Phase Transformations in the Lower Continental Crust and its Seismic Structure
- Robert F. Mereu,
- Stephan Mueller and
- David M. Fountain
Published Online: 9 APR 2013
Copyright 1989 by the American Geophysical Union
Properties and Processes of Earth's Lower Crust
How to Cite
Sobolev, S. V. and Babeyko, A. Yu. (1989) Phase Transformations in the Lower Continental Crust and its Seismic Structure, in Properties and Processes of Earth's Lower Crust (eds R. F. Mereu, S. Mueller and D. M. Fountain), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM051p0311
- Published Online: 9 APR 2013
- Published Print: 1 JAN 1989
Print ISBN: 9780875904566
Online ISBN: 9781118666388
Data on lower crustal xenoliths and high grade metamorphic terrains suggest that the lower crust of tectonically stable continental regions can consist of anhydrous mafic rocks with equilibration temperatures higher than 700°C. These rocks are chemically unstable in the cool continental crust and survive only due to the extremely low reaction rates at T<700°C.
The method of calculation of density-depth and seismic wave velocity-depth relations in the mafic lower crust is developed. the calculations are made in two steps. Firstly, we determine chemically stable mineral assemblages in pressuretemperature diagram using Gibbs energy minimization technique for multicomponent systems and compute corresponding densities and seismic wave velocities of the rocks. Secondly, we estimate the deviations of densities and seismic wave velocities from those at equilibrium using a linearized kinetic model of solid state phase reactions in rocks and models of thermal evolution of the lithosphere.
The method suggested is applied to a number of problems pertaining to the seismic structure of the lower crust. It is shown that quartz tholeiite chemical composition of the lower crust is consistent with seismic data if kinetics of solid state phase transformations is considered. the emplacement of thin mafic intrusions into the mafic lower crust produces seismic layering even if the intrusions have the same chemical composition as a bulk of the lower crust. Phase transformations in the mafic lower crust coupled with the mechanical instability at the crust-mantle boundary limit the thickness of the crust. the “extra” crustal material goes into the mantle and participates in the process of recycling.