Multi-stage metasomatism revealed by trace element and Li isotope distributions in minerals of peridotite xenoliths from Allègre volcano (French Massif Central)

International audienceThe modal, chemical, and isotopic compositions of mantle peridotite are largely modified by metasomatic processes, which may affect them repeatedly. Xenoliths are commonly used to characterize those metasomatic processes along with the structure, and chemical and isotopic compositions of mantle domains. Nevertheless, the original mantle signatures born by mantle xenoliths are potentially obscured by the interactions occurring between the host magma and the xenolith itself. Here we attempt to identify to which degree the original Li content and isotopic composition, as well as other trace element contents of mantle xenoliths, can be modified by interaction with the host magma. Peridotite xenoliths that have suffered extensive exchange with the entraining magma were sampled in the solidified lava lake of Allègre, Southern French Massif Central, in order to decipher the signature related to peridotite-melt interaction, and to further unravel the evolution of the sub-continental lithospheric mantle. In-situ trace element analyses of clinopyroxene (Cpx) were performed via LA-ICP-MS, and the Li content and isotopic composition of pyroxene and olivine (Ol) via SIMS. Negative HFSE anomalies (Ti/Eu ratios as low as 437) and markedly high LREE/HREE ratios ((La/Yb)N as high as 79) are characteristic of mantle metasomatism at depth. Lithium isotope systematics indicates that at least two different metasomatic events affected the peridotite. Exceptionally high Li contents in Cpx (up to 50 ppm) and slight Li enrichment of Ol rims are ascribed to diffusive Li influx with a positive δ7Li value (+ 3.2‰) from the host magma after entrainment. Conversely, Ol cores preserve extremely light Li isotopic compositions (δ7Li as low as - 25‰) with high Li contents (up to 4.4 ppm) compared to normal mantle, indicating a metasomatic event that occurred before xenolith entrainment. The negative δ7Li signature of this early metasomatism may be related to subduction-related fluids released during the Variscan orogeny. Trace element distributions in minerals reveal that the HFSE and REE composition of Cpx and the negative δ7Li signature in Ol cores were not acquired simultaneously. Therefore at least three successive metasomatic events affected the Allegre peridotites, as revealed through the use of detailed in-situ Li isotopic analyses to trace melt-rock interactions