Ultrahigh-pressure Conditions Research Articles

Relic Oceanic Crust at Sub‐Arc Depth: An Example from UHP Eclogites Enclosed in Serpentinites from the Southwestern Tianshan, China

Ultra‐high pressure (UHP) eclogites that derive from subducted oceanic crust are rarely found at the Earth's surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation (Hermann et al., 2000; Guillot et al., 2001). Under normal subduction geotherms, serpentinites break down just before UHP conditions are reached and therefore most of the exhumed eclogites representing subducted oceanic crust formed under fore‐arc conditions. We investigated eclogite blocks enclosed into serpentinites that occur in the southwestern Tianshan oceanic subduction, China. A previous study proved that the serpentinites derive from altered oceanic crust and experienced UHP metamorphism at low temperatures of 510–530°C (Shen et al., 2015).Three relatively fresh eclogite samples were studied in detail. Sample 129–7 shows the retrograde mineral assemblage of amphibole + biotite + albite + chlorite + minor titanite and peak metamorphic relics of omphacite + garnet ± chlorite. Sample C107‐23 is mainly composed of amphibole + albite + chlorite + zoisite + muscovite + minor titanite as a retrograde assemblage and garnet + phengite as the peak metamorphic relics with omphacite only found as inclusions in garnet. Similar to sample C107‐23, sample C11066 preserves large‐grained euhedral to subhedral garnet relics with omphacite inclusions, and epidote, diopside, amphibole, muscovite, chlorite, albite and biotite are in the matrix belong to the retrograde assemblage. These three retrograde eclogite samples were modelled using thermodynamic calculations in the MnNCKFMSHO (MnO‐Na2O‐CaO‐K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O‐Fe2O3) system. Based on the peak assemblage of omphacite+garnet and the crossing of the grossular and pyrope isopleths in garnet, peak P‐T conditions of ∼460–470 °C, 28–29 kbar (129–7), 450–500 °C, 28–35 kbar (C107–23), ∼475–505 °C, 26–29 kbar (C11066) were calculated. The retrograde assemblages indicate near isothermal decompression resulting in a clockwise P‐T evolution of these eclogites. The peak metamorphic pressures at 500°C are well within UHP conditions (coesite stability field) and are within error the same as peak conditions of the host serpentinites (Shen et al., 2015). This provides evidence that eclogites and serpentinites shared the same evolution. We infer that the subducted low‐density serpentinites were assembled with the high‐density eclogites during subdution and helped the latter to exhume back to the surface. The studied eclogites thus represent rare examples of relics of oceanic crust that was subducted to sub‐arc depth.

High-pressure rotational deformation apparatus to 135 GPa.

A large-strain, torsional deformation apparatus has been developed based on diamond anvil cells at high pressures, up to 135 GPa with a help of hard nano-polycrystalline diamond anvils. These pressure conditions correspond to the base of the Earth's mantle. An X-ray laminography technique is introduced for high-pressure in situ 3D observations of the strain markers. The technique developed in this study introduces the possibility of the in situ rheological measurements of the deep Earth materials under ultrahigh-pressure conditions.

Modification of an ancient subcontinental lithospheric mantle by continental subduction: Insight from the Maowu garnet peridotites in the Dabie UHP belt, eastern China

Orogenic mantle-derived peridotites commonly originate from the subcontinental lithospheric mantle (SCLM) and thus provide a key target to investigate the modification of the SCLM by a subducting slab. The Maowu ultramafic rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt have formerly been debated as representing cumulates or mantle-derived peridotites. Detailed petrological and geochemical data presented in this study provide new constraints on the origin and formation of the peridotites involving melt depletion in the ancient SCLM and deep crustal metasomatism. The Maowu garnet dunites have refractory bulk compositions characterized by high Mg# (91.9–92.0) and Ni (2537–2892ppm) values and low Al2O3 (0.26–0.76wt.%), CaO (0.05–0.32wt.%), TiO2 (<0.03wt.%), Pd/Ir (0.40–0.46) and 187Os/188Os (minimum 0.11461) values. The Paleoproterozoic model ages (TRD=2.1Ga, TMA=2.3Ga) of the most refractory dunites represent minimum estimates for the age of the initial melt extraction. The extremely depleted nature, high olivine Fo (92.7–93.9), high Cr# (82–87) of spinel, and Re–Os isotopic data suggest that the Maowu garnet dunites are the residues of ~40% partial melting and represent a Paleoproterozoic fragment of the SCLM beneath the southeastern margin of the North China craton. Many garnet orthopyroxenite veins crosscutting the Maowu dunites preserve abundant metasomatic textures and show variable enrichment in incompatible elements. Mineral and whole-rock chemistry indicate that these veins represent metasomatic products between the wall dunites and silica-rich hydrous melts under UHP conditions. The veins show large variations in platinum-group element (PGE) signatures and Re–Os isotopes. The garnet-poor orthopyroxenite veins are characterized by low Al2O3 (<2wt.%) and S (<31ppm) contents and have PGE patterns and 187Os/188Os ratios similar to the wall dunites, whereas the garnet-rich orthopyroxenite veins have high Al2O3 (>6wt.%) and S (99–306ppm) contents and show melt-like PGE patterns and high 187Os/188Os ratios (up to 0.36910). These features, combined with the occurrence of interstitial sulfides in the garnet-rich orthopyroxenite veins, suggest that crust-derived sulfur-saturated silicate melts may have significantly modified the PGE signature and destroyed the Re–Os systematics of the SCLM. However, when the crust-derived silicate melts became sulfur-depleted, such melts would not significantly modify the PGE patterns, radiogenic Os-isotope compositions or the Re-depletion model ages of the SCLM. Consequently, deep crust-mantle interactions in continental subduction zones could induce high degrees of Os isotopic heterogeneity in the SCLM wedge.

The Case for UHP Conditions in the Cuaba Terrane, Río San Juan Metamorphic Complex, Dominican Republic

This paper is essentially a review of previously published data and interpretations for ultramafic and eclogitic rocks from the Cuaba terrane in northern Dominican Republic. Ultrahigh pressure (UHP) conditions are indicated for the Cuaba terrane on the basis of phase relationships in garnet-bearing ultramafic rock. Dikes and orthocumulate textures indicate a magmatic origin. Mineral assemblages define a line of descent controlled by fractional crystallization. The original estimate of the magmatic conditions (P>3.4GPa, T>1550°C) was inferred previously from available high-P melting experiments in the CMAS system and high-P experimental determination of the sapphirine-out reaction in the MAS system. Revised estimates of magmatic conditions (P>3.2GPa, T>1500°C) take into account the influence of other components, especially Fe. We propose an origin in the mantle-wedge above a subduction zone. The rock was delivered to the subduction zone by forced convection in the mantle wedge (corner-flow), coupled with erosion of the hanging wall. Thermobarometry indicates >850°C and >3.4GPa when the ultramafic rock was incorporated into eclogite (deep-subducted oceanic crust). Evidence for UHP conditions in the retrograded eclogite is not obvious. Two types of symplectic intergrowths, plagioclase + clinopyroxene (Sym-I) and plagioclase + epidote (Sym-II), are interpreted as the products of the decomposition of two types of omphacite, Omp-I and Omp-II. Theoretically, Omp-II formed as the result of a retrograde reaction of the form, Omp-II + coesite = Omp-I + kyanite + /- garnet, according to which the maximum pressure for Omp-II is between ~2.8GPa (~850°C) and ~4.2GPa (~950°C), consistent with subsolidus conditions for the garnet-bearing ultramafic rocks. For eclogite, the highest-pressure mineral assemblage would have been Omp-I + kyanite + garnet + coesite.

Total synthesis of (-)-aritasone via the ultra-high pressure hetero-Diels-Alder dimerisation of (-)-pinocarvone.

This paper describes a total synthesis of the terpene-derived natural product aritasone via the hetero-Diels-Alder [4 + 2] cyclodimerisation of pinocarvove, which represents the proposed biosyntheic route. The hetero-Diels-Alder dimerisation of pinocarvone did not proceed under standard conditions, and ultra-high pressure (19.9 kbar) was required. As it seems unlikely that these ultra-high pressures are accessible within a plant cell, we suggest that the original biosynthetic hypothesis be reconsidered, and alternatives are discussed.

MG-CR-TYPE SPINEL PERIDOTITES IN THE WESTERN PART OF THE CENTRAL ASIAN OROGENIC BELT (ZHELTAU MASSIF, SOUTHERN KAZAKHSTAN): THE FIRST DATA ON P-T PATHS AND PROTOLITHS

Ultramafic and mafic lithologies, attributed to the orogenic terranes and formed under ultrahigh-pressure (UHP) and high-pressure (HP) conditions, have been intensively studied for the last decades. It is mainly related to a particular significance of these rocks for geodynamics, since they contain an important information on the fluid-rock interactions and element redistribution in the subduction-collision zones and could shed the light on the tectonic evolution of the studied region.

Ultrahigh pressure-assisted enzymatic extraction maximizes the yield of longan pulp polysaccharides and their acetylcholinesterase inhibitory activity in vitro

An extraction method employing ultrahigh pressure-assisted enzymatic treatment was developed and optimized by response surface methodology to increase the yield of longan pulp polysaccharides (LP-UE). A maximum polysaccharides yield of 8.55% was obtained under the optimal conditions of 407MPa ultrahigh pressure maintained for 6min with an enzyme to pretreated material ratio of 1:100, an enzymolysis time of 1.7h and a water to pretreated material ratio of 42ml/g. Subsequently, the physicochemical properties and acetylcholinesterase (AChE) inhibitory activity of LP-UE were compared to those of longan pulp polysaccharides (LP) extracted by hot water (LP-H), ultrahigh pressure (LP-U) or enzymatic treatment (LP-E). Results demonstrated that the extraction yield, hexuronic acid content and AChE inhibitory activity of LP-UE was the highest among the four LP samples. LP-UE was primarily made up of arabinose, glucose, and galactose and was linked mainly by β-type glycosidic linkage. The FTIR spectrum of LP-UE was very similar to those of LP-H, LP-U, and LP-E. In summary, ultrahigh pressure-assisted enzymatic treatment is a more efficient technique for extracting LP with considerable improvement of both yield and memory enhancement function.

Comment on “Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications”. Tectonophysics, 662, 219–242 (2015) by Ishwar-Kumar et al.

Determining the possible tectonic regimes active during the Neoproterozoic is crucial for the knowledge of the evolution of the super-continent Gondwana. In Madagascar, that occupies a key position in Gondwana, there is an on-going debate regarding the location of possible suture zones and the implications in terms of paleo-geography. Recognizing high-pressure to ultra-high pressure conditions in mafic rocks is commonly viewed as a strong argument for paleo-subduction zones. Ishwar-Kumar et al. (2015) report unusual high pressure conditions (24kbar) in Neoproterozoic to Cambrian rocks from North-Central Madagascar (Andriamena Complex). They propose a geodynamic model in which exhumation of the high pressure terranes from up to ~80km to ~40km occurred via vertical extrusion during the collision of various crustal blocks after subduction and closure of an oceanic domain during the formation of Gondwana in the late Neoproterozoic to Cambrian. We question this model and in particular the (ultra-)high pressure conditions reported, because their estimation is based on a misinterpretation of the petrography and inaccurate thermodynamic modeling for the crucial metabasite sample. The authors suggest that garnet-quartz coronas around orthopyroxene and ilmenite coexist with clinopyroxene. The postulated garnet-clinopyroxene-quartz assemblage is interpreted to document an eclogite facies overprint. However, the presence of abundant plagioclase in the sample and the lack of high jadeite content in clinopyroxene clearly refute the postulated eclogite facies conditions. According to the presented photographs clinopyroxene is part of the rock matrix. We therefore suggest that the sample represents a common two-pyroxene granulite, formed at mid- to low-pressure granulite facies conditions of >700°C and <6kbar, consistent with PT data of former studies for the Andriamena Complex. Garnet-quartz-bearing coronas produced at the expense of the granulite-facies assemblage could have been produced during isobaric cooling. Although a subduction zone may have been active during this period of time in Madagascar, this interpretation is not justified by the data presented by Ishwar-Kumar et al. (2015).

Timing of eclogite-facies metamorphism of mafic and ultramafic rocks from the Pohorje Mountains (Eastern Alps, Slovenia) based on Lu–Hf garnet geochronometry

The metamorphic series of the Pohorje Mountains represents a part of the Eastern Alpine realm that was subjected to ultrahigh-pressure conditions during the Cretaceous Eo-Alpine orogenic cycle. The Slovenska Bistrica Ultramafic Complex located in the south-eastern Pohorje Mountains is an 8km wide serpentinite body that contains lenses of garnet-bearing ultramafites and eclogites. It is embedded in and part of a mixed continental unit of metapelitic gneisses, orthogneisses, and eclogites. We present Lu–Hf garnet chronometry coupled with geochemical and petrological data from three samples: one garnet lherzolite, one eclogite from within the ultramafic complex, and one eclogite from the surrounding mixed unit. All obtained ages are identical within error, i.e. 96.6±1.2Ma and 94.8±5.1Ma, respectively, for the two eclogites and 91.6±4.1Ma for the garnet lherzolite. Garnet of all samples shows homogeneous concentrations of major bivalent elements due to high temperature re-equilibration. It does, however, preserve growth-related zoning with respect to Lu in all three samples implying that Lu–Hf ages still record garnet growth. The coincidence of ages suggests that the ultramafic complex and the surrounding continental mixed unit share the same subduction history, i.e. the complex was part of the subducting plate during and after the garnet growth stages.

High temperature glacial meltwater–rock reaction in the Neoproterozoic: Evidence from zircon in-situ oxygen isotopes in granitic gneiss from the Sulu orogen

To seek the occurrence of negative δ18O magmas in the Neoproterozoic, we conducted in-situ zircon O isotope analysis and U–Pb dating for granitic gneiss from the northeastern Sulu orogen, east-central China. Zircon U–Pb dating yields protolith ages of 753±15Ma to 780±13Ma and metamorphic ages of 209±3 to 244±7Ma. The Neoproterozoic cores with concordant U–Pb ages exhibit a wide δ18O range from −11.0‰ to 5.8‰, which is nearly the same as those for cores with discordant U–Pb ages. The Triassic rims of some samples have homogeneous δ18O values of around −10‰ whereas the rims of the other samples show a wider range from −9.8‰ to 5.0‰. The δ18O values as negative as −11.0‰ for zircons with concordant Neoproterozoic U–Pb ages are reported for the first time, representing the primary record of negative δ18O magma in the Neoproterozoic. The continental subduction-zone metamorphism in the Triassic did not erase the abnormal δ18O record in the protolith zircon cores despite metamorphic dehydration and partial melting under high-pressure to ultrahigh-pressure conditions. A conservative estimate suggests that the hydrothermal fluid reacted with the rocks should have δ18O values lower than −9.2‰, corresponding to the meteoric water in cold paleoclimate or the meltwater of local continental glaciation. The spatial variation in the O isotope compositions of Neoproterozoic zircons is a manifestation of the O isotope heterogeneity in the extinct hydrothermal-magmatic system. The hydrothermal alteration during the Neoproterozoic was incongruent, which was lately recorded by the wide range of δ18O values in the metamorphic zircons of Triassic age. The extensive O isotope exchange between the surface water and the deep rock requires high temperature and high water–rock ratios in continental rifting zones. This is ascribed to Neoproterozoic splitting of the South China Block from the Rodinia supercontinent.

Two episodes of partial melting in ultrahigh-pressure migmatites from deeply subducted continental crust in the Sulu orogen, China

Partial melting plays an important role in the geodynamics of continental subduction zones. This is identified from a suite of ultrahigh-pressure (UHP) migmatites in the Sulu orogen through a combined study of zircon U-Pb ages, trace elements, and oxygen isotopes, as well as rock-forming mineral and inclusion compositions. The results indicate two episodes of partial melting in the subducted continental crust during continental collision, providing insights into subduction channel processes. The first episode of anatexis is indicated by the occurrence of nanogranites, not only in zircon, garnet, and monazite from diatexite, but also in zircon cores from leucosome. The anatectic zircon exhibits U-Pb ages of 230−227 Ma and flat rare earth element (REE) patterns with weak or no negative Eu anomalies, and it contains mineral inclusions of coesite and garnet + amphibole. Newly grown zircon grains in the diatexite and zircon cores in the leucosome exhibit high δ 18 O values of 8.3‰−17.3‰, indicating a metasedimentary protolith. The host rocks show high A/CNK (= molar ratio of Al 2 O 3 /[CaO + Na 2 O + K 2 O]) values and the occurrence of peritectic garnet in the diatexite. Thus, the diatexite was produced by partial melting of metasedimentary rocks. The Ti-in-zircon thermometry, the garnet-phengite Fe-Mg partition thermometry for mineral inclusions in the zircon, and the occurrence of coesite inclusions in zircon indicate partial melting at 650−800 °C and 2.5−3.0 GPa, corresponding to high-pressure (HP) to UHP conditions. On the other hand, the second episode of anatexis is recorded by newly grown zircon grains in metatexite and zircon rims in leucosome, which show U-Pb ages of 218−214 Ma, oscillatory zoning, steep heavy (H) REE patterns with negative Eu anomalies, low temperatures of 550−700 °C, and significant variations in Th, U, Nb, and Ta contents. The Zr-in-titanite thermometry for nanogranite-bearing titanite and the garnet-phengite Fe-Mg partition thermometry for mineral inclusions in the leucosome zircon rims indicate anatexis at 800−850 °C and 1.0−1.5 GPa. The zircon in the metatexite exhibits low δ 18 O values of −1.5‰−3.5‰ and Neoproterozoic U-Pb ages for relict magmatic cores, indicating the protolith of a low δ 18 O UHP metagranite. The two episodes of anatexis yield zircon domains with a series of differences not only in U-Pb age, but also in geochemical composition. Thus, protoliths with different origins were involved in the anatexis, with a possible difference in spatial positions. The UHP metasedimentary rocks atop the deeply subducted continental crust would have undergone the first episode of anatexis during the final subduction, whereas their underlying metagranite would have undergone the second episode of anatexis during the exhumation of deeply subducted crust. In either case, the breakdown of UHP hydrous minerals during exhumation is the key for the partial melting of UHP metamorphic rocks in the continental subduction channel.

Argon, oxygen, and boron isotopic evidence documenting 40ArE accumulation in phengite during water-rich high-pressure subduction metasomatism of continental crust

The Luliang Shan area of the North Qaidam high pressure (HP) to ultrahigh pressure (UHP) metamorphic terrane in northwestern China features thick, garnet- and phengite-rich metasomatic selvages that formed around gneiss-hosted mafic eclogite blocks during HP conditions. Here we present new 40Ar/39Ar, δ18O, and δ11B results from a previously studied 30 m, 18 sample traverse that extends from the host gneiss into a representative eclogite block. Previous thermobarometry and new mica-quartz oxygen isotope thermometry from the traverse reveal that the phengite-rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure. Quartz and white mica δ18O data from the selvage cannot be explained by simple mixing of gneiss and eclogite, and indicate a fluid/rock ratio >1 during regional-scale infiltration of high δ18O (ca. 14‰) fluids. Heavy δ18O overgrowths of metamorphic zircon over lighter δ18O detrital grains indicate that the gneiss was similarly affected. Starkly contrasting boron content and δ11B compositions for the host gneiss and the selvage also cannot be explained by local-scale devolatilization of the gneiss to form the selvage. Instead, the boron systematics are best attributed to two distinct phases of fluid infiltration: (1) low-boron selvage phengite with δ11B from −10 to −30‰ grew under HP conditions; and (2) tourmaline and boron-rich muscovite with generally positive δ11B crystallized in the host gneiss under subsequent lower pressure epidote–amphibolite facies conditions as the Luliang Shan gneiss terrane was exhumed past shallower portions of the subduction channel. Consistent with observations made worldwide, we were able to identify uptake of excess argon (40ArE) in phengite as a high pressure phenomenon. Phengite 40Ar/39Ar ages from massive eclogite exceed the ca. 490 Ma zircon U–Pb age of eclogite metamorphism by a factor of 1.5. However, phengite ages from the more permeable schistose selvage were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40Ar/39Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40ArE concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low phengite/fluid partition coefficients for argon (Dphg/fluidAr=10−3to10−5) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40ArE partitions are able to accumulate within phengite. We alternatively propose that phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10−2 to allow phengites to accumulate large amounts of 40ArE from aqueous fluids under HP to UHP conditions.

Propagation characteristics of induced shock waves generated by diesel spray under ultra-high injection pressure

The phenomenon of shock waves can be commonly seen in fuel spray under ultra-high injection pressure conditions, where the spray jet penetrates at speeds greater than Mach 1. This paper focuses on the induction of shock waves and their propagation characteristics generated by a supersonic fuel jet at 400MPa, and simultaneously considers the impact of jet atomization. A specially designed atomizer, utilizing diaphragm rupture, was adopted to replace the traditional electronic valves fuel system to meet the ultra-high injection pressure. Visualization of the spray field by using a high-speed camera was conducted, and a Schlieren apparatus was equipped for shock capturing based on a variable environmental density distribution. The experimental results indicated that the leading edge shock wave shows two forms in the early stage, and each penetrates in a unique way. Along with the spray development, multiple expansion waves were sequentially generated, followed by the leading shock wave. The evolution of wave velocity over time was calculated from the penetration results, and the results showed that the velocity first increased rapidly and then slowly decreased as a result of the effect of air resistance, with a gradual tendency of decreasing for the four peak velocities of the intensity weakened waves. In addition, the spray convex flow path had an inhibitive effect on the deceleration of the first expansion wave. These results give an insight into diesel spray and propagation process of induced shock waves at an under ultra-high injection pressure of 400MPa.

ContrastingP-Tpaths within the Barchi-Kol UHP terrain (Kokchetav Complex): Implications for subduction and exhumation of continental crust

![Figure][1] The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950–1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800–900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900–950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. U-Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528–521 Ma), peak (528–522 Ma), and peak to retrograde metamorphism (503–532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20–25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models. [1]: pending:yes

Ultrahigh-pressure polyamorphism in GeO2 glass with coordination number >6

Knowledge of pressure-induced structural changes in glasses is important in various scientific fields as well as in engineering and industry. However, polyamorphism in glasses under high pressure remains poorly understood because of experimental challenges. Here we report new experimental findings of ultrahigh-pressure polyamorphism in GeO2 glass, investigated using a newly developed double-stage large-volume cell. The Ge-O coordination number (CN) is found to remain constant at ∼6 between 22.6 and 37.9 GPa. At higher pressures, CN begins to increase rapidly and reaches 7.4 at 91.7 GPa. This transformation begins when the oxygen-packing fraction in GeO2 glass is close to the maximal dense-packing state (the Kepler conjecture = ∼0.74), which provides new insights into structural changes in network-forming glasses and liquids with CN higher than 6 at ultrahigh-pressure conditions.

Blueschist- to eclogite-facies rocks: from HP to UHP

Impressed by the beauty of a metamorphic rock from the Saualpe, Austria, Rene-Just Hauy (1743–1822) created the name “eclogite” (the chosen rock), which he derived from the Greek word eκλογή (choice). The name was used for the first time in 1822 in the second edition of Hauy’s “Traite de mineralogie”. However, the first “scientific” description dates back to the second half of the 18th century, when Horace-Benedict de Saussure (1740–1799) mentioned a “beautiful rock that is not described yet” from the Rhone Valley in his famous book “Voyages dans les Alpes”. Eclogite is a metabasic rock dominated by the minerals omphacite and garnet, and in the 20th century eclogite was recognized as a high-pressure (HP) metamorphic rock. Concurrently, it was discovered that metabasic blueschist, dominated by blue sodium-rich amphiboles, represented another HP metamorphic rock formed at lower metamorphic grade than eclogite. Then, in the mid nineteen-eighties, metamorphic coesite (and somewhat later even diamond) was discovered in eclogite-facies rocks, indicating pressures even higher than had been considered possible up to that time. The term “ultrahigh-pressure (UHP) metamorphism” was born. The term describes metamorphism at depths corresponding to pressure–temperature conditions exceeding the lower limit of the coesite stability field. The significance of UHP metamorphism is most striking for rocks that initially formed in the crust and were then subducted to UHP conditions, but our understanding of mantle processes has been enriched through study of mantle-derived garnet peridotites at UHP conditions as well. These findings generated numerous new research projects with enormous implications for petrology, geodynamics, global tectonics, seismology, and geochemical recycling. This special issue contains a selection of contributions which were presented at the 11th International Eclogite Conference (IEC), in accordance with the theme “Blueschist-to eclogite-facies rocks: from HP to UHP”. The meeting took place near Rio San Juan …

Tracking the incidence of excess argon in white mica Ar–Ar data from UHP conditions to upper crustal levels in the Dora-Maira Massif, Western Alps

The age of peak-metamorphic conditions of the ultrahigh-pressure ( UHP ) metamorphic rock sequence constituting the Dora-Maira Massif is well established at 35 Ma. In order to understand the behaviour of excess argon during sequential pressure–temperature stages of exhumation from >3.5 to less than 0.5 GPa, distinct generations of chemically and petrographically well-characterized white mica were studied with the multigrain, stepwise-heating 40Ar/39Ar dating technique. Mica separates were obtained from boudins of coesite-bearing UHP pyrope quartzite and phengite schist enclosed within them, from retrograde assemblages evolving from the pyrope quartzite, and from biotite–phengite orthogneiss surrounding the boudins. Assuming the 35-Ma age as a benchmark, it is possible to reconstruct directly the quantity of excess argon during these various stages of the exhumation history. The amount of excess Ar roughly correlates with the Si content of phengite, which is an indicator for the pressure at which phengite incorporated excess Ar. All mica samples crystallized at pressures >1.0 GPa exhibit significant excess argon and aberrant ages significantly higher than 35 Ma. Between 1.0 and 0.5 GPa excess Ar diminishes and realistic ages are approached. Phengite (Si3.55) separates from two localities of fresh coesite-bearing pyrope quartzite equilibrated at ca . 3.8 GPa show similar U-shaped spectra with high apparent ages in the first or second degassing steps, dropping to a plateau-like value of 71.0±1.1 Ma in one case and yielding an apparently trustworthy plateau age of 103.4±1.0 Ma in the second. Retrograde phengite generations from pyrope quartzite crystallized between 3.2 and 1.5 GPa also display a U-shaped spectrum with a broad depression corresponding to a minimum age of 96.9±1.6 Ma. Retrograde phlogopite, representing part of the breakdown assemblage of pyrope and formed between 1.9 and 1.2 GPa, documents several sharp steps in age values from 548 Ma towards 850 Ma, whereas further degassing steps are characterized by a nearly linear increase of age values from 852 to 973 Ma. A plateau age of 56.3±0.9 Ma was calculated for phengite from a phengite-schist inclusion within pyrope quartzite. The spectra of phengite from the country-rock biotite–phengite gneiss surrounding the pyrope quartzite boudins are hump-shaped. The total degassing ages decrease with decreasing grain size (and decreasing metamorphic pressure from 1.6 to 0.3 GPa, as signalled by a concomitant decrease of Si per formula unit) from 43.25±0.60 Ma (1–2 mm) to relatively realistic values of 33.50±0.50 Ma (90–180 μm) and 31.55±0.50 Ma (<100 μm). The latter age correlates with very low calculated excess argon values. The data demonstrate that even retrograde micas are affected by excess argon, and that “ages” derived from inverse isochron diagrams (36Ar/40Ar vs . 39Ar/40Ar) as well as “perfect” plateau ages need to be treated with caution and constrained by other geochronological systems.

Late Cretaceous UHP metamorphism recorded in kyanite–garnet schists from the Central Rhodope Mountains, Bulgaria

In this study, we report the first discovery of microdiamond inclusions in kyanite–garnet schists from the Central Rhodope Mountains in Bulgaria. These inclusions occur in garnets from metapelites that are part of a meta-igneous and meta-sedimentary mélange hosted by Variscan (Hercynian) orthogneiss. Ultra-high-pressure (UHP) conditions are further supported by the presence of exsolved needles of quartz and rutile in the garnet and by geothermobarometry estimates that suggest peak metamorphic temperatures of 750–800°C and pressures in excess of 4GPa. The discovery of UHP conditions in the Central Rhodopes of Bulgaria compliments the well-documented evidence for such conditions in the southernmost (Greek) part of the Rhodope Massif. Dating of garnets from these UHP metapelites (Chepelare Shear Zone) using Sm–Nd geochronology indicates a Late Cretaceous age (70.5–92.7Ma) for the UHP metamorphic event. This is significantly younger than previously reported ages and suggests that the UHP conditions are associated with the Late Mesozoic subduction of the Vardar Ocean northward beneath the Moesian platform (Europe). The present-day structure of the RM is the result of a series of subduction–exhumation events that span the Cenozoic, alongside subsequent post-orogenic extension and metamorphic core complex formation.

Kumdykolite, kokchetavite, and cristobalite crystallized in nanogranites from felsic granulites, Orlica-Snieznik Dome (Bohemian Massif): not evidence for ultrahigh-pressure conditions

A unique assemblage including kumdykolite and kokchetavite, polymorphs of albite and K-feldspar, respectively, together with cristobalite, micas, and calcite has been identified in high-pressure granulites of the Orlica-Snieznik dome (Bohemian Massif) as the product of partial melt crystallization in preserved nanogranites. Previous reports of both kumdykolite and kokchetavite in natural rocks are mainly from samples that passed through the diamond stability field. However, because the maximum pressure recorded in these host rocks is <3 GPa, our observations indicate that high pressure is not required for the formation of kumdykolite and kokchetavite, and their presence is not therefore an indicator of ultrahigh-pressure conditions. Detailed microstructural and microchemical investigation of these inclusions indicates that such phases should instead be regarded as (1) a direct mineralogical criteria to identify former melt inclusions with preserved original compositions, including H2O and CO2 contents and (2) indicators of rapid cooling of the host rocks. Thus, the present study provides novel criteria for the interpretation of melt inclusions in natural rocks and allows a more rigorous characterization of partial melts during deep subduction to mantle depth as well as their behavior on exhumation.

Characterization of natural carbon particles formed at low temperature UHP conditions

Micro-to-nanosized carbon particles forming symmetrically oriented trails in ultrahigh pressure (UHP) garnet have been studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Micrometer-sized particles include two distinct morphological types: minor rod-shaped particles, and abundant equidimensional particles, interpreted on the basis of the Raman spectra, as graphite and diamond, respectively. The graphite spectra show structural disorder greater than that found in graphite of other UHP rocks. Most diamond spectra shows additional Raman features due to sp2 carbon phases and weaker and broader Raman bands than metamorphic diamond. The SEM images reveal clear topotactic relationships between garnet, micrometer-sized rods of graphite and equidimensional particles of diamond. The TEM study shows that garnet also contains multiwall nanotubes, nanocrystalline diamond particles and polyhedral carbon+silicate structures. The morphology and the orientation relationships between garnet and micro-to-nanometer graphite-like particles are compatible with formation through co-precipitation of garnet and metastable carbon allotropes. It is suggested that precipitation of disordered sp2 carbon can act as a precursor for diamond formation within the stability field of diamond. The presented data reveal the importance that the metastable carbon structures can have on the formation of diamond.