Textures Induced by the Coesite‐Stishovite Transition and Implications for the Visibility of the X‐Discontinuity

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 23; no. 10
• Main Authors: Krug, Matthias, Saki, Morvarid, Ledoux, Estelle, Gay, Jeffrey P., Chantel, Julien, Pakhomova, Anna, Husband, Rachel, Rohrbach, Arno, Klemme, Stephan, Thomas, Christine, Merkel, Sébastien, Sanchez‐Valle, Carmen
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.10.2022; AGU and the Geochemical Society; Wiley
• Series: Geochemistry, Geophysics, Geosystems
• Subjects: Anomalies; Basalt; Chemical Sciences; Coefficients; coesite‐stishovite transition; Compression; Condensed Matter; Crystallography; Depth; Detection; Diamonds; Geophysics; Impedance; Lasers; mantle heterogeneities; Material chemistry; Materials Science; Microstructure; microstructures; multigrain crystallography; P-waves; Phase transitions; Physics; reflection coefficients; Seismic activity; Seismic propagation; Seismic wave propagation; Seismic wave velocities; Seismic waves; Silica; Upper mantle; Velocity; Visibility; X‐discontinuity; X-discontinuity; multigrain crystallography; microstructures; mantle heterogeneities; reflection coefficients; coesite-stishovite transition
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010544

The coesite‐stishovite phase transition is considered the most plausible candidate to explain the X‐discontinuity observed at around 300 km depth in a variety of tectonic settings. Here, we investigate the microstructure in SiO2 across the coesite‐stishovite transition in uniaxial compression experiments. We apply the multigrain crystallography technique (MGC) in a laser‐heated diamond‐anvil cell (LH‐DAC) to identify the seismic signature of the transition and the amount of SiO2 in the mantle. While coesite displays weak lattice‐preferred orientations (LPO) before the transition, stishovite develops strong LPO characterized by the alignment of [112] axes parallel to the compression direction. However, LPO has little effect on the impedance contrast across the transition, which is up to 8.8% for S‐waves in a mid‐ocean ridge basalt (MORB) composition at 300 km depth along a normal mantle geotherm, 10 GPa‐1700 K. Therefore, 10–50 vol.% of a MORB component, corresponding to 0.6–3.2 vol.% SiO2, mechanically mixed with the pyrolytic mantle would be required to explain the range of impedance (and velocity) contrasts observed for the X‐discontinuity. Based on the reflection coefficients computed for the coesite‐stishovite transition, we show that the incidence angle or epicentral distance is critical for the detection of silica‐containing lithologies in the upper mantle, with highest detection probabilities for small incidence angles. The intermittent visibility of the X‐discontinuity may thus be explained by the seismic detectability of the coesite‐stishovite transition rather than by absence of the transition or chemical heterogeneities in some specific tectonic settings. Plain Language Summary Seismic studies report widespread occurrence of velocity anomalies at ∼300 km depth, whose origin is still not well understood. Here, we performed experiments to check whether a phase transition in SiO2 silica can explain these observations and the reasons for their widespread but not global occurrence. We reproduced the pressure and temperature conditions at 300 km depth in the laboratory and applied an advanced X‐ray diffraction technique to monitor changes in the orientation of grains (i.e., microstructure) in the sample across the transition. We observe that the randomly oriented grains in the low‐pressure phase display strong preferred orientation after the transition. Further, we computed the effect of grain orientations on the propagation of seismic waves and the velocity changes across the phase transitions. We conclude that 10–50 vol.% of crustal rocks embedded in the mantle are needed to explain the observed anomalies. Moreover, we compute seismic parameters associated to the phase transition to guide future exploration of mantle structures. We propose that the intermittent observation of this anomaly is related to the seismic sampling strategy rather than to lack of silica anomalies (and hence the absence of the transition) in some specific mantle settings. Key Points Strong lattice‐preferred orientation develops across the coesite‐stishovite transition at mantle conditions 10–50 vol.% mid‐ocean ridge basalt mixed with pyrolite explains the impedance contrast of the X‐discontinuity Intermittent visibility of the X‐discontinuity can be explained by probing geometry