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  • 1
    Publication Date: 2022-05-25
    Description: Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 272 (2008): 501-512, doi:10.1016/j.epsl.2008.03.063.
    Description: Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation. We present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the Josephine Peridotite (southwest Oregon). We find that the LPO in harzburgites re-orients from a pre-existing LPO outside the shear zone to a new LPO with the olivine [100] maximum aligned sub-parallel to the shear direction between 168% and 258% shear strain. The strain at which [100] aligns with the shear plane is slightly higher than that observed in experimental samples, which do not have an initial LPO. While our observations broadly agree with the experimental observations, our results suggest that a pre-existing LPO influences the strain necessary for LPO alignment with the shear direction. In addition, olivine re-alignment appears to be dominated by slip on both (010)[100] and (001)[100], due to the orientation of the pre-existing LPO. Fabric strengths, quantified using both the J- and M- indices, do not increase with increasing shear strain. Unlike experimental observations, our natural samples do not have a secondary LPO peak. The lack of a secondary peak suggests that subgrain rotation recrystallization dominates over grain boundary migration during fabric re-alignment. Harzburgites exhibit girdle patterns among [010] and [001] axes, while a dunite has point maxima. Combined with the observation that harzburgites are finer grained than dunites, we speculate that additional phases (i.e., pyroxenes) limit olivine grain growth and promote grain boundary sliding. Grain boundary sliding may relax the requirement for slip on the hardest olivine system, enhancing activation of the two easiest olivine slip systems, resulting in the [010] and [001] girdle patterns. Overall, our results provide an improved framework for calibration of LPO evolution models.
    Description: This work was partly supported by NSF grants EAR-0230267 and EAR-0409609. Funding for fieldwork was provided by the WHOI Academic Programs Office as part of a 2003 field class run by P.B.K. and G.H.
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 2
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 114 (2009): B12203, doi:10.1029/2008JB006186.
    Description: Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to 〈1 km and even 〈0.1 m length scales. Examination of Nd isotopes in relation to modal, trace, and major element compositions indicate that the length scales and amplitudes of heterogeneities in abyssal peridotites reflect both ancient mantle heterogeneity and recent modification by melting, melt-rock reaction and melt crystallization. The isotopic and trace element compositions of pyroxenite veins in this study indicate that they are not direct remnants of recycled oceanic crust, but instead are formed by recent melt crystallization. Combined with existing data sets, the results show that the average global isotopic composition of peridotites is similar to that of mid-ocean ridge basalts, though peridotites extend to significantly more depleted 143Nd/144Nd and 87Sr/86Sr. Standard isotope evolution models of upper mantle composition do not predict the full isotopic range observed among abyssal peridotites, as they do not account adequately for the complexities of ancient and recent melting processes.
    Description: This research was supported by WHOI Academic Programs funding to J.M.W; EAR0115433 and EAR0106578 to N.S; OCE9907630, OCE0526905, and OPP0425785 to H.J.B.D; and COE-21 funding to E.N.
    Keywords: Mantle ; Abyssal peridotite ; MORB
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    Massachusetts Institute of Technology and Woods Hole Oceanographic Institution
    Publication Date: 2022-05-25
    Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2007
    Description: I provide constraints on mantle convection through observations of the rheology and composition of the oceanic upper mantle. Convection cannot be directly observed, yet is a fundamental part of the plate tectonic cycle. Relative motion among plates is accommodated by localized deformation at their boundaries. I demonstrate that in the ductile regime, strain localization occurs when different mineral phases are mixed together, limiting grain annealing. Upper mantle flow is by dislocation creep, resulting in seismic anisotropy due to mineral alignment. I use a shear zone in the Josephine Peridotite to quantify the relationship between mineral orientation and shear strain, providing an improved framework for the interpretation of seismic anisotropy. The upper mantle is generally assumed to be homogeneous in composition. From detailed isotopic and chemical analyses of abyssal peridotites from the Southwest Indian Ridge, I show that the mantle is heterogeneous at a range of length-scales. Abyssal peridotites recovered at ocean ridges are generally interpreted as the depleted residues of melt extraction. I find that melt-rock reaction is a significant part of the melt extraction process, modifying the composition of the lithospheric mantle. The generation of heterogeneous lithosphere provides a source for asthenospheric heterogeneity, via subduction and mantle convection.
    Description: This work has been supported financially by a variety of sources, including a Hollister Fellowship from WHOI, the Richard Vanstone Fund at WHOI and student travel assistance funds at both WHOI and MIT. Funding from the National Science Foundation was provided by grants OCE-0526905 and OCE-0624408 to H.J.B.D., EAR-0230267, EAR- 0405709 and EAR-0409609 to G.H., and EAR-0115433 and EAR-0106578 to N.S. Research at ISEI, Japan, was supported by COE-21 funding to E.N.
    Keywords: Ocean bottom ; Core-mantle boundary ; Melville (Ship) Cruise Vancouver Expedition Leg 7 ; Islas Orcadas (Ship) Cruise IO11/76 ; Agulhas (Ship) Cruise AG22 ; Polarstern (Ship) Cruise PS86 ; Knorr (Ship : 1970-) Cruise KN162
    Repository Name: Woods Hole Open Access Server
    Type: Thesis
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  • 4
    Publication Date: 2022-05-25
    Description: Author Posting. © The Authors, 2009. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Journal of Petrology 51 (2010): 395-423, doi:10.1093/petrology/egp096.
    Description: Ranges in clinopyroxene trace elements of 2-3 orders of magnitude occur over 〈2 cm distance in peridotite samples from the Atlantis II Fracture Zone on the Southwest Indian Ridge. This represents the smallest length-scale at which clinopyroxene trace element concentrations have been observed to vary in abyssal peridotites. Due to the absence of any accompanying veins or other macroscopic features of melt-rock interaction, these peridotites are interpreted as being the result of cryptic metasomatism by a low volume melt. The small length-scale of the variations, including porphyroclastic clinopyroxene grains of 2 mm diameter with an order of magnitude variation in light rare earth elements, precludes an ancient origin for these anomalies. Calculation of diffusive homogenization timescales suggests that for the trace element variations to be preserved, metasomatism occurred in the oceanic lithospheric mantle at 1000-1200°C and 10-20 km depth. This observation provides constraints for the on-axis thickness of the lithospheric mantle at an ultra-slow spreading ridge. Trace amounts of plagioclase are present in at least two of the metasomatized samples. Textural and trace element observations indicate that it formed following the trace element metasomatism, indicating that the mantle can be infiltrated multiple times by melt during the final stages of uplift at the ridge axis. The peridotites in this study are from two oceanic core complexes on the Atlantis II Fracture Zone. Our observations of multiple late-stage metasomatic events in the lithospheric mantle agree with current models and observations of melt intrusion into the mantle during oceanic core complex formation. These observations also indicate that heterogeneous lithospheric mantle can be created at ultra-slow spreading ridges.
    Description: This research was supported by EAR0115433 and EAR0106578 (NS) and the WHOI Academic Programs Office (JMW).
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 5
    Publication Date: 2022-05-26
    Description: The goal of this cruise was to deploy 51 ocean bottom seismometers (OBS) and conduct rock dredges at the Quebrada/Discovery/Gofar (QDG) transform fault systems, with a focus on the Gofar system. QDG is located in the equatorial east Pacific on the East Pacific Rise (Figure 1). Sections of the Gofar and Discovery systems rupture with large (Mw 〉 5.3) earthquakes every 5-6 years, while only one event of this size has occurred on Quebrada in the last 35 years. Variations in along-strike earthquake behavior during these seismic cycles are further constrained by results from a 2008 OBS experiment on QDG. The 2008 experiment revealed the presence of ‘rupture zones’, which fail quasi-periodically in M6 earthquakes, and ‘rupture barriers’, which repeatedly stop large ruptures, yet undergo intense foreshock sequences. The current OBS deployment on Gofar is thus aimed at recording variations in stress build up, stress release, and fault strength within the context of a well-known seismic cycle.
    Description: 2022-03-03
    Repository Name: Woods Hole Open Access Server
    Type: Technical Report
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  • 6
    Publication Date: 2016-09-07
    Description: Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 7
    Publication Date: 2021-08-17
    Description: Abstract
    Description: This dataset is supplemental to the paper Wallis et al. (2020) and contains data derived from syn-chrotron X-ray diffraction, electron backscatter diffraction (EBSD), high-angular resolution electron backscatter diffraction (HR-EBSD), and scanning transmission electron microscopy (STEM). The da-taset consists primarily of measurements of the effect of annealing on stress heterogeneity meas-ured by X-ray diffraction; maps of lattice orientation measured by EBSD; maps of lattice rotations, densities of geometrically necessary dislocations (GNDs), and heterogeneity in residual stress measured by HR-EBSD; and images of dislocations obtained by STEM. Data are provided as 66 tab delimited text files organised and labelled by the figure in which they first appear within Wallis et al. (2020). Table 1 of the data description file presents an overview of the datasets and Table 2 provides a description of each data file. Data types are also indicated in the file names.
    Keywords: Low-temperature plasticity ; olivine ; synchrotron X-ray diffraction ; electron backscatter diffraction ; EBSD ; high-angular resolution electron backscatter diffraction ; HR-EBSD ; scanning transmission electron microscopy ; STEM ; geometrically necessary dislocation ; GND ; residual stress ; EPOS ; multi-scale laboratories ; rock and melt physical properties ; EARTH SCIENCE 〉 SOLID EARTH 〉 ROCKS/MINERALS/CRYSTALS 〉 IGNEOUS ROCKS 〉 IGNEOUS ROCK PHYSICAL/OPTICAL PROPERTIES 〉 HARDNESS ; EARTH SCIENCE 〉 SOLID EARTH 〉 ROCKS/MINERALS/CRYSTALS 〉 MINERALS 〉 MINERAL PHYSICAL/OPTICAL PROPERTIES 〉 COMPOSITION/TEXTURE ; EARTH SCIENCE 〉 SOLID EARTH 〉 ROCKS/MINERALS/CRYSTALS 〉 MINERALS 〉 MINERAL PHYSICAL/OPTICAL PROPERTIES 〉 HARDNESS ; EARTH SCIENCE 〉 SOLID EARTH 〉 TECTONICS 〉 PLATE TECTONICS 〉 STRAIN ; EARTH SCIENCE 〉 SOLID EARTH 〉 TECTONICS 〉 PLATE TECTONICS 〉 STRESS ; Hardness ; Multi Anvil ; olivine ; Strength 〉 Yield Strength ; Triaxial
    Type: Dataset , Dataset
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  • 8
    Publication Date: 2017-02-01
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 9
    Publication Date: 2017-03-01
    Print ISSN: 0003-004X
    Electronic ISSN: 1945-3027
    Topics: Geosciences
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  • 10
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