Geoscience Reference
In-Depth Information
(a)
(b)
Age (Ma)
0
0
10
20
30
sea
Density
(kg m )
Depth
(km)
-
3
10
0
1.5
3.5
sea
hydrothermally
altered
1000
2800
3270
20
komatiitic
basalt
3200
fresh
30
8.5
3210
3300-3350
olivine cumulates
40
3190
50
base of crust
strong mantle
16.5
3230
60
3170
o
21.5
900- C isotherm
3150
3150-3200
70
weak mantle
80
thermal boundary layer at ~60 km
below which density ~3150 kg m
−
3
Figure 10.65.
(a) Komatiitic Hadean oceanic crust, 15 km thick, assuming an
asthenospheric temperature of 1700
◦
C. Upper crust would be komatiitic basalt with
dykes and pillow lavas, and lower crust would be cumulates. (b) A cooling model of
the Archaean oceanic lithosphere. The density of Archaean mantle at 1700
◦
C was
assumed to be 3150 kg m
−3
. The mechanical base of the plate was arbitrarily chosen
as 900
◦
C for refractory mantle. (After Nisbet and Fowler (1983).)
o
Temperature ( C)
B
A
0
1000
2000
0
B
A
100
Diamonds
200
zone of refractory mantle
?level of density crossover
magma shell?
300
Figure 10.66.
A diagram of some speculations about the Archaean upper mantle.
Geotherm A at the right-hand side is for mid-ocean ridges; geotherm B is for cool
continents. (From Nisbet (1985).)
produced the fascinating speculation that olivine may have floated above a buried
magma ocean of melt (on the modern Earth, melts, less dense everywhere than
crystal residue, rise). Such a gravitationally stable deep-magma 'ocean' has been
nicknamed the LLLAMA (large laterally linked Archaean magma anomaly). If
such a density difference existed in the late Hadean to earliest Archaean, a magma
shell could have surrounded the Earth, as in Fig. 10.66.Itwould have been overlain
by alayer of olivine. All of this is the subject of debate, but it illustrates how very
