skip to main content


Search for: All records

Award ID contains: 1625032

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract

    The effect of small concentrations of intracrystalline water on the strength of olivine is significant at asthenospheric temperatures but is poorly constrained at lower temperatures applicable to the shallow lithosphere. We examined the effect of water on the yield stress of olivine during low‐temperature plasticity using room‐temperature Berkovich nanoindentation. The presence of water in olivine (1,600 ppm H/Si) does not affect hardness or yield stress relative to dry olivine (≤40 ppm H/Si) outside of uncertainty but may slightly reduce Young’s modulus. Differences between water‐bearing and dry crystals in similar orientations were minor compared to differences between dry crystals in different orientations. These observations suggest water content does not affect the strength of olivine at low homologous temperatures. Thus, intracrystalline water does not play a role in olivine deformation at these temperatures, implying that water does not lead to weakening in the coldest portions of the mantle.

     
    more » « less
  2. Abstract

    Shear localization in the upper mantle, a necessity for plate tectonics, can have a number of causes, including shear heating, the presence of melt, the development of a strong crystal preferred orientation, and the presence of water. The Josephine Peridotite of southwestern Oregon contains shear zones that provide an excellent opportunity to examine the initiation of shear localization. These shear zones are relatively small scale and low strain compared to many shear zones in peridotite massifs, which typically have extreme grain size reduction indicating extensive deformation. We use major, trace, and volatile element analyses of a large suite of harzburgites from the Fresno Bench shear zones to evaluate the mechanisms leading to shear localization. Lithological evidence and geochemical transects across three shear zones show a complex history of melting, melt addition, and melt‐rock interaction. The distribution of aluminum and heavy rare earth elements across the shear zones suggest that melt flow was focused in the centers of the studied shear zones. Water concentrations in orthopyroxene grains of 180–334 ppm H2O indicate a comparatively high degree of hydration for nominally anhydrous minerals. The correlation of water with aluminum and ytterbium in orthopyroxene is consistent with a melt source for this hydration, suggesting that water equilibrated between the melt and peridotite. The presence of melt and hydration of the host rock provide mechanisms for initial weakening that lead to localized deformation.

     
    more » « less
  3. Abstract

    Seismic anisotropy arises in the upper mantle due to the alignment of olivine crystal lattices and is often used to interpret mantle flow direction. Experiments on the evolution of olivine crystal‐preferred orientation (CPO) have found that the texture that develops is dependent on many factors, including water content, differential stress, preexisting CPO, and deformation kinematics. To evaluate the role of these factors in naturally deformed samples, we present microstructural transects across three shear zones in the Josephine Peridotite. Samples from these shear zones exhibit a mixture of A‐type textures, which have been associated with dry conditions and primary activation of the olivine [100](010) slip system, and of E‐type textures, which have been associated with wetter conditions and primary activation of the [100](001) slip system. CPOs with characteristics of both A‐type and E‐type textures are also present. CPO type does not evolve systematically as a function of either strain or water content. We used a micromechanical model to evaluate the roles of preexisting texture and kinematics on olivine CPO evolution. We find that the preexisting texture controls CPO evolution at strains up to 5 during simple shear. Kinematics involving a combination of simple shear and pure shear can explain the olivine CPOs at higher strain. Hence, preexisting CPOs and deformation kinematics should be considered in the interpretation of CPOs measured in naturally deformed rocks and of large‐scale patterns in upper‐mantle seismic anisotropy.

     
    more » « less
  4. null (Ed.)
  5. Hydrogen is a rapidly diffusing monovalent cation in nominally anhydrous minerals (NAMs, such as olivine, orthopyroxene, and clinopyroxene), which is potentially re-equilibrated during silicate melt-rock and aqueous fluid-rock interactions in massif and abyssal peridotites. We apply a 3D numerical diffusion modeling technique to provide first-order timescales of complete hydrogen re-equilibration in olivine, clinopyroxene, and orthopyroxene over the temperature range 600-1200°C. Model crystals are 1-3 mm along the c-axis and utilize H+ diffusion coefficients appropriate for Fe-bearing systems. Two sets of models were run with different boundary compositions: 1) “low-H models” are constrained by mineral-melt equilibrium partitioning with a basaltic melt that has 0.75 wt% H2O and 2) “high-H models,” which utilize the upper end of the estimated range of mantle water solubility for each phase. Both sets of models yield re-equilibration timescales that are identical and are fast for all phases at a given temperature. These timescales have strong log-linear trends as a function of temperature (R2 from 0.97 to 0.99) that can be used to calculate expected re-equilibration time at a given temperature and grain size. At the high end of the model temperatures (1000-1200°C), H+ completely re-equilibrates in olivine, orthopyroxene, and clinopyroxene within minutes to hours, consistent with previous studies. These short timescales indicate that xenolith NAM mantle water contents are likely to be overprinted prior to eruption. The models also resolve the decoupled water-trace element relationship in Southwest Indian Ridge peridotites, in which peridotite REE abundances are reproduced by partial melting models whereas the relatively high NAM H2O contents require later re-equilibration with melt. At temperatures of 600-800°C, which correspond to conditions of hydrothermal alteration of pyroxene to amphibole and talc, H+ re-equilibration typically occurs over a range of timescales spanning days to years. These durations are well within existing estimates for the duration of fluid flow in oceanic hydrothermal systems, suggesting that peridotite NAM water contents are susceptible to diffusive overprinting during higher temperature hydrothermal alteration. Thus, diffusion during aqueous fluid-rock interactions may also explain NAM H2O contents that are too high to reflect residues of melting. These relatively short timescales at low temperatures suggest that the origin of water contents measured in peridotite NAMs requires additional constraints on sample petrogenesis, including petrographic and trace element analyses. Our 3D model results also hint that H+ may diffuse appreciably during peridotite serpentinization, but diffusion coefficients at low temperature are unconstrained and additional experimental investigations are needed. 
    more » « less