Geochem icalJournal, V ol. 23, pp. 255 to 268, 1989
Chemical properties and evolution of mid-ocean ridge hydrotherm alsystem s flow system approach
H O D AK A K A W A H ATA
G eological Survey of Jap an, 1-1-3 H igashi, T sukuba lbaraki, 305, Jap an
(Received Septem ber 13. 1989;A ccepted January 11. 1990)
L ow w ater/rock ratios (about I in w eight)in discharge zone appearto be an im portant characteris- tic ofsu bm arine hydrotherm al system s associated w ith axialspreading centers. T hese ratios, w hich are stable over tim e, are linked w ith the chem istry of en d-m em ber hydrotherm al solutions (original hydrotherm al solution before m ixing with am bient seaw ater) and the chem istry and distribution of greenstones. T hese features are considered to result from the properties of an open fio w system . T his paper is an attem pt to analyze a subm arine hydrotherm al system , using an ideal open flo w m odel. In this m o del, seaw ateris percolating through a rock colum n w hich is divided into a num ber of cells.In each cell, both rocks and solutions com e to equilibrium , based on strontium isotope exchange. T he fundam ental features of w ater/rock interaction in this fiow system difer from those inferred from closed system ; (1) chem ical com position ofthe discharged solution can be kept constant for som e w hile although large volum e of recharged solution bringslarge changein the bulk chem ical com position ofthe system .(2) T he dif erence betw een chem icalcom position ofrecharged and discharged solutionsis com pensated by the large ch ange of rock com position near recharge zone at earlier stage of hydrother- m alsystem .(3) T he values of w ater/rock ratio deduced from rock orsolution chem istry are,in general,
diferent from the integrated volu m e w ater/rock ratio. T hese results are applied to the naturalsubseafloor hydrotherm alsystem . The features are classified in relation to three evolutionary stages: (1) E arly stage: chlorite-qu artz-(C Q -) and C Q -rich greenstones occur only in recharge zone w hile C Q -poor greenstones occurin the rest of the system .(2)Interm ediate stage:the increase of fluid fiow prom otes replacem ent of C Q -poor with C Q -rich and som e of C Q-rich w ith C Q -greenstones,respectively.(3) L ate stage: C Q -poor greenstones are com pletely replaced by C Q - and C Q -rich greenstones. T heintegrated volum e w ater/rock ratio ([W / R]FLOW) constrained by the energy of heatsourceis es- tim ated to be u p to 4. So itis suggested that hydrotherm al activity dies out by the interm ediate stage.
T he chem ical com position of end-m em ber hydrotherm alsolutionsstays constant through the early an d interm ediate stage. A n increase of integrated w ater volum e circulating through the system does not necessarily lead to change the chem istry of hydrotherm al solution and host rock through the system .
m al solutions (C orliss et al., 1979; A lbarede et
IN TR O D UC TIO N al., 1981; E dm ond et al., 1982, 1985; V on
In thelast decade,severalaxialhydrotherm al D am m et al., 1985). E dm ond et a/. (1985) have system s along m id-ocean ridges have been show n that the chem ical com position of end- discovered (e.g. E dm ond et al., 1979; Rise Pro- m em ber hydrotherm al solutions from 21'N,
E PR has rem ained constant over the six years ject G roup, 1980; H ekinian et al., 1980; E ast sin ce they w ere first m easured in 1979. Further- P acific Rise Study G roup, 1981; M ichard et a/., m ore, the sulfur isotope values of sulfide 1981; M cC onachv et al., 1986). A n im portant and interesting characteristic ofthese system s is deposits from 21'N , E PR, Juan de Fuca, Ex- the narrow range of w ater/rock ratios (1 to 5 in plorer Ridge, and A xial Seam ount lie in a nar- weight) estim ated from end-m em ber hydrother- row range (~34S = 0 to 50100)(Shanks et al., 1984;
255-
256 H. Kawahata
Table 1. C haracteristics on rock chemistry, solution chemistry, and secondary m inerals versus waterlrock ratio (fW lRJCHEM)
CQ-poor CQ-rich CQ W / R ratio O-IO lO-50 >50 Rock chemistry
Sm all gain or loss of M g Large gain of M g Sm allloss of C a Largeloss of Ca G ains of N a G ain orloss of N a Loss of Fe Gain of Fe M ineralogy Rich in A ct + / - Ep Poorin A ct+ Ep O nly C h]+ Qtz Solution chem istry
Mg O A ppreciable pH M oderate acid Very acid Heavy m etal low high (C Q: chlorite-quartz) (A ct.'A ctino!ite. Ep: Epidote. C hl: C h!orite. Qtz: Quartz) (from M ott/, 1983).
Zierenberg et a/., 1984) and indicate that the change the bulk chem ical com position of the
sulfides precipitated from hydrotherm al solu- hydrotherm alsystem . T his changeis expected to
tions at low w ater/rock ratios (about I to 5), afect the chem istry and m ineral assem b lages of
based on the m odels by Shanks et a/. (1984) and altered rocks and hot spring chem istry.
Zierenberg et al. (1981). In addition to present A num ber of closed-system basalt-seaw ater
seafioor hydrotherm al fields, strontium isotopic experim ents at high tem p eratures (150-500'C)
data from D SD P H ole 504B, drilled into a 500 m show that the chem istries of altered rocks and
thick greenstone sequencein the C osta Rica Rift, m odified solutions are controll ed m ainly by
indicatethatthe greenstonesin a discharge zone the param eter w ater/rock ratio (in w eight ,
w ere form ed under alow w ater/rock ratio of I.6 [W /R]CHEM) as w ell as by tem perature (Bischof (K aw ahata et a/., 1987). These lines of eviden ce and D ickson, 1975; H ajash, 1975; M ottl and suggest that low w at er/rock rati os of e nd- H olland, 1978; Seyfried and M ottl, 1982; m e m ber hydrotherm alsolutions and gr eenstones Bowers and T aylor, 1985). T hese w orks w ere so in discharge zones w ere stable with tim e du ring successfulthat M ottl(1983) w as abletoinferthe the period of hydrotherm al activity associated relationship betw een the chem istries of several with axial spreadi ng cen ters. kinds of m etabasalts and hydrotherm alsolutions For the axial system at 21'N , E P R, neither (T able 1). The presentstudy extendsthe w ork of an ordinary closed system nor an open system M ottl(1983) by com bining data deduced from a can explain the production of narro w rang e of closedsystem with fundam entalchem ical proper- the chem ical a nd isotopic com positi on s of ties expected from an open flow system to quan- hydrotherm alsolutions and their constancy with tify his m o del. tim e (Edm ond et a/., 1985). Seaw ater per- Fortunately, initial oceanic crust and colating in oceanic crust reacts with surrounding seaw ater, befor e hydrotherm al alteratio n, ha ve rocks, changing the chem ical com position of fairly uniform isotopic ratios and chemicalcom - both rocks and w ater. Since the chem ical com - positions. T he birth of a hot spring solution and position of recharged seaw ater is diferent from the evolution of a hydrotherm al system m ay be that of discharged hydrotherm al solution (E d- sim ulated by sim plif ying the natu ral system to m ond et al., 1985), an increase in the volum e of anideal flow m odel.In this m odel,seaw at er per- w ater circulating through the system should colates through a rock colum n w hich is divided
M id-ocean ridge hydrotherm al system s 257
into a num ber of cells. In each cell, both rocks system and solutions com e to chem ical and isotopic Seaw ater enters the convection system in a equilibrium . H erethe strontium isotopic values, recharge zone within a few kilom eters from the w hich arelinked with w ater/rock ratio, aretrac- spreading axis (Spooner and Fyfe, 1973; W olery ed with tim e through the system . and Sleep, 1976). T his dow nw elling seaw ater in
The objectives ofthis paper are (1)to clarify an axial hydrotherm al system traverses an ex- the diference between flow and closed system s, trem ely steeptem perature gradient within the up-
(2) to indicate the diference betw een a per oceanic crust, from about O to 250-450"C , w ater /rock rat io dedu ced from therock orsolu- causing seaw ater sulfate to precipitate as tion chem istries ([W /R]CHEM) and an integrated anhydrite (M ottl, 1983). A t such high tem pera- w ater/r ock ra tio ([W /R]FL ow) (3) to em p hasize tures, it is suggested that solutions react with a capa city and process to keep the chem ical and rocks in relatively short tim e (Bischof and isotopic com positions of discharged hydrother- Dickson, 1975, Seyfried and Bischof, 1977, m alsolutions constant,(4)to disc ussits applica- 1981; Seyfried and M ottl, 1982). T he m ain flow tion to the natural subs eafioor hydrotherm al pathis assum ed to go dow n to about I to 2.5 km system . subbottom depth, to the upperm ost gabbroic layer, based on little strontium contam ination of gabbroic sequence (Spooner etal.,1977a,b) and ID EA L F L O W M OD EL A ND SIM U LA TIO N ontem perature and chem istry of hotspring solu- OF M ID-O CEA N RID G E tion (Bischof, 1980; H ekinian et al., 1980; V on H Y D RO TH ER M AL S YSTE M D am m et al., 1985). If the solution cools P hysicalconstraints of hydrotherm al circulation adiabatically asit rises(Bischof, 1980),the tem -
1 2 -~F (N-1) N Sea water - J> t=1 -~ 2 Colum n of fresh basalt and seawater Ce[l -- ~ Rock[i,(t-1)] ~ Rockri,tl - ~ W,aterL(i-I),(t-1)] W aterri,tl ,t, / '~ / ¥ / / ~ ¥ / ¥ / , ¥ / ¥ / i lbt / ¥ / ¥ / t-1 ¥ ,, Sea water - > -> t=t 1 2 i (N -1) N
- :> F
Fig. 1. M odelfor num erically comp uting increm ental equilibrium elem ent an dlo risot ope exchange i n an iso the rm a/flow sy stem. A n ini tially fresh colum n of basal t a nd seawat erissplitinto cells (1 .. N). S olution
flow is m odeled by the passage of successive increm ents (1.. t) of seawater (initially fr esh com po sition) through the colum n, Th e reaction b et wee n rock a nd s o lu tion in each cell att ains ch em ica/ and isotopic
equil ibria, then t he solution p roce edsto the nex t cell.
258 H. Kawahata
perature and pressure at the base ofthe upflow tions. T hus solutions should achieve local
zoneis probably in the range 350 to 370'C and equilibrium w hilecirculating atelevated tem pera-
500 to 700 bars for a typical axial hydrotherm al tu re. (3) T he distribution and chem istry of
system . These ranges are sim ilarto conditions of altered r ocks from the recharge zone in the
greenstone form ation (E ast P acific Rise Study T roodos and Liguria ophiolite suites in C yprus
G roup, 1981; M ottl, 1983). H ot spring solution can be explained using a sim ilarideal flow m o del
mixes with cold seaw ater on, or within the crust (Spooner et a/., 1977a, b).
and form s sulfide/oxide deposits. T he reaction betw een rock and seaw ater at
low tem perature (< 50'C)isso slow that a detec-
ldealfl o w m ode/ table change in chem ical com position of In this study chem ical properties of flow w eathered rocks requires tim e on the order of a paths are analyzed by adopting an ideal flow m illion years (H art, 1970; 1973). T husit is sug- m odel w herein crem ent al equili brium is att ain ed gested that seaw ater arrives w ith little or no during the reaction betw een rock an d solu tion in change in its chem ical com position at a place a dife rential portion ofthe system (Fi g. 1) . T he w here an extrem ely steep tem perature gradient diferential portion is called a cell here. S eaw at er exists. T heideal flow m odel ofthis study begins is percolatin g through a rock colum n w hich is atthis point. T he hydrotherm alsolution rises up divided into N cells. E ach cell has a length I*. rapidly and adiabatically. The discharge zone of T he reaction betw een solution and rock attains this study corresponds to the bottom of an chem ical and isotopic equ ilibriu m in eac h cell. upfiow zone. Following equilibrium in each ce ll, the reacted
solution proc eedsto the nextcell, w h ereit reacts D efi nition of waterlrock ratio with the rocks and reeq uilibr a tes. The con- For a naturalhydrotherm alsystem taken as a tinuous inflow of recharged s eaw ater and w hole,the integrated w ater/rock ratios m ay be outflow of discharged solution m ake solu tion ad- defined asthetotal m ass of w aterthat has passed vance in the direction F. This sequ enc e takes through the system , integrated through tim e, placein all c el ls (Fig. 1). divided by the total m ass of altered rocks within The m odelisjustified based upon the follow - the system ([W /R]FLow) (form ula (1) (M ottl, ing considerations: (1) basalt-seaw ater ex- 1983)): perim ents indicate that the reaction betw een [W /R]FLow = W / R, (1) basalt and seaw ater attainssteadystate or alm ost
equilibrium in a short tim e (within one year) w here W is the w eight of convected solution
above 300 'C (Seyfried, 1977; Seyfried and w hich has passed throughthesystem an d R i st he
Bischof, 1977, 1981; M ottl and Seyfried, 1980; w eight of the altered r ocks in the total sys tem
Seyfried and M ottl, 1982).T heir results,in spite that have been afected by this solution . In the
of closed system s, are consistent with chem ical m odel calculation in this paper, [W /R]FLOW
properties of end-m em ber hydrotherm al solu- represents the ratio of the total m ass of w ater
tions form ed in natural flow system s (M ottl, having passed through the colu m n to the tota l
1983; V on D am m et al., 1985). (2) D SD P H ole m ass of altered rocks through tim e.
504B isthe only hole thatcontains hydrotherm al- T he w ater/rock ratio ([W / R]CHEM) for the ly altered rocks of greenschist facies in all the closed system experim ents (B isc hof an d
D SD P H oles drilled in typical oceanic crust D ickson, 1975; H ajash, 1975; Seyf rie d , 1977;
(A nderson et a/., 1982). T he bulk perm eability Seyfried and Bischof, 1977, 1981; M ot tl and ofthegreenschist-altered sheeted dike zone from H olland, 1978; M ottl and Seyfried, 1980;
D SD P H ole 504B is very low (less than 10-18 m 2 Seyfried and M ottl, 1982) is sim ply t he rati o of
(lO m darcy)) (Becker et a/., 1983), w hich im plies the m ass of w atertothat o frockin itially c on tain- very slow flow velocities for circulating solu- ed in the experim ental charge.
M id-ocean ridge hydrotherm al system s 259
T he w ater/rock ratio (in w eight) in relation cussed later.T heisotoperatio ofthe rock ofi'th to strontium isotopic ratiosisgiven bythe follow - cell after equilibrium exchange with the w ater of ing equation taken from A lbarede et al. (1981): tpassage is given by: [W / R]CHEM R(i,t) = (C WATER/C RocK)x (W (i - I,t - 1) ([87Sr/8 6Srl~ocK _ [87Sr/86Sr]PocK) x C ROCK - W (i,t))/r + R(i,t - 1), (3) ([87Sr/86Sr]~ ATER - [87Sr/86Sr]FW ATER) x C W ATER ' where (2) R(i,t)is the final87Sr/86Sr ratio ofthe rock in
where equilibrium with the w aterin i'th cellattim et,
W (i,t)isthe final87Sr/86Sr ratio ofthe w aterin I= the initial values of fresh basalt and equili brium with the w aterin i'th cellattim et, seaw ater, w hich is equal to R(i,t) F = the final values after basalt-seaw ater in- teraction, and and ris the w ater/rock ratio in w eight. C = the strontium concentration in w ater or In the form ula (3), C WATER' C RocK, and r are con- rock. stant and W (i- 1,t- 1) and R(i,t- 1) are know n, so R(i,t) can be calculated. Strontium isotope exchange betw een rocks and Initial condition of fl ow system solution in each cell and rock colum n Spooner et a/. (1977a) adopted a sim ilar Theevolution oftherock colum n can betrac- m odel to discuss the contam inat ion of altered ed by calculating the w ater/rock ratios, using rocks in an ophiolite by seaw ater com ponents, eq. (2), from the com positions of rocks and assum ing that no w ater w as in the system prior seaw ater in each cell at a given tim e. In other to the hydrotherm al activity. H ow ever, this in- w ords, a pseudofiow is usedto m ake good use of i tial conditio n is not realistic because perm eabil- experim ents and therm odynamic calculations ity of the upper oceanic crust (volcanics and donein closed system s. H ere,the distribution of sheeted dike zones)is high enough for seaw ater strontium isotopic ratiosis investigated because to penetrate. In ad dition , their calculation pro- their fractionation is alm ost independent of co- duced a hydrotherm al solution with an ex- existing m inerals, tem perature, and pressure. trem ely low w ater/rock ratio of about zero. Further,it has a relation to a w ater/rock ratio Thus, in m y m odel, space am ong pillow lavas ([W /RlcHEM) by a m ass balance calculation. and sheeted dikesis filled with fresh seaw ater at The strontium concentration of end-m em ber the beginning ofthe hydrotherm al activity. hydrotherm al solutions is alm ost the sam e as I nitial 87Sr/86Sr ratios of fresh basalt and that ofseaw ater (C orliss eta/.,1979; Albarede et seaw ater are 0 .70265 (H art et al., 1974) and a/., 1981; V on D am m et a/., 1985), w hich m eans 0.7090 (Veizer and C om pton, 1974; B urk et a/. that neither addition nor loss of strontium is 198 2), respectively. Strontium , ca lcium , and found during hydrotherm al alteration near sulfate concentrations are 8, 400, and 2760 ppm discharge zone (Spooner et al., 1977b). It cor- in seaw ater and 8, 1000, and O ppm in a respondsto the exchange ofstrontium isotope be- hydrotherm alsolution, respectively (Al barede et tw e en rock and w ater for a cell. al., 1 981; V on D am m et al., 198 5). Strontium W hen w ater ofthe fiow system show nin Fig. an d calc ium oxide (C aO ) contents of rocks are 1 enters the next cell after equilibrium with 93 ppm and 11.88 wt.o/o (H art et al., 1974), re- rocks, the values of param eter t increases by spectively. T hese values are representative of one. T he duration tim e of each cell will be dis-
260 H. Kawahata 5 param eter (r) in Fig. 2. The w eight ratio (r) is assum ed to be 1.0 because end-m em ber hydrotherm al solutions from subm arine 4 hydrotherm al fields (21"N , E P R, 87'W , oc G alapagos Spreading C enter, D SD P H ole 504B) ,, w ere form ed under a sim ilar w ater/rock ratio of ~-o:' 3 ul about unity. T he tem perature through the rock ~ 1:' a: L0o' 2 colum n is assum ed to be 350'C. T he steep tem - ~ perature gradient in the dow nflow zone (M ottl, ; ca' ~ u] 1983)suggeststhat m inorerrors m ay beinvolved S 1 in the calculations with this asum ption. The m ost poorly know n param eter is N ,the O totalnum ber ofcells. Sinceitis assum ed thatthe O 1 2 3 4 5 solution m oves into the next cell after local r equilibrium , the param eter N depends m ainly Fig. 2. The relation between param eter (r) and upon fiow velocity and reaction kinetics. M ost fW /R JCHEM Of disc harge d solution during earl y an d w orkers considering flow velocities have used a interm ediate evolutionary stages. The param eter r crack m odel (e.g. Lister, 1974; Low ell, 1975). repre sen tsthe weight r atio of seaw ate rto ro ck in each cell. If e quilibri um is attained i n each cell , ch em ical L ow ell(1975) points outthat flow velocity varies an d isotopic c om positions o f di scharg ed solu tion are asthe cube ofthe crack thickness,so any di､cus- l inked with [W /RJCHEM' sion of such velocities m ust depend on estim a-
tion of crack width and frequency. Circulation tim e is estim ated to range from 4 (Ribando et natural subm arine hydrotherm al system . T here a/., 1976) to 104years (Low ell, 1975)(with large are m any argum ents on anhydrite form ation in uncertainty). Ribando et a/. (1976) considers recharge zon es of hydrothe rm al system s (e.g. fiow velocities ofthe order of 5 x 10~3 cm /s and
Bischof and Dickson, 1975; M cD u f and E d- circulation tim e of 4 years, not unreasonable for m ond, 1982; M ottl, 1983). T hus, calculations a crack width of 0.05 m m . If the velocity w ere done as sum ing both anhydrite form ation (1 x lO~4cm /s) ofthe cracking front penetrating and no anhydrite form ation. It appears that through hot rock is sim ilar to flow velocity, the precipitation of anhydrite has a neglig ible efect circulation tim e is estim ated to be 200 years i n the following interpretation, because the (Lister, 1974). T he fiow rate of hot spring solu- diference in strontium isotopic ratios betw een tions at 21'N, E P R is 214 cm /s with an area of the tw o cases is less than 0.00004. W hen l03cm 3 (C onverse et al., 1984). If it is assum ed anhydrite is form ed, calcium m ust be supplied that(1)seaw ater entersthe hydrotherm alsystem from rocks as well as from seaw ater to in an area of I km 2 around the hydrotherm al precipitate all seaw ater sulfate (28.8 m M ). A t m ounds, that (2) hydrotherm al solutions pour that t im e strontium is also leached from the out atthesam e velocity and in the area as 21'N , rocks at a weight ratio of 0.0011 to calcium E PR, and that (3)the porosity of upper oceanic w hichisthesam e order ofm agnitude asthat of a crustis 150/0 to 200/0 (Rise Project G roup, 1980 fres h ro ck (H um phris and T hom pson, 1978). and referencestherein), then the fiow rate is esti-
The strontium concentration of anhydrite is m ated as 4 x 10-4cm /s with one circulation tim e calculated based upon the partition ing ex- of about 50 years. These estim ationssuggestthat perim ents (Shikazono and H olland, 1983). ittakes a few to a few hundred years for one cir-
W ater reacts with rock in the cell with the culation. weight ratio (r). The chem istry of end-m em ber W hen flow velocities and a length of one cell solution is calculated as a function of the aretaken as v and L,it takes L /v (years)to pass
M id-ocean ridge hydrotherm al system s 261 through one cell, and N x L /v (years) through ratio ([W /R]FLow) changes, since com plete mix- the rock col um n, respectively. F or exam ple, us- ing and equilibrium can be attained. In the open ing a 50-year period for one circulation and one sys tem , on the other hand,thechem icalcom posi- ye ar for localequilibrium of basalt-seaw aterin- tion of the solution can be kept constant for a teraction at 350'C , the total num ber of cells is w hile in spite of large changes in the bulk
50. chem ical com position of the system .
T he m echanism to keep the chem ical com -
position of discharged solution constant for
R ESUL TS A N D D ISC USSIO N m uch ofthelifetim e ofthe hydrotherm alsystem
Characteristics of fl ow system is a very im portant process of the flow system .
Sim ulation of the above m odelis show n in This property also results in the presence of
Figs. 3 and 4. T he fundam ental features of the chlorite quar tz (C Q -)poor greenstones w hich profiles can be described in a context of three escape m etasom atic p rocesse s in the discharge evolutionary stages w hich are sum m arized in zone. A continuous influx of seaw ater will not
T abl e 2.T he pr ofiles ofstrontium isotopicratios afect rock chem istry in the discharge zone and w ater/rock ra tios ([W /RlcHEM) are show n because seaw ater loses its ability to change the with a para m eter ofthe tota l num ber of cells N rock chem istry, having already passed through
(Fig. 3). Fortunatelythe tw o conditions(N = 50, and reacted with surrounding rocks. T he
10 0) afe ctthe profilesso littlethat only m inor er- diference between the chem icalcom positions of rors a re introduced in the following interpreta- recharge and discharge solutionsis com pensated tion Large chem ical efect of seaw ater com - not by the hom ogeneous changes of the w hole po nents added to crystalline rocks is (1)Iim ited system but by the large change ofthe rock com - to arecharge zonein the early evolutionary stage position only nearthe recharge zone until an in-
(Fig.3a);(2)reachesup to the centralzonein the tegrated volu m e w ater/rock ratio ([W /R]FLow) i nter m edi ate evolutionary stage (Fig. 3b); and be com es around 4.0 (early and interm ediate
(3)covers alm ost allthe system in thelate e volu- evolution stages). tionary stage (Fig. 3c). The chem ical com positions of both rocks
Figure 4 com pares the chemical changes of and solutions change s ystem atically in the flow the discharge solution deduced from the present system . The([W /R] CHEM ) values as a function of ifow m odel with that of a closed system m odel as tim e are com pared with t hree parts of rock col- a function of ti m e. In t he close d system , the um n in Fig. 4. These results ind icate that the c he m ic al com position of the solution should values of w ater/rock ratio deduced from change as the integrated volum e w ater/rock chemistry ([W /R]cHEM) are,in general, diferent
Table 2. Characteristics offlow system versus dlf erentevolution stages and portionsinfl ow colum n
Evolution stage Early Interm ediate Late [W / R]FSLYOSW l.O 4.0 10.0 [W /R]CHEM and rock types of greenstone Recharge zone High Very high Very high CQ-rich CQ- C Q-
Center zone Low (1.O) From low to high Very high C Q-poor CQ -poor and -rich CQ- Discharge zone Low (1.O) Low (1.0) From low to high C Q-poor C Q-poor CQ -poor and C Q-rich
(C Q: chlorite-quartz)
262 H. Kawahata
CQ -rich CQ- CQ -poor ,7 1 O
S.W. L 1OO a) 50 tt .708 L=iJ ~i = co o t .706 co f~ 10 1 ac t ~ ;,~ 5 1 J~ t1 N : 50 ~ ~. /7t~.- .704 co 1 N=100 Basalt .7o2 O .1 .2 .3 .4 .5 .6 .7 .8 9 1.O Ear[y stage i/N
CQ-rich cQ - CQ - poor .71O S.W. 100 b) 50 IL1 .708 :: LaJ l VL) o:: .71;, .7 0 6 co(JD a'~c 10 N=100 ¥~ N=50 .¥ ;.~ 5 ~; '~ .70 4 GE¥o 1 Basalt .702 O .1 .2 .3 .4 .5 .6 .7 .8 9 1.0 Interm ediate stage i/N
Fig. 3. Strontium isotope exchange in the ideal flow m ode!. D istribution of strontium isotopic ratios of altered rocksin rock colum n at dlf erentintegrated volum e waterlrocksin rock colum n at dlf erentintegrated volum e waterlrock ratios(fW /RJ;~LY~w)'Solid and brokenlinesindicatetheprofilesforthetotalce!lnum ber(N) to and 100, respectively.a)E arly evo!ution stage(fW /RJSFLY~w= 1), b)interm ediatestage(fW /RJ~I~sow= 4), and c) Iate stage (fW /RJSFLYSOW= 10). from the integrated volum e w ater/rock ratio (Fig. 3B). T here is only one point (A ) w here
([W / R]FLow). For exam ple, in the interm ediate [W / R]CHEM and [W /R]FLow) are e qual. In evolutionary stage ([W /R]FLow = 4,Iine ain Fig. general,the strontium isotopic value a nd [W /R ]
4),[W /R]CHEM isvery high (> 50)in the recharge CHEM arelow erin the discharge zone than in t he zone,w hereasitislow (1.O)in the discharge zone recharge zone, and rock and solution have a
M id-ocean ridge hydrotherm alsystem s 263
C Q - rlCh C Q - C Q -p oor .71o S.W, C) 100 50 .70 8 = J~ UJ (o = N=100 ' N=50 .706 co oco 10 ¥ ¥ ~:, ~; ;~ 5 t~ ~ .704 co 1 Basalt .702 O .1 .2 .3 .4 .5 .6 .7 .8 .9 1.O Late stage i/N Recharge zone Discharge zone
S.W. .709 100 / ilN= " i/N = 'f iIN = ,'/ .708 50 O.2i' 0.5f' 0.8 ,1' ~ ~l! ~ff .707 , a' , :ｷ o / ~; =U:J c :i , _l .706 aCoO f~ ' 'l ,, OC 10 J'l'_l ~; ;,, CIosed '/ " system l' //' '~ I tl' .705 ta~D l', / 5 / A~~ / / / / ! l .704 ;'/' 1 Discharge .l solution .703 Basatt
O 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
(W / R)FSLYOSW Fig. 4. Comparison of fW /RJCHEM ratio (or 87Srl86Sr ratio) versus fW /RJSFLY~w ratio plot of discharge hydrotherm alsolution between theflow (solid and broken lines) and closed systems (dash-dotline). Broken linesrepresentvariation curvescorresponding to rechargezone(i/N = 0.2),centerofthesystem (i/N = 0.5), and dischargezone(i/N = 0.8).Line aindicatesthatthe two dintinct waterlrock ratios([W /RJs~LYSow' fW /RJCHEM) show an identica/value of 4 only atpointA.
264 H. Kawahata
minim um value (1.0) of [W /R]CHEM' T hese and
results are consistent w ith that b oth greenstones
and hot spring solutions from the hy drotherm al CROCK is the strontium concentration in rock.
system s along m id-ocean ridges ind icate w ater- Sincethe87Sr/86Srratio ofseaw ateris higher
rock ratios higher t han I.O. than that of discharge solution,som e87Sr m oves
from the solution to rock, w hile som e 86Sr also m oves from rock to the solution. T he degree of Application of the ideal flow m ode/ to sub- m arine hydrotherm alfields 87Sr enrichm ent varies depending upon types of greenstone, forexam ple,87Sr/86Srratio increases l. The relation between m ass fl ux from in the order of chlorite-quartz-(C Q -)poor< C Q-
hydrotherm al system and alte ra tion of rich < C Q -greenstones.In theinterm ediate evolu- oceanic crust tion stage, C Q-poor rocks cover about 650/0 of The m ass fiux of strontium from hydrother- thesystem , but gain a relativ ely sm all am ount of m al solutions to the ocean is im portant. Stro n- 87Sr from the solution. O n the other hand, C Q - tium w hich is retain ed in or released fro m rocks rich and C Q-greenstones occupy only about duringthe hydrotherm al alteration should corres- 350/0 of the system , but hold m ore than 650/0 of
pond to the net hydrotherm al flux to the ocean, the net gained 87Sr from the solution (Fig. 5).
w hich can be described bythe following form ula These results suggest that the C Q -rich and C Q-
(4) and corresponds to the shaded region ofFig. greenstones, w hich are not directly in 5. equilibrium with end-m em ber hydrotherm alsolu- tions, play a veryim portantrolein thestrontium FLU X NET= V x L x J [R(n,t)- 0.702651 budget ofthe hydrotherm alsystem .T his proper- ty could apply to the m ass flux of otherelem ents x CRocKd(i/N), (4) because it is linked w ith the param eter of w ater/rock ratio ([W /R]CHEM). where V is the volum e of each cell 2. E nergy constraints of subm arine hydrother- L is the length of each cell m a/ system s Seafioor hydrotherm al fields at 21'N and 13'N , East Pacific Rise, w here m any activevents
.71 O S.W. .7o8 ~5 CO ss.~':': CO .7o 6 J~ e~;~~:~~~ CQ-poor GhO .704 Basalt .702 O .1 ,2 .3 .4 .5 .6 .7 .8 .9 1,0
ReCharge z on e j/ N DisCh arg e zo n e
Fig. 5. Strontium isotoperatios of dlf erenttypes of altered rocksin rock colum n attheinterm ediatestage.
Ch/orite- quartz- (C Q-)rich and CQ-greenstones occupy about350/0 of the system, but hold m orethan 650/0 of the 87 Srgai ned from se aw ter.
M id-ocean ridge hydrotherm al system s 265
EA RLY STAG E INTERM EDIATE STAG E -- ~ AXIS Seafloor Start of Seaw ater ~l recharge zone IL / Low Tem perature / 'L'L ¥ Low T. E lb~, !x CQ - High Tem perature Hi9h T' CQ - cu~)i C Q -rich - Discharge CQ -rich i zone CQ-poor CQ-poor
Magma cham ber
Fig. 6. Schem atic m odelof theevolution of mid-ocean ridge hydrotherm a/system.(]) Early stage: chlorite- quartz-(C Q- )poor rocks occupy the m a jority of t hesystem. C Q-and CQ-rich rocks appear neartherecharge zone.(2) Interm ediatestage: The area of CQ-poorrocksdiminishesinthesystem. CQ-ric k or C Q- rocks occu py a gre aterpar t ofthesys tem, butth echemistry oft hed ischarge s olutio n showsn o change. Thehydrotherm a!ac- tivity declinesas m agm a,them ain heatsource,solidlfiesand coolsdown.(3)Latest age: t hisstagei s n otseen i n n atura /axis hydroth erm a/syste m s becau ses olidl fied m agm a had exh aust ed heat energy to drive hydrotherm al conv ection sy st em. and relatively sm all am ounts of fresh sulfide T he life tim e ofthe subm arine hydrotherm al deposits are found, m ay belong to a fairly early system sis probably controlled bythe heatsource evolutionary stage. O n the ot her hand, Clam - energy. If the heat em anated from a m agm a bake I (on e o f th e hydrotherm al vent fields on cham ber drives hydrotherm al circulation (M ac- the G ala pagos Rift 86'W ), the outer portion of donald et al., 1980; M acdonald, 1982), then lg w hich isinactive, suggeststhat this ve nt, as w ell of basaltic m agm a cooling from 1200'C dow n to as others having m ajor dead areas, m ay be 350'C m akes aboutI g ofhydrotherm alsolution already showing dow n and will die out (Crane heated from O up to 350'C (Spooner and Fyfe, and Ballard,198 0). H ydrotherm al activity w hich 1973). If gabbroic (ancient m agm a cham ber) form ed the green stones of D SD P H ole 504B layer is 4 km thick (Rise Project G roup, 1980) died out about 6 m illion years ago (K aw ahata and seaw ater circulates m ainly in the upper an d Furut a, 1 985; A lt et a l., 1985). T hese four oceanic crust(about 2 km thick)(Spooner, 1979; h ydr othe rm al fields probably represent evolu- Bischof, 1980;Ito and Clayton, 1983), and ifall tionary stages of hydrotherm al system s and m agm a is solidified near the spreading axis and indicate w ater-rock interaction with low heat is exchanged betw een m agm a and cir- w a ter/rock ratios. So rocks with low culating w ater com pletely,theintegrated volum e w ater/rock ratios (ca. I.O) occupy the discharge w ater/rock ratio ([W /R]FLow) w ould be about zone as lo ng as hydroth er m al cir culation is ac- 2.0. Although the assum ption is m ade that all tive. oceanic crust attains equilibrium w ith hotspring
266 H. Kawahata
solutions along the m ain paths of circulation corporates m ajorrock types associated with sub-
flow, another im portant factor m ust be con- m arine hydrotherm alsystem s, w ater/rock ratios
sidered. T hat is, there are usually relicts of and strontium isotope ratios. The m odel predic ts
plagioclase and/or clinopyroxenein greenstone, the following properties:
w hich indicates that som e portions of rocks (1) T hree evolutionary stages of hydrother-
resist hydrotherm al alteration (H um phris and m a l alteration. Early stage: chlorite-quartz-
Thom pson, 1978; K aw ahata and Furu ta. 1985; (C Q -)rich greenstones occur only in the recharge
A lt et a/., 1985). For exam ple, if only 500/0 of zone w hile C Q-poor rocks occu r in the rest of
rock in volum e is altered to secondary m inerals, the system . In term ediate stage: the increase o f
the substantial integrated volum e w ater/rock fiuid fiow through rock ([W /R]FLow) Prom otes
ratio (4.0) w ould be tw ice aslarge as that ofthe replacem ent of C Q -poor w ith C Q-ri ch and C Q-
ideal condition. G reenstones recovered from rich with C Q -greenstones, respectively. Late
890 m to over 1350 m subbottom depth at the stage: C Q-poor greenstones a re com pletely
G alapagos Spreading C enter contain on average replaced by C Q - a nd C Q -rich greenstones. All
330/0 Of unaltered m inerals (K aw ahata et a/., three stages are accom panied by progressive
1987). T hus an integrated volum e w ater/rock changesin 87Sr/86Sr ratios.
ratio ([W /R]FLow)is estim ated to be up to 3.In (2) The chem ical com position of end-
fact, all m agm a could not cool dow n as low as m em ber hydrotherm alsolutions willrem ain con-
350'C nor heat exchange can be com plete, so stant unti l the system proceeds to the in-
these values are probably an upper lim it. These term ediate evolutionary stage.
values suggest that a hydrotherm al system will (3) T he w ater/rock ratio for a closed system
die out at the interm ediate evolutionary stage. deduced from chem istry ([W /R]CHEM) is, in
T he schem atic m odel of a hydrotherm al general, diferent from the integrated volum e
system is show n in Fig. 6. H ydrother m al circula- w ater/rock ratio ([W /R]FLow).
tion begins with a supply of heatenergy from an (4) The quanti ty of elem ents or isotopes
underlying m agm a cham ber. T he d ischarging w hich are fixed in or released from ro cks durin g
end-m em ber hydrotherm al solution with hydrotherm al alteration correspond s to the net
w ater/rock ratio of 1.0 m eets cold seaw ater to hydrotherm al flux to the ocean, except for
form sulfide/oxide deposits on or within the de position on or within the crust. C Q-rich an d
oceanic crust. A s tim e passes, chlorite-quartz- C Q -greenstones, w hich constitute a relatively
(C Q -)rich rocks replace C Q -poor ones and oc- sm all portion of the system and are not directl y
cup ylargerpart ofthe hydrotherm alsystem . B ut in equilibrium with end-m em ber hydr otherm al
C Q-poor rocks still exist near the discharge solutions, play an im portant role in m ass flux
zone, w hich control the constancy of the calculation for the interm ed iate stag e.
chem icalcom position ofend-m em ber hydrother- (5) T he integrated volum e w ate r/r ock ratio
m alsolutions. W hen m agm a solidifies and cools ([W /R]FLow) constrained by the energy of heat
dow n,the hydrotherm al activity declines. source is estim ated to be up to 4 based upon
theoretical calculations and ob servatio ns in the
drill core of H ole 504B. Th is su gg ests that SU M M A RY A N D C O N CLUSIO NS hydrotherm al activity dies out at about the in - T he significant characteristics of subm arine term ediate evolution stage. hydrotherm al system s associated with spreading axes arethe sim ilaritiesin w ater/rock ratios and A cknowledgm ents- The author w ould like to express the chem istry ofend-m em ber hydrotherm alsolu- his ap preciation to Prof. J. T. Iiyam a for valuable
discussions and instructive suggestions. H e thanks tions, and greenstones, and the stability ofthese D rs. N . Shikazono, N . Takeno, M . J. M ottl and T. F. properties with tim e. The natural system has M cC onachy for rea ding the m anus crip t and valuable been sim plified to an ideal fiow m odel w hich in- criticism . H e is m uch i ndebted to D rs. T. U rabe, K.
M id-ocean ridge hydrotherm al system s 267
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