THE ROLE OF INITIAL CONDITION IN CONTROLLING THE VERTICAL STABILITY OF DEEP MANTLE COMPOSITIONAL RESERVOIRS

One of the leading hypotheses for the presence of the Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle is the presence of compositionally distinct reservoirs of intrinsically more-dense material than the background mantle. If mantle plumes are generated from these reservoirs, this hypothesis agrees with the observed difference in trace element geochemistry between ocean island basalts (OIBs) and mid ocean ridge basalts (MORBs). Geodynamic models show such compositional reservoirs may take one of two forms; passive, vertically stable thermochemical piles, or more active, vertically unstable thermochemical superplumes. The formation of these features is largely controlled by the density contrast between the reservoirs and the background mantle. Here we find the initial thermal condition plays an important role in the stability of the features created by these compositional reservoirs. Earth’s thermal history is not well understood, but it is assumed the Earth’s mantle has been cooling throughout geological history. Contrary to this assumption, numerical calculations normally start with a compositionally layered system starting at thermal equilibrium. Laboratory experiments start with a compositionally layered system that is gradually heated during the experiment. In this study we use numerical calculations to explore the effects of density differences and initial thermal conditions on the stability of compositional reservoirs. We find that an initially hot mantle allows the compositional reservoirs to remain vertically stable at relatively small density differences. As the mantle continues to cool we see these compositional reservoirs develop more topography, and start stirring with the background mantle due to entrainment in plumes. An initially cold mantle, or a mantle at thermal equilibrium makes these compositional reservoirs vertically unstable at relatively small density differences. It is only at larger density differences these compositional reservoirs become vertically stable. In summary, an initially hot mantle allows compositional reservoirs to remain stable with a variety of density differences.