Evidence and significance of wehrlitization within continental lithospheric mantle

Wehrlitization refers to the mantle process that mantle peridotite interacts strongly with silica-undersaturated melts. This process usually triggers significant changes in mantle physico-chemical properties and thus is intimately linked to the thinning and destruction of cratonic lithosphere. Here, we first review the mechanism, texture and composition variations on wehrlitization and its impact on continental stability and then quantitatively characterize this process via thermodynamic modeling. It has been shown that wehrlitization may involve both modal and stealth metasomatism that usually introduce apatite, amphibole, phlogopite and carbonate into the peridotite and alter the modal abundance of rock-forming phases at the same time. Besides, non-equilibrium textures, including sponge texture, melt pockets (now presented as finer-grained clinopyroxene, olivine and other accessory minerals) and mineral zoning, are also typical wehrlitization. Thermodynamic modeling shows that the effects from melt impregnation on mantle peridotites are jointly controlled by the nature of the initial rock and the infiltrating melts as well as melt/rock ratio. Reaction with silica-poor (e.g., nephelinitic) melts usually facilitate the formation of clinopyroxene-rich peridotites and eventually wehrlite especially at high pressures (2.5Ga and 4.0GPa), regardless of the initial compositions of the peridotites (e.g., refractory harzburgite or fertile lherzolite). In contrast, the reaction with silica-rich (e.g., MORB) melts cannot produce wehrlite under the possible pressure range of continental lithosphere. Furthermore, the melt-rock reaction will always increase the density of mantle peridotites no matter of the melt compositions. The density of infiltrated fertile mantle may exceed that of the Preliminary Reference Earth Model (PREM), and this will eventually result in lithospheric instability. Collectively, our study quantitatively reveals the preferred physical conditions for wehrlitization and provides a quantitative theoretical framework for the previous conceptual model that peridotite-melt interaction can cause the thinning and destruction of the lithospheric mantle and has significant implications for continental stability.