Liquid Immiscibility Research Articles

The use of trace elements in apatite to trace differentiation of a ferrobasaltic melt in the Sept-Iles Intrusive Suite, Quebec, Canada: Implications for provenance discrimination

The Sept Iles Intrusive Suite, Quebec, Canada, comprises one of the world’s largest mafic layered intrusions. It is a natural laboratory to study differentiation processes of a ferrobasaltic melt whereby extreme fractional crystallisation of the mafic melt resulted in residual, felsic magma. Furthermore, liquid immiscibility played a role in forming Fe–Ti–P mineralisation (Fe-Ti oxides and apatite) in the apatite-bearing Critical Zone near the top of the mafic sequence. In this study, we explore and develop the use of trace elements in apatite as a petrogenetic indicator of these magmatic processes. A full suite of 31 trace elements was determined in apatite by LA-ICP-MS and electron microprobe from the mafic Layered Series and felsic Upper Series of the Sept-Iles Intrusive Suite. We show that the trace element content of cumulus apatite records the changing melt composition of the fractionating ferrobasalt from the mafic to the felsic rocks (increase in REE + Y, Th, U, K, Pb and Rb, and decrease in Sr, Ba, V and Mg of apatite), in accordance with the evolution of Sept-Iles liquid line of descent. In the mineralised Critical Zone, the fractionation trend overprints systematic chemical differences in apatite and plagioclase composition between alternating Fe–Ti–P-rich and -poor layers. This is attributed in part to partitioning behaviour of trace elements between immiscible Fe-rich and Si-rich silicate melts and the different cotectic proportions of apatite crystallising from these melts, which was calculated to be 5 to 8 % and 1 % for the Fe–Ti–P-rich and -poor, respectively. Crystal settling and accumulation is then responsible for the elevated proportion of apatite (7 to 36 vol%) observed in Fe–Ti–P-rich layers. Furthermore, a series of new discrimination diagrams is proposed to help identify the provenance of igneous rocks using apatite chemistry. At Sept Iles, apatite from mafic rocks is distinguished from the felsic rocks using REE + Y, Sr and the Eu anomaly. Intercumulus apatite, which crystallised from the interstitial liquid, are readily distinguishable from cumulus apatite by enrichment in highly incompatible elements (higher Cl/F and Th/Lu ratios). This suggests that apatite could be used as an indicator mineral for the exploration of Fe–Ti–P mineralisation in mafic layered intrusions using detrital apatite.

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.

Formation of HFSE-Rich Glimmerites by Silicate Liquid-Liquid Immiscibility, Suzhou Pluton, Eastern China

Abstract Understanding the formation of high field strength element (HFSE) mineralization remains a challenge. Processes ranging from melt generation to final crystallization have been invoked to play a key role in the enrichment of HFSEs in magmatic-hydrothermal systems, yet the importance of crystal-liquid fractionation, liquid immiscibility, and/or fluid unmixing remains debated. Here, we present results of a study of granites and HFSE-mineralized glimmerites from the granitic Suzhou pluton, eastern China. We provide whole-rock major and trace element compositions, a description of mineral assemblages, and a detailed description of zircon textures, zircon Raman spectra, zircon major and trace element and δ18O compositions, and U-Pb ages. The granites and glimmerites have been dated at ~126 to 124 Ma and are thus coeval. The granites range from biotite and zircon poor to biotite and zircon rich and are in contact with glimmerites. The glimmerites form lenses and layers close to and along the margin of the Suzhou granite, comprising up to ~37 vol % biotite and up to ~2 vol % zircon and other HFSE-rich accessory minerals. The biotite-poor granites contain a single type of zircon (type-A: single crystals, oscillatory zoned, fully crystallized structure, relatively poor in trace elements, and relatively high δ18O), whereas the biotite-rich granites and the glimmerites contain two zircon types (type-A crystals: same features as in the biotite-poor granites; type-B crystals: clustered, unzoned, partially metamict structure, rich in trace elements, and relatively low δ18O). Both granite types are Si, Al, and Na rich, whereas the glimmerites are Fe, Ti, Mn, Mg, Ca, P, F, and HFSE rich and, compositionally, fall off simple fractionation trends. We interpret the textural, mineralogical, and compositional relationships to indicate that the glimmerites are the products of Fe-, F-, and HFSE-rich immiscible melts that unmixed from an alkali-rich, moderately reducing (~QFM + 0.5 and ~QFM + 1.0; QFM = quartz-fayalite-magnetite buffer) Suzhou magma system at low crustal pressure. In addition, we note that the zircon textures and compositions are important recorders of the processes and conditions that led to the HFSE mineralization.

Genetic model of the El Laco magnetite-apatite deposits by extrusion of iron-rich melt

Magnetite-apatite deposits are important sources of iron and other metals. A prominent example are the magnetite lavas at the El Laco volcano, Northern Chile. Their formation processes remain debated. Here, we test the genetic hypothesis that an Fe-rich melt separated from silicate magma and ascended along collapse-related fractures. We complement recent analyses with thermodynamic modelling to corroborate Fe-Si liquid immiscibility evident in melt inclusions at El Laco and present viscometry of Fe- and Si-rich melts to assess the time and length scales of immiscible liquid separation. Using a rock deformation model, we demonstrate that volcano collapse can form failure zones extending towards the edifice flanks along which the ore liquid ascends towards extrusion driven by vapour exsolution despite its high density. Our results support the proposed magmatic genesis for the El Laco deposits. Geochemical and textural similarities indicate magnetite-apatite deposits elsewhere form by similar processes.

Effects of Ni Content on Particle Size of Secondary Phase in Cu-Based Alloys with Liquid Immiscibility by Laser-Melting

Effects of Ni Content on Particle Size of Secondary Phase in Cu-Based Alloys with Liquid Immiscibility by Laser-Melting

The innards of the cell: studies of water dipolar relaxation using the ACDAN fluorescent probe

This article reviews the use of the 6-acetyl-2-(dimethylamino)naphthalene (ACDAN) fluorophore to study dipolar relaxation in cells, tissues, and biomimetic systems. As the most hydrophilic member of the 6-acyl-2-(dimethylamino)naphthalene series, ACDAN markedly partitions to aqueous environments. In contrast to 6-lauroyl-2-(dimethylamino)naphthalene (LAURDAN), the hydrophobic and best-known member of the series used to explore relaxation phenomena in biological (or biomimetic) membranes, ACDAN allows mapping of spatial and temporal water dipolar relaxation in cytosolic and intra-organelle environments of the cell. This is also true for the 6-propionyl-2-(dimethylamino)naphthalene (PRODAN) derivative which, unlike LAURDAN, partitions to both hydrophobic and aqueous environments. We will (i) summarize the mechanism which underlies the solvatochromic properties of the DAN probes, (ii) expound on the importance of water relaxation to understand the intracellular environment, (iii) discuss technical aspects of the use of ACDAN in eukaryotic cells and some specialized structures, including liquid condensates arising from processes leading to liquid immiscibility and, (iv) present some novel studies in plant cells and tissues which demonstrate the kinds of information that can be uncovered using this approach to study dipolar relaxation in living systems.

Ultralarge-scale anomalous phase separation discovered in a Cr-Fe-Co-Ni-Zr high-entropy metallic glass system

Anomalous phase separation in metallic glasses is a longstanding issue that has been controversial since it was first implied in 1969 due to the influence of microstructure artifacts and the lack of direct large-scale (over nanoscale) evidence. This study confirms an ultralarge scale anomalous glass phase separation phenomenon in a Cr-Fe-Co-Ni-Zr high-entropy metallic glass in which any pair of atomic components has a nonpositive heat of mixing using interferencefree visible-light, atomic force, scanning electron, and double spherical aberration-corrected atomic resolution analytical electron microscopes. The second glassy phase separated from the glassy matrix has an ultralarge size of tens of micrometers, which can be controlled by the cooling rate during sample preparation. A mechanism of before-monotectic-like liquid immiscibility is introduced to interpret the phenomenon, which helps understand the anomalous phase separation in metallic glasses and in other fields, such as liquid-liquid phase separation in cells.

Late Palaeozoic tectonic evolution of the eastern Palaeo-Asian Ocean: new evidence from the early Permian arc magmatic suites in the Kulun region

AbstractLate Palaeozoic igneous rock associations in response to subduction, accretion, and final closure of the eastern Palaeo-Asian Ocean play a significant role in understanding the geodynamic evolution of the southeastern Central Asian Orogenic Belt. Previous studies have identified a Permian arc magmatic belt associated with the southward-dipping subduction of the eastern Palaeo-Asian Ocean along the Solonker–Changchun suture zone. The genetic mechanism and associated geodynamic settings are of great importance in deciphering the evolution of the eastern Palaeo-Asian Ocean. This paper presents zircon U–Pb–Hf isotope and whole-rock geochemical analyses for a suite of magmatic rocks including the early Permian diorite porphyrites (ca. 281.0 Ma), andesites (ca. 276 Ma) and rhyolites (ca. 275 Ma) in the Kulun region. The diorite porphyrites and andesites have high SiO2 and total alkali contents, and low MgO contents and Mg no. values, with enrichments in large ion lithophile elements and depletions in high-field-strength elements. These geochemical characteristics, together with low-Sr and high-Yb contents, a weak concave-upward shape of middle rare earth elements and negative Eu anomalies, suggest that these intermediate igneous rocks were generated by partial melting of amphibolitic lower crust at a crustal depth of 30–40 km. The rhyolites have heterogeneous isotopic compositions, with ϵHf(t) values and TDM2 ages of –20.8 to +0.5 and 3578∼1494 Ma, implying that they were likely derived from partial melting of a mixed source dominated by recycled ancient crust with minor juvenile crustal materials. The rhyolites show potassic affinity with relatively high K2O and very low Na2O, which was attributed to liquid immiscibility of felsic magma and subsequent limited fractional crystallization of plagioclase. The regional igneous associations, metamorphic events, and coeval sedimentary rocks along the Solonker–Changchun suture zone indicate that the early Permian igneous rocks were formed in an active continental arc environment in response to southward subduction of the eastern Palaeo-Asian Ocean.

Genetic link between saline and carbonatitic mantle fluids: The system NaCl-CaCO3-MgCO3 ± H2O ± Fe0 at 6 GPa

Melt inclusions in kimberlitic minerals and diamonds indicate that chlorides are important constituents of mantle carbonatite melts. Besides, alkaline chlorides are important constituents of saline high-density fluids (HDFs) found in diamonds from kimberlites and placers around the world. Continuous compositional variations suggest that saline and carbonatitic HDFs could be genetically linked. However, the essence of this link remains unclear owing to the lack of data on phase relations in the chloride-carbonate systems under pressure. Here we studied subsolidus and melting phase relations in the system NaCl–CaCO3–MgCO3 at 6 GPa and 1000–1600 °C using a Kawai-type multianvil press. We found that at 1000 °C, subsolidus assemblage consists of halite, magnesite, and aragonite. At higher temperatures, the stabilization of dolomite splits the subsolidus area into two partial ternary fields: halite + magnesite + dolomite and halite + dolomite + aragonite. The minimum on the liquidus surface corresponds to the halite-dolomite-aragonite ternary eutectic, situated at 1100 °C. The eutectic melt has Ca# 89 and contains 30 wt.% NaCl (26 mol% 2NaCl). The system has two ternary peritectics: halite + dolomite = magnesite + liquid located near the ternary eutectic and magnesite + dolomite = Mg-dolomite + liquid situated between 1300 and 1400 °C. Although under dry conditions incipient melting yields carbonate-dominated melt, the addition of water facilitates the fusion of NaCl and expands the liquid field to NaCl-rich compositions with up to 70 wt.% NaCl. The obtained results favor the idea that hydrous saline melts/fluids (brines) found as inclusions in diamonds could be a lower temperature derivative of mantle carbonatite melts and disagree with the hypothesis on chloride melt generation owing to the chloride-carbonate liquid immiscibility since no such immiscibility was established. We also studied the interaction of the NaCl–CaCO3–MgCO3 system with iron metal and found that carbonate reduction produces C-bearing species (Fe0, Fe-C melt, Fe3C, Fe7C3, C0) and wüstite containing Na2O, CaO, and MgO. Besides, a carbonate chloride compound, Ca2Cl2CO3, was established among the reaction products. The interaction between NaCl-bearing carbonate melt shifts its composition toward Mg-poor and NaCl-rich. Given the above, an alternative hypothesis can be proposed, according to which the interaction of alkaline chloride-bearing carbonate melts formed in the subduction zones with the reduced mantle should be accompanied by diamond crystallization and shift the composition of the melt from carbonatitic to alkali-rich saline.

Olivine in Kimberlites: Magma Evolution from Deep Mantle to Eruption

Abstract Elucidating the origin, composition and physical properties of primary kimberlite melts is crucial to our understanding of their source, petrogenesis, ascent mechanisms and ultimately the origin of diamonds. Recently, there has been a growing interest in the study of olivine, which is one of the most abundant minerals in kimberlites with xenocrystic, metamorphic (mantle) and magmatic origins. Olivine is one of the earliest minerals to crystallise in kimberlite magmas, and the presence of ubiquitous zoning (e.g. cores, internal zones, transitional zones, rims, rinds, outmost rinds) and different generations (i.e. primary, pseudosecondary and secondary) of crystal/melt/fluid inclusions in euhedral olivine grains has been shown to provide fundamental insights into the composition and evolution of kimberlite melts. In this contribution, we review and evaluate the following: (1) the widely accepted notion that kimberlite olivine has two distinct origins—xenocrystic and magmatic. We present detailed electron microprobeX-ray element maps of well-preserved and zoned euhedral olivine microcrysts from the Koala and Mark (Lac de Gras, Canada) and Udachnaya-East (Siberia, Russia) kimberlites to show that the cores of olivine occasionally adopt euhedral shapes, which is commonly defined by the distribution of Ni. We present a scenario in which mantle olivine was recrystallised by the early (or proto-) kimberlite melt/fluid infiltrating through the lithospheric mantle to form euhedral ‘pyrocrysts’ (i.e. olivine that formed via re-crystallisation in the mantle in the presence of a melt), which in turn become cores for the subsequent crystallisation of magmatic olivine during kimberlite magma ascent and emplacement. (2) The evolution of ideas using different geochemical, petrological, experimental and melt inclusion approaches to constrain the composition of the primary/parental kimberlite melt. Based on our assessment of available data, in particular using melt inclusions, we propose that kimberlites originated from melts that were initially Si-poor, and Na-K-F-Cl-P-S-bearing and Ca-Mg-carbonate-rich. With this model composition for the primary/parental kimberlite melt considered, we emphasise the implications for the evolution of olivine and its role in the kimberlite petrogenesis. Furthermore, we present a comprehensive model outlining the key stages involved in the petrogenesis of kimberlites, ranging from the generation of the proto kimberlite melt in the mantle, its interaction with mantle silicates during ascent, the role of liquid immiscibility in driving magma differentiation and CO2 degassing and its emplacement and modification in the crust. Finally, we discuss prospective directions that may further guide the future of kimberlite petrological research.

The Paleo-Mesoarchaean Gondpipri Mafic-Ultramafic Intrusions, Western Bastar Archaean Craton, Central India: Insights from Bulk-Rock Geochemistry and Sm-Nd and S Isotope Studies on the Formation of Ni-Cu-PGE Mineralization

Abstract Magmatic Ni-Cu-platinum group element (PGE)-Te mineralization in the Gondpipri mafic-ultramafic layered intrusion of ca. 3323 ± 74 Ma age, western Bastar craton, central India, is one of the most prospective exploration targets for magmatic sulfides in India. The Gondpipri layered intrusion is divided into two distinct groups of rocks based on their mineralization potential, which include (1) mineralized layered gabbro and pyroxenite and (2) a barren olivine gabbro intrusion. The host rocks show Cu + Ni concentrations up to 5,000 ppm with a Cu/Ni ratio <1 and all PGE values between 0.1 and 1.1 ppm. Mineralization occurs in two modes: type I mineralization occurring as blebs, specks, and dissemination and type II mineralization occurring as stringers and minor veins. The geochemical data suggest that the parental magma of the host rock was generated at depths between spinel and garnet peridotite mantle source regions and subsequently modified by assimilation fractional crystallization (AFC) of the continental crust. High large ion lithophile elements, Th/Yb ratios of the studied rocks, and Sm-Nd isotope studies are consistent with a depleted mantle source. The geochemical proxies such as Th versus Ba/Th and (Ta/La)PM versus (Hf/Sm)PM and higher Sr/Nd (2.21–82.58) ratios indicate involvement of fluid-related subduction metasomatism and enrichment processes in an island-arc tectonic setting. Mineral assemblages and textural relationships between platinum group minerals (PGMs) and base metal sulfides suggest that sulfide-silicate liquid immiscibility was brought about by the precipitation of magnetite/Cr magnetite resulting in sulfide saturation in the melt by decreasing S solubility. Sulfur isotope compositions (δ34S: 1.61–3.30‰) and Sm-Nd geochemistry suggest that the sulfur was added in the tholeiitic magma by magmatic process. Crustal contamination played a significant role in sulfide saturation and in bringing about PGE and Te, As, Bi, Sb, Se (TABS) mineralization. PGM-NiTeBi developed at relatively low temperatures, whereas moncheite (PtPd)Te2 and merenskyite (PdTe) were formed at 650°C. The identification of Ni-Cu-PGM-Te in the margin of the western Bastar craton boosts deeper subsurface exploration.

Origin of carbonatites—liquid immiscibility caught in the act

Carbonatites are rare but worldwide occurring igneous rocks and their genesis remains enigmatic. Field studies show a close spatial but controversially debated genetic relationship with alkaline silicate rocks, and petrological and experimental studies indicate liquid immiscibility from mantle-derived magmas being one viable model for the generation of carbonatites. However, unaltered carbonatitic melts are rare and the composition of primary carbonate liquids and their silicate conjugates is poorly constrained. Here we show an example of primary Ca-carbonatitic melt formed by liquid immiscibility from a phonolitic magma of the Laacher See volcano (Eifel, Germany). The conjugate blebs of carbonate-silicate liquids are found in hauyne-hosted melt inclusions. The Ca-carbonatite melts are moderately alkali-rich and contain high F and Cl at elevated SiO2 and Al2O3 concentrations. Such carbonatite liquids are viable parental magmas to the globally dominating intrusive Ca-carbonatite complexes and may provide the missing link to extrusive Na-carbonatitic magmas.

The system KCl−CaCO3−MgCO3 at 6 GPa: A link between saline and carbonatitic diamond-forming fluids

Alkaline chlorides and carbonates are abundant in melt inclusions in diamonds, mantle xenoliths, and igneous minerals from kimberlites. Despite this, the phase relationships in chloride‑carbonate systems remain poorly constrained. Here we studied phase equilibria in the system KCl–CaCO3–MgCO3 in multianvil experiments at 6 GPa and 1000–1600 °C. It was found that at 1000 °C, subsolidus assemblage consists of KCl, magnesite, and aragonite. At higher temperatures, the stabilization of dolomite splits the system into two partial ternaries: KCl + dolomite + aragonite and KCl + magnesite + dolomite. Both ternaries start to melt near 1200 °C. The melting of the first ternary is controlled by the KCl-dolomite-aragonite eutectic situated at 30 mol% (24 wt%) KCl and Ca# 92, where Ca# = 100∙Ca/(Ca + Mg). The melting of the second ternary is controlled by the peritectic, KCl + dolomite = magnesite + liquid, producing chloride‑carbonate melt with 33 mol% (27 wt%) KCl and Ca# 69.The established melting temperature of the KCl + Ca-Mg carbonate assemblages is 200–500 °C higher than the subduction geotherms. This indicates that chlorides and carbonates can survive subduction to a depth of 200 km. However, the stagnation and warming of slabs to the ambient mantle temperature must result in partial melting of KCl + Ca-Mg carbonate assemblages producing a KCl-rich low-Mg carbonate melt. Percolation of this melt into the subcontinental lithospheric mantle should yield crystallization of KCl and CaMg carbonates. In the presence of water, KCl will form hydrous saline melt or fluid, while Ca and Mg carbonates, which are poorly soluble in water, will remain solid. The obtained results imply that the hydrous saline fluids (brines) found as inclusions in diamonds are a lower temperature derivative of mantle carbonatite melts and do not support the hypothesis of chloride melt generation owing to the chloride‑carbonate liquid immiscibility since no such immiscibility was established.

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.

Mica compositional constraints on the petrogenesis and mineralization of the syenite–carbonatite complex in the Maoniuping REE deposit, SW China

Mica, as a long-lasting mafic mineral in the magmatic–hydrothermal evolution of alkaline–carbonatite complexes, is an ideal monitor of petrogenesis and mineralization processes. Here, the compositions of mica in the bulk syenite, carbonatite and their related hydrothermal veins from the Maoniuping rare earth elements (REE) deposit (SW China) were analyzed in detail, in order to reveal the petrogenesis and REE mineralization processes of the Maoniuping syenite–carbonatite complex. Micas in the bulk syenite and carbonatite are common phlogopite with indistinguishable compositions, while micas in the hydrothermal veins of syenite and carbonatite are quite different from that in the bulk syenite and carbonatite and compositionally close to ideal tetraferriphlogopite. The common phlogopite is interpreted as orthomagmatic xenocryst crystallizing from parent magma system, whereas tetraferriphlogopite precipitates in the REE-enriched hydrothermal fluids. The compositional variations of mica show that Fe and F contents increase, whereas Al and Ti contents and Nb/Ta ratios decrease evolving from common phlogopite to tetraferriphlogopite. Compared to common phlogopite, tetraferriphlogopite has higher levels of REE (up to 77 ppm), Ba (up to 5000 ppm), Sr (up to 700 ppm), Li (up to 14000 ppm), Rb (up to 1700 ppm), and Na (up to 1200 ppm). The overlapped chemical compositions of common phlogopite in the bulk syenite and carbonatite support that the liquid immiscibility of syenitic and carbonatitic melts from a common parent magma, which may happen due to moderate crystal fractionation induced by decreasing temperature/pressure in the crustal magma chamber. Meanwhile, the presentation of tetraferriphlogopite in the hydrothermal veins of syenite and carbonatite indicates that syenitic and carbonatitic melts subsequently experienced independent magmatic–hydrothermal evolution path after separation, leading to REE mineralization of syenite and carbonatite respectively. The tetraferriphlogopite in the hydrothermal veins of carbonatite has higher oxygen fugacity and higher contents of incompatible elements (e.g., Li, Rb, Na, F, Sr, Ba, and REE) than that in the hydrothermal veins of syenite, which indicates that the carbonatite-derived fluids have higher REE mineralization potential than the syenite-derived fluids. Therefore, we conclude that liquid immiscibility and separated magmatic–hydrothermal evolution may be the key processes controlling the formation of the giant Maoniuping REE deposit.

Condensation of cometary silicate dust using an induction thermal plasma system

Glass with embedded metal and sulfides (GEMS), the major components of chondritic-porous interplanetary dust particles (CP-IDPs), is one of the most primitive materials in the Solar System and may be analogous to the amorphous silicate dust observed in various astronomical environments. Mineralogical characteristics of GEMS should reflect their formation process and condition. In this study, synthetic experiments in the sulfur-bearing system of Fe–Mg–Si–O–S were performed with a systematic change in redox conditions using thermal plasma systems to reproduce the mineralogy and textures of GEMS. The resulting condensates were composed of amorphous silicates with Fe-bearing nano-inclusions. The Fe content and texture in the amorphous silicates as well as the mineral phases of the nanoparticles correlate with redox conditions. Fe dissolved in the amorphous silicate as FeO in oxidizing conditions formed Fe-metal nanoparticles in intermediate redox conditions, and gupeiite (Fe3 Si) nanoparticles in reducing conditions. In intermediate to reducing redox conditions, Fe-poor amorphous silicate formed a biphasic texture with Mg- and Si-rich regions, indicating liquid immiscibility during the melt phase. Most Fe-metal particles were surrounded by FeS and formed on the surface of amorphous silicate grains. Condensates produced in intermediate to slightly reducing redox conditions resemble GEMS in that they have similar mineral assemblages and chemical compositions to amorphous silicate, except that the Fe-metal grains are absent from the interior of the amorphous silicate grains. This textural difference can be explained by the sulfidation at high temperatures in this study, in contrast to sulfidation occurring at low temperatures in the presence of H2 in natural GEMS formation. Based on the two-liquid structures observed in the experimental products and in GEMS, also recognized in infrared spectra, we propose that GEMS condensed as silicate melt under limited redox conditions followed by incorporation of multiple metal grains into the silicate melt or by aggregation of coreshell structured grains before sulfidation of the metallic iron. Condensates produced in oxidizing conditions are similar to GEMS-like material in the matrices of primitive carbonaceous chondrite meteorites, indicating the possibility that they form by direct condensation from nebula gas in relatively oxidizing conditions compared to GEMS.

Constraints on the Genesis of Cobalt Deposits: Part II. Applications to Natural Systems

AbstractIn a companion paper in this issue, the authors reviewed the properties of cobalt, its mineralogy, and the processes that concentrate it to exploitable levels. Using this information and knowledge of the geology of the principal types of cobalt deposits, the present paper assesses the conditions and controls of cobalt transport and deposition and develops/refines plausible models for the genesis of these deposits. Economic cobalt deposits owe their origins to the compatible nature of Co2+, which causes it to concentrate in the mantle, mainly in olivine, and to be released to magmas only after high degrees of partial melting (i.e., to komatiitic and basaltic magmas). Thus, there is a very close association between cobalt deposits and mafic and ultramafic rocks. Magmatic deposits, in which Co is subordinate to Ni, develop through sulfide-silicate liquid immiscibility as a result of the very strong preference of these metals for the sulfide liquid. Predictably, these deposits reach their highest grades where hosted by olivine-rich ultramafic rocks. Approximately 60% of the world’s cobalt resource is of hydrothermal origin and is contained in sediment-hosted copper deposits in the Democratic Republic of the Congo. Using a combination of thermodynamic data and geologic information, we have refined a model in which Co is leached from mafic and ultramafic rocks by oxidized, chloride-rich hydrothermal fluids, derived from evaporation, and deposited in response to decreasing fO2 in carbonaceous sediments that accumulated in intracratonic rift basins. Economic Co deposits also develop as hydrothermal vein systems, in which Co is the primary ore metal. In the only deposits of this type that are currently being exploited (Bou Azzer, Morocco), the source of the Co was an adjacent serpentinized peridotite. The ore fluid was an oxidized, high-salinity brine derived from evaporites, and deposition occurred in response to pH neutralization by the felsic to intermediate igneous host. The final major class of Co deposits is laterite-hosted and develops on olivine-rich ultramafic rocks or their serpentinized equivalents. Our thermodynamic modeling shows that Co is leached from an ultramafic source by mildly acidic fluids as Co2+ and is transported down the laterite profile, eventually concentrating by a combination of adsorption on Mn oxides, incorporation in the structure of absolane (an Mn oxide), and precipitation as heterogenite (HCoO2). The dissolution of cobalt at the surface and its deposition at depth are controlled mainly by pH, which decreases downward; oxygen fugacity, which also decreases downward, has the opposite effect, inhibiting dissolution of cobalt at the surface and promoting it at depth. It is our hope that this introduction to the economic geology of cobalt and the processes responsible for the formation of cobalt-bearing ores will help guide future studies of cobalt ore genesis and strategies for the exploration of this critical metal.

Style of Fractional Crystallization in Basalts from the Paleoarchean Western Iron Ore Group of Singhbhum Craton, Eastern India: Implications from One Atmosphere Experimental Studies

Abstract One atmosphere experimental study on fresh basaltic rocks of Paleoarchean western Iron Ore Group (IOG) basin of Singhbhum Craton, Eastern India gives insight of the crystallization behavior of the parent magma, liquid lines of descent and appearance of the different constituent phases. The natural starting material is sufficiently ferroan (FeOt: 14.28 wt%) and corresponds to ‘ferro-basalt’. Regarding the experimental run products, clinopyroxene shows Al2O3 rich character (ranging from 2.23 – 6.94 wt%; formed due to fluctuating silica activities in the magma) while plagioclase reveals typical Fe-Mg enrichment (that corresponds to higher KdFe-Mg between plagioclase and the parent liquid caused due to immiscible silicate melt inclusion in plagioclase where Mg enters more readily in plagioclase than Fe). The run-product glass corresponds to both high and low iron contents suggesting liquid immiscibility in the parent magma at particular temperature-domains. This study suggests that the crystallization of the western Iron Ore Group basalts represents a broad ‘Skaergaard analogue’ involving depression/shift of eutectic point in the diopside-anorthite system during crystallization.

Experimental constraints on truly conjugate alkaline silicate – carbonatite melt pairs

Physical separation of immiscible carbonatites from CO2-bearing alkaline silicate melts is frequently invoked to explain the common presence of carbonatites in alkaline rock suites. We present new isobaric 1.0 GPa crystallization and liquid immiscibility experiments between 1250 and 925°C on chemically complex CO2-bearing subvolcanic basanitic, nephelinitic and nepheline-syenitic compositions from the intrusive complex on Brava Island, Cape Verdes. In the experiments, carbonatites exsolve from silicate melt along the basanite–nephelinite melt fractionation pathway throughout an unmixing temperature window from 1100 to 950°C. Liquid immiscibility is suppressed by 950°C by the onset of alkali-feldspathoid crystallization, which deflects the residual silicate melt away from the two-liquid field. While evolving along the two-liquid field boundary, silicate melt increases in SiO2 from 46 to 51 wt% and decreases in CaO / (Na2O + K2O) (by weight) from 0.83 to 0.12, reproducing the observed compositional array of alkaline silicate subvolcanic rocks from Brava. The conjugate carbonatite melts are moderately alkaline Ca-carbonatites with 5 to 9 wt% SiO2 and CaO / (Na2O + K2O) that decreases from 5.64 at 1100°C to 1.60 at 950°C. These coexisting silicate and carbonatite melt compositions show that previous high-pressure experimental studies have not yielded truly conjugate melt pairs, mainly due to the use of modified starting materials resulting in melt compositions too enriched in alkalis to match the natural alkaline silicate rock record.

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.