High-pressure single-crystal studies of pyroxene minerals in metastable compression regime with relevance to cold subducting slabs

Our understanding of transformation pathways of rock-forming minerals as a function of depth in the Earth interior derives from analysis of their crystal structures at high pressure and temperature conditions and from the relations between different polymorphic forms of the same material. From this perspective, it is often valuable to consider metastable compression behavior of principal rock-forming minerals in the metastable regime, beyond the thermodynamic stability limits and outside of the PT path established by standard geotherm models. Such metastable compression regime can provide useful crystal chemical clues offering insights into intermediate stages of major geophysically-relevant phase transformations and clarifying the most important aspects of the compression mechanisms. Beyond the crystallographic and crystal chemical aspects, the metastable compression experiments may provide information applicable to deep Earth environments that significantly deviate from the standard geotherm, such as subduction zones. Seismic imaging data indicate that in subduction zones the temperatures in the cold subducted oceanic slab, which is dragged beneath the continental plate can be lower than the geotherm by as much as thousands of degrees. Recent seismic travel time tomography evidence indicates that some of the subduction zones, particularly in the Southeast Asia region, extend within the mantle well beyond the 660 km, discontinuity, perhaps as deep as 800 km. The main source of information about the compression behavior of minerals, the crystallographic experiments at high pressure and temperature, have been constrained by the limitations of the experimental in situ techniques and mostly limited to pressure range below 10 GPa. Recent developments in synchrotron-based high-pressure single-crystal diffraction, which will be reviewed in this presentation, opened possibilities to extend these studies well into the pressure range beyond 50 GPa. I will discuss the latest technical developments for in situ crystallographic experiments at the GSECARS facility and present results of metastable compression experiments on a range of most geologically-important pyroxene compositions which reveal important new clues about the cpx-pv transformation pathway.