1-D electrical conductivity structure beneath the Philippine Sea

Physics of the Earth and Planetary Interiors 162 (2007) 2–12
1-D electrical conductivity structure beneath the Philippine Sea:
Results from an ocean bottom magnetotelluric survey
Nobukazu Seama a,d,e,∗ , Kiyoshi Baba b , Hisashi Utada b , Hiroaki Toh c ,
Noriko Tada d , Masahiro Ichiki e , Tetsuo Matsuno d
a
Research Center for Inland Seas, Kobe University, Kobe 657-8501, Japan
Earthquake Research Institute, The University of Tokyo, Tokyo 113-0032, Japan
c Department of Earth Sciences, University of Toyama, Toyama 930-8555, Japan
d Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan
e IFREE, Japan Marine Science and Technology Center, Kanagawa 237-0061, Japan
b
Received 6 August 2004; received in revised form 6 July 2006; accepted 2 February 2007
Abstract
Eight-months of observation using Ocean Bottom Electro-Magnetometers (OBEMs) have allowed us to estimate the regional
electrical conductivity structure beneath the Philippine Sea. Six OBEMs were deployed along a line crossing the Philippine Sea from
NW to SE and five of them recorded useful data. The raw time series data were cleaned up before we estimated the magnetotelluric
(MT) impedance tensor. Conductivity structure at five sites is estimated using 1-D Occam’s inversion to fit the determinant average
of each MT impedance tensor after a correction for the effect of topography. We examined effect from two dimensionalities on the
1-D conductivity structure and the robustness of solutions. The results of the 1-D conductivity structural model are strongly related
to tectonic setting and the crustal age beneath each site. The structure beneath the spreading axis of the Mariana Trough shows a
distinct low conductivity structure at depths of 50–150 km and it probably reflects the upwelling dynamics operating beneath the
spreading axis. These low values are comparable with that of olivine with low hydrogen content, implying that (1) the melting
process extracts water from minerals such as olivine, and (2) the melt beginning depth in the Mariana Trough is deeper than that of
the typical MORB source region. The off-axis conductivity profiles infer the existence of a high conductivity peak or a conductivity
gradient change at mid-depth. The depth level of the peak increases with crustal age, suggesting that the conductivity structure is
related to a geothermal structure and that these conductivity profiles are explained by the temperature gradient change, possibly
combined with the presence of partial melt. Our results suggest that further ocean bottom EM study has high potential to investigate
the temperature gradient change and amount of hydrogen (water) and melt in the upper mantle.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Philippine Sea; Upper mantle conductivity structure; Crustal age; Water; Mantle temperature; Marine magnetotelluric
∗
Corresponding author at: Research Center for Inland Seas, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan. Tel.: +81 78 803 5798;
fax: +81 78 803 5757.
E-mail addresses: [email protected] (N. Seama), [email protected] (K. Baba), [email protected] (H. Utada),
[email protected] (H. Toh), [email protected] (N. Tada), [email protected] (M. Ichiki), [email protected] (T. Matsuno).
0031-9201/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.pepi.2007.02.014
N. Seama et al. / Physics of the Earth and Planetary Interiors 162 (2007) 2–12
1. Introduction
The Philippine Sea is one of the major marginal seas in
the western Pacific and consists of three major basins: the
West Philippine Basin, the Shikoku-Parece Vela Basin,
and the Mariana Trough. Previous studies indicate that
these basins have been formed by successive episodic
opening from west to east (e.g., Karig, 1971; Hall et
al., 1995), although a more complicated history of the
West Philippine Basin was proposed by Okino et al.
(1999), using detailed geophysical mapping. The crustal
age under the Philippine Sea varies from the present at
active back-arc spreading axis in the Mariana Trough
to around 60 Ma (e.g., the DSDP site 445 in the West
Philippine Basin shows the basement age of 59.0 ± 3 Ma,
according to Ozima et al. (1980)). This variety of crustal
ages allows us to examine the upper mantle structure
beneath the ocean floor related to its crustal age.
The active back-arc spreading axis in the Mariana
Trough is one of the plausible targets to reveal the role
of water in the melting process through the upwelling
dynamics, because H2 O-enriched basalts exist in the
Mariana Trough (Stolper and Newman, 1994). The water
content affects the depth at which partial melting initiates, and the initial depth in the typical MORB source
region is estimated as ∼115 km using petrological constraints (Hirth and Kohlstedt, 1996), while the depth in
the source region of back-arc type MORB (higher water
content) is estimated as ∼250 km (Karato and Jung,
1998). The extraction of water from minerals such as
olivine through the melting process reduces the viscosity of olivine, strongly influencing the mantle dynamics
beneath the spreading axis (Karato, 1986; Hirth and
Kohlstedt, 1996). Thus, water is believed to play a key
role in the melting process. In other words, geophysical
constraints in higher water content regions are considered to be crucial in understanding the melting process
beneath the spreading axis related to water content.
The Philippine Sea is a suitable target to study seafloor
age dependence of various depth parameters that characterizes the dynamics of the back-arc spreading, because
of the variety of its crustal ages. The Philippine Sea
shows a linear relationship between the seafloor depth
and the square root of age (Park et al., 1990), which has
been found for other major oceanic floors (Davis and
Lister, 1974; Parsons and Sclater, 1977). The basement
depth of the Philippine Sea is, however, about 800 m
deeper than that of the other major ocean floors of the
same age (Park et al., 1990). The reason for this distinct
deeper depth is still one of the major scientific questions.
Moreover, the depth of the low velocity zone beneath
the Pacific oceanic floor, which was obtained by surface
3
wave techniques, shows a linear relationship against the
square root of age (Forsyth, 1977), and that of the high
electrical conductive zone also shows a similar relationship (Oldenburg, 1981; Oldenburg et al., 1984). On the
other hand, the depth of the Gutenberg discontinuity,
where there is a sharp decrease in seismic wave (particularly shear wave) velocities, does not change with age,
but its depth in back-arc regions is somewhat deeper than
those in the typical oceanic upper mantle (Karato and
Jung, 1998). Thus, the comparison of the relationship
between these depth parameter and age from the Philippine Sea back-arc region with that from major oceanic
basins will be useful to understand the dynamic process
of back-arc spreading.
The ocean bottom electromagnetic (EM) data using
the magnetotelluric (MT) method allows us to estimate
the electrical conductivity structure beneath the ocean
floor. The outcome of this method is dependent on temperature, the presence of melt, the influence of hydrogen
such as water, and the direction of preferred orientation in the case of crystal anisotropy. The relationship
between temperature and electrical conductivity of dry
olivine is well documented by Standard Olivine 2 (the
SO2 model; Constable et al., 1992), which has been
determined by various laboratory conductivity measurements. The presence of a melt fraction in a subsolidus
matrix strongly affects the electrical conductivity values
(Shankland and Waff, 1977; Sato et al., 1989). Furthermore, hydrogen content in olivine also increases the
electrical conductivity, which is estimated from its solubility and diffusivity through experimental data by using
the Nernst–Einstein relation (Karato, 1990; Lizarralde
et al., 1995). Moreover, the a-axis of olivine in an
anisotropic fabric possibly causes an additional enhancement in the conductivity (Lizarralde et al., 1995). Thus,
the ocean bottom EM study is considered a sensitive
tool to probe temperature, the presence of melt, hydrogen (water) content, and anisotropy in the upper mantle
beneath the ocean floor.
In this paper, we will first show our ocean bottom
EM observations from the Philippine Sea. Secondly, the
MT impedance tensor will be estimated at each observation site. The MT impedance tensor will be corrected
for the effect of topography and one-dimensionality
will be examined using the Rho+ algorithm (Parker
and Booker, 1996). Then, the one-dimensional (1-D)
conductivity structure beneath the Philippine Sea will
be estimated at each observation site. Effect from two
dimensionalities on the 1-D conductivity structure and
the robustness of the solutions will also be examined.
Finally, we will interpret the 1-D conductivity structure.
The conductivity structure near the spreading axis of
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N. Seama et al. / Physics of the Earth and Planetary Interiors 162 (2007) 2–12
the Mariana Trough will be used to discuss the melting process beneath the spreading axis related to water
content. The results from the off-axis locations will be
examined with respect to tectonic setting and crustal age
difference.
2. Ocean bottom EM observation and data
analysis
We conducted an ocean bottom EM array jointly with
a broadband seismic array as a part of the Ocean Hemisphere Project, Japan (Kawakatsu et al., 1998). One of
the aims of this experiment is to reveal the large-scale
upper mantle structure beneath the Philippine Sea. Our
EM observations were based on Ocean Bottom ElectroMagnetometers (OBEMs). Six OBEMs were deployed
along a line crossing the Philippine Sea from NW to
SE during the Dai-5 Kaikou-maru cruise in November, 1999; two OBEMs in the northern West Philippine
Basin, two in the northern Parece Vela Basin, and two
in the central Marina Trough (Fig. 1 and Table 1). Two
types of OBEMs were used and we call them type-A
and -B hereafter (Fig. 2). The type-A OBEM is made of
three glass-spheres with a geomagnetic field measurement resolution of 0.1 nT, which has also been used in
the MELT experiment at the southern East Pacific Rise
(Evans et al., 1999). The type-B OBEM has been developed during the Ocean Hemisphere Project and has two
glass-spheres with a geomagnetic field measurement resolution of 0.01 nT. For about 8 months, both types of
the OBEMs measured two horizontal components of the
electric field and three components of the geomagnetic
field at 1-min intervals. All the OBEMs were recovered during the Shintatu-maru cruise in July 2000 and
fairly high-quality data were obtained with the following
exceptions: (1) The OBEM3 data are not useful, because
the OBEM3 had been flooded when it was recovered and
Fig. 1. Locations of the observation sites (solid circles with their site
names).
recorded data only for 1 month. (2) The OBEM1 data
show bad electric field data for about 50 initial days. (3)
The OBEM2 data have a data gap of 12 days.
The raw time series data were cleaned up through
the following procedure. First, we removed the drifts
using a cubic function applied to the data with a time
window length of 3 days. The electric field data show
much larger drifts than geomagnetic field data, which
are mainly due to electrodes. Secondly, the unusual noise
spikes in the electric field were carefully edited; i.e., the
abnormal signals were picked out manually and then
interpolated by appropriate values. Although the electric field data are contaminated by some noise spikes, the
Fig. 2. Photographs of (a) type-A Ocean Bottom Electro-Magnetometer (OBEM) and (b) type-B OBEM.
N. Seama et al. / Physics of the Earth and Planetary Interiors 162 (2007) 2–12
5
Table 1
Memo of each observation site
Site
Location
Water depth (m)
Instrument
Note
OBEM1
OBEM2
OBEM3
OBEM4
OBEM5
OBEM6
16◦ 34.4 N, 144◦ 41.7 E
17◦ 47.2 N, 143◦ 24.5 E
20◦ 12.9 N, 140◦ 46.0 E
22◦ 33.6 N, 138◦ 07.3 E
25◦ 00.0 N, 135◦ 08.3 E
27◦ 11.4 N, 132◦ 25.0 E
3259
4082
4604
5102
5245
5431
Type B
Type A
Type A
Type A
Type B
Type A
Bad E field data for 50 days
Data gap of 12 days
Not useful (flooded)
geomagnetic field data are found to be very high quality.
Thirdly, the OBEM’s clock was corrected by measuring
the time difference between the clocks of each OBEM
and the GPS before the deployment and after the recovery. This correction was based on an assumption of linear
time drift. Since all the OBEMs had been placed by free
fall, each component was transformed into local Cartesian coordinate system with x, y, and z axes pointing
to the geomagnetic north, east, and downward directions, respectively. The tilt data and the absolute value
of each component (obtained from the geomagnetic field
data) are used for transformation. The OBEM1 data is
an exception, because the tilt value is more than that of
the measurement range of 8.5◦ and no tilt data are available for transformation. For the OBEM1, the three axes
were rotated so that declination and inclination at the
observation site coincide with those given by the International Geomagnetic Reference Field (IGRF; IAGA
Division V Working Group VMOD, 2005). Finally, we
removed signals of tidal components by a least squares
method.
We estimated the MT impedance tensor, which is a
transfer function from magnetic field to electric field
variation, as a function of period. Five seafloor MT
impedances with jackknife error bars are obtained from
original time series data using the robust remote reference method (Chave and Thomson, 1989). The apparent
resistivity and its phase calculated from the impedance
tensor of OBEM1 are shown in Fig. 3, for example. All
the data show relatively small error bars between 500
and a few tens of thousand seconds in period. Once the
MT impedance is obtained, the electric field can be predicted from the magnetic field. The coherences between
observed and predicted electric field were used to check
the reliability of the MT impedances.
We corrected the observed MT impedance for the
effect of topography using flattening surface 3-D modeling (FS3D; Baba and Seama, 2002) and the correction
equation of Nolasco et al. (1998), which is the most
appropriate for topographic correction (Matsuno et al., in
press). Following Nolasco et al. (1998), horizontal elec-
tric and magnetic fields distorted by seafloor topography
(E and B) and without topographic distortions (Em and
Bm ) are represented by
E = Em + CE,
B = Bm + DE,
where C and D are 2 × 2 complex tensors to represent
electric and magnetic topographic effects including both
galvanic and inductive distortions. Then, the topographic
correction equation is
Zc = (I − C)(I − Zo D)−1 Zo ,
where I is the identity tensor, Zo an observed impedance
tensor, and Zc is a corrected impedance tensor. For this
correction, we first calculate the electric and magnetic
fields using FS3D forward modeling in two cases that
are with seafloor topography and with flat-lying seafloor.
Then, an observed impedance tensor is corrected from
these electric and magnetic fields through this correction equation. A conductivity structure, which needs to
be assumed for the calculation, is taken as a uniform
half-space of 100 m. The area for each calculation
was taken as a square, 6400 km on a side, which covers both land and ocean, because Heinson and Constable
(1992) showed that the electromagnetic coast effect is
of paramount importance in modeling oceanic MT data.
The area is discretized into numerical rectangular blocks
whose side length varies; it is 1 km near the center (the
observation site) but it becomes larger as it gets farther from the center. After topographic correction, the
apparent resistivities for the diagonal impedance tensor
elements become much smaller than ones for the offdiagonal elements, showing one- or two-dimensionality
compared to ones before the correction (Fig. 3). The
apparent resistivity and its phase (Fig. 4) are calculated
from the determinant average of each impedance tensor
after the topographic correction.
We tested whether or not the apparent resistivity
and its phase data are consistent with the modeling assumption of one-dimensionality using the Rho+
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N. Seama et al. / Physics of the Earth and Planetary Interiors 162 (2007) 2–12
Fig. 3. All four elements of apparent resistivities and phases at the OBEM1 site. Observations (circles) are compared with data after topographic
effect correction (dots) and with calculations from a model (solid lines). The model is a 1-D conductivity structure (Fig. 4) below 3-D bathymetry
and land–ocean distribution. The x and y axes are the geomagnetic north and east directions, respectively. Error bars of 2S.D. are only shown for
the observations.
algorithm (Parker and Booker, 1996), which gives the
minimum possible RMS misfit model the same as
the D+ algorithm (Parker, 1980). All the OBEM data
after topographic correction passed it at the 95% confidence level, although the OBEM2 data before the
topographic correction failed this test. This failure is
due to the fact that the OBEM2 was located very close
to a marked topographic feature of the West Mariana
Ridge (remnant arcs), and any 1-D structural model cannot explain the OBEM2 data before the topographic
correction.
The 1-D electrical conductivity structure beneath
each observation site (Fig. 4) is estimated to fit the
determinant average of each MT impedance tensor using
Occam’s inversion (Constable et al., 1987), which gives
a smooth conductivity model. For the Occam’s inversion, both the first and second derivatives of roughness
are minimized with the RMS misfit of 1.0 and the
coincidence of the two models supports the interpretation over the depth range. The separation of the two
models in the deeper part suggests that the acceptable limit of the depth range is about 400 km (Fig. 4).
N. Seama et al. / Physics of the Earth and Planetary Interiors 162 (2007) 2–12
7
Fig. 4. The apparent resistivity (top), phase (middle), and 1-D conductivity structural profiles (bottom) of each OBEM site. The observed apparent
resistivity and phase with error estimates are calculated from the determinant average of each magnetotelluric impedance. The error bars are 1S.D.
The lines are based on the conductivity structural models, which are obtained by Occam’s inversion (Constable et al., 1987). Occam’s inversion
gives smooth profiles in two ways: first derivative roughness (solid lines) and second derivative roughness (dashed lines). Dashed arrows in the
OBEM1 profile indicate a low conductivity zone. Arrows in the OBEM2, OBEM4, OBEM5, and OBEM6 profiles show peaks of high conductivity.
See text for detail.
Although each 1-D electrical conductivity structure was
obtained by inversion using the determinant average of
each MT impedance tensor after the topographic correction, a model at each site, which is 1-D electrical
conductivity structure (Fig. 4) below 3-D seafloor topography and land–ocean distribution (the same area for
the topographic correction), is well supported by the
data; the model impedance explains all four elements
of the observed MT impedance at each site, as one
example shown in Fig. 3. Moreover, we re-calculated
the topographic effects using the resultant 1-D electrical conductivity structural model instead of the simple
100 m oceanic crust and mantle model. But, these two
corrected MT impedance, using the different conductivity structural models, become almost the same within
the error bars. This is probably because the topographic
effects are weakly coupled with the underlying oceanic
crust and mantle structure as suggested by Nolasco et al.
(1998) and because the correction equation of Nolasco
et al. (1998) is robust to a resistivity structure assumed
for the correction (Matsuno et al., in press).
3. Results
The results of the 1-D conductivity inversion for each
observation site (Fig. 4) indicate common and different
features among the sites as follows.
(1) The structure of the OBEM1 site is quite different from the other sites. Following a moderate high
at shallow depth, the OBEM1 conductivity profile
shows distinct low at a depth of 50–150 km (dashed
arrows in Fig. 4) and then a gradual increase in the
deeper part.
(2) The conductivity structural profiles of the OBEM2,
OBEM4, OBEM5, and OBEM6 sites show a similar
pattern. For all the sites, a high conductivity peak
at intermediate level (arrows in Fig. 4) follows the
lowest conductivity values at shallow depth. In the
deeper part, the conductivity values increase with
depth.
(3) The depth of the high peak in each conductivity
profile is 80 km for the OBEM4 site, 100 km for
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N. Seama et al. / Physics of the Earth and Planetary Interiors 162 (2007) 2–12
Fig. 5. 2-D inversion result of conductivity structure along the observation line (top) and comparison of 1-D conductivity structural profiles at each
OBEM site (bottom). The location of each OBEM is shown by triangle with label of its site name. The 1-D conductivity structural profiles are from
1-D Occam’s inversion results with first derivative roughness (solid lines; Fig. 4) and from the 2-D inversion result (dashed lines).
the OBEM5 and OBEM6 sites, and 200 km for the
OBEM2 site.
(4) The conductivity profiles of the OBEM5 and
OBEM6 sites are almost identical.
We performed a simple 2-D inversion of all the data
along the OBEM observation line to justify our results
from 1-D model at each site. As pointed out in a previous section, all impedance tensors showed one- or
two-dimensionality after a topographic correction. This
means that 1- and 2-D inversion of observed impedances
will give consistent conductivity-depth profiles for each
OBEM site, if lateral variations of conductivity are gradual. Conversely, there will be significant discrepancy
between profiles estimated by 1- and 2-D inversion, if
there is a sharp lateral variation of conductivity and the
observation site is located close to its boundary. The
2-D electrical conductivity structure beneath the observation line (Fig. 5) is estimated to fit the off-diagonal
elements (Fig. 6) of the MT impedance tensor after the
topographic correction using the data space Occam’s
inversion (DASOCC inversion; Siripunvaraporn and
Egbert, 2000). The strike of the 2-D electrical conductivity structure is not parallel to the regional strike at
each site, which was estimated from the MT impedance
tensor after the topographic correction using the method
of Groom and Bailey (1989); the strike of the 2-D electrical conductivity structure is assumed to be N43◦ E,
while the regional strikes are N17◦ E, N36◦ W, N19◦ W,
N25◦ E, N43◦ E, at OBEM1, OBEM2, OBEM4, OBEM5,
and OBEM6, respectively. Furthermore, the number of
the present available data sets is limited and its coverage
is sparse (only five sites along the observation line of
1700 km length). Thus, the 2-D electrical conductivity
structure itself is not suitable for interpretation, but it is
useful to check effect from two dimensionalities on the
1-D conductivity structure and to justify our results from
1-D model at each site.
A conductivity-depth profile for each OBEM site was
obtained from the inverted 2-D profile and was compared with each conductivity profile from 1-D inversion.
This comparison (bottom in Fig. 5) shows two similar-
N. Seama et al. / Physics of the Earth and Planetary Interiors 162 (2007) 2–12
9
OBEM4, OBEM5, and OBEM6 sites, but not at the
OBEM2 site in this study.
We also examined the robustness of the solutions by
comparing inversion results for different levels of RMS
misfit, ranging from RMS = 0.9 which is the smallest
obtainable, to RMS = 1.3 at which the structure becomes
smoothest and only robust structures can survive. The
results of these examinations (Fig. 7) clearly show that
the high conductivity peak of the OBEM4 site is a persistent part of the profile. On the other hand, the high
conductivity peaks of the OBEM5 and OBEM6 sites
become a saddle point when the RMS level becomes
higher, indicating that these peaks are weak and might
only show a change in conductivity gradients. Moreover,
the structure beneath the OBEM1 site, which shows a
distinct low conductivity region at intermediate depth
(50–150 km), is robust and well constrained by the data.
Fig. 6. Off-diagonal elements of apparent resistivities and phases at
the OBEM1 site. Observations (circles) with error bars of 1S.D. are
compared with calculations from a model (solid lines). The x and y
axes are parallel and perpendicular to the observation line, respectively.
ities between the conductivity structural profiles. First,
both profiles beneath the spreading axis of the Mariana
Trough (the OBEM1 site) show distinct low conductivity
at depths of 50–150 km. Second, both of off-axis conductivity profiles (the OBEM4, OBEM5, and OBEM6 sites)
infer existence of a high conductivity peak at middle
depth level, and the depth level shows similar tendency.
On the other hand, one distinct difference appears in the
conductivity structural profiles at the OBEM2 site, suggesting that this 1-D model is not fully supported by the
present data set. Thus, we will interpret the results from
the 1-D conductivity structural profile at the OBEM1,
4. Discussions
The results of the 1-D conductivity structure are well
conformed to the tectonic setting and the crustal age
beneath each site. The OBEM1 site is only 20 km away
from the spreading axis and the crustal age beneath
the OBEM1 site is estimated at 1 Ma using the vector
geomagnetic anomaly data obtained from the surface
geophysical survey (Iwamoto et al., 2002). In comparison to OBEM1, the other sites are located far from the
spreading axis. Crustal ages of the other OBEM sites are
reported as follows. The crustal age beneath the OBEM4
site is placed at 21 Ma corresponding to the geomagnetic
anomaly 6A results (Okino et al., 1999). The DSDP sites
445 and 446 near the OBEM5 site show a basement
age of 59.0 ± 3 and 56.5 ± 2 Ma, respectively (Ozima
et al., 1980). Furthermore, the geomagnetic anomaly 24
Fig. 7. 1-D Occam’s inversion results, with first derivative roughness at each OBEM site for RMS values varying from 0.9 to 1.3. The dashed line
shows a preferred misfit level; the profile is shown in Fig. 4.
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(55 Ma) is identified to be located closer to the Central
Basin Ridge, the fossil spreading axis, than the OBEM5
site (Hilde and Lee, 1984). Therefore, the crustal age
is estimated as around 60 Ma at the OBEM5 site. The
crustal age beneath the OBEM6 site is not well studied
but probably older than that of the OBEM5 site, because
the former is located farther from the Central Basin Ridge
in deeper water than the latter (Table 1). The age difference is, however, not significant because of the small
change in geographical distance and depth.
The OBEM1 site is located near the spreading axis of
the Mariana Trough and its conductivity structure probably reflects the upwelling dynamics existing beneath
this back-arc spreading axis. The distinct low conductivity of the OBEM1 is compared with the conductivity
value of dry olivine (the SO2 model of Constable et
al. (1992)) as well as the influence of dissolved hydrogen within the isotropic olivine mantle. The estimation
of conductivities in the presence of different hydrogen
contents is based on the Nernst–Einstein relation following previous analyses (Karato, 1990; Lizarralde et
al., 1995), even though this is still a hypothesis as there
are no published laboratory measurements on this work.
A potential temperature of 1300 ◦ C is used for these calculations. The result indicates the reduction of hydrogen
contents at intermediate depth beneath the OBEM1 site
(Fig. 8). These results suggest that the extraction of water
from minerals such as olivine through melting (Karato,
1986; Hirth and Kohlstedt, 1996) may explain the low
conductivity values of the OBEM1 profile. The melting
initiates at a depth of nearly 250 km where the trend of
the OBEM1 conductivity profile becomes different from
that of the conductivity profile calculated for an isotropic
olivine mantle with a constant hydrogen content (Fig. 8).
Although the resolution of our result is not high enough,
the presence of this low conductivity region is robust in
depths of 50–150 km as we have examined (Fig. 7), indicating that the melting begins at a depth deeper than at
least 150 km. This depth is significantly greater than that
of the typical MORB source region (∼115 km; Hirth and
Kohlstedt, 1996), which is probably due to higher water
content in back-arc regions (Karato and Jung, 1998).
The OBEM4, OBEM5, and OBEM6 sites are located
far from spreading axis, and their conductivity profiles
show a similar pattern; a high conductivity peak or a
change in conductivity gradients exists at mid-level. The
depth of the peak or the gradient change shows a relationship between depth and crustal age beneath each
site. The OBEM4 site has a younger crustal age (21 Ma)
and shows a shallower depth for the high conductivity peak (80 km) compared to those of the OBEM5 and
OBEM6 sites (around 60 Ma and 100 km). The OBEM5
Fig. 8. 1-D conductivity structural profiles of the OBEM1 site (thin
solid and dashed lines as shown in Fig. 4), compared with the conductivity profile of dry olivine (the SO2 model of Constable et al.
(1992), grey dashed line) and with a profile calculated for an isotropic
olivine with different hydrogen contents (grey lines; 1000 ppm corresponds to 1000 H/106 Si) and with hydrogen saturated olivine (grey
dotted line). The estimation of conductivities in the presence of different hydrogen contents is based on the Nernst–Einstein relation
following previous analyses (Karato, 1990; Lizarralde et al., 1995),
and high Hashin–Shtrikman bounds on conductivity (Shankland and
Duba, 1990) are shown. A potential temperature of 1300 ◦ C is used for
these calculations.
and OBEM6 sites have similar crustal ages and their
structural profiles are almost identical, suggesting that
crustal age is the main parameter for the conductivity
structure. The whole evidence supports a relationship
between crustal age and the depth of the high conductivity peak or the conductivity gradient change; the peak
depth increases as the age becomes older. The relationship is well related to the geothermal model of Parsons
and Sclater (1977), suggesting that the age dependency
of the peak depth is explained by the thermal structure in
terms of lithospheric cooling, possibly with the presence
of partial melt. Temperature change is a primary parameter for the conductivity value. In the geothermal model
of Parsons and Sclater (1977), the isotherms run deeper
and the thermal gradient becomes gentler as the mantle gets older. The depth of the high conductivity peak
beneath the OBEM4, OBEM5, and OBEM6 sites coincide with the depth of the 1300 ◦ C isotherm calculated
from Parsons and Sclater (1977). Further, the steep conductivity gradient of the OBEM4 profile and the gentle
conductivity gradient of the OBEM5 and OBEM6 profiles qualitatively agree with the gradient change with
age of the model of Parsons and Sclater (1977). Moreover, partial melt may be required for the OBEM4 site to
explain the high conductivity peak, which is the robust
N. Seama et al. / Physics of the Earth and Planetary Interiors 162 (2007) 2–12
feature and is different from those of the other sites
(Fig. 7). A high conductivity peak cannot be produced
only by the thermal structure associated with the lithospheric cooling alone, but by the limited depth range that
partial melt can exist. The limited depth range occurs
because the thermal gradient is small below the partial
melt region while the pressure increases linearly with
depth. The compositional boundary model for the base
of an oceanic plate can be a candidate, but the difficulty
of this model is that another mechanism is required to
explain the age dependency of the peak depth.
5. Conclusions
Our study is the first attempt to investigate the largescale features of the electrical conductivity distribution
in the mantle beneath the Philippine Sea back-arc basins
by 1-D inversion of magnetotelluric data from five
seafloor stations. The observed tensor impedance at
each site has been shown to be well explained by a 1D model with 3-D effects of seafloor topography and
land-ocean contrast. Further, a comparison between 1D model and the simple 2-D inversion model justified
the 1-D treatment at four sites (the OBEM1, OBEM4,
OBEM5, and OBEM6 sites). Considering the crustal age
of each site, geodynamic processes in the upper mantle,
and effects controlling the mantle conductivity by temperature and fluid contents, the following conclusions
have been obtained by interpretation of the inversion
results.
(1) The structure nearly beneath the spreading axis of
the Mariana Trough (the OBEM1 site) probably
reflects the upwelling dynamics operating beneath
the spreading axis and shows distinct low conductivity structure at depths of 50–150 km. These low
values are comparable with that of olivine with
low hydrogen content, implying that (1) the melting process extracts water from minerals such as
olivine, and (2) the depth of melt beginning in the
Mariana Trough is deeper than that of the typical MORB source region (∼115 km; Hirth and
Kohlstedt, 1996). Although these interpretations are
based on one observation site, it is suggested that
further ocean bottom EM study has the high potential to investigate the amount of hydrogen (water)
content and melt in upwelling dynamics operating
beneath the spreading axis.
(2) The off-axis conductivity profiles (the OBEM4,
OBEM5, and OBEM6 sites) show the existence
of a high conductivity peak or conductivity gradient change at middle depth level, and the depth
11
level increases with the crustal age. We are the first
to show the relationship between depth level and
crustal age in back-arc basins and it indicates that the
conductivity structure is well related to the geothermal model of Parsons and Sclater (1977), suggesting
that the high conductivity peaks are explained by
a temperature gradient change, possibly with the
presence of partial melt.
Acknowledgements
We would like to thank to H. Shiobara (Chief Scientist), M. Kikuchi, H. Miyano, K. Nakase, and K. Tashiro
for their help during the deployment and/or recovery
cruises. Anthony Toigo and Haider Zaman read the
manuscript and offered helpful improvements. We are
also thankful to two anonymous reviewers for reviewing our paper and giving us valuable comments. This
work was supported by the Ocean Hemisphere Project
(Scientific Grant 11NP1101 and 12NP1101), by Grantin-Aid for Scientific Research (B) (1) (No. 15340149),
Japan Society for the Promotion of Science, and partly
by “The 21st Century COE Program of Origin and Evolution of Planetary Systems” and “Stagnant Slab Project
(No. 17037003)” in the Ministry of Education, Culture,
Sports, Science and Technology (MEXT). We strongly
acknowledge the support provided by these organizations. The GMT software (Wessel and Smith, 1998) was
extensively used in this study.
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