Experimental deformation of diopside and websterite

Diopside single-crystals, oriented favorably for twin gliding on both systems: (001) 100] and (100)001] have been deformed in a Griggs apparatus using talc as pressure medium. The latter mechanism is dominant at temperatures (T) below 1050° C at strain rates () of 10⁻³ sec⁻¹, and below 800° C at ; at higher temperatures translation gliding on (100)001] accompanied by syntectonic recrystallization is dominant but other glide systems also operate. Tests at a single set of conditions, T- and -incremental tests and stress-relaxation experiments have been carried out on websterite (68% CPX, 32% OPX), both in talc (“wet”) and talc-AlSiMag (“dry”) assemblies. Most tests were performed in the high-T regime, where syntectonic recrystallization and “relatively nonselective” glide are dominant. The mean size of recrystallized clinopyroxenes (D, μm) appears to be related to stress (σ, kb) as D = 60σ^−0.9. The mechanical data fit the power law exp(-Q/RT)σⁿ, where for the “wet” experiments A = 10^5.9kb⁻ⁿsec⁻¹, Q = 91.2 kcal/mole, n = 5.3; for σ < 3.5 kb n appears to decrease to 3.3. For the “dry” experiments A = 10^2.2, Q = 77.9, and n = 4.3 for σ < 7.0 kb. Clinopyroxene in the upper mantle occurs as ca. 0–15% mixed phase in peridotites and websterites occur as thin layers. Stresses in these materials will then be near those in the olivine-rich matrix. At , the equivalent viscosity of dry websterite is less than that of dry dunite at depths to 60 km but it increases rapidly at higher pressures; at 240 km it is 10⁶ greater than that of dunite. This may account for the low strains and passive behavior observed for clinopyroxene crystals in most peridotites and websterites, that presumably have formed at great depth. Attenuated folds of websterite in peridotite—evidence of more ductile behavior—may then have formed at shallower levels; alternatively they may have formed under “wet” conditions.