Isobaric Heating Research Articles

Intermittent terrane arrival induces pulses of inland tectonic cycles

Pulsing volcanotectonic cycles characterized by short-lived (10 s Myr) switches in magmatic and tectonic patterns have been broadly identified in active margins. However, the specific mechanism that causes these switches remains ambiguous, i.e., whether the subduction continuity and/or terrane arrival (accretion, underthrusting, or subduction of buoyant continental/oceanic blocks with a thicker crust than their surrounding oceanic plate) plays a crucial role in controlling the observed volcanotectonic cycles remains controversial. Here, by modeling subduction processes involving the sequential arrival of buoyant terranes, we show that 1) in scenarios where the oceanic plate is weakly coupled with terranes, the entraining of the terrane into the subduction induces slab breakup to occur between the partially subducted terrane and its adjacent oceanic slab, with the terrane then rebounding and moving away from the trench. This evolution causes switches in the magmatic and tectonic patterns within the overriding plate. 2) In these models, the exposed terrane materials can preserve characteristic pressure-temperature-time trajectories, i.e., nearly isothermal compression to isothermal decompression and/or isobaric heating after partial exhumation. 3) slab breakup does not guarantee the occurrence of a trench jump; only when the terrane has a medium scale (∼300 km) will a new trench tend to develop prominently behind the rebounded terrane. 4) in scenarios where the composite slab (terrane and oceanic portions) resists yielding deformation and the terrane density is close to that of the oceanic plate (<0.6% density contrast), continuous subduction will occur. In this latter scenario, slab deformation (revealed by subduction angle and slab curvature), instead of slab breakup, will control the magmatic and tectonic patterns in the overriding plate. By further comparing model results with observations, we demonstrate that intermittent subduction interrupted by the subduction of terranes can be a tectonic driver for episodes of compression-to-extension transformations and magmatism (or piston-like volcanotectonic cycles) in two representative accretionary belts — with or without trench jumps (exemplars in south-central-Tibet and eastern-Mediterranean). In contrast, continuous subduction with strongly coupled upper and subducting plates could have contributed to similar cycles in the example without accretion (e.g., the Altiplano in the Central Andes). Therefore, over 10 s of Myr, the scale and frequency of terrane arrivals could essentially control the specific motion pattern of the subducting plate, creating the observed short-lived volcanotectonic switches at these three subtypes of active margins.

P-T paths of contact metamorphism constraints on emplacement mechanism of magmas of the zoned Fangshan granitic Plutons, Zhoukoudian, Western Hills of Beijing, China

P-T path of a contact aureole can provide important insights into the magma emplacement mechanism for nearly concentrically zoned granitic plutons. The Mesoproterozoic Xiamaling formation immediately adjacent to the southern Fangshan granitic Plutons in the Western Hills of Beijing, China, developed three contact metamorphic prograde staurolite zone, sillimanite zone and K-feldspar zone. These zones preserved corroded pink-andalusite wrapped by pale-andalusite, corroded garnet inclusion inside the K-feldspar porphyroblasts and staurolite, which add to well-established sequences of mineral growth. These sequences allow reconstruction of the complete P-T path for each prograde zone on the pseudosection diagram obtained by full system composition-based phase equilibrium modeling The resultant prograde P-T paths are composed of two segments, with the first one isobaric heating and the second distinct compression and heating. The staurolite zone shows the P-T path initiating from ∼2.0 kbar/350 °C to ∼1.8 kbar/510 °C and ending at ∼3.3 kbar/580 °C. The sillimanite zone path began from ∼2.0 kbar/350 °C to ∼1.5 kbar/500 °C then ended at ∼3.8 kbar/580 °C. The K-feldspar zone path started from ∼2.0 kbar/350 °C to ∼1.7 kbar/510 °C, then terminated at ∼3.5 kbar/670 °C, approaching the initial partial melting temperature condition, then documented a retrograde path cooling down to ∼630 °C. The two prograde P-T path segments of the three zones suggest a in situ heating followed by a ductile return flow from ∼7 km down to ∼11 km depth of the metapelites from the Xiamaling formation, responsable to a diapiric emplacement of the magmas for the zoned composite Fangshan Plutons.

Metamorphic P-T-t evolution of ultrahigh-temperature granulite facies metapelites from the Lundazi terrane, Irumide Belt, Eastern Zambia

The Lundazi terrane is part of the Mesoproterozoic Irumide Belt of Central Africa which together with other terranes of similar ages are interpreted to record collisional events relation to the amalgamation of the Rodinia Supercontinent. Pelitic granulites from the area preserve evidence of ultrahigh-temperature metamorphism and partial melting with complex textures involving garnet + sillimanite + spinel + quartz ± cordierite ± orthopyroxene, rare corundum, and ternary feldspars. Garnet porphyroblasts preserve inclusions of spinel + quartz and sillimanite + feldspar which indicate high temperatures were reached during the metamorphic peak. The metamorphic evolution of these rocks has been investigated using phase equilibria modelling and complemented by ternary feldspar thermometry and garnet-orthopyroxene thermobarometry which indicate peak conditions of up to ∼900–1000 °C and 6–9.5 kbar. The preservation of peak assemblages and textural features of these rocks suggest substantial melt loss during their prograde evolution. Melt-reintegrated pseudosections indicate isobaric heating prior to exhumation and subsequent cooling. Monazite EPMA dating identifies the thermal peak occurred at 1056–1028 ± 8 Ma, followed by retrograde recrystallization at 1003 ± 8–975 ± 7 Ma. The high-grade metamorphism is interpreted in terms of clockwise P-T-t paths with retrograde recrystallization marked by the partial resorption of garnet rims to form plagioclase, the growth of late cordierite around garnet and sillimanite, and alteration of monazite. The absence of a Neoproterozoic overprint in the area indicates the Lundazi terrane is part of the Irumide Belt s.s rather than the Southern Irumide Belt, and a revision of the boundary between the two regions is proposed.

Evidence of Paleoproterozoic metamorphism in Vestfold Hills, East Antarctica: Insights from phase equilibria modelling and monazite CHIME dating

The Vestfold Hills sector on the coastal fringe of the Princess Elizabeth Land forms part of an Archean to Paleoproterozoic aged cratonic nucleus of the East Antarctic Shield. A charnockite-granite association from the Mossel Gneiss Group in the northern region of the Vestfold Hills is investigated in this work to characterise the metamorphic-magmatic evolution of the area. Conventional thermometry and phase equilibria modelling indicate that the charnockite formed as a result of ultra-high temperature metamorphism at low-intermediate pressure (∼4 kbar). Phase equilibria modelling, reveals that the melt-integrated charnockite composition is a restitic product of a protolith of quartz diorite composition which underwent isobaric heating (peak temperature up to 960 °C) at 4 kbar pressure followed by anatexis. The anatexis of the quartz diorite protolith also resulted in the formation of the associated granitic melt. U–Th-PbTotal ages obtained from monazites of the granite integrated with CHIME ages extracted using the isochron method indicate an emplacement age of ∼2200 Ma and two younger events are recorded at ∼2000 Ma and ∼1700 Ma. The younger ages are ascribed to the resetting during the emplacement of younger basalt dykes that cross cut the charnockite-granite sequence. Vestfold Hills’ geological correlations with Indian and Australian cratons are discussed considering this new data.

Early Cretaceous subduction of an oceanic plateau at the Northern Andes; geochemical, metamorphic, and cooling age constraints of the Raspas Metamorphic Complex

The Raspas Metamorphic Complex is a high-pressure unit that includes eclogites, blueschists, and metapelites, with minor garnet amphibolites and greenschists. Although its geochemistry and ages are relatively well known, metamorphic constraints and the origin of its protolith remain unclear. In this study, we present new geochemical, metamorphic, and cooling age data to understand the protolith, metamorphic conditions, and the tectonic implications of this feature for the Northern Andes.Geochemical analyses reveals rocks with N- to E-MORB signatures and trace element ratios typical of a heterogeneous mantle source, like oceanic plateau lavas. Blueschists originated from a deep enriched source, similar to primitive mantle, while eclogites are similar to MORB. Iterative thermodynamic modeling of an eclogite indicates a low T/P gradient of ∼406 °C/GPa during the formation of the assemblage garnet (core), quartz, katophorite, and omphacite cores at ∼598 °C and 1.43 GPa, followed by the formation of atoll garnets and zoisite at 661 °C and 1.63 GPa, around 133 Ma. Peak metamorphic conditions were attained after isobaric heating at 709 °C and 1.61 GPa, resulting in the assemblage of garnet (rim), zoisite (rim), omphacite (rim), and katophorite. 40Ar/39Ar ages around 129 Ma indicate fast exhumation rates of 1.25 to 0.5 cm/yr.We propose that the Raspas Metamorphic Complex represents the interaction of the South American plate with an oceanic plateau during the Early Cretaceous. During the early stages of subduction, it created a flat slab geometry around 130 Ma. As the plateau transformed to eclogite, the decrease in buoyancy caused an increase in subduction dip during a regional roll-back at 129–123 Ma, which triggered the exhumation along the high-pressure belt.

Metamorphic P–T paths of Archean granulite facies metasedimentary lithologies from the eastern Beartooth Mountains of the northern Wyoming Province, Montana, USA: constraints from quartz-in-garnet (QuiG) Raman elastic barometry, geothermobarometry, and thermodynamic modeling

Abstract. Metamorphic pressure and temperature (P–T) paths in late-Archean high-grade rocks of the eastern Beartooth Mountains of Montana (USA), a portion of the Wyoming Province, are established by a combination of imaging, analytical, and modeling approaches. Garnet inclusion mechanical and chemical thermobarometry, applied to several granulite-facies migmatites and an iron formation, is particularly useful in constraining the prograde P–T conditions. Quartz-in-garnet (QuiG) elastic Raman barometry was used on quartz inclusions in garnet for all samples studied. For a smaller subset of four representative samples, QuiG constraints were used in conjunction with Ti-in-quartz (TitaniQ) and Ti-in-biotite (TiB) thermometry to establish unique prograde inclusion entrapment P–T conditions. Ti measurements of garnet hosts and cathodoluminescence (CL) imagery of inclusion and matrix quartz grains were employed to check for Ti loss/diffusion. Lastly, inclusion studies were supplemented with thermodynamic modeling and matrix chemical thermobarometry to examine garnet nucleation temperatures and peak metamorphic conditions. Disagreement between the volume strain and elastic tensor methods used to calculate quartz inclusion pressures implies that quartz inclusions studied are under strong differential strain. Prograde entrapment results from the two inclusion thermobarometry pairs used are distinct: 0.55–0.70 GPa and 475–580 ∘C (QuiG–TitaniQ) versus 0.85–1.10 GPa and 665–780 ∘C (QuiG–TiB). Garnet modal isopleth modeling indicates that the majority of garnet growth occurred at ∼ 450–600 ∘C, implying that P–T conditions of garnet growth are interpreted to be most reliably represented by QuiG–TitaniQ inclusion thermobarometry. Normal distributions of calculated QuiG inclusion pressures and the concentration of mineral inclusions in garnet cores suggest that the majority of garnet inclusions were entrapped during a single stage of porphyroblast growth. A general lack of evidence from CL imagery for post-entrapment mechanical or chemical modifications to quartz inclusions suggests that quartz inclusions used to calculate entrapment P–T largely preserve their initial entrapment conditions. Biotite inclusions preserve higher temperatures than quartz inclusions in the same garnets, likely due to Fe–Mg exchange with garnet hosts that allowed Ti content of biotite to change after entrapment. Pseudosection modeling and matrix chemical thermobarometry of multiple, independent lithologies examined during inclusion studies suggest a range of peak granulite facies conditions of ∼ 0.50–0.70 GPa and 730–800 ∘C. Peak metamorphic P–T modeling work from this study, together with evidence of regional amphibolite facies overprinting in immediately adjacent samples, indicates an overall clockwise metamorphic P–T path with nearly isobaric prograde heating to peak temperatures. Interpreted P–T path reconstructions are consistent with metamorphism developed in a more modern-style continental arc subduction zone and are observed in portions of the northern Wyoming Province as exemplified by metasupracrustal lithologies of the eastern Beartooth Mountains.

Glass Polymorphism in Hyperquenched Aqueous LiCl Solutions

We investigate the glass polymorphism of dilute LiCl-H2O in the composition range of 0-5.8 mol % LiCl. The solutions are vitrified at ambient pressure (requires hyperquenching with ∼106 K s-1) and transformed to their high-density state using a special high-pressure annealing protocol. Ex situ characterization was performed via isobaric heating experiments using X-ray diffraction and differential scanning calorimetry. We observe signatures from a distinct high-density and a distinct low-density glass for all solutions with a mole fraction xLiCl of ≤ 4.3 mol %, where the most notable are (i) the jumplike polyamorphic transition from high-density to low-density glass and (ii) two well-separated glass-to-liquid transitions Tg,1 and Tg,2, each pertaining to one glass polymorph. These features are absent for solutions with xLiCl ≥ 5.8 mol %, which show only continuous densification and relaxation behavior. That is, a switch from water-dominated to solute-dominated region occurs between 4.3 mol % LiCl and 5.8 mol % LiCl. For the water-dominated region, we find that LiCl has a huge impact only on the low-density form. This is manifested as a shift in halo peak position to denser local structures, a lowering of Tg,1, and a significant change in relaxation dynamics. These effects of LiCl are observed both for hyperquenched samples and low-density samples obtained via heating of the high-density glasses, suggesting path independence. Such behavior further necessitates that LiCl is distributed homogeneously in the low-density glass. This contrasts earlier studies in which structural heterogeneity is claimed: ions were believed to be surrounded by only high-density states, thereby enforcing a phase separation into ion-rich high-density and ion-poor low-density glasses. We speculate the difference arises from the difference in cooling rates, which are higher by at least 1 order of magnitude in our case.

The water cavitation line as predicted by the TIP4P/2005 model.

The formation of vapor bubbles in a metastable liquid, cavitation, is an activated process due to the free energy cost of having both phases at contact. Such an energetic penalty enables the existence of the liquid beyond its thermodynamic borders. Establishing the stability limits of a liquid as ubiquitous as water has important practical implications and has thereby attracted a lot of attention. Different experimental strategies and theoretical analyses have been employed to measure and predict the cavitation line, or the pressure-temperature kinetic stability border of liquid water. Understanding the location of the cavitation line requires knowing the cavitation rate dependence on pressure and temperature. Such dependency is difficult to obtain in experiments, and we use molecular simulations with the TIP4P/2005 model to fill this gap. By deeply overstretching liquid water below the saturation pressure, we are able to observe and quantify spontaneous cavitation. To deal with a lower overstretching regime, we resort to the Seeding technique, which consists of analyzing simulations of a liquid containing a vapor bubble under the theoretical framework of Classical Nucleation Theory. Combining spontaneous cavitation with Seeding, we get a wide overview of the cavitation rate. We study two different temperatures (450 and 550K) and complement our perspective with the results previously obtained at 296.4K [Menzl et al., Proc. Natl. Acad. Sci. 113, 13582 (2016)] to establish a broad simulation-experiment comparison. We find a good agreement between simulations and both isobaric heating and isochoric cooling experiments using quartz inclusions. We are, however, unable to reconcile simulations with other experimental techniques. Our results predict a decrease in the solid-liquid interfacial free energy as the liquid becomes increasingly overstretched with a temperature independent Tolman length of 0.1nm. Therefore, the capillarity approximation underestimates the nucleation rate. Nonetheless, it provides a fair indication of the location of the cavitation line given the steep rate vs pressure dependence. Overall, our work provides a comprehensive view of the water cavitation phenomenon and sets an efficient strategy to investigate it with molecular simulations.

Tracing the cryptic Sardic (Ordovician) metamorphism across Alpine Europe: the Krndija region in the Slavonian Mountains, Croatia

Results of a combined petrological, geochemical and geochronological study suggest that metasedimentary rock units in the Krndija region of the Slavonian Mountains, Croatia, were affected by at least three major tectonometamorphic imprints: during the Middle Ordovician (Sardic event), the early Carboniferous (Variscan event), and the Cretaceous (Alpine event). All three metamorphic phases are established by electron microprobe-based in-situ U–Th–Pb dating of monazite grains. The Sardic metamorphic event is additionally confirmed by a precise Lu–Hf garnet-whole-rock isochron age of 466.0 ± 2.3 Ma. Taken together, the data unveil a relatively large and well-preserved piece of the cryptic Sardic orogen in central Krndija, that we name the Kutjevo Zone. A Sardic subduction-related metamorphic event (ca. 540-580 ℃, 8–11 kbar) at ca. 466 Ma is manifested in the mineral paragenesis Ca-rich garnet plus rutile. A low degree of retrograde reequilibration suggests a subsequent fast exhumation. Low-Ca cores in some garnets and staurolite relics record a pre-HP metamorphic event that involves isobaric heating from 570 to 610 ℃ at ~ 7 kbar. We attribute this (so far undated) event to mid-crustal contact metamorphism caused by early Sardic magmatism. Southern parts of Krndija (the Gradište Zone) experienced an (additional?) clockwise PT evolution in Variscan times at ca. 350 Ma. Garnet formed with ilmenite during a PT increase from 580 ℃/5 kbar to 600 ℃/6 kbar and underwent later strong retrograde resorption. Slow Variscan exhumation resulted in andalusite formation at < 550 ℃/ < 3.8 kbar. Penetrative Alpine metamorphism was observed in low-grade phyllites in the north. The lithology and metamorphic history of the Kutjevo Zone is similar to what has been reported from the Sardic Strona-Ceneri Zone in the western Alps. Both areas expose metapelitic (metagreywacke) rocks with a pre-middle Ordovician formation age. These metasedimentary rocks are inter-layered with numerous small amphibolitic units as well as metagranitoids and were likely deposited along the active Gondwana margin, perhaps in a fore-arc position, prior to their subduction during the middle Ordovician. According to recent palaeogeographic reconstructions, both the Kutjevo Zone and the Strona-Ceneri Zone have once resided in an eastern sector of the northern Gondwana margin (i.e., in E-Armorica). We conclude that in the Middle Ordovician, important subduction activities took place in this E-Armorican segment of north Gondwana, which is today exposed in the Alps. The W-Armorican segment of north Gondwana (now exposed in the French, German, and Czech Variscides) had probably already mutated from a (Cadomian) subduction setting to an extensional (transtensional–transpressional) setting by the late Cambrian.

Benzophenone glass, supercooled liquid, and crystals: elastic properties and phase transitions.

The elastic properties of glass α- and β-modifications of benzophenone (C6H5)2CO are determined for the first time by the ultrasonic method at high pressures up to 1 GPa and in the temperature range of 77 K< T < 293 K. Four states of benzophenone are experimentally observed in the investigated temperature range of 77-293 K: glass, supercooled liquid, and α- and β-crystalline phases. The boundaries of phase transitions during isobaric heating are determined. The bulk and shear moduli B and G, Poisson's ratio σ, and density ρ are calculated. Glassy benzophenone has much lower elastic moduli than the α-modification at 77 K (the shear modulus G of glass is about half the value for the crystal). The crystalline α-modification demonstrates a strong softening of both moduli with an increase in the temperature in the range of 77-293 K. Heating of glass leads to the formation of a very viscous supercooled liquid, which crystallizes into the metastable β-modification. A further increase in the temperature leads to a monotropic phase transition β → α.

Thermobaric history as a tool to govern properties of glasses: case of dipropylene glycol.

The elastic properties of high- and low-pressure glasses of dipropylene glycol were determined for the first time under conditions of isothermal compression up to 1 GPa at 77 K and isobaric heating of 77-300 K at 0.05 GPa and 1 GPa. A strong dependence of the elastic properties of glasses on their thermobaric history has been revealed: glasses obtained at high pressure have not only higher densities (3.9%), but also noticeably higher elastic moduli. This effect is especially pronounced in the shear modulus: high-pressure glass has a 30% higher shear modulus than low-pressure glass. The behavior of elastic moduli during the glass-to-liquid transition also depends on the thermobaric history. Glass produced at low pressure but heated at high pressure has anomalous temperature dependences of the elastic moduli. Heating dipropylene glycol glasses at different pressures allowed us to refine the Tg(P) dependence.

A small fraction of defects can order a crystal

Based on calculations of the activation process parameters (vacancy formation and self-diffusion) for gold at the temperature range T = 10–1330 K, the correlations of the activation process parameters si(T) ∼ hi(T) and si(T) ∼ vi(T), along two isobars. P = 0 and 24 GPa, are studied. Here hi is the enthalpy, si is the entropy, and vi is the volume of the activation process, i.e. for the formation of an electroneutral vacancy (i = v) or for the self-diffusion of atoms (i = d). It is shown that if, during isobaric heating, an energy less than a certain value (hi < hsi) is required to create a defect (vacancy or diffusing atom), or if the resulting defect has a volume less than a certain value (vi < vsi), then this defect has a negative entropy, i.e. this defect orders the crystal. However, if a larger energy value is required to create a defect (hi > hsi) or this defect has a noticeable volume (vi > vsi), then this defect has a positive entropy, i.e. this defect disorders the crystal. It is shown that at a low concentration of defects, they order the crystal, and only starting from a certain concentration (Xsi) does the defect formation entropy pass into the positive region, where defects disorder the crystal. The changes in the functions of hsi, vsi, and Xsi with increasing pressure are studied.

Ultrasonic study of the elastic properties of propylene glycol oligomers in the glassy state and during the glass–liquid transition

ABSTRACT Using an ultrasonic technique under pressure, we investigated the dependence of elastic properties of propylene glycol, dipropylene glycol and tripropylene glycol on the concentration of hydrogen bonds and the mass of molecules. Studies were carried out up to 1.8 GPa in the glassy state and at isobaric heating during the glass–liquid transition. Propylene glycol in glassy state has the highest modules, and its interaction potential is more central than in di- and tripropylene glycol. Both the shear modulus G and the bulk modulus B are practically identical for glassy di- and tripropylene glycol.

To the possibility of experimental estimation of the diffusion spinodal position of binary mixture with LCST via pulsed heating method

In this paper, we present new data on non-stationary heating of a platinum wire probe, immersed in a binary partially-miscible liquid. A pressure value and a mass fraction of the a polymer in the mixture were the experimental parameters. The characteristic heating time was from 5 to 15 ms. The object of the research was the water/polypropylene glycol-425 (PPG-425) mixture having lower critical solution temperature (LCST). The position of the diffusion spinodal was estimated based on the obtained data on the liquid-liquid binodal in the framework of the Flory-Huggins approximation. An experimental technique to estimate the position of the spinodal of two-component mixtures with LCST on the scale of the mixture component ratio was developed. It was shown that the method of isobaric pulse heating can be used for this purpose. This representation is based on the threshold change in the heat transfer pattern when crossing the phase coexistence curve and the diffusion spinodal.

Modeling the propagation of a high-average-power train of ultrashort laser pulses.

We investigate the interpulse thermal interaction of a train of ultrashort laser pulses and develop a model to describe the isobaric heating of air by a train of pulses undergoing filamentation. We calculate the heating of air from a single laser pulse and the resulting refractive index perturbation encountered by subsequent pulses, and use this to simulate the propagation of a high-power pulse train. The simulations show deflection of laser filaments by the thermal refractive index consistent with previous experimental measurements.

Temperature measurements of long sparks in air using time-resolved moiré deflectometry

This paper presents an original investigation into time-resolved moiré deflectometry (MD) for gas temperature measurements of long sparks in air. In order to perform a rigorous comparative investigation with the widely used quantitative schlieren (QS) method, we set up an orthogonal optical measurement system. The impacts of spatial resolution and exposure time on the measurement accuracy are investigated by comparing the measured radial distribution of the gas temperature and its time evolution during the same spark discharge event. It is found that the time-resolved MD with an exposure time of 1.0 μs can accurately measure the gas temperature evolution during the isobaric heating and relaxation process of long sparks. The measured results of the expansion rate of cross-section and average temperature T g confirms that the energy loss is dominated by thermal conduction during the isobaric heating and relaxation process after the extinction of discharge current. The MD can achieve a finer spatial resolution than the QS method, which is beneficial to the reconstruction of steep radial temperature distribution and the suppression of measured temperature fluctuation. Moreover, the MD can obtain a sub-microsecond exposure time by increasing the power of continuous-wave laser source, which shows its potential to capture the rapid changes in gas temperature during the fast-heating process.

Near Infrared Spectroscopy As an Effective Way of Studying Hydrogen Bonding in a LiCl–H2O–CO2 Ternary Mixture

Abstract The possibility of using near IR spectroscopy to analyze the effect isobaric heating has on hydrogen bonding in an aqueous solution of LiCl in equilibrium with supercritical carbon dioxide (a LiCl–H2O–scCO2 ternary mixture) is demonstrated in a wide range of electrolyte concentrations. It is shown that this approach is highly efficient when studying ion and molecular systems with different types of interparticle interactions. The use of near IR spectroscopy allows distinguishing spectral contributions from hydrogen bonded n-mers of bulk water and water molecules in the solvation shells of ions or in ion–water chains like those formed in solutions with extremely high electrolyte concentrations that do not contain bulk water. It is shown for the studied ternary mixture that raising the concentration of electrolyte completely neutralizes the destructive effect of carbon dioxide on the formation of a hydrogen bonded structure of water. The latter is stabilized under the influence of an ion field, which also substantially weakens the temperature effect.

Eburnean/Trans-Amazonian orogeny in the Nyong complex of southwestern Cameroon: Meta-basite geochemistry and metamorphic petrology

New and published geochemical, petrologic, and geo-thermobarometric data of meta-basites were combined to constrain the geodynamic evolution of the Paleoproterozoic Nyong complex (NyC) in southwestern Cameroon, a part of the Central African tectonic belt. Their geochemical signatures, including (La/Yb)N = 0.53–56.26, slightly positive Eu anomalies (0.21–1.04), abundance of Pb and U, negative Nb–Ta anomalies, and REE depletion, suggest various degrees of partial melting of mostly spinel-bearing mantle sources. In addition, the meta-basite protoliths can be classified as IAB-, E-MORB-, N-MORB-, and exceptionally OIB-compositional types with minor crustal contamination. Petrologically, the meta-basites are characterized by coronitic textures, marked by microblasts and porphyroblasts of garnet surrounded by plagioclase or by both plagioclase and clinopyroxene, indicating metamorphic recrystallization under decompression. Our new geo-thermobarometric data outline various segments of an overall clockwise pressure‒temperature path. In the different meta-basite samples, the pressure peak at eclogite-facies conditions (∼625 °C, 11.7 kbar) was followed by nearly isobaric heating to amphibolite-facies conditions (600–690 °C, 11.5 kbar) or decompression and heating to amphibolite- (∼740 °C, 10.8 kbar) and granulite-facies (∼800 °C, 10.0 kbar) conditions. The retrograde path evolved from granulite- (850–750 °C, 11.3–7.6 kbar) and amphibolite- (790–550 °C; 12.4–3.0 kbar) to epidote-amphibolite- (∼600‒490 °C, 8.0–7.6 kbar) and greenschist-facies (∼400 °C, 4.2 kbar) conditions. Combining the new with published, in general high to middle temperature-middle pressure data from various rock types, with published high temperature-high pressure data from eclogites highlights distinct burial-exhumation paths for the NyC rocks: these paths document a clockwise pressure-temperature evolution, typical for a collisional setting associated with the Rhyacian-Orosirian Eburnean/Trans-Amazonian orogeny in Cameroon. The eclogite data outline a deep burial–isothermal decompression evolution, typical for a subduction setting predating the collisional setting.

Upper Mantle Dynamic and Evolution beneath Lake Nyos Volcano (Cameroon Volcanic Line), Documented by Peridotite Xenoliths Enclosed in Alkali Basalts

Volcanic deposits from the lake Nyos contain ultramafic xenoliths: lherzolites, harzburgites and wehrlites, sometimes containing amphiboles and phlogopites. The lithospheric mantle beneath Nyos, as inferred from chemical diagrams, has experienced partial melting and variably cryptic and modal metasomatism of the two groups of samples that have been distinguished: Group 1 samples are characterized by spoon-shaped REE patterns, and Group 2 samples show light (L) REE-enriched patterns. Metasomatic events were associated with pervasive infiltration of volatile (Ti, CO2, H2O) or alkali-rich small melts fractions and fluids. Later on, hydrous phases, Ti-rich Cpx, CaO-rich Ol, Ti-rich Ol, Cr poor and low values of NiO and FO (%) in wehrlite compared to other xenoliths, precipitated from alkali enrichments due to the percolation of the mantle by basaltic magmas. The metasomatic liquid which percolates the Nyos mantle column was a dense alkaline silicate rich in volatile, displaying low HFSE abundances in the metasomatic hydrous melts compared to the LILE. It is suggested that 1) cryptic metasomatism affected Group 1 samples, 2) the spinel-free wehrlite is a Group 2 sample corresponding to a cumulate of a similar melt and 3) amphibole may be a potassium-bearing mineral in addition to phlogopite at shallower levels of Nyos upper mantle. P-T estimated indicates that xenoliths were initially equilibrated in the garnet stability field, at depth of 85 Km, and then they were re-equilibrated in the spinel field owing to isobaric heating up to 1000℃. Adiabatic decompressions occur from 85 to 50 Km materialized by sample NK14 showing transitional porphyroclastic to equigranular texture and displaying pyroxene-Cr spinel symplectites, and from 50 to 30 Km corresponding to 8 - 18 Kbar, pressures in which most xenoliths were incorporated in the host lavas. Therefore, the presence of rising mantle plumes from 85 to 50 Km (sample NK14) and from 50 to 30 Km (all studied samples) is probably related to the evolution of heterogeneous translithospheric mantle diapirs beneath this section of the continental Cameroon Volcanic Line. Local diapirs here may be interpreted as rift-zone initiators.

Eclogite with unusual atoll garnet from the southern Armorican Massif, France: Pressure-temperature path and geodynamic implications

Eclogites form in different geotectonic environments such as subduction zones or roots of overthickened continental crust. The specific environment can be identified by the construction of pressure-temperature (P-T) trajectories. We determined the P-T path of eclogite from the Les Essarts high-pressure unit, Southern Armorican zone in western France. The eclogite contains unusual atoll garnet characterized by a younger generation (Grt2) that replaced the core of an older generation (Grt1). Large-area (cm2) X-ray mapping with the electron microprobe was applied to determine the quantities of Grt1 and Grt2 in the studied eclogites. Detailed characterization of the mineral chemistry was undertaken followed by thermodynamic modelling. The assemblage of amphibole, quartz, rutile, and ilmenite enclosed in Grt1 formed in the amphibolite facies during a first metamorphic event. Grt1 grew at P-T conditions around 20 kbar and 540 °C probably after nearly isothermal burial during a second metamorphic cycle. An isobaric heating to ca. 650 °C followed at which Grt2 and, thus, atoll garnet formed. A time period of ca. 15 million years for exhumation in the temperature range 620–655 °C was estimated by diffusional modelling of the contact between Grt1 and Grt2. We correlate the Southern Armorican zone with the Malpica-Tuy zone (NW Spain) that contains eclogite and related rocks with a similar metamorphic evolution. We outline a geodynamic scenario in which the eclogite-facies rocks of both zones were tectonically eroded from the upper continental plate by a subducting plate to reach maximum depths of 70–90 km in the Late Devonian. Exhumation to depths of ca. 45–50 km occurred in a subduction channel just before continent-continent collision started.