Tehostekuvassa fysiikan laitosta etuoven suunnalta

6e. Research of deep drillcores: Outokumpu formation (Finland)

Participating scientists

  • T. Elbra
  • L.J. Pesonen
  • I. Lassila
  • E. Haeggström
  • R. Karlqvist

Highlights

The Paleoproterozoic Outokumpu region is located in the Fennoscandian Shield, in eastern Finland, close to the Archean- Proterozoic boundary zone and is known for its occurrences of early Proterozoic Cu-Zn-Co sulphide ore deposits. The Outokumpu ores are associated with a ophiolitic complex which is embedded in mica schists and -gneisses. To characterize the geophysical nature of the Paleoproterozoic crustal section of eastern Finland and to find lithological and geophysical interpretations to the distinct crustal reflectors as observed in seismic surveys the petrophysical, in particular seismic velocity, measurements of the Outokumpu Deep Drill Core (depth 2.5 km) were carried out. The results show that different lithological units can be identified based on the petrophysical data. The density of the samples remained nearly constant throughout the drilled section. Only diopsidetremolite skarns and black schists exhibit higher densities. The samples are dominated by the paramagnetic behaviour with occasional ferromagnetic signatures caused by serpentinitic rocks. Large variations in seismic velocities, both at ambient pressure and under in situ crustal conditions are observed. The porosity of the samples, which is extremely low, is either intrinsic by nature or caused by decompaction related to fracturing during the core retrieval. It is noteworthy that these microfractures have dramatically lowered the VP and VS values. From the measured velocities and density data we have calculated the seismic impedances, Young's modulus and Poisson's ratios for the lithological units of the Outokumpu section and from these data the reflection coefficients for the major lithological boundaries, evident in the surveyed section, were determined. The data show that the strong and distinct reflections visible in wide-angle seismic surveys are caused by interfaces between diopside-tremolite skarn and either serpentinites, mica schist or black schist (Elbra et al., 2011).

Instrumentation Laboratory

Fig. 2: Instrumentation

Recent publications

  • Elbra, T, Karlqvist, R, Lassila, I.J., Haeggström, E., Pesonen, L.J., 2011. Laboratory measurements of the seismic velocities and other petrophysical properties of the Outokumpu deep drill core samples, eastern Finland. Geophysical Journal International, 184(1), 405-415.
  • Elbra, T., Karlqvist, R., Lassila, I., Haeggström, E., Pesonen, L.J., 2011. P- and S-wave velocities of rocks from the upper 1.5 km crustal section sampled by the Outokumpu Deep Drilling Project, Finland. In: Kukkonen, I. (ed.), Outokumpu Deep Drilling Project 2003-2010. Special Paper 51, Geological Survey of Finland, Espoo, p. 95-104.
  • Pesonen, L.J., Elbra, T., Karlqvist, R., Lassila, I., Haeggström, E., 2008. Seismic velocities of the Outokumpu deep drill core and FIRE profile samples: what do the rocks tell us? In: Korja, T., Arhe, K., Kaikkonen, P., Korja, A., Lahtinen, R. and Lunkka, J.P. (Eds.), Lithosphere 2008, Fifth Symposium on the Structure, Composition and Evolution of the Lithosphere in Finland. Programme and Extended Abstracts, Oulu, Finland, November 5-6, 2008. Institute of Seismology, University of Helsinki, Report 53, 87-90.