Search

Discordant iron-rich ultramafic pegmatites (IRUPs) intersect the UG2 chromitite at many places in the Bushveld Complex. The effects of IRUP interactions on the UG2 ore mineralogy and PGE grade are assessed at the Thaba mine, north-west Bushveld, based on a borehole profile through the UG2 layer and detailed analysis of mineral textures and compositional variations across the UG2-IRUP contacts using micro-XRF element mapping. The UG2-IRUP interaction operated at different scales and probably by different mechanisms. At the local scale (2 and lower Mg#, and there is an anomalously low abundance of interstitial plagioclase. The IRUP-affected UG2 shows relatively abundant secondary hydrous silicates, replacement of PGE sulfides by PGE alloys and PGE-As–Sb–Bi–Te–Pb phases, and formation of secondary Ni–Cu–Fe sulfides after pentlandite and chalcopyrite. These large-scale effects are attributed to hydrothermal fluids derived from IRUP melts. The IRUP bodies at the Thaba mine caused redistribution of PGE within the UG2 layer but did not significantly reduce the overall grade. However, significant changes in the ore mineral assemblage and an increased abundance of secondary silicates can reduce the efficiency of PGE recovery.

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 δD 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‰ (δ18O 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 δD values require a second step of contamination, which we attribute to brines from the marine sediments in the Transvaal Basin at the emplacement level.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 δD 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‰ (δ18O 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 δD values require a second step of contamination, which we attribute to brines from the marine sediments in the Transvaal Basin at the emplacement level.

The Main Zone of the Bushveld Complex in South Africa is the most voluminous but least studied part of the world’s largest igneous intrusion. Modal layering is poorly developed compared with the units above and below (Upper and Critical Zones, resp.), and most of the ca. 3000 meter-thick Main Zone consists of monotonous gabbronorite, occasionally grading into norite and anorthosite. An exception is the ultramafic “Pyroxenite Marker” near the top of the Main Zone, which is present regionally in the complex and represents a major event of magma recharge into the chamber. However, studies of drillcore through the Main Zone in the Bushveld Northern limb (Ashwal et al., 2005; Hayes et al., 2017) found evidence for layering by periodic variations in rock density at vertical length-scales of 40 to 170 m. This implies there were many more episodes of magma recharge than previously thought.Our study in the Eastern Limb of the complex tests if cryptic layering in the Main Zone is a local phenomenon or is regionally developed like the Pyroxenite Marker. The first step, reported here, was a vertical profile of bulk density data (Archimedes method) for a 1450 m section of the upper Main Zone below the Pyroxenite Marker. Samples were taken at 1 to 5 m intervals and the results show several intervals of density variations at length-scales of 30 to 120 m, comparable to those previously described in the Northern Limb. Periodicity in density changes is not so well developed as in the earlier study, and we identified several 50 to 75 m intervals where density variations are below 0.05 g/cm3. The second step of the study will use multispectral and laser-induced breakdown spectroscopy (LIBS) scanning to provide modal mineralogy profiles of the same drillcore samples used for density measurement. After cryptic modal layering is documented in this way, follow-up petrologic-geochemical studies at the layer boundaries will aim to characterize the composition and temperature of the magmas involved.For this project the Bushveld Complex Drilling Project (BVDP) provided access to the BH7771 borehole, donated by Impala Platinum’s Marula mine.References:Ashwal, L..D., Webb, S.J. and Knoper, M.W. (2005) S. Afr. Jour. Geol., 108, 199-232.Hayes, B., Ashwal, L.D., Webb, S.J. and Bybee, G.M. (2017) Contrib. Mineral. Petrol., 172, 13.

Fluorite mineralization associated with different types of magmatism is common in the Western Transbaikalia. This study deals with Arshan, Yuzhnoe, and Ulan-Ude fluorite occurrences, which are the most significant examples of carbonatite-related fluorite mineralization in the region. The present paper focused on new fluorite geochemistry and fluid inclusion data, is aimed at revealing conditions of the fluorite mineralization formation, highlighting its genetic relationship with magmatism, compared to other deposits of this type. All the three locations belong to the Late Mesozoic Central Asian carbonatite province. Fluorites here are characterized by high rare earth elements (REE), Sr, and elevated La/Yb values. Fluid inclusions data imply that the formation of fluorite mineralization is a long process extending from late magmatic to the hydrothermal stage. Early fluorite crystallized from sulfate-carbonate orthomagmatic fluids at temperatures up to 500 °C. True hydrothermal fluorite was formed from the same fluids that were probably mixed with meteoric waters, which caused the temperature to drop to below 420 °C and led to an increase in the chloride component. The REE compositions of fluorite from the studied locations are similar to compositions of REE-rich fluorites from carbonatite-related deposits around the world.

There is still debate whether Large Igneous Provinces (LIPs) are caused by high mantle temperatures induced by thermal plumes or by other factors that enhance melt production from the mantle. A prerequisite for assessing the thermal plume model is a reliable estimate of liquidus temperatures of the magmas produced, preferably based on more than one method of geothermometry. The study reported here compares multiple geothermometers for the Etendeka LIP, which is among the largest Phanerozoic examples and one that shows several features suggestive of a plume origin (continental flood basalt province linked via an age-progressive volcanic ridge to an active hotspot). Magnesium (Mg)-rich magmas emplaced as dikes in NW Namibia are the most primitive rocks known from this province and are thus best suited to determine the composition and melting conditions of their mantle source. Earlier studies of the Etendeka Mg-rich dikes reported high liquidus temperatures based on olivine-melt Mg–Fe equilibria. We extend that work to a larger set of samples and compare the results of olivine-melt Mg–Fe thermometry with other methods based on spinel-melt and spinel–olivine equilibria (Al-in-olivine thermometry), as well as olivine-melt trace-element exchange (Sc/Y thermometry and V oxybarometry). All methods used the same starting assumptions of nominally anhydrous melts and a crystallization pressure of 0·5 GPa. Only mineral-melt or mineral-mineral pairs consistent with compositional equilibrium were used for calculating temperatures. The trace-element compositions of olivine are also used to discuss the relative proportion of peridotite and pyroxenite in the mantle source for these magmas. Twelve dike samples were studied, with whole-rock MgO concentrations ranging from 8·4 to 19·4 wt %. Diagnostic element ratios of transition metals in olivine (e.g., Mn/Fe, Mn/Zn, Zn/Fe) indicate a peridotite-dominated mantle source for the magmas, which is consistent with the other indicators based on whole-rock data e.g., 10 000×Zn/Fe, CaO–MgO trend, FeO/MnO and FC3MS (FeO/CaO–3×MgO/SiO2). The temperature variations show a positive correlation with the Fo-content of host olivines, and values from high-Fo olivine agree well with olivine and spinel liquidus temperatures calculated from thermodynamic models of bulk-rock composition. All methods and most samples yielded a temperature range between 1300 °C and 1400 °C. An exceptional few samples returned temperatures below 1300 °C, the minimum being 1193 °C, whereas several samples yielded temperatures above 1400 °C, the upper range being 1420–1440°C, which we consider to be a robust estimate of the maximum liquidus temperatures for the high-Mg magmas studied. The conversion to mantle potential temperatures is complicated by uncertain depth and degree of melting, but the functional relationship between Tp and primary melt MgO contents, using melt inclusions from olivine phenocrysts with of Fo > 90, indicate a Tp range from 1414 to 1525 °C ( 42 °C), which is 100–150°C higher than estimates of ambient upper mantle Tp in the South Atlantic today.

With a homogenization experiment (700°C, 2 kbar) on melt inclusions in polished thick sections of emeralds from the Habachtal, it could be demonstrated that the primary crystallization of emerald started at significantly higher temperatures than previously assumed. This experiment confirms the prediction in the precursor publication on this subject. Supercritical conditions around 700°C and 5 kbar fix the start point for the emerald crystallization.

The Upper Group 2 (UG2) chromitite layer in the upper Critical Zone of the Bushveld Complex, South Africa, is the world’s largest PGE orebody. The UG2 is typically 0.5 to 1.5 m thick and it consists of 75–90 modal % chromite with interstitial silicate and sulfide minerals. Although a minor component, phlogopite is important because it is a hydrous phase. It has been noted that the UG2 chromitite contains more abundant phlogopite (locally > 1 modal %) than the surrounding pyroxenite layers (Mathez and Mey, 2005). More and more studies suggest that water plays an important role in the UG2 chromite formation and in PGE enrichment or remobilization (e.g., Li et al., 2004; Mathez and Mey, 2005; Schannor et al., 2018). The source of the water is controversial, and this motivated our ongoing study of hydrous minerals in the UG2.We determined the chemical composition and hydrogen isotope ratio of phlogopite from the chromitite layer and from the surrounding pyroxenite in drill cores from two sites the eastern and western Bushveld (Nkwe and Khuseleka, respectively). The δD values of phlogopite in chromitite from the eastern site are -38.2 to -25.5‰ (mean = -29.7‰, n = 6). The corresponding values from the western site are similar, with -34.6 to -31.6‰ (mean = -33.2‰, n = 6). The δD values of phlogopite from pyroxenite are more variable, ranging from -43.1 to -26.1‰ for the eastern site (mean = -32.9‰, n = 4) and from -38.7 to -26.1‰ for the western site (mean = -31.7‰, n = 3).Published whole-rock δD values for silicate cumulate rocks in the upper Critical Zone are -93 to -55‰ (Mathez et al., 1994), which are similar to mantle values (-70±10%; Boettcher and O'neil, 1980) and are interpreted as magmatic. In comparison, our δD values of phlogopite from UG2 are much higher and suggest a significant contribution of crustal fluids. Harris and Chaumba (2001) estimated a δD value of -22‰ for paleo-meteoric water in the Bushveld Complex. Given the relative homogeneity of the phlogopite δD data in both sites of the complex, and the primary appearance of the grains in thin section, we argue that the crustal fluids were incorporated in the magma before the crystallization of the UG2 layer. Triple oxygen isotopes will test our hypothesis further. References: Boettcher & O'neil (1980) Amer. Jour. Sci. 280A, 594–621. Harris & Chaumba (2001) J. Petrol. 42, 1321–1347. Li et al. (2004) Econ. Geol. 99, 173–184. Mathez et al. (1994) Econ. Geol. 89, 791–802. Mathez & Mey (2005) Econ. Geol. 100, 1616–1630. Schannor et al. (2018) Chem. Geol. 485, 100–112.

Variations of mineral chemistry and whole-rock compositions were studied in detail, at millimetre to centimetre intervals, in two vertical drill core profiles through the platiniferous UG2 chromitite layer in the western and eastern limbs of the Bushveld Complex, South Africa. Analytical methods included electron microprobe and LA-ICP-MS analyses of the main rock-forming minerals, orthopyroxene, plagioclase and interstitial clinopyroxene. One profile was also studied by synchrotron-source XRF. Statistical analysis of crystal size distribution of chromite was also performed at different levels in the chromitite layer and in adjacent silicate rocks. The results provide new evidence for chemical and textural late magmatic re-equilibration in the UG2 layer and in the silicate rocks at the contact zones. The chromite crystal size distributions imply extensive coarsening of that mineral within the main chromitite seam, which has erased any textural evidence of primary deposition features such as recharge or mechanical sorting of crystals, if those features originally existed. The mineral compositions in chromitite differ from those in adjacent silicate rocks, in general agreement with predictions of chemical re-equilibration with evolved, residual melt (the trapped liquid shift effect). In detail, the geochemical data imply, however, that the conventional trapped liquid shift model has shortcomings, due to the effects of material transport driven by chemical gradients between modally contrasting layers of crystal mush undergoing re-equilibration reactions. In the presence of such gradients, selective open-system conditions may hold for alkalis and hydrogen because of their higher diffusion rates in silicate melts. Differential mobility of components in the interstitial melt can also sharpen the original modal layering by causing minerals to crystallise in one layer and dissolve in another. Detailed trace element profiles by synchrotron XRF reveal an uneven vertical distribution of incompatible elements which implies that the permeability of the chromitite layer may have been significant, even at the latest stages of interstitial crystallization.

A detailed study of the Sr- and O- isotopic composition of minerals in the Upper Group 2 (UG2) unit of the eastern Bushveld Complex revealed significant variations in the pegmatoidal footwall pyroxenite and the UG2 chromitite layer itself. Initial 87Sr/86Sr ratios (Sri) of interstitial plagioclase in the footwall rocks are significantly higher than in adjacent rocks of the Critical Zone. The δ18O values of plagioclase-pyroxene and plagioclase-chromite mineral pairs in footwall pyroxenite and chromitite are consistent with oxygen isotope exchange from crystallization temperatures down to 700 °C and 800 °C, respectively, whereas in other parts of the UG2, plagioclase and pyroxene δ18O values are consistent with O isotope equilibration at liquidus crystallization temperatures of approximately 1150 °C. We propose that the higher initial 87Sr/86Sr of plagioclase in the UG2 chromitite and the footwall pyroxenite reflects infiltration of fluid derived from dehydrating country rocks beneath the Bushveld Complex. Fluids could have been introduced either late or during chromitite crystallization. The chromitite layer and its pegmatoidal footwall could have acted as barriers to entrap late stage crustal fluids. Alternatively, reaction of fluids with the chromite-bearing crystallizing mush layer would induce hydration melting of pyroxene and plagioclase supplying Cr and Al to the melt, thus invoking further chromite formation. This dissolution-reaction releases Cr from pyroxene, which along with pre-existing disseminated chromite could refine the chromitite layers. The pegmatoidal UG2 footwall constitutes the reaction front-line of this process, which recrystallized to a coarse-grained rock upon cooling. This study emphasizes the role of crustal fluids and late- and post-magmatic processes in layered intrusions taking place at subsolidus temperatures, whereby isotopic and chemical compositions may be significantly changed.

© 2017 Elsevier B.V. Phase equilibrium experiments have been performed on an extremely high-Ti (5.4 wt.% TiO2) picrite from the base of the Paleogene (~55 Ma) East Greenland Flood Basalt Province. This sample has a high CaO/Al2O3 ratio (1.14), a steep rare-earth elements (REE) profile, is enriched in incompatible trace elements, and is in chemical equilibrium with highly primitive olivine. This all suggests that the picrite is a near-primary melt that did not suffer major chemical evolution during ascent from the mantle source and through the crust. Near-liquidus phase relations were determined over the pressure range of 1 atm, 1 to 1.5 GPa and at temperatures from 1094 to 1400°C. They provide an important constraint on the petrogenesis of these lavas. The high-Ti picritic melt is multi-saturated with olivine (Ol) + orthopyroxene (Opx) at 1 GPa but has only Ol or Opx on the liquidus at lower and higher pressures, respectively. This indicates the primitive melt was last equilibrated with its mantle source at relatively shallow pressure (~ 1 GPa). Melting probably started at 2-3 GPa and the picritic melt was produced by 15-30% melting of the mantle source. Such a degree of partial melting requires a mantle with a high potential temperature (1480-1530˚C). The relatively low CaO content and high FeO/MnO ratios of the most primitive East Greenland picrites, the high Ni content of olivine phenocrysts and the presence of low-Ca pyroxene (i.e., pigeonite) at high pressure in our experiments all suggest that the mantle source contained a major component of garnet pyroxenite. Residual garnet in the source could adequately explain the low Al2O3 content (7.92 wt.%) and steep REE patterns of the picrite sample. However, simple melting of a lherzolitic source, even with a major pyroxenite component, cannot explain the formation of magmas with the very high Ti contents observed in some East Greenland basalts. We therefore propose that magmas highly-enriched in Ti were produced by melting of a metasomatized mantle source containing Ti-enriched amphibole and/or phlogopite. ispartof: LITHOS vol:300 pages:20-32 status: published

The nature of petrogenetic links between carbonatites and associated silicate rocks is a matter of discussion for several decades and still remains controversial among igneous petrologists. The Belaya Zima plutonic complex in southern Siberia is a typical intrusion, in which carbonatites are spatially and temporally associated with the ijolite series rocks and nepheline syenites. In this study we use whole-rock compositions and trace element characteristics of the major and accessory minerals for reconstructing the magmatic evolution of the complex and clarifying the origin of carbonatites. We conclude that the observed gradual increase of Zr, Nb and REE concentrations in the magma is consistent with the process of extensive fractional crystallization and gradual transition from silicate rocks to carbonatites, and inconsistent with the formation of carbonatites by liquid immiscibility. Magma differentiation by fractional crystallization continued during the carbonatite formation. The textural and analytical evidence indicates that the early calcitic carbonatites evolved to dolomitic and ankeritic carbonatites. In addition, maximum Nb and Zr concentrations occur in the calcitic carbonatite, whereas the REE content increases in the dolomitic and ankeritic facies.

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.

© 2016 Elsevier B.V.. The Bushveld Complex (South Africa) is the largest layered intrusion on Earth and plays a considerable role in our understanding of magmatic differentiation and ore-forming processes. In this study, we present new geochemical data for apatite-hosted multiphase inclusions in gabbroic cumulates from the Bushveld Upper Zone. Inclusions re-homogenized at high-temperature (1060-1100 °C) display a range of compositions in each rock sample, from iron-rich (35 wt.% FeOtot; 28 wt.% SiO2) to silica-rich (5 wt.% FeOtot; 65 wt.% SiO2). This trend is best explained by an immiscible process and trapping of contrasted melts in apatite crystals during progressive cooling along the binodal of a two-liquid field. The coexistence of both Si-rich and Fe-rich immiscible melts in single apatite grains is used to discuss the ability of immiscible melts to segregate from each other, and the implications for mineral and bulk cumulate compositions. We argue that complete separation of immiscible liquids did not occur, resulting in crystallization of similar phases from both melts but in different proportions. However, partial segregation in a crystal mush and the production of contrasting phase proportions from the Fe-rich melt and the Si-rich melt can be responsible for the cyclic evolution from melanocratic (Fe-Ti-P-rich) to leucocratic (plagioclase-rich) gabbros which is commonly observed in the Upper Zone of the Bushveld Complex where it occurs at a vertical scale of 50 to 200 m. ispartof: EARTH AND PLANETARY SCIENCE LETTERS vol:443 pages:108-117 status: published

Small-scale variations in mineral chemistry, textures and platinum group element (PGE) mineralization were investigated in the Lower and Middle Group chromitite layers LG6, LG6a, MG1, MG2, MG2 II from vertical drill core profiles at the Thaba mine in the northwestern limb of the Bushveld Complex. We present detailed geochemical profiles of chromite composition and chromite crystal-size distribution curves to shed light on the processes of chromite accumulation and textural modification as well as mineralization. Multiple samples within each layer were assayed for platinum-group element concentrations and the respective platinum-group mineral association was determined by mineral liberation analysis (MLA). There is strong evidence for post-cumulus changes in the chromitites. The crystal size distribution curves suggest that the primary chromite texture was coarsened by a combination of adcumulus growth and textural equilibration, while compaction of the crystal mush played only a minor role. Mineral compositions were also modified by post-cumulus processes, but because of the very high modal amount of chromite and its local preservation in orthopyroxene oikocrysts, that phase retained much primary information. Vertical variations of chromite composition within chromitite layers, and from one layer to another do not support the idea of chromite accumulation from crystal-rich slurries nor crystal settling from a large magma chamber. Instead, we favor a successive buildup of chromitite layers by repeated injections of relatively thin layers of chromite-saturated magmas, with in-situ crystallization occurring at the crystal mush-magma interface. The adcumulus growth of chromite grains to form massive chromitite required addition of Cr to the layers, which we attribute to downward percolation from the overlying magma. The PGE concentrations are elevated in all chromitite layers compared to adjacent silicate rocks, and show a systematic increase upwards from LG6 (ave. 807 ppb Ir+Ru+Rh+Pt+Pd+Au) to MG2 II (ave. 2062 ppb). There are also significant internal variations in all layers, with enrichments at hanging and/or footwalls. The enriched nature of chromitites in PGE compared to host pyroxenites is a general feature, independent of the layer thickness. The MLA results distinguish two principal groups of PGE mineral associations: the LG6, LG6 and MG1 are dominated by the malanite series, laurite and PGE sulfarsenides, while the MG2 and MG2 II layers are characterized by laurite and PGE sulfides, as well as Pt-Fe-Sn and PGE-Sb-Bi-Pb alloys. Differences in the PGE associations are attributed to post-cumulus alteration of the MG2 and MG2 II layer, while the chromitites below, particularly LG6 and LG6a, contain the primary association.

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.

The Upper Critical Zone (UCZ) of the Bushveld Igneous Complex displays spectacular layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence of silicate cumulates in the order, from bottom to top, pyroxenite–norite–anorthosite. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses of chromite and silicate minerals in layers between the UG2 chromitite and the Merensky Reef reveal variations in major and trace element compositions that defy explanation with existing models of cumulate mineral– melt evolution. The anomalous features are best developed at sharp contacts of chromitite with adjacent anorthosite and pyroxenite cumulates. Here, chromite compositions change abruptly from high and constant Mg/(Mg þ Fe 2þ ) and Fe 2þ /Fe 3þ ratios in chromitite layers to variable and generally lower values in chromite disseminated in silicate layers. Furthermore, the composition of disseminated chromites varies depending on the host silicate assemblage; for example, in Ti, V and Zn contents. Importantly, the abrupt change in chromite composition across the chromitite–silicate layer contacts is independent of the thickness of the chromitite layer and the estimated mass proportions of chromite to intercumulus liquid. Chemical variations in plagioclase are also abrupt and some are hard to reconcile with conventional models of re-equilibration with intercumulus liquid. Among those features is the decoupling of alkalis from other incompatible lithophile elements. In comparison with cumulus plagioclase, intercumulus poikilitic plagioclase in chromitite layers is enriched in rare earth elements but strongly depleted in equally incompatible Li, K and Rb. Strong alkali depletion is also observed in intercumulus pyroxene from ultramafic cumulates and chromitite layers. To explain these features, we propose a new model of post-cumulus recrystallization, which intensifies the modal layering in the crystal–liquid mush, producing the observed sequence of nearly monomineralic layers of chromitite, pyroxenite and anorthosite that define the cyclic units. The crucial element of this model is the establishment of redox potential gradients at contacts between chromite-rich cumulates and adjacent silicate layers owing to peritectic reactions between the crystals and intercumulus melt. Because basaltic melts are ionic electrolytes with Na þ as the main charge carrier, the redox potential gradient induces electrochemical migration of Na þ and other alkali ions. Selective mobility of alkalis can explain the enigmatic features of plagioclase composition in the cyclic units. Sodium migration is expected to cause remelting of previously formed cumulates and major changes in modal mineral proportions, which may eventually result in the formation of sharply divided monomineralic layers. The observed variations in ferric/ferrous iron ratios in chromite from the cyclic units and Fe distribution in plagioclase imply a redox gradient of

Here we report the results of an experimental study aimed at testing the existence of stable, super-liquidus 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.

The origin of iron oxide-apatite deposits is controversial. Silicate liquid immiscibility and separation of an iron-rich melt has been invoked, but Fe–Ca–P-rich and Si-poor melts similar in composition to the ore have never been observed in natural or synthetic magmatic systems. Here we report experiments on intermediate magmas that develop liquid immiscibility at 100 MPa, 1000–1040 °C, and oxygen fugacity conditions (fO2) of ∆FMQ = 0.5–3.3 (FMQ = fayalite-magnetite-quartz equilibrium). Some of the immiscible melts are highly enriched in iron and phosphorous ± calcium, and strongly depleted in silicon (, The origin of iron oxide-apatite deposits remains enigmatic and controversial. Here, the authors perform experiments on intermediate magmas and show that increasing aH2O and fO2 enlarges the two-liquid field thus allowing the Fe–Ca–P melt to separate easily from host silicic magma and produce iron oxide-apatite ores.

Models for the nelsonite formation are currently highly contentious, with liquid immiscibility and fractional crystallization as frequently proposed formation mechanisms. The nelsonites in the Damiao massif anorthosite complex in the North China Craton are revisited here together with experimental evidence for the existence of silica-free CaO-FeO-Fe 2 O 3 -TiO 2 -P 2 O 5 immiscible nelsonitic liquids. Our results of differential scanning calorimetry (DSC) and internally heated pressure vessel (IHPV) demonstrate that nelsonite with the composition of one-third apatite and two-thirds Fe-Ti oxides by weight 1) completely melts well above 1450 °C at dry condition, which is in good agreement with numerous experimental studies of the CaO-P 2 O 5 -Fe x O system for metallurgical purposes; 2) does not melt at the temperature up to 1200 °C with presence of considerable amount of volatiles, e.g., fluorine and water. Therefore, the composition of the nelsonite cannot be molten at temperatures relevant for crystallization of the Damiao magma. A review of experimental studies of liquid immiscibility and analyses of natural immiscible glasses show that all the liquids on the Fe- and P-rich side of the miscibility gap have at least 20 wt.% of aluminosilicate components. Analyses of apatite-hosted melt inclusions from Damiao nelsonite provide further constraints on the nelsonite formation. The inclusions range from ~ 3 to 200 μm in diameter. They are ubiquitous and meet all the morphological criteria of primary melt inclusions crystallized into assemblages of daughter minerals. Almost all assemblages contain vermiculite and/or chlorite, and some contain biotite, amphibole and Fe-Ti oxide. Melt compositions of individual inclusions has been estimated based on modal proportions and mineral compositions of daughter phases. The inclusions show large variations in SiO 2 (20.8–50.2 wt.%) and FeO t (13.4–32.8 wt.%). With a few exceptions, the inclusions are very low in CaO ( 2 O 3 (10.0–21.2 wt.%). The low total values indicate very hydrated nature of the melt inclusions. The melt inclusions were also re-heated at 1200 °C by IHPV. Glasses from partly to fully homogenized melt inclusions show similar compositional characteristics as the bulk compositions from the crystallized melt inclusions but incorporated excessive melting of the host apatite. It appears that the cumulus apatite crystallized from hydrated late-stage immiscible Fe-rich melt formed by liquid immiscibility occurred in the relatively early stage, i.e. prior to the arrival of aqueous fluid phase. We propose that the inclusions at Damiao record a trend of late-stage melt evolution, which have been through silicate liquid immiscibility but most likely have been strongly affected by separation of a magmatic fluid phase and loss of alkali and Ca silicate components from the melt into the fluid.

Late-stage microstructures developed during the last stages of solidification of the Skaergaard intrusion comprise a wide array of reactive and non-reactive intergrowths. Reactive microstructures ascribed to open-system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts, fish-hook pyroxenes and mafic symplectites. They form by the addition of Fe and Ca and removal of alkalis and silica. Other microstructures include those formed by (internally generated) redox reactions between olivine, Fe-Ti oxides and pyroxene. Non-reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite-rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids. The open-system reactive microstructures occur predominantly in the cumulates on the chamber floor, appearing in LZb and disappearing (to be replaced by a granophyre-ilmenite-rich intergrowth association) in the Upper Zone. They are not common in the Marginal Border Series. Their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a Si-rich immiscible component from the interstitial liquid. The loss of the Si-rich component causes the remaining Fe-rich liquid to react with the primocrysts. Bulk-rock major element data are consistent with little or no preferential loss of the Si-rich liquid from the mush, but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma. The first appearance of the paired conjugate non-reactive intergrowths in MZ* (Marginal Border Series) points to the early onset of immiscibility in the bulk liquid. © The Author 2010. Published by Oxford University Press. All rights reserved.

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 2 O-FeO-Fe 2 O 3 -Al 2 O 3 -SiO 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 °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% SiO 2 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.

ARTICLE I NFO 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.

This experimental study examines relationships between alternative evolution paths of basaltic liquids (the so-called Bowen and Fenner trends), and silicate liquid immiscibility. Synthetic analogues of natural immiscible systems exhibited in volcanic glasses and melt inclusions were used as starting mixtures. Conventional quench experiments in 1atm gas mixing furnaces proved unable to reproduce unmixing of ferrobasaltic melts, yielding instead either turbid, opalescent glasses, or crystallization of tridymite and pyroxenes. In contrast, experiments involving in situ high-temperature centrifugation at 1000g (g 1⁄4 9 8 m/s) did yield macroscopic unmixing and phase separation. Centrifugation for 3^4 h was insufficient to complete phase segregation, and resulted in sub-micron immiscible emulsions in quenched glasses. For a model liquid composition of the Middle Zone of the Skaergaard intrusion at super-liquidus temperatures of 1110^11208C, centrifugation produced a thin, silicic layer (64 5 wt% SiO2 and 7 4 wt% FeO) at the top of the main Fe-rich glass (46 wt% SiO2 and 21 wt% FeO). The divergent compositions at the top and bottom were shown in a series of static runs to crystallize very similar crystal assemblages of plagioclase, pyroxene, olivine, and Fe^Ti oxides.We infer from these results that unmixing of complex aluminosilicate liquids may be seriously kinetically hampered (presumably by a nucleation barrier), and thus conventional static experiments may not correctly reproduce it. In the light of our centrifuge experiments, immiscibility in the Skaergaard intrusion could have started already at the transition from the Lower to the Middle Zone.Thus, magma unmixing might be an important factor in the development of the Fe-enrichment trend documented in the cumulates of the Skaergaard Layered Series.

Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as cross-bedding, trough structures or layer termination. It is now accepted that basaltic magma chambers mostly crystallize in situ in slightly undercooled boundary layers formed at the margins of the chamber. As a consequence, most known existing layering cannot be ascribed to a simple crystal settling process. Based on detailed field relationships, geochemical analyses as well as theoretical and experimental studies, other potential mechanisms have been proposed in the literature to explain the formation of layered igneous rocks. In this study, we review important mechanisms for the formation of layering, which we classify into dynamic and non-dynamic layer-forming processes.

Marsh (Contrib Miner Petrol 166:665–690, 2013) again claims that crystal-free basalt magmas are unable to differentiate in crustal magma chambers and regards layered intrusions as primarily due to the repeated emplacement of crystal suspensions. He ignores an earlier critique of his unconventional inferences (Latypov, J Petrol 50:1047–1069, 2009) as well as a wealth of petrographic, geochemical and experimental evidence supporting the dominant role of fractional crystallization in the solidification of layered intrusions. Most tellingly, the cryptic variations preserved in the Skaergaard and many other basaltic layered intrusions would require an exceedingly implausible sequence of phenocrystic magmas but are wholly consistent with in situ fractional crystallization. A major flaw in Marsh’s hypothesis is that it dismisses progressive fractional crystallization within any magma chamber and hence prohibits the formation of crystal slurries with phenocrysts and melts that change systematically in composition in any feeder system. This inherent attribute of the hypothesis excludes the formation of layered intrusions anywhere.

The results of a study of isotopic fractionation of boron between silica-rich (LS) and borate-rich (LB) immiscible liquids in the system Si–B–Al–Ca–Na are reported. Partitioning experiments were performed on glasses produced by quenching immiscible coexisting Si-rich and B-rich borosilicate melt pairs that had been equilibrated at high temperature and separated with the aid of a high temperature centrifuge furnace [Veksler, I.V., Dorfman, A.M., Dingwell, D.B., Zotov, N., 2002. Element partitioning between immiscible borosilicate liquids: a high-temperature centrifuge study. Geochim. Cosmochim. Acta 66, 2603–2614]. Here B/Si and 11 B/ 10 B ratios have been determined using secondary ion mass spectrometry (SIMS). The determinations encountered unexpectedly strong matrix effect (up to 12‰) due to contrasting chemical compositions of the immiscible melts. Employment of − 60 V high-voltage offset (not used during routine SIMS B isotope measurements) effectively eliminated the initially observed matrix effect. With this improved SIMS technique, we obtained the result that the coexisting quenched immiscible liquids have the same 11 B/ 10 B ratio within ± 1.2‰ (2 σ ) analytical uncertainty. Thus little or no significant two-liquid fractionation of 10 B and 11 B isotopes was observed in the temperature range between 950 and 1350 °C. This is despite the observation that significantly different proportions of trigonal vs. tetrahedrally coordinated B in these immiscible liquids have been determined by Raman spectroscopic analyses. The essential observations (i.e., a strong matrix effect and a lack of significant B isotope fractionation between the glasses) were both supported by independent thermal ionization mass spectrometry (TIMS) analyses of one selected centrifuged pair. Those glasses showed no difference in B isotopic composition within the quoted 2 σ analytical error of ± 1.0‰.

Aluminum orthoposphate (AlPO 4 ) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to α-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 °C and 0.1–0.2 GPa H 2 O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5–7 mol% SiO 2 on the phosphate side of the solvus and to the maximum of 1 mol% AlPO 4 on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinite-quartz 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.

The Skaergaard intrusion, Greenland, is the type locality for Skaergaard-type mineralizations. Mineralization levels are perfectly concordant with igneous layering, up to 5 m thick, internally fractionated, and contain crystallized sulphide droplets and precious metal alloys, sulphides, arsenides and telluride. Immiscible Cu-rich sulphide droplets, formed in a mush zone below the roof, scavenged precious metals. They were subsequently dissolved and transported to the floor in late-formed, immiscible, Fe-rich mush melts. Mineralized stratigraphic intervals of floor gabbro formed in ‘proto-macrolayers‘, owing to local sulphide saturation in melt concentrated between floating plagioclase and sinking clinopyroxene. The floor mineralization is divided into four stratigraphic sections. Formation of the Lower Platinum Group Element Mineralization (LPGEM) involved: (1) crystallization of the bulk liquid liquidus paragenesis and in situ fractionation; (2) sulphide saturation and formation of sulphide droplets in melt in the upper part of ‘proto-macrolayers‘. After further in situ fractionation, the following steps occurred: (3) the onset of silicate–silicate immiscibility and the consequent loss of buoyant and immiscible Si-rich melt; (4) dissolution of unprotected droplets of sulphide melt present in the Fe-rich mush melt; (5) compaction-driven upwards loss of residual mush melt enriched in, for example, Au. The LPGEM preserves upward increasing bulk Pd/Pt (∼6–13) owing to a continued supply of PGE and Au, with high Pd/Pt. The further development of the LPGEM ceased as the supply of precious metals to the floor waned. The Upper PGE Mineralization (UPGEM) subsequently formed from precious metals recycled in the floor. The UPGEM is characterized by increasing Au substitution in PGE phases, and a decrease in total PGE and Pd/Pt owing to upward fractionation in migrating mush melts and exhaustion of Pd and Pt. An upper Au-rich mineralization level (UAuM) was caused by late remobilization of Au and deposition on grain boundaries in fully crystallized gabbro. Cu concentrations (∼150 ppm) are not correlated with PGE and Au. Repeated Cu mineralization levels (CuM), attaining >1000 ppm, occur above the Au levels, caused by local mush layer sulphide saturation. PGE, Au and Cu distributions in the floor mineralization reflect sub-liquidus, but supra-solidus, processes and reactions in mushes at the roof, wall and floor. Constraints provided by a new model for the mineralization provide the basis for re-evaluation of the solidification processes in the Skaergaard intrusion. We have identified the importance of extensive in situ fractionation and intrusion-wide elemental redistributions in immiscible Fe- and Si-rich silicate melts. Our model characterizes the floor cumulates as bulk liquid orthocumulates containing an upwards-increasing proportion crystallized from Fe-rich, immiscible mush melt. The roof-rocks are complementary to the floor, with downwards increasing proportions crystallized from the conjugate Si-rich melt. Petrographic observations and the relative timing of crystallization support the hypothesis that crystallization was restricted to marginal mush zones. Bulk melt remaining in the magma chamber evolved not, as generally assumed, as a result of loss of crystals grown from the bulk melt, but as the consequence of mixing with recycled and evolved melt expelled from the mush by compaction. Redistribution of Fe in immiscible melts may be common to mafic intrusions and puts into question the validity of petrogenetic modelling of bulk liquids in mafic intrusions based only on consideration of floor cumulates.

More and more evidence for the development of silicate liquid immiscibility during cooling of magmas in layered intrusions have been presented. Here, we review some theoretical principles with a focus on the separation of two silicate melts, i.e. silica-rich vs. iron-rich. We discuss the role of melt structure and present phase equilibria relevant to stable and metastable immiscibility. The understanding of immiscibility in magmas has strongly benefited from recent progress in experimental approaches. Kinetics studies evidence the importance of nucleation barriers in producing unmixing , coarsening and potential separation of equilibrium melts. Improvement of analytical tools has also enabled detailed study of major and trace element partitioning. The study of immiscible emulsion in volcanic rocks also brings important information on the evolution of plutonic systems and on the potential formation of compositional gap along liquid lines of descent. We then present the most recent evidence for immiscibility in some major layered intrusions, i.e. the Skaergaard, Sept Iles, intrusions of the Emeishan province, and the Bushveld complex. Paired melts are identified as contrasted melt inclusions trapped in apatite and their segregation can be responsible for the formation of Fe–Ti–P-rich rocks. We finally discuss more broadly the potential effect of immiscibility in interstitial melt and the implications on the evolution of the crystal mush.

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 fractionated magma is the predominant process responsible for the formation of the apatite ores.

The main objectives of this study were to investigate conditions for stable and metastable liquid immiscibility in dry borosilicate synthetic systems and to evaluate effects of temperature and bulk melt composition on two-liquid element partitioning and boron speciation. To distinguish between the stable immiscibility and quench heterogeneity, we used high-temperature centrifuge phase separation. For the case of stable liquid immiscibility, silica-rich (LS) and borate-rich (LB) conjugate liquids formed two distinct layers separated by a sharp meniscus. The liquids were quenched into glasses, which were analysed by electron microprobe. Some of the glasses were also studied by Raman spectroscopy. We used several synthetic mixtures along the danburite-anorthite (CaB2Si2O8-CaAl2Si2O8) and danburite-reedmergnerite (CaB2Si2O8-NaBSi3O8) joins. In addition, we studied four complex, six-component, Mg-bearing compositions with variable Na2O and Al2O3 contents. The experiments show that the width of the LS-LB miscibility gap decreases more rapidly with the B-Al substitution (in the danburite-anorthite join) than with the Ca-Na substitution, implying that interactions between network-forming elements have a greater effect on borate-silicate unmixing than the nature of network-modifying cations. Ca and Mg partition strongly to the depolymerised borate-rich liquid with LB-LS partition coefficients of ∼40 and higher. On the other hand, two-liquid partition coefficients of Na and Al in most cases are close to 1 and show complex variations with temperature and bulk melt composition. Raman spectra of LB glasses quenched at different temperatures suggest that the proportion of trigonal boron in bulk boron content decreases with decreasing temperature. The change in boron speciation appears to affect Al and Na two-liquid partitioning in such a way that at low temperatures, the latter element becomes more compatible with LS.

Extreme enrichment in H2O, B, P and F is characteristic of many evolved granites and pegmatites. We report experimental phase relations of a synthetic peraluminous pegmatite spiked with P2O5 ,B 2O3 and F (5 wt% of each), Rb2O, Cs2O (1 wt% of each) and Li2O (0.5 wt%). Experiments were carried out in H2O-satu- rated conditions in cold-seal rapid-quench pressure vessels at 0.1-0.2 GPa. Crystallisation starts at about 820 C with berlinite and topaz. Quartz appears at 700- 750 C. Topaz is replaced by muscovite at about 600 C. At near-solidus temperatures (450-500 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 Na3AlF6 and H3BO3 components. Experi- mental evidence of the liquid immiscibility and mineral reactions documented in our study offers new explana- tions of many enigmatic features of natural pegmatites.

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 °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 CO2-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 H2O–CO2-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.

Summary Geophysical data have been instrumental in the exploration and exploitation of the Bushveld Complex, South Africa. Surprising new economic discoveries are still being made, for example in the Northern Lobe, and major advances in exploration and analytical tools promise many more. We are still in the first-order characterization phase of understanding large igneous provinces and there is much to be learned on a wide range of topics. Little work has been published on linking borehole geophysical measurements with extensive geophysical data sets.

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 (CaCO3, MgCO3 and Na2CO3) 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 °C in the two-phase (fluid + melt) stability region. Both quenched melt and quenched fluid were dissolved in dilute HCl 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 DNa = 0.35 ± 0.02, DCa = 0.09 ± 0.02, and DMg = 0.13 ± 0.01. Between 700 and 900 °C, the effect of temperature on partitioning is negligible. However, DNa 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.

The origin of carbonatites remains a contentious topic. However, INTRODUCTION an important role for liquid immiscibility between silicate and Silicate–carbonate liquid immiscibility has often been carbonate liquids has often been proposed. To understand and proposed as a possible mechanism for the origin of constrain the role this process may play, it is important to have carbonatites associated with ultramafic alkaline rocks in trace element partitioning data available. Few experimental studies composite volcanic and subvolcanic complexes. Field on trace element partitioning between silicate and carbonate liquids observations (Ferguson & Currie, 1971; Kjarsgaard & have been undertaken, reflecting both analytical and experimental Peterson, 1991; Church & Jones, 1995) and data on difficulties. To achieve better phase separation new two-liquid melt inclusions in minerals (Rankin & Le Bas, 1974; experiments have been performed utilizing the rotating centrifuge Romanchev & Sokolov, 1979) support the role of liquid autoclave. Trace elements in the run products were analysed in situ immiscibility in carbonatite petrogenesis. Since the piusing laser ablation microprobe–inductively coupled plasma mass oneering work of Koster van Groos & Wyllie (1963) spectrometry. Partition coefficients (D) have been determined for the separation of carbonatitic immiscible liquids from selected rare earth elements (La, Nd, Sm, Tb, Er, Tm), high field carbonated silicate melts has been demonstrated exstrength elements (Zr, Hf, Nb, Ta), and for Sr, Ba and Y. Most perimentally in many synthetic silicate–carbonate systems of the rare earth elements partition preferentially into the silicate and for natural rock compositions [e.g. see review by liquid. La, Sr and Ba, however, strongly partition into the carbonate Kjarsgaard & Hamilton (1989)]. However, there is no liquid. The high field strength elements, although all preferentially consensus about the role of liquid immiscibility in the partitioning into the silicate liquid, are characterized by a wide development of carbonatite magmas (see Gittins, 1989). range of D values. Zr and Hf have similar D values, which are In this connection, quantitative experimental data on one to two orders of magnitude lower than those of Nb, Ta and trace element partitioning between the immiscible liquids Ti. Ti and Nb behave similarly, whereas Ta demonstrates behaviour are important for constraining the origin of carbonatites. intermediate to that of Zr and Hf. Nb/Ta ratios are strongly Experimental studies in silicate–carbonate systems are fractionated by two-liquid partitioning. hampered by the rapid crystallization of carbonate-rich

Cumulus olivine, clinopyroxene, melilite and perovskite from silicate INTRODUCTION rocks and carbonatites of the Gardiner (East Greenland) and Kovdor Carbonatite-bearing ultramafic alkaline complexes gen(Kola Peninsula) ultramafic alkaline complexes contain primary erally exhibit chaotic field relations because of explosive melt inclusions crystallized into aggregates of daughter minerals. magmatism, metasomatic alteration and high activity of The petrography and homogenization temperatures of the inclusions volatiles. In these complexes the primary characteristics constrain the fractionation paths and the formation of carbonatites of the parental melts and the primary liquidus asin the complexes. Carbonated melanephelinite was parental to both semblages may be obscured by post-magmatic reactions, complexes and early cumulates (dunite, olivinite, peridotite and and volatile components are easily lost during crysmelteigite) are comparable. The common occurrence of phlogopite tallization together with components soluble in fluids. and amphibole in the inclusions and in the host rocks indicates Melt and fluid inclusions in liquidus minerals constitute that these were important liquidus phases. In both complexes the microscopic natural autoclaves and their study is espefractionation of phlogopite and amphibole drove the melts towards cially powerful in volatile-rich systems. The petrography Ca-rich (larnite-normative) compositions. At the ijolite stage the and composition of inclusions constrain petrogenetic evolutionary trends are believed to separate and the evolved larnitemodels for ultramafic alkaline complexes and carbonatite normative melt produced calcite-bearing ijolite in Kovdor and formation. Carbonatites and related silicate rocks have melilitolite in Gardiner. The two assemblages are related by debeen the subject of melt and fluid inclusion studies for carbonation reactions. A fractional crystallization origin is suggested several decades (e.g. Roedder, 1984, pp. 406–411), and for the Kovdor carbonatite, whereas the Gardiner carbonatite formed such studies have revealed traces of strongly alkaline by liquid immiscibility in the course of melilite fractionation. carbonatite melt associated with alkali-poor plutonic carNa–K–Ca and Na–Mg carbonates are common daughter phases bonatite (Aspden, 1981; Le Bas & Aspden, 1981; Kogarko and especially abundant in late-stage inclusions. Thus, all caret al., 1991). bonatite melts in the two complexes are alkaline. Calcite carbonatites Here we present results of an investigation of crysappear to be cumulates. tallized melt inclusions in rock-forming minerals from

The evolution of nephelinitic melts in equilibrium with mica-bearing liquidus assemblages and melting relations have been studied on two silica-undersaturated joins of the KAlSiO4– Mg2SiO4– Ca2SiO4– SiO2– F system at atmospheric pressure by quench runs in sealed platinum capsules. Fluorine has been added to the batch compositions by the direct exchange of fluorine for oxygen (2F− = O2−). The first join is the pseudo-ternary Forsterite – Diopside – KAlSiO3F2 system. Forsterite, diopside, F-phlogopite and leucite crystallisation fields and a fluoride-silicate liquid immiscibility solvus are present on the liquidus surface of the join. Sub-liquidus and sub-solidus phases include akermanite, cuspidine, spinel, fluorite and some other minor fluorine phases. The second system is the pseudo-binary Akermanite – F-phlogopite join that intersects the Forsterite – Diopside – KAlSiO3F2 join. Akermanite, forsterite, diopside, F-phlogopite, leucite and cuspidine are found to crystallise on the join. Forsterite (fo) and leucite (lc) are related to F-phlogopite (phl) by a reaction with the fluorine-bearing liquid: fo + lc + l = phl, and the reaction proceeds until forsterite or leucite are completely consumed. The reaction temperature and resulting phase association depend on batch composition. Thus, leucite is not stable in the sub-solidus of the Akermanite – F-phlogopite join, but is preserved in a part of the Forsterite – Diopside – KAlSiO3F2 system where forsterite reacts out, or does not crystallise at all. The phlogopite-in reaction has an important effect on the composition of the coexisting liquid. The liquids initially saturated in forsterite evolve to extremely Ca rich, larnite-normative residuals. The experimental data show that larnite-normative melilitolites can crystallise from evolved melilititic melts generated from “normal” melanephelinitic parental magmas with no normative larnite. The evolution towards melilitites requires fractionation of phlogopite-bearing assemblages under volatile pressure.

Article history: Large Igneous Provinces (LIPs) have been proposed to trigger mass-extinction events by the release of large quantities of volcanic gases which results in major climatic perturbations causing worldwide ecological stress and collapse. A prerequisite for understanding the proposed link between LIP volcanism and biological crisis is reliable information about the total gas emissions during these events. In this paper we present the first estimations of total F, Cl and S emissions from the Parana-Etendeka LIP in the South Atlantic. Data from this province are of special interest because it is among the world's largest LIPs but is not associated with a mass extinction event. We have determined pre-eruption concentrations of F, Cl and S by in situ analysis of melt inclusions preserved in olivine phenocrysts from basaltic dikes in the Etendeka province of NW Namibia. The melt inclusions have Mg-rich basaltic bulk compositions with about 8 to 18 wt.% MgO, overlapping the compositional range of the host rocks. A major feature of the melt inclusions is their wide variation in major and minor element concentrations, including F, Cl and S. This is attributed to trapping of variably-mixed melt fractions during crystallization of olivine in the roots of the dike system. Fluorine concentrations vary from about 190 to 450 μg/g, Cl from

The Henties Bay-Outjo dyke swarm (HOD) in NW Namibia presents a well-exposed example of the magma feeder system for continental flood basalts. This study uses bulk-rock, mineral and melt inclusion compositions from dyke samples to define pressure–temperature crystallization conditions and thus contributes to the understanding of the magma plumbing and storage beneath the province. The thermobarometry calculations for near-equilibrium mineral–melt pairs (the latter proxied by whole-rock) indicate olivine crystallization at ~ 1170 to 1350 °C and lower, overlapping, temperatures for clinopyroxene and plagioclase (~ 1070 to 1210 °C) in keeping with the order of crystallization inferred from petrographic observation. The dykes yielding the highest temperatures (> 1300 °C) are in a specific region of the HOD near the litho-tectonic boundary of the Neoproterozoic Kaoko and Damara Belts, where magma permeability of the crust may have been enhanced. Pressure estimates from clinopyroxene–melt pairs range from 0 to 10 kbar overall (13 dykes) indicating polybaric crystallization. The lowest pressures are recorded by clinopyroxene oikocrysts intergrown with plagioclase, which likely represent the dyke emplacement depths. Clinopyroxene phenocrysts and plagioclase-free oikocryst cores yield a higher range of crystallization pressures at 4–6 kbar, corresponding to mid-crustal depths of 11–17 km. There is no spatial pattern in the pressure variations, suggesting a rather uniform level of magma stagnation and crystallization in all areas of the HOD. Partial crystallization at intermediate depths is consistent with the inference of entrained crystals and with geochemical evidence for crystal accumulation in some of the dykes. Comparison of model magma densities and the crustal density derived from seismic velocity profiles suggest the dyke magmas had positive buoyancy in the lower- and middle crust and near-neutral buoyancy in the upper crust. The depth of magma stagnation or pooling at 11–17 km depth may relate to this or to the brittle–ductile transition in the felsic Damara crust.

Liquid–liquid partitioning of 42 elements between synthetic silicate melts and immiscible fluoride, chloride, carbonate, phosphate and sulfate liquids was studied at temperatures of 650–1100 °C, pressures 72–100 MPa, with 0–11 wt.% H2O. One series of experiments was performed in a rotating internally-heated autoclave where separation of the immiscible liquids was assisted by centrifugal forces. An analogous series of experiments was done in static rapid-quench cold-seal pressure vessels. The experimentally determined liquid–liquid distribution coefficients (D’s) vary over several orders of magnitude, as a result of variable Coulombic interactions between cations and anions. For alkaline, alkaline earth and rare earth elements ther is a strong and systematic dependence of the liquid/liquid D values on the ionic potential Z/r for all the examined systems. In contrast, highly charged cations (e.g., HFSE) show no systematic relationships between the D’s and Z/r. New experimental constraints on the carbonate/silicate liquid–liquid D values presented here confirm that rare metals such as Nb, Zr, REE, Th and U concentrate in silicate liquids, and therefore carbonatites that carry economical rare metal mineralization are not likely to have formed by liquid immiscibility. The comparison between experimentally-determined carbonate–silicate liquid–liquid D values and bulk-rock natrocarbonatite vs. nephelinite compositions at the Oldoinyo Lengai in Tanzania reveals significant discrepancies for Cs, Rb, Ba, Be, Zn, heavy REE, Ti, Mo and W, thus rendering a simple, one-stage immiscibility model for Oldoinyo Lengai questionable.

Karoo LIP in Antarctica (Heinonen and Luttinen, 2010), and the Parana-Etendeka LIP in northwest Namibia (Thompson and Gibson, 2000), the subject of this paper. Shared features of ultra-magnesian olivines in these provinces are that they are rare, occurring in just a few localities, and they are too Mg rich to be considered equilibrium phenocrysts in the basaltic host rocks. It is signifi cant that these olivines coexist in the same hand specimens with much more abundant, less magnesian crystals (Fo 85‐90 ), the compositions of which are close to equilibrium with the Mg# of the host rocks, and can therefore be considered as phenocrysts. It follows that the ultra-magnesian olivines are not phenocrysts in the present host, but represent xenocrysts. The high CaO contents in these olivines clearly distinguish them from entrained crystals of mantle olivine, and most studies have concluded that the ultra-magnesian crystals originated from an unerupted komatiitic melt and were incorporated into a second, more evolved magma. We emphasize that this conclusion is based only on olivine and bulk-rock compositions. However, olivine compositions constrain only the Mg# [MgO/(MgO + FeO) in molar percent] of the equilibrium liquid and not its MgO concentration, on which the inference of a komatiitic affi nity and high temperature of the parental melts critically depend. To calculate the melt MgO requires information on its FeO content. This study presents an example where original melt compositions are recovered directly by analysis of inclusions trapped in the ultra-magnesian olivines. This removes the dependence of the results on the bulk rock composition and

The presence of Fe- and Si-rich liquids found as melt inclusions in apatite and olivine in the Upper Zone of the Skaergaard intrusion, East Greenland, demonstrates the occurrence of liquid immiscibility in the late-stage evolution of tholeiitic magmas in a plutonic setting. However, it remains unclear at which stage of crystallization unmixing began. To constrain the onset and the petrological importance of liquid immiscibility in the Skaergaard and tholeiitic magmas in general, we have studied crystallized melt inclusions entrapped in early primocryst plagioclase. Such melt inclusions become abundant from the top of the Lower Zone and upwards in the Layered Series, in primocryst plagioclase of composition An54^26. The daughter phase assemblage is the same in all the inclusions, although the modal proportions of the daughter phases are highly variable: plagioclase (42^59%), clinopyroxene (28^41%), ilmenite (4^9%), magnetite (3^10%), apatite (1^9%) and accessory phases (51%). Accordingly, the bulk compositions of reheated and homogenized melt inclusions show large variations in SiO2 (40^54 wt %), FeO t (7^23 wt %), P 2O5 (0^1 ·9 wt %) and K2 O( 0^2·8 wt %), and have variable CaO/Al2O3 ratios.These variations are best explained by trapping of varying proportions of immiscible iron- and silica-rich melts and demonstrate that liquid immiscibility started in the upper part of the Lower Zone. We conclude that a significant part of the Skaergaard intrusion crystallized from an emulsion of Fe- and Si-rich immiscible melts.The heterogeneous trapping of a mixture of Fe- and Si-rich immiscible liquids in primocryst plagioclase indicates that the dispersed droplets in the Lower and Middle Zones were smaller than the size of the inclusion (5500mm). In the Upper Zone, most of the inclusions in apatite are composed of the conjugate end-member liquids, indicating a larger size for the dispersed droplets. Metre-sized pods and layers of melanogranophyre in the upper part of the intrusion are believed to represent pooled bodies of the immiscible Si-rich liquid. Differentiation of an emulsified magma must be considered in petrogenetic models for the Skaergaard intrusion.

Liquid immiscibility is common in glass-forming oxide melts, and interfacial tension between the conjugate liquid phases is an important property that determines the kinetics of phase separation. Here we present, to the best of our knowledge, the first experimental measurements of interfacial tension between immiscible melts in the alkaline earth – boron oxide binary systems; MgO-B 2 O 3 , CaO-B 2 O 3 and BaO-B 2 O 3 . We have also measured surface tension for the conjugate borate liquids. The measurements were carried out at 1100-1250 °C by the maximum detachment force method employing a vertical cylinder geometry and using a gravimetric balance system. The values of interfacial tension between immiscible melts of the binary systems are 28 ± 3.5, 12 ± 2.8 and 2.1 ± 0.5 mN/m, for the Mg-, Ca- and Ba-bearing systems respectively. These results yield a positive, linear correlation with the width of miscibility gap expressed in molar units. We recognise no direct, simple relationship between the liquid-liquid interfacial tensions and the individual surface tensions of the conjugate liquids in air.

This short technical note describes a simple technique for synthesising an optimised binary glass for performing coarse mass table calibrations of secondary ion mass spectrometers. A Ge-Bi glass doped with five selected trace elements was used to anchor the entire mass table up to m/z ≥ 209 at an accuracy of < 0.5 dalton. Using this material it has been possible to calibrate coarsely the mass table of a Cameca ims 6f instrument in less than five minutes, thereby greatly simplifying the switch from low mass range to high mass range measurements. Although not suitable for the quantification of geological materials, the ease of synthesis and its utility for rapidly performing what is an otherwise time consuming process makes this material a useful tool for SIMS instruments which offer a multiple mass range capability. Cette courte note technique presente une technique simple permettant de synthetiser un verre binaire specifiquement adaptea la calibration en masse rapide des spectrometres de masses a ions secondaires. Un verre de germanium - bismuth dope avec 5 elements en trace choisis, a ete utilise pour caler toute la table jusqu'a des m/z ≥ 209 avec une precision < 0.5 dalton. En utilisant ce materiau il a ete possible de calibrer grossierement la table des masses d'un instrument Cameca ims 6f en moins de 5 minutes, ce qui simplifie grandement le passage des masses legeres aux masses lourdes. Bien que ce verre ne soit pas adaptea la quantification de materiaux geologiques, sa facilite de synthese et son utilite pour effectuer rapidement ce qui est sinon fort long, font de ce verre un outil tres utile pour les SIMS qui peuvent couvrir un eventail de masses tres large.

This paper examines phase equilibria and mass balance constraints on Fe enrichment in tholeiitic liquids in plutonic environments. The peak of Fe enrichment is thought to roughly coincide with magma saturation in Fe–Ti oxides; and olivine starts to react with the liquid at about the same time. This crucial stage of crystallization is examined in detail using a compilation of chemical analyses of 64 experimental charges that comprise liquids (quenched glasses) equilibrated with the liquidus assemblage of olivine, plagioclase, high-Ca pyroxene, and low-Ca pyroxene. Some samples also contain Fe–Ti oxides. It is shown that the 4-phase liquidus assemblage does not constrain a narrow range of liquid compositions. The concentrations of SiO 2 in the selection of experimental glasses vary broadly from 42 to 66 wt.%. Silica content shows strong negative correlations with FeO and CaO/Al 2 O 3 , and strong positive correlation with alkalis. Extreme Fe enrichment above 22 wt.% FeO is observed only in alkali-free or alkali-poor liquids. Broad compositional variations for the multiply-saturated liquids are attributed to strong non-ideality and complex concentration-activity relationships in ferrobasaltic melts. Liquid immiscibility characteristic of Fe-rich silicate liquids is the ultimate consequence of non-ideality. Petrogenetic implications of phase equilibria and mass balance constraints are discussed for a classical example of the Skaergaard intrusion in East Greenland, where the trend of extreme Fe enrichment has been in contention. It is proposed that seemingly conflicting results of experiments on Skaergaard natural cumulate rocks and model melt compositions can be reconciled if it is assumed that silicate liquid immiscibility in Skaergaard started not at the very end of crystallization but earlier, soon after the start of ilmenite and magnetite crystallization.