Constraints on melting, temperature and chemical composition from full waveform tomographic images

Abstract:

The recent development of full waveform tomography on continental scales has provided new insights into the seismic structure of the lithosphere and asthenosphere. In particular, we can map shorter wavelength, high-amplitude velocity anomalies which would traditionally be damped and spatially smeared using classical methods. Quantitative interpretation of these anomalies – expressed as absolute rather than relative velocities – may open up the possibility of identifying important dynamic processes such as melting, that would otherwise go undetected or unconstrained. In this study we focus on the S-wave speed structure beneath Europe, as obtained from full waveform inversion. On average, the European continent is slow compared to 1-D reference models such as AK135, but of particular interest are regions beneath the Atlantic and Iberian Peninsula which are 6-8% slower. Traditional interpretation of a regional tomography model would assume a fixed chemical composition, and from this estimate lateral temperature variations. In our case we allow both the temperature and composition to vary at random within very broad ranges, and generate thousands of different thermochemical structures in a Monte Carlo procedure. We then convert these thermochemical structures into S-wave speeds using thermodynamic modelling, and including a correction for temperature-dependent intrinsic attenuation. Although we cannot uniquely define the chemical composition in a given location, due to trade-offs between different chemical components and the temperature, this does not affect our interpretation of the very slow regions. We find that in order to generate such low velocities without melting, either prohibitively high temperatures (above the melting temperature) or extremely low Q values (in contradiction with seismic constraints) are required. Our study demonstrates: 1. The importance of using absolute rather than relative seismic wave speeds in constraining the thermochemical structure and 2. That interpreting Vs structure alone without further observables (such as Vp or density) may still provide important insights into mantle physical properties.