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Here we report the results of an experimental study aimed at testing the existence of stable, superliquidus immiscibility between silica- and Fe-rich multicomponent melts at temperatures above 1100 °C. Four pairs of the potentially immiscible compositions were tested in a 1-atm gas-mixing furnace (Ar/H2-CO2 gas mixture) at 1150 and 1200 °C and at the oxygen fugacity corresponding to that of the QFM buffer. Pre-synthesized pairs of the silica-rich and Fe-rich starting compositions were loaded in Pt wire loops, fused separately at 1300 °C, then brought in contact and kept at constant experimental temperature for more than 24 h. Three pairs of compositions out of four used in this study did not mix. Some temperature-dependent chemical re-equilibration was observed in the Fe-rich liquid phase but, in the cases of immiscibility, the two liquids remained compositionally distinct and showed sharp compositional gradients at contacts. One pair of liquids crystallized some tridymite, whereas the other compositions were clearly above the liquidus. Overall, the results of the experiments are in good agreement with the earlier centrifuge study and confirm the existence of stable, super-liquidus immiscibility in some Fe-rich basaltic-andesitic compositions at temperatures up to 1200 °C. [ABSTRACT FROM AUTHOR]

The Fanshan intrusion in the North China Craton (NCC) is concentrically zoned with syenite in the core (Unit 1), surrounded by ultramafic rocks (clinopyroxenite and biotite clinopyroxenite; Unit 2), and an outer rim of garnet-rich clinopyroxenite and orthoclase clinopyroxenite and syenite (Unit 3). The intrusive rocks are composed of variable amounts of Ca-rich augite, biotite, orthoclase, melanite, garnet, magnetite and apatite, with minor primary calcite. Monomineralic apatite rocks, nelsonite and glimmerite exclusively occur in Unit 2. Geochemically, the Fanshan rocks are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in high field strength elements (HFSE), and have a limited range of Sr-Nd-O isotopic compositions. The similar mineralogy, mineral compositions, and trace element characteristics of the three units suggest that all the rocks are co-magmatic. The parental magma is ultrapotassic and is akin to kamafugite. Very low-degree partial melting of metasomatized lithospheric mantle best explains the geochemistry and petrogenesis of the parental magmas of the Fanshan intrusion. We propose that the mantle source may have been metasomatized by a hydrous carbonate-bearing melt, which has imprinted the enriched Sr-Nd isotopic signature and incompatible element enrichment with conspicuous negative Nb-Ta-Zr-Hf-Ti anomalies and LREE enrichments. The mantle source enrichment may be correlated with oceanic sediment recycling during southward subduction of the Paleo-Asian oceanic plate during the Carboniferous and Permian. We propose that crystal settling and mechanical sorting combined with repeated primitive magma replenishment and mixing with previously fractionatedmagma is the predominant process responsible for the formation of the apatite ores. [ABSTRACT FROM AUTHOR]

Abstract: This experimental study examines the kinetics of silicate liquid immiscibility, and the effects of melt chemistry on phase separation. We used three synthetic compositions in the systems K2O–FeO–Al2O3–SiO2 and K2O–CaO–FeO–Al2O3–SiO2. Two of them lie inside the known miscibility gaps on the joins fayalite–orthoclase–silica and fayalite–hedenbergite–orthoclase–silica; the third one lies on the anorthite–fayalite cotectic, well outside any known region of stable or metastable unmixing. In order to distinguish between sub-micron exsolutions due to quenching and those that were formed by immiscibility above liquidus, we performed high temperature in situ centrifugation experiments in addition to conventional static runs. Both types of experiments were performed at the atmospheric pressure and super-liquidus temperatures. Run durations varied from a few hours to a few days, and melts were quenched to glasses in air at the end of each run. Exsolution textures were observed in all the glasses. Droplet sizes varied widely in the micron to sub-micron range, with the smallest exsolutions being visible only with the aid of a transmission electron microscope (TEM). Those immiscible droplets that settled over macroscopic distances during high temperature centrifugation were interpreted, regardless of their size, as products of stable unmixing. Of particular interest is the settling of very fine, 20–30 nm emulsion droplets in the composition on the anorthite–fayalite cotectic, in which stable unmixing was previously unknown. Our results show that unmixing of melts with high contents of Ca aluminosilicate components may develop slowly, even at high temperatures above liquidus, and remain in a latent, sub-micron form for a long time. Textures of quenched charges imply that interfacial energies between immiscible liquids are low, and this fact is likely to significantly impede the coarsening of the emulsions. Slow growth of immiscible droplets and the uncertainty about interpretation of very fine, sub-micron emulsions raise the possibility that regions of silicate immiscibility, including those in natural basaltic melts, may be broader than presently thought, with the implication that the petrogenetic role of immiscibility should be revisited. [Copyright &y& Elsevier]

Mafic–ultramafic layered rocks of the Lower and Critical Zones of the Bushveld Complex are believed to have formed from a parental magma usually labelled B1 with SiO2 content at 55 wt.%, MgO at about 12 wt.% and Cr2O3 concentration at 0.15 wt.%. Previous experimental study at 300 MPa and nominally dry conditions showed that this composition had orthopyroxene on liquidus between 1250 and 1300 °C, and the mineral was joined by plagioclase and clinopyroxene at and below 1166 °C. The objective of this study was to investigate the effects of variable additions of H2O (2, 4 and 6 wt.% in starting compositions) on liquidus and sub-liquidus phase equilibria. Main attention was given to the solubility of Cr-spinel and Cr distribution between melt and crystalline phases. Six experiments were carried out in the internally heated pressure vessel (IHPV) at 300 MPa and in the temperature interval 1100–1300 °C. Redox conditions were not controlled and thermal dissociation of the H2O component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products. The effective values of fO2 in experiments were calculated using SPINMELT 2.0 software bases on the regression of experimental data on the spinel-melt equilibrium. According to the calculations, H2O additions in isothermal series of experiments increased fO2 by 0.8–1.6 log-units from the level in a nominally dry run. Overall, the experiments showed that H2O additions suppressed crystallization of silicates, shifted olivine-orthopyroxene peritectic equilibrium towards higher olivine stability (olivine first appears in H2O-bearing compositions at 1175 °C) and increased the proportion of magnetite component in spinel solid solution. The addition of 4 wt.% H2O to the starting composition at 1200 °C resulted in complete melting of orthopyroxene, which left Cr-spinel as the only liquidus phase. Equal amount of H2O at 1125 °C leads to complete melting of plagioclase, orthopyroxene and clinopyroxene, which leaves only olivine and spinel stable and increases the liquid mass fraction from 0.62 in the nominally dry compositions to 0.9. The results of this study appear to support the idea that chromitite layers in the Critical Zone of the Bushveld Complex were formed by episodes of hydration melting of silicate cumulates. [ABSTRACT FROM AUTHOR]

There is abundant evidence for significant H2O in evolved melts from the platinum-rich UG2 chromitite and the Merensky Reef of the Bushveld Complex (South Africa), but there is no consensus about the source of H2O. We report triple-oxygen and hydrogen isotope ratios of interstitial, late-magmatic phlogopite from three localities of the UG2 layer. The phlogopite yielded dD values of -43‰ to -23‰, which is >30‰ higher than previously known from Bushveld rocks and far above the mantle values of ~-75‰. The phlogopite triple-oxygen isotope ratios are the first to be reported for Bushveld rocks, with values of Δ'17O0.5305 (17O excess relative to the reference line 0.5305) from -0.069‰ to -0.044‰ (d18O 5.2‰-6.2‰). The oxygen data support existing models of as much as 30%-40% contamination of mantle-derived magmas in the lower to middle crust. However, the high dD values require a second step of contamination, which we attribute to brines from the marine sediments in the Transvaal Basin at the emplacement level. [ABSTRACT FROM AUTHOR]

Here we present an experimental study of the distribution of a broad range of trace elements between carbonatite melt, calcite and fluorite. The experiments were performed in the CaCO3 + CaF2 + Na2CO3 ± Ca3(PO4)2 synthetic system at 650-900 °C and 100 MPa using rapid-quench cold-seal pressure vessels. Starting mixtures were composed of reagent-grade oxides, carbonates, Ca3(PO4)2 and CaF2 doped with 1 wt% REE-HFSE mixture. The results show that the distribution coefficients of all the analyzed trace elements for calcite and fluorite are below 1, with the highest values observed for Sr (0.48-0.8 for calcite and 0.14-0.3 for fluorite) and Y (0.18-0.3). The partition coefficients of REE gradually increase with increasing atomic number from La to Lu. The solubility of Zr, Hf, Nb and Ta in the synthetic F-rich carbonatitic melts, which were used in our experiments, is low and limited by crystallization of baddeleyite and Nb-bearing perovskite. [ABSTRACT FROM AUTHOR]

Interfacial tension between immiscible liquids is an important thermodynamic parameter of silicate melt unmixing and a property that determines the kinetics of phase separation. In this study, we present experimental measurements of interfacial tension between immiscible Fe-rich and silica-rich melts in the system K2O-FeO-Fe2O3-Al2O3-SiO2. We have also measured densities and surface tensions of the individual immiscible liquid phases. The measurements were carried out in air at 1500-1550 °C by the maximum detachment force method employing vertical cylinder geometry and using a gravimetric balance system. We have chosen the most oxidized and contrasting liquid compositions containing 73 and 17 wt% SiO2 and 14 and 80 wt% FeOt, respectively, that have been shown to coexist in air at and above 1465 °C. Interfacial tension between the synthetic immiscible liquids decreases with increasing temperature from 16.4 ± 3.1 mN/m at 1500 °C to 7.8 ± 1.1 mN/m at 1550 °C. Interfacial tension between natural, less compositionally contrasting ferrobasaltic and rhyolitic melts should be even lower by a factor of 2 or 3. Very low interfacial tension implies easy nucleation of immiscible liquid droplets and very slow coarsening of resulting silicate emulsions. [ABSTRACT FROM AUTHOR]