What seismic measurements really tell us about the composition of the lower crust

We present a new method to quantitatively assess the relationship between the composition and seismic velocity of igneous rocks. Equilibrating phase assemblages are calculated for all igneous composition space at various pressure and temperature conditions. Seismic velocities for each assemblage are estimated from mixing theory using laboratory measurements of the elastic parameters for pure mineral phases. The resultant velocities are used to derive a direct relationship between Vp and major element composition valid to +/-0.12 km/s for pressure and temperature conditions along a normal crustal geotherm in the depth range of 5--50 km. Finally, we use the calculated velocities to invert for major element chemistry as a function of seismic velocity assuming only the in situ temperature and pressure conditions are known. The resulting compositional bounds for a specific velocity are found to be broad. Compiling typical velocity-depth profiles for the lower continental and oceanic crust, we calculate compositional bounds for each of these geologic environments. We find that the acceptable compositional range for the lower continental crust is extremely large, ranging from basaltic to dacitic compositions, and conclude that seismic velocity measurements alone are insufficient to provide fundamental constraints on the composition of the lower continental crust. In the case of oceanic crust, observed lower crustal velocities are 0.2--0.3 km/s lower than velocities calculated based on the average bulk composition of gabbros in drill cores and exposed ophiolite sequences. We attribute this discrepancy to residual porosity at crustal depths less than ~10 km, and suggest caution when inferring mantle melting parameters from observed velocities in the lower oceanic crust.