Contrasts of Fluid Chemistry, Isotope Compositions

Contrasts of Fluid Chemistry, Isotope Compositions and Temperature in Modern Seafloor Hydrothermal Systems N.S. Bortnikov IGEM, Russian Academy of Sciences, 119017 Staromonetny 35, Moscow, Russia Abstract- A comparative study of fluid inclusions, He and hydrocarbons in fluid inclusions trapped in sulfides and S isotopes in sulfides from modern sulfide edifices on the Mid-Atlantic ridge (Rainbow, Logachev, Broken Spur, TAG), East Pacific Rise (EPR 9 N, 21 N) and in the Manus and Lau back-arc basins (Pacific) was carried out. Variations in fluid salinity were found. Methane through butane occur in fluids. R/Ra ratio indicates a contribution of helium from upper mantle and recycled seawater to fluids. The S isotope ratio suggest a mixing of sulfur derived from different sources.

I.

INTRODUCTION

The submarine sulfide edifices are of great interest because they have many features in common with massive sulfide ores in ancient mobile belts, which are among the main sources of base and precious metals. A study of modern seafloor hydrothermal systems may provide insights for understanding the genesis of the massive sulfide ores deposited at spreading centres of the world ocean and those found on land. The reconstruction of a fluid chemistry seems to be important because metal concentrarions depends on the fluid salinity. Our knowledge on chemistry of mineral-forming fluids deposited sulfide edifices at oceanic spreading zones is mainly based on results of direct temperature measurements using submersibles, analysis of the fluids venting onto the seafloor from sulfide chimneys, and data of thermodynamic and experimental modeling. Monitoring of active hydrothermal fields located on mid-oceanic ridges and back-arc spreading zones showed that the temperature, pH, and salt content in fluids change with time and geological environments [7, 8, 15]. A decrease in temperature (by 10 to 15◦C), pressure, and chlorine and dissolved volatile contents over several years was revealed. A study of the sequence of deposition of mineral assemblages in seafloor sulfide edifices revealed successive mineral replacements and allowed us to assume a decrease in mineral formation temperatures from 370 to 240◦ C or lower during the growth of one edifice and even one chimney [2]. These changes are apparent because the direct measurements allow one to estimate the evolution of the physicochemical parameters and composition of fluids for short periods (several years), while sulfide edifices grow over several thousands to tens of thousands of years. It is of interest to find out to what extent the fluid temperature and composition change in active seafloor systems during the period of their activity. These data can be obtained by study of fluid inclusions in minerals, because this technique permits quite reliable estimation of physicochemical parameters of hydrothermal systems. In order to compensate for the lack of available data, we studied fluid inclusions in minerals from sulfide edifices located on the slow-spreading Mid-Atlantic Ridge (the Logachev, Rainbow, and Broken Spur hydrothermal fields) in different geological settings, fast- to intermediate spreading East Pacific Rise (EPR 9oN and EPR 21oN), and in the Manus, Lau and Woodlark basins of the southwestern Pacific (Fig. 1). Several hydrothermal fields associated with rocks of different composition were found on the Mid-Atlantic Ridge. Some of them (Lucky Strike, Broken Spur, TAG, and Snake Pit) are confined to neovolcanic zones and linked with domes composed of normal basalts of mid-oceanic ridges (N-MORB). Other hydrothermal fields (Rainbow and Logachev) are related to serpentinites uplifted to the seafloor along rifts and faults. The examined samples were collected during dives of Mir submersibles during cruises of the R/V “Akademik Mstislav Keldysh”. II. GENERAL CHARACTERISTICS More than 300 sites of hydrothermal activity have been discovered on the ocean floor. About 100 of them vent hydrothermal fluids responsible for the precipitation of high-temperature massive sulfide deposits [10]. They occur at mid-ocean ridges, in back-arc basins, and on submarine volcanic arcs. These mineralization are related to mid-ocean ridge basalt, ultramafic intrusive rocks, and more evolved volcanic suites in both oceanic and continental crust. The majority of known sulfide edifices occur on fast-spreading mid-ocean ridges, but the largest massive sulfide deposits are located at intermediate- and slow-spreading centers, at ridge-axis volcanoes, in deep back arc basins, and in sedimented rifts adjacent to continental margins.

Fig. 1 Distribution of the main sulfide deposits in the World Ocean. Names of the studied edifices are underlined.

The most typical hydrothermal sulfide edifices are mounds oval or elongated in plan several tens to 200–400 m in size and from several to 50 m in height. Mounds are commonly surmounted with numerous chimneys from 10–20 cm to 2–10 m (rarely to 20–30 m) in height. Some active chimneys and spire- and cone shaped edifices occur on truncated cone basements or on thin (20– 30 cm) sulfide crusts. The basement consists of sulfide ore fragments, accreted sulfide chimneys, or low-temperature hydrothermal deposits. Individual sulfide and sulfate–sulfide edifices and their clusters and chains are also found within some hydrothermal fields. Stockwork zones composed of sulfides and sulfate–sulfide veins are also known. The modern sulfide edifices have relatively simple mineral composition. They are mainly composed of pyrrhotite, pyrite, marcasite, chalcopyrite, isocubanite, sphalerite, as well as gangue anhydrite, barite, and amorphous silica. However, pyrrhotite and isocubanite were not found in some edifices. Bornite, galena, tennantite–tetrahedrite, and Pb and As sulfosalts occur in several hydrothermal sites. Some rare minerals identified only in one or two edifices are as follows: native bismuth, native gold, loellingite, argentite, arsenopyrite, pentlandite, and millerite [3]. “Invisible gold” was detected in sulfide minerals. The sulfide chimneys have a concentrically zoned structure around feeders or smaller channels. Individual zones differ in mineral assemblages and grain shapes of crystallizing sulfides. An alternation of zones of porous, zonal colloform, spherulitic, and dendritic fine-grained sulfide aggregates with zones of euhedral sulfide crystals is typical of these edifices. Sometimes, the bands are composed of the same minerals of different generations. Structural and textural differences may indicate different mechanisms and conditions of mineral crystallization. The sequence of mineral crystallization, evolution of mineral composition, and structural–textural features of mineral aggregates show that the history and mechanisms of mineral precipitation differ in individual sulfide edifices due to a variable rate of mixing of high-temperature hydrothermal fluids with cold seawater and to different physicochemical conditions in edifices: some of them are high-temperature, while others are medium-temperature hydrothermal formation [2]. III. FLUID INCLUSIONS Generally, fluid inclusions in minerals from the modern sulfide edifices are extremely rare. They are found in anhydrite, opal, and barite. Most of the inclusions are small (5-25 µm) and diameters ≥30 µm are exceptional. Fluid inclusions of primary, pseudosecondary, and secondary origin were identified following the criteria outlined by Roedder (1984). Two-phase, liquid-rich fluid inclusions have been commonly classified by their phases observable at room temperature. One exception is anhydrite from the Logatchev field, in which three compositional types fluid inclusions were identified: (1) type I contains three phases: an aqueous phase, vapor, and solid, (2) type II contains liquid and vapor, and (3) type III inclusions are vapor-rich. The liquid-rich and vapor-rich inclusions are confined to the same mineral growth zones.

Fig. 2 Fluid salinity in fluid inclusions vs. homogenization temperatures. (a) Sulfide edifice in the Logachev-1 field: (1) type I fluid inclusions in anhydrite; (2–4) type II fluid inclusions in anhydrite I; (5) anhydrite II; (6) salinity of seawater under standard conditions (25°C). (1–3) After Bortnikov et al., 1997; (4, 5) this study. (b) Sulfide edifices in the Rainbow field: (1) anhydrite from the anhydrite–marcasite–chalcopyrite assemblage; (2) anhydrite from the chalcopyrite–sphalerite–anhydrite assemblage; (3) salinity of seawater under standard conditions (25°C). (c) Sulfide edifices in the Broken Spur field: (1) sample 3348-1; (2) sample 3348-2a; (3) sample 3348-2b. (d) Anhydrite from chimneys EPR: (1) 21° N EPR (sample 4683-2), (2) 21° N EPR (sample 4683-6), (3) 9° N EPR (sample 4669-2). (e) Vienna Wood sulfide edifice: (1) anhydrite; (2) barite; (3) opal; (4) salinity of seawater under standard conditions (25°C).

All two-phase fluid inclusions in anhydrite were homogenized into liquid. Three phase fluid inclusions (type I) were homogenized into liquid at 363◦ C. Salt crystals in these fluid inclusions identified as halite by their optical properties disappeared by +25◦C. We can assume that the salt content in the trapped fluid reached 26 wt % NaCl-equiv. A salt content in the fluid inclusions homogenized at 248 to 355◦ C was estimated to have ranged from 4.2 to 16.2 wt % NaCl-equiv. (Fig. 2a). It should be noted that fluids with higher salt contents are higher temperature, while low saline fluids are lower temperature that is evidence for mixing of contrasting fluids. The homogenization temperatures of the vapor-rich fluid inclusions in vapor phase were identical to those for the liquid-rich fluid inclusions. This indicates their simultaneous trapping and formation by phase separation of one parental fluid into two phases upon a pressure drop. Thus, the study of fluid inclusions showed that several immiscible phases and compositionally distinct fluids circulated in the mineral-forming system of the Logachev hydrothermal field. A high-temperature aqueous fluid containing up to 26 wt % NaClequiv. was among them. Its maximum temperature could reach 365◦C. In addition to the liquid-rich fluid, a high-temperature vapor also circulated in the system. Its chemical composition has not been determined precisely, but we believe that aqueous vapor prevailed in this fluid. In general, the anhydrite deposition occurred from a fluid with salt contents of from 4.5 to 6.5 wt % NaCl-equiv. at 270 to 350°C. Fluids with salinities of 4.1 to 7.7 wt % NaCl-equiv. were trapped in fluid inclusions in anhydrite from the Rainbow sulfidesulfate edifices that were homogenized at 143 to 372◦C, mainly, at 143 to 186◦C and 282 to 365◦C (Fig. 2b). Fluid inclusions found in aragonite trapped a low-salinity fluid (2.7 to 3.7 wt % NaCl-equiv.) at 144 to 315◦C. Because the ocean depth at the sampling site is about 2300 m, the true temperatures of anhydrite deposition in the chalcopyrite-sphalerite-anhydrite aggregates were about 160 to 370◦ C. Therefore, the salinity of fluids depositing the sulfide-sulfate mineralization in the Rainbow hydrothermal field was twice as high as the salinity of seawater. A wide variation in the crystallization temperature is typical of mineral formation in the Rainbow hydrothermal field. The fluid inclusions in anhydrite from sample from Broken Spur field trapped the fluid with salt content of 3.0 to 6.3 wt % NaCl-equiv. The homogenization of fluid inclusions occurred at 271 to 398◦C (Fig 2c). The direct measurements showed that the maximum temperature of the fluids discharged onto the seafloor at the Broken Spur hydrothermal field was 366°C. This is much lower than the temperatures obtained by fluid inclusion study. Fluid inclusions in anhydrite from chimney P at EPR 9° N were typically homogenized at 320-376°C, more rarely at 270310°C and fluid salinity estimated from ice melting varies from 4.0 to 12.6 wt % NaCl equiv. (Fig. 2d). Fluid inclusions in anhydrite from the chimneys at EPR 21° N were homogenized within a wider temperature range of 142 to 380°C. The salinity of fluid entrained is nearly constant (5.0 to 7.8 wt % NaCl equiv), though the salinity occasionally sharply changes and decreases to 2.3 wt % or increases to 9.8 wt % NaCl equiv. This study of fluid inclusions indicates that variation in the fluid salinity and temperature in the EPR hydrothermal systems studied is significantly wider than was shown by direct measurements periodically conducted using the Alvin submersible. This study demonstrates an exceptionally wide evolution of the fluid salinity even during formation of one sulfide chimney, i.e., less than 15 years in the EPR 9° N hydrothermal system. These data also contradict the generally accepted viewpoint that the chemistry and temperature of the fluid in the EPR 21° N hydrothermal system has remained relatively stable for more than 20 years. Fluid inclusions in anhydrite, barite, and opaline silica from the Vienna Wood edifice contained fluids of different salinity encapsulated at the wide temperature range. A fluid entrained by inclusions in anhydrite homogenized usually at 224 to 268°C contains 5.3 to 7.2 wt % NaCl-equiv. (Fig. 2e). Fluid inclusions homogenized up to 308°C were also found in anhydrite. If the homogenization temperatures of fluid inclusions increase, the salt concentrations in the trapped fluid also increase. Barite trapped the primary fluid inclusions with the aqueous fluid with the salt concentration of about 6.6 wt % NaCl-equiv., which where homogenized at 216°C. Secondary fluid inclusions in this mineral homogenized predominately at 165 to 210°C contain the fluid with the salinity of 4.7 to 7.6 wt % NaCl-equiv. Fluids in fluid inclusions in opaline silica encapsulated the diluted fluid with the salt content of 1.6 to 4.2 wt % NaCl-equiv. These fluid inclusions were homogenized at low temperature of 102 to 118°C. Primary fluid inclusions in barite from the sulfide-sulfate edifice on the Franklin seamount trapped the fluid with salinity of 2.7 to 6.9 wt % NaCl-equiv. at 185 to 302°C. Fluid composition and homogenization temperatures of fluid inclusions in the inner and outer growth zones of barite are different: the inner zone contains inclusions trapped the low-salinity fluid (2.7 to 3.3 wt % NaClequiv.) at 200 to 205°C, while fluid inclusions in the outer zone contain the fluid of higher salinity (5.4 to 6.9 wt % NaCl-equiv.), and their homogenization occurred at higher temperatures of 185 to 302°C. The pressure-corrected temperatures for barite crystallization range from 203 to 316°C. The study of fluid inclusions has revealed that the salinities of the hydrothermal fluids depositing the sulfide–sulfate ores of the Rainbow edifice (up to 8.5 wt % NaCl-equiv.) and the Vienna Wood edifice (up to 7.7 wt % NaCl-equiv.) were two times higher than the salt content in seawater. An even higher salinity of the mineralforming fluid was recorded in the Logachev-1 hydrothermal system, where highly concentrated hydrothermal fluids with a salt content five times higher than that in seawater were found. The data obtained indicate that the fluids discharging from sulfide edifices on the ocean floor differ in composition from seawater, which assumed to be the major component of the mineralforming fluid in generally accepted models of seafloor

hydrothermal systems. Finding that fluid inclusions in minerals from the Vienna Wood edifice display salinities that range to both higher- and lower-than seawater may infer phase separation of the hydrothermal fluid resulting in the formation of two compositionally different phases. A discovery of coexisting liquid- and vapor-rich fluid inclusions in anhydrite from the Logachev site is a first record of fluid phase separation in the “black smoker” hydrothermal system. IV. LIGHT HYDROCARBONS Saturated and unsaturated hydrocarbons were found in the fluids [5, 6]. The saturated hydrocarbons are methane, ethane, propane, and butane, while unsaturated hydrocarbons are represented by ethylene (C2H4), propylene (C3H6), and butylenes (C4H8 and i-C4H8). The hydrocarbon contents vary in different samples. Their content in fluid inclusions varies considerably (Fig. 3). The CH4 content and С2-С5 hydrocarbon contents in fluid inclusions in sulfides from fields on the MAR range from 19.2 to 1181.0х10-6 cm3 (STP)/g and from 0.44 to 441.5х10-6 cm3 (STP)/g. The fluid inclusions in sulfides deposited in back-arc environments are generally poor in hydrocarbons relative to those precipitated in the hydrothermal systems at Mid-Atlantic Ridge. In general, the CH4 content in fluid inclusions in sulfides from the edifices at the back-arc spreading zones ranges from 1.59 to 74х10-6 cm3 (STP)/g), while the С2-С5 hydrocarbon content in these fluid inclusions varies from 0.15 to 105.7х10-6 cm3 (STP)/g). The CH4/ΣC2-C5 ratio ranges from 1.6 to 35.8, 2.3 to 39.6, 2.1 to 43.6, and 1.6 to 20.5 in fluid inclusions in sulfides from Broken Spur, Mir, Rainbow, and Logatchev, respectively. These values in fluid inclusions from sulfides deposited in back arc environment are 1.0 to 13.5. The following contents were found in samples from EPR 9° N (cm3/g · 10–6): 0.76-1035 CH4, 0.07148 C2H4, 0.18-191 C3H6, 35 i-C4H10, 89.2 n-C4H10. Samples from EPR 21° N contain 38.2-980 CH4, 2.42-79.6 C2H4, 1.27-111 C3H6, 0.22-19.6 i-C4H10, and 0.32-50.7 n-C4H10. The methane distinctly dominates among hydrocarbons (up to 80-90%) and shows positive correlation with the sum of heavy hydrocarbons. The above values are absolute quantities of volatiles per gram of samples. The measurement of water is needed to estimate the mole proportions of the various hydrocarbon gases. Interpretation of the data must take into account a presence of more than one generation of inclusions in samples. This makes it difficult to compare the above data with those measured for venting fluids. Nevertheless, they indicate that variations took place both within a single vent and in different vents.

Fig. 3 CH4 vs. ∑C2+C5 (a) and . ∑C2+C8 (b) contents in fluid inclusions trapped by sulfides from modern sulfide edifices. (a) Explanations: M-Manus basin, NL-north Lau basin, CL-central Lau basin, BS-Broken Spur, Mir-Mir zone (TAG), R-Rainbow, L-Logatchev. (b) EPR: (1) 9° N EPR, (2) 21° N EPR.

V. HELIUM ISOTOPES 3

4

3

4

The R/Ra = ( Не/ Не)/ ( Не/ Не) air ratios in fluids trapped by fluid inclusions in sulfides vary considerably in different geodynamic settings and from a site to a site (Fig. 4). The R/Ra ratio in the fluids deposited the Rainbow sulfide edifices varies from 5.2 to 7.2. This value in fluids capsulated in sulfides from the Logatchev field is 5.6 to 8.3. A chimney sample from the Logatchev field is characterized by an increase in the R/Ra ratio from the outer zone (6.3) towards the core (8.0). The R/Ra ratio in fluids from the Broken Spur field vary from 4.6 to 7.1, and those from the “Mir” mounds range from 4.6 to 7.3. This ratio increases in the fluids in the chimneys related to serpentinized ultramafic rocks [4, 5]. The R/Ra ratio of fluid trapped in

inclusions in sulfides from the Logachev and Rainbow fields is comparable to the R/Ra ratio suggested for the upper mantle, i.e. 7-8, and may indicate a significant contribution of the upper mantle helium into the hydrothermal mineral-forming system.

Fig. 4. 3He (a) and R/Ra (b) ratio vs. 4He content in fluid inclusions trapped by sulfides in modern edifices. (a) – (1) 9° N EPR, (2) 21° N EPR (our data), (3) 21° N EPR [14]. (b) Explanations: M-Manus basin, NL-north Lau basin, CL-central Lau basin, BS-Broken Spur, Mir-Mir zone (TAG), R-Rainbow, L-Logatchev.

The R/Ra ratio is mainly from 7.7 to 8.4 (with the exception of one R/Ra ratio equal to 4.61) in fluid entrained by sulfides from chimneys of EPR 9° N, while this figure is from 5.8 to 7.1 in fluids from chimneys at EPR 21° N (Fig. 4 a). The ratio was 1.73 in the anhydrite-hosted fluid from chimneys at 21° N EPR. It should be noted that the fluid from vents, which make up sulfatesulfide chimneys at 21° N EPR, seemed to be slightly enriched in radiogenic isotope 4He relative to fluid from the EPR 9° N hydrothermal system and relative to previously obtained data on fluid inclusions from sulfides of EPR 21° N (R/Ra = 7.1-8.0) and fluid taken from the active vent (R/Ra = 7.8)]. The R/Ra values of 7.7 and 8.4 obtained for the fluid that produced sulfides of chimney P at EPR 9°N are close to the upper mantle 3He/4He ratio of 8 ± 1. This suggests that He was supplied from the upper mantle owing to its degassing. The released radiogenic helium could reduce the R/Ra ratio in the mineral-forming fluid to 5.5 or less. Since the EPR 21°N hydrothermal system has been operating over a significantly longer period than the EPR 9°N counterpart, the content of radiogenic He is higher in the former system. However, mantle He significantly predominates in this system. The R/Ra ratio in fluids responsible for sulfides precipitation in the “Vienna Wood” field is 3.05 to 4.3, while others edifices such as “White Tower” and “Died Wood” were formed from fluids with higher R/Ra ratios of 6.7, 5.1 and 7.7, respectively. Fluids trapped in sulfides from edifices in the central Lau Basin are characterized by the R/Ra ratio of 5.3, while those trapped in sulfides from the northern Lau Basin have values from 3.1 to 4.5 (Fig. 4b). These results indicate a contribution of mantle He to the hydrothermal fluids regardless of the tectonic setting. Hydrothermal fluids from deposits in a back-arc tectonic setting are clearly enriched in radiogenic 4He relative to those venting from EPR chimneys suggesting an involvement of He from recycled seawater or pore water from sediments recycled in the subduction zone. Fluids venting in the from Mid-Atlantic Ridge are enriched radiogenic 4He isotope relative to EPR chimneys as well. Thus, the helium isotopic composition of mineralizing fluid is variable. This could be related to mixing of helium derived from different sources (seawater, basalts, degassing mantle) or to phase separation of fluid. VI. SULFUR ISOTOPE SYSTEMATICS 34

The δ S values in sulfides from the edifices formed in the modern hydrothermal fields at the Mid-Atlantic Ridge and the back arc environment varies from –8.0 to +15.0 ‰ (Fig. 5). For basalt-hosted massive sulfide deposits on the Mid-Atlantic Ridge, the δ34S ratios range from -4.4 to +8.2 ‰, while those obtained for sulfides related to serpentinites are enriched in the heavier 34S

isotope (+0.7 to +13.8‰). Sulfur isotope composition in minerals from EPR 9oN varied from +0.5 to +6.3‰, while that EPR 21oN ranged from +0.8 to +3.0‰. Considerable variations of the δ34S values were also found in sulfides deposited in back arc environments. They vary from –7.7 to +10.9‰ [11, this study]. The sulfur isotope composition was found to vary considerably in most of the different hydrothermal fields. These variations were found to be wider than was previously suggested from studies of “black smokers” on the EPR (11° to 21° N) [1, 9; 17], i.e., +1.2 to +5.5‰. The δ34S value range in sulfides from edifices in various geodynamic settings are different. These values are more stable in sulfides of fast-spreading ridges, i.e., from +0.2 to +6.2‰. The δ34S values vary from –4.4 to +13.3‰ in sulfide at the slowspreading ridges. The δ34S is also variable in sulfides in spreading centers of backarc basins, i.e., from –7.7 to +10.9‰. Sulfur isotopic ratios also differ in different edifices. They are most variable in sulfides of edifices in the slow-spreading oceanic ridges and spreading centers of backarc basins. Some of the edifices show narrow δ34S variations (–2 to –3‰). Such values were found in the following edifices: Snake Pit, a Mir relict edifice and an active edifice in the TAG field (MAR), Vienna Wood and PACMANUS edifices in the Manus Basin, and a 15°23’S edifice in the Lau Basin. Sulfides of the Hine Hina edifice in the Lau Basin and edifices in the Logachev and Rainbow fields show wider δ34S variations. Sulfides of some edifices are enriched in 34S. Sulfides from edifices associated with serpentinites (Logachev and Rainbow), Fig. 5 The δ34S values in sulfides from selected modern seafloor hydrothermal fields. Explanations: L–Logatchev, IL–Irina, Logatchev, R– edifices in back arc basins (Vai Lili in the Lau Basin), have Rainbow, M–Markov’s trough, MT–Mir zone (TAG), SP–Snake pit, BS– the highest δ34S values. Broken Spur, VW–Vienna Wood, NL–north Lau basin, CL–central Lau Sulfur isotopic composition shows regular variations in basin. some edifices. Significant variations of δ34S values were found for minerals of one generation. Variations within the chalcopyrite zones exceed 3‰. Chalcopyrite filling the conduit is richer in 34S than chalcopyrite grown on its walls. Sphalerites from walls of neighboring channels vary in δ34S from +6.6 to +9.5‰ or from +3.4 to 5.6‰. Sulfur isotopic ratios also vary at a distance from the conduit: δ34S values are +9.2‰ for dendritic sphalerite; +12.3‰ for chalcopyrite of the next zone, 2–3 mm thick; and +9.2‰ for pyrite overgrown by this chalcopyrite. Vertical variations in the conduit are also observed, and chalcopyrite in the uppermost part of a chimney is the richest in heavy isotope (δ34S = +9.3 to +12.7‰). The δ34S values are lower at the chimney base (+5.0 to +5.8‰). Copper–iron sulfides in the Broken Spur hydrothermal field are richer in heavy sulfur than iron sulfides. These relationships are opposite on equilibrium crystallization of these minerals. VII. DISCUSSION The fluid inclusion study found significant differences in fluid chemistry and homogenization temperature for minerals both between different sites and within individual hydrothermal fields. The participation of low-temperature (130-170°С) mineralforming fluids and considerable fluid salinity variations were also recorded. Fluids with the salinity that range to both higher- (the Rainbow and Logatchev hydrothermal systems) and lower-than seawater (the “Vienna Wood” hydrothermal system) deposited sulfide minerals. Evidence for phase separation such as coexisting vapor-rich and liquid-rich inclusions was observed in anhydrite from the Logatchev field. Their formation may result in phase separation of the supercritical fluid. This results in the formation of two fluids: one vapor-rich with low salinity and a brine. Venting of a salt-rich fluid at Rainbow is likely generated by phase separation. Finding that fluid inclusions in minerals from the “Vienna Wood” edifice display salinities that range to both higher- and lower-than seawater may infer phase separation of the hydrothermal fluid resulting in the formation of two compositionally different phases. Light hydrocarbon gases are an important constituent of the hydrothermal fluids in the seafloor systems. In hydrothermal systems related to sediment-starved environments they are considered to be “abiogenic” in origin being derived from basalts or mafic rocks, from seawater CO2 reduction during water/rock interaction or from magmatic outgassing [16]. Experimental study using "spiked" carbon isotopes showed that some fluids contained methane that was released from olivine [12]. The CH4/3He x 106 in fluids from the MAR systems range from 24 to 1,525. These values are intermediate between 3,000 suggested

for mantle and bioorganic methane, respectively [16]. Thus, hydrocarbon species of multiple origin can be transported by these fluids. The R/Ra values in vent fluids (5.2-7.2) indicate a significant contribution of mantle-derived helium (R/Ra = 8.1±1) into the mineral-forming system. Helium from the upper mantle could have been generated by ultramafic-seawater interaction or be released during mantle degassing that preceded magma eruption. Low R/Ra ratio suggests a significant contribution of radiogenic isotope 4He. Established variations in the 3He/4He ratio indicate probably mixing of He (and fluids) from different sources in the mineral-forming systems. The sulfur isotopic composition in minerals from basalt-related deposits suggest the mixing of “mantle” and seawater-derived sulfur or isotope shift due to fluid phase separation, while that from serpentinite-related systems infer a significant contribution of seawater derived sulfur [13], and the influence of fluid phase separation as well. Involvement of magmatic sulfur and disproportionation of the SO2 gas may play an important role in certain deposits, where a sulfur isotope shift to lighter values has been observed without obvious involvement of bacterial sulfate reduction [11]. ACKNOWLEDGMENT Author acknowledges a financial support from the Russian Academy of Sciences. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

G. J. Bluth and H. Ohmoto, “Sulfide–Sulfate Chimneys on the East Pacific Rise, 11° and 13° N Latitudes. Part II: Sulfur Isotopes,” Can. Mineral. 26, 1988, pp. 505–515 (). N.S. Bortnikov, and A.P. Lisitsyn, “Conditions of Formation of Modern Sulfide Edifices in Spreading Zones of Lau and Manus Back-Arc Basins, Pacific Ocean”, Geologiya i mineral'nye resursy Mirovogo okeana (Geology and Mineral Resources of the World's Ocean), St. Petersburg: VNIIOkeangeologiya, 1995, pp. 158-173. N.S. Bortnikov and I.V. Vikent’ev, “Modern Base Metal Sulfide Mineral Formationin the World Ocean” Geology of Ore Deposits, vol. 47, № 1, 2005, pp. 13–44. N. S. Bortnikov, S. B. Ikorskii, I. L. Kamenskii, et al., “Helium Isotopic Composition and Hydrocarbons in Fluid Inclusions from Serpentinites and Sulfides of the Logachev and Rainbow Hydrothermal Fields, Mid- Atlantic Ridge,” Dokl. Akad. Nauk [Dokl. Earth Sci.], 375 (3), 2000, pp. 375–379. N. S. Bortnikov, S. V. Ikorskii, I. L. Kamenskii, et al., “Modern Sulfide Ores at Mid-Atlantic Ridge and Pacific Back-Arc Basins: Fluid Inclusion, Hydrocarbon and He, Ar, and S Isotope Studies,” in Mineral Exploration and Sustainable Development. Millpress, Rotterdam, 2003, pp. 115–118. N.S. Bortnikov, A.M. Sagalevish, V.A. Simonov, S.V. Ikorskii, E.O. Terenya, I.L. Kamenskii, et al., “Evolution of Mineral-Forming Fluids at the 9–10° N and 21° N East Pacific Rise Hydrothermal Systems: Fluid Inclusion Data on Minerals” Dokl. Akad. Nauk [Dokl. Earth Sci.], Vol. 413A, No. 3, 2007, pp. 437–440. C. J. E. de Ronde, G. J. Massoth, E.T. Baker, J. E. Lupton, “Submarine Hydrothermal Venting Related to Volcanic Arcs”, Society of Economic Geology Special Publication, 10, 2003, pp. 91-110. C. J. E. de Ronde, “Fluid Chemistry and Isotopic Characteristics of Sea-Floor Hydrothermal and Associated VMS deposits: Potential for Magmatic Contributions”, Mineralogical Association of Canada Short Course Series, vol. 23, 1995, pp. 479-509. U. M. Graham, G. J. Bluth, and H. Ohmoto, “Sulfide– Sulfate Chimneys on the Pacific Rise, 11° and 13° N Latitudes. Part I: Mineralogy and Paragenesis,” Can.Mineral. 26, 1988, pp. 487–504. M. D. Hannington, C. E. J. de Ronde, S. Petersen, “Sea-Floor Tectonics and Submarine Hydrothermal Systems”, Economic Geology, 100th Anniverasary Volume, 2005, pp. 111-141. P. M. Herzig, M. D. Hannington, and A. Jr. Arribas, “Sulfur Isotope Composition of Hydrothermal Precipitates from the Lau Back-Arc: Implications for Magmatic Contributions to Hydrothermal Systems”, Miner. Deposita, vol. 33, 1998, pp. 226-237. T. M. McCollom, & J. S. Seewald, “A reassessment of the potential for reduction of dissolved CO2 to hydrocarbons during serpentinization of olivine”, Geochim. Cosmochim. Acta, 65, 2001, pp. 3769-3778. W. C. Shanks III, “Stable Isotope in Seafloor Hydrothermal Systems,” in Stable Isotope Geochemistry (Mineral. Soc. America, Washington), 2001, pp. 469– 526. F. M. Stuart, G. Turner, R. C. Duckworth, and A. E. Fallick, Geology 22, 1994, p. 823. K. L. Von Damm, L. G. Buttermore, and S. E. Costing, “Direct Observation of the Evolution of a Seafloor 'Black Smoker' from Vapor to Brine”, Earth Planet. Sci. Lett., vol. 149, nos. 1-4, 1997, pp. 101-111. J. A. Welhan, “Origins of methane in hydrothermal systems”, Chem. Geol., 71, 1988, pp. 183-198. L. G. Woodruff and W. C. Shanks III, “Sulfur Isotope Study of Chimney Minerals and Vent Fluids from 21° N, East Pacific Rise: Hydrothermal Sulfur Sources and Disequilibrium Sulfate Reduction”, J. Geophys. Res. 93, 1988, pp. 4562–4572.