Preservation of water concentration in mantle xenoliths: a case study from Allègre and Ray Pic volcanoes (French Massif Central)

Abstract. Water in the form of hydrous point defects in the crystal structure of pyroxenes from mantle xenoliths is frequently used to trace the water content in the lithospheric mantle. However, little is known about the mechanism that allows xenoliths to preserve deep hydrogen signatures and if we can avoid a complete or partial reset by reaction with the host magma during transport. In particular, it is unknown how much of the water content of xenoliths is modified during the emplacement of lava and cooling at the surface and by the eruption mode (effusive versus explosive). In this work, we attempted to address to these matters. We analysed the water content of peridotite xenoliths from two localities in the French Massif Central, Allègre and Ray Pic, using Fourier transform infrared (FTIR) spectroscopy. We performed point analyses and profile measurements in olivine (ol), clinopyroxene (cpx), and orthopyroxene (opx) crystals derived from 17 xenoliths. The two localities have different types of outcrops. In Allègre, the xenoliths are present in a frozen lava lake with a vertical structure, while in Ray Pic, xenoliths are hosted both in pyroclastic deposits and in a lava flow running over more than 20 km along a riverbed. Both studies on xenoliths in different parts of the lava flow show that the solidification and cooling of the basalt at the surface do not significantly affect the water content of pyroxenes in the xenoliths. The xenoliths do not show the presence of diffusion profiles, and the water content is independent of the location of the xenoliths within the lava bodies. However, the comparison of the water content of xenoliths from the pyroclastic deposit and within the lava flow at Ray Pic shows that the water concentrations are strongly impacted by the degree of degassing of the magma before the eruption. The concentrations of xenoliths in the degassed lava flow are much lower, by a factor of > 10 in ol and ≥ 2 in cpx and opx, compared to xenoliths from the pyroclastic deposits. Therefore, water content measured in pyroxenes can only represent minimal values, even for xenoliths hosted in rapidly cooled volcanic products (e.g. explosive eruptions). The amount of water in mantle xenoliths does not reflect the original amount of water in the lithospheric mantle. On the contrary, this study indicates that xenoliths picked up within the same lava flow, even at a few metres' distance, can exhibit pyroxenes with different spectral signatures. This suggests that the spectral signatures have been acquired before the emplacement of the lava flow and were not affected by the late degassing that occurred just before the eruption of the lava flow.