skip to main content


Title: The Effect of Secondary‐Phase Fraction on the Deformation of Olivine + Ferropericlase Aggregates: 2. Mechanical Behavior
Abstract

To study the mechanical behavior of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (fPer) between 0.1 and 0.8. Each sample was deformed in torsion atT = 1523 K,P = 300 MPa at a constant strain rate to a final shear strain ofγ = 6 to 7. The stress‐strain data and calculated values of the stress exponent,n, indicate that Ol in our samples deformed by dislocation‐accommodated sliding along grain interfaces while Per deformed via dislocation creep. At shear strains ofγ < 1, the strengths of samples withfPer > 0.5 match model predictions for both phases deforming at the same stress, the lower‐strength bound for two‐phase materials, while the strengths of samples withfPer < 0.5 are greater than predicted by models for both phases deforming at the same strain rate, the upper‐strength bound. These observations suggest a transition from a weak‐phase supported to a strong‐phase supported regime with decreasingfPer. Aboveγ = 4, however, the strength of all two‐phase samples is greater than those predicted by either the uniform‐stress or the uniform‐strain rate bound. We hypothesize that the high strengths in the Ol + Per system are due to the presence of phase boundaries in two‐phase samples, for which deformation is rate limited by dislocation motion along interfacial boundaries. This observation contrasts with the mechanical behavior of samples consisting of Ol + pyroxene, which are weaker, possibly due to impurities at phase boundaries.

 
more » « less
Award ID(s):
2011401
NSF-PAR ID:
10407245
Author(s) / Creator(s):
 ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Solid Earth
Volume:
128
Issue:
4
ISSN:
2169-9313
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    To study the microstructural evolution of polymineralic rocks, we performed deformation experiments on two‐phase aggregates of olivine (Ol) + ferropericlase (Per) with periclase fractions (fPer) between 0.1 and 0.8. Additionally, single‐phase samples of both Ol and Per were deformed under the same experimental conditions to facilitate comparison of the microstructures in two‐phase and single‐phase materials. Each sample was deformed in torsion atT = 1523 K,P = 300 MPa at a constant strain rate up to a final shear strain of γ = 6 to 7. Microstructural developments, analyzed via electron backscatter diffraction (EBSD), indicate differences in both grain size and crystalline texture between single‐ and two‐phase samples. During deformation, grain size approximately doubled in our single‐phase samples of Ol and Per but remained unchanged or decreased in two‐phase samples. Zener‐pinning relationships fit to the mean grain sizes in each phase for samples with 0.1 ≤ fPer≤ 0.5 and for those with 0.8 ≥ fPer ≥ 0.5 demonstrate that the grain size of the primary phase is controlled by phase‐boundary pinning. Crystallographic preferred orientations, determined for both phases from EBSD data, are significantly weaker in the two‐phase materials than in the single‐phase materials.

     
    more » « less
  2. Abstract

    To understand the effects of secondary minerals on changes in the mechanical properties of upper mantle rocks due to phase mixing, we conducted high‐strain torsion experiments on aggregates of iron‐rich olivine + orthopyroxene (opx) with opx volume fractions offopx = 0.15, 0.26, and 0.35. For samples with larger amounts of opx,fopx = 0.26 and 0.35, the value of the stress exponent decreases with increasing strain fromn ≈ 3 for γ  5 ton ≈ 2 for 5  γ  25, indicating that the deformation mechanism changes as strain increases. In contrast, for samples withfopx = 0.15, the stress exponent is constant atn ≈ 3.3 for 1  γ  25, suggesting that no change in deformation mechanism occurs with increasing strain for samples with smaller amounts of opx. The microstructures of samples with larger amounts of opx provide insight into the change in deformation mechanism derived from the mechanical data. Elongated grains align subparallel to the shear direction for samples of all three compositions deformed to lower strains. However, strain weakening with grain size reduction and the formation of a thoroughly mixed, fine‐grained texture only develops in samples withfopx = 0.26 and 0.35 deformed to higher strains of γ  16. These mechanical and associated microstructural properties imply that rheological weakening due to phase mixing only occurs in the samples with largerfopx, which is an important constraint for understanding strain localization in the upper mantle of Earth.

     
    more » « less
  3. Abstract

    Synthesized polycrystalline samples composed of enstatite and olivine with different volumetric ratios were deformed in compression under anhydrous conditions in a Paterson gas‐medium apparatus at 1150–1300°C, an oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 or 450 MPa (protoenstatite or orthoenstatite fields). Mechanical data suggest a transition from diffusion to dislocation creep with increasing differential stress for all compositions. Microstructural analyses by optical and scanning electron microscopy reveal well‐mixed aggregates and homogeneous deformation. Crystallographic preferred orientations measured by electron backscatter diffraction are consistent with activation of the slip systems (010)[100] and (010)[001] for olivine and (100)[001] and (010)[001] for enstatite, as expected at these conditions. Nonlinear least‐squares fitting to the full data set from each experiment allowed the determination of dislocation creep flow laws for the different mixtures. The stress exponent is 3.5 for all compositions, and the apparent activation energies increase slightly as a function of enstatite volume fraction. Within the limits of experimental uncertainties, all two‐phase aggregates have strengths that lie between the uniform strain rate (Taylor) and the uniform stress (Sachs) bounds calculated using the dislocation creep flow laws for olivine and enstatite. Calculation of the Taylor and Sachs bounds at strain rate and temperature conditions expected in nature (but not extrapolating in pressure) indicates that using the dislocation creep flow law for monomineralic olivine aggregates provides a good estimate of the viscosity of olivine‐orthopyroxene rocks deforming by dislocation creep in the deeper lithosphere and asthenosphere.

     
    more » « less
  4. Abstract

    Large earthquakes and slow slip events typically nucleate along plate boundaries near the depth limit of the seismogenic zone, which is also recognized as the brittle‐plastic transition zone (BPT). HighVp/Vsratios are commonly observed at the BPT in subduction zones, indicating the presence of aqueous fluid in pore spaces. We conducted experiments to investigate the rheology of quartz with different fluid fractions at deformation conditions that cross the BPT. The strengths of quartz aggregates with fluid‐filled porosities of 5–25 vol% are significantly lower than predicted by wet quartzite flow laws, and decrease with increasing fluid fraction. Recovered samples deformed in the ductile regime exhibit S‐C´ mylonitic structures characterized by elongate grains, shear localization and fluid segregation. Variations in strength are explained by a combination of a constitutive law for dislocation creep that includes the geometric effects of fluid fraction, a friction law that includes the effect of fluid fraction through its role on the real area of contact, and an empirical function to describe the smooth brittle‐plastic transition. Our results indicate that the presence of fluid‐filled porosity promotes significant weakening in shear zones, and that variations in fluid fraction (together with temperature) can explain transitions in the spectrum of slip behaviors observed along plate boundaries.

     
    more » « less
  5. Abstract

    Novel fluid medium pressure cells were used to deform antigorite under constant stress creep conditions at low temperature, low strain rate (10−9 − 10−41/s), and high pressure (1 GPa) in a Griggs‐type apparatus. Antigorite cores were deformed at constant temperatures between 75°C and 550°C, by applying 8–12 stress‐strain steps per temperature. The microstructures of deformed samples share features documented in previous work (e.g., shear microcracks), and highlight the importance of basal shear and kinks to antigorite plasticity. Rheological data were fit with a low temperature plasticity law, consistent with a deformation mechanism involving large lattice resistance. When applied at geologic stresses and strain rates, the extrapolated viscosity agrees well with predictions based on subduction zone thermal models.

     
    more » « less