Zircon Inclusions Research Articles

Experimental subsolidus phase relations in the system CaCO3–CaMg(CO3)2 up to 6.5 GPa and implications for subducted marbles

The dissociation of dolomite to aragonite + magnesite and the reaction of dolomite + aragonite to Mg-calcite have been experimentally determined by piston cylinder experiments in the range of 650–1000 °C and 2–6.5 GPa. Friction decay within the salt sleeves used as pressure medium was monitored by continuously logging the travel distance of the piston. Piston movement ceased after the first 24 h of the experiment, and all experiments were kept at the steady-state conditions for an additional 24–196 h. The reaction of dolomite to aragonite + magnesite has a positive slope and has been bracketed at 650 °C between 5.1 and 5.8 GPa and at 750 °C between 5.7 and 6.5 GPa. The reaction is sluggish with a 0.6-GPa interval where all three phases coexist. The reaction dolomite + aragonite to Mg-calcite has been determined in 0.5-GPa intervals from 660 °C at 2 GPa to 940 °C at 6.5 GPa. The composition of Mg-calcite at the reaction curve is buffered and changes systematically from X(Ca) of 0.90 at 2 GPa to 0.57 at 6.5 GPa. The solvus between dolomite and Mg-calcite shrinks slightly with increasing pressure, providing evidence for a small negative slope of the critical curve dolomite = Mg-calcite. The degree of disorder in dolomite has been determined in some of the retrieved samples by Rietveld refinement based on X-ray diffraction patterns. The order parameter S decreases gradually from 1 to 0.7 as a function of temperature as the critical curve is approached and then drops dramatically after crossing it. No ordering on quench has been observed. The experimental results permit the construction of a petrogenetic grid for the system dolomite, aragonite, Mg-calcite, and magnesite, which has an invariant point at ~7.5 GPa and 980 °C, representing the maximum pressure stability of dolomite. X(Ca) isopleths of Mg-calcite coexisting with dolomite serve mainly as a thermometer, whereas in the presence of aragonite they are a useful barometer. The experimental results have been applied to diamond-bearing marbles of the Kokchetav Massif, where Mg-calcite inclusions in garnet and zircon with X(Ca) of 0.76 most likely formed during prograde metamorphism close to the graphite–diamond transition.

Empirical constraints on shock features in monazite using shocked zircon inclusions

Abstract Shock deformation microstructures in monazite have been systematically characterized for the first time in grains from the Vredefort impact structure in South Africa. Electron backscatter diffraction mapping has identified 12 unique orientations of monazite deformation twins, including 7 orientations that have not previously been described in experiments or nature. Other shock features include planar deformation bands and strain-free neoblasts, which have been shown to date deformation. Shock-twinned zircon inclusions within the deformed monazite require pressures of 20 GPa, thus providing critical empirical constraints on formation conditions, confirming a hypervelocity impact origin of the monazite microstructures. The Vredefort monazite grains described here represent the first case of using shocked mineral inclusions to empirically calibrate shock microstructures formed in the host mineral. These results conclusively establish monazite as a recorder of shock deformation, and highlight its use in identifying and dating impact structures.

U–Pb zircon geochronology and phase equilibria modelling of a mafic eclogite from the Sumdo complex of south-east Tibet: Insights into prograde zircon growth and the assembly of the Tibetan plateau

The Sumdo complex is a Permian–Triassic eclogitic metamorphic belt in south-east Tibet, which marks the location of a suture zone that separates the northern and southern Lhasa terranes. An integrated geochronological and petrological study of a mafic eclogite from the complex has constrained its tectonometamorphic history and provides a case study of zircon growth in eclogite as a product of prograde dissolution–precipitation. In situ U–Pb geochronology indicates that the eclogite contains a single population of zircon with a crystallisation age of 273.6±2.8Ma. The morphology and chemistry of the zircon grains are consistent with growth by dissolution–precipitation of protolith magmatic zircon. The presence of zircon grains as inclusions in the cores of peak phases indicates that zircon dissolution–precipitation occurred during prograde metamorphism, and calculated pressure and temperature conditions over which mineral inclusions in zircon are stable suggest that the zircon most likely precipitated at ~15.5–16.5kbar and 500–560°C. Subsequent peak metamorphism is calculated to have reached pressure–temperature conditions of 27±1kbar and 670±50°C. Previous studies, which have documented a range of peak metamorphic conditions from high- to ultrahigh-pressure at c. 266–230Ma, indicate that the Sumdo complex is a composite belt that experienced protracted eclogite exhumation. The results of this study are consistent with this interpretation, and extend the age range of high-pressure metamorphism in the complex to over 40Myr. Analysis of published pressure–temperature–time data indicates two systematic behaviours within this spread. First, peak metamorphic temperatures declined over time. Second, eclogite exhumation occurred in two discrete intervals: soon after formation, and during the demise of the subduction zone. The latter behaviour serves as a reminder that eclogite exhumation is the exception rather than the rule.

U–Pb dating and composition of inclusions in zircon from ophiolitic gabbro of the Klyuchevsk massif (Middle Urals): Results and geological interpretation

The U–Pb (SHRIMP) dating of zircon from the layered complex of ophiolitic gabbro in the Klyuchevsk massif yielded an age of 456 ± 6 Ma corresponding within the limits of error to zircon dates obtained for other petrographic varieties from this massif. The investigation of the composition of silicate inclusions in dated zircon grains revealed that they are represented by typical metamorphic minerals: albite, zoisite, and secondary amphiboles. The data indicate that zircon was crystallized during metamorphic transformations of gabbroids and its U–Pb age (Late Ordovician–Silurian) is characteristic of all rocks in the ophiolite association of the Klyuchevsk massif indicating the age of metamorphism, not their formation time.

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.

Multi-stage evolution of xenotime–(Y) from Písek pegmatites, Czech Republic: an electron probe micro-analysis and Raman spectroscopy study

The chemical variability, degree of radiation damage, and alteration of xenotime from the Pisek granitic pegmatites (Czech Republic) were investigated by micro-chemical analysis and Raman spectroscopy. Dominant large xenotime–(Y) grains enriched in U, Th and Zr crystallized from a melt almost simultaneously with zircon, monazite and tourmaline. Xenotime is well to poorly crystalline depending on its U and Th contents. It shows complex secondary textures cutting magmatic growth zones as a result of its interaction with F,Ca,alkali-rich fluids during the hydrothermal stage of the pegmatite evolution. The magmatic xenotime underwent intense secondary alteration, from rims inwards, resulting in the formation of inclusion-rich well crystalline xenotime domains of near end-member composition. Two types of recrystallization were distinguished in relation to the type of inclusions: i) xenotime with coffinite-thorite, cheralite and monazite inclusions and ii) xenotime with zirconcheralite and zircon inclusions. Additionally, inner poorly crystalline U,Th-rich xenotime domains were locally altered, hydrated, depleted in P, Y, HREE, U, Si and radiogenic Pb, and enriched in fluid‐borne cations (mainly Ca, F, Th, Zr, Fe). Interaction of radiation-damaged xenotime with hydrothermal fluids resulted in the disturbance of the U–Th–Pb system. Alteration of radiation-damaged xenotime was followed by intensive recrystallization indicating the presence of fluids >200 °C. Subsequently other types of xenotime formed as a consequence of fluid-driven alteration of magmatic monazite, and Y,REE,Ti,Nb-oxides or crystallized from hydrothermal fluids along cracks in magmatic monazite and xenotime.

ContrastingP-Tpaths within the Barchi-Kol UHP terrain (Kokchetav Complex): Implications for subduction and exhumation of continental crust

![Figure][1] The Barchi-Kol terrain is a classic locality of ultrahigh-pressure (UHP) metamorphism within the Kokchetav metamorphic belt. We provide a detailed and systematic characterization of four metasedimentary samples using dominant mineral assemblages, mineral inclusions in zircon and monazite, garnet zonation with respect to major and trace elements, and Zr-in-rutile and Ti-in-zircon temperatures. A typical diamond-bearing gneiss records peak conditions of 49 ± 4 kbar and 950–1000 °C. Near isothermal decompression of this rock resulted in the breakdown of phengite associated with a pervasive recrystallization of the rock. The same terrain also contains mica schists that experienced peak conditions close to those of the diamond-bearing rocks, but they were exhumed along a cooler path where phengite remained stable. In these rocks, major and trace element zoning in garnet has been completely equilibrated. A layered gneiss was metamorphosed at UHP conditions in the coesite field, but did not reach diamond-facies conditions (peak conditions: 30 kbar and 800–900 °C). In this sample, garnet records retrograde zonation in major elements and also retains prograde zoning in trace elements. A garnet-kyanite-micaschist that reached significantly lower pressures (24 ± 2 kbar, 710 ± 20 °C) contains garnet with major and trace element zoning. The diverse garnet zoning in samples that experienced different metamorphic conditions allows to establish that diffusional equilibration of rare earth element in garnet likely occurs at ~900–950 °C. Different metamorphic conditions in the four investigated samples are also documented in zircon trace element zonation and mineral inclusions in zircon and monazite. U-Pb geochronology of metamorphic zircon and monazite domains demonstrates that prograde (528–521 Ma), peak (528–522 Ma), and peak to retrograde metamorphism (503–532 Ma) occurred over a relatively short time interval that is indistinguishable from metamorphism of other UHP rocks within the Kokchetav metamorphic belt. Therefore, the assembly of rocks with contrasting P-T trajectories must have occurred in a single subduction-exhumation cycle, providing a snapshot of the thermal structure of a subducted continental margin prior to collision. The rocks were initially buried along a low geothermal gradient. At 20–25 kbar they underwent near isobaric heating of 200 °C, which was followed by continued burial along a low geothermal gradient. Such a step-wise geotherm is in good agreement with predictions from subduction zone thermal models. [1]: pending:yes

Chemistry and origin of the Mayo Kila sapphires, NW region Cameroon (Central Africa): Their possible relationship with the Cameroon volcanic line

Mineralogical, chemical and geochronological studies constrained the origin of sapphires from Mayo Kila, Northwest Cameroon. The sapphires are mostly blue, with sizes ranging from 2 to 5 mm. The pale blue grains are transparent, whereas, other corundums are transparent to translucent and/or opaque. The sapphires are dominantly euhedral to sub-hedral with few polished lustrous grains, acquired features during moderate to short distance transport from a proximal source rock. Solid inclusions are limited to rutile and zircon.Trace element analysis of sapphires shows significant concentration (in ppm) in some elements: Fe (2208–14,473), Ti (82–1783), Ga (77–512), Mg (0.9–264.9), Cr (b.d.l −168) and V (1.3–82). The other elements (e.g. Sn, Nb, Ta, Th, Zr, Ni, Ce) are generally below 10 ppm. The calculated ratios for some of the selected elements show an extreme variation: Fe/Mg (43–3043), Fe/Ti (2–76), Ti/Mg (1–328), and Ga/Mg (0.4–363). They are dominantly corundum crystallized in alkaline magma (s) with few from metamorphic source (s).Trace elemental features with Hf (13,354–26,238 ppm), Th (4018–45,584 ppm) and U (7825–17,175 ppm), and Th/U (0.39–2.65) found in zircon inclusions are compatible with quantified values in magmatic crystallized zircons. The Cenozoic age (mean of 30.78 ± 0.28 Ma) obtained for zircon inclusions is close to the age of some igneous rocks found within the Cameroon Volcanic Line (e.g. rocks of the Mount Oku: 31–22 Ma), showing the same period of formation. The most probable source of the zircon host sapphires is the Oku Mountain located SW of Mayo Kila.

Preservation of primary mineral inclusions and secondary mineralization in igneous zircon: a case study in orthogneiss from the Blue Ridge, Virginia

A wide variety of minerals occur as inclusions in igneous zircon and can provide valuable evidence of source magma composition from originally magmatic zircons in metamorphic and clastic sedimentary rocks. However, it is not clear the extent to which zircons preserve the primary composition of their inclusions through a variety of geologic processes. This paper documents a case study of inclusion-rich, originally igneous zircon from an orthogneiss in the Blue Ridge of southwest Virginia. Zircon inclusions isolated from cracks contain 4 % clearly metamorphic phases (mainly in hosts with disturbed U–Pb systems) and otherwise retain distinct plagioclase chemistry, K-feldspar/plagioclase pairs, and biotite with much wider-ranging Mg/(Mg + Fe) than biotite in the rock matrix. A clearly secondary mineralization suite filling cracks in the zircons consists of quartz, biotite, albite, and epidote. Overall, these zircons preserve mineral inclusions distinct from their current host rock (except when exposed to external environments via cracks), demonstrating that non-metamict zircons may preserve their primary inclusion assemblages through later amphibolite to lower granulite facies metamorphism.

Cenozoic exhumation history of Sulu terrane: Implications from (U–Th)/He thermochrology

The Qinling–Dabie–Sulu orogen is the most prominent Phanerozoic orogenic belt in China. The discovery of ultra-high pressure (UHP) minerals in zircon inclusions suggests that the crust was subducted to deeper than 120km into the mantle and then exhumed to shallow crustal. Recently, low temperature thermochronology has been applied to constrain the final exhumation of Dabie Shan, while there are few studies describing the Cenozoic exhumation history of the Sulu belt. Here we report some (U–Th)/He ages for various lithologies from Sulu Orogenic belt and its northern part-Jiaobei terrane. The single grain He ages range between 18 and 154Ma, and most of the samples having large intra-sample age scattering. Several reasons such as invisible U/Th-rich inclusions, grain size effect, slow cooling rate, and zonation of parent nuclide or radiation damage effect may account for this dispersion. For all samples, the pattern of the single grain age data exhibits a peak at ~45Ma which is consistent with the borehole fission-track age pattern in adjacent Hefei Basin. Both (U–Th)/He and fission track ages of the Sulu area suggest an enhanced exhumation/cooling in Early-Middle Eocene in the southern part of Tan-Lu fault zone. This enhanced cooling event coincides with rapid subsidence of North China Basin and rapid uplift of its surrounding reliefs, which indicates basin-mountain coupling. This Eocene event is widespread in central China and could be far-field consequence of India–Asia collision. The convergence rate between Pacific Plate and Eurasia decreased substantially during early Tertiary and reached a minimum in Eocene (~30–40mm/yr) while at the same time the collision between India and Asia was completed. Therefore, the Cenozoic exhumation history of the Sulu Orogenic Belt was a combined result of far-field effect of India–Asia collision and declined subduction rate of the Pacific Plate under Eurasia.

An apatite for progress: Inclusions in zircon and titanite constrain petrogenesis and provenance

Apatite has recently gained considerable attention as a mineral with many uses within the Earth and planetary sciences. Apatite chemistry has recently given new insight into a wide range of geological processes and tools, for example, magmatism, metasomatism, planetary geochemistry, and geochronology. We expand the utility of apatite here by presenting a novel way to fingerprint magma chemistry and petrogenesis using apatite inclusions within robust titanite and zircon. We present trace element data from apatite mineral inclusions shielded within magmatic zircon and titanite. Importantly, apatite inclusion and host titanite chemistries detailed in this study allow estimation of the whole-rock Sr and SiO2. We show how these data can be used to assess the degree of fractionation of the host magma and to calculate key trace element abundances and ratios. We demonstrate that the inclusions can be linked to discrete periods in the crystallization history of the host phases, thus providing insight into petrogenesis. The results highlight that apatite compositions might discriminate modern granitoids (younger than 2.5 Ga) from Archean–Proterozoic transitional granitoid compositions (sanukitoid signatures). Development of such a petrological tool has important potential for interpretation of provenance and a better understanding of the secular evolution of the continental crust, including that of early Earth.

The youngest eclogite in central Himalaya: P–T path, U–Pb zircon age and its tectonic implication

Relict omphacite inclusions have been discovered in mafic granulite at Dinggye of China, confirming the existence of eclogite in central Himalayan orogenic belt. Detailed petrological studies show that relict omphacite occur as inclusions in both garnets and zircons, and the peak mineral assemblage of eclogite-facies should be garnet, omphacite, rutile, muscovite and quartz which was strongly overprinted by granulite-facies minerals during the exhumation. Phase equilibria modeling and associated geothermometer predict that the minimum P–T conditions for peak eclogite-facies stage are 720–760°C and 20–21kbar, and those of overprinted granulite-facies are 750°C and 7–9kbar in water-undersaturated condition. Thus, a near isothermal decompression P–T path for central Himalayan eclogite has been obtained. Zircon SHRIMP U–Pb dating of two studied eclogite samples at Dinggye yields the peak metamorphic ages of 13.9±1.2Ma and 14.9±0.7Ma, respectively, which indicates that the Dinggye eclogite should be the youngest eclogite in Himalayan orogenic belt. Geochemical characteristics and zircon analyses show that the protoliths of eclogite in Dinggye are predicted to be continental rift-related basaltic rocks. The eclogite at Dinggye in central Himalaya should be formed by the crustal thickening during the long-lasting continental overthrusting by Indian plate beneath Euro-Asian continent, and its exhumation process may be related with channel flow and orogen-parallel extension. In the middle Miocene (~14Ma), Indian continental crust had reached at least ~65km depth in southern Tibet.

Zircon U–Pb ages of Paleoproterozoic mafic granulites from the Huai’an terrane, North China Craton (NCC): Implications for timing of cratonization and crustal evolution history

The North China Craton (NCC) is considered to have been incorporated into the Columbia supercontinent during late Paleoproterozoic. The Huai’an terrane located in north-central part of the NCC is an important tectonic window to investigate the Paleoproterozoic crustal evolution history of this craton, particularly due to the wide occurrence of high-grade metamorphic rocks including retrograded eclogites and high-pressure granulites. Available data show two controversial peak metamorphic ages of these rocks as 1.95–1.90 or ∼1.85–1.80Ga. Thus, determining the precise timing of metamorphism of the high-pressure granulites is important in reconstructing the collisional history. In this study, we present zircon U–Pb ages, bulk chemistry, and mineral inclusions in zircons and matrix mineral assemblages from the high-pressure granulites to evaluate the timing of metamorphism. Our results reveal ca. 2.05–2.0Ga ages from magmatic zircons in these rocks representing the timing of emplacement of mafic magmas. The metamorphic zircons yield two groups of ages at 1.95–1.90 and ∼1.85∼1.80Ga. Clinopyroxene, plagioclase, titanite, and apatite occur as common mineral inclusions within the ∼1.85–1.80Ga metamorphic zircons. The chemical compositions of clinopyroxene and plagioclase occurring as inclusions are very similar to those of the rim compositions of those minerals in the matrix, suggesting their formation during retrograde granulite metamorphism, and are distinctly different from those of the core domains of the minerals in the matrix which represent high-pressure granulite metamorphism. These results indicate that the metamorphic zircons formed during retrograde granulite metamorphism dated as ∼1.85–1.80Ga and the age range of ∼1.95–1.90Ga corresponds to high-pressure granulite metamorphism. The dual nature of the geological and geochronological features suggest that the Archean gray gneisses in the Huai’an terrane represent the basement rocks detached from the Khondalite Belt of the NCC after collision. Integrating with published data, a double-side collision with scissor-like suturing model is proposed for the Paleoproterozoic cratonization of the NCC.

Water-present melting in the middle crust: The case of the Ollo de Sapo gneiss in the Iberian Massif (Spain)

The Variscan migmatitic region of Sanabria, in the inner zone of the Iberian Massif (Spain), provides a unique place to study the geochemistry and rheology of anatectic melts formed by water-present conditions. In Sanabria, the Ollo de Sapo gneiss underwent partial melting at 0.5–0.66GPa and 690–770°C. Leucosomes, leucogranitic bodies within the migmatite and leucogranites are widespread throughout the migmatite, with the leucogranitic bodies being concordant to discordant to the foliation. Mass balance calculations indicate that the partial melting of the gneiss produced 62.5wt.% of melt. Experiments and pseudosections made at the peak metamorphic P–T conditions under fluid-absent conditions yield much lower volumes of melt and therefore a fluid-present scenario is proposed. Major and trace element composition and structures of leucosomes and leucogranites support this scenario. Trace element composition of leucosomes is variable, with low to high REE, Zr, Y and Th content and positive to negative Eu anomalies. The composition of the most felsic, REE-depleted, Eu-rich leucosomes is closely similar to that of experimental melts with added water at T<800°C, implying both that these leucosomes represent pure melts and that partial melting took place under water-present conditions. A comparison between the composition of the most felsic leucosomes and melts modelled in pseudosections suggests that 4wt.% free water was present during partial melting, an amount of water that agrees with that needed to produce the calculated melt volume. The variability in the trace element composition of leucosomes and leucogranites is explained by the entrainment of residual phases, mainly biotite with inclusions of apatite and zircon. The source of the water is unknown and two candidates are proposed: water released from crystallization of mafic magmas emplaced in the migmatite and water fluxed from subhorizontal shear zones, with the latter option very likely being the main source. Migration of melt through the migmatite is supported by the location of leucosome bodies in strain shadows and in veins parallel to the main foliation and by mass balance calculations, which show that migmatites at the present outcrop level lost between one fourth and one half of the total amount of melt produced. The remaining melt is present as leucogranitic bodies within the migmatite and as the Hermisende–Padornelo leucogranitic pluton, a residuum-rich granitoid outcropping at the southern boundary of the migmatites that shows a continuous gradation from the migmatite to diatexites and the diatexitic granite. Leucosomes represent 37–55wt.% of the migmatites, very close to the second critical rheological boundary, which is interpreted as an upper limit of melt retention: when the local melt amount exceeded this rheological boundary, the stromatic structure broke and melt coalesced to form the residuum-bearing leucogranites found within the migmatite. The geology of the migmatites at Sanabria supports a model in which melts formed under water-present conditions are cool, leucocratic and REE-poor, that their geochemistry is dominated by entrainment of variable amounts of residuum and that the magmas are mobile only in the vicinity of the source.

SHRIMP U–Pb zircon ages, mineral compositions and geochemistry of placer nephrite in the Yurungkash and Karakash River deposits, West Kunlun, Xinjiang, northwest China: Implication for a Magnesium Skarn

The most highly valued placer nephrite deposits in the world occur in the Yurungkash and Karakash Rivers (also known as the White and Black Jade Rivers) in Hetian, Xinjiang, Northwest China. However, the ages of these deposits and the geological environment in which they formed are not known, and their genesis, mineral compositions, and geochemical and petrographic characteristics are poorly constrained. More than 50 samples of white, green, brown, and black nephrites were examined from the placers with the aim of understanding the timing and processes of formation of the nephrite and its archeological significance. The samples collected comprise fine-grained tremolite with highly variable Fe2O3 (total) contents (green nephrite=0.77–3.97wt.%; black nephrite=0.56–16.23wt.%) and FeO concentrations (green=0.67–3.18wt.%; black=0.48–9.55wt.%). XRD and petrographic studies indicate that placer nephrite is more mineralogically complex than primary nephrite and contains tremolite (most samples >80.1wt.%), diopside (≤21.3wt.%), chlorite (≤10.5wt.%), zircon, titanite, apatite, spinel, garnet, epidote, barite, actinolite, sphalerite, pyrrhotite, graphite, calcite, and iron-hydroxide. The black nephrite contains significant amounts of graphite and Fe(OH)3 as mineral inclusions. The presence of strong negative Eu anomalies (0.04–0.26), flat REE patterns, and δ18O=1.3‰–8.4‰ (330°C) and δD=−17.7‰ to −87.1‰ (350–650°C) for the ore-forming fluids suggests that the deposit is a Mg-skarn formed by the metasomatism of dolomite by fluids derived from local granite/granodiorite intrusions with quite few samples showing geochemical characteristics of serpentinite-related nephrite.Zircons inclusions in the placer nephrite are oval in shape, <200μm long and <100μm wide. Their geochemical characteristics are typical of magmatic zircons, with high Th/U ratios (>0.1), positive Ce anomalies (Ce/Ce*) of ≥1.31. Zircons from both Alamas and two placer rivers have similar geochemical signatures, suggesting that the zircon grains in the placer nephrite were derived from the magmatic rocks that host the nephrite deposits. Zircon grains from two samples of black nephrite yield concordant SHRIMP U–Pb ages of 389±4Ma, 397.1±3.5Ma, and 440.7±4.4Ma. Zircon grains from one sample of green nephrite yield concordant SHRIMP U–Pb ages of 377.8±6.2Ma. These ages are roughly close to those of the emplacement ages of the Alamas and Buya granodiorites and probably represent the recrystallization of zircons or formation ages of the nephrite within the Hetian Nephrite Belt. Some igneous zircon grains yield ages of ~600Ma and were probably sourced from the wall rocks and entrained in the ~400Ma magma.Petrographic studies indicate that the Mg-skarns that host the primary nephrite have been thermally metamorphosed. The nephrite formed during early retrogression from diopside skarn→tremolite skarn (coarse-grained), epidote skarn→tremolite skarn and coarse-grained tremolite→fined-grained tremolite. The first step in the formation of nephrite was alteration of the Mg-dolomite to form an assemblage of diopside, epidote, and minor grossular. This was followed by metasomatism of the skarn assemblage by pneumatolitic/hydrothermal fluids derived from local granitoid intrusions, which, combined with later brittle deformation, formed the fine-grained tremolite (nephrite). Zircon inclusions in the primary and placer nephrite at Hetian have similar geochemical characteristics, implying that the nephrites are genetically related. However, the placer nephrite contains more complex phases or associated minerals than does the primary nephrite. Primary nephrite is generally regarded as the main source of the downstream placer nephrite deposits. The fine grain-size of the tremolite and small quantities of accessory minerals are the main characteristics of high-quality placer nephrite. A combination of transport by glacial melt-water and the relatively high altitude (the primary source region and the placer deposits differ in elevation by 3km) led to the formation of the two extensive placer nephrite deposits at Hetian.

CHEMICAL EVOLUTION AND LATE-STAGE RE-EQUILIBRATION OF Zr-Hf-U-BEARING COLUMBITE-GROUP MINERALS IN THE KOKTOKAY NO. 1 GRANITIC PEGMATITE, ALTAI, NORTHWESTERN CHINA

The Koktokay No. 1 pegmatite in northwestern China is a highly fractionated Li-Cs-Ta (LCT) - type granitic pegmatite. As main Nb - Ta oxides in this pegmatite, columbite-group minerals (CGMs) not only display variations in Nb/Ta and Fe/Mn, but also are typified by noticeable Zr, Hf, and U contents. Here, we report on the chemical evolution and late-stage re-equilibration of CGMs in the processes of crystallization and evolution of this pegmatite. Columbite-group minerals record large variations in Ta/(Nb + Ta) values (0.06–0.98) and moderate variations in Mn/(Fe + Mn) values (0.58–0.99), which could be a result of fractional crystallization of pegmatite magma. The concentrations of ZrO 2 , HfO 2 , and UO 2 in CGMs from the contact zone inwards to zone II of this pegmatite are uniformly less than 0.35 wt.%. However, primary tantalite-(Mn) with Ta/(Nb + Ta) and Mn/(Fe + Mn) values of 0.63–0.98 and 0.86–0.99, respectively, show relatively high contents of ZrO 2 , HfO 2 , and UO 2 , up to 1.68, 0.73, and 1.25 wt.%, respectively. The contents of ZrO 2 , HfO 2 , and UO 2 are positively correlated with variations in Ta/(Nb + Ta) values within different grains and single grains of primary tantalite-(Mn). Crystal-chemical considerations suggest that Zr, Hf, and U are possibly present in the tantalite-(Mn) structure due to the substitution 3(Zr,Hf,U) 4+ = 2(Nb,Ta) 5+ + (Fe,Mn) 2+ . In some areas within the primary tantalite-(Mn) crystals in zone III, specific tantalite-(Mn) has been identified with sharp but irregular contacts with the host tantalite. Such tantalite-(Mn) is chemically depleted in Zr, Hf, and U, but contains many small inclusions of zircon, the MnZrTa 2 O 8 phase, uraninite, and U-rich member of the microlite group of minerals. In a similar way, the primary Zr-Hf-U-bearing tantalite-(Mn) experienced late-stage local re-equilibration, giving rise to the formation of secondary Zr-Hf-U-depleted tantalite-(Mn) and other Zr- or U-rich mineral inclusions in an open fluid-mediated system. The behaviors of Zr, Hf, and U in CGMs in the Koktokay No. 1 pegmatite are indicative of Zr, Hf, and U enrichment during fractional crystallization of a highly fractionated pegmatite magma on one hand and imply late-stage activities of fluids on the other. Considerable enrichment of U and Hf in CGMs indicates that this type of mineral may act as a target for in situ LA-ICP-MS U-Pb dating and Lu-Hf isotope measurements, which may provide key information for understanding the source of the magma and the evolution mechanism of the Nb-Ta mineralization.

A revisit to the Yorii jadeite–quartz rock, the Kanto Mountains, central Japan: Implications for petrogenesis

The jadeite–quartz rock (or jadeitite) from the Yorii area, the Kanto Mountains, central Japan occurs in greenstone mélange subjected to metamorphism related to the Sanbagawa event. This rock was previously suggested to have formed through “vein precipitation” process contemporaneous with the Jurassic accretionary event. Mineral inclusions in zircons from this jadeite–quartz rock were re-examined in this study. The results clearly showed that these zircons contain mineral inclusions of both primary (igneous) and secondary (metasomatic) origin. The former are not present in matrix of the jadeite–quartz rock and the latter are pseudo-inclusions. These zircons are thus inherited or incompletely recrystallized ones. The geochemical characteristics of these zircons, including Th/U ratio and Ce anomaly, are also in accord with this conclusion. The jadeite–quartz rock should therefore have formed through “wholesale metasomatic replacement” process at an age younger than 141Ma from a protolith of probable igneous origin aged at 162.2±0.6Ma.Based on the available data, a few criteria, including texture, mineral inclusion, Th/U ratio and Ce anomaly of zircon, were proposed to categorize zircons in jadeitite into inherited, incompletely recrystallized and metasomatic ones. The results may be used to infer the formation mechanism of the hosting jadeitite. Some possible ambiguities in applying the proposed criteria were discussed. Following the proposition, it was shown that previous genetic conclusions for some jadeitites from various occurrences around the world would be subjected to question and should be re-examined by careful investigation of mineral inclusions in zircons, similar to this study, in the future.

Optical processes in YVO4:Eu3+across zircon-to-scheelite phase transition

The luminescence processes across zircon-to-scheelite phase transition in undoped YVO4 and Eu3+-doped YVO4 are investigated by measuring the Raman, excitation and emission spectra and decay characteristics of the compounds with respect to high hydrostatic pressure up to 200kbar. The scheelite polymorphs obtained after releasing the high pressure show no luminescence when undoped, even down to 10K, but when activated with Eu3+ exhibit the characteristic scheelite-related Eu3+ emission but with low intensity. This emission is produced upon UV excitation in zircon inclusions that remain after the pressure treatment.

Refining late Quaternary plunge pool chronologies in Australia's monsoonal 'Top End'

Plunge pool deposits from Australia's 'Top End' are considered as important archives of past monsoonal activity in the region. The available chronology of these deposits was so far based on thermoluminescence (TL) dating and indicated maximum flood magnitudes during the Last Glacial Maximum in contrast with more arid conditions as deduced from other archives of the region. This study revisits plunge pool deposits at Wangi Falls by applying multiple and single-grain Optically Stimulated Luminescence (OSL) dating of quartz and high-resolution gamma spectrometry, supported by radiocarbon dating of organic material. The aim is to reappraise the existing chronology and investigate if the deposits are affected by partial bleaching, post-depositional mixing and/or problems related to annual dose determination. The latter seems to have a minor impact on the ages at most. Equivalent Dose (De) distributions are broad, in particular for single grains, but apparently not result from partial bleaching or post-depositional mixing. Rather, microdosimetry caused by radiation hotspots in the sediment and zircon inclusions in the quartz grains is considered problematic for these sediments. The results presented here imply that the previous TL chronology overestimated the real deposition age of the sediments.

Age Determination of Zircon Inclusions in Faceted Sapphires

Age Determination of Zircon Inclusions in Faceted Sapphires