Diagrams of electrochemical equilibria of the system copper-potassium ethylxanthate-water at 25°C

On the basis of thermodynamic calculations, diagrams of metastable electrochemical equilibria of the system copper-potassium ethylxanthate-water, at 25°C were constructed. The equilibria equations and diagrams for the total activity of [EtX^?] + [HEtX] + 2 [(EtX)₂] (equalling 10^?1, 10^?4 and 10^?7 mol/dm³), are presented. The practical conclusions resulting from these diagrams for the flotation of native copper are discussed.     

A re-assessment of phase equilibria involving two liquids in the system K2O-Al2O3-FeO-SiO2

In previously published papers the presentation of experimental results involving two liquids has not adequately addressed the quaternary aspects of the phase equilibria. A theoretical examination of the types of quaternary and pseudo-ternary phase diagrams to be expected is prestend here. When applied to much of the previous data, such quaternary diagrams cope with the variation in the K₂O∶Al₂O₃ ratio of coexisting liquids, and all the previous data are shown to be in agreement with the existence of the assemblage tridymite-fayalite-two liquids for a limited temperature range along the field boundary between primary tridymite and primary fayalite. The absolute values of temperature in the different investigations are not easily reconciled without appealing to differences in experimental techniques, to differences in oxygen pressure, or to differences in K₂O loss during the experiments, as possible explanations of temperature differences greater than reasonable interlaboratory variations.     

The system Mg2SiO4-Ca2SiO4-CaAl2O4-NaAlSiO4-SiO2: one atmosphere liquidus equilibria of analogs of alkaline mafic lavas

One atmosphere liquidus relationships in the system Mg₂SiO₄ (Fo)–Ca₂SiO₄ (La)–NaAlSiO₄ (Ne)–CaAl₂O₄ (CA)–SiO₂ (Sil) are presented as analogs for alkaline mafic lavas. Liquidus diagrams are constructed from electron microprobe analyses of quenched liquids and coexisting mineral phases produced in melting experiments and they are depicted in terms of sub-projections within the pseudo-quaternary system Fo–La–[Ne,CA]–Sil. The Ne and CA components are combined to create a normative feldspathoid component defined as Ne#=Ne/[Ne+CA]. Liquidus relations at Ne#=0.5 from this study are compared to relations at Ne#=0.0 and 1.0 from previous studies. In general, liquidus temperatures decrease and positions of liquidus boundaries involving feldspathic phases shift toward the [Ne, CA] component as Ne# increases. The pseudoinvariant points fo+di+pl+mel+l and fo+pl+mel+sp+l exist at Ne#=0.5. These equilibria between forsterite-plagioclase-melilite-liquid are not present in the system when Ne#=1.0 because a boundary curve (fo+di+ne+l) separates the plagioclase and melilite liquidus fields. The liquidus diagrams provide useful analogs for the crystallization sequences of natural primary alkali olivine basalts, basanitoids, basanites, olivine nephelinites, olivine-melilite nephelinites and olivine melilitites.     

On the importance of minding one’s Ps and Ts: metamorphic processes and quantitative petrology

This Special Issue comprises a selection of the papers given at a two‐day discussion meeting held at the University of Melbourne, Australia in June 2009 to celebrate Roger Powell’s 60th birthday. At this milestone, it is fitting to review Roger’s career to date. He has published ～200 scientific papers on topics that range from low‐ to high‐grade metamorphism, from low‐ to ultrahigh‐pressure (UHP) metamorphism, and from thermodynamics to kinetics. Most of Roger’s papers are multi‐authored and address important questions in the petrogenesis of metamorphic rocks. Roger is widely known for his work with Tim Holland to develop the most complete internally consistent dataset of thermodynamic properties of end members of phases necessary to undertake calculations on the conditions of formation and modification of metamorphic rocks. Additionally, Roger and Tim have developed activity–composition models for many of these phases, building on their important methodological developments in formulating such models. Roger is also responsible for the ongoing development of thermocalc , a thermodynamic calculation software package that may be used to undertake a wide range of phase diagram calculations, including P–T projections, P–T, P–X and T–X, compatibility diagrams and μ–μ diagrams. Together, Roger and Tim have changed the way we carry out quantitative phase equilibria studies. However, Roger’s contributions to metamorphic petrology go well beyond the development of phase equilibria methods and mineral thermodynamics. He has contributed significantly to our understanding of a range of metamorphic processes, and with an extensive array of co‐authors has shown how phase equilibria can be used to understand the evolution of metamorphic rocks in general terms as well as in specific terranes. The papers in this Special Issue cover the range from the stabilization of the continents to understanding the formation of orogenic gold deposits, from the stability of sapphirine–quartz‐bearing assemblages to the crystallization of melt in migmatites, from the effects of ferric iron and sulphur on the stability of metamorphic mineral assemblages in general to the effects of ferric iron and H₂O on the stability of eclogite in particular, and to the quantification of UHP metamorphism. It is our hope that in reading these contributions, you will be stimulated to seek a better understanding of metamorphic processes and to improve our quantification of the variables in metamorphism.     

Petrology of marble and peridotite in the Seiad ultramafic complex, northern California, USA

Abstract Peridotite and infolded marble of the Seiad ultramafic complex were recrystallized in the upper amphibolite facies as part of the regional progressive metamorphism of the Rattlesnake Creek terrane. Field relations, including the occurrence of metarodingites, and metasomatic zones between dissimilar rock types, demonstrate that the metasediments and serpentinized ultramafic rocks were juxtaposed prior to regional, barrovian metamorphism. Temperatures are estimated to have reached 760–800°C at pressures of 7–8 kbar during the peak of metamorphism. Four low-variance parageneses have been identified within a small (3 km²) area of the complex, which may reasonably be assumed to have formed under the same P and T conditions. Isobaric T-X co₂ diagrams of appropriate equilibria are presented for three different internally consistent sets of thermodynamic data. Despite the seemingly small numerical differences between the standard state thermodynamic properties of the data sets, only one diagram allows the four observed assemblages to coexist within a reasonable temperature range. All three phase diagrams require differences in fluid composition on the scale of a thin section; strong evidence for effective control of pore fluid composition by local mineral reactions during metamorphism.     

Physicochemical Environment during the Formation of the Mozumi Skarn-type Pb–Zn–Ag Deposit at the Kamioka Mine, Central Japan: A Thermochemical Study

Abstract: The physicochemical environment during the ore formation of the Mozumi skarn-type Pb–Zn–Ag deposit, Kamioka mine, Central Japan is discussed with silicate phase equilibria using calculated phase diagrams. The mineral assemblages, mineral chemistry, and fluid inclusion data are coupled with thermodynamic calculation to estimate the stability fO2–XCO2condition of the mineral assemblages at each ore formation stage. The skarn was approximated by the model system CaO–A12O3–FeO–SiO2–O2–CO2–H2O including grossular-andradite garnet and clinozoisite-epidote solid solutions. The solid solutions are combined into the calculation using "pseudocompound approximation", and real boundary of the mineral assemblages other than simple activity corrected diagram was provided. The diagrams also show isoplethal contour for garnet and epidote.     

False metamorphic events inferred from misinterpretation of microstructural evidence and P–T data

Geometrical relationships involving inclusions and partial inclusions in metamorphic microstructures can be inadequate for inferring an order of crystallization and hence a metamorphic reaction. Unique spatial and/or chemical relationships need to be defined for mineral inclusions, in the context of a reference paragenesis, commonly the matrix assemblage. Corona microstructures are reliable indicators of metamorphic reactions, but require considerable care when used to infer reactions or changes in P–T conditions, owing to kinetic problems, as well as to changes in the effective reaction volume during changes across relatively broad P–T stability fields of assemblages. Mineral equilibria models, most commonly implemented through P–T pseudosections, may allow the order in which different minerals become stable along a given P–T path to be inferred. However, the order in which two minerals become stable may be different from the order in which two grains of these minerals nucleate. Furthermore, such diagrams cannot make predictions about which minerals will form porphyroblasts and which minerals will form inclusions in porphyroblasts. An evaluation of three examples from the Australian Proterozoic shows that modelling, in combination with inclusion‐host relationships, is a powerful tool for understanding the metamorphic evolution of a rock, but involves considerable uncertainty.     

The effect of garnet cell volume on the distribution of Mg and Fe in garnet exchange equilibria

The distribution of Mg in garnet exchange equilibria has been examined in three regional metamorphic and four eclogitic parageneses. Garnet cell volume has been found to have a significant effect on the distribution of Mg between garnet and coexisting ferromagnesian minerals. When the activity of Mg is corrected for the variation in cell volume, the range in variation of the distribution coefficients is strongly reduced within each group. Also the relative positions of the different groups in the distribution diagrams are in better accordance with geological evidence of crystallization temperatures. It is proposed that the effect of the garnet cell volume on the Mg distribution may be used as a geobarometer. The distribution of Ca between garnet and plagioclase is also discussed and it is found that the Ca partitioning is not suitable for geobarometry, as this partioning is dependent on the Mg/Fe ratio of the garnet.     

Modelling of mineral equilibria in ultrahigh-temperature metamorphic rocks from the Anápolis–Itauçu Complex, central Brazil

A new quantitative approach to constraining mineral equilibria in sapphirine‐bearing ultrahigh‐temperature (UHT) granulites through the use of pseudosections and compatibility diagrams is presented, using a recently published thermodynamic model for sapphirine. The approach is illustrated with an example from an UHT locality in the Anápolis–Itauçu Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000 °C. The early formed, coarse‐grained assemblage is garnet–orthopyroxene–sillimanite–quartz, which was subsequently modified following peak conditions. The retrograde pressure–temperature (P–T) path of this locality involves decompression across the FeO–MgO–Al₂O₃–SiO₂ (FMAS) univariant reaction orthopyroxene + sillimanite = garnet + sapphirine + quartz, resulting in the growth of sapphirine–quartz, followed by cooling and recrossing of this reaction. The resulting microstructures are modelled using compatibility diagrams, and pseudosections calculated for specific grain boundaries considered as chemical domains. The sequence of microstructures preserved in the rocks constrains a two‐stage isothermal decompression–isobaric cooling path. The stability of cordierite along the retrograde path is examined using a domainal approach and pseudosections for orthopyroxene–quartz and garnet–quartz grain boundaries. This analysis indicates that the presence or absence of cordierite may be explained by local variation in a_H2O. This study has important implications for thermobarometric studies of UHT granulites, mainly through showing that traditional FMAS petrogenetic grids based on experiments alone may overestimate P–T conditions. Such grids are effectively constant a_H2O sections in FMAS‐H₂O (FMASH), for which the corresponding a_H2O is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on a_H2O is that local variations in a_H2O may explain the formation of cordierite without significant changes in P–T conditions, particularly without marked decompression.     

A curious case of agreement between conventional thermobarometry and phase equilibria modelling in granulites: New constraints on P–T estimates in the Antarctica segment of the Musgrave–Albany–Fraser–Wilkes Orogen

The Windmill Islands region in Wilkes Land, east Antarctica, preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable P–T estimates from conventional thermobarometry and mineral equilibria modelling. This is uncommon in granulite facies terranes, where conventional thermobarometry and phase equilibria modelling generally produce conflicting P–T estimates because peak mineral compositions tend to be modified by retrograde diffusion processes. In situ U–Pb monazite geochronology and calculated metamorphic phase diagrams show that the Windmill Islands experienced two phases of high thermal gradient metamorphism during the Mesoproterozoic. The first phase of metamorphism is recorded by monazite ages in two widely separated samples and occurred at c. 1,305 Ma. This event was regional in extent, involved crustally derived magmatism and reached conditions of ~3.2–5 kbar and 690–770°C corresponding to very high thermal gradients of >150°C/kbar. The elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back‐arc setting that existed prior to c. 1,330 Ma. The first metamorphic event was overprinted by granulite facies metamorphism at c. 1,180 Ma that was coeval with the intrusion of charnockite. This event involved peak temperatures of ~840–850°C and pressures of ~4–5 kbar. A phase of granitic magmatism at c. 1,250–1,210 Ma, prior to the intrusion of the charnockite, is interpreted to reflect a phase of compression within an overall back‐arc setting. Existing conventional thermobarometry suggests conditions of ~4 kbar and 750°C for M₁ and 4–7 kbar and 750–900°C for M₂. The apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly, or that the P–T ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of P–T conditions obtained in this study. However, without phase equilibria modelling, the veracity of existing conventional P–T estimates cannot be evaluated. The calculated phase diagrams from this study allow the direct comparison of P–T conditions in the Windmill Islands with phase equilibria models from other regions in the Musgrave–Albany–Fraser–Wilkes Orogen. This shows that the metamorphic evolution of the Wilkes Land region is very similar to that of the eastern Albany–Fraser Orogen and Musgrave Province in Australia, and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the ~5,000 km strike length of this system.     

Thermodynamic equilibrium, kinetics, activation barriers, and reaction mechanisms for chemical reactions in Karst Terrains

Chemical reactions pertinent to karst systems divide broadly into (a) speciation reactions within aqueous solutions, (b) dissolution/precipitation and other acid/base reactions between aqueous solutions and solid minerals, and (c) redox reactions involving various carbon and sulfur-bearing species. As a backdrop against which other chemistry can be evaluated, selected phase diagrams and equilibrium speciation diagrams were calculated for the system Ca—Mg—O—H—C—S. The kinetics of reactions within this system span time scales from milliseconds for homogeneous reactions in solution through hundreds of hours for carbonate mineral dissolution reactions, to geologic time scales for reactions such as the aragonite/calcite inversion or the oxidation/reduction of native sulfur. In purely inorganic systems, kinetic barriers, typically on the order of tens of kJ/mole, are set by nucleation processes and by activated complex formation. Biological processes impact the purely inorganic chemistry by the following mechanisms: (a) Secretions and waste products from biological activity or consumption of CO₂ by organisms changes the chemistry in the microenvironments of reaction surfaces. Oxidation potentials, pH, and ion activities may be modified, thus shifting equilibria. (b) Reaction rates may be increased due to modification of activated complexes and thus the activation barriers to reaction. (c) Organic compounds or microorganisms may act as substrates, thus lowering nucleation barriers. The preservation of microorganisms in cave deposits does not necessarily prove a cause and effect relationship.     

Evaluation of two multidimensional discrimination diagrams from beach and deep-sea sediments from the Gulf of Mexico and their application to Precambrian clastic sedimentary rocks

Two discriminant-function-based multidimensional major-element diagrams for the tectonic discrimination of siliciclastic sediments were recently published from a coherent statistical methodology of log_e-ratio transformation and linear discriminant analysis. These diagrams were constructed based on worldwide examples of Neogene–Quaternary siliciclastic sediments from known tectonic settings. In this work, these two tectonic discrimination diagrams were first successfully tested from Holocene (<0.0117–0 Ma) beach and deep-sea sediments from the Gulf of Mexico. These diagrams were used to decipher tectonic settings of 11 case studies of the Precambrian clastic sedimentary rocks (~512–2800 Ma) from Argentina, USA, Ghana, Spain, Norway, India, China, and Australia. The test and application results obtained from these discrimination diagrams were generally consistent with the geology of the Precambrian source areas. Therefore, the two multidimensional diagrams can be considered as a useful tool for successfully discriminating the tectonic setting of older sedimentary basins, which may consist of one or more tectonic assemblages. Comparison of results of this study with the previously published tectonic discrimination diagrams is illustrated and the probable reasons for some inconsistent inferences were also discussed.     

MORB数据挖掘:玄武岩判别图反思

板块构造学说推动了玄武岩理论的完善和进步,从20世纪70年代兴起的玄武岩构造环境判别方法成为学术界关注的热点,虽然现在逐渐降温了,但在国内仍然倍受青睐。大洋中脊玄武岩(MORB)是拥有数据最多、数据质量最好、研究得最详细的玄武岩类,是构建玄武岩理论的基石。本文利用Pet DB数据库资料(http://www.earthchem.org/petdb/search),对全球MORB数据进行了初步的挖掘,发现早先的玄武岩构造环境判别方法的理论、思路需要进一步的完善。早先研究认为,N-MORB是强烈亏损的。但本文研究却显示,MORB成分具有较宽的变化范围,包含了从MORB到OIB及IAB的组分,即部分MORB并不是亏损的,而是相对富集的。因此,玄武岩构造环境判别理论及方法有待进一步的完善。     

关于岩石微量元素构造环境判别图解使用的有关问题

针对目前应用愈来愈广泛的不同岩石,特别是岩浆岩的微量元素构造环境判别图解使用过程中存在的问题,从这些判别图解建立的原理,介绍了微量元素构造环境判别图解的使用原则。强调指出所采集的样品必须新鲜(无蚀变或极弱蚀变)、非堆晶的岩石;选择的判别图解必须与判别的岩石类型相一致,即对花岗岩类要用花岗岩的判别图解,不能用玄武岩的判别图解;对特殊类型岩石要选择专门用于该类型岩石的判别图解,如碱性花岗岩,钾质火成岩;要应用多种图解综合判断;不能用单个样品,而应作多个样品分析;要注意所选择判别图解的特别说明等。此外,一些构造环境判别图解还能给出岩石的成岩过程和源区。     

相图在透闪石和透辉石锦砖配方设计中的应用

文章指出了陶瓷生产中应用相图的要点和设计配方时应注意的问题,并着重从CaO-MgO-Al2O3-SiO2四元系统相图和K2O-Al2O3-SiO2三元系统相图讨论了透闪石和透辉石用于陶瓷锦砖生产中之新体系配方设计的思路。认为纯粹用这两种原料为助熔剂生产锦砖节能效益不十分明显。在传统的锦砖配方中加入适当的透闪石或透辉石可明显地降低产品的烧成温度,但会使坯体的烧成范围变窄。文章强调具有实际意义的配方是具有相当宽烧成范围的低温配方。     

An internally consistent thermodynamic data set for phases of petrological interest

The thermodynamic properties of 154 mineral end-members, 13 silicate liquid end-members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature-dependent thermal expansion and bulk modulus, and the use of high-pressure equations of state for solids and fluids, allows calculation of mineral–fluid equilibria to 100  kbar pressure or higher. A pressure-dependent Landau model for order–disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha–beta quartz transition to be handled more satisfactorily. Several melt end-members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data.     

Phase equilibria of several tungsten deposits in southern China

This paper reviews the features of several tungsten ore deposits in southern China, including the Shizhuyuan, Yaogangxian, Xihuashan, Dangping, Piaotang and Dajishan mines. The authors think that the formation depth of the greisen-type deposits is intermediate between the pegmatite-and porphyry-type deposits. Using phase diagrams of the model system K₂O-Al₂O₃-SiO₂-H₂O-HF, the formation mechanism of greisens has been argued. The variation of mineral assemblages from K-feldspar, muscovite to topaz reflects a gradual increase in acidity. This variation can be used as a criterion to appreciate the depth of mineralization. Phase equilibria of tungsten minerals in grcisen and skarn deposits can be depicted on chemical potential diagrams of the model system CaO-FeO-WO₃-CO₂-F₂O_?1. The assemblage wolframite + fluorite is shown to have ben transformed to scheelite + magnetite with decreasingμ _HF. Depending on CO₂ activity in the fluids, there are two distinct phase diagrams. One shaws that wolframite is incompatible with calcite, and the other demonstrates that scheelite is incompatible with rhodochrosite. On the phase diagrams of the model system FeO-MnO-WO₃-F₂S_?1, huebnerite is transformed to ferberite with increasingμF₂O_?1 orμF₂S_?1. During the replacement of wolframite by scheelite, the wolframite residue must have been enriched in Mn.     

The effect of Tschermak's substitution on assemblages in aluminous dolomites

A calculated petrogenetic grid for the system CaO-MgO-Al₂O₃-SiO₂-CO₂-H₂O (CaMASCH), incorporating Tschermak's substitutions in amphibole, chlorite, talc and clinopyroxene, is used to examine phase relationships in aluminous marbles. A series of diagrams illustrating the effect upon stable mineral assemblages of increasing the aluminium content of a bulk composition is used to show the way aluminous minerals enter mineral assemblages in progressively more aluminous rocks. The effects of changing pressure and the incorporation of Fe into the bulk composition on the stable mineral assemblages are also examined. The calculated equilibria are shown to be in reasonable agreement with natural assemblages, and the incorporation of new experimental data on amphiboles into the existing dataset is shown to improve the agreement between observed and natural amphibole compositions.     

Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system

The type and kinetics of metamorphic CO₂-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic CO₂ cycle. This paper focuses on CO₂ production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO–FeO–Al₂O₃–SiO₂–CO₂–H₂O (CFAS–CO₂–H₂O) system are investigated using P–T phase diagrams at fixed fluid composition, isobaric T–X(CO₂) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS–CO₂–H₂O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant CO₂ source during prograde heating and/or early decompression. However, if the system remains closed, fluid–rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of CO₂-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic CO₂ cycle.     

Do major oxide tectonic discrimination diagrams work? Evaluating new log‐ratio and discriminant‐analysis‐based diagrams with Indian Ocean mafic volcanics and Asian ophiolites

Many geochemical diagrams exist that classify old volcanic terranes of ambiguous provenance into various modern plate tectonic settings, with variable success. Recently proposed diagrams, based on log‐ratios and linear discriminant analysis with large datasets of major oxides, were tested here with data for ocean island, arc and mid‐ocean ridge lavas from the Indian Ocean. Success rates are 45–100%, with misclassifications potentially caused by alteration, although alteration demonstrably need not cause misclassification. The diagrams were further applied to some Asian ophiolites, representing Tethyan and Indian ocean crusts, to see if the diagrams confirm their tectonic setting inferred from trace and isotopic data. Lower success rates (30–60%, but 75–100% for specific suites) are not surprising in view of the ubiquitous and complex alteration in ophiolites. Log‐ratio transformation and linear discriminant analysis appear to be powerful methods when discrimination diagrams are based on major oxide data alone.