Chemical Geology 263 (2009) 82–88
Contents lists available at ScienceDirect
Chemical Geology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o
Halogen diffusion in magmatic systems: Our current state of knowledge
Don R. Baker a,b,⁎, Hélène Balcone-Boissard a,c
a
b
c
Earth and Planetary Sciences, GEOTOP-UQAM-McGill Research Centre, McGill University, 3450 rue University, Montreal, QC, Canada H3A 2A7
Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italia
Institut de Physique du Globe de Paris, CNRS, Equipe Géologie des Systèmes Volcaniques, 4 place Jussieu, Case courrier 89, 75252 Paris, France
a r t i c l e
i n f o
Article history:
Accepted 4 October 2008
Keywords:
Halogens
Diffusion
Silicate melts
Magmatic processes
Volcanic gas
a b s t r a c t
Diffusion of halogens has the potential to influence petrogenetic processes in magma chambers and conduit
degassing processes. This contribution reviews our current state of knowledge concerning halogen diffusion
and the influence of halogens on the diffusion of major elements in silicate melts. The addition of halogens to
silicate melts at common, natural concentration levels will have little effect on the diffusion of major
elements. However, the differences between the diffusivity of water, the diffusivities of halogens, and the
diffusivity of sulfur are significant enough that during melt inclusion entrapment, or during rapid bubble or
crystal growth, diffusive fractionation between water and the halogens, and between halogens and sulfur, are
expected to occur and can influence the compositions of melt inclusions, crystals and volcanic gases.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Volatiles are important in igneous processes. They exert significant
effects on transport (e.g., diffusion and viscosity) and equilibrium
(e.g., phase equilibria) properties of melts. They also trigger volcanic
eruptions and control the eruptive style. Volatile species dissolved in
melts may reach saturation in response to a pressure decrease or to
variations of the melt composition, due, for example, to crystallization.
At saturation the magma vesiculates and bubbles expand as volatiles
diffuse to the growing bubble and partition into the fluid phase.
Understanding the diffusion of volatiles in silicate magmas provides
the framework necessary for the quantitative modeling of processes
such as bubble formation and growth that can ultimately lead to
violent volcanic eruptions.
The volatiles commonly found at the highest concentrations in
magmatic systems are water, carbon dioxide, sulfur species, and to a
lesser extent fluorine, chlorine, bromine and iodine. Although H2O is
commonly the dominant volatile in magmas and the second most
abundant volatile is CO2 (Roedder, 1965, 1979; Symonds et al., 1994),
halogen concentrations can approach weight percent levels in melts
(Aiuppa et al., 2009-this issue) and are significant components of
volcanic gases (Aiuppa, 2009-this issue). They are important constituents of hydrous minerals and occasionally are found as halogenbearing minerals in igneous rocks (e.g., apatite, fluorite, topaz, etc.).
⁎ Corresponding author. Earth and Planetary Sciences, GEOTOP-UQAM-McGill
Research Centre, McGill University, 3450 rue University, Montreal, QC, Canada H3A
2A7. Tel.: +1 514 398 7485.
E-mail addresses: [email protected] (D.R. Baker), [email protected]
(H. Balcone-Boissard).
0009-2541/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemgeo.2008.10.010
Halogens expelled from volcanoes react to form atmospheric species
that produce detrimental effects on both a local and a global scale
(Gerlach, 2004; von Glasow, 2009-this issue). Recently, halogens have
become routinely measured at volcanoes and provide one of the tools
that may be used to predict eruptions (Aiuppa, 2009-this issue).
This contribution presents a brief introduction to the importance of
diffusion in petrogenetic processes and summarizes our current
knowledge of halogen diffusion.
2. The importance of diffusion in magmatic processes of bubble
growth, crystal growth and melt inclusion entrapment
Diffusion in magmatic systems is expected to operate at a limited,
local scale. This spatial restriction is because diffusion in silicate melts
is slow. The simple relationship,
pffiffiffiffiffiffiffi
x = 2 Dt;
ð1Þ
where x is the distance, D is the diffusion coefficient and t is the time
can be used to calculate the diffusion length. Using a typical diffusion
coefficient for a high-temperature silicate melt of 10− 10 m2 s− 1 and a
time of 10,000 years (3.16 × 1011 s), the calculated diffusion distance is
on the order of 10 m. Obviously, given these values, a simple diffusive
process appears unlikely to significantly affect the composition of a
magma whose volume is greater than approximately one cubic
kilometer. However many processes in magma chambers and in
conduits operate at a much shorter spatial scale where diffusion can
be an important mechanism affecting the evolution of magmas and
our interpretations of their differentiation.
D.R. Baker, H. Balcone-Boissard / Chemical Geology 263 (2009) 82–88
The dynamics of bubble growth controls the physics of volcanic
eruptions (cf., Sparks et al., 1994). Two processes control bubble
growth: the first is diffusion of volatiles from the melt into growing
bubbles and the second is the expansion of bubbles due to
decompression (Sparks et al., 1994). Both processes occur simultaneously during ascent of a volatile-saturated magma. The simple
diffusive growth law defined for bubble growth by Scriven (1959) is:
R = 2β
pffiffiffiffiffiffiffi
Dt;
ð2Þ
where R is the radius of the bubble, t the time, D the diffusion coefficient
and β is a rate constant. Bubble growth in volcanic systems is more
complicated and recent models consider additional parameters such as
advection, viscous resistance, decompression rate, volatile supersaturation, and variable volatile concentrations (e.g., Proussevitch et al., 1993;
Proussevitch and Sahagian, 1998; Blower et al., 2001; Lensky et al.,
2004). If bubbles grow rapidly by either decompression or by the fast
diffusion of water, then the concentrations of gases that diffuse more
slowly in the melt are predicted to differ from their equilibrium values
(cf., Smith et al., 1955). An example of the importance of diffusion in
affecting bubble compositions was presented by Alletti et al. (2007),
who demonstrated the efficacy of diffusive fractionation (see also
Balcone-Boissard et al., 2009-this issue).
Diffusion also appears important in controlling the composition of
melt inclusions. As demonstrated by Baker (2008), ions with diffusion
coefficients below those of Si–Al interdiffusion may be fractionated
significantly by diffusion during rapid crystal growth. This phenomenon can lead to the entrapment of melt inclusions that are
anomalously enriched in halogens, especially the larger halogens
with lower diffusion coefficients.
Diffusion also affects the compositions of rapidly grown crystals. At
growth rates as slow as 10− 9 m s− 1 the compositions of elements in
crystals can be modified in comparison to expected equilibrium values
(cf., Smith et al., 1955). Such a process could conceivably affect the
halogen concentrations in hydrous minerals and also lead to melt
saturation in halogen minerals, e.g. fluorite, at the interfaces between
rapidly growing crystals and melts.
Thus, although diffusion distances may be short, diffusion
processes still have the ability to modify the compositions of gases,
melts and crystals. These diffusive modifications may have significant
ramifications on our interpretation of magmatic processes and must
be understood. Quantifying the diffusivities of halogens in melts is a
fundamental step in the investigation of their diffusive affects on
petrogenetic processes.
3. The state of knowledge concerning halogen diffusion
Shaw appears to have been the first geoscientist to measure
volatile diffusion in silicate melts when he investigated water
diffusion in a rhyolitic melt (Shaw, 1974). Since that time many
studies of water, carbon dioxide, and sulfur diffusion have been
performed (see reviews by Watson, 1994; Baker et al., 2005).
However, the study of halogen diffusion has lagged significantly
behind the more common volatiles, although recent experiments have
begun to address this topic.
Diffusion measurements can be performed either in the absence of
a chemical potential gradient, or in its presence (Shewmon, 1989;
Chakraborty, 1995). In the first case the movement of ions at trace
element concentrations can be well-modeled as a random walk
process; this is tracer diffusion, which is similar to self-diffusion. In the
second case transport is complicated by interactions between the ion
and other ions caused by the chemical potential gradient; this is
chemical diffusion. Because natural melts virtually always contain
chemical potential gradients, most investigations of halogen diffusion
studied chemical diffusion, but many have tried to measure diffusion
at concentrations low enough to detect the presence of compositional
83
effects on halogen diffusion, as seen for water (Shaw, 1974; Zhang and
Stolper, 1991; Zhang et al., 1991; Zhang and Behrens, 2000). Detailed
discussions on the types of diffusion and descriptions on the
experimental techniques used to study diffusion are beyond this
paper, but provided in Baker (1993a,b) and Watson (1994).
Published studies of halogen diffusion in magmatic melts
demonstrate that their diffusion displays Arrhenian behavior at
magmatic temperatures. Therefore, the general form of the Arrhenius
equation provides an excellent description of halogen diffusion:
D = Do exp
−Ea
;
RT
ð3Þ
where D is the diffusion coefficient, in m2 s− 1, Do is the pre-exponential
factor, also in m2 s− 1, Ea is the activation energy, in J mol− 1, R is the gas
constant, 8.3143 J K− 1 mol− 1, and T is the temperature, in K. All
measurements of halogen diffusion in melts of geological interest
demonstrated Arrhenius behavior, and no influence of halogen
concentration on the halogen diffusivities has been observed. Thus,
one Arrhenius equation is sufficient to describe the diffusive behavior
of halogens at typical natural concentration levels. The pre-exponential factors, Do's, and activation energies, Ea's, determined for halogen
diffusion are listed in Table 1, and Arrhenius diagrams for fluorine
diffusion (Fig. 1) and chlorine and bromine diffusion (Fig. 2) are
provided. No data on iodine diffusion exists for magmatic melts.
Uncertainties in Ea are included in Table 1 when they are present in the
cited source, but the uncertainties in Do are not.
3.1. Fluorine diffusion
Dingwell and Scarfe (1985) exposed a fluoridated albitic, jadeitic,
and peraluminous aluminosilicate melts to air at high temperatures
and 1 atm pressure, resulting in the diffusive exchange of fluorine in
the melt with oxygen in the atmosphere. The Arrhenius equation for
the chemical diffusion of F they determined for albitic melt is
Dfluorine = 1:8 × 10
−9
exp
−120; 900
;
RT
ð4Þ
exp
−143; 900
;
RT
ð5Þ
for jadeitic melt is
Dfluorine = 3:2 × 10
−7
and for peraluminous melt is
Dfluorine = 8:1 × 10
−7
exp
−179; 900
:
RT
ð6Þ
Dingwell and Scarfe (1984) measured fluorine and oxygen
chemical interdiffusion in anhydrous jadeite melt at high pressure
using a diffusion couple in which one half of the couple was composed
of jadeite melt and the other half was composed of a jadeite melt in
which some of the oxygen was replaced by fluorine. In the
temperature range of 1200 to 1400 °C and pressure range of 1.0 to
1.5 GPa, the diffusion of F in jadeite melt was found to be pressure
insensitive and to follow the Arrhenius equation:
Dflourine = 5:9 × 10
−6
exp
−159; 000
:
RT
ð7Þ
The small difference in the Arrhenius equations for F diffusion at
1 atm and at 1.0 to 1.5 GPa (Eqs. (5) and (7); Fig. 1) results in an order
of magnitude difference in Dfluorine at 1300 °C. This difference is
attributed to either changes in the fluorine diffusion mechanism
between the two types of experiments, or to a small affect of pressure
between 1 atm and 1.0 GPa, or to differences in the experimental
techniques (i.e., loss of F at 1 atm as SiF4), or to a small amount of water
in the high-pressure melts.
84
D.R. Baker, H. Balcone-Boissard / Chemical Geology 263 (2009) 82–88
Table 1
Arrhenius parameters for Halogen diffusion.
Melt
T (°C)
P (MPa)
H2O (wt.%)
Do (m2 s− 1)
Ea (kJ mol− 1)
Reference
Fluorine
Albite
Jadeite
Peraluminous
Jadeite
Hawaiitic basalt
Hawaiitic basalt
K phonolite
K phonolite
K phonolite
Na phonolite
Na phonolite
Na phonolite
1200–1400
1200–1400
1200–1400
1200–1400
1250–1450
1250–1450
1250–1450
1250–1450
1250–1450
1250–1450
1250–1450
1250–1450
0.001
0.001
0.001
1000–1500
500–1000
500–1000
500–1000
500–1000
500–1000
500–1000
500–1000
500–1000
0
0
0
0
0
3
0
2
5
0
2
5
1.8 × 10− 9
3.2 × 10− 7
8.1 × 10− 7
5.9 × 10− 6
5.9 × 10− 4
1.4 × 10− 4
3.4 × 10− 5
4.5 × 10− 8
3.5 × 10− 7
1.2 × 10− 6
5.5 × 10− 7
2.2 × 10− 6
120.9 ± 37.2
143.9 ± 14.2
179.9 ± 30.9
159.0 ± 11.2
218.2 ± 33.5
191.9 ± 30.7
215.0 ± 29.5
94.3 ± 32.1
108.9 ± 36.6
133.2 ± 14.8
122.3 ± 42.7
134.1 ± 45.3
Dingwell and Scarfe (1985)
Dingwell and Scarfe (1985)
Dingwell and Scarfe (1985)
Dingwell and Scarfe (1984)
Alletti et al. (2007)
Alletti et al. (2007)
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Chlorine
Hawaiitic basalt
Hawaiitic basalt
Granite
Granite
K phonolite
K phonolite
K phonolite
Na phonolite
Na phonolite
Na phonolite
Na–Ca–Al–Si–O melt
1250–1450
1250–1450
800–1450
650–900
1250–1450
1250–1450
1250–1450
1250–1450
1250–1450
1250–1450
1100–1300
500–1000
500–1000
0.001–450
200
500–1000
500–1000
500–1000
500–1000
500–1000
500–1000
600–1800
0
3
0
Hydrousa
0
2
5
0
2
5
0
3.3 × 10− 2
2.3 × 10− 2
3.2 × 10− 9
6.5 × 10− 7
2.2 × 10− 6
6.7 × 10− 1
5.8 × 10− 1
4.7 × 10− 5
1.3 × 10− 6
2.3 × 10− 5
3.4 × 10− 4
277.2 ± 8.1
259.4 ± 8.6
86.19
110.9
164.0 ± 33.1
330.3 ± 111.2
306.7 ± 103
215.7 ± 54.4
148.4 ± 53.8
178.3 ± 63.3
207
Alletti et al. (2007)
Alletti et al. (2007)
Bai and Koster van Groos (1994)
Bai and Koster van Groos (1994)
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Balcone-Boissard et al. (2009-this
Watson and Bender (1980)
1250–1450
500–1000
0
7.5 × 10− 5
191.1 ± 33.3
Alletti et al. (2007)
Bromine
Hawaiitic
basalt
issue)
issue)
issue)
issue)
issue)
issue)
issue)
issue)
issue)
issue)
issue)
issue)
a
Experiments were water and NaCl brine saturated.
Gabitov et al. (2005) measured fluorine diffusion in a haplogranitic
melt during the dissolution of a fluorite crystal at 100 MPa, 900 and
1000 °C; they found that increasing the water concentration in the
melt from approximately 1 wt.% to 3.5 wt.% increased Dfluorine from 2.3
to 6.6 × 10− 12 m2 s− 1 at 1000 °C. Gabitov et al.'s (2005) fluorine
diffusivities are approximately two orders magnitude greater than
those calculated from the Arrhenius equation for F diffusion in
anhydrous albite and peraluminous melts at 1 bar (Eqs. (2) and (4)) of
Dingwell and Scarfe (1985). This enhancement is easily explained by
Fig. 1. Diffusion of fluorine in melts of geological significance as a function of
temperature. There appears to be no significant pressure affect at crustal to upper
mantle conditions. The Arrhenius lines for albite, jadeite and the peraluminous melt at
1 atm are from Dingwell and Scarfe (1985) and for jadeite at high pressure from
Dingwell and Scarfe (1984). Fluorine diffusion data for the hawaiitic basalt (labeled
basalt in the figure) were determined by Alletti et al. (2007) and for the phonolites by
Balcone-Boissard et al. (2009-this issue). The equations for the Arrhenius lines
describing halogen (F, Cl, and Br) diffusion in this and subsequent figures are in Table 1.
the presence of water in their experiments and its absence in the 1 bar
experiments of Dingwell and Scarfe (1985).
Alletti et al. (2007) measured F diffusion in a basaltic melt from
Etna at 0.5 and 1.0 GPa under anhydrous and hydrous, 3 wt.% water,
conditions at temperatures from 1250 to 1450 °C. They investigated
the diffusion of F, Cl, and Br individually and together in the melt; the
addition of other diffusing halogens did not significantly affect the
diffusion of fluorine. Addition of 3 wt.% water approximately doubled
Dfluorine. The affect of pressure on the diffusion coefficients was found
to be small over the pressure range investigated.
Fig. 2. Diffusion of chlorine in melts of geological significance as a function of temperature.
There appears to be no significant pressure effect at crustal to upper mantle conditions. The
Arrhenius line for chlorine diffusion in the hawaiitic basalt is from Alletti et al. (2007), the
line for the granitic melt is from Bai and Koster van Groos (1994), and for the phonolites
from Balcone-Boissard et al. (2009-this issue). Also shown is the data for bromine diffusion
in a dry basaltic melt (Alletti et al., 2007). See Table 1 for the Arrhenius equations.
D.R. Baker, H. Balcone-Boissard / Chemical Geology 263 (2009) 82–88
Balcone-Boissard et al. (2009-this issue) studied F diffusion in a Narich phonolite and a K-rich phonolite. Their study used similar conditions
and techniques as Alletti et al. (2007) and investigated melts with 0, 2
and 5 wt.% water. They found that fluorine had a higher activation energy
for diffusion in the K-phonolite than in Na-phonolite under anhydrous
conditions, but that Dfluorine was slightly lower in the K-phonolite. The Ea
calculated for F diffusion in the Na-phonolite (133 kJ/mol) is similar to
that for other sodic compositions (120 kJ/mol in albite and 144 or 159 kJ/
mol in jadeite). No influence of pressure on F diffusion was found.
Addition of water resulted in Dfluorine values and Arrhenius equations
that were similar in both melt compositions (Table 1). The addition of
water enhanced fluorine diffusion, but only by a relatively small amount
compared to the affect of water on major- and trace-element cation
diffusion in other compositions; for example, the addition of 5 wt.% water
to the Na-phonolite only increased F diffusion at 1250 °C from 2.7 × 10− 11
to 5.3 ×10− 11 m2 s− 1, instead of the orders of magnitude increase seen for
elements such as Zr and P (Watson, 1994). This enhancement of Dfluorine
with water addition is quantitatively similar to the results of Alletti et al.
(2007), even though the phonolitic melts are more polymerized than the
basaltic ones and the effect of water on breaking bridging oxygens might
expected to be more significant, resulting in a greater increase in Dfluorine
for equivalent amounts of water addition.
3.2. Chlorine diffusion
Watson and Bender (1980) made the first measurements of
chlorine tracer diffusion in an anhydrous, geologically relevant melt, a
simple Na–Ca–Al–Si–O composition with 56 wt.% silica, at 1100 to
1300 °C, and 0.6 to 1.8 GPa. They fit their data at 0.6 GPa and produced
an Arrhenius equation of:
Dchlorine = 3:4 × 10
−4
exp
−207; 000
RT
ð8Þ
and found no pressure dependence of Cl diffusion over the pressure
range investigated.
Watson continued these studies (Watson, 1991) and measured Cl
tracer diffusion in rhyolitic and dacitic melts containing 8 wt.% water at
1.0 GPa. Because experimental temperatures only varied from 850 to
1100 °C he did not define an Arrhenius relation, but demonstrated that
Dchlorine varied from 3×10− 12 at low temperature to 4×10− 11 m2 s− 1 at
high.
Bai and Koster van Groos (1994) studied the diffusion of chlorine in a
haplogranitic melt with 75 wt.% SiO2 and a natural obsidian melt with
78 wt.% SiO2 at pressures from 1 atm to 460 MPa by equilibrating them
with molten NaCl. At anhydrous conditions their Cl chemical diffusion
measurements between 850 and 1400 °C in the haplogranitic melt
produced an Arrhenius equation with a surprising low activation energy:
Dchlorine = 3:16 × 10
−9
exp
−86; 190
:
RT
ð9Þ
Comparing the activation energies for Cl diffusion measured by
Watson and Bender (1980) with those of Bai and Koster van Groos
(1994) demonstrates that with increasing SiO2 concentrations the
activation energy decreases.
The activation energy for chlorine chemical diffusion measured by
Bai and Koster van Groos (1994) in a hydrous, granitic melt saturated
with a NaCl brine at 200 MPa, 650 to 900 °C is higher than their
anhydrous measurements:
Dchlorine = 6:46 × 10
−7
exp
−110; 900
:
RT
ð10Þ
In contrast to the results of Watson and Bender (1980) and Watson
(1991), Bai and Koster van Groos found that the Cl diffusion coefficient
is pressure dependent and decreased by approximately a factor of 2
over the pressure range of 200 to 460 MPa.
85
Alletti et al. (2007) studied chlorine diffusion in the same basaltic
melt and at the same conditions used for their F diffusion study. They
found that in general Dchlorine was a little lower than Dfluorine at
temperatures below 1400 °C, but due to its higher activation energy
(Table 1) became similar to Dfluorine at high temperatures. Addition of
3 wt.% water to the melt increased Dchlorine by a factor of two and had a
small affect on the Arrhenius parameters (Table 1). Interestingly, they
found that their activation energies for chlorine diffusion in a basaltic
melt were greater than those found by Bai and Koster van Groos for a
rhyolitic melt, but similar to those of Watson and Bender (1980) for a
haplobasaltic melt.
Balcone-Boissard et al. (2009-this issue) found that chlorine
diffusion in the Na- and K-rich phonolites they studied were very
similar to each other. Addition of water to these melts increased both
Dfluorine and Dchlorine in both compositions, but the affect on Cl was
much greater than F. For equivalent amounts of added water, Dchlorine
was enhanced more in phonolites than in the basalt of Alletti et al.
(2007), as expected considering the more-polymerized nature of the
phonolitic melts. Interestingly, the addition of water increased the
activation energy for Cl diffusion in the K-phonolites, reminiscent of
the results of Bai and Koster van Groos (1994). Chlorine diffusion in the
K-phonolite also displays a larger enhancement in Dchlorine between 2
ant 5 wt.% H2O than between 0 and 2 wt.%, unlike observations for
most elements in silicate melts, where the diffusivity increases the
most when small amounts of water are added (Watson, 1994).
3.3. Other halogens
The study of the diffusion of the other naturally occurring
halogens, Br and I, lags far behind those of F and Cl. A literature
search found no studies of iodine diffusion in natural melts, and the
only geologically relevant melt in which bromine diffusion has been
studied is the hawaiitic basalt of Alletti et al. (2007). These authors
measured bromine diffusion under anhydrous conditions in their
basaltic melt at the same temperatures and pressures as their studies
of F and Cl diffusion. They demonstrated that Dbromine was a little
slower than Dchlorine, but that its activation energy was similar to that
of fluorine (Table 1, Fig. 2).
3.4. Comparisons of halogen diffusivities
The diffusivities of chlorine are typically lower than those of fluorine
at magmatic temperatures and bromine appears to be even slower
(Fig. 3). The effect of size on halogen diffusion appears to become more
significant with increasing SiO2 in the melt (Balcone-Boissard et al.,
2009-this issue). This trend is attributed to the structural roles (or “sites”
or coordinating ions) of F and Cl (and Br) in silicate melts. BalconeBoissard et al. (2009-this issue) hypothesize that the differences
between the behaviour of F and Cl are due to the roles F and Cl play in
the melt structure: F replacing coordinating oxygen, and Cl associating
with alkali cations and water (Schaller et al., 1992; Zeng and Stebbins,
2000; Liu and Nekvasil, 2002; Mysen et al., 2004; Sandland et al., 2004;
Kiczenski and Stebbins, 2006), possibly forming a Cl-alkali-H2O complex
outside of the melt structure, perhaps some form of nano-emulsion, but
importantly not a separate immiscible phase.
The contrasting behavior of F and Cl as a function of pressure, melt
composition and water addition provide some tantalizing hints as to
their diffusion mechanisms. There may be a greater affect of anhydrous
melt composition on Dchlorine than on Dfluorine as melts vary from
polymerized to depolymerized (Table 1), but admittedly the influence is
not clear given the small amount of data and the necessity of combining
the results from different studies using different techniques. This
possible reduced sensitivity might be explained by the similarity of F
environments in all magmatic melts. The pressure effect on Cl diffusion
measured by Bai and Koster van Groos (1994) is consistent with Cl
diffusion depending upon the free-volume in the melt, which decreases
86
D.R. Baker, H. Balcone-Boissard / Chemical Geology 263 (2009) 82–88
Fig. 3. Comparison of diffusivities in basaltic melts. The Arrhenius line for total water
and single point for CO2 (filled square) are from Zhang and Stolper (1991). Si–Al
interdiffusion is calculated based upon Baker (1992). The dotted line for CO2 diffusion is
for a haplobasaltic melt (Watson et al., 1982). Halogen diffusivities are from Alletti et al.
(2007) and sulfur diffusivities are from Freda et al. (2005).
with increasing pressure. This dependence, in turn suggests that
diffusion may be occurring by a vacancy mechanism, which is consistent
with the presence of an alkali–Cl–H2O species discussed above. The lack
of similar effect for F suggests diffusion by an exchange mechanism, but
we recognize that until comprehensive studies are performed using the
same techniques over a wide pressure range these ideas remain untested
hypotheses.
3.5. The potential for diffusive fractionation of halogens
Differences in diffusion coefficients can lead to diffusive fractionation during rapidly occurring petrogenetic processes. The degree of
diffusive fractionation is controlled by the rate of the process and the
differing diffusion coefficients of the melt species. The diffusion
coefficients of non-alkali, elements are coupled in most cases (cf.,
Watson, 1982, 1994), but volatile species are possibly decoupled. The
fractionation of one volatile species from another is controlled by their
relative diffusion coefficients, the greater the difference the greater
the extent of diffusive fractionation.
In basaltic melts total water diffuses most rapidly, approximately
one order of magnitude above fluorine diffusion at magmatic
temperatures (Fig. 3). Fluorine diffusion is similar to Si–Al interdiffusion calculated from melt viscosity (Baker, 1992); the diffusivities of
chlorine, bromine (both from Alletti et al., 2007) and total CO2 (Zhang
and Stolper, 1991; Watson 1991) are similar and only about a factor of
two (2) less then Si–Al interdiffusion. Sulfur diffusion (Freda et al.,
2005) is almost one order of magnitude slower than Si–Al interdiffusion in anhydrous basaltic melts. In these melts the potential for
diffusive fractionation between water and halogens is high, and
between the individual halogens is moderate, as calculated by Alletti
et al. (2007).
In hydrous silicic melts with 6 wt.% H2O the diffusion of CO2 (Watson,
1991) and total water (Zhang and Behrens, 2000) are similar and rapid
(Fig. 4), as is the diffusion of fluorine in Na- and K-phonolites with 5 wt.%
H2O (Balcone-Boissard et al., 2009-this issue). Chlorine diffusion in
rhyolite (Bai and Koster van Groos, 1994) and in Na- and K-phonolites
with 5 wt.% H2O (Balcone-Boissard et al., 2009-this issue) are one to
one-and-a-half orders of magnitude below water and CO2 diffusion at
1000 °C. But Si–Al interdiffusion in these melts is almost two orders of
magnitude slower, and sulfur diffusion is another one-half an order of
magnitude slower than Si–Al interdiffusion. If the melt water concentration falls to 2 wt.%, water diffusion and Si–Al interdiffusion drop by
approximately 1 order of magnitude. Based upon Balcone–Boissard's
results, chlorine diffusion in silicic magmas will also decrease by an
order of magnitude, but the decrease in fluorine diffusion will be much
less (not shown in Fig. 4). In these melts the possibility for diffusive
fractionation between water and halogens appears small, however
fractionation between halogens and sulfur appears possible.
The diffusion coefficients displayed in both Figs. 3 and 4 demonstrate
large enough differences in the diffusive behavior of the species to cause
diffusive fractionation, especially during bubble growth in a volcanic
conduit. Discussion of such models is beyond the scope of this
contribution. An illustrative model of how diffusive fractionation can
affect the compositions of bubbles is discussed in Alletti et al. (2007).
Baker (2008) discussed diffusive fractionation during melt inclusion
entrapment, and Balcone-Boissard et al. (2009-this issue) discuss how
diffusive fractionation may yield incorrect estimates of fluid-melt
partitioning based upon natural samples. All of these examples point
to the necessity of considering the potential influence of halogen
diffusion on magmatic processes and their records that we examine
(e.g., melt inclusions, volcanic gas compositions, crystal compositions).
4. Effect of added halogens on the diffusion of major elements
Only a few, limited studies have investigated the affects of halogens
on the diffusion of major elements in magmatic melts (Fig. 5). Baker
(1993a,b) investigated Si–Al interdiffusion in two sets of peralkaline
melts, each with approximately 1 wt.% of either fluorine or chlorine.
Even though these halogen concentrations are near the maximum
values found in nature, the influence of the halogens on Si–Al
interdiffusion was minor and no decoupling of major-element
diffusion was observed. The measured diffusion coefficients were
often within uncertainty of each other for the volatile-free melt, the Clbearing melt, and the F-bearing melt (Fig. 5), but Baker (1993a,b) found
that addition of F decreased the activation energy for diffusion, while Cl
increased it. Baker (1993a,b) discussed the possibility that Cl caused
polymerization of the melt because of its coordination with an alkali
Fig. 4. Comparison of diffusion coefficients in hydrous silicic (almost exclusively
rhyolitic) melts. The plotted diffusivities of F and Cl in the Na-phonolite are similar to
those in K-phonolite (Balcone-Boissard et al., 2009-this issue). The line for Cl diffusion
is a combination of the high temperature results of Watson (1991) and the low
temperature results of Bai and Koster van Groos (1994). Water diffusion, for 2 and 6 wt.
% total, is calculated from Zhang and Behrens (2000). CO2 diffusion is calculated from
Watson (1991) in a melt with 8 wt.% dissolved water. S diffusion is a combination of the
high-temperature results of Watson et al. (1993), see also Watson (1994), and the lowtemperature results of Baker and Rutherford (1996). The apparent activation energies
for S and Cl diffusion may be an artifact of combining data from two different studies.
Lines for Si–Al interdiffusion in silicic melts containing either 3 or 6 wt.% H2O are
extrapolated using the Arrhenius relationships determined by Baker (1991).
D.R. Baker, H. Balcone-Boissard / Chemical Geology 263 (2009) 82–88
87
ments of halogen diffusion in different melt compositions (e.g.,
granitic melts) are clearly needed and hopefully will soon become
available. These experimental measurements will then form the basis
for empirical models that not only predict the diffusion of halogens,
but also the effect of halogens on the diffusion of major elements.
Acknowledgments
The authors thank the reviewers of this manuscript who
contributed to the clarity of this presentation. This work was
supported by an NSERC Discovery grant to D.R.B.. This is GEOTOP
contribution #2008-0021 and IPGP contribution #2365.
References
Fig. 5. The effect of halogen addition on Si–Al interdiffusion. Sources for the Arrhenius
lines are discussed in the text.
cation and conversion of an Al-bearing Q3 species into a Q4 one,
resulting in higher activation energies for Si–Al interdiffusion. Baker
and Bossányi (1994) measured the influence of the combined addition
of water and 1 wt.% fluorine on Si–Al interdiffusion in the same
peralkaline melts studied by Baker (1993a,b). They found that addition
of 3 wt.% water to a melt with 1 wt.% F greatly increased diffusion
coefficients; these values were even greater than Si–Al interdiffusion in
a metaluminous melt with 3 wt.% H2O. However the addition of 3 more
wt.% water, to create a total of 6 wt.%, to the fluoridated melt had little
affect on diffusion, and in this case the Si–Al diffusivities were less than
in the metaluminous melt with 6 wt.% H2O (Fig. 5). Baker and Bossányi
(1994) investigated two methods for the estimation of Si–Al
interdiffusion in hydrous and fluoridated silicic melts. One method
involved empirical correlations between both the pre-exponential
factor and the activation energy with the ratio of F + OH in the melt to
the oxygens in the anhydrous melt; although this method was
successful, the uncertainties in the calculated diffusion coefficients
were large. Baker and Bossányi (1994) also demonstrated that the
Eyring equation combined with methods of predicting melt viscosity
available at that time allowed them to predict diffusivities to within a
factor of three of the measured ones. Application of new methods for
the prediction of melt viscosities (Giordano et al., 2008) should be
applied to these old diffusion results to see if the accuracy of the Eyring
model is improved.
Baker et al. (2002) investigated the effect of 1.2 wt.% fluorine
addition or 0.35 wt.% chlorine addition on Zr diffusion away from
dissolving zircons in hydrous, metaluminous granitic melts. They
measured only small differences in the diffusion coefficients and in
the zircon saturation values between hydrous melts and halogenbearing, hydrous melts. Although the differences in the diffusion
coefficients were small, Baker et al. (2002) found that the activation
energies for Zr diffusion in melts with 4.5 wt.% H2O were lower than in
the same hydrous melt with the halogens added, resulting in lower Zr
diffusion coefficients at magmatic temperatures.
5. Conclusions
Our knowledge of halogen diffusion in silicate melts of geological
interest is still surprisingly small, even though we have long
recognized the importance of halogens in magmatic processes,
metallogenic processes, and, more recently, their impact on atmospheric chemistry. This review of halogen diffusion has summarized
the studies to date and discussed the potential role of halogen
diffusion during the evolution of magmatic systems. Further measurements on the heavier halogens, Br and I, as well as more measure-
Aiuppa, A., 2009. Degassing of halogens from basaltic volcanism: insights from volcanic
gas observations. Chem. Geol. 263, 99–109 (this issue).
Aiuppa, A., Webster, J.D., Baker, D.R., 2009. Halogens in volcanic systems. Chem. Geol.
263, 1–18 (this issue).
Alletti, M., Baker, D.R., Freda, C., 2007. Halogen diffusion in a basaltic melt. Geochim.
Cosmochim. Acta 71, 3570–3580.
Bai, T.B., Koster van Groos, A., 1994. Diffusion of chlorine in granitic melts. Geochim.
Cosmochim. Acta 58, 113–123.
Baker, D.R., 1991. Interdiffusion of hydrous dacitic and rhyolitic melts and the efficacy of
rhyolite contamination of dacitic enclaves. Contrib. Mineral. Petrol. 106, 462–473.
Baker, D.R., 1992. Interdiffusion of geologic melts: calculations using transition state
theory. Chem. Geol. 98, 11–21.
Baker, D.R., 1993a. The effect of F and Cl on the interdiffusion of peralkaline intermediate
and silicic melts. Am. Mineral. 78, 316–324.
Baker, D.R.,1993b. Measurement of diffusion at high temperatures and pressures in silicate
systems. In: Luth, R.W. (Ed.), Mineral. Assoc. Canada Short Course Handbook on
Experiments at High Pressure and applications to the Earth's Mantle, pp. 305–355.
Baker, D.R., 2008. The fidelity of melt inclusions as records of melt composition. Contrib.
Mineral. Petrol 57, 377–395.
Baker, D.R., Bossányi, H., 1994. The combined effect of F and H2O on interdiffusion
between peralkaline dacitic and rhyolitic melts. Contrib. Mineral. Petrol. 117,
203–214.
Baker, L.L., Rutherford, M.J., 1996. Sulfur diffusion in rhyolite melts. Contrib. Mineral.
Petrol. 123, 335–344.
Baker, D.R., Conte, A.M., Freda, C., Ottolini, L., 2002. The effect of halognes on Zr diffusion
and zircon dissolution in hydrous metaluminous granitic melts. Contrib. Mineral.
Petrol. 142, 666–678.
Baker, D.R., Freda, C., Brooker, R.A., Scarlato, P., 2005. Volatile diffusion in silicate melts
and its effects on melt inclusions. Annal. Geophys. 48, 699–717.
Balcone-Boissard, H., Baker, D.R., Villemant, B., Boudon, G., 2009. F and Cl diffusion in
phonolitic melts: influence of the Na/K ratio. Chem. Geol. 263, 89–98 (this issue).
Blower, J.D., Mader, H.M., Wilson, S.D.R., 2001. Coupling of viscous and diffusive controls
on bubble growth during explosive volcanic eruptions. Earth Planet. Sci. Lett. 193,
47–56.
Chakraborty, S., 1995. Diffusion in silicate melts. In: Stebbins, J.F., McMillan, P.F.,
Dingwell, D.B. (Eds.), Structure, Dynamics and Properties of Silicate Melts, Rev.
Mineral., vol. 32, pp. 411–503.
Dingwell, D.B., Scarfe, C.M., 1984. Chemical diffusion of fluorine in jadeite melt at high
pressure. Geochim. Cosmochim. Acta 48, 2517–2525.
Dingwell, D.B., Scarfe, C.M., 1985. Chemical diffusion of fluorine in melts in the system
Na2O–Al2O3–SiO2. Earth Planet. Sci. Lett. 73, 377–384.
Freda, C., Baker, D.R., Scarlato, P., 2005. Sulfur diffusion in basaltic melts. Geochim.
Cosmochim. Acta 69, 5061–5069.
Gabitov, R., Price, J.D., Watson, E.B., 2005. Diffusion of Ca and F in haplogranitic melt
from dissolving fluorite crystals at 900 °–1000 °C and 100 MPa. Geochem. Geophys.
Geosystems 6, Q03011. doi:10.1029/2004GC000832.
Gerlach, T.M., 2004. Volcanic sources of troposphere ozone-depleting trace gases.
Geochem. Geophys. Systems 5, Q09007.
Giordano, D., Russell, J.K., Dingwell, D.B., 2008. Viscosity of magmatic liquids. A model.
Earth Planet Sci. Lett 271, 123–134.
Kiczenski, T.J., Stebbins, J.F., 2006. The effect of fictive temperature on the structural
environment of fluorine in silicate and aluminosilicate glasses. J. Am. Ceram. Soc. 89
(1), 57–64.
Lensky, N.G., Navon, O., Lyakhovsky, V., 2004. Bubble growth during decompression of
magma: experimental and theoretical investigation. J. Volcan. Geotherm. Res. 129,
7–22.
Liu, Y., Nekvasil, H., 2002. Si–F bonding in aluminosilicate glasses: inferences from ab
initio NMR calculations. Am. Mineral. 87, 339–346.
Mysen, B.O., Cody, G.D., Smith, A., 2004. Solubility mechanisms of fluorine in peralkaline
and meta-aluminous silicate glasses and in melts to magmatic temperatures.
Geochim. Cosmochim. Acta 68 (12), 2745–2769.
Proussevitch, A.A., Sahagian, D.L., Anderson, A.T., 1993. Dynamics of diffusive bubble
growth in magmas: isothermal case. J. Geophys. Res. 98B, 22,283–22,307.
Proussevitch, A.A., Sahagian, D.L., 1998. Dynamics and energetics of bubble growth in
magmas: analytical formulation and numerical modeling. J. Geophys. Res. 103B,
18,223–18,251.
88
D.R. Baker, H. Balcone-Boissard / Chemical Geology 263 (2009) 82–88
Roedder, E., 1965. Liquid CO2 inclusions in olivine-bearing nodules and phenocrysts
from basalts. Am. Mineral. 50, 1746–1782.
Roedder, E., 1979. Origin and significance of magmatic inclusions. Bull. Mineral. 102,
487–510.
Sandland, T.O., D.L., Stebbins, J.F., Webster, J.D., 2004. Structure of Cl-containing silicate
and aluminosilicate glasses: a 35Cl MAS-NMR study. Geochim.Cosmochim. Acta 68
(24), 5059–5069.
Schaller, T., D.D.B., Keppler, H., Knöller, W., Merwin, L., Sebald, A., 1992. Fluorine in
silicate glasses: a multinuclear nuclear magnetic resonance study. Geochim.
Cosmochim. Acta 56 (2), 701–707.
Scriven, L.E., 1959. On the dynamics of phase growth. Chem. Eng. Sci. 10, 1–13.
Shaw, H.R., 1974. Diffusion of H2O in granitic liquids: Part I. Experimental data; Part II. Mass
transfer in magma chambers. In: Hofmann, W., Giletti, B.J., Yoder, H.S., Yund, R.A.
(Eds.), Geochemical Transport and Kinetics. Carnegie Institution of Washington,
Washington, pp. 139–170.
Shewmon, P., 1989. Diffusion in Solids. The Minerals, Metals & Materials Society,
Warrendale, p. 246.
Smith, V.G., Tiller, W.A., Rutter, J.W., 1955. A mathematical analysis of solute
redistribution during solidification. Can. J. Phys. 33, 723–744.
Sparks, R.S.J., Barclay, J., Jaupart, C., Mader, H.M., Phillips, J.C., 1994. Physical aspects
of magma degassing I. Experimental and theoretical constrains on vesiculation.
In: Carroll, M.R., Holloway, J.R. (Eds.), Volatiles in Magmas, Rev. Mineral., vol. 30,
pp. 413–445.
Symonds, R.B., Rose, W.I., Bluth, G.J.S., Gerlach, T.M., 1994. Volcanic-gas studies:
methods, results, and applications. In: Carroll, M.R., Holloway, J.R. (Eds.), Volatiles
in Magmas. Rev. Mineral., vol. 30, pp. 1–66.
von Glasow, R., Bobrowski, N., Kern, C., 2009. The effects of volcanic eruptions on
atmospheric chemistry. Chem. Geol. 263, 131–142 (this issue).
Watson, E.B., 1982. Basalt contamination by continental crusts: some experiments and
models. Contrib. Mineral. Petrol. 80, 73–87.
Watson, E.B., 1991. Diffusion of dissolved CO2 and Cl in hydrous silicic to intermediate
magmas. Geochim. Cosmochim. Acta 55, 1897–1902.
Watson, E.B., 1994. Diffusion in volatile-bearing magmas. In: Carroll, M.R., Holloway, J.R.
(Eds.), Volatiles in Magmas. Rev. Mineral., vol. 30, pp. 371–411.
Watson, E.B., Bender, J.F., 1980. Diffusion of cesium, samarium, strontium, and chlorine
in molten silicate at high temperatures and pressures. Geol. Society America
Abstracts with Program, vol. 12, p. 545.
Watson, E.B., Sneeringer, M.A., Ross, A., 1982. Diffusion of dissolved carbonate in
magmas: experimental results and applications. Earth Planet. Sci. Lett. 61, 346–358.
Watson, E.B., Wark, D.A., Delano, J.W., 1993. Initial report on sulfur diffusion in magmas.
EOS Trans.Amer. Geophys. Union 74, 620.
Zeng, Q., Stebbins, J.F., 2000. Fluoride sites in aluminosilicate glasses: high-resolution
19
F NMR results. Am. Mineral. 85, 863–867.
Zhang, Y., Stolper, E.M., 1991. Water diffusion in basaltic melts. Nature 351, 306–309.
Zhang, Y., Stolper, E.M., Wasserburg, G.J., 1991. Diffusion of water in rhyolitic glasses.
Geochim. Cosmochim. Acta 55, 441–456.
Zhang, Y., Behrens, H., 2000. H2O diffusion in rhyolitic melts and glasses. Chem. Geol.
169, 243–262.