Geology Reference
In-Depth Information
Fig. 1.5
Asthenosphere flow beneath a mid-ocean ridge
(
black lines
) and melting regime (
green triangle
).
Red
dashed lines
represent melt migration flow lines. The
oceanic crust is shown in
yellow
.The
small black rectan-
gle
at the spreading center represents the volume of new
crust generated per unit time. The
large black rectangles
at dry and wet solidi depths show the volumes of mantle
entering respectively the higher-degree and lower degree
melting regime zones during the same time interval, which
are necessary to produce that amount of new crust
Figure
1.4
shows the mantle adiabatic geotherm
for a potential temperature
T
p
D
1,280
ı
C. In
this example, the melt temperature ranges from
1,310
ı
C at the dry peridotite solidus (55 km
depth) to
1,260
ı
C at the surface. We note
that the melt adiabatic geotherm has different
slope and potential temperature. The reason is
that (@
T
/@
z
)
S
Š
1.0
ı
K/km for a basaltic magma,
approximately twice the slope of the mantle
geotherm.
Let us consider now the solidus curves of
peridotite rocks (Fig.
1.4
). Hirschmann (
2000
)
showed that the dry solidus of peridotite between
0 and 10 GPa can be constrained experimentally
to follow a simple parabolic law:
to 330 km, producing 0.03-0.3 % carbonatite
liquids, as a consequence of the addition of
small amounts of CO
2
to peridotite. However,
most of melts form at depths shallower than
55 km, where the mantle adiabat crosses
the dry solidus of peridotite (Fig.
1.4
). The
zone of upwelling and melting can be as wide
as 100 km at 50-70 km depth. At shallower
depths melting proceeds at enhanced rate, with
extensive extraction of water from the solid
phase. Figure
1.5
illustrates the flows within and
around the so-called
melting regime
, which is the
region below a mid-ocean ridge where melts are
generated and carried to the surface.
The melting regime can be defined as the
region above the peridotite wet solidus where
the asthenosphere flow has a vertical component
of velocity (Plank and Langmuir
1992
). There-
fore, its lateral boundaries are marked by the
zone where the mantle flows almost horizontally,
so that adiabatic decompression ceases or heat
conduction prevails. In stationary conditions, it
can be shown that the maximum velocity of
upwelling is
u
max
D
v
/ ,where
v
is the spreading
rate (Phipps Morgan
1987
). Therefore, if
w
is
the width
of
the higher-degree melting regime
zone and F is the average degree of melting,
then the generation of a sliver of oceanic crust
having thickness
H
requires a melting regime
T
s
.p/
D
ap
2
C
bp
C
c
(1.12)
where
a
Š
5.14
ı
CGPa
2
,
b
Š
132.90
ı
C
GPa
1
,and
c
Š
1, 120.66
ı
C. This solidus is
strongly affected by the presence of volatiles
(water, CO
2
, etc.), which always reduce the
melting point temperature of these rocks. For
example, Fig.
1.4
shows the effect of the addition
of water to a dry peridotite system (Katz et al.
2003
). Therefore, partial melting may start at
considerable depth beneath a spreading center.
Dasgupta and Hirschmann (
2006
) have recently
shown that deep melting must occur at depths up
