Coesite Inclusions Research Articles

Raman Spectroscopy of Mineral Inclusions in Diamonds from Yakutia

New data on the study of residual pressure in mineral inclusions in diamonds from kimberlite pipes and placers of Yakutia obtained using Raman Spectroscopy are presented. Calculated values of residual pressure in olivine and coesite inclusions in kimberlite diamonds according to works of Israeli et al [1] and Sobolev et al [2] indicate that the host crystal crystallized at the depth not less than 165 km, at pressure of 4.8-5.0 GPa and temperature T=1200 °C. Comparative analysis of Raman spectra of olivine inclusions in diamonds from placers of Ebelyakh river on inclusions, contained in diamond and then brought to the surface, showed that olivine inclusions are less stressed. The structural orientation of olivine inclusions in diamonds from placers of Ebelyakh river possibly plays a role in these differences of Raman spectra.

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

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

Prograde Metamorphic History of UHP Granulites from the Moldanubian Zone (Bohemian Massif) Revealed by Major Element and Y + REE Zoning in Garnets

Major and trace element distribution in garnet crystals from felsic granulites of the Kutná Hora Complex provides evidence of two metamorphic events, the first related to continental subduction at ultrahigh-pressure (UHP) conditions and the second to a granulite-facies overprint that occurred at mid- to lower crustal levels. To reconstruct P–T paths of both metamorphic events, pseudosection modeling was combined with major and trace element zoning in garnet. The granulites contain lenses and boudins of mantle-derived garnet peridotite and eclogite. UHP conditions are confirmed by the presence of inclusions of micro-diamond and coesite in garnet and zircon in the granulites. Compositional zoning in garnet provides evidence for a pre-granulite-facies high-pressure (HP)–UHP metamorphic history of the rocks. The two distinct metamorphic events are documented by high Y + HREE concentrations in the garnet cores and annuli in the mantle part of the garnet grains. The cores of large garnets with relatively high Ca contents and bell-shaped Mn profiles suggest formation during a prograde low- to medium-temperature metamorphic event that was coeval with HP–UHP metamorphism. Decompression and cooling during exhumation of the rocks led to partial resorption of garnet and release of trace elements into the matrix. The new garnet with high Y + HREE in the annuli was formed during the granulite-facies event at crustal levels.

Discussion on “Coesite-bearing eclogite breccia: implication for coseismic ultrahigh-pressure metamorphism and the rate of the process” by Yang et al. (Contrib. Mineral. Petrol., 2014a, 167: 1013)

Recently, Yang et al. (Contrib Mineral Petrol 167:1013, 2014a) proposed that “cataclasites,” “breccias” and “pseudotachylytes” of eclogites at Yangkou in the Chinese Sulu UHP metamorphic belt are formed by “a compression stress wave of earthquake.” They suggested that the intergranular coesite resulted from a rapid cooling from about 700 °C to below 375–400 °C and a “sudden pressure release of seismic wave” from 3.3 to 1.2 GPa in some hours. However, the earthquake-induced UHP metamorphism under fluid-deficient conditions, proposed by these authors, remains inconclusive and inconsistent with the available data from the UHP metamorphic belt. The coesite inclusions within garnet, zircon and omphacite, and intergranular coesite grains between these minerals can be preserved by the presence of the pressure vessel effect, the lack of fluid infiltration, and the presence of a low-temperature and nonhydrostatic deformation environment during rapid exhumation.

Jadeite- and dolomite-bearing coesite eclogite from western Tianshan, NW China

Jadeite associated with omphacite has been reported from high-pressure, low-temperature (H P -L T ) rocks worldwide. However, the upper pressure stability of this association is not clear in nature. This study describes an immiscible texture of clinopyroxenes at ultrahigh-pressure (UH P )-L T conditions from western Tianshan, NW China. The host eclogite mainly consists of quartz, garnet, carbonate, albite, omphacite, white mica and Na-Ca amphibole. The modal contents of constituent minerals vary from place to place, and quartz-rich and carbonate-rich domains are developed. Jadeite (Jd 70–91 Aeg 2–16 ) only occurs as inclusion in garnet, mostly accompanied by patchy omphacite. Micro-Raman analyses show that jadeite and omphacite have different diagnostic peaks, consistent with the quantitative microprobe results. Jadeite is locally replaced by secondary sodic phases like paragonite and albite. A composite inclusion occurs in garnet and consists of lath-like paragonite, quartz, omphacite and titanite, with or without glaucophane. Matrix porphyroblastic albite usually encloses paragonite, titanite and barroisite, with or without omphacite, constituting a multi-phase aggregate. These two types of sodic phase-rich aggregates within and outside garnet are interpreted to be pseudomorphic replacements after jadeite at different conditions during retrogression. In addition to coesite in garnet, inclusions of coesite and its pseudomorphs without radial fractures were found in ankeritic dolomite. This first reported paragenesis of dolomite + coesite at low temperatures suggests that dolomite can be stabilized at depths of up to 90 km in a cold subduction zone. This contribution not only reports a new occurrence of Tianshan UH P rocks, but also suggests that UH P -L T metamorphism has impacted a variety of lithologies in addition to paraschist and basaltic eclogite, thus supporting the internal coherence of the UH P terrain of the Chinese western Tianshan.

Grain‐scale pressure variations and chemical equilibrium in high‐grade metamorphic rocks

AbstractIn the classical view of metamorphic microstructures, fast viscous relaxation (and so constant pressure) is assumed, with diffusion being the limiting factor in equilibration. This contribution is focused on the only other possible scenario – fast diffusion and slow viscous relaxation – and brings an alternative interpretation of microstructures typical of high‐grade metamorphic rocks. In contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals, a multi‐anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time‐scales in a polycrystalline material. In such a mechanical context, exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses (>10 kbar) at geological time‐scales. Furthermore, it is argued that the existence of grain‐scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims. Assuming zero net flux across the microstructure, an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning. This new barometric method is applied to plagioclase rims around kyanite in felsic granulite (Bohemian Massif, Czech Republic), yielding a grain‐scale pressure variation of 8 kbar. In this approach, kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade.

The problem of depth in geology: When pressure does not translate into depth

We review published evidence that rocks can develop, sustain and record significant pressure deviations from lithostatic values. Spectroscopic studies at room pressure and temperature (P-T) reveal that in situ pressure variations in minerals can reach GPa levels. Rise of confined pressure leads to higher amplitude of these variations documented by the preservation of α-quartz incipiently amorphized under pressure (IAUP quartz), which requires over 12 GPa pressure variations at the grain scale. Formation of coesite in rock-deformation experiments at lower than expected confined pressures confirmed the presence of GPa-level pressure variations at elevated temperatures and pressures within deforming and reacting multi-mineral and polycrystalline rock samples. Whiteschists containing garnet porphyroblasts formed during prograde metamorphism that host quartz inclusions in their cores and coesite inclusions in their rims imply preservation of large differences in pressure at elevated pressure and temperature. Formation and preservation of coherent cryptoperthite exsolution lamellae in natural alkali feldspar provides direct evidence for grain-scale, GPa-level stress variations at 680°C at geologic time scales from peak to ambient P-T conditions. Similarly, but in a more indirect way, the universally accepted’ pressure-vessel’ model to explain preservation of coesite, diamond and other ultra-high-pressure indicators requires GPa-level pressure differences between the inclusion and the host during decompression at temperatures sufficiently high for these minerals to transform into their lower pressure polymorphs even at laboratory time scales. A variety of mechanisms can explain the formation and preservation of pressure variations at various length scales. These mechanisms may double the pressure value compared to the lithostatic in compressional settings, and pressures up to two times the lithostatic value were estimated under special mechanical conditions. We conclude, based on these considerations, that geodynamic scenarios involving very deep subduction processes with subsequent very rapid exhumation from a great depth must be viewed with due caution when one seeks to explain the presence of microscopic ultrahigh-pressure mineralogical indicators in rocks. Non-lithostatic interpretation of high-pressure indicators may potentially resolve long-lasting geological conundrums.

Petrology and U–Pb zircon dating of coesite-bearing metapelite from the Kebuerte Valley, western Tianshan, China

This paper deals with the petrology and U–Pb dating of coesite-bearing garnet–phengite schist from the Kebuerte Valley, Chinese western Tianshan. It mainly consists of porphyroblastic garnet, phengite, quartz and chlorite with minor amounts of paragonite, albite, zoisite and chloritoid. The well preserved coesite inclusions (∼100μm) in garnet are encircled by a narrow rim of quartz. They were identified by optical microscopy and confirmed by Raman spectroscopy. Using the computer program THERMOCALC, the peak metamorphic conditions of 29kbar and 565°C were obtained via garnet isopleth geothermobarometry. The predicted UHP peak mineral assemblage comprises garnet+jadeite+lawsonite+carpholite+coesite+phengite. The metapelite records prograde quartz–eclogite-facies metamorphism, UHP coesite–eclogite-facies peak metamorphism, and a late greenschist-facies overprint. Phase equilibrium modeling predicts that garnet mainly grew in the mineral assemblages garnet+jadeite+lawsonite+chloritoid+glaucophane+quartz+phengite and garnet+jadeite+lawsonite+carpholite+glaucophane+quartz+phengite. SHRIMP U–Pb zircon dating of the coesite-bearing metapelite yielded the peak metamorphic age 320.4±3.7Ma. For the first time, age data of coesite-bearing UHP metapelite from the Chinese western Tianshan are presented in this paper. They are in accord with published ages obtained from eclogite from other localities in the Chinese western Tianshan and the Kyrgyz South Tianshan and therefore prove a widespread occurrence of UHP metamorphism.

Petrological and zircon evidence for anatexis of UHP quartzite during continental collision in the Sulu orogen

AbstractPetrological evidence is provided for anatexis of ultrahigh‐pressure (UHP) metamorphic quartzite in the Sulu orogen. Some feldspar grains exhibit elongated, highly cuspate shapes or occur as interstitial, cuspate phases constituting interconnected networks along grain boundaries. Elongated veinlets composed of plagioclase + quartz ± K‐feldspar also occur in grain boundaries. These features provide compelling evidence for anatexis of the UHP quartzite. Zircon grains from impure quartzite are all metamorphic growth with highly irregular shape. They contain inclusions of coesite, jadeite, rutile and lower pressure minerals, including multiphase solid inclusions that are composed of two or more phases of muscovite, quartz, K‐feldspar and plagioclase. All zircon grains exhibit steep REE patterns, similar U–Pb ages and Hf isotope compositions with a weighted mean of 218 ± 2 Ma. Most grains have similar δ18O values of −0.6 to 0.1‰, but a few fall in the range −5.2 to −4.3‰. Thus, these grains would have grown from anatectic melts at various pressures. Zircon O isotope differences indicate that anatectic melts were derived from different sources with contrasting O isotopes, but similar Hf isotopes, that is, one from the quartzite itself and the other probably from the country‐rock granitic gneiss. Zircon grains from pure quartzite contain relict magmatic cores and significant metamorphic overgrowths. Domains that contain eclogite facies minerals exhibit flat HREE patterns, no Eu anomalies and concordant U–Pb ages of c. 220 Ma. Similar U–Pb ages are also obtained for domains that contain lower pressure minerals and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite facies conditions but suprasolidus growth at lower pressures. Zircon enclosed by garnet gave consistent U–Pb ages of c. 214 Ma. Such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record multistage growth and metasomatism by anatectic melts. Therefore, the anatexis of UHP metamorphic rocks is evident during continental collision in the Triassic.

Two fresh types of eclogites in the Dabie-Sulu UHP metamorphic belt, China: Implications for the deep subduction and earliest stages of exhumation of the continental crust

Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are discernible in large eclogite bodies surrounded by country rock gneisses from the Dabie (大别)-Sulu (苏鲁) UHP metamorphic zone. They are different in mineral assemblage, texture and structure at various scales. The massive eclogite has a massive appearance with a metamorphic inequigranular and granoblastic texture, which consists mainly of nominally anhydrous minerals such as garnet, omphacite, rutile with inclusions of coesite and rare microdiamond. Massive eclogites which formed at the peak UHP metamorphic conditions (∼3.1–4.0 GPa, 800±50 °C) within the coesite to diamond stability field recorded the deep continental subduction to mantle depths greater than 100 km during the Triassic (∼250–230 Ma). The diagnostic UHP minerals, mineral assemblages and absence of notable macroscopic deformation indicate the peak metamorphic’ forbidden-zone’ P-T conditions, an extremely low geothermal gradient (≤7 °C·km−1) and low differential stress. The foliated eclogite is composed of garnet+omphacite+rutile+phengite+kyanite+zoisite+talc+nyboite±coesite/quartz pseudomorphs after coesite. It is quite clear that the foliated eclogite bears relatively abundant hydrous mineral, and shows well-developed penetrative foliation carrying mineral and stretching lineation reflecting intense plastic deformation or flow of eclogite minerals. The foliated eclogite occurred at mantle levels and recorded the earliest stages of exhumation of UHP metamorphic rocks. At a map scale, the foliated eclogites define UHP eclogite-facies shear zones or high-strain zones. Asymmetric structures are abundant in the zones, implying bulk plane strain or dominant non-coaxial deformation within the coesite stability field. The earliest stages of exhumation, from mantle depths to the Moho or mantle-crust boundary layering, were characterized by a sub-vertical tectonic wedge extrusion, which occurred around 230-210 Ma. The three-dimensional relationship between the massive and foliated eclogites is well displayed a typical ‘block-in-matrix’ rheological fabric pattern indicating the partitioning of deformation and metamorphism in the UHP petrotectonic unit. The existing data support the now widely accepted concept of deep continental subduction/collision and subsequent exhumation between the Yangtze and Sino-Korean cratons. The pressure is a constitutive geological variable. The influence of tectonic overpresure on UHP metamorphism is rather limited.

Northward extrusion of the ultrahigh‐pressure units in the southern Dabie metamorphic belt, east‐central China

AbstractThe Dabie orogenic belt in central China extends along the Triassic suture between the Sino–Korean and Yangtze cratons. The structure of the metamorphic rocks have been investigated in the Shima–Wumiao area near the southeastern end of the Dabie orogenic belt. Detailed mapping revealed that the metamorphic sequence in this area can be subdivided into four subhorizontal lithotectonic units. These units are denoted by I, II, III, and IV in the order of structural bottom to top. Unit I is composed of felsic gneiss, eclogite, and minor pelitic gneiss. Unit II consists of felsic gneiss and eclogite with coesite pseudomorphs. Unit III consists of felsic gneiss, pelitic gneiss, metacarbonate, and eclogite with occasional coesite inclusion. Unit IV is composed of felsic gneiss, amphibolite, and minor pelitic gneiss. Those metamorphic rocks underwent penetrative ductile deformation which is denoted by gently south‐dipping or subhorizontal foliation and NNW‐trending mineral lineation. Deformation microstructures in oriented samples from Units I and II indicate consistent northward displacements of the hanging wall and those from Units III and IV indicate southward displacements. The subhorizontal structure and opposite shear directions between Units I–II and Units III–IV suggest northward extrusion of UHP metamophic units (II and III). The subduction polarity could be opposite to the extrusion trend. The UHP metamorphism in the Dabie orogenic belt is attributable to southward subduction of the Sino–Korean craton beneath the Yangtze craton.

Synexhumation anatexis of ultrahigh-pressure metamorphic rocks: Petrological evidence from granitic gneiss in the Sulu orogen

Petrological evidence is provided for partial melting of ultrahigh-pressure (UHP) metamorphic granitic gneiss in the Sulu orogen. Petrographic observations show the occurrence of elongated, highly cuspate feldspars in grain boundaries, interstitial cuspate feldspars in triple junctions, felsic veinlets mainly consisting of K-feldspar+quartz, and feldspar crystal faces against quartz. These features indicate that the feldspar and quartz would have grown from anatectic melts in the granitic gneiss. Zircon domains grown from these melts were identified based on CL images, mineral inclusions and REE patterns. Some large zircon grains (>100μm) contain small relict domains of magmatic origin, suggesting nearly complete dissolution of the protolith zircon during the anatexis. All newly grown zircon domains are categorized into two groups based on the presence or absence of coesite inclusions. One group of domains contain no coesite inclusion and exhibit high U contents but low Th/U ratios (<0.1), steep REE patterns with strong negative Eu anomalies, and UPb ages of 217±2 to 224±2Ma. The other group of domains contain coesite inclusions and exhibit low U contents and very low Th/U ratios (<0.01), steep REE patterns with strong negative Eu anomalies, and UPb ages of 221±5 to 226±3Ma. The two groups of zircon domains are thus interpreted as growing from the anatectic melts at different pressures because they exhibit marked negative Eu anomalies that are absent for metamorphic zircons grown from aqueous fluids. The zircon UPb ages of 217±2 to 226±3Ma are close to, but slightly younger than, known ages for major UHP metamorphism in the Sulu orogen. Therefore, the UHP gneiss experienced incipient melting during the initial exhumation subsequent to the peak UHP metamorphism and extensive anatexis later at lower pressures. Muscovite relicts coexist with cuspate feldspars, suggesting that the anatectic melts originate from dehydration melting due to decompression breakdown of phengitic muscovite in the UHP granitic gneiss. As the degree of partial melting increases with temperature, significant fractions of the anatectic melts would be produced in the regional gneiss provided that the UHP rocks experienced “hot” exhumation. Such melts can be gathered together, being eventually emplaced as synexhumation granitic intrusions.

Mineralogy, petrology, U-Pb geochronology, and geologic evolution of the Dabie-Sulu classic ultrahigh-pressure metamorphic terrane, East-Central China

The Dabie-Sulu Triassic collisional orogen in eastern Asia was created by northward subduction of the Yangtze continental-crust capped plate beneath the Sino-Korean craton. Eclogites, garnet peridotites, and surrounding country rock gneisses and marbles were all subjected to in situ UHP metamorphism, as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks, as well as by virtually identical metamorphic ages of various UHP rock types. Metamorphic P-T estimates, combined with investigations of mineral exsolution textures and high- P polymorphs, indicate that recovered depths of continental subduction may have exceeded 200 km. Parageneses of mineral inclusions in zoned zircon domains combined with U-Pb ages delineate a well-constrained P-T -time path, suggesting exhumation rates of 5–10 km/Myr. A similar P-T -time trajectory has been established for the microdiamond-bearing Kokchetav Massif. Thus far, however, diamond inclusions have not been confirmed from coesite-bearing zircon domains of Dabie-Sulu UHP rocks despite numerous detailed studies. Oxygen isotopes of minerals from many outcrop samples and the Chinese Continental Scientific Drilling (CCSD) project main hole cores indicate that Delta; 18 O depletion took place in a volume of Proterozoic protoliths exceeding 100 000 km 3 along the northern edge of the Yangtze craton. Evidently, passive-margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent Rodinia were subjected to extensive meteoric water-rock interactions attending terminal Neoproterozoic Snowball Earth conditions. Such hydrothermal alteration volatilized and depleted C from the relatively oxidized protoliths, accounting for the rare occurrences of graphite and apparent lack of microdiamond in Dabie-Sulu UHP rocks.

From oceanic subduction to continental collision: An overview of HP–UHP metamorphic rocks in the North Qaidam UHP belt, NW China

The North Qaidam UHPM belt is unique among “continental type” subduction zones in showing well-preserved subducted oceanic relics that predate continental subduction and collision. We review petrologic, thermobarometric, geochemical and geochronological studies for the Yuka, Luliang Shan, Xitieshan and Dulan terranes in this belt. UHP conditions are demonstrated by coesite inclusions in eclogite and country gneiss, and diamond inclusions from garnet peridotite. The relict subducted oceanic lithology crops out in the Shaliuhe cross-section in Dulan terrane; it originated as the floor of a Paleo-Qilian Ocean that existed between Qaidam and Qilian blocks before the early Ordovician. Whereas most eclogites of this belt are derived from mafic rocks of either a Neoproterozoic continental rift or incipient oceanic basin setting, which were subducted along with continental rocks in the early Paleozoic. The data, especially the two protolith sources for eclogites, demonstrate tectonic evolution of the North Qaidam UHPM belt from oceanic subduction to continental collision.

Inclusions under remnant pressure in diamond: a multi-technique approach

Measurement of the remnant pressure sustained by a mineral inclusion within a diamond can allow calculation of the source pressure and temperature conditions of the diamond formation. While Raman spectroscopy (point analyses and 2D mapping) has been the most commonly used non-destructive method for measuring this remnant pressure, two new techniques (quantitative birefringence analysis and in situ X-ray diffraction) have recently been developed. In this paper we apply all of these techniques to the study of two diamonds. The first is a diamond twin (macle) from Shandong (China), containing two olivine inclusions. Analysis of the largest inclusion by the birefringence and X-ray techniques returned compatible values for the remnant pressure of 0.1–0.2 GPa. However, while 2D Raman mapping of the diamond host qualitatively confirmed the presence of a radial stress field, the low remnant pressures values were too small to be detected by pressure shifts in the olivine’s Raman spectrum. The second sample studied was a rounded dodecahedral diamond (unknown origin) containing several coesite inclusions. Its morphology prevented quantitative birefringence analysis or Raman mapping being performed but Raman analyses on three inclusions (2.2–2.5 GPa) provided similar results to those obtained by X-ray diffraction from a single inclusion (2.7 GPa); all of these values lie within the range of previous published remnant pressures for coesite in diamond. Calculation of the source pressure and temperature conditions for both diamond samples returned anomalously low values, both below the diamond stability field for a range of mantle temperatures (950–1350 °C). The reasons for this are due to violations of the fundamental assumptions upon which such studies of these elastic effects in diamond are based. Although previous studies showed that olivine inclusions have the potential to record reasonable formation pressures under favourable conditions, the present study re-iterates the serious concerns of using the coesite-in-diamond geobarometer. It is inferred that, as the remnant pressures in the coesite-diamond pair are so high, there is a high probability that the diamond will undergo some plastic deformation, thus reducing the elastic behaviour and leading to severe underestimation of entrapment pressure.

First record and timing of UHP metamorphism from zircon in the Xitieshan terrane: Implications for the evolution of the entire North Qaidam metamorphic belt

The Xitieshan terrane is one of four metamorphic terranes in the North Qaidam metamorphic belt, which is an early Paleozoic high-pressure to ultrahigh-pressure (HP-UHP) metamorphic belt in NW China. However, conclusive evidence and precise timing of UHP metamorphism in the Xitieshan terrane have not been well documented. In this study, we report an integrated study of zircon grains from an amphibolite in the Xitieshan terrane in terms of mineral inclusions, trace elements, and U-Pb age systematics. The first record of coesite is reported as an inclusion in a metamorphic zircon, which provides unambiguous evidence for the UHP metamorphism of the Xitieshan terrane. The metamorphic zircon domains have weak or no zoning, very low Th/U ratios, insignificant Eu anomalies, and flat HREE patterns. Zircon grains contain inclusions of garnet, omphacite and rutile, in addition to the coesite inclusion. These inclusions indicate that the metamorphic zircon grains formed under UHP metamorphic conditions. The metamorphic zircon grains were dated by the SIMS and LA-ICPMS methods and yield weighted mean ages of 432 ± 14 and 441 ± 9 Ma, respectively. Combined with previous results, the HP-UHP metamorphism of the Xitieshan terrane may have lasted 460-440 Ma with the peak UHP metamorphism at 441 ± 9 Ma. A compilation of the reported geochronological data reveals that all four terranes of the North Qaidam metamorphic belt might have experienced coeval UHP metamorphism during the early Paleozoic (420-450 Ma), and thus may have suffered a coherent subduction, UHP metamorphism, and exhumation cycle.

U–Pb zircon geochronology of coesite‐bearing eclogites from the southern Dulan area of the North Qaidam UHP terrane, northwestern China: spatially and temporally extensive UHP metamorphism during continental subduction

AbstractCoesite‐bearing eclogites from &gt;100 km2in the southern Dulan area, North Qaidam Mountains (NQM) of western China, contain zircon that records protolith crystallization and ultra high pressure (UHP) metamorphism. Sensitive High‐Resolution Ion Microprobe (Mass Spectrometer) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry U–Pb analyses from cathodoluminescence (CL)‐dark zircon cores in a coesite‐bearing eclogite yield an upper intercept age of 838 ± 50 Ma, and oscillatory zoned cores in a kyanite‐bearing eclogite gave a weighted mean206Pb/238U age of 832 ± 20 Ma. These zircon cores yield steep heavy rare earth element (HREE) slopes and negative Eu anomalies that suggest a magmatic origin. Thus,c. 835 Ma is interpreted as the eclogite protolith age. Unzoned CL‐grey or ‐bright zircon and zircon rims from four samples yield weighted mean ages of 430 ± 4, 438 ± 2, 446 ± 10 and 446 ± 3 Ma, flat HREE patterns without Eu anomalies, and contain inclusions of garnet, omphacite, rutile, phengite and rare coesite. These ages are interpreted to record 16 ± 5 Myr of UHP metamorphism. These new UHP ages overlap the age range of both eclogite and paragneiss from the northern Dulan area, suggesting that all UHP rock types in the Dulan area belong to the same tectonic unit. Our results are consistent with slow continental subduction, but do not match oceanic subduction and diapiric exhumation UHP model predictions. These new data suggest that, similar to eclogites in other HP/UHP units of the NQM and South Altyn Tagh, protoliths of the eclogites in the Dulan area formed in a continental setting during the Neoproterozoic, and then subducted to mantle depth together with continental materials during the Early Palaeozoic.

Quantifying strain birefringence halos around inclusions in diamond

The pressure and temperature conditions of formation of natural diamond can be estimated by measuring the residual stress that an inclusion remains under within a diamond. Raman spectroscopy has been the most commonly used technique for determining this stress by utilising pressure-sensitive peak shifts in the Raman spectrum of both the inclusion and the diamond host. Here, we present a new approach to measure the residual stress using quantitative analysis of the birefringence induced in the diamond. As the analysis of stress-induced birefringence is very different from that of normal birefringence, an analytical model is developed that relates the spherical inclusion size, Ri, host diamond thickness, L, and measured value of birefringence at the edge of the inclusion, to the peak value of birefringence that has been encountered; to first order. From this birefringence, the remnant pressure (Pi) can be calculated using the photoelastic relationship, where qiso is a piezo-optical coefficient, which can be assumed to be independent of crystallographic orientation, and n is the refractive index of the diamond. This model has been used in combination with quantitative birefringence analysis with a MetriPol system and compared to the results from both Raman point and 2D mapping analysis for a garnet inclusion in a diamond from the Udachnaya mine (Russia) and coesite inclusions in a diamond from the Finsch mine (South Africa). The birefringence model and analysis gave a remnant pressure of 0.53 ± 0.01 GPa for the garnet inclusion, from which a source pressure was calculated as 5. 7 GPa at 1,175°C (temperature obtained from IR analysis of the diamond host). The Raman techniques could not be applied quantitatively to this sample to support the birefringence model; they were, however, applied to the largest coesite inclusion in the Finsch sample. The remnant pressure values obtained were 2. 5 ± 0.1 GPa (birefringence), 2.5 ± 0.3 GPa (2D Raman map), and 2.5-2.6 GPa (Raman point analysis from all four inclusions). However, although the remnant pressures from the three methods were self-consistent, they led to anomalously low source pressure of 2.9 GPa at 1,150°C (temperature obtained from IR analysis) raising serious concerns about the use of the coesite-in-diamond geobarometer. © 2010 Springer-Verlag.

UHP metamorphic evolution of coesite-bearing eclogite from the Yuka terrane, North Qaidam UHPM belt, NW China

Coesite, recognized petrographically and confirmed by Raman spectroscopy, is reported from a coarse-grained eclogite from the Yuka terrane, North Qaidam UHP metamorphic belt. This represents the first record of UHP metamorphism in the western part of this belt. In addition, inclusions of Ab + Qtz assemblage in gamet with strong evidence for volume expansion may have derived from a Jd + Coe precursor. Two types of eclogite, coarse-grained and fine-grained, are exposed in the Yuka terrane. The eclogites experienced three distinguishable metamorphic stages: (1) a pre-peak metamorphic stage recorded by the mineral assemblage of Amp + PI + Qtz in the garnet core at P = 0.8-1.0 GPa, T = 450-560 °C for the coarse-grained eclogite and P = 1.0-1.2 GPa, T = 450-510 °C for the fine-grained eclogite, and by the mineral assemblage of Cpx + Phn in the garnet mantle at P = 2.25-2.65 GPa, T = 550-610 °C for the coarse-grained eclogite; (2) the peak stage characterized by the mineral assemblage of Grt + Omp + Phn + Qtz/Coe + Rt at P = 3.0 GPa. T = 652 °C for the coarsegrained eclogite, and Grt + Omp + Phn + Rt + Qtz at P = 2.9-3.2 GPa, T = 566-613 °C for the fine-grained eclogite; and (3) a retrograde stage recorded by the garnet reaction rims, the breakdown of omphacite, and the pervasive retrograde mineral assemblage of Grt + Amp + Pl + Qtz at P = 0.7-1.1 GPa, T = 550-590 °C for the coarse-grained eclogite. The coesite inclusion and the calculated P-T paths suggest that the Yuka UHPM terrane experienced fast and deep subduction (∼100 km) under a cool geothermal gradient (6-7 °C/km) followed by rapid exhumation.

Paleozoic U-Pb age of rutile inclusions in diamonds of the V–VII variety from placers of the northeast Siberian platform

1151 Diamonds attributed according to the classifica tion of Yu.L. Orlov to the V and VII varieties are wide spread in rich placers of the northeastern part of the Siberian Platform [6]. Mineralogical investigation (certification) of these diamonds showed that they represent a unified genetical type subdivided by Orlov according to the following formal characteristics: dia monds of the V variety are represented by monocrys tals, and diamonds of the VII variety are represented by intergrowths. Thus, we unite them into one variety V–VII [2]. Diamonds of this joint variety are easily distinguished among other diamonds from placers of the northeastern part of the Siberian platform for the following reasons: (1) these diamonds are represented by crystals of octahedral habitus (rarely) or their inter growths; (2) they occur in the forms of dissolution of dodecahedroids and intermediate forms between octahedrons and dodecahedroids (often); (3) crystals are overfilled with black flaky inclusions (graphite along the walls of flattened vacuoles with fluids; (4) the surface of crystals is covered by scars and fissures of leaching along the vacuoles brought out into the open surface during dissolution (Fig. 1a); (5) these dia monds are characterized by increased size (crystals less than 1 mm are practically absent); (6) lightened isotope composition of carbon, δ13С varies from –19 to –25‰, which corresponds to the characteristics of crustal carbon of organic origin established for a sig nificant part of diamonds from eclogite paragenesis [10]; (7) extremely high structural admixture of nitro gen mostly in the A form (up to 1200 ppm); (8) abundance of fluid inclusions (mostly such compo nents as СО2, Н2О, N); (9) reduced density (3.500– 3.508 g/cm3, diamonds of the I variety show density values about 3.51543 g/cm3). Eclogite paragenesis of these diamonds is proved by occurrences of coesite inclusions [7]. Such diamonds are absent in the known kimberlites and lamproites of different ages in the Siberian platform and worldwide.