Crystallographic preferred orientation and elastic anisotropy of high-pressure rocks from the Eclogite Zone of the Tauern Window, Austria

So far, it has not been possible to image the fine scale internal structure of subduction channels with geophysical methods. In part, this is due to a lack of knowledge regarding elastic wave velocities, anisotropy, and textures (crystallographic preferred orientation, or CPO) of the rocks. Subduction channel fills are likely metabasalts of oceanic origin, and metamorphosed deep-sea sediments. These rock types contain polyphase mineral assemblages, for which CPO is difficult to obtain. In this study, the mineral assemblage of high pressure rocks was assessed by microprobe analysis and subsequently, the CPO was determined from time-of-flight neutron spectra applying full pattern fit method ('Rietveld texture analysis'). With this method, it is possible to investigate polyphase samples, since the CPOs of all mineral phases can be determined simultaneously despite overlapping Bragg reflections in the spectra. From the CPO, 3D models for P-wave velocity anisotropies were calculated. Since subduction channels are not directly accessible, samples were collected in the Eclogite Zone of the Tauern Window, Austria. This is a paleo-subduction channel of the Alpine orogen, which originally formed in the Tertiary during subduction of the Penninic ocean beneath the Adriatic continent. It comprises eclogites, blueschists and greenschists, as well as gneisses, micaschists, marbles and quartzites. The rocks have been exposed to P-T-conditions of 20-25 kbar and 600 +/- 30°C, and were exhumed in a very short time span of 1-2 Ma. Microprobe analysis reveals a complex metamorphic and tectonic history. Some of the eclogites still exhibit their high pressure mineral assemblage, while others were strongly overprinted during exhumation. CPO analysis shows that in some of the eclogites, omphacite, the main constituent (37-49%), exhibits a well pronounced CPO, while in others it is relatively weak. The same applies for retrograde hornblende, if present. In the eclogites, with original high pressure assemblages, omphacite CPO determines the maximum P-wave velocity, leading to small anisotropies of 0.8-1.5%. With the growth of more retrogressed mineral assemblages, specifically hornblende, anisotropies generally increase to up to 3 %. The paragneisses and micaschists contain phengitic mica showing that the metasediments also experienced high pressure metamorphism. Mica always has a strong preferred orientation with its basal plane aligned within the foliation. Quartz exhibits a strong CPO in some samples, while in others it is weak, but distinct. When quartz CPO is strong, it determines the maximum P-wave velocity. In metasediments with weak quartz CPO, the slowest velocity is normal to the mica foliation. Anisotropies in the metasediments lie between 5.2 and 7.4%. These results allow conclusions on seismic properties of rocks in subduction zones, as well as deeper structural levels of the Alps.