Reconstructing alpine massifs paleoaltimetry using stable isotopic composition of fluid inclusions in quartz veins

We collected quartz veins and meteoric waters at alike altitudes on the Mont Blanc and the Chenaillet massifs in the French Alps to reconstruct the paleoaltimetry of these massifs. We analyzed 17 modern meteoric waters in total; the isotopic ratios of the MBM samples show a negative correlation with sample elevation defining a linear trend varying from the low values (δ 2 H = −144.3‰; δ 18 O = −18‰) at high elevation (3759 m) to higher values (δ 2 H = −33.3 ‰; δ 18 O = −4.9‰) at lower altitudes (1200 m) comparable to the Global Meteoric Water Line. The isotopic composition of Chenaillet samples collected at elevations ranging from 2160 to 2600 meters showed lower dispersion, with δ 18 O values ranging from −11.8‰ to −15.9‰ and δ 2 H values ranging from −83 ‰ to −108.1 ‰. The quartz veins from the Mont Blanc massif (MBM) and the Chenaillet massif (CM) used in this work cements tectonic fissures formed during compressive or extensive events associated with the exhumation of the massifs. To recover the water from quartz FIs and obtained their isotopic composition, we used two distinct extraction methods (thermal decrepitation and crushing) and two distinct analytical methods (the conventional one: equilibration/reduction and the one developed in this study combining H and O analysis). Three separate laboratories, the LGL-TPE and ISA in Lyon and the CRPG in Nancy performed the analyses. The FIs contained in the quartz of the MBM provide δ 18 O values ranging from −5 ‰ to −10.5 ‰ and δ2H values ranging from −33 ‰ to −76 ‰. The FIs from the CM have an isotopic composition ranging from −6.4 to −16.9% for the δ 18 O values and from −83 ‰ to −120.3 ‰ for the δ2H values. The results indictae that the water in the FIs is meteoric in origin. The presence of adularia associated with quartz in the MBM samples allow to date the formation of the veins between 14 and 11 Ma. Based on data from the literature, we suggest an age of 22 Ma for the formation of the veins in the CM. A comparison of the isotopic compositions of the FIs waters with modern precipitations suggests that the MBM was at low elevation (between 0 and 1156 m) during the Middle Miocene, while the CM was already uplifted at an elevation between 2250 m and 3750 m. Finally, we demonstrate that H and O analysis of aqueous fluid inclusions from quartz veins is an effective method for mountain range paleo-altimetry reconstruction.