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| Titel |
Probing the Oldest Geodynamo |
| VerfasserIn |
R. D. Cottrell, J. A. Tarduno, R. K. Bono |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250070024
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| Zusammenfassung |
| The evolution of Earth during the Hadean and Archean, and its relationship to the
development of life, remains a frontier research area where the most advanced field and
analytical techniques must be harnessed to glean details from the ancient geologic record.
During this time, the early geomagnetic field shielded Earth from intense solar winds from
the rapidly rotating young Sun. Therefore, the onset and strength of the earliest field are of
prime interest for understanding evolution of the planet. However, there are considerable
obstacles preventing the easy retrieval of primary paleomagnetic information from the most
ancient rocks. Even the best-preserved Paleoarchean rocks have seen low-grade
metamorphism. The thermal and chemical effects render many, if not most, whole rocks
unsuitable for analysis. Our approach to these challenges is to focus on the study of single
silicate crystals hosting minute magnetic inclusions [1]. Recent investigations of Archean
single silicate crystals from the Kaapvaal craton, using highly sensitive SQUID
magnetometers and CO2 laser demagnetization, have allowed definition of geomagnetic field
intensities at 3.2 Ga [2], 3.4 Ga and 3.45 Ga [3]. In our current work, we extend
this time line to 3.47 Ga through the study of single silicate crystals from granitic
rocks that are subvolcanic feeders to the Duffer Formation of the Pilbara craton [4].
Preliminary paleointensity data suggest that a protective magnetic field was present.
Some of the first physical evidence for life has been reported in similarly aged
rocks. However, the new measured field intensities are only ~25% of the modern
value.
Extending the field to the Hadean necessitates study of ancient minerals held in younger
conglomerate units. We discuss new paleomagnetic data from the Jack Hills conglomerate
[e.g. 5] that host Hadean-age zircons. Data from whole rocks pass a preliminary
conglomerate test suggesting some clasts could retain a primary magnetization. We will
outline an approach (currently underway) using single silicate crystal analysis toward
definition of these potential Hadean-age magnetizations.
[1] Tarduno J. A. et al. (2006) Rev. Geophys., 44, RG1002. [2] Tarduno J. A. et al. (2007)
Nature, 446, 657-660. [3] Tarduno J.A. et al. (2010) Science, 327, 1238-1240. [4] Van
Kranendonk et al. (2002) Economic Geol., 97, 695-732. [5] Valley, J.W. (2009) Elements, 2,
201-204. |
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