Zircon Inclusions Research Articles

Garnet EoS: a critical review and synthesis

All available volume and elasticity data for the garnet end-members grossular, pyrope, almandine and spessartine have been re-evaluated for both internal consistency and for consistency with experimentally measured heat capacities. The consistent data were then used to determine the parameters of third-order Birch–Murnaghan EoS to describe the isothermal compression at 298 K and a Mie–Grüneisen–Debye thermal-pressure EoS to describe the PVT behaviour. In a full Mie–Grüneisen–Debye EoS, the variation of the thermal Grüneisen parameter with volume is defined as gamma = {gamma }_{0}{left(frac{V}{{V}_{0}}right)}^{q}. For grossular and pyrope garnets, there is sufficient data to refine q which has a value of q = 0.8(2) for both garnets. For other garnets, the data do not constrain the value of q and we therefore refined a q-compromise version of the Mie–Grüneisen–Debye EoS in which both γ/V and the Debye temperature θ D are held constant at all P and T, leading to left( {{raise0.7exhbox{{partial C_{{text{V}}} }} !mathord{left/ {vphantom {{partial C_{{text{V}}} } {partial P}}}right.kern-nulldelimiterspace} !lower0.7exhbox{{partial P}}}} right)_{{text{T}}} = 0, parallel isochors and constant isothermal bulk modulus along an isochor. Final refined parameters for the q-compromise Mie–Grüneisen–Debye EoS are: PyropeAlmandineSpessartineGrossularV0 (cm3/mol)a113.13115.25117.92125.35K0T (GPa)169.3 (3)174.6 (4)177.57 (6)167.0 (2)K^{prime}_{{0{text{T}}}}4.55 (5)5.41 (13)4.6 (3)5.07 (8)θ D0771 (28)862 (22)860 (35)750 (13)γ01.185 (12)1.16 (fixed)1.18 (3)1.156 (6)for pyrope and grossular, the two versions of the Mie–Grüneisen–Debye EoS predict indistinguishable properties over the metamorphic pressure and temperature range, and the same properties as the EoS based on experimental heat capacities. The biggest change from previously published EoS is for almandine for which the new EoS predicts geologically reasonable entrapment conditions for zircon inclusions in almandine-rich garnets.

Multi-stage metasomatic Zr mineralization in the world-class Baerzhe rare earth element Nb-Zr-Be deposit, China

AbstractMagmatic and metasomatic zircon occurs in many alkaline igneous rocks and both are potential economic reservoirs of Zr, and in some places, rare-earth elements. The Baerzhe deposit in China is an example of a system where both types of zircon occur. Previous studies recognized deuteric and variably altered magmatic zircon in a transsolvus miaskitic granite, as well as four types of metasomatic zircon in a transsolvus agpaitic granite. In this study, the relationships among, and origins of, zircon and how these relate to models for rare-metal mineralization are assessed. In situ backscattered electron (BSE) and cathodoluminescence (CL) imaging, Raman spectroscopy (including mapping), and chemistry of zircon from the agpaitic granite were conducted, combined with evaluation of published data on zircon from Baerzhe. Their textural, spectroscopic, and chemical characteristics suggest that the four types of metasomatic zircon in the agpaitic granite were not subjected to metamictization or intense alteration, with trace-element accommodation largely following a xenotime substitution mechanism. The most abundant type of metasomatic zircon in the agpaitic granite occurs in zircon-quartz pseudomorphs and exhibits comparable CL, Raman spectral, and chemical features to rare zircon that has partially replaced elpidite. This confirms that the pseudomorphs formed by complete replacement of elpidite. The pseudomorph zircon occurs in association with snowball quartz that contains inclusions of zircon, aegirine, and albite, and with secondary quartz containing aegirine. This is consistent with their coeval formation during Na metasomatism. The restriction of Na metasomatism to the agpaitic granite indicates that this event and the associated zircon formation resulted from early autometasomatism of the agpaitic phase. REE- and Be-rich zircon that replaced magmatic amphibole crystallized as a result of reaction with a REE- and Be-rich fluid that most likely was responsible for the later REE-Nb-Be mineralization that affected both the miaskitic and agpaitic granites. The miaskitic granite contains deuteric and altered magmatic zircon with different chemical characteristics to the four types of metasomatic zircon in the agpaitic granite. This suggests that secondary Zr mineralization in the miaskitic granite formed from different fluids to those that metasomatized the agpaitic granite and may also have resulted from autometasomatism. This study reveals a complex picture for the formation of zircon at Baerzhe, the character of which can vary significantly, both temporally and spatially. Such variable chemistry of the various types of zircon resulted not only from their different origins (magmatic vs. metasomatic), but also from localized water-rock interaction that involved multiple stages of fluids. Zircon in both the miaskitic and agpaitic phases was mainly the product of autometasomatism that was constrained to their parental granites.

Zircon Inclusions in Emeralds from Lake Manyara, Tanzania

Zircon Inclusions in Emeralds from Lake Manyara, Tanzania

AGE AND DURATION OF CRYSTALS GROWTH IN CHAMBERS OF VOLYN PEGMATITES (UKRAINIAN SHIELD)

Formation of chamber pegmatites is associated with tectonomagmatic activation of the region of their distribution. It should be expected that the granitic chamber pegmatites of Volyn were formed by products of degassing of acidic magma. However, in the growth of quartz of the late generation, the beginning of crystallization of which was about 200ºC, CO2-fluids were involved. They are products of degassing of basic magma, which also took part in the growth of crystals at higher temperatures (˃573ºC). Crystallization of minerals in chambers was long: from 1.75±0.10 billion years (age of protogenetic inclusions of zircon and uraninite in topaz crystals) to 1.39 billion years (age of galena inclusions in late-generation quartz). So, for the first time, it was substantiated that the duration of crystal growth in the chambers of Volyn pegmatites was at least 360±100 million years. Therefore, it is assumed that the growth of crystals in chambers Volyn pegmatites lasted for quite a long time.

Raman Spectroscopy of Zircon Inclusions in Unheated Pink Sapphires from Ilakaka, Madagascar: Opening New Perspectives

Raman Spectroscopy of Zircon Inclusions in Unheated Pink Sapphires from Ilakaka, Madagascar: Opening New Perspectives

Raman Spectroscopy of Zircon Inclusions in Unheated Pink Sapphires from Ilakaka, Madagascar: Opening New Perspectives

Raman Spectroscopy of Zircon Inclusions in Unheated Pink Sapphires from Ilakaka, Madagascar: Opening New Perspectives

Deciphering Cryptic Multi-Stage Crystal-Melt Separation during Construction of the Tonglu Volcanic–Plutonic Complex, SE China

Abstract Revealing the origin of explosive eruptions of silica-rich magma is of paramount importance for understanding the evolution of continental crust and volcanic hazards. However, it remains controversial as to how the erupted magmas form and how they connect with plutonic realms, partly owing to the rarity and the obscurity of the ‘cumulate’ complementary to such eruptions of silica-rich magmas. Here the issues are explored by comparing the volcanic rocks (rhyodacite + rhyolite) and their associated subvolcanic intrusions (monzodiorite + monzonite + quartz monzonite) within Tonglu volcanic basin (SE, China). The Tonglu plutonic and volcanic units are consistent with each other in age (~130 Ma), space and source materials (e.g. Sr-Nd-Hf isotopes), strongly suggesting that they are cogenetic. Mineral mapping demonstrates that abundant plagioclase clusters (and chains in monzodiorite) occur in plutonic units, implying the processes of crystal gathering and/or accumulation. Rhyolite-MELTS modeling, and geochemical studies coupled with textural observations suggest that the Tonglu volcanic and plutonic rocks represent the residual melts and the complementary cumulate residues, respectively. The compositional and mineral variations in the plutonic rocks can be explained by two-stage, low-pressure crystal-melt separation of a dacitic magma. The monzodiorite represents the first-stage cumulate that was unsaturated in zircon and biotite/K-feldspar on the basis of low Zr and Ba concentrations and the occurrence of these two phases in the interstices between plagioclase and amphibole. The monzonite and quartz monzonite are the second-stage cumulates after saturation of zircon and biotite/K-feldspar as indicated by abrupt increases in Zr and Ba concentrations and zircon inclusions within euhedral biotite. Mass balance calculation and textural estimation indicate that the cumulates actually are a mixture of crystals and melt, containing ~40 vol% interstitial melt. Interstitial zircons from monzodiorite, largely crystallized from trapped melt, show contrasting trace-element trends (e.g. Ti, Zr/Hf, Eu/Eu*, Gd/Yb) to those of the other rock types (i.e. monzonite, quartz monzonite, rhyodacite and rhyolite), consistent with fractional crystallization (FC). We interpret these divergences to heating-induced partial dissolution of a basal crystal framework of monzodiorite due to recharges of hot mafic magmas, but the limited dissolution fails to rejuvenate the crystal mush. The Tonglu volcanic–plutonic system demonstrates that compositional distillation via crystal–liquid separation within the upper crust is an effective mechanism by which a potentially eruptible rhyolitic cap can be generated. Recharges of hot mafic magmas at shallow crustal levels may prolong the lifespan of granitic magma reservoirs. A combined study including texture, geochemistry, thermodynamic modeling and mass-balance calculations can help us identify the fingerprints of cumulates in felsic magma systems and thus track the processes responsible for producing large eruptions of silica-rich magmas.

Hadean zircon formed due to hydrated ultramafic protocrust melting

Abstract Hadean zircons, from the Jack Hills (Western Australia) and other localities, are currently the only window into the earliest terrestrial felsic crust, the formation of which remains enigmatic. Based upon new experimental results, generation of such early crust has been hypothesized to involve the partial melting of hydrated peridotite interacting with basaltic melt at low pressure (<10 km), but it has yet to be demonstrated that such liquids can indeed crystallize zircons comparable to Jack Hills zircon. We used thermodynamic and geochemical modeling to test this hypothesis. The predicted zircon saturation temperatures of <750 °C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element (REE) contents at 700 °C, δ18OVSMOW (Vienna standard mean ocean water) signatures, and co-crystallizing mineral assemblage were compared to those of the Jack Hills zircon. This comparison was favorable with respect to crystallization temperature, most trace-element contents, and mineral inclusions in zircon. The discrepancy in δ18OVSMOW signatures may be explained by hotter conditions of Hadean protocrust hydration. Our work supports the idea that felsic magma generation at shallow depths involving a primordial weathered ultramafic protocrust and local basaltic intrusions is indeed a viable mechanism for the formation of felsic crust on early Earth.

Multi-Stage Metamorphism of the South Altyn Ultrahigh-Pressure Metamorphic Belt, West China: Insights into Tectonic Evolution from Continental Subduction to Arc–Backarc Extension

Abstract The South Altyn ultrahigh-pressure (UHP) metamorphic belt is claimed to host the deepest subducted continental crust based on the discovery of former stishovite, and thus can provide unique insights into the tectonic evolution from deep continental subduction and exhumation to arc–backarc extension. In this article, we present detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U–Pb dating for three representative samples involving garnet amphibolite (A1531 & A1533) and associated garnet-biotite gneiss (A1534) from the UHP belt. Three phases of metamorphism are inferred for the rocks. The first phase high-pressure (HP)–UHP-type eclogite facies is represented by the mineral assemblages of garnet and phengite inclusions in zircon and garnet cores with the high grossular (XGrs = 0.33–0.34). The Si contents of 3.40–3.53 and 3.24–3.25 p.f.u. in phengite inclusions yield pressure conditions of >1.7–2.3 GPa for A1533 and 2.5–2.55 GPa for A1534 at a fixed temperature of 770°C. The second phase medium-pressure (MP)-type overprinting of garnet amphibolite facies shows P–T conditions of 0.8–1.2 GPa/750–785°C based on the stability fields of corresponding mineral assemblages, the measured isopleths of Ti contents in biotite and amphibole cores, and XGrs in garnet. The third phase low-pressure (LP) type overprinting includes early-stage heating to peak granulite facies followed by cooling towards a late-stage amphibolite facies. The peak granulite facies is represented by the high Ti amphibole mantle, high Zr titanite and the intergrowths of clinopyroxene + ilmenite in A1533 & A1531, with P–T conditions of 800–875°C/0.80–0.95 GPa. The late-stage is defined by the solidus assemblages, giving P–T conditions of 0.5–0.7 GPa/720–805°C. U–Pb geochronology on metamorphic zircons from A1533 and A1534 gives three ages of c. 500 Ma, c. 482 Ma and c. 460 Ma. They are interpreted to represent the HP-UHP, MP and LP types of metamorphism respectively, based on cathodoluminescence (CL) images, mineral inclusions and trace element patterns. Combining the regional geology and metamorphic evolution from the Altyn Orogen, a tectonic model is inferred, including the following tectonic scenarios. The small Altyn Microcontinent (AM) was subducted to great mantle depths with dragging of the surrounding vast oceanic lithosphere to undergo the HP-UHP eclogite facies metamorphism during the early subduction stage (c. 500 Ma) of the Proto-Tethys Ocean. Then, the subducted slabs were exhumed to a thickened crust region to be overprinted by the MP-type assemblages at c. 482 Ma. Finally, an arc–backarc extension was operated within the thickened crust region due to the retreat of subduction zones. It caused evident heating and the LP-type metamorphic overprinting at c. 460 Ma, with a fairly long interval of 30–40 Myr after the HP-UHP metamorphism, distinct from the short interval of <5–10 Myr in the Bohemian Massif.

Shock impedance amplified impact deformation of zircon in granitic rocks from the Chicxulub impact crater

Zircon is a precise chronometer and prominent recorder of impact deformation. However, many impact-induced features in zircon are poorly calibrated, sometimes due to contradicting experimental data, in other instances due to the lack of systematic studies of impact-deformed zircon. To resolve issues with the shock petrographic use of zircon, we classified impact deformation features in 429 zircon grains in a continuous drill core of uplifted, granitic bedrock in the peak ring of the 200-km-diameter K-Pg Chicxulub impact structure. Following initial identification in backscattered electron (BSE) images, Raman spectroscopy and electron backscatter diffraction confirmed one reidite-bearing zircon grain. Quartz-based shock barometry indicates the host rock of this zircon-reidite grain experienced an average shock pressure of 17.5 GPa. A survey of BSE images of 429 ZrSiO4 grains found brittle deformation features are ubiquitous, with planar fractures in one to five sets occurring in 23% of all zircon grains. Our survey also reveals a statistically significant correlation of the occurrence of planar fractures in zircon with the types of host materials. Compared to zircon enclosed in mafic, higher density mineral hosts, felsic, low-density minerals show a much higher incidence of zircon with planar fractures. This finding suggests amplification of pressure due to shock impedance contrasts between zircon and its mineral hosts. Using the impedance matching method, we modeled the shock impedance pressure amplification effect for zircon inclusions in Chicxulub granitic hosts. Our modeling indicates shock impedance could have amplified the average 17.5 GPa shock pressure in a zircon inclusion in quartz or feldspar in the Chicxulub granitic rocks to 24 ± 1 GPa, suggesting that reidite in these rocks formed between 17.5 and 25 GPa. In essence, our study of impedance-induced shock pressure amplification in zircon assemblages, including the onset of reidite formation, details how shock impedance in mineral associations can be quantified to refine shock pressure estimates.

Buchan type metamorphism in the Pingtan-Dongshan metamorphic belt, SE China: Evidence from combined EMP monazite and U-Pb zircon ages of mica schists

Detailed age information of the Pingtan-Dongshan Metamorphic Belt in the coastal Cathaysia Block is important in understanding geodynamics of the Late Mesozoic South China continental margin. Geochronological determinations on monazite and zircon inclusions and zircon separates from five mica schists and two granites intruding schists are used to constrain the timing of metamorphism and sediment sources. Common ages of monazite and zircon inclusions within major constituents (quartz, muscovite, sillimanite and K-feldspar) in schists define a high temperature/low pressure metamorphic event at ~100 Ma. Along with zircon ages of granites (106 and 101 Ma), a Buchan type metamorphism is proposed regarding a heat source from the post-orogenic magmatism in the neighboring igneous province. Based on the age distribution patterns of discrete zircons, two groups of schist are distinguished by sources of sediments prior to metamorphism. One group has a simple age cluster (145–130 Ma) inferring proximal magmatic rocks in the Cathaysia Block. The other group shows a widespread age spectrum with two particular populations of ~1.8 Ga and ~190 Ma that are uncommon in the Cathaysia Block but comparable to schists in the Tailuko Metamorphic Belt of eastern Taiwan. This may suggest a general source between these two belts in which a now-concealed microcontinent would play a role. Such a new tectonic framework can account for the two provenances of sediments in schists of this belt, and its structural configuration is influenced by a high temperature regime caused by mantle upwelling as a reflection of slab rollback of the subducted Paleo-Pacific Plate during 110–90 Ma.

Alteration mineralogy and pathfinder element inventory in the footprint of the McArthur River unconformity-related uranium deposit, Canada

ABSTRACTPathfinder elements associated with the exploration footprint of the McArthur River unconformity-related U deposit include U, radiogenic Pb, V, Ni, Co, Cu, Mo, As, Zn, and rare earth elements. In this study, the mineralogical and paragenetic context for their occurrence was established by integrating in situ mineral chemistry and laser ablation mass spectrometry chemical mapping of interstitial assemblages, detrital grains, and cements with whole-rock analyses of drill core samples from the diagenetically altered background and the hydrothermally altered sandstone host rocks. Diagenetically altered background sandstones contain a matrix assemblage of illite and dickite, with trace to minor aluminum-phosphate-sulfate (APS) minerals, apatite, and Fe-Ti oxide minerals. Aluminum-phosphate-sulfate minerals account for the majority of the Sr and light rare earth element concentrations, whereas early diagenetic apatite, monazite, and apatite inclusions in detrital quartz and detrital zircon contribute significant U and heavy rare earth elements to samples analyzed with an aggressive leach (partial digestion) such as aqua regia. Hydrothermally altered sandstone host rocks also contain variable assemblages of Al-Mg chlorite (sudoite), alkali-deficient tourmaline, APS minerals, kaolinite, illite, Fe-oxide, and sulfide minerals. Late pre-mineralization chlorite accounts for a significant portion of the observed Ni concentrations, whereas Co, Cu, Mo, and Zn occur predominantly in cryptic sub-micron sulfide and sulfarsenide inclusions within clay mineral aggregates and in association with Fe-Ti oxides. Elevated concentrations of U were observed in cryptic micro-inclusions associated with sulfides in quartz overgrowths, with Fe-Ti oxide micro-inclusions in kaolinite, and in post-mineralization Fe-oxide veins. The distribution of pathfinder elements throughout the deposit footprint appears to be less related to the primary dispersion of alteration minerals from the hydrothermal system than to the secondary dispersion of elements post-mineralization. Their occurrence throughout pre-, syn-, and post-mineralization assemblages further demonstrates the limitations to defining geochemical footprints from pathfinder element concentrations expressed in lithogeochemical data sets without structural, lithological, and mineralogical context.

Metamorphic PT path, U-Pb zircon dating and tectonic implications of High-pressure Pelitic Granulites from the Kharta region, Southern Tibet, China

HP pelitic granulites associated with eclogites from the Kharta Gneiss are found in the Thongmön area, central Himalaya, which provides significant constraints for the tectonic evolution of the Himalayan orogeny. Three generations of garnets are identified in pelitic granulites (DR15P15). The first generation of garnet is core sections of euhedral to subhedral porphyroblasts (Grt-I core) followed by growth of Grt-I mantle during the heating stage. The third generation of garnet is represented by thin rims of Grt-I and small grains (Grt-II) in the matrix, which likely nucleated and grew in response to peritectic reaction during exhumation. Petrographic evidence and P–T pseudosection calculation reveal a four-stages metamorphic evolution: 1) a prograde metamorphic stage (M1) represented by Grt-I core and its mineral inclusions rutile, ilmenite and quartz with metamorphic conditions of 740–760 °C and 13–16 kbar; 2) a near isobaric heating stage with slightly decompression to HP granulite-facies (M2, 820–840 °C, 13–14 kbar) indicated by the composition of Grt-I mantle and the increase of Zr concentrations of rutile in it; 3) a heating decompression stage (M3) supported by a decrease in grossular content in the Grt-I rim and higher XAn in plagioclase, yielding T > 860 °C and P = 10–11 kbar in the sillimanite stability field; 4) a final decompression stage followed by cooling (M4, T ~ 800 °C and P = 6–8 kbar) indicated by biotite replacing sillimanite and lower Ti content in biotite in the matrix. Based on the zircon trace element geochemistry, zircon inclusions, and the REE partitioning between garnet and zircon, the metamorphic zircon rims with middle Miocene age (16–13 Ma) probably grew during Grt-I rim/Grt-II formation at granulite facies stage (M3). The P–T history of the pelitic granulites of the Kharta Gneiss indicates that they were subducted beneath the Asian plate and exhumed to middle-lower crustal depths prior to 14–13 Ma due to the activity of STDS.

The contribution of elastic geothermobarometry to the debate on HP versus UHP metamorphism.

Characterizing the pressure and temperature (P–T) histories of eclogite facies rocks is of key importance for unravelling subduction zone processes at all scales. Accurate P–T estimates provide constraints on tectonic and geochemical processes affecting subduction dynamics and help in interpreting the geophysical images of present‐day converging plates. Conventional equilibrium geothermobarometers are challenged in ultra high pressure (UHP) metamorphic terranes, as minerals may undergo re‐equilibration along their exhumation path. Elastic geobarometry applied to host‐inclusion systems is a complementary method to determine P–T conditions of metamorphism independent from chemical equilibrium. Because only a single measurement, the inclusion strain, is made, only a line in P–T space of possible entrapment conditions, the entrapment isomeke, can be determined. Thus, the entrapment pressure along an isomeke can only be determined if the entrapment temperature is known. An alternative is to calculate entrapment conditions for two types of inclusions that are believed, from petrological evidence such as being in the same garnet growth zone, to have been entrapped at the same time. The intersection between the two sets of isomeke calculated on multiple quartz and zircon inclusions demonstrates that measuring different inclusion phases trapped inside a single host allows unique P–T conditions for the host rock to be determined. Here, we combine Zr‐in‐Rutile thermometry and thermodynamic modelling with micro‐Raman measurements on quartz and zircon inclusions trapped in garnet to obtain pressures and temperatures of equilibration of a quartz–garnet vein from the Proterozoic Ulla gneiss basement and of garnet–kyanite gneiss from the Caledonian Blåhø nappe, both in the Fjørtoft UHP terrane, Norway. We find that the quartz–garnet vein formed at high pressure (1.5–2.5 GPa and 750–800°C) and recrystallized at ~1.2 GPa and 880°C. In contrast, the garnet–kyanite gneiss followed an anticlockwise path with peak P–T at 1.2 GPa and 880°C: these estimates are consistent with previous thermodynamic modelling and suggest that the Ulla gneiss and the Blåhø nappe came into contact at these last conditions. We also discuss a new method to detect hydrostatic versus Non‐hydrostatic stresses near quartz and zircon inclusions in garnet.

Quantifying F and Cl concentrations in granitic melts from apatite inclusions in zircon

Quantifying F and Cl concentrations in granitic melts from apatite inclusions in zircon

Peritectic minerals record partial melting of the deeply subducted continental crust in the Sulu orogen

AbstractThe deeply subducted continental crust commonly undergoes partial melting during its exhumation from mantle to crustal depths. Peritectic minerals are commonly produced during decompressional melting, providing an important record of the timing, P–T conditions, and mechanisms of crustal anatexis in continental subduction zones. A combined study of petrography, mineral inclusions, mineral major and trace element compositions, phase equilibrium modelling, zircon and titanite U–Pb dating, and zircon Hf–O isotopes was carried out to characterize peritectic minerals in ultrahigh‐pressure (UHP) metamorphic gneisses from the Sulu orogen, China. Field and petrographic observations reveal that the crustal anatexis was extensive and is recorded by folded leucosomes, corroded phengite relicts, cuspate K‐feldspar and quartz, pseudomorphs of melt films along grain boundaries, and multiphase crystal inclusions in zircon, garnet, and phengite. The multiphase crystal inclusions represent the crystallized product of anatectic melt droplets trapped in the peritectic zircon and garnet as well as the residual UHP phengite. The peritectic origin of both zircon and garnet is identified by their compositional characteristics, and the peritectic origin of garnet is further supported by phase equilibrium modelling which shows an increase in garnet contents as partial melting proceeds. The modelled phase diagrams also indicate that the two peritectic minerals were produced through two episodes of phengite dehydration melting. The first occurred at 237 ± 3 Ma under P–T conditions of ≥2.6 GPa and 810–880°C, corresponding to thermobaric ratios of ~312–338℃/GPa. The second took place at 222 ± 2 Ma under P–T conditions of 1.5–2.1 GPa and 730–760℃, corresponding to thermobaric ratios of 362–487℃/GPa. In comparison to peak UHP metamorphic P–T conditions of 2.8–4.0 GPa and 720–870℃ for thermobaric ratios of 218–257℃/GPa, it appears that the decompressional melting is associated with an increase in thermal gradients during the exhumation in the different stages. Whereas the first is caused by decompression during initial exhumation of the deeply subducted continental crust, the second is caused by decompression during relamination of the UHP slices into the collision‐thickened orogenic lower crust. Peritectic amphibole and titanite were also recognized by comparing their petrographic textures with natural samples and experimental products. They were produced through water‐fluxed melting at 217 ± 3 Ma under lower P–T conditions, suggesting the liberation of aqueous fluids from the underlying exhuming UHP rocks. All of the peritectic minerals generally show compositional inheritance from their precursor minerals. Nevertheless, they commonly underwent dissolution when anatectic P–T conditions deviate significantly from those of peritectic reactions. This is indicated by the dissolution of peritectic zircon and garnet and by the occurrence of anhedral and embayed peritectic amphibole. As a consequence, the behaviour of peritectic minerals provides a snapshot of crustal anatexis for geochemical differentiation in collisional orogens.

Potassium-rich granite melt inclusions in zircon from gabbro-hosted felsic stringers, Mid-Atlantic Ridge at 13°34′N: E-MORB connection

We present new data on crystallized micro-inclusions in zircon from clinopyroxene-bearing felsic stringers within gabbronorite of gabbro-peridotite oceanic core complex at Mid-Atlantic Ridge, 13°34′ N. A relatively large (about 49 × 22 μm) crystallized inclusion exposed in situ in thin section consists of the muscovite + biotite + plagioclase + K-feldspar + quartz assemblage. Smaller inclusions (≤20 μm) found in zircon separates consist of silicate minerals, interstitial water solution, and gas bubbles. At temperature close to 800 °C and estimated pressure about 1.5 kbar the inclusions completely homogenize into a K2O-rich (3.9 ± 0.1 wt%), H2O-bearing (about 4 wt%) granite (SiO2 70 ± 2 wt%) melt. Calculations employed DOMINO software show that complete closed-system crystallization of the analyzed melt exactly reproduces mineral assemblage of the large crystallized inclusion (+H2O) at about 1.5 kbar below 600 °C. Granitoid mineralogy of felsic stringers hosting zircon does not correspond to that of the crystallized melt inclusions, with a significantly lower K2O in the former. The K2O loss might result from interaction of the melt with pyroxenes of the host gabbro. This implies that considering felsic granitoid veins, often present in the oceanic lower crustal sections, as simply “frozen melt” is not always justified. Compositional variations of basalt lava glasses from the 13°25′–14° portion of the Mid-Atlantic Ridge show continuous geochemical trends from D-MORB to E-MORB with K2O enrichment up to 0.9 wt%, suggesting mixing from two sources. The most K2O-enriched E-MORB composition (K2O = 0.9 wt%, H2O = 0.7 wt%, K2O/TiO2 = 0.42, (La/Sm)N = 2.45) was assumed as a source of the granitoid inclusions. Computer simulation of equilibrium crystallization of this E-MORB composition returned a granite residual melt closely matching the average composition of the zircon-hosted melt inclusions at 820 °C, 1.5 kbar and melt fraction 5.2 vol%. The result suggests a direct link between the E-MORB magma occurring in the studied area, and the origin of the K-rich granite melt in zircon-hosted inclusions.

Raman spectroscopic investigation of zircon in gem‐quality sapphire: Application in origin determination

AbstractIn this study, zircon inclusions in selected 115 unheated sapphires originating from metamorphic deposits were studied by confocal micro‐Raman spectroscopy. By comparing Raman features of zircon inclusions in gem‐quality sapphires from Myanmar, India (Kashmir), Sri Lanka, and Madagascar, it could be established that those of younger age (sapphires from Myanmar and India [Kashmir]) contain zircon inclusion which exhibit relatively low ʋ1 and ʋ3 band positions and also smaller FWHM (ʋ3) than those in the older sapphires from Sri Lanka and Madagascar. Binary plotting of ʋ1 versus ʋ3 frequencies (Figure 3) and ʋ3 wavenumber versus FWHM of the studied zircon inclusions provide a method to distinguish young sapphires formed during Alpine‐Himalayan orogeny (Kashmir, Myanmar) from those related to the Pan‐African orogeny (Sri Lanka, Madagascar). This study shows the potential of the non‐destructive method on zircon inclusions in sapphires to be used to distinguish their origin as a service to the commercial gem trade.

Construction of P–T–t paths for eclogite in the Tongbai orogen by combining phase equilibria modelling with zircon inclusion composition

AbstractThe interpretation of metamorphic ages for zircon domains in eclogite remains challenging because of the uncertainties in interpreting their rare earth element (REE) patterns and mineral inclusions. This has seriously hindered the construction of robust pressure (P)–temperature (T)–time (t) paths for eclogites from collisional orogens. In this study, a composite approach is used to obtain a reliable P–T path for eclogite from the Tongbai orogen and quantitatively attribute zircon U–Pb ages to specific P–T intervals. An integrated study using isochemical phase diagram section (pseudosection) modelling, multi‐equilibrium thermobarometry, and the Zr‐in‐rutile thermometry with petrographic observations indicates that the target eclogite experienced multistage metamorphic evolution. The evolution is characterized by a clockwise P–T path with compressional heating for peak eclogite facies metamorphism at 588 ± 20°C and 26.7 ± 2.8 kbar and two stages of isothermal decompression from an early stage of amphibole eclogite facies metamorphism at 15–10 kbar to a late stage of amphibolite facies metamorphism at pressures of <10 kbar. Metamorphic zircon domains in eclogite and quartz vein show relatively flat patterns of heavy REE without notable Eu anomalies but different cathodoluminescence responses and middle REE (MREE) contents. This indicates their growth in different stages of eclogite facies metamorphism, which are categorized into two groups. The first group is composed of bright rims or homogenous grains that are characterized by low MREE contents, suggesting the existence of abundant amphibole during the metamorphism. The compositional isopleths of amphibole inclusions indicate that this group of zircon domains would form by hydration during crustal exhumation to amphibole eclogite facies at 240 ± 2 Ma. The second group consists of dark sector mantles and shows high MREE contents, indicating the prograde to peak metamorphism with significant decomposition of amphibole or lawsonite during crustal subduction to eclogite facies at 245 ± 2 Ma. Therefore, the combination of the isopleth thermobarometry of mineral inclusions with the Zr‐in‐rutile thermometry and the zircon REE partitioning is an effective means to link the metamorphic P–T conditions to the ages of metamorphic zircons in the different stages of collisional orogeny.

Glass microspheres in road dust of the city of Kielce (south-central Poland) as markers of traffic-related pollution

Glass microspheres are widely used as reflective components in road and pavement marking materials. They commonly occur in road dust mixed with different particles of anthropogenic and geogenic origin. This paper describes different methods that enable discrimination of glass microspheres from other morphologically similar particles that occur in road dust of Kielce. The individual glass microspheres vary from 30 to 1350 µm in diameter and consist of recycled Ca-, Na-, Mg-silica glass with a minor admixture of Al, Fe, K and S. Because of their stability and chemical composition, glass microspheres are good markers of traffic-related pollution in different environmental archives. Of different elements present in Kielce road dust, the fraction < 0.125 mm was distinctly abundant in zirconium, the main constituent of disk brakes or brake pads and a potential marker of road traffic pollution. However, the statistically significant positive correlation of the Si–Zr pair (R = 0.54) in the 1.0–2.0 mm fraction is linked to the presence of some detritic quartz grains with zircon inclusions. The other metals determined provide ambiguous traffic-related signatures and may be derived from different pollution sources.