Classification of Volcanoclastic Deposits Using Gamma-ray- and Magnetic Susceptibility Measurements

Iris Gehring (gehring@geologie.uni-wuerzburg.de)¹, Pierfrancesco Dellino (dellino@lgxserve.ciseca.uniba.it)² & Bernd Zimanowski (zimano@mail.uni-wuerzburg.de)¹

¹ Physikalisch-Vulkanologisches Labor, Institut für Geologie, Pleicherwall 1, 97070 Würzburg, Germany

² Dipartimento Geomineralogico, Università di Bari, Via Orabona 4, 70125 Bari, Italy

Hazard assessment at active volcanic systems must be based on the understanding of the specific volcano's behaviour in the past. Therefore, the quantitative description (erupted volumes and spacing) at least of its proximal pyroclastic deposits is mandatory. However, the high variability of facies types within single deposits in many cases do not allow a stratigraphical classification by sedimentological or geochemical features only. Here we report on a successful application of geophysical field and laboratory methods in the case of La Fossa di Vulcano, Isola di Vulcano, S-Italy. We used gamma-ray-measurements taken in the outcrops layer by layer in combination with grain-size dependent magnetic susceptibility measurements in our laboratory to define marker horizons in the stratigraphy.

Results of the susceptibility measurements show, that ferromagnetic minerals are concentrated in the grain sizes between 0.5 to 0.25 and 0.25 to 0.125 mm. This grain-sizes are not significantly affected by the specific transport and depositional mechanisms of La Fossa. Therefore, systematic variations of the susceptibility values could be used to define stratigraphical marker beds. In combination with these results, the respective gamma-ray values support the origin of these systematic fluctuations in magma chamber processes.

In the case of La Fossa, stratigraphic complexities of the proximal pyroclastic deposits erupted during the past 6000 years, could be solved by the combined application of these methods.

Cordierite I: The Co-ordination of Fe²⁺

Charles A. Geiger (chg@min.uni-kiel.de)¹, Thomas Armbruster², Vladimir Khomenko³ & Simona Quartieri⁴

¹ Institut für Geowissenschaften der CAU zu Kiel, Olshausenstr. 40, D-24098 Kiel, Germany

² Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland

³ Ukrainian Academy of Science, pr. Palladine 34, 252142 Kiev, Ukraine

⁴ Dipartimento di Scienze dell Terra, Università di Messina, Salita Sperone 31, 98166 S. Agata die Messina, Italy

Cordierite has the ideal formula (Mg,Fe)₂Al₄Si₅O₁₈.(H₂O,CO₂). Natural cordierites can be described by the general formula (Na,K,Fe2+,Fe3+)_0-1(Mg,Fe2+,Mn2+,Li)₂(Si,Al,Be,Fe3+)₉O₁₈ .x(H₂O,CO₂,Ar) (Schreyer, 1985). One of the complications concerns the role of Fe²⁺. It is well known that most Fe²⁺ occupies the octahedral M-site. However, some Fe²⁺ can occupy an additional structural site.

The incorporation of Fe²⁺ has been investigated in four natural cordierite samples. ⁵⁷Fe Mössbauer, single-crystal electronic absorption, and X-ray absorption spectroscopy were used, as well as X-ray single-crystal diffraction. ⁵⁷Fe Mössbauer and XAS spectroscopy show that Fe²⁺ is incorporated on two different structural sites in two Mg-rich samples. The spectroscopic data are consistent with small amounts of Fe²⁺ substituting on a tetrahedral site and not in channel cavities or in the six-membered ring. Mössbauer measurements give the best quantitative measure of the amounts of Fe²⁺, but the electronic absorption spectra are the most sensitive for determinations at low concentrations and at high-bulk Fe-concentrations in cordierite. X-ray single-crystal refinements on the two Mg-rich cordierites show a very small excess electron density on T11. We interpret these data as indicating that small amounts of Fe²⁺ (0.01 to 0.02 atoms per formula unit) can replace tetrahedral Al in cordierite, where charge balance is achieved by placing Na in the center of the six-membered rings. The identification and assignment of small amounts of Fe²⁺ on T11 is not possible with normal microprobe analysis methods and requires spectroscopic determination.

Schreyer W, Bull. Mineral, 108, 273-291, (1985).

Si ­ A Major Constituent in the Earth's Core?

C. K. Gessmann (christine.gessmann@bristol.ac.uk)¹, M. R. Kilburn², B. J. Wood¹ & D. C. Rubie³

¹ Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, U.K.

² Max-Planck-Institut Mainz, Mainz, Germany

³ Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany

Since the recognition that the Earth's outer core is significantly less dense than pure Fe, it is believed that the core contains about 10% of one or more light elements, which were presumably dissolved into the Fe-Ni-rich metal during separation from the mantle. Among others, Si is one of the likely light elements in the core. Assuming that the contents of non-volatile elements in the Earth approximate those of CI chondrites as suggested by Allègre et al. [1], the relative depletion of Si in the mantle implies that the core contains 7wt% Si. In order to test this hypothesis, we determined the solubility of Si in liquid Fe-rich metal up to 23 GPa and 2400°C over a range of oxygen fugacities using piston-cylinder and multianvil apparatus. Starting materials were metal powders, doped with various elements, contained in MgO-capsules. Results show that the solubility of Si in liquid metal varies systematically with P, T and fO₂. The solubility of Si increases significantly with decreasing oxygen fugacity (at constant P, T; [2]), with increasing pressure (at constant T, fO₂) and, with increasing temperature (at constant P, fO₂). These results suggest that significant amounts of Si are soluble in liquid metal even at 'moderate fO₂', e.g. 4.5 wt% Si at 18 GPa, 2200ºC, IW-3.5. The experimental data were recalculated to IW-2 (for consistency with the FeO content of the mantle), fitted by thermodynamic models and extrapolated to 30 GPa. At this pressure, 7wt% Si dissolve in metal at temperatures which are plausible for core formation, thus supporting Si as a possible light element in the core. Extrapolation of the models to higher pressures, however, suggests that Si, dissolved in metal during core formation at conditions discussed above, will exsolve from the metal at core-mantle boundary conditions.

Allègre CJ, Poirier J-P, Hummler E & Hofmann AW, Earth Planet. Sci. Lett., 134, 515-526, (1995).

Kilburn MR & Wood BJ, Earth Planet. Sci. Lett, 152, 139-148, (1997).