Simultaneous replacement of plagioclase by albite and K-feldspar: natural evidence and hydrothermal experiments

Replacement of feldspars occurs ubiquitously during fluid-rock interaction in crusts, and the formation of micro- to nano- pores along with the replacement potentially provides significant impacts on hydrological properties within the crust (e.g. Plümper et al., 2017; Yuguchi et al., 2019). In this contribution, we report the novel texture of the plagioclase replacement by K-feldspar and albite and showed the conditions of such replacement. The mafic schists near the pegmatitic quartz diorite within the Kinkasan Island, NE Japan show extensive feldspar alteration at various stages, involving Na-rich and K-rich fluids, respectively. Interestingly, during the later K-rich fluid infiltration at 400-570 ˚C at 0.3–0.45 GPa, plagioclase (An35-60) was replaced by K-feldspar (An0Ab1Or99) and albite (An4Ab94Or2) intergrowth, meaning that simultaneous K-feldspathization and albitization, and nano- to microscale pore network developed preferentially along with albite, resulting in an increase of the bulk rock porosity up to 1.34±0.14%.

To understand the relationship between K-feldspar and albite formations within the same plagioclase grain, we conducted the hydrothermal experiments on the feldspar replacement by using different pairs of starting minerals (anorthite, An96Ab4; labradorite, An66Ab33Or1; albite, An1Ab99) and fluid compositions (2M KCl and/or NaCl aqueous solutions) for 4-8 days. AIn all runs, the replacement processes of feldspars developed the distinct reaction front and pores formation close to the reaction front with porosity up to ~7%. In the experiments with KCl solution, the reaction front migrated twice faster than those with the mixture of KCl and NaCl. The most intense replacement occurred in the run of Labradorite-KCl solution, where large cavities were formed in the center of the labradorite grain with developing albite exsolution, and homogenous rim of K-feldspar precipitation. Such occurrences are similar to the replacement texture observed in the mafic schist within the Kinkasan Island and suggest the preferential removal of Ca and the fixed Na during K-feldspar formation. Our experimental results indicate the primary controls of the fluid composition on the replacement texture, pore formation, and the reaction rate.

Keywords feldspar replacement, micropores, fluid transport, hydrothermal experiment, Kinkasan