Geology Reference
In-Depth Information
6
Post - Depositional Diagenesis and
Chemical Remanent Magnetization
The theme of this topic is the accuracy and the reliabil-
ity of paleomagnetic results from sedimentary rocks.
In this chapter we'll examine how diagenesis, both
early and late, can affect the paleomagnetic signal in
sediments and sedimentary rocks. As Tarling points
out in the introduction to his topic
Palaeomagnetism and
Diagenesis in Sediments
(Tarling 1999 ), historically
paleomagnetists were most concerned about the incli-
nation error discovered in the early re-deposition
experiments discussed in Chapter 4; diagenetic
effects were thought to be minor. This view was
changed signifi cantly by Bob Karlin's observation of
magnetite dissolution due to early diagenesis in recent
hemipelagic marine sediments from the Gulf of Cali-
fornia and coastal Oregon (Karlin & Levi 1983; Karlin
1990 ).
When secondary magnetic minerals fi rst form they
are too small to carry a stable remanence; they are
superparamagnetic grains with magnetizations that
can change direction due to external fi elds on times-
cales of only seconds or minutes. As they grow larger
and pass through a very narrow size range, they
quickly attain stability to external magnetic fi elds with
relaxation times of the order geological timescales (10
9
years). This size is known as the 'blocking volume'
because the grains growing through this size have
'blocked in' a geologically stable magnetization. When
they reach this grain size, the grains are magnetized by
a chemical (or crystallization) remanent magnetiza-
tion (CRM). Both Tauxe ' s (2010) topic
Essentials of
Paleomagnetism
and Butler ' s (1992) topic
Paleomagnet-
ism: Magnetic Domains to Geologic Terranes
have excel-
lent treatments of the theoretical aspects of CRM
acquisition. The pertinent fact we need to keep in mind
for our discussion is: when magnetic minerals grow at
some time after the deposition of sediments, they will
pick up a secondary magnetization at the point in time
when their particle grain size grows large enough to
become paleomagnetically stable. This grain size is
usually of submicron - micron order (e.g. 0.05 - 1 μ m
for magnetite; Butler & Banerjee 1975).
What this means for the accuracy and reliability of
sedimentary rock paleomagnetism is that the direction
of a CRM may be a very accurate record of the geomag-
netic fi eld when the magnetic grain grew through its
blocking volume, but the magnetization will have an
age younger than the depositional age of the rock. How
much younger depends on the post-depositional chem-
ical environment of the sediments, i.e. whether early
or late diagenesis occurs, and over what time period the
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