Evidence of increased density of LLVPs from vote map constrained density inversion

The Large Low Velocity Provinces (LLVP) are two antipodal regions of reduced seismic velocity that extend about 800 km into the mantle from the core-mantle boundary. The LLVPs might affect the generation of plumes and organize large-scale plate motions.

However – except for the reduced velocity – almost all properties of the LLVPs are the subject of vigorous debate. The LLVPs could simply be hot upwellings, or they could be chemically different from normal mantle. They could be a transient feature, exist since the Early Earth or be the result of continuous accumulation as a result of plate tectonics. To some extent, determining the density of the LLVPs could help to distinguish between these scenarios. However, most seismic methods are only weakly sensitive to density and so far both negative and positive density anomalies have been proposed based on seismology. A more direct means of assessing the density structure comes from inverting the gravity field.

While density inversions are inherently non-unique, this can be somewhat alleviated by constraining the geometry of potential sources of the gravity anomalies. In this contribution, we use vote maps to constrain the geometry. A vote map is based on a collection of seismic tomographies and highlights areas of agreement between the seismic tomographies.

We find that the LLVPs possess a slight positive density anomaly between 0.1 and 0.6 %. The variation results from how the lithosphere is treated, since we use both an isostatic model and seismically determined Moho depths, with the isostatic model resulting in smaller LLVP densities. The combination of increased density and reduced velocity can only be explained if the LLVPs are somewhat chemically different from ‘normal’ pyrolitic mantle. Using petrophysical data bases we estimate that an enrichment of 1-1.5% iron oxide content together with a temperature increase of 260 – 380 K with respect to an adiabatic temperature curve can explain the density increase and velocity decrease. Alternatively, the LLVPs would have to contain 40-60 % Mid-Oceanic Ridge Basalt and be 870 – 960 K hotter in order to explain our findings.