Long-term evolution of the geomagnetic dipole moment
ABSTRACT The geomagnetic field intensity measured at the surface of the planet is a potential indicator of the dynamo activity in the conducting liquid core of the Earth. Rapid field variations must be generated by the rapid fluid motions within the core, whereas long-term changes could be associated with other processes such as changes in boundary conditions at the core–mantle interface and/or at the outer (liquid)–inner (solid) core boundary. For about 50 years, paleomagnetists gathered records of absolute paleointensity using the magnetization of lava flows distributed over the globe in order to reconstruct the field intensity changes during the past 3000Ma. Despite recent acquisition of paleointensity records, the temporal distribution of the results remains limited (except within a few time intervals) to extract coherent features of the time-averaged field intensity over periods of a few million years. However, significant informations can be gained from the recent updated paleointensity database by averaging the virtual dipole moment over very long time intervals. The results suggest the existence of a long-term evolution of the dipole field intensity during the past 3 billion years (from 3 × 1022Am2 at 1000–2000Ma to 8 × 1022Am2 at present times).
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ABSTRACT: It is generally accepted that geodynamo is mainly driven by compositional convection due to liquid core crystallization which began about 1.5±0.5 Ga. Therefore geomagnetic paleointensity should have a low value before 1 Ga, because it was determined only by an inefficient thermal convection. This is in contradiction with the paleomagnetic records. This contradiction could be resolved if we suggest that the sufficient part of the modern Earth's solid core has never been formed due to liquid core crystallization, but represents the small relic of the protocore on which heterogenic accretion has begun. This protocore consists of heavy (Fe, Ni) and light components (chondrite silicates). The protocore is dissolved under influence of the liquid core. We suggest that concentration of the light component of protocore is decreased from ~75% down to ~10% towards the protocore's center. The floating of the light protocore component in liquid core during protocore dissolution causes the composition convection, which supports geodynamo. According to the model calculations this process could have maximal magnitude at about 2.5±1Ga and so the geomagnetic intensity could have the modern value or larger. When the protocore decreases down to the size at which concentration of the light component becomes less than ~15%, the protocore dissolution stops and the liquid core crystallization can begin. At the time of this transition about 1.5±1Ga the estimated geomagnetic intensity should be smaller than the modern value. After this period the paleointensity could rise again up to its modern value due to another kind of compositional convection which is now determined by the liquid core crystallization.
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ABSTRACT:  Rock-magnetic and Thellier-Thellier-Coe paleointensity results are reported for a new collection of samples from the 2.78 Ga Modipe Gabbro of Botswana. The magnetic properties are very favorable, leading to an unusually high success rate and a well-constrained result of 36–40 µT (95% confidence interval). We discuss the long-standing problem of allowing for the enormous differences between the natural and laboratory cooling rates, and apply a temperature-dependent correction derived from first-order reversal curve (FORC) data. Whereas pure single-domain (SD) corrections often lead to quite large decreases in paleointensity estimates (sometimes exceeding 50%), we find a modest increase of about 10%. The Earth's Magnetic Dipole moment in the late Archaean is thereby estimated to have been ∼6×1022 Am2.Geochemistry Geophysics Geosystems 07/2013; 14(7). · 3.05 Impact Factor
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ABSTRACT: The Vredefort impact crater in South Africa is one of the oldest and largest craters on Earth, making it a unique analog for planetary basins. Intense and randomly oriented remanent magnetization observed in surface samples at Vredefort has been attributed to impact-generated magnetic fields. This possibility has major implications for extraterrestrial paleomagnetism since impact-generated fields have been proposed as a key alternative to the dynamo hypothesis for magnetization on the Moon and asteroids. Furthermore, the presence of single-domain magnetite found along shock-generated planar deformation features in Vredefort granites has been widely attributed to the 2.02 Ga impact event. An alternative hypothesis is that the unusual magnetization and/or rock magnetic properties of Vredefort rocks are the products of recent lightning strikes. Lightning and impact-generated fields can be distinguished by measuring samples collected from below the present surface. Here we present a paleomagnetic and rock magnetic study of samples from two 10 m deep vertical boreholes. We show that the magnetization at depth is consistent with a thermoremanent magnetization acquired in the local geomagnetic field following the impact, while random, intense magnetization and some of the unusual rock magnetic properties observed in surface rocks are superficial phenomena produced by lightning. Because Vredefort is the only terrestrial crater that has been proposed to contain records of impact-generated fields, this removes a key piece of evidence in support of the hypothesis that paleomagnetism of the Moon and other extraterrestrial bodies is the product of impacts rather than past core dynamos.Journal of Geophysical Research Atmospheres 01/2012; 117(E1):1007-. · 3.44 Impact Factor