Immiscibility Field Research Articles

High‐temperature Mass Spectrometric Study and Modeling of the Thermodynamic Properties of the TiO2‐SiO2‐ZrO2 System

AbstractDevelopment of the advanced materials based on the TiO2‐SiO2‐ZrO2 system is hindered by lack of the information on its thermodynamic properties and phase equilibria. In the present study, the samples in the TiO2‐SiO2‐ZrO2 system were prepared by solid‐state synthesis and analyzed by X‐ray microanalysis, fluorescence, and diffraction techniques. The thermodynamic properties in this system were evaluated by Knudsen effusion mass spectrometric (KEMS) method and optimized within the generalized lattice theory of associated solutions (GLTAS). The main vapor species over the silica‐containing samples vaporized from the tungsten cell at the temperature 1973 K were SiO, O, WO2, and WO3. The vapor over the samples in the TiO2‐ZrO2 system at 2356 K was composed mainly of TiO, TiO2, O, WO2, and WO3. The activity of components in the samples and the partial pressures of the identified vapor species were determined. At 1973 K, the TiO2‐SiO2‐ZrO2 system is characterized by positive deviations from the ideality. The observed weak dependence of the SiO2 activity on the SiO2 content suggests the presence of a broad field of immiscibility on the phase diagram in the concentration range 40–100 mol . % SiO2. The thermodynamic data on the TiO2‐SiO2‐ZrO2 system obtained by KEMS can be of great value for calculation of the phase equilibria.

Carbonatite formation in continental settings via high pressure – high temperature liquid immiscibility

The goal of this study is to compare compositions of high temperature silicate-carbonatite immiscible melts, known from melt inclusions and experiments, to compositions of silica-undersaturated volcanic rocks from continental settings, in order to improve understanding of the formation of calcite carbonatite rocks worldwide. Melt inclusions in this study are abundant in perovskites crystals from magnetite-perovskite cumulates sampled at the Kerimasi volcano in the East African Rift System. The temperature of complete dissolution of daughter minerals in the melt inclusions and the high CO2−content of the silicate melt (5.4–9.8 wt%) support early formation of the rock and entrapment of melts at high temperatures (∼1100 °C) and pressures (≥1 GPa). Heated-quenched melt inclusions indicate the presence of immiscible mafic-melilitite and Ca-Na-K-carbonatite melts together with a fluid phase at entrapment. Melilitite melts are low in SiO2 (29.4–33.5 wt%), moderate in MgO (4.0–5.9 wt%) but high in CaO (16.3–24.4 wt%), FeOT (9.0–13.5 wt%), Na2O (6.8–12.7 wt%) and K2O (1.8–3.2 wt%). Coexisting carbonatite melts also show high CaO-content (28.4–39.0 wt%), along with moderate-to-high Na2O (8.2–20.2 wt%) + K2O (4.1–6.6 wt%) compositions (total alkalis ranges between 13.1 and 24.3 wt%).We compared the studied silicate melts in the inclusions with a global dataset of 146 continental melilitite and 640 nephelinite compositions (GEOROC database). We argue that the studied calcite-saturated melilitite melts formed in a continental rift setting and were able to exsolve carbonatite melts that crystallized voluminous calcite carbonatite rocks during their evolution. In contrast, magnesian melilitite and nephelinite volcanic rocks from intracontinental settings are compositionally far away from any immiscibility field at feasible pressures and were only able to unmix carbonatite melts during late-stage evolution, leaving little opportunity for calcite crystallization. CaO- and alkali-rich, but extremely SiO2-undersaturated melilitite melts play a key role in early silicate-carbonatite immiscibility, can only be preserved in melt inclusions and cannot be represented by volcanic rocks. The parental melts of the studied melilitite-carbonatite melts probably formed via low-degree partial melting of modally metasomatized continental lithosphere.

Petrogenesis of Early Cretaceous High Ba-Sr Granitoids in the Jiaodong Peninsula, East China: Insights into Regional Tectonic Transition

Element geochemistry, Sr and Nd isotope, and LA-ICP-MS zircon U-Pb isotope data have been obtained for the granitoids of Dazesan pluton in the Jiaodong Peninsula, East China, and their intermediate microgranular dark enclaves so as to reveal their petrogenesis and tectonic implications. These granitoids have high SiO2 (68.25–71.56 wt.%), K2O (3.44–5.50 wt.%), total alkalis (K2O + Na2O = 7.29–9.00 wt.%), Sr (451–638 ppm), Ba (1157–2842 ppm) and light rare earth elements (LREEs) (131.57–210.08), with strong depletion both in heavy rare earth element (HREE) and high field strength element (HFSE) concentrations as well as unclearly Eu anomalies, showing typical signatures of high Ba-Sr granitoids. They possess high (La/Yb)N (32–50) and Sr/Y (50–79) values and low MgO (0.76–1.11 wt.%), Cr (9.9–19.6 ppm) and Ni (4.51–7.04 ppm) concentrations. All the above geochemical compositions are similar to those of late Early Cretaceous granitoids, in combination with zircon LA-ICP-MS U-Pb ages of 119.6 ± 1.3 to 120 ± 1 Ma for these granitoids obtained in this study, indicating c. 120 Ma probably represents the lower limit of ages when late Early Cretaceous granitoids emplaced in the Jiaodong Peninsula. The microgranular dark enclaves, forming a linear trend with their host granitoids on the oxide against SiO2 plots, display higher MgO contents of 3.05–4.39 wt.% at lower SiO2 concentrations of 54.25–56.84 wt.% and possess a zircon LA-ICP-MS U-Pb age of 119 ± 2 Ma, identical to those of these granitoids, indicating the acid magma and intermediate magma were coeval. Furthermore, dark enclaves and their host granitoids have indistinguishable (87Sr/86Sr)i values of 0.709523–0.70972 and 0.709361–0.709858, respectively, and plot within a two-liquid immiscible field on the Greig pseudoternary phase diagram. In addition, they have markedly parallel REE patterns, with the dark enclaves having much greater REE and HFSE abundances than those of their host granitoids. Therefore, it is suggested that liquid immiscibility is a viable model to explain the chemical compositional variations between the Dazeshan granitoids and their dark enclaves. Based on the element geochemistry, geochronology and Sr- and Nd-isotope of the Dazeshan granitoids and their dark enclaves, it is envisaged the crust-derived acid melts due to partial melting of ancient continental lower crust in the Jiaodong Peninsula (mainly Neoarchean-Palaeoproterozoic basement in the Jiaobei terrane) containing a subduction-related material, resulting from the addition of the enriched subcontinental lithospheric mantle-derived melts, assimilated the lithospheric mantle-derived basic melts and formed the homogeneous magma chamber at the crust base, then split into two immiscible liquids, with one granitic liquid producing the Dazeshan granitoids and the other intermediate one forming the dark enclave during its ascent. Combined with previous studies, the identification of a lithospheric mantle-derived material in the Dazeshan granitoids suggests a catastrophic lithospheric thinning at c. 120 Ma, reflecting an abrupt change in the direction of Palaeo-Pacific plate subducting and the corresponding regional tectonic transition from E–W extension to NW–SE extension.

Fluid inclusion and stable isotope (C, O and S) constraints on the genesis of the high-grade Langdu Cu skarn deposit in Yunnan, SW China

The Langdu Cu skarn deposit is the highest grade Cu deposit (average 6.49% Cu) in the Zhongdian polymetallic district in SW China. Skarns and ore bodies occur primarily in the contact zone of Upper Triassic Qugasi Formation (Fm.) and Late Triassic (ca. 217 Ma) intrusions. Disseminated, massive, stockwork, and vein-like sulfide ores occur in the skarn, quartz-calcite veins, porphyry and marble. Skarn minerals include pyroxene, garnet, actinolite, and epidote. Four types (six subtypes) of fluid inclusions (FIs) are identified in the sulfide-bearing quartz veins. Type-I liquid-rich two phase FIs have varying vapor/liquid ratios, and yielded homogenization temperatures of 110–338 °C and salinities of 1.7–29.5 wt% NaCl eqv. Type-II vapor-rich FIs contain CH4-N2 bubbles and homogenized to vapor at 262–425 °C. Type-III FIs are single phase at room temperature (20 °C). A vapor phase is present at below −120 °C, and the FIs homogenized at −117 to −114 °C. Type-IV daughter mineral-bearing FIs homogenized by halite disappearance at 295–392 °C, and have salinities of 37.8–46.6 wt% NaCl eqv. Such characteristics suggest that FIs were trapped by fluid immiscibility and phase separation. The δ34SCDT values of chalcopyrite and pyrrhotite are of −5.3 to 0.7‰. On the δ34S histogram, all Langdu data show a cluster of 32S-rich (δ34SCDT = −5.3 to −4.6‰) and a cluster of 34S-rich (δ34SCDT = −1.1 to 0.7‰). Type-I calcite (in skarn) and type-II calcite (in quartz-sulfide vein) have δ13CPDB and δ18OSMOW values of −8.4 to −7.8‰, −6.3 to −6.1‰ and 10.1 to 12.3‰ and 12.0 to 13.0‰, respectively. Type-III calcite from calcite-sulfide veins has δ13CPDB = −5.6 to 0.2‰ and δ18OSMOW = 12.5 to 16.3‰. Biotite, garnet and pyroxene, syn-ore quartz, and chlorite yielded δ18OSMOW values of 6.9‰, 4.1–5‰, 13.2–15.3‰, and 6.8‰, respectively. Sulfur, carbon and oxygen isotopic features of these hydrothermal minerals and the FIs microthermometric data suggest that the ore-forming fluids were magmatic sourced and contaminated by meteoric/formation water during ascent and/or fluid/wallrock interactions. Low δ34S (=−5.3 to −4.6‰) in sulfides and the CH4 in the FIs are interpreted to reflect the reaction between the magmatic fluids and the carbonaceous slate in Qugasi Fm. The methane likely expanded the vapor/liquid immiscibility field and reduced the sulfates (SO42−) to sulfides (S2−). And extensive water/rock reaction and fluid mixing may have occurred, leading to the high-grade Cu mineralization at Langdu.

Package fluids. Part 5: The NaCl-H2O system in fluid inclusion research and applications of the software AqSo_NaCl (Bakker, 2018)

The software AqSo_NaCl (Bakker, 2018) can be used to characterize the properties of phases that may exist in the binary H2O-NaCl system up to 1000 °C and 500 MPa. The model replaces all available correlation equations and other equation of state that are restricted to narrow temperature, pressure, and composition intervals. Liquid-vapour, liquid-solid, and vapour-solid immiscibility fields emerge with increasing salinity at relative high temperatures, and already occur at mass fractions of a few μg per gram. The liquid-vapour immiscibility is split in two fractions in solutions between x(NaCl) = 2·10−5 and 0.00025, at salinities that cannot be detected by microthermometry. Properties such as density, molar volume, composition, and volume fraction of liquid, vapour, and solid phases in fluid inclusions can be calculated in heating and freezing experiments. Isochores can be calculated within the homogeneous fluid field, within immiscibility fields, and along the SLV (solid-liquid-vapour) curve. Iso-Th lines are obtained by modifying the total volume of fluid inclusion with the volumetric properties of quartz. Bulk properties of fluid inclusions can be directly calculated from microthermometric data.

Numerical Simulation of Multiphase Magnetohydrodynamic Flow and Deformation of Electrolyte–Metal Interface in Aluminum Electrolysis Cells

A computational fluid dynamics based multiphase magnetohydrodynamic (MHD) flow model for simulating the melt flow and bath–metal interface deformation in realistic aluminum reduction cells is presented. The model accounts for the complex physics of the MHD problem in aluminum reduction cells by coupling two immiscible fluids, electromagnetic field, Lorentz force, flow turbulence, and complex cell geometry with large length scale. Especially, the deformation of bath–metal interface is tracked directly in the simulation, and the condition of constant anode–cathode distance (ACD) is maintained by moving anode bottom dynamically with the deforming bath–metal interface. The metal pad deformation and melt flow predicted by the current model are compared to the predictions using a simplified model where the bath–metal interface is assumed flat. The effects of the induced electric current due to fluid flow and the magnetic field due to the interior cell current on the metal pad deformation and melt flow are investigated. The presented model extends the conventional simplified box model by including detailed cell geometry such as the ledge profile and all channels (side, central, and cross-channels). The simulations show the model sensitivity to different side ledge profiles and the cross-channel width by comparing the predicted melt flow and metal pad heaving. In addition, the model dependencies upon the reduction cell operation conditions such as ACD, current distribution on cathode surface and open/closed channel top, are discussed.

Statistical and analytical review of worldwide CO2 immiscible field applications

CO2 immiscible flooding is an important enhanced oil recovery (EOR) technology that has demonstrated great potential under varying reservoir and fluid conditions. This paper provides a comprehensive review of worldwide CO2 immiscible experiences by collecting and analyzing data of 41 field applications from more than 60 publications, including books, DOE reports, AAPG databases, Oil and Gas Journal surveys, field reports, and SPE publications. About 100 papers have been reviewed. Two major parts are included in this paper. The first part explores where CO2 immiscible could be applied, in which screening guidelines have been established and updated by applying statistical methods. Boxplots and histograms were used to detect special cases and to interpret the main distributions of reservoir/fluid properties. The second part discusses the influences of operation to the productions, the performances of each field, and the existing operational problems by using analytical methods, which include injection strategies, gas injection compositions, CO2 utilization, CO2 injection efficiency, incremental oil recovery, and incremental oil production rate per well. Results show that CO2 immiscible flooding could produce an additional 4.7%–12.5% of oil with 10.07 Mscf/stb average CO2 injection efficiency.

A stand-alone Co mineral deposit in northeastern Hunan Province, South China: Its timing, origin of ore fluids and metal Co, and geodynamic setting

The Hengdong cobalt (Co) deposit, located in northeastern Hunan Province of South China, is hosted by the low-grade metamorphic volcaniclastic sedimentary rocks of the early Neoproterozoic Lengjiaxi Group. The Co orebodies strictly controlled by the NE- to ENE-trending Changsha-Pingjiang deep fault zone (CPDFZ) and its secondary structures. Occurring in altered breccias and cataclasites with similar mineral assemblages, Co mineralization is characterized by zoned alteration with predominant silicification and chloritization proximal to the orebodies, and sericitization and carbonatization distal from the mineralization. The integrated field and microscope observations reveal three hydrothermal stages marked by quartz + pyrite + muscovite ± chalcopyrite of the early-stage mineralization (E-stage), quartz + polymetallic sulfides + chlorite of the middle-stage mineralization (M−stage), and quartz + chlorite + carbonate of the late-stage mineralization (L-stage). Muscovite from both the E-stage Co-bearing altered breccia and the CPDFZ mylonite yield 40Ar–39Ar plateau ages of 124.7 ± 0.6 Ma (1σ) and 130.3 ± 1.4 Ma (1σ), respectively, indicating an early Cretaceous mineralization likely associated with the last movement of CPDFZ strike-slip shearing. The δ34S values of pyrite and chalcopyrite ranging from −1.5 to −15.9% with a majority between −7.5 and −15.9%, and the lead isotope compositions of the pyrite (206Pb/204Pb = 18.156–18.761, 207Pb/204Pb = 15.645–15.662 and 208Pb/204Pb = 38.469–39.172) overlapping with those of upper crust, indicate a main crust-derived source. The chemical compositions of pyrite further indicate the ore fluids and metal Co of the Hengdong deposit are most likely linked to the meta-mafic and volcanic rocks of the Neoarchean to Paleoproterozoic Lianyunshan Group, but with a contamination by the wall rocks of Lengjiaxi Group and Lianyunshan granitoids. Fluid inclusion investigations from Hengdong deposit reveal the decreasing homogenization temperatures from 250 to 320 °C (peak of 280–300 °C) at the E stage, through ∼220–320 °C (peak of 270–300 °C) at the M stage, and to ∼150–230 °C at the L stage without obviously salinity (7.0–15 wt% NaCl equiv.) changed. In the M−stage, the presence of coexisting LV inclusions and V-rich inclusions with the similar homogenization temperature, consistent with the chlorite geothermometry data, is interpreted to be the result of fluid immiscibility, which was caused by cyclic pressure release during fault-zone movement. Combined with the Late Mesozoic tectonism of South China, the present data support the Hengdong deposit formed under an extension-associated tectonic regime most likely induced by slab roll-back of the subducted Paleo-Pacific Plate during the early Cretaceous. This extensional event not only caused the reactivation of the pre-existing structures as manifested by the CPDFZ characteristic of stress transformation from compression to extension but also likely resulted in the release of large amounts of Co-bearing ore fluids from the Proterozoic or older volcanogenic rocks. When the ore fluids migrated along the CPDFZ and its secondary faults, the decompression (adiabatic cooling) of the hydrothermal fluids shifted the ore fluid to the immiscibility field, significantly reduced the degree of cobalt undersaturation, and caused cobalt to precipitate, which finally formed the Hengdong Co mineral deposit.

Distribution of niobium and lanthanum between two immiscible melts in the system LaPO4–SiO2–NaF–Nb2O5–Fe2O3

The system LaPO4–SiO2–NaF–Nb2O5–Fe2O3 is characterized by immiscibility fields in the liquid state region. Addition of iron expands fields of immiscibility of melts and decreases the temperature of their coexistence. A fraction of 87–90% of niobium is extracted into iron silicate melt, and 92–98% of lanthanum is extracted into phosphate salt melt.

Deep differentiation of alkali ultramafic magmas: Formation of carbonatite melts

The study of melt microinclusions in olivine megacrysts from meimechites and alkali picrites of the Maimecha–Kotui alkali ultramafic and carbonatite province (Polar Siberia) revealed that the melt compositions corrected for loss of olivine due to post-entrapment crystallization of olivine on inclusion walls (differentiates of primary meimechite magma) match well to the composition of nephelinites and olivine melilitites belonging to carbonatite magmatic series. Modeling of fractional crystallization of meimechite magmas results in the high-alkali melt compositions corresponding to the silicate–carbonate liquid immiscibility field. The appearance of volatile-rich melts at the base of magma-generating plume systems at early stages of partial melting can be explained by extraction of incompatible elements including volatiles, by near-solidus melts at low degrees of partial melting, and meimechites are an example of such magmas. Subsequent accumulation of CO2 in the residual melt results in generation of carbonate magma.

Phase Equilibria of the Ternary Sn-Pb-Co System at 250°C and Interfacial Reactions of Co with Sn-Pb Alloys

The isothermal section of the ternary Sn-Pb-Co system at 250°C was experimentally determined through a series of the equilibrated Sn-Pb-Co alloys of various compositions. The equilibrium phases were identified on the basis of compositional analysis. For the Sn-Co intermetallic compounds (IMCs), CoSn3, CoSn2, CoSn and Co3Sn2, the Pb solubility was very limited. There exist five tie-triangle regions. The Co-Pb system involves one monotectic reaction, so the phase separation of liquid alloys near the Co-Pb side occurred prior to solidification. The immiscibility field was also determined. Additionally, interfacial reactions between Co and Sn-Pb alloys were conducted. The reaction phase for the Sn-48 at.%Pb and Sn-58 at.%Pb at 250°C was CoSn3 and CoSn2, respectively. Both of them were simultaneously formed in the Sn-53 at.%Pb/Co. The formed IMCs were closely associated to the phase equilibria relationship of the liquid-CoSn3-CoSn2 tie-triangle. Furthermore, with increasing temperatures, the phase formed in equilibrium with Sn-37 wt.%Pb was found to transit from CoSn3 to CoSn2 at 275°C. We propose a simple method of examining the phase transition temperature in the interfacial reactions to determine the boundaries of the liquid-CoSn3-CoSn2 tie-triangles at different temperatures.

Carbonate, silicate, and sulfide melts: heterogeneity of the UHP mineral-forming media in calc-silicate rocks from the Kokchetav massif

We present data on carbonatite, silicate, and sulfide melts and their immiscibility at different stages of ultrahigh-pressure metamorphism of rocks of the Kokchetav Massif (northern Kazakhstan). The identified silicate, silicate-carbonate, and sulfide inclusions are regarded as crystallization products of high-pressure melts. The detected reactionary garnet-K-feldspar-allanite-calcite symplectite structures as inclusions in garnet and as identical structures around it evidence that they resulted from carbonatite melt crystallization. Carbonate melting was probably triggered by the present free fluid phase (mostly H2O) and/or a high content of alkalies in the system. The coexistence of carbonate and silicate inclusions testifies to the immiscibility of carbonatite and silicate melts. The presence of K-cymrite in the polyphase inclusions indicates that the minimum pressure of silicate melt intake is ~4.5GPa. The maximum pressure of this intake is 6–7GPa at 1000–1100°C and corresponds to the peak of metamorphism of the Kokchetav Massif rocks. Most likely, the field of immiscibility of carbonatite and silicate melts lies within 4.5-7GPa and 950–1100°C. The carbonatite melt can dissolve up to 18wt.% SiO2, and the silicate melt, up to 4.5vol.% CaCO3.

Geochemical zoning and magnetic mineralogy at Fe,Ni‐alloy–troilite interfaces of three iron meteorites from Morasko, Coahuila II, and Mundrabilla

AbstractWe combined high‐resolution and space‐resolved elemental distribution with investigations of magnetic minerals across Fe,Ni‐alloy and troilite interfaces for two nonmagmatic (Morasko and Mundrabilla) IAB group iron meteorites and an octahedrite found in 1993 in Coahuila/Mexico (Coahuila II) preliminarily classified on Ir and Au content as IIAB group. The aim of this study was to elucidate the crystallization and thermal history using gradients of the siderophile elements Ni, Co, Ge, and Ga and the chalcophile elements Cr, Cu, and Se with a focus on magnetic minerals. The Morasko and Coahuila II meteorite show a several mm‐thick carbon‐ and phosphorous‐rich transition zone between Fe,Ni‐alloy and troilite, which is characterized by magnetic cohenite and nonmagnetic or magnetic schreibersite. At Morasko, these phases have a characteristic trace element composition with Mo enriched in cohenite. In both Morasko and Coahuila II, Ni is enriched in schreibersite. The minerals have crystallized from immiscible melts, either by fractional crystallization and C‐ and P‐enrichment in the melt, or by partial melting at temperatures slightly above the eutectic point. During crystallization of Mundrabilla, the field of immiscibility was not reached. Independent of meteorite group and cooling history, the magnetic mineralogy (daubreelite, cohenite and/or schreibersite, magnetite) is very similar to the troilite (and transition zone) for all three investigated iron meteorites. If these minerals can be separated from the metal, they might provide important information about the early solar system magnetic field. Magnetite is interpreted as a partial melting or a terrestrial weathering product of the Fe,Ni‐alloy under oxidizing conditions.

Crystallization paths in SiO2-Al2O3-CaO system as a genotype of silicate materials

The phases trajectories in the fields of primary crystallization of cristobalite (SiO2cr), tridymite (SiO2tr), mullite (3Al2O3-2SiO2) and in a field of liquid immiscibility are analyzed on a basis of computer model for T-x-y diagram of SiO2-Al2O3-CaO system. The concentration fields with unique set of microconstituents and the fields without individual crystallization schemes and microconstituents are revealed.

Crystallization paths in SiO2-Al2O3-CaO system as a genotype of silicate materials

The phases trajectories in the fields of primary crystallization of cristobalite (SiO2cr), tridymite (SiO2tr), mullite (3Al2O3-2SiO2) and in a field of liquid immiscibility are analyzed on a basis of computer model for T-x-y diagram of SiO2-Al2O3-CaO system. The concentration fields with unique set of microconstituents and the fields without individual crystallization schemes and microconstituents are revealed.

Flux of carbonate melt from deeply subducted pelitic sediments: Geophysical and geochemical implications for the source of Central American volcanic arc

We determined the fluid‐present and fluid‐absent near‐solidus melting of an Al‐poor carbonated pelite at 3–7 GPa, to constrain the possible influence of sediment melt in subduction zones. Hydrous silicate melt is produced at the solidi at 3–4 GPa whereas Na‐K‐rich carbonatite is produced at the solidi at ≥5 GPa for both starting compositions. At ≥5 GPa and 1050°C, immiscible carbonate and silicate melts appear with carbonate melt forming isolated pockets embedded in silicate melt. Application of our data to Nicaraguan slab suggests that sediment melting may not occur at sub‐arc depth (∼170 km) but carbonatite production can occur atop slab or by diapiric rise of carbonated‐silicate mélange zone to the mantle wedge at ∼200–250 km depth. Flux of carbonatite to shallower arc‐source can explain the geochemistry of Nicaraguan primary magma (low SiO2and high CaO, Ba/La). Comparison of carbonate‐silicate melt immiscibility field with mantle wedge thermal structure suggests that carbonatite might temporally be trapped in viscous silicate melt, and contribute to seismic low‐velocity zone at deep mantle wedge of Nicaragua.

ORIGIN OF PLATINUM-GROUP MINERALS IN MAFIC AND ULTRAMAFIC ROCKS: FROM DISPERSED ELEMENTS TO NUGGETS

Based on the studies of paragenetic assemblages of platinum-group minerals (PGM) from placers and chromitite ores associated with zonal-ring dunite intrusions in the Aldan Shield, on the Siberian platform, a novel model for the PGM formation is offered, which implies separation of a Cr-rich oxide liquid with significant concentration of the platinum-group elements (PGE) from a picritic magma. The presence of PGM alloys in sulfide-poor chromitite ores from layered intrusions and zonal gabbro–dunite massifs indicates that PGE mineralization is genetically related not just to sulfide formation but to chromite formation as well. A hypothetical (MgO + FeO + CaO) – SiO2 – (Cr2O3 + Fe2O3 + Al2O3) triangular diagram shows that fractional crystallization of olivine results in the enrichment of the residual melt in Cr, PGE, a range of metals, and easily fusible and volatile elements, and in magma separation into a light silicate-rich liquid and a dense PGE–Cr-rich oxide liquid. In fluid-saturated magma, a Cr-enriched oxide melt completely separates from a silicate melt, its crystallization temperature is lower, and the solubility of PGE is higher. It is likely that the presence of PGE in the Cr-rich oxide liquid takes the form of sybotactic domains, clusters surrounded by ligands of easily fusible and volatile elements, which prevents the PGE from removal via an untimely crystallization from the melt as micronuggets. Crystallization of a PGE-, Cr-rich oxide melt can be traced in the schematic chromite – Os – Pt ternary diagram proposed, which shows a wide field of immiscibility existing between chromite and the PGE. Owing to the low solubility of PGE in chromite, crystallization of the Cr-rich melt results in accumulation of PGE in the interstitial liquid. With IPGE prevailing over PPGE, the crystallization trend does not reach the immiscibility field, ending in synchronous crystallization of chromite and small grains of Pt–Ru–Ir–Os alloys. In case of Pt dominance, the crystallization trend reaches the immiscibility field, Pt gradually accumulates in the residual melt, with final crystallization occurring at a triple eutectic point, which creates favorable conditions for the formation of large Pt nuggets from tens of grams to a few kilograms in weight, which are characteristic only of PGE–chromite deposits of the Uralian–Alaskan and Aldanian types.

Thermodynamic properties of silicate glasses and melts: VII. System MgO-B2O3-SiO2

Processes of evaporation and thermodynamic properties of glasses and melts of the system MgO-B2O3-SiO2 in the range of 1550–1800 K were studied for the first time by high-temperature mass-spectrometry. The resulting activities and Gibbs energies of components and also corresponding excess values point to negative deviations from ideal behavior and to the absence of immiscibility fields in glasses and melts of the system under study in the studied temperatures and concentration intervals.

Peralkaline nephelinite–natrocarbonatite immiscibility and carbonatite assimilation at Oldoinyo Lengai, Tanzania

This study presents petrographic and compositional data for coexisting peralkaline silicate glass and quenched natrocarbonatite melt in nepheline phenocrysts from the 24 September 2007 and July 2008 eruptions of the natrocarbonatite volcano Oldoinyo Lengai (Tanzania). Data are also given for peralkaline residual glass in combeite nephelinite ash clasts occurring in the March–April 2006 large volume natrocarbonatite flow. These data are considered to demonstrate the occurrence of liquid immiscibility between strongly peralkaline Fe-rich nephelinite melt and natrocarbonatite at Oldoinyo Lengai. Compositional data for coexisting silicate–carbonate pairs in conjunction with previous experimental studies suggest that the size of the field of liquid immiscibility for carbonated nephelinitic magmas is a function of their peralkalinity. It is shown that peralkaline combeite wollastonite nephelinite was present at Oldoinyo Lengai prior to, and during, the 24 September 2007 ash eruption. It is postulated that the driving force for this major eruption was assimilation and decomposition of previously emplaced solid natrocarbonatite. Assimilation resulted in the formation of the unusual hybrid nepheline–andradite–melilite–combeite–phosphate magma represented by the 24 September 2007 ash.

Phase diagrams for the [Th(NO3)4(TBP)2]-decane-[UO2(NO3)2(TBP)2] liquid ternary system

We report a phase diagram (on the mole fraction scale) for the [Th(NO3)4(TBP)2]-decane-[UO2(NO3)2(TBP)2](1-2-3) ternary liquid system, where TBP stands for tributyl phosphate, at T = 298.15 K. This system is characterized by a homogeneous solution field and a two-liquid field (immiscibility field); one phase (phase I) is enriched in [Th(NO3)4(TBP)2] and [UO2(NO3)2(TBP)2], and the other (phase II) is enriched in decane. Molecular interaction parameters and excess Gibbs energies G ex were calculated for the binary systems and the ternary liquid system along the binodal curve proceeding from miscibility in the [Th(NO3)4(TBP)2]-decane system and the ternary system and using equations of the NTRL model. For the ternary system, G ex > 0. G ex decreases in the following order of pairs of liquids: (1, 2) > (2, 3) > (1, 3).