Crystallization of primitive basaltic magmas at crustal pressures and genesis of the calc-alkaline igneous suite: experimental evidence from St Vincent,Lesser Antilles arc

Near-liquidus crystallization experiments have been carried out on two basalts (12.5 and 7.8 wt% MgO) from Soufriere, St Vincent (Lesser Antilles arc) to document the early stages of differentiation in calc-alkaline magmas. The water-undersaturated experiments were performed mostly at 4 kbar, with 1.6 to 7.7 wt% H₂O in the melt, and under oxidizing conditions (ΔNNO = −0.8 to +2.4). A few 10 kbar experiments were also performed. Early differentiation of primitive, hydrous, high-magnesia basalts (HMB) is controlled by ol + cpx + sp fractionation. Residual melts of typical high-alumina basalt (HAB) composition are obtained after 30–40% crystallization. The role of H₂O in depressing plagioclase crystallization leads to a direct relation between the Al₂O₃ content of the residual melt and its H₂O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo_89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H₂O) and very oxidized (ΔNNO = +1.5–2.0) HMB liquids at middle crustal pressures and temperatures from ∼1,160 to ∼1,060°C. Hornblende played no role in the early petrogenetic evolution. Derivative HAB melts may contain up to 7–8 wt% dissolved H₂O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H₂O concentrations (2–5 wt%).     

Water content of primitive low-K tholeiitic basalt magma from Iwate Volcano,NE Japan arc: implications for differentiation mechanism of frontal-arc basalt magmas

The water content of low-K tholeiitic basalt magma from Iwate volcano, which is located on the volcanic front of the NE Japan arc, was estimated using multi-component thermodynamic models. The Iwate lavas are moderately porphyritic, consisting of ~8 vol.% olivine and ~20 vol.% plagioclase phenocrysts. The olivine and plagioclase phenocrysts show significant compositional variations, and the Mg# of olivine phenocrysts (Mg#78–85) correlates positively with the An content of coexisting plagioclase phenocrysts (An_85–92). The olivine phenocrysts with Mg# > ~82 do not form crystal aggregates with plagioclase phenocrysts. It is inferred from these observations that the phenocrysts with variable compositions were primarily derived from mushy boundary layers along the walls of a magma chamber. By using thermodynamic calculations with the observed petrological features of the lavas, the water content of the Iwate magma was estimated to be 4–5 wt.%. The high water content of the magma supports the recent consensus that frontal-arc magmas are remarkably hydrous. Using the estimated water content of the Iwate magma, the water content and temperature of the source mantle were estimated. Given that the Iwate magma was derived from a primary magma solely by olivine fractionation, the water content and temperature were estimated to be ~0.7 wt.% and ~1,310 °C, respectively. Differentiation mechanisms of low-K frontal-arc basalt magmas were also examined by application of a thermodynamics-based mass balance model to the Iwate magma. It is suggested that magmatic differentiation proceeds primarily through fractionation of crystals from the main molten part of a magma chamber when it is located at <~200 MPa, whereas magma evolves through a convective melt exchange between the main magma and mushy boundary layers when the magma body is located at >~200 MPa.     

Concentrations,sources, and losses of H2O,CO2, and S in Kilauean basalt

Basaltic glasses included in olivine phenocrysts from Kilauea volcano contain concentrations of H₂O, CO₂, and S similar to glassy Kilauean basalt dredged from the deep sea floor and greater than vesicular, subaerial Kilauean basalt. Our result contrasts with earlier reports that inclusions of basaltic glass in phenocrysts have little or no H₂O and large ratios of $\text{CO}{}_2\text{H}{}_2\text{O}$. Our analysed inclusions of glass are larger than 100 micrometers thick and similar in chemical composition to the host glass surrounding the olivine crystals indicating that the trapped melts are representative of the bulk liquid from which the crystals grew. Crystallization of about 2–8% of olivine from the melts after they were trapped is indicated by slight departures from the experimentally established equilibrium distribution of Mg and Fe between olivine and liquid. The measured concentrations of CO₂ correspond to phenocryst crystallization pressures of about 1.3 kbar for a subaerial basalt and about 5 kbar for a submarine basalt, consistent with geophysical models of Kilauea volcano. The compositions of volcanic gas predicted from our analyses are consistent with restored compositions of actual Kilauean gases. The rate of sulfur emission predicted from our analyses is greater than the sulfur dioxide emission rate observed during repose, but probably consistent with total degassing including eruptive episodes. The concentrations of H₂O, K₂O, Cl, and P in parental Kilauean basalt can be derived from upper mantle phlogopitic mica, pargasitic amphibole and apatite with compositions close to those of natural primary minerals in ultramafic xenoliths from continental kimberlites, or solely from apatite and phlogopitic mica with $\text{H}{}_2\text{O}\text{K}{}_2\text{O}$ near 0.47 ± 0.03, slightly higher than the range of values reported. The amounts of phlogopitic mica and pargasitic amphibole contributing volatiles to Kilauean tholeiite is about 10 percent by mass of the parental liquid, or about 5% if the source does not include amphibole. In view of an estimated 20% of partial melting of mantle source rock to produce Kilauean tholeiites, there may be about 2 weight percent of mica plus amphibole in part of the mantle beneath Kilauea, or about 1 weight percent of phlogopitic mica if amphibole is absent.     

花岗岩岩浆中H2O含量测定方法: 综述

花岗岩岩浆中的H₂O含量通过影响熔体物理化学性质, 进而控制了花岗岩岩浆的结晶粒度、岩浆侵位深度以及某些金属元素迁移、富集的过程。因此, 对花岗岩熔体包裹体开展H₂O含量的定量研究具有重要的地质意义。目前, 测试花岗岩岩浆中H₂O含量的方法可分为间接估算和直接测量两种方法。间接估算法, 如利用花岗岩熔体的粘度和岩浆中H₂O溶解度模型进行岩浆H₂O含量估算, 其H₂O含量数据的准确性严重依赖于花岗岩熔体组成、熔体包裹体精确的温压参数; 直接测量法, 如利用傅里叶变换红外光谱(FTIR)、电子探针(EPMA)、二次离子探针(SIMS)等对熔体包裹体H₂O含量直接开展原位微区分析, 这些测试技术具有样品制备繁琐、H₂O容易泄漏、红外光谱分析光斑大, 测试结果受控因素较多等特点, 容易降低测试精度。激光拉曼测试法作为直接测量法中的重要技术, 具有样品制备简单, 原位、无损分析测试的特点, 本文认为激光拉曼在花岗岩岩浆H₂O含量的定量研究中具有较好的应用和推广前景。今后, 可尝试建立热液金刚石压腔+激光拉曼原位检测熔体包裹体H₂O含量的测试方法。

     

The role of H2O during crystallization of primitive arc magmas under uppermost mantle conditions and genesis of igneous pyroxenites: an experimental study

Exposed, subduction-related magmatic arcs commonly include sections of ultramafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the effects of variable H₂O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H₂O concentration in the liquid changes the crystallization sequence. Low H₂O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H₂O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas. Density calculations on the residual solid assemblages indicate that ultramafic plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm³. The combination of high pressure and high H₂O concentration in the liquid suppresses plagioclase crystallization, so that ultramafic plutonic rocks form over a significant proportion of the crystallization interval (up to 50% crystallization of ultramafic rocks from initial, mantle-derived liquids). This suggests that in subduction-related magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO₂, Al₂O₃) of igneous crust in arcs.     

An experimental study of the solubility of molybdenum in H2O and KCl-H2O solutions from 500 °C to 800 °C, and 150 to 300 MPa

The solubility of molybdenum (Mo) was determined at temperatures from 500 °C to 800 °C and 150 to 300 MPa in KCl-H₂O and pure H₂O solutions in cold-seal experiments. The solutions were trapped as synthetic fluid inclusions in quartz at experimental conditions, and analyzed by laser ablation inductively coupled plasma mass spectrometry (LA ICPMS).Mo solubilities of 1.6 wt% in the case of KCl-bearing aqueous solutions and up to 0.8 wt% in pure H₂O were found. Mo solubility is temperature dependent, but not pressure dependent over the investigated range, and correlates positively with salinity (KCl concentration). Molar ratios of ∼1 for Mo/Cl and Mo/K are derived based on our data. In combination with results of synchrotron X-ray absorption spectroscopy of individual fluid inclusions, it is suggested that Mo-oxo-chloride complexes are present at high salinity (>20 wt% KCl) and ion pairs at moderate to low salinity (<11 wt% KCl) in KCl-H₂O aqueous solutions. Similarly, in the pure H₂O experiments molybdic acid is the dominant species in aqueous solution. The results of these hydrothermal Mo experiments fit with earlier studies conducted at lower temperatures and indicate that high Mo concentrations can be transported in aqueous solutions. Therefore, the Mo concentration in aqueous fluids seems not to be the limiting factor for ore formation, whereas precipitation processes and the availability of sulfur appear to be the main controlling factors in the formation of molybdenite (MoS₂).     

Sources, degassing, and contamination of CO2, H2O, He, Ne, and Ar in basaltic glasses from Kolbeinsey Ridge, North Atlantic

New volatile data (CO₂, H₂O, He, Ne, and Ar) are presented for 24 submarine basaltic glasses from the Kolbeinsey Ridge, Tjörnes Fracture Zone and Mohns Ridge, North Atlantic. Low CO₂ and He contents indicate that magmas were strongly outgassed with the extent of degassing increasing toward the south, as expected from shallower ridge depths. Ne and Ar are significantly more abundant in the southernmost glasses than predicted for degassed melt. The strong atmospheric isotopic signal associated with this excess Ne and Ar suggests syn- or posteruptive contamination by air. Degassing, by itself, cannot generate the large variations in δ¹³C values of dissolved CO₂ or coupled CO₂-Ar variations. This suggests that δ¹³C values were also affected by some other processes, most probably melt-crust interaction. Modelling indicates that degassing had a negligible influence on water owing to its higher solubility in basaltic melt than the other volatiles. Low H₂O contents in the glasses reflect melting of a mantle source that is not water-rich relative to the source of N-MORB.Before eruption, Kolbeinsey Ridge melts contained ∼400 ppm CO₂ with δ¹³C of −6‰, 0.1 to 0.35 wt.% H₂O, ³He/⁴He ∼11 R_A, and CO₂/³He of ∼2 × 10⁹. We model restored volatile characteristics and find homogeneous compositions in the source of Kolbeinsey Ridge magmas. Relative to the MORB-source, He and Ne are mildly fractionated while the ⁴⁰Ar/³⁶Ar may be low. The ³He/⁴He ratios in Tjörnes Fracture Zone glasses are slightly higher (13.6 R_A) than on Kolbeinsey Ridge, suggesting a greater contribution of Icelandic mantle from the south, but the lack of ³He/⁴He variation along the Kolbeinsey Ridge is inconsistent with active dispersal of Icelandic mantle beyond the Tjörnes Fracture Zone.     

The upper thermal stability of chlorite + quartz: an experimental study in the system MgO-Al2O3-SiO2-H2O

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H₂O, chlorite + quartz = talc + cordierite + H₂O, and talc + kyanite + quartz = cordierite ± H₂O by reversed runs in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). These reaction curves intersect at an invariant point (IP1) at PH₂O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H₂O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H₂O at water pressures is clearly positive. The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg_4.85Al_1.15[Al_1.15Si_2.85O₁₀](OH)₈ over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg_2.94Al_0.06[Al_0.06Si_3.94O₁₀](OH)₂ at PH₂O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H₂O = 21 kbar. As a consequence of the new experimental data, the existing phase topologies of the MASH-system and K₂O-MASH-system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H₂O = chlorite + cordierite are stable. For this reason, the development of medium- to high-temperature metamorphic rocks compositionally approximating the MASH-system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe²⁺ into magnesian chlorite extends its stability field in the presence of quartz by 10°-15°C in comparison with pure Mg-chlorite.     

H2O contents and D/H ratios of nominally anhydrous minerals from ultrahigh-pressure eclogites of the Dabie orogen, eastern China

Garnet and omphacite from ultrahigh-pressure eclogites from the Dabie orogen, eastern China were investigated by Micro-FTIR. The results show that all garnet and omphacite grains contain structural water occurring as hydroxyl (OH), with H₂O contents varying from 14 to 1915 ppm (H₂O wt) and from 105 to 695 ppm, respectively. Within the same sample, the water contents are either homogeneous at the grain scale or vary systematically from higher in the core to lower in the rim. Low water contents at crystal rims possibly result from hydroxyl exsolution after pressure decrease upon rock exhumation. The δD values of omphacites are between −108.4‰ and −114.2‰, and independent of water contents. Heterogeneous water contents of garnet occur at the centimeter scale and fluid mobility during UHP metamorphism was very limited. The estimated whole-rock water content based on mineral H₂O contents is between 260 and 750 ppm, thereby implying that eclogitic rocks formed during continental subduction have the potential to recycle (at least) several hundreds ppm water to mantle depths. The preserved chemical differences likely indicate that the eclogitic rocks resided at mantle conditions for a limited time span, and imply that they were exhumed shortly after subduction. The water released during decompression might represent the early-stage retrograde fluid.     

Polymerization of silicate and aluminate tetrahedra in glasses,melts and aqueous solutions—II. The network modifying effects of Mg2+ , K+, Na+, Li+, H+, OH−, F−, Cl−, H2O,CO2 and H3O+ on silicate polymers

The effect of the group IA and VIIA ions, as well as Mg²⁺, and the molecules H₂O, CO₂, H₃O⁺ and OH^? on the energy of the Si-O bond in a H₆Si₂O₇ cluster has been calculated using semiempirical molecular orbital calculations (CNDO/2). Three types of elementary processes, i.e. substitution, addition, and polymerization reactions have been used to interpret data on the dynamic viscosity, surface tension and surface charge, hydrolytic weakening, diffusivity, conductivity, freezing point depression, and degree of polymerization of silicates in melts, glasses, and aqueous solutions. As a test of our calculational procedure, observed X-ray emission spectra of binary alkali silicate glasses were compared with calculated electronic spectra. The well known bondlength variations between the bridging bond [Si-O(br)] and the non-bridging bond [Si-O(nbr)] in alkali silicates are shown to be due to the propagation of oscillating bond-energy patterns through the silica framework. A kinetic interpretation of some results of our calculations is given in terms of the Bell-Evans-Polanyi reaction principle.     

The microdetermination of H2O,CO2, and SO2 in glass using a 1280°C microscope vacuum heating stage,cryopumping, and vapor pressure measurements from 77 to 273 K

A new microscope vacuum heating stage and gas analyzer has been developed for measurement of H₂O, CO₂, SO₂, and noncondensable gas (H₂, CO, N₂, Ar, CH₄, etc.) evolved from samples, particularly natural glass, at temperatures up to 1280°C. The gas evolved upon heating to 1280δC is collected in a liquid nitrogen cold trap. Gas components are identified by the characteristic vapor pressure and temperature ranges over which solid and vapor are in equilibrium during sublimation of individual components. The masses of CO₂, SO₂, and H₂O derived from samples and blanks are calculated using the ideal gas law, the molecular weights of the components, and the gauge constant (i.e. the ratio of the number of moles of a gas to its partial pressure in the constant volume). Results obtained by repeated determinations of H₂O, CO₂, and SO₂ evolved from a submarine basaltic glass from Kilauea volcano, Hawaii, (average sample mass = 3 × 10^?3 g) gave probable errors for the determinations of H₂O (0.23%), CO₂ (0.025%), and S (0.071%) equal to 4, 10, and 8% respectively, of the concentrations. Determinations of H₂O in smaller samples of H₂O-poor basaltic pumice show a linear proportionality (0.063%) between the measured H₂O and the sample mass over the range 0.1 × 10^?6 to 1.7 × 10^?6 g H₂O. Comparisons of H₂O determinations by this technique with those obtained by Penfield, gas chromatic, microcoulometric, and vacuum fusion techniques used elsewhere show reasonably good agreement. Determinations of SO₂ by this technique agree reasonably well X-ray fluorescence and electron microprobe determinations of sulfur. Determinations of CO₂ by the present technique are reproducible but cannot be compared directly to measurements made in other labs because of differences in samples analyzed. The principle advantages of this analytical technique are the very small sample required, the simultaneous determination of H₂O, CO₂, SO₂ and noncondensable gas, the avoidance of calibration procedures dependent on chemical standards, and the visual observations that can be made during sample outgassing.     

Enrichment of SiO2 in rhyolites by fractional crystallization: An experimental study of peraluminous granitic rocks from the St. Francois Mountains, Missouri, USA

Experiments were conducted to test the hypothesis that higher silica rhyolites of the St. Francois Mountains, Missouri, USA are products of fractional crystallization of lower silica granitic magmas. Experiments were carried out at pressures of 0.5 and 1.0 kb under water-saturated conditions, temperatures ranged from 800° to 925°C and the oxygen fugacity for all experiments were maintained at, or near, the nickel-nickel oxide (NNO) buffer level.

Results of experiments with Butler Hill granite indicate that fractionation of near liquidus silicate phases, orthopyroxene and plagioclase, causes an enrichment of SiO₂ in the residual melts. Mineralogical and chemical compositions of the experimental charges are similar to that of higher silica Grassy Mountain rhyolites of the St. Francois Mountains. Experiments also show that at pressures of 1.0 Kb or higher orthopyroxene reacts with the hydrous melt to produce biotite which is a common phase in the plutonic rocks of the St. Francois Mountains.     

Evolution of H2O content in a polymetamorphic terrane: the Plattengneiss Shear Zone (Koralpe, Austria)

The Koralpe of the Eastern European Alps experienced high-temperature/low-pressure metamorphism (∼650 °C and 6.5 kbar) during the Permian and eclogite facies metamorphism (∼700 °C and 14 kbar) during the Eo-Alpine (Cretaceous) metamorphic event. In the metapelitic Plattengneiss shear zone that constitutes much of the Koralpe, the second metamorphism caused only partial re-equilibration of the assemblages formed during the first metamorphism. It is shown here that the Eo-Alpine mineral assemblage, garnet + biotite + muscovite + plagioclase + quartz (with or without kyanite), formed under low water activity conditions that are consistent with the level of dehydration that occurred during the Permian event. This implies that the rocks were essentially closed-system from the peak of the Permian metamorphism through the Eo-Alpine event. The evolution of water content of the rocks is traced through time: that prograde dewatering during the Permian metamorphic event terminated at the metamorphic peak with a water content around 3–4 mol.%. The water content remained then constant and led to water-undersaturation during the subsequent Eo-Alpine metamorphism. From the water content and activity evolution a post-peak isothermal decompression path close to the solidus is inferred for the Eo-Alpine event.     

Mineral-solution equilibria—I. An experimental study of complexing and thermodynamic properties of aqueous MgCl2 in the system MgO-SiO2-H2O-HCl

Speciation of aqueous magnesium in the system MgO-SiO₂-H₂O-HCl in supercritical aqueous fluids has been investigated using standard rapid-quench hydrothermal techniques and a modification of the Ag + AgCl buffer method (Frantz and Eugster, 1973. Am. J. Sci.267, 268–286). A concentric double-capsule charge was utilized. The outer gold capsule contained the assemblage talc + quartz + Ag + AgCl + H₂O-MgCl₂ fluid; the inner platinum capsule, Ag + AgCl + H₂O-HCl fluid. During the experiments, and thus $\text{?}{}_{\mathrm{HCl}}$ equilibrated between the two capsules. After quenching, measurement of the chloride concentration in the fluid in the inner capsule and total magnesium in the fluid in the outer capsule defines the concentrations of HCl and Mg that coexist with talc + quartz in the outer capsule. Changes in the measured molality of HCl as a function of the total magnesium concentration at constant P and T were used to identify the predominant species of magnesium in the hydrothermal fluid. Experimental results showed that at 2000 bar, MgCl°₂ is the predominant species above 550°C and Mg²⁺, below 400°C. Data at intermediate temperatures when combined with the dissociation constant for HCl were used to obtain the dissociation constant for MgCl°₂. The results of these experiments were combined with results from experiments using Ag + AgCl in conjunction with the oxygen buffer, hematite-magnetite, to obtain the equilibrium constant for the reaction $\text{1}\text{3}\text{Talc + 2HC1° H}{}_2\text{O MgCl°}{}_2\text{+}\text{4}\text{3}\text{Quartz +}\text{4}\text{3}\text{H}{}_2\text{O}$ from which the difference in Gibbs free energy of MgCl°₂ and HC1° was obtained as a function of temperature at 1000, 1500 and 2000 bar pressure, Solubility constants for brucite. forsterite, chrysotile, and talc were calculated.     

Dualite, Na30(Ca,Na,Ce,Sr)12(Na,Mn,Fe,Ti)6Zr3Ti3MnSi51O144(OH,H2O,Cl)9, a new zircono-titanosilicate with a modular eudialyte-like structure from the Lovozero alkaline Pluton, Kola Peninsula, Russia

Dualite has been found at Mount Alluaiv, the Lovozero Pluton, the Kola Peninsula in peralkaline pegmatoid as sporadic, irregularly shaped grains up to 0.3–0.5 mm across. K-Na feldspar, nepheline, sodalite, cancrinite, aegirine, alkaline amphibole, eudialyte, lovozerite, lomonosovite, vuonnemite, lamprophyllite, sphalerite, and villiaumite are associated minerals. Dualite is yellow, transparent or translucent, with conchoidal fracture. The new mineral is brittle, with vitreous luster and white streaks. The Mohs hardness is 5. The measured density is 2.84(3) g/cm₃ (volumetric method); the calculated density is 2.814 g/cm₃. Dualite dissolves and gelates in acid at room temperature. It is nonfluorescent. The new mineral is optically uniaxial and positive; ω = 1.610(1), ɛ = 1.613(1). Dualite is trigonal, space group R3m. The unit cell dimensions are a = 14.153(9), c = 60.72(5) ?, V = 10533(22) ?, Z = 3. The strongest reflections in the X-ray powder pattern [d, ? (I,%)(hkl)] are as follows: 7.11(40)(110), 4.31(50)(0.2.10), 2.964(100)(1.3.10), 2.839(90)(048), 2.159(60)(2.4.10, 0.4.20), 1.770(60)(2.4.22, 4.0.28, 440), 1362(50)(5.5.12, 3.0.42). The chemical composition (electron microprobe, H₂O calculated from X-ray diffraction data) is as follows, wt %: 17.74 Na₂O, 0.08 K₂O, 8.03 CaO, 1.37 SrO, 0.29 BaO, 2.58 MnO, 1.04 FeO, 0.79 La₂O₃, 1.84 C₂O₃, 0.88 Nd₂O₃, 0.20 Al₂O₃, 51.26 SiO₂, 4.40 TiO₂, 5.39 ZrO₂, 1.94 Nb₂O₅, 0.58 Cl, 1.39 H₂O,-O = 0.13 Cl₂; they total is 99.67. The empirical formula calculated on the basis of 106 cations as determined by crystal structure is (Na_29.79Ba_0.1K_0.10)_Σ30(Ca_8.55Na_1.39REE_1.27Sr_0.79)_Σ12 · (Na_3.01Mn_1.35Fe_0.87²⁺Ti_0.77)_Σ6(Zr_2.61Nb_0.39)_Σ3 (Ti_2.52Nb_0.48)_Σ3(Mn_0.82Si_0.18)_Σ1(Si_50.77Al_0.23)_Σ51 O₁₄₄[(OH)_6.54(H₂O)_1.34·Cl_0.98]_Σ8.86). The simplified formula is Na₃₀(Ca,Na,Ce,Sr)₁₂(Na,Mn,Fe,Ti)₆Zr₃Ti₃ MnSi₅₁O₁₄₄ (OH,H₂O,Cl)₉). The name dualite is derived from Latin dualis (dual) alluding to the dual taxonomic membership of this mineral, which is at the same time zirconosilicate and titanosilicate. The crystal structure is characterized by two module types (alluivite-like and eudialyte-like) alternating along a threefold axis with a doubled c period relative to eudialyte and close chemical affinity to rastsvetaevite (Khomyakov et al., 2006a) and labyrynthite (Khomyakov et al., 2006b). According to the authors’ crystal chemical taxonomy of the eudialyte group, the new mineral belongs to one of three subgroups characterized by a 24-layered structural framework. Dualite is a mineral formed during the final stages of peralkaline pegmatite formation. The type material of dualite is deposited at the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow. Original Russian Text ? A.P. Khomyakov, G.N. Nechelyustov, R.K. Rastsvetaeva, 2007, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2007, Pt CXXXVI, No. 4, pp. 68–73. Approved by the Commission on New Minerals and Mineral Names, International Mineralogical Association, July 8, 2005.     

H2S fluxes from Mt. Etna, Stromboli, and Vulcano (Italy) and implications for the sulfur budget at volcanoes

We present here new measurements of sulfur dioxide and hydrogen sulfide emissions from Vulcano, Etna, and Stromboli (Italy), made by direct sampling at vents and by filter pack and ultraviolet spectroscopy in downwind plumes. Measurements at the F0 and FA fumaroles on Vulcano yielded SO₂/H₂S molar ratios of ≈0.38 and ≈1.4, respectively, from which we estimate an H₂S flux of 6 to 9 t · d⁻¹ for the summit crater. For Mt. Etna and Stromboli, we found SO₂/H₂S molar ratios of ≈20 and ≈15, respectively, which combined with SO₂ flux measurements, suggest H₂S emission rates of 50 to 113 t · d⁻¹ and 4 to 8 t · d⁻¹, respectively. We observe that “source” and plume SO₂/H₂S ratios at Vulcano are similar, suggesting that hydrogen sulfide is essentially inert on timescales of seconds to minutes. This finding has important implications for estimates of volcanic total sulfur budget at volcanoes since most existing measurements do not account for H₂S emission.     

Partition of Sr,Ba, Ca,Y, Eu2+, Eu3+, and other REE between plagioclase feldspar and magmatic liquid: an experimental study

Plagioclase feldspar/magmatic liquid partition coefficients for Sr, Ba, Ca, Y, Eu²⁺, Eu³⁺ and other REE have been determined experimentally at 1 atm total pressure in the temperature range 1150–1400°C. Natural and synthetic melts representative of basaltic and andesitic bulk compositions were used, crystallizing plagioclase feldspar in the composition range An₃₅–An₈₅. Partition coefficients for Sr are greater than unity at all geologically reasonable temperatures, and for Ba are less than unity above approximately 1060°C. Both are strongly dependent upon temperature. Partition coefficients for the trivalent REE are relatively insensitive to temperature. At fixed temperature they decrease monotonically from La to Lu. The partition of Eu is a strong function of oxygen fugacity. Under extreme reducing conditions D_Eu approaches the value of D_Sr.     

The system H2O-NaCl. Part II: Correlations for molar volume, enthalpy, and isobaric heat capacity from 0 to 1000 °C, 1 to 5000 bar, and 0 to 1 XNaCl

A set of correlations for the volumetric properties and enthalpies of phases in the system H₂O-NaCl as a function of temperature, pressure, and composition has been developed that yields accurate values from 0 to 1000 °C, 1 to 5000 bar, and 0 to 1 X_NaCl. The volumetric properties of all fluid phases from low-density vapor to hydrous salt melts and single-phase binary fluids at high pressures and temperatures, can be described by a simple equation

     

The system H2O-NaCl. Part I: Correlation formulae for phase relations in temperature-pressure-composition space from 0 to 1000 °C, 0 to 5000 bar, and 0 to 1 XNaCl

Realistic simulations of fluid flow in geologic systems have severely been hampered by the lack of a consistent formulation for fluid properties for binary salt-water fluids over the temperature-pressure-composition ranges encountered in the Earth’s crust. As the first of two companion studies, a set of correlations describing the phase stability relations in the system H₂O-NaCl is developed. Pure water is described by the IAPS-84 equation of state. New correlations comprise the vapor pressure of halite and molten NaCl, the NaCl melting curve, the composition of halite-saturated liquid and vapor, the pressure of vapor + liquid + halite coexistence, the temperature-pressure and temperature-composition relations for the critical curve, and the compositions of liquid and vapor on the vapor + liquid coexistence surface. The correlations yield accurate values for temperatures from 0 to 1000 °C, pressures from 0 to 5000 bar, and compositions from 0 to 1 X_NaCl (mole fraction of NaCl). To facilitate their use in fluid flow simulations, the correlations are entirely formulated as functions of temperature, pressure and composition.