Seismic properties of the upper mantle beneath Lanzarote (Canary Islands): Model predictions based on texture measurements by EBSD

Abstract

We present a petrophysical analysis of upper mantle xenoliths, collected in the Quaternary alkali basalt fields (Series III and IV) from the island of Lanzarote. The samples consist of eight harzburgite and four dunite nodules, 5 to 15 cm in size, and exhibit a typical protogranular to porphyroclastic texture. An anomalous foliation resulting from strong recovery processes is observed in half of the specimens. The lattice preferred orientations (LPO) of olivine, orthopyroxene and clinopyroxene were measured using electron backscatter diffraction (EBSD). In most samples, olivine exhibits LPOs intermediate between the typical single crystal texture and the [100] fiber texture. Occasionally, the [010] fiber texture was also observed. Simultaneous occurrence of both types of fiber textures suggests the existence of more than one deformation regime, probably dominated by a simple shear component under low strain rate and moderate to high temperature. Orthopyroxene and clinopyroxene display a weaker but significant texture. The LPO data were used to calculate the seismic properties of the xenoliths at PT conditions obtained from geothermobarometry, and were compared to field geophysical data reported from the literature. The velocity of P-waves (7.9 km/s) obtained for a direction corresponding to the existing seismic transect is in good agreement with the most recent geophysical interpretation. Our results are consistent with a roughly W-E oriented fastest P-wave propagation direction in the uppermost mantle beneath the Canary Islands, and with the lithosphere structure proposed by previous authors involving a crust-mantle boundary at around 18 km in depth, overlaid by intermediate material between 11 and 18 km.

Subcontinental lithosphere reactivation beneath the Hoggar swell (Algeria): Localized deformation, melt channeling and heat advection

“…They were also observed in the subduction-related Murray Ridge massif (Canadian Cordillera - Tommasi et al, 2006) and in xenoliths from the Canaries and Kerguelen islands (Bascou et al, 2008;Vonlanthen et al, 2006;). Several processes have been suggested to explain the development of the [010]-fiber CPO symmetry:…”

“…Dunites experimentally sheared under high temperature, low pressure, dry conditions display CPO with orthorhombic or [100]-fiber symmetry (Bystricky et al, 2000;Zhang and Karato, 1995). Such olivine CPO was observed in ophiolites, orogenic peridotite massifs, and oceanic and continental mantle xenoliths (e.g., Ben Ismaïl and Mainprice, 1998;Tommasi et al, 2004;Vonlanthen et al, 2006 (Tommasi et al, 1999).…”

Olivine fabrics and tectonic evolution of fore‐arc mantles: A natural perspective from the Songshugou dunite and harzburgite in the Qinling orogenic belt, central China

“…This result implies that the SPO of olivine (subperpendicular to the SPO of spinel) is probably anomalous and marks a false foliation. This anomalous foliation of olivine was also reported in some mantle xenoliths from the island of Lanzarote (Canary Islands) [ Vonlanthen et al ., ] and Marie Byrd Land in West Antarctica [ Chatzaras et al ., ], and it is interpreted as a result of recovery [ Vonlanthen et al ., ]. Namely, the [100]‐slip edge dislocations first migrate and accumulate to form a set of widely spaced SGBs parallel to the (100) plane in large olivine crystals; and then the subgrains rotate and form new grain boundaries which segment the original large‐sized olivine into smaller grains.…”

An introduction to the application of X-ray microtomography to the three-dimensional study of igneous rocks

“…Enough information to separate touching crystals can usually be obtained using a petrographic microscope, but a complete description requires use of a universal stage (whose use is now rarely taught). Another commonly applied method is electron backscatter diffraction (EBSD) and its complement, orientation contrast imaging (Feinberg et al, 2004;Prior et al, 1999;Vonlanthen et al, 2006). This method uses a scanning electron microscope with a special detector.…”