The cratonic lithospheric mantle is the buoyant 200–250 km deep root that enabled ancient continents (cratons) to survive for billions of years. The root stabilised due to extensive melting, erupting magnesian magmas at surface. However, peridotite xenolith samples from the cratonic mantle exhibit far more variable olivine/orthopyroxene and MgO/SiO2 ratios than expected from residues after melting of fertile mantle, exemplified by enigmatic dunites and orthopyroxene-rich harzburgites. Similarly, Archaean ultramafic lavas (komatiites) also show wider MgO/SiO2 variability than could be expected from shallow olivine fractionation. In addition, many komatiites have unexpectedly complex radiogenic isotope systematics, inconsistent with high-degree melting of refractory mantle. Thermodynamic modelling has suggested that these complexities could arise during deep (2–5 GPa) melt-solid reactions. Here, we present high-pressure layered experiments (3 GPa) involving the reaction between komatiite juxtaposed against harzburgite. The melt-bearing reactions yielded layered charges, wherein the initial komatiite layer (original, unreacted composition) was transformed into orthopyroxene-rich harzburgite following the reaction, and the original harzburgite layer resulted in the formation of dunite, with mineral modes and Mg-number depending on temperature. The solid residues of the reacted layers (orthopyroxene-rich and olivine-rich) are compositionally and mineralogically analogous to natural cratonic harzburgites and dunites, respectively. Thermodynamic modelling indicates that the MgO/SiO2 variability of the residues could be further increased by reacting the komatiite with fertile lherzolite instead of harzburgite. Solid-melt reactions markedly altered the composition of the incoming komatiite melt, with basaltic komatiite and komatiitic basalt formed at lower temperatures and modified high-Mg komatiites at higher temperatures. The melts exhibit the very wide range of MgO/SiO2 seen in ultramafic lavas in Archaean greenstone belts. Consideration of rare earth element and Hf partitioning suggests that assimilation of protocratonic ancient harzburgite into invading komatiite will be much more strongly seen in radiogenic Hf than Nd (or Ce) isotope ratios. Although both the original solid (harzburgite) and the incoming melt (komatiite) are ultramafic, their chemical contrast is sufficient to drive metasomatic reaction, likely partly due to the presence of a percolating melt. The reactions require higher temperatures than attained in the post-Archaean mantle, explaining both the sharp demise of komatiites after 2.5 Ga and the highly refractory nature of Archaean harzburgites and dunites. Ancient proto-cratonic mantle is one plausible candidate harbouring radiogenic signatures of Hadean silicate differentiation. The reason why these signatures are most commonly found in Archaean mafic and ultramafic igneous rocks may relate to the secular drop in maximum plume temperature, which has compromised the capacity of younger melts to sample this reservoir via assimilation.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)