Rheological Weakening of Olivine + Orthopyroxene Aggregates Due To Phase Mixing: Part 2. Microstructural Development

To understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior, we conducted torsion experiments on samples of iron‐rich olivine plus orthopyroxene. The experiments were conducted at a temperature, T, of 1200°C and a confining pressure, P, of 300 MPa using a gas‐medium deformation apparatus. Samples composed of olivine plus 26% orthopyroxene were deformed to outer radius shear strains up to γ ≈ 26. In samples deformed to lower strains of γ ≲ 4, elongated olivine and pyroxene grains form a compositional layering. Already by this strain, mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain. The ratios of olivine to pyroxene grain size in deformed samples follow the Zener relationship, indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth. Due to the reduction in grain size, the dominant deformation mechanism changes as a function of strain. The microstructural development forming more thoroughly mixed, fine‐grained olivine‐pyroxene aggregates can be explained by the difference in diffusivity among Me (Fe or Mg), O, and Si, with transport of MeO significantly faster than that of SiO2. These mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks.

[1]  D. Kohlstedt,et al.  Rheological Weakening of Olivine + Orthopyroxene Aggregates Due to Phase Mixing: 1. Mechanical Behavior , 2017 .

[2]  D. Bercovici,et al.  Grain damage, phase mixing and plate-boundary formation , 2017 .

[3]  J. Précigout,et al.  Water pumping in mantle shear zones , 2017, Nature Communications.

[4]  G. Fraysse,et al.  Strength of orthoenstatite single crystals at mantle pressure and temperature and comparison with olivine , 2016 .

[5]  V. Toy,et al.  Recrystallisation, phase mixing and strain localisation in peridotite during rapid extrusion of sub-arc mantle lithosphere , 2016 .

[6]  D. Kohlstedt,et al.  Creep behavior of Fe‐bearing olivine under hydrous conditions , 2015 .

[7]  G. Hirth,et al.  The stress dependence of olivine creep rate: Implications for extrapolation of lab data and interpretation of recrystallized grain size , 2015 .

[8]  D. Kohlstedt,et al.  Evolution of the rheological and microstructural properties of olivine aggregates during dislocation creep under hydrous conditions , 2015 .

[9]  L. Hansen,et al.  Quantifying the effect of pyroxene on deformation of peridotite in a natural shear zone , 2015 .

[10]  M. Herwegh,et al.  Small quantity but large effect — How minor phases control strain localization in upper mantle shear zones , 2015 .

[11]  P. Skemer,et al.  Dynamic recrystallization and phase mixing in experimentally deformed peridotite , 2014 .

[12]  K. Michibayashi,et al.  Influence of mineral fraction on the rheological properties of forsterite + enstatite during grain size sensitive creep: 3. Application of grain growth and flow laws on peridotite ultramylonite , 2014 .

[13]  D. Kohlstedt,et al.  Protracted fabric evolution in olivine: Implications for the relationship among strain, crystallographic fabric, and seismic anisotropy , 2014 .

[14]  David Bercovici,et al.  Plate tectonics, damage and inheritance , 2013, Nature.

[15]  T. Hiraga,et al.  Olivine crystals align during diffusion creep of Earth’s upper mantle , 2013, Nature.

[16]  S. Karato,et al.  Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization , 2013, Proceedings of the National Academy of Sciences.

[17]  M. Tasaka,et al.  Influence of mineral fraction on the rheological properties of forsterite + enstatite during grain‐size‐sensitive creep: 1. Grain size and grain growth laws , 2013 .

[18]  M. Zimmerman,et al.  Influence of mineral fraction on the rheological properties of forsterite + enstatite during grain‐size‐sensitive creep: 2. Deformation experiments , 2013 .

[19]  M. Richards,et al.  The Relation Between Mantle Dynamics and Plate Tectonics: A Primer , 2013 .

[20]  P. Tackley.  The Quest for Self‐Consistent Generation of Plate Tectonics in Mantle Convection Models , 2013 .

[21]  P. K. Shukla,et al.  Alfvénic tornadoes in a magnetized plasma , 2013 .

[22]  D. Kohlstedt,et al.  The influence of microstructure on deformation of olivine in the grain-boundary sliding regime , 2012 .

[23]  P. Raterron,et al.  High‐temperature deformation of enstatite aggregates , 2011 .

[24]  A. Berger,et al.  The role of second phases for controlling microstructural evolution in polymineralic rocks: A review , 2011 .

[25]  R. Heilbronner,et al.  Quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creep , 2011 .

[26]  W. Heinrich,et al.  Growth of multilayered polycrystalline reaction rims in the MgO–SiO2 system, part II: modelling , 2011 .

[27]  M. Herwegh,et al.  Evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation , 2011 .

[28]  R. Wirth,et al.  Growth of multilayered polycrystalline reaction rims in the MgO–SiO2 system, part I: experiments , 2011 .

[29]  N. Ohashi,et al.  Grain growth systematics for forsterite ± enstatite aggregates: Effect of lithology on grain size in the upper mantle , 2010 .

[30]  D. Kohlstedt,et al.  Effect of iron content on the creep behavior of olivine: 1. Anhydrous conditions , 2009 .

[31]  R. Cooper,et al.  Crystallographic preferred orientation produced by diffusional creep of harzburgite: Effects of chemical interactions among phases during plastic flow , 2008 .

[32]  R. Wirth,et al.  Growth kinetics of enstatite reaction rims studied on nano-scale, Part I: Methodology, microscopic observations and the role of water , 2007 .

[33]  S. Karato,et al.  Deformation fabrics of the Cima di Gagnone peridotite massif, Central Alps, Switzerland: evidence of deformation at low temperatures in the presence of water , 2006 .

[34]  T. Langdon.  Grain boundary sliding revisited: Developments in sliding over four decades , 2006 .

[35]  S. Karato,et al.  The misorientation index: Development of a new method for calculating the strength of lattice-preferred orientation , 2005 .

[36]  D. Mainprice,et al.  The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite , 2004 .

[37]  Dana S. Henry,et al.  Microstructural Evidence for Grain Size Sensitive Deformation Mechanisms in Naturally Deformed Peridotites , 2002 .

[38]  S. Chakraborty,et al.  Si and O diffusion in olivine and implications for characterizing plastic flow in the mantle , 2002 .

[39]  M. Handy,et al.  Strain localization by fracturing and reaction weakening — a mechanism for initiating exhumation of subcontinental mantle beneath rifted margins , 2002, Geological Society, London, Special Publications.

[40]  B. Evans,et al.  A few remarks on the kinetics of static grain growth in rocks , 2001 .

[41]  R. Yund,et al.  Silicon diffusion in forsterite aggregates: Implications for diffusion accommodated creep , 2000 .

[42]  W. Lamb,et al.  Deformation processes in a peridotite shear zone: reaction-softening by an H2O-deficient, continuous net transfer reaction , 1999 .

[43]  P. A. Manohar,et al.  Five Decades of the Zener Equation , 1998 .

[44]  S. Karato,et al.  Mechanisms of shear localization in the continental lithosphere: inference from the deformation microstructures of peridotites from the Ivrea zone, northwestern Italy , 1998 .

[45]  M. Handy,et al.  The evolution of high-temperature mylonitic microfabrics: evidence from simple shearing of a quartz analogue (norcamphor) , 1996 .

[46]  S. Karato,et al.  Lattice preferred orientation of olivine aggregates deformed in simple shear , 1995, Nature.

[47]  P. Kelemen,et al.  Focused melt flow and localized deformation in the upper mantle: Juxtaposition of replacive dunite and ductile shear zones in the Josephine peridotite, SW Oregon , 1995 .

[48]  T. Langdon.  A unified approach to grain boundary sliding in creep and superplasticity , 1994 .

[49]  H. O’Neill,et al.  An experimental study of Fe-Mg partitioning between olivine and orthopyroxene at 1173, 1273 and 1423 K and 1.6 GPa , 1993 .

[50]  M. Ashby,et al.  Interface controlled diffusional creep , 1983 .

[51]  N. Christensen,et al.  Pyroxene orientation within the upper mantle , 1982 .

[52]  R. Twiss.  Theory and applicability of a recrystallized grain size paleopiezometer , 1977 .

[53]  Robert L. Coble,et al.  A Model for Boundary Diffusion Controlled Creep in Polycrystalline Materials , 1963 .

[54]  Conyers Herring,et al.  Diffusional Viscosity of a Polycrystalline Solid , 1950 .

[55]  R. Oppermann,et al.  THE AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS. , 1920, Science.