Insights into the tectonic history of the Western Alps through dating of fissure monazite in the Mont Blanc and Aiguilles Rouges Massifs

Dating tectonic activity in the Lepontine Dome and Rhone-Simplon Fault regions through hydrothermal monazite-(Ce)

“…The SIMS spot measurements were distributed on the basis of domains visible in BSE images, among these the cen- Leu (1986); (d) tectonic section through the eastern Northern Steep Belt, redrawn after Wiederkehr et al (2008); (e) tectonic section across the Forcola normal fault; see also Meyre et al (1998) and Berger et al (2005). Profiles (b)-(e) are not to scale with map (a).…”

“…Whenever age clusters were found on the basis of these groups, weighted mean domain ages were calculated (Fig. 5), as these could be shown to generally date tectonic activity (Berger et al, 2013;Grand'Homme et al, 2016;Bergemann et al, 2017Bergemann et al, , 2018Bergemann et al, , 2019bRicchi et al, 2019). Since any new crystallization or alteration associated with a change in chemical composition must have happened in equilibrium with the surrounding fluid, any age cluster within a chemical group must be due to the simultaneous formation or alteration of those crystal parts.…”

“…The penetration of meteoric water into a fissure will typically lead to a dilution of the fissure fluid and an end of monazite crystallization. This means that while an age in an individual crystal might, in rare cases, record fluid flow unassociated with deformation, previous studies suggest that this is not the norm (Berger et al, 2013;Grand'Homme et al, 2016;Bergemann et al, 2017Bergemann et al, , 2018Bergemann et al, , 2019bRicchi et al, 2019). Consequently, individual ages found in one crystal of an area have therefore an uncertainty in what they date; deformation is most likely dated if several crystals of an area record the same age.…”

“…This enables precise dating of individual domains using secondary ion mass spectrometry (SIMS). Thereby allowing researchers to resolve growth duration and identify single events and phases of tectonic activity (e.g., Berger et al, 2013;Bergemann et al, 2017Bergemann et al, , 2018Bergemann et al, , 2019b.…”

“…These processes may be active as long as conditions in the cleft stay within the monazite-(Ce) formation temperature window and stability field and appear to be largely temperature independent within this temperature window, with only slightly increasing reaction speeds with increasing temperature (Budzyn et al, 2011). Therefore, several (re)crystallization or dissolutionprecipitation cycles may occur over the active lifetime of a monazite-(Ce) crystal (e.g., Bergemann et al, 2018Bergemann et al, , 2019b. The dissolution-reprecipitation/recrystallization events commonly induce porosity and fractures in the monazite-(Ce) and catalyze later reactions by bringing an increased crystal volume into direct contact with the fluid (Putnis, 2002(Putnis, , 2009.…”