Laboratory deformation experiments were carried out on diopside single crystals. Creep tests were made in a dead load apparatus at temperatures T=1020 to 1320 ¿C, axial compressive stresses &sgr;=50 to 170 MPa with strain rates (˙ϵ) ranging from 2¿10-9 s-1 to 4¿10-7 s-1. The specimens were oriented such that mechanical twining was not possible. The experiments were designed to activate {110}1/2⟨a¿b⟩ slip systems. Our results demonstrate that they are the major operative systems for T>1000¿C and appear to be predominant over the (100)<c> slip system. The mechanical data are fitted to a power law ϵ˙=A &sgr;n exp(-E*/RT). An inversion method was used to determine the parameter ln(A), the activation energy E*, and the stress exponent n. Below a critical temperature Tc≂1130--1140 ¿C, E* is found to be 440¿30 kJ/mol associated with the {110}1/2⟨a¿b⟩ slip systems, activated symmetrically without contribution from the (100)/<c> system (orientation <2>); E* is found equal to 740¿30 kJ/mol if all three systems have the same resolved shear stresses (orientation <1>).

For T>Tc, values of E* drop to 50¿15 and 85¿30 kJ/mol for orientations <2> and <1>, respectively, so that the creep law of diopside becomes nearly temperature independent. The law derive from the orientation <2> crystals could be consistent with natural deformations. For this orientation and within the range of temperatures covered, n is 6.5¿0.4. The strong decrease in the activation energy at Tc may be ascribed unambiguously to occurrence of partial melting leading to microdroplets which pin the mobile dislocations (Ingrin et al., this issue). The high value found for the stress exponent (n) in the creep law suggests that clinopyroxene is softer than olivine at high stresses (at T=900 ¿C, stress &sgr;≥ 20 MPa and ϵ˙≥10-15 s-1), while olivine is softer at lower stresses. This competence inversion is predicted to occur within the ranges of stresses and strain rates expected in the lower crust and upper mantle. ¿American Geophysical Union 1991