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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]

The jadeite jade in Guatemala exerts remarkable commercial quality, which has attracted wide attention. Guatemalan jadeite jade displays a rich variety of colors; however, the color formation of this jadeite jade has not been systematically investigated to date. In this paper, we study different colors of jade samples to trace the compositions and color-causing mechanisms through petrography, X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy (LRS), and UV-visible absorption spectroscopy (UV-Vis), as well as electron probe microanalysis (EPMA). The results show that jadeite and omphacite are the main mineral compositions of Guatemalan jadeite jade, together with minor albite and other impurities. The color of Guatemala jadeite jade is mainly related to Cr3+, Fe2+, and Fe3+, of which a small amount of Cr3+ causes the jadeite jade to be emerald green. Moreover, 1~2% FeO contents can lead to the blue or gray color of the samples, while the Fe3+ makes the sample dark green. The green color of some Cr3+-free jadeite is caused by the electron transition between bands of Fe3+, and the green color is related to the iron content. Moreover, the chemical composition analysis shows that some metallic elements existed in Guatemalan jadeite jade, such as Ca, Ti, Al, Si, Ni, Fe, Mn, Cr, Na, Mg, and Sr, and some trace elements were lost or unevenly distributed, which may lead to the heterogeneity of the color of the samples. Our present investigation provides insights into color discrimination, quality evaluation, and identification of Guatemala jadeite jade. [ABSTRACT FROM AUTHOR]

The jadeite jade in Guatemala exerts remarkable commercial quality, which has attracted wide attention. Guatemalan jadeite jade displays a rich variety of colors; however, the color formation of this jadeite jade has not been systematically investigated to date. In this paper, we study different colors of jade samples to trace the compositions and color-causing mechanisms through petrography, X-ray fluorescence spectroscopy (XRF), Fourier transform infrared spectroscopy (FTIR), laser Raman spectroscopy (LRS), and UV-visible absorption spectroscopy (UV-Vis), as well as electron probe microanalysis (EPMA). The results show that jadeite and omphacite are the main mineral compositions of Guatemalan jadeite jade, together with minor albite and other impurities. The color of Guatemala jadeite jade is mainly related to Cr3+, Fe2+, and Fe3+, of which a small amount of Cr3+ causes the jadeite jade to be emerald green. Moreover, 1~2% FeO contents can lead to the blue or gray color of the samples, while the Fe3+ makes the sample dark green. The green color of some Cr3+-free jadeite is caused by the electron transition between bands of Fe3+, and the green color is related to the iron content. Moreover, the chemical composition analysis shows that some metallic elements existed in Guatemalan jadeite jade, such as Ca, Ti, Al, Si, Ni, Fe, Mn, Cr, Na, Mg, and Sr, and some trace elements were lost or unevenly distributed, which may lead to the heterogeneity of the color of the samples. Our present investigation provides insights into color discrimination, quality evaluation, and identification of Guatemala jadeite jade.

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]

Electrical conductivity of water-rich omphacite and garnet in eclogite was measured at 1 GPa and 200–800°C in a piston cylinder press and by a Solartron-1260 impedance/gain-phase analyzer at 106-1 Hz frequency. The water content of pre-annealed omphacite and garnet was 775–2,000 and 705–1,460 ppm H2O, respectively. Sample chemistry and water contents remained unchanged during conductivity runs. At otherwise identical conditions, the conductivity of both minerals increases with both temperature and water content, and the water content exponent is ∼1.45 and 1.12 for omphacite and garnet, respectively. The activation enthalpy is ∼70 kJ/mol for omphacite and 84 kJ/mol for garnet and is broadly independent of sample water content. Combining with previous work, the conductivity dependence of omphacite on water content differs between water-rich and water-poor conditions, due to different types and mobility of water in samples that are closely related to its incorporation mechanism; in contrast, the conductivity dependence of garnet with a similar type of water is comparable over a wide range of water contents. The estimated bulk conductivity of eclogite at water-rich conditions is very high, up to ∼0.01–0.1 S/m at 600–900°C. Geophysically resolved high resistivity of subducting crusts at 70–120 km depth suggests extremely low water contents of omphacite and garnet in the eclogitized slab. The data provide support to the model based on omphacite and garnet conductivity at water-poor conditions that the amount of water recycled by crust subduction to the deep mantle is probably limited.

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]

Omphacite found in eclogites from the Arquía Complex (Colombia) was characterized using scanning electron microscopy, electron probe microanalysis and single-crystal X-ray diffraction. The sample is chemically homogeneous. Omphacite usually shows symplectitic intergrowths or a relictic character partially and totally included in amphibole. The transformation of omphacite to symplectites colonies characterizes the initiation of the eclogites retrograde metamorphism. The reaction history can be summarized as the decomposition of omphacite into symplectites as a product of decompression through the reaction: omphacite + quartz = plagioclase + clinopyroxene (low in Na), which can be considered as a discontinuous precipitation reaction. The structure of the studied omphacite single-crystal is successfully refined in the P2/c space group. This structure is interpreted as an intermediate towards omphacite with space group P2/n. The structure of omphacite shows silicate chains formed by two crystallographically different silicon tetrahedra (T1 and T2) with an O2-O3-O2 angle of 169.002 (2)°, indicating that the chain is slightly distorted. The Na and Ca cations occupy the octahedral and 8-coordination sites. Fe occurs as Fe2+ and Fe3+ and are distributed over the octahedral sites M1 and M11, respectively.

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]

ABSTRACT: As the important component of the eastern Tethys tectonic region, the Jinsha River-Ailao Mountain suture zone is often considered to be an ophiolitic mélange belt. However, the P-T-t path and chronological framework of the metamorphic evolution in the collisional orogenetic process of this zone are still poorly understood owing to the lack of metamorphism research of symbolic high-pressure rocks. During a regional geological survey on a scale of 1 : 50000 in Gonjo County, Tibet Autonomous Region involved in this paper, (retrograde) eclogites lenses of different scales were found in Jinsha River suture zone, eastern Tibet for the first time. The (retrograde) eclogites can be divided into garnet-albite-chlorite-actinolite schists and eclogites according to retrograde degrees. The mainly mineral components of eclogites include garnet (45%–50%), clinopyroxene (about 25%), and hornblende (5%–10%) primarily, and biotite, quartz, rutile, and muscovite secondarily. According to the data of electron probe micro analysis (EPMA), clinopyroxenes feature high content of Na2O (5.6%–6%) and corresponding jadeite (Jd) molecules of 37%–44%, and they fall within the omphacite region in Quad-Jd-Ae diagram. The temperature and pressure of the metamorphism at peak are P≈2.2–2.34 GPa and T≈622–688 °C respectively as measured with geobarometry Grt-Omp-Phe and geothermometer Grt-Omp. This will provide a new reference for the understanding of Paleo-Tethyan evolution. In this paper, two samples of eclogites were chosen for LA-ICP-MS zircon U-Pb dating and their 206Pb/238U ages obtained are 240 ± 3 Ma and 244 ± 1 Ma respectively. Furthermore, the zircons feature extremely low Th/U ratio (

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]

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.

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.

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]

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]

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.

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 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.

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]

Clinopyroxene (Cpx) is commonly believed to be the best structural water (hydrogen) carrier among all major upper mantle nominally anhydrous minerals (NAMs). In this study, we have measured the single-crystal elastic properties of a Cpx, a natural omphacite with ~710 ppm water at ambient pressure (P) and temperature (T) conditions. Utilizing the single-crystal X-ray diffraction (XRD) and electron microprobe data, the unit cell parameters and density were determined as a = 9.603(9) Å, b = 8.774(3) Å, c = 5.250(2) Å, β = 106.76(5)o, V = 255.1(4) Å3, and ρ = 3.340(6) g/cm3. We performed Brillouin spectroscopy experiments on four single crystals along a total of 52 different crystallographic directions. The best-fit single-crystal elastic moduli (Cijs), bulk and shear moduli were determined as: C11 = 245(1) GPa, C22 = 210(2) GPa, C33 = 249.6(9) GPa, C44 = 75.7(9) GPa, C55 = 71.2(5) GPa, C66 = 76(1) GPa, C12 = 85(2) GPa, C13 = 70(1) GPa, C23 = 66(2) GPa, C15 = 8.0(6) GPa, C25 = 6(1) GPa, C35 = 34.7(6) GPa, and C46 = 8.7(7) GPa, KS0 = 125(3) GPa, and G0 = 75(2) GPa, respectively. Compared with the anticipated elastic properties of an anhydrous omphacite with the same chemical composition, our results indicate that the incorporation of ~710 ppm structural water has no resolvable effect on the aggregate elastic properties of omphacite, although small differences (up to ~9 GPa) were observed in C13, C25, C44, and C66.

[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]

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]

This study describes the strain geometry, crystal-plastic deformational features, isotopic age of metamorphism, and calculated seismic properties of two medium-temperature eclogite types from the Malpica-Tui Allochthonous Complex of Variscan NW Iberia. The eclogite types are eclogites with coronitic garnets and eclogites with a planolinear fabric. Both of them were buried, deformed and recrystallized under maximum pressure and temperature of 2.6 GPa and 610-640°C, and subsequently exhumed in a late Devonian subduction channel. The metamorphic peak of the subduction-exhumation cycle occurred ~375 Ma ago. Omphacite petrofabric ties eclogites with coronitic garnet to noncoaxial constrictional strain and eclogites with planolinear fabrics to noncoaxial flattening strain and stretching along the lineation. We also used omphacite crystallographic preferred orientations to calculate and constrain the seismic properties of the eclogites. The slight variations in petrophysical properties observed are interpreted to result from variations in the strain regime recorded by pristine eclogites, or from variations in the modal proportions of the constituent high-pressure minerals. We foresee that eclogite in subduction metamorphic complexes might be either seismically undetectable or detected as planar features with high impedance contrasts relative to their host rocks. [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]

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]

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]