Alkali Basalts Research Articles

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

Abstract High-MgO (>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 (>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 (>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 (>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.

Dispersion of sandy sediments during marine–continental transition: An integrated study from the Late Paleozoic western Ordos Basin

The western Ordos Basin is located at the intersection of multiple tectonic units and is a key area in the restoration of the palaeogeographic pattern of the East Asia region. The stability of the area and well-developed Paleozoic strata in the basin provide ideal conditions for revealing the relationships among the tectonic processes apparent there. However, within the constraints of extensive post-depositional modification, poor-quality seismic data, and a lack of wells, sediment dispersion differences, source attribution, and source-to-sink processes are still unclear. In this study, we integrated sedimentology, mineral, and geochemical data obtained from marine, continental, and marine-continental transition sedimentary environments in the study area in order to reveal the evolution from source-to-sink. The results show that the western Ordos Basin was predominantly supplied by the Alxa block, Yinshan orogenic belt, Qilian orogenic belt, and the North Qinling orogenic belt during the Late Paleozoic era. As a result of the closure of the Paleo Asian Ocean, the Alxa block and Yinshan orogenic belt became the primary source areas to the north, being composed of granite and alkaline basalt and some sedimentary rocks. In the southern area, the Qilian and the Northern Qinling orogenic belts were subjected to the PaleoTethys remnant ocean basin subduction during the Early Permian. Granite and alkaline basalt were the source rocks of the North Qinling orogenic belt, which became the source area during the Shanxi period. The Qilian orogenic belt contains some sedimentary rocks. The continuous uplift of the southern and northern orogenic belts. The tectonic setting of the northern source area transitions from a passive continental margin to an active continental margin with a mixture of continental arc. The tectonic setting of the southern source area transitions from a mixture of passive continental margin and active continental margin to an active continental margin and continental island arc. The climate changes gradually from humid to semi-arid and then to arid, and meanwhile, the sea level shows a decreasing trend, leading to an increasing distance of advancement for each source-to-sink system, and gradually converged in the central and eastern regions of the basin. Tidal and fan delta sandy sediments in the study area were transformed into fluvial delta sandy sediments in the northern two source-to-sink systems. The southern source-to-sink systems gradually changed from scattered sandbarrier and coastal sandy sediment to fluvial sandy sediment after the Shanxi Formation.

Zircon and cassiterite U-Pb geochronology and petrochemical characteristics of early Tertiary tin mineralization at Mayo Darlé, Cameroon Volcanic Line

Tin mineralization at Mayo-Darlé in the Cameroon Volcanic Line is associated with highly evolved anorogenic ferroan granite magmatism with a pervasive hydrothermal overprint. Mineralization occurs as cassiterite-quartz-muscovite stockwork and disseminated in quartz-mica and quartz-topaz greisen in low-Ca alkali feldspar granite with a composition near the 1 kbar haplogranitic thermal minimum. The trace element composition of zircon indicates elevated temperature (880 ± 55 °C) and very low oxidation state below the quartz-fayalite-magnetite buffer. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) identifies a partly disturbed U-Pb system for igneous zircon (U and Th contents >1000 ppm) but defines Tera-Wasserburg concordia ages and common-lead-corrected weighted 206Pb/238U ages of about 54 Ma, similar to a previous 87Rb/86Sr vs 87Sr/86Sr regression with a date of 56.1 ± 1.7 Ma (MSWD = 58; n = 9) and an initial 87Sr/86Sr ratio of 0.7085 ± 0.0035 (2 s). Hydrothermal cassiterite gives a pooled LA-ICPMS Tera-Wasserburg concordia age of 56.7 ± 0.6 Ma on two samples (each about 20 points, MSWD = 0.54 and 1.3). These new age data define a period of early Cenozoic intraplate calc-alkalic felsic magmatism as a precursor to abundant Oligocene to Recent alkali basalt volcanism along the Cameroon Volcanic Line. Geochemical and Sr–Nd–Hf isotopic data suggest that the granitic rocks from Mayo Darlé were largely derived by partial melting of Mesoproterozoic basement and modified by strong differentiation, all likely powered by heat (and possibly fluids) from the mantle. The Tertiary age of the rare-metal magmatism sets this system apart from the widespread Late Neoproterozoic to Early Paleozoic rare-metal pegmatites and granites in the Pan-African collisional belts of Africa, and also from the anorogenic Mesozoic ring complexes in Central Nigeria.

Early Eocene Arctic volcanism from carbonate-metasomatized mantle

Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New 40Ar/39Ar chronology indicates their emplacement between ~ 56 and ~ 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at ~ 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component (“A”; 3He/4He ~ 8 RA, εNdi ~ + 3 εHfi ~ + 7, γOsi ~ 0). The second was an enriched component (“B” 3He/4He < 3 RA, εNdi < – 1 εHfi < + 3, γOsi > + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at ~ 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere.

Road performance evaluation of prestressed high-strength concrete pile waste powder as alternative filler in asphalt concrete

As a kind of solid waste, using Prestressed High-Strength Concrete Pile Waste Concrete (PPWC) as the replacement for limestone filler in asphalt concrete can not only reduce the accumulation of PPWC and increase its utilization but also avoid the increased road construction costs and environmental degradation associated with limestone mining. This study aims to investigate the effect of using PPWC filler to replace limestone filler on the road performance of asphalt concrete. Firstly, PPWC was ground into filler particles with a diameter less than 0.075 mm. The particle characteristics such as surface morphology, particle size distribution and chemical composition of PPWC filler and limestone filler were compared. Then, PPWC filler was used to replace limestone filler with different volume fractions to prepare asphalt concrete, and the water damage resistance, high-temperature rutting resistance, low-temperature crack resistance, fatigue resistance and adhesion performance of asphalt concrete were tested. The results showed that PPWC filler has a smaller particle size and rougher surface than limestone filler, and it contains Ca(OH)2 produced by hydration. The addition of PPWC filler can effectively improve the mechanical properties of asphalt concrete without reducing its water damage resistance. PPWC filler can improve the high-temperature rutting resistance and low-temperature crack resistance of asphalt concrete, but reduce its low-temperature fatigue resistance. The low content of PPWC filler will enhance the adhesion between asphalt mortar and aggregate. However, when the content of PPWC filler exceeds 50%, Ca (OH)2 in PPWC will reduce the adhesion between acid asphalt mortar and alkaline basalt aggregate. Therefore, the use of PPWC as filler in asphalt mixtures provides a reliable solution for the sustainable development of road materials.

Mantle Oxidation Induced by Recycled Carbonate: Insights From Mg‐Zn‐Fe‐Cu Isotopic Systematics of Intraplate Basalts

AbstractMantle oxidation plays an essential role in volatile cycling between Earth’s interior and exterior. However, the conceivable oxidizer remains enigmatic and needs to be explored by geochemical tools. Here, we examine the potential of recycled carbonates in inducing redox changes of the deep mantle through combined isotopic studies of Mg (δ26Mg), Zn (δ66Zn), Fe (δ56Fe), and Cu (δ65Cu) on a suite of intraplate alkaline basalts proposed to have originated from the mantle transition zone (∼440–660 km). Compared with mid‐ocean ridge basalts (MORB), these basalts have variably lower δ26Mg and higher δ66Zn, the typical features of marine carbonates, indicating the presence of various amounts of deeply recycled carbonates in their sources. As the estimated amounts of recycled carbonates in sources increase (i.e., lower δ26Mg and higher δ66Zn), the basalts have higher δ56Fe and lower δ65Cu compared with MORB. These correlations cannot be explained alone by binary mixing between recycled carbonates and the pristine mantle because of the much lower Fe and Cu contents of marine carbonates than those of the mantle and instead require redox‐driven isotope fractionation induced by mantle carbonate metasomatism. During partial melting of an oxidized mantle source, isotopically heavy ferric iron and isotopically light sulfide were preferentially incorporated/dissolved into the melts that are enriched in heavy Fe and light Cu isotopes. These observations highlight that recycled carbonates serve as an important oxidizer for the deep mantle and facilitate the extraction of mantle sulfides, which can be effectively tracked by Fe and Cu isotopic systematics of intraplate basalts.

Unveiling the formation and magma transport dynamics of the Ulanqab maar volcanic cluster in the western North China Craton: Insights from 40Ar-39Ar geochronology, mineral chemistry, and Sr-Nd-Mg isotopes

The chemical composition of eastern North China Craton (NCC)'s alkaline basaltes likely influenced by subducting Pacific Plate. Nonetheless, the influence of these processes on the western craton remains uncertain due to the lack of geochemical evidence. The recent discovery of Ulanqab maar volcanic cluster (UMVC) in the western NCC has become significant research windows. However, their petrogenesis and magmatic processes remain poorly understood. In the present study, 40Ar/39Ar, whole rock and mineral geochemistry, and Sr–Nd–Mg isotope data for the UMVC in the western NCC are reported. These data reveal that the UMVC was formed during the Neogene (7.60 ± 0.04 Ma), diverging from Quaternary basalts or as part of the Hannuoba basalts (eastern Ulanqab). The UMVC rocks, featuring typical oceanic island basalts traits with moderate (87Sr/86Sr)i values (0.70487–0.70524) and εNd(t) values from −4.95 to + 0.82, likely originated from an enriched EMI-type mantle source during the mid-Proterozoic. This involved mixing melts from 30 to 50 % partial melting of garnet lherzolite and 5–15 % partial melting of garnet pyroxenite in a deep magma chamber. Crystallization of clinopyroxene and garnet in this chamber created high-Ti alkaline lavas, with limited presence in erupted lavas due to sluggish magma ascent in the profound lithospheric mantle. In shallower lithospheric mantle regions, interactions between alkaline magma and orthopyroxenite improved transport kinetics, enabling clinopyroxene and olivine crystallization under lower pressure. These magmas integrated mafic crystal mushes from accumulation chambers. The study also identified a low Mg isotope composition (δ26Mg = − 0.56 ‰ to − 0.42 ‰) in samples, suggesting a hybrid source influenced by early decarbonation in Precambrian subduction zones. This finding indicates the incorporation of Mg-rich carbonate minerals from marine sediments into the magma source, contributing to the observed variations in eastern China's alkaline basalts.

Fractional crystallization of garnet in alkali basalts at >1.8 GPa and implications for geochemical diversity of Large Igneous Provinces

A suite of lavas from northwest Mull, British Palaeocene Igneous Province, exhibit major and trace element characteristics that are best explained by very high pressure (>1.8 GPa) crystallization of an assemblage comprising aluminous clinopyroxene (Al-Cpx) plus garnet. The resulting series of consanguineous magmas are mildly Si-undersaturated. The trace element effects of this crystallizing assemblage are manifested in increasing light rare earth element (LREE) enrichment and heavy REE (HREE) depletion with decreasing whole-rock MgO, a predictable consequence of the bulk distribution coefficient (D) of this assemblage being > > 1 for HREE but <<1 for the LREE. Early crystallization of Al-Cpx and replacement of plagioclase feldspar by garnet in the crystallizing assemblage also results in increasing or near constant Al2O3 abundances in cotectic compositions with MgO < ∼7 wt%. Garnet as opposed to plagioclase crystallization is also reflected in the incompatibility of Sr and compatibility of Y resulting in very high Sr/Y lavas (up to 100) with high Sr (∼1200 ppm) and low Y contents (∼12 ppm). Lavas also have high Zr/Y (up to 30). Major element constraints suggest the crystallizing assemblage comprised ∼40% garnet and 60% Al-Cpx.Magmas that fractionated at ∼1.8 GPa rose to the surface without interacting with the continental crust. Immediately underlying basalts and picrites show evidence of crustal contamination and assimilated fusible portions of the crust, therefore effectively lining the plumbing system and allowing later magmas to rise to the surface without crustal contamination. A local change in tectonic regime from extension to passive appears to be linked to a change from low pressure to very high-pressure crystallization of magmas. Whilst evidence for garnet fractionation in continental flood basalts is very rare, this paper provides a characterization of its geochemical consenquences.

The origin of the Daly gap by fractional crystallization in a transcrustal plumbing system in the Passa Quatro alkaline complex (SE Brazil)

ABSTRACT The surface gap of tephriphonolite compositions in the Passa Quatro alkaline complex, in SE Brazil, is attributed to non-buoyancy of these high viscosity intermediate compositions during magmatic fractional crystallization at different depths. Thermodynamic and Rayleigh models for three alkaline suites of a strongly silica-undersaturated series were integrated to demonstrate that parental (basanite, nephelinite and alkaline basalt) and residual (phonolites) magmas are cogenetic by fractional crystallization at pressures of 10 kbar and 5 kbar, i.e. at magma chambers located above and below the Conrad discontinuity within the Upper Cretaceous-Palaeogene 40 km-thick continental crust in SE Brazil. The fractional crystallization of the parental magmas produced an ultramafic residual solid (wehrlite, olivine-bearing clinopyroxenite and clinopyroxenite) and residual liquids (evolved basanite and alkaline basalt, and phonotephrite). The latter moved up and built magma chambers at lower depths in the continental crust where those magmas initially crystallized clinopyroxenite extracts and high viscosity residual liquid that was entrapped and prevented to ascent towards the surface. Then, an abrupt change in the extract mineralogy produced a feldspathic-rich extract (leucocratic leucite monzosyenite, mesocratic syenite or leucocratic monzodiorite) and a residual phonolitic liquid with low viscosity. Following the feldspar crystallization, the magma viscosity decreased, acquiring adequate buoyancy that resulted in shallow-crust magma chambers in which phonolite diversification took place. These processes occurred due to the existence of a transcrustal, plumbing architecture where the high viscosity intermediate tephriphonolites were prevented to reach shallower depths, resulting in the Daly gap observed in the Passa Quatro Alkaline Complex.

Petrogenesis of alkaline magmas across a continent to ocean transect, northern Ross Sea, Antarctica

The West Antarctic Rift System (WARS) is one of the world's largest active rift systems and alkaline magmatic provinces. The alkaline magmatism crosscuts oceanic and continental lithosphere but maintains generally similar Sr-Nd-Pb isotope signatures. Here we present new major-, trace- and highly siderophile-element (Os, Ir, Ru, Pt, Pd, Re) abundances and Sr-Nd-Pb-Os isotope data for a Miocene to recent WARS alkaline basalt suite from a continental-oceanic transect in the northwest Ross Sea. The samples are geochemically enriched similar to ocean island basalts, with oceanic lavas having higher absolute incompatible trace element abundances than continental lavas. The suite exhibits a large range in measured 187Re/188Os (1.7 to 1305) and age-corrected 187Os/188Os (0.1254 to 1.055) that has a positive, highly curved relationship with measured 87Sr/86Sr (0.70287–0.70331), but a negative, highly curved relationship with both measured 143Nd/144Nd (0.51280–0.51300) and 206Pb/204Pb (19.237–20.237). These data suggest that contamination by a mafic continental lithospheric component has occurred to varying degrees and exerts some control on the geochemistry of the continental lavas. Primitive lavas from the oceanic Adare Basin that have experienced negligible contamination are used to constrain the primary source of northwest Ross Sea alkaline lavas and to assess previously proposed hypotheses regarding the origin of magmatism. Explanations for the origin of WARS alkaline lavas range from plume magmatism to partial melting of continental lithospheric mantle metasomatized by subduction fluids prior to cessation of subduction of the Phoenix Plate underneath the Gondwana margin. Samples from the continental-oceanic transect, however, neither exhibit classic arc-lava geochemical signatures nor provide any evidence to support an active mantle plume source. Rather, they are geochemically enriched magmas generated through partial melting of previously subducted oceanic lithospheric mantle concomitant with continental lithospheric thinning during the Cenozoic.

Kinetic partitioning of trace cations between zoned clinopyroxene and a variably cooled-decompressed alkali basalt: Thermodynamic considerations on lattice strain and electrostatic energies of substitution

We present kinetic partitioning data for trace cations measured in zoned clinopyroxene crystals obtained from a variably cooled and decompressed olivine basalt erupted at Mt. Etna volcano in Italy. Supersaturation effects and compositional heterogeneities at the interface melt lead to the development of sector zoning, concentric zoning, and patchy zoning in clinopyroxene crystals. Apparent partition coefficients between compositionally different growth layers and adjacent melts (Di) for isovalent groups of trace elements are tested for internal consistency on the thermodynamic basis of lattice strain (ΔGstrain) and electrostatic (ΔGelec) energies of substitutions. The excess energy of partitioning (ΔGpartitioning) for trace cations in zoned crystals accounts for a kinetic incorporation control leading to large enthalpic effects through distortion of the lattice and changes in the electrostatic forces. ΔGpartitioning depends upon the complementary relationship between ΔGstrain and ΔGelec, which is the most appropriate thermodynamic description for the accommodation of rare earth elements and high field strength elements in the lattice site of zoned crystals. Polyhedral sectors, skeletal forms, and overgrowth zones have Di values settled by the number of charge-balanced and -imbalanced configurations taking place in the lattice site as a function of aluminium in tetrahedral coordination, and crystal structural changes produced by heterovalent cation substitutions. In an energetically unstable macroscopic system ruled by cooling and decompression, thermodynamic requirements for the crystallochemical control of Di encompass the attainment of local equilibrium at the crystal-melt interface via the establishment of small-volume reaction kinetics. The requisite of local interface equilibrium is however susceptible to the anisotropic growth velocity of each specific clinopyroxene surface, thereby giving reason to different energetic properties of the crystallographic site. This axiomatic control requires that transition metal cations partition also in consideration of electronic effects related to the crystal field stabilization energy. The overriding implication is that Di values for trace cations having different size, charge, and electronic configuration serve as sensitive probes of the different crystal growth mechanisms, surface incorporation sites, and arrangements of atoms at the lattice-scale. In this perspective, fractional crystallization modeling of 2011–2013 bulk rock data from lava fountains indicates that the compositional evolution of magmas erupted at Mt. Etna cannot be described by a unique equilibrium value of Di for a given clinopyroxene-melt interface. The leverage of interface kinetics is distinctively dominant along the subvolcanic plumbing system, thereby requiring that values of Di differ for structurally and compositionally distinct zones in clinopyroxene phenocrysts. To successfully interpret the trace element signature of Etnean magmas, the archetypal constancy of partition coefficient at bulk thermodynamic equilibrium must be in some measure reappraised in favor of the establishment of a local interface equilibrium upon highly dynamic crystallization and growth conditions.

Late Cretaceous bimodal volcanic rocks in Shuanghu induced by lithospheric delamination beneath the Southern Qiangtang, Tibet

The southern Qiangtang (SQT) in central Tibet is characterized by sporadic Late Cretaceous magmatic rocks whose petrogenesis and tectonic implications remain unclear. Here, new zircon UPb ages and Hf isotope compositions, together with whole-rock element and isotope data have been collected for the Duoma volcanic rocks from the SQT, central Tibet. Zircon UPb dating indicates that the Duoma volcanic rocks crystallized at ca. 98 Ma. The basalts of the Duoma volcanic rocks have low SiO2 (51.49–53.36 wt%), high total alkalis (4.65–6.25 wt%), and display OIB-like isotopic signatures (87Sr/86Sr)i = 0.7051 to 0.7058, εNd(t) = +0.04 to +1.10), overall similar to intra-plate alkali basalts. Moreover, basalts exhibit slight depletion of high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti), but enrichment of light rare earth (LaN/YbN = 6.7–7.6) with no negative Eu anomalies (Eu/Eu* = 0.91–0.99). These geochemical features, together with their high La/Nb (1.56–1.70) and Zr/Hf (44.70–45.87) but low La/Ba (0.08–0.09) and Ca/Al (0.43–0.65) ratios, suggest that the Duoma basalts originated from partial melting of garnet clinopyroxenite that was metasomatized by depleted mantle-derived melts. The coexisting rhyolites have high SiO2 (69.78–69.92 wt%) and exhibit significant enrichment in large-ion lithophile elements (LILEs) and depletion in HFSEs (Nb, Ta, and Ti). Furthermore, the Duoma rhyolites are characterized by the decoupled Nd (εNd(t) values ranging from −4.4 to −11.1) and Hf (ɛHf(t) values ranging from −0.7 to +2.7) compositions, that character indicates that they were probably derived from the partial melting of the lower continental crust. The compositional bimodality and heterogeneous source of the Late Cretaceous volcanism indicate that they were formed in an extension setting, presumably triggered by lithospheric delamination beneath the SQT.

Within-plate magmatism in the southern Borborema Province (NE Brazil): Mantle plumes associated with the Nuna-Columbia and Rodinia breakup?

Formation and fragmentation of landmasses, such as the Nuna/Columbia and Rodinia supercontinents, are influenced by mantle plume activity during rifting and breakup. In the Borborema Province, NE Brazil, within-plate magmatism at ca. 1.6 and at ca. 0.8 Ga is evidenced by the formation of mafic-ultramafic volcanic and plutonic rocks, mafic-ultramafic layered intrusions, and A-type granite intrusions. The Paulistana and Santa Filomena complexes, located in the Riacho do Pontal Orogen, southern Borborema Province, correspond, respectively, to metaplutonic/volcanic-sedimentary and metavolcanic-sedimentary sequences. These sequences are composed of metamorphic rocks of the greenschist to amphibolite facies whose basement corresponds to the Paleoproterozoic rocks of the Itaizinho Complex, which is located in the southern portion of the São Pedro Terrane. The combined analysis of new field, petrographic and geochemical data obtained for the metavolcanic rocks of the São Pedro Terrane and Riacho do Pontal Orogen aided the determination of their tectonic setting. Whole-rock geochemical analyses revealed the predominance of andesites, basaltic andesites and tholeiitic alkali basalts generated in a within-plate environment (E-MORB) with shallow melting of the metavolcanic rocks of the Itaizinho Complex. In the Riacho do Pontal Orogen, basaltic to alkali-basaltic tholeiites are the dominant metavolcanic rocks of the Paulistana and Santa Filomena complexes. Tectonic setting discriminant diagrams indicate within-plate (E-MORB/OIB), deep and shallow melting environments, respectively. This study, combined with data available in the literature, suggests that the metavolcanic rocks of the Itaizinho Complex and metavolcanic rocks of the Paulistana/Santa Filomena complexes were formed in an extensional setting related to Calymmian (1631 Ma) and Tonian (882 Ma) mantle plumes, respectively, precursor to the oceanic crust between the São Pedro Terrane and the São Francisco Craton, represented by the metabasalts of the Monte Orebe complex, which underwent re-accretion and reworking during the Brasiliano-Pan-African Orogeny.

The early Late Triassic Yumen mafic rocks in the eastern Tethyan Himalaya: A record of the progressive opening of the Yarlung Zangbo Neo-Tethyan Ocean

The timing of the opening of the Yarlung Zangbo Neo–Tethyan Ocean (YZNTO) is debated. In this contribution, geochemical and geochronological analyses of diabase, pillow basalt and layered basalt from the Upper Triassic Yumen tectonic mélange (T3Y) of the eastern Tethyan Himalaya in South Tibet are used to elucidate the timing and the magmatic evolution preceding the development of the YZNTO. Zircon U-Pb geochronology indicates that the Yumen mafic rocks crystallized in the Late Triassic (diabase, c. 225 Ma). These rocks display alkaline basalt affinities and have variable enrichment in Ba, U, Pb, Sr and Ti and depletion in Rb, Th and Nb. The diabase and pillow basalt samples have compositions transitional between enriched mid–ocean ridge basalt and ocean island basalt (OIB); layered basalt is more compatible with an OIB affinity. Whole–rock εNd (t) ratios of the mafic rocks range from +5.5 to +11.5 and zircons from a diabase have εHf(t) values varying from – 1.1 to +10.0. Whole–rock trace element concentrations indicate that the parental magmas of the Yumen mafic rocks were derived from the mantle at variable depths extending from the garnet to spinel stability fields. The Yumen mafic rocks originated from partial melting of a heterogeneous magma source composed of dominant depleted asthenospheric magmas and the addition of some components of an ancient and enriched lithospheric mantle; negligible crustal contamination occurred. Combined with previous studies, we conclude that the Yumen mafic rocks were probably produced in an intercontinental rift setting with extremely thin continental lithosphere in the early Late Triassic prior to the opening of the eastern YZNTO. The YZNTO progressively opened along the northern edge of the Indian plate from west to east in the Phanerozoic.

Tephra from the Island Intra-Plate-Oceanic Volcano Cumbre-Vyaha (Eruption 2021)

Comprehensive mineralogical and geochemical studies of a representative sample of tephra from the Cumbre-Vyaha volcano (2020–2021 eruption), which has a gravel-psammitic granulometric composition and is characterized by an anomalously high degree of particle vesicularity, have been carried out. In terms of bulk chemical composition, this tephra corresponds to the transition from alkaline picrobasalts to alkaline basalts, differing fundamentally from the tephra ashes of marginal continental volcanoes. The studied tephra contains 45 trace elements with a total content of up to 2333 ppm, which exceeds that in the tephra of island-arc volcanoes. According to the ratio of geochemical criteria, tephra from Cumbre-Vyaha correspond to the average values for within-plate volcanoes in the oceans. The geochemical feature of the studied tephra is a strong enrichment in lanthanides, noble and platinoid metals. H2, CO, CO2, H2O, CH4, C2H4, C2H6, C3H6, C3H8 were found in the composition of the lithogenic gas phase separated by heating from the studied tephra. The proportions between inorganic components in the gas phase generally correspond to the boundary region between crustal and mantle-crustal derivatives, but a relatively low water content is found. Olivine of composition Fo73–84, essentially diopside clinopyroxene, andesine-bytovnite plagioclases, chrome spinels, phase-homogeneous solid solutions of ilmenite in magnetite, phases of Ni-Cu-bearing native iron, quartz, and sodium hydroxyl chlorides were identified and studied in microlites. In addition, dispersed carbon matter with carbon isotopic composition δ13СPDB = –30…–24‰ was found in the Cumbre-Vyaha tephra, which corresponds to the carbon isotope composition in abiogenic carbonaceous substances of volcanic origin. The totality of the results of mineralogical and geochemical studies characterizes the Cumbre-Vyaha volcano as a typical representative of within-plate oceanic plume volcanoes.

Early Ordovician seamounts preserved in the Canadian Cordillera: Implications for the rift history of western Laurentia

Abstract The breakup of the supercontinent Rodinia and development of the western Laurentian rifted margin are in part recorded by Neoproterozoic to mid-Paleozoic igneous and sedimentary rock successions in the Canadian Cordillera. New bedrock mapping and volcanic facies analysis of Early Ordovician mafic rocks assigned to the Menzie Creek Formation in central Yukon allow reconstruction of the depositional environment during the volcanic eruptions, whole-rock geochemical data constrain the melting depth and crust-mantle source regions of the igneous rocks within the study area, and zircon U-Pb age studies provide determination of the precise timing of submarine eruptions. Menzie Creek Formation volcanic rocks are interlayered with continental slope strata and show lithofacies consistent with those of modern seamount systems. Representative seamount facies contain several kilometers of hyaloclastite breccia and pillow basalt with rare sedimentary rocks. Menzie Creek Formation seamounts form a linear array parallel to the Twopete fault, an ancient extensional or strike-slip fault that localized magmatism along the nascent western Laurentian margin. Zircon grains from two volcanic successions yielded high-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) dates of ca. 484 Ma (Tremadocian), which are interpreted as the age of eruption. Menzie Creek Formation rocks are alkali basalt and have oceanic-island basalt–like geochemical compositions. The whole-rock trace element and Nd-Hf isotope compositions are consistent with the partial melting of subcontinental lithospheric mantle at ~75–100 km depth. Post-rift, Early Ordovician seamounts in central Yukon record punctuated eruptive activity along a rift-related fault, the separation of a continental fragment from western Laurentia, or the oblique post-breakup kinematics from the counterclockwise rotation of Laurentia that facilitated local extension in the passive margin.

Ascent rate of the Udachnaya-East kimberlite melts from olivine diffusion chronometry

Kimberlite melts are low volume, volatile-rich melts formed in the asthenosphere at depth >150-250 km. Due to their low viscosity and the exsolution of CO2 triggered by their interaction with the sub-cratonic mantle, these melts are highly buoyant and rise rapidly through the lithosphere, ultimately reaching the surface and erupting at sonic to super-sonic speed. Studies based on fluid/melt dynamics, fracturing and disaggregation of the transported mantle-derived cargo, as well as on H diffusion in olivine and Ar diffusion in magmatic phlogopite showed that the ascent rate of kimberlite melts can vary between 0.1 and 30 m/s. However, these rates are extrapolated from processes starting at different depths, and thus may not be representative for the whole ascent path of kimberlite melts through the sub-cratonic lithosphere. To shed light on this issue, we applied diffusion chronometry to the deepest feature ascribable to the liquid line of descent of the melts, i.e. early liquidus olivine formed around mantle-derived xenocrystic cores from the Udachnaya-East pipe (Siberian craton). Our results showed that the time elapsed between the formation of the Internal Zone II (early olivine on the liquidus) or rims (typical magmatic olivine) around xenocrystic cores and final cooling is between 5 and 74 days. Considering travelled distances of 80-110 km in accordance with thermobarometric calculations, this implies overall average ascent rates of kimberlite melts of 0.02 to 0.23 m/s, which are comparable to those of typical alkali basalts, albeit starting at much greater depth. This indicates that kimberlite melts start with moderate ascent rates and only later increase their speed, likely due to massive CO2 exsolution triggered by decarbonation reactions during melt/fluid-rock interactions in the lithospheric mantle. The overall timescales of 5 to 74 days obtained in our study overlap with the lowest values reported in the literature from chemical zoning of mantle-derived minerals (25 days to >100 years), confirming that the time elapsed between kimberlite-related mantle metasomatism and the ascent of kimberlite melts was short. Importantly, our data show that Fe-Mg interdiffusion in olivine can be used as a trustable chronometer in kimberlite systems, opening new perspectives on the study of ascent dynamics in kimberlites worldwide.

Petrogenesis of the Hoy lava field, a long-lived continental mafic volcanic province in eastern Australia

Small-scale continental basaltic fields can erupt with little warning and bring deep undegassed magmas to the surface rapidly. To explore the lifetime, petrogenesis and plumbing system architecture feeding such basaltic lava fields and compare them with large-scale shield volcanoes, we have focused on the Hoy lava field, central Queensland, Australia. 40Ar/39Ar geochronology, elemental and isotopic whole-rock geochemistry and mineral chemistry on selected Hoy samples reveal long-lived volcanism of ca 50 Ma and magma storage at mantle depths, notably different from the comparatively short duration (3–5 Myr) and crustal magma storage depths of shield volcanoes. In this study, four Hoy lava-field eruptive intervals spanning ca 50 Ma were investigated: 67.5 ± 0.3 Ma, 32.3 ± 0.6–31.6 ± 0.7 Ma, 21.9 ± 0.5 Ma and 18.1 ± 0.3 Ma. In all four eruptive events, samples are porphyritic alkali basalts and trachybasalts (11.41–6.45 wt% MgO) with incompatible element concentrations and Sr–Nd–Pb isotope ratios dominantly derived from a metasomatised sub-continental lithospheric mantle (SCLM) source with an enriched mantle I (EMI) signature. Complex crystal populations show major- and trace-element variations reflecting fractional crystallisation, magma recharge, magma mixing and mantle xenocryst entrainment. Clinopyroxene–melt thermobarometry indicates magma storage in SCLM reservoirs at ∼30–47 km depths. The nearby larger but shorter-lived (3–5 Myr) Buckland central volcano has similar source compositions; however, magma storage is limited and concentrated in the crust, resulting in increased crustal contamination. The results suggest that basaltic centres of contrasting scale and longevity are linked to distinct magma production mechanisms, fluxes, ascent and differentiation.

Apatite CO2 and H2O as Indicators of Differentiation and Degassing in Alkaline Magmas

Abstract Apatite can incorporate significant amounts of H2O and CO2, potentially recording volatile abundance and behavior during magma evolution. Here we conducted in situ measurements of CO2 and H2O concentrations in apatite, along with elemental compositions, from two contemporaneous alkaline volcanic suites (Tianbao and Tudiling) in the South Qinling belt in central China to better understand the CO2 and H2O contents and behavior in evolving alkaline melts. Apatite from alkali basalts in Tianbao contains variable CO2 contents ranging from &amp;lt;100 ppm to 2.7 wt.% and H2O contents ranging from 0.1 to 0.6 wt.%. Apatite from REE-enriched trachytes, which evolved from alkali basalt, shows significantly lower CO2 contents and a continuous decrease in H2O during magma fractionation. The observations suggest that CO2 loss commenced at the early stages of magma evolution, whereas significant H2O loss occurred during subsequent magma fractionation in tandem with REE-enrichment. In comparison, apatite grains from the Tudiling trachyte, which is genetically linked with carbonatite, contain higher CO2 contents (0.6 to 1.5 wt.%) but lower REE concentrations than the Tianbao trachytes. Apatite in the Tudiling trachyte is inferred to have crystallized from a carbonated alkaline magma prior to significant CO2 loss and the separation of Tudiling carbonatitic melts, where subsequent liquid immiscibility led to REE enrichment into the carbonatitic melts. The volatile characteristics of apatite from the two volcanic suites provide valuable insights into two different evolutionary processes of alkaline/carbonatitic magmas, the behavior of CO2 and H2O, and the enrichment of REE in alkaline systems.

The effect of elemental diffusion on the application of olivine-composition-based magmatic thermometry, oxybarometry, and hygrometry: A case study of olivine phenocrysts from the Jiagedaqi basalts, northeast China

Abstract Olivine compositions are widely used to constrain magmatic thermodynamic conditions such as magmatic temperature, oxygen fugacity, and H2O content. However, elemental diffusion may change the initial compositions and lead to large uncertainty on the estimation of these thermodynamic conditions. In this study, we conducted LA-ICP-MS elemental mapping and EPMA analysis of olivine phenocrysts and olivine-hosted spinel from the Jiagedaqi (JGD) alkaline basalts in northeast China to evaluate the influence of elemental diffusion on olivine-composition-based geothermometry, oxybarometry, and hygrometry. The JGD olivines show normal Fo [Mg/(Mg + Fe) × 100 in moles] zoning, with cores having Fo of 77–87 and rims having Fo of 67–73. The constant P contents from core to rim indicate that these compositional zonings were caused mainly by diffusion. Because Al is a slow-diffusing element and its content is relatively constant from core to rim, the temperature calculated by the Al-in-olivine thermometer is not influenced by elemental diffusion and preserves the JGD olivine crystallization temperature up to 1150 °C. The temperatures calculated using the Sc/Y-in-olivine thermometer, the oxygen fugacity calculated using the olivine–spinel oxybarometer, and the H2O content calculated on the basis of Ca partitioning between olivine and melt are strongly influenced by the diffusion of Fo, Sc/Y, and Ca. However, the compositional plateaus in olivine cores, which were not influenced by elemental diffusion, preserve the magmatic temperature (1150 °C), oxygen fugacity (QFM+1.4), and H2O content (4 wt%) that applied during the formation of the JGD olivines. Together, these findings suggest that the mantle source of the JGD basalts was metasomatized by fluids released from the subducted slab. This study highlights that elemental diffusion in olivine phenocrysts can strongly affect the application of olivine-composition-based geothermometers, oxybarometers, and hygrometers. However, primitive olivine cores that have not been influenced by diffusion preserve the initial magmatic thermodynamic conditions.