Fractional Crystallization Research Articles

Geochemical modeling of basic pegmatites from Foz do Iguaçu in the Itaipu region, western Paraná, Brazil

In the westernmost portion of the state of Paraná, in the city of Foz do Iguaçu, basic pegmatites occur within inflated pahoehoe basaltic flows of the Paraná Igneous Province. Both rock groups are composed of similar mineral assemblages which include labradorite, augite, ilmenite and titano-magnetite, but pegmatites are medium to coarse grained, whereas basalts are fine grained. Host rocks are geochemically classified as basalts (s.s.), while pegmatitic segregations are basaltic andesite and basaltic trachy-andesite, according to the TAS [(Na2O + K2O) x SiO2] diagram. Based on the AFM [(Na2O + K2O) – (FeOtot) – (MgO)] diagram, all rocks are included in the tholeiitic series, classified as high-Fe tholeiite basalts in the cation plot [Al x (Fetot + Ti) x Mg]. Variation diagrams indicate enrichment of SiO2, TiO2, FeO, Na2O, K2O e P2O5 coupled with depletion of Al2O3, MgO e CaO from basalts to pegmatites. La/LuN ratios range between 5.4 to 6.5 for basalts and 6.3 to 7.9 for pegmatites reflecting a greater degree of fractionation in the pegmatitic segregations and, therefore, their more evolved character. Plagioclase crystals in pegmatites have higher sodic contents when compared to those found in host basalts, while CaO content in basaltic pyroxenes is higher than those from pegmatitic segregations. Trace element geochemical modeling results show that it is possible to generate basic pegmatites from basalts comprised of 55 wt.% plagioclase, 35 wt.% clinopyroxene and 10 wt.% titanium and iron oxides. The process occurs as fractional crystallization with 50% residual melt.

Mantle plumes sample heterogeneous mixtures of oxidized and reduced lithologies

Constraining the heterogeneity of oxygen fugacity (fO2) in the upper mantle is important for understanding the role of the deep Earth in determining the redox state of the atmosphere. Measuring the fO2 of primitive, mantle-derived basalts is a means with which to constrain mantle fO2 heterogeneity. Trace element systematics of 721 primitive olivine crystals from fifteen Icelandic picrites are reported, and these data are used with previously published bulk rock compositions to constrain their magmatic fO2. These fO2 values were compared with previously reported ocean island basalt (OIB) fO2 data calculated using identical methods, showing that fO2 is negatively correlated with parental magma MgO and positively correlated with bulk rock La/Yb, trends which were not evident in previous, smaller, global ocean island basalt datasets. These correlations cannot reasonably be the result of fractional crystallization, differential volcanic degassing, or differential partial melting of a typical homogenous mantle peridotite. Instead, differential partial melting of a heterogeneous source which preferentially sampled oxidized fusible lithologies at low melt fractions and diluted such signatures with depleted peridotite melts at high melt fractions likely caused the observed fO2-MgO-isotopic trends. These conclusions further reinforce that the mantle should not be modeled as a homogenous peridotite, but instead as a heterogeneous, non-equilibrated mixture of enriched “blebs” in a matrix of depleted peridotite, and that degree of partial melting is an important control on what portions of the mantle are detectable in igneous rocks.

Potential of porphyry copper mineralization in the Triassic Yunxian-Jinggu arc, in the Sanjiang orogen, SW China

Porphyry copper deposits are commonly associated with porphyries generated in arc environments. Assessing the contemporaneous arc volcanic rocks provides crucial insights into the potential for porphyry copper mineralization. The Triassic Yidun arc in the northern part of the Sanjiang orogen in southwest China, which formed in association with the subduction of the Paleo-Tethys Oceanic slab, hosts several porphyry copper deposits. However, the potential for porphyry copper mineralization in the Yunxian-Jinggu volcanic arc in the southern part of the Sanjiang orogen, which was also linked to the subduction of the Paleo-Tethys Oceanic slab, remains unclear. This study reports zircon U–Pb ages, trace element compositions and Hf isotopes, and whole-rock element compositions and Sr–Nd–Pb isotopes of the basaltic-andesitic-dacitic rocks in the Yunxian-Jinggu volcanic arc. Zircon SHRIMP U–Pb dating of the Wenyu basalts and andesites yielded ages of 244.3 ± 1.9 Ma and 242.5 ± 2.5 Ma, respectively. The basalts have arc geochemical affinities with εHf (t) values of 2.3 to 9.3, εNd(t) values of –0.6 to 0.2, and (87Sr/86Sr)i values of 0.7065 to 0.7068. They have high Ba concentrations (∼ 550 ppm), highly variable Ba/La ratios (0.01 to 0.10), and a narrow range of Th/Yb ratios (1.2 to 1.9). These characteristics suggest that the basalts were derived from mantle metasomatized by fluids likely released from the subducted Paleo-Tethys slab. The Wenyu andesites, with εHf (t) values range from 1.6 to 7.0, εNd (t) value of –0.4, and (87Sr/86Sr)i value of 0.7031, along with distinct Pb isotopes, were probably produced through basaltic magmas undergoing crustal fractional crystallization. The Minle dacites, sharing similar εHf (t) value (–1.6 to 10.7) but lower εNd (t) values (–3.1 to –4.9) compared to Wenyu volcanics, are interpreted as derived from partial melting of basaltic magmas with contamination from ancient crustal components. The zircon Eu/Eu* ratios (mostly < 0.4), zircon ΔFMQ value (–0.6 ± 1.1), and bulk-rock Sr/Y ratios (< 10.3) for the Yunxian-Jinggu volcanic rocks indicate that co-genetic intrusions, if present, would likely be less hydrous and less oxidized, making them less favorable for porphyry copper mineralization.

Petrology and geochemical modeling (Ba-W-Sn) of the Peña do Seo granitoids (West Asturian-Leonese Zone, León, Spain)

The Cadafresnas granites and granitoid breccias (Peña do Seo, W of León) are small post-tectonic stocks of Variscan age related to W-Sn mineralizations. According to their mineralogical geochemical characteristics, they are classified as granites-syenogranites, granodiorites and granitoids rich in quartz, of peraluminous/metaluminous composition. The Rb-Ba-Sr relationships allow assigning a metapelitic source for the granites and a metagreywacke source for the granitoid breccias. Using geochemical modeling (partial melting), the contribution of these sources to explain the Ba, W and Sn contents of the granites and granite breccias is investigated. The results indicate that the partial melting of these lithologies explains part of the content of these elements in the studied rocks, requiring additional processes of fractional crystallization and/or hydrothermal injections.

Fibre alignment in fresh concrete with coarse aggregates and its visualization in transparent model concrete

Steel fibres distributed in concrete can greatly improve the mechanical performance of concrete, especially flexural and tensile behavior, if their orientations align to the direction of principal tensile stress. The study attempts to find diversified characters of fibre alignment in fresh concrete with consideration of adding coarse aggregates. A transparent concrete model, consisting of Carbopol gel, glass beads and crystals, is designed for the observation of fibre orientation. It shows good transparency and visibility, and can consider the effect of coarse aggregates since the volume fraction and particle size distribution of crystals are the same as coarse aggregates in real concrete mixture. The rheological properties of the model can be adjusted by the dosage of Carbopol and glass beads. Using this concrete model, the distribution and orientation of fibres can be directly observed and analyzed, considering the effect of aggregate sizes, concrete rheology and vibration time. The results show that coarse aggregates could limit the distribution space of fibres and lead to the decrease of fibre orientation coefficient. Fibre dispersion coefficient also decreases with the increase of aggregate sizes.

Constraints of boron and oxygen stable isotopes on dehydration fluids, sediment-derived melts, and crustal assimilation of the Toba volcanic system (Indonesia)

Abstract Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with &amp;lt;87 mol% Fo has highly variable (4.8–7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of &amp;lt;0.1 wt% fluids to the mantle, &amp;gt;40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ~0.02‰. Approximately 6–9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab—an application that can be applied to arc magmas worldwide.

Magmatic evolution of the Schiel Alkaline Complex, Bushveld large igneous province, South Africa

The Paleoproterozoic Schiel Alkaline Complex (SAC) in the Limpopo Belt is a composite pluton that coevally formed with the layered intrusion of the Bushveld large igneous province. The SAC is mainly composed of alkaline felsic rocks, pyroxenite, gabbro, syenodiorite/−gabbro, and minor carbonatite and gabbroic dikes. Here we present a new geochemical, geochronological, and isotopic study of the felsic and (ultra)mafic rock suites of the western SAC. The whole rock SmNd isotope study of the felsic rocks revealed εNd2059 Ma values between −6.5 and − 3.8, while those of the syenodiorite/−gabbros and gabbros vary from −7.9 to −8.8, and − 4.3 to +1.1, respectively. The pyroxenites have εNd2059 Ma values of +7.4 to +2.7. The LuHf isotope study displays εHf2059 Ma values of −15.9 to −9.0 for the syenites, alkali granites and syenodiorite/−gabbros, and significantly higher εHf2059 Ma of −3.1 to +3.4 for the gabbros. LA-SF-ICPMS UPb zircon dating yields 2079 ± 10 Ma and 2088 ± 30 Ma for the syenodiorite/−gabbros, and 2086 ± 51 Ma and 2107 ± 30 Ma for the gabbros. In conjunction with published data from the eastern SAC, we propose a close petrogenetic link between the mafic and felsic rocks of the SAC. The positive εNd and εHf values of the pyroxenite and gabbro favor a depleted mantle source, while variable εNd and εHf values, and trace element ratios of the gabbros are ascribed to crustal contamination during magma ascent. The felsic rocks, however, mainly formed by fractional crystallization from the gabbros as revealed by major and trace element correlation trends, isotopic data and thermodynamic modelling.

The lithospheric cycle of ore-forming ingredients and the resultant recurrent ‘flare-ups’ of porphyry Cu deposits: Examples from the Yidun Arc, eastern Tibet

The porphyry Cu deposits with distinct mineralisation ages frequently occur as recurrent ‘flare-ups’ in a narrow region; however, the reasons controlling their occurrence are still debated. The eastern Yidun Arc in eastern Tibet produced a cluster of porphyry Cu deposits of the late Triassic and late Cretaceous ages, making it the ideal location for conducting this research. As a result, we use the late Triassic Xuejiping Cu–(Au) and late Cretaceous Hongshan–Hongniu Cu–Mo deposits as research objects, compiling new and published geochronological, elemental, and Sr–Nd–Hf isotopic data from their ore-forming porphyries to shed light on the above issue. The zircon U–Pb ages of the Xuejiping and Hongshan–Hongniu porphyries are 217.8 – 217.9 Ma and 78.7 Ma, respectively, and display adakitic characteristics. The Xuejiping porphyries present enriched Sr–Nd–Hf isotopes (0.70529 – 0.70590, −3.30 – −1.09, 0.40 – 2.20, respectively), low (Gd/Yb)N (2.24 – 3.20) and Nb/Ta (12.8 – 15.9) ratios, relatively high Nb (14.9 – 20.0 ppm) and Zr/Sm (23.4 – 41.5) values, indicating their origin in the partial melting of enriched mantle that is metasomatised by slab-melts and fluids in subduction zone. Furthermore, the Hongshan–Hongniu porphyry is adakitic, like the late Triassic igneous rocks in this area. Its Sr–Nd–Hf isotopes (0.69873 – 0.70996, −10.7 – −6.72, −14.9 – −0.50, respectively) are close to the isotopic evolution line of the late Triassic Xuejiping intrusions and show a trend towards the regional crystalline basement, which signify that the Hongshan–Hongniu porphyry mainly originated from the late Triassic juvenile crust, with some contributions from crystalline basement. One highly fractionated sample (i.e. HZK17-11-76) has extremely evolved Sr–Nd isotopes (0.70996 and −10.7, respectively), indicative of assimilation of sedimentary rocks. The new genetic models of the late Triassic and late Cretaceous porphyries in the Geza Arc show that the westward subduction of the Garzê–Litang oceanic crust in the late Triassic triggered the occurrence of the highly oxidised and Cu–S–H2O rich adakitic melts in the middle–lower crust, which evolved to amphibole cumulates and porphyry Cu–(Au) deposits (i.e. Xuejiping) in the upper crust. In the late Cretaceous, the extensional environment triggered the re-melting of late Triassic juvenile cumulates and Mo-rich crystalline basement, resulting in Cu–Mo–S–H2O-rich, highly oxidised magma. Through fractional crystallisation and assimilation of Mo-rich sedimentary rocks, the magma eventually transformed into Hongshan–Hongniu porphyry-(skarn) type Cu–Mo deposits. In summary, we conclude that the periodic recycling of the ore-forming ingredients on a lithospheric scale from slab subduction to post-subduction periods controls the recurrent ‘flare-ups’ of PCDs in this confined area.

Oligocene High-MgO Alkali Basalts in Central Tibet: Implications for Magma–Mush Mixing and Mantle Processes

Abstract High-MgO (&amp;gt;9 wt %) basaltic rocks can be primary magmas and are used to constrain the geochemistry and temperature of the mantle. However, high MgO contents can also result from mixing between evolved melts and antecrysts or xenocrysts, and thus, the whole-rock composition might not represent the solidified equivalents of primary magma. Whether such mixing with crystals can result in erroneous interpretations of mantle processes remains unclear. This study presents a petrological and geochemical investigation of the post-collision high-MgO (&amp;gt;9 wt %) Lugu volcanic rocks in the southern Qiangtang terrane, central Tibet. The Lugu volcanic rocks comprise porphyritic and intersertal alkali basalts. Zircon U–Pb ages and 40Ar/39Ar dating suggest that the two types of alkali basalts were erupted at c. 29 Ma. Based on detailed petrographic observations and geochemical analysis, the porphyritic alkali basalts may represent near-primary melts, which are characterised by low SiO2 contents (40.9–45.1 wt %), high CaO/Al2O3 ratios (1.1–1.5), and arc-like trace element patterns. We suggest these basalts were derived by partial melting of enriched garnet peridotite (&amp;gt;3 GPa) in the presence of H2O and CO2. These geochemical features are different from those of the c. 30-Ma (ultra)-potassic rocks in the Qiangtang terrane, indicating that a heterogeneous lithospheric mantle existed beneath the Qiangtang terrane during the Oligocene. In contrast, although the intersertal alkali basalts have high MgO contents (&amp;gt;9 wt %), evidence from mineral chemistry indicates that the whole-rock compositions of the intersertal alkali basalts represent mixtures of evolved residual melts and cumulate crystals. They were the product of polybaric fractional crystallisation and the subsequent mixing of crystals and residual melts in a magmatic plumbing system. Furthermore, when intersertal alkali basalts are assumed to be primary melts, they would have been derived by partial melting of shallow (~2.5 GPa) CO2-poor pyroxenite or peridotite. These conditions are different from interpretations of the nature of the mantle source and melting conditions for porphyritic alkali basalts. Our results highlight that the interpretation of petrogenetic processes should be preceded by detailed mineralogical investigations.

Origin of carbonatite-related niobium deposits: Insights from pyrochlore geochemistry

The carbonatite-related Nb deposits of the Alto Paranaíba Igneous Province (APIP) in central Brazil, currently account for ∼92 % of the global Nb production. In the APIP, pyrochlore is abundant in magnetite–apatite–tetraferriphlogopite ± carbonate rocks or phoscorites, occurring as interbedded layers with carbonatites in the lower hypogene zone, feeding dike swarms of phoscorite and calcite carbonatite, and late-stage carbothermal veins in the upper hypogene zone. The origin of the phoscorite-carbonatite association can be explained by three hypotheses: (1) crystal segregation from fractional crystallization, (2) liquid immiscibility, and/or (3) phoscoritic magma formation after basement metasomatism (fenitization). However, it is not well understood whether pyrochlore formation is limited to a carbonatitic event, carbohydrothermal, or both, and this gap of knowledge is addressed in this work. To investigate the petrogenesis of pyrochlore-rich phoscorite, cathodoluminescence (CL) images, chemical maps, and LA-ICP-MS data were acquired of pyrochlore crystals from magmatic and carbothermal rocks from the Boa Vista Nb mine, Catalão II Complex. In the Boa Vista mine, oscillatory and patchy zoning were identified as primary pyrochlore textures commonly recorded at the lower hypogene zone, while secondary dissolution, skeletal and zonation-free textures are registered at shallower depths in the upper hypogene zone. Calciopyrochlore is the dominant Nb phase at the Boa Vista mine, with only two kenopyrochlore outliers. The pyrochlore CI chondrite-normalized REE distribution is consistent with geochemical results of the carbonatite and phoscorite rocks, indicating a magmatic origin for pyrochlore and the presence of pyrochlore antecrysts in carbothermal veins. The Sr/Y vs La and Na vs Ce diagrams in pyrochlore indicate a continuous fractionation pattern, with some mixtures of antecrysts and primary phases. An examination of intercumulus calcite using CL provide evidence of carbonatitic magma residues within tetraferriphlogopite phoscorite dikes and suggests that alkaline–carbonate-rich fluids played a role in transporting heavy minerals (i.e., magnetite, apatite, pyrochlore). Consequently, the textural and chemical evidence in the Boa Vista Nb mine indicates that the origin of pyrochlore-rich phoscorites is the result of physical segregation of heavy minerals from a carbonatite magma by fractional crystallization, leading to the emplacement of pyrochlore-rich carbonatite and phoscorite dikes. The implications at Catalão II may extend to other APIP alkaline-carbonatite complexes, as they share a genetic connection, and should motivate further studies focusing on pyrochlore geochemistry in other carbonatite-related Nb deposits, which will be crucial for advancing our knowledge of global Nb metallogenesis.

Optical and scintillation properties of modified LSO crystals with different ions doping grown by a rapid CZ system

LSO:Ce is a well-known scintillation crystal that is widely used in nuclear medicine, security checks, and high-energy physics. The scintillation properties of LSO can be modified by doping to meet the ever-growing requirements of radiation detection. However, exploring new doping techniques for LSO crystals with self-designed scintillation features remains time-consuming, especially by the Czochralski (CZ) method. In this paper, we established a high-efficiency crystal growth system based on the CZ method for the rapid growth of LSO crystals. The system is regulated by a high-accuracy auto-diameter controller to grow small boules of ϕ 20 mm × 100 mm. By incorporating an arc bottom crucible and optimizing the temperature field, the crystallization fraction of the as-grown crystal can be increased to 95% with the growth period compressed to 48 h. With the aid of the growth system, LSO crystals with Lu3+ partially substituted by optical inertia RE ions (Sc3+, Y3+, La3+, Gd3+), metal cations co-doping (Ca2＋, Li1+, Yb3+), and different luminescent ions doping (Pr3+, Nd3+, Eu3+, Tb3+, Dy3+, Yb3+) are grown. Comparative analysis shows that the Ca co-doped LSO:Ce exhibits the best scintillation performance, taking into account both light output and decay time, among our samples. Moreover, Tb-doped LSO features relatively good scintillation efficiency with various wavelengths of visible light emission.

Petrology and source characteristics of the Arbarastakh alkaline ultramafic carbonatite-phoscorite complex, the Aldan-Stanovoy Shield

Late Neoproterozoic Arbarastakh alkaline ultramafic carbonatite-phoscorite complex in the southern margin of the Siberian craton (Aldan-Stanovoy shield), includes carbonatites and phoscorites closely associated with pyroxenites-ijolites and ultramafic lamprophyres. Major and trace elements data, Sr, Nd and Pb isotope compositions for the Arbarastakh rocks have been obtained to characterize the sources involved in their formation, primary melt composition and to build the petrogenetic model.All rock varieties, excluding nepheline syenites, are characterized by incompatible elements enrichments, including light rare earth elements, and strong fractionation of REEs. The initial isotope ratios of the analyzed samples, calculated at 645 Ma, display limited variations: εNd from +5.9 to +6.9 and 87Sr/86Sr from 0.70225 to 0.70272, excluding nepheline syenite with εNd +5.4. The initial Pb isotope ratios for the most studied samples overlap with each other within uncertainties. They yield an age of 642 ± 5 Ma. The Nd and Sr isotope data of the Arbarastakh rocks generally fit the patterns of the other Neoproterozoic alkaline ultramafic carbonatite complexes of the southwestern and southern margins of the Siberian craton. Observed Sr, Nd and Pb isotope variations indicate mixing of the asthenosphere and the depleted mantle components.It is supposed that the primary melts for the Arbarastakh rocks were generated directly by low-degree melting of metasomatic phlogopite‑carbonate veins with apatite and Ti-oxides in garnet peridotite, which formed shortly before the onset of melting. Ultramafic lamprophyre (aillikite) is closest to the primary melt composition in terms of high #MgO, Cr and Ni. The aillikite and latter pyroxenite crystallized from primitive melt by fractionating the olivine, phlogopite and clinopyroxene dominated mineral assemblage that was free of feldspar. This fractionation forced Na-enrichment in the magmas resulted in liquid silicate‑carbonate immiscibility. Major and trace element and isotope data indicate that the nepheline syenites are unlikely to be related with other alkaline silicate rocks through fractional crystallization and could have been formed through pre-emplacement interaction with the ambient crustal materials. Emplacement of calcite carbonatites resulted in metasomatism of surrounding pyroxenites with formation of clinopyroxene-phlogopite-calcite carbonatites. Subsequent portions of Fe-P-enriched carbonatite magmas crystallized phoscorites and apatite-dolomitic carbonatites.

Effect of redox conditions of glass melting on the structure and the properties of titanium-containing gahnite glass-ceramics

In order to develop glass-ceramics containing ions of variable valence in lower oxidation states, it is important to know how changing the redox conditions of glass melting affects structure and properties of glass-ceramics. The zinc aluminosilicate glass nucleated by TiO2 was melted with and without addition of As2O3 and heat-treated from 720° to 1350°C to obtain gahnite-based glass-ceramics. DSC, XRD analysis, Raman spectroscopy and TEM studies revealed that variation of glass melting redox conditions affects kinetics of liquid phase separation and rutile crystallization, composition and structure of gahnite and rutile, crystallization of the residual glass, and does not affect kinetics of gahnite crystallization, gahnite and rutile fractions and structure of glass-ceramics. In glass-ceramics prepared from glasses melted without As2O3, absorption in the visible and near-IR spectral ranges is due to octahedrally coordinated Ti3+ ions in gahnite nanocrystals. The study is important for development of rare-earth-free phosphors.

Study on flow and heat transfer characteristics of salt solution ice slurry

As a phase change material, ice slurry allows the storage of thermal energy in the form of latent heat. This has great advantages in terms of reducing energy consumption and environmental protection, and is being widely used. In this study, based on the Eulerian-Eulerian CFD model and the Population Balance Model (PBM), the flow and heat transfer characteristic of salt solution ice slurry was simulation studied, which included the pressure drop, friction coefficient, volume fraction, distribution of ice crystals and local heat transfer coefficient during the flow and solidification of salt solution ice slurry in different conditions. And the experimental system is set up to verify the results of the model and analyze the variation rules of these parameters. The results showed that in salt solutions with different concentrations, when the flow rate and solid phase volume fraction increase, the pressure drop and local heat transfer coefficient of the ice slurry will increase. The surface heat transfer coefficient reached to 2.5 × 103 W m-2 K-1, 2.9 × 103 W m-2 K-1, and 3.2 × 103 W m-2 K-1 when ice crystal volume fraction was 5 %, 10 %, and 15 %, respectively. The study will contribute to understanding of the changes in basic characteristics of salt solution slurry during the flow and phase transition processes.

Plumbing System Architecture and Differentiation Processes of the Nyiragongo Volcano, DR Congo

ABSTRACT The Nyiragongo volcano is one of the most alkali-rich volcanic centres on the planet (Na2O + K2O generally &amp;gt;10 wt.%, agpaitic index up to 1.34), characterized by a semi-permanently active lava lake which hosts silica-undersaturated (SiO2 &amp;lt; 40 wt.%), low viscosity lavas. To improve our understanding of this unique magmatic system, we present a set of 291 samples, acquired during new field excursions between 2017 and 2021. The major and trace element composition of all samples was measured, revealing a lithological range extending from primitive picrites (Mg# 82) erupted from parasitic cones to a variety of highly evolved nephelinites, leucitites, and melilitites erupted from the main edifice as recently as 2002, 2016, and 2021. We measured major and trace element compositions from the full spectrum of minerals present in all sampled lithologies. From these we calculated that the main magma reservoirs feeding Nyiragongo are at approximately 9–15 and 21–33 km depth, in agreement with recent seismic observations. Fractional crystallization modelling using observed mineral compositions and proportions was performed to quantitatively link the lithologies to specific residual liquid fractions assuming evolution from an olivine-melilite parental melt. Our modelling indicates that fractionation and cumulate formation in deep chambers reduces the remaining melt fraction to ~60%, after which melts are injected into upper, liquid dominated magma chambers where fractionation and accumulation of clinopyroxene, melilite, and feldspathoids dominates. Characterisation of mineral textures and geochemistry reveals high crystal mobility in a repeatedly recharging plumbing system split between liquid-dominated, evolved magma chambers and more solid-dominated, primitive mushes, decreasing in liquid fraction with depth.

Apatite geochemical composition of Mesozoic granitoids in the eastern Jiangnan Orogen, S. China: insights into petrogenesis and intrinsic magmatic variables

ABSTRACT The geochemical compositions of apatite are commonly used to provide reliable constraints on the intrinsic magmatic variables of the parental magma. There is a widespread distribution of Mesozoic granitic rocks in the eastern Jiangnan Orogen (EJNO), which include I-type and A-type granitoids. This study aims to investigate the geochemical composition of apatite in 11 representative I-type and A-type granitic intrusions from the EJNO to understand their petrogenesis, intrinsic magmatic variables, and polymetallic mineralization potential. The results show that apatites in these granitoids were characterized by oscillatory zonation and homogeneous texture, and exhibited high F, and low Cl contents, as well as high REE, Y, and Th contents, indicating magmatic origin. These apatites were divided into two groups depending on their geochemical characteristics and original rock types. In Group I apatites (in the I-type granitoids), the absence of an abrupt change in major and trace element concentrations from the core to the rim of the apatite crystals, combined with their low levels of MgO, Sr, and HREY (HREE+Y), serves as evidence to classify the I-type granitoids as low Sr-Y adakitic granitoids with a low Mg#, which can be attributed to the partial melting of the thickened lower crust. In Group A apatites (in the A-type granites), the normal increase in incompatible elements from the core to the rim, as well as the presence of a flat REE pattern with pronounced Eu anomalies, can be attributed to crystal fractionation of plagioclase, allanite, and zircon. Furthermore, Group I apatites have higher SO3, Cl, and molar OH/F and Cl/F ratios compared to Group A apatites. Quantitative calculation by apatite-melt partitioning and modelling suggests that the I-type granitic magmas were more volatile-rich and more oxidized than A-type granitic magmas. Taken together, based on the relationship between the intrinsic magmatic variables and the behaviour of ore-forming elements, our study demonstrates that early I-type granitoids of the EJNO have a strong W-Mo-Cu polymetallic mineralization potential, while late A-type granites may possess rare metal mineralization potential.

Petrogenesis of the Wushi carbonatites in the northwestern Tarim Basin: Implications to deep carbon recycling

Carbonatites are rarely exposed on the Earth's surface, and most of them are supposed to have been mantle-derived. However, in the most recent years, carbonatites with pronounced crustal signatures had also been recognized. Although carbonatites have attracted the attention of many researchers, their genesis remains controversial. In this study, we present a systematic C–O–Sr–Nd–Mg isotopic investigation on the carbonatites exposed in Wushi County located at the northwestern margin of the Tarim Basin to determine their petrogenesis. The Wushi carbonatites, containing the xenoliths of wall rocks, intruded into Precambrian sandstones as dykes. Carbonatites predominantly comprise calcite and accessory minerals, such as dolomite, apatite, mica and quartz, and thus are classified as calciocarbonatites. Trace element geochemistry indicates that the total rare earth element (REE) concentrations in Wushi carbonatites range from 13.6 to 57.3 ppm, with an average of 30.9 ppm. The chondrite-normalized REE patterns of the test samples indicate that light rare earth elements (LREE) in the samples have been enriched relative to their heavy rare earth elements (HREE) and that the LREE/HREE ratios vary from 6.0 to 9.1. The patterns also show negative Eu anomalies (δEu = 0.65–0.75) and weak negative Ce anomalies (δCe = 0.75–0.99). The primitive mantle-normalized trace element spider diagrams display the enrichment of large ion lithophile elements (LILE), such as Pb, Sr and Ba, and the strong depletion of high field strength elements (HFSE), such as Nb, Ta, Zr, Hf and Ti. The δ18OV-SMOW (15.9‰–18.2‰) and δ13CV-PDB (−2.97‰–3.36‰) values are more consistent with those of crust-derived carbonatites than with mantle-derived carbonatites. The values of δ18OV-SMOW and δ13CV-PDB are consistent with the high values of 87Sr/86Sr (0.7096–0.7110) and low values of 143Nd/144Nd (0.5118–0.5120). Typically, δ26Mg of the carbonatites (−1.76‰ to −1.27‰) is much lighter than the δ26Mg of the mantle (−0.25‰ ± 0.04‰) but heavier than the δ26Mg of sedimentary carbonates (−2.72‰ to −2.15‰) obtained from the neighbouring Western Tianshan orogenic belt. The light Mg isotope anomalies cannot be explained using secondary alteration, continental crustal contamination, kinetic isotope fractionation and fractional crystallization. Thus, these anomalies could be due to the isotope exchange reactions occurring between the oceanic crust and sedimentary carbonates during subduction. In this study, we propose a two-stage model to explain Wushi carbonatite formation based on its isotope characteristics. During the early stage of carbonatite formation, sedimentary carbonate rocks located along the northern margin of the Tarim Craton and oceanic crust were subducted into the deep mantle. Isotope exchange and carbonation occurred during subduction to form carbonated eclogite. In the second stage of carbonatite formation, the heat produced by the Permian mantle plume in the Tarim Basin melted the carbonated eclogite to produce carbonatite melt. Thus, the petrogenesis of the Wushi carbonatites revealed by the presence of multiple isotopes indicates that the carbonatite produced by carbonated eclogite in the deep mantle would exhibit typical crustal isotopic characteristics and provide new insights into deep carbon recycling.

Petrological and Geochemical Constraints on the Origin of Strongly Peraluminous Granitoids from the Triassic Guangtoushan Pluton in South Qinling

AbstractAs an important part of the early Mesozoic granites in the South Qinling tectonic belt (SQTB), the Guangtoushan pluton provides a material basis for research on the composition of magma sources and the effects of peritectic assemblage entrainment (PAE) on the changes in the granite composition. As shown by the results of LA‐ICP‐MS zircon U‐Pb dating, the Guangtoushan pluton was emplaced during the Late Triassic (214–212 Ma) and was formed in the post‐collision stage between the SQTB and the Yangtze plate. The collected samples had high SiO2 content and low Cr and Ni contents, indicating that the magmas did not undergo significant crust‐mantle mixing during their evolution. The Guangtoushan granitoids were distributed along the trend line of magmatic fractional crystallization in the F–An–Or diagram. This result, combined with the relatively homogeneous Sr‐Nd isotopic composition, implies that the Guangtoushan pluton underwent slight assimilation and contamination. As can be inferred from the comparison between the compositions of the Guangtoushan granitoids and various fluid‐absent experimental melts, the magma sources of the Guangtoushan granitoids contain a variety of materials, such as graywackes, pyroclastic graywackes, and pelites and are not derived from lower crustal mafic rocks. The correlation between the maficity and the major and trace elements further indicates that the strongly peraluminous granitoids from the Guangtoushan pluton was formed by the partial melting of biotite‐bearing crustal rocks and its magmatic evolution was accompanied by the entrainment of clinopyroxenes and accessory minerals.

Petrogenesis of the Late Eocene to Early Oligocene Yao'an Shoshonitic Complex, Southeastern Tibet: Partial Melting of an Ancient Continental Lithospheric Mantle beneath the Yangtze Block

AbstractCenozoic potassic–ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau. Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications. Almost all of the Cenozoic extrusive and intrusive rocks in the Yao'an area, western Yunnan Province, SW China, are geochemically shoshonitic, collectively termed here the Yao'an Shoshonitic Complex (YSC). The YSC is located in the (south) easternmost part of the ENE–WSW‐trending, ∼550 km‐long and ∼250 km‐wide Cenozoic magmatic zone; the latter separates the orthogonal and oblique collision belts of the India–Eurasia collision orogen. Previously published geochronological and thermochronological data revealed that the rocks of the YSC were emplaced over a short timespan of 34–32 Ma. This and our new data suggest that the primary magma of the YSC likely was formed by partial melting of ancient continental lithospheric mantle beneath the Yangtze Block. This part of the continental lithospheric mantle had likely not been modified by any oceanic subduction. Fractionation crystallization of an Mg‐ and Ca‐bearing mineral and Ti‐Fe oxides during the magmatic evolution probably account for the variable lithologies of the YSC.

Origin and Tectonic Implications of I-type Granites, North Delhi Fold Belt, NW, India: Insights from Whole Rock Geochemistry and Mineral Compositions

Abstract A comprehensive whole-rock geochemical and mineral chemistry study was carried out of granites from the Narnaul and surrounding isolated hills of North Delhi Mobile Belt, NW India to constrain their genesis, and tectonic environment. The distinctive features of granites are meta to peraluminous nature and decreasing trend of P2O5 with high SiO2 (&amp;gt;71%) ascribed to its metaigneous origin. In detail the chemical diversity of the studied samples is reflected by their affinity to Calc-alkaline to shoshonite series with high K2O/Na2O (mostly&amp;gt;1), low MgO (&amp;lt; 0.11%) and variation in Mg# (upto 0.5) and maficity with increasing SiO2. The positive Eu anomalies, high LILE, highly enriched LREE (La/Sm = 4.13–19.33) pattern and comparatively elevated concentrations of Cs, La, Ta, Dy, Zr, Pb and Yb suggest different episodes of magmatic differentiation. Our data indicate that melting of older subduction-related igneous rocks and the underwent fractional crystallization in hydrous conditions is the possible mechanism for the generation of I-type granites in collisional tectonic setting.