Rates of Olivine Grain Growth During Dynamic Recrystallization and Postdeformation Annealing

We performed deformation and grain growth experiments on natural olivine aggregates with olivine water contents (C-OH = 600 +/- 300 H/10(6) Si) similar to upper mantle olivine, at 1000-1200 degrees C and 1,400 +/- 100 MPa confining pressure. Our experiments differ from published grain growth studies in that most were (1) conducted on natural olivine cores rather than hot-pressed aggregates and (2) dynamically recrystallized prior to or during grain growth. We combine our results with similar experiments performed at 1200-1300 degrees C and fit the data to a grain growth relationship, yielding a growth exponent (p) of 3.2, activation energy (E-G) 620 +/- 145 kJ mol(-1) (570 +/- 145 kJ mol(-1) when accounting for the role of temperature on water content), activation volume (V-G) similar to 5 x 10(-6) m(3) mol(-1), and rate constant (k(0)) 1.8 x 10(3) m(p) s(-1). Our E-G is within uncertainty of that predicted for dislocation creep of wet olivine (E* = 480 +/- 40 kJ mol(-1)). Grain size in strain rate-stepping samples adjusted to the olivine piezometer within 1.3-7.9% strain. The active grain boundary migration processes during deformation and dynamic recrystallization affect the kinetics of postdeformation grain growth, as grain boundary migration driven by strain energy density (rho GBM) may delay the onset of grain growth driven by interfacial energy (gamma GBM). We compared our postdeformation grain growth rates with data from previously published hydrostatic annealing experiments on synthetic olivine. At geologic timescales, the growth rates are much slower than predicted by the existing wet olivine grain growth law.