Ursachen magnetischer Anomalien in Vulkaniklastika des Messel Maar-Diatrems
Final Report Abstract
In 2001 the 433 m deep Messel 2001 borehole was drilled in the centre of the Messel Pit, 25 km south of Frankfurt (Germany). Geoscientific results from this drilling clarified the origin of the circular-shaped basin as a maar-diatreme-structure. Recovered deposits consist of lacustrine sediments (0-240 m) and volcaniclastic rocks such as lapilli tuffs (240-373 m) as well as rocks of the underlying diatreme breccia (373-433 m). The lapilli tuffs, as main interest here, show little differentiation on a macro- and microscopic scale and appear as a massive and unsorted volcaniclastic body with dominating juvenile lapilli and accidental clasts mostly in the range of (sub)millimetres to centimetres in diameter. Rock magnetic properties measured on core samples of the volcaniclastic units can explains the origin of downhole magnetic anomalies detected during the drilling project in 2001. Magnetic behaviour of the erupted material is related to fine-grained, Fe-rich (titano-) magnetite, which are dispersed within the juvenile lapilli. Temperature-dependent susceptibility experiments, isothermal remanent magnetisation and hysteresis investigations demonstrate similar ferrimagnetic properties throughout the volcaniclastic material, in terms of composition, coercivity and grain size (pseudosingle- domain particles) of the ferrimagnetic minerals. Thus, during emplacement of the erupted material, the ferrimagnetic minerals had the same remanence acquisition potential. However, demagnetisation experiments show different magnetic stability behaviour of the acquired natural remanent magnetisation (NRM). Heating experiments prove the acquisition of thermal remanent magnetisation (TRM) dominated by temperature effects which could have been occurred during eruption and deposition of volcanic material, forming the Messel maar-diatreme. It is assumed that the upper half of the lapilli tuffs was deposited at relatively low depositional temperatures (<300 °C), whereas the material of the lower half took advantage of higher temperatures (>>300 °C). To understand the rock magnetic character within the Messel maar-diatreme-facies, particle grain sizes, the degree of the relative fraction dominance and the shape of the juvenile fragments have been studied in more detail. Image analytical methods as well as major and trace element analyses on the juvenile fraction support the clear subdivision of the lapilli tuffs. These findings in combination with rockmagnetic data indicate a separation into a relatively hot, geochemically undifferentiated eruption phase and a colder, differentiated phase. A two-condition eruption stage at the end of the Messel volcanic activity is suggested. The juvenile particles account for the temperature evolution and heat conditions during deposition of the Messel tuffs and contribute to the origin of magnetic field anomalies. Based on gravity parameters and the results of magnetisation properties, the potential field 3Dmodel of the Messel subsurface explains the negative ground anomalies, calculates the mass and volume parameters of the drilled lithozones and shows the asymmetric appearance of the diatremestructure.
Publications
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(2005): Detailed investigation of preserved maar structures by combined geophysical surveys. - Bulletin of Volcanology, 68, (2): 95 - 106
Schulz, R., Buness, H., Gabriel, G., Pucher, R., Rolf, R., Wiederhold, H. & Wonik, T.
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(2005): Erste Ergebnisse von Suszeptibilitätsmessungen an Vulkaniklastika der Forschungsbohrung Messel 2001. - Zeitschrift der Deutschen Geologischen Gesellschaft, 155, (2-4): 343-352
Nitzsche, T., De Wall, H. & Rolf, C.
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(2005): Magnetische Untersuchungen an Kernen der Forschungsbohrung Messel 2001 - Erste Erkenntnisse zum Erwerb magnetischer Remanenzen in Pyroklastika. - In: Harms, F.-J. & Schaal, S. (eds.): Current Geological and Paleontological research in the Messel Formation. Courier Forschungsinstitut Senckenberg: 47-55; Frankfurt am Main
Rolf, C., Pucher, R., Schulz, R. & Wonik, T.
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(2006): Origin of magnetic anomalies in volcaniclastic units of the Messel maar-diatreme (Germany). - In: Goth, K. & Suhr, P. (eds.): Geophysics of maars and diatrems. Zeitschrift der Deutschen Geowissenschaftlichen Gesellschaft: 373-385
Nitzsche, T., Rolf, C. & De Wall, H.
