Upper mantle xenoliths as sources of geophysical information: the Perşani Mts. area as a case study

Abstract

The aim is to give an overview on how geochemical and petrological data, obtained from upper mantle xenoliths, could be utilized to provide information on the geophysical properties of the upper mantle at their origin. First we demonstrate how a tentative lithospheric column may be constructed based on the equilibrium temperature of upper mantle xenoliths and the area specific depth-temperature curves. Then it is described how the speed of seismic waves at the given pressure and temperature conditions could be calculated from the modal composition and geochemistry of major rock forming minerals of upper mantle xenoliths (e.g. olivine and orthopyroxene). It is also discussed how the lattice preferred orientation of minerals in upper mantle xenoliths provides information on the seismic anisotropy of the upper mantle, and how this information could be used to calculate the orientation and thickness of the anisotropic layer in the upper mantle if one anisotropic layer is assumed. Structural hydroxyl (or most commonly referred to as ‘water’) incorporated in nominally anhydrous minerals plays a critical role in determining the electrical conductivity and rheology of the upper mantle. Finally, it is presented how electrical conductivity and effective viscosity of the upper mantle could be approximated based on the structural hydroxyl content in olivine, the most abundant mineral constituent of the upper mantle. Our study area, the Persani Mountains is situated in the Carpathian Bend area (Romania) which is geologically one of the most active areas in Europe. Abundant upper mantle xenoliths from the Persani Mountains (Eastern Carpathians) will serve as examples how meaningful geophysical information can be obtained for the upper mantle. Furthermore, it is shown how these pieces of information may be utilized in interpreting geophysical and geodynamic challenges of this area.