Experimental studies of Ta2O5 and columbite–tantalite solubility in fluoride solutions from 300 to 550°C and 50 to 100 MPa

It is well established that the fractionation of Li–F granitic magmas at depth leads to the accumulation of flux elements such as F and Li, and metal cations such as Ta and Nb in residual melts. However, it remains to be determined whether magmatic fractionation is sufficient to concentrate Nb and Ta into economically significant quantities, and what role hydrothermal–metasomatic processes play in the formation of such ore deposits. In the literature, reliable data about the solubility of Ta and Nb in hydrothermal solutions is missing or incomplete. This study provides a quantitative experimental estimation of the possible contribution from hydrothermal processes in Ta enrichment in cupolas of albitized and greisenized Li–F granite. Experimental studies of Ta₂O₅ and columbite–tantalite (Mn,Fe)(Nb,Ta)₂O₆ solubility were carried out in fluoride solutions consisting of HF, NaF, KF, and LiF. At low fluoride concentrations (0.01 and 0.1 m), Ta₂O₅ solubility at 550°C and 100 MPa under Co–CoO oxidizing conditions is low (near 10^?5–10^?4 m) in all fluoride solutions (HF, NaF, KF, LiF). At high fluoride concentrations (1 and 2 m) the highest Ta₂O₅ concentrations (10^?1 m) were detected in HF solutions. In KF, NaF, and LiF solutions, the Ta₂O₅ solubility is also high (10^?3–10^?2 m). The dependence of columbite–tantalite (Nb₂O₅-59 wt. %, Ta₂O₅-18 wt. %) solubility as a function of solution composition, T, and P has also been investigated. Tantalum and Nb concentrations have the highest values in HF solutions at reduced conditions (up to 10^?3 to 10^?2 m Ta in 1 m HF). In 1 m NaF solutions, the concentrations of Nb and Ta are, respectively, 2.5 and 3 orders of magnitude less than those in the 1 m HF solutions. Solubility of Ta and Nb in KF solutions has intermediate values. It is established that in NaF and KF solutions the dependence of solubility on pressure is distinctly negative. The Nb and Ta contents increase with increasing concentrations of HF and KF in solution, however, they do not change with increasing NaF concentration. In NaHCO₃, Na₂CO₃, and HCl solutions columbite–tantalite solubility is low. Even in 1 m chloride solutions the content is within the limits of 10^?5 m for Nb and 10^?6 to 10^?8 m for Ta. We conclude that hydrothermal transport of Ta and Nb is possible only in concentrated fluoride solutions.     

Partitioning of trace elements between rutile and silicate melts: Implications for subduction zones

Mineral/melt trace element partition coefficients were determined for rutile (TiO₂) for a large number of trace elements (Zr, Hf, Nb, Ta, V, Co, Cu, Zn, Sr, REE, Cr, Sb, W, U, Th). Whilst the high field strength elements (Zr, Hf, Nb, Ta) are compatible in rutile, other studied trace elements are incompatible (Sr, Th, REE). In all experiments we found D_Ta > D_Nb, D_Hf > D_Zr and D_U > D_Th. Partition coefficients for some polyvalent elements (Sb, W, and Co) were sensitive to oxygen fugacity. Melt composition exerts a strong influence on HFSE partition coefficients. With increasing polymerization of the melt, rutile/melt partition coefficients for the high field strength elements Zr, Hf, Nb and Ta increase about an order of magnitude. However, D_Nb/D_Ta and D_Hf/D_Zr are not significantly affected by melt composition. Because D_U ? D_Th, partial melting of rutile-bearing eclogite in subducted lithosphere may cause excesses of ²³⁰Th over ²³⁸U in some island arc lavas, whereas dehydration of subducted lithosphere may cause excesses of ²³⁸U over ²³⁰Th. From our partitioning results we infer partition coefficients for protactinium (Pa) which we predict to be much lower than previously anticipated. Contrary to previous studies, our data imply that rutile should not significantly influence observed ²³¹Pa-²³⁵U disequilibria in certain volcanic rocks.     

Trace element geochemistry of scheelite and rutile from metaturbidite-hosted quartz vein gold deposits, Meguma Terrane, Nova Scotia, Canada: genetic implications

Scheelite and rutile from several metaturbidite-hosted gold-bearing quartz vein deposits of the Meguma Terrane of Nova Scotia were analyzed for trace elements including rare earth elements, niobium and tantalum. Scheelites have high concentrations of Sr, Nb, Y and rare earth elements (REE) with bell-shaped chondrite-normalized REE patterns accompanied by both positive and negative Eu anomalies. They also have high Nb/Ta ratios (80–300). Three distinct trace element types of the scheelites are interpreted to reflect chemical differences in the pulses of hydrothermal fluids. Hydrothermal rutiles have high contents of W (up to 4.2 wt.% WO₃), are rich in Ta compared to Nb and have a very low Nb/Ta ratio (~0.3). Hydrothermal fluids which produced both scheelite with a high Nb/Ta and rutile with a low Nb/Ta ratio are an efficient medium for fractionation of this ratio although these two minerals play an important role during the process.     

Petrogenesis of Maobei rutile eclogites from the southern Sulu ultrahigh-pressure metamorphic belt, eastern China

The Maobei complex in the southern Sulu ultrahigh‐pressure (UHP) metamorphic belt, eastern China, mainly consists of layered eclogites, garnet peridotites and orthogneisses. Based on the modal mineral and whole‐rock compositions, eclogites from the Maobei complex are divided into quartz eclogite, quartz‐rich eclogite, rutile eclogite, rutile‐rich eclogite and eclogite. The distinct spatial changes in the lithology and related chemical compositions indicate that this complex includes 10 rhythmic layers. The rutile eclogites have high TiO₂ (2.4–5.9 wt%), commonly coupled with high P₂O₅ (up to 4.1 wt%) contents; most show fractionated REE patterns with slight positive Eu anomalies. The rutile‐rich eclogites have very high TiO₂ (3.3–5.7 wt%), FeO^T (17.5–25.3 wt%), V (126–1163 ppm) and Co (14–132 ppm), and very low SiO₂ (38.0–42.3 wt%), Zr (24–85 ppm), Nb (0.3–6.9 ppm), Ta (<0.1–0.6 ppm) and total REE (10.7–334.0 ppm) contents, variable degree of LREE depletion, and positive Eu anomalies (Eu/Eu* = 1.1–2.9), and the Ti is decoupled from other high‐field‐strength elements. These characteristics are consistent with Fe‐Ti gabbros of typical layered intrusions, implying a cumulate of plagioclase, clinopyroxene and abundant accessory magnetite in an evolved basaltic magmatic chamber. Based on a normal stratigraphic sequence, the Maobei complex shows an iron‐enrichment trend, followed by alkaline enrichment with increasing fractionated crystallization and stratigraphic height. These facts, together with SHRIMP U‐Pb zircon ages of 773.7 ± 8.0 Ma, indicate that the protolith of the Maobei complex is a Neoproterozoic layered intrusion consisting of a base of peridogabbro, a main body of gabbro and minor granodiorite. Unusually high Ti, V and P contents in three rutile eclogite layers suggest that they are potential economic ore deposits.     

Petrogenesis and mantle source characteristics of Cenozoic alkaline diabase,Jiangxi Province,southeastern China

Major element, trace element, and Sr–Nd–Pb isotopic compositions of Cenozoic diabase in southeastern China provide insights into the nature of their mantle sources and processes. The diabases are alkaline in lithochemistry (Na₂O + K₂O = 4.37–5.19 wt.%) and have overall oceanic island basalt-like trace element patterns, without negative Nb–Ta anomalies. In addition, they are characterized by lower La/Nb (<1.5) and La/Ta (<22), and higher Ce/Pb (>15) and Nb/U (>30) ratios, indicating an origin in the asthenospheric mantle. The relatively lower ¹⁴³Nd/¹⁴⁴Nd (0.512632–0.512648) and ²⁰⁶Pb/²⁰⁴Pb (18.20–18.22), but intermediate ⁸⁷Sr/⁸⁶Sr (0.7061–0.7063) ratios of the diabases are similar to enriched mantle type 1, suggesting crustal contamination or mixing with metasomatized lithsopheric mantle. However, the low Th and U contents and lack of correlations of Nd isotope compositions and MgO preclude significant crustal contamination. Alternatively, the moderate TiO₂ contents (2.01–2.09 wt.%) and high Cr concentrations (>240 ppm) suggest interaction between asthenosphere-derived melts and metasomatized lithospheric mantle. Petrological modelling suggests that the diabases were generated from a low degree (~3–5%) of partial melting of lherzolite with ~2–3% garnet. Jiangxi diabase was generated in a within-plate extensional regime, probably related to the far effect of the Himalaya–Tibetan orogen.     

Phase-equilibrium constraints on titanite and rutile activities in mafic epidote amphibolites and geobarometry using titanite–rutile equilibria

Activities of titanite (Ttn, CaTiSiO₅) and/or rutile (Rt, TiO₂) phase components were calculated for 45 well‐characterized natural titanite‐ or rutile‐undersaturated epidote–amphibolites by using the equilibria: (i) 3 anorthite + 2 zoisite/clinozoisite + rutile + quartz = 3 anorthite + titanite + water (referred to as TZARS) and (ii) anorthite + 2 titanite = grossular + 2 rutile + quartz (referred to as GRATiS). In titanite‐bearing and rutile‐absent samples a_Rt is 0.75 ± 0.26. In titanite‐absent, rutile‐bearing samples a_Ttn is 0.89 ± 0.16. Mean values derived for a_Rt/a_Ttn are 0.92 ± 0.12 for rutile + titanite‐bearing samples and 0.42 ± 0.27 for samples lacking both titanite and rutile. Use of these values with TZARS yields pressure estimates for epidote–amphibolites that differ on average by <0.5 kbar from those recorded by established mineral barometers, even where both titanite and rutile are lacking. Despite rather large uncertainties in the average values obtained for a_Rt, a_Ttn or a_Rt/a_Ttn, application of TZARS yields pressure estimates that agree with independent estimates to within ±0.5 kbar for titanite‐ and/or rutile‐saturated samples, and to within ±0.8 kbar for samples that contain neither Ti‐phase. The accuracy and precision of the TZARS barometer are comparable to that of many well‐calibrated barometers. TZARS offers a much‐needed barometer for mafic rocks metamorphosed at epidote‐bearing amphibolite and blueschist facies conditions. In addition, the results provide a basis for application of other thermobarometers, such as Ti‐in‐zircon, where rutile activity is required as input.     

Partition coefficients of Nb and Ta between rutile and anhydrous haplogranite melts

Partition coefficients (D) for Nb and Ta between rutile and haplogranite melts in the K₂O-Al₂O₃-SiO₂ system have been measured as functions of the K₂O/Al₂O₃ ratio, the concentrations of Nb₂O₅ and Ta₂O₅, the temperature, in air and at 1 atmosphere pressure. The Ds increase in value as the K^* [K₂O/(K₂O + Al₂O₃)] molar ratio continuously decreases from highly peralkaline [K^* ∼ 0.9] to highly peraluminous [K^* ∼ 0.35] melts. The D values increase more dramatically with a unit decrease in K^* in peraluminous melts than in peralkaline melts. This compositional dependence of Ds can be explained by the high activity of NbAlO₄ species in peraluminous melts and the high activity of KONb species (or low activity of NbAlO₄ species) in peralkaline melts. A coupled substitution, Al⁺³ + Nb⁺⁵ (or Ta⁺⁵) = 2Ti⁺⁴, accounts for the Ds of Nb (Ta) being much greater in peraluminous melts than in peralkaline melts because this substitution allows Nb (Ta) to enter into the rutile structure more easily. The Ds of Ta between rutile and melt are greater than those of Nb at comparable concentrations because the molecular electronic polarizability of Ta is weaker than that of Nb. The Nb⁺⁵ with a large polarizing power forms a stronger covalent bond with oxygen than Ta⁺⁵ with a small polarizing power. The formation of the strong bond, Nb-O, distorts the rutile structure more severely than the weak bond, Ta-O; therefore, it is easier for Ta to partition into rutile than for Nb. These results imply that the utilization of the Nb/Ta ratio in liquid as a petrogenetic indicator in granitic melts must be done with caution if rutile (or other TiO₂-rich phases) is a liquidus phase. The crystallization of rutile will increase the Nb/Ta ratio of the residual liquid because the Ds of Ta between rutile and melts are greater than those of Nb. Received: 28 December 1998 / Accepted 27 September 1999     

Devonian F-rich peraluminous A-type magmatism in the proto-Andean foreland (Sierras Pampeanas,Argentina): geochemical constraints and petrogenesis from the western-central region of the Achala batholith

A new LA-ICP-MS crystallization age of 370?±?8 Ma is presented for monzogranite from the Achala batholith, the largest Devonian igneous body in the Sierras Pampeanas, confirming previous U-Pb zircon ages and indicating emplacement within a relatively short episode. Granitic rocks from the central area of the batholith display restricted high SiO₂ contents (69.8–74.5 wt.%). Major element plots show ferroan and alkaline-calcic to calc-alkaline compositions with an A-type signature. High concentrations of the high field-strength elements such as Y, Nb, Ga, Ta, U, Th, and flat REE patterns with significant negative Eu anomalies, are also typical of A-type granites. The aluminium saturation index (1.10–1.37) indicates aluminous parent magmas which are further characterised by high FeO/MgO ratios (2.6–3.3) and F contents of igneous biotites (0.9–1.5 wt%), as well as relatively high Al^IV (2.39–2.58 a.p.f.u.) in biotites and the occurrence of primary muscovite. Petrogenetic modelling supports a source enriched in plagioclase and progressive fractional crystallization of feldspar. The central area of the batholith displays small-scale bodies composed predominantly of biotite (80 %), muscovite (10 %) and apatite (10 %), yielding rock compositions with 2.3–5.4 wt. % P₂O₅, and 6–7 wt.% F, together with anomalous contents of U (88–1,866 ppm), Zr (1081–2,581 ppm), Nb (257–1,395 ppm) and ΣREE (1,443–4,492 ppm). Previous studies rule out an origin of these bodies as metasedimentary xenoliths and they have been interpreted as cumulates from the granitic magma. An alternative flow segregation process is discussed here.     

Geochemical features of the Matomb alluvial rutile from the Neoproterozoic Pan-African belt,Southern Cameroon

The Matomb region constitutes an important deposit of detrital rutile. The rutile grains are essentially coarse (> 3 mm), tabular and elongated, due to the short sorting of highly weathered detritus. This study reports the major, trace, and rare-earth element distribution in the bulk and rutile concentrated fractions. The bulk sediments contain minor TiO₂ concentrations (1–2 wt%), high SiO₂ contents (∼77–95 wt%) and variable contents in Al₂O₃, Fe₂O₃, Zr, Y, Ba, Nb, Cr, V, and Zn. The total REE content is low to moderate (86–372 ppm) marked by high LREE-enrichment (LREE/HREE ∼5–25.72) and negative Eu anomalies (Eu/Eu^* ∼0.51–0.69). The chemical index of alteration (CIA) shows that the source rocks are highly weathered, characteristic of humid tropical zone with the development of ferrallitic soils. In the concentrated fractions, TiO₂ abundances exceed 94 wt%. Trace elements with high contents include V, Nb, Cr, Sn, and W. These data associated with several binary diagrams show that rutile is the main carrier of Ti, V, Nb, Cr, Sn, and W in the alluvia. The REE content is very low (1–9 ppm) in spite of the LREE-abundance (LREE/HREE ∼4–40). The rutile concentrated fractions exhibit anomalies in Ce (Ce/Ce^* ∼0.58 to 0.83; ∼1.41–2.50) and Eu (Eu/Eu^* ∼0.42; 1.20–1.64). The high (La/Sm)_N, (La/Yb)_N and (Gd/Yb)_N ratios indicate high REE fractionation.     

Zr-in-rutile thermometry of eclogites from the Karakaya Complex in NW Turkey: Implications for rutile growth during subduction zone metamorphism

Eclogites occur as a tectonic slice within a metabasite-phyllite-marble unit of the Karakaya Complex in northwest Turkey. The high-pressure mineral assemblage in eclogite is mainly composed of garnet + omphacite + glaucophane + epidote + quartz. Trace element characteristics of rutile and Zr-in-rutile temperatures were determined for eclogites from the Karakaya Complex. Core-rim analyses of rutile grains yield remarkable trace element zoning with lower contents of Zr, Nb and Ta in the core than in the rim. The variations in Zr, Nb and Ta can be ascribed to growth zoning rather than diffusion effects. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents, which could be ascribed to the effect of metamorphic dehydration in subduction zones on rutile Nb/Ta differentiation. The rutile grains from eclogites in the Karakaya Complex are dominated by subchondritic Nb/Ta and Zr/Hf ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.The Zr contents of all rutile grains range between 81 and 160 ppm with an average of 123 ppm. The Zr-in-rutile thermometry yields temperatures of 559–604 °C with an average temperature of 585 °C for eclogites from the Karakaya Complex. This average temperature suggests growth temperature of rutile before peak pressure during the subduction. However, some rutile grains have higher Zr contents in the outermost rims compared to the core. Zr-in-rutile temperatures of the rims are about 20 °C higher than those of the cores. This suggests that the outermost rims would have grown from a distinct fluid at higher temperatures than that of the cores. Moreover, Zr contents and calculated temperatures in both inclusion rutile and matrix rutile from eclogites are identical, which suggests that eclogites within the Karakaya Complex belong to the same tectonic slice and underwent similar metamorphic evolution.     

Petrology of Late Jurassic allochthonous ultramafic lamprophyres within the Batain Nappes,Northeastern Oman

ABSTRACT

Late Jurassic ultramafic lamprophyre (UML) sills and dikes occur as 3 km-long intrusions within the allochthonous Whara Formation of the Batain nappes, eastern Oman. The sills and dikes comprise macrocrystic phlogopite and spinel-bearing aillikite and damtjernite. Aillikite is a light grey, massive fine-grained tuffaceous rock with euhedral laths of mica, while damtjernite is a dark grey, medium- to coarse-grained rock with abundant pelletal lapilli and globular segregationary textures. Both lithologies are composed of calcite, phlogopite, apatite, magnetite, spinel, diopside, and richterite. Orthoclase occurs only within damtjernite. The rocks are strongly silica undersaturated (17.6–33.7 wt.% SiO₂), with low MgO (4.7–10.2 wt. %) and high Al₂O₃ (3.5–8.6 wt.%). The aillikites are distinguished from the damtjernites by their lower SiO₂, Al₂O₃, and Na₂O abundances, and their higher MgO, CaO, and P₂O₅ contents. The rare earth element (REE) patterns of both rock types are similar and show strong light REE (LREE) enrichment. Both are enriched in Ba, Th, U, Nb, and Ta, with normalized concentrations of up to 1000 times those of primitive mantle. Relative depletions are apparent for high REE (HREE), K, Rb, Pb, Sr, P, Zr, and Hf. The rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70435–0.70646, whereas initial ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0 · 512603 and 0 · 512716 (εNd_i 2.6–3.2). Pb isotopic ratios are more varied among the aillikites and damtjernites: ²⁰⁸Pb/²⁰⁴Pb_i = 38.97–39.39 and ²⁰⁷Pb/²⁰⁴Pb_i = 15.35–15.58, ²⁰⁶Pb/²⁰⁴Pb_i = 18.08–18.96. The abundance of phlogopite, apatite, and rutile and enrichment in LREEs, Ba, Th, U, Nb, and Ta in the Sal UMLs suggest metasomatic enrichment of these rocks following a low degree of partial melting of a depleted source region. Ar–Ar age dating of phlogopite macrocrysts from the aillikites and damtjernites (154 and162 Ma, respectively) correlates with large-scale tectonic events recorded in the proto-Indian Ocean at 140–160 Ma.     

Petrology and geochemistry of the Juzzak Sill,western Chagai arc (Pakistan): implications for petrogenesis and emplacement

The Juzzak Sill occurs in the western part of the east-west trending, subduction-related magmatic belt known as the Chagai arc. The sill is concordantly emplaced in the Paleocene Juzzak Formation and locally cross-cuts the Early to Middle Eocene Robat Limestone and Eocene Saindak Formation. The sill is a porphyritic pyroxene diorite that grades into a porphyritic andesite (60.12–61.57 wt% SiO₂) along the chilled margins. It comprises phenocrysts of hypersthene and plagioclase (An_32–45) in a medium- to fine-grained groundmass of these minerals, opaque oxide, and apatite. The rocks are high-K (2.37–2.86 wt% K₂O) calc-alkaline with low Mg# (42–55), Cr (51–80 ppm), and Ni (22–30 ppm) contents. Mantle-normalized trace element patterns, exhibited by marked negative Nb anomalies and positive spikes for Sr, Rb, and Zr and are akin to island arc signatures. The relatively higher ratios of Zr/Y (3.57–6.58), Ti/V (46.05–54.36), Ta/Yb (0.14–0.15), and Th/Yb (2.56–2.65) and high ⁸⁷Sr/⁸⁶Sr ratio (0.70524) suggest the role of continental crust materials, thus implying continental margin-type arc affinity. The source diagnostic ratios including K/Ba, P/Zr, and La/Ce of Juzzak Sill andesite and Eocene andesite from the Chagai arc are more or less similar, but the former has a much higher K/Y and Ba/Y ratios, which suggests assimilations of the host sediments during intrusion.     

Trace element variations in olivine phenocrysts from Ugandan potassic rocks as clues to the chemical characteristics of parental magmas

Olivine phenocrysts in ugandite and leucite basanite from the western branch of the East African Rift have been analysed for up to 34 trace elements by Laser-ICP-MS with detection limits as low as 1 ppb. A combination of point analyses with varying ablation crater diameters and line scans allow the identification of subtle zonations from core to rim, as well as characterization of the chemical effects of contamination along cracks. Trace element concentrations are remarkably uniform between large and small phenocrysts; fractionated leucite basanites (Mg# 59) have higher D _Ca and D _Al, and less fractionated LREE/HREE than MgO-rich ugandites (Mg# 75–80). Minor zonation is seen in elements with cation charges from 5+ to 2+ (P, Ti, Zr, Cr, Al, Sc, V, Cu, Mn, Ni) and show correlation between Ti and Al, but not P. Early phenocryst cores with high Li or Ni, low Mn, or enrichments in many trace elements can be identified, whereas xenocrysts have exceptionally low Na, Cr, Ti, V and Co. Partition coefficients for Ni are 31–35, less than in lamproites, with which they demonstrate an approximately linear correlation with K₂O content, K₂O/Al₂O₃ and K₂O/Na₂O of the melt, but none with SiO₂ content or Mg#. D-values for Cr, Mn and Co overlap with those of basalts, whereas those for Sc (0.011–0.018), Zn (0.44–0.49) and Ga (0.006–0.007) are lower. D _V of various potassic rocks (0.015 in the Ugandan rocks) confirms the dependence on fO₂ calibrated by the Fe³⁺/(Fe³⁺+Fe²⁺) of spinels; the Ugandan potassic rocks crystallized at fO₂ = FMQ to FMQ + 1. The ugandite olivines have some trace element characteristics reminiscent of those in metasomatized Kaapvaal peridotites, but not ocean islands. Line scan analyses are contaminated in Al, Ca, Cu, Ga, Sr, Zr, Nb, La and Ce, elements that are also concentrated in microcracks between subgrains, indicating smearing out during polishing, and demonstrating that large spot analyses produce the best results.     

Geochronology and geochemistry of Early Devonian intrusions in the Qimantagh area,Northwest China: evidence for post-collisional slab break-off

The Qimantagh area of Northwest China lies in the western part of the East Kunlun Orogenic Belt and is characterized by extensive magmatism, particularly in the Triassic. However, recent research has shown that Devonian magmatism was also widespread in this area and has a genetic relationship with mineralization. This article presents a detailed study of three types of Early Devonian intrusions: high-K calc-alkaline granites, A-type granites, and mafic intrusions, all from the Qimantagh region. These rocks were subjected to precise zircon U–Pb dating, major and trace element analyses, and Sr–Nd isotope measurements, focusing on the Lalingzaohuo (eastern Qimantagh) and Yemaquan (central Qimantagh) monzogranites, as well as the coeval Tanbeixuefeng (western Qimantagh) mafic dike swarm. To better understand the Early Devonian igneous activity in the Eastern Kunlun, data for other coeval granitoids were compared with our data. The Yemaquan monzogranite yielded a mean zircon U–Pb age of 400.5 ± 1.4 Ma. These rocks are metaluminous to slightly peraluminous, with Al₂O₃ contents of 13.10–14.16 wt.%, high alkali contents (total K₂O + Na₂O) of 6.89–7.68 wt.%, relatively low Sr contents (79–192 ppm), and high (La/Yb)_N ratios, all of which indicate an I-type granite affinity. The Lalingzaohuo monzogranites yielded mean zircon U–Pb ages of 396.2–402.2 Ma. These rocks have higher SiO₂ and alkali contents than the Yemaquan monzogranite, with high 10,000 Ga/Al ratios, high Zr + Nb + Ce + Y contents, high Fe₂O₃^T/MgO ratios, and high Y contents, indicating an A-type granite affinity. These two monzogranites have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.703–0.706 and εNd(t) values of –0.1 to –0.7. The Sr–Nd isotopic data require a significant input of a mantle component in the petrogenesis of these granites. The Tanbeixuefeng diabase dikes formed at ~396 Ma and have a continental tholeiitic affinity, as evident from small Ti–Nb–Ta anomalies and high contents of light rare earth and large-ion lithophile elements. We propose that post-collisional slab break-off was responsible for the generation of these Early Devonian intrusions in the Qimantagh area.     

Altered rocks of the Onguren carbonatite complex in the Western Tansbaikal Region: Geochemistry and composition of accessory minerals

The paper discusses the mineralogy and geochemistry of altered rocks associated with calcite and dolomite–ankerite carbonatites of the Onguren dyke–vein complex in the Western Transbaikal Region. The alteration processes in the Early Proterozoic metamorphic complex and synmetamorphic granite hosting carbonatite are areal microclinization and riebeckitization; carbonates, phlogopite, apatite, and aegirine occur in the near-contact zones of the dolomite–ankerite carbonatite veins; and silicification is displayed within separated zones adjacent to the veins. In aluminosilicate rocks, microclinization was accompanied by an increasing content of K, Fe³⁺, Ti, Nb (up to 460 ppm), Th, Cu, and REE; Na, Ti, Fe³⁺, Mg, Nb (up to 1500 ppm), Zr (up to 2800 ppm), Ta, Th, Hf, and REE accumulated in the inner zone of the riebeckitization column. High contents of Ln _Ce (up to 11200 ppm), U (23 ppm), Sr (up to 7000 ppm), Li (up to 400 ppm), Zn (up to 600 ppm), and Th (up to 700 ppm) are typical of apatite–phlogopite–riebeckite altered rock; silicified rock contains up to (ppm): 2000 Th, 20 U, 13000 Ln _Ce, and 5000 Ва. Ilmenite and later rutile are the major Nb carriers in alkali altered rocks. These minerals contain up to 2 and 7 wt % Nb₂O₅, respectively. In addition, ferrocolumbite and aeschynite-(Ce) occur in microcline and riebeckite altered rocks. Fluorapatite containing up to 2.7 wt % (Ln _Ce)₂O₃, monazite-(Ce), cerite-(Ce), ferriallanite-(Ce), and aeschynite-(Ce) are the REE carriers in riebeckite altered rock. Bastnäsite-(Ce), rhabdophane-group minerals, and xenotime-(Y) are typical of silicified rock. Thorite, monazite-(Ce), and rhabdophane-group minerals are the Th carriers.     

Niobium and Tantalum Concentrations in NIST SRM 610 Revisited

Niobium and Ta concentrations in MPI‐DING and USGS (BCR‐2G, BHVO‐2G, BIR‐1G) silicate rock glasses and the NIST SRM 610–614 synthetic soda‐lime glasses were determined by 193 nm ArF excimer laser ablation and quadrupole ICP‐MS. Measured Nb and Ta values of MPI‐DING glasses were found to be consistently lower than the recommended values by about 15% and 25%, respectively, if calibration was undertaken using commonly accepted values of NIST SRM 610 given by Pearce et al. Analytical precision, as given by the 1 s relative standard deviation (% RSD) was less than 10% for Nb and Ta at concentrations higher than 0.1 μg g^?1. A significant negative correlation was found between logarithmic concentration and logarithmic RSD, with correlation coefficients of ‐0.94 for Nb and ‐0.96 for Ta. This trend indicates that the analytical precision follows counting statistics and thus most of the measurement uncertainty was analytical in origin and not due to chemical heterogeneities. Large differences between measured and expected Nb and Ta in glasses GOR128‐G and GOR132‐G are likely to have been caused by the high RSDs associated with their very low concentrations. However, this cannot explain the large differences between measured and expected Nb and Ta in other MPI‐DING glasses, since the differences are normally higher than RSD by a factor of 3. Count rates for Nb and Ta, normalised to Ca sensitivity, for the MPI‐DING, USGS and NIST SRM 612–614 glasses were used to construct calibration curves for determining NIST SRM 610 concentrations at crater diameters ranging from 16 (im to 60 μm. The excellent correlation between the Nb/Ca_1μgg‐1 signal (Nb represents the Nb signal intensity; Ca_{1μg g‐1} represents the Ca sensitivity) and Nb concentration, and between the Ta/Ca_{1μg g‐1} signal (where Ta represents the Ta signal intensity; Ca_{1μg g‐1} represents the Ca sensitivity) and Ta concentration (R²= 0.9992–1.00) in the various glass matrices suggests that matrix‐dependent fractionation for Nb, Ta and Ca was insignificant under the given instrumental conditions. The results confirm that calibration reference values of Nb and Ta in NIST SRM 610 given by Pearce et al. are about 16% and 28% lower, respectively. We thus propose a revision of the preferred value for Nb from 419.4 ± 57.6 μg g^?1 to 485 ± 5 μg g^?1 (1 s) and for Ta from 376.6 ± 77.6 μg g^?1 to 482 ± 4 μg g^?1 (Is) in NIST SRM 610. Using these revised values for external calibration, most of the determined average values of MPI‐DING, USGS and NIST SRM 612–614 reference glasses agree within 3% with the calculated means of reported reference values. Bulk analysis of NIST SRM 610 by standard additions using membrane desolvation ICP‐MS gave Nb = 479 ± 6 μg g^?1 (1 s) and Ta = 468 ± 7 μg g^?1 (1 s), which agree with the above revised values within 3%.     

金红石微量元素电子探针分析

金红石电子探针微量元素分析一般以人工合成的氧化物来作为监测标样,尚较缺乏对金红石标样进行系统地测试分析。本文运用CAMECA SXFive电子探针对金红石标样R10进行微量元素分析,根据金红石中主要微量元素在地质学中的应用,本次共分析了Al、Si、Ti、Fe、Cr、Zr、V、Nb、Ta等9个元素,Ti、Si元素作为本次分析的监测元素。本文通过调整加速电压和电流、背景和峰值测试时长以及干扰谱峰处理等来提高微量元素分析精度和准确度。分析结果显示,其中,Zr(780±29×10~(-6))(1SD,n=25)、Nb(2799±66×10~(-6))、V(1276±33×10~(-6))、Fe(4309±34×10~(-6))、Cr(718±31×10~(-6))的分析结果与二次离子质谱(SIMS)和激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)的推荐值在误差范围内一致。大部分元素数据波动范围在10%以内,V、Fe元素的数据波动范围仅在5%以内。V、Nb和Fe测试精度比前人电子探针分析结果有较大提高。金红石Zr测试误差传递给金红石Zr温度计给出的温度误差一般22℃。本文还对金红石Zr温度计应用、提高Ta元素分析精度和准确度、金红石Fe~(3+)分析等问题进行了探讨。     

Experimental study of partitioning of tantalum,niobium, manganese,and fluorine between aqueous fluoride fluid and granitic and alkaline melts

This study presents a new set of quantitative experimental data on the partitioning of Ta, Nb, Mn, and F between aqueous F-bearing fluid and water-saturated, Li- and F-rich haplogranite melts with varying alumina/alkali content at T = 650–850 °C and P = 100 MPa. The starting homogeneous glasses were preliminary obtained by melting of three gel mixtures of K₂O-Na₂O-Al₂O₃-SiO₂ composition with a variable Al₂O₃/(Na₂O+K₂O) ratio, ranging from 0.64 (alkaline) and 1.1 (near-normal) to 1.7 (alumina-rich). Ta, Nb, and Mn were originally present in glass only, whereas F was load in both the glass and the solution. The solutionto-glass weight ratio was 1.5–3.0. The compositions of quenched glass were measured by an electronic microprobe, and those of the aqueous solution, with the ICP-MS and ICP-AES methods. The F concentration in the quenched solution was calculated from the mass balance. Under experimental conditions the partition coefficients of Ta, Nb, and Mn between the fluid and the granitic melt (weight ratio ^fluid C _Ta/^melt C _Ta = ^fluid/melt D _Ta) are shown to be extremely low (0.001–0.008 for Ta, 0.001–0.022 for Nb, and 0.002–0.010 for Mn); thus, these metals partition preferentially into the melt. The coefficients ^fluid/melt D _Ta and ^fluid/melt D _Nb generally increase either with increasing alumina ratio A/NKM in the glass composition, or with rising temperature. The experiments also demonstrated that F preferentially concentrates in the melt; and the partition coefficients of F are below 1, being within the range of 0.1–0.7.

     

V–Cr–Nb–W-Bearing Rutile in Metamorphic Rocks of the Slyudyanka Complex,Southern Baikal Region

The paper presents data on accessory rutile in Cr–V-rich metamorphic rocks of the Slyudyanka crystalline (granulite) complex in the southern Baikal region. The geochemical features of the studied rutile are unique in combining isomorphic admixtures typical of the mineral from either mafic (Cr) or felsic igneous and associated metasomatic rocks (W), as well as the Nb and V contents, which are abnormally high for metamorphic rocks. The highest concentrations of these elements are as follows, wt %: 15.38 V₂O₃, 4.33 Cr₂O₃, 11.09 Nb₂O₅, 12.36 WO₃. These high W and V contents have never been measured in natural rutile. The studied rutile is compared to that from other Cr–V-bearing rocks in the world and various genetic types. The optimal isomorphic substitutions and probable conditions of their realization are discussed.     

Geochemistry and Zircon U–Pb–Hf Isotopic Systematics of the Sanchahe Quartz Monzonite Intrusion in the North Qinling Tectonic Zone,Central China： Implications for its Petrogenesis and Tectonic Setting

The Sanchahe quartz monzonite intrusion is situated in the middle segment of the North Qinling tectonic belt, Central China mainland, and consists chiefly of sanukitoid–like and granodioritic-monzogranitic rocks. The sanukitoid–like rocks are characterized by quartz monzonites, which display higher Mg#（55.0–59.0）, and enrichments in Na2 O＋K2 O（7.28–8.94 %）, Ni（21-2312 ppm）, Cr（56-4167 ppm）, Sr（553-923 ppm）, Ba（912-1355 ppm） and LREE（（La/Yb）N =9.47–15.3）, from negative to slightly positive Eu anomalies（δEu=＋0.61 to ＋1.10）, but also depletion in Nb, Ta and Ti. The granodioritic-monzogranitic rocks diaplay various Mg#of 6.00-53.0, high Na2 O＋K2 O（7.20– 8.30%）, Sr（455–1081 ppm） and（La/Yb）N（27.6–47.8）, with positive Eu anomalies（δEu=1.03–1.57） and depleted Nb, Ta and Ti. Laser ablation inductively coupled plasma mass spectrometry（LA-ICPMS） zircon U-Pb isotopic dating reveals that the sanukitoid-like rocks were emplaced at two episodes of magmatism at 457±3 Ma and 431±2 Ma, respectively. The monzogranites were emplaced at 445±7Ma. Sanukitoid–like rocks have their εHf（t） values ranging from ＋0.3 to ＋15.1 with Hf–depleted mantle model ages of 445 to 1056 Ma, and the monzogranite shows its εHf（t） values ranging from 21.6 to ＋10.8 with Hf–depleted mantle model ages of 635 to 3183 Ma. Petrological, geochemical and zircon Lu –Hf isotopic features indicate that the magmatic precursor of sanukitoid–like rocks was derived from partial melting of the depleted mantle wedge materials that were metasomatized by fluids and melts related to subduction of oceanic slab, subsequently the sanukitoid magma ascended to crust level. This emplaced mantle magma caused partial melting of crustally metamorphosed sedimentary rocks, and mixing with the crustal magma, and suffered fractional crystallization, which lead to formations of quartz monzonites. However, the magmatic precursor of the granodioritic-monzogranitic rocks were derived from partial melting of subducted oceanic slab basalts. Integrated previous investigation for the adackitic rocks in the south of the intrusion, the Sanchahe intrusion signed that the North Qinling tectonic zone was developed in an early Paleozoic transitionally tectonic background from an island arc to back–arc.