Isobaric Heating Research Articles

Local isobaric heating above an extensional detachment in the middle crust of a Variscan allochthonous terrane (Órdenes complex, NW Spain)

In one of the most studied Variscan exposures, the Ordenes allochthonous complex (NW Spain), the transition between medium-pressure (MP) and high-pressure (HP) units in the SW of the complex has been identified as an extensional shear zone: the Fornas detachment. Migmatitic paragneisses crop out discontinuously along that boundary, at the base of the MP ensemble (O Pino unit). The metamorphic reaction sequence, mass balance calculations, and phase diagram modeling investigated in these paragneisses are interpreted in terms of an approximately isobaric heating path (8 ± 0.8 kbar), from ∼650 °C to 740 °C, crossing into the melt- and K-feldspar–bearing stability fields. These anatectic conditions are evidenced by the presence of leucosomes through progressive muscovite and biotite melting reactions. Our results indicate that the heating path evidenced by the migmatitic paragneisses is directly related to the subtractive nature of the Fornas detachment, with heat transferred from the footwall to the hanging-wall unit.

Coupled phengite40Ar–39Ar geochronology and thermobarometry:P-T-tevolution of Andros Island (Cyclades, Greece)

AbstractAndros is a key island for understanding both the timing of high-pressure–low-temperature (HP-LT) metamorphism and the dynamics of crustal-scale detachment systems exhuming high-grade units in the Cyclades (Greece). Using phengite40Ar–39Ar geochronology coupled with thermobarometry, as well as data from literature, we constrain the pressure–temperature–time (P-T-t) paths of the Makrotantalon and Attic–Cycladic Blueschist units on Andros. Peak conditions of the HP-LT episode in the Makrotantalon unit are 550°C and 18.5 kbar, dated at 116 Ma. We correlate this episode with Early Cretaceous blueschist facies metamorphism recognized in the Pelagonian zone of continental Greece. This is a new argument favouring a Pelagonian origin for the Makrotantalon unit. In the Attic–Cycladic Blueschist unit, theP-T-tpath is characterized by: (1) exhumation after peak conditions in HP-LT conditions between 55 and 35 Ma; (2) isobaric heating at 7 kbar until 30 Ma; and (3) isothermal decompression until 21 Ma. This thermal evolution and timing are similar to those of the neighbouring Tinos Island, emphasizing major thermal re-equilibration at the transition between stable and retreating subduction. Modifications of the crustal thermal state played a major role in the evolution of the North Cycladic Detachment System, below which Andros HP-LT units were exhumed.

Two-stage granulite formation in a Proterozoic magmatic arc (Ongole domain of the Eastern Ghats Belt, India): Part 1. Petrology and pressure–temperature evolution

The Ongole domain of the Proterozoic Eastern Ghats Belt, India, is dominated by charnockites and enderbites with enclaves of migmatitic metapelites. Reaction textures indicate two generations of mineral growth. Spinel–hercynite solid solution+quartz-bearing Fe–Al granulites provide critical evidence for an ultrahigh-temperature (UHT) metamorphism. The association of spinel and quartz, now separated by garnet and sillimanite coronas, suggests that the rocks underwent T>950°C peak metamorphism at P=6.5–7kbar, as constrained from P–T pseudosection. Coarse-grained orthopyroxene in garnet+cordierite-bearing metapelites shows the highest Al2O3 content up to 8.2wt.%, suggesting T=950–1000°C. Temperatures estimated from mesoperthite and plagioclase pairs and other geothermometers (T=900–1000°C) further support ultrahigh-temperature metamorphism in the Ongole domain. The absence of cordierite in retrograde mineral assemblages points to isobaric cooling, giving rise to a near isobaric heating–cooling trajectory. The second generation of garnet, occurring as thick overgrowths over coarse-grained garnet porphyroblasts, in metapelites and especially in charnoenderbites have higher grossular contents than the porphyroblasts. Thermobarometry and P–T pseudosections indicate that they formed at a higher pressure but lower temperature (ca. 780°C, 9.5kbar) during a second metamorphic event. Orthopyroxene+sillimanite±kyanite±spinel symplectites replacing cordierite is most likely formed during this second metamorphic event. The presence of sillimanite, kyanite as well as andalusite along with coarse-grained retrograde biotite in the same rock marks the last stage of the second metamorphic event characterized by isobaric cooling just below the aluminosilicate triple point. This further suggests near-isothermal decompression from higher pressures. UHT metamorphism at low pressures during the first metamorphic event is most likely caused by magma emplacement. The higher pressure but lower temperature conditions achieved during the second metamorphic event is due to crustal thickening during collision of continental blocks. Near-isothermal decompression to ca. 4kbar points to subsequent rapid exhumation of the over thickened crust. The two-stage evolutionary history of the Ongole domain fits well to the expected evolution of a magmatic arc, in which the first can be attributed to magmatic heat advection during arc growth and the second to crustal thickening during collision.

High pressure-temperature polymorphism of 1,1-diamino-2,2-dinitroethylene

1,1-diamino-2,2-dinitroethylene (FOX-7) is a low sensitivity energetic material with performance comparable to commonly used secondary explosives such as RDX and HMX. At ambient pressure, FOX-7 exhibits complex polymorphism with at least three structurally distinct phases (α, β, and γ). In this study, we have investigated the high pressure-temperature stability of FOX-7 polymorphs using synchrotron mid-infrared (MIR) spectroscopy. At ambient pressure, our MIR spectra and corresponding differential scanning calorimetry (DSC) measurements confirmed the known α → β (~110 °C) and α → β (~160 °C) structural phase transitions; as well as, indicated an additional transition γ → (~210 °C), with the δ phase being stable up to ~251 °C prior to decomposition. In situ MIR spectra obtained during isobaric heating at 0.9 GPa, revealed a potential α → β transition that could occur as early as 180 °C, while β → β+δ phase transition shifted to ~300 °C with suppression of γ phase. Decomposition was observed slightly above 325 °C at 0.9 GPa.

Heat transfer in supercritical water under impulse isobaric heating

Heat transfer in supercritical water under impulse isobaric heating

Inverse melting in a two-dimensional off-lattice model.

We carry out computer simulations of a simple, two-dimensional off-lattice model that exhibits inverse melting. The monodisperse system comprises core-softened disks interacting through a repulsive square shoulder located inside an attractive square well. By systematically varying the potential parameters, we increase the pressure range over which the liquid freezes to a crystal upon isobaric heating. The effect is largely controlled by the extent of the shoulder. Despite occurring in two dimensions, the melting transition is first order and to a liquid, rather than to a hexatic or quasicrystal phase. We also provide comment on a commonly employed correlation function used to determine the degree of translational ordering in a system.

Heating-induced glass-glass and glass-liquid transformations in computer simulations of water

Water exists in at least two families of glassy states, broadly categorized as the low-density (LDA) and high-density amorphous ice (HDA). Remarkably, LDA and HDA can be reversibly interconverted via appropriate thermodynamic paths, such as isothermal compression and isobaric heating, exhibiting first-order-like phase transitions. We perform out-of-equilibrium molecular dynamics simulations of glassy water using the ST2 model to study the evolution of LDA and HDA upon isobaric heating. Depending on pressure, glass-to-glass, glass-to-crystal, glass-to-vapor, as well as glass-to-liquid transformations are found. Specifically, heating LDA results in the following transformations, with increasing heating pressures: (i) LDA-to-vapor (sublimation), (ii) LDA-to-liquid (glass transition), (iii) LDA-to-HDA-to-liquid, (iv) LDA-to-HDA-to-liquid-to-crystal, and (v) LDA-to-HDA-to-crystal. Similarly, heating HDA results in the following transformations, with decreasing heating pressures: (a) HDA-to-crystal, (b) HDA-to-liquid-to-crystal, (c) HDA-to-liquid (glass transition), (d) HDA-to-LDA-to-liquid, and (e) HDA-to-LDA-to-vapor. A more complex sequence may be possible using lower heating rates. For each of these transformations, we determine the corresponding transformation temperature as function of pressure, and provide a P-T "phase diagram" for glassy water based on isobaric heating. Our results for isobaric heating dovetail with the LDA-HDA transformations reported for ST2 glassy water based on isothermal compression/decompression processes [Chiu et al., J. Chem. Phys. 139, 184504 (2013)]. The resulting phase diagram is consistent with the liquid-liquid phase transition hypothesis. At the same time, the glass phase diagram is sensitive to sample preparation, such as heating or compression rates. Interestingly, at least for the rates explored, our results suggest that the LDA-to-liquid (HDA-to-liquid) and LDA-to-HDA (HDA-to-LDA) transformation lines on heating are related, both being associated with the limit of kinetic stability of LDA (HDA).

Ultrahigh-temperature metamorphism under isobaric heating: New evidence from the North China Craton

The Khondalite Belt within Inner Mongolia Suture Zone (IMSZ) in the North China Craton (NCC) preserves evidence for extreme crustal metamorphism under ultra-high temperature (UHT) conditions at ca. 1.92Ga, associated with the subduction–collision tectonics between the Yinshan and Ordos Blocks. Here we report a new locality in Hongsigou where cordierite- and spinel-bearing granulites record UHT metamorphism. The prograde, peak, and retrograde mineral assemblages in these pelitic granulites have been identified based on petrography and mineral chemistry as: Bt1+Grt1+Sil1+Kfs1+Pl1+Ilm+Qtz1, Grt1+Sil2+Kfs2+Pl2+Spl+Ilm+Qtz2+Liq, and Crd+Grt2+Sil3+Kfs2+Pl2+Ilm+Qtz2 respectively. The peak metamorphic conditions of the pelitic granulite were estimated as 930–1050°C and 6.5–7.5kbar based on pseudosection analysis in the system NCKFMASHTO, suggesting extreme thermal metamorphism. We report LA-ICPMS zircon U–Pb data from the granulite which show weighted mean 207Pb/206Pb age of 1881±6.6Ma, marking the timing of UHT metamorphism. Lu–Hf analyses of the zircons show εHf(t) values within a restricted range of −4.2 to 0.3 and together with Hf model ages, a Paleoproterozoic arc magmatic source is inferred for the detrital zircons. The estimated P–T path for the UHT granulite suggests isobaric heating followed by cooling and decompression along a clockwise trajectory, different from the anti-clockwise P–T paths defined in earlier studies for the 1.92Ga UHT rocks from the IMSZ. The younger age and the isobaric heating trajectory suggest that the Hongsigou UHT rocks are related to heat input from underplated mafic magmas following continental collision.

Comparative analysis of the v(OD) and 2v 3(H2O) spectral bands of H2O-D2O system under isobaric heating conditions

The results of an investigation of the influence of isobaric heating on distribution of hydrogen bonds in the H2O-D2O system on the basis of comparative complex analysis of the v(OD) and 2v3(H2O) spectral bands are presented. In the case of analysis of the band assigned to the 2v3(H2O) overtone, the possibilities of obtaining more detailed information on the influence of external factors on the formation of local water structure widen significantly. In particular, there are noticeable differences in the quality of the obtainable data for these bands, which are especially manifested in their low-frequency regions, which are determined by the presence of strong hydrogen bonds characteristic of H-bonded high order water n-mers.

Consequences of open-system melting in tectonics

Partial melting and melt drainage from deep suprasolidus crust in orogens has important consequences for tectonics. Melt extraction along prograde segments of clockwise P – T paths reduces fertility and increases the density and strength of residual crust, which has implications for further melt production during decompression. Using calculated P – T phase diagrams, implications of stepwise melt loss along clockwise P – T paths for pelite and greywacke are assessed, and density of the progressively more residual source and the potential role of buoyancy in the exhumation of deep crustal rocks are evaluated. Two model P – T paths are considered: isobaric heating at 1.2 GPa followed by decompression to 0.4 GPa at 750, 820 and 890 °C, and prograde heating from the fluid-present solidus at 1.2 GPa to 860 °C at 1.8 GPa followed by isothermal decompression to 0.4 GPa. Both closed-system (undrained) and conditionally open-system (drained by intermittent melt loss) conditions are assessed. If melt is drained along clockwise P – T paths in suprasolidus crust then lower quantities of melt will be generated during decompression than sometimes inferred in tectonic models. Instead, the role of melt transfer through suprasolidus crust and melt accumulation at shallow levels in the anatectic zone should be considered rather than simply invoking the generation of large volumes of melt in decompressing crust.

P‐T‐D histories from quartz: A case study of the application of the TitaniQ thermobarometer to progressive fabric development in metapelites

This study investigates the ability of quartz to record segments of the pressure‐temperature‐deformation (P‐T‐D) path in poly‐deformed metamorphic terranes and the associated application of the Ti‐in‐quartz thermobarometer (TitaniQ). Metapelites from the Strafford Dome (eastern Vermont) were selected for this study because they record progressive fabric development during prograde metamorphism and have well constrained P‐T‐D histories from previous work. Results of this investigation are in agreement with previous studies and demonstrate that quartz in these samples records additional distinct intervals of the P‐T‐D path. Six preserved quartz equilibration events include: (1) early prograde growth during burial; (2) kyanite‐in quartz‐producing reactions; (3) prenappe emplacement equilibration during isobaric heating; (4) precipitation of quartz in quartz‐feldspar domains during crenulation cleavage development due to solution transfer; (5) quartz‐producing (chlorite‐out) reactions during heating postnappe emplacement; and (6) retrograde quartz overgrowths from an influx of siliceous fluids. Modeling quartz volume allows for the identification of quartz‐producing reactions in P‐T space; the same may be done for Ti‐phases to better constrain activities during reequilibration events. An integration of cathodoluminescence imaging, microstructural and petrographic investigation, isochemical forward stability modeling, and thermobarometry allows identification and interpretation of zoning patterns in quartz to unravel histories that would otherwise not be obtainable.

Polymorphs of 1,1-diamino-2,2-dinitroethene (FOX-7): Isothermal compression versus isobaric heating

Raman spectroscopy was used to examine polymorphic changes in 1,1-diamino-2,2-dinitroethene (FOX-7) single crystals under: isothermal compression to 15GPa and isobaric heating to 500K. Changes in the Raman spectra were observed at ∼2.0 and ∼4.5GPa, and at ∼390K and ∼450K. These onsets are in general accord with previously reported onsets from IR measurements under isothermal compression and from X-ray diffraction measurements under isobaric heating, respectively. In contrast to recent suggestions, we show that the high pressure polymorphs have different vibrational structures, and likely different crystal structures, than the high temperature polymorphs.

Petrology and phase equilibrium of newly found eclogites from Kekesu Valley in eastern part of southwest Tianshan HP-UHP metamorphic belt, China, and its tectonic significance

Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt, Xinjiang, northwest China. After detailed petrological study, three samples including one fresh eclogite TK003, one blueschist sample TK026-8 and one retrograded eclogite TK027, were selected for phase equilibrium modeling under NC(K)MnFMASHO (N2O-CaO-K2O-MnO-FeO-MgO-Al2O3-SiO2-H2O-O) system, by thermocalc 3.33 software. Composition analyses of garnets in these three samples show typical growth zoning with Xpy and Xgrs increasing, Xspss decreasing from core to rim. Pseudosection modeling of the garnet zonation reflects that the eclogites and blueschist experienced a similar P-T evolution trajectory, with a near iso-baric heating in the early stage, and reached eclogite facies metamorphic field with peak P-T regime of 480–515°C, 2.00–2.30 GPa. Subsequently the rocks experienced an early iso-thermal decompression retrograde stage with P-T conditions of 515–519°C, 1.78–1.93 GPa. Variations of mineralogy and modes of these rocks are probably due to different retrograde paths as a consequence of different bulk-rock composition, as well as a variation in fluid activity during exhumation. P-T calculation and a peak geothermal gradient of 6–7°C/km indicate HP rocks in the Kekesu Valley experienced cold subducted eclogite facies metamorphism. Thus a huge oceanic subduction eclogite facies metamorphic belt in southwest Tianshan has been recognized, extending from the Kekesu Valley in the east to the Muzhaerte Valley in the west for nearly 200 km. However, UHP evidence has not been found in the Kekesu terrane, perhaps because the slab in east part of southwest Tianshan did not subduct into such a great depth.

Tectonic evolution of high-grade metamorphic terranes in central Vietnam: Constraints from large-scale monazite geochronology

Several metamorphic complexes in Southeast Asia have been interpreted as Precambrian basement, characterized by amphibolite to granulite facies metamorphism. In this paper, we re-evaluate the timing of this thermal event based on the large-scale geochronology and compositional variation of monazites from amphibolite to granulite facies metamorphic terranes in central Vietnam. Most of the samples in this study are from metamorphic rocks (n=38) and granitoids (n=11) in the Kontum Massif. Gneisses (n=6) and granitoids (n=5) from the Hai Van Migmatite Complex and the Truong Son Belt, located to the north of the massif, were also studied. Two distinct thermal episodes (245–230Ma and 460–430Ma) affected Kontum Massif gneisses, while a single dominant event at 240–220Ma is recorded in the gneisses from the Hai Van Complex and the Truong Son Belt. Monazites from granitoids commonly yield an age of 240–220Ma. Mesoproterozoic ages (1530–1340Ma) were obtained only from monazite cores that are surrounded by c. 440Ma overgrowths. Thermobarometric results, combined with concentrations of Y2O3, Ce2O3, and heavy rare earth elements in monazite, and recently reported pressure–temperature paths suggest that Triassic ages correspond to retrograde metamorphism following decompression from high- to medium-pressure/temperature conditions. Ordovician–Silurian ages reflect low-pressure/temperature metamorphism accompanied by isobaric heating during prograde metamorphism. Some samples were affected by both metamorphic events. We conclude that high-grade metamorphism observed in so-called Precambrian basement terranes in central Vietnam occurred during both the Permian–Triassic and the Ordovician–Silurian, while peraluminous granitoid magmatism is Triassic. Additionally, our preliminary analyses for U–Pb zircon age and whole-rock chemistry of granitic gneisses from the Truong Song Belt suggests the presence of the Ordovician–Silurian volcanic arc magmatism in the region. Based on the pressure–temperature–time–protolith evolutions, metamorphic rocks from central Vietnam provide a continuous record of subduction–accretion–collision tectonics between the South China and Indochina blocks: in the Ordovician–Silurian, the region was characterized by active continental margin tectonics, followed by continental collision during the Late Permian to Early Triassic and subsequent exhumation during the Late Triassic. The results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other Southeast Asian terranes should be verified to achieve a better understanding of the Precambrian to Early Mesozoic tectonic history in Asia.

Structure of hydrogen-bonded associates in supercritical water under low and high pressures

The character and structural features of hydrogen-bonded associates in sub- and supercritical water are studied by analyzing distributions of the dipole moments of water molecules at P = 40, 80, and 100 MPa and T = 373–773 K, calculated using Car-Parrinello molecular dynamics. The main types of hydrogen-bonded structures and their changes upon isobaric heating are determined. It is shown that clusters with tetrahedral configurations exist in supercritical water only under high pressure.

Ultrasonic study of solid-phase amorphization and polyamorphism in an H2O-D2O (1: 1) solid solution

The elastic moduli and volume of H2O-D2O (1: 1) isotopically mixed ice (solid solution) have been studied at the solid-phase amorphization of normal 1h ice under compression at a temperature of 77 K and at the transition from high-density amorphous ice to low-density amorphous ice with subsequent successive crystallization to cubic (1c) and hexagonal (1h) ice at isobaric (0.05 GPa) heating. Comparison of the results with the respective data for H2O and D2O ices indicates that the observed concentration (in the isotopic composition) dependences of the elastic moduli and their derivatives for different phases of ice at isotopic hydrogen substitution in the H2O, H2O-D2O (1: 1), and D2O chain can be both monotonic and significantly nonmonotonic.

Application of a comprehensive approach to analysis of the IR spectra of hydrogen-bonded fluids

Application of a comprehensive approach to analyzing complex spectral bands by the example of the ν(OD) band of the HDO molecule in a [D2O]: [H2O] = 1: 60 mixture under isobaric heating is demonstrated. To analyze the spectral changes, the following approaches were applied: analysis of asymmetry, differential analysis of spectral bands, and method of difference spectra, as well as matrix analysis, including 2D-cross-correlation analysis and principal component analysis. The results can be used as initial data in the expansion of experimental spectral bands into constituent spectral components. This may be important for problems involving the interpretation of poorly resolved or strongly overlapping spectral bands, the analysis of which is nontrivial.

High-pressure–temperature phase diagram and the equation of state of beryllium

X-ray diffraction of beryllium in a laser-heated diamond anvil cell provides experimental insight into its behavior at high pressure and temperature. We measure the cold compression of Be in helium and NaCl pressure media up 192 GPa, and its thermal expansion up to 82 GPa and 2630 K. The new measurements form a $P\ensuremath{-}V\ensuremath{-}T$ data set which is fit by the Vinet-Debye form to establish a Be experimental equation of state. We compare the results to several theoretical models. The crystal structure of Be is determined up to 205 GPa and 4000 K; no evidence for the predicted high-temperature transition to a cubic phase is found. Finally, the maximum temperature stability of the solid phase along isobaric heating ramps gives a lower bound for the melting curve.

The cordierite‐bearing anatectic rocks of the higher Himalayan crystallines (eastern Nepal): low‐pressure anatexis, melt productivity, melt loss and the preservation of cordierite

AbstractCordierite‐bearing anatectic rocks inform our understanding of low‐pressure anatectic processes in the continental crust. This article focuses on cordierite‐bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite‐bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two‐mica metapelite/metagreywacke to biotite‐metapelite) has been investigated at low‐pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low‐pressure conditions, the two‐mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P‐T paths with a peculiar dP/dT slope of about 15–18 bar °C−1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low‐P regional anatectic terrane. The cordierite‐bearing migmatites may well represent the source rocks for the Miocene andalusite‐bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low‐P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.

1,1-diamino-2,2-dinitroethylene under high pressure-temperature

The structural phase stability of 1,1-diamino-2,2-dinitroethylene (FOX-7) has been studied up to 10 GPa through isothermal compression at 100 °C and 200 °C using synchrotron mid- and far-infrared spectroscopy. During isothermal compression at 100 °C changes are observed in vibrational spectra with increase in pressure that are indicative of significant distortion to monoclinic α phase or a possible structural transformation to a high pressure α(') phase at 2.2 GPa and α(") phase at 6.1 GPa. At 200 °C, for the far- and mid-IR regimes, the similar changes were observed at 2.1 (2.0) GPa and 5.3 (5.5) GPa, respectively. The observed change is nearly isobaric, consistent with previously reported high pressure and room temperature values, up to the highest temperature of 200 °C reached in our experiments. Over the total P-T range investigated, up to ∼10 GPa and 200 °C, we observed no evidence of sample decomposition. The observed changes are partially reversible with only slight evidence of the high pressure distortion remaining upon complete decompression. Additional isobaric heating at 1.07 GPa was performed in the mid-IR regime, which clearly revealed an onset of decomposition at 360 °C. Further x-ray or neutron diffraction, which are needed to fully resolve the cause of observed changes above 2 and 5 GPa, are ongoing.