Anhydrous Melts Research Articles

Highly siderophile element systematics of the 3.3 Ga Weltevreden komatiites, South Africa: Implications for early Earth history

Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundances, Re–Os isotopic systematics, and major-, minor-, and lithophile trace element abundances are determined for a suite of remarkably fresh komatiites from the Weltevreden Formation (Fm.) of the Barberton Greenstone Belt, South Africa. The komatiite lavas examined are calculated to have contained ca. 31% MgO upon emplacement, and are among the most MgO-rich lavas ever erupted onto Earth's surface. Based on the partitioning behavior of V, indicating reduced conditions of magma formation, as well as the low calculated H 2O abundance of ~ 0.02% in the mantle source, anhydrous melting for the Weltevreden Fm. komatiites is inferred. The high calculated liquidus temperatures of the emplaced komatiite lavas of ca. 1600 °C require an unusually hot (~ 1800 °C) mantle source, > 100 °C hotter than the ambient contemporaneous mantle. These observations are most consistent with formation of the Weltevreden komatiites via deep (~ 530 km) melting in a mantle plume. The Re–Os isotopic data for twelve whole-rock samples and an olivine separate define a precise Re–Os isochron age of 3266 ± 8 Ma (2σ mean ) and an initial 187Os/ 188Os of 0.10440 ± 5 (γ 187Os = − 0.13 ± 0.05). The Re–Os data, coupled with the relatively high calculated HSE abundances in the mantle source totaling ~ 80% of those in modern Primitive Mantle (PM) estimates, indicate that at least some portions of the deep, early Archean mantle evolved with essentially chondritic Re/Os and absolute HSE abundances that are typical of the sources of late Archean komatiites. If the HSE budget of the terrestrial mantle was established via late accretion of several large planetesimals following the cessation of major chemical interaction between the core and the mantle, our data indicate that by 3.3 Ga, these materials were well homogenized within the mantle domains sampled by the Weltevreden komatiites and, by inference, within the mantle as a whole.

A general viscosity model of Campi Flegrei (Italy) melts

Viscosities of shoshonitic and latitic melts, relevant to the Campi Flegrei caldera magmas, have been experimentally determined at atmospheric pressure and 0.5 GPa, temperatures between 840 K and 1870 K, and H 2O contents from 0.02 to 3.30 wt.%. The concentric cylinder technique was employed at atmospheric pressure to determine viscosity of nominally anhydrous melts in the viscosity range of 10 1.5 − 10 3 Pa s. The micropenetration technique was used to determine the viscosity of hydrous and anhydrous melts at atmospheric pressure in the high viscosity range (10 10 Pa s). Falling sphere experiments were performed at 0.5 GPa in the low viscosity range (from 10 0.35 to 10 2.79 Pa s) in order to obtain viscosity data of anhydrous and hydrous melts. The combination of data obtained from the three different techniques adopted permits a general description of viscosity as a function of temperature and water content using the following modified VFT equation: l o g η = − a + b T − c + d T − e ⋅ e x p g ⋅ w T where η is the viscosity in Pa·s, T the temperature in K, w the H 2O content in wt.%, and a, b, c, d, e, and g are the VFT parameters. This model reproduces the experimental data (95 measurements) with a 1σ standard deviation of 0.19 and 0.22 log units for shoshonite and latite, respectively. The proposed model has been applied also to a more evolved composition (trachyte) from the same area in order to create a general model applicable to the whole compositional range of Campi Flegrei products. Moreover, speed data have been used to constrain the ascent velocity of latitic, shoshonitic, and trachytic melts within dikes. Using petrological data and volcanological information (geometrical parameters of the eruptive fissure and depth of magma storage), we estimate a time scale for the ascent of melt from 9 km to 4 km depth (where deep and shallow reservoirs, respectively, are located) in the order of few minutes. Such a rapid ascent should be taken into account for the hazard assessment in the Campi Flegrei area.

Trace element partitioning between ilmenite, armalcolite and anhydrous silicate melt: Implications for the formation of lunar high-Ti mare basalts

We performed a series of experiments at high pressures and temperatures to determine the partitioning of a wide range of trace elements between ilmenite (Ilm), armalcolite (Arm) and anhydrous lunar silicate melt, to constrain geochemical models of the formation of titanium-rich melts in the Moon. Experiments were performed in graphite-lined platinum capsules at pressures and temperatures ranging from 1.1 to 2.3 GPa and 1300–1400 °C using a synthetic Ti-enriched Apollo ‘black glass’ composition in the CaO–FeO–MgO–Al 2O 3–TiO 2–SiO 2 system. Ilmenite–melt and armalcolite–melt partition coefficients ( D) show highly incompatible values for the rare earth elements (REE) with the light REE more incompatible compared to the heavy REE ( D La Ilm – melt 0.0020 ± 0.0010 to D Lu Ilm – melt 0.069 ± 0.010 for ilmenite; D La Arm – melt 0.0048 ± 0.0023 to D Lu Arm – melt 0.041 ± 0.008 for armalcolite). D values for the high field strength elements vary from highly incompatible for Th, U and to a lesser extent W (for ilmenite: D Th Ilm – melt 0.0013 ± 0.0008, D U Ilm – melt 0.0035 ± 0.0015 and D W Ilm – melt 0.039 ± 0.005, and for armalcolite D Th Arm – melt 0.008 ± 0.003, D U Arm – melt 0.0048 ± 0.0022 and D W Arm – melt 0.062 ± 0.03), to mildly incompatible for Nb, Ta, Zr, and Hf (e.g. D Hf Ilm – melt 0.28 ± 0.05 and : D Hf Arm – melt 0.76 ± 0.07). Both minerals fractionate the high field strength elements with D Ta/ D Nb and D Hf/ D Zr between 1.3 and 1.6 for ilmenite and 1.3 and 1.4 for armalcolite. Armalcolite is slightly more efficient at fractionating Hf from W during lunar magma ocean crystallisation, with D Hf /D W = 12–13 compared to 6.7–7.5 for ilmenite. The transition metals vary from mildly incompatible to compatible, with the highest compatibilities for Cr in ilmenite ( D ∼ 7.5) and V in armalcolite ( D ∼ 8.1). D values show no clear variation with pressure in the small range covered. Crystal lattice strain modelling of D values for di-, tri- and tetravalent trace elements shows that in ilmenite, divalent elements prefer to substitute for Fe while armalcolite data suggest REE replacing Mg. Tetravalent cations appear to preferentially substitute for Ti in both minerals, with the exception of Th and U that likely substitute for the larger Fe or Mg cations. Crystal lattice strain modelling is also used to identify and correct for very small (∼0.3 wt.%) melt contamination of trace element concentration determinations in crystals. Our results are used to model the Lu–Hf–Ti concentrations of lunar high-Ti mare basalts. The combination of their subchondritic Lu/Hf ratios and high TiO 2 contents requires preferential dissolution of ilmenite or armalcolite from late-stage, lunar magma ocean cumulates into low-Ti partial melts of deeper pyroxene-rich cumulates.

The origin of high-Mg magmas in Mt Shasta and Medicine Lake volcanoes, Cascade Arc (California): higher and lower than mantle oxygen isotope signatures attributed to current and past subduction

We report the oxygen isotope composition of olivine and orthopyroxene phenocrysts in lavas from the main magma types at Mt Shasta and Medicine Lake Vol- canoes: primitive high-alumina olivine tholeiite (HAOT), basaltic andesites (BA), primitive magnesian andesites (PMA), and dacites. The most primitive HAOT (MgO ( 9 wt%) from Mt. Shasta has olivine d 18 O( d 18 OOl) values of 5.9-6.1%, which are about 1% higher than those observed in olivine from normal mantle-derived magmas. In con- trast, HAOT lavas from Medicine Lake have d 18 OOl values ranging from 4.7 to 5.5%, which are similar to or lower than values for olivine in equilibrium with mantle-derived magmas. Other magma types from both volcanoes show intermediate d 18 OOl values. The oxygen isotope composi- tion of the most magnesian lavas cannot be explained by crustal contamination and the trace element composition of olivine phenocrysts precludes a pyroxenitic mantle source. Therefore, the high and variable d 18 OOl signature of the most magnesian samples studied (HAOT and BA) comes from the peridotitic mantle wedge itself. As HAOT magma is generated by anhydrous adiabatic partial melting of the shallow mantle, its 1.4% range in d 18 OOl reflects a heter- ogeneous composition of the shallow mantle source that has been influenced by subduction fluids and/or melts sometime in the past. Magmas generated in the mantle wedge by flux melting due to modern subduction fluids, as exemplified by BA and probably PMA, display more homogeneous composition with only 0.5% variation. The high-d 18 O values observed in magnesian lavas, and prin- cipally in the HAOT, are difficult to explain by a single- stage flux-melting process in the mantle wedge above the modern subduction zone and require a mantle source enriched in 18 O. It is here explained by flow of older, pre- enriched portions of the mantle through the slab window beneath the South Cascades.

Experimental study of trace element partitioning between lunar orthopyroxene and anhydrous silicate melt: Effects of lithium and iron

Orthopyroxene (Opx) was present during a significant portion of crystallisation of the lunar magma ocean, but its influence on co-existing melt trace element contents is not well quantified. We performed high-pressure ( P, 1.1 to 3.2 GPa), high-temperature ( T, 1400 to 1600 °C) experiments on synthetic Fe-rich compositions at reducing conditions relevant to the lunar mantle to constrain trace element partitioning between Opx and anhydrous silicate melts. Opx–melt partition coefficients ( D Opx-melt) for a wide range of trace elements (LILE: Li, Ba; REE: La, Ce, Nd, Sm, Dy, Eu, Er, Tm Y, Yb, Lu; HFSE: Zr, Nb, Hf, Ta, Th, U); and transition metals (Sc, V, Mn, Co, Mo) show only very minor variations across the considered P– T range. REE partition coefficients increase from D La opx-melt = 0.0014 ± 0.0008 to D Lu opx-melt = 0.051 ± 0.007. D values for highly charged elements vary from D Th opx-melt = 0.0013 ± 0.0008 through D Nb opx-melt = 0.0018 ± 0.0006 and D U opx-melt = 0.0015 ± 0.0006 to D Ti opx-melt = 0.068 ± 0.0010. D Lu opx-melt/ D Hf opx-melt values of 6.3 ± 2.4 are at the high end of reported values for minerals that played a role during crystallisation of the lunar magma ocean, and higher than previously reported for Opx under identical oxygen fugacity conditions, implying Opx-rich cumulates in the lunar mantle have highly superchondritic Lu–Hf ratios. Lattice strain modelling of our REE partitioning data suggests that varying the concentration of divalent Fe in Opx very slightly decreases the ideal cation radius for M 3+ elements entering the M2 site, r 0 M2 , whereas the partitioning of M 3+ elements entering the M1 site is unaffected. A subtle increase in the maximum partition coefficient for M 3+ elements entering both the M1 and M2 sites with decreasing T is identified, when experiments carried out at similar reducing oxygen fugacities are considered. The presence of Li at concentrations of up to ~ 350 ppm does not have a measureable effect on the Opx–melt partitioning behaviour of REE or any other element, showing that charge-balancing of M 3+, M 4+ and M 5+ elements in Opx is likely dominated by a vacancy mechanism.

Radon diffusion in an anhydrous andesitic melt: a finite difference solution

Radon-222 diffusion in an anhydrous andesitic melt was investigated. The melts were glass discs formed artificially from melted volcanic materials. Solutions of the relevant diffusion equations were done by the explicit finite difference method. Results were compared to analytical solutions reported in the literature and good agreement was found. We have shown that the explicit finite difference method is effective and accurate for solving equations that describe 222Rn diffusion in andesitic melts, which is especially important when arbitrary initial and boundary conditions are required.

Long‐Aliphatic‐Segment Polyamides: Salt Preparation and Subsequent Anhydrous Polymerization

AbstractLong‐aliphatic‐segment polyamides were prepared based on hexamethylenediamine and α,ω‐(CH2)x biosynthetic diacids (x = 10, 11, 12). The pertinent monomers (salts) were isolated as solids, thoroughly characterized for the first time, and then submitted to an anhydrous melt prepolymerization technique. The obtained prepolymers exhibited $\overline {M} _{{\rm n}} $ in the range of 5 100–11 800 g · mol−1, and the molecular weight was further increased by up to 55% through solid‐state finishing. The suggested overall polyamidation cycle was conducted at short melt‐reaction times, so as to avoid any thermal degradation, and was proved efficient, indicating similar reactants polymerizability independently of the methylene content.magnified image

Two Contrasting H2O-rich Components in Primary Melt Inclusions from Mount Shasta

In addition to the abundant andesite and dacite lavas of the Mt. Shasta stratocone, primitive mafic lavas have erupted during the Quaternary from the Mt. Shasta region. Two types of basic lavas are the focus of this study: nearly anhydrous high-aluminium olivine tholeiites, produced by decompression melting, and basaltic andesites, produced by melting of a metasomatized mantle source (i.e. modified by various amounts of H2O-rich components). Here we provide further insight into the origin and the coexistence of these two types of magma based on a detailed study of the dissolved volatile contents in melt inclusions trapped in magnesium-rich olivine crystals from both tholeiites and basaltic andesites. The melt inclusions fall on the primitive extension of their respective host lava compositions: (1) the tholeiite melt inclusions are nearly anhydrous melts with low and clustered volatiles compositions; (2) the basaltic andesite melt inclusions are ‘wet’ melts with higher and more variable volatile contents (up to 2·6 wt% H2O, 820 ppm CO2, 1270 ppm Cl, 1220 ppm F and 6280 ppm S) than those of the tholeiite melt inclusions. The basaltic andesite melt inclusions are enriched in fluid-mobile elements and in incompatible trace elements: they display a stronger signature of slab-derived components than their host lavas. The selective enrichment of the basaltic andesite melt inclusions in fluid-mobile elements such as K, F, Cl, Ba and B as well as their contrasting δ11B (from −10·2 ± 1·3‰ to −3·2 ± 0·9‰ for the melt inclusions from sample 95-15 and from −4·9 ± 1·1‰ to +4·4 ± 1·1‰ for the melt inclusions from samples 85-1a and 85-47) show the imprints of two distinct slab-derived components C1 and C2. Using trace element ratios, we modelled the compositions of these two H2O-rich components (C1 with Cl/F of 1·1 is rich in both in high field strength elements and incompatible trace elements, whereas C2 with Cl/F of 4·0 is poorer in trace elements) and show that they represent mixing between sediment melts and dehydration fluids from the altered oceanic crust.

Calibration of a diamond capsule cell assembly for in situ determination of liquid properties in the Paris–Edinburgh press

We describe the calibration of a Paris–Edinburgh press cell assembly, centred around a single-crystal diamond sample capsule. The combination of low X-ray absorbance and diffraction, limited reaction with the sample material, and extreme incompressibility preserving the sample geometry makes single-crystal diamonds ideal for in situ measurements of liquid properties such as density, viscosity, and short-range structure. Here, we use in situ synchrotron X-ray diffraction measurements at pressures up to 5.5 GPa and temperatures up to ∼ 2000 K to show that diamond incompressibility significantly impairs pressure transmission to sample materials at low temperature. At room temperature, diffraction patterns of pressure calibrants situated inside the diamond sample capsule show that sample pressure remains constant at ∼ 0.3 GPa, even when the same pressure calibrants positioned outside the capsule show assembly pressure reaches up to ∼ 3.0 GPa. During heating, inside and outside pressures converge, with full pressure transmission achieved at 1150–1250 K. Our results demonstrate that diamond sample capsules are suitable for property determinations of most anhydrous silicate and iron-rich metal melts, but that sample pressures in low-temperature experiments (e.g. focusing on properties of hydrous fluids and melts) could be significantly overestimated.

Cassiterite-saturated minimum melting behavior within Sn-SnO2-SiO2 at 1 atm and 10 kbar

Anhydrous minimum-melting equilibria saturated with SnO 2 form a pseudo-binary within Sn- SnO 2 -SiO 2 close to the join SnO-SiO 2 . This relationship, previously observed in various studies at 1 atmosphere and confirmed here, is explored more uniformly across the full compositional range of the system at pressure of ca. 10 kbars using Re capsules. Eutectic points and curves increase significantly in temperature with 10-11.6 kbars of pressure with only modest shifts of eutectic composition from that at ca. 1 bar pressure. No silicate liquid immiscibility was encountered in the minimum-melting regime although it may develop with increasing pressure in SiO 2 -rich compositions. Possible configurations for the equilibria at temperatures greater than minimum melting suggest wide fields of Sn-SiO 2 and SnO 2 -SiO 2 liquid immiscibility on either side of the SnO-SiO 2 join. The SnO-SiO 2 pseudo-binary involves a trough between the two thermo-compositional immiscibility maxima that reflects the thermal stability and volatility of the SnO dimer and its complexes. In general, these data support high solubility of tin in anhydrous silicate melts, at geologically relevant temperatures and somewhat low oxygen fugacities, as the tin contents of the run products ranged from 24-62 and 51-81 wt%, respectively, for ca. 1 bar and 10 kbar experiments.

Viscosity of crystal-bearing melts and its implication for magma ascent

Experiments were performed at high temperature and pressure to determine the effective viscosity of a crystal- bearing andesite using the falling sphere method. Because viscosity experiments with partly crystallized sam- ples are difficult to realize (i.e., due to high sensitivity of phase equilibria to P, T and water content), we have added zircon crystals to adjust precisely the volume fraction and the size of crystals in a magma. Using this ap- proach, the anhydrous melt composition does not vary significantly with temperature and water content of the melt and, hence, the effects of crystals on effective viscosity can be worked out accurately. The investigated systems (magma analogue) were composed of an andesitic melt containing 0.5 wt% to 4.1 wt% H2O and 15 vol% to 40 vol% of zircon crystals with grain size <100 µm. Most experiments were performed at 300 MPa and 1523 K. The ZrO2 content dissolved in the melt under these conditions is 1.61 wt% ± 0.16 wt% (0.5-4.0 wt% H2O in melt) with no significant dependence on water content, which is about twice the value predicted by the model of Watson and Harrison (1983). The falling velocity of large platinum spheres was measured in the experimental magmas. The radius of the spheres (between 130 µm and 510 µm) was always much larger than the crystal sizes and the inter-grain distances, implying that the falling velocity of the spheres can be used to calculate the effective viscosity of the magmas. For magmas containing 15 vol% zircon, the measured viscosity was 0.7 log to 1.6 log units higher than the melt viscosity. The effective magma viscosity is higher than expected from literature models. The spheres did not move in systems containing 30 vol% to 40 vol% of crystals, even after 16 h at 1523 K. We at- tribute this observation to the presence of yield strength of more than 100 Pa, i.e., a threshold of accelerating force needs to be passed before the sphere can move in the magma.

Absorption spectra of cobalt(II) chloride and nitrate complexes in aqueous calcium nitrate–ammonium nitrate melts: The influence of solvent composition

Influence of solvent composition, i.e. the salt mole fraction and water content on absorption spectra of cobalt(II) chloride and nitrate in xCa(NO 3) 2· zH 2O–(1 − x)NH 4NO 3 systems was investigated by spectrophotometric method in the wavelength range 400–800 nm at 55 °C. On the basis of absorption spectra of cobalt(II) chloride and nitrate complexes and overall molar absorption coefficients obtained in 13 investigated systems, the geometry of the complexes formed was studied. Comparing of these spectra with those obtained in anhydrous melts and aqueous solutions, possible coordination of water molecules with cobalt(II) and formation of mixed complexes was considered.

Polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts

In order to describe and quantify the reactivity of silicate melts, the ionic notation provided by the Temkin formalism has been historically accepted, giving rise to the study of melt chemical equilibria in terms of completely dissociated ionic species. Indeed, ionic modelling of melts works properly as long as the true extension of the anionic matrix is known. This information may be attained in the framework of the Toop-Samis (1962a,b) model, through a parameterisation of the acid-base properties of the dissolved oxides. Moreover, by combining the polymeric model of Toop and Samis with the «group basicity» concept of Duffy and Ingram (1973, 1974a,b, 1976) the bulk optical basicity (Duffy and Ingram, 1971; Duffy, 1992) of molten silicates and glasses can be split into two distinct contributions, i.e. the basicity of the dissolved basic oxides and the basicity of the polymeric units. Application to practical cases, such as the assessment of the oxidation state of iron, require bridging of the energetic gap between the standard state of completely dissociated component (Temkin standard state) and the standard state of pure melt component at P and T of interest. On this basis it is possible to set up a preliminary model for iron speciation in both anhydrous and hydrous aluminosilicate melts. In the case of hydrous melts, I introduce both acidic and basic dissociation of the water component, requiring the combined occurrence of H+ cations, OH- free anions and, to a very minor extent, of T-OH groups. The amphoteric behaviour of water revealed by this study is therefore in line with the earlier prediction of Fraser (1975).

Volatile diffusion in silicate melts and its effects on melt inclusions

A compendium of diffusion measurements and their Arrhenius equations for water, carbon dioxide, sulfur, flu- orine, and chlorine in silicate melts similar in composition to natural igneous rocks is presented. Water diffusion in silicic melts is well studied and understood, however little data exists for melts of intermediate to basic com- positions. The data demonstrate that both the water concentration and the anhydrous melt composition affect the diffusion coefficient of water. Carbon dioxide diffusion appears only weakly dependent, at most, on the volatile- free melt composition and no effect of carbon dioxide concentration has been observed, although few experi- ments have been performed. Based upon one study, the addition of water to rhyolitic melts increases carbon dioxide diffusion by orders of magnitude to values similar to that of 6 wt% water. Sulfur diffusion in intermedi- ate to silicic melts depends upon the anhydrous melt composition and the water concentration. In water-bearing silicic melts sulfur diffuses 2 to 3 orders of magnitude slower than water. Chlorine diffusion is affected by both water concentration and anhydrous melt composition; its values are typically between those of water and sulfur. Information on fluorine diffusion is rare, but the volatile-free melt composition exerts a strong control on its dif- fusion. At the present time the diffusion of water, carbon dioxide, sulfur and chlorine can be estimated in silicic melts at magmatic temperatures. The diffusion of water and carbon dioxide in basic to intermediate melts is on- ly known at a limited set of temperatures and compositions. The diffusion data for rhyolitic melts at 800°C to- gether with a standard model for the enrichment of incompatible elements in front of growing crystals demon- strate that rapid crystal growth, greater than 10 −10 ms −1 , can significantly increase the volatile concentrations at the crystal-melt interface and that any of that melt trapped by the formation of melt inclusions may not be rep- resentative of the bulk melt. However, basaltic melt inclusions trapped at 1300°C are more likely to contain bulk melt concentrations of water and carbon dioxide.

Experimental and computational study of trace element distribution between orthopyroxene and anhydrous silicate melt: substitution mechanisms and the effect of iron

Although orthopyroxene (Opx) is present during a wide range of magmatic differentiation processes in the terrestrial and lunar mantle, its effect on melt trace element contents is not well quantified. We present results of a combined experimental and computational study of trace element partitioning between Opx and anhydrous silicate melts. Experiments were performed in air at atmospheric pressure and temperatures ranging from 1,326 to 1,420°C in the system CaO–MgO–Al2O3–SiO2 and subsystem CaO–MgO–SiO2. We provide experimental partition coefficients for a wide range of trace elements (large ion lithophile: Li, Be, B, K, Rb, Sr, Cs, Ba, Th, U; rare earth elements, REE: La, Ce, Nd, Sm, Y, Yb, Lu; high field strength: Zr, Nb, Hf, Ta, Ti; transition metals: Sc, V, Cr, Co) for use in petrogenetic modelling. REE partition coefficients increase from $$ D_{\text{La}}^{{\text{Opx}} {\hbox{-}} {\text{melt}}} \sim 0.0005 $$ to $$ D_{\text{Lu}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.109 $$ , D values for highly charged elements vary from $$ D_{\text{Th}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0026 $$ through $$ D_{\text{Nb}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0033 $$ and $$ D_{\text{U}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0066 $$ to $$ D_{\text{Ti}}^{{\text{Opx}} {\hbox{-}} {\text{melt}}} \sim 0.058 $$ , and are all virtually independent of temperature. Cr and Co are the only compatible trace elements at the studied conditions. To elucidate charge-balancing mechanisms for incorporation of REE into Opx and to assess the possible influence of Fe on Opx-melt partitioning, we compare our experimental results with computer simulations. In these simulations, we examine major and minor trace element incorporation into the end-members enstatite (Mg2Si2O6) and ferrosilite (Fe2Si2O6). Calculated solution energies show that R2+ cations are more soluble in Opx than R3+ cations of similar size, consistent with experimental partitioning data. In addition, simulations show charge balancing of R3+ cations by coupled substitution with Li+ on the M1 site that is energetically favoured over coupled substitution involving Al–Si exchange on the tetrahedrally coordinated site. We derived best-fit values for ideal ionic radii r 0, maximum partition coefficients D 0, and apparent Young’s moduli E for substitutions onto the Opx M1 and M2 sites. Experimental r 0 values for R3+ substitutions are 0.66–0.67 Å for M1 and 0.82–0.87 Å for M2. Simulations for enstatite result in r 0 = 0.71–0.73 Å for M1 and ~0.79–0.87 Å for M2. Ferrosilite r 0 values are systematically larger by ~0.05 Å for both M1 and M2. The latter is opposite to experimental literature data, which appear to show a slight decrease in $$ r_{0}^{{{\text{M}}2}} $$ in the presence of Fe. Additional systematic studies in Fe-bearing systems are required to resolve this inconsistency and to develop predictive Opx-melt partitioning models for use in terrestrial and lunar magmatic differentiation models.

Visualization-based analysis of structural and dynamical properties of simulated hydrous silicate melt

We have explored first-principles molecular dynamics simulation data for hydrous MgSiO3 liquid (with 10 wt% water) to gain insight into its structural and dynamical behavior as a function of pressure (0–150 GPa) and temperature (2,000–6,000 K). By visualizing/analyzing a number of parameters associated with short- and mid-range orders, we have shown that the melt structure changes substantially on compression. The speciation of the water component at low pressures is dominated by the isolated structures (with over 90% hydrogen participated) consisting of hydroxyls, water molecules, O–H–O bridging and four-atom (O–H–O–H and H–O–H–O) groups, where every oxygen atom may be a part of polyhedron or free (i.e., bound to only magnesium atom). Hydroxyls favor polyhedral sites over magnesium sites whereas molecular water is almost entirely bound to magnesium sites, and also interpolyhedral bridging (Si–O–H–O–Si) dominates other types of bridging. Water content is shown to enhance and suppress, respectively, the proportions of hydroxyls and molecular water. As compression increases, these isolated structures increasingly combine with each other to form extended structures involving a total of five or more O and H atoms and also containing threefold coordination species, which together consume over 80% hydrogen at the highest compression studied. Our results show that water lowers the mean coordination numbers of different types including all cation–anion environments. The hydrous melt tends to be more tetrahedrally coordinated but with the Si–Si network being more disrupted compared to the anhydrous melt. Protons increase the content of non-bridging oxygen and decrease the contents of bridging oxygen as well as oxygen triclusters (present at pressures above 10 GPa). The calculated self-diffusion coefficients of all atomic species are enhanced in the presence of water compared to those of the anhydrous melt. This is consistent with the prediction that water depolymerizes the melt structure at all pressures. Our analysis also suggests that proton diffusion involves two processes—the transfer of H atoms (requiring the rupture and formation of O–H bonds) and the motion of hydroxyls as hydrogen carriers (requiring the rupture and formation of Si–O and/or Mg–O bonds). Both the processes are operative at low compression whereas only the first process is operative at high compression.

Origin of primitive high-Mg andesite: Constraints from natural examples and experiments

The occurrence in arcs of high-magnesium andesites with Mg#'s of around 0.7 implies that such rocks are generated in, or equilibrated with, mantle peridotite under conditions which do not pertain beneath ridges. In this paper we use a review of experimental data on anhydrous peridotite melting to show that these primitive high-Mg andesites cannot be generated by anhydrous melting of lherzolite. Addition of H 2O to peridotitic compositions has been shown, in numerous studies, to displace partial melts from basaltic towards more andesitic compositions leading to the possibility that high-Mg andesites are “wet” melts of lherzolite. Our review of the experimental data demonstrates however, that addition of H 2O alone cannot explain the increases in both SiO 2 and MgO contents (on an anhydrous basis) required to shift from basaltic to high-Mg andesitic melts in equilibrium with lherzolite residue. A much more plausible alternative is that these melts are extracted from a harzburgite residue, a model which we develop in more detail. We performed experiments at 0.6 GPa in which high Mg-andesitic melts were equilibrated with an olivine + orthopyroxene residue and find (in the absence of H 2O) that MgO and SiO 2 contents increase in the ratio 2:1 as the degree of undersaturation in clinopyroxene increases. This is the right sign and magnitude of effect to explain the compositions of primitive high-Mg andesites. Data from earlier studies of the CaO–MgO–SiO 2 system at 1 atm and natural compositions at 1.5–2 GPa are in excellent agreement with our observations. When the effects of H 2O and clinopyroxene undersaturation are added together we obtain a line in Pressure–H 2O space which describes the conditions under which a given high Mg-andesite could be in equilibrium with a harzburgite residue. Application to rocks from White Island (New Zealand), Amphlett Island (Papua New Guinea), Setouchi Belt (Japan), Mt. Shasta (USA), Adak Island and Piip volcano (Aleutians, USA) yield, for crustal thicknesses > 20 km, H 2O contents of the melts of 2–7%, in generally good agreement with the available compositions of melt inclusions.

Mid-ocean ridge and supra-subduction geochemical signatures in spinel–peridotites from the Neotethyan ophiolites in SW Turkey: Implications for upper mantle melting processes

The Lycian and Antalya ophiolite complexes in SW Turkey represent fragments of oceanic lithosphere emplaced following the closure of the Neotethys Ocean during the Late Cretaceous. The peridotites from both of these ophiolites have compositions ranging from relatively undepleted lherzolites to highly depleted harzburgites and display a diverse suite of geochemical signatures indicative of both anhydrous, mid-ocean ridge (MOR)-type and hydrous, supra-subduction zone (SSZ)-type melting regimes. Whole-rock major and trace element systematics and mineral chemistry indicate that the MOR- and SSZ-type peridotites represent the residues from 5–9% and 13–25% of mantle melting, respectively, and display evidence for a multi-stage evolution of an oceanic lithosphere. Olivine–orthopyroxene–spinel equilibria indicate that the clinopyroxene-bearing, MOR-type peridotites are moderately reduced with their oxygen fugacity (fO2) ranging from −2.22 to −1.44 log units relative to the FMQ (fayalite–magnetite–quartz) buffer and are similar to abyssal peridotites, while more refractory, SSZ-type harzburgites and dunites (<4% modal clinopyroxene) are more oxidized with their higher oxygen fugacity (FMQ −0.72 to +1.02). The latter can readily be explained by interaction with oxidizing melts/fluids originating from a subducted slab in a SSZ environment. Precise determination of trace element compositions of residual clinopyroxenes by LA-ICPMS indicates that the SSZ peridotites are strongly depleted in Ti and HREE and enriched in Zr and LMREE, compared to the most depleted MOR-type peridotites. The results of quantitative model calculations show that moderate degrees of anhydrous mantle melting accounts for the composition of MOR peridotites, while SSZ peridotites are likely to have originated from hydrous, orthopyroxene-dominated remelting of previously depleted mantle. For the case of hydrous melting, it is inferred that flux of a slab-derived fluid component sustained further melting of MORB-depleted peridotite, resulting in higher degrees of depletions in the SSZ mantle than the anhydrous melting in the MOR setting. Interaction of SSZ-type melts with depleted peridotites enriched the source mantle in more incompatible trace elements, resulting in elevated Zr/Ti and LMREE/HREE ratios in peridotites. The occurrence of both MOR and SSZ styles of melting regimes indicates that these ophiolites contain mantle residues from discrete stages of oceanic lithosphere generation.

Hydrogen partitioning between nominally anhydrous upper mantle minerals and melt between 3 and 5 GPa and applications to hydrous peridotite partial melting

To extend our knowledge of partitioning of hydrogen between nominally anhydrous mantle minerals and basaltic melts to higher pressures, we conducted 8 experiments at 3–5 GPa and 1350–1440 °C on a modified mid-ocean ridge basalt (MORB) + 2 wt.% H 2O. H contents in olivine, orthopyroxene, clinopyroxene, and garnet were measured by counting 16O 1H ions with low-blank SIMS, and total H 2O concentrations in coexisting glasses were measured by FTIR, SIMS, and electron microprobe. The glasses have water contents ranging from 4.1 to 8.9 wt.%. Eleven measured pyroxenes have H 2O concentrations ranging from 430 to 1430 ppm, two garnets have concentrations of 73 and 260 ppm, and one olivine has a concentration of 29 ppm. Mineral/melt hydrogen partition coefficients are as follows: D H cpx/melt ranges from 0.014–0.021, D H opx/melt ranges from 0.009–0.019, D H gt/melt ranges from 0.001–0.0033, and D H ol/melt has a value of 0.0006. Mineral/mineral hydrogen partitioning data are as follows: D H cpx/opxranges from 1.2–2, D H cpx/gt has values of 5.5 and 14, and a single D H cpx/ol has a value of 27. D H pyx/melt correlates with the concentration of Al 2O 3 in pyroxene and with the concentration of Al on tetrahedral sites. Combining the mineral/melt hydrogen partitioning data from this study, and estimates of mineral modes and compositions along the solidus, we estimate that D H perid/melt increases from 0.006 at 1 GPa up to 0.01 near the garnet lherzolite/spinel lherzolite transition, owing to the maximum in Al 2O 3 concentration in pyroxenes. With increasing pressure D H perid/melt for garnet lherzolite diminishes to 0.005 at 5 GPa, as pyroxenes become less aluminous and less abundant. As a result, incipient hydrous melts beneath oceanic ridges with bulk mantle H 2O contents of 50–200 ppm will have solidus partial melts with H 2O contents between 0.5 and 3.8 wt.%, while oceanic island mantle with 300–1000 ppm will have solidus partial melts with H 2O content between 3 and 20 wt.%.

Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle

The origin of komatiites, volcanic rocks formed by unusually extensive melting of mantle rocks, mainly during the Archaean, is the subject of much debate. They are thought to have been produced either by anhydrous melting of anomalously hot mantle, or by hydrous melting at temperatures only a little higher than those of today. Berry et al. have now determined the oxidation state of iron in pristine samples of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe. Their findings are consistent with near-anhydrous melting of a source with an oxidation state similar to that of present-day ocean-floor basalt. The results suggest that the Belingwe melt was a product of high mantle temperatures of about 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archaean. The original iron isotope ratios of a 2.7 billion-year-old komatiitic magma from Belingwe, Zimbabwe have been determined. These measurements are consistent with near-anhydrous melting of a source with similar oxidation state to that of present-day ocean-floor basalt. The results support the identification of the Belingwe komatiite as a product of high mantle temperatures of ∼ 1,700 °C, rather than melting under hydrous conditions, confirming the existence of anomalously hot mantle in the Archean. Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle1,2,3 or hydrous melting at temperatures only modestly greater than those found today4,5. Here we determine the original Fe3+/ΣFe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe6, preserved as melt inclusions in olivine, to be 0.10 ± 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe3+/ΣFe value, together with a water content of only 0.2–0.3 wt% (ref. 7), excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe3+/ΣFe ratio would be ∼1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (∼1,700 °C), rather than melting under hydrous conditions (3–5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.