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The interaction of natural eclogite (Ecl) with synthetic hydrous carbonate melts with Na:K = 0:1 (KH2) and 1:1 (NKH2) was studied in multianvil experiments at 3-6 GPa and 850-1250 °C. The interaction with KH2 consumes garnet and clinopyroxene producing phlogopite and calcite-dolomite solid solution. Besides, the interaction yields a decrease in the jadeite component of clinopyroxene, evolving eclogite toward pyroxenite. This is consistent with a metasomatic alteration of eclogite xenoliths, manifested as Na-poor “spongy” clinopyroxene, replacing primary omphacite, and kelyphitic rims around garnet, containing phlogopite and carbonates. The interaction with NKH2 also produces phlogopite and carbonate, but the latter is more magnesian and represented by magnesite, above the solidus, and magnesite + dolomite below the solidus. The interaction with NKH2 increases the jadeite component in clinopyroxene and grossular component in garnet, evolving eclogite Group A to eclogite Group B. The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite eclogite under fluid-absent conditions. Based on the obtained results its solidus is situated near 1050 °C at 3 GPa and decreases to 950 °C at 6 GPa. Thus, hydrous K- and Na-K-carbonatite melts can coexist with eclogite in SCLM at depths exceeding 120-170 km, and solidify as temperature decreases below 950-1050 °C according to the following solidus reactions: pyrope + diopside + melt → phlogopite + dolomite, below 6 GPa, and pyrope + diopside + melt → phlogopite + magnesite + grossular, at 6 GPa. The melting reaction, involving phlogopite and dolomite, suggests the partial melting at the peak of ultrahigh-pressure metamorphism (UHPM) during continent-continent plate collision. The prograde P-T path of UHPM crosses the solidus of clinopyroxene + garnet + phlogopite + dolomite assemblage at 4.7-5.2 GPa and 970-990 °C and yields the formation of hydrous K-carbonatite melt-fluid in situ. This melt could be responsible for the formation of K-bearing clinopyroxenes and microdiamonds in the UHPM marbles in the Kokchetav massif, Kazakhstan. The retrograde P-T path intersects the solidus that has a negative Clapeyron slope in the diamond stability field. Thus, the hydrous K-carbonatite melt should disappear soon after the peak of metamorphism reacting with garnet to produce Ca-Mg carbonates and phlogopite.

Omphacite is a diagnostic mineral of high- and ultrahigh-pressure rocks, and its association with garnet is characteristic of eclogites from subduction-related massifs and volcano-entrained xenoliths. Omphacite can accommodate significant amounts of water as structurally bound hydroxyl (OH) groups, and is able to convey water into Earth's interior. We show, for the first time, experimental evidence that the infrared absorption patterns of water in omphacite are temperature sensitive. This provides a new framework for discriminating water in natural omphacite equilibrated at different temperatures. We also demonstrate that in low-temperature omphacite, the integral absorbance ratio between the infrared OH absorption bands at 3620 cm−1 and 3450 cm−1 is linearly related to temperature. Water in omphacite of massif eclogites records the temperature of OH equilibrium, allowing reconstruction of the fluid-involved thermal history and tectonics of rock evolution.

The recent paper by Chaves and Porcher published in Gondwana Research (2020, v.87, 224–237) asserts the presence of 2.2 Ga eclogite facies rocks in the Itaguara Sequence, Sao Francisco craton (SFC), Brazil. They present images in which the diagnostic high-pressure minerals either occur on cracks (rutile), or as fragmentary matrix grains with unclear mineral relationships (omphacite and phengite). Here I discuss the sample petrology in order to identify the equilibrium assemblage, and utilise a P-T phase modelling approach, integrating the major mineral compositions, to show that these rocks are not retrogressed eclogites. The modelling presented here indicates that peak conditions were likely in the amphibolite facies, reaching maximum pressures of no more than 8 kbar and peak temperatures of ca. 700 °C (at 5 kbar). This result is consistent with that found by Massonne (2020, JSAMES, v.99, 102495) who suggested that mica schists from Itaguara reached maximum pressures of 13–14 kbar, and possibly lower than 10 kbar at 600 °C (i.e. within the medium to high-pressure amphibolite facies). This work shows that declarations of eclogite facies conditions, particularly from Paleoproterozoic terrains, should be accompanied by carefully evaluated petrography and thermodynamic investigations based on minerals (or mineral replacement textures) that indicate equilibrium relationships.

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 mm 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.

The comments are given on the article authored by M.V. Mints and K.A. Dokukina – The Belomorian Eclogite Province (Eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic? (Geodynamics & Tectonophysics 2020, 11 (1), 151–200). The Belomorian (White Sea) province of the Fennoscandia Shield is a key site for studying the tectonics of the early periods because numerous Precambrian eclogites have been found there. It was not anticipated, but the problem of age determinations of the eclogite metamorphism of gabbroids in the White Sea mobile belt has turned out to be extremely relevant not only for this region, but also for the Precambrian geology in general. The reason is that a number of authors determine the age of eclogites as Archean (2.7–2.8 Ga), which makes the White Sea mobile belt the only example of the Archean eclogite metamorphism in the world and, therefore, the only dated evidence in support of the plate tectonic model of the evolution of the Earth’s crust at the earliest stage of its formation. The article consistently provides a critical analysis of the arguments put forward by the supporters of the Archean age of the eclogites of the White Sea mobile belt. Special emphasis is made on the isotope geochronological and geochemical features of the composition of zircons from eclogite samples, as well as on the phase and chemical compositions and distribution patterns of mineral inclusions. Considering the age of eclogite metamorphism that led to the formation of eclogites in the White Sea mobile belt, we propose our interpretation based on a set of independent isotope geochemical dating methods, including the local U- Pb method for heterogeneous zircons with magmatic cores and eclogite rims, the Lu-Hf and Sm-Nd methods for the minerals of eclogite paragenesis (garnet and omphacite). And this age interpretation is fundamentally different from the one described in the commented article: all the three methods independently determine the eclogite metamorphism as Paleoproterozoic and yield the same age of circa 1.9 Ga. According to our data, the eclogites of the White Sea mobile belt are among the most ancient high-pressure rocks, their reliably established age of metamorphism is circa 1.9 Ga, and the age of the magmatic protolith is the range of 2.2–2.9 Ga.

High-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P–T conditions of 15.8–18.2 kbar/625–690 °C (M1), 11.8–14.5 kbar/788–806 °C (M2), and 5.4–6.4 kbar/613–668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240–244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.

The paper pertains to the studies carried out on the eclogitic xenoliths of KL-2 kimberlite of Kalyandurg kimberlite cluster in south India. Petrographic studies revealed bi-mineralic and kyanite-bearing eclogitic xenoliths in KL-2 kimberlite. The bimineralic and kyanite-bearing eclogites of Kalyandurg KL-2 kimberlite pipe show variation in modal proportion of garnet, omphacite, clinopyroxene and kyanite. The paper reports discovery of native gold grains and Au-Pt alloy in the kyanite-bearing eclogite xenoliths of KL-2 kimberlite. The flaky gold grains occurring in the matrix of kyanite-bearing eclogite are homogeneous and two grains of Au-Pt alloy with Au and Pt in the proportion of 9.8:1.2 are also present. This is the first report of gold and gold-platinum alloy specs from eclogitic xenoliths of Indian kimberlites.

In the Variscan Schwarzwald metabasic rocks form small bodies included within anatectic plagioclase-biotite gneisses. Many metabasites first underwent an eclogite-facies metamorphism at about 2.0 GPa and 670–700 °C, resulting in the assemblage garnet + omphacite + rutile + quartz ± epidote ± amphibole ± kyanite. Since these eclogites are nearly free of an OH-bearing phase, they underwent almost complete dehydration during subduction, suggesting formation along an average to warm top-of-the-slab geotherm of 10–13 °C/km. The age of the Variscan high-P/high-T metamorphism is > 333 Ma. After partial exhumation from ~ 65 to ~ 15 km depth, the eclogites were overprinted under increasing activity of H2O by a number of retrograde reactions. The degree of this overprint under amphibolite-facies conditions (0.4–0.5 GPa/675–690 °C) was very different. Up to now, only retrograde eclogites have been found, but some samples still contain omphacite. Kyanite is at least partially transformed to aggregates of plagioclase + spinel ± corundum ± sapphirine. On the other hand, there are amphibolites that are extensively recrystallized and show the assemblage amphibole + plagioclase + ilmenite/titanite ± biotite ± quartz ± sulphides. The last relic phase that can be found in such otherwise completely recrystallized amphibolites is rutile. After the amphibolite-facies metamorphism at ~ 333 Ma, the metabasites underwent a number of low-temperature transformations, such as sericitization of plagioclase, chloritization of amphibole, and formation of prehnite. The intimate association of metabasite bodies with gneisses of dominantly meta-greywacke compositions suggests derivation from an active plate margin. This view is corroborated by bulk-rock geochemical data. Excluding elements that were mobile during metamorphism (Cs, Rb, Ba, K, Pb, Sr, U), the concentrations of the remaining elements in most of the metabasites are compatible with a derivation from island-arc tholeiites, back-arc basin basalts or calc-alkaline basalts. Only some samples have MORB precursor rocks.

High/ultrahigh-pressure (HP/UHP) metamorphic complexes, such as eclogite and blueschist, are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones. Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary melange in the Changning-Menglian suture zone, at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau. The authors report the result of petrological, mineralogical and metamorphism investigations of these rocks, and discuss their tectonic implications. The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists. The major mineral assemblage of the eclogites includes garnet, omphacite, glaucophane, phengite, clinozoisite and rutile. Eclogitic garnet contains numerous inclusions, such as omphacite, glaucophane, rutile, and quartz with radial cracks around. Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation. Four stages of metamorphic evolution can be determined: The prograde blueschist facies (M1), the peak eclogite facies (M2), the decompression blueschist facies (M3) and retrograde greenschist facies (M4). Using the Grt-Omp-Phn geothermobarometer, a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658°C was determined, which is typical of low-temperature ultrahigh-pressure metamorphism. The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic melange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean. The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone. ©2021 China Geology Editorial Office.

Paleoproterozoic retrogressed eclogite (retroeclogite) occurs in the Itaguara Sequence included in the suture zone formed by collision between the Archean Divinopolis and Campo Belo/Bonfim Complexes in the southern Sao Francisco Craton, which represents the South American counterpart of the African Congo Craton. The Itaguara retroeclogite contains scarce omphacite and phengite but abundant garnet porphyroblasts embedded in a fine-grained, amphibole, biotite and quartz-bearing matrix. The 2.20 ± 0.05 Ga eclogitization event (garnet and whole rock Sm-Nd isochronic age) of the E-MORB protolith (TDM ~ 2.47 Ga) is recorded by omphacite formation during high-pressure prograde stage in amphibole eclogite facies due to ~70 km depth subduction process. Amphibole eclogite facies metamorphic peak stage of 17–20 kbar and 600–700 °C occurred during ~2.1 Ga continental collision. Tectonic exhumation-related decompression during collision probably triggered partial melting of the eclogitic rock. Finally, decompression late stage estimated between 5 and 8 kbar and 550–650 °C under amphibolite facies overprint during orogenic collapse was responsible for appearance of kelyphitic reaction rims (symplectite) around garnet crystals. As its Paleoproterozoic contemporary analogues from Congo Craton, the Itaguara retroeclogite is one of the oldest records of the modern-style plate tectonics.

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is considered as the main suture of the Paleozoic Paleo-Tethys that separates Gondwana-derived continental fragments from Eurasia-derived ones. Understanding the evolutionary history of this orogenic belt is of critical importance in the reconstruction of the tectonic history of Paleo-Tethys. The CMOB preserves well-exposed blueschist facies rocks, albeit their tectonometamorphic history and protolith signatures remain poorly constrained. Here we present, for the first time, results from a detailed investigation of lawsonite-bearing blueschist rocks, including epidote-magnesioriebeckite schist and garnet-ferroglaucophane schist from the CMOB and involving petrological, mineralogical, thermodynamic modeling, whole-rock geochemical, and geochronological studies. The epidote-magnesioriebeckite schist samples and garnet-ferroglaucophane schist samples display OIB- and E-MORB-like geochemical affinities, respectively, and have whole-rock eNd (t) values in the range of −5.4–+4.4, suggesting that their protoliths were mainly oceanic crust with limited degree of crustal assimilation. Magmatic zircon grains from the garnet-ferroglaucophane schist samples yield protolith ages of 253–250 Ma. Combined with previous data, our data represent the youngest ages of rock formation in the Paleo-Tethyan Ocean reported to date. The epidote-magnesioriebeckite schist samples show a peak assemblage of magnesioriebeckite + lawsonite + augite-aegirine + phengite + titanite + allanite, with peak P–T conditions of 12.4–16.8 kbar and 350–406 °C. In contrast, the garnet-ferroglaucophane schist samples preserve a peak assemblage of garnet + ferroglaucophane + omphacite + lawsonite + phengite + titanite/ilmenite/rutile ± allanite and yield peak P–T conditions of 19.5–22.6 kbar and 490–510 °C. The epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists record a nearly complete clockwise P–T loop characterized by a steep prograde P–T path with a low thermal gradient of ~5–8 °C/km followed by cooling or overprinting by isothermal decompression. Reconstruction of the metamorphic P–T evolution, together an evaluation of previous age data, allows us to propose rapid subduction of cold oceanic lithosphere to depths of 50–75 km at a rate of ~6.3–9.3 km/Myr during Early-Middle Triassic (248–240 Ma), followed by exhumation in the Late Triassic (231–214 Ma). The short time lag (

Automated electron backscattered diffraction (EBSD) data yield abundant information on the lattice-preferred orientations (LPOs) and deformation microstructures and mechanisms of minerals in a rock, which aid in our understanding of the tectonic conditions and history of a region. Additionally, this information allows us to interpret seismic anisotropies in the crust and mantle. However, great care must be taken in the collection and production of crystallographic orientation data via automated EBSD to more precisely interpret LPOs, deformation microstructures, and seismic anisotropies because petro-fabrics can be different depending on sampling methods such as all-points-per-grain (whole) data and one-point-per-grain (one orientation per grain) data. Here, we report a detailed comparison of the crystallographic orientation data produced using both all-points-per-grain and one-point-per-grain techniques to analyze eclogites from the Yuka terrane in the North Qaidam ultrahigh-pressure metamorphic belt of northwestern China. By comparing eclogite crystallographic orientations between different sampling methods, we found that there was no marked difference in the LPOs, fabric strengths (J-index and M-index), and seismic anisotropies for relatively small mineral grains (e.g., garnet and omphacite). However, there were large differences in the LPOs, fabric strengths, and seismic anisotropies of relatively large minerals grains (e.g., amphibole). This result can be attributed to the acquisition method of crystallographic orientation data, which can influence the LPO, fabric strength, and seismic anisotropy of minerals and rocks. Therefore, the pole figures of minerals should be constructed from one point per grain data in order to avoid the oversampling of large grains for samples with highly heterogenous grain size distributions, as well as to permit the comparison of crystallographic orientations obtained using different tools and in other studies.

We report mineral textures of retrogressed eclogites from the gneiss–amphibolite complex, which is represented by a series of isolated bodies that were thrusted over the greenschist-facies Rakovec group in the Gemer unit (Western Carpathians). Peak pressure and temperature conditions of about 2.0 GPa and 650 °C are estimated for the eclogite-facies assemblage (omphacite, garnet and rutile). The eclogites show strong amphibolite-facies re-equilibration that is characteristic for the rest of gneiss–amphibolite complex rocks. Pressure and temperature conditions for both eclogite and subsequent amphibolite-facies stages are comparable with those reported from the retrogressed eclogites in the Vepor and Tatra units. The underlying Rakovec group rocks indicate greenschist-facies conditions, but relicts of high-pressure (blueschist facies) metamorphism are also preserved. The Pre-Alpine age of the eclogite-facies metamorphism in the gneiss–amphibolite complex and blueschist-facies metamorphism in the Rakovec group is confirmed by the presence of their pebbles in the overlaying upper Carboniferous conglomerates. The origin of eclogites and blueschists in the Gemer unit and their relationship to eclogites in the Vepor and Tatra units are discussed with respect to the subduction zone along which they originated, but also in relation to the high-pressure rocks occurring along the European Variscan Belt. As the whole Gemer unit underwent Alpine metamorphism, the retrogressed eclogites and associated gneisses were also studied to clarify possible degree of this metamorphic overprint in the gneiss–amphibolite complex rocks.

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.

As the important component of the eastern Tethys tectonic region, the Jinsha River-Ailao Mountain suture zone is often considered to be an ophiolitic melange 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 (

Guatemalan jadeite has a long history, and much Guatemalan jadeite can be found on the market today with a variety of colors and species diversity. Seven varieties of green pyroxenic jadeite from Guatemala with greyish green, yellow green, brilliant green, blue green, dark green, black green and mottled green colors were investigated by combining the methods of XRD, Raman spectroscopy, cathodoluminescence, EPMA and μ-XRF mapping. The results showed that according to the composition, Guatemalan pyroxenic jadeite can be divided into three categories: jadeite jade, omphacite jade, and jadeite-omphacite jade. According to the characteristics of cathodoluminescence, it can be speculated that the formation of jadeite undergoes three stages, and the formation of jadeite jade is mainly due to the crystallization (Jad I) of the early jadeite fluid, along with the second-stage fluid metasomatism/crystallization (Jad II). In the later stage, the fluid that is rich in Ca-Mg-Fe components can replace early Jad I/Jad II, or it can coexist with Jad II, and metamorphism occurs to produce omphacite jade. The jadeite-omphacite jade can be formed when the omphacite fluid with incomplete metasomatism, with uneven texture and reduced transparency.

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.

The Beloretsk Metamorphic Complex in the SW Urals formed at a convergent eastern margin of Baltica during the Neoproterozoic-Early Cambrian Timanide orogeny. It comprises three major units with lenses of facies-critical metabasites within metasedimentary rocks: A lowermost eclogite unit, an intermediate garnet amphibolite unit and an upper amphibolite-greenschist unit. Pressure (P)-temperature (T)-paths of four rocks from the two lowermost units were determined mainly by PT pseudosection techniques showing similar clockwise loops at different peak metamorphic, water-saturated conditions: A phengite-bearing eclogite shows peak PT conditions of 16.5–18.5 kbar/525–550 °C (stage I) followed by stage II at 11.5–13.0 kbar/585–615 °C. A garnet amphibolite from the intermediate unit yields lower peak conditions of 11.7–14.5 kbar/480–510 °C (stage I) followed by stage II at 9.5–11.0 kbar/535–560 °C. However, a granite gneiss in the eclogite unit shows similar maximum pressures as the eclogite, but higher temperatures at 15.6–16.2 kbar/660–675 °C, whereas a garnet micaschist contains comparable high pressure relicts, but underwent an advanced midcrustal reequilibration at 7.5–9.0 kbar/555–610 °C. We dated the eclogite by a 7-point Rb/Sr mineral isochron (phengite, omphacite, apatite) at 532.2±9.1 Ma interpreted as age of crystallisation of the eclogitic peak PT assemblage. This age is the youngest compared to the known Timanide metamorphic and magmatic ages.

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: 10 vol.% pre‐eclogitic hydrous minerals, is not preserved in eclogite but liberated to the mantle wedge., Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

The early evolution of the southern Brasiliano Orogen is recorded in the Sao Gabriel Terrane, which consists of two Neoproterozoic (Tonian and Cryogenian) juvenile arcs, ophiolite complexes and supracrustal rocks. This study aims at providing new insights into the geodynamics processes and subduction process of this orogen by investigating the first Tonian eclogite (named Tres Vendas eclogite) discovered in the southern Brasiliano Orogen. To determine the tectono-metamorphic evolution, we integrated field work with petrography, scanning electron microscopy, electron microprobe analyses of minerals and in situ LA-ICP-MS U-Pb geochronology with Lu-Hf and rare earth elements in zircon. The preserved high-grade metamorphic assemblage in these rocks has garnet, rutile, phengite, and omphacite, indicating that these rocks are (retro) eclogites. The presence of omphacite with up to 50% jadeite indicates minimum burial pressures of ≥15 kbar that are consistent with forward modeling of the garnet compositions. Omphacite was altered to fine-grained symplectites of hornblende, oligoclase, and diopside or augite with minor quartz during rapid exhumation. The second stage of retrogression involved reaction of garnet and diopside to plagioclase and hornblende symplectites (kelyphites) at amphibolite facies conditions. Kelyphitic hornblende-garnet pairs indicate peak temperatures of 710–760 °C. Whole rock composition and Zr–Y characteristics are consistent with tholeiitic island arc basalts, while their garnet compositions point to subduction zone eclogites. U-Pb ages of zircon cores yielded 909.5 ± 5.3 Ma and metamorphic rims 891.9 ± 8.2 Ma. Zircon age-corrected eHf isotope compositions range between +4 and +10, reflecting the moderately juvenile nature of these rocks. Zircon cores are distinguished by higher rare earth element (REE) contents and steeper middle REE to heavy REE (MREE-HREE) patterns, higher Th/U ratios and higher Sr and Ca contents than the metamorphic rims, and the rims are likely coeval with the kelyphites. The combined data indicate that the Tres Vendas eclogites belong to a Tonian arc and were buried to >45 km, most likely in a subduction channel. The data from the first eclogite in southern Brazil have significant consequences for the interpretation of the Tonian evolution of the Brasiliano Orogen and, by extension, the Rodinia framework.

Eclogite‐facies breccias from the Monviso metaophiolite complex (N‐Italy and W‐Alps) represent a unique opportunity to study the triggering mechanisms for transient brittle deformation at eclogite‐facies conditions in ductilely deforming subducted oceanic lithosphere. Here the intact foliation of Fe‐Ti‐rich and Mg‐Al‐rich metagabbro clasts, cemented by unfoliated eclogitic matrices (paragenesis: omphacite ± garnet ± lawsonite), demonstrates pristine brecciation at eclogite‐facies conditions. Successive generations of high‐pressure veins and eclogitic matrices reveal multiple brittle rupture events. Pseudosection modeling, textural observations, and geochemical data suggest that minerals of both prograde veins and first brecciation event (M1 matrix) crystallized in presence of fluids buffered by the surrounding metagabbro minerals, while M2 matrix composition suggests an incipient infiltration of external fluids. Trace element composition of the third M3 matrix, associated to its impressive lawsonite pseudomorphs, clearly points to the massive ingression of external, serpentinite‐derived fluids. Therefore, these rocks record the progressive increase in the scale of fluid circulation along multiple, stepwise eclogitic brecciation events, from locally released fluids (closed system) to kilometer‐scale fluid infiltration. The successive brecciation steps promoted permeability creation (at least transiently), which resulted in progressive opening of the system to large‐scale sustained fluid circulation. Eclogite‐facies brecciation also controlled the initial stages of strain localization, which highlights the importance, in ductilely deforming low‐permeability eclogite‐facies rocks, of brittle deformation events for both strain localization initiation and creation of large‐scale fluid circulation pathways. Thus, Monviso breccia blocks record the complete, stepwise development of a major shear zone, via progressive strain localization, permeability creation, and increasing scale of fluid circulation.

Melt inclusions in kimberlitic and metamorphic diamonds worldwide range in composition from potassic aluminosilicate to alkali-rich carbonatitic and their low-temperature derivative, a saline high-density fluid (HDF). The discovery of CO2 inclusions in diamonds containing eclogitic minerals are also essential. These melts and HDFs may be responsible for diamond formation and metasomatic alteration of mantle rocks since the late Archean to Phanerozoic. Although a genetic link between these melts and fluids was suggested, their origin is still highly uncertain. Here we present experimental results on melting phase relations in a carbonated pelite at 6 GPa and 900–1500 °C. We found that just below solidus K2O enters potassium feldspar or K2TiSi3O9 wadeite coexisting with clinopyroxene, garnet, kyanite, coesite, and dolomite. The potassium phases react with dolomite to produce garnet, kyanite, coesite, and potassic dolomitic melt, 40(K0.90Na0.10)2CO3·60Ca0.55Mg0.24Fe0.21CO3 + 1.9 mol% SiO2 + 0.7 mol% TiO2 + 1.4 mol% Al2O3 at the solidus established near 1000 °C. Molecular CO2 liberates at 1100 °C. Potassic aluminosilicate melt appears in addition to carbonatite melt at 1200 °C. This melt contains (mol/wt%): SiO2 = 57.0/52.4, TiO2 = 1.8/2.3, Al2O3 = 8.5/13.0, FeO = 1.4/1.6, MgO = 1.9/1.2, CaO = 3.8/3.2, Na2O = 3.2/3.0, K2O = 10.5/15.2, CO2 = 12.0/8.0, while carbonatite melt can be approximated as 24(K0.81Na0.19)2CO3·76Ca0.59Mg0.21Fe0.20CO3 + 3.0 mol% SiO2 + 1.6 mol% TiO2 + 1.4 mol% Al2O3. Both melts remain stable to at least 1500 °C coexisting with CO2 fluid and residual eclogite assemblage consisting of K-rich omphacite (0.4–1.5 wt% K2O), almandine-pyrope-grossular garnet, kyanite, and coesite. The obtained immiscible alkali‑carbonatitic and potassic aluminosilicate melts resemble compositions of melt inclusions in diamonds worldwide. Thus, these melts entrapped by diamonds could be derived by partial melting of the carbonated material of the continental crust subducted down to 180–200 km depths. Given the high solubility of chlorides and water in both carbonate and aluminosilicate melts inferred in previous experiments, the saline end-member, brine, could evolve from potassic carbonatitic and/or silicic melts by fractionation of Ca-Mg carbonates/eclogitic minerals and accumulation of alkalis, chlorine and water in the residual low-temperature supercritical fluid. Direct extraction from the hydrated marine sediments under conditions of cold subduction would be another possibility for the brine formation.

The peritectic mechanisms controlling the ultramafic–mafic evolution of magmatism during fractional crystallization and the genesis of peridotite–pyroxenite–eclogite rock series of the garnet–peridotite facies of the upper mantle were substantiated experimentally. The melting phase relations of differentiated mantle material in the olivine–clinopyroxene/omphacite–corundum–coesite multicomponent system were studied by the polythermal sections method; their boundary phases reproduced the compositions of peridotitic and eclogitic minerals. The peritectic reaction between orthopyroxene and melt with the formation of clinopyroxene proceeds at the liquidus of the olivine–orthopyroxene–clinopyroxene–garnet system as the mechanism for “clinopyroxenization of orthopyroxene,” which yields the regressive olivine + clinopyroxene + garnet + melt monovariant cotectic reaction. The further evolution of magmatism was studied experimentally at 6 GPa in the ultramafic–mafic olivine–diopside–jadeite–garnet system with a variable composition of the diopside–jadeite solid solutions (clinopyroxene ↔ omphacite). The peritectic reaction between olivine and melt with the formation of garnet was detected on the liquidus of the triple system olivine–diopside–jadeite as the of olivine garnetization mechanism yielding the omphacite + garnet + melt cotectic reaction with the formation of bimineral eclogite. The structure of the liquidus of the olivine–diopside–jadeite–garnet system was defined, as well as its critical role as the “physicochemical bridge” between the ultramafic olivine-bearing peridotite–pyroxenite composition and mafic silica-saturated eclogitic composition of matter within the garnet–peridotite facies. The experimental physicochemical results illustrate the genetic links between ultramafic and mafic rocks and the mechanisms of continuous fractional magmatic evolution and petrogenesis from olivine-bearing peridotite–pyroxenite rocks to silica-saturated eclogite–grospydite rocks. This explains the complete petrochemical trends for the rock-forming components in clinopyroxenes and garnets from differentiated rocks of the garnet–peridotite facies.

We present the results of analogue experiments carried out in a piston–cylinder apparatus at 750–900°C and 2.9 GPa aimed to simulate metasomatic transformation of the fertile mantle caused by fluids and melts released from the subducting sediment. A synthetic H2O- and CO2-bearing mixture that corresponds to the average subducting sediment (GLOSS, Plank and Langmuir (1998)) and mineral fractions of natural lherzolite (analogue of a mantle wedge) were used as starting materials. Experiments demonstrate that the mineral growth in capsules is controlled by ascending fluid and hydrous melt (from 850°C) flows. Migration of the liquids and dissolved components develops three horizontal zones in the sedimentary layer with different mineral parageneses that slightly changed from run to run. In the general case, however, the contents of omphacite and garnet increase towards the upper boundary of the layer. Magnesite and omphacite (±garnet ± melt ± kyanite ± phengite) are widespread in the central zone of the sedimentary layer, whereas SiO2 polymorph (± kyanite ± phengite ± biotite ± omphacite ± melt) occurs in the lower zone. Clinopyroxene disappears at the base of lherzolite layer and the initial olivine is partially replaced by orthopyroxene (± magnesite) in all experiments. In addition, talc is formed in this zone at 750°C, whereas melt appears at 850°C. In the remaining volume of the lherzolite layer, metasomatic transformations affect only grain boundaries where orthopyroxene (± melt ± carbonate) is developed. The described transformations are mainly related to a pervasive flow of liquids. Mineral growth in the narrow wall sides of the capsules is probably caused by a focused flow: omphacite grows up in the sedimentary layer, and talc or omphacite with the melt grow up in the lherzolite layer. Experiments show that metasomatism of peridotite related to a subducting sediment, unlike the metasomatism related to metabasites, does not lead to the formation of garnet-bearing paragenesis. In addition, uprising liquid flows (fluid, melt) do not remove significant amounts of carbon from the metasedimentary layer to the peridotite layer. It is assumed that either more powerful fluxes of aqueous fluid or migration of carbonate-bearing rocks in subduction melanges are necessary for more efficient transfer of crustal carbon from metasediments to a mantle in subduction zones.

Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids. In this paper, we present the helium isotopic compositions of the Songduo eclogites in the Lhasa terrane, Tibet. We found that garnet and omphacite in the eclogites have different helium retention characteristics. The 4He content of most omphacite grains are about 10-20 times of that of garnet, suggesting that omphacite has a higher ability to capture 4He than garnet. Similarly, there is about 10-20 times difference in 3He content between omphacite and garnet in the same eclogite samples. The 3He/4He ratios of garnet and omphacite in these rocks range from 0.27 to 0.60 Ra (relative to the modern air 3He/4He ratio, 1.4×10-6). These ratios are within the range of both mantle-and crust-derived helium, suggesting mixed sources. The Songduo eclogites have much higher 3 He/4He ratios than those observed in the Dabie eclogites of eastern China. Such high ratios are typically thought to be associated with deep mantle sources. We cautiously conclude that deep mantle materials might have been involved during the formation of the Songduo eclogites.

The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indicate a multi-stage metamorphic history of the granulitized eclogites: (1) an earlier eclogite facies metamorphism (P>18.5 kbar, T> 830 °C) is deduced from omphacite relics in the matrix and rare omphacite inclusions within garnet. The possible assemblage is garnet+omphacite+rutile+ quartz; (2) the early stage of high pressure granulite facies assemblages (garnet+clinopyroxene+ plagioclase+rutile+quartz+liquid) developed in the early decompression process has a P-T regime of 17.5 kbar and 852–858 °C, constrained by plagioclase and clinopyroxene inclusions in garnet. The late stage of high pressure granulite assemblages (garnet+clinopyroxene+amphibole+plagioclase+rutile+quartz+liquid) records an isothermal decompression process with the pressure successively declining from 17.5 to 14.7 kbar and to 11.3 kbar at 858 °C; (3) the later medium pressure granulite facies assemblage (garnet+ orthopyroxene+ clinopyroxene+amphibole+plagioclase+ilmenite+liquid+quartz) indicates a drop in pressure and rise in temperature at P-T conditions of 7.6–7.7 kbar and 878–883 °C; (4) retrogressive amphibolite facies stage, which is represented by amphibole+plagioclase kelyphitic rims around garnet, formed under conditions of

The Sumdo eclogite-bearing (U)HP metamorphic belt extends over 100 km across the middle part of the Lhasa terrane in southern Tibet, which forms a Permian-Triassic oceanic subduction zone between the south and the north Lhasa sub-terranes, leading to the reinterpretation of the tectonic evolution of the Lhasa terrane in the Tibetan-Himalayan orogeny. Previous studies show that there are significant differences in temperature and pressure conditions of the eclogites in four areas, e.g., Sumdo, Xindaduo, Bailang and Jilang areas. Studying the peak metamorphic P-T conditions and path of eclogite in the Sumdo belt is of great significance to reveal the subduction and exhumation mechanism of Paleo-Tethys Ocean in the Lhasa terrane. In this contribution, eclogite in the Jilang area of the Sumdo belt is chosen as an example to study its metamorphic evolution. The mineral assemblage of the eclogite is garnet, omphacite, phengite, hornblende, epidote, quartz and minor biotite. Garnet has a "dirty" core with abundant inclusions such as epidote, amphibole, plagioclase and a "clear" rim with few inclusions of omphacite and phengite. From the core to the rim, pyrope content in garnet increases while grossular content decreases, showing typical growth zoning. The rim of garnet is wrapped by the pargasite+plagioclase corona, showing amphibolite facies overprint during retrogression. Three stages of metamorphism are inferred as (1) prograde stage, represented by the core of garnet and mineral inclusions therein; (2) peak stage, represented by the garnet rim, omphacite, lawsonite, phengite, and quartz; (3) retrograde stage characterized by decomposition of lawsonite to zoisite, followed by symplectite of omphacite and corona rimmed garnet. A P-T pseudosection contoured with isopleths of grossular and pyrope contents in garnet is used to constrain the near peak P-T condition at 2.85 GPa, 575 C. In general, the Jilang eclogite shows a clockwise P-T path with a near isothermal decompression process during exhumation. Combined with the age peaks of 583, 911, and 1 134 Ma from the detrital zircons of the country metaquartzite, a continental margin material involving exhumation process at shallow depth after the subduction channel exhumation is inferred for the Jilang eclogite and may further indicate that the subduction direction of the Sumdo eclogite belt is from north to south.

The Sumdo metamorphic belt, located in the Lhasa terrane of Tibet, is a key area for unraveling the Paleo-Tethys evolution. In order to identify the fluid evolution in oceanic subduction zones, eclogite from the Sumdo metamorphic belt, has been investigated in a combined study of petrography, mineral chemistry, fluid inclusions and oxygen isotopes. On the basis of petrography and mineral chemistry, two kinds of garnet were distinguished. The core part of Grt-I shows increasing grossular content and contains prograde mineral inclusions, suggesting that it grew during prograde metamorphism, while the rim of Grt-I shows varying grossular and pyrope contents, representing subsequent overgrowth. Grt-II displays a composition comparable with the outermost part of Grt-I, indicating that Grt-II grew at the peak or initial retrograde stage. Garnet trace element zoning patterns and P-T estimates suggest that the Sumdo eclogite experienced a heating stage during an initial exhumation after the peak metamorphic event. Based on textural criteria and host minerals, three types of fluid inclusions were distinguished: Type-I are primary NaCl dominated intermediate to high-salinity (10–22 wt% NaCl) fluid inclusions hosted by omphacite, which were most likely released during dehydration of the subducting oceanic crust and reflect the composition of eclogite-facies fluids; Type-II are defined as primary low-salinity (1–6 wt% NaCl) fluid inclusions hosted by epidote/clinozoisite, representing fluids that evolved towards low-salinity during uplift; Type-III are secondary low-salinity (0–7 wt% NaCl) fluid inclusions hosted by matrix quartz (i.e., minerals other than the mineral inclusions) and rarely by omphacite and must have formed at a very late stage. In addition, the eclogite displays δ18O values (δ18O = +5.0 to +8.9‰) similar to its protolith, low-temperature altered oceanic crust (δ18O = +4.9 to +12.7‰). The equilibrium oxygen isotope fractionations (Δ18OQtz-Min) between quartz and garnet/omphacite suggest isotopic equilibrium and a closed fluid system during eclogite facies metamorphism. Our data indicate that the Sumdo eclogite experienced “hot” exhumation, during which large amounts of fluids were released by dehydration. Fluids in the Sumdo eclogite were typically aqueous fluids of varying salinities and now preserved as fluid inclusions in HP metamorphic minerals. In comparison with continental subduction zones, we confirm previous conclusion that the fluid regime in oceanic subduction zones is relatively simple (i.e. mainly NaCl bearing brines) with seawater-altered slabs as the dominant source.

co-auteur étranger; International audience; 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% Al2O3) 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. H2O (determined as OH). We infer that intertwined lamellae of hydrous garnet (600-960 ppm wt. H2O) and minor (2O) formed at mantle conditions due to progressive exsolution, without external fluids. Recalculated whole rock water contents range from ~320 to 970 ppm wt. H2O. 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.

Erzgebirge eclogite occurs in three high-pressure (HP) units, including one UHP unit (Unit 1). Each sample investigated contains either magmatic or metamorphic zircon but never both. This is in contrast to many literature examples of eclogite with more than one generation of zircon. We infer that metamorphic zircon formed at the expense of a magmatic precursor, and that the process either went to completion (consuming all magmatic zircon) or did not occur at all. Metamorphic zircon is only present in UHP eclogite and one HP eclogite from Unit 2. Another sample from Unit 2 and one from Unit 3, which experienced the lowest metamorphic PT conditions of all samples, contain only magmatic zircon. The U-Pb SIMS age of c. 540 Ma of igneous zircon, being somewhat younger than average Hf model ages (i.e., 567 and 591 Ma), is attributed to crystallization in the basaltic or gabbroic precursor that formed from a juvenile mantle source. The time of peak metamorphism is recorded by only two zircons from UHP eclogite giving ages of 360.5 ± 5.4 Ma and 359.7 ± 5.3 Ma that well agree with a previous three-point Sm-Nd isochron of 360 ± 7 Ma. All other grains gave younger ages, mainly between 340 and 330 Ma but covering a range of c. 350–315 Ma, which is attributed to retrogression at amphibolite-facies conditions. We infer (i) that this post-eclogitic zircon replaced peak-metamorphic grains (only in UHP eclogite) as well as magmatic zircon and (ii) that peak-metamorphic zircon did not form in non-UHP eclogite. Growth of peak and post-peak zircon was governed by fluid and/or melt. The latter is related to decompressional melting in felsic host rocks (and possibly also to late-orogenic granitic magmatism). Fluid is either of external origin (melt-derived or from dehydration reactions in subducting footwall units) or due to decompressional release of internal fluid (molecular water from fluid inclusions or structural water in omphacite and garnet). The new and former age data can best be reconciled with a two-stage exhumation process. Initial, buoyancy-driven exhumation was fast and terminated as the HP/UHP slices reached lower crustal levels where the rocks presumably rested for 10–20 million years, followed by slow exhumation associated with gravitational collapse of the Variscan orogen. The majority of metamorphic zircon (with ages of 340–330 Ma) grew at lower crustal levels, where the rocks presumably rested for 10–20 million years, and other grains (younger than 330 Ma) during the final, slow exhumation that partly overlapped with the time of granitic magmatism.

International audience; The Queyras-Monviso traverse of the Schistes Lustrés complex, is a stack of underplated nappes of oceanic rocks subducted from blueschist- to eclogite-facies conditions.This study reports on salinities and gas contents of primary fluid inclusions trapped in high pressure veins from metasediments and metamafic rocks (445 fluid inclusions in 22 samples). These data provide snapshots of the compositions of fluids present at peak burial conditions, varying from ~30 to 80 km depth and illustrate the evolution of fluid composition with burial along a cold subduction zone.Fluid inclusions trapped in lawsonite- and carpholite-bearing veins in metasediments contain moderately saline aqueous fluids (average salinity of 4.6 wt% NaCl eq.) with subordinate amounts of CO2 and CH4 in the vapor phase. The observed salinity decrease with increasing grade is interpreted as reflecting progressive dilution of initial seawater-like pore fluid by low-salinity fluids released locally by dehydration reactions. Less-frequently measured higher salinities in the uppermost metasedimentary-dominated tectonic unit suggest brine infiltration from embedded blocks of margin units containing evaporites. CO2 and CH4 (and scarce potential hydrocarbons) appear to be locally released from fluid interaction with carbonates and carbonaceous matter-rich pelitic horizons, respectively.Fluid inclusions in high pressure omphacite veins in metagabbros record higher salinities (mean salinity about 17 wt% NaCl eq.) with small amounts of N2 in eclogitic veins only, and a variety of minerals (calcite, white mica, salts) indicative of complex chemical systems. These high salinities are interpreted as partly inherited from seafloor high-temperature hydrothermal alteration of gabbros, resulting in phase separation and brine formation. Progressive breakdown of hydrothermal Cl-rich amphibole to glaucophane (blueschist-facies) and then omphacite (eclogite-facies) and release of brines trapped in fluid inclusions could account for the high salinity fluids. Therefore, in metagabbros, fluid inclusions record progressive release of Cl in the fluid phase with increasing grade.Fluid signatures of metasediments and metamafics appear characteristic of each rock type. Local fluid signatures and redox conditions were preserved within units, possibly due to restricted and transient fluid circulation (at the hm-scale at most).Fluid inclusions in Alpine metasediments show salinities and gas contents comparable with other subducted fragments of oceanic lithosphere worldwide, whereas fluid salinities of Alpine metagabbros are higher than salinities recorded elsewhere, either due to (1) higher-temperature hydrothermal alteration and brine formation in Alpine metagabbros (compared to metavolcanics) or (2) more restricted infiltration by sediment-derived fluids compared to block-in-mélange subduction complexes.

Raman and photoluminescence (PL) mapping is a non-destructive method which allows gemologists and scientists to evaluate the spatial distributions of defects within a gem, it can also provide a method to quickly distinguish different species within a composite gem. This article provides a summary of this relatively new technology and its instrumentation. Additionally, we provide a compilation of new data for various applications on several gemstones. Spatial differences within diamonds can be explored using PL mapping, such as radiation stains observed on the rough surface of natural green diamonds. Raman mapping has proven useful in distinguishing between omphacite and jadeite within a composite of these two minerals, identifying various tourmaline species within a heterogeneous mixture, and determining the calcium carbonate polymorphs in pearls. Additionally, it has potential to be useful for country-of-origin determination in blue sapphires and micro-inclusion analysis. As new avenues of research are explored, more applications for gem materials will inevitably be discovered.

This study proposes new data on the rare pyroxene, variety violane, sampled from its type locality, Praborna manganese deposit, near Saint Marcel (Val d’Aosta, Italy). Violane is very appreciated as a gemstone for its different hues of violet-blue color and is characterized by its diopsidic or omphacitic composition. To assess the possible causes of color, electron-probe microanalysis (EMPA) and laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) were used to establish the chemical composition. Raman and Fourier transform infrared (FTIR) spectroscopy, ideal for the non-destructive analysis, identified the different present phases directly on gemstone. Raman and FTIR spectra highlighted the presence, in the same sample, of diopside and omphacite showing almost inappreciable violet hue difference. The two minerals were easily differentiated by microprobe analyses and showed a compositional heterogeneity not linked to the different colors. The best way to detect the color-causing elements in microcrystalline violanes resulted in the analysis of trace elements and their quantification. An enrichment of Ti and Li characterizes darker violet omphacite and that of V and rare-earth elements (REE) the lilac-lavender or light violet-blue diopside. In general, our results led us to say that the color changes, previously proposed as due to Mn both in divalent and trivalent oxidation state, may be controlled by trace elements or by concentration of minor elements, such as Fe, and their oxidation state. REE patterns showed a negative anomaly of Ce that could be ascribed to the variation of the oxygen chemical potential occurring in the ore. The new data, combined with previous results, may provide new constrains on the processes that generated the Mn-rich deposit of Praborna.

Detailed study of kimberlites and mantle xenoliths from pipes of Nyurbinskaya, Botuobinskaya and Mayskaya Nakyn field revealed their unusual, interesting and important mineralogical features, which indicate the reduced nature of asthenospheric impact on the mantle substrate of the area (absence of megacrystic picroilmenites of is compensated by presence of large orange-red titanous pyropes of “megacryst” type), development of subduction processes (picroilmenite, chrome-omfacite) and crystallization of mantle melts (chrome-diopside), as well as on the impact of the processes of “calcium” (chromium-calcium and titanium-chromium-calcium) mantle metasomatosis (green garnet). The exotic nature of the mantle substrate emphasizes the presence of MARID and alcremite xenoliths. Pipes of Nakyn field are unique in terms of mineralogical features of kimberlites and mantle xenoliths, both among other pipes of Yakutia Diamond-bearing Province, and among kimberlite tubes of the World.

International audience; In Holsnøy (Bergen Arcs, Norway), metastable granulite facies anorthosite rocks are partially eclogitised within hydrous shear zones, that have been interpreted as widening over time with fluid influx and strain. We here present a detailed petrological description of two different metre-scale shear zones from this area. Within a few tens of centimetres of each shear zone, the granulite protolith (initially plagioclase + garnet + two pyroxenes) is transformed into a albite + zoisite + garnet + clinopyroxene assemblage. The outer edge of the shear zones consists in a fine-grained heterogeneous assemblage of omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz. The core of the shear zones is an homogeneous eclogite assemblage of coarser omphacite + kyanite + garnet + zoisite + phengite ± quartz. As the shear zones widened over time, this lateral evolution from the edge to the core of the shear zones reflects the temporal evolution of the granulite from the beginning to the end of the eclogitisation reaction. The outer omphacite + zoisite + kyanite + garnet + phengite ± albite ± quartz assemblage therefore represents a transient eclogite facies assemblage. Field and petrological observations suggest that this transient assemblage is mechanically weaker than both the starting strong granulite and the final eclogite. We here investigate the impact of transient weakening during syn-tectonic metamorphism using a one-dimensional numerical model of a fluid-fluxed, reacting shear zone. We show that transient weakening is required to explain our field and petrological observations. Furthermore, although the widening of the shear zones was primarily controlled by fluid infiltration, we show that strain hardening during the end of the eclogitisation reactions sequence had a noticeable widening effect on the shear zones.

Coexisting fine-grained (meta-volcanic) and coarse-grained (meta-plutonic) mafic rocks in a high-pressure (P)/low-temperature (T) complex (Sivrihisar, Turkey) preserve different prograde, peak, and retrograde mineral assemblages, providing an opportunity to evaluate controls on mineral assemblages in metabasites that experienced the same P–T conditions. Fine-grained metabasalts are garnet-bearing lawsonite blueschist and eclogite with similar assemblages that vary on a mm- to cm- scale in mode of glaucophane vs. omphacite. In contrast, metagabbro consists of a disequilibrium mineral suite of relict igneous clinopyroxene partially replaced by omphacite or hydrous phases (lawsonite + tremolite or glaucophane) in a matrix of fine-grained lawsonite, omphacite, tremolite, white mica, very rare garnet, and retrograde minerals (e.g., epidote, albite, and titanite), with later chlorite and calcite. Pseudosection modeling predicts similar peak P–T conditions (490–530 °C, 1.8–2.0 GPa) for both glaucophane-rich (blueschist) and omphacite-rich (eclogite) layers of the metabasalt and similar to slightly higher conditions (490–600 °C, 1.9–2.5 GPa) for metagabbro. The range of modelled H2O content at peak P–T conditions in metabasalt (2.0–5.4 wt%) is significantly lower than in metagabbro (6.4–8.7 wt%) due to the higher modal abundance of hydrous minerals in the latter. At the relatively similar peak P–T conditions, metagabbro experienced different reaction histories from coexisting metabasalt, thereby developing distinctive HP/LT mineral assemblages and modes (e.g., scarce garnet) owing to its more Mg-rich bulk composition (XMg = 0.58–0.84 vs. 0.50), higher H2O content, and coarser grain-size. This study is the first petrologic analysis of Sivrihisar metagabbro and the first systematic study of H2O content in metabasites from this locality, which is one of the best-preserved examples of lawsonite eclogite and blueschist in the world.

Investigating the seismic properties of natural eclogite is crucial for identifying the composition, density, and mechanical structure of the Earth&rsquo, s deep crust and mantle. For this purpose, numerous studies have addressed the seismic properties of various types of eclogite, except for a rare eclogite type that contains abundant olivine and orthopyroxene. In this contribution, we calculated the ambient-condition seismic velocities and seismic anisotropies of this eclogite type using an olivine-rich eclogite from northwestern Flems&oslash, ya in the Nord&oslash, yane ultrahigh-pressure (UHP) domain of the Western Gneiss Region in Norway. Detailed analyses of the seismic properties data suggest that patterns of seismic anisotropy of the Flem eclogite were largely controlled by the strength of the crystal-preferred orientation (CPO) and characterized by significant destructive effects of the CPO interactions, which together, resulted in very weak bulk rock seismic anisotropies (AVp = 1.0&ndash, 2.5%, max. AVs = 0.6&ndash, 2.0%). The magnitudes of the seismic anisotropies of the Flem eclogite were similar to those of dry eclogite but much lower than those of gabbro, peridotite, hydrous-phase-bearing eclogite, and blueschist. Furthermore, we found that amphibole CPOs were the main contributors to the higher seismic anisotropies in some amphibole-rich samples. The average seismic velocities of Flem eclogite were greatly affected by the relative volume proportions of omphacite and amphibole. The Vp (8.00&ndash, 8.33 km/s) and Vs (4.55&ndash, 4.72 km/s) were remarkably larger than the hydrous-phase-bearing eclogite, blueschist, and gabbro, but lower than dry eclogite and peridotite. The Vp/Vs ratio was almost constant (avg. &asymp, 1.765) among Flem eclogite, slightly larger than olivine-free dry eclogite, but similar to peridotite, indicating that an abundance of olivine is the source of their high Vp/Vs ratios. The Vp/Vs ratios of Flem eclogite were also higher than other (non-)retrograded eclogite and significantly lower than those of gabbro. The seismic features derived from the Flem eclogite can thus be used to distinguish olivine-rich eclogite from other common rock types (especially gabbro) in the deep continental crust or subduction channel when high-resolution seismic wave data are available.

A coesite-bearing diamondiferous eclogite from the Udachnaya kimberlite (Daldyn field, Siberian craton) has been studied to trace its complex evolution recorded in rock-forming and minor mineral constituents. The eclogite sample is composed of rock-forming omphacite (60 vol%), garnet (35 vol%) and quartz/coesite (5 vol%) and contains intergranular euhedral zoned olivine crystals, up to 200 &micro, m long, coexisting with phlogopite, orthopyroxene, clinopyroxene (secondary), K-feldspar, plagioclase, spinel, sodalite and djerfisherite. Garnet grains are zoned, with a relatively homogeneous core and a more magnesian overgrowth rim. The rim zones further differ from the core in having higher Zr/Y (6 times that in the cores), ascribed to interaction with, or precipitation from, a kimberlite-related melt. Judging by pressure-temperature estimates (~1200 &deg, C, 6.2 GPa), the xenolith originated at depths of ~180&ndash, 200 km at the base of the continental lithosphere. The spatial coexistence of olivine, orthopyroxene and coesite/quartz with K-Na-Cl minerals in the xenolith indicates that eclogite reacted with a deep-seated kimberlite melt. However, Fe-rich olivine, orthopyroxene and low-pressure minerals (sodalite and djerfisherite) likely result from metasomatic reaction at shallower depths during transport of the eclogite by the erupting kimberlite melt. Our results demonstrate that a mixed eclogitic-peridotitic paragenesis, reported previously from inclusions in diamond, can form by interaction of eclogite and a kimberlite-related melt.

Variable pressure–temperature (P–T) conditions and paths have previously been reported from eclogites and blueschists from the Acatlan complex, Mexico; yet, the large range of P–T estimates, and uncertainties related with conventional thermobarometry suggest that the P–T evolution of the Acatlan complex remains incompletely understood hindering reliable tectonic interpretations. In this paper, we focus on two high-pressure samples: a phengite eclogite and a garnet-epidote blueschist. We couple phase equilibrium modelling and Zr-in-rutile thermometry to re-evaluate the P–T conditions from these rocks and discuss their implications for the Acatlan complex. The phengite eclogite displays a granoblastic texture with a peak mineral assemblage of garnet + omphacite + phengite + rutile + quartz, and an early retrograde mineral assemblage of amphibole + zoisite + plagioclase + titanite. The garnet-epidote blueschist display a nematoblastic texture with a peak mineral assemblage glaucophane + phengite + epidote + quartz + plagioclase + chlorite + rutile + garnet. Our results indicate that the phengite eclogite followed a clockwise P–T path from peak conditions at ~22 kbar and ~690 °C to re-equilibration conditions at ~14.5 kbar and ~660 °C. Similarly, the garnet-epidote blueschist followed a clockwise P–T path. Phengite zoning indicates two different possibilities for the P–T evolution: (1) from peak conditions at ~19 kbar and ~505 °C or (2) from ~13 kbar and ~480 °C, to re-equilibration conditions at ~8.5 kbar and ~487 °C. Epidote + phengite + albite aggregates (pseudomorphs after lawsonite) and phengite zoning constrain the prograde path within the lawsonite stability field, indicating that the complex evolved through a colder subduction zone (gradient of ~6 °C km−1) than pre-existing estimates. Our results indicate that the eclogite and blueschist reached depths higher than previously thought, ~70 and ~50 km respectively, and experienced a near-isothermal exhumation (~0.15–0.60 °C km−1 for the blueschist, and ~0.9 °C km−1 for the eclogite). These new results, combined with published geochronology, result in a burial rate of ~5.4 km Myr−1, and an exhumation rate of ~3.4 km Myr−1.

International audience; The deformation of eclogites and the driving forces for their fabric development are an important topic, potentially allowing to determine deformation rates and stresses in subduction zones, where the greatest number of large earthquakes occurs. Here, fabric studies of grain size and shape, texture, and chemical composition from two locations of Variscan and Alpine eclogites are presented. All samples show a well-developed crystallographic preferred orientation (CPO) of omphacite with a strong maximum of [001] in the lineation direction and a weaker maximum of poles to (010) normal to foliation. Garnet shows no systematic CPO. Anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde (in terms of pressure change) P,T-path. The individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal mis-orientations. Chemical, microstructural, and CPO data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction. Dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures. The dominant deformation is inferred to be diffusion creep, where dissolution of material took place in reacting mafic phases (plagioclase, pyroxene) and precipitation took place in the form of new eclogite facies minerals (omphacite, garnet, zoisite). This type of diffusion creep deformation represents a transformation process involving both, deformation and metamorphic reactions. It is emphasized that the weakening is directly connected to the transformation and therefore transient. The weakening facilitates diffusion creep deformation of otherwise strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. The results imply low stresses during the deformation of eclogite blocks in subduction zones. These results can be applied to other rock types, too.

Lawsonite is an abundant hydrous mineral in oceanic crust, sediments, and metasomatic rocks at depths of ~45 to 300 km in most subduction zones, but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism. Owing to the significance of lawsonite for water and element cycling in subduction zones, occurrences of fresh lawsonite in blueschist and, more rarely, eclogite provide important opportunities to determine lawsonite composition, zoning, and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation. In this review, we use new and published data to document lawsonite composition in eight of the nine known lawsonite eclogite localities in which fresh lawsonite coexists with garnet + omphacite in the rock matrix, as well as the composition of lawsonite inclusions in six of seven known sites in which lawsonite occurs only as inclusions in garnet in eclogite-facies rocks that lack matrix lawsonite. As lawsonite blueschist is much more common than lawsonite eclogite, we survey the composition of lawsonite in representative localities of blueschist, including blueschist associated with eclogite (lawsonite-bearing, epidote-bearing), and blueschist not associated with eclogite at current exposure levels. Included in this review are metabasaltic rocks, silica- and carbonate-rich metasedimentary rocks, metasomatic rocks, and lawsonite-rich veins. This dataset demonstrates that lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation, and specifically of fluid–rock interaction, with implications for element cycling in subduction zones. Furthermore, most exhumed lawsonite eclogite records slab-surface conditions that correspond to the location where the slab-mantle interface transitions from decoupled to coupled, and therefore provides key insights into the thermal history and dynamics of subduction zones. © 2020 The Authors

The Tsäkkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the μm-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz ± kyanite ± paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich microchannels are also delimited by low angle (, National Science Center, Poland National Science Centre, Poland 2014/14/E/ST10/00321 2019/33/N/ST10/01479, Polish National Agency for Academic Exchange scholarship PPN/IWA/2018/1/00046/U/0001 PPN/IWA/2018/1/00030/U/00001

This paper reports the results of high-pressure experimental modeling of interaction between glaucophane schist and harzburgite or websterite for the evaluation of the influence of mantle material on the input–output of components and character of metasomatic transformations at the crust–mantle boundary in the subduction zone. In all experiments, glaucophane schist (proxy for oceanic crust) containing volatile components (H2O and CO2) incorporated in hydrous minerals (amphiboles, phengite, and epidote) and calcite was loaded into the bottom of each capsule and overlain by mantle material. During the experiments at a temperature of 800°C and a pressure of 2.9 GPa, which correspond to the conditions of a hot subduction zone, the schist underwent partial (up to 10%) eclogitization with the formation of the anhydrous assemblage omphacite + garnet + quartz ± magnesite ± potassic phase. Carbonate and a potassic phase were formed only in the experiments with websterite in the upper layer. A reaction zone was formed at the base of the websterite layer, where newly formed omphacite, quartz, and orthopyroxene replaced in part initial pyroxenes. Orthopyroxene and phlogopite (or an unidentified potassic phase) were formed in the reaction zone at the base of the harzburgite layer; among the initial minerals, only orthopyroxene relicts were preserved. Above the reaction zones produced by diffusion metasomatism, new phases developed locally, mainly at grain boundaries: newly formed orthopyroxene and magnesite were observed in harzburgite, and omphacite and quartz, in websterite. Alterations along grain boundaries extended much further than the reaction zones, which indicates that fluid infiltration dominated over diffusion in the experiments. The experiments demonstrated that the H2O–CO2 fluid with dissolved major components released from the glaucophane schist can produce mineral assemblages of different chemical compositions in mantle materials: Na-bearing in websterite and K-bearing in harzburgite. The complementary components, K2O and CO2 for the websterite layer and Na2O for the harzburgite layer, are fixed in the initial glaucophane schist layer. The distinguished separation of alkalis and CO2 at the crust–mantle boundary can affect the character of metasomatism in the mantle wedge, primary magma compositions, and the chemical evolution of the rocks of the subducting slab.

Garnet single-grain analysis is an often used and well established tool in sedimentary provenance studies, especially when metamorphic source rocks are involved. So far, however, solely the geochemical composition of detrital garnets is considered to draw conclusions concerning probable source rocks. The gained information is often limited by (i) geochemical overlap of garnets derived from different lithology and metamorphic grade, (ii) similar probabilities of belonging to more than one source rock type, and (iii) the limitations of discriminating different protolith compositions. Here we present the first attempt of using mineral inclusions in detrital garnet as a provenance tool. We analyzed the inclusions of ~300 fine to medium sand-sized detrital garnets from two proximal modern sand samples taken in the HP/UHP Western Gneiss Region of SW Norway. All mineral inclusions ≥2 μm were identified by Raman spectroscopy, showing that (i) most garnets from HP/UHP metamorphic source rocks contain mineral inclusions ≥2 μm, (ii) Raman spectroscopy is a very powerful tool to characterize the inclusion types, and (iii) less stable mineral phases like kyanite, omphacite, diopside, enstatite, coesite, amphibole group, and epidote group minerals occur as inclusions in garnet. These minerals, which are important for provenance studies, can thus be preserved in the sedimentary record as long as garnet is stable. The combination of inclusion types in garnet and geochemical garnet classification shows that (i) inclusions well reflect the geological characteristics of the sampled catchments, implying that they are useful indicators for HP/UHP provenance, and (ii) inclusions in garnet can be used to support and enhance the provenance information obtained by garnet geochemistry.

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.

When plotting the representative FTIR spectra of garnet in Fig. 3d and e, an error was made with the thickness normalization, and the spectra were wrongly normalized. We reproduce the correct figure below.

Microinclusion-bearing diamonds offer the opportunity to investigate relationships between mantle metasomatism, diamond formation and kimberlite eruptions in intracratonic provinces. We have analyzed a suite of 7 microinclusion-bearing diamonds from the Finsch Group II kimberlite, South Africa, and identified two diamond populations: ‘Finsch IaA’ diamonds have nitrogen solely in A-centers and contain saline high-density-fluid (HDF) microinclusions, while ‘Finsch IaAB’ diamonds have nitrogen in both A- and B-centers (25–35% B-centers) and are characterized by carbonatite HDF compositions. Based on nitrogen aggregation states and estimates for mantle residence temperatures, we conclude that ‘Finsch IaA’ diamonds formed during a young saline metasomatic event that preceded kimberlite eruption by ~50 kyr to 15 Myr. The possible timing of metasomatism and formation of ‘Finsch IaAB’ diamonds by carbonatite HDFs is less constrained, and could have taken place between ~15 Myr and 2 Gyr before eruption. Two of the diamonds encapsulated omphacite microinclusions in association with saline or low-Mg carbonatitic-like HDF. We observe compositional differences for Al2O3 vs. CaO between these metasomatised omphacites, and also compared to omphacites in mantle eclogites which were identified as metasomatised by kimberlite or high-Mg carbonatite; suggesting a possible relationship between Al2O3 and CaO in metasomatised omphacite and the type of fluid/melt it interacted with. The combined data for microinclusion-bearing diamonds from the Finsch Group II kimberlite and the neighbouring Group I kimberlites at Koffiefontein and De Beers Pool indicate that a substantial volume of the southwest Kaapvaal deep lithosphere was impacted by saline metasomatism during Cretaceous time, and a direct relationship between saline metasomatism, diamond formation and the Kaapvaal late-Mesozoic ‘kimberlite bloom’. We therefore conclude that saline HDFs play a key role in the buildup of metasomatic mantle sources leading to kimberlite eruptions.