1

56.6212.568.05

0.694.282.582.321.190.16

Site 787Unit III

3

54.07

12.046.660.36

8.082.54 ,2.041.710.39

Site 787Unit iVA

1 1

55.87±5.1413.40±1.09

7.91±1.780.50±0.156.24±4.513.51±1.052.53±0.68

1.57±1.450.23+0.06

Site 787Unit IVB

2

55.14

11.83

9.10.535.025.482.020.820.23

Site 788Unit IIA

1

53.2611.0111.25

0.894.094.651.951.150.29

Site 790Unit I

4

63.87±2.64

11.42±0.766.08+0.39

0.52±0.103.65+.0.932.10±0.352.82±0.321.68±0.45

0.18±0.02

Site 790Unit II

4

59.17±1.2612.21±0.88

6.85±1.330.54±0.066.68±1.852.69±0.41

2.30±0.111.72±0.51

0.20±0.03

Site 791Unit I

6

57.72±5.6810.18±0.74

4.90±0.320.48±0.054.49±3.671.77*0.272.65±0.411.58±0.420.15±0.03

Notes: Major elements in weight percent, (n) = number of samples analyzed.

O • r


Unit 40 60 0 200 100 1 0 200 100 1 0 40 %

Q.CDQ

200-



Unit 40 60 0 20 0 1 0 0 1 0 20 0 10 0 1 0 4i0-r

.αE

CDQ

500-

800-

•

•

•

•

•

••

•

•

•

•

•

•

••

λ•

•••••

••••• *•

••••

•••••••*•

••

••

•

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•

•

4•••

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•

•t•

•

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496



Site 791Unit II

18

56.33+3.5610.14*1.295.38*1.400.43*0.116.87*5.682.15±0.802.22*0.321.78*0.410.21 ±0.07

Site 792Unit I

8

53.01±2.3414.01*1.847.73*1.500.56*0.126.22*1.162.47*0.562.37*0.210.97*0.230.17*0.02

Site 792Unit II

3

53.5213.747.910.485.012.532.250.930.24

Site 792Unit III

3

51.1611.956.420.254.833.291.933.030.51

Site 792Unit IV

1 1

48.77+4.4412.22*1.908.01*1.700.45±0.132.79*2.086.07*1.312.17*1.071.36±0.980.46*0.20

Site 793Unit IA

1

55.6510.575.060.274.541.852.632.250.18

Site 793Unit IB

3

57.6110.735.140.267.021.722.761.710.2

Site 793Unit III

7

53.52*3.9312.70*1.116.72*1.430.41*0.134.50*2.382.81*0.872.00*0.261.43*0.180.24*0.10

Site 793Unit IV

3

52.3814.636.890.431.592.541.612.260.94

Site 793Unit V

14

51.06*2.9516.06*2.937.56*1.850.42*0.104.55*1.574.02*2.112.47*1.231.28*0.790.23*0.07

magnetostratigraphy and biostratigraphy yield ages of 22.6 -23.5 Maand 27.0-27.5 Ma for the two samples, respectively. This coincidenceindicates pelagic sedimentation of calcium carbonate in this unit.

Terrigenous Components in Chemical Composition

In the Fe2O3-MgO-TiO2 triangles, the Oligocene sequences offorearc sites show high contents of MgO, compared with those of theother sedimentary sequences (Fig. 8). At Site 792, the chemicalcompositions are clearly separated in the triangle and are arrangedfrom the Quaternary unit (with low MgO contents) to the Oligoceneunit (with high MgO contents). The MgO content of the Oligocenesequence is higher, with some samples exceeding 7% (Table 2). Highmagnesian boninitic and andesitic rocks with MgO contents exceed-ing 20% occur in the Chichijima Island of the Ogasawara (Bonin)Ridge (Kuroda and Shiraki, 1975; Umino, 1985). The Oligocenebasement volcanic rock sequences of forearc Sites 792 and 793 attainMgO contents up to 12% (Taylor, Fujioka, et al., 1990). In contrast,MgO contents in the late Miocene to Quaternary sequences of theforearc sites are as small as those of the Quaternary sequences at thebackarc sites, which are lower than 4% (Table 2).

The sedimentation rates (even the net sedimentation rates ofmuddy sediments) of the Oligocene sequences were calculated toseveral tens to hundred meters per million years, except for the lateOligocene to early Miocene Unit m of Site 792 and Unit IV of Site 793,both of which have very slow rates (e.g., 4-14 m/m.y.). The sedimen-tation rates of the Oligocene sequences are distinctly larger than those inoceanic pelagic environments (Table 5), which suggests that the contri-bution of terrigenous inputs to hemipelagic sediments is probably severaltimes as large as those in pelagic environments. Therefore, the chemicalcomposition of hemipelagic sediments directly reflects those of thesource area. The MgO enrichment in the Oligocene sequences of theforearc sites shows the influence of the chemical composition of theimmature Paleogene arc with high magnesian rock series to those of thehemipelagic sediments.

A similar relationship was found in the coarser grained sediments.The chemical composition of sandstone in the Oligocene sequencesof the forearc shows the same tendency, with the high MgO contentsreflected in those of the source area (Hiscott and Gill, this volume).

The REE patterns of the sediments in Figures 9 and 10 show thatthe absolute abundance of REEs in the sediments is generally higherthan that of oceanic basalts and that all of the sediments are enrichedin light REEs. Enrichment in light REEs in the sediments is moremarked in the backarc than in the forearc. Ikeda and Yuasa (1989)showed that volcanic rocks from the Sumisu Rift and its vicinitygenerally have two types of REE patterns, that is, enriched (E) anddepleted (N) types. Most of the volcanic rocks from the arc ridge andthe intraridge in the Sumisu Rift possess an N-type pattern and are

depleted-to moderate in light REEs with 0.66-1.50 (Ce/Yb)n (Ikedaand Yuasa, 1989;Freyeretal., 1990;Hochstaedteretal., 1990). However,volcanic rocks from the backarc seamounts northwest of the Sumisu Riftdisplay an E-type pattern with 1.7-3.23 (Ce/Yb)n (Ikeda and Yuasa,1989). The sediment samples from the Sumisu Rift exhibit stronglylight-REE-enriched patterns with 2.31-4.33 (Ce/Yb)n, suggestingthat they probably originated from the enriched-type rocks (Figs. 9and 11). The MgO contents of the sediments suggests that forearcsediments are highly affected by high magnesian rocks, as mentionedabove. However, the REE data of the forearc sediments displays ahigh abundance and a different pattern from the boninitic rocks.Boninites from the Bonin Island contain a very low abundance inREEs with 0.88 (Ce/Yb)n and a dish-shaped pattern (Hickey and Frey,1982) (Fig. 11). To know the origin of the difference in the REEpattern between the backarc and forearc sediments, more work fo-cused on the sedimentary environment and the sedimentary processof hemipelagic sediments is needed.

Hydrothermal Effects on Chemical Compositionin the Sumisu Rift

In the Izu-Bonin Arc, crusts of manganese oxides with hydrother-mal origins were found on the slope area of active volcanoes alongthe volcanic front (Usui et al., 1986; Nakao et al., 1990). In the SumisuRift, high manganese contents of surface hemipelagic sedimentsexceeding 1% are characteristic (Nishimura et al, 1988; Nakao et al.,1990). Moreover, on the ridges between the north and south basins ofthe Sumisu Rift, dead but very young chimneys composed of silicaand barites were found (Taylor et al., 1990), which can be a source ofmanganese in the surface sediments of the Sumisu Rift basin (Urabeand Kusakabe, 1990). In this study, however, the MnO contents in thesediments of the Sumisu Rift are low, with the highest value at only0.4% (Table 1). This suggests that the hydrothermal influence on thechemical composition of hemipelagic sediments is negligible through-out the core sequences of Sites 790 and 791. It should be pointed outthat manganese ions were released from the bottom through thediagenetic reductive processes, which are related to the higher organiccarbon contents of sediments and the higher sedimentation rate in theSumisu Rift. Surface sediments of the Sumisu Rift with high MnOcontents have a dark brown color (Nishimura and Murakami, 1988),but the samples analyzed in this study possess an olive gray color. TheREE analyses of the samples at Site 791 also do not indicate thehydrothermal effect on the hemipelagic sediments in the Sumisu Rift.

Authigenic Components in Chemical Composition

The MnO contents are influenced by hydrothermal activity andsedimentary environments. In both the forearc and backarc sites of

497

A. NISHIMURA, N. MITA, M. NOHARA

Table 4. Contents of rare-earth elements of sediment samples of Sites 791 and 792.

Core, Section,Interval (cm)

La

Ce

Pr

Nd

Sm

Eu

Gd

TbDy

HoEr

Tm

Yb

Lu

126-791A-4H-4130-150 cm

15.1331.723.8114.623.210.933.520.623.430.782.040.472.080.40

126-791A-22H-645-65 cm

10.1223.253.1113.453.341.134.270.694.301.062.850.482.540.49

126-791A-39X-123-25 cm

15.0533.394.1116.643.711.094.340.673.980.912.320.422.560.43

126-791B-13R-197-99 cm

13.6627.303.5313.362.741.003.920.643.150.842.000.502.210.42

126-791B-23R-46-7 cm

11.5122.362.7610.322.120.752.720.472.750.621.460.381.350.32

126-791 B-34R-2139-141 cm

11.1423.993.2112.823.101.023.930.734.391.092.790.602.900.55

Note: Trace elements in parts per million (ppm).

Leg 126, we cannot find a large anomaly in the manganese oxidecontent, which would suggest a hydrothermal origin, as mentionedabove. However, the vertical distribution of manganese oxide con-tents in the core sequences coincides with the lithologic change in theforearc sites (Figs. 2-4). Higher manganese contents were observedin Unit IV of Site 793 and in Unit III of Site 792, which have distinct,small sedimentation rates in the Leg 126 site sequences. Matsumotoet al. (1985) discussed the relationship between manganese contentsand sedimentation rates of continental margin sediments and abyssalbasin sediments. They concluded that an inverse relationship betweenmanganese contents and sedimentation rates is clear and that thisrelation can be explained by the dilution effects of terrigenousmaterials on hydrogenous manganese oxides with a constant rate of

Table 5. Sedimentation rates of lithologic units, Leg 126.

Site

787

788

790

791

792

793

Unit

IIIIII

IVAIVB

IAIBIIAMB

1II

1II

1IIIIIIV

IAIBIIIIVV

Depth(mbsf)

0 - 21.421.4 - 40.340.3 - 118.9

118.9 - 281.7281.7 - 320.1

0 - 229.2229.2 - 249

249 - 278.6278.6 - 374

0 - 165165 - 266.6

0 - 428.4428.4 - 834

0 - 183.7183.7 - 357.4357.4 - 429.3429.3 - 783.4

0 - 32.532.5 - 98.998.9 - 735.7

735.7 - 759.0759.0 - 1373.1

Apparentsedimentation

rate (m/m.y.)

679

26,120120

115145-23039-282

282,73,192

ca.100090

ca.2200344

81,120,12243,2314,4

30, ca.300

105

707

80, ca.250

Sedimentaionrate of muddy

sediments (m/m.y.)

150, 59*60*

56*

1428f

728|,43t

Notes: * = based on visual descriptions, and t = based on the FMS columns (Hiscott etal., this volume).

supply. We recalculated the sedimentation rates of muddy sediments(hemipelagic and pelagic sediments) in both forearc and backarc sites,excluding ash layers, debris-flow deposits, and turbidites (Table 5),based on original onboard visual core descriptions (Taylor, Fujioka,et al., 1990) and FMS columns (Hiscott et al., this volume). Therecalculated sedimentation rates and the manganese oxide contentsshow an inverse relationship (Fig. 12), like that found by Matsumotoet al. (1985) and Sugisaki (1984).

On the REE patterns of the sediments (Figs. 9 and 10), a fewsamples have a weak negative Ce anomaly, which can be attributedto reduction in the seawater or sediment column (De Baar et al., 1985;Tanaka et al., 1990). Moreover, all samples have a zigzag feature inHo-Er-Tm-Yb-Lu with variable degrees of curvature. Masuda andIkeuchi (1979) and Tanaka et al. (1990) recognized a zigzag featurein the heavy REE patterns of seawater, and they interpreted thisfeature to be a reflection of a partially cut-off lanthanide tetrad effect.Although seawater would have an affect, in part, on the abundancesof heavy REEs in the sediments, these zizag patterns are possibly aresult of analytical uncertainties because of the very low abundancesof Ho, Tm, and Lu in the sediments.

Sedimentary Environments and Diagenesis

The SiO2 contents of the Pliocene-Pleistocene sequence in thebackarc sites are higher than those in the forearc sites with corre-sponding age (Fig. 7). Higher SiO2 contents and higher ratios of SiO2

to A12O3 can possibly be attributed to biogenic silica (i.e., to radio-larians and diatoms) and to acidic volcanogenic materials. Becausesmear slide observations suggest that siliceous biogenic contributionsto the chemical composition are negligible, as mentioned already, weconclude that finer grained acidic volcanic materials affected the SiO2

content of the sediments. Quaternary acidic volcanic materials de-rived from the arc volcanoes have contents of 72%- 80% SiO2 and11%-14% A12O3 (Nishimura et al, this volume; Fujioka et al., thisvolume). The rift basin is surrounded by the rift flanks, and suspendedparticles are trapped in the basin. On the other hand, the forearc basinis an open basin and almost all of the suspended particles are blownaway from the basin. The differences between the sedimentary envi-ronments of forearc and backarc basins are suggested by the sedimentsequences of thick pumice deposits. The thick pumice deposits of thebackarc sites have thick, finer grained (silt- to clay-size) ash in the

498



126-791B-43R-131-32 cm

15.6932.353.9714.793.110.873.680.603.280.802.060.372.230.40

126-792A-4H-6100-102 cm

5.4111.861.666.801.840.842.550.512.520.711.700.431.720.37

126-792B-9X-213-15 cm

6.7216.092.329.282.530.973.480.633.420.902.230.562.530.47

126-792E-19R-2120-122 cm

4.068.911.465.871.670.822.830.542.800.791.880.502.080.44

126-792E-37R-262-63 cm

8.4319.772.7812.352.971.203.980.764.061.072.620.592.780.54

126-792E-52R-194-96 cm

8.4916.832.6211.472.721.113.880.673.600.962.620.542.330.45

126-792E-66R-3105-107 cm

4.818.911.496.161.600.812.500.492.460.721.690.481.710.40

upper part of the sequences, which were trapped in the basin. Theforearc sites, however, have no finer grained ash in the upper part ofthe sequences (Nishimura et al., this volume).

The minimum CaO contents of the Site 792 and 793 sequences(Figs. 3 and 4) coincide with the turning point of CaCl2 in the porewater (Egeberg et al., 1990). The origin of the unusually high CaCl2

content in the lower part of the Site 792 and 793 sequences is thoughtto be related to the alteration of volcanogenic materials, which reflectsthe chemical composition of sediments with a low CaO content. Thesedimentary samples of this area throughout the sequences havegenerally high contents of CaO, suggesting that they would be highlyaffected by the high CaO contents in the volcanic rocks as a sedimentsource. Moreover, the presence of gypsum-filled fractures in thesequences (Taylor, Fujioka, et al., 1990) implies the possibility of CaOenrichment from secondary minerals formed through a diageneticalteration.

CONCLUSIONS

The chemical composition of hemipelagic and pelagic sedimentsin the Izu-Bonin Arc shows the following features:

1. The chemical composition of the hemipelagic and pelagicsediments of the sequences of Leg 126 is highly variable. The contentsof SiO2 and MgO are very high in several samples, and those of Cu,Co, and Ni are constantly very low, compared with those of sedimentsfrom ocean basins and the Philippine Sea.

2. The major chemical compositions of hemipelagites are highlyaffected by that of the source land area. As for the Izu-Bonin region,the source has been the Izu-Bonin Arc itself. The change in nature ofthe arc is indicated by the vertical change of MgO content of sedi-ments in the forearc sites. The Oligocene sequences with high MgOcontents reflect the effect of the Paleogene immature arc volcanicrocks. Pliocene to Pleistocene sediments have low MgO contentssimilar to acidic volcanic rock in the present arc. The REE analysesalso demonstrate the large influence of volcanic rocks to the chemicalcomposition of the sediment.

3. No influence of hydrothermal activity on the chemical featuresof hemipelagites can be found in the core sequences of the SumisuRift, although high MnO contents in the surface sediments andhydrothermal vents were observed in the rift.

4. The MnO values are not so large throughout the sequences, butthe values do show an inverse correlation with sedimentation rates.

The highest values are recorded in the chalk with a very smallsedimentation rate deposited in the pelagic environment.

5. A contrast in chemical composition between the forearc andbackarc sites is suggested by the SiO2 content. The values of SiO2 inthe backarc are higher than in the forearc, which is probably relatedto the topography of the rift basin, which is surrounded by rift flanksthat trap suspended finer grained acidic volcanogenic materials.

ACKNOWLEDGMENTS

We thank Dr. R. Hiscott for permission to use FMS columns torecalculate the sedimentation rates and references of sandstone geo-chemistry in this area. We would like to thank S. Nakao, M. Yuasa,K. Rodolfo, K. Marsaglia, and K. Fujioka for extensive and fruitfuldiscussions regarding the sedimentation and geologic history of theIzu-Bonin Arc. We are very grateful to R. Sugisaki and T. Plank forreviewing the manuscript.

REFERENCES

Brown, G., and Taylor, B., 1988. Sea-floor mapping of the Sumisu Rift,Izu-Ogasawara (Bonin) Island Arc. Bull. Geol. Surv. Jpn., 39:23-38.

Burke, W. H., Denison, R. E., Hetherington, E. A., Koepnic, R. B., Nelson, H.E, and Otto, J. B., 1982. Variation of seawater 87Sr/86Sr throughoutPhanerozoic time. Geology, 10:516-519.

De Baar, H.J.W., Bacon, M. P., Brewer, P. G., and Bruland, K. W., 1985. Rareearth elements in the Pacific and Atlantic Oceans. Geochim. Cosmochim.Acta, 49:1943-1959.

Egeberg, P. K., and Leg 126 Shipboard Scientific Party, 1990. Unusual composi-tion of pore waters found in the Izu-Bonin fore-arc sedimentary basin. Nature,344:215-218.

Fryer, P., Taylor, B., Langmuir, C. H., and Hochstaedter, A. G., 1990. Petrologyand geochemistry of lavas from the Sumisu and Torishima backarc rifts.Earth Planet. Sci. Lett., 100:161-178.

Hess, J., Bender, M. L., and Schilling, J.-G., 1986. Evolution of the ratio ofstrontium-87 to strontium-86 in seawater from Cretaceous to Present. Science,231:979-984.

Hickey, R. L., and Frey, F. A., 1982. Geochemical characteristics of boniniteseries volcanism: implications for their source. Geochim. Cosmochim.Acta, 46:2099-2115.

Hochstaedter, A. G., Gill, J. B., and Morris, J. D., 1990. Volcanism in theSumisu Rift, IL Subduction and non-subduction related components. EarthPlanet. Sci. Lett., 100:195-209.

Honza, E., and Tamaki, K., 1985. The Bonin Arc. In Nairn, A.E.M., Stehli, F. G.,and Uyeda, S. (Eds.), The Ocean Basins and Margins (Vol. 7): The PacificOcean: New York (Plenum Press), 459-499.

499


Site 790

Unit I oUnit II 1 0

15

15 20 25 30 35AI 2 O 3

Site 787

Unit II DUnit IIIUnit IVAUnit IVB

80

70

15 20 25 30 35AI2O3

Site 792

Unit I °Unit II π 1 0

Unit IIIUnit IV

15/ Δ ^ . ft \.75

65

15 20 25 30 35AI2O3

Site 793

Unit IUnit IIIUnit IVUnit V

65

30 35 AI 2 O 3

Figure 7. SiO2-Al2O3-10Tiθ2 triangles of Leg 126 samples.

500

Site 790

Unit I oUnit II

60

50

20 30 40 50 MgO

Site 791

Unit I oUnit II

20 30 40 50 MgO


Site 787

Unit II ÜUnit III AUnitlVA ΔUnit IVB v 2 0

20 30 40 50 MgO

Site 792

Unit I o 10Unit II DUnit III *Unit IV Δ 20

20 30 40 50 MgO

Site 793

Unit I o 10

Unit III o

Unit IV A

Unit V Δ

50

20 30 40 50 MgO

Figure 8. Fe2O3-MgO-TiO2 triangles of Leg 126 samples.

501


100 100

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 9. Chondrite-normalized REE patterns for sediment samples at Site 791.1 = Sample 126-791A-4H-4,130-150cm;2=Sample 126-791A-22H-6,45-65 cm;3 = Sample 126-791A-39X-1,23-25 cm; 4 = Sample 126-791B-13R-1,97-99 cm;5 = Sample 126-791B-23R-4,6-7 cm; 6 = Sample 126-791B-34R-2,139-141 cm;and 7 = Sample 126-791B-43R-1, 31-32 cm.

100

CDT3

COCD

10

•

o•

1

8

9

10

i

D

•

Δ

11

12

13

•

i i i i i i i I

T3

O

Q.

03


Figure 10. Chondrite-normalized REE patterns for sediment samples at Site 792.8 = Sample 126-792A-4H-6,100-102 cm; 9 = Sample 126-792B-9X-2,13-15 cm;10 = Sample 126-792E-19R-2, 120-122 cm; 11 = Sample 126-792E-37R-2,62-63 cm; 12 = Sample 126-792E-52R-1, 94-96 cm; and 13 = Sample 126-792E-66R-3, 105-107 cm.

Ikeda, Y., and Yuasa, M., 1989. Volcanism in nascent back-arc basins behindthe Shichito Ridge and adjacent areas in the Izu-Ogasawara arc, north-west Pacific: evidence for mixing between E-type MORB and island arcmagmas at the initiation of back-arc rifting. Contrib. Mineral. Petrol,101:377-393.

Kuroda, N., and Shiraki, K., 1975. Boninite and related rocks of Chichijima,Bonin Islands, Japan. Rep. Fac. Sci., Shizuoka Univ., 10:145-155.

Leinen, M., 1987. The origin of paleochemical signatures in North Pacificpelagic clays: partitioning experiments. Geochim. Cosmochim. Acta,51:305-319.

Masuda, A., and Bceuchi, Y, 1979. Lanthanide tetrad effect observed in marineenvironment. Geochem. J., 13:19-22.

Matsumoto, R., Minai, Y, and Iijima, A., 1985. Manganese content, ceriumanomaly, and rate of sedimentation as aids in the characterization andclassification of deep-sea sediments. In Nasu, N., Kobayashi, K., Uyeda, S.,Kushiro, I., and Kagami, H. (Eds.) Formation of Active Ocean Margins:Tokyo (Terra Scientific), 913-939.

CDT3

CDCD

Φ

^ 10 -

Q_


Figure 11. Chondrite-normalized REE patterns for selected sediment samplescompared with volcanic rocks of the Izu-Bonin Arc. 1 = dacite from the backarcseamount (Sample D623-2; Ikeda and Yuasa, 1989); 2 = basalt from the arcridge (Sample D640-1; Ikeda and Yuasa, 1989); 3 = boninitic rock from BoninIsland (Sample 2983; Hickey and Frey, 1982); 4 = backarc sediment sample(Sample 126-791B-34R-2,139-141 cm; this study); and 5 = forearc sedimentsample (Sample 126-792E-66R-3, 105-107 cm; this study).

1.0

3 0.5o

O

100

Sedimentation Rate (m/m.y.)

200

Figure 12. Relationship between MnO content and sedimentation rate. The areabetween two solid lines shows modern sediments in Matsumoto et al. (1985).

Mita, N., Nakao, S., and Kato, K., 1982. Minor chemical composition ofbottom sediments from the Central Pacific Wake-Tahiti Transect. CruiseRep., Geol. Surv. Jpn., 18:313-337.

Murakami, F., 1988. Structural framework of the Sumisu Rift, Izu-OgasawaraArc. Bull. Geol. Surv. Jpn., 39:1-21.

Nakao, S., Yuasa, M., Nohara, M., and Usui, A., 1990. Submarine hydrother-mal activity in the Izu-Ogasawara Arc, western Pacific. Rev. Aquatic Sci.,3:95-115.

Nishimura, A., and Murakami, F., 1988. Sedimentation in the Sumisu Rift,Izu-Ogasawara Arc. Bull. Geol. Surv. Jpn., 39:39-61.

Nishimura, A., Yamazaki, T, Yuasa, M., Mita, N., andNakao, S., 1988. Bottomsample and heat flow data of Sumisu and Torishima Rifts, Izu-OgasawaraIsland Arc. Geol. Surv. Jpn., Mar. Geol. Map Sen, 31 (scale 1:200,000).

Nohara, M., 1980. Geochemical history of Japan Trench sediments sampledduring Leg 56, Deep Sea Drilling Project. In Langseth, M., Okada, HL, etal., Init. Repts. DSDP, 56, 57, Pt. 2: Washington (U.S. Govt. PrintingOffice), 1251-1257.

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Date of initial receipt: 2 January 1991Date of acceptance: 19 August 1991Ms 126B-151

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Determination of Major and Minor Chemical Elements

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