Strong tin enrichment in a pegmatite-forming melt

To investigate processes of magmatic tin enrichment and cassiterite deposition, we studied the abundances of major, trace, and volatile elements in a large number of rehomogenized silicate melt inclusions in quartz and topaz from a pegmatite body at the Ehrenfriedersdorf Sn–W deposit. This deposit is associated with evolved Variscan granites of the central Erzgebirge, southeast Germany. The melt inclusions are peraluminous; the molar aluminum saturation index (ASI) ranges from 1.15 to 2.0, and many inclusions are characterized by a very high content of fluxing components and volatiles. Some inclusions contain more than 20 wt% of H₂O, F, Cl, and P₂O₅, plus Li as well as very high levels of Sn. Some rare, highly evolved fractions of late-stage pegmatite-forming liquid at Ehrenfriedersdorf contained up to 7000 ppm Sn. The presence of hydrogen and methane in addition to water and carbon dioxide in the vapor phase of the melt inclusions suggests a very low oxygen fugacity for some fractions of magma. The extreme levels of tin, volatiles, and fluxing components in this magma had an important influence on processes of melt movement and cassiterite precipitation. Melts, like these, that are high in volatiles and alkalis (sum of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O is >8 wt%) have low densities (≤1.8 g/cm³), low viscosities (<10 Pa.s at 700 °C), facilitate relatively rapid diffusion of ions through melts, and hence are excellent solvents for extracting and transporting ore-forming elements. Received: 1 February 1999 / Accepted: 19 January 2000     

The behaviour of boron in a peraluminous granite-pegmatite system and associated hydrothermal solutions: a melt and fluid-inclusion study

Detailed analyses of melt and fluid inclusions combined with an electron-microprobe survey of boron-bearing minerals reveal the evolution of boron in a highly evolved peraluminous granite-pegmatite complex and the associated high- and medium-temperature ore-forming hydrothermal fluids (Ehrenfriedersdorf, Erzgebirge, Germany). Melt inclusions in granite represent embryonic pegmatite-forming melts containing about 10 wt% H₂O and 1.8 wt% B₂O₃. These melts are also enriched in F, P, and other incompatible elements such as Be, Sn, Rb, and Cs. Ongoing differentiation and volatile enrichment drove the system into a solvus, where two pegmatite-forming melts coexisted. The critical point is at about 712 °C, 100 MPa, 20 wt% H₂O and 4.1 wt% B₂O₃. Cooling and concomitant fractional crystallisation from 700 to 500 °C induced development of two conjugate melts, an H₂O-poor (A-melt) and an H₂O-rich melt (B-melt) along the opening solvus. Boron is a major element in both melts and is preferentially partitioned into the H₂O-rich melt. Temperature-dependent distribution coefficients D_boron^{B - melt/A - melt} D_{{\rm{boron}}}^{{\rm{B - melt/A - melt}}} are 1.3 at 650 °C, 1.5 at 600 °C, and 1.8 at 500 °C. In both melts, boron concentrations decreased during cooling because of exsolution of a boron-rich hypersaline brine throughout the pegmatitic stage. Boromuscovite containing up to 8.5 wt% was another sink for boron at this stage. The end of the melt-dominated pegmatitic stage was attained at a solidus temperature of around 490 °C. Fluid inclusions of the hydrothermal stage reveal trapping temperatures of 480 to 370 °C, along with varying densities and highly variable B₂O₃ contents ranging from 0.20 to 2.94 wt%. A boiling system evolved, indicating a complex interplay between closed- and open-system behaviour. Pressure switched from lithostatic to hydrostatic and back, generating hydrothermal convection cells where meteoric waters were introduced and mixed with magmatic fluids. Boron-rich solutions originated from magmatic fluids, whereas boron-depleted fluids were mainly of meteoric origin. This highlights the potential of boron for discriminating fluids of different origin. Tin is continuously enriched during the evolution because tin and boron are cross-linked by formation of boron-, fluorine- and tin-fluorine-bearing complexes and is finally deposited within quartz-cassiterite veins during the transition from closed- to open-system behaviour. Boron does not only trace the complex evolution of the Ehrenfriedersdorf complex but exerts, together with H₂O, F and P, an important control on the physical and chemical properties of pegmatite-forming melts, and particularly on the formation of a two-melt solvus at low pressure. We discuss this with respect to experimental results on H₂O solubility and the critical behaviour of the haplogranite-water system which contained variable concentrations of volatiles.     

Experimental investigation of H2O and Cl− solubilities in F-enriched silicate liquids; implications for volatile saturation of topaz rhyolite magmas

To interpret the degassing of F-bearing felsic magmas, the solubilities of H₂O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C. Chloride solubility in these liquids increases with decreasing H₂O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl⁻ exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate liquids containing as little as 1 wt% H₂O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H₂O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl⁻ in H₂O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H₂O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H₂O versus Cl⁻ in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H₂O and Cl⁻ concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive H₂O and Cl⁻ concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior to eruption. Received: 6 November 1995 / Accepted: 29 January 1998     

The miarolitic pegmatites from the K?nigshain: a contribution to understanding the genesis of pegmatites

In this paper, we show that the crystallization of miarolitic pegmatites at K?nigshain started at about 700°C, in melts containing up to 30 mass% water. Such high water concentration at low pressures (1–3 kbar) is only possible if the melts are peralkaline. Such peralkaline melts are highly corrosive, and reacted with the wall rock—here the granite host—forming the graphic granite zone, in part via a magmatic–metasomatic reaction. With cooling, the water concentration in some melt fractions increased up to 50 mass% H₂O. The melt-dominated system ends below 600°C and passes into a fluid-dominated system, the beginning of which is characterized by strong pressure fluctuations, caused by the change of OH and CO₃ ²⁻ in the melt, to molecular water and CO₂. We note two generations of smoky quartz, one crystallized above the β–α-transition of quartz (≈573°C), and one below, both of which contain melt inclusions. This indicates that some melt fraction remains during at least the higher-temperature portion of the growth of minerals into the miarolitic cavity, contradicting the view that minerals growing into a pegmatite chamber only do so from aqueous fluids. We show that the K?nigshain miarolitic pegmatites are part of the broad spectrum of pegmatite types, and the processes active at K?nigshain are representative of processes found in most granitic pegmatites, and are thus instructive in the understanding of pegmatite formation in general, and constraining the composition and characteristics of pegmatite-forming melts. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The influence of water on melting of mantle peridotite

This experimental study examines the effects of variable concentrations of dissolved H₂O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet. Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H₂O dissolved in the liquid. The effects of increasing the concentration of dissolved H₂O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the concentrations of SiO₂, FeO, MgO, and CaO. The concentration of Al₂O₃ is unaffected. The lower SiO₂ contents of the hydrous melts result from an increase in the activity coefficient for SiO₂ with increasing dissolved H₂O. The lower concentrations of FeO and MgO result from the lower temperatures at which H₂O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated SiO₂/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO₂ rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the peridotite melting process in subduction zones is strongly influenced by the composition of the H₂O-rich component introduced into the mantle wedge from the subducted slab. Received: 7 April 1997 / Accepted: 9 January 1998     

Boron in granitic magmas: stability of tourmaline in equilibrium with biotite and cordierite

Experiments at 750 °C, 200 MPa_(H2O), a _(H2O)=1, and fO₂∼Ni-NiO established that the equilibrium among tourmaline, biotite, cordierite, and melt (± spinel, aluminosilicate, or corundum) occurs with ∼2 wt% B₂O₃ in strongly peraluminous melt with an aluminosity, measured by the parameter ASI, of >1.2. The experiments demonstrate the relationship of tourmaline stability to the activity product of the tourmaline components boron and aluminum, which are inversely related to one another. Tourmaline is unstable in metaluminous to mildly peraluminous melts (ASI <1.2) at 750 °C regardless of their boron content. For a given aluminosity, addition of components such as F requires a greater boron content of melt at this equilibrium. The stability of tourmaline increases with decreasing temperatures below 750 °C. At the inception of melting, tourmaline breaks down incongruently to assemblages containing crystalline AFM silicates (biotite, cordierite, garnet, sillimanite), aluminates (spinel, corundum), and B-enriched but Fe-Mg-poor melt. Granitic melts are likely to be undersaturated in tourmaline from the start of their crystallization, and their initial boron contents will be limited by the abundance of tourmaline in their source rocks. Quartzofeldspathic (gneissic, metapelitic) rocks that reached conditions of the granulite facies and still contain (prograde) tourmaline are rare, and probably have never yielded a partial melt. Most leucogranitic magmas will initially crystallize biotite, cordierite, or garnet, but not tourmaline. With crystallization, the Fe-Mg content of melt decreases, and the B₂O₃ content increases until the tourmaline-biotite and/or tourmaline-cordierite (or garnet) equilibria are attained. The B₂O₃ content of melt is buffered as long as these equilibria continue to operate, but low initial Fe-Mg contents of the magmas limit the quantity of boron that can be consumed by these reactions to <1 wt% B₂O₃. Normally, leucogranitic magmas contain insufficient Fe and Mg to conserve all boron as tourmaline and thus lose a large fraction of magmatic boron to wallrocks. Leucogranites and pegmatites with tourmaline as an early and only AFM silicate mineral probably contained >2 wt% B₂O₃ in their bulk magmas. Received: 6 August 1996 / Accepted: 21 July 1997     

Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr₂O_3, and 20 to 220 ppm H₂O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM₂₁O₃₈, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi₂O₆), and a new oxide mineral, carmichaelite (MO_2−x(OH)_x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb₂O₅), Cr (up to ∼6 wt% Cr₂O₃), and OH (up to ∼0.9 wt% H₂O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks. Received: 20 December 1997 / Accepted: 15 October 1998     

Multiphase carbonate-salt immiscibility in carbonatite melts: data on melt inclusions from the Krestovskiy massif minerals (Polar Siberia)

Minerals of olivine–melilite and olivine–monticellite rocks from the Krestovskiy massif contain primary silicate-salt, carbonate-salt, and salt melt inclusions. Silicate-salt inclusions are present in perovskite I and melilite. Thermometric experiments conducted on these inclusions at 1,230–1,250°C showed silicate–carbonate liquid immiscibility. Globules of composite carbonate-salt melt rich in alkalies, P, S, and Cl separated in silicate melt. Carbonate salt globules in some inclusions from perovskite II at 1,190–1,200°C separated into immiscible liquid phases of simpler composition. Carbonate-salt and salt inclusions occur in monticellite, melilite, and garnet and homogenize at close temperatures (980–780°C). They contain alkalies, Ca, P, SO₃, Cl, and CO₂. According to the ratio of these components and predominance of one of them, melt inclusions are divided into 6 types: I—hyperalkaline (CaO/(Na₂O+K₂O)≤1) carbonate melts; II—moderately alkaline (CaO/(Na₂O+K₂O)>1) carbonate melts; III—sulfate-alkaline melts; IV—phosphate-alkaline melts; V—alkali-chloridic melts, and VI—calc-carbonate melts. Joint occurrence of all the above types and their syngenetic character were established. Some inclusions demonstrated carbonate-salt immiscibility phenomena at 840–800°C. A conclusion in made that the origin of carbonate melts during the formation of intrusion rocks is related to silicate–carbonate immiscibility in parental alkali-ultrabasic magma. The separated carbonate melt had a complex alkaline composition. Under unstable conditions the melt began to decompose into simpler immiscible fractions. Different types of carbonate-salt and salt inclusions seem to reflect the composition of these spatially isolated immiscible fractions. Liquid carbonate-salt immiscibility took place in a wide temperature range from 1,200–1,190°C to 800°C. The occurrence of this kind of processes under macroconditions might, most likely, cause the appearance of different types of immiscible carbonate-salt melts and lead to the formation of different types of carbonatites: alkali-phosphatic, alkali-sulfatic, alkali-chloridic, and, most widespread, calcitic ones.     

Coexistence of two distinct mantle sources during formation of ophiolites: a case study of primitive pillow-lavas from the lowest part of the volcanic section of the Troodos Ophiolite, Cyprus

We present a detailed mineralogical, petrological and melt inclusion study of unusually fresh, primitive olivine + clinopyroxene phyric Lower Pillow Lavas (LPL) found near Analiondas village in the northeastern part of the Troodos ophiolite (Cyprus). Olivine phenocrysts in these primitive LPL show a wide compositional range (Fo_82–92) and have higher CaO contents than those from the Upper Pillow Lavas (UPL). Cr-spinel inclusions in olivine are significantly less Cr-rich (Cr/Cr + Al = 28–67 mol%) compared to those from the UPL (Cr# = 70–80). These features reflect differences in melt compositions between primitive LPL and the UPL, namely higher CaO and Al₂O₃ and lower FeO* compared to the UPL at a given MgO. LPL parental melts (in equilibrium with Fo₉₂) had ∼10.5 wt% MgO and crystallization temperatures ∼1210 °C, which are significantly lower than those previously published for the UPL (14–15 wt% MgO and ∼1300 °C for Fo₉₂). The fractionation path of LPL parental melts is also different from that of the UPL. It is characterized initially by olivine + clinopyroxene cotectic crystallization joined by plagioclase at ∼9 wt% MgO, whereas UPL parental melts experienced a substantial interval of olivine-only crystallization. Primitive LPL melts were formed from a mantle source which was more fertile than that of tholeiites from well-developed intra-oceanic arcs, but broadly similar in its fertility to that of Mid-Ocean Ridge Basalt (MORB) and Back Arc Basin Basalts (BABB). The higher degrees of melting during formation of the LPL primary melts compared to average MORB were caused by the presence of subduction-related components (H₂O). Our new data on the LPL coupled with existing data for the UPL support the existing idea that the LPL and UPL primary melts originated from distinct mantle sources, which cannot be related by progressive source depletion. Temperature differences between these sources (∼150 °C), their position in the mantle (∼10 kbar for the colder LPL source vs 15–18 kbar for the UPL source), and temporal succession of Troodos volcanism, all cannot be reconciled in the framework of existing models of mantle wedge processes, thermal structure and evolution, if a single mantle source is invoked. Possible tectonic settings for the origin of the Troodos ophiolite (forearc regions of intra-oceanic island arc, propagation of backarc spreading into arc lithosphere) are discussed. Received: 20 May 1996 / Accepted: 25 March 1997     

Genesis of tin and tantalum mineralization in pegmatites from the Bynoe area,Pine Creek Geosyncline,Northern Territory

The tin‐ and tantalum‐bearing pegmatites of the Bynoe area are located in the western Pine Creek Geosyncline. They are emplaced within psammopelitic rocks in the contact aureole of the Two Sisters Granite. The latter is a Palaeoproterozoic, fractionated, granite with S‐type characteristics and comprises a syn‐ to late‐orogenic, variably foliated, medium‐grained biotite granite and a post‐orogenic, coarse‐grained biotite‐muscovite granite. The pegmatites comprise a border zone of fine grained muscovite + quartz followed inward by a wall zone of coarse grained muscovite + quartz which is in turn followed by an intermediate zone of quartz + feldspar + muscovite. A core zone of massive quartz is present in some occurrences. Feldspars in the intermediate zone are almost completely altered to kaolinite. This zone contains the bulk of cassiterite, tantalite and columbite mineralization. Fluid inclusions in pegmatitic quartz indicate that early Type A (CO₂ + H₂O ± CH₄) inclusions were trapped at the H₂O‐CO₂ solvus at P～100 MPa, T～300°C (range 240–328°C) and salinity ～6 wt% eq NaCl. Pressure‐salinity corrected temperatures on Type B (H₂O + ～20% vapour), C (H₂O + < 15% vapour) and D (H₂O + halite + vapour) inclusions also fall within the range of Type A inclusions. Oxygen and hydrogen isotope data show that kaolin was either formed in isotopic equilibrium with meteoric waters or subsequent to its formation, from hydrothermal fluid, underwent isotopic exchange with meteoric waters. Fluid inclusion waters from core zone quartz show enrichment in deuterium suggesting metamorphic influence. Isotope values on muscovite are consistent with a magmatic origin. It is suggested that the pegmatites were derived from the post‐orogenic phase of the Two Sisters Granite. Precipitation of cassiterite took place at about 300°C from an aqueous fluid largely as a result of increase in pH due to feldspar alteration.     

The temperature dependence of the speciation of water in NaAlSi3O8-KAlSi3O8 melts: an application of fictive temperatures derived from synthetic fluid-inclusions

 The speciation of water dissolved in glasses along the join NaAlSi₃O₈-KAlSi₃O₈ has been investigated using infrared spectroscopy. Hydrous melts have been hydrothermally synthesized by chemical equilibration of cylinders of bubble-free anhydrous start glasses with water at 1040^° C and 2 kbar. These melts have been isobarically and rapidly (200^° C/s) “drop”-quenched to room temperature and then subsequently depressurized. The speciation of water in the quenched glasses reflects the state of water speciation at a temperature (the so-called fictive temperature) where the quenched-in structure of the glasses closely corresponds to the melt structure at equilibrium. This fictive temperature is detectable as the macroscopically measureable glass transition temperature of these melt compositions. A separate set of experiments using vesicular samples of the same chemistry has precisely defined the glass transition temperature of these melts (±5^° C) on the basis of homogenization temperatures for water-filled fluid inclusions (Romano et al. 1994). The spectroscopic data on the speciation of water in these quenched glasses has been quantified using experimentally determined absorptivities for OH and H₂O for each individual melt composition. The knowledge of glass transition temperatures, together with quantitative speciation data permits an analysis of the temperature dependence of the water speciation over the 113^° C range of fictive temperatures obtained for these water-saturated melts. The variation of water speciation, cast as the equilibrium constant K where K = [H₂O] [O _m ]/[OH]² is plotted versus the fictive temperature of the melt to obtain the temperature dependence of speciation. Such a plot describes a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange of K for Na has little influence on melt speciation of water. The enthalpy derived from temperature dependence is 36.5(±5) kJ/mol. The results indicate a large variation in speciation with temperature and an insensitivity of the speciation to the K–Na exchange. Received: 8 March 1995/Accepted: 6 June 1995     

Fluid evolution of rare-element and muscovite granitic pegmatites from central Galicia, NW Spain

Fluid inclusions have been studied in three pegmatite fields in Galicia, NW Iberian Peninsula. Based on microthermometry and Raman spectroscopy, eight fluid systems have been recognized. The first fluid may be considered to be a pegmatitic fluid which is represented by daughter mineral (silicates)-rich aqueous inclusions. These inclusions are primary and formed above 500 °C (dissolution of daughter minerals). During pegmatite crystallization, this fluid evolved to a low-density, volatile-rich aqueous fluid with low salinity (93% H₂O; 5% CO₂; 0.5% CH₄; 0.2% N₂; 1.3% NaCl) at minimum P–T conditions around 3 ± 0.5 kbar and 420 °C. This fluid is related to rare-metal mineralization. The volatile enrichment may be due to mixing of magmatic fluids and fluids equilibrated with the host rock. A drop in pressure from 3 ± 0.5 to 1 kbar at a temperature above 420 °C, which may be due to the transition from predominantly lithostatic to hydrostatic pressure, is recorded by two-phase, water-rich inclusions with a low-density vapour phase (CO₂, CH₄ and N₂). Another inclusion type is represented by two-phase, vapour-rich inclusions with a low-density vapour phase (CO₂, CH₄ and N₂), indicating a last stage of decreasing temperature (360 °C) and pressure (around 0.5 kbar), probably due to progressive exhumation. Finally, volatile (CO₂)-rich aqueous inclusions, aqueous inclusions (H₂O-NaCl) and mixed-salt aqueous inclusions with low Th, are secondary in charac- ter and represent independent episodes of hydrothermal fluid circulation below 310 °C and 0.5 kbar. Received: 14 October 1999 / Accepted: 5 October 1999     

Columbite solubility in granitic melts: consequences for the enrichment and fractionation of Nb and Ta in the Earth's crust

The behaviour of niobium and tantalum in magmatic processes has been investigated by conducting MnNb₂O₆ and MnTa₂O₆ solubility experiments in nominally dry to water-saturated peralkaline (aluminium saturation index, A.S.I. 0.64) to peraluminous (A.S.I. 1.22) granitic melts at 800 to 1035 °C and 800 to 5000 bars. The attainment of equilibrium is demonstrated by the concurrence of the solubility products from dissolution, crystallization, Mn-doped and Nb- or Ta-doped experiments at the same pressure and temperature. The solubility products of MnNb₂O₆ (K_sp ^Nb) and MnTa₂O₆ (K_sp ^Ta) at 800 °C and 2 kbar both increase dramatically with alkali contents in water-saturated peralkaline melts. They range from 1.2 × 10⁻⁴ and 2.6 × 10⁻⁴ mol²/kg², respectively, in subaluminous melt (A.S.I. 1.02) to 202 × 10⁻⁴ and 255 × 10⁻⁴ mol²/kg², respectively, in peralkaline melt (A.S.I. 0.64). This increase from the subaluminous composition can be explained by five non-bridging oxygens being required for each excess atom of Nb⁵⁺ or Ta⁵⁺ that is dissolved into the melt. The K_sp ^Nb and K_sp ^Ta also increase weakly with Al content in peraluminous melts, ranging up to 1.7 × 10⁻⁴ and 4.6 × 10⁻⁴ mol²/kg², respectively, in the A.S.I. 1.22 composition. Columbite-tantalite solubilities in subaluminous and peraluminous melts (A.S.I. 1.02 and 1.22) are strongly temperature dependent, increasing by a factor of 10 to 20 from 800 to 1035 °C. By contrast columbite-tantalite solubility in the peralkaline composition (A.S.I. 0.64) is only weakly temperature dependent, increasing by a factor of less than 3 over the same temperature range. Similarly, K_sp ^Nb and K_sp ^Ta increase by more than two orders of magnitude with the first 3 wt% H₂O added to the A.S.I. 1.02 and 1.22 compositions, whereas there is no detectable change in solubility for the A.S.I. 0.64 composition over the same range of water contents. Solubilities are only slightly dependent on pressure over the range 800 to 5000 bars. The data for water-saturated sub- and peraluminous granites have been extrapolated to 600 °C, conditions at which pegmatites and highly evolved granites may crystallize. Using a melt concentration of 0.05 wt% MnO, 70 to 100 ppm Nb or 500 to 1400 ppm Ta are required for manganocolumbite and manganotantalite saturation, respectively. The solubility data are also used to model the fractionation of Nb and Ta between rutile and silicate melts. Predicted rutile/melt partition coefficients increase by about two orders of magnitude from peralkaline to peraluminous granitic compositions. It is demonstrated that the γNb₂O₅/γTa₂O₅ activity coefficient ratio in the melt phase depends on melt composition. This ratio is estimated to decrease by a factor of 4 to 5 from andesitic to peraluminous granitic melt compositions. Accordingly, all the relevant accessory phases in subaluminous to peraluminous granites are predicted to incorporate Nb preferentially over Ta. This explains the enrichment of Ta over Nb observed in highly fractionated granitic rocks, and in the continental crust in general. Received: 9 August 1996 / Accepted: 26 February 1997     

Liquidus temperatures and phase compositions in the system Qz-Ab-Or at 5 kbar and very low water activities

Liquidus phase relations have been experimentally determined in the systems Qz-Ab-Or-(H₂O), Qz-Ab-(H₂O) and Qz-Or-(H₂O) at H₂O-undersaturated conditions (a _H2O = 0.07) and P = 5 kbar. Starting materials were homogeneous synthetic glasses containing 1 wt% H₂O. The liquidus temperatures were bracketed by crystallization and dissolution experiments. The results of kinetic studies showed that crushed glasses are the best starting materials to overcome undercooling and to minimize the temperature difference between the lowest temperature of complete dissolution (melting) and the highest temperature at which crystallization can be observed. At P = 5 kbar and a _H2O = 0.07, the Qz-Ab eutectic composition is Qz₃₂Ab₆₈ at 1095 °C (±10 °C) and the Qz-Or eutectic is Qz₃₈Or₆₂ at 1030 °C (±10 °C). The minimum temperature of the ternary system Qz-Ab-Or is 990 °C (±10 °C) and the minimum composition is Qz₃₂Ab₃₅‐ Or₃₃. The Qz content of the minimum composition in the system Qz-Ab-Or-H₂O remains constant with changing a _H2O. The normative Or content, however, increases by approximately 10 wt% with decreasing a _H2O from 1 to 0.07. Such an increase has already been observed in the system Qz-Ab-Or-H₂O-CO₂ at high a _H2O and it is concluded that the use of CO₂ to reduce water activities does not influence the composition of the minima in quartz-feldspar systems. The determined liquidus temperature in melts with 1 wt% H₂O is very similar to that obtained in previous nominally “dry” experiments. This discrepancy is interpreted to be due to problems in obtaining absolutely dry conditions. Thus, the hitherto published solidus and liquidus temperatures for “dry” conditions are probably underestimated. Received: 27 March 1997 / Accepted: 1 October 1997     

Parental melts of melilitolite and origin of alkaline carbonatite: evidence from crystallised melt inclusions, Gardiner complex

Perovskite and melilite crystals from melilitolites of the ultramafic alkaline Gardiner complex (East Greenland) contain crystallised melt inclusions derived from: (1) melilitite; (2) low-alkali carbonatite; (3) natrocarbonatite. The melilitite inclusion (1) homogenisation temperature of 1060 °C is similar to liquidus temperatures of experimentally investigated natural melilitites. The compositions are peralkaline, low in MgO (ca.␣5 wt%), Ni and Cr, and they are low-pressure fractionates of more magnesian larnite-normative ultramafic lamprophyre-type melts of primary mantle origin. Low-alkali carbonatite compositions (2) homogenise at 1060–1030 °C and are compositionally similar to immiscible calcite carbonatite dykes derived from the melilitolite magma. Natrocarbonatite inclusions (3) homogenise between 1030 and 900 °C and are compositionally similar to natrocarbonatite lava from Oldoinyo Lengai. Nephelinitic to phonolitic dykes which are related to the calcite carbonatite dykes, are very Zr-rich and agpaitic (molecular Na₂O + K₂O/Al₂O₃ > 1.2) and resemble nephelinites of Oldoinyo Lengai. The petrographic, geochemical and temporal relationships indicate unmixing of carbonatite compositions (ca. 10% alkalies) from evolving melilitite melt and continued fractionation of melilitite to nephelinite. It is suggested that the natrocarbonatite compositions represent degassed supercritical high temperature fluid formed in a cooling body of strongly larnite-normative nephelinite or evolved melilitite. The Gardiner complex and similar melilitolite and carbonatite-bearing ultramafic alkaline complexes are believed to represent subvolcanic complexes formed beneath volcanoes comparable to Oldoinyo Lengai and that the suggested origin of natrocarbonatite may be applied to natrocarbonatites of Oldoinyo Lengai. Received: 18 January 1996 / Accepted: 2 September 1996     

Phase relations and chemistry of Ti-rich K-richterite-bearing mantle assemblages: an experimental study to 8.0 GPa in a Ti-KNCMASH system

Experiments have been conducted in a peralkaline Ti-KNCMASH system representative of MARID-type bulk compositions to delimit the stability field of K-richterite in a Ti-rich hydrous mantle assemblage, to assess the compositional variation of amphibole and coexisting phases as a function of P and T, and to characterise the composition of partial melts derived from the hydrous assemblage. K-richterite is stable in experiments from 0.5 to 8.0 GPa coexisting with phlogopite, clinopyroxene and a Ti-phase (titanite, rutile or rutile + perovskite). At 8.0 GPa, garnet appears as an additional phase. The upper T stability limit of K-richterite is 1200–1250 °C at 4.0 GPa and 1300–1400 °C at 8.0 GPa. In the presence of phlogopite, K-richterite shows a systematic increase in K with increasing P to 1.03 pfu (per formula unit) at 8.0 GPa/1100 °C. In the absence of phlogopite, K-richterite attains a maximum of 1.14 K pfu at 8.0 GPa/1200 °C. Titanium in both amphibole and mica decreases continuously towards high P with a nearly constant partitioning while Ti in clinopyroxene remains more or less constant. In all experiments below 6.0 GPa ΣSi + Al in K-richterite is less than 8.0 when normalised to 23 oxygens+stoichiometric OH. Rutiles in the Ti-KNCMASH system are characterised by minor Al and Mg contents that show a systematic variation in concentration with P(T) and the coexisting assemblage. Partial melts produced in the Ti-KNCMASH system are extremely peralkaline [(K₂O+Na₂O)/Al₂O₃ = 1.7–3.7], Si-poor (40–45 wt% SiO₂), and Ti-rich (5.6–9.2 wt% TiO₂) and are very similar to certain Ti-rich lamproite glasses. At 4.0 GPa, the solidus is thought to coincide with the K-richterite-out reaction, the first melt is saturated in a phlogopite-rutile-lherzolite assemblage. Both phlogopite and rutile disappear ca. 150 °C above the solidus. At 8.0 GPa, the solidus must be located at T≤1400 ^°C. At this temperature, a melt is in equilibrium with a garnet- rutile-lherzolite assemblage. As opposed to 4.0 GPa, phlogopite does not buffer the melt composition at 8.0 GPa. The experimental results suggest that partial melting of MARID-type assemblages at pressures ≥4.0 GPa can generate Si-poor and partly ultrapotassic melts similar in composition to that of olivine lamproites. Received: 23 December 1996 / Accepted: 20 March 1997     

The determination of partial melt compositions of peridotitic systems by melt inclusion synthesis

An experimental method of melt inclusion synthesis within olivine crystals has been developed to determine the composition of the melt present in a partially molten peridotite assemblage. Trace element doped peridotite was equilibrated with 5 wt% of a C-O-H volatile source at 20 kbar/1175 °C in a piston-cylinder apparatus under buffered oxygen and sulphur fugacity conditions [log(f _O2) ∼ IW +1 log unit, log (f _S2) ∼ Fe/FeS > +1 log unit]. A single crystal of olivine, which had been cut to a disc shape, was included in the sample capsule. At run conditions the peridotite charge formed olivine, orthopyroxene, clinopyroxene, Fe-Ni sulphide and a volatile-bearing melt. The melt phase is preserved as homogeneous glass inclusions up to 50 μm in size, trapped in situ in the olivine disc. The major element composition of the glass inclusions showed them to be of broadly basaltic character, but with a low Mg/(Mg + ΣFe), which is associated with precipitation of olivine from the melt inclusion onto the walls of the olivine disc during quenching. Thus the equilibrium melt composition has been calculated from the glass inclusion composition by addition of olivine component using the Fe/Mg exchange coefficient of Roeder and Emslie (1970); the desired Mg/(Mg + ΣFe) being determined from the composition of olivine formed at run conditions in the peridotite section of the charge. The melt composition obtained is close to the trend for dry melting established by Falloon and Green (1988), and it is evident that although the reduced volatiles in this case have induced a liquidus depression of some 250 °C, there has been only a small shift in melt composition. Trace element, carbon and hydrogen contents of thirteen melt inclusions have been determined by secondary ion mass spectrometry (SIMS). The trace element signature is consistent with ∼29% melting in equilibrium with a lherzolitic assemblage. The equilibrium melt has a C/H of 0.48 by weight. Carbon solubility in partial melts is thus significant under reducing conditions in the presence of dissolved “water components” and establishes a major melt fluxing role for carbon in the upper mantle. The ubiquitous presence of carbon and hydrogen in basaltic magmas underscores the importance of determining both the position of vapour-present solidi and the composition of melts generated, when developing petrogenetic models. Received: 1 July 1996 / Accepted: 25 June 1997     

Interdiffusion of Na,K,and Ca Between Granitic Melts and NaCl Liquid

This study is aimed at determining the diffusion coefficient of net-work modifiers (mainly Na, K, and Ca) in a two-phase melt-NaCl system, in which the melts are granitic and the system is NaCl-rich in composition. The diffusion coefficients of Na, K, and Ca were measured at the temperatures of 750 – 1400°C, pressures of 0.001 × 10⁸ – 2 × 10⁸ Pa, and initial H₂O contents of 0 wt% –6.9 wt% in the granitic melts. The diffusion coefficients of Fe and Mg were difficult to resolve. In all experiments a NaCl melt was present as well. In the absence of H₂O, the diffusion of net-work modifiers follows an Arrhanious equation at 1 × 10⁵ Pa: lgD_ca=−3. 88−5140/T, lgD_k =−3. 79−4040/T, and lgD_Na, =−4.99−3350/T, where D is in cm² /s andT is in K. The diffusion coefficients of Ca, Na, K, and Fe increase non-linearly with increasing H₂O content in the melt. The presence of about 2 wt% H₂O m the melt will lead to a dramatical increase in diffusivity, but higher H₂O content has only a minor effect. This change is probably the result of a change in the melt structure when H₂O is present. The diffusion coefficients measured in this study are significantly different from those in previous works. This may be understood in terms of the “transient two-liquid equilibrium” theory. Element interdiffusion depends not only on its concentration, but also on its activity co-efficient gradient, which is reflected by the distribution coefficient, of the two contacting melts.     

Models of corundum origin from alkali basaltic terrains: a reappraisal

Corundums from basalt fields, particularly in Australia and Asia, include a dominant blue-green-yellow zoned “magmatic” suite (BGY suite) and subsidiary vari-coloured “metamorphic” suites. The BGY corundums have distinctive trace element contents (up to 0.04 wt% Ga₂O₃ and low Cr/Ga and Ti/Ga ratios <1). Different melt origins for BGY corundums are considered here from their inclusion and intergrowth mineralogy, petrologic associations and tectonic setting. Analysed primary inclusion minerals (over 100 inclusions) cover typical feldspars, zircon and Nb-Ta oxides and also include hercynite-magnetite, gahnospinel, rutile-ilmenite solid solution, calcic plagioclase, Ni-rich pyrrhotite, thorite and low-Si and Fe-rich glassy inclusions. This widens a previous inclusion survey; New England, East Australia corundums contain the most diverse inclusion suite known from basalt fields (20 phases). Zircon inclusion, intergrowth and megacryst rare earth element data show similar patterns, except for Eu which shows variable depletion. Temperature estimates from magnetite exsolution, feldspar compositions and fluid inclusion homogenization suggest that some corundums crystallized between 685–900 °C. Overlap of inclusion Nb, Ta oxide compositions with new comparative data from niobium-yttrium-fluorine enriched granitic pegmatites favour a silicate melt origin for the corundums. The feasibility of crystallizing corundum from low-volume initial melting of amphibole-bearing mantle assemblages was tested using the MELTS program on amphibole-pyroxenite xenolith chemistry from basalts. Corundum appears in the calculations at 720–880 °C and 0.7–1.1 GPa with residual feldspathic assemblages that match mineral compositions found in corundums and their related xenoliths. A model that generates melts from amphibole-bearing lithospheric mantle during magmatic plume activity is proposed for BGY corundum formation. Received: 3 January 1997 / Accepted: 8 July 1998     

The origin and evolution of the parental magmas of frontal volcanoes in Kamchatka: Evidence from magmatic inclusions in olivine from Zhupanovsky volcano

The paper presents data on naturally quenched melt inclusions in olivine (Fo 69–84) from Late Pleistocene pyroclastic rocks of Zhupanovsky volcano in the frontal zone of the Eastern Volcanic Belt of Kamchatka. The composition of the melt inclusions provides insight into the latest crystallization stages (∼70% crystallization) of the parental melt (∼46.4 wt % SiO₂, ∼2.5 wt % H₂O, ∼0.3 wt % S), which proceeded at decompression and started at a depth of approximately 10 km from the surface. The crystallization temperature was estimated at 1100 ± 20°C at an oxygen fugacity of ΔFMQ = 0.9–1.7. The melts evolved due to the simultaneous crystallization of olivine, plagioclase, pyroxene, chromite, and magnetite (Ol: Pl: Cpx: (Crt-Mt) ∼ 13: 54: 24: 4) along the tholeiite evolutionary trend and became progressively enriched in FeO, SiO₂, Na₂O, and K₂O and depleted in MgO, CaO, and Al₂O₃. Melt crystallization was associated with the segregation of fluid rich in S-bearing compounds and, to a lesser extent, in H₂O and Cl. The primary melt of Zhupanovsky volcano (whose composition was estimated from data on the most primitive melt inclusions) had a composition of low-Si (∼45 wt % SiO₂) picrobasalt (∼14 wt % MgO), as is typical of parental melts in Kamchatka and other island arcs, and was different from MORB. This primary melt could be derived by ∼8% melting of mantle peridotite of composition close to the MORB source, under pressures of 1.5 ± 0.2 GPa and temperatures 20–30°C lower than the solidus temperature of “dry” peridotite (1230–1240°C). Melting was induced by the interaction of the hot peridotite with a hydrous component that was brought to the mantle from the subducted slab and was also responsible for the enrichment of the Zhupanovsky magmas in LREE, LILE, B, Cl, Th, U, and Pb. The hydrous component in the magma source of Zhupanovsky volcano was produced by the partial slab melting under water-saturated conditions at temperatures of 760–810°C and pressures of ∼3.5 GPa. As the depth of the subducted slab beneath Kamchatkan volcanoes varies from 100 to 125 km, the composition of the hydrous component drastically changes from relatively low-temperature H₂O-rich fluid to higher temperature H₂O-bearing melt. The geothermal gradient at the surface of the slab within the depth range of 100–125 km beneath Kamchatka was estimated at 4°C/km.