Shimojuku et al. Earth, Planets and Space 2014, 66:2
http://www.earth-planets-space.com/content/66/1/2

LETTER

Open AccessElectrical conductivity of brine-bearing quartzite
at 1 GPa: implications for fluid content andsalinity of the crust
Akira Shimojuku1,2*, Takashi Yoshino1 and Daisuke Yamazaki1

Abstract
The electrical conductivity of brine-bearing quartzite with fluid fractions of 0.19 to 0.30 and salinity of 3 to 17 wt.%
was measured at 800 to 1,100 K and 1 GPa. The conductivity of the brine-bearing quartzite increases with salinity
and fluid fraction, but is almost independent of temperature. Our results suggest that regions of high conductivity(10−3 to 10−2 S/m) in the crust could be explained by the presence of quartzite with fluids of salinity similar to thatof seawater. To account for those regions with the highest conductivity of 10−1 S/m, quartzite with fluid of highsalinity (>10 wt.%) is required.
Keywords: Electrical conductivity; Fluid; Brine; High-conductivity anomaly

Correspondence/Findings
Introduction

Recent magnetotelluric surveys have revealed that there
are regions with anomalously high electrical conductivity
in the middle to the lower crust (e.g., Ogawa et al. 2001;
Wannamaker et al. 2009). The conductivity of these regions ranges from 10−3 to 10−1 S/m, which is at least 1 to
2 orders of magnitude higher than the conductivity of dry
crustal rocks (e.g., Olhoeft 1981; Marquis and Hyndman
1992). Several explanations for this high conductivity havebeen suggested, which include the presence of interconnected graphite (e.g., Duba and Shankland 1982; Frostet al. 1989), aqueous fluids (e.g., Hyndman and Hyndman
1968; Hyndman and Shearer 1989; Glover and Vine 1994),
partial melts (e.g., Hermance 1979; Wannamaker 1986;
Gaillard et al. 2004), sulfides (e.g., Ducea and Park 2000),
and magnetite (Stesky and Brace 1973).
Of these, the presence of aqueous fluids has been
considered the most plausible candidate (e.g., Marquisand Hyndman 1992; Glover and Vine 1994). However,Shimojuku et al. (2012) demonstrated that the electricalconductivity of fluid-bearing quartzite under deep crustal* Correspondence: simojuku@eri.u-tokyo.ac.jp1
Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori
682-0193, Japan
2
Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku,
Tokyo 113-0032, Japan

conditions was definitely lower than that of the highconductivity anomalies, even when the quartzite contained a large fluid fraction (0.32). This suggests that otherdissolved species such as Na+, Cl−, and Al3+ are requiredin the fluids to account for the high-conductivity anomalies. NaCl is commonly found in fluid inclusions in rocks
from the lower crust (e.g., Banks et al. 1991). In addition,
the high-conductivity anomalies observed in the crust
above the subducting plate are closely related to active
faults (e.g., Wannamaker et al. 2009). Thus, it is important
to measure the electrical conductivity of brine-bearing
rocks. Glover and Vine (1994) reported on the electricalconductivity of brine-bearing rocks at 0.2 GPa. However,their measurement is limited to a salinity of approximately
3 wt.%. In addition, the exact fluid fraction of theirrecovered samples was unclear. Thus, it is impossible
to constrain the plausible fluid fraction and salinity inthe crust from their data set.
In the present study, we measure the electricalconductivity of brine-bearing quartzite as a functionof salinity and fluid fraction at 1 GPa and 800 to 1,100 K
and discuss the salinity and fluid fractions needed torealize the crustal high-conductivity anomalies. Althoughnatural lower crustal rocks are composed of granulite,
gabbro, and amphibolite, we used quartzite as a simplified
analogue for the crustal rocks in order to focus on the© 2014 Shimojuku et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly credited.

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effects of the salinity and fluid fraction on the bulk
conductivity.
Experimental proceduresHigh-pressure and high-temperature experiments were performed using the DIA-type apparatus installed at the Institute for Study of the Earth’s Interior, Okayama University.
The cell assembly was identical to that used by Shimojuku
et al. (2012). It should be noted that we used a cylindricalsleeve of single crystal quartz as a capsule, which effectively
prevents water loss from the sample during the conductivity measurements (Shimojuku et al. 2012).
The starting materials were prepared by mixing thereagents SiO2, SiO2·nH2O, and NaCl powders. By changing the mixing ratio, we produced four types of starting
material. Because the n value of the SiO2·nH2O powder
can differ on a daily basis, the exact water contents of thestarting materials are unclear. Thus, the fluid fraction(water content) of the samples was calculated from therecovered samples by counting the areal fraction of poresin the secondary electron image (SEI) using ImageJ software. First, the area of fluid and quartz in the SEI was distinguished by setting the function ‘Threshold’ in ImageJ.
In this function, the gray-scale image was converted into abinary image with only two values: 0 for white (quartz),and 255 for black (fluid). Then, the volume fraction of thefluid was calculated based on the ratio between the total
and fluid areas using the function ‘Measure’ in ImageJ.
The number of analyzed SEIs was at least 15 for eachsample. The average fluid fraction was adopted, and theuncertainty was 1σ from the averaged value.
The SEIs were obtained with a field-emission scanningelectron microscope (FE-SEM, JEOL JSM-7001 F, Tokyo,
Japan) operated at an accelerating voltage of 15 kV
and a current of 12 nA. Salinity was estimated fromthe weight ratio of the amount of fluid determined from(a) Fluid fraction:0.19; Salinity: 3 wt.% (Run A2354)
SiO2

the porosity in the SEI and the amount of NaCl powder
added to the starting material, based on the assumptionthat fluid did not escape during the conductivity measurements. This is deemed reasonable because the capsule ofthe single crystal quartz effectively prevents water loss
from the sample (Shimojuku et al. 2012). The uncertainties of salinity are estimated by propagating the uncertainties of the amount of fluid.
The electrical resistance of the sample was measured
by an impedance gain-phase analyzer combined with a
Solartron 1296 interface with amplitude of 1 V and frequency range of 10−1 to 106 Hz. The electrical conductivity σ (S/m) was derived directly from the equation σ = l/SR, where l is the length of the sample (m), S is a crosssectional area of the sample (m2), and R is the resistanceof the sample (Ω).
First, the sample was pressurized to 1 GPa at room
temperature, and the electrical conductivity was measured at elevated temperatures of 300, 600, 900, and1,100 K (the heating path). The sample was then held at
1,100 K for at least 12 h to reach textural equilibrium.
Then, conductivity was measured at intervals of 25 K as
the temperature fell to 800 K (the cooling path). The
sample was rapidly reheated to 1,100 K to confirm thereproducibility of the conductivity at 1,100 K. Finally,
the sample was quenched from 1,100 K by shutting off
the power supply to the heater. The recovered samples
were quickly covered by epoxy and placed into a heater
under vacuum to solidify the epoxy. Subsequently, thesample was prepared as a polished section for FE-SEM
analysis.
Results

Typical SEIs of the recovered samples with the nominal
fluid fractions of 0.19 (run A2354) and 0.29 (run A2343)
are shown in Figure 1a,b. The fluid portions are(b) Fluid fraction: 0.29; Salinity: 5 wt.% (Run A2343)
SiO2

SiO2

fluid
SiO2
fluid

SiO2

SiO2
fluid

SiO2

fluid
SiO2

Figure 1 Representative SEIs of recovered samples. (a) Run A2354 with fluid fraction of 0.19 and salinity of 3 wt.%. (b) Run A2343 withfluid fraction of 0.29 and salinity of 5 wt.%. The dark-gray and light-gray regions correspond to the fluid portions (filled with epoxy) and quartz,
respectively.

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observed as pores filled with epoxy resin, which are located along grain boundaries or at grain corners. Somepores at the grain corners are present as large pools.
Fluid fractions and salinity ranged from 0.19 to 0.30 and
3 to 17 wt.%, respectively (Table 1).
Typical Cole-Cole plots for the samples with fluid
fractions of 0.19 (run A2354) and 0.30 (run A2341) are
shown in Figure 2a,b. Also shown are the equivalentcircuits (R-C parallel circuit) expected from the obtained
impedance spectra. All the impedance spectra arecomposed of a semicircle and an additional tail at low
frequencies (Figure 2). Roberts and Tyburczy (1999)
deduced that the electrical circuit was different in termsof the connectivity of partial melts in the rock. In thecase of the presence of an interconnected melt pathway,
the circuit was composed of the response of the graininterior and basaltic melts in a parallel circuit. On theother hand, in the absence of an interconnected melt
pathway, the circuit was composed of the response of
the grain interior and the basaltic melt in a series circuit.
Thus, it was expected that the impedance spectra of asingle semicircle would be obtained when the fluid
phase forms an interconnecting network, as observedin Shimojuku et al. (2012), because the resistivity of thegrain interior (Yoshino and Noritake 2011) is at least 3 orders of magnitude higher than that of the interconnecting
fluid phase at 800 to 1,100 K. Thus, the presence of thesingle semicircle would indicate that the fluid establishesinterconnection and that the semicircular shape is controlled by ionic conduction in the interconnected fluids.
The low-frequency tail is probably due to electrodesample interface effects (e.g., ten Grotenhuis et al. 2005).
The slope with 45° can be interpreted as Warburg impedance, which occurs via diffusion of charge species near theelectrode interface.
Caricchi et al. (2011) obtained impedance spectra
comprising two semicircles in melt-bearing olivine aggregates. They discussed that high-frequency and lowfrequency arcs were responsible for melt pockets and interconnected melt in long-range scales, respectively. In
the present study, we could not obtain the two semicircles as observed in Caricchi et al. (2011). This suggeststhat the volume fraction of isolated fluid pockets is negligibly small.
Table 1 Fluid fraction and salinity of the samples
Salinitya (wt.%)

Run

Fluid fraction

A2354

0.19 (0.05)

3 (1)

A2359

0.19 (0.03)

10 (2)

A2343

0.29 (0.03)

5 (1)

A2341

0.30 (0.04)

17 (2)

Numbers in parentheses represent the uncertainties. aThe values with respect
to the water content.

The resistivity of the brine-bearing quartzite was determined by fitting the semicircular part of the impedance
data, excluding the low-frequency tail. The size and shapeof the semicircle were very similar at 900 and 1,100 K(Figure 2a,b). This indicates that the conductivity ofbrine-bearing quartzite is not strongly dependent on
temperature. Uncertainties that arose from the fitting
process led to an uncertainty of ±0.03 log units inthe electrical conductivity values.
The dihedral angle of the SiO2-H2O-NaCl system
was reported to be less than 60° at 1 GPa and 1,273 K(Watson and Brenan 1987). This small value of dihedral
angle is further evidence that the fluid forms an interconnected network along grain boundaries in the quartz
aggregates.
Figure 3 shows variations in the electrical conductivityof brine-bearing quartzite (runs A2343 and A2354)
during the heating and cooling paths as a function ofreciprocal temperature. The conductivity data in theheating path are scattered (Figure 3). This is because theconductivity was affected by absorbed water in the specimen assembly (e.g., Yoshino 2010). Therefore, we did
not use the conductivity data obtained in the heating
path. During the heating path, SiO2·nH2O would dehydrate and then NaCl would dissolve into the fluid. Afterreaching 1,100 K, the sample was held for at least 12 h toreach textural equilibrium. The conductivity decreased by
0.02 to 0.7 orders of magnitude and reached a constantvalue during this period (Figure 3). This indicates that theabsorbed water in the specimen assembly was completely
purged and that the samples reached textural equilibrium.
Then, the conductivity was measured at intervals of 25 K
as the temperature fell to 800 K. The sample was rapidly
reheated to 1,100 K to confirm the reproducibility of
the conductivity at 1,100 K (Figure 3). The similar
conductivity values indicate that both the fluid fraction and salinity maintain constant values. The conductivity values obtained from the cooling path were adopted
for the analysis.
Figure 4 shows the temperature effect on electricalconductivity of brine-bearing quartzite obtained fromthe cooling path. The conductivity is nearly constant at
temperatures of 800 to 1,100 K, and it increases withboth the fluid fraction and salinity. The conductivity of
the NaCl-free fluid-bearing quartzite (Shimojuku et al.
2012) is 1 and 2 orders of magnitude lower than that of
the brine-bearing quartzite with salinities of 5 and 17 wt.%,
respectively, in similar fluid fractions (0.29 to 0.32). This
indicates that the presence of NaCl in fluids strongly
increases the conductivity. The α-quartz to β-quartz
transition occurs at about 1,073 K at 1 GPa (e.g., Ostrovsky1967). Because we did not observe a change or jump of
conductivity in this condition, the transition might notaffect the fluid fraction and salinity.

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Page 4 of 9(a) Fluid fraction:0.19; Salinity: 3 wt.% (Run A2354)(b) Fluid fraction: 0.30, Salinity: 17 wt.% (Run A2341)

30000

500

Electrode reaction

Electrode reaction
Fluid

Fluid

ZW

25000

20000

Grain interior

ZW

400

Rct

Rct
Grain interior

Cdl

Cdl

300

15000

1100 K
900 K

10000

1100 K
900 K

200

100
5000

0

0
0

5000

10000

15000

20000

25000

30000

0

Z' (Ohm)

100

200

300

400

500

Z' (Ohm)

Figure 2 Representative impedance spectra. (a) Run A2354 with fluid fraction of 0.19 and salinity of 3 wt.%. (b) Run A2341 with fluid fractionof 0.30 and salinity of 17 wt.%. The straight segments indicate the lines fitted to the data points. The insets show the expected circuit. The
low-frequency tail is possibly approximated by Randles circuit (Randles 1947), which includes a double-layer capacitor (Cdl), charge transfer reaction
resistance (Rct), and Warburg impedance (Zw).

Run A2343

Run A2354

Figure 3 Variation of the electrical conductivity during the heating path (crosses) and cooling path (solid circles). After completing themeasurements of the cooling path, the sample was reheated to 1,100 K. Then, we measured the conductivity again (open circles) to confirm
the reproducibility.

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Temperature (K)
1100

1000

900

800

brine-bearing quartzite (This study)

1.0

NaCl-free fluid-bearing quartzite (Shimojuku et al. 2012)log conductivity(S/m)=0.30 (NaCl:17 wt.%)

0.0= 0.19 (NaCl:10 wt.%)=0.29 (NaCl:5 wt.%)-1.0=0.19 (NaCl:3 wt.%)

=0.32

-2.0
=0.048=0.022-3.0=0.00043=0.0025

-4.0
9.0

10.0
4

11.0

12.0

13.0

-1

10 /Temperature (K )Figure 4 Arrhenius plots of electrical conductivity for brine-bearing quartzite at 1 GPa (solid circles). The solid lines show the smoothed
fit to the data points. The attached numbers indicate the fluid fractions (ϕ) and salinity. The dashed lines indicate the electrical conductivity of
NaCl-free fluid-bearing quartzite measured by Shimojuku et al. (2012) with fluid fractions shown in italics.

Discussion

The main charge carriers for the electrical conduction in
the brine-bearing quartzite are Na+, Cl−, and soluble
ionic species derived from SiO2. As SiO2 dissolves into
fluid mainly as neutral hydroxide of Si(OH)4 (e.g.,
Wasserburg 1958; Wood 1958), the majority of thecharge carriers are Na+ and Cl− ions. The conductivity of the NaCl-free fluid-bearing quartzite increases
with temperature, whereas that of the brine-bearing
quartzite is nearly constant with temperature (Figure 4).
This indicates that the concentrations of Na+ and Cl− ionsin the fluid are almost constant over the measured
temperature ranges. This is reasonable because NaCl dissolves into the aqueous fluid irrespective of temperature.
Nesbitt (1993) reported on the variation of the resistivities of KCl solution with salinities of 3.6, 13.5, and24.7 wt.% at 0.1 to 0.3 GPa and 273 to 773 K. The results
indicated that resistivity was almost constant from 473 to
773 K at salinities of 13.5 and 24.7 wt.%. The temperature
insensitivity was explained by the constancy of the mobility of ions (Nesbitt 1993). Although temperature insensitivity was not clearly observed in the salinity of 3.6 wt.% in
Nesbitt (1993), our data showed that resistivity was almost
constant, even in the low salinity of 3 wt.%. Because ourexperimental pressure and temperature conditions werehigher than those in Nesbitt (1993), the mobility of
Na and Cl ions in aqueous fluid might be insensitive
to temperature under high-pressure and high-temperature
conditions.
The electrical conductivity of rocks generally increases
with the increasing volume fraction of melt and/or fluid
phases, which have a constant number of charge carriers(e.g., ten Grotenhuis et al. 2005; Yoshino et al. 2010;
Shimojuku et al. 2012). In order to evaluate the effect of
fluid fraction on the bulk electrical conductivity, various
geometrical models have been proposed, such as theHashin-Shtrikman upper and lower bound models(Hashin and Shtrikman 1962), the cube model (Waff 1974),
and the tube model (Schmeling 1986).
The Hashin-Shtrikman upper bound (HS+) and cube
models are applicable to cases in which fluid is distributed along the grain boundaries and triple junctions.
The difference between the two models relates towhether the shape of the resistive phase is spherical orsquare. The two models usually yield similar values ofbulk conductivity (e.g., ten Grotenhuis et al. 2005). The
tube model describes cases in which fluid is not distributed along the grain boundaries, but in a network along

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the triple junctions. The Hashin-Shtrikman lower boundmodel addresses cases in which fluid is present as
isolated pockets and does not form an interconnected
network.
The microstructure and shape of the impedance spectra
demonstrated that the fluid phase was interconnected.
Thus, the interconnected HS + and cube models are themost appropriate for reproducing the texture of the brinebearing quartzite. In the HS + model, the bulk conductivity of fluid-bearing rocks (σb) is expressed as (Hashin and
Shtrikman 1962):
σb ¼ σf þð1−ϕ Þ
;1=ðσ s −σ f Þ þ ϕ=3σ fð1Þwhere ϕ is the fluid fraction, and σs and σf are the conductivity of the solid phase and fluid phase, respectively.
In the cube model, σb is expressed as follows:hi2
σ b ¼ 1−ð1−ϕ Þ3 σ f :ð2ÞArchie’s law is also known to address the relationshipbetween σb and ϕ (Archie 1942):
σ b ¼ Cϕ m σ f ;ð3Þwhere C and m are constants. This law is not a geometrical model, but an empirically derived equation (e.g.,
Watanabe and Kurita 1993; ten Grotenhuis et al. 2005).
We calculated the σf values from Equations 1 to 3
based on the σb and ϕ determined in the present study
at 800 K. The value reported by Yoshino and Noritake(2011) (approximately 3.0 × 10−7 S/m) was used as the σsvalue in Equation 1. The m value in Equation 3 wasassumed as 1. The calculated σf values at salinities of 3,5, 10, and 17 wt.% from the three models are summarized in Table 2. Using σf, the relationships between σband ϕ at the various salinities of 3, 5, 10, and 17 wt.% inthe three models were calculated (Figure 5). Figure 5
shows that the three models yield similar values of bulk
conductivity. Thus, the value of the fluid fraction needed
to realize the high-conductivity anomalies would be
similar in the three models. Although the uncertainty inthe bulk conductivity arising from the extrapolationfrom the high fluid fraction (0.19 to 0.30) to the lower
fluid fraction would be quite large, we will discuss thesalinity and fluid fractions needed to realize the crustal
high-conductivity anomalies.
In the case of salinity similar to seawater (3 wt.%),those regions with conductivities of 10−3 to 10−2 S/mcan be realized by brine-bearing quartzite with a fluid
fraction of 0.002 to 0.02. To account for those regionswith the highest conductivity of 10−1 S/m, however, an
unrealistically large fluid fraction exceeding 0.2 is required.
Thus, an electrical conductivity of 10−1 S/m is unlikely to
be explained by the presence of seawater-like fluids. Whensalinity is approximately three times higher than that ofseawater (10 wt.%), a fluid fraction of 0.04 is required toachieve the conductivity of 10−1 S/m. Thus, those regionswith 10−1 S/m could be the result of very high salinity
fluids. High salinity has been reported in area where evaporite is present (e.g., Yardley and Graham 2002). The
Arima-type thermal water originating from the slabderived fluid is characterized by high salinity (e.g., Masuda
et al. 1985). If the salinity is 17 wt.%, those regions with
conductivity of 10−1 S/m could be explained by evensmaller fluid fractions of <0.01. Salinities much higher
than that of seawater could be achieved by magmatic
activity through the formation of a hydrous phase duringcooling (e.g., Roedder 1992; De Vivo et al. 1991). Thus,
brine-bearing quartzite might account for a large part of
the high-conductivity anomalies in regions where fluid has
high salinity.
As shown in Figure 4, the conductivity of brinebearing quartzite is almost independent of temperature,indicating that the conductivity values are sensitive todifferences in the salinity and/or fluid fraction rather thanchanges in temperature. Therefore, although Figure 5
shows the relationship between the bulk conductivity andfluid fraction at 800 K, the constraints of salinity and fluid
fraction mentioned above could be applicable at evenlower temperatures.
The σf values estimated from the present results appear
to be lower than for both NaCl (Quist and Marshall 1968)
and KCl solutions (Nesbitt 1993) with similar salinity(Figure 5). For instance, the σf with salinity of 3 wt.% isapproximately 1.5 orders of magnitude lower than the
conductivity of a KCl solution with salinity of 3.6 wt.% at
0.3 GPa reported by Nesbitt (1993) (Figure 5). As the conductivities of the KCl and NaCl solutions rise with pressure (Quist and Marshall 1968), the difference becomeslarger at the present experimental pressure of 1 GPa.

Table 2 σf with different salinities derived from HS+, cube models, and Archie’s law
Geometrical model

Salinity of 3 wt.%

Salinity of 5 wt.%

Salinity of 10 wt.%

Salinity of 17 wt.%

HS+ model

0.8 (0.2)

1.4 (0.1)

3.8 (0.6)

20 (3)

Cube model

0.8 (0.2)

1.4 (0.2)

4.0 (0.7)

21 (3)

Archie’s lawa

0.5 (0.1)

1.0 (0.1)

2.7 (0.4)

15 (2)

Numbers in parentheses represent the uncertainties calculated based on propagation of uncertainties of the fluid fraction and the conductivities (σb, σs). σf,
conductivity of fluid. aThe m value of Archie’s law was assumed to be 1.

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Figure 5 Bulk conductivity versus fluid fraction at various salinities at 800 K and 1 GPa. The variables were calculated from the HS + model(dotted lines), cube model (solid lines), and Archie’s law (dashed-dotted lines). The solid circles show experimental data points obtained in thisstudy. The open circle and open squares show the conductivities of NaCl with salinity of 0.3 wt.% at 0.4 GPa and 800 K (Quist and Marshall 1968)
and KCl with salinities of 3.6, 13.5, and 24.7 wt.% at 0.3 GPa and 773 K (Nesbitt 1993), respectively. The gray region indicates the range of thecrustal high-conductivity anomalies (10−3 to 10−1 S/m).

One possible reason for this discrepancy is that a certain amount of Na+ and/or Cl− could be adsorbed on
the surface of the quartz grains, resulting in a decreasein the numbers of charge carriers within the fluid. The
adsorption of ionic charge on the quartz surface has
been commonly reported in quartz-fluid systems (e.g., Li
and De Bruyn 1966). If this effect were large in brinebearing rocks, performing conductivity measurementson the rock and fluid system would be crucial for evaluating the effects of the fluid fraction on the bulk electricalconductivity. Therefore, using a procedure whereby thebulk electrical conductivity is derived from the electricalconductivities of the rock and solution end-members (e.g.,
Hyndman and Shearer 1989) could be problematic.
Furthermore, the geometrical distribution of fluid in
our samples might not be fully consistent with the HS +and cube models. As shown in Figure 1, some parts of thefluid are present at the grain corners as large pools. This
heterogeneous fluid distribution might reduce the effective
pathway for the charge carriers (Yoshino et al. 2006). As aresult, the σf values determined in the present study could
be lower than might be expected from the HS + and cube

models. Based on Figure 5, the bulk conductivities derived
from the HS + and cube models are consistent with thosefrom Archie’s law with m ≈ 1 in Equation 3. Because thevalue of m is commonly around 2 in natural porous rocks(Archie 1942) and because a larger m value leads to alarger σf value, we calculated the σf values from Archie’s
law with m = 2 to derive the possible range of reduction ofthe σf values. The calculation showed that the σf values
derived from Archie’s law with m = 2 were about half anorder of magnitude larger than those derived from the
HS + and cube models. However, the σf values are still
lower than the NaCl (Quist and Marshall 1968) and KClsolutions (Nesbitt 1993), even when considering the halforder-of-magnitude difference. Thus, the effect of the
precipitation or absorption of Na+ and/or Cl− on the mineral surfaces could be a more plausible reason to accountfor the discrepancies between our σf values and the NaClor KCl solutions.
The origin of the conductivity anomalies beneath theHimalayan-Tibetan belt is a widely debated issue (e.g.,
Gaillard et al. 2004). Recently, Hashim et al. (2013) discussed whether saline fluids could explain the conductivity

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anomalies. Based on the salinity of the fluid inclusions in
the rocks from this region (25 wt.%) and conductivity dataof the KCl solution (Nesbitt 1993), they discussed that a
fluid content of approximately 0.8 vol.% was required to
explain the values of the conductivity anomalies beneaththe Himalayan-Tibetan belt (0.1 to 0.3 S/m). Because thisfluid content is essentially lower than the fluid content
constrained from the seismic data (at least 10 vol.%), they
concluded that the saline fluid hypothesis was not plausible and that the conductivity anomalies beneath theHimalayan-Tibetan belt were better explained by thepartially molten rocks. Because their discussion was basedon the conductivity data of the KCl solution (Nesbitt1993) and our σf values are lower than the KCl solution(Figure 5), we considered whether their conclusion was
valid, even if using our σf values. Although the highest
salinity of our data is restricted to 17 wt.%, the required
fluid content to explain the values of the conductivity
anomalies appears to be approximately 2 vol.%. This fluid
content is still lower than the fluid content constrained
from the seismic data (at least 10 vol.%). Thus, the saline
fluid hypothesis, based on brine-bearing quartzite, mightalso not be plausible in explaining the conductivity
anomalies beneath the Himalayan-Tibetan belt.
In the present study, we used quartzite as a solid
analogue phase of fluid-bearing crustal rock. However,
Glover and Vine (1994) demonstrated that the conductivity of brine-bearing rocks depends on the rock type.
Thus, the conductivity in fluid-bearing rocks could vary
according to the presence of other minerals. The lower
crust is believed to consist of granulite, gabbro, and amphibolite in which the main constituents are plagioclase,pyroxene, feldspar, and amphibole. Further experiments
to investigate the effects of other ionic species, derived
from these minerals, on bulk conductivity are needed toconstrain more precisely the effects of fluid fraction
and salinity.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
AS carried out the experimental and analytical works and drafted themanuscript. TY and DY organized the project and helped draft themanuscript. All authors read and approved the final manuscript.
Acknowledgements
We thank E. Ito for valuable discussions regarding the manuscript. We also
thank the two anonymous reviewers for their constructive comments thathave helped us improve the manuscript significantly. This work was
supported by a Grant-in-Aid for Scientific Research on Innovative Areas(Research in a Proposed Research Area), ‘Geofluids: Nature and Dynamics
of Fluids in Subduction Zones’ from the Japan Society for Promotion of
Science (No. 2109003). AS is supported by the JSPS Research Fellowships
for Young Scientists.
Received: 16 October 2013 Accepted: 26 February 2014
Published: 1 April 2014

Page 8 of 9

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Cite this article as: Shimojuku et al.: Electrical conductivity of brine-bearing
quartzite at 1 GPa: implications for fluid content and salinity of the crust.
Earth, Planets and Space 2014 66:2.

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