In the past 4 years U.S. workers have brought to bear a great array of experimental, numerical modeling, rock magnetic and observational techniques to better understand the underlying microscopic processes of magnetic fabric development. Examples of these new approaches will be covered in the review of individual studies that will follow in separate sections of this review. In this section, new sampling and measurement techniques will be discussed.
Ellwood et al. [1993a] have had success sampling friable or poorly lithified material using two masonry blades mounted on an electric drill. The dual masonry blades cut slots in the rock or sediment 2 cm apart. A second cut at right angles to the first allows plastic sample boxes to be fit over the resulting pedestal of rock material. A pilot study of the Bandelier tuff showed sampling with this technique increased within site precision for AMS measurements when compared to drilling cylindrical samples, the more traditional sampling technique.
A promising new technique could emerge from the demonstration that the susceptibility measured for electrically conducting diamagnetic and paramagnetic materials (copper, biotite, clays) is strongly frequency dependent in the frequency range from 50 Hz to 5 kHz [ Ellwood et al., 1993b]. Ferrimagnetic materials (magnetite) do not show this frequency dependence. Hence, the AMS tensor which originates from diamagnetics and paramagnetics could be separated from the AMS carried by ferrimagnetics. The electromagnetic susceptibility tensor (EMS) measured at high frequency is mathematically subtracted from the AMS tensor measured at low frequencies, thus isolating the AMS of ferrimagnetic minerals.
Thomas et al. [1992] have developed the use of a magnetic
first-order gradiometer connected to a SQUID for high-resolution
measurement of the vertical magnetic induction (B
). With this
instrument the magnetic remanence and susceptibility of geological
thin sections can be imaged with a spatial resolution of less than 1
mm. Thomas et al. suggest that this approach will be useful for
determining the origin of AMS from pyroclastic flows. Based on a
pilot study of a single sample from the Bandelier tuff, it appears that
secondary Fe-Ti oxides deposited in vesicle walls carry the bulk
susceptibility. High resolution imaging like this could also be
important to remanence anisotropy and NRM studies.
Finally, although not a new technique, Lienert [1991] has used
Monte Carlo simulation of errors in AMS data to show that the
Hext/Jelinek statistical analysis technique is a satisfactory method of
estimating errors in eigenvector distributions. The technique developed
by Hext [1963] and Jelinek [1978] is based on multivariate
analysis and allows estimation of the sizes and orientations of the
confidence ellipses around the means for the three AMS ellipsoidal
axes (AMS eigenvectors). The use of the Hext/Jelinek approach would
allow the comparison of the statistical quality of different AMS, or
any magnetic fabric measurement, data sets much like Fisher
[1953] statistical analysis of natural remanent magnetization (NRM)
data. Lienert used Monte Carlo perturbations of synthetic AMS
êtensors to show that when the perturbation size is increased to as
large as 100% of the minimum eigenvalue difference the Hext/Jelinek
method underestimates the dispersion by about 10
, for better
datasets the Hext/Jelinek technique is quite accurate.