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The cations of an ordered omphacite from the Tauern window were gradually disordered in piston cylinder experiments at temperatures between 850 and 1150 °C. The samples were examined by X-ray powder diffraction and then investigated using low-temperature calorimetry and IR spectroscopy. The low-temperature heat capacity data were used to obtain the vibrational entropies, and the line broadening of the IR spectra served as a tool to investigate the disordering enthalpy. These data were then used to calculate the configurational entropy as a function of temperature. The vibrational entropy does not change during the cation ordering phase transition from space group C2/c to P2/n at 865 °C but increases with a further temperature increase due to the reduction of short-range order. [ABSTRACT FROM AUTHOR]

Earth's subduction zone processes and surface environments are intricately governed by mass transfer phenomena at plate convergent boundaries. The determination of their rates and timings from high-pressure metamorphic rocks (e.g., eclogite), or remnants of ancient convergent boundaries, remains an ongoing challenge. Here, we proposed the potential and versatility of ordering transformation kinetics of omphacite, an essential mineral found in eclogite, as a dynamic recorder of the metamorphic history. Through macroscopic phase-field simulation, we explored the growth of antiphase domains (APDs) in metastable disordered omphacite, discussing the feasibility of constraining metamorphic reaction kinetics based on the size and morphology of omphacite APDs in eclogitized oceanic crust. Our simulation corroborated that omphacite nucleating later during the prograde metamorphism can exhibit an incompletely ordered state with sparsely distributed ordered domains, which suggests their usefulness in estimating the recrystallization timing of the omphacite. Additionally, we confirmed that the APD formation dynamics are significantly influenced by the initial cation configuration of the disordered matrix. This implies the APD morphology in natural omphacite under slab-surface conditions may reflect their precipitation kinetics. These findings provide valuable insights into the microtextural evolution of omphacite due to its ordering transformation, thereby enhancing our ability to interpret morphological features. [Display omitted] • The growth of antiphase domains in omphacite was explored with phase-field modeling. • Incompletely ordered omphacite may allow to resolve short-span geological events. • The morphology of ordered domains should reflect the recrystallization kinetics. [ABSTRACT FROM AUTHOR]

Gemmologists distinguish two rock types in the jade group: nephrite and jadeite jade (or jadeitite). Both are extremely tough monomineralic rocks. New Guinea is favourable for jade occurrence. Giant faults of the New Guinea Mobile Belt (NGMB) extend the 2200 km length of the island and are associated with extensive linear belts of tectonised and serpentinised ultramafic rocks and metabasites, blueschist rocks, intrusive masses of serpentinite containing tectonic inclusions, tremolitised rocks and talc. Geological conditions are favourable for serpentinite-related metasomatic and carbonate replacement nephrite deposits and serpentinite-related vein and pod jadeite jade deposits. Jade is found in eastern New Guinea in the Suckling–Dayman massif, where widespread nineteenth-century use of nephrite tools, including adze blades, axe blades and lime spatulas, occurred in surrounding coastal and inland villages in the Cloudy Bay, Collingwood Bay, Holincote Bay and Musa River region. Several quarries are located along the Mai'iu Fault. Nephrite is also recorded in the Kumusi River area, east of Kokoda. In the Central Fold Belt, jadeite jade, omphacitic pyroxene, rodingite veins and dykes and tremolitised metabasite occur in the headwaters of the April River, Salumei River, Karawari River and Korosameri River regions, and jadeite jade and locally abundant chloromelanite occur in the headwaters of the Leonard Schultze River in the eastern and western sections, respectively, of the April–Lagaip megashear. Numerous quarries are located in the Wario River area. At the Ya'li'me quarries in the Irian Jaya Ophiolite Belt, chloromelanite was quarried for stone-tool manufacture. Jade is also found in the Cyclops Mountains–Lake Sentani ultramafic belt where blocks of chloromelanite for tool manufacture were quarried in the upper Torare River area. This paper describes the history of discoveries, presents an overview of jade petrogenesis and describes the geological settings of NGMB jade occurrences. The geology of New Guinea is favourable for jade occurrence. Jade is found in extensive linear belts of tectonised and serpentinised ultramafic rocks and metabasites, blueschist rocks, intrusive masses of serpentinite, tremolitised rocks and talc. The use of jade was widespread in parts of eastern New Guinea in the nineteenth century. Nephrite tools (adze blades, axe blades and lime spatulas) were common in coastal and inland villages in an area including Cloudy Bay, Collingwood Bay, Holincote Bay and the Musa River. Jade is known from three localities in the Central Fold Belt of New Guinea. Jadeite jade, locally abundant chloromelanite and omphacitic pyroxene are found in the April–Lagaip megashear. Chloromelanite is found in the Ya'li'me quarries, located in the Irian Jaya Ophiolite Belt. Chloromelanite was quarried in the Cyclops Mountains–Lake Sentani region. Blocks used in the manufacture of adze blades and other implements were sourced from upper Torare River. Some tools were traded widely, including across the Bismarck Archipelago to Emirau Island. [ABSTRACT FROM AUTHOR]

Major and trace elements in omphacite, including hydrogen, were determined in eclogites from two Variscan basement complexes in Germany: Erzgebirge (EG) and Fichtelgebirge (FG). Erzgebirge eclogite is derived from three units, showing different peak pressure (P) and temperature (T) conditions (Unit 1: 840–920°C/≥30 kbar, Unit 2: 670–730°C/24–26 kbar, Unit 3: 600–650°C/20–22 kbar). The peak conditions of FG eclogite (690–750°C/25–28 kbar) resemble those of EG Unit 2. Coesite eclogite occurs in EG Unit 1, and quartz eclogite in all other units. Omphacite from all samples shows four infrared (IR) absorption bands. Two prominent, sharp bands occur at 3,455 ± 10 cm−1 (band II) and 3,522 ± 10 cm−1 (band III). Band II is usually more prominent than band III, except for few samples with low jadeite content. A further, broad band is centred between 3,270 and 3,370 cm−1 (band I) and a fourth, minor band at 3,611–3,635 cm−1 (band IV). Bands II and III are due to hydrogen bound as structural OH− ions in omphacite. In most cases, this also applies to band IV. However, some spectra with extremely large type IV bands reflect phengite inclusions. The ambiguous band I may be due to different H2O species (molecular water, structural OH, and water in phengite). Omphacite of quartz eclogite has lower contents of TiO2, Zr, Hf, and REE, compared with that from coesite eclogite. By contrast, omphacite in quartz eclogite from both EG (H2O sample averages: 465–852 ppm) and FG (546–1,089 ppm) contains the same amount of structural OH (concentrations given in wt.‐ppm H2O) as omphacite in coesite eclogite (492–1,140 ppm). The obtained difference in the garnet‐omphacite H2O partition coefficient between quartz (0.01–0.03) and coesite eclogite (0.08–0.11) results from different H2O contents in garnet (coesite eclogite: 50–150 ppm; quartz eclogite: <2–50 ppm; Gose & Schmädicke, 2018). The total content of structural OH in omphacite is unrelated to its major and trace element composition. However, treating the individual IR bands separately, a relation between OH and mineral composition is observed. The OH amount defined by band II is positively correlated to Ti and tetrahedral Al, and that of band III shows a positive correlation with Ca and a negative one with Na (and jadeite). Both the total OH content of omphacite and the partial contents deduced from individual IR bands are unrelated to PT conditions. This implies that omphacite incorporated its structural H2O mainly in the quartz stability field, presumably during initial omphacite growth. Conversely, most OH in garnet was derived from the final breakdown of the last remaining calcic amphibole close to or within the coesite stability field. Our data suggest that coesite eclogite is able to transport a significant amount of H2O (average 550 ppm, maximum 730 ppm), corresponding to that in 3–4 vol.% calcic amphibole, via subduction to depths beyond 100 km. However, the majority of water liberated by dehydration reactions during subduction, including the breakdown of 5–10 vol.% eclogite facies and >10 vol.% pre‐eclogitic hydrous minerals, is not preserved in eclogite but liberated to the mantle wedge. [ABSTRACT FROM AUTHOR]

Low-temperature omphacite has peculiar microstructures called "antiphase domains (APDs)," which can be formed via phase transition from disordered C2/c to ordered P2/n structure during cooling. Hence morphological analyses of the APDs of undeformed omphacite have a potential to unravel the temperature-time (T-t) histories of the eclogite. We investigated five omphacite inclusions in a euhedral garnet porphyroblast obtained from low-temperature eclogite in Syros. The garnet (~6 µm in size) exhibits a distinct prograde chemical zoning and contains abundant mineral inclusions. Transmission electron microscope (TEM) observations of the focused ion beam (FIB) foils confirmed a heterogeneous distribution of equiaxed APDs (10-280 nm in diameter) and columnar APDs. Size distributions of the equiaxed APDs are characterized by kurtosis values of -0.45-3.91, which are larger than those in the matrix omphacite. The columnar APDs are subdivided into two types: dislocation-related (Type I) and inclusion-host interfacial (Type II). The presence of Type I APDs may suggest the inclusions were deformed prior to the host garnet growth. In contrast, Type II APDs, which are characterized by a bundle of stripe-like APDs (~40 nm in width) aligned perpendicular to the host garnet, imply the simultaneous growth of omphacite and garnet in a non-deformation state. The presence of these two contrasting APDs of omphacite inclusions in the single prograde-zoned garnet prevents a simple application of geospeedometry based on APD sizes. Nevertheless, our observations demonstrate that APDs are keys to understanding thermodynamic equilibrium states and the mineral growth kinetics during eclogitization. [ABSTRACT FROM AUTHOR]

Identifying and locating the geochemical and geophysical heterogeneities in the Earth's interior is one of the most important and challenging tasks for the deep Earth scientists. Subducted oceanic crust metamorphizes into the dense eclogite in the upper mantle and is considered as a major cause of geochemical and geophysical heterogeneities in the deep Earth. In order to detect eclogitic materials inside the Earth, precise measurements of the high pressure‐temperature single‐crystal elasticity of major minerals in eclogite are thus exceedingly important. Omphacite, a Na,Al‐bearing clinopyroxene, constitutes up to 75 vol% of eclogite. In the present study, we performed the first high pressure‐temperature single‐crystal elasticity measurements of omphacite using Brillouin spectroscopy. Utilizing the finite‐strain approach, we obtained the following thermoelastic parameters for omphacite: KS0' = 4.5(1), G0' = 1.53(5), ∂KS0/∂T = −0.029(5) GPa/K, ∂G0/∂T = −0.013(5) GPa/K, with KS0 = 123(3) GPa, G0 = 74(2) GPa, and ρ0 = 3.34(1) g/cm3. We found that the seismic velocities of undeformed eclogite are similar to pyrolite at the depths of 200–300 and 410–500 km, thus eclogite is seismically invisible at these depths. Combined with the lattice‐preferred orientations of the omphacite in naturally deformed eclogites, we also modeled seismic anisotropy of eclogite at various pressure‐temperature conditions. A 10 km thick subducted eclogitic crust can result in ∼0.2 s shear wave splitting in the Earth's upper mantle. Plain Language Summary: Omphacite, as a type of clinopyroxene, is one of the main mineral phases in the deeply sinking basaltic materials recycled to the deep Earth. To better constrain seismic properties of the subducted oceanic crust, we experimentally measured elastic properties of omphacite under high pressure‐temperature conditions. Using our results, the calculated seismic anisotropy of deformed basaltic oceanic crust is on the level of 4%–5% in the uppermost mantle. If the subducted basaltic materials remained undeformed, their isotropic seismic velocities are identical to the ambient mantle at depths between 200–300 and 410–500 km. Key Points: Single‐crystal elastic properties of omphacite were measured for the first time at high P‐T conditionsUndeformed eclogite is seismically invisible at 210–300 and 400–500 km depth rangeModeled seismic anisotropy of deformed eclogite can be as high as 4%–5% in the uppermost mantle [ABSTRACT FROM AUTHOR]

Omphacite microstructures are important to decipher the metamorphic pressuretemperature (P-T) conditions of eclogite during subduction and exhumation processes. Here we present a systematic microstructural investigation of omphacite using transmission electron microscopy (TEM) in an ultrahigh-pressure (UHP) eclogite sample from Shuanghe, Dabie orogen, China. The omphacite can be divided into two phases: (1) Omp-1, exhibits as the main part of omphacite with P2/n space group containing small amount of dislocations and numerous antiphase domains (APDs). (2) Omp-2, shows as banded subgrains with C2/c space group containing large amount of dislocations. These two phases of omphacite have almost the same crystallography orientation and bounded by dislocation walls. Along these dislocation walls, we found some barroisite and albite microcrystals. The barroisite microcrystals contain large amount of dislocations and show a topological relation with host omphacite. While the albite microcrystals contain small amount of dislocations and does not show topological relation with host. These microstructures could give us the following metamorphic information to constrain the P-T path of eclogite: Firstly, this two omphacite space groups should be evolved from the same precursor C2/c omphacite, which had been underwent strongly plastic deformation during the syn- to late-peak metamorphic stage. Secondly, the precursor C2/c omphacite began to be retrograded and altered by small amount of barroisite microcrystals along its dislocation walls under the P-T condition of 2.2–2.6 GPa and 650–700 °C at the early amphibole eclogite stage. Thirdly, large amount of Na and other elements were exsolved from some precursor C2/c omphacite subgrains and crystallized numerous albite microcrystals at their boundaries, which is the necessary for the space group of C2/c surviving under the lower P-T conditions (1.7–1.9 GPa and 630–690 C) during the middle amphibole eclogite stage. Lastly, the rest of precursor C2/c omphacite subgrains were transformed into P2/n polymorph and formed the APDs structures under the P-T condition of ~1.5 GPa and 650–680 °C during the late amphibole eclogite stage. [ABSTRACT FROM AUTHOR]

Magnetotelluric surveys revealed high‐conductivity layers in the lower crust beneath Tibet and in the shallow part of the upper mantle relevant to continental collision extending to the Dabie‐Sulu ultrahigh‐pressure metamorphic (UHPM) belts of eastern China, which have been interpreted by the presence of aqueous fluids/partial melts or hydrous phases. However, these explanations are not consistent with their petrological features and seismic properties. Hydrogen‐bearing omphacite could be a probable candidate to explain such high‐conductivity anomalies due to its high water‐partitioning coefficient versus coexisting garnet in eclogite. In this study, we investigated electrical conductivities of Fe‐free and Fe‐bearing omphacite as a function of water content (0.005–0.122 wt.%) at 3 GPa and 500–1300 K. Our results show that water significantly enhances the electrical conductivity of omphacite, while iron facilitates conductivity by accelerating hydrogen diffusivity and lowering its activation enthalpy. Assuming a heat flow of 70 mW/m2, the high electrical anomalies observed beneath the Dabie‐Sulu UHPM belts and the Tibetan Plateau can be reasonably explained by omphacite containing 0.07 wt.% water since water content higher than 0.07 wt.% in omphacite was frequently reported in naturally collected eclogite. Key Points: We investigated the electrical conductivities of Fe‐free and ‐bearing omphacite as a function of water content at 3 GPa and 500‐1300 KH2O greatly enhances the EC of omphacite, while Fe facilitates EC by accelerating H diffusivity and lowering its activation enthalpyFe‐bearing omphacite with 0.1 wt% H2O is a strong candidate to explain the conductivity anomalies in the Dabie‐Sulu UHPM belts and Tibet [ABSTRACT FROM AUTHOR]

Omphacite is a major mineral phase of eclogite, which provides the main driving force for the slab subduction into the Earth's interior. We have measured the single‐crystal elastic moduli of omphacite at high pressures for the first time up to 18 GPa at ambient temperature using Brillouin spectroscopy. A least squares fit of the velocity‐pressure data to the third‐order finite strain equation of state yields KS0′ = 4.5 (3), G0′ = 1.6 (1) with ρ0 = 3.34 (1) g/cm3, KS0 = 123 (3) GPa, and G0 = 74 (2) GPa. In addition, the synchrotron single‐crystal X‐ray diffraction data have been collected up to 18 GPa and 700 K. The fitting to Holland‐Powell thermal‐pressure equation of state yields KT0′ = 4.6 (5) and α0 = 2.7 (8) × 10−5 K−1. Based on the obtained thermoelastic parameters of omphacite, the anisotropic seismic velocities of eclogite are modeled and compared with pyrolite between 200 and 500 km. The largest contrast between the eclogite and pyrolite in terms of seismic properties is observed between ~310 and 410 km. Key Points: Thermal equation of state and single‐crystal elasticity measurements is performed on omphacite crystalsWith the newly obtained thermoelastic parameters of omphacite, anisotropic seismic properties of eclogite are modeledThe maximum Vp, Vs, and anisotropy contrast between pyrolite and eclogite is between 310 and 410 km [ABSTRACT FROM AUTHOR]

We examined the microfabrics of omphacite and garnet in foliated eclogite to determine the influence of the layered structure on seismic observations in subduction zone. The analyzed eclogite, from the Lanterman Range, northern Victoria Land, Antarctica, is characterized by layering in which the modal abundances of garnet and omphacite vary. For garnet, the low aspect ratios, similar angular distribution of long axes relative to the foliation in both layers, uniform grain size distribution, near-random crystallographic preferred orientations (CPOs), and misorientation angle distributions are indicative of passive behavior during deformation. In contrast, omphacite shows relatively high aspect ratios, a low angle between the long axes of crystals and the foliation, a wide grain-size distribution, and distinctive CPOs, suggesting dislocation creep as the main deformation mechanism. The results of fabric analyses are consistent with strain localization into omphacite or omphacite-rich layers rather than garnet or garnet- rich layers. The single-crystal seismic anisotropy of garnet is very weak (AVP = 0.2%, AVS = 0.5-0.6%), whereas that of omphacite is much stronger (AVP = 3.7-5.9% and AVS = 2.9-3.8%). Seismic anisotropy of the omphacite-rich layers shows an increase of 329% for AVP and 146% for AVS relative to the garnet-rich layers. Our results demonstrate the importance of the layered structure in strain localization and in the development of the seismic anisotropies of subducting oceanic crust. [ABSTRACT FROM AUTHOR]

The India-Eurasia collision zone in eastern Ladakh, India, is marked by the Tso Morari Crystalline Complex (TMCC), which underwent continental subduction, eclogite grade metamorphism, and rapid exhumation during the Himalayan orogeny. The TMCC is characterized by eclogitic enclaves, metamorphic evolution of which has been extensively studied in the past. Detailed petrography, mineral chemistry, Raman spectroscopy, and P-T-X pseudosection modelling of two such eclogite samples from the core and western margin of TMCC were carried out to address the differences and discrepancies from previous studies. Our study suggests peak metamorphism at ∼2.75 ± 0.1 GPa and ∼550 ± 50 °C, followed by a decompression stage at ∼1.50 ± 0.1 GPa and ∼580 ± 50 °C and subsequent high-temperature overprint at ∼0.85 ± 0.1 GPa and ∼680 ± 50 °C. Fine-grained grain boundary omphacite + garnet + amphibole symplectite colonies indicate exhumation along with fluid infiltration. The transition of omphacite into a more calcic variety and its subsequent replacement with amphibole are also observed. Presence of diopside + plagioclase symplectite indicates post-peak decompression and fluid infiltration. Carbonates in the matrix as veins and within fractures indicate late-stage fluid infiltration and metasomatism. Our study shows that lawsonite and/or Na-amphibole have not played any significant role in the metamorphic evolution of TMCC. In contrast to most of the previous studies carried out in TMCC, we opine that omphacite breakdown and metasomatism through externally derived carbonaceous fluids played a vital role in the exhumation and retrogression. We further opine that our mineral chemistry data and metamorphic modelling resemble a continental eclogite and mark differences from oceanic eclogites. • Mineral chemistry, Raman spectroscopy and P-T pseudosection modelling of Tso Morari eclogite of trans-Himalaya carried out. • Tso Morari eclogite are of continental subduction setting with no role of lawsonite or Na-amphibole in its evolution. • Exhumation is characterized by omphacite breakdown, Na-Ca amphibole to Ca-amphibole transition and symplectite formation. • Majority of Carbonates of Tso Morari eclogite are metasomatic formed by fluid infiltration from Zildat Ophiolitic Mélange. [ABSTRACT FROM AUTHOR]

Water contents were analyzed by infrared spectroscopy for structural hydroxyl in randomly oriented cuboids and doubly polished wafers of single crystal omphacite from an ultrahigh-pressure eclogite in the Dabie orogen. The results are quantitatively compared with bulk water contents measured by the TC/EA-MS continuous flow technique. This enables recognition of various hydrous species in omphacite and their bearings on metamorphic dehydration during continental subduction. The two cuboids yield constant maximal structural water contents of ∼378 ppm (H 2 O wt.) using the omphacite-specific infrared absorption coefficient, which are much lower than the bulk water contents of ∼1496 ppm. The excessive water is contributed by the presence of other hydrous species in the form of hydrous mineral inclusions, hydrous symplectite and submicron fluid inclusions. Since the hydrous mineral and/or fluid inclusions may be captured at different metamorphic stages during subduction and exhumation of continental crust and the hydrous symplectite is the secondary rather than the primary, it is suggested that the bulk water contents represent a mixing of various hydrous species with distinct origins. In this regard, it is critical to examine whether water in these secondary hydrous species is of internal origin; if not, it has nothing to do with the transport capacity of water in deeply subducted crustal rocks. Upon stepwise heating up to 700 °C, omphacite exhibits a gradual decrease in bulk water contents but an increase in δD values, indicating the preferential loss of D-depleted hydrous species as a whole due to the decompressional dehydration of UHP metamorphic rocks during exhumation. Therefore, the D-depleted hydrous species may be dominated in the hydrous symplectite and inclusion water. [ABSTRACT FROM AUTHOR]

A key question concerning the water budget of Earth's mantle is how much water is actually recycled into the mantle by the subduction of eclogitized oceanic crust. Hydrous phases are stable only in quartz eclogite not coesite eclogite so that water transport to greater depths is mainly governed by structural water in omphacite and garnet. Here we explore if garnet can be used as a proxy to assess the amount of this water. Available data on the water contents of garnet in coesite eclogite vary over orders of magnitude, from a few up to ca. 2000 ppm. By implication, the maximum bulk-rock water contents are unrealistically high (wt% level). New data from the Erzgebirge indicate moderate amounts of structural H2O stored in garnet (43-84 ppm), omphacite (400-820 ppm), and in the bulk coesite eclogite (ca. 280-460 ppm). Higher garnet water contents occur, but these are not primary features. They are related to molecular water in fluid inclusions that can be attributed to eclogite-facies fluid influx postdating the metamorphic peak. Fluid influx also caused the uptake of additional structural water in garnet domains close to fluid inclusions. Such secondary H2O incorporation is only possible in the case of primary water-deficiency indicating that garnet hosted less water than it was able to store. This is insofar astonishing as comparably high H2O amounts are liberated by the breakdown of prograde eclogite-facies hydrous minerals as a result of ultrahigh-pressure (UHP) metamorphism. Judging from Erzgebirge quartz eclogite, dehydration of 5-10% hydrous minerals (±equal portions of zoisite+calcic amphibole) produces 1500-3000 ppm water. We infer that the largest part of the liberated water escaped, probably due to kinetic reasons, and hydrated exhuming UHP slices in the hanging-wall. Depending on the physical conditions, water influx in eclogite during exhumation (1) produces fluid inclusions and simultaneously enhances the structural water content of nominally anhydrous minerals--as in the Erzgebirge--and/or (2) it may give rise to retrograde hydrous minerals. We conclude that eclogite transports moderate quantities of water (several hundred parts per million) to mantle depths beyond 100 km, an amount equivalent to that in ca. 1% calcic amphibole. [ABSTRACT FROM AUTHOR]

[Display omitted] • Garnet contains both structural hydroxyl and molecular H 2 O in tiny fluid inclusions. • Molecular H 2 O can be either intrinsic or external origin in eclogitic garnet. • Inter-transformation between structural hydroxyl and molecular H 2 O in garnet. • Hydroxyl at higher wavenumber in garnet are more stable and more enriched in D. Water in nominally anhydrous minerals may exist as structural hydroxyl and molecular H 2 O, and their inter-transformation can occur during subduction zone metamorphism. This issue is examined here by a systematic study on water in garnet and omphacite for ultrahigh-pressure metamorphic eclogites from the Dabie orogen. IR spectra of garnet at different preheating temperatures imply that the broad absorption band at 3400–3450 cm−1 mainly represents molecular H 2 O. Molecular H 2 O in garnet can be intrinsic origin, but is mainly originated from external fluid during exhumation. The positive correlation between molecular H 2 O and structural hydroxyl in garnet suggests their inter-transformation. Molecular H 2 O that was formerly included in garnet or derived from decomposition of hydrous minerals was transformed gradually to structural hydroxyl in garnet during subduction. OH− modes at higher wavenumber in garnet are more stable and more enriched in D, and d -poor molecular H 2 O is preferentially lost during dehydration. Hydroxyl in both anhydrous and hydrous minerals would be transformed to molecular H 2 O during exhumation and redistributed into garnet or retrograde minerals. The addition of molecular H 2 O into garnet during exhumation causes increase of structural hydroxyl but decrease of δD value, and affects accurate estimate of water content in peak garnet. Different from the large variation of garnet water contents, omphacite tends to be water-saturated at high-pressure conditions and its water content is mainly controlled by Ca-Eskola. The subducting mafic crust can transport a considerable amount of water to subarc depths to result in the formation of relatively water-rich mantle regions. [ABSTRACT FROM AUTHOR]

Tectonic blocks and slabs of mafic–ultramafic rocks are distributed discontinuously in the Yuli metamorphic belt of Taiwan. The blocks include rare omphacite metagabbros and garnet–epidote blueschists in the Wanjung and Juisui (Tamayen) areas, respectively. Such high-pressure (HP) mineral assemblages have been attributed to a mid-Miocene subduction event. However, the surrounding psammitic, pelitic and chloritic schists are the dominant greenschist-facies lithologies of the Yuli belt. In the Chinshuichi area, tectonic blocks are enclosed in garnet-bearing metapelites, suggesting elevated pressures. In this area, we recently discovered meta-plagiogranite containing the assemblage glaucophane + omphacite (X Jd up to 0.39) + rutile + quartz, indicating P–T conditions near 13 kbar/550 °C. New equilibrium phase modeling of a garnet–paragonite mica schist and compositional isopleths for peak assemblage minerals garnet and phengite (Si = 3.33–3.37 pfu) indicate metamorphic conditions of 15.5–17 kbar/530–550 °C. These P–T estimates are higher than previously reported in the Yuli belt and suggest that both tectonic blocks and host metapelites underwent HP metamorphism. The juxtaposition of tectonic blocks and metapelites apparently occurred during the formation of a subduction–accretionary complex, followed by exhumation facilitated by a collisional event. These new findings imply that HP metamorphism was not limited to tectonic blocks, and instead played a significant role attending orogenesis in eastern Taiwan. [ABSTRACT FROM AUTHOR]

Electron backscattered diffraction (EBSD) and crystallographic preferred orientations (CPO) studies on Himalayan high- and ultrahigh-pressure eclogites are reported. Subduction-related peak eclogite facies (omphacite, garnet, quartz, rutile) and exhumation-related post-eclogite facies (hornblende, epidote, titanite) minerals were investigated to understand subduction and exhumation-related deformation regimes. Petrographic features from the studied eclogite samples show preferably shape-oriented omphacites coexisting with random to weakly shape-oriented garnets, and overgrown by hornblende and symplectites. Quartz grains are also randomly shape-oriented in the eclogite matrix whereas epidote grains display shape-preferred orientations. Based on EBSD data, an L-type CPO was dominant in omphacites in the studied eclogites, exhibited by the maximum density of [001] axes parallel to the lineation and the high density of poles to the (010) and (110) planes forming girdles perpendicular to the foliation. This CPO likely developed in a constrictional strain regime during the peak eclogite-facies conditions due to dislocation creep and is associated with an intra-crystalline (110) [001] and (110) <110> slip systems. EBSD data of garnets exhibit complex fabric, suggesting they most likely behaved like rigid objects in the presence of omphacite, indicating insignificant plastic deformation. EBSD data of quartz in high-pressure eclogites show irregular CPO but in ultrahigh-pressure eclogites the fabric is relatively strong. EBSD data of rutile show CPO of [001] axes strongly aligned along the lineation, suggesting its deformation was coeval to the omphacite. Titanite, mainly overgrown around rutile, indicates a random CPO different from rutile, suggesting its growth during late-stages. Hornblende in both types of eclogites shows identical fabric to that of omphacite, displaying CPO of [001] axes parallel to the lineation and poles of (010) and (110) planes form girdles perpendicular to the foliation, suggesting topotactic replacement over omphacite during exhumation. The EBSD data, combined with textural features, indicate syn-deformation peak metamorphism as well as preserve records of subsequent exhumation. • Crystallographic Preferred Orientations (CPO) of Himalayan HP/UHP eclogites. • Data indicate petrofabric developed during peak eclogite facies stage. • Clinopyroxene with an L-type CPO suggests an intra-crystalline plasticity in constrictive strain regime. • Garnet with random fabric suggests rigid objects rotation. • Hornblende, with CPO similar to that of clinopyroxene, display topotactic growth over clinopyroxene during retrogression. [ABSTRACT FROM AUTHOR]

Omphacite, a clinopyroxene mineral with two distinct crystallographic sites, M1 and M2, and composition intermediate between diopside and jadeite, is abundant throughout the Earth's upper mantle and is the dominant mineral in subducted oceanic crust. Unlike the end-members, omphacite exists in two distinct phases, a P2/ n ordered phase at low temperature and a high-temperature C2/ c disordered phase. The crystal structure and full elastic constants tensor of ordered P2/ n omphacite have been calculated to 15 GPa using plane-wave density functional theory. Our results show that several of the elastic constants, notably C, C, and C deviate from linear mixing between diopside and jadeite. The anisotropy of omphacite decreases with increasing pressure, and at 10 GPa, is lower than that of either diopside or jadeite. The effect of cation disorder is investigated through force-field calculations of the elastic constants of special quasi-random structures supercells with simulated disorder over the M2 sites only, and over both cation sites. These show that cation order influences the elasticity, with some components displaying particular sensitivity to order on a specific cation site. C, C, and C are sensitive to disorder on M1, while C is softened substantially by disorder on M2, but insensitive to disorder on M1. This shows that the elasticity of omphacite is sensitive to the degree of disorder, and hence the temperature. We expect these results to be relevant to other minerals with order-disorder phase transitions, implying that care must be taken when considering the effects of composition on seismic anisotropy. [ABSTRACT FROM AUTHOR]

An in situ single-crystal high-temperature X-ray diffraction study was performed on clinopyroxene crystals along the jadeite, (NaAlSiO Jd)-diopside (CaMgSiO Di) join. In particular, natural samples of jadeite, diopside, P2/ n omphacite and three C2/ c synthetic samples with intermediate composition (i.e., Jd, Jd, Jd) were investigated. In order to determine the unit-cell volume thermal expansion coefficient ( α), the unit-cell parameters for all these compositions have been measured up to c.a. 1,073 K. The evolution of the unit-cell volume thermal expansion coefficient ( α) along the Jd-Di join at different temperatures has been calculated by using a modified version of the equation proposed by Holland and Powell (J Metamorph Geol 16(3):309-343, ). The equation $$ a_{{{\text{V}}\;(303{\text{K}},1{\text{bar}})}} = 2.68(3) \times 10^{ - 5} + [1.1\left( 1 \right) \times 10^{ - 8} \times X_{\text{Jd}} ] - [7.1\left( {1.7} \right) \times 10^{ - 10} \times X_{\text{Jd}}^{2} ] $$ obtained from the α at room- T (i.e., α) allows us to predict the room- T volume thermal expansion for Fe-free C2/ c clinopyroxenes with intermediate composition along the binary join Jd-Di. The observed α value for P2/ n omphacite α = 2.58(5) × 10 K was compared with that recalculated for disordered C2/ c omphacite published by Pandolfo et al. (Phys Chem Miner 1-10, ) [ α = 2.4(5) × 10 K]. Despite the large e.s.d.'s for the latter, the difference of both values at room- T is small, indicating that convergent ordering has practically no influence on the room- T thermal expansion. However, at high- T, the smaller thermal expansion coefficient for the C2/c sample with respect to the P2/n one with identical composition could provide further evidence for its reduced stability relative to the ordered one. [ABSTRACT FROM AUTHOR]

A significant portion of the Earth's water is found at trace levels in nominally anhydrous rock forming minerals in the crust and mantle. Its distribution has far-reaching implications in melt-generating processes, mantle rheology and the stability of the lithosphere. We report petrographic, major and trace element, Fourier transform infrared spectroscopy and secondary ion mass spectrometry data on four eclogite xenoliths from the Obnazhennaya kimberlite, Siberia that contain hydrous minerals at upper mantle conditions. Al-rich (>9.9 wt% Al 2 O 3) omphacitic clinopyroxene, a major mineral of eclogites, contains crystallographically controlled exsolutions of CaO-rich (up to 23.6 wt%) garnet (coronae, lenses), garnet-zoisite (lamellae) or amphiboles (needle/blade-like exsolutions). The omphacite hosting the exsolved lamellae shows enrichment or depletion in major (Al, Si, Mg) and trace (e.g., HREE) elements compatible with garnet and zoisite, proportional to the abundance of exsolutions. We argue that zoisite likely formed concomitantly with garnet exsolution from a water- and Al-rich precursory omphacite, without any fluid addition. The compositional gradients are consistent with diffusion-controlled partitioning in a closed system, and their preservation indicates the exsolution likely took place shortly prior to eclogite entrapment by the kimberlite. Large, non-exsolved, omphacite grains contain minimum ~870–1500 ppm wt. H 2 O (determined as OH). We infer that intertwined lamellae of hydrous garnet (600–960 ppm wt. H 2 O) and minor (<1%) zoisite (2 wt% H 2 O) formed at mantle conditions due to progressive exsolution, without external fluids. Recalculated whole rock water contents range from ~320 to 970 ppm wt. H 2 O. These estimates exceed by far those for the surrounding peridotitic mantle and suggest that, although a minor component, eclogites may locally be an important water reservoir in the cratonic mantle. • Rare eclogites from Obnazhennaya kimberlite contain water-rich exsolutions. • Zoisite stability at mantle conditions is demonstrated for the first time in natural samples. • Bulk water concentrations of eclogites exceed those of the surrounding peridotites. • Eclogites may be unique carriers and storage for water in the lithospheric mantle. [ABSTRACT FROM AUTHOR]

Raman spectroscopy was used for the characterization of seven gem quality green 'jade' samples and three green 'jade' samples of archaeological importance. The results were also compared with those acquired by other nondestructive techniques such as classical gemology, energy-dispersive X-ray fluorescence (EDXRF), ultraviolet-visible-near infrared (UV-Vis-NIR) in absorption, Fourier transform infrared spectroscopy (FTIR) in absorption and micro-FTIR in reflectance. Five samples of gem quality and two samples of archaeological interest were found to be 'jadeite jade', whereas two samples of gem quality and one sample of archaeological interest were 'omphacite jade'. Raman spectroscopy is found to be the most efficient method for their characterization. The results were confirmed with EDXRF and micro-FTIR in reflectance. Data acquired using classical gemology, UV-Vis-NIR absorption and FTIR absorption spectroscopy were similar on 'omphacite jade' and 'jadeite jade'. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Distributions of the rare-earth elements (REE) in omphacite and garnet and REE behaviors during metamorphic processes were discussed. The REE concentrations of garnet and omphacite in six eclogite samples from the Dabie Mountain, central China, were measured by inductively coupled plasma-mass spectrometry (ICP-MS). The correlation of δEu ratios between garnet and omphacite indicated that chemical equilibrium of REE distribution between garnet and omphacite could be achieved during ultra-high pressure (UHP) metamorphism. Most of the partition coefficients ( K=C/C) of light rare-earth elements (LREE) are higher than 1. However the partition coefficients of heavy rare-earth elements (HREE) are lower than 1. This indicated that the LREE inclined to occupy site M2 in omphacite, but the HREEs tended to occupy eightfold coordinated site in garnet during the eclogite formation. The REE geochemistry of the eclogites indicated that LREE could be partially lost during the prograde metamorphic process of protolith, but be introduced into the rocks during the symplectite formation. LREE are more active than HREE during the UHP metamorphism. The results are favorable to highlighting the REE behavior and evolution of UHP metamorphic rocks. [ABSTRACT FROM AUTHOR]

We report mineral assemblages from three clasts in the Northwest Africa 801 CR chondrite. The clasts, 1-3 mm in size, are ellipsoidal to irregular shaped, and show similar granular texture. The constituent minerals in the clasts are omphacite-rich clinopyroxene and pyrope-rich garnet, in addition to olivine and orthopyroxene. The omphacite contains jadeite (34 mol%) and diopside-hedenbergite (37%), and a significant amount of an enstatite-ferrosilite component (19%), which distinguishes it from terrestrial omphacite. The omphacite has a disordered C2/c structure. Graphite, phlogopite, chlorapatite, Fe-Ni metal, troilite, and pentlandite are present as minor minerals in the clasts. The minerals commonly found in chondrites, such as plagioclase and spinel group minerals, are not found in these clasts. Aluminum and sodium in the clasts are completely partitioned into omphacite and garnet. The mineral assemblages and compositions of the clasts are similar to those in terrestrial eclogite, except for the occurrence of olivine and some mineral chemistry, and this is the first discovery of an extraterrestrial eclogitic mineral assemblage. The clasts formed under high-pressure conditions, 2.8-4.2 GPa and 940-1080 °C, as estimated from a set of conventional geothermobarometers, indicative of formation in a large parent body. Another possibility is impact-induced origin, although the formation conditions would have been different from those for known shock veins. Meteorites usually consist of minerals that formed under low-pressure conditions, except for ultrahigh-pressure minerals found in shock veins. However, this study suggests that the pressure conditions for meteorite formation vary much wider than previously understood. [ABSTRACT FROM AUTHOR]

Abstract: Lattice preferred orientations (LPOs) are commonly interpreted to form by dislocation creep. Consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable, especially if those minerals show a shape fabric. Here we show that LPOs can occur through mimicry of a pre-existing LPO, so they formed statically, not during deformation. Omphacite and glaucophane LPOs occur in eclogite facies rocks from the Zermatt-Saas Unit of the Northwest Italian Alps. Barroisite grew during greenschist facies retrogression and has an LPO controlled significantly by the eclogite facies omphacite and glaucophane LPOs, rather than directly by deformation. Using spatially resolved lattice orientation data from the three key minerals, collected using electron backscatter diffraction, we deploy a new technique of interphase misorientation distribution analysis to prove this. Barroisite LPO develops by mimicry of omphacite (via a particular lattice orientation relationship) and by direct topotactic and epitactic replacement of glaucophane. LPO in turn influenced anisotropic grain growth, resulting in a barroisite grain shape fabric. Thus regional retrogression during exhumation of the Zermatt-Saas high-pressure rocks was, in large part, static, rather than dynamic as previously interpreted. In general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation, in both crust and mantle. [Copyright &y& Elsevier]

The compressibility at room temperature and the thermal expansion at room pressure of two disordered crystals (space group C2/ c) obtained by annealing a natural omphacite sample (space group P2/ n) of composition close to JdDi and JdDi, respectively, have been studied by single-crystal X-ray diffraction. Using a Birch-Murnaghan equation of state truncated at the third order [BM3-EoS], we have obtained the following coefficients: V = 421.04(7) Å, K = 119(2) GPa, K′ = 5.7(6). A parameterized form of the BM3 EoS was used to determine the axial moduli of a, b and c. The anisotropy scheme is β ≤ β ≤ β, with an anisotropy ratio 1.05:1.00:1.07. A fitting of the lattice variation as a function of temperature, allowing for linear dependency of the thermal expansion coefficient on the temperature, yielded α = 2.64(2) × 10 K and an axial thermal expansion anisotropy of α ≫ α > α. Comparison of our results with available data on compressibility and thermal expansion shows that while a reasonable ideal behaviour can be proposed for the compressibility of clinopyroxenes in the jadeite-diopside binary join [ K as a function of Jd molar %: K = 106(1) GPa + 0.28(2) × Jd], the available data have not sufficient quality to extract the behaviour of thermal expansion for the same binary join in terms of composition. [ABSTRACT FROM AUTHOR]

A single-crystal X-ray diffraction (XRD) study, using a diamond-anvil cell at high pressure and room temperature, was performed on a crystal from a natural space group P2/n omphacite sample with composition very close to Jd55Di45 and with a high degree of order in cation distribution. Unit-cell parameters were determined at 13 different pressures up to about 7.5 GPa. A third-order Birch-Murnaghan equation of state (BM3-EoS) fitted to the P-V data yielded V0 = 421.43(4) ų, KT0 = 122(1) GPa, and K' = 5.1(3). The KT0 value for this sample lies between the data obtained for the two end-members jadeite and diopside, and describes a slight positive curvature trend. During the same experiment, intensity data were collected and crystal structures were refined at 5 pressures up to 7.3 GPa. Both M1 and M2 polyhedra volumes showed a slight but significant change in slope at about 4 GPa. This behavior can likely be explained in terms of tilt angle variation of TA and TB tetrahedral, which also showed a change in slope with pressure, rather than in terms of bond length compression anomaly. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution transmission electron microscopic data have revealed the existence of nanoscale intragranular amorphous phase in omphacite of eclogite from the Dabieshan UHP metamorphic belt, Eastern China. The results of profiles in the omphacite across the amorphous phase and micro-fractured regions indicate variations in compositions and OH band intensity, respectively. This implies that the occurrence of amorphization in the omphacite appears to be pressure induced and controlled by kinetics. OH may diffuse outward and further react with the atom at M2 site during more distorted plastic deformation, which occurs as the omphacite crystalline structure collapses to a denser amorphous phase. The reported propensity to amorphization and the strong influence on the outcome of the initial structure and of the pressurization protocol indicate the presence of metastability and large kinetic barriers. [Copyright &y& Elsevier]

Abstract: To investigate the magnitude and mechanism of inter-mineral Mg isotope fractionation at high temperatures, we report high-precision analyses of Mg isotopes for 10 whole rocks and 13 mineral separates for a set of eclogites from Bixiling in the Dabie orogen, China. Magnesium isotopic compositions of whole rocks (δ26Mg of −0.44 to −0.26‰) are similar to the estimated δ26Mg values of the mantle, suggesting Mg isotopic inheritance from a gabbroic protolith with limited Mg isotope fractionation during eclogite-facies metamorphism. By contrast, mineral separates are highly heterogeneous, with δ26Mg values ranging from +0.30 to +0.60‰ in phengite, from +0.16 to +0.40‰ in omphacite and from −0.95 to −0.74‰ in garnet. Phengite and omphacite are >1‰ heavier in δ26Mg than coexisting garnet, indicating large high-temperature inter-mineral Mg isotope fractionations. The constant Δ26Mgomphacite–garnet (=δ26Mgomphacite −δ26Mggarnet) value (1.14±0.04‰), together with homogeneous mineral chemistry and equilibrium oxygen isotopic partitioning between omphacite and garnet, suggests an equilibrium Mg isotope fractionation, controlled by the difference in coordination number of Mg between omphacite (six) and garnet (eight). The 1.14‰ fractionation is the largest high-temperature equilibrium inter-mineral Mg isotope fractionation observed so far and makes the omphacite-garnet Mg isotope fractionation a potential geothermometer. By contrast, Mg isotope fractionations between phengite and garnet and between phengite and omphacite vary from 1.25 to 1.47‰ and from 0.14 to 0.32‰, respectively. This implies Mg isotopic disequilibria between phengite and garnet/omphacite, which might result from the Mg isotopic variation in phengites due to Mg isotope exchange between phengites and retrograde fluids in Bixiling eclogites. [Copyright &y& Elsevier]

Eclogite rheology, an essential control on the mechanics of subduction zones, is probably governed by the mechanical properties of the volumetrically dominant constituent, omphacite. Triaxial compression tests were performed in a solid-medium apparatus on synthetic aggregates with five compositions along the solid-solution series jadeite-- diopside. The bulk water content of the synthetic material is of the order of 10-2H/Si. The deformation experiments were performed at a confining pressure of 2.5 GPa and temperatures between 900 and 1100°C (i.e. conditions in the stability field of jadeite). Strain rates ranged from 10-6 to 10-4 s-1. For a given composition, strength decreases with increasing temperature and decreasing strain rate. The relation between strength and composition changes with increasing temperature. Whereas jadeite is the weaker endmember at all explored conditions, intermediate compositions are stronger than either endmember at 900°C and 1000°C. Samples with calcic compositions deformed at differential stresses above 500MPa have abundant mechanical (100)[001] twins. In contrast, almost all deformation steps on jadeite aggregates yield strengths below 500MPa and the resulting microstructures are dominated by well-developed subgrain boundaries and locally recrystallized grains. A weak crystallographic preferred orientation is developed in the deformed samples. Combining mechanical data and observed microstructures we hypothesize that the rate-controlling deformation mechanism differs for the two compositional endmembers. Deformation of diopside aggregates is controlled by dislocation glide, whereas the kinetics of dislocation climb controls the deformation of jadeite aggregates at the explored conditions. Reasonable parameters are found when fitting corresponding, micromechanically motivated flow laws to our data.The variation in strength with composition obeys scaling based on the dependence of melting temperature and shear modulus on composition. Extrapolation of the constrained flow laws to geological strain rates indicates considerable strength for omphacite aggregates at temperatures up to 1000°C. Continuous deformation of eclogites by dislocation creep therefore appears unlikely in subduction zones. We suspect that exhumed eclogites exhibiting microstructures indicative of crystal plastic deformation experienced high-stress episodes related to seismic events. [ABSTRACT FROM AUTHOR]

Aragonite and omphacite-bearing metapelite occurs in the albite–biotite zone of the Togu (Tohgu) area, Besshi region, Sambagawa metamorphic belt, central Shikoku, Japan. This metapelite consists of alternating graphite-rich and graphite-poor layers that contain garnet, phengite, chlorite, epidote, titanite, calcite, albite, and quartz. A graphite-poor layer contains a 1.5-cm ivory-colored lens that mainly consists of phengite, calcite, albite, and garnet. Aragonite, omphacite, and paragonite occur as inclusions in the garnet of the ivory lens. The aragonite has a composition that is close to the CaCO3 end-member: the FeCO3 and MnCO3 components are both less than 0.3 mol% and the SrCO3 component is about 1 mol%. The aragonite + omphacite + quartz assemblage in garnet indicates equilibrium conditions of P > 1.1–1.3 GPa and T = 430–550°C. Quartz grains sealed in garnet of the aragonite and omphacite-bearing sample and other metapelites in the Togu area preserve a high residual pressure that is equivalent to the Sambagawa eclogite samples. These facts suggest that: (i) the Togu area experienced eclogite facies metamorphism; and (ii) thus, eclogite facies metamorphism covered the Sambagawa belt more extensively than previously recognized. [ABSTRACT FROM AUTHOR]

Quantitative analysis of the structural evolution of jadeite-quartzite, a rare ultra-high pressure (UHP) rock type from the Dabie Mountains of eastern China, sheds light on the formation and evolution of UHP orogenic belts worldwide. Geological mapping of the Shuanghe area, where jadeite-quartzites crop out, was carried out to determine the spatial relationships between different UHP rocks within this orogen. The deformation mechanisms of jadeite-quartzite, geodynamical parameters (stress, strain, strain rate), and microstructure including lattice preferred orientation (LPO) were determined from six jadeite-quartzite samples from the Shuanghe area. LPOs of clinopyroxene (jadeite and omphacite), garnet, rutile and quartz from these jadeite-quartzite samples are compared with those of three eclogites preserving different degrees of deformation from the Shuanghe area. Microstructural LPOs of jadeite, omphacite, garnet, rutile and quartz were determined using electron backscattered diffraction (EBSD) analysis. Quartz fabrics were largely recrystallized during late, low-grade stages of deformation, whereas garnet shows no strong LPO patterns. Rutile fabrics show a weak LS fabric along [001]. Jadeite and omphacite show the strongest eclogite facies LPO patterns, suggesting that they may provide important information about mantle deformation patterns and control the rheology of deeply subducted continental crust. Microstructural data show that the jadeite LPO patterns are similar to those of omphacite and vary between L- and S-types, which correlate with prolate and oblate grain shape fabrics (SPO); quartz LPOs are monoclinic. Microstructural analysis using TEM shows that the dominant slip systems of jadeite in one sample are (100)[001], (110)[001] and (1 1 0)1/2[110], while in another sample, no dislocations are observed. Abundant dislocations in quartz were accommodated by the dominant slip system (0001)[11 0], indicating basal glide and represents regional shearing during the exhumation process. This suggests that dislocation creep is the dominant fundamental deformation mechanism in jadeite under UHP conditions. The protoliths of jadeite-quartzite, metasedimentary rocks from the northern passive continental margin of the Yangtze craton, experienced the same deep subduction and were deformed under similar rheological conditions as other UHP eclogite, marble and paragneiss. Experimental UHP deformation of quartzo-feldspathic gneiss with a chemical composition similar to the bulk continental crust has shown that the formation of a jadeite–stishovite rock is associated with a density increase of the host rock similar to the eclogite conversion from basaltic protoliths. The resulting rock can be denser than the surrounding mantle pyrolite up to depths of 660 km (24 GPa). Thus, processes of deep continental subduction may be better-understood through understanding the rheology and mechanical behaviour of jadeite. Jadeite-quartzites such as those from the Shuanghe may be exhumed remnants of deeply-subducted slabs of continental crust, other parts of which subducted past the ‘depth of no return’, and remain in the deep mantle. [ABSTRACT FROM AUTHOR]

Abstract: We investigated several mineral phases and their replacement products which occur as inclusions in garnets from felsic and mafic granulites of the Gföhl Unit in the Moldanubian Zone. The most important mineral inclusions, Ti-rich muscovite and omphacite, were used for the reconstruction of the metamorphic history of granulites. Some inclusions were transformed during high-temperature granulite facies metamorphism, partial melting and decompression to other phases, and so the original mineral can only be deduced from the inclusion morphology and reaction products. These inclusions have columnar shapes and consist of K-feldspar+kaolinite, albite+Fe-oxide, plagioclase+Fe-oxide, or albite+K-feldspar, respectively. The pseudomorphs with albite/plagioclase occur in a Ca-rich garnet that shows prograde zoning. Pressure–temperature (PT) evolution, derived from mineral assemblages in granulite and based on the inclusions, suggests a prograde metamorphism from amphibolite through eclogite to granulite facies conditions with subsequent amphibolite facies overprint during exhumation. The estimated PT trajectory for the studied granulites, which also host lenses or boudins of eclogites and garnet peridotites, allows reconstruction of the complete clockwise metamorphic path that is consistent with subduction geotherm prior to the tectonic amalgamation within the continental collisional root. [Copyright &y& Elsevier]

Abstract: In the Western Alps, the ocean-derived Lago di Cignana Unit of the Piemonte Zone has experienced ultrahigh-pressure metamorphism. We have studied the minerals of two eclogite samples from this unit using transmission electron microscopy (TEM) in order to characterise their microstructures. These microstructures are the result of deformation, phase transformation and reactions and allow conclusions on the processes that had generated them. The goal of this process-oriented paper is to contribute to the knowledge of the formation and exhumation of ultrahigh pressure eclogites. In our TEM-study we found omphacite, amphibole (barroisite, glaucophane, hornblende), clinozoisite, garnet, albite, and thin layers of chlorite and mica in amphibole. The omphacite is well-ordered and has the space-group P2/n. We observed antiphase domains (APDs), dislocations that are organised in small-angle grain boundaries, and – very rare – crystallographic shear planes parallel to (010) and (110). Deformation twin lamellae on (100) have not been observed. Most interesting is the first observation of faults parallel to (110) in a natural omphacite. They are due to deformation. Chain multiplicity faults are common in the amphibole grains. In one case, the orientation of crystallographic shear planes was not only parallel to (010), but also parallel to (130) and (−110). Clinozoisite showed deformation twin lamellae on (100) with widths of a few nm up to about 50 nm. Dislocations organised into a small-angle grain boundary have been found, which have not been described before. The garnet is almost free of dislocations. While there are differences regarding the deformation microstructures from the UHP Lago di Cignana eclogite and other HP occurrences, it is unlikely that these are due to higher pressures. It is much more likely that the whole formation and exhumation history of the different geological units and the individual positions of the collected rocks within them are reflected in their microstructures. A tentative correlation of the omphacite microstructures with the P–T path of the Lago di Cignana Unit is presented. An important point is the early crystallisation of C2/c-omphacite at a temperature below 500 °C, perhaps around 400 °C. The low temperature prevents rapid diffusion-controlled ordering to P2/nomphacite and thus enables deformation of the metastable C2/c-omphacite. Since C2/c-omphacite is more easily to deform than P2/n-omphacite, this may influence the interplate coupling. The onset of the transition to P2/n-omphacite, however, will increase the shear strength of eclogite and the coupling with the overriding plate, which may slow-down the subduction and finally stop it. [Copyright &y& Elsevier]

Early Precambrian retrogressed eclogites are abundant in the Archean Belomorian Province of the Fennoscandian Shield. Archean zircons with inclusions of omphacite have been found in these eclogites. Similar Archean zircons from the retrogressed eclogites also contain garnet inclusions. The Archean zircons display no negative Eu anomaly, which indicates their crystallization in plagioclase-free rock. Garnet, omphacite and clinopyroxene-plagioclase symplectite as a proxy of omphacite compose ≥75% of the studied rocks, with garnet and omphacite being major constituents and associating with rutile and quartz. These data strongly suggest that the studied rock is eclogite. In the majority of petrogenetic grids, P-T parameters calculated for these rocks fall in the eclogite-facies field. Thus, these findings and data provide evidence that eclogite-facies metamorphism occurred in the Neoarchean. [ABSTRACT FROM AUTHOR]

Volodichev et al. (Volodichev et al., 2021) reported on the first finding of omphacite (23%–25% Jd) inclusions in 2.68 Ga metamorphic zircons from Gridino eclogites and presented it as evidence for Archean eclogite-facies metamorphism in the Belomorian Mobile Belt. We believe that the Archean age of the garnets referred to by the above authors was estimated incorrectly. Our interpretation is that omphacite origin is related to Archean high-pressure granulite-facies metamorphism. [ABSTRACT FROM AUTHOR]

Abstract: Omphacites are very important in petrology because of their occurrence in rocks metamorphosed under high-pressure conditions. They have a wide thermobaric range of stability. Raman spectra of a natural, iron-bearing, ordered omphacite have been studied and compared with the experimentally disordered spectra. The order–disorder phase transition causes extra modes in the spectra. The ordering state affects the full width at half maximum (FWHM) of the main omphacite peak at 680cm−1. Ordered omphacite shows a split peak around 1010cm−1 caused by the slightly different orientation of tetrahedra in the monoclinic chain. [Copyright &y& Elsevier]

Abstract: Mining of Cenozoic alluvial deposits at Copeton and Bingara (Eastern Australia) has produced two million macrodiamonds (0.25 ct median size). Raman spectroscopy is used to identify included minerals within uncut Copeton diamonds, with sealed chamber remnant pressures of 31.7 to 35.6 kbar for coesite, 13.6 and 22.7 kbar for clinopyroxene, and 7.6 kbar for grossular garnet. Assuming elastic behaviour, these values generate inclusion entrapment PT loci which intersect, restricting diamond formation conditions: from 250 °C, 43 kbar to 800 °C, 52 kbar. Larger than error (±100 °C and ±4 kbar), this range shows a systematic variation in inclusion composition with diamond zoning and N properties. Published research shows 1) Copeton and Bingara diamonds are unique, and 2) modern alluvium in the Bingara district carries mantle-formed garnet, captured by post-tectonic alkali basalt from an extensive diamondiferous ultrahigh pressure (UHP) terrane that stalled at depth because it is dominated by mafic eclogite. The combined Raman and geological results indicate two sets of subduction UHP diamond formation conditions/protolith are required, firstly cooler oceanic slab and secondly including higher temperature continental crust. The Copeton and Bingara stones are UHP macrodiamonds, and Carboniferous 40Ar/39Ar age dates on clinopyroxene inclusions should be interpreted as ages of crystallisation, representing the termination of subduction. The characteristic features of ruptured inclusions and etched percussion marks on Copeton and Bingara diamond indicate volcanic delivery to the earth''s surface. Alluvial deposits elsewhere in Eastern Australia may carry similar diamond along with diamond of different origin. [Copyright &y& Elsevier]

Abstract: Dehydroxylation of omphacites in eclogites from UHP Dabie-Sulu terrain has been investigated using a combined method of petrological observations, mineral chemistry, heat treatment, Mössbauer, and micro-FTIR spectroscopy. Progressive heating of omphacite in air up to 1100 °C results in complex, multi-stage processes involving oxidation and dehydrogenation at different temperatures. During heating, Fe2+ at the M1 site is oxidized to Fe3+, accompanied by a decrease in the intensity of the 3460 cm−1 OH band. This reaction can be described as Fe2+(M1)+(OH)− =Fe3+(M1)+O2−(O3)+1/2H2, and most likely represents loss of H bonded to O1 adjacent to the M1 site. At higher temperatures, continued oxidation results in further dehydroxylation due to H loss from OH bands 3530 cm−1 and 3630 cm−1. These bands represent H defects with higher thermal stability/slower diffusivity. In this case, oxidation of Fe2+ provides a means for charge-balancing H loss over the whole crystal structure, rather than locally. OH bands at 3530 and 3630 cm−1 are consistent with H incorporation onto O2, with vibration of OH dipoles towards adjacent tetrahedral vacancies. Rapid loss of H during annealing at even low temperatures implies that significant H loss from eclogites occurs during exhumation. Furthermore, changes in relative OH band intensity are expected during exhumation due to different thermal behaviour of H defects in omphacite. As such, the ratio of OH bands in omphacite spectra could be used to deduce exhumation conditions. [Copyright &y& Elsevier]

Abstract: We discuss the relationship of omphacite lattice preferred orientation (LPO) patterns in eclogite and kinematic indicators in the matrix surrounding the eclogite during the very rapid exhumation of the deeply buried Eclogite Zone in the Tauern Window of the Eastern Alps. LPO patterns are presented from profiles parallel to the eclogite stretching lineation across the Eclogite Zone. The omphacite textures show symmetric patterns across the entire Eclogite Zone; only in few cases slight asymmetries are observed. The patterns are characterized by an alignment of {110} with the foliation and a maximum of 〈001〉 close to the lineation and are indicative of deformation by intracrystalline glide on the (010) [001] and (110) [001] slip systems. The patterns have close resemblance to S-type LPO patterns and probably developed during the burial and accretion of the eclogites. We argue that the LPO patterns are not related to deformation associated with the very rapid exhumation of the Eclogite Zone. We conclude that exhumation-related deformation was concentrated in the metasedimentary rocks that surround the eclogites, allowing the eclogites to preserve their burial LPO signature during the exhumation process. Kinematic indicators show top-N thrusting at the base of the Eclogite Zone and sinistral strike-slip faulting at the top. Most of the kinematic indicators record blueschist- and greenschist-facies metamorphic conditions. However, because of the very rapid exhumation of the Eclogite Zone with rates exceeding 36kmMyr−1, the kinematic indicators are interpreted to reflect the entire exhumation process from eclogite-facies conditions. We discuss a model according to which the Eclogite Zone was exhumed in an oblique extrusion wedge. [Copyright &y& Elsevier]

Silica lamellae in eclogitic clinopyroxene are widely interpreted as evidence of exsolution during decompression of eclogite. However, mechanisms other than exsolution might produce free silica, and the possible mechanisms depend in part on the nature and definition of excess silica. ‘Excess’ silica may occur in both stoichiometric and non-stoichiometric pyroxene. Although the issue has been debated, we show that all common definitions of excess silica in non-stoichiometric clinopyroxene are internally consistent, interchangeable, and therefore equivalent. The excess silica content of pyroxene is easily illustrated in a three-component, condensed composition space and may be plotted directly from a structural formula unit or recalculated end-members. In order to evaluate possible mechanisms for the formation of free silica in eclogite, we examined the net-transfer reactions in model eclogites using a Thompson reaction space. We show that there are at least three broad classes of reactions that release free silica in eclogite: (i) vacancy consumption in non-stoichiometric pyroxene; (ii) dissolution of Ti-phases in pyroxene or garnet; (iii) reactions between accessory phases and either pyroxene or garnet. We suggest that reliable interpretation of the significance of silica lamellae in natural clinopyroxene will require the evaluation not only of silica solubility, but also of titanium solubility, and the possible roles of accessory phases and inclusions on the balance of free silica. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the rheology of hot-pressed polycrystalline omphacite (Di58Jd42, space group C2/c) at strain rates of 10−4–10−5/s, temperatures of 1300–1500K and a pressure of 3GPa, using a 5GPa Griggs-type deformation apparatus. The rheological constitutive equation of omphacite is determined with a stress exponent of 3.5±0.2 and an activation energy of 310±50kJ/mol. Our study shows that: (1) the creep strength of omphacite falls between those of diopside and jadeite; (2) experimental omphacite microfabrics are indistinguishable from natural ones: S-type fabric and its mesoscopic foliation develop under conditions of axially symmetric shortening, whereas L-type fabric and its mesoscopic lineation develop under conditions of general strain, with L parallel to the greatest elongation; (3) as in naturally deformed omphacite, the deformation substructure is characterized by the dominant slip systems {110}1/2〈110〉, {110}[001] and (100)[001]. We conclude that both S- and L-type omphacite fabrics are produced by these slip systems; fabric differences arise solely from variation of the geometry and orientation of the finite strain ellipsoid. We also confirm the previous suggestion that Na-bearing pyroxenes are significantly weaker than Na-free pyroxenes. [Copyright &y& Elsevier]

Microstructural and petrological analysis of samples with increasing strain in high-pressure (HP) shear zones from the Haram garnet corona gabbro give insights into the deformation mechanisms of minerals, rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions. Scanning electron microscopy with electron backscattering diffraction (SEM–EBSD), compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization. Plagioclase with a shape-preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the (0 0 1)[1 0 0] slip system. In omphacite, crystallographic preferred orientations indicate slip on (1 0 0)[0 0 1] and (1 1 0)[0 0 1] during deformation. Fine-grained garnet deformed by diffusion creep and grain-boundary sliding. Ilmenite deformed by dislocation glide on the basal and, at higher strains, prism planes in the a direction. Relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500–650 °C and 0.5–1.4 GPa and continued during prograde metamorphism to ultra-high-pressure (UHP) conditions. Both products and reactants show evidence of syn- and post-kinematic growth indicating that prograde reactions continued after strain was partitioned away. The restriction of post-kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn-kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away, demonstrating that deformation played an important role in enhancing metamorphic reactions. Two important consequences of deformation observed in these shear zones are: (i) the homogenization of chemical composition gradients occurred by mixing and grain-boundary migration and (ii) composition changes in zoned metamorphic garnet by lengthening diffusion distances. The application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100–150 MPa and lower limits for strain rates of 10−12 to 10−13 s−1 as deformation conditions for the shear zones in the Haram gabbro that were produced during subduction of the Baltica craton and resulted in the production of HP and UHP metamorphic rocks. [ABSTRACT FROM AUTHOR]

Abstract: The Concepenido eclogites are part of an ordered rock sequence in the upper tectonic unit of the Cabo Ortegal Complex (NW Spain). They form a continuous and about 200 m thick layer separating other mafic rocks from quartzo-feldspathic gneisses. The whole sequence has been deformed and metamorphosed at high pressure and high temperature during the development of an early Paleozoic subduction zone. In this study, microfabrics of these eclogites are quantitatively analysed, in particular crystallographic preferred orientations (CPO) in omphacite and garnet measured by electron back-scatter diffraction (EBSD). From field observations and from detailed microstructural analysis we estimate that the high pressure deformation, which produced the crystallographic and shape fabrics in omphacite, has occurred under a strain regime close to bulk coaxial flattening. Both omphacite lineations and omphacite c[001] concentrations scatter in geographical coordinates and do not show a single preferred orientation that would indicate a predominant direction of extension. We suggest that the high pressure and high temperature environment during deformation, the coaxial nature of strain and the significant thinning of the layer and rock sequence may relate to flattening deformation of a subducting slab with extension slightly preferred in a down-dip direction. [Copyright &y& Elsevier]

Ultra high-pressure (UHP) eclogites from Sulu region (China) represent mafic components of the continental crust, which were first subducted to mantle depths greater than 100 km and then exhumed to the earth''s surface. Detailed investigation of microstructures, chemical compositions, petrofabrics and seismic properties of the UHP eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle. We present here results from field, optical and TEM observations, electron back-scattered diffraction (EBSD) measurements and numerical computations of the seismic properties of UHP eclogites collected from fresh surface outcrops at the drill site (Maobei, Donghai County, Jiangsu Province) of the Chinese Continental Scientific Drilling Program (CCSD). Two types of eclogites have been distinguished: Type-1 (coarse-grained) eclogites deformed by recovery-accommodated dislocation creep at the peak metamorphic conditions, and Type-2 (fine-grained) eclogites which are composed of reworked Type-1 materials during recrystallization-accommodated dislocation creep in shear zones which were active during the exhumation of the UHP metamorphic rocks. Both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the UHP metamorphic conditions. Plasticity of eclogites under UHP conditions can effectively facilitate channeled flow along the interplate shear zone. The preservation of the relict crustal materials within the continental lithosphere may produce regionally extensive, strong, seismic reflections in the upper mantle. This may explain the origin of mantle reflections observed in many areas of the world. [Copyright &y& Elsevier]

We present microstructural data and lattice preferred orientations (LPOs) of omphacites from a suite of eclogites, from the Adula/Cima Lunga nappe (Central Alps). Our work shows a surprisingly strong correlation between the measured LPO and the ordering state of cations in omphacite. Estimates of deformation temperature from metamorphic petrology, together with measured omphacite compositions and LPOs, determine the field (ordering state), on the omphacite phase diagram, into which each sample falls. LPOs dominated by L-type and S-type signatures are developed in samples that fall in the P2/n field (ordered structure) and C2/c field (disordered structure), respectively.Dislocations with b=1/2〈−110〉 or b=[001] are observed in the transmission electron microscope (TEM) in all samples. The former change from a perfect dislocation in the C2/c structure to a partial in P2/n. Any movement of a partial dislocation requires the formation or growth of a stacking fault. Furthermore, in order to pass an obstacle a partial dislocation has to constrict to a unit dislocation. The energy to form a constriction is high in omphacite due to the large separation width. Thus, the activity of the b=1/2〈−110〉 dislocation is hindered in the P2/n structure relative to the C2/c structure, which change the balance between the two and might give rise to the different LPOs. [Copyright &y& Elsevier]

Abstract Observations of oriented SiO2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the Alpe Arami garnet peridotite massif, Ticino, Switzerland, and petrological studies of the eclogitic mineral assemblages, suggest that this rock was subjected to higher-pressure metamorphism than previously realized. We employed various calibrations of the Fe2+ - Mg exchange thermometer and calculations of equilibria with thermodynamic data, considering the calcium–Tschermak's component (CaAl2 SiO6 ), of garnet-pyroxene pairs. From these calculations, it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following: Stage I (prograde) c. 2.4 GPa; 700 °C; Stage IIa (maximum recorded grade) c. 7.0 GPa; 1100 °C; Stage IIb (retrograde) c. 3.7 GPa; 900 °C; Stage III (retrograde) c. 2.1 GPa; 750 °C. Because of the preservation of Stage I, a relatively rapid subduction and exhumation of Alpe Arami eclogite is suggested. The exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the Alpe Arami garnet lherzolite proposed previously by several authors based upon a variety of different observations, although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km. [ABSTRACT FROM AUTHOR]

Mineral inclusions in Sao Luiz diamonds have been characterized using angle dispersive X-ray diffraction and Raman spectroscopy. We identified two different garnet phases coexisting with an omphacitic pyroxene. They represent disintegration products of a former homogeneous majorite-rich garnet phase. The two garnets have significantly different cell parameters but are tightly intergrown with their unit cells parallel to each other. The garnets are oriented relative to the diamond host with [100]garnet parallel to [110]diamond. Combining the measured cell parameters of the garnet inclusions with chemical analyses of similar inclusions from the same source allows the extraction of a residual pressure between 1 and 3 GPa, depending on the exact chemical composition assumed. Depth-resolved Raman spectra at the diamond–garnet interface indicate a residual pressure of about 1 GPa. Such a low residual pressure is unexpected at first glance for a garnet assemblage from the transition zone. The inclusion pressure is lowered due to pyroxene crystals, which surround the garnet inclusions and act as cushions reducing the residual inclusion pressure. [Copyright &y& Elsevier]

We present the results of high-resolution transmission electron microscope (HRTEM) studies on jadeite crystals of jadeite quartzite from the Dabie ultra-high-pressure (UHP) metamorphic terrane, China. Although jadeite is known to have only C2/c crystal structure, [100], [110], [112], [010] diffraction patterns and HRTEM images reveal C2/c and P21/n structures in 500 nm microdomains of single jadeite crystals, that we recognized in all the above four orientations. The cell parameters of C2/c crystal structure correspond to those of jadeite, whereas the cell parameters of P21/n structure are equivalent to those of omphacite. The presence of the P21/n structure shows that jadeite partially changed into omphacite on a nano-scale during retrograde metamorphism and that this reaction failed to reach equilibrium. It suggests uplift of the host rock during exhumation accompanied by complex non-equilibrium retrogressive process in the jadeite, with significant implications for the derivation of P–T paths and tectonic interpretation of the Dabie UHP metamorphic terrane. [Copyright &y& Elsevier]

Early Precambrian retrogressed eclogites are abundant in the central and northern parts of the Belomorian Province of the Fennoscandian Shield (Gridino + Keret and Salma + Kuru-Vaara study areas, respectively). Older and younger eclogites are recognized and their Archean and Paleoproterozoic ages are argued. Archean eclogites are intensely retrogressed and occur in amphibolite boudins in the tonalite-trondhjemite-granodiorite (TTG) gneiss matrix of the Archean Gridino eclogite-bearing mélange. Less retrogressed Paleoproterozoic eclogites form patches in mafic dikes and some amphibolite boudins; their Paleoproterozoic age is supported by U-Pb/SIMS data on zircons depleted in heavy rare earth elements (REE) with omphacite, garnet, and kyanite inclusions, and Sm-Nd and Lu-Hf mineral isochrons. Archean eclogites contain Archean heavy rare-earth elements (REE)-depleted zircons with garnet and zoisite inclusions and Archean garnets. No omphacite inclusions were found in these zircons, and this fact was considered as evidence against the existence of Archean eclogites. This study reports on the first finding of omphacite (23–25% Jd) inclusions in 2.68 Ga metamorphic zircons from eclogites from the Gridino eclogite-bearing mélange. The zircons are poorly enriched in heavy REE and display a weak negative Eu-anomaly but a poor positive Ce-anomaly typical of eclogitic zircons. Thus, zircons with these decisive features provide evidence for an Archean eclogite-facies metamorphism. [ABSTRACT FROM AUTHOR]

Various rock phases, including those in subducting slabs, impact seismic anisotropy in subduction zones. The seismic velocity and anisotropy of rocks are strongly affected by the lattice-preferred orientation (LPO) of minerals; this was measured in retrograded eclogites from Xitieshan, northwest China, to understand the seismic velocity, anisotropy, and seismic reflectance of the upper part of the subducting slab. For omphacite, an S-type LPO was observed in three samples. For amphibole, the <001> axes were aligned subparallel to the lineation, and the (010) poles were aligned subnormal to foliation. The LPOs of amphibole and omphacite were similar in most samples. The misorientation angle between amphibole and neighboring omphacite was small, and a lack of intracrystalline deformation features was observed in the amphibole. This indicates that the LPO of amphibole was formed by the topotactic growth of amphibole during retrogression of eclogites. The P-wave anisotropy of amphibole in retrograded eclogites was large (approximately 3.7–7.3%). The seismic properties of retrograded eclogites and amphibole were similar, indicating that the seismic properties of retrograded eclogites are strongly affected by the amphibole LPO. The contact boundary between serpentinized peridotites and retrograded eclogites showed a high reflection coefficient, indicating that a reflected seismic wave can be easily detected at this boundary. [ABSTRACT FROM AUTHOR]