Lattice-preferred orientation of olivine found in diamond-bearing garnet peridotites in Finsch, South Africa and implications for seismic anisotropy

Abstract

a b s t r a c tSeismic anisotropy in the upper mantle provides important constraints on mantle dynamics, continental evolution and global tectonics and is believed to be produced by the flow-induced lattice-preferred orientation (LPO) of olivine. Recent experimental studies at high pressure and temperature have suggested that the LPO of olivine is affected by pressure in addition to water and stress. However, there has been no report yet for the pressure-induced LPO of natural olivine because samples from the deep upper mantle are rare and often unsuitable for study due to ambiguous foliation and lineation. Here we show evidence of the pressure-induced LPO of natural olivine in diamond-bearing garnet peridotites from Finsch, South Africa. We found that the [010] axes of olivine are aligned subnormal to foliation and that the [001] axes are aligned subparallel to lineation, which is known as B-type LPO of olivine. The equilibrium pressure of the samples, as estimated using geobarometer, was greater than 4 GPa, indicating that the samples originated from a depth greater than~120 km. In addition, FTIR spectroscopy of the olivine showed that the samples are dry, with a water content of less than 90 ± 20 ppm H/Si (5.5 ± 1.2 ppm wt. H 2 O). These data suggest that the samples are the first natural examples of olivine displaying B-type LPOs produced due to high pressure under dry condition. Our data indicate that the trench-parallel seismic anisotropy observed in many subduction zones in and below subducting slabs at depths greater than~90 km under dry condition may be attributed to the pressure-induced olivine fabrics (B-type LPO) and may be interpreted as the entrainment of the sub-lithospheric mantle in the direction of subduction rather than anomalous trench-parallel flow.

Axial‐type olivine crystallographic preferred orientations: The effect of strain geometry on mantle texture

“…The presence of only one sample with B‐type olivine CPO pattern in the analyzed suite of xenoliths does not allow for definitive predictions concerning the relationship between this CPO and strain geometry. We note, however, that our observations are in agreement with the results of Lee and Jung [], which also show correlation between the olivine B‐type CPO and flattening strain. Furthermore, the B‐type CPO is predominantly formed in direct shear experiments [ Jung and Karato , ; Holtzman et al ., ]; samples are subjected to transpressional deformation, which predominantly produces oblate strain ellipsoids.…”

“…Increased activity of the (010)[001] slip system with increased pressure (>30 kbar) [ Couvy et al ., ; Raterron et al ., ; Jung et al ., ; Lee and Jung , ] or water content (>200 H/10 6 Si) [ Jung and Karato , ; Mizukami et al ., ; Karato et al ., ] may lead to the development of the B‐type and the axial‐[010] CPOs. Despite the uncertainty included in the determination of the geotherm for the Marie Byrd Land lithospheric mantle, the xenoliths with axial‐[010] and B‐type CPOs are characterized by equilibration pressures of less than 21 kbar and contain relatively dry olivine.…”

“…Deformation experiments have established that olivine CPO pattern depends on the degree of activation of different olivine slip systems [ Durham and Goetze , ; Bai et al ., ], which in turn depends on temperature, pressure, differential stress, and water content (Figure a) [ Carter and Avé Lallemant , ; Jung and Karato , ; Couvy et al ., ; Katayama et al ., ; Karato et al ., ; Demouchy et al ., ; Raterron et al ., ]. Studies of naturally deformed rocks support the results of deformation experiments [ Vauchez et al ., ; Mizukami et al ., ; Lee and Jung , ] and in many cases extrapolate experimental results to nature using olivine CPO to infer deformation conditions [e.g., Saruwatari et al ., ; Katayama and Korenaga , ].…”

Seismic anisotropy and mantle flow below subducting slabs

“…The di culty in modelling trench-parallel flow has prompted a number of alternate hypotheses to explain the splitting data; these exploit the fact that does not always equate with the mantle flow direction (e.g., Savage, 1999, and references therein). For example, under simple shear deformation, olivine B-type fabrics have normal to flow (e.g., Jung et al, 2006), leading to the suggestion of B-type fabric in the sub-slab mantle (Jung et al, 2009;Ohuchi et al, 2011;Lee and Jung, 2015). The relationship between flow and also depends on the geometry of deformation (e.g., simple shear vs. pure shear; Ribe, 1992;Tommasi et al, 1999;Di Leo et al, 2014), for example trench-parallel could be caused by pure shear deformation (Faccenda and Capitanio, 2012;Li et al, 2014).…”

Magnitude and symmetry of seismic anisotropy in mica‐ and amphibole‐bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast Tibetan Plateau

“…The CPO of each mineral, which is induced by differential stress and ductile deformation, depends on the prevailing deformation mechanism (e.g., dislocation slip system); the magnitude, geometry, and history of strain (e.g., coaxial or noncoaxial strain); and the conditions during deformation (e.g., temperature, pressure, differential stress, and fluid content) [e.g., Hansen et al , ; Miyazaki et al , ; Raterron et al , ]. As a consequence, the seismic properties of naturally deformed rocks consisting dominantly of trigonal (e.g., α‐quartz and calcite), hexagonal (e.g., β‐quartz) [ Mainprice and Casey , ; Naus‐Thijssen et al , ; Ward et al , ; Zhao et al , ], orthorhombic (e.g., olivine and orthopyroxene [ Ji et al , , ; Jung et al , ; Lee and Jung , ; Park and Jung , ; Saruwatari et al , ]), monoclinic (e.g., amphibolite and clinopyroxene [ Barberini et al , ; Barruol and Mainprice , ; Ji et al , , ; Ko and Jung , ; Tatham et al , ]), and triclinic (e.g., plagioclase [ Ji and Mainprice , ; Ji and Salisbury , ; Ji et al , ; Satsukawa et al , ]) minerals are often of complex geometry.…”