Search

Columbite-group minerals (CGMs) have been widely used in U-Pb geochronological analysis of granite and pegmatite Nb-Ta deposits due to their high U and low common Pb characteristics. Due to the equivalent replacement of elements in the mineral structure and complex geological processes, CGMs often exhibit strong, local, and complex chemical zoning. Multiple types of composition zonation of CGMs can reflect the process of niobium-tantalum mineralization. However, the laser resolution of the currently established U-Pb methods cannot be used for in situ dating of CGMs with zonation with a width less than 20 mm. Additionally, there is no consensus on whether significant matrix effects could be observed among CGMs when nonmatrix-matched calibrations were performed. These limitations hinder the advancing understanding of niobium-tantalum differentiation and enrichment mechanisms in the formation of CGMs. In this study, we utilized the widely used geochronological reference material Coltan139 as the primary standard, established a high-resolution LA-MC-ICP-MS U-Pb analysis method using homogeneous CGM samples from the Dakalasu and Jingerquan deposits in the Altai and East Tianshan orogenic belts, Xinjiang, China. This method has a resolution of up to 10 mm. We applied this method to sample JEQ-2, which was collected from the Jingerquan Li-Be-Nb-Ta deposit and exhibited a variety of compositional zonations. Through a comparison of the ages calibrated using the ferrocolumbite Coltan139 and ferrotapiolite CT1 standards, we propose that the major matrix effect for U-Pb dating of CGMs correlates with the Ta/(Nb + Ta) ratios under small beam spot conditions. The matrix effect is likely to increase with the increase in Ta/(Nb + Ta) differences between reference materials and the samples. Furthermore, the U-Pb age results of sample JEQ-2 confirm that CGM grains with normal and oscillatory zonings are magmatic contemporaneous products. A new in-house standard material sample DKLS-27 was also reported with a reliable and stable U-Pb age for CGM U-Pb dating analysis. The 206Pb/238U weighted mean age obtained from long-term monitoring is 250.2 ± 0.3 Ma (2 s, MSWD = 0.52, n = 141). This study offers a new analysis method for the fine geochronology of rare metal deposits. [ABSTRACT FROM AUTHOR]

Columbite-tantalite is predominantly found in rare-metal granites and pegmatites, alkaline and carbonatitic rocks, and columbite-tantalite-bearing peraluminous granites. It is recognized as a potentially valuable mineral for use in U-Pb isotopic dating. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) can be used in the U-Pb dating of columbite-tantalite. LA-ICP-MS requires matrixmatched reference materials for isotopic fractionation/mass bias correction. Some columbite-tantalite reference materials contain common Pb. However, columbite-tantalite reference materials with low contents of common Pb are scarce. Thus, in this study, we proposed a new potential reference material, OXF, with a low common Pb content for use in LA-ICP-MS U-Pb dating of columbite-tantalite. LA-ICPMS analysis revealed that the average U, Th, and Pb concentrations in OXF are 428 ± 156 ppm (2s), 6.3 ± 2.5 ppm (2s), and 20.0 ± 10.0 ppm (2s), respectively, with a total weight of 336.6 g. Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) revealed that the weighted mean 206Pb/238U age of OXF is 262.85 ± 0.64 Ma (MSWD = 1.8, 2s). The LA-ICP-MS U-Pb ages of the four laboratories in the China University of Geosciences (Wuhan) (CUGW), the China University of Geosciences (Beijing) (CUGB), Wuhan Sample Solution Analytical Technology Co. Ltd, China (WHSS), and the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) were determined. The 206Pb/238U ages of the CUGW, the CUGB, WHSS, and the IGGCAS were 262.71 ± 0.33 Ma (2s, n = 151), 263.3 ± 2.3 Ma (2s, n = 46), 263.5 ± 1.3 Ma (2s, n = 42) and 264.3 ± 2.4 Ma (2s, n = 25), respectively. The weighted mean 206Pb/238U age of the four laboratories obtained using LA-ICP-MS and by combining the U-Pb data obtained from the laboratories was 262.83 ± 0.29 Ma (2s, n = 358), which aligns with its value obtained using CA-ID-TIMS after allowing for analytical uncertainties. OXF demonstrated homogeneous U-Pb ages with minimal Pb contributions, indicating its potential for serving as a primary reference material in LA-ICP-MS calibrations. [ABSTRACT FROM AUTHOR]

Columbite group minerals generally incorporate considerable amounts of U, making them promising minerals for in situ U–Pb dating and constraining the time of Nb–Ta mineralization. Columbite group minerals may have uranium-rich inclusions and metamict domains and may show complex internal textures with recrystallization. To avoid such disturbances, isotopic measurements with high spatial resolution are needed. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique, which is widely used for U–Pb dating with high spatial resolution, requires homogeneous reference materials for external calibration. Herein, we present in situ U–Pb ages, ranging from ∼2050 Ma to ∼136 Ma and chemical compositions for ten columbite group minerals considered potential reference materials. Our study indicates that CT3, Buranga, Rongi and SN3 contain very limited amounts of common Pb and can be used as primary reference materials for in situ U–Pb dating. Coltan17, ZKW, DDB, HND and RL2 show variable common Pb contents but have robust U–Pb ages that allow them to be used as primary reference materials. Among them, Coltan17 needs to be further characterized by applying the isotope dissolution thermal ionization mass spectrometry (ID-TIMS) method. A new ID-TIMS weighted average 206Pb/238U age of Coltan139 of 507.8 ± 1.3 Ma (2s, MSWD = 0.9; n = 6) is reported. This study demonstrates that Coltan139 contains relatively high and variable common Pb contents. Therefore, special attention should be given to the selected measurement areas when using Coltan139 as the primary reference material. [ABSTRACT FROM AUTHOR]

Copyright of Mineralogy & Petrology is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Due to their relatively high Hf contents (i.e., several hundreds to thousands of ppm), columbite-group minerals (CGMs) are suitable for Lu–Hf isotope analysis, which can provide new insights into the source of such minerals in combination with the U–Pb age of rare-metal deposits. However, few studies have reported Lu–Hf isotope data for CGMs. Tantalum is a major element in CGMs (12–48 wt% Ta2O5) and has a significant effect on measurements of Hf isotopes using MC-ICP-MS. This effect was first evaluated in this study by analysis of mixed Hf–Ta standard solutions with different Ta/Hf ratios and LA-MC-ICP-MS analysis of various CGMs. This analytical artifact reflects serious tailing of the large 181Ta signal onto the 180Hf and 179Hf masses during analysis. Therefore, in order to obtain accurate Hf isotope data for CGMs by MC-ICP-MS, we present an improved chemical separation procedure for Hf from Ta, and also a novel analytical protocol for LA-MC-ICP-MS analysis of CGMs. Accurate in situ176Hf/177Hf ratios for CGMs can be obtained by normalization to 178Hf/177Hf = 1.4672, rather than 179Hf/177Hf = 0.7325, using the exponential law. This approach was validated by solution and laser ablation MC-ICP-MS analysis of four CGM samples from China and Africa. The Lu–Hf isotopic composition of CGMs provides a new geochemical tracer for rare-metal deposits. [ABSTRACT FROM AUTHOR]

This paper focuses on reasons for the appearance of tetrad effects in chondrite-normalized REE distribution patterns of granitoids (Li-F granites, peralklaine granites, ongonites, fluorine-rich rhyolites, and granitic pegmatites). The analysis of published data showed that the alteration of such rocks by high- and/or low-temperature metasomatic processes does not result in most cases in the appearance or enhancement of M-type tetrad effects in REE patterns. These processes are accompanied by the removal or addition of lanthanides, a W-type sag appears between Gd and Ho, and negative or positive Ce anomalies develop sometimes in REE patterns. The formation conditions of peculiar rocks enriched in Ca and F from the Ary Bulak ongonite massif (eastern Transbaikalia) and the character of REE distribution in these rocks and melt inclusion glasses were discussed. Based on the obtained data and the analysis of numerous publications, it was concluded that REE tetrad effects in rare-metal granitoids are caused by fluoride-silicate liquid immiscibility and extensive melt differentiation in the accumulation chambers of fluorine-rich magmas. A considerable increase in fluorine content in a homogeneous granitoid melt can cause its heterogenization (liquation) and formation of fluoride melts of various compositions. The redistribution of lanthanides between the immiscible liquid phases of granitoid magma will result in the formation of M-type tetrad effects in the silicate melts, because the REE patterns of fluoride melts exhibit pronounced W-type tetrad effects. The maximum M-type tetrad effect between La and Nd, which is observed in many rare-metal granitoids, is related to the character of REE partitioning between fluoride and silicate melts and F- and Cl-rich magmatic fluids. The low non-chondritic Y/Ho ratio (<15) of many rare-metal granitoids may be indicative of a contribution of fluoride melts to the differentiation of F-rich silicic magmas, from which these rocks were formed. The influence of high-temperature F-Cl-bearing fluids on melts and/or granitoid rocks results in an increase in Y/Ho ratio owing to the elevated solubility of Ho in such fluids. [ABSTRACT FROM AUTHOR]

The distribution of mass anomalies within the crust of the Archean Western Superior Province has been investigated with forward and inverse gravity modeling routines. The gravity models indicate that in most of the Western Superior Province, significant mass anomalies occur only within the top 10 km of the crust, where they are generally related to dense metavolcanic rocks and low-density granitoid plutons, and at the crust-mantle boundary, where they are linked to undulations of this interface. An exception is the region encompassing the central Wabigoon Subprovince and the segments of the Quetico and Wawa belts south of it, where implied large intracrustal mass anomalies are associated with a segment of denser lower crust, and with overlying depressions of the interfaces between the main crustal layers. We interpret the predominant lateral mass homogeneity observed at deep crustal levels as evidence of mass redistribution processes associated with a major postaccretionary episode of thermal softening of the Western Superior crust. The incongruent presence of a denser lower crustal segment may reflect the subsequent modification of the lower crust through magmatic intra and underplating, possibly in relation with the Mesoproterozoic midcontinent rifting event. [ABSTRACT FROM AUTHOR]

A high spatial resolution Ar–Ar dating study of compositionally zoned micas using UV laser extraction has been used to investigate the effect of composition and compositional boundaries upon argon diffusion in mica. The crystals are characterised by muscovite cores and Li–F–Rb–Cs-rich lepidolite rims produced by the interaction of a residual melt-vapour phase with the original muscovite during the late stages of pegmatite crystallisation. Single crystals exhibit dramatic intra-grain age variations, with a maximum range of 2,880–2,117 Ma. Backscattered electron images combined with UV-laser analyses reveal that apparent ages younger than 2,600 Ma are directly associated with the compositionally distinct rims, suggesting that Ar diffusion was faster in the lepidolite than in muscovite. Although it is not possible to be fully quantitative using the present data, it seems that argon diffusion rates in lepidolite are similar to those in biotite. Major rearrangements of the crystal structure needed to accommodate high concentrations of lithium, fluorine and large interlayer cations (Rb+, Cs+) in muscovite account for lower argon retentivity of the crystal rims. In most cases the age profiles show no disturbance at the compositional boundary, despite the transition from a di-octahedral to tri-octahedral mica, indicating that in general the boundaries do not act as fast diffusion pathways. However, in some cases there is a clear drop in ages at the compositional boundary, which acts as an effective sub-grain boundary. The difference between the two types may be related to the internal structure of the compositional boundaries or their degree of contact with the grain boundary network of the rock. [ABSTRACT FROM AUTHOR]

U-Pb isotopic analyses using TIMS and Laser Ablation-Multi Collector-ICP-MS were carried out on columbite-tantalite minerals from three suites of rare-element (Li, Cs, Ta) pegmatites in the Superior Province of Canada. Conventional TIMS analyses of these columbite-tantalite crystals produce scattered data and reverse discordance even after HF leaching of the grains prior to dissolution, possibly reflecting the incomplete removal of the open-system metamict segments during sample preparation. LA-MC-ICP-MS analyses of unleached, primary columbite free from inclusions and alteration give consistent and precise (<0.5%) Pb-Pb ages, demonstrating the utility of this approach. However, normal and reverse discordance is also observed in U-Pb data from LA-MC-ICP-MS analyses. This discordance represents either U-Pb mobilisation during recent weathering, sample preparation and/or an analytical artefact originating from variable elemental fractionation between U and Pb during ablation and ionisation that itself may have its origin in the contrasting metamictization of the dated columbite and the monazite standard used. Best age estimates of columbite from pegmatites in the Superior Province are; 2670±5 Ma for the Pakeagama Lake pegmatite, 2644±7 Ma for the Separation Rapids group, and 2665±8 Ma for the Mavis Lake group. The ages broadly show that the rare-element pegmatites are temporally synchronous with adjacent peraluminous granites. [ABSTRACT FROM AUTHOR]

Presents field, petrographical and geochronological results from the Northern Marginal Zone (NMZ) of the Limpopo tectonic Belt in Zimbabwe. Duration of tectonic events and thermal anomalies in the NMZ; Evidence of isotrophic disequilibrium among the retrograde assemblages; Geological framework of the NMZ `sensu stricto.'