Geology Reference
In-Depth Information
Box 3.5 Diffusion and cooling rate: applications to meteorites
at temperatures below 900°C, the continuous high-tem-
perature solid solution between metallic iron and nickel
divides into two solid solutions, separated by a two-phase
region below a solvus (Figure 3.5.1a). accordingly many
iron meteorites exhibit complex exsolution intergrowths of
two separate Fe-Ni alloys, kamacite and taenite, devel-
oped during cooling through this solvus (Figure 11.1.1).
Consider the cooling of a crystal of alloy X which at
nearly 900°C is a homogeneous metallic solid solution. at
about 700°C the alloy encounters the solvus, below which
taenite becomes supersaturated with Fe, and expels it in
the form of platelets of a separate kamacite phase, whose
composition is indicated by drawing a tie-line to the other
limb of the solvus (Ni ~ 6%). the upper inset shows a pro-
file of the Ni content in a representative cross-section of
the crystal (on the scale of a few mm) at 680°C: slender
lamellae of low-Ni kamacite have appeared in the initially
homogeneous taenite.
the Fe-Ni solvus is unusual in that both limbs slope in
the same direction (cf. Figure 2.6). as the temperature
falls, therefore,
both
immiscible phases grow more Ni-rich.
the lever rule (Box 2.3) indicates that their relative propor-
tions must also change: the middle and lower insets show
that kamacite lamellae grow at the expense of taenite,
forming ever-thicker plates.
this process relies on the diffusion rates of Fe and Ni
atoms in the two phases. Laboratory experiments show
that diffusion rates are slower in taenite than in kamacite,
and thus Ni is expelled from kamacite more rapidly than it
can diffuse into the interior of the adjacent taenite crystal.
Consequently the Ni distribution in taenite lamellae devel-
ops an M-shaped profile, with Ni concentrated at the
edges. the more rapidly the meteorite is cooled, the more
pronounced is the central dip in the Ni profile. Calculations
based on diffusion profiles allow estimation - from the
shape of the M-profiles in iron meteorites - of the cooling
rates they experienced during the early development of
the solar system (Figure 3.5.1b). these estimates, com-
monly between 1°C and 10°C per million years, suggest
that iron meteorites are derived from relatively small par-
ent bodies (diameter < 400 km); large planetary bodies
would cool more slowly (hutchison, 1983).
Scan
across la
mellae
(b)
(a)
Calculated diffusion profiles
680°C
900
Kamacite lamellae
X
Cooling rates
T
T
T
620°C
0.01 °C
per 10
6
yr
0.1 °C
per 10
6
yr
1.0 °C
per 10
6
yr
TAENITE
680°C
K
T
K
T
620°C
525°C
K
T
525°C
500
K
K
TAENITE + KAMACITE
T
T
mm
10
20
Ni content/weight %
30
40
Fe
Ni
Figure 3.5.1
(a) phase relations for Fe-Ni alloys showing the subsolidus fields of kamacite and taenite and the 2-phase
field (ruled) between them (see Goldstein and Short, 1967). (b) Calculated diffusion profiles across a taenite platelet as
a function of cooling rate, based broadly on Wood (1964). (Source: adapted from hutchison (1983)). See
http://www.
Search WWH ::
Custom Search