DIFFUSE, LOW-TEMPERATURE HYDROTHERMAL DEPOSITS ON THE JUAN DE FUCA RIDGE AND PLATE
Catherine Erma Channing B.Sc., Carleton University, 2001
A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of
MASTER OF SCIENCE
In the Department of Earth and Ocean Sciences
O Catherine E. Channing, 2004 University of Victoria
All rights reserved. This thesis may not be reproduced in whole or in part, by photocopy or other means, without permission of the author. Supervisor: Dr. Kathryn M. Gillis
ABSTRACT
Hydrothermal circulation in ocean crust results in significant geochemical
exchanges between hydrosphere and lithosphere. This process begins at the mid-ocean
ridge and continues as basaltic crust ages and is subducted, significantly altering the
chemical composition of both fluid and rock. In the on-axis environment, heated crustal fluids with a composition altered from that of seawater vent as either high temperature
(> 100 C), focused flow or low temperature (< 100 oC) diffuse flow. Reaction between warm fluids and basalt results in the alteration of the rock, manifested as the breakdown of glass and primary minerals and the deposition of secondary minerals. In the off-axis environment (crust > 1 Ma), crustal fluids discharge locally at seamounts, where extensive manganese oxides can precipitate. Both types of mineral deposits record the time-integrated history of diffuse fluid-rock interaction, and in addition, Mn-oxide deposits are useful for estimating the longevity of hydrothermal activity.
The effects of low-temperature diffuse fluids on the basaltic crust was examined at both young (Axial Volcano) and mature (Main Endeavour field) on-axis hydrothermal sites. In general, alteration was very minor (< 2%), with the MEF basalts showing slightly more abundant and diverse mineral assemblages that that at Axial, due to the presumed longer period of low-temperature basalt-water reaction. Interaction of basalt with diffuse, low-temperature fluids resulted in only minor chemical changes in basalt. Chemical fluxes for basalt alteration at Axial Volcano is insignificant when compared to the cumulative on-axis fluid flux and fluxes associated with older, off-axis basalt alteration. Diffuse fluids from Axial were examined using geochemical modeling to determine how parameters such as fluid temperature, pH and degree of mixing and water-rock reaction influence alteration mineral precipitation. Results show that precipitation of the observed alteration assemblages requires a combination of minerals precipitating directly from diffuse fluids, from mixing of fluids and seawater (at < 20 % seawater) and fi-om diffuse fluid - basalt reaction at water-rock ratios > 200: 1.
Manganese oxide crusts were examined at Baby Bare seamount to investigate the history of hydrothermal venting. Baby Bare acts to focus crustal fluids, which precipitate extensive Mn-oxide deposits. Textural, mineralogical and chemical evidence indicates a hydrothermal origin for these deposits, with a diagenetic signature as well.
Minimum ages of Mn-oxide crusts, using calculated growth rates (324 to - 1800 mm/Ma) and manganese outcrop thicknesses, indicate that Baby Bare has been hydrothermally active for at least 0.5 Myr, and possibly since its formation at - 1.7
Myr.
TABLE OF CONTENTS .. ABSTRACT ...... 11
TABLE OF CONTENTS ...... v ... LIST OF TABLES ...... VIII
LIST OF FIGURES ...... ix
ACKNOWLEDGEMENTS...... x
1 . INTRODUCTION ...... 1
1.1. Oceanic Hydrothermal Systems ...... 1
1.2. Study Areas ...... 5
1.3. Diffuse Hydrothermal Fluids ...... 8
1.4. Thesis Objectives ...... 9
1.5. Contributions and Publication Plan ...... 10
Z . ALTERATION AND MASS TRANSFER AT AXIAL, LOW-TEMPERATURE
DIFFUSE HYDROTHERMAL SITES ...... 11
2.1. Introduction ...... 11
a 1.1. Geologic Setting and Sample Suites ...... 12
2.1.2. Dfluse Hydrothermal Fluids ...... 16
2.2. Methods ...... 19
2.2.1. Analytical Methods ...... 19
2.2.2. Geochemical Modeling Methods ...... 21
2.3. Results ...... 26
2.3.1.Alteration Mineralogy ...... 26
a3.2.Bulk Rock Chemistry...... 29
2.3.3. Chemical Change Calculations...... 33 2.3.4. Geochemical Models ...... 34
2.3.4.1. Model I - Speciation of Dzfluse Fluids ...... 34
2.3.4.2. Model 2 - Dguse Fluid - Seawater Mizing Model ...... 37
2.3.4.3. Model 3- Dguse Fluid - Basalt Reaction Model ...... 42
2.4. Discussion ...... 45
2.4.1. Alteration ofBasaltic Crust...... 45
2.4.9. Chemical Fluxes ...... 48
2.5. Conclusions ...... 53
3 . HYDROTHERMAL MANGANESE OXIDE DEPOSITS FROM BABY BARE
SEAMOUNT IN THE NORTHEAST PACIFIC OCEAN ...... 54
3.1. Introduction ...... 54
3.2. Regional Setting ...... 55
3.2. 1. Baby Bare Geologic Setting...... 55
3.3. Sample Suite ...... 59
3.4. Analytical Methods ...... 64
3.5. Results ...... -65
3.5.1. Mineralogy ...... 65
3.5.2. Bulk Chemistry ...... 66
3.6. Discussion ...... 70
3.6. I . Classzficationas Diagenetically Inzuenced Hydrothermal
Crusts ...... 70
3.6.2. Elemental Sources and Growth-Conditions...... 75
3.6.3. Calculation of Crust Growth Rates ...... 80
3.7. Conclusions ...... 83 vii
4. CONCLUSIONS ...... 84
FUTURE WORK ...... 86
REFERENCES CITED ...... 87
APPENDIX A: SAMPLE LOCATION SUMMARY FOR AXIAL VOLCANO
AND MAIN ENDEAVOUR FIELD ...... 106
APPENDIX B: SAMPLE DESCRIPTIONS ...... 108
APPENDIX C: PRECISION AND ACCURACY ...... 115
APPENDIX D: ELECTRON MICROPROBE DATA ...... 119
APPENDIX E: LASER ABLATION ICP-MS RESULTS ...... 127
APPENDIX F: GEOCHEMICAL MODELING METHODS ...... 129
F. 1 . Introduction ...... 129
F.2. Initial Steps ...... 129
F.3. Sensitivity Calculations ...... 133
F.4. Mineral Suppression...... 134
F.5. Geochemist's Workbench Suite...... 135
F.6. Calculation Check ...... 139
APPENDIX G: ALTERATION MINERALOGY SUMMARY FOR AXIAL
VOLCANO AND MAIN ENDEAVOUR FIELD ...... 141
APPENDIX H: RESULTS OF CHEMICAL CHANGE CALCULATIONS ...... 143
APPENDIX I: GEOCHEMICAL MODELING RESULTS ...... 146 viii
LIST OF TABLES
Number
Table 2.1. Fluid chemistry used for geochemical models ...... 18
Table 2.2. Basalt chemistry used for geochemical models ...... 25
Table 2.3. List of minerals suppressed in geochemical models ...... 25
Table 2.4. Alteration minerals and fluid temperatures during basalt collection ...... 28
Table 2.5. Bulk rock chemistry of basalt from Axial Volcano ...... 30
Table 2.6. Bulk Chemistry of basalt from the Main Endeavour Field ...... 31
Table 2.7. Conditions at which fluid-seawater mix became seawater dominated ...... 41
Table 2.8. Comparison of flux calculations in the Axial region ...... 51
Table 3.1. Sampled Mn-oxide crust locations at Baby Bare seamount ...... 61
Table 3.2. Major and trace element chemistry for Mn-oxide crusts ...... 67
Table 3.3. Comparison of Baby Bare Mn crusts with those from other locations ...... 74
Table 3.4. Calculated growth rates and ages for Mn-oxide crusts ...... 82 LIST OF FIGURES
Figure 1.1. Hydrothermal circulation in the axial region ...... 3
Figure 1.2. Hydrothermal circulation in ocean crust with age ...... 4
Figure 1.3. Map of sampled sites ...... 7
Figure 2.1. Maps of Axial Volcano and the Main Endeavour Field ...... 15
Figure 2.2 Solubility plot for SiOq,, ) for all hydrothermal fluids at 15 OCvs . pH ...... 36
Figure 2.3 Results of diffuse fluid-seawater mixing model for Cloud (2998)...... 38
Figure 2.4 Results of water-rock reaction model for diffuse fluid from Cloud (1998).... 43
Figure 3.1. Location of Baby Bare seamount ...... 58
Figure 3.2. Photographs of Mn-oxide deposits at Baby Bare seamount ...... 60
Figure 3.3. Photographs of Mn-oxide crust samples from Baby Bare seamount ...... 63
Figure 3.4. Chondrite-normalized rare earth element plot for Mn- oxide crusts ...... 69
Figure 3.5. Ternary diagrams classifying Mn-oxide crusts ...... 73
Figure 3.6: Cu + Co + Ni concentrations (grade) versus calculated growth rates in
Mn-oxide crusts ...... 76
Figure 3.7: Chondrite normalized REE plot for average Mn crusts, spring fluids and
Baby Bare bottom seawater ...... 78 ACKNOWLEDGEMENTS
I wish to thank first and foremost Kathy Gillis for being a great mentor, and for her help and guidance while supervising this work. I really enjoyed my time here and I am thankful for the opportunities she gave me to experience life on the ocean. Russ
McDuff is also thanked for agreeing on short notice to be my external examiner.
Comments by Meg Tivey, Russ McDuff, Dave Butterfield, Dante Cad, Kevin Telmer and Kathy Gillis greatly improved the final draft. Technical assistance from Richard
Cox, Steve Calvert, Brent Gowen, Mati Raudsepp and Sergei Mateev are greatly appreciated. This work also benefited from advice from and discussions with Meg
Tivey, William Seyfried Jr., Susan Humphris, Dave Butterfield, Karen Von Damm, Jay
Cullen and Laurence Coogan. I would also like to thank J.D. Chadwick and Michael
Perfit for generously sharing data from their work at Axial Volcano.
I would also like to thank Bob Embley and Paul Johnson for inviting me on their oceanographic research cruises; Matt Heinz, Will Sellers and other members of the
ROV Jason I1 group and the ROPOS team for assistance with sample collection at sea; crews of the R/V Thomas G. Thompson and R/V Atlantis; Kevin Telmer for his assistance with the geochemical modeling program; Dante Canil for use of his laboratory for sample preparation; Verena Tunnicliffe for sharing her cruise reports fi-om previous cruises to Axial Volcano; and Brian Cousens for inspiring me to do ocean science in the first place.
Thank you also goes to my friends: Laurie and Ian Gallagher, Colleen Delaney,
Heather Paul, Bill Martin, Karen Patterson and Mitch Selly, whose friendships have made my time here absolutely fabulous. Finally, I could not have completed this work without the love and support, both financial and emotional, from Kyle Fitzgerald and my parents.
This study was financially supported through the National Science and
Engineering Research Council Discovery Grant to Kathy Gillis. Chapter I
1. INTRODUCTION
1.1. Oceanic Hydrothermal Systems
Hydrothermal circulation in ocean crust is one of the largest geochemical cycles
on earth and is the major method of geochemical exchange between the oceanic
lithosphere and hydrosphere (Delaney et al., 1998). This process removes heat and
chemical species from ocean crust as it ages (Stein and Stein, 1994), acts to cool the
lithosphere, and exerts a control on the chemistry of the oceans (Fisher and Becker,
2000). Heat that drives hydrothermal circulation is supplied at the mid-ocean ridge by a
magma source, and in the off-axis (crust > 1 Myr old), by the cooling of the lithosphere.
Uppermost ocean crust at mid-ocean ridge spreading centres is porous and permeable,
which permits cold, oxygenated seawater to convect rapidly in and out of the crust
(Mottl, 1983). Seawater penetrates basement through faults and fractures, is heated
while descending through the crust and reacts with the surrounding rock, altering the
chemical compositions of both the fluid and rock. These fluids are eventually emitted at
the seafloor with a chemical composition that is altered from that of seawater
(Butterfield et al., 1990). Basalt alteration is manifested by the breakdown of primary minerals and glass and deposition of secondary (alteration) phases in the rock. With age, porosity and permeability in the crust decreases as alteration minerals precipitate in void spaces, reducing the vigor of the hydrothermal circulation process (Davis, 1992).
Crustal circulation may persist in the off-axis environment to an age of - 65 Myr (Stein and Stein, 1994; Stein et al., 1995). In the axial region, hydrothermal vents are typically located in the neo-volcanic
zone and are either high or low temperature (Fig. 1.1). High temperature (> 100 oC)
vents have focused flow at high velocities where fluids are emitted from sulfate or
sulfide chimney structures. Low temperature (< 100 oC) vents are diffuse, where fluids
are emitted through holes and cracks in the seafloor. These fluids may be spatially
associated with high temperature sites where fluids seep through sulfides or basalt, or
more distal from high temperature sites where they vent only through basalt.
In the off-axis ridge-flank environment (crust > 1 Myr), hydrothermal
circulation is less vigorous, fluid temperatures are much lower (< 60 oC) and fluid
chemistry is much closer to that of seawater (Butterfield et al., 1997). Hydrothermal fluids are diffuse, venting either directly from the basaltic seafloor or diffuse through the overlying sediment cover (Fig. 1.2) (Butterfield et al., 1997; Delaney et al., 1992; Mottl et al., 1998; Wheat et al., 2002). Figure 1.1: Hydrothermal circulation at the mid-ocean ridge axis. Seawater that penetrated basement is heated in the subsurface, rises to the seafloor, and is vented from either high- temperature sulfide or sulfate structures, or low-temperature, diffuse sites on the seafloor
(Kelley et al., 2002). Distance from Ridge Axis (km) Figure 1.2: Eastern flank of the Juan de Fuca Ridge. As the crust ages, it becomes progressively
covered with sediments, which act to restrict the free interaction between seawater and the ocean
crust. Also with age, porosity and permeability are reduced in the crust due to secondary minerals precipitating in void spaces. These act to reduce the vigor of hydrothermal circulation in the off-axis
(Davis, 1995). High temperature mineral precipitation at vent sites has been extensively studied in the axial region (Tivey, 1995a, b; Tivey et al., 1995; Tivey and McDuff,
1990), whereas in the diffuse environment, it has been largely ignored. Diffuse fluids in the on-axis may contribute as much or up to three times more heat flux as high temperature flow (McDuff et al., 2002), and is certainly the major heat contributor in the off-axis region (Davis et al., 1997).
1.2. Study Areas
Three study areas were examined, which represent three different stages of evolution of a hydrothermal system. All areas are located in the Northeast Pacific ocean
(Fig. 1.3) and are associated with the Juan de Fuca plate. Lava ages provide the distinction between young and mature sites. Axial Volcano and the Main Endeavour
Field (MEF) represent on-axis hydrothermal sites (situated on the Juan de Fuca ridge).
Axial Volcano represents a young hydrothermal system (5 years), and is currently both seismically and magmatically active. The MEF represents a mature, on-axis hydrothermal system (5000-8000 years old (Volpe and Goldstein, 1990), is also seismically active, but is currently in a period of tectonic, rather than magmatic spreading (Delaney et al., 1992). Finally, Baby Bare seamount (-1.7 Myr) represents a mature, off-axis site, located - 100 km from the Endeavour segment of the Juan de Fuca ridge.
Two types of diffuse, low-temperature fluid flow were examined at these three sites. The first was associated with the on-axis sites, where seawater-derived fluids heated to temperatures > 300 OCmixes with fresh, oxygenated seawater in the subsurface before venting at the seafloor as low-temperature diffuse flow (Butterfield et al., 1990). The second was associated with the off-axis site, where seawater-derived fluids < 25 OCvent through basalt and sediment cover at the summit of the seamount.
In this region, seawater is recharged into basement locally (Fisher et al., 200.3) and fluid temperatures are much lower as they are associated with older and cooler crust.
Hydrothermal circulation in the off-axis results in two-thirds more heat loss than that at ridge crests, and nearly 20% of the total predicted global ocean heat flux (Elderfield et al., 1999; Fisher and Becker, 2000; Stein and Stein, 1994). Figure 1.3: Map showing locations of Axial Volcano, the Main Endeavour
Field and Baby Bare seamount on the Juan de Fuca Ridge and plate (Wheat and Mottl, 2000). 1.3. Diffuse Hydrothermal Fluids
In the on-axis environment, diffuse fluids are primarily a result of mixing of high
temperature fluids with fresh, oxygenated seawater in the subsurface (Butterfield et al.,
1990, in press). Over-printing of mineral precipitation/dilution reactions and
biologically mediated processes also add to the chemical signature of diffuse fluids
(James and Elderfield, 1996; Sedwick et al., 1992; Tivey, 199513). Diffuse fluids may also
result from conductive cooling of high temperature fluids or conductive heating of
seawater (Cooper et al., 2000), however, at Axial Volcano, there is no evidence for this
in the diff~lsefluids (Butterfield et al., in press). Evidence for the dilution process comes
from the presence of chemical species and minerals in diffuse fluids that result only from
high-temperature reaction of fluids with basalt (Butterfield et al., 1990, 2001, in press).
Reaction with basalt causes most of the chemical changes that occur in the seawater- derived fluids, so basalt plays an important role in determining fluid chemistry (Von
Damm, 1995). Many elements are exchanged between the fluid and basaltic rock during the hydrothermal circulation process. For example, Mg, K and sulfate are removed from solution, whereas elements such as Ca, Si, Fe and Mn are extracted from rock through dissolution and exchange reactions (Butterfield et al., 1997; Mottl, 1983; Seyfried Jr. et al., 1988; Seyfried Jr. and Bischoff, 1979). Any Mg present in diffuse fluids is inferred to be the result of high temperature fluids mixing with fresh seawater, as high temperature reactions at temperatures > 300 OCresults in the complete removal of Mg into chlorite
(Butterfield et al., in press).
In the off-axis environment, diffuse fluids are a result of the subseafloor heating of seawater by the cooling of the lithosphere. Here, chemical exchange between these fluids, basaltic basement and the sediment cover influence diffuse fluid chemistry (Mottl et al., 2000; Wheat et al., 2000, 2002, 2003; Wheat and Mottl, 2000). These exchanges
typically occur at temperatures below 100 "C, with seawater losing elements such as
Mg, Na and K, and gaining Ca and Mn from oceanic crust. Elements such as Mn, Fe,
Na, Ca and K are also transferred from the sediment cover to the diffuse fluids (Wheat
and Mottl, 2000).
1.4. Thesis Objectives
The overall goal of this study was to examine low-temperature mineral
precipitates from diffuse hydrothermal vent site. This involved two types of diffuse
hydrothermal sites: on-axis sites where low-temperature fluids vent through basaltic
seafloor and sulfide/basalt talus mounds; and an off-axis site, where diffuse fluids vent
through sediments and precipitate manganese oxide deposits. At the on-axis young and
mature sites (Axial Volcano and the MEF, respectively); goals were to characterize the
alteration mineral assemblage, to determine the chemical changes associated with basalt
alteration and to determine how these are influenced by differences in such parameters
as tectonic setting and system age. In addition to this, the compositions of diffuse fluids
from Axial Volcano were thermodynamically modeled in order to examine what fluid
characteristics and types of reactions influence the precipitation of alteration minerals.
At the mature, off-axis site (Baby Bare seamount), the goal was to determine the origin
and evolution of the manganese oxide deposits near the summit area where low-
temperature hydrothermal fluids once vented freely from the sediment cover.
At all hydrothermal vent sites, low-temperature alteration minerals provide information on the chemical reactions that occur between the hydrosphere and lithosphere. By comparing these different locations at different stages of evolution, some important questions will be addressed, such as: how does age of the hydrothermal system influence mineral precipitation and chemical fluxes between fluid and rock?
What role does geologic setting play in the extent and nature of fluid-rock reactions?
What are the chemical fluxes between the fluids and rock and what parameters control these fluxes?
1.5. Contributions and Publication Plan
Chapter 2 - First author, submitted to Geochimica et Cosmochimica Acta, July 3oth,
2004. D.A. Butterfield supplied fluid data for this study.
Chapter 3 - First author, submitted to Marine Geology, June 2sth,2004. Chapter 2
2. ALTERATION AND MASS TRANSFER AT AXIAL, LOW TEMPERATURE
DIFFUSE HYDROTHERMAL SITES ON THE JUAN DE FUCA RIDGE
2.1. Introduction
Movement of heated seawater through ocean crust is the major pathway of
geochemical exchange between the oceanic lithosphere and hydrosphere. The upper
basaltic ocean crust is porous and permeable, and permits seawater to penetrate to deep
levels, where it is heated by a heat source at mid-ocean ridges. Hot, buoyant fluids rise
to the seafloor, where they vent from hydrothermal vents at both diffuse, low
temperature (< 100 oC) and focused, high-temperature (> 150 oC) sites. In axial vent
fields, diffuse flow is typically distributed over a much larger cross-sectional area than
focused, high temperature venting and may contribute as much as or up to three times
more heat flux than high temperature venting (Veirs et al., 2001). The chemical
composition of diffuse fluids is primarily determined by mixing of hot fluids with cold
seawater-like fluids beneath the seafloor and low-temperature water-rock reaction produce secondary effects (Edmond et al., 1979a ; Butterfield et al., 1997; in press).
Based on the prevalence and areal extent of diffuse venting, it is clear that the mass flux associated with this type of venting is significant. A critical unresolved issue is whether the rates of low-temperature water-rock reaction at the seafloor are fast enough to have a significant impact on chemical fluxes at mid-ocean ridges.
As diffuse fluids react with basalt, primary minerals breakdown and secondary minerals precipitate. The chemical and mineralogical effects of reactions between diffuse fluids and rock have not been explicitly examined, and it is not known, for example, how parameters such as fluid chemistry and the age of the hydrothermal system influence
mineral precipitation or the extent and nature of basalt alteration and chemical
exchange. Understanding the chemical evolution of diffuse fluids and the impact on
basaltic crust is important to resolve if we are to accurately predict the mass flux that
occurs between the hydrosphere and lithosphere in these environments.
For this study, alteration and chemical changes associated with basalt
interacting with warm, diffuse fluids are examined at young and mature axial hydrothermal sites. Alteration mineralogy and bulk geochemistry are examined for basalt samples recovered from Axial Volcano and the Main Endeavour Field, Juan de
Fuca Ridge. In addition, diffuse fluid compositions from three sites at Axial were used for geochemical reaction path modeling to determine how temperature, fluid composition, degree of mixing and other factors affect basalt alteration. Comparison of minerals predicted in different models to those observed provide insights about the relative importance of seawater mixing and water-rock reaction on the formation of the a1 teration assemblages.
2-1.I. Geologic Setting and Sample Suites
The Juan de Fuca is an intermediate spreading ridge (-29 mm/yr) located in the
Northeast Pacific Ocean (Fig. 2. la). Both young and mature, on-axis hydrothermal sites are found along the ridge, represented by Axial Volcano and the Main Endeavour Field, respectively. Axial Volcano is a young (5 years old) hydrothermal system that lies at the ridge intersection with the Cobb-Eickelberg seamount chain. In January 1998, an eruption occurred along the South Rift Zone, in the southeast portion of the volcano's three-sided caldera (Dziak and Fox, 1999; Embley et a]., 1999), where the East hydrothermal field is hosted. Erupted lavas extend over 9 km along the rift zone in a
primarily diffusely venting area that was previously hydrothermally active (Embley et
al., 1999). Basalt samples were collected from four diffuse vents in 2002 in the new lava
flow: Cloud, Village, Snail and Marker 33 (Fig. 2. lb and Appendix A). Fluid
temperature, composition, and flow velocity at Cloud, Marker 33 and Snail vents has been well documented since the eruption (Butterfield et al., in press) and basalts were collected in contact with diffuse fluids, where possible. Cloud vent is located in an area of collapsed lava tubes and pillars, where - 20 OCfluids emanate from a 1-meter wide hole in the basalt carapace. Snail and Marker 33 both vent from cracks in sheet flow lavas. Fluids are now < 25 OC at both Snail and Marker 33 vents, but Marker 33 was one of the hottest and most vigorous vents after the eruption, with fluid temperatures up to 78 "C in 1999 (Butterfield et al., in press). Village vent, discovered in 2002, is a - 2 meter high pile of basaltic talus, where - 40 OC fluids vent through cracks between basaltic rubble. Rocks collected from the 1998 eruption zone provide the unique opportunity to study alteration where basalt age is unequivocally known and fluid compositions are available.
The Main Endeavour Field (MEF) is the largest of 5 vent fields found on the
Endeavour segment and represents a mature end-member of a hydrothermal system, as lavas are 5000-8000 years old (Volpe and Goldstein, 1990). In the MEF, 5 vents sites were sampled: S&M, Southeast Hulk, Milli-Q, West Grotto and Easter Island (Fig.
2.lc). S&M and Milli-Q are high-temperature sulfide structures with extensive talus mounds of sulfide, basalt talus and hydrothermal sediments. Basalt samples from these vents were collected in or near diffuse fluid flow (< 20 oC) at the base of the structures in the talus mounds, lying loose in the hydrothermal sediments. These rock samples were also associated with organisms such as tubeworms, allvenilid worms and white filamentous bacteria. S.E. Hulk, W. Grotto and Easter Island are all diffuse sites, venting from the basaltic seafloor. The basalt from S.E. Hulk was collected - 10 meters southeast of the Hulk sulfide structure from a diffusely venting crack in sheet flow, where fluids were < 40 oC. Biological organisms such as tubeworms and white filamentous bacteria were associated with venting fluids here. Basalt from W. Grotto was collected from a diffusely venting pile of basalt talus found between the Grotto sulfide structure and the west axial valley wall, in contact with fluids < 40 C. Basalt at this vent was completely covered in white filamentous bacteria. Finally, Easter Island is a diffusely venting area -80 m2 in sheet flow and pillow lavas. Basalt was collected from the southeast area of the vent, lying loosely in hydrothermal sediments approximately 1 meter away from the venting crack. Fluids near where this sample was collected were <
50 oC.
At all sampled diffuse vents from both young and mature sites, the presence of active diffuse venting, biological communities (bacterial mats and filaments, tubeworms and allvenilid worms) and/or surface mineral stainingdcoatings on basalt indicate that samples were at one point in contact with diffuse fluids. Basalt samples were collected in
2002 during R/V Thomas G. Thompson cruise TN-149 using the ROV ROPOS; in
2003 during the R/V Atlantis voyage 7, leg 20 using the ROV Jason 11; in 2003 during the R/V Thomas G. Thompson cruise TN-158 using the ROV Jason-11; and finally, in
2003 during the R/V Thomas G. Thompson cruise TN-159 using the ROV ROPOS.
Appendix B lists detailed sample descriptions for basalts from both Axial and the MEF. I'W I
Figure 2.1: Maps of the Juan de Fuca Ridge, Axial Volcano and the MEF. A: Map of the Juan de Fuca ridge showing locations of Axial and MEF (Baker et al., 2004). B: Map of Axial Volcano showing sampled diffuse vents in the East Field: 1: Marker 33 and Snail vents; 2: Cloud and Village vents. 1998 Lava flow indicates the extent of eruptive material (Embley et al., 1999). C:
Map of the MEF showing sampled diffuse vent sites (Lilley et al., 200s). Sampled diffuse vent sites are marked with a star. 9.1.9. Dguse Hydrothermal Fluids
In the axial region, diffuse fluids form primarily from the dilution of zero-Mg,
sulfide-rich high-temperature fluids with seawater in the subseafloor (Butterfield et al.,
1997, 2001, in press; Edmond et al., 1979a, b). Their chemistry can generally be
accounted for by the mixing of ambient bottom seawater with high-temperature fluids
(Edmond et al., 1979a), with specific sites being affected by mineral precipitation,
microbial activity (Butterfield et al., 1997; Butterfield and Massoth, 1994) and/or
dissolution of existing hydrothermal minerals (James and Elderfield, 1996). Evidence
for the high temperature component in axial diffuse fluids includes depleted Mg and
elevated concentrations of chemical species and minerals that result only from high-
temperature reaction of fluids with basalt (Butterfield et al., 1997, 2001, in press;
Edmond et al., 1979a, a; Lowell et al., 200s).
Diffuse fluid chemical data for Cloud, Snail and Marker 33 vents from Axial are described in detail by Butterfield et al. (in press). Fluids were collected from Cloud and
Marker 33 vents one year before the eruption and for three years afterwards. Only data from the year 2000 was available for the Snail vent. In general, major element chemistry for these fluids is very similar between sites and years, with significant variations seen in concentrations of Has, COe, SiOq,,) and the trace elements Mn, Fe, A1 and Ba between vents (Table 2.1). All diffuse fluids sampled in the Axial S.E. caldera eruption area have chloride concentrations at or below ambient seawater, which indicates that they contain a vapour-rich component and phase separation has occurred at depth.
Hydrothermal fluids commonly pass through two-phase conditions at depth in the seafloor and separate into a low-chlorinity vapour phase and a high-chlorinity liquid or brine phase (Butterfield et al., 1990). The Mg depletion and Si enrichment, indicating the presence of a high-temperature fluid component (Butterfield et al., in press). Fluid temperatures were measured at the time of basalt collection and are generally similar to those measured in the diffuse fluids over the four years of fluid sampling with the exception of Marker 33 in 1999, where fluids were much hotter immediately after the eruption. No diffuse fluid data was available for sites of basalt sampling at the MEF. Table 2.1: Fluid Chemistry used for geochemical models."
Axial Bottom Marker Marker Vent Marker33 Marker33 Cloud Cloud Cloud Cloud Snail Seawater 33 33 Sample # Year Maximum T (OC) Average T (OC) pH (measured at 22 OC) Alkalinity (meqlkg) cr Na' so4'- SiO~(aq) M~~+ ca2+ K+ H~SW ~e'+(umoUkg) ~n'+(umollkg) ~a'+(umollkg) ~1~'(umoUkg)
&(a,, (umollkg) Calculated logjOzcg)
*Data from Butterfield et al. (in press). Values are in mmoVkg unless otherwise stated. aValue of Mn taken from Snail 2000 sample R547b19 because no Mn data was available for this particular sample, which had the most complete chemistry. %slues ofPz(,, were calculated assuming equilibrium with HZ(aq) 2.2. Methods
9-2.I. Analytical Methods
For basalt samples from Axial Volcano, the light-grey, "fresh interiors were
analyzed along with its paired altered outer rims, for major and trace element
compositions. The altered rims were several mm thick, and included the sample exterior
where surficial alteration minerals were scraped off: The rims were separated from the
cores based on colour differences, using a diamond saw. For the MEF basalts, altered
rock halos were < 2 cm thick. Fresh glass was used as an analogue for the fresh interiors for these samples, which was picked off samples by hand.
Major elements and select trace elements (Cu, Cr, Ni, and Zn) were analyzed by
X-Ray Fluorescence (XRF) with a Philips PW244.0 4 kW automated XRF spectrometer at McGill University in Montreal, Quebec. Fused beads were prepared from a 1:5 sample: Lithium tetraborate mixture. Precision is within 0.5% relative for each element and accuracy is within 0.5 % for silica, 1 % for other major elements and 5 % for trace elements (Cu, Cr, Ni, and Zn). Select trace elements (V, Ba, Sc, Rb, Sr, Y, Zr, Nb, Cs, Hf,
Ta, Th and U) and rare earth elements (REE's) were analyzed by Inductively-Coupled
Plasma Mass Spectrometry using a Thermo Instruments PQII ICP-MS with a Gilsonm auto-sampler and peristaltic pump at the University of Victoria, in Victoria, British
Columbia. Dissolution procedure follows the hotplate methods of Taylor et al. (2002), except that HF was added only once during dissolution. In some samples, a fine, white, insoluble fluoride precipitate appeared. These samples were centrifuged and only the clear solute that formed was used for analysis. Final solutions were mixed to a final weight of 50 g in polycarbonate Falcon tubes using 1% HN03 and 1 rnl of a complexing agent consisting of 0.1IN HF, O.45N Boric and 0.22N Oxalic acids. Reproducibility for Sr, Y, Zr, Nb, Ba, U and most REEs is < 10% (Eu, Tb, Ho, Tm and Lu were 15-58%)
and 11-28% for Sc, V, Cr, Rb, Cs and Th. Accuracy is I 10 % for Sc, V, Rb, Sr, Y, Zr
and Ba; 11 to 40 % for Cr, Ni, Cs, Gd, Dy, Ho, Er, Eu, Y, Hf, Th and U; and > 40 % for
Tb, Tm, Lu, Ta and Pb (Appendix C).
Major elements compositions for MEF glasses were analyzed by microprobe
using a JXA-8900 Superprobe at the University of Alberta, Calgary, Alberta, at 15 kV
acceleration voltage with a 15 nA beam current and a beam diameter of 1 pm. Reported
results are an average of two analyses (Appendix D). Trace elements were analyzed
using Laser Ablation Inductively-Coupled Plasma Mass Spectrometry using a Thermo
Instruments PQII ICP-MS at the University of Victoria. Operating conditions followed
those of Chen et al. (~ooo),except that 20 Hz laser output frequency and He carrier gas were used (Appendix E). Precision is better than 10 % and accuracy is within 10 % relative for all elements, except Ti (30 %) and Sc (20 %) using MPI-DING glass standards (Canil et al., 2003).
Alteration minerals coating exterior and fracture surfaces were identified using three methods. First, where enough material permitted, coatings were analyzed using a
Siemens D.5000 powder X-Ray Diffractometer at the University of British Columbia, at
40 kV, 30 d Cu-K alpha with a monochromatized scan from 3-50 2 00; samples were run glycolated and unglycolated to identify clay phases. Second, for very small quantities, coatings were analyzed using Scanning Electron Microscope (SEM) with both a Philips X30, with a Princeton Gamma-Tech EDS system at the University of
British Columbia, and a Hitachi S-3500N with an Oxford ISIS EDS at the University of
Victoria. Elemental spectra were used to identify potential minerals. Finally, grain mounts of minerals coatings and fresh glass were analyzed using the JXA-8900 Superprobe at the University ofAlberta, at 15 kV acceleration voltage with a 15 nA beam current and a beam diameter of 1-3 pm. Thin sections hosting secondary minerals, and plagioclase crystals in all samples were also analyzed by electron microprobe
(Appendix D).
aaz. Geochemical Modeling Methods
Seawater basalt interaction at mid-ocean ridges has been explored by using chemical equilibria and mass transfer computer models, but most studies have focused on high-temperature systems (Bowers and Taylor, 1985; Janecky and Seyfried Jr., 1984;
Janecky and Shanks 111, 1988; Tivey, 1995a; Tivey, 199.513; Tivey et al., 1995; Tivey and
McDuff, 1990). These models have also been used to examine the effects of host rock composition on secondary mineralogy and fluid chemistry in the subsurface (McCollom and Shock, 199'7; Wetzel and Shock, 2000). One purpose of these types of models is to understand the interaction between hydrothermal fluids, seawater and basalt by examining coexisting fluids and deposits at active sites where both can be directly sampled (Tivey, 1995a; Tivey et al., 1995). In this study, low-temperature water-rock reaction was examined using reaction path modeling in order to examine the conditions under which observed alteration minerals precipitated. Models were performed using diffuse fluid compositions from Cloud, Snail and Marker 33 vents at Axial Volcano only, as no fluid data was available for Village vent or the MEF. Reaction path modeling was carried out using the Geochemist's Workbench software package (Bethke, 2002), utilizing the thermo.dat database compiled by the Lawrence Livermore National
Laboratory (Delany and Lundeen, 1990). The database used for modeling provides the thermodynamic data for aqueous species, minerals and gases, equilibrium constants for reactions to form these species, and data required to calculate the activity coefficients of species in solution. The extended form of the Debye-Huckle equation was used to calculate activity coefficients of aqueous species and the activity of water was set to 1.
More detailed explanation of modeling methods are discussed in Appendix F.
Default pressure in the program is 1 bar for temperatures less than 100 OC
(Bethke, 1996). Generally, a pressure difference of a few hundred bars does not significantly affect the thermodynamic properties of most mineral phases. Molar compressibility's for most minerals can be assumed to be zero up to - 1 kbar, with the exception of sulfate minerals, and the effect of pressure in reactions involving only solid phases can be assumed to be negligible (Langmuir, 1997). The change in log K values is considered to be small when compared to the error on these values. A 170 bar database, supplied by M.K. Tivey, used for comparing log K values at 1 bar and 170 bar, showed that the change in log K due to this pressure increase was negligible (Appendix F).
Finally, changes in log K values for seawater are negligible up to 1 kbar at 25 OC
(Millero, 1973). Pressure does have dramatic effects on gases, however, which is accounted for in the models by setting the partial pressure (fugacity) of oxygen, by assuming equilibrium between Ha(aq)and He0 based on the reaction
He + '/Z 02+ He0 and using measured He(,) in the diffuse fluids. Setting the oxygen fugacity determines the redox state of the system.
The goal of all model types was to investigate what conditions are required in order for the observed minerals assemblage to precipitate in the low-temperature diffuse environment at Axial. Three modeling approaches were used to investigate mineral precipitation in the diffuse environment at the three Axial vent sites. Results of all models are determined using a set of concentrations that satisfy equilibrium equations
for each possible reaction in the system (Bethke, 1996). The first model involved
speciating the diffuse fluids, which determined the distribution of species in solution,
mineral saturation states, pH, Eh and gas fugacities. Results of this model illustrate an
equilibrium attained within the fluid, but one that is meta-stable with respect to mineral
precipitation (Bethke, 1996); not all minerals predicted to be supersaturated will
precipitate (Tivey, 1995a). The second model involved mixing diffuse fluids with
seawater, simulating the natural environment where diffuse fluids exit the vent orifice.
This model was run using the diffuse fluid as the initial system and seawater as a
reactant, and again, in reverse, using seawater as the initial system and the diffuse fluid
as the reactant to ensure different mineral phases would not precipitate. The final model
involved the titration of 100 g of basalt into various quantities of diffuse fluids in order
to simulate the reaction of fluid and rock at the vent orifice, which is a fluid dominated
environment.
Diffuse fluids were constrained by entering the fluid mass, in-situ measured values of temperature, pH (measured at 22 oC), alkalinity, and concentrations of major components and the trace components Mn", Few, A13+ and Ba2+(all in mol/kg). For the water-rock reaction models, 100 grams of fresh basalt with a major element chemistry based on a Ti02 free rock was entered as weight of oxides in grams (Table
2.2). Titanium was not measured in the diffuse fluids as samplers are made of Ti, so this element had to be excluded from the composition of the rock.
Suppression of certain mineral phases was required in the models in order to have those mineral phases known to precipitate in the low-temperature environment form (Table 2.9). Evaluation of which minerals could be suppressed was based on mineral stabilities, environment of formation and reaction kinetics. A limitation of the program is that phases known to precipitate at higher temperatures may appear in models that were run at low-temperatures, due to the fact that the program converges on the lowest AG assemblage. These high temperature phases may be stable at low temperatures, slow kinetics prevent them from precipitating. hat form only at low temperatures. Table 2.2: Basalt chemistry used for geochemical models.*
Vent Marker 33 Cloud Snail Sample 52-08-09 R674-18 R549-12-1 Si02 50.33 50.25 50.26 A1203 14.86 14.85 14.87 FeO 11.30 11.34 11.29 MnO 0.20 0.20 0.2 1 MgO 7.65 7.68 7.69 CaO 12.51 12.52 12.5 1 NazO 2.86 2.87 2.90 K20 0.22 0.2 1 0.22 Ba 0.003 0.003 0.003 co2 0.07 0.08 0.06 Total (g) 100 100 100
*Bulk chemistry is based on a TiOz free basis and scaled to total 100 g. All values reported as weight percent.
Table 2.3: List of suppressed minerals in geochemical models. Mineral Phases Reason for Suppression Quartz, Tridymite, Cristobalite, Allows for the formation of amorphous silica Chalcedonya Dolomite Precipitation too slow at low temperatures Pyrite Allows for the formation of Fe-oxyhydroxides Pyrrhotite Allows for the formation of Fe-oxyhydroxides Hematite, Magnetite Allows for the formation of Fe-oxyhydroxides
Kaolinite, Illite, Muscovite, ~-felds~ar~Not typically found in seafloor environment Dawsonite*' Not typically found in seafloor environment allows for the formation of goethte and it is Tr~ilite*~ only found in meteorites ~hen~ite*~ Not twicallv found in seafloor environment
*These phases were suppressed only in water-rock models. aMottl and McConachy, 1990 b~urray,1988 'Deer et al., 1992 9.3. Results
2.3.1. Alteration Mzneralogy
Basalt samples from both young and mature sites are very fresh with alteration manifest primarily as surficial coatings that were generally < 3 mm thick. Some samples have slight internal alteration, visible in hand sample as a dark-grey halo, when compared to the fresh, light-grey coloured interior of the sample. Also, secondary minerals partially line vesicles and fractures in some basalts in halos. Surficial coatings on MEF basalts were generally thicker (2-3 mm) than those found on Axial samples (<1 mm) and comprise a diverse mineral assemblage. Saponite and nontronite were the most abundant alteration phases at both locations (Table 2.4 and Appendix G).
All Axial basalt samples are very fresh (visual estimate of haloes is < $2 % altered, whereas alteration for the entire sample is < 1 %) with minor vesicle linings of Fe-Si oxyhydroxide and glass being only slightly devitrified. Clinopyroxene and olivine are very fresh. An extensive but discontinuous white coating on all samples at Axial consists primarily of saponite, anhydrite and a magnesium-silicate phase, very close in composition to talc or stevensite. The common assemblage (in order of decreasing abundance) is saponite + nontronite + Mg-silicate + particulate sulfur + anhydrite + zeolites + Fe-oxyhydroxide (Table 2.4 and Appendix G).
Basalt samples from the MEF are fairly fresh (visual estimate of alteration is <
10 % in haloes and < 4 % for entire sample) with glass that is partially devitrified and rock that has minor vesicle linings of Fe-Si oxyhydroxide, pyrite and a saponite- celadonite-beidellite mix. Clinopyroxene and olivine are very fresh. The common assemblage on surfaces (in order of decreasing abundance) is Mn-oxide + saponite + Fe- oxyhydroxide -t. barite + nontronite + anhydrite + zeolites f unidentified Al-silicates +- pyrite (Table 2.4 and Appendix G). The Al-silicates, identified by SEM, have traces of
Ca, K, Na and Fe in various combinations and may possibly be zeolites.
Minerals common to both young and mature hydrothermal sites include zeolites, saponite and nontronite. Particulate sulfur and an Mg-silicate are observed only at
Axial, while Mn-oxide is observed as a major mineral phase only at the MEF. Saponite, nontronite and Fe-oxyhydroxides are the most abundant phases at both locations. Table 2.. klteration minerals and fluid temperatures at the time of basalt collection. Fluid T Chemical Changes - Chemical Changes - Site Vent Surface Coatings Vesicle Linings I (OC) gained bv altered rock lost bv fresh rock Anh, Sap, Fe-oxy, S, Bar, Non, Eri, Cal, SO2,Fe203, MgO, Axial Fe-Si oxy Ba Mg-Si-0 phase CaO K20
S, Fe-oxy, Anh, Sap, Non, Phil, Mg-Si-0 SO2,Fe203, MgO, CaO, 'loud phase Na20, K20
Marker 33 Sap, Non SO2,TiO,, MgO, Na20
S, Sap, Anh, possibly Phil, Non, Mg-Si- Si02,Fe203, MnO, Snail CaO, Na20 0 phase MgO, Ba
Mn-ox, Am.Si, Sap, Bar, Anh, Fe-Si Fe-Si oxy, Sap-cel, SO2,Ti02, Fe203, S&M MnO, Ba MEF oxy, Al-Non, Al-Fe-Si-0 phase Sap-cel-beid CaO
l~e-S-0phase, Sap, Mn-Fe-ox, Bar, Am. Si., Al-Na-Si-0 phase, Mg-S-0 phase, Si02,Ti02, Fe203, Sap-cel-beid, Py MnO, Ba Milli-Q IFe-oxy, Fe-Si oxy, Al-Fe-Si-0 phase, CaO, Na20 Al-Non Chl, Fauj, Anh
Mn-ox, Non, Sap, Bar, Am. Si., Fe-Si Si02,Ti02, Fe203, S'E' Hulk Fe-Si oxy MnO, MgO, K20,Ba oxy, Gor, Phil, HyTc CaO, Na20 Mn-ox, Na-Al-Ca-Si-0 phase, Sap, Anh, SiOz, Ti02,Fe203, W. Grotto MnO, MgO, K20,Ba Phil CaO, Na20 Bar, Anh, Sap, Fe-Si oxy, Mn-ox, Non, Py, Sap-cel, Fe-Si laster Island Al-Na-Si-0 phase, Fe-oxy, Py, Na-Mg- MnO, MgO, K20,Ba Si-0 phase, Gor, Chl, Eri OXY I Minerals are in no particular order. Abbreviations: A1-Si phase = unidentified Al-silicate; Al-Non = Al-rich Nontronite, Am-Si = Amorphous Silica, Anh = anhydrite, Bar = Barite, Cal = Calcite, Chl= Chlorite, Eri = Erionite, Fauj = Faujasite, Fe-oxy = Fe oxyhydroxide or Ferrihydrite, Fe-Si oxy = either a mix of Fe-oxy and Am-Si., or a distinct Fe-Si oxyhydroxide phase, Gor = Gormanite, HyTc = Hydrotalcite, Mn-ox = Mn oxide, Non= nontronite, Phil = Phillipsite, Py = pyrite, S = elemental Sulfur, Sap = Saponite, Sap-cel-beid = saponite-celadonite-high A1 Beidellite mix, Sap-cel = Saponite-Celadonite mix. 2,3,2.Bulk Rock Chemistry
Bulk rock compositions for Axial samples are presented in Table 2.5 and MEF
samples in Table 2.6. Axial basalt samples analyzed in this work are very similar
chemically to basalt samples collected in the new lava flow, away from diffuse vents
(Chadwick et al., 2000). Our MEF samples are chemically identical (within analytical
uncertainty) to other basalts from the Endeavour area (Karsten et al., 1995). Axial
basalts are transitional between E and N-MORB (Rhodes et al., 1990), whereas MEF
basalts are typical E-MORB (Karsten et al., 1995). Groundmass plagioclase in samples
from both locations have igneous values, with the An content of cores ranging from
0.79 to 0.90 and rims from 0.70 to 0.84.
At both Axial Volcano and the MEF, low-temperature alteration of the basalts resulted in only minor chemical changes in basalts. At Axial, major element concentrations between fresh and altered pairs differed only slightly, whereas essentially no change was observed in the trace elements, with the exception of Ba (Table 2.5). At the MEF, slightly greater changes were observed in the major elements, and differences in most trace elements and REE's are negligible (Table 2.6). In general, the variation in elemental concentrations between fresh and altered rock in basalt samples from both young and mature hydrothermal sites are only slight. Table 2.5: Bulk rock chemistry of basalt from Axial Volcano.
Vent Site Marker 33 Marker 33 Snail Snail Village Village Cloud Cloud Type altered fresh altered fresh altered fresh altered fresh -Sample 08-09E 08-09i 12-1E 12-li 674-13E 674-131 674-183 674-181 SiOz TiOz A1203 Fez03 MnO MgO CaO NazO K20 p2os co2 LO1 Total
Cu Ni Zn v Cr Ba Sc Rb Sr Y Zr Nb Cs La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Y b Lu Hf Ta Th u - 0.12 0.22 0.14 0.37 *< dl1 is lesis than detection limits. Trace elements for Cloud (altered) are an average of two analyses. Major elements are reported as weight percent and trace elements as ppm. Blank spaces = no data. Table 2.6: Bulk Chemistry of basalt from the Main Endeavour Field.
Vent Site S.E. Hulk S.E. Hulk W. Grotto W. Grotto Easter Easter Easter Easter M-Q M-Q M-Q M-Q S&M S&M S&M S&M Sample 68-153 68-151 68-163 68-163 68-173 68-171 717-23 717-2i 717-43 717-4i 717-33 717-33 717-1E 717-li 730-1E 730-li SiOz 49.53 49.68 49.84 50.68 49.46 49.75 49.59 50.47 49.31 49.95 49.40 50.35 49.21 50.07 49.29 49.91 Ti02 1.76 1.73 1.66 1.67 2.17 2.14 2.18 2.23 1.76 1.99 1.77 1.81 2.18 2.18 1.76 1.87 AlzO3 14.88 14.55 14.82 14.62 14.44 14.34 14.44 14.47 14.85 14.34 14.95 14.52 14.52 14.37 14.77 14.52 Fe203 11.37 11.52 10.07 11.01 12.29 12.31 11.99 12.31 11.27 12.01 10.94 11.69 11.94 12.29 11.51 11.71 MnO 0.19 0.18 0.26 0.14 0.24 0.19 0.22 0.19 0.18 0.15 0.19 0.17 0.20 0.17 0.20 0.18 MgO 7.30 7.04 7.22 6.94 6.36 6.25 6.33 6.28 7.21 6.57 7.04 6.99 5.89 6.33 7.35 6.99 CaO 11.73 11.81 11.78 11.64 10.71 10.84 10.76 10.65 11.71 11.37 11.79 11.63 10.79 10.91 11.69 11.87 NazO 2.83 2.83 2.79 2.83 3.12 3.17 3.21 3.21 2.88 3.15 2.94 3.01 3.27 3.20 2.88 2.89 K20 0.41 0.38 0.50 0.48 0.60 0.58 0.66 0.58 0.44 0.48 0.43 0.38 0.61 0.57 0.43 0.40 LO1 < dl nla 0.66 nla < dl nla 0.05 nla 0.02 da 0.20 nla 0.57 da dl nla Total 100.28 98.83 99.95 99.18 99.78 98.69 99.82 99.45 99.91 99.12 99.92 99.65 99.78 99.15 100.16 99.45
*n/a indicates that element was not analyzed. Major elements are reported in weight percent and trace elements in ppm. "E: analysis was on the altered exterior of the basalt b. I: analysis was on fresh interior of basalt Table 2.6: Bulk Chemistrv of basalt from the Main Endeavour Field (continued). \ / Vent Site i.E. Hulk S.E. Hulk W. Grotto W. Grotto Easter Easter Easter Easter M-Q M-Q M-Q M-Q S&M S&M S&M S&M Sample 68-153 68-151 68-163 68-16i 68-173 68-173 717-2E 717-21 717-43 717-43 717-33 717-31 717-1E 717-li 730-1E 730-li Cu nla 98.00 n/a 110.00 n/a 107.00 112.00 da 95.00 nla 102.00 nla Ni nla 31.00 nla 30.00 Zn n/a 43.00 nla 90.00 Co nla 38.00 n/a 39.00 v nla 333.00 nla 332.00 Ba 40.1 1 114.00 67.52 150.00 Sc 45.1 1 27.53 40.44 37.01 Rb 2.59 5.84 4.32 5.82 Sr 155.85 182.65 172.15 237.57 Y 25.15 26.58 31.60 27.83 Zr 94.01 162.74 136.70 219.99 Nb 8.22 17.54 13.60 22.89 Cs 0.03 0.05 0.04 0.00 La 6.26 14.13 10.22 13.09 Ce 11.75 33.40 17.94 32.26 Pr 1.89 4.44 2.78 4.54 Nd 9.77 21.22 13.96 19.41 Sm 2.74 5.67 3.74 5.26 Eu 0.92 1.46 1.17 1.26 Gd 3.18 7.41 4.17 6.44 Tb 0.5 1 1.13 0.66 1.13 DY 3.49 7.58 4.49 6.04 Ho 0.70 1.45 0.90 1.21 Er 2.04 4.23 2.56 3.73 Tm 0.29 0.61 0.36 0.53 Y b 2.00 3.87 2.54 3.41 Lu 0.28 0.60 0.36 0.47 H f 1.76 5.06 2.41 5.02 Ta 0.45 1.64 0.79 1.97 Th 0.42 1.43 0.76 1.31 u 0.07 0.48 0.13 0.92 2.3,3. Chemical Change Calculations
The chemical change that occurred during alteration, between fresh and altered
rock, were calculated using Gresens' equation (Gresens, 196'7) modified by Grant
(1986). This method normalizes chemical data to determine relative elemental gains and
losses from fresh rock. Grant (1986) modified this method to relate the concentration
components in altered rock to that in the original, fresh rock, such that
c; = MYM* (c:+ ACJ
where CiA = concentration of component i in altered rock; Mo = mass of fresh rock; MA
= mass of altered rock; Cio = concentration of component i in fresh rock; and ACi = change in concentration of component i. Elements that are immobile during alteration defines the mass change term (MO/MA). Combining this with the chemical changes calculated in the basalt determined how much of each element was gained or lost by the fresh rock during alteration. At both Axial Volcano and the MEF, Ale03 was used to calculate the mass change term.
Results of chemical change calculations are discussed as the relative difference from fresh rock, in percent, in order to illustrate the magnitude of chemical change. For the Axial samples, altered rock haloes were compared to fresh, grey interiors. Altered halos in most samples generally lost Fee03 (< 2 %), gained NaaO (< 3%) whereas SiOe
(k 1 %), MgO (k 1 %), K,O (+ 25 %), Ba and Sr (+ 50 % each) were variable. Based on these chemical changes, basalts from Axial Volcano have experienced < 1 % alteration.
For MEF basalts, altered rock haloes were compared to fresh glass, which was used as an analogue for fresh interiors. The MEF samples show more element mobility than the
Axial suite, where altered halos show a loss of SiOa (< 5 %), Ti02 (< 15 %), Fee03 (3-10
%), CaO (< 3 %), NaeO (< 12 %) and a gain of MnO (< 83 %) and Kg0 (< 15 %). Of the trace elements, Ba, Rb and Sr showed the greatest mobility, with altered rock gaining
up to 200 % generally for each element. One exception is the S&M sample 'i 17-1, where
Ba changed by - 2200 %. Changes in the REEs and some other trace elements were
variable in the MEF samples, but generally REEs were gained by altered rock as was
Hf, Y, Zr, Nb and Cs. Based on the chemical change calculations, basalt from the MEF
have experienced < 4 % chemical change due to low-temperature alteration.
A general trend between both young and mature sites is a loss of SiOe and Fee03
from fresh rock and a gain of Ba and Sr to altered rock. The largest chemical changes at
both young and mature sites are observed in Ba and Sr. Essentially no chemical change
occurred in most trace elements in Axial basalts, whereas altered rock showed an
overall gain of most trace elements at the MEF.
2.3.4. Geochemical Models
Geochemical modeling was conducted to investigate the processes that lead to
precipitation of low-temperature alteration minerals on the Axial basalt samples. The
three model approaches used here consider minerals that precipitate directly from diffuse fluids, as a result of mixing of fluids and seawater, and by reaction of basalt with diffuse fluids. Emphasis is on the Axial suite only, as diffuse fluid data was available over a four-year period and the rocks have a precise age and known history. For all models, the predicted mineral assemblage was compared to the observed alteration assemblage.
2.3.4.1. Model I - Speciation of D @use Fluids
Diffuse fluids were speciated in this mode1 in order to determine what minerals phases would precipitate directly from them. Results show that there is essentially no oxygen in solution, as the oxygen fugacity was very low (ranging from - lo-" to 10-70) in the diffuse fluids. Oxygen fugacity for bottom seawater was calculated to be lo-*.". All diffuse fluid speciation results are listed in Appendix I.
When fluids were speciated with mineral precipitation disabled, diffuse fluids were supersaturated with many phases, such as clinoptililite, muscovite, beidellite, pyrophyllite, mordenite, illite and kaolinite. Supersaturation with many phases is commonly observed in low-temperature fluids (Bethke, 1996). When any minerals were allowed to precipitate, barite, pyrite, quartz and either muscovite or kaolinite precipitated in all diffuse fluids. Finally, fluids were speciated with mineral precipitation enabled, but with some phases suppressed (Table 2.3). This resulted in barite and mordenite precipitation, and silica and pyrite supersaturation. Amorphous silica precipitated in addition to these phases from the Marker 33 (2001) fluid. A SiOq,,) stability plot reveals that all fluids are near saturation with respect to amorphous silica
(Fig. 2.2).
Parameters such as temperature, pH and@qg) were varied from measured values in this model to investigate how changes in these control mineral precipitation.
In all diffuse fluids, temperatures greater than 50 OCand pH increases (up to 9) resulted in the precipitation of clays such as Mg-rich saponite, beidellite and talc. In addition, barite disappeared in all but the Marker 33 fluids and amorphous silica precipitated in the Marker 33 (1999) fluid (only at 50 oC). When temperatures were increased to > 125 oC, anhydrite precipitated in all fluids, in some cases with saponite and talc.
Temperature increases combined with a pH decrease resulted in only barite and mordenite precipitation. Measured temperatures accompanied with a pH increase to neutral values resulted in talc, barite and mordenite forming, while a pH decrease resulted in barite only. Finally, increases inflqg) values resulted in the precipitation of barite, mordenite and nontronite in all diffuse fluids.
In summary, diffuse fluid speciation results show barite and mordenite precipitate fi-om fluids at Marker 33, Cloud and Snail vents, at the conditions measured at the time of fluid sample collection. In addition to these phases, amorphous silica precipitated in the Marker 33 (200 1) fluid. Of all the predicted minerals, only zeolite is observed at Cloud and Snail vents at Axial Volcano. None of the phases predicted were observed at Marker 33 vent (Table 2.4), possibly because other processes not considered in this particular model have influenced mineral precipitation at this vent.
Amorphous silica
-2% 2 4 6 8 I0 12
Figure 2.2: Solubility plot for SiOq,,) vs. pH at 25
C.All diffuse fluids plot on the boundary between
amorphous silica and SiOq,,). 2.3.4.2. Model 2 - Dguse Fluid and Seawater Mixing Models
To simulate mixing of bottom seawater with diffuse hydrothermal fluids as they
exit the vent orifice, a model was used that illustrates possible mixing ratios between
these two fluids. This permits us to determine the mixing ratios required to produce the
observed mineral assemblages. Seawater was speciated first and added to the diffuse
fluid (the initial system) as a reactant. Models were also run in reverse to ensure that no
different phases were not precipitated. Results of mixing illustrate a sequence of mineral
assemblages that precipitate from the mixed fluid as the proportion of seawater
increases (Fig. 2.3A). Minerals listed in Table 2.3 were suppressed and models were
closed system, where precipitated minerals were permitted to back-react with the fluid
and dissolve. A sliding temperature path was used, where the initial and final
temperatures were those measured in the diffuse fluids and bottom seawater,
respectively.
Results show that, in general, similar minerals precipitated in each diffuse fluid-
seawater mixture and quartz and dolomite are always supersaturated (Appendix I). In
all mixtures, barite and mordenite precipitated, with barite remaining stable at all
mixing ratios in all fluids. Mordenite, in contrast, precipitated at all mixing ratios in the
Snail fluid and all Marker 33 fluids, with the exception of 1998. In all Cloud fluids and
the Marker 33 (1998) fluid, mordenite precipitated only when there was < 5-20 % seawater, temperatures were > 10-1 8 oC, pH was > 5.7-6.8 and$Iqg was < (Fig.
2.3B). Na-rich nontronite, pyrolusite and goethite precipitated in these fluids when no mordenite was present. Nontronite precipitated at all mixing ratios in the Snail fluid and the Marker 33 (1999) fluid. In Marker 33 (2000) and ZOO^), nontronite precipitated only at < 20 % seawater, temperatures > 22 "C, pH < 6 andflqg) < lWO;pyrolusite A Temperature ('C)
0 Diffuse Fluid - Seawater Ratio
Mixing fraction
Figure 2.3: Results of mixing between seawater and Cloud
(1998) diffuse fluid. A: mineralogical changes that occurred in the diffuse fluid as seawater reacted into it. B: changes in
Pe(gj, and pH as seawater is reacted into the diffuse fluid. Goethite, nontronite and pyrolusite precipitate once mordenite dissolves, whenJ3qg)begins to increase to seawater values. Talc precipitated when goethite dissolved. precipitated at > 20 % seawater, temperatures > 14 "C, pH < 6.1 and $la(g) < 10-lo; and goethite did not precipitate. Talc precipitated in all Cloud fluids and in the Marker 33
(1999) and (2001) fluids when goethite dissolved, shown by the reaction
4 FeO(0H) + 4.5 Mg2++ 6 SiOq,q) + 1.5 MgsSi40lo(OH)a + H+ + 4 Fez+ + Oq,,) and when temperatures were < 6 oC, pH > 6 andflq,) at lo-+..".Finally, amorphous silica precipitated in the Marker 33 (2001) fluid at seawater < 35 %, temperatures > 17
"C, pH < 6.1 andflqg) < In general, mordenite, nontronite and amorphous silica precipitated at reducing conditions at < 35 % seawater, whereas pyrolusite, goethite and talc precipitated under more oxidizing conditions at > 20 % seawater. In all models, only very small amounts (-0.001 g) of the predicted minerals precipitated from the mixed fluids, requiring a time-integrated effect for hydrothermal minerals to be evident on rock surfaces.
As seawater concentration increased in the mixtures, concentrations of SiOq,,),
Mgw, Few and Mnw decreased due to dilution with seawater, or their consumption by precipitating minerals; concentrations of other species were relatively stable. The amount of seawater required in the mixtures forflqg) to reach seawater values increased generally with increasing concentrations of He(,,) and HzS(,,) in the diffuse fluids, and with decreasing pH (Table 2.7).
Mixing of diffuse fluid and seawater precipitated barite, mordenite, nontronite, pyrolusite, goethite, talc and amorphous silica in various combinations: barite, nontronite and mordenite precipitated in all fluids; pyrolusite and goethite only in
Cloud fluids and the Marker 33 (1998) fluid; talc only in Marker 33 (1999) and (2001); and amorphous silica only in the Marker 33 (2001) fluid. Of these phases, nontronite and the Mg-silicate are observed at all three vents and zeolite is observed at Cloud and
Snail vents. No other predicted phases are observed. Table 2.7: Conditions at which diffuse fluid-seawater mix became seawater- dominated*.
Fluid H2S(,,) Hz(aq) Diffuse Fluid Sample seazter p~ (mmol~kg) (mmol~kg) U•‹C) P2M ppI Snail 2000 7 6.1 0.09 0.07 14.7 -68.92 Cloud 200 1 15 6.9 0.02 0.26 9.6 -71.32 Cloud 1999 16 5.8 0.49 4.2 1 19.9 -68.17 Cloud 2000 25 6.0 0.12 0.50 15.8 -68.94 Cloud 1998 28 5.3 0.75 10.16 20.6 -67.73 Marker 33 2000 32 6.2 0.70 2.25 33.4 -67.13 Marker 33 1998 3 3 4.9 1.94 6.08 19 -67.65 Marker 33 200 1 44 5.6 0.20 8.84 30 -66.88 Marker 33 1999 47 4.83 2.32 11.88 68.3 -56.45 *Seawater dominated conditions in the mixtures are when pH and reached seawater values of 7.8 and 10-+5,respectively. The parameters listed here are those measured in the diffuse fluids. 2.3.4.3. Model 3- Dzfuse Fluid - Basalt Reaction
Reaction of basalt with diffuse fluid is simulated in this model, where conditions
are fluid dominated (large amounts of fluid flow by and interact with small amounts of
rock). Diffuse fluids were first speciated and basalt was then added as a reactant. Water-
rock ratios ranging from 10:1 to 1000:1 were examined using 100 g of basalt (reacted
homogeneously) and 1 kg of diffuse fluid (however ratios as low as 1: 1 were examined;
at ratios less than lo: 1, many mineral phases that were not observed precipitated).
Mineral suppressions (Table 2.3) were made before reaction with basalt. The model
assumes local equilibrium and is closed system, with the reaction path tracing chemical
and mineralogical changes that occur in the diffuse fluid as basalt is reacted into it.
At all water-rock ratios and for all diffuse fluids, saponite nontronite, mordenite
and barite precipitated (Fig. 2.4A and Appendix I). Saponite, nontronite and mordenite
were the most abundant phases, producing up to 15 g of each mineral per 100 g of basalt
reacted. Barite (0.001 g per 1 kg fluid) and gibbsite (0.001 g per 1 kg fluid) were
produced in very small amounts. The predicted mineral assemblage best matched the
observed assemblages when water-rock ratios were greater than 200: 1 and pH < 7.5 in
all fluids, with the exception of Marker 33 (1999). Saponite, nontronite, mordenite,
gibbsite and barite all precipitated at these high water-rock ratios. At ratios less than
this, phases not observed (calcite, alabandite, rhodochrosite) precipitated. In the Marker
33 (1999) fluid, the predicted assemblages best matched the observed at water-rock ratios > 100:1, where saponite, mordenite, Mg-rich beidellite, nontronite, barite and gibbsite precipitated. Talc precipitated in the Marker 33 (2000) fluid at water-rock ratios > 500: 1 and amorphous silica precipitated in the Marker 33 (2001) at water-rock ratios > Water-rock ratio
Mass reacted (grams)
Figure 2.4: Results of reaction between basalt and Cloud
(1998) diffuse fluid. 100 g of fresh basalt was reacted into 1 kg of diffuse fluid. A: mineralogical changes that occurred in the diffuse fluid as basalt was reacted into it. Phases precipitated upon rock reaction. B: jQg) decreases steadily as pH increases from left to right. 1000: 1. In all models,fOq,) decreased in solution with decreasing water-rock ratio and
pH increased (Fig. 2.4B).
Barite and gibbsite were predicted to precipitate in all water-rock models,
however, these phases were not observed on any Axial basalts. Discrepancies between
the models and nature may be due to kinetic factors in nature, not taken into account in
the models; poor thermodynamic data at the temperatures/conditions of interest; or
incorrect assumptions about what conditions observed minerals precipitated at.
However, barite and gibbsite were the least abundant phases, precipitating in an amount
3 orders of magnitude smaller than saponite and nontronite. It is possible that they
were produced in such small quantities on basalt samples that they were not observed.
Compositional changes in the diffuse fluids are also monitored as water-rock
reaction progresses. In all fluids, generally concentrations of SiOq,,), Few, Mg2+ and
Mn2+increased in solution as water-rock ratios increased, whereas AlS+ and Bag+
decreased. All other components were relatively stable as reaction progressed. It is
inferred that those elements that decreased in solution are consumed by precipitating
alteration phases and that those that increased are derived from the breakdown of basalt.
Finally, the reaction of basalt with diffuse fluids was taken one step further, where 1 kg of seawater was added to the already reacted diffuse fluids. This resulted in the same mineral phases precipitating and the observed assemblages at similar mixing ratios.
Results of water-rock reaction models show that the predicted mineral assemblage best matches the observed assemblages at water-rock ratios higher than
ZOO: 1, consistent with the fluid dominated environment observed at diffuse vents. 2.4. Discussion
2.4.1. Alteration of Basaltic Crust
Seafloor surface interaction of basalt with warm, diffuse hydrothermal fluids at
Axial Volcano and the MEF resulted in only minor alteration and chemical changes.
The older MEF site shows more extensive vesicle linings and thicker surficial coatings than those from Axial. Saponite and nontronite are the most abundant alteration minerals at both sites. In addition to these, an Mg-silicate and particulate sulfur were abundant phases at Axial, whereas Mn-oxide was an abundant phase at the MEF. A more diverse mineral assemblage was observed at the MEF, possibly due to the older lava ages (> 5000 years versus 5 years) and presumed longer period of interaction with diffuse, warm fluids. However, the young Axial basalts were 1 % altered, whereas the
MEF basalts were only 4 % altered.
We use the observed mineral assemblages in combination with geochemical modeling results, to infer the conditions under which the basalts were altered at Axial
Volcano. Fe-oxyhydroxides are indicative of oxidative alteration (Hunter et al., 1998), consistent with model observations where goethite precipitated from an oxygenated mixed diffuse fluid-seawater solution. This phase was identified at most sites and typically precipitates from diffuse fluids that transport little or no HaS (Hekinian and
Fouquet, 1985; Tunnicliffe et al., 1986). Fluids from Axial Volcano generally have high
HeS/Fe ratios (Table 2.1). Cloud fluids have relatively high Fe and low HeS with Fe- oxyhydroxides present, whereas none were observed at Snail and Marker 33 vents, where fluids have very low Fe and high HeS. Saponite and nontronite precipitate under more reducing conditions (Hunter et al., 1998), consistent with modeling results (Figs.
2.3 and 9.4). Zeolites are typical low-temperature alteration phases (Coombs et al., 1959) and precipitated in the models under both reducing and oxidizing conditions. The
presence of anhydrite indicates that fluid temperatures were once much hotter than
those measured during fluid collection, as temperatures > 150 OCare required for
anhydrite precipitation (Bischoff and Seyfried Jr., 1978). Indeed, anhydrite precipitation
was limited to temperatures > 125 OCin the speciation models, and hotter fluids may
have vented at Axial after the eruption (Butterfield et al., 1997; in press). Water-rock
reaction is required for clay and zeolite phases to precipitate in significant quantities at
Axial Volcano. Other phases such as talc (Mg-silicate), goethite (Fe-oxyhydroxides) and
anhydrite precipitated directly from diffuse fluids or from fluids mixing with seawater.
Particulate sulfur, a phase not predicted but observed, may require bacterial action to
precipitate (Butterfield et al., in press), or may result from the incomplete oxidation of
HeS. These processes were not considered in the models. In diffuse fluids at Axial, there
is generally insufficient Fe in solution to bind HaS in order to produce metal sulfides,
leaving HeS free to be oxidized and particulate sulfur to form by
Has + '/z 02-+ S(s)+ He0
Particulate sulfur is also observed suspended in the diffuse fluids (Butterfield et al., in press). Thus, precipitation directly from diffuse fluids; from the mixing between diffuse
fluids and seawater; and from water-rock reaction is required to produce the observed
alteration assemblage.
Chemical changes are also compared to geochemical modeling results and the alteration assemblage at Axial to determine if elemental sources and sinks can be
identified. In each model, changes in bulk fluid composition were tracked as reaction occurred, showing that the concentrations of SiOq,,), Mn2+,Few and Mgw decreased in
solution in mixing models as the amount of seawater present increased. This was due either to their dilution with seawater, or their consumption by precipitating minerals
whose abundances increased with increasing seawater concentration. In the water-rock
models, concentrations of these elements increased as the water-rock ratio increased at
ratios greater than - 40: 1. At ratios less than this, concentrations of these elements
remained relatively unchanged. Axial basalts show a loss of SiOe and Fen03 from
chemical change calculations, suggesting that these losses occurred at diffuse fluid-
seawater mixing ratios > 60:40 and/or water-rock ratios > 40: 1. The loss of FeeOs, determined by chemical change calculations, is consistent with the observation in the diffuse fluids that there is evidence of dissolution of an iron phase (Butterfield et al., in press). No Mn phases were observed at Axial, consistent with the fact that MnO was immobile in the chemical change calculations. This is not consistent with modeling results, which shows Mn phases precipitating during diffuse fluid-seawater mixing. No trend was observed in the chemical change of Mg at Axial, but Mg-silicates and saponite were in the observed assemblage, suggesting that the Mg in these minerals was derived from seawater. In the models, Mg was consumed by saponite and/or talc.
Finally, Ba was generally lost from basalts at Axial and no barite was observed, inconsistent with the models, where barite occurred in all model types. Village vent was the only vent in which altered rock gained Ba and barite was observed.
At the MEF, the alteration assemblages may be compared to the chemical changes that occurred during alteration. At all vents, basalt lost SiOe and FeeO3, MnO and Ba were gained by altered rock, whereas MgO was variable. SiOe and Fee03 may have been taken up the precipitation of saponite and nontronite, whereas Mn was taken up from the fluid into the Mn-oxides. Saponite consumed MgO, whose variable chemical change indicates that Mg was supplied by both the fluid and rock at the seafloor. These chemical changes are consistent with results from water-rock reaction experiments,
where reaction at 70 OCand 1 bar resulted in a loss of SiOe and a gain of Mg and Ba
(Mottl, 1983; Seyfried Jr., 1979). Amorphous silica was identified at several MEF sites,
which is thought to precipitate from neutral fluids that are conductively cooled and have
seen minimal mixing (Haymon and Kastner, 198 1; Janecky and Shanks 111, 1988;
Juniper and Fouquet, 1988). The presence of chlorite and anhydrite indicates that
diffuse fluids were once much hotter than those measured during fluid collection.
Earthquake disturbances at the MEF have been shown to dramatically affect
hydrothermal venting and may be the cause of increased fluid temperatures (Johnson et
al., 2000; Liiley et d., 2003).
9.4.2. Chemical Fluxes
The direction and magnitude of chemical fluxes between seawater and the ocean
crust were calculated to determine if the mass flux associated with near-surface
interaction between diffuse fluids and basalt is significant to global geochemical budgets
at Axial Volcano. Local chemical fluxes associated with Axial Volcano were calculated
using the average chemical changes for basalts from Cloud, Marker 33 and Snail vents,
combined with the estimated average volume of the 1998 eruption (4.7 x 107 m3;
(Embley et al., 1999), crustal density (2800 kg/m3), our estimate of the extent of
alteration (1 %) (based on results of chemical change calculations) and basalt age (5.5. years). In addition, we assumed that 5 % of the lava flow was in contact with diffuse fluids, based on the visual estimate of the abundance of diffuse vents at the seafloor in this region (Butterfield, pers. comm. 2004). The resulting flux is termed the on-axis basalt flux, which represents the local flux of chemical species resulting from diffuse fluid-rock interaction at the seafloor at Axial Volcano (Table 2.8). No flux was
calculated for Al, Mn, Ca and P, as little or no chemical change in these elements was
evident for the altered basalts. Results of the Axial basalt flux calculations show that
the breakdown of basalt at the seafloor by low-temperature alteration processes releases
Fe and Mg and consumes Si, Na, K, Ba and Sr (Table 2.8).
The chemical changes in basalts at Axial Volcano were extrapolated to a global
scale by combining these with the global crustal production rate (3.45 x 106 mVyr)
(Parsons, 198 1); crustal density (2800 kg/m5); thickness of layer 2 basalts at mid-ocean
ridges (600 m) and the global estimated abundance of hydrothermal activity along mid-
ocean ridges (40 %) (Baker et al., 1996) (e.g. Bach et al., 2003). We assumed that 5 % of
the global ridge system is hydrothermally active with respect to diffuse venting and that
basalts were 1 % altered. Results of this calculation show that global on-axis basalt
fluxes resulting from the interaction of basalt with low-temperature diffuse fluids are on average 2-3 orders of magnitude greater than the local on-axis basalt flux at Axial
Volcano (Table 2.8).
Another approach to evaluate on-axis chemical fluxes utilizes diffuse fluid compositions, which record the integrated chemical exchange along the entire hydrothermal reaction path, as diffuse fluids are primarily the result of high temperature fluids mixing with seawater in the subsurface. A cumulative on-axis fluid flux was calculated using the diffuse fluid chemistry to determine the difference in elemental concentrations between the diffuse fluids and seawater (e.g. M. Mottl in (Kadko et al.,
1994)), combined with the estimated global water flux associated with 20 OCdiffuse venting in the axial region (6.4 x 1014 kg/yr) (Schultz and Elderfield, 1997) (Table 2.8).
The on-axis diffuse fluid flux, which represents primarily high temperature fluid reactions, is a source of Mg and Na to the crust and a sink for Si, Fe, K, Ba and Sr. Mg and Na are taken up into the crust by the formation of chlorite and the breakdown of plagioclase, respectively, at high temperatures in the subsurface. Comparison of these fluxes with the on-axis basaltic flux show that fluxes of Fe and Na are in the same direction; however cumulative fluid fluxes are 2-5 orders of magnitude greater than those associated with the on-axis basaltic flux (Table 2.8). Table 8: Comparison of flux calculations in the Axial region to ODP Hole 504B.
On-Axis On-Axis Cumulative On-Axis ODP Hole 504B Basaltic Fluxa Basaltic Fluxb Fluid Fluxc Basaltic Flwd (local scale) (global scale) (global scale) (global scale) 5.97Ef04 1.45E+07 -6.87Efll 3.10E+ 1 1 -l..7lE+05 -3.18E+07 -$.09E+O9 1.80E+ 11 -3.7 1E+04 -8.99E+06 2.90E+ le $.08E+ 11 1.35E+05 3.87E+07 1.05E+13 e.5o~+lo 1.$?7E+04< 3.08E+06 -l.S5E+ll 6.50E+ 10 e. 1 sE+02 5.28E+04 -8.19E+08 n.d.
n.d. indicates that no data was available. a Flux (mol/yr) = chemical change (mol/kg) * crustal density (2800 kg/m3) * extrusion volume (m3)* 5% * 1% / basalt age (5.5 years). Positive values indicate a gain to the crust, negative values a loss. bF1ux (mol/yr) = chemical change (mol/kg) * crustal production rate (3.45 x lo7m3/yr) * crustal density (2800 kg/m2) * thickness of layer a at the ridge (600 m) * 5 % * 1 % * 40 %. Fluid chemistry from Butterfield et al. (in press) dData from Alt et al. (1996) The next step is to assess the significance of these global on-axis basalt and fluid
fluxes to those that result from low temperature alteration in the off-axis environment.
On-axis basaltic and fluid fluxes were compared to those calculated for the volcanic
section of ODP Hole 504B where basalts are - 6.9 Ma old and have been affected by
low-temperature alteration processes. Chemical fluxes were calculated by measuring the
abundance of all rock and alteration types and veins in drill cores and combining these
with chemical analyses of these in a mass balance (Alt et al., 1996). The off-axis basaltic
fluxes are significantly larger than the on-axis basaltic flux. Si, Na and K are taken up
by the crust during low-temperature alteration both on and off-axis (Table 2.8). The
magnitudes of off-axis basaltic fluxes are comparable to the on-axis cumulative fluid
flux. Both show an uptake of Mg and Na by the crust, however, the magnitude of the
fluxes due to high-temperature reactions are larger for these elements (Table 2.8). In
the off-axis, altered basalt from Hole 504B actually lost Mg, however, the Mg was
retained by the crust in saponite veins (Alt et al., 1996). The cumulative fluid fluxes
show a loss of Si, Fe and K from the crust, whereas Hole 504B shows an uptake of these elements, which were incorporated into low-temperature clay phases (Alt et al., 1996).
Results of chemical exchange calculations show that low-temperature alteration at the seafloor in the axial region is not a significant source or sink for most major elements, Ba and Sr. The direction of Fe and Na fluxes in the axial region do correlate between the on-axis basaltic and cumulative fluid fluxes, suggesting that these elements are lost from the crust in the axial region due to both high and low temperature reactions. 2.5. Conclusions
Diffuse, low-temperature hydrothermal venting at Axial Volcano and the Main
Endeavour field resulted in only minor alteration of basalts and chemical change at the
seafloor. At the more mature (5000-8000 years) MEF site (< 4 % altered), basalt
coatings were thicker and had a more diverse mineralogy than the young (5.5 years)
Axial site (< 1 % altered), presumably due to a longer period of low-temperature water-
rock interaction. The presence of trace high temperature alteration minerals at many
sites indicates that diffuse fluids were once much hotter than at the time of fluid sample
collection. Chemical changes in basalts from both young and mature sites are small,
implying that basalt alteration by low-temperature diffuse fluids is slow and the
contribution of low-temperature reactions at the axis to global geochemical cycles is
small. In contrast, chemical fluxes to the ocean from the diffuse fluids are significant,
which indicates that high temperature reactions at depth in the crust are more
important than low-temperature reactions at the seafloor, to crust-ocean exchange.
Geochemical modeling of diffuse fluids from Axial Volcano shows that the observed alteration mineral assemblages developed by precipitating directly from diffuse fluids, from mixed diffuse fluid-seawater solutions and by the reaction of basalt with diffuse fluids. Particulate sulfur and anhydrite precipitated directly from diffuse fluids, whereas clays, zeolites and talc precipitated from mixtures of diffuse fluid and seawater and as a result of reaction with basalt at water-rock ratios > 200: 1.
Geochemical model results are a step toward understanding the early stages of basalt alteration at the ridge axis. Additional studies need to address how variation in fluid compositions and temperatures may affect the chemical fluxes at the ridge axis. Chapter 3
3. HYDROTHERMAL MANGANESE OXIDE DEPOSITS FROM BABY
BARE SEAMOUNT IN THE NORTHEAST PACIFIC OCEAN
3.1. Introduction
Circulation of seawater through basaltic ocean crust is one of the largest geochemical cycles on earth and significantly alters the chemical and biological states of the crust and oceans. In the ridge-flank environment, seawater convection driven by heat associated with the cooling of the lithosphere may persist for millions of years, and removes more than 20% of the total heat flux through the seafloor (Elderfield and
Schultz, 1996; Mottl et al., 1998; Stein and Stein, 1994). Basement topographic highs and seamounts are common features in the ocean basins (Villinger et al., 2002; Wessel,
2001) and may act as sites for crustal fluid discharge (Mottl and Wheat, 1994; Mottl et al., 1998; Wheat and McDuff, 1995) or recharge (Fisher et al., 2003). Baby Bare seamount is one such topographic high, located on the eastern flank of the Juan de Fuca ridge, where warm, diffusely venting fluids vent naturally from the summit (Mottl et al.,
1998). Baby Bare is one of several isolated seamounts in the area, which rise above the regionally continuous sediment cover. The presence of basement outcrops such as Baby
Bare has important consequences for regional heat flow and hydrothermal circulation as they may act as direct conduits between the oceanic lithosphere and ocean (Becker et al.,
2000; Mottl et al., 1998). On the eastern flank of the Juan de Fuca ridge, the presence of these outcrops has resulted in the delay of basement sealing and prolonged off-axis advective heat loss from the oceanic crust (Becker et al., 2000). Baby Bare seamount is of interest because it is the only known seamount older than 1 Myr that is hydrothermally active (Mottl et al., 1998; Wheat and Mottl, 2000).
There is excellent regional knowledge of the area, with investigations involving heat flow surveys (Davis, 1992, 1999), fluid-sediment interaction (Buatier et al., 2001;
Giambalvo et al., 2002), crustal permeability (Becker and Fisher, 2000; Fisher et al.,
1997)~crustal seismic stratigraphy (Rohr, 199+), fluid and geochemical transport
(Elderfield et al., 19991; Wheat et al., 1996,2000, 200.3)~trace element chemistry of pore fluids (Monnin et al., 2001; Mottl et al., 2000) and basalt alteration (Hunter et al., 1998;
Marescotti et al., 2000). There have also been studies of geophysical and geological properties of Baby Bare (Becker et al., 2000; Karsten et al., 1998) and the heat flow and hydrothermal venting associated with the seamount (Mottl et al., 1998; Wheat and
Mottl, 2000; Wheat et al., 2002).
For this study, manganese oxide crusts were recovered from Baby Bare seamount to investigate their origin and the history of hydrothermal venting. Mn-oxide crusts from Baby Bare are stratabound deposits (cementing sediments or lying within sediments) and found near the summit of the seamount where natural warm springs have been observed. We will show, using mineralogical, chemical and morphological criteria, that these crusts are hydrothermal in origin and estimate the longevity of hydrothermal activity at Baby Bare Seamount.
3.2. Regional Setting
3.2. r. Baby Bare Geologic Setting
Manganese oxide deposits were recovered from the summit of Baby Bare
Seamount, on the eastern flank of the Juan de Fuca ridge, in the Northeast Pacific (Fig. 3. IA). Baby Bare seamount formed by off-axis volcanism at - 1.7 Myr, sits on top of a
3.5 Myr old buried basement ridge that parallels the Juan de Fuca ridge axis and rises
70 m above the regional sediment cover (Davis et al., 1997). The eastern flank of the
Juan de Fuca plate was buried rapidly by continentally derived turbidites and muds
(Davis, 1992) which consist of quartz and clay minerals such as kaolinite, illite, chlorite
and saponite (Buatier et al., 2001; Underwood and Hoke, 2000).
The eastern flank of the Juan de Fuca ridge has been an area of interest mainly
for regional heat flow and hydrologic studies. It was first investigated for these
properties with the FlankFlux study in 1992 (Davis, 1992), and subsequently drilled in
1997 by the Ocean Drilling Program (ODP) Leg 168 with a transect of 10 holes across
the flank (Davis et al., 1997). Results of these studies show that the regional sediment
cover insulates the crust and permits fluids to circulate in the basement at elevated
temperatures for prolonged periods of time (Butterfield et al., 1997; Davis, 1992; Mottl,
2002; Thomson et al., 1995). Baby Bare seamount acts to focus and vent crustal fluids in
the region, which vent naturally near the summit through lavas and a thin (<0.7 m)
sediment cover (Mottl et al., 1998). Natural warm springs were discovered in 1998, -
50 m south of the summit where 25 OCbasement fluids vented that had cooled conductively from - 63 OC(Davis et al., 1997; Mottl et al., 1998). Although no discharge was observed during expeditions to Baby Bare in 2002 and 2003, < 15 OCfluids were emitted from sediments once they were disturbed (Johnson, 2003). Fisher et al. (2003) determined that fluids venting from Baby Bare were recharged at Grizzly Bare seamount, - 52 km to the southwest, found along the same buried ridge.
As fluids upwell through sediments at the summit of Baby Bare they precipitate
Mn-oxide layers and crusts (Wheat and Mottl, 2000) up to 0.5 meters thick (extent visible above the sediments). Samples of this Mn-oxide material were collected in the once actively venting area in 2002 using the R/V Thomas G. Thompson (TN-158)
(Johnson, unpublished cruise reports, 2002) and in 2003 with R/V Atlantis (voyage 7, leg 2o), using the remotely operated vehicle Jason I1 (Fig. 3.1B). Figure 3.1: A: Location of Baby Bare seamount on the eastern flank of the Juan de Fuca
Ridge in the Northeast Pacific Ocean. B: Location of natural warm springs observed in
1998 and the area of Mn-oxide crust sampling near the summit of Baby Bare seamount
(Becker et al., 2000). 3.3. Sample Suite
The summit area of Baby Bare is primarily sediment covered with few outcrops
of basalt and several outcrops of Mn-oxide crust (Fig. 3.2A and B). Pieces of Mn-oxide
of varying size (approximately 5 to 30 cm) also lie loosely on top of sediment in some
areas. Sediment colour ranged from light to dark brown on the surface, but appeared
grey below the seawater-sediment interface. Biological organisms such as spider crabs,
crinoids, brittle stars, and purple Pacific octopus are found in the area, indicating that
the area is still hydrothermally active. The Mn-oxides here are primarily stratabound
(Hein et al., 1990), where they partially cement sediments that overly the summit of
Baby Bare.
Six Mn-oxide crust samples were recovered from near the summit area where natural springs were venting in 1998 (Table 3.1). Samples 04-02, 04-03 and 62-04 were broken off Mn-oxide outcrops while samples 05-04,05-05 and 63-1 1 were lying loose on or in sediments. All crusts are hard and bluish-black to grey-black in colour with a submetallic luster, typical of many hydrothermal crusts (Eckhardt et al., 1997) and are dense with low porosity (< 5%). Upper surfaces of crusts are coated with a thin (< 1 cm) layer of light brown sediment and patches of biological material. In addition, most samples are coated in a thin (< 3 mm) non-shiny, Mn-oxide coating that may be hydrogenous. Lower surfaces are coated with a thin (< 1 cm) coating of dark brown sediments. Some sediment has been incorporated into the Mn-oxide crusts in small clumps, presumably as the oxide accumulated. Small (< 1 cm in diameter and thickness) patches of green and yellow Figure 3.2: A: Extensive bulbous Mn-oxide deposits near the summit of Baby Bare seamount covered by biological organisms, including crabs, crinoids, brittle stars, rays and Pacific octopus. B: Outcrop of
Mn-oxide deposits. Note arm of ROV Jason in lower right. Table 3.1: Sampled Mn-oxide crust locations at Baby Bare seamount.
Sample # Depth (m) Latitude (W) Longitude (OW) 52-04-02 2596 47 42.596 127 47.140 52-04-03 52-05-04 52-05-05 52-62-04 52-63-11 clays and orange Fe-oxyhydroxides are also found in thin coatings on the upper surfaces of several samples.
Four samples (04-02,04-03,05-04 and 63-1 1) have a massive texture (Fig.
3.3A); sample 63-1 1 had clumps of radiating fibers on fresh interior surfaces and four fine (< 0.5 mm thick) laminations the base of the crust. Two samples (05-05 and 62-04) are completely laminated and were subdivided for chemical and mineralogical analysis on the basis of texture and colour. Sample 05-05 was divided into 2 samples: dark grey massive outer ride (05-05A) and an inner core that consisted of grey clumps of fine radiating needles (05-o~B).Sample 62-04 was divided into 3 sub-samples (Fig. 3.3B): -
1 cm thick top horizon (62-04T) which consisted of 6-10 bluish-black discontinuous laminations < e mm thick; a grayish-black more massive middle horizon 2.5 cm thick
(62-04M) and a black bottom horizon that had a cusp-like texture and consisted of 2 laminations < 4 mm thick (62-o~B).There is no relationship between texture of Mn- oxides and whether or not they were collected from outcrops or from in or on sediments. Figure 3.3: A: Massive Mn-oxide crust (sample 63-1 1).
Radiating fibers of todorokite are visible on the fresh surface. Sample is '7 cm long and 3.5 cm wide. B: Sub- division of sample 62-04 into three horizons based on textural and colour differences. Sample is 11 cm long and
6 cm wide. 3.4. Analytical Methods
All Mn crusts were analyzed in bulk, with the exception of 05-05 and 62-04,
which were sub-divided on the basis of colour and texture (see Section 3.2). For
preparation of all samples, sediment and hydrogenous Mn material on the outer surfaces
were avoided, as were sediments that were incorporated into crust interiors during
growth. Samples were ground into a fine powder using a tungsten carbide ring mill and
an agate pestle and mortar.
Major elements and select trace elements (Co, Cu, Ni, V and Zn) were analyzed by X-Ray Fluorescence (XRF) with a Philips PW24.40 4 kW automated XRF spectrometer at McGill University in Montreal, Qukbec. Fused beads were prepared from a 1:5 samp1e:lithium tetraborate mixture. Accuracy is within 0.5% for silica, 1% for other major elements and 5% for trace elements; precision is within 0.5% relative for each element. Other trace elements and the REEs were analyzed by Inductively-
Coupled Plasma Mass Spectrometry using a Thermo Instruments PQII ICP-MS with a
Gilsonm auto-sampler and peristaltic pump at the University of Victoria in Victoria,
British Columbia. Dissolution procedure follows the hotplate methods of Taylor et al.
(2002)~except HF was added only once during dissolution. Final solutions were mixed to a final weight of 50 g in polycarbonate Falcon tubes using 1% HNOs and 1 rnl of a complexing agent consisting of 0.1 IN HF, 0.45N Boric and o.22N Oxalic acids.
Reproducibility for Sr, Ba, Zr, Na, Cr, Hf, Th, U and most REE's is < 10% (Eu, Tb, Ho,
Tm and Lu were 20-55%) and 14-28% for Sc, V, Cr, Ni, Rb, Cs (Appendix C). Barium occurs in high concentrations in these crusts and has a high oxide bonding energy which causes interferences in the determination of Eu (Dulski, 1994). Eu concentrations were corrected by subtracting the measured Ba160 from the total measured Eu. This
resulted in Eu being below detection limits for all samples except 62-04B.
Bulk mineralogy was determined using a Siemens D5000 powder X-Ray
Diffractometer at the University of British Columbia, in Vancouver, British Columbia.
Samples were run at 40 kV, 30 d Cu-K alpha with a monochromatized scan from 3-50
2 08 and were run glycolated and unglycolated to identify clay phases.
3.5. Results
3.5: 1. Mineralogy
All samples are composed primarily of ~oAmanganate todorokite
CMn"4Mn"+aOle 3 (H20)] which has characteristic x-ray diffraction peaks at 9.6 A,
4.8 and 3.4 A. Pyrolusite CMn++Oa] is also a dominant phase in samples 62-04 and
63-1 1, with peaks at 3.13 A, 2.41 A, 2.21 A, 2.1 1 A and 1.98 A. Minor phases include saponite and nontronite in many crusts (saponite was identified in all but 05-05 and nontronite in 05-04 and 62-O4M) as well as barite, which was identified in sample 05-
05A and may be present in samples 04-02, 04-03 and 05-04. Identification of barite in the latter samples was inconclusive as the primary barite peaks were missing or very small. Of the samples that were subdivided, 62-o4B had a slightly larger primary pyrolusite peak and smaller primary todorokite peak than 62-04T and 62-o4M while only 05-05A was analyzed for XRD.
Todorokite and pyrolusite are Mn-oxide minerals that are common in low- temperature hydrothermal deposits. Both form a framework structure containing tunnels and chains; todorokite forms platy or fibrous morphologies and the tunnels are large enough to incorporate trace elements such as Ba, Sr, Co, Cu and Ni. The mechanism of todorokite formation is not well understood (Post, 1999) and some experiments suggest that biological processes may play a role in its precipitation
(Mandernack and Tebo, 1993). Pyrolusite commonly forms acicular crystals, is usually almost pure MnOe and has tunnels that are too small to incorporate trace elements. It is the most abundant and stable polymorph of MnO2 and precipitates in low-temperature hydrothermal deposits or as replacements after other Mn-oxide minerals such as ramsdellite and manganate (Post, 1999).
3.5.2.Bulk Chemistry
Baby Bare Mn-oxide crusts have relatively uniform compositions with respect to the major elements (Table 3.2). Mn and Fe concentrations range from -39 to 54 wt. % and
- 1.7 to 2.6 wt. %, respectively. Mn and Fe are highly fractionated, with Mn/Fe ratios ranging from -7 to 32. Si and Fe are slightly higher in samples 04-09, 04-03 and 05-04, which is likely due to incorporation of more sediment, relative to other samples as XRD peaks for nontronite are the strongest in these three samples. Si/Al ratios generally range from -4 to 9, similar to sediments (- 1 to 6.5) collected from hole 1026C close to the Baby Bare edifice (Buatier et al., 2001). Table 3.2: Maior and trace element chemistrv for Mn-oxide crusts. Sample # Analysis bulk bulk bulk rind core ~OD middle bottom bulk Si Ti Al Fe Mn Mg Ca Na K P LO1 MnIFe SVAI Cr Ba Cu Ni Zn Co v Sc Rb Sr Y Zr Nb Cs La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu H f Ta Th U 0.87 3.22 * are within the range that is typical of hydrothermal crusts, as hydrogenetic values are generally > 3000 ppm (Ingram et al., 1990). However, it is not uncommon to see a wide range of concentrations of these elements (Hein et al., 1996). Crusts have variable Co/Zn ratios (0.3-1.35) and Co concentrations are < 300 ppm. Of the trace elements, the most variation is observed in Ba, where concentrations range from -2400 to - 16 000 ppm. The highest Ba concentration is observed in sample 05-05, which includes barite as a minor phase (Table 2). Total REE concentrations are low with C REE ranging from - 13 to 52 pprn The crusts are LREE enriched with L~N/S~Nratios ranging from 2.72 to 4.28 and D~N/Y~N ranging from 0.45 to 1.12. Crusts exhibit a prominent negative Ce anomaly (Ce* = ceN/(La~+ Pr~)/2),with values of Ce* ranging from 0.3 1 to 0.86 (Fig. 3.4). Sample 62- 04B also shows a prominent negative Eu anomaly (note that Eu is below detection limits in the other samples, which suggests that all samples have a negative Eu anomaly). Of the sub-divided samples, there is slightly more Cu and Ni and slightly less Ba, Zn and Co in 05-05A compared to 05-05B. Of the three horizons analyzed for sample 62-04, the top and bottom are more alike in their major and trace element chemistry than the middle. Concentrations of Ba, Cu, Ni, Zn, Co, V, Rb, Y, Zr, Nb and REE's are slightly higher in the top and bottom than in the middle section. Inter-element relationships were also examined in the Mn-oxide crusts. Positive correlations were observed between Mn and Co, Zn, Sr and Ba and between Fe and Si, Ti, Al, and Cu. Negative correlations were observed between Mn and Fe, Ti, Zn, Ni, Cu, Ce* and V. Figure 3.4: Chondrite-normalized rare earth element plot for Mn- oxide crusts from Baby Bare seamount (Sun and McDonough, 1989). All samples but 04-02 and 05-04 have pronounced negative Ce anomalies; sample 62-04B also has a strong negative Eu anomaly; Eu in all other samples is below detection limits. 3.6. Discussion 3.6.1. Classzfication as Diagenetically Influenced Hydrothermal Crusts Manganese oxide deposits occur in many tectonic settings throughout the world's oceans. These can be classified as diagenetic, hydrogenetic or hydrothermal in origin, based on chemical composition and tectonic setting (Hein et al., 1997). Diagenetic deposits typically form nodules that precipitate slowly (< 1 mm/Myr) from un-enriched, diagenetically altered sediment pore waters along continental margins in shallow waters or on the abyssal seafloor, away from hydrothermal sources (Bonatti et al., 1972; Calvert and Price, 1970; Klinkhammer et al., 1982; Manheim and Lane- Bostwick, 1988). Hydrogenous crusts also precipitate slowly (< 10 mm/Myr), but do so directly from seawater in the form of crusts or pavements, precipitating on sediment- free, hard substrates (Halbach et al., 1983; Ingram et al., 1990; Manheim and Lane- Bostwick, 1988). Finally, deposits may be classified as hydrothermal, where Mn-oxides precipitate rapidly (typically > 1000 mm/Myr) and directly from low-temperature hydrothermal fluids (Hein et al., 1997; Ingram et al., 1990). These deposits typically have a laminated texture and are stratabound (cementing sediments) (Hein et al., 1997). Hydrothermal Mn-oxide deposits have been recovered from volcanic arc settings (Bolton et al., 1988; Cronan et al., 1982; Hein et al., 1990), back arc spreading centers (Halbach et al., 1989; Herzig et al., 1990) and mid-ocean ridges (Grill et al., 198 1). They have also been sampled from midplate hot spot volcanoes associated with calderas of Hawaiian volcanoes (De Carlo et al., 1983) and at the McDonald, Pitcairn and Teahitia- Mehetia hotspots in the South Pacific (Hodkinson et al., 1994; Puteanus et al., 1991; Stoffers et al., 1993). The majority of these deposits are iron-rich oxides, with more manganese-rich deposits found along active Hawaiian rifts associated with midplate seamounts (Hein et al., 1996) and at Palinuro Seamount, in a semi-enclosed basin in the Tyrrhenian sea (Eckhardt et al., 1997), both seamount settings. Of the three types of Mn-oxide deposits, hydrogenous crusts are most easily identified using chemical characteristics (Varnavas et al., 1988), whereas it is difficult to distinguish hydrothermal and diagenetic deposits based only on mineralogical and chemical properties. For example, hydrogenous crusts are primarily composed of amorphous manganese oxide (6-MnOz or vernadite) and iron-oxyhydroxides. These crusts have Mn/Fe ratios near 1, high trace metal (> 3000 ppm typically) and REE concentrations, and positive Ce anomalies (Bolton et al., 1988; Hein et al., 1996, 1997; Ingram et al., 1990; Lonsdale et al., 1980; Toth, 1980; Usui and Nishimura, 1992; Usui and Someya, 1997). In contrast, diagenetic and hydrothermal crusts are composed of todorokite and/or birnessite, have high Mn/Fe ratios (> 10) and low trace metal contents; (Bolton et al., 1988; Cronan et al., 1982; Hein et al., 1994, 1996; Usui and Nishimura, 1992; Varentsov et al., 1991). Due to these similarities, it is necessary to use information on crust morphology, tectonic setting and growth rate to distinguish between diagenetic and hydrothermal origins (Kuhn et al., 1998; Varnavas et al., 1988). Manganese oxide crusts from Baby Bare are first classified on the basis of the relative abundance of Mn, Fe and select trace elements. The most common discrimination diagram for Mn-oxides uses the relative abundances of Mn, Fe and (Co + Ni + Cu) x 10 (Bonatti et al., 1972). Baby Bare crusts plot where the diagenetic and hydrothermal fields overlap, at the Mn pinnacle (Fig. 3.5A), which may reflect both hydrothermal and diagenetic influences on these crusts. To distinguish between hydrothermal and diagenetic sources for the Baby Bare deposits, another commonly used ternary diagram was used (Toth, 1980). The majority of Baby Bare crusts plot in the hydrothermal field with some crusts trending towards the diagenetic field (Fig. 3.5B). This trend may either represent a real diagenetic influence on these crusts, or may simply illustrate the increased Si and Fe contents in samples 04-02,04-O.!? and 05- 04 due to the higher concentration of clay phases. Textural evidence also points to a hydrothermal origin. The layered structure of these crusts is common in hydrothermal oxide deposits, reflecting variation in intensity of hydrothermal discharge (Lalou, 1983), or changes in acidity or redox conditions during growth of the crusts (Eckhardt et al., 1997). The stratabound nature of the crusts is also consistent with a hydrothermal origin (Hein et al., 1997). Chemical, mineralogical and textural properties and information on the tectonic setting of Baby Bare indicate that the Mn-oxide crusts formed by hydrothermal processes. In particular, the chemistry of the Mn-oxides is consistent with average chemical compositions of hydrothermal crusts collected in other tectonic settings (Table 3.3). Based on fluid chemistry, it has been predicted that spring and pore fluids have both a crustal and sediment source for some elements (Mottl et al., 2000; Wheat et al., 2002), which may be reflected in the crusts that precipitated from these fluids, further discussed below. The diagenetic influences on the Mn-oxide crusts from Baby Bare warrant their classification as diagenetically influenced hydrothermal Mn-oxide deposits. Figure 3.5: A: Classification of Mn oxide deposits based on Mn-Fe- (Cu+Co+Ni x 10) from Baby Bare seamount as a: diagenetic; b: hydrogenous (Hein et al., 1992) or c: hydrothermal. The diagenetic and hydrothermal fields are after Bonatti et al. (1972) and these fields overlap. B: Classification of Mn oxide deposits based on Mn - Fe - Si x 2 (Toth, 1980) as a: diagenetic; b: hydrogenous or c: hydrothermal for samples from Baby Bare seamount. Table 3.3: Average chemical compositions of hydrothermal Mn-oxide deposits from various locations in the Pacific Ocean, and the Palinuro seamount in the Tyrrhenian Sea. Lau Locatio Baby Hawaii Valu Fa Tonga Mariana Pitcairn Explorer Palinur Basin n Barea Riftb Ridgeb A~C~ b c 1slandd Ridgee o smtf Depth 0 Si Ti Al Fe Mn Mg Ca Na K P MnIFe Ba Cu Ni Zn Co La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Total Major elements are reported in weight percent, trace elements in ppm a: This study b: Hein et al. (199'7) c: Cronan et al. (2002) d: Glasby et al. (1997) e: Grill et al. (1981) f: Eckhardt et al. (1997). 3.6.2. Elemental Sources and Growth- Conditions Manganese oxide crusts recovered from Baby Bare precipitated within and on top of sediment near the summit of the seamount, where warm spring fluids exit the seafloor through both basalt and sediment cover. The sediment pore fluids are a mixture of upwelling spring fluids and seawater, and it is from these pore fluids that the hydrothermal Mn-oxide crusts likely precipitated. In this environment, discharging reducing, warm spring fluids mix with oxygenated bottom seawater at or near the seawater-sediment interface causing a change in redox conditions (Wheat et al., 2002). This results in the oxidation of Mnw to Mn4+ in particulate form through the reaction Mnw + 00p(q)= Mnw02 which accumulates to form Mn-oxide crusts (Mandernack and Tebo, 1993; Post, 1999 and references therein). Todorokite is an end-product of Mn2+ oxidation (Golden et al., 1986) and is the primary mineral phase in Mn-oxide crusts from Baby Bare. Trace element contents are controlled by the amount available from the hydrothermal fluid and by competition between these and Mn and Fe for sites on accreting surfaces. Rapid precipitation of hydrothermal Mn-oxides and high Mn to minor-element ratios act to inhibit trace element accumulation (Toth, 1980)~which is illustrated by the negative correlation between Cu, Co and Ni (termed "grade") with growth rate (Fig. 3.6) (calculation of growth rate is discussed in the section 3.6.3). Growth Rate (mm/Ma) Figure 3.6: Comparison of Cu + Co + Ni concentrations (grade) and calculated growth rates in Mn-oxide crusts. With the exception of sample 04-03, there is a strong correlation between growth rate and grade, showing that the faster the growth rate, the more trace metals are excluded from the Mn crusts. The composition of Mn-oxide crusts from Baby Bare is in part controlled by the chemical composition of both spring and sediment pore fluids. Spring fluids that vent from Baby Bare are enriched in Mn compared to seawater, due to reactions occurring within the oceanic crust, whereas reactions within the sediment produces and consumes Mn and Fe (Wheat and Mottl, 2000). Spring fluids at Baby Bare are also enriched in Co, Ni and Zn and depleted in Cu and REE (excluding Ce) compared with seawater (Wheat et al., 2002). Spring fluids and Mn-oxide crusts have similar DYN/Y~Nratios (spring fluid is 0.85; Mn-oxide crusts are 0.45 to 1.12) and Ce* (spring fluid is 0.24; Mn-oxide crusts are 0.15 to 0.43) (Fig. 3.7). One exception is sample 05-05B, which has a Ce* of 0.03, similar to bottom seawater at Baby Bare. In contrast, L~N/S~Nratio of the spring fluid (5.5) is higher than those for the Mn-oxide crusts (2.7 to 4.3). Thus, the HREE patterns of Mn-oxide crusts were likely inherited fiom the spring fluids, whereas the LREE signature was not. Mn-oxides have a preference for LREE over HREE (De Carlo and McMurtry, 1992; Sholokovitz et al., 1994), suggesting that either the LREE signature in the crusts reflects a source other than the spring fluids, or that over time, sedimentary processes have acted to consume LREE preferentially over HREE, thus leaving LREE unavailable for adsorption onto accreting Mn-oxide surfaces. LREE may have also been preferentially leached from Mn-oxides crusts after their formation. / 1 +Bottom Seawaterat BabyBare +Spring at Marker 17 1 1 +Average Mn-oxide crust -+S.E.Pacific Bottom Water Figure 3.7: Chondrite normalized REE plot for average Mn-oxide crusts, spring fluids and Baby Bare bottom seawater (Wheat et al., 2002). Data of SE Pacific bottom seawater (2500 m water depth) from (Klinkhammer et al., 1983). Fluid values have been multiplied by 1000 to bring values up to the scale of the oxide deposits. Processes occurring in the sediment would also influence the chemistry of Mn- oxide crusts, as pore waters are mixed with discharging spring fluids. Microbial action near the oxic-anoxic interface within sediments leads to the breakdown of organic matter and mobilizes Mn in the sediment section at Baby Bare (Mandernack and Tebo, 1993; Wheat et al., 2002). Iron and the trace metals Cu, Ni and Zn are also released from the sediment into pore fluids (Wheat and Mottl, 2000). The porous and fine- grained texture of the Mn-oxides results in a high surface area that is ideal for the adsorption of trace elements and REEs (Sholokovitz et al., 1994). High Ba concentrations in sediment pore fluids, relative to spring fluids, and high sedimentary Ba (up to - 3000 ppm) (Buatier et al., 2001) likely contributed to high Ba contents in the Mn-oxide crusts. Sedimentary Ba is directly related to the amount of organic matter present in sediments, as it accumulates as poorly crystalline, Sr-rich barite within decaying organic matter or fecal pellets (Dymond et al., 1992; Paytan and Kastner, 1996). Sedimentary Ba at Baby Bare is complexed with a labile organic ligand during organic matter decay, which leads to enhanced marine barite solubility that results in high pore water Ba concentrations. Mn-oxide crusts and authigenic barite, which may be found in the sediment section at Baby Bare, may both act as sinks for this remobilized Ba (Monnin et al., 2001). Finally, the incorporation of clay phases from sediment also influences Mn crust compositions. The Si/Al ratios of Mn crusts and the surrounding sediments are similar, suggesting that the primary source of these elements to the crusts was sediments. Positive correlation between Si, Al, Fe and Ti indicates the source of these elements was the incorporation of clay phases into the crusts as they grew. By contrast, Mn and Fe are negatively correlated, supporting the assumption that these elements likely do not have a common source. 3.6.3. Calculation of Crust Growth Rates It is possible to estimate the rate at which Mn-oxide deposits precipitated using empirically-derived growth equations (Manheim and Lane-Bostwick, 1988). Here we use the "cobalt chronometer" derived by Manheim and Lane-Bostwick (1988) for hydrothermal deposits, which utilizes the concentrations of Fe, Mn and Co in Mn-oxide crusts to estimate growth rate. The main factor that influences Co concentrations in Mn-oxide crusts is the growth rate, as the flux of Co into different types of Mn-oxide crusts is relatively constant (Halbach et al., 1983). Co is adsorbed onto MnOe substrates as a result of oxidation of Co(I1) to Co(II1) upon the release of fluids to seawater. A normalized value of Co concentration (in ppm) Con, is calculated from the measured concentration of Co, whereby Con = Co x 50/ Fe + Mn and Fe + Mn = normalized per cent by weight of Fe + Mn. Fe + Mn was chosen as the normalizing factor because Fe and Mn are the building blocks of all oxide crusts and these elements are analyzed in nearly all samples (Manheim and Lane-Bostwick, 1988). The equation then used to calculate the growth rate of the Mn crusts is R = 6.8 x lo-' / in mm/Myr, which has been found to apply consistently for all types of Mn-oxide crusts (Hein et al., 1994). One limitation of the calculation is that the equation does not consider a hiatus in hydrothermal venting, therefore the calculated rates represent a maximum value and derived ages (using sample thicknesses) are minimum values (Hein et al., 1990; McMurtry et al., 1994). Calculated growth rates for Baby Bare crusts range from 324 to 1836 mm/Myr, consistent with a hydrothermal origin (i.e. > 1000 mm/Myr). Maximum crustal ages were determined using the growth rate and the thickness for each sample (Table 3.4). Ages range from 11 713 to 51 030 years. Using the observed outcrop thickness of 0.5 m, the deposit ages range from approximately 272 000 to 666 000 years, with sample 62-04B showing a maximurn age of 1.5 Myr. The oldest age of 1.5 Myr suggests that Baby Bare may have been hydrothermally active since the time it was formed. It is notable that, in general, Th/U ages of other deposits agree well with ages calculated using the cobalt chronometer and the crust thicknesses (Bolton et al., 1988; Cronan et al., 1982; Hodkinson et al., 1994; Lalou et al., 1983; Moore and Vogt, 1976). Thus, these growth rates and ages determined for hydrothermal Mn-oxide crusts from Baby Bare estimate the longevity of hydrothermal venting at Baby Bare to be at least 0.5 Myr, and possibly since the seamount was created. Table 3.6: Calculated growth rates* and ages for Mn-oxide crusts from Baby Bare seamount. Growth Rate Sample thickness Age - Crust Scale Age - Outcrop (mm/Myr) (mm) (~rs) Scale (yrs) *Growth rates calculated using the empirical equation by Manheim and Lane-Bostwick (1988). Ages at the crust scale were calculated using the growth rate and individual sample thicknesses. Ages at the outcrop scale were calculated using growth rates and the observed maximum outcrop thickness of 0.5 meter. 3.7. Conclusions Mn-oxide crusts recovered from Baby Bare seamount precipitated near the summit of the seamount from discharging, warm, hydrothermal fluids. These crusts, composed primarily of todorokite (with trace pyrolusite and barite), precipitated at the seafloor from sediment pore fluids that are a mixture of both upwelling spring and sediment pore fluids that were influenced by crustal and sedimentary reactions, respectively. In addition, Ba, Fe, Cu, Ni, Zn and Ce were liberated from the sediments due to organic matter decay. Mn-oxide crusts have inherited the HREE and Ce signature of the spring fluids, however, the LREE were affected by other processes. The hydrothermal and diagenetic influences on the disc%~argingfluids from which the Mn- oxide crusts precipitated warrants classification of the Mn-oxides as diagenetically influenced hydrothermal manganese precipitates. Growth rate and age calculations indicate that Baby Bare may have been hydrothermally active for at least 500 000 years and possibly since it was formed. Chapter 4 4. CONCLUSIONS Interaction between diffuse, warm hydrothermal fluids and basalt results in chemical exchanges between seawater and ocean crust which significantly alters the chemistry of both fluid and rock. In the on-axis environment, low-temperature fluid- rock interaction results in only minor basalt alteration (< 2%) and chemical changes in basalt at both young (Axial Volcano - 5 years) and mature (Main Endeavour field - 5000 to 8000 years) hydrothermal sites. High temperature minerals were observed at many sites, indicating that diffuse fluids were once much hotter than at the time of fluid collection. These fluids, however, were not consistently hot, as low temperature phases predominate the alteration assemblage. The mature site showed a more extensive and diverse alteration mineral assemblage and slightly more chemical change due to the presumed longer period of low-temperature basalt-fluid interaction. Chemical changes in basalts from both young and mature sites are limited, which implies that low- temperature basalt alteration at the seafloor is slow and that the contribution of low- temperature reactions at the axis to global geochemical cycles is small. In contrast, the diffuse fluid fluxes are significant, which indicates that high temperature reactions at greater depth in the crust are more significant contributors to global geochemical cycles in the on-axis environment. Geochemical modeling of diffuse hydrothermal fluid from Axial Volcano was used to determine how parameters such as fluid temperature, pH and degree of mixing and water-rock reaction influence alteration mineral precipitation. Modeling results show that alteration mineral precipitation requires a combination of minerals precipitating directly from diffuse fluids, from mixing of fluids and seawater and from diffuse fluid - basalt reaction. In these models, anhydrite precipitated directly from diffuse fluids when they were much hotter, whereas clays, zeolites and talc precipitated from mixtures of diffuse fluid and seawater (< 20 % seawater); and from reaction of basalt with diffuse fluids at water-rock ratios > 200: 1. As water-rock ratios increase, SiOe(aq),Fe2+, Mnfi and Mg" show an increase in concentration in solution. Chemical change calculations show a loss of these elements fi-om fresh basalt, suggesting that alteration did occur at high water-rock ratios. Manganese oxide crusts recovered from the summit area of Baby Bare seamount are hydrothermal in origin, as they precipitated from warm, discharging crustal fluids. These deposits are composed primarily of todorokite (with trace pyrolusite and barite) and precipitated at the seafloor from sediment pore fluids that are a mixture of both upwelling spring and sediment pore fluids that were influenced by crustal and sedimentary reactions, respectively. The diagenetic signature of the Mn-crusts comes from the incorporation of Ba, Fe, Cu, Ni, Zn and Ce, which were liberated from sediment due to organic matter decay. Mn-oxide crusts have inherited the HREE and Ce signature of the spring fluids, however, the LREE were affected by other processes. The hydrothermal and diagenetic influences on the discharging fluids from which the Mn-oxide crusts precipitated warrants classification of the Mn-oxides as diagenetically influenced hydrothermal manganese precipitates. Minimum ages of Mn-oxide crusts, using calculated growth rates (324 to - 1800 mm/Ma) and manganese outcrop thicknesses, indicate that Baby Bare has been hydrothermally active for at least 0.5 Myr, and possibly since its formation at - 1.7 Myr. FUTURE WORK Further investigation of diffuse fluid - basalt reaction is needed for many more axial diffuse sites in order to determine if seafloor basalt alteration contributes significantly to global elemental budgets from various mid-ocean ridge hydrothermal sites. Work at Axial has shown that, although the alteration and chemical changes in basalts occur slowly, the flux of certain elements may be significant when compared to high-temperature flow and that associated with older, off-axis basalts. This is intriguing and warrants investigation of seafloor, low- temperature alteration at other on-axis sites where fluid chemical data is available. Manganese oxide crusts are useful tools for estimating the longevity of hydrothermal processes at off-axis seamounts. Further work may include investigation of these deposits that have been recovered from other hydrothermally active seamounts. In addition, the diagenetic signature of crusts from Baby Bare suggests that further investigation may provide insights into organic processes occurring in the sediment column at Baby Bare. REFERENCES CITED Alt, J.C. et al., 1996. Ridge-flank alteration of upper oceanic crust in the Eastern Pacific: Synthesis of results for volcanic rocks of Holes 504B and 896A. In: J.C. Alt, H. Kinoshita, L. Stokking and P. Michael (Editors), Proc. ODP Sci. Results, 148, College Station, TX (Ocean Drilling Program), pp. 435-450. Baker, E.T., Lowell, R.P., Resing, J.A., Feely, R.A., Embley, R.W., Massoth, G.J. and Walker, S.L. 2004. Decay of hydrothermal output following the 1998 seafloor eruption at Axial Volcano: Observations and models. J. Geophys. 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Composition of pore and spring waters from Baby Bare: Global implications of geochemical fluxes from a ridge flank hydrothermal system. Geochim. Cosmochim. Acta, 64: 629-642. Wheat, C.G., Mottl, M.J. and Rudnicki, M., 2002. Trace element and REE composition of a low-temperature ridge-flank hydrothermal spring. Geochim. Cosmochim. Acta, 66: 3693-3705. Appendix A SAMPLE LOCATIONS The following table lists sample locations, water depths, diffuse fluid temperatures and information on the vent area for basalt samples collected from diffuse vents at both Axial Volcano and the Main Endeavour Field. Sample numbers are in the format (ROV) - (dive number) - (sample number), where "R is for ROV ROPOS and "52" for ROV Jason-11. Sample Latitude Longitude Depth Fluid Location Vent Description of Area No. ("N) ("w (m) T PC) Collected at edge of actively vent crack, not in direct contact with fluids at Axial Marker 33 the time of collection. Biological organisms surro~tndthe area. Collected at edge of actively venting hole, in contact with diffuse fluids at the Cloud time of collection. Hydrothermal sediments and biological organisms surround the area. Collected at edge of actively vent crack. Unknown if it was in contact with Snail fluids or not. Hydrothermal sediments and biological organisms surround the area. Collected at the top of actively venting basalt talus pile, in contact with fluids Village at the time of collection. Hydrothermal sediments and biological organisms surround the area. Collected from Eastern base of a cliff, - 20 m beneath the basalt/sulfide talus mound of the S&M black smoker. Rock was not in direct contact with diffuse MEF S&M fluids at the time of collection, but fluids were venting in the general area, which is covered with hydrothermal sediments and biological organisms. Collected from Eastern base of a cliff, - 20 m beneath the basalt/sulfide talus mound of the S&M black smoker. Rock was not in direct contact with diffuse S&M fluids at the time of collection, but fluids were venting in the general area, which is covered with hydrothermal sediments and biological organisms. Collected from atop hydrothermal sediments, - lm away from the actively venting crack in the seafloor. The general area is heavily covered with Easter hydrothermal sediments and biological organisms, however, the sample was not in direct contact with diffuse flow at the time of collection. Collected at margin of actively venting area, in contact with hydrothermal Easter sediments and biological organisms that extensively cover the area. The sample was not in contact with diffuse fluids at the time of collection. Collected from the Southwest base of the sulfide/basalt talus mound of the Milli-W black smoker. Very little diffuse flow is evident in the area, but the Milli-Q area is heavily covered with hydrothermal sediments and biological organisms. Collected from the Southwest base of the sulfide/basalt talus mound of the Milli-W black smoker. Very little diffuse flow is evident in the area, but the Milli-Q area is heavily covered with hydrothermal sediments and biological Appendix B SAMPLE DESCRIPTIONS Listed here are descriptions of basalt samples collected from both Axial Volcano (B. 1) and the Main Endeavour Field (B.2). Also listed are descriptions of Mn-oxide crusts collected from Baby Bare seamount (B.3). Tables list sample type, size and hand sample descriptions. Appendix B.l: Descriptions of basalt samples from Axial Volcano Sample Length Width Thickness Location Vent Sample Sample Description No. (cm) (cm) (cm) Large blocky piece of broken pillow basalt; (< 4 % altered); n mm thick zone of light green material penetrates the rock and is visible when the rock is broken open, followed by a zone of brown Fe-oxyhydroxide staining 1 cm thick. Rock is - 5% vesicular; vesicles are small (< 0.5 mm diameter) and unlined; sparsely phyric with plagioclase phenocrysts < a mm in length. fresh Axial Village basalt 40 15 15 Thin, discontinuous white material thinly (- lmm) coats sample exterior. Top glass layer is < 1 cm thick, bottom layer is < 4 mm thick. Patchy beige clays coated top of sample. Patches are < 2 cm in diameter and < 5 mm thick. Sample is 1-2 % fractured, fractures are unlined. Discontinuous, interior glass layer is < 4 cm thick, and pinches out on one side of the sample. Blocky piece of broken pillow basalt; (< 5% altered); sparsely phyric with plagioclase phenocrysts < 5 mm long; < 5 % vesicular; vesicles are small, (-0.5 mm), round and not infilled or lined. There are larger round vesicles < 1 cm in diameter. Exsolution front - "Lm in width, - 1 cm into the rock from the top, comprised of elongate, irregularly shaped vesicles, < a mm long and 1 mm wide. Fe-staining extends 1 cm into the interior Marker fresh of the sample. Glass layer on sample top coated in thin (- 1 Axial 16 13 l3 33 basalt mm) discontinuous, amorphous white coating. This glass layer is < 3 mm thick, whereas the bottom glass layer is < 4 mm thick. Bottom glass layer is Fe-stained, with the staining penetrating fractures in the glass. Patchy beige clay material on top glass layer, with patches being < cm in diameter and < 5 mm thick. Entire sample is 1-2 % fractured (< a mm wide and < 8 cm long) which are < 10 % filled with Fe-oxyhydroxides. Appendix B.1: Descriptions of basalt samples from Axial Volcano (continued) Sample Sample Length Width Thickness Location Vent Sample Description No. Type (4 (4 (cm) Blocky piece of broken pillow basalt. Alteration < 5 %); sparsely phyric with plagioclase phenocrysts < 5 mm long; < 5% vesicular, with pipe shaped vesicles < 1 mm wide and 3 mm long. These are partially lined with orange Fe- fresh oxyhydroxides. These Fe-oxyhydroxides coat the rock Axial Cloud basalt portion of the sample. Glass rind < 1 cm thick and upper surface has few patches of orange Fe-oxyhydroxides c 3 mm thick. Sample is - 1% fi-actured, with fractures < lmm wide and filled with dark orange Fe-oxyhydroxides. The bottom of the sample has a thin veneer of glass < 5 mm thick that was partially sloughed otT. Blocky chunk of pillow basalt. i5 % altered. with a thin (< emm thick), fresh glass rind; i5% vesicular; vesicles are round, small (ie mm in diameter) and are either unlined or partially lined with white amorphous material; rock is < 1% slightly fractured, with these partially filled with the white, Axial Snail altered amorphous material. Vesicle exsolution front - 1 cm into the basalt sample interior from the glass layer. These vesicles are < 2 mm long and i 1 mm wide. White amorphous mineralogical material partially coats most of the rock portion of the sample and tills fixtures in glass. Appendix B.2: Descriptions of basalt samples from Main Endeavour Field Sample Sample Length Width Height ,ocation Vent Site Sample Description No. Type (cm) (cm) (cm) Irregularly shaped broken pillow basalt; < 5 % altered; aphyric; 1-9 % vesicular. Exsolution front with vesicles < 5 mm long and 9 mm wide, slightly unlined found 1 cm into the interior of the rock. Small (1x4 cm) patch of MEF S.E. Hulk altered 19 15 9 black Mn-oxide on side of rock, underneath layer of glass. Glass rinds on top basalt and bottom of sample are - 1 mm thick and very fresh. Thin (< Pmm), discontinuous orange Fe-oxyhydroxide material coats sample. Various coloured oxides/clays found in 9x5 cm patch on the side of the rock (< 3 mm thick). Irregularly shaped broken piece of pillow basalt; < 5 % altered; aphyric and 1-2 % vesicular. Very fi-esh, a cm thick glass layer. Thin (< e mm), black Mn- slightly oxide material coats entire sample, including glass. Underneath this coating MEF W.Grotto altered e 1 13 1s is a thin (< 3 mm) coating of beige/golden clay material, also covering the basalt whole sample. Thin (< a mm) patch (1x1 cm) of orange Fe-oxyhydroxide found on rock, and beige/green clay found in patch 1x10 cm and < 5 mm thick. Irregularly shaped broken pillow basalt; < 10% altered; aphyric and 1-2% vesicular. Patchy black Mn-oxide (< 1 mm thick) coat portions of the rock. Glass on top and bottom of sample is fi-esh and < 2.5 cm thick. Vesicles are slightly Easter very small (< 1 mm in diameter) and unlined. Underside of the sample has altered 8'1 15 MEF Island exsolution front with vesicles < 5x9 cm in size. Side and underside of rock basalt are coated in light and dark orange Fe-oxyhydroxides i1 cm thick, and beige/white mineral also discontinuously coats the sample, in patches < 8 mm thick. Rectangular piece of broken pillow basalt; < 10 % altered; aphyric; < 5 % vesicular. Glassy rind is somewhat fresh and 3-5 mm thick, coated in a slightly Easter discontinuous, thin (< a mm) veneer of white mineralogical material on one altered ao 15 9 MEF Island side of the sample. Patches of orange Fe-oxyhydroxide coat the sample, basalt which are 2x2 cm and 9 mm thick. Also, there are very small patches of vellow clay 1x1 cm and < 1 mm thick. Appendix B.2: Descriptions of basalt samples from Main Endeavour Field (continued) Sample Vent Sample Length Width Thickness Location Sample Description No. Site Type Icm) (cm) Icm) Triangular piece of broken pillow basalt;< 10 % altered; aphyric and is < 5 % vesicular. Fresh, discontinuous glass rind < e mm thick on one side of slightly sample. Brown/red alteration coating < 1 mm thick coats half of rock. MEF altered Patches of green material, with a fuzzy appearance < e cm in diameter and basalt < e mm thick -may be biological. Also, dark brown and white mineralogical partially coats rock, < & mm thick. Most surfaces of the crystalline rock are stained with orange Fe-oxyhydroxide. Triangular piece of broken pillow basalt; < 10% altered; aphyric; < e % vesi.cular. Sample is almost entirely coated in brown/orange Mn and Fe slightly oxides < lmm thick, with patches of white material underneath this in some MEF altered places. Glass rind (< 2mm thick) is fairly fresh, except for a thin (< Qmm), basalt discontinuous orange oxide coating. One corner of sample has white/dark grey sulfates in a patch 5x9 cm in size, < 1 cm thick. Rectangular piece of broken pillow basalt; < 10 % altered; aphyric; < 5 % vesicular. Thin (< &mm)brown/black Mn mineral coats almost entire sample. Underneath this there is a patchy, thin (< & mm) beige coating. Glass is slightly altered and < e mm thick on one side of sample. slightly Beige/green clay (< lmm thick) and orange F-oxyhydroxides coat fracture MEF altered surfaces directly beneath the glass layer. Fractures penetrate I! cm into the basalt rock interior and are < 1 mm wide. Rock is < & % fractured. White mineralogical material discontinuously coats glass, and is < Imm thick. Vesicles are irregularly shaped and < 1.5 mm in diameter generally. One fragment broken off this sample has an orange oxidation halo. Broken piece of pillow basalt; < 10% altered; aphyric; < 2 % vesicular. Reaction rim penetrates 1.5 cm into sample interior and is green/yellow in colour. This rim grades into white/orange clays. Vesicles are < lmm in slightly diameter, round and unlined. Glass is somewhat altered, e mm thick and a MEF altered 29 white mineral layer is found between the rock and glass layer. Also, basalt fractures in glass are lined with white and yellow/green clay like material. Patchy orange and white clays and some black Mn oxide coat glass. These patches are < 3 mm thick generally. Rock is < & % fi-actured. Appendix B.3: Descriptions of Mn-oxide crusts from Baby Bare Seamount Sample No. Oval shaped piece of hydrothermal Mn-oxide crust. Crust has a layered texture (layers were too thin to separate). Crust is - 9 cm thick. The bottom 3 cm consists of black, non-shiny hydrogenous Mn-oxide. Crust is hard, bluish-black in colour with a submetallic luster and porosity < 5%. Sample shows cracking due to dehydration. Upper surface coated discontinuously with orange Fe-oxyhydroxides and light brown Baby N/A Mn-oxide 55 8 Bare clays, presumably bottom sediments, < 5 mm thick. Thin (< 3 mm) layer of black Mn oxide, with a bulbous texture also coats top of crust. Also, top surface of crust has patches of green/yellow clays and Fe-oxyhydroxides < 1 cm in diameter and thickness. Light brown sediments were incorporated into the sample in a horizon < 1 cm thick. Sponge organisms are found on the top of the sample. Bottom surface is coated with light and dark brown sediments < 1 cm thick. Irregularly shaped piece of hydrothermal Mn-oxide crust. Crust has a layered texture (too thin to separate) and has dehydration cracks. There are - 6 discontinuous laminations, each c 5 mm thick at the base of the crust. Crust is hard, dense and has a sub-metallic luster, is bluish-black in colour and is < 5 % porous. Sample top is Baby N/A Mn-oxide 90 5 Bare partially coated in dark brown sediments < 5 mm thick, with some sponge organisms present. Bottom of crust is coated in beige-brown sediments < 6 mm thick and patches of orange Fe-oxyhydroxides < 4 mm thick. There are discontinuous patches of black, hydrogenous Mn-oxide all over the crust (< 3 rnm thick). Blocky chunk of hydrothermal Mn-oxide crust with a massive texture and dehydration cracking. Crust is hard, dense, grey-black in colour with submetallic luster and < 5 % porosity. Upper surface is covered with sponge organisms and is Baby N/A Mn-oxide 50 18 25 coated in black, non-shiny Mn-oxide material i4 mm thick that has a bulbous Bare texture. Light and dark brown sediments and gold coloured clay were incorporated into the sample, visible on broken surfaces. ~heseinterior layers are - lcm thick. Bottom surface of crust is light brown/ beige sediment < 5 mm thick. Appendix B.3: Descriptions of Mn-oxide crusts from Baby Bare Seamount (continued) Sample Length Width Thickness Location Vent Sample Sample Description No. (cm)., (cm). , (cm). , Oval shaped piece of hydrothermal Mn-oxide crust with a laminated texture and dehydration cracking. crusts is hard, dense and has porosity < 5 %. Two distinct horizons are visible and crust was sub-divided into 2 sections based on colour Baby variation: A: dark grey, massive outer rind that was < L! cm thick and B: dark brown Bare inner core that consisted partially of fine needles or dendrites of Mn-oxide. Top of sample is coated in a layer of dark brown sediments < mm thick, while the bottom of the sample is coated in a < 5 mm thick layer of beige and light brown sediments. Blocky piece of hydrothermal Mn-oxide crust with a layered texture and dehydration cracking. Crust is hard, dense and bluish-black to grey-black in colour, with a submetallic luster and porosity < 5%. Sample has three distinct horizons and was subdivided into three sub-sections based on colour variation: T: - lcm thick top Baby horizon, consisting of 6-10 bluish-black, discontinuous laminations < 2 mm thick; M: N/A Mn-oxide 28 20 7 -2.5 cm thick middle horizon, grey-black in colour; B: black bottom horizon with a Bare cusp-like texture and e fine laminations < 4 mm thick. Top of crust is coated in black, non-shiny Mn oxide < 5 mm thick, light brown bottom sediments < 2 mm thick and sponge organisms. Bottom is coated in dark brown bottom sediments < 4 mm thick. Blocky piece of hydrothermal Mn-oxide crust, massive texture and dehydration cracking. Crust has clumps of radiating fibers on the fresh interior surface, and four Baby fine laminations at the base of the crust than were less than 0.5 mm thick each. Crust N/A Mn-oxide I2 10 Bare is hard, dense and dark grey in colour with porosity < 5%. Top surface is coated in a black, non-shiny hydrogenous Mn coating - Imm thick, and in sponge organisms. Bottom is coated in dark brown sediments < 4 mm thick. Appendix C PRECISION AND ACCURACY Sample repeats were analyzed for one basalt and one Mn-oxide sample during solution ICP-MS and XRF analyses. Samples analyzed using XRF were run in batches, so the same repeat samples were not used for each batch.XRF precision and accuracy are determined by the laboratory (McGill University), while these were calculated for the ICP-MS and LA-ICP-MS analyses carried out at the University of Victoria using standard and the repeated analyses. For ICP-Solution work, the BCR-8 analyses were used to calculate precision and accuracy. Appendix C.l: Precision and Accuracy for ICP-MS Solution Appendix C.2: ICP-MS Solution Duplicate Analyses for Basalt and Mn-Oxide Crust Appendix C.3: XRF Duplicate Analyses for Basalt and Mn-Oxide Crust ~ - - - ..- - .. . .- . .- Appendix C. 1: Precision and AccuracvJ for ICP-MS Solution work using- BCR-2 Standards* Standard BCR-2 BCR-2 BCR-2 BCR-2 BCR-2 BCR-2 Standard Average Precision Reported/ Accuracy Sub-sample 192-1 202-1 192-2 202-2 202-3 Reported Deviation Measured % Error Measured AVB. % Error < d/l indicates that concentrations were less than the detection limit. 117 Appendix C.2: ICP-MS Solution Duplicate Analyses for Basalt and Mn-Oxide Crust* Sample 674-18 674-18 05-04 05-04 CC-14 CE-15 repeat CC-3 CE-16 repeat Cloud Cloud Baby Bare Baby Bare % Yo Analysis Fresh Basalt Fresh Basalt Difference Mn-oxide Mn-oxide Difference 25.58 29.26 -7% 0.24 0.20 analysis. Appendix C.3: XRF Duplicate Analyses for Basalt and Mn-Oxide Crust* Sample R674-18 R674-18 62-04B 62-04B Sub-sample CC-14 CE-15 repeat CC-48 CE-16 repeat Vent Site Cloud Cloud Baby Bare Baby Bare Analysis Fresh Basalt Fresh Basalt % Difference Mn-oxide Mn-oxide % Difference SiOz 48.77 48.91 -0.14% 2.25 2.5 1 -5.46% TiOz 1.49 1SO -0.20% 0.02 0.02 -2.13% A1203 14.40 14.40 0.00% 0.66 0.72 -4.35% Fez03 12.31 12.29 0.08% 2.69 2.70 -0.19% FeO 11.08 11.06 0.08% 2.42 2.43 -0.19% MnO 0.20 0.20 -0.50% 65.69 65.32 0.28% MgO 7.50 7.50 0.00% 2.55 2.55 0.00% CaO 12.13 12.17 -0.16% 1.19 1.20 -0.42% Na20 2.82 2.83 -0.18% 1.53 1.57 -1.29% K20 0.24 0.25 -2.04% 0.25 0.26 - 1.96% P205 0.14 0.14 0.00% 0.18 0.18 0.00% Crz03 382 393 -1.42% n/a n/a nla Total 100.06 100.25 -0.09% 99.20 99.27 -0.04% LO1 ELECTRON MICROPROBE DATA Microprobe analyses were performed using a JXA-8900 Superprobe at the University of Alberta. Operating conditions were at 15 kV acceleration voltage with a 15 nA beam current and a beam diameter of 1 ym for glass, plagioclase and sulfide phases, while 3 ym was used for clay phases. Instrument calibration was performed on natural standards and ZAF corrections were applied to all analyses. Grain mounts are minerals picked from rock surfaces. Microprobe data for the following phases are summarized D. 1: Secondary minerals (Grain Mounts) from Main Endeavour Field D.2: Secondary minerals (Thin Section) from Main Endeavour Field D.3: Barites from the Main Endeavour Field D.4: Plagioclase from Axial Volcano and Main Endeavour Field D.5: Fresh Glass from Main Endeavour Field No microprobe work was performed on samples from Baby Bare seamount, and alteration phases in thin section on samples from Axial were in quantities large enough for microprobe analyses. Appendix D.l: Electron microprobe results for select secondary phases in grain mounts from Main Endeavour Field* Sample No. 68-17 717-2 717-2 68-17 68-17 68-17 717-4 717-3 717-4 717-1 Sub-sample 50 229 229 54 54 54 235 206a 233 202 Easter Easter Easter Easter Easter Easter Vent Milli-Q Milli-Q Milli-Q S&M Island Island Island Island Island Island SiOZ 22.23 35.50 36.03 70.02 49.16 51.15 46.96 43.57 TiOz 0.09 0.07 0.04 0.34 0.08 0.39 A1203 9.50 1.32 1.32 1.33 21.32 18.09 0.26 9.66 FeO 44.44 39.48 38.66 0.04 5.83 7.86 30.14 18.80 MnO 0.01 0.03 0.02 0.01 0.04 0.10 0.00 0.10 MgO 0.44 1.61 1.49 4.48 1.05 2.97 2.67 3.62 CaO 1.82 0.63 0.5 1 6.44 0.44 0.38 0.09 1.53 Na20 3.28 2.01 1.99 11.92 0.61 6.08 0.44 1.16 KzO 0.39 1.96 2.00 0.98 1.58 0.21 3.22 2.43 Total 82.11 82.63 82.08 95.25 80.36 86.84 83.86 81.27 Octahedral 4.29 4.11 5.15 4.91 Total Grain dark red orange dark orange dark orange dark orange white white green beige orange oxide Description oxide oxide oxide oxide oxide mineral mineral mineral grain under Mn- under Mn- under Mn- coating coating oxide oxide glass glass with beige coating Occurrence coating glass oxide coating glass glass coating on coating on coating coating clays rock on glass glass glass sap-cel- Si-Na-Ca- Si-Na-Ca- Si-Na-Ca- Si-Al-Fe- Identification Fe-oxy Fe-Si Oxy Fe-Si Oxy high Al Non high A1 Mg phase Mg phase Mg phase Na phase mix *Blank spaces - no octahedral reported for phases that are not clays. Fe-oxy: Fe-oxyhydroxide; Fe-Si Oxy: Fe-Si oxyhydroxide; Si-Na-Ca-Mg phase: unidentified silicate phase; Non: nontronite; sap-cel-high A1 mix: mixture of saponite, celadonite and high-A1 beidellite; high Al: high-A1 beidellite. All data is reported as weight percent. Appendix D.2: Electron microprobe results for select secondary phases in thin section from Main Endeavour Field* Sample No. Sub-sample Vent Si02 31.38 2 1.56 30.19 15.61 29.06 Ti02 0.12 0.00 0.33 0.98 0.40 0.44 1.O8 0.43 A203 4.43 0.26 7.34 7.93 8.39 5.89 3.46 4.28 FeO 40.67 55.20 22.04 20.1 1 17.23 15.68 8.69 19.21 MnO 0.04 0.05 0.12 0.09 0.13 0.09 0.06 0.10 MgO 1.12 1.68 1.49 1.32 1.70 4.69 2.77 3.54 CaO 2.72 1.86 1.78 2.33 2.13 9.82 17.37 10.35 NazO 1.27 0.29 1.35 1.71 1.25 0.96 0.63 0.94 K20 1.02 0.48 1.16 0.91 1.47 0.75 0.21 0.22 p205 2.72 5.64 0.18 0.10 0.11 0.09 0.07 0.08 Total 85.50 87.03 66.20 63.93 68.16 60.19 50.16 68.59 Octahedral 5.36 5.16 4.80 4.50 2.07 4.30 Total orange and orange and orange and orange orange silicate silicate green green green silicate lining Analysis material material lining in lining in material material material in vein lining vein lining vein vein vein lining vein lining vein lining vein sap or sap- sap cel or sap-cel or sap-cel-high sap-cel-high sap-cel-high Identification Fe-Si oxy Fe-Si oxy cel " satl-highu A1 A1 Mix A1 Mix A1 Mix *Blank spaces - no octahedral reported for phases that are not clays. Fe-Si Oxy: Fe-Si oxyhydroxide; Si-Na-Ca-Mg phase: unidentified silicate phase; Non: nontronite; sap-cel: mixture of saponite and celadonite; sap-cel-high A1 mix: mixture of saponite, celadonite and high-A1 beidellite; high Al: high-A1 beidellite. All data is reported as weight percent Appendix D.2: Electron microprobe results for select clay phases in thin section from Main Endeavour Field (continued) Sample No. 717-2 717-2 717-2 717-2 717-2 717-4 717-4 717-4 Sub-sample 717-2-2 717-2-2 717-2-2 717-2-2 717-2-2 717-4-1 717-4-1 717-4-1 Easter Easter Easter Easter Easter Vent Milli-Q Milli-Q Milli-Q Island Island Island Island Island Si02 47.87 48.47 23.70 35.82 24.93 Ti02 0.15 0.40 0.23 0.19 0.23 0.26 0.54 A1203 0.86 2.32 1.52 1.54 4.81 1.99 4.12 FeO 48.96 48.45 31.20 30.34 18.16 11.84 17.79 MnO 0.06 0.04 0.02 0.02 0.07 0.16 0.07 MgO 1.O5 1.86 2.32 2.34 3.05 2.39 4.70 CaO 1.89 2.86 0.97 1.O7 11.21 12.32 9.72 Na20 1.17 0.25 0.5 1 0.33 0.91 0.30 0.78 K20 0.71 0.68 1.92 2.12 0.37 0.18 0.38 pzo5 1.81 1.5 1 0.07 0.14 0.05 0.08 0.09 Total 81.42 84.24 86.67 86.61 62.73 65.45 63.27 Octahedral 5.32 5.25 4.17 3.12 4.56 Total orange orange orangelgreen orangelgreen orangelgreen orange orange orange material material material material material Analysis material material material filling filling filling filling filling lining vein lining vein lining vein fracture fracture fracture fracture fracture sap-cel-high sap-cel-high sap-cel-high Identification Fe-Si oxy Fe-Si oxy sap-cel sap-cel sap-cel A1 Mix A1 Mix A1 Mix I *Blank spaces - no octahedral reported for phases that are not clays. Fe-Si Oxy: Fe-Si oxyhydroxide; Si-Na-Ca-Mg phase: unidentified silicate phase; Non: nontronite; sap-cel: mixture of saponite and celadonite; sap-cel-high A1 mix: mixture of saponite, celadonite and high-A1 beidellite; high Al: high-A1 beidellite. All data is reported as weight percent. Appendix D.4: Electron microprobe results for select plagioclase analyses in thin section from Axi a1 Volcano and the Main Endeavour Field Sample Analysis Vent Si02 A1203 FeO MnO MgO CaO Na20 K20 Total An# -- 08-08 Village 08-08 Village 08-08 Village 08-08 Village 08-09 Village 08-09 Marker 33 08-09 Marker 33 549-12-1 Snail 549-12-1 Snail 549-12-1 Snail 674-18 Cloud 674-18 Cloud 674-18 Cloud 68-16 W. Grotto 68-16 W. Grotto 68-16 W. Grotto 68-17 Easter 68-17 Easter 717-2 Easter 717-2 Easter 717-2 Easter 7 17-3 Milli-Q 7 17-3 Mini-Q 717-3 Milli-Q 7 17-3 Milli-Q 7 17-4 Milli-Q 717-4 Milli-Q 717-4 Milli-Q 7 17-4 - Milli-Q 51.07 31.05 0.62 0.01 0.26 14.83 3.51 0.12 101.54 0.82 ,ioc:lase rnicrolite; "homogeneous phenocrysts; ~phenocrystcore; "henocryst rim. All data is reported as weight percent Appendix D.5: Electron microprobe results for fresh glass analyses in grain mounts from the Main Endeavour Field - - I Sub-sample 1 6815-gl-1 6815-g1-2 6816-gl-1 6816-gl-2 6817-g1-1 6817-gl-2 717-2-gl-1 717-2-gl-2 Easter Easter Easter Easter Vent Hulk Hulk W. Grotto W. Grotto Island Island Island Island Ti02 FeO MgO MnO CaO Na20 K20 BaO SO3 Total All data is reported as weight percent. Appendix D.5: Electron microprobe results for fresh glass analyses in grain mounts from the Main Endeavour Field (continued) Sample 717-3 717-3 7 17-4 717-4 717-1 717-1 730-1 730-1 Sub-sample 717-3-gl-1 717-3-g1-2 717-4-gl-1 717-4-gl-2 717-1-gl-1 717-1-g1-2 730-1-gl-1 730-1-gl-2 Vent Milli-Q Milli-Q Milli-Q Mim-Q S&M S&M S&M S&M SiOz 50.1 1 50.58 49.94 49.95 50.12 50.01 50.09 49.73 Ti02 1.83 1.78 2.09 1.88 2.20 2.15 1.79 1.95 FeO 10.53 10.51 11.09 10.52 11.15 10.98 10.57 10.51 MgO 7.05 6.93 6.20 6.94 6.29 6.36 7.08 6.89 MnO 0.17 0.16 0.14 0.15 0.16 0.18 0.17 0.19 CaO 11.75 11.51 10.89 11.85 10.96 10.86 11.97 1 1.76 NazO 3 .07 2.95 3.21 3.08 3.27 3.14 2.97 2.81 K20 0.41 0.36 0.55 0.40 0.59 0.55 0.40 0.41 BaO LASER ABLATION ICP-MS RESULTS The following table lists all analyses of fresh glasses collected from basalts from the Main Endeavour Field. Apy ndix E.1: LA-ICP-MS results for fresh glass from the Main Endeavour Field Sample No. Milli- Milli- Milli- Milli- W. W. Vent S&M S&M S&M S&M Easter Easter S'E' S'E' Easter Easter Q Q Q Q Hulk Hulk Grotto Grotto *r indicates that the analysis is a repeat. All data is reported in ppm. Appendix F GEOCHEMICAL MODELING METHODS I?. 1: Introduction Geochemical models of diffuse hydrothermal fluids may be used to directly link hydrothermal fluid chemistry with alteration phases observed on basalt samples, by determining what conditions affect mineral precipitation. For this study, the Geochemist's Workbench software package (Bethke, 2002) with the thermo.dat database (compiled by the Lawrence Livermore National Laboratory (Delany and Lundeen, 1990)) was used. Three model techniques were utilized to investigate mineral precipitation using chemical data on diffuse hydrothermal fluids from Cloud, Snail and Marker 33 vents at Axial Volcano. No fluid data was available for other sites. These models simulated processes that occur in the natural environment at low-temperature diffuse vents. The first model considered mineral precipitation directly from diffuse fluids, the second considered mineral precipitation that resulted from mixing of diffuse fluid with seawater, and the third considered precipitation as a result of fluid-rock interaction. Minerals that are predicted in these models were compared to the observed alteration assemblages on basalts from three diffuse vents at Axial, in order to elucidate what parameters affect precipitation of low-temperature alteration minerals. The following sections discuss the steps taken to model, assumptions made in the models and the computer codes that were used for the modeling. F.2: Initial Steps To begin modeling, the initial step was to constrain the diffuse fluids using the fluid mass, in-situ measured values of temperature, pH (measured at 22 oC), alkalinity, and concentrations of major components and trace components of interest (Mn2+, Few, AP+ and Ba2+),entered as mol/kg. The concentration of C1- was adjusted automatically by the program to account for charge balance. Oxidation State Next, the oxidation state of the system was set by constrainingfi>qg).The program React requires that oxygen be present in the system, which was not measured (as there is essentially no oxygen in these fluids) (Butterfield et al., in press). 09(aq) was constrained using the He(,,) species (which was measured in these fluids) assuming that H%(q)and H2O are in equilibrium. This assumption is considered to be valid, since it is known that the reaction He(,,) + 02(aq) = He0 is a "quick reaction (M.K. Tivey, pers. comm., goo+). Fugacity of oxygen is set simply by "swapping" H2(aq)for Oa(aq)and then setting the measured He(,,) concentration for the diffuse fluids. fi>qg) for bottom seawater was calculated using the measured Oe(aq)concentration (D. Butterfield, pers. comm., 2004). Pressure It was assumed for all models that pressure does not significantly affect mineral precipitation at the low-temperatures of the Axial system. In React, pressure is automatically set to 1 bar at temperatures below 100 "C - it is not a parameter that can be varied. Thermodynamic databases are commonly compiled at 25 OC and 1 bar, as there is little need for compiling data at other pressures while working at low temperatures, because changes in log K values with pressure for most reactions is considered small compared to the uncertainty in this value. In addition, the compressibility of seawater is considered negligible at pressures below 1 kbar (Millero, 1973) and the thermodynamic data for aluminosilicates at higher pressures is poorly defined. Generally with many mineral phases, the change in log K is considered to be negligible when compared to the error associated with these calculated values (http://gwb.geo.stonybrook.edulgeneral/ques-48.html). This web page is the Geochemist's Workbench User's discussion site, where the React programmer (C. Bethke) addresses questions on pressure. Technically, the program can work at any pressure, but only if log K values for all reactions are adjusted in the thermodynamic database for the pressure of interest. It was preferred to run these models at 150 bar (the pressure at 1500 m water depth at Axial Volcano), but recalculation of log K values was beyond the scope of this project. A thermodynamic database at 170 bar was acquired from M.K. Tivey which contained mostly high-temperature mineral phases. The 1 bar and 170 bar databases, however, do have species and minerals common to both. The log K values for these phases are listed in Table F. 1 and are very similar, illustrating that indeed pressure does not greatly affect the results of these models. Table F.1: Comparison of log K values for select species and minerals at 1 bar and 170 bar. Soecies 1 1 bar 170 bar OH- 1 13.9868 13.929 C02,aq) HSOi H,S(aq) talc barite am. silica Hz% C02cg) 02,) H2(d H2Sk) Generally, pressure concerns refer to the partial pressure of a gas, as pressure can have dramatic effects on gases (http://gwb.geo.stonybrook.edu/general/generalq8.htm). We get around this issue by directly setting the partial pressure (or fugacity) of Oa(g),(the gas of concern here) in the diffuse fluids at Axial. To further explore the impact of pressure variability on mineral precipitation reactions, results of diffuse fluid speciation models were plotted on stability plots at both 1 bar and 150 bar. Figures F. 1A and F. 1B illustrate that a pressure increase from 1 to 150 bar does not change the boundaries between stable species. 100 Amorphous slltca Amorphous s~l~ca 50 Figure F.l: SiOp(,,) stability plot versus pH for A: 1 bar and B: 150 bar. All fluids plot in the same general region, on the line bordering stability of amorphous silica and SiOs(,,). One final assumption in all models was that the system was closed. Although this is certainly not the case in the natural environment, this allows for a simplified representation of mineral precipitation reactions. Reaction proceeds in all models until all reactants are exhausted or the fluid reaches saturation with the reactant. Model output shows the calculated saturation state of minerals, gas fugacities, activities of aqueous species, Eh, pH and ionic strength of the solution. F.3: Sensitivity Calculations It is important to examine the sensitivity of the system to properly investigate what parameters influence mineral precipitation. All models were initially run using the measured parameters. In addition, models were run by altering some of these measured characteristics to investigate what parameters cause certain minerals to form. For example, models were performed with changes in the amount of fluid and/or rock present in the system, temperature, pH, andjOqg). Results of these variations are discussed in Chapter 2. Such calculations help to determine what conditions are necessary in order for the observed minerals to precipitate. The sensitivity of mineral precipitation was also examined by suppressing various mineral phases. Iterations were made until the minimum number of minerals was suppressed in order to produce results that were consistent with observations. Minerals were suppressed in varying combinations, to see what phases, if any, would replace the suppressed mineral. Conditions were also altered in order to attempt to get minerals that were observed to precipitate, if they were not predicted in the models using measured properties. For example, anhydrite is observed at the diffuse vents at Axial, but not predicted at the low temperatures measured in the diffuse fluids. Anhydrite does precipitate from the diffuse fluids once they are heated to 150 OCin the models, illustrating that the anhydrite observed on the basalt samples either resulted from much hotter fluids venting at the seafloor, or anhydrite precipitated at depth and was carried to the seafloor. Finally, conditions were altered in order to determine what mineral phases were insensitive to changes in these parameters. F.4: Mineral Suppression In order to precipitate the observed mineral assemblage, and to have only low- temperature minerals precipitate, certain phases have to be suppressed. The evaluation of which minerals should be suppressed was based on mineral stabilities, information on the environment of formation and kinetics. For example, all SiOa polymorphs were suppressed in order to allow amorphous silica to form, as it is thought that these phases precipitate too slowly at these low temperatures (Mottl and McConachy, 1980). Minerals that are known to precipitate at temperatures much higher than those measured at diffuse sites were also suppressed (i.e. chalcopyrite). These phases precipitate at low temperatures in React because the program converges on the lowest AG assemblage and does not regard details such as temperature of mineral stability. A table summarizing the minerals suppressed in all models is listed in Chapter 2. It is important to research why alteration phases that are observed at the diffuse sites were not predicted in the models. Possibilities include 1) poor thermodynamic data at the temperature and pressure conditions that are being considered; 2) kinetics; and/or 3) incorrect assumptions made about possible conditions under which samples were altered (e.g., maybe the rock actually altered under different pressure, temperature and redox conditions than what was measured in the diffuse fluids). F.5: Geochemist's Workbench Suite Act The Act program is used to graphically represent diffuse fluid speciation results using mineral stability diagrams. Results of fluid speciation by React may be transferred to the Act program directly. Pressure may be adjusted in Act. On the stability diagrams, solid lines represent a 50:50 line between the species on either side such that, the farther away a fluid plots from the lines between species, the less stable the fluid is with respect to the species on the far side of the line. Rxn The program Rxn calcuIates log K values for reactions, and automatically balances reactions. Pressure cannot be adjusted in this program so log K values calculated are for 1 bar. This program is useful for visualizing the species that are involved in reactions that precipitate low-temperature minerals of interest. React Model I - Speciation of Dzfise Fluids and Seawater In the first model technique, using React, diffuse fluids were constrained and allowed to speciate (i.e. reach equilibrium). Results illustrate an equilibrium attained within the fluid, but one that is meta-stable with respect to mineral precipitation (Bethke, 1996) and not all minerals predicted to be supersaturated will precipitate (Tivey, 1995a). In this model, the program calculates the system equilibrium state but does not trace a reaction path. The model output is a prediction of the distribution of dissolved species in solution, fluid saturation state with respect to minerals, and gas fugacities. In the react.txt output, the top block is the results of the first calculation, where the fluid is in a metastable state. The bottom block gives the results if minerals were allowed to precipitate, which results in a change in the bulk fluid composition. Model 2 - Titration Model - Flash Mixing of Dzfuse Fluids and Seawater The second model technique simulates the mixing of diffuse fluids with seawater. In this case, the "flash model was used, which reacts fluid and seawater at all possible mixing ratios. This is useful to examine what mixing ratios between diffuse fluids and seawater are required for the observed minerals to form. In this model, seawater is speciated first and is then added as a reactant to the diffuse fluids, which are the initial system. This model was run in reverse as well, to ensure that different minerals would not precipitate if seawater was the initial system. Seawater was added to the diffuse fluid in progressive, equal increments until the system attains equilibrium, or the reactant is exhausted. The resulting reaction path illustrates a sequence of mineral assemblages that precipitate and dissolve in the mixed fluid as the proportion of seawater increases from left to right across the diagram (Figure 5b). This model was isothermal, so the temperatures of the initial system and reactants were set using measured temperatures in these fluids. Model 3 - Titration Model - Reaction between Dguse Fluids and Basalt In this model, similar to the Flash model, a reactant was gradually added to an initial system. In this case, a small amount of basalt was added to the diffuse fluids gradually, and in equal amounts, over the course of the reaction path. The program repeatedly adds a small amount of reactant and recalculates the equilibrium states. You may use a set amount of rock and fluid initially in the program, and then vary the amount as you progress through the calculations by specifying how much you want to use (e.g. reactants times 10 = 10 times the amount of seawater you first equilibrated). These models were isothermal - they were run at the temperatures that were measured in the fluid. They were also run with cooling and heating the fluid before reaction with basalt. Basalt was constrained into React using the major elements as weight of oxides, which is easy since the measured concentrations (using XRF) can be converted to grams from weight percent. Basalt may also be entered into the program by normalizing the basalt chemistry to represent the mineralogy of the rock. This may be done using any basalt normalizing program (eg. Magma), where you simply enter the basalt major element chemistry, and the program calculates what minerals would be present in this rock based on the major elements you specified. The former method was not utilized in these models, as I felt it did not accurately represent the rock and did not utilize all the elements of interest, such as Ba. In addition, not all of the normative minerals (such as titanomagnetite) are represented in the thermo.dat database. One final rock issue is that Ti is not measured in hydrothermal fluids because titanium samplers are used to collect fluid samples. Therefore, Ti had to be excluded from the basalt chemistry when rock was constrained. To do this, Ti02 was excluded from the weight of the rock and the remaining elements were normalized to sum 100 g based on a Ti free basis. One hundred grams of basalt was used in this model. Conditions at the vent orifice are fluid dominated, where large amounts of diffuse fluids pass by small amounts of rock as they exit the vent orifice. This model simulates this by reacting more fluid than rock. Also, when too much rock is reacted with little fluid, the system becomes too supersaturated and the program cannot finish the equilibrium state calculations. The amount of fluid used in the model was varied in order to investigate what water-rock ratios are required to precipitate the observed assemblage. Water-rock ratios from 1: 1 to 1000:1 were examined. In each case, the basalt is reacted with the diffuse fluid homogeneously, as no provision is made for some parts of the rock altering more readily than others (Bethke, 1996). The resulting reaction path illustrates changes that occurred in the diffuse fluids as rock was reacted into it. This model also considered what influence the addition of seawater would have on the diffuse fluid-rock reactions. In all cases, when seawater was added after rock had reacted into diffuse fluid, the same minerals were produced. The only mineralogical difference that resulted from the presence of seawater was the precipitation of the manganese mineral rhodochrosite, which formed at higher water-rock ratios than in the model where no seawater was present. Other Model Types Available Not Utilized Flush model In this model, fluid that is added as a reactant displaces an existing fluid from the system. This traces from the reference frame of the rock through which the fluid migrates. I didn't use this model because I didn't feel it represented the system well, as seawater mixes with diffuse fluids at the seafloor, and doesn't necessarily displace it. Flow-Through Model This model is similar to a titration model, but in this instance minerals do not dissolve once they precipitate. This isolates minerals as they form, so they cannot dissolve into the fluid, by removing mass from the system, essentially creating an open system. I felt this model did not represent the diffuse system very accurately, as it is known that certain mineral phases (e.g. anhydrite) re-dissolve at lower fluid temperatures. F.6: Calculation Check It is important to show that the program has calculated the system correctly. This is done by comparing the calculated activity coefficients and species activity in the output to the component concentrations initially entered. The program calculates activities of the complexes of the components that were constrained initially. For example, Na+ is entered as a component measured in the diffuse fluids, but the equilibrium output shows Na+ as an isolated ion and in all of its complexed forms (i.e. NaC1). As a check, you have to arrive back at your input constraint by examining the output of the React program. To do this, the calculated activities and activity coefficients of all of the Na+ species must be added together. These should sum to the input concentration using the equation a = ym, where "a" is the activity of the species, y is the activity coefficient of that species, and m is the concentration. Table F.2 gives an example of this calculation, using sulfur species. Table F.2: Calculation check for sulfur species for Cloud 1998 Fluid speciation* Species Molality ~0~~-0.0 154 1 MgS04 0.005 122 CaS04 0.0007565 NaSOi 0.005704 KSOi 0.0001 73 1 BaS04- I .85E-07 FeS04 1.24E-06 MnS04 7.238-07 HS0i 3.4OE-07 HzS@q) 1.15E-07 Also4+ 3.36E-08 Total 0.02717 Total moles entered 0.02717 *model was run with mineral suppressions (minerals listed in Table 2.3). Appendix G ALTERATION MINERALOGY SUMMARY FOR AXIAL VOLCANO AND MAIN ENDEAVOUR FIELD The following table summarizes all alteration minerals observed at both Axial Volcano and the Main Endeavour Field. Also specified is the method by which these phases were identified. Sample Probe XRD (select Vent Site SEM (surface coatings) No. Probe surface coatings) (vesicle linings) surface coatings) 52-008-09 aarker 33 jap Sap, Non R674-18 Cloud j, Fe-oxy, Anh, Sap, Mg-Si-0 phase Sap, Non, Phil Sap, Non, Anh, R549-12-1 Snail j, Sap, Anh, Mg-Si-0 phase Phil R674-13 Village Ynh, Sap, Fe-oxy, S, Bar, Mg-Si-0 phase Fe-Si oxy Sap, Non, Cal, Eri Fe-Si oxy, Sap-cel, vln-ox, Am. Si, Sap, Bar Al-Non, Fe-Si oxy, A1-Si phase R717-1 S&M Sap-cel-beid R730-1 S&M hh, Mn-ox, Am-Si., Bar, Fe-Si oxy, possibly Sap Bar Fe-S-0 phase, Sap, Bar, Mn-Fe ox, Na-Al-Si-0 phase, R717-3 Milli-Q A1-Si Dhase. Bar Am. Si. Sap, Non, Chl, Sap, Mg-S-0 phase, Fe-oxy, Am. Si, Bar, Fe-S-0 R717-4 Milli-Q Al-Non, Non, Bar, AI-Si phase Anh,Fauj. possibly phase, Fe-Si oxy Easter Bar, Anh, Sap, Mn-ox, Tubular Fe-Si oxy, Al-Si phase Fe-Si oxy, Py, Sap- R717-2 Fe-Si oxy, A1-Si phase Island (Non), Na-Al-Si-0 phase cel Easter Fe-Si oxy, Fe-Mn oxide, Na-Mg- Sap, Non, Bar, 52-068-17 Mn-ox, Fe-oxy, Py, Fe-Si oxy, Sap Fe-Si oxy Island Si-0 phase, Fe-oxy Chl, Eri, Gor Mn-ox, Sap, Bar, Am. Si, Fe-Si oxy, Fe-Al-Si-0 phase Sap, Gor, Phil, 52-068-15 S.E. Hulk Fe-Si oxy Fe-Si oxy (No4 HyTc 52-068-16 W. Grotto Mn-ox, Ca-Na-Al-Si-0 phase Phil, Sap, Anh cular order. Abbreviations: A1-Si phase = unidentified Al-silicate; Al-Non = Al-rich Nontronite, Am-Si = Amorphous Silica, Anh = anhydrite, Bar = Barite, Cal = Calcite, Chl= Chlorite, Eri = Erionite, Fauj = Faujasite, Fe-oxy = Fe oxyhydroxide or Ferrihydrite, Fe-Si oxy = either a mix of Fe-oxy and Am-Si., or a distinct Fe-Si oxyhydroxide phase, Gor = Gormanite, HyTc = Hydrotalcite, Mn-ox = Mn oxide, Non= Nontronite, Phil = Phillipsite, Py = pyrite, S = elemental Sulfur, Sap = Saponite, Sap-cel-beid = saponite-celadonite-high A1 Beidellite mix, Sap-cel = Saponite-Celadonite mix. Blank spaces indicate that no data was available. Appendix W RESULTS OF CHEMICAL CHANGE CALCULATIONS Changes in chemical species between fresh and altered rock were calculated using Gresens' equation (Gresens, 1967) modified by (Grant, 1986). This method normalizes the chemical data so the relative elemental gains and losses that occurred during alteration can be determined. Following the methods of Humphris et al. (1998), those elements that maintain proportionality during alteration (not whether the absolute abundances are the same) were identified. Combining the mass change term, defined by the immobile element, with the chemical change that occurred between fresh and altered rock, determines how much of each element was lost or gained by the fresh rock during alteration. A1203 was used for basalt from both Axial Volcano and the MEF. Results are reported as weight percent for major elements and ppm for trace elements (H. I). Chemical changes are also expressed as percent relative difference (H.2) in order to illustrate the magnitude of the chemical change. Negative values reflect a loss of that element from the fresh basalt due to alteration, and positive values a gain by altered rock. Appendix H.1: Results of chemical change calculations* i 8 0 L. -.m & v 2 u 8 3 am m -m g - 44 + u .-Q .-Q E E Vent z m E 1 - - s 'a s G G 8 Wa W 3 3 2 24 Sample No. Mass Change Term SiOz TiOz A1203 Fe203 MnO MgO CaO Na20 K2O P20~ Ba Sc Rb Sr Y Zr Nb Cs La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu H f Ta Th u 2.01 *Values are expressed as weight percent for major elements and ppm for trace elements. Negative values indicate a loss of that element from the fresh rock, and positive a gain. Blank spaces indicate that no flux was calculated. A~~endixH.2: Results of chemical chan~ecdculations* Vent Sample No. Mass Change Term Si02 Ti02 -41203 Fez03 MnO MgO CaO NazO KzO pzos Ba Sc Rb Sr Y Zr Nb Cs La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Hf Ta Th u *Values are expressed as % difference of fresh rock in order to illustrate the magnitude of chemical exchange. Negative values indicate a loss of that element from the fresh rock, and positive a gain. Blank spaces indicate that no flux was calculated. Appendix Z GEOCHEMICAL MODELING RESULTS The following lists the results of all geochemical models of diffuse fluids from Axial Volcano. L 1: Speciation Model Results Listed here is the output text from React, produced when diffuse fluids are speciated (i.e. brought to equilibrium). These results are only for runs where measured parameters (e.g. temperature, pH) were used. Results list solution properties, saturated minerals, species distribution, supersaturated mineral phases and gas fugacities, each in separate blocks. Cloud 1998 - Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 Xi = 0.0000 Temperature = 20.6 C Pressure = 1.013 bars pH = 5.337 logP2 = -67.733 Eh = -0.0675 volts pe = -1.1581 Ionic strength = 0.607998 Activity of water = 0.983229 Solvent mass = 1.000000 kg Solution mass = 1.033844 kg Solution density = 1.030 g/cm3 Chlorinity = 0.52737 1 molal Dissolved solids = 32736 mg/kg sol'n Rock mass = 0.000000 kg Minerals in system moles log moles grams volume (cm3) -- - Barite 6.785e-008 -7.168 1.584e-005 3.535e-006 Mordenite-K 5.982e-007 -6.223 0.0002707 0.0001289 (total) 0.0002865 0.0001324 Aqueous species molality mg/kg sol'n act. coef. log act. C1- Na+ ME;++ S04-- K+ MgCl+ NaS04- Ca++ MgS04 CaCl+ NaCl CO&(aq) CaS04 SiO2(aq) HCO3- KSO4- NaHC03 KC1 MgHC03+ H+ CaHCO3+ Mn++ Fe++ HeS(aq) HSO4- FeC1+ MnS04 FeSO4 MnCl+ MnC12 Ba++ FeCl2 HS- 6.357e-008 0.002033 0.6505 -7.3836 NaH3SiO4 6.125e-008 0.006996 1.0000 -7.2 129 Bas04 4.361e-008 0.009845 1.0000 -7.3604 MnC13- 3.111e-008 0.004854 0.6704 -7.6808 H3Si04- 2.153e-008 0.00198 1 0.6704 -7.8405 MgCO3 1.408e-008 0.001 148 1.0000 -7.85 14 BaCl+ 1.208e-008 0.002018 0.6704 -8.09 17 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K Pyrite 4.1786s/sat CaS04"1/2H~O(bet -2.0060 Quartz 0.7939dsat Sulfur-Rhmb -2.1927 Tridymite 0.6240s/sat Sanidine high -2.3 195 Chalcedony 0.5 185s/sat Mirabilite -2.4030 Cristobalite 0.2330s/sat Gibbsite -2.4236 Barite 0.0000 sat Albite low -2.4886 Mordenite-K 0.0000 sat Albite -2.4887 Clinoptil-K -0.0 137 Halite -2.6003 AmrphAsilica -0.5295 Epsomite -2.7033 Gypsum -0.9919 Kaolinite -2.7916 Maximum Microcli -1.0904 Witherite -2.8322 K-feldspar -1.09 15 Pyrophyllite -2.868 1 Anhydrite -1.1980 Magnesite -2.8835 Bassanite -1.8319 Hexahydrite -2.9657 Mordeni te-Na - 1.9099 (only minerals with log Q/K > -3 listed) fugacity log fug. --- 0.05203 -1.284 0.02339 -1.63 1 1.809e-005 -4.743 3.446e-009 -8.463 2.879e-012 -11.541 2.352e-019 -18.629 1.850e-068 -67.733 Cloud 1999 - Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 Xi = 0.0000 Temperature = 19.9 C Pressure = 1.013 bars pH = 5.800 logj02 = -68. 174 Eh = -0.0976 volts pe = -1.6782 Ionic strength = 0.610463 Activity of water = 0.983 169 Solvent mass = 1.000000 kg Solution mass = 1.034022 kg Solution density = 1.030 g/cm3 Chlorinity = 0.5294 15 molal Dissolved solids = 32903 mg/kg sol'n Rock mass = 0.000000 kg Minerals in system moles log moles grams volume (cm3) ------Barite 1.058e-007 -6.976 2.468e-005 5.5 10e-006 Mordenite-K 1.598e-007 -6.796 7.233e-005 3.445e-005 (total) 9.702e-005 3.996e-005 Aqueous species molality mg/kg sol'n act. coef. log act. ------C1- 0.5147 1.765ef004 0.6281 -0.4905 Na+ 0.4449 9892. 0.6701 -0.5256 Mg++ 0.0368 1 865.3 0.3192 -1.9299 SO&-- 0.01556 1446. 0.1749 -2.5651 K+ 0.009651 364.9 0.6281 -2.2 174 MgCl+ 0.008653 500.1 0.6701 -2.2367 NaS04- 0.0058 10 669.0 0.6701 -2.4096 Ca++ 0.005752 223.0 0.25 11 -2.8403 MgSO4 0.005267 613.1 1.0000 -2.2785 CaCl+ 0.003899 284.8 0.6701 -2.5829 NaCl 0.002 160 122.1 1.0000 -2.6655 C02(aq) 0.001616 68.76 1.0000 -2.7917 CaS04 0.0008059 106.1 1.0000 -3.0937 HCO3- 0.0005682 33.53 0.6882 -3.4078 SiO2(aq) 0.0003690 2 1.44 1.1665 -3.3661 NaHC03 0.000 1745 14.18 1.0000 -3.7582 KSO4- 0.0001722 22.51 0.6701 -3.9378 ME;HCO3+ 6.904e-005 5.697 0.6701 -4.3348 KC1 4.699e-005 3.388 1.0000 -4.3280 CaHC03+ 1.245e-005 1.217 0.7108 -5.0530 Fe++ 3.649e-006 0.1971 0.25 11 -6.0380 H+ 1.990e-006 0.001940 0.7970 -5.7997 Mn++ 1.875e-006 0.09960 0.251 1 -6.3273 H2S(aq) 9.817e-007 0.03235 1.0000 -6.0080 FeCI+ 8.559e-007 0.07557 0.6701 -6.2414 HSO4- 5.410e-007 0.05079 0.6701 -6.4406 FeS04 3.954.e-007 0.05808 1.0000 -6.4030 MnS04 2.268e-007 0.033 12 1.0000 -6.6443 Ba++ 1.505e-007 0.01998 0.2139 -7.4924 NaH3SiO4 1.48 1e-007 0.01692 1.0000 -6.8293 MnCI+ 1.439e-007 0.01258 0.6701 -7.0156 FeC12 9.73le-008 0.01193 1.0000 -7.0119 MnCl2 8.804e-008 0.01071 1.0000 -7.0553 MgC03 8.522e-008 0.006948 1.0000 -7.0695 HS- 7.083e-008 0.002265 0.6502 -7.3367 H3SiO4- 5.168e-008 0.004753 0.6701 -7.4606 co3-- 5.054e-008 0.002933 0.1946 -8.0073 Bas04 4.2 l3e-008 0.009509 1.0000 -7.3754 MgH3Si04+ 2.35 1e-008 0.0027 15 0.6701 -7.8026 CaC0.3 2.158e-008 0.002089 1.0000 -7.6659 Mg2CO3++ 1.828e-008 0,001920 0.2139 -8.4079 NaC03- 1.607e-008 0,001290 0.6701 -7.9679 FeHCO3+ l.213e-008 0.00137l 0.6701 -8.0900 BaCl+ 1.165e-008 0.001947 0.6701 -8.1075 MnCI3- 1.148e-008 0.001791 0.6701 -8.1139 MgOH+ 1.098e-008 0.0004388 0.6701 -8.1331 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K Pyrite 4.0883s/sat Magnesite -2.1083 Quartz 0.7336s/sat Sanidine high -2.2015 Tridyrnite 0.5630dsat Calcite -2.2321 Chalcedony 0.4575s/sat Mirabilite -2.3603 Cristobalite 0.17 los/sat Albite low -2.3775 Mordenite-K 0.0000 sat Albite -2.3776 Barite 0.0000 sat Aragonite -2.3974 Clinoptil-K -0.0140 Gibbsite -2.5763 ArnrphAsilica -0.5954 Halite -2.5943 Maximum Microcli -0.9678 Dolomite -2.6795 K-feldspar -0.9689 Dolomite-ord -2.6795 Gypsum -0.9694 Epsornite -2.7010 Anhydrite -1.1824 Phengite -2.7 126 Bassanite -1.8164 Sulfur-Rhmb -2.7145 Mordenite-Na -1.9205 Pyrrhotite -2.7359 CaS0&"1/2H2O(bet -1.99 14 Hexahydrite -2.9689 Witherite -2.0416 (only minerals with log Q/K > -3 listed) fugacity log fug. ------0.03871 -1.412 0.02240 -1.650 6.963e-006 -5.157 4.330e-009 -8.364 6.837e-012 -1 1.165 1.877e-020 -19.727 6.703e-069 -68.174 Cloud 2000- Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 Xi = ~.~~~~ Temperature = 15.8 C Pressure = 1.013 bars pH = 5.949 logp2 = -68.940 Eh = -0.0978 volts pe = -1.7067 Ionic strength = 0.617 123 Activity of water = 0.982941 Solvent mass = 1.000000 kg Solution mass = 1.034419 kg Solution density = 1.033 g/cm3 Chlorinity = 0.537362 molal Dissolved solids = 33274 mg/kg sol'n Rock mass = 0.000001 kg Minerals in system moles log moles grams volume (cm3) ...... ------Barite 7.581e-007 -6.120 0.0001769 3.950e-005 Mordenite-K 7.800e-007 -6.108 0.0003530 0.000 1681 (total) 0.0005299 0.0002076 Aqueous species molality mg/kg sol'n act. coef log act Ba++ 1.27'ie-007 0.01695 0.2141 -7.5633 H2S(aq) 1.153e-007 0.003799 1.0000 -6.938 1 MgCO3 1.15 1e-007 0.009380 1.0000 -6.9390 H3Si04- 1.055e-007 0,009702 0.6689 -7.15 13 co3-- 7.378e-008 0.004280 0.1948 -7.8425 MgH3SiO4-t 5.5 13e-008 0.006365 0.6689 -7.4332 FeHC03+ 4.853e-008 0.005482 0.6689 -7.4887 MnC13- 4.126e-008 0.006434 0.6689 -7.5591 Bas04 3.418e-008 0.007712 1.0000 -7.4662 CaCO.3 2.895e-008 0.002802 1.0000 -7.5383 Mg2CO3++ 2.658e-008 0.002791 0.2141 -8.2448 NaC03- 2.626e-008 0.002 107 0.6689 -7.7554 MnHCO3+ 1.925e-008 0.002 158 0.6689 -5.8901 MgOH+ 1.066e-008 0.0004257 0.6689 -8.1469 BaCl+ 1.003e-008 0.001675 0.6689 -8.1733 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K Pyrite 3.3 185s/sat CaS04"1/2H2O(bet -2.0500 Quartz 1.0263dsat Calcite -2.1011 Tridyrnite 0.85 18s/sat Nontronit-Na -2.145 1 Chalcedony 0.7464s/sat Mirabilite -2.1666 Cristobalite 0.4544s/sat Nontronit-Mg -2.2668 Mordenite-K 0.0000 sat Aragonite -2.2670 Barite 0.0000 sat Nontronit-K -2.3398 Clinoptil-K -0.0 153 Nontronit-Ca -2.4202 AmrphAsilica -0.3363 Dolomite -2.4286 Gypsum -0.9810 Dolomite-ord -2.4286 Anhydrite -1.2347 Halite -2.5727 Maximum Microcli -1.5415 Epsomite -2.6785 K-feldspar -1.5429 Sanidinehigh -2.8032 Witherite -1.8679 Siderite -2.8929 Bassanite -1.8698 Phengite -2.9060 Mordenite-Na - 1.9916 Hexahydrite -2.9766 Magnesite -2.0 142 (only minerals with log Q/K > -3 listed) Gases fugacity log fug. co2(g) 0.02932 -1.533 Steam 0.01731 -1.762 HeS(g) 6.852e-007 -6.164 Hem 1.986e-009 -8.702 CH4($) 9.975e-013 -12.001 3.278e-022 -2 1.484 O2(g) 1.14.9e-069 -68.940 Cloud eooi - Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 Xi = 0.0000 Temperature = 9.6 C Pressure = 1.013 bars pH = 6.860 logP2 = -71.317 Eh = -0.1528 volts pe = -2.7242 Ionic strength = 0.616628 Activity of water = 0.982955 Solvent mass = 1.000000 kg Solution mass = 1.034460 kg Solution density = 1.037 g/cm3 Chlorinity = 0.537919 molal Dissolved solids = 333 12 mg/kg sol'n Rock mass = 0.000000 kg Minerals in system moles log moles grams volume (cm3) ------Barite 7.14le-007 -6.146 0.0001667 3.72 1e-005 Mordenite-K 5.600e-007 -6.252 0.0002534 0.0001207 (total) 0.0004201 0.0001579 Aqueous species molality mg/kg sol'n act. coef. log act. ------c1- 0.52 12 l.786e+004 0.6254 -0.4868 Na+ 0.4512 1.003e+004 0.6669 -0.52 16 Mg++ 0.03645 856.5 0,3198 -1.9334 So4-- 0.01554 1443. 0.1761 -2.5626 R/IgCl+ 0.01040 600.8 0.6669 -2.1589 K+ 0.009894 374.0 0.6254 -2.2084 NaS04- 0.005636 648.6 0.6669 -2.4250 Ca++ 0.005 179 200.6 0.252 1 -2.8843 MgSO4 0.004930 573.6 1.OOOO -2.307 1 CaCl+ 0.004530 330.8 0.6669 -2.5198 NaCl 0.001733 97.89 1.000o -2.7613 HC03- 0.00133 1 78.50 0.6847 -3.0403 CaS04 0.0006854 90.20 1.0000 -3.1640 NaHC03 0.000503 1 40.86 1.0000 -3.2983 SiO2(aq) 0.0004886 28.38 1.1766 -3.2405 co2(aq) 0.0003798 16.16 1.0000 -3.4205 KSO4- 0.0001737 22.69 0.6669 -3.9362 MgHCO3-t 0.000 1549 12.78 0.6669 -3.9858 KC1 4.129e-005 2.975 1.0000 -4.3842 CaHC03+ 2.556e-005 2.498 0.7071 -4.7430 Fe++ 4.766e-006 0.2573 0.2521 -5.9204 Mn++ 4.056e-006 0.2 154 0.252 1 -5.9904 NaH3Si04 1.674e-006 0.191 1 1.0000 -5.7762 MgC03 1.429e-006 0.1165 1.0000 -5.8450 C03- 1.075e-006 0.06235 0.1958 -6.6770 FeCl+ 7.3 18e-007 0.06459 0.6669 -6.31 15 H3Si04- 5.677e-007 0.05219 0.6669 -6.42 18 FeS04 5.15 1e-007 0.07564 1.0000 -6.288 1 NaC03- 4.478e-007 0.03593 0.6669 -6.5248 MnS04 4.117e-007 0.06009 1.0000 -6.3855 Mg2CO3++ 3.695e-007 0.03880 0.2150 -7.1000 CaCO.3 3.600e-007 0.03484 1.0000 -6.4437 MgHsSiO++ 3.591e-007 0.04145 0.6669 -6.6207 MnC1+ 3.168e-007 0.02768 0.6669 -6.675 1 MnCl2 1.945e-007 0.02366 1.0000 -6.7 112 H+ l.745e-007 0.0001700 0.7920 -6.8596 Ba++ 9.406e-008 0.01249 0.2 150 -7.6942 FeCl2 8.442e-008 0.01034 1.0000 -7.0736 FeHC03+ 4.921e-008 0.005560 0.6669 -7.4838 MgOH+ 4.72 1e-008 0.001885 0.6669 -7.5019 HWaq) 4.351e-008 0.001433 1.0000 -7.3614 HS04- 3.650e-008 0.003424 0.6669 -7.6 137 OH- 3.142e-008 0.0005 166 0.6472 -7.69 17 MnC13- 2.569e-008 0.004006 0.6669 -7.766 1 MnHC03+ 2.446e-008 0.002742 0.6669 -7.7874 Bas04 2.437e-008 0.005498 1.0000 -7.6 13 1 CaH3Si04+ 2.284e-008 0.002985 0.6669 -7.8 172 HS- 2.149e-008 0.0006868 0.6472 -7.8568 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K ------Pyrite 3.7567dsat Phengite -1.4371 Quartz 1.055"2s/at Maximum Microcli -1.58 12 Tridymite 0.8'743dsat K-feldspar -1.5833 Chalcedony 0.769 ls/sat Mirabilite -1.8730 Cristobalite 0.4686s/sat Dolomite-dis -1.8950 Mordenite-K 0.0000 sat Bassanite - 1.9500 Barite 0.0000 sat Monohydrocalcite - 1.9706 Clinoptil-K -0.0 172 Saponite-Na -2.0862 Nontronit-Na -0.0$66 Mordenite-Na -2.0902 Nontronit-Mg -0.2069 Rhodochrosite -2.1 117 Nontronit-K -0.2 191 CaS04A1/2H20(bet -2.1382 Dolomite-ord -0.2333 Siderite -2.1702 Dolomite -0.2333 Saponite-Mg -2.2447 Nontronit-Ca -0.3522 Saponite-K -2.2643 ArnrphAsilica -0.361 1 Pyrrhotite -2.3476 Witherite -0.7000 Saponite-Ca -2.39 18 Talc -0.7024 Goethite -2.4079 Magnesite -0.9638 Halite -2.5521 Gypsum -0.9964 Epsomite -2.6464 Calcite -0.9965 Sanidine high -2.8866 Aragonite -1.1633 Hexahydrite -2.9843 Anhydrite -1.3 130 (only minerals with log Q/K > -3 listed) Gases fugacity log fug. Steam 0.01 157 -1.937 conk) 0.006885 -2.162 HeS(g) 1.946e-007 -6.7 11 Hz@ 2.284e-009 -8.641 CH4(g) 4.082e-012 -1 1.389 Sew 4.3 17e-024 -23.365 02(g) 4.818e-072 -71.317 Marker 33 1998 - Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 Xi = 0.0000 Temperature = 19.0 C Pressure = 1.013 bars pH = 4.925 logp2 = -67.631 Eh = -0.0348 volts pe = -0.6005 Ionic strength = 0.598843 Activity of water = 0.983463 Solvent mass = 1.000000 kg Solution mass = 1.033379 kg Solution density = 1.031 g/cm3 Chlorinity = 0.520020 molal Dissolved solids = 32301 mg/kg soh Rock mass = 0.000001 kg Minerals in system moles log moles grams volume (cm3) Barite 9.9 18e-007 -6.004 0.00023 15 5.167e-005 Mordenite-K 1.499e-006 -5.824 0.0006784 0.0003231 (total) 0.0009099 0.0003747 Aqueous species molality mg/kg sol'n act. coef log act. ------c1- 0.5056 1.735e+004 0.6292 -0.4974 Na+ 0.4358 9694. 0.6708 -0.5341 Mg++ 0.03641 856.3 0.3203 -1.9333 SO+- 0.01515 1408. 0.1765 -2.5730 K+ 0.009586 362.7 0.6292 -2.2 196 MgCl+ 0.008559 494.9 0.6708 -2.2410 NaS04- 0.005562 640.8 0.6708 -2.428 1 Ca++ 0.005559 215.6 0.2524 -2.8529 MgS04 0.005 101 594.1 1.0000 -2.292% CaCl+ 0.003795 277.4 0.6708 -2.5941 Cwaq) 0.002324 98.97 1.0000 -2.6338 NaCl 0.002042 115.5 1.0000 -2.6899 SiO2(aq) 0.001002 58.28 1.1638 -2.9331 CaSO4 0.0007636 100.6 1.0000 -3.1 171 KSO4- 0.0001676 21.92 0.6708 -3.9492 HC03- 0.0001079 6.370 0.6887 -4.1290 KC1 4.539e-005 3.275 1.0000 -4.3430 NaHC03 3.307e-005 2.689 1.0000 -4.4805 H+ 1.491e-005 0.01454 0.7966 -4.9252 MgHC03+ 1.296e-005 1.070 0.6708 -5.0609 Fe++ 1.180e-005 0.6379 0.2524 -5.5258 Mn++ 8.67 1e-006 0.4610 0.2524 -5.6598 HSO4- 3.878e-006 0.3642 0.6708 -5.5848 FeCl+ 2.633e-006 0.2326 0.6708 -5.7529 CaHC03+ 2.287e-006 0.2238 0.71 11 -5.7887 HeS(4 1.506e-006 0.04966 1.0000 -5.822 1 FeS04 1.260e-006 0.1853 1.0000 -5.8995 MnS04 1.019e-006 0.1489 1.0000 -5.9919 MnCI+ 6.58 1e-007 0.05756 0.6708 -6.3551 MnCl2 3.965e-007 0.04829 1.0000 -6.401 7 FeC12 2.953e-007 0.03622 1.0000 -6.5297 Ba++ 1.467e-007 0.01950 0.2153 -7.5004 NaH3SiO4 5.127e-008 0.005860 1.0000 -7.2901 MnCl3- 5.083e-008 0.007934 0.6708 -7.4673 Bas04 4.028e-008 0.009097 1.0000 -7.3949 H3Si04- 1.818e-008 0.001673 0.6708 -7.9139 H S- 1.391e-008 0.0004450 0.651 1 -8.0432 BaCl+ 1.124e-008 0.001880 0.6708 -8.1225 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K ------Pyrite 3.641ls/sat Bassanite -1.8455 Quartz 1.1830s/sat Maximum Microcli -1.8645 Tridymite 1.0116s/sat K-feldspar -1.8657 Chalcedony 0.906 ls/sat Mordenite-Na - 1.941 1 Cristobalite 0.6 184s/sat CaS04" 1/2H20(bet -2.02 16 Mordenite-K 0.0000 sat Sulfur-Rhmb -2.1277 Barite 0.0000 sat Mirabilite -2.3422 Clinoptil-K -0.0 139 Halite -2.6072 ArnrphAsilica -0.1533 Epsomite -2.7060 Gypsum -0.989 1 Hexahydrite -2.98 10 Anhydrite -1.2 113 (only minerals with log Q/K > -3 listed) Gases fugacity log fug. COW 0.0544 1 -1.264 Steam 0.02119 -1.674 H2S(g) 1.028e-005 -4.988 H2(g) 1.616e-009 -8.791 CH4(g) 2.569e-013 -12.590 SL2(g) 2.380e-019 -18.623 Ow 2.338e-068 -67.63 1 Marker 33 1999 - Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 xi = 0.0000 Temperature = 68.3 C Pressure = 1.013 bars pH = 4.824 logp2 = -56.443 Eh = -0.0602 volts pe = -0.8887 Ionic strength = 0.542550 Activity of water = 0.984822 Solvent mass = 1.000000 kg Solution mass = 1.030418 kg Solution density = 0.995 g/cm3 Chlorinity = 0.477288 molal Dissolved solids = 29520 mg/kg sol'n Rock mass = 0.000001 kg Minerals in system moles log moles grams volume (cm3) Barite 8.260e-007 -6.083 0.0001928 4.303e-005 Mordenite-K 8.569e-007 -6.067 0.0003878 0.0001847 (total) 0.0005806 0.0002277 Aqueous species molality mg/kg soh act. coef. log act. - - c1- 0.4657 1.602e+004 0.6176 -0.541 1 Na+ 0.3974 8866. 0.6606 -0.5809 Mg++ 0.03 149 742.7 0.2994 -2.0256 S04-- 0.0 1098 1023. 0.1596 -2.7565 K+ 0.00975 1 370.8 0.6176 -2.2 193 Ca++ 0.007622 296.5 0.2327 -2.75 11 MgCI+ 0.004594 266.4 0.6606 -2.5 179 NaCl 0.00448 1 254.2 1.0000 -2.3486 NaS04- 0.004466 5 16.0 0.6606 -2.5301 MgSO4 0.004356 508.8 1.0000 -2.3609 SiO2(aq) 0.003525 205.6 1.1170 -2.4047 CaCl+ 0.002390 175.2 0.6606 -2.8017 Co2(aq) 0.002012 85.95 1.0000 -2.6963 CaS04 0.001 104 145.8 1.0000 -2.957 1 KSO4- 0.0001597 20.94 0.6606 -3.9768 HC03- 9.100e-005 5.388 0.6791 -4.2090 KC1 7.827e-005 5.663 1.0000 -4.1064 Mn++ 2.863e-005 1.527 0.2327 -5.1763 H+ 1.892e-005 0.01850 0.7920 -4.8244 HS04- 1.796e-005 1.692 0.6606 -4.9259 MgHCO3+ 1.234e-005 1.022 0.6606 -5.0887 NaHCO3 1.04le-005 0.8486 1.0000 -4.9826 MnS04 5.552e-006 0.8 136 1.0000 -5.2556 CaHC03+ 3.922e-006 0.3848 0.7024 -5.5599 H2S(aq) 2.894e-006 0.09570 1.0000 -5.5385 Ba++ 8.045e-007 0.1072 0.1968 -6.8005 FeCl+ 6.986e-007 0.06190 0.6606 -6.3359 Fe++ 4.793e-007 0.02598 0.2327 -6.9526 NaHSSi04 3.252e-007 0.03727 1.0000 -6.4878 Bas04 2.395e-007 0.05424 1.0000 -6.6208 HSSiO4- 1.603e-007 0.01480 0.6606 -6.975 1 HS- 7.603e-008 0.002440 0.640 1 -7.3 127 FeCl2 6.255e-008 0.007695 1.0000 -7.2038 MgOH+ 4.603e-008 0.001846 0.6606 -7.5 170 FeS04 3.950e-008 0.005824 1.0000 -7.4034 OH- 1.525e-008 0.00025 16 0.6401 -8.0106 MnHC03+ 1.505e-008 0.001694 0.6606 -8.0025 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K Quartz 0.9973s/sat Bassanite -1.3049 Pyrite 0.8685s/sat CaS04"1/2H2O(bet -1.4249 Tridymite 0.8663s/sat Maximum Microcli -1.6098 Chalcedony 0.7604s/sat K-feldspar - 1.6154 Cristobalite 0.5305s/sat Albite -2.3781 Mordenite-K 0.0000 sat Albite low -2.378 1 Barite o.oooo sat Sanidine high -2.5623 Clinoptil-K -0.0042 Sulfur-Rhmb -2.5651 ArnrphAsilica -0.0267 Clinoptil-Na -2.5849 Anhydrite -0.6704 Clinoptil-Ca -2.68 15 Gypsum -0.9035 Halite -2.7454 Mordenite-Na -1 287 Magnesite -2.79 12 (only minerals with log Q/K > -3 listed) Gases fugacity log fug. Steam 0.2813 -0.551 Co2(g) 0.1255 -0.901 H2S(g) 7.956e-005 -4.099 Hew 1.017e-007 -6.993 CH4(g) 2.679e-012 -1 1.572 S2(g) 6.646e-017 -16.177 3.606e-057 -56.443 Marker 33 2000 - Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 Xi = 0.0000 Temperature = 24.8 C Pressure = 1.013 bars pH = 6.150 logfl2 = -67.128 Eh = -0.1258 volts pe = -2.1287 Ionic strength = 0.609359 Activity of water = 0.983049 Solvent mass = 1.000000 kg Solution mass = 1.033999 kg Solutiondensity = 1.027 g/cm3 Chlorinity = 0.533020 molal Dissolved solids = 32881 mg/kg sol'n Rock mass = 0.000001 kg Minerals in system moles log moles grams volume (cm3) Barite 2.597e-006 -5.586 0.0006061 0.0001353 Mordenite-K 3. 7 10e-007 -6.43 1 0.000 1679 7.995e-005 (total) 0.0007740 0.0002 152 Aqueous species molality mg/kg soh act. coef. log act. C1- Na+ Mg++ S04-- K+ MgCl+ Ca++ NaS04- MgS04 CaCI+ NaCl SiO2(aq) COQ(aq) CaS04 HC03- NaHC03 KS04- MgHCO3-t KC1 CaHCO3+ Mn++ MnS04 Fe++ NaH3Si04 MnCI+ H+ MnCl2 H2S(aq) FeCl+ H3Si04- MgC03 HSO4- Ba++ 2.04le-007 0.0271 1 0.2126 -7.3627 co3-- 1.932e-007 0.01 121 0.1933 -7.4277 MgHsSiO++ 1.618e-007 0.01 869 0.6706 -6.9645 FeS04 1.480e-007 0.02 175 1.0000 -6.8297 CaCO3 1.057e-007 0.01023 1.0000 -6.9759 MnC13- 9.795e-008 0.01528 0.6706 -7.1825 HS- 9.346e-008 0.002989 0.6505 -7.2 16 1 Mg2CO3++ 6.674e-008 0.00701 1 0.2126 -7.848 I MnHC03+ 6.639e-008 0.007445 0.6706 -7.3515 NaC03- 5.413e-008 0.004345 0.6706 -7.4401 Bas04 5.330e-008 0.01203 1.0000 -7.2733 FeCl2 4.989e-008 0.006116 1.0000 -7.3020 MgOH+ 3.684e-008 0.001472 0.6706 -7.6072 OH- 2.119e-008 0.0003486 0.6505 -7.8606 BaCl+ 1.582e-008 0.002643 0.6706 -7.9745 CaH3Si04+ 1.407e-008 0.001839 0.6706 -8.0252 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K Pyrite 3.4970s/sat Maximum Microcli -1.8 192 Quartz 1.152'is/sat K-feldspar -1.8202 Tridymite 0.9869s/sat Mordenite-Na -1.8666 Chalcedony 0.88 12s/sat CaS04"1/2H2O(bet -1.906 1 Cristobalite 0.6012s/sat Talc -1.9655 Barite 0.0000 sat Phengite -2.2 156 Mordenite-K o.oooo sat Rhodochrosite -2.23 12 Clinoptil-K -0.0128 Nontronit-Na -2.4525 AmrphAsilica -0.1379 Nontronit-Mg -2.5245 Gypsum -0.9394 Monohydrocalcite -2.5443 Anhydrite -1.1040 Halite -2.6026 Dolomite -1.3669 Mirabilite -2.6348 Dolomite-ord -1.3669 Nontronit-K -2.6706 Witherite -1.4197 Nontronit-Ca -2.6768 Magnesite -1.4549 Pyrrhotite -2.7 126 Calcite -1.5429 Epsomite -2.7968 Aragonite -1.7075 Dolomite-dis -2.9136 Bassanite -1.7372 (only minerals with log Q/K > -3 listed) Gases fugacity log fug Steam 0.03016 -1.521 Co2(g) 0.02874 -1.541 H2S(g) 4.065e-006 -5.39 1 Ha(& 8.936e-009 -8.049 CH4(g) 1.672e-011 -10.777 s2(g) 4.583e-02 1 -20.339 O2(d 7.449e-068 -67.128 Marker 33 2001 - Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 Xi = 0.0000 Temperature = 25.0 C Pressure = 1.013 bars pH = 5.609 l0gP2 = -66.~~0 Eh = -0.09 16 volts pe = -1.5492 Ionic strength = 0.599803 Activity of water = 0.983253 Solvent mass = 1.000000 kg Solution mass = 1.033528 kg Solutiondensity = 1.027 g/cm3 Chlorinity = 0.5270 11 molal Dissolved solids = 32441 mg/kg soh Rock mass = 0.000014 kg Minerals in system moles log moles grams volume (cm3) ArnrphAsilica 0.0002 18 1 -3.661 0.013 10 0.006324 Barite 2.45 1e-006 -5.61 1 0.0005719 0.0001277 Mordenite-K 6.000e-007 -6.222 0.00027 15 0.0001293 (total) 0.01395 0.00658 1 Aqueous species molalit~ mg/kg soh act. coef log act. C1- Na+ Mg++ SO+-- K+ Ca++ MgCl+ NaS04- CaCl+ MgS04 NaCl CO2(aq) SiO.~(aq) CaSO4 HC03- KSO4- NaHC03 KC1 MgHC03+ Mn++ CaHC03+ H+ MnS04 H2S(aq) Fe++ MnCl+ MnCle HSO4- FeCI+ NaHsSi04 Ba++ H3Si04- 1.748e-007 0.01608 0.6746 -6.9285 FeS04 1.7 lee-007 0.025 17 1.0000 -6.7664 HS- 1.438e-007 0.004601 0.6545 -7.0263 MnC13- 1.401e-007 0.02 187 0.6746 -7.0244 MgHSSiO4+ 6.152e-008 0.007 108 0.6746 -7.38 19 FeCl2 6.105e-008 0.007487 1.0000 -7.2143 Bas04 5.456e-008 0.01232 1.0000 -7.263 1 MgC03 4.86 1e-008 0.003966 1.0000 -7.3 133 MnHCO3+ 4.705e-008 0.005278 0.6746 -7.4984 co3-- 2.826e-008 0.001641 0.1945 -8.2599 CaC03 1.864e-008 0.001806 1.0000 -7.7294 BaCl+ 1.680e-008 0.002808 0.6746 -7.9457 MgOH+ 1.025e-008 0.0004097 0.6746 -8.1602 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K Pyrite 4.019 ls/sat Mordenite-Na -1.8622 Quartz 1.2857s/sat Maximum Microcli -2.0840 Tridymite 1.1199s/sat K-feldspar -2.0840 Chalcedony 1.0145s/sat Witherite -2.2437 Cristobalite 0.7352s/sat Calcite -2.3 144 Mordenite-K 0.0000 sat Magnesite -2.3232 ArnrphAsilica 0.0000sat Sulfur-Rhmb -2.4066 Barite 0.0000 sat Aragonite -2.4793 Clinoptil-K -0.0047 Halite -2.61 14 Gypsum -0.888 1 Mirabilite -2.671 1 Anhydrite -1.05 15 Epsomite -2.8407 Bassanite -1.6841 Pyrrhotite -2.847 1 CaS04"l /~HeO(bet-1.8527 Rhodochrosite -2.9160 (only minerals with log Q/K > -3 listed) Gases fugacity log hg. --- 0.04927 -1.307 0.03079 -1.5 12 2.209e-005 -4.656 7.607e-009 -8.119 1.348e-011 -10.870 1.950e-019 -18.710 1.319e-067 -66.880 Snail 2000 - Mineral precipitation enabled with some phases suppressed (Table 2.3 in Chapter 2). Step # 0 xi = 0.0000 Temperature = 14.7 C Pressure = 1.013 bars pH = 6.069 10gF2 = -68.923 Eh = -0.0993 volts pe = -1.7381 Ionic strength = 0.614323 Activity of water = 0.983002 Solvent mass = 1.000000 kg Solution mass = 1.034313 kg Solution density = 1.034 g/cm3 Chlorinity = 0.535465 molal Dissolved solids = 33 175 mg/kg sol'n Rock mass = 0.000000 kg Minerals in system moles log moles grams volume (cm3) ------Barite 2.869e-007 -6.542 6.696e-005 1.495e-005 Mordenite-K 5.200e-007 -6.284 0,0002353 0.0001121 (total) 0.0003023 0.0001270 Aqueous species molality mg/kg soh act. coef. log act. co3-- 1.154e-007 0.006695 0.1953 -7.6471 FeS04 9.678e-008 0.0142 1 1.0000 -7.0142 MgHsSiO4+ 7.122e-008 0.008222 0.6688 -7.322 1 CaCO3 4.5 11e-008 0.004365 1.0000 -7.3457 NaCO3- 4.227e-008 0.009392 0.6688 -7.5486 Mg2CO3++ 3.948e-008 0.004146 0.2 1+6 -8.072 1 Bas04 3.225e-008 0.007278 1.0000 -7.4914 FeC12 1.985e-008 0.002433 1.0000 -7.702 1 MnCI3- 1.788e-008 0.002788 0.6688 -7.9224 H2S(aq) 1.584e-008 0.00052 19 1.0000 -7.8002 MgOH+ 1.238e-008 0.0004945 0.6688 -8.0819 MnHCO3+ 1.02 1e-008 0.001 144 0.6688 -8.1659 (only species > le-8 molal listed) Mineral saturation states log Q/K log Q/K Quartz 1.0480s/sat Bassanite -1.8756 Pyrite 0.91 86s/sat Calcite -1.9073 Tridymite 0.8723dsat Mordenite-Na -2.0108 Chalcedony 0.7670s/sat CaS04"1/2H2O(bet -2.0572 Cristobalite 0.4735dsat Dolomite -2.0623 Barite 0.0000 sat Dolornite-ord -2.0623 Mordenite-K 0.0000 sat Aragonite -2.0733 Clinoptil-K -0.0 156 Mirabilite -2.1 157 AmrphAsilica -0.3239 Halite -2.5708 Gypsum -0.9754 Epsomite -2.6826 Anhydrite -1.2402 Phengite -2.752 I Maximum Microcli -1.58 18 Sanidine high -2.85 12 K-feldspar -1.5833 Monohydrocalcite -2.8885 Witherite -1.6712 Hexahydrite -2.9883 Magnesite -1.8487 (only minerals with log Q/K > -3 listed) fugacity log fug. ------0.02663 -1.575 0.01614 -1.792 8.961e-008 -7.048 l.238e-009 -8.907 2.047e-013 -12.689 1.105e-023 -52.957 1.193e-069 -68.923 12: Diffuse Fluid - Seawater Mixing Results Listed here are the output reaction path plots from React, produced when diffuse fluids are mixed with seawater. These results are only for runs where measured parameters (e.g. temperature, pH) were used. Results show changes in the diffuse fluid as seawater is reacted into it - illustrating the precipitation and dissolution of mineral phases as mixing proceeds (A). Also shown are changes in mineral precipitation with changing temperature (B), changes inflq,) (C) and pH (D). Solution composition changes from a diffuse fluid composition to a seawater composition from left to right across the diagrams. 13: Diffuse Fluid - Basalt Reaction Results Listed here are the output reaction path plots from React, produced when fresh basalt was reacted with diffuse fluids. These results are only for runs where measured parameters (e.g. temperature, pH) were used. Results show mineralogical changes in the diffuse fluid as basalt is reacted into it (A), changes inflq,) (B), pH (C) and fluid bulk composition (only for those elements that showed change in concentration during reaction).