Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

Qi, Chongchong; Hansen, Louis S.; Wallis, David; Holtzman, B. K.; Kohlstedt, D. L.

doi:10.1002/2017gc007309

Cited by 54 publications

(84 citation statements)

References 79 publications

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“…It is noteworthy that comparison between the CPO of the recrystallized domains and of the bulk rock in strongly recrystallized peridotite 16FN40 highlights that the recrystallized domains have olivine CPO with a stronger fiber‐[010] tendency. This change in olivine CPO pattern is consistent with previous observations on peridotites deformed in presence of melts both in nature and experiments (Higgie & Tommasi, , ; Qi et al, ). Weak fiber‐[010] olivine CPO was also described in troctolites formed by reactive melt percolation in the shallow mantle beneath the Mid‐Atlantic Ridge (Drouin et al, ).…”

Section: Discussionsupporting

confidence: 92%

Deformation, Annealing, Melt‐Rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle

et al. 2019

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We analyzed a set of mantle xenoliths from the Fernando de Noronha archipelago to constrain the roles of deformation, cooling, annealing, and melt percolation in the evolution of the mantle lithosphere of the equatorial Atlantic. The peridotites are dominantly lherzolites with coarse granular or porphyroclastic microstructures. Equilibrium temperatures range between 850 and 1,000 °C. Olivine crystal preferred orientations (CPO) have mainly orthorhombic patterns characterized by clear [100] and [010] point maxima or a fiber‐[100] tendency; these patterns imply deformation by dislocation creep with dominant activation of the [100](010) slip system. Olivine fiber‐[010] patterns are less common; they probably result from recrystallization or melt‐rock interaction. Annealing after deformation partially recovered the microstructures and reduced the olivine CPO strength. Pyroxene CPOs often have weak consistency with olivine CPO, implying post‐deformation refertilization. Chemical compositions are systematically more fertile than those from abyssal peridotites from the Mid‐Atlantic Ridge. These compositions together with the microstructures suggest two stages of melt‐rock interaction. An early refertilization of the base of the lithosphere due to continuous percolation of small melt fractions and later, more important, but local chemical changes (both refertilization and dunitization) as well as recrystallization due to reactive melt percolation likely associated with the Cenozoic volcanism. By comparing the calculated seismic properties with existing seismological data we argue that the moderate seismic anisotropy displayed by the Fernando de Noronha mantle xenoliths likely records past horizontal asthenospheric flow parallel to the spreading direction frozen in the lithospheric mantle as the plate cooled.

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Section: Discussionsupporting

confidence: 92%

Deformation, Annealing, Melt‐Rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle

et al. 2019

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“…Lastly, we note that increased melt retention could explain mantle seismic anisotropy that is oblique to the paleospreading direction beneath the JdF plate interior (VanderBeek & Toomey, ). Laboratory studies on fabric development (Hansen et al, ; Qi et al, ) show that olivine aggregates deformed in the presence of partial melt may generate azimuthal seismic anisotropy that trends >60° from the shear direction. While our interpretation of the JdF age trend is speculative, we consider thermal anomalies or mantle alteration less likely.…”

Section: Interpretation and Discussion Of Tomographic Resultsmentioning

confidence: 99%

Pn Tomography of the Juan de Fuca and Gorda Plates: Implications for Mantle Deformation and Hydration in the Oceanic Lithosphere

VanderBeek

Toomey

2019

JGR Solid Earth

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Tomographic analysis of Pn arrivals—the guided P wave propagating within the lithospheric mantle—is ideal for studying uppermost mantle structure. While plate‐scale seismic images of Pn velocities are common beneath the continents, similar scale studies have not been possible within ocean basins due to the sparse distribution of seismic stations. The Cascadia Initiative (CI) data set provides the first opportunity to image spatial variations in lithospheric structure from accretion to subduction across an entire oceanic plate. We invert Pn travel times from local earthquakes for 3‐D variations in isotropic and anisotropic P wave velocity and event hypocentral parameters. Despite surficial evidence of extensive faulting, we find that the velocity structure of the Gorda uppermost mantle is remarkably consistent with predictions from a conductive cooling model. Limited brittle deformation at mantle depths is supported by seismic anisotropy measurements, which show the fast direction of P wave propagation rotates in concert with the magnetic anomaly lineations. This rotation may be explained by local plate kinematics without internal deformation and hydration of the shallow mantle. In contrast to Gorda, the seismic velocity structure of the Juan de Fuca (JdF) plate does not exhibit a clear age dependence. Anomalously slow mantle velocities are found along the southern edge of the JdF plate and are spatially associated with the termini of pseudofaults. We attribute these velocity reductions to mantle alteration by seawater. Our results highlight the heterogeneous nature of the oceanic mantle lithosphere and show that patterns of mantle alteration are not readily discerned from surficial indicators of seafloor deformation.

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“…For example, fabrics other than olivine CPO may develop at mid‐ocean ridges and impart intrinsic anisotropy. In particular, deformation experiments conducted on olivine aggregates with melt present (e.g., Hansen et al, , 4% melt; Qi et al, , 7% melt) produce fabrics that appear to be a combination of CPO and shape‐preferred orientation (Hansen et al, , 2016; Holtzman et al, ; Qi et al, ). At the much lower melt fractions typical of mid‐ocean ridges, anisotropy due to this melt‐present fabric is still likely to be dominated by the CPO component aligned in the shear direction, but the strength of effective seismic anisotropy may be reduced (Hansen et al, ; Zhong et al, ).…”

Section: Discussionmentioning

confidence: 99%

“…Measurements of seismic anisotropy provide a means to learn about oceanic plate formation and mantle flow. This is possible because the evolution of lithospheric anisotropy is sensitive to a variety of factors including preexisting mantle fabric (Boneh & Skemer, ; Skemer et al, ), the amount of strain the lithosphere experiences (e.g., Hedjazian & Kaminski, ; Ribe, ; Zhang & Karato, ), the magnitude of shear strain relative to the rate of rotation of the strain axes (Kaminski & Ribe, ), and the presence of melt during deformation (Hansen et al, ; Qi et al, ). At the same time, this variety of sensitivities can make the interpretation of anisotropy difficult.…”

Section: Discussionmentioning

confidence: 99%

Azimuthal Seismic Anisotropy of 70‐Ma Pacific‐Plate Upper Mantle

et al. 2019

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Plate formation and evolution processes are predicted to generate upper mantle seismic anisotropy and negative vertical velocity gradients in oceanic lithosphere. However, predictions for upper mantle seismic velocity structure do not fully agree with the results of seismic experiments. The strength of anisotropy observed in the upper mantle varies widely. Further, many refraction studies observe a fast direction of anisotropy rotated several degrees with respect to the paleospreading direction, suggesting that upper mantle anisotropy records processes other than 2‐D corner flow and plate‐driven shear near mid‐ocean ridges. We measure 6.0 ± 0.3% anisotropy at the Moho in 70‐Ma lithosphere in the central Pacific with a fast direction parallel to paleospreading, consistent with mineral alignment by 2‐D mantle flow near a mid‐ocean ridge. We also find an increase in the strength of anisotropy with depth, with vertical velocity gradients estimated at 0.02 km/s/km in the fast direction and 0 km/s/km in the slow direction. The increase in anisotropy with depth can be explained by mechanisms for producing anisotropy other than intrinsic effects from mineral fabric, such as aligned cracks or other structures. This measurement of seismic anisotropy and gradients reflects the effects of both plate formation and evolution processes on seismic velocity structure in mature oceanic lithosphere, and can serve as a reference for future studies to investigate the processes involved in lithospheric formation and evolution.

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Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

Cited by 54 publications

References 79 publications

Deformation, Annealing, Melt‐Rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle

Deformation, Annealing, Melt‐Rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle

Pn Tomography of the Juan de Fuca and Gorda Plates: Implications for Mantle Deformation and Hydration in the Oceanic Lithosphere

Azimuthal Seismic Anisotropy of 70‐Ma Pacific‐Plate Upper Mantle

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