Chemistry of shallow submarine warm springs in an arc-volcanic setting: Vulcano Island, Aeolian Archipelago, Italy

ELSEVIER

Marine Chemistry 53 (1996) 147-161

Chemistry of shallow submarine warm springs in an arc-volcanic setting: Vulcan0 Island, Aeolian Archipelago, Italy Peter Sedwick a, Doris Stiiben b ’

Antarctic

h Institutftir

CRC, Unirersity Petrographic

of Tasmania,

und Geochemie

GPO Box 252C, Hobart.

der Uniuersitiit

Karlsruhe,

Tasmania D-76128

7001, Australia Karlsruhe.

German?

Received 27 October 1994; accepted 29 June 1995

Abstract Results are presented from a geochemical survey of active submarine warm springs off the island of Vulcano. an active island-arc volcanic centre in the Aeolian Archipelago, Italy. Water samples were collected from submarine ‘fumaroles’ which discharge hot ( _ SO-lOO”C), acidic, gas-rich, low-chlorinity fluids into the shallow embayment of Port0 di Levante. adding dissolved Si, K, Li, Rb, Fe, Mn, NH, and H,S into surrounding seawater. These fluids are interpreted as a mixture of seawater and low-salinity groundwater which has undergone high-temperature ( > 100°C) hydrothermal alteration, followed by mixing with cool seawater in the sub-seafloor and during venting. The fluid compositions also suggest the chemical ‘overprint’ of reactions resulting from the input of significant amounts of the volcanic gases CO,, SO2 and H,S at this site, specifically, the attack of igneous silicate phases by dissolved CO,, and the hydrolysis and oxidation of SO, and H,S to SO:-, H+ and elemental sulphur. These overprinting reactions have been proposed for other gas-rich submarine hydrothermal fluids collected from shallow island-arc and hotspot volcanoes, and may be typical of such settings. Several water samples were also collected from a site where warm (< 30°C) fluids seep from volcanic sand, and are enriched in dissolved Si, K, Li, Rb, Mg, Ca, and particularly Fe and Mn relative to ambient seawater. These solutions are interpreted as

the result of low-temperature (< 100°C) hydrothermal alteration of seawater, again overprinted by the addition of acidic volcanic gases; the warm, acidic fluids then leach Si and metal cations, including Mg”, from the volcanic sands. The elevated H,S concentrations and low pH of ‘ambient’ embayment water relative to typical surface ocean waters suggest that this type of hydrothermal activity significantly alters the redox and pH conditions of local seawater.

1. Introduction Numerous processes affect the composition of seawater in the Mediterranean, including fluvial inputs of weathering products, atmospheric and volcanic inputs, and recently municipal, industrial and agricultural pollution. One potentially important process which has received relatively little geochemical study in the region is submarine hydrothermal activity, such as that associated with arc volcanism in the 0304.4203/96/$15.00 PII

Tyrrhenian and Aegean Seas (e.g., Vamavas, 1989: Gabbianelli et al., 1990; Baubron et al., 1990; Varnavas and Cronan, 1991). Submarine hydrothermal vents typically discharge fluids which are warm-tohot, acidic, reducing and metal-rich, and. on a global scale, are thought to exert significant controls on the major-ion composition of seawater (Thompson, 1983; Von Damm et al., 1985). In the Mediterranean Sea. shallow-water hydrothermal activity is of particular interest because it may strongly affect the composi-

Copyright 0 1996 Elsevier Science B.V. All rights reserved All rights reserved

SO304-4203(96)00020-5

148

P. Sedwick, D. Stiiben/Marine

tion of biologically-important and often poorlyflushed coastal surface waters. The fluid chemistry of such shallow-water hydrothermal systems is likely quite different from that of deep-sea hydrothermal vents due to the increased effects of magmatic degassing and phase separation under reduced hydrostatic pressure (e.g., Sedwick et al., 1992; McMurtry et al., 19931, and from that of well-studied subaerial geothermal systems because fluids may be derived

Chemistry 53 (1996) 147-161

from seawater rather than groundwater and magmatic fluids. Here we present the results of a geochemical survey of one such system off the island of Vulcano in the Tyrrhenian Sea. The island of Vulcan0 (Fig. la) is an active volcanic centre of the Aeolian island-arc dating from the Late Pleistocene (Barberi et al., 1973, 1974; Keller, 1974, Ventura, 1994). The current locus of volcanic activity is Gran Cratere (Caldera della

Vulcanello

/.

beach fumardes

4

il Faraglione

Fig. 1. (a) Location of Vulcan0 Island and the Aeolian Levante (map modified after Baubron et al., 1990).

Archipelago.

(b) Map of Vulcano,

showing

the two sampling

sites in Port0 di

P. Sedwick, D. Stiiben/

Marine Chemistry 53 (19961 147- 161

Fossa), rising approximately 390 m above sea level and composed of alkalic lavas ranging in composition from leucite to alkali rhyolite (Keller, 1980). Fumaroles with temperatures of up to 600°C are active around the crater rim, and fumaroles on the beach and ‘drowned’ in shallow waters of Port0 di Levante (Fig. lb) discharge gases and hot water at temperatures of around 100°C (Mazor, 1985; Baubron et al., 1990; Bolognesi and D’Amore, 1993). Numerous geochemical studies have addressed fluids from geothermal wells on the island, which currently discharge fluids with temperatures of up to 65°C (e.g., Carapezza et al., 1983; Sommaruga, 1984; Ricchiuto et al., 1986; Dongarra’ et al., 1988; Mazor et al., 1988; Capasso et al., 1991; Bolognesi and D’Amore, 1993). Most recently, these fluids have been interpreted as deriving from meteoric and magmatic fluids, with no modem seawater component (Bolognesi and D’Amore, 1993; Tedesco, 1994). To our knowledge, previous geochemical studies of fluids vented from the submarine fumaroles in Port0 di Levante are restricted to analyses of the vented gases (Mazor, 1985; Baubron et al., 1990), which are 2 98% volcanic COZ (Baubron et al., 1990). We collected actively venting warm fluids from two sites in Port0 di Levante in May 1993: (1) from two hot ( N 50- 100°C) submarine fumaroles several metres from the main beach, at N 30 cm water depth (10 samples, ‘beach-fumarole’ site, Fig. lb); and (2) where warm (N 25°C) fluids were seeping from sand below a small beach at the southern end of the bay, at N 15 cm water depth (2 samples, ‘hot-sand’ site, Fig. lb). Four l-l samples of ‘ambient’ bay water were also collected, approximately 10 m from the shoreline at N 30 cm water depth (three samples near the beach-fumarole site and one sample near the hot-sand site). At the beach-fumarole site, fluids and gas bubbles were vigorously vented from cobblesized lavas coated in bacterial mats and deposits of elemental sulphur. A typical rock collected from this site is a hydrothermally altered vesicular lava, with X-ray fluorescence analysis suggesting an initial composition similar to leucite (being richer in K than Na; R. Miihr, pers. commun.); except for the elemental sulphur deposits, no secondary alteration phases are apparent. The hot-sand site is at the termination of a major fissure in the steep northern flank of the Caldera della Fossa (Fig. 1b). Immediately above the

149

small beach, rocks along the fissure are visibly bleached and altered, presumably due to venting of acidic volcanic gases. Sand collected from the beach at this site is primarily fresh, dark, volcanic glass; the only secondary mineralisation apparent at this site was orange iron oxyhydroxide deposits which coated rocks in the shallow waters nearby.

2. Sample collection and analysis Water samples were collected by submerging acid-cleaned l-l polyethylene bottles (ambient and ‘hot-sand’ samples), or by placing an _ 30-cmdiameter polyethylene funnel connected to N 30 cm of silicone tubing over the vent orifice thus directing the hot fluids into the sample bottles held above sea level (‘beach-fumarole’ samples). Sample temperatures were measured with thermometers immediately before capping the bottles, and efforts made to minimise headspace gas in all samples, adding small volumes of ambient bay water where necessary. Additional samples were collected from the beachfumarole site in N 60-ml gas-tight glass bottles with no headspace; these samples were preserved with zinc acetate solution for later determination of H 2S, or used for on-site measurement of pH and dissolved 0,. Water samples were then transferred to the research vessel Poseidon for processing, which was completed within 6-12 h of sample collection. The water samples collected and chemical data are presented in Table 1. On board the Poseidon, aliquots were taken from selected samples for pH measurement, and from each sample, aliquots of around 40 ml were syringefiltered through 0.2~pm-pore Nuclepore filters and frozen in high-density polyethylene bottles for later determination of nutrients. The remaining solutions were pressure-filtered through 0.2~pm-pore Nuclepore filters under a class-100 clean-air bench using filtered nitrogen gas and each collected in a 60-ml high-density polyethylene bottle (without headspace) and a 250-ml high-density polyethylene bottle. To the 250-ml filtered subsamples were added 500 p,l of concentrated quartz-distilled HNO, (for later determination of trace metals and minor species); 5 ml aliquots from these filtered acidified subsamples were diluted ten-fold with deionised water for later deter-

BFA BFA BFI BFI BFI BFI HS HS HSA BF2 BF2 BF2 BFA BFA BFI BFA BFI BFA BFI BF2 BF2 BF2

s/5/93 5/s/93 5/5/93 5/5/93 5/5/93 5/5/93 10/5/93 10/5/93 1o/5/93 10/5/93 10/5/93 10/5/93 10/5/93 5/5/93 5/5/93 5/5/93 5/5/93 5/s/93 5/5/93 10/5/93 10/5/93 1o/5/93

Na’ (7)

510 506 480 477 483 490 493 497 506 488 471 505

VI V2 v3 v4 V5 V6 VI V8 v9 VI0 Vll VI7 VI8 VFI VF2 VIS vzs VOI vo2 VF3 VI6 vo3

Sample number

VI v2 v3 v4 V5 V6 v7 V8 v9 VI0 VII VI8

(WI 26.6 31.9 43.2 40.5 44.5 39.7 59.7 49.0 32.6 33.9 35.7 29.7

(t)

11.3 11.6 12.6 12.6 12.4 12.2 12.7 12.6 11.4 11.6 Il.4 11.9

1.59 1.68 5.83 5.79 4.96 3.96 7.79 7.21 2.25 3.01 3.32 1.83

ctt>

Rbf

N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 0.28 0.04 N.D. N.D. 0.11

0, (mmol/kg)

in Port0 di Levante.

56.49 56.54 51.95 52.13 53.40 54.58 59.15 58.16 56.88 53.93 52.93 56.80

(t) 11.20 11.18 IO.47 10.53 10.83 11.17 12.36 12.97 11.19 II.05 10.93 11.12

(t)

Ca2 +

6.5 6.5 5.6 5.9 5.7 6.1 6.2 6.3 7.5 5.5 5.7 5.5 7.1 6.1 5.2 N.D. N.D. N.D. N.D. 5.4 N.D. N.D.

N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. 153 273 N.D. N.D. N.D. 166 N.D.

Mg’+

PH (NBS)

H,S (pmol/kg)

Vulcan0

1.28 1.32 19.9 20.0 16.5 12.0 84.8 79.6 9.68 6.38 Il.2 0.78

(tt) 1.19 1.30 8.92 5.33 5.77 7.92 58.1 48.2 0.52 4.97 2.66 0.8 1

(W) 66 66 52 50 55 57 63 62 65 61 60 66

Mn (Yl-)

33.9 32.3 32.3 31.9 30.2 30.4 32.4 33.3 32.8 30.4 29.4 29.4 31.3 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

so; (mmol,‘kg)

Fe

N.D.

N.D.

586.3 585.5 550.6 549.4 561.0 570.6 582.7 583.8 585.5 566.9 549.7 552.2 587.7 N.D. N.D. N.D. N.D. N.D. N.D. N.D.

(mmol/kg)

Cl_

SrZ+

2.58 2.54 1.92 1.95 2.1 1 2.32 2.01 2.05 2.41 2.09 1.92 1.95 2.54 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

(nzeqjkg)

88.2 87.5 1630 1420 1240 770 529 484 75.5 631 801 46.6

(t) = concentrations in mmol/kg; (l_t) = concentrations in pmol/kg. N.D. = not determined; < DL = less than limit of detection. = Ke!: BFA = beach-fumdrole ambient; BFI = beach-fumarole 1; BF2 = beach-fumarole 2; HS = hot sands; HSA = hot-sands ambient * 1 Sub-sample taken for 02, HIS or pH measurement only.

Lit

K+

*’ ** ** ** - ’ ** ** ***

21.5 21.5 65.5 63.0 54.0 44.0 25.5 24.5 19.0 40.0 54.5 51.5 19.0 N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D.

type *

(d/m/y)

-

T (“Cl

Sample

Sample number

Date

Comoosition of water sarnoles collected

Table I

17.2 21.1 625 643 428 247 29.3 27.7 3.5 322 371 12.7

0.17 0.08 0.05

NO; WI

0.01

NO; (-ti-1

0.04 0.04 1.07 I .20 0.57 0.13 0.05 0.04 0.10 0.73 1.23 0.55

(tt)

PO: -

: 2

P. Sedwick, D. Stiiben/Marine

mination of dissolved Si. Alkalinity titrations were performed on 25ml aliquots from the 60-ml filtered subsamples. The analytical techniques and estimated analytical uncertainties (in brackets, expressed as percent relative standard deviation on the mean) used to obtain data reported here are as follows: Cl- calculated from chlorinity as determined by AgNO, titration with colourimetric endpoint ( < 0.2%); SO:- by ion chromatography (< 3%); total alkalinity (A,) by potentiometric titration with HCl against combination pH electrode, calculated with Gran function (< 1%); pH by potentiometry with combination pH electrode against NBS buffers ( < 0.1 pH unit); 0, by Idronaut polarographic O,-analyser ( < 1%); H,S by colourimetry with the methylene blue method on the ZnS precipitate ( < 10%); Na+ by charge-balance calculation of major cations and anions (< 0.5%); Kt by flame atomic absorption spectrophotometry (< 5%); Li+ and Rb+ by flame atomic absorption spectrophotometry using standard additions ( < 5%); Ca2+ by complexometric EGTA titration with colourimetric endpoint ( < 0.2%); Mg’+ calculated from total alkaline earths as determined by complexometric EDTA titration with colourimetric endpoint coupled plasma(< 0.2%); Sr’+ by inductively atomic emission spectroscopy ( < 10%); dissolved Fe and Mn by flameless atomic absorption spectrophotometry (< 15%); Si, NO;, NO;, NH: and PO:by flow-through colourimetry (< 5%). Note that there are likely additional uncertainties in the A,, pH, Oz and H, S values reported for the Vulcan0 beach-fumarole samples due to the high CO, content of the fluids (for A,; see Butterfield et al., 1990) and resultant degassing of the samples before measurement/preservation (for pH, 0, and H,S); no attempt has been made to correct the data for these processes.

3. Results 3.1. Seawater tracer

mixing

and use of Si as a mixing

All of the warm fluids collected from Port0 di Levante have been diluted to some degree with cool ‘ambient’ seawater, either during venting or in the

Chemistp 53 (1996) 147-161

Si

151

1200

(wnol/W 800

T (“Cl Fig. 2. Dissolved Si concentration vs estimated sample temperature for water samples from Vulcano, showing model I linear regression fit to the data. Equations for regression lines are: beach-fumaroles: y = 31.0x -601, r’ = 0.92; hot sand: y = 71.3x1280, r* = 0.997.

shallow sub-seafloor prior to discharge. Assuming that such mixing is rapid, sample temperature provides a conservative tracer for this dilution process, reflecting the proportion of cool seawater admixed with the warm hydrothermal fluids. A conservatively mixing chemical species can also be used as a mixing tracer, given a sufficient concentration difference between hydrothermal fluid and seawater, and has the advantage that it need not be measured immediately upon sampling. All of the warm fluid samples collected from Port0 di Levante are highly enriched in dissolved Si relative to ‘ambient’ seawater in the bay, reaching 1630 pmol/kg Si in a 65.5”C sample from the beach-fumarole site (ambient seawater = 47-88 p,mol/kg Si). The Si vs T data for the beach-fumarole samples define a single straight line within the uncertainty of the temperature measurements (around _+5”C), whereas the three data points from the hot-sand site define a steeper Si vs T relationship (Fig. 2). The simplest interpretation of me beach-fumarole Si vs T trend is that the data define a mixing line between the warm Si-rich fluids and ambient bay water, with dissolved Si behaving approximately conservatively during this process, as observed for other low-temperature submarine warm springs (e.g., see Edmond et al., 1979; Sedwick et al., 1992). Dissolved Si concentration is thus chosen as a mixing tracer, allowing examination of mixing trends for other dissolved species in the samples:

152

P. Sedwick, D. Stiiben/Marine

where species concentrations plot linearly against dissolved Si concentration, conservative behaviour is suggested for that species during mixing of the hydrothermal fluids with seawater; where a non-linear relationship is observed, non-conservative mixing behaviour is suggested. For samples from the hotsand site, a linear Si vs T relationship of greater slope than that of the beach-fumarole samples is assumed (see Fig. 2). 3.2. Dissolved gases (0,

and H,S)

and pH

Hydrogen sulphide was smelled throughout the embayment, particularly around the beach-fumarole site. A sample of ‘ambient’ bay water contained 153 Fmol/kg H,S (much higher than the sub-micromolar concentrations typical of oxygenated seawater), whereas two fluid samples taken from the beachfumarole site contained 273 and 166 p,mol/kg H,S, which should be considered as lower limits on the H, S concentration of the fumarole fluids. No H zS samples were collected from the hot-sand site, although the warm fluids collected there also smelled strongly of H 2S. Vent fluids from the beach-fumarole site are clearly depleted in dissolved oxygen relative to ambient bay water (0.28 mmol/kg O,>, and field measurements of 0.04 and 0.11 mmol/kg 0, for the fluids should be regarded as upper limits. No dissolved 0, measurements were made for fluids from the hot-sand site. The warm fluids from the beachfumarole site and the hot-sand site are quite acidic (pH 5.5-6.3, measured in the laboratory) relative to ambient samples at both sites (pH 6.5-7.5), with field pH measurements suggesting even lower values (ambient bay water pH N 6.1, beach-fumarole fluid N 5.2-5.4). No total dissolved carbon dioxide were performed for the fluids, & T measurements although it is assumed that the beach-fumarole fluids, which were effervescing when sampled, are saturated with respect to CO, (the major component of gases vented at this site; Baubron et al., 1990). The C, concentration calculated with the computer code SOLVEQ (Spycher and Reed, 1990) using measured T (65.5”C), A, (1.92 meq/kg) and pH (5.2) values are consistent with this assumption. A similar calculation for the hot-sand fluids (T = 25S”C, A, = 2.01 meq/kg, pH = 6.2) suggests an estimated C, of around 4 mmol/kg.

Chemistry 53 (1996) 147-161

3.3. The anions: Cl -, SO: _ and A, For samples from the beach-fumarole site, dissolved Cl- is depleted by more than 6% relative to ambient bay water and negatively correlated with Si, although there are apparently different mixing trends for the two sampling dates (Fig. 3a); the two warm samples from the hot-sand site are also slightly depleted in Cl-, although observed depletions are close to the analytical uncertainty. Titration alkalinities of the warm fluids are depleted over ambient bay water by over 20%, and negatively correlated with dissolved Si for the three sample sets (Fig. 3b); as for Cl-, the beach-fumarole data define a distinct mixing trend for each sampling date. The SOi- data

1

a.

Fig. 3. (a) Dissolved chloride concentration vs dissolved Si concentration for water samples from Vulcano, with linear regression fits: beach-fumarole, May 5: y = - 0.0241 x + 588, r* = 0.98; beach-fumarole, May 10: y = -0.0491x +591, r2 = 0.93. (b) Titration alkalinity vs dissolved silica concentration for water samples from Vulcano, with linear regression fits: beach-fumarole, May 5: y = -0.000423x + 2.61, r’ = 0.99; beach-fumarole, May 10: y = 0.000823x + 2.58, r’ = 0.99; hot sand: J = - 0.000884x + 2.48, t-’ = 0.999. (c) Dissolved sulphate concentration vs dissolved Si concentration for water samples from vu1can0.

P. Sedwick. D. Stiiben /Marine

show no trend versus Si for the three sample within the analytical uncertainty (Fig. 3~).

Chemistry 53 119961 147-161

IS3

sets

3.4. The alkali metals: Na ‘, K +, Li + and Rb ’ The Naf concentrations (calculated by charge balance) in both the beach-fumarole and hot-sand fluids are depleted relative to ambient bay water (Fig. 4a). The maximum depletions of more than 6% for the beach-fumaroles are comparable to those observed for chloride, and again two distinct mixing trends are observed for the beach-fumarole samples. The dissolved K+ data show no clear trend versus Si within the analytical uncertainty (Fig. 4b). Dissolved Li+ and Rb+ are both significantly enriched over ambient bay water in the beach-fumarole fluids (by over 50% and 250%, respectively) and the hot-sand

caz+ (mmol/kg)

20’ 0

400

800

1200

1600

I

si W-W)

4501

81

70

10

';j; 60

';;a

e T,

SO

c 53

5 + 3

30

40

20

Fig. 5. (a) Dissolved magnesium concentration vs dissolved Si concentration, for water samples from Vulcano, with linear regression fits: beach-fumarole, May 5: y = -0.00302x +56.8, r* = 0.99; beach-fumarole, May 10: y = -0.00566x +57.09, r* = 0.93; hot sand: y = 0.00427x +56.5, r* = 0.88. (b) Dissolved calcium concentration vs dissolved Si concentration, for water samples from Vulcano, with linear regression fits: beach-fumarole, May 5: y = -0.000460x + 11.30, r* = 0.84; beach-fumarole, May 10: y=-00.CKB263x+11.14, r2=0.72; hot sand: _v= 0.00328 x + 10.98, r* = 0.82. (c) Dissolved strontium concentration vs dissolved Si concentration, for water samples from Vulcane.

54 + 92 0

800

Si fmnd/kg)

1600

0

0

800

1600

St Onnd/k@

Fig. 4. (a) Calculated dissolved sodium concentration vs dissolved Si concentration, for water samples from Vulcano, with linear regression tits: beach-fumarole, May 5: y = - 0.0202 x + 508.5, r* = 0.95; beach-fumarole, May 10: y = - 0.0422x f508.3, r* = 0.92; hot sand: y = -0.0249x+508.0, r* = 0.95. (b) Dissolved potassium concentration vs dissolved Si concentration, for water samples from Vulcano. (c) Dissolved lithium concentration vs dissolved Si concentration, for water samples from Vulcano, with linear regression tits: beach-fumarole, May 5: y = 0.00941 x f29.5, r* = 0.82; beach-fumarole, May 10: y = 0.00763x + 29.3, r* = 0.99; hot sand: y = 0.0518x +28.3, r2 = 0.91. (d) Dissolved rubidium concentration vs dissolved Si concentration, for water samples from Vulcano, with linear regression fits: beach-fumarole, May 5: y = 0.00286x + 1.47, r* = 0.99; beach-fumarole, May 10: y = 0.00197x + 1.74, r2 = 0.998; hot sand: y = 0.0122x + 1.33, r2 = 0.999.

fluids (by over 80% and 200%, respectively); both species correlate highly with Si, with indistinguishable trends (within error) for the two beach-fumarole sample sets (Fig. 4c). 3.5. The alkaline earths: Mg 2 +, Ca2 + and Sr2 + The alkaline earths Mg and Ca display opposite trends relative to seawater for the two sites sampled. The beach-fumarole fluids are depleted in Mg2+ by nearly 10% relative to ambient bay waters (Fig. 5a), whereas the two warm samples from the hot-sand site are enriched in Mg2+ by several percent relative to ambient seawater, a situation generally atypical

154

P. Sedwick, D. Stiiben/Marine

for seawater-derived hydrothermal fluids. Dissolved Ca’+ is slightly (up to - 7%) depleted over ambient bay water in the beach-fumarole samples, and significantly enriched (up to - 16%) in the hot-sand samples (Fig. 5b). The Sr*+ versus Si data suggest a depletion in the beach-fumarole fluids relative to ambient bay water, although the apparent depletions are within the analytical uncertainty of the Sr*+ determinations (Fig. 5~).

Chemistry 53 (lY96) 147-161

(umol/ko)

urn

0. 0

0

mm

0

b. NHq+

3.6. The transition metals: Fe and Mn

WmoVko)

Dissolved Fe is enriched by almost an order of magnitude over ambient values in the beach-fumarole samples, and is highly enriched (by more than 100 X ) in the hot-sand samples (Fig. 6a). Dissolved Mn is

1.2-i po43-

0.6

(umovkg)

c.

I

z

P

0.4

I 0

06 0

400

600

1200

1600

Si OnnoVkg) Fig. 7. (a) Dissolved nitrate + nitrite concentration vs dissolved Si concentration, for water samples from Vulcano. (b) Dissolved ammonium concentration vs dissolved Si concentration, for water samples from Vulcano, with linear regression fits: beach-fumarole, May 5: y = 0.411 x -29.2, r* = 0.97; beach-fumarole, May 10: v = 0.531 x - 12.6, r’ = 0.999; hot sand: .v = 0.0578 x -0.793, ;’ = 0.999. (c) Dissolved phosphate concentration vs dissolved Si concentration, for water samples from Vulcano.

b. also enriched by around an order of magnitude relative to ambient concentrations in both the beachfumarole and hot-sand fluids, and correlates highly with Si (Fig. 6b). 3.7. The nutrients: NO, t- NO,,

NH4’ and PO:-

Si Olrnol/kg) Fig. 6. (a) Dissolved iron concentration vs dissolved Si concentration, for water samples from Vulcano, with linear regression fits: beach-fumarole, May 5: y = 0.00400x + 1.57, r2 = 0.69; beachfumarole, May 10: y = 0.00415x +0.984, r2 = 0.54; hot sand: y = 0.123x - 8.98, r2 = 0.995. (b) Dissolved manganese concentration vs dissolved Si concentration, for water samples from Vulcano, with linear regression fits: beach-fumarole, May 5: y = 0.0128x f0.675, r’ = 0.98; beach-fumarole, May 10: 4’ = 0.0131x-0.175, I-* =0.94; hot sand: y=O.l68x-2.86, r’= 0.999.

Dissolved NO; + NO; concentrations, as well as NO; and NO, concentrations alone, show no consistent trend against dissolved Si (Fig. 7a). In the beach-fumarole samples, NH: is significantly enriched (up to - 30 X ) over ambient bay water and highly correlated with dissolved Si, whereas the hot-sand samples are roughly ten-fold enriched (Fig. 7b). The PO:vs Si data for the beach-fumarole samples suggest that these fluids are enriched in

P. Sedwick, D. Stiiben / Marine Chemistv

PO:relative to ambient seawater, although fluid concentrations from the hot-sand site are uniformly low (Fig. 7~). Some of the scatter in the NO; + NO; and PO:- data may reflect biological processes occurring after sample collection, given the abundant bacterial mats surrounding the beach-fumaroles.

4. Discussion 4.1. Comparison jluids

with other island-arc

geothermal

The composition of the warmest ‘ beach-fumarole’ and ‘hot-sand’ water samples are compared with other hot-spring fluids from island-arc tectonic settings in Table 2. In terms of major-ion composition, the Port0 di Levante fluids are much closer to seawater than to subaerial geothermal well fluids from Vulcano, which contain less than 100 mmol/kg Cl(Bolognesi and D’Amore, 1993). In comparison to subaerial geothermal fluids from other arc-volcanic settings (e.g., see Giggenbach, 1992, and references therein), the Port0 di Levante fluids are quite similar in major-ion composition to saline brines and coastal thermal waters of Japan, which have been interpreted as deriving from geothermal alteration of mixtures of meteoric water, seawater and ‘residual’ fluids (Matsubaya et al., 1973). However, the Japanese coastal thermal waters generally have lower SOi-/Clratios than unaltered seawater (Matsubaya et al., 19731, in contrast to the Port0 di Levante fluids for which this ratio is greater than or equal to that of seawater. There have been few reported studies of submarine hydrothermal fluids in island-arc environments with which to compare our data. Vamavas and Cronan ( 1991, and references therein) present chemical data for fluids collected from shallow submarine warm springs off the islands of Nisiros, Kos and Santorini in the Hellenic Volcanic Arc (Aegean Sea), although individual sample temperatures and chloride concentrations are not reported. They report a maximum fluid temperature of 33°C a minimum pH of 6.7, and significant enrichments in dissolved Na, K, Mg, Ca, Fe and Mn relative to seawater. Such enrichments in Na+, K+, Mg*+ and Ca*+ are not observed in our beach-fumarole samples, although

53 (1996) 147-161

155

samples from the hot-sand site are enriched over seawater in Mg*’ and Ca’+. Warm fluids collected from submarine arc volcanoes in the Mariana Islands (western Pacific) at water depths of 402 m and 1140 m display enrichments in dissolved Si, alkalinity, CO,, H2S, Li, Rb, Sr, Fe and Mn, depletions in pH and Cl-, variable behaviour in SO:-, Na+, Mg” and Ca*+, and no trend for K’, relative to ambient seawater (MCMUQ et al., 1993). These few examples suggest that there are significant variations in the major-ion composition of submarine hydrothermal solutions in arc-volcanic settings, as is observed for the composition of subaerial geothermal fluids from different island-arc locations. 4.2. Origin of the beach-fumarole

jluids

The fluids sampled from the beach-fumaroles likely represent some hydrothermal component with a temperature near 100°C or higher, which has mixed with ambient seawater during venting or prior to venting in the shallow sub-seafloor. If it is assumed that fluid samples from each vent represent a single hydrothermal endmember variously admixed with cool, unaltered seawater, then the composition of the hydrothermal endmember may be estimated by back-extrapolation of mixing trends to some ‘known’ endmember condition of temperature or composition (e.g, see Edmond et al., 1979). This back-extrapolation procedure assumes that species behave conservatively upon mixing of the hydrothermal endmember with seawater (i.e., mixing trends are linear) in the sub-seafloor and that conductive cooling of the ascending hydrothermal fluids is negligible. For the beach-fumarole fluids, this proposed hydrothermal endmember may derive from alteration of groundwater and magmatic fluids, as suggested for subaerial geothermal fluids on the island (Bolognesi and D’Amore, 1993), from alteration of seawater, or from alteration of some mixture of these two source fluids. The low salinity of the beach-fumarole fluids relative to ambient seawater at this site suggests that low-salinity groundwater has contributed to the fluids. If such low-salinity fluids are the principal source of the hydrothermal component, then the composition and temperature of the endmember hydrothermal fluid should be similar to that of subaerial

a

25.5 529 (7.0 X 1 -SWh -SW 2.01 (- 16.6%) 6.2 - 4 (calculated) k NR 493 (- 2.6%) -SW 59.7 (+ 83%) 7.79 (3.5 x J 59.15 (+4.0%) 12.97 (+ 15.9%) -SW 58.1 (112x1 84.8 (8.8 x 1 29.3 (8.4 X 1

Vulcan0 (submarine)

Hot-sand

61 NR g 662 trace NR 6.4 NR NR 486 41.3 NR NR 0.68 53.0 NR NR NR NR

seawater

expressed

60 NR 366 17.1 NR 6.4 NR NR 309 10.3 NR NR 26.7 11.2 NR NR NR NR

Japan (coastal) (subaerial)

Japan (brine) (subaerial) 39 1820 (88.7 x ) 515.0 (-3.8%) 31.8 (+ 16.9%) 31.9(13.5x) 5.2 262 (calculated) 1.6 463 (+0.9%) -sw28.1 (+ 12.4%) 3.33 (2.7 x 1 65.83 (+ 26.9%) 3.62 (0.36x) 95.9 ( + 11.8%) 5.96 (103X) 47.6 (2070 X ) NR

’

9.3 381 (3.3 X) -SW -SW 2.58 (+ 4.0%) 7.5 NR NR 459 f - 0.6%) SW -SW 1.96 ( + 5 1.9%) 51.53 (--0.85%0) 11.3 (+ 10.2%) 92.0(+5.6%) 20.6 (278 X) 19.2 (1280x1 NR

Mariana Arc (submarine)

Kasuga-3

’ for depletion, ‘ + ’ for enrichment) or proportion

< 33 NR NR 29.3 (+ 1.3%) i NR 2 7.1 NR NR 530 (+ 10.2%) 11.4 (+ 7.2%) NR NR 61.3 (+ 12.4%) 14.6 (+ 38.6%) 119 (+ 22.4%) 3.1 (57x) 0.51 (14.1 xl NR

Mariana Arc (submarine)

Hellenic Arc (submarine)

’

Kasuga-2

Yali Bay Ad

as percent (‘ -

’

settings

Ibusuki-6

Arima- 10 ’

to fluids from other island-arc

65 1980 (59.5 x ) 86.0 (0.14x) 28.8 (-4.8%) 10.4 (4.1 x1 j 6.4 NR NR 107 (0.20 x 1 12.7 (+ 23.6%) NR NR 10.1 (0.17x) 6.51 (-38.3%) NR NR NR 52(4.1x)

Vulcan0 (subaerial)

Well W2 b

of Port0 di Levante fluids compared

This study. Bolognesi and D’Amore, 1993. Matsubaya et al., 1973; no information reported for ambient seawater. Varnavas and Cronan, 1991. McMurtry et al., 1993. f Numbers in parentheses refer to enrichment or depletion of species relative to ‘ambient’ of seawater value. g NR = not reported. h No discernible difference from ambient seawater within analytical uncertainty. ’ Reported as S. ’ Reported as HCO;. k See text: ambient value not determined.

a ’ ’ d e

Fe (pmol/kgJ Mn (pmol/kgl NH: (p,mol/kg)

PH C r (mmol/kgI H,S (pmol/kgl Na+ (mmol/kgJ K+ (mmol/kg) Li+ (p,mol/kg) Rb+ (hmol/kg) Mg’+ (mmol/kg) Ca2+ (mmol/kg) Sr2+ (pmol/kgl

A, (meq/kg)

65.5 1630(18.7xJf 549.7 (- 6.5%) -SW 1.92(- 25.6%) 5.2 saturated k 273 (+78%) 471 (-6.7%) -SW 44.5 (+67%) 5.83 (3.7 x 1 51.95(-8.1%) 10.47 (- 6.5%) -SW 8.92 (7.5 X) 20.0 (15.6~) 643 (37.3 x 1

Vulcan0 (submarine)

Location

T(“C) Si (pmol/kg) Cl- (mmol/kgl SO:- (mmol/kg)

Beachfumarole a

Fluid

Table 2 Major and minor species compositions

P. Sedwick, D. Stiiben/Marine Table 3 Beach-fumarole mixing-line extrapolations for endmember with T = 2OO”C, using Si = 5600 p,mol/kg, assuming conservative dilution by seawater species

Extrapolated

Si

5600 pmol/kg non-linear trend against temperature 3 16 to 453 mmol/kg - 2.0 to 0.24 meq/kg no trend against temperature 272 to 395 mmol/kg no trend against temperature 72.0 to 82.2 pmol/kg 12.7 to 17.5 pmol/kg 25.4 to 39.9 mmol/kg 8.71 to 9.67 mmol/kg no trend against temperature 24.0 to 24.2 pmol/kg 73.1 pmol/kg non-linear trend against temperature 2270 to 2960 pmol/kg non-linear trend against temperature

PH Cl_ AT

&So;Na+ K+ Li’ Rb+ Mg2+ Ca*+ Sr*+ Fe Mn NO; + NO; NH; PO: -

concentration

range

geothermal waters from the island. Based on deep geothermal well fluids sampled near the beach at Port0 di Levante in the 1950’s (Sommaruga, 19841, Bolognesi and D’Amore (1993) suggest a deep geothermal aquifer at N 200°C and containing N 110 mmol/kg Cl-, a higher chloride concentration than reported for any other geothermal well on the island. Extrapolation of the linear mixing trends for the beach-fumaroles to a temperature of 200°C which corresponds to 5600 pmol/kg Si (using the Si-T relationship in Fig. 2), yields a fluid of composition shown in Table 3. The calculated ‘endmember’ Clconcentration of 3 16-453 mmol/kg is considerably higher than that for geothermal well fluids on the island or that proposed for the deep geothermal aquifer by Bolognesi and D’Amore (1993); even higher extrapolated Cl- concentrations result if a lower endmember temperature is assumed. Steam loss after boiling or addition of volcanic HCl to the fluids could increase the extrapolated chlorinity of the endmember fluid; however, the simplest interpretation is that the high extrapolated Cl- concentrations reflect a seawater-derived contribution of around 50% at the hydrothermal endmember. Given the near-boiling temperature of the vented fluids, an alternative explanation of the fluid origin could involve an entirely seawater-derived hy-

Chemistry 53 (1996) 147-161

157

drothermal endmember, which has undergone phase separation and partial phase segregation in the subseafloor. The simplest model for this process would be that used by Buttertield et al. (1990) to describe low-salinity fluids vented from Axial Seamount on the Juan de Fuca Ridge, whereby sub-critical phase separation (boiling) and subsequent partial phase segregation produce fluids enriched in the low-salinity vapour component of the phase-separated fluid. Such a model is consistent with the high gas content of the beach-fumarole fluids, since dissolved gases partition into the vapour phase during phase separation. However, we favour ‘groundwater’ input as the most likely explanation for the low salinity of the beach-fumarole fluids, given that low-salinity geothermal fluids have been collected within 100 m of the vents. An isotopic investigation of the beachfumarole fluids may provide a definitive answer in this regard, as SD values < - 5%~,are proposed for subaerial geothermal fluids from Vulcano, whereas and local seawater has 6D N 7-%O (Bolognesi D’Amore, 1993). The different mixing trends suggested for the May 5 and May 10 beach-fumarole samples, most apparent in the Cl- vs Si plot (Fig. 3a), may reflect a change in the relative proportions of the proposed ‘fresh’ and ‘saline’ source fluids at the hydrothermal endmember, or alternately, variation in the extent of phase separation/segregation at the hydrothermal endmember, as proposed for vent fluids from Loihi Seamount, Hawaii (Sedwick et al., 1992).

4.3. Effect of volcanic gases on fluid chemistry The observation that lavas around the beachfumaroles are hydrothermally altered and encrusted with elemental sulphur suggests that the ascending hydrothermal fluids have continued to react with host rocks during and after mixing with cool seawater. Such ‘low-temperature’ reactions likely result from the high content of volcanic gases in the fluids, which were actively degassing when sampled. We have already mentioned the possible addition of magmatic HCl to the fluids, which may contribute chloride and acidity to the fluids. Other volcanic gases typical of this tectonic setting are CO,, H,S and SO, (Allard, 19821, which may exert important

P. Sedwick, D. Stiiben / Marine Chemistry 53 (1996) 147-161

158

controls on the fluid chemistry. The principal gas in the beach-fumarole fluids is probably volcanic CO,, based on analyses reported by Baubron et al. (1990) for this site. High concentrations of CO, in hydrothermal fluids may drive so-called ‘chemical weathering’ of igneous silicates and aluminosilicates by carbonic acid (Garrels and Mackenzie, 1971), in reactions of the type: Mg,SiO,(s)

MgZ+ (mmol/kg)56

+ 4CO,(aq)

= 2Mg 2+ (aq) + 4HCO;

(aq)

+ SiO,(aq)

\ 481 10

Such reactions have been proposed for other gasrich, low-temperature ( < 150°C) fluids venting from submarine hotspot and arc volcanoes (e.g., Sedwick et al., 1992; McMurtry et al., 1993), and will contribute metal cations (e.g., alkali metals, alkaline earths and transition metals), bicarbonate and silica into solution. Saturation-state calculations were performed using the computer code SOLVEQ (Spycher and Reed, 1990) for the composition of our warmest beachfumarole sample (V3, 65S”C, 1 atm, Table l), forcing saturation with respect to CO, and using our highest measured H,S concentration of 273 p,mol/kg. These calculations suggest an in-situ pH of 5.18, close to our lowest measured value of pH 5.2, and oversaturation with respect to the commonly occurring phases chalcedony, cristabolite, quartz and pyrite. The fluid is calculated as being undersaturated with respect to all other silicate and aluminosilicate minerals in the SOLVEQ data base, indicating the thermodynamic favourability of the proposed ‘low-temperature’ chemical-weathering reactions. Other likely components of volcanic gases in the beach-fumarole fluids which may significantly affect the solution chemistry are the sulphur species H,S and SO, (Mazor, 1985; Bolognesi and D’Amore, 1993). Sulphur dioxide is expected to undergo hydrolysis to form H, S, H, SO, and elemental sulphur under the conditions of cooling hydrothermal fluids (Holland, 19651, and H,S will be oxidised to elemental sulphur and sulphate under the acidic conditions as the fluids mix with oxic seawater (Stumm and Morgan, 1981). These reactions will contribute acidity and sulphate to the fluids. Ammonium in the beach-fumarole fluids, which is present at concentrations well in excess of that which could

30

50

70

T (‘Cl Fig. 8. Dissolved magnesium concentration vs estimated sample temperature, for beach-fumarole water samples from Vulcano, with linear regression fits: May 5: J = -0.1036x +58.84, r’ = 0.991; May 10: y= -0.1221x+59.00, I’ =0.95.

be supplied by reduction of seawater nitrate and nitrite, may also be derived from the volcanic gases. The chemical ‘overprint’ of such ‘low-temperature’ gas-driven reactions which occur after the hydrothermal endmember has mixed with cool seawater will invalidate back-extrapolations (Table 3) for participating species, and may also invalidate various geothermometers which involve alkali metals, alkaline earths, and silica (Henley et al., 1984; Von Damm, 1988). For the beach-fumarole fluids we consider the magnesium- and quartz-geothermometers, which may be applied to submarine hydrothermal fluids where a high-temperature (> 150°C) hydrothermal endmember has mixed rapidly with ambient seawater. The Mg-geothermometer assumes quantitative removal of Mg *+ during hydrothermal alteration of fluids at temperatures greater than m 15O”C, so that extrapolation of the Mg vs temperature mixing line to zero-Mg yields the endmember temperature (Von Damm, 1990). Extrapolation of the beach-fumarole Mg vs T mixing trends (Fig. 8) to zero-Mg yields T = 483-565°C which seem unreasonably high and thus suggest addition of Mgzf to the hydrothermal fluids after mixing with seawater due to ‘low-temperature’ chemical-weathering reactions. The quartz geothermometer/geobarometer assumes equilibrium with respect to quartz at the hydrothermal endmember, so that extrapolation of the Si vs T mixing line to intersect the quartz solubility

P. Sedwick. D. Stiiben/Matine

Chemistry 53 (1996) 147-161

close to saturation with respect to anhydrite tion index - 0.67).

159

(satura-

4.4. Origin of the hot-sand fluids

,...... 0

I

I

200

I

i

400

W’C) Fig. 9. Extrapolated regression fit to beach-fumarole Si vs temperature data, showing intersection with solubility curves for quartz in distilled water at various pressures (Kennedy, 1950); curves for quartz solubility in sodium chloride solutions of seawater ionic strength are not significantly different. Figure modified after Mottl (1983).

curves constrains the pressure-temperature conditions of the endmember. The beach-fumarole fluids are supersaturated with respect to quartz, and the Si vs T mixing trend extrapolates to intersect the quartz solubility curves at around 200°C for pressures below 500 bar (Fig. 9). This result is consistent with our proposed endmember temperature, although it should be regarded with caution due to the possible effect of the ‘low-temperature’ reactions and other equilibria on the Si content of the fluids. The hydrolysis and oxidation of SO, and H,S may also significantly overprint the fluid chemistry, in providing a source of SO:for the ascending fluids. Solutions formed from the hydrothermal alteration of SO:-- and Ca*+-bearing fluids at temperatures above N 150°C are thought to lose SOi- as anhydrite as temperature is increased (anhydrite solubility decreases with increased temperature) and Ca’+ is leached from the host rock (Seyfried and Bischoff, 1979; Seyfried and Mottl, 1982). Solutions derived from the high-temperature alteration of seawater are generally depleted in SOi- and enriched in Ca2+ relative to seawater (Von Damm, 1990). The beach-fumarole fluids show no clear depletion of sulphate over seawater and are slightly depleted over seawater in Ca*’ (Fig. 3c, 5b). This may reflect ‘low-temperature’ addition of SO,‘- to the fluids (from SO, and H,S), removing Ca*+ from the fluids via precipitation of anhydrite in the sub-seafloor, a suggestion supported by the saturation-state calculations which put the 65.W beach-fumarole fluid

Our interpretation of the fluid chemistry for the hot-sand site is limited by the small number of samples. Given that the chlorinities of the warm fluid samples are indistinguishable from ambient seawater, and that there are no subaerial geothermal springs in the immediate vicinity, it is likely that the warm fluids result from the hydrothermal alteration of seawater which penetrates into the warm volcanic sand at this location. The warm fluids are highly enriched in Si, Li+, Rb+, Fe and Mn relative to ambient seawater, and exhibit significantly greater enrichments in these species, relative to temperature, than observed in the beach-fumarole fluids. The hot-sand solutions are also enriched in Mg*+ relative to ambient seawater, which is generally atypical for seawater-derived hydrothermal fluids (Bischoff and Seyfried, 1978; Mottl, 1983). Given that the sampling site is at the base of a fissure extending from the Caldera della Fossa, it is likely that the chemistry of the fluids reflects exhalation of acidic volcanic gases such as CO,, H,S, SO, and HCl at this site, which contribute acidity to the fluids. The acidic fluids may then attack the volcanic sand at relatively low temperature ( < 1OO”C>,leaching the above-mentioned species into solution. Although the Cl- concentrations of the fluids are close to ambient seawater, the calculated depletion in Na+ suggests addition of a low-salinity component such as groundwater; in this case the addition of a low-chlorinity component may be ‘masked’ by addition of volcanic HCl. Saturation-state calculations performed with SOLVEQ for a water sample from this site (V7, 25.5”C 1 atm. Table 1) are consistent with our suggestion that the acidic fluids are attacking igneous phases in the sand: the fluid is calculated as being oversaturated with respect to the commonly occurring phases chalcedony, cristabolite and quartz only, and undersaturated with respect to all other silicate and aluminosilicate minerals in the thermodynamic data base.

5. Conclusion This study has identified two chemically-distinct submarine hydrothermal fluids vented into shallow

160

P. Sedwick, D. Stiiben/Marine

seawater around an active island-arc volcano. Hot, gas-rich fluids vented from the ‘drowned’ fumaroles appear to be derived from the high-temperature (> 100°C) hydrothermal alteration of a mixture of seawater and low-salinity groundwater, overprinted by the reactions of volcanic CO,, SO, and H,S in the fluids. The chemistry of warm fluids which seep from volcanic sand at another site may reflect lowtemperature hydrothermal alteration of seawater, also overprinted by reaction of acid volatiles. Although our data do not allow chemical flux estimates for these submarine vents, the ‘ambient’ values of pH (6.1) and sulphide concentration (153 pmol/kg) measured for waters in the embayment Pot-to di Levante provide a qualitative indication of the effect of this hydrothermal activity on the redox and pH conditions of local seawater. The significant chemical differences between fluids from these two nearby sites, and from other submarine vent fluids in similar tectonic settings, encourage further geochemical investigations of submarine hydrothermal activity in areas of arc-volcanism.

Acknowledgements The authors wish to thank Elena Ferretti, Jonathan Barnes, Chandra Nauth-Misir, and the captain and crew of the Poseidon for assistance with sample collection. David Cooke is thanked for help with the saturation-state calculations. Pa010 Colantoni provided information on submarine venting around the Aeolian Islands. An anonymous reviewer provided detailed and helpful comments on the manuscript. Peter Sedwick acknowledges the generous support of the Alexander von Humboldt Foundation during this work, and the organisers of the 13th Chemistry of the Mediterranean Symposium where some of these results were presented. This research was funded by the EC-MAST 1 programme.

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