Estimating particle size and coercivity distributions of pigmentary
hematite in red chert with thermal fluctuation tomography
Abstract
Pigmentary hematite carries important signals in paleomagnetic and
paleoenvironmental studies. However, weak magnetism and the assumption
that it has high magnetic coercivity prevents prevents routine
identification of the size distribution of pigmentary hematite,
especially for fine particle sizes. We present a strategy for estimating
joint hematite particle volume and microcoercivity (f (V, Hk0))
distributions from low-temperature demagnetization curves and thermal
fluctuation tomography (TFT) of pigmentary hematite in bulk samples of
Triassic-Jurassic Inuyama red chert, Japan. The coercivity of the
pigmentary hematite increases exponentially with decreasing temperature,
following a modified Kneller’s law, where microcoercivity has a wide but
approximately symmetric distribution in logarithmic space from
~1 tesla to tens of tesla. All of the red chert samples
contain stable single domain (SSD) hematite with 35 - 160 nm diameter; a
significant superparamagnetic (SP) hematite population with sizes down
to several nanometers also occurs in Jurassic samples. The SP/SSD
threshold size is estimated to be 8 - 18 nm in these samples. The fine
particle size of the pigmentary hematite is evident in its low median
unblocking temperature (194 °C to 529 °C) and, thus, this hematite may
contribute to all four paleomagnetic components identified in published
thermal magnetization studies of the Inuyama red chert. In this work,
uniaxial anisotropy and magnetization switching via coherent rotation
are assumed. Uniaxial anisotropy is often dominant in fine-grained
hematite, although the dominant anisotropy type should be evaluated
before using TFT. This approach is applicable to studies that require
knowledge of coercivity and size distributions of hematite pigments.