Paleomagnetism indicates that primary magnetite in zircon records a strong Hadean geodynamo

Determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind. The absence or presence of the geomagnetic field also provides a unique gauge of early core conditions. Evidence for a geomagnetic field 4.2 billion-year (Gy) old, just a few hundred million years after the lunar-forming giant impact, has come from paleomagnetic analyses of zircons of the Jack Hills (Western Australia). Herein, we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select Hadean zircons have escaped magnetic resetting since their formation. New paleointensity and Pb-Pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4.1 to 4.0 billion years ago (Ga) that may represent efficient convection related to chemical precipitation in Earth’s Hadean liquid iron core.

Improving the measurement accuracy of magnetometer algorithmic correction of its instrumental errors

In many applications of the magnetic method it is advantageous to have information about the geomagnetic field strength and its gradients. The direct measurement of gradients does not depend on geomagnetic variations or normal field values. Special equipment is required for this purpose. The results of the development and construction of a three-component magnetometer-gradiometer are presented. The device is designed to measure on the earth's surface the absolute values of the three components of the geomagnetic field vector and the corresponding three components of the gradient. Installation of additional measuring sensors in the device – accelerometers, allows you to calculate the orientation of these vectors in space. The device of a magnetometer-gradiometer is described, its functional scheme and operating principle are presented. A set of instrumental errors that occur in the manufacture of three-axis systems of ferrosondes and accelerometers for measuring the components of the geomagnetic field strength and determining the orientation of the device is considered. The paper presents a method for determining instrumental errors and algorithmic correction of information signals coming from measuring sensors to significantly improve the accuracy of measurements. Examples of field tests of the device are given. The presented magnetometer-gradiometer can be used for accurate localization of previously identified ore bodies and determining the details of their structure.

Instability of thermoremanence and the problem of estimating the ancient geomagnetic field strength from non-single-domain recorders

Data on the past intensity of Earth’s magnetic field (paleointensity) are essential for understanding Earth’s deep interior, climatic modeling, and geochronology applications, among other items. Here we demonstrate the possibility that much of available paleointensity data could be biased by instability of thermoremanent magnetization (TRM) associated with non-single-domain (SD) particles. Paleointensity data are derived from experiments in which an ancient TRM, acquired in an unknown field, is replaced by a laboratory-controlled TRM. This procedure is built on the assumption that the process of ancient TRM acquisition is entirely reproducible in the laboratory. Here we show experimental results violating this assumption in a manner not expected from standard theory. We show that the demagnetization−remagnetization relationship of non-SD specimens that were kept in a controlled field for only 2 y show a small but systematic bias relative to sister specimens that were given a fresh TRM. This effect, likely caused by irreversible changes in micromagnetic structures, leads to a bias in paleointensity estimates.

Absolute geomagnetic intensity determinations on Formative potsherds (1400–700 BC) from the Oaxaca Valley, Southwestern Mexico

New Thellier-Coe archeointensity determinations have been measured on 15 potsherds from the Oaxaca Valley belonging to three of the four Formative Periods (Pre-Classical) of Mesoamerica, spanning 1400–700 BC. Seven of these are considered to be reliable and indicate a geomagnetic field strength of about 30 μT. This value is some 75% of the present geomagnetic field strength but is in agreement with the absolute intensities predicted from global models for this time and location, and consistent with coeval published determinations. These data thus provide significant evidence for the geomagnetic field strength in an area and for a time that was previously poorly constrained, thus providing an important contribution towards establishing a local master curve for the last 3500 yr. When established, such a curve would be a useful dating tool and also enable establishing for field strength correlations with climatic events and civilization evolutions in a region that is particularly strong in archeological and geological features. Such potential is examined for aridity events, although such observations can only be considered tentative at this stage.