Petrographic, petrological, and mineralogical characteristics of basalts from the Manihiki plateau, South Pacific Ocean

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[Data

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Pctrographic,

petrological, and

mineralogical

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basalts from the Þ.hnihiki plateau, SouLh Pacific Occan

Introduction

Hiroshi SATO

School ofRusincss Administration, Senshu University, Kawasaki, Kanagawa, 214-8580 ,Japan

29

Lilrge igll制u.s pro川町出h"出火er, LIPs) arc 巴haractcriz.吋by erupt.iOn3 of lar伊川町m市ct

mapぬ" during山latively short pcriod, which ilrで∞rlsidel'oo lo be relatt>d to th巴 幻sup('rplum�

activities_ Early CretacピJUStime i5 charactcrizcd hv thc fûrmfltion of n u mel'O山LIh,and theyおrmed

ccぞ込mじ pl込teaus at prescnt Pacific Occuo i nclu des, r-干ぞ�3S,Magellan 札口d Shat5ky Risc罰and Hikt斗ffWgl

!\'lanihiki, and Ontong ,Jnv凡platCfl出(Fig. .l J\j. Sぞveral Cretaじ刷出l'aciflc LIPσh附ch何,n suggested

to h<è prooue�rl by 8 single "印perplume" (e.g. Larson, 1991), and l'cccnt in凶tigatìω'lS ha刊discl品以l

that thrce of Pa白日c. ,日旬éiTl1じplllteéiuiò (OJ1', MP, and HPl fnrmed as a 日inglH LTP (つ�yl()γ 2006;

\Vnγthington et..31., 20凶j

Tbe 1Ianihiki plateau (1<'j民 1 Bl cnvcl'� an il.ア日of 0.77 x 1 OG km� (Co[fin and Elciholm, 1994ì 引"

' ""

c,関iot.d �,lh,r:1.tr, (rr,) Fig.1 (A) Location n崎p showing the di�tributions 01 Large Igneous Provinces (LlPs) 01↑he P院出C 印刷∞r. HR

Hess Rise: M-PM: Mid-Pacil旧Mc川mtains: SR: Shaぬky R日se: MP: Manihiki P凶teau: OJP: Ontコ9 J:司" 山岡凶(B) predicted bathymetry 01 the Manih山円四au and locations 01 dr凶gesiles durin,州0301

Leg 5 cruisc by RN Hakuho-maru. Dala is from 9臥川品"什l町//Wv.仰9凹warc-online.colTiIつ and map

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30 Bulletin of the Institute of Natura一 Sciences, Senshu Universlty No.38

plateau is divided into three parts by the Danger Islands Troughs. Igneous rocks recovered from Site

317 on the Highplateau (Jackson et a1., 1976), and a few dredge performed in the Danger Islands

Troughs (Clague, 1976) and two seamounts on the eastern flank of the High plateau (Beiersdorfet a1.,

1995). Previous workers suggested geochemical similarities between Site 317 and OJP rocks,

particularly in their radiogenic isotope compositions, but values for rocks fromthe Danger Islands Troughs are distinct (Mahoney and Spencer, 1991).

Additional sampling and data are needed to understand the orlgln Of the plateau and its

relationship to other LIPs, particularly the HP and OJP. We carried out fわur dredge hauls in the

Danger Islands Troughs during R/V Hakuho maru cruise KH 0301-Leg 5 in August 2003. In this

paper, we present the petrographic, petrological, and mineraloglCal characteristics of dredged volcanic

rocks from the Manihiki plateau, althoughsample volumes were small, limiting the types of analyses

we could perfわrm. Additional geochemical data including trace element and isotope compositions and

40Ar-39Ar age data and their tectonic and petrogenetic slgnificances will be reported and discussed by

lngle et al. (in press).

Dredge sites and recovered materials

Four dredge operations (D 1 to D 4) were performed during R/V Hakuh0-maru cruise KH 0301-Leg

5. Detail locations of these sites are listed on Table 1. Dredge sites are located at northern and eastern

scarp of the High plateau (Fig. 1 B). Recovered materials during each dredge operations are

summarized in Table 2. Sea floor and scarp along the Danger Islands Troughs are seems to be covered with thick sediments, dredge basket were丘lled by large amounts of sediment rather than lgneOuS

materials. Only small pieces of igneous materials were recovered from the sites D 2, D 3, and D 4.

Tab一e 1. Location of dredge sites duri【q KHO305-Leq5 cruise rR〃 Hakuh0-marul.

site 柳la 友宥VFR long 冲ude 要GF メ FW&FW劔免VF la 友宥VFRfV& lon 没宥VFRGF メwaterdepth

degree 蒙也WFW2 degree 猛 nutes 猛 degree 蒙也WFW2 degree 蒙也WFW2 m

KHO301-D1 澱 47.49S 都c2 47,54W 929 澱 47.52S c2 47.23W 鉄 r

KHO301-D2 湯 14.98S cB 19.24W 939 湯 14.96S 都cB 78.93W sCb

KHO301-D3 唐 27.01S c2 50.43W 424 唐 26.98S c2 50.07W 塔

8

r

2

163 鉄

Er

#"

8

r

2

163 鼎偵sUr

3956

KHO301-D4 澱

42.50S

c"

45.43W

130 澱

42.49S 都c"

45.78W 鼎s迭

'ltall'c data is from shipls record, i.e. water depth was mesured by PDR.

Water depth by PDR is often ca. 300m shallower than SeaBeam measurement.

Petrographic description

Petrographic descriptions of dredge volcanic materials are as follows. PhotomicrogTaPh of

representative part of each sample is in Fig. 2.

KHO301ID2-001 (Fig. 2A) : Doleritic texture with slightly altered plagopclase and pyroxene grains.

Relatively larger (<1 mm) grains ofplagioclase are sparsely scattered.

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Data report: Petrographic,petrological, and mineralogical characteristics ofbasalts from the Manihiki plateau, South Pacific Ocean Sl O ■;= .∫= 亡ロ て) q) ⊂ (q I_ l q) き て⊃ d> L q) ■一一 石 J= ⊥= ハR粐Kメイツ」メメ2ネツツ2nrツメBメ¥rBヨR,ツツ2ネケ⊂ .⊆ ■l・.■ くつ L q) ■●■■■ くつ め ∽ _壁 ど _壁 .1 U O L O3 ⊂ L O t●l O a) ≡ ○ 帆 BツyBR,ツ,Hメヤ3u) _聖 ≡ q め C> L■ くつ 石 > ・●■■ ∼ 管 .二と .S2 .⊂ - ⊂> ⊂> ⊂> N ネメ⊂) 寸 ツ⊂⊃ N 汎テW⊂) ∼ 綿r 卜- 猛の ツの X∼ ツ ∼ t■ 綿2寸 ツの ツeT) Ln ⊂> ー 汎テLL) 綿r寸 汎ツ寸 窒脾 -メ 唸ネ・●・.■■ ェR闔「■●.- ハR・●■-■■ 「耳ネメ2カツ,ツ鶻ツq) > 0 .工⊃ くつ の ィメB也) > O J⊃ くつ め To q 鈴{「(ロ ■市 rE) 傍CO ∽ 【ロ メ∽ くつ 紐逸∽ q *リ,"∽ (8 剿 ≡ 冢rq) ∈ J⊃ 定4x5" ⊥⊂⊃ 定ニツ_⊂I ⊂ ツ⊂ ツ⊂ ツ⊂ 犯ツ簫B(q U) 儻B-(ロ ∽ i O L. 亦ツき O L イツき O L bツi O L 亦,Hメ■.I- ○ 綿ネ爾「■●・. ○ .上⊃ クメ.1⊃ 抱ト.⊥⊃ 畑メ.工⊃ ・●.'.■ 宙8メくつ て】 C> ,Hメて⊃ a) D「⊃ a) Hメて⊃ q) 帝¥r.⊂l (ロ 定4x5".エコ (ロ L 」 」 L 也) q) 也) 冢r也) 飛L 版」 ・.llJ -Z5 啌ハR粐2■・.■■ くつ ク韭r葛 ェZX爾逸■●・.■■ 【ウ +Rニツ■●- × -I ハR粨爾B■・..一 つく 也) ▼■■- n LL) 宙ヤr卜ー クイ▼■ 棉"∼ ⊂⊃ ネメ⊂⊃ ネメ⊂) ツ⊂) ネメ⊂〉 ツ⊂⊃ ⊂⊃ ネメ⊂⊃ ツ⊂⊃ ネ"⊂⊃ ツ▼■ N 寸 【⊃ I ▼■ ⊂) の ⊂) :工 :ど l l l l l 1⊃ 噸RメツKXツ4sメて⊃ 板ヨ,HメBcLi; U O L O Rツ 肇メR u) -C> ラ l4) O q) O Rヽ●■■■■ (ツ簫 (ロ 宙x_壁 辻C> ■-■ メ - B- 釘ツU 痴b q) L 呈q) 」 紐(q O 苓ヒ2 め .也) BU) 也) 冢rツl ⊂ 白ツ ⊂ 凪⊂ 滅2○ イ .≧ -5 辻モ8メ○ 爾⊂ (ト. ツ*粐 ー■. L- X 付 宙8メq 定sメくq ・■l・.■ ツ粨爾 :⊃ O 謄ィメR=⊃ U Bq) ≡ u) 4) B∽ q) ■て⊃ ,Hメ > 凪> メq) U) -剽 d) ■・.一 くつ L XツC> tl..一 くつ L 貞3",ツ 剴 I: .≡ ,Bツくじ 1⊃ ○ ≡ 凪ツイツ- 辻定4x5"テトツ ∽ B ∽ ○ イ屯ツRリ," l l ⊂ ツ⊂ ツ上と U 2ャr逸.上と 0 停謫「.上と U .l= ・∼ 定ツR賁R粐J= ・一一 定sメ粨爾.上= ■一・一 價= ー 肇メ剃定爾.1= ■■■ l宕 l ネメネx爾⊂つ く上) 牝ィ,"イ⊂> ⊂⊃ ⊂) の テ*リ4r⊂> N ネメニネ,"X⊂) ⊂⊃ 0⊃ テトツ)ィ !o "0 披の ツー 綿x爾tt) 1 l 板"勺 ツO) 忠e2寸 リ,"め (王) l リ,"卜ー ツの Tl 冉リ"く工) Xク⊂> ▼■ .上と ,Hメ くJ O L. て) 」 q) (ロ .∫= 釘 ・■●・.■■ q _⊂) 亦 ∽ イ ∽ ○ " 一■■一 〇 綿ツメイ (8 ネ.メ*リ8メ _⊂l rO L 仲ツ ・●・..一 >く 友ツ鶻粨メ q) l l l ,Bb3RC2 8,b て⊃ 也) 」 ,B2■て) q) 」 BnrネB q) ■-■ 冢rRクq) ■■■■■ 冢rて⊃ q) 璽「 」 くつ 宙8メ(ロ 仲q) .・.■ 「元 TR Q) ■■一 q) くつ L■ ネツ■●■■ くつ 」 啌クネ-- B ■q) q) q) 一・・一 ツ ○ イ○ イ 紐 ∈ ・●■■ XハR粐≡ ・●-∫ ∈ ○ ツメ飛ツ5" (ロ 彦-而 リrxキツO ._e 冢r2 の くロ 紐*リ,"U) q B*リ,"i ○ 偃RイJ= i 爾 _l⊃ 也) 定8メ.エコ a) 呈=C> 屯3追 ⊂) 宙8メ ⊂ ツ⊂ ツ■l・.■ I 啌メsメ_C> 噸 -o 辻 宙4Rq) >ヽ ∽ 也) 召 u) 4) > i 4) l◆..- ツ*リメハRツ耳爾ツsメRツ ΔsRツウメ」メィメR粨爾メツ,ツツ⊥= く乃 ⊂ i O L .エ⊃ ≡ ≡ n O t●.J =I て⊃ Q) ⊂ q L l イイ u) q) 窒nrO L .∫⊃ BBB∽ q) ち 白イ ⊂ 飛O U 儖 飛飛 ィトR q L 由u) a) > 由¥rく乃 q> 窒uD A) ツB.▲= O ツめ ■...■ ⊂ O 凪O 凪> ) ー_ 畔○ 敦ツ也) U q) u) Rq) め BCq) 帆 Ba) の 冢r-tl.・.一 =⊃ .壁 R粐2O O

rE) L O *リ,"」 くつ 凪イし くで 凪イL (ロ *リ,"O ・●■■ ○ 定4x7"O u) q) 板ニニニニネ⊂) ∼ トツ

U 由め ツ∽ ツの B⊂ 肇メ耳爾> 宙8メI X q) 」 白メL 版L■ q) q) q) C> 板d> L d> q) ⊂ 冢rツq) ⊂ ツa) ⊂ ツq) ⊂ ツa) ⊂ q) 4) C> 砲C> 4) > 凪> 凪> 凪> 凪> ⊂ ○ ⊂ ツ⊂ ツ⊂ ツ⊂ ツ⊂ ツ⊂ ○ ⊂ J= ・■-∫ 白爾.= ■.・.一 定ツク.∫= 一・.一 偵メXハR闔「.▲= t●■■■■■■ 定3メX_⊂ ■一■ ⊂⊃ ツ⊂) ネメl∫つ 汎ツ⊂> ツ⊂) ツ⊂⊃ ネメ の n 剖B▼■ Wー X▼- ヤB寸 " LL) ∼ ツn 披▼■ ▼■■ WLL) ▼■ リ,"LO の Rメ LL) の ツ0 挽の LL) N 汎ツ寸 ヤB Cr) ヨ、BくD ヤB寸 LL) rO ツの テFツメ卜ヽ ツ 一■..一 唸ハR・●・.■ 唸ネ闔「・●■■ ハR粨爾■l..■■ 白メ■-■ 坪*リ,"-テ葛 の q J⊃ 」 くつ =コ (J u) q) > メツ瀞(メB2め -a) =I てフ ○ ⊂ ⊂ ≡ q) N め I q) 云 J⊃ q) 唸ツリr韭r-ィメ│rヤ,ツ,Hメ褪CXネネメ"メ馭 lZ5 宙峵く勺 リ,"くつ 苓ヒ2一元 03 B「コ q) リrー ⊂) 定sメ耳爾「Hリイ

の く屯 .上⊃ B58メu) rB .上⊃ 由G2∽ CO .上⊃ 紐,X8メ8メu) fE5 .1⊃ 紐'Bネメめ rQ .上⊃ 粐リL q) ■・.■■ RイハR

.S2 」 .⊂ 飛モメツU '= .∫= モメメO L .∫= 飛ツツ.S2 」 .I 飛ツU L J= 鈴{「」メ03 I 窒リメナツツの .エコ eO L ・.●.一一 >く 忠?Rxメ$ツRハR粐メ

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32 Bulletin of the Institute of Natural Sciences, Senshu University No.38

Fig. 2 Photomicrographs of thin sections of recovered volcanic rocks from the Manihiki plateau. All photos are

taken under plane polarized light. (A) KH O301-D2-001. (B) KH O301-D3-001. (C) KH O301-D3-002.

(D) KH O3011D3-005. (E) KH O301-D4-003. (F) Pyroxene phenocrysts in basalt KH O301-D3-001. Dashed lines indicate trace of osci"atory zonlng. Circles indicate analyzed position by EPMA with Mg#

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Data report: Petrographic, petrological, and mineralogical characteristics of basalts from the Manihiki plateau, South Pacific Ocean SS

Phenocrysts mostly consist of pyroxene, and microphenocrysts (<0.5 mm) of plagioclase are sparsely

scattered.

KH O301lD3-002 (Fig. 2 C) : Highly altered aphyric basalt with intersertal texture.

KH O301ID31005 (Fig. 2 D) : Altered aphyric basalt with intersertal texture. Plagioclase grains (<1

mm) are acicular.

KHO3011D41003(Fig. 2E): Highly altered volcanic rock. It seems to be volcanic rocks with

intersertal texture containing olivine and plagioclase aggregates orlglnally.

Whole-rock malOr element compositions

Five KH O301 Leg 5 samples were selected for X-ray fluorescence (XRF) analysis of the major

elements, plus Ni, Co, and Cr, at the Ocean Research Institute, University of Tokyo uslng a Rigaku 3270 XRF. After removing ferromanganese crusts, the samples were coarse-crushed. Chips with filled

vesicles and other visibly highly altered areas were removed prlOr tO bead preparation and

measurement fわllowing methods described in Haraguchi et al. (2003). Powdered sample were dried at llOo C fわr about 24 h and weighed a氏er complete cooling to obtain H20. The dried samples were then ignited at 9500 C fわr about 6 h and weighed to obtain loss on ignition (Lot). Fused disks were prepared with a lithium tetraborate flux (Li2B407) in a dilution ratio of 1:10.

Whole-rock major element chemical compositions are listed in Table 3.

Sample D2-001 is sub-alkalic from its low alkali contents (Fig. 3). Althoughcompositions of

Tab一e 3. Whole-rock com ositions of vo一canic rocks from Manihiki

KHO301-Leg 5 (RⅣ Hakuh0-ma「u)

001    002    004

maior element oxide (wt%)

SiO2     50.69 TiO2      0.57 Al203    14.69 Fe203t MnO MgO CaO Na20 K20      0.32 P205      0.05 tota1     1 00.32 minor element (ppm) Co      53 Cr      25 Ni       75 H20 (%)  1.43   1.26 0.12    1.82

All Fe measured as Fe203t・

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Bulletin of the institute of Natural Sciences, Senshu Universlty No.38

Fig. 3 SiO 2-total alkalies diagram. Dashed line indicates boundary between alkaline and

sub-alkalien by Kuno (1966).

sample D41003 falls into alkaline, its potassium content has been eleva.ted by alteration process

because it has high weight H20 and loss on ignition (LOI) or 3.3 wt% and 4.8 wt%, respectively.

Basalts from site D3 are considered to be alkalic basalts.

Minera一 compositions

Mineral compositions were analyzed on a JEOL JXA-8900R Superprobe, equlpped with five wavelength dispersive spectrometers (WDS) at the Ocean Research Institute, University of Tokyo.

Silicate and oxide minerals were analyzed uslng a fbcused beam at 15 kV and 12 nA. All elements

were analyzed with a counting time or 10 seconds at peak position and 5 seconds at background.

Only sample KH O301ID3-001 Contains phenocrysts and microphenocrysts available for analysts. Another sample is aphyric and/or highly altered. Dominant mineral in sample KH 0301-D 3-001 is

clinopyroxene fわllowed by plagioclase and only small amount of spinel.

Clinopyroxene : Chemical compositions of pyroxene grains are listed in Table 4. Most of phenocrysts

and microphenocrysts have relative homogeneous compositions in each single grain. Mg# (=Mg/(Mg+ Fe2')x 100) ranges from 82 to 88, and averagely they have a composition Fs7En46Wo46, indicating that

they are classified into diopside. It is noteworthy that they are partly enriched in chromium (ca. >1.0

wt%), i.e. Cr-diopside. In the larger grains, however, have oscillatory and compositional zoning (e.g.,

Fig. 2 F). In this grain, inner part has low Mg# (ca. 82), while peripheral part has high Mg# (ca. 86).

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36 Bulletin of the Institute of Natural Sciences, Senshu Universlty No.38

processes from magma,

Plagioclase: Most of plagioclase grains were

altered. Analyzed phenocrysts and

microphenocrysts have compositions An50-70 Ab30-460r1-2 in average.

Spinel: Only small grains Of spinel occur in

grain boundary of glomeroporphyritic

assemblages. Their compositions are listed in Table 5.

Conc山ding remarks

Small pieces Of volcanic rocks from the

Manihiki plateau, one of Cretaceous Pacific

LIPs, Were investigated in terms of petrography,

petrology, and mineralogy. Although volcanic

rocks from the plateau were highly altered,

petrographic texture show variation; doleritic,

glomeroporphyritic, and aphyric. Volcanic rocks from dredge sites KHO3011D2 and KHO301-D4

are sub-alkalic basalts, where as those from

site KH 0301-D3 are alkalic basalt.

Phenocrysts and microphenocrysts fh)m sample

Table 5. Chemical com ositions ofs inel grains.

Cruise KHO301lLeg 5 Sample旧 Grain No. 些!塾 TiO2 Al203 FeO★ MgO Cr203 Total O=32 D3-001 86      65 Fe3+★ Fe2+★ Ti AI Mg Cr Calculated wt% FeO★ Fe203★ totaI Mg# Cr# Fe3+# 2.24 23.70 22.41 13.34 34.50 97.28 15.38   16.12 6.70     6.99 99.22    97.98 0.62     0.60 0.45     0.49 0.08     0.09 ★: Calcurated based on Droop (1987).

KH 0301-D3-001 (alkalic basalt) are available

for chemical analysts. Cr-diopside is dominant mineral phase followed by plagioclase and small

amounts of splnel. High chromium content in diopside is believed to become a key to understanding the orlgln Of the Manihiki magma

Only small amounts of samples were studied in this study. More samples enough to discuss the

variation of rock series and petrogenesis will be needed for the future study. Acknowledgements

The author expresses his thanks to the captain and crew of ⅣV Hakuh0-maru during KH O301-Leg 5 f♭r their excellent support of the research program. Discussions with Dr. Stephanie lngle and

Prof. Mike Coffin are much appreciated. Dr. Teruaki Ishii is appreciated for the help for petrologlCal

and mineraloglCal works. Analysts in this study was supported by "Cooperative Research f♭r Visiting

Scientists" program by Ocean Research Institute, University of Tokyo. This study was financially

supported by Grant-in-Aid of Senshu University fわr "Petrological Study of back-arc basin basalts and development of techniques fわr trace element analysis (2004)." GMT 3.0 (Wessel and Smith, 1998)

was used to map bathymetric data.

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Data report: Petrographic, petrological, andmineralogical characteristics of basalts from the Manihiki plateau, South Pacific Ocean S7

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