Behaviour of argon during high pressure metamorphic fluid-rock interaction
Final Report Abstract
40Ar/39Ar mass spectrometry is one of the most powerful tools to determine absolute ages as well as rates of geo-processes. Furthermore, Ar as a fluid-mobile trace element can be used to study fluid-rock interaction and thus quantify metamorphic mineral growth and modification. However, as the element transport properties as well as the incorporation of Noble gases into metamorphic minerals is difficult to predict we compared the results of 40Ar/39Ar age determinations with the concentrations and isotopic composition of boron and lithium in a set of metamorphic samples. Boron and lithium were chosen because of their properties during fluid-rock interaction. Furthermore we developed a numerical modeling code that enables combined thermodynamic-geochemical simulations of fluid-rock interaction in order to quantify the observed compositional chatacteristics of our samples. The combined observation of younger 40Ar/39Ar ages and boron loss, yielding comparable values of both parameters only in cores and rims of different samples, is best explained by a selective metasomatic overprint. In low permeability samples, this overprint caused recrystallization of phengite rims, whereas higher permeability in other samples led to complete recrystallization of phengite grains. We interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone. This event also caused partial resetting of the Ar isotope system and further B loss in more permeable rocks of the adjacent unit. We conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of 40Ar/39Ar data in partially metasomatized rocks. These results indicate the importance of recrystallization processes for the isotopic equilibration of phyllosilicates during metamorphism. This is a fundamental aspect for the interpretation of geochronological investigations. In combination with the Rb/Sr isotopic dating of the same sampleset we could show that the comparison of Rb/Sr and 40Ar/39Ar data reveals that equilibration of the Rb/Sr system during deformation does not necessarily cause re-equilibration of the K/Ar system. Instead, older apparent 40Ar/39Ar ages point to the presence of extraneous 40Ar in the investigated samples from older high-pressure crystallisation, highlighting the importance of rock permeability for re-equilibrating Ar isotopes. These results are among the first ones treating Ar isotopic compositions in metamorphic rocks under geochemical aspects rather that as simple geochronometers. The published works should therefore serve as motivation for general research in this direction.
Publications
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(2018) Rb-Sr and in situ 40Ar/39Ar dating of exhumation-related shearing and fluid-induced recrystallization in the Sesia zone (Western Alps, Italy). Geosphere 14 (4) 1425–1450
Ralf Halama; Johannes Glodny; Matthias Konrad-Schmolke; Masafumi Sudo
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2013: AGU San Francisco. Long-lasting (25 Ma) re-crystallization during exhumation recorded by white mica in situ 40Ar/39Ar ages (Sesia-Lanzo Zone, Western Alps)?
Ralf Halama, Matthias Konrad-Schmolke, Masafumi Sudo, Cees-Jan De Hoog, Alexander Schmidt
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(2014) . Effects of fluid-rock interaction on40Ar/39Ar geochronology in high pressure rocks (Sesia-Lanzo zone, Western Alps). Geochimica et Cosmochimica Acta, 126, 475-494
Halama, R., Konrad-Schmolke, M. Sudo, M., Marschall, M.R. and Wiedenbeck, M.
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(2014). Combined thermodynamic-geochemical modeling in metamorphic geology: Boron as tracer of fluid-rock interaction. Lithos, 208, 393-414
Konrad-Schmolke, M., and Halama, R.
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2014: TSK Potsdam. Unravelling timing of metamorphism and associated element transfer by combining in situ geochronological (Ar/Ar) with geochemical and isotopic information in phengitic white mica
Halama, R., Konrad-Schmolke, M., Sudo, M., Schmidt, A., De Hoog, J.C.M., Wiedenbeck, M.
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2015: IEC Dominican Republic. Fluid infiltration in an interlayered blueschist-greenschist sequence (Coastal Cordillera, Chile)
R. Halama, M. Konrad-Schmolke
