The complex hydrothermal history of granitc rocks

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The Complex Hydrothermal History of Granitic
Rocks: Multiple Feldspar Replacement Reactions
under Subsolidus Conditions
OLIVER PLU« MPER* AND ANDREW PUTNIS
INSTITUT FU« R MINERALOGIE, UNIVERSITY OF MU« NSTER, CORRENSSTR. 24, D-48149 MU« NSTER, GERMANY
RECEIVED NOVEMBER 13, 2008; ACCEPTED APRIL 10, 2009
ADVANCE ACCESS PUBLICATION MAY 18, 2009
Recurring subsolidus re-equilibration of granitic feldspars induced by
fluid infiltration events provides a record of fluid^rock interactions
that affect large volumes of the Earth’s continental crust.This has a
direct bearing on the interpretation of the present-day granitic rock
mineralogy and geochemistry. We examine Palaeoproterozoic grey
and red-stained granitoids from the Simpevarp and Laxemar areas
in SE Sweden, particularly focusing on consecutive feldspar replace-
ment reactions, to provide an in-depth understanding of subsolidus
re-equilibration of granitic rocks with hydrothermal fluids. The
apparently most unaltered grey granitoids contain highly porous oli-
goclase grains that enclose crystallographically continuous microcline
relicts.This texture suggests that the oligoclase is already secondary
and may be a replacement product of original microcline. Oligoclase
is progressively replaced by albite (�An9) along polysynthetic twin-
ning and intragranular fractures. The features of this replacement
are characteristic of a dissolution^reprecipitation mechanism. Fine-
grained mica (sericite) is closely associated with the albite porosity
within micron-sized pores observable with scanning electron micros-
copy as well as in nanopores imaged with transmission electron
microscopy. The reddening phenomenon in the vicinity of fractures
is contemporaneously related to the K-feldspathization of sericite,
which is restricted to the altered oligoclase. Submicron size hematite
precipitation within orthoclase pores at the replacement front results
in the red coloration. The complex associations between the fluid^
feldspar reactions indicate that the replacement reactions may be
due to sequential fluid infiltration events and that the granitoids
have undergone extensive subsolidus re-equilibration, changing the
original magmatic mineralogy. Therefore, the effects of large-scale
re-equilibrations of granitic rocks through hydrothermal convection
systems should be more closely considered.
KEY WORDS: dissolution^reprecipitation; feldspar replacement; fluid^
rock interaction; granites; hydrothermal alteration
I NTRODUCTION
The hydrothermal alteration of granitic rocks indicates
considerable interaction between external fluid circulation
systems and crustal rocks on an enormous scale, as shown
by oxygen isotope studies (Taylor, 1977; Taylor & Forester,
1979; Hoefs & Emmermann, 1983). Recent research into
granitic rocks attributes their mineralogical and geochemi-
cal characteristics to purely magmatic processes and typi-
cally invokes multiple parental magma sources from the
mantle and crust as well as magma mixing and crystal
fractionation (Clemens & Vielzeuf, 1987; Millar et al.,
2001; Yang et al., 2004; Kemp et al., 2007). However, these
studies neglected to consider the effect of pervasive, large-
scale fluid^rock interaction and the resulting subsolidus
re-equilibration of granitic rocks with hydrothermal
fluids, despite clear indications of the hydrothermal reset-
ting of oxygen isotopes.
Hydrothermal activity is likely to be a more common
phenomenon in the continental crust than is currently
documented, as it is a natural consequence of any thermal
perturbation in fluid-rich rocks (Fyfe et al., 1978; Yardley
& Shmulovich, 1995). Pervasive fluid circulation systems
can extend throughout the crust and may influence several
cubic kilometres of rock. Conditions of pervasive metaso-
matism involving large rock volumes are well known in
mid-oceanic ridge environments (e.g. Alt et al., 1986), and
*Corresponding author. Present address: Physics of Geological
Processes (PGP), University of Oslo, Sem Selands vei 24, N-0316 Oslo,
Norway. E-mail: oliver.pluemper@fys.uio.no
� The Author 2009. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oxfordjournals.org
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are also responsible for the generation of many of the most
important ore deposits, including examples such as the
Olympic Dam Mine in South Australia (Haynes et al.,
1995; Barton & Johnson, 1996). The presence of crustal
hydrothermal activity is usually not accounted for, unless
it is associated with the formation of new minerals that
are characteristic of fluid infiltration, or when succes-
sive stages of replacement can be identified. A variety of
studies have been performed that address subsolidus re-
equilibration of granitic rocks by deuteric fluids, focusing
mainly on the coarsening of cryptoperthite to patch
perthite. This coarsening provides a simple petrographic
marker for fluid^rock interaction at subsolvus tempera-
tures (Parsons, 1978; Parsons & Brown, 1984;Worden et al.,
1990; Walker et al., 1995; Lee & Parsons, 1997; Parsons &
Lee, 2000; Nakano et al., 2002, 2005). Remarkably, more
general investigations of subsolidus re-equilibration of
granitic rocks in geochemical studies, using a broader vari-
ety of petrographic markers for fluid^rock interaction,
have not been undertaken. The albitization of plagioclase
and alkali feldspars is a possible petrographic marker as it
is one of the most frequently observed metasomatic alumi-
nosilicate reactions in the upper crust of the Earth, as has
recently been reviewed by Perez & Boles (2005).
Albitization has received limited attention, even though
this process probably marks the circulation of fluids of var-
ious origins on a very large scale. More generally, the
replacement of one feldspar by another is commonly asso-
ciated with fluid^rock interactions and metasomatism, as
Harlov et al. (1998) and Putnis et al. (2007a) have demon-
strated the replacement of plagioclase by K-feldspar.
Furthermore, the association of plagioclase alteration and
sericitization is a form of hydrothermal alteration found
extensively in granitoids (Que & Allen, 1996, and refer-
ences therein).
The red clouding in granitic rocks has long been attribu-
ted to the presence of ferric iron oxides in alkali feldspar
minerals (Boone, 1969; Taylor, 1977; Smith & Brown, 1988;
Nakano et al., 2002). Putnis et al. (2007a) recently demon-
strated that this red staining is due to the precipitation of
hematite from a fluid within the feldspar pores. The gener-
ation of porosity has been proposed as a fundamental
feature of a dissolution^reprecipitation mechanism for re-
equilibrating minerals in the presence of a fluid phase
(Putnis, 2002; Putnis et al., 2005, 2007, 2007a; Putnis &
Putnis, 2007b). Turbidity, a characteristic of replaced feld-
spars, originates from porosity and is almost ubiquitously
developed to varying extents within plutonic rocks
(Montgomery & Brace, 1975; Parsons, 1978; Worden et al.,
1990). Thus, the red clouding of feldspars can be used
as another direct marker for large-scale, subsolidus, re-
equilibrating crustal fluid flow.
Here we present work on granitoids from the Simpevarp
and Laxemar areas, SE Sweden, providing new data on
the re-equilibration of granitic rocks with crustal hydro-
thermal systems. Our study particularly emphasizes feld-
spar replacement reactions around fractures and their
textural and chemical changes. Numerous earlier studies
in the area and at the nearby A« spo« Hard Rock
Laboratory have illustrated that reddening in the vicinity
of fractures is the most prominent style of alteration of the
original grey granitoid (Drake et al., 2008, and references
therein). Initially the main aim of this work was to gain
an in-depth understandingof the changes in mineralogy
between the grey and red granitoids to evaluate the red-
dening phenomenon. However, detailed optical micros-
copy as well as scanning and transmission electron
microscopy provided evidence that the grey granitoids
have already undergone intensive alteration with respect
to feldspar replacement reactions prior to the fracturing
causing the reddening.We illustrate the affiliations and rel-
ative timings between the multiple feldspar replacement
reactions and demonstrate that their complex association
with fluids implies that the replacements may be due to
sequential fluid infiltration events. In addition, these obser-
vations suggest that such granitic rocks are secondary, sub-
solidus products created by fluid-induced re-equilibration
of their original magmatic mineralogy. This should be
taken into closer consideration when studying granitic
rocks.
GEOLOGICAL SETT ING AND
SAMPLES
The bedrock in the Simpevarp and Laxemar areas 30 km
north of Oskarshamn, SE Sweden is dominated by the
Sm�land granitoids and dioritoids of the Trans-
Scandinavian Igneous Belt (TIB) (Fig. 1). Formation of
TIB granitoids can be subdivided into three major pulses
of magmatism between 1�85 and 1�66 Ga; TIB 1 (1�81^1�77
Ga), TIB 2 (1�72^1�69 Ga) and TIB 3 (1�69^1�66 Ga)
(Larson & Berglund, 1992). U^Pb dating of the Sm�land
granitoids from the Simpevarp and Laxemar areas reveals
that they are between 1�79 and 1�80 Ga in age, and there-
fore belong to the TIB 1 generation (Kornfa« lt et al., 1997;
Wahlgren et al., 2004). The TIB is about 1600 km long, up
to 150 km wide, nearly north^south in orientation and situ-
ated between older Svecofennian (c. 1�9 Ga) crust in the
ENE and younger rocks from the Swedish Gneiss Region
in the SW (—ha« ll & Larson, 2000). Further A-type granitic
intrusions are found to the north and south of the investi-
gated areas: the Go« temar granite (1�45 Ga) and the
Uthammar granite (1�44 Ga), respectively (Kresten &
Chyssler, 1976).
The Simpevarp area is dominated by three major rock
types: (1) fine-grained dioritoid in the southern part of the
peninsula and in the central part of the A« vro« granite; (2)
medium-grained quartz monzodiorite in the eastern part
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of the peninsula; (3) porphyritic granite to quartz monzo-
diorite in the north and at H�lo« and A« vro« (Wahlgren
et al., 2004). The dominant rock types have overlapping
compositional variations and are mainly distinguished by
texture and grain size. Minor intrusions and enclaves of
intermediate to mafic composition (diorite to gabbro) are
common. Contact associations and mixing as well as min-
gling relationships between the different rock types in the
Simpevarp area imply that they were formed almost syn-
chronously (Kornfa« lt et al., 1997; Wahlgren et al., 2004).
This is confirmed by U^Pb zircon and titanite geochronol-
ogy that gives ages of 1802�4 Ma (zircon) for the quartz
monzodiorite and 1793�4 Ma (titanite) and 1800� 4 Ma
(zircon and titanite) for the A« vro« granite (Wahlgren et al.,
2004). Based on field relationships the following chronos-
tratigraphy for the dominant and subordinate rock types
is suggested (beginning with the youngest): (1) fine- to
medium-grained granite and pegmatite; (2) fine-grained
mafic rock; (3) medium- to coarse-grained granite;
(4) A« vro« granite; (5) quartz monzodiorite; (6) diorite to
gabbro; (7) fine-grained dioritoid as the oldest rock type
(Wahlgren et al., 2004).
Up to 80% of the Laxemar area is dominated by
the A« vro« granite, which comprises a suite of porphyritic
rocks that varies in composition from quartz
monzodiorite to granite and includes quartz dioritic and
quartz monzonitic varieties. As in the Simpevarp area, a
characteristic feature of the A« vro« granite is the occurrence
of enclaves of intermediate to mafic composition. The
A« vro« granite has been observed to intimately mix with the
equigranular quartz monzodiorite. Gradual contact rela-
tionships indicate that the two rock types formed
contemporaneously.
Within the study area several localized shear zones of
ductile and brittle^ductile nature occur. Red staining
appears to have affected larger rock volumes in the
Simpevarp area than in the Laxemar area (Nilsson et al.,
2004) and is mainly concentrated along fractures, although
Fig. 1. (a) Geological map of the Simpevarp and Laxemar area showing the locations of the investigated drillcores KSH03 and KLX04 (mod-
ified from Drake et al., 2008). (b) Simplified overview of the main geological units in southern Sweden highlighting the location of the investi-
gated area (black square) and theTrans-Scandinavian Igneous Belt (TIB) (modified from Kornfa« lt et al., 1997).
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in many places it has also affected the rock between meso-
scopic fractures (Wahlgren et al., 2004). Drake et al. (2008)
considered that the hydrothermal alteration that caused
the red staining occurred at temperatures between 250
and 4008C. This is based on analysis of low-temperature
mineral parageneses in fractures and associated altered
granitoids; the upper temperature is limited by the stabil-
ity of prehnite and chlorite (Deer et al., 1992; Liou et al.,
1983; Spear, 1993), whereas the lower temperature limit is
defined by the breakdown of prehnite below �2508C
(Liou, 1971; Liou et al., 1983). Furthermore, the lower tem-
perature estimation is consistent with the observation that
laumonite is absent in sample KSH03 (Drake et al., 2008,
and references therein). In this area prehnite is commonly
replaced by laumonite, which has an upper stability of
around 220^2808C (Liou, 1970; Frey et al., 1991; Bucher &
Frey, 2002).
Drillcores were obtained during site investigation by the
Swedish Nuclear Fuel andWaste Management Co. (SKB)
for a nuclear waste repository in the Oskarshamn region.
Samples were obtained from several drill sites.We focused
on two cores (KLX04, KSH03) that contain both grey
and red-stained granitoids, showing a clear association
between the red staining and fractures (Fig. 2). The most
prominent red staining is observed in drillcore KSH03.
Several polished thin sections and thick sections were
prepared incorporating both the red and grey granitoids
as well as the transition zone between them. In addition,
rock samples of the macroscopically least altered
granitoid were studied from a shallower drillcore level,
located several tens to hundreds of metres away from
the fracture. The geological map in Fig. 1a shows the loca-
tions from which the cores were taken; KSH03 is a grano-
diorite drillcore from 102�55m depth and KLX04 a
quartz monzodiorite drillcore from 322�25m below the
present-day surface. Detailed descriptions of the drillcores
have been given by Nilsson et al. (2004) and Carlsten et al.
(2006).
ANALYT ICAL AND
THEORET ICAL TECHNIQUES
Analytical techniques
To investigate the nature of the hydrothermal alteration
the two drillcores were studied with transmitted light
microscopy, scanning electron microscopy (SEM), particu-
larly focusing on back-scattered electron (BSE) imaging
and energy-dispersive X-ray (EDX) analysis, and finally
using transmission electron microscopy (TEM).
Quantitative electron probe microanalysis (EPMA) and
laser ablation high-resolution inductively coupled plasma
mass spectrometry (LA-HR-ICP-MS) analysis was also
performed to examine the elemental gains and losses that
occurred during the feldspar replacement reactions, in
particular the albitization of oligoclase.
For phase identification, especially of the alkali feld-
spars, Raman spectra were collected with a high-resolution
JobinYvon HR800 Raman spectrometer using the 532 nm
line of a 14 mW Nd^YAG laser. Thescattered Raman
light was analysed with a 100� objective lens in a 1808
backscattering geometry and a charged-coupled device
(CCD) detector. After the light passed a 100 mm entrance
slit it was dispersed by a grating of 1800 grooves per mm.
A confocal hole of 1000 mmwas used for all measurements.
Quantitative EPMA was performed using a JEOL JXA
8600M Superprobe equipped with wavelength-dispersive
spectrometers to obtain mineral compositions.The acceler-
ating voltage was 15 kV and the counting time 5^10 s on
peak. When possible, minerals were analysed with a defo-
cused beam to prevent loss of alkalis and to gain an aver-
age quantitative analysis of the altered plagioclase grains.
Standardization was carried out using a selection of natu-
ral mineral standards, including hypersthene for Si, kya-
nite for Al, fayalite for Fe, diopside for Ca, jadeite for Na,
sanidine for K, rhodonite for Mn and rutile for Ti. The
ZAF correction procedure was applied to correct for
matrix effects.
Fig. 2. Photograph of the polished drillcore KSH03, which is representative of the grey (left side) and red-stained (lighter colour, right side)
granitoids with direct association with a sealed fracture (arrow). Prehnite is the main fracture filling.Traces of red staining can be found further
from the fracture than is macroscopically visible.
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Trace element concentrations were analysed by LA-ICP-
MS utilizing a New Wave UP103HE 193 nm ArF high-
energy excimer laser coupled to a single-collector magnetic
sector ICP mass spectrometer (Finnigan Thermo
Element2). The spot size was varied from 2 to 20 mm to
obtain optimal spatial resolution and to avoid mixed sig-
nals. An average analysis of albite and sericite was pro-
duced by selecting a spot size between 10 and 20 mm.
Laser frequency and laser energy were set to 5Hz and
9 J/cm, respectively. An NIST silicate glass SRM 612 was
used for standardization. Data reduction was performed
using the GLITTER� software (Access Macquarie Ltd.).
Selected samples from each drillcore were prepared for
optical, SEM and TEM analyses. Thin sections (30 mm in
thickness) were made for polarized transmitted light
microscopy. Before SEM analyses the thin sections were
coated with a carbon layer of 15^20 nm thickness and
then examined with a SEM JEOL 840 and a field emission
(FE) SEM JEOL JSM 6300F, both equipped with a BSE
detector and EDX spectrometer (OX2000 from Oxford
Instruments, INCA system). Total porosity distribution
was calculated from two-dimensional images created
using the SEM.
For TEM analysis, several 3mm discs were removed
from the thin sections and glued to copper grids using dif-
ferent sizes of gratings and slots. Final thinning was exe-
cuted with a Gatan Duo-mill ion polishing system. TEM
analyses were performed in a JEOL 3010 EX TEM operat-
ing at 300 kV, equipped with an EDX spectrometer (link
ISIS) and a post-column elemental mapping device
(Gatan Image Filter, GIF). Crystallographic orientation
and identification of submicron inclusions in the alkali
feldspars were determined from diffraction patterns.
All analytical measurements were performed at the
Institut fu« r Mineralogie, University of Mu« nster in conjunc-
tion with the Interdisciplinary Centre of Electron
Microscopy and Microanalysis (ICEM), with the excep-
tion of Raman spectroscopy, which was conducted at the
Institut fu« r Anorganische und Analytische Chemie,
University of Mu« nster.
Theoretical approach to feldspar equilibria
To gain a better understanding of equilibria between feld-
spars and fluids an appropriate graphical model needed to
be established. Examining the change in molarity of Naþ
and Kþ in a fluid and the concurrent effects on the coexist-
ing minerals is one possible method. Real fluid^solid
phase systems (e.g. the hydrothermal feldspar replacement
reaction we have studied) are in constant flux and thus
never reach true thermodynamic equilibrium. However,
we can apply thermodynamic models to systems close to
equilibrium and therefore to areas of local equilibrium,
such as at reaction interfaces, and to residual fluid in
pores. As any disequilibrium condition relaxes
instantaneously to an equilibrium state, we can assume that
the system remains in local equilibrium (Knapp,1989).
We have generated equilibrium constants for aqueous
species dissociation reactions at appropriate temperatures
and pressures for the hydrothermal alteration processes
observed utilizing SUPCRT92 (Johnson et al., 1992) with
the slop98 database (GEOPIG, 1998). Using these equilib-
rium constants activity diagrams were constructed
between albite, sericite and K-feldspar. The pressure effect
on feldspar equilibrium was also investigated.
Thermodynamic equilibrium modeling using SUPCRT92
has been used previously to good effect when modelling
various fluid^feldspar interactions (e.g. Wibberley &
McCaig, 2000).
PETROLOGY AND MINERAL
CHEMISTRY
Grey granitoids
Microcline islands in single oligoclase grains
The dominant feldspar phase in the investigated granitoids
is oligoclase. These grains are equigranular, fine- to
medium-grained (up to �5mm) crystals with embayed
grain boundaries (Fig. 3a) with compositions in the range
of An24Ab76 to An30Ab70 (average An26Ab74) for KLX04
and An27Ab72 to An30Ab68 (average An28Ab72) for
KSH03 (Table 1). In back-scattered SEM images the oligo-
clase phase has a high porosity distribution of up to 3%
(Fig. 4). During analysis we noticed that the samples also
contain irregular-shaped, porosity-free, cross-hatched
twinned K-feldspar islands, varying from 100 mm up to
1cm (Fig. 3), which are consistently found in almost every
oligoclase grain in the granitoids. The bulk compositions
of these islands are in the range of Ab10Or90 to Ab3Or97
(Table 1). The islands are usually distributed within the
central section of a single oligoclase grain in both unal-
tered and altered areas (Fig. 3), but can also be found in
the outer parts of oligoclase grains. Under cross-polarized
light all the K-feldspar islands in a single oligoclase grain
are optically continuous and extinguish in the same direc-
tion (Fig. 3b and c). Additionally, most of the islands’
cross-hatched twinning is parallel to the polysynthetic
twinning of the surrounding oligoclase. Back-scattered
electron imaging of the K-feldspar islands showed no
porosity or intragranular fractures and only minor perthi-
tic exsolution (Fig. 3d). Raman spectroscopy of the K-feld-
spar islands identified them as microcline; these were
clearly distinguishable from other K-feldspar structural
types, such as sanidine and orthoclase, because of the dif-
ference in their Al^Si ordering (Mernagh, 1991; Freeman
et al., 2008).
Progressive replacement of oligoclase by albite and sericite
The transformation of the observed oligoclase to albitic
feldspar (average An9Ab91) occurs at sharp interfaces
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along polysynthetic twinning (Fig. 4a) and, to a smaller
extent, along intragranular micro-scale, albite-filled frac-
tures that cut inconsistently through the oligoclase grains
(Fig. 4d). Albitization along the polysynthetic twinning
can be subdivided into different stages. The first stage is a
crystallographically defined replacement of oligoclase by
albite along several polysynthetic twins (Fig. 4a). In the
second stage progressive replacement along the oligoclase
twinning combines to form albitic patches (Fig. 4b and c).
The well-defined oligoclase^albite replacement interface
in the BSE images is also detectable as a sharp composi-
tional change from An26Ab72 to An9Ab91 (Fig. 5). The
albite is noticeably porous and associated with numerous
crystals of fine-grained mica (sericite) thatare up to �40
mm in length (Fig. 6a). Epidote is also observed and is
restricted to the albitized areas (Fig. 4a). Subsequent TEM
investigations of the replaced areas show that the albite
also contains scattered sub-micron inclusions of sericite
(Fig. 6b). Thus, it is likely that the K content of albite
determined by electron microprobe has been overesti-
mated and therefore the Na content is underestimated
(Fig. 5d).
To estimate elemental gains and losses during the albiti-
zation of oligoclase, sericite was also included in the min-
eral analyses because it is intricately associated with the
albite. To evaluate fluctuations in composition the
Gresens’ equation for metasomatic changes was used
(Gresens,1967; Grant,1986).This model evaluates composi-
tional variations assuming constant volume, mass or the
constant concentration of one component. Average repre-
sentative electron microprobe analyses of oligoclase (olg)
and albite (ab) (Table 2) were used to balance the reaction
100golgþ0�8gSiO2þ0�04gFeO
þ0�5gNa2Oþ2�4gK2O
¼93gab ðincludingmsÞþ0�33gAl2O3þ4�1gCaO
ð1Þ
assuming constant volume and with sericite (ms) included
in the albite.
Transmission electron microscopy: focusing on sericite
As illustrated by the back-scattered electron imaging
(Fig. 6a), sericite grains are commonly associated with the
porosity generated during albitization. Figure 6b shows
Fig. 3. Occurrence of scattered microcline relicts (Or93) in highly porous oligoclase. (a) Optical photomicrograph under crossed polars giving
an overview of the distribution of microcline relicts in a single oligoclase grain. (b, c) Magnified view of (a), to illustrate the crystallographic
continuity of the microcline relicts by rotating the microscope stage (crossed polars). (d) Back-scattered electron image showing the contrast
between the oligoclase grain and the irregularly shaped microcline relict in (a). Mc, microcline; Pl, plagioclase; Ab, albite; Ms, muscovite (ser-
icite) (after Kretz, 1983).
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one example of sericite intimately related to an albite pore.
However, subsequent detailed TEM investigations of the
albite patches revealed a more complex association
between albite and sericite that is dominated by a complex
diffraction contrast. This contrast is extensive in the albi-
tized patches as seen in Fig. 6c and d. The sericite grains
appear blurred in the TEM images (Fig. 6c) as a result of
the intricacy of the high contrast, but can be clearly identi-
fied with EDX analysis. This makes it difficult to ascertain
the nature of the association between the sericite and the
albite pore. The origin of this contrast in the albite patches
is unclear, as it could be dependent on lattice strain,
which could be due to the presence of tiny sericite particles,
from a high defect density or from elastic strain associated
with the replacement process.
Red granitoids
K-feldspathization and associated Fe-oxide inclusions
The most obvious alteration in the Simpevarp and
Laxemar areas is the red staining located in the vicinity
of fractures. To investigate this, several thin sections of the
interface between the grey and red granitoids were
prepared, as well as sections from the red granitoid itself
for comparison. Subsequent transmitted light microscopy
and BSE imaging of the red-stained oligoclase reveals that
they have undergone intensive K-feldspathization (Fig. 7).
Within a single oligoclase grain the formation of secondary
K-feldspar is restricted to areas that have previously under-
gone albitization and sericitization, leaving the remaining
oligoclase unaffected (Fig. 7a and b). K-feldspathization
appears to have pseudomorphically replaced the oligoclase
where the albitization of oligoclase occurs almost through-
out an entire original grain. Further high-magnification
back-scattered (BSE) imaging of these areas revealed that
the K-feldspar has replaced the sericite in a pseudomorphic
manner (Fig. 7c). A complex texture has developed where
the relationship between albite, sericite and K-feldspar is
not clearly visible and the determination of parent and
product phases is insufficient to fully interpret the replace-
ment texture.
The secondary K-feldspars contain numerous inclusions,
resulting in a turbid appearance in thin section, which
cannot be resolved by optical microscopy. At high magnifi-
cation in back-scattered electron images these inclusions
Table 1: Representative electron microprobe analyses of oligoclase, albite and microcline relicts in KLX04 and KSH03
Sample: KLX04 KSH03
Analysis no.: 10 73 71 72 45 50 65 67 66 81 10 84
Mineral: Olg Olg Ab Ab Mc relict Mc relict Olg Olg Ab Ab Mc relict Mc relict
wt%
SiO2 62�13 61�94 67�49 66�38 65�87 65�69 62�95 63�01 69�74 67�93 66�24 65�95
Al2O3 23�92 24�18 21�08 20�85 18�05 18�13 23�70 23�59 19�72 20�43 17�87 18�04
FeO 0�03 0�06 0�00 0�00 0�00 0�08 0�10 0�09 0�01 0�00 0�00 0�03
CaO 5�17 5�28 1�74 1�68 0�01 0�01 5�92 5�54 0�21 1�77 0�02 0�01
Na2O 8�02 7�97 9�84 10�26 0�35 0�77 8�27 8�31 11�88 10�55 0�70 0�61
K2O 0�16 0�08 0�10 0�09 16�04 15�19 0�17 0�19 0�18 0�11 15�68 15�38
Total 99�43 99�51 100�25 99�26 100�32 99�87 101�11 100�73 101�74 100�79 100�51 100�02
Structural formula based on 5 cations (a.p.f.u.)
Si 2�76 2�75 2�94 2�93 3�02 3�02 2�76 2�77 2�99 2�95 3�03 3�03
Al 1�25 1�27 1�08 1�08 0�98 0�98 1�22 1�22 1�00 1�05 0�96 0�98
Fe 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00 0�00
Na 0�69 0�69 0�83 0�88 0�03 0�07 0�70 0�71 0�99 0�89 0�06 0�05
Ca 0�25 0�25 0�08 0�08 0�00 0�00 0�28 0�26 0�01 0�08 0�00 0�00
K 0�01 0�00 0�00 0�00 0�94 0�89 0�01 0�01 0�01 0�00 0�92 0�90
mol%
An 26�0 26�6 8�8 8�3 0�0 0�0 28�1 26�6 1�0 8�4 0�1 0�0
Ab 73�0 72�8 90�6 91�2 3�2 7�2 71�0 72�3 98�1 90�9 6�3 5�7
Kfs 1�0 0�5 0�6 0�5 96�8 92�8 0�9 1�1 1�0 0�6 93�6 94�3
Olg, oligoclase; Ab, albite; Mc, microcline.
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are directly associated with the replacement interface, seen
as the brighter back-scattering spots (Fig. 7c), and were
revealed as Fe oxides by EDX analysis. Raman spectros-
copy can distinguish between different feldspars and thus
was conducted on several secondary K-feldspars from
both drillcores, identifying them as orthoclase (Fig. 8).
The Raman spectra obtained show typical orthoclase
modes along with a second order band at �1320 cm^1,
which is characteristic of a second-order scattering by
hematite. To further investigate the relationship between
the orthoclase interface and the iron oxide inclusions, as
well as to determine any microstructural relationships
between the inclusions and the feldspar, transmission elec-
tron microscopy samples were prepared from the optical
thin sections. TEM imaging revealed open pores in the
secondary orthoclase matrix of several hundred nano-
metres in cross-section that contain numerous polycrystal-
line inclusions of plate-like rosettes and needles (Fig. 9a).
Electron diffraction patterns verify that the inclusions are
hematite (Fig. 9b and c; see also Golla-Schindler et al.,
2006; Putnis et al., 2007a), supporting the Raman spectra
observations (Fig. 8). Interconnection and density estima-
tions of the orthoclase pores were not possible for these
samples, even though pores that are observable in a typical
TEM field of view (several mm2) can be found in every
thinned section.
DISCUSS ION
Although our initial aim was to gain a better understand-
ing of the reddening phenomenon by investigating the
changes in feldspar mineralogy between the red and grey
granitoids, using the latter as a reference, we unexpectedly
discovered that the grey granitoids themselves had already
undergone multiple feldspar replacement reactions.
Consequently, further work was undertakento obtain a
better insight into the multiple metasomatic feldspar
replacement reactions observed and the relative timings
between them. In the following section, we discuss the feld-
spar reactions in the sequence in which they occur, as indi-
cated by the microstructural observations.
Fig. 4. Back-scattered electron images of the microstructural development in oligoclase grains as albitization progresses along polysynthetic
twinning and intragranular fractures. (a) The progressive albitization along several polysynthetic twins. The close association between albite,
fine-grained mica (sericite) and epidote should be noted. (b) The beginning of albitic patch formation by expanding replacement along twin-
ning, resulting in patches such as (c). (d) Intragranular fracturing can initiate further fluid infiltration, also resulting in albitization. Pl, plagio-
clase; Ab, albite; Ms, muscovite (sericite); Ep, epidote (after Kretz, 1983).
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Feldspar replacement reactions
Searching for the protolith: relict microcline in porous
oligoclase
To understand the origin of the crystallographically con-
tinuous microcline relicts found in the highly porous oligo-
clase three possible scenarios need to be taken into
consideration: (1) the microcline relicts represent mag-
matic mineral inclusions; (2) the islands are post-magmatic
exsolution products from oligoclase grains; or (3) the
growth of oligoclase is at the expense of microcline, leav-
ing intragranular relicts of pristine microcline behind.
First, crystallographically continuous inclusions scat-
tered throughout a single grain are not likely to form
from melts because communication between nuclei during
crystallization in intragranular pores is limited and would
not produce the texture observed. It is expected that
microcline crystallized from a melt would preferentially
remain at three-grain junctions with melt^solid dihedral
angles of less than 608 (Holness, 2006). This has recently
been observed for microcline in migmatites, which crystal-
lizes from melt films at plagioclase grain boundaries and
not within single grains (Holness & Sawyer, 2008).
Fig. 5. (a) Back-scattered electron image of an oligoclase^albite intracrystalline replacement interface chosen for element distribution mapping
via electron probe micro-analysis and for elemental traverse analysis. Element distribution maps of (b) Na and (c) Ca of the reaction interface.
It should be noted that the transition from the parent oligoclase to the product albite is sharp on the micrometre scale. Bright spots in (c) are
epidote grains. (d) Chemical profile along a traverse, dashed white line in (a), showing the abrupt change in Ca and Na content at the oligo-
clase^albite interface, acquired by spot analyses with a focused beam. All elemental data are displayed as mole percent. The slight increase in
K content beginning at the intracrystalline oligoclase^albite interface proceeding into albite should be noted. It is obvious that the K content
determined by electron microprobe has been overestimated, and therefore the Na content (albite) underestimated, as a result of scattered sub-
micron inclusions of sericite. Pl, plagioclase; Ab, albite; Ms, muscovite (sericite) (after Kretz, 1983).
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In contrast, in the rocks from this study, the inclusions are
intragranular with random shapes and sizes. In addition,
a slow crystallizing melt that forms microcline would
have reacted with the surrounding mineral producing
reaction rims; however, BSE imaging has shown no evi-
dence of reaction with the oligoclase grains (Fig. 3d).
An origin by solid-state post-magmatic exsolution would
create a better defined textural relationship between the
distribution of the microcline islands and the host oligo-
clase, controlled by diffusion distances and strain distribu-
tion [see, for example, exsolution textures in alkali
feldspars (Brown & Parsons, 1988)]. Instead, the distribu-
tion of microcline inclusions within the oligoclase is essen-
tially random.
The high porosity of the oligoclase grains compared
with the porosity-free microcline also argues against a con-
temporaneous origin. Previously reported porosity distri-
butions in plagioclase grains, ranging from 1 to 2% by
volume, are thought to represent former fluid inclusions
Fig. 6. (a) Back-scattered electron image showing the contrast between albite and sericite as well as the close association of the latter with albite
porosity.The black spots in (a) are pores. (b^d) Bright-fieldTEM images: (b) the detailed microstructure of two sericite (ms) grains in different
orientations in direct association with an open albite pore; (c) the complex image contrast at the interface between sericite and albite; (d)
moire¤ fringes showing the position of sericite grains. [Note the lack of any sharp interface between sericite and albite in (b)^(d).] Ab, albite;
Ms, muscovite (sericite) (after Kretz, 1983).
Table 2: Representative Gresens analysis of the albitization
of oligoclase for sample KLX04 assuming constant volume
Olg Ab þ Ms G/L
average average
(n¼ 7) (n¼ 5)
wt%
SiO2 61�90 62�70 0�83
Al2O3 23�95 23�62 �0�33
FeO 0�08 0�04 �0�04
CaO 5�52 1�57 �4�08
Na2O 7�98 8�48 0�51
K2O 0�15 2�48 2�40
Density r 2�65 2�73
G/L, gain/loss; Olg, oligoclase; Ab þ Ms, albite including
sericite.
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or cleavages (Montgomery & Brace,1975). However, Smith
& Brown (1988) suggested that most plagioclase pores are
of secondary origin and are generated by deuteric or
hydrothermal alteration (Parsons, 1978). The characteristi-
cally high porosity of up to 3% in the oligoclase grains
and the crystallographic continuity of relict microcline
suggest that the oligoclase crystallized at the expense of
the microcline. Experiments recently conducted by
Niedermeier et al. (2009) examined the production of
orthoclase via the re-equilibration of albite in a KCl-
bearing hydrothermal fluid and have shown that the
transformation is consistent with an interface-coupled
dissolution^reprecipitation mechanism (Putnis & Putnis,
2007b). Investigations on the natural albitization of plagio-
clase from the Bamble Sector in South Norway also
support the re-equilibration of feldspars in the presence
of a fluid phase via this process (Engvik et al., 2008).
These observations suggest that the replacement of
microcline by oligoclase could also occur by this mechan-
ism, especially as the generation of porosity is an impor-
tant consequence of this process (Putnis, 2002; Putnis &
Putnis, 2007b). Porosity is generated whenever the vol-
ume of the parent mineral dissolved is less than that of
the volume reprecipitated. This volume change is depen-
dent not only on differences in molar volume between the
solid phases, but also on their relative solubilities. Thus it
is possible to generate porosity even when the molar
volume of the product solid is higher than that of the
parent (Putnis et al., 2005). An increase in porosity would
be expected upon the dissolution of microcline and repreci-
pitation of oligoclase as the molar volume decreases
by �8%.
Fig. 7. Back-scattered electron images of an oligoclase grain in the red-stained granitoid showing the complex relationship that develops during
the replacement of sericite by orthoclase. (a) Overview image illustrating the restriction of K-feldspathization to areas previously albitized
and sericitized. The two relict microclines (both crystallographically continuous) in the lower left part of the oligoclase grain shouldbe noted.
(b) Close-up of the area affected by K-feldspathization in (a). (c) A high-magnification image [outlined area in (b)] reveals the pseudomorphic
replacement of sericite by orthoclase.The two Fe oxide inclusions (brighter back-scattering spots in the white ellipse), which form at the reaction
interface between sericite and orthoclase and become incorporated into the growing orthoclase, should be noted. Further Fe oxide inclusions
are found in the lower left and upper right corners. The black spots are pores. Pl, plagioclase; Mc, microcline; Ab, albite; Ms, muscovite (seri-
cite); Ep, epidote; Or, orthoclase (after Kretz, 1983).
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Fig. 9. (a) A typical TEM image of polycrystalline hematite needles in an open pore within the red-stained granitoid orthoclase (Or). (b) The
electron diffraction pattern can be clearly indexed as hematite in (c), using the EMS software package (Stadelmann,1987).
Fig. 8. (a) Raman spectra of hematite-bearing orthoclase with hematite second-order scattering mode at �1320 cm^1. The phases can be iden-
tified through comparison with the reference spectra (b) and (c), from the RRUFF Project (Downs, 2006). (d) Raman spectra of a microcline
relict shown in Fig. 3, which can be distinguished unambiguously from orthoclase because of the characteristic band positions and the evolution
of the peak triplet between 450 and 520 cm^1 with increasing Al^Si order (Freeman et al., 2008).
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The microcline^oligoclase transformation observed in
trondhjemite from Alabama, USA (Drummond et al.,
1986) and Sicily, Italy (Fiannacca et al., 2005, 2008) points
to a scenario where metasomatism and fluid flow cause
the almost entire replacement of microcline by oligoclase.
In such a situation the replacement may result in an alkali
ion exchange process, as suggested by Orville (1963),
where Kþmoves to low-temperature and Naþ to high-tem-
perature regions. Therefore, fluids moving in the initial
stages of a cooling granitic complex will tend to enrich
feldspars in Naþ and transfer their Kþ to the fluid.
From these arguments we propose that microcline was
replaced by oligoclase in the early high-temperature
stages of alteration.
Oligoclase^albite transformation and the role of sericite
Albitization is a well-known feldspar replacement phe-
nomenon that has been described in a range of rock types
from various geological settings. Numerous experimental
investigations on the replacement of one feldspar by
another in the presence of a fluid phase illustrate that
these processes are rapid even on a laboratory timescale
(e.g. O’Neil & Taylor, 1967; Labotka et al., 2004;
Niedermeier et al., 2009; Ho« velmann et al., in preparation).
Therefore, whenever feldspars in rocks come into contact
with hydrothermal fluids it is virtually inevitable that
such re-equilibration reactions will take place.
However, the contemporaneous appearance of sericite
with albite has been almost completely neglected even
though its formation is one of the most common types of
hydrothermal alteration found in granitic rocks (Que &
Allen, 1996). Further progress in understanding the rela-
tionship between the formation of albite and associated
sericite requires the identification of textural relationships,
the pathways taken by the fluid and its chemical evolution
with regard to input and output fluxes, as well as the influ-
ence of the fluid on the origin of sericite. The first step
towards these aims is to identify the microstructural evolu-
tion of the albitization and associated formation of sericite,
as it provides important information about the mechanism
of these processes.
The most obvious textural relationship is the close asso-
ciation of sericite with the albite porosity. To understand
the process by which this texture arises, the transformation
of oligoclase to albite needs to be examined in greater
detail. The replacement of oligoclase by albite illustrates
the most important characteristics of fluid-induced min-
eral replacement reactions:
(1) a sharp replacement interface, even on the micro-
metre scale, as illustrated by BSE images (Figs 4 and
5a) and element distribution maps (Fig. 5b and c),
shows no evidence for significant solid-state diffusion
between parent and product phases;
(2) porosity development in the reaction product;
(3) preservation of the original external volume occupied
by the oligoclase crystals and their crystallographic
orientation;
(4) additional phases, in this case sericite and epidote,
co-precipitate during the albitization reaction.
The observation of the features described above, in con-
junction with the results of recent investigations into the
albitization of metagranitic rocks (Sandstro« m et al., 2009)
and tonalites (Engvik et al., 2008), indicates that this pro-
cess can be ascribed to an interface-coupled dissolution^
reprecipitation mechanism (Putnis & Putnis, 2007b).
Generating porosity is a key feature of this mechanism, as
it allows the fluid to infiltrate the product phase and
access the reaction interface. The newly generated pores
also provide sites for the nucleation and growth of co-preci-
pitates. For this to arise the pores must be interconnected
during the time of reaction. SEM and TEM observations
were not able to provide further information about pore
connectivity in the present samples, but observations in
analogue experiments involving salt systems reveal that
porosity produced during mineral replacement is coar-
sened and eventually eliminated by textural equilibration,
which follows the chemical equilibration in the mineral
(Putnis et al., 2005). Further detailed TEM investigations
of the relationship between sericite and albite show that it
is more complex (Fig. 6c and d) than was observed with
previous analytical techniques. The distinct association of
sericite with albite pores in the BSE images (Fig. 6a) is no
longer clear inTEM images because of the unusual diffrac-
tion contrast in the albite, which is probably due to disloca-
tions, a high point defect population and elastic strain in
this area. This unusual strain contrast is also evident in
natural albitized oligoclase from tonalites (Engvik et al.,
2008) and has been recently observed to arise in experi-
mental investigations on the albitization of plagioclase
feldspars (Ho« velmann et al., in preparation). However,
areas where sericite can be clearly attributed to open
albite nanopores were identified (Fig. 6b and c).
To gain additional insight into the interlinked evolution
of albite and sericite an investigation into element mobili-
ties during replacement reactions and changes in fluid
activity was undertaken, which included the consequences
for mineral stability. One method of determining quantita-
tive element mobility is by evaluating mass transfer on the
basis of gains and losses (Gresens, 1967; Grant, 1986). This
can be performed in an isocon diagram (Grant, 1986),
which represents a graphical solution to the original com-
position^volume relationships in metasomatism as out-
lined by Gresens (1967). In a Gresens diagram,
concentrations of components in the altered mineral or
rock are displayed versus those in the original mineral or
rock. The actual concentrations of these components in
the altered mineral are then compared with those calcu-
lated from the assumptions of constant volume, constant
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mass or constant concentration of one component (here we
use Al). From the displacement of the data points relative
gains and lossescan be obtained. In this study all three of
the above-mentioned possibilities are shown in Fig. 10.
The presence of sericite inclusions in close association
with albite indicates that Al3þ may be released from the
oligoclase, as suggested by Leichmann et al. (2003). Al3þ
mobility during metasomatism has also been documented
by Nijland & Touret (2001). As albitization is the dominant
alteration process it is reasonable to assess element mass
transfer by comparing the formation of albite both
accounting for and disregarding sericitization (Fig. 10).
Under the assumption of constant volume Al3þ appears to
be immobile when sericite is accounted for, but is revealed
as mobile when it is neglected. This implies that the Al3þ
released from the oligoclase during albitization was incor-
porated into the associated sericite. Furthermore, it sup-
ports a cogenetic evolution of albite and its inclusions, as
it is consistent with the textural relationship observed in
BSE andTEM images (Fig. 6). As the albitization of oligo-
clase (An20) involves a fluid phase, which introduces Na
þ
and Si4þ and releases mobile Al3þ and Ca2þ, the following
reaction is an appropriate description of this alteration:
Na0�8Ca0�2Al1�2Si2�8O8 ðolgÞ þ 0�2Na
þ
ðaqÞ
þ0�2H4SiO4 ðaqÞ
¼ NaAlSi3O8 ðabÞ þ 0�2Ca
2þ
ðaqÞ
þ0�2Al3
þ
ðaqÞ þ 0�8OH�ðaqÞ:
ð2Þ
Reaction (2) assumes that silica was supplied by the pore
fluid in the form of Si(OH)4 (Merino, 1975), rather than
from quartz. This is further supported by the observation
that quartz grains in all samples have not been heavily dis-
solved or deformed during the hydrothermal alteration.
Assuming constant volume, Gresens analysis of albitized
parts (including sericite) and oligoclase indicates that per
100 g of oligoclase, 4�1g of CaO is released. This and other
components that are released during the associated break-
down of biotite and hornblende are incorporated into the
epidote that coexists with the albitized sections of
oligoclase.
Even if all of the Al3þ released from oligoclase was
incorporated into ideal sericite KAl2AlSi3O10(OH)2 there
would be insufficient K to produce the amount found in
these samples. This discrepancy is also evident in the K
content of the albite^sericite intergrowths by comparing
the chemical analyses given inTables 1 and 2, as well as in
the net gains and losses of the oligoclase vs albite^sericite
intergrowths (Fig. 10). A more feasible source for the K
concentration required to initiate sericitization in the oli-
goclase has been rarely discussed. Engvik et al. (2008) sug-
gested that the externally derived fluids containing the
Naþ needed for albitization could also bring in Kþ. The
activities of Kþ (aKþ) and Na
þ (aNaþ) in this fluid
have important effects on the process of albitization
and the associated sericitization, as well as concurrent
repercussions on the albite and sericite mineral stabilities.
Temperature and pressure, which are key parameters con-
trolling these alterations (Hemley & Jones, 1964), also
influence activities. Using SUPCRT92 (Johnson et al.,
1992) we calculated equilibrium constants to construct an
activity diagram that relates fluid composition to albite,
sericite and K-feldspar in equilibrium at 200MPa and var-
ious temperatures (Fig. 11a). Using this diagram we can
follow the changes in fluid composition during the feldspar
replacement reactions. The removal of Naþ from the fluid
during the albitization of oligoclase would be expected to
locally decrease the aNaþ. Furthermore, the incomplete
replacement reaction to pure albite (An0), approximately
An9 in all analysed albite patches, points to a limited Na
reservoir. As the pore volumes in albite are small, we
assume that local equilibrium is reached within the pores.
Therefore, fluid remaining in the albite pores shortly after
the reaction would be low enough in aNaþ to enter the seri-
cite stability field (Fig. 11a; arrow 1). Sericite would then
precipitate because the aKþ in the externally derived fluid
is also expected to be in the stability field of sericite. This
is also supported by the observation that with decreasing
temperature the stability field of albite shrinks, favouring
sericitization.
Textural and chemical observations, as well as theoreti-
cal considerations based on thermodynamic equilibrium
modeling, suggest that albitization and sericitization are
directly linked to each other and develop as cogenetic
alteration products.
K-feldspathization and reddening around fractures
The phenomenon of reddening in the vicinity of fractures
in the granitoid is directly related to the K-feldspathization
of sericite (Fig. 7c). The formation of secondary orthoclase
is favoured by high aKþ=aHþ in the fluid (Fig. 11a; arrow
2), which implies that conditions of high aKþ or pH are
required. Metasomatic replacement of sericite was partly
accomplished by the following reaction:
KAl2AlSi3O10ðOHÞ2 ðmsÞþ6SiO2 ðqtzÞþ2KClðaqÞ
¼ 3KAlSi3O8 ðorÞþ2HClðaqÞ:
ð3Þ
Fracturing would produce a pressure change from litho-
static to hydrostatic pressure, which also favours the feld-
spar side of the equilibrium and the production of HCl.
This effect is evident from the calculated equilibrium con-
stant increase for the sericite^orthoclase reaction at 3008C
by approximately one and a half orders of magnitude
when a drop in pressure from 200 to 70MPa is considered
(Fig. 11b).
The production of the red staining itself is caused by
hematite precipitation during the K-feldspathization (Figs
7c and 9a). The observed microtexture of hematite needle
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formation in orthoclase pores could not be produced by
the solid-state exsolution of hematite from a feldspar
matrix, nor is it consistent with the argument of co-crystal-
lization from a magma.The presence of hematite in associ-
ation with the replacement interface between sericite and
orthoclase, as well as in pores throughout the orthoclase
grains, indicates that there is a direct relationship between
the replacement reaction and the formation of hematite.
Putnis et al. (2007a) recently showed that hematite rosettes
and needles formed in pores at the reaction interface
between K-feldspar and plagioclase, with no evidence of
exsolution or co-crystallization processes. The formation
of hematite within pores in alkali feldspars is consistent
with a subsolidus replacement origin, where hematite pre-
cipitation is a coproduct dependent on the Fe content of
the fluid. The question arises about the location of the Fe
Fig. 10. Isocon diagram after Grant (1986), showing net gains and losses of oligoclase vs albite, (a) disregarding and (b) accounting for sericite.
It should be noted that Al is mobile in (a) when only albite is considered under the conditions of constant volume or mass, but appears to
remain constant in (b) when sericite is accounted for with albite.
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source for hematite formation. Three different origins can
be examined: (1) an Fe source exists within the rock (e.g.
biotite and hornblende breakdown); (2) the infiltrating
hydrothermal fluid responsible for secondary feldspar for-
mation also contains significant amounts of Fe; (3) re-equi-
libration of a feldspar containing Fe in solid solution (Fe3þ
substituted for Al3þ in the tetrahedral sites) releases Fe
into solution to precipitate as hematite in pores linked
with the feldspar replacement process (Nakano et al.,
2005). The last option is not likely to be applicable in the
investigated rocks as the albite and sericite do not contain
enough Fe for the extensive hematite precipitation
observed (Table 1). Drake et al. (2008) conducted
Mo« ssbauer spectroscopy to determine changes in the Fe3þ/
Fetot ratio betweenthe red-stained and grey granitoids,
and showed that the total amount of Fe remains very simi-
lar, indicating an in situ source. Therefore, the Fe source
probably originates from the chloritization of biotite and
the breakdown of hornblende, which are observed in con-
junction with the feldspar replacement reactions.
Hydrothermal fluids
Fluid system and origin
An evaluation of the fluid system is crucial in any discus-
sion of major element metasomatism caused by fluid flow
because the extent of metasomatism is directly related to
compositional changes in the fluid along the flow pathway.
Fluid composition may vary along the flow path, even in
a homogeneous rock, because the composition of a fluid
in equilibrium with a fixed mineral assemblage varies
with temperature and pressure (Dipple & Ferry, 1992).
Despite the extensive research into fluid composition
based on fluid inclusion analyses, drill-hole sampling
(Touret, 2001; Yardley & Graham, 2002; Yardley et al.,
2003) and fluid^rock interaction models (Reed, 1982;
Dolejs & Wagner, 2008), which are often limited to a
restricted range of compositions, our understanding of the
fluid chemistry that drives fluid^mineral reactions in the
Earth’s crust is incomplete. This is also the case for the
granitic rocks studied here. Hence, we have made a quali-
tative prediction of the fluid composition by extrapolating
information about the fluid from comparisons of mineral
reaction products and their parent phases using composi-
tional as well as textural changes. Overall, the observed
complex metasomatism does not require the infiltration of
a chemically exotic fluid, but also cannot be easily
explained by a single fluid infiltration event. A sequential
infiltration of multicomponent aqueous fluids consti-
tutes a more likely explanation for the several feldspar
replacement reactions found in these plutonic rocks.
Fig. 11. (a) Activity^activity diagram showing the stability of albite, sericite and orthoclase at various temperatures and 200MPa. The dia-
gram was constructed using equilibrium constants calculated by the SUPCRT92 program (Johnson et al., 1992). Inset (b) illustrates the effect of
pressure on the replacement reaction of sericite by orthoclase where the equilibrium constant K of reaction (3) gradually increases with decreas-
ing pressure within the range of a lithostatic to hydrostatic pressure drop.
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The introduction of a single fluid containing sufficient Ca,
Na and K to drive the replacement of microcline by oligo-
clase and subsequently transform the oligoclase to albite
and sericite is questionable. Furthermore, the fracturing
and K-feldspathization observed, especially the presence
of the red hematite precipitate, point to the possibility of
sequential infiltration events. Variability in the degree of
sericitization throughout the granitoids suggests that fluid/
rock ratios may have varied geographically within the plu-
tonic complex, or even on the mesoscopic and microscopic
scale. This would be dependent on the degree of fluid
access.
Combining mineral chemical analyses and textural
observations for different mechanisms of fluid infiltration,
discussed below, we suggest a scenario of sequential fluid
infiltration events. However, the origin of the fluids caus-
ing the hydrothermal alteration is at present unknown.
Drake et al. (2008) suggested that, on the basis of wall-
rock and fracture dating as well as similarities in element
concentration, the majority of the red staining in the area
is related to fluid circulation during the emplacement of
the nearby Go« temar and Uthammar granites. It is uncer-
tain whether the two intrusions are initiators of the entire
sequence of feldspar replacement reactions observed or if
they play a key role in only the reddening event.
To unravel the history of the fluid causing the feldspar
replacement reactions, further detailed studies, especially
of fluid inclusions, are needed. For example, Nijland &
Touret (2001) successfully used fluid inclusions associated
with the replacement of graphic pegmatite by a graphic
albite^actinolite^clinopyroxene intergrowth from
Mj�vatn, southern Norway to gain insight into the fluid
composition and temperature. Although during the pres-
ent study no fluid inclusions were identified, the high
porosity and microfracturing observed does not preclude
their presence. Furthermore, fluid inclusions in quartz can
provide valuable information about feldspar alteration
when they can be correlated with adjacent feldspars
(Fig. 3a, large quartz grain at the lower left side).
Combined with recent achievements in oxygen isotope
analysis by in situ ion-microprobe techniques (Cole et al.,
2004; Bowman et al., 2009), systematically obtained fluid
inclusion analysis could be used to gain detailed informa-
tion about the fluid regime and temperature path of hydro-
thermal alteration when applied to granitic feldspar
replacement reactions. It will therefore be of great interest
to conduct fluid inclusion and oxygen isotope analyses in
future studies.
Mechanism of fluid infiltration
The intensive reddening in the vicinity of fractures indi-
cates that such fractures can act as channels for penetrat-
ing hydrothermal fluids on the outcrop scale. This
staining is not only restricted to these areas, but is also
found between the mesoscopic fractures (Wahlgren et al.,
2004). Fracturing is a well-established mechanism for fluid
infiltration in low-permeability rocks both in deep crustal
(Austrheim, 1987; Jamtveit et al., 1990) and shallow crustal
settings (Bons, 2001; Engvik et al., 2005). Furthermore,
fluid infiltration aided by fracturing on the micrometre
scale has been demonstrated by several workers (Fitz
Gerald & Stunitz, 1993; Oliver, 1996; Engvik et al., 2001;
Jamtveit et al., 2009). Microtextures observed in the grani-
toids from the Laxemar and Simpevarp areas emphasize
the importance of fluid infiltration along microcracks as
one way of providing fluid for mineral replacement reac-
tions. This is illustrated by the transformation of oligoclase
to albite along intragranular microcracks (Fig. 4d) and
the observation that K-feldspathization and pale reddening
are located within the grey granitoid away from the origi-
nal macroscopic fractures and their intense staining.
However, the introduction of fluids via microcracks alone
cannot be the only mechanism that results in an almost
completely replaced oligoclase grain. More pervasive
fluid infiltration is provided by the instantaneously gener-
ated porosity that is a crucial consequence of reactions
governed by an interface-coupled dissolution^reprecipita-
tion mechanism. Pre-existing high porosity in oligoclase
from the replacement of microcline also ensures that con-
tact is established between the reactive fluid and the
parent phase. Selective albitization along single polysyn-
thetic twins in the oligoclase crystals (Fig. 4a and b) sug-
gests that the crystallographic orientation of twinning
controls the mineral reaction and acts as another pathway
for fluid infiltration into the grains. Continuous fluid infil-
tration along the twinning is likely to form isolated albite
patches, as in Fig. 4c. When considered in three dimen-
sions, these patches may interconnect to others several
tens of micrometres away, leading to the formation of
larger albitized zones. Engvik et al. (2008) have also
observed this selective replacement along polysynthetic
twinning in TEM investigations of oligoclase grains and
showed that it is linked to complex diffraction contrast
caused by dislocation formation. Dislocation structures
induced by deformation increase the rate of mineral reac-
tions (White, 1975), producing the selective albitization
observed.
Although separately these processes would not pro-
vide enough fluid for the degree of replacement
observed,together microcracking, porosity generation
and crystallographically controlled infiltration along twin-
ning produce a pervasive mechanism for the introduction
of fluids at the micrometre scale and ensure that the con-
tact between the reactive fluid and parent mineral is main-
tained. Furthermore, macroscopic fracturing, such as that
related to the reddening, ensures further fluid infiltration
via fracture flow. The combination of pervasive and frac-
ture flow acted as the overall mechanism for fluid
infiltration.
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SUMMARY AND CONCLUSIONS
Based on the observed replacement textures and their con-
secutive relationships, we can summarize the observed
hydrothermal alteration using the following proposed evo-
lutionary sequence (Fig. 12).
(1) Based on the microtextural observations made in this
study we propose the following scenario for the for-
mation of relict microcline in oligoclase. The granitic
intrusion, which crystallized microcline as the domi-
nant alkali feldspar, released fluids during a late
stage of magmatic crystallization, which initiated the
replacement of pristine microcline by oligoclase. The
original protolith is observed in the studied samples
as crystallographically continuous microcline relicts
within highly porous oligoclase grains. However, fur-
ther work is needed to fully unravel the origin of the
relict microcline.
(2) An externally derived Na-enriched hydrothermal
fluid established the progressive albitization of oligo-
clase. The contemporaneous association of sericite
within albite pores indicates a requirement for further
K introduction and is linked to both albite stability
and aNaþ in the fluid. This intricate relationship also
implies that albite and sericite are of cogenetic origin.
(3) Fracturing and the infiltration of a K-bearing fluid
caused the K-feldspathization of sericite and hematite
precipitation in orthoclase pores, producing the red
coloration. Observed K-feldspathization is restricted
to previously albitized and sericitized areas and
hence occurred after the albitization process. In addi-
tion, it is evident that although the red coloration
observed here is clearly associated with fractures, it is
not necessarily restricted to them. It is likely that the
reddening is a more pervasive phenomenon of feld-
spar replacement reactions in the presence of a fluid
throughout a granitic pluton. A direct relationship
between red feldspars in granitic rocks and hydrother-
mal activity has been suggested before, based on field
observations (Boone, 1969). This is also consistent
with low d18O values observed in many Precambrian
brick red granitic rocks, which are indicative of subso-
lidus re-equilibration with meteoric^hydrothermal
fluids (Taylor, 1977; Hoefs & Emmermann, 1983;
Fiebig & Hoefs, 2002).
Consistently throughout this evolutionary sequence
porosity generation and the mechanism of dissolution and
reprecipitation, as described by Putnis et al. (2002), play
crucial roles in the re-equilibration of granitic feldspars in
the presence of a fluid phase.
The present study has provided new microtextural and
chemical insights into the subsolidus re-equilibration of
granitic rocks with hydrothermal fluids, with particular
emphasis on feldspar replacement reactions and their
sequential occurrence along the hydrothermal alteration
path of granitic rocks. This hydrothermal history is cer-
tainly complex and in many aspects a puzzling process.
Further work is required to gain a full understanding of
this history, with particular emphasis on the absolute
timing of the overall replacement and each of the feldspar
replacement stages (i.e. albitization, K-feldspathization)
as well as their corresponding fluid chemistry.
Although our understanding of the mineralogical and
geochemical changes during the re-equilibration of grani-
tic rocks is restricted to the investigated area, it is a logical
Fig. 12. Evolutionary sequence of multiple feldspar replacement reactions summarizing the observed hydrothermal alteration of the Simpevarp
and Laxemar granitoids. The primordial high-temperature stage of the alteration caused the replacement of microcline by oligoclase, leaving
intragranular, crystallographically continuous relict microcline behind. Hydrothermally derived fluid established the subsequent replacement
of oligoclase by albite and contemporaneously associated sericite precipitation in albite pores. Later fracturing and fluid infiltration caused the
K-feldspathization of sericite, and hematite precipitation in orthoclase pores resulted in reddening.
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progression that hydrothermal fluids could affect larger
volumes of the Earth’s crust than is currently acknowl-
edged. Furthermore, fluid inclusions within feldspar
minerals in the upper crust have been calculated to con-
tain the same amount of water as is incorporated in all
hydrous minerals (Johnson & Rossman, 2004).
Consequently, the interactions of gigantic hydrothermal
systems with crustal rocks should be taken into closer con-
sideration in future studies of granitic rocks and their pet-
rological history.
ACKNOWLEDGEMENTS
We are grateful to H. Drake (Go« teborg) for providing the
drillcore samples, and H. Austrheim for discussion.
A. Janssen and M. MacKenzie are thanked for help with
the transmission electron microscope, J. Berndt for assis-
tance in microprobe and mass spectrometry analyses, and
M. Menneken for operating the Raman spectrometer.
Helpful comments from H. King improved and clarified
the manuscript. Furthermore, the manuscript benefited
from valuable comments and suggestions given by the
reviewers D. E. Harlov, C. C. Lundstrom and J. L. R.
Touret. The laboratory facilities in Mu« nster are supported
by the German Research Council (Deutsche
Forschungsgemeinschaft).
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