JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 1
PAGES 37^59
2009
doi:10.1093/petrology/egn071
Fe^Ni^Co^O^S Phase Relations in
Peridotite^Seawater Interactions
FRIEDER KLEIN AND WOLFGANG BACH*
GEOSCIENCE DEPARTMENT, UNIVERSITY OF BREMEN, KLAGENFURTER STRAE, 28359 BREMEN, GERMANY
RECEIVED SEPTEMBER 8, 2008; ACCEPTED NOVEMBER 28, 2008
Serpentinization of abyssal peridotites is known to produce extremely
reducing conditions as a result of dihydrogen (H2,aq) release upon
oxidation of ferrous iron in primary phases to ferric iron in
secondary minerals by H2O. We have compiled and evaluated thermodynamic data for Fe^Ni^Co^O^S phases and computed phase
relations in fO2,g^fS2,g and aH2,aq^aH2S,aq diagrams for temperatures between 150 and 4008C at 50 MPa. We use the relations
and compositions of Fe^Ni^Co^O^S phases to trace changes in
oxygen and sulfur fugacities during progressive serpentinization and
steatitization of peridotites from the Mid-Atlantic Ridge in the
158200 N Fracture Zone area (Ocean Drilling Program Leg 209).
Petrographic observations suggest a systematic change from awaruite^magnetite^pentlandite and heazlewoodite^magnetite^pentlandite assemblages forming in the early stages of serpentinization to
millerite^pyrite^polydymite-dominated assemblages in steatized
rocks. Awaruite is observed in all brucite-bearing partly serpentinized rocks. Apparently, buffering of silica activities to low values
by the presence of brucite facilitates the formation of large amounts
of hydrogen, which leads to the formation of awaruite. Associated
with the prominent desulfurization of pentlandite, sulfide is removed
from the rock during the initial stage of serpentinization. In contrast,
steatitization indicates increased silica activities and that highsulfur-fugacity sulfides, such as polydymite and pyrite^vaesite solid
solution, form as the reducing capacity of the peridotite is exhausted
and H2 activities drop. Under these conditions, sulfides will not
desulfurize but precipitate and the sulfur content of the rock increases.
The co-evolution of fO2,g^fS2,g in the system follows an isopotential
of H2S,aq, indicating that H2S in vent fluids is buffered. In contrast,
H2 in vent fluids is not buffered by Fe^Ni^Co^O^S phases, which
merely monitor the evolution of H2 activities in the fluids in
the course of progressive rock alteration.The co-occurrence of pentlandite^awaruite^magnetite indicates H2,aq activities in the
interacting fluids near the stability limit of water. The presence
Mantle peridotite is commonly exposed at the seafloor of
ultraslow- and slow-spreading mid-ocean ridges by detachment faulting that initiated close to the spreading axes (e.g.
Cann et al., 1997; Tucholke et al., 1998). Retrograde metamorphic hydration of these rocks as a result of reaction
with seawater (i.e. serpentinization) is widespread in
these settings. Serpentinization strongly influences the
rheology of the oceanic lithosphere (Escart|¤ n et al., 1997),
the geochemical budgets of the oceans (Thompson &
Melson; 1970; Snow & Dick, 1995), and microbial processes
within, at, and above the seafloor (Alt & Shanks, 1998;
O’Brien et al., 1998; Kelley et al., 2001). A remarkable feature of serpentinization is the strongly reducing nature of
the interacting fluids, as indicated by the occurrence of
native metals or alloys in serpentinites (Nickel, 1959;
Chamberlain et al., 1965; Lorand, 1985; Abrajano &
Pasteris, 1989). Fluids issuing from ultramafic massifs, for
instance in the Coast Range and Samail ophiolites, exhibit
exceptionally high concentrations of dissolved dihydrogen
(H2,aq) (Thayer, 1966; Barnes et al., 1967; Neal & Stanger,
1983). Serpentinite-hosted seafloor hydrothermal systems
also show high H2,aq concentrations of 10^15 mmol/kg
*Corresponding
author.
Telephone:
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Fax: 0049-421-218-65429. E-mail: [email protected]
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of a hydrogen gas phase would add to the catalyzing capacity of
awaruite and would facilitate the abiotic formation of organic
compounds.
KEY WORDS: serpentinization; ODP Expedition 209; sulfide;
oxygen fugacity; sulfur fugacity; hydrothermal system; metasomatism;
Mid-Atlantic Ridge
I N T RO D U C T I O N
JOURNAL OF PETROLOGY
VOLUME 50
(Charlou et al., 2002; Douville et al., 2002; Kelley et al.,
2005; Proskurowski et al., 2006). These high H2,aq concentrations are due to the oxidation of Feþ2 in the host rock by
water to Feþ3 in magnetite that forms along with serpentine and brucite during serpentinization. Seyfried et al.
(2007) have presented experimental data suggesting that
incorporation of Feþ3 in serpentine may also generate considerable amounts of hydrogen.
The combined effects of fluid^rock interaction as a function of rock composition, water to rock ratio, and temperature can be examined by peridotite^seawater reaction path
models (e.g. Wetzel & Shock, 2000; Palandri & Reed, 2004;
McCollom & Bach, 2008), but detailed knowledge of the
mineral^fluid equilibria that govern fluid compositions is
required. Experimental and theoretical studies (e.g.
Seyfried & Ding, 1995; Seyfried et al., 2004, 2007) indicate
that specific mineral^fluid reactions buffer H2,aq and also
H2S,aq in hydrothermal solutions venting at the seafloor.
In basalt systems, for instance, the pyrite^pyrrhotite^magnetite (PPM) buffer commonly sets H2,aq and H2S,aq in
the interacting fluids (Seyfried & Ding, 1995). The reactions that control H2,aq and H2S,aq in peridotite^
water systems are less well known. Seyfried et al. (2004)
suggested that a bornite^chalcopyrite^magnetite buffer
controls H2,aq and H2S,aq in fluids issuing from submarine peridotite-hosted hydrothermal systems. In other studies it has been suggested, however, that the phase
relations actually observed in altered peridotites indicate
the presence of H2,aq concentrations of the order of
hundreds of millimoles in serpentinization fluids (Sleep
et al., 2004; Bach et al., 2006; Frost & Beard, 2007). These
high predicted concentrations of hydrogen are corroborated by similarly high H2,aq concentrations in fluids
from hydrothermal experiments (Janecky & Seyfried,
1986; Berndt et al., 1996; Horita & Berndt, 1999;
McCollom & Seewald, 2001; Allen & Seyfried, 2003;
Seyfried et al., 2007).
The use of mineral^fluid equilibria calculations in
examining peridotite^water interactions and associated
H2,aq and H2S,aq activities in hydrothermal solutions is
currently limited by the lack of thermodynamic data for
many of the phases that are abundant in serpentinite (e.g.
pentlandite, awaruite, godlevskite, polydymite, violarite,
vaesite; idealized formulae of opaque minerals in serpentinized peridotites are given inTable 1). Eckstrand (1975) and
Frost (1985) worked out the phase relations in the Fe^Ni^
O^S system and concluded that native metals and alloys
[e.g. awaruite (Ni3Fe), which is commonly observed in serpentinites] indicate extremely low oxygen fugacities (e.g.
eight orders of magnitude below PPM at 3008C) along
with low sulfur fugacities [e.g. 10 orders of magnitude
below PPM (Frost, 1985)]. Alt & Shanks (1998) recognized
that awaruite is a common phase forming in the early
stages of serpentinization of abyssal peridotites. Awaruite
NUMBER 1
JANUARY 2009
Table 1: Idealized formulae of opaque minerals in serpentinized peridotites
Mineral
Chemical formula
awaruite
Ni3Fe
tetrataenite
NiFe
pentlandite
(FeNi)9S8
godlevskite
Ni9S8
heazlewoodite
Ni3S2
millerite
NiS
polydymite
Ni3S4
violarite
FeNi2S4
magnetite
Fe3O4
pyrite
FeS2
pyrrhotite
FeS
linnaeite
Co3S4
cattierite
CoS2
jaipurite
CoS
wairauite
CoFe
chalcopyrite
CuFeS2
and many other phases, however, were not considered in
previous theoretical studies dealing explicitly with seafloor
hydrothermal systems because of a scarcity of thermodynamic data. To overcome these difficulties, we added thermodynamic data for some crucial phases of the Fe^Ni^Co^
O^S system to the SUPCRT92 (Johnson et al., 1992) database and created a new EQ3/6 (Wolery, 1992) isobaric
(50 MPa) thermodynamic database for temperatures from
0 to 4008C in 258C increments. Values of the dissolution
reaction constant (log K) for phases for which heat capacity data are unavailable were estimated using the van’t
Hoff relation. Our approach is to document carefully
Fe^Ni^Co^O^S phase relations in altered peridotites from
the Ocean Drilling Program (ODP) Leg 209, MidAtlantic Ridge (MAR) 158N area, and determine phase
stabilities in the aH2,aq^aH2S,aq plane. Following Frost
(1985), we use these phase relations to deduce the evolution
paths of peridotite^water interaction. Our goal is to estimate H2,aq and H2S,aq concentrations in fluids associated
with the assemblages observed so as to compare the predicted concentrations with those measured in field
and experimental studies (Seyfried & Dibble,1980; Janecky
& Seyfried,1986; McCollom & Seewald, 2001; Charlou et al.,
2002; Douville et al., 2002; Allen & Seyfried, 2003;
Proskurowski et al., 2006; Seyfried et al., 2007).
GEOLOGIC A L S ET T I NG
The area of the 158200 N Fracture Zone (FZ, Fig. 1) at the
slow-spreading (53 cm/year, full rate) MAR has been
38
KLEIN & BACH
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
Fig. 1. Location of the study area in the vicinity of the 158200 N Fracture Zone. Investigated samples are from Sites 1268, 1270, 1271, and 1274;
redrawn from Kelemen et al. (2004c).
Sulfides are abundant in particular in the lower half of
the section.
Site 1270 is the southernmost of all the drill sites, located
on the eastern flank of the MAR in the vicinity of the
Logatchev hydrothermal field (148450 N) on the eastern
median valley wall. Peridotites from Hole 1270C are pervasively serpentinized and steatized adjacent to gabbroic
veins. Although alteration of peridotites from Holes 1270C
and 1270D mainly took place under static conditions,
some peridotites are closely related to strongly deformed,
schlieren-like gabbroic intrusions that have been completely altered to chlorite and tremolite. In addition, Holes
1270C and 1270D contain minor carbonate and oxide
veins.
Site 1271 is located on the inside corner high of the MAR
spreading segment south of the 158200 FZ. Drill core 1271A
is mainly composed of completely serpentinized dunite.
Drill core 1271B comprises variably serpentinized dunite
and harzburgite. Steatitization is minor in these rocks.
Site 1274 is located 31km north of the 158200 N FZ on the
western rift valley wall at 3940 m water depth and 700 m
west of the termination of the detachment fault. Hole
1274A penetrates 156 m into the basement and recovered
35 m of core that comprises 77% harzburgite, 20%
dunite, and 3% gabbro. Peridotite from this hole represents the least altered rock from Leg 209 with up to 35%
of the original minerals preserved. For a comprehensive
description of all the drill sites and a more detailed
description of the alteration mineralogy and chemistry we
refer the reader to Kelemen et al. (2004a, 2007), Bach et al.
(2004, 2006) and Paulick et al. (2006).
explored in detail by numerous surveys (e.g. Rona et al.,
1987; Bougault et al., 1988; Cannat et al., 1997; Casey et al.,
1998; Escart|¤ n & Cannat, 1999; Escartin et al., 2003;
Fujiwara et al., 2003). Basaltic volcanism is sparse north of
the 148N region and the volcanic rocks are mainly
enriched-type mid-ocean ridge basalts (E-MORB) (Dosso
& Bougault, 1986; Dosso et al., 1993). Two magmatic centers
at 14 and 168N coincide with negative mantle Bouguer
gravity anomalies, whereas the region between these gravity bull’s-eyes shows a gravity high consistent with the predominance of mantle rocks at the seafloor (e.g. Cannat
et al., 1997; Escart|¤ n & Cannat, 1999; Fujiwara et al., 2003).
Extensive outcrops of serpentinized peridotites and gabbroic rocks on both rift flanks are indicative of a heterogeneous lithosphere, a high ratio of tectonic to magmatic
extension, crustal thinning and the formation of oceanic
core complexes along long-lived low-angle detachment
faults (Escart|¤ n & Cannat, 1999). ODP Leg 209 drilled 19
holes at eight sites north and south of the 158200 FZ into
variably serpentinized peridotites intruded by gabbroic
rocks (Kelemen et al., 2004a, 2007). The following brief
description of drill holes and rock alteration is based on
the work of Kelemen et al. (2004a) and Bach et al. (2004),
and focuses on those holes from which samples were
selected for this study.
Hole 1268A lies on the western rift valley wall south of
the 158200 FZ and extends 147.6 m into completely altered
harzburgite, dunite, strongly altered late magmatic dikes
and mylonitic shear zones. The rocks were affected by pervasive serpentinization and locally a superimposed pervasive replacement of serpentine by talc (steatitization).
39
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 1
JANUARY 2009
In the following subsections, we describe the thermodynamic data for the Fe^Ni^Co^O^S phases, which are not
part of the SUPCRT92 mineral set. Uncertainties in these
data and their propagation in the calculation of phase
boundaries are hard to quantify. Standard state thermodynamic data for minerals, aqueous and gaseous species, as
well as high-temperature heat capacity data for minerals
and interaction parameter for solid solutions are often
poorly known. Consequently, these data must be considered preliminary.
A N A LY T I C A L M E T H O D S
Microscopy and electron microprobe
analysis
Thin sections were optically investigated in transmitted
and reflected light using a Leica DM RXP HC oil immersion microscope. Mineral compositions were analyzed
with a ‘JEOL Superprobe JXA 8900 R’ electron microprobe at the University of Kiel (Germany), equipped with
five wavelength-dispersive spectrometers. Minerals were
analyzed with an accelerating voltage of 20 kV for a beam
current of 20 nA and a fully focused 1 mm beam diameter.
Both synthetic and natural mineral standards were used.
The raw data were corrected using the ZAF method.
Micro-scale element mapping and backscattered electron
images of serpentine meshes and nickeliferous opaque
mineral assemblages were used to complement petrographic observations. In total, 915 single point analyses of
nickeliferous opaque minerals were obtained. Although
many of these analyses had low totals because of the small
grain size of most of the Fe^Ni^Co^O^S minerals, their
evaluation helped us understand the changing phase relations with progressive serpentinization.
Pentlandite
Berezovskii et al. (2001) conducted low-temperature heat
capacity measurements for synthetic pentlandite
(Fe460Ni454S8) and reported a standard entropy (S8) of
4749 J/mol per K and H29815 H0 of 76280 kJ/mol.
Using these data, we calculated a standard enthalpy of formation (iH8f) of 8470 kJ/mol for stoichiometric pentlandite (Fe45Ni45S8), using standard enthalpies of
formation for troilite (FeS) and millerite (NiS) from
Robie & Hemingway (1995). Cemic› & Kleppa (1987)
reported a iH8f of 83737 1459 kJ/mol, which is in
agreement with our results. An apparent Gibbs energy of
formation (iG8f) of 8363 kJ/mol was derived using the
standard molar entropies of Fe, Ni and S given by Robie &
Hemingway (1995). This number is consistent with
iG8f ¼ 8352 kJ/mol from Craig & Naldrett (1971).
Because high-temperature heat capacity data are lacking,
we used the van’t Hoff extrapolation and SUPCRT92
data for Fe, Ni and S to compute log K values for dissolution of pentlandite. A molar volume (V8) of 1533 cm3/mol
was calculated for a natural pentlandite from cell constants
given by Kouvo et al. (1959).
Thermodynamic calculations
Thermodynamic calculations were conducted using the
SUPCRT92 (Johnson et al., 1992) computer code. The
database of SUPCRT92 consists of standard-state
(29815 K and 105 Pa) thermodynamic parameters, Maier^
Kelley coefficients, and equation of state parameters for
pure minerals, aqueous species and gases for the calculation of equilibrium constants (log K values) for temperatures and pressures up to 10008C and 500 MPa. The
database used for this study combines all upgrades from
the slop98.dat and the speq02.dat database (Wolery &
Jove-Colon, 2004). Phase diagrams were constructed using
Geochemist’s Workbench (GWB ) version 7.0.2 (Bethke,
2007). A thermodynamic database for GWB was
assembled for a pressure of 50 MPa and temperatures of 0,
25, 100, 200, 250, 300, 350, and 4008C. Log K values in that
database were either computed by SUPCRT92 or calculated using a van’t Hoff temperature extrapolation, while
ignoring the effect of pressure (see below). Log K values
for the dissolution of minerals are given in Table 2.
Activity coefficients for H2,aq were calculated following
Drummond (1981) for CO2,aq, whereas the activity coefficient for H2S,aq was assumed to be unity at all temperatures (see Helgeson et al., 1970). Actual fugacity^
concentration relations for H2S and H2 from Kishima
(1989) and Kishima & Sakai (1984) suggest some deviation
from ideal behavior. However, corrections to the log
K values for equilibrium between dissolved and gaseous
species were not applied, because (1) fugacity^concentration
relations are unavailable for T53008C and (2) corrections
are negligible (5015 log units) between 300 and 4008C.
Heazlewoodite
iG8f, iH8f and S8 for heazlewoodite (Ni3S2) were taken
from Robie & Hemingway (1995). We used high-temperature heat capacity data from Stlen et al. (1991) to calculate
Maier^Kelley coefficients. V8 (40655 cm3/mol) for heazlewoodite was calculated using cell constants given by Parise
(1980).
Awaruite
Howald (2003) reported iG8f, iH8f and S8 for awaruite
(Ni3Fe). We calculated log K values for awaruite by means
of the van’t Hoff extrapolation and SUPCRT92 data for Fe
and Ni, because high-temperature calorimetric data are
unavailable. V8 (2696 cm3/mol) was calculated from cell
constants given by Anthony et al. (1990).
Tetrataenite
Howald (2003) also reported iG8f, iH8f and S8 for tetrataenite (NiFe). We calculated log K values for tetrataenite
by means of the van’t Hoff extrapolation and SUPCRT92
data for Fe and Ni, because high-temperature calorimetric
40
KLEIN & BACH
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
Table 2: Equilibrium constants for dissolution of opaque minerals (P ¼ 50 MPa)
Reaction
Mineral
no.
log K
08C
258C
1008C
2008C
2508C
3008C
3508C
4008C
1
awaruite
23170
19680
16191
12006
10474
9177
8046
2
tetrataenite
11943
10367
8338
6190
5405
4741
4163
7221
3738
3
pentlandite
5771
5673
5635
5909
6167
6515
7008
7177
4
heazlewoodite
5
godlevskite
3068
2661
1694
773
393
032
323
540
8745
8442
7962
7871
8001
8244
8650
8729
6
millerite
913
883
842
849
873
909
964
984
7
polydymite
12100
11388
9944
8924
8654
8508
8496
8359
8
violarite
11814
11042
9459
8335
8031
7857
7821
7667
9
vaesite
1548
1463
1344
1345
1390
1462
1572
1665
10
Wairauite
12048
10925
8427
6291
5511
4863
4279
3857
11
cobaltpentlandite
8222
7911
7366
7180
7265
7464
7829
7883
12
jaipurite
825
800
764
772
794
829
883
903
13
linnaeite
11241
10566
9184
8196
7930
7780
7756
7574
1691
14
cattierite
1794
1681
1498
1441
1464
1517
1611
15
H2S,aq
728
686
637
653
682
722
778
849
16
H2O,l
5066
4630
3635
2758
2431
2153
1909
1684
Reaction no.
1
Ni3Fe þ 8 Hþ þ 2 O2,aq ¼ 3 Ni2þ þ Fe2þ þ 4 H2O
2
NiFe þ 4 Hþ þ O2,aq ¼ Fe2þ þ Ni2þ þ 2 H2O
3
Fe45Ni45S8 þ 10 Hþ ¼ 45 Ni2þ þ 45 Fe2þ þ 8 HS þ H2,aq
4
Ni3S2 þ 4 Hþ þ 05 O2,aq ¼ 3 Ni2þ þ 2 HS þ H2O
5
Ni9S8 þ 10 Hþ ¼ 9 Ni2þ þ 8 HS þ H2,aq
6
NiS þ Hþ ¼ Ni2þ þ HS
7
Ni3S4 þ 4 Hþ ¼ Ni2þ þ 2 Ni3þ þ 4 HS
8
FeNi2S4 þ 4 Hþ ¼ Fe2þ þ 2 Ni3þ þ 4 HS
9
NiS2 þ H2,aq ¼ Ni2þ þ 2 HS
10
CoFe þ 4 Hþ þ O2,aq ¼ Fe2þ þ Co2þ þ 2 H2O
11
Co9S8 þ 10 Hþ ¼ 9 Co2þ þ 8 HS þ H2,aq
12
CoS þ Hþ ¼ Co2þ þ HS
13
Co3S4 þ 4 Hþ ¼ Co2þ þ 2 Co3þ þ 4 HS
14
CoS2 þ H2,aq ¼ Co2þ þ 2 HS
15
H2S,aq ¼ HS þ Hþ
16
H2O,l ¼ H2,aq þ 05 O2,aq
data are lacking. V8 (1384 cm3/mol) was calculated from
cell constants given by Albertsen et al. (1978).
standard molar entropies for Ni and S given by Robie &
Hemingway (1995). V8 for a natural godlevskite (1487
cm3/mol) was calculated from cell constants given by
Fleet (1987).
Godlevskite
Thermodynamic properties of godlevskite (Ni9S8) were
obtained from heat capacity measurements by Stlen et al.
(1994) for Ni7S6 and Ni3S2. A iH8f of ^80292 kJ/mol was
calculated from H29815 H0 (74920 J/mol) using the standard enthalpy of formation of NiS given by Robie &
Hemingway (1995). iG8f was derived from iH8f and
Millerite
The standard state thermodynamic properties of millerite
(NiS) were taken from Robie & Hemingway (1995). The
transition of b-millerite at 3798C to -millerite was
41
JOURNAL OF PETROLOGY
VOLUME 50
accounted for in the calculation of the equilibrium constants at 4008C.
NUMBER 1
JANUARY 2009
R E S U LT S
Petrography
We distinguish two types of rock alteration: serpentinization of peridotite and steatitization of serpentinite. At Site
1274, peridotites are partially to fully serpentinized,
whereas at Sites 1270, 1271 and 1268, partially to fully serpentinized peridotites have undergone additional steatitization to variable degrees (see Bach et al., 2004).
Microtextures of the serpentinized peridotites range from
pseudomorphic mesh and hourglass textures after olivine
to transitional ribbon texture to non-pseudomorphic and
interlocking textures. Typical in partially serpentinized
rocks (65^98% altered) are mesh-textures with fresh olivine relicts in the mesh centers and serpentine/brucite-rich
mesh rims. Magnetite is concentrated away from the olivine kernels along former grain boundaries and in areas
of complete serpentinization (e.g. Bach et al., 2006).
Completely serpentinized rocks have serpentine/brucite/
iowaite centers, which often appear dark in transmitted
light. Most samples are extensively veined by paragranular
and transgranular serpentine veins. Paragranular veins
form an anastomosing network that is usually foliation parallel and wraps around porphyroclasts, whereas transgranular veins crosscut porphyroclasts (Kelemen et al., 2004a).
Late isotropic picrolite veins are also transgranular.
Serrate chrysotile veins occur almost exclusively in nonpseudomorphic interlocking textures. In proximity to gabbroic intrusions steatitization is strongest and often invades
adjacent serpentinite by replacing former transgranular
serpentine veins. Even in strongly steatized rocks the original serpentine micro-texture is commonly preserved, indicating alteration under static conditions (Bach et al., 2004).
Cr-spinel is widespread but is clearly a magmatic relict
that, as mentioned by Eckstrand (1975), serves only as a
nucleus to secondary overgrowth of magnetite. Incipient
alteration of Cr-spinel is indicated by occasional overgrowths of grains by a thin (5^10 mm) ferrit-chromite rim.
The following petrographic description focuses on
opaque mineral assemblages in variably serpentinized and
steatized peridotites. Because of the small grain size of
most nickeliferous opaque minerals, their identification by
reflected light microscopy was often impossible. To overcome this problem and to document better the changing
Fe^Ni^Co^O^S phase relations with progressive serpentinization we analyzed the chemical compositions of opaque
phases by electron microprobe and used the compositional
data for phase identification.
Vaesite
iH8f, S8 and heat capacity data for vaesite (NiS2) were
taken from NIST (Chase, 1998). iG8f (1248 kJ/mol) was
derived using iH8f and the standard molar entropies of Ni
and S. V8 was taken from Smyth & McCormick (1995).
Violarite
We used iG8f and S8 of violarite (FeNi2S4) reported by
Craig (1971). iH8f was taken from Cemic› & Kleppa
(1987). As no heat capacity data are available we calculated
log K values for dissolution of violarite using the van’t Hoff
extrapolation and SUPCRT92 data for Fe, Ni and S. V8
was taken from Smyth & McCormick (1995).
Polydymite
iH8f, S8 and heat capacity data for polydymite (Ni3S4)
were taken from NIST (Chase, 1998). iG8f
(2918 kJ/mol) was derived using standard molar entropies of Ni and S given by Robie & Hemingway (1995).
V8 was taken from Smyth & McCormick (1995).
Cobaltpentlandite
iH8f (85479 kJ/mol) and S8 (46317 J/mol per K) data for
the Co-endmember (Co9S8) of the pentlandite solid solution series were taken from Rosenqvist (1954). iG8f
(83643 kJ/mol) was computed using iH8f and standard
molar entropies for Co and S given by Robie &
Hemingway (1995). High-temperature heat capacity data
were adopted from Kelley (1949). The standard molar
volume (1471cm3/mol) was calculated using cell parameters from Rajamani & Prewitt (1975).
Wairauite
iG8f and iH8f for wairauite (CoFe) are listed in the
scientific group thermodata binary compounds database
(Dinsdale, 1991). We calculated dissolution constants by
means of the van’t Hoff extrapolation and SUPCRT92
data for Co and Fe. The molar volume (1409 cm3/mol)
was calculated from cell constants given by Bayliss (1990).
Linnaeite, cattierite, and jaipurite
iH8f, S8 and high-temperature heat capacity data for all
three phases were taken from Mills (1974). The iG8f of all
three minerals was calculated using iH8f and standard
molar entropies for Co and S given by Robie &
Hemingway (1995). The molar volumes of linnaeite
(Co3S4) and cattierite (CoS2) were taken from Robie &
Hemingway (1995), and that of jaipurite (CoS) from
Naumov et al. (1974).
Primary sulfides
Peridotites from ODP Leg 209 are strongly melt-depleted
as indicated by their modal and trace element composition
(Paulick et al., 2006; Seyler et al., 2007). Intercumulus sulfide^oxide ‘blebs’ of clear magmatic origin, as described
by Eckstrand (1975) for the Dumont serpentinite, or sulfide
42
KLEIN & BACH
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
Veins
inclusions hosted by olivine or pyroxene, as described by
Lorand (1989), were not recognized in samples investigated
here.
Primary sulfides are commonly completely replaced by
secondary sulfides or alloys formed during serpentinization. Seyler et al. (2007) reported local occurrences of magmatic sulfides (polyhedral blebs of pentlandite, bornite,
and chalcopyrite with concave inward grain boundaries)
probably introduced during late melt impregnation of the
lithospheric mantle. Chalcopyrite was found only in samples from Site 1268; no Cu-sulfides were present in the limited number of samples investigated from Sites 1270, 1271,
and 1274. Sulfide grains residual to melting in ultramafic
rocks often consist of pentlandite and minor pyrrhotite.
Although pyrrhotite occurs in many serpentinized peridotites described in the literature (e.g. Shiga, 1987; Abrajano
& Pasteris, 1989; Lorand, 1989), it is absent in all the samples we investigated from Leg 209. Miller (2007) reported
the occurrence of pyrrhotite together with pentlandite in
samples from Hole 1268A. This paragenesis is probably of
magmatic origin and related to gabbroic intrusions frequently found in the lower part of this hole. Rare euhedral
pentlandite grains with octahedral cleavage (10^30 mm in
diameter), which were found in samples from Site 1271
(Figs. 2a and e), may also be primary. They usually occur
in porphyroclasts of former orthopyroxene (bastite), but no
pentlandite inclusions were found in fresh pyroxene.
In paragranular serpentine veins, abundant anhedral to
weakly subhedral magnetite is aligned along tracks of a
few tens of micrometers thickness, which are up to several
hundred micrometers long. Pentlandite, heazlewoodite,
awaruite, and godlevskite are preferentially located in the
central part of the veins together with magnetite. In places
pentlandite þ awaruite þ magnetite and cobaltian pentlandite þ awaruite þ magnetite occur in the same vein.
In larger transgranular (isotropic picrolite) veins, typically 05^1mm thick, magnetite occurs as either patchy
single tracks or as double tracks discontinuously along
both sides of the vein. In transgranular veins, nickeliferous
opaques and their different replacement products are
easily identified, because their grain size is typically
larger (up to 50 mm in diameter) than in meshes or paragranular veins.
In transgranular veins of partly serpentinized peridotites pentlandite is often intergrown with awaruite and
mantled by magnetite, suggesting that pentlandite and
awaruite perhaps grew in equilibrium and were subsequently mantled by magnetite (Fig. 2d). In other cases,
pentlandite is clearly replaced by awaruite and magnetite
(Fig. 2b). This is indicated by the elevated Co and Ni contents of the magnetite (see below), mantling pentlandite.
Remarkably, awaruite and not wairauite replaces cobaltian pentlandite. Pentlandite is frequently associated with
heazlewoodite and/or magnetite (Figs. 2f and g). In some
apparently pure heazlewoodite^magnetite assemblages in
transgranular veins of almost fully serpentinized peridotites micro-scale element mapping revealed the presence
of relic cobaltian pentlandite. It occurs as inclusions typically smaller than 1 mm within relatively coarse-grained
heazlewoodite or between heazlewoodite and magnetite,
suggesting that heazlewoodite and magnetite grew at the
expense of cobaltian pentlandite. Heazlewoodite co-occurring with godlevskite and magnetite may also contain
small cobaltian pentlandite inclusions, which are lacking
in godlevskite. This indicates that godlevskite did not
directly grow at the expense of cobaltian pentlandite.
Because godlevskite is exclusively found in fully serpentinized rocks, it is more likely that godlevskite replaces heazlewoodite (Fig. 2h) in the final stage of serpentinization.
In some completely serpentinized peridotites from Hole
1268A magnetite in veins is partially replaced by pyrite.
Secondary opaque phases
The investigated serpentinites contain 501vol.% nickeliferous opaque minerals. The principal opaque minerals in
partly serpentinized peridotites include, in order of
decreasing abundance, magnetite, cobaltian pentlandite,
pentlandite, and heazlewoodite. Awaruite is common but
occurs only in minor amounts. In general, nickeliferous
opaque minerals appear as finely disseminated grains in
the serpentine matrix. Their grain size ranges from 51 to
50 mm. By far the most abundant mineral assemblages are
pentlandite þ awaruite þ magnetite and pentlandite þ
heazlewoodite þ magnetite (Figs. 2b and f).
Mesh rims
In pseudomorphic serpentine mesh rims, disseminated
opaque phases are generally 51 mm in diameter and thus
their mineralogy could not be determined by reflected
light oil immersion microscopy or conventional quantitative electron microprobe analysis. Semi-quantitative
micro-scale element mapping revealed the presence of
magnetite, pentlandite, heazlewoodite, and minor awaruite (the presence of awaruite could only be deduced by
strong enrichments in Ni in places with low sulfur). In
completely serpentinized areas magnetite forms threads
along former olivine grain boundaries or pre-serpentinization intra-grain cracks.
Bastite
Serpentine veins crosscutting bastite (serpentine pseudomorphic after pyroxene) are devoid of magnetite. In contrast to pentlandite co-occurring with awaruite and/or
magnetite, the pentlandite occurring as a solitary phase
in bastite exhibits a distinct octahedral cleavage (Fig. 2a).
Where serpentinization is advanced, pentlandite in veins
crosscutting bastite is rimmed by and/or intergrown with
43
JOURNAL OF PETROLOGY
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NUMBER 1
JANUARY 2009
Fig. 2. Back-scattered electron images of representative sulfide, oxide, and native metal assemblages in variably serpentinized and steatized
peridotite samples from ODP Leg 209. (a) Pentlandite in bastite serpentine with an octahedral cleavage (sample 1271B-7R1-15^22; scale bar
5 mm). (b) Grain of cobaltian pentlandite (medium grey) located in a paragranular vein, partly altered to awaruite (light grey) and magnetite
(dark grey) (sample 1274A-15R1-106^114; scale bar 10 m m). (c) Euhedral grain of cobaltian pentlandite (medium grey) intergrown with and
rimmed by awaruite (light grey); located in a transgranular vein crosscutting bastitic serpentine (sample 1274A-17R1-121^129; scale bar 3 mm).
(d) Pentlandite (medium grey) intergrown with awaruite (light grey) and mantled by magnetite (dark grey) located in a transgranular
serpentine vein of sample 1274A-10R1-3^10 (scale bar 10 mm). (e) Euhedral grain of cobaltian pentlandite in bastitic serpentine with flame-like
appendages on each corner (sample 1274A-18R1-83^93; scale bar 3 mm). (f) Pentlandite (medium grey) and heazlewoodite (light grey) rimmed
by magnetite (dark grey) in sample 1271B-17R1-98^102 (scale bar 10 mm). (g) Heazlewoodite (light grey) and magnetite (medium grey) in a
transgranular vein of sample 1274A-20R1-121^126 (scale bar 10 mm). (h) Grain of heazlewoodite (light grey), which is partly replaced by
godlevskite (medium grey) and mantled by magnetite (dark grey) (sample 1271B-7R1-15^22; scale bar 10 mm). (i) Porous polydymitess that has
completely replaced pentlandite associated with magnetite (sample 1271B-7R1-15^22; scale bar 50 mm). (j) Grain of heazlewoodite (light grey),
godlevskite (light to medium grey), pentlandite (medium grey), millerite (medium dark grey) and magnetite (dark grey); heazlewoodite has
replaced pentlandite during serpentinization. Godlevskite probably replaced heazlewoodite as serpentinization neared completion, whereas the
initiation of transformation of heazlewoodite and godlevskite to millerite is most probably related to steatitization (sample 1271B-10R1-30^35;
scale bar 30 mm). (k) Opaque vein crosscutting partly talc altered bastitic serpentine along pseudomorphic cleavage plane. Magnetite
(dark grey) is in sharp contact with pyrite (medium grey), which hosts millerite and polydymite-ss (sample 1268A-20R1-8^12; scale bar 100 mm).
44
KLEIN & BACH
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
awaruite (Fig. 2c), suggesting that awaruite replaced pentlandite. Awaruite in bastite is exclusively found in association with pentlandite.
These pyrite-bearing serpentinites show the first signs of
steatitization, preferentially of bastite. The assemblages
observed change with increasing extent of serpentinization
from pentlandite þ awaruite þ magnetite to pentlandite þ
heazlewoodite þ magnetite to heazlewoodite þ godlevskite þ magnetite, and continue to change with progressive
steatitization manifested in Hole 1268A to magnetite þ
pyrite þ millerite (magnetite þ pyrite, if Ni is locally lacking) to pyrite þ millerite þ polydymite-ss to pyrite þ polydymite-ss (to pyrite þ vaesite as indicated by chemical
analyses; see below).
Steatitization
Serpentine veins host magnetite that is gradually transformed to pyrite with increasing degree of steatitization
(Fig. 2k). Completely steatized rocks contain pyrite, which
is locally replaced by hematite and/or goethite (see Alt
et al., 2007). Awaruite, pentlandite, heazlewoodite, and
godlevskite are relics of serpentinization and scarce in
partly steatized rocks (e.g. Fig. 2j). Where present, they
are mantled by magnetite, protecting them from reaction
to millerite or other higher sulfur-fugacity phases. Relics of
the assemblage pentlandite þ awaruite þ magnetite are
found in partly serpentinized peridotites that have undergone steatitization, whereas relics of the assemblages pentlandite þ heazlewoodite þ magnetite and pentlandite þ
godlevskite þ magnetite were found in fully serpentinized
peridotites that have undergone steatitization. With
increasing degree of steatitization, sulfur-poor Ni sulfides
are progressively replaced by sulfur-rich Ni sulfides
(Figs. 2i^k). Millerite is the most abundant Ni sulfide in
partly steatized samples, where it replaces heazlewoodite
or godlevskite. Where steatitization is advanced, minerals
of the violarite^polydymite solid solution (polydymite-ss)
grow at the expense of pentlandite or millerite and magnetite (Figs. 2i and k). In completely steatized serpentinites
magnetite is completely replaced by pyrite. Chalcopyrite
is absent in samples from Sites 1270, 1271 and 1274. It
occurs only within a few steatized samples from Hole
1268A together with pyrite. Miller (2007) found chalcopyrite associated with pyrrhotite or minerals from the monosulfide solid solution in samples from the lower half of Hole
1268A, but the occurrence of these minerals is clearly
related to gabbroic intrusions.
Mineral chemistry
Awaruite
In partly serpentinized peridotites from Hole 1274A the Ni
and Fe contents of awaruite vary between 630 and
735 mol % and 213 and 293 mol %, respectively
(Supplementary Data Table A1at http://petrology.oxfordjournals.org/). Co is present only in minor amounts,
usually 535 mol %. In one sample from Hole 1274A an
awaruite grain has an elevated copper content of 28 mol
%; in all other awaruite grains the copper concentration
was below the detection limit of the electron microprobe
(300 ppm). Awaruite is scarce in Hole 1268A and has Ni
contents ranging from 640 to 715 mol %. The Fe content
varies between 260 and 279 mol % and, although generally a minor constituent, Co contents can range up to
66 mol %. Awaruite is absent in samples from Hole
1271B. Compositional variability of awaruite seems to be
independent of pentlandite composition, as the Co content
is rather uniform and Ni/Fe atomic ratios of awaruite exhibit no systematic relation with pentlandite composition.
Wairauite (CoFe) was described by Chamberlain et al.
(1965) and Abrajano & Pasteris (1989) in serpentinites, but
could not be found in the samples investigated here. The
transformation of cobaltian pentlandite to awaruite suggests that the stability field of awaruite is much larger
than that of wairauite.
Relations with increasing degree of
serpentinization and steatitization
Pentlandite
The assemblage type changes with increasing extent of serpentinization (Table 2). Pentlandite þ awaruite þ magnetite is exclusively found in partly serpentinized peridotite,
whereas pentlandite þ heazlewoodite þ magnetite is
usually found in fully serpentinized peridotites. The assemblages pentlandite þ awaruite (in bastite), magnetite þ
heazlewoodite, and heazlewoodite þ godlevskite þ magnetite (in fully serpentinized rocks) are common, but less
abundant (Fig. 2c, g and h). Locally, pentlandite occurs as
a solitary phase (Fig. 2a). All examined samples lack cobaltian minerals other than cobaltian pentlandite (and cattierite in pyrite; see below). The sulfur-rich Ni sulfides
developed exclusively in steatized rocks are millerite,
vaesite (in pyrite; see below), and minerals of the polydymite-ss. Pyrite replacing magnetite veins was found
in some fully serpentinized samples from Hole 1268A.
Pentlandite displays a wide compositional range that may
indicate a solid solution between Co9S8 and (FeNi)-pentlandite with approximately equal proportion of both
metals (Supplementary Data Table A2; Fig. 3). The cobalt
content of pentlandite varies from virtually Co-free
(503 mol %) to Co-rich (442 mol %). The atomic
metal/sulfur ratio of most pentlandite grains is close to 9/8
(i.e. 1125). The full range, however, is between 106 and
165. Rather than real variations in pentlandite composition, the elevated ratios are related to finely intergrown
awaruite or heazlewoodite in pentlandite. The slightly
lower atomic metal/sulfur ratios of the pentlandite fall
within the metal/sulfur range for natural pentlandites
reported by Harris & Nickel (1972) and synthetic pentlandites reported by Kaneda et al. (1986). In Fig. 3 Co-rich
45
JOURNAL OF PETROLOGY
VOLUME 50
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JANUARY 2009
proposed by Fleet (1987). Where godlevskite replaces
heazlewoodite, metal/sulfur ratios are slightly elevated,
whereas low metal/sulfur ratios correspond to godlevskite
associated with millerite. Common impurities in godlevskite from Leg 209 are Fe and Co, ranging between 07
and 58 mol % and 00 and 08 mol %, respectively. In
places elevated Fe contents correlate with low totals, suggesting that some magnetite was included in the analyses.
Millerite
The atomic metal/sulfur ratio of millerite (Supplementary
Data Table A5) in Hole 1268A ranges between 097 and
106. Where millerite replaces godlevskite the metal/sulfur
ratio is slightly elevated. The Fe content varies from 03 to
50 mol % with a bimodal distribution depending on associated minerals. Millerite with low Fe content (512 mol %)
is always associated with pyrite and polydymite-ss,
whereas millerite with high Fe content (415 mol %) cooccurs with magnetite. Usually millerite associated with
pyrite or polydymite-ss contains some Co, but in most
cases this is below 03 mol %. Remarkably, millerite associated with relic cobaltian pentlandite in samples from
Hole 1271B has elevated Co contents of 511mol %, indicating that millerite grew at the expense of cobaltian
pentlandite.
Fig. 3. Ternary pentlandite diagram redrawn from Kaneda et al.
(1986). Pentlandite forms a continuous Co9S8^Fe45Ni45S8 solid solution at temperatures above 3008C. Bimodal Co distribution in pentlandites from Sites 1271 and 1274 indicates temperatures below
2008C, whereas those from Site 1268 indicate temperatures 43008C.
pentlandites trend towards the Ni-rich side. Samples from
Holes 1268A and 1271A contain only Co-rich pentlandite,
whereas samples from Site 1270 contain only Co-free pentlandites. In Holes 1271B and 1274A Co-rich and Co-free
pentlandites are found together in some thin sections or
even in the same serpentine vein.
Polydymite^violarite solid solution (polydymite-ss)
In polydymite-ss from Hole 1268A the molar metal/sulfur
ratio varies between 072 and 080, whereas polydymite-ss
from Hole 1271B has higher metal/sulfur ratios as a result of
intergrowths with magnetite (Supplementary Data
Table A6). Compositions are intermediate between polydymite and violarite, with Fe content ranging from 71 to
123 mol %. Polydymite-ss grains associated with millerite
have a mean Fe content of 77 mol, whereas those that
occur with pyrite contain on average 112 mol % Fe. The
Co content is mostly below 03 mol %, but one grain associated with millerite contains 37 mol % Co. Because of the
porous texture of polydymite-ss (and intergrowths with
magnetite in samples from Hole 1271B) most electron
microprobe analyses have low totals (Supplementary
Data Table A6).
Heazlewoodite
The atomic metal/sulfur ratio of heazlewoodite is mostly
near stoichiometric and varies between 140 and 169.
Considerable amounts of Fe (5506 mol %) and small
amounts of Co (5056 mol %) were detected
(Supplementary Data Table A3) in heazlewoodite from
Hole 1274A. However, the presence of minute inclusions of
pentlandite in heazlewoodite, as mentioned above, could
increase the apparent Fe and Co content, but would also
lower the atomic metal/sulfur ratio systematically.
Samples from Hole 1271B, however, have abundant heazlewoodite. Variable amounts of Fe (05^76 mol %) and small
amounts of Co (5032 mol %) were detected in these
grains. High Fe contents together with low totals suggest
that some magnetite was included in the analyses. In contrast, where analyses approach 100 wt % higher metal/
sulfur ratios are positively correlated with a higher Fe content, possibly indicating small-scale intergrowths with
awaruite.
Godlevskite
The atomic metal/sulfur ratio of godlevskite ranges
between 108 and 125 (Supplementary Data Table A4).
Most analyses are consistent with a stoichiometry of Ni9S8
Magnetite
Magnetite is rather uniform in composition with small
amounts of NiO (see Supplementary Data Table A7;
529 mol %) and CoO (503 mol %). Magnetite replacing
cobaltian pentlandite is slightly enriched in Co and Ni
compared with magnetite that is not associated with cobaltian pentlandite. Copper is below the detection limit of the
electron microprobe (300 ppm) in all magnetite analyses
of rocks from Hole 1274A. For Hole 1268A magnetite analyses reveal slightly elevated copper and zinc contents
(5005 mol %).
46
KLEIN & BACH
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
Pyrite
investigated, we show the phase boundaries of tetrataenite
as grey continuous lines to account for the manifested
coexistence of pentlandite þ awaruite þ magnetite.
Log activity^activity diagrams for the Co^Fe^O^S
system in the H2,aq^H2S,aq plane are shown for temperatures between 150 and 4008C at 50 MPa (Fig. 6). Because of
the smaller entropy of cobaltpentlandite compared with
that of pentlandite, the size of the pentlandite stability
field is strongly dependent on the Co content. The higher
the Co content of pentlandite the more the field expands
towards lower H2,aq and H2S,aq activities. As awaruite
and not wairauite replaces cobaltian pentlandite, it should
be expected that the stability field of awaruite is larger
than that of wairauite. This observation is in apparent contrast to the actual stable phase relations (Fig. 6). However,
the greater abundance of Ni relative to Co in the system
will promote the stability of nickeliferous phases relative
to cobaltian phases. In addition to cobaltian pentlandite,
cattierite coexisting with pyrite was the only other Co
phase we detected in our samples. Jaipurite is metastable
relative to cobaltian pentlandite and linnaeite, and does
therefore not project.
The molar metal/sulfur ratio of pyrite varies between 048
and 054 (Supplementary Data Table A8). Nickel contents
in pyrite range from 00 to 74 mol %. The Ni content of
pyrite is low if it is associated with low-Ni minerals (e.g.
Co-rich pentlandite or magnetite). In contrast, the Ni content of pyrite is high if it is associated with Ni sulfides such
as millerite or polydymite-ss. The majority of pyrite grains
have high proportions of a vaesite component (4^8 mol %).
Typical Co contents are503 mol %, but can be as high as
61mol % (equivalent to 1845 mol % cattierite) in a few
spots. Copper in pyrite is below the detection limit of
300 ppm.
Chalcopyrite
Chalcopyrite in talc altered rocks from Hole 1268A exhibits a stoichiometric composition (Supplementary Data
Table A9). Small amounts of Co and Ni (5004 mol %)
were detected.
Phase diagrams
We constructed diagrams illustrating the Fe^Ni^O^S
phase relations in the log fO2 vs log fS2 and log aH2,aq vs
log aH2S,aq plane. Phase boundaries were all calculated
for a pressure of 50 MPa, temperatures between 150 and
4008C and aH2O ¼1 (Figs 4^6). Minerals that are
obviously lacking in natural samples (bunsenite, native
nickel, native iron, and wu«stite) were omitted from the diagrams to accommodate for metastable equilibrium of kinetically favored minerals.
Seyfried et al. (2004) constructed an activity^activity
diagram, which depicts the phase relations in the NiO^
H2S^H2O^HCl system at 4008C and 50 MPa. In addition
to the stability fields of the minerals considered by Seyfried
et al. (2004), we show stability fields of vaesite, polydymite,
godlevskite, and the Fe-bearing phases tetrataenite, awaruite and pentlandite. Violarite is metastable with respect to
millerite, godlevskite, polydymite and vaesite, and therefore does not project in the phase diagrams shown. The stability region of millerite as suggested by Seyfried et al.
(2004) is significantly reduced in size when pentlandite
and vaesite are considered. Godlevskite is expected to be
more stable over a wide range in H2,aq and H2S,aq activities and would exclude millerite as a stable phase.
Millerite is common, however, and millerite and godlevskite even co-occur in a single opaque grain. We therefore
show the phase boundaries of godlevskite as dotted lines
to allow for examination of the perhaps metastable phase
relations. The stability field of awaruite is larger than that
of native nickel shown by Seyfried et al. (2004) and shifts
the appearance of heazlewoodite to higher H2S,aq activities. Remarkably, the stability field of tetrataenite suggests
that awaruite would form metastably from pentlandite.
Because tetrataenite is lacking in the samples we
DISCUSSION
Fe^Ni^Co^S phase relations and
temperature estimates of fluid^rock
interaction
The phase diagrams displayed in Figs. 4^6 indicate considerable temperature dependences of the positions of invariant points and univariant reaction lines in the H2,aq^
H2S,aq activity plane. Hence, before the H2 and H2S concentrations of the interacting fluids can be estimated constraints on the prevailing temperatures of fluid^rock
interaction are required. These can be estimated using
phase relationships (Bach et al., 2004) or oxygen isotope
compositions (Alt et al., 2007). Phase relations, specifically
the replacement of olivine by serpentine, brucite and magnetite in the presence of fresh clinopyroxene from the
upper half of Hole 1274A, have been interpreted to indicate
low temperatures of serpentinization (5200^2508C; Bach
et al., 2004). Using whole-rock oxygen isotope data, Alt
et al. (2007) estimated variable serpentinization temperatures of peridotites from Leg 209. Consistent with the
phase relation estimate, those workers proposed rather
low alteration temperatures (51508C) based on the high
18O (up to 81ø) of samples from Hole 1274A. In contrast, higher alteration temperatures (250^3508C) are indicated by low 18O whole-rock values (26^44ø) at Site
1268.
The phase relations in the Fe^Ni^Co^O^S system can
reveal additional information about the formation temperatures (Craig, 1971; Kaneda et al., 1986; Alt & Shanks,
2003; Kitakaze & Sugaki, 2004). We investigated thin
47
JOURNAL OF PETROLOGY
VOLUME 50
0
JANUARY 2009
0
150°C
200°C
–1
Pyrite
–2
Vaesite
Pyrrhotite
–3
er
Pyrrhotite
Pentlandite
–4
–5
Godlevskite
Tetrataenite
M
ill
er
ite
Pentlandite
ite
–3
–4
Vaesite
ill
–2
Pyrite
M
–1
log a H2S,aq
NUMBER 1
–5
Tetrataenite
Magnetite
Awaruite
Magnetite
Hematite
–6
–5
–1
Pyrite
–2
Vaesite
a
He
–4
–3
–2
–7
Hematite
Heazlewoodite
–1
0
–8
1 –8
–7
0
300°C
–6
–5
–4
–3
–2
ite
Pyrrhotite
Vaesite
0
1
Pyrrhotite
ly
Po
–2
Pentlandite
Pentlandite
–3
Tetrataenite
Tetrataenite
–3
Magnetite
Hematite
Heazlewoodite
–5
–4
–3
–2
–1
–5
Hematite
0
–6
1 –6
–5
0
400°C
–4
–3
–2
m
ite
–1
Pentlandite
Tetrataenite
–2
Po
M
ill
er
P
M olyd
ill
er ym
ite ite
ite
Tetrataenite
Magnetite
–3
LHF
RHF
–3
Magnetite
Godlevskite
–5
–5
–4
–3
–2
–1
0
Awaruite
Heazlewoodite
Awaruite
Hematite
Godlevskite
–4
1
Pyrrhotite
Pentlandite
–2
0
Vaesite
Pyrrhotite
dy
–1
ly
Vaesite
Heazlewoodite
Pyrite
Pyrite
–1
Awaruite
Awaruite
M
Magnetite
–7
–7
–6
0
350°C
Godlevskite
ill
–4
M
ill
er
Godlevskite
ite
–6
ite
–4
–5
log a H2S,aq
–1
Pyrite
–1
er
log a H2S,aq
–8
–8
–7
0
250°C
w
zle
m
e
dit
oo
–7
Awaruite
–6
Godlevskite
dy
–6
–4
Hematite
Heazlewoodite
–5
–5
1
log a H2,aq
–4
–3
–2
–1
0
1
log a H2,aq
Fig. 4. Activity^activity diagrams depicting redox phase equilibria in the Fe^Ni^O^S system from 150 to 4008C at 50 MPa. Dashed lines are
the boundaries of the magnetite, hematite, pyrrhotite, and pyrite stability fields (field labels in italics); continuous lines are boundaries of awaruite, pentlandite, heazlewoodite, millerite, polydymite, and vaesite stability fields. Because the stability fields of godlevskite and tetrataenite
cover the stability fields of millerite and polydymite and awaruite (in part), respectively, we show their phase boundaries as dotted and grey
lines. Phase boundaries represent equal activities of the minerals in adjacent fields. In the 3508C panel, white and grey circles represent the H2
and H2S concentrations of fluids from the Logatchev (LHF) and Rainbow (RHF) hydrothermal fields (Charlou et al., 2002; Douville et al.,
2002).
48
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
, main
a[
Py
Py
0
, main
a[
KLEIN & BACH
Vs
Pd
Mi
–5
Pyrite
Vaesite
Polydymite
Mi
Hz
Millerite
–10
log fS2,g
Pn
Mt
Pentlandite
log aH2S,aq = −3
Pyrrhotite
–15
Hematite
Heazlewoodite
Pn
Pn
Hz
Magnetite
–20
Mt
Awaruite
Aw
–25
350°C Mt
50 MPa
–35
–30
–25
–20
log f O2,g
Fig. 5. Fugacity^fugacity diagram depicting redox phase equilibria in the Fe^Ni^O^S system at 3508C and 50 MPa. Dashed lines are boundaries of magnetite, hematite, pyrrhotite, and pyrite stability fields (field labels in italics); continuous lines are boundaries of awaruite, pentlandite, heazlewoodite, millerite, polydymite, and vaesite stability fields. The H2S isopotential (bold dash line) is for an activity of 1mmol/kg. It is
calculated for the equilibrium S2,g þ H2O,l ¼ O2,g þ H2S,aq using SUPCRT92 and assuming unity activity of water. The chemographs depict
the changing Fe^Ni^O^S phase assemblages observed. A trend from lower left to upper right represents the evolution of oxygen and sulfur
fugacity with increasing extent of peridotite^water interaction. It follows the H2S isopotential, suggesting that H2S activities in the interacting
fluids may be buffered to values around 1mM.
sections from the same samples, for which 18O whole-rock
data are available (Alt et al., 2007; see Table 3). The contrasting 18O composition between rocks from Holes
1268A and 1274A is also reflected in systematic differences
in Fe^Ni^Co^O^S phase relations and mineral compositions. Both can be used in deriving rough estimates of
alteration temperature. In particular, the compositions of
pentlandite and polydymite-ss may reveal temperature
information. Kaneda et al. (1986) reported that pentlandite
forms a complete solid solution between (Fe, Ni)9xS8 and
Co9xS8 in the 300^6008C temperature range. At 2008C,
there appears to be a solvus that would allow cobaltian
pentlandite to coexist with an Fe^Ni endmember pentlandite (Fig. 3). Cobaltian and non-cobaltian endmember pentlandites do indeed co-occur in veins in some samples from
Hole 1274A, indicating low formation temperatures of
2008C or lower (Supplementary Data Table A2). Wholerock 18O values for these samples (48^74ø, Alt et al.,
2007) corroborate these rather low alteration temperatures.
A single sample from Hole 1271B (10-R1-30^35) also features both pentlandites, which would seem inconsistent
with alteration temperatures 43508C deduced by Alt et al.
(2007) based on 18O of rocks from Hole 1268A and similarities to rocks from Site 1271 in sulfide contents and 34S.
Pentlandite from sample 1274A-15-R1-106^114 and most
pentlandite from Hole 1268A fall just outside the 3008C
range in Fig. 3. Locally, alteration temperatures may have
exceeded 3008C even in Hole 1274A. Unfortunately, no
18O data exist for that sample. All pentlandite in rocks
from Hole 1268A is cobaltian so that alteration temperatures apparently were 43008C, consistent with the low
18O values of those samples (Alt et al., 2007).
Polydymite and violarite form a continuous solid solution at 3008C (Craig, 1971). The composition of the polydymite-ss grains in rocks from Hole 1268A are consistent with
a (Fe,Ni)3S4 phase stable at 4008C, but they are too rich in
Ni to have formed at 4508C. The polydymite-ss composition of the Hole 1268A samples is consistent with the
rather high alteration temperatures of 3508C and higher
deduced from oxygen isotope data.
Redox conditions during serpentinization
and steatitization
Previous studies revealed that sulfides, oxides and alloys in
the Fe^Ni^O^S system are indicative of the redox conditions during serpentinization (e.g. Eckstrand, 1975; Frost,
1985; Alt & Shanks, 1998). Our petrographic investigations
reveal that serpentinization of abyssal peridotites from
49
JOURNAL OF PETROLOGY
VOLUME 50
200°C
150°C
–1
–1
Pyrite
–2
–2
Pyrite
Cattierite
Cattierite
ite
–3
Pyrrhotite
–4
Pyrrhotite
Li
nn
–4
Li
ae
ite
nn
ae
–3
–5
Cobaltpentlandite
Wairauite
–6
Magnetite
–8
–8
0
He
m
at
ite
–7
–7
–6
–5
–4
Cobaltpentlandite
–3
–2
Magnetite
Wairauite
–5
–6
Cobalt
–7
Hematite
–1
0
1
–8
–8
0
–7
–6
–5
–4
–3
–2
–1
0
1
300°C
250°C
–1
–1
Pyrite
Cattierite
Pyrite
ite
–2
Pyrrhotite
–2
Pyrrhotite
nn
ae
Cattierite
–3
ite
Li
nn
–4
ae
–3
Hematite
Cobaltpentlandite
Li
log a H2S,aq
JANUARY 2009
0
0
log a H2S,aq
NUMBER 1
–4
Cobaltpentlandite
–5
Wairauite
–6
Cobalt
Hematite
–6
–5
–4
–3
–2
–1
0
1
–6
–6
0
350°C
–5
–4
–3
–2
–1
0
1
400°C
–1
Pyrite
–1
Pyrite
Cattierite
Pyrrhotite
Cattierite
Pyrrhotite
–2
nn
ae
ite
Li
nn
ae
i
–2
Li
log a H2S,aq
Cobalt
te
–7
–7
0
–5
Wairauite
Magnetite
Magnetite
–3
Cobaltpentlandite
–3
Cobaltpentlandite
Magnetite
–5
–5
Cobalt
–4
–3
–2
–1
0
Wairauite
Hematite
–4
Wairauite
Magnetite
–4
Hematite
Cobalt
1
log a H2,aq
–5
–5
–4
–3
–2
–1
0
1
log a H2,aq
Fig. 6. Activity^activity diagrams depicting redox phase equilibria in the Fe^Co^O^S system from 150 to 4008C at 50 MPa. Dashed lines
are boundaries of magnetite, hematite, pyrrhotite, and pyrite stability fields; continuous lines are boundaries of cobalt, wairauite,
Cobaltpentlandite, linnaeite, and cattierite stability fields. Jaipurite is metastable relative to Cobaltpentlandite and linnaeite, and does therefore
not project.
50
KLEIN & BACH
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
Table 3: Opaque phase assemblages and 18O isotope data for the studied samples
Hole:
1274 A
1274 A
1274 A
1274 A
1274 A
1274 A
1274 A
1274 A
1274 A
Core:
10
15
15
16
17
18
20
22
27
Section:
1
1
2
1
1
1
1
1
2
Depth (cm):
3–10
106–114
39–46
44–52
121–129
83–93
121–126
24–32
5–11
Depth (mbsf):
4933
7506
7586
8414
8951
9413
10411
12234
14765
Rock type:
Du
Hz
Hz
Hz
HZ
HZ
Du
Hz
Hz
Lab. code:
AP-88
AP-92
AP-93
AP-94
AP-95
AP-96
AP-98
AP-99
AP-103
Pentlandite
þþ
þ
þ
Co-pentlandite
þþþ
þþþ
þþþ
þ
þþ
þþþ
þþþ
Awaruite
þþþ
þþþ
þþþ
þ
þþ
þþ
Heazlewoodite
þþþ
þ
Godlevskite
þþþ
þ
þ
þ
þ
þ
þþþ
Millerite
Polydymitess
Magnetite
þþþ
þþþ
þþþ
þþþ
þþþ
þþþ
þþþ
Serpentine
þþþ
þþþ
þþþ
þþþ
þþþ
þþþ
þþþ
Brucite
þþþ
þ
þ
þ
þ
þþ
þþþ
6
48
54
þþ
þ
57
54
Pyrite
Chalcopyrite
Talc
þ vein
þ
18O
74
Hole:
1268 A
1268A
1268A
1268 A
1268 A
1271 A
1271 B
1271 B
1271 B
Core:
2
2
4
13
20
4
7
10
17
Section:
1
2
3
1
1
1
1
1
1
Depth (cm):
10–16
108–115
26–35
46–55
8–12
105–110
15–22
30–35
98–102
Depth (mbsf):
1410
1648
2804
6874
10365
2955
3635
508
8549
Rock type:
Hz
Hz
Hz
Hz
Hz
Du
Du
Du
Hz
Lab. code:
AP-02
AP-03
AP-08
13R1
none
AP-55
AP-61
AP-63
AP-67
þþ
þ
Pentlandite
Co-pentlandite
þ
Awaruite
þ
þþþ
þþ
þþ
Heazlewoodite
þþþ
þþþ
þþþ
Godlevskite
þþ
þþ
Millerite
þþþ
þþþ
Polydymite-ss
þþ
þþ
Magnetite
þ
þ
þ
þþ
þþ
Pyrite
þþþ
þþþ
þþ
þþþ
þþþ
þþþ
þ
Chalcopyrite
þ
þþ
þþþ
þþþ
þþþ
þþþ
þ
Serpentine
þþþ
þþþ
þþþ
þ vein
þ
þ
Brucite
Talc
þþþ
þ
þþþ
18O
59
37
48
41
18
O isotope data are from Alt et al. (2007).
þ, scarce; þþ, abundant; þþþ, very abundant; Du, dunite; Hz, harzburgite; mbsf, metres below sea floor.
51
þ
JOURNAL OF PETROLOGY
VOLUME 50
ODP Leg 209 is accompanied by changing Fe^Ni^Co^
O^S phase assemblages. In partly serpentinized peridotites, pentlandite þ awaruite þ magnetite and pentlandite
þ heazlewoodite þ magnetite are the dominant assemblages. Fe^Ni^O^S minerals in serpentinites from ODP
Leg 209 are volumetrically insignificant (501 vol.%) and
hence incapable of buffering H2,aq. Changes in Fe^Ni^O^
S assemblage mineralogy apparently monitor changes in
H2,aq activity superimposed by reactions between seawater-derived fluids and phases in the MgO^FeO^Fe2O3^
SiO2^H2O system.
A reaction commonly observed in thin section is the
desulfurization of pentlandite to awaruite and magnetite
that is driven by the large quantities of H2,aq released
during serpentinization:
Ni45 Fe45 S8 þ 4H2 ,aq þ 4H2 O ¼
15Ni3 Fe þ Fe3 O4 þ 8H2 S,aq:
wherease awaruite breaks down by the reaction
3Ni3 Fe þ 6H2 S,aq þ 4H2 O !
Fe3 O4 þ 3Ni3 S2 þ 10H2 ,aq:
ð3Þ
Both reactions indicate increasing oxygen fugacities; the
latter also suggests increasing sulfur fugacities compared
with conditions during the breakdown of pentlandite to
awaruite. Although there is ample evidence for pentlandite
breakdown to heazlewoodite and magnetite in thin section
(Fig. 2f), the actual awaruite breakdown reaction is not
recorded in a specific assemblage, so it is uncertain if reaction (3) does indeed take place.
At 4008C the awaruite stability field expands into the
pyrrhotite field. Here, the assemblage pentlandite þ awaruite þ magnetite is not stable.
Redox conditions during steatitization
Ni3 S2 þ H2 S,aq ! H2 ,aq þ 3NiS
ð4Þ
Ni9 S8 þ H2 S,aq ! H2 ,aq þ 9NiS:
ð5Þ
The replacement of magnetite by pyrite is represented by
the reaction
ð1Þ
ð2Þ
JANUARY 2009
partly to fully serpentinized peridotites. With increasing
degree of steatitization magnetite is replaced by pyrite
and sulfur-poor Ni sulfides are progressively replaced by
sulfur-rich Ni sulfides. These transitions indicate increasing
oxygen and sulfur fugacities (see Eckstrand, 1975; Frost,
1985). During steatitization of serpentinized peridotites
millerite grows at the expense of the sulfur-poor Ni sulfides
heazlewoodite and godlevskite (Fig. 2j). The direct replacement of pentlandite or awaruite by millerite was not
observed, although it may take place if fO2 increases
rapidly:
Fe3 O4 þ 6H2 S,aq !
The reaction indicates extremely low oxygen and sulfur
fugacities in the system.
Pentlandite þ awaruite þ magnetite equilibria in the
absence of heazlewoodite imply hydrogen concentrations
close to or at the solubility of dihydrogen in water, in particular between 200 and 3508C (Fig. 4). Awaruite and heazlewoodite never co-occur in one assemblage, although
they may co-occur in the same thin section. The assemblage consisting of pentlandite, heazlewoodite, and magnetite indicates H2,aq activities just below dihydrogen
saturation of the fluids. This assemblage suggests that pentlandite breakdown is now by the reaction
Ni45 Fe45 S8 þ 6H2 O !
15Ni3 S2 þ 15Fe3 O4 þ H2 ,aq þ 5H2 S,aq
NUMBER 1
2H2 ,aq þ 4H2 O þ 3FeS2 :
ð6Þ
Reactions (4)^(6) indicate decreasing H2,aq and increasing
H2S,aq activities. With progressive steatitization the replacement of millerite by polydymite-ss (Fig. 2k), indicates a
further decrease in H2,aq and an increase in H2S,aq
activities:
3NiS þ H2 S,aq ! H2 ,aq þ Ni3 S4
ð7Þ
6NiS þ Fe3 O4 þ 6H2 S,aq !
2H2 ,aq þ 4H2 O þ 3FeNi2 S4 :
ð8Þ
In partly steatized serpentinites magnetite þ millerite þ
pyrite polydymite is the dominant assemblage (Fig. 2k).
Although this is not an equilibrium assemblage per se, those
phases do represent a small range in H2,aq^H2S,aq activities at 3508C (Fig. 4).
The solubility of vaesite in pyrite at temperatures
54008C is below 2 mol %, whereas the maximum solubility of vaesite in pyrite is 75 mol % at temperatures
around 7008C (Clark & Kullerud, 1963). The high Ni concentrations in pyrite from Hole 1268A (up to 744 mol %)
in combination with alteration temperatures around 3508C
suggest that the solution of Ni in pyrite is metastable. This,
in turn, implies that H2,aq^H2S,aq activities were in the
stability region of vaesite. The Ni-rich pyrite in Hole
1268A hence is the phase representing the highest sulfur
fugacitiesçor lowest aH2,aq / highest aH2S,aq conditions
(Figs. 4^6). This type of mineralization appears tightly
linked to the formation of talc in veins and steatitization
of serpentinite.
In addition to forming vaesite from polydymite in the
course of increasing sulfur fugacities,
The opaque phase assemblages we found in steatized serpentinites are completely different from those found in
Ni3 S4 þ S2 ,g ! 3NiS2
52
ð9Þ
KLEIN & BACH
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
vaesite-rich pyrite along with millerite may replace polydymite-ss:
Fex Ni3x S4 ! Fex Ni1x S2 þ 2NiS:
buffer a major fluid species such as dissolved H2, except
perhaps in a mineralized upflow zone. In a subsequent
paper we will show that the levels of dissolved H2 in the
Logatchev and Rainbow hydrothermal fluids are entirely
consistent with serpentinization reactions at 4008C. Our
preliminary conclusion is that there is no unique H2,aq^
H2S,aq buffer in peridotite-hosted systems, but H2S,aq
should be set by phase equilibria to values around 1mM
at temperatures around 350^4008C. However, we do not
seem to have a sample of a rock that represents a reaction
zone of a high-temperature vent fluid. Rocks from Hole
1274A reveal low alteration temperatures, whereas rocks
from Hole 1268A have Fe^Ni^O^S phase relations inconsistent with the H2,aq^H2S,aq systematics of the vent
fluids. The problem of whether or not there is a unique
H2,aq^H2S,aq buffer and what that buffer might be
requires further examination.
ð10Þ
Our thermodynamic analyses indicate that this reaction is
favorable with decreasing temperatures (Fig. 4).
Implications for a potential H2S,aq buffer
in serpentinite-hosted hydrothermal
systems
We next examine if the sulfide assemblages observed in
veins (i.e. unit water activities) are consistent with
H2S,aq concentrations measured in high-temperature
vent fluids from ultramafic-hosted hydrothermal systems.
Fluids venting at the Rainbow or Logatchev hydrothermal
fields have uniform H2S concentrations of around 1mmol/
kg (mM) (Charlou et al., 1998, 2002; Schmidt et al., 2007).
We can relate the aqueous H2S concentrations of interacting fluids with estimated sulfur and oxygen fugacities from
phase relations, if we explore the equilibrium of the
reaction
S2 ,g þ 2H2 O,l ¼ 2H2 S,aq þ O2 ,g
Sulfur metasomatism
Total sulfur contents of rocks from ODP Leg 209 range
from 0003 to 21wt % (Paulick et al., 2006; Alt et al.,
2007). They are variably depleted or enriched compared
with the depleted upper mantle (0012 wt %; Salters &
Stracke, 2004). As our petrographic observations reveal,
main-stage serpentinization results in desulfurization of
primary sulfide (see Alt & Shanks, 1998). Consequently,
sulfur should be lost from the rocks during serpentinization. Indeed, sulfur concentrations in many serpentinite
samples are below 0012 wt % (Fig. 7). In a plot of SiO2 vs
S data partly serpentinized peridotites with modal brucite (those with SiO2 540 wt %) have distinctly lower
S compared with completely serpentinized and steatized
rocks. The latter can have sulfur contents of the order of
2 wt %. What is the source of that sulfur and why do sulfides become enriched in the course of steatitization?
Hydrogen produced in copious amounts during serpentinization will keep sulfur fugacities low and push sulfur
out of primary sulfides into dissolved H2S:
ð11Þ
(see Frost, 1985). To determine which phases could be in
stable coexistence with a fluid with 1mM H2S at 3508C,
we plotted an H2S,aq isopotential of 1mM in Fig. 5.
Remarkably, at 3508C and 50 MPa the 1mM H2S,aq isopotential follows the fS2 / fO2 evolution trend revealed by
the succession of Fe^Ni^O^S phase relations observed. It
can thus be suggested that H2S,aq in serpentinite-hosted
hydrothermal systems is buffered by equilibria between
the Fe^Ni^O^S phases. H2S concentrations in the alkaline
lower temperature Lost City hydrothermal vent fluids are
less well constrained, but exploratory data (Kelley et al.,
2001; Dulov et al., 2005) are suggestive of H2S activities
that are are in the range of several mmol/kg. Such low
H2S activities are entirely consistent with buffering by pentlandite breakdown reactions at estimated reaction zone
temperatures (Allen & Seyfried, 2004) of 2008C (Fig. 4).
An alternative explanation was provided by Seyfried
et al. (2004), who hypothesized that the high H2,aq and
low H2S,aq concentrations found in these systems suggest
magnetite þ bornite þ chalcocite þ fluid equilibria at
4008C and 50 MPa. Although this interpretation is entirely
consistent with the thermodynamic data, we have not yet
observed the magnetite þ bornite þ chalcocite assemblage
in altered peridotite. Perhaps the serpentinites and soapstones drilled from the area around Logatchev are unlike
rocks in the reaction / upflow zones underneath the
Logatchev vent field. Our preferred interpretation, however, is that H2S,aq is set by pentlandite-desulfurization
reactions.
Metal sulfides are trace components in altered peridotite
(in particular Cu sulfides), so it appears unlikely that they
S2 ,g þ 2H2 ,aq ¼ 2H2 S,aq:
ð12Þ
Primary sulfides in fresh peridotite will desulfurize and
become obliterated during serpentinization:
FeSðin primary sulfideÞ þ H2 ,aq ¼
H2 S,aq þ Feðin awaruiteÞ:
ð13Þ
When serpentinization nears completion the conditions
become less reducing and reaction (12) proceeds to the left,
allowing mineralization with high sulfur fugacity assemblages such as observed in Hole 1268A to develop. One possible explanation for the sulfur enrichment in completely
serpentinized peridotites is a moving serpentinization
front. Sulfur is leached from the peridotite during active
serpentinization, removed by the serpentinization front
and reprecipitated in rocks where serpentinization is
53
JOURNAL OF PETROLOGY
NUMBER 1
JANUARY 2009
Hole
1268A
1270A
1270B
1270C
1270D
1271A
1271B
1272A
1274A
2.0
1.5
S wt.%
VOLUME 50
1.0
0.5
0.0
30
35
40
45
50
55
60
65
70
SiO2 wt.%
Fig. 7. Whole-rock concentrations of sulfur vs silica for samples from ODP Leg 209. Partly serpentinized peridotites (540 wt % SiO2) have
sulfur concentration that are slightly enriched or markedly depleted relative to depleted mantle peridotites (0012 wt % S). Sulfur is strongly
enriched in silica-metasomatized (i.e. brucite-free) serpentinites (440 wt %) and steatites from Hole 1268A. Data plotted are from the literature
(Kelemen et al., 2004b; Paulick et al., 2006; Alt et al., 2007). (See text for details.)
serpentinites have exceedingly high sulfur contents (Fig. 7;
see Paulick et al., 2006) indicating that the sulfur-metasomatism preceded the silica-metasomatism. In those samples, steatitization starts in serpentine veins or where
serpentine pseudomorphs pyroxene (bastite). In contrast,
serpentine replacing olivine is apparently unaffected by
steatitization. Because bastite and vein serpentine are
usually devoid of brucite they can be readily transformed
into talc, whereas brucite intergrown with serpentine in
the mesh-textured groundmass sucks up the silica before
serpentine can be transformed into talc. Apparently the
introduction of silica to the system leads to increased
oxygen and sulfur fugacities that, in turn, promote sulfide
precipitation. Aqueous silica plays a role in setting oxygen
fugacities, as hydrogen production tied to magnetite formation will be facilitated at low silica activities; for
example,
complete. Perhaps the parts of the system that undergo
active serpentinization are regions where percolating
fluids pick up H2S,aq that they will subsequently dump in
sulfides in areas where increased sulfur and oxygen fugacities prevail. The H2S,aq front, however, would have to be
fairly subdued, as even the low sulfur fugacity phases
would buffer H2S,aq concentration to values of the order
of 1mM (see above). To create the sulfide accumulations
observed in Hole 1268A, one would need to flux a substantial amount of serpentinization fluids through a zone that
has externally controlled high sulfur and oxygen fugacities.
Such a zone could potentially be the peripheral parts of
hydrothermal upflow zones, where upwelling reduced
fluids mix with entrained seawater. Not only would the
physicochemical changes in those areas of fluid mixing be
conducive to sulfide precipitation, but also seawater sulfate
would constitute an external source of sulfur, which could
be reduced thermogenically by dihydrogen dissolved in the
hot upwelling fluids. Indeed, the sulfur isotope composition of hydrothermal sulfide veins from Hole 1268A
(34S ¼ 5^11ø; Alt et al., 2007) indicates that reduced seawater sulfate is a significant source of sulfur besides sulfide
leached from the basement. Similarly, Beard & Hopkinson
(2000) found marcasite accumulations associated with a
fossil vent in Hole 1068 (Leg 173; Iberian Margin) and concluded that the sulfide was precipitated from seawater
interacting with reducing fluids venting from the
serpentinite.
It appears that both sulfur- and silica-metasomatism are
somehow related. However, some weakly steatized
3Fe2 SiO4 þ 2H2 O !
2Fe3 O4 þ 3SiO2 ,aq þ H2 ,aq
ð14Þ
or
Fe3 Si2 O5 ðOHÞ4 !
Fe3 O4 þ SiO2 ,aq þ H2 O þ H2 ,aq:
ð15Þ
As long as brucite is present, it will keep silica activities
low; that is, at the brucite^serpentine buffer, which is 3^4
orders of magnitude below quartz saturation (e.g. Frost &
Beard, 2007). As silica activity goes up, reaction (15) may
be reversed and Fe-rich serpentine forms. That reaction
54
KLEIN & BACH
KLEIN AND BACH PERIDOTITE^SEAWATER INTERACTIONS
1.5
would provide a sink for H2,aq, required to pyritize magnetite [see reaction (5)].
Because talc does discriminate against Fe much more
than serpentine, the sulfide impregnation peaks during
silica metasomatism immediately after brucite has reacted
out, but before replacement of serpentine by talc is complete. The source of silica is most probably gabbroic intrusions (Bach et al., 2004); such intrusions have also been
proposed to explain the sulfur and S isotopes systematics
(Alt et al., 2007). Both silica- and sulfur-enrichments in
rocks from Hole 1268A can, therefore, be best explained
by the involvement of gabbroic lithologies, which are frequently found in Hole 1268A and elsewhere in the 15820’N
Fracture Zone area (see Kelemen et al., 2007).
Log aH2, aq
0.5
25
0
a
10
−0.5
MP
H2,aq solubility
−1
Possible existence of a free H2-rich vapor
phase
Aw-Hz-Pn-Mt
control
200
300
Temperature °C
400
500
Fig. 8. Comparison of the amount of H2,aq corresponding to H2O^
awaruite^heazlewoodite^pentlandite^magnetite equilibria (comapre
invariant point in Fig. 4) and the amounts of H2,aq soluble at pressures
indicated by the numbers in italics. It is assumed that the partial pressure of hydrogen in the gas phase corresponds to the total hydrostatic
pressure. Solution curves terminate just short of the two-phase boundary. It should be noted that a hydrogen gas phase could potentially
develop at pressures below 50 MPa.
As demonstrated above, pentlandite þ awaruite þ magnetite equilibria imply hydrogen concentrations close to or at
the solubility of dihydrogen in water, in particular between
2008C and 3508C (Fig. 5). Because temperature estimates
for alteration of rocks from ODP Leg 209 largely overlap
with this temperature range, a free H2-rich vapor phase
may exist in abyssal serpentinization systems. In continental settings active serpentinization produces H2-rich gas
emanations (e.g. Thayer, 1966; Coveney, 1971; Barnes et al.,
1978; Yurkova et al., 1982; Coveney et al., 1987; Abrajano
et al., 1988; Sturchio et al., 1989). Although the hydrostatic
pressure in the deep sea will increase the solubility of
dihydrogen, serpentinization of abyssal peridotites may
produce a free H2-rich vapor phase. This has been proposed previously, based on experimental work
(McCollom & Seewald, 2001, 2006) and theoretical considerations (Sleep et al., 2004). Our calculations provide additional support for the idea that H2 concentrations close to
or exceeding hydrogen solubilities may develop during serpentinization. Figure 8 compares the H2 concentrations
corresponding to awaruite^pentlandite^magnetite^heazlewoodite equilibrium
9 13 H2 ,aq þ 2 13 Fe3 O4 þ 4Ni3 S2
0
50
−1.5
100
Ni45 Fe45 S8 þ 9 13 H2 O þ 2 12 Ni3 Fe ¼
10
1
(Proskurowski et al., 2006) are undersaturated at a hydrogen partial pressure of 75 MPa (ambient pressure at Lost
City) by only a factor of five.
Awaruite has been assigned a critical role in methanogenesis and Fischer^Tropsch-type synthesis of organic
compounds in hydrothermal systems (Horita & Berndt,
1999; McCollom & Seewald, 2001). We suggest that the
presence of awaruite in a rock indicates that the interacting
fluids may have exsolved a H2 gas phase, which would also
make a very efficient catalyst for organic synthesis reactions (McCollom & Seewald, 2006). Whether or not a
hydrogen-rich gas phase forms during serpentinization
depends on the pressure (Fig. 8). Our calculation results
suggest that a free hydrogen gas phase could potentially
develop at pressures 550 MPa.
H2- and CH4-rich fluid inclusions observed in deepseated gabbroic rocks from the ocean crust (e.g. Kelley,
1997; Kelley & Fru«h-Green, 2001) may provide evidence
for the development of fluid exsolution. Further examination of this hypothesis will rely on improved estimates of
the pressure^temperature conditions of peridotite^fluid
interaction.
ð16Þ
with that of equilibrium H2,g ¼ H2,aq. As indicated in the
phase diagrams presented above, awaruite-bearing assemblages represent extremely high H2,aq concentrations close
to saturation, in particular in the 200^3008C temperature
range. The effect of pressure is also considered in the calculations and illustrated in Fig. 8.
Hydrogen concentrations actually measured in fluids
venting from peridotite-hosted hydrothermal systems (e.g.
Charlou et al., 2002) are one to two orders of magnitude
lower than the maximum values expected. However,
Lost City fluid hydrogen concentrations of 15 mM
CONC LUSIONS
We propose that Fe^Ni^Co^O^S phase relations provide a
very useful monitor for the evolution of temperature and
the fugacities of sulfur and oxygen during peridotite^seawater interaction. Our results demonstrate that peridotites
55
JOURNAL OF PETROLOGY
VOLUME 50
NUMBER 1
JANUARY 2009
sponsored by the US National Science Foundation (NSF)
and participating countries under management of Joint
Oceanographic Institutions (JOI), Inc. This work was supported with funds from the Special Priority Program 1144
of the German Science Foundation (BA 1605/1-1 and BA
1605/1-2) and by the DFG-Research Center/Excellence
Cluster ‘The Ocean in the Earth System’.
from Hole 1274A were serpentinized at uniformly reducing
conditions and relatively low temperatures of 53008C.
Steatitization superimposed on serpentinization resulted
in increasing oxygen and sulfur fugacities.
Sulfur-metasomatism affecting fully serpentinized peridotites is related to steatitization. The evolution of SiO2, H2
and H2S activities is coupled. Dihydrogen drops when brucite is exhausted by reaction with high-aSiO2 fluids probably derived from interactions with gabbroic bodies that
make up a large fraction of the lithosphere in the 158200 N
Fracture Zone area. As H2,aq activity drops, high-sulfurfugacity phases such as pyrite and polydymite precipitate.
The sequence of events leads to early pervasive sulfide leaching followed by late and localized sulfur enrichment.
Associated with the prominent desulfurization of pentlandite, sulfide is removed from the rock during the initial
stage of serpentinization. In contrast, steatitization indicates increased silica activities, and high-sulfur-fugacity
sulfides, such as polydymite and pyrite^vaesite solid solution, form as the reducing capacity of the peridotite is
exhausted and H2 activities drop. Under these conditions,
sulfides will not desulfurize but precipitate, leading to an
enrichment of S, as seen in rocks from Site 1268.
The co-evolution of fO2,g^fS2,g in the system follows an
isopotential of H2S,aq, indicating that H2S in vent fluids is
buffered to about 1mmol/kg at 350^4008C and to micromolal quantities at 150^2008C. These predicted concentrations are consistent with concentrations observed in active
peridotite-hosted hydrothermal systems.
The development of pentlandite þ awaruite þ magnetite assemblages implies hydrogen concentrations close to
or at solubility of dihydrogen in water, in particular
between 200 and 3008C. The common occurrence of awaruite indicates that an H2-rich vapor phase may develop in
abyssal serpentinization systems, if the pressures of water^
rock interaction are 550 MPa. The presence of such a gas
phase would greatly facilitate the abiotic synthesis of
organic compounds. The phase petrological constraints on
redox could be tightened if high-temperature calorimetry
data were available for awaruite, pentlandite, and violarite.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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AC K N O W L E D G E M E N T S
We thank Michael Hentscher for his help in setting up the
thermodynamic database. Many thanks go to Niels Jo«ns
and Ron Frost for stimulating discussions. We thank
Barbara Mader and Peter Appel for their assistance with
the electron microprobe analyses. Carlos Garrido and
Holger Paulick provided sample material and thin sections. Carlos Garrido also is thanked for sharing information on Hole 1268A opaque phase assemblages. Insightful
reviews by Bernhard Evans, James Beard and Dionysis
Foustoukos as well as helpful comments by editor Ron
Frost are much appreciated. This research used samples
supplied by the Ocean Drilling Program (ODP). ODP is
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