Assessing the effects of heat-producing element enrichment and mantle thermal conductivity on the stability of primordial reservoirs

Thermo-chemical mantle convection models featuring heterogeneous thermal conductivity indicate that heat-producing element (HPE) enrichment in large low shear velocity provinces (LLSVPs) significantly impacts the long-term stability of these regions. Because the rate of internal heating was more significant in the past, thermal conductivity's influence on thermal buoyancy (and bulk erosion) must have also been more substantial. Consequently, their initial volume may have been significantly larger than their present-day volume. Energy balance calculations suggest that a smaller initial mantle volume fraction of LLSVP material supports more HPE enrichment than a larger mantle volume fraction to maintain the mantle's internal heat budget. For example, an initial layer thickness of 160km (~3% mantle volume) implies present-day HPE enrichment factors greater than ~45 times the ambient mantle heating rate (compared with more conservative factors of 10 to 20 for similar initial conditions employed in previous studies of thermo-chemical pile stability). Thus, HPE enrichment may have been significantly underestimated in earlier models of LLSVP evolution. Conversely, and assuming that LLSVPs formed from a much larger reservoir, HPE enrichment may be overestimated based on the present-day LLSVP volume. Our study considers LLSVPs with a primordial geochemical reservoir composition (consistent with an undegassed 4He/3He signature and HPE enrichment). We present thermo-chemical mantle convection models that feature time-dependent internal heating rates and HPE enrichment (implied by initial mantle volume fraction). In this new context, we re-examine, in particular, the impact of a fully heterogeneous lattice thermal conductivity (derived from conductivity measurements of upper and lower mantle minerals). Furthermore, in light of recent developments with radiative conductivity, we also examine the added effect of a strongly temperature-dependent radiative conductivity component on the stability of LLSVPs. Using tomographic filtering on our simulations, with LLSVPs' present-day volume and core-mantle boundary coverage as a constraint, we examine potential initial conditions, heating scenarios, and thermal conductivity for an Earth-like model.