Hydrothermal sediments associated with a relict back-arc spreading center in the Shikoku Basin,...

Research ArticleHydrothermal sediments associated with a relict back-arc spreading center in the Shikoku Basin, recovered from the

Nankai accretionary prism, Japan

JANE L. ALEXANDER,1,2,* KEVIN T. PICKERING 1 AND ELIZABETH H. BAILEY 2,†

1Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT and2Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK

Abstract The hemipelagic mudrocks of the Nankai accretionary prism, Japan, containhydrothermal deposits associated with a relict spreading center in the Shikoku Basin.Initial work on core samples from Ocean Drilling Program site 808 found several sampleswith elevated concentrations of calcium, magnesium, iron and manganese, at depths ofbetween 1060 and 1111 m below sea floor. However, the origin of these sediments wasuncertain, due to a lack of data. There was no recorded evidence of whether these elevatedconcentrations were present throughout this interval of core, or if they were present asdiscrete layers with the background hemipelagic mudrocks in between. In the presentstudy the core was resampled, and the sediments with anomalous chemical compositionswere found to be present in discrete layers. This fact, along with a detailed interpretationof their geochemistry, has allowed them to be identified as hydrothermal sediments, associated with the relict spreading center in the Shikoku Basin. The lower (older) twolayers display a chemical composition typical of umbers, while the upper (younger) twolayers are metalliferous mudrocks typical of deposits found further from the spreadingcenter.

Key words: accretionary prism, back-arc basin, hydrothermal, Japan, mudrock, NankaiTrough, plate tectonics, sedimentology, umber.

Nankai Trough is relatively shallow, with a waterdepth of less than 4900 m. This is due to the youngage of the Shikoku basin, its thermal buoyancy andthe thick pile of sediments accumulated in thetrench (Shipboard Scientific Party 1991a). Thesediments are being actively accreted to the accre-tionary prism, as the Philippine Sea Plate is sub-ducted towards the northwest. Evidence of ancientsubduction in this region is found on land in theShimanto Belt, which comprises a Cretaceous–lower Miocene accretionary prism (Taira et al.1989).

The Shikoku Basin formed as a back-arc basinbehind the Izu–Bonin Arc, from the Oligocene tothe middle Miocene (Shipboard Scientific Party1991a). A spreading center was active in aneast–west direction at the center of the basin until15 Ma. There is some uncertainty as to whetherthe spreading ceased after 15 Ma (Okino et al.

INTRODUCTION

The Nankai Trough and accretionary prism lie tothe southeast of Japan, and mark the boundarybetween the Shikoku Basin and the Honshu Arc(Fig. 1). The Shikoku Basin forms part of thePhilippine Sea Plate, and is being subductedbeneath the Honshu Arc (Eurasian Plate) at a rateof between 2 and 4 cm/year (Shipboard ScientificParty 1991a).

In comparison to other deep sea trenches, the

Accepted for publication December 1998.*Present Address: Department of Earth and Planetary Sciences,

Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA. Email:<[email protected]>

†Present Address: Division of Environmental Science, University of Nottingham, Sutton Bonington Campus, Leicestershire LE12 5RD, UK.

© 1999 Blackwell Science Asia Pty Ltd.

The Island Arc (1999) 8, 281–292

282 J. L. Alexander et al.

1994), or continued in a north–south direction until12 Ma (Chamot-Rooke et al. 1987). The relictspreading center then collided with the Nankaiaccretionary prism at ca 15 Ma (Pickering et al.1993a), and is currently being subducted close toOcean Drilling Program (ODP) site 808 (Fig. 1;Shipboard Scientific Party 1991b).

Ocean Drilling Program leg 131, site 808, wasdrilled in 1990 through the toe of the Nankai accre-tionary prism. It was located on the landwardtrench slope, less than 150 m above the trenchfloor. Drilling reached a depth of 1327 m below thesea floor (m.b.s.f.), penetrating the frontal thrust,décollement zone and the basaltic oceanic base-ment (Shipboard Scientific Party 1991b). The prox-imity of the relict spreading center affects thethermal regime of the subducting sediments(Wang et al. 1995). Heat flow is higher than thatmeasured in other accretionary prisms, increasingthe rate of sediment diagenesis in the form ofchanges in chemistry and clay mineralogy (Under-wood et al. 1993b).

STRATIGRAPHY AT ODP SITE 808

The Nankai accretionary prism is a clastic-domi-nated accretionary prism (Taira & Pickering 1991).The thick trench fill shows an overall coarseningupward sequence, typical of subduction systems.The oldest recovered sediments are hemipelagic

muds, which gradually become interbedded withturbidites and then coarsen up into entirely ter-rigenous sands (Pickering et al. 1993a). Thestratigraphy at site 808 can be divided into six dis-crete lithological units, two of which have beenfurther divided into subunits (Fig. 2). There issome repetition of subunits IIb and IIc across the frontal thrust (Shipboard Scientific Party1991b).

Unit IV comprises the hemipelagic deposits ofthe Shikoku Basin. Sedimentation rates were muchslower than in the trench wedge, and variedbetween 45 and 110 m/million years (ShipboardScientific Party 1991b). It is divided into two subunits. Subunit IVa comprises volcanic ash andtuff, interbedded with hemipelagic muds. SubunitIVb is composed almost entirely of bioturbatedhemipelagic muds (Underwood et al. 1993a).However, there are some metalliferous sediments(mudrocks with slightly enriched levels of iron andmanganese) and umbers (‘iron, manganese andtrace-metal enriched mudstones of volcanic exhala-tive origin’ Robertson 1975) lower in the sequence,interpreted as being due to the proximity of therelict spreading center (Pickering et al. 1993b).The décollement zone is contained within subunitIVb, and does not appear to be associated with anysignificant change in lithology.

Pickering et al. (1993b) have already shown thatthere are a variety of discrete rare earth element(REE) signatures in the sediments from theNankai accretionary prism. There are also severalsuspected umbers between 1060 and 1110 m.b.s.f.,which are enriched in all REE, as well as cal-cium, iron, magnesium, manganese, phosphorus,yttrium, strontium, thorium, scandium, copper and barium (Pickering et al. 1993b). These areinterpreted as being due to the proximity of therelict spreading center. However, Underwood et al. (1993b) report a general region with elevatedlevels of calcium, magnesium, iron and manganesebetween 1087 and 1111 m.b.s.f.. They suggest thatthis was the result of fluid circulation within thesediments after they had been deposited, a viewsupported by Kastner et al. (1993).

Underwood et al. (1993b) stated ‘We believe thata detailed program of X-ray diffraction and chem-ical analyses, with a much closer sample spacingand focused targets, must be completed before theabsolute chemical contributions of Ca-bearing and Fe-bearing carbonates, barite, and the otherminerals can be quantified’. The present study fol-lowed that approach, and addressed the questionsposed by the initial work (Pickering et al. 1993b;

Fig. 1 Location of Ocean Drilling Program (ODP) site 808, relative toplate boundaries, the Nankai Trough and the relict spreading center of theShikoku Basin (modified after Shipboard Scientific Party 1991a).

Underwood et al. 1993b) by examining the natureof the hydrothermal deposits, and determiningtheir origin.

METHODS

SAMPLING

A total of 79 samples were selected after inspect-ing the core from ODP site 808C. They were collected at more frequent intervals than in pre-vious geochemical studies, to allow for a moredetailed investigation of the variations betweenbackground mudrocks and anomalous layers. Thethickness of specific layers was measured and anysubtle color or textural changes were noted. Thesamples include background mudrocks, two bandswhich were distinctly lighter in color (808C-80R-02, 78–80 and 808C-85R-01, 66–68), and othersamples with subtly different color or fractur-ing characteristics to those of the background

mudrocks. All were collected from subunit IVb,between 910.99 and 1102.6 m.b.s.f. For a full listingand description of these samples, see Alexander(1998). The samples were processed and analyzedusing a variety of techniques to determine theirmineralogy and geochemistry. The size of individualODP samples was limited to 10 mL, and this in-fluenced the amount of work that could be done.

Samples were dried and crushed, before beingdigested in acid for chemical analysis. Selectedsamples were also analyzed by X-ray diffraction(XRD) and electron microprobe to determine theirmineralogy. Major and trace elements were ana-lyzed using inductively coupled plasma atomicemission spectrometry (ICP-AES), to determinetheir bulk chemistry and provide an indication oftheir mineralogy. Potassium results were poorusing this technique, and so were re-analyzedusing atomic absorption spectroscopy (AAS). Rareearth elements, uranium and thorium were ana-lyzed using inductively coupled plasma mass spec-trometry (ICP-MS).

Shikoku Basin hydrothermal sediments 283

Fig. 2 Stratigraphy of the Nankai accretionary prism at Ocean Drilling Program (ODP) site 808, compiled using data from Shipboard Scientific Party(1991b), Pickering et al. (1993a) and Underwood et al. (1993a). m.b.s.f., meters below sea floor.


DRYING AND CRUSHING

Samples from the ODP cores were dried for 1 weekat 50°C. At higher temperatures, there is a risk ofaltering the surface structure of clay minerals andfixing exchangeable ions (Lewis & McConchie1994). Preliminary crushing using a fly press withhardened steel plates reduced the sample grainsize to less than 5 mm. Contamination is minimalwith this technique, due to the straight crushingaction involved (Fairchild et al. 1988). Each samplewas then crushed for 30 s in a Tema® disk mill(Tema Systems Inc., Cincinati, OH, USA) with anagate barrel, resulting in a fine powder (grain size~ 40 µm). Silicon and lead are possible contami-nants from agate, but neither element was used inthe present study. The agate surfaces were thor-oughly cleaned between samples, by washing inwater followed by acetone. Finally, the powderswere dried for a further 24 h at 50°C and stored inclear plastic tubs.

X-RAY DIFFRACTION AND ELECTRON MICROPROBEANALYSIS

A small selection of powders and clay mineral sep-arations were analyzed using a position-sensitivedetector X-ray diffractometer, to investigate their mineralogy and compare it with the resultsof bulk chemical analysis. This was limited to the identification of the main minerals present,without measuring the percentage of each.Samples were chosen to include the suspectedumbers and samples with high REE concentra-tions, as well as several background mudrocks forcomparison. Polished mounts of the same sampleswere analyzed using a Hitachi S-2500 scanningelectron microscope (Hitachi Instruments Inc.,San Jose, CA, USA), to examine accessory mineralphases and their influence on the anomalous REElevels in some mudrocks.

ACID DIGESTION

Acid digestions were performed to allow thesamples to be analyzed by ICP-MS, ICP-AES and AAS. For each sample, 0.2 g of powder wasdigested in an open polytetrafluoroethylene(PTFE) beaker. Before use, the beakers werewashed, rinsed three times in de-ionized H2O (18 MW), soaked in 10% Aristar™ HNO3 (BDHLaboratory Supplies, Poole, Dorset, UK), andfinally rinsed a further three times in de-ionized H2O (18 MW). Aristar™ acids (BDH,

Poole, UK) were used throughout the digestion procedure.

Concentrated HNO3 (1.5 mL) was added to eachsample and evaporated to dryness on a heatedsand-bath, to oxidize any organic matter present.This was followed by fuming the sample with 12 mL HF (40%) and 2 mL HClO4 for 2–3 h, untilit had reduced to a gel-like mass. This stagedecomposes silicates and carbonates, with siliconbeing removed as the volatile silicon tetrafluoridecomplex. The HClO4 ensures oxidizing conditionsduring this process. The residue was then digestedin 3 mL concentrated HNO3 and 18 mL de-ionizedwater (18 MW) for 2 h. The samples were filtereddirectly into cleaned volumetric flasks usingWhatman no. 42 filter papers (Whatman Interna-tional Ltd, Maidstone, Kent, UK), made up toexactly 100 mL and stored in high-density poly-ethylene (HDPE) bottles.

This digestion technique may not dissolve allheavy minerals (Totland et al. 1992). However, aselection of filter papers was examined under anoptical microscope, and most showed no residue ofminerals. A few papers contained one or two tinyminerals, probably zircon and monazite. Also, bulksamples examined using the electron microprobeshowed very few or no heavy minerals, and thosepresent were all smaller than 4 µm in diameter.

ICP-AES ANALYSIS

All acid digestions were analyzed for major andtrace elements using an ARL 3410 Minitorchinductively coupled plasma atomic emission spec-trometer (ARL/Fisons Instruments, Beverly, MA,USA) at the Natural History Museum, London.The acid digestions were diluted in a ratio of one part sample to four parts de-ionized water (18 MW), so that element concentrations werewithin the calibration range. Some samples withhigh concentrations of particular elements had tobe diluted further and re-analyzed.

Drift samples containing all the elements ofinterest were run after every fifth sample tocorrect for instrument drift during each run.Several acid blanks and reference standards(SGR-1 (shale) and NBS1633a (fly ash)) were alsorun (Table 1). The blanks were below the detectionlimits and reference standards were, except forpotassium, in good agreement with published data(Govindaraju 1989). Potassium results were variable, probably due to problems with the driftsamples, and so analyses were repeated usingAAS.

ICP-MS ANALYSIS

All acid digestions were analyzed for the REE,uranium and thorium, using a VG Elemental Plas-maQuad inductively coupled plasma mass spec-trometer (Ion Path, Winsford, Cheshire, UK) atthe University of Bristol. Each sample was spikedwith a 100-ppb Re and Ru internal standard tocorrect for instrument drift, by adding 5 mL of a200-ppb Re and Ru stock solution in 5% HNO3 to5 mL of digested sample. No further dilutions ofthe acid digestions were necessary, because con-centrations fell within the calibration limits of theinductively coupled plasma mass spectrometer.Results are an average of three replicate runs.Several acid blanks and reference standards(SGR-1 (shale) and NBS1633a (fly ash)) were alsorun (Table 1). The blanks showed no contaminationwith REE, U or Th, and reference standards werein good agreement with published data (Govin-daraju 1989).

ATOMIC ABSORPTION SPECTROMETRY ANALYSIS

Acid digestions were analyzed for potassium usinga Varian SpectrAA-20 atomic absorption spec-trometer (Palo Alto, CA, USA), at the Universityof Nottingham. Several acid blanks and reference

standards (SGR-1 (shale) and NBS1633a (fly ash))were also run (Table 1) and results were in goodagreement with published data (Govindaraju1989). The AAS results were used in preference tothe ICP-AES analysis of potassium, which hadunacceptably large errors.

ESTIMATION OF ERRORS

Systematic errors in the results from both ICP-AES and ICP-MS were eradicated by using eitherdrift correction or internal standards. This cor-rection was confirmed by checking with referencesamples (SGR-1 (shale) and NBS1633a (fly ash);Table 1). The effects of random errors are repre-sented by the error bars shown on all graphs. Theyare a combination of errors in analysis, samplemass and sample volume. Analytical errors werecalculated from the standard deviations of repeatruns for ICP-MS and AAS analyses, and detectionlimits for ICP-AES analysis.

RESULTS

BACKGROUND MUDROCKS

The major element chemistry of the hemipelagicmudrocks shows little variation with depth (Fig. 3).


Table 1 Results from reference standards (average and standard deviation) and comparison with published data (Govin-daraju 1989)

NBS %Al2O3 %Fe2O3 %MnO %MgO %CaO %Na2O %K2O %TiO2 S Zr Sr Ba1633a (ppm) (ppm) (ppm) (ppm)

Actual 27.02 13.44 0.023 0.75 1.55 0.23 2.26 1.33 1800 310 830 1500Average 27.5 14.3 0.024 0.81 1.64 0.29 2.22 1.35 1726 190 803 1413SD 1.5 0.5 0.002 0.03 0.10 0.08 0.12 0.06 174 16 24 68

NBS La Ce Pr Nd Sm Eu Tb Dy Ho Er Tm Yb Lu1633a (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Actual 83 180 20 77 16.7 4 2.4 15.2 2.8 8.2 1.8 7.6 1.12Average 76.6 169.5 18.5 71.8 14.8 3.81 2.22 13.2 2.55 7.12 1.00 6.41 0.98SD 5.6 11.1 1.3 6.5 1.8 0.28 0.20 1.3 0.35 0.89 0.16 0.91 0.15

SGR-1 %Al2O3 %Fe2O3 %MnO %MgO %CaO %Na2O %K2O %TiO2 S Zr Sr Ba(ppm) (ppm) (ppm) (ppm)

Actual 6.52 3.03 0.034 4.44 8.38 2.99 1.66 0.264 15 300 53 420 290Average 6.68 3.05 0.032 4.50 8.73 3.22 1.64 0.22 13 521 < 50 390 298SD 0.15 0.05 0.002 0.07 0.13 0.22 0.12 0.01 1 198 7 12

SGR-1 La Ce Pr Nd Sm Eu Tb Dy Ho Er Tm Yb Lu(ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Actual 20.3 36 3.9 15.5 2.7 0.56 0.36 1.9 0.38 1.11 0.17 0.94 0.14Average 19.7 37.1 3.8 13.8 2.2 0.68 0.32 1.6 0.28 0.91 0.12 0.77 0.12SD 0.6 2.3 0.4 1.6 0.1 0.11 0.04 0.3 0.02 0.09 0.03 0.12 0.01


Fig. 3 Major and trace element concentrations versus depth (meters below sea floor (m.b.s.f.)), all measured during the present study. (s), old umber(1098.96 m.b.s.f.); (u) young umber (1060.08 m.b.s.f.); (+), old metalliferous mudrock (1032.05 m.b.s.f.); (¥), young metalliferous mudrock (1025.11m.b.s.f.). All other data points are background mudrocks.

Most have a composition of between 15 and 20%Al2O3, ~ 2.5% MgO, 2% Na2O and 3.5% K2O, andless than 1% MnO (Table 2). X-ray diffractionresults indicate that the major mineral phases are quartz, feldspar and mixed-layer clays, con-sistent with the major element chemistry. Mostmudrocks in the décollement zone have the samecomposition as the background hemipelagicmudrocks, despite being brecciated. Several of thebackground mudrocks have anomalously high con-centrations of iron and sulphur (Fig. 3). Thesesamples contain pyrite (FeS2), which has decom-posed in places to form jarosite (KFe3(OH)6(SO4)2;Alexander 1998). Others have higher calcium con-centrations due to the presence of calcite (Fig. 3;Alexander 1998).

Trace element concentrations are also relativelyconstant with depth. Titanium and zirconium arepresent at low levels (< 4000 ppm Ti and < 100 ppmZr: cf. Gromet et al. 1984; Condie 1991), suggest-ing that there are very few detrital minerals. Thiswas confirmed by electron microprobe analysis(Alexander 1998). Ti/Al ratios varied little betweensamples because all background mudrocks had avery similar composition, and no ash layers weresampled in this part of the core (Alexander 1998).The REE concentrations and chondrite normal-ized patterns for the hemipelagic mudrocks aresimilar to those for average shales (Fig. 4; Taylor& McLennan 1985).

UMBERS

Two samples at 1060.08 and 1098.96 m.b.s.f., notedas being distinctly lighter in color during sampling,have been interpreted as umbers from their bulk chemistry. They were present as discretelayers within the background mudrocks, horizon-tally bedded with indistinct upper and lowerboundaries, and were ~ 2–4 cm thick. Quartz andillite/smectite mixed layer clays were identified by XRD, as in the background mudrocks. Theamorphous precipitates present in this type ofdeposit, such as iron and manganese oxyhydrox-ides, cannot be identified using this technique.However, the presence of manganese oxidesand/or oxyhydroxides is indicated by the highmanganese concentrations measured by ICP-AES. The only accessory minerals, seen duringexamination using the electron microprobe, werebarite (BaSO4) and a very manganese-rich calcite((Ca,Mn)CO3).

The umbers are depleted in aluminium, magne-sium, sodium and potassium, due to the lower proportion of clay minerals present. They areenriched in manganese, calcium and strontium,and the lower sample is enriched in barium (Fig.3; Table 2). They also have high uranium (~ 20 ppm)and low thorium (~ 5 ppm) concentrations. Bothumbers are enriched in all REE, which is typicalof this type of deposit (Robertson & Fleet 1986).


Table 2 Major and trace element concentrations in background mudrocks, metalliferous mudrocks and umbers

%Al2O3 error† %Fe2O3 error† %MnO error† %MgO error† %CaO error† %Na2O error†

Background mudrocks 17.3 0.8 6.40 0.35 0.17 0.08 2.53 0.10 1.22 1.28 2.00 0.13Metalliferous mudrock 14.0 0.3 8.79 0.14 8.55 0.13 2.51 0.04 2.50 0.04 1.61 0.19

(1025.11 m.b.s.f.)Metalliferous mudrock 14.4 0.3 7.28 0.11 8.30 0.13 2.49 0.04 1.61 0.02 1.69 0.19

(1032.05 m.b.s.f.)Umber (1060.08 m.b.s.f.) 8.5 0.2 5.31 0.08 8.22 0.13 1.75 0.03 17.59 0.26 1.35 0.19Umber (1098.96 m.b.s.f.) 5.2 0.1 5.81 0.09 16.98 0.24 1.40 0.02 20.22 0.30 0.94 0.18Suhaylah umber* 1.87 9.15 9.47 0.64 22.64 0.4Suhaylah umber‡ 7.1 26.3 7.3 2.4 1.7 0.3

%K2O error† %TiO2 error† S (ppm) error† Zr (ppm) error† Sr (ppm) error† Ba (ppm) error†

Background mudrocks 3.32 0.16 0.64 0.03 1021 882 77 8 117 36 563 52Metalliferous mudrock 2.61 0.00 0.50 0.01 0 68 51 121 3 695 18

(1025.11 m.b.s.f.)Metalliferous mudrock 2.84 0.00 0.52 0.01 0 64 51 90 3 694 18

(1032.05 m.b.s.f.)Umber (1060.08 m.b.s.f.) 1.64 0.00 0.31 0.01 1857 1027 55 51 486 8 935 21Umber (1098.96 m.b.s.f.) 1.07 0.00 0.20 0.01 5526 1081 0 751 12 15 416 233Suhaylah umber* 0.25 0.08 27 6768 151Suhaylah umber‡ 1.7 0.3 94 493 216

† Errors given as standard deviation for background mudrocks, absolute errors for metalliferous mudrocks and umbers.* Data from Robertson and Fleet (1986); ‡ data from Robertson and Boyle (1983).Values shown are the average of all background mudrocks and actual values for individual metalliferous mudrocks and umbers; m.b.s.f., meters below sea floor.


The older umber also displays a positive europiumanomaly when normalized to chondrite values(Fig. 4; Table 3).

METALLIFEROUS MUDROCKS

A further two samples, at 1025.11 and 1032.05m.b.s.f., are enriched in manganese and iron com-pared to the background hemipelagic mudrocks,hence the name ‘metalliferous’ (Fig. 3; Table 2).They were also slightly lighter in color than thebackground mudrocks and were present as hori-zontally bedded discrete layers with a thickness of2–4 cm. Quartz, feldspar and illite/smectite mixedlayer clays were again identified using XRD, butas with the umbers, amorphous minerals cannot bedetected using this method. The main accessory

minerals, calcite (CaCO3) and pyrite (FeS2), wereidentified using the electron microprobe.

These metalliferous mudrocks have lower concentrations of aluminium and potassium thanthe background mudrocks, due to the lower pro-portion of clay minerals present. They are alsoslightly enriched in calcium and strontium (Fig. 3;Table 2). Rare earth element concentrations arehigher than in the background hemipelagicmudrocks, but lower than in the umbers (Fig. 4;Table 3).

DISCUSSION

Four of the samples collected from ODP site 808Chave chemical signatures suggesting that they are

Table 3 Rare earth element concentrations in background mudrocks, metalliferous mudrocks and umbers

La* error† Ce* error† Pr* error† Nd* error† Sm* error† Eu* error† Tb* error†

Background mudrocks 31.3 3.4 72.3 7.3 6.6 0.5 23.3 1.8 4.1 0.4 1.4 0.2 0.651 0.117Metalliferous mudrock 47.3 0.7 135.0 2.1 12.9 0.2 53.2 0.4 11.9 0.5 3.6 0.1 1.909 0.034

(1025.11 m.b.s.f.)Metalliferous mudrock 32.1 0.4 81.3 0.4 7.7 0.1 29.8 0.4 5.8 0.2 1.9 0.1 0.993 0.017

(1032.05 m.b.s.f.)Umber (1060.08 m.b.s.f.) 186.7 1.7 301.9 2.8 23.4 0.1 86.6 0.7 14.4 0.1 5.7 0.1 4.104 0.039Umber (1098.96 m.b.s.f.) 168.4 0.8 349.3 1.5 29.7 0.3 121.3 1.1 22.7 0.4 21.2 0.3 4.982 0.042

Dy* error† Ho* error† Er* error† Tm* error† Yb* error† Lu* error†

Background mudrocks 3.445 0.516 0.582 0.086 1.846 0.253 0.230 0.059 1.644 0.212 0.269 0.037Metalliferous mudrock 10.424 0.098 1.754 0.015 4.319 0.043 0.504 0.024 2.885 0.083 0.492 0.014

(1025.11 m.b.s.f.)Metalliferous mudrock 5.316 0.186 0.886 0.011 2.629 0.005 0.351 0.017 2.406 0.050 0.446 0.032

(1032.05 m.b.s.f.)Umber (1060.08 m.b.s.f.) 24.269 0.193 5.611 0.016 15.816 0.062 2.037 0.040 11.485 0.052 1.826 0.051Umber (1098.96 m.b.s.f.) 29.193 0.065 5.915 0.092 15.669 0.216 1.986 0.028 11.658 0.146 1.953 0.030

* Concentrations given in ppm; † errors given as standard deviation for background mudrocks, absolute errors for metalliferous mudrocks and umbers.Values shown are the average of all background mudrocks and actual values for individual metalliferous mudrocks and umbers; m.b.s.f., meters below sea floor.

Fig. 4 Chondrite-normalized rare earthelement concentrations (Taylor & McLen-nan 1985). Individual umber ((-s-),1098.65 m.b.s.f.; (-u-), 1060.08 m.b.s.f.;(—n—), Suhaylah umber (Robertson &Fleet 1986)) and metalliferous mudrock((–+–), 1032.05 m.b.s.f.; (–¥–), 1025.11m.b.s.f.) samples are plotted, whereas datafor (-r-) the background mudrocks areaverage values with error bars representingthe standard deviations. (—m—) Blacksmoker fluid (Mills & Elderfield 1995),values on right-hand axis.

related to the hydrothermal system at the spread-ing center of the Shikoku Basin. The lower (older)two, at 1060.08 and 1098.96 m.b.s.f., have a similargeochemistry to ancient and modern umbers(Robertson & Fleet 1976). The upper (younger)two, at 1025.11 and 1032.05 m.b.s.f., are enriched inmetals but not in the other elements associatedwith umbers. All are enriched in REE, to varyingdegrees. It is not possible to consider the three-dimensional structure of these hydrothermaldeposits, because data are available only from thissingle core.

Underwood et al. (1993b) found a general regionwith elevated levels of Ca, Mg, Fe and Mn between1087 and 1111 m.b.s.f.. They suggested that it wasthe result of fluid circulation within the sediments,either while the sediments were near the spread-ing center and influenced by the hydrothermalsystem, or at a later stage related to the subduc-tion of the sediments. However, the more detailedsampling used here shows that these increasedlevels are present in discrete layers, 2–4 cm thick.The surrounding mudrocks have the same chem-istry and mineralogy as the other backgroundhemipelagic mudrocks. Hence, large-scale fluidcirculation is unlikely to be responsible, and it isprobable that these layers were deposited at thesame time as the mudrocks. Pickering et al.(1993b) identified similar samples from this core asumbers, based on their major-element chemistry,and the present study supports this hypothesis.Kastner et al. (1993) suggested that the zone ofchemical anomalies may be related to the migra-tion of low Cl–, D-depleted and 18O-enriched fluidsfrom deeper within the prism. The present studywould indicate that the bulk chemical anomalies inthe discrete layers sampled are unrelated to thisfluid flow, and that they are also unrelated tocalcite veins with clay inclusions that formed muchmore recently and are found in the same region ofthe core.

The hydrothermal deposits are contained withinthe sedimentary sequence, unlike most ophioliticumbers such as Cyprus or Oman (Robertson &Boyle 1983), which directly overlie pillow lavas.This is due to the high rate of sediment accumula-tion, compared to a normal mid-ocean ridge,because the Shikoku Basin received detritus fromthe nearby Honshu arc. The surrounding sedi-ments have been dated using nannofossils, and arebetween 11 and 12 Ma (Shipboard Scientific Party1991b). This is around the time that activity at thespreading center stopped (Chamot-Rooke et al.1987).

UMBERS

The umber samples have a much lower claycontent than the background mudrocks, as indi-cated by their low aluminium concentrations (Fig.3; Table 2). They have high manganese concentra-tions (up to 18% MnO), and are also enriched incalcium, strontium and phosphorus (Fig. 3; Table2). Their major element chemistry is similar toumbers and oxide-sediments found above the axiallavas at Suhaylah, Oman (Table 2, Robertson &Boyle 1983; Robertson & Fleet 1986). The lowerumber is also enriched in barium, in the form ofbarite (BaSO4).

All of these elements may be precipitated fromthe type of hydrothermal plume found at mid-ocean ridges (Robertson & Fleet 1976). Sulphidesare the first minerals to precipitate from theplume, and hence are found close to the source.Then sulphates precipitate, followed by iron andmanganese oxides and oxyhydroxides, at a greaterdistance from the ridge (Feely et al. 1996). Thisexplains the presence of barite (BaSO4) in thelower umber (1098.96 m.b.s.f.), which was closer tothe spreading center. Barite can spread over largedistances, as shown by Arrhenius (1966). However,the initial amount of barium expelled at this back-arc basin was much less than is found at Arrhe-nius’ example of the East Pacific Rise (EPR; theBa/Al mass concentration ratio is less than aquarter of that at a similar distance from theEPR), probably resulting in precipitation closer tothe source. Trace element concentrations arealtered over time in the plume by scavenging andoxidative dissolution.

The lower umber is ~ 200 m above the igneousbasement, at 1289.9 m.b.s.f. (Shipboard ScientificParty 1991b). Given a sediment accumulation rateof between 45 and 110 m/million years (ShipboardScientific Party 1991b) and a spreading rate of 2–3 cm/year (Okino et al. 1994), a very approximatecalculation suggests that the umber was deposited~ 30 km from the spreading center. Present-dayhydrothermal plumes associated with spreadingcenters may be detected up to 100 km from thesource (Feely et al. 1996), so the umbers couldeasily have been precipitated from a similar plume.

The umbers are enriched in REE, again similarto the umbers from Suhaylah, Oman (Fig. 4,Robertson & Boyle 1983; Robertson & Fleet 1986),but with a few specific differences. The deeperumber (1098.96 m.b.s.f.), which was depositedcloser to the ridge, is more enriched in REE thanthe shallower umber (1060.08 m.b.s.f.). It has a



chondrite-normalized REE pattern that is verysimilar in shape to that of modern vent fluids (Fig.4, James et al. 1995; Mills & Elderfield 1995;James & Elderfield 1996). Unlike the umbers atSuhaylah, it has a distinctive positive europiumanomaly. This is typical of an oceanic hydrother-mal system (McLennan 1989), and has been pre-served in this umber due to the presence of barite,as the Eu2+ ion substitutes for Ba2+ in the baritelattice (Guichard et al. 1979; Mills & Elderfield1995).

Both umbers from the Nankai Trough haveslight positive cerium anomalies, whereas umbersand other ridge sediments usually have negativeanomalies (Piper & Graef 1974; Matsumoto et al.1985). The negative anomalies occurring as man-ganese nodules near the ridge scavenge any Ce4+,depleting concentrations in seawater and other sediments (Murray et al. 1990). However, the manganese remaining in the plume will also scavenge Ce4+ by hydrogenenous precipitation, andbecause of the distance traveled before precipita-tion, this process probably gave these umbers aslightly positive cerium anomaly. Such scavengingof light REE, and Ce in particular, by hydrother-mal plumes was discovered by Klinkhammer et al.(1983). Similar scavenging was later confirmed by Thomson et al. (1984), Ruhlin & Owen (1986),German et al. (1990) and German et al. (1997). Inparticular, Ruhlin and Owen (1986) describe thescavenging of REE by iron in a hydrothermalplume from the East Pacific Rise, where the nega-tive Ce anomaly becomes smaller with distancefrom the ridge. The higher levels of manganese inthe Shikoku Basin plume would strengthen thisprocess, resulting in a positive Ce anomaly. Theshallower (younger) umber has a larger chondritenormalized anomaly (Ce/Ce* = 1.158), than thedeeper (older) umber (Ce/Ce* = 1.070), which wasnearer the source of the plume.

METALLIFEROUS MUDROCKS

The two metalliferous mudrocks, at 1025.11 and1032.05 m.b.s.f., are enriched in manganese andiron, and have a lower percentage of clay mineralsthan the background hemipelagic mudrocks (Fig.3; Table 2). The term ‘metalliferous’ derives fromthe fact that they are enriched in iron and man-ganese, and it is used to distinguish them from theumbers, which are also enriched in other elements.They are similar in composition to oxide-sedimentshigher in the sequence at Suhaylah, Oman(Robertson & Boyle 1983; Robertson & Fleet

1986). Unlike the umbers they are not enriched inother elements associated with hydrothermaldeposits, such as calcium, barium and strontium.This, along with their stratigraphically higherposition, suggests that they were depositedfurther from the spreading center, but were prob-ably still associated with the hydrothermal system.At the most distal part of the plume, the only min-erals remaining to be precipitated would be ironand manganese oxides (Feely et al. 1996). Theupper metalliferous mudrock is ~ 260 m above theigneous basement (Shipboard Scientific Party1991b), suggesting an approximate distance of 40 km from the spreading center.

The metalliferous mudrocks are enriched inREE, but to a lesser extent than the umbers, andthey do not exhibit the same oceanic hydrothermalsystem patterns (Fig. 4). Rare earth element con-centrations are similar to other ferromanganeseoxyhydroxide crusts (Fleet 1984; De Carlo &McMurtry 1992). Both samples exhibit slight posi-tive cerium anomalies. These would have formeddue to scavenging of Ce4+ by manganese in theplume, in the same way as in the umbers. However,due to the larger distance traveled, and hence a longer time in the plume, the anomalies are larger.The lower (older) metalliferous mudrock has asmaller chondrite normalized anomaly (Ce/Ce* =1.189) than the upper (younger) one (Ce/Ce* =1.282), because it was closer to the spreading center.

CONCLUSIONS

Two umbers and two metalliferous mudrocks fromthe Nankai accretionary prism were collected fromODP site 808, and analyzed for major and rareearth elements. These hydrothermal sedimentsare important as examples of the spatial variabil-ity in a relatively young and well-constrainedspreading center associated with a back-arc basin.They were of particular interest because there hasbeen some disagreement about their origin.

The umbers and metalliferous mudrocks arepresent as discrete bands in the core, separated bybackground hemipelagic mudrocks, suggestingthat they were deposited from a hydrothermalplume, rather than fluids circulating within thesediments. The lower (older) sediments have astronger hydrothermal signature in both REE andmajor elements. The upper (younger) sedimentswere deposited further from the spreading center,and hence were a more distal product of ahydrothermal plume.

ACKNOWLEDGEMENTS

This work was carried out as part of Jane Alexan-der’s PhD studies at University College Londonand the Natural History Museum, London, andwas funded by a NERC studentship (referencenumber GT4/94/214/G). Samples were supplied bythe International Ocean Drilling Program. V. Dinprovided assistance with ICP-AES analyses andXRD analyses were carried out by M. Batchelder,both at the Natural History Museum. The ICP-MSfacilities were provided by the Department ofGeology at the University of Bristol, with the assis-tance of A. J. Kemp, and AAS analysis was pro-vided by the University of Nottingham. J. M.Gieskes and R. Matsumoto contributed helpful andconstructive reviews of the first draft.

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