Behaviour of argon during high pressure metamorphic fluid-rock interaction

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40Ar/39Ar mass spectrometry is one of the most powerful tools to determine absolute ages and rates of geo-processes. Furthermore, Ar as a fluid-mobile trace element can be used to quantify fluid fluxes and fluid-rock interaction, if Ar transport parameters during metamorphism are known. Our preliminary work has identified, in natural high-pressure rocks from the Western Alps, that fluid infiltration has led to three different modification types of the mineral assemblage: (1) intra-grain solution-reprecipitation, (2) volume diffusion and (3) neocrystallisation of K-bearing and K-free phases that form in reaction sites and in overgrowth on existing grains. Partial overprinting of mineral grains allows to determine the isotopic composition and abundances of Ar in relict as well as modified parts of phengitic white mica and amphiboles. Additionally, concentrations of fluid-sensitive elements, such as Li, Be, B and large ion lithophile elements (LILE) will be determined in situ. These data, together with thermodynamic and trace element forward models will be used to constrain the relationship between rates and magnitudes of fluid influx, recrystallisation mechanisms and intracrystalline trace element and isotopic re-equilibration. Combined with the information from the well-known trace elements, the newly won in situ Ar data will allow us to establish the partitioning, transport and diffusive behaviour of Ar during metamorphism: data that are sorely lacking.