ELEMENTAL MYSTERIES: A DEEP DIVE INTO MANTLE MATTERS. The impact of chemical composition on upper mantle density and mineral assemblage

Abstract

Geophysical and geodynamic modeling consistently increases our understanding of the Earth’s internal structure and processes. This thesis investigates the effects on rifting processes of mantle densities resulting from differences in mantle chemistry and mineralogy. Mantle densities were calculated utilizing Gibbs energy minimization algorithms and the effects on continental as well as oceanic rifting processes were quantified using a finite difference model.

Based on literature data, examples of ten mantle compositions pertaining to roughly five different mantle rock-types were chosen for investigation. As the first part of the work in this thesis, a four-dimensional visualization tool was developed to capture how density and mineral assemblage differ between the chosen compositions for the pressure range of 10-40 kbar and temperature range of 900-1800 °C.

For the second part of this thesis, four of the ten compositions were chosen for further comparison in the geophysical/geodynamic modeling software GeoMet. It was found that composition is clearly linked to bulk rock density for conditions and compositions found in the upper mantle regime. Finite difference modeling shows that this is highly relevant for geophysical/geodynamic modeling contexts, as the disregard or generalization of the upper mantles chemical composition leads to significant differences in density, which directly affects seismic velocities. While this has implications for the interpretation of seismic velocity anomalies, it also affects the geophysical and geodynamic models we make to understand and investigate the inner workings of our Earth.