Microstructures and seismic properties of south Patagonian mantle xenoliths (Gobernador Gregores and Pali Aike)

CLAUDIA BEATRIZ ZAFFARANA; ANDREA TOMMASI; ALAIN VAUCHEZ; MICHEL GREGOIRE

Cordoba

Congreso; XIX Congreso Geológico Argentino; 2014

The structure and seismic properties of the South Patagonian subcontinental mantle lithosphere was investigated through a microstructural and Electron Backscattered Diffraction (EBSD) study of a suite of 35 peridotite xenoliths brought to the surface by Paleocene alkaline volcanic rocks from Gobernador Gregores and Pali Aike (Gorring et al. 1997, D´Orazio et al. 2000). These kinds of studies in mantle xenoliths are useful to constrain the interpretation of shear wave splitting data. Olivine and pyroxene crystal preferred orientations (CPO) were determined at Geosciences Montpellier using a JEOL 5600 scanning electron microscope equipped with an EBSD system from Oxford HKL Technology. Microstructural and EBSD studies were performed on polished thin sections cut in a random orientation, because the foliation and lineation could not be determined in most macroscopic samples. To allow easy comparison among different samples, we rotated all CPO into a common orientation, in which the maximum concentration of olivine [100] axis was placed in the E?W direction and the [010] axis was placed in the N-S direction of the pole figure. Numerical simulation studies show that the maximum concentration of [100] olivine axes is parallel to the X structural direction, whereas the maximum concentration of olivine [010] axes is parallel to the Z structural direction (Wenk et al. 1991; Tommasi et al. 2000) and normal to grain elongation in the samples which show tabular texture. . Seismic properties were calculated for ambient pressure and temperature conditions using the MTEX software (Mainprice et al. 2011). All xenoliths are middle to coarse-grained, and most samples have equigranular textures characterized by roughly equidimensional olivine crystals with polygonal grain boundaries forming 120° triple junctions. A relict tabular texture, characterized by aligned elongated olivine grains (average aspect ratio of 1:3) defining a foliation is observed in 80% of the peridotites. Olivine crystals display few widely spaced, but well-developed subgrain boundaries, which in tabular crystals are normal to the elongation. The middle to coarse-granular or tabular textures, the low density of intracrystalline deformation features in olivine and pyroxenes and the common polygonal grain boundaries with 120° triple junctions suggest that all xenoliths from Gobernador Gregores and Pali Aike were affected by extensive annealing. The EBSD study showed that all samples show a well-developed olivine and pyroxene crystallographic preferred orientation (CPO), consistent with deformation by dislocation creep with dominant activation of the low-temperature, low-stress [100](010) glide system in olivine. There are three main types of olivine crystallographic fabric: orthorhombic, fiber-[100] and fiber-[010] (Ben Ismaïl and Mainprice 1998). Orthorhombic olivine CPO is characterized by three orthogonal point concentrations of [100], [010] and [001] axes, fiber-[100] olivine CPO is characterized by a strong point concentration of [100] with the other two axes distributed along incomplete girdles normal to it, and fiber-[010] olivine CPO is characterized by a concentration of [010] normal to the foliation (coinciding with the Z structural axis) and girdles of [100] and [001] in the foliation plane, with weaker maxima at 90° to each other. Orthorhombic and fiber-[100] fabrics predominate in Gobernador Gregores xenoliths, whereas fiber-[010] olivine CPO is, however, very common among Pali Aike peridotites. Orthorhombic and fiber-[100] olivine CPO result from deformation regimes characterized by a well-defined extension direction: simple shear, extrusion, transpression or transtension (Tommasi et al. 1999). The common association of fiber-[010] olivine CPO and tabular microstructures suggests the best candidate to explain the axial-[010] symmetry is recrystallization. The fact that these CPO are associated with the tabular microstructure suggests that anisotropic grain boundary properties controlled grain growth during annealing, resulting in strongly anisometric grain shapes and in an increase of the volume fraction of crystals with [010] normal to the foliation. The analyzed samples show modal compositions enriched in clinopyroxene (in the case of Gobernador Gregores) and in orthopyroxene (in the case of Pali Aike) with respect to partial melting trends, suggesting that they experienced refertilization through metasomatism. Moreover, previous geochemical studies show that the mantle beneath Gobernador Gregores and Pali Aike has been extensively metasomatized, though the nature of the metasomatic fluids differs in each locality (Laurora et al. 2001; Aliani et al. 2004; Rivalenti et al. 2004a,b; Bjerg et al., 2005; Dantas 2007). The relative timing of metasomatic pyroxene addition with respect to the high-temperature deformation in olivine was investigated through the comparison of olivine and pyroxene CPO data. There are two kind of samples: samples where the [100] olivine axis maximum is parallel to the [001] pyroxene axis, and samples where the maximum concentration of pyroxenes [001] axes either is very weak or coincides with the [001] olivine axis maximum. The lack of correlation between olivine and pyroxene CPO would imply post-deformation pyroxene addition through reactive melt percolation. In contrast, for the samples that show correlation between olivine and pyroxene CPOs two interpretations are possible: either the pyroxenes are pre- to synkinematic or pyroxene enrichment occurred post-kinematically through overgrowth of primary pyroxenes, which preserved their preexisting CPO. Seismic properties at room temperature conditions of average samples for each site are very homogeneous. The pattern of seismic velocities and anisotropies is very similar in the two localities, with the maximum splitting for S waves propagation in the foliation plane at high angle to the [100] axis concentration (parallel to the X structural direction) and the minimum for those propagating at low angle to this direction. The larger proportion of peridotites with fiber-[010] fabric in Pali Aike relative to Gobernador Gregores does not seem to influence much this pattern. 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