A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge

A deep tow magnetic survey of Middle Valley,

Juan de Fuca Ridge

Jeffrey S. GeeScripps Institution of Oceanography, La Jolla, California 92093, USA ( [email protected])

Spahr C. WebbLamont Doherty Earth Observatory, Palisades, New York 10964, USA ([email protected])

Jeffrey Ridgway, Hubert Staudigel, and Mark A. ZumbergeScripps Institution of Oceanography, La Jolla, California 92093, USA ( [email protected]; [email protected];

[email protected])

[1] Abstract: We report here results from a deep tow magnetic survey over Middle Valley, Juan de

Fuca Ridge. A series of track lines are combined to generate a high-resolution map of the magnetic

field anomaly within a 10 � 12 km region surrounding the Bent Hill massive sulfide (BHMS)

deposit. A uniformly magnetized body (5 A/m) with a cross section approximating the body inferred

from Ocean Drilling Program (ODP) drilling can account for the observed near-bottom magnetic

anomaly amplitude. Assuming this magnetization is entirely induced, the average susceptibility (0.11

SI) corresponds to �3.5% magnetite + pyrrhotite by volume, consistent with the abundance of thesephases observed in drill core samples. However, this uniform magnetization model significantly

underestimates the magnetic anomaly measured a few meters above the seafloor by submersible,

indicating that the upper portion of the sulfide mound must have a significantly higher magnetization

(�10% magnetite + pyrrhotite) than at deeper levels. On a larger scale, the near-bottom magneticanomaly data show that basement magnetizations are not uniformly near zero, as had been inferred

from analysis of the sea surface anomaly pattern. We interpret this heterogeneity as reflecting

primarily differences in the degree of hydrothermal alteration. Our results highlight the potential of

magnetic anomaly data for characterizing hydrothermal deposits where extensive drill core sampling

is not available.

Keywords: Magnetic anomaly; hydrothermal alteration; mid-ocean ridge.

Index terms: Magnetic anomaly modeling; spatial variations attributed to seafloor spreading; hydrothermal systems;

midocean ridge processes.

Received April 24, 2001; Revised July 20, 2001; Accepted July 25, 2001; Published November 15, 2001.

Gee, J., S. Webb, J. Ridgway, H. Staudigel, and M. Zumberge, A deep tow magnetic survey of Middle Valley,

Juan de Fuca Ridge, Geochem. Geophys. Geosyst., 2, 10.1029/2001GC000170, 2001.

1. Introduction

[2] At sedimented ridges, hydrothermal circu-

lation is confined beneath relatively imperme-

able sediments, which restrict heat and fluid

flux out of the system [Davis and Fisher,

1994], and consequently discharge is focused

G3G3GeochemistryGeophysics

Geosystems

Published by AGU and the Geochemical Society

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

GeochemistryGeophysics

Geosystems

Article

Volume 2

November 15, 2001

Paper number 2001GC000170

ISSN: 1525-2027

Copyright 2001 by the American Geophysical Union

into widely separated vent fields. Hydrothermal

activity in these settings can produce signifi-

cant massive sulfide deposits that may be of

economic significance. Key factors leading to

the development of massive sulfide deposits

include the interaction of seawater with hot

oceanic crust and often the subsequent reaction

of these fluids with organic-rich sediments.

Such ‘‘Kieslager’’ or ‘‘Besshi’’ type massive

sulfide deposits are commonly found in the

geological record and may be extremely large

(e.g., Ducktown, Tennessee; Windy Craggy,

British Columbia). The most prominent mod-

ern Besshi-type deposits are found in the

Guaymas basin [Gieskes et al., 1982], the

Escanaba Trough (Gorda Ridge [Zierenberg et

al., 1993]) and in Middle Valley (Juan de Fuca

Ridge [Davis et al., 1987]).

[3] Middle Valley, a sediment covered axial

valley at the northern end of the Juan de Fuca

Ridge (Figure 1), has been the target of exten-

sive geophysical investigations and was a

major focus of Ocean Drilling Program (ODP)

Legs 139 and 169. The rift valley has been

buried by from 200 m to more than 1000 m of

turbiditic and hemipelagic sediments derived

from the adjacent continental margin during

the Pleistocene sea level low stand. A number

of hydrothermal centers have been documented

in Middle Valley. In this paper, we focus

primarily on the inactive massive sulfide

mound located south of Bent Hill, a 500-m

diameter feature that rises �60 m above thesurrounding seafloor. The Bent Hill massive

sulfide (BHMS) deposits were produced by

high-temperature fluids (�350–4008C [Good-fellow and Peter, 1994; Peter et al., 1994]),

with a significant volume of the deposit likely

already formed 200–140 kyr ago [Mottl et al.,

1994]. Despite its proximity to Bent Hill, a

variety of lines of evidence indicate that the

BHMS deposit is not genetically related to the

intrusions that resulted in the uplift of Bent Hill

[Mottl et al., 1994].

[4] We collected near-bottom magnetic anom-

aly data over the BHMS in order to corrobo-

rate the overall dimensions and hemispherical

shape of the deposit inferred from drill core

data [Fouqet et al., 1998] and to provide

additional constraints on the integrated proper-

ties of this ore deposit. The reduced to the

pole magnetic anomaly is nearly circular, with

a width of 150 m EW and 190 m NS at half

peak value. Forward magnetic models approx-

imating the hemispherical source and with an

induced magnetization of �5 A/m (suscepti-bility 0.11 SI) match the observed anomaly.

Moreover, this same source model with a

density contrast of 2300 kg/m3 yields a grav-

ity signal of 3.5 mgal, close to estimates from

a previous on bottom gravity survey [Ballu et

al., 1998]. Comparison of our near-bottom

magnetic anomaly data with a previous sub-

mersible survey a few meters above the sul-

fide mound [Tivey, 1994a] indicates that the

upper portion of the sulfide mound must have

a significantly higher magnetization than at

deeper levels.

[5] Near-bottom magnetic anomaly variations

away from Bent Hill also provide insights

into the effects of hydrothermal alteration on

the magnetization of the seafloor basalts.

Analyses of sea surface magnetic surveys

suggest near-zero magnetization within Mid-

dle Valley [Currie and Davis, 1994; Davis

and Lister, 1977a]. The low magnetizations

in this and other sedimented ridge settings

have been attributed to either the destruction

of remanence-carrying phases during exten-

sive hydrothermal alteration or to the demag-

netizing effect of elevated temperatures

[Larson et al., 1972; Levi and Riddihough,

1986; Currie and Davis, 1994]. Our deep

tow magnetometer results indicate finite but

small magnetization throughout much of the

valley. Together with other geophysical data,

these results allow us to investigate the

relative importance of temperature and alter-

GeochemistryGeophysicsGeosystems G3G3 gee et al.: deep tow magnetic survey 2001GC000170

ation in affecting magnetization in the

region.

2. Middle Valley

[6] Middle Valley is the northernmost segment

of the Juan de Fuca Ridge (Figure 1). It is

terminated by the Sovanco transform fault to

the north, with the NE-trending Nootka fault

forming a tectonically complicated triple junc-

tion between the Pacific, Explorer, and Juan de

Fuca plates [Davis and Currie, 1993]. The full

spreading rate of the Juan de Fuca Ridge is 6

cm/yr, but most of the recent extension at the

northern end of the Juan de Fuca Ridge is

focused in West Valley, just west of Middle

Valley as a result of a recent westward ridge

jump. Normal faults mark the valley boundaries

to the east and west. An additional west-facing

normal fault in the center of the valley sepa-

rates the central rift graben from a shallow

basement bench to the east [Davis and Villin-

ger, 1992]. The thickness of the turbidite sedi-

ments ranges from over 1 km to the north to

less than a hundred meters to the south with

thinner sediments found to the east of the

central fault [Davis and Villinger, 1992].

0 5km

SOVANCO F Z

STUDY AREA

WESTVALLEY

ENDEAVOURSEGMENT

(Bro

ad Z

one)

(Broad Zone)

NOOTKA

FAULT

50o

130o 127o

48o

Cascadia

Subduction

Zone

MIDDLEVALLEY

PacificPlate

Juan de FucaPlate

ExplorerPlate

48o25'N

48o30'N

128o55'W 128o50'W 128o45'W 128o40'W 128o35'W

-240

0

-240

0

-240

0

-240

0

-240

0

-240

0

-240

0

-230

0

-240

0

-2500

-250

0

-230

0

-240

0

Dead Dog858/1036

Bent Hill856/1035

857855

Figure 1. Bathymetry and location of ODP drill sites (circles) in Middle Valley. Box indicates the areacontaining the near bottom survey lines. Inset shows simplified tectonic map with location of Middle Valley.


[7] During ODP Legs 139 and 169, four sites

were drilled in the eastern part of the valley

to study various aspects of the hydrothermal

system beneath the sediments [Davis et al.,

1992; Fouqet et al., 1998]. Site 855, located

near the eastern boundary fault, recovered

basaltic flows with minor alteration that char-

acterize the low-temperature fluid recharge

for the hydrothermal circulation. Sites 856

and 1035 were dedicated primarily to sam-

pling the relict high temperature (350–

4008C) BHMS deposit. Results from two

sampling transects revealed an �100 m thick,hemispherical shaped massive sulfide deposit

underlain by a copper rich feeder zone.

Mineralogical, textural, and geochemical

studies indicate that the BHMS deposit

formed by mineral precipitation near or

above the seafloor, rather than by replace-

ment of sediments.

[8] Site 858 is within the Dead Dog Vent field,

a large active vent field discharging fluids as

hot as 2768C from at least 20 active vents[Ames et al., 1993]. The vent field overlies a

small basement high that acts to focus hydro-

thermal fluids to the Dead Dog vent field

[Davis and Fisher, 1994]. Site 857, located

�1.5 km south of the Dead Dog vent field,was designed to reach the permeable hydro-

logical basement where high-temperature cir-

culation was inferred to occur beneath an intact

sediment cover. Drilling revealed that the per-

ceived basement-sediment interface in seismic

refraction records [Rohr and Schmidt, 1994]

was the top of an extensive sequence of inter-

bedded sills and turbidite sediments that must

underlie much of the valley [Langseth and

Becker, 1994].

[9] The location of active high-temperature

vents is evident in a map of heat flow in the

valley (Figure 2). Although no active venting

was found at the BHMS deposit, the smaller

ODP and Lone Star vents 300–400 m south

were actively venting high-temperature fluids

(�2658C). At this site and at the Dead Dogvent field, a shallow subseafloor silicification

zone forms a caprock for the present hydro-

thermal reservoir. This horizon divides the

hydrothermal system into two nearly independ-

ent systems above and below the caprock [Stein

et al., 1998; Stein and Fisher, 2001]. As noted

in previous regional studies [e.g., Davis and

Lister, 1977a], heat flow in the valley is

approximately inversely proportional to sedi-

ment thickness, suggesting that temperature

variations at the basement-sediment interface

are likely to be subdued as a result of circu-

lation within the relatively permeable basement

[Davis and Villinger, 1992; Bessler et al.,

1994].

.4

W / m2.20 .4 .6 .8 1 2 448o30'N

128o40'W128o45'W

4.9

855

1.4

.6.6

858/1036

856/1035

857

.8

.8

1.0

.6.6

.4.2

.6

24

6.6

.8.8

.6

.4

48o28'N

48o26'N

48o24'N

Figure 2. Heat flow data from Middle Valley.Active high temperature vents in the Dead Dog andBent Hill areas are accompanied by high heat flow.Location of ODP drill sites indicated by circles.Contour interval 0.2 for values <1.0 W/m2 and 2 forvalues above 2.0 W/m2. Modified from Davis andVillinger [1992].


[10] A recent ocean bottom seismometer study

tracked the seismicity (due to thermal contrac-

tion) associated with the mining of heat beneath

the Dead Dog vent field to investigate the

source region of the vent fluids (C. E. Golden,

S. C. Webb, and R. A. Sohn, Hydrothermal

microearthquake swarms beneath active vents

at Middle Valley, Juan de Fuca ridge, submitted

to Journal of Geophysical Research, 2001).

Earthquakes occurred primarily in large, tightly

grouped swarms, but the swarms extended

many kilometers from the vent field forming

a ramp-like structure that deepened to the north

to depths of 2.5 km (below seafloor) away from

the vent field. These results suggest a large

volume of rock provides heat for the fluids at

the vent field.

3. Deep Tow Survey

[11] The gravity or magnetic signal from a two-

dimensional feature with a wavelength of L is

attenuated by exp(�2pz/L), where z is the

height of the observer above the source. Thus

narrow geologic features with wavelengths

shorter than the ocean depth will be so strongly

attenuated as to be virtually undetectable from

surface surveys. To examine the structure of the

narrow (�100 m) BHMS deposit, we thereforetowed an instrument package with both a

magnetometer and gravimeter near the seafloor.

The towed deep-ocean gravimeter was devel-

oped to enable high-resolution surveys of sea-

floor geological features [Zumberge et al.,

1997]. The meter is towed 50–100 meters

above the seafloor at a speed of 1.5 knots

(0.77 m/s). The gravity results will be reported

elsewhere. Here we report results obtained

from a magnetometer towed above the grav-

imeter package.

[12] Both the magnetometer and gravimeter

were deployed from the ship’s 1.73 cm

(0.680 inches) electrically conducting armored

cable, via a telemetry interface (called the

Deep Sea Instrument Interface, or DSII)

(Figure 3). Control communications, the grav-

ity and magnetic data, and acoustic naviga-

tion signals were multiplexed onto this cable

along with power to the instruments. The

DSII navigation package carried multiplexers

and power separation electronics along with

an acoustic interrogation transponder (10–12

KHz bandwidth) that was used to navigate

the towed sensors. Acoustic transponders

placed on the ocean bottom in the survey

area exchanged signals with a transducer

on the DSII to provide precise navigation

of the package. The package also carried

a CTD package. The height of the instrument

over the seafloor was measured with a 3.5

kHz down-looking echo sounder on the DSII.

The vertical position of the instrument was

determined by recording pressure and convert-

ing to a depth assuming a seawater density

profile.

[13] The deep tow magnetometer system was

based on a low cost miniature angular orienta-

tion sensor built by Applied Physics Systems

(model 544). The system consists of a three

axis fluxgate magnetometer and a three axis

accelerometer and was designed to provide roll,

pitch, and azimuth from combinations of the

accelerometer and magnetometer data. The

specified fluxgate noise level and alignment

are 0.4 nT and ±0.28. The direct magnetometeroutput (sampled at 2–3 times per second)

proved too noisy for direct detection of mag-

netic anomalies because of excessive motion of

the magnetometer package during towing. To

improve resolution of the magnetic anomalies

the data were smoothed by applying a 100 s

(full width, 6 sigma) Gaussian filter to the root

mean square total field calculated from the raw

data.

[14] The small size of the Bent Hill sulfide body

and of other targets within Middle Valley made


accurate navigation of the tow body critical to

the interpretation of the data. We deployed

acoustic transponders on short moorings (100

m) at four sites within Middle Valley at the

beginning of the cruise. The mooring locations

were determined to within an absolute accuracy

of a few meters by combining P-Code GPS

ship locations and acoustic ranges between the

ship and moorings obtained during an initial

survey of the transponders. Throughout the

cruise we navigated the ship and the tow fish

by acoustic ranging between the DSII deep tow

navigation package, the moorings, and a trans-

ducer mounted on the ship’s hull. Ship and

deep tow package positions were determined

roughly every 1 min with an accuracy of a few

meters during each tow.

[15] The magnetic data in this paper were

obtained from three lowerings of the tow

vehicle during a cruise on the R/V Thompson

in July 1998. A constant depth of observation

of 2420 m for the DSII was chosen for the bulk

of the survey, with the intent that the gravimeter

would never be closer than �20 m to theseafloor. The magnetometer was flown �30m above the DSII (Figure 3), so its tow depth

was mostly near 2390 m. A watch stander

monitored the depth of the DSII and controlled

the ship’s 1.73 cm traction winch to keep the

magnetometer on tether with float

0.680" electro-mechanical cable

Interface Package Down-looking sonar Transponder Interrogator Pressure Gauge

gravimeter

nylon tetherTransponder

Figure 3. Tow configuration showing relative positions of the towed magnetometer, gravimeter, interfacepackage (DSII).


DSII depth constant as changes in the ship’s

speed caused slow fluctuations in wire angle.

This was generally successful within a window

of less than 10 m except during turns when the

excursion in depth could exceed a few hundred

meters. These depth excursions are not a sig-

nificant problem for interpretation of the mag-

netometer data.

[16] We discovered through an analysis of cross

over errors in the resulting smoothed data

stream that there was an apparent dependence

of the total field value on the azimuth (u) of themagnetometer. This heading dependence can

be attributed to small errors (<0.2%) in the gain

and orientation (<0.058) of the individual mag-netometer components. In the data shown in

this paper the azimuthal error has been

removed by fitting sine and cosine components

in azimuth to the data at the track crossing

points, using only those crossovers where the

difference in instrument elevation was <50 m.

A small second harmonic in azimuth depend-

ence was also observed in the cross over point

data and terms in (cos(2u), sin(2u)) were fit andremoved from the data. RMS cross over errors

were reduced from 113 to 32 nT for tow 3 and

from 197 to 48 nT for the first two tows. The

first and second tows also exhibited an offset

from tow 3 of 103 nT. Fortunately, the large

number of track line crossings in this data set at

a wide variety of headings (Figure 4) allows the

azimuthal error to be accurately determined.

The procedure is essentially a mathematical

system of ‘‘boxing’’ the compass and is a

useful technique to apply to any towed mag-

netic survey data. The IGRF field value at Bent

Hill (48.458N, 128.688W) for 1998.6 of 54500nT was subtracted from all total field data to

calculate the magnetic anomalies.

4. Results Near Bent Hill

[17] The observed magnetic anomaly data from

the combination of the three track lines is

shown in Figure 4. The ‘‘race track’’ character

of the track lines is the result of trying to fly

closely spaced track lines repeatedly over the

vicinity of the Bent Hill massive sulfide

(BHMS) deposit while holding long intervals

of constant heading to obtain stable Eotvos

corrections for the gravimeter. The magnetic

anomaly is negative to the west of the eastern

bounding faults, implying low magnetization

beneath the sediments. The color scale in this

figure has been chosen to emphasize anomaly

variations within the sediment-filled valley and

saturates above the strongly magnetized rocks

outside of the graben that are associated with

positive anomalies of several thousand nT. The

Bent Hill sulfide mound is visible as a small

local anomaly maximum near 1288410W,408260N.

[18] The magnetic anomaly associated with the

BHMS deposit is more easily seen in a higher

resolution figure of the magnetic anomaly

(Figure 5). In this figure the field has been

reduced to the pole, assuming the magnetiza-

tion is entirely induced (i.e., ambient field and

induced magnetization both have an azimuth of

0208 and an inclination of 698). The anomalyhas been smoothed (gridded from continuous

curvature splines in tension [Smith and Wessel,

1990]) and interpolated to 10 m spacing. The

amplitude (maximum �525 nT) and shape ofthe BHMS anomaly is well resolved by the ten

track lines crossing the body. The outlines of

the BHMS, ODP, and Bent Hill have been

drawn on Figure 5. The reduction to the pole

realigns the magnetic anomaly to lie directly

over the top (bathymetric expression) of the

BHMS mound. The magnetic field profiles

observed along north-south and east-west slices

through the anomaly map show the Bent Hill

anomaly is slightly wider in the north-south

direction (190 m width at half amplitude) then

in the east-west direction (150 m (Figure 6)).

The smaller ODP deposit �300 m south ofBent Hill is also seen in the figure as a small


local magnetic anomaly high. The strong neg-

ative anomaly in a broad region around these

small positive anomalies will be discussed in

section 5. Bent Hill, a region of raised sedi-

ments above an instrusion, is not directly asso-

ciated with a significant magnetic anomaly.

[19] The magnetic data constrain the width of

Bent Hill sulfide body. With additional con-

straints on magnetization it would also be

possible to constrain the volume, and more

weakly, the depth of the body. Previous model-

ing has used this approach, incorporating

limited drill core data from ODP Leg139 and

assuming the sulfide mound had the simple

geometry of a sphere or rod [Tivey, 1994a].

Additional drilling during ODP Leg 169 pro-

vides a more complete picture of the sulfide

mound and allows us to more directly model

the magnetic anomaly. The Bent Hill deposit is

a large mound with steep flanks. The base of

the massive sulfides is nearly horizontal at a

depth of �100 m (Figure 6) as determined fromcores recovered from a total of 12 drill holes

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200

100

200 300

400

300

100

128o50'W 128o40'W

48o28'N

48o22'N

0 5 km-800 -600 -400 -200 0 200 >350 Anomaly (nT)

Figure 4. Near-bottom magnetic anomaly data and basement temperatures estimated from heat flow data.Color scale for anomaly data chosen to highlight variations within Middle Valley; note that anomalies abovebasement exposures in the east reach several thousand nT. Basement temperature contours from Davis andVillinger [1992].


along east-west and north-south transects

through the deposit from ODP Legs 139 and

169 [Fouqet et al., 1998; Zierenberg and

Miller, 2000].

[20] We begin by modeling the Bent Hill mas-

sive sulfide deposit as a stack of flat lying,

uniformly magnetized circular disks of varying

diameter that approximate the cross section

revealed by drilling (Figure 6). The top disk

lies at the seafloor, and the bottom of the stack

lies 100 m below seafloor. Model results are

presented assuming an average magnetometer

tow height of 70 m. With this geologically

constrained model, a uniform magnetization

of 5 A/m matches the observed amplitude of

525 nT. The results slightly overpredict the

width of the anomaly in the east-west direction

128o42'W 128o40'W128o41'W

48o26'N

48o25'N

-400 -300 -200 -100 0 100 200 300 400 Anomaly (nT)

-100

-100

-100

-100

2 00

1000

0

0

-200

-300

BHMS

Bent Hill

ODP

LS

Figure 5. High-resolution near-bottom magnetic anomaly data near Bent Hill area. Anomaly data havebeen reduced to the pole [Blakely, 1995]. Outline of Bent Hill, BHMS and smaller Lone Star (LS) andODP mound vents to the south are shown in red (locations from Fouqet et al. [1998]). Note that outlinesof these features have been shifted eastward by �50 m to account for an offset between the transpondernavigated tracks and the locations of features determined by the drilling ship. Near-bottom track linesshown as fine lines.


300

0

100

200

N

S

E

W

observedE - W Line

observedN - S LineAnomaly at 70 m

(reduced to pole)

depth varying model

uniform magnetizationmodel

Anomaly at 5 m(ambient field direction)

uniform magnetizationmodel

depth varyingmodel

-300 -200 -100 0 100 200 300

Distance (m)

MagnetometerTrack

Dep

th (

m)

2350

2450

2550

70 m

outline of circularcylinders

W E

massivesulfidemound

deep copper zone

sulfidefeederzone

Dep

th b

elow

sea

floor

(m

)

100

80

60

40

20

0

0 5 10 15

Magnetization A/m

Ano

mal

y (n

T)

6000

4000

2000

0

Ano

mal

y (n

T)

Figure 6. Geometry of BHMS deposit and forward models of magnetic anomaly at two different altitudes.(bottom) Geometry of the sulfide deposit along a W-E projection derived from drill core data [Fouqet et al.,1998]. The source for the magnetic models is a series of cylindrical disks (outline shown as dotted line) thatapproximate the shape of the BHMS deposit determined from drill core data. Inset shows magnetization fordepth varying model. (middle) Comparison of observed near bottom anomaly (altitude of �70 m) withforward models generated using the cylindrical disk source. Observed N-S and E-W profiles are slicesthrough the reduced to pole anomaly map (Figure 5). (top) Forward model predictions for anomaly at 5 mabove the seafloor using the same source.


and underpredict the width in the north-south

direction. The predicted magnetic anomaly is

necessarily circular since we began with a stack

of circular disks. If we had started with a

slightly non-radially symmetric body we could

fit both axes more closely. The fit in the north-

south direction would also be improved by

including the long wavelength field component

associated with the surrounding and deeper

structure.

[21] A magnetization of 5 A/m implies an

average volume susceptibility of 0.11 SI units

assuming an entirely induced magnetization in

the ambient field of 54,500 nT. This value

compares well with the average susceptibility

measured on sulfide samples from ODP drill

cores (arithmetic mean = 0.17 ± 0.15 SI; geo-

metric mean = 0.08 SI [Korner, 1994]). All

samples from the sulfide mound contain pre-

dominantly pyrite (90%), with 2–10% magnet-

ite and smaller amounts of hematite, pyrrhotite

and other sulfides [Korner, 1994]. Because the

susceptibility of both magnetite and pyrrhotite

exceeds that of pyrite or hematite by �3 ordersof magnitude [Hunt et al., 1995], the suscept-

ibility of these samples should predominantly

be related to the proportion of magnetite and

pyrrhotite. Moreover, hysteresis parameters

suggest that the magnetite within the sulfide

mound occurs primarily as coarse, multidomain

grains (saturation remanence/saturation mag-

netization = 0.01–0.15 [Korner, 1994]).

Although a significant remanent contribution

to the anomaly cannot be excluded, these

results suggest that the magnetization may be

adequately modeled as entirely induced. The

susceptibility of 0.11 S.I. suggests that the

sulfide mound contains an average of �3.5%magnetite + pyrrhotite by volume.

[22] The magnetic anomaly (Figure 6) predicted

by the uniform susceptibility model provides a

close match, both in amplitude and shape, to

the anomaly measured �70 m above the sea-

floor by the towed magnetometer. However,

this model greatly underpredicts the anomaly

(6000 nT) measured previously over the

BHMS mound at a height of �5 m by amagnetometer on Alvin in 1990 [Tivey,

1994a]. Two additional deep tow profiles col-

lected in 1988 reveal amplitudes of �600 nT atan elevation of 50 m and 1250 nT at 25 m

above bottom [Tivey, 1994a]. These values are

higher than would be predicted by our uni-

formly magnetized model for these elevations,

even though neither near-bottom profile

crossed the center of the sulfide mound. The

magnetic anomaly data at 70 m and those

collected by submersible much nearer the sea-

floor may be reconciled only if the upper,

narrower portion of the sulfide mound has

significantly higher magnetization than the

deeper layers. This is consistent with the ODP

results, which show a significant gradient in

susceptibility with depth, with susceptibilities

as high as 0.47 S.I. at the top of the deposit and

much lower values deeper in the deposit

[Korner, 1994].

[23] Both the deep tow and the Alvin data may

be fit using a linear gradient in magnetization

from a value of 12 A/m at the top of the deposit

tapering to 3 A/m at 30 m depth and with

constant magnetization (3 A/m) below. The

measurements from nearest the seafloor are

controlled by the high magnetization shallow

in the deposit, whereas the measurements from

higher above the seafloor depend more on the

average magnetization of the body (Figure 6).

Any model with values near 12 A/m shallow in

the deposit will fit the submersible data, and

models with average magnetizations near 5 A/

m over the bulk of the deposit will fit the data

from the 70 m tow. The magnetization of the

deepest layers is only very weakly constrained

by the data since these layers are farther from

the magnetometer. The higher magnetization

shallow in the deposit corresponds to a much

higher fraction of magnetite and pyrrhotite


(�10% by volume), but this value is still withinthe range of values determined from the core

material. Extensive low-temperature oxidation

of pyrrhotite to an assemblage of pyrite +

magnetite was noted in cores from the BHMS

[Duckworth et al., 1994]. Although no system-

atic variation in the degree of oxidation with

depth was evident, we speculate that enhanced

seawater circulation in the upper portions of the

deposit may have enriched magnetite relative to

pyrrhotite. Because magnetite has a higher

spontaneous magnetization and Curie temper-

ature than pyrrhotite, oxidation to magnetite

might be responsible for the enhanced magnet-

ization of the upper portion of the deposit.

5. Hydrothermal Effects on

Magnetization in Middle Valley

[24] Sea surface magnetic anomaly data reveal a

broad magnetic low over Middle Valley, with

higher values outside of the valley (Figure 7).

Currie and Davis [1994] have reviewed the

possible origins of this negative anomaly in

Middle Valley. The Bruhnes-Matuyama boun-

dary lies some 5 km beyond the valley boun-

dary faults, and so this low is not the result of

reversely magnetized crust. Rather, they asso-

ciate the anomaly with very low magnetization

of the crust below Middle Valley compared to

typical oceanic ridge basalt. A model fit to the

sea surface anomaly data places zero magnet-

ization over an �13 km wide region beneaththe valley with values near +14 A/m outside of

the valley [Currie and Davis, 1994].

[25] Two primary explanations have been

advanced for low magnetization values, and

correspondingly subdued anomaly amplitudes,

in sedimented ridge environments (see review

by Levi and Riddihough [1986]). Curie temper-

atures as low as 1008–2008C are observed forunoxidized titanomagnetites in spreading cen-

ter basalts [e.g., Johnson and Atwater, 1977;

Marshall, 1978]. Although more recent studies

have documented a broader range in titanomag-

netite compositions and Curie temperatures in

mid-ocean ridge basalts [Gee and Kent, 1997;

Zhou et al., 2000], Ti-rich titanomagnetites

with relatively low Curie temperatures are

likely to be the volumetrically dominant mag-

netic phase in most unaltered seafloor lavas.

Temperatures beneath Middle Valley are high

enough to significantly reduce the spontaneous

magnetization, and hence the remanence, of

any minerals with such low Curie temperatures.

However, as noted by Levi and Riddihough

[1986], the existence of very low amplitude

anomalies over crust as old as 3 Ma in sedi-

mented areas such as the Gorda Ridge/Esca-

naba Trough and the Gulf of California argues

against such thermal demagnetization as a

general explanation. Moreover, low tempera-

ture (<1008C) alteration during the initialstages of burial [Davis and Wang, 1994] should

have resulted in the transformation to cation-

deficient titanomaghemites with higher Curie

temperatures that would be less susceptible to

such thermal effects [Levi and Riddihough,

1986].

[26] A more likely cause for the absence of

lineated magnetic anomalies over sedimented

ridges is pervasive hydrothermal alteration

under the thick sediment blanket [Levi and

Riddihough, 1986]. Hydrothermal alteration

results in the replacement of titanomagnetites

by nonmagnetic phases (dominant volumetri-

cally) and a smaller amount of magnetite [Ade-

Hall et al., 1971; Pariso and Johnson, 1991;

Fujimoto and Kikawa, 1989]. As a result of this

hydrothermal alteration, remanence and satura-

tion magnetization (a direct measure of the

proportion of magnetite) can be reduced by

an order of magnitude or more [Woolridge et

al., 1990; Pariso and Johnson, 1991] with the

remaining magnetic phase being near end-

member magnetite [Shau et al., 1993]. Com-

parison of magnetizations of basalt samples


recovered from Site 855 (located on the eastern

boundary fault) and Sites 857 and 858 (near the

Dead Dog vent field and beneath the BHMS,

respectively) provide direct confirmation of the

effect of hydrothermal alteration on Middle

Valley basalts. A small number of samples

from the eastern site are characterized by high

magnetization (�10 A/m) and cation-deficienttitanomaghemites similar to basalts from non-

sedimented ridges [Fukuma et al., 1994]. In

129o30'W 129o00'W 128o30'W 128o00'W48o00'N

48o30'N

49o00'N

-800 -600 -400 -200 0 200 400 600 800

Anomaly (nT)

Sovanco FZ

Nootk

a Fa

ult

HeckelSeamount

Chain

Jara

mill

o

Old

uvai

Bru

nhes

Figure 7. Sea surface magnetic anomaly data in the vicinity of Middle Valley. Box indicates the areacontaining the near bottom survey lines. Locations of selected near ridge faults shown for reference. Note thatBrunhes/Matuyama boundary occurs just east of the eastern bounding faults.


contrast, samples from Sites 857 and 858 have

substantially lower magnetizations (�0.1 A/m)carried predominantly by pure magnetite

[Fukuma et al., 1994; Fouqet et al., 1998].

[27] The degree of alteration of basement

rocks might be expected to vary greatly

because there are large spatial variations in

heat flow within Middle valley (Figure 2).

A direct extrapolation from surface heat flow

measurements to basement depths predicts

temperatures varying by more than 2008Cat the sediment basement interface and

temperatures exceeding 4008C at the base-ment beneath both Dead Dog and Bent Hill

(Figure 4) [Davis and Villinger, 1992].

Elsewhere beneath Middle Valley, these

extrapolated basement temperatures range

mostly between 1008 and 2008C, with values<1008C more typical for the shallow base-ment to the east of the eastern bounding faults

(variations in basement depth are shown

in Figure 8a). This method yields accurate

basement temperatures only if vertical con-

duction is the dominant mechanism of heat

loss and if the physical properties of the

sediments and the thickness of the sediments

are well known. In areas with high heat flow

(>1 W/m2) where hydrothermal discharge

occurs, both nonvertical heat conduction and

advective transport will modify the temper-

ature field, and basement temperature esti-

mates are likely to be erroneous [Davis and

Villinger, 1992].

[28] Although the pattern of temperatures at the

basement-sediment interface shown in Figure 4

may be qualitatively correct, subsequent results

suggest that present-day maximum tempera-

tures in the hydrothermal reservoir beneath

both Dead Dog vent field and the ODP mound

south of Bent Hill are somewhat lower. The

temperature of fluids currently venting at these

two sites are �2758C [Ames et al., 1993;

Zierenberg and Miller, 2000]. This value is

close to that inferred for the reaction zone in

the sill/sediment complex (250–3008C [Stakesand Schiffman, 1999; Peter et al., 1994]).

Davis and Wang [1994] estimate a temperature

at top of the shallowest sill to be �2808C fromextrapolation of the gradient seen in the top of

Hole 857D south of Dead Dog. Basement

temperatures comparable to that of the active

vents (250–2808C) have also been estimatedfor Hole 857C using the observed heat flow

and grain conductivities of 2.6–3.2 W/m8K[Villinger et al., 1994]. The consistency of

these temperature estimates suggests that base-

ment temperatures in the immediate vicinity of

both Dead Dog and Bent Hill are presently

250–2808C.

[29] Lateral variations in basement temperatures

are also likely to be less than inferred from the

extrapolated heat flow data. The general

inverse correlation between heat flow and sedi-

ment thickness is thought to reflect a variable

thickness, low permeability sediment blanket

overlying a highly permeable and approxi-

mately isothermal basement [Davis and Lister,

1977b]. This model is supported by the small

variations in basement temperature (108–208C)inferred for the eastern flank of the Juan de

Fuca Ridge, where heat flow is well correlated

with basement topography [Davis et al., 1989].

The more pronounced basement relief in Mid-

dle Valley (Figure 8a) is likely to result in

somewhat more heterogeneous basement tem-

peratures (variation of 308–608C [Bessler et

al., 1994]). On the basis of two-dimensional

thermal modeling, these authors conclude that

basement permeabilities greater than 10�13 to10�12 m2 would be required to maintain base-ment temperatures within a 208C range. How-ever, measured permeabilities in Middle Valley

are typically 1–2 orders of magnitude lower

(10�14 to 10�13 m2) although permeabilities infault zones are locally as high as 10�10 m2

[Becker et al., 1994]. We therefore suggest that

the temperature of basement in Middle Valley


may vary considerably and that the isotherms in

Figure 4 provide a reasonable qualitative

description of this temperature distribution.

[30] Because higher temperatures are likely to

be associated with more pervasive hydrother-

mal alteration and will also facilitate thermal

demagnetization, one might expect that varia-

tions in basement temperatures would also be

reflected in the pattern of basement magnet-

ization. However, previous sea surface mag-

netic anomaly surveys have insufficient

resolution to discern whether significant varia-

tions in magnetization might be associated with

variable degrees of alteration within the valley.

The coarse spacing of survey lines (�10 km) inthis region, together with the inherent loss of

short wavelength information in sea surface

anomaly data, allows only broad scale features

of the crustal magnetization to be determined.

128o45'W 128o14'W 128o37'W

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 >12.0

48o23'N

48o24'N

48o25'N

48o26'N

48o27'N

48o28'N

48o29'N

48o30'N

Mag. (A/m)Basement Depth (m)

128o45'W 128o14'W

3900 3700 3500 3300 3100 2900 2700 2500 2300

48o23'N

48o24'N

48o25'N

48o26'N

48o27'N

48o28'N

48o29'N

48o30'N

A B

Dead DogVent Field

Bent HillMassiveSulfide

858/1036

856/1035

857855

858/1036

856/1035

857855

2400

3100

3500

2600

2900

2800

2700

68

410

6

6

4

2

6

8

14

Figure 8. Depth to basement and magnetization solution from inversion of near-bottom anomaly data inMiddle Valley. (a) Contour map of sediment-basement interface used for magnetic inversion (after Davis andVillinger [1992]). (b) Magnetic anomaly data in Figure 4 were smoothed using a 100 m block median filterand gridded using continuous curvature splines under tension [Smith and Wessel, 1990]. The magnetic sourcewas a constant thickness (0.5 km) layer draped beneath the sediment-basement interface. The inversion wascarried out assuming an ambient field direction from the IGRF at the site (006/69) and a remanentmagnetization expected from an axial geocentric dipole (000/66). The passband for this analysis was 1–20km (wavelength) with tapers between 20 and 10 km and between 2 and 1 km. The original data are reflectedinto adjacent quadrants to reduce wrap-around effects at the edges. The magnetometer tows were sufficientlylevel that the tow depth could be assumed to be constant in the inversion without significant error. Sufficientmagnetic annihilator has been added to the solution to make the magnetizations positive everywhere.Locations of ODP drill sites (circles) and near bottom survey tracks are shown for reference.


On the basis of the sea surface anomaly data,

Currie and Davis [1994] have modeled the

Middle Valley anomaly low as a broad zone

of essentially nonmagnetic crust. Our near-

bottom magnetic anomaly data allow us to

evaluate whether the hydrothermal alteration

in this area has uniformly reduced the magnet-

ization.

[31] The deep tow magnetic field data (Figure

4) show large variations over Middle Valley

with some of the lowest field values associated

with the Dead Dog vent field and with the

region around the BHMS deposit. These

anomaly variations are the result of both

varying seafloor magnetization as well as

differences in the depth to the magnetized

basement. There is more than 1 km of top-

ography on the basement-sediment interface

associated with the large normal faults within

the valley (Figure 8a). To account for this

variable altitude above the magnetic source,

we have inverted for the magnetization within

a uniform thickness layer (500 m) draped from

the sediment-basement interface [Parker and

Huestis, 1974]. We have calculated the anni-

hilator [Parker and Huestis, 1974; Blakely,

1995] for this basement topography and added

a sufficient multiple of it to the inversion so

that magnetization is everywhere positive,

consistent with the location of Middle Valley

entirely within the Brunhes. Adding the mini-

mum annihilator for positivity also results in

magnetizations as high as �14 A/m east of theboundary faults. This value is well within the

range of magnetization determined from com-

parable age basalts samples from the Juan de

Fuca [e.g., Johnson and Holmes, 1989;

Fukuma et al., 1994] and is consistent with

magnetizations inferred from regional mag-

netic inversions [Tivey, 1994b].

[32] Because our data coverage is sparse, some

care should be exercised in interpreting the

resulting magnetization solution (Figure 8b).

The inversion depends on Fourier methods,

and there are obvious problems with Gibbs

phenomena (ringing and overshoot) particu-

larly in regions without data. The map pro-

duced by the inversion procedure can only be

usefully interpreted in regions near deep tow

lines. We apply spatial filtering to accommo-

date the very nonuniform distribution of data in

space, but the results depend on the passbands

of the spatial filter used in the analysis. Even

with this filtering, the large field gradient over

the eastern faults generates large oscillations

(ringing) in the eastern edge of the inversion.

The labels for the Dead Dog and Bent Hill vent

fields obscure two sections of the inversion,

which are completely unconstrained by data.

Despite these problems, we believe the inver-

sion is useful in the densely sampled center

section of the model between 488250N and488280N.

[33] Our inversion solution (Figure 8b) sug-

gests that magnetizations within Middle Valley

are not uniformly near zero, as had been

inferred from analysis of the sea surface

anomaly pattern [Currie and Davis, 1994].

Some ridge-parallel patterns in the magnet-

ization solution (e.g., near the eastern boun-

dary fault) are closely related to basement

structures, and variations in geochemistry,

paleofield, or source layer thickness may also

contribute to the complex magnetization pat-

tern. However, we interpret this magnetic

heterogeneity as reflecting primarily differen-

ces in the degree of hydrothermal alteration.

For example, a large region around the Bent

Hill sulfide mound is characterized by near

zero magnetization (<3 A/m), suggesting that

this area was extensively altered in the process

that formed the deposit. Similarly, low mag-

netizations inferred from a narrow magnetic

anomaly low near the TAG hydrothermal field

have also been interpreted in terms of perva-

sive hydrothermal alteration [Tivey et al.,

1993]. The magnetization beneath Dead Dog


vent field is also very low, although perhaps

slightly higher than that seen under Bent Hill,

with values between 3 and 4 A/m reflecting a

somewhat lower integrated degree of alteration

of the basement rocks. The remainder of the

region between the central graben fault and

the eastern boundary faults has low but finite

magnetization of between 4 and 6 A/m.

[34] Substantially higher magnetizations (�10A/m) are present west of the central graben

fault. Although the extent of track lines is

limited, all deep tow profiles in this region

show little variation across and to the west of

the Dead Dog vent field (Figure 4). These

higher magnetization values are primarily the

result of the much greater depth to basement

west of the central fault in Middle Valley

(sediment thickness rapidly increases to more

than 1 km west of this fault). The higher

magnetization in the down dropped block west

of the central fault suggests these rocks may

have experienced lower temperature or shorter

duration hydrothermal alteration and conse-

quently less reduction in magnetization.

[35] Although the difference in magnetization

inferred for Bent Hill and Dead Dog is small,

we suggest that the lower magnetization sur-

rounding Bent Hill is significant and that it

reflects more intense hydrothermal alteration

resulting from the higher paleotemperatures

associated with this sulfide deposit. Present-

day basement temperatures are similar at both

Bent Hill and Dead Dog, and so it is unlikely

that the difference in magnetization is due to

differences in the degree of thermal demagnet-

ization. Alteration mineral assemblages, oxy-

gen isotopic data, and fluid inclusion studies all

yield a remarkably consistent estimate of max-

imum alteration temperatures (�2758C) forsamples from Sites 857 and 858 [Stakes and

Schiffman, 1999]. Together with the absence of

significant sulfide accumulations, these data

indicate that basement temperatures surround-

ing the Dead Dog vent field were never much

above 2758C [Davis and Fisher, 1994]. In

contrast, the fluids responsible for the massive

sulfide accumulation at Bent Hill had substan-

tially higher temperatures (350–4008C [Good-fellow and Peter, 1994; Peter et al., 1994]). We

suggest that the higher fluid temperatures asso-

ciated with the Bent Hill sulfide deposit and/or

more protracted exposure to hydrothermal flu-

ids have resulted in more intense hydrothermal

alteration and hence a lower average magnet-

ization than evident near the Dead Dog vent

field.

[36] The magnetization of basement beneath the

Dead Dog vent field (Site 858) is nominally

lower than that near Site 857. This contrast is

surprising because these two areas are presently

hydrologically connected and have experienced

similar alteration conditions [Davis and Fisher,

1994]. Moreover, sills at Site 857 are more

pervasively altered than are corresponding

basement samples from Site 858 [Stakes and

Schiffman, 1999]. From these observations, one

might expect that magnetizations near Site 858

would be higher than near Site 857. Despite the

similarity of present alteration conditions at the

two sites, we suggest that the lower magnet-

ization at Site 858 may reflect a higher inte-

grated degree of alteration.

[37] There are a few additional features in the

inversion solution that are worth noting. The

very low magnetization around Bent Hill

extends to the ESE to a point directly adjacent

to the normal fault bounding the valley. No

significant heat flow anomaly is currently seen

above the site, but the magnetic anomaly low is

clearly seen on all three deep tow tracks cross-

ing overhead (Figure 4), although Fourier edge

effects may result in an overprediction of this

magnetization low (make it more negative than

it is) in the inversion solution. We suggest that

this region must have experienced an episode

of high-temperature alteration and that some


sulfide mineralization could be present within

the sediments beneath the surface. A small area

of low magnetization is also seen near

48827.30N, 128838.30E, which does seem tobe associated with a current day heat flow

anomaly of 6 W/m2.

6. Conclusions

[38] A deep tow magnetometer survey of Mid-

dle Valley, Juan de Fuca Ridge has been used to

develop a high-resolution map of magnetic

field anomaly in the region around the Bent

Hill Massive sulfide deposit. The shape and

extent of the BHMS mound is well resolved by

the deep tow magnetometer survey. Using a

magnetic source whose geometry is constrained

by the drilling data, the amplitude of the

anomaly can be fit assuming an average mag-

netization of 5 A/m. This magnetization is

consistent with average susceptibilities meas-

ured from ODP drill cores from the sulfide

mound and a combined fraction of magnetite

and pyrrhotite near 3.5%. Fitting the much

larger magnetic field anomaly detected previ-

ously in observations from 5 m above the

seafloor from Alvin [Tivey, 1994a] apparently

requires a strong gradient in the magnetization

with depth with surface values near 12 A/m.

However, this is also consistent with ODP core

results that show higher susceptibility at the top

of the deposit associated with a higher fraction

of magnetic minerals (10% magnetite + pyr-

rhotite). These results highlight the utility of

magnetic methods, particularly when anomaly

data are available at multiple levels, as an

exploration tool for mineral deposits in the

marine environment.

[39] The deep tow magnetometer survey at a

larger scale shows low but finite magnetization

in the basement rocks over most of the eastern

part of the Middle Valley graben. We interpret

the variable magnetization as reflecting hetero-

geneous hydrothermal alteration of the base-

ment. The lowest magnetization is found in the

region adjacent to the Bent Hill sulfide deposit

where the magnetization must be essentially

zero. The Dead Dog vent field is also associ-

ated with weak magnetizations, although

slightly higher values than observed near the

BHMS deposit. The slightly lower magnetiza-

tions associated with the BHMS deposit likely

result from the higher fluid temperatures

responsible for the sulfide mineralization and/

or a longer exposure to hydrothermal fluids.

Magnetization variations in this sedimented

ridge environment provide an indication of

the integrated hydrothermal alteration of the

basement and may prove valuable in locating

regions of past alteration that would otherwise

be difficult to detect.

Acknowledgments

[40] Partial support for this study was provided through

NSF grants OCE97-12027 (J.G.), OCE98-19779 (S.W.),

and OCE96-18325 (M.Z. and H.S.). We thank two

anonymous reviewers for helpful comments that im-

proved the manuscript.

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Documents

A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge