FeO MgO Research Articles

The fate of carbonate in oceanic crust subducted into earth's lower mantle

We report on laser-heated diamond anvil cell (LHDAC) experiments in the FeO–MgO–SiO2–CO2 (FMSC) and CaO–MgO–SiO2–CO2 (CMSC) systems at lower mantle pressures designed to test for decarbonation and diamond forming reactions. Sub-solidus phase relations based on synthesis experiments are reported in the pressure range of ∼35 to 90 GPa at temperatures of ∼1600 to 2200 K. Ternary bulk compositions comprised of mixtures of carbonate and silica are constructed such that decarbonation reactions produce non-ternary phases (e.g. bridgmanite, Ca-perovskite, diamond, CO2–V), and synchrotron X-ray diffraction and micro-Raman spectroscopy are used to identify the appearance of reaction products. We find that carbonate phases in these two systems react with silica to form bridgmanite ±Ca-perovskite + CO2 at pressures in the range of ∼40 to 70 GPa and 1600 to 1900 K in decarbonation reactions with negative Clapeyron slopes. Our results show that decarbonation reactions form an impenetrable barrier to subduction of carbonate in oceanic crust to depths in the mantle greater than ∼1500 km. We also identify carbonate and CO2–V dissociation reactions that form diamond plus oxygen. On the basis of the observed decarbonation reactions we predict that the ultimate fate of carbonate in oceanic crust subducted into the deep lower mantle is in the form of refractory diamond in the deepest lower mantle along a slab geotherm and throughout the lower mantle along a mantle geotherm. Diamond produced in oceanic crust by subsolidus decarbonation is refractory and immobile and can be stored at the base of the mantle over long timescales, potentially returning to the surface in OIB magmas associated with deep mantle plumes.

Genesis of Diamonds and Paragenetic Inclusions under Lower Mantle Conditions: The Liquidus Structure of the Parental System at 26 GPa

The regularities of the joint genesis of diamonds and their paragenetic inclusions under lower mantle conditions are controlled by the melting relations at the liquidus of the multicomponent diamond-forming system. The boundary compositions of this system are evident from the generalized data on the analytical mineralogy of paragenetic inclusions in lower-mantle diamonds. The structure of the liquidus of the diamond-forming system was studied in a physicochemical experiment for P–T parameters typical of the depths of 670–800 km. The compositions of parental melts/solutions for diamonds and paragenetic inclusions correspond to the multicomponent system MgO–FeO–СаО–SiO2–MgCO3–FeCO3–CaCO3–Na2CO3–C. Its primary melting is controlled by the peritectic phase relations at the solidus, which was shown as a result of experimental studies of polythermal sections of the system during a study of their phase diagrams. Of key importance is the effect of the “stishovite paradox,” i.e., the peritectic reaction between the ultrabasic bridgmanite phase and melt with the formation of basic oxide associations of periclase–wustite solid solutions and stishovite. The peritectic reaction of bridgmanite is a fundamental feature of the diamond-forming system and determines the major peculiarity in its liquidus structure. Structure of the peritectic liquidus provides the physicochemical basis for the evolution of growth melts of diamonds and their paragenetic minerals. Based on the experimental data, we have constructed a fractional diagram of syngenesis of diamonds and inclusions clearly illustrating the solution–melt mechanism of diamond genesis and sequence of growth trapping of primary inclusions by diamonds under the lower-mantle conditions. The physicochemical factors of the genesis of diamonds and primary inclusions are agreed and generalized in a compositional diagram of lower-mantle diamond-forming media, and they provide a natural basis for the genetic classification of inclusions of rock-forming and accessory minerals in diamonds

Research on the oxygen content in 304L stainless steel during VOD-LF-CC process based on IMCT

The total oxygen in 304L stainless steel produced by the VOD-LF-CC process is investigated in this research. The equilibrium oxygen content in the molten steel during the smelting process is calculated based on the ion-molecule coexistence theory. The results show that it is feasible to use the developed thermodynamic model for calculating the equilibrium oxygen content in CaO–SiO2–Al2O3–MgO–FeO–Cr2O3 slags equilibrated with 304L molten steel. The variation law for total oxygen is in good agreement with the equilibrium oxygen content. The oxygen content of the oxide-inclusion decreases with the number density of inclusions, which decrease at different stages in the process. The content of FeO and Cr2O3 in the slag has no obvious effect on the equilibrium oxygen content when the basicity of the slag remains constant. Combining the proposed method with industrial practice, the optimum VOD process parameters for obtaining a low equilibrium oxygen content are as follows: 1600–1650 °C of the temperature after the VOD stage, 0.4–0.5% of the Si content after reduction, and 2.2–2.5 of the slag basicity.

High-pressure and high-temperature stability of chlorite and 23-Å phase in the natural chlorite and synthetic MASH system

A series of experiments was conducted on the decomposition of natural and chemically mixed chlorites to examine the stable hydrous phases in the MgO–FeO–Al2O3–SiO2–H2O (MFASH) system under 5–12GPa and 700–1100°C. The upper pressure and temperature limits of the stability region of chlorite are consistent with those observed in previous studies. The hydrous aluminum bearing pyroxene (phase HAPY) and Mg-sursassite (Sur) were observed just above the temperature stability region of chlorite (Chl); clinohumite (cHm) was observed coexisting with phase HAPY at 6GPa and 800°C and coexisting with the 23-Å phase at 7GPa and 800°C, which may suggest the transportation of water through Chl→(HAPY→cHm)→23-Å phase along a relatively warm slab. The 23-Å phase has a wider stability region in the pure MASH system (up to 12GPa and 1100°C) than it does in the MFASH system (7–10GPa, up to 1000°C). The stability of the 23-Å phase beyond the chlorite breakdown pressure indicates that it may play an important role in transporting water into the deep Earth and even into the mantle transition zone.

Low‐Pand high‐Tmetamorphism of basalts: Insights from the Sudbury impact melt sheet aureole and thermodynamic modelling

AbstractLow‐pressure and high‐temperature (LP–HT) metamorphism of basaltic rocks, which occurs globally and throughout geological time, is rarely constrained by forward phase equilibrium modelling, yet such calculations provide valuable supplementary thermometric information and constraints on anatexis that are not possible to obtain from conventional thermometry. Metabasalts along the southern margin of the Sudbury Igneous Complex (SIC) record evidence of high‐grade contact metamorphism involving partial melting and melt segregation. Peak metamorphic temperatures reached at least ~925°C at ~1–3 kbar near theSICcontact. Preservation of the peak mineral assemblage indicates that most of the generated melt escaped from these rocks leaving a residuum characterized by a plagioclase–orthopyroxene–clinopyroxene–ilmenite‐magnetite±melt assemblage. Peak temperatures reached ~875°C up to 500 m from theSIClower contact, which marks the transition to metabasalts that only experienced incipient partial melting without melt loss. Metabasalts ~500 to 750 m from theSICcontact are characterized by a similar two‐pyroxene mineral assemblage, but typically contain abundant hornblende that overgrew clino‐ and orthopyroxene along an isobaric cooling path. Metabasalts ~750 to 1,000 m from theSICcontact are characterized by a hornblende–plagioclase–quartz–ilmenite assemblage indicating temperatures up to ~680°C. Mass balance and phase equilibria calculations indicate that anatexis resulted in 10–20% melt generation in the inner ~500 m of the aureole, with even higher degrees of melting towards the contact. Comparison of multiple models, experiments, and natural samples indicates that modelling in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2(NCFMASHTO) system results in the most reliable predictions for the temperature of the solidus. Incorporation of K2O in the most recent amphibole solution model now successfully predicts dehydration melting by the coexistence of high‐Ca amphibole and silicate melt at relatively low pressures (~1.5 kbar). However, inclusion of K2O as a system component results in prediction of the solidus at too low a temperature. Although there are discrepancies between modelling predictions and experimental results, this study demonstrates that the pseudosection approach to mafic rocks is an invaluable tool to constrain metamorphic processes atLP–HTconditions.

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.

Geochemical Constraints on the Cold and Hot Models of the Moon’s Interior: 1–Bulk Composition

The variations of the bulk composition of the silicate Moon (crust + mantle = Bulk Silicate Moon, BSM) depending on the thermal state are explored based on the joint inversion of gravitational, seismic, and petrologic data within the Na2O–TiO2–CaO–FeO–MgO–Al2O3–SiO2 system. The mantle bulk temperature Tmean determining the mineral composition and physical properties of the Moon is adopted as the integral characteristic of thermal state. By parameter Tmean, all thermal models of the Moon can be conventionally broken down into the “cold” with Tmean ~ 690–860°C and the “hot” with Tmean ~ 925–1075°C. The estimations of refractory oxide abundance in lunar rocks depending on the thermal state are included in two different groups. Cold models of BSM are comparable by the bulk content of Al2O3 ~ 3.0–4.6 wt % to those for the silicate Earth (Bulk Silicate Earth, BSE), while hot models of BSM are significantly enriched with Al2O3 ~ 5.1–7.3 wt % (Al2O3 ~ 1.2–1.7 × BSE) as compared with BSE. On the contrary, independent of the temperature distribution, both types of BSM models are characterized by nearly constant values of bulk concentrations of FeO ~ 12–13 wt % and magnesian number MG# 80–81.5 (MG# = [MgO/(MgO + FeO) × 100]), which differ markedly from those for BSE (FeO ~ 8% and MG# 89). It means that for all possible temperature distributions, the silicate fraction of the Moon is FeO-enriched and MgO-depleted in relation to BSE. These arguments discard the possibility of the Moon’s formation out of the material of the Earth’s primitive mantle. In spite of the almost complete coincidence of the isotopic systems, this apparently undeniable fact has no adequate explanation in the existing canonical models of the Moon’s origin and should result in additional constraints on the dynamic processes in models of the formation of the Earth–Moon system. However, the problem of the similarity of and/or difference between compositions of the Moon and the Earth regarding the abundance of refractory elements, which is very important for the geochemistry of the Moon and the Earth’s mantle, remains unresolved and requires further study.

Effect of Al2O3/TiO2/Na2O on enrichment of phosphorus in P-bearing steelmaking slag

To explore the influence of Al2O3/TiO2/Na2O on the mineralogical structure and phosphorus enrichment of converter slag, phosphorus enrichment and distribution in the CaO–SiO2–Fe2O3–MgO–Al2O3/TiO2/Na2O slag system were experimentally examined. The results show that phosphorus mainly exists in an n2CaO·SiO2–3CaO·P2O5 (nC2S–C3P) solid solution. Al2O3/TiO2/Na2O is beneficial for phosphorus enrichment in the CaO–SiO2–Fe2O3–MgO slag system to various degrees. This allows screening and separation of the enriched phase in the slag. When Al2O3 content was increased from 0 to 15%, the P2O5 content in the P-rich phase increased from 13.88 to 21.32%, and the area of the P-rich phase decreased from 57.50 to 37.87%. When the TiO2 content was increased from 0 to 10%, the content of P2O5 in the P-rich phase increased from 13.88 to 22.90%, and the area of the P-rich phase decreased from 57.50 to 39.20%. When the contents of Al2O3 and TiO2 were increased, the previously formed nC2S–C3P solid solution transformed into n′C2S–C3P (n < n′) with higher phosphorus content. Na2O had no significant effect on phosphorus enrichment, but exerted significant influence on the area of the P-rich phase. When the Na2O content was increased from 0 to 8%, the previously formed nC2S–C3P solid solution gradually became Na2Ca4(PO4)2SiO4, and the area of the P-rich phase increased from 57.50 to 68.50%. With increasing the content of ATN (a mixture with a mass ratio of Al2O3/TiO2/Na2O = 15:4:3) from 0 to 30%, the content of P2O5 increased from 13.88 to 42.10% and the area of the P-rich phase decreased from 54.92 to 18.40%.

Multielemental composition and arsenic speciation in low rank coal from the Velenje Basin, Slovenia

Coal is one of the main fossil fuel resources and remains the most important contributor to global power generation but coal utilization has severe negative impacts, mostly due to the release of CO2 and toxic elements into the environment. This means that determining the quality of coal is important to address the environmental and health problems related to coal combustion. Coal samples from the Velenje coalmine, one of the largest actively mined coal basins in Central Europe, were divided into organic rich and inorganic rich fractions according to the percentage of carbon. Oxides of the major elements (SiO2, Al2O3, Fe2O3 MgO, CaO, Na2O, K2O, TiO2, P2O5, MnO, Cr2O3), toxicologically and environmentally relevant elements (As, Ba, Co, Cu, Hg, Mo, Ni, Pb, Th, U, V, Zn, Se) and other trace elements (Ce, Cs, Dy, Er, Eu, Gd, La, Nb, Nd, Pr, Rb, Sm, Sr, Tb, Y, Zr) were measured in sample digests using inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of the majority of analyzed elements were either equal to or below the global average for coal. Exceptions were Sr (1090 ± 510 μg g−1, 9 times higher), Ba (301 ± 184 μg g−1, 2 times higher) and Pb (9.12 ± 17.0 μg g−1, 1.4 times higher) in inorganic rich coal and Mo (7.76 ± 4.76 μg g−1, 3.5 times higher) and U (5.24 ± 3.23 μg g−1, 1.8 times higher) in organic rich samples. Eighteen elements (Ag, Au, Be, Bi, Cd, Ga, Hf, Ho, Lu, Sb, Sc, Se, Sn, Ta, Ti, Tm, Yb, W) were below the limit of detection in >70% of the samples. Speciation analysis revealed the presence of several organoarsenic compounds in the organic rich samples, with the tetramethylarsonium ion (TETRA, 0.01–1.10 μg g−1) and trimethylarsine oxide (TMAO, 0.01–0.29 μg g−1) as the most abundant. A comparison with coal samples from the Senovo, Kanižarica, and Trbovlje coal mines in Slovenia and from the Sokolov Basin, (Czech Republic) revealed that Velenje contains much higher amount of organoarsenic compounds (34.8 ± 16.9%) in comparison to the others (4.45 ± 4.19%).

Origin, age, and significance of deep‐seated granulite‐facies migmatites in the Barrow zones of Scotland, Cairn Leuchan, Glen Muick area

AbstractPetrological modelling of granulite‐facies mafic and semipelitic migmatites from Cairn Leuchan, northeast Scotland, has provided new constraints on the pressure (P) and temperature (T) conditions of high‐grade metamorphism in the type‐locality Barrow zones. Phase diagrams constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 system have constrained the P−T conditions of peak metamorphism in the Glen Muick region of the upper sillimanite zone (Sill+Kfs) to have been at least ∼840°C at ∼9 kbar (high‐P granulite facies). These conditions are ∼120°C and ∼3 kbar higher than those recorded by lower sillimanite zone (Sill+Ms) units located only a few kilometres away to the southeast at Glen Girnock, indicating the presence of a significant thermal and barometric high exposed within the Scottish Dalradian, and supporting previous suppositions of a potential tectonic break between the two regions. U–Pb zircon geochronology performed on these mafic migmatites produced ages of c. 540–470 Ma from grains with both igneous and metamorphic morphological characteristics. Their basaltic protoliths likely formed during a period of volcanism dated at c. 570 Ma, associated with passive‐margin extension prior to the onset of Iapetus Ocean closure, and high‐grade metamorphism and partial melting is interpreted to have taken place at c. 470 Ma, synchronous with sillimanite‐grade metamorphism recorded elsewhere in the Dalradian. These high‐grade Cairn Leuchan lithologies are interpreted as representing a fragment of Grampian Terrane lower crust that was exhumed via displacement along a steeply dipping tectonic discontinuity related to the Portsoy–Duchray Hill Lineament, and are not pre‐Caledonian Mesoproterozoic basement, as suggested by some previous studies. Veins within some mafic migmatites in the Cairn Leuchan area, composed almost entirely (&gt;80%) of garnet, with minor quartz, plagioclase, amphibole, and clinopyroxene, are interconnected with leucosomes and are interpreted to represent former garnet‐bearing melt segregations that have been locally drained of almost all melt. Thus, mafic components of the lower crust, currently underlying relatively lower grade metasediments exposed to the southeast, may represent a potential source rock for widely documented, post‐orogenic felsic plutons, sills, and dykes that occur throughout the Grampian Terrane.

Magnesium isotopic composition of altered oceanic crust and the global Mg cycle

To investigate the behavior of Mg isotopes during low-temperature alteration of oceanic crust and to further understand its role in the global Mg cycle, we measured the Mg isotopic compositions (25Mg/24Mg and 26Mg/24Mg) of a set of samples of altered oceanic crust (AOC) recovered from the Ocean Drilling Program Hole 801C, the reference site for old crust (∼170 Ma) subducting in the Pacific. The measured δ26Mg values range from −1.70‰ to 0.21‰, deviating from that of pristine oceanic basalts (−0.25 ± 0.07‰). Composite samples of volcanoclastic breccia that have experienced relatively intense alteration have larger variation in δ26Mg values (−1.01‰ to 0.15‰) than composite samples of massive basaltic flows (−0.53‰ to −0.04‰), indicating significant Mg isotope fractionation during low-temperature alteration of the oceanic crust. Moreover, the upper off-axis basement has on average lower δ26Mg values (−1.70‰ to −0.04‰) than the lower on-axis basement (−0.16‰ to 0.21‰). These findings, combined with the co-variations between MgO content and FeO∗/CaO ratio and between δ26Mg and FeO∗/CaO ratio, suggest that formation of Mg-bearing minerals (i.e., saponite and calcite) during low-temperature alteration of the oceanic crust accounts for the highly variable δ26Mg of AOC. Early formation of saponite under anoxic condition preferentially takes up heavy Mg isotopes and accounts for Mg enrichment and relatively high δ26Mg in the on-axis basement. Subsequent precipitation of carbonates results in the dilution of Mg and relatively low δ26Mg in the off-axis basement. In addition, accumulation of carbonate-rich interflow sediments in the upper basement may contribute further to the low δ26Mg. A weighted average δ26Mg value of 0.00 ± 0.09‰ is estimated for the AOC at Site 801, implying that low-temperature alteration of oceanic crust drives the ocean to a lighter Mg isotopic composition, and thus requires additional carbonate precipitation to maintain a steady-state Mg isotopic composition of seawater. A mass balance calculation suggests that the Mg output flux due to low-temperature alteration of the oceanic crust equals ∼12% of the annual Mg riverine input, indicating that AOC is a significant sink of Mg in seawater. Our study further highlights that recycling of AOC with highly variable δ26Mg along with overlying marine sediments into the mantle through subduction may generate Mg isotopic heterogeneity in the mantle at small scales.

Cordierite-bearing granulites from Ihosy, southern Madagascar: Petrology, geochronology and regional correlation of suture zones in Madagascar and India

Madagascar, a major fragment of Gondwana, is mainly composed of Precambrian basement rocks formed by Mesoarchean to Neoproterozoic tectono-thermal events and recording a Pan-African metamorphic overprint. The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent. Here we present detailed petrology, mineral chemistry, metamorphic P–T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone. Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism. The peak mineral assemblage is interpreted as garnet + sillimanite + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite, which is overprinted by a retrograde mineral assemblage of biotite + garnet + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite. Phase equilibria modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) indicates peak metamorphic conditions of 850–960 °C and 6.9–7.7 kbar, and retrograde P–T conditions of <740 °C and <4.8 kbar, that define a clockwise P–T path. Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma, indicating dominantly Paleoproterozoic sources. The lower intercept age of 514 ± 33 Ma probably indicates the timing of high-grade metamorphism, which coincides with the assembly of the Gondwana supercontinent. The comparable rock types, zircon ages and metamorphic P–T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.

Phase equilibrium modelling and SHRIMP zircon U–Pb dating of medium-pressure pelitic granulites in the Helanshan complex of the Khondalite Belt, North China Craton, and their tectonic implications

The Helanshan complex in the Khondalite Belt, North China Craton (NCC) is characterized by the occurrence of both medium-pressure (MP) and high-pressure (HP) pelitic granulites. This paper presents new results from phase equilibrium modelling and SHRIMP zircon U–Pb dating on the MP garnet-sillimanite granulites, in order to better understand the relationship between the MP and HP pelitic granulites, the tectonic implications of the MP pelitic granulites for the formation of the Khondalite Belt, and their geodynamic implications for the Paleoproterozoic orogenesis. Based on petrographic observations, the metamorphic evolution can be divided into four stages (M1–M4). The M1–M3 stages are suprasolidus as reflected by the migmatitic structure on the outcrop. The peak metamorphic stage (M1) is represented by the rim of garnet porphyroblasts along with inclusions of sillimanite + quartz ± biotite, and rutile relicts + plagioclase + K-feldspar in the matrix; the M2 is defined by the ilmenite that have partially replaced rutile; and the M3 is characterized by the replacement of garnet by cordierite. The subsolidus mineral assemblage is garnet + biotite + plagioclase + K-feldspar + sillimanite + cordierite + ilmenite + quartz (M4). Combined with the phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 chemical system, the MP pelitic granulites record a clockwise P–T path, which is characterized by near-isothermal decompression (ITD) from ∼10.1 kbar at ∼830 °C to ∼5.8 kbar at ∼800 °C and subsequent close-to-isobaric cooling (IBC). SHRIMP zircon U–Pb dating yielded a metamorphic age of 1931 ± 21 Ma, interpreted as the time of the retrograde cooling. Integrated with previously reported results, we propose that slab break-off subsequent to Yinshan-Ordos collision at ca. 1.95 Ga led to uplift of the MP and HP granulites in the Khondalite to the middle crustal level in an extensional regime as indicated by their ITD evolution, which was followed by slow cooling at ca. 1.93 Ga as evidenced by their nearly IBC evolution.

High‐temperature, decompressional equilibration of the eclogite facies orogenic root (Western Gneiss Region, Norway)

AbstractGarnet–clinopyroxene mafic rocks have been investigated in the outer coastal area of the northwestern‐most part of the Western Gneiss Region (WGR), South Norway. The garnet–clinopyroxene rocks occur as lenses with amphibolitized and deformed margins ranging in size from 1 m2 up to 2–3 km2. They are regionally widespread and included in migmatitic gneisses, mica schists and amphibolites. The mafic lenses vary from fine‐ to coarse‐grained with a strain variation from massive to coaxial S &gt; L tectonite fabric. Garnet is Alm42‐53Prp17‐35Grs20‐33Sps0‐3. Clinopyroxene is a Na–Al diopside (En34‐43Fs8‐17Wo48‐52) with Al up to 0.50 a.p.f.u. and Na content up to Jd24. Garnet and clinopyroxene occur in an assemblage with edenitic‐pargasitic amphibole (Ti &lt; 0.32 a.p.f.u.), plagioclase (An16‐43Ab57‐71), quartz, locally biotite (Mg# = 0.0.46–0.56; Ti = 0.51–0.59 a.p.f.u.), calcite, epidote and accessory rutile, ilmenite, zircon and apatite. Garnet porphyroblasts occur commonly as euhedral crystals, and locally with corroded rims surrounded by a corona of plagioclase or amphibole–plagioclase. Growth of secondary garnet is locally observed in S &gt; L tectonite rock. Clinopyroxene occurs as elongated subhedral crystals forming a strong fabric, or as a coarse symplectite with plagioclase. Amphibole is present as matrix grains in the garnet–clinopyroxene assemblage, but occurs also in coronas on garnet as symplectite with plagioclase, or as replacement textures on clinopyroxene. Secondary titanite is produced on rutile, and spinel+plagioclase on ilmenite. The P–T evolution is modelled by P–T pseudosections (TheriakDomino software), thermobarometry and by mapping of garnet chemistry. Garnet porphyroblasts show a decrease in CaO and MnO, an increase in MgO and variable FeO with resulting increasing Mg# from core to rim, indicating growth under increasing temperatures and decompression. Calculation of garnet+clinopyroxene+plagioclase+quartz+rutile stability combined with garnet and clinopyroxene isopleths of grossular and Mg# composition yields a maximum temperature metamorphism of 1.4–1.8 GPa and &gt;900°C. The P–T modelling supports high‐P granulite facies conditions for the equilibration of the garnet–clinopyroxene‐dominated mafic lenses. The maximum temperature metamorphism is associated with partial melting. In addition, an outermost small Mn increase, and local reversal of the Mg# ratio and CaO‐flattening in garnet of the mafic lenses are interpreted as retrogression into amphibolite facies. This is in accordance of mineral replacement of clinopyroxene to amphibole and titanite growth on rutile. The data support an evolution where the eclogite facies crust in the northwestern‐most coastal part of WGR underwent decompression during heating into high‐P granulite facies conditions, followed by cooling and amphibolitization. Our investigation gives a regional petrological documentation and illustrates an extensive high‐T equilibration in the Caledonian root zone subsequent to the deep crustal burial.

Evaluation of the effective bulk composition (EBC) during growth of garnet

The effective bulk composition (EBC) for the initial growth of garnet that has nucleated only after considerable overstepping of the equilibrium garnet-in reaction has been calculated for eight samples from central Vermont. The method developed assumes that the composition of garnet is determined by the maximum driving force (maximum change in Gibbs free energy) at the assumed conditions of nucleation. The method of intersecting isopleths is used to find the EBC where the isopleth intersection matches the P–T conditions of nucleation.The calculated EBC for all samples (four epidote-free and four epidote-bearing) contains less MnO and FeO (or lower FeO/MgO) than the whole rock bulk composition. The CaO content of the EBC is lower than the whole rock bulk composition in all epidote-free samples and in two of the four epidote-bearing samples. It is proposed that the differences between the EBC and whole rock bulk composition are caused by the sluggish kinetics of dissolution of reactants rather than diffusive transport of nutrients to the garnet.

Investigation of the kinetic mechanism of the demanganization reaction between carbon-saturated liquid iron and CaF2–CaO–SiO2-based slags

The demanganization reaction kinetics of carbon-saturated liquid iron with an eight-component slag consisting of CaO–SiO2–MgO–FeO–MnO–Al2O3–TiO2–CaF2 was investigated at 1553, 1623, and 1673 K in this study. The rate-controlling step (RCS) for the demanganization reaction with regard to the hot metal pretreatment conditions was studied via kinetics analysis based on the fundamental equation of heterogeneous reaction kinetics. From the temperature dependence of the mass transfer coefficient of a transition-metal oxide (MnO), the apparent activation energy of the demanganization reaction was estimated to be 189.46 kJ·mol–1 in the current study, which indicated that the mass transfer of MnO in the molten slag controlled the overall rate of the demanganization reaction. The calculated apparent activation energy was slightly lower than the values reported in the literature for mass transfer in a slag phase. This difference was attributed to an increase in the “specific reaction interface” (SRI) value, either as a result of turbulence at the reaction interface or a decrease of the absolute amount of slag phase during sampling, and to the addition of calcium fluoride to the slag.

The usage of newly developed glass fibre in cement structure and their characterization

In this work, recently developed Zrn1 glass fibres belonging to the SrO–Mn2O3–Fe2O3–MgO–ZrO2–SiO2 (SMFMZS) system have been used as reinforcing materials in cement. The highest (2.610 N/mm2) compressive strength value was obtained with the addition of 3 wt% of Zrn1 fibres into Portland cement. The microstructural behaviour of these fibres along with commercially available ones in the cement structure were investigated using a scanning electron microscope (SEM). Microstructures were fully compatible with mechanical results obtained. Eventually, it was determined that Zrn1 fibre can be a good candidate for cement reinforcement leading to high strength values.

Multicomponent diffusion in basaltic melts at 1350 °C

Nine successful diffusion couple experiments were conducted in an 8-component SiO2–TiO2–Al2O3–FeO–MgO–CaO–Na2O–K2O system at ∼1350 °C and at 1 GPa, to study multicomponent diffusion in basaltic melts. At least 3 traverses were measured to obtain diffusion profiles for each experiment. Multicomponent diffusion matrix at 1350 °C was obtained by simultaneously fitting diffusion profiles of diffusion couple experiments. Furthermore, in order to better constrain the diffusion matrix and reconcile mineral dissolution data, mineral dissolution experiments in the literature and diffusion couple experiments from this study, were fit together. All features of diffusion profiles in both diffusion couple and mineral dissolution experiments were well reproduced by the diffusion matrix. Diffusion mechanism is inferred from eigenvectors of the diffusion matrix, and it shows that the diffusive exchange between network-formers SiO2 and Al2O3 is the slowest, the exchange of SiO2 with other oxide components is the second slowest with an eigenvalue that is only ∼10% larger, then the exchange between divalent oxide components and all the other oxide components is the third slowest with an eigenvalue that is twice the smallest eigenvalue, then the exchange of FeO + K2O with all the other oxide components is the fourth slowest with an eigenvalue that is 5 times the smallest eigenvalue, then the exchange of MgO with FeO + CaO is the third fastest with an eigenvalue that is 6.3 times the smallest eigenvalue, then the exchange of CaO + K2O with all the other oxide components is the second fastest with an eigenvalue that is 7.5 times the smallest eigenvalue, and the exchange of Na2O with all other oxide components is the fastest with an eigenvalue that is 31 times the smallest eigenvalue. The slowest and fastest eigenvectors are consistent with those for simpler systems in most literature. The obtained diffusion matrix was successfully applied to predict diffusion profiles during mineral dissolution in basaltic melts.

The Eocene corundum-bearing rocks in the Gangdese arc, south Tibet: Implications for tectonic evolution of the Himalayan orogen

The genesis of Liangguo corundum deposit in the southern Gangdese magmatic arc, east-central Himalaya, remains unknown. The present study shows that the corundum-bearing rocks occur as lenses with variable sizes in the Eocene gabbro that intruded into marble. These corundum-bearing rocks have highly variable mineral assemblage and mode. The corundum-rich rocks are characterized by containing abundant corundum, and minor spinel, ilmenite and magnetite, whereas the corundum-poor and corundum-free rocks have variable contents of spinel, plagioclase, sillimanite, cordierite, ilmenite and magnetite. The host gabbro shows variable degrees of hydration and carbonization. The corundum grains are mostly black, and rarely blue, and have minor FeO and TiO2. The spinel is hercynite, with high FeO and low MgO contents. The corundum-bearing rocks have variable but high Al2O3, FeO and TiO2, and low SiO2 contents. Inherited magmatic and altered zircons of the corundum-bearing rocks have similar U–Pb ages (∼47 Ma) to the magmatic zircons of the host gabbro, indicating corundum-bearing rock formation immediately after the gabbro intrusion. We considered that emplacement of gabbro induced the contact metamorphism of the country-rock marble and the formation of silica-poor fluid. The channeled infiltration of generated fluid in turn resulted in the hydrothermal metasomatism of the gabbro, which characterized by considerable loss of Si from the gabbro and strong residual enrichment of Al. The metasomatic alteration probably formed under P–T conditions of ∼2.2–2.8 kbar and ∼650–700 °C. We speculate that SiO2, CaO and Na2O were mobile, and Al2O3, FeO, TiO2 and high field strength elements remained immobile during the metasomatic process of the gabbro. The Liangguo corundum deposit, together with metamorphic corundum deposits in Central and Southeast Asia, were related to the Cenozoic Himalayan orogeny, and therefore are plate tectonic indicators.

Petrology and age of Precambrian Aksu blueschist, NW China

The first occurrence of blueschist in the Neoproterozoic signifies a transition in thermal regime from a warmer Archean–Proterozoic Earth to modern colder subduction characterized by blueschist facies or ultrahigh pressure metamorphism. To evaluate the thermal regime of the transition, we apply phase equilibrium modeling to the Aksu blueschist in northwestern China to constrain the pressure (P) and temperature (T) conditions of the peak mineral assemblages and to calculate the thermal regime recorded by the blueschist. Phase equilibrium modeling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–O system indicates that the peak mineral assemblage (glaucophane/riebeckite + epidote + phengite + albite + quartz + chlorite) was developed at P–T conditions of 6.8–8.7 kbar and 320–410 °C for a stilpnomelane-bearing epidote blueschist (AK04) and of 6.5–8.0 kbar and 310–380 °C for an epidote blueschist (AK06). These P–T data define an apparent thermal gradient of ∼470 °C/GPa for the peak stage which is typical of the high-pressure intermediate type metamorphic facies series. In addition, we present new LA–ICP–MS U–Pb ages from detrital zircons from two blueschists (AK04 and AK06) and SHRIMP U–Pb ages from zircon from a cross-cutting post-metamorphic mafic dyke (AK134) to constrain the age of the metamorphism. The youngest concordant detrital zircon ages give a maximum depositional age for the blueschist protoliths of 806 ± 10 (AK04) and 803 ± 10 Ma (AK06), respectively, and the SHRIMP U–Pb ages of zircons from the mafic dyke yields a weighted mean age of 769 ± 19 Ma interpreted to date crystallization. These results constrain the age of Aksu blueschist facies metamorphism to between 769 ± 19 and c. 805 Ma.