ORIGINAL PAPER

Mineralogical and strontium isotopic record of hydrothermalprocesses in the lower ocean crust at and near the East Pacific Rise

Timo M. Kirchner • Kathryn M. Gillis

Received: 3 October 2011 / Accepted: 7 February 2012 / Published online: 22 February 2012

� Springer-Verlag 2012

Abstract Tectonic exposures of upper plutonics ([800 m)

that are part of a contiguous section of young East Pacific

Rise (EPR) crust at the Hess Deep Rift provide the first

regional-scale constraints on hydrothermal processes in the

upper plutonic crust formed at a fast-spreading ridge.

Submersible-collected samples recovered over a 4-km-

wide region show that the sheeted dike complex is largely

underlain by a 150- to 200-m-thick gabbro unit, followed

by a more primitive gabbronorite unit. Gabbroic samples

are variably altered by pervasive fluid flow along fracture

networks to amphibole-dominated assemblages. The gab-

broic rocks are significantly less altered (average 11%

hydrous phases) than the overlying sheeted dike complex

(average 24%), and the percentage of hydrous alteration

diminishes with depth. Incipient, pervasive fluid flow

occurred at amphibolite facies conditions (average 720�C),

with slightly higher temperatures in the lower 500 m of the

section. The extent of subsequent lower-temperature

alteration is generally low and regionally variable. The

gabbroic samples are slightly elevated in 87Sr/86Sr relative

to fresh rock values (0.7024) and less enriched than the

overlying sheeted dike complex. 87Sr/86Sr for the perva-

sively altered gabbroic samples ranges from 0.70244 to

0.70273 (mean 0.70257), tonalites is 0.7038, and pyroxene

hornfels ranges from 0.70259 to 0.70271. 87Sr/86Sr does

not vary with depth, and there is a strong positive corre-

lation with the percentage of hydrous phases. Strontium

contents of igneous and hydrothermal minerals, combined

with bulk rock 87Sr/86Sr, indicate that Sr-isotopic exchange

is largely controlled by the uptake of fluid 87Sr/86Sr in

hydrous minerals and does not require Sr gain or loss. The

minimum, time-integrated fluid–rock ratio for the sheeted

dike complex and upper plutonics is 0.55–0.66, and the

fluid flux calculated by mass balance is *2.1 to 2.5 9

106 kg m-2, 30–60% higher than fluid fluxes calculated in

the same manner for sheeted dike complexes on their own

at Hess and Pito Deeps, and Ocean Drilling Program

Hole 504B. Alteration patterns within the upper plutonics

evolved in response to axial magma chamber (AMC)

dynamics at the EPR, such that magma replenishment led

to assimilation and thermal metamorphism of the country

rock, and the position of the hydrothermal root-zone

tracked the vertical migration of the AMC. The freshness

of the lowermost gabbroic rocks suggests that pervasive

fluid flow does not lead to significant fluid and heat fluxes

at and near fast-spreading ridges.

Keywords East Pacific Rise � Gabbros � Hydrothermal

systems � Sr isotopes � Pervasive fluid flow � Hess Deep

Introduction

Hydrothermal fluid flow and fluid–rock reactions throughout

the life cycle of an ocean plate act to cool the lithosphere

and modulate the geochemistry of the ocean crust and

oceans. High crustal temperatures at mid-ocean ridges

(MORs) lead to globally significant heat and chemical

fluxes, even though the volume flux is far greater in the

ridge flanks (i.e., C1 million year crust) (e.g., Sleep 1991;

Communicated by J. Hoefs.

Electronic supplementary material The online version of thisarticle (doi:10.1007/s00410-012-0729-5) contains supplementarymaterial, which is available to authorized users.

T. M. Kirchner � K. M. Gillis (&)

School of Earth and Ocean Sciences, University of Victoria,

PO Box 3065 STN CSC, Victoria, BC V8W 3V6, Canada

e-mail: kgillis@uvic.ca

123

Contrib Mineral Petrol (2012) 164:123–141

DOI 10.1007/s00410-012-0729-5

Mottl 2001). The contribution of fluid flow and fluid–rock

reactions in the upper ocean crust to heat and chemical

fluxes has been estimated using a variety of approaches;

however, the contribution of the lower crust is poorly

constrained.

Much of what we know about axial hydrothermal sys-

tems in fast- to intermediate-spreading crust comes from

the combination of investigations of active hydrothermal

systems (e.g., vent fluid chemistry, time series data, Von

Damm 2000) and hydrothermally altered ocean crust (see

Alt 2004 for review). Crustal studies have largely focused

on sheeted dike complexes exposed at submarine escarp-

ments (Pito and Hess Deeps, Gillis et al. 2001; Heft et al.

2008) and in numerous ophiolites (e.g., Harper et al. 1988;

Nehlig and Juteau 1988; Schiffman et al. 1991), and

recovered by drilling (Holes 504B and 1256D, e.g., Alt

et al. 1996, 2010), as this is where the bulk of fluid–rock

interaction at MORs is thought to take place. These studies

show that fluid–rock interaction is heterogeneous in space

and time, requiring re-evaluation of long-held conceptual

models of fluid flow and reaction (e.g., Coogan 2008).

Axial fluid–rock interaction is not restricted to sheeted

dike complexes, as the base of axial hydrothermal systems,

hereafter called hydrothermal root-zones, at least tempo-

rarily, migrates into the uppermost gabbros (e.g., Wilcock

and Delaney 1996). Migration of hydrothermal root-zones

is thought to be coupled to the position of axial magma

chambers (AMCs), shallowing and deepening in response to

periods of magma replenishment and crystallization,

respectively. To date, the time-integrated effects of both

axial and off-axis fluid–rock interactions in the upper plu-

tonics have been best studied in ophiolites (e.g., Gregory

and Taylor 1981; Nehlig and Juteau 1988; Stakes and

Taylor 1992), as tectonic exposures of fast-spreading lower

crust are rare and drilling has so far led to the recovery of

short cores (Gillis et al. 1993; Wilson et al. 2006). Similar to

sheeted dike complexes, regional studies in the upper plu-

tonics in ophiolites document heterogeneous alteration

patterns that vary laterally and with depth (e.g., Gregory and

Taylor 1981; Nehlig and Juteau 1988; Stakes and Taylor

1992; Bickle and Teagle 1992). Such studies have served as

the basis for models of the hydrothermal evolution of the

lower crust in space and time. Testing the applicability of

these models to the modern ocean crust is critical, as the

crustal structure and tectonic evolution of ophiolites differ

in some respects from in situ ocean crust and the compo-

sition of seawater has varied over time.

Hydrothermally altered rocks provide a time-integrated

view of the chemical fluxes associated with axial hydro-

thermal systems and provide insight into the explicit reac-

tions that govern chemical exchange (e.g., Alt et al. 1996;

Laverne et al. 2001). If fluid flow pathways and fluid–rock

reaction mechanisms are known, then the magnitude of

chemical fluxes can theoretically be translated into fluid and

heat fluxes by modeling geochemical tracers that are sen-

sitive to the magnitude of hydrothermal exchange, such

as Sr isotopes (e.g., Bickle and Teagle 1992). To date, time-

integrated fluid fluxes have only been calculated using

bulk rock 87Sr/86Sr for sheeted dike complexes at inter-

mediate- to fast-spreading ridges and are between 1.5 and

2.6 9 106 kg m-2 for Hess and Pito Deeps, and Hole 504B

(Barker et al. 2008, revised from Gillis et al. 2005 and

Teagle et al. 2003).

In this paper, we present the first comprehensive,

regional-scale constraints on hydrothermal processes

within the upper plutonic crust formed at a fast-spreading

ridge. Our focus is a section of upper plutonics that are part

of a contiguous crustal section formed at the fast-spreading

East Pacific Rise (EPR) and exposed along the northern

submarine escarpment at the Hess Deep Rift. Metamorphic

and geochemical characteristics and Sr-isotopic composi-

tions of a suite of submersible-collected samples distin-

guish alteration patterns that vary regionally, reflecting

spatial variations in alteration conditions over time.

Strontium contents of igneous and hydrothermal minerals,

combined with bulk rock 87Sr/86Sr, indicate that Sr-isotopic

exchange is largely controlled by the uptake of fluid87Sr/86Sr in hydrous minerals and does not require Sr loss

or gain. Relationships at the Hess Deep Rift provide a

rigorous geological test of conceptual models for the evo-

lution of the magma–hydrothermal transition and show that

the extent of fluid–rock interaction diminishes rapidly with

depth in the upper plutonics, suggesting that pervasive fluid

flow does not lead to significant fluid and heat fluxes at and

near fast-spreading ridges.

Geological background

The Hess Deep Rift is an east–west oriented, submarine rift

valley located in the equatorial Pacific at the western tip

of the Cocos-Nazca ridge (Fig. 1b). The Cocos-Nazca

spreading center is propagating westward toward the EPR at

a rate of *65 mm year-1, exposing young, fast-spreading

(*135 mm year-1) EPR crust (Lonsdale 1988). The EPR

at the latitude of the Hess Deep Rift is made up of many

short segments, and thus, the crust exposed at the Hess Deep

Rift likely formed within a short-segment, perhaps at a

segment end (Lonsdale 1988).

The field area for this study is the northern escarpment

of the Hess Deep Rift where a section of *1 Ma EPR crust

is well exposed (Fig. 1a). The volcanic sequence, sheeted

dike complex, and uppermost plutonic sequence (*800-m-

thick) were mapped and sampled using the submersible

Alvin in a 4-km-wide area during cruises R/V Atlantis II

125-6 and R/V Atlantis 32-3, respectively (P. Lonsdale,

124 Contrib Mineral Petrol (2012) 164:123–141

123

unpublished data, 1992; Karson et al. 1992, 2002). The

focus of this study is the upper plutonic sequence, whose

upper boundary is generally marked by a narrow zone of

intercalated dikes and gabbros indicative of a gradational

boundary (Karson et al. 2002), although in one location the

presence of thermally metamorphosed dikes suggests that

gabbros intruded into the base of the sheeted dike complex

(Gillis 2008). Previous studies of the upper gabbros have

largely focused on their magmatic evolution, including the

role of assimilation and late-stage magmatic degassing

(Coogan et al. 2002; Gillis et al. 2003; Hanna 2004;

Natland and Dick 1996, 2009) and the history of hydro-

thermal alteration using a much smaller sample suite

(Agrinier et al. 1995; Gillis 1995; Manning et al. 1996).

A comparable suite of gabbroic rocks was recovered by

the Ocean Drilling Program (ODP) from the crest of a

intrarift ridge (Leg 147, Site 894), located *29 km SE of

the field area studied here (Gillis et al. 1993) (Fig. 1a).

Petrological and cooling rate estimates indicate the intrarift

section is composed of very shallow level gabbros (e.g.,

Coogan et al. 2007a; Pedersen et al. 1996), likely from

immediately beneath the sheeted dike complex.

Analytical methods

Thirty-one gabbroic rocks representative of all rock types

and their regional distribution and alteration characteristics

were selected for trace element analysis. Oxidized edges

were removed, and macroscopic alteration haloes adjacent

to veins or fractures were avoided. Bulk rock trace element

concentrations were analyzed using a Thermo X Series II

inductively-coupled-plasma-mass-spectrometer (ICP-MS)

at the University of Victoria following a modified proce-

dure similar to that of Eggins et al. (1997). Rock standards

BIR-1, W2-A, BVHO-2 were analyzed for calibration at

the beginning and end of the analyses (Online Resource 1).

Our trace element data for a subset of previously studied

samples are generally within 10% of reported values

(Hanna 2004; Natland and Dick 1996).87Sr/86Sr was determined for 26 unleached bulk rock

powders, two epidote separates and two fresh plagioclase

separates. Mineral separates were leached in hot 6N HCl for

30 min prior to digestion. For isotopic analysis, Sr was

separated using standard cation exchange techniques

(Weis et al. 2006) and then analyzed in a Triton thermal

NScarp

500 km

30°S

0°

120°W 90°W

Pacific

PlateCocos

Plate

EP

R

Nazca

Plate

2°20'N

2°10'N

101°40'W 101°30'W 101°20'W

Hess Deep

Cocos-Nazca Ridge

*Site 894

Lavas

Sheeted dike complex

Plutonic sequence

mbsl

2000

4000

3000Area shown in Fig. 2

Talus

(a)

(c)

(b)Fig. 1 a Location and

b schematic tectonic map of the

Hess Deep Rift (after Lonsdale

1988). The box in a identifies

the study area along the

northern escarpment where a

well-exposed crustal section of

young (*1 Ma) EPR crust has

been extensively mapped; the

location of ODP Site 894 is

shown. c A schematic cross-

section of the study area

showing the distribution of the

crustal lithologies (Karson et al.

2002). The volcanic sequence

and sheeted dike complex range

in thickness from 200–800 m

and 200–1000 m, respectively,

and the subjacent-exposed

gabbroic section is up to 800 m

thick. The transitions between

these units are gradational, with

dikes intruding into both the

volcanic and plutonic sections.

The box in c identifies the

region shown in Fig. 2

Contrib Mineral Petrol (2012) 164:123–141 125

123

ionization mass spectrometry (TIMS) at the Pacific Centre

for Isotopic and Geochemical Research, University of British

Columbia. Analyses of the SRM987 standard yielded values

of 0.710242 ± 0.000009 (2r; n = 11) and 0.710247 ±

0.000018 (2r; n = 15) for two separate runs with a reference

value of 0.710248. Three duplicates reproduced the analyses

within ±0.00001 (average ±0.000007).

Strontium, Ba, and Nb concentrations of igneous and

metamorphic minerals were determined using a laser

ablation ICP-MS at the University of Victoria. NIST 615,

NIST 613, and NIST 611 were used as external standards

for calibration and as instrumental drift monitors. 42Ca or43Ca was used as internal standards for the analyses.

Plagioclase glasses (An0, An25, An50, An75, and An100), run

at the beginning and end of each analytical session, were

used for calibration of anorthite content using Ca/Al ratios

for analytical sessions focusing on the Sr content of

plagioclase. Measured Sr, Nb, and Ba contents are within

\5% of preferred values for NIST 613 and NIST 611 and

*10% for NIST 615 (Online Resource 3). Major and

minor element compositions of igneous and metamorphic

minerals were determined using a Cameca SX-50 electron

microprobe at the University of British Columbia. The

microprobe was operated at 15 kV accelerating voltage, a

20 nA beam current, and a beam diameter of 5 lm.

Modal mineralogy was determined by point counting

950–1,000 spots on standard-sized thin sections. Secondary

plagioclase was identified using a Hitachi S-4800 field

emission scanning electron microscope at the University

of Victoria and a Cambridge Cathode Luminescence

Instrument (CITL 8200 mK4) at the University of British

Columbia.

Results

Regional igneous petrology

The upper plutonic section exposed along the northern

escarpment is dominated by gabbronorite, with lesser

amounts of Fe–Ti oxide and hornblende gabbro, varitex-

tured gabbro, olivine gabbronorite, Fe–Ti oxide gabbron-

orite, and rare tonalite (this study; Hanna 2004; Natland

and Dick 1996) (Fig. 2a). These lithologies are similar to

those recovered at ODP Site 894G (Gillis et al. 1993).

In the central region of the study area (dives 2213, 2218,

3369, 3370), a 150- to 200-m-thick gabbro unit directly

underlies the sheeted dike complex and overlies a[500-m-

thick gabbronorite unit (Fig. 2a). Exposures of the plutonic

section to the east and west are much thinner than in the

central region, with *135 m of gabbronorite (dive 3374)

and *100 m of gabbro (dive 2212), respectively (Fig. 2a).

Within the gabbro unit, varitextured gabbros occur in

the westernmost part of the study area (dive 2212), and

Fe–Ti oxide (±hornblende) gabbro is concentrated in the

central region (dives 3369 and 3370). The gabbronorite

unit is subjacent to the gabbro unit in the central region,

directly underlies the sheeted dike complex in the east

(dive 3374), and may underlie the gabbros in the west (one

sample at base of dive) (Fig. 2a). This unit contains rare

amphibole and olivine gabbronorite, as well as isolated

samples of Fe–Ti oxide amphibole gabbro, amphibole

gabbronorite, and rare tonalite (Fig. 2a). Pyroxene horn-

fels, which are either recrystallized gabbroic or dike sam-

ples, and two gabbronorite samples with textures indicative

of partial recrystallization are located in the central

region of the study area (dives 2213, 2218, and 3370) (see

section ‘‘Regional hydrothermal alteration’’ for complete

description).

Dikes are distributed throughout the upper plutonic

section and were recovered from depths of up to [500 m

below the base of the sheeted dike complex. These dikes

are fine-grained, similar to the sheeted dike complex (Gillis

et al. 2001).

Hydrothermal alteration characteristics and conditions

Regional hydrothermal alteration

The upper plutonic section at Hess Deep has undergone

two stages of hydrothermal alteration. The first is associ-

ated with high-temperature interaction with hydrothermal

fluids at or near the EPR and, as such, is considered to be

representative of fast-spreading crust (Coogan et al. 2002;

Gillis 1995; Manning et al. 1996); this early stage is the

primary focus of this study. The second, low-temperature

stage is associated with tectonic unroofing and is specific to

the Hess Deep Rift. Along the northern escarpment, low-

temperature alteration within the plutonic section is mini-

mal and is manifest largely as oxidation haloes and rare

occurrences of clay minerals in the groundmass and filling

fractures. This contrasts with samples from the intrarift

ridge, which have a greenschist to zeolite facies overprint

associated with extensive brittle deformation (e.g., Fruh-

Green et al. 1996).

Three types of high-temperature alteration are observed:

(1) groundmass alteration associated with pervasive fluid

flow along microfractures and, in a few samples, macro-

scopic fractures; (2) localized alteration in a region of

focused fluid flow within fault zones; and (3) partial to

complete recrystallization of dikes and gabbroic rocks to

fine-grained, granoblastic pyroxene hornfels. Pervasive

alteration dominates the plutonic suite, with isolated

occurrences of pyroxene hornfels (Fig. 2b) and localized,

126 Contrib Mineral Petrol (2012) 164:123–141

123

focused fluid flow. Alteration associated with focused fluid

flow in fault zones is not considered here.

Alteration assemblages in the pervasively altered gab-

broic samples are primarily amphibole-dominated, with

chlorite-rich samples (defined as chlorite comprising[25%

of total hydrous secondary minerals) largely located in the

western part of the field area (dive 2212) (Fig. 2b).

Clinopyroxene is variably replaced (\2 to *70%) by

calcic amphibole ± chlorite ± magnetite ± minor secondary

clinopyroxene. Orthopyroxene is replaced by intermixed

Fe–Mg amphibole ? talc ± magnetite or calcic amphibole ±

magnetite and shows a wider range of replacement

than clinopyroxene (\5–100%). Magmatic plagioclase

(An35–An70) is replaced by secondary plagioclase (An\5 to

An[80) with minor to rare amphibole locally filling mi-

crofractures. Albitic secondary plagioclase, identified using

backscattered electron images, was found in \40% of the

samples where it generally represents \5%, but up to

10–15%, of the plagioclase in a given sample. Anorthitic

(An70–An96) secondary plagioclase, observed in only one

sample using cathodoluminescence images, makes up

30–40% of the plagioclase. Secondary plagioclase with

intermediate anorthite contents (*An30–An60) was iden-

tified by Fe depletion in some samples; however, its

-3500-3500

-3000

-3000

-2500

2212

3369

3370

2218

3374

2° 21' 00"

2° 21' 30"

(a)

(b)

101° 16' 30" 101° 16' 00" 101° 15' 30"

-2500

101° 15' 00"W

2213

Sheeted dike complex

Plutonic section

500 m

E

GabbroFe-Ti-oxide &/or amp gabbroGabbronoriteFe-Ti-oxide &/or amp gabbronoriteTonalitePyroxene hornfelsBasaltic dikeCataclastite

Sheeted dike complexPlutonic sequenceTalusSheeted dike–plutonicboundary

Lithologies

-3500-3500

-3000

-3000

-2500

2212

3369

3370

2218

3374

2° 21' 00"

2° 21' 30"

101° 16' 30" 101° 16' 00" 101° 15' 30"

-2500

101° 15' 00"W

2213

Sheeted dike complex

Plutonic section

500 m

E

Amphibole-dominatedChlorite-richPx-Hornfels

Low (<10%)Medium (10-20%)High (>20%)

Percentage of hydrous phases

Alteration Types

Fig. 2 Simplified geological maps of the study area showing dive

tracks and the interpreted lithological boundary (dashed line) between

the sheeted dike complex and the plutonic sequence (Karson et al.

2002) [for full dive tracks within the volcanic sequence and sheeted

dike complex see Gillis et al. (2001)]. Reference to the central study

area in the text includes dives 2213, 2218, 3369, and 3370, the

western field area includes dive 2212, and the eastern field area

includes dive 3374. The E–W trending exposures in the study area are

perpendicular to the general strike of the EPR; thus, the study area

represents *60,000 years of spreading history (spreading rate is

*135 mm year-1). Depth below sea level decreases from south to

north; thus, the lower plutonic and uppermost lava sequences are

exposed in the south and north, respectively. The 1990 dive tracks

start with ‘‘22’’ and the 1999 dive tracks start with ‘‘33’’. a Igneous

rock types: symbols along each dive track indicate the location and

igneous rock type of studied samples. b Hydrothermal alteration

characteristics: symbols along each dive track show locations of

studied samples, the style of hydrothermal alteration, and an index of

the percentage of hydrous phases. Contour interval: 100 m

Contrib Mineral Petrol (2012) 164:123–141 127

123

abundance could not be accurately quantified. Therefore,

reported percentages of secondary plagioclase for all

samples are minimum values.

Tonalite is the most altered lithology ([90%). Plagio-

clase is strongly mottled and replaced by secondary

plagioclase ? epidote ? chlorite, hornblende is partially

to completely replaced by actinolite, and alteration patches

contain assemblages of prismatic epidote ? quartz or

chlorite ? actinolite.

Microscopic veins (\40 lm wide) along grain bound-

aries or cutting primary grains are ubiquitous and contain

amphibole or chlorite. Macroscopic hydrothermal veins

([40 lm wide), found in \10% of the gabbroic samples,

are filled with amphibole or quartz ± chlorite ± amphi-

bole. Alteration haloes adjacent to these veins are restricted

to a distance of 1–2 mm from the vein margin and have a

higher percentage replacement of clinopyroxene than

elsewhere in the groundmass.

Pyroxene hornfels samples include dikes and gabbro/

norite that have been recrystallized to clinopyroxene ?

orthopyroxene ? plagioclase ? Fe–Ti oxides, with local

preservation of magmatic textures of plagioclase ±

pyroxene (Table 1). One sample (2213-1351) with a dike

protolith is located at the sheeted dike–gabbro transition.

Another sample (3370-1328) has a sharp transition

(\2 mm) between fine- and medium-grained domains, with

a slight grain size reduction adjacent to the contact in the

fine-grained domain, suggesting the sample represents a

hornfelsed dike–gabbro contact. It is not known whether

the other pyroxene hornfels with gabbroic and basaltic

protoliths were recrystallized in place within mixed zone of

dikes and gabbros or were xenoliths in the upper plutonic

sequence. Both of these relationships are common in the

upper 500–800 m of the plutonic sequence at the Oman

and Troodos ophiolites (Coogan et al. 2003; France et al.

2009; Gillis 2008). Two gabbronorite samples appear to be

partially recrystallized, such that there are patches of gra-

noblastic clinopyroxene and orthopyroxene and/or aggre-

gates of grains whose form mimics relict pyroxene grains.

Pyroxene hornfels are largely anhydrous, with minor

localized hydrous alteration occurring as veins and partial

replacement of the mafic phases.

The distribution of the alteration assemblages described

above documents regional variation within the gabbroic

sequence itself and relative to the overlying sheeted dike

complex. In the central region of the field area, amphibole is

the dominant hydrous phase in the basal dikes and subjacent

gabbros within 200–300 m of the dike samples. In the wes-

tern area (dive 2212), amphibole is also the dominant

hydrous phase in the basal dikes and subjacent gabbros, with

chlorite being more abundant in the gabbros relative to both

the immediately overlying dikes and gabbroic rocks else-

where in the field area. It is not known whether the basal dike

assemblages track into the subjacent gabbros everywhere, as

we lack gabbro samples beneath localized, chlorite-domi-

nated regions within the basal dikes within the central region.

Percentage of hydrous minerals

The percentage of hydrous minerals was visually estimated

using thin sections for the entire sample suite (Fig. 2b); the

degree of alteration for a subset of these samples (for which

Sr-isotope data were obtained) was also quantitatively

determined by point counting (Online Resource 2). Visual

estimates were found to be generally within 5% of point-

counted values. For both approaches, secondary plagio-

clase was not included because accurate identification of

secondary plagioclase cannot be obtained using a standard

petrographic microscope. As such, the percentage of

hydrous minerals represents a minimum value for the

overall degree of alteration. As the percentage of secondary

plagioclase identified using backscattered electron images

is low, the degree of alteration would be only slightly

higher (\1%) than the percentage of hydrous phases.

Overall, the pervasively altered gabbroic samples are rel-

atively fresh, with the percentage of hydrous minerals ranging

from \5 to *30% (average 10.7%, excluding tonalites)

(Fig. 3a; Table 1). The plutonic sequence shows a gradual

decrease in the percentage of hydrous minerals with depth,

with the upper*200 m showing the broadest range (Fig. 3a).

Gabbros are generally more altered than gabbronorites

(average 16 versus 7%). Gabbroic rocks with chlorite-rich

assemblages are more altered than those that are amphibole-

dominated. Pyroxene hornfels are generally anhydrous, and

tonalites are the most altered (C90%) lithology. Dikes hosted

in the upper plutonic section show substantially higher

hydrous alteration percentage ([50%) than their host gab-

broic rocks and the overlying sheeted dike complex (average

24 ± 14% hydrous minerals) (Gillis et al. 2001).

Temperature constraints

The temperature of amphibole formation in the pervasively

altered, amphibole-bearing, gabbroic rocks are constrained

using the edenite ? albite = richterite ? anorthite exchange

geothermometer of Holland and Blundy (1994), assuming a

pressure of 0.1 GPa. This thermometer is calibrated for

a temperature range of 500–900�C, with an uncertainty of

39�C (Holland and Blundy 1994). Shifts in the composition

of plagioclase adjacent to amphiboles (e.g., Fe depletion)

and rapid hydrothermal reaction rates at amphibolite

facies conditions (Wood and Walther 1983) suggest that

equilibrium was achieved on a local scale (see also

Manning et al. 1996, 2000). Calculated temperatures for

hydrothermal amphibole range from 651 to 814�C, with

mean value of 720�C (1r = 39�C, n = 62) (this study;

128 Contrib Mineral Petrol (2012) 164:123–141

123

Coogan et al. 2002; Manning et al. 1996; Weston 1998).

Calculated peak temperatures have a broader range in the

upper 300 m of the gabbroic sequence, with a mean tem-

perature of 710�C (±42�C, n = 36), compared to 733�C

(±32�C, n = 26) in the lower 500 m (Fig. 4a).

Where equilibrium pairs are lacking, or for temperatures

\500�C, the AlIV content in hydrothermal amphibole may

be used to qualitatively assess temperature, as experimental

studies show that Al2O3, TiO2, and Na2O contents increase

with increasing temperature (e.g., Liou et al. 1974), such

that magnesio-hornblende (AlIV contents[0.5; based on 23

oxygens) is indicative of amphibolite facies conditions

([450 to *700�C), whereas actinolite (AlIV contents\0.5)

suggests lower temperatures (down to 250�C) (e.g., Liou

et al. 1974; Schiffman and Fridleifsson 1991). Based on these

constraints, hydrothermal amphibole formed from [700

to *250�C (Fig. 4b), with temperatures \450�C largely

focused in the upper 300 m of the section.

Minimum temperatures of fluid–rock interaction in the

pervasively altered samples may be estimated based on the

Table 1 Summary of sample characteristics

D basaltic dike, G gabbro, GN gabbronorite, O Fe–Ti oxide, A amphibole, T tonalite, PH pyroxene hornfels, f fine, c coarse, part partially, prot.protolith, NA not availablea Sample numbers: first number denotes the dive number, the second number the time collectedb Meters below sea levelc Meters below the lava–dike transition; this datum is used as there is significant local variation in the thickness of the lava sequenced See Online Resource 1 for data source

Contrib Mineral Petrol (2012) 164:123–141 129

123

presence or absence of chlorite and actinolite. The presence

of discrete chlorite and actinolite indicates minimum

temperatures of 160–270 and *250�C, respectively, and

maximum temperatures of *450�C for both phases (e.g.,

Liou et al. 1974; Schiffman et al. 1991). Chlorite-rich

assemblages are concentrated in the western part of the

field area (dive 2212) and, except for two gabbroic sam-

ples, chlorite-bearing samples are restricted to the upper

150 m of the plutonic sequence. Actinolite is prevalent

throughout the plutonic sequence, although it is signifi-

cantly less abundant in the lower 500 m (Fig. 4b).

Pyroxene hornfels recovered from the central part of the

field area were recrystallized at temperatures between 756

and 1,024�C (average = 929 ± 44�C, n = 33, Gillis 2008).

Bulk rock chemistry

Major and trace element compositions

The plutonic sequence has a wide range in Mg#

(0.76–0.30), with gabbronorite being the most primitive

and Fe–Ti oxide ± amphibole gabbro being the most

evolved end-members (this study; Hanna 2004; Natland

and Dick 1996) (Online Resource 1). The more evolved

samples (Mg# \0.6) are generally concentrated in the

upper gabbro unit, with isolated samples within the

underlying gabbronorite unit. In general, plutonic rocks

from the northern escarpment are more evolved than those

recovered from the intrarift ridge and Site 894 (Coogan

et al. 2002; Hekinian et al. 1993; Pedersen et al. 1996).

The extent of hydrothermal alteration appears to be

related to magmatic differentiation, such that more differ-

entiated rocks are altered more strongly. The percentage of

hydrous secondary phases is weakly negatively correlated

with Al2O3, CaO, and Mg# and positively correlated with

incompatible, immobile trace elements, such as Y, Nb,

or La (Fig. 5a).

Strontium contents of pervasively altered gabbroic rocks

range from 60 to 137 ppm. One epidotized tonalite sample

has a Sr content of 227 ppm, reflective of the high modal

abundance of epidote (average Sr content = 570 ppm).

There is no correlation between Sr content and magmatic

Dep

th b

elow

lava

-dik

e tr

ansi

tion

(m)

G amp-domG chl-richGN amp-domGN chl-richPHT epi-richDike

-300

-100

100

300

500

700

900

1100

0 10 20 30 40

% hydrous phases

0.7026 0.7030 0.7034 0.7038

87Sr/ 86Sr

Plutonic Sequence

Sheeted Dike Complex

Lava–sheeted diketransition zone

Fres

h M

OR

B

(a) (b)

Fig. 3 a Percentage of hydrous phases and b bulk rock 87Sr/86Sr

versus depth below the lava-sheeted dike transition (this datum is

used because the paleoseafloor depths are not known across the field

area due to local mass wasting). The vertical black line in a is the

average % hydrous phases in the sheeted dike complex, the gray boxindicates ±1r (Gillis et al. 2001); the gray box in b identifies the

range in fresh rock values; and the fine black line in b joins data for

the fine- and medium-grained parts of sample 3370-1328. The dashedlines in a and b mark the average depths of lithological boundaries.87Sr/86Sr data for the sheeted dike complex from Gillis et al. (2005).

See text for discussion. G gabbro, GN gabbronorite, PH pyroxene

hornfels, T tonalite, amp-dom amphibole-dominated, chl-rich chlo-

rite-rich, epi-rich epidote-rich

130 Contrib Mineral Petrol (2012) 164:123–141

123

differentiation proxies (e.g., Mg#, Y) (Fig. 5b), and there is

no systematic change in Sr content with depth below the

sheeted dike complex. Amphibole-dominated samples

show a weak positive correlation between Sr content and

the percentage of hydrous phases, whereas chlorite-rich

samples show no correlation (Fig. 5c). This suggests that

replacement of primary phases by amphibole can lead to Sr

uptake in the gabbroic rocks.

Bulk rock 87Sr/86Sr ratios

Due to the distinct Sr-isotopic signature of fresh MORB

and seawater, bulk rock 87Sr/86Sr can be used to quantify

the degree of fluid–rock interaction in MOR hydrothermal

systems (e.g., Bickle and Teagle 1992). In order to assess

the extent of isotopic exchange, it is first necessary to

constrain fresh rock values. The 87Sr/86Sr of acid-leached

plagioclase separates from the least altered gabbronorites

range from 0.70243 to 0.70251 (Table 1; Barker et al.

2008). Fresh MORB glasses from the equatorial Pacific

have 87Sr/86Sr from 0.70235 to 0.70275 (average 0.70255)

(Petrological Database for the Ocean Floor; http://petdb.

ldeo.columbia.edu/petdb). Based on new and published

data, and considering the lowest bulk rock value is 0.70244

and the possibility that petrographically fresh plagioclase

separates underwent minor reaction with a hydrothermal

fluid, fresh rock 87Sr/86Sr values have a range of 0.70240–

0.70245. This range is in agreement with an inferred value

for gabbroic rocks recovered during Leg 147 (0.7024;

Lecuyer and Gruau 1996), determined by extrapolating the87Sr/86Sr—percentage of alteration minerals correlation

(see their Fig. 2) to zero percent alteration.

Bulk rock 87Sr/86Sr was measured for 26 samples that

are representative of the range in rock types, percentage

of hydrous phases and geographic distribution (Table 1).

The 87Sr/86Sr of most samples is elevated relative to fresh

rocks, but only slightly toward seawater values (Fig. 3b).

The 87Sr/86Sr of gabbroic rocks altered by pervasive fluid

flow ranges from 0.70244 to 0.70273 (mean = 0.70257 ±

0.00007, n = 20) (Table 1). This range is comparable to

Sr-isotopic ratios of plutonic rocks from ODP Holes 894G

and 894F from the intrarift ridge, ranging from 0.70247

to 0.70309 (mean = 0.70263 ± 0.00017, n = 15; outlier

close to macroscopic vein not included, Lecuyer and

Gruau 1996). Amphibole-dominated samples have a mean87Sr/86Sr value of 0.70256 (n = 17), and chlorite-rich

400

500

600

700

800

900

1000

1100

1200

650 700 750 800

Temperature (°C)

Dep

th b

elow

lava

-dik

e tr

ansi

tion

(m)

GabbonoriteGabbroPyroxene hornfels

0 0.5 1 1.5 2

Al iv in hydrothermal amphibole

Sheeted dikecomplex

Upper plutonicsequence

(b)(a)

Fig. 4 a Calculated peak temperatures for hydrothermal amphibole

versus depth below the lava-sheeted dike complex; error barsrepresent ±1r (this study; Coogan et al. 2002; Manning et al. 1996;

Weston 1998), and b AlIV content of hydrothermal calcic amphibole

versus depth below the lava-sheeted dike complex. Amphiboles were

recalculated on the basis of 23 anhydrous using the 15eNK method,

which minimizes the Fe3? content (Leake et al. 1997). Dashed line in

upper right of b marks the average location of the sheeted dike

complex–plutonic boundary. Hydrothermal amphiboles were distin-

guished by either their Nb content (see text) or \1.0 wt% TiO2,

following (Coogan et al. 2002). Data from: this study; Coogan et al.

(2002); K. Gillis (unpublished data)

Contrib Mineral Petrol (2012) 164:123–141 131

123

samples have a mean 87Sr/86Sr value of 0.70261 (n = 3).

The tonalite samples have the most elevated 87Sr/86Sr

(0.7038). The 87Sr/86Sr of a pyroxene hornfels sample with

fine-grained dike and medium-grained gabbro domains is

0.70259 and 0.70271, respectively. The 87Sr/86Sr of the

dike sample is higher (0.70283) than most of the perva-

sively altered gabbroic samples and pyroxene hornfels and

falls within the range of the sheeted dike complex.

The crustal section along the northern escarpment at the

Hess Deep Rift shows a decrease in bulk rock 87Sr/86Sr

with depth (Fig. 3b). Pervasively altered gabbroic rocks

have slightly lower values than the overlying sheeted dike

complex and show no systematic isotopic shift with depth

with the plutonic section (Fig. 3b). Bulk rock 87Sr/86Sr

shows a weak correlation with Sr for the amphibole-

dominated samples (Fig. 6a), whereas in the overlying

sheeted dike complex Sr is negatively correlated with87Sr/86Sr (Gillis et al. 2005).

Bulk rock 87Sr/86Sr of the pervasively altered gabbroic

rocks is influenced by both the extent of alteration and the

alteration assemblage. There is a strong positive correlation

0

20

40

60

80

100

0 5 10 15 20 25

% hydrous phases

Y (

ppm

)

0

20

40

60

80

100

40 60 80 100 120 140

Sr (ppm)

Y (

ppm

)

50

60

70

80

90

100

110

0 5 10 15 20 25

% hydrous phases

Sr

(ppm

)(a)

(b)

(c)

G amp-domG chl-richGN amp-domGN chl-richPHG/GN Site 894

Fig. 5 a Y, and b Bulk rock Y versus Sr content and c Sr contents

versus percentage of hydrous phases. Note that the dominant

secondary mineral assemblage for specific samples in the Hole

894G data is not known; the dominant secondary hydrous phase is

amphibole (Lecuyer and Gruau 1996). G gabbro, GN gabbronorite,

PH pyroxene hornfels, amp-dom amphibole-dominated, chl-richchlorite-rich

(b)

(a)

50

70

90

110

130

Sr

(ppm

)

0

10

20

30

40

50

0.7024 0.7025 0.7026 0.7027 0.702887Sr/86Sr

% h

ydro

us p

hase

s

0

20

40

60

80

0.7024 0.7029 0.7034 0.70398687Sr/ Sr

% h

ydro

us p

hase

s

Fig. 6 Bulk rock 87Sr/86Sr versus a Sr content and b percentage of

hydrous phases. The relationships in a are interpreted to be a result of

differential Sr uptake into chlorite and amphibole. The good

correlation between 87Sr/86Sr and percentage of hydrous alteration

in b suggests that hydrous alteration controls or follows the

Sr-isotopic shift. Note that the gabbronorite sample that falls below

the trend, with high 87Sr/86Sr and low hydrous phases, is interpreted to

be influenced by assimilation. Data for ODP Site 894 (Lecuyer and

Gruau 1996) are plotted in b. Inset in b shows all data; symbols as in

Fig. 5

132 Contrib Mineral Petrol (2012) 164:123–141

123

between 87Sr/86Sr and the percentage of hydrous phases

(Fig. 6b). The more differentiated gabbros and samples

with chlorite-rich alteration assemblages have slightly

higher hydrous alteration percentages at a given 87Sr/86Sr

value.

Sr mobility

Interpretation of bulk rock 87Sr/86Sr requires knowledge of

the behavior of Sr during fluid–rock reactions in the upper

plutonic sequence. To assess Sr mobility, the Sr contents of

igneous minerals are compared with those of their repla-

cive secondary phases (Online Resource 2). Strontium

contents of six samples were calculated by mass balance,

and compared to measured bulk rock Sr contents, to ensure

that all of the key reactions are accounted for (Table 2).

The six samples are representative of the percentage of

hydrous alteration, alteration assemblage, and igneous rock

type (Table 2).

Comparison of the Sr content of igneous and secondary

plagioclase shows that change in Sr contents is influ-

enced by anorthite content. Igneous plagioclase contains

115–230 ppm Sr, with up to 70 ppm variation within

individual samples (Fig. 7, Online Resource 3). Albitic

(An0–40) secondary plagioclase has lower Sr contents than

igneous plagioclase in the same sample (Online Resource

3) and shows a weak positive correlation of Sr content and

anorthite content (Fig. 7). Anorthitic (An70–96) secondary

plagioclase has similar Sr contents to igneous plagioclase

in the same sample (Online Resource 3). Strontium

behavior of intermediate plagioclase is not known, as it

was not possible to distinguish igneous and secondary

optically. For comparison, Sr contents of igneous and

secondary plagioclase in the sheeted dike complex are

Table 2 Mineral Sr contents

and modal abundance used to

calculate bulk rock Sr contents

Plag plagioclase, Cpxclinopyroxene, Opxorthopyroxene, Mag ampmagmatic hornblende, hyhydrothermal amphibole, Chlchlorite, NA not availablea Magmatic and hydrothermal

amphibole were distinguished

by their Nb content, such that

magmatic amphibole has

Nb [ 1, hydrothermal

amphibole has Nb \ 1,

following (Gillis et al. 2003)

Sample # Rock

type

Minerala Average

Sr content

(ppm) ± 1 SD

Modal

abundance

(%)

Calculated

Sr content

(ppm)

Measured

Sr content

(ppm)

2212-1338 GN Plag 185 ± 14.9 52 99.5 ± 1.5 90

Cpx 6.6 ± 0.8 22

Opx 0.1 ± 0.1 3

Hy Ca-amp NA 2

Hy Fe–Mg-amp 11.4 8

Chl 1.7 ± 2.3 7

2212-1409 G Plag 174 ± 6.3 38 70 ± 0.6 86

Cpx 7.8 ± 0.8 37

Hy Ca-amp 12.4 10

Chl 1.2 ± 1.0 5

2218-1048 GN Plag 134 ± 5.6 36 52 ± 0.3 60

Cpx 7.3 ± 3.4 50

Opx 1.3 ± 0.5 12

Hy Ca-amp NA 2

2218-1440 AGN Plag 172 ± 13.6 26 56 ± 0.9 61

Cpx 6.4 1.5

Opx 1.1 ± 1.1 9

Mag Amp 25.5 ± 24.6 54

Hy amp NA 1

3369-1321 GN Plag 162 ± 4.8 52 87 ± 0.5 88

CPx 6.3 ± 0.4 31

Opx 0.5 6

Hy Ca-Amp 4.6 ± 2.8 9

Amp NA 1

3374-1031 GN Plag 217 ± 1.0 44 100 ± 1.5 98

Cpx 7.5 ± 1.3 34

Opx 0.4 ± 0.4 4

Hy Ca-amp 3.4 ± 0.8 10

Hy Fe–Mg-amp 3.0 ± 1.5 1

Contrib Mineral Petrol (2012) 164:123–141 133

123

indistinguishable (Gillis et al. 2005). The absence of a

strong correlation between Sr and anorthite contents sug-

gests that factors other than crystal chemistry (e.g., Blundy

and Wood 1991; Lagache and Dujon 1987), such as fluid

chemistry, controlled Sr mobility during plagioclase

replacement.

Replacement of clino- and orthopyroxene by amphibole

or chlorite leads to no change or an increase in Sr contents.

Clinopyroxene contains 4–12 ppm Sr, falling within the

range of published clinopyroxene data for Hess Deep

(Coogan et al. 2002). Calcic amphibole replacing clinopy-

roxene has slightly lower to significantly higher Sr contents

than its precursor, meaning clinopyroxene replacement can

lead to Sr loss or gain. Sr contents of calcic amphibole are

positively correlated with AlIV (Fig. 8), suggesting Sr

uptake into hydrothermal amphibole may be temperature

dependent. The cause of this is not clear, as Sr can be

partitioned into the both the A and M4 sites in calcic

amphibole (Tiepolo et al. 2007). Chlorite replacing clino-

pyroxene contains \1–5 ppm Sr (Online Resource 3),

indicating that chlorite replacement leads to no change or Sr

depletion. Orthopyroxene has low Sr contents (\1–3 ppm).

Fe–Mg amphibole (cummingtonite) replacing orthopyrox-

ene commonly has slightly to higher Sr contents than

orthopyroxene, acting to enrich the rocks in Sr.

Bulk rock Sr values calculated for the six samples using

average Sr contents of the igneous and hydrothermal

minerals, and their modal proportions, reproduce the

measured bulk rock Sr content within 20%, three samples

reproduced it within 10%. There is no correlation between

modal abundance of Fe–Ti oxides and the deviation

between calculated and measured bulk rock Sr content,

suggesting that Sr resides in the major silicate minerals and

bulk rock Sr values are not significantly impacted by the

abundance of Fe–Ti oxides and accessory phases such as

apatite. Thus, only reactions involving the major primary

phases and their replacive hydrothermal minerals need to

be considered in assessing changes in Sr content and Sr-

isotopic exchange.

Discussion

The upper plutonic sequence exposed along the northern

escarpment of the Hess Deep Rift displays regional-scale

alteration patterns that reflect the spatial and temporal

evolution of hydrothermal processes active in fast-spread-

ing ocean crust. We start this section by first discussing the

magmatic processes that contribute to bulk rock 87Sr/86Sr

and fluid–rock reactions that influence Sr mobility and,

potentially, Sr-isotopic exchange. Time-integrated fluid/

rock ratios and fluxes are then calculated using bulk rock87Sr/86Sr. Finally, the geological framework of the field

area provides the basis for developing a conceptual model

for the development of hydrothermal alteration patterns in

the upper plutonic sequence, at and near the fast-spreading

EPR axis.

Controls on Sr mobility and Sr-isotopic exchange

Suprasolidus conditions

Assimilation is the primary igneous process that has the

potential to modify the mantle-derived Sr-isotopic com-

position of the melt. Assimilation is a fundamental process

at intermediate- to fast-spreading ridges, such that stop-

ing of hydrothermally altered dikes ± gabbroic rocks ±

hydrothermal fluids into AMCs acts to modify the magma

compositions in a variety of ways. Evidence for this comes

0

50

100

150

200

250

0 20 40 60 80 100

An (%)

Sr

(ppm

)

PrimarySecondary

Fig. 7 Anorthite versus Sr content in primary and secondary

plagioclase. Secondary plagioclase with low anorthite content gen-

erally has lower Sr contents than primary plagioclase in the same

sample. One sample (2218-1440) with anorthitic, secondary plagio-

clase shows no difference in Sr content between primary and

secondary plagioclase

0

2

4

6

8

10

12

0 0.5 1 1.5 2

Al iv hydrothermal amphibole

Sr

(ppm

)

Previous dataThis study

Fig. 8 Amphibole Sr content versus AlIV in hydrothermal calcic

amphibole (data from this study; K. Gillis 2003, unpublished data)

134 Contrib Mineral Petrol (2012) 164:123–141

123

from EPR normal-mid-ocean ridge basalts (n-MORB),

which are overenriched in Cl relative to n-MORB from

slow-spreading ridges (Michael and Schilling 1989), and

dacitic lavas from the EPR and Juan de Fuca Ridge whose

trace element and O-isotopic compositions are best

explained by partial melting and assimilation of hydro-

thermally altered crust (Wanless et al. 2011). Assimilation

into AMCs is attributed to episodic upward migration of

AMCs, likely due to magma replenishment, and to a lesser

degree, subsidence of seismic layer 2 across the width of

AMCs, leading to recycling of *20% of the crust across

the sheeted dike–gabbro transition (Coogan et al. 2003).

At the Hess Deep Rift, massive enrichment in Cl and

moderate enrichment in B in magmatic amphibole indicate

that assimilation was prevalent at least within the upper

*800 m of the gabbroic sequence (Gillis et al. 2003).

Thus, it is reasonable to expect that the Sr-isotopic com-

position of magmas would be shifted toward higher87Sr/86Sr than their mantle-derived signal, as potential

assimilants would have elevated 87Sr/86Sr. Assuming a

contaminant 87Sr/86Sr of 0.70265 (average altered dike87Sr/86Sr, Gillis et al. 2005) and a mantle-derived melt87Sr/86Sr of 0.7024, recycling of 20% of altered oceanic

crust would lead to an apparent fresh rock 87Sr/86Sr of

0.70245, the upper limit of predicted fresh rock values (see

section ‘‘Bulk rock 87Sr/86Sr ratios’’). It is only possible to

evaluate the effects of assimilation on 87Sr/86Sr in samples

with low abundances of hydrous minerals (\5%), that is,

samples with minimal isotopic overprinting by subsequent

hydrothermal alteration. Four of the low hydrous mineral

abundance samples have elevated 87Sr/86Sr relative to fresh

values and lie below the trend defined by 87Sr/86Sr versus

percentage of hydrous phases (Fig. 6b). Of these four, one

gabbronorite shows the influence of assimilation (2218-

1048). The remaining three samples are pyroxene hornfels

whose isotopic enrichment is attributed to hydrothermal

alteration and isotopic enrichment prior to recrystallization

(see section ‘‘Conceptual model for the evolution of

hydrothermal alteration patterns in the upper plutonic

sequence’’). Thus, assimilation can influence 87Sr/86Sr of

the melt and, importantly, our estimates of mantle-derived

melt compositions from fresh rock values.

Subsolidus conditions

The exchange and transport of the Sr-isotopic signal

between hydrothermal fluids and minerals at subsolidus

conditions may be controlled by diffusion and/or dissolu-

tion–reprecipitation. To evaluate whether Sr-isotopic

exchange between the fluid and rock occurred via diffusion,

only plagioclase is considered as it is the dominant

Sr-bearing phase and diffusion rates in other primary phases

are much slower (Giletti and Casserly 1994; Sneeringer

et al. 1984). Diffusive exchange of Sr between plagioclase

and hydrothermal fluids was modeled using equation 8.6.11

from Albarede (1995) for radial diffusion in a sphere, and

the plagioclase diffusion coefficient for an anorthite content

of 0.5 from Giletti and Casserly (1994). We assume fresh

plagioclase had a radius of 500 lm, a 87Sr/86Sr of 0.7024,

and Sr content of 160 ppm, and the hydrothermal fluid had a87Sr/86Sr of 0.7040 and Sr content of 11 ppm. Diffusion

modeling for a duration of 1,000 years at 750�C leads to a87Sr/86Sr shift from 0.70240 to 0.70247, which is close to

the uncertainty of fresh rock 87Sr/86Sr. Thus, the effect of

diffusion on the Sr-isotopic composition of the hydrother-

mally altered plutonic rocks is interpreted to be negligible,

suggesting that dissolution–precipitation dominates the

replacive mechanism.

The dissolution–precipitation reactions contribute to the

loss or gain of Sr and Sr-isotopic exchange in different

ways. The correlation of 87Sr/86Sr with the percentage of

hydrous minerals in samples from the northern escarpment

and Hole 894 (Fig. 6b) suggests that the Sr-isotopic signal

in the gabbroic rocks is largely influenced by the replace-

ment of clinopyroxene and orthopyroxene by amphibole

and/or chlorite. Assuming equilibrium, the 87Sr/86Sr of the

hydrous phases would be equal to that of the fluid, and fluid87Sr/86Sr would be equivalent to a rock altered to 100%

hydrous phases (0.7034; Fig. 6b, inset). This means that

bulk rock 87Sr/86Sr is fixed by 2-component mixing of

igneous and hydrous minerals. Replacement of primary

plagioclase by secondary plagioclase appears to play at

most a minor role in facilitating Sr-isotopic exchange,

primarily because secondary plagioclase is such a minor

phase in these rocks.

Reactions involving pyroxene replacement also appear

to be the dominant mechanism of isotopic exchange in

other gabbroic suites, based on Sr-isotopic data for bulk

rock, and plagioclase and clinopyroxene mineral separates

for the same sample (ODP Sites 921–924, Kempton and

Hunter 1997; ODP Site 735, Kempton 1991). Clinopy-

roxene generally shows isotopic enrichment relative to

coexisting plagioclase, due to partial replacement by

amphibole, and plagioclase has slightly lower 87Sr/86Sr

than the bulk rocks (ODP Sites 921–924, Kempton and

Hunter 1997; ODP Site 735, Kempton 1991). Alteration

conditions in these cores spanned initial amphibole to

greenschist conditions (Stakes et al. 1991).

Comparison of the Sr contents of igneous minerals and

their replacive hydrothermal minerals indicates that the

hydrothermal Sr budget is influenced by the prevailing

alteration conditions. Mineral Sr data suggest that rocks

dominated by lower temperature, greenschist facies assem-

blages (chlorite, actinolite, albite) would lose Sr during

fluid–rock reaction, whereas rocks dominated by higher

temperature, amphibolite facies assemblages (hornblende,

Contrib Mineral Petrol (2012) 164:123–141 135

123

labradorite) would show no change or gain Sr, due to the

uptake of Sr into hydrothermal amphibole. The prediction

that Sr is lost during fluid–rock interactions at low temper-

atures is supported by Sr depletion in sheeted dikes that are

variability altered to greenschist facies assemblages (Barker

et al. 2008; Gillis et al. 2005). Change in Sr contents during

higher-temperature reactions has not been confirmed using

bulk rock data, due to the variability of initial Sr contents of

the gabbroic rocks.

The lack of significant correlation between Sr and87Sr/86Sr in the gabbroic rocks suggests that Sr loss or gain

is not required for isotopic exchange to take place or that

the initial Sr contents of the gabbroic rocks are sufficiently

heterogeneous to mask a correlation. In comparison, the

negative correlation between Sr content and 87Sr/86Sr in the

sheeted dike complex suggests that Sr loss is accompanied

by isotopic exchange (Gillis et al. 2005). This is not the

case for all sheeted dike complexes, however, as the dikes

at the Pito Deep Rift are similarly altered, but with less Sr

depletion and a weak Sr and 87Sr/86Sr correlation (Barker

et al. 2008). Collectively, these relationships indicate that

formation of isotopically enriched hydrous phases plays a

significant role in shifting 87Sr/86Sr, and no gain or loss of

Sr is required to elevate the 87Sr/86Sr of hydrothermally

altered rocks.

Fluid–rock ratios and fluid fluxes

Strontium isotopic compositions of hydrothermally altered

samples can be used to quantify the amount of fluid a rock

has interacted with during hydrothermal alteration. Fluid/

rock ratios for a closed system are calculated by mass

balance, such that:

Fl

R¼

87Sr=86SrFR � 87Sr=86SrI

R

87Sr=86SrFFl � 87Sr=86SrI

Fl

� ��

CIR

� �Sr

CFFl

� �Sr

ð1Þ

where CIR is the initial rock Sr concentration, CF

Fl is the final

fluid Sr concentration, I initial, F final, R rock, Fl fluid.

The approach taken here is to treat the sheeted dike

complex and upper gabbroic sequence as one hydrothermal

system, as the compositions of fluids entering and exiting

the sheeted dike complex are reasonably well constrained.

The average initial and final rock Sr content and Sr-isotopic

ratio values are used, weighted by lithological thickness,

for the combined sheeted dike complex and upper gab-

broic sequence (initial 87Sr=86SrFR = 0.70240–0.70245,

CIR = 90 ppm, final 87Sr=86SrF

R = 0.702688). The initial

fluid entering the sheeted dike complex is seawater whose

isotopic composition has been slightly modified (0.7086)

to account for exchange with basaltic Sr within the lava

pile, and the final fluid is the global average vent fluid

composition (0.7038, see (Barker et al. 2008) for data

compilation). It is assumed that Sr is not mobile on a

crustal scale; thus, the initial Sr concentrations of the fluid

(8 ppm) and rock (90 ppm) remain constant. With these

values, the range of closed-system fluid/rock ratio for the

upper crustal section exposed at Hess Deep is 0.55–0.66.

The documented loss of Sr in the sheeted dike complex

would act to increase the fluid/rock ratio by 0.1–0.2

(Barker et al. 2008), whereas the predicted gain of Sr in the

gabbroic sequence would act to lower the fluid/rock ratio.

Moreover, if magma 87Sr/86Sr were affected by assimila-

tion, mantle-derived fresh rock values would be lower than

the range used here, leading to higher fluid–rock ratios.

The time-integrated fluid flux for the upper crust may be

calculated by multiplying the fluid–rock ratio (0.55–0.66)

by the mass per m2 column of crust. The mass of the

column of crust is calculated by assuming a crustal density

of 2,700 kg m-3 and a column thickness of 1,300 m (dike

complex and upper gabbros). The calculated fluid flux is

*2.1 to 2.5 9 106 kg m-2, which is higher than the

sheeted dike complex on its own (1.5 9 106 kg m-2).

Conceptual model for the evolution of hydrothermal

alteration patterns in the upper plutonic sequence

Alteration patterns documented in the upper plutonic

sequence at the Hess Deep Rift evolved as the crust was

built at the EPR and subsequently moved off-axis. At the

EPR, geophysical data suggest that AMC properties are

dynamic, in that their position in the crust migrates verti-

cally on eruptive cycle timescales (100 to 102 years), pre-

sumably in response to episodes of magma replenishment

and crystallization (e.g., Hooft et al. 1997). In this section,

we examine the evidence for AMC migration in the Hess

Deep upper plutonics and explore how these dynamics led

to the development of the observed alteration patterns.

Whether or not AMC migration was active when the

alteration patterns at Hess Deep were developing can be

assessed from field relationships and the petrochemical

characteristics of specific plutonic rocks. Gabbroic rocks

whose compositions have been affected by assimilation are

found throughout the upper plutonics, requiring at least one

episode of upward AMC migration into hydrothermally

altered, isotopically enriched crust. Development of the

pyroxene hornfels also requires upward migration of the

AMC into hydrothermally altered crust for two reasons.

First, pyroxene hornfels have higher Sr-isotopic ratios than

fresh rock values despite having almost zero secondary

hydrous phases (Fig. 6b), in contrast to other lithologies in

which hydrous secondary mineral abundance and 87Sr/86Sr

correlate. This means these samples were hydrother-

mally altered prior to recrystallization, consistent with

interpretations based on textural evidence in ophiolites

(e.g., France et al. 2009; Gillis and Roberts 1999). Second,

136 Contrib Mineral Petrol (2012) 164:123–141

123

close proximity (likely\50 m) to magmatic heat is needed

to drive the thermal metamorphism (Gillis 2008); thus, the

position of the pyroxene hornfels is closely linked to the

paleo-positions of the AMC.

Migration of the hydrothermal root-zone would result in

a 87Sr/86Sr profile in the upper plutonics that reflects the

time-integrated record of overprinting recharging and dis-

charging fluids. The lack of 87Sr/86Sr change with depth

cannot be readily explained if isotopic exchange occurred

as recharging fluids flowed downwards into the crust. This

is because the 87Sr/86Sr of recharging fluids would become

more rock-dominated with depth, resulting in a decrease in

rock 87Sr/86Sr (Bickle and Teagle 1992), which is not

observed. If, as proposed above, the upper plutonics were

subjected to multiple cycles of fluid–rock interactions as

hydrothermal root-zones migrate, these overprinting reac-

tions would act to homogenize the Sr-signal over time. In

the same way, the lack of a 87Sr/86Sr depth profile in

sheeted dike complexes from Hess and Pito Deeps, and

ODP Hole 504B is attributed to reaction along both

recharge and discharge fluid pathways (Barker et al. 2008).

Within this dynamic framework, incipient, pervasive

fluid flow along microfracture networks occurred through-

out the upper plutonics at[650�C, with a slight increase in

mean temperature from the upper 300 m to lower 500 m

(710–733�C) (Fig. 4a). At these high temperatures, ther-

modynamic reaction path models predict that the percentage

of hydrothermal amphibole is controlled by temperature,

such that fluid–rock reactions form less amphibole and

retain more igneous phases with increasing temperature

(McCollom and Shock 1998). This suggests that the

observed decrease in the percentage of hydrous phases

(Fig. 3a) was, at least in part, temperature controlled.

The observed decrease in the percentage of hydrous

phases with depth in the upper plutonics is attributed to two

additional factors, stratigraphic position and retrograde

overprinting. The upper gabbros, which have more hydrous

alteration, would be more susceptible to fluid–rock inter-

action than deeper gabbros due their proximity to the

sheeted dike-hosted hydrothermal system. Vertical migra-

tion of hydrothermal root-zones, in response to fluctuations

in the magmatic state of the AMC, would be followed by

downward migration as the crustal section cooled and

moved off-axis. Pervasive fluid flow continued down to

B450�C, with lower-temperature fluid–rock reactions lar-

gely concentrated in the upper 300 m of the plutonic

sequence where the percentage of hydrous phases is highest.

The timing of hydrothermal alteration in the upper plu-

tonic sequence associated with pervasive flow fluid is not

well constrained. Clearly some fluid–rock interaction took

place on axis, as evidenced by the pyroxene hornfels and

gabbros influenced by assimilation; however, the duration

of alteration cannot be deduced from the petrochemical

characteristics of the sample suite presented here. Cooling

rates determined for two samples from the Hess Deep field

area are comparable to the upper gabbros at Wadi Abyad

gabbro section in the Oman ophiolite (Coogan et al. 2007a),

implying similar rapid cooling (0.01–0.1�C year-1) in the

uppermost gabbros. Using these cooling rates, the calcu-

lated *160�C range of high-temperature amphibole for-

mation, and assuming magma temperatures of 1,150�C,

incipient high-temperature amphibole formation occurred

over 1,600–16,000 years. At a half-spreading rate of

65 mm year-1, this equates to \1.5 km of spreading. Pre-

vious statistical analysis of the temperature distribution for

incipient fracturing in gabbros from Hess Deep, in combi-

nation with thermal models, predicts that incipient high-

temperature alteration took place within B6,000 years

(B4 km from ridge axis) (Manning et al. 1996). Detailed

chemical profiles in plagioclase adjacent to a high-tem-

perature amphibole vein in core from Hole 894G suggest

that fluid flow and reaction along this fracture lasted

B100 years (Coogan et al. 2007b). These constraints indi-

cate that fractures were locally sealed on short timescales

and that high-temperature fluid–rock interaction started at

the ridge axis and may have continued after the crustal

section left the axial region.

Regional-scale geological and petrochemical data for the

Hess Deep Rift allow for the development a conceptual

model for fluid–rock interaction in the upper plutonics at

and near the EPR that builds on models for the magma–

hydrothermal transition based on ophiolite observations,

geophysical data, and theoretical modeling (e.g., Gillis and

Roberts 1999; Lowell et al. 1995; Wilcock and Delaney

1996). The conceptual framework is as follows. The

hydrothermal root-zone, located within the lowermost

sheeted dikes or uppermost plutonics, is separated from the

AMC that drives hydrothermal convection by a thin (\50 m)

conductive boundary layer (Fig. 9a). The position of the

hydrothermal root-zone and conductive boundary layer are

coupled to the AMC, and they shallow and deepen in

response to periods of magma replenishment and crystalli-

zation, respectively. As the AMC advances upwards into the

base of the sheeted dikes, it intrudes into hydrothermally

altered crust, stoping and assimilating the overlying roof

rocks, leading to chemical modification of the magma.

Moreover, within the overlying conductive boundary layer,

thermal metamorphism creates a thin, impermeable contact

aureole composed of hornfelsic country rock. Depending on

the local thermal conditions, the country rock may partially

melt (e.g., Gillis and Coogan 2002; Koepke et al. 2008). As

AMCs crystallize and subside, so does the hydrothermal

root-zone, bringing hydrothermal fluids to deeper crustal

levels (Fig. 9b). Later injection of dikes into earlier formed

plutonics, vertically from a deeper seated AMC or laterally

from a distal AMC, creates a mixed zone of gabbro and

Contrib Mineral Petrol (2012) 164:123–141 137

123

dikes. This mixed zone may be subsequently intruded if the

AMC shoals again, leading to contact metamorphism of

the dikes and gabbros (Fig. 9c). A section of crust has the

potential to undergo many cycles of AMC migration, if the

magmatic evolution of AMCs evolves on eruptive time-

scales. Once outside of the axial zone, the hydrothermal

root-zone migrates downwards, overprinting the fluid–rock

record set in the axial region.

The freshness of the lowermost gabbros at the Hess

Deep Rift suggests that downward propagation of the

hydrothermal root-zone into the plutonic sequence along

fracture networks, as originally envisioned by Lister

(1974), does not lead to significant mineralogical or

chemical change and, thus, heat exchange as the crust

moves off-axis. This suggests that the most significant fluid

and heat fluxes in the lower crust are associated with

focused fluid flow, for example, along axial faults (Coogan

et al. 2006).

Summary

Exposures of the upper plutonic sequence within a 4-km-

wide region along the northern escarpment of the Hess

Deep Rift provide a record of fluid–rock interaction within

crust formed at the fast-spreading EPR. Mineralogical

and chemical characteristics and Sr-isotopic data for sub-

mersible-collected samples reveal alteration patterns within

the upper plutonics and across the sheeted dike complex–

plutonic transition. The plutonics were altered by pervasive

fluid flow along micro- and macroscopic fracture networks

to amphibole-dominated assemblages. Incipient alteration

throughout the section occurred at [650�C, with slightly

higher temperatures in the lowermost 500 m. The lower

500-m gabbros are less altered, due to the combined effects

of higher temperatures at the axis and less lower-temper-

ature overprint. The distribution of gabbros showing evi-

dence for assimilation and thermally metamorphosed

hornfels, coupled with the lack of change in 87Sr/86Sr with

depth, indicates that the hydrothermal root-zone migrated

into the upper plutonics on axis, perhaps during several

cycles of waxing and waning of magmatism.

The Sr-isotopic composition of the plutonic rocks has

been shifted only slightly toward seawater values by fluid–

rock interactions, with lower 87Sr/86Sr than the overlying

sheeted dike complex. Changes in Sr contents are influ-

enced by alteration assemblages, with no change or Sr gain

at amphibolite facies conditions and Sr loss at lower

AMC

(b)(a)

AMC

(c)

Sheeted dikes

contact aureole

AMC

crystal mush

HRZ

HRZ

Sheeted dikes

contact aureole

Gabbro

contact aureole

Sheeted dikes

Gabbro

HRZ

Fig. 9 Conceptual model for the development of alteration patterns

in the upper plutonic crust formed at the fast-spreading East Pacific

Rise. a An axial magma chamber (AMC) intrudes into the base of the

sheeted dike complex, leading to stoping of the hydrothermally

altered dikes into the AMC (light gray blebs are xenoliths) and

development of a thin contact aureole immediately above the AMC.

This contact aureole acts as the boundary between the hydrothermal

root-zone (HRZ) above and the subjacent magma chamber, the

so-called conductive boundary layer. b As an AMC subsides, the

hydrothermal root-zone migrates down into the upper plutonics,

leading to high-temperature hydrothermal alteration of the gabbroic

rocks (black blebs). c Replenishment of the AMC at a deeper

stratigraphic level than a leads to dike intrusion into the overlying

gabbroic rocks, creating a zone of mixed dikes and gabbros. A thin

contact aureole develops immediately above the AMC, leading to the

formation of pyroxene hornfels within the upper plutonics. At the

EPR, a section of crust may undergo multiple cycles (a–c). Off-axis,

the hydrothermal root-zone would migrate down into the upper

plutonics. Thick arrows mark the direction of movement of the AMC;

thin arrows mark fluid pathways [modified from Gillis and Roberts

(1999), Gillis (2008)]

138 Contrib Mineral Petrol (2012) 164:123–141

123

temperature, greenschist facies conditions. Isotopic

exchange is largely controlled by the replacement of

igneous minerals by hydrous hydrothermal phases and does

not require Sr gain or loss. Time-integrated fluid–rock

ratios and fluid fluxes for the sheeted dike complex and

upper 800 m of the plutonic sequence are 30–60% higher

than those calculated for the sheeted dike complex on its

own.

Acknowledgments We grateful to the following colleagues for

their analytical assistance: J. Spence (LA-ICP-MS); M. Raudsepp

(electron microprobe); G. Dipple (cathode luminescence); and

E. Humphrey (SEM). We thank D. Kelley, D. Canil and L. Coogan

for their input during the project, and L. Coogan for discussions and

review of an early version of this manuscript. Helpful reviews by

J. Alt and A. McCaig are appreciated. The Hess Deep cruises were

supported by the National Foundation for Science grants to J. Karson,

P. Lonsdale, E. Klein and S. Hurst; KMG gratefully acknowledges

these PIs for their invitations to participate in their Hess Deep Alvin

cruises, and many discussions. This project was supported by a

Natural Sciences and Engineering Research Council discovery grant

(KMG).

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