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smectite-illite transformation was the source of the
Fe for the secondary magnetite. Before illitization is
widely accepted as a remagnetization mechanism,
more detailed studies need to be conducted like those
of Zwing et al . and Tohver et al . which date the illitiza-
tion event.
CONCLUDING THOUGHTS: ACCURACY
OF A CRM REMAGNETIZATION
Since the theme of this topic is accuracy of the paleo-
magnetic signal, it is worth looking at the laboratory
experimental work on the accuracy of a CRM. There
have been several important laboratory CRM studies.
Stokking & Tauxe (1990a, b) performed early experi-
ments in which a CRM was carried by secondary hem-
atite crystals grown from nothing. In this case, the
hematite carried an accurate record of the magnetic
fi eld during their growth. When Stokking and Tauxe
grew hematite in a series of experiments (i.e. 'multi-
generational' hematite) in the presence of sequentially
perpendicular fi elds, they found that the resulting CRM
was complex and could be either be parallel or antipar-
allel to the growth fi elds or intermediate between the
fi elds.
Walderhaug (1992) heated basic igneous rock
samples so that strongly magnetic titanomagnetites
were altered to either magnetite or hematite. The
resulting CRM in the secondary hematite was interme-
diate in direction between the pre-existing NRM and
the magnetic fi eld during growth for some samples,
and parallel to the laboratory fi eld in other cases.
Madsen et al . (2002) followed this work with the
heating of igneous rocks with NRMs at different angles
to the magnetic fi eld during heating, mimicking a CRM
acquired in a fold carrying a pre-folding magnetiza-
tion. In all cases, the secondary CRM was intermediate
between the CRM fi eld and the NRM direction of the
sample.
Cairanne et al . (2004) heated a pyrite - rich claystone
at low temperatures for an extended period of time and
observed the oxidation of pyrite fi rst to magnetite and
then to hematite. The samples were heated in fi elds of
successively different polarities and the CRM acquired
an accurate record of the last polarity fi eld during the
heating.
What appears to be emerging from these experi-
ments is that, for magnetic grains that grew from
'nothing' i.e. with no magnetic precursor minerals, the
resulting CRM is an accurate record of the magnetic
fi eld during growth. Since pyrite does not carry a rema-
nence, the magnetic minerals that grow from the oxi-
dation of pyrite can carry an accurate CRM. However,
for those magnetic minerals that grow from the altera-
tion of a pre-existing magnetic mineral phase, i.e. oxi-
dation of magnetite to hematite or multiple generations
Other remagnetization mechanisms:
Hydrocarbon migration and dolomitization
In their comprehensive review of remagnetization,
McCabe & Elmore (1989) also mention hydrocarbon
migration and maturation as a cause of remagnetiza-
tion in addition to their main focus on orogenically-
driven fl uids. McCabe & Elmore (1989) indicate that
Donovan et al . (1979) were the fi rst to identify a rela-
tionship between hydrocarbon migration and the pos-
sible formation of magnetite. Donovan et al . implicate
hydrocarbon migration in the formation of secondary
magnetite from the reduction of hematite in an Okla-
homa oilfi eld although, as McCabe & Elmore (1989)
indicate, the work came under close scrutiny that lead
Reynolds et al . (1985) to suggest that the magnetite
was merely the result of drilling contamination.
More recent work has documented a secondary
CRM with hydrocarbon migration in two carbonate
units: the Mississippian Deseret Limestone in Utah and
the Pennsylvania Belden Formation carbonates in
Colorado. In both units, a CRM remagnetization was
identifi ed that was the same age as the modeled time
of hydrocarbon migration in the rocks (Banerjee et al .
1997 ; Blumstein et al . 2004 ). Hydrocarbon migration
was also implicated in the formation of secondary
magnetite, pyrrhotite and specular hematite from the
reduction dissolution of hematite in Montana's Chug-
water Formation red beds (Kilgore & Elmore 1989).
The specular hematite carried a secondary CRM.
Both dolomitization, in which Mg is added to
calcite during mudrock burial dewatering, and de-
dolomitization in which the Mg is removed, can cause
the growth of secondary magnetic minerals. Addison
et al . (1985) show that in the Pendleside Formation
carbonates from the Craven Basin in England, un-
dolomitized limestones have magnetite with a primary
magnetic fabric and that dolomitzed carbonates have a
CRM residing in secondary hematite. Secondary hema-
tite is also the culprit in de-dolomitized carbonates
(McCabe & Elmore 1989 ).
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