Mobility of major and trace elements during sulfuric

acid-rich hydrothermal alteration of basaltic

andesites from the Desmos caldera, Manus Basin.

Kaul GENA*1 Toshio MJZUTA*1 Daizo 1SHIYAMA81 Tesluro URABE*2

Fresh and altered basaltic andesites sampled from the Desmos caldera during the Manusflux

In 1995 and BIOACCESS cruises in 1996 were studied Jo determine the mincralogical and

geochemical changes thai may result from interaction of basaltic andesite with sulfuric acid-rich

hoi hydrothermal solution (pH; 2.1, temperature: 88 to 120*0).

The mineralogical changes observed in Hie altered rocks are dissolution and leaching of rock

forming miners of clinopyroxene ･ orthopyroxene ･ plagioclase s olivine and precipitation of

secondajy minerals of quarts, native sulfur, anhydrite and gypsum. On the basis of mineralogy

and degree of alteration, the altered rocks were separated into outer rim and core for chemical

analysis and microscopic studies T1>c <iuari2≪suifu>*rlcli outer rims and margins of altered

basaltic andesites shows the most chemical changes while the anliydrite-gypsum poor core show

small change in composition compared to fresh basaltic andesites.

The major elements of CaO, NajO, A1j03, MgO, KjO, Fc2Os, P^Oj. MnO and TiO? are lost in

the altered rocks. While Si02 is gained up to about 78.2 wt% and 57,7 wi% in the outer rim and

core, respectively. The average content of major elements in the fresh rocks are CaO -9 8 wt%,

Na20 -2.9 M%. Al203-I5.0 wt%, MgO -6.0 wt%, Kp -0,74 wt%, Pe^03-9.<1 wt%, MnO -0.14 vw%.

Ti02-0.61 wt%. ~ 014 wt% and SiO^ -55.34 wt%, The light rare earth elements

(LRHE) of la,

Ce. Nd, Sm, Eu and Tb are also lost with strong negative Ce and Eu anomaly in the altered rocks.

The significant lost of the major and trace elements in the altered rocks is quite different to what

has been observed for altered rocks from oceanic scafloor. The Si02 content in the altered rock

is gained up to 65 g/100 cm3 while CaO, Na^O, A]203. MgO, MnO and Fc2Os are lost. TiOs. P20$

and K^O are gained in some samples while it lost in others. Most of the REE in the altered

samples are significantly lost while the large ion lithophile elements, high field strength elements

and transition elements are either gained or lost.

The mechanism of alteration in the Desmos caldera is quite different to what has been

observed for altered rocks from mid-oceanic ridge system and back arc basin spading centers

Element* which are believed to depicted (SiO,.) are enriched while elements consider to be

cnriched (MgO. Fe203, CaO) or immobile (Al^.TiO^) are strongly depleted.

Keywords: Desinos caldera, Manus. dissolution, mid-oceanic ridge system, back arc basin

Spreading centers. RKE.

＊1　111 ｓlillltｃりrApplicdl 汕｢111S ぐicIlccｓ.Akila University

＊2(; ａ)IogicalS ｕｒｖｅy01Jap ｕll.Ｔｓｕkｕbａ

JAMSTEC Ｊ. Deep Sea Ｒｅｓ.，14（1愬8）1啣

1. INTRODUCTION

Geological evidences reveal that hydrothormal circu-

lation of cold bottom seawater is presently occurring

abng mid-oceanic ridges and back arc basin environment.

As a result of tins mechanism, the seawater interact with

basalt at elevated temperature to produce basalt of varying

metamorphic grade (Humphris and Thompson. 197?) and

metabasalt greenstone (Hckinlan and Aumento, 1973),

These observations has prompted a lot of workers to do

laboratory experiment (eg. Seyfried and Mottl, 1982)

where powdered basalt were reacted with seawater at

different temperature, pressure and water/rock ratios.

From these experiments it was observed that, seawater

components (MgO, H20) were removed and incorporated

into the secondary clay minerals (Seyfried and Mottl,

1982; Humphris and Thompson, 1978) whereas major

elements such as Si02 and CaO are leached from the

basalt while no trend was observed for Na20 and K^O

(Humphris and Thompson. 1978).

Manus basin in the eastern part of the Bismarck sea is

a back-arc bssin with respect to the currently aciivc New

Britian^rc trench system (Fig. 1A). The spreading* activity

and tectonic framework has been discussed by Taylor

(1979), Both et a)., (1986) and Martinez and Taylor (1996).

The hydrothcrmal activity, massive sulfide deposits and

associated chemosynthctic organisms of Vienna Wood

and Pacmanus site associated with this activity have been

documented by a lot of workers (eg. Both et a), 1986:

Tufar, 1990: Bimis and Scott, 1993; Scott and Binns, 1995

and GamoetaL 1993,1997).

During 15th October and 13th November, 1995, the

Japanese-French New Starmer Program carried out a

muftRf'scipffuary survey in the Vienna Wood, Pacmanus

and Desmos caldera using the submej*sible "Shinkal 6500"

(Auzende et aL 1995; Gamo et al., 1997; Gena et. al.. 1997).

A follow up cruise (BIOACCKSS) to the same sites was

undertaken in 1996 using the research vessel "Natsushima"

and submersible "Shinkai 2000" (Ohta et al.. 1997),

During these cruises a total of 11 dives were made in

(he iJcismos calderu (Fig. Itt) to explore for hydrothermal

activity and associated massive sulfide deposits. From

these dives, iho hydrolhormal activity discovered was hot

<SK-12 ≫C> and acidic (pH ≪2 1). ii was named Onsen

hydroiheJ i≪hI site (Claino H al.. I9')7j. The- Onsen site has

tH4

neither chimney nor sulfide mound as observed in the

nearby Vienna Wood and Pacmanus hydrothermnl fields.

The rocks around the Onsen site are hvdrothemially altered

and enriched in native sulfur, silica and clay minerals. The

hydro thermal fluids, fresh and altered rocks around the

Onsen hydrothermal site were sampled for farther on-land

studies. Part of the studies done on the altered rocks have

been reported by Gena et al.. (1997).

The purpose of this paper is to describe whether the

niincraOogical u*anstormalion> magnitude and diiection of

chemical changes across Ihe cores and outer rims of altered

basaltic andesite tinder sulfuric acid rich hydrothermal

solution is similar to altered rocks from mid-oceanic ridge

system and back -are basin spreading centers.

2. SAMPLE SELECTION AND ANALYTICAL

TECHNIQUES

The analytical data used in this paper represent a suite

of fresh .and h yd rot hernial altered basaltic andesite

collected from Che northwestern wall 0/ ihe Desmos

caldera during the Manusflux and BIOACCESS cruises.

The rocks were selected and analyzed for rruyor, minor

and ti*acc elements. Some of (he rock samples used in

this study were described by Gena etal.. (1997),

The altered rocks from the Onsen site shows a rela-

tively fresh core with minor secondary anhydrite and

gypsum crystals. But a highly altered outer rica with

native sulfur and silica showing the best evidence (hat (he

rocks have been exposed to alteration by the sulfuric acid-

rich hydro thermal fluid (Fig. 2A). Geochemical analysis

and mineralogical studies across the core and rim (Pig.

2B) of altered samples provide useful information to the

mineralogical transformation, and magnitude and

direction of geochemical changes thai has occurred due

(0 alteration. The fresh basaltic andesite from (he same

site have been also selected (0 determine the precui*sor

and (he tectonic seuitig using chemical daw.

The rock samples were cut and separated into outer

rim. core or bulk depending on the degree of alteration

About 500 milligram of each samples were placed inside a

stainless steel mortar and crushed using a stainless steel

pestle. The crushed samples wciv taken aiirl placed in an

electrical driven agate rotaiy (will mill and powdered for 20

minutes. Major awl minor elements wciv determined al

JAMST6C J Deep Sea Res, 14 09m

1θ5

JAMSTEC J. Deep Ｓｅａ Res.. 14 11911 ）

J;iB. I (A) lln≫ tfrnorul tmotiic ≪cliiMK vf Jl≫c Maim* back<uo ixism wuh loc.ittoii ol Wnn<i Wootl. I'.icmanits ≪>n<l ))o)"os 'Hto

Miiim* cnHcisiiii'jJ'iMli'il ･≫ W.Hk wlmli mi by ilac≪ Willamm'2 Dyaul and Welti" iiniisfoiTO fault (Modified livm

Kinnsvi ul.. ;mcl Cona ei ≪L <I*J An .H'rifi! viow oi tlio Ih'juiios oaWcra, x%lnly rtoayjakxl

in ilie noi'lli-nonH'vt.j direction

wiiltaofalxiw ≪ 21<m *1 hi* Onsen <iU' i> iiKlk.ncil wijli a si.ir ieo≫tmu* in HieMs)

/陌;

X-iK- 2 <A) A Mrtlktn of ihe altered sample showing ihe degree of leaching in tin' outer rim and tore. <Ti> Sertoli of i>hrtto≪rajjli A with

hydi-ot hernial i>i ecipuaic≪ of native sulfur diid amorphous silica comeming basafiic aade^iic elast.

JAMSTgC J. Deep Sea Res., 14 １１ｑＳ ）

Tsukuba and Aklta University by X-Ray Fluorescence

analysis using pressed pellet techniques. The trace

elements were determined by Induced Neutron Activation

Analysis (INAA) technique at Research Institute of

Reactor, Kyoto University. The accuracy of the trace

elements was monitored by concurrent analysis of

standard rocks from Geological Survey of Japan. The

concentration of major, minor and trace elements are

reported in Table 1.

3. BJILK CHEMICAL COMPOSITION

3.1 Major elements

Based on the geochemical data of the fresh volcanic

rocks, the suiie of lava from the Desmos caldera belongs

to fhe volcanic arc ba$ah I)7?e which discriminates

it from the mid-oceanic ridge bas-ali (MORB) and within

plate basalt (WPB) type. The volcanic rocks plot towards

the calcalkaKne end-member discriminating it from the

tholeiitic basalt end-mcniber and high K calc-alkaline

Tabic 1 Chemical composition in fresh and altered basaltic andesite from the Desmos caldcra. Manus Basin.

Den$ii> 2.01 1.54 2 0!

Hcte. #P!& dive R ■ rock(t): Tola) iron as Pc?0,n d.; roi detected

JAMSTEC J. Deep Sea Ses , 14 uwwj

Fresh Samples

98.81 101.1? 100.9? 100.85 99.14 98.28 98.49 100.06

2.07

107

series (Fig. 3A). On the KjO vs. Si02 diagram (Fig. 3B)

the lavas from the Desmos caldera plot in ihe basaltic

andesite field. The low KgO content of these suit of lava

discriminate them from basalt and andesite from the

nearby Pacmanus field (Binns and Scott, 1995).

S1O2 (Wi%>

K.O vs silica diagram for iii<- suMivWon of sobalka)in<?locks. Tti ̂IY<*sh sample (rod) tho pl ≪i in the

enriched in seawater component such as MgO. Figure 4

shows the relationship between concentrations of MgO

and other major elements in the fresh and altered rock

samples. The de-line indicates (he same sample separated

into core and outer rim depending on the degree of

alteration. The outer rims and cores of the altered rocks

have very low MgO contents compared to the fresh

samples. The trend of increasing silica and decreasing

AI2Os, CaO, Na20, ICgO, MnO. P20^. Ti02, Fe2Os< MgO in

the rims and core of the altered basaltic andesite can be

clearly observed on Fig. 4. It appears that the altered

samples are strongly oxidixed compared to the fresh

samples collected around the Onsen site. The enrichment

of silica up to 78 wi% is the highest observed in normal

submarine settings

3.2 Trace elements

The elemeats of the. *Atere4 basaltic andesta wec^

partly discussed by Gena ct al. (1997). In this paper, the

rare earth elements of both fresh and altered rocks will be

discussed. The chondiitc-normalized values for the fresh

and altered rocks are plotted in Fig. 5A. From Fig. SA (not

labeled), it can be observed that, the rare earth elements

(REE) In the fresh rocks have a linear trend indicating

that, there is either light nor heavy REE fractionation.

The REE are very stable and resistant to alteration

(Pcarcc, l£83). However, the REE in (he outer rini and core

of the altered rocks (Fig. SA) are strongly depleted

compared to the fresh samples. On the chondrite*

normalized diagram, the i*are earth elements are strongly

depleted with a strong negative Eu and Ce anomaly. These

shows that the I.REG in the altered samples are highly

mobile compared to the LREE concentration in fresh

samples (Fig. SA}. The HREE are alto because

the concentration of the HREG plot below the HREE

content in the fresh samples (Fig. 5A). The depletion of the

RGG in the altered samples indicated that the REE are

mobile during water-rock interaction at very low pH. The

concentration of REG in the altered rocks were divided by

the fresh i*ock (#298 R 01) and their ratio are plotted on

l-ig SB The ratios ol the RGG dements fall below the

ratio line of 1 (dashed) with profound negative Ce and Ku

anomaly illustrating that the KKK are l"tally lost m the

altered rocks, if (he RhJC ai*e immobile in Oie altered rocks

JAMST6C J Deep Sea Res., 14 OWW

(15; Hoi of silica a≪ai≫Kl K./) resliii'i iM' i-ocks in 1I10

bd^allic

')1ir hulk chemical composition in the altered and fresh

rock samples w<* refx>ilcd In Table I. Altered cocks from

sojiwakT supersaturatect enviionnH'Pt ≪tre normally

/OH

Correlation between MgO ami Si03. Ti03. FczOit <0. MnO. CaO. Na20. It/). P,0S. conlenls of fresh and hydroihermally

altered basaltic andesiies. Triangles *rc fi≪h basaltic audesitcs. g|ien circle arc altered basaliic andesites while the ue Hues

represent the same samples sepaiatcd into cores and onto i hits.

all the REE will plot on the ratio line of 1 indicating That

the concentration of REE remain constant befoif and after

alteration However. the ratios fall beiow the ratio of one

indicating that ail of REE are depleted in the altered.

3.3 Efement Fluxes

The bulk chcmical composition of the rocks (Table

Dwere used to calculate ihe chemical gains and losses in

(lie core and nm of altered rocks during hyd rot hernial

aHcraiioii. Nonnalmng to constaul aluminum and ticanumi

(Humphris ami lliompson. 1978a) can not be used for (he

JAMST6C J Deep Sea 14 Wl

Desmos samples because aluminum and

titanium fire

in die rocks. Vteiebr^, the Ruxes OS rhe

major, minor and tracc elements were calculated

by

normalizing them lo constant Yb (ppm) and comparing

the composition of the balk, core and lim of the altered

samples to the fresh sample (#29$ R 02A). The gains

and looses arc reported in g/lOOcc and ;ig/100cc for ihe

major elements and trace elements respectively as shown

in equations 1 and 2. 'Hie density measurements, used in

I he gain and loss calculation ;irc reported on (he bottom of

T: ≫bk≫ j. deusjly wciv usivl U> rolrutoi^ ;J>e

lot/

≫ V|#> ≫

(A) Chondriic norina|5*e<l diagram for (resh am! allied

Wsallic afl<lesite show (Hal the REE in (he altered rocks arc

sirongly depleted with negative Cc ami Eu anomaly (labeled

w|t)i sample number). The REE content in the fi'csli samples

where C. i, a. p, f, represent concentration, element, altered,

density and fresh respectively. The factor 100 is applied in

trace element calculation to convert the concentration in ppm

to microgram Table 2 shows the chemical gain And loss

during alteration of the basaltic andesite. The major

elements composition of the fresh basaltic andesite

samples does not vary much. Therefore, the original rock

composition was based on fresh samples collected from

the Onsen site (#298-R-02 A, Gena et al., 1997).

From Table 2, it can be noted that most of the jnajor

elements in altered rocks are strongly depleted compared

to the fresh basaltic andesite (#298-R-02A). The silica

content in the altered rocks are strongly enriched up to

about 65.2 g/100 cm* compared to the fresh sample (#298

- R- 02A. The silica gain of 65.2 g/100 cm3 is probably one

of the highest for submarine condition. Humphris and

Thompson (1978b) mentioned that SiO^ Fe203, CaO are

lost in the altered samples collected at Mid-Atlantic ridge

whereas elements like MgO, Ti02, FeO are gained. The

A1203 and CaO in the altered samples from Desmos caldera

tend to loss up to 6.7 and 10.2 g/100 cm3 respectively.

Na20, CaO, Al2Os. MgO and Fe203(t) tend to be lost in all

the samples irrespective of outer rim or core of the altered

samples. However, Ti02 and P205 tend to gain in (he

altered rocks. The precision of T102 and P205 gained is

about 0.07 to 0.15 and 0.02 to 0.04 g/100 cm3 respectively,

which is ± 10% of concentration in fresh sample (#298-R

02A) so the gain is minor. The changcs in gains or losses

of the major elements in the core and rim of altered

samples are plotted on Fig. CA. Most of the major elements

in both the core and rim of the altered samples arc

significantly lost. However, silica is strongly gained in (he

rims and cores.

Hie gains and losses for the REE elements are plotted

in Fig. 6B. It can be obseived from these bar graphs that,

the RKE elements in the outer rim of the altered samples

are totally lost. LREE of Ce and Ru lost up to fil5 and 62

pg/100 em*, respectively. The other I.REE of La. Nd and

Sen are also lost in the outer rims while it is gained up to

about 430 ng/l(H) cm1 in some (-ores Hie magnitude ol

HRKK lost in the cores arp <|uit<* small compared to (he

outer rims. This is p:irltally due i<≫ immobility of HRKF

compared to LRKF under similar conditions.

The othfi* irace elements <l$a. (X Kb. Sr. Tit. U. Hf. Ta.

JAMSTEC J Deep Sea Res., T4 (JW*>

upper trend) (ft). The ratios of Altered samples/fiosrt

sample (0298 It 02) ptoi lictow rlic ralio line ol one (clash)

incltcaicd ilia) the REE aj≪ si) cungly depleted in the altered

basaltic amlcsite

gain or loss intenns grains of oxide per 100 cin;i of jxk1<

using the formulae:

Gain/loss = [{C*. * ,*) (Cf, * fi* * * *')/CyiJ x/?1)]

S/lOOcc. (I>

For lb<* (race elements. lb<* and loss wot calculated

usin≪

Gain/loss ≫ ＼(C-＼ *//･'* i(J0> (C', x/ZflDO xC,'Yh *

*//Hug/1 Wee (2)

//tf

Table 2 Gains (･≫) or losses (･) during: hydrotfiernwl slieralion of basaltic andesite assuming constant Yb (major elements "g/iGC em3,

irace clemenU-ang/lQC cm*).

Sc. Cr, Co, Ni. Zn, Cu, Zn) are either gained or lost in the

core and outer rim as shown on Fig. 6C and D. The

magnitude of trace elements gained is quite small

irrespective 0/ rims and cores compared to the major and

RKli elements lost in the outer rims and cores.

4. DISCUSSION

4.1 Major elements

Generally, ahcred basalt front mid-oceanic ridge ancl

back-arc basin spreading centers show that. SiO, and CaO

are leached while MgO and H.>0 arc taken up. There was

no general trend obsctved from Na.,0 and K/> although il

was believed that they display some variation on the rim

and core analysis of allured basalt. (Humpliris and

Thompson. iy?fri) Soy fried and Mottl (1982). obsmvd

that Mg i< i*cmt≫vr<i fn>m <eawater and incorporated into

che clay minerals. At higher temperature (3001^). they

･spoiled that, Na. Ca, K. Ba, Mn, Cu and 2n ai* leached

from the silkatcs.

The altered rocks froiri iho Onsen site show an extreme

enrichment m Si02 but depletion in CnO. M^O. MnO.

AI^Oj. Na,0. K20. Fe203. TUXand PjOr>. Tlie depletion of

the major elemenis and enrichment of silica indicate that, the

chemist iy of fluid responsible for the alteration is slightly

iibtHn nut! wnfyxivd 10 ilx* fluids ihal <?/e n'spoiis/bh for

aHcmtion of basalt dredged from mid-occauic ridge systems

01* back sue basin spreading cenlci*s. The weakly acidic to

mid-pi I fluids fiom MOR systems and bsick-siro basin

spreading centers are vei*y hoi. "Hie only cxplsnialion foi

depletion of tnsijoi elements in the t>esmos samples is due

u> the voi*y low pH of 2.1. The temperalui'es of 8S 120'C

for hyd roi hernial solutions in the Desinos c^klera is low

III

for the depletion.

The results of this study indicates that, all the major

elements except silica ure depleted in the altered rocks.

The depletion of these major elements can be attributed to

the vej-y low pH and hot hydrothermal solutions which

dissolves all kinds of rock forming minerals resulting in

mobility of the major elements. The major rock forming

minerals of plagioclase, pyroxene and olivine dissolve very

easily in acidic conditions (Gena et tl, 1997; Oelkers and

Schott, 1995: Wogelius and Walther. 1991). Also other

workers (eg. Brady and Walther.1989). mentioned the pH-

dependence protonation on aluminum, magnesium and

Alteon sites in feldspar and olivine. The low pH hydrothenria)

fluid at the Onsen site will play a dominant role in the

protonation of the respective elements and its mobility.

Both outer rims and cores of the altered rocks gain

significant quantities of SiO? due to the precipitation of

secondary quartz and amorphous silica. Since the amount

of silica in both the core and rim are strongly enriched up

la 7? v'1% compared to the fresh sample (55 wt%). ≪n

additional source of silica is necessary and this could be

contributed by a fluid that has undergone previous reaction

where silica is leached and transported as aqueous silica

(HjSiO^). Upon quenching of the hydro-thermal fluid by

cold bottom seawater, the silica are deposited as amorphous

silica on the surface of the altered rocks. Associated with

amorphous silica are native sulfur, mavcasite and pyiite.

The center of the present hydrethermal activity can be

cl&ttttfted as a silica sinter because the altered rocks

collected from the Onsen site has high silica content

compared to the fresh basaltic andesites. The enrichment

of silica is caused by dissolution and leaching of th≪? rocks

by the hot acidic hydrotheimal fluid leaving behind a

silica residue. (Jena ct al ,(3998) reported high sulfidatio"

tyfle of mineral Assemblages of WTOphyltttc. atuniH-.

<iuartz, cristobaliie, muscovite. pyrite. enargite and

covolllite in altered samples collected 100 meters south of

the present hydrothermal activity.

4 2 Rare earth elements

'Hie U*KV. Yemenis concern ran on in the ouu ≫r rim of

lhe aUer^l rocks art* vr ＼y low (eg. U - 2.8 |>]>J≪. CV = tU)

PIHH. Sm = 0.9 ppm) in ())<' coros siikI fresh lock

≪tfim|)lr≪s (fff Isi = f>.f) ppm. (V -- !<>.<> i>pm. Sm - 2.4 pptn).

JAMSTEC J Deep Sea *es MOW

compared i<> ink)･oceanic ridge ftyslem or b≫ck-aiv basin

*;pjv<idinir ccnlerv IJowevr. (hose low iomi><*raunvA c<m

play a minor rok* in lh≪ Heincni mobility and can aocoum

il≪. (≫ (A). Miw.≪ of liic en iijor cIcrDrnis jiv l<>sJ in il≪- altered bas<i!iie

inuJcNiU c-xwoi silica whicli are- siumficaiiily ikWciI. <H) M.i≪

fluids of KM: clcnx-nis in ilic nidifiin ami coir ≪du>w ihai

U ≪-Ji nrc lnsi <CJ>J Mass (luxes "( minor frloaiiojns in lite

miix'ia) .infl c op* of nlb-red lx≫≪wtllU iiiulosjjc hi <' tflltii )o≪i 01

iMmt-d.

Ce and £u have strong negative anomalies. These

anomalies and depletion of the LREE in the whole rock

can be partially explained by the hoi acidic hydro thermal

fluid-rock interaction, resulting In decoupling of Ce. Ru

and other LREE compared to the heavy REE. At the

known pH of 2.1. temperatures of 88 to 120*0 and high

oxygen fugacity, all the LREE will be in the divalent state

(Wood, 1990). The main controlling factor for the LREE

speciation would be the low pH, temperature and the

completing agents. In most acidic water (pH-2.0) free

ions ai*c the major species if salinity and SO^/Cl are low

(Lewis ct al., 1998). Bau (1991)

reported that decoupling

of Eu from the other REE will develop a negative or

positive Eu anomaly. Sverg≪nsky (1984) also mentioned

that significant fraction of Eu relative to other i-are earth

elements occur in MOR system during high temperature

hydrothermal system. The depletion of the LREE with

strong negative Ce and Eu anomalies m the

Desmos

sample indicated that most of the REE elements are either

transported as free ions or sulfate complexes. Gamo et al.

(199?) reported a high sulfate content (9.7 mM) for the

Desmos fluid but a very low CI concentration. In high

sulfate water with low concentration of other Suitable

complexing agent, Lewis ct a 1. (1908) mentioned that the

RlvE a≪e present as complexes with sulfate or as free ions.

Since the other complexing agents such as F, CI. CO., arc

very low in the Desmos fluid, the likely complexing agent

responsible for the REE depletion would be sulfate.

Dtewvlved sulfate ions readily complex with (He REE

under acidic and oxidizing environment

(Brooks. 1989).

However, free ions would be also dominant at very law pH

(lewis et al.. 199R).

The depletion of LlUiK in the rocks

indicates thai fractionation has occurred aei*oss the RKE

series during the water-rock interaction There is a

possibility that there will be a slight enrichment of UiliV.

and Ku anomaly in (he

Desmos fluid. Tin- quantitative

striping of IJ<I＼K. decoupling of

Ku from the feldspars and

BiwiOmass caused depletion

in the altered rocks.

The Desmos fluid collected in 1995 and 1996 lias vei-y

low concentration of the truce elements compared

to

fluids (iciti nthei localities such

as Pacmanus. Vienna

Wood. Kasl Pacific Rise and I an Ka*in (ishibashi per.

comm.. HW8) The irace eloneiiis m lhe jilicix*cl ･■>cks arc

slwntfy <}et>k-U-d huiioiliiin tlmt

lhe fluid deiiviMl fmm

JAMSTgc J OeepSeaRes., 14

the water≫rock interaction should be enriched in trace

elements. However, the depletion of U*ace elements in the

fluid reveals thai the trace elements have been protonated

and transported out of the rock within a short span of

lime. The first fluids dischavged from the Onsen site as

soon as the hydrothernial activity commenced will have

high trace element contents because of Increasing

solubility of the trace elements with decreasing pH.

5. CONCLUSION

The major, minor and REE clement concentrations and

patterns for the fresh basaltic andesites and altered rocks

from the Onsen site reflect a number of p>x>cesses. The

majors minor and KEE elements are clearly mobile under

very low pH conditions during water-rock interaction.

Comparison of major, minor and REE elements concent-

ration in the altered rocks to the fresh rocks show that all

of these elements are strongly removed except S≫02

which is eniic/ieif antoiphous siiica. The Major, minor and

REE aro released into solution during dissolution of the

primary minerals and volcanic glasses from the basaltic

andesite during circulation and interaction of the low pH

hydrothermal fluid. The overall dissolution, leaching and

mobility of elements in the basaltic andesjfe are strongly

dependent on the solution pH. oxygen fugacity, water-rock

ratio and

Hie mobility of major, minor and REE elements at the

Onsen site is quite unique compared to the mid-oceanic

ridge systems and back-arc basin spreading centers

Elements which are believed to

be immobile in altered

samples front mid-oceanic ridge systems and

back-arc

basin spreading centers are mobile at the

Onsen site

which suggest that the mechanism of Alteration at Onsen

site is quite different.

The evidence obscivcd in this study suggests that. (1)

l>H is a major factor affecting the ability of a hyd)*othcrmal

fin id lo mobilize and transport major, minor and rare canh

elements, (2) l-RKE arc strongly decoupled with negative

Ce and Ku anomaly compared to HRK1-, (3) in rocks

subjected l<> alteration by low pH (2.1) hydrothermal fluid

under *eawater supo-saturaU'd condition, silica is enriched

while MgO and othei major elements aiv depleted.

Acknowledgments

We would like to acknowledge ihe following people for

their help in ihis study. Dr. Jitsuya Takada of Kyoto

University of J?eaclor for his support wirb INAA. The

operation team of "Shmkal 6500", "Shinkai 2000 ' and crcw

members of the R/V "Yokosuka" and "Natsushima" for

their help during sampling in the Manus Basin.

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