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.     

An improved and extended internally consistent thermodynamic dataset for phases of petrological interest,involving a new equation of state for solids

The thermodynamic properties of 254 end‐members, including 210 mineral end‐members, 18 silicate liquid end‐members and 26 aqueous fluid species are presented in a revised and updated internally consistent thermodynamic data set. The PVT properties of the data set phases are now based on a modified Tait equation of state (EOS) for the solids and the Pitzer & Sterner (1995) equation for gaseous components. Thermal expansion and compressibility are linked within the modified Tait EOS (TEOS) by a thermal pressure formulation using an Einstein temperature to model the temperature dependence of both the thermal expansion and bulk modulus in a consistent way. The new EOS has led to improved fitting of the phase equilibrium experiments. Many new end‐members have been added, including several deep mantle phases and, for the first time, sulphur‐bearing minerals. Silicate liquid end‐members are in good agreement with both phase equilibrium experiments and measured heat of melting. The new dataset considerably enhances the capabilities for thermodynamic calculation on rocks, melts and aqueous fluids under crustal to deep mantle conditions. Implementations are already available in thermocalc to take advantage of the new data set and its methodologies, as illustrated by example calculations on sapphirine‐bearing equilibria, sulphur‐bearing equilibria and calculations to 300 kbar and 2000 °C to extend to lower mantle conditions.     

Transition enthalpies and entropies of high pressure zinc metasilicates and zinc metagermanates

The enthalpies of transition at T= 298 K between zinc metasilicate assemblages, measured by molten oxide solution calorimetry, are:

Semi-empirical Gibbs free energy formulations for minerals and fluids for use in thermodynamic databases of petrological interest

The P–T partition function in statistical thermodynamics can be used to derive semi-empirical formulations of the Gibbs free energy G for minerals and fluids. Parameterization of these equations includes simultaneous regression of experimental heat capacity and molar volume data, allowing fitting, appraisal and optimization of various data sources, as required in the construction of internally consistent petrological data bases. This approach can also be extended to minerals with -transitions and to fluids by considering the Gibbs free energy as a function of pressure P, temperature T and an ordering parameter X, so that accurate modelled representation and extrapolation of the thermodynamic properties of large numbers of petrologically significant minerals and coexisting fluids can be attained. The ordering parameter is chosen to denote the equilibrium mole fraction (thermodynamic probability) of ordered clusters (structural units) in a substance when G(T,P, X)=min. The procedure is tested on existing experimental data for the system MgO–SiO₂–H₂O. The proposed Gibbs free energy formulation permits thermodynamic properties of minerals, fluids and phase equilibria to be described and extrapolated over a wide range of pressure (0–800 kbar) and temperature (20–3000 K), thus allowing effective use in thermodynamic data bases of petrological interest.     

The causes and petrological significance of cathodoluminescence emissions from alkali feldspars

The cathodoluminescence (CL) of a variety of alkali feldspars from South Greenland has been examined in an attempt to understand the causes of the CL and its petrological significance. Analytical methods have included CL spectroscopy, secondary ion mass spectrometry (SIMS) and electron paramagnetic resonance (EPR) to correlate the presence of certain CL emissions to the presence of certain trace element and point defects. Where possible, blue and red luminescent fractions of the same rock samples have been separated and analysed separately. Blue CL appears to relate to the presence of electron holes on bridging oxygens, particularly on the Al-O-Al bridge, as determined from EPR studies. No correlation with other proposed activators for blue CL such as Eu²⁺, Ga³⁺ or Ti⁴⁺ was observed. Some blue luminescent feldspars also have an emission in the infra-red (IR), invisible during normal visible CL petrography. The red and IR CL emissions correspond to features in EPR spectra attributed to Fe³⁺ and support previous suggestions that Fe³⁺ is related to this emission. However, our studies indicate that the visible red CL relates specifically to Fe³⁺ on the T1 site, whereas the equivalent CL from disordered feldspars lies in the IR. The difference between red and IR CL emissions therefore relates to the state of Fe³⁺ order across the tetrahedral sites. These data allow more meaningful interpretations of CL as a petrographic tool in alkali feldspar-bearing rocks. Received: 5 March 1998 / Accepted: 23 November 1998     

Determination of the enthalpies of formation of some platinum antimonides and their phase diagrams under standard conditions

Using the method of direct synthesis calorimetry, we determined the standard enthalpy of formation of PtSb (stumpflite), Δ _f H°_298.15 (PtSb, cr) =–105.16 ± 0.84 kJ/mol and PdSb₂ (geversite), Δ _f H°_298.15 (PtSb₂,cr) =–160.92 ± 0.84 kJ/mol. Isothermal (298.15 K, p = 1 bar) phase diagrams were computed for the Pt–Sb–S and Pt–Sb–O ternary systems in the coordinates composition of the Pt–Sb binary system versus fugacity of a gaseous volatile component (O₂, S₂).     

The effect of Poisson's ratio and the far-field boundary conditions on the accuracy of finite element calculations

It is shown that a finite element calculation which approximates an ‘infinite medium’; problem by a mesh with finite boundaries will yield greater accuracy when stress boundary conditions are applied on the far-field boundary than is obtainable with displacement boundary conditions. In particular, with Poisson's ratio close to 0.5, the accuracy of the latter model is severely impaired, whereas the stress boundary condition model is unaffected for Poisson's ratio of 0.49 and a reasonable mesh. The eight-node quadratic isoparametric element displays superb accuracy for the axisymmetric thick cylinder with either type of boundary condition.     

Extraction and accuracy assessment of high-resolution DEM and derived orthoimages from ALOS-PRISM data over Sahel-Doukkala (Morocco)

In Sahel-Doukkala, which is characterized by lands of a relatively low relief, global DEMs and DEMs generated from digitizing topographic maps, have been the primary source of several multidisciplinary researches. Although these products present a great value of the conducted research, the level of the given accuracy is not sufficient enough for detailed geospatial analysis. These requirements led us to generate a high-resolution DEM as an alternative of available global DEMs or/and DEMs generated from digitizing topographic maps. In this study, we present a workflow to extract high-resolution DEM at 5 m resolution and derived orthoimages from ALOS-PRISM data over Sahel-Doukkala, through photogrammetric techniques, using a variation of GCPs obtained from topographic maps at scale 1:25,000. The accuracy of the generated products is reported according to NSSDA standards. Using ten GCPs, a PRISM-DEM with 3.88 m vertical accuracy and 11.60 m horizontal accuracy, both at 95% confidence level is obtained. This DEM will serve as base dataset for further detailed geospatial analysis and mapping applications in order to identify the relationship between surface parameters and groundwater, and also to assess and understand all factors influencing the development of karst landscapes and consequently subsurface stability in the investigated area.     

准噶尔盆地西北缘晚石炭系-早二叠系火山岩岩石学和地球化学研究

晚石炭纪-早二叠纪,在准噶尔盆地西北缘,发育区域性的火山岩建造,岩性以酸性英安岩和流纹岩为主,同时发育少量中性玄武安山岩和安山岩,多属于中-低钾的钙碱性系列。样品全碱(Na₂O+K₂O)含量为0.99％ ～8.12％,K₂O的含量较低,为3.01％ ～0.05％,稀土元素总量(∑ REE)较低。稀土元素轻重分异,轻稀土元素(LREE)相对于重稀土元素(HREE)富集:(La/Yb)_N=0.76～5.18,且部分样品显示轻微的Eu负异常和Ce正异常:δEu=0.533～1.148,δCe=0.979～1.224。微量元素中大离子亲石元素(LILE)富集,高场强元素Nb、Ta亏损。分析认为石炭系末期-二叠系早期,研究区所处构造环境为洋内弧附近的前弧盆地,发育的火山岩是母源岩浆经历了一定程度分异作用后的产物,以岛弧环境为主,兼具有部分板内大陆环境特征。     

The major petrological and geochemical characters of the 3,600 m.y. Uivak gneisses from Labrador

The 3,622±72 m.y. Uivak gneisses of northern Labrador consist of two major rock suites: regionally developed layered granodioritic gneisses (Uivak I suite) interpreted as derived from earlier tonalitic igneous parents by the massive introduction of potassium and rubidium, and a less extensive group of iron-rich porphyritic granodiorites and ferro-diorites (Uivak II suite) emplaced after at least one major period of deformation and migmatisation had affected the Uivak I gneisses. Samples from both suites fall on the 3,622 m.y. Rb/Sr whole-rock isochron.The age is provisionally interpreted as that of Rb metasomatism and homogenisation affecting both suites. It is suggested that the low initial ratio of 0.7014±0.0008 obtained from both suites need not be diagnostic of a short crustal residence if the evidence for massive addition of Rb to this level of the crust at 3,622 m.y. is accepted.     

K-rich glass-bearing wehrlite xenoliths from Yitong,Northeastern China: petrological and chemical evidence for mantle metasomatism

 Ultramafic xenoliths in Cenozoic alkali basalts from Yitong, northeast China comprise three types in terms of their modal mineralogy: lherzolite, pyroxenite and wehrlite. The wehrlite suite always contains interstitial pale/brown glass which occupies several per cent by volume of the whole rock. The texture of the wehrlites is porphyroclastic with some large strained grains of olivine (0.5–1 mm) scattered in a very fine grained matrix (0.1 mm), implying a metamorphic origin for the protolith rather than an igneous origin. The host minerals are compositionally zoned, showing evidence of reaction with a melt. Petrological evidence for resorption of spinel (lherzolite) and orthopyroxene (wehrlite) by infiltrating melt further supports the hypothesis that the wehrlites result from interaction between a partial melting residue and a melt, which preferentially replaced primary spinel, Cr-diopside and enstatite to produce secondary clinopyroxene (cpx) + olivine (ol) ± chromite ± feldspar (fd). The composition of the mineral phases supports this inference and, further indicates that, prior to melt impregnation, the protoliths of these wehrlites must have been subjected to at least one earlier Fe-enrichment event. This explanation is consistent with the restricted occurrence of glasses in the wehrlite suite. The glass is generally associated with fine-grained (0.1 mm) minerals (cpx+ol+chromite ±fd). Electron microprobe analyses of these glasses show them to have high SiO₂ content (54–60 wt%), a high content of alkalis (Na₂O, 5.6–8.0%; K₂O, 6.3–9.0%), high Al₂O₃ (20–24%), and a depletion in CaO (0.13–2.83%), FeO (0.89–4.42%) and MgO (0.29–1.18%). Ion probe analyses reveal a light rare earth element-enrichment in these glasses with chondrite normalised (La)_n = 268–480. The high K₂O contents in these glasses and their mode of occurrence argue against an origin by in-situ melting of pre-existent phases. Petrographic characteristics and trace element data also exclude the possibility of percolation of host-basalt related melts for the origin of these glasses. Thus the glasses must have resulted from local penetration of mantle metasomatic melts which may have been produced by partial melting of peridotites with involvement of deep-seated fluids. Such melts may have been significantly modified by subsequent fractional crystallization of ol, cpx and sp, extensive reaction with the mantle conduit and the xenolith transport process. Received: 1 August 1995 / Accepted: 19 June 1996     

Quantitative petrological evidence for the origin of K-feldspar megacrysts in dacites from Taapaca volcano,Chile

K-feldspar megacrysts are common in granitoids, but relatively rare in chemically equivalent volcanic rocks. Dacites from Taapaca volcano have euhedral sanidine megacrysts up to 5 cm long. Small crystals, where present, are rounded. Growth of the megacrysts engulfed plagioclase and amphibole crystals. Crystal size distributions (CSD) of sanidine megacrysts are hump shaped. All these data show that megacrysts developed from the host magma by coarsening: this was enabled by the cycling of magma temperature around the sanidine liquidus temperature in response to injections of more mafic magma and subsequent magmatic overturns. Plagioclase crystals enclosed in the megacrysts are small and have short, steep, straight CSDs, which contrasts with the CSDs of plagioclase in the groundmass which are shallower and extend to larger sizes. This shows that plagioclase was also coarsened approximately synchronously with sanidine, in response to the same temperature conditions.     

Sensitivity and uncertainty analysis of mixing and mass balance calculations with standard and PCA-based geochemical codes

One of the most important observations that can be obtained from the study of an aquifer system dominated by mixing is the contribution of each end-member water to the chemical composition of every water parcel in the aquifer. Once the first-order effect of mixing has been taken into account via the mixing proportions, water–rock interaction can be used to explain the remaining variability. There are many sources of uncertainty that can prevent the accurate calculation of the mixing proportions of a mixing-dominated system, but the type and intensity of the chemical reactions that have taken place as a consequence of mixing is one of the most critical. Here the uncertainty in the computed mixing proportions of samples from a “synthetic” aquifer system derived from the actuation of different chemical reactions are assessed (always remembering that the chemical reactions are a second-order effect). These uncertainties are explored using two different geochemical codes in order to infer the limits of both methodological approaches: PHREEQC, as an example of a standard geochemical code; and M3, as an example of a Principal Component-based geochemical code. Several synthetic water samples are created with the direct approach of PHREEQC, both by pure mixing and including different types of chemical reactions. Together with the chemical information of the end-member waters, these samples are then fed into PHREEQC (inverse modelling) and M3 and the mixing proportions and mineral mass transfers are computed. PHREEQC calculations give a reasonable estimate of the real mixing proportions and the chemistry of the groundwaters. However, similar mixing proportions and mass transfers can be obtained using different sets of reactions, indicating a source of uncertainty that should be overcome with additional chemical information. For M3, where synthetic samples have been included in a real data set of groundwater samples from the Scandinavian Shield, mixing proportions are only mildly affected either by the number of compositional variables or the number of samples used for the Principal Component Analysis (PCA). However, the robustness of the output is quite sensitive to whether only conservative compositional variables are used or both conservative and non-conservative compositional variables. Mass balance calculations in M3 are much more sensitive to non-conservative compositional variables and the recommendation here is not to use non-conservative variables with PCA-based codes if any information about reactions is to be obtained.     

The lateral and vertical reflectance and petrological variation of a heat-affected bituminous coal seam from southeastern British Columbia, Canada

Thermally altered pods of coal of very high rank have been observed in a high-volatile-bituminous coal seam in the eastern side of Eagle Mountain, Elk Valley Coalfield, British Columbia. Rank changes have been measured over a strike distance of 7.5 m from 1.24% to 7.1% R_{o max}, corresponding to a rank gradient of 0.78% R_om⁻¹.Petrologically, unaltered to extremely altered vitrinite showing nongranular (basic) anisotropy, mosaic-textured liptinite and pyrolytic carbon are the most abundant components. The limited presence of mosaic on vitrinite is an indication that the coal seam may have been weathered prior to being heat-affected.Evidence points to localized temperatures as high as 1,000°C, which could have been caused by a lightning strike. The eastern side of Eagle Mountain has experienced higher temperatures than the western side, and it appears that the heat ‘front’ and zone of alteration have an irregular pattern, pointing to saturation of parts of the coal seam by water.Four types of pyrolytic carbon having distinct morphology, anisotrophy and optical path with increasing temperature were observed. Reflectance of pyrolytic carbon falls within the zone of heat-affected coals, whereas the optical path of heat-affected Seam 15 samples is different from that of fresh coal with increasing rank.Finally, the reflectance of vitrinite in heat-affected coal is higher than the reflectance of vitrinite in carbonaceous shale in the Seam 15 section.     

The north-eastern Polish anorthosite massifs: petrological, geochemical and isotopic evidence for a crustal derivation

ABSTRACT Deeply buried 1.5 Ga Polish anorthosites, accessible only by bore holes, reveal diagnostic features of some massif-type anorthosites (polybarism, jotunitic parent magma), diapirically emplaced in the mid crust together with the rapakivi granites of the EW-trending Mazury complex, intruded along a major crustal discontinuity. Geochemical modelling and isotope data corroborate recent experimental work on the basaltic system in dry conditions: the source rock of the parental magma is a gabbronorite, necessarily lying in the lower crust. Since no Archaean crust is known in the region, high initial ¹⁸⁸Os/¹⁸⁷Os ratios for sulphide-oxide isochrons and negative ε_Nd values are best accounted for by melting a ∼ 2.0 Ga mafic crust.     

Evolution of the petrological and seismic Moho-implications for the continental crust-mantle boundary

The chemical and petrological composition of mafic rocks from the lower continental crust are discussed by comparing mafic granulites and meta-gabbroic rocks from the Ivrea Zone and the Northern Hessian Depression (NHD) xenolith suite. Both regions contain contrasting types of meta-mafic lithologies (i) former basaltic rocks with trace element patterns ranging from MORB-Iike to subduction-related or intra-plate tholeütes and (ü) Ca-and Al-enriched, plagiodase-dominated gabbroic rocks showing positive Eu-anomalies generated by complex deep crustal magmatic processes such as fractionation, accumulation of plagiodase and pyroxene, and crustal contamination. The absence of typical garnet-omphadte parageneses in these rocks indicates that the eclogite stability field was not reached during Palaeozoic orogenic processes. A compilation of experimentally determined P-wave velocities and densities for mafic granulites, gabbroic rocks, eclogites and peridotites is used to evaluate key physical properties of lower crustal mafic rocks during crystal thickening caused by continent-continent collision. In a step-by-step scenario it is demonstrated that the position of the seismic Moho (defined as a first-order velocity discontinuity) and the petrological Moho (defined as the boundary between non-peridotitic crustal rocks and olivine-dominated rocks) is not identical for the case that mafic rocks are transformed into edogites at the base of orogenically thickened crust. P-wave velocities of eclogites largely overlap with those of peridotites, although their densities are significantly higher than common upper mantle rocks. As a consequence, refraction seismic field studies may not detect edogites as crustal rocks. This means that the seismic Moho detected by refraction seismic field studies appears at the upper boundary between edogites and overlying crustal units. Since edogites generally have higher densities than peridotites, they might be recycled into the deeper lithosphere thereby transferring excess Eu into the upper mantle. This process could be a due for understanding the negative Euanomaly in the upper continental crust which is apparently not balanced quantitatively by the abundance of common mafic crustal rocks.     

The distribution of uranium in some basic igneous cumulates and its petrological significance

The distribution of uranium in the minerals and rocks of the Skaergaard, Rhum and Bushveld layered igneous intrusions has been investigated by the delayed neutron analytical technique. The uranium concentration in the minerals depends, in part, upon the nature of the cumulate from which they were separated. In those minerals from adcumulates, pyroxene contains the most uranium and olivine the least. The uranium content of a basic igneous cumulate is shown to be useful as an indicator of the relative amount of mesostasis in the rock, provided uranium-bearing cumulus minerals such as apatite are absent. The amount of mesostasis sometimes changes rapidly from one layer to another.     

伊吾Eucrite陨石的岩石学、矿物学特征及其成因

Howardite-Eucrite-Diogenite(HED)族陨石是地球上目前发现数量最多的无球粒陨石类型,已发现的 HED族陨石主要来自于南极和非洲西北部的沙漠,本研究的 HED族陨石国际命名为Yiwu 001 ,发现于中国新疆维吾尔自治区伊吾县境内.系统的矿物学和岩石学研究显示,该陨石属于单矿物碎屑角砾岩 Eu...