Geological and geochemical studies with Dolphin 3K

on North Hiyoshi seamount, Izu-Bonin-Mariana arc

Chih-Hsien SUN81 Robert J. STERN*1 Jh-o NAKA*2

Izumi SAKAMOTO*2 Makoto ARIMA*3

Shoshoniles (SiOs ≪ 55 to GO wi%; 100 = 3.0 lo 5.2 wi%) we recovered by R0V "Dolphin 3K'

on North Hiyoshi seamouiu (Dives 350 and 351) in the northernmost Mariana Aj-c. The major

mineial phase? in ihcse rocks are: olivine. clmopyroxene. and plagioclase with subordinate

sanidtne, magnetite, and biotile. These samples have elevated concentrations of lai-ge ion

lilhophile elements with respect lo high field strength elements. Light rare earth elements are

highly enriched, with <U/Yl ≫v = 13.0-K9. Their chemical characteristics and radiogenic (Sr-.

Nd. and Pb) is<*topic compositions are similar to other (avas from the Alkalic Volcano Province.

Glass inclusions in olivine and clino|≫yroxene are more felsic (SiOs = 61-74 wify and potassic

(KsO ･ 4 5-6.S wi%) 1 han the host lavas. Systematic valuations >≫ the compositions of glass

inclusions suggest that magma mixing controlled the petrogenesis of North HiyoShi magmas.

Keywords: fzu^Bomn-Manana, subducrion. shosliomie. Alkalic Volcano Province, glass inclusion

* I CenU'i foi Ulhosplu'rio Slmlios. Utiivorsily of 'IVxasal Pallas, I ISA

* 2 I)ocp S<*a Research itmom.Jai>an Marine*Soieiw -md 'IVclmoloyvCviuci.Jajwm

*'S (icolosical IhsUimU'. Yokohama National I'ninTsiiv.Japan

JAMSTEC J. Oeep Se* Res . J4 U!KW>

1. Introduction

Convergent margins arc The most complex regions In

plate tectonics. Petrogenetically, the complexity is

induced by interactions between encountering plates and

subducting material, and between mantle wedge and fluid

derived from dehydration of the subducted plate. Each

has important influences on magma compositions. To

better understand the importance of each factor, it is best

to study a simple arc system. "Andean-type" arcs, though

easily accessible and widely studied, do not satisfy the

requirement, because contamination of mantle-derived

magma by continental crust may obscure the effects of

other controlling factors. For this reason, studies of

convergent margin pedogenesis focus increasingly on

intraoceanic arcs, Intraoceanic arcs provide the simplest

setting due to their entirely oceanic nature. Studies of

intraoceanic arcs have mostly focused on subaerial

edifices, which represent only a minor portion of the arc

system. Submarine edifices, the less mature portion of

intraoceanic arc systems, are poorly understood. Detailed

studies on the submerged portions of intraoceanic arcs

should provide a better understanding of convergent

margin magmatic systems. Modern marine technology,

such as the ROV "Dolphin 31? provides a valuable too) to

study submarine volcanoes. This report concerns

application of ROV "Dolphin 3fC' to studying one such

submarine volcano.

The hu-Bonin-Mariana (IBM) arc in the western Pacific

is an outstanding example of an intJ*aoceanic arc (Figure

1A). The two plates controlling the arc system are

oceanic, so that possible continental crustal contamination

can be neglected North Hiyoshi seamount lies in the

northernmost part of Mariana arc (Figure IB), about 1.100

kin south of Tokyo. Geochemical and penological studies

of North Hiyoshi and other volcanoes of the Alkalic

Volcano Province (AVP) m the centra! IBM arc. including*

Iwo Jima, Fukutoku-oka-nobu. Fukutoku seamount.

Central Hiyoshi. South Hiyoshi, and Ko-Hiyoshi show

chemical characteristics different from lavas of most IBM

volcanoes, which are tholehtic or calc-alkalic (Bloomer et

al.. 19R9b; Jin et al, 1989). The most striking difference of

AVP lav≪s arc their elevated Mi. and U*KK contents,

which show shoshonitie affinities.

The origin of shoshoniiic magmas is controversial.

MO

Attempts to interpret the origin of these melts in the IBM

Arc include, for instance, source mixing with an OlB'like

enriched mantle ([.in et al.. 1989), and involvement of

subduction components in the mantle wedge (Lin. 1992).

Stem et al. (1988) argued that propagating rifting of the

Mariana Trough backarc basin unroofed an enriched

region at the top of asthenosphere and triggered

shoshonitic volcanism. In this contribution we present

new geochemical and penological data for lavas collected

with ROV "Dolphin 3K' and use these to interpret the

roagmatic evolution of North Hiyoshi volcano.

2, Sampling and Analytical Methods

Observation and sampling was conducted on Nov.l,

1997 by the unmanned ROV "Dolphin 3K ＼ and mother

ship R/V "Natsushimo'. lis capability of underwater

operation noi only has yielded valuable samples, bui also

allows us to investigate the morphology and volcanology

of the submarine volcano. Two dives, Dives 350 and 351.

were performed before rough sea associated with a

typhoon halted operations.

Lava samples were sawed to remove Mn-coated and

altered outer rinds and then slabbed

Each slab was

cleaned in IN HCI for one hour and in de-ionized

water for

a half hour in ultrasonic bath. Sample powders were

prepared for major element analyses in a tungsten carbide

canister; for trace element and isotope compositions in an

alumina canister to avoid Ta and Nb contamination. All

microprobe sections wei*e doubly polished.

Major and trace element (including KEE) analyses were

carried out at the University of Kansas by ICP-ARS

(lnductively≪coupled plasma atomic emission speetrometiy)

and ICP-MS (Inductivcly≫coupled plasma mass spectro*

metry). respectively. Sample preparation for major

elemeni chemical analysis used 30 nig of powder and a

standard procedure, fusion with Uihium inelaborate

(UBOs) in a graphite ciuciblc, quenching and dissolution

in 0.1N HNO.i, and dilution to a total volume of 50 ml by

0.)N HNO:<. For trace element analysis, 100 mg of powder

was decomposed in HNO* and Hl ＼ evaporated, re-

dissolved completely in 4N HNOs. and diluted to a total

mass of 150g

Approximately 100 mg of powder was dissolved for

radiogenic isotopie compositions. Sr. Nd. and Pb were

JAMSTEC J. Ofiep Sea fies. 14{l<K)8)

rig. I (A) Tectonic setting of the l≫i≪Honin-M.tri≪oia arc in ihc western Pacific. Water depth less than 3kni is outlined by shaded partem

PjuIhhI ivctnnglc shows Hip location of Flpuro IB. (B) Bathymc-tnc map of Otc Alkslic Volcano Province g^VPl- Only on? volcano.

Minami Iwo Jii≫a, dor-> not show alkalic affinity. 'Itlis i"≪y> also shows (wo drwljfp tracks. D53 & W. on Noith Myosin by R/V Thomas

VtoMpsoH in tf>85 during cru*e Tr-192 (Bitumen <! a].. 19S&0. I5iv<? siles in this stu<ly I* wiiliiii die d!*Jie<l rectanylv d≪aik<t in

Figure 2 Maud names arc given in italics; oilier AVP edifices also sliown

separated by conventional ion*exehange techniques. Pb

was further purified by the single-bead technique

(Maiaon. 1986). S>＼ Nd-. and Pb isotopes were determined

using the Fitinigan MAT261 mullicotleetor mass

spectrometer at the University of Texas at Dallas (UTD).

Sr data ace fraction at ioii<o riveted to *RSr/ShSr = 0.1194

and noi inalixcd lo "'Sr/^Sr - 0.70800 for K&A SrCOs

standard. Five analyses of K&A Si CO * dining this study

yielded a mean *"Sr/ShSr of 0.70801? ± 0.000017 (total

range). M'*Nd/,1JNd are fractiomitioivcoiiecU'd to

M'vNcf/,,|*1N<l- 0.7^l<)0. Th<* mean ,nN<l/ ≪*≫Nd is

0.511857 s 0.000009 foi four measurements of ihel* Julia

JAMSTEC j. Deep Sea Ret., 14 (ISWRl

standard during this study- (Nd was calculated by using

theNd<i ≪otopicdataotPid'^al (19R9) forth?UJollaNd

standard and bulk earth to compute the bulk earth

'･^Nd/'^N'd value appropriate for the UTD lab. Pb

isotopes were analyzed

at 1.350 V and the ratios were

corrected for a 0.15% / amu thermal fraction at ion. Six

analyses of NBS 931 during ilus study gave a mean

ao≪p|,/20ipb. 16.930 a 0.004 (total range). 207Pb/-04Pb-

1 f>m* OM. ^Pb/^'Pb- 36.728 ± 0.020.

Minora) and glass inclusion compositions were

deu ≫i mined by using (he wavelength-dispersive sj^tcni of

(he JROl. .IX

A SfiOO eledron micro probe a l U'lIX An

i-n

accelerating voltage of 15 )<V and a beam curreni o/10 nA

was used The beam diameter varied with deferent targets.

Olivine and pyroxene were analyzed using a beam diameier

of 1 nm; feldspar and glass inclusions were analyzed using a

beam diameter of5ym to minimize Na≫loss.

3. ResuJts

3,1 General Geology

North Hiyoshi seamount is a dormant edifice composed

of one major peak and five subordinate peaks, with

summit depths ranging from less than 200 to almost 600m

(Figure 2), These volcanic peaks extend approximately

10km E-W from 141 °39'E to about 141'45'E. Such a

cross-chain arrangement is a notable feature in portions of

the Mariana arc (Bloomer et aL, 1989a), Several neighboring

volcanoes (e.g. Iwo Jima and Central Hiyoshi Knoll, around

North Hiyoshi Seamount) show similar structures.

The major peak (14JR43'E) and two subordinate peaks

tl2

were surveyed by Dives 350 and 351. The two track* were

separated by a distance of about 4 km and both traversed

NW-SE. 350 started at a depth of 522m on a small

depression between the major peak and a subordinate

peak to the north, and climbed to 216m along a distance of

about 2 km. The region surveyed is mostly covered by

lava flow or pyroclastic deposits {Figure 3A-C). The

un sorted grain size and irregular shapes o/ clasts imply

that they are near the source. Sediment cover is thin and

only found in some gentle-sloping regions or depressions.

Nearly all exposed rocks have a thin coating of Mn-oxide.

Five samples were obtained from 4 localities in this (live

(Table i).

Two subordinate peaks wem investigated by Dive 351.

The dive began at 641m and traversed across the

northwestern peak, passed across a 674m deep saddle,

and ended on the slope of the western subordinate peak.

The traversed area has a rugged surface covcrcd most

JAMSTCC J Peep Sea Res., n

Table. 1　 Ｈａｎ(lｓｐｅｃｉｍ剛dcs ｃriplionof ｓajllpleｓrｅｆｉ)ｖｅredby “Z)alμxiｓ 3K~ 350 ajld 351divcs.

Sample Mn-coating Comments3K350-1 film Porphyritic with plagioclase phenocrysts up to 1 cm long.

3K350-2 film

This rock may have been slightly altered. White interstitial

material can be seen between grains.

3K350-3A film

Vesicular rock; The dominant minerals, pyroxene and

plagioclase, can be seen easily on fresh surfaces. The outer

portion of this rock has been intensely altered

3K350-3B film More vesicular than 3k350-3A.

3K350-4 110 Vesicular and outer portion is altered. Subhedral pyroxenes

can be observed on fresh surfaces.

3K351-1 thick Volcanic tuff and biological remains cemented by Mn-oxide.3K351-2 no Andesitic lava. The rock is highly altered, but a few minerals

(biotite and clinopyroxene) remain unaltered.

3K351-3A3K.351-3B3K351-3C

thick Same as 3K351-1

ＪＡＭ ＳＴＥＣ Ｊ．Ｄｅｅｐ Ｓｅａ Ｒｅｓ．．1411! 〕Ｓ吋Ｍ?

likely by thin Mn-oxicle (Figure 3D). Five samples were

recovered, which mostly consist of Mnoxide-cemeuted

volcanic tuff and biological remains. The only lava sample

(3K3S1-2) was collected on the northwestern cone and is

heavily altered, 'Ihe slabby appearance of some outcrops

(Figure 3£~F) reflects the original surface of the

pyrodaslic deports.

Bathymetric data obtained by surveying before the dives

was compared with US NAVY SASS batliymetiy reported

by Bloomer ct al. (1989a). The earlier survey reported a

summit depth of 349m, whereas the R/V "Natsiishima"

survey indicates a summit depth of <200iri. Given that the

volcano has not einptcd in the recent past, the difference

indicates either a technical problem with the earlier sujvey

or a remarkable amount (>150 m) of tectonic uplift. We

prefer the fu*st explanation. A few narrow fissures were

observed during the dives (Figure 3E-P). No

hydrotherma) vents were found along the fissures

144JAMSTEC J Dw> Sea Res. 14 (UWK)

3.2 Petrography

Igneous rocks from the two dives have mostly

porphyritcc and vesicular textures, <ind

outer portions

exhibiting different degrees of alteration (Figure 4).

Two phenocryst assemblages were noted; plagioclasc *

clinopyroxene + K≫ feldspar (sanidine) in 3K350-1, and

plagioclase + clinopyroxene + olivine * biotite + magnetite

* K-fcldspar (sanidine) in 3K350-2, 3a. 3b. and 4.

Typical textures of these lavas are shown In Figure 5.

Plagioclase commonly occurs as glomerocrysts with

clinopyroxene. Grain stees range up to 1 cm. Lamellar

twinning is common in plagioclase: oscillatory zoning is

observed in a few coarser grains Due to alteration, some

plagiocla&e grains arc partly replaced by secondly phases.

Samdine occurs as phenocrysts in The ground mass

(Figure 5A). Single or simple twinning are the common

habits. Sanidine has not been reported previously in North

Hlyoslu lavas.

Biotite generally forms euhedral phenocrysts or

inclusions in plagioclase (Figure 5B). Olivine is homo-

geneous without optical zoning, and occurs as euhedral to

subhedral grains (figure 5C~D). Groundmass typically

consists of |>lagioclase microlites and glass.

Sample 3K351-2. the only lava sample recovered on

Dive 351, is intensely altered, with residual biotite and

clinopyroxene.

Brownish glass inclusions are found in a few olivine and

clinopyroxene phenocrysts These inclusions show

I'ig. 4 Sample plic>l<M. (A) I'rosli iwji i>h>rrilk amluwiv wiiJi IcMsimi ptieuoci y>>K ojh ≪i lotn itmj* (RMC) M<ki samples h.nv alioird

omo jwmloiah aiul Mn tkxklo cuais on tJioir surface. <1>) Typical Mn oxidf cmxi oji Nonli Hiyoslii iaimuuni. While iwrnons

ai'P biological li.tce*.

JAMSTEC ≪)･ Deep Sea Res , 14 O'isA)

Ktg. 5 Photomicrograph# of typical North Hiyoshi lavas collccietl clunng Dives 360 & 361. (A) Sample 3K- 350-1. (13)-(D) Sample

3K3S0 2. Flag ･ |i(agioclasc, Cyx ･ clinopyroxonc. 01" olivine.

variable textures even within an individual host mineral

Glass inclusions in olivine are spherical

to ellipiical

(Figure 6A & C), but those in pyroxene range

in shape

fi'Cin #i)hencHJ tr> veiy httgvteir fi^giirc £>33 Sr I)), 'Jln'ir

dimensions range* from 10 jjiti (o lOOym. Gas bubbles are

common in the inclusions. Th* diatncleis of these bubbles

are iclatlvely constant al about 113-20 ＼ufi-

3.3 Bulk rock compositions

Chemical (lata arc limbed lo igneous samples from Dive

'*550 l)jw 350 lavas ai*o iiuei'mediale in com pnsj lion (55 lo

60 StO; Table 'D and are diaiaUCi'izod by high

cmUtfiils. so lhal Ihe rocks He m Uic slioshoniU* field on a

SiO; fO<) diagram (HKiiro 7). All samples have

K<0/Nay()*) and aie approximmdy >≫imnled In silica,

willi up lu A ) molfof noim alive email/. Hie low Nj and

Mb

Cr concern rniions. atxl Mg# indicaic these samples are

highly fracJionatcd.

Previous studies subdivide lavas in ihc IBM arc inio iwo

majoi categories: cnlculkaline-lholeiilic and shosbonitic

series (Meijer. 197C; Siern. 1979; Dixon and Batiza. 19?9:

Meijer and Keagan, 1981. 1983: Dixon and Stem. 15)83.

Hole et <?/.. 1984; Slern ft at.. 1984: Slein el a I.. 198S:

Hloonicr et al 1989b; l,in ft oi, 1989). The alkalic affinities

of Dive 350 iavsis arc similai lo bill sligblly more felsic

ih ≪n i Ik* samples JVom TJ' U>2 1)53 reported by Bloomer

<tt af (1989b), The mafic samples recovered by 'IT 192

1)54 aiv nol ropresonled in ihc J?350 suiie.

Ugh I rare enrlb clement (l.RKK) enrichment 5k one ol

th<* lypkiil ch.ii'aclcriMic* of ,nv lavas Chondrh*--

uomiaUx^d Rl＼K gjiiUn'us iu 'be suing les display <c ≪i＼r

l,RKk mm hmi'iil with a slitfhUy nrgalive (> anomaly

JAMST6C J Oeci) Sea Res, 14

Pig (3 Scanning election photoflraplis of ^!as≪ inclusion*. .mil

lioa olivine six) woxeue. OlkVuic-hoftie<l gUs* inclusions (pliotos A And

R) commonly have sj>]i<?rical io subsphciiesil ≫hajjcs: while glass inclusions in clinopyroxeno liavc sulrtphertcal to irregular

forms. (A)&(C) Sample 3K -3S0 2. <I3K*(1>> Sample 3K 350 4 01 - Olivine: Cpx ≪ clino-pyroxwe: Mt ･ niagneiitc.

(Cc/Ce**0.9) (Figure SA). The (U/Yb)N values range

from 13 9 lo 14.9, comparable to previously documented

values of A VP lavas (8.5-16.7, Liu et al.. 1989). HRKK

patterns are nearly flat. which is a common chaiticleri sties

of all IBM arc lavas. Another important geochenueal

feature of North Hiyoshi lavas is their enrichment in lai*ge

ion lithophile (MI.) elements. As shown in N-MORU-

noDualized "Spidergram" (figure SB) > Norili Hiyoshi

lavas display strong enrichments in I.II, (sticli as Kb. Ha.

and Th) and Pb with respect to high field strength

elements (HFSE) They are also depleted in Nb> Ta. ami

Ti relative to adjacent elemenls on the spidergram.

"Sr/ ≫Sr of the sample* cluster wilhln a narrow nntge.

from 0.7c>3?^ to rt.7u3#f>. again overlapping the AVP field

(Figure J>A). Nd rsoloptc oomiiosiiion arc less radiogenic

JflMSTEC J. Oaep Sea Res.. 14 mm

than Those of the Choleiitic to ealc-alkahue scries and

mostly lie wlihiii the lower pari of the AVP field. Pb

isotopir compositions are similar lo other

AVP lavas

(Figure W)

3,4 Mineral and glass inclusion compositions

Chemical variations in Norlli Hiyoshi plienociysts ait

modest. Cfinopyroxene phenuoysls range in composition

from !･ W 043 Fsn to KmaWo.isFsw. Com position a)

zoning was not obsewed Our clinopyioxenc analyses aa-

similar to previously reported dala lor Norlh Hiyoshi

(Bloomer ct at.. 1989b). Olivines also show a fairly

uniform compositions of Fo;a, somewhai less magncston

Ih≪m reported for IT 192 l)f>4 basnlis. I'lagioclaso

aiv commmilv zoned. The variations of compos)lion

147

MH

Table. 2 Chemical anil ≫soiopic analyses of "Dolphin 2fC 3S0 dive samples.

TiOj

Al,0,

FejOs*

MnO

CaO

MgO

Na?0

K30

P,0

Sum

UO.I

Mg*

Rb (ppm)

Sr

Ba

Zr

Y

La

Ce

Pr

Nd

Sm

Eu

Gcf

Tb

Dy

Ho

£r

Yb

Lu

Hf

Nb

Ta

Pb

Sc

V

Cr

Co

Ni

Cu

7n

Ca

Cs

Th

U

Cc/Ce*

MSr/"$r

lfl,Nd/M,N<3

£Nd

2MPt/ °*Pb

≫≫Pb/"4Pb

0.5017.355.010.144.471.894.1752!0.22

99.320.91

0.45

149

79478124126.755.995.010,636.16.271.835.090.854.570.852.422.620.435.4015.90.8514.61091145 }41096.25J7 A61.317,12.53252

7 6414.4092

0.703740.512762.38

38 SOI15.62518.973

0.7017.427.390.166.Z 83.253.644.060.40

100.150.33

0.49

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0 703830.512803 14

38 82715.64819.022

0.6817.48

7.01O.IS7.24

3.4$3.55

3.950.3599.020.50

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78224528.054.497,411.441.47,442.056.180.9?4,900.932.522.57OA I5.2515.40.7910.917.5

21316.7121.714459972 5P.O

＼ 9017.95.3514.30.92

0 703850.512803.16

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0.6817.557.180.157.0*

3.421& ＼4,03

0.3799.780.43

0.51

1131056

77424128.454.497.911.541.57.632.056.500.995.090.962.5C2.620.425.2214 80.7210.418.222412.5822 814069075 217.51.82

17.65.1714.00.92

0.703840.512813.28

38,83515.62919.004

0.6717667.170.157.12

3 243.573.02

0 3299 590.61

0 50

1151057

76324726.95J.291.110.558.06.981.975.950.924.710.892.422.490.405.2514.90.7310.517.91899.4520 012.166.170.116.61.8817.65.151390 92

0 703820.512803.06

32.87715645IV 01 ?

JAMSTEC J Oeen Sea Res. 14 cJWW

Fir. 7 SiOa-KzO plot for whole rock (open clrdcs). ddjustctl olivme≪hostc>d (shaded squares) an<l clinopyroxene-hosted glass

inclusions (sj>o(te<l diamonds). compared 10 other IBM air lavas, lavas from TT 152 D53 and D54 are encuded by stippled

lines Data Tor AJkalic Volcano Province (including Pukuioku-okA-no-ba) lavas are taken from Sun and Stem (1998) and Suri el al

(190S): <la!,i for other IBM arc lavas ai*e from IJIovhkt el til. (1980b) and Un el ≪/. (19S9). Adjusted compositions of olivine-

hosted glass inclusions is discussed in text. All gl.iss inclusion analyse* are nui tnalized lo 100

between core and rink in a single grain can be as much as

10 mole % An. The plagioclase analyses in Table 3 range

from An73 10 An??, in the middle of (he broad range

defined by TT 192 D53 and D54 (Answn).

Microprobe analyses of glass inclusions are presented

in Table 3. Hie results show ihal the inclusions are more

fclsic (Si02 = 61―74 w(% ) titan (he whole rocks and are

more felsic than any AVP lava. These are classified as

shoshonites and banakites on the SiCb K2O diagram

(Figure 7). These inclusions are characterized by high

K>0 content and fo()/Na*0 ratio but low Mj*0 and CaO

contents. Normative compositions indicate these glasses

arc strongly oversalu rated in silica (5 1-42.3 mole%

quartz). All filusS inclusions aie highly (Yactionntcd. with

(Vfg# < 51 Pyroxene-hosted glass inclusions, as a whok*.

JAMSTEC J. Oeeo $*a R≪s . 14 (＼<m)

have higher SiOs than olivine-hosted ones. Glass

inclusions napped in olivine have higher CaO and MgO,

*nd generally lower T10* and AJsO*.

4. Discussion

One of the main contributions of this research is the

recognition of evolved alkalic glass inclusions within

olivines and pyroxenes in North Hiyoshi lavas.

Glass inclusions {01) are known lo provide useful

records of iritigina evolution (Anderson. 1Q76; Donaldson

find Brown. 1977). Glass inclusions in pbcnoerysis show

vaiions compositions, which are observed not only in

different host minerals but also m the same host pliases

from dilleivm samples. The variability has two possible

explanations: (I) Gl coniposilions weie modified by (>t≫st≪

)■!!)

Fig. 8 (A) N-MORB-normalized rare earth element (REE) patterns North Iliyoshi lavas have highly enriched LRF.fi and flal UREE

patterns similar to those of Alkalic Volcano Province lavas. The dominant IBM cale-alkalic anit ihtoleiitic lavas are much less

enriched in LR£B, Data for Alkalic Volcano Province arc from Sun anil Stern (unpublished dala); for Average IBM arc lavas are

from LSn et ai (1989), Elltnu el ui (1997), and Pc-ate and Pearcc (1998). (B) Flemcni comimtiWliiy diagiam. Data for Alkalic

Volcano Province and Average IBM arc lavas are compiled /rom Lin et at. (1989), Btoomer et al. (1989W. Elliott et ai (1997), Sun

H cl. (199&). Peate and Pearce (1998), Sun d ai (1998) and Sun and Stem (unpublished dauh

entrapment crystallization; or (2) GI reflect melt

compositions captured during different stages of magma

evolution. A handful of articles discuss (he chemical

modification of glass inclusions by post-entrapment

crystallisation (e.g. Dnngan and Rhodes, 1978; Kamenctsky

et al., 1995; Lee and Stern. 1998). The possibility of post-

entrapment crystallisation for Gf in olivine can be

assessed by calculating the Fe- Mg Kj> (Kp - (Fed/Mgoi)

/(Feii<i/Mgiiri) | of host mineral-glass inclusion pairs

(apparent Kjj) and comparing this with the value of 0.30

expected at equilibrium (Roeder and Emslie, 1970).

Olivine hosted glass inclusions in caloalkaline and

th ol eii tic lavas of (he IBM arc have apparen i Kus of 0 14 to

0.20 (Ice and Stern, 1998). Apparent K)>s of North Hiyoshi

glass Inclusions i tinge from 0.10 to 0.21, indicating significant

<H≫)nciH u'ysiaUisalion. The- compositions of olivjnc-

hosied glass inclusions in this study werf adjusted to the

ISO

equilibrium Kn value of 0.30 (Roeder and Emslie, 1970) by

adding 2 - '3% of olivine. This indicates thai post-

entrapment crystallization is a minor effect. The corrected

results are tabulated along w[(h the raw data in Table 3.

Connection for post-entrapment ciystullization in pyroxene-

hosted glass inclusions is currently not feasible clue to a

lack of procedure for evaluating equilibrium between

melt-clinopyroxene pair*. However, the effect of post*

entrapment crystallization in clinopyroxcne is not

negligible. 'Hie decrease of CaO/AlsOs in dinopyroxene-

hosted inclusions may be due to post ≪entra)>ment

crystallization of di no pyroxene Notably. Che significant

differences in Na and total oxides between olivine-hosted

and pyroxene-hosted melt inclusions probably resulted

from Na-loss during micro pi obt> analyses, especially for

(hose analyses with low total (<9? wt%)

JAMSTEC J Oe≪f> Sea Res., 14

I;kfi. 0 (A) fNd versus 8?Sr/*6Sr diagram showing lhr> North Hiyoshi data of this study relative to other IBM arc lavas. The data arc

from: Alkalic Volcano Province (Sun et aL 1998 ; Sun and Stem. uniHiblishcd (lata):

Oiher IBM arc lavas (Di≪ni and Sicni, 1083:

Stem and Bibec. 1981. Woodhead and Prater. 19S5: Uo and Stern, im Woodhead. 19S9; Un ct al. 1990). (B) Pb isotopic

compositions of Nonh Hiyoshi lavas compared to the oilier IBM arc bvas. The field for AJkalic Volcano Province (Sun et oi, in

press; Sun and Sscrn, unpublished data) is shown, along with Uto field for other IBM arc lavns (Mcijer. 197& Woodhead el al,

1987: Slern el al. 1993: Elliou et qL 199?).

Although glass inclusions have up to 14% more Si02

than the host lavas, they define a relatively continuous

trend with the whole rock data on a Si02-K*0 diagram

(Figure 7). A subset of the most fclsie glass inclusions fall

off the trend of increasing KiO and SrC? and fie in tfie

h'gh-K calcalkaline field It is noteworthy that these

anomalous Gl all are clinopyroxene-hosted G ＼ in sample

3K350-1.

The continuous trend among host rocks and glass

inclusions might result fi-oni factional crystalljnation. To

evaluate this, major elements and CaO/AhOs mtios of CU

<11* plotted in Harker dtagianis along with the whole rocks

(figure 10). Al'Ox C.tO. MgO, and FeO* inversely

correlate wah SiO:'. sn^^siine magmatic evolution was

controlled by fractional cryatafHstaiion. Olivinc-liostcd

compositions plot between bulk

rocks and clmopyroxeno-

hosted inclusions. As crystallisation proceeded. j| depleted

JAMST6C J Deep Sea ≪･≫.. H <!≪*>

such elements as Mg, Fe, Ca by crystallizing olivine and

clioopyroxene, Na was lately consumed by plagioclase

accumulation.

Felsic melts In the IBM arc are not very common. Host

lavas and glass Inclusions define a cafc-afkaline trend

toward high total alkali in AFM diagram (Fig 11),

Illustrating magma evolution. Lee et al

(1995) and l-ee

and Stern (1998) argued that felsic tephra and glass

inclusion hi Mariana Arc may be a result of li-actionation

from mafic melts. Meen ct al, (1998) argued that North

Hiyoshi lavas evolved by fractional crystallisation along

the olivineplagioclase-augite thermal divide. Norm

compositions of lavas and glass inclusions in Dive 350 are

piojected from Di onto a 01 Plag Qz/Ne Quaiternajy

diagram (Ilgiuv 12), Host lavas are quana-saturated and

basielly fall on ihe thermal divide, indicating fractional

crystallization of phcuocrysi phases of o!iv|ne≫nugito

tSl

152

Table. 3 Representative mineral and glass inclusion compositions

(,) Raw analytical data.

{2> For calculation of Mg#, Fe2V(Fe2* + Fe3* ) - 0.9.

<3)01ivine-hosled glass inclusion compositions adjusted to Kd = 0.3,

where Kj> ■ [(Fc<n/Mgoi)/(Fenq/Mgnq)] (Roederand Emslie, 1970).,li} Raw analytical data.

JAMSTEC J. Oeep SeR Res., 14 (1D98)

Fig, 10 Marker diagrams showing relationships ber＼v≪en host rocks and glass inclusions. Whote rocks. adjusted oJivicie-hosted and

clmopyroxe/w-hosuxl glass inclusions are 1 epresenled by oi>et> circlcs, shaded squares. ajid sported diamonds, respectively.

plagioclase. Glass Inclusions, on the other hand, form a

trend from near (he Qz apex, through the Plag+Aug+low-

Ca Pyx. to the OI*Plag*Aug field. This trend is not

compatible with fractional crystallization of either

Plag+Aug+low-Ca Pyx or 01 -Plag-Aug alone. A plausible

explanation is that magma mixing, instead of fractional

ciystallization, create the liquid trend. An alkaline magma

mixed with low-K silicic magma may possibly generate a

serial fieri vant magma as shown in these North Hiyoshi

glass inclusions. Tlie alkaline endmember could be

potassic and sNiea-undersaturated, in order to pixxince

these alkalic Norih Hiyoshi lavas: while the felsic

endmember is most likely a subalkaline rhyolite Such a

process had been proposed by Meen et ai (in press) lo

interpret the* pedogenesis of Central and South Hiyoshi

lavjis. Fractional crystallization may be ihe dominant

process among the host lavas in North Hiyoshi. But.

evidence from (he glass inclusions implies that magma

mixing lias controlled th<* i*vokil5ou of Ihe whole magmsi

system beneath North Hiyoshi. as well ihe other

Hiyoshi volcanoes.

5. Conclusions

Lavas collected from North Hiyoshi seamount in (his

study are characterized by highly enriched R£E and L1L

contents. Their relatively rariwgerirc

Pb end $r isotapic

compositions and non-radiogenic

Nd aj* like those of

other AVP lavas.

Glass Inclusions within olivine and clinopvroxene are

more felsic than hosi lavas but illustrate a continuous

evolutionary trend The feature may be interpreted by

magma mixing between a fractional crysiiiUination of

olmne-pyroxenc-plagioclase-iriagiietile. This implies thai

magmaftc evolution ol N011I1 Hiyoshi lavas may bp mainly

con (rolled by lintiional crystallization

Acknowl ed ge ments

Wc are grateful to Uie captain and crew of the R/V

"NavsushinuT (01 iheir veiy professional effort? dining the

com*sc of this invcsiication. We ihank T. Plank, smd C Dan

Fig. 11 AFM diagram show* a calc-aJkatine fractionation trend ol North Hiyoshi whole rocks and jilass inclusions, compared to whole

rock compositions of AVPand other IBM arc lavas Data far AJkalic Volcano Province are taken from Sun a≫<l Sicm (1906): Sun

ei for other IBM arc lauas arc compiled from Bloomer el ai (1989b) ami Lin at (19S9). Symbols are same as tlial in

Figure 10.

Fig 12 Ol-Plag-Qfc/Ne quaternary diagram projected from Di. Host rocks and glass inclusions arc plotted and compared with lavas

from IT-192 DS3 & DS4 (Bloomer ef aLl9$9b). The daia of lavas and tflass inclusions approximately form a trajectory of

fractional crystallization. Ol-olivine. Plaff-plagioclase. Qz-quartT, N'c≫)Cpl)≪ii≫≪. Di≪d ioesitlc. Aug-augite. low-Ca Pyx-low C ≫

pyroxene.

for the help in ICP-AES and ICP- MS. and D. Draper for

technical assistance with microprobc analyses. The

p-icdtipaAwn Saat> arid Stern rrttde possible by a gram

from NOAA through ihc US-Japanese Submersible Science

Progi-jiri This is UTD Geoscicnces conlnhulion A89I

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JAMST6C J Deep Sea Res, 14 <)')<≫)