Preferred orientation in experimentally deformed stishovite: implications for deformation mechanisms

Stishovite is a polymorph of SiO2 present in the mantle, making its deformation properties relevant to Earth science. Stishovite was first produced experimentally (Stishov and Popova 1961) and found to have a rutile structure (space group P42/mnm, Stishov and Belov 1962; Sinclair and Ringwood 1978; Ross et al. 1990) with its silicon in octahedral rather than tetrahedral coordination, as is characteristic of lower-pressure SiO2 polymorphs. Stishovite has since been documented in meteorites such as the Shergotty (El Goresy et al. 2004); at meteorite impact sites such as Ries Crater, Germany (Shoemaker and Chao 1961; Chao and Littler 1963), Meteor Crater, Arizona, USA (Chao et al. 1962), and Vredefort crater, South Africa (Martini 1978, 1991); and in high-pressure metamorphic environments (Liu et al. 2007). First principles calculations (e.g., Cohen 1991; Karki et al. 1997; Teter et al. 1998; Lee and Gonze 1997), Landau theory (Carpenter et al. 2000), Raman spectroscopy experiments (e.g., Kingma et al. 1995), and X-ray diffraction experiments (e.g., Tsuchida and Yagi 1989; Ross et al. 1990; Kingma et al. 1996; Andrault et al. 1998; Hemley et al. 2000) find stishovite to be stable from ~10 to 50 GPa, corresponding to 300–1,200 km depth in the Earth, so a significant portion of the upper and lower mantle. Stishovite is inferred to exist in the upper mantle from inclusions in diamond (Joswig et al. 1999) where quartz is likely transported in subducting slabs as a component in sedimentary layers and mid-ocean ridge basalts and converted to stishovite (Irifune and Ringwood 1993). Acoustic velocity measurements through a single crystal find stishovite to be elastically anisotropic (Yoneda et al. 2012). Thus, if stishovite develops crystallographic preferred orientation (CPO), it may contribute to seismic anomalies in the mantle (Vinnik et al. 2001; Asahara et al. 2013), yet little is known Abstract Although the crystal structure of the high-pressure SiO2 polymorph stishovite has been studied in detail, little is known about the development of crystallographic preferred orientation (CPO) during deformation in stishovite. Insight into CPO and associated deformation mechanics of stishovite would provide important information for understanding subduction of quartz-bearing crustal rocks into the mantle. To study CPO development, we converted a natural sample of flint to stishovite in a laser-heated diamond anvil cell and compressed the stishovite aggregate up to 38 GPa. We collected diffraction patterns in radial geometry to examine in situ development of crystallographic preferred orientation and find that (001) poles preferentially align with the compression direction. Viscoplastic self-consistent modeling suggests the most likely slip systems at high pressure and ambient temperature are pyramidal and basal slip.