Geomagnetic field records from volcanic glasses

Reliable paleomagnetic data, giving information about both direction and intensity of the Earth`s magnetic field, is crucial to understand the evolution of this field throughout the geological past. Fundamental knowledge about past geomagnetic field variations, in turn, is essential to scrutinize the present decay of the geomagnetic field moment and the underlying geodynamo processes within the Earth`s interior. Furthermore, it provides important ramifications to other disciplines, such as geochronology and paleoclimatology. Obtaining reliable data, in particular, reliable absolute palaeointensity information, however, is hampered by several mechanisms that can cause failure or bias in the determinations. Among these are magnetomineralogical changes during geological time and laboratory treatment, magnetic anisotropy of the remanence, magnetic domain state bias affecting the different remanence acquisition processes in nature and in the laboratory, and different heating/cooling histories. It is found that volcanic glasses appear to be an ideal material for such paleomagnetic records, because all biasing effects which are usually hard or impossible to detect in other recording media, are either absent or can be corrected for by using mineralogical and rock magnetic constraints. It is found that volcanic glasses are often pristine and contain a stable magnetic remanence carried by single domain magnetite, two characteristics which already exclude prominent biasing effects. In collaboration with volcanologists the structural properties of volcanic glass, particularly the glass transition and the natural cooling rate across this transition are investigated by relaxation geospeedometry. Glass transition temperatures need to be well above the magnetic blocking temperatures in order to prevent related bias. Determination of cooling rate dependency of the thermoremanence allows for an extrapolation to natural cooling rates and, thus, a correction of absolute paleointensity determinations conducted on laboratory time scales. It is proposed to investigate a collection of glass yielding different age, composition, facies and quenching environment. In dependency of these conditions, the ideal character of volcanic glass regarding paleomagnetic research will be verified. So far, we particulary investigated silicic volcanic glasses. Results from Holocene and Pleistocene samples from Italy, Tenerife and New Zealand support the ideal character, at least of this mineralogical group and provide high quality paleomagnetic records from yet only sparsely investigated time periods and localities. In the continuation of the project we will significantly enhance our collection regarding the above conditions. Using the combined petrophysical and paleomagnetic approach we will test the possible influence of rock magnetic and mineralogical conditions on the determination of paleomagnetic field vectors from a variety of different volcanic glasses. The obtained paleomagnetic records will be added to a collection of published archeomagnetic results, and, thus provide the essential basis for reconstructing the past global geomagnetic field evolution during the Holocene. Furthermore, our measurements will provide very well characterized samples, in terms of mineralogy and magnetic grain sizes, which are of high thermal stability due to the pristine character of volcanic glass. Such natural materials are the ideal candidates for improving analyzis techniques of, in particular, paleointensity determination. These aspects will also be approached within the here requested project.

The principle aim of the FWF project P21221-N19 Geomagnetsim and Volcanic glass is to improve our understanding of geomagnetic field variation throughout Earths history including all aspects from its origin, evolution and consequences. In order to contribute to this fundamental question four major research aims were defined within this project. Firstly, we projected to investigate rock and paleomagnetic properties of volcanic glass and characterize its suitability as recording medium for obtaining information on past geomagnetic field variations. For this purpose a broad scientific approach including mineralogical, volcanological and magnetic investigation technquies was applied. We could identify a series of possible biasing mechanisms like devitrification, hydration, and post-emplacement rotations and suggest techniques to identify or correct for such drawbacks. Altogether three peer-review articles were devoted to this aspect.After proving the suitability of such glasses a second aim of the project was to extract reliable paleointensity information. Materials from several different sources were studied for this purpose. Glasses from Tenerife (Spain), Iceland, Armenia and from New Zealand provided very well defined records of past geomagnetic field strength. The results were summarized in two peer-reviewed articles and others are in preparation.Reconstructions of the global geomagnetic field are a third aim of this project. An iterative inversion technique was improved and applied to records from Iceland and data from the paleomagnetic database. It provided a conclusive model for the Laschamp excursion 41 kyr ago. Two articles are published on this aspect.Finally, we focused on improving paleomagnetic analysis techniques, in particular for extracting geomagnetic field intensity information. Within the project we developed and tested an improved experimental approach including its theoretical background and analysis routine (documented in one article). We also worked on some important issues related to the grain size of magnetic recording media and effects of cooling rate differences between laboratory experiments and natural cooling of the lavas. As a result we obtained several unique findings, published in two article, which will improve further work on past geomagnetic field analysis. Beside the main aims we applied our developed techniques and results for several projects outside the projected scientific field and made important contributions on dating, archeomagnetism and volcanology.