Geoscience Reference
In-Depth Information
Figure 10.64.
(a) A model
of 2.8-Ga Archaean
continental crust.
(b) Model equilibrium
Archaean continental
geotherms based on (a).
For geotherm 1, the heat
flow into the base of the
crust is 63 mW m
−2
, and
the conductivity is
3.3 W m
−1 ◦
C
−1
; this
geotherm has
T
= 550
◦
C
at 15 km depth, as
determined from the
metamorphic
assemblages. Geotherm 2
is a possible geotherm in
an old cold continent as
implied by the existence
of Archaean diamonds.
The box shows the
pressure-temperature
field inferred from mineral
inclusions in diamonds.
The dashed line shows
the position of the crustal
solidus. (After Nisbet
(1984; 1987).)
not have been valid everywhere. Some regions may have been relatively hot, others
relatively old and cool (geotherm 2). These temperatures can be interpreted as
differences between young (hot) and old (cold) continents. The hot, newly formed
continents may have had a lithosphere 80 km or less in thickness, while coexisting
colder continental regions may have had a 150-200-km-thick lithosphere.
