11:30 AM - 11:45 AM
[SMP25-10] The effect of fluid compositions on pore formation during plagioclase replacement under supercritical conditions
Keywords:Na-K rich fluid, feldspar replacement, fluid transport, hydrothermal experiment, Kinkasan
Multistage feldspar replacement by Na-K rich fluid is frequently found in mid-crustal depth (e.g. Plümper et al., 2009; Yuguchi et al., 2019) at which micro- to nano- pores fluid pathways developed intensively. Such feldspar replacement was also found in mafic schist from Kinkasan Island, NE Japan, as a part of the South Kitakami Belt. Multiple plagioclase replacement by infiltration of Na-rich and K-rich fluid was recorded. In the K-rich fluid alteration, plagioclase (An35-60) was altered to Kfs (An0Ab1Or99) and albite (An4Ab94Or2) intergrowth along with pores development that increases bulk rock porosity up to 1.34±0.14%. The replacement was induced by pegmatite-related Na-K-rich fluid and took place at 400-570°C at 0.3–0.45 GPa. That shows that Na-K-rich fluid may induce multiple secondary feldspar and micro- to nano- pores generations. However, the role of these fluids governs the replacement process is still poorly understood.
We conduct feldspar replacement by Na-K rich fluids to see if the Na-K ratio may induce the secondary product and pore formations to compare to natural sample in Kinkasan. We carried out plagioclase replacement by aqueous 2M KCl and/or NaCl under 600°C & 150 MPa for 4-8 days. Plagioclase starting materials are anorthite (An; An96Ab4), labradorite (Lab; An66Ab33Or1), and albite (Ab; An1Ab99) that each of them reacted with the fluid with and without silica presence to progress feldspar replacement. Reaction results show the dissolution-reprecipitation process with a sharp reaction front and induced pores formation up to ~7%. That micro-size pores are unlikely to always relate to the grain boundaries. Reactions of KCl-feldspar in undersaturated silica show a faster reaction rate and varied porosity properties compare to NaCl and/or KCl fluid with feldspar in silica saturated conditions. Labradorite-KCl reaction shows the most intense pore formations and alkali feldspar product which is the closest texture of plagioclase replacement by K-rich fluid in Kinkasan sample.
That result indicates the role of Na-K rich fluid ratio and silica saturation toward feldspar replacement. The reaction of feldspar with the higher K ratio fluid and limited silica leads to intense pore generations as the main fluid channel. The different ratios of Na-K and silica presence brought the feldspar replacement into different pore properties.
We conduct feldspar replacement by Na-K rich fluids to see if the Na-K ratio may induce the secondary product and pore formations to compare to natural sample in Kinkasan. We carried out plagioclase replacement by aqueous 2M KCl and/or NaCl under 600°C & 150 MPa for 4-8 days. Plagioclase starting materials are anorthite (An; An96Ab4), labradorite (Lab; An66Ab33Or1), and albite (Ab; An1Ab99) that each of them reacted with the fluid with and without silica presence to progress feldspar replacement. Reaction results show the dissolution-reprecipitation process with a sharp reaction front and induced pores formation up to ~7%. That micro-size pores are unlikely to always relate to the grain boundaries. Reactions of KCl-feldspar in undersaturated silica show a faster reaction rate and varied porosity properties compare to NaCl and/or KCl fluid with feldspar in silica saturated conditions. Labradorite-KCl reaction shows the most intense pore formations and alkali feldspar product which is the closest texture of plagioclase replacement by K-rich fluid in Kinkasan sample.
That result indicates the role of Na-K rich fluid ratio and silica saturation toward feldspar replacement. The reaction of feldspar with the higher K ratio fluid and limited silica leads to intense pore generations as the main fluid channel. The different ratios of Na-K and silica presence brought the feldspar replacement into different pore properties.