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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 SiO.sub.2, Al.sub.2O.sub.3 and P.sub.2O.sub.5 by equation: [Formula omitted]where [Formula omitted] is a ratio of K and Fe mole fractions in the silica-rich (s) and Fe-rich (f) immiscible liquids: [Formula omitted] and [Formula omitted] is a difference in mole fractions of a network-forming oxide i between the liquids (s) and (f): [Formula omitted]. 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., Author(s): Alexander Borisov [sup.1], Ilya V. Veksler [sup.2] [sup.3] Author Affiliations: (1) grid.4886.2, 0000 0001 2192 9124, Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of [...]

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 SiO.sub.2 content at 55 wt.%, MgO at about 12 wt.% and Cr.sub.2O.sub.3 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 H.sub.2O (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 H.sub.2O component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products. The effective values of fO.sub.2 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, H.sub.2O additions in isothermal series of experiments increased fO.sub.2 by 0.8-1.6 log-units from the level in a nominally dry run. Overall, the experiments showed that H.sub.2O additions suppressed crystallization of silicates, shifted olivine-orthopyroxene peritectic equilibrium towards higher olivine stability (olivine first appears in H.sub.2O-bearing compositions at 1175 °C) and increased the proportion of magnetite component in spinel solid solution. The addition of 4 wt.% H.sub.2O 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 H.sub.2O 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., Author(s): Ilya V. Veksler [sup.1] [sup.2], Tong Hou [sup.3] [sup.4] Author Affiliations: (1) grid.23731.34, 0000 0000 9195 2461, GFZ German Research Centre for Geosciences, , Telegrafenberg, 14473, Potsdam, Germany (2) [...]

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 CaCO.sub.3 + CaF.sub.2 + Na.sub.2CO.sub.3 ± Ca.sub.3(PO.sub.4).sub.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, Ca.sub.3(PO.sub.4).sub.2 and CaF.sub.2 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., Author(s): Dmitry A. Chebotarev [sup.1] , Ilya V. Veksler [sup.1] [sup.2] [sup.3] , Cora Wohlgemuth-Ueberwasser [sup.2] , Anna G. Doroshkevich [sup.1] [sup.4] , Monika Koch-Müller [sup.2] Author Affiliations: (Aff1) 0000 [...]

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 [K.sub.2]O-FeO-[Fe.sub.2][O.sub.3]-[A1.sub.2][O.sub.3]-Si[O.sub.2]. We have also measured densities and surface tensions of the individual immiscible liquid phases. The measurements were carried out in air at 1500-1550[degrees]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% Si[O.sub.2] and 14 and 80 wt% FeOt, respectively, that have been shown to coexist in air at and above 1465[degrees]C. Interracial tension between the synthetic immiscible liquids decreases with increasing temperature from 16.4 [+ or -] 3.1 mN/m at 1500[degrees]C to 7.8 [+ or -] 1.1 mN/m at 1550[degrees]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 interracial tension implies easy nucleation of immiscible liquid droplets and very slow coarsening of resulting silicate emulsions. Keywords: Immiscibility, interface, tensiometry, nucleation DOI: 10.2138/am.2010.3456

Byline: Ilya V. Veksler (1), Alexander M. Dorfman (2,3), Leonid V. Danyushevsky (4), Jakob K. Jakobsen (5), Donald B. Dingwell (3) Abstract: This study investigates partitioning of elements between immiscible aluminosilicate and borosilicate liquids using three synthetic mixtures doped with 32 trace elements. In order to get a good spatial separation of immiscible liquids, we employed a high-temperature centrifuge. Experiments were performed at 1,050--1,150degC, 1 atm, in sealed Fe and Pt containers. Quenched products were analysed by electron microprobe and LA ICP-MS. Nernst partition coefficients (D's) between the Fe-rich and Si-rich aluminosilicate immiscible liquids are the highest for Zn (3.3) and Fe (2.6) and the lowest for Rb and K (0.4--0.5). The plots of D values against ionic potential Z/r in all the compositions show a convex upward trend, which is typical also for element partitioning between immiscible silicate and salt melts. The results bear upon the speciation and structural position of elements in multicomponent silicate liquids. The ferrobasalt--rhyolite liquid immiscibility is observed in evolved basaltic magmas, and may play an important role in large gabbroic intrusions, such as Skaergaard, and during the generation of unusual lavas, such as ferropicrites. Author Affiliation: (1) 4.1, GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany (2) Vernadsky Institute of Geochemistry, Kosygin Str., 19, 117975, Moscow, Russia (3) Geo & Umwelt, Universitat Munchen, Theresienstrasse 41, 80333, Munich, Germany (4) Centre for Ore Deposit Research and School of Earth Sciences, University of Tasmania, Private Bag 79, Hobart, TAS , 7001, Australia (5) Department of Earth Sciences, University of Aarhus, C.F. Mollers Alle 110, 8000, Aarhus C, Denmark Article History: Registration Date: 06/07/2006 Received Date: 20/09/2005 Accepted Date: 05/07/2006 Online Date: 10/08/2006 Article note: Communicated by J. Hoefs

This study investigates partitioning of elements between immiscible aluminosilicate and borosilicate liquids using three synthetic mixtures doped with 32 trace elements. In order to get a good spatial separation of immiscible liquids, we employed a high-temperature centrifuge. Experiments were performed at 1,050-1,150[degrees]C, 1 atm, in sealed Fe and Pt containers. Quenched products were analysed by electron microprobe and LA ICP-MS. Nernst partition coefficients (D's) between the Fe-rich and Si-rich aluminosilicate immiscible liquids are the highest for Zn (3.3) and Fe (2.6) and the lowest for Rb and K (0.4-0.5). The plots of D values against ionic potential Z/r in all the compositions show a convex upward trend, which is typical also for element partitioning between immiscible silicate and salt melts. The results bear upon the speciation and structural position of elements in multicomponent silicate liquids. The ferrobasalt-rhyolite liquid immiscibility is observed in evolved basaltic magmas, and may play an important role in large gabbroic intrusions, such as Skaergaard, and during the generation of unusual lavas, such as ferropicrites.

Aluminum orthoposphate (AlP[O.sub.4]) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to [alpha]-quartz. We report berlinite-quartz solid solutions to crystallize from a synthetic P-rich peraluminous granitic melt, similar in composition to the most volatile-rich silicate melt inclusions found in pegmatites. The crystallization took place in experiments performed in cold-seal pressure vessels at 450-700 [degrees]C and 0.1-0.2 GPa [H.sub.2]O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5-7 mol% Si[O.sub.2] on the phosphate side of the solvus and to the maximum of 1 mol% AlP[O.sub.4] on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinitequartz solid solutions are strongly zoned and show complex intergrowths between the P-rich and silica-rich phases. They were studied by electron microprobe, transmission electron microscopy, and Raman spectroscopy. In the light of our new experimental results, the extreme P enrichment reported earlier for some natural quartz-hosted melt inclusions may be explained as a post-entrapment contamination by the berlinite-bearing host.

Extreme enrichment in [H.sub.2]O, B, P and F is characteristic of many evolved granites and pegmatites. We report experimental phase relations of a synthetic peraluminous pegmatite spiked with [P.sub.2][O.sub.5], [B.sub.2][O.sub.3] and F (5 wt% of each), [Rb.sub.2]O, [Cs.sub.2]O (1 wt% of each) and [Li.sub.2]O (0.5 wt%). Experiments were carried out in [H.sub.2]O-saturated conditions in cold-seal rapid-quench pressure vessels at 0.1-0.2 GPa. Crystallisation starts at about 820 [degrees]C with berlinite and topaz. Quartz appears at 700-750 [degrees]C. Topaz is replaced by muscovite at about 600 [degrees]C. At near-solidus temperatures (450-500 [degrees]C) amblygonite, lacroixite and a Cs-bearing aluminosilicate crystallise. In all charges aluminosilicate melt coexists with low-density hydrous fluid and hydrosaline melt. The latter is strongly enriched in [Na.sub.3]Al[F.sub.6] and [H.sub.3]B[O.sub.3] components. Experimental evidence of the liquid immiscibility and mineral reactions documented in our study offers new explanations of many enigmatic features of natural pegmatites.

We present an experimental study of synthetic peraluminous granite doped with [H.sub.2]O, B, P, and F, which confirms that aluminosilicate melt, hydrous fluid, and hydrosaline melt (high-temperature brine) can stably coexist at 450-900 [degrees] C and 0.1-0.2 GPa in bulk compositions similar to those of natural granitic pegmatites. Hydrosaline melt is not quenchable, unstable at room conditions, and requires special techniques for synthesis and preservation. Raman spectroscopy and electron microprobe analyses of hydrosaline melt synthesized in our experiments show that it is composed of [H.sub.3]B[O.sub.3], [Na.sub.3]A1[F.sub.6], A1P[O.sub.4], [H.sub.2]O, and aluminosilicate components. Aluminosilicate melt saturated in both hydrosaline liquid and hydrous fluid at 850 [degrees] C and 0.2 GPa contains 3.6 wt% F, 4.2 wt% [P.sub.2][O.sub.5], and 4 wt% [B.sub.2][O.sub.3]. Natural hydrosaline melts have previously been found as inclusions trapped in rock-forming minerals. They are not restricted to granites and can be effective agents for enhanced crystal growth, metasomatism, and ore formation. In addition, hydrosaline melts may account for many characteristic features of rare-element and miarolitic pegmatites, such as giant size and perfect shapes of crystals in pegmatite cores, diverse mineralogy, and strong enrichment in rare elements.

Natrocarbonatite flows in the crater of the volcano Oldoinyo Lengai (Tanzania) are the only carbonatite magmas observed to erupt and have provided strong arguments in favor of a magmatic origin for carbonatite. The currently favored explanation for the genesis of these carbonatites by liquid immiscibility between a silicate and a carbonatite melt is questioned based on the extremely low eruption temperatures of 544-593 [degrees] C and compositional and mineralogical characteristics not in agreement with experimental constraints. Experimental investigations of the relationship between Oldoinyo Lengai natrocarbonatite and related silicate rock compositions do indicate that alkali-bearing peralkaline carbonatite with liquidus calcite can form by liquid immiscibility. At the same time, these experiments result in evidence which speaks against a liquid immiscibility origin for the highly alkaline and peralkaline Oldoinyo Lengai natrocarbonatite. On the carbonatite side of the miscibility gap, fractional crystallization cannot account for a liquid evolution from alkali-bearing peralkaline carbonatite to highly alkaline natrocarbonatite. Such an evolution does not seem to be compatible with the liquidus mineral assemblages and the chemistry of Oldoinyo Lengai natrocarbonatite. No natural silicate magma is known to produce natrocarbonatite compositions by liquid immiscibility. The best interpretation of the Oldoinyo Lengai natrocarbonatite flows involves expulsion of a cognate, mobile, alkaline, and C[O.sub.2]-rich fluid condensate. This conclusion is supported by recent studies of silicate and carbonatite melt inclusions in minerals of ultramafic alkaline complexes, trace element partitioning, isotopic constraints, and by experimental data on major element partitioning between coexisting [H.sub.2]O-C[O.sub.2]-rich fluid and carbonatitic melt. In contrast to all other suggested modes of formation, an origin of Oldoinyo Lengai natrocarbonatite from cognate fluid appears best to be in agreement with the field observations, the petrography, mineralogy, and geochemistry of Oldoinyo Lengai natrocarbonatite and the dynamics of the Oldoinyo Lengai natrocarbonatite extrusion.

Experimental studies of the element distribution between carbonatite melts and hydrous fluids are hampered by the fact that neither the fluid nor the melt can be isochemically quenched in conventional high-pressure vessels. In order to overcome this problem, we used a double-capsule technique to separate immiscible fluid and melt phases during and after the runs. The inner platinum capsules were charged with carbonate mixtures (CaC[O.sub.3], MgC[O.sub.3] and [Na.sub.2]C[O.sub.3]) and placed inside the outer capsules charged with distilled water and diamond powder. The latter was used as an inert trap for solids precipitating from the fluid on quenching. Carbonate melt and hydrous fluid equilibrated through a small hole left in the upper end of the inner capsule. The runs were performed in rapid-quench cold-seal pressure vessels at 0.1-0.2 GPa and 700-900 [degrees] C in the two-phase (fluid + melt) stability region. Both quenched melt and quenched fluid were dissolved in dilute HC1 and analysed by inductively coupled plasma atomic emission spectroscopy. The results show that under all conditions investigated, fluid/melt partition coefficients for Ca and Mg are similar and several times smaller than those for Na. At 0.1 GPa and a water/carbonatite ratio of 1 (by weight), the partition coefficients are [D.sub.Na] = 0.35 [+ or -] 0.02, [D.sub.ca] = 0.09 [+ or -] 0.02, and [D.sub.Mg] = 0.13 [+ or -] 0.01. Between 700 and 900 [degrees] C, the effect of temperature on partitioning is negligible. However, [D.sub.Na] increases significantly with decreasing water/carbonatite ratio in the system. Our data show that the release of a hydrous fluid enriched in sodium and simultaneous crystallisation of calcite can transform an alkaline, vapour-saturated carbonatite melt into a body of pure calcite surrounded by zones of sodium metasomatism. Thus, it is quite possible that carbonate magmas with substantial amounts of alkalies were common parental liquids of plutonic carbonatites.

A study was conducted to analyze the effect of volatiles on the crystal fractionation path of ultramafic alkaline magmatic systems at moderate pressure. It also aimed to determine the link between the stability of liquid phlogopite and the development of larnite normative melilitite melts which may form from the late-stage residual melts by phlogopite, amphibole, clinopyroxene and Fe-Ti oxide fractionation. However, the results were limited in explaining the effect of H2O-F ratio on the crystallization of silicate melts.