Oxygen isotopes reveal crustal contamination and a large, still partially molten magma chamber in Chaîne des Puys (French Massif Central)
Résumé
The two main magmatic properties associated with explosive eruptions are high viscosity of silica-rich magmas and/or high volatile contents. Magmatic processes responsible for the genesis of such magmas are differentiation through crystallization, and crustal contamination (or assimilation) as this process has the potential to enhance crystallization and add volatiles to the initial budget. In the Chaîne des Puy series (French Massif Central), silica- and H2O-rich magmas were only emitted during the most recent eruptions (ca. 6-15 ka). Here, we use in situ measurements of oxygen isotopes in zircons from two of the main trachytic eruptions from the Chaîne des Puys to track the crustal contamination component in a sequence that was previously presented as an archetypal fractional crystallization series. Zircons from Sarcoui volcano and Puy de Dôme display homogeneous oxygen isotope compositions with δ18O = 5.6 ± 0.25‰ and 5.6 ± 0.3‰, respectively, and have therefore crystallized from homogeneous melts with δ18Omelt = 7.1 ± 0.3‰. Compared to mantle derived melts resulting from pure fractional crystallization (δ18Odif.mant. = 6.4 ± 0.4‰), those δ18Omelt values are enriched in 18O and support a significant role of crustal contamination in the genesis of silica-rich melts in the Chaîne des Puys. Assimilation-fractional-crystallization models highlight that the degree of contamination was probably restricted to 5.5-9.5% with Rcrystallization/Rassimilation varying between 8 and 14. The very strong intra-site homogeneity of the isotopic data highlights that magmas were well homogenized before eruption, and consequently that crustal contamination was not the trigger of silica-rich eruptions in the Chaîne des Puys. The exceptionally strong inter-site homogeneity of the isotopic data brings to light that Sarcoui volcano and Puy de Dôme were fed by a single large magma chamber. Our results, together with recent thermo-kinetic models and an experimental simulation (Martel et al., 2013), support the existence of a large ( 6-15 km3), still partially molten mid-crustal reservoir (10-12 km deep) that is filled with silica-rich magma. Calculated oxygen isotope compositions of the trachytic melts that crystallized the analyzed zircons for Puy de Dôme, Sarcoui dome, and Sarcoui phreatomagmatic deposits, and the range of values for each analyzed zircon grain. The range for trachytes obtained by pure fractional crystallization of mantle melts is given for comparison. See text for details on calculations. Chemical differentiation trend of Chaîne des Puys magmas (data from Boivin et al., 2009), and results of the fractional crystallization models presented herein and in Table 3. L1 is obtained after the first step of differentiation, and L2 after the second. The composition of Sarcoui trachytes is identified by an X. S3.1. Core-rim variations for oxygen isotope compositions of the studied zircons. S3.2. Oxygen isotope compositions of the various zircon domains observed with cathodoluminescence imaging (dark versus bright), and for zircons with different types of zoning (oscillatory versus sector). No systematic variation is observed.