Chemical Geology (Isotope Geoscience Section), 101 ( 1992 ) 63-70 Elsevier Science Publishers B.V., Amsterdam

63

Laser microprobe study of sulfur isotope variation in a sea-floor hydrothermal spire, Axial Seamount, Juan de Fuca Ridge,

eastern Pacific

Douglas E. Crowe and John W. Valley Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53 706, USA

(Received March 7, 1991; revised and accepted September 6, 1991 )

ABSTRACT

Crowe, D.E. and Valley, J.W., 1992. Laser microprobe study of sulfur isotope variation in a sea-floor hydrothermal spire, Axial Seamount, Juan de Fuca Ridge, eastern Pacific. In: R.S. Harmon and R.W. Hinton (Guest-Editors), Frontiers in Isotope Geosciences. Chem. Geol. (Isot. Geosei. Sect.), 101: 63-70.

The Axial Seamount site is located on the Juan de Fuca Ridge near the intersection of the Cobb-Eikenberg-Brown Bear seamount chain and the axial rift of the ridge. Several large (to 12 m ) silica + sulfide + sulfate hydrothermal spires reflect high-temperature (to 300 ° C) hydrothermal activity. Laser microprobe analyses along traverses of five temporally distinct fluid conduits within an inactive high-temperature spire reveal significant temporal variation in ~34S, both within individ- ual conduits and between conduits. The maximum intra-conduit t~34S variation (on wurtzite) is 5%0 and the maximum inter-conduit variation (on sph~lerite) is 7.40/00. Inter-conduit ~34S variation occurs primarily due to variable amounts of seawater mixing with hydrothermal fluids within the spire; generally t~a4Smine~ decreases through time due to lesser amounts of seawater that invade more mature, heavily mineralized spires. Intra-conduit t~34S variation has not been documented at this scale (5%o within l ram), and the potential mechanisms responsible for this variation include: ( 1 ) within-spire sea- water-hydrothermal fluid mixing, which also produces the inter-conduit variations, or (2) more deep-seated convection and redox processes in the sea-floor subsurface that alter the ~34Snui d.

Mixing of hydrothermal fluid with seawater cannot produce a range of d34Smi,er~-values of this magnitude (5°/0o), and deeper subsurface processes are required. Such deep-seated processes may involve the opening of new fluid conduits in the subsurface, exposing fresh basalt which will increase the reduction potential of the rock system. This in turn will promote increased reduction of seawater sulfate in the hydrothermal fluid and attendant increases of ~345nuid. The fluids will ulti- mately precipitate 34S-enriched sulfide phases, although this excursion in d34S-values is ephemeral and will last only until the fluid has exhausted the reducing potential of the new conduit. At this point, sulfate reduction will be sharply reduced and ~34S-values will correspondingly decrease. This process may explain the major (to + 5%0) isotope excursions that occur within individual conduits on a very small ( < l mm) scale.

1. Introduction

The laser microprobe is a new technique that provides stable isotope ratio measurements with high spatial resolution and precision ap- proaching that of conventional isotope meth- ods. To date, several research groups have de- veloped techniques to analyze stable isotope

Correspondence to: Douglas E. Crowe, Department of Geology, University of Georgia, Athens, GA 30602, USA.

ratios of sulfur (Crowe et al., 1990a; Kelley and Faliick, 1990), carbonate carbon and oxygen (Smalley et al., 1989) and silicate oxygen (Sharp, 1990; Elsenheimer et al., 1990, 1992 in this issue). The importance of high spatial resolution sampling cannot be overstated; con- ventional sample preparation techniques com- monly homogenize individual mineral do- mains, which in turn causes homogenization of isotope signatures. Ion probe studies (e.g., De- loule et al., 1986; Chaussidon et al., 1987;

0168-9622/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

64 D.E. CROWE AND J.W. VALLEY

--L-

VIDEO CAMERA

SPLfTTER I j~"/MIC RO SC O PE OCULARS

HR MIRROR " ~ ~ </]-

VIDEO MONITOR

SEAM EXPANDE~ [

Nd-YAG LASER

WISCONSIN LASER

MICROPROBE SYSTEM

IOX OBJECTIVE /" '~

LENS ~ > ~ T A ~

REFLECTED ~ VACUUM STOPCOCK CAPACITANCE VACUUM LIGHT\~ [ ~ MANOMETER GAUGE

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CHAMBER'" ;-- IJllll ~ - ~ SPECTROMETER MICROSCOPE STAGE II V II

, ,

CRYOGENIC TO - TRANSMITTED LIGHT TO VACUUM TRAP VACUUM

VACUUM

Fig. 1. Schematic of laser microprobe system at the University of Wisconsin. See Crowe et al. (1990a) for detailed de- scription of the system and technique.

McKibben and Eldridge, 1989; Valley and Graham, 1991 ) have documented extreme isotope ratio variations on scales of 2-30 #m, but are presently limited to precision of roughly + 1%o ( la) , making them unsuitable for doc- umenting small, yet significant, isotope varia- tions. Our laser microprobe system (Fig. 1 ) has demonstrated reproducibility of + 0.15%o ( 1 a) tbr sulfur, and is therefore capable of accu- rately measuring ~34S-values that can be used to study zoning in fine-grained materials and to calculate temperatures of mineral deposi- tion. The state of the art at this time is such that fundamental questions relating to the physical aspects of laser combustion or abla- tion of minerals remain unanswered, but the highly reproducible results promise to produce exciting new lines of research. A detailed de- scription of hardware, procedures, standard- ization and preliminary results from several

sulfide ore deposits and other sulfide-bearing lithologies are presented by Crowe et al. (1990a). This contribution outlines prelimi- nary research of sea-floor hydrothermal sam- pies, and documents previously unknown sul- fur isotope variations at scales not attainable with conventional extraction methods.

2. Analytical techniques

Laser microprobe 634S data were collected using the University of Wisconsin laser micro- probe facility. Analyses were performed using the technique outlined in Crowe et al. (1990a). Samples to be analyzed were first cleaned ul- trasonically in trichlorethylene for 5 min, fol- lowed by ultrasonic cleaning in distilled water for 5 rain. Samples were then placed in a 120°C vacuum desiccator and degassed (typically 12- 24 hr.). This procedure also removed native

SULFUR ISOTOPE VARIATION IN A SEAFLOOR HYDROTHERMAL SPIRE 65

sulfur, thereby eliminating a potential source of contamination during subsequent laser combustion.

All analyses are reported in standard per mil (%o) notation relative to the Canyon Diablo troilite (CDT) reference standard. Analytical precision is + 0.15%o ( I a) for all J34S data.

3. Axial Seamount

The Axial Seamount site is located on the Juan de Fuca Ridge at 45°57'N, 130°02'W near the intersection of the Cobb-Eikenberg- Brown Bear seamount chain and the axial rift of the ridge (Fig. 2 ). Low-temperature ( < 35 °C) hydrothermal fluids currently ema- nate from numerous vents, cracks and fissures within the seamount caldera, and several large (to 12 m) silica + sulfide + sulfate hydrother- real spires reflect previous higher-temperature (to 300°C) hydrothermal activity. The sulfide assemblage in the spire is comprised mainly of sphalerite + wurtzite + marcasite (Hannington and Scott, ! 988). One of the inactive spires was collected during a 1985 CASM dive, and forms the basis of this study. Hannington (1986) and

,-'-" taU" 128" 126 ° 5 3 " ~ . , . 116"

50'

48" ENDEA~iUR f S E G ~ . ~ 4z,, UAN. ~M TED t'DE ",/11 ....... I=7 46" " SE aUNT " ~. AXIAL

I I 45" ~F.SOUTHERN JUAN De FUCA

44. i. GeNO ~ G I ~ PolvmetoIlic 43" 1 f ~sulfides

42* f ~ t ~ and \ ORE vent fields 41" ~ G[ /200 nautical miles !

z i ( 370 km ) ,,o" t 39. Paci[ic ' Ocean =RidQe a,ls • ~ ./Major tr?nsforrp__

Fig. 2. Location of the Axial Seamount site. Figure from Hannington and Scott (1988).

Hannington and Scott (1988) provide de- tailed descriptions of samples, mineralogy and whole-rock chemistry of the spire, and prelim- inary laser microprobe tp4S results from the same spire are reported in Crowe et al. (1990b).

4. Results and discussion

Laser microprobe analyses along traverses of five temporally distinct fluid conduits within the inactive higher-temperature spire (Figs. 3 and 4) reveal significant temporal variation in t~34S, both within individual conduits and be- tween conduits. The maximum intra-conduit d34S variation (on sphalerite) is 5%0, and the maximum inter-conduit variation is 7.4%0.

Inter-conduit ~34S variation is primarily due to variable amounts of seawater mixing with hydrothermal fluios within the spire; generally £~34Ssphalerite decreases toward the interior of the spire as lesser amounts of seawater invade the inner p~rtions of heavily mineralized spires. Sulfide precipitated near the exterior of the spire likely incorporates sulfate from 34S-en- riched seawater. Ten analyses of sphalerite precipitated in the core of the spire have an av- erage ~34S-value of + 3.20o. Thirty-one anal- yses of sphalerite toward the exterior of the spire have an average t~34S of +4.30/00 (Fig. 5 ). Sulfur isotope zonation across an individual chimney structure has previously been docu- mented at the > l-mm scale (Woodruff and Shanks, 1988) and is consistent with our ob- servation that interior sulfide phases reflect minimal mixing with seawater, while sulfides precipitated near the exterior of a chimney are 34S-enriched due to seawater interaction.

Laser microprobe traverses were also per- formed across thin wurtzite-lined fluid con- duits to evaluate intra-conduit sulfur isotope variations at a scale of < 1 ram. The variations seen, on the order of 5%0 within 1 mm (Fig. 6) have never been documented within sea-floor samples, and suggest that significant short-term variations of tp4Snuid have occurred. The vari-

66 D.E. CROWE AND J.W. VALLEY

>

Fig. 3. Photograph of a vertical section through the hydrothermal sulfide + sulfate + silica seafloor spire examined in this study. Figs. 4-6 show horizontal cross-sectional views of the spire as well as the inter- and intra-conduit laser microprobe J34S data. Figure from Hannington and Scott (1988).

SULFUR ISOTOPE VARIATION IN A SEAFLOOR HYDROTHERMAL SPIRE 67

Fig. 4. Photograph of cross-section (taken at the saw cut visible in Fig. 3) of spire shown in Fig. 3. Core of spire is to lower left, outer wall to upper right. Figs. 5 and 6 show laser microprobe data collected on this sample, and are shown at the same scale. Photograph courtesy of Mark Hannington.

ation may be due to: ( 1 ) within-spire seawater mixing with hydrothermal fluid, which also produces the inter-conduit variations, (2) more deep-seated convection and redox pro- cesses in the sea-floor subsurface that alter the ~34Snuid, or a combination of ( 1 ) and (2). Mo- delling of within-spire fluid mixing (Janecky and Shanks, 1988) has shown that seawater sulfate reduction can only account for some of the sulfur isotope variation documented in this study. Mixing of heated hydrothermal fluids with ambient-temperature seawater can result in reduction of some seawater sulfate by the reaction:

8Fe 2+ + 10H + +SO~-

8Fe 3+ +H2S + 4 H 2 0

as suggested by Woodruff and Shanks (1988). However, the reduction potential of the hydro- thermal fluid is finite in the sea-floor near-sur- face environment. Values of ~34Swuazi,e as high

as + 9%0, as seen in this study, cannot be ex- plained by mixing models of a hydrothermal fluid (~34SH2s=+ 1.5%0) and seawater (~34Sso4=+21°/o0), assuming adiabatic or conductive cooling and sulfide-sulfate isotope disequilibrium (Janecky and Shanks, 1988). Sulfat~ reduction processes operative below the seawater-basalt interface must therefore be considered.

The opening of new fluid conduits in the subsurface, exposing fresh basalt, will increase the reducing potential of the hydrothermal sys- tem. Simplified reactions that typify this pro- cess are proposed by Janecky and Shanks ( 1988 ), where reaction of !abradorite + fayal- ite to Fe-chlorite oxidizes ferrous iron in fay- alite to ferric iron in chlorite, and thus reduces SO4 in the hydrothermal fluid. Such increased reduction of seawater sulfate in the hyd~other- mal fluid will produce attendant increases in the t~34Saum-vahles. These reactions operate in concert with the pyrrhotite sulfidation rent-

68

5c

f J

D.E. CROWE AND J.W VALLEY

k )

t

Fig. S. Schematic of inter-conduit laser microprobe ~34Ssphalerite data. Laser pits averaged 100 p.m in diameter; numbers are centered over location where analysis was performed. Note that the average ¢~34 SsphaP.r~e-values increase outward from the core (avg. J3~S= + 3.2°/o0) to the outer wall (avg. j34S= +4.30/00). Hatched zones represent fossil sulfate walls (pre- dominantly barite) as defined by Hannington (1986). Same scale as Figs. 4 and 6.

V - 7 ~ E : ' i ~ / / / ~ S i l i c a + Marcasite ± Barite L P : ; : !.-. - ~ \

2ram / "~ 2ram

2 rnm 2ram /

Silica + Marcasi te-

Fig. 6. Schematic of intra-conduit laser microprobe data. Note large shifts in ~3"S-values across several of the wurtzite lined cavities. The wurtzite rinds are typically < l- mm wide. The ~34S.~.=i~e variation reflects changes in subsurface geochemical processes discussed in text. Hatched zones and scale are the same as in Figs. 4 and 5.

tion to pyrite proposed by Shanks et al. ( 1981 ) that reduces seawater SO 4 to HzS deeper in the hydrothermal system. The fluids will ulti- mately precipitate 34S-enriched sulfide phases in the near-surface and chimney environ- ments. The excursion in ~34S-values from the typical + 1.5%o-value of most sea-floor hydro- thermal fluids is ephemeral and will last only until the fluid has exhausted the reducing po- tential of the new conduit. No measurements of venting fluids from active hydrothermal systems have shown ~34SH,s-values of this magnitude ( + 9%o); we assume therefore that this phenomenon is short-lived. Sulfate reduc- tion of this magnitude will be sharply reduced in a short period of time and ~34S-values will correspondingly decrease. Woodruff and Shanks (1988) suggest growth rates of 0.3 mm yr- ~ for sphalerite in a chimney structure at the 21 °N site, thus the thin cavity lining ZnS ana-

SULFUR ISOTOPE VARIATION IN A SEAFLOOR HYDROTHERMAL SPIRE 69

lyzed in the Axial Seamount samples could have precipitated rapidly, recording very short- lived events in the subsurface. Conventional sampling traverses would have homogenized the variations elucidated with the laser micro- probe, illustrating the need to examine micro- scale isotope variations.

5. Conclusions

The variation in ~34S seen within a hydro- thermal sulfide spire from the Axial Seamount site suggests an interplay of stockwork zone processes with surface fluid mixing, processes that occur within the spire on the sea floor. Subsurface reaction of labradorite + fayalite to Fe-chlodte results in an increase in the sulfate reduction potential of the fluid due to reduc- tion of SO4 by Fe 2÷ in the hydrothermal fluid, and subsequent formation of Fe3+-bearing chlorite. This will result in elevation of "typi- cal" ( ~ 3 4 8 n 2 S - - "4- 1.50/00) hydrothermal fluid sulfur isotope values to roughly + 5%0. Within- spire reduction of additional sulfate by mixing of heated hydrothermal fluid and cool seawa- ter serves to increase the t~34Snuid, and ulti- mately results in the precipitation of 34S-en- riched sulfide as seen in this study. The ephemeral nature of this combination of pro- cesses is reflected by the laser microprobe data, where short-lived positive excursions of~34Sfluid to ~ + 9%0 are recorded in very thin wurtzite cavity linings with elevated ~34S-values. Vari- ations of this sort cannot be measured by con- ventional techniques due to the size of the sample domain to be measured, ,rod illustrate the need for microsampling techniques like the laser microprobe.

Acknowledgements

We wish to thank Dr. Mark Hannington of the Geological Survey of Canada and Dr. Steve Scott of the University of Toronto for provid- ing samples and photographs from the Axial Seamount site for this study as well as valuable

insight into sea-floor hydrothermal systems. Discussions with Pat Shanks, Laurel Woodruff and J.K. BiShlke of the U.S. Geological Survey greatly enhanced the senior author's under- standing of sulfur isotope systematics in sea- floor environments. Kevin Baker is acknowl- edged for his invaluable help during the devel- opment of the laser microprobe facility at the University of Wisconsin. The final stages of manuscript preparation were made while the senior author was a National Research Coun- cil Post Doctoral Fellow at the U.S. Geological Survey in Reston, Va. This research was sup- ported by the Gas Research Institute (5086- 260-1425) and NSF (EAR 89-05101 ).

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70 D.E. CROWE AND J.W. VALLEY

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