Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 126

126
GUÐMUNDUR PÁLMASON
from northern and eastem Iceland to show how and where this
change in depth takes place.
It is of some interest to try to estimate the temperatnre to he ex-
pected at the 3-4 boundary. In SW- and W-Iceland the surface tem-
perature gradient, where it is undisturbed by water movement, ap-
pears to be in the range 100 to 150°C/km although hoth higher and
lower values have been found. Layer 4 is at a depth of 8—9 km. If
the surface temperature gradient is used to extrapolate the tempera-
ture down to this depth, temperatures in the range 800—1300°C are
obtained. This is close to or within the melting range of basalts. If
heat sources such as cooling intrusions exist above this depth, the
temperature gradient decreases on the average with depth and some-
what lower temperatures would be obtained at the 3—4 boundary.
Nevertheless it appears likely that the 3—4 boundary in SW- and W-
Iceland is at a temperature close to the melting range of basalts.
In northem Iceland the undisturbed temperature gradient appears
to be lower, on the average perhaps ahout 60°C/km. The depth to
layer 4 appears to be somewhat greater than in SW- and W-Iceland,
perhaps 14-15 km. This gives a temperature at the 3-4 boundary in
the range 850-900°C. In the eastem part of southern Iceland the
depth to layer 4 is 14-15 km. If the temperature gradient in the
Prestbakki borehole is taken as typical for this part of Iceland, a
temperature of 650-700°C is obtained at the 3—4 boundary. This
boundary then appears in N- and SE-Iceland to be at a temperature
below the melting range of basalts.
Fig. 47 shows probable temperatures at the depths of layers 3 and
4 for those parts of Iceland where data are available. The data on
layer 3 correspond closely to Fig. 43 and the data on layer 4 are those
discussed above. The melting range of basalts is taken from Yoder
and Tilley (1962). This would be lowered somewhat if the water
vapour pressure in the basalts were sufficiently high.
Layer 4 with a P-wave velocity of about 7.2 km/sec is probably
to be equated to the anomalous upper mantle as observed under the
crestal zones of the mid-ocean ridges. The velocity in this anomalous
mantle is commonly found to be in the range 7.2-7.6 km/sec. Several
suggestions have been offered as to the nature of this layer. Ewing and
Ewing (1959) considered it to be a “mantle-crust mix”. Bott (1965b)
suggested that partial fusion is probably the dominant cause of the
low-density region in the upper mantle beneath ocean ridges. Ox-
burgh and Turcotte (1968) considered that the material of layer 4