Chemical composition of the continental crust: Insights from a quantitative interpretation of the Vp/Vs ratio

Publikation: Bidrag til tidsskriftTidsskriftartikelfagfællebedømt

The processes driving continental crust formation are not yet fully understood. One of the fundamental keys necessary to investigate the enigma is represented by crustal composition. The Vp/Vs ratio from seismic receiver functions or tomography studies is a powerful tool to constrain the crustal composition. However, to date only qualitative relationships between Vp/Vs and composition have been proposed. We present a quantitative interpretation of the Vp/Vs in terms of major oxide components, based on thermo-elastic constrained modelling of rock phase equilibria and physical properties. The geophysical-petrological approach is implemented in the new release of the software package LitMod, which now allows for integrated and self-consistent modeling of the entire lithosphere (crust + lithospheric mantle) and upper mantle. Forward modelling of the Vp/Vs, based on petrology and thermodynamics, reveals that, as expected, mafic compositions have higher Vp/Vs than felsic ones. However, in high temperature settings (surface heat flow > 75 mW/m2), the quartz alpha / quartz beta transition strongly increases Vp, leaving Vs almost unaltered, leading to SiO2-rich compositions displaying Vp/Vs values higher than those associated with mafic compositions. Additionally, we highlight the importance of H2O, the presence of which stabilizes amphibole (in place of pyroxene), characterized by a relatively low Vp/Vs. If H2O is present, mafic compositions show Vp/Vs ratios that are comparable to those produced by anhydrous SiO2-rich compositions. The destabilization of amphibole (in favour of pyroxene) generates a sharp seismic discontinuity, potentially detectable by, for example, seismic refraction and receiver function investigations. We invert the Vp/Vs ratio for composition and hydrous state of the crust in the Southern African cratons. Our results show that the Kaapvaal craton, Archean in age, has an intermediate (SiO2 60 wt%) composition. The finding has implications on our understanding of the mechanisms responsible for crustal formation as an intermediate bulk composition is potentially generated by subduction-related processes. We conclude that, already before the Meso-Archean (3.2 Ga), subduction related crust-forming processes were active in the area and potentially also in other cratons.
OriginalsprogEngelsk
TidsskriftAmerican Geophysical Union, Fall Meeting 2017
Vol/bind11
StatusUdgivet - 1 dec. 2017
Eksternt udgivetJa

ID: 303367604