ORIGINAL PAPER

The legacy of crystal-plastic deformation in olivine:high-diffusivity pathways during serpentinization

Oliver Plumper • Helen E. King • Christian Vollmer •

Quentin Ramasse • Haemyeong Jung •

Hakon Austrheim

Received: 20 May 2011 / Accepted: 13 September 2011

� Springer-Verlag 2011

Abstract Crystal-plastic olivine deformation to produce

subgrain boundaries composed of edge dislocations is an

inevitable consequence of asthenospheric mantle flow.

Although crystal-plastic deformation and serpentinization

are spatio-temporally decoupled, we identified composi-

tional readjustments expressed on the micrometric level

as a striped Fe-enriched ( �XFe = 0.24 ± 0.02 (zones);

0.12 ± 0.02 (bulk)) or Fe-depleted ( �XFe = 0.10 ± 0.01

(zones); 0.13 ± 0.01 (bulk)) zoning in partly serpentinized

olivine grains from two upper mantle sections in Norway.

Focused ion beam sample preparation combined with

transmission electron microscopy (TEM) and aberration-

corrected scanning TEM, enabling atomic-level resolved

electron energy-loss spectroscopic line profiling, reveals

that every zone is immediately associated with a subgrain

boundary. We infer that the zonings are a result of the

environmental Fe2?Mg-1 exchange potential during an-

tigorite serpentinization of olivine and the drive toward

element exchange equilibrium. This is facilitated by

enhanced solid-state diffusion along subgrain boundaries in

a system, which otherwise re-equilibrates via dissolution-

reprecipitation. Fe enrichment or depletion is controlled by

the silica activity imposed on the system by the local

olivine/orthopyroxene mass ratio, temperature and the

effect of magnetite stability. The Fe-Mg exchange coeffi-

cients KAtg=OlD between both types of zoning and antigorite

display coalescence toward exchange equilibrium. With

both types of zoning, Mn is enriched and Ni depleted

compared with the unaffected bulk composition. Nanome-

ter-sized, heterogeneously distributed antigorite precipi-

tates along olivine subgrain boundaries suggest that water

was able to ingress along them. Crystallographic orientation

relationships gained via electron backscatter diffraction

between olivine grain domains and different serpentine vein

generations support the hypothesis that serpentinization

was initiated along olivine subgrain boundaries.

Keywords Crystal-plastic deformation � Diffusion �Dislocation � Dissolution-reprecipitation � Element

exchange � Serpentinization

Introduction

Mineral deformation and hydration are interrelated pro-

cesses and may dramatically alter the physical and chem-

ical properties of a mineral. Hence, they play a crucial role

in the dynamical and geochemical evolution of the Earth.

For example, crystal-plastic deformation of olivine at high

temperatures ([1,000�C) is an inevitable consequence of

asthenospheric mantle flow (e.g., Nicolas and Christensen

1987). Although olivine is nominally anhydrous, diffusion

Communicated by J. Hoefs.

O. Plumper (&) � H. Austrheim

Physics of Geological Processes (PGP), University of Oslo,

P.O. Box 1048, 0316 Blindern, Oslo, Norway

e-mail: oliver.pluemper@fys.uio.no

H. E. King � C. Vollmer

Institut fur Mineralogie, University of Munster,

Correnstrasse 24, 48149 Munster, Germany

Q. Ramasse

SuperSTEM, Daresbury Laboratory, Keckwick Lane, Daresbury,

Cheshire WA4 4AD, UK

H. Jung

School of Earth and Environmental Sciences, Seoul National

University, 311 Ho, 25-1 Dong, San 56-1, Sillim-dong,

Gwanak-gu, Seoul 151-747, Republic of Korea

123

Contrib Mineral Petrol

DOI 10.1007/s00410-011-0695-3

of water in trace amounts (as a hydrogen source) can

dramatically enhance plastic deformation (e.g., Blacic

1972; Chopra and Paterson 1984; Mei and Kohlstedt 2000),

with consequences for the strength of the lithosphere. The

impact of water on olivine plasticity at high temperatures is

also evident in the marked changes that Jung and Karato

(2001) observe in the relationship between flow geometry

and seismic anisotropy when water was added to olivine.

The presence of hydrogen also lowers the kinetic barrier of

olivine transformation to high-pressure polymorphs (Kubo

et al. 1998; Ando et al. 2006) and enhances Fe-Mg inter-

diffusion (Hier-Majumder et al. 2005).

At low to intermediate temperatures (50–600�C), the

presence of hydrothermal fluids (with water as the domi-

nant constituent) results in olivine hydration to form

serpentine minerals (Mg3Si2O5(OH)4), substantially modi-

fying the petrophysical and geochemical properties of the

oceanic lithosphere (e.g., Toft et al. 1990; Ranero et al.

2003; Bach and Frueh-Green 2010). In this regime, olivine

yields via brittle failure to stress perturbations caused by

tectonic forces or stress generated by the reaction-induced

volume change, significantly weakening the oceanic lith-

osphere (Escartin et al. 1997, 2001; Iyer et al. 2008b;

Jamtveit et al. 2008). Generally, olivine is thought to be

incapable of readjusting compositionally via solid-state1

diffusion at low temperatures, and thus, the system re-

equilibrates with infiltrating fluid via the dissolution of

olivine and consequent precipitation of serpentine miner-

als. At the low end of the temperature scale (50 to

*350�C), olivine serpentinization typically involves

hydrogen production during the oxidation of Fe2? from

olivine by water forming Fe3? that is subsequently incor-

porated into magnetite (Fe2þFe3þ2 O4) or serpentine (e.g.,

Klein et al. 2009). The generation of abiogenic hydrogen

during magnetite formation appears to have important

implications for the origin of life (e.g., Sleep et al. 2004).

The mechanisms of system re-equilibration are dictated

by the temperature of the system and the presence of fluids.

At high temperatures ([1,000�C), diffusion is the dominant

mineral re-equilibration mechanism (e.g., Mackwell and

Kohlstedt 1990; Watson and Baxter 2007), whereas at

lower temperatures upon the ingress of water, dissolution-

reprecipitation will be the fastest mechanism (Putnis 2009

and references therein). A crossover of the two regimes can

be observed during serpentinization at temperatures

between 300 and 600�C. For example, the observed

absence of magnetite and enrichment of Fe in olivine

during serpentinization to antigorite led Evans (2010) to

suggest that olivine can compositionally readjust during

serpentinization at temperatures above *400�C due to

faster Fe-Mg bulk diffusion. Elemental re-equilibration

may also occur when both reactant and product phases are

recrystallizing such as during prograde dehydration of an-

tigorite to form secondary olivine.

Mantle-derived olivine has invariably endured crystal-

plastic deformation, yielding by dislocation creep to form

low-angle subgrain boundaries (dislocation walls) (e.g.,

Carter and Ave Lallemant 1970). Diffusion along such

intracrystalline defects is faster than bulk diffusion, as

recently reviewed by Dohmen and Milke (2010). As tem-

perature decreases, the role of high-diffusivity pathways

becomes increasingly important in facilitating intragranular

element mobility. A number of studies in different mineral

systems have pointed to the importance of intracrystalline

defects as high-diffusivity pathways that enhance element

mobility and compositional re-equilibration (Yund et al.

1981; Hacker and Christie 1991; Lee 1995; Kramar et al.

2003; Keller et al. 2006; Reddy et al. 2006; Konrad-

Schmolke et al. 2007; Mark et al. 2008). Boudier et al.

(2010) recently suggested that crystal-plastic deformation-

induced subgrain boundaries have the potential to chan-

nelize fluid entry and hence may play a critical role in the

initial stages of upper mantle hydration. Thus, although

high-temperature crystal-plastic deformation and low-

temperature hydration are spatio-temporally distinct pro-

cesses, the legacy of crystal-plastic deformation plays a

crucial role during serpentinization.

Our study examines partly serpentinized olivine from

two sites in Norway that exemplify the importance of

crystal-plastic deformation and associated formation of

high-diffusivity pathways along subgrain boundaries for

olivine reactivity during serpentinization. Along these

pathways, the original olivine composition is altered to

adhere to equilibrium element partitioning, even though the

main mechanism of mineral re-equilibration in a fluid-

controlled environment is dissolution-reprecipitation (there

is ample evidence from natural and experimental investi-

gations that this is the general case, e.g., Putnis 2009;

Putnis and Austrheim 2010 and references therein). This

has implications for the interpretation of metamorphic

reactions because they show that ‘frozen-in’ partial

replacement reactions and related element exchange cannot

be treated in terms of a classical metamorphic mm-cm

scale phase assemblage. However, local equilibrium can be

obtained via enhanced diffusion along intracrystalline

defects. Our observations also provide new insight into the

interplay of intensive parameters, especially silica activity

(Frost and Beard 2007) and the environmental Fe2?Mg-1

exchange potential (Evans 2008), which control the

occurrence of magnetite. We demonstrate how water is

channelized along subgrain boundaries to result in ser-

pentine nucleation and use the relationship between olivine

orientation and different generations of serpentine veins to

1 Thereafter, diffusion will always refer to solid-state diffusion unless

otherwise stated.

Contrib Mineral Petrol

123

evaluate the control of crystal-plastic deformation–induced

microstructures on serpentinization.

Geologic settings and sample locations

The main investigation of this study was conducted in the

upper mantle section of the Leka Ophiolite Complex

(LOC), Norway (Fig. 1). The LOC is the most completely

preserved ophiolite present in the Scandinavian Caledo-

nides and part of the Uppermost Allochthon of the Nor-

wegian tectonostratigraphy (Furnes et al. 1988). Ophiolite

formation took place in a suprasubduction setting of the

North Iapetus Ocean, and it currently occurs in a pull-apart

structure resulting from postorogenic transtension (Titus

et al. 2002). The LOC has undergone serpentinization to

varying degrees from partial to complete. Samples were

taken from several localities across the complex, as indi-

cated in Fig. 1b. To test whether our findings are repre-

sentative of widespread olivine hydration phenomena,

further samples were taken in the Feragen Ultramafic Body

(FUB), Norway (Fig. 1a). The FUB is exposed over an area

of *5 km2 situated 25 km east of the town of Røros in the

southern part of the Trondheim basin and is part of a chain

of ultramafic bodies in this area (Moore and Hultin 1980).

The majority of the FUB is extensively serpentinized.

Samples for this study were taken from meta-harzburgite

interlayered with meta-dunite.

Analytical techniques

Polished thin sections were studied using polarized light

microscopy combined with a universal stage (Leitz) to

determine olivine grain orientations. Back-scattered elec-

tron (BSE) imaging and quantitative element analyses were

performed in a Cameca SX 100 electron microprobe (EMP)

equipped with wavelength-dispersive X-ray (WDX) spec-

trometers (Department of Geosciences, University of Oslo,

Norway). The accelerating voltage was 15 kV at a beam

current of 10–20 nA. Standardization was carried out using

a selection of natural and synthetic mineral standards,

including enstatite-ferrosilite, kyanite, fayalite, diopside,

jadeite, sanidine, rhodonite, rutile, MgO and NiO for Si, Al,

Fe, Ca, Na, K, Mn, Ti, Mg and Ni, respectively. Matrix

corrections followed a routine implemented in the Cameca

PAP-program (Pouchou and Pichoir 1984). Several areas

along individual zones of the striped zonings were analyzed

to ensure representative element concentrations. Oversam-

pling due to contributions from the bulk olivine within the

Fig. 1 a Geological map of the

LOC and surrounding

geological units (modified from

Prestvik 1972), displaying a

typical ophiolite sequence. The

inset shows the geographical

location of the LOC and the

FUB, from which samples were

taken for comparison. Typical

localities in which Fe-enriched

and Fe-depleted zoned olivine

occurs are displayed in b (meta-

dunite) and c (meta-peridotite),

respectively. Note the presence

of meta-orthopyroxenite dikes

in b, which are absent in

c. Holes in b are drill core

sampling localities

Contrib Mineral Petrol

123

activation volume during microprobe analysis was mini-

mized by reducing the beam current. A 2-lm defocused

beam was used for serpentine analyses to prevent beam

damage. Element distribution maps (120 lm2) were

obtained using operating conditions of 15 kV and 40 nA.

For serpentine polymorph identification (e.g., Rinaudo et al.

2003) and detection of brucite (e.g., Dawson et al. 1973),

Raman spectra were collected with a high-resolution Jobin–

Yvon T6400 Raman spectrometer using the 532-nm line of

a 14-mW Nd:YAG laser (Norwegian Center for Raman

Spectroscopy, University of Oslo, Norway). The scattered

Raman light was analyzed using a 1009 objective lens in a

180� backscattering geometry and a charged-coupled

device (CCD) detector. After the light passed a 100-lm

entrance slit, it was dispersed by a grating of 1,800 grooves

per mm. Crystallographic orientation relationships were

analyzed by scanning electron microscopy (SEM) enabling

electron backscatter diffraction (EBSD) analysis in a JEOL

6380 operating at 20 kV and equipped with an Oxford

Nordlys II detector (School of Earth and Environmental

Sciences, Seoul National University, Republic of Korea).

EBSD analyses were set individually and indexed manually

to ensure an accurate solution. Pattern indexing and data

processing were executed using the HKL CHANNEL5

software. Several electron-transparent thin foils were pre-

pared across the striped zoning for (scanning) transmission

electron microscopy ((S)TEM) by using the focused ion

beam (FIB) technique in a FEI FIB200 (GeoForschungs-

Zentrum (GFZ) Potsdam, Germany) and in a Zeiss Cross-

Beam ESB1540 (Institut fur Mineralogie, University of

Munster, Germany). This technique enables site-specific

sampling from a standard thin section by Ga-ion milling,

with the ability to maintain optimal spatial correlation

between features observed optically and by BSE imaging

with those subsequently studied at the nanometer level in

the TEM (Wirth 2004, 2009). TEM investigations were

made in a JEOL 2000FX and 2010F, both operating at

200 kV (Department of Physics, University of Oslo, Nor-

way), as well as in a Zeiss LIBRA 200FE TEM/STEM

operating at 200 kV and equipped with a Thermo Noran

energy-dispersive X-ray (EDX) detector (Institut fur Min-

eralogie, University of Munster, Germany). The latter was

also used for high-angle annular dark-field (HAADF)

imaging and EDX profiling. The HAADF signal is highly

sensitive to specimen compositional variations, where the

heavier the atoms, the brighter the HAADF contrast in the

image (Z-contrast images). High-resolution (HR) STEM

bright-field (BF) and HAADF images were acquired in an

aberration-corrected Nion UltraSTEM 100, capable of

achieving a spot size of\0.1 nm. This dedicated STEM is

equipped with a cold field-emission gun (FEG) emitter that

was operated at 100 kV, a Nion Quadrupole-Octopole

corrector allowing for full correction of up to sixfold

astigmatism and a Gatan Enfina electron energy loss (EEL)

spectrometer. Smart-acquisition EEL spectroscopic line

profiles (150 points) were acquired across the subgrain

boundary, while the locally incurred total electron dose was

distributed parallel to the grain boundary to minimize beam

damage (Sader et al. 2010). To map the EEL signal of O-K

edge, Fe-L2,3 edge and Mg-K edge simultaneously, the

channel dispersion was set to 0.7 eV, although the instru-

ment native energy resolution in the EEL spectra is

\0.35 eV. The beam dwell time and beam intensity were

reduced (typically 15 ls and 8–10 pA, respectively) to

restrict the otherwise rapidly occurring beam damage.

Furthermore, with an estimated probe size of *0.8–0.9 A,

the pixel size for imaging was adjusted to 0.5 A to yield an

oversampling of 2 and further mitigate the effects of beam

damage during observation. Further details about strategies

to achieve high-resolution, low-dose STEM can be found

in Buban et al. (2010). BF and HAADF-HR images were

filtered for visual clarity using a radial Wiener filter (e.g.,

Kilaas 1998). Mineral abbreviations in this paper follow

suggestions by Whitney and Evans (2010).

Results

During our investigations, we identified an intracrystalline

chemical alteration that occurs in conjunction with antig-

orite formation during olivine hydration. The alteration

produced marked changes in the major (Mg and Fe) and

minor (Mn and Ni) element composition along specific

crystallographic orientations of olivine grain domains

resulting in a striped zoning. The formation of this zoning

is a ubiquitous phenomenon within olivine grains of the

upper mantle section in the LOC and in meta-harzburgites

of the FUB (Fig. 1). Two different types of zoning have

been identified:

Type (1) Fe-enriched striped zoning, which is present at

both locations, in olivine grain domains within meta-dunite

in close vicinity to meta-orthopyroxenite dikes at the LOC

(Fig. 1b) and in olivine domains within meta-harzburgite

that are interlayered with meta-dunite at the FUB.

Type (2) Fe-depleted striped zoning was observed in

olivine grain domains within meta-peridotite and is specific

to the LOC (Fig. 1b).

Type (1): Fe-enriched striped zoning in olivine

The meta-orthopyroxenite dikes associated with the

Fe-enriched zoning in the LOC have no identifiable pri-

mary ortho- or clinopyroxene grains but comprise serpen-

tine, talc, amphibole and secondary olivine grains (Iyer

et al. 2008a). The meta-dunite, where the Fe-enriched

zoning is exclusively detected, is composed of olivine

Contrib Mineral Petrol

123

grains penetrated by crosscutting antigorite blades, sepa-

rating the olivine grains into domains that retain optical

and hence structural continuity across an area of *3 mm2.

No brucite was found to be present during Raman spec-

troscopic analysis. Each olivine domain displays typical

kink bands and optical undulose extinction (Fig. 2a). Using

polarized light microscopy combined with a universal

stage, the domain orientations were determined to be either

[010] or [001] normal to the thin section surface. Juxta-

position of optical (Fig. 2) and BSE images (Fig. 3)

showed that the optically visible kink bands are parallel to

zones of elevated BSE intensity, representing the striped

zoning. Quantitative EMP analyses revealed that the

increase in BSE intensity results from an enrichment in Fe

as well as in Mn compared with the bulk olivine ( �XFe =

0.24 (Fe-enriched zones), 0.12 (bulk); Mn = *5,500 ppm

(Fe-enriched zones), *2,300 ppm (bulk)) (Table 1). Some

individual zones developed an anticorrelation between Mn

and Ni, where Ni is as low as *800 ppm (Fe-enriched

zones) compared with *1,300 ppm (bulk). Other Fe-

enriched zones remained unaffected in terms of Ni deple-

tion. Antigorite blades associated with the Fe-enriched

zones are also elevated in Fe ( �XFe = 0.08) and have Mn

and Ni concentrations of *1,000 and *900 ppm,

respectively (Table 1). When present, magnetite grains are

sparsely dispersed as interlayers within the antigorite

blades (Fig. 3a). None is found within the olivine domains.

The Fe-enriched zones are parallel and transect olivine

domains that exhibit the same optical orientation (Fig. 3a,

c). The zones either pass entirely through an individual

olivine domain or terminate inside a domain (Fig. 3a,

lower half of the image). Within a single domain, the zones

have a homogeneous distribution. However, the spacing

varies from 1 to 9 lm (average 4.4 lm) among different

domains. The zoning is most pronounced where antigorite

blades penetrate the olivine grains, extending into the

olivine away from the antigorite-olivine interface (Fig. 3b).

In some cases, a pervasive Fe enrichment affects entire

olivine domains (Fig. 3c), where either (1) closely spaced,

parallel striped zones overlap to result in a bulk enrichment

(Fig. 3b) or (2) fully Fe-enriched olivine occurs. Figure 3d

displays the latter case where a homogenously Fe-enriched

olivine with interstitial antigorite inclusions and small

pores has a sharp microstructural contact to a primary

olivine fragment with striped zoning. It is noteworthy that

the interfacial area of the primary olivine became enriched

in Fe and that the striped zoning leads away from this area

to enrich the bulk olivine grain. Interfacial areas of a few

lm within the olivine domains show elevated Fe concen-

trations in comparison with the bulk, creating Fe-enriched

halos around antigorite inclusions and blades (Figs. 3b, 4).

The Fe content in these halos is comparable to that of the

Fe-enriched zones in the striped zoning.

A similar Fe-enriched zoning is observed in the FUB

(Fig. 5) but is restricted to meta-harzburgite layers that are

dominated by fully serpentinized orthopyroxene grains

surrounded by areas of partially serpentinized olivine

grains (Fig. 5a). Similar to the LOC, the main serpentine

polymorph associated with the Fe-enriched olivine

domains is antigorite. However, lizardite was also identi-

fied. The FUB olivine striped zoning is less distinct than

that of the LOC olivine ( �XFe = 0.11 (Fe-enriched zones);

0.09 (bulk)) but it exhibits an increase in Mn (*1,800 ppm

(Fe-enriched zones); *1,100 ppm (bulk)) and a locally

developed decrease in Ni (*3,000 ppm (Fe-enriched

zones); *3,500 ppm (bulk)) (Table 2). Associated antig-

orite blades have �XFe of 0.04 with Mn and Ni contents of

*750 and *1,200 ppm, respectively. Unlike the LOC,

antigorite in the FUB meta-harzburgite often penetrates the

Fig. 2 Optical photomicrographs under crossed polars giving an

overview of the partly serpentinized olivine grains from the LOC.

Note the fine spacing of undulose extinction in both photomicro-

graphs. BSE imaging (Fig. 3) and EMP analyses reveal that areas

parallel to the undulose extinction are either enriched a or depleted

b in Fe. Note the occurrence of magnetite in b which is nearly absent

in a. Ol olivine, Atg antigorite, Mag magnetite

Contrib Mineral Petrol

123

olivine domains for several lm along individual striped

zones (Fig. 5e). As in the LOC, completely Fe-enriched

olivine is found either as a complex intergrowth with ser-

pentine (Fig. 5c, upper left part of image) or as an appar-

ently pseudomorphic replacement of the primary olivine

(Fig. 5c, lower part of the image). Bulk olivine composi-

tions within the meta-harzburgite coincide with those of the

meta-dunite. However, olivine domains within the meta-

dunite in the immediate vicinity of meta-harzburgite in the

FUB display neither Fe enrichment nor association with

antigorite. In this case, the serpentine polymorph is lizar-

dite ( �XFe = 0.03) with disseminated brucite (verified by

Raman spectroscopy), magnetite and awaruite (Fig. 5b).

Type 2: Fe-depleted striped zoning in olivine

The modal mineralogy associated with the two types of

zoning differs due to the abundance of magnetite in areas

where the Fe-depleted zoning is present (Fig. 2b). Mag-

netite in this case is found both within the olivine domains

Fig. 3 BSE images of olivine

striped zoning and associated

antigorite from the LOC.

a–d display Fe-enriched zones in

olivine fragments (high BSE

intensity). If magnetite is present,

it occurs interlayered with the

antigorite blades in minor

amounts or is otherwise absent.

FIB thin foils for TEM

investigations were obtained

across several striped zonings as

indicated in (a). b Magnified BSE

image of a single olivine fragment

exhibiting fine-spaced Fe-

enriched zones, giving rise to an

overlapping appearance. Note the

development of Fe-enriched halos

of high BSE intensity around

antigorite inclusions (arrows).c Overview of an area where

several olivine fragments display

Fe-enriched striped zones (image

center), surrounded by an area of

intensively Fe-enriched olivine

fragments. d Contact between a

striped zoned olivine fragment

and a homogeneously Fe-

enriched olivine. Note the

porosity within the antigorite

blades (white arrow). e–h Display

the microstructures associated

with Fe-depleted zoning. e Finely

spaced occurrence of Fe-depleted

zones (low BSE intensity). The

dark line across the zones in e is a

relic from microprobe profiling.

Association between Fe-depleted

striped zoning and the appearance

oflm-sized antigorite precipitates

aligned along the zones is shown

in (f) and (g). Magnetite occurs

along intragranular olivine

fractures. FIB thin foils were

taken across the zonings as

displayed in (f). h Dilated Fe-

depleted zones with an array of

serpentine precipitates in the zone

center. Ol olivine, Atg antigorite,

Mag magnetite

Contrib Mineral Petrol

123

Table 1 Electron microprobe analyses of Fe-enriched and Fe-depleted zones with corresponding bulk olivine and antigorite compositions from

the LOC

Fe-enriched zoning

Averaged analyses (1 r) Single olivine-antigorite pairs

Ol (zone) Ol (bulk) Atg Ol (zone) Ol (bulk) Atg Ol (zoning) Ol (bulk) Atg

(na = 65) (n = 50) (n = 65)

wt%

SiO2 38.25 ± 0.88 40.23 ± 0.68 43.74 ± 1.20 39.92 39.41 43.34 37.10 39.74 43.86

TiO2 b.d.l.b b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Al2O3 b.d.l. b.d.l. 0.21 ± 0.14 b.d.l. b.d.l. 0.18 b.d.l. b.d.l. 0.316

Cr2O3 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

FeO(tot) 21.73 ± 2.01 11.62 ± 1.79 6.74 ± 2.06 21.28 11.70 5.41 23.75 12.14 5.51

NiO 0.18 ± 0.04 0.16 ± 0.07 0.11 ± 0.04 0.11 0.14 0.13 0.11 0.20 0.10

MgO 38.89 ± 2.01 47.83 ± 1.92 37.13 ± 0.75 38.62 47.20 37.23 37.12 47.05 37.14

MnO 0.69 ± 0.10 0.30 ± 0.08 0.13 ± 0.12 0.74 0.21 0.08 0.74 0.31 0.048

CaO b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Na2O b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Total 99.78 ± 1.44 100.19 ± 0.78 87.24 ± 0.82 100.67 98.67 86.39 98.80 99.44 86.98

Structural formula based on 4 oxygens for olivine & 6.823 oxygens and Fe3?/Fetot = 0 for antigorite (m = 17c)

Si 0.996 ± 0.014 0.992 ± 0.008 2.022 ± 0.011 1.031 0.986 2.011 0.985 0.990 2.020

Al 0.006 ± 0.004 0.005 0.009

Fe 0.473 ± 0.081 0.240 ± 0.46 0.228 ± 0.022 0.460 0.245 0.210 0.527 0.253 0.212

Ni 0.004 ± 0.001 0.003 ± 0.001 0.004 ± 0.001 0.002 0.003 0.005 0.002 0.004 0.004

Mg 1.510 ± 0.212 1.758 ± 0.048 2.535 ± 0.028 1.487 1.760 2.575 1.469 1.747 2.549

Mn 0.016 ± 0.002 0.006 ± 0.002 0.004 ± 0.002 0.016 0.004 0.003 0.017 0.006 0.002

XMgd 0.76 ± 0.02 0.88 ± 0.02 0.92 ± 0.01 0.76 0.88 0.92 0.74 0.87 0.92

XFee 0.24 ± 0.02 0.12 ± 0.02 0.08 ± 0.01 0.24 0.12 0.08 0.26 0.13 0.08

Fe-depleted zoning

Averaged analyses (1 r) Single olivine-antigorite pairs

Ol (zone) Ol (bulk) Atg Ol (zone) Ol (bulk) Atg Ol (zone) Ol (bulk) Atg

(n = 50) (n = 50) (n = 50)

wt%

SiO2 40.70 ± 0.29 39.97 ± 0.40 44.25 ± 0.73 40.81 40.15 44.348 40.64 39.71 42.54

TiO2 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Al2O3 b.d.l. b.d.l. 0.74 ± 0.41 b.d.l. b.d.l. 0.55 b.d.l. b.d.l. 0.67

Cr2O3 b.d.l. b.d.l. b.d.l. b.d.l. 0.02 b.d.l. b.d.l. b.d.l. 0.03

FeO(tot) 9.56 ± 0.66 12.11 ± 0.64 2.58 ± 0.44 9.54 12.52 2.52 9.77 12.78 3.50

NiO 0.15 ± 0.08 0.25 ± 0.05 0.04 ± 0.03 0.14 0.17 0.06 0.19 0.28 0

MgO 49.30 ± 0.66 47.08 ± 0.70 39.59 ± 0.49 49.18 46.60 39.72 48.46 46.38 37.93

MnO 0.48 ± 0.14 0.33 ± 0.10 0.06 ± 0.02 0.53 0.34 0.05 0.52 0.29 b.d.l.

CaO b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Na2O b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Total 100.00 ± 0.63 99.75 ± 0.79 87.35 ± 0.95 100.24 99.80 87.31 99.58 99.44 86.27

Structural formula based on 4 oxygens for olivine & 6.823 oxygens and Fe3?/Fetot = 0 for antigorite (m = 17c)

Si 0.997 ± 0.006 0.993 ± 0.007 2.015 ± 0.011 0.998 0.999 2.015 1.002 0.992 2.068

Al 0.020 ± 0.011 0.015 0.039

Fe 0.196 ± 0.014 0.252 ± 0.014 0.098 ± 0.017 0.195 0.261 0.096 0.202 0.267 0.142

Ni 0.003 ± 0.001 0.005 ± 0.001 0.001 ± 0.001 0.003 0.003 0.002 0.004 0.006

Mg 1.793 ± 0.016 1.743 ± 0.014 2.689 ± 0.026 1.792 1.729 2.690 1.782 1.728 2.749

Contrib Mineral Petrol

123

and along crosscutting intragranular microfractures

(Fig. 3g). The dominant, associated serpentine mineral is

antigorite ( �XFe = 0.04) ± brucite (verified by Raman

spectroscopy); however, minor lizardite is also identified.

As for the Fe-enriched zoning, Fe depletion ( �XFe = 0.10

(Fe-depleted zones); 0.13 (bulk)) is evident in BSE images

as an alternating BSE intensity within olivine domains,

where in this case zones of low intensity are correlated with

the optical undulose extinction (Fig. 3 e–h). Olivine

domains are also optically continuous over several mm2 in

areas that exhibit Fe depletion (Fig. 2b). Compared with

the Fe-enriched zones, analyses of minor element con-

centrations reveal a similar Mn enrichment (*3,700 ppm

(Fe-depleted zones); *2,500 (bulk)) but a stronger Ni

depletion (*1,200 ppm (Fe-depleted zones); *2,000 ppm

(bulk)) (Table 1). Slightly elevated Al concentrations in

antigorite after olivine are likely due to the contempora-

neous alteration of abundant spinel and locally occurring

pyroxene. A common feature along the Fe-depleted zones

are precipitate trails composed of antigorite (1–2 lm in

size) (Fig. 3 f–h). Optical defocusing through these zones

revealed that the trails are 2-dimensional planar, subregu-

larly-spaced accumulation of precipitates. Where trails

become extensive, the Fe depletion expands into the oliv-

ine domains for several tens of lm (Fig. 3h); otherwise,

Fe-depleted zones are restricted to a few lms only

(Fig. 3 e–g).

Location of the striped zoning and its relation

to subgrain boundaries

As illustrated by the BSE images (Figs. 3, 4, 5), both types

of zoning follow a specific crystallographic orientation

within the olivine domains. FIB sample preparation across

several zones was applied to gain further insight into the

relationship of the intracrystalline chemical modifications

with structures at the nanometer level. Conventional TEM

investigation revealed that each striped zone (Fe-enriched

and Fe-depleted) coincides with a low-angle tilt boundary

with angular misorientation of 1� to 4� between the

Fig. 4 a BSE image of a partly serpentinized olivine area (LOC),

exhibiting Fe-enriched zoning and corresponding WDX element

distribution maps of Fe (b) and Mg (c). The square in a denotes the

area mapped. Note the abrupt Fe offset in (b) between olivine and

antigorite. Halos (*1 lm in size) of Fe enrichment (corresponding

Mg depletion) developed at interfacial areas (arrows) in olivine, both

in contact with antigorite blades or surrounding intracrystalline

antigorite inclusions. Ol olivine, Atg antigorite

Table 1 continued

Fe-depleted zoning

Averaged analyses (1 r) Single olivine-antigorite pairs

Ol (zone) Ol (bulk) Atg Ol (zone) Ol (bulk) Atg Ol (zone) Ol (bulk) Atg

(n = 50) (n = 50) (n = 50)

Mn 0.010 ± 0.003 0.007 ± 0.002 0.002 ± 0.001 0.011 0.007 0.002 0.011 0.006

XMgd 0.90 ± 0.01 0.87 ± 0.01 0.96 ± 0.01 0.90 0.87 0.97 0.90 0.87 0.95

XFee 0.10 ± 0.01 0.13 ± 0.01 0.04 ± 0.01 0.10 0.13 0.03 0.10 0.13 0.05

a n = number of analysisb b.d.l. = below detection limitc m = polysomatic number for antigorite structure after temperature dependence considerations by Wunder et al. (2001)d XMg = Mg/(Mg ? Fe)e XFe = 1 - XMg

Contrib Mineral Petrol

123

subgrains (Figs. 6a, 7b). The subgrain boundaries are

composed of edge dislocations with [100](010) or

[100]{0kl} slip systems and hence are parallel to (100) in

agreement with EBSD analyses (not shown). However,

(101) subgrain boundaries have also been identified. The

dislocation density is on the order *107–108 cm-2 for both

types of zones. To gain an initial closer comprehension of

the spatial relationship between the zoning and the sub-

grain boundaries, BSE images of thin foil localities before

and after preparation were compared with BF-TEM over-

view images of the associated thin foils and confirmed, by

simple counting, that every zone corresponds to a subgrain

boundary. Figure 6 displays a representative FIB thin foil

across two Fe-enriched zones. The BF image (Fig. 6a)

exposes two (100) subgrain boundaries, and the corre-

sponding chemically sensitive Z-contrast HAADF image

(Fig. 6b) reveals elevated intensity in comparison with the

surrounding bulk olivine immediately at the subgrain

Fig. 5 a Thin section overview of a contact between a meta-

harzburgite and meta-dunite within the FUB. Note the abundant

occurrence of fully hydrated orthopyroxene grains (hydr. Opx) within

the meta-harzburgite. b BSE image of partly serpentinized olivine

fragments in association with lizardite, disseminated brucite (phase

with lowest backscattering contrast in (b) and verified by Raman

spectroscopy) and magnetite. Note the absence of chemical variations

in the olivine fragments. Within the meta-harzburgite, olivine

fragments developed an Fe-enriched striped zoning as shown in the

BSE images (c–e). Recrystallized homogeneously Fe-enriched olivine

occurs in (c), either as rims in a sharp contact to primary olivine

(lower part of image) or as a complex intergrowth with serpentine

(top part of the image). In some cases (e) antigorite penetrates along

the zoning into the olivine. Ol olivine, Opx orthopyroxene, Atgantigorite, Brc brucite, Mag magnetite

Contrib Mineral Petrol

123

boundaries, indicating a heavier average atomic weight of

the material at the subgrain boundary. An ultrathin

(\60 nm) FIB foil across two Fe-enriched zones was pre-

pared for aberration-corrected STEM imaging and EEL

spectroscopic line profiling. Only the Fe-enriched zoning

was examined because the chemical changes between the

zones and the bulk olivine are considerably higher than

those associated with the Fe-depleted zones. Figure 7a and

b display raw and filtered BF-HRSTEM images, showing

the spatial extent of the boundary-induced lattice distor-

tion. Corresponding HAADF-HRSTEM images (Fig. 7c,

d) reveal increased HAADF contrast decorating the sub-

grain boundary in nanometer-sized patches, suggesting Fe

atom clustering at the boundary. In total, ten EEL spec-

troscopic smart-acquisition line profiles across the subgrain

boundaries were acquired and found to show a consistent

behavior. Together with an increase in HAADF intensity,

an increase in Fe and decrease in Mg were recorded at the

boundary (Fig. 7e). The discrepancy between the zone

width in BSE images (width of BSE intensity maximum is

*200 nm) in comparison with the smaller width (*5 nm)

in the HAADF-HR images and EEL spectroscopic line

profiles is due to the spatial extent of the BSE generating

region. This causes a considerable convolution in the BSE

image. Inclination of the subgrain boundary to the polished

thin section surface will also contribute to a broadening of

the BSE intensity. Nevertheless, EMP chemical analyses of

the zones display consistent compositional characteristics

when compared to larger areas of enrichment such as in

Fig. 3c and d. Results for Fe-depleted zones can be

regarded as similar because microprobe analyses clearly

display Fe depletion along zones that are revealed as sub-

grain boundaries by subsequent TEM investigations

(Fig. 8). Furthermore, HAADF intensity in STEM mode

was observed to decrease along subgrain boundaries shown

in BF-TEM images in Fig. 8a.

Table 2 Electron microprobe analyses of Fe-enriched zones with corresponding bulk olivine and antigorite compositions within meta-harz-

burgite from the FUB

Fe-enriched zoning

Averaged analyses (1 r) Single olivine-antigorite pairs

Ol (zone) Ol (bulk) Atg Ol (zone) Ol (bulk) Atg Ol (zone) Ol (bulk) Atg

(na = 45) (n = 52) (n = 53)

wt%

SiO2 40.66 ± 0.23 40.85 ± 0.34 42.11 ± 2.06 40.97 40.97 42.57 40.51 40.84 43.44

TiO2 b.d.l.b b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Al2O3 b.d.l. b.d.l. 0.58 ± 0.60 b.d.l. b.d.l. 0.27 b.d.l. b.d.l. 0.25

Cr2O3 b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

FeO(tot) 11.36 ± 0.51 8.90 ± 0.55 2.92 ± 0.57 11.89 8.18 2.20 11.90 9.18 2.20

NiO 0.41 ± 0.05 0.42 ± 0.05 0.16 ± 0.16 0.36 0.36 0.03 0.48 0.41 0.05

MgO 47.93 ± 0.48 49.78 ± 0.50 41.13 ± 1.76 47.33 49.79 40.38 47.52 49.51 41.71

MnO 0.23 ± 0.03 0.15 ± 0.03 0.10 ± 0.05 0.31 0.10 0.11 0.22 0.17 0.12

CaO b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. 0.13 b.d.l. b.d.l. b.d.l.

Na2O b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l. b.d.l.

Total 100.60 ± 0.50 100.13 ± 0.76 87.13 ± 1.00 101.1 99.45 85.72 100.65 100.17 87.99

Structural formula based on 4 oxygens for olivine & 6.823 oxygens and Fe3?/Fetot = 0 for antigorite (m = 17c)

Si 0.998 ± 0.004 0.996 ± 0.006 1.936 ± 0.074 1.004 1.003 1.970 0.985 0.990 1.960

Al 0.016 ± 0.017 0.007 0.007

Fe 0.233 ± 0.011 0.182 ± 0.011 0.112 ± 0.023 0.244 0.167 0.085 0.527 0.253 0.083

Ni 0.008 ± 0.001 0.008 ± 0.001 0.006 ± 0.006 0.006 0.007 0.001 0.002 0.004 0.002

Mg 1.755 ± 0.013 1.810 ± 0.012 2.822 ± 0.143 1.740 1.817 2.785 1.469 1.747 2.805

Mn 0.005 ± 0.001 0.003 ± 0.001 0.004 ± 0.002 0.005 0.002 0.004 0.017 0.006 0.004

XMgd 0.88 ± 0.01 0.91 ± 0.01 0.96 ± 0.01 0.88 0.92 0.97 0.736 0.874 0.97

XFee 0.12 ± 0.01 0.09 ± 0.01 0.04 ± 0.01 0.12 0.08 0.03 0.264 0.126 0.03

a n = number of analysisb b.d.l. = below detection limitc m = polysomatic number for antigorite structure after temperature dependence considerations by Wunder et al. (2001)d XMg = Mg/(Mg ? Fe)e XFe = 1 - XMg

Contrib Mineral Petrol

123

Nanometer-sized serpentine precipitates along Fe-

depleted olivine subgrain boundaries

In conjunction with micrometer-sized trails of antigorite

precipitates in the BSE images along Fe-depleted zones

(Fig. 3f-g), TEM investigations of FIB thin foils show

pseudo-periodic, nanometer-sized serpentine precipitates

along the subgrain boundaries (Fig. 8). Measurement of the

dihedral angle of two precipitates in Fig. 8c suggests an

angle of *70� between the precipitates and the subgrain

boundary. From analysis of the lattice fringes in the

HRTEM image of the serpentine inclusion (Fig. 9a) and

identification of larger serpentine precipitates within oliv-

ine domains from Raman spectra, it can be inferred that the

nanometer-sized precipitates are also antigorite. The pre-

cipitate exhibits an asymmetrical shape with a straight side

parallel to the subgrain boundary and an opposite, bulging

side penetrating into the olivine subgrain (Fig. 9a). Within

the olivine around the precipitate (Fig. 9a) and in close

vicinity to the subgrain boundary (Fig. 9b), areas of dif-

ferent lattice spacing (7.4 A) than the olivine spacing are

observed (Fig. 9), which likely represent the beginning of

serpentine nucleation within olivine. An EDX intensity

profile generated across the precipitate displays chemically

sharp interfaces at the nanometer level as illustrated by the

abrupt increase in Mg and decrease in Si as well as Fe

compared with the surrounding olivine (Fig. 8c).

Crystallographic control on olivine serpentinization

To identify whether a microscopic orientation relationship

exists between the olivine separation and the formation of

different serpentine vein generations, we have investigated

a representative area composed of olivine grain domains

that are penetrated by a serpentine vein network, using the

EBSD technique (Fig. 10). The relative chronological

order of the serpentine vein network was determined by

cross-cutting relationships and morphological criteria.

These criteria imply that the vertically aligned, thick ser-

pentine veins were formed prior to the thin subhorizontally

orientated veins. Olivine domains show kink bands and

undulose extinction orientated parallel to the thick ser-

pentine veins. EBSD analyses of the olivine grains in

relation to the serpentine veins show that the thick ser-

pentine veins are subparallel to the (100)ol plane and the

thinner serpentine veins are parallel to the (010)ol plane for

areas A to C (Fig. 10b) as well as normal to the thick

serpentine vein in the BSE image. Olivine domains in areas

E and D (Fig. 10a) display the same orientational rela-

tionship; however, the well-developed parting parallel to

the (010)ol plane deviates from being normal to the thickest

serpentine vein.

Discussion

The origin of striped zoning along olivine subgrain

boundaries: Linking solid-state diffusion and mineral

replacement reactions

To minimize the long-range stress field generated during

high-temperature, high-pressure crystal-plastic deforma-

tion, dislocations align into planar arrays to form subgrain

boundaries modifying the microstructure of practically

every mantle-derived olivine (e.g., Boullier and Nicolas

1970; Carter and Ave Lallemant 1970). In conjunction with

this, there is also evidence that crystal-plastic deformation

affects not only the microstructure but also the micro-

chemistry of olivine. Previously described striped zoning in

Fig. 6 Representative FIB thin foil across the Fe-enriched striped

zoning in olivine. a BF-TEM image of two (100) subgrain boundaries

(arrows) composed of edge dislocations. Every striped zoning evident

in the BSE images can be related to a subgrain boundary.

b Chemically sensitive Z-contrast HAADF image of the same area

as in a, displaying a HAADF intensity increase at both subgrain

boundaries. The spherical brighter zones at the bottom of the foil are

an artifact due to beam damage during sample preparation

Contrib Mineral Petrol

123

olivine ( �XFe = 0.094 (Fe-enriched zones); 0.082 (bulk)) is

thought to originate from a physico-chemical feedback

mechanism during plastic deformation, where the align-

ment of edge dislocations drags a ‘Cottrell atmosphere’

(Cottrell and Bilby 1949) of solute atoms (in this case Fe)

into the subgrain boundary (Kitamura et al. 1986; Ando

et al. 2001). This stress-assisted segregation of a cation

onto edge dislocations has been observed directly using

state-of-the-art aberration-corrected STEM, which showed

that excess flux of solute atoms (in this case Pb in SrRuO3)

rapidly decays to zero for distances beyond 1 nm from a

dislocation core (Arredondo et al. 2010). Thus, solute

concentrations at an array of dislocations (i.e., subgrain

boundary) would be unlikely to exceed several wt% via

this mechanism. Therefore, the exact mechanism of Fe

enrichment at olivine subgrain boundaries remains unclear.

Previously investigated peridotite bodies exhibiting only

Fe-enriched striped zoning have been found in rocks that

are apparently unaffected by hydrothermal alteration (e.g.,

San Carlos xenolith, USA, Kitamura et al. 1986; Uenzaru

peridotite, Japan, Ando et al. 2001) and in serpentinized

meta-peridotites (Conical and South Chamorro Seamounts,

Mariana forearc, Murata et al. 2009). In contrast, the

samples investigated here derive from hydrated upper

mantle rocks, displaying opposing types of striped zoning

reflected by (1) an enrichment and (2) a depletion in Fe. By

integrating BSE imaging (Fig. 3) with FIB sample prepa-

ration and subsequent (S)TEM analyses (Figs. 6, 7, 8), it

was determined that each of the striped zones visible at the

micrometer level in BSE images represents a subgrain

boundary at the nanometer level. In our samples, a distinct

correlation between the fluid-mediated replacement reac-

tion of olivine by antigorite and the development of striped

zoning is observed, leading to the hypothesis that the

chemical modifications along the subgrain boundaries and

the olivine-antigorite replacement are linked. This is sup-

ported by the observation that the zoning is most pro-

nounced where antigorite penetrates the olivine grain

(Fig. 3b), implying that fluid infiltration into olivine grains

is related to the striped zoning development, as also

recently suggested by Murata et al. (2009). Furthermore,

minor element contents of Mn and Ni (Table 1) associated

with the zoning exhibit an opposing signature to those

found in apparently dry Uenzaru peridotite (Ando et al.

2001, Table 1). Mn is strongly enriched, whereas Ni is

typically depleted along the olivine subgrain boundaries, in

Fig. 7 Aberration-corrected STEM imaging and EEL spectroscopic

line profiling of a Fe-enriched olivine subgrain boundary. a and b are

raw and filtered BF-HRSTEM images displaying the spatial extent of

the boundary-induced lattice distortion (subgrain boundary orientated

NW-SE). c and d Corresponding raw and filtered chemically sensitive

Z-contrast HAADF-HRSTEM images of the subgrain boundary. Note

the patchy occurrence of increased HAADF intensity along the

subgrain boundary (two are marked by white arrows), suggesting a

clustering of Fe at the boundary. e HAADF intensity and lattice-

resolved EEL spectroscopic line profiles across the subgrain bound-

ary, illustrating the increase in HAADF intensity as well as increase in

Fe and decrease in Mg at the subgrain boundary. The Fe (respectively,

Mg) elemental line profile was obtained by integrating the EEL

intensity over a 26 eV (respectively, 65 eV) window after the Fe-L2,3

(respectively Mg-K) edge energy onset. The background was

removed in both cases using a power law

c

Contrib Mineral Petrol

123

Fig. 8 a BF-TEM overview image of a representative FIB thin foil

over several Fe-depleted striped zones in olivine. Each of the striped

zones visible at the micrometer level in BSE images represents a

subgrain boundary (arrows) at the nanometer level. The white squarein the BF-TEM in (a) represents the area magnified in (b). A trail of

precipitates is clearly visible along the two subgrain boundaries. The

faint dark oval lines in (a) and (b) are hole edges of the carbon

substrate upon which the thin foil rests. The dark spot is an carbon

film artifact. c Association between two serpentine precipitates and

the subgrain boundary as well as their dihedral angle. The inserted

HRTEM image (30 nm image width; with fast Fourier transform

(FFT) pattern) displays the subgrain boundary in between the two

precipitates

Fig. 9 a HRTEM image of a serpentine (antigorite) precipitate along

an olivine subgrain boundary displayed in Fig. 8. Insert is FFT pattern

across the whole image including the antigorite precipitate (arrows)

and the olivine. Note the heterogeneous contrast inside the precipitate

which is due to beam damage during TEM investigation. b Magnified

HRTEM image of the interface between the precipitate and the

olivine as well as the relation to the subgrain boundaries. FFT pattern

generated across the olivine is inserted. c A STEM/EDX intensity

profile across the precipitate in (a), displaying an abrupt Mg increase

and Si (and Fe) decrease at the olivine-antigorite interfaces

Contrib Mineral Petrol

123

agreement with previous observations of Mn and Ni par-

titioning between olivine and serpentine in prograde meta-

serpentinites (Trommsdorff and Evans 1972) but contrary

to the Uenzaru peridotite where Mn and Ni concentration

remains unaffected in the Fe-enriched zones. The evidence

that the Fe-enriched and Fe-depleted zones in our samples

are associated with subgrain boundaries as well as their

relationship to the olivine-antigorite replacement reaction

indicates that defects play a crucial role in the chemical

changes observed within olivine grains in these rocks.

Olivine re-equilibration in the presence of a fluid phase is

governed by several processes that occur contemporane-

ously. In general, diffusion is the slowest of these processes

(Evans 2010) and fluid-mediated replacement reactions,

such as dissolution-reprecipitation, the fastest (Putnis

2009). When bulk diffusion is considered at conditions of

the fastest serpentinization rate (*300�C; Martin and Fyfe

1970) and at the upper limit of serpentinization within a

dunite (*400�C), the interdiffusion coefficient of Fe and

Mg (DFeMg) in olivine (XFe = 0.1) along the fastest crys-

tallographic direction (i.e., [001]) can be approximated via

log DFe Mgm2�

s� �

¼ �8:27

�226000þ P� 105

� �� 7� 10�6

2:303RTþ 3XFe ð1Þ

where P is pressure (Pa), R universal gas constant

(Jmol-1K-1), T temperature (K) and XFe the molar

fraction of Fe [Fe/(Fe ? Mg)] (Dohmen and Chakraborty

2007). This yields a DFeMg of 10-28.9 and 10-25.78 (m2 s-1)

and results in a characteristic diffusion distance x via

x ¼ffiffiffiffiffiDtp

ð2Þ

of only *0.02 and *0.72 lm over 1 Ma at 500 MPa,

respectively. However, changes in chemical composition

that transect the investigated olivine grain domains show

that dislocation walls can serve as a lamellar network of

high-diffusivity pathways for diffusing atoms. On the scale

of an isolated dislocation, this phenomenon is known as

dislocation pipe diffusion (Love 1964) and originates from

the disordered dislocation core region, which lowers the

activation energy for diffusion (e.g., Huang et al. 1989).

Recent in situ experiments with a simple alloy observed

diffusivity enhanced by up to three orders of magnitude

along a single dislocation core compared with bulk diffu-

sion (Legros et al. 2008). This is supported by atomistic

modeling indicating that in an ionic solid (MgO), the dif-

fusivity associated with dislocation pipe diffusion is *100

times higher than that of bulk diffusion at *500�C (Zhang

et al. 2010). There are ample indications that in many

solids the parameters of dislocation pipe diffusion are

similar to those of diffusion along high-angle grain

boundaries and thus both can be treated by similar phe-

nomenological approaches (Mehrer 2007; Dohmen and

Milke 2010). Indeed, it has been observed that synthesized

high-angle grain boundaries in olivine bicrystals are com-

posed of dislocation arrays at angular misorientations as

high as 21� (Heinemann et al. 2005). Diffusion along these

boundaries in a solid-state, fluid-free environment is

regarded as the main mechanism of mass transport,

regardless of whether it is a grain (i.e., olivine-olivine) or

Fig. 10 a BSE image of a partially serpentinized meta-peridotite

showing areas (A–E squares) investigated via EBSD analyses to

obtain olivine crystallographic orientations in relationship to the thick

serpentine vein (right side of the BSE image). b Pole figures showing

the orientation of olivine. N represents the number of points measured

on olivine fragments representing former whole grains. The pole

figures are presented in the upper hemisphere by using an equal-area

projection. The reference frame of the pole figures is chosen to

coincide with the X0 (horizontal axis) and Y0 (vertical axis) of the

BSE image. We used a half width of 30� to draw the pole figures. The

color coding represents the density of data points (pole figure

contours correspond to the multiples of uniform distribution). Redcolor shows the strong alignment of crystallographic axes. Note that

the (100)ol plane (the locus of edge dislocation piling to form subgrain

boundaries) is parallel to the thick serpentine veins, whereas the

(010)ol parting plane is parallel to the thinner serpentine veins. The

latter are more perfectly developed. Ol olivine, Srp serpentine, Didiopside, Mag magnetite

Contrib Mineral Petrol

123

interphase (i.e., olivine-pyroxene) boundary. If no subgrain

boundaries are present, diffusion into the bulk grain can

only occur via a direct diffusive flux from the grain/inter-

phase boundary. In contrast, Klinger and Rabkin (1999)

suggest that the presence of subgrain boundaries acts as

additional pathways for diffusion allowing leakage of a

diffusant from the dislocation cores into greater areas of the

bulk crystal, leading to a more pervasive chemical change.

The higher diffusivity associated with intracrystalline

defects and grain/interphase boundaries means that in the

absence of a fluid phase, particularly at temperatures too

low for bulk diffusion to occur, mass transport via diffusion

will take place along these pathways. Nevertheless, in a dry

diffusion-dominated system at low temperatures, this will

probably only affect a crystal along a very limited length

scale, i.e., one that is much shorter than the scale of

microstructures (Putnis and Austrheim 2010). When a

fluid phase is present, mass transport can take place via

advection and the diffusion of elements through an

interconnected three-dimensional fluid network. Mineral

re-equilibration in this environment is dominated by dis-

solution-reprecipitation, with rates much faster than diffu-

sion even on a laboratory time-scale (e.g., Martin and Fyfe

1970; Putnis 2009; King et al. 2010; Raufaste et al. 2011).

Upon dissolution, elements that are released into the fluid

create an elemental reservoir, from which some elements

are removed during the precipitation of new phases and

some remain in solution. The retention of elements in the

fluid phase is evident, for example, in Fe- and Mn-rich

sediment pore water from serpentinite mud volcanoes

in the Mariana forearc (Hulme et al. 2010). Although

dissolution-reprecipitation will be the main process of re-

equilibration in a fluid environment, a variety of experi-

mental studies have shown that grain boundary diffusion is

significantly enhanced in the presence of water (e.g., Yund

1997). As water (H2O molecules or dissociated as H? and

OH-) is incompatible with olivine, it is likely that it

strongly segregates into grain boundaries, with repercus-

sions for diffusion mechanisms and rates of element

transport. The exact rates of water diffusion along grain

boundaries or dislocations are unknown, especially in the

range of temperatures and pressures where serpentinization

takes place. Recent experimental investigations into rates

of H grain boundary diffusion in olivine suggest that it is

only marginally faster at high temperature ([1,000�C)

compared with bulk H diffusion (Demouchy 2010). How-

ever, the diffusion of H along (sub)grain boundaries would

become increasingly important as temperatures decrease

and thus bulk diffusion slows. Hence, we propose that the

striped zoning in olivine originates from a complex inter-

play between the fluid-mediated replacement reaction of

olivine by antigorite. This reaction releases elements,

which are then partitioned into the olivine crystal along the

subgrain boundaries from a fluid reservoir at conditions

otherwise inadequate for substantial bulk diffusion to

occur.

The insufficiency of bulk diffusion is exemplified by the

extent of Fe enrichment observed at contacts with antig-

orite, which create marginal Fe-rich halos protruding into

the bulk olivine (Fig. 3b). To produce these halos via bulk

diffusion, a distance of *1 lm at a pressure of 500 MPa

would take *2 Myr at 400�C and *10,000 years at 500�C

using Eq. 1 (Dohmen and Chakraborty 2007). Although

temperatures of 400�C and more can be reached over a

metamorphic cycle of 10 Myr, as in regionally metamor-

phosed Tethyan ophiolites in the Alps (Evans 2010), it is

the supply, dwell time and composition of the fluid that

will control the metamorphic reaction rate (e.g., Baxter

2003; Jamtveit and Austrheim 2010; Putnis and Austrheim

2010) and as a consequence the degree of intracrystalline

modification along olivine subgrain boundaries.

Compositional adjustments of olivine

during serpentinization

Major element exchange of Fe and Mg

In the classical treatment of a metamorphic system, equi-

librium phase relationships are commonly displayed in

pressure-temperature-composition (P–T–X) phase diagrams

or in pseudosections that illustrate the stability fields of

different equilibrium mineral assemblages for a single bulk-

rock composition (e.g., Spear 1993; Bucher and Frey 2002).

Reactions encompassing solid-solution minerals are com-

monly written to conserve oxygen (i.e., no oxidation of

metals such as Fe) and are illustrated by isobaric tempera-

ture-composition (T–X) or isothermic pressure-composition

(P–X) phase loops. Such phase loops represent system-wide

equilibrium behavior and require reactant and product

minerals to concordantly adjust their composition to con-

serve the exchange coefficient KD. Kunugiza (1982) used

such a T–X phase loop diagram to argue that Fe-enriched

olivine in antigorite serpentinites from the Ryumon peri-

dotite, Japan, was the result of equilibrium exchange at

400–500�C. Furthermore, Evans (2010) recently proposed

that during high-temperature antigorite serpentinization

(*400–600�C), residual olivine can contemporaneously

adjust its composition via diffusion to adhere to the Fe-Mg

exchange coefficient

KAtg=OlD ðFe�MgÞ ¼

XFe=Mg

� Atg

Fe=Mgð ÞOl ð3Þ

(not considering Fe3? in antigorite), leading to the pre-

cipitation of Mg-rich serpentine and Fe enrichment of the

residual olivine. Evans (2010) argues that at low temper-

atures (50–300�C), lizardite serpentinization cannot obey

Contrib Mineral Petrol

123

the classic model due to the sluggish Fe-Mg bulk diffusion

and hence olivine fails to adjust compositionally. Thus, the

inability of Fe to partition back into olivine results in a

supersaturation of the fluid with respect to a Fe-bearing

phase, typically magnetite (e.g., Bach et al. 2006).

A critical validation used in phase equilibrium experi-

ments is that the system-wide equilibrium attained between

phases is reversible via elemental exchange. By comparing

reassessed experimental data of Fe-Mg exchange between

garnet and clinopyroxene (Pattison and Newton 1989) with

diffusional exchange, Pattison (1994) concluded that

equilibration in the presence of a fluid phase occurs via

dissolution-reprecipitation. Thus, true dynamic equilibrium

between reactant and product solid phases, where an

equilibrium KD can be applied, would be achieved if the

equilibrium mechanism is solid-state diffusion. This has

profound implications for the way thermodynamic data are

extracted from experiments and subsequently applied to

metamorphic systems. As recently pointed out by Putnis

and Austrheim (2010), when a fluid is involved in a

metamorphic reaction and the main re-equilibration

mechanism is dissolution-reprecipitation, the interpretation

of microstructures and phase assemblage in terms of a

system-wide metamorphic equilibrium paragenesis, hence

the assumption of an equilibrium KD between two phases,

becomes problematic. A ‘frozen-in’ partial replacement of

olivine by antigorite does not imply that the parent and

product phase coexist or that they are in system-wide

equilibrium unless both reactant and product phases are

recrystallizing, e.g., during prograde metamorphism.

However, if high-diffusivity pathways are present during

replacement reactions where the reactant dissolves without

recrystallizing, the system can locally obtain an equilib-

rium distribution of elements via diffusional exchange

between the two phases.

It is a logical progression that the striped zoning is an

attempt of the system to achieve element exchange equi-

librium that is restricted to a local scale along subgrain

boundaries where diffusion is enhanced. The overall sys-

tem, however, is re-equilibrating with infiltrating fluid via

dissolution of the bulk olivine, which fails to readjust

compositionally via diffusion, and precipitation of antig-

orite. The coalescence of the two types of zoning

(enrichment and depletion) toward a local elemental

exchange equilibrium between antigorite and olivine

becomes especially apparent when illustrated in a Rooze-

boom diagram (Fig. 11). If an ideal partition coefficient is

considered, as defined by the KAtg=OlD ðFe�MgÞ isocurve

derived from Eq. 3, it can be seen that the average

Oliv

ine

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.100.00

0.05

0.10

0.15

0.20

0.25

0.30

XF

e

XAntigorite

Leka Ophiolite Complex (LOC)Fe-depletion (n=65)avg. Fe-depletion

Fe-enrichment (n=65)avg. Fe-enrichment

Feragen Ultramafic Body (FUB)Fe-enrichment (n=45)avg. Fe-enrichment

0.01

ideal KD isocurve

polynomial KD isocurve

initial olivine(LOC)

initial olivine(FUB)

towards exchange equilibrium

Fe

K D=0.30

Fig. 11 Roozeboom diagram of Fe-Mg exchange between antigorite

and olivine along subgrain boundaries resulting in Fe-enriched or

Fe-depleted striped zoning, respectively. Both the LOC and the FUB are

displayed. As antigorite is a result of olivine hydration, initial olivine

compositions were taken from the bulk olivine (i.e., olivine areas

between individual Fe-enriched or Fe-depleted zones) composition and

plotted on the y-axis, indicated by stars, as no antigorite would have

been present. All zones whether enriched or depleted in Fe converge

toward Fe-Mg exchange equilibrium. An ideal KAtg=OlD ðFe�MgÞ

isocurve of 0.3 and a polynomial isocurve to account for the dependence

of the KAtg=OlD ðFe�MgÞ on XFe of olivine (Evans pers. comm.) are

shown. Fe-enriched zones of the LOC are in good agreement with both

ideal and polynomial isocurves, whereas Fe-depleted zones of the LOC

and Fe-enriched zones of the FUB likely display a ‘frozen-in’ trend

toward exchange equilibrium

Contrib Mineral Petrol

123

composition of the Fe-enriched and Fe-depleted zones

converges toward a KAtg=OlD value of *0.3–0.4 ( �K

Atg=OlD =

0.28 ± 0.03 (Fe enrichment); 0.37 ± 0.08 (Fe depletion)).

However, recent advances in understanding the equivalent

Fe-Mg partitioning in prograde metamorphosed meta-per-

idotites suggest that partitioning is more accurately repre-

sented by the polynomial KD isocurve in Fig. 11 (Evans

2010, Evans pers. comm.). This polynomial fit takes into

account that the KAtg=OlD ðFe�MgÞ is a function of XFe of

olivine. The average composition of the Fe-enriched zones

and associated antigorite from the LOC plot in the imme-

diate vicinity of the point where the ideal and polynomial

KD isocurves cross, implying Fe-Mg exchange equilibrium

has been obtained. In contrast, at the composition of the

LOC Fe-depleted zones, the ideal and polynomial isocur-

ves do not coincide with one another. Some of the points

plot on the ideal KD isocurve; however, it can be inferred

from the graph that there is a progressive trend toward the

exchange equilibrium depicted by the polynomial isocurve.

Caution must be taken when considering this trend because

it is not clear which composition of the antigorite is the true

equilibrium value leading to a possibility of excess scatter

in the antigorite composition data. Furthermore, one has to

question over which spatial range exchange equilibrium

was achieved, especially in cases of local hydration.

Despite this, bulk olivine compositions obtained in the

immediate vicinity of the zonings do not shift in compo-

sition from the initial value and thus remain out of equi-

librium with the associated antigorite, displaying no

evolution toward KAtg=OlD ðFe�MgÞ. Compositions of the Fe

enrichment in the FUB and associated antigorite are also

displayed in the Roozeboom diagram (Fig. 11), showing

the same evolution toward element exchange equilibrium

as the LOC Fe depletion. We are lacking an adequate

solid-solution model for antigorite and olivine in order to

evaluate any temperature dependence or ‘frozen-in’ com-

position trends toward exchange equilibrium. Nevertheless,

even if we could describe the Fe-Mg antigorite and olivine

solid-solution, phase equilibrium considerations would

view the system as a compositional readjustment via dif-

fusion or full recrystallization of reactant and product

phase following a T–X phase loop and could not account

for re-equilibration via dissolution alone.

Both types of zoning appear in the same complex (LOC)

within a range of less than a few hundred meters (depen-

dent on the scale of sampling, real spatial correlation could

be much smaller); thus large differences, for example

varying temperatures, during the serpentinization event that

could lead to different types of zoning are not likely to

explain the chemical difference in Fe enrichment and Fe

depletion. Although determining the temperature is not

trivial, estimates based on the general occurrence of

antigorite and brucite within the LOC indicate that

hydration and evolution of both types of zoning probably

occurred between *300 and 400�C (e.g., Fig. 5 in Evans

2004), in agreement with previous temperature estimates

(Bucher-Nurminen 1991). It is more likely that the chem-

ical environment and chemical potentials within the fluid

control the evolution of the striped zoning. The primary

difference between the two types of zoning is their spatial

correlation to orthopyroxene-bearing domains (Fig. 1b)

and the presence or absence of magnetite (Fig. 2). Hydra-

tion of orthopyroxene can lead to a net loss of silica (e.g.,

Bach et al. 2006; Putnis and Austrheim 2010) under open

system conditions during an isovolumetric serpentinization

reaction

4MgSiO3 þ 2Hþ þ H2O! Mg3Si2O5ðOHÞ4 þMg2þ

þ 2SiO2ðaqÞ ð4Þ

(approximate molar volumes are 25 and 100 cm3 mol-1 for

enstatite and serpentine, respectively). Silica loss to the

fluid is supported by microstructural observations of

undisturbed clinopyroxene exsolution lamellae and only

minor talc formation in hydrated orthopyroxene in the

FUB, indicating that orthopyroxene serpentinization in the

investigated samples occurred isovolumetrically (Viti et al.

2005). However, if orthopyroxene hydrates to serpentine

and talc

6MgSiO3 þ 3H2O! Mg3Si2O5ðOHÞ4þMg3Si4O10ðOHÞ2 ð5Þ

the subsequent reaction of talc to serpentine can also

release silica into solution

Mg3Si4O10ðOHÞ2 þ H2O! Mg3Si2O5ðOHÞ4 þ 2SiO2ðaqÞ:

ð6Þ

Microstructural relationships between talc and serpentine

in the LOC suggest that initially formed talc was replaced

by serpentine releasing SiO2(aq) (Iyer et al. 2008a). Frost

and Beard (2007) argued that the activity of SiO2(aq)

aSiO2ðaqÞ

� controls whether magnetite is absent (high

aSiO2ðaqÞ) or present (low aSiO2ðaqÞ), based on theoretical

phase petrology considerations (Fig. 12). Indeed, recent

natural observations provide evidence for the critical role

of low silica activity on magnetite formation and hydrogen

production (e.g., Klein et al. 2009). Further support is

provided by experimental observations of Fe-silica

interactions in solutions, which show that elevated

concentrations of SiO2(aq) inhibit Fe-Fe polymerization

and solid-phase formation increasing the Fe stability in

solution, particularly for Fe2? (Doelsch et al. 2002;

Pokrovski et al. 2003). If aSiO2ðaqÞ is sufficiently high (i.e.,

low Ol/Opx mass ratio), olivine may hydrate directly to

Contrib Mineral Petrol

123

antigorite, without the co-evolution of brucite (Frost and

Beard 2007). Indeed, adjacent to orthopyroxene-bearing

domains, olivine hydration leads to the direct formation of

antigorite without brucite and with very minor to no

magnetite. The crucial reaction that defines this is (Frost

and Beard 2007)

2Fe2þ3 Si2O5ðOHÞ4ðin atgÞ ! 2Fe3O4 þ 4SiO2ðaqÞ

þ 2H2Oþ 2H2 ð7Þ

implying that with an increase in SiO2(aq) chemical

potential the formation of high-Fe antigorite is promoted,

i.e., it leads to an increase in the Fe2?Mg-1 exchange

potential Dl Fe2þMg�1

� �(Evans 2008). Sluggish bulk

diffusion under the given temperature conditions (300–

400�C) would normally prevent olivine from composi-

tional readjustment to adhere to the Dl Fe2þMg�1

� �, but

due to the legacy of crystal-plastic deformation in the

creation of high-diffusivity pathways, the drive toward

Fe2?Mg-1 exchange equilibrium between antigorite and

olivine causes the enrichment of olivine along the subgrain

boundaries. Strictly speaking, it is the limitation of diffu-

sivity that establishes local Fe2?Mg-1 exchange equilibria,

where only diffusion into subgrain boundaries is fast

enough. The development of the Fe-enriched zoning is

restricted to areas dominated by the supply of silica, hence

areas that have an elevatedaSiO2ðaqÞ . This dependence on

silica also explains the differences in the magnitude of

enrichment observed between the LOC and the FUB. In the

LOC swarms of meta-orthopyroxenite dikes (Fig. 1b) act

as a substantial silica reservoir, creating a steep gradient in

chemical potential lSiO2ðaqÞ

� into the surrounding meta-

dunite (cf. Klein et al. 2009), whereas in the meta-harz-

burgites of the FUB, the silica is intrinsically limited by the

modal orthopyroxene abundance (Fig. 3a). In both cases,

however, instantaneous consumption of silica during oliv-

ine hydration restricts silica mobility to areas around the

orthopyroxene-dominated portions of the rock. This is

especially highlighted by the lack of Fe-enriched olivine in

the meta-dunite in immediate contact with the meta-harz-

burgite layer (Fig. 3b). Furthermore, Fe enrichment

reflected in olivine domains in meta-harzburgites

(XFe = 0.12–0.14 (Fe-enriched zones); XFe & 0.10 (bulk))

from the Mariana forearc seamounts, western Pacific

(Murata et al. 2009), is enriched to the same order of

magnitude as those in the FUB meta-harzburgite.

In contrast to the Fe-enriched scenario, communication

between orthopyroxene-bearing portions and the sur-

rounding olivine-bearing portions of the rock is lacking and

thus olivine hydration is isochemical for Si and the aSiO2ðaqÞ

is decreased because Si is consumed by olivine serpentini-

zation. At lower aSiO2ðaqÞ magnetite is stable (Frost and Beard

2007) and in conjunction with the Dl Fe2þMg�1

� �imposed

on the system by the presence of olivine, serpentinization

forms a high-Mg antigorite ± brucite with the co-evolution

of magnetite. The precipitation of magnetite from the fluid

acts as a significant Fe ‘sink’, creating a substantial gradient

in the chemical potential of Fe2? within the system. The

antigorite compositions in the case of the Fe-depleted

zoning determined here ( �XFe = 0.04) are consistent with the

global compilation of serpentine compositions from mantle

peridotite (Evans 2008). The formation of such Mg-rich

serpentines in our samples is in agreement with the

hypothesis of Evans (2008) that this is an attempt of the

system to achieve the environmental Fe2?Mg-1 exchange

equilibrium imposed upon it. Hence, even though the

overall system re-equilibration will be governed by the bulk

dissolution of olivine yielding higher-Mg antigorite and

magnetite, local compositional adjustments along olivine

subgrain boundaries result in the depletion of Fe.

At this point, it is relevant to consider the role of the

fluid in the element exchange process. Although there are

differences during seafloor weathering of peridotites (Snow

and Dick 1995), most investigations into peridotite alter-

ation conclude that it is nearly isochemical for all major

aSiO2 internally set (Ol hydration)

Mag + Brc

XMg

Fe-rich Srp w/o Mag Fe-enriched Ol zoning

Fe-poor Srp + Mag Fe-depleted Ol zoning

Brc + Srp

Brc

Mag + Srp

Srp

+Ol, H2O; f = fixedH2O

Mag

log

aS

iO2

high aSiO2 (SiO2 addition from Opx hydration)

Fig. 12 XMg-aSiO2 diagram (isothermal, isobaric) illustrating the

possible associations that control the generation of magnetite during

serpentinization and its dependence on silica activity (modified from

Frost and Beard 2007). Increased silica activity due to Opx hydration

facilitates olivine hydration to Fe-rich serpentine without (w/o)

magnetite (star). Under these conditions, increased solid-state diffu-

sion along subgrain boundaries enriches the olivine in Fe to adhere to

the environmental Fe-Mg exchange equilibrium. Serpentinization of

olivine with limited or no communication to hydrating Opx domains

results in low (internally set) silica activity and magnetite formation

(hexagons). In this scenario, Fe depletion occurs along olivine

subgrain boundaries because Fe is incorporated into abundant

magnetite

Contrib Mineral Petrol

123

components except for the addition of H2O (and CO2) (e.g.,

Coleman and Keith 1971; Shervais et al. 2005). There are

indications of major element mobility over larger areas in

LOC but these are restricted to Ca originating from

clinopyroxene breakdown to form rodingite assemblages

(grossular ? vesuvianite ? diopside) (Austrheim and

Prestvik 2008) or secondary diopside within shear zones

that have no immediate association with primary pyroxene

grains. In the investigated samples, there was no evidence

for major element metasomatism, e.g., a Fe contribution

from the meta-orthopyroxenite dikes. Nonetheless, intrinsic

silica transport did occur in the vicinity of hydrating

orthopyroxene grains but was immediately consumed by

the direct formation of antigorite from olivine. Hence,

overall the fluid can be regarded as having played a dom-

inantly catalytic role, functioning as an elemental reservoir

for the elements released from the olivine to exchange,

rather than transporting elements over large distances.

Minor element exchange of Mn and Ni

Enhanced diffusional element exchange due to pathways

created by crystal-plastic deformation is not only reflected

in the partitioning of major elements but also of minor

elements, especially Mn and Ni. During olivine crystalli-

zation from a melt, Mn (depending on the oxidation state)

and Ni commonly partition into olivine (e.g., Takahashi

1978). Under dry bulk diffusion conditions at high tem-

peratures in olivine, diffusion rates and activation energies

for the divalent cations Mn and Ni have been found to be

very close to those of Fe-Mg (Petry et al. 2004; Holzapfel

et al. 2007). It is interesting to note that under conditions of

serpentinization and with regard to elemental partitioning,

Mn is strongly enriched in olivine regardless of Fe

enrichment or depletion. Mn has limited compatibility in

serpentine and is mobile during serpentinization, as

exemplified by elevated Mn concentrations at serpentinite

mud volcanoes across the Mariana forearc (Hulme et al.

2010). To partition into the olivine Mn must remain

divalent after release into solution during initial olivine

dissolution. Mn2? is known to be stable over a wide range

of pH and redox conditions (e.g., Crerar and Barnes 1974).

Thus, if element transport is limited, Mn can partition back

into olivine. In general, Mn enrichment and Ni depletion in

relict olivine grains from serpentinized meta-peridotite are

in agreement with previous observations (Trommsdorff and

Evans 1974). However, Ni is homogeneously and more

strongly depleted when related to Fe-depleted zoning,

whereas in association with Fe-enriched zoning, Ni

depletion is minor and only heterogeneously distributed. In

the case of the Fe-depleted zoning, the greater Ni depletion

suggests that Ni-bearing alloys such as awaruite (Ni3Fe)

may be present, although they were not explicitly identified

within the studied LOC samples. The formation of these

alloys is favored in an environment where oxidation of

Fe2? to Fe3? by water causes the generation of hydrogen

leading to strongly reducing conditions (e.g., Klein and

Bach 2009).

Serpentine along olivine subgrain boundaries: evidence

for molecular water diffusion?

The importance of the high-diffusivity pathways extends to

more than just element exchange, but also water (as H2O

molecules and potentially dissociated to H? and OH-) has

been proposed to be channeled through subgrain bound-

aries into olivine (Boudier et al. 2010). Further support is

provided by the observations of serpentine along olivine

subgrain boundaries displayed in Figs. 8 and 9, strength-

ening the argument that water is mobile along dislocations

and can ingress along olivine dislocation walls during

serpentinization.

Even though the development of serpentine precipitates

along dislocation walls is restricted to zones of Fe deple-

tion, there are indications that externally nucleated antig-

orite can preferentially penetrate along Fe-enriched

subgrain boundaries as observed in the FUB meta-harz-

burgites (Fig. 5e) and in Mariana forearc seamounts (Mu-

rata et al. 2009). Formation of a solid phase at a dislocation

wall would be favorable because the Gibbs free energy

barrier for nucleation at defects is lower than that in a

defect-free bulk crystal (Putnis 1992). Common examples

of heterogeneous nucleation at defects sites are provided by

high-temperature, solid-state phase transformations, such

as the nucleation of wadsleyite and ringwoodite at olivine

grain boundaries or intracrystalline shear-induced stacking

faults (Kerschhofer et al. 1996). There are also a number of

examples where the permeation of fluids along dislocations

leads to the nucleation of fluid-filled bubbles (e.g., Bakker

and Jansen 1994) or where fluid permeation along nano-

channels or nano-porosity (essentially crystal defects)

nucleates a new, more stable phase (e.g., Harlov et al.

2005).

A first insight into the origin of the individual serpentine

precipitates is provided by the dihedral angle h of a pre-

cipitate along a grain boundary. This angle controls the

connectivity of the fluid phase and hence the permeability

of the fluid-bearing rock (e.g., Watson and Brenan 1987;

Holness 2006). In the simplest form, it is defined as the

angle formed by two intersecting walls of a pore at a

junction with two solid grains and is controlled by the

relative value of the interfacial energies. If h\ 60�, the

stable geometry is an interconnected tube of fluid, whereas

at h[ 60�, isolated precipitates/pockets will form. Watson

and Brenan (1987) found a h of *65� for pure H2O at

high-angle grain boundaries in olivine, thus producing

Contrib Mineral Petrol

123

isolated fluid pockets. Although we are lacking a repre-

sentative analysis of the dihedral angle of the observed

serpentine precipitates, first simple measurements point to

h & 70� (Fig. 8c), in agreement with the occurrence of

isolated precipitates. The dihedral angle is also sensitive to

pressure and temperature conditions, so a continuous film

at some conditions might become isolated at lower pres-

sures and temperatures. However, the structure of the dis-

location wall between precipitates (Fig. 8c) implies that

nucleation along the subgrain boundary occurred as

pockets.

The precipitate pseudo-periodicity is probably due to

spatial relationships of the chemical potentials generated

by nucleation, where the local depletion of elements due to

nearby nucleation creates quasi-random nucleation points

along the subgrain boundaries. Additionally, TEM obser-

vations do not indicate a volume increase in relation to the

serpentine precipitates, expressed as an absence of strain

contrast around the precipitates or preferential fracturing

along the dislocation walls. For this to be the case, the

isovolumetric replacement of olivine by serpentine requires

that Mg is lost from the system. The serpentinization

reaction under constant volume resulting in Mg loss can be

written as (approximate molar volumes are 50 and 100 cm3

mol-1 for forsterite and serpentine, respectively)

2Mg2SiO4 þ 2Hþ þ H2O! Mg3½Si2O5�ðOHÞ4 þMg2þ:

ð8Þ

In this case, Mg could diffuse away from the serpentini-

zation site along the subgrain boundary. The excess Mg

released via the reaction in Eq. 8 would aid chemical

adjustments along the subgrain boundary by compensating

for perturbations in the Fe-Mg interdiffusion associated

with the loss of Fe. This would partially negate the

requirement of Mg interdiffusion from the fluid reservoir.

The exact mechanism for the formation of these inclusions

requires further research due to the limited knowledge of

species mobility along dislocation arrays and grain

boundaries as well as hydrous nano-phase nucleation

especially along defects. However, the lack of an evident

diffusion profile at the olivine-antigorite interface (Fig. 9c)

suggests that reaction and growth of the antigorite precip-

itate might have progressed via a fluid-mediated interface-

coupled dissolution-reprecipitation mechanism, producing

a chemically sharp interface.

The role of a pre-existing olivine fabric

during serpentinization

Observations by Boudier et al. (2010) suggest that the

plastic deformation–induced olivine fabric can control the

evolution of the serpentine network. Specifically, from

their observations, the authors conclude that antigorite

serpentinization is potentially initiated along the (100)ol

plane, which is the locus of edge dislocation pile-ups that

results in the formation of subgrain boundaries and is

subsequently followed by parting along the (010)ol plane.

In our samples, olivine subgrain boundaries act as high-

diffusivity pathways during serpentinization. There are also

considerable indications, in the form of serpentine precip-

itates, that water was able to ingress along these intra-

crystalline imperfections. At the nanometer level in the

precursor stage of this microstructurally controlled process,

some of the individual serpentine precipitates display a loss

of coherency, suggesting the initiation of a preferentially

orientated replacement (Fig. 9). Figure 9 implies that

although serpentinization was initiated along the (100)ol

subgrain boundaries, parting along the (010)ol plane sub-

sequently became dominant, possibly because parting in

this direction is easier to develop as it causes minimal

disruption of the olivine crystal. Specifically, only a few of

the weaker Mg-O bonds need to be broken with minor

disturbance of the bulk atomic arrangement in comparison

with other crystal planes (e.g., de Leeuw et al. 2000).

Furthermore, natural observations of cleaved olivine also

show that preferential parting occurs along the (010)ol

plane because of its minimal fracture surface energy

(Swain and Atkinson 1978). EBSD investigations into the

role of crystallographic orientational relationships between

olivine grains and different chronological generations of

serpentine veins support the observation that serpentiniza-

tion in the LOC probably initiated along (100)ol subgrain

boundaries and was followed by further parting along

(010)ol planes (Fig. 10). Future investigations into the

impact of the role of inherited peridotite fabric during

serpentinization are needed to fully understand this pro-

cess, particularly in terms of its implications for the

anisotropy of seismic wave propagation as suggested by

Boudier et al. (2010).

Summary and conclusions

We have investigated the legacy of olivine crystal-plastic

deformation during serpentinization, which uses subgrain

boundaries composed of edge dislocations as high-diffu-

sivity pathways. Furthermore, we have gained new insights

into the crossover of two fundamentally different mecha-

nisms of mineral re-equilibration, solid-state diffusion and

dissolution-reprecipitation. Based on our observations, we

can draw certain conclusions that are summarized here:

1. Crystal-plastic deformation-induced dislocation walls

can act as high-diffusivity pathways, due to their lower

activation energy for diffusion even during low-

Contrib Mineral Petrol

123

temperature (possibly as low as 300�C), fluid-induced

mineral replacement reactions exemplified by the

replacement of olivine by antigorite. In this system,

bulk olivine cannot readjust its composition via solid-

state diffusion and instead re-equilibrates to the

chemical environment in the presence of a fluid phase

via dissolution and reprecipitation of serpentine.

However, subgrain boundaries enhance local diffu-

sional element transport, driven by the convergence to

equilibrium, to result in the development of striped

zonings.

2. Within the zones Fe was either enriched or depleted

depending on the chemical environment. Mn was

found to be strongly enriched in both types of zoning,

whereas Ni was depleted but showed a more strongly

developed depletion in relation to the Fe-depleted

zoning. We infer that the changes in Fe concentrations

are a result of the Fe2?Mg-1 exchange potential during

serpentinization of olivine, where the Fe enrichment or

depletion is controlled by the silica activity imposed

on the system by the hydration of orthopyroxene and

the role of silica activity on magnetite stability (Frost

and Beard 2007; Evans 2008). Comparison of XFe

from both types of zoning and associated antigorite

displays a convergent evolution toward equilibrium KD

values, whereas the composition of the bulk olivine

remained unaffected. Determined KAtg=OlD ðFe�MgÞ of

the LOC Fe-enriched zoning agrees well with a

polynomial isocurve taking into account that

KAtg=OlD ðFe�MgÞ is a function of the olivine compo-

sition. In contrast, FUB Fe enrichment and LOC Fe

depletion likely display a trend toward exchange

equilibrium.

3. Trails of serpentine precipitates along olivine subgrain

boundaries have been identified and when combined

with microscopic relationships between olivine orien-

tation and different generations of serpentine veins,

support the hypothesis that water can ingress along

dislocation walls (Boudier et al. 2010), potentially

playing a critical role during early hydration stages to

initiate serpentinization.

Additional investigations are required to fully under-

stand the processes occurring along intracrystalline defects

during mineral replacement reactions and the role of the

exothermic nature of serpentinization, which may help to

overcome energetic barriers to reaction. Experimental

investigations to constrain the exact rates of defect-assisted

diffusion and its competition with dissolution-reprecipita-

tion are challenging as solid-state diffusion at low tem-

perature is slow on the laboratory time-scale. However,

recent advances in thin film preparation (e.g., Watson and

Dohmen 2010) and high-resolution analytical methods

have the potential to further our understanding of these

processes. Overall, this study helps to bridge the gap

between systems where fluid-mediated mineral replace-

ment reactions are dominant and those where solid-state

diffusion governs elemental re-equilibration. In the pres-

ence of a fluid phase, re-equilibration will occur via dis-

solution-reprecipitation. However, the high-diffusivity

pathways created by intracrystalline defects, such as the

dislocation walls studied here, also allow the dissolving

crystal to adapt compositionally via solid-state diffusion to

the elemental exchange equilibrium imposed on the sys-

tem. There is ample evidence that most metamorphic

reactions in the lithosphere involve fluids (Putnis and

Austrheim 2010); therefore, the implications of this study

can have consequences for a variety of metamorphic

reactions where both processes are simultaneously ongoing

but vary in relative rates.

Acknowledgments This research was funded by the European

Commission through the Marie Curie Initial Training Network Delta-

Min (Mechanisms of Mineral Replacement Reactions) contract no.

PITN-GA-2008-215360. We thank A. Beinlich, P. Meakin, A. Putnis,

J. Mathiesen, J. Semprich, J. Hovelmann and B. Jamtveit for

numerous discussions and stimulating thoughts about the topic. The

manuscript benefited from valuable comments and suggestions given

by the reviewers B.W. Evans and F. Klein. We thank K. Iyer for

providing his thin section collection. M. Erambert and Ø. Prytz are

thanked for technical assistance. Anja Schreiber at GFZ Potsdam,

Germany, is thanked for FIB cut preparation. O. Plumper acknowl-

edges his scientific mobility opportunities at the Institut fur Miner-

alogie, University of Munster, Germany. H. Jung was supported by

the Mid-career Research Program through NRF grant funded by the

MEST (No. 3345-20100013).

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