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TRANSCRIPT
How do metamorphic fluids move
through rocks?
An investigation of timescales, infiltration mechanisms and
mineralogical controls
Barbara I. Kleine
©Barbara I. Kleine, Stockholm University 2015 ISBN 978-91-7649-120-1
Cover picture: A relict of glaucophane from Fabrika Beach.
Printed in Sweden by Holmbergs, Malmö 2015
Distributor: Publit
III
Abstract
This thesis aims to provide a better understanding of the role of mountain building in the
carbon cycle. The amount of CO2 released into the atmosphere due to metamorphic processes
is largely unknown. To constrain the quantity of CO2 released, fluid-driven reactions in
metamorphic rocks can be studied by tracking fluid-rock interactions along ancient fluid flow
pathways. The thesis is divided into two parts: 1) modeling of fluid flow rates and durations
within shear zones and fractures during greenschist- and blueschist-facies metamorphism and
2) the assessment of possible mechanisms of fluid infiltration into rocks during greenschist- to
epidote-amphibolite-facies metamorphism and controlling chemical and mineralogical factors
of reaction front propagation.
On the island Syros, Greece, fluid-rock interaction was examined along a shear zone and
within brittle fractures to calculate fluid flux rates, flow velocities and durations. Petrological,
geochemical and thermodynamic evidence show that the flux of CO2-bearing fluids along the
shear zone was 100-2000 times larger than the fluid flux in the surrounding rocks. The time-
averaged fluid flow velocity and flow duration along brittle fractures was calculated by using
a governing equation for one-dimensional transport (advection and diffusion) and field-based
parameterization. This study shows that fluid flow along fractures on Syros was rapid and
short lived.
Mechanisms and controlling factors of fluid infiltration were studied in greenschist- to
epidote-amphibolite-facies metabasalts in SW Scotland. Fluid infiltration into metabasaltic
sills was unassisted by deformation and occurred along grain boundaries of hydrous minerals
(e.g. amphibole) while other minerals (e.g. quartz) prevent fluid infiltration. Petrological,
mineralogical and chemical studies of the sills show that the availability of reactant minerals
and mechanical factors, e.g. volume change in epidote, are primary controls of reaction front
propagation.
IV
Sammanfattning
Denna avhandling syftar till att öka förståelsen för bergskedjebildningens roll i kolcykeln.
Mängden CO2 som släpps ut i atmosfären till följd av metamorfa processer under orogens är i
stort sett okända. För att bestämma den mängd CO2 som frigörs, kan fluiddrivna reaktioner i
metamorfa bergarter studeras genom att spåra växelverkan mellan fluid och berggrund längs
tidigare flödesvägar. Avhandlingen är uppdelad i två delar: 1) modellering av
flödeshastigheter och deras varaktighet i skjuvzoner och sprickor under grönskiffer och
blåskiffer facies-metamorfos och 2) undersökning av möjliga mekanismer för fluiders
infiltrering i bergarter under grönskiffer till epidot-amfibolit facies-metamorfos, samt kemiska
och mineralogiska kontrollfaktorer som påverkar reaktions frontens utbredning.
På den grekiska ön Syros undersöktes fluid-berggrundsinteraktionen längs en skjuvzon och i
sprickor för att beräkna flödeshastigheter och deras varaktighet. Bevis från petrologiska,
geokemiska och termodynamiska studier visar att flödet av CO2-bärande fluider längs
skjuvzonen var 100-2000 gånger större än flöden i det omgivande berget. Med hjälp av en
styrande ekvation för endimensionell transport (advektion och diffusion) och parametrar
framtagna i fält kunde den genomsnittliga tidsflödeshastigheten och varaktigheten av flöden
längs sprikorna, vilka skär igenom kvarts-glimmer skiffrar, beräknas. Denna studie stödjer att
fluidflöden längs sprickor på Syros var snabba och kortlivade.
Mekanismer och kontrollerande faktorer för fluiders infiltrering studerades i grönskiffer till
epidot-amfibolit facies-metabasalter i sydväst Skotland. Bevis som presenteras i denna studie
visar att fluidinfiltreringen i metabasalter skedde längs korngränser av vattenhaltiga mineral
(t.ex. amfibol) medan andra mineral (t.ex. kvarts) förhindrar infiltrering. Infiltreringen skedde
utan påverkan från deformation. Stöd från mineralogiska och kemiska profiler genom dessa
metabasalter, och de reaktions texturer som bevarats inom dem visar att tillgången på
reagerande mineral och mekaniska faktorer, såsom volymförändringar i epidot kontrollerar
fluiddrivna reaktionsfronters utbredning under metamorfos.
V
List of papers/ manuscripts
This thesis is comprised of a summary that outlines the main aims and implications of this
doctoral dissertation and the key results. The following manuscripts are included in the thesis.
Paper I is reprinted with the permission of the Oxford University Press Journals. Papers II
and III are submitted and paper IV is a manuscript.
Paper I
Kleine, B.I., Skelton, A.D.L., Huet, B., Pitcairn, I.K. (2014). Preservation of blueschist-facies
minerals along a shear zone by coupled metasomatism and fast-flowing CO2-bearing fluids.
Journal of Petrology, 55, 1905-1939.
Paper II
Kleine, B.I., Zhao, Z., Skelton, A.D.L. (submitted to American Journal of Science). Rapid
fluid flow along fractures at greenschist-facies conditions on Syros, Greece.
Paper III
Kleine, B.I., Pitcairn, I.K., Skelton, A.D.L. (submitted to American Mineralogist). The
mechanism of infiltration of metamorphic fluids recorded by hydration and carbonation of
epidote-amphibolite-facies metabasaltic sills in the SW Scottish Highlands.
Paper IV
Kleine, B.I., Pitcairn, I.K., Skelton, A.D.L. (manuscript). Pre-metamorphic controls on the
propagation of fluid-driven reaction fronts at greenschist-facies metamorphic conditions.
B. Kleine was the main contributor in terms of field work, analyses and the writing of all four
manuscripts, with help from all of the co-authors. For paper II, Zhihong Zhao derived the
transport equations for inverse modeling of fluid flow along fractures. The ideas for paper I
were developed primarily by B. Kleine and A. Skelton. B. Kleine and Z. Zhao proposed the
ideas for paper II. The ideas behind papers III and IV were developed by B. Kleine and I.
Pitcairn.
VI
Table of Contents
Abstract .................................................................................................................................... III
Sammanfattning ....................................................................................................................... IV
List of papers/ manuscripts ....................................................................................................... V
Introduction ................................................................................................................................ 1
Metamorphic fluids .................................................................................................................... 2
Formation and migration of metamorphic fluids .................................................................... 2
Fluid-rock interactions and reaction textures ......................................................................... 2
Isochemical reactions .......................................................................................................... 3
Metasomatic reactions ........................................................................................................ 3
Pseudomorphism ................................................................................................................. 4
T-XCO2 diagrams and pseudosections ..................................................................................... 5
Calculation of metamorphic fluid fluxes ................................................................................ 7
Time-integrated fluid fluxes................................................................................................ 8
Time-averaged fluid fluxes ................................................................................................. 8
Controls on metamorphic fluid flow ...................................................................................... 8
Study sites .................................................................................................................................. 9
The Cycladic archipelago in Southern Greece ....................................................................... 9
Geological background ....................................................................................................... 9
Metamorphic fluid flow in the Cyclades .......................................................................... 11
Fabrika Beach ................................................................................................................... 12
Delfini ............................................................................................................................... 12
The SW Scottish Highlands .................................................................................................. 12
Geological background ..................................................................................................... 12
Metamorphic fluid flow in the SW Scottish Highlands .................................................... 13
Loch Stornoway ................................................................................................................ 14
The Ard of Port Ellen ........................................................................................................ 15
Methods .................................................................................................................................... 16
Sampling ............................................................................................................................... 16
Analytical techniques ........................................................................................................... 16
Petrography ....................................................................................................................... 16
Geochemistry .................................................................................................................... 16
Geothermobarometric modeling ........................................................................................... 18
Results ...................................................................................................................................... 18
Quantification of metamorphic fluid fluxes (papers I and II) .............................................. 18
Timing of fluid flow on Syros .......................................................................................... 21
Mineralogical controls on fluid flow (papers III and IV) ..................................................... 21
Conclusions .............................................................................................................................. 24
Future prospects ....................................................................................................................... 24
Acknowledgements .................................................................................................................. 25
References ................................................................................................................................ 27
1
Introduction
This thesis aims to broaden our general understanding of how metamorphic fluids move
through rocks. This is necessary as metamorphic fluid flow is a significant controlling factor
for mineral reactions, heat and mass transfer and deformation within the Earth’s crust (e.g.
Bickle and McKenzie 1987; Ague 1994a, 1994b, 2003a; Ferry 1994; Oliver 1996; Cartwright
1997; Ferry and Gerdes 1998). As metamorphic fluids reach shallow levels within Earth’s
crust, they enter into hydrothermal and groundwater systems. Therefore, these systems can
interact with the hydrosphere and atmosphere (see review of Ague 2003a and references
therein).
Several studies showed that greenhouse gases (e.g. CH4 and CO2) that were mobilized by
metamorphic fluid flow could have a significant influence on the Earth’s climate (e.g. Kerrick
and Caldeira 1993; Bickle 1996; Kerrick and Caldeira 1998; Wallmann 2001a, 2001b; Evans
et al. 2008). Bickle (1996) demonstrated that the global climate is controlled by degassing of
CO2 during metamorphic decarbonation and CO2-uptake during silicate weathering. In the
Himalayas, Evans et al. (2008) showed that metamorphic processes provide a source of CO2
that is larger than the uptake of CO2 during weathering processes. Estimates of CO2 released
during metamorphic reactions were calculated to be on the order of ~ 1017
kg/Myr (Kerrick
and Caldeira 1993, 1998; Wallmann 2001a, 2001b). However, it is not known if this release
occurs slowly and at a constant rate or as a series of rapid pulses is unknown. The release of
high quantities of metamorphic CO2 could affect the Earth’s climate. Therefore, it is
important to understand the influence which metamorphic CO2 has on the global carbon
cycle.
The calculation of fluid fluxes and how metamorphic fluids actually move through rocks is a
subject that has received much attention in recent years. The widely cited observations that
metamorphic fluid flow is pervasive at the grain scale and channeled along specific lithologies
and structures at the regional scale imply that mineralogical and textural factors (e.g. foliation,
fractures) affect how metamorphic fluids move through rocks (e.g. Oliver 1996).
This thesis answers the following issues:
How do fluids infiltrate and move through rocks?
How quickly do fluids migrate through the bulk rock and how fast do they migrate
through zones of structural weakness such as shear zones and fractures?
How do pre-metamorphic chemical and mineralogical heterogeneities influence the
position of fluid-induced reaction fronts?
How does deformation influence fluid infiltration?
2
Metamorphic fluids
Formation and migration of metamorphic fluids
In the boundaries between crystals, in hydrous minerals or in pore spaces of rocks, volatiles
such as H2O and CO2 are often found in varying amounts. If these “wet” rocks are
compressed and metamorphosed at elevated temperatures and pressures, dehydration and
decarbonation reactions will lead to the release and liberation of these volatile species (see
review of Ague 2003a and references therein). This most often occurs in subduction zones,
for example the Hellenic subduction zone, where massive amounts of fluids are released from
the wet oceanic plate as it reaches greater depths and higher temperatures (e.g. Selverstone et
al. 1991, 1992; Bröcker et al. 1993; Getty and Selverstone 1994; Barnicoat and Cartwright
1995; Ganor et al. 1996; Ague 2000, 2003a, 2007; Putlitz et al. 2000; Breeding et al. 2003).
The liberated fluids will start to migrate upwards, due to their low density and viscosity
compared to the surrounding rocks. However, fluid mobility and resulting fluid flow requires
an interconnected porosity and permeability, which is either intrinsic to the rock or created
during deformation. Several studies have shown that permeability and porosity are controlled
by mineralogy (e.g. Holness and Graham 1991, 1995; Ferry et al. 2005, 2013). Studies of
monomineralic solid aggregates such as marbles and quartzites show that these materials do
not have a stable interconnected fluid-filled network (Watson and Brenan 1987; Holness and
Graham 1991, 1995; Holness 1993; Price et al. 2004; Schumacher et al. 2008b). At certain P-
T conditions, high dihedral angles between the fluid and crystal in these rocks inhibit further
fluid advancement. However, if porosity and permeability exist, the fluid may move either
pervasively upwards through the rock (e.g. Schliestedt and Matthews 1987; Ferry 1992; Léger
and Ferry 1993) or will use pathways of structural weakness, such as through fault zones (e.g.
McCaig et al. 1995; Abart et al. 2002), shear zones (e.g. Selverstone et al. 1991; Gupta and
Bickle 2004; Kleine et al. 2014), and fold axes (e.g. Skelton et al. 1995; Graham et al. 1997;
Pitcairn et al. 2010) to reach higher levels in Earth’s crust.
Fluid-rock interactions and reaction textures
On their way to higher levels in Earth’s crust, fluids might interact or equilibrate with the
surrounding rocks. Fluids can enhance both prograde and retrograde metamorphism by
carrying dissolved ions and causing chemical reactions. This implies that metamorphism is
not just controlled by changes in pressure and temperature (e.g. Schliestedt and Matthews
1987; Straume and Austrheim 1999; Beinlich et al. 2010; Putnis and Austrheim 2010). In the
Tianshan Mountains in western China volatiles were “sucked” from the host rock into a vein
system (Beinlich et al. 2010). This caused dehydration of “wet” blueschist-facies rocks along
a vein and the formation of a “dry” eclogite-facies rim tapering that vein. Kyanite eclogites
from Ulsteinvik in Norway underwent partial retrogression to granulite- and amphibolite-
facies which was driven by the infiltration of a hydrous fluid (Straume and Austrheim 1999).
Also in the Greek Cyclades, retrogression of blueschist- to greenschist-facies rocks during
exhumation has been considered to have been a consequence of regional and pervasive fluid
infiltration (Schliestedt and Matthews 1987). However, in this thesis (paper I), it is shown that
retrograde blueschist- to greenschist-facies metamorphism can also be inhibited by the
infiltration of CO2-bearing fluids and involved chemical reactions (Kleine et al. 2014).
3
Fluid-rock interaction can result in a change in mineralogy and/or chemistry of the bordering
rock along the fluid pathway. Reaction textures in minerals are one of the most convenient
traces that fluids leave behind after interaction with the surrounding rocks and can be used to
identify special types of fluid-rock interactions. The change in the rock can occur in two
different ways of reaction: 1) isochemically in respect to all but the volatile species or 2)
metasomatic which involves a change in the bulk rock chemistry. These types of reactions are
tightly linked. Reactions can be isochemical on a large scale either regional or even in hand
specimen. However, most reactions involve metasomatic changes where mobilization of
major and trace elements occurs at the crystal scale. Examples for both types of reactions are
given below.
Isochemical reactions
Fluid-induced isochemical mineral reactions leave the chemical bulk rock composition
unchanged and involve the addition or removal of volatile species such as CO2 and H2O.
Addition of CO2 and H2O, by diffusion outwards from a fluid pathway, results in carbonation
(figures 1a, b) and hydration reactions (figure 1c) of the adjacent rock.
In the SW Scottish Highlands, the infiltration of CO2-H2O-fluids caused largely isochemical
carbonation reactions in the margins of metabasaltic sills (figure 1a; Skelton et al. 1995). This
resulted in the precipitation and replacement of the original mineral assemblage by carbonates
(e.g. calcite) and hydrous minerals (e.g. chlorite). In the Greek Cyclades hydration reactions
during retrograde greenschist-facies metamorphism caused the isochemical replacement of
glaucophane by chlorite and albite (figure 1c). The same hydrous fluid was also responsible
for local decarbonation of the former HP-mineral assemblage (Kleine et al. 2014). Figure 1d
shows an example of the decarbonation reaction, where the former carbonated epidote is
replaced by the “dry” mineral albite. This reaction was isochemical on a large scale
(isochemical hydration and decarbonation during retrograde greenschist-facies
metamorphism) and metasomatic on a small scale (metasomatic addition of Na and removal
of Ca).
Metasomatic reactions
Metasomatism is defined as a process where the rock re-equilibrates while involving a change
in chemical composition (e.g. Putnis and Austrheim 2010). This change in chemical
composition of the rock requires an external flux of material carried by a fluid phase. Usually,
the end product of this process is the creation of new minerals by the substitution, removal, or
addition of chemical ions. Numerous studies have reported metasomatic reactions along fluid
pathways (e.g. Selverstone et al. 1991; Ague 1994a, 1994b, 2011; Penniston-Dorland and
Ferry 2008; Beinlich et al. 2010; Ferry et al. 2013; Kleine et al. 2014). On Syros, e.g., the
addition of Na and Si and the simultaneous removal of Ca and Al by a fluid phase caused the
stabilization of the Na-bearing amphibole glaucophane (Kleine et al. 2014). This was seen in
the replacement of phengite by glaucophane and in the breakdown of epidote (figures 1e, f).
4
Figure 1. Microphotographs (crossed polarized light). (a) Replacement of actinolite by calcite
(carbonation). (b) Replacement of epidote by calcite (carbonation). (c) Replacement of glaucophane by
chlorite and albite (hydration). (d) Replacement of carbonated epidote by albite (decarbonation). (e)
Replacement of white mica (phengite) by glaucophane (metasomatic K-Na exchange). (f) Replacement of
epidote by a Na-K bearing phase (metasomatic Na-K-Ca exchange). act = actinolite; cc = calcite; ep =
epidote; ab = albite; chl = chlorite; phg = phengite.
Pseudomorphism
Often, only the shape of the parent crystal is preserved, while the parent mineral is completely
replaced by a new phase (figure 2). This is clearly seen in the replacement of garnet by
chlorite at Loch Stornoway in the SW Scottish Highlands. There, the former outline of the
garnet is perfectly preserved by the crystallographically controlled new growth of chlorite
(figure 2). This so-called pseudomorphic replacement is most likely caused by an interface-
coupled dissolution-precipitation mechanism where the reactive fluid passes through the
parent crystal causing precipitation of the new phase and the generation of porosity within
that phase (Putnis et al. 2005; Austrheim et al. 2008; Putnis 2009; Putnis and Austrheim
5
2010). The generation of porosity not only permits fluid access to the replacement front but
also provides pathways for addition or removal of other (non-fluid) components. Carmichael
(1969) demonstrated that metasomatic cation-exchange reactions may occur at a scale of
individual crystals but the overall transformation from one mineral to another was
isochemical at the hand specimen scale. This mechanism shows that isochemical and
metasomatic reactions are intimately linked.
Figure 2. Microphotographs (plane polarized light). Pseudomorphic replacement of garnet by chlorite at
different stages.
T-XCO2 diagrams and pseudosections
The influence of fluid composition on the stability of minerals and/ or fluid-induced reactions
at certain pressures and temperatures can be studied by using thermobarometric modeling. For
this purpose the chemistry of metamorphic fluids is usually simplified, by reducing the
number of components.
For this study, the fluid phase is assumed to be a binary (2-component) fluid that is composed
of H2O and CO2. Reactions and stability of mineral assemblages at certain temperature and/or
pressure at the presence of a binary H2O-CO2-fluid can be illustrated by T-(P)-XCO2 diagrams
and T-(P)-XCO2 pseudosections, respectively. XCO2 stands for the molar proportion of CO2 in
the binary fluid. XCO2 ranges from 0 to 1, with 0 being equal to 0% CO2 in the fluid and 1 is
equal to a fluid containing 100% CO2. The XH2O represents the proportion of H2O in the fluid
and is defined as 1 – XCO2. Furthermore, these diagrams are calculated in a simplified system,
6
where the number of elemental components is usually reduced. For example, the T-XCO2
diagram in figure 3 was calculated in a CMASH + CO2 system which means that the chemical
components that were included are Ca, Mg, Al, Si, H2O and CO2. The reduction to a very
simplified system is so far justified, as these diagrams serve only to illustrate the process. For
the pseudosection (figure 4), the components of Na and Fe were added to the system
(NCFMASH + CO2) to gain more compatibility to the natural system.
With the help of a T-XCO2 diagram the behavior of de- and carbonation as well as de- and
hydration reactions can be studied in detail at a fixed pressure (see Winter 2001 and
references therein). The idea of a T-XCO2 diagram as with all other phase diagrams is founded
in Le Châtelier’s principle which states:
“If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in
a direction that tends to reduce that change.”
Figure 3 illustrates the shape of a hydration reaction (A + H2O = B) and two coupled
carbonation and hydration reactions (A + CO2 + H2O = B) in a T-XCO2 diagram. If Le
Châtelier’s principle is applied to these reactions the mentioned “change” which is imposed
on the system would be to increase T, and increase or decrease XCO2. As a response the
reactions would shift along the curve to either lower or higher XCO2 to reduce the change. In
this study T-XCO2 diagrams were used to identify carbonation and hydration in metabasaltic
rocks, and to constrain possible reaction pathways caused by fluid infiltration.
Unlike in a T-XCO2 diagram which shows all possible equilibrium relationships between
minerals of fixed composition, a T-XCO2 pseudosection illustrates the stability of different
mineral assemblages in a rock of known composition as a function of temperature and XCO2 at
a fixed pressure (figure 4). The pseudosections in this study were used to demonstrate that
certain mineral assemblages are stable at higher XCO2 and to model how these mineral
assemblages will behave along different thermal gradients.
Figure 3. T-XCO2 diagrams (at fixed pressure of 10 kbar in a CMASH + CO2 system) showing a hydration
reaction (py + tr + H2O = clin + cz + q) and two coupled carbonation and hydration reactions (py + tr +
CO2 + H2O = clin + q + cc and cz + tr + CO2 + H2O = clin + q + cc). py = pyrope (Ca-garnet); tr =
tremolite (Ca-amphibole); clin = clinochlore (Mg-chlorite); cz = clinozoisite (Al-endmember of epidote); q
= quartz; cc = calcite.
7
To construct T-XCO2 diagrams and to find metamorphic reactions at certain temperature,
pressure and/or XCO2 conditions the computer software package THERMOCALC was used.
This software allows performing thermodynamic and phase diagram calculations for multi-
component systems. The calculations are based on thermodynamic databases. For this study,
the updated version ds55 of the database of Holland and Powell (1998) was used. The
activities of the minerals entered into THERMOCALC were calculated by the computer
program AX (version 2). The input data for AX was mineral chemistry obtained by electron
microprobe (EMP) analyses.
Pseudosections for this study were calculated using the software package PerPlex 6.6.6
(Connolly 1990, 2005, 2009), with the thermodynamic data set HP02 (Holland and Powell
1998) and several solution models for chlorite, garnet, amphiboles and carbonates. In contrast
to the calculations performed in THERMOCALC, here, actual bulk rock compositions (in
mol%) where used as input data.
Figure 4. Pseudosection at fixed pressure of 8 kbars in a NCFMASH + CO2 system. This pseudosection
shows that a carbonated blueschist-facies mineral assemblage is stable at these relatively low P-T
conditions in the presence of a CO2-bearing fluid. gl = glaucophane; cc = calcite; chl = chlorite; pa =
paragonite (white mica); ab = albite; dol = dolomite; gt = garnet; act = actinolite; cz = clinozoisite (Al-
endmember of epidote).
Calculation of metamorphic fluid fluxes
Reaction textures along fluid pathways can give knowledge about the chemistry of a
metamorphic fluid. Broadened reaction zones where fluid-rock interaction has occurred, can
be used to calculate the amount of fluid which was needed to produce the observed changes in
the affected rocks. Throughout the last few decades numerous studies tackled the problem of
quantifying metamorphic fluid flow from geological data. This can be done by calculating
time-integrated and time-averaged fluid fluxes. While time-integrated fluid fluxes (m3m
-2)
give information about the total volume of fluid that has passed through a unit area of rock
throughout an entire metamorphic event, time-averaged fluid fluxes (m3m
-2s
-1) provide some
8
constraints about the rate and duration of metamorphic fluid flow (e.g. Walther and Orville
1982; Brady 1988; Connolly and Thompson 1989; Peacock 1989; Skelton 2011; Zhao and
Skelton 2013).
Time-integrated fluid fluxes
The first attempts to estimate time-integrated fluid fluxes were 1-dimensional (1-D)
calculations based on the propagation of stable isotope fronts (Bickle and Baker 1990),
broadening of reaction fronts (Skelton et al. 1995) and reaction progress (Ferry 1988;
Baumgartner and Ferry 1991). These models were further developed to consider 2- or 3-D
time-integrated fluid fluxes, so as to incorporate geometric parameters, by assuming that fluid
flow was along a temperature gradient (Ague 2007; Penniston-Dorland and Ferry 2008),
along thrust planes (McCaig et al. 1995; Abart et al. 2002) or along fold hinges (Skelton et al.
1995). The calculated values for time-integrated fluxes for regional pervasive fluid flow fall
in the range of 10 to 104
m3m
-2 (Ferry 1992; Léger and Ferry 1993; Skelton et al. 1995; Evans
and Bickle 1999; Ague 2003a). Time-integrated fluid fluxes can become several orders of
magnitude larger if fluid flow is channelized within shear zones, fold hinges, lithological
layers or fractures (e.g. Ferry and Dipple 1991; Ague 1994a, 1994b; Skelton et al. 1995; Ague
1997; Breeding and Ague 2002; Ague 2003b; Penniston-Dorland and Ferry 2008; Ague 2011;
Kleine et al. 2014).
Time-averaged fluid fluxes
Time-averaged fluid fluxes, which involve a time component, have been solved geometrically
(in 2- or 3-D) and calculated based on coupled advective displacement and diffusive
broadening of geochemical fronts (Skelton 2011; Zhao and Skelton 2013). Calculations of
time-averaged fluid fluxes are always compared with a process where the rate is known. This
can be, e.g. diffusion rates that were measured in a laboratory. Wark and Watson (2004)
measured the interdiffusion of H2O and CO2 in metamorphic fluids at 490 to 690° C and at 1
GPa. Using these diffusion coefficients Skelton (2011) calculated time-averaged fluid fluxes
for the SW Scottish Highlands during greenschist-facies metamorphism. Regional
metamorphic fluid flow occurred over a relatively short geological timescale (less than 104
years) and was surprisingly fast (10-10
to 10-11
m3m
2s
-1). Time-averaged fluid fluxes along
shear zones were calculated to be 100-2000 times faster than in the surrounding rocks (Kleine
et al. 2014). However, until today it is not possible to distinguish between steady state fluid
flow and fluid flow occurring in short rapid pulses.
Controls on metamorphic fluid flow
Several studies imply that mineralogy, chemistry of the bulk rock and deformation commonly
affect fluid flow and the reaction progress. These controls might have an influence on the
positions of reaction fronts which could imply that fluid fluxes were calculated several orders
of magnitudes too high or low. For example, Skelton et al. (2000) showed that a reaction front
is broadened by chemical diffusion, mechanical dispersion and kinetically limited exchange
processes analogous to a simple chromatographic model. This could imply that in such cases
the factors controlling how metamorphic fluids move through rocks are properties inherent to
the metamorphic rock before fluid-rock interaction occurred. However, higher reaction
progress in some rock layers does not automatically mean that this was caused by higher fluid
9
fluxes. This has been shown by Ferry et al. (2013) in a study of marls from Northern New
England. They demonstrated that higher reaction progress in some layers was caused by
different bulk compositions, such as variations in the abundance of reactants from layer to
layer, instead of larger fluid fluxes. Arghe et al. (2011) showed that the position of syn-
metamorphic reaction fronts within metabasaltic sills in the SW Scottish Highlands was
strongly controlled by pre-metamorphic alteration. Here, they suggested that diffuse layers of
epidote formed during pre-metamorphic spilitization deflected the infiltrating fluid and
protected the interior of the sill from more extensive carbonation. This was possible because
epidote undergoes a series of volume changes and can therefore generate local porosity at
greenschist-facies conditions (Grapes and Hoskin 2004; Arghe et al. 2011). Arghe et al.
(2011) showed that fluid flow was enhanced in areas of the rock where porosity was higher.
Increased porosity may also result from interface-coupled dissolution-precipitation (e.g.
Putnis et al. 2005; Putnis 2009; Putnis and Austrheim 2010) or be induced by deformation-
fluid events associated with metamorphism, i.e. hydraulic fracturing (e.g. Thompson and
Connolly 1992; Ague 1994b; Oliver 1996; Putnis and Austrheim 2010). As porosity is
commonly low in metamorphic rocks (Etheridge et al. 1984; Oliver 1996; Manning and
Ingebritsen 1999; Ague 2003a), deformation is commonly invoked as a cause of enhanced
fluid access to the rock during metamorphism (Sharp and Buseck 1988; Yardley et al. 2000,
2004; Putnis and Austrheim 2010). At a smaller scale, the development of foliation induced
by deformation can enhance fluid flow (Rumble and Spear 1983; Skelton et al. 1995; Ague
2007). On a large scale, fluid-induced retrogression is in most cases associated with crustal
shear zones (Yardley et al. 2000). This has been shown in the Greek Cyclades, where regional
greenschist-facies retrogression was induced by fluids using broad shear zones as pathway
(Ring et al. 2010).
In summary, changes in mineralogy and chemistry may control fluid-induced reaction
progress, while deformation and deformation-induced mineral alignment, such as the
development of foliation have a strong influence on fluid flow within metamorphic rocks.
However, is it active deformation that channels fluid flow or does deformation “set the stage”
for fluid flow by creating foliation?
Study sites
The Cycladic archipelago in Southern Greece
Geological background
The studies presented in paper I and paper II were conducted on the Greek island Syros, in the
Cycladic archipelago, in the Aegean Sea (figure 5). The island is part of the Attic-Cycladic
metamorphic core complex and is now located in the back-arc of the active Hellenic
subduction zone (Aubouin and Dercourt 1965; Dürr et al. 1978; Jacobshagen et al. 1978;
Jolivet and Brun 2010). Most of the rocks on Syros belong to the Cycladic Blueschist Unit
(CBU). The CBU is comprised from bottom to top of: 1) a volcano-sedimentary sequence
and; 2) a structurally higher meta-ophiolitic mélange composed of eclogites, glaucophanites,
meta-gabbros, ultrabasic rocks, felsic gneisses and jadeitites (Dixon and Ridley 1987; Bröcker
10
and Enders 2001). In the southeastern part of Syros a third unit from the Middle to late
Cretaceous (Maluski et al. 1987; Tomaschek et al. 2000) known as the Vari Gneiss occurs.
However, the tectono-stratigraphic position of this third unit is debated as some authors
interpreted the Vari unit as the structurally uppermost unit on Syros (Maluski et al. 1987;
Ring et al. 2003; Keiter et al. 2011; Soukis and Stockli 2013) while others have seen the Vari
unit as part of the Variscan basement that underlies the CBU (Philippon et al. 2011). With the
exception of the Vari gneiss, the rocks on Syros contain high-P mineral assemblages which
record eclogite-facies peak metamorphism at pressures exceeding 1.5 GPa and temperatures
around 500º C (Dixon and Ridley 1987; Okrusch and Bröcker 1990; Trotet et al. 2001a,
2001b).
Figure 5. (a) Location of the Cycladic archipelago in Greece. (b) Simplified geological map of Syros
(modified after Keiter et al. 2011). The arrows indicate the location of the study areas: Fabrika Beach
(paper I) and Delfini (paper II).
HP-LT metamorphism on Syros has been dated at 52 Ma (Maluski et al. 1987; Tomaschek et
al. 2003; Putlitz et al. 2005; Lagos et al. 2007). The P-T evolution of the CBU on Syros
remains controversial. Whereas some authors favor one single unit sharing the same P-T
evolution with peak conditions around 1.5 GPa and 500° C (Schumacher et al. 2008a and
references therein), Trotet et al. (2001a, 2001b) describe three metamorphic sub-units
separated by normal shear-zones, each with peak metamorphism at 1.9 GPa and 525° C, but
with different retrograde P-T paths. Retrogression at greenschist-facies conditions on Syros
has occurred in Miocene times (Altherr et al. 1979; Wijbrans et al. 1990; Avigad 1993;
Bröcker et al. 1993; Trotet et al. 2001a; Bröcker et al. 2004) at 0.9 – 0.4 GPa and 500 – 350°
11
C (Avigad et al. 1992; Bröcker et al. 1993; Trotet et al. 2001a; Parra et al. 2002; Schumacher
et al. 2008a).
Metamorphic fluid flow in the Cyclades
Evidence of prograde, retrograde and post-metamorphic fluid flow events have been
described in rocks from the Cycladic archipelago.
During prograde HP-LT metamorphism, fluid flow was highly channeled which resulted in
locally variable fluid fluxes (e.g. Selverstone et al. 1991, 1992; Bröcker et al. 1993; Getty and
Selverstone 1994; Barnicoat and Cartwright 1995; Ganor et al. 1996; Ague 2000; Putlitz et al.
2000; Breeding et al. 2003; Ague 2007). The fluids that were released during the
decompression of the subducting slab preferentially leached LILEs, U and Pb as they passed
through the subducting metasedimentary rocks (Breeding et al. 2004). During or after peak
metamorphism, rocks on Syros underwent locally pervasive carbonation (Kleine et al. 2014).
Retrograde metamorphism from blueschist to greenschist-facies conditions was at least partly
accompanied by pervasive metamorphic fluid flow (Schliestedt and Matthews 1987; Avigad
et al. 1992; Bröcker et al. 1993; Parra et al. 2002; Schumacher et al. 2008a). This fluid was 18
O-enriched (Schliestedt and Matthews 1987), presumably hydrous (Schumacher et al.
2008b) and variably saline (Barr 1990).
Late stage, post-metamorphic swarms of carbonate and quartz veins cross cut variably well-
preserved and retrograded high P – low T metamorphic rocks on Sifnos, Tinos and Kythnos
(Ganor et al. 1994). These veins have low δ13
C values (ranging from -12 ‰ to -8 ‰) which
can be either attributed to oxidation of organic carbon or circulation of surface water through
extensional fractures generated during late stage exhumation (Ganor et al. 1994; Famin et al.
2004).
Figure 6. (a) Field photography showing the steeply dipping normal shear zone that cuts through
retrogressed greenschist-facies rocks at Fabrika Beach. (b) Close-up view of the shear zone with type I
and II blue haloes and surrounding greenschist (GS).
12
Fabrika Beach
The study site for paper I is located at Fabrika Beach which is on the SE coast of Syros close
to the village of Vari (figure 6). The rocks belong to the CBU (figure 5; Trotet et al. 2001a,
2001b; Keiter et al. 2011).
The outcrop of interest centres on a brittle to ductile high-angle normal shear zone within the
metavolcanic sequence. Greenschist-facies metabasites (chlorite + epidote + actinolite +
albite) in the footwall of the shear zone are juxtaposed against garnet-mica psammites (quartz
+ white mica + garnet) in its hanging wall. The shear zone is bordered by a dark blue halo
within the greenschist-facies metabasites. This dark blue halo can be further distinguished into
two types: type I and type II blue halo. The type I blue halo is immediately adjacent to the
shear zone and is comprised of glaucophane schist (glaucophane + phengite). The type II blue
halo is comprised of glaucophane, phengite, calcite, epidote and garnet and is observed
further away from the shear zone.
Delfini
The studied outcrop for paper II is located along the northern shoreline of a small peninsula
north of the village Delfini, on the northwest side of Syros. Also here, the rocks belong to the
metamorphosed volcano-sedimentary subunit of the CBU and consist of quartz-mica schists.
The outcrop contains steeply dipping conjugate fractures (0.2 – 2 cm in width) which are
filled with quartz and carbonates. The most conspicuous features of this locality are
symmetrical dark halos flanking these fractures (figure 7). The widest halos have constant
widths of ~ 60 cm. Thinner haloes show discernible tapering upwards.
Figure 7. (a) Field photography of the outcrop at Delfini. (b) Fracture with symmetrical dark reaction
halo. (c) A set of several fracture-halo systems.
The SW Scottish Highlands
Geological background
The metasedimentary rocks in the SW Scottish Highlands belong to the Argyll Group of the
Dalradian Supergroup (figure 8; Harris and Pitcher 1975) and were deposited during late
13
Precambrian to Cambrian times under shallow marine conditions (Roberts and Treagus 1977).
The Argyll group comprises metapsammites, metapelites and metacarbonates. These
sedimentary units have been intruded by mafic sills at 595 4 Ma (Halliday et al. 1989).
During an early stage of the Caledonian Orogeny in the Ordovician, the rocks of the SW
Scottish Highlands underwent multiple phases of deformation. Primary deformation (D1 to
D2) was associated with a penetrative axial planar foliation and produced the NW-facing Islay
Anticline, the upwards facing Tayvallich and Kilmory Bay Synclines (Loch Awe Syncline)
and the SE-facing Ardrishaig Anticline (Roberts and Treagus 1977). Textural features of the
deformation event D3 during peak metamorphic temperatures (Harte et al. 1984) are not
observed in the SW Scottish Highlands (Skelton et al. 1995). Secondary deformation (D4)
caused the collapse of the Ardrishaig Anticline, which resulted in the formation of the Tarbert
Monoform, Cowal Antiform, Ben Ledi Monoform and the synformal Aberfoyle Anticline.
This deformation event is associated with the development of a crenulation cleavage which is
axial planar to folds of bedding and primary cleavages (Roberts and Treagus 1977).
The rocks of the SW Scottish Highlands were metamorphosed at greenschist- to epidote-
amphibolite conditions (Graham et al. 1983; Skelton et al. 1995) with peak-metamorphic PT-
conditions of 0.9 – 1.1 GPa and 550° - 580° C (Vorhies and Ague 2011). A near-isothermal
decompression of the rocks followed on peak-metamorphism (Vorhies and Ague 2011).
Figure 8. Simplified geological map of SW Scotland (modified after Roberts 1974). Arrows indicate the
location of the study areas: Loch Stornoway (paper III) and Port Ellen (paper IV). The small window
shows a simplified cross section of the Islay Anticline (IS), Loch Awe Syncline (LAS) and Ardrishaig
Anticline (AA) after secondary deformation. TS = Tayvallich Syncline; KBS = Kilmory Bay Syncline.
Metamorphic fluid flow in the SW Scottish Highlands
Three main fluid-rock interaction events have been documented in the Dalradian rocks of the
SW Scottish Highlands. These are 1) pre-metamorphic spilitization; 2) syn-metamorphic
carbonation of metabasaltic sills and; 3) post-metamorphic quartz-carbonate-sulphide veins.
Pre-metamorphic spilitization of the metabasaltic sills was caused by the interaction of basalt
with hydrous, saline fluids at or near the ocean floor (Graham 1976; Skelton et al. 2010). This
led to the formation of epidote segregations, enrichment of Na2O in the rock, high modal
plagioclase and variable depletion in CaO (Graham 1976). Calcic plagioclase was replaced by
14
albite without involving a change in volume by the release of Ca. The released Ca was
incorporated partly into epidote, calcite and/or other Ca-bearing phases which deposit in
fractures and amygdales (Graham 1976).
During greenschist-facies metamorphism a CO2-bearing, hydrous fluid generated by
devolatilisation of the underlying metamorphic pile, infiltrated the Dalradian rocks. Fluid flow
was structurally focused through metapelites (Skelton et al. 1995; Pitcairn et al. 2010) with
which the fluid was near equilibrium (Skelton et al. 1995). The fluid caused carbonation of
the metabasaltic sills which resulted in a characteristic mineral assemblage zonation of the
sills with carbonate-free interiors and carbonate-rich margins (Harte and Graham 1975;
Skelton et al. 1995). Extensive carbonation is mostly seen in metabasalts metamorphosed at
greenschist-facies conditions (Skelton et al. 1995).
As the Dalradian rocks cooled during exhumation post-peak metamorphic fluid infiltration
caused widespread formation of veins of quartz, carbonates and sulphides (Parnell et al.
2000a, 2000b; Anderson et al. 2004; Baron and Parnell 2005). Anderson et al. (2004) showed
that fluid infiltration occurred at high temperature (> 300° C) and significantly low
temperature (< 200° C). The high temperature fluid was moderately saline and was in isotopic
equilibrium with the metasediments. The low temperature fluid had a similar salinity but
showed enrichment in its 18
O-signature (Parnell et al. 2000a; Parnell et al. 2000b; Anderson et
al. 2004; Baron and Parnell 2005).
Loch Stornoway
Paper III focuses on a group of metabasaltic sills at Loch Stornoway on the mainland
peninsula of Knapdale in the SW Scottish Highlands (figure 9). These metabasaltic sills
which are several tens of meters in width are separated by metasedimentary layers. The whole
sequence was metamorphosed to epidote-amphibolite-facies conditions.
Figure 9. Simplified sketch of the studied locality at Loch Stornoway.
The metabasaltic sills are massive with generally unfoliated interiors. The sill margins are
commonly strongly foliated and partly carbonated. They are similar in appearance to the
15
greenschist-facies metabasaltic sills which are found elsewhere in the SW Scottish Highlands
(Skelton et al. 1995; Arghe et al. 2011). However, because of their higher metamorphic grade,
sills at Loch Stornoway exhibit a different mineral assemblage zonation. Sill interiors are
primarily comprised of coarse-grained amphibole, epidote, plagioclase, biotite, garnet, quartz
and minor calcite. Foliated sill margins contain fine-grained amphibole, epidote, chlorite,
quartz, calcite and relicts of biotite.
The Ard of Port Ellen
The study site of paper IV is located on a small peninsula called The Ard, south of Port Ellen
village on the island Islay in SW Scotland. At this locality there are five metabasaltic sills (2
of which may represent the same sill repeated by folding), which range from several meters
up to ca. 50 m in width. The sills are separated by thick layers of metasediments and consist
of massive metabasalts metamorphosed at greenschist-facies conditions (figure 10).
During greenschist-facies metamorphism, infiltration of CO2-H2O-bearing fluids caused a
distinct zonation in all of the metabasaltic sills with carbonate-free interiors and strongly
carbonated margins (Skelton et al. 1995). The focus in paper IV was on the two centralmost
sills which additionally exhibit large plagioclase phenocrysts (up to 3 cm) in their interiors.
These sills also show a distinct distribution of large Fe-oxides crystals (up to 2 cm; figure 3c)
with high abundances in the sill margins, while sill interiors are nearly completely free of Fe-
oxides. Epidote segregations formed during spilitization are most easily recognized from sets
of parallel cracks, up to 50 cm in length. These cracks record the later contraction during
greenschist metamorphism, as the density of epidote increases (Arghe et al. 2011). Epidote
segregations are often aligned and parallel to foliation in the sill margin and the
metasedimentary layering.
Figure 10. (a) Field photography from the Ard of Port Ellen showing a metabasaltic sill and the
underlying metasediments. (b) Sill interior with large plagioclase phenocrysts in fine-grained matrix. (c)
The fine-grained sill margin with large Fe-oxide crystals.
16
Methods
Sampling
Fieldwork for this study has been performed in two different regions which expose
spectacular sites for studying metamorphic fluid flow. One region is the Greek island Syros,
which belongs to the Cycladic archipelago in the Aegean Sea (papers I and II) and the second
region are the Southwestern Scottish Highlands (papers III and IV). Sampling was carried out
in a similar manner in both areas. Profiles were constructed outgoing from a likely fluid
pathway into the adjacent rocks. On Syros, the fluid pathways were along shear zones and
brittle fractures. The adjacent rock was visibly changed by fluid-rock interaction. Samples
were taken along profiles from the deformational zones and across reaction halos into the
unaltered host rocks. In the SW Scottish Highlands, fluid flow occurred within
metasedimentary layers and affected metabasaltic sills which intruded layer parallel into the
lithologies. Several profiles were constructed from the boundary between metasediments and
metabasalts across all of the sills.
Analytical techniques
All samples have been prepared for petrographic, electron microprobe (EMP) and X-ray
fluorescence analysis.
Petrography
Mineral modes were estimated by point counting of 1000 evenly spaced points in thin
sections from each of the profiles to provide a quantitative basis for determining possible
mineral reactions. For point counting a polarization microscope (Leica) with a point counting
stage driven by the computer program PetrogLite (Version: V2.37) was used (papers I and
III). Later, the point counting stage on the polarization microscope was exchanged with an
automated stage driven by a PELCON automatic point counter and associated software
(papers II and IV). Uncertainties were calculated following the approach of Van der Plas and
Tobi (1965). Modal calcite for papers I, III and IV was determined using a field-based method
(“fizz-o-metry”) which was first described by Skelton et al. (1995). A small volume of
crushed sample is reacted with HCl in a sealed vessel and the pressure of evolved CO2 is used
to determine modal calcite. The instrument is calibrated using samples with known modes of
calcite. The precision of this method is equivalent to mineral modes obtained by counting
2000 points (Skelton et al. 1995). Modal calcite determined by fizz-o-metry will show some
discrepancy to modal data obtained by point-counting. In the latter case mineral modes are
estimated from a 2 x 4 cm thin section, whereas modal calcite from fizz-o-metry is estimated
from a fist-sized volume of rock. Mineral chemistry was determined by electron microprobe
analyses (EMPA, type: JEOL JXA-8530 Field emission-EMPA; software: JEOL PC-EMPA
application software, version: 1.5.0.4; conditions: 15kV, 20 nA; beam: 0.5-10 μm). The
analyses have been carried out at the EMPA facilities of the Department of Earth Sciences at
Uppsala University.
Geochemistry
Major element chemistry of the samples was determined using X-ray fluorescence (XRF)
analysis (type: Rigaku ZSX Primus II Sequential X-Ray Fluorescence Spectrometer) at the
17
Department of Geological Sciences at Stockholm University. For XRF-analysis, accuracy was
controlled through repeated analyses of international reference materials and is better than
1%.
Trace elements for samples from paper II and IV have been determined by using Laser
Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at the Department of
Geological Sciences at Stockholm University. A spot size of 120 μm was used, laser pulse
frequency was tuned to 10 Hz and a laser energy density of 7 J.cm-2
was chosen.
Quantification was performed using Si as internal standard and the international standard
reference material NIST SRM 612 was used as external calibration standard with the sample-
standard bracketing method. Accuracy was controlled by repeating analyses of the
international glass reference material BCR-2G. Data reduction was performed using the
public domain software Iolite (Hellstrom et al. 2008; Paton et al. 2011). The trace element
chemistry of samples from paper I was determined following lithium-metaborate-tetraborate
fusion by inductively-coupled plasma mass spectrometry at Activation Laboratories in
Canada. Accuracy was controlled through repeated analyses of international reference
materials and is better than 1% and 5% for major and trace elements respectively.
Additionally, chemical variation along profiles at Delfini (paper II) was studied using an
Olympus Innov-X Delta handheld XRF analyzer (figure 11). No drilling is required for this
analytical method and chemical analysis is done in the field. This field-based method
generates a chemical analysis of the rock surface with a penetration depth of 1 – 3 mm. The
spot diameter is 1 cm.
Figure 11. The portable XRF-analyzer and how it is used in the field.
One shortcoming of this method is that elements lighter than Mg cannot be analyzed. A
second shortcoming is that the volume of rock that is analyzed is small compared to the
crystal size. This heterogeneity generates scatter in the data which can be partly overcome by
reducing sample spacing along each profile. Another way of handling this issue is to use the
portable XRF analyzer to study ratios of elements which tend to substitute for another, rather
than absolute concentrations of specific elements. This reduces scatter caused by
heterogeneous mineral distribution. A third shortcoming arises because analyses obtained
using the field-based method are affected by processes affecting the chemistry of a rock
surface. The rock surface that was studied is largely free from weathering. However, sea salts
18
were precipitated on the rock surface from sea spray and these needed to be subtracted from
portable XRF measurements. The sea salts were removed from our measurements by
assuming that these were NaCl and Na2SO4 and recalculating accordingly.
Geothermobarometric modeling
For geothermobarometric modeling in paper I the computer program PerPlex 6.6.6 with the
thermodynamic dataset HP02 (Holland and Powell 1998) was used. The same program was
also used to construct a set of T-XCO2 pseudosections for paper I. These calculations
considered solution models for chlorite (Holland et al. 1998), garnet (Holland and Powell
1998), amphiboles (Dale et al. 2005) and carbonates (Holland and Powell 2003). In paper III,
the computer program THERMOCALC (version 3.33) was used together with mineral
chemistry and modal variations estimated by point counting to identify probable reactions
responsible for the observed reaction textures. Mineral endmember activities were calculated
from microprobe data by using the software AX (© Holland 2008). T-XCO2 diagrams were
constructed using the computer program THERMOCALC (© Powell and Holland) with the
database ds55 (Holland and Powell 1998).
Results
Quantification of metamorphic fluid fluxes (papers I and II)
The studied localities (Fabrika Beach and Delfini) on the island Syros in the Greek Cyclades
provide spectacular relationships between fluid flow, deformation and fluid-rock interaction.
At Fabrika Beach, the extension of a dark blue halo surrounding a shear zone was used to
model fluid fluxes along that structural zone of weakness. The dark blue halo can be divided
into two types (types I and II). The inner type I halo is narrow (< 20 cm) and consists of
nearly pure glaucophane schist. It was formed by the metasomatic addition of Na, Si, K and
LILEs and removal of Ca, Al and Mn. The outer type II halo comprises of a carbonated
blueschist-facies mineral assemblage (glaucophane + calcite + phengite + epidote + garnet +
quartz) and only experienced slight metasomatic addition of K and LILEs. These relatively
mobile elements are not major components in the rock-forming minerals seen in this type of
halo. Furthermore, no new growth of blueschist-facies minerals occurred as it would have
been expected if the type II halo was of pure metasomatic origin. Instead, modal data and
observed reaction textures suggest that replacement of a blueschist mineral assemblage by a
greenschist-facies mineral assemblage occurred in the outer parts of halo type II. Therefore,
retrogression was accompanied by decarbonation, which was best exemplified by the
replacement of carbonated epidote by albite (figure 1d). Based on these observations, the type
II halo is a preserved blueschist-facies rock. The mechanism of blueschist preservation was
initiated by a fast flowing fluid within the shear zone (figure 12). This fluid had an elevated
concentration of CO2 relative to the more hydrous fluid that caused pervasive decarbonation
during greenschist-facies retrogression. It flowed simultaneously but relatively slowly through
the adjacent bulk rock. Progress of the decarbonation reaction was inhibited by the higher
fraction of CO2 in the fast flowing fluid, causing the carbonated blueschist-facies assemblage
to be preserved close to the shear zone.
19
Figure 12. Model for localized preservation of a carbonated blueschist-facies mineral assemblage along
the shear zone.
A simplified P-T vs. XCO2 pseudosection (figure 13) confirmed that preservation of
carbonated blueschist-facies rocks is possible at greenschist-facies conditions in the presence
of CO2-bearing fluids. Mass balance calculations reveal that for the preservation of blueschist-
facies minerals in a halo of the width of 1 m, the fluid flux within the shear zone is required to
be at least 100 to 2000 times faster than in the surrounding rocks.
Figure 13. P-T vs. XCO2 diagrams (NCFMASH + CO2 system) for three thermal gradients (a) 300°/GPa,
(b) 350°/GPa and (c) 400°/GPa. Glaucophane is stabilized at low T and P in the presence of CO2-bearing
fluids. ab = albite; act = actinolite; cc = calcite; chl = chlorite; cz = clinozoisite; gl = glaucophane; gt =
garnet (Ca-endmember); pa = paragonite (white mica).
20
At Delfini, quartz and carbonate filled fractures that are surrounded by symmetrical dark
reaction halos were used to calculate time-averaged fluid velocities in a scenario where the
direction of fluid flow is known in at least two dimensions, i.e. along the fractures (figure 14).
This was possible because the reaction halos showed discernible tapering upwards.
Figure 14. Illustration of the sampled profiles at different halo widths along the continuation of a fracture.
Fluid flow occurred along the fracture (advective) and diffused into the host rock.
The reaction halos were formed by carbonation reactions that were induced by the diffusion
of CO2 outwards from the fracture where a hydrous fluid passed through. Fluid flow along
this fracture also caused minor metasomatic changes in the concentration of relatively mobile
elements (e.g. K, Na, Cs, Pb and Sr) in the country rock. However, the concentrations of
major and less mobile trace elements were largely unaffected by metasomatism. This implies
that carbonation was isochemical with respect to volatiles.
Besides conventional XRF analyzes of the bulk rock in-situ bulk rock analyzes were carried
out along profiles across five different fracture halo systems by using a portable XRF
analyzer. This technique makes it possible to collect high resolution geochemical profiles
(sample spacing < 2 cm) and thereby to collect sufficient data to constrain the shape of
geochemical fronts. These fronts were later used for the parameterization of metamorphic
fluid flow along the fractures. For this study, the ratio of Sr/Ca that was derived from the
geochemical analyzes from the portable XRF, served as a tracer for fluid-rock interaction.
Strontium is mobile in metamorphic fluids and it is therefore likely that advective transport of
Sr occurred along the fracture, and that the transverse diffusive transport of Sr occurred
outwards from the fracture into the country rock. Inverse modeling of the Sr/Ca ratios
revealed an early stage of fracture-controlled fluid flow within the quartz-mica schists at
Delfini. Fluid flow velocities ranged from 10-6
to 10-5
ms-1
and lasted from 0.1 to 400 years
for porosities of 10-2
to 10-4
, volumetric fluid/ solid partition coefficients for Sr of 10-2
to 10-4
and fracture widths of 1 to 10 mm. Halo widths at Delfini do not exceed 60 cm, and this
implies that a steady state developed after the critical halo widths had been reached (figure
15). Along fractures for which steady state fluid flow was reached, fluid velocities were
significantly lower (10-8
to 10-6
ms-1
) and fluid flow lasted 100 to 15000 years.
21
Figure 15. (A) CO2 diffuses from the fluid flowing in the fracture outwards into the adjacent country rock
causing the formation of a reaction halo. This reaction halo extents until it has reached a critical width.
(B) Thereafter, steady state fluid flow over a longer period occurred.
Timing of fluid flow on Syros
At Fabrika Beach fluid flow must have occurred during an early stage of greenschist-facies
retrogression. During this period pressure and temperature conditions were still sufficiently
high to stabilize blueschist-facies minerals in the presence of a CO2-bearing hydrous fluid. At
Delfini, flow of CO2-bearing fluids would have been sufficiently fast to preserve HP-LT
minerals. However, besides high Si-values in phengites which could be inherited from HP-
facies conditions (Bröcker et al. 1993), no other evidence for the presence of HP-LT minerals
could be found. Fluid infiltration caused the formation of greenschist-facies minerals such as
chlorite, biotite and albite. This and the brittle nature of the fractures point to the conclusion
that fluid infiltration at Delfini occurred during a late stage of retrograde greenschist-facies
metamorphism.
Mineralogical controls on fluid flow (papers III and IV)
In the SW Scottish Highlands, the exposure of metabasaltic sills that are affected by several
fluid-rock interaction events can be used to study mineralogical, chemical and deformational
controls on metamorphic fluid flow.
The metabasaltic sills at Loch Stornoway are metamorphosed at a higher grade than usual for
the SW Scottish Highlands. Here, the sills were metamorphosed at epidote-amphibolite-facies
conditions and contain abundant garnet. The garnets preserve excellent textural evidence for
the mechanisms of fluid-driven reactions (carbonation and hydration), and what the
controlling factors of fluid infiltration into these rocks were. The sill interiors contain coarse-
grained amphibole, epidote, plagioclase, biotite, garnet, quartz and minor calcite. The sill
margins are usually foliated and are comprised of fine-grained amphibole, epidote, chlorite,
quartz, calcite and relicts of biotite.
The garnets are pseudomorphically replaced by chlorite and calcite which are interpreted to
have occurred by an interface-coupled dissolution-precipitation mechanism (figure 2).
Porosity generated in the product chlorite most likely enhanced fluid access to the
replacement front. Alteration of garnet was not only restricted to the sill margins where it is
22
most extensive but also occurred throughout the sill. Furthermore, examination of the mineral
grains that surround the replaced and pristine garnets showed that the occurrence of hydrous
elongated and fine-grained minerals might have an influence on the degree of alteration of
garnet. Replaced garnets are often surrounded by chlorite, amphibole and epidote whereas
calcite and quartz surround pristine garnets. The latter minerals might seal off the individual
garnet grains from the infiltrating fluid and in this way inhibit progression of hydration and
carbonation reactions. In addition the grain size of amphibole, for example, is coarser in areas
where garnets are better preserved while in areas such as in the sill margins amphibole is finer
grained and aligned and garnet is strongly altered. From these observations it is concluded
that fluid access was enhanced in areas that contain abundant fine-grained elongate minerals.
The fluid exploited the pre-existing foliation in the sill margins but was not necessarily
hindered by the lack of foliation in the sill interior. The perfectly preserved former outline of
the replaced garnets even in foliated parts of the sill margins indicates that infiltration of the
fluid causing hydration and carbonation occurred unassisted by deformation. Some of the
preserved garnets show a penetrative foliation preserved by inclusion trails within their
interiors. This is interpreted to mean that the foliation of the sill margins developed
contemporaneously with garnet growth during prograde to peak metamorphism. That is
before fluid infiltration. It is concluded that fluid infiltration that caused carbonation of the sill
margins in the SW Scottish Highlands occurred after peak metamorphism and was not
accompanied by deformation (figure 16). Instead, alignment of the minerals was controlled by
deformation before the fluid flow occurred, and the flow was controlled by the mineral
orientation. This sequence of events may apply to other studies in the SW Scottish Highlands.
Figure 16. Schematic sketch showing the sequence of events. Development of foliation in the sill margins
during prograde to peak metamorphism was contemporaneous with garnet growth. Infiltration of H2O-
CO2 fluid occurred after peak metamorphism.
23
On the Ard of Port Ellen metamorphic sills are separated by metasedimentary layers. The
whole sequence was metamorphosed at greenschist-facies conditions. The infiltration of H2O-
CO2 fluids at greenschist-facies conditions caused a mineral zonation with carbonate-poor sill
interiors and carbonate-rich sill margins. These sills also contain large abundant phenocrysts
of plagioclase, which is atypical for sills in the SW Scottish Highlands. Extensive magmatic
flow differentiation concentrated these phenocrysts in the sill interior. Magmatic flow
differentiation was also responsible for enrichment of Ti, Fe, P, HFSEs and REEs in the sill
margins. However, the sill interiors were enriched in Al, Na and LILEs.
Subsequent spilitic alteration caused albitization of the plagioclase phenocrysts. This led to
the preservation of these phenocrysts throughout greenschist-facies metamorphism, and the
production of layers of epidote segregations. Metamorphic reaction fronts spatially correlate
with pre-metamorphic mineralogical zonation and epidote segregations (figure 17). For
example, carbonation fronts did not advance beyond the boundaries between the phenocrysts-
poor sill margins and phenocrysts-rich interiors. This is because carbonation reactions did not
involve plagioclase as a reactant phase. Modes of reactant minerals such as amphibole and
epidote would be reduced in the sill interior because of the rock volume being occupied by
plagioclase crystals. The sills on the Ard of Port Ellen would have been much more
extensively carbonated if the sills were not mineralogically and chemically zoned prior to
metamorphism, by magmatic flow differentiation.
Figure 17. Extent of the carbonation fronts (from fizz-o-metry data) compared to the modes of optically
discernible plagioclase phenocrysts and modes of optically discernible Fe-oxides in (A) sill 1 and (B) sill 2.
In some sills, epidote segregations occur at the boundary between the margin of the sill and
the interior. The segregations are aligned parallel to the sill margins. The Fe-content of
epidote increased during greenschist-facies metamorphism (Arghe et al. 2011), which led to
an increase in density causing contraction and the production of parallel fractures in the
segregations. This porosity generation in layer parallel segregations of epidote could deflect
metamorphic fluid flow so as to flow parallel to the sill margin within these epidote-rich
patches. This inhibited further advancement of the reaction front. Based on these
mineralogical and chemical observations, the availability of reactant minerals and mechanical
factors, such as volume change in epidote control fluid-driven reaction front propagation
during metamorphism.
24
Conclusions
The main conclusions of the thesis are:
Metamorphic fluid flow does not only drive both prograde and retrograde
metamorphism but also inhibits metamorphic reactions. High fluid fluxes with
elevated XCO2 locally preserved blueschist-facies minerals alongside a shear zone at
Fabrika Beach on the Greek Island Syros. However, this proposed mechanism for
blueschist preservation is dependent upon the occurrence of carbonate minerals in the
HP-LT mineral assemblage.
On Syros, fluid flow along brittle fractures was relatively fast (10-6
to 10-5
ms-1
)
compared to fluid passing through the bulk rock (10-8
to 10-6
ms-1
). Fluid flow was
active over a period of years (0.1 to 400 years). This is shorter than previous estimates
of metamorphic fluid duration and implies that metamorphic fluid flow might be a
short term phenomenon which is capable of influencing surface systems in societal-
relevant timescales.
Fluid infiltration may occur unassisted by significant deformation. Previous
deformation causes a mineralogical control of fluid flow. Fluid flow uses a pre-
existing foliation, by taking advantage of mineral alignment, which allows fluid access
into the rock. Metamorphic fluid flow postdates primary deformation in the SW
Scottish Highlands.
Metamorphic reaction fronts are spatially correlated with pre-metamorphic
mineralogical zonation that was produced by magmatic flow differentiation and
spilitization. Fluid-driven reaction front propagation was partly controlled by the
availability of reactant minerals and metamorphic volume changes during mineral
transformations in metabasaltic sills in the SW Scottish Highlands.
Future prospects
A lot of work has been performed on metamorphic fluid flow however, several issues remain
where further research is needed.
The proposed mechanism whereby a blueschist-facies mineral assemblage was preserved due
to elevated XCO2 caused by fast flowing fluid (see paper I) should be applied in a different
geological setting. This can be demonstrated on the island Naxos in the Greek Cyclades. Here,
a thick marble layer overlies mica schists that were metamorphosed at greenschist-facies
conditions. In these rocks, blueschist-facies minerals occur in a narrow range in direct contact
with the marble layer. Elevated XCO2 in the fluid phase that caused retrogression might also be
responsible for the preservation of HP-LT minerals in this outcrop. The higher CO2
concentration in the fluid is likely derived from the interaction of the fluid with the marble at
the lithological boundary. If preservation was caused by a CO2-bearing fluid in this geological
25
setting it would reveal that fluid flow velocity plays a minor role in the whole preservation
process.
In the SW Scottish Highlands, metamorphic fluid flow and fluid-induced reaction progress
was not necessarily deformation controlled (paper III). Instead, fluid infiltration was
dependent on the development of an interconnected porosity and the alignment of specific
minerals or volume change. However in what timescales does fluid infiltration and reaction
progress occur in this geological setting? This could be assessed by experimental work where
deformed and undeformed rock species are infiltrated by a supercritical CO2-H2O fluid phase
at varying temperatures and pressures. Measurements of diffusion rates and knowledge about
the development and influence of porosities on fluid infiltration that can be derived from these
experiments would advance timescale models for fluid infiltration.
The calculation of fluid fluxes provides constraints about the time-averaged rate and
maximum duration of metamorphic fluid flow. However, these calculations do not give any
information about instantaneous fluid fluxes, where fluid flow was continuous over a period
of time or if fluid flow occurred in a series of rapid pulses. Therefore, one interesting question
about metamorphic fluids is to calculate the instantaneous fluid flux during episodic fluid
flow. A way to answer this question could be to investigate kinetic broadening of reaction
fronts in fluid flow systems. This is because kinetic broadening will depend on the
instantaneous, rather than time-averaged, fluid flux. This is expressed by the Damköhler
number which can be obtained by chromatographic modeling of reaction fronts.
Another area to investigate is to quantify surface release of metamorphic fluids. This has been
attempted in studies of hot springs in orogenic belts (e.g. Evans et al. 2008). Our studies from
Syros (paper I and II) show evidence of metamorphic fluids crossing the brittle ductile
transition and entering the shallow brittle crust. This points to the likelihood that metamorphic
fluids enter the surface system. Rapid release of deep (and metamorphic) fluids might occur in
the aftermath of an earthquake. Quantification of the metamorphic fluid component in hot
springs after an earthquake could help us to monitor pulses of fluid flow over time and could
give information about the nature and timescales of fluid flow, even deep within the Earth’s
crust.
Acknowledgements
First of all, I would like to express my sincere thanks to my main supervisor Alasdair Skelton.
Thank you so much for constructive discussions, enjoyable field work in pouring rain (Syros)
and sunshine (Islay), encouraging words after struggling with editors and reviewers,
numerous drams of whisky, the trust that I will finish my Ph.D. and well, listening to my
music in the car despite sincere doubts about my taste of music. Many thanks to my co-
supervisor, Iain Pitcairn, for great discussions and help on bursting questions on all different
kind of subjects, for inventing amusing shortcuts for our field areas such as APE (Ard of Port
Ellen) and LOST (LOch STornoway) and for always finding new thin sections for point-
counting. Special thanks also to Sandra Piazolo who I could contact as often as I wanted even
if she is far away in Australia. For financial support I would like to thank the EU Initial
26
Training Network DeltaMIN (Mechanisms of Mineral Replacement Reactions) grant and the
Navarino Environmental Observatory (NEO), Messinia in Greece, a cooperation between
Stockholm University, the Academy of Athens and TEMES S.A..
I am very grateful to Dan Zetterberg, Marianne Ahlbom, Cora Wohlgemuth-Überwasser and
Runa Jacobsson for their endless patience during field work, sample preparation and analyses.
Cora is also thanked for proof-reading the final thesis and nice excursions to the Northern
Lights and Kullaberget. Elin Tollefsen and Joakim Mansfeld are thanked for the Swedish
translation of the abstract of this thesis. I also would like to thank Eve Arnold for the great
support throughout the years as a Ph.D. student.
A big thank you goes to my “roomies” Jing, Alex and Hagen. It has been a great time among
the numerous boxes, bicycles, lava lamps, rock samples and gymnastic balls with fun talks
about nearly every topic and sometimes even about science. Special thanks I would like to
address to Francis for long sun walks over the campus and losing on purpose in badminton, to
Cliff for examining my lunch box (if I had one) and great dinner parties, to Reuben for proof-
reading the final thesis and being a reliable Frescati-burger-mate, to Rob for his trustworthy
weather fore-casts and having solutions for nearly each problem, to Catherine for not skiing
too fast, to Stefano for being such a good company throughout the years and for fighting and
struggling together until the end, to Nut for keeping track on the rules during numerous
Citadels sessions at Profex, to Kweku for great life discussions in bus 540 and sincere
shuffling of the cards in Citadels, to Isabell and Alfred for great times with Pricken and Teza,
TT and toast, Tatort and wine. Thanks to Elin, Daniele, Francesco, Nolwenn, Nathalie, Ernest,
Alexander, Liselott, Pedro, Natalia, Engy, Matthias and to all the others I have not mentioned
here but have been there for numerous chats in the office, on the hall way, in the lunch room,
during fika, geopub, Gröna Villan, on excursions and during field work.
I also would like to thank my friends outside university, especially Lena, Johanna S., Johanna
F., Sarah, Latina, Hedvig, Pia, Jenny and Diman, for reminding me that there is a big great
world behind metamorphic fluid flow.
Special thanks I would like to express to my great friend Moo. Sawadeeka for amazing talks,
the cheering up, for sharing great moments, and for not losing contact even though more than
8000 km of land and ocean are separating us. Without you, times would not have been always
so easy.
Last but not least I would like to give my warmest thanks to my family. To Anne, Georg and
Martin for your open ears, unforgettable hiking tours and great visits in Stockholm. My
sincere thanks, I would like to express to my parents for the great support throughout the
years, believing in me and their trust that I will always find the right path.
Thanks to all of you for making my time here in Stockholm
to such a great experience!
27
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