B-type olivine fabric in the mantle wedge: Insights from high-resolution non-Newtonian subduction zone models

Kneller, E. A.; Keken, Peter E. van; Karato, Shun‐ichiro; Park, Jeffrey

doi:10.1016/j.epsl.2005.06.049

Cited by 249 publications

(241 citation statements)

References 61 publications

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“…However, we conclude that the most likely explanation for the trench-parallel anisotropy in the wedge is a model that combines corner flow in the wedge with a B-type olivine fabric. Although we observe trench-parallel fast directions farther into the backarc than predicted by early modeling work on the B-type hypothesis (Kneller et al, 2005), we think it is possible that our splitting results can be reconciled with such models by an adjustment of model parameters or scaling relations of laboratory results. The B-type fabric model may also be consistent with the minority of trenchperpendicular and intermediate fast directions observed in part of the dataset if the associated raypaths sample regions of the backarc mantle that are dominated by A-, C-, or E-type fabric, or if they sample the transition region between the fabric regimes.…”

Section: The Most Plausible Model For Ryukyu Anisotropymentioning

confidence: 56%

“…However, new laboratory results have shown that when olivine aggregates are deformed under high-stress, lowtemperature, water-rich conditions, the fast axes of individual olivine crystals tend to align 90 • from the flow direction (Jung and Karato, 2001). These B-type olivine fabrics, in conjunction with trench-perpendicular flow in the mantle wedge, may explain trench-parallel fast directions in some regions (Karato, 2003;Kneller et al, 2005). It is not clear, however, if the stresses, temperatures, and volatile concentrations needed to produce B-type olivine fabric are relevant to large volumes of the mantle wedge.…”

Section: Introductionmentioning

confidence: 99%

“…Examples of Btype fabric in mantle-derived rocks have since been documented by Mizukami et al (2004) in samples from the Higashi-akaishi peridotite body in southwest Japan, just northeast of the Ryukyu arc. Subsequent modeling work by Kneller et al (2005) indicates that B-type fabric conditions may occur in part of the mantle wedge. They find that the forearc mantle may have conditions that favor a B-type fabric (and therefore trench-parallel splitting), and they predict a rapid transition to trenchperpendicular fast directions (associated with A-, C-, or E-type olivine fabric) toward the backarc.…”

Section: Corner Flow With B-type Olivine Fabricmentioning

confidence: 99%

“…Only a slight shift in this transition point would be required to explain our observations with the B-type model, especially if the anisotropy we observe is generally located on the near-surface portion of the raypaths. It is not immediately obvious what change(s) in model parameters would be required to move the transition point farther into the backarc; we are currently undertaking a detailed comparison of splitting observations in Japan with the models of Kneller et al (2005), and further modeling of B-type fabric may also be required to elucidate this point. It also remains to be demonstrated through modeling and experiments that a region of B-type fabric in the mantle wedge can produce split times on the order of the ∼1.5 s or more that we observe.…”

Section: Corner Flow With B-type Olivine Fabricmentioning

confidence: 99%

“…2). Kneller et al (2005) envision a transition from trenchparallel to trench-perpendicular fast directions located at the volcanic front for a generic subduction zone model. However, we also observe trench-parallel fast directions for rays that sample a larger part of the backarc (see Fig.…”

Section: Corner Flow With B-type Olivine Fabricmentioning

confidence: 99%

See 4 more Smart Citations

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

Long

Hilst

2006

Physics of the Earth and Planetary Interiors

141

148

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We present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the Japanese F-net array located in the Ryukyu arc. We obtained high-quality splitting measurements for 70 event-station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction. Splitting times for individual measurements ranged from 0.25 to 2 s; most values were between 0.75 and 1.25 s. Both the fast directions and the split times were similar to results for teleseismic S(K)KS and S wave splitting at the same stations, which suggests that the anisotropy is located in the mantle wedge above the slab. We considered several mantle deformation scenarios that would result in predominantly trench-parallel fast directions, and concluded that for the Ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with B-type olivine fabric. In this model, the flow direction in the wedge is perpendicular to the trench, but the fast axes of olivine crystals tend to align perpendicular to the flow direction, resulting in trench-parallel shear wave splitting.

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Section: The Most Plausible Model For Ryukyu Anisotropymentioning

confidence: 56%

Section: Introductionmentioning

confidence: 99%

Section: Corner Flow With B-type Olivine Fabricmentioning

confidence: 99%

Section: Corner Flow With B-type Olivine Fabricmentioning

confidence: 99%

Section: Corner Flow With B-type Olivine Fabricmentioning

confidence: 99%

See 3 more Smart Citations

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

Long

Hilst

2006

Physics of the Earth and Planetary Interiors

141

148

View full text Add to dashboard Cite

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Pyroxenes as tracers of mantle water variations

2014

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The concentration and distribution of volatiles in the Earth's mantle influence properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H 2 O, P, and F were measured in olivine, orthopyroxene, and clinopyroxene in mantle peridotites by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H 2 O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and coexisting pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high-temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral-mineral partition coefficients. Clinopyroxenes have 60-670 ppm H 2 O, while orthopyroxenes have 10-300 ppm, which gives calculated olivine concentrations of 8-34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 1019 Pa s at 1250°C and~15 km depth, compared to a dry effective viscosity of 2.5 × 10 21 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for reequilibration.

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Three-dimensional flow in the subslab mantle

Paczkowski

Montési

Long

et al. 2014

Geochem. Geophys. Geosyst.

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Three-dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench-parallel subslab seismic anisotropy. Subslab flow becomes inherently threedimensional when slab-driven flow interacts with a wide variety of ambient background mantle flow conditions. This interaction depends on slab geometries, mechanical coupling parameters, and lower mantle viscosities. Deflection of subslab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient, background mantle flow. Background mantle flow can become trench-perpendicular or trench-parallel subslab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or toward the edge of the slab. The first case is especially prominent in models with short slabs that do not penetrate into the lower mantle. The second case is especially prominent in models with long, steep slabs. The results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it. High levels of coupling create a boundary layer of trench-perpendicular entrained flow, pushing the deflection due to the obstructing slab away from the slab. We compare our subslab flow model predictions with a global set of seismic anisotropy fast directions in the subslab mantle, and find generally good agreement between the anisotropy observations (dominantly trench-parallel or trench-perpendicular) and the mantle flow directions predicted for decoupled systems.

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B-type olivine fabric in the mantle wedge: Insights from high-resolution non-Newtonian subduction zone models

Cited by 249 publications

References 61 publications

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

Pyroxenes as tracers of mantle water variations

Three-dimensional flow in the subslab mantle

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