Geology Reference
In-Depth Information
Fig. 3.16
Ternary
diagram of the
iron-titanium oxides and
their solid-solution series.
Va r i a b l e
x
is the
composition parameter (Ti
content). The ratio of ferric
iron (Fe
C3
) to ferrous iron
(Fe
C2
) increases from left
to right. The Ti content
increases from
bottom
to
top
. The diagram is
normalized to one cation
Fe, so that oxidation lines
(increasing Fe
3C
/Fe
2C
ratio) are horizontal
The distribution of grain sizes, hence the structure
of the magnetic domains, depends strongly
from the cooling rate. Titanomagnetites that
form in rapidly cooling volcanic rocks (e.g.,
oceanic pillow lavas) are fine-grained, because
they generally contain a significant fraction of
grains with size of 1 m or smaller. Conversely,
the grain size is larger in the case of slowly
cooled intrusive rocks, where it may exceed
100 m. Paleomagnetists consider fine-grained
ferromagnetic particles as the best magnetic
recorders (e.g., Butler
1992
). Therefore, volcanic
rocks are generally preferred over intrusive rocks
as targets for paleomagnetic studies.
The number of magnetic domains is an in-
creasing function of the grain size. When the
grains are sufficiently small, they will contain
just one domain. These grains, which are referred
to as
single
-
domain
(SD) grains, have magnetic
properties that are dramatically different from
those of
multi
-
domain
(MD) grains. The thresh-
old grain diameter,
d
0
, below which we have only
SD grains, depends essentially from the grain
shape and from the saturation magnetization
M
s
.
For hematite,
d
0
D
15 m, thereby in most cases
these minerals are SD. Conversely, only fine-
grained magnetite is SD. In general, cubic mag-
netite particles must have a diameter
d
< 0.1 m
to be SD, although elongated SD particles can
have a length of up to 1 m. Our interest into
SD grains arises from their property of being
Fig. 3.17
Variation of room-temperature saturation mag-
netization (
solid line
) and Curie temperature (
dashed line
)
with composition (
x
parameter) in the titanomagnetites.
End members are magnetite (
x
D
0) and ulvospinel (
x
D
1)
(Redrawn from Hunt et al. (
1995
))
portion of ferromagnetic minerals, although in
highly silicic or highly oxidized igneous rocks
hematite can give a major contribution to the
rock ferromagnetism. Furthermore, hematite
is the dominant or exclusive ferromagnetic
mineral in red beds, which represent an important
sedimentary source of paleomagnetic data. The
general formula of titanohematites is Fe
2
x
Ti
x
O
3
,
where hematite (
x
D
0) and ilmenite (
x
D
1) are
the end-members of the series. For a more in-
depth discussion about the mineral physics of
titanomagnetites and titanohematites, the reader
is referred to Butler (
1992
).
Both titanomagnetites and titanohematites
crystallize early, at a temperature of
1,300
ı
C.
