PRECAMBRIAN GEOMAGNETIC FIELD: INTENSITY, MORPHOLOGY AND STABILITY

Paleomagnetic data have been instrumental in studying Precambrian paleogeography. Interpretation of the data, however, is ultimately based on our knowledge of the characteristics of geomagnetic field during that time period. Knowing the geometry, stability, and intensity of the field is also crucial for understanding early geodynamo evolution. Space-time characteristics of the Precambrian field, such as the relative significance of the dipole and non-dipole components, could have been significantly different from their Phanerozoic counterparts. In the absence of strict theoretical constraints, paleomagnetic data become a principal source of information about the Precambrian field. The field geometry can be estimated by combining paleomagnetic data with independent latitudinal indicators such as evaporites and glacial deposits. Another problem is the field stability; it has been suggested that the hotter Earth and the absence of the inner core could have resulted in higher variation of the field, including more frequent excursions. The current magnetostratigraphic database is insufficient to test whether the early geodynamo reversed its polarity with the same range of frequencies as in the Mesozoic-Cenozoic interval. Precambrian rocks may preserve directional information useful for constraining paleosecular variation (PSV). We will discuss the estimates of PSV based on the Precambrian paleomagnetic database. Paleointensity data provide information on the energy state of geodynamo; in particular, an abrupt increase in field strength could have accompanied the onset of inner core formation, although this is not necessarily the case. Many Precambrian rock sequences have been affected by alteration, however, which hinders the preservation and measurement of paleointensity using bulk rock samples. Alternatively, single silicate crystals may be used as paleointensity recorders. Such crystals are much less susceptible to alteration in nature and during paleointensity experiments. Data from plagioclase crystals separated from mafic dikes, together with directional data from whole rocks, indicate a dipole-dominated field at 2.5–2.7 Ga. The bulk of available data indicate that on a long-term scale the Proterozoic field was not grossly different from the present-day field.