Timing of metal–silicate differentiation in the Eagle Station pallasite parent body

The time of the metal–silicate differentiation of the Eagle Station pallasite (ESP) parent body was investigated using the ²⁶Al–²⁶Mg short-lived chronometer (half-life of 0.72 Myr). The Mg isotope ratios were measured in ESP olivines by both MC–SIMS and HR-MC–ICPMS, allowing us to check the consistency between the results given by two different analytical protocols and data reduction processes. Results show that the two datasets are consistent, with a (δ²⁶Mg*)_av. value of –0.003 (± 0.005)‰ (2 s.e., n = 89). Such a value, associated with data from the ¹⁸²Hf–¹⁸²W short-lived systematics (half-life of 8.9 Myr), indicates an ESP parent body metal–silicate differentiation occurring most likely at least at ∼ 2 Ma, but possibly 4 Ma, after CAI formation. From the ²⁷Al/²⁴Mg ratios measured in ESP olivines using MC–SIMS, the duration of the olivine crystallization process was inferred to have lasted over ∼ 275 kyr if the core has differentiated as early as 2 Ma after CAIs, while in the case of a core differentiation occurring 4 Ma after CAIs, the silicate–silicate differentiation should have lasted for another 4 Myr.     

Geochemistry of large river suspended sediments: silicate weathering or recycling tracer?

This study focuses on the major and trace element composition of suspended sediments transported by the world’s largest rivers. Its main purpose is to answer the following question: is the degree of weathering of modern river-borne particles consistent with the estimated river dissolved loads derived from silicate weathering?In agreement with the well known mobility of elements during weathering of continental rocks, we confirm that river sediments are systematically depleted in Na, K, Ba with respect to the Upper Continental Crust. For each of these mobile elements, a systematics of weathering indexes of river-borne solids is attempted. A global consistency is found between all these indexes. Important variations in weathering intensities exist. A clear dependence of weathering intensities with climate is observed for the rivers draining mostly lowlands. However, no global correlation exists between weathering intensities and climatic or relief parameters because the trend observed for lowlands is obscured by rivers draining orogenic zones. An inverse correlation between weathering intensities and suspended sediment concentrations is observed showing that the regions having the highest rates of physical denudation produce the least weathered sediments. Finally, chemical and physical weathering are compared through the use of a simple steady state model. We show that the weathering intensities of large river suspended sediments can only be reconciled with the (silicate-derived) dissolved load of rivers, by admitting that most of the continental rocks submitted to weathering in large river basins have already suffered previous weathering cycles. A simple graphical method is proposed for calculating the proportion of sedimentary recycling in large river basins. Finally, even if orogenic zones produce weakly weathered sediments, we emphasize the fact that silicate chemical weathering rates (and hence CO₂ consumption rates by silicate weathering) are greatly enhanced in mountains simply because the sediment yields in orogenic drainage basins are higher. Hence, the parameters that control chemical weathering rates would be those that control physical denudation rates.     

Equation of state of silicate melts with densified intermediate-range order at the pressure condition of the Earth’s deep upper mantle

The knowledge from the compression behavior of densified SiO₂ glass suggests that SiO₂ melt may behave as a single phase having a densified network structure (intermediate-range order) at the pressure condition of the Earth’s deep upper mantle, including the transition zone. A simple and easy-to-use equation of state of silicate melts which is applicable to a wide range of chemical composition at the pressure condition of the deep upper mantle is proposed based on the assumption that SiO₂ component is in its densified state (or phase). The equation of state proposed in this study is consistent with all the available density data of silicate melts with an SiO₂ content of about 35–55 mol% measured with large-volume presses at pressures between 8 and 22 GPa. The equations of state in previous studies differ considerably from each other. The main reason for the discrepancies seems to be that the compression behavior of multiple states (or phases) of silicate melts has been described in most cases with a single equation of state. It is necessary to consider that silicate melts are in their densified states (or phases) in the deep upper mantle.     

Tin–rare-earth element greisens of the Solnechnoe cassiterite–silicate deposit,Russian Far East

The paper considers for the first time the morphology, composition, and conditions of formation of the greisens of the Solnechnoe deposit (Komsomol’sk ore district), a typical cassiterite–silicate assemblage. The greisens are localized in the root parts of the deposit and represent a system of veins and veinlets formed in the contraction fractures of the metasomatically altered roof of the monzogranite intrusion (age of 94–92 Ma). The cassiterite–chlorite–carbonate–muscovite–quartz composition of the greisens with admixture of topaz, fluorite, and apatite reflects the composition of the monzogranites. The greisens are close in age (85.3 Ma on muscovite) to the granitic aplites (80–85 Ma on the whole-rock and biotite) of the final phase of the intrusive magmatism. The fluid regime of their formation differs from that of the economic ores in higher temperature, pressure, and salinity. One distinguishing feature of the greisens is elevated contents of LREE, U, and Th, which are incorporated in the REE fluorcarbonates, thorite, and uranothorite crystallizing together with cassiterite.     

Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within aplite dikes

Aplite dikes intruding the Proterozoic 1.42(±?3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P₂O₅, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride?+?phosphate (+?S?+?CO₂) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride?+?phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(±?25) and 1.442(±?8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, ε_Nd1.42Ga values for the granites and related granitic pegmatites range from ??3.3 to ??4.7 (average ??3.9), and differ from the values for both the aplites and REE-rich segregations, which range from ??1.0 to ??2.2 (average ??1.6). Furthermore, the granites and pegmatites have (La/Yb)_N <50 with significant negative Eu anomalies, which contrast with higher (La/Yb)_N >100 and absence of an Eu anomaly in both the aplites and segregations. These data are consistent with the aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher ε_Nd1.42Ga values for the aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.     

Diffusive reequilibration of quartz-hosted silicate melt and fluid inclusions: Are all metal concentrations unmodified?

Experiments were conducted to determine the extent and mechanism by which the composition of quartz-hosted silicate melt inclusions (SMI) and aqueous fluid inclusions (FI) can undergo post-entrapment modification via diffusion. Quartz slabs containing assemblages of SMI and FI were reacted with synthetic HCl bearing and metalliferous aqueous fluids at T = 500-720 °C and P = 150-200 MPa. SMI from the single inclusion assemblages were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and electron probe microanalysis (EPMA) before and after the experiments. Analyses revealed that rapid diffusion of the univalent cations Na⁺, Li⁺, Ag⁺, Cu⁺ and H⁺ occurred through the quartz from the surroundings, resulting in significant changes in the concentrations of these elements in the inclusions. Concentrations of other elements with an effective ionic radius larger than that of Ag⁺, or multiple valence states were not modified in the inclusions during the experiments. Our results warn inclusion‘‘ researchers that the interpretation of Na, Li, Cu and Ag concentrations from quartz-hosted SMI and FI should be treated critically.     

Simulation of molecular mass distributions and evaluation of O<Superscript>2−</Superscript> concentrations in polymerized silicate melts

A new statistical model is proposed for the molecular mass distributions (MMD) of polymerized anions in silicate melts. The model is based on the known distribution of Q ⁿ species in the MeO-Me₂O-SiO₂ system. In this model, chain and ring complexes are regarded as a random series of Q ⁿ structons with various concentrations of bridging bonds (1 ≤ n ≤ 4, Q ⁰ corresponds to SiO ₄ ^4? ). This approach makes it possible to estimate the probability of formation of various ensembles of polymer species corresponding to the general formula (Si _i O_3i+1?j )^2(i+1?j)?, where i is the size of the ion, and j is the cyclization number of intrachain bonds. The statistical model is utilized in the STRUCTON computer model, which makes use of the Monte Carlo method and is intended for the calculation of the composition and proportions of polyanions at a specified degree of polymerization of silicate melts (STRUCTON, version 1.2; 2007). Using this program, we simulated 1200 MMD for polyanions in the range of 0.52 ≤ p ≤98, where p is the fraction of nonbridging bonds in the silicon-oxygen matrix. The average number of types of anions in this range was determined to increase from three (SiO ₄ ^4? , Si₂O ₇ ^6? , and Si₃O ₁₀ ^8? ) to 153, and their average size increases from 1 to 7.2. A special option of the STRUCTON program combines MMD reconstructions in silicate melts with the formalism of the Toop-Samis model, which enables the calculation of the mole fraction of the O^2? ion relative to all anions in melts of specified composition. It is demonstrated that, with regard for the distribution and average size of anion complexes, the concentration of the O^2? ion in the MeO-SiO₂ system is characterized by two extrema: a minimum at 40–45 mol % SiO₂, which corresponds to the initial stages of the gelenization of the polycondensated silicate matrix, and a maximum, which is predicted for the range of 60–80 mol % SiO₂.     

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

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

Application of IR and Raman spectroscopy for the determination of the role of oxygen fugacity in the formation of N–С–О–Н molecules and complexes in the iron-bearing silicate melts at high pressures

Large-scale melting of the Earth’s early mantle under the effect of global impact processes was accompanied by the generation of volatiles, which concentration was mainly controlled by the interaction of main N, C, O, and H gas-forming elements with silicate and metallic melts at low oxygen fugacity (fO₂), which predominated during metallic segregation and self-oxidation of magma ocean. The paper considers the application of Raman and IR (infrared) Fourier spectroscopy for revealing the mechanisms of simultaneous dissolution and relative contents of N, C, O, and H in glasses, which represent the quench products of reduced model FeO–Na₂O–Al₂O₃–SiO₂ melts after experiments at 4 GPa, 1550°C, and fO₂ 1.5–3 orders of magnitude below the oxygen fugacity of the iron—wustite buffer equilibrium (fO₂(IW)). Such fO₂ values correspond to those inferred for the origin and evolution of magma ocean. It was established that the silicate melt contains complexes with N–H bonds (NH₃, NH ₂ ⁺ , NH ₂ ^- ), N₂, H₂, and CH₄ molecules, as well as oxidized hydrogen species (OH^– hydroxyl and molecular water H₂O). Spectral characteristics of the glasses indicate significant influence of fO₂ on the N–C–O–H proportion in the melt. They are expressed in a sharp decrease of NH ₂ ⁺ , NH ₂ ^- (O–NH₂), OH^–, H₂O, and CH₄ and simultaneous increase of NH ₂ ^- (≡Si–NH₂) and NH₃ with decreasing fO₂. As a result, NH₃ molecules become the dominant nitrogen compounds among N–C–H components in the melt at fO₂ two orders of magnitude below fO₂(IW), whereas molecular СН₄ prevails at higher fO₂. The noteworthy feature of the redox reactions in the melt is stability of the ОН^– groups and molecular water, in spite of the sufficiently low fO₂. Our study shows that the composition of reduced magmatic gases transferred to the planet surface has been significantly modified under conditions of self-oxidation of mantle and magma ocean.     

Peridotite–eclogite–carbonatite systems at 7.0–8.5 GPa: concentration barrier of diamond nucleation and syngenesis of its silicate and carbonate inclusions

Diamond crystallization in multicomponent melts of variable composition is studied. The melt carbonates are K₂CO₃, CaCO₃?MgCO₃, and K-Na-Ca-Mg-Fe-carbonatites, and the melt silicates are model peridotite (60 wt.% olivine, 16 wt.% orthopyroxene, 12 wt.% clinopyroxene, and 12 wt.% garnet) and eclogite (50 wt.% garnet and 50 wt.% clinopyroxene). In the experiments carried out under the PT-conditions of diamond stability, the carbonate-silicate melts behave like completely miscible liquid phases. The concentration barriers of diamond nucleation (CBDN) in the melts with variable proportions of silicates and carbonates have been determined at 8.5 GPa. In the system peridotite–K₂CO₃–CaCO₃?MgCO₃–carbonatite they correspond to 30, 25, and 30 wt.% silicates, respectively, and in the analogous eclogite–carbonate system, 45, 30, and 35 wt.%. In the silicate-carbonate melts with higher silicate contents seed diamond growth occurs, which is accompanied by the crystallization of thermodynamically unstable graphite phase. In the experiments with melts compositionally corresponding to the CBDN at 7.0 GPa and 1200–1700 °C, a full set of silicate minerals of peridotite (olivine, orthopyroxene, clinopyroxene, garnet) and eclogite (garnet, clinopyroxene) parageneses was obtained. The minerals occur as syngenetic inclusions in natural diamonds; moreover, the garnets contain an impurity of Na, and the pyroxenes, K. The experimental data indicate that peridotite-carbonate and eclogite-carbonate melts are highly effective for the formation of diamond (or unstable graphite) together with syngenetic minerals and melts, which agrees with the carbonate-silicate (carbonatite) model for the mantle diamond formation.     

Effect of network-forming cations on the oxygen isotope fractionation between silicate melts: Experimental study at 1400–1570°C

We have experimentally studied the behavior of oxygen isotope composition in silicate melts with a wide range of network-forming cations. Isotopic equilibration of the Di-An eutectic melts modified by addition of Si, Al, Ti, and Fe was carried out in a vertical tube furnace within a temperature range from 1400 to 1570°C. It was established that the value ^{10 3}Lnα between silicic and basic melts at 1400 and 1450°C systematically increases with increase of SiO₂ content, reaching ≈1‰ at 20% melt silica enrichment. The effect of the Fe₂O₃, TiO₂, and Al₂O₃ contents was studied at 1500°C. An increase in Fe₂O₃ from 5 to 20 wt % causes a 0.4‰ increase of δ¹⁸O. An increase in Ti and Al contents results in the non-linear behavior of δ¹⁸O, which decreases in the region of the highest TiO₂ (28.4%) and Al₂O₃ (29.3 %) contents. In the region of moderate Fe₂O₃, TiO₂, and Al₂O₃ contents, the values of δ¹⁸O show monotonous linear dependence on the oxide contents. Methods of estimations of oxygen isotope fractionation coefficients at T > 1400°C in the studied range of network-forming oxides are considered on the basis of experimental data. The calculation of fractionation coefficients with the use of I¹⁸O index showed that experimental values with increase of SiO₂ content deviate from calculated values by 0.3‰ for basic melts and 0.5–0.6‰ in the region of silicic melts. Similar pattern is observed during approximation of a melt by normative mineral composition. The calculation with the Garlick index leads to the systematic underestimation (on average, by 0.3‰) of 10³Lnα as compared to the experimental data. The NBO/T ratio appeared the best parameter to describe 10³Lnα in the melt-melt system, including the region of high-Fe melts. Analysis of experimental data leads us to conclude that the degree of polymerization of the melts in the studied temperature-composition region is the most important factor affecting the oxygen-isotope fractionation in the melt-melt system. Empirical index similar to the Garlick index was proposed to take into account oxygen associated with T-cations: $$I^m = (C_{Si} + aC_{Al} + bC_{Ti} + cC_{Fe^{3 + } } )/\Sigma C_i ,$$ where a, b, and c constants are empirically established coefficients: 0.75, 0. 70, and 1.75, respectively.     

Microanalysis of Fe3+/ΣFe in oxide and silicate minerals by investigation of electron energy-loss near-edge structures (ELNES) at the Fe M 2,3 edge

The Fe M _2,3-edge spectra of solid solutions of garnets (almandine-skiagite Fe₃(Al_1–xFe_x)₂[SiO₄]₃ and andradite-skiagite (Fe_1–xCa_x)₃Fe₂[SiO₄]₃), pyroxenes (acmite-hedenbergite (Ca_1–xNa_x)(Fe²⁺ _1−xFe³⁺ _x)Si₂O₆), and spinels (magnetite-hercynite Fe(Al_1–xFe_x)₂O₄) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission electron microscope (TEM). The Fe M _2,3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge feature varies depending on Fe³⁺/ΣFe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship between Fe³⁺/ΣFe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M _2,3 edge is observed. Since the partial cross sections of the Fe M _2,3 edges are some orders of magnitude higher than those of the Fe L _2,3 edges, the Fe M _2,3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated by Fe M _2,3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that shows different edge shapes consistent with variations in the Fe³⁺/ΣFe ratio over distances on the order of 100 nm. Received: 14 April 1998 / Revised, accepted: 8 March 1999     

Thermodynamic controls on element partitioning between titanomagnetite and andesitic–dacitic silicate melts

Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO₂) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺) and Cu²⁺/Cu⁺ with increasing fO₂ between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO₂ implies a reciprocal relationship between a(Fe²⁺O) and a(Fe³⁺O_1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO₂ in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO₂ in silicic, but less so in mafic bulk systems.     

Trace element partitioning between amphibole and hydrous silicate glasses at 0.6–2.6 GPa

Partitioning behavior between amphibole and silicate glass of thirty-three minor and trace elements (Sc, Ti, V, Cr, Co, Rb, Sr, P, Y, Zr, Nb, Cs, Ba, K, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th, and U) have been determined experimentally. Products of crystallization of hydrous basalt melts from 0.6 GPa/860 °C up to 2.6 GPa/970 °C were obtained in a multi-anvil apparatus. Major and trace element compositions of amphibole and glass were determined with a combination of electron microprobe and laser ablation inductively coupled plasma mass spectrometry. The main mineral phase is calcic amphibole, and the coexisting glass compositions are tonalite, granodiorite, and granite. The compatibility of rare earth elements increase at 915 °C and then decrease at 970 °C, but the compatibility of most of these elements shows a continued, significant increase with increasing pressure. For high-field strength elements, large ion lithophile elements, actinide compatibility decrease with increasing temperature or pressure, but transition metals show a continued increase in compatibility within the temperature–pressure conditions. From mathematical and graphical fitting, we determined best-fit values for the ideal ionic radius (r₀, 1.01–1.04 Å), the strain-free partition coefficient (D₀, 1.18–1.58), and apparent Young’s modulus (E, 142–370 GPa) for the M4 site in amphibole according to the lattice strain model. The \(D_{0}^{M4}\) for rare earth elements rises at 915 °C and then drops at 970 °C at 0.6 GPa. However, the \(D_{0}^{M4}\) values are positively proportional to the pressure for rare earth elements in the amphibole-glass pairs at 0.6–2.6 GPa and 970 °C. Furthermore, the derived best-fit values for \(r_{0}^{M4}\) and \(E^{M4}\) are almost constant and trend to increase with rising temperature and pressure, respectively. The partition coefficient is distinctly different for different melt compositions. The rare earth elements become more enriched in amphibole if the quenched glass is granodiorite or granite compared to the tonalitic glasses.

     

Formation of baotite - a Cl-rich silicate - together with fluorapatite and F-rich hydrous silicates in the Kval?ya lamproite dyke, North Norway

Baotite occurs as a late phase in the Kval?ya lamproite dyke and in the fenitized granite adjacent to the dyke, suggesting that baotite formed during reactions between rock and fluids derived from a volatile-rich lamproitic magma. Most of the analyzed grains of baotite from the Kval?ya lamproite show compositions close to the ideal Nb-free end-member Ba₄Ti₈Si₄O₂₈Cl. Compilation of all published baotite analyses suggests that the major compositional variations of baotite occur between the Nb-free end member Ba₄Ti₈Si₄O₂₈Cl, and a Nb-rich end member Ba₄Ti₂Fe2⁺ ₂Nb₄Si₄O₂₈Cl. However, a Pb-bearing baotite, showing significant concentrations of Ca, Sr, Pb and K, and approximately 3 Ba p.f.u., was also identified from the Kval?ya lamproite. Euhedral fluorapatite formed as an early phase during crystallization of the lamproite magma, while anhedral REE-rich fluorapatite overgrowths on the euhedral grains formed during reactions with the late magmatic fluid. Fluorapatite contains up to 1.2?F p.f.u., but only traces of Cl. Other F-rich, but Cl-poor minerals of the lamproite include fluoro-potassic-magnesio-arfvedsonite, fluoro-phlogopite, and yangzhumingite. The presence of baotite together with a range of high-F, but low-Cl mineral phases suggests that the minerals formed in equilibrium with a high-F, Cl-bearing hydrous fluid. The high Cl-content of baotite demonstrates that Cl is strongly partitioned into this mineral in the presence of a Cl-bearing F-rich hydrous fluid. We suggest that a combination of high a_Si, a_Ti, a_Ba, and f_O2, but low a_Ca of the fluid enabled baotite formation.     

Ferric/ferrous ratio in silicate melts: a new model for 1 atm data with special emphasis on the effects of melt composition

The effect of MgO and total FeO on ferric/ferrous ratio in model multicomponent silicate melts was investigated experimentally in the temperature range 1300–1500 °C at 1 atm total pressure in air. We demonstrate that the addition of these weak network modifier cations results in an increase of Fe³⁺/Fe²⁺ ratio in both mafic and silicic melts. Based on present and published experimental data, a new empirical equation is proposed to predict the ferric/ferrous ratio as a function of oxygen fugacity, temperature and melt composition. In contrast to previous equations, the compositional effect of melts on the Fe³⁺/Fe²⁺ ratio is not only modeled by the sum of the molar fraction of the individual oxide components. Additional interactions terms have also been incorporated. The main advantage of the proposed model is its applicability for a wide compositional range. However, its application to felsic melts (>?68 wt% SiO₂) is not recommended. Other advantages of this equation and differences when compared with previous models are discussed.     

Petrology of Al- and Cr-rich ophiolitic chromitites from the Muğla,SW Turkey: implications from composition of chromite,solid inclusions of platinum-group mineral,silicate, and base-metal mineral,and Os-isotope geochemistry

Ultramafic rocks around the city of Muğla in SW Turkey are represented by mantle peridotites depleted to various degrees, ranging from cpx-rich harzburgites to depleted harzburgite and dunite. Cpx-rich harzburgites are thought to be the residua left after extraction of MORB-type basalt, from which high-Al chromitite [49.2 < Cr# = 100 × Cr/(Cr + Al) < 53.5] crystallised with a higher proportion of ¹⁸⁷Os/¹⁸⁸Os (average of 0.1361). However, depleted harzburgites are assumed to be the residua left after extraction of hydrous boninitic melt produced by second stage partial melting of already depleted mantle due to a subducting slab, from which high-Cr chromitites (64.2 < Cr# < 85.9) with lower and heterogeneous ¹⁸⁷Os/¹⁸⁸Os ratio (average of 0.1324) were crystallised as a result of melt–rock interaction in a supra-subduction environment. Dunites around the chromite deposits are considered to be the product of melt–peridotite interaction. Most of the chromitites contain high-Cr chromite and display enrichment in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), with PGE concentrations between 61 and 1,305 ppb. Consistently, laurite-erlichmanite series minerals with various Os concentrations are found to be the most abundant PGM inclusions in chromite. Os–Ir–Ru alloy, irarsite, and kashinite, as well as Pt–Fe alloy and Pt-oxide, which are not common in ophiolitic chromitites, were also detected as magmatic PGM inclusions. Pentlandite, millerite, and, rarely heazlewoodite form the magmatic inclusions of base-metal sulphide. The presence of olivine and clinopyroxene, as well as hydrous silicate inclusions such as amphibole and phlogopite, in high-Cr chromitite supports the idea that high-Cr chromitites were formed in a supra-subduction environment.     

Crystal chemical characteristics of α-CaSi2O5, a new high pressure calcium silicate with five-coordinated silicon synthesized at 1500°C and 10 GPa

The crystal structure of α-CaSi₂O₅ synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P , using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca_1.02Si_1.99O₅ by EPMA analysis and a=7.243(2) Å, b=7.546(4) Å, c=6.501(4) Å, α=81.43(5)°, β=84.82(4)°, γ=69.60(3)°, V=329.5(3) ų, yielding the density value, 3.55 g/cm³. The structure is closely related to that of titanite, CaTiSiO₅ and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Å. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of α-CaSi₂O₅ at room pressure.