Kinetics of near hydrous solidus reaction through dehydration melting of nominally anhydrous minerals: an analogue experiment in jadeite-spodumene system
Abstract
It is widely believed that mineral reactions in nominally anhydrous rocks occur by solid-state diffusion and/or by a small amount of aqueous fluid-mediated dissolution-precipitation. Here we provide an experimental observation that mineral reactions of nominally anhydrous minerals (NAMs) just above the hydrous solidus take place by dissolution-precipitation processes thorough incipient melting controlled by water storage capacity. We juxtaposed solid-solution end-members, jadeite (Jd: NaAlSi2O6) and spodumene (Spd: LiAlSi2O6), both of which are NAMs but have different storage capacity of water. A single crystal of natural Jd was placed in natural Spd powders without adding free water; reactions of these pyroxenes were investigated at 2 GPa, 700°C using a piston-cylinder apparatus. The P-T condition is below hydrous solidus of both pyroxenes. All the experimental products run showed that reaction zone of the intermediate solid-solution composition was formed, having a sharp interface with relict Jd. The polyhedral pores partially filled with silicate glass were observed at the reaction front and within the reaction zone. The rate of replacement was orders of magnitude faster than the cation interdiffusion in clinopyroxene. These features indicate that mineral replacement has progressed via melt-mediated dissolution-precipitation rather than solid-state diffusion. It was confirmed via FT-IR measurements that the concentration of structural water in the newly formed pyroxene with an intermediate composition was lower than that of Jd. These results suggest that the incipient hydrous eutectic melting at Jd-Spd grain boundaries occurred due to the decreased storage capacity of water in the solid-solution of the intermediate composition. The incipient melting might have been triggered by the Na-Li diffusion at Jd-Spd grain boundaries. The hydrous melt then could have worked as a solvent and enhanced rapid mineral replacement via dissolution-precipitation. Our analogues Jd-Spd system is not directly applied to natural setting, but demonstrates that storage capacity-controlled incipient melting can enhance re-equilibration of rocks near hydrous solidus conditions such as granulite and eclogite facies. This work was supported by the JSPS Japanese-German Graduate Externship.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43E0145T
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY;
- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY