Time and metamorphic petrology: calcite to aragonite experiments.

Area of Science:

• Geochemistry
• Mineralogy
• Materials Science

Background:

• Equilibrium phase relations in mineral systems are established, but transformation kinetics, especially in polycrystalline rocks, remain largely unknown.
• Previous studies on calcite to aragonite transformation kinetics focused on powdered or single-crystal calcite, potentially not reflecting natural rock behavior.

Purpose of the Study:

• To investigate the kinetics of the calcite to aragonite phase transformation in fully dense polycrystalline marble.
• To compare the transformation mechanisms and rates in polycrystalline marble with those in powdered and single-crystal calcite.
• To assess the implications of these kinetic findings for mineral transformations on geological time scales.

Main Methods:

• Experimental investigation of the calcite to aragonite transformation in polycrystalline marble samples.
• Analysis of transformation mechanisms, temperature dependence, and reaction rates.
• Extrapolation of experimental kinetic data to geological time scales.

Main Results:

• The calcite to aragonite transformation in polycrystalline marble occurs via different mechanisms compared to single-crystal or powdered calcite.
• The transformation rate in polycrystalline marble is markedly slower and exhibits a different temperature dependence.
• Kinetic hindrance significantly impacts the transformation process in natural rock analogues.

Conclusions:

• Kinetic studies on fully dense polycrystalline aggregates are crucial for understanding natural mineralogic phase changes.
• The calcite to aragonite transformation is unlikely to occur below 200°C in the absence of volatiles on geological time scales.
• Kinetic limitations are expected to be even more significant for more complex mineral transformations at higher temperatures.