Rates of olivine grain growth during dynamic recrystallization and post-deformation annealing

Abstract

We performed deformation and grain growth experiments on natural olivine aggregates with moderate olivine water contents (COH = 600±300 ppm H/Si) at 1000-1200°C and a confining pressure of 1400±100 MPa. 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. The active grain boundary migration processes during deformation and dynamic recrystallization affect the kinetics of post-deformation grain growth, as grain boundary migration driven by strain energy density (⍴GBM) may delay the onset of grain growth driven by interfacial energy (γGBM). We compared our post-deformation grain growth rates with data from previously published hydrostatic annealing experiments on synthetic olivine. At geologic timescales, the growth rates are slower than predicted by the existing wet olivine grain growth law.

Microstructural Shift due to Post‐Deformation Annealing in the Upper Mantle

“…We did not measure the water content of our sample after deformation; FTIR analyses of olivine grains from the samples studied by Speciale et al. (2020), deformed at 1100°C at 1 GPa, show water contents of 37 ± 19 ppm by weight.…”

“…(2011) and Speciale et al. (2020). The initial water content of this material is estimated based on mass loss (approximately 0.4 wt.%) upon drying cores of the dunite starting material in a controlled atmosphere furnace at 1000°C for 10 h (CO:CO 2 = 1:5), suggesting trace amounts of hydrous alteration phases along grain boundaries (Speciale et al., 2020).…”

“…(2020). The initial water content of this material is estimated based on mass loss (approximately 0.4 wt.%) upon drying cores of the dunite starting material in a controlled atmosphere furnace at 1000°C for 10 h (CO:CO 2 = 1:5), suggesting trace amounts of hydrous alteration phases along grain boundaries (Speciale et al., 2020). FTIR on optically clear olivine grains in the starting material indicate an initial water content of approximately 20 ppm by weight (Speciale et al., 2020).…”

“…The initial water content of this material is estimated based on mass loss (approximately 0.4 wt.%) upon drying cores of the dunite starting material in a controlled atmosphere furnace at 1000°C for 10 h (CO:CO 2 = 1:5), suggesting trace amounts of hydrous alteration phases along grain boundaries (Speciale et al., 2020). FTIR on optically clear olivine grains in the starting material indicate an initial water content of approximately 20 ppm by weight (Speciale et al., 2020). Experiments were performed using molten salt (NaCl + KCl) as the pressure medium (e.g., Green & Borch, 1989) at a pressure of 1030 ± 20 MPa and temperatures measured by type S thermocouples (schematic illustration of the experimental assembly used is shown in Figure in the Supporting Information).…”

“…Although the water content in experiment W2306 presented here was not measured, analyses of Balsam Gap deformed under similar experimental conditions ( P = 1 GPa and T = 1200°C) using the same assembly show a water content of ∼35 ppm wt. that originates from dehydration of a small amount of hydrous minerals in the initial sample at run conditions (Speciale et al., 2020).…”