Earth and Planetary Science Letters, 76 (1985/86) 35-44 35 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands [6] An estimate of hydrothermal fluid residence times and vent chimney growth rates based on 210 Pb/Pb ratios and mineralogic studies of sulfides dredged from the Juan de Fuca Ridge David Kadko ~, Randolph Koski 2 Mitsunobu Tatsumoto 3 and Robin Bouse 2 i College of Oceanograpt~v, Oregon State Unieersio,, Cort,allis, OR 97331 (U.S.A.) : U.S. Geological Survey, 345 3~iiddlefield Road, Menlo Park, CA 94025 (U.S.A.) -~ U.S. Geological Survey, Denver. CO 80225 (U.S.A.) Received January 22, 1985; revised version received August 26, 1985 The 2u~Pb/Pb ratios across two sulfide samples dredged from the Juan de Fuca Ridge are used to estimate the growth rate of the sulfide material and the residence time of the hydrothermal fluid within the oceanic crust from the onset of basalt alteration. 21°Pb is added to the hydrothermal fluid by two processes: (1) high-temperature alteration of basalt and (2) if the residence time of the fluid is on the order of the 22.3-year half-life of 210 Pb, by in-situ growth from 222 Rn (Krishnaswami and Turekian, 1982). Stable lead is derived only from the alteration of basalt. The 21°Pb/Pb ratio across one sample was - 0.5 dpm/10 6 g Pb, and across the other it was - 0.4 dpm/10 6 g Pb. These values are quite close to the 23SU/Pb ratios of basalts from the area, suggesting that the residence time of the hydrothermal fluid from the onset of basalt alteration is appreciably less than the mean life of 21°pb. i.e.. the time required for ingrowth from the radon. An apparent growth rate of 1.2 cm/yr is derived from the slope of the 21~JPb/Pb curve for one of the samples. This is consistent with its mineralogy and texture which suggest an accretionary pattern of development. There is no obvious sequential growth pattern, and virtually no gradient in 21°Pb/Pb across the second sample. This is consistent with alteration of the original 21°pb/Pb distribution by extensive remobilization reactions which are inferred from the mineralogic and textural relationships of the sample. 1. Introduction de Fuca Ridge are found to contain significant quantities of Cu, Zn, Ag, and Cd [6-9]. From a The discovery of large chemical anomalies in resource perspective, it is desirable to know how the hydrothermal fluids which emanate from vents the vent sulfide material forms and how rapidly it associated with seafloor spreading centers has had accumulates. The preservation of massive sulfide a significant impact on our understanding of the deposits on the seafloor is dependent on the nature chemical budget of the ocean [1,2]. To compre- of the depositional mechanisms, the rates of accu- hend the influence of seawater hydrothermal sys- mulation by these mechanisms, and on the rate of tems on the composition of ocean water, it is chemical weathering prior to burial. necessary to understand the rate, duration, and The substantial excess 21°pb (tl/2 = 22.3 years) possible periodicity of hydrothermal activity at over its parent 226Ra (tl/2 = 1622 years) found in individual vents, as well as the residence time of hydrothermal vent material is a potential tool for ocean water within these systems. Similarly, the determining the growth rate and lifespan of such significance of the very high rate of heat loss via deposits. Finkel et al. reported significant excess the black smoker vents, as it pertains to the ridge 2~°Pb concentrations in sulfide precipitates and crest heat budget and possibly the lifetime of the vent material from the East Pacific Rise (EPR) at axial magma chamber, depends on how long such 21°N [10]. These authors compared the 21°Pb and activity persists at a given locality [3-5]. 2t°Po contents of active and inactive vent material Sulfide chimney or chimney-type fragments and suggested that the cycle of buildup, cessation, from the East Pacific Rise at 21°N and the Juan and decay by oxidation of these sulfide chimneys 0012-821x/85/$03.30 © 1985 Elsevier Science Publishers B.V. 36 49Okl is measured in tens to a few hundred years. Lalou I and Brichet treated the 21°pb/Pb ratio of venting MJcidle VaRey particles as an initial ratio and compared this to the 2mPb/Pb ratio measured on vent wall material. Assuming instantaneous growth of the chimneys, :ndeovour sulfide they found the age of several active vents to be on 4e the order of tens of years [11]. Chung et al. mea- sample sured the 2mpb/Pb ratio across a vent wall and .~Dlit Smt suggested that the growth of such deposits could TT152-11 occur in tens of years or less [12]. These relatively rapid growth rates are consistent with the steep temperature and chemical gradients found at many 47 discharge sites, and with some visual observations 2-21 &'~" [13]. The ages of molluscs collected near hot springs of the Galapagos Rise and the EPR at 21°N have been found to range from 3 to 27 years [14 16]. These ages suggest that some vents may indeed be 46 = short-lived on a geologic time scale. Excess 2mpb in the hydrothermal fluid may also be used to determine the residence time of the hydrothermal fluid within the crust. Krishnaswami and Turekian measured the concentration of 2mpb in vent waters of the Galapagos Rise and sug- 45" gested that when compared to the rate of 2mpb production from in-situ decay and basalt 222Rn i • • . ° alteration, this information could yield an estimate of the crustal residence time of the hydrothermal fluid from the onset of basalt alteration [17]. 44 ° I 1 In this work, we present 2]°pb data for two 131 °W 130 ° 129 ° 128 = 127 ° sulfide and four basalt samples dredged from the Fig. 1. Location of samples from the Juan de Fuca Ridge Juan de Fuca Ridge. The 21°Pb/Pb ratios, along analyzed in this study. Sulfide samples are indicated by a star, with the mineralogic, textural, and chemical char- basalts by a solid square. Contour lines indicate the magnetic anomaly pattern, in gammas [37]. Stippled area is + 500 gam- acteristics of the sulfides, is used to estimate vent mas. growth rates, fluid residence time, and possible growth histories of these deposits. versity of Washington's expedition to the En- 2. Sample location deavour site in 1984 contributed to the interpreta- tion of our data. The locations of the basalts The sulfide samples analyzed in this work were analyzed in this work are also indicated in Fig. 1. recovered from two locations on the Juan de Fuca Ridge (JDF) (Fig. 1). Sulfide material was dredged 3. Methods from the southern Juan de Fuca Ridge (44°40'N, 130°22'W; water depth 2200 m) in September Mineral identifications and textural interpreta- 1981 by the U.S. Geological Survey vessel "S.P. tions were made using reflected and transmitted Lee" [8,9,18], and from the Endeavour segment of light microscopy, scanning electron microscopy, the JDF, north of the Cobb Offset (47°57'N, electron microprobe, and X-ray diffraction tech- 129°06'W; water dep.th -2200 m) in October niques. Polished thin sections for petrographic 1982 by the University of Washington research studies were prepared along transects perpendicu- vessel "T.G. Thompson" [19 22]. Observations lar to layering in the southern JDF and Endeavour conducted from DSRV "Alvin" during the Uni- sulfide samples (Fig. 2). Details of the mineralogic 37 studies are presented elsewhere [9,21,22]. by this method were not greatly different from The 21°pb analysis of the sulfides and basalt those values obtained by mass spectrometry (Table material was made by counting its daughter prod- 1). uct, 21°po (tl/2 = 138.4 days). Finkel et al. have shown that disequilibrium may exist between 210 Pb 4. Composition, texture, and growth mechanisms and 21°po in venting sulfide particles [10]. How- ever, the samples within this study were not fresh 4.1. Southern Juan de Fuca Ridge sample particles but were from seafloor constructional sulfide deposits and were analyzed for the The massive sulfide sample from the southern polonium 1-2 years after the collection date. Dur- Juan de Fuca Ridge (WF-22D-6) is an angular ing this time, any 2t°pb-21°Po disequilibrium would slab approximately 8 cm in thickness, composed be largely eliminated. Approximately 200 mg of -largely of granular, dark gray layers rich in Zn each sulfide subsample (obtained along the same sulfide (sphalerite + wurtzite) with thin interlayers transect from which the thin sections were pre- and a crust of Fe sulfide (pyrite + marcasite) (Fig. pared) was dissolved in a mixture of HNO 3, 2a). The Fe-sulfide crust is constructed from over- HC104, and HF to which approximately 25 dpm lapping arcuate segments, with the underturned 2°8po standard was added. After the solution was layer extending a short distance into the zone rich first purged of radon, it was enclosed for a known in Zn sulfide. The thin (-30 #m) veneer of period of time, and the ingrown radon collected and counted to determine the 226Ra content. The radon is counted within a zinc-sulfide cell housed against a photomultiplier tube. The solution was then taken to dryness, redissolved in 1.5N HCI, and the polonium was directly plated onto a silver disk which had been placed onto the bottom of the beaker.