Silicate Liquid Immiscibility Research Articles

Recycling carbonated hydrous sediments into the mantle source of carbonatite-associated rare-earth-element deposits

Decoding the petrogenesis of carbonatites is of great importance for our understanding of the Earth’s deep carbon cycling and formation of rare-earth-element (REE) deposits. Among global carbonatites, the Cenozoic Mianning-Dechang (MD) carbonatites in the western Yangtze Craton are unique because of their development in a post-collisional zone and host numerous giant-large REE deposits. However, most of previous studies have focused on the late magmatic to hydrothermal stages, while the nature of the mantle source and early-stage magmatic evolution remain largely unexplored. Here we present a combined petrographic, chemical and Sr-Nd-Pb isotopic study on the MD lamprophyres to address these issues. A genetic linkage is revealed between these lamprophyres and the MD carbonatites and syenites by their geochemical similarities and close spatial and temporal associations. Our data suggest a phlogopite-rich carbonated subcontinental lithospheric mantle (SCLM) source metasomatized by subducted carbonated hydrous sediment-derived melts/fluids. Significant fractional crystallization of olivine, clinopyroxene and biotite (±Ti-Fe oxides) from hydrous, K-rich and carbonated primary melts caused the carbonate–silicate liquid immiscibility in the highly evolved melts and thus formation of the MD carbonatites and syenites at shallow depths. Melting of subducted carbonated hydrous sediments not only provides carbonatitic liquids for the SCLM carbonation, but also supplies sufficient water and alkalis that are critical for accumulating and transporting REE in carbonatitic fluids. Post-collisional lithospheric thinning also played a critical role in the formation of carbonatites via triggering the melting of the carbonated SCLM. This study provides new insights into the deep processes during the formation of carbonatites and associated REE deposits in post-collisional zones.

Genesis of Fe–Ti Oxide-Bearing Ultramafic Intrusions in the Duluth Complex, Minnesota, USA

Abstract The Duluth Complex is a large mafic intrusive system located in northeastern Minnesota emplaced as part of the 1.1-Ga Midcontinent Rift. Several Fe–Ti oxide-bearing ultramafic intrusions are hosted along the Western Margin of the Duluth Complex, and are discordant bodies present in a variety of geometries, hosted in multiple rock types, and dominated by peridotite, pyroxenite, and semi-massive to massive Fe–Ti oxide rock types. Their origin has been debated, and here we present geochemical evidence and modeling that supports a purely magmatic origin for the Titac and Longnose Fe–Ti oxide-bearing ultramafic intrusions. Ilmenite and titanomagnetite textures indicate a protracted cooling process, and δ34S values of sulfides reveal little assimilation of the footwall Virginia Formation, a fine-grained pelitic unit that contains sulfide-rich bands. We model the crystallization of a hypothetical parental magma composition to the host intrusion of Longnose using Rhyolite-MELTS and demonstrate that the accumulation of Fe–Ti oxides in the discordant intrusions cannot be explained by density-driven segregation of crystallized Fe–Ti oxides. Instead, we show that the development of silicate liquid immiscibility, occurring by the unmixing of the silicate melt into conjugate Si- and Fe-rich melts, can result in the effective segregation and transportation of the Fe-rich melt. The Fe-rich melt is ~2 orders of magnitude less viscous than the Si-rich melt, allowing the Fe-rich melt to be more effectively segregated and transported in the mush regime (crystallinities >50%). This suggests that viscosity, in addition to density, plays a significant role in forming the discordant Fe–Ti oxide-bearing ultramafic intrusions. We propose a genetic model that could also be responsible for the Fe–Ti oxide-rich layers or bands that are hosted within the igneous stratigraphy of mafic intrusions of the Duluth Complex.

High-pressure minerals and new lunar mineral changesite-(Y) in Chang’e-5 regolith

Forty-five years after the Apollo and Luna missions, China’s Chang’e-5 (CE-5) mission collected ∼1.73 kg of new lunar materials from one of the youngest basalt units on the Moon. The CE-5 lunar samples provide opportunities to address some key scientific questions related to the Moon, including the discovery of high-pressure silica polymorphs (seifertite and stishovite) and a new lunar mineral, changesite-(Y). Seifertite was found to be coexist with stishovite in a silica fragment from CE-5 lunar regolith. This is the first confirmed seifertite in returned lunar samples. Seifertite has two space group symmetries (Pnc2 and Pbcn) and formed from an α-cristobalite-like phase during “cold” compression during a shock event. The aftershock heating process changes some seifertite to stishovite. Thus, this silica fragment records different stages of an impact process, and the peak shock pressure is estimated to be ∼11 to 40 GPa, which is much lower than the pressure condition for coexistence of seifertite and stishovite on the phase diagram. Changesite-(Y), with ideal formula (Ca8Y)□Fe2+(PO4)7 (where □ denotes a vacancy) is the first new lunar mineral to be discovered in CE-5 regolith samples. This newly identified phosphate mineral is in the form of columnar crystals and was found in CE-5 basalt fragments. It contains high concentrations of Y and rare earth elements (REE), reaching up to ∼14 wt. % (Y,REE)2O3. The occurrence of changesite-(Y) marks the late-stage fractional crystallization processes of CE-5 basalts combined with silicate liquid immiscibility. These new findings demonstrate the significance of studies on high-pressure minerals in lunar materials and the special nature of lunar magmatic evolution.

Compositional analysis of Apollo 12 Granitic Breccia 12013: Insights into protoliths and formation

We report bulk rock composition and mineral chemistry of previously unstudied fragments and thin sections of lunar granitic breccia 12013. Instrumental Neutron Activation Analysis data on 25 fragments indicate that the 12013 breccia can be described as a three-component system comprising granitic, rare earth element (REE)-rich, and mafic components. The granitic component is low in FeO and REE but rich in K2O, Th, and associated incompatible elements. The REE-rich component has an elevated concentration of REE, moderate FeO, and low concentrations of those elements that are high in the granitic component, in other words, its composition is complementary to that of the granitic component. The mafic component is richest in FeO, and low in those elements that are concentrated in the granitic and REE-rich components. Petrographically, the REE-rich component is an impact melt breccia with a composition unique to Apollo 12 impact melt breccias. Trace-element concentrations in the REE-rich component indicate that its protolith is possibly monzogabbro, although its composition is significantly more magnesian than other known lunar monzogabbros. The mafic component is dominated by fine-grained pyroxene and plagioclase that appear to have recrystallized after impact. However, preserved lithic clasts with an igneous texture occur in the mafic component and are inferred to represent its protolith. The textures of the preserved mafic lithic fragments and lack of Mg-Fe zoning in pyroxene indicate that the protolith of the mafic component formed either as a crustal intrusion or at the bottom of a thick lava flow. The composition of the mafic component is unlike any previously studied mare basalt samples. Textures of this complex breccia suggest that the granitic breccia was first incorporated with the mafic component to form the “gray breccia”, and that the gray breccia was then incorporated with the REE-rich component while the components were still in a hot, plastic state. The complementary trace-element compositions, including the REE patterns, indicate that the petrogenesis of the granitic and REE-rich components are related. We infer that silicate-liquid immiscibility is likely not the formation mechanism for the 12013 components. Instead, bimodal volcanism owing to basaltic underplating may have led to the formation of the 12013 components.

Trace elements in ilmenite, titanomagnetite and apatite unravel the petrogenesis of Fe-Ti-P-(+/-Zr) rich rocks and associated nelsonite from the Raftsund intrusion, Vesterålen-Lofoten AMCG suite, Northern Norway

The formation of Fe-Ti-P- (+/− Zr)-rich rocks and nelsonite (Fe-Ti oxide-apatite +/− zircon-rich rocks mostly devoid of silicates) remains poorly understood. Accumulation of mafic minerals during the evolution of magma by fractional crystallization and silicate-liquid immiscibility are the two main processes considered. However, the latter process is difficult to identify in intrusive rocks as melt compositions are rarely preserved. In this study, we analyzed the trace element content of ilmenite, titanomagnetite, and apatite from the Raftsund intrusion, a 1800 Ma monzonitic to syenitic batholith belonging to the Lofoten-Vesterålen anorthosite-mangerite-charnockite-granite suite in order to test whether liquid-immiscibility was responsible for the formation of Fe-Ti-P-rich rocks. In addition, we extend the study to a new nelsonite occurrence. Detailed microtextural characterization of apatite in the monzonite shows evidence of metasomatic alteration in the form of thin rims of allanite, whereas apatite in the nelsonite contains small monazite inclusion along fractures. Ilmenite and titanomagnetite, not affected by later events, record enrichments in elements that should have behaved compatibly (Mg, Sc, Co, Cu, V) and incompatibly (Zr, Mn, Zn) in a fractionating system where zircon and sphalerite are absent from the mineral assemblage. This observation supports the liquid immiscibility hypothesis. Similar enrichments are observed in other parts of the system, including the nelsonite. However, the high field strength elements are affected by the stability and crystallization of abundant zircon and ilmenite, resulting in lower concentration of these element in the Fe-Ti oxides. The Fe-Ti oxides in the nelsonite record enrichments that are more pronounced than the Fe-Ti oxides in the surrounding rocks, which indicate that not only liquid immiscibility is responsible for the genesis of the nelsonite, but that the Fe-rich melt from which the Fe-Ti oxides crystallized was more enriched in trace elements than the Fe-Ti-P-rich rocks in the rest of the intrusion. Enrichment in transition metals (Co, V, Cu) is related to mingling with a less evolved monzodioritic magma, which most likely triggered the liquid immiscibility that formed the nelsonite. Fractionation and segregation of Fe-rich silicates, such as augite and fayalite, are necessary to explain the formation of the nelsonite.

Microstructures in the Baima layered intrusion, SW China: Constraints on the late-stage magmatic processes during solidification of a crystal mush

Magmatic microstructures in layered intrusions record key petrographic evidence for the late-stage magmatic processes that took place during solidification of a crystal mush. Systematic examination of the magmatic microstructures can reveal dynamic evolution of interstitial liquid in crystal mush, which however has been only conducted in a few studies on layered intrusions by far. In order to constrain the late-stage magmatic processes related to the Fe-Ti oxide-bearing layered intrusions, we investigated the magmatic microstructures of the Baima intrusion in the Panxi region (SW China). The intrusion is divided into three major units from the base upwards, including the lower zone (LZ), middle zone (MZ) and upper zone (UZ). Through detailed examination of a series of drill core samples, we found several different types of reactive microstructures, such as replacive symplectites, fish-hook pyroxene/olivine/amphibole and amphibole rim. Replacive symplectites include fine-grained, lamellar intergrowths of An-rich plagioclase + olivine/clinopyroxene/amphibole/phlogopite/biotite that are rooted to Fe-Ti oxides and replacing plagioclase primocrysts (Type 1) and intergrowths of An-rich plagioclase + orthopyroxene/olivine/amphibole that are rooted to olivine and replacing plagioclase primocrysts (Type 2). These microstructures are particularly abundant in the LZ except for the Type 2 symplectites, which are rare throughout the intrusion. An-rich plagioclase in the replacive symplectites and fish-hook textures have An contents up to 96, much higher than those of plagioclase primocrysts. Overall, fish-hook amphibole and amphibole rims contain 0.19−4.24 wt.% TiO2, higher than those of amphibole lamellae in the replacive symplectites that varying from 2.38 wt.% TiO2 to below the detection limit. These microstructures were developed during solidification of the intrusion due to the reaction between plagioclase primocrysts and ambient Fe-rich liquid, accompanying with addition of Fe, Ti and Ca and removal of alkalis and silica in an open system. The abundance of such reactive microstructures in the LZ are therefore attributed to silicate liquid immiscibility within a crystal mush, in which the segregation of the buoyant Si-rich component left behind the dense Fe-rich liquid in the lower part of magma chamber. The common presence of amphibole and phlogopite/biotite in the replacive symplectites indicates that continuous fractionation of the Fe-rich liquid leads to hydration of interstitial liquid during the late-stage solidification.

Evidence for silicate liquid immiscibility in recharging, alkali-rich tholeiitic systems: The role of unmixing in the petrogenesis of intermediate, layered plutonic bodies and bimodal volcanic suites

The role of silicate liquid immiscibility in the magmatic evolution of basalts and accounting for the scarcity of intermediate compositions on Earth's surface (Daly Gap) remains a matter of debate. Here we report the first finding of natural, immiscible Fe- and Si-rich silicate melts entrapped as apatite-hosted silicate Melt Inclusions (MIs) in the sub-volcanic Limeira Intrusion, a basic-intermediate layered occurrence associated with the Paraná Magmatic Province (PMP), southeastern Brazil. At least three nested magma pulses constitute this igneous body, in which compositional and textural features of the youngest one indicate the effective redistribution of a Si-rich liquid to the top and the sinking of a paired Fe-Ti-P-rich one to the bottom. Compared to other well-known natural immiscible liquids through multivariate statistics (Principal Component Analysis), MIs and whole-rock compositions of the Limeira Intrusion strongly support the role of silicate liquid immiscibility in the evolution of this intrusion. The incomplete unmixing or the mixing between varying proportions of the immiscible liquids containing early-formed crystals within a mush is the best mechanism explaining the origin of intermediate compositions in this intrusion, possibly induced by multiple magma batches. Based on whole-rock compositions and thermodynamic models, when the liquid line of descent reaches relatively high amounts of alkalis (Na2O + K2O ≥ 5–6 wt%) and the SiO2 (55–57 wt%), residual liquids can unmix to give rise to a paired Fe-rich and Si-rich immiscible liquids if the temperature is kept at relatively high values during prolonged periods. To test this hypothesis, experimental studies were conducted on two natural basic and intermediate samples, representing the first basaltic pulse that initiated the intrusion and the third/last basaltic-andesitic batch, respectively. Experiments were conducted under atmospheric conditions and oxygen fugacities close to the Fayalite-Magnetite-Quartz (FMQ) buffer, and immiscible liquids were produced only in isothermal conditions (ca. 1010 °C) starting with the intermediate sample. Our findings suggest that silicate liquid immiscibility can be significant in making intermediate compositions in this recharging basaltic-fed system, owing to the incomplete/inefficient unmixing of paired immiscible liquids and heat maintenance in the magma reservoir. Additionally, our interpretations can be extended to the whole PMP and contribute to our understanding of the Daly Gap for the high-Ti, alkali-rich tholeiitic compositions, suggesting large-scale magmatic differentiation due to liquid immiscibility along the tholeiitic liquid line of descent.

Origin of Amphibole-Biotite-Fluorite-Rich Enclaves from Gabal El-Ineigi Fluorite-Bearing Granite, Central Eastern Desert of Egypt: Insights into Fluoride–Calcium and Silicate Liquid Immiscibility

Gabal El-Ineigi fluorite-bearing rare-metal granite with A-type affinity, located in the Central Eastern Desert of Egypt, is distinguished by its abundance of large fluorite-quartz veins and mafic enclaves. Plagioclase (labradorite to oligoclase), Mg-rich biotite, and Mg-rich hornblende are the main components of mafic enclaves, with significant amounts of fluorite as essential phases, and titanite and Fe-Ti oxides (Nb-free rutile and ilmenite-rutile solid solution) as the main accessories. These enclaves are monzodioritic in composition, Si-poor, and highly enriched in Ca, Fe, Mg, and F compared to the host alkali feldspar F-poor Si-rich granites. Given the conflicting evidence for a restitic, xenolithic, magma mixing/mingling, cumulate, or bimodal origin for these enclaves, we propose that the mafic enclaves and felsic host granites are two conjugate liquids, with contrasting compositions, of a single parental melt. This is inferred by the normalized REE patterns that are similar. As a result, liquid immiscibility is proposed as a probable explanation for this mafic–felsic rock association. These enclaves can be interpreted as transient melt phases between pure silicate and calcium-fluoride melts that are preserved from the early stages of separation before evolving into a pure fluoride (Ca-F) melt during magma evolution. Due to element partitioning related to melt unmixing, the enclaves are preferentially enriched in Ca, F, Li, Y, and REE and depleted in HFSE (such as Zr, U, Th, Ta, Nb, Hf, and Ga) in comparison to the host granites. Furthermore, mafic enclaves exhibit W-type tetrad effects, while host granites exhibit M-type tetrad effects, implying that the REE partitioning, caused by liquid immiscibility, is complementary.

Characterizing a new type of nelsonite recognized in the Damiao anorthosite complex, North China Craton, with implications for the genesis of giant magmatic Fe-Ti oxide deposits

Abstract Nelsonite (Fe-Ti oxide-apatite rock) devoid of silicates offers a rare opportunity to investigate the magma processes for the formation of magmatic Fe-Ti oxide deposits. Both fractional crystallization and silicate liquid immiscibility have been put forward, but the lack of robust evidence has hindered unambiguously distinguishing the role of these two processes in Fe-Ti mineralization. The nelsonite and associated Fe-Ti-P-rich rocks hosted in the Proterozoic Damiao anorthosite complex represent a typical example for studying Fe-Ti ore-forming processes. We recognized a new type of nelsonite (type-I) in the Damiao complex, which is distinct from the two known types of nelsonite (type-II and type-III) from the same complex. The type-I nelsonite is characterized by its coexistence with oxide-apatite gabbronorite and granite in the same dike, and all these rocks have identical emplacement ages (1740 ± 7 Ma), subparallel REE patterns, and major-element compositions lacking intermediate compositions, suggesting derivation from conjugate Fe- and Si-rich melts generated by silicate liquid immiscibility. The large type-II nelsonite bodies form irregular dikes along fractures in anorthosite and constitute the major ore type. The type-III nelsonite occurs as conformable layers or pods within oxide-apatite gabbronorite and pyroxenite, and occupies the end part of the type-II dike. The latter two types of nelsonites formed by extensive fractional crystallization of residual magma with crystal accumulation and subsequent hydrothermal replacement. During residual magma evolution, silicate liquid immiscibility was crucial for Fe-Ti-P enrichment, fractional crystallization was responsible for enhancing oxide-apatite concentrations, and hydrothermal replacement was effective for mobilizing oxide-apatite concentrations. Our newly recognized nelsonite provides an unambiguous, outcrop-scale, field evidence for the operation of silicate liquid immiscibility process. We show that giant magmatic Fe-Ti oxide orebodies can form by a combination of processes involving silicate liquid immiscibility, fractional crystallization and hydrothermal mobilization.

Experimental study of high-Ti and low-Ti basalts: liquid lines of descent and silicate liquid immiscibility in large igneous provinces

Experimental study of high-Ti and low-Ti basalts: liquid lines of descent and silicate liquid immiscibility in large igneous provinces

Significance of silicate liquid immiscibility for the origin of young highly evolved lithic clasts in Chang’E-5 regolith

Highly evolved lithology distributes across the Moon sparsely but serves as a critical record of the extensive differentiation processes of lunar magmas. In contrast to the Apollo-returned highly evolved rocks that essentially formed before the end of the Nectarian period, silicic lithologies detected by remote sensing within the nearside Procellarum KREEP Terrane (PKT) have cratering model age as young as ∼2.5 Ga (Chevrel et al., 2009). The formation mechanism of the young silicic magmatism remains enigmatic. Here we present a detailed study of lithic clasts with highly evolved compositions from the northwestern PKT returned by Chang’E-5 mission. Two different types of highly evolved lithic clasts were recognized: (a) Type A clasts predominately consist of granophyric intergrowths of K-feldspar and quartz. They are highly depleted in incompatible elements (except for K, Rb, Cs, and Ba) and have a V-shaped REE pattern, which can be explained by silicate liquid immiscibility (SLI) following the fractionation of merrillite from a KREEP-like melt. The microtextural features of quartz in Type A clasts indicate that they could have crystallized through relatively slow cooling at temperature below 870 °C, supporting a shallow intrusive origin. The silicic intrusion exposed in the interior, rim, and ejecta of Aristarchus crater has a cratering model age of ∼2.5–3.7 Ga, which could be the source for Type A clasts; (b) Type B clast has little MgO, high incompatible element concentrations, and an REE pattern inclined to the right. Thermodynamic calculations indicate that Type B clast likely formed through SLI of the ∼25% residual melt of Em3 basalts in the Chang’E-5 landing region. This is consistent with the crystallization age of 2.57 ± 0.26 Ga for the zirconolite in Type B clast. The highly evolved samples from Chang’E-5 regolith provide new evidence that SLI may have played an important role in the young highly evolved intrusive bodies’ formation on the Moon. Furthermore, our thermodynamic modeling results show that compared to KREEP basalt, partial melting of quartz monzodiorite/monzogabbro at ∼930–1000 °C can produce melts with composition close to lunar granites and felsites. Thus, if a series of silicic volcanisms distributed mostly within the PKT was generated through this mechanism, quartz monzodiorite/monzogabbro may also widely distribute within the lunar nearside upper crust.

Detailed petrogenesis of the unsampled Oceanus Procellarum: The case of the Chang'e-5 mare basalts

Lunar mare basalts provide a probe to study the magmatic and thermal evolution of the Moon. The Chang'e-5 (CE-5) mission returned samples from a young and hitherto unsampled mare terrain, providing fresh opportunities to understand lunar volcanic history. A detailed petrologic survey was conducted in this study on basalt fragments and glasses from the returned CE-5 soil samples. Relatively large-sized (100–400 μm) basaltic fragments were hand-picked and examined for texture, mineral assemblage and mineral chemistries. Basaltic fragments exhibit dominantly subophitic textures and are phenocryst-free, with low to intermediate-Ti (2.1–5.5 wt%) and low Mg# (Mg/(Mg + Fe) × 100, 19–47, with an average whole-rock Mg# of 33) consistent with olivine-melt equilibrium calculation (Mg# = 34). A range of highly evolved basaltic materials have been identified, in which abundant fayalitic olivine, symplectitic intergrowths, and Si + K-rich mesostasis co-exist were found resulting from late-stage silicate liquid immiscibility. Basaltic glass compositions largely overlap with basaltic fragment compositions suggesting they are locally derived. The CE-5 basalts have a relatively limited range of eruption temperatures of 1150–1230 °C. Based on their petrographic and geochemical characteristics, some CE-5 mare basalts are highly evolved and some of the resultant basaltic melt products underwent high crystallization. Thermodynamic modeling using MELTS suggests highly evolved basaltic magma was produced by a low-pressure and simple fractional crystallization under reduced conditions. This may have occurred at the surface in the inflated Em4/P58 flow with a thickness of ~50 m. The low degree of partial melting mantle source of the parental melts is the late-stage lunar magma ocean cumulates in a similar manner to some evolved low-Ti mare basalt meteorites, although the source of CE-5 basalts may have been slightly more Ti-rich.

Analysis and experimental investigation of Apollo sample 12032,366‐18, a chemically evolved basalt from the Moon

Abstract Sample 12032,366‐18 is a 41.2 mg basaltic rock fragment collected during the Apollo 12 mission to the Moon. It is enriched in incompatible trace elements (e.g., 7 ppm Th), but does not have a bulk composition that would be considered a KREEP (enriched in potassium, rare earth elements, and phosphorous) basalt. The sample is of particular interest because it may be representative of some of the mare basalts within Oceanus Procellarum that are inferred to be Th‐rich, based on remote sensing data. The major mineral assemblage of 12032,366‐18 is pyroxene, plagioclase, olivine, and ilmenite, and the bulk composition has 4.2 wt% TiO2, 11.7 wt% Al2O3, and 0.25 wt% K2O. The sample contains regions of late‐stage crystallized minerals and glass (collectively termed mesostasis), including K‐feldspar, apatite, rare earth (RE) merrillite, ilmenite, troilite, silica, and relatively sodic plagioclase adjacent to ferroan pyroxene. The mesostasis also occurs in several areas that are highly enriched in silica and intergrown with K‐feldspar and very fine‐grained, high‐mean‐atomic‐number phases. We explore the petrology of this sample, including the origin of the Si‐K‐rich mesostasis to assess whether the mesostasis had formed by silicate liquid immiscibility (SLI). We used experiments to determine if the bulk composition of 12032,366‐18 is representative of a bulk liquid composition, how the residual liquid evolves, and to investigate the partitioning of elements between phases as the melt evolves. Experiments support that the mesostasis formed by SLI after crystallization of minerals closely matches the major‐mineral assemblage of 12032,366‐18. Experiments bracket the onset of SLI and merrillite saturation between 1024 and 1002 °C. Some high field strength elements, such as Zr and P, partition preferentially into the Fe‐rich liquid. From the experiments, we infer that the bulk composition of 12032,366‐18 represents the magma from which it crystallized. Based on the Th‐rich and KREEP‐bearing chemistry of this sample, along with experimental evidence showing that the sample is representative of a bulk liquid composition and not a cumulate, we conclude that basalt fragment 12032,366‐18 was delivered to the Apollo 12 landing site as ejecta from a distant impact and could represent an Oceanus Procellarum basalt. Missions to Oceanus Procellarum, such as Chang’E 5, have the potential to confirm whether some of those basalts are indeed enriched in Th and other incompatible trace elements as indicated by remote sensing.

Geochronology and C-O-Sr-Nd isotopic study of the Ulgii Khiid carbonatite from the Southern Mongolian alkaline Province and its implications for the mantle sources

The Ulgii Khiid carbonatite-alkaline rock complex is located within the well-known Alkaline Province of Southern Mongolia, Central Asia. However, the complex's origin is poorly constrained, and its tectonic environment remains controversial (i.e., mantle plume vs. subduction-related post-collision). Here, we report apatite U-Pb dating, major and trace elements, and C-O-Sr-Nd isotopic data to constrain the origin and mantle source of the carbonatite and associated alkaline syenite rocks. The apatites yield an age of 166 ± 25 Ma, coeval to the related alkaline syenite's age reported previously. The carbonatite show higher P2O5, Sr, and REE but lower alkalis and HFSE (Nb, Ta, Zr, and Hf) contents. The carbonatite and alkaline syenite show restricted and similar Sr [(87Sr/86Sr)i: 0.7041–0.7049 and 0.7044–0.7048] and Nd [εNd(t): 4.1–4.8 and 4.2–4.7, respectively] isotopic compositions, indicating a common mantle source. Combined with the previous melt inclusion data, it is, therefore, suggested that the Ulgii Khiid carbonatite was formed by carbonate–silicate liquid immiscibility from a carbonated mantle source. The δ13CVPDB and δ18OVSMOW values of the carbonatites vary from −2.6 to −9.4‰ and 10.6 to 23.2‰, respectively and show a positive C-O linear correlation between the mantle and sedimentary carbonates, which imply the incorporation of the subducted sedimentary carbonates into the mantle source. We, therefore, propose that the Ulgii Khiid, and other carbonatite complexes in the Alkaline Province of Southern Mongolia, were produced via low-degree melting of the underlying carbonated lithospheric mantle, which is heterogeneously modified by the subducted Paleo-Asian Ocean.

Immiscible silicate liquids: K and Fe distribution as a test for chemical equilibrium and insight into the kinetics of magma unmixing

Silicate liquid immiscibility leading to formation of mixtures of distinct iron-rich and silica-rich liquids is common in basaltic and andesitic magmas at advanced stages of magma evolution. Experimental modeling of the immiscibility has been hampered by kinetic problems and attainment of chemical equilibrium between immiscible liquids in some experimental studies has been questioned. On the basis of symmetric regular solutions model and regression analysis of experimental data on compositions of immiscible liquid pairs, we show that liquid–liquid distribution of network-modifying elements K and Fe is linked to the distribution of network-forming oxides SiO2, Al2O3 and P2O5 by equation: log K_{{text{d}}}^{{text{K/Fe}}} = , 3.796Delta X_{{{text{SiO}}_{2} }}^{{{text{sf}}}} + , 4.85Delta X_{{{text{Al}}_{2} {text{O}}_{3} }}^{{{text{sf}}}} + , 7.235Delta X_{{{text{P}}_{2} {text{O}}_{5} }}^{{{text{sf}}}} - , 0.108,where K_{{text{d}}}^{{text{K/Fe}}} is a ratio of K and Fe mole fractions in the silica-rich (s) and Fe-rich (f) immiscible liquids: K_{d}^{{text{K/Fe}}} = , left( {X_{{text{K}}}^{s} /X_{{text{K}}}^{f} } right)/ , left( {X_{{{text{Fe}}}}^{s} /X_{{{text{Fe}}}}^{f} } right) and Delta X_{{text{i}}}^{sf} is a difference in mole fractions of a network-forming oxide i between the liquids (s) and (f): Delta X_{i}^{sf} = X_{i}^{s} - X_{i}^{f}. We use the equation for testing chemical equilibrium in experiments not included in the regression analysis and compositions of natural immiscible melts found as glasses in volcanic rocks. Departures from equilibrium that the test revealed in crystal-rich multiphase experimental products and in natural volcanic rocks imply kinetic competition between liquid–liquid and crystal–liquid element partitioning. Immiscible liquid droplets in volcanic rocks appear to evolve along a metastable trend due to rapid crystallization. Immiscible liquids may be closer to chemical equilibrium in large intrusions where cooling rates are lower and crystals may be spatially separated from liquids.

Textural and Geochemical Evidence for Multiple, Sheet-like Magma Pulses in the Limeira Intrusion, Paraná Magmatic Province, Brazil

Abstract Quantitative petrographic, structural, and textural parameters are integrated with geological, geochemical, and Sr-isotope data to examine the emplacement, growth processes, and magmatic evolution of the high-Ti tholeiitic Limeira Intrusion, in the Paraná Magmatic Province of SE Brazil. Our data strongly support a multiple-stage evolution, owing to the nested emplacement of distinct crystal-bearing magma pulses that probably evolved independently, except at their boundaries. A stage of cooling and crystallization between magma injections results in a stepwise T–t path, leading to variations in the plagioclase residence times and effective growth rates inwards, also occasioning sudden changes in crystal shape and size at the boundaries of each magma pulse. The time delay between pulses allows preservation of internal ‘chilled margins’ and the development of near-rigid surfaces at their contacts, increasing the alignment and clustering of crystals during magma replenishment. Isotopic and textural data demonstrate a complex assembly history, in which the appearance of mixed plagioclase populations between magma pulses coincides with the onset of initial Sr isotope ratio increase, which can be attributed to a locally enhanced cooling rate, and the extraction of residual melts from the previous crystallizing batches and mixing with the younger pulses. Typical C- and S-shaped MgO (wt%) compositional profiles within individual pulses indicate that the first one probably evolved by in situ fractional crystallization followed by melt migration inward, whereas the younger ones have contributions from both compaction of the lowermost crystallization front and compositional convection. Mafic globular structures are found at the boundaries of magma pulses and constituting the mafic-rich layers in layered rocks. They are interpreted as evidence for chemical disequilibrium, arguably associated with the trigger of silicate liquid immiscibility. The upwards compositional convection of the silica-rich residual liquid and the accumulation of the Fe–Ti–P-rich crystalbearing end-member in the bottom of the latest magma pulses might represent the most significant mechanism of differentiation in the Limeira Intrusion.

Experiments on phosphate–silicate liquid immiscibility with potential links to iron oxide apatite and nelsonite deposits

The formation of phosphorus–iron oxide (P–Fe) immiscible melts and their possible connection to the genesis of Kiruna-type and Nelsonite deposits was experimentally investigated by adding phosphoric acid (H3PO4), water, and sulfur, to andesite at 100–450 MPa, 500–900 °C, at the NiNiO and magnetite-hematite fO2 buffers using internally heated gas vessels. The addition of up to 8.02 wt% of H3PO4 to the andesite causes crystallization of apatite. At higher concentrations of H3PO4 whitlockite crystallizes, and at concentrations above ~ 11.4% H3PO4 (at 800 °C, 385 MPa) an immiscible P–Fe melt forms. Adding sulfur at low fO2 (NiNiO) causes an additional immiscible Fe–S melt to form. Increasing the fO2 to the hematite-magnetite buffer causes the sulfur-rich melt to shift in composition to a Ca–S–O melt, and the coexisting P-Fe melt to incorporate large amounts of SO4. Immiscible P-Fe melts can form at temperatures above 1100 °C down to 600 °C (at 400 MPa). Mass balance calculations show that some experimentally produced P-Fe rich immiscible liquids may result in mineral assemblages similar to those found at some Kiruna-type deposits, such as actinolite-rich dikes, and apatite-rich veins. Depending on the geological conditions and the composition the fractionation of a P-Fe melt may result in the formation of nelsonites at high pressures, high temperatures, and low fO2 or Kiruna-type deposits at lower temperatures and higher fO2.

Mineralogical and Geochemical Study on the Yaojiazhuang Ultrapotassic Complex, North China Craton: Constraints on the Magmatic Differentiation Processes and Genesis of Apatite Ores

The differentiation process of ultrapotassic magmas is enigmatic and poorly understood. The Yaojiazhuang ultrapotassic complex is concentrically zoned by late-intruded syenite in the core and early emplaced clinopyroxenite in the periphery, combining a “bi-modal” lithology. Spatially, apatite and iron oxide-apatite (IOA) ores, glimmerite and pseudoleucite occur in the upper part of clinopyroxenite. The syenite and clinopyroxenite are composed of variable amounts of clinopyroxenite, biotite, K-feldspar, magnetite, apatite with minor analcite, titanite and primary calcite. The pseudoleucite clinopyroxenite contains mainly clinopyroxene, biotite and garnet in the matrix, and nepheline–K-feldspar intergrowth with muscovite and minor celestine in the leucite pseudomorph. Geochemically, rocks of the Yaojiazhuang complex are significantly enriched in potassium (K), light rare earth elements (LREE) and large ion lithophile elements (LILE). Crustal contamination by Archean tonalite–trondhjemite–granodiorite (TTG) gneisses basement may play an important role to convert the syenitic melts from silica-undersaturation to saturation. Fractionation crystallization is supported by the mineral crystallization sequence to explain the bimodal lithologies instead of silicate liquid immiscibility. During the magmatic evolution, decompression, fractionation of volatile-poor clinopyroxene and the enhancement by CO2 may result in the exsolution of an aqueous fluid. The late-stage interactions between existing minerals and magmatic fluids in the crystal mush could be a key process in the generation of both leucite pseudomorphs and apatite/IOA ores.

Viscous deformation as a measure of heat work during coal ash fusibility testing

Coal ash fusibility is important in evaluating coal quality and managing coal ash in boilers and gasifiers. Ash fusibility test (AFT) data constitute the basis for many ash and slag management models. Coal ash is a mixture of inorganic compounds derived from sedimentary minerals. As is the case for the bending of pyrometric cones used to control the firing of traditional ceramics, the deformation of coal ash cones is a measure of heat work and not temperature. Heat work, the combined effect of time and temperature, is measured by cone deformation that results from the integration of viscous strain over time. The effective viscosity of 4726 coal ash samples from the U.S. Geological Survey Coal Quality (COALQUAL) database were evaluated at their AFT critical temperatures and found to be lower than the working range of glass, which is consistent with the heat work interpretation of AFT critical temperatures. Thermophysical model data also showed strong positive linear correlations between AFT critical temperatures and the liquidus temperature, as was also the case for pyrometric cones, but not with their solidus temperatures. The finding provides a thermophysical basis for AFT models based on composition while also explaining the spurious correlation between the critical viscosity temperature (TCV) in coal ash slags and AFT critical temperatures. Silicate liquid immiscibility and phase inversion between iron-rich and iron-poor phases results in a sharp change in the effective viscosity of the slag emulsion. High effective viscosities magnify the effect of heating rate variability on AFT critical temperature measurements.

Silicate Liquid Immiscibility as a Result of Fenner-Type Crystal Fractionation of Wangtian’e Tholeiitic Melts, Northeast China

The results of a study of mineral-hosted melt inclusions were used to reconstruct the evolution of tholeiitic melts that produced the rocks of the shield edifice of Wangtian’e volcano, Northeast China, including the P–T parameters of their crystallization. Primary melt inclusions were studied in plagioclase of tholeiitic basalts. They contain “dry” Fe-rich silicate glass or a crystallized fine-grained mineral aggregate, feldspar rim, and globules of various compositions: water-bearing Fe-rich globules, water-bearing Si-rich globules and “dry” Si-rich globules. In the groundmass of tholeiitic basalts, “dry” Si-rich and Fe-rich glasses, as well as hydrated Fe-rich glass with an H2O content of up to 10–15 wt.% were identified. The fractional crystallization path of Wangtian’e tholeiitic melts was calculated by the COMAGMAT-5.2.2 software using glass compositions of homogenized melt inclusions in plagioclase from the basalts. It was shown that the calculated temperatures of the beginning of plagioclase crystallization are in good agreement with temperatures obtained in thermometric experiments with melt inclusions in plagioclase, and correspond to 1180–1200°C. Based on our studies, several stages of the melt differentiation during the formation of Wangtian’e volcanic rocks were identified. The first stage includes the Fenner-type crystal fractionation of the melt due to the appearance of plagioclase on the liquidus at the beginning of magma crystallization. The separation of the ferrobasaltic melt into Si-rich and Fe-rich silicate immiscible liquids (“dry” and hydrated) corresponds to the second stage of differentiation. The occurrence of silicate liquid immiscibility follows from the presence of Fe-rich and Si-rich glasses in the intergranular space of the basalts and in the melt inclusions in plagioclase of these rocks.