mineralogical and strontium isotopic record of hydrothermal processes in the lower ocean crust at...
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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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