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Chemical Properties and Evolution of Mid-Ocean Ridge Hydrotherm Alsystem S Flow System Approach

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Chemical Properties and Evolution of Mid-Ocean Ridge Hydrotherm Alsystem S Flow System Approach 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 (~3 4S = 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).
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