The role of the mantle decompaction layer in Hadean volcanism

How the very early Earth lost its internal heat remains a subject of debate. Early Earth may have been characterized by extensive magmatism due to a hot mantle, which then acted as the primary heat loss mechanism, or been more volcanically quiescent, where heat conduction through the lithosphere served as the primary heat loss mechanism. The primary mode of early Earth heat loss would then strongly influence tectonics and crust formation, the long-term thermal evolution of the interior, and surface environments where life could originate in the Hadean or Eoarchean.

Our recent crustal evolution model suggests that mantle melt production and mafic extrusive volcanism must have been limited prior to 3.6 Ga to remain consistent with Hf isotope data. We hypothesized that these geochemical constraints require a 'quiescent Earth' with a low melt production rate (<0.6 mm/yr). However, the actual magma supply is governed by complex geodynamic factors: specifically, the mechanics of melt generation, ascent, and accumulation at the mantle-crust boundary. Understanding these physical mechanisms is critical, particularly when evaluating high-flux regimes such as heat-pipe tectonics, which may be incompatible with the low rates inferred from the geochemical record.

A critical phenomenon affecting this supply is the formation of a decompaction layer beneath the mantle-crust interface (Sparks and Parmentier, 1991). Since the crust acts as a rigid thermal boundary, temperatures drop rapidly near this interface. Consequently, ascending melt encounters a freezing horizon that acts as a permeability barrier, causing it to accumulate. Within this zone, the decompaction layer and accumulated magma generate significant melt overpressure relative to the solid matrix, driving magma into the plumbing system and initiating ascent. Therefore, characterizing the dynamics of the decompaction layer is crucial for understanding the physical controls on melt supply.

Recent numerical modeling of Io, an active heat-pipe body (OReilly and Davies, 1981), demonstrates that crustal thermal structure is controlled by the physics of two-phase melt transport (Spencer et al., 2020). This model suggests that magma transport is driven by mantle overpressure at the decompaction layer but limited by solidification within the plumbing system. Applying this physical framework to the Hadean, we tested which mantle dynamics and temperature ranges are compatible with the restricted melt fluxes required by the geochemical record. Preliminary results demonstrate the existence of a decompaction layer, where both effective pressure and extraction rates increase significantly with porosity. By systematically exploring the parameter space, we identify the specific mantle geodynamic conditions required to align plateau melting tectonics with the Hf isotope constraints.