Modelling the anisotropic seismic properties of partially molten rocks found at mid-ocean ridges

Abstract

The problem of modelling the seismic properties of mid-ocean ridge rocks of the axial magma chamber (AMC) and the low-velocity triangle (LVT) immediately below it has been addressed using two samples from Oman ophiolite as examples. The specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis. The seismic properties have been simulated at a temperature of 1200°C and pressure of 200 MPa so that the basalt melt is above its solidus. Various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation (CPO) and an introduced anisotropy due to oriented melt filled inclusions. Calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity. The self-consistent scheme (SCS) and differential effective medium (DEM) are compared in some detail. A tensorial model was developed using a poro-elastic method of Gassman at low seismic frequency and a standard DEM with isolated basalt inclusions at high frequency. At low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform, whereas at high frequency basalt inclusions are isolated. Assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation. Calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction. The symmetry of the background anisotropy due to CPO is preserved, but gradually reduced with increasing melt fraction. For the seismic velocities observed for the axial magma chamber (AMC) with V p of 3.5 to 4.7 km/s the model predicts 50–70% melt. For the low-velocity triangle (LVT) below the AMC with V p of 4.8–5.6 km/s the model predicts 35–50% melt. Predictions from the observed attenuations ( Q −1) at the AMC of 0.05-0.02 and in the LVT of 0.02-0.01 are around 60% and 45% melt respectively. Seismic anisotropy has been modelled with ellipsoidal basalt ‘pancake’ shaped inclusions with their circular sections in the ( XY) foliation plane. Only a 2–3% of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium. There is a rapid decrease in V p and increase of Q −1 for propagation normal to the foliation ( Z) with increasing axial ratio of the inclusions. With increasing axial ratio the model predicts decreasing amounts of melt for given velocity V p in the Z direction. For the LVT only 15–25% melt is predicted for a 10:1 inclusion instead of the 35–50% of the spherical inclusion.