Thermodynamic modelling and chronometric dating of melting and refertilization in the subcontinental lithospheric mantle

Geochemically distinguishing the products of melt depletion from refertilization and constraining the timing of such mantle-melt interactions in the subcontinental lithosphere (SCLM) remain outstanding research issues. Here we utilize alphaMELTS thermodynamic modeling of both partial melting and refertilization to revisit the origins of the Lherz mantle rocks from the Pyrenean orogenic mantle massifs. Thermodynamic modeling reveals subtle but critical differences between refertilization (i.e., elevated TiO₂/Al₂O₃ and higher HREE in both whole rocks and clinopyroxenes) and partial melting processes. Harzburgites from the Lherz massif comprise predominantly pristine residues of partial melting and subordinately refertilized products with low melt/rock ratios. The Lherz lherzolites that show close spatial associations with olivine websterite layers represent secondary rocks, derived from refertilization involving the pristine, refractory harzburgites and upwelling N-MORB-like melts. Lherzolites with no intimate spatial association to olivine websterites are open to an additional origin, i.e., via stationary cooling at the base of lithosphere after moderate adiabatic upwelling of asthenospheric mantle. The Re-depletion model ages (T_RD) of refractory harzburgites yield a systematic peak melting age of 2.0 Ga. We have developed a novel solution for constraining the relatively ancient age of refertilization (∼ 1.5 – 2.0 Ga) through the approach of an adapted percolation model to assess the behavior of Re-Os and Lu-Hf isotopic systems. Together with a comprehensive dataset of global on- and off-cratonic SCLM, this study has successfully distinguished silicate-melt induced refertilization from partial melting on elemental level, demonstrated the contribution of different melting mechanisms to the distinctive SCLM compositional evolution in the history, and highlighted how profound a role that refertilization has played on the variations of geochemical buoyancy and mechanical robustness and eventually on the stability and longevity of the ancient SCLM.