Uncertainties of geochemical codes and thermodynamic databases for predicting the impact of carbon dioxide on geologic formations

Numerical codes are applied to calculate chemical reactions following geologic carbon sequestration in deep formations and CO₂ leakage in shallow formations. However, using different thermodynamic databases generates variations in the simulation results, which are referred to as the model uncertainty. The PHREEQC and The Geochemist's Workbench codes were used to simulate anorthite dissolution for storage, retention, transfer, and near-surface formation waters in the respective geological units. For each of the formation waters, a simple one-dimensional scenario was simulated using eight different thermodynamic databases. Groundwaters in shallow aquifers commonly exhibit low ionic strengths (<0.5 mol/kgw) and low temperatures, whereas storage formation waters are characterized by high ionic strength (>1.0 mol/kgw) and high temperatures. In storage formations, mineral trapping is the most efficient process for long-term CO₂ storage. However, with respect to the geological formations and the time needed for anorthite dissolution, the model uncertainties associated with using different combinations of numerical codes and thermodynamic databases were largest (∼90%) for the storage formation waters at 58 °C and I = 6.5 mol/l. Conversely, in near-surface formation waters, the model uncertainty was less than 1%. Due to CO₂ dissolution, the calculated pH of the formation waters decreased to a range between pH 4.0 and 5.5. In this pH range, the dissolution mechanism of anorthite switches from the slow neutral mechanism to the faster acid mechanism, causing dissolution time length variations. The calculated pH variation further increased with rising ionic strength. A detailed examination of the reasons revealed the activity coefficient calculation method of the main aquatic species to have the largest impact on the simulated model results. The calculation method of the CO₂ activity coefficient had the second largest impact. Via calibration with the experimental data, a specific thermodynamic database can be chosen to represent these experimental results. However, the calibration of thermodynamic databases is not possible for all potential reactions in more complex geological systems at large ranges of temperature, ionic strength and pressure conditions. The uncertainties associated with using thermodynamic databases quantified in this study for CO₂ storage systems will therefore persist independently from previously conducted calibrations of thermodynamic databases with experimental or field data. In view of these model uncertainties, the modeller is encouraged to include a routine in the simulations for quantification of the model uncertainty depending on the specific scenario or to assess the simulation results as a range of values that represent a soft outcome.     

Searching for the perfect surface area normalizing term—a comparison of BET surface area-, geometric surface area- and mass-normalized dissolution rates of anorthite and biotite

Dissolution rates were calculated for a range of grain sizes of anorthite and biotite dissolved under far from equilibrium conditions at pH 3, T = 20 °C. Dissolution rates were normalized to initial and final BET surface area, geometric surface area, mass and (for biotite only) geometric edge surface area. Constant (within error) dissolution rates were only obtained by normalizing to initial BET surface area for biotite. The normalizing term that gave the smallest variation about the mean for anorthite was initial BET surface area. In field studies, only current (final) surface area is measurable. In this study, final geometric surface area gave the smallest variation for anorthite dissolution rates and final geometric edge surface area for biotite dissolution rates.     

Transport properties of high albite crystals,near-endmember feldspar and pyroxene glasses,and their melts to high temperature

Thermal diffusivity (D) was measured using laser-flash analysis (LFA) from oriented single-crystal albite and glasses near LiAlSi₃O₈, NaAlSi₃O₈, CaAl₂Si₂O₈, LiAlSi₂O₆ and CaMgSi₂O₆ compositions. Viscosity measurements of the supercooled liquids, over 2.6 × 10⁸ to 8.9 × 10¹² Pa s, confirm strongly non-Arrhenian behavior for CaAl₂Si₂O₈, and CaMgSi₂O₆, and near-Arrhenian behavior for the others. As T increases, D _glass decreases, approaching a constant near 1,000 K. Upon crossing the glass transition, D decreases rapidly. For feldspars, D for the melt is ~15% below D of the bulk crystal, whereas for pyroxenes, this difference is ~40%. Thermal conductivity (k _lat = ρC _P D) of crystals decreases with increasing T, but k _lat of glasses increases with T because heat capacity (C _P ) increases with T more strongly than density (ρ) and D decrease. For feldspars, k _lat for the melt is ~10% below that of the bulk crystal or glass, whereas this decrease for pyroxene is ~50%. Therefore, melting substantially impedes heat transport, providing positive thermal feedback that could promote further melting.     

Cation ordering in BaAl2Ge2O8-feldspar: implications for the phase transition in anorthite

BaAl₂Ge₂O₈-Feldspar undergoes an order-disorder phase transition I2/c↔C2/m at T _tr ≈1690 K. The thermodynamics of the Al,Ge cation ordering process is described in terms of the compressible Ising model in mean field approximation. The mean field potential predicts a first order character of the phase transition. This is compared to antiferromagnetic ordering in a two-dimensional square Ising model with NN-pair interactions and four-spin interactions on alternating squares. Calculated order parameters and short range ordering are in good agreement with the corresponding properties observed in BaAl₂Ge₂O₈-feldspar by means of X-ray diffraction, hard mode infrared spectroscopy and TEM. Using known calorimetric data a similar model is postulated for Al,Si ordering in anorthite, CaAl₂Si₂O₈, for which the derived potential describes a transition with slightly stronger first order character at T _tr ≈1928 K. Received: 30 January 1998 / Revised, accepted: 29 August 1998     

CaAl2Si2O8 polymorphs: Sensitive geothermometers and geospeedometers

Plagioclase is the major rock-forming mineral constituting the Earth’s crust, whereas anorthite (CaAl₂Si₂O₈) is a common minerals in lunar highlands crust, meteorites, possibly in some comets and on Mercury. Besides anorthite, two high-temperature polymorphs of CaAl₂Si₂O₈ are known: dmisteinbergite and svyatoslavite, which are found in burnt coal dumps, meteorites and pseudotachylytes. Here we present the results of detailed studies (quenching experiments, elemental analysis, Raman spectroscopy and in situ high temperature single crystal X-ray diffraction (up to 1000 °C)) on naturally co-occurring CaAl₂Si₂O₈ polymorphs (anorthite, dmisteinbergite and svyatoslavite) from a burnt coal dump in Kopeisk, Russia. New polymorphs were found in all natural samples and obtained upon heating of dmisteinbergite (unquenchable β-dmisteinbergite and quenchable γ-dmisteinbergite). It was shown that Ca coordination differs significantly in CaAl₂Si₂O₈ polymorphs, resulting in a different capacity to host Ba and possibly other large ion lithophile elements. Combining our data on natural samples with the previously published data on natural and synthetic compounds, we propose a new scheme of CaAl₂Si₂O₈ polymorphs stability. Our results indicate that CaAl₂Si₂O₈ polymorphs could be used for temperature estimations for both Earth and planetary sciences.     

西藏普兰地幔橄榄岩中尖晶石内的钙长石包裹体及其成因

西藏普兰超镁铁岩体之东南缘与玄武岩接触界线附近的地幔橄榄岩中除有粒状半自形的钙长石产出外,还在尖晶石中发现有呈蠕虫状、浑圆状的钙长石包裹体存在.研究发现两种产状的钙长石An值都大于95且均无环带构造,说明钙长石从高Ca/Al比值的熔体中结晶时具有结晶时间短、结晶速度快的特点,可能形成于地壳较浅部位.从化学成分来看,包裹体形态的钙长石具有较高的Cr2O3含量,其寄主矿物尖晶石的Cr#值低且TiO2含量比深海橄榄岩中的尖晶石低得多,推断钙长石包裹体与寄主矿物尖晶石是在液相条件下几乎同时结晶的产物.综合研究表明钙长石包裹体的成因可能是玄武岩熔体在地壳较浅部位侵入方辉橄榄岩时,高温的玄武质熔体提供热源,使得方辉橄榄岩中尖晶石内的Cpx+ Opx细粒矿物包裹体在高温环境下发生熔融,发生Opx+ Cpx+ Sp→Ol+ Pl的反应,由于这种情况下尖晶石有剩余,故新生成的橄榄石和钙长石矿物仍然包裹于尖晶石内,从而形成尖晶石内部呈蠕虫状的钙长石包裹体.     

Effect of anorthite on granite phase relations: Experimental data and models

New experimental data on the effect of anorthite (An) on liquidus phase equilibria in the system Qz–Ab–Or are presented. The data were obtained for 5 wt% An added to variable Qz/Ab/Or compositions at 300 MPa and under H₂O-saturated conditions. Crystal–liquid equilibria were determined for 13 synthetic glass compositions made from gels in experiments performed between 660 and 750 °C in cold-seal pressure vessels. Forward and reversal experiments were systematically conducted on each composition to demonstrate equilibrium. A total of 51 charges was examined. Three crystalline phases, quartz, alkali feldspar and plagioclase appear on the H₂O-saturated liquidus surface. The determined minimum liquidus 5 wt% An “piercing” point (39% Qz, 33% Ab, 28% Or) is shifted away from the Ab apex toward the Qz–Or sideline when compared with the An-free 300 MPa H₂O-saturated minimum. This shift is of the same type as that observed at 100 MPa in the same system and at 200 MPa in a rhyolitic system. The new experimental results are used to test both empirical and thermodynamic models for silicic magmas. Empirical models reproduce reasonably well the new experimental data, although more sophisticated calculations schemes appear to be required to improve their accuracy. The new experimental results in the haplogranodiorite system are not well reproduced with the model of Holland and Powell (2001), mainly because plagioclase stability appears greatly enhanced in the model. Rhyolite-MELTS satisfactorily reproduces the Qz-, Pl- and Af-liquid phase equilibria, but model H₂O solubilities are significantly lower and crystallization temperatures higher than in experiments.