Coesite Stability Field Research Articles

Quartz-in-garnet (QuiG) under pressure: insights from in situ Raman spectroscopy

Elastic geothermobarometry relies on the contrast between the thermal expansion and compressibility of a mineral inclusion and its surrounding host, leading to a residual pressure in the inclusion (Pinc) that may differ significantly from the external pressure. Quartz-in-garnet (QuiG) inclusion-host systems are widely used in elastic geothermobarometry to estimate the inclusion entrapment conditions and thus the rock petrogenesis. To elucidate the behaviour of QuiG at elevated pressures, we have applied in situ high-pressure Raman spectroscopy on three QuiG samples having Pinc close to 0 GPa at room temperature. We demonstrate that upon pressure increase, the garnet host acts as a shield to the softer quartz inclusion. Consequently, the Pinc increases with a smaller rate compared to that of the external pressure. Up to 2.5 GPa, the evolution of Pinc calculated from the Raman data agrees very well with prediction from the equations of state. Furthermore, the behaviour of a quartz inclusion in a relatively thin host specimen was explored up to external pressures of 7 GPa. Our results indicate that the shielding effect of the host (even if only partial because of the insufficient distance between the inclusion and the host surface) can keep the quartz inclusions thermodynamically stable up to about 2 GPa above the equilibrium quartz–coesite phase boundary. In addition, the partial shielding leads to the development of anisotropic symmetry-breaking stresses and quartz inclusions undergo a reversible crossover to a lower symmetry state. Given that the presence of non-hydrostatic stress may influence the quartz-to-coesite phase boundary, especially at elevated temperatures relevant for entrapment conditions, our results emphasize the importance of elastic anisotropy of QuiG systems, especially when quartz inclusion entrapment occurs under conditions close to the coesite stability field.

Petrology of UHP eclogite-facies felsic schist in the Western Tianshan subduction zone, China

Although quartzo-feldspathic metasedimentary rocks are widespread in high pressure-ultrahigh pressure (HP-UHP) metamorphic belts worldwide, their petrogenesis and metamorphic evolution is poorly understood. We discovered an UHP eclogite-facies felsic schist in the Western Tianshan metamorphic belt, China. Petrological observations and phase equilibria modeling both indicate the felsic schist experienced UHP metamorphism in the coesite stability field. In particular, it experienced prograde metamorphism at 21–24 kbar, 445°C–470°C, a pressure peak at 25–28 kbar and 490°C–525°C, and eventually heating with decompression to 20 kbar and 560°C. The obtained clockwise P-T path was consistent with those of other lithologies (eclogite and pelitic schist) from the same belt, which provides new evidence for the coherent exhumation of the UHP unit of the Western Tianshan metamorphic belt. The final uplift of the Western Tianshan oceanic crust to the surface is attributed to fluid activity and late tectonic deformation.

A new UHP unit in the Western Alps: First occurrence of coesite from the Monviso Massif (Italy)

AbstractThe Western Alps are one of the most studied exhumed subduction-accretionary complexes worldwide. Ultrahigh-pressure (UHP) metamorphism has been documented there since the 1980s. We now report the first discovery of coesite in the meta-ophiolitic suite of the Monviso Massif, corresponding to the fourth UHP unit defined on the Western Alps. Previous petrographic studies and results from thermodynamic modeling already suggested that these Alpine units may have experienced UHP metamorphism, but no occurrences of index minerals, such as coesite, have been reported to date. The newly discovered coesite inclusions from the Monviso Massif occur as intact single crystals (10–60 µm) hosted by garnet. The observations suggest that they have escaped re-equilibration and maintained all the original features from the trapping time. The reduced size of the crystals and the lack of re-equilibration significantly differ from the typical textural features described in past findings (i.e., radial cracks, palisade texture of quartz surrounding coesite relicts). Detailed garnet inclusions analysis and thermodynamic modeling constrained the metamorphic peak conditions at P = 2.8–2.9 GPa and T = 500–520 °C within the coesite stability field.The Lago Superiore Unit represents the fourth UHP unit discovered on the Western Alps. The UHP metamorphism on the Western Alps was considered rare due to the escape process of unusual units from mantle depths. In our view, the implication of our discovery provides new insight into UHP processes that seem to be more common than expected. Further tectonic reconstructions should take into account the common features observed in the UHP units to better constrain the subduction- and exhumation-related mechanisms that drove the actual stacking of mountain belts.

A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira Massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit. It is only a few tens of metres thick and consists of graphite-rich, garnet–chloritoid micaschists, whose protoliths may be black shales of Silurian age. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P–T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25–27 kbar 470–500 °C) (stage 1), up to the coesite stability field (27–28 kbar 510–530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500–515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite–ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P–T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P–T conditions are comparable to the ones of the Tian Shan coesite–chloritoid-bearing rocks.

H2O in omphacite of quartz and coesite eclogite from Erzgebirge and Fichtelgebirge, Germany

AbstractMajor and trace elements in omphacite, including hydrogen, were determined in eclogites from two Variscan basement complexes in Germany: Erzgebirge (EG) and Fichtelgebirge (FG). Erzgebirge eclogite is derived from three units, showing different peak pressure (P) and temperature (T) conditions (Unit 1: 840–920°C/≥30 kbar, Unit 2: 670–730°C/24–26 kbar, Unit 3: 600–650°C/20–22 kbar). The peak conditions of FG eclogite (690–750°C/25–28 kbar) resemble those of EG Unit 2. Coesite eclogite occurs in EG Unit 1, and quartz eclogite in all other units. Omphacite from all samples shows four infrared (IR) absorption bands. Two prominent, sharp bands occur at 3,455 ± 10 cm−1 (band II) and 3,522 ± 10 cm−1 (band III). Band II is usually more prominent than band III, except for few samples with low jadeite content. A further, broad band is centred between 3,270 and 3,370 cm−1 (band I) and a fourth, minor band at 3,611–3,635 cm−1 (band IV). Bands II and III are due to hydrogen bound as structural OH− ions in omphacite. In most cases, this also applies to band IV. However, some spectra with extremely large type IV bands reflect phengite inclusions. The ambiguous band I may be due to different H2O species (molecular water, structural OH, and water in phengite). Omphacite of quartz eclogite has lower contents of TiO2, Zr, Hf, and REE, compared with that from coesite eclogite. By contrast, omphacite in quartz eclogite from both EG (H2O sample averages: 465–852 ppm) and FG (546–1,089 ppm) contains the same amount of structural OH (concentrations given in wt.‐ppm H2O) as omphacite in coesite eclogite (492–1,140 ppm). The obtained difference in the garnet‐omphacite H2O partition coefficient between quartz (0.01–0.03) and coesite eclogite (0.08–0.11) results from different H2O contents in garnet (coesite eclogite: 50–150 ppm; quartz eclogite: <2–50 ppm; Gose & Schmädicke, 2018). The total content of structural OH in omphacite is unrelated to its major and trace element composition. However, treating the individual IR bands separately, a relation between OH and mineral composition is observed. The OH amount defined by band II is positively correlated to Ti and tetrahedral Al, and that of band III shows a positive correlation with Ca and a negative one with Na (and jadeite). Both the total OH content of omphacite and the partial contents deduced from individual IR bands are unrelated to PT conditions. This implies that omphacite incorporated its structural H2O mainly in the quartz stability field, presumably during initial omphacite growth. Conversely, most OH in garnet was derived from the final breakdown of the last remaining calcic amphibole close to or within the coesite stability field. Our data suggest that coesite eclogite is able to transport a significant amount of H2O (average 550 ppm, maximum 730 ppm), corresponding to that in 3–4 vol.% calcic amphibole, via subduction to depths beyond 100 km. However, the majority of water liberated by dehydration reactions during subduction, including the breakdown of 5–10 vol.% eclogite facies and >10 vol.% pre‐eclogitic hydrous minerals, is not preserved in eclogite but liberated to the mantle wedge.

A refined zirconium-in-rutile thermometer

Abstract The zirconium-in-rutile thermometer enjoys widespread use, but confidence in its accuracy is limited because experiments were conducted at higher temperatures than many rutile-bearing rocks and calibration uncertainties have not been quantitatively assessed. Refined calibrations were developed using bootstrap regression to minimize residuals in the natural logarithm of the equilibrium constant, based on experiments only (n = 32) and on a combined compilation of experiments and natural data (n = 94, total). Rearranging the regression to solve for T, and expressing Zr concentration (C) in parts per million (μg/g), the calibrations in the α-quartz stability field are: Experimental data set: T ( C ∘ ) = 68740 + 0 . 441 · P ( bars ) - 0 . 114 · C ( ppm ) 129 . 76 - R · ln [ C ( ppm ) ] - 273 . 15 . Combined data set: T ( C ∘ ) = 71360 + 0 . 378 · P ( bars ) - 0 . 130 · C ( ppm ) 130 . 66 - R · ln [ C ( ppm ) ] - 273 . 1 . Thermodynamics of the quartz-coesite transition as applied to the calibration for α-quartz yields calibrations for the coesite stability field: Experimental data set T ( C ∘ ) = 71290 + 0 . 310 · P ( bars ) - 0 . 114 · C ( ppm ) 128 . 76 - R · ln [ C ( ppm ) ] - 273 . 15 . Combined data set: T ( C ∘ ) = 73910 + 0 . 247 · P ( bars ) - 0 . 130 · C ( ppm ) 129 . 65 - R · ln [ C ( ppm ) ] - 273 . 15. Propagated temperature uncertainties are ±20–30 °C (2σ) for the experimental data set calibration, and ±10–15 °C (2σ) for the combined data set. Compared to previous experimental calibrations, the refined thermometer predicts temperatures up to 40 °C lower for T ≤ 550 °C, and systematically higher temperatures for T > 800 °C. With careful attention to distributions of Zr in rutile grains, precisions of ±5 °C and accuracies ~±15 °C may be possible, although a poor understanding of how to select compositions for thermometry will typically lead to larger uncertainties. The ZiR calibration promises continued high-precision and accurate thermometry, and possibly improved thermodynamic properties, but the sources of compositional variability in rutile warrant further scrutiny.

Fossil subduction recorded by quartz from the coesite stability field

Abstract Metamorphic rocks are the records of plate tectonic processes whose reconstruction relies on correct estimates of the pressures and temperatures (P-T) experienced by these rocks through time. Unlike chemical geothermobarometry, elastic geobarometry does not rely on chemical equilibrium between minerals, so it has the potential to provide information on overstepping of reaction boundaries and to identify other examples of non-equilibrium behavior in rocks. Here we introduce a method that exploits the anisotropy in elastic properties of minerals to determine the unique P and T of entrapment from a single inclusion in a mineral host. We apply it to preserved quartz inclusions in garnet from eclogite xenoliths hosted in Yakutian kimberlites (Russia). Our results demonstrate that quartz trapped in garnet can be preserved when the rock reaches the stability field of coesite (the high-pressure and high-temperature polymorph of quartz) at 3 GPa and 850 °C. This supports a metamorphic origin for these xenoliths and sheds light on the mechanisms of craton accretion from a subducted crustal protolith. Furthermore, we show that interpreting P and T conditions reached by a rock from the simple phase identification of key inclusion minerals can be misleading.

Two-stage garnet growth in coesite eclogite from the southeastern Papua New Guinea (U)HP terrane and its geodynamic significance

Mineral compositions and textures of Late Miocene coesite eclogite from Tomagabuna Island were investigated to constrain P–T conditions during UHP metamorphism and subsequent exhumation. Two stages of garnet growth (core and rim), at eclogite-facies conditions, were documented. In addition to core garnet (I), peak assemblages include omphacite, coesite, phengite, and rutile. Rim garnet (II), amphibole, paragonite, plagioclase, quartz, and accessory biotite and spinel were formed after peak pressure conditions. Although a compositional gradient is present at the core–rim garnet interface, the garnet core has a relatively flat compositional profile, suggesting its crystallization in the coesite stability field. Together with minerals formed subsequent to peak pressure (UHP) conditions, the composition of the garnet rim indicates heating of the eclogite during its decompression into the amphibolite facies mineral stability field. Based on compositional zoning in garnet and application of diffusion modelling, we propose that eclogite-facies metamorphism of the mafic protolith and its host lithologies occurred at, or near, UHP conditions. The core garnet (I) formed at 650 °C at 8 Myr in the coesite stability field and was partially resorbed during the onset of exhumation. We infer that garnet rim growth at < 7.1 Ma occurred at P–T conditions corresponding to the boundary between the eclogite and amphibolite facies fields. Diffusion modelling for the garnet core–rim boundary compositions suggests a transient heating event (0.3 Myr) occurred at 1.5 GPa that we infer resulted from heat transport within the Australian—Woodlark plate boundary zone as the coesite eclogite was exhumed. Results caution that apparent P–T paths documented for many UHP terranes may not result from isothermal decompression corresponding to peak temperatures. Instead, paths may link P–T points’ set during transient mineral growth events when the UHP rocks form within the subduction channel, and during subsequent heating events, when UHP rocks are exhumed from mantle depths.

The Discovery of Mesoproterozoic Oceanic Crust In North Qaidam UHP Belt, NW China

AbstractThe North Qaidam is an Early Paleozoic UHP metamorphic belt located at the north margin of the Tibet plateau. Eclogites in this belt contain both continental‐and oceanic‐type ones. In which, the continental‐type eclogites have protolith ages of 750–850 Ma and WPB or CFB geochemical signatures and are believed to have formed in a continental rift or an incipient oceanic basin setting related to the breakup of the Rodinia supercontinent, their metamorphic ages (421–458 Ma) and P–T paths are comparable to their host gneisses; oceanic‐type eclogites have cumulate gabbro or E‐MORB geochemical signatures, their protolith and metamorphic ages are 510–516 Ma and 425–450 Ma, respectively(Zhang et al., 2008). Therefore, the North Qaidam UHP belt was thought to record the whole Neoprotoerozoic–Paleozoic Wilson cycle (Song et al., 2014). In this study, we reported three new kinds of eclogites: kyanite‐bearing eclogite, lawsonite pseudomorph‐bearing eclogite and double mineral eclogite. They occur as big lentoid blocks in regional granitic gneiss in the western part of the belt. Phase equilibrium modelling and zircon LA‐ICPMS U‐Pb dating show that all these three eclogites experienced a clockwise P–T path with peak metamorphic conditions close to or fall in the coesite stability field, and their peak metamorphic age were around 436‐439 Ma, similar to those continental‐type eclogites in this belt. But their protolith ages are between 1273 and 1070 Ma, and some of them recorded an amphibolite facies metamorphic age of 927 Ma, and geochemical data and zircon Lu‐Hf and O isotope analysis indicate these eclogites have features of present day N‐MORB. Combined with the existing results, we propose that the North Qaidam is a polycyclic composite orogenwhich recorded tectonic evolution of Mesoproterozoic ocean floor spreading, assembly and breakup of Rodinia supercontinent, Early Paleozoic oceanic deep subduction and subsequently continental deep subduction.

Quartz paramorphs after former stishovite in UHP eclogite from the South Altyn Tagh, Western China and its Significance

AbstractThe long prism/needle‐shaped polycrystalline quartz aggregates and square/parallelogram‐shaped singlephase quartz inclusions in omphacite and garnet of ultrahigh pressure eclogite were first discovered from the Jiangalesayi area, South Altyn UHP belt. Based on their morphology, these quartz inclusions are quartz paramorphs after stishovite. The minimum peak pressure of the eclogite is estimated to be &gt;8–9 GPa at 800– 1000 °C based on the stability field of stishovite. This new evidence, together with previous stishovite exsolution microstructure in the gneiss from the same region, suggests an ultra‐deep subduction and exhumation of the South Altyn continental rocks to/from mantle depths in stishovite stability field. Evidence of ultra‐deep subduction of continental materials might be more common and diverse than previous thought.Exhumation of subducted continental rocks from≥300 km has been considered impossible because they are denser than mantle at these depths. How did the stishovite bearing continental rocks of the South Altyn exhumated?As we all know, the densities of stishovite (4.3 g/cm3) are much higher than coesite (2.9 g/cm3), and stishovite transforms into coesite with temperature increases. Density calculations were performed for subducted continental rocks along phase transition of stishovite to coesite, using the third‐order Birch‐Murnaghan equation of state based on mineral fractions obtained from experiments and Perple_X. The results show that the density of Siliceous rocks decrease remarkably, lower than the surrounding mantle in coesite stability field, whereas the density of Oligosiliceous and Silicon unsaturated rocks is higher than surrounding mantle.Thus, we propose that the thermal induced transformation could provide an initial driven force for the exhumation of ultra‐deep subducted silica‐enriched felsic continental rocks. Temperature increase could be derived from an increased geothermal gradient from convective mantle or mantle plume. Mafic to ultra‐mafic rocks and silica‐deficient rocks may be captured by the upwelling subducted continental rocks and exhumated together.

An experimentally calibrated thermobarometric solubility model for titanium in coesite (TitaniC)

Experiments were conducted to quantify the temperature and pressure effects on the solubility of titanium in coesite. Powdered amorphous silica, titania (anatase), zirconia, and water were added to silver capsules and run in the coesite stability field (at 32, 35, and 40 kbar) from 700 to 1050 °C using a piston–cylinder apparatus. Crystallization of coesite, rutile, and zircon from silica-, titania-, and zircon-saturated aqueous fluids was confirmed by Raman spectroscopy. Cathodoluminescence images and electron microprobe measurements showed that coesite crystals are relatively homogenous. The Ti concentrations of coesite crystals are significantly higher than concentrations predicted using the Ti-in-quartz calibration (Wark and Watson in Contrib Mineral Petrol 152:743–754, 2006. https://doi.org/10.1007/s00410-006-0132-1 ; Thomas et al. in Contrib Mineral Petrol 160:743–759, 2010. https://doi.org/10.1007/s00410-010-0505-3 ). Titanium K-edge X-ray absorption near edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in coesite at all conditions studied. A model was calibrated to describe the effects of pressure and temperature on the solubility of titanium in coesite by using a least-squares method to fit Ti concentrations in coesite to the simple expression: $$RT\ln X_{{{\text{TiO}}_{2} }}^{\text{coesite}} = - 55.068 + 0.00195 \times T\;({\text{K}}) - 1.234 \times P\;({\text{kbar}}) + RT\ln a_{{{\text{TiO}}_{2} }}^{\text{rutile}} ,$$ where R is the gas constant 8.3145 × 10−3 kJ/K, P is pressure in kbar, T is temperature in kelvin, $$X_{{{\text{TiO}}_{2} }}^{\text{coesite}}$$ is the mole fraction of TiO2 in coesite, and $$a_{{{\text{TiO}}_{2} }}^{\text{rutile}}$$ is the activity of TiO2 in the system referenced to rutile. Ti-in-coesite solubility can be used as a thermobarometer for natural samples when used in combination with another indicator of temperature or pressure, such as another thermobarometer in a cogenetic mineral (e.g. rutile) or other phase equilibria (e.g. graphite = diamond). Applications of the Ti-in-coesite thermobarometer to samples from the western Alps and Papua New Guinea are presented.

HP–UHP metamorphism and tectonic evolution of orogenic belts: introduction

Abstract High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role for understanding the tectonic evolution of orogenic belts (Johnson, M.R.W. &amp; Harley, S.L. 2012. Orogenesis: The Making of Mountains . Cambridge University Press, Cambridge). They have typically experienced complex changes during subduction and exhumation processes, arising from recrystallization, deformation, element redistribution, fluid–rock interactions and even partial melting, and may therefore carry a valuable record of evolving geodynamic systems in an orogenic belt. Until now, more than 20 UHP metamorphic belts, i.e. belts which contain rocks that experienced pressure–temperature ( P–T ) conditions exceeding the lower limit of the coesite stability field, have been identified all over the world (Carswell, D.A. &amp; Compagnoni, R. 2003. Ultrahigh Pressure Metamorphism . Eotvos University Press, European Mineralogical Union, Notes in Mineralogy, Budapest; Liou, J.G., Ernst, W.G., Zhang, R.Y., Tsujimori, T. &amp; Jahn, B.M. 2009. Ultrahigh-pressure minerals and metamorphic terranes – the view from China. Journal of Asian Earth Sciences, 35 , 199–231; Zheng, Y.F., Zhang, L.F., McClelland, W.C. &amp; Cuthbert, S. 2012. Processes in continental collision zones: preface. Lithos , 136 , 1–9). New and innovative studies from different geoscience disciplines have been invaluable in developing a better understanding of the geodynamic evolution of orogenic belts. This special issue contains 15 papers, most of which were presented as part of the session ‘HP–UHP metamorphism and tectonic evolution of orogenic belts’, held at the 35th International Geological Congress in Cape Town, South Africa during 27 August to 4 September, 2016.

Tso Morari coesite eclogite: pseudosection predictions v. the preserved record and implications for tectonometamorphic models

Abstract Ultrahigh-pressure eclogites of the Tso Morari area, NW Himalaya (Ladakh, India), have been intensively investigated petrographically and petrologically with surprisingly different results. Metamorphic subduction paths based on mineral isopleths in pressure–temperature pseudosections in some studies claim concave (to the temperature axis) pressure–temperature paths predicting significant Ca–Mg–Fe garnet growth in the lawsonite and glaucophane fields: a prediction at odds with abundant epidote/clinozoisite and sodic-calcic amphibole inclusions in garnet interiors more probable along a convex path. One study deduced strong heating still at high pressures and proposed a felsic diapir rising through the mantle wedge: an explanation strongly at odds with well-documented glaucophane cores to barroisite replacing matrix omphacite requiring a cold exhumation most likely back up the subduction channel. In addition, matrix magnesite rimmed by dolomite suggests pressures well into the coesite (if not diamond) stability field: something neglected in most studies. Despite the application of modern analytical and thermodynamic modelling tools, the peak conditions attained by Tso Morari ultrahigh-pressure rocks are often poorly deduced and at odds with simple observations. Is this problem perhaps hindering the reliable identification of new ultrahigh-pressure terranes?

Isotopic-geochemical evidence for crustal contamination of eclogites in the Kokchetav subduction-collision zone

This paper reports isotopic and geochemical studies of eclogites from the western ultrahigh pressure (UHP) and eastern high-pressure (HP) blocks of the Kokchetav subduction-collision zone. These HP and UHP eclogites exhumed in two stages: (1) The rocks of the western block metamorphosed within the field of diamond stability (e.g., Kumdy-Kol and Barchy); (2) In contrast, the metamorphic evolution of the eastern block reached the pressure peak within the stability field of coesite (e.g., Kulet, Chaglinka, Sulu-Tyube, Daulet, and Borovoe). The eclogites vary widely in the ratios of incompatible elements and in the isotope ratios of Nd (143Nd/144Nd = 0.51137-0.513180) and Sr (87Sr/86Sr = 0.703930.78447). The Sulu-Tyube eclogites display isotope-geochemical features close to N-MORB, while those from the other sites are compositionally similar to E-type MORB or island arc basalts (IAB). The model ages TNd(DM) of eclogites vary between 1.95 and 0.67 Ga. The Sulu-Tyube eclogite yields the youngest age; it has the values of εNd(T) (7.2) and 87Sr/86Sr (0.70393) close to the depleted mantle values. The crustal input to the protolith of the Kokchetav eclogites is evident on the εNd(T)-86Sr/87Sr and εNd(T)-T plots. The eclogites make up a trend from DM to country rocks. Some eclogites from the Kulet, Kumdy-Kol, and Barchy localities display signs of partial melting, such as high Sm/Nd (0.65-0.51) and low (La/Sm)N (0.34-0.58) values. The equilibrium temperatures of these eclogites are higher than 850 °C. The geochemical features of eclogites testify to the possibility of the eclogite protolith formation in the tectonic setting of passive continental rift margin subducted to depths over 120 km.

Integrating X‐ray mapping and microtomography of garnet with thermobarometry to define theP–Tevolution of the (near)UHPMiędzygórze eclogite, Sudetes,SWPoland

AbstractDetailed X‐ray compositional mapping and microtomography have revealed the complex zoning and growth history of garnet in a kyanite‐bearing eclogite. The garnet occurs as clusters of coalesced grains with cores revealing slightly higher Ca and lower Mg than the rims forming the coalescence zones between the grains. Core regions of the garnet host inclusions of omphacite with the highest jadeite, and phengite with the highest Si, similar to values in the cores of omphacite and phengite located in the matrix. Therefore, the core compositions of garnet, omphacite, and phengite have been chosen for the peak pressure estimate. Coupled conventional thermobarometry, averageP–T, and phase equilibrium modelling in theNCKFMMnASHTsystem yieldsP–Tconditions of 26–30 kbar at 800–930°C. Although coesite is not preserved, theseP–Tconditions partially overlap the coesite stability field, suggesting near ultra‐high–pressure (UHP) conditions during the formation of this eclogite. Therefore, the peak pressure assemblage is suggested to have been garnet–omphacite–kyanite–phengite–coesite/quartz–rutile. Additional lines of evidence for the possibleUHPorigin of the Międzygórze eclogite are the presence of rod‐shaped inclusions of quartz parallel to the c‐axis in omphacite as well as relatively high values of Ca‐Tschermak and Ca‐Eskola components. Late zoisite, rare diopside–plagioclase symplectites rimming omphacite, and minor phlogopite–plagioclase symplectites replacing phengite formed during retrogression together with later amphibole. These retrograde assemblages lack minerals typical of granulite facies, which suggests simultaneous decompression and cooling during exhumation before the crustal‐scale folding that was responsible for final exhumation of the eclogite.

UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P–T path and tectonic implications

AbstractThe Seve Nappe Complex (SNC) of the Scandinavian Caledonides records a well‐documented history of high pressure (HP) and ultra‐high pressure (UHP) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jämtland and Norrbotten. The Jämtland eclogites and associated rocks are well‐studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P–T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high‐Na core and two rims: inner, medium‐Na and outer, low‐Na. Garnet consists of a high‐Ca euhedral core, low‐Ca inner rim and medium‐Ca outer rim. A similar pattern occurs within phengite, where high‐Si cores are enveloped by medium and low‐Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz‐in‐Garnet geobarometry, coupled with Zr‐in‐rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5–1.6 GPa and 620–660°C. The eclogite peak‐pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P–T conditions of 2.8–3.1 GPa and 660–780°C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMMnASHT system. Later, lower‐pressure stages E2 and E3 record conditions of 2.2–2.8 GPa, 680–780°C and 2.1 GPa, 735°C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole–quartz–plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at &lt;1.5 GPa (referred as E4 stage). The obtained P–T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P–T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8°C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.

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.

Ca-Eskola incorporation in clinopyroxene: limitations and petrological implications for eclogites and related rocks

Clinopyroxene is an essential mineral in eclogitic rocks. It commonly contains minor amounts of the defect-bearing Ca-Eskola (CaEs, Ca0.5□0.5AlSi2O6) component, with higher concentrations generally considered to indicate a high-pressure origin at least within the coesite stability field. Changes in pressure and temperature conditions can lead to exsolution of this component as a free SiO2 phase, which may have a number of petrological implications. This makes it important to understand the factors that maximize CaEs incorporation in clinopyroxene. We have undertaken a series of experiments at high pressures and temperatures (4–10 GPa and 1000–1350 °C) to further investigate the systematics of CaEs incorporation in eclogite-like clinopyroxene and the factors responsible for maximizing CaEs contents. Two simple chemical systems were chosen that allow unambiguous interpretation of the results: (1) CMAS + H2O and (2) two compositions in the NCMAS system. All experimental products contained clinopyroxene and garnet along with either a free SiO2 phase or a silicate melt. Coexisting garnet is grossular-rich, generally with X gr ≥ 0.67. Compositional variations are attributable to the presence or absence of melt and changes in modal amounts of garnet at different pressure–temperature conditions. Even small amounts of H2O lower the solidus temperature and the presence of a melt reduces the SiO2 activity, which destabilizes the CaEs component in clinopyroxene. The CaEs and the Ca-Tschermaks (CaTs, CaAl2SiO6) components in clinopyroxene decrease with increasing jadeite mole fraction, which is also a function of pressure and bulk Al content. Modeling X-ray powder diffraction data yields a molar volume for the CaEs endmember of V CaEs = 60.87(63) cm3, which reasonably agrees with a literature value that was estimated from natural samples. In the presence of coexisting coesite, the CaEs and CaTs do not vary independently of each other, being controlled by the internal equilibrium 2CaEs = CaTs + 3SiO2 (coesite). This relation, observed in simple systems (i.e., CMAS ± Na), is also obeyed by clinopyroxene in more complex, natural analog bulk compositions. An assessment of available experimental data reveals a maximum of 15–18 mol% CaEs in eclogitic clinopyroxene at conditions corresponding to 130–180 km depth. CaEs contents are maximized at high temperatures; i.e., at or near the solidus in the presence of coesite. Thus, this study supports the role of CaEs exsolution in contributing to melt generation during upwelling of eclogite bodies in the mantle, albeit with some caveats. Somewhat higher maximum CaEs contents (~20 mol%) are found in Ca and Al-rich bulk compositions, such as grospydite xenoliths. Such bulk compositions also seem to require the coexistence of kyanite. Other Ca and Al-rich rock types, like rodingites, should have the potential of containing CaEs-rich clinopyroxenes, except that they are SiO2-undersaturated. This emphasizes the further role of bulk composition, in addition to high temperatures, in achieving maximum CaEs contents in high-pressure clinopyroxene.

Eclogite varieties and petrotectonic evolution of the northern Guatemala Suture Complex

ABSTRACTField and petrologic characteristics of two new eclogite localities within the Guatemala Suture Complex (GSC) north of the Motagua Fault are presented. The Tuncaj Hill locality exposes a coherent body of retrogressed eclogite hundreds of metres long that is associated with serpentinite of the North Motagua Unit. The Tanilar River locality exposes numerous bands and lenses of eclogite hosted in sialic gneisses of the Chuacús Complex. The Tuncaj eclogite has a two-stage prograde evolution containing the peak assemblage Grt + Omp + Ttn + Czo + Zo ± Am, formed at temperatures <720°C. In contrast, eclogites of the Tanilar unit are characterized by the paragenesis Omp + Grt + Rt ± Phg ± Qtz ± Ep giving higher peak conditions of T = 720–830°C and P = 2.1–2.7 GPa, near the stability field of coesite. Previously obtained data and our thermobaric calculations suggest distinct petrotectonic evolutions for the various types of eclogites within the suture. The lawsonite eclogites south of the Motagua Fault were probably produced in a mature Farallon subduction zone during the Early Cretaceous. The northern high-pressure (HP) blocks in serpentinite mélange and coherent amphibolite bodies with eclogite relics were generated by the Early Cretaceous subduction of the proto-Caribbean lithosphere under the Great Caribbean Arc. A continental block, the North American passive margin, reached the arc’s trench in the Campanian and was subducted to ca. 80 km depth, producing the eclogites of the Chuacús Complex. As the slab was delaminated and partially exhumed, the continental Chuacús became tectonically juxtaposed with HP blocks of the proto-Caribbean that had been accreted to the Caribbean plate forming the North Motagua Unit. The juxtaposed group migrated to mid-crustal level and was contemporaneously retrogressed under epidote-amphibolite facies conditions.

Stresses and pressures at the quartz‐to‐coesite phase transformation in shear deformation experiments

AbstractCoesite was found in quartz aggregates, experimentally deformed at confining pressures of 1.0–1.5 GPa and temperatures between 600°C and 900°C. The confining pressure (Pc) and, in most cases, the mean stress (σm) of the experiments were below those of the quartz‐to‐coesite phase transformation. Yet coesite formed when the maximum principal stress (σ1) was within the P‐T range of the coesite stability field. In one sample, the euhedral coesite grains were corroded indicating that coesite started to transform back to quartz. It is inferred that this sample started to deform with σ1 above the quartz‐to‐coesite phase transformation and, with ongoing deformation, σ1 decreased to values in the quartz stability field due to strain weakening. In all cases, σ1 triggered the quartz‐to‐coesite reaction as well as the reverse reaction, suggesting that σ1 is the critical parameter for the quartz‐to‐coesite transformation—not Pc or σm. With progressive deformation, the coesite laths rotated toward the shear plane as more rigid particles with the sense of shear. In case of back reaction, new quartz grains exhibit no systematic crystallographic relationship with respect to old coesite. The experiments cover different degrees of pressure “overstepping,” different temperatures, and different experimental durations at P and T, and deformation always enhances the reaction kinetics. The observation that σ1 is critical for a pressure‐dependent phase transformation (also for reversals) poses questions for the thermodynamic treatment of such phase transformations.