Earth and Planetary Science Letters 459 (2017) 227–237
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Seawater storage and element transfer associated with mantle
serpentinization in magma-poor rifted margins: A quantitative
approach
Victor Hugo G. Pinto a,b,∗ , Gianreto Manatschal a , Anne Marie Karpoff a , Marc Ulrich a ,
Adriano R. Viana c
a
b
c
Institut de Physique du Globe de Strasbourg, CNRS UMR7516, Université de Strasbourg, Strasbourg, France
Petrobras S.A, E&P-EXP, Applied Geology to Exploration, Rio de Janeiro, Brazil
Petrobras S.A, E&P-EXP, Applied Geophysics to Exploration, Rio de Janeiro, Brazil
a r t i c l e
i n f o
Article history:
Received 16 March 2016
Received in revised form 12 November 2016
Accepted 14 November 2016
Available online 5 December 2016
Editor: M. Bickle
Keywords:
mantle exhumation
serpentinization
element transfer
water storage
magma-poor rifted margins
ocean continent transitions
a b s t r a c t
Continental breakup in magma-poor rifted margins can develop, in some instances, after the formation
of a wide exhumed domain that can be several hundreds of km wide. As exhumation of the continental
mantle occurs serpentinization, due to seawater circulation, can extend as far down as 5–6 km, as
observed in refraction seismic data. The impact caused by the process of serpentinization within the
evolving ocean may have the potential to change: (i) seawater chemistry; (ii) sustain the evolution of
primitive life; (iii) control depositional environments; and (iv) form weak zones preferentially used during
the formation, reactivation and subduction of distal rifted margins. Based on geological observations, and
geophysical and geochemical data from present-day and fossil zones of exhumed continental mantle, we
present a first-order quantification showing that approximately 0.380 km3 of water per km2 can be stored
in the mantle. Using simple methods, it can be shown that serpentinization may account for a significant
loss of Si, Mg, Fe, Mn, Ca, Ni and Cr during serpentinization of mantle rocks. In particular during latest
stages of rifting, when basins are often restricted and seaways are not yet connected, exhumation and the
serpentinization of large areas of continental mantle may result in a major transfer of elements between
the main Earth reservoirs, such as the mantle and seawater.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
It is well known that serpentinized mantle can store large
volumes of water (Skelton et al., 2005; Skelton and Jakobsson,
2007). Moreover, serpentinites drilled or dredged at zones of
exhumed continental mantle, slow spreading mid-ocean ridges
as well as sampled in ophiolites have allowed the characterization of the reactions and mass transfer linked to the process
of serpentinization (e.g. Skelton et al., 2005; Bach et al., 2006;
Boschi et al., 2006). Fluids sampled in situ at present day active ultramafic-hosted hydrothermal vents associated with oceanic
core complexes located at slow to ultraslow spreading mid-ocean
ridges, have also showed that serpentinization is responsible for a
significant mass transfer (Boschi et al., 2006; Schmidt et al., 2007;
Seyfried Jr. et al., 2007; Edmonds, 2010 and references therein).
*
Corresponding author at: Institut de Physique du Globe de Strasbourg, CNRS
UMR7516, Université de Strasbourg, 1 rue Blessig, 67084, Strasbourg Cedex, France.
E-mail address: vhpinto@me.com (V.H.G. Pinto).
http://dx.doi.org/10.1016/j.epsl.2016.11.023
0012-821X/© 2016 Elsevier B.V. All rights reserved.
Analysis of the compositions of these fluids reveals that they are
often enriched in H2 and CH4 gases and have varying concentrations of dissolved Si, Mg, Ca, Fe, Mn, Ni and Co (Kelley et al., 2001;
Fouquet et al., 2010). Although the remnants of fossil rifted margins in the Alps are not directly comparable with slow spreading ridges (e.g. Manatschal and Müntener, 2009), they also show
large amounts of hydrothermal cherts (SiO2 ) with botryoidal Mnminerals and Fe–Ni alloys (Perseil and Latouche, 1989) that are directly linked to the serpentinization process (Pinto et al., 2015). Sirich fluids are also expelled during the process of serpentinization
as observed during controlled laboratory experiments (Daval et al.,
2011; Ogasawara et al., 2013). In addition, it has been observed
that the serpentinization results in the depletion of Ca, Mg and
Si as described from drill hole data following the Ocean Drilling
Program (ODP) along the Iberia margin (e.g. Gibson et al., 1996;
Milliken and Morgan, 1996). Using complex geochemical assumptions, Skelton et al. (2005) calculated for exhumed and serpentinized domains the fluxes of H2 O, H2 and CH4 . In this study,
we use a different method to calculate not the flux but the total
volume of water stored in the exhumed continental mantle. The
228
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
Fig. 1. Map of domains from the Central Western Iberian margin. The map was
modified from Sutra and Manatschal (2012) and expanded to the south where is
localized the IAM-9 section. Yellow circles are ODP Sites close to the IAM-9. Reflection seismic lines CAM-144 and ISE-1 were used to define the limits of the rift
domains.
method is not only based on geochemical analyses, but also on geological and geophysical observations from the Iberia margin and,
its analogue, the Alpine Tethys margins exposed in the Alps. Taking the direct relationship between seismic velocity, rock density
and the degree of serpentinization, we can estimate the adsorption
of water. Furthermore because serpentinization leads to element
losses (Pinto et al., 2015), simple mass balance calculations can be
proposed. We present estimates of mass balance calculations for
the exhumed and serpentinized domains of the Western Iberia and
fossil Alpine Tethys margins. At present, these are the only margins
where drill hole and refraction seismic data, as well as full access
to exhumed continental mantle rocks exist to allow us to constrain and quantify the element transfer during serpentinization.
Based on these data sets, we present a mass balance calculation
that has been performed to quantify element transfer due to serpentinization in the zone of exhumed continental mantle at these
magma-poor rifted margins.
2. Exhumed mantle along the Western Iberia margin
The zone of exhumed continental mantle along the magmapoor Iberia rifted margin (Fig. 1) has been constrained by drill hole
data (ODP; e.g. Tucholke and Sibuet, 2007) and by the interpretation of refraction and reflection seismic data (Sutra et al., 2013
and references therein). At its widest along the Iberian margin, exhumed continental mantle comprises a zone up to 180 km. The
transition from the exhumed mantle to the unequivocal oceanic
crust has been a matter of debate over the last two decades. Based
on seismic velocity, drill hole data, gravity and magnetic anomalies, it is considered that the J-anomaly, which encompasses M3 to
M0, marks the oceanward termination of exhumed subcontinental
mantle (Bronner et al., 2011). Minshull et al. (2014) repositioned
these anomalies in the refraction seismic sections of Dean and
Minshull (2000), allowing a better constrain of the oceanward limit
of exhumed continental mantle.
Fig. 2 synthesizes major geological observations, seismic velocity data and the reinterpreted reflection and refraction seismic section IAM-9. In Fig. 2B, the velocity intervals attributed to serpentinites are observed below the hyperextended continental crust,
between km 240 and km 270, and along the zone of exhumed
continental mantle (km 240–100). This zone is made of 5–6 km
thick serpentinized mantle that progressively grades to an embryonic oceanic crust (km 100–30).
3. Relationship between V p , density, water content and degree of
serpentinization
Serpentine polymorphs are formed at different pressure–temperature (P–T) conditions and they have different water contents (i.e.,
hydroxyl [OH] groups in crystal lattice). Antigorite forms at higher
P–T conditions (∼20 kbar and ∼500 ◦ C) compared to lizardite and
chrysotile (Deschamps et al., 2013). Antigorite is often related to
subduction zones, and it has ∼10 wt.% of water (H2 O). Lizardite
and chrysotile are by far the most frequent minerals in exhumed
continental mantle along continental margins. In this tectonic setting, lizardite and chrysotile form at temperatures and pressures
lower than 350 ◦ C and 10 kbar, respectively (Milliken and Morgan, 1996; Müntener et al., 2010), and contain 13wt.% of water.
Although most of the water in serpentinized mantle rocks may be
within serpentine minerals, other hydrated minerals such as talc,
brucite, chlorite or clays may influence the water budget as well.
However, their occurrence is minor compared to serpentines. This
is corroborated by the analysis of serpentinite rocks, which show
an average water content of 12.38 ± 2.99 wt.% (calculated on 109
samples, Deschamps et al., 2013). This suggests that the additional
mineral phases that may occur in serpentinite rocks do not significantly affect the water budget.
A correlation between seismic velocity (V p ), density and degree
of serpentinization (Fig. 3A–B) is given by Miller and Christensen
(1997). We used a data set of serpentinized peridotites for which
the degree of serpentinization is defined by petrological criteria,
e.g., the presence of olivine/serpentine (Müntener et al., 2010;
Pinto et al., 2015). Because these rocks are free of carbonates, the
loss on ignition is related to water content only. Therefore, the
correlation of the loss on ignition with variable degrees of serpentinization is related to the hydration of peridotites (Fig. 3C). By the
relationship between V p , density, serpentinization and hydration,
it is possible to propose the model shown in Fig. 3D.
The correlation between V p and degree of serpentinization was
used to transform the velocity contours to serpentinite contours in
the IAM-9 seismic section (Dean and Minshull, 2000) (Fig. 2). The
density values were used to calculate the loss of elements, and
the correlation between serpentinites and hydration was used to
calculate the volume of water absorbed in the mantle along the
zone of exhumed mantle at the Iberia distal margin.
4. Method for calculation of the element transfer and water
absorbed in the zone of exhumed continental mantle
A method is proposed to quantify the mass of element transfer
and water absorption in the zone of exhumed continental mantle.
The term element transfer is used because the flux of seawater in
the mantle leads both to absorption of seawater in the mantle during serpentinization and to outflow of seawater that reacted with
the mantle. The outflowing fluids carry dissolved mantle-derived
elements. To quantify the element transfer in the mantle, a method
referred to as P –M diagram (Potdevin and Marquer, 1987) was
used. This method enables the estimation of the loss of elements
due to serpentinization. The steps of the calculations are presented
in Tables 1 to 3. Because the calculation of both element transfer
and absorbed volume of water in the mantle share the same input
data (e.g. seismic refraction data and mapped extent of exhumed
serpentinized mantle; Fig. 2) they are described together.
4.1. Calculation of the loss of elements in mass fraction caused by
serpentinization
The data used to perform all the calculations of mass transfer in the zone of exhumed continental mantle are shown in
the median columns of Table 1. The median was used instead
of the average values in-order to diminish the risk of calculating
extremely high or low concentrations that may be related to either analytical error, or inappropriate sampling. The median was
calculated from samples of serpentinized peridotites from the Malenco and the Platta nappes in the Alps (Müntener et al., 2010;
Pinto et al., 2015), which are also the samples used in Fig. 3C. The
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
229
Fig. 2. Seismic section IAM-9 and related geophysical data and interpretations. (A) Refraction data (from Dean and Minshull, 2000) showing the velocities and the zones used
to estimate the amount of water absorbed during serpentinization. Note the zones (Z1, Z2, etc.) differentiated by colors used in the calculation. (B) Interpreted reflection
seismic section used to define the domains and limits of the rifted margin. The seismic velocities from refraction data are used to correlate with serpentinites according to
Fig. 3A.
samples from the Alps were taken instead of the ones from Iberia
because the Alpine samples, although mineralogically and compositionally very similar to those of Iberia (Müntener and Manatschal,
2006), cover a wider spectrum of little to completely serpentinized
peridotites. Moreover, the ease of access to world-class outcrops
of exhumed continental mantle in the Alps enabled us to better
choose the appropriate samples and to describe their relationship
to the surrounding structures and lithologies. The samples were
assembled in three groups by open and closed intervals according
to their degree of serpentinization. Samples with degrees of serpentinization of ≤10% (Group 1: weakly serpentinized), samples in
the interval 10 to 60% (Group 2: moderately serpentinized), and
samples more than 60% serpentinized (>60%), (Group 3: highly
serpentinized).
The gain or loss of elements were calculated for all the samples by the following relationships: Group 1 (weakly serpentinized)
were assumed to represent the protolith; the transformation (alteration) from Group 1 to Group 2 (moderately serpentinized);
and the transformation from Group 2 to Group 3 (highly serpentinized).
Eq. (1) gives the variation of concentration C relative to
the initial concentration of elements or chemical compounds (C ),
where loss or gain of elements are a function of the density (ρ )
and of the f -factor (for details see Potdevin and Marquer, 1987):
C = [C a /C o ] · [ρa /ρo ] f − 1
(1)
where C is concentration, ρ is density, the subscripts a and o stand
for the altered and original rock respectively, and f treats changes
in volume or concentration. Concentration values are taken from
the whole-rock geochemical analyses presented in Table 1.
Following the results of numerous studies (see Pinto et al., 2015
and references therein), the assumption was made that Ti and Al
behaved as immobile elements during serpentinization. The linear
relationship between Ti and Al contents plotted for the samples
used in this study (Fig. 4) supports this assertion. However, some
data are not aligned along the linear trend, suggesting that Ti and
Al may not have behaved in all samples perfectly immobile. Thus,
assuming Al and Ti are immobile, the f -factor can be calculated
(Eq. (1)). This is done by setting C = 0 and given calculated densities of rocks (see equation in Fig. 3B), and the concentration of
230
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
Fig. 3. Petrological and petrophysical data. (A) Seismic velocity (V p ) vs. degree of serpentinization (data from Miller and Christensen, 1997). (B) Density vs. degree of
serpentinization (data from Miller and Christensen, 1997). (C) Percentage of hydration vs. degree of serpentinization (data from Müntener et al., 2010 and Pinto et al., 2015).
(D) Proposed global model that results from the combination of the previous ones. Equations in figures A, B, C were used for calculations (see text).
losses (d) in Table 1 can be found by multiplying the C values (c)
by the median values of the assumed protolith (Group 1). Note the
superscript symbols (c and d) in the header of Table 1. The minus
sign (–) stands for loss of elements.
4.2. Estimation of the loss of elements caused by serpentinization in a
2D section
Fig. 4. TiO2 vs. Al2 O3 . values from mantle samples ranging from 2 to 95% of serpentinization. Data from Müntener et al. (2010) and Pinto et al. (2015). The good linear
correlation between Ti and Al supports the assumption that they can be considered
as immobile or little mobile elements during serpentinization and formation of the
zone of exhumed continental mantle.
Al and Ti from the altered and the original rock. The calculated
f -factors for both Al and Ti can be used to estimate the variation
in concentrations of the other elements, see Table 1. The variations of these other elements (Table 1) all differ assuming Ti or Al
as immobile. Thus, both values provide an estimation of the range
of C for other elements and therefore of the mass that can be
lost or gained during serpentinization. The differences are a measure of the uncertainties. The percentage or the mass of gains or
Table 2 gives all the information necessary to calculate the
losses of elements during serpentinization and the amount of absorbed water that remains in the serpentinized mantle. The zones
identified in the IAM-9 seismic section (Fig. 2A) are shown at the
very top of Table 2. The hyperextended domain corresponds to
Z1, the zone of exhumed continental mantle encompasses Z2 to
Z9, and the embryonic oceanic domain encompasses Z10 to Z13,
which in the calculations stops at the M0 magnetic anomaly. The
top and base seismic velocities (V p -top and V p -base) for each zone
are reported in Table 2. The volume line (superscript symbol †)
is in fact a calculated area of these zones with a lateral continuity of 1 km used in order to have a volumetric value. The degree
of serpentinization shown in Table 2 was calculated by taking the
average between V p -top and V p -base and applying the equation
shown in Fig. 3A. The density (ρ ) is calculated by using the equation shown in Fig. 3B. The percentage of hydration is calculated
using the equation shown in Fig. 3C.
The mass (ton) and the percentage of loss of elements (%)
were calculated for each zone (Z1, Z2, etc.) using Ti or Al as assumed immobile elements. To perform this calculation, the results
from Table 1 were considered, following the criteria: (i) the values
corresponding to alteration from Group 1 to 3 were used when
the degree of serpentinization is higher than 60% (V p < 6 km/m);
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
231
Table 1
Gain and loss estimation.a
Elements
SiO2
TiO2
Al2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
Cr2 O3
NiO
LOI
Ba
Ce
Co
Cr
Cs
Cu
Eu
Gd
Nb
Nd
Ni
Pb
Rb
S
Sc
Sm
Sr
Ta
Tb
Th
Tm
U
V
Y
Yb
Zn
Zr
Serpentinization
(%)
Group 1
Group 2
Group 3
Serpentinization Group 1 to Group 2
Median
values
<10b
Median
values
]10–60]b
Median
values
>60b
Ref. Al2 O3
]10–60]c
Ref. TiO2
]10–60]c
Ref. Al2 O3
(%wt)
(or g)d
Ref. TiO2
(%wt)
(or g)d
Ref. Al2 O3
>60c
Ref. TiO2
>60c
Ref. Al2 O3
(%wt)
(or g)d
Ref. TiO2
(%wt)
(or g)d
43.44
0.05
2.11
7.94
0.12
42.75
1.72
0.04
0.02
0.01
0.38
0.35
1.65
1.03
0.22
105
2712
0.01
51
0.03
0.12
0.06
0.19
2205
0.05
0.14
200
10.00
0.07
3.03
0.01
0.03
0.00
0.02
0.00
51.00
1.11
0.14
41.00
1.26
10
41.72
0.07
2.29
7.83
0.13
39.27
1.84
0.00
0.00
0.01
0.34
0.32
5.82
0.22
0.30
96
2496
1.24
22.50
0.05
0.17
0.03
0.29
2011
–
0.24
550
9.00
0.11
5.05
0.00
0.03
0.01
0.02
0.00
51.50
1.36
0.14
44.50
1.45
50
40.10
0.12
3.43
7.70
0.13
34.97
2.36
0.00
0.00
0.10
0.40
0.26
10.21
4.26
0.53
99
2687
0.78
21
0.11
0.38
3.10
0.57
1887
0.45
0.41
550
11.00
0.23
6.60
0.05
0.06
0.00
0.05
0.01
73.00
3.26
0.30
49.50
6.30
80
−0.12
0.36
0.00
−0.09
−0.01
−0.16
−0.02
−1.00
−1.00
−0.10
−0.20
−0.17
2.24
−0.81
0.26
−0.16
−0.15
161.38
−0.59
0.30
0.29
−0.63
0.39
−0.16
–
0.58
1.53
−0.17
0.39
0.53
−0.85
0.18
0.52
0.03
−0.75
−0.07
0.12
−0.11
−0.00
0.06
−0.35
0.00
−0.26
−0.33
−0.27
−0.38
−0.27
−1.00
−1.00
−0.34
−0.41
−0.39
1.38
−0.86
−0.07
−0.38
−0.38
118.60
−0.70
−0.04
−0.05
−0.73
0.02
−0.38
−5.12
0.02
0.00
−0.75
−0.00
−6.68
−0.03
−0.04
−0.02
−0.00
−0.08
−0.06
3.70
−8 × 10−5
6 × 10−6
−2 × 10−3
−4 × 10−2
1 × 10−4
−3 × 10−3
1 × 10−6
3 × 10−6
−4 × 10−6
7 × 10−6
−4 × 10−2
–
8 × 10−6
3 × 10−2
−2 × 10−4
3 × 10−6
2 × 10−4
−1 × 10−6
4 × 10−7
2 × 10−7
5 × 10−8
−3 × 10−7
−4 × 10−4
1 × 10−5
−2 × 10−6
−1 × 10−5
7 × 10−6
−15.22
0.00
−0.55
−2.64
−0.03
−16.18
−0.47
−0.04
−0.02
−0.00
−0.16
−0.14
2.29
−9 × 10−5
−1 × 10−6
−4 × 10−3
−1 × 10−1
8 × 10−5
−4 × 10−3
−1 × 10−7
−6 × 10−7
−5 × 10−6
4 × 10−7
−8 × 10−2
−0.43
0.52
0.00
−0.40
−0.33
−0.50
−0.16
−1.00
−1.00
5.10
−0.36
−0.55
2.80
1.54
0.46
−0.42
−0.39
67.35
−0.75
1.04
0.95
28.75
0.83
−0.47
4.58
0.80
0.69
−0.32
1.00
0.34
1.69
0.57
−0.31
0.43
−0.23
−0.12
0.80
0.34
−0.26
2.07
−0.63
0.00
−0.34
−0.61
−0.56
−0.67
−0.45
−1.00
−1.00
3.01
−0.58
−0.70
1.50
0.67
−0.04
−0.62
−0.60
43.97
−0.83
0.34
0.28
18.57
0.21
−0.65
2.67
0.19
0.11
−0.56
0.32
−0.12
0.77
0.03
−0.55
−0.06
−0.49
−0.42
0.18
−0.12
−0.51
1.02
−18.81
0.03
0.00
−3.21
−0.04
−21.27
−0.27
−0.04
−0.02
0.05
−0.14
−0.19
4.62
2 × 10−4
1 × 10−5
−4 × 10−3
−1 × 10−1
5 × 10−5
−4 × 10−3
3 × 10−6
1 × 10−5
2 × 10−4
2 × 10−5
−1 × 10−1
2 × 10−5
1 × 10−5
1 × 10−2
−3 × 10−4
7 × 10−6
1 × 10−4
2 × 10−6
1 × 10−6
−1 × 10−7
9 × 10−7
−9 × 10−8
−6 × 10−4
9 × 10−5
5 × 10−6
−1 × 10−3
3 × 10−4
−27.23
0.00
−0.72
−4.83
−0.07
−28.62
−0.77
−0.04
−0.02
0.03
−0.22
−0.25
2.48
7 × 10−5
−8 × 10−7
−6 × 10−3
−2 × 10−4
3 × 10−5
−4 × 10−3
1 × 10−6
3 × 10−6
1 × 10−4
4 × 10−6
−1 × 10−1
1 × 10−5
3 × 10−6
2 × 10−3
−5 × 10−4
2 × 10−6
−4 × 10−5
9 × 10−7
9 × 10−8
−2 × 10−7
−1 × 10−7
−2 × 10−7
−2 × 10−3
2 × 10−5
−2 × 10−6
−2 × 10−3
1 × 10−4
–
0.16
0.86
−0.39
0.02
0.13
−0.89
−0.13
0.12
−0.24
−0.81
−0.32
−0.18
−0.35
−0.27
−0.22
Serpentinization Group 1 to Group 3
–
2 × 10−6
2 × 10−2
−4 × 10−4
2 × 10−7
4 × 10−5
−1 × 10−6
−3 × 10−7
5 × 10−8
−5 × 10−7
−3 × 10−7
−2 × 10−3
−2 × 10−5
−5 × 10−6
−1 × 10−3
−3 × 10−5
a
Negative values are losses and positive values are gains. The median values in bold was calculated using data from Müntener et al. (2010) and Pinto et al. (2015). See
table in Suplementary Material.
b
Oxides are in %wt, and elements are in ppm.
Concentration variation (C values) calculated by using Eq. (1). The calculated f -factors from Group 1 to Group 2 are 1.015 and 0.747 for Al and Ti respectively. The
calculated f -factors from Group 1 to Group 3 are 0.736 and 0.484 for Al and Ti respectively.
c
d
Percentage or the mass of gains or losses.
(ii) the values corresponding to the alteration from Group 1 to 2
were used when the degree of serpentinization is lower than 60%
(V p > 6 km/s).
To illustrate the calculation steps, the zone Z1 is presented as
an example. The average degree of serpentinization in Z1 is 34%.
The percentages of SiO2 lost of 5.12% and 15.22%, calculated from
Al and Ti respectively, are multiplied by 255 × 109 tons of serpentinized rock. This is equal to 13 × 109 or 38 × 109 tons of silica lost
based on Al or Ti as immobile. The percentage of elements that are
losses are plotted below the calculated mass, because they were
also used for estimation of the loss of elements along the Central
Iberia margin, using the map shown in Fig. 1.
4.3. Estimation of the loss of elements caused by serpentinization in 3D
Table 3 presents the results of stored water and mass of predicted elements lost from the mantle, due to serpentinization, in
the Central Western Iberia margin. In order to obtain a volumetric
calculation for this area, the map of exhumed mantle (Sutra and
Manatschal, 2012) was extended to the location of the IAM-9 line.
The map shown in Fig. 1 was used to calculate the area of the hyperextended domain, the zone of exhumed continental mantle and
the embryonic ocean domain. Because the location of the transition from the zone of exhumed mantle to first oceanic crust is
uncertain, the embryonic ocean domain out to magnetic anomaly
M0 was also taken into account in the calculation reported in
Table 3. The thickness shown in Table 3 is an average value calculated for each domain using Fig. 2A. The volume of serpentinites
was obtained multiplying the area by these thicknesses. The mass
of serpentinites is given by multiplying the volume by the average density of the rocks (Table 2). The volume of absorbed water
was estimated by multiplying the average values of water by the
volume of serpentinized rock in each domain. In order to estimate
the overall element losses in the Central Western Iberia margin,
the percentage of losses from Table 2 was multiplied by the volume of serpentinized peridotite. The results, considering both Al
232
Table 2
Calculation along the IAM-9 seismic section.
Zones
Vp-Top (km/s)
Vp-Base (km/s)
Volume (km3 )†
Statistics
Z12
Z11
Z10
4
5
5
6
6
7
7
7.9
27.27
0.98
13.31
2.54
69.31
9.22
9.22
−18.81
−27.23
−13.04
−18.87
−3.21
−4.83
−2.23
−3.35
−0.04
−0.07
−0.027
−0.046
−21.27
−28.62
−14.74
−19.83
−0.27
−0.77
−0.19
−0.53
−0.10
−0.14
−0.072
−0.100
−0.11
−0.16
−0.074
−0.113
56.7
0.80
10.95
2.68
152
16.64
16.64
−18.81
−27.23
−28.59
−41.4
−3.21
−4.83
−4.88
−7.34
−0.04
−0.07
−0.059
−0.100
−21.27
−28.62
−32.33
−43.5
−0.27
−0.77
−0.417
−1.17
−0.10
−0.14
−0.159
−0.219
−0.11
−0.16
−0.161
−0.247
179.61
0.48
6.86
2.92
524.84
36.01
36.01
−5.12
−15.22
−26.89
−79.86
−0.75
−2.64
−3.93
−13.88
−0.00
−0.03
−0.008
−0.169
−6.68
−16.18
−35.07
−84.94
−0.03
−0.47
−0.140
−2.48
−0.04
−0.08
−0.188
−0.443
−0.04
−0.10
−0.220
−0.537
98.91
0.18
2.98
3.15
311.69
9.29
9.29
−5.12
−15.22
−15.97
−47.43
−0.75
−2.64
−2.33
−8.24
−0.00
−0.03
−0.005
−0.101
−6.68
−16.18
−20.83
−50.45
−0.03
−0.47
−0.083
−1.47
−0.04
−0.08
−0.111
−0.263
−0.04
−0.10
−0.131
−0.319
AVE
Zone of exhumed continental mantle
SUM
362.49
8.52
2.82
1058
71.15
71.15
−11.97
−21.23
−1.98
−3.74
−0.02
−0.05
−13.98
−22.40
−0.15
−0.62
−0.07
−0.11
−0.07
−0.13
−84.48
−187.57
0.00
−13.37
−32.81
−0.099
−0.416
−102.98
−198.73
−0.831
−5.66
−0.531
−1.025
−0.586
−1.216
Statistics
Z9
Z8
Z7
Z6
Z5
Z4
Z3
Z2
4
5
5
6
6
7
7
7.4
7.4
7.9
4.3
5
5
6
7.6
7.9
32.98
0.98
13.31
2.54
83.82
11.15
11.15
−18.81
−27.23
−15.77
−22.83
−3.21
−4.83
−2.69
−4.05
−0.04
−0.07
−0.032
−0.055
−21.27
−28.62
−17.83
−23.99
−0.27
−0.77
−0.23
−0.65
−0.10
−0.14
−0.088
−0.121
−0.11
−0.16
−0.089
−0.136
28.02
0.80
10.95
2.68
75.12
8.22
8.22
−18.81
−27.23
−14.13
−20.46
−3.21
−4.83
−2.41
−3.63
−0.04
−0.07
−0.029
−0.049
−21.27
−28.62
−15.98
−21.5
−0.27
−0.77
−0.206
−0.578
−0.10
−0.14
−0.079
−0.108
−0.11
−0.16
−0.080
−0.122
174.35
0.48
6.86
2.92
509.47
34.95
34.95
−5.12
−15.22
−26.10
−77.53
−0.75
−2.64
−3.82
−13.47
−0.00
−0.03
−0.008
−0.164
−6.68
−16.18
−34.05
−82.46
−0.03
−0.47
−0.14
−2.41
−0.04
−0.08
−0.182
−0.430
−0.04
−0.10
−0.214
−0.521
47.77
0.26
4.00
3.09
147.66
5.91
5.91
−5.12
−15.22
−7.56
−22.47
−0.75
−2.64
−1.11
−3.90
−0.00
−0.03
−0.002
−0.048
−6.68
−16.18
−9.87
−23.9
−0.03
−0.47
−0.040
−0.699
−0.04
−0.08
−0.053
−0.125
−0.04
−0.10
−0.062
−0.151
241.52
0.12
2.16
3.20
772.75
16.71
16.71
−5.12
−15.22
−39.59
−117.59
−0.75
−2.64
−5.79
−20.43
−0.00
−0.03
−0.012
−0.249
−6.68
−16.18
−51.64
−125.07
−0.03
−0.47
−0.207
−3.66
−0.04
−0.08
−0.276
−0.652
−0.04
−0.10
−0.324
−0.791
45.55
0.98
13.31
2.54
115.77
15.40
15.40
−18.81
−27.23
−21.77
−31.53
−3.21
−4.83
−3.72
−5.59
−0.04
−0.07
−0.045
−0.076
−21.27
−28.62
−24.63
−33.13
−0.27
−0.77
−0.32
−0.891
−0.10
−0.14
−0.121
−0.167
−0.11
−0.16
−0.123
−0.188
82.24
0.80
10.95
2.68
220.47
24.13
24.13
−18.81
−27.23
−41.47
−60.04
−3.21
−4.83
−7.08
−10.65
−0.04
−0.07
−0.085
−0.145
−21.27
−28.62
−46.9
−63.1
−0.27
−0.77
−0.605
−1.696
−0.10
−0.14
−0.231
−0.318
−0.11
−0.16
−0.234
−0.359
84.75
0.08
1.75
3.22
273.20
4.79
4.79
−5.12
−15.22
−14
−41.57
−0.75
−2.64
−2.05
−7.22
−0.00
−0.03
−0.004
−0.088
−6.68
−16.18
−18.26
−44.22
−0.03
−0.47
−0.073
−1.293
−0.04
−0.08
−0.098
−0.231
−0.04
−0.10
−0.115
−0.280
† The volume is in fact a calculated area of these zones with a lateral continuity of 1 km. HE stands for hyperextended domain.
AVE
SUM
HE
Statistics – all
Z1
Total
AVE
STDEV
7.88
4.43
−9.69
−19.22
7.10
6.24
−1.57
−3.37
1.28
1.13
−0.014
−0.04
0.019
0.02
−11.55
−20.33
7.57
6.45
−0.11
−0.57
0.13
0.15
−0.06
−0.10
0.04
0.03
−0.06
0.03
6
7.9
737.18
7.91
2.86
2198.25
121.27
121.27
−11.97
−21.23
−1.98
−3.74
−0.02
−0.05
−13.98
−22.40
−0.15
−0.62
−0.07
−0.11
−0.07
−0.13
−180.4
−394.01
0.00
−28.66
−68.95
−0.219
−0.876
−219.14
−417.36
−1.815
−11.87
−1.127
−2.152
−1.240
−2.548
84.18
0.34
5.02
3.03
255.12
12.81
12.81
−5.12
−15.22
−13.07
−38.82
−0.75
−2.64
−1.91
−6.75
−0.002
−0.032
−0.004
−0.082
−6.68
−16.18
−17.05
−41.29
−0.027
−0.473
−0.068
−1.207
−0.036
−0.084
−0.091
−0.215
−0.042
−0.102
−0.107
−0.261
1183.85
3511.21
205.24
205.24
−277.95
−620.4
−43.94
−108.50
−0.322
−1.373
−339.17
−657.38
−2.71
−18.74
−1.75
−3.39
−1.93
−4.02
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
Degree of serpentinization
Hydration (%)
Density (g/cc)
Mass – rock (109 ton)
Mass – seawater (109 ton)
Volume – water (km3 )
SiO2 – Ref. Al (%)
SiO2 – Ref. Ti (%)
SiO2 – Ref. Al (109 ton)
SiO2 – Ref. Ti (109 ton)
FeO – Ref. Al (%)
FeO – Ref. Ti (%)
FeO – Ref. Al (109 ton)
FeO – Ref. Ti (109 ton)
MnO – Ref. Al (%)
MnO – Ref. Ti (%)
MnO – Ref. Al (109 ton)
MnO – Ref. Ti (109 ton)
MgO – Ref. Al (%)
MgO – Ref. Ti (%)
MgO – Ref. Al (109 ton)
MgO – Ref. Ti (109 ton)
CaO – Ref. Al (%)
CaO – Ref. Ti (%)
CaO – Ref. Al (109 ton)
CaO – Ref. Ti (ton)
Ni – Ref. Al (%)
Ni – Ref. Ti (%)
Ni – Ref. Al (ton)
Ni – Ref. Ti (ton)
Cr – Ref. Al (%)
Cr – Ref. Ti (%)
Cr – Ref. Al (ton)
Cr – Ref. Ti (ton)
Embryonic oceanic domain
Z13
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
Table 3
Volume of absorbed water and mass of lost element.
2
Area iberia (km )
Ave-thick (km)
Vol (km3 )
Hydration (%)
Volume H2 O (km3 )
Mass serpentinite (ton)
SiO2 – Ref. Al2 O3 (ton)
SiO2 – Ref. TiO
FeO – Ref. Al2 O3
FeO – Ref. TiO
MnO – Ref. Al2 O3
MnO – Ref. TiO
MgO – Ref. Al2 O3
MgO – Ref. TiO
CaO – Ref. Al2 O3
CaO – Ref. TiO
Ni – Ref. Al2 O3
Ni – Ref. TiO
Cr – Ref. Al2 O3
Cr – Ref. TiO
233
Table 4
Percentage and mass of gain and loss.
Hyperextended
domain
Exhumed +
Embryonic
domaina
Total
11872
2.96
35141
5.02
1765
1 × 1014
−5 × 1012
−2 × 1013
−8 × 1011
−3 × 1012
−2 × 109
−3 × 1010
−7 × 1012
−2 × 1013
−3 × 1010
−5 × 1011
−4 × 1010
−9 × 1010
−4 × 1010
−1 × 1011
33134
5.46
180912
8.52
15377
5 × 1014
−6 × 1013
−1 × 1014
−1 × 1013
−2 × 1013
−1 × 1011
−2 × 1011
−7 × 1013
−1 × 1014
−8 × 1011
−3 × 1012
−4 × 1011
−6 × 1011
−4 × 1011
−7 × 1011
45006
–
216053
–
17142
6 × 1014
−7 × 1013
−1 × 1014
−1 × 1013
−2 × 1013
−1 × 1011
−3 × 1011
−8 × 1013
−1 × 1014
−8 × 1011
−4 × 1012
−4 × 1011
−7 × 1011
−4 × 1011
−8 × 1011
a
This area is the sum of the zone of exhumed mantle plus embryonic oceanic
domain until the M0 magnetic anomaly.
or Ti as immobile, provide the range of values of losses caused
by serpentinization that we consider realistic. The summation, average and standard deviation were calculated in order to give an
estimate of the range of masses of water and elements involved in
each zone.
5. Amount of water absorbed in the zone of exhumed
continental mantle calculated based on geological and
geophysical data
The IAM-9 reflection and refraction seismic data were used to
estimate the amount of hydration and element transfer related
to the formation of the zone of exhumed continental mantle. Although the exhumed mantle domain may contain other igneous
rocks, such as basalts or gabbros, ODP drilling showed that these
rocks are rare or absent continentwards of the J-anomaly. Thus,
the serpentinization is assumed to start at km 270 and extend to
km 30, i.e. from the mantle underlying the hyperextended crust to
the J-anomaly (Fig. 2).
From refraction seismic data, Minshull et al. (2014) showed
that the thickness of the serpentinized mantle is about 5–6 km
(Fig. 2A). Although the gradient of serpentinized peridotites
changes along the zone of exhumed continental mantle, the first
3 km below the top of the exhumed serpentinized basement (until 6.4 km/s) corresponds to a degree of serpentinization of more
than 50%. The average values of wt.% of water along the IAM-9 section (Table 2) range from 5% in the hyperextended domain, 8% in
the zone of exhumed continental mantle, to 9% in the embryonic
oceanic domain. The volume of H2 O for each domain is calculated by multiplying the area by the thickness by the percentage
of water (hydration) shown in Table 3. The total volume of H2 O absorbed in the mantle at the Central Iberian Margin is in the order
of 17 × 103 km3 (Tables 3 and 4). Given that the area of serpentinized mantle is 45,006 km2 and the volume of absorbed water is
17 × 103 km3 , about 0.38 km3 of water is absorbed per km2 of exhumed mantle. Thus, if serpentinization occurred in a closed basin,
without addition or evaporation of seawater, the sea-level would
drop by 380 m due to mantle serpentinization. It is important to
point out that this volume of trapped water in the serpentinized
mantle is lower than the value of a total flux of water, going in and
out of the mantle during the serpentinization reactions (Skelton et
IAM-9 seismic section
Water
SiO2
MgO
CaO
FeO
MnO
Ni
Cr
Central iberia margin
Avg. gain
and loss
(%)
STDEV
gain and
loss
(%)
Total mass
gained
and lost
(ton)
Minimum
mass gained
or lost
(ton)
Maximum
mass gained
or lost
(ton)
7.88%
−16.10%
−17.67%
−0.38%
−2.77%
−0.03%
−0.09%
−0.10%
4.43%
8.09%
8.14%
0.28%
1.49%
0.02%
0.04%
0.04%
2 × 1011
−4 × 1011
−5 × 1011
−1 × 1010
−8 × 1010
−8 × 108
−3 × 109
−3 × 109
–
2 × 1013
−1 × 1014
−1 × 1014
−4 × 1012
−2 × 1013
−3 × 1011
−7 × 1011
−8 × 1011
−7 × 1013
−8 × 1013
−8 × 1011
−1 × 1013
−1 × 1011
−4 × 1011
−4 × 1011
al., 2005). This explains the misfit between the numbers proposed
by Skelton et al. (2005) (calculated total flux) and the numbers
presented in this study (volume of absorbed water in the mantle).
Note that these results are impacted by the accuracy of the seismic velocity models and the assumption that the section is only
made of serpentinized mantle. While errors may exist the velocities interpreted by Dean and Minshull (2000) are derived from
the OBS stations which are also calibrated by well data. Therefore
the results presented here may provide a valuable quantification
of the volume of water stored in the zone of exhumed continental
mantle along the Central Western Iberia margin. Another important assumption is that the mineralogy of hydrated peridotites is
controlled mainly by serpentines. The occurrence of chamosite (8%
wt.% of water) and/or clinochlore (12% wt.% of water) would lower
the estimated volume of water. However, both minerals are rare
in the serpentinites drilled offshore Iberia and if present, the most
common member of the chlorite group is clinochlore (Beard and
Hopkinson, 2000), which has a similar content of water to serpentine.
6. Mass budget and element transfer in the zone of exhumed
continental mantle
A simple mass balance calculation was done for element transfer during serpentinization and formation of a zone of exhumed
continental mantle using data from Iberia and the Alps. At present,
the remnants of the zone of exhumed continental mantle of the
fossil Tethyan margin exposed in the Alps are the best field analogues for present-day zones of exhumed continental mantle at
magma poor rifted margins. This is well supported by the comparison between drilled mantle samples from Iberia and outcrop
samples from the Alps (Manatschal and Müntener, 2009). Using
samples from the Alps, the P –M diagrams enabled the calculation of the percentage of element losses due to different degrees
of serpentinization. The results (Table 4) show that the values
calculated from aluminum are lower than those calculated from
titanium. The chemical changes in such highly altered environments are complex and the term “immobile” refers to elements
that have relatively lower mobility compared to others. Nevertheless, despite differences between zones of exhumed continental
mantle and slow spreading ridges (e.g. thermal state, thickness and
nature of the lithosphere, and magma budget), it is interesting to
note that the results presented here are similar to those of midocean ridges (e.g. fluid enriched in Mg and Ca; Kelley et al., 2001;
Cannat et al., 2010; Edmonds, 2010). Laboratory experiments,
which simulate serpentinization also produce a fluid enriched in
these elements (Daval et al., 2011; Ogasawara et al., 2013). However, in contrast to present-day mid-ocean ridges, the zones of exhumed continental mantle are formed during early stages of plate
separation, i.e. at stages when these domains were not necessarily
connected to the global oceans. Therefore, the implications of the
It was assumed the volume of world oceans as 1.34 × 109 km3 .
d
Volume of basin at ∼112 Ma is 4.5 × 105 km3 , which is the sum of hyperextended and exhumed mantle domains.
Volume of basins at 112 Ma is 1.7 × 105 km3 and at 117 Ma is 1.2 × 105 km3 based on GPlate reconstruction model.
c
Values from river discharge are from: Si minimum (Tréguer et al., 1995), Si maximum (Dürr et al., 2011), Mg and Ca (Wilkinson and Algeo, 1989). These fluxes are reported in mol/yr or mol/My. We transformed in mass/yr in
order to compare them with our results.
a
6 × 1013
8 × 1013
3 × 1012
2 × 1013
2 × 1011
7 × 1011
8 × 1011
Si
Mg
Ca
Fe
Mn
Ni
Cr
b
0.28
0.14
0.40
nd
nd
nd
nd
0.10
0.09
0.36
nd
nd
nd
nd
371.00
184.80
531.20
nd
nd
nd
nd
136.64
124.80
483.60
nd
nd
nd
nd
24.45
33.26
1.10
7.10
0.09
0.28
0.33
13.07
19.84
0.24
3.54
0.03
0.17
0.18
48.90
66.52
2.20
14.20
0.18
0.57
0.66
26.14
39.69
0.48
7.08
0.07
0.33
0.36
17.27
23.50
0.78
5.02
0.07
0.20
0.23
9.23
14.02
0.17
2.50
0.02
0.12
0.13
34.55
46.99
1.55
10.03
0.13
0.40
0.47
18.47
28.04
0.34
5.00
0.05
0.24
0.25
88.63
120.56
3.98
25.74
0.33
1.03
1.20
47.38
71.93
0.87
12.83
0.12
0.61
0.65
177.25
241.12
7.97
51.48
0.67
2.06
2.40
94.76
143.85
1.73
25.67
0.25
1.21
1.29
−3 × 106
−4 × 106
−1 × 105
−8 × 105
−1 × 104
−3 × 104
−4 × 104
−2 × 106
−2 × 106
−3 × 104
−4 × 105
−4 × 103
−2 × 104
−2 × 104
−6 × 106
−8 × 106
−3 × 105
−2 × 106
−2 × 104
−7 × 104
−8 × 104
Min
3 × 1013
5 × 1013
6 × 1011
8 × 1012
8 × 1010
4 × 1011
4 × 1011
−3 × 106
−5 × 106
−6 × 104
−8 × 105
−8 × 103
−4 × 104
−4 × 104
Min
Max
(×10 ton/yr)
Min
(20 Ma)
Max
(10 Ma)
(ton/yr/km )
Min
(10 Ma)
Max
(10 Ma)
Min
(10 Ma)
Max
(10 Ma)
Min
(10 Ma)
Minimum
(10 Ma)
Max mass
lost
Min mass
lost
(ton/yr)
Maximum
(10 Ma)
Minimum
(20 Ma)
Maximum
(20 Ma)
3
(ton/yr/km )
Min
(20 Ma)
Max
(20 Ma)
3
(ton/yr/km )
Min
(20 Ma)
Max
(20 Ma)
3
117 Ma = 1.2 × 105 km3
112 Ma = 4.5 × 104 km3
Serpentinization fluxes
Loss of elements due
to serpentinization
(ton)
Central Iberiaa
where T e is the total element transfer, v b the volume of the basin
and tm the time of mantle exhumation. The total element transfer
is taken from Table 3, calculated weight percentage of elements
rather than oxides.
Taking the example of minimum mass of lost Si, it can be estimated that Si = −3.14 × 1013 ton/33,134 km3 /10 Myr, which result
in T = −95 ton km−3 yr−1 . Because the timing of mantle exhumation and serpentinization is not well constrained, two end member
values have been chosen: 10 Myr for fast exhumation and 20 Myr
for slow exhumation (values in Table 5).
The paleogeography of the North Atlantic at the time of mantle
exhumation points to an open seawater circulation (see Fig. 1 from
Tucholke and Sibuet, 2007), which may have resulted in element
exchange with the global ocean. The importance of the element
transfer due to serpentinization may therefore be more important
in a closed and restricted basin that was separated from the global
ocean, such as the central South Atlantic prior to its breakup.
Therefore, this basin was used as an example for testing the impact of these values and their potential implications. At Aptian
time, the proto-South Atlantic was a restricted basin (Dias, 2005;
Moulin et al., 2010), where seawater circulation was controlled by
the Rio Grande–Walvis high at the south and the Maceio high at
the north (Azevedo, 2004). The total area of the evaporite basin
(e.g. GPlate reconstruction model; Fig. 5) was 1.2 × 106 km2 and
1.7 × 106 km2 at 117 Ma and 112 Ma respectively. The deposition of evaporites occurs at extreme conditions of evaporation at
shallow water depth (usually at less than 100 m; Christeleit et al.,
2015 and references therein). Using this information and applying Eq. (2), the concentration of elements can be calculated within
the basin, assuming mantle exhumation and serpentinization similar to that reported from the Iberia margin (Unternehr et al., 2010;
Sutra et al., 2013). Our calculations show that in restricted basins,
such as the proto-central South Atlantic at Aptian times, the transfer of Si due to serpentinization would be up to 175 times higher
than the dissolved global Si derived from river discharge. For Mg
and Ca it is up to 475 and 5 times higher, respectively (Table 5).
112 Ma = 1.7 × 105 km3
South Atlanticb
(2)
Table 5
Rate of element transfer from the mantle to the seawater and element discharge from rivers.
T = T e / v b /tm
Max
(20 Ma)
A comparative analysis between element transfer due to serpentinization and the dissolved Si, Mg and Ca from river discharge
in the World Ocean (Wilkinson and Algeo, 1989; Tréguer et al.,
1995; Dürr et al., 2011) is presented in Table 5. As an example,
the fluxes of Si and Mg related to serpentinization calculated for
the Iberia margin are two orders of magnitude lower than the
fluxes of the World’s river discharge, and three to four orders of
magnitude for Ca. Note that the time range and the size of basins
(i.e. oceans, seas, restricted basins) may be very different. Therefore, standard ratios must be used in both cases, here ton/km3 /yr.
In fact, this ratio expresses the dissolved elements within these
basins. To calculate these values, it is necessary to know the time
span over which the basin formed and volume of the basin. In the
Central Western Iberia margin, mantle exhumation occurred between Barremian to Aptian time, which corresponds approximately
to 10–20 Myr (Sutra et al., 2013). The area of exhumed mantle
corresponds to approximately 33,134 km2 . Although not directly
constrained, the average paleo-water depth across the margin was
probably less than 1 km at the stage of exhumation (Esedo et al.,
2012). We assume 1 km water depth. This gives a seawater volume
of 33,134 km3 . The element transfer (T ) in ton/km3 /yr is estimated
using Eq. (2):
6
World river
6.1. A comparative analysis of mantle-derived elements and river
discharge elements
Discharge fluxesc
Dissolved in
oceansd
(ton/yr/km3 )
mass transfer described here are only discussed for early stages of
plate separation.
Max
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
Element
234
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
235
Fig. 5. Plate reconstruction of the South Atlantic according to GPlates model software (www.gplates.org). Two maps are made for estimating the area of the basin, because
of age uncertainties and the temporal variation of salt deposition along the margins. (A) at 117 Ma the restricted basins (yellow) has ∼1.2 × 106 km2 and ∼1.2 × 105 km3 .
(B) at 112 Ma the restricted basins (yellow) has ∼1.7 × 106 km2 and ∼1.7 × 105 km3 . The volume of water in the basin was calculated with a water column of 100 m. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
7. Implications of mantle exhumation and serpentinization
during early plate separation
7.1. Element budget in nascent oceanic basins
Serpentinization may be an important source of elements such
as Si, Mg and Ca (Table 5) in nascent oceanic basins (prior to
seafloor spreading) yet disconnected from the global oceans. The
importance of mantle serpentinization is supported by direct observations, including: (i) hydrothermal cherts deposited over exhumed mantle in the Alpine Tethys (Perseil and Latouche, 1989);
and (ii) eodiagenetic quartz related to serpentinization fluids in the
Alps and Iberia (Haggerty and Germann, 1988; Pinto et al., 2015).
This is in contrast to the mid-Atlantic ridge, where the transfer of
these elements occurs over long time frames (Kelley et al., 2001;
Edmonds, 2010 and references therein) and the elements are diluted in the global oceans (Saito et al., 2013). During the early
stages of plate separation, the expulsion of these elements may occur over a short time interval, and in certain geological scenarios,
such as the central Southern Atlantic, under restricted conditions
(Azevedo, 2004; Dias, 2005; Moulin et al., 2010). Nevertheless, further studies are necessary to quantify the mass of mantle-derived
elements dissolved or precipitated as well as their residence time
in these nascent oceanic basins to better understand the environmental impact that serpentinization may have caused during
lithospheric breakup.
7.2. Depositional environment
The results shown in Table 5 provide a first order estimate
of the fluxes of elements derived from the mantle due to serpentinization during formation of a zone of exhumed continental
mantle. These elements can be either dissolved in seawater and/or
be precipitated in the sedimentary basins. In the Alpine Tethys,
for example, the radiolarian cherts occur over the zone of exhumed continental mantle as well as in the shallower parts of
the proximal margins, which suggests that they were not, as commonly interpreted (Bernoulli and Jenkyns, 2009), related to calcite
dissolution below the CCD. Radiolarian cherts are Si rich sediments that overlap in time with the late Middle to early Late
Jurassic mantle exhumation and serpentinization in the Alpine
domain. In addition to the Radiolarian cherts, we observe that
syn-exhumation siliciclastic sediments are highly silicified which
also point to early cementation caused by Si-enriched fluids in
the Alps during Middle Jurassic time (Pinto et al., 2015). Similar
silicification is described in the central Iberia margin, where silicification (eodiagenesis) is observed in Aptian carbonate-bearing
rocks (Haggerty and Germann, 1988). In both cases, silicification is
accompanied by oxyhydroxides and/or carbonates of Fe, Mn, Mg
and Ca. Based on mineral stability and isotopic data, the temperature of the hydrothermal fluids associated with serpentinisation fluids can be up to 135 ◦ C (Haggerty and Germann, 1988;
Pinto et al., 2015). These observations together with other geochemical, geological and geophysical analyses, led Pinto et al.
(2015) to conclude that these fluids are produced by serpentinization during mantle exhumation in the zone of exhumed continental mantle of the Alpine Tethys.
Apart from the age and the size of the basins, there are first order similarities between the Alpine Tethys and in the proto-Central
South Atlantic at a pre-oceanic stage. Because the proto-central
South Atlantic was a restricted basin, this may have implications
for the depositional environments, diagenesis and the biosphere
in areas where mantle exhumation occurred during Aptian time
(Unternehr et al., 2010). Some basins in the deep Brazilian margins are marked by a thick Aptian sequence of peculiar limestones
and dolomites of up to 800 m (Buckley et al., 2015). Some sedimentary structures in this sequence points to both inorganic and
organic origin (Wright, 2012; Petersohn et al., 2013). Whatever the
nature of these carbonate rocks, the question remains whether the
Ca and Mg necessary to build the limestones and dolomites over
a time span of less than 10 Myr was related to fluids derived
from serpentinization. Moreover, it is interesting to note that these
carbonates show a strong syn-depositional to early diagenetic silicification caused by Si-rich fluids (Petersohn et al., 2013).
There is a growing database gained from the analysis of sediments deposited within deep water basins, exposed in the Alps
and/or drilled in Iberia or in the Southern Central Atlantic, which
hint at Si-rich environments. The question arises whether there is
a correlation between the excess of Si, Mg and Ca in the deposition of carbonates and radiolarian cherts, eodiagenetic processes,
and the exhumation of mantle and serpentinization during the
formation of the zone of exhumed continental mantle. Our study
provides first estimates that this could be the case, however, more
detailed work is necessary to further constrain the rates, timing
and the provenience of the fluids that may strongly influence the
depositional environments, diagenesis, production and preservation of organic matter and therefore impacting the elements of
petroleum systems (e.g., reservoir, seal and source rock).
236
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
7.3. Paleoclimate studies
Another implication of the results presented here (Table 4) is
related to paleoclimate research. Mg/Ca ratios are used as a proxy
for paleo-temperature evaluation (e.g., Evans and Müller, 2012).
Therefore, the amount of Mg, and to a lesser extent Ca, released
into the hydrosphere should be taken into account for paleoclimate studies. For example, Mg loss can be 30 to 83 times higher
than Ca loss during exhumation and serpentinization (Table 5).
This may suggest that Mg/Ca ratios from the Early to Middle Cretaceous were higher than in the Cenozoic (Ligi et al., 2013).
Although not quantified in this study, it is known that serpentinization can lead to a considerable release of CH4 (Kelley et
al., 2001; Früh-Green et al., 2004; Cannat et al., 2010). Because
more than 50% of passive margins involved mantle exhumation
during the Cretaceous (Reston and Manatschal, 2011), the release
of CH4 due to mantle serpentinization could have had an impact
on climate change, because methane is a 20 times more efficient
heating-trap than carbon dioxide (Beerling et al., 2009).
7.4. Rheology
Massive serpentinization (hydration) has also an impact on the
mantle rheology since it weakens the uppermost part of the mantle where the yield strength of 250 MPa in peridotites is reduced to
about 130 MPa in serpentinite (see Reston and Manatschal, 2011
and references therein), which corresponds to a reduction of the
yield strength of about 52%. This may exert a control on fault
systems and influence the style of extension associated with mantle exhumation or subsequent uplift (Pérez-Gussinyé and Reston,
2001; Skelton and Jakobsson, 2007).
7.5. Chemical exchange between Earth reservoirs
Last but not least, exhumed mantle domains may eventually
be subducted back into the mantle and, as a consequence, the
stored water may be released in the subduction zone impacting
not only on the early evolution of orogens (Reston and Manatschal,
2011) but also the magmatic budget along arcs. The H2 O retained
is not only an important fluid source triggering arc magmatism
(Deschamps et al., 2013), but it is also an efficient transport-agent
to recycle chemical components in the upper crust and deeper in
the mantle (Manning, 2004; Deschamps et al., 2013). The cyclic
mantle exhumation accompanied by serpentinization, water absorption, element transfer and its later subduction must therefore
have a substantial impact on changing the geochemistry of major
Earth reservoirs during the evolution of the planet. Although this
is generally accepted, so far little has been done to quantify these
budgets related to mantle serpentinization in zones of exhumed
continental mantle.
8. Conclusions
In this work, we determine the volume of water retained in serpentinized mantle formed during exhumation of continental mantle. Refraction and reflection seismic and drill hole data from the
present day Iberia margin was used to estimate the volume of
serpentinized mantle calibrated by peridotites with wide ranges
of serpentinization from the fossil Alpine Tethys margin exposed
in the Alps. The calculations allow estimates of the amount of
element transfer from the mantle into the overlying reservoirs
(sediments and seawater) during the serpentinization and the exhumation of the continental mantle during early plate separation.
Although the approach developed here is simple, it is important
to note that it uses a unique data set and is based on the study
of the two best investigated zones of exhumed continental mantle
worldwide exposed in the Alps and drilled offshore Western Iberia.
The results may be extrapolated to other magma-poor rifted margins, which are about 50% of the present-day rifted margins. As
discussed here, the impact of water stored and element transfer
from the mantle during serpentinization in zones of exhumed continental mantle may be significant, especially for basins that were
restricted and separated from the global ocean during their formation (e.g. Central South Atlantic). The observations to date suggest
that serpentinization and the related transfer of mantle-derived
elements (e.g. Si, Mg, Ca) may play a major role in the evolution of these closed embryonic oceanic basins. The impact may
be particularly important for restricted and isolated environments,
as suggested for the central South Atlantic during the embryonic
stage of its formation. However, enhanced serpentinization during
formation of zones of exhumed continental mantle may also have
a global impact on some environmentally important elements (e.g.
Si, Mg, Ca, Fe, Ni, Cr) over short time spans (10 to 20 Myr) causing local changes in sedimentary environments, and their early
diagenetic evolution (e.g. dolomitisation and silicification). The serpentinization in zones of exhumed continental mantle may control
subduction initiation due to the occurrence of weak, low frictional
material (serpentinite). Furthermore, the de-watering of serpentinites during early stages of subduction may have an impact on
the H2 O budget that may be an important factor controlling the
rheology and magmatic evolution of incipient subduction systems.
Acknowledgements
The authors are grateful to the financial support of Petrobras
S.A. and to the useful remarks of the reviewers and editor. We also
thank the editor and Philip Ball for improving the English and for
comments.
Appendix A. Supplementary material
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2016.11.023.
References
Azevedo, R.L.M., 2004. Paleoceanografia e a evolução do Atlântico Sul no Albiano.
Bol. Geociênc. PETROBRAS 12, 231–249.
Bach, W., Paulick, H., Garrido, C.J., Ildefonse, B., Meurer, W.P., Humphris, S.E., 2006.
Unraveling the sequence of serpentinization reactions: petrography, mineral
chemistry, and petrophysics of serpentinites from MAR 15◦ N (ODP Leg 209, Site
1274). Geophys. Res. Lett. 33, 4–7.
Beard, J.S., Hopkinson, L., 2000. A fossil, serpentinization-related hydrothermal vent,
Ocean Drilling Program Leg 173, Site 1068 (Iberia Abyssal Plain): some aspects
of mineral and fluid chemistry. J. Geophys. Res. 105 (B7), 16,527–16,539. http://
dx.doi.org/10.1029/2000JB900073.
Beerling, D., Berner, R.A., Mackenzie, F.T., Harfoot, M.B., Pyle, J.A., 2009. Methane
and the CH4 related greenhouse effect over the past 400 million years. Am. J.
Sci. 309 (2), 97–113. http://dx.doi.org/10.2475/02.2009.01.
Bernoulli, D., Jenkyns, H.C., 2009. Ancient oceans and continental margins of the
Alpine-Mediterranean Tethys: deciphering clues from Mesozoic pelagic sediments and ophiolites. Sedimentology 56, 149–190.
Boschi, C., Früh-Green, G.L., Delacour, A., Karson, J.A., Kelley, D.S., 2006. Mass transfer and fluid flow during detachment faulting and development of an oceanic
core complex, Atlantis Massif (MAR 30◦ N). Geochem. Geophys. Geosyst. 7, 1–39.
http://dx.doi.org/10.1029/2005GC001074.
Bronner, A., Sauter, D., Manatschal, G., Péron-Pinvidic, G., Munschy, M., 2011. Magmatic breakup as an explanation for magnetic anomalies at magma-poor rifted
margins. Nat. Geosci. 4, 549–553.
Buckley, J.P., Bosence, D., Elders, C., 2015. Tectonic setting and stratigraphic architecture of an Early Cretaceous lacustrine carbonate platform, Sugar Loaf High,
Santos Basin, Brazil. Geol. Soc. (Lond.) Spec. Publ. 418, 175.
Cannat, M., Fontaine, F., Escartín, J., 2010. Serpentinization and associated hydrogen
and methane fluxes at slow spreading ridges. In: Rona, P.A., Devey, C.W., Dyment, J., Murton, B.J. (Eds.), Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges. In: Geophys. Monogr. AGU, pp. 241–264.
V.H.G. Pinto et al. / Earth and Planetary Science Letters 459 (2017) 227–237
Christeleit, E.C., Brandon, M.T., Zhuang, G., 2015. Evidence for deep-water deposition
of abyssal Mediterranean evaporites during the Messinian salinity crisis. Earth
Planet. Sci. Lett. 427, 226–235.
Daval, D., Sissmann, O., Menguy, N., Saldi, G.D., Guyot, F., Martinez, I., Corvisier,
J., Garcia, B., Machouk, I., Knauss, K.G., Hellmann, R., 2011. Influence of amorphous silica layer formation on the dissolution rate of olivine at 90 ◦ C and
elevated pCO2 . Chem. Geol. 284, 193–209. http://dx.doi.org/10.1016/j.chemgeo.
2011.02.021.
Dean, S., Minshull, T., 2000. Deep structure of the ocean–continent transition in the
southern Iberia Abyssal Plain from seismic refraction profiles: the IAM-9 transect at 40◦ 20 N. J. Geophys. Res. 105, 5859–5885.
Deschamps, F., Godard, M., Guillot, S., Hattori, K., 2013. Geochemistry of subduction zone serpentinites: a review. Lithos 178, 96–127. http://dx.doi.org/10.1016/
j.lithos.2013.05.019.
Dias, J.L., 2005. Tectônica, estratigrafia e sedimentação no Andar Aptiano da margem
leste brasileira. Bol. Geociênc. PETROBRAS 13, 7–25.
Dürr, H.H., Meybeck, M., Hartmann, J., Laruelle, G.G., Roubeix, V., 2011. Global spatial distribution of natural riverine silica inputs to the coastal zone. Biogeosciences 8, 597–620. http://dx.doi.org/10.5194/bg-8-597-2011.
Edmonds, H.N., 2010. Chemical signatures from hydrothermal venting on slow
spreading ridges. In: Rona, P.A., Devey, C.W., Dyment, J., Murton, B.J. (Eds.), Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges. In: Geophys.
Monogr. AGU, pp. 27–42.
Esedo, R., Wijk, J., Van Coblentz, D., Meyer, R., 2012. Uplift prior to continental breakup: indication for removal of mantle lithosphere? Geosphere 8,
1078–1085. http://dx.doi.org/10.1130/GES00748.1.
Evans, D., Müller, W., 2012. Deep time foraminifera Mg/Ca paleothermometry: nonlinear correction for secular change in seawater Mg/Ca. Paleoceanography 27,
1–11. http://dx.doi.org/10.1029/2012PA002315.
Fouquet, Y., Cambon, P., Etoubleau, J., Charlou, J.L., Ondréas, H., Barriga, F.J.A.S.,
Cherkashov, G., Semkova, T., Poroshina, I., Bohn, M., Donval, J.P., Henry, K., Murphy, P., Rouxel, O., 2010. Geodiversity of hydrothermal processes along the
Mid-Atlantic Ridge and ultramafic-hosted mineralization: a new type of oceanic
Cu–Zn–Co–Au volcanogenic massive sulfide deposit. In: Rona, P.A., Devey, C.W.,
Dyment, J., Murton, B.J. (Eds.), Diversity of Hydrothermal Systems on Slow
Spreading Ocean Ridges, AGU. In: Geophys. Monogr., pp. 321–367.
Früh-Green, G.L., Connolly, J.A.D., Plas, A., Kelley, D.S., Grobéty, B., 2004. Serpentinization of oceanic peridotites: implications for geochemical cycles and biological activity. In: Wilcock, W.S.D., et al. (Eds.), The Subseafloor Biosphere at
Mid-Ocean Ridges, pp. 119–136.
Gibson, I.L., Beslier, M.-O., Cornen, G., Milliken, K.L., Seifert, K.E., 1996. Major- and
trace-element seawater alteration profiles in serpentinite formed during the development of the Iberia Margin, Site 897. In: Whitmarsh, R.B., Sawyer, D.S.,
Klaus, A., Masson, D.G. (Eds.), Proc. ODP, Sci. Results, vol. 149, pp. 519–527.
Haggerty, J.A., Germann, S.H., 1988. Resedimentation and diagenesis, including silicification, of Barremian-Aptian shallow-water carbonates from the Galicia Margin,
Eastern North Atlantic, at Ocean Drilling Program Site 641. In: Boillot, G., Winterer, E.L., Meyer, A.W., Applegate, J. (Eds.), Proc. ODP, Sci. Results, vol. 103,
pp. 513–530.
Kelley, D.S., Karson, J.A., Blackman, D.K., Früh-Green, G.L., Butterfield, D.A., Lilley,
M.D., Olson, E.J., Schrenk, M.O., Roe, K.K., Lebon, G.T., Rivizzigno, P., 2001. An offaxis hydrothermal vent field near the Mid-Atlantic Ridge at 30◦ N. Nature 412,
145–149. http://dx.doi.org/10.1038/35084000.
Ligi, M., Bonatti, E., Cuffaro, M., Brunelli, D., 2013. Post-Mesozoic rapid increase of
seawater Mg/Ca due to enhanced mantle–seawater interaction. Sci. Rep. 3, 2752.
http://dx.doi.org/10.1038/srep02752.
Manatschal, G., Müntener, O., 2009. A type sequence across an ancient magma-poor
ocean–continent transition: the example of the western Alpine Tethys ophiolites. Tectonophysics 473, 4–19. http://dx.doi.org/10.1016/j.tecto.2008.07.021.
Manning, C.E., 2004. The chemistry of subduction-zone fluids. Earth Planet. Sci.
Lett. 223, 1–16. http://dx.doi.org/10.1016/j.epsl.2004.04.030.
Miller, D.J., Christensen, N.I., 1997. Seismic velocities of lower crustal and upper
mantle rocks from the slow-spreading Mid-Atlantic Ridge south of the Kane
Transform Zone (MARK). In: Karson, J.A., Cannat, M., Miller, D.J., Elthon, D. (Eds.),
Proc. ODP, Sci. Results, vol. 153, pp. 437–454.
Milliken, K., Morgan, J., 1996. Chemical evidence for near-seafloor precipitation of
calcite in serpentinites (Site 897) and serpentinite breccias (Site 899), Iberia
Abyssal Plain. In: Whitmarsh, R.B., Sawyer, D.S., Klaus, A., Masson, D.G. (Eds.),
Proc. ODP, Sci. Results, vol. 149, pp. 553–558.
Minshull, T.A., Dean, S.M., Whitmarsh, R.B., 2014. The peridotite ridge province
in the southern Iberia Abyssal Plain: seismic constraints revisited. J. Geophys.
Res. 119, 1580–1598. http://dx.doi.org/10.1002/2014JB011011.
237
Moulin, M., Aslanian, D., Unternehr, P., 2010. A new starting point for the South and
Equatorial Atlantic Ocean. Earth-Sci. Rev. 98, 1–37.
Müntener, O., Manatschal, G., 2006. High degrees of melt extraction recorded by
spinel harzburgite of the Newfoundland margin: the role of inheritance and
consequences for the evolution of the southern North Atlantic. Earth Planet. Sci.
Lett. 252, 437–452. http://dx.doi.org/10.1016/j.epsl.2006.10.009.
Müntener, O., Manatschal, G., Desmurs, L., Pettke, T., 2010. Plagioclase peridotites
in ocean–continent transitions: refertilized mantle domains generated by melt
stagnation in the shallow mantle lithosphere. J. Petrol. 51, 255–294. http://
dx.doi.org/10.1093/petrology/egp087.
Ogasawara, Y., Okamoto, A., Hirano, N., Tsuchiya, N., 2013. Coupled reactions and silica diffusion during serpentinization. Geochim. Cosmochim. Acta 119, 212–230.
http://dx.doi.org/10.1016/j.gca.2013.06.001.
Pérez-Gussinyé, M., Reston, T.J., 2001. Rheological evolution during extension at
nonvolcanic rifted margins: onset of serpentinization and development of detachments leading to continental breakup. J. Geophys. Res. 106 (B3), 3961–3975.
http://dx.doi.org/10.1029/2000JB900325.
Perseil, E.A., Latouche, L., 1989. Découverte de microstructures de nodules polymétaliques dans les minéralisations manganésifères métamorphiques de Falotta et
de Parsettens (Grisons-Suisse). Miner. Depos. 24, 111–116.
Petersohn, E., Abelha, M., Pedrosa, L., 2013. Brasil pre-salt 1: Libra. In: 1◦ Licitação
de Partilha de Produção. ANP, Rio de Janeiro.
Pinto, V.H., Manatschal, G., Karpoff, A.M., Viana, A., 2015. Tracing mantle-reacted
fluids in magma-poor rifted margins: the example of Alpine Tethyan rifted
margins. Geochem. Geophys. Geosyst. 16, 3271–3308. http://dx.doi.org/10.1002/
2015GC005830.
Potdevin, J.L., Marquer, D., 1987. Quantitative methods for the estimation of mass
transfers by fluids in deformed metamorphic rocks. Geodin. Acta 1, 193–206.
Reston, T., Manatschal, G., 2011. Rifted margins: building blocks of later collision. In:
Brown, D., Ryan, P.D. (Eds.), Arc-Continent Collision, Frontiers in Earth Sciences.
Springer-Verlag, Berlin, Heidelberg, pp. 3–21.
Saito, M.A., Noble, A.E., Tagliabue, A., Goepfert, T.J., Lamborg, C.H., Jenkins, W.J., 2013.
Slow-spreading submarine ridges in the South Atlantic as a significant oceanic
iron source. Nat. Geosci. 6, 775–779. http://dx.doi.org/10.1038/ngeo1893.
Schmidt, K., Koschinsky, A., Garbe-Schönberg, D., de Carvalho, L.M., Seifert, R., 2007.
Geochemistry of hydrothermal fluids from the ultramafic-hosted Logatchev hydrothermal field, 15◦ N on the Mid-Atlantic Ridge: temporal and spatial investigation. Chem. Geol. 242, 1–21. http://dx.doi.org/10.1016/j.chemgeo.2007.01.023.
Seyfried Jr., W.E., Foustoukos, D.I., Fu, Q., 2007. Redox evolution and mass transfer during serpentinization: an experimental and theoretical study at 200 ◦ C,
500 bar with implications for ultramafic-hosted hydrothermal systems at MidOcean Ridges. Geochim. Cosmochim. Acta 71, 3872–3886. http://dx.doi.org/
10.1016/j.gca.2007.05.015.
Skelton, A., Jakobsson, M., 2007. Could peridotite hydration reactions have provided
a contributory driving force for Cenozoic uplift and accelerated subsidence along
the margins of the North Atlantic and Labrador Sea? Norwegian J. Geol. 87,
241–248.
Skelton, A., Whitmarsh, R., Arghe, F., Crill, P., Koyi, H., 2005. Constraining the rate
and extent of mantle serpentinization from seismic and petrological data: implications for chemosynthesis and tectonic processes. Geofluids 5, 153–164.
Sutra, E., Manatschal, G., 2012. How does the continental crust thin in a hyperextended rifted margin? Insights from the Iberia margin. Geology 40, 139–142.
http://dx.doi.org/10.1130/G32786.1.
Sutra, E., Manatschal, G., Mohn, G., Unternehr, P., 2013. Quantification and restoration of extensional deformation along the Western Iberia and Newfoundland
rifted margins. Geochem. Geophys. Geosyst. 14, 2575–2597. http://dx.doi.org/
10.1002/ggge.20135.
Tréguer, P., Nelson, D.M., Van Bennekom, A.J., Demaster, D.J., Leynaert, A., Quéguiner,
B., 1995. The silica balance in the world ocean: a reestimate. Science 268,
375–379.
Tucholke, B.E., Sibuet, J., 2007. Leg 210 synthesis: tectonic, magmatic, and sedimentary evolution of the Newfoundland–Iberia rift. In: Tucholke, B.E., Sibuet, J.-C.,
Klaus, A. (Eds.), Proc. ODP, Sci. Results, vol. 210, pp. 1–56.
Unternehr, P., Peron-Pinvidic, G., Manatschal, G., Sutra, E., 2010. Hyper-extended
crust in the South Atlantic: in search of a model. Pet. Geosci. 16, 207–215.
Wilkinson, B., Algeo, T., 1989. Sedimentary carbonate record of calcium–magnesium
cycling. Am. J. Sci. 289, 1158–1194.
Wright, V.P., 2012. Lacustrine carbonates in rift settings: the interaction of volcanic
and microbial processes on carbonate deposition. Geol. Soc. (Lond.) Spec. Publ.,
39–47.