Evidence for aqueous activity on comet 81P/Wild 2 from sulfide mineral assemblages in Stardust samples and CI chondrites

The discovery of nickel-, copper-, and zinc-bearing iron sulfides from comet 81P/Wild 2 (Wild 2) represents the strongest evidence, in the Stardust collection, of grains that formed in an aqueous environment. We investigated three microtomed TEM sections which contain crystalline sulfide assemblages from Wild 2 and twelve thin sections of the hydrothermally altered CI chondrite Orgueil. Detailed structural and compositional characterizations of the sulfide grains from both collections reveal striking similarities. The Stardust samples include a cubanite (CuFe₂S₃) grain, a pyrrhotite [(Fe,Ni)_1−xS]/pentlandite [(Fe,Ni)₉S₈] assemblage, and a pyrrhotite/sphalerite [(Fe,Zn)S] assemblage. Similarly, the CI-chondrite sulfides include individual cubanite and pyrrhotite grains, cubanite/pyrrhotite assemblages, pyrrhotite/pentlandite assemblages, as well as possible sphalerite inclusions within pyrrhotite grains. The cubanite is the low temperature orthorhombic form, which constrains temperature to a maximum of 210 °C. The Stardust and Orgueil pyrrhotites are the 4C monoclinic polytype, which is not stable above ∼250 °C. The combinations of cubanite and pyrrhotite, as well as pyrrhotite and pentlandite signify even lower temperatures. The crystal structures, compositions, and petrographic relationships of these sulfides constrain formation and alteration conditions. Taken together, these constraints attest to low-temperature hydrothermal processing.Our analyses of these minerals provide constraints on large scale issues such as: heat sources in the comet-forming region; aqueous activity on cometary bodies; and the extent and mechanisms of radial mixing of material in the early nebula. The sulfides in the Wild 2 collection are most likely the products of low-temperature aqueous alteration. They provide evidence of radial mixing of material (e.g. cubanite, troilite) from the inner solar system to the comet-forming region and possible secondary aqueous processing on the cometary body.     

In situ identification of a CAI candidate in 81P/Wild 2 cometary dust by confocal high resolution synchrotron X-ray fluorescence

We detected additional CAI-like material in STARDUST mission samples of comet 81P/Wild 2. Two highly refractory cometary dust fragments were identified in the impact track 110 [C2012, 0, 110, 0, 0] by applying high resolution synchrotron induced confocal and conventional XRF analysis (HR SR-XRF). The use of a polycapillary lens in front of the detector for confocal spectroscopy dramatically improves the fidelity of particle measurements by removing contribution from the surrounding aerogel. The high spatial resolution (300 × 300 nm²; 300 × 1000 nm²) obtained allowed the detailed non-destructive in situ (trapped in aerogel) study of impacted grains at the sub-μm level.For the two largest particles of the track, the terminal particle and a second particle along the impact track, Ca concentration is up to 30 times higher than CI and Ti is enriched by a factor of 2 compared to CI. High resolution (HR) SR-XRF mapping also reveals that the highest concentrations of Ca, Ti, Fe (and Ni) measured within each grain belongs to different areas of the respective maps which indicate that the particles are composed of several chemically diverse mineral phases. This is in agreement with the finding of a complex phase assemblage of highly refractory minerals in the first ever detected Stardust mission CAI grain “Inti” of Track 25.Principle component analysis (PCA) is a powerful tool for extracting the dominant mineral components and was applied to the two grains indicating that regions in the terminal particle and the second particle are consistent with anorthite or grossite and gehlenite, monticellite or Dmitryivanovite (CaAl₂O₄), respectively.Our new findings demonstrate that the HR SR-XRF with confocal geometry and PCA analysis is capable of identifying CAI-like fragments without the need to extract particles from the aerogel matrix which is a time-consuming, complex and destructive process.Furthermore, the detection of new CAI-like fragments in the coma dust of comet 81P/Wild 2 strengthens the observation that strong mixing effects and, therefore, mass transport before or during comet formation must have occurred at least up to the region where Kuiper Belt comets formed (30 AU).     

Isotopic signatures of deposition and transformation of Lower Cambrian saliferous rocks in the Irkutsk amphitheater: Communication 2. Strontium isotopic composition

Rb-Sr data were obtained for different Lower Cambrian rocks from boreholes Bel’sk and Zhigalovo drilled in the Irkutsk amphitheater. Shales in the lower part and insoluble residue of marls throughout the entire (up to 2 km thick) sequence define the first transformation stage of silicate rocks dated at 580 Ma with a large uncertainty. Sulfate and carbonate rocks are the main Sr carriers in the Lower Cambrian rocks. Owing to a negligible amount of silicate impurities in them, the input of radiogenic Sr exerted no significant influence on the Rb-Sr isotope system. However, almost all carbonate and sulfate rocks studied are contaminated by radiogenic Sr relative to its initial isotopic composition in the sedimentation basin. Hence, the rocks experienced epigenetic (probably, repeated) recrystallization and one or several stages of dissolution.     

Occurence of abundant diradicaloid moieties in the insoluble organic matter from the Orgueil and Murchison meteorites: a fingerprint of its extraterrestrial origin?

The highly aromatic structure of the macromolecular organic matter (OM) of the Murchison and Orgueil meteorites was recently shown to contain free organic radicals which are concentrated in micro-regions in contrast with terrestrial samples which always show an homogeneous distribution of radicals. An additional signature is revealed, in the present study, by the evolution of the radical concentration with temperature. Whereas in terrestrial samples, this concentration is independent of temperature (Curie magnetism), a significant increase is observed above 150 K in the two meteorites. Based on the electronic structure of organic radicals, calculated by Extended Hückel and Density Functional methods, this behavior was assigned to the occurrence of diradicaloid moieties hosted by aromatic structures of 10 to 15 rings and having a quinoidal structure. They represent 40 and 25% of the total radicals in Orgueil and Murchison, respectively. The search for the cosmochemical interpretation of this unique observation should open a new field of experimental investigations.     

Investigation of matrix effects in the MC-ICP-MS induced by Nb, W, and Cu: Isotopic case studies of iron and copper

Some elements normally occur at trace levels while the majority of natural geological materials may be exceedingly enriched in some special cases, such as the Bayan Obo ore deposit where REE and Nb are extremely enriched. These elements may not be removed completely during purification. Therefore, matrix effects will be caused during stable isotope ratio measurement in the MC-ICP-MS. Experiments have shown that the established methods of chromatographic separation of Cu, Fe, and Zn using AG MP-1 Anion Exchange Resin cannot make ef-fective separation of Nb, W, and Cu from Fe using 20 mL 6 M HCl. It is also observed that the elution curves of W and Cu overlap at working conditions and thus W is present in measurable amounts in some sample solutions. Matrix effects in the MC-ICP-MS induced by Nb, W, and Cu during Fe isotope ratio measurements and by W during Cu isotope ratio measurements were thus investigated by examining their changes in delta values between doped and undoped standards. The results show that the effects of the matrix elements Nb, W, or Cu on Fe isotope ratio measurements are minimal in the case of m(Nb)/m(Fe)<0.005, m(W)/m(Fe)<0.01, or m(Cu)/m(Fe)<0.6. This finding, combined with the extremely low levels of W and Cu, and the fact that nearly 90% of Nb can be removed during purification, demonstrates that the methods of chromatographic separation of Fe established before are suitable for Bayan Obo ore samples and that the methods can be simplified when Cu elution is unnecessary. The effects of the matrix element W on Cu isotope determinations are minimal in the case of m(W)/m(Cu)<0.7. Therefore, W exerts no significant effect on the measurements of Cu isotopes for the majority of natural geological materials.     

Carbon sequestration and decreased CO2 emission caused by terrestrial aquatic photosynthesis: Insights from diel hydrochemical variations in an epikarst spring and two spring-fed ponds in different seasons

Whether carbonate weathering could produce a stable carbon sink depends primarily on the utilization of dissolved inorganic carbon (DIC) by aquatic phototrophs (the so-called Biological Carbon Pump-BCP effect). On this basis, water temperature (T), pH, electrical conductivity (EC) and dissolved oxygen (DO) were synchronously monitored at 15-min resolution for one and two days respectively in January and October 2013 in Maolan Spring and the spring-fed midstream and downstream ponds in Maolan Nature Reserve, China. A thermodynamic model was used to link the continuous data to allow calculation of CO₂ partial pressures (pCO₂) and calcite saturation indexes (SI_C). A floating static chamber was placed on the water surface successively at all sites to quantify CO₂ exchange flux between atmosphere and water so as to evaluate the BCP effect. Results show that, in both winter and autumn, remarkable diel variations of hydrochemical parameters were present in the midstream pond where DO, pH, and SI_C increased in the day and decreased during the night while EC, $\left[{{\text{HCO}}_3}^{-}\right]$, [Ca²⁺] and pCO₂ showed inverse changes mainly due to the metabolic processes of the flourishing submerged plants, with photosynthesis dominating in the day and respiration dominating at night. However, hydrochemical parameters in the spring and downstream pond show less change since few submerged plants developed there. It was determined that the BCP effect in the midstream pond was 285 ± 193 t C km⁻² a⁻¹ in winter and 892 ± 300 t C km⁻² a⁻¹ in autumn, indicating a potential significant role of terrestrial aquatic photosynthesis in stabilizing the carbonate weathering-related carbon sink.     

KT Boundary Chromites Determined to be Terrestrial: Cr Isotopic Evidence for Excavation and Ejection of Mafic/Ultramafic Rocks by the KT Boundary Impact

Evidence for a mantle and/or basaltic component in KT boundary distal ejecta is apparently inconsistent with ejection from Chicxulub Crater since it is located on~35km thick continental crust(De Paolo et al.,1983;Montanari et al.,1983;Hildebrand and Boynton,1988,1990).Evidence for mafic/ultramafic target rocks was reinforced by discovery of chromites,some with shock planar deformation features(PDF),in impact layer samples from sites in southern Colorado and eastern Wyoming(Bohor et al.,1990).However,until now it was unclear whether the chromites originated with an impactor or with terrestrial target rocks.To this end,high-precision 54Cr/52Cr isotope ratios were measured on KT boundary chromites along with known terrestrial chromites.We find a terrestrial 54Cr/52Cr ratio in KT boundary chromites from impact layer samples collected at the above sites over the last several years(Fig.1).Ejected terrestrial chromites suggest the impact sampled terrestrial mafic and/or ultramafic target rocks not known to exist in the Chicxulub target area.     

Estimation of colored dissolved organic matter and salinity fields in case 2 waters using SeaWiFS: Examples from Florida Bay and Florida Shelf

Estimates of water quality variables such as chlorophylla concentration (Chl), colored dissolved organic matter (CDOM), or salinity from satellite sensors are of great interest to resource managers monitoring coastal regions such as the Florida Bay and the Florida Shelf. However, accurate estimates of these variables using standard ocean color algorithms have been difficult due to the complex nature of the light field in these environments. In this study, we process SeaWiFS satellite data using two recently developed algorithms; one for atmospheric correction and the other a semianalytic bio-optical algorithm and compare the results with standard SeaWiFS algorithms. Overall, the two algorithms produced more realistic estimates of Chl and CDOM distributions in Florida Shelf and Bay waters. Estimates of surface salinity were obtained from the CDOM absorption field assuming a conservative mixing behavior of these waters. A comparison of SeaWiFS-derived Chl and CDOM absorption with field measurements in the Florida Bay indicated that although well correlated, CDOM was underestimated, while Chl was overestimated. Bottom reflectance appeared to affect these estimates at the shallow central Bay stations during the winter. These results demonstrate the need for new bio-optical algorithms or tuning of the parameters used in the bio-optical algorithm for local conditions encountered in the Bay.     

腾冲火山区壳内岩浆囊现今温度:来自温泉逸出气体 CO2、CH4间碳同位素分馏的估计

温度是岩浆囊的重要物理参数,获取温度参数并监测其变化对更好地理解岩浆囊的物理化学性质和行为、评价火山的活动性和喷发危险性具有重要的理论和现实意义.本文通过对温泉逸出气体CO2和CH4碳同位素样品的采集、分析测试,利用Horita通过实验矫正的Richet的平衡分馏系数的理论计算数据,通过拟合得到的CO2-CH4碳同位素平衡分馏方程T(K)=3880.3(10001nα(CO2-CH4))-0.5984,计算了腾冲火山区现存3个岩浆囊的温度.结果表明:在整个腾冲火山区,由CO2和CH4的碳同位素分馏值计算的气体源区平衡温度最低397℃,最高1163℃,平均615℃.腾冲火山区的3个岩浆囊中,南部五合-龙江-浦川岩浆囊的现今温度在464～1163℃,平均773℃,温度最高;中部腾冲-和顺-热海岩浆囊的现今温度在438 ～773℃间,平均达566℃,温度次之;北部马站-曲石-永安岩浆囊的现今温度在397～651℃间,平均达524℃,温度最低.我们认为,腾冲火山区地下岩浆囊顶部气体富集区目前的温度变化范围为400～ 1200℃,岩浆囊的实际温度应高于平均值615℃.3个岩浆囊的边缘温度可能在400 ～600℃间,中心温度可能在700～1200℃间.3个岩浆囊中心的现今温度已达到流纹岩浆(600～900℃)、安山岩(800 ～ 1100℃)和玄武岩浆(1000～1250℃)的形成温度,进一步说明腾冲火山区目前3个岩浆囊是客观存在的.另外,我们发现在用δ-△图解法判断CO2-CH4间碳同位素分馏平衡的过程中,在保持2条拟合直线的斜率符号相反的条件下,无论如何剔除数据点,都不能使δ13CCO2-ΔCO2-CH4拟合直线的截距bCO2和δ13 CCH4-ΔCO2-CH14拟合直线的截距bCH4之差bΔ小于0.1245,说明2条拟合直线的交点不能落于δ轴上,而这一数值与Horita方程的常数项接近,这说明CO2-CH4间碳同位素分馏方程中确实存在常数项,CO2和CH4间在再高的温度下都存在碳同位素的分馏.δ-△图解法判断CO2-CH4间碳同位素分馏平衡准则应修正为:在保持2条拟合直线的斜率符号相反的条件下,δ13 CCO2-ΔCO2-CH4拟合直线和δ13CCH4-ΔCO2-CH4拟合直线应相交于δ轴附近截距差0.1245处.     

Reactive flow of mixed CO2–H2O fluid and progress of calc-silicate reactions in contact metamorphic aureoles: insights from two-dimensional numerical modelling

Previous models of hydrodynamics in contact metamorphic aureoles assumed flow of aqueous fluids, whereas CO₂ and other species are also common fluid components in contact metamorphic aureoles. We investigated flow of mixed CO₂–H₂O fluid and kinetically controlled progress of calc‐silicate reactions using a two‐dimensional, finite‐element model constrained by the geological relations in the Notch Peak aureole, Utah. Results show that CO₂ strongly affects fluid‐flow patterns in contact aureoles. Infiltration of magmatic water into a homogeneous aureole containing CO₂–H₂O sedimentary fluid facilitates upward, thermally driven flow in the inner aureole and causes downward flow of the relatively dense CO₂‐poor fluid in the outer aureole. Metamorphic CO₂‐rich fluid tends to promote upward flow in the inner aureole and the progress of devolatilization reactions causes local fluid expulsion at reacting fronts. We also tracked the temporal evolution of P‐T‐X_CO2conditions of calc‐silicate reactions. The progress of low‐ to medium‐grade (phlogopite‐ to diopside‐forming) reactions is mainly driven by heat as the CO₂ concentration and fluid pressure and temperature increase simultaneously. In contrast, the progress of the high‐grade wollastonite‐forming reaction is mainly driven by infiltration of chemically out‐of‐equilibrium, CO₂‐poor fluid during late‐stage heating and early cooling of the inner aureole and thus it is significantly enhanced when magmatic water is involved. CO₂‐rich fluid dominates in the inner aureole during early heating, whereas CO₂‐poor fluid prevails at or after peak temperature is reached. Low‐grade metamorphic rocks are predicted to record the presence of CO₂‐rich fluid, and high‐grade rocks reflect the presence of CO₂‐poor fluid, consistent with geological observations in many calc‐silicate aureoles. The distribution of mineral assemblages predicted by our model matches those observed in the Notch Peak aureole.     

Land uplift induced by injection: a feasible method to evaluate the security of CO<Subscript>2</Subscript> capture and sequestration projects

Technology of CO₂ capture and sequestration (CCS) is one of the many solutions to reduce greenhouse gases and alleviate the current global warming, but its security is important and needs to be evaluated. A simulator which links TOUGHREACT and FLAC^3D was used to simulate the process of coupled temperature-hydrologic-mechanics (THM) in CCS. A test on laboratory scale was set up and water was injected into compacted sand covered by low permeability clay to study the land uplift displacement. The results were used to verify the accuracy of the simulator for calculating the THM coupling. The effects of injection quantity, injection time, and injection mode on land uplift were also studied on the constructed model. At last, a land uplift evaluation system was built to quantify the CO₂ escape if any. The evaluation process can be divided into five steps: model generalization, acquisition of model parameters, numerical modeling, simulation and analysis, monitor comparison, and evaluation of model results. The major output of this study will provide a feasible method for quantitative analysis of CO₂ leakage in CCS projects.     

Low-pressure granulite facies metapelitic assemblages and corona textures from MacRobertson Land, east Antarctica: the importance of Fe2O3 and TiO2 in accounting for spinel-bearing assemblages

Low-pressure granulite facies metasedimentary gneisses exposed in MacRobertson Land, east Antarctica, include hercynitic spinel-bearing metapelitic gneisses. Peak metamorphic mineral assemblages include spinel + rutile + ilmenite + sillimanite + garnet, spinel + ilmenite + sillimanite + garnet + cordierite, ortho-pyroxene + magnetite + ilmenite + garnet, spinel + cordierite + biotite + ilmenite and orthopyroxene + cordierite + biotite, each with quartz, K-feldspar and melt. The presence of garnet + biotite- and cordierite + orthopyroxene-bearing assemblages implies crossing tie-lines in AFM projection for the K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (KFMASH) system. This apparent contradiction, and the presence of spinel, rutile and ilmenite in the assemblages, is acounted for by using the KFMASH-TiO₂-O₂ system, i.e. AFM + TiO₂+ Fe₂O₃. We derive a petrogenetic grid for this system, applicable to low-pressure granulite facies metamorphic conditions. Retrograde assemblages are interpreted from corona textures on hercynitic spinel and Fe-Ti oxides. The relative positions of the peak and retrograde metamorphic assemblages on the petrogenetic grid suggest that corona development occurred during essentially isobaric cooling.     

Expansivity and compressibility of strontium fluorapatite and barium fluorapatite determined by in situ X-ray diffraction at high-T/P conditions: significance of the M-site cations

Strontium fluorapatite (SrF-Ap) and barium fluorapatite (BaF-Ap) were synthesized by using a piston-cylinder apparatus at 1.0 GPa and 800 °C. Their thermal expansivity and compressibility were subsequently investigated by using in situ high-T powder X-ray diffraction method (up to 1,000 °C at ambient P) and by using a diamond-anvil cell coupled with synchrotron X-ray radiation (up to about 5 GPa at room T), respectively. The derived thermal expansion coefficients and bulk moduli were combined with the literature data for calcium fluorapatite (CaF-Ap) and lead fluorapatite (PbF-Ap) to constrain the influence of the M-site cations on the elasticity of the fluorapatites (MF-Ap). The results show that both the size of the M cations and their polarizability affect the thermal expansion coefficients of the MF-Ap. For the axial and volumetric bulk moduli, the size of the M cations plays a dominant role, with some crystallographic features of the MF-Ap being potentially important as well.     

Fixation of CO2 by chrysotile in low-pressure dry and moist carbonation: Ex-situ and in-situ characterizations

A detailed study of low-pressure gas-solid carbonation of chrysotile in dry and humid environments has been carried out. The evolving structure of chrysotile and its reactivity as a function of temperature (300-1200 °C), humidity (0-10 mol %) and CO₂ partial pressure (20-67 mol %), thermal preconditioning, and alkali metal doping (Li, Na, K, Cs) have been monitored through in-situ X-ray photoelectron spectroscopy, isothermal thermogravimetry/mass spectrometry, ex-situ X-ray powder diffraction, and water and nitrogen adsorption/desorption. Based on chrysotile crystalline structure and its nanofibrilar orderliness, a multistep carbonation mechanism was elaborated to explain the role of water during chrysotile partial amorphisation, formation of periclase, brucite, and hydromagnesite crystalline phases, and surface passivation thereof, during humid carbonation. The weak carbonation reactivity was rationalized in terms of incongruent CO₂ van der Waals molecular diameters with the octahedral-tetrahedral lattice constants of chrysotile. This lack of reactivity appeared to be relatively indifferent to the facilitated water crisscrossing during chrysotile core dehydroxylation/pseudo-amorphisation and surface hydroxylation induced product stabilization during humid carbonation. Thermodynamic stability domains of the species observed at low pressure have been thoroughly discussed on the basis of X-ray powder diffraction patterns and X-ray photoelectron spectroscopy evidence. The highest carbon dioxide uptake occurred at 375 °C in moist atmospheres. On the basis of chrysotile fresh N₂ BET area, nearly 15 atoms out of 100 of the surface chrysotile brucitic Mg moiety have been carbonated at this temperature which was tantamount to the carbonation of about 2.5 at. % of the total brucitic Mg moiety in chrysotile. The carbonation of brucite (Mg(OH)₂) impurities coexisting in chrysotile was minor and estimated to contribute by less than 17.6 at. % of the total converted magnesium. The presence of cesium traces (3 Cs atoms per 100 Mg atoms) was found to boost chrysotile carbonation capacity by a factor 2.7.     

The system H2O-NaCl. Part I: Correlation formulae for phase relations in temperature-pressure-composition space from 0 to 1000 °C, 0 to 5000 bar, and 0 to 1 XNaCl

Realistic simulations of fluid flow in geologic systems have severely been hampered by the lack of a consistent formulation for fluid properties for binary salt-water fluids over the temperature-pressure-composition ranges encountered in the Earth’s crust. As the first of two companion studies, a set of correlations describing the phase stability relations in the system H₂O-NaCl is developed. Pure water is described by the IAPS-84 equation of state. New correlations comprise the vapor pressure of halite and molten NaCl, the NaCl melting curve, the composition of halite-saturated liquid and vapor, the pressure of vapor + liquid + halite coexistence, the temperature-pressure and temperature-composition relations for the critical curve, and the compositions of liquid and vapor on the vapor + liquid coexistence surface. The correlations yield accurate values for temperatures from 0 to 1000 °C, pressures from 0 to 5000 bar, and compositions from 0 to 1 X_NaCl (mole fraction of NaCl). To facilitate their use in fluid flow simulations, the correlations are entirely formulated as functions of temperature, pressure and composition.     

A thermodynamic model for the prediction of phase equilibria and speciation in the H2O-CO2-NaCl-CaCO3-CaSO4 system from 0 to 250 °C, 1 to 1000 bar with NaCl concentrations up to halite saturation

A thermodynamic model is developed for the calculation of both phase and speciation equilibrium in the H₂O-CO₂-NaCl-CaCO₃-CaSO₄ system from 0 to 250 °C, and from 1 to 1000 bar with NaCl concentrations up to the saturation of halite. The vapor-liquid-solid (calcite, gypsum, anhydrite and halite) equilibrium together with the chemical equilibrium of H⁺,Na⁺,Ca²⁺, , , and CaSO_4(aq) in the aqueous liquid phase as a function of temperature, pressure and salt concentrations can be calculated with accuracy close to the experimental results.Based on this model validated from experimental data, it can be seen that temperature, pressure and salinity all have significant effects on pH, alkalinity and speciations of aqueous solutions and on the solubility of calcite, halite, anhydrite and gypsum. The solubility of anhydrite and gypsum will decrease as temperature increases (e.g. the solubility will decrease by 90% from 360 K to 460 K). The increase of pressure may increase the solubility of sulphate minerals (e.g. gypsum solubility increases by about 20-40% from vapor pressure to 600 bar). Addition of NaCl to the solution may increase mineral solubility up to about 3 molality of NaCl, adding more NaCl beyond that may slightly decrease its solubility. Dissolved CO₂ in solution may decrease the solubility of minerals. The influence of dissolved calcite on the solubility of gypsum and anhydrite can be ignored, but dissolved gypsum or anhydrite has a big influence on the calcite solubility. Online calculation is made available on www.geochem-model.org/model.     

The mixing properties of melts and glasses in the system NaAlSi3O8-KAlSi3O8: Comparison of experimental data obtained by knudsen cell mass spectrometry and solution calorimetry

The apparent differences between heats of mixing determined by solution calorimetric measurements on quenched samples in the system NaAlSi₃O₈-KAlSi₃O₈ and heats of mixing determined at high temperatures by Knudsen cell mass spectrometry are caused by thermal effects related to the glass transition. When these effects are allowed for, the calorimetric data and Knudsen cell measurements are in good agreement.