Selective transfer of Li-Al-rich phyllosilicate to metamorphic veins (Western Alps): Laser Induced Breakdown Spectroscopy (LIBS) compositional profiles and microstructural characterization

International audienceIn convergent settings, fluid-rock interactions generally result in quartz and calcite preferential transfer to metamorphic veins in classical metamarls, while phyllosilicates tend to remain in the host-rock. However, the mechanisms responsible for such a selective mass transfer are poorly discussed in the literature. Here, we study Alpine metabauxites in which phyllosilicates (Li-Al-rich chlorite called cookeite, followed by pyrophyllite) were preferentially transferred to veins at blueschist peak P-T conditions, by a dissolution-diffusion-precipitation process without any fluid infiltration or associated reaction. Cookeite fibrous en-echelon veins formed by extensional shear, and part of them evolved towards thicker fluid-filled veins with euhedral cookeite crystallization. We performed chemical profiles across host-rocks between successive cookeite veins, using Laser Induced Breakdown Spectroscopy (LIBS), associated to a microstructural study. Flat LIBS Li profiles show that about half of the initial cookeite remains homogeneously distributed in host-rocks, which suggests a diffusion distance of 2–4 cm for Li. The availability of an aqueous fluid during most of the metamorphic cycle is demonstrated here. A simple 1D reaction-diffusion model, assuming Li diffusion through a connected fluid-filled porosity network, is able to account for the observed lithium distribution assuming Li diffusion coefficients and mineral-water interaction rates consistent with literature values in fluid-bearing systems. Chemical potential gradients that drove phyllosilicate transfer to veins can be either interfacial energy driven (i.e., Ostwald ripening), the anhedral phyllosilicate microsheets maintaining high supersaturation levels in the small host-rock pores compared to veins, or stress-induced: phyllosilicates present the highest solubility variations with pressure in the Vanoise bauxites (contrary to quartz-bearing rocks), which may account for their unusual selective transfer to veins. Therefore, mineral solubility variation with pressure seems to be the chief controlling parameter for pressure-solution creep. Cookeite (over pyrophyllite) transfer to veins was favoured by faster Li diffusion (compared to Si and Al). The shift from cookeite to pyrophyllite transfer, before host-rock cookeite-depletion, could reflect the onset of the retrograde P-T path, with pyrophyllite crystallization in response to P-T decrease. Lithium, which is a strategic element, was observed to preferentially migrate and segregate into veins during metamorphic processes, which may be of importance for exploration purposes