Liquid immiscibility between silicate,carbonate and sulfide melts in melt inclusions hosted in co-precipitated minerals from Kerimasi volcano (Tanzania): evolution of carbonated nephelinitic magma

The evolution of a carbonated nephelinitic magma can be followed by the study of a statistically significant number of melt inclusions, entrapped in co-precipitated perovskite, nepheline and magnetite in a clinopyroxene- and nepheline-rich rock (afrikandite) from Kerimasi volcano (Tanzania). Temperatures are estimated to be 1,100°C for the early stage of the melt evolution of the magma, which formed the rock. During evolution, the magma became enriched in CaO, depleted in SiO₂ and Al₂O₃, resulting in immiscibility at ~1,050°C and crustal pressures (0.5–1 GPa) with the formation of three fluid-saturated melts: an alkali- and MgO-bearing, CaO- and FeO-rich silicate melt; an alkali- and F-bearing, CaO- and P₂O₅-rich carbonate melt; and a Cu–Fe sulfide melt. The sulfide and the carbonate melt could be physically separated from their silicate parent and form a Cu–Fe–S ore and a carbonatite rock. The separated carbonate melt could initially crystallize calciocarbonatite and ultimately become alkali rich in composition and similar to natrocarbonatite, demonstrating an evolution from nephelinite to natrocarbonatite through Ca-rich carbonatite magma. The distribution of major elements between perovskite-hosted coexisting immiscible silicate and carbonate melts shows strong partitioning of Ca, P and F relative to Fe_T, Si, Al, Mn, Ti and Mg in the carbonate melt, suggesting that immiscibility occurred at crustal pressures and plays a significant role in explaining the dominance of calciocarbonatites (sövites) relative to dolomitic or sideritic carbonatites. Our data suggest that Cu–Fe–S compositions are characteristic of immiscible sulfide melts originating from the parental silicate melts of alkaline silicate–carbonatite complexes.     

Experiments on silicate melt immiscibility in the system Fe2SiO4–KAlSi3O8–SiO2–CaO–MgO–TiO2–P2O5 and implications for natural magmas

The effect of CaO and MgO, with or without TiO₂ and P₂O₅, on the two-melt field in the simplified system Fe₂SiO₄–KAlSi₃O₈–SiO₂ has been experimentally determined at 1,050°–1,240°C, 400 MPa. Despite the suppressing effect of MgO, CaO, and pressure on silicate melt immiscibility, our experiments show that this process is still viable at mid-crustal pressures when small amounts (0.6–2.0 wt%) of P₂O₅ and TiO₂ are present. Our data stress that the major element partition coefficients between the two melts are highly correlated with the degree of polymerisation (nbo/t) of the SiO₂-rich melt, whatever temperature, pressure, or exact composition. Experimental immiscible melt compositions in natural systems at 0.1 MPa from the literature (lunar and tholeiitic basalts) plot on similar but distinct curves compared to the simplified system. These relations between melt polymerisation and partition coefficients, which hold for a large range of compositions and fO₂, are extended to various volcanic and plutonic rocks. This analysis strengthens the proposal that silicate melt immiscibility can be important in volcanic rocks of various compositions (from tholeiitic basalts to lamprophyres). However, the majority of proposed immiscible compositions in plutonic rocks are at least not coexisting melts, but may have suffered accumulation of early crystallized minerals.     

Differentiation of nelsonitic magmas in the formation of the ~1.74 Ga Damiao Fe–Ti–P ore deposit, North China

The ~1.74 Ga Damiao anorthosite complex, North China, is composed of anorthosite and leuconorite with subordinate melanorite, mangerite, oxide-apatite gabbronorite, perthite noritic (i.e., jotunitic) and ferrodioritic dykes. The complex hosts abundant vein-, pod- and lens-like Fe–Ti–P ores containing variable amounts of apatite (10–60 modal%) and Fe–Ti oxides. In addition to Fe–Ti–P ores, there are also abundant Fe–Ti ores which are closely associated with Fe–Ti–P ores in the deposit. Most of Fe–Ti–P ores are dominated by Fe–Ti oxides and apatite, devoid of silicate minerals, mineralogically similar to the common nelsonites elsewhere. In contrast, Fe–Ti ores are dominated by Fe–Ti oxides with minor apatite (<5 modal %). The parental magma of these ores, estimated from olivine and apatite compositions using mineral-melt partition coefficients, has composition similar to the ferrodioritic dykes. Fe–Ti–P ores have variable Fe–Ti oxides and apatite proportions, indicating that they are cumulates. Their simple assemblage of Fe–Ti oxides and apatite and local net-texture suggest that the Fe–Ti–P ores in Damiao have formed from nelsonitic melts immiscibly separated from the ferrodioritic magma during late-stage differentiation. Fe–Ti ores are also cumulates and have mineral compositions similar to Fe–Ti–P ores. The close association between Fe–Ti and Fe–Ti–P ores indicates that the Fe–Ti ores may have also formed from the nelsonitic melts. We proposed that differentiation of nelsonitic melts accompanied by gravity settling is responsible for the formation of Fe–Ti and Fe–Ti–P ores. Such a differentiation process in nelsonitic melts is well supported by variations of Sr, Y, Th, U, REE and Eu/Eu* of apatite in Fe–Ti–P ores. Using oxides/apatite ratio of 2:1 and compositions of apatite and calculated primary oxides, we estimate the composition of the nelsonitic melt as ~52.0 wt% Fe₂O₃t, ~18.5 wt% CaO, ~14.2 wt% P₂O₅, ~8.7 wt% TiO₂, ~4.0 wt% Al₂O₃ and ~1.1 wt% MgO with minor SiO₂, K₂O, Na₂O and F. Such a nelsonitic melt is suggested to be possibly conjugated with Si-rich melts compositionally similar to the Damiao jotunitic dykes (~50 wt% SiO₂ and ~15 wt% Fe₂O₃t) which may subsequently evolve to mangeritic rocks in Damiao. Our modeling also indicates that the onset of immiscibility occurs at a time when the evolved melt has ~44 wt% SiO₂, ~21 wt% Fe₂O₃t, ~3.0 wt% TiO₂ and ~2.6 wt% P₂O₅. High oxygen fugacity and phosphorous content in magmas may play important roles in the immiscibility of nelsonitic magmas, including promoting iron enrichments and widening the two-liquid field.     

The effect of titanium and phosphorus on ferric/ferrous ratio in silicate melts: an experimental study

The effect of TiO₂ and P₂O₅ on the ferric/ferrous ratio in silicate melts was investigated in model silicate melts at air conditions in the temperature range 1,400–1,550 °C at 1-atm total pressure. The base composition of the anorthite–diopside eutectic composition was modified with 10 wt % Fe₂O₃ and variable amounts of TiO₂ (up to 30 wt %) or P₂O₅ (up to 20 wt %). Some compositions also contained higher SiO₂ concentrations to compare the role of SiO₂, TiO₂, and P₂O₅ on the Fe³⁺/Fe²⁺ ratio. The ferric/ferrous ratio in experimental glasses was analyzed using a wet chemical technique with colorimetric detection of ferrous iron. It is shown that at constant temperature, an increase in SiO₂, TiO₂, and P₂O₅ content results in a decrease in the ferric/ferrous ratio. The effects of TiO₂ and SiO₂ on the Fe³⁺/Fe²⁺ ratio was found to be almost identical. In contrast, adding P₂O₅ was found to decrease ferric/ferrous ratio much more effectively than adding silica. The results were compared with the predictions from the published empirical equations forecasting Fe³⁺/Fe²⁺ ratio. It was demonstrated that the effects of TiO₂ are minor but that the effects of P₂O₅ should be included in models to better describe ferric/ferrous ratio in phosphorus-bearing silicate melts. Based on our observations, the determination of the prevailing fO₂ in magmas from the Fe³⁺/Fe²⁺ ratio in natural glasses using empirical equations published so far is discussed critically.     

Oxidised phase relations of a primitive basalt from Grenada,Lesser Antilles

A series of liquidus determinations is reported for a primitive arc basalt (15.4 wt % MgO, 45.5 wt % SiO₂) from Grenada, Lesser Antilles, at anhydrous, H₂O-undersaturated and H₂O-saturated conditions in the pressure range 1 atm to 1.7 GPa. \(\hbox{Fe}^{3+}/\Upsigma\hbox{Fe}\) of high-pressure experimental glasses as measured by μXANES ranges from 0.44 to 0.86, corresponding to oxygen fugacities (fO₂) between 3.2 and 7.8 log units above the nickel–nickel oxide redox buffer (NNO). 1-atm experiments conducted from NNO ? 2.5 to + 3.8 show that increasing fO₂ mainly increases the forsterite content (Fo) of olivine and has little effect on phase relations. The crystallisation sequence at lower crustal pressures for all water contents is forsteritic olivine + Cr-rich spinel followed by clinopyroxene. The anhydrous liquidus is depressed by 100 and 120 °C in the presence of 2.9 and 3.8 wt % H₂O, respectively. H₂O-undersaturated experiments at NNO + 3.2 to + 4.5 produce olivine of equivalent composition to the most primitive olivine phenocrysts in Grenadan picrites (Fo_91.4). We conclude that direct mantle melts originating beneath Grenada could be as oxidised as ~NNO + 3, consistent with the uppermost estimates from olivine–spinel oxybarometry of high Mg basalts. μXANES analyses of olivine-bearing experimental glasses are used to develop a semi-empirical oxybarometer based on the value of \({{K}_{D}}_{\rm ol-melt}^{\rm Fe-Mg}\) when all Fe is assumed to be in the Fe²⁺ state (\({K}_{D}^{{\rm Fe}_T}\)). The oxybarometer is tested on an independent data set and is able to reproduce experimental fO₂ to ≤1.2 log units. Experiments also show that the geochemically and petrographically distinct M- and C-series lavas on the island can be produced from hydrous melting of a common picritic source. Low pressures expand the olivine stability field at the expense of clinopyroxene, enriching an evolving melt in CaO and forcing differentiation to take place along a C-series liquid line of descent. Higher pressure conditions allow early and abundant clinopyroxene crystallisation, rapidly depleting the melt in both CaO and MgO, and thus creating the M-series.     

Review Section

ABSTRACT

The bulk composition of silicic igneous rocks hovers directly over the ‘minimum melt’ located on the NaAlSi₃O₈–KAlSi₃O₈–SiO₂ ternary. It is universally accepted that these rocks are igneous, reflecting the thermodynamic equilibrium between minerals and melt. However, there is a contradiction between the use of this phase diagram and current models of differentiation: fractional crystallization or partial melting (or any mechanical separation process) implies granites were melts extracted from a quartz-bearing residue.

Here, I propose that a thermodynamically controlled process, wet thermal migration, provides a more consistent explanation for producing silicic rocks. This process, when coupled with slow incremental emplacement of sills, allows magmatic differentiation to take place without mechanical separation and produces minimum-melt compositions from input magmas not saturated in quartz. Examination of phase equilibria in the SiO₂–Al₂O₃–Na₂O–K₂O–H₂O system, as well as assessment of thermodynamic models, provides a template for understanding the wet thermal migration process (diffusion-based differentiation of crystal-mush in a temperature gradient). Phase equilibria in SiO₂–Na₂O–K₂O–Al₂O₃ indicate a cotectic surface between quartz and alkali feldspar slopes down-temperature as melt peralkalinity increases. Experiments show quartz and two feldspars coexist with a single water-rich (>40 wt.% H₂O) melt at 400°C and 0.1 GPa. Modelling suggests that development of water-rich melt at the hot end of a temperature gradient drives the process. Given the propensity of alkalis to rapidly diffuse down-temperature gradients and form these melts, small amounts (<5%) of interconnected melt form, leading to the differentiation of partially molten materials by wet thermal migration.

Because the quartz-feldspar cotectic ranges from 650°C to 330°C, granites can reflect formation by a process that never involves the existence of rhyolitic melt. If this is correct, the process has implications for understanding the formation of plutons and batholiths and, therefore, the continental crust.     

Expansivity and compressibility of wadeite-type K2Si4O9 determined by in situ high T/P experiments, and their implication

Wadeite-type K₂Si₄O₉ was synthesized with a cubic press at 5.4 GPa and 900 °C for 3 h. Its unit-cell parameters were measured by in situ high-T powder X-ray diffraction up to 600 °C at ambient P. The T–V data were fitted with a polynomial expression for the volumetric thermal expansion coefficient (α_T = a ₀ + a ₁ T), yielding a ₀ = 2.47(21) × 10^?5 K^?1 and a ₁ = 1.45(36) × 10^?8 K^?2. Compression experiments at ambient T were conducted up to 10.40 GPa with a diamond-anvil cell combined with synchrotron X-ray radiation. A second-order Birch–Murnaghan equation of state was used to fit the P–V data, yielding K _T = 97(3) GPa and V ₀ = 360.55(9) Å³. These newly determined thermal expansion data and compression data were used to thermodynamically calculate the P–T curves of the following reactions: 2 sanidine (KAlSi₃O₈) = wadeite (K₂Si₄O₉) + kyanite (Al₂SiO₅) + coesite (SiO₂) and wadeite (K₂Si₄O₉) + kyanite (Al₂SiO₅) + coesite/stishovite (SiO₂) = 2 hollandite (KAlSi₃O₈). The calculated phase boundaries are generally consistent with previous experimental determinations.     

Unique Clinkers and Paralavas from a New Nyalga Combustion Metamorphic Complex in Central Mongolia: Mineralogy,Geochemistry, and Genesis

The paper presents mineralogical and geochemical data on clinkers and paralavas and on conditions under which they were formed at the Nyalga combustion metamorphic complex, which was recently discovered in Central Mongolia. Mineral and phase assemblages of the CM rocks do not have analogues in the world. The clinkers contain pyrogenically modified mudstone relics, acid silicate glass, partly molten quartz and feldspar grains, and newly formed indialite microlites (phenocrysts) with a ferroindialite marginal zone. In the paralava melts, spinel microlites with broadly varying Fe concentrations and anorthite–bytownite were the first to crystallize, and were followed by phenocrysts of Al-clinopyroxene ± melilite and Mg–Fe olivine. The next minerals to crystallize were Ca-fayalite, kirschsteinite, pyrrhotite, minerals of the rhönite–kuratite series, K–Ba feldspars (celsian, hyalophane, and Ba-orthoclase, Fe³⁺-hercynite ± (native iron, wüstite, Al-magnetite, and fresnoite), nepheline ± (kalsilite), and later calcite, siderite, barite, celestine, and gypsum. The paralavas contain rare minerals of the rhönite–kuratite series, a new end-member of the rhönite subgroup Ca₄Fe ₈ ²⁺ Fe ₄ ³⁺ O₄ [Si₈Al₄O₃₆], a tobermorite-like mineral Ca₅Si₅(Al,Fe)(OH)O₁₆ · 5H₂O, and high- Ba F-rich mica (K,Ba)(Mg,Fe)₃(Al,Si)₄O₁₀F₂. The paralavas host quenched relics of microemulsions of immiscible residual silicate melts with broadly varying Si, Al, Fe, Ca, K, Ba, and Sr concentrations, sulfide and calcitic melts, and water-rich silicate–iron ± (Mn) fluid media. The clinkers were formed less than 2 Ma ago in various parts of the Choir–Nyalga basin by melting Early Cretaceous mudstones with bulk composition varies from dacitic to andesitic. The pyrogenic transformations of the mudstones were nearly isochemical, except only for volatile components. The CM melt rocks of basaltic andesitic composition were formed via melting carbonate–silicate sediments at temperatures above 1450°C. The Ca- and Fe-enriched and silicaundersaturated paralavas crystallized near the surface at temperatures higher than 900–1100°C and oxygen fugacity \(f_{O_2 }\) between the IW and QFM buffers. In local melting domains of the carbonate–silicate sedimentary rocks and in isolations of the residual melts among the paralava matrix the fluid pressure was higher than the atmospheric one. The bulk composition, mineral and phase assemblages of CM rocks of the Nyalga complex are very diverse (dacitic, andesitic, basaltic andesitic, basaltic, and silica-undersaturated mafic) because the melts crystallized under unequilibrated conditions and were derived by the complete or partial melting of clayey and carbonate–silicate sediments during natural coal fires.     

Application of magnetic adsorbent with silicate and phenyl polymers to adsorb Rhodamine B

The extensive use of Rhodamine B (RhB) for textile, paper, pigment, food, cosmetic, and drug manufacturing and its indiscriminate disposal leads to serious human, biological, and environmental hazards. A magnetic adsorbent with silicate and phenyl polymers (Ph/SiO₂/Fe₃O₄) has been prepared to absorb RhB. The morphology and structure of the adsorbents have been characterized by TGA, XRD, FTIR, and adsorption–desorption measurement. The results revealed that Ph/SiO₂/Fe₃O₄ exhibited a paramagnetic behavior and could easily and quickly be separated from a suspension. The RhB adsorption behavior was almost pH independent due to the adsorption between the phenyl ring of Ph/SiO₂/Fe₃O₄ and RhB by π–π electron-donor–acceptor interactions. The adsorption behavior of RhB adsorption was in good agreement with the Langmuir adsorption isotherm, and the maximum adsorption capacity was 142.186 mg g^?1. Good desorption performance of Ph/SiO₂/Fe₃O₄ showed that this novel magnetic adsorbent cannot only be activated by ethanol extraction process but also reuse by the recovery of magnetic force.     

Synchrotron radiation study on the high-pressure and high-temperature phase relations of KAlSi3O8

In situ X-ray diffraction study on KAlSi₃O₈ has been performed using the cubic type high pressure apparatus, MAX90, combined with synchrotron radiation. We determined the phase relations of sanidine, the wadeite-type K₂Si₄O₉+kyanite (Al₂SiO₅)+coesite (SiO₂) assemblage, and hollandite-type KAlSi₃O₈, including melting temperatures of potassic phases, up to 11 GPa. Our data on subsolidus phase boundaries are close to the recent data of Yagi and Akaogi (1991). Melting relations of sanidine are consistent with the low pressure data of Lindsley (1966). The breakdown of sanidine into three phases reduces melting temperature, and wadeite-type K₂Si₄O₉ melts first around 1500° C in three phase coexisting region. Melting point of hollandite-type KAlSi₃O₈ is between 1700° C and 1800° C at 11 GPa. If these potassic phases host potassium in the earth's mantle, the true mantle solidus temperature will be much lower than the reported dry solidus temperature of peridotite.     

Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study

We experimentally investigated the phase relations of a peralkaline phonolitic dyke rock associated with the Ilímaussaq plutonic complex (South Greenland). The extremely evolved and iron-rich composition (magnesium number = 2, alkalinity index = 1.44, FeO* = 12 wt%) may represent the parental magma of the Ilímaussaq complex. This dyke rock is therefore perfectly suited for performing phase-equilibrium experiments, since in contrast to the plutonic rocks of the complex, no major cumulate formation processes complicate defining a reasonable starting composition. Experiments were carried out in hydrothermal rapid-quench cold-seal pressure vessels at P = 100 MPa and T = 950–750 °C. H₂O contents ranging from anhydrous to H₂O saturated (~5 wt% H₂O) and varying fO₂ (~ΔlogFMQ ?3 to +1; where FMQ represents the fayalite–magnetite–quartz oxygen buffer) were applied. Reduced and dry conditions lead to substantial crystallization of alkali feldspar, nepheline, hedenbergite-rich clinopyroxene, fayalite-rich olivine and minor amounts of ulvøspinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and H₂O-rich conditions, however, suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation. Accordingly, combined low fO₂ and aH₂O force the evolution of the residual melt toward decreasing SiO₂, increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO₂ and aH₂O produce residual melts with relatively low FeO*, high SiO₂ and a relatively constant alkalinity index. We show that variations of aH₂O and fO₂ lead to contrasting trends regarding the liquid lines of descent of iron-rich silica-undersaturated peralkaline compositions. Moreover, the increase in FeO* and alkalinity index (reduced and dry conditions) in the residual melt is an important prerequisite to stabilize late-magmatic minerals of the dyke rock, for example, aenigmatite (Na₂Fe₅TiSi₆O₂₀), coexisting with the most evolved melts at 750 °C. Contrary to what might be expected, experiments with high aH₂O and interlinked high fO₂ exhibit higher liquidus T’s compared with experiments performed at low aH₂O and fO₂ for experiments where magnetite is liquidus phase. This is because ulvøspinel-poor magnetite crystallizes at higher fO₂ and has a higher melting point than ulvøspinel-rich magnetite, which is favored at lower fO₂.     

Aluminum coordination in metaaluminous and peralkaline silicate melts

Incremental amounts of Na₂O and K₂O added to immiscible melts in the MgO-CaO-TiO₂-Al₂O₃ SiO₂ system cause a decrease in critical temperature, phase separation and change in the pattern of Al₂O₃ partitioning. Al₂O₃, which is concentrated in the low SiO₂ immiscible melts in the alkali-free system, is increasingly partitioned into the high-SiO₂ immiscible melt as the alkali/aluminium ratio is increased. However, K₂O is more effective than Na₂O in stabilizing Al₂O₂ in the SiO₂-rich melt. The coordination changes occurring in the aluminosilicate melts upon the addition of the alkali oxides are described by CaAl₂O₄+2SiOK=2KAlO₂+SiOCaOSi where K (or Na) displaces Ca as the charge-balancing cation for the networkforming AlO₄ tetrahedra. The increased stability of the AlO₄ species in the highly polymerized SiO₂-rich melt and the consequent shrinkage of the miscibility gap is ascribed to positive configurational entropy and negative enthalpy changes associated with the formation of K, Na-AlO₄ species. Element partition systematics indicate that (Na, K)AlO₂ species favor the more polymerized, CaAl₂O₄ and TiO₂ species, the less polymerized silicate structure in the melt.     

The role of P2O5 in silicate melts

Phase equilibria data in the systems SiO₂-P₂O₅, P₂O₅-M_xO_y, and P₂O₅-M_xO_y-SiO₂ are employed in conjunction with Chromatographic and spectral data to investigate the role of P₂O₅ in silicate melts. Such data indicate that the behavior of P₂O₅ is complex. P₂O₅ depolymerizes pure SiO₂ melts by entering the network as a four-fold coordinated cation, but polymerizes melts in which an additional metal cation other than silicon is present. The effect of this polymerization is apparent in the widening of the granite-ferrobasalt two-liquid solvus. In this complex system P₂O₅ acts to increase phase separation by further enrichment of the high charge density cations Ti, Fe, Mg, Mn, Ca, in the ferrobasaltic liquid. P₂O₅ also produces an increase in the ferrobasalt-granite REE liquid distribution coefficients. These distribution coefficients are close to 4 in P₂O₅-free melts, but close to 15 in P₂O₅-bearing melts.The dual behavior of P₂O₅ is explained in a model which requires complexing of phosphate anions (analogous to silicate anions) and metal cations in the melt. This interaction destroys Si-O-M-O-Si bonds polymerizing the melt. The higher concentration of Si-O-M-O-Si bond complexes in immiscible ferrobasaltic liquids relative to their conjugate immiscible granite liquids explains the partitioning of P₂O₅ into the ferrobasaltic liquid.     

Effects of strong network modifiers on Fe<Superscript>3+</Superscript>/Fe<Superscript>2+</Superscript> in silicate melts: an experimental study

The effect of CaO, Na₂O, and K₂O on ferric/ferrous ratio in model multicomponent silicate melts was investigated in the temperature range 1450–1550?°C at 1-atm total pressure in air. It is demonstrated that the addition of these network modifier cations results in an increase of Fe³⁺/Fe²⁺ ratio. The influence of network modifier cations on the ferric/ferrous ratio increases in the order Ca?<?Na?<?K. Some old controversial conceptions concerning the effect of potassium on Fe³⁺/Fe²⁺ ratio in simple model liquids are critically evaluated. An empirical equation is proposed to predict the ferric/ferrous ratio in SiO₂–TiO₂–Al₂O₃–FeO–Fe₂O₃–MgO–CaO–Na₂O–K₂O–P₂O₅ melts at air conditions.     

Hydration of mantle olivine under variable water and oxygen fugacity conditions

The incorporation of H into olivine is influenced by a significant number of thermodynamic variables (pressure, temperature, oxygen fugacity, etc.). Given the strong influence that H has on the solidus temperature and rheological behavior of mantle peridotite, it is necessary to determine its solubility in olivine over the range of conditions found in the upper mantle. This study presents results from hydration experiments carried out to determine the effects of pressure, temperature, and the fugacities of H₂O and O₂ on H solubility in San Carlos olivine at upper mantle conditions. Experiments were carried out at 1–2 GPa and 1,200 °C using a piston-cylinder device. The fugacity of O₂ was controlled at the Fe⁰–FeO, FeO–Fe₃O₄, or Ni⁰–NiO buffer. Variable duration experiments indicate that equilibration is achieved within 6 h. Hydrogen contents of the experimental products were measured by secondary ion mass spectrometry, and relative changes to the point defect populations were investigated using Fourier transform infrared spectroscopy. Results from our experiments demonstrate that H solubility in San Carlos olivine is sensitive to pressure, the activity of SiO₂, and the fugacities of H₂O and O₂. Of these variables, the fugacity of H₂O has the strongest influence. The solubility of H in olivine increases with increasing SiO₂ activity, indicating incorporation into vacancies on octahedral lattice sites. The forsterite content of the olivine has no discernible effect on H solubility between 88.17 and 91.41, and there is no correlation between the concentrations of Ti and H. Further, in all but one of our experimentally hydrated olivines, the concentration of Ti is too low for H to be incorporated dominantly as a Ti-clinohumite-like defect. Our experimentally hydrated olivines are characterized by strong infrared absorption peaks at wavenumbers of 3,330, 3,356, 3,525, and 3,572 cm^?1. The heights of peaks at 3,330 and 3,356 cm^?1 correlate positively with O₂ fugacity, while those at 3,525 and 3,572 cm^?1 correlate with H₂O fugacity.     

The crystal chemistry of lithium and Fe3+ in synthetic orthopyroxene

Lithian ferrian enstatite with Li₂O = 1.39 wt% and Fe₂O₃ 7.54 wt% was synthesised in the (MgO–Li₂O–FeO–SiO₂–H₂O) system at P = 0.3 GPa, T = 1,000°C, fO₂ = +2 Pbca, and a = 18.2113(7), b = 8.8172(3), c = 5.2050(2) Å, V = 835.79(9) Å³. The composition of the orthopyroxene was determined combining EMP, LA-ICP-MS and single-crystal XRD analysis, yielding the unit formula ^M2(Mg_0.59Fe _0.21 ²⁺ Li_0.20) ^M1(Mg_0.74Fe _0.20 ³⁺ Fe _0.06 ²⁺ ) Si₂O₆. Structure refinements done on crystals obtained from synthesis runs with variable Mg-content show that the orthopyroxene is virtually constant in composition and hence in structure, whereas coexisting clinopyroxenes occurring both as individual grains or thin rims around the orthopyroxene crystals have variable amounts of Li, Fe³⁺ and Mg contents. Structure refinement shows that Li is ordered at the M2 site and Fe³⁺ is ordered at the M1 site of the orthopyroxene, whereas Mg (and Fe²⁺) distributes over both octahedral sites. The main geometrical variations observed for Li-rich samples are actually due to the presence of Fe³⁺, which affects significantly the geometry of the M1 site; changes in the geometry of the M2 site due to the lower coordination of Li are likely to affect both the degree and the kinetics of the non-convergent Fe²⁺-Mg ordering process in octahedral sites.     

Amphibole stability in primitive arc magmas: effects of temperature,H<Subscript>2</Subscript>O content,and oxygen fugacity

The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% SiO₂, 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200–800 MPa, temperature range of 915–1,070 °C, and oxygen fugacities varying from the nickel–nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% SiO₂, 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515–533; 2003) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved H₂O on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and H₂O content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. H₂O-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of H₂O. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975–1,025 °C. With increasing H₂O pressure (\({P}_{\text {H}_2{\rm O}}\)), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and \(f_{\text {O}_2}\):

$$ P_{\text{H}_{2}{\rm O}}({\rm MPa})=\left[{\frac{{\rm Mg\#}}{52.7}}-0.014 * \Updelta {\rm NNO}\right]^{15.12} $$

This barometer gives a minimum \({P}_{\text{H}_{2}{\rm O}}\) recorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high H₂O pressures (500–900 MPa) and high pre-eruptive magmatic H₂O contents (10–14 wt% H₂O) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil (2010).

     

Lightning-strike fusion of gabbro and formation of magnetite-bearing fulgurite, Cornone di Blumone, Adamello, Western Alps, Italy

The Adamello gabbro exposed on the summit of Cornone di Blumone, Western Alps, Italy, has been fused by lightning strikes to form magnetite-rich fulgurites produced by melting of magnetite, hornblende, calcic plagioclase and minor clinopyroxene. The composition of quench magnetite in the fulgurite is 44.4 Fe₃O₄; 27.5 MgFe₂O₄; 15.1 FeAl₂O₄; 7.9 Fe₂TiO₄; 2.5 Fe₂SiO₄; 1.9 CaFe₂O₄; 0.8 MnFe₂O₄ and is inferred to have crystallized from a low-Si, Fe-rich melt under high oxidation conditions of about 1 log unit below the log₁₀?O₂ of hematite–magnetite. The low Si, Fe-rich melt is considered to have been produced from fusion of magnetite + hornblende-rich areas of the host gabbro and/or possible separation of an immiscible high Fe₂O₃/FeO Fe-rich, low-Si melt from a more siliceous glass during superheating. Skeletal-dendritic morphologies of magnetite in the fulgurite indicate crystallization under conditions of extreme supercooling. Juxtaposition of areas exhibiting different growth habits and crystal sizes of magnetite may reflect compositionally different local melt domains and/or small differences in the delicate balance between nucleation and growth in domains that had slightly different, although ultrafast, cooling rates.     

Experimental phase relations of a low MgO Aleutian basaltic andesite at XH2O = 0.7–1

We conducted melting experiments on a low MgO (3.29 wt.%) basaltic andesite (54.63 wt.% SiO₂) from Westdahl volcano, Alaska, at XH₂O = 0.7–1 and fO₂ ~ Ni–NiO, at pressures = 0.1–180 MPa and temperatures = 900–1,200 °C. We examine the evolution of the melt along a liquid line of descent during equilibrium crystallization at high H₂O and fO₂ conditions, starting from a high FeOt/MgO, low MgO basaltic andesite. Ti-magnetite formed on the liquidus regardless of XH₂O, followed by clinopyroxene, plagioclase, amphibole, and orthopyroxene. We observe slight but significant differences in the phase stability curves between the XH₂O = 1 and 0.7 experiments. Early crystallization of Ti-magnetite and suppression of plagioclase at higher pressures and temperatures resulted in strongly decreasing melt FeOt/MgO with increasing SiO₂, consistent with a “calc-alkaline” compositional trend, in agreement with prior phase equilibria studies on basalt at similar H₂O and fO₂. Our study helps quantify the impact of small amounts of CO₂ and high fO₂ on the evolution of melts formed during crystallization of a low MgO basaltic andesite magma stored at mid- to shallow crustal conditions. Like the prior studies, we conclude that H₂O strongly influences melt evolution trends, through stabilization of Ti-magnetite on the liquidus and suppression of plagioclase at high P–T conditions.     

Fluids in the peridotite–water system up to 6 GPa and 800°C: new experimental constrains on dehydration reactions

The phase assemblages and compositions in a K-free lherzolite + H₂O system were determined between 4 and 6 GPa and 700–800°C, and the dehydration reactions occurring at subarc depth in subduction zones were constrained. Experiments were performed on a rocking multi-anvil apparatus using a diamond-trap setting. The composition of the fluid phase was measured using the recently developed cryogenic LA–ICP–MS technique. Results show that, at 4 GPa, the aqueous fluid coexisting with residual lherzolite (~85 wt% H₂O) doubles its solute load when chlorite transforms to the 10-Å phase between 700 and 750°C. The 10-Å phase breaks down at 4 and 5 GPa between 750 and 800°C and at 6 GPa between 700 and 750°C, leaving a dry lherzolite coexisting with a fluid phase containing 58–67 wt% H₂O, again doubling the total dissolved solute load. The fluid fraction in the system increases from 0.2 when a hydrous mineral is present to 0.4 when coexisting with a dry lherzolite. Our data do not reveal the presence of a hydrous peridotite solidus below 800°C. The directly measured fluid compositions demonstrate a fundamental change in the (MgO + FeO) to SiO₂ mass ratio of fluid solutes occurring at a depth of ca. 120–150 km (in the temperature window of 700–800°C), from (MgO–FeO)-dominated at 4 GPa [with (MgO + FeO)/SiO₂ ratio of 1.41–1.56] to SiO₂-dominated at 5–6 GPa (ratios of 0.61–0.82). The mobility of Al₂O₃ increases by more than one order of magnitude across this P–T interval and demonstrates that Al₂O₃ is compatible in an aqueous fluid coexisting with the anhydrous ol-opx-cpx ± grt assemblage. This shift in the fluid composition correlates with changes in the phase assemblage of the residual silicates. The hitherto unknown fundamental change in (MgO + FeO)/SiO₂ ratio and prominent increase in Al₂O₃ of the aqueous fluid with progressive subduction will likely inspire novel concepts on mantle wedge metasomatism by slab fluids.