Transforming understanding of paleomagnetic recording in igneous rocks:
Insights from aging experiments on lava samples and the causes and
consequences of ‘fragile’ curvature in Arai plots
Abstract
The theory for recording of thermally blocked remanences predicts a
quasi-linear relationship between low fields like the Earth’s in which
rocks cool and acquire a magnetization. This serves as the foundation
for estimating ancient magnetic field strengths. Addressing
long-standing questions concerning Earth’s magnetic field require a
global paleointensity dataset, but recovering the ancient field strength
is complicated because the theory only pertains to uniformly magnetized
particles. A key requirement of a paleointensity experiment is that a
magnetization blocked at a given temperature should be unblocked by
zero-field reheating to the same temperature. However, failure of this
requirement occurs frequently and the causes and consequences of failure
are poorly understood. Recent experiments demonstrate that the remanence
in many samples typical of those used in paleointensity experiments is
unstable, and exhibits an ”aging’ effect in which the unblocking
temperature spectrum changes over only a few years resulting in
non-ideal experimental behavior. While a fresh remanenence may conform
to the requirement of equality of blocking and unblocking temperatures,
aged remanences may not. Blocking temperature spectra can be unstable
(fragile), which precludes reproduction of the conditions under which
the original magnetization was acquired. This limits our ability to
acquire accurate and precise ancient magnetic field strength estimates
because differences between known and estimated fields can be
significant (up to 10 μT) for individual specimens, with a low field
bias. Fragility of unblocking temperature spectra appears to be related
to grain size and may be related to features observed in first-order
reversal curves.