Ocean Ridge Hydrothermal Vent Fluids Alison Marie Bray University of New Hampshire, Durham

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Winter 2001 The geochemistry of boron and lithium in mid - ocean ridge hydrothermal vent fluids Alison Marie Bray University of New Hampshire, Durham

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. THE GEOCHEMISTRY OF BORON AND LITHIUM IN MID-OCEAN RIDGE HYDROTHERMAL VENT FLUIDS

Alison Marie Bray Bachelor of Arts, University of San Diego, 1995 Master of Science, University of New Hampshire, 1998

Submitted to the University of New Hampshire in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy

Earth Sciences

December 2001

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. This dissertation has been examined and approved

----- Dissertation Director, Dr. Karen L. Von Damm Professor of Geochemistry and Earth, Oceans and Space

_____ -A______Dr. Jo Laird Associate Professor of Geology

Dr. Susan Humphris Senior Scientist Department of Geology and Geophysics Woods Hole Oceanographic Institution

Dr. flfegdry Ravizza Associate Scientist Department of Geology and Geophysics Woods Hole Oceanographic Institution

Dr. Lui-Heung 1 Professor Department of Geology and Geophysics Louisiana Sate University

1 2 0 0 1 Date

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The lure of the sea has enticed explorers to probe the mysteries of that vast, sparkling wilderness, probably for as long as there have been human beings. Our origins are there, reflected in the briny solution coursing through our veins and in the underlying chemistry that links us to all other life.

Dr. Sylvia A. Earle Sea Change: A Message of the Oceans

iii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

I am grateful to Professor Karen Von Damm for her supervision over the past six

years. As my advisor, she has provided me with amazing opportunities and has helped

me to learn about the process of scientific research and discovery. I would also like to

thank Professor Jo Laird, Dr. Susan Humphris and Dr. Greg Ravizza for many helpful

discussions, advice and encouragement during my residence as a graduate student. I also

gratefully acknowledge Dr. Lui-Heung Chan who devoted endless hours to coaxing Li

results from the mass spectrometer. Without her dedication, patience and expertise, this

project would never have been completed.

Many people have provided scientific assistance and much needed camaraderie,

both on land and at sea. Among the many are: M. Ferguson, K. O’Grady, R. Gallant, T.

Blanchard, D. Fomari and E. Olsen. Dr. E. Rose is acknowledged for providing the

boron ion chromatography method and Dr. M. Perfit is thanked for providing the fresh

basalt samples. Mid-ocean ridge research is fundamentally the work of a team and these

people have been vital to my education as a scientist. In addition, the officers, crew and

Alvin group of the R/V Atlantis are acknowledged for their dedication and expertise that

makes work at the bottom of the sea possible. This work was supported by the National

Science Foundation (OCE-9911612) and a Hubbard Grant from the University of New

Hampshire.

I thank my family (Mom, Dad and Andrew) for instilling in me a spirit of debate

and love for science and most of all for constant encouragement. Finally, I express my

deepest gratitude to my husband Tim who has very patiently stood by me. His support

and faith in me have been essential to my success.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ACKNOWLEDGEMENTS...... iv LIST OF TABLES...... vii LIST OF FIGURES...... viii ABSTRACT...... ix

CHAPTER

I. INTRODUCTION...... 1 1.1 Geochemistry of Boron and Lithium ...... 2 1.2 Hydrothermal Processes ...... 3 1.3 Previous Studies ...... 6 1.4 Objectives of this Study ...... 13 1.5 Study Site Descriptions ...... 14

II. THE ROLE OF PHASE SEPARATION AND WATER-ROCK REACTION IN CONTROLLING THE BORON CONTENT OF MID-OCEAN RIDGE HYDROTHERMAL VENT FLUIDS Abstract...... 21 2.1 Introduction ...... 22 2.2 Methods ...... 23 2.3 Results and Discussion ...... 28 2.4 Implications for the Hydrothermal Flux of Boron ...... 39 2.5 Conclusions ...... 42

III. CONTROLS ON LITHIUM IN HIGH AND LOW TEMPERATURE AXIAL HYDROTHERMAL FLUIDS: INSIGHTS FROM LITHIUM ISOTOPES Abstract...... 44 3.1 Introduction ...... 46 3.2 Methods ...... 50 3.3 Lithium Isotopes in Solid Samples ...... 53 3.4 Lithium Isotopes in High Temperature Hydrothermal Fluids ...... 55 3.5 Lithium Isotopes in Diffuse Hydrothermal Fluids ...... 69 3.6 Hydrothermal Fluids and the Balance of Lithium Isotopes in the Ocean ...... 79 3.7 Conclusions ...... :...... 82

IV. CONCLUDING REMARKS Summary of Conclusions ...... 84 Future Study ...... 88

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix A Boron Ion Chromatography Method Description ...... 91 Appendix B Analytical D ata ...... 94 Appendix C Lithium Isotope Method Comparison ...... 108 Appendix D Calculations for Mixing of Seawater and Basaltic Lithium: Effects of Down Flow Reactions ...... I ll REFERENCES...... 118

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

1-1. Previous Determinations of Boron in Hydrothermal Fluids ...... 8 1-2. Previous Determinations of Lithium Isotopes in Hydrothermal Fluids. 12 2-1. Boron and Chloride End Members ...... 26 2-2. Sources and Sinks of Boron in the Oceans ...... 41 3-1. End Member Compositions for Lithium Isotopes ...... 48 n 3-2. Lithium Concentrations and 8 Li for Solid Samples ...... 49 3-3. Diffuse Flow 87Li Results ...... 49 3-4. Bio9 Riftia Mixing Scenarios...... 74 3-5. Lowell Diffuse Flow and the Intermediate Fluid Composition ...... 76 3 -6. Hydrothermal Lithium Fluxes ...... 80

vii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

1 -1. Hydrothermal Circulation ...... 4 1 -2. Phase Diagram for Seawater...... 5 1 -3. Map of the World showing vent sites ...... 15 1 -4. Site map for 17-22°S East Pacific Rise (EPR) ...... 16 1-5. Site map for 9-10°N EPR...... 18 1 -6. Site map for Lucky Strike, Mid-Atlantic Ridge ...... 19 2-1. Map of Vent Sites ...... 24 2-2. Boron Concentrations Relative to Seawater ...... 30 2-3. Boron/Chloride V ersus Chloride ...... 31 2-4. Boron and Boron/Chloride Time Series for 9-10°N EPR ...... 35 3-1. Map of Vent Sites ...... 47 3-2. 5 Li Versus Mg/Li Mixing Plot for High Temperature Hydrothermal Fluids ...... 54 3-3. End Member Lithium Isotope Data ...... 56 3-4. Hydrothermal Fluids Compared to Basalt and Seawater ...... 57 3-5. End Member Lithium Isotope Data Versus Time for 9-10°N EPR 60 3-6. Mixing Plot for Diffuse Hydrothermal Fluids ...... 72 3-7. Mixing Scenario for Bio9 Riftia ...... 73 3-8. Mixing Scenario for Lowell Diffuse Flow ...... 77

viii

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

THE GEOCHEMISTRY OF BORON AND LITHIUM IN MID-OCEAN RIDGE HYDROTHERMAL VENT FLUIDS

Alison M. Bray

University of New Hampshire, December, 2001

Examining the chemistry of hydrothermal vent fluids can provide insight into

processes occurring below the seafloor along the mid-ocean ridge axis. Boron and

lithium in hydrothermal vent fluids were studied because previous results indicated that

these elements might be useful tracers of water-rock processes. The B concentration and

the Li isotopic signature of hydrothermal fluids from four areas on the mid-ocean ridge

were examined in order to better constrain the physical and chemical controls on these

elements.

The results for B suggest that water-rock reaction and phase separation are

important controls on the B content of hydrothermal fluids. Boron concentrations vary

over a relatively narrow range (~0.7 to 1.5 times the seawater value) and show little

variation with time as hydrothermal systems age. Unlike most other elements including

Cl and Li, B is not significantly fractionated between the vapor- and liquid-phases during

phase separation, resulting in little spatial or temporal variability in B content. While the

majority of the fluids sampled have B concentrations greater than the seawater value

(-415 pmol/kg), fluids from the Irina vent at the ultramafic-hosted Logatchev

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. hydrothermal site are unique in that the B is depleted with respect to seawater by -28%,

likely due to low temperature reaction with serpentinites in the down flow zone.

The results for Li demonstrate that the 57Li signature of high temperature

hydrothermal fluids is remarkably constant with a global average value of 7.5±1.6%o.

This contrasts the variability seen for many other chemical parameters in hydrothermal n fluids including Li concentration. The 5 Li signature of the hydrothermal fluids is

interpreted as an equilibrium value between the fluids and secondary alteration mineral

products as previously suggested by Chan et al. [1993], Very subtle variations in the 57Li

isotopic signature (near the level of the analytical precision) are noted in the fluids

sampled immediately after a volcanic eruption, likely resulting from non-equilibrium

conditions during this period.

Measurements of Li isotopes in low temperature diffuse flow fluids are consistent

with diffuse flow originating from mixing of high temperature (>300°C) or intermediate

temperature (~140-170°C) crustal fluids with ambient seawater. This suggests little low

temperature interaction with the rock is occurring and that the fluids have very short

(days to months) residence times within the oceanic crust.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter One

INTRODUCTION

To understand the chemical and isotopic balance of the oceans, the sources and

sinks of elements and isotopes to the oceanic system need to be identified and quantified.

Of these potential sources and sinks, hydrothermal venting along the mid-ocean ridge has

proven to be extremely important to the geochemical balance [Edmond et al. 1979]. In

our efforts to understand the physical and chemical processes that occur at mid-ocean

ridges, researchers have tried to identify elements or chemical species that might be

useful as geochemical tracers. One application of geochemical tracers is the calculation

of water/rock ratios as done in early studies of seafloor hydrothermal systems. These

values, interpreted as the amount of rock that had reacted with a given volume of water,

were critical to the interpretations of early hydrothermal fluid studies [Edmond et al.

1979; Von Damm et al. 1985; Von Damm and Bischoff 1987], While well defined in

hydrothermal experiments, the concept of water/rock ratios is much more ambiguous in

natural opening flowing systems. In early studies the alkali elements, such as Li and K,

were used as a basis for the determination of water/rock ratios as these elements were

believed to behave as soluble elements, leached quantitatively into circulating fluids

during reaction with the basalt. Water/rock ratios cannot be accurately calculated from

an element controlled by equilibrium with a solid phase (i.e. non-soluble elements). As

hydrothermal sites have continued to be discovered and sampled over the past two

decades, many elements previously used as indicators of water/rock ratios are now

known to be controlled by equilibrium (e.g. K, Rb and Cs) [Bray 1998; Bray and Von

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Damm submitted]. Therefore, we continue to look for elemental and/or isotopic species

that can be used as tracers for processes occurring in these systems.

Boron and Li may be very useful as tracers as they are incompatible in basalts and

can be easily leached into hydrothermal solutions. Also, the chemistry of B and Li may

be simpler than elements such as transition metals because B and Li are not affected by

changes in redox conditions, do not precipitate as sulfides, and are likely unimportant to

biological activity. The goal of the research presented in this thesis is to gain a better

understanding of the processes that control Li and B in mid-ocean ridge hydrothermal

systems, and to evaluate the usefulness of these two elements as possible geochemical

tracers for understanding processes occurring in seafloor hydrothermal systems.

l.f Geochemistry of Boron and Lithium

Although Li and B share some geochemical similarities, they are different in

several fundamental ways. Lithium is the most metallic known element, whereas B is a

non-metal. Lithium primarily exists as the Li+ ion in solution, whereas B is never found

as a lone ion but is associated with hydroxyl groups as either B(OH)3° or B(OH) 4 '. At pH

less than ~9, B is primarily three-coordinated. Lithium and B each have two stable

isotopes, 6Li/ 7Li and 10B/ nB. Both elements behave conservatively in the oceanic water

column, changing concentrations in proportion to variations in salinity. The

concentration of B in seawater is 412 pmol/kg while the Li concentration in seawater is

only 26 pmol/kg. Both elements are considered incompatible in basalts and the

concentration of B and Li in fresh basalts is extremely low, -0.02-0.3 mmol/kg and ~0.4-

1 mmol/kg, respectively [Ryan and Langmuir 1987; Ryan and Langmuir 1993],

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.2 Hydrothermal Processes

Seawater percolating through the oceanic crust at mid-ocean ridges is chemically

changed due to reaction between the circulating fluids and the rock (either basalt or

ultramafic rock and, in some cases, overlying sediments). Reaction begins in the down

flow zone as Ca and SO4 are removed through the formation of anhydrite at temperatures

of ~150°C (Fig. 1-1). Removal of Li and B from the fluids, to low temperature alteration

minerals can potentially occur in parts of the down flow zone, generally at temperatures

less than 150°C [Seyfried et al. 1984]. As the fluid moves deeper into the crust, higher

temperature reactions progress and many elements, including Li and B, are leached from

the rock into the circulating hydrothermal fluids. Both Li and B are trace elements in

basalt, and their incompatible nature in mineral structures allows them to be easily

leached into high temperature hydrothermal fluids. This primarily occurs in the reaction

zone, resulting in enrichments of these elements relative to seawater in most

hydrothermal fluids.

As the hydrothermal fluid continues to be heated, it may phase separate

depending on temperature and pressure conditions deep within the system. The two-

phase curve for seawater divides temperature and pressure space into a region where only

liquid is stable and a region where liquid and vapor coexist (Fig. 1-2). If at any point in

its evolution a hydrothermal fluid crosses the two-phase curve, phase separation occurs

resulting in the formation of a low chlorinity vapor and a high chlorinity liquid phase

(brine) [Bischoff and Rosenbauer 1987], Phase separation below the seawater critical

point (407°C and 298 bars) is termed sub-critical and is analogous to boiling, whereas

super-critical phase separation (condensation) occurs above the critical point. Phase

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U <405° C Vent

Seawater k Chimney 2°C Seawater

Oceanic Crust I (a) Down Flow Zone

(b) Reaction Zone

Heat Source Magma Chamber or Dike

Figure 1-1. Schematic figure of a seafloor hydrothermal system. Seawater percolates through cracks in the seafloor and moves downward reacting with the basalt at low temperatures (a). In the reaction zone (b) water-rock reactions occur at high temperatures (>400°C). Phase separation may also occur in the reaction zone. As the fluid is heated it becomes less dense and rises, through the upflow zone (c) until it exits the seafloor as a hydrothermal vent.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50

Vapor + Halite 100

150 w

0) :3c_-- 250 CO (A 2 300 C.P. 0 . 350 Liquid + Vapor _

400 Liquid 450

500 250 300 350 400 450 500 Temperature (°C)

Figure 1-2. The phase diagram for seawater showing the liquid, liquid/vapor, and vapor + halite stability fields. “CP” designates the critical point for seawater (407°C, 298 bars) (Modified from Von Damm, 2000, based on data from Bischoff and Rosenbauer, 1985 and Bischoff, 1991).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. separation is accepted as a primary control on the chemical composition of mid-ocean

ridge hydrothermal fluids [Von Damm 1995] and it is necessary to consider with respect

to the behavior of Li and B in these systems. As hydrothermal fluids continue to be

heated, their buoyancy increases and eventually the fluids return to the seafloor via the up

flow zone and are vented. Based on the difference between measured temperatures and

the conditions required by phase separation, cooling of hydrothermal fluids in the up flow

zone likely occurs. Unlike many other elements, B and Li are unaffected by the changes

in redox conditions or the precipitation of sulfide minerals often induced by the cooling

of hydrothermal fluids in the up flow zone. Hydrothermal fluids sampled at the seafloor

represent the integrated chemical signature of the processes occurring in all parts of the

hydrothermal system.

1.3 Previous Studies

Experimental studies have suggested that B, and to a lesser degree Li, can behave

as soluble species under some high temperature alteration conditions [Seyfried et al.

1984], These experiments have shown that both Li and B behave differently under low

versus high temperature alteration conditions. At low temperatures (<150°C), B and Li

are removed from solution, whereas at high temperatures (>150°C), B and Li can be

leached from the rocks into circulating fluids.

1.3.1 Boron

Previous results for B in hydrothermal fluids have shown that its concentration

ranges from 370 (TAG site, Mid-Atlantic Ridge, MAR) to greater than 2000 pmol/kg in

fluids from the sediment covered Escanaba Trough area [Spivack and Edmond 1987;

Campbell et al. 1988; Butterfield and Massoth 1994; Butterfield et al. 1994; You et al.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1994] (Table 1-1). Fluids from sedimented-hosted ridge systems are much higher in B

concentration than those from bare basalt systems [Spivack et al. 1987; Palmer 1991].

Examination of B isotopes in many of these fluids demonstrated that B in hydrothermal

fluids was a mixture of seawater B and various amounts of B extracted from the rock

[Spivack and Edmond 1987]. While B has been analyzed in a variety of hydrothermal

fluids, fluids from fast, ultra-fast and ultramafic-hosted hydrothermal sites have not been

previously investigated.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1-1. Chloride and B concentrations in fluids from mid-ocean ridge and back-arc basin sites'. Location Cl (mM) B (mM) Seawater 540 0.42 11 °N EPR 11 EPR-6 346 0.48 11 EPR-5 702 0.46 11 EPR-4 576 0.49 13-NEPR 13EPR-3 888 0.45 13 EPR-1 735 0.47 21 ° N EPR OBS -81 500 0.52 OBS -85 511 0.51 SW -81 507 0.52 SW -85 537 0.51 HG -81 507 0.52 HG -85 518 0.55 NGS -81 592 0.52 NGS -85 589 0.50 Guaymas Basin S-Field 581 1.60-1.68 E-Hill 603 1.55 Escanaba Trough, Gorda 668 2.16 Cleft Segment. Juan de Fuca Ridge (JDFR) Pipe Organ 1245 0.524 Monolith-90 908 0.482 Monolith-91 875 0.468 Brigadoon 852 0.465 Fountain 880 0.46 Plume 1115 0.50 Axial Seamount. JDFR Inferno 638 0.60 Hell 563 0.62 Virgin Mound 180 0.50 Endeavour See.. JDFR Hulk 518 0.698 Grotto 442 0.744 Crypto-84 478 0.716 Crypto-88 480 ' 0.698 TP 448 0.706 Dante 457 0.715 LOBO-84 426 0.753 LOBO-87 423 0.757 LOBO-88 429 0.771 Dudley 349 0.754 S&M 334 0.798 Peanut 253 0.723 North 477 0.725 MARK, MAR MARK-86 572 0.52 MARK-90 0.56 TAG. MAR 2179 660 0,36 2187 0.40 Lau Basin LB-1 712 0.77 LB-2 804 0.87 LB-3 790 0.83 Mariana Trough MT-1 557 0.83 MT-2 0.83 MT-3 0.77 Okinawa Trough 423-3 528 3.44 423-2 485 2.04 427-1 514 2.73 427-3 509 2.77 ^ ' Tr- . - T \'7_ . riAA

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Campbell et al. [1988] and Palmer [1991] determined B concentrations in

hydrothermal fluids from the TAG and MARK sites on the slow spreading MAR. Fluids

from TAG were found to have B concentrations less than seawater (~0.36-0.40 mM).

These are the only hydrothermal fluids with B concentrations less than seawater B (-0.42

mmol/kg) likely resulting from losses of seawater B in low temperature down flow

reactions [Campbell et al. 1988; Palmer 1991]. Concentrations of B in the MARK fluids

were similar to those in fluids from the East Pacific Rise (EPR) (Table 1-1). While B

concentrations in the MARK fluids were not unique, Campbell et al. [1988] noted that the

isotopic signature of the MARK fluids was distinct from EPR vents (8nB~ 27%o for

MARK vs. ~32%o at 21°N EPR). While a specific B isotopic signature or B

concentration for the source rocks reacting with the MARK fluids could not be identified,

these authors suggested that the unique isotopic signature in these fluids might result

from reaction .with basalts that had been previously altered at low temperatures. A

similar model of reaction with previously altered crust has been proposed to explain the

concentrations of Rb and Cs in hydrothermal fluids [Palmer and Edmond 1989], but

further work has shown that Rb and Cs data from the 9-KEN EPR site cannot be

explained by this type of model [Bray, 1998; Bray and Von Damm, submitted].

Experimental studies of both super- and sub-critical phase separation have shown

that the B concentration in the liquid phase is higher than the concentration of B in the

vapor phase, similar to observations for the alkali metals [Bischoff and Rosenbauer 1987;

Bemdt and Seyfried 1990; Spivack et al. 1990]. However, unlike the alkali metals which

form chloro-complexes, B is more likely to bond with hydroxyl groups forming the

neutral B(OH) 3 ° molecule [Culberson et al. 1967; Spivack and Edmond 1987; Bemdt and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Seyfried 1990]. In both super- and sub-critical phase separation experiments, B/Cl molar

ratios are much different in the vapor and liquid phases. Because B does not form

chloro-complexes, boron/Cl ratios in the vapors are much greater (by as much as 2Ox)

than in the liquids, despite the absolute concentration of B being greater in the liquid

phase. This behavior of B is different from the alkalis, as alkali/chloride ratios are

uniform in the vapor and liquid phases of these experiments.

1.3.2 Lithium

The Li concentration in sampled hydrothermal fluids ranges from ~4 to 2350

pmol/kg [c.f. Von Damm 1995], These studies have shown that Li in most hydrothermal

fluids is greatly enriched over the seawater concentration due to leaching of Li from the

rock. Lithium has two stable isotopes, 6Li and 7Li, and the -15% mass difference

between these two isotopes may cause isotopic fractionation to occur during geochemical

reactions. Traditionally, Li isotopes have been reported as 86Li, representing the 6/7 ratio

relative to the L-SVEC Li-carbonate standard, but in this thesis the values will be

reported as 57Li or the 7/6 ratio relative to L-SVEC, to be consistent with the reporting

conventions for other isotopic systems. Lithium isotopes have been measured in very

few seafloor hydrothermal fluid samples (Table 1-2). Chan et al. [1993] determined the

Li isotopic composition of hydrothermal fluids and rocks from 21°N EPR, 11-13°N EPR

and the MARK site on the MAR. The fluids from both the EPR and MAR sites are 3-7%o

heavier than the source rocks from each of these areas. These authors therefore

suggested that Li is not quantitatively removed from the source rock and that the

formation of secondary alteration minerals preferentially retains 6Li, resulting in

isotopically heavier hydrothermal fluids [Chan et al. 1993]. The Li isotopic composition

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of hydrothermal fluids from sedimented hydrothermal systems, including Guaymas Basin

and Escanaba Trough, has also been investigated (Table 1-2) [Chan et al. 1994]. The

results from these sites are generally similar to those from bare basalt hydrothermal

systems, with no distinct signature of sediment-water interaction recorded in the Li

isotopes.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1-2. Lithium and chloride concentrations and Li isotopic composition of hydrothermal fluids from mid-ocean ridge sites1.

Location Li Cl 87Li Vent pmol/kg mmol/kg %0 Seawater 26 540 32.4 1981 21°N EPR SW 902 496 9.7 ±1.0 8.5 ±1.3 OBS 887 489 9.3 ±1.3 NGS 1039 579 10.1 ±0.7 HG 1336 496 6.6 ±0.8

1985 21 °N EPR SW 979 525 8.7+1.0 OBS 904 500 8.4±0.9 8.9+1.2 HG 1421 506 6.9±0.7 11-13°N EPR Vent 1 617 718 11.0±0.8 Vent 4 904 563 8.2 ±0.8 Vent 6 477 338 10.8±0.9

23N° MAR MARK 839 563 8.6+ 1.0 858 563 8.5± 1.0 6.3± 1.0 1982 Guaymas Basin S. Field 1072 580-600 5.0± 0.7 N. Field 721 637 10.1+0.8 C. Field 955 589 8.6±1.1 E. Hill 882 599 7.7±0.7 1985 Guaymas Basin S. Field 993 580 2.6± 0.8 E. Hill 830 603 10.1±1.0 E. Hill 853 603 10.3±1.0

Escanaba Trough, Gorda Ridge

#2036-6 1275 668 8 . 1 ± 1 .0 #2036-9 1241 668 6.6±1.0 J a n ____ ^ > r i al. [1988, 1994]; Bowers et al. [1988]. SW= South West, OBS= Ocean Bottom Seismometer, NGS= National Geographic Smoker, HG= Hanging Gardens.

Experimental studies of super-critical phase separation provide no evidence for

fractionation of the alkali metals relative to Cl during this process [Bemdt and Seyfried

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1990]. In these experiments, which contained no rock, both the K/Cl and Li/Cl ratios

were equal to the seawater ratio in both the vapor and brines, which is very different from

the behavior of B. James et al. [1999] measured the Li isotopic composition in fluids

from super-critical phase separation experiments [described in Bemdt et al. 1996] and

determined that fractionation of Li isotopes does not occur. As part of this thesis, the

effect of phase separation on Li isotopes in field samples is investigated.

1.4 Objectives of this Study

While we cannot currently drill active bare basalt hydrothermal systems,

understanding the chemical composition of hydrothermal fluids can provide insight into

the chemical and physical processes occurring below the seafloor. Lithium and Boron

may be useful as geochemical tracers of hydrothermal processes, particularly water/rock

ratios. However, in order to evaluate the usefulness of these elements as tracers, the

processes which may affect these elements need to be more completely understood.

Additionally, understanding how these elements behave in hydrothermal systems is also

important to our greater understanding of elemental cycling in the oceanic system. In

order to better understand the geochemistry of these two elements, this research

investigates the B content and Li isotopic signature of hydrothermal fluids in order to

address four major questions:

1. Is there temporal variability in Li isotopes and B concentration from a

single vent? If so, what is the cause of this variability?

2. What is the spatial variability of Li isotopes and B in fluids from

several mid-ocean ridge locations? Is variability in the substrate

reflected in the fluid chemistry for Li or B?

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3. How does phase separation affect Li isotopes and B in hydrothermal

fluids?

4. What are the major controls on Li, Li isotopes and B in mid-ocean

ridge hydrothermal fluids?

5. Are Li and B useful as geochemical tracers in seafloor hydrothermal

systems?

1.5 Study Site Descriptions

This study determined the B concentration and the Li isotopic composition of

hydrothermal vent fluids from four locations on the mid-ocean ridge (Fig. 1-3). Samples

from four sites were analyzed because they represent unique geological and geophysical

characteristics of the mid-ocean ridge, providing distinct but complementary information

on how B and Li behave in these systems.

The first site, 17-22°S East Pacific Rise (EPR), is located on an ultra-fast

spreading ridge (~15 cm/yr full rate) at a depth of up to 2860 meters. Forty high

temperature vents (Fig. 1-4) were sampled on this ridge section during the SouEPR cruise

in Oct.-Nov. 1998 and of these two were selected for Li isotope analysis and 10 for B

analysis. Samples of the Lowell diffuse flow (low temperature fluids) were also analyzed

for Li isotopes. The most interesting vent at this site is Brandon vent. With a measured

temperature of 405°C, Brandon is the hottest vent ever sampled [Von Damm et al.,

submitted]. Brandon is also exceptional because it is venting both vapors and liquids

from separate orifices on the same sulfide structure, allowing for a study of phase

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 1-3. Locations (indicated by stars) of the four hydrothermal vent areas studied.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N Sfuaker St nle*

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I I I I I | r - ] -■

1400 1640 1880 2120 2360 2600 2B40 3080 3320 3560 3899

Figure 1-4. Bathymetric map of the Southern East Pacific Rise from 17°S to 21°40'S (depth in meters) showing the locations of the hydrothermal vents discussed in the following chapters. Data from Scheirer and Macdonald (unpublished) and the SouEPR cruise (Lilley and Von Damm). Map courtesy o f D. Fomari.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. separation effects on B and Li without post-phase separation re-equilibration of the fluids

with aluminosilicate minerals.

The second location is the fast spreading (11 cm/yr) [Carbotte and Macdonald

1994] northern EPR between 9-10°N (Figure 1-5). This site allowed study of B and Li in

a suite of hydrothermal fluids encompassing, in some cases, a 7- year time series as these

vents have developed following volcanic eruptions in 1991 and 1992. Other processes

known to affect the 9-10°N EPR vents such as magmatic degassing as well as samples of

diffuse flow fluids were examined.

The third site, the Lucky Strike hydrothermal field, is located on the slow

spreading Mid-Atlantic Ridge (MAR) (Fig. 1-6) and may be influenced by its proximity

to the Azores hot spot. This site is significantly shallower than the first two sites (-1600-

1730 m), and the Lucky Strike site is unique in that it may not only have a signature of

enriched crust (due to the hot spot influence), but based on their chemical signatures, the

fluids appear to have reacted with a highly altered substrate [Langmuir et al. 1997; Yon

Damm et al. 1998]. The presence of hydrothermally altered hyaloclastic slab found

extensively covering this site may also be suggestive of hydrothermal activity extending

over a long time period. Thus, these fluids were investigated to understand the potential

chemical influence of the altered substrate.

The final site studied was the Logatchev site located at 14°45’N on the MAR at a

depth of 2930-3010 m. Logatchev is an unusual site in that the fluids are formed by the

reaction of seawater with an ultramafic source rock instead of basalt as at the other three

systems studied [Batuyev et al. 1994; Bogdanov et al. 1995]. Fluids from one Logatchev

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Biovent

Ma 9°51.5' BS 9°49.0’

Tube Worm 112 Pillar

Aa & 9°44,8’ La

9°37,1* D 9*34.9’ D&E El 9° 32. 7 *

9° 241 ’

9° 17 '

104°IS,W

Figure 1-5. Location of hydrothermal vents in the 9-10°N East Pacific Rise study area. The vents are indicated by capital letters or names on the map. Letters in boxes refer to the division of the ridge into 4th order segments. Shaded segments (B1 and B2) are the site of the volcanic eruptions in 1991/2. Bathymetry is given in meters. From Haymon et al. [1991] and Oosting and Von Damm [1996].

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2608 Vent

Eiffel Tower

Marker #4 Crystal Vent

1600

Figure 1-6. Locations of hydrothermal vents analyzed in this study at the Lucky Strike hydrothermal site. From Von Damm et al. [1998],

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. vent, Irina, were studied for both Li isotopes and B in order to determine how this other

substrate might affect hydrothermal fluid chemistry.

The format of this thesis consists of two main chapters (Two & Three), each of

which represents a paper that has been or will be submitted for publication. Each chapter

contains its own abstract, introduction, methods, results, and conclusions. Chapter Two

describes the results from the study of B concentration in the hydrothermal fluids, and

Chapter Three describes the results of the Li isotopic study of these fluids. A final

chapter, entitled “Concluding Remarks” includes a review of the major conclusions of

this study as well as proposed topics for future research. A complete list of references as

well as analytical data and method details are included as appendices at the end of this

thesis.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter Two

THE ROLE OF PHASE SEPARATION AND WATER-ROCK REACTION IN CONTROLLING THE BORON CONTENT OF MID-OCEAN RIDGE HYDROTHERMAL VENT FLUIDS*

ABSTRACT

To further understand the role of hydrothermal systems in B cycling in the oceans,

we report here the first measurements of B in hydrothermal vent fluids from fast and

ultra-fast spreading as well as ultramafic-hosted mid-ocean ridge hydrothermal systems.

This study expands our knowledge of B distribution in fluids from these varied

hydrothermal settings, provides insight into the temporal variability of B concentrations

in evolving high temperature vents and furthers our understanding of the geochemical

processes affecting B. This understanding is necessary in order to constrain more

completely the hydrothermal flux of B to the oceans.

Our results, consistent with those previously reported, suggest that water-rock

reaction and phase separation are important controls on the B content of hydrothermal

fluids. Boron concentrations vary over a relatively narrow range (~0.7 to 1.5 times the

seawater value) and show little variation in time series samples on the scale of 7 years

following a volcanic eruption. An unusual feature in our dataset is the extraordinarily

high B/Cl ratios resulting from very low chlorinity concentrations in fluids vented

following volcanic events at 9-10°N East Pacific Rise. For hydrothermal systems,

elemental data are typically ratioed to Cl to account for the effects of phase separation

and to evaluate the effect of other processes such as water-rock reaction and biological

* This chapter has been submitted as a paper to Geochimica et Cosmochimica Acta.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. activity. However, unlike most other elements including Cl, B is not significantly

fractionated between the vapor- and liquid-phases during phase separation, thereby

resulting in little spatial or temporal variability in B content despite widely varying Cl

contents.

While the majority of the vent fluids analyzed in our study have B concentrations

greater than seawater, fluids from the Irina vent at the ultramafic-hosted Logatchev

hydrothermal site are unique in that the B is depleted with respect to seawater by -28%.

This most likely indicates loss of seawater B due to reaction with serpentinites. Boron-

depleted fluids like those from Irina and those previously reported from the TAG system

[Palmer 1991] are important considerations to the hydrothermal flux. The results of flux

estimates suggest that hydrothermal processes at slow spreading ridges, particularly

ultramafic-hosted sites, could potentially be a sink for B, in contrast to fast spreading and

sediment-hosted sites, which are in most cases B sources. Loss of B at slow spreading

and ultramafic mid-ocean ridge settings may therefore play an important role in balancing

the large fluxes of B to the oceans from rivers and ridge flanks.

2.1 Introduction

Sampling of hydrothermal systems over the past two decades has shown that

hydrothermal fluid chemistry is often highly variable in both space and time [c.f. Von

Damm 1995] and this variability is an important consideration when refining estimates of

hydrothermal flux [Von Damm et al. in prep]. Boron concentrations in hydrothermal

fluids, ranging from 370 pmol/kg (TAG) to 2160 pmol/kg (Escanaba Trough), have

been previously reported from a variety of hydrothermal regimes including intermediate

and slow spreading ridges as well as sedimented systems and back-arc basins [Spivack

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and Edmond 1987; Campbell et al. 1988; Palmer 1991; Butterfield and Massoth 1994;

Butterfield et al. 1994; You et al. 1994]. However, these studies did not include fluids

from fast spreading, ultra-fast spreading or ultramafic-hosted hydrothermal sites.

Similarly, limited time series data for B previously existed. We report B concentrations

in hydrothermal fluids from four previously unstudied MOR sites including the ultra-fast

spreading southern East Pacific Rise (EPR) (17-22°S), the fast spreading and eruption

impacted northern EPR (9-10°N), the highly altered and hot spot influenced Lucky Strike

site at 37°N Mid-Atlantic Ridge (MAR), and the ultramafic hosted Logatchev site at

15°N MAR (Fig. 2-1). The objectives of this study are to examine the effects of phase

separation, volcanic eruptions and substrate type on the B content of hydrothermal fluids

as well as to determine how B concentrations in hydrothermal fluids vary both spatially

and temporally. Understanding the effects of these processes is important to refining

estimates of B flux in axial mid-ocean ridge hydrothermal systems.

2.2 Methods

Water samples were collected in titanium syringe samplers using the DSV Alvin

or ROV Jason. (See Von Damm et al. [1985] for a detailed description of sample

collection and handling.) Only fluids from the “majors” samplers were used, as we

determined that B was lost from fluid samples during gas extraction from the gas tight

bottles. Each sample was filtered and acidified with Ultrex HC1 prior to analysis. Boron

was measured in the hydrothermal fluids using a Dionex model 500 Ion Chromatograph

with a CD20 conductivity detector. An ICE-AS1 ion exclusion column with a 0.1 M

mannitol eluent and no suppression was used. Standards were prepared from a National

Institute of Standards and Technology (NIST) traceable primary B standard.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V «■ uth A rt pll

m Ais

Fig. 2-1. (a) Map of the global mid-ocean ridge system with the location of hydrothermal vent sites included in this study indicated with stars, (b) Map of the 17-22°S EPR area with stars indicating the location of the vent field areas included in this study. Vent field names and individual vent names are listed to the left, (c) Map of vent locations at 9- 10°N EPR. Individual vents are indicated by capital letters on the map. 4th order ridge segments as designated by Haymon et al. [1991] are indicated by letters to the right. Segments B1 and B2 (shaded) are the areas of the 1991/2 eruptions. [After Haymon et al. 1991; Oosting and Von Damm 1996]. (d) Location of the hydrothermal vents at the Lucky Strike site [Von Damm et al.1998].

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. International Association for Physical Sciences of the Ocean (IAPSO) seawater was also

run every 10 samples to adjust for instrumental drift. This method is described in detail

in Appendix A, and analytical data are presented in Appendix B. End member vent fluid

compositions were determined by a least squares linear regression of B versus Mg

assuming a 0 mmol/kg Mg end member [Von Damm 2000], Errors reported (Table 2-1)

are the larger values of either the analytical error or the error on the intercept of the

regression. Chloride analyses are as described in Oosting and Von Damm [1996],

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 2-1. Boron and chloride end member concentrations, temperature and sampling date for the hydrothermal fluids analyzed in this study. n.m.= not measured

Vent Sampling Temperature Cl B B/Cl Date °C mmol/kg pmol/kg pmol/mmoi EPR, 9-10°N Seawater1 2 540 412 0.763 B V .l2 21-Oct-94 366 326 +2 550 ±20 1.69 ±0.05 BV.2 22-Nov-95 356 324 +2 490 ±20 1.51 ±0.05 BV.5 10-Nov-97 345 336 ±2 5,10 ±20 1.52 +0.05 BV.6 13-Dec-97 343 339 +2 540 ±20 1.59 +0.05 Ma. 1 29-Feb-92 321 245 +2 450 ±10 1.82 ±0.06 Ma.2 13-Mar-92 321 285 ±1 450 ±10 1.58 ±0.05 Ma.3 10-Mar-94 337 554 ±3 430 ±10 0.77 ±0.02 Ma.4 30-Mar-94 342 563 ±3 430 ±10 0.77 ±0.02 Ma.5 29-Oct-94 353 580 ±3 440 ±10 0.75 ±0.02 Ma.6 14-Nov-95 361 592 ±3 440 ±10 0.75 ±0.02 Ma.8 !5-Nov-97 , 365 577 ±3 450 ±10 0.77 ±0.02 B9.1 1-Apr-9 3 368 154 +1 430 ±10 2.81 ±0.09 B9.2 6-Mar-92 >388 76 + 1 430 ±10 5.7 ±0.2 B9.3 28-Dec-93 365 212 ±2 440 ±10 2.05 ±0.06 B9.4 10-Mar-94 359 263 ±7 460 ±10 1.73 ±0.07 B9.5 29-Mar-94 363 267 +1 460 ±10 1.71 ±0.05 B9.7 24-Oct-94 359 330 ±2 460 ±10 1.39 ±0.04' B9.8 25-Nov-95 364 498 +2 470 ±10 0.95 ±0.03 B9.9 4-Nov-97 373 400 ±3 450 + 10 1.14 +0.04 B9.10 10-Nov-97 371 401 ±2 470 ±10 1.18 ±0.04 P.l 7-Apr-91 369 135 +1 460 ±10 3.40 ±0.11 P.3 9-Mar-92 392 41.6 +0.4 510 ±20 12.3 ±0.4 P. 4 18-Dec-93 364 262 ±1 480 ±10 1.83 ±0.06 P. 5 18-Mar-94 350 347 ±2 460 +10 1.33 ±0.04 P. 6 27-Mar-94 377 352 +2 480 +10 1.35 ±0.04 P.7 25-Oct-94 359 530 ±3 470 +10 0.88 ±0.03 P. 8 16-Nov-95 360 622 ±3 500 ±20 0.82 ±0.03 P.10 28-Nov-95 367 620 ±3 510 ±20 0.82 ±0.03 P.13 11-Nov-97 372 529 +3 510 ±20 0.97 ±0.03 TWP.l 26-Oct-94 351 235 +3 540 ±20 2.31 ±0.07 TWP.2 27-Oct-94 351 237 ±1 580 ±20 2.44 ±0.07 TWP.3 11/29,30/95 341 301 +4 570 ±20 1.90 ±0.06 TWP.4 12-Nov-97 306 337 +2 560 ±20 1.66 ±0.05 TWP.6 10-Dec-97 304 344 ±3 550 ±20 1.68 ±0.05 Aa.l 10-Apr-91 390 81 +3 440 ±10 5.42 ±0.23 Aa.2 17-Apr-91 396 30.5 +0.6 500 ±20 16.4 ±0.6 Aa.3 24-Apr-91 403 43.3 +2.6 560 ±20 12.9 ±0.9 Aa.4 8-Mar-92 332 286 ±1 510 ±20 1.77 ±0.05

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Vent Date Temperature Cl B B/Cl °C mmol/kg pmol/kg pmol/mmol Aa.5 10-Mar-92 332 282 ±15 500 ±20 1.7 ±0.1 Aa.6 12-Mar-94 329 398 +2 470 ±10 1.19 ±0.04 Aa.7 26-Mar-94 330 397 ±2 480 ±10 1.20 ±0.04 Aa.10 24-Apr-96 259 600 ±3 490 ±20 0.82 ±0.03 A a.ll 9-Dec-97 n.m. 679 ±3 490 ±20 0.72 ±0.02 La.l 17-Apr-91 388 114 + 1 510 ±20 4.5 ±0.1 La.2 10-Mar-92 380 62.1 ±0.3 450 ±10 7.3 ±0.2 La.4 26-Mar-94 347 492 ±2 430 +10 0.88 ±0.03 La.7 3-Dec-97 n.m. 599 ±3 440 ±10 0.74 ±0.02 D.l 2-Apr-91 290 846 ±4 550 ±20 0.65 ±0.02 D.2 18-Apr-91 308 801 ±4 540 ±20 0.68 ±0.02 D.5 19-Mar-94 264 851 ±4 520 ±20 0.61 ±0.02 E.l 2-Apr-91 280 859 ±4 560 ±20 0.65 ±0.02 E.3 2-Mar-92 188 858 ±4 560 ±20 0.65 ±0.02 E.4 14-Mar-94 260 860 ±4 540 +20 0.63 ±0.02 K.l 13-Apr-91 263 556 ±3 540 ±20 0.97 ±0.03 K.2 31-Mar-94 253 573 ±3 440 ±10 0.77 ±0.02 F.l 16-Apr-91 388 46.2, ±1.4 460 ±10 9.9 ±0.4 F.2 15-Mar-94 351 846 ±4 620 ±20 0.73 ±0.02 EPR, 17-22°S Seawater 2 540 412 0.763 21°33-34’S B a.l3 19-Oct-98 404 317 ±2 430 ±10 1.37 ±0.04 Ba.2 20-Oct-98 404 338 ±2 430 ±10 1.28 ±0.04 Ba.3 3-Nov-98 404 339 ±2 430 ±10 1.28 ±0.04 Bc.4 4-Nov-98 403 304 ±2 430 ±10 1.40 ±0.04 Be. 5 7-Nov-98 n.m. 321 ±4 430 ±10 1.35 ±0.04 Bd.4 4-Nov-98 401 297 ±4 430 ±10 1.44 ±0.05 Bd.5 7-Nov-98 405 330 ±2 430 ±10 1.31 ±0.04 Bb.3 3-Nov-98 368 558 +3 460 ±10 0.83 ±0.03 Be. 5 7-Nov-98 376 557 ±3 470 ±10 0.85 ±0.03 Krasnov 10-Oct-98 368 752 ±4 470 ±10 0.62 ±0.02 21°24-26’S Natasha 13-Oct-98 354 390 ±2 440 ±10 1.13 ±0.03 Jasmine 14-Oct-98 349 371 ±2 430 ±10 1.16 ±0.04 Tweety 16-Oct-98 343 119 +6 400 ±10 3.36 ±0.20 17°24-27’S Stanley 25-Oct-98 363 412 ±3 490 ±20 1.19 ±0.04 North Smoker 25-Oct-98 276 410 ±2 490 ±20 1.19 ±0.04 Nadir 28-Oct-98 343 476 ±2 470 ±10 0.99 ±0.03 Dumbo 29-Oct-98 260 1090 ±7 480 ±10 0.44 ±0.01 Gumbo 29-Oct-98 294 915 ±5 500 ±20 0.55 ±0.02 MAR, Lucky Strike Seawater 4.5 548 418 0.762 Sintra. 1 31-May-93 212 530 ±3 480 ±10 0.91 ±0.03

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Vent Date Temperature Cl B B/Cl °C mmol/kg pmol/kg pmol/mmol Sintra.2 21-Jul-96 222 532 ±3 460 ±10 0.87 ±0.03 Sintra.3 30-Jun-97 219 531 ±3 464 ±10 0.87 ±0.03 Eiffel Tower. 1 3-Jun-93 325 438 ±2 450 ±10 1.02 ±0.03 Eiffel Tower.2 1-Jul-96 323 441 ±2 450 ±10 1.02 ±0.03 Eiffel Tower.3 30-Jun-97 318 440 ±2 470 ±10 1.08 ±0.03 Marker 4.1 3-Jun-93 297 440 ±2 470 ±10 1.07 ±0.03 Marker 4.2 26-Jul-96 318 441 ±2 460 ±10 1.04 ±0.03 2608 Vent.l 29-Jul-96 328 526 +3 460 ±10 0.88 ±0.03 2608 Vent. 2 29-Jun-97 n.m. 521 +3 460 ±10 0.89 ±0.03 Crystal. 1 30-Jul-96 281 535 ±3 430 ±10 0.80 ±0.03 Crystal.2 29-Jun-97 290 538 +3 440 ±10 0.80 ±0.03 MAR, Logatchev Seawater 415 Irina 27-Jul-97 350 524 ±3 300 ±10 0.57 ±0.02 'As seawater salinity varies, the values for local ambient seawater at each hydrothermal site are reported for comparison. 2At locations where a time series exists for a vent, the sampling chronology is indicated by a number, e.g. BV.l is the first sample from Biovent, and BV.2 is the second sampling etc. BV= Biovent, B9=Bio9 Vent, TWP= Tube Worm Pillar. 3Ba, Be and Bd indicate the Brandon vapor phase fluids, and Bb and Be are Brandon liquid phase fluids (brines).

2.3 Results and Discussion

The B concentrations in the hydrothermal fluids are in most cases enriched

relative to the seawater concentration (-415 pmol/kg) (Table 2-1) reflecting leaching of

B from the rock into the circulating hydrothermal fluids. The two exceptions are fluids

from the Tweety vent from 17-22°S EPR which are equal to the ambient seawater

concentration and fluids from the Logatchev site which are depleted with respect to the

seawater concentration by 28%. For hydrothermal fluids, elemental data are often

presented as element-to-chloride ratios, which accounts for the variations in Cl caused by

phase separation and allows other processes, such as water-rock reaction and biological

activity, to be examined. Thus, both B concentrations as well as B/Cl ratios are

examined. The results for each of the four sites as well as the processes that affect the

geochemistry of B are described in detail below.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.3.1 17-22°S East Pacific Rise

The EPR between 17-22°S latitude is an ultra-fast spreading ridge (-15 cm/yr full

rate) [Cormier and Macdonald 1994] with water depths up to 2860 meters (Fig. 2-1). Of

the 40 vents sampled along this ridge during the SouEPR cruise (Fall 1998), fluids from

10 vents from three areas, 17°25-27’S, 21°24-26’S, and 21°33-34’S, were analyzed for B

(Table 2-1, Fig. 2-2). The 17°25-27’S area is an inflated section of ridge where seismic

measurements imply a very shallow melt lens [Detrick et al. 1993]. Previous fluid

sampling of vents in this area suggests recent volcanic eruptions between 1984 and 1993

[Charlou et al. 1996]. In contrast, the morphology from 21°24-26’S is suggestive of a

tectonically governed regime, and the fluid chemistry (e.g. Cl and Si) suggests these

fluids react at relatively deep depths within the oceanic crust [O’Grady, 2001]. The

21°33-34’ S area also appears to be tectonically controlled [Von Damm et al., submitted].

The southern EPR fluids have a very limited range in B concentration (-400-500

pmol/kg) despite a wide range in chemical composition (e.g. Cl=l 16-1090 mmol/kg;

Li=250-1197 pmol/kg) (Fig. 2-3). Although B is a very soluble element, the lack of

correlation of concentration of B with temperature (R2=0.4), suggests that the

concentrations are not controlled by leaching efficiency from the rock as a function of

temperature. Instead, the low and relatively homogeneous concentrations likely reflect

the limited availability of B in fresh basalt (-20-40 pmol/kg) [Spivack and Edmond

1987]. The constancy of the B concentrations may also reflect control by steady-state or

equilibrium with alteration mineral products. In hydrothermal solutions, most elements

do not behave as truly soluble elements but appear to be controlled by steady-state if not

true thermodynamic equilibrium with alteration mineral phases [c.f. Von Damm ,1995;

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. B pmol/kg 300 350 400 450 500 550 600 650

9-10°N EPR Fluids Eruption impacted No Recent Eruption

17-22"S EPR Fluids

Lucky Strike- MAR Fluids

Logatchev- MAR Fluids

Figure 2-2. Range of B concentrations measured in high temperature hydrothermal fluids. The concentration ranges for all four sites overlap and with the excpetion of the Logatchev fluids are generally greater than the seawater value.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.0 4.0- 17-22°S EPR (b)

o o 3.0-- |£ 0 1 ® 2.0

o Osz c o o CO

0.0 0 .0 - 200 400 600 800 1000 1200 1400 200 400 600 800 1000 1200 1400

Chloride (mmol/kg) Chloride (mmol/kg)

4.0 MAR (d) 9-10°N EPR 3.5

o 15-- 0 £ I £o 10--O 5© o sz Oc o 00o

0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400

Chloride (mmol/kg) Chloride (mmol/kg)

Figure 2-3. Boron/Chloride versus Chloride (a) Previously reported bare basalt mid-ocean ridge hydrothermal fluids. Squares^ Mid-Atlantic Ridge, Triangles^ Juan de Fuca Ridge, Circles = East Pacific Rise. Line of best fit was determined for the fluids in (a) and this trend line is overlayed in (b-d). [Data sources: Spivack and Edmond 1987; Von Damm and Bischoff 1987; Bowers et al. 1988; Campbell et al. 1988; Butterfield et al. 1990; Palmer 1991; Butterfield and Massoth, 1994; Butterfield et al., 1994; Campbell et al. 1994; You et al. 1994; Edmonds and Edmond 1995] (b) 17-22°S EPR fluids. Open circles are 17°24-27'S vents, open squares are 21°33-34'S vents and closed squares are 21°24-26’S vents. SW= seawater (c) 9-10°N East EPR fluids. Closed symbols are eruption area vents, open symbols are south of the 1991 eruption area. Note that the upper limit of B/Cl ratios is much greater than reported in previously sampled fluids, (d) MAR fluids. Open symbol is Logatchev, closed symbols are Lucky Strike.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bray and Von Damm, submitted; O’Grady 2001]. It should be considered that B may not

be a completely soluble element, though the mineral phase that may control B

concentrations is currently unknown.

The B/Cl molar ratio in fluids from 17-22°S EPR varies by almost an order of

magnitude (Table 2-1). These ratios are similar to the B/Cl versus Cl trend for previously

reported hydrothermal vent fluid data (Fig. 2-3). Tweety vent (21°26’S) is venting very

low chlorinity fluids (-116 mmol/kg) and has the lowest B concentration (identical to

seawater within the analytical error) but the highest B/Cl ratio of all the fluids at this

location.

Brandon vent (21°33.7’S) is unique in that its pressure and temperature conditions

at the seafloor (405°C, 283 bars) place it very close to the critical point for seawater

(407°C, 298 bars), and it is venting both vapor (low chlorinity) and liquid (brine, high

chlorinity) fluids simultaneously, from the same structure, from orifices less than two

meters apart [Von Damm et al. submitted]. Because phase separation is occurring within

the sulfide structure, fluids from Brandon provide insights into the behavior of B during

phase separation without additional (post-phase separation) water-rock reaction. While

the vapor phase fluids from Brandon are only approximately 7% lower in B content than

the liquid phase fluids, the B/Cl ratio in the vapors is -40% greater than in the liquid

phase fluids. At Brandon vent, the liquid phase is enriched in B by only a factor o f-1.1

relative to the vapor phase, whereas Cl and all of the other major elements are different

by a factor of 1.6-1.9 between the vapor and liquid [Von Damm et al. submitted]. This is

conclusive field evidence that there is minimal fractionation of B between the two phases

during phase separation, consistent with experimental results [Bischoff and Rosenbauer

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1987; Bemdt and Seyfried 1990, You et al. 1994], In contrast in these same experiments,

which did not include any water-rock reaction, the K/Cl and Li/Cl ratios were nearly

identical in the vapor and liquid phases, while the absolute concentration of dissolved

species was greater in the liquid than in the vapor phase. Chloride and elements that

form chloro-complexes are strongly fractionated between the vapor and liquid phases

during phase separation, resulting in the extreme high and low chlorinity fluids that have

been sampled from hydrothermal vents [Bemdt and Seyfried 1990; You et al. 1994].

There is little fractionation of B between the liquid and vapor phases as B does not form

chloro-complexes but is instead covalently bonded to hydroxyl groups.

2.3.2 9-10°N East Pacific Rise

The 9-10°N EPR hydrothermal site is located on a fast spreading section of ridge

(~11 cm/yr full rate) [Klitgord and Mammerickx 1982] with an average depth of 2500-

2600 meters (Fig.2-1). Volcanic eruptions in 1991 and 1992, north of 9°45’N (hereafter

referred to as the “northern area”) resulted in rapid and extreme chemical changes in the

hydrothermal fluids venting at this site [Von Damm 2000]. Fluids from seven vents in

this area were analyzed for B (Table 2-1). Resampling of many of these vents on a nearly

annual basis between 1991 and 1997 has allowed us to follow the chemical evolution of

this area following the eruptions. Several vents south of 9°45’N (hereafter referred to as

the “southern area”) where no eruption is known to have taken place were also sampled

in 1991, 1992 and 1994 and their B contents were also determined. F vent, located

approximately 60 km south of the 1991/92 eruption area, is chemically more similar to

the northern area vents. Visual observations in this area suggest an eruption may have

occurred here within a few years before the 1991 sampling [Von Damm et al. 1997]. All

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of the fluids sampled at 9-10°N EPR are enriched in B by as much as 49% relative to the

local seawater concentration (-412 pmol/kg), indicating that water-rock reactions leach B

from the basalt (Fig. 2-2). There is no trend with measured fluid temperature (R2=2xl0'3)

and no consistent temporal trends (Fig. 2-4) in the B concentrations, which vary over a

relatively limited range (-430-620 pmol/kg). Previous studies have utilized B

concentrations to determine water/rock ratios, as B is a highly soluble element, easily

leached from the rock [Seyfried et al. 1984; Spivack and Edmond 1987], Flowever, the

lack of temporal variability in the evolving 9-10°N EPR system suggests that B

concentrations cannot be used to track the extent of water-rock interaction or changing

water/rock ratios as a hydrothermal system ages. Other chemical evidence such as very

low Li, K and Si concentrations suggests a shallow reaction zone and that minimal water-

rock reaction was occurring in the immediate post-eruptive period [Bray 1998; Von

Damm 2000], As the system ages, increased water-rock reaction is evident from

chemical tracers (e.g. increasing Li and Si contents). This evolution is not reflected in the

B concentration of the hydrothermal fluids, suggesting other processes are affecting B in

these fluids.

Unlike the absolute B concentrations, the B/Cl molar ratios show significant

temporal variability (Fig. 2-3). In the northern area, with the exception of Biovent, the

B/Cl ratios are highest in the first two years following the volcanic eruption (Table 2-1).

The B/Cl ratios in the early part of the time series are the highest yet measured in seafloor

hydrothermal fluids. These extreme B/Cl ratios are associated with the very hot, low

chlorinity fluids vented at 9-10°N EPR in the period immediately following the volcanic

eruptions. As the absolute B concentrations of these immediate post-eruptive period

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9-10°N EPR Eruption Area

650

O Ma BV O Ma 600 V B9 B TWP V P TWP □ Aa a> ♦ ta 550 ♦ La

c o 500 CO VO

450

400 0 5 10 15 20 500 1000 1500 2000 2500 3000 5 10 15 20 500 1000 1500 2000 2500 3000

Days Since 1 April 1991 Days Since 1 April 1991

9-10°N EPR- South of Eruption Area

650

600 o 14 E E o 550 E a. TJa> 500

c o o 450 £0

400 3000 0 5 10 15 20 500 1000 1500 2000 2500 3000

Days Since 1 April 1991 Days Since 1 April 1991

Figure 2-4. (a) Boron versus Days Since 1 April 1991 (time), the approximate date of the first volcanic eruption at 9-10°N EPR. (The solid horizontal line indicates the seawater B concentration and a representative error bar is shown for scale.) There is no consistent trend in B concentrations with time, (b) B/Cl versus time for hydrothermal fluids from the area of the 1991/2 eruptions at 9-10°N EPR. (The solid line indicates the B/Cl ratio for seawater). B/Cl ratios are highest in the early part of the time series and then decrease, (c) B versus time and (d) B/Cl versus time for vents at 9-10°N south of the eruption area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fluids are not atypical for hydrothermal fluids, these unusual B/Cl ratios result primarily

from the extremely low Cl concentrations. In the northern area after 1992 the B/Cl ratios

decrease to values less than 2.4 pmol/mmol. For some vents, particularly in the southern

area, the B/Cl ratios are less than the seawater ratio.

F vent at 9-10°N also, like Brandon vent provides insights into the behavior of B

during phase separation. When first sampled in 1991, F vent was venting low chlorinity

hydrothermal fluids (Cl=46 mmol/kg). Sampling in 1994 revealed a high chlorinity fluid,

compositionally the conjugate pair (i.e. the liquid) to the previously sampled low chloride

vapor-phase fluid [Von Damm et al. 1997]. While the brine (liquid phase) is enriched in

Cl by a factor o f-18 and Li by a factor of -53, relative to the vapor phase fluids, B is

only enriched in the brine by a factor of 1.4. This again demonstrates that while phase

separation may slightly enrich the liquid phase in B, the fractionation of B between the

two phases is very minor compared to most other elements.

2.3.3 Logatchev, Mid-Atlantic Ridge

The Logatchev hydrothermal site is located at 14°45’N on the MAR at a depth of

2930-3010 meters [Bogdanov et al. 1995] (Fig. 2-1). This site is one of only two known

mid-ocean ridge hydrothermal sites where hydrothermal fluids are formed by reaction

between seawater and ultramafic rock [Batuyev et al. 1994]. Fluids from the Irina vent at

Logatchev were sampled in 1997 and have a B concentration of 300 pmol/kg (Table 2-1),

-28% less than the seawater value, and lower than the value previously reported for the

TAG hydrothermal fluids [Palmer 1991].

These unusually low B concentrations likely reflect reaction with the ultramafic

substrate found here and could result from losses of seawater B during down flow

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reactions as previously suggested for other MAR sites [Edmond et al. 1995]. Palmer

[1996] concluded from B isotopic data that -50% of seawater B was lost from

hydrothermal fluids at MAR sites, due interactions of the fluids with deep ultramafic

rocks. Palmer [1996] also proposed that deeper reaction paths on the MAR might

contribute to the losses of seawater B. Hydrothermal reaction paths are generally

assumed to be longer and deeper on the MAR (several kilometers) relative to fast-

spreading ridges where relatively shallow (-1.5 km) and more continuous steady-state

magma chambers limit the depth of fluid circulation. In contrast, B iso topic

measurements suggest that less than 10% of the seawater B is lost in the down flow zone

for EPR vents [Spivack and Edmond 1987]. Our results suggest that the Logatchev fluids

may have lost a large proportion of their original seawater B in the down flow zone,

during reaction between the fluid and serpentinized peridotites at low temperatures. If a

large portion of the seawater B is lost, the B observed in the final, high temperature

hydrothermal solution is most likely the result of high temperature leaching in the

reaction zone and is therefore almost completely rock derived. Preliminary data show

that the Logatchev fluids have a very light isotopic signature (5nB~ 7%o) [E. Rose,

unpublished data] closer to the basalt value (~ -3%o) and very different from previously

reported data for B in hydrothermal fluids (- 35%o) [Spivack and Edmond 1987].

Oceanic serpentinites can be enriched in B by more than an order of magnitude relative to

unaltered peridotites, and serpentinized peridotites are much more enriched in B than

altered basalts [Bonatti et al. 1984; Spivack and Edmond 1987], The loss of seawater B

is enhanced in the down flow zone at the Logatchev site due to reaction of the fluids with

ultramafic rocks. Bonatti et al. [1984] found an inverse relationship between the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. temperature of serpentinization and the boron content of the rocks, suggesting removal of

seawater B to serpentinites at low temperatures. Similarly, in hydrothermal experiments,

Seyffied and Dibble [1980] found that B decreased in solution due to uptake into

alteration products at low temperatures and concluded that the enrichment of B in

serpentinites might be the result of reaction between serpentinites and seawater at

temperatures as low as 2°C. The Logatchev results demonstrate the importance of the

reacting rock composition as well as the potentially deeper reaction paths on the MAR to

the final B concentration of the hydrothermal solutions venting at the seafloor. Loss of B

through serpentinization reactions has been previously proposed as a sink for B in an

effort to balance the geochemical budget of B in the oceans [Seyfried et al. 1984; Kadko

et al. 1995], and our results provide evidence that this process is occurring at the

Logatchev site.

2.3.4 Lucky Strike, Mid-Atlantic Ridge

Lucky Strike seamount, located at 37°17’N on the slow spreading MAR, (Fig. 2-

1) is a relatively shallow hydrothermal site (-1600-1730 meters) influenced by the

presence of the Azores hot spot [Langmuir et al. 1997]. Hydrothermal fluids were

sampled from the Lucky Strike vents in 1993, 1996 and 1997. It has been proposed that

low metal contents in the Lucky Strike fluids may be indicative of a waning/cooling

hydrothermal system [Klinkhammer et al. 1995]. However, Von Damm et al. [1998]

argued that the hydrothermal fluids resulted from super-critical phase separation at depths

>1.3 km below the seafloor and reflect reaction with a highly altered crust. This suggests

that this slow spreading site may have been active over long time scales. Boron

concentrations are equal to or slightly enriched (up to 13%) compared to the local

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. seawater concentration (418 pmol/kg) (Table 2-1). These are much smaller enrichments

of B relative to the seawater value compared to 17-22°S EPR and especially 9-10°N EPR.

This could reflect the highly altered substrate of Lucky Strike if most of the available B

has been removed through previous high temperature hydrothermal activity. However

this seems unlikely as hydrothermally altered rocks from OOP Hole 504B were at most

depleted in B by 50% relative to fresh basalt concentrations [Ishikawa and Nakamura

1992]. More likely, the relatively low B concentrations and low B/Cl ratios at Lucky

Strike may reflect B loss in the down flow zone during low temperature reactions as

described above for Logatchev and other MAR sites. However, unlike Logatchev, the

fluids from Lucky Strike are never less than the seawater B concentration. The down

flow loss of seawater B at Lucky Strike may not be as extreme as that at the Logatchev

site because Lucky Strike is a basalt, not ultramafic-hosted, hydrothermal system.

2.4 Implications for the HydrothermalFlux of Boron

Several previous studies have attempted to reconcile the sources and sinks of B in

the oceans; however, the magnitude of hydrothermal fluxes for all elements continues to

be refined. The fluxes of many elements from hydrothermal systems to the ocean likely

vary with time due to the evolution of hydrothermal vent fluid chemistry following

magmatic events, as a result of changing temperatures and phase separation [Von Damm

2000; Von Damm et al. in prep.]. This will result in modifications of the flux estimates

for certain elements such as decreases in the alkali metal flux, and increases in the

transition metal fluxes. Boron, in contrast, is relatively unaffected by temperature and

changes in chlorinity resulting from phase separation; hence its flux will remain relatively

constant during the temporal evolution of a hydrothermal system.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The results of this and previous studies have found that some fluids from the

MAR (i.e. TAG and Logatchev) have the lowest B contents of the fluids sampled to date.

What may be more important for the B cycle is the proportion of fluids exiting from slow

versus fast spreading hydrothermal regimes, particularly where hydrothermal fluids react

with ultramafic rocks, which may be closer to the surface at slow spreading ridges.

While it is currently impossible to accurately quantify the proportion of MOR

hydrothermal fluids that react with ultramafic rocks, we present several flux estimates

that constrain the upper and lower limits of the B hydrothermal flux and compare them to

the other known sources and sinks for B. All hydrothermal fluxes were calculated by

subtracting the seawater concentration (415 pmol/kg) from the hydrothermal fluid

concentration and multiplying this value by the average global water flux (3x1013 kg/yr)

[Elderfield and Schultz 1996]. Only high temperature fluids were considered as the

contribution of diffuse flow to the hydrothermal flux is very poorly constrained.

In the simplest case, the range of B concentrations for unsedimented hydrothermal

sites was used (300-620 pmol/kg). The net flux ranges from -0.4x1 Oi0 to 0.6 xlO10 mol

B/yr, indicating that hydrothermal processes could be a net source or sink for B (Table 2-

2). This represents the extreme case as it is unreasonable that the entire MOR system is

represented either by the low values from the ultramafic-hosted Logatchev or the highest

concentrations observed on the EPR.

A second approach is to calculate the flux for slow and fast spreading ridges

separately. Slow spreading ridges, with full rates less than 5 cm/yr account for -55% of

the total mid-ocean ridge length and fast spreading ridges (>5 cm/yr) account for the

other -45%. For these calculations, we assume that hydrothermal fluids from the slow

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. spreading ridges (with a B content of 300-560 pmol/kg) account for 55% of the total

water flux and the fast spreading ridges (B= 400-620 pmol/kg) for 45%. The results

suggest a flux of -0.2 to 0.2x1010 mol B/yr for slow spreading ridges and -0.02 to

0.3x1010 mol B/yr for the fast spreading ridges and a net total flux for the entire ridge

system of -0.2xl010 to 0.5xl010 mol B/yr. This range encompasses the estimate by

Spivack and Edmond [1987] based on the crustal availability of B (0.04 to 0.08x1010 mol

B/yr). Unlike previous estimates however, the results presented here suggest that the

MOR, in particular slow spreading ridges, may be a net sink of B in the oceans.

Table 2-2. Estimated fluxes for the major sources and sinks of B to the oceans. Flux (10jumol/yry Data Source Axial Hydrothermal Vents -0.4 to 0.61

Slow Spreading Ridge Fraction3 -0.2 to 0.2 This study Fast Spreading Ridge Fraction -0.02 to 0.3 This study and You et al. [1994], Fast Spreading Fraction including 0 to 0.3 This study & You et al. Sediment Hosted Sites4 [1994] Total Hydrothermal Flux Based on -0.2 to 0.5 Division of the Ridge into Slow and Fast Categories Ridge Flanks 1.7 Wheat and Mottl [2000]

Rivers 3.5 Lemarchand et al. [2000]

Fluids expelled at accretionary prisms 0 .2 You et al. [1993]

Oceanic Crust Alteration -2.5 Smith et al. [1995]

Sediments -1.5

Adsorption -0.9 Spivack et al. [1987] Carbonate Precipitation -0.6 Vengosh et al. [1991] 1 x 7 _ . n ______r~ i * i 2Total hydrothermal flux using the concentration range of 300-620 pmol/kg and a water flux of 3x10° kg/yr. JSlow spreading ridges are considered <5cm/yr full rate and fast spreading are considered >5 cm/yr. 4 Assuming 1 % of the fast spreading ridge water flux is represented by sediment hosted type hydrothermal fluids with an average concentration of 1750 pmol/kg [You et al. 1993; You et al. 1994].

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fluids from sedimented-hosted hydrothermal sites were excluded from the above

estimates. Sedimented sites represent only a very small fraction of the MOR, however

fluids from these sites are greatly enriched in B (as much as 5 times greater than

seawater), and thus their contribution to the hydrothermal flux should be considered.

Assuming sediment-influenced hydrothermal sites account for 1% of the fast spreading

ridges, suggests a flux of 0 to 0.3x1010mol B/yr for the fast spreading portions of the

MOR and a total flux of -0.2 to 0.5xl010mol B/yr (Table 2-2).

A sink for B as a result of hydrothermal activity at slow spreading ridges could

help to resolve the budget of B in the oceans. While previous estimates have suggested

that the B budget is balanced [Lemarchand et al. 2000], hydrothermal fluids with B

concentrations less than seawater, as well as the flux on ridge flanks, were not

considered. The sources of B to the ocean (>5xl010 mol/yr) are not balanced by the

estimated sinks (-4 xlO10 mol/yr) (Table 2-2). If all venting at slow spreading ridges

were represented by hydrothermal fluids like those at Logatchev, this sink could account

for a maximum of 0.2x1010 mol B/yr, partially, resolving the apparent imbalance. This

suggests that either the sources of B are too large or that unidentified B sinks remain.

2.5 Conclusions

Analysis of B in hydrothermal fluids from four contrasting spreading centers

indicates that phase separation and water-rock reactions are important controlling factors

for the concentration of this element, consistent with previous studies [Spivack and

Edmond 1987; Campbell et al. 1988; Palmer 1991; You et al. 1994], This suite of fluids

exhibits a relatively narrow range of B concentrations (-0.7 to 1.2 times the seawater

value) and shows little temporal variability as the vents age following a volcanic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. eruption. With the exception of Tweety and Irina vents, the fluids all have B

concentrations greater than the seawater value (-415 pmol/kg), reflecting leaching of B

into the hydrothermal fluids.

Unlike most other elements which partition preferentially into the liquid phase as

chloro-complexes, B fractionates only minimally between the vapor and liquid phases.

Thus, while phase separation resulted in very low chlorinity fluids in 1991 and 1992 at 9-

10°N EPR, B concentrations varied over only a narrow range, resulting in the highest

B/Cl ratios yet measured in hydrothermal fluids. Boron concentrations do not vary as a

function of the measured fluid temperatures, suggesting that the leaching efficiency of B

from the rock is not very temperature sensitive. The lack of temporal variability at the 9-

10°N EPR site (on a time scale of 7 years) suggests that despite its possible behavior as a

soluble element, the B concentration in the fluids cannot be used as a simple tracer of the

water/rock ratio of the system.

The spatial and temporal distribution of B has important implications for

estimates of the hydrothermal flux. Differences in B concentrations between the

hydrothermal sites may reflect relative amounts of seawater B loss through down flow

reactions, which is likely most important on slow spreading ridges, especially where

fluids react with ultramafic rocks, as is inferred at the Logatchev site. Total hydrothermal

flux estimates range from -0.1 to 0.6xl010 mol B/yr. While fast spreading ridges are in

most cases a B source, the slow spreading ridges could be a sink for B. This sink would

help to balance the large sources of B into the oceans.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter Three

CONTROLS ON LITHIUM IN HIGH AND LOW TEMPERATURE AXIAL HYDROTHERMAL FLUIDS: INSIGHTS FROM LITHIUM ISOTOPES

Abstract

To reconcile the balance of Li isotopes in the ocean and to resolve the oceanic

cycle of Li, it is necessary to understand the processes controlling Li in hydrothermal

systems. In order to address outstanding questions about the behavior of Li in

hydrothermal systems, this study reports the Li isotopic signature for high temperature

and diffuse (low temperature) hydrothermal fluids from four geologically contrasting

mid-ocean ridge hydrothermal systems on the East Pacific Rise (EPR) and Mid-Atlantic

Ridge (MAR). The results, in addition to previously reported values, demonstrate that

n the 8 Li signature of high temperature hydrothermal fluids is remarkably constant with a

global average value of 7.5±1.6%o. This constancy is in stark contrast to the variability

seen for many other chemical parameters in hydrothermal fluids including Li

concentration. These data provide important constraints on the potential effects of phase

separation, volcanic eruptions, magmatic degassing and substrate variability on Li

-j isotopes. The 8 Li isotopic signature does not vary systematically with C1-, Li-content or

measured fluid temperature. The 8 7Li signature also does not appear to vary

systematically with substrate based on data from the Lucky Strike and Logatchev sites,

where fluids react with highly altered and ultramafic substrates, respectively. Only very

subtle variations in the 8 Li isotopic signature (near the level of the analytical precision)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. are noted in the early post-eruptive fluids from the 9-10°N EPR site, likely resulting from

non-equilibrium conditions during this period.

Fractionation of the Li isotopes is inferred to occur as the isotopic signature of the

hydrothermal fluids differs from that of the fresh basalt measured in this and previous

studies. Analyses of the vapor and liquid phases, which were venting simultaneously

from the Brandon vent, demonstrate that phase separation does not result in fractionation.

Thus, the Li isotopic signature of the hydrothermal fluids is interpreted as an equilibrium

value between the fluids and secondary alteration products, consistent with previous

interpretations [Chan et al. 1993]. These results demonstrate unequivocally that Li does

not behave as a completely soluble element. Furthermore, the results imply that the

pressure and temperature conditions of the reaction zone for these four geologically

diverse hydrothermal sites are likely similar.

Our measurements of Li. isotopes in axial diffuse flow fluids, the first reported,

are consistent with diffuse flow originating from mixing of high temperature (>300°C) or

intermediate temperature (~140-170°C) crustal fluids with ambient seawater, and

therefore demonstrate that these fluid types may not require separate consideration in

calculations of the axial hydrothermal flux of Li. These results also indicate that little

low temperature interaction with the rock is occurring and suggest that the fluids have

very short (on the scale of days to months) residence times within the oceanic crust. The

hydrothermal flux of Li is estimated to be 8.0xl0 9 -9.9xl09 mol Li/yr, in good agreement

with other recent flux estimates. This flux, in addition to the river input of Li, can

balance the heavy isotopic signature of the oceans (32.4%o) if the only major sink for 6Li

occurs in low temperature weathering of basalts.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.1 Introduction

Although the concentration of Li has been measured in nearly every hydrothermal

fluid collected to date (with a global range of ~4 to 2350 pmol/kg), the chemical controls

on Li in mid-ocean ridge (MOR) hydrothermal fluids as well as the balance of Li isotopes

in the ocean have remained unresolved. In early studies of hydrothermal fluids,

concentrations of Li (and other alkali elements) were used to estimate the volume of

water and rock reacting (i.e. water/rock ratio) in these systems, as these elements were

believed to approximate soluble element behavior [e.g. Edmond et al. 1979; Ellis 1979].

However, further examination has consistently shown that many of the elements

previously considered “soluble” are controlled by equilibrium with solid alteration

products and are thus inappropriate for determining water/rock ratios.

This study reports Li isotopic data for MOR hydrothermal fluids from four sites:

17-22°S on the East Pacific Rise (EPR) and 9-10°N EPR, Lucky Strike and Logatchev

sites on the Mid-Atlantic Ridge (MAR), as well as for several samples of solid substrate

from these sites (Fig. 3-1) (Tables 3-1,3-2,3-3). These data provide important constraints

on the behavior of Li during phase separation as well as insights into the temporal and

spatial variability of the Li isotopic signature of hydrothermal fluids. This knowledge is

critical to furthering our understanding of the controls on Li in these systems, including

its utility in determining water/rock ratios, and helps us to better constrain the Li cycle in

the ocean. Additionally, the potential contribution of low temperature axial diffuse flow

fluids to the total hydrothermal flux has been poorly assessed. We examined four

samples of diffuse fluid to determine if the Li isotopic composition (1) could provide

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60'N 60’N

/Juan de -uca Plate"

30’N 30‘N Pacific Plate

Loo ait

South America Wazca T-22 "S EPR Plate

30”S — H 3 0 *s

ScoTra' Jglale_

60"S 7 60’S

Figure 3-1. Map of the global mid-ocean ridge system with the location of hydrothermal vent sites included in this study indicated with stars.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. clues to the genesis of these fluids, and ( 2 ) constrain their contribution to the axial

hydrothermal flux.

Table 3-1. End member compositions for high temperature hydrothermal vent fluid samples.

Vent Plot Sampling Temp 57Li Li Cl Li/Cl Symbol Date °C %0 pmol/kg mmol/kg pmol/mmol Seawater SW 2 32.4 ±1.0 26 540 0.048 EPR- 9-10° N A a .l1 Aa.l 10-Apr-91 390 5.3 ±1.1 62.7 80.9 0.775 Aa.2 Aa.2 17-Apr-91 396 7.3 ±1.4 15.6 30.5 0.511 Aa.3 Aa.3 24-Apr-91 403 4.32 ±7.9 10.5 43.3 0.242 Aa.4 Aa.4 08-Mar-92 332 9.7 ±1.3 99.6 286 0.348 Aa.6 Aa.6 12-Mar-94 330 5.8 ±1.0 515 398 1.23 Aa.10 Aa.10 24-Apr-96 259 6.8 ±1.0 719 600 1.20 B9.1 B9.1 01-Apr-91 368 5.1 ±1.0 97.9 154 0.636 B9.3 B9.3 06-Mar-92 365 6.1 ±1.0 183 212 0.863 B9.5 B9.5 29-Mar-94 363 5.9 ±1.0 240 267 0.899 B9.8 B9.8 25-Nov-95 364 6.9 ±1.0 438 • 498 0.880 D .l D.l 02-Apr-91 290 6.9 ±1.0 1220 846 1.44 F.I F.I 16-Apr-91 388 5.5 ±1.1 30.5 46.2 0.66 F.2 F.2 15-Mar-94 351 8.9 ±1.0 1620 846 1.91 EPR-17-22°S B a.l3-vapor Ba.l 19-Oct-98 404 7.0 ±1.0 296 317 0.93 Ba.2 -vapor Ba.2 20-Oct-98 405 5.9 ±1.0 335 338 0.99 Bb.3 -liquid Bb.3 3-Nov-98 368 6.8 ±1.0 489 558 0.88 Be.5 -liquid Be.5 17-Nov-98 376 6.9 ±1.0 488 557 0.88 Marker 72 M72 24-Oct-98 343 7.6 ±1.0 951 670 1.42 MAR-Lucky Strike Crystal C 29-Jun-97 290 8.4 ±1.0 337 538 0.63 Vent 2608 8V 29-Jun-97 328 7.2 ±1.0 361 520 0.70 Eiffel Tower ET 30-Jun-97 318 6.8 ±1.0 311 441 0.70 MAR-Logatchev Irina I 27-Jul-97 350 7.3 ±1.0 281 524 0.54 !At locations where a time series exists for a vent, the sampling chronology is indicated by a number, e.g. Aa.1 is the first sampling date for Aa vent and Aa.2 is the second sampling etc. 2The 57Li value for Aa.3 cannot be considered significantly different from the values at the other time points due to the large uncertainty on this value. 3Ba indicates the Brandon vapor phase fluids and Be and Bb are Brandon liquid phase fluids (brines). Individual orifices on Brandon vent were sampled on different days.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3-2. Lithium concentrations and 57Li for solid samples

Sample Description Location Sampling Li Li 57Li (%o) Date (ppm) (pmol/kg) EPR-9-10° N 2360-7 Fresh basalt from 9°46.5’N EPR 10-Apr-91 5.51 794 5.6±1.0 talus pile1 (Aa vent) 2500-4 Glassy sheet flow 9°46.5’N EPR 8-March-92 5.57 802 4.6±1.0 (west edge of axial summit collapse trough)

MAR-Lucky Strike 183-5-1GM Basalt groundmass Southwest 29-July-96 3.83 552 5.2±1.0 comer of Lucky Strike lava lake 2608-3-3 Slab conglomerate Near Eiffel 3-June-93 3.23 465 3.8±1.0 with euhedral Tower vent at pyrite, barite, amp. Lucky Strike silica and devitrified glass2 ‘M. Perfit, pers. comm. 2S. Humphris and M. K. Tivey, pers. comm.

Table 3-3. Lithium isotopic signature of diffuse flow fluids Sample Plot Sampling Temp 57Li Li Cl Mg Symbol Date °C %0 pmol/kg mmol/kg mmol/kg Seawater SW 2 32.4 ±1.0 26 540 52.2 EPR-9-100 N Bio9R in 1994 R.94 27-Mar-94 29.9 24.7 ±1.0 35.1 528 51.7 Bio9R in 1995 R.95 20-Nov-95 33.3 17.7 ±1.0 63.5 541 48.2 EPR-17-22°S Lowell LDF 24-Oct-98 90.7 16.9 ±1.0 56.1 541 50.7 Lowell LDF 24-Oct-98 90.7 18.5 ±1.0 53.2 541 50.6

Lithium has two stable isotopes, 7 Li and Li,f\ and due to the relatively large mass

difference (-15%) between these two isotopes, fractionation can occur in natural systems.

Because the isotopic signatures of fresh basalt and seawater, the two sources of Li to

hydrothermal fluids, are unique (5 7 Li— ~3-6%o and 32.4%o, respectively) examining the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. isotope systematics in addition to the Li concentration can potentially provide further

constraints on the behavior of Li in these systems. Lithium isotopic determinations have

been made for a few samples of hydrothermal fluids from a limited subset of MOR sites

[Chan et al. 1993]. As more MOR sites, with increased geographic, geophysical and

chemical variability have now been sampled, we re-examine Li isotopes in a larger suite

of hydrothermal fluids in order to more fully understand the geochemical behavior of Li

in these systems.

3.2 Methods

3.2.1 Hydrothermal Fluids

All fluids used for Li isotopic analysis were collected in titanium “majors”

samplers [Von Damm et al. 1985] using the DSV Alvin or the ROV Jason. In some cases,

the bottles were attached to the NO A A manifold sampler [Massoth et al. 1988]. Fluid

temperatures were measured using the Alvin high temperature probe, the manifold

temperature probe or the inductively coupled link (ICL) temperature probes. All samples

were filtered through 0.4 pm Nucleopore™ filters and were acidified with Ultrex™ HC1.

Lithium concentrations were determined by flame atomic absorption spectrophotometry

(FAAS) using National Institute of Standards and Technology (NIST) traceable standards

with a precision of ±1%. For Li concentrations at or below the seawater value (26

pmol/kg), standard additions using FAAS were used with a precision of ±3%. Analytical

data for the fluid samples are reported in Appendix B. For the high temperature fluids,

hydrothermal end member Li concentrations were determined by linear regression and

extrapolation to zero-Mg as described in Von Damm [2000].

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission 3.2.2 Solid Samples

Powdered rock samples were dissolved in a mixture of hydrofluoric and

perchloric acids on a hot plate [Chan et al. 1992]. Samples were filtered through a 0.4

pm Nucleopore™ filter using sub-boiled water, and the Li concentration was determined

by standard additions using flame emission spectroscopy with a precision of ± 2 %.

3.2.3 Isotopic Measurements

Lithium isotopic measurements for both fluids and solids were made by

measuring the 6/7Li ratio using Li 3 PC>4 as the ion source. The details of the mass

spectrometry method are described in detail in You and Chan [1996] and the chemical

separation process is briefly summarized here. To separate Li from the sample matrix,

each sample (100 ng Li) was passed through cation exchange columns (1 cm diameter, 15

cm length) packed with Bio-Rad AG50-WX8 resin. The columns were prepared by

washing with 80 mis of quartz-distilled 6 N HC1 and 80 mis of quartz-distilled sub-boiled

water. Samples were loaded onto the columns in water, and Li was eluted with 0.5N

HC1. The eluent was evaporated to dryness, redissolved in 40 mis of sub-boiled water

and then irradiated overnight with ultra-violet light in quartz tubes. Subsequently, the

samples were evaporated to a small drop and transferred to 1 ml Teflon beakers.

Approximately 50 pi of 0.025 N phosphoric acid were added to the Teflon beaker, and

the samples were allowed to react on a hot plate for at least 5 hours to produce LisPCL.

Samples were loaded onto double rhenium filaments and were analyzed at

Louisiana State University using a Finnigan MAT 262 multi-collector thermal ionization

mass spectrometer [You and Chan 1996]. 9-10°N EPR bottom seawater was used as an

absolute standard and was analyzed on every run. For ease of comparison to previous

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. data, the 57Li values are reported as per mil deviations from a L-SVEC Li-carbonate

standard value of 0.0828. While the in-run precision was less than 1 per mil, the overall

method error is estimated at ±l%o based on replicate analyses of several samples and

seawater.

3.2.4 57Li End Member Calculations for High Temperature Fluids

The process of sampling hydrothermal fluids invariably entrains some amount of

ambient seawater; thus the sampled fluid can be treated as a two-component mixture of

ambient seawater and high temperature hydrothermal end member fluid (Figure 3-2).

The sampled fluids contain both Li present in the hydrothermal end member fluid as well

as Li from the entrained seawater, each with a unique isotopic signature. Magnesium is

quantitatively removed from high temperature hydrothermal fluids through the formation

of Mg-hydroxy silicates. The 57Li for a zero-Mg end member fluid was calculated for

individual samples accounting for the isotopic contribution of seawater Li entrained

during sampling according to equations 1 and 2 :

c7 T j A useawaier k N1 measured fseawater f r •! *8 T iseawaier V, L Isample 81Liendmember / r r 1 3 (1) 1- ■fseawaier eawater V L ^\sa m p le

J seawater 1 V / L o Iseawater

where fseawater is the fraction of seawater entrained in the sample, [Li]seawater is 26

pmol/kg and $Liseawater is 32.4%o. If more than one sample for a vent on the same

sampling date was analyzed, end members were calculated individually and the results

were averaged. Hydrothermal fluids originate from seawater and thus may contain some

52 with permission of the copyright owner. Further reproduction prohibited without permission. amount of seawater-derived Li. This is different from the Li resulting from entrainment

of seawater during the sampling process. Only the amount of Li resulting from seawater

contamination during sampling is removed using the described Mg-zero correction.

Errors reported are cumulative, including errors on the Li and Mg FAAS analyses as well

as the Li isotopic determinations. In the case of the Aa.3 sample (Table 3-1), the

cumulative error is very large due to the poor sample quality (Mg>30 mmol/kg)

coincident with a very low Li end member concentration.

3.3 Lithium Isotopes in Solid Samples

Lithium isotopes in two very fresh basaltic glasses from the 9-10°N EPR 1991

volcanic eruption area were measured (Table 3-2). These samples were collected in 1991

(within weeks of the eruption) and 1992 (one year after the eruption), and their reaction

n with ambient seawater was minimal. The 5 Li of these samples is 5.6±l%o and 4.6±l%o,

identical within the analytical error and in agreement with previous values determined by

Chan et al. [1992] for basalts from the MAR and EPR. A sample of basalt from Lucky

Strike was also analyzed. This basalt contained large plagioclase phenocrysts, which

were separated from the groundmass prior to analysis. The plagioclase crystals contained

very low Li concentrations (<0.3 ppm, <43 pmol/kg) and therefore the Li isotopes were

not measured. The 57Li of the basalt groundmass was 5.2%o, identical to the EPR basalts

and within the range of previously analyzed MAR basalts (3.4-6.3%o) [Chan et al. 1992].

Hyaloclastic hydrothermal “slab” covers much of the area of the Lucky Strike site, and

reaction of hydrothermal fluids may occur with this substrate. The slab sample analyzed

had a Li concentration of 3.23 ppm (465 pmol/kg) and a 8 7Li of 3.8%o, ~l-2%o lighter

than the basalts. This light value, while similar to previously measured MAR basalts

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. s w 30

Aa.3 25

0.0 0.5 2.0 2.5 3.0 Mg/Li (mmol/nmol)

-2

72 a.10 ie.S

a.10 _Ba.2

Fresh Basalt Value from 9-10°N EPR

0.0 0.1 0.2 0.3 0.4 0.5 Mg/Li (mmol/nmol)

» 7 > > • Figure 3-2. (a, b) 8 Li versus Mg/Li mixing plots for the high temperature vent fluids. 7 « . • 8 Li is plotted against the Mg/Li ratio to account for the isotopic contribution of both the hydrothermal end member and seawater entrained during sampling. Data plotted are measured values, not end members. Plot symbols as in Table 3-1. In (b) solid line is drawn through F.I, B9.1 and Aa.l all of the samples with Mg < 8 mmol/kg collected at 9- 10°N EPR in 1991 in areas where recent volcanic eruptions had occurred. Note that these three samples extrapolate to a hydrothermal endmember identical to the analyzed fresh basalt value (~5%o).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. [Chan et al. 1992], may reflect the retention of 6Li during high temperature alteration

processes.

3.4 Lithium Isotopes in High Temperature Hydrothermal Fluids

High temperature hydrothermal fluids as well as low temperature diffuse flow

fluids were analyzed for Li isotopes. The results for high temperature fluids for

individual sites are described below, followed by a discussion of the processes

controlling Li and Li isotopes. The diffuse flow results and discussion are presented

separately in section 3.5. The 57Li isotopic signature of the end member high

temperature fluids is remarkably constant both spatially and temporally with an average

value of 6.7+1.2%o (n=22) (Fig. 3-3). Chan et al. [1993] determined the Li isotopic

signature of hydrothermal fluids from 11-13°N EPR, 21°N EPR and the MARK site on

the MAR, with a range of 6.6-11.0%o, and concluded that secondary alteration products

retained 6 Li, resulting in isotopic signatures similar to, but heavier than, the fresh basalt

value (Fig. 3-4). The results of Chan et al. [1993] are in general isotopically heavier than

the fluids examined here, and this is most likely an analytical artifact. Comparative

studies of both methods for hydrothermal fluids and seawater suggest that in some cases

the borate method [Chan 1987] results in slightly heavier isotopic values than the

phosphate method [You and Chan 1996], although the difference is close to the analytical

error (Appendix C). In combination with the results presented here, the 8 7Li content of

MOR hydrothermal fluids varies over a relatively narrow range, with a global average of

7.5±1.6%o (n=36).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bfe$^ae,3 Aa.10 2 Aa.6 B9.B9.33a.Aa F/fe.l09.

600 900 1200 400 600

Li (pmol/kg) Cl (mmol/kg)

LW (C)

14 -

12 -

10 -

8 - Aal2l'l^ « lA a.1 0 D 71 Aa.10 D.1 6 - Aa 6 4 -

2 - 0 - 0 5 1.0 1.5 2.0 250 300 350 400

Li /Cl (pmol/mmot) Measured Fluid Temperature (°C)

1600

Aa.10

* ET

1 Aa.4

200 400

Cl (mmol/kg) Figure 3-3. End member lithium isotope, Li, Cl and temperature data for the high temperature fluids analyzed, (a) 5 Li versus Li concentration. A ±l%o error bar (shown) is applicable to most samples, though the error for the Aa.3 sample is much larger, (b) §7Li versus Cl concentration, (c) 57Li versus Li/Cl ratio, (d) 57Li versus measured fluid temperature, (e) Li concentration versus Cl concentration. Plot symbols as in Table 3-1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 7 Li %o 0 2 4 6 8 1 0 7 / 25 35 i i i i i - r

9-10'N EPR Fluids Eruption Impacted No Recent Eruption ■1 .. » ■ Fresh 17-22'S EPR 1 Fluids

> | i > .... MORE

Lucky Strike- MAR i Fluids i Hyaloclastite Slab ;

Logatchev- MAR Fluids m

Chan et al. [1993] EPR Fluids MAR Fluids

7 Figure 3-4. Range of 5 Li signatures for high temperature hydrothermal fluids samples compared to seawater and fresh mid-ocean ridge basalt (MORB). The hyaloclastite slab sample from Lucky Strike is also shown. Note that the 5 Li signature from the hydrothermal fluids at four sites overlap and are in general isotopically heavier than the fresh basalt value.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.4.1 East Pacific Rise, 17-22°South

Hydrothermal vent fluids were sampled along the ultra-fast spreading southern

EPR between 17-22°S during the SouEPR cruise in October-November 1998 (Fig. 3-1).

Two of the 40 high temperature vents sampled from the southern EPR, Brandon and

Marker 72, were analyzed for Li isotopes (Table 3-1). Marker 72 is located at 18°26.0’S

latitude and has high chlorinity fluids, interpreted to be the result of super-critical phase

separation [O'Grady 2001]. The 57Li of the Marker 72 fluids is 7.6±1.0%o (Table 3-1;

Fig. 3-3), similar to many of the values reported by Chan et al. [1993]. Brandon vent,

located at 21°33.7’S, is unlike any other hydrothermal vent previously sampled from a

MOR environment. The pressure and temperature conditions at the seafloor (405°C, 283

bars) place it very close to the critical point of seawater (407°C, 298 bars), and it is

venting both vapor (low chlorinity) and liquid (brine, high chlorinity) fluids

simultaneously from separate orifices located on one sulfide structure [Von Damm et al.,

submitted]. Phase separation occurs within the sulfide structure, thus fluids from

Brandon provide important constraints on the behavior of Li isotopes during phase

separation without overprinting by post-phase separation water-rock reaction with the

aluminosilicate substrate. The Li isotopic composition of the Brandon vapor and liquid

phase fluids are identical within the analytical error (Table 3-1). This is conclusive field

evidence that Li isotopes do not fractionate during phase separation. The Brandon results

are consistent with the experimental results of James et al. [1999], who determined that

Li isotopes do not fractionate during super-critical phase separation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.4.2 East Pacific Rise, 9-10°North

The EPR between 9-10°N is a fast spreading ridge (11 cm/yr) [Carbotte and

Macdonald 1994], with an average depth between 2500-2600 meters (Fig. 3-1). This site

has been intensely studied since a volcanic eruption occurred in 1991. This provided a

time-zero point from which the evolution of a hydrothermal system could be observed

with unprecedented constraints on the age of the system. Hydrothermal fluid samples

have been collected at this site on a near-annual basis between 1991 and 1997, providing

an opportunity to examine temporal evolution of Li isotopes in hydrothermal fluids.

Lithium isotopes were measured in fluids from four vents: Aa, Bio9, D and F vents (Fig.

3-3, 3-4, 3-5). Bio9 and Aa vents are both in the area of the 1991 eruption, and these

vents have been sampled on multiple occasions since 1991 [Von Damm et al. 1995; Von

Damm 2000]. Bio9 vent was also impacted by a second, smaller eruption in 1992. Both

vents had exceptionally low Cl and Li concentrations in 1991 (Table 3-1) and have

generally evolved to higher Cl and Li concentrations over the time series (Fig. 3-5).

Because Cl is a major variable in hydrothermal fluids, alkali-to-chloride ratios can

be used to compare hydrothermal fluids of varying chlorinities. Variations from the

seawater ratio indicate that processes other than phase separation, such as water-rock

reaction, are occurring. Lithium-to-chloride ratios greater than the seawater value

indicate that basaltic Li is being leached into the circulating hydrothermal fluids. The

Li/Cl ratios in the Aa and Bio9 vent fluids were lowest in 1991 and 1992 and rose later in

the time series (Fig. 3-5). The low alkali-to-chloride ratios in these fluids from early in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (a)

E t 3 § 3 9 B9 V Aa

y ' A 0 10 20 30 500 1000 1500 0 10 20 30 500 1000 1500

Days Since 1 April 1991 Days Since 1 April 1991

1000

800

g> 600

400

9 B9 200 V Aa ■ D O F

0 10 20 30 500 1000 1500 0 10 20 30 500 1000 1500

Days Since 1 April 1991 Days Since 1 April 1991

Figure 3-5. End member time series data for the 9-10°N EPR high temperature hydrothermal vents showing their chemical evolution since April 1991. The chemistry of

B9, Aa and F vents were all affected by recent volcanic activity, (a) 8 7Li versus time, (b) Li concentration versus time, (c) Cl concentration versus time, (d) Li/Cl ratio versus time. In most cases, the error bars are smaller than the symbols.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the time series are indicative of minimal water-rock interaction due to shallow, short f' reaction paths [Bray 1998; Bray and Von Damm, submitted; Von Damm 2000], D vent,

located at 9°33.5’N, was not affected by the eruption and thus provides a comparison to

those vents impacted by the eruption. The D vent fluids have high chlorinity (846

mmol/kg) with a very high Li concentration (1220 pmol/kg); and sampling in 1991, 1992

and 1994 revealed that its chemistry remained relatively constant during this period. F

vent, the most southerly vent at the 9-10°N EPR site, is -60 km south of the area where

the 1991/1992 eruptions occurred. However, when fluids were first sampled from F vent

in 1991, they were chemically similar to those in the area of the new eruption with very

low Cl (46.5 mmol/kg) and Li contents (18 pmol/kg). Re-sampling of F vent in 1994

indicated that it had evolved to a brine (Cl=846 mmol/kg, Li=1620 pmol/kg). Von

Damm et al. [1997] showed that compositionally the 1991 F vent fluids were the

conjugate pair to the 1994 brines, formed by the sub-critical phase separation of seawater.

Despite the widely varying Cl- and Li-contents and Li/Cl ratios of the 9-10°N

EPR fluids, the Li isotopic signature of these fluids is nearly constant with an average

n value of 8 Li=6.4±l .5%o (n=13). The variability observed is just slightly larger than the

error of the analyses. While close to the level of the analytical error, several of the 1991

9-KEN EPR fluids (Aa.l, B9.1, F.I) have Li isotopic signatures that are systematically

lighter compared to the samples from later in the time series, and are identical to the fresh

basalt signature (Fig. 3-3, 3-4). After 1991, the 8 7Li values of the fluids are generally

heavier and more similar to previously published values [Chan et al. 1993].

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.4.3 Mid-Atlantic Ridge

Hydrothermal fluids from the Lucky Strike and Logatchev vent fields (Fig. 3-1)

on the slow spreading (2.5 cm/yr) MAR were analyzed to determine if reaction with the

different substrates found at these two locations is reflected in their Li isotopic signatures.

The Lucky Strike hydrothermal field, located at 37°17’N, is relatively shallow (1618-

1726 meters), situated between the three cones of the Lucky Strike volcano. The oceanic

crust here is enriched due to its proximity to the Azores hot spot [Langmuir et al. 1997],

but the fluid chemistry suggests that the hydrothermal fluids react with a substrate that

has been extensively altered at high temperatures [Von Damm et al. 1998]. The Li

concentration of the vent fluids is low (292-417 pmol/kg) compared to other vent

locations, particularly on a chloride-normalized basis.

The fluids at the Logatchev site (14°45’N)(Fig. 3-1) are formed by reaction of

seawater with an ultramafic source rock [Bogdanov et al. 1995; Gebruk et al. 2000] and

have relatively low Li concentrations (281 pmol/kg) as well as low Si and B contents and

high H2 contents (>10 mmol/kg) [Bray and Von Damm, submitted; K.L. Von Damm,

unpubl. data, M.D. Lilley, unpubl. data]. The 57Li signature of the hydrothermal fluids

from these two MAR sites is nearly identical to the fluids from the EPR and does not

reflect the different substrate found at the MAR locations (Table 3-1, Fig. 3-4). Reaction

of hydrothermal fluids on the MAR with substrate previously weathered at low

temperatures has been suggested as potentially important to the cycling of the alkali

elements and boron [Campbell et al. 1988; Palmer and Edmond 1989], Isotopically

heavy Li from seawater is taken up by basalt during low temperature weathering; thus

reaction of the hydrothermal fluids with weathered crust should result in heavier isotopic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. values in the fluids [Seyfried et al. 1998]. Isotopically heavy fluids are not observed at

the MAR sites, suggesting that recycling of seawater Li through this process is not

occurring in these high temperature hydrothermal systems, consistent with the results of

Chan et al. [1993].

3.4.4 Controls on Li and Li Isotopes in High Temperature Hydrothermal Fluids

There are three major processes that potentially affect the concentration of Li and

the Li isotopic signature of hydrothermal fluids: magmatic degassing, phase separation

and water-rock interactions.

3.4.4.1 Magmatic Degassing

Direct degassing of the magma chamber is unlikely to affect Li, as it is not a

volatile metal. Results from the Bio9 vent fluids at 9-10°N EPR support this conclusion.

Extremely high gas contents in the Bio9 vent fluids beginning in 1993 are suggestive of

direct magmatic degassing [M.D. Lilley, unpublished data]. While the Li concentration

increased in the Bio9 fluids between 1992 and 1994 by -30%, much of this increase is a

n result of increasing Cl concentration. The Li/Cl ratio increased by only 4% and the 8 Li

isotopic signature remained unchanged within the analytical error, suggesting that

magmatic degassing has little effect on the Li chemistry and that other processes are

controlling Li in these fluids.

3.4.4.2 Phase Separation

For hydrothermal fluids, Li concentrations generally vary with changes in Cl

concentration (Fig. 3-3). Phase separation is the primary control on Cl content, and the

relationship between Li and Cl clearly indicates the important roles that chloro-

complexing and phase separation have on Li concentrations in hydrothermal fluids. In

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. experimental studies of phase separation, Bemdt and Seyfried [1990] found that Li

concentrations were greater in the liquid than in the vapor phase, though the Li/Cl ratio

was conserved during phase separation. They suggested that the low electronegativity of

the alkalis and the ionic nature of the bond between the alkalis and Cl in chloro-

complexes cause these metals to move primarily into the phase with more Cl. While

phase separation is a very important control on the concentration of Li in hydrothermal

fluids, evidence from Brandon vent demonstrates that phase separation does not result in

any measurable fractionation in the Li isotopes.

3.4.4.3 Water-Rock Interaction

Hydrothermal fluid originates as seawater that is then chemically modified by

high and low temperature reactions during its residence time in the oceanic crust.

Therefore, there are two potential sources of Li to hydrothermal fluids: seawater and

basalt. Some of the seawater Li may be lost through low temperature down flow

reactions. However, hydrothermal fluids are in most cases greatly enriched (as much as

90 times) over the seawater value. As a result, the vast majority of Li present in high

temperature hydrothermal solutions must be basaltic in origin, in agreement with 57Li

signatures that are very similar to the basaltic value. Calculations of two-component

mixing between seawater and basaltic Li (Appendix D) show that even large losses

(-50%) in downflow reactions have little impact on the final isotopic signature of the

hydrothermal fluids. With the exception of several 1991 9-10°N EPR fluids, the

hydrothermal fluids studied do not have an isotopic signature identical to the basalt value,

suggesting that fractionation of the Li isotopes is occurring.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fractionation of stable isotopes is fundamentally dependent on pressure and

temperature conditions, and fractionation of Li isotopes can potentially occur during any

of several processes in the formation of high temperature hydrothermal fluids.

Preferential leaching of one Li isotope from the rock may occur. Isotopes could also be

fractionated between the aqueous species. In hydrothermal solutions, Li exists as Li+ or

L iC F aq, and the relative proportion of these species is determined by a temperature and

pressure dependent reaction (Li++Cr<-»LiCl°aq). Only Li+ is available to participate in

mineral reactions, and variations in temperature and pressure will change the equilibrium

constant for this reaction and thus the relative proportion of these species. Geochemical

modeling of hydrothermal solutions suggests Li is partitioned as 60% Li+, 40% LiCl°aq at

395°C and 370 bars [Bray, unpublished data], Schauble et al. [2001] predicted

significant equilibrium fractionations of Fe isotopes between coexisting aqueous

complexes for temperatures between 0-300°C, caused by variations in bond strength and

oxidation state. While the appropriate experimental studies of Li isotope fractionation

related to aqueous speciation have not yet been done, this fractionation mechanism

warrants consideration especially given the large mass difference between the Li

isotopes. The proximity of fluid reaction conditions to the critical point may be

important to this fractionation mechanism because all aqueous species must be neutral

(i.e. charged species are completely complexed) at the critical point [Helgeson 1964].

As suggested by Chan et al. [1993], isotopic fractionation may also occur during

the formation of secondary hydrothermal alteration products, with the solids

preferentially retaining 6 Li. Using a fractionation factor (aminerai-fiuid) of 0.996 [Chan et al.

1993] and the average 57Li value for hydrothermal fluids, suggests an isotopic signature

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of 57Li =3.5%o for the secondary alteration minerals. This value is lighter than the fresh

EPR basalt value but is consistent with light isotopic values measured in hydrothermally

altered rocks from ODP Hole 504B [Chan et al., in preparation]. This fractionation factor

is also identical to our measured isotopic fractionation between the hydrothermal fluids

and the 57Li signature of the Lucky Strike hydrothermal slab, which may represent the

best example of high temperature mineral alteration products. Experimental studies have

shown that some Li is present in secondary alteration products, particularly zeolites,

smectites and chlorites [Seyffied et al. 1984; Berger et al. 1988]. Additionally, the

retention of the lighter 6Li isotope is consistent with the observed direction of Li isotope

fractionation in ion exchange experiments [Taylor and Urey 1938; Oi et al. 1991]. The

final isotopic signature observed in the hydrothermal fluids may reflect a combination of

these processes.

The isotopically light values observed in several of the early 9-10°N EPR fluids

may result from differences in the fractionation processes and reaction conditions of the

immediate post-eruptive period. These data provide new insights into the relative

importance of the proposed fractionation mechanisms. The post-eruptive fluids had

extremely high measured temperatures (<403°C) and are likely the result of very shallow

(<300m) hydrothermal circulation and extreme conditions of sub-critical phase separation

[Von Damm et al. 1995; Von Damm 2000], While phase separation itself does not result

in isotopic fractionation, the proximity of these very hot fluids to the critical point of

seawater (407°C, 298 bars) will affect the relative distribution of Li+ and LiCl°aq. As

suggested earlier, fractionation of Li isotopes between the aqueous species of Li may

occur. The extreme temperature and pressure conditions of the early 9-10°N EPR fluids

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. could result in isotopic fractionations different from those sampled later in the time

series.

As the Brandon fluids (21°S EPR) also have measured temperatures very close to

the critical point but do not have unusually light isotopic signatures, these results suggest

that proximity to the critical point is not the source for the unusual isotopic signatures in

the early 9-10°N fluids. The lighter isotopic signatures at 9-10°N EPR most likely reflect

isotopic disequilibria of the immediate post-eruptive fluids. Geochemical modeling has

demonstrated that in the immediate post-eruptive period, the Aa vent fluids were not at

equilibrium with respect to Na- and K- containing minerals, such as muscovite and

paragonite, but ~3 years after the eruption the fluids had achieved equilibrium with these

or similar alteration minerals [Bray 1998; Bray and Von Damm, submitted]. Similar

modeling for Li could not be done, as the appropriate thermodynamic data do not exist

[Bray 1998; Bray and Von Damm, submitted]. The lighter 57Li values from the Aa.l,

B9.1 and F.l fluids are identical to a pure basaltic isotope signature with no isotopic

fractionation occurring, most likely reflecting the non-equilibrium/non-steady-state

conditions in the immediate post-eruptive period. Low Li/Cl, K/Cl, Rb/Cl and Cs/Cl

ratios from these samples, as well as other chemical data from the 9-KEN EPR fluids, are

consistent with a shallow reaction zone and short reaction path in which the fluids did not

have time to fully react with the rock and to reach equilibrium or steady-sate [Bray 1998;

Von Damm, 2000]. As these fluids did not achieve chemical equilibrium, they could not

n have achieved isotopic equilibrium. The 8 Li signatures of these vent fluids therefore

likely reflect kinetic, rather than equilibrium, controlled isotopic processes.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Unlike the 1991 Aa.l vent fluid which is isotopically lighter than most of the

fluids studied, the Aa.4 fluid (1992) has the heaviest isotopic signature of all the fluids

analyzed (Table 3-1). These fluids also have a low Li/Cl ratio, indicative of less Li

enrichment from the basalt. The Aa.4 fluids may represent a mixture of shallow dike-

derived hydrothermal fluids and some amount of deeper derived hydrothermal fluids as

the system recovers from the volcanic eruption in 1991 and the reaction zone begins to

deepen. Increased Si contents in the 1992 fluids are consistent with a deepening reaction

zone, and small but significant chemical variability between repeat samplings (on the

time scale of days) also suggests that in 1992 the Aa vent system was still unstable and

had not yet re-equilibrated following the eruption [Von Damm et al. 1995]. Therefore,

7 * . . the heavy 5 Li signature, coincident with a relatively low Li/Cl ratio, may reflect the

continued state of non-equilibrium one year after the volcanic eruption.

With the exception of the early post-eruptive fluids from 9-10°N EPR, the.

remarkable consistency in the Li isotopic signatures in the high temperature hydrothermal

fluids from these four sites suggests that isotopic equilibrium between the hydrothermal

fluids and mineral alteration products has been achieved. This conclusion is in agreement

with previous studies [Chan et al. 1993]. Lithium therefore, like the other alkali metals,

does not appear to behave as a completely soluble element and this should be considered

in any future calculation of water/rock ratios using Li. Equilibrium isotopic fractionation

is fundamentally a function of temperature and pressure conditions, and in the case of Li

it appears that the isotopic signature is a result of the establishment of equilibrium

between the fluids and an alteration mineral assemblage, possibly including greenschist

minerals, such as chlorite. Thus, the global constancy of the 57Li signature suggests that

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the reaction zone conditions at these four hydrothermal vent fields are similar, despite

their locations on ridges with very different spreading rates, water depths and substrates.

3.5 Lithium Isotopes in Diffuse Hydrothermal Fluids

In addition to the analysis of high temperature hydrothermal fluids, Li isotopes

were also measured in diffuse, low temperature axial hydrothermal fluids. Previous

studies have not analyzed these types of fluids, although some authors have suggested

they are a potentially important component of the hydrothermal flux to the ocean [Schultz

et al. 1992; Baker et al. 1993; Schultz and Elderfield 1997], Diffuse flow fluids in this

study are defined as unfocused flow emanating directly from basalt, rather than sulflde-

sulfate constructional features (chimneys), along the ridge axis with a measured

temperature <100°C. In past studies, axial diffuse flow fluids have generally been

assumed to be a dilution of high temperature “black smoker” fluids with seawater

[Edmond et al. 1979; Butterfield and Massoth 1994; Von Damm et al. 1996], in part

because extrapolation of the original Galapagos diffuse fluids so accurately predicted the

chemistry of the high temperature fluids later discovered at 21°N EPR. More recent

evidence suggests that this simple mixing scenario may not be accurate for all elements

as losses of bioactive elements such as EhS, as well as Fe and Mn and gains of CH 4

between adjacent high temperature vents and diffuse flow fluids have been observed

[Von Damm et al. 1996; Lilley et al. 1996], If diffuse flow fluids are a simple dilution of

high temperature fluids with seawater, for a given heat flux they are relatively

unimportant to hydrothermal flux calculations for elements that behave conservatively

during seawater mixing [Kadko et al. 1995], Alternatively, if these low temperature

fluids are formed through processes different from the formation of high temperature

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fluids, they are not being incorporated correctly into flux estimates. The nature and

origin of these diffuse fluids must be understood to refine estimates of the hydrothermal

flux. As Li is not likely to be affected by biological activity nor is it sensitive to

precipitation as sulfides, but can be taken up during low temperature reactions, it is a

useful tracer for understanding the origins of diffuse flow. The 57Li signature of four

diffuse flow samples was measured (Table 3-3). These diffuse samples were selected

based on the availability of high temperature fluids from the same sampling time and

location so that comparisons could be made between the high and low temperature fluid

chemistries. All chemical results presented for the diffuse fluids are measured values and

are compared to the calculated end member values for the adjacent high temperature

fluids.

3.5.1 Bio9-Riftia

Lithium isotopes were determined in diffuse flow fluids emanating from a clump

of tubeworms at the Bio9-Riftia (B9R) site sampled in March 1994 and November 1995.

Fluids from the adjacent high temperature Bio9 vent were also sampled during these two

time periods. Chemical changes occurred in the high temperature fluids during this

period including increased Cl and Li concentrations, resulting from a deepening of the

reaction zone in response to a crustal cracking event in March 1995 [Fomari et al. 1998;

Sohn, 1998]. In 1994, the B9R fluids were 29.9°C and had a chlorinity 2.3% less than

the seawater value and a Li concentration 35% greater than the seawater concentration

(Table 3-3). When re-sampled in 1995, the B9R fluids were 33.3°C with a chlorinity

equal to the seawater concentration and a Li concentration more than twice the seawater

value.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A mixing curve between high temperature fluid and seawater end members

suggests that these diffuse flow fluids may be a simple mixture of these two fluids (Fig.

3-6). The results of mass balance calculations using fluid enthalpies (determined for the

measured temperatures for pure water) suggest that at both sampling times, the B9R

fluids could result from mixing of -7-8% Bio9 fluids and -92-93% ambient seawater,

assuming no cooling or heating of the fluid took place after mixing (Fig. 3-7).

The Li concentration and 8 7Li expected in the diffuse fluids if simple mixing had

occurred can be calculated based on the above proportions and the chemistry of the high

temperature Bio9 vent for each sampling period. The calculated results (Table 3-4)

suggest Li losses of at most 14% from the concentration expected by simple conservative

mixing. For the 9-10°N B9R fluids, the Li isotope values are consistent with mixing of

high temperature fluids with seawater with only a very small amount of Li loss at low

temperatures. Lithium is removed from solution at temperatures <150°C [Seyfried et al.

1984], and low temperature losses of Li would result in isotopic fractionation. If 6Li

were being preferentially retained in the alteration products [Chan et al. 1992], then the

diffuse fluids should be isotopically heavy, yet the samples measured here are <3%o

heavier than the 5 Li signature expected from simple mixing. This suggests minimal low

temperature reaction is occurring between the diffuse fluids and the basalt. In

experimental reactions of seawater and basaltic glass at 150°C, Seyfried et al. [1984]

observed losses of Li from solution (-15%) after approximately 70 days. The very

limited losses of Li observed in the B9R samples may suggest a relatively short residence

time for these fluids in the oceanic crust, on the scale of several months and that the fluid

temperatures are likely greater than 150°C.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 5

25 R94

LDF R95 LDF

0.0 0.5 1.0 1.5 2.0 2.5 3.0 Mg/Li (mmol/fimol)

n Figure 3-6. 8 Li versus Mg/Li mixing plot show the high temperature fluids (grey) and the diffuse flow values (black). SW= seawater. Data plotted are measured values, not end members. Plot symbols as in Table 3-1, 3-3.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Bio9 Bio9 Riftia 363-364°C 29.9-33.9°C Mg=0 mmol/kg Mg=48.2-51.7 mmol/kg Li=240-438 jamol/kg Li=35.1-63.5 jiimol/kg 87Li=5.8-6.9%o Seafloor 87Lf=17.7-24.7%o

Sdb-garfacc Mixing

Seawater 2°C ----- Mg=52.2 mmol/kg Li=26 jxmol/kg 57Li=32.4%o

Figure 3-7. Mixing scenario for the Bio9 Riftia fluids in 1994 and 1995 showing sub surface mixing between the high temperature Bio9 vent and seawater to form the diffuse fluids. Using enthalpy, simple mixing would require 6.8-7.7% Bio9 fluids with 93.2- 92.3% seawater to form fluids with the measured temperature of Bio9 Riftia. See Table 3-4 for a comparison of the predicted chemical composition for the Bio9 Riftia fluids based on the mixing scenario with the measured values.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3-4. Lithium isotopic signatures predicted by simple mixing of seawater with high temperature Bio9 vent fluids compared to measured values for Bio9 Riftia diffuse fluids. Sample Plot Sampling Measured Calculated Measured Calculated Measured Calculated Measured Symbol Date Temp 87Li' S7Li Li Li M g Mg °C pm ol/kg pm ol/kg mmol/kg mmol/kg Seawater SW 2 32.4%o 26 52.2 Bio9 Hi T in B9.5 29-Mar-94 363 5.9%» 240 0 1994 Bio9 Hi T in B9.8 25-N ov-95 364 6.9%o 438 0 1995 B io9R in R.94 27-Mar-94 29.9 21.7%» 24.7%o 40.6 35.1 48.7 51.7 1994 B io9R in R.95 20-Nov-95 33.3 17.4%o 17.7%o 57.7 63.5 48.0 48.2 1995 __—■J4— — ■■ ■ ■■...... ■■■...... ■ ■ nr...... -r.. . — — ...... Based on mass balance calculations using 6.8-7.7% Bio9 fluids and 92.3-93.2% seawater. Proportions of seawater and Bio9 fluids reflect the amount of seawater and high temperature fluid required to achieve the Bio9R measured temperature (calculated based on the enthalpy).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.5.2 Lowell Diffuse Flow

The Lowell Diffuse Flow (LDF) fluids from 18°26’S EPR are considerably hotter

(90.7°C) than the B9R fluids but have Mg concentrations only -1% less than seawater

(Table 3-3). Two samples of LDF collected on Alvin dive 3293 were analyzed and have

Li concentrations of 56.1 and 53.2 jimol/kg and S7Li equal to 16.9 and 18.5%o,

respectively. For comparison, borehole fluids sampled from ODP Hole 504B have

similar measured temperatures (80-162°C), Li concentrations (29-53 pmol/kg) and Li

isotopic signatures (57Li of 18-25%o) compared to LDF, but have a greater range of Mg

concentrations (54.0-26.9 mmol/kg) [Magenheim et al. 1995].

The LDF is located 11 meters from the Marker 72 high temperature vent. Like

the B9R fluids the LDF fluids fall on the mixing line between seawater and high

temperature fluids (Fig. 3-6). Flowever, both geochemical modeling and mass balance

calculations definitively demonstrate that the LDF is not Marker 72 fluid that has simply

cooled or mixed with seawater [O'Grady 2001], As simple mixing and cooling of known

high temperature fluids with seawater cannot explain the LDF, an alternative model is

that the LDF results from a mixture of seawater with an intermediate temperature,

partially reacted hydrothermal fluid (Table 3-5, Fig.3-8). Partially reacted “intermediate

fluids” (IF) have previously been suggested by Ravizza et al. [2001] to explain the Sr

isotopic composition of some hydrothermal fluids from the 9-10°N EPR site.

Hydrothermal fluids with temperatures between 100-200°C have only rarely been

sampled from the MOR axis, but the 504B borehole fluids can be used as a possible

analog to the proposed IF fluid. If the IF fluid is assumed to have a temperature similar

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3-5. Chemical composition of a hypothetical intermediate fluid (IF) predicted by mixing of seawater and an IF 1 fluid (140-170°C) to form the LDF. The 504B bore hole fluids are shown for comparison [Magenheim et al. 1995]. Sample Plot M easured Calculated M easured Calculated M easured Calculated M easured Symbol Temp 57Li' 5?Li Li Li Mg M g °C pmol/kg pm ol/kg mmol/kg mmol/kg Seawater SW 2 32.4%o 26 52 Lowell Diffuse LDF 90.7 16.9- 53.2-56.1 50.7-50.6 Flow 1&.5%o Intermediate Fluid IF 364 14-15%o 71-82 50.1-50.3 504B Fluids 29.9 18-25%o 32-53 27-54 ‘Intermediate fluid composition necessary to produce the measured LDF chemistry by mixing with seawater. Mixing proportions used were 36-48% seawater and 52-64% IF and the IF fluid was assumed to be 140-170°C.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lowell Diffuse Mow 90.7°C Mg= 51 mmol/kg Li= 55 fimol/kg 87Li=18%o Seafloor t Sdb-§arfacg Mixing Seawater Intermediate Fluid 2°C 140-170°C Mg=52.2 mmol/kg ^ Mg= ? Li=26 p,mol/kg Li=? 57Li=32.4%o 87Li=?

Figure 3-8. Mixing scenario showing sub-surface nixing between seawater and a hypothetical intermediate fluid (IF.) to form the Lowell Diffuse Flow. The IF. may have a temperature similar to bore hole fluids sampled from ODP hole 504B and the chemical composition predicted for the IF. is reported inTable 3-5.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to the hottest 504B borehole fluids (~140-170°C), the LDF could result from mixing -52-

64% IF and -36-48% ambient seawater. Using these mixing proportions, seawater and

IF fluid could form the LDF if the IF had a Li concentration of 50-80 pmol/kg and a 57Li

signature of 13-15%o. This requires leaching of basaltic Li into the IF and reaction

temperatures >150°C. The Mg concentration of the IF would be -49 mmol/kg, higher

than the values previously seen in most of the borehole fluids [Magenheim et al. 1995],

The LDF may therefore be a result of mixing between seawater and an IF that has gained

basaltic Li but lost very little Mg. In low temperature experiments of glass and seawater

reaction at 150°C, -30% of the original Mg was lost within the first 3 days of the

experiment [Seyfried and Bischoff 1979]. The very small losses of Mg in the LDF may

suggest that the residence time of the LDF in the oceanic crust is very short, on a time

scale of less than -3 days.

3.5.3 Controls on Li Isotopes in Diffuse Flow Fluids

The 8 7Li signatures of the EPR diffuse flow suggest that these fluids could result

from mixing of seawater with high temperature vent fluids (at 9-10°N EPR) or mid

temperature intermediate fluids (at 21°S EPR). Large low temperature losses of Li are

inconsistent with the measured isotopic signature of all of these diffuse fluids. The

observation that little Li is lost from the axial diffuse fluids suggests that the residence

time for these fluids in the oceanic crust may be relatively short (on the scale of days to

several months). These axial diffuse fluids are therefore fundamentally different from the

warm spring fluids observed in ridge flank settings where the residence time of the fluids

is much longer (thousands of years), and large amounts (-65%) of Li are lost through low

temperature reaction [Wheat and Mottl 2000]. The diffuse described here could result

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. from dilution of high or intermediate temperature fluids with seawater. If most diffuse

fluids are formed by dilution of high temperature vent fluids like the Bio9R, for elements

like Li that mix conservatively, diffuse venting does not require separate consideration in

estimates of hydrothermal flux from axial hydrothermal systems. The effect on

hydrothermal flux from intermediate fluids is unknown but warrants further examination.

3.6 Hydrothermal Fluids and the Balance of Li Isotopes in the Ocean

Previous studies [Stoffyn-Egli and Mackenzie 1984; Chan et al. 1992; Chan et al.

1994; Huh et al. 1998] have attempted to reconcile the heavy Li isotopic signature of the

ocean (32.4%o) with the relatively light sources of Li to the oceans: rivers (23.5%o) [Huh

et al. 1998] and high temperature hydrothermal vent fluids (7.5%o) [Chan et al. 1993; this

study]. The recent calculation of the Li isotope budget by Huh et al. [1998] suggested

that an unidentified 6Li sink such as authigenic clay formation exists, or that the

hydrothermal flux of Li is much smaller than previous estimates. Early estimates of the

Li flux from the MOR were very large (14xl010 mol/yr) [Edmond et al. 1979; Von

Damm et al. 1985; Chan et al. 1992], whereas more recent estimates are nearly an order

of magnitude lower [Elderfield and Schultz, 1996; Von Damm et al., in prep] (Table 3-6).

As the results of this study have shown that the 57Li signature of high temperature

hydrothermal fluids is very constant, the isotopic mass balance of the ocean can now be

readdressed using the approach of Chan et al. [1992] and Huh et al. [1998]. Equation 3

represents an isotope balance for Li in the ocean at steady-state:

6 Li 1 v 'Lij seawater V J seawater

V V "Li,./ input v "Liy, output

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3-6. Estimates of Li flux from rivers, MOR flanks and axial hydrothermal activity. Li Source or Flux (mol Li/yr) Data Note Sink Source Li-River 2.8xl09-1.4xl0lu 1,2 Most recent estimates of river flux Li-Ridge Flank -1.8xl09 3 From Baby Bare site, Juan de Fuca Ridge Li-Hydrothermal 1.6xlOn 4 From extrapolation of the low temperature Galapagos fluids Li-Hydrothermal 1.2-1.9x10" 5 Based on high temperature fluids at 2]°N EPR Li-Hydrothermal 1.4x10'° 6 Based on the amount of available Li in new oceanic crust Li-Hydrothermal 1.2-3.9x10'° 7 Based on geophysical estimates of water flux and high temperature fluid chemistry Li-Hydrothermal 3.7 x 109- 2 .4x 10'° 8 As in 7 but accounts for increased water flux and low Li contents of the immediate post-eruptive period at 9-KEN EPR Li-Hydrothermal 8.0x 109- 9.9x 109 9 Assuming a steady-state Li isotopic signature for the oceans T- i- J M . 2Huh et al. [1998] 3 Wheat and Mottl [2000] ''Edmond et al. [1979] 5Von Damm et al. [1985] 6Ryan and Langmuir [1987] 7Elderfield and Schultz [1996] 8Von Damm et dl. [in preparation] 9This study

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. where alpha (a) is the isotopic fractionation factor required to maintain the isotopic

balance of the oceans [Chan et al. 1992; Huh et al. 1998]. This equation assumes a

steady-state ocean where the sources of Li are balanced by the sinks; therefore 6Li must

be preferentially removed in order to balance the heavy sources of Li to the oceans.

Lithium behaves conservatively in the oceans with a residence time on the order of one

million years and the oceanic Li isotopic composition of seawater is spatially constant

[Chan and Edmond 1988; Huh et al. 1998]. The isotopic signature of the Li input to the

ocean can be represented by:

(«v L,,,=A (^£0,,+/,(Vi o , (4)

where F .is the fraction of hydrothermal (h) and river (r) input, respectively. If the

hydrothermal and river fluxes are known, these equations can be used to determine the

fractionation factor required to maintain the isotopic composition of seawater.

Alternatively, these equations can be used to calculate the hydrothermal flux by assuming

a fractionation factor for the Li sinks [Huh et al. 1998].

Two sinks for Li in the ocean system include low temperature (~2°C) basalt

alteration and mid-temperature (~60°C) reactions on the ridge flanks [Wheat and Mottl

2000], We have calculated the hydrothermal flux two ways, assuming each of these two

mechanisms to be the major Li sink. In the first case, we assume that the only sink for Li

in the ocean is low temperature alteration of basalt with a fractionation factor of 0.9813

[Chan et al. 1992]. Using a 8 7Li of 7.5%o for hydrothermal fluids and 23.5%o for river

water [Huh et al. 1998], results in a hydrothermal flux of 8.0x10 9 mol Li/yr. A second

approach is to assume that the primary sink for Li is in flank reactions. The isotopic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. signature of flank fluids is poorly constrained but a fractionation factor for 60°C is

estimated to be 0.9850 [James et al. 1999]. Using this fractionation factor and the same

hydrothermal and river signature as in the previous calculation suggests a hydrothermal

flux of 9.9x10 9 mol Li/yr. Both calculations suggest very similar values for the

hydrothermal flux and the results are in good agreement with recently refined

geophysical estimates of Li flux [Elderfield and Schultz 1996; Von Damm et al. in prep.],

as well as estimates made based upon the amount of Li available in newly emplaced

MOR basalt [Ryan and Langmuir 1987].

3.7 Conclusions

Analysis of Li isotopes in high temperature hydrothermal fluids from four

geologically different sites on both the MAR and EPR reveal that the 57Li signature is

remarkably constant, with an average value of 7.5±1.6%o, despite a wide range in

hydrothermal fluid compositions. The exception to this constant value is from the

recently volcanically impacted 9-KEN EPR site. While there is no consistent temporal

trend in the 8 Li signature, several of the hydrothermal fluids from the immediate post-

eruptive period at the 9-10°N EPR site are isotopically lighter than the other fluids and

identical to a basaltic signature. The isotopic signature of these fluids reflects the

disequilibria of the hydrothermal system in the days and weeks following a volcanic

eruption.

n The 8 Li signature of the hydrothermal fluids analyzed is generally isotopically

heavier than a pure basaltic value indicating that isotopic fractionation is occurring.

Fluids from Brandon vent provide conclusive evidence that phase separation is not the

source of this fractionation. The 8 7Li signature of the hydrothermal fluids is interpreted as

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. an equilibrium value between the fluids and secondary alteration mineral products as

previously suggested by Chan et al. [1993]. Contrary to traditional paradigms, Li, like

the other alkali metals, does not behave as a truly soluble element and cannot be used to

determine water/rock ratios as was done in previous studies. Further, the constancy of

the Li isotopic signature suggests that the reaction zone conditions at these four MOR

sites are similar, despite different spreading rates and water depths.

The first analyses of Li isotopes in low temperature, diffuse flow fluids from two

EPR sites are consistent with mixtures of seawater with either high temperature

hydrothermal fluids or intermediate temperature, partially-reacted hydrothermal fluids.

The results suggest that these diffuse flow fluids experience very little low temperature

(<150°C) water-rock interaction, indicative of a short (days to months) residence time

within the oceanic crust. For conservative elements like Li, axial diffuse venting may not

require separate consideration in estimates of hydrothermal flux.

Using the global average value for 57Li of high temperature hydrothermal fluids,

and assuming that low temperature basalt alteration is the only important sink, the Li

isotopic signature of the oceans can be maintained at steady-state with an axial

hydrothermal flux of 8.0xl0 9-2.6xl0 10 mol Li/yr. This result, while an order of

magnitude smaller than original estimates, is in good agreement with recent geophysical

estimates for the Li flux.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Chapter Four

CONCLUDING REMARKS

4.1 Summary of Conclusions

The objective of this research, as outlined in Chapter 1, was to determine the B

concentration and Li isotopic signature in a suite of hydrothermal fluids from four mid

ocean ridge sites (MOR) with geologically diverse settings. This was done to understand

how these chemical parameters differed in fluids from several vents at the same site and

in fluids from vents at different MOR sites, as well as in time series samples of individual

vents on time scales as long as seven years. The larger goal was to better understand the

chemical controls on these two elements and to determine how these elements might be

used as geochemical tracers of processes occurring within hydrothermal systems. The

axial hydrothermal flux for Li and B to the oceans was also addressed.

4.1.1 Boron

The results of this study show that the majority of the hydrothermal fluids

analyzed are enriched in B relative to seawater by as much as 50% due to leaching of B

through water-rock reactions. However, the B contents vary over a relatively narrow

concentration range (0.7-1.5 times the seawater concentration). This is very different

from elements (such as Li) that can show order of magnitude variation in hydrothermal

fluids. Also in contrast to many other elements (e.g. Cl and Li), there is no consistent

pattern in the evolution of the B concentration following a volcanic eruption at 9-10°N

East Pacific Rise (EPR). The lack of temporal variability in the B concentration in fluids

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. from this evolving system indicates that while B may be a highly soluble element; it is

not a useful tracer of water/rock ratios.

Of the samples analyzed, fluids from the Irina vent at the ultramafic-hosted

Logatchev system were the only fluids significantly depleted (-28%) from the seawater B

concentration. Preliminary isotopic analysis suggests a 8 nB signature in the Irina fluids

much closer to the basalt value and very different from previously analyzed hydrothermal

fluids from unsedimented mid-ocean ridge sites. The results suggest large losses of

seawater B in the down flow zone due to reaction of the fluids with the ultramafic rocks.

Losses of seawater B may also occur at the Lucky Strike site resulting in relatively small

B enrichments over the seawater value (<13%) compared to fluids from the fast

spreading sites (<50% enrichment).

Analyses of vapor and liquid phase fluid pairs from the Brandon vent (21°33.7’S

EPR) clearly demonstrate that B fractionates only very minimally between the liquid and

vapor phases, consistent with experimental results [Bischoff and Rosenbauer 1987;

Bemdt and Seyfried 1990]. This behavior of B during phase separation has important

implications for its distribution in hydrothermal fluids. For example, while the B

concentrations did not vary with time at 9-10°N EPR, extraordinarily high B/Cl ratios

were observed in several of the immediate post-eruptive fluids from 9-10°N EPR. This

resulted from B concentrations that are typical of hydrothermal fluids coincident with

extremely low Cl concentrations in this young and developing hydrothermal system. The

relatively uniform B concentrations in the hydrothermal fluids, as well as the variations

seen in the B/Cl ratios, result from differences in behavior during phase separation for B

compared to those elements that form chloro-complexes. Thus, while chloro-complexed

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. elements exhibit very wide concentration ranges in MOR hydrothermal fluids, B

concentrations are comparatively constant.

Analysis of B in hydrothermal fluids from fast, ultra-fast as well as ultramafic-

hosted ridge sites provided important constraints on the distribution of B in fluids from

systems that were previously not included in the global inventory. A sink for B at slow

spreading ridges could help to reconcile the apparent imbalance of sources and sinks of B

to the oceans. Total hydrothermal flux estimates range from -0.1 to 0.6xl0 10 mol B/yr.

While hydrothermal systems at fast spreading and sediment-hosted MORs are likely a

source of B, losses of B at slow spreading ridges, particularly if reaction with ultramafic

rock proves common, could help to balance the large influx of B to the oceans from

rivers and ridge flanks.

4.1.2 Lithium

This study showed that the 57Li isotopic signature of high temperature

hydrothermal fluids is extremely constant globally(7.5±1.5%o), despite Li concentrations

ranging over two orders of magnitude. No consistent trend in the §7Li signature with Li

or Cl content or measured fluid temperature was observed. Unlike the results for B

concentration that showed differences between the fast and slow spreading ridges, the

57Li signature from the Lucky Strike and Logatchev sites were identical to those from the

EPR sites. Further, there is no indication of reaction of hydrothermal fluids with

previously weathered oceanic crust, as has been suggested to explain observed variations

in the other alkali metals (Rb and Cs), especially in slow spreading environments [Palmer

and Edmond 1989]. While there are no consistent temporal trends in the Li isotope

values, several fluids from the immediate post-eruptive period at 9-KEN EPR (1991 and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1992) had isotopic signatures that were different from the average value of 7.5%o,

reflecting the non-equilibrium conditions and evolving nature of this volcanically

impacted hydrothermal system.

The high temperature hydrothermal fluids, with the exception of several fluids

from the immediate post-eruptive period from 9-10°N EPR, did not have an isotopic

signature identical to the basalt value, indicating that isotopic fractionation is occurring.

The results from Brandon vent provide conclusive field evidence that phase separation

does not result in fractionation. While several other potential sources of isotopic

fractionation were considered, the isotopic signature of the hydrothermal fluids is

interpreted as an equilibrium value between the fluids and secondary hydrothermal

alteration products, with 6Li retained preferentially in the solid phases, consistent with the

results of Chan et al. [1993]. Equilibrium control for Li precludes its use as a tracer of

water/rock ratios, and the constancy of the 5 Li signature of the hydrothermal fluids

suggests that the equilibration conditions of the four sites studied are similar despite

major differences in their geologic settings.

The results of analysis of diffuse flow samples from the EPR indicate that diffuse

fluids could result from mixing of high or intermediate temperature hydrothermal fluids

with ambient seawater. Low temperature water-rock reactions, where isotopic

n fractionations would be large, are inconsistent with the 5 Li signature of the diffuse

fluids. While other authors have recently emphasized the potential contribution of

diffuse fluids to the hydrothermal flux, the results suggest that for conservative elements

like Li, the axial hydrothermal flux can be determined from the high temperature fluid

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. chemistry for a given heat flux, and the diffuse fluids may not need separate

consideration.

Flux estimates for Li were made by balancing the isotopic budget for Li in the

oceans, assuming that uptake of Li to the oceanic crust at low temperatures was the only

important sink. This suggests a hydrothermal flux of 8.0xl0 9-2.6xl0 10 mol Li/yr, in good

agreement with geophysical estimates of the Li flux.

4.2 Future Study

While this research has answered many questions about the geochemical behavior

of Li and B in hydrothermal systems, many new questions have now been raised. Future

research directions should include:

♦> Analysis ofB isotopes in a subset of the hydrothermal fluids studied here.

> Because the B isotopic signature of basalt, seawater and vent fluid are known to

be distinct, analyzing a subset of the fluids discussed in Chapter Two could

provide further insight into the behavior of B isotopes during phase separation,

and might provide more information on losses of seawater B during low

temperature down flow reactions. A small suite of hydrothermal fluid samples, as

described below, could address many of the outstanding questions about the

hydrothermal geochemistry of B.

■ A selection of Lucky Strike fluids should be analyzed to determine if their

relatively low B concentration is a result of seawater B loss during down flow

reactions, and to understand if the altered substrate found at the Lucky Strike

system is reflected in the 5nB signatures of the hydrothermal fluids.

Preliminary analyses show that the Logatchev fluids have a 5nB signature

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. similar to the rock isotopic signature and much different from values

• previously reported for unsedimented MOR hydrothermal fluids ( 8 n B -26-

37%o), suggesting much seawater B was lost from these fluids in the down

flow zone. Analysis of the Lucky Strike fluids could provide insight into the

importance of seawater B loss in hydrothermal fluids vented at basalt-hosted

systems on slow spreading ridges.

■ A set of time series samples from 9-10°N EPR, such as the Aa vent time

series should be analyzed. Limited time series data for B isotopes in

hydrothermal fluids currently exist and there are no data available on possible

variations in the B isotopic signature of fluids following a volcanic eruption.

* Analysis of the vapor and liquid fluid pairs from the Brandon vent would

provide field confirmation of experimental studies that suggested phase

separation (super-critical) did not result in any fractionation of B isotopes

[Spivack et al. 1990].

■ In addition to Brandon vent, a subset of samples from the 17°S and 21°S EPR

areas should also be analyzed. The B concentrations were lower at 21°S,

possibly reflecting a deeper reaction zone at this site compared to the 17°S

area, and isotopic data might provide clues to the cause of this difference,

such as losses of seawater B.

Additionally, the results for B underscore the potential importance of ultramafic-

hosted MORs to the elemental flux. These sites remain chronically under

sampled in the global dataset, and further exploration and study of ultramafic

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. systems is necessary to identify the occurrence of this type of setting and to better

quantify their contribution to the global fluxes.

♦> Experimental studies to investigate the processes of Li isotopic fractionation.

> Several possible mechanisms for the fractionation of Li isotopes were discussed

in Chapter 3 (fractionation during leaching, fractionation resulting from aqueous

complexing, and fractionation between the fluids and secondary alteration

products). Controlled laboratory studies are required to understand the direction

and magnitude of all of these possible fractionations more completely. The

constancy of the 57Li signature of the hydrothermal fluids analyzed suggests that

the equilibrium conditions at the four MOR sites may be very similar. Thus,

included in this should be determination of how pressure, temperature and

solution composition affect these fractionations, which could potentially provide

insight into the equilibrium conditions experienced by the hydrothermal fluids.

❖ Analysis of Li isotopes in additional diffuse flow samples.

> As the 8 7Li isotopic signature of high temperature hydrothermal fluids is very

constant, measurement of Li isotopes in many more samples is not necessary.

However, the results here also showed that Li isotopes may provide some insight

into the origins of low temperature axial diffuse fluids. More samples of these

fluids should be analyzed to determine if their chemistry can be explained by

mixing of high temperature fluids with seawater. In addition, Li isotope analysis

of low temperature fluids from the ridge flanks (such as those from the Baby Bare

site) should also be done to understand how these fluids compare to axial low

temperature venting.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX A

Boron Ion Chromatography Method Description

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Boron was measured using a Dionex Model 500 ion chromatograph (IC) with a

CD20 conductivity detector (Table A-l). A Dionex ICE-AS1 ion exclusion column was

used. In some cases an Alltech All-Guard GA-1 anion guard column was placed in line

prior to the ICE column, although it made no difference to the analytical results. A 0.1 M

mannitol eluent was used, prepared from solid mannitol powder. Often, a stronger eluent

stock was prepared and was diluted with 18 megohm water using the gradient mixer of

the IC. Samples were diluted by with an appropriate amount of 18 megohm water to

maintain Cl concentrations equivalent to a 100:1 seawater dilution (-5.4 mmol/kg NaCl).

A standard curve of four standards ranging in concentration from -3-26 pmol/kg as well

as a standard blank were prepared in -5.4 pmol/kg NaCl solution from a National

Institute of Standards and Technology (NIST) traceable liquid boron standard. A 100

fold dilution of International Association for Physical Sciences of the Ocean (IAPSO)

seawater was run every ten samples to normalize for instrumental drift of the IC. All

runs were corrected to an IAPSO value of 455.6 pmol/kg, the average value for 19 runs.

As IAPSO is stored in glass ampoules, the B concentration may be affected by leaching

from the borosilicate glass and thus cannot be used as an absolute standard. An

additional standard (NIST traceable) of known concentration (-5 pmol/kg) was run like a

sample, on most runs as well to adjust for any offset induced from drift in the standard

curve. Samples colleted in the gas tight water samplers were not analyzed for this study

as comparative analyses of several samples indicated anomalously low B concentrations

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. in the gas tight samples, resulting from the shipboard gas extraction procedure used to

process these fluids.

This method was originally designed for the analysis of river water samples (E.

Rose, pers. comm.). Analyses of high salt samples such as seawater and vent fluids by

this method result in build-up of Na on the ion exchange sites reducing the column’s

affinity for B and the useful life of the column. Using a guard column such as the Alltech

column used here may help to prolong the life of the ICE column by trapping some Na.

Table A-l. DX-500 Ion Chromatograph Settings

Eluent Flow Rate Data Collection Time Data Collection Rate Suppressor

0.80 ml/min 13 min 5 Hz O ff

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX B

Analytical Data

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Analytical data for Mg, B, Li and 57Li (phosphate method only). The 9-10°N EPR data, organized sequentially by cruise is presented first, followed by the southern EPR SouEPR cruise data and finally the Mid-Atlantic Ridge data (for 1993, 1996 and 1997) including Lucky Strike and Logatchev.

Table B -l. 9-10°N EPR, AdVenture 1 (April 1991) analytical data1. Dive Bottle Vent Mg B Li 87Li mmol/kg pmol/kg pmol/kg %0

2351 5C B9.1 4.03 403 93.9 5.59 7C B9.1 14.5 428 76.5 GT3 B9.1 25.2 62.4 2352 1C D.l 4.6 553 1140 6.91 2C D.l 4.09 519 1173 6.90 11C E.l 21.1 482 12C E.l 20.0 502 GT2 D.l 1.07 1136 2357 4 P.l 9.11 422 12 P.l 26.8 464 5C P.l 27.5 439 7C P.l 21.4 435 2360 4 Aa.l 27.3 431 43.0 14 Aa.l 10.9 435 58.1 1C Aa.l 7.91 429 56.6 7.18 3C Aa.l 45.3 30.9 4C Aa.l 47.8 27.8 6C. Aa.l 22.2 43.6 9C Aa.l 39.6 433 34.8 GTS Aa.l 41.4 31.8 GT5 Aa.l 43.5 25.6 2362 11 K.l 46.1 423 12 K.l 46.7 431 5C K.l 38.5 445 1C K.l 42.1 459 2365 4 F.l 8.40 450 28.2 12 FT 5.54 434 28.5 8.06 14 FT 12.7 445 33.0 1C F.l 46.5 445 23.5 2C F.l 41.4 432 26.4 5C F.l 23.2 26.2 1C F.l 30.5 27.1 GT2 F.l 45.3 9.04 GT3 F.l 40.9 18.0 GT4 FT 42.5 10.7

'In this and all of the following tables, B was reported for all samples analyzed. Lithium isotopes were only analyzed in a small fraction o f samples however, Li concentrations are reported for all samples for the vents/time points when Li isotopes were analyzed. No entry in the table indicates that the sample was either not analyzed (in the case o f B) or the data were not discussed in this thesis (for Li).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %0 2366 1C La.l 34.2 445 2C La.l 43.0 432 4C Aa.2 27.5 494 19.9 6C Aa.2 19.8 484 19.8 9C Aa.2 33.6 23.1 11C Aa.2 8.60 449 17.1 7.18 GT3 Aa.2 44.4 24.7 GT4 Aa.2 45.2 16.9 2367 4 D.2 20.7 479 5C D.2 2.09 545 2368 12 Q.l 6.89 6.89 4C Q.l 27.1 27.1 11C Q.l 19.9 19.9 12C Q.l 25.5 25.5 2373 11 Aa.3 38.6 22.0 5C Aa.3 38.3 452 20.1 6C Aa.3 32.4 464 20.4 7C Aa.3 18.4 16.0 9C Aa.3 29.1 21.3 lie Aa.3 33.2 467 20.5 26.9 12C Aa.3 29.3 19.2 GT2 Aa.3 49.1 22.1 GT3 Aa.3 46.1 16.7

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-2 9-10°N EPR, AdVenture 2 (Feb-March, 1992) analytical data. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %0

2492 1C Ma.l 47.0 425 12C Ma.l 25.7 428 2494 6C E.3 4.17 533 9C E.3 22.1 513 11C E.3 29.9 479 2498 6C B9.2 21.6 458 9C B9.2 39.4 430 11C B9.2 6.80 427 12C B9.2 51.1 427 2499 6C TWP.l 45.8 442 9C TWP.l 50.9 416 2500 1C Aa.4 49.8 26 2C Aa.4 48.5 28 6C Aa.4 24.7 471 62 14.3 9C Aa.4 26.3 446 63 14.1 10 Aa.4 46.2 445 28 GT4 Aa.4 50.8 23 GT5 Aa.4 7.04 92 2501 9C P.3 23.7 475 11C P.3 26.3 462 12C P.3 20.4 465 2502 1C La.2 21.6 447 2C La.2 4.80 444 6C ■Aa.5 3.68 481 11C Aa.5 11.5 487 12C Aa.5 3.71 507 2505 11C Ma.2 11.5 442 12C Ma.2 18.5 437

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-3. 9-10°N EPR, AdVenture 3 (Dec. 1993) analytical data. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %0

2685 9 P.4 13.5 435 14 P.4 22.4 449 15 P.4 23.5 451 2693 4 B9.3 5.00 439 159 6.41 6 B9.3 33.5 432 64.1 10 B9.3 6.87 420 163 13 Seawater 51.3 427 14 Seawater 51.4 428

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-4. 9-10°N EPR, AdVenture 4 (March-April 1994) analytical data. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %0

2734 4 Ma.3 2.85 435 5 Ma.3 2.09 431 9 Ma.3 2.04 426 10 Ma.3 2.61 422 11 Ma.3 2.93 432 2735 4 B9.4 4.08 460 10 B9.4 4.51 445 2736 4 A-a.6 3.04 478 483 5 Aa.6 2.86 467 489 5.81 9 Aa.6 3.20 473 487 10 Aa.6 3.23 463 488 GT5 Aa.6 2.91 505 GT7 Aa.6 3.02 501 2738 10 E.4 3.12 532 11 E.4 5.86 533 12 E.4 3.99 518 2739 5 F.2 52.7 6 F.2 8.24 617 1346 9 F.2 4.33 616 1470 11 F.2 6.63 623 1409 8.98 12 F.2 6.10 616 1433 14 F.2 14.3 1209 15 F.2 17.6 1096 GT2 F.2 - 19.9 1006 GT3 F.2 8.18 1396 GT5 F.2 8.36 1358 GT6 F.2 9.89 1322 2743 5 P.5 18.8 6 P.5 3.51 463 9 P.5 43.6 11 P.5 51.7 12 P.5 51.6 14 P.5 4.25 459 15 P.5 5.01 454

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg jimol/kg %0 2744 6 D.5 5.85 508 9 D.5 2.11 529 14 D.5 5.80 502 2751 4 Aa.7 2.15 475 5 La.4 29.0 427 6 La.4 9.33 432 9 La.4 3.05 428 10 Aa.7 2.01 466 11 Aa.l 6.39 475 12 Aa.7 1.82 473 2752 6 B9R 51.7 35.1 24.7 9 P.6 5.28 435 10 P.6 3.52 468 11 P.6 3.73 481 13 P.6 17.2 442 2754 5 B9.5 5.52 443 207 9 B9.5 2.13 469 223 11 B9.5 2.18 451 236 6.49 12 B9.5 2.37 448 231 GT6 B9.5 5.76 227 GT7 B9.5 1.62 240 2755 5 Ma.4 10.4 413 13 Ma.4 2.73 432 16 Ma.4 3.94 432 2756 5 K.2 4.41 443 6 K.2 3.75 434 13 K.2 3.25 442

100

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-5. 9-10°N EPR, AdVenture 5 (Oct. 1994) analytical data. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %> 2840 5 Seawater 50.4 422 9 Seawater 51.7 424 2843 9 BV.l 3.88 540 13 BV.l 3.74 536 14 BV.l 6.18 538 15 BV.l 7.21 .533 2846 6 B9.7 3.00 449 9 B9.7 4.88 449 14 B9.7 2.05 463 15 B9.7 3.64 460 2849 3 P.7 2.58 461 6 P.7 21.0 452 9 P.7 4.20 470 10 P.7 7.38 460 14 P.7 13.7 446 15 P.7 17.7 454 2850 6 TWP.2 24.5 496 10 TWP.2 17.7 508 14 TWP.2 23.2 441 15 TWP.2 17.9 512 2851 3 TWP.3 4.89 564 9 TWP.3 5.90 559 2853 3 Ma.5 4.50 435 6 Ma.5 15.4 427 9 Ma.5 4.17 432 10 Ma.5 15.2 452 14 Ma.5 13.8 422 15 Ma.5 17.2 435 2854 3 B 9\3 4.90 463 6 B9’.3 15.0 456 9 B9’.3 1.98 463 10 B9’.3 7.60 440 14 B9’.3 6.76 464 15 B9’.3 10.5 455

101

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-6. 9-10°N EPR, AdVenture 6 (Oct.-Nov. 1995) analytical data. Dive Bottle Vent Mg B Li 87Li mmol/kg pmol/kg pmol/kg %o

3019 4 Ma.6 2.25 425 5 Ma.6 3.72 439 9 Ma.6 13.1 463 10 Ma.6 36.5 437 11 Ma.6 8.62 434 13 Ma.6 8.67 434 15 Ma.6 14.2 425 3021 4 P.8 18.0 476 5 P. 8 19.8 460 9 P.8 35.3 460 10 P. 8 27.3 482 11 P.8 10.0 498 14 P.8 7.60 499 15 P.8 9.58 467 3025 11 B9R 48.2 3027 11 BV.2 4.82 482 13 BV.2 3.67 482 15 BV.2 5.16 483 3030 9 B9.8 3.94 458 11 B9.8 1.82 480 15 B9.8 2.68 471 3031 9 Seawater 52.2 3033 9 P.10 20.2 478 15 P.10 20.4 467 3034 4 TWP.4 17.3 531 5 TWP.4 48.8 524 9 TWP.4 28.1 496 11 TWP.4 8.41 545 13 TWP.4 4.88 537 3034 15 TWP.4 16.6 531 3035 11 TWP.4 17.9 600 3046 9 Q.4 3.88 457

102

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-7. 9-10°N EPR, Legacy (Oct.-Nov. 1995) analytical data. Dive Bottle Vent Mg B Li 87Li mmol/kg nmol/kg pmol/kg %o “3073 5*~ Aa.10 4?78 660 9* Aa.10 22.5 424 13 Aa.10 4.67 473 652 7.30 14 Aa.10 4.47 492 658 6.22 ♦Dead volumes filled with distilled water. Data above have been corrected for the volume of distilled water based on measurements o f bottle dead volumes; data below are original analytical data. Dive Bottle Vent Mg B Li 57Li 3073 mmol/kg pmol/kg pmol/kg %o — ...... ' 4L73 653 ...... 9* Aa.10 22.4 421

103

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-8. 9-10°N EPR, AdVenture 7 (Oct.-Nov. 1997) analytical data. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %o 3157 1 B9.ll 14.0 419 13 B 9 .ll 20.2 438 9C B 9 .ll 40.7 422 3163 4 BV.5 3.80 505 10 B9.12 4.99 438 11 B9.12 3.21 476 12 B9.12 1.67 466 13 B9.12 8.21 457 15 BV.5 1.80 509 9C BV.5 4.30 498 3164 4 P. 13 4.05 518 15 P. 13 3.77 498 10C P. 13 3.02 507 3168 4 Ma.8 3.71 432 15 Ma.8 3.07 447 9C Ma.8 3.06 449 3177 4 L.7 1.90 444 12 L.7 3.50 425 13 L.7 3.20 440 15 L.7 4.70 441 3183 13 Aa.l 1 3.21 433 15 Aa.l 1 17.7 468 3184 12 TWP.6 10.0 542 15 TWP.6 9.10 514 3187 12 BV.6 4.60 541- 13 BV.6 11.9 493

104

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-9 17-22°S East Pacific Rise, SouEPR Cruise (Oct.-Nov. 1998) analytical data. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %o 3283 1C Natasha 3.17 436 9C Natasha 33.6 447 10C Natasha 2.10 442 3284 12 Jasmine 2.68 432 13 Jasmine 2.88 422 lc Jasmine 2.15 434 3285 9c Seawater 52.0 422 10c Seawater 52.1 417 3286 12 Tweety 2.50 398 6c Tweety 1.86 403 3289 4 Ba.l 2.83 448 277 10 Ba.l 2.81 451 280 16 Ba.l 1.47 455 287 7.10 GT8 Ba.l 1.03 296 GT10 Ba.l 1.22 294 3290 4 Krasnov 8.24 470 12 Ba.2 4.30 429 289 6.96 13 Ba.2 3.67 430 345 16 Krasnov 31.8 439 12c Krasnov 2.83 450 GT13 Ba.2 31.5 138 GT15 Ba.2 25.5 173 3293 4 Lowell 50.7 56.1 16.9 10 Lowell 50.6 53.2 18.5 12 Mk 72 5.35 847 13 Mk 72 3.04 898 11c Mk 72 3.13 897 12C Mk72 2.21 918 7.59 3294 12 Stanley 3.17 490 13 Stanley 1.97 481 11c N. Smoker 2.72 484 10 Stanley 8.77 482 3297 12 Nadir 2.51 468 13 Nadir 2.16 469 9c Nadir 3.02 469 3298 6C Dumbo 12.1 464 11C Gumbo 21.0 466 12C Gumbo 14.2 473 16 Dumbo 36.9 428 3302 12 Bb.3 5.36 456 442 6.90 13 Bb.3 17.9 446 336 1C Ba.3 3.65 434 6C Ba.3 2.12 434 9C Bb.3 9.55 456 407 10C Bb.3 10.2 451 403 11C Ba.3 5.10 426 12C Ba.3 6.10 430

105

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %0 GT12 Bb.3 16.7 336 GT15 Bb.3 39.9 128 3303 12 Bd.4 2.25 436 13 Bc.4 1.33 427 1C Be.4 5.58 424 9C Bd.4 3.55 424 10C Bd.4 3.19 424 11C Bc.4 34.9 423 12C Bc.4 28.2 419 3306 12 Bd.5 9.95 429 13 Bd.5 46.4 415 6C Ba.5 24.8 427 9C Bc.5 33.6 418 IOC Bc.5 1.20 432 11C Be.5 8.18 456 404 7.06 12C Be.5 7.08 459 423 6.89 GT10 Be.5 26.0 273

106

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table B-10. Lucky Strike (1993, 1996, 1997) and Logatchev (1997), Mid-Atlantic Ridge Dive Bottle Vent Mg B Li 57Li mmol/kg pmol/kg pmol/kg %0 2605 10 Sintra. 1 11.4 464 14 Sintra. 1 26.9 454 2608 4 ET.l 6.17 441 8 ET.l 3.35 445 10 Mkr. 4.1 22.6 470 14 Mkr. 4.1 3.94 456 J177 5 ET.2 2.79 442 9 ET.2 2.59 445 10 ET.2 2.25 448 16 ET.2 2.54 461 J179 5 Sintra.2 4.59 457 J180 5 Mkr 4.2 1.88 464 9 Mkr 4.2 16.6 457 13 Mkr 4.2 3.26 460 14 Mkr 4.2 3.10 446 J183 5 2608.1 3.97 472 13 2608.1 6.11 448

• 14 2608.1 4.21 449 J 183-2 5 Crystal. 1 3.82 426 9 Crystal. 1 3.56 425 14 Crystal. 1 2.18 425 3114 11 2608.2 1.62 429 354 7.15 9c 2608.2 3.14 456 341 4 Crystal.2 1.81 439 323- 10 Crystal.2 1.64 427 322 8.40 11 2608.2 1.62 354 12 Crystal .2 3.20 435 330 13 Crystal.2 3.24 434 313 GT2 2608.2 15.9 344 GT6 Crystal.2 12.9 303 3115 10 Sintra.3 6.16 457 330 9c Sintra.3 8.26 459 320 11 ET.3 2.03 470 297 12 ET.3 1.36 471 307 6.75 13 ET.3 2.14 474 298 3133 4 Irina 4.98 306 260 7.58 10 Irina 4.82 316 255

107

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX C

Lithium Isotope Method Comparison

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. As noted in Chapter 3, the Li isotopic measurements for high temperature

hydrothermal fluids in this study are generally lighter (average =6.7±1.2%o) compared to

the fluids measured by Chan et al. [1993] (average= 8.9±1.3%o). This offset was

attributed to utilizing different analytical techniques. The data presented in this thesis

were measured using a thermal ionization mass spectrometry (TIMS) method where

Li3P 0 4 is used as the ion source and the 6Li/7Li is measured directly [You and Chan

1996]. In the original study of lithium isotopes in hydrothermal fluids, [Chan et al. 1993]

a tetraborate TIMS method was used [Chan 1987]. This method measures Li as Li 2B 0 2+

at masses of 56 and 57. In a comparative study of the two methods for a variety of

geologic samples, You and Chan [1996] found that in most cases the two methods

showed excellent (<±l%o) agreement. For this thesis, samples of hydrothermal fluids

were run by both the borate and phosphate methods, allowing for comparison of the two

methods for this specific sample matrix (Table C-l). In general, the borate method

resulted in slightly heavier isotopic values relative to the phosphate method, although the

offset is in most cases close to the level of the analytical precision (±l%o).

Table C-l. 57Li isotopic analytical results for hydrothermal fluids by the borate and phosphate TIMS methods. Sample ID Li 87Li Borate Method 57Li Phosphate Method pmol/kg 2360-lc Aa.l 56.7 10.9 7.2 2366-1 lc Aa.2 17.2 19.5 13.6 3289-16 Ba.l 287 10.0 7.2 3133-4 Irina 260 8.7 7.6 3302-12 Bb.3 442 7.9 7.0 1160-11 HG 1284 6.6' 6.8 1158-16 OBS 869 9.2' 6.4 ‘Borate analysis from Chan et al. [1993]

109

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The results of this study show that the Li isotopic signature of hydrothermal fluids

is very constant temporally and spatially. However, the results reported by Chan et al.

[1993 ] were in general isotopically heavier than the results of the current study (Figure C-

1 ). While it is possible that the samples studied by Chan et al. [1993] are truly

geochemically unique from the all other locations studied for Li isotopes on the mid

ocean ridge, most likely that the offset between the two data sets is an analytical artifact

of the two different methods used.

12 -

2 -

0 0 200 400 600 800 1000 Cl (mmol/kg)

Figure C-1. Comparison of 57Li end member values for hydrothermal fluids from this study (circles) and Chan et al. [1993] (squares) versus Cl concentration.

110

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX D

Calculations for Mixing of Seawater and Basaltic Lithium: Effects of Down Flow Reactions

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lithium in hydrothermal fluids is a mixture of seawater and basaltic derived Li,

each with a distinctive isotopic signature (32.4 and 5.1%o, respectively). The Li

concentration in seawater is low (26 pmol/kg) and most vent fluids are greatly enriched

in Li. Thus, the seawater isotopic signature in hydrothermal fluids is generally

overwhelmed by basaltic Li. This is reflected in the Li isotopic signature of

hydrothermal fluids, which is similar to the basalt value. Some amount of seawater

derived Li may be lost through low temperature uptake to alteration minerals in the down

flow zone. The results in Chapter 2 suggest that down flow reactions may be very

important to .the B composition of hydrothermal fluids and these reactions should plso be

considered with respect to Li.

In order to assess the possible effect of down flow losses of Li on the final

isotopic signature of the fluids, the isotopic signature of each high temperature

hydrothermal fluid was calculated as a two-component mixture of seawater and basaltic

Li, with variable amounts of low temperature Li loss. In each case, the amount of

seawater Li was determined based on the end member Cl concentration and assuming

that the Li/Cl ratio of seawater was maintained during phase separation. The amount of

seawater Li was subtracted from the total measured Li and any excess was assumed to be

basaltic Li with an isotopic signature of 5.1%o. The amount of seawater derived Li and

basaltic Li were multiplied by their respective isotopic values to calculate a final 57Li for

the hydrothermal fluid. These values were compared to the end member values

determined for the hydrothermal fluids.

112

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Four cases were examined. In each case the basalt isotopic value was fixed at

5.1%o, based on an average fresh basalt signature (Chapter 3). In Case 1, no down flow

losses of Li were assumed. In Cases 2, 3 and 4, variable amounts of Li were assumed to

be lost (10, 25 and 50%, respectively). In order to account for the isotopic fractionation

in during these low temperature fluid losses, the seawater component was modified to

reflect a heavier isotopic signature, dependent on the amount of Li loss. The calculations

assumed a low temperature fractionation factor of 0.9995, based on the results of Chan et

al. [1992] for low temperature weathering of basalts. For simplicity, it was assumed that

no fractionation of the Li leached from the basalt occurred.

The results show that in most cases the isotopic signature of the hydrothermal

fluids cannot be exactly replicated based on this mixing model, reflecting the

oversimplification of the system. The calculated results for the four cases are in some

instances heavier than the measured signature and in other instances, the calculated

values are isotopically light relative to the measured value. However, in the majority of

the cases, the calculated results are within ±l-2%o of the measured value, close to the

level of the analytical uncertainty. More importantly, the results of these calculations

show that the final isotopic signature of the hydrothermal fluids is relatively insensitive to

losses of Li in the down flow zone, even in the case of 50% loss. The signature of the

hydrothermal fluids is dominated by the basaltic signature and the contribution of the

seawater Li and potential down flow Li losses to the final isotopic signature of the fluids

is minimal. This is different from the results for B where down flow reactions appear to

be important to the final hydrothermal fluid composition, in part due to the relatively high

concentration of B in ambient bottom seawater.

113

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.1 1.5 0.1 0.1 0.5 0.1 3.1 0.2 0.8 0,7 0.5 -0.3 -0.2 -0.6 -0.4 -0.6 -0.7 -1.6 -0.4 -2.2 -7.5 Measured- 57Li 7.5 %0 5.8 6.6 6.5 6.4 6.6 6.6 8.9 6.1 6,6 6.8 7.7 Calc 57Li 8.37.3 7.2 7.1 7.0 6.7 6.7 9.6 6.9 6.0 6.05.8 6.6 7.3 Meas. 14.1 85.8 90.5 5.0 7.2 1,179 172,8 310.6 289,5 6.7 7.0 918.7 7.6 6.0 461.2 318.7 5.9 280,7 7.0 from Rock SW Li 1.5 2.2 28.3 5.5 7.1 3.9 58.8 5,3 16.3 13.8 10.2 15.3 19.2 495.8 5.7 12.9 227.1 32.3 25.2 256.1 25.9 24.0 414.0 6.8 26.9 462.1 40.7 40.7 1,579 8.9 28.9 690.1 6.7 6.2 from pmol in kg1 Meas-Li from SW %0 Li 1.57 1.03 26.8 0.29 7.4 0.09 0.06 0.74 0.39 CIV|/C1S1/ 524 0.97 286 0.53 398 846 212 Cl End End 183 362311 520 441 0.96 0.82 25.0 21.2 336.5 337 538281 1.00 515 335489 338 558 0,63 1.03 240 267 0.49 719 600 1.11 296 317 0.59 438 498 0.92 15.6 30.5 99.6 97.9 154 30.5 46.2 1620 846 1.57 62.7 80.9 0.15 1220 Li lumol/kg mmol/kg %0 8.9 9.6 5.7 5.8 7.3 5.0 5.5 7.0 6.8 7.6 951 670 1.24 5.9 7,3 5.3 6.7 6.9 57Li

%o 5.1 %o 32.4 F.l F.2 D .l Ba.l Be.5 6,7 488 557 B9.8 Ba.2 Aa,6 B9.1 Aa. 1 Aa.4 B9.5 B9.3 6.0 Bb.3 6.7 Aa.2 Aa. 10 Crystal 8.3 Vent 2608Logatchev 7.1 M arker 72 Eiffel Tower 6,7 Logatchev. Mid-Atlantic Ridge & the measured value. *VF= Vent Fluid, SW=Seawater**!talics indicates that the calculated value is heavierPlain by morefont thanindicates l%o relativeagreement to the between measured the value measured and bold and fontcalculated indicates value that of the+/-l%o calculated value is lighter by more than comparedl%o to Lucky Strike 9-10°N EPR Case 1 Rock= Assume no loss of Li in down flow zone and noSeawater= fractionation.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1.3 1.6 0,2 0.2 0.3 0.6 0.4 -0,1 -0.1 0.9 -0.5 -0.6 -1.6 -0.3 -2.0 -1.4 Measured- 6.9 7.5 6.9 6.46.5 -0.4 %0 6.0 1.0 7.0 5.8 3.2 7.1 7.6 6.16.2 -0.3 0.6 6.7 8.46.8 7.3 7.1 7.2 7.6 6.0 %0 8.9 6.9 6.5 5.95.5 6.5 7.0 6.5 6.9 Meas. 57Li Calc 57Li 14.3 7.3 59.2 5.3 313.2 258.6 173.8 6.1 6.6 922.0 291.6 320.4464.8 5.9 6.8 416.4 6.9 6,5 228.4 693.0 6.8 1583.3 1183.3 2.0 28.5 9.2 6.7 91.2 5.1 19.1 14.6 13.7 282.3 17,2 497.8 5.8 22.7 12.4 87.2 9,7 8.7 22,5 339.0 24.2 36.7 24.1 463.9 36.7 21.6 Li from SW Li from Rock 1.24 29.0 1,03 1.03 0.97 0.63 0.92 0.29 0.09 0.39 0.15 3.5 0.53 0.74 civf/cu* ■ ■ 0.96 524 520 - 338 670 846 1.57 557 846 1.57 498 600 1.11 26.0* 46.2 relative to the measured value and bold font indicates that the calculated value is lighter by more than compared l%o to l%> 281 311 441 0.82 488 515719 398 1220 30.5 97.9 154 1620 99.6 286 Li End Cl End pmol/kg mmol/kg pmol in 1 kg Meas-Li from SW %0 5.9 240 267 0.49 11.6 5.1 8.9 5.3 62.7 80.9 5.5 7.3 7.39.7 15.6 30.5 0,06 1.3 5.96,8 335 489 558 5.8 6.9 7.6 951 6.9 6.9 438 6.8 67Li

%* %, 5.1 34.4 F.l F.2 D.l Ba.2 Ba.lBe.5 7.0 296 317 0.59 B9.1 B9.3 6.1 183 212 Bb.3 Aa. 1 B9.5 B9.8 Aa,2 Aa.4 Aa.6 Aa. 10 Crystal 8.4 337 538 1,00 23.3 Vent 2608 7.2 362 Logatchev M arker 72 Eiffel Tower 6.8

the measured value. *VF= Vent Fluid, SW=Seawater**Italics indicates that the calculated value is heavier by more than Plain font indicates agreement between the measured and calculated value of+/-!%». Luckv Strike & Logatchev. Mid-Atlantic Ridge 9-10°N EPR Case 2 Rock= Fractionated Seawater Assume 10% loss ofLi in down flow zone and low temperature fractionation in down flow reactions

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. compared to l% o 1.4 1.7 1.2 1.0 0.4 0.0 0.0 0.7 0.3 0.4 1.1 0.5 0.0 0.7 -0.4 -0.4 -0.5 ■1.4 ■1.9 Measured- 7.3 %0 6.8 6.5 5.7 7.0 6.5 7.4 -0.1 6.6 %0 7.3 7.1 6.4 7,6 5.9 6,8 6.8 6.86.9 6.5 5.1 6.1 6,96.96.9 6.5 5.9 6,9 9.7 8.5 5.86.8 6.0 6.1 -0.2 Meas. 57Li Calc 87Li SW 29 14.5 7.3 92.3 1189 1589 6.9 59.8 5.3 175.3 342.7294.8 7.2 926.8 230.4 5.9 6.4 284.6 468.9 467.9 500.6 420.0 697,3 Meas-Li from

2.9 5.6 • 19.4 317.1 8.4 7.0 18.8 11.4 15.9 10.3 89.3 14.4 20.1 24.2 20.2 21.7 Li from SW Li from Rock 1.00 1.11 1.03 1.24 1.57 30.6 0.82 0,97 18.9 262.4 0.060.53 1.1 0.29 0,09 1.7 Clvf'CU* relative to the measured value and bold font indicates that the calculated value is lighter by more than 1%o 154 558 1.03 398 0,74 212 0.39 7.7 317338 0.59 0.63 12.2 322.8 5,9 6.3 538 520 0.96 524 286 267 0.49 9.6 498 0.92 18.0 80.9 0.15 30.5 46.2 Cl End mmol/kg pmol in 1 kg 183 296 489 337 361 281 719 600 438 1220 846 Li End pmol/kg %0 7.3 15.6 6.1 5.9 240 5.9 335 8.9 1620 846 1.57 30.6 7.2 5.5 30.5 5.3 62.7 5.8 515 9.7 99.6 6.8 5.1 97.9 6.97.6 488 951 557 670 6.9 6.8 6.9 6.87,3 311 441 67Li

%o 5.1 38.0 %« F.l F.2 D.l Ba.2 Be,5 Ba.l 7.1 Aa. 1 Aa.4 Aa.2 B9.1 B9.3 B9.5 B9.8 Bb,3 Aa.6 Aa.10 Crystal 8.4 Marker 72 Vent 2608 Logatchev Eiffel Tower loss of Li in down flow zone and low temperature fractionation in down flow reactions

the measured value. **Italics indicates that the calculated value is heavier by more than Luckv Strike & Logatchev. Mid-Atlantic Ridge * VF= * Vent Fluid, SW=Seawater Plain font indicates agreement between the measured and calculated value of+/-l%o. Rock= Fractionated Seawate Assume 25% Case.3

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 0.1 1.8 0.3 1.8 0.4 1.6 1.1 0.7 1.2 0.7 1.3 0.9 1.7 0.5 0.4 0.8 0.7 0.0 - -0.2 -0.3 - - Measured- 87Li 6,5 %0 6.2 6.1 6.6 5.6 5.9 6.4 Calc

S7Li DO 7.3 6.9 8.47.2 6.7 6.5 % >

6.8 6.2 SO 7.3 7.0 0.3 5.86.8 5.1 5.9 6.6 9.7 7.9 Meas. 14.9 29.4 6.9 268.7 323.5 349.0 934.9 7.6 5.8 326.9 5.9 288.4 7.1 6.1 704.6 505.4 from Rock Li 8.1 7.6 10.6 300.1 9.6 6.4 233.6 5.9 6,2 12.6 13.4 474.6 6.9 13.4 475.6 14.4 20.4 1199.6 6.9 5.8 20.4 1599.6 6.9 from SW . . 16.1 Li pmol in I kg Meas-Li from SW 1.00 13.0 1.03 1.24 1.57 1.03 1.57 1.11 0.960.82 12.5 0.74 0.53 6.9 92.7 0.39 5.1 177,9 6.1 6.2 0.29. 3.-7 94.2 0.15 1.9 60.8 5.3 ' ' 0,06 0.7 154 524 ' 0.97 538 44! 670 557 846 846 267 0.49 498 0.92 12.0 426.0 6,9 6.2 286 600 80.9 mmol/kg End Cl End Clvf/CLC 183 212 361311 520 335 338 0.63 296 317 0.59 281 719 515 398 15.6 30.5 438 99.6 1220 62.7 Li pmol/kg %0 7.3 5.9 5.5 30.5 46.2 0.09 1.1 7.3 9.7 6.1 5.8 6.9 488 8.9 1620 6.8 489 558 6,9 5.3 5.1 97.9 57Li 5.1 %o %o 5.1 45.8 % 45.8 F.2 F.l D.l Be.5 Ba.lBa.2 7.1 B9.5 5.9 240 Aa. 1 Aa.2 Aa.4 B9.3 Aa.6 B9.1 B9.8 6.9 Bb.3 Aa.10 6.8 Crystal 8.4 337 Logatchev Marker 72 7.6Vent 2608 951 7.2 Eiffel Tower 6.8

the measured value. *VF= Vent Fluid, SW=Seawater Luckv Strike & Logatchev. Mid-Atlantic Ridge **Italics indicates that the calculated value is heavierPlain by fontmore indicates than relative l%o agreement to the betweenmeasured the value measured and bold andfont calculated indicates valuethat theof+/-l%o. calculated value is lighter by more than compared l%o to Southern EPR 9-10°N F.PR Fractionated Seawater Case 4 Assume 50% loss ofRock= Li in down flow zone and low temperature fractionation in the down flow zone

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