Volcanic glass - an ideal paleomagnetic recording material?

Volcanic glass is often considered an ideal recording material for
paleointensities. Experiments to determine the ancient field
intensity are time consuming and mostly have low success rates.
Studies have shown that the usage of glassy samples can increase
success rates very much as the remanence carriers are in or close
to the single domain range. Further, effects like magnetic
anisotropy and cooling rate correction can be corrected for. The
aim of this thesis is to clarify whether an ideal behavior can be
expected when working on volcanic glass. Studies were done on
samples of different compositions (phonolite, pantellerite and
rhyolite) and varying degrees of devitrification/hydration. Rock
magnetic measurements were done to determine the remanence carriers
of remelted glass samples of phonolitic composition. Single domain
(SD) titanomagnetites were identied to carry the stable remanence
and in the course of paleointensity experiments the validity of a
cooling rate correction method that makes use of the natural
cooling rate of the samples as determined from relaxation
geospeedometry was shown. After correction the samples reproduce
the intensity of the previously applied field. For the unhydrated
samples from Montana Blanca, Tenerife (phonolitic composition, 2 ka
old) and from Mayor Island, New Zealand (pantelleritic composition,
8 ka old) remanence carriers in or close to the SD range were found
and high quality well defined paleointensities were obtained.
Success rates of the paleointensity experiments were 70%. The
results compare very well with other paleointensities from close-by
studies at the respective times. These data support the ideal
recording behavior of volcanic glass. Yet, rock magnetic and
paleointensity experiments on devitrified and hydrated samples give
contrary results: It was found that hydration and devitrification
result in a loss of magnetic stability and remanence carriers, an
increase in grain size of magnetic particles and a decrease of the
"apparent" paleointensity. A possible explanation for these
obsevations is a partial or full overprint of the original
thermoremanent magnetization by a chemical remanence. It is
therefore obvious that suchlike altered glasses are far from being
ideal recorders. Great care has thus to be taken when sampling
volcanic glass. If pristine glass is sampled, an ideal behavior
during paleointensity experiments is probable and - as the here
presented data suggest - a good estimate of the ancient field
intensity is likely to be gained. If, however, the glass is
unknowingly altered the determined paleointensities are prone to
underestimate the true field value.