The density of magmatic, ultrametamorphic rocks of the Ukrainian shield in model deep conditions of the Earth’s crust (experimental data)

Abstract

Analysis of the materials of geophysical studies using the data on the physical properties of rocks at various depths contributes to more substantiated understanding the structure of the Earth's crust horizons which are not accessible for drilling. In this regard, our method of petrophysical thermobaric modeling (PTBM) is a successful tool for this purpose. Its basic principle is a comparison (taking into account surface geology) of experimental data on changes in the physical parameters of rocks (in particular, the velocity of elastic longitudinal waves (V_p=f (Н)) and density (ρ=f (Н)) under РT-conditions of thermobaric deep regimes of the Earth’s crust of the study regions and DSS and gravimetric information. Continuing the study of the elastic-density characteristics of rocks and minerals under high pressures and temperatures, we present the results of experiments to examine changes in the density of igneous and ultrametamorphic rocks. They are widespread in the Ukrainian shield, mostly often found in the form of large blocks and massifs with the thickness of up to several tens of kilometers. Our long-term studies convincingly show that temperature significantly influences the relationships of V_p and ρ under conditions of great depths. As experiments demonstrate, they vary according to nonlinear laws with a depth indicating maxima and minima. The simultaneous effect of pressure and temperature on the mineral substance corresponding to the interval of depths of the Earth's crust of 5—20 km results in inversion zones of the elastic waves velocity and density. The zones are similar to the low velocity zones (LVZs) found in the Earth’s crust by DSS methods and are characterized by rocks decompaction. Like the LVZs of reduced density (according to experimental data), the inversion zones are sensitive to the temperature conditions of the Earth’s crust. With increasing deep heat flux, the rock density decreases, the ability of rocks decompaction increases, their permeability and hygroscopicity increase that can activate the process of fluid movement and their localization. Such crustal anomalies must be taken into account in refining the relationship of V_p=f (ρ) at various depths (the most important relation in seismic-gravity modeling and PTBM). In most cases, this relationship for all specimens of rocks at certain depths is far from linear one contrary to popular opinion. First of all, this observation is explained by the complex dependences of the elastic and density characteristics of rocks with depth. The materials of an experimental study of the elastic-density characteristics of igneous and ultrametamorphic rocks of the Ukrainian shield under model deep conditions of the Earth’s crust are widely used for petrophysical thermobaric modeling of specific sections of the deep horizons of the Earth’s crust of various regions of the planet.