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(b)
(a)
14
0
150
100
1400
12
10
75
30
40
50
20
10
20
1360
10
8
30
5
1320
6
40
1280
4
50
2
60
0
70
0
20
40
60
80
100
120
140
160
0
5
10
15
20
25
Melt fraction below ridge axis (%)
Plate-separation rate (mm yr )
--1
Figure 9.14.
(a) The predicted melt fraction in the mantle beneath a mid-ocean ridge
calculated for a mantle potential temperature of 1300
◦
C. The curves are labelled
with the half-spreading rate (mm yr
−1
). For very slow spreading rates, below
1cmyr
−1
, there is a marked drop in the percentage of the melt fraction in the mantle
and a restriction on the depth interval over which melting takes place as the depth of
the upper limit on melting increases. (b) The predicted crustal thickness calculated
for potential temperatures of 1280-1320
◦
C appropriate for normal oceanic mantle
and 1360-1400
◦
C for mantle in the vicinity of a hotspot. Crustal thicknesses
determined by seismic experiments are shown as dots. (See White
et al
.(2001) for
other models.) (Reprinted from
Earth Planet. Sci. Lett.
,
121
, Bown, J. W. and White,
R. S., Variation with spreading rate of crustal thickness and geochemistry, 435-49,
copyright 1994, with permission from Elsevier.)
the maximum melt fraction of 15%-18% localized between 8 and 15 km depth
(Fig. 9.14).
Oceanic crust produced at ridges with very slow spreading rates (half-rate
2cmyr
−
1
)isvery much thinner than normal (4-5 km only
on the Southwest Indian Ridge, which is spreading at 0.8 cm yr
−
1
, and less in the
Arctic). These are the spreading rates below which the loss of heat by conduction
from the ascending mantle becomes crucial. The consequences of very slow
spreading are that the depth to the top of the melting zone increases and the
melt fraction is reduced. Together these factors mean that, at very slow spreading
rates, the total production of melt is much reduced and so the crust is thinner
(Fig. 9.14). The 1800-km-long Gakkel Ridge (Figs. 2.2 and 9.18)which crosses
the Arctic Ocean beneath the ice from northwest of Svalbard to the Lapatev Sea
is the slowest-spreading mid-ocean ridge on the Earth (the half-spreading rate
decreases from 0.75 cm yr
−
1
1cmyr
−
1
, full rate
<
<
0.3 cm yr
−
1
in the east). It may
spread so slowly at its eastern end (where it is very close to the rotation pole)
that that there is effectively no basaltic crust, very enriched magmas erupt and
peridotite may be exposed over large areas.
in the west to
∼
