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| Titel |
Comparison between the domain-state-corrected multispecimen and microwave methods using historical lavas from La Palma, Spain |
| VerfasserIn |
Marilyn Monster, Lennart de Groot, Andrew Biggin, Mark Dekkers |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250048228
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| Zusammenfassung |
| Reliable palaeointensities (PIs) are necessary to determine the full vector variation of Earth’s
magnetic field as a function of geological time, and hence to provide constraints for
geodynamo models. Knowledge of PIs also has implications for cosmic abundance ratios, as
the strength of Earth’s magnetic field influences the amount of radiation entering the
atmosphere. Most current palaeointensity determination methods are based on the classic
Thellier-Thellier protocol. These protocols are usually fairly laborious and are characterised
by rather low success rates.
A new method of determining palaeointensities was proposed by Dekkers and Böhnel in
2006. In this ‘multispecimen parallel differential pTRM method’ (MSP-DB) an ancient TRM
is overprinted by an laboratory pTRM pointing at the same direction, at a single low
measurement temperature. The method is based on the linearity of pTRM with inducing field,
a property that is independent of domain state. It uses multiple specimens to ensure that all
specimens experienced the same magnetic history. However, especially for intermediate grain
sizes the MSP-DB protocol seems to overestimate palaeointensities. To solve this problem,
Fabian and Leonhardt (2010) proposed a new, domain-state-corrected protocol (MSP-DSC)
based on a statistical theory of weak-field thermoremanence and a phenomenological
thermoremanence model that quantifies the domain-state-induced overestimate. The DSC
protocol uses three additional steps in comparison to the original DB protocol, which
isolate the overprinted pTRM, make an estimate of the domain state and detect
alteration.
Both the MSP-DB and the DSC protocols have been tested on historical lavas from La Palma
and compared to microwave results obtained using the Aitken protocol and pTRM checks.
Nine flows were sampled, including the 1949 and 1971 flows, which were sampled at 5
and 3 sites respectively. These twentieth-century flows are particularly important,
as their PI results can be compared to IGRF values. All samples were within the
pseudo-single-domain range, but their hysteresis ratios varied widely, from close to the
single-domain range to very near the multi-domain range. Curie temperatures varied from 80
to 540 -C and differed between sites within one flow. Palaeomagnetic directions
determined using thermal and alternating-field demagnetisation were within error
of the IGRF values. When applying the MSP-DB protocol, the three sites with
high Curie temperatures (540 -C) all yielded PI underestimates within 10% of
the IGRF value, whereas the sites with low (135 -C) to very low (80 -C) Curie
temperatures yielded large under- or overestimates (30 to 50%), perhaps fortuitously.
The overestimates were reduced considerably by the MSP-DSC protocol, but were
still quite high. Sparse data, in conjunction with results from Mount Etna by De
Groot (2009) indicate that underestimates occur as well, but this aspect remains
enigmatic.
The microwave method was successful for six out of eight sites and generally yielded the best
results, within 4 to 11% of the IGRF values. Differences between the results from
multispecimen and microwave measurements were in some cases as large as 40%. This is
interesting, because in an earlier study by Böhnel et al. (2009), microwave and multispecimen
results were found to be very similar; the multispecimen results generally being slightly
higher than those obtained using the microwave.
References
Böhnel et al. (2009) Geophys. J. Int. 177, 383–394
Dekkers and Böhnel (2006) EPSL 248, 508-517
Fabian and Leonhardt (2010) EPSL 297, 84-94
De Groot (2009) IAGA 11th Scientific Assembly, Sopron, Hungary |
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