Geoscience Reference
In-Depth Information
which indicates that the axial depth is dependent on the thermal structure of
the underlying mantle. Where the mantle is hottest (and the ridge axis shallow-
est), the rising mantle material intersects the solidus earlier, at greater depth
(Fig. 7.17), and so melts more. Maximum differences in temperature of some
200
◦
Cinthe subsolidus mantle may be necessary to account for this observed
correlation. Results of seismic and electromagnetic studies (MELT) of the East
Pacific Rise suggest that small amounts of melt (
2%) are present over a very
wide region within the mantle. The melt then converges on the narrow active
ridge axis. Variations in chemistry of erupted lavas are attributed to the details of
the path taken by the melt from source to surface. The fine details of the melting,
rising, interaction, solidification and eruption processes are still matters of current
research.
In order to calculate how much partial melting occurs when mantle at a tem-
perature
T
upwells and so is decompressed, it is necessary first to express the
solidus and liquidus temperatures for the mantle as functions of pressure (e.g.,
Fig. 7.17). For a garnet-peridotite mantle the solidus temperature
T
s
(in
◦
C) and
the pressure
P
(in GPa) are related by the following equation:
<
T
s
−
1100
136
10
−
4
e
1
.
2
×
10
−
2
(
T
s
−
1100
)
P
=
+
4
.
968
×
(9.1)
The liquidus temperature
T
l
(in
◦
C) also varies with pressure
P
(in GPa):
T
l
=
1736
.
2
+
4
.
343
P
+
180 tan
−
1
(
P
/
2
.
2169)
The degree of melting, as a fraction by weight of the rock,
x
,isthen given by
x
−
0
.
5
=
T
(
T
2
−
0
.
25)(0
.
4256
+
2
.
988
T
)
(9.2)
where
T
,adimensionless temperature, is defined as
T
−
(
T
s
+
T
l
)
/
2
T
l
−
T
=
(9.3)
T
s
Surprisingly, there is no clear evidence for variation of the degree of melting
x
(
T
) with pressure.
The thickness of the oceanic crust is almost constant on all the plates; 7
1km
(Table 9.2), provided that seismic measurements are made away from fracture
zones where the crust is on average some 3 km thinner (Section 9.5.2), and plumes
where the crust is thicker (e.g., Iceland). In order to generate 7-km-thick oceanic
crust, 7 km of melt is needed from the upwelling mantle. Equations (1)-(3) show
that, to produce this amount of melt, the potential temperature (Eq. (7.95)) of the
source region needs to be about 1280
◦
C (McKenzie and Bickle 1988). Rising
mantle will then cross the solidus at 1300
◦
Catadepth of about 45 km, and
melt will reach the surface at 1200
◦
C(Fig. 7.17). These values yield an average
melting depth of 15 km and a melt fraction of 10%-15%. The magma would
be about 10% MgO, and the melt fraction would not exceed 24%. Estimates
of melting based on matching crustal thickness are broadly similar: a normal
potential temperature of 1300
±
±
20
◦
C, with melting starting at 50 km depth and
