TRANSIENT AND STEADY-STATE CREEP OF PURE FORSTERITE AT LOW STRESS

Thirty-one single crystals of synthetic forsterite, Fo100, were deformed in 69 compressional creep tests in a 0.1-MPa confining atmosphere of H2 + Ar. Temperature ranged from 1753 to 2023 K and stress σ (= σ1 - σ3) from 1.5 to 37.8 MPa. Steady-state creep under these conditions follows an empirical law of the form: strain rate ε{lunate} ̇ = Azigma;nexp( -Q RT) where A, n, and Q are constants. General characteristics of Fo100 creep - uniformity of strain, shape change as a function of orientation of σ, relative deformation resistance of different orientations - match those of natural olivine single crystals of composition Fo92. Specific constants in the flow law, however, are distinctly new: for σ oriented along [111]c (equidistant from the three principal crystallographic axes), values for Fo100 are n = 2.9 ± 0.2 and Q = 0.67 ± 0.03 MJ/mol (160 ± 7 kcal/mol). A single law covers the range 3 < σ < 30 MPa and 1753 < T < 1953 K. Steady-state deformation is preceded by a transient period of strain softening. High strain rates at σ ≥ 10 MPa render the transient barely resolvable; it apparently displaces the steady-state flow law by approximately -0.5% in strain. At σ ≤ 7.8 MPa, the amount of strain imparted to a sample of the [101]c orientation is typically <0.1% after one hour. © 1979.