Coesite‐bearing eclogites from >100 km2 in the southern Dulan area, North Qaidam Mountains (NQM) of western China, contain zircon that records protolith crystallization and ultra high pressure (UHP) ...metamorphism. Sensitive High‐Resolution Ion Microprobe (Mass Spectrometer) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb analyses from cathodoluminescence (CL)‐dark zircon cores in a coesite‐bearing eclogite yield an upper intercept age of 838 ± 50 Ma, and oscillatory zoned cores in a kyanite‐bearing eclogite gave a weighted mean 206Pb/238U age of 832 ± 20 Ma. These zircon cores yield steep heavy rare earth element (HREE) slopes and negative Eu anomalies that suggest a magmatic origin. Thus, c. 835 Ma is interpreted as the eclogite protolith age. Unzoned CL‐grey or ‐bright zircon and zircon rims from four samples yield weighted mean ages of 430 ± 4, 438 ± 2, 446 ± 10 and 446 ± 3 Ma, flat HREE patterns without Eu anomalies, and contain inclusions of garnet, omphacite, rutile, phengite and rare coesite. These ages are interpreted to record 16 ± 5 Myr of UHP metamorphism. These new UHP ages overlap the age range of both eclogite and paragneiss from the northern Dulan area, suggesting that all UHP rock types in the Dulan area belong to the same tectonic unit. Our results are consistent with slow continental subduction, but do not match oceanic subduction and diapiric exhumation UHP model predictions. These new data suggest that, similar to eclogites in other HP/UHP units of the NQM and South Altyn Tagh, protoliths of the eclogites in the Dulan area formed in a continental setting during the Neoproterozoic, and then subducted to mantle depth together with continental materials during the Early Palaeozoic.
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
To constrain the water solubility of coesite (Coe) at typical temperatures of subduction zones, a series of Coe coexisting with an aqueous fluid was synthesized at 3–6 GPa and at 600–800 °C. Most ...experiments were performed in the system SiO2-H2O, and some were performed with small amounts of boron addition. With very long heating durations (120–336 h), all these experiments successfully produced Coe crystals of large grain size, ranging from ∼100 to 1300 μm. For every experimental product, multiple unpolarized FTIR spectra were collected on randomly-selected Coe crystals. We have found that type-I hydrogarnet substitution (Si4+(Si2) + 4O2− = 4□(Si2) + 4OH−) is the major water-incorporation mechanism whereas B-related defect (H+ + B3+ ↔ Si4+) makes no much contribution. The water solubility of Coe, ranging from 3(1) to 47(12) wt ppm, positively correlates with both P and T. It can be well described by the empirical equation cH2O = −49(17) + 6.0(21) × P + 0.06(2) × T (cH2O representing water content in wt ppm, P pressure in GPa and T temperature in °C) and by the thermodynamic expression cOH=exp∆S1barRfH2O2exp−∆H1bar−∆VsolidPRT (cOH representing water content in H/106 Si, fH2O water fugacity in GPa, R the gas constant, P pressure in GPa, T temperature in K, ∆S1 bar reaction entropy as 18.5(509) J/mol/K, ∆H1 bar reaction enthalpy as −10.7(516) kJ/mol, and ∆Vsolid the volume change of Coe during hydroxylation as 23.6(42) cm3/mol). Consequently, the water solubility of Coe in subduction zones should be ∼0–123 wt ppm. When Coe with deep origin becomes metastable (i.e., approaching the P-T locus of the Coe-quartz (Qz) reaction), its water content is likely less than ∼10 wt ppm. This trace water may be quickly lost along with further exhumation process, and metastable Coe becomes completely dry, as observed in the Coe discovered in all exhumed ultrahigh-pressure (UHP) metamorphic rocks. Since structurally-bonded water substantially speeds up the Coe-to-Qz phase transition, zero water in metastable Coe may be the key to the preservation of Coe in the UHP metamorphic rocks.
•The water solubility of coesite at realistic P-T conditions of subduction zones were directly experimentally constrained.•The type-I hydrogarnet substitution is the major water-incorporation mechanism and B-related water defects are uncritical.•The H2O solubility of coesite positively correlates with P and T, as described by an empirical and a thermodynamic equation.•The water solubility of Coe in subduction zones should be 0–123 wt ppm.•A zero H2O in Coe helps its metastable preservation in the exhuming ultra-high pressure rocks.
Worldwide, Ultrahigh Pressure (UHP) oceanic units are rare and to date only three were recognized: Tianshan (China), and Lago di Cignana and Lago Superiore Unit (Western Alps, Italy). The UHP oceanic ...units represent the only geological object directly exhumed from mantle depth and they record fundamental information about processes occurring in the deepest portions of the subduction interface.
In this work, we describe the occurrence of a new UHP oceanic unit within the Western Alps (mid Susa Valley, Internal Piedmont Zone). We here report the finding of a UHP index mineral, i.e., coesite and we provide a detailed study of garnet inclusions in metapelites and metabasites part of the meta-sedimentary cover of the meta-ophiolites cropping out in the mid Susa Valley. The samples were investigated via optical microscope, Raman spectroscopy, Elastic Geothermometry and classic thermometry (Zr-in-rutile) in order to constrain the P-T evolution of the area.
The finding of a new UHP unit in the Internal Piedmont Zone, together with the already described Lago di Cignana and Lago Superiore units, points towards the possible existence of a UHP oceanic slice that reached a similar peak-P (i.e., the return point) along the same subduction gradient. This slice was then removed from the slab and dismembered during exhumation, and it is now exposed as coesite-bearing units juxtaposed with lower pressure eclogite-facies ophiolites. Moreover, the large occurrence of coesite along the entire Internal Piedmont Zone significantly increases the extension of the UHP oceanic units. Hence, the model of a localized non-lithostatic pressure is at the state of the art difficult to apply to the oceanic units of the Western Alps.
•We report a new occurrence of coesite within the eclogitic meta-ophiolites (Internal Piedmont Zone) of the Western Alps.•Elastic Geobarometry and Zr-in-rutile thermometry results confirm the PT path proposed in literature with pseudosections.•We propose a unique UHP level inside oceanic slab by linking the three UHP oceanic units outcropping on the Western Alps.
Hardness of polycrystalline SiO 2 coesite Kulik, Eleonora; Nishiyama, Norimasa; Higo, Yuji ...
Journal of the American Ceramic Society,
05/2019, Letnik:
102, Številka:
5
Journal Article
Recenzirano
Abstract
We measured elastic moduli and hardness of polycrystalline SiO
2
coesite. Translucent polycrystalline bulk coesite with a grain size of about 10 micrometers was fabricated at 8 GPa and ...1600°C using a Kawai‐type multianvil apparatus. The obtained bulk and shear moduli are 94(1) and 60.2(3) GPa, respectively. The resulting Vickers and Knoop hardness values are 10.9(7) and 9.6(4) GPa, respectively, at an indentation load of 4.9 N. Coesite is as hard as other fourfold coordinated silica materials such as quartz and densified silica glasses. The hardness values of coesite and the fourfold coordinated silica materials are about one‐third of those of sixfold coordinated silica materials, stishovite, and seifertite, which are the hardest known oxides.
Contrasting metamorphic conditions determined by chemical geothermobarometric investigations of ultrahigh-pressure (UHP) lenses surrounded by high-pressure (HP) and medium-pressure (MP) felsic ...country rocks are an enigmatic feature of UHP terranes. One of the major questions arising is whether the UHP lenses and the country rocks are a product of different peak metamorphic conditions corresponding to different maximum depth or whether country rocks also experienced UHP conditions but equilibrated and/or re-equilibrated at a different metamorphic stage. Here we address this question to the central Saxonian Erzgebirge in the northwestern Bohemian Massif, Germany. In order to screen the variety of garnet from lithologies occurring in the study area, we analyzed the detrital garnet record from seven modern stream sands. In addition to 700 inclusion-bearing garnet grains previously studied from the 125–250 μm grain-size fraction, we analyzed the 63–125 and 250–500 μm fractions and extended the dataset to overall 2100 inclusion-bearing grains. The new findings of coesite and diamond inclusions in several garnet grains, which are in compositional contrast to garnet of the known UHP lenses but match with those of the felsic country rocks, show that considerable parts of the country rocks underwent UHP metamorphism. Melt inclusions containing cristobalite, kokchetavite, and kumdykolite in garnet derived from the country rocks point to partial melting and re-equilibration during exhumation at HP/HT conditions. Although an amalgamation of rocks which reached different maximum depth may be responsible for some of the contrasting peak metamorphic conditions, the mineralogical evidence for UHP conditions in the felsic country rocks surrounding the UHP lenses proves a largely coherent slab subducted to UHP conditions. Furthermore, the presence of coesite in the subducting voluminous felsic crust and its transformation to quartz during exhumation have great implications for buoyancy development during the metamorphic cycle, which may explain the high exhumation rates of UHP terranes.
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•Screening for ultrahigh-pressure (UHP) rocks by detrital garnet inclusion analysis•Country rocks hosting UHP lenses also underwent UHP metamorphism.•UHP rocks were subducted as a largely coherent slab.•First report of cristobalite, kokchetavite, and kumdykolite in the Erzgebirge
Abstract
In this study, we present an example of phase equilibrium modeling of medium-temperature–ultrahigh-pressure (MT–UHP) eclogites that includes consideration of the influence of ferric iron (O) ...and H2O on the phase equilibria. As a case study, we focus on the intergranular coesite-bearing eclogites at Yangkou in the Sulu Belt. Based on phase equilibrium modeling of four eclogites, we monitor changes in phase relations during deep subduction and exhumation, and investigate fluid behavior during decompression. To determine the appropriate O and H2O contents to use in calculating P–T pseudosections for these eclogites, we use an iterative process in which calculated temperature/pressure (T/P)–O/H2O phase diagrams are combined with constraints from petrological observations. P–T pseudosections were calculated for each of the four eclogites to constrain the P–T conditions. The highest P–T conditions retrieved were P > 5·5 GPa at T > 850°C, although variation in mineral compositions suggests that the maximum P–T conditions could have been higher. A P–T path was reconstructed based on microstructural evidence, mineral compositions that constrain P–T conditions within phase assemblage fields, average P calculations and mineral thermobarometry. During exhumation, the retrograde P–T path passed through metamorphic conditions of P = 4·0–3·4 GPa at T = 850–800°C and P = 2·4–1·7 GPa at T = 800–750°C, before reaching crustal levels at P = 1·3–0·9 GPa at T = 730–710°C. The prograde evolution is suggested to have followed a high dT/dP path during the early stage of subduction, followed by a low dT/dP segment to the metamorphic peak. During exhumation, the eclogites at Yangkou became domainal, made up of host-rock with low a(H2O) in which garnet and omphacite have partially re-equilibrated and intergranular coesite has been preserved, cut by veins and veinlets where a(H2O) was higher and new mineral assemblages have developed. In the veins, the new assemblage comprises coarse phengite and quartz with symplectites of K-feldspar + plagioclase + biotite + quartz around the phengite. By contrast, the veinlets comprise symplectites of hornblende + plagioclase ± quartz ± clinopyroxene after omphacite; similar symplectites occur at the edges of the phengite–quartz veins against host eclogite. We interpret the coarse phengite and quartz, which previously could have been coesite, to have formed by precipitation of solutes from fluid migrating under UHP conditions, whereas we interpret the symplectites around the phengite to have formed by local melting and crystallization during exhumation from HP eclogite- to HP amphibolite-facies conditions. The symplectites in the veinlets and along the edges of the phengite–quartz veins are interpreted to have formed by reaction of local grain-boundary fluid with the host under HP amphibolite-facies conditions.
•SEM and μRaman analysis reveal coesite plus PDF-bearing quartz intergrowths.•TEM shows that quartz is in direct contact with euhedral coesite grains.•These observations indicate solid-state ...quartz-to-coesite transformation.•Recurrent iso-orientation with 11¯1* of quartz parallel to 0 1 0* of coesite.•Martensitic structural shifting of 1¯011 quartz planes into coesite (0 1 0) planes.
Coesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its discovery in impact rocks in the 1960s. Electron diffraction analysis coupled with scanning electron microscopy and Raman spectroscopy of shocked silica grains from the Australasian tektite/microtektite strewn field reveals fine-grained intergrowths of coesite plus quartz bearing planar deformation features (PDFs). Quartz and euhedral microcrystalline coesite are in direct contact, showing a recurrent pseudo iso-orientation, with the 11¯1* vector of quartz near parallel to the 0 1 0* vector of coesite. Moreover, discontinuous planar features in coesite domains are in textural continuity with PDFs in adjacent quartz relicts. These observations indicate that quartz transforms to coesite after PDF formation and through a solid-state martensitic-like process involving a relative structural shift of {1¯011} quartz planes, which would eventually turn into coesite (0 1 0) planes. This process further explains the structural relation observed between the characteristic (0 1 0) twinning and disorder of impact-formed coesite, and the 101¯1 PDF family in quartz. If this mechanism is the main way in which coesite forms in impacts, a re-evaluation of peak shock pressure estimates in quartz-bearing target rocks is required because coesite has been previously considered to form by rapid crystallization from silica melt or diaplectic glass during shock unloading at 30–60 GPa.
Phase transformation and optical properties of silica (silicon dioxide, SiO2) quartz sand under high pressure/temperature has been of interest in geology and optical physics for many years. In this ...study, besides high pressure/temperature, high plastic strain is simultaneously applied to the quartz sand by high‐pressure torsion (HPT) processing. The material shows oxygen vacancy formation and transformation to (a) a denser nanocrystalline quartz phase, (b) a high‐temperature amorphous phase and (c) a high‐pressure coesite phase. These structural and microstructural changes lead to light absorbance, electron spin resonance, photoluminscence and photocatalytic activity, while these changes are enhanced by increasing strain. This study introduces a possible pressure‐temperature‐strain‐based mechanism for the formation of naturally observed vacancies and coesite phase in SiO2‐based minerals and sands.
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