Mass transfer and element redistribution during chloritization of metamorphic biotite in a metapelite: insights from compositional mapping

Abstract

Fluid-driven reactions are common in the upper crust, leading to mineral replacement and element redistribution on a kilometer to a micron scale. One common fluid-induced reaction is the chloritization of biotite. Chloritization can lead to the production of multiple phases, such as K-feldspar, titanite, and rutile, depending on local chemistry and fluid composition. Here, we compare different approaches to mass balance calculations using an altered garnet-bearing metapelite collected from Ny Friesland, Svalbard Archipelago, Norway. In the outcrop, amphibolite facies schists are cut by joints that acted as fluid pathways that led to the development of a narrow (~5 cm) alteration zone. Alteration involved the replacement of biotite by chlorite + K-feldspar ± rutile ± titanite. Mass balance calculations based on whole-rock composition and assuming immobile Zr indicate the addition of H2O, Si, Ti, Fe, or Mg and the removal of K or Ca. In contrast, mass balance calculated for the pseudomorphic replacement of Bt by Chl + Kfs ± Rt ± Ttn using X-Ray maps for three focused areas indicates the removal of Si or Ti rather than addition. Moles of product phase and elements gained or lost during this reaction varies between individual areas. This variation suggests that fluid heterogeneity and local chemistry governed the progression of the replacement reaction. All the product phases involved in the biotite breakdown are geochronometers and/or geothermometers, providing new opportunities for the petrochronology of fluid-induced reactions.

Keywords

• fluids
• mass balance calculations
• xmap tools
• biotite breakdown