318 Earth and Planetary Sctence Letters, 107 (1991) 318-327 Elsevier Science Pubhshers B V, Amsterdam [XleP] Gas-rich submarine exhalations during the 1989 eruption of Macdonald Seamount J.-L. Chemin6e a, p. Stoffers b, G. McMurtry c, H. Richnow d, D. Puteanus b and P. Sedwick c c, Observatowes Volcanologtques, InstltUt de Physique du Globe de Parts, 75252 Paris Cedex 05, France b Geologtsches-Palaontologisches Insutut und Museum, Unwersttat Ktel, D-2300 Ktel, FRG Department of Oceanography, School of Ocean and Earth Sctence and Technology, Unwerstty of Hawatt, Honolulu, Hawau 96822, USA a Geologlsch-Palaontologtsches Instttut und Museum, Unwersttat Hamburg, D-2000 Hamburg, FRG Received January 31, 1991, revision accepted August 14, 1991 ABSTRACT In January 1989 we observed submarine eruptions on the summit of Macdonald volcano during a French-German dlvmg programme with the IFREMER submersible Cyana Gas-streaming of large amounts of CH 4, CO 2 and SO 2 from sumrmt vents, inferred from water column anomahes and observed by submersible, was accompamed on the sea surface by steam bursts, turbulence, red-glowing gases, and black bubbles compnsmg volcamc ash, sulphur and sulphldes Chloride depleUon of water sampled on the floor of an actively degassmg summit crater suggests either boiling and phase separation or addmons of magmatlc water vapour. Submersible observations, m-sltu samphng and shipboard geophysical and hydrograpbac measure- ments show that the hydrothermal system of this hotspot volcano is dlstmgmshed by the influence of magmatm gases released from its shallow sumrmt 1. Introduction Macdonald Seamount, at the southern terminus of the Austral Island volcanic cham, is one of the most active submarine volcanoes in the world. The seamount was discovered in 1967 by detection of seismic events [1] and its activity has since been monitored by the Polynesian selsnuc network [2]. Macdonald Seamount Is about 4200 high and comes to within 40 m of the surface (Fig. 1). The volcano was intermxttently active between June 1987 and December 1988 [2] and, following a brief pause of two weeks, became active again on 19 January 1989. After a 4-day-long quiet period, eruptions began again just as the submersible sup- port ship N.O. Le Surolt arrived at Macdonald (Fig. 2). Glowing red gases, probably caused by H 2 burning and coloured by high-temperature oxidation of FeO to Fe203 in Included volcanic ash, were frequently observed. Hydrogen sulphide vapours were also apparent, especially during epi- sodes of intense turbulence m the water, steam burs(s and gas discharge. A green discolouration of the surface water appeared, spreading over an area at least one nautical mile an diameter with a thtckness of about 25 m. At the same time, the Polynesmn seismic network in Papeete recorded the explosive activity on Macdonald from T-wave arrivals [2], and we recorded the noise of the explosions and degasslng on board with a sub- merged geophone. After a 24-hour period (Fig. 2) of no surface and httle selsrmc activity, while the Cyana was diving to observe the active summit zone, water fountains and steam were observed at the surface accompanied by large black bubbles that produced a cohesive, metalhc-grey shck (Fig 3a). Underwater, the Cyana observed abundant gas release along fissures in the floor of one of the summit craters (Fig 3b). As the submersible was caught up in the eruption, observations of small submarine emissions of pyroclastlc and phreato- magmatlc material were made, although strong turbulence and greatly reduced visibility made fur- ther observations impossible. Over the total 5-day period of the eruption, strong degassing events with columns of water and steam lasting for up to 3 hours occurred five times (Fig. 2). These were accompanied twice by red-glowing gas emission Elsevier Science Pubhshers B V
Transcript
318 Earth and Planetary Sctence Letters, 107 (1991) 318-327 Elsevier Science Pubhshers B V , Amsterdam
[XleP]
Gas-rich submarine exhalations during the 1989 eruption of Macdonald Seamount
J.-L. Chemin6e a, p. S toffers b, G. M c M u r t r y c, H. R i c h n o w d, D. P u t e a n u s b and P. Sedwick c
c, Observatowes Volcanologtques, InstltUt de Physique du Globe de Parts, 75252 Paris Cedex 05, France b Geologtsches-Palaontologisches Insutut und Museum, Unwersttat Ktel, D-2300 Ktel, FRG
Department of Oceanography, School of Ocean and Earth Sctence and Technology, Unwerstty of Hawatt, Honolulu, Hawau 96822, USA a Geologlsch-Palaontologtsches Instttut und Museum, Unwersttat Hamburg, D-2000 Hamburg, FRG
Received January 31, 1991, revision accepted August 14, 1991
ABSTRACT
In January 1989 we observed submarine eruptions on the summit of Macdonald volcano during a F r e n c h - G e r m a n dlvmg programme with the I F R E M E R submersible Cyana Gas-s t reaming of large amounts of CH 4, CO 2 and SO 2 from sumrmt vents, inferred from water column anomahes and observed by submersible, was accompamed on the sea surface by steam bursts, turbulence, red-glowing gases, and black bubbles c o m p n s m g volcamc ash, sulphur and sulphldes Chloride depleUon of water sampled on the floor of an actively degassmg summit crater suggests either boiling and phase separation or addmons of magmatlc water vapour. Submersible observations, m-sltu samphng and shipboard geophysical and hydrograpbac measure- ments show that the hydrothermal system of this hotspot volcano is d ls tmgmshed by the influence of magmatm gases released from its shallow sumrmt
1. Introduction
Macdonald Seamount, at the southern terminus of the Austral Island volcanic cham, is one of the most active submarine volcanoes in the world. The seamount was discovered in 1967 by detection of seismic events [1] and its activity has since been monitored by the Polynesian selsnuc network [2].
Macdonald Seamount Is about 4200 high and comes to within 40 m of the surface (Fig. 1). The volcano was intermxttently active between June 1987 and December 1988 [2] and, following a brief pause of two weeks, became active again on 19 January 1989. After a 4-day-long quiet period, eruptions began again just as the submersible sup- port ship N.O. Le Surolt arrived at Macdonald (Fig. 2). Glowing red gases, probably caused by H 2 burning and coloured by high-temperature oxidation of FeO to Fe203 in Included volcanic ash, were frequently observed. Hydrogen sulphide vapours were also apparent, especially during epi- sodes of intense turbulence m the water, steam burs(s and gas discharge. A green discolouration of the surface water appeared, spreading over an
area at least one nautical mile an diameter with a thtckness of about 25 m. At the same time, the Polynesmn seismic network in Papeete recorded the explosive activity on Macdonald from T-wave arrivals [2], and we recorded the noise of the explosions and degasslng on board with a sub- merged geophone. After a 24-hour period (Fig. 2) of no surface and httle selsrmc activity, while the Cyana was diving to observe the active summit zone, water fountains and steam were observed at the surface accompanied by large black bubbles that produced a cohesive, metalhc-grey shck (Fig 3a). Underwater, the Cyana observed abundant gas release along fissures in the floor of one of the summit craters (Fig 3b). As the submersible was caught up in the eruption, observations of small submarine emissions of pyroclastlc and phreato- magmatlc material were made, although strong turbulence and greatly reduced visibility made fur- ther observations impossible. Over the total 5-day period of the eruption, strong degassing events with columns of water and steam lasting for up to 3 hours occurred five times (Fig. 2). These were accompanied twice by red-glowing gas emission
Elsevier Science Pubhshers B V
140 ° 120°W
G A S - R I C H S U B M A R I N E E X H A L A T I O N S D U R I N G T H E 1989 E R U P T I O N O F M A C D O N A L D S E A M O U N T 319
plays extremely low pH values compared to open-ocean seawater. N1 and Co were also mea- sured, but displayed no measurable enrichment in the effluent. The very concentrated topmost C H 4
and dissolved Mn anomahes at a depth of 120-150 m were not observed in companion hydrocasts taken near the summit 3 days earlier. However, the 120-250 m width of the CH 4 and dissolved Mn rich effluent layer was detected. This lmphes that the very intense topmost layer was introduced during the summit eruptions.
A single water sample was taken by Cyana on the floor of an actively degassmg summit crater at a depth of 156 m (Fig. 3b). Compared with am- bient seawater, the crater bottom water is 11% depleted in C1, 20% depleted in L1, about 10% depleted in Na, K, Rb, Mg, Ca and SO4, and highly enriched in Fe, Mn, S1 and Ba (Table 1).
3. Mineralogy and chemistry of the slick material
I~'U~ Ib I~-0~1~ W
Fig I (a) General location map showing Austral volcamc chain and Macdonald volcano (arrow) After [6] (b) Seabeam bathymetry map of Macdonald Seamount with 250 m contour intervals Box shows location of summit shipboard and sub- mersible lnvestlganons (Fig lc) After [6] (c) Detailed bathymetry of Macdonald summit region showing location of Cyana dive TH 30 track and hydrocast stations Station CY36- MS is the dot, arrow indicates drift Active eruption zone is
stippled Contour interval is 50 m
and large black bubbles on the surface Green water masses containing patches of metallic-grey slick were observed three times. Explosive shocks were strongly felt on board Le Surolt on the morning of 28 January. After 28 January, the eruptive activity apparently ceased.
2. Hydrocast sampling
Hydrocast samphng (CY 36-MS, 29 ° 01.840 S, 140 °15.798 W) over the Macdonald summit on 27 January revealed an intense hydrothermal plume at a depth of 120-250 m (Fig. 4). The effluent is enriched m C H 4 , H z S , Fe, Mn and Si, and dis-
Samples of the metalhc-grey slick were re- covered by skimming the surface seawater from a small boat. Microscopic and X-ray diffraction analyses of the recovered material reveal that it is primarily composed of fine-grained, amorphous volcanic glass, crystalline c~-sulphur and pyrite. There are also indications of cubatlne (CuFezS3, 5 lines), cinnabar (HgS, 6 lines) and quenstedtlte, a hydrated iron sulphate (7 lines). Quenstedtlte may have formed after sampling or when the slick was exposed on the sea surface. Small amounts of cubanlte are possible, because the slick material is enriched in Cu relative to basalt glass (Table 2). Mercury is extremely volatile and is found in volcanic gases [3-5]; reaction of Hg with sulphide would explain the presence of cinnabar.
Scanning electron microscope studies reveal numerous c~-sulphur spherules ranging from < 10 to 50 #m in diameter (Fig. 5A). Some sulphur spherules bear coatings of small ( < 1/zm) iron-rich particles that are probably pyrite (Fig. 5B) Sulphur isotopic analysis of the bulk sample yields a 8 34S value of - 3.1%o, which is consistent with a magmatlc origin for the sulphur. These spherules probably formed in the H2S-rlch eruptive plume. Sulphur also occurs as coatings on the volcanic glass particles (Fig. 5C). The glass particles occur in both dense and highly vesicular forms, the latter reflecting the gas-rich nature of the Mac-
320 .1 - L C H E M I N E E E T A L
I I I I I I I . . . . . . . . I • I • I
I 0 T-wave recording I
O Observed eruptwe event o o
z
o 1
N m N • • m
o o o o o
! °°
z
. , . , . . . . . . . , - , - . - , - . . ,
18 19 20 21 22 23 24 25 26 27 28 29
Juhan Day
Fig 2 Time-line of the January 1989 eruption of Macdonald volcano and major samphng and observational events
donald emissions (Fig. 5D). Based on the chemical composition of volcanic glasses of the eruption of May 1986 recovered from dredge samples of the summit surface ([6], Table 2), the glass is probably an alkali basalt.
The bulk chemical composition of the slick material indicates substantial concentratzons of S, Si, Fe, Zn and Cu (Table 2). These data generally reflect the particle mineralogy, although no Zn sulphades were found and the bulk Cu concentra-
TABLE 1
Chermstry of Macdonald summit crater bottom water *
Component Crater bottom Cl-normahzed Ambient Bottom water water crater bottom seawater * * * enrichment ( + ) or
* Water samples were processed on board ship by filtration through prewashed 0 2 ~m Nuclepore filters followed by pH adjustment to _< 2 0 with ultrapure mtnc acid Ll, Na, K, Rb, and Fe and Mn for the crater bottom water, were deterrmned by flame AAS Estimated preczslons based on multiple deternnnatxons of samples and standards are within 3% for Lz, Na, K, Rb, Fe and Mn Mg ( < 0 2%) and Ca ( < 0 2%) were deterrmned by EGTA and EDTA titration Ba (15%), and Fe and Mn (15%) for the ambient water, were determined by flameless AAS Dissolved Sl (1%) was determined colonmetncally as sahcomolybdate Sulphate (1%) was determined by ion chromatography Chloride ( < 0 2%) was determined by AgNO 3 t~tratlon with electrometnc end point. * * Values normahzed to the ambient C1 concentratmn, assuming conservative behaviour for CI * * * Ambient seawater from hydrocast CY32-MS-3 (29°01 840 S, 140°15 798 W), water depth =154 m
G A S - R I C H S U B M A R I N E E X H A L A T I O N S D U R I N G T H E 1989 E R U P T I O N O F M A C D O N A L D S E A M O U N T 321
Fig 3 (a) Metalhc-grey shck on the sea surface from a sumrmt eruption of Macdonald Seamount m the Austral volcanic chain The slick material is primarily composed of fine-grained, amorphous volcanic glass, crystalhne a-sulphur and pyrite Bright circular areas are gas bubbles trapped in the shck material (b) Photograph of a Macdonald sumrmt crater floor taken from submersible Cyana
during an erupttve cycle Note copious gas-streaimng from numerous small vents m pyroclastlc debris Water depth is 156 m
O,
J -L C H E M 1 N E E LT A L
50"
100
250
322
g l .
150
200
Diss. Si02 (Bmol/kg)
0 5 I0 15 20 25
CH4 (Bmol]kg)
100 200 300
50
100
150 '
200
250
Total Fe (p.mol/kg) Diss. Mn (l~mol/kg)
0 1 2 3 0 1 2 3
J
H2S (l~mol/kg) pH
0 2 4 6 8 10 12 5 6 65 7 75 8 85
• , . , . . , , . ,
Fig 4 Profiles of CH4, total Fe, dissolved Mn, dissolved S1, H2S and pH for Macdonald sumnut hydrocast CY36-MS Shaded area indicates bottom Water samples for major and nunor inorganic constituents were processed on board sbap either by filtration through prewashed 0 2 /zm Nuclepore fdters followed by pH adjustment to _< 2 00 with ultrapure mtrlc acid (for &ssolved basis) or by pH adjustment to _< 2 0 with ultrapure nitric acid (for total basis) Fe and Mn were determined by flameless atomic absorption spectrophotometry (AAS) Precislons for Fe and Mn are wRhan 15% Dissolved SI (1%) was determined colonmetrically as sdicomolybdate CH 4 was deterrmned by gas chromatography H2S was preserved shipboard by precipitation with Zn acetate, ZnS was collected by centrlfugat~on and measured colorlmetrically using phenylene dlamlne reaction Sample pH was deterrmned
mamedmtely after recovery on board
GAS-RICH SUBMARINE EXHALATIONS DURING THE 1989 ERUPTION OF MACDONALD SEAMOUNT 323
T A B L E 2
Chermcal c o m p o s m o n of Macdona ld Seamoun t surface slick
mater ia l *
Shck Basalt Enr i chmen t
mater ia l glass * * factor * * *
Element (wt %) N a 2 0 1 82 3 62 1 10
K 20 0.45 1.23 0.80
M g O 4 46 5.33 1.82
CaO 4 88 11 37 0 94
A1203 7 09 15 45 1 00
$102 19 66 43 80 0 98
Fe203 11 61 14 00 1 81
17205 0 22 0.53 0 91
T 1 0 2 1 72 4 19 0 90
SO 3 23.56 - -
Subtotal 75 47 99 52 -
Element (ppm) Mn 574 1400 0 89
V 185 - -
Cr 261 39 14 6
N1 368 56 14 3
Cu 783 89 19 2
Zn 1568 164 20.8
G a 16 22 1 59
As 7 - -
Rb 11 27 0 89
Sr 739 697 2 31
Y 11 27 0 89
Zr 96 243 0 86
N b 20 55 0 79
Ba 131 347 0 82
La 20 - -
Ce 34 - -
N d 20 - -
Sm 2 - -
Sc 8 - -
Pb 28 - -
Th 16 - -
* All analyses by X-ray f luorescence spec t romet ry
* * F r o m 1989 Macdona ld erupt ion [6] Whole- rock analysis
of a volcanic rock collected on Cyana dive T H 3 0 shows that
the composi t ion of these rocks is near ly identical [44]
* * * En r i chmen t factor = [ ( X / A 1 ) s h c k / ( X / A 1 ) b a s a l t , where
X = e lement concent ra t ion
tlon is low relative to the detectmn of cubamte. Such apparent discrepancies between sample splits are not unusual, however, given the heterogeneous nature of the slick material. Enrichment factors calculated relative to Macdonald basalt glass show large values for Zn, Cu, Ni and Cr (Table 2), in accordance with their chalcophile (Zn, Cu, Ni) and slderophile (Ni, Cr) nature [7]. The Ni enrichment
may be unique to magmatlc systems, as it is usually only found as a sulphide in vesicle linings [8]. Chromium may be scavenged from seawater under reducing condmons as the oxide [9,10]. The slick material contained 970 d p m / g 2 ] ° p o a t the time of collection relative to a parent 21°pb activ- ity of 10 dpm/g , or an activity ratio to 21°pb of 97 (J.D. Macdougall, pers. commun., 1989). This en- richment accounts for a portion of the 2t°po that is totally lost by the Macdonald magma during its eruptions [11].
4. Interpretation
The Macdonald hydrothermal plume is distinct when compared with plumes detected over active spreading ridges [12-15] and other submarine hotspot volcanoes such as Loihl and Teahitia seamounts [16-19]. Craig et al. [19] reported very h i g h 8 3 H e values of 639%, o r R / R A = 7.4, I n
hydrocasts taken during a Macdonald summit erupuon in 1987. The large dissolved S~ anomalies reported here have only been previously reported for the megaplume observed over the Juan de Fuca Ridge [20]. To our knowledge, this is the first report of H z S in an open-ocean hydrothermal plume. Table 3 presents comparisons of the most intense Macdonald effluent sample at 136 m with selected vent water samples from Loihi Seamount and mid-ocean ridge hydrothermal sites. We have normahzed the chemical data to the dissolved Mn and $1 anomalies. This approach assumes con- servative behaviour for Mn and S1. For Mn, there ~s no difference within the analytical uncertainty between the total dissolvable and <0 .2 ~m filtered (dissolved) samples in hydrocast CY36- MS, indicating that this element is at least behav- ing conservatively in the near-field plume. The dissolved S1 may not behave as conservatively; however, these data are presented for comparison.
Relative to dissolved Mn, CH 4 shows more than a 4000-fold enrichment compared with the Loihi vent water values, and nearly 7000- and 20,000-fold enrichments over the Galapagos and East Pacific Rise (EPR) at 21°N vent waters; CH4/Si ratios display similar enrichments. The H z S / M n ratios are 6 to 18 times higher than at the Galapagos and 21°N EPR sites, whereas H2S is extremely enriched compared with the Loihl vent waters, which have such low concentrations
324 J -L CHEMINEE ET AL
Fig 5 Scanning electron microscope photographs of surface slick material (A) Spherules of a-sulphur (B) Sulphur spherule with coating of small Fe-nch particles (pynte'~). (C) Volcanic glass particles with sulphur coatings (D) Volcanic glass particle showing
highly vesicular form
TABLE 3
Chermstry of Macdonald Seamount hydrocast CY36-MS and comparison with selected deep-sea hydrothermal vent sites
Molar ratio Macdonald Lolht * Galapagos * * EPR 21 ° N * * * C H 4 / M n 1385 0 34 0 19-0 21 0 067 ECO2/Mn 11,000 a 14,200 80-250 6 5 b H 2 S / M n 40 0 2 2-6 9 c 5.4 F e / M n 13 48 0 21-0 66 - S1/Mn 83 55 15-47 23
* Pele vents (30°C) [23] * * Galapagos Rift composite vent water data from [10,34,41,42] * * * 304°C vent water from sulptude chimney [42]
Calculated ECO 2 from pH measurement and assumed alkalinity of 2 35 meq /kg (see text) b 3ZCO 2 value from [43] normahzed to Mn and S1 values [42] c Ldley et al [42], tughest values § Ratios × 1000
G A S - R I C H S U B M A R I N E E X H A L A T I O N S D U R I N G T H E 1989 E R U P T I O N O F M A C D O N A L D S E A M O U N T 325
in recovered samples that they are essentially un- measurable (detection limit of 0.1/~mol/kg, [21]). The pH anomaly cannot be due solely to the measured water column H2S concentration (Fig. 3). No direct measurements of total dissolved CO 2 (ECO2) were made; however, if the pH anomaly is assumed to be the result of injection of dissolved CO2, ttus corresponds to a minimum water col- umn ECO 2 of approximately 4.7 mmol/kg, given a water column alkalinity near 2.35 meq /kg [pre- hminary data from SONNE cruise 65 supports this assertion; [22], P. Sedwlck (unpubl. data)]. The calculated )~CO2/Mn anomaly ratio in Table 1 is comparable to Lolhi vent waters, and is en- riched relative to the mid-ocean ridge sites. The estimated ECO 2 added to the water column is 70 times greater than that calculated for the plume over Loihl Seamount [16]. The inferred CO 2 en- richment and measured CH 4 and H2S anomalies in the Macdonald plume probably result from shallow summit magma reservoir degassing that is enhanced by the eruptive activity, in contrast to the deeper, passive hydrothermal emissions found on Loihi [23]
The inferred great CO 2 and sulphur outgassing of the Macdonald eruption is not unreasonable. In Kllauea volcano in March 1983, for example, the CO 2 content in dry gases was about 12%, with about 44% SO 2 [24]. One year later, the CO 2 flux was 3-5 × 103 tonnes/day, and the SO 2 flux was 18-27 × 103 tonnes /day [25] For the active volcano Piton de la Fournalse, the CO 2 content in dry gases is 45%, with about 49% SO 2 [26]. From the known terrestrial magmatic flux of these gases [24-28] it is referred that large amounts of CO 2 and SO 2 were introduced to the water column during the Macdonald erupUon. The large CO 2 addition suggested by the water column pH anomaly is supported by ECO 2 measurements of a hydrothermal plume at the same site in Novem- ber, 1989. The plume ECO 2 present during this seisnucally inactive period (up to 4.1 mmol/kg, as measured by pH and alkalinity, and by coulome- try) is surprisingly similar to that indicated in the plume during the January eruption. Correspond- mgly large amounts of methane but no H2S or surface slick were detected on November 13, sug- gesting that the major difference during the January eruption is the addition of SO 2. Upon cooling, the SO 2 has probably created the large
amounts of observed H2S and elemental sulphur by hydration reactions (4SO 2 + 4H20 = H2S + 3H2SO 4 and 3SO 2 + 2H20 = [S] + 2H2SO4) simi- lar to those proposed for coohng hagh-temperature hydrothermal fluids with relatively low H 2 fugaci- ties [29]. Dissociation of the H2SO 4 produced would contribute H +, which could lower our estimate of the Macdonald dissolved CO 2 ad- dition based on pH measurements. However, even for the maximum measured H2S concentration m the Macdonald plume (about 10 /~mol/kg) the stoichlometric production of 30 ~tmol/kg H2SO 4 would not significantly alter the pH given the buffering capacity of seawater.
The Fe, Mn, Si and Ba enrichments of the bottom water sample indicate that the sample is a mixture of vented hydrothermal solution and am- bient seawater. Removal of C1 into alteration products during seawater-basalt interaction has been documented above 400°C [30], a solution temperature that could only be reached at depths approaching 4.5 km (450-500 bar) [31]. The shal- low summit elevation of Macdonald together with the observational evidence of copious degassing therefore suggest that the C1 depletion results from either: (a) addition of vapour-rich, high-tempera- ture ( > 200°C, the boiling point of seawater at 156 m; [32]) hydrothermal fluid that has under- gone partial phase segregation [33] before venting from the crater; or (b) added magmat~c U20 that is low in C1. Water vapour becomes an increas- ingly important contributor to magmatlc gases at depths of < 2 km [34,35], but Michard et al. [36] invoked magmatlc processes to explain high C1 concentrations in vent waters at 13°N EPR.
If the crater bot tom water C1 depletion is a result of either (a) or (b), then normalislng to the ambient C1 concentration allows a first-order cor- rection for partial phase segregation of water vapour addition, assuming conservative behavlour for the elements m Table 3 during these processes. The Cl-normahsed Na, K, Rb and SO 4 concentra- tions are analytically indistinguishable from am- bient seawater values, whereas Cl-normahsed Ca and Mg are 3% enriched over ambient seawater (significantly greater than the analytical uncer- tainty) and L1 is 11% depleted (also significantly greater than the analytical uncertainty). The Mg and Li trends are puzzhng, since Mg is thought to be removed from solution during all seawater-
326 J -L CHEMINEE ET AL
basalt reactions above 70°C, and Li is known as a soluble element during hydrothermal interactions [37,38]. Low-temperature basalt alteration may be responsible for LI uptake, and could explain the apparent Mg enrichment [39]. The 23°Th and Sr isotopic evidence suggest that basalt alteration occurred prior to the Macdonald eruptions [11], but the exact processs is presently unknown. It is clear, however, that the shallow, eruptave summit of Macdonald Seamount is contributing large amounts of magmatic gases to the surrounding seawater that are in turn influencmg the chemistry of many of the observed &ssolved and particulate components. Our study further indicates that the composition of the gases emitted from a sub- marine hotspot volcano can change with eruption intensity, which is probably related in turn to changing height of the magma reservoir, m accor- dance with models of gas exsolution [34,35].
Acknowledgements
We thank the captain and crew of N.O. Le Surolt and the Cyana submersible team for their able assistance under sometimes stressful condi- tions. The cruise was jointly funded by IFREMER, the Bundesmmlstermm fiir Forschung und Techno- logw and the Deutsche Forschungsgernemschaft. GM acknowledges the support of the U.S. N O A A - U H Sea Grant College Programme. HR acknowledges the support of W. Michaehs. We thank D. Karl for helpful discussion and for pro- vtding the H2S measurements, N. Binard for the surface slick photograph, and F. Goff, A. Mala- hoff and M. Mottl for helpful comments.
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