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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

We report the occurrence of coesite in a white mica-garnet-bearing quartzite from the metasedimentary cover of the meta-ophiolites exposed in the Orco Valley, Western Alps (Italy). This discovery is an addition to the growing number of ultra-high-pressure (UHP) meta-ophiolite localities in this portion of the Alps, and it indicates that the hosting rock has reached depths exceeding the quartz-coesite transition (⥠2.8 GPa, 80-100 km) during subduction. Here, the petrological and mineralogical observations on garnet-hosted inclusions of the sample are reported and used to qualitatively constrain the metamorphic evolution of Orco Valley, also in relation to the other UHP units. At the scale of the Alpine fossil subduction zone, the UHP evidence occurs locally and discontinuously along strike, with exposures that are patchy rather than continuous (e.g., Lago di Cignana, Ala Valley, Susa Valley, Lago Superiore); however, when compared, the different units show similar metamorphic and structural features, suggesting similar P-T evolutions. This finding supports the interpretation that UHP meta-ophiolites of the Western Alps represent remnants of a former level that underwent comparable conditions in the coesite stability field within the oceanic slab. The frequent new identification of coesite likely reflects both improvements in micro-analytical techniques and increasing attention to smaller isolated inclusions.

The zirconium-in-rutile thermometer enjoys widespread use, but confidence in its accuracy is limited because experiments were conducted at higher temperatures than many rutile-bearing rocks and calibration uncertainties have not been quantitatively assessed. Refined calibrations were developed using bootstrap regression to minimize residuals in the natural logarithm of the equilibrium constant, based on experiments only (n = 32) and on a combined compilation of experiments and natural data (n = 94, total). Rearranging the regression to solve for T, and expressing Zr concentration (C) in parts per million (µg/g), the calibrations in the α-quartz stability field are: Experimental data set: T (° C) = 68740 + 0.441·P (bars) - 0.114·C (ppm) / 129.76 - R·ln [C(ppm)] - 273.15. Combined data set: T (° C) = 71360 + 0.378 ·P(bars) - 0.130·C(ppm) / 130.66 - R·ln [C(ppm)] - 273.1. Thermodynamics of the quartz-coesite transition as applied to the calibration for α-quartz yields calibrations for the coesite stability field: Experimental data set T (° C) = 71290 + 0.310·P(bars) - 0.114·C (ppm) / 128.76 - R·ln [C(ppm)] - 273.15. Combined data set: T (° C) = 73910 + 0.247·P(bars) - 0.130 ·C(ppm) / 129.65 - R·ln [C(ppm)] - 273.15. Propagated temperature uncertainties are ±20-30 °C (2σ) for the experimental data set calibration, and ±10-15 °C (2σ) for the combined data set. Compared to previous experimental calibrations, the refined thermometer predicts temperatures up to 40 °C lower for T ≤ 550 °C, and systematically higher temperatures for T > 800 °C. With careful attention to distributions of Zr in rutile grains, precisions of ±5 °C and accuracies ∼±15 °C may be possible, although a poor understanding of how to select compositions for thermometry will typically lead to larger uncertainties. The ZiR calibration promises continued high-precision and accurate thermometry, and possibly improved thermodynamic properties, but the sources of compositional variability in rutile warrant further scrutiny.

We examined the influence of Al.sub.2 O.sub.3 and H.sub.2 O on the position of the coesite-stishovite transition by means of in situ X-ray diffraction measurements with the large-volume press at the PETRA III synchrotron in Hamburg. The position of the transition was determined by several reversal experiments and was found to be shifted almost in parallel by about 1.5 GPa to lower pressures compared to results for the pure SiO.sub.2 system reported by Ono et al. (2017). Two further reversal experiments with either additional Al.sub.2 O.sub.3 or additional H.sub.2 O added to SiO.sub.2 showed smaller changes compared to the results of Ono et al. (2017), indicating the effect of the coupled Al and H incorporation in coesite and stishovite on their transition. Further investigations of the solid quenched products and of products from additional multi-anvil experiments performed at the GFZ Helmholtz-Zentrum für Geoforschung in Potsdam were done by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), electron probe micro-analysis (EPMA), and Fourier transform infrared (FTIR) and Raman spectroscopy. Generally, the recovered samples of the in situ experiments contained less stishovite than expected from the last in situ XRD pattern before quenching. Thus, these investigations clearly show that hydrous, Al-rich stishovite that formed at high pressure (P) and temperature (T) could, at least partly, not be quenched to room conditions and transformed to coesite with unusually high (Al, H) contents. As result of this, conventional quench experiments would lead to erroneous results of the transition in the (Al, H)-bearing SiO.sub.2 system. We observed two kinds of coesite in the experiments: one relatively Al-poor coesite transformed under equilibrium conditions at P and T from stishovite over a certain time frame and an Al-richer one, sometimes pseudomorphically replacing former stishovite during the decompression process to room conditions. Within both types of coesite, nanometre-sized kyanite inclusions and relicts or remnants of stishovite were observed by TEM. These observations resemble those of Yang et al. (2007) on ophiolites with identical textures and phases and were interpreted as result of a stishovite transition back to coesite during retrograde metamorphism. Our results clearly indicate that the coesite-stishovite transition is sharp but can considerably vary in depth by the addition of Al and H to the SiO.sub.2 system. This has consequences for the assignment and interpretation of the depth variation of the seismic X discontinuity.

The East Kunlun Orogen (EKO) is the NW part of the Central China Orogenic Belt, which records the evolutionary history of the Proto- and Palaeo-Tethys Oceans from the Cambrian to the Triassic. An Early Palaeozoic eclogite belt has been recognized in recent years, which extends discontinuously for ∼500 km as three eclogite-bearing terranes. In this study, we report an integrated study of zircon grains from mica-schists accompanying the eclogites, in terms of mineral inclusions, U-Pb age systematics and P-T conditions. The presence of coesite is identified, as inclusions within the metamorphic domain of zircons, which provides unambiguous evidence for subducted terrigenous clastic rocks of the Proto-Tethys Ocean exhumed from coesite-forming depths. U-Pb dating of the metamorphic zircons yields a concordia age of 426.5 ± 0.88 Ma, which is likely to be the time of ultrahigh-pressure metamorphism in the Kehete terrane. P-T calculations suggest that metapelite may have experienced a clockwise P-T path with peak P/T conditions of 685 ± 41 °C and >28 kbar, and equilibrated at 482-566 °C and 5.6-8.9 kbar during subsequent exhumation. The high-pressure - ultrahigh-pressure (HP-UHP) metamorphic belt within the EKO may have formed by collision between the Qaidam Block and the South Kunlun Block, as a consequence of the closure of the Proto-Tethys Ocean.

Amorphous–amorphous transformations under pressure are generally explained by changes in the local structure from low- to higher-fold coordinated polyhedra 1 – 4 . However, as the notion of scale invariance at the critical thresholds has not been addressed, it is still unclear whether these transformations behave similarly to true phase transitions in related crystals and liquids. Here we report ab initio-based calculations of compressed silica (SiO 2 ) glasses, showing that the structural changes from low- to high-density amorphous structures occur through a sequence of percolation transitions. When the pressure is increased to 82 GPa, a series of long-range (‘infinite’) percolating clusters composed of corner- or edge-shared tetrahedra, pentahedra and eventually octahedra emerge at critical pressures and replace the previous ‘phase’ of lower-fold coordinated polyhedra and lower connectivity. This mechanism provides a natural explanation for the well-known mechanical anomaly around 3 GPa, as well as the structural irreversibility beyond 10 GPa, among other features. Some of the amorphous structures that have been discovered mimic those of coesite IV and V crystals reported recently 5 , 6 , highlighting the major role of SiO 5 pentahedron-based polyamorphs in the densification process of vitreous silica. Our results demonstrate that percolation theory provides a robust framework to understand the nature and pathway of amorphous–amorphous transformations and open a new avenue to predict unravelled amorphous solid states and related liquid phases 7 , 8 . Amorphous–amorphous phase transitions in silicon dioxide are shown to proceed through a sequence of percolation transitions, a process that has relevance to a range of important liquid and glassy systems.

Experiments were conducted to quantify the temperature and pressure effects on the solubility of titanium in coesite. Powdered amorphous silica, titania (anatase), zirconia, and water were added to silver capsules and run in the coesite stability field (at 32, 35, and 40 kbar) from 700 to 1050 °C using a piston–cylinder apparatus. Crystallization of coesite, rutile, and zircon from silica-, titania-, and zircon-saturated aqueous fluids was confirmed by Raman spectroscopy. Cathodoluminescence images and electron microprobe measurements showed that coesite crystals are relatively homogenous. The Ti concentrations of coesite crystals are significantly higher than concentrations predicted using the Ti-in-quartz calibration (Wark and Watson in Contrib Mineral Petrol 152:743–754, 2006. https://doi.org/10.1007/s00410-006-0132-1; Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010. https://doi.org/10.1007/s00410-010-0505-3). Titanium K-edge X-ray absorption near edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in coesite at all conditions studied. A model was calibrated to describe the effects of pressure and temperature on the solubility of titanium in coesite by using a least-squares method to fit Ti concentrations in coesite to the simple expression: \\[RT\\ln X_{{{\\text{TiO}}_{2} }}^{\\text{coesite}} = - 55.068 + 0.00195 \\times T\\;({\\text{K}}) - 1.234 \\times P\\;({\\text{kbar}}) + RT\\ln a_{{{\\text{TiO}}_{2} }}^{\\text{rutile}} ,\\] where R is the gas constant 8.3145 × 10−3 kJ/K, P is pressure in kbar, T is temperature in kelvin, \\[X_{{{\\text{TiO}}_{2} }}^{\\text{coesite}}\\] is the mole fraction of TiO2 in coesite, and \\[a_{{{\\text{TiO}}_{2} }}^{\\text{rutile}}\\] is the activity of TiO2 in the system referenced to rutile. Ti-in-coesite solubility can be used as a thermobarometer for natural samples when used in combination with another indicator of temperature or pressure, such as another thermobarometer in a cogenetic mineral (e.g. rutile) or other phase equilibria (e.g. graphite = diamond). Applications of the Ti-in-coesite thermobarometer to samples from the western Alps and Papua New Guinea are presented.

Pressure and temperature estimates of metamorphic terranes are increasingly retrieved from forward thermodynamic modeling coupled with isopleth thermobarometry. Although geodynamic reconstructions are mathematically derived, thermobarometric estimates are limited by a lack of quantitative constraints, especially on their uncertainties. This work explores the application of the statistically based approach of Bingo‐Antidote on a chloritoid‐garnet‐bearing micaschist from a high‐pressure unit in the continental Western Alps, Italy. Calculations highlighted a good match between the observation and the model at conditions that overlay the quartz‐coesite polymorphic reaction. This result called for targeted investigations, which ultimately led to the recognition of coesite inclusions in the garnet domain modeled to be equilibrated at such conditions, pinpointing the pressure peak at ultra‐high‐pressure conditions. Our results show a so‐far unexplored application of Bingo‐Antidote as a straightforward and fast tool for guiding targeted investigations, such as the search for potential occurrences of otherwise elusive phases like coesite, which might be time‐consuming if randomly applied. In addition, this contribution infers a possible architecture of the Dora‐Maira Massif paleomargin, discussing it in the framework of existing geodynamic simulations.

In low-temperature, high-pressure hydrothermal environments coesite transforms into hydrous forms of stishovite. We studied hydrous stishovite produced from hydrothermal treatment of silica glass as initial SiO2 source at temperatures of 350-550 °C and pressures around 10 GPa. The P-T quenched samples were analyzed by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermal analysis, and IR and magic-angle spinning (MAS) NMR spectroscopy. The presence of significant amounts of H2O (ranging from 0.5 to 3 wt%) is shown from thermogravimetric measurements. PXRD reveals that at temperatures below 400 °C, hydrous stishovite is obtained as two distinct phases that may relate to the solid ice-VII environment present at prevailing P-T conditions. Initially formed hydrous stishovite is metastable and dehydrates over time in the low-temperature, high-pressure hydrothermal environment. The primary mechanism of H incorporation in stishovite is a direct substitution of 4H+ for Si4+ yielding unique octahedral hydrogarnet defects. In IR spectra this defect manifests itself by two broad but distinct bands at 2650 and 2900 cm-1, indicating strong hydrogen bonding. These bands are shifted in the deuteride to 2029 and 2163 cm-1, respectively. Protons of the octahedral hydrogarnet defect produce 1H MAS NMR signals in the 9-12 ppm region. The presence of multiple resonances suggests that the octahedral defect is associated with various proton arrangements. At elevated temperatures, the NMR signals narrow considerably because of proton dynamics.