Rheology of plagioclase transforming under high pressure and temperature conditions: insights from deformation experiments

<p>In convergent zones, plagioclase, the main rock forming mineral of the lower continental crust, breakdowns with increasing pressure via two reactions, respectively involving its calcic (anorthite) and sodic (albite) end-members. In the presence of water, anorthite starts to breakdown at 1.5 GPa and 700&#176;C into an assemblage of zoisite, kyanite and quartz, while<span>&#160; </span>albite breakdowns into jadeite and quartz at higher pressure (~ 2 GPa). These two reactions involve large volume reduction (~15%), as well as the nucleation and growth of new minerals with mechanical properties different from the initial plagioclase, both of which may have important mechanical consequence if the rock transforms while deforming. </p><p>The aim of our study is to understand and further quantify the impact of these transformations on the rheology of the continental lower crust. Deformation experiments were performed on plagioclase aggregates in a 3rd generation Griggs-type apparatus (Moarefvand et al. 2021), equipped with an acoustic emission monitoring system. Focussing on the lower pressure reaction (an + H2O = zo+ky+qz), expected to be of paramount importance at laboratory timescales because of the fast nucleation of Ca-Al silicates, samples were deformed at various <em>P-T</em> conditions (1 GPa to 2 GPa<span>&#160; </span>and 700&#176;C to 800&#176;C) and two different strain rates (4e-5 /s an d 4e-6 /s). In addition, various initial water content were tested in order to study the mechanical behavior of the aggregate under different reaction kinetics.</p><p>Our results suggest that under these experimental conditions, the rheology of plagioclase is controlled by the complex feedbacks between hydration, reaction kinetics and strain rate. Under &#8216;wet&#8217; conditions, the reaction is fast and the rheology observed is consistent with that of &#8216;wet&#8217; plagioclase. Additional softening is observed when the reaction takes place, during which plagioclase behaves as a weak assemblage of albite and zoisite (+-mica). Under dry conditions, the reaction is much slower and an inverse ductile to brittle transition was observed, i.e. embrittlement was observed at the slowest strain rate only, conditions under which stress may build up (&#8216;dry&#8217; rheology) while the reaction is fast enough to trigger embrittlement of the sample.</p><p>These results further our understanding of the lower crust rheology and in particular, of the conditions under which intermediate-depth seismicity linked to eclogitization of the lower continental crust may be observed in convergent zones.</p>