5:15 PM - 6:45 PM
[SMP24-P13] Platinum-coated plagioclase replacement; preliminary method to evaluate pseudomorphism in dissolution-precipitation process
Keywords:plagioclase replacement, pseudomorphism, platinum coating, overgrowth
Mineral replacement, particularly in feldspar, is characterized by pseudomorphism and the occurrence of microporosity which is significant for the fluid transport even at the great depth. Pseudomorphism refers to the preservation of external volume of the parent mineral as the consequence of couple dissolution-precipitation process. Bulk rock and fluid chemistry control the extent of isovolumetric of minerals after replacement. Experimental studies of labradorite, with reactive fluid at 600°C and 150 MPa for 1-8 days, showed the formation of porous anorthite and Kfs overgrowth which indicate the spatially dissolution-transport-reprecipitation process (Nurdiana et al., 2024). However, without any marks in the grain boundary, identifying pseudomorphism in single-crystal reaction poses a challenge. An alternative approach involves conducting experiments on polished polycrystalline aggregates, but complex replacements among mineral assemblages can occur. Thus, this study aims to track the initial outline of the parent mineral by utilizing platinum coating to determine whether the replacement is isovolumetric or involves overgrowth.
We observe the replacement product in platinum-coated labradorite replacement within the quartz-present and quartz-absent bulk rock. The hydrothermal experiments were conducted using pairs of starting coated minerals or polycrystalline grain (labradorite, An66Ab33Or1; hornblende schist with labradorite An67-78Ab33-28; gabbro with labradorite An40-70Ab60-30) and 2M KCl aqueous solutions with W/R=2. The vessel condition is 600°C and 150-200 MPa for 2 and 5-day experiments. The samples were coated with platinum powder with thicknesses of 30 nm and 90 nm.
In the absence of quartz, the replacement in single crystal and polycrystalline show a similar result of ~20% porous anorthite and homogenous Kfs overgrowth (~20 um length) for ~40% of the initial volume, while the porosity generated 10-30% for 5 days. Secondary minerals found in the coated samples indicate the platinum remains unreacted and Kfs predominantly overgrew, enclosing the platinum layer.
In quartz-present gabbro, amorphous silica grains are found on the platinum layer (~5 um) and there is no observed mineral overgrowth. A thicker platinum layer suggests increased capability in limiting the fluid infiltration to the middle of the aggregate despite the layer exhibiting granular texture after the experiment.
The replacement of labradorite in quartz absent fluid is beneficial to evaluate the pseudomorphism. The result suggests that in the presence of a platinum layer, fluid chemistry at the reaction interface can be maintained, allowing micropore generation while still enabling the tracing of the external volume.
This experiment reveals the platinum coating is effective in determining whether the dissolution is spatially coupled with precipitation. This finding provides a better understanding of the replacement mechanism in the metamorphic process, particularly in single-crystal experimental studies.
We observe the replacement product in platinum-coated labradorite replacement within the quartz-present and quartz-absent bulk rock. The hydrothermal experiments were conducted using pairs of starting coated minerals or polycrystalline grain (labradorite, An66Ab33Or1; hornblende schist with labradorite An67-78Ab33-28; gabbro with labradorite An40-70Ab60-30) and 2M KCl aqueous solutions with W/R=2. The vessel condition is 600°C and 150-200 MPa for 2 and 5-day experiments. The samples were coated with platinum powder with thicknesses of 30 nm and 90 nm.
In the absence of quartz, the replacement in single crystal and polycrystalline show a similar result of ~20% porous anorthite and homogenous Kfs overgrowth (~20 um length) for ~40% of the initial volume, while the porosity generated 10-30% for 5 days. Secondary minerals found in the coated samples indicate the platinum remains unreacted and Kfs predominantly overgrew, enclosing the platinum layer.
In quartz-present gabbro, amorphous silica grains are found on the platinum layer (~5 um) and there is no observed mineral overgrowth. A thicker platinum layer suggests increased capability in limiting the fluid infiltration to the middle of the aggregate despite the layer exhibiting granular texture after the experiment.
The replacement of labradorite in quartz absent fluid is beneficial to evaluate the pseudomorphism. The result suggests that in the presence of a platinum layer, fluid chemistry at the reaction interface can be maintained, allowing micropore generation while still enabling the tracing of the external volume.
This experiment reveals the platinum coating is effective in determining whether the dissolution is spatially coupled with precipitation. This finding provides a better understanding of the replacement mechanism in the metamorphic process, particularly in single-crystal experimental studies.