JOURNAL OF
PETROLOGY
Journal of Petrology, 2015, Vol. 56, No. 2, 299–324
doi: 10.1093/petrology/egv001
Advance Access Publication Date: 11 February 2015
Original Article
Halogen Element and Stable Chlorine Isotope
Fractionation Caused by Fluid–Rock Interaction
(Bamble Sector, SE Norway)
Christof Kusebauch1*, Timm John1,2, Jaime D. Barnes3, Andreas Klügel4
and Håkon O. Austrheim5
1
Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149 Münster, Germany,
Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin 12249, Germany, 3Jackson School of
Geoscience, University of Texas, Austin, TX 78759, USA, 4FB-Geowissenschaften, University of Bremen, Bremen
28359, Germany and 5Department of Geoscience, University of Oslo, Oslo 0316, Norway
2
*Corresponding author. Telephone: þ49 251 83 33506. Fax: þ49 251 83 38397. E-mail: [email protected]
Received December 3, 2013; Accepted January 2, 2015
ABSTRACT
Pervasive interaction between rock and fluid produces regional-scale alteration and/or metasomatism of rock units. The middle and upper crustal rocks from the Bamble sector (SE Norway) show a
complex history of metamorphic and metasomatic events, accompanied by ore deposit formation,
which provides an excellent context for investigation of regional-scale fluid–rock interaction.
Halogens are important agents of rock alteration and ore metal transportation in such hydrothermal regimes. Here we use F, Cl, Br and I concentrations and stable chlorine isotope ratios (d37Cl) to
trace the metasomatic evolution of two gabbroic bodies and understand the interplay between
localized and pervasive fluid flow in the upper crust. An alteration sequence was sampled from
pristine gabbro to an amphibolitic shear zone, showing progressive amphibolitization and scapolitization caused by pervasive fluid ingress. Halogen concentrations and ratios suggest the evolution
of a single fluid, causing enrichment of Cl, Br and I in samples nearest the shear zone. Owing to the
differences in fluid–mineral distribution coefficients, fluid–mineral interaction resulted in either enrichment or depletion of halogens in the fluid as it reacted pervasively with the gabbro. Although
scapolite formation leads to progressive desalination of the fluid near the shear zone, increasing Cl
concentrations in amphibole near the gabbro reflect an increase in salinity towards the unaltered
gabbro owing to desiccation of the fluid. The formation of alteration minerals mainly governs
changes in the halogen chemistry of the fluid, showing that halogens do not behave fully conservatively during fluid–rock interaction. Nevertheless, the evolving fluid has a feedback on the stability
of alteration minerals such as scapolite and biotite, which form only close to the shear zone. The
Br/Cl and I/Cl ratios of the most highly altered samples are 3 10–3 and 25 10–6, respectively, and
overlap with the range of ratios measured for marine pore fluids. Remobilization of evaporites
would have produced distinctly lower halogen ratios and can be ruled out as a fluid source.
The unaltered gabbro has d37Cl values near 0% and a similar value is inferred for the infiltrating
fluid. Minimally altered samples, noted by the presence of <20 vol. % amphibole, have negative
d37Cl values (average ¼ –06 6 01%). The d37Cl values increase (up to þ 1% nearest the shear zone)
with increasing evidence of fluid–rock interaction towards the shear zone. Mixing of different Cl
reservoirs (i.e. gabbro and fluid) cannot explain the observed changes in the d37Cl values. Instead,
isotopic fractionation owing to Rayleigh distillation with a fractionation factor of 10010 can explain
the observed negative d37Cl values of the alteration sequence by the evolution of a single fluid
during fluid–rock interaction. An additional kinetic component during fractionation may have
C The Author 2015. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: [email protected]
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Journal of Petrology, 2015, Vol. 56, No. 2
enhanced the observed changes in chlorine isotope values. We speculate that these processes are
the reason for the large spread of d37Cl values found in many stable Cl studies for single reservoirs.
Key words: Bamble sector; Cl isotopes; fluid evolution; halogen ratios; Rayleigh fractionation
INTRODUCTION
Fluid–rock interaction causes element mobilization and
in some cases fractionation of their isotopes. Fluid–rock
interaction is further responsible for the formation of
economic mineral deposits and has consequences for
the petrophysical properties of the affected rocks and
the geodynamics of the related large-scale systems.
Mass transport is most efficient in channelized flow
systems, where fluid flow controls not only the redistribution but also the quantities of the elements (WilliamsJones & Heinrich, 2005; Zack & John, 2007; Ague, 2011;
Richards, 2011; Williams-Jones et al., 2012).
Mobilization of economically valuable elements (usually occurring in trace amounts in rocks), as well as regional-scale alteration or metasomatism of rock units,
also requires pervasive interaction between rock and
fluid. Field evidence suggests that fluid–rock interaction
in various geodynamic settings may occur pervasively
on regional scales, but is in fact locally concentrated
along and adjacent to veins and shear zones
(Austrheim, 1987; Bach & Frueh-Green, 2010; Engvik
et al., 2011; John et al., 2012). It is thus of importance to
understand how fluid–rock interaction occurs with respect to flux of fluid, number of fluid flow events and
potential sources of fluid. Chemical tracers (e.g. halogens) and fluid-composition indicative minerals (e.g.
amphibole, biotite, apatite) help to better understand
fluid flow patterns and reaction front propagation leading to the formation of alteration minerals.
Furthermore, they provide information about fluid
pulses and their origins.
Halogens (F, Cl, Br, I) can be used to characterize
these fluids as they are major anions in metasomatic
fluids, but only minor or trace elements in the bulk solid
(e.g. Kullerud, 1996; Kullerud & Erambert, 1999; Markl &
Bucher, 1998; Svensen et al., 1999; John et al., 2011).
Therefore, halogen concentrations and ratios in mineral
phases that formed during fluid–rock interaction reflect
fluid characteristics, rather than features inherited from
the pristine solid. The strongly different ionic radii of
halogens (F < Cl < Br < I) lead to different partitioning
between minerals and fluid. As F– and Cl– are relatively
small, they are capable of replacing OH– in hydrous
minerals (e.g. biotite, amphibole, apatite). In contrast, I–
and Br– are larger in size and are less compatible in silicate minerals. Halogen ratios are characteristic of different halogen reservoirs such as pore fluids, evaporites,
mantle fluids and sedimentary fluids (e.g. John et al.,
2011; Kendrick et al., 2011) and can be used to trace the
source of the metasomatic fluids derived from these
reservoirs. Owing to the different partitioning behaviour
of halogens, their ratios (e.g. Br/Cl, I/Cl) are potentially a
powerful tool to investigate fluid–rock reactions.
Chlorine isotopes provide another means to trace
fluid–rock interactions. As Cl concentrations in fluids
are relatively high compared with those in silicates,
a recorded isotopic signal probably reflects fluid
source characteristics rather than inherited silicate characteristics. Chlorine isotope compositions, given in
standard d notation, where d37Cl ¼ {[(37Cl/35Cl)sample –
(37Cl/35Cl)SMOC]/(37Cl/35Cl)SMOC} 1000 and SMOC is
Standard Mean Ocean Chloride, for most geologically
relevant reservoirs vary between –8 and þ3% (e.g.
Barnes & Sharp, 2006; John et al., 2010; Sharp et al.,
2013, and references therein). The limited range and
overlapping values for different rock reservoirs hinders
the application of Cl isotopes as a source tracer for unaltered rock. Fractionation of Cl isotopes occurs mainly
during fluid–rock interaction (e.g. Eggenkamp et al.,
1995; Markl et al., 1997; Gleeson & Smith, 2009;
Amundson et al., 2012; Barnes & Cisneros, 2012;
Selverstone & Sharp, 2013) and is indicated by hydrothermally altered rocks. Accordingly, changing Cl isotope ratios within one suite of altered rocks reflect
changes in the fluid.
The Bamble sector (SE Norway) is a well-studied
high-grade metamorphic terrane (Bingen et al., 2005),
which experienced metasomatic alteration during the
Sveconorwegian orogeny (c. 1090–1040 Ma) (e.g.
Engvik et al., 2011). Extensive fluid infiltration during
this event led to several mineral replacement reactions
(Nijland & Touret, 2001; Harlov et al., 2002; Engvik et al.,
2008, 2009; Engvik & Austrheim, 2010), almost all occurring in intrusive and metamorphic rocks to an extent
that the mineral assemblage of the protolith cannot be
identified. Metasomatism is linked to the formation of
several iron-oxide and phosphate ore deposits in the region (Nijland et al., 1998b, and references therein).
Gabbros of early Sveconorwegian age are affected by
this fluid infiltration and exhibit various metasomatic
features. These include a penetrative amphibolitization
adjacent to shear zones, a scapolitization event altering
pristine plagioclase to scapolite, and an albitization reaction front replacing plagioclase and scapolite with
albite (Nijland et al., 1998a; Engvik et al., 2008, 2011).
The later alteration also shows formation of prehnite,
pumpellyite and analcime, indicating a possible lowgrade metamorphism of unknown age. Although the
size of the alteration zones varies from centimetres to
tens of metres in different outcrops, the related reaction
sequences are always the same (Engvik et al., 2011).
Although the metamorphic history of this high-grade
terrane is largely understood (Bingen et al., 2008b), the
Journal of Petrology, 2015, Vol. 56, No. 2
impact of metasomatism on the related metamorphic
reactions is still poorly constrained. Here, we use halogen concentrations, ratios and chlorine isotope ratios to
investigate the fluid history of the Bamble sector. We
focus on the origin and evolution of the fluids causing
the major alteration (i.e. amphibolitization and scapolitization) observed in gabbroic bodies. Furthermore, possible mechanisms leading to fractionation of chlorine
isotopes are discussed and implications formulated.
GEOLOGICAL SETTING
The Bamble sector (SW Norway), part of the
Precambrian basement of southern Norway, consists of
amphibolite- to granulite-facies metamorphosed rocks.
Together with the Kongsberg sector, which shows similar lithologies, the Bamble sector (Fig. 1a) is interpreted
as the collision zone between the Telemarkia terrane
(western Norway) and the Iddefjorden terrane (eastern
Sweden) (Bingen et al., 2005, 2008b). Most lithologies in
the Bamble sector are ortho- and paragneisses formed
during the Sveconorwegian orogeny (Fig. 1b). The oldest orthogneisses of the sector formed between 157
and 152 Ga. Embedded in the SW–NE-striking metasedimentary rocks are gabbro-tonalitic and charnockitic
rocks intruded between 1200 and 1150 Ma (Ransom
et al., 1995). Granulite-facies rocks have been dated at
1150–1125 Ma by monazite U–Pb dating (Cosca et al.,
1998; Bingen et al., 2008a). Estimated peak metamorphic conditions are 07 GPa and 800 C, inferred
from garnet–pyroxene thermobarometry (Harlov, 2000).
A younger amphibolite-facies overprint (1105–1080 Ma)
is observed in many metapelites and amphibolites
(Cosca et al., 1998).
High fluid flux events metasomatized most metamorphic and intrusive rocks during the late
Sveconorwegian (1090–1040 Ma) (Engvik et al., 2011).
Scapolitization and albitization are widespread alteration features in many lithologies (Nijland et al., 1998b).
Notably, gabbroic bodies display two or more distinguishable alteration fronts (Engvik et al., 2011).
Scapolitization is observed in many gabbroic bodies
and occurs as reaction fronts starting from shear zones
and veins. Pristine gabbro in these altered bodies exists
as lenses between shear zone networks. Temperature
and pressure conditions for the scapolitization of the
Bamble sector are estimated to be 600–700 C and
02–04 GPa (Nijland & Touret, 2001; Engvik et al., 2011).
Rb–Sr ages, determined from phlogopite, date the scapolitization at 1040–1068 Ma and therefore at a late
stage of the Sveconorwegian orogenesis (Engvik et al.,
2011).
Albitites occur as larger bodies (3 km length,
100–300 m thickness) in mafic and felsic rocks (Engvik
et al., 2008) on a regional scale. The albitites occur as
products of metasomatism and are formed as alteration
rims along cracks or as penetrating veins in host-rocks.
Areas with a high density of veins underwent almost
complete albitization, and only minor remnants of the
301
host-rock can be found (Engvik et al., 2008). Nijland &
Touret (2001) estimated the temperature for the albitization in the Bamble area to be at 350–450 C.
Assemblages of prehnite, pumpellyite and analcime
found coexisting with the albite suggest even lower
temperatures for the last alteration event. Although no
age data for the albitization in the Bamble sector are
available, a Sveconorwegian age can be assumed as
albitization in the Modum complex of the Kongsberg
terrane (also SE Norway), which has a similar metamorphic history, has an age of 1080 Ma based on U–Pb
chronology (Munz et al., 1994).
Fluid inclusions from granulite-facies rocks are usually CO2 rich, whereas inclusions from amphibolitefacies rocks show a more aqueous or saline (from 30 wt
% NaCl to halite saturation) composition (Hoefs &
Touret, 1975; Touret, 1985). Both CO2 and NaCl strongly
influence the water activity of the fluid as well as the
stabilities of coexisting minerals indicative of metamorphic facies, which is of importance as changes in
the fluid composition lead to changes in the mineral assemblage of metamorphic rocks.
The Bamble sector contains several important ironore and apatite deposits that were mined in past centuries (Nijland et al., 1998b) and are interpreted to originate from hydrothermal interaction of host-rocks with
metasomatic fluids.
FIELD RELATIONS AND SAMPLE DESCRIPTION
Fluid flow in the Bamble area reveals both a pervasive
and a channelized pattern depending on the scale of observation. Herein, the term pervasive is used to describe
a penetrative fluid flow causing alteration (fully or
partly) in a rock volume of interest. The distinction between pervasive and channelized fluid flow is not necessarily mutually exclusive, as fluid flow that looks
pervasive on a large scale can be channelized on a
smaller scale. Fluid flow in the Bamble area appears
pervasive on a regional scale (100 km by 20 km) as
hydrothermal alteration affects almost every rock type.
On the outcrop scale (10–1000 m) the fluid flow pattern
becomes a fluid network with a high density of shear
zones and veins of variable size in which fluid flow is
localized. On a decimetre to centimetre scale these
localized fluids pervasively infiltrate the host-rock and
cause alteration starting at the wall-rock next to the
main fluid pathways (Fig. 2a–c). On a microscopic scale,
no filled veins but rather fractures providing possible
pathways for the infiltrating fluid are observed.
Additionally, pervasive fluid flow along grain boundaries is inferred from altered phases that are not connected to fractures. To understand the interplay
between localized (in shear zones) and pervasive fluid
flow (decimetre to centimetre scale) in the Bamble area,
two different metagabbros were sampled.
The Valberg metagabbro is located 2 km north of
Kragerø and is 15 km in diameter (Fig. 1b). The complex consists mainly of altered gabbro containing shear
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Journal of Petrology, 2015, Vol. 56, No. 2
Fig. 1. (a) Regional map showing the location of the Bamble area (B); (b) geological map of the northeastern part of the Bamble
area showing the sample sites (stars) near Kragerø (Valberg) and at the shore of Langøy [from Norwegian Geological Survey
(NGU) databases].
zones and veins that range from centimetres to metres
in thickness. Lenses of pristine olivine–orthopyroxene–clinopyroxene gabbro vary from 05 to 30 m in length
and thickness (Fig. 2a and b). The primary gabbro is pervasively altered from the shear zones towards the centre
of the lenses. Metre-thick veins and shear zones consist
of coarse-grained albite and amphibole, whereas smaller
veins mainly consist of pargasitic amphibole. An alteration sequence was sampled from the Valberg metagabbro consisting of five samples showing different
alteration stages (pristine gabbro, amphibolitized gabbro, scapolitized gabbro, shear zone) according to their
distance from the shear zone (Fig. 2b).
The Langøy metagabbro is exposed over >1 km2
along the coastline. Similar to the Valberg metagabbro,
the protolith gabbro at Langøy was continuously
amphibolitized and scapolitized, with the highest alteration always adjacent to shear zones and veins.
Compared with Valberg, the Langøy samples show a
lesser extent of deformation and are highly localized in
shear zones. Larger veins and shear zones contain
almost monomineralic amphibolite, whereas smaller
veins are usually filled with scapolite. To investigate the
alteration sequence of the Langøy metagabbro in detail,
a 30 cm 20 cm 10 cm sized block was sampled
(Fig. 2c). The block shows an alteration sequence,
including scapolitization and amphibolitization, with an
increasing degree of alteration towards the shear zone.
To gain information on small-scale changes within the
alteration sequence, the block was first sliced perpendicular to the shear zone to prepare thin sections.
Afterwards the block was sliced into 15 pieces (plus one
pristine gabbro sample for comparison) of 15–2 cm
thickness parallel to the shear zone for whole-rock and
mineral separates analyses.
ANALYTICAL METHODS
Halogen extraction
Sliced rock samples were washed in ultra-pure water
(18 MX), then dried and crushed with a hardened steel
hammer. One aliquot of crushed rock sample was
Journal of Petrology, 2015, Vol. 56, No. 2
303
Fig. 2. Field relations and location of sample specimens: (a) metre-sized blocks of unaltered gabbro (in grey) surrounded by
‘augen’-like shear-zone structures and an albitic vein (grey lines), Valberg; (b) metre-sized block of unaltered gabbro (grey) with
increasing alteration towards amphibolitic shear zones; circles indicate sample positions in the Valberg outcrop; (c) block (27 cm in
length) of metagabbro from the second sampling location on the island of Langøy containing the whole alteration sequence including amphibolitization, scapolitization and the amphibolitic shear zone; (d) thin sections of the complete alteration sequence of
Langøy sample (c) showing an increasing amount of amphibole (green) and biotite (brown) with decreasing distance to the
amphibolitic shear zone (right side); (e) Langøy sample: modal abundances of major minerals according to their distance from the
shear zone shown in (d) [with zero being the boundary between wall-rock and shear zone; values on the right side of zero belong to
the shear zone, and those to the left side to the altered wall-rock; pristine gabbro (PG) as reference].
powdered using an agate mortar. Another aliquot of
each rock sample was washed again in ultra-pure water,
dried and split into different sized portions by sieving.
After magnetic separation, amphibole, biotite and
scapolite were handpicked from sieve fraction
125–250 mm and pulverized using an agate mortar. Rock
and mineral powders were washed five times in ultrapure water for 10 min and slowly dried at 60 C overnight.
304
Halogens were extracted using the pyrohydrolysis
method described by Barnes & Sharp (2006), in which
rock and/or mineral powders were melted in a continuous flow of ultra-pure water vapor. The water vapor,
containing the released halogens, was condensed
and collected in 005N NaOH and 0005N Na2SO3 solution (Schnetger & Muramatsu, 1996). The solution was
split into aliquots for isotope and concentration
analyses.
Chlorine isotopes
Chlorine isotope ratios were measured using the technique of Barnes & Sharp (2006) as modified from
Eggenkamp et al. (1994). Extracted chlorine was converted to AgCl via reaction with an excess of AgNO3.
AgCl was filtered from the sample solution, dried and
reacted with CH3I under vacuum to produce CH3Cl. The
gaseous CH3Cl was purified of excess CH3I on a gas
chromatographic column and analysed using a
ThermoElectron MAT253 isotope ratio mass spectrometer at the University of Texas in Austin. The long-term
reproducibility for d37Cl values is 602%, based on the
average of three internal seawater standards and an internal serpentinite rock standard.
Halogen concentrations
Halogen concentrations in the extraction solution were
determined following the procedure described by John
et al. (2011). Fluorine and Cl were analysed by ion chromatography (IC) (Metrohm 761 Compact IC system) at
the University of Münster. Detection limits of F and Cl
via ion chromatography were 100 ng ml–1 and
50 ng ml–1, respectively. Almost all sample solutions
have F– concentrations slightly above or below the detection limit. Concentrations of Cl, Br and I were measured by inductively coupled plasma mass spectrometry
(ICP-MS) using a ThermoFinnigan Element2 system at
the University of Bremen equipped with a self-aspiring
micro-flow nebulizer and a PEEK cyclonic spray chamber. To minimize memory effects, samples and blanks
were prepared with a 5% NH4OH solution (p.a. quality),
and a blank was analysed before each sample (see Bu
et al., 2003). Extraction solutions were diluted 1:1 with
NH4OH and spiked with 1 mg ml–1 Te as internal standard. The isotopes 35Cl, 79Br and 127I were analysed in
medium resolution and 79Br additionally in high resolution; both 79Br determinations yielded essentially
identical results. Calibration used five solutions with
concentrations up to 10 ng ml–1 for Br and I and
1000 ng ml–1 for Cl.
The lower limit of detection (3r) for the extraction solutions is 16 ng ml–1 for Cl, 04 ng ml–1 for Br and
006 ng ml–1 for I. Nine replicate analyses of a representative aliquot having Cl, Br and I concentrations of 3830,
75 and 07 ng ml–1, respectively, yielded a precision of
13–22%. Halogen concentrations in rock and mineral
samples were calculated from the concentration of the
extraction solutions. John et al. (2011) determined the
Journal of Petrology, 2015, Vol. 56, No. 2
quantitative yield by multiple analysis of reference material (JB-2) following this method to be within 10%
error.
Electron microprobe analysis (EMPA)
Quantitative analyses of mineral compositions were
carried out using a JEOL 8900 electron microprobe at
the University of Münster, equipped with four wavelength-dispersive spectrometers. Typical beam conditions were 20 kV acceleration voltage, a 15 nA beam
current, a focused beam of 1 mm spot size and 5–10 s
counting time on the peak. Natural as well as synthetic
minerals were used for standardization of major elements. Standardization of halogens was done using natural tugtupite (for Cl) and fluorite (for F).
X-ray diffraction (XRD)
Abundances of phases were measured by powder XRD
of bulk-rocks using a Phillips X’pert 9 diffractometer and
X’pert ‘Highscore’ software package for quantification.
Oxygen isotope measurements
Amphibole mineral separates were prepared for oxygen isotope analysis by coarsely crushing 10 g of gabbro sample. The crushed material was then sieved,
washed in deionized water and rinsed with HCl, and
amphibole mineral grains were handpicked using a binocular microscope to ensure purity. Approximately
20 mg of separated amphibole was measured using the
laser fluorination method of Sharp (1990). Samples
were separately heated by a CO2 laser in the presence
of BrF5 and then cryogenically purified using the silicate
extraction line at the University of Texas at Austin.
Once the O2 was purified, the d18O values were measured on a ThermoElectron MAT 253 gas source mass
spectrometer. To check for precision and accuracy, garnet standard UWG-2 (d18O value ¼ þ58%) (Valley et al.,
1995) and two in-house quartz standards, Lausanne-1
(d18O
value ¼ þ181%)
and
Gee
Whiz
(d18O
18
value ¼ þ126%), were run. All d O values are reported
relative to SMOW, where the d18O value of NBS-28 is
þ965%. Precision is 601%.
RESULTS
Petrography and mineral chemistry
Gabbro protolith
The pristine gabbro consists mainly of plagioclase (pl
with An60–70), orthopyroxene (opx with En70–75), clinopyroxene (cpx with Di65–70) and olivine (ol with Fo68–75).
Apatite (ap) of heterogeneous composition (XCl ¼
02–09, XOH ¼ 01–08, XF < 001) and ilmenite are minor
phases. Olivine shows kelyphitic coronae of orthopyroxene locally intergrown with spinel that are in turn surrounded by a fine-grained amphibole–spinel corona
(Fig. 3a). Coronitic and matrix opx are compositionally
indistinguishable from each other. De Haas et al. (2002)
interpreted opx coronae between pl and ol in gabbros
from the Arendal area (SW Norway) to be of magmatic
Journal of Petrology, 2015, Vol. 56, No. 2
305
Fig. 3. Back-scattered electron images and Si, Na and Cl element maps showing replacement reactions: (a) reaction rims between
olivine and plagioclase in least altered parts of the sequence near the gabbro; (b) replacement of plagioclase by scapolite (6 spinel)
indicating first scapolitization; (c) replacement of ilmenite by biotite leaving a corona of zircons, indicating the former grain shape
of the ilmenite; (d) micrometre-sized halite crystal found in the least altered part of the sequence (26 cm distance to the shear zone).
(super-solidus) origin. Amphibole–spinel coronae of the
same assemblage are interpreted to originate from
interaction of the opx coronae with late-stage magmatic
fluids during cooling of the gabbro. Amphibole
from the pristine gabbro has Cl concentrations of
02–04 wt % determined by EMPA (Supplementary
Data) and is pargasitic in composition with high TiO2
(up to 2 wt %) and K2O (<06 wt %) concentrations.
Pinkish plagioclase (An60–70) exists as millimetre-sized
euhedral laths.
The inner part of the sample contains the least altered
metagabbro. Olivine is mostly replaced by pyroxene, but
spinel can still be found as remnants of the gabbro assemblage as inclusions in pyroxene and amphibole. Opx
and cpx have coronae of Cl-rich amphibole (Cl > 10 wt
%) occurring as micrometre-sized crystals with random
orientation or with palisade-structure between pyroxene
and plagioclase. In general, secondary amphibole contains between 2 and 3 atoms per formula unit (a.p.f.u.) Al
and >07 a.p.f.u. Na, making it pargasitic in composition
(Fig. 4). The highest Al concentrations are found in
amphiboles surrounding opx, whereas lower Al concentrations are found in amphiboles close to cpx. TiO2 concentrations (01–04 wt %) in secondary amphiboles are
lower than those in the magmatic amphiboles.
Plagioclase from this zone is not altered and has the
same composition as plagioclase from the protolith. The
reaction of plagioclase with pyroxene to form amphibole
marks the beginning of the amphibolitization process.
Samples from this early alteration sequence contain
micrometre-sized halite crystals (Fig. 3d). To preserve
salt crystals during sample preparation, samples were either sawn only in oil or investigated as cracked fragments. Halite crystals observed in samples prepared
with both techniques were situated in irregular-shaped
holes and cracks between amphibole grains. They do not
originate from a fluid inclusion within a single mineral as
fluid inclusions are not observed in the whole sample sequence. KCl, a typical high-temperature salt (Markl &
Bucher, 1998), was not found.
Amphibole zone
The amphibole zone is characterized by the progressive
replacement of the two pyroxenes by Al-rich amphibole. Modal abundance of amphibole increases from
12% (unaltered gabbro) to 55% (completely amphibolitized gabbro) with decreasing distance to the shear
Journal of Petrology, 2015, Vol. 56, No. 2
amphibolitization
scapolitization
amphibolitization
scapolitization
1
3
0.8
2
0.6
0.8
6
0.7
5.5
#Mg
0.9
6.5
2
1
1
0.5
1
1.8
0.5
1.6
PG −25 −20 −15 −10
−5
0
PG −25 −20 −15 −10
−5
0
K2O (wt.%)
0.6
Cl (wt.%)
Ca (a.p.f.u)
TiO2(wt.%)
Si (a.p.f.u)
Al (a.p.f.u)
4
Na (a.p.f.u)
306
0
distance to shear zone (cm)
Fig. 4. Element distribution in amphibole from the Langøy alteration sequence with increasing distance from the shear zone
towards the least altered gabbro (PG); box (horizontal line indicates mean; size of box indicates standard deviation) and whisker
bars (1r) representation of EMPA data for each sample (outliers as þ); a.p.f.u. and Mg# calculated from concentration data on the
basis of 23 O.
zone; simultaneously, the abundance of magmatic
mafic mineral relicts decreases (px 32% in pristine gabbro to 0% in altered gabbro; Fig. 2e). Although amphibole is mainly pargasitic in composition it shows highly
variable Mg# (060–075) and TiO2 (01–14 wt %), Al2O3
(12–20 wt %) and K2O (02–05 wt %) concentrations
(Fig. 4). Microprobe analyses of Cl in amphibole reveal
decreasing concentrations towards the shear zone,
from more than 1 wt % on average (maximum 13 wt %)
to 04 wt % on average for each section. Fluorine
concentrations of amphibole were below the detection
limits of EMPA. Plagioclase from this zone shows slight
sericitization in the mineral cores and incipient replacement by scapolite at the rims. Cl-rich apatite is replaced
by homogeneous OH-rich apatite (XCl ¼ 02, XOH ¼ 08,
XF < 005).
Scapolite zone
The transition from the amphibole to the scapolite zone
is defined by the appearance of scapolite (Ma57–65) and
Ti-rich biotite (14–17 wt % TiO2) as major phases. The
modal abundance of scapolite increases within this
zone from 5% nearest the amphibole zone to 36% nearest the shear zone. Simultaneously, the abundance of
biotite increases from 0% to 17%. Biotite has Cl
concentrations (by EMPA) of 02–03 wt % and F concentrations below detection limit. Scapolite has low S with
a maximum 01 wt % SO4, but different amounts of Cl
and CO3 (defining the end members marialite
[Na4Al3Si9O24(Cl)] and meionite [Ca4Al6Si6O24(CO3)].
Chlorine concentrations (by EMPA) in scapolite vary between 20 and 28 wt % and show a slight trend of
increasing Cl towards the shear zone. CO2 concentrations were calculated using the meionite content of scapolite, which in turn was calculated following Hassan &
Buseck (1988) by using the coupled exchange of
(Na4.Cl)Si2 and (NaCa3.CO3)Al2 to be 0378 6 0034.
Calculated CO2 concentrations are of the order of
197 6 018 wt %. Amphibole from the scapolite zone is
coarser-grained and more homogeneous in composition than amphibole from the amphibolitization zone.
On average, it has slightly lower Al2O3 (13–16 wt %, corresponding to 24–26 a.p.f.u.) and Na2O (24–31 wt %,
corresponding to 06–08 a.p.f.u) concentrations, but
higher TiO2 concentrations (04–07 wt %) compared
with amphibole formed in the first alteration zone (Fig.
4). EMPA of Cl in amphibole gives average concentrations of 04 wt %. Magmatic plagioclase and ilmenite
are absent. Apatite from the scapolite zone is heterogeneous (XCl ¼ 01–02, XOH ¼ 05–09, XF ¼ 0–03) within
single crystals and shows the highest F concentrations
Journal of Petrology, 2015, Vol. 56, No. 2
3
Scapolite
2.5
wt.% Cl (EMPA)
of 15 wt % close to the shear zone. Albite, analcime, calcite and tourmaline are accessory minerals that occur
close to the shear zone, indicating the transport of B, Na
and CO2 during metasomatism. The latter assemblage
possibly reflects the albitization event, as albite and
analcime typically replace scapolite. Analcime and albite also grow on the 001 planes of single biotite grains
(Fig. 3c), producing a swelling structure within biotite
grains. Although albitization is an important regional
metasomatic event, it is minimal in the studied rocks
and might have happened at a later stage.
307
2
1.5
Biotite
1
Shear zone
The shear zone consists of monomineralic amphibole
of pargasitic composition (Al 14–15 wt %, corresponding to 24–25 a.p.f.u; Na 27–30 wt %, corresponding to
07–08 a.p.f.u) with high TiO2 (07–09 wt %) concentration, Cl of 04 wt % and a Mg# of 07.
Halogen chemistry
In the following section halogen data for solid phases
are calculated from the halogen concentrations of pyrohydrolysis extraction solutions measured by IC (F) and
ICP-MS (Cl, Br, I), if not explicitly stated otherwise.
Solids used for pyrohydrolysis were either bulk samples or picked mineral separates that represent a homogenized fraction of possibly heterogeneous minerals
from one sample. Comparison of average Cl concentrations obtained by EMPA and pyrohydrolysis for different minerals and mineral separates of the same sample
give identical values within uncertainties for most pairs
(Fig. 5). Differences between the two methods for scapolite might result from a contamination of hand-picked
separates with plagioclase intergrown with scapolite. In
this case, the amount of scapolite melted for extraction
would be lower than inferred for calculations, leading to
an underestimation of Cl concentration. The similarity
of EMPA and ICP-MS analyses provides an additional
indicator of the absence of fluid inclusions in amphibole, as pyrohydrolysis would liberate halogens from
fluid inclusions whereas EMPA measurements would
be not affected by fluid inclusions.
Amphibole zone
Bulk-rock halogen concentrations vary widely along the
sequence (Fig. 6, Table 1). The pristine gabbro has relatively low halogen concentrations (F ¼ 104 mg g–1; Cl ¼
1495 mg g–1; Br ¼ 32 mg g–1; I ¼ 005 mg g–1) compared
with concentrations in the least altered samples from
the amphibole zone (F ¼ 159 mg g–1; Cl ¼ 3801 mg g–1;
Br ¼ 141 mg g–1; I ¼ 046 mg g–1). However, the Br/Cl and I/
Cl ratios are similar (Fig. 6e and f). Metagabbros from
the amphibole zone reveal only a small spread in F, Cl
and Br concentrations (F ¼ 49–122 mg g–1; Cl ¼ 3801–
4900 mg g–1; Br ¼ 88–141 mg g–1), except for two samples close to the scapolite zone, which have very high F
concentrations up to 964 mg g–1. This sudden increase in
F is detected only in bulk samples, which may contain
Amphibole
0.5
0
0
0.5
1
1.5
2
2.5
wt.% Cl (pyrohydrolysis)
3
Fig. 5. Comparison of Cl concentration measured by EMPA
(average Cl content of biotite, amphibole and scapolite with 1r
error) and by ICP-MS of extraction solution (with 1r error of
nine replicates) of mineral separates from the same sample.
undetected minor phases (e.g. fluorite, hydrogarnet)
that might incorporate F. Secondary hydrogarnets
found in other metamorphic rocks from the Bamble sector can contain high F concentrations of up to 16 wt %
(Visser, 1993), but are not observed in the studied sample sequence. The only observed F-bearing mineral in
the whole sequence is apatite, which in the amphibole
zone is, however, F-poor <05 wt %). Therefore, a nonuniform distribution of apatite or an undetected F-bearing mineral could account for local high F concentrations. The maximum iodine concentration (087 mg g–1)
was detected in the centre of the amphibole zone.
Chlorine and Br concentrations slightly increase from
4900 to 8060 mg g–1 and from 101 to 185 mg g–1, respectively, towards the scapolite zone, whereas the I concentration remains constant (Fig. 6d). The similar
distribution of Cl and Br leads to a limited variation in
Br/Cl ratios (19 10–3 to 37 10–3). As bulk I and Cl
concentrations show little variation in this part of the sequence, I/Cl ratios do not change systematically and
vary between 70 10–6 and 120 10–6 (Table 1).
Amphibole is a major sink for halogens, especially
for Cl, in this zone. As the infiltrating fluid reacts with
the gabbro, newly formed amphibole dominates the Cl
budget. Amphibole in the pristine gabbro has higher Br
and F concentrations, but similar I and lower Cl concentrations, compared with amphiboles from the first alteration zone. Amphibole from the least altered samples
has a high Cl content of 11 wt %; single amphibole
grains incorporate >13 wt % Cl, measured by EMPA.
Although bulk Cl concentrations are constant in the
amphibolitization zone, Cl concentrations in amphibole
decrease towards the shear zone, in the same way as
the modal abundance of amphibole increases (Fig. 3e).
At the transition to the scapolitization zone amphibole
308
Journal of Petrology, 2015, Vol. 56, No. 2
bulk
amph
scapo
bio
Cl(wt%)
2
1.5
b)
a)
100
10
1
Br(µg/g)
2.5
0.5
1
c)
d)
1
100
0.1
I(µg/g)
F(µg/g)
1000
10
f)
1000
1
0.1
100
PG −25
−20
−15
−10
−5
0
distance to shear zone (cm)
PG −25
−20
−15
−10
−5
0
I/Cl (*10-6)
Br/Cl (*10-3)
e)
10
distance to shear zone (cm)
Fig. 6. Chlorine, Br, F and I concentrations, and Br/Cl and I/Cl ratios of bulk samples and mineral separates from the Langøy alteration sequence.
has Cl concentrations of 040–045 wt %. Bromine
concentrations in amphibole show a similar trend to Cl
concentrations, with the highest values (54 mg g–1) in
the least altered part of the sequence and lower values
(17 mg g–1) close to the second (scapolitization) alteration zone. Iodine concentrations in amphibole vary
between 02 and 07 mg g–1 in the amphibolitization
zone, without a distinct trend. Fluorine in amphibole
has concentrations between 89 and 180 mg g–1. Halogen
ratios for amphibole are 035 10–3 to 051 10–3 for
Br/Cl and 38 10–6 to 113 10–6 for I/Cl.
Apatite appears as an accessory phase (modal abundance <1% determined from optical observations) in all
samples and was not picked as mineral separates; however, single apatite grains were analysed by EMPA.
Apatite from the amphibolitization zone has Cl concentrations between 08 and 14 wt % and low F concentrations of less than 05 wt %.
Scapolitization zone
Bulk-rock concentrations of Cl and Br increase from the
transition to the amphibolitization zone towards the scapolitization zone to values of 1 wt % for Cl and
27–33 mg g–1 for Br and vary little within the scapolitization zone. Although iodine concentrations (03–
04 mg g–1) vary less than Cl concentrations for the whole
scapolitization zone, the increasing Cl concentrations
lead to decreasing I/Cl ratios (from 93 10–6 to
27 10–6) from the amphibolitization zone towards the
shear zone. Simultaneously, Br/Cl ratios increase from
19 10–3 to 3 10–3 towards the shear zone.
Amphiboles from the scapolitization zone show little
halogen variability, in the range of 041–044 wt % for Cl,
12–27 mg g–1 for Br and 021–064 mg g–1 for I. Therefore,
Br/Cl ratios vary little between 034 10–3 and
064 10–3 and I/Cl ratios between 47 10–6 and
130 10–6. Fluorine is elevated (with the exception of
one measurement) compared with the amphibolitization zone, with concentrations ranging from 184 to
245 mg g–1.
In scapolite halogen concentrations increase towards
the shear zone. Average F, Cl, Br, and I concentrations
range from 43 mg g–1, 19 wt %, 77 mg g–1, and 073 mg g–1
close to the amphibolitization zone to 97 mg g–1, 24 wt
%, 102 mg g–1 and 1 mg g–1 close to the shear zone, respectively. As scapolite is a solid solution between
marialite (Cl-rich) and meionite (CO3-rich), the increase
in Cl concentration needs to be charge balanced, probably by a decrease of CO3 in scapolite. Halogen ratios of
all scapolite samples fall within a small range (Br/
Cl ¼ 36 10–3 to 42 10–3; I/Cl ¼ 34 10–6 to 42 10–6).
Biotite is another halogen-bearing mineral phase
from the scapolitization zone, but incorporates less Cl
(018–020 wt %) compared with amphibole and scapolite. The concentrations of F, Br and I in biotite increase
with decreasing distance to the shear zone, from 131 to
336 mg g–1 F, 11 to 55 mg g–1 Br and 053 to 279 mg g–1 I
(Fig. 6d). In this context, the highest concentrations of F,
Br and I and exceptionally high halogen ratios
(Br/Cl ¼ 23 10–3 and I/Cl 1169 10–6) are found in the
sample closest to the shear zone, which shows some
alteration of biotite (i.e. growth of analcime between
Journal of Petrology, 2015, Vol. 56, No. 2
309
Table 1: Halogen concentrations, d37Cl and d18O values in bulk-rocks, amphibole, biotite and scapolite from Langøy and Valberg
alteration sequences
Locality
Sample
Assemblage
cm to
F
Cl
Br
I
SZ
(mg g–1) (mg g–1) (mg g–1) (mg g–1)
F/Cl
(103)
Br/Cl
(103)
I/Cl
(106)
d37Cl d18O
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Valberg
Valberg
Valberg
Valberg
Valberg
Amphibole
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Langøy
Valberg
Valberg
Valberg
Valberg
Valberg
Biotite
Langøy
Langøy
Langøy
Langøy
Langøy
Valberg
Scapolite
Langøy
Langøy
Langøy
Langøy
Langøy
Valberg
Valberg
BAM 3-5
BAM 3-4-1
BAM 3-4-2
BAM 3-4-3
BAM 3-4-4
BAM 3-4-5
BAM 3-4-6
BAM 3-4-7
BAM 3-4-8
BAM 3-4-9
BAM 3-4-10
BAM 3-4-11
BAM 3-4-12
BAM 3-4-13
BAM 3-4-14
BAM 3-4-15
BAM 21-1
BAM 21-2-III
BAM 21-5
BAM 21-3-III
BAM 21-3-I
ol–px–amph–pl
px–amph–pl
px–amph–pl
px–amph–pl
px–amph–pl
amph–pl
amph–pl
amph–pl
amph–pl–scp–bio
amph–pl–scp–bio
amph–pl–scp–bio
amph–scp–bio
amph–scp–bio
amph–scp–bio
amph–scp–bio
amph
ol–px–amph–pl
amph–pl
amph–scp–bio
amph–scp
amph
—
264
24
218
198
177
159
14
114
94
76
57
42
25
07
–08
147
47
27
16
0
104
159
85
99
122
73
49
397
964
162
131
140
106
159
131
187
122
550
231
98
215
1495
3801
5098
5097
4803
4554
4960
4900
5419
6555
8060
10213
9727
11036
9864
5534
777
7136
11430
16410
10902
32
141
129
98
92
88
125
101
109
125
185
276
277
334
294
79
10
88
455
1093
41
0047
0460
0358
0402
0569
0872
0579
0456
0424
0390
0321
0376
0359
0298
0366
0398
0081
0393
0369
0382
0375
6938
4192
1669
1940
2532
1608
990
8102
17795
2470
1625
1374
1088
1436
1331
3380
15761
7705
2018
600
1968
216
370
254
192
191
193
252
206
202
191
230
270
285
303
298
142
123
123
398
666
037
3158
12101
7023
7885
11858
19148
11670
9310
7820
5944
3977
3682
3691
2703
3706
7186
10455
5512
3226
2326
3443
–01
BAM 3-5
BAM 3-4-3
BAM 3-4-4
BAM 3-4-5
BAM 3-4-6
BAM 3-4-7
BAM 3-4-8
BAM 3-4-9
BAM 3-4-10
BAM 3-4-11
BAM 3-4-12
BAM 3-4-13
BAM 3-4-14
BAM 3-4-15
BAM 21-1
BAM 21-2-III
BAM 21-5
BAM 21-3-III
BAM 21-3-I
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
Amphibole
—
218
198
177
159
14
114
94
76
57
42
25
07
–08
147
47
27
16
0
348
157
113
119
89
180
125
135
245
201
17
198
184
383
405
165
128
151
132
7042
10683
8887
8181
7594
6313
4441
4122
4057
4409
4417
4361
4234
4725
3200
5942
4505
4969
10971
82
54
38
34
33
23
17
13
16
17
15
22
27
21
16
12
32
57
10
0393
0669
0341
0417
0470
0712
0583
0218
0537
0238
0212
0640
0432
0574
0179
1005
0522
0693
0637
4936
1470
1271
1460
1166
2846
2813
3284
6048
4568
396
4544
4357
8108
12658
2776
2846
3036
1200
116
051
042
041
043
036
038
031
040
038
034
050
064
044
050
020
070
115
010
5578
6259
3841
5098
6187
11272
13129
5296
13228
5407
4789
14677
10211
12144
5601
16917
11598
13937
5810
03
–06
–04
–02
00
03
04
00
01
03
05
BAM 3-4-10
BAM 3-4-11
BAM 3-4-12
BAM 3-4-13
BAM 3-4-14
BAM 21-5
Biotite
Biotite
Biotite
Biotite
Biotite
Biotite
76
57
42
25
07
27
131
192
176
159
336
51
1828
1939
2009
1949
2382
2965
14
11
19
26
55
22
0583
0529
0815
0657
2787
0574
7191
9926
8743
8139
14093
1715
075
058
096
131
232
073
31899
27300
40564
33684
116986
19378
00
02
04
BAM 3-4-10
BAM 3-4-11
BAM 3-4-12
BAM 3-4-13
BAM 3-4-14
BAM 21-5
BAM 21-3-III
Scapolite
Scapolite
Scapolite
Scapolite
Scapolite
Scapolite
Scapolite
76
57
42
25
07
27
16
43
43
61
63
97
33
724
19113
21441
22378
23774
24090
23748
22377
771
799
809
929
1016
1191
1621
1618
0727
0823
1000
0956
0897
0785
225
198
272
264
402
141
3236
403
373
361
391
422
502
724
8468
3389
3677
4208
3968
3779
3508
02
00
02
–06
–02
–02
–02
07
05
06
04
00
03
05
07
05
01
10
12
05
08
09
00
04
–03
–02
00
51
37
54
51
34
55
55
68
72
72
67
70
74
73
10
04
02
–07
–05
SZ, shear zone.
biotite laths). This alteration might be the reason for a
decoupling of Cl and Br in this sample (Fig. 6e). For all
other samples Br/Cl ratios vary between 058 10–3 and
13 10–3 and I/Cl ratios between 273 10–6 and
405 10–6.
Apatite has highly variable F concentrations (by
EMPA) from less than 01 wt % to 15 wt % with a trend
of increasing values towards the shear zone, whereas
Cl concentrations are constant with an average value of
10 6 03 wt %. Although apatite might affect the F
budget of the bulk-rock, it plays only a minor role in the
budget of Cl (and probably Br and I) as its abundance is
too low compared with all other halogen-bearing
phases.
310
Journal of Petrology, 2015, Vol. 56, No. 2
1.5
2
scapolitization
amphibolitization
a)
δ 37Cl
1
1
scapolitization
amphibolitization
b)
1σ error
0.5
0
0
-0.5
bulk
-1
-1.5
amph
scapo
bio
1σ error
PG
-25
-20
-15
-10
-5
distance to shear zone (cm)
0
-1
-2
-147
-47
-27 -16
0
distance to shear zone (cm)
Fig. 7. d37Cl values of (a) Langøy sample sequence and (b) Valberg sample sequence for bulk-rocks, amphibole, scapolite and biotite according to their distance to the shear zone.
Chlorine isotope geochemistry
Pristine gabbro
The unaltered Langøy gabbro has a d37Cl value of
–01 6 02% (Fig. 7a), similar to values from a depleted
mantle reservoir (Sharp et al., 2007, 2013; John et al.,
2010). Amphibole separated from the pristine
gabbro has a slightly higher d37Cl value of þ03 6 02%,
but still within analytical uncertainty of the bulk
measurement.
Sequence
The d37Cl values of bulk samples from the sequence
vary between –06 and þ 07%, with a clear tendency of
lower values towards the unaltered zone (Fig. 7a,
Table 1). With increasing degree of alteration, d37Cl values from the amphibole zone increase until a maximum
of þ06% is reached close to the transition to the scapolite zone. Samples across this transition show decreasing values (to a minimum of 0%). However, within the
scapolite zone d37Cl values increase again and reach a
maximum (þ07%) close to the shear zone, which itself
has a d37Cl value of þ05%.
Mineral separates
The d37Cl values of amphibole separates range from
–06 to þ1%, similar to the bulk samples. The amphibole d37Cl values are lowest in the least altered part of
the section, also similar to the bulk samples (Fig. 7a).
The d37Cl values increase towards the shear zone, but
show a local minimum (0%) at the transition between
the scapolite zone and amphibole zone.
Separated scapolite shows minimal variation in its
isotopic composition throughout the sequence and has
an average value of þ02 6 01%. The d37Cl values of
biotite separates range from 0 to þ1% and decrease
with increasing distance from the shear zone (Fig. 7a).
Bulk d37Cl values are consistent with the weighted
average d37Cl of each of the mineral phases. Therefore,
the dominant Cl-bearing phase of each zone controls
the overall bulk d37Cl value. The small variation in d37Cl
for different minerals (i.e. amphibole, scapolite and
biotite) in one single sample indicates the absence of
a significant fractionation of Cl isotopes between
different silicates. However, an isotopic fractionation
between fluid and silicates cannot be excluded.
Oxygen isotopes
d18O values of amphiboles (Table 1) from each sample
were measured to estimate fluid:rock ratios (f/r) during
alteration. Amphibole from the pristine gabbro has a
d18O value of þ51%, which falls in the isotopic range
for mantle-derived rocks. Owing to minimal oxygen isotope fractionation between silicate melt and crystallizing minerals at magmatic temperatures (typically <1%
for most phases), most mantle-derived minerals will
have similar d18O values to the bulk upper mantle
(þ55%) (Eiler, 2001; Sharp, 2007). In the amphibolitization zone, the d18O values of amphibole range from
þ34 to þ55%, and d18O shows higher values in the
scapolitization zone, as well as the shear zone, ranging
from þ67 to þ74%.
Valberg metagabbro
Samples from the Valberg metagabbro show the same
alteration sequence as the Langøy samples. Adjacent to
the amphibolitic shear zone, the scapolitization zone is
characterized by the mineral assemblage scapolite,
amphibole and biotite. Close to the shear zone biotite
forms isolated grains, whereas further from the shear
zone it forms clusters, similar to biotite in the Langøy
samples. Furthermore, the width of the scapolitization
zone is larger (30 cm wider) than that of the scapolitization zone in the Langøy sample sequence. The
amphibolitization zone adjacent to the scapolitization
zone is also larger, as amphibole-rich metagabbro is
still present 1 m away from the shear zone. Shearing,
indicated by a high density of micrometre-sized cracks
and veins, is more prominent in the Valberg samples
compared with the Langøy sample and may be the reason for the enlarged alteration zones, as the higher fluid
flux in microveins accelerates pervasive fluid flow (Zack
& John, 2007).
Halogens
Halogen concentrations and d37Cl values of the Valberg
samples are in the same range as those in the Langøy
sequence (Table 1). Although the Valberg sequence
Journal of Petrology, 2015, Vol. 56, No. 2
bulk
amph
scapo
bio
Cl(wt%)
2
1.5
b)
a)
100
10
1
Br(µg/g)
2.5
311
0.5
1
c)
d)
100
0.1
0.01
10
e)
Br/Cl (*10-3)
I(µg/g)
1
f)
100
1
0.1
-147
amphibolitization
-47
scapolitization
amphibolitization
-27 -16 0 -147
distance to shear zone (cm)
-47
scapolitization
-27 -16 0
I/Cl (*10-6)
F(µg/g)
1000
10
distance to shear zone (cm)
Fig. 8. Halogen concentrations and ratios of Valberg metagabbro samples.
was sampled in less detail, changes in halogen concentrations (Fig. 8) and d37Cl values within the various zones
follow similar trends to the Langøy samples (Fig. 7b).
DISCUSSION
A detailed investigation of the mineral reaction history
is crucial to understand the geochemical evolution of
the fluid. Observed mineral reactions are caused by
changed chemical potentials owing to infiltration of a
metasomatic fluid under constant temperature and
pressure conditions. The continuing fluid–rock interaction will not only alter the composition of the rock,
but also progressively change the fluid composition, especially if the water:rock ratio is low. Here, we present a
case study in which a highly saline fluid is infiltrating a
dry gabbro, causing various mineral replacement reactions. These reactions lead to a continuing desiccation
of the fluid until halite is precipitated and the fluid is
completely consumed. Chlorine concentration in
amphibole is used to monitor the changing salinity of
the fluid. Changes in d37Cl values with distance to the
shear zone probably result from Rayleigh fractionation
of Cl isotopes during incorporation of Cl from the fluid
into the forming mineral phases (Gleeson & Smith,
2009). Oxygen isotope data reveal fluid:rock ratios (f/r)
of <02 for the amphibolitization zone, indicating a rockdominated environment, and 2–14 for the scapolitization zone, indicating a more fluid-dominated regime.
Correlations among the changing mineral assemblages,
the mineral chemistry, and the chemical evolution of
the fluid on a small scale are critical to understanding
metasomatism and related ore genesis on a regional
scale (Smith et al., 2013).
We focus on the Langøy sequence for the discussion
as the two sample sites indicate essentially identical
mineral reactions and changing mineral compositions
with increasing metasomatism. The sample sequence
from Langøy is more detailed compared with that
from Valberg and alteration is clearly caused by
pervasive fluid infiltration adjacent to the main fluid
pathway.
Mineral replacement reaction
The alteration reactions leading to the various zones in
the sequence are similar to those described by Engvik
et al. (2011). All alteration reactions are referred to as
mineral replacement reactions because newly formed
minerals start growing either as coronae around magmatic crystals or at the expense of pristine coronae,
until the primary phases are completely consumed.
Opx (6 spinel) coronae around olivine, as well as
amphibole–spinel coronae around pre-existing opx
(6 spinel) coronae (Fig. 3a), are present in the most pristine samples and are probably of magmatic origin (de
Haas et al., 2002). However, less altered samples (indicated by a low abundance of amphibole) are almost
olivine free. The rare olivine appears to be replaced by
pyroxene first, which is then replaced by amphibole, rather than the olivine being directly reacted to amphibole. Orthopyroxenes of probably different origin (i.e.
magmatic coronae or products of olivine replacement)
312
Journal of Petrology, 2015, Vol. 56, No. 2
are chemically indistinguishable from each other.
However, all pyroxenes show amphibole coronae
(Fig. 3a) caused by the reactions
Clinopyroxene ðDi65–70 Þ þ Plagioclase ðAn60–70 Þ þ H2 O
! Pargasite
(1a)
Orthopyroxene ðEn70–75 Þ þ Plagioclase ðAn60–70 Þ þ H2 O
! Pargasite:
(1b)
Reactions (1a) and (1b) are characteristic for the
amphibolitization zone and lead to the mineral
assemblage pargasite þ plagioclase 6 apatite 6 ilmenite.
Increasing alteration leads to a complete replacement
of all pyroxene, removing all coronitic textures.
Alteration occurring in the second alteration zone
(scapolitization) includes the following reaction:
Ilmenite þ Kþ þ Mg2þ þ Al3þ þ Si4þ þ H2 O
! Biotite þ Ti4þ :
(2)
Biotites grown from this reaction have high TiO2
concentrations (up to 2 wt %) and contain many
micromtre-sized zircon inclusions (Fig. 3c). The zircons
form a three dimensional framework, which reflects the
former grain boundaries of the replaced ilmenite
(Austrheim et al., 2008).
Scapolite was formed by a reaction of magmatic
plagioclase with a highly saline, CO2-containing fluid:
PlagioclaseðAn60–70 Þ þ Naþ þ Cl– þ CO2
! Scapolite ðMa57–67 Þ:
(3)
Local Al excess owing to the replacement reaction
leads to the formation of sub-micrometre- to micrometre-sized spinel crystals at the reaction front (Fig. 3b).
Greenish spinel is found only in the narrow transition
zone between the two alteration zones and is no longer
observed when all plagioclase is replaced. The formation of other Al-rich phases (sapphirine, corundum) during plagioclase to scapolite replacement (Engvik &
Austrheim, 2010) is a metasomatic reaction feature and
occurs only on small spatial scales. The formation of
scapolite with a meionite component of 33–43% indicates the presence of CO2 in the fluid in addition to the
high halogen content.
Amphiboles in the scapolitization zone are coarser
grained and compositionally homogeneous compared
with the amphibolitization zone in which several different pargasitic amphibole types can be distinguished by
composition (e.g. different Ti, Na, Cl, and Al concentrations) and by origin (e.g. magmatic cooling products,
coronae surrounding opx or cpx). Therefore, it is likely
that a recrystallization process chemically homogenized
the amphibole.
Fluid:rock ratio
Fluid:rock ratios (f/r) were estimated using the d18O
values of the amphibole that originates from
fluid–rock interaction. The f/r (in mole fraction) can be
defined as
f
df rock di rock
¼ i
r d fluid ðdf rock Drock–fluid Þ
(4)
where dirock, dfrock and difluid are the isotope ratios of the
initial rock, final rock and initial fluid, respectively, and
Drock–fluid is the equilibrium fractionation between the
final rock and the fluid. This equation was used to calculate f/r for each sample assuming a single pass of fluid
through the rock (e.g. Sharp, 2007). To calculate f/r
ratios for the Bamble alteration sequence, dirock is
assumed to be equal to the isotopic ratio of pristine
amphibole (þ51%) and dfrock is assumed to be equal to
average measured d18O values of amphibole in the
altered zone (þ102%). The Drock–fluid was calculated following Zheng (1993) to be –28% for the equilibrium
fractionation between pargasite and H2O at the
assumed temperature of metasomatism (600 C).
Although difluid cannot be measured directly, it can be
estimated in two ways. First, the d18O value of shear
zone amphibole can be used to calculate a fluid value
as these amphiboles should have experienced the most
interaction with fluid. Shear zone amphibole has a d18O
value of þ73%, corresponding to a fluid with þ101%
taking equilibrium partitioning at 600 C into account.
Second, data from the literature were used to compare
the oxygen isotope composition of the shear zone
amphibole with other phases. Broekmans et al. (1994)
reported d18O values for calcite found in scapolite-bearing veins that cross-cut metagabbros from the Bamble
area to range from þ102 to þ235%. As a best approximation of their data, we calculated a d18O value for the
Bamble fluid of þ93%, assuming an equilibrium fractionation of þ09% (Hu & Clayton, 2003) between calcite
with the lowest values (d18O ¼ 102%) and fluid at
600 C. Although the value calculated from the literature
is lower than that calculated from the shear zone amphibole, they agree reasonably well, as the second value
was calculated from the lowest d18O value and, therefore, represents a minimum value for the fluid. Further
calculations of f/r ratios were carried out using a difluid
of 101%.
Amphibole from the amphibolitization zone has almost identical d18O values to amphibole from pristine
gabbro. Consequently, calculated f/r values assuming a
single-pass case of the fluid are low (<02), indicating a
rock-dominated regime for the amphibolitization zone.
Fluid:rock ratios of samples from the scapolitization
zone vary between 25 and 14, indicating a clearly fluiddominated regime if a single-pass case is assumed.
Assuming a multi-pass case (or an evolving fluid)
lowers the calculated f/r significantly to a maximum of
11 for the scapolitization zone and of <01 for the
amphibolitization zone.
Fluid:rock ratios might be overestimated because
using the d18O values of the shear zone amphibole to
represent difluid carries the implicit assumption that
these were formed under completely fluid-dominated
Journal of Petrology, 2015, Vol. 56, No. 2
Mantle
(Kendrick et al. 2012)
SW
sca
po
liti
zat
ion
fore arc sediments
(Kendrick et al. 2011)
(John et al. 2011)
S apoScapop
tes
tes
lites
sw e
v
traje apor.
ctory
5
amphibolitization
Bio
t
ites
1
Evaporites
The pervasive metasomatism in the Bamble sector
occurs on a regional scale and affects not only gabbros, but almost all magmatic and metamorphic rocks
(Engvik et al., 2008, 2011). Therefore, a large quantity
of external fluid is required to allow this extensive alteration. On a metre scale, fluid–rock interaction is
localized in zones of enhanced permeability, such as
veins and shear zones, and their adjacent wall-rocks.
Fluid inclusions from neighbouring metasediments
(Fig. 1b) and quartz veins have high NaCl concentrations (up to 25 wt % and higher as indicated by NaCl
daughter crystals found within the inclusions) (Touret,
1985; Nijland et al., 1998a). This saline fluid was also
enriched in K, Mg, B and P, as can be inferred from
newly formed minerals (Engvik et al., 2011). The origin
of these brines is still under debate. Possible sources
are as follows: (1) deeply infiltrating fluids derived
from overlying sediment sequences (Munz et al., 1995;
Nijland & Touret, 2001); (2) fluids from prograde metamorphosed, deeply buried sediments (Engvik et al.,
2011); (3) late-stage magmatic fluids; (4) fluids derived
from the mobilization of evaporites (Touret, 1985;
Engvik et al., 2011). Tourmalines from different alteration zones (i.e. scapolitization and albitization) of
Bamble rocks have d11B values of –6 to þ26%, indicating a fluid component derived from a marine environment (Bast et al., 2014).
Chlorine concentrations in the upper mantle are low
(1–4 mg g–1) (Saal et al., 2002), but are higher and extremely variable (1–4000 mg g–1) in basaltic melts
derived from a mantle source (Kendrick et al., 2012).
The most pristine gabbro sample from Langøy has a Cl
concentration of 1495 mg g–1, which is within the Cl concentration range for mantle-derived magmas. However,
an additional gain of Cl from interaction with the infiltrating hydrothermal fluid cannot be excluded. Bromine
and iodine concentrations (Br 32 mg g–1; I 004 mg g–1) in
-3
Origin of the fluid
the pristine gabbro are elevated compared with averaged literature data for mid-ocean ridge basalt (MORB;
Edmond et al., 1979; Schilling et al., 1980; Deruelle
et al., 1992; Jambon et al., 1995; John et al., 2011), but
are similar to concentrations in enriched (E)-MORB
glasses from Macquarie Island (Br 038–45 mg g–1; I
001–0109 mg g–1) (Kendrick et al., 2012). Br/Cl and I/Cl
ratios (Fig. 9) in the least altered sample are similar to
those for E-MORB from Macquarie Island, which
Kendrick et al. (2012) considered is representative of
present-day mantle. Therefore, the halogen ratios of the
most pristine Langøy gabbro may represent the halogen ratios of its mantle source.
Bulk Br/Cl and I/Cl ratios of altered samples result
from interaction of halogen-poor, pristine gabbro with a
halogen-rich metasomatic fluid, which leads to the formation of minerals incorporating different amounts of
halogens. The characteristics of the infiltrating metasomatic fluid are best represented by the most strongly
altered samples adjacent to the shear zone, as these
were fluid-dominated and in contact with the initial fluid
composition. Halogen concentrations and ratios of the
fluid are given by the partition coefficient (Dmin ¼ chalmin/
chalfluid) between mineral and fluid. If the Dmin values
are known, a bulk partition coefficient (Dbulk) can be calculated. Partition data for Br and I between fluid and
minerals of interest (i.e. amphibole, scapolite and biotite) are lacking, but are probably smaller than for Cl.
Following lattice strain theory, one would expect
DCl > DBr > DI for amphibole, scapolite and biotite.
Without knowing the exact partition coefficients, we can
constrain the halogen ratios of the initial fluid by applying the relative order (DCl > DBr > DI) of partition coefficients to the measured halogen concentrations of the
most strongly altered samples. Consequently, Br/Cl
Br/Cl (*10)
conditions. Although this agrees with a difluid calculated
using the lowest reported value of calcite, Broekmans
et al. (1994) argued that the low values (102–106%) of
the vein calcite are probably the result of the strong
interaction of fluid with the wall-rock (metagabbro). If
the high d18O values of 20% observed for other vein
calcite from the same metagabbro were used for the
calculations, f/r of both alteration zones would be <<1
and the scapolitization zone would also be strongly
rock-dominated. Even though the estimation of the isotopic composition of the initial fluid and its evolution
owing to fractionation create uncertainties in the f/r calculations, a strongly rock-dominated regime for the
amphibolitization zone and a less rock-dominated regime for the scapolitization zone is a robust conclusion.
This is further expressed by the different degree of alteration in the amphibolitization zone (low) and scapolitization zone (complete), which indicates a decreasing
amount of fluid towards the pristine part of the
sequence.
313
Amphiboles
highly altered
least altered
100
1000
-6
I/Cl (*10 )
Fig. 9. Br/Cl vs I/Cl concentration ratios (by weight) of bulk-rock
samples and mineral separates from Langøy compared with
different fluid sources: pore fluids (Kendrick et al., 2011), evaporites (Holser, 1979), pelitic fore-arc sediments (John et al.,
2011), mantle (Kendrick et al., 2012) and seawater (SW); light
grey arrow indicates halogen ratio trend in scapolite zone; dark
grey arrow indicates halogen ratio trend in amphibole zone.
314
Journal of Petrology, 2015, Vol. 56, No. 2
ratios and I/Cl ratios must be higher than 3 10–3 and
25 10–6, respectively, measured for samples from the
scapolite zone. When comparing these ratios with those
of geologically important fluid sources, they fall in the
range of marine pore fluids (Fig. 9) and marine sediments, which have high Br/Cl and I/Cl ratios (John et al.,
2011). Therefore, a fluid produced by prograde metamorphism of marine sediments together with liberation
of coexisting pore fluids should reflect the high I/Cl
ratios of the sediments and the high Br/Cl of the pore
fluids (Kendrick et al., 2011). In contrast, evaporites produce fluids with very low I/Cl and Br/Cl ratios (Holser,
1979), neither of which match the ratios of the Bamble
fluid. A late-stage magmatic origin of the fluid is also
unlikely as magmatic fluids from ore deposits have I/Cl
ratios and Br/Cl ratios similar to mantle ratios (Kendrick
et al., 2001, 2011), which are too low if the partitioning
between fluid and alteration minerals is considered.
Cl
Cl
The observed correlation XBio
033XAmph
(Markl &
Piazolo, 1998; Markl et al., 1998) allows the calculation
of NaCl activity from amphibole composition by measuring the Cl concentration in biotite and amphibole and
assuming ideal mixing of OH– and Cl– in the biotite and
amphibole structures.
A similar approach was used to calculate NaCl
activities in fluids from the Langøy metagabbro. The
reaction
OH-Tremolite þ Enstatite þ Anorthite þ NaClðaqÞ
þSiO2 ðaqÞ ! Cl-Tremolite þ H2 O þ Albite
(8a)
þSpinel þ Diopside
Ca2 Mg5 Si8 O22 ðOHÞ2 þ 1 5 Mg2 Si2 O6
þ2 CaAl2 Si2 O8 þ 2 NaCl ðaqÞ þ 3 SiO2 ðaqÞ
! Ca2 Mg5 Si8 O22 Cl2 þ H2 O þ 2 NaAlSi3 O8
(8b)
þMgAl2 O4 þ 2 CaMgSi2 O6
Calculation of fluid salinity
Besides scapolite, amphibole is a major sink for Cl
during fluid–rock interaction in the scapolitization
zone and the main Cl-bearing phase in the amphibolitization zone. Amphibole from the two alteration
zones always originates from the reaction of the pervasively infiltrating fluid with the unaltered gabbro.
Therefore, the halogen activities of the fluid should be
reflected in the halogen concentrations of the
amphibole.
Based on the thermodynamically calculated partitioning of F–Cl–OH between micas and hydrothermal
fluids (Zhu & Sverjensky, 1991, 1992) and observed correlations of XCl in biotite and coexisting amphibole,
Markl et al. (1998) used the Cl concentration of amphibole to calculate the NaCl activity of fluids in a lower
crustal environment. This approach is based on the following equilibrium exchange reaction for biotite:
OH Biotite þ NaClðaqÞ þ Anorthite
! Cl Biotite þ Albite þ Grossular þ H2 O þ Kyanite
(5a)
KMg3 AlSi3 O10 ðOHÞ2 þ 2NaClðaqÞ þ 15CaAl2 Si2 O8
! KMg3 AlSi3 O10 Cl2 þ 2NaAlSi3 O8 þ 5Ca3 Al2 Si3 O12
þH2 O þ 9Al2 SiO5 :
(5b)
The equilibrium constant for the reaction is
2
K¼
5
9
aH2 O ðaClBio aAb aGrs aKy Þ
:
15
2
ðaOHBio aAn
aNaCl
Þ
(6)
It follows that a specific biotite composition relates to a
specific fluid composition by the expression
aOH-Bio aH2 O 1 2 5 9 15
¼ 2 K aAb aGrs aKy aAn :
aCl-Bio
aNaCl
(7)
2
5
9
15
The term K 1 aAb
aGrs
aKy
aAn
is a constant for a given
temperature, pressure, and mineral assemblage and
was estimated to be 1891 (Markl et al., 1998).
relates the mineral assemblage found in the metagabbro to the coexisting NaCl fluid. The equilibrium constant for this reaction is
Kð9Þ ¼
2
2
aDi
aSp Þ
aH2 O ðaCl-Tr aAb
2
2 a 15 a 3
aNaCl
ðaOH-Tr aAn
En SiO2 Þ
(9)
and
aOH-Tr aH2 O 1 2 2
2 15 3
¼ 2 Kð9Þ aAb aDi aSp aAn
aEn aSiO2 :
aCl-Tr
aNaCl
(10)
As thermodynamic data for this reaction are lacking,
thermodynamic models (e.g. supcrt92) are not applicable to calculate the equilibrium constant K(9). However,
1 2
2
2 15 3
the term Kð9Þ
aAb aDi
aSp aAn
aEn aSiO2 can be estimated
2
empirically by using a known activity ratio (aH2 O =aNaCl
)
for one fluid and the corresponding activity ratio (aCl-Tr/
aOH-Tr) for amphibole. As long as T, P, and the mineral
1 2
2
2
assemblage are constant, the term Kð9Þ
aAb aDi
aSp aAn
15 3
aEn aSiO2 is a constant and can be used to calculate activity ratios. In the Langøy sequence, the least-altered
sample showing first signs of amphibolitization provides an assemblage that can be used to calculate this
term. Newly formed amphibole from this zone has Cl
contents as high as 13 wt % (measured by EMPA).
Assuming an ideal two-site mixing model between Cl–
Amph
and OH– for amphibole, XCl
is 015 and activities for
OH-tremolite and Cl-tremolite are 07225 and 00225, respectively. Halite coexisting with the most Cl-rich
amphiboles (Fig. 3d) indicates a fluid saturated in NaCl
for these samples. Saturation concentrations (and activity ratios) for the system NaCl–H2O can be calculated by
using the correlation formulae of Driesner & Heinrich
(2007). For a given temperature of 600 C and pressure
of 02 GPa for the Bamble metasomatic event (Nijland &
Touret, 2001), a saturated fluid has a XH2 O [¼H2O/
(NaCl þ H2O)] of 055 (73 wt % NaCl), which corresponds
2
to aH2 O =aNaCl
¼ 149. The activity data for this sample,
which contains halite crystals and the highest Cl
Journal of Petrology, 2015, Vol. 56, No. 2
315
concentrations in amphibole, were then used to calcu1 2
2
2 15 3
late the term Kð9Þ
aAb aDi
aSp aAn
aEn aSiO2 , which is 2155.
The expression
2
aH2 O XOH
¼ 2 -Tr =21 55:
2
aNaCl
XCl-Tr
(11)
links the fluid composition to the amphibole
composition.
For this calculation, Cl– is assumed to be speciated
only with Naþ in the fluid, but theoretically it could be
also speciated with other ions, such as Kþ and Ca2þ.
Although K is important for the formation of alteration
phases (i.e. biotite), Markl et al. (1998) showed that by
far the most dominant uncharged species in their alteration system is NaCl. Furthermore, the observed salt
crystals in the least altered sample are pure halite, indicating that NaCl was present at all alteration stages.
The fact that no sylvite (KCl) was found, a common
high-temperature salt, suggests that either KCl saturation was not reached, which implies that the concentration of K in the fluid was low, or the amount of K was
not sufficient to produce a recognizable amount of sylvite. From petrographical observations, the former
seems to be more likely as there are no stable K-rich
minerals in the amphibolitization zone.
Calculations based on equation (8) utilize the exchange of Cl– and OH– in amphibole within a Na-buffered reaction. A reaction buffered in K could be used in
a similar manner and yield similar results. In contrast to
the NaCl-buffered reaction, where the saturation of halite is used to calculate a reference NaCl/H2O ratio, there
is no reference KCl/H2O ratio because KCl was not detected in the sample set, which in turn makes a calculation of KCl concentration impossible. However, even if
we assume that KCl was present, it would be saturated
at a fluid composition of XH2 O ¼ 044 (Chou et al., 1992).
If we further assume that the most Cl-rich amphibole
(13 wt % Cl) was formed in equilibrium with sylvite and
fluid, rather than halite and fluid, the equilibrium constant used would shift from 2155 for NaCl to 5201 for
KCl, resulting in a shift towards slightly higher values
for calculated salinities. A pure NaCl-bearing fluid with
an XH2 O of 078 (48 wt % NaCl) would have an XH2 O of
070 (63 wt % KCl) as a hypothetical fluid in the binary
system H2O–KCl. Although mobility of Ca2þ and Fe2þ is
not observed, these divalent cations may also play a
role in speciating Cl–.
Evolution of the fluid salinity and the effect of Clconsuming vs desiccation reactions
A fluid in equilibrium with shear-zone amphiboles has
2
an XNaCl of 022 (aH2 O =aNaCl
¼ 132), giving an initial salinity of 48 wt % NaCl for the infiltrating fluid. Fluid inclusions in metasediments from Bamble also record a
high-salinity composition of the fluids; hence are in accordance with our findings (Touret, 1985).
Amph
Constant XCl
in amphibole from the scapolitization zone indicates little to no change in fluid salinity
during amphibole formation (Fig. 6). The decreasing Cl
concentrations in scapolite with increasing distance to
the shear zone (Fig. 6) could be an indicator of changing
salinity, but the capability of using scapolite as a fluid
probe for NaCl concentrations is limited. Ellis (1978)
investigated the exchange equilibrium of scapolite, calcite, and fluid at 750 C and 04 GPa and formulated an
equation to calculate fluid compositions from coexisting scapolite. However, the application of Ellis’ formula
to natural examples has been shown to be difficult
(Mora & Valley, 1989; Oliver et al., 1992; Markl &
Piazolo, 1998) as it lacks corrections for pressure, temperature and CO2 activity. Although scapolite cannot be
Fluid
used to quantify the true XNaCl
, it may display changes
in fluid composition and, hence, the NaCl/CO2 ratio at
constant pressure and temperature. Consequently, the
observed decrease in Cl concentration in scapolite with
increasing distance to the shear zone (Fig. 6) can be explained by a decreasing salinity at constant CO2 activity,
or increasing CO2 activity at constant salinity. Scapolite
formed early during pervasive fluid infiltration of the
unaltered gabbro was in equilibrium with a shear zone
bulk fluid with a certain initial NaCl/CO2 ratio, hence it
shows the highest Cl concentration. Owing to the formation of this Cl-rich scapolite, the NaCl/CO2 ratio in the
fluid decreases. Scapolite that forms later or further
away from the shear zone will therefore contain less Cl
and more CO2.
In the amphibolitization zone, where scapolite is not
present, amphibole is the major sink for halogens and
is used as a salinity probe. Average Cl concentration in
amphibole increases from 04 wt % (01 a.p.f.u.) to
11 wt % (03 a.p.f.u) with increasing distance to the
Fluid
shear zone. Using equation (11), calculated XNaCl
increases from 022 (48 wt % NaCl) to 040 (69 wt % NaCl)
(Fig. 10). In the scapolite zone Cl partitions into the
solid, whereas in the amphibole zone Cl is more compatible in the fluid compared with the solid. Therefore, a
different process governs salinity changes in this zone.
The formation of amphibole preferentially consumes
OH– over Cl– from the fluid [equation (1)]; consequently,
Fluid
XNaCl
will increase. The formation of Cl-rich amphiboles
found in other localities of lower crustal fluid–rock interaction sequences shows a similar history and was
described as being the result of desiccation reactions
(Kullerud, 1996; Markl et al., 1998). Hence, the controlling factor for salinity changes in the sequence is the
amount of newly formed amphibole (i.e. reaction progress) and the amount of water that is consumed by
this process. This desiccation reaction leads to the
observed enrichment of NaCl in the fluid until the saturation point and the formation of halite in the least
altered samples (Fig. 3d). It is likely that the consumption of fluid limits the alteration progress in the Langøy
metagabbro.
Fractionation of stable chlorine isotopes
d37Cl values from the Langøy sample sequence display
distinct trends towards lower d37Cl values with
316
Journal of Petrology, 2015, Vol. 56, No. 2
wt% NaCl
800
93
0.2 GPa
69
83
a
[Fluid]
N
Cl
s
T (°C)
n
t io
ra
500
u
at
700
600
45
0.4 GPa
[Salt]
+
[Fluid]
decreasing
alteration
Markl et al. (1998)
this study
400
0
0.2
0.4
0.6
0.8
1
XH2O
Fig. 10. Salinity evolution of a saline fluid, calculated from
amphibole composition (see text for details of the calculation).
NaCl saturation lines after Driesner & Heinrich (2007);
XH2 O ¼ H2O/(NaCl þ H2O).
increasing distance from the shear zone (Fig. 7). The
d37Cl values of amphibole from the scapolitization zone
decrease from 09% in the shear zone to 0% close to the
transition to the amphibolitization zone. The transition
zone itself is characterized by slightly higher d37Cl values
of 05%. However, in the amphibolitization zone, d37Cl
values decrease constantly with distance from the shear
zone, from 05 to –06% in the least altered sample.
Chlorine concentrations in all altered samples are at least
25 times higher than Cl concentrations in the pristine
gabbro; hence, most Cl is brought into the system by a
single fluid event of constant isotopic composition causing scapolitization and amphibolitization. Therefore,
observed changes in the isotopic composition of the
altered samples must be related to changes in the fluid
composition and variations require a fractionation process capable of changing the d37Cl value of the fluid.
Two fractionation processes are discussed below, both
of which can lead to the observed trend in d37Cl values
and help to understand the Cl isotope system.
Rayleigh fractionation
Equilibrium isotope fractionation owing to different
bonding energies was first observed for Cl in evaporation experiments, in which the precipitating salts have
higher d37Cl values than the original saline water
(Eggenkamp et al., 1995). Barnes & Cisneros (2012)
described a positive correlation between d37Cl values
and the amphibole content of samples from altered oceanic crust that can be explained by a fractionation of Cl
isotopes between minerals, where 37Cl preferentially
partitions into the amphibole. These findings are consistent with theoretical calculations based on vibration
data for Cl in minerals, which estimate d37Cl values to
be 2–3% higher (at 25 C) for minerals where Cl is
bonded to þ2 cations (e.g. micas, amphiboles) than
those where Cl is bonded to þ1 cations (e.g. halite)
(Schauble et al., 2003). Although fractionation decreases with increasing temperature, calculations show
that even at 300 C and higher it is still 1% between
minerals. Additionally, fractionation of Cl isotopes between Cl strongly bonded in a crystal lattice and Cl– dissociated in a fluid is calculated to be of the order of 06
to 15% at 300 C depending on the bonding partner
(Schauble et al., 2003).
In the studied sample sequence, chlorine is primarily
bonded to either þ2 cations in amphibole and biotite or
þ1 cations in scapolite. Therefore, minor fractionation
of Cl isotopes between Cl– dissociated in aqueous solution and different minerals is expected even at the rather high temperatures of the Bamble metasomatic
event depending on the bonding partner. A direct observation of this fractionation between fluid and minerals is impossible as no suitable fluid inclusions exist
in the sample sequence. However, the trends of
decreasing d37Cl values with increasing distance to the
shear zone (Fig. 7) may be explained by a Rayleigh fractionation model. Rayleigh fractionation will lead to an
enrichment of one isotope in one phase and a simultaneous depletion of the same isotope in another phase.
For example, in most cases minerals growing from a
fluid preferentially incorporate heavier isotopes, leading
to relative enrichment of lighter isotopes in the fluid
(e.g. Eggenkamp et al., 1995; Marschall et al., 2009;
Barnes & Cisneros, 2012). The general Rayleigh fractionation model can be formulated in terms of delta
notation for Cl isotopes as
d37 Cl ¼ ð1000 þ d37 Cli ÞF a–1 –1000
(12)
where d37Cli and d37Cl are the initial and the final d37Cl
values of the fluid phase, respectively, F is the fraction
of Cl remaining in the fluid and a is the equilibrium
fractionation factor between the phases at a given
temperature.
Calculations based on equation (12) are used to
evaluate the role of Rayleigh fractionation and to model
the isotopic evolution of the Bamble fluid. For parameterization, three a values (10005, 10007 and 10010)
were approximated based on the calculations of
Schauble et al. (2003) assuming a temperature of 600 C
(Fig. 11). The d37Cli of the fluid can be calculated from
averaged values of the bulk-rock and amphibole determined from the shear zone samples. The d37Cli values
using the three a values are þ02%, 0% and –03%, respectively, and are all close to 0%. The fraction of Cl remaining in the fluid (F) is a critical parameter, which
cannot be measured directly, but can be constrained in
various ways. As a first assumption, F is taken as the
fraction of the Cl inventory at a certain distance to the
shear zone relative to the overall Cl inventory of the section. This necessarily assumes that all fluid entering the
alteration sequence and the dissolved Cl are consumed.
This assumption is justified by the observed desiccation
of the fluid during amphibole formation in a
Journal of Petrology, 2015, Vol. 56, No. 2
317
1.5
1.5
bulk rock
0.5
δ 37 Cl
δ 37 Cl
0.5
0
−0.5
Rayleigh fract. based on:
−1
−20
1.0
007
α=
1.00
10
α=
−25
Progress of:
Amphibolitization
α=
1.0
00
5
Cl inventory
−1.5
−2
amphibole dominated
scapolite dominated
1
amphibole
−15
0
−0.5
−1
Rayleigh fractionation
1
Rayleigh fractionation due to:
Amphibolitization
Scapolitization
combination of
both processes
−1.5
Scapolitization
−10
−5
0
distance (cm)
Fig. 11. Calculated d37Cl values along the sampling transect by
Rayleigh fractionation using equation (12) with varying fractionation coefficients (a) and two models to calculate the fraction of Cl (F). Model 1 (continuous lines): considering the
consumption of Cl from the fluid, F is calculated from the Cl
budget of the whole sequence; model 2 (dotted lines): the progress of each of the alteration reactions, F is calculated from
the modal abundances of Cl-consuming minerals, amphibole
and scapolite; d37Cl values of Bamble bulk-rocks (circles) and
amphibole (diamonds) are shown for comparison.
rock-dominated environment until halite saturation. A
polynomial equation was fitted to the measured bulk Cl
concentration data to calculate the inventory of Cl for
various distances to the shear zone. Based on these
data, the modelled Rayleigh fractionation trend shows
decreasing d37Cl values (Fig. 11) with increasing distance to the shear zone in the range of measured d37Cl
values and is able to explain the low d37Cl values in the
amphibolitization zone. Although this simple model
matches some of the observed isotopic features, some
considerations need to be taken into account. As the
number of mineral reactions involved in a Rayleigh process increases, the system becomes substantially more
complicated. In the presented sequence, the Cl content
of the fluid is mainly controlled by two reactions (scapolite and amphibole formation). The simple model presented considers only one process, a linear
consumption of Cl from the fluid during pervasive reaction with the bulk solid. To obtain a more realistic case
an additional model is needed that links the fraction of
remaining Cl (F) to the progress of each of the mineral
reactions and not only to Cl concentrations measured in
the samples.
Although the relative timing and rates of the two reactions are unknown, petrographical observations
show that amphibole [equation (1)] always forms before
scapolite [equation (3)] and is coarsened during scapolitization. Therefore, a contemporaneous formation of
amphibole in the amphibolitization zone with scapolite
and amphibole in the scapolitization zone is likely. In
turn, this will cause consumption of Cl and H2O by the
two alteration reactions simultaneously, but spatially at
different places within the sequence until all fluid and Cl
is consumed. The amount of Cl and H2O consumed is
directly proportional to the reaction progress, which is
indicated by the modal abundance of newly formed
−2
−25
−20
−15
−10
−5
0
distance (cm)
Fig. 12. d37Cl values in the sample sequence as calculated from
contemporaneous Rayleigh fractionation (a ¼ 10010) in
amphibole-dominated and scapolite-dominated parts of the alteration sequence. d37Cl values are calculated for bulk-rocks in
equilibrium with the metasomatic fluid; d37Cl values of Bamble
bulk-rocks (circles) and amphiboles (diamonds) are shown for
comparison.
minerals. The remaining fraction of Cl not incorporated
in minerals (F) can then be inferred from the reaction
progress, which is expressed by modal abundances of
scapolite and amphibole (Fig. 3e). If we assume contemporaneous formation of amphibole and scapolite,
high modal abundances of alteration minerals (amphibole in the amphibolitization zone and scapolite in the
scapolitization zone) represent a high reaction progress
and lead to high values of F as there is a sufficient quantity of fluid and Cl to form these alteration minerals.
During formation of alteration minerals fluid and Cl will
be consumed and F will decrease as there will be a
lesser amount of Cl in the remaining, but decreasing
amount of fluid. Consequently, the subsequently
smaller abundance of alteration minerals indicates a
decreasing reaction progress and gives a small number
for F. By using the fraction F derived from the reaction
progress, the Rayleigh fractionation model can be divided into two fractionation curves for the two specific alteration zones; that is, the amphibolitization zone and
the scapolitization zone, respectively (Fig. 11). The d37Cl
values of minerals calculated by the second model are
in agreement with the measured d37Cl values of the
bulk-rocks and amphibole in the sequence and show
the same trends of decreasing d37Cl with increasing distance to the shear zone for both specific parts of the
overall alteration zones (Fig. 12). Although scapolite
from different samples shows no distinct trend (Fig. 7),
d37Cl values agree within given uncertainties with isotope data for other minerals. Only the sample adjacent
to the shear zone has an unexpectedly low value for
scapolite compared with other Cl-bearing phases.
Assuming a changing fluid to cause the observed
trends in amphibole and biotite, a flat pattern of d37Cl in
scapolite would mean that it has formed in equilibrium
with a fluid of constant composition. In turn, this would
argue for disequilibrium between scapolite and amphibole. The general observation that scapolite has slightly
lower d37Cl values than other phases might be
10000
8000
6000
4000
2000
1
0.5
0
δ 37 Cl
explained by the bonding of Cl to Na and the resulting
lower equilibrium fractionation factor (Schauble et al.,
2003).
The Cl isotope composition of the infiltrating fluid
can be calculated using realistic fractionation factors
ranging from 10005 to 10010 (Schauble et al., 2003;
Barnes & Cisneros, 2012). The infiltrating fluid had a
d37Cl value near 0% (–03 to þ02%), which progressively decreased in value during fluid–rock interaction
(to as low as –16 to –11%). The d37Cl values of marine
pore fluids vary over a large range, between 0% for unchanged primary fluids and –8% for fluids that have
undergone isotope fractionation at low-temperature
caused by fluid–rock interaction, ion filtration or
diffusion (Ransom et al., 1995; Spivack et al., 2002;
Godon et al., 2004a; Bonifacie et al., 2007). The
observed d37Cl values for the Bamble fluid fall within
the upper bound of this range, which is consistent
with the previously concluded origin as a marine pore
fluid. Furthermore, the relatively high initial d37Cl value
of 0% indicates a primary pore fluid that experienced
little to no low-temperature alteration prior to hightemperature evolution during the Bamble metasomatic
event.
Journal of Petrology, 2015, Vol. 56, No. 2
Cl (µg/g)
318
−0.5
−1
−1.5
Model 1
−2
−60
Model 2
bulk rock
β = 0.018
β = 0.036
−50
amphibole
−40
−30
−20
−10
0
distance (cm)
Fig. 13. Results of kinetic modelling of d37Cl evolution. Model 1,
simple diffusion model; model 2, diffusion model including a
moving boundary to consider reactive fluid flow (advection
term). Significant negative d37Cl values can be observed at high
b values (i.e. large kinetic fractionation between isotopes) and
with a fast-moving boundary (i.e. fast alteration reactions); a
b value of 0036 corresponds to a 02% fractionation of 35Cl compared with 37Cl; d37Cl values of Bamble bulk-rocks (circles) and
amphiboles (diamonds) are shown for comparison.
sample sequence. Using a time-independent numerical
solution for Fick’s second law
Kinetic fractionation
Diffusion-related processes are well known to cause
variations in isotopic ratios of different isotopic systems
(e.g. Li, Ca, Mg; DePaolo, 2004; Richter et al., 2006; Teng
et al., 2006; Dohmen et al., 2010; John et al., 2012).
Owing to mass-related differences in their diffusion coefficients (D), lighter isotopes travel faster in liquid and
solid media than heavier isotopes of the same element
(Richter et al., 2006). This kinetic fractionation is large
for isotopic systems with large relative mass differences (i.e. H, Li). For the Cl system, experimental investigations show variations in d37Cl values of several per
mil depending mainly on concentration gradients and
temperature
(Eggenkamp
&
Coleman,
2009).
Comparison of experimentally derived fractionation factors (Dlight/Dheavy) with natural data shows good agreement and models based on these fractionation factors
give reasonable results for low-temperature systems
(0–80 C) of salt and brine migration (Eggenkamp et al.,
1994; Eggenkamp & Coleman, 2009; Amundson et al.,
2012). Unfortunately, fractionation factors for hightemperature applications are lacking. Although diffusion coefficients of elements and species are highly
dependent on temperature, calculated fractionation factors for Cl isotopes (i.e. the ratios of diffusion coefficients of 35Cl and 37Cl) show little temperature
dependence (Desaulniers et al., 1986; Eggenkamp et al.,
1994; Eggenkamp & Coleman, 2009). Therefore, fractionation data from low-temperature experiments might
be used in principle to model high-temperature processes, such as for the Bamble metasomatic event.
A numerical diffusion model was set up to validate a
possible kinetic fractionation of Cl isotopes in our
dc
d2 c
¼D 2
dt
dx
(13)
(where D is diffusion coefficient, c is concentration and
x is distance), for each of 35Cl and 37Cl, we calculated
the d37Cl value and the Cl concentration as the ratio and
the sum of both isotopes, respectively, within the same
run. As the model is not used to obtain any time constraints, the ratios of the diffusion coefficients (given by
the fractionation factor) rather than their actual values
are relevant. Therefore, D37 was calculated for various
fractionation factors from the literature (Richter et al.,
2006; Eggenkamp & Coleman, 2009). The kinetic fractionation model was defined using the measured Cl
concentration in unaltered gabbro (using a d37Cl value
of –01%), the measured Cl concentration in the most
strongly altered gabbro sample (using a d37Cl value of
08%) and a profile length of 06 m. After modelling concentrations of each of the isotopes, d37Cl values were
obtained by the standard formulation relative to the
seawater standard (37ClSW/35ClSW ¼ 0319766; Shields
et al., 1962; Godon et al., 2004b).
The results of this simple model show that for the
given parameters kinetic fractionation would lead to
decreasing d37Cl values from 08 to –1% for a fractionation factor of 1002 (Fig. 13); that is, the lighter 35Cl isotope is 02% enriched over the heavier 37Cl. Generally, b
values, which are empirically derived from the expression D1/D2 ¼ (m2/m1)b, are used to describe the magnitude of kinetic fractionation. In our case, a fractionation
factor of 1002 corresponds to a b value of 0036 (as
used in Fig. 13), if only Cl– is considered, or 0059 if the
species NaCl is considered to be the moving species.
Journal of Petrology, 2015, Vol. 56, No. 2
A more realistic model that considers an advective flow
10 times higher than the diffusive flow by computing
numerically a moving boundary produces slightly more
negative d37Cl values (Fig. 13). Although the modelled
kinetic fractionation is able to produce d37Cl values similar to those observed in the Bamble sequence, several
points argue against a purely kinetic fractionation process for the Bamble setting, as follows.
1. Concentration and d37Cl values must be modelled
simultaneously (Eggenkamp & Coleman, 2009); consequently, both calculated concentration and d37Cl
values must fit to the measured concentration and
d37Cl values at each point of the natural sequence.
With kinetic fractionation, we are only able to fit either the Cl concentration or d37Cl value.
2. As kinetic fractionation is a continuous process, it
cannot produce the two trends (decreasing d37Cl
values in scapolitization and amphibolitization zone)
observed in the sequence (Fig. 7).
3. The observed salinity evolution showing highest
NaCl concentration in the least altered gabbro argues
against a purely diffusion-controlled process of Cl
fractionation, because concentration gradients in Cl
would lead to diffusion in the opposite direction.
Nevertheless, kinetic fractionation may be a process
that causes negative d37Cl values in other geological
settings, also at elevated temperatures.
For the studied system, it seems likely that the negative trends of d37Cl with increasing distance to the shear
zone are dominantly caused by a Rayleigh fractionation-like process in a rather static system.
Consequently, reaction rates must be slower than the
rate of pervasive fluid ingress, but still fast enough to
ensure observed fluid evolution during progressive
fluid–rock interaction. Additional fractionation of Cl isotopes by a kinetic process would be capable of enhancing the calculated Rayleigh fractionation, so that the
effect of each of the fractionation mechanisms could be
smaller. This would allow the application of smaller
fractionation factors to our calculations. The exact contribution of each fractionation process to the overall
fractionation is not known, but depends on the complex
interplay of mineral reaction rates and diffusion rates.
Implications for using halogens and stable Cl
isotopes to decipher fluid–rock interaction and
fluid-source systematics
Contrary to what was initially expected (John et al.,
2010, 2011; Selverstone & Sharp, 2013), halogen concentrations and also d37Cl values do not behave fully
conservatively during fluid–rock interaction. Smallscale heterogeneities develop adjacent to the major
fluid pathways, which reflect large variability in the
halogen signal on a centimetre to decimetre scale.
Halogen concentrations and ratios in solids that have
undergone fluid–rock interaction depend strongly on
mineral-specific distribution coefficients between fluid
and mineral. These are highly variable; hence different
319
minerals (e.g. amphibole, mica, scapolite, apatite) will
show different halogen ratios (Fig. 9). Consequently,
changes in the bulk halogen ratios of rocks may be due
to changes in the abundance of halogen-bearing phases
and not necessarily caused by changes in the halogen
source. Using halogen ratios as a fingerprint of fluid
sources can, therefore, be applied only with knowledge
of the complete alteration history of the rock.
Chlorine isotopes are less affected by inter-mineral
fractionation factors than by Rayleigh and kinetic fractionation during fluid–rock interaction, as shown here
and previously argued from fluid inclusion data by
Gleeson & Smith (2009). For other isotopic systems (e.g.
Li, B, Fe, Mg), it has been shown that fractionation processes are able to change d values over a large range
(Teng et al., 2006; Marschall et al., 2007a, 2007b; Li et al.,
2010; Penniston-Dorland et al., 2010; John et al., 2012).
For example, Marschall et al. (2007b) argued that low
d7Li values recorded in eclogites are the result of kinetic
fractionation during diffusive influx of light Li from
country rocks and measured d7Li values are not representative for the major rock type. Dynamic processes
such as fluid flow, fluid–mineral reactions and diffusion
govern these systems and have major impacts on the
behaviour of stable isotopes and thus the measured d
values of the rock. The data presented here and modelled fractionation processes also suggest a similar dependence on dynamic factors for the Cl isotope system.
In fact, it may explain partly the large spread observed
for the various Cl reservoirs (Barnes & Sharp, 2006;
Sharp et al., 2007; Bonifacie et al., 2008; John et al.,
2010; Selverstone & Sharp, 2011), especially in those affected by fluid–rock interactions.
Our results have ramifications for sampling strategies. If samples are taken for reservoir studies, it is important to know the history and geometry of fluid–rock
interaction in the sample area. Sampling should cover a
large volume, preferably far away from any fluid pathways, to avoid or integrate over possible alterations
(Fig. 14). However, alteration processes will always
introduce a certain bias in d37Cl values. For studies concerning fluid flow and fluid evolution the sampling target is shifted towards fluid pathways (Fig. 14). To learn
about the effects of the fluid on the wall-rock system
sampling should be perpendicular to the flow structure,
but if the interest is set more on the evolution of the
fluid, sampling along the fluid-dominated systems
(veins or shear zones) is essential.
SUMMARY AND CONCLUSIONS
Halogen concentrations and ratios can be powerful
tools in the investigation of metasomatic settings, if
careful sampling is applied. They can be used to trace
fluid sources and fluid–rock interaction processes.
Halogen source studies might be easily affected by a
sampling bias of rock prone to fluid–rock interaction. In
the Bamble area, the halogen concentrations and ratios
(e.g. Br/Cl, I/Cl) of the scapolitized samples suggest that
320
Journal of Petrology, 2015, Vol. 56, No. 2
δ37Cl
+1 ‰
δ37Cl
0‰
δ37Cl
-1 to +1‰
0‰
from -1 to +1 ‰
pristine
gabbro
+1 ‰
metagrabbro
in shear zones
- progressive alteration reactions
- δ37Cl fractionation
Fig. 14. Schematic illustration (not to scale) of fluid flow in the Bamble metagabbros. Isotopic fractionation owing to intense fluid–rock interaction causes centimetre- to decimetre- scale heterogeneities in d37Cl values where fluid flow is pervasive.
fluids most probably originated from marine pore fluids
and marine sediments during prograde metamorphism
of their host-rocks. Remobilization of meta-evaporites
or a mantle source for these fluids for the Bamble area
can be excluded as these sources have very different
halogen ratios.
The altered metagabbro sequence shows how mineral formation was able to cause significant changes in
the halogen chemistry of the involved fluid. Assuming
that a brine (48 wt % NaCl) pervasively infiltrated the
unaltered gabbro first, the subsequent formation of Clrich scapolite caused a decrease of the fluid salinity.
The formation of amphibole during continuing fluid ingress led to increasing salinity owing to desiccation of
the fluid until all fluid was consumed and the alteration
process was terminated. The most evolved fluid, coexisting with the least altered gabbro, reached Cl saturation as indicated by halite formation.
Journal of Petrology, 2015, Vol. 56, No. 2
321
Both the pristine gabbro and infiltrating fluid had
similar d37Cl values, close to 0%. High d37Cl values of
samples and mineral separates from close to the shear
zone can be explained by equilibrium fractionation of Cl
isotopes between fluid and alteration phases formed by
reaction of the wall-rock with this fluid. Decreasing d37Cl
values with increasing distance to the shear zone originate from fractionation of Cl isotopes caused either by
pure Rayleigh fractionation or a combination of
Rayleigh and kinetic fractionation. These processes
seem to work so efficiently that they are able to change
d37Cl values by 2% on centimetre to decimetre scales.
We speculate that these processes are the reason for
the rather large spread of d37Cl values found in many
stable Cl-isotope studies.
ACKNOWLEDGEMENTS
The authors thank Alexandra Reschka and Jasper
Berndt-Gerdes for help with IC and electron microprobe
measurements, Toti E. Larson for help with d18O measurements, Rebecca Bast and Ralf Dohmen for fruitful
discussions, and the workshop in Münster for sample
preparation. H. Marschall, M. Smith, G. Layne and an
anonymous reviewer are thanked for their constructive
comments that helped to improve this paper. R. Gieré is
thanked for editorial handling.
FUNDING
This work was supported by the
Forschungsgemeinschaft (JO 349/3-1).
Deutsche
SUPPLEMENTARY DATA
Supplementary data for this paper are available at
Journal of Petrology online.
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