Deformation experiments under conditions of the blueschist- to eclogite-facies transition: implications for the upper Wadati-Benioff plane of seismicity

Abstract:

Blueschist samples were experimented on a Griggs, a piston-cylinder (PC) and a D-DIA apparatus. Starting materials were sieved powders (<30 µm) from a natural sample collected on Alpine Corsica which initially contained approx. 60 % glaucophane and 40 % lawsonite. After mineral separation, we produced powders with a high (80 % glaucophane, 20 % lawsonite) and a low (30 % glaucophane, 70 % lawsonite) glaucophane content. Samples of cold-pressed powders of the two compositions were deformed in the Griggs and D-DIA apparatus at low temperatures (<550 °C) at which lawsonite is stable but glaucophane should decompose into jadeite and talc, and at high temperatures (up to 850 °C) at which lawsonite dehydrates. Powders were also isostatically annealed at comparable experimental conditions in the PC-apparatus to provide a microstructural reference.

The use of synchrotron X-rays during the D-DIA experiments enables us to follow the strain evolution (radiography) while deforming the sample, but also to monitor phase changes in-situ using X-ray diffraction. We also monitored acoustic emissions (AE). Depending on the sample mineralogy and the applied pressure, AEs were either observed at low temperature around 380-420 °C or in a range from 610-850 °C. In some samples AEs were triggered in both ranges. Microstructural observations (SEM, Raman, Microprobe, X-ray tomography) on retrieved samples reveal that samples deformed under deviatoric stress exhibit faults almost exclusively, whereas those under hydrostatic conditions do not.

Our preliminary interpretation of stress, strain, microstructural and mineralogical evolution during deformation, is that the two temperature ranges of AE activity correspond to transformational faulting due to the glaucophane breakdown into jadeite and talc and lawsonite dehydration. We interpret these micro-earthquakes as possible laboratory analogs of intermediate depth earthquakes occurring within the upper Wadati-Benioff plane of seismicity observed between 60 and 100 km deep in cold subduction zones.