A. F. Koster van Groos

CHLORINE STABLE ISOTOPE FRACTIONATION IN EVAPORITES

Abstract Chlorine isotope fractionation ( 37 Cl 35 Cl ) between NaCl, KCI, and MgCl2·6H2O and their saturated solutions was determined in laboratory experiments at 22 ± 2°C. The results are as follows: 10 3 ln α( NaCl—solution ) = +0.26 ± 0.07 (1δ) 10 3 ln α( KCl—solution ) = −0.09 ± 0.09 (1δ) 10 3 ln α( MgCl 2 ·6H 2 O—solution ) = −0.06 ± 0.10 (1δ) where fractionation factor a is defined as: α = − ( 37 Cl / 35 Cl ) precipitate ( 37 Cl / 35 Cl ) solution These data were used to approximate the isotope fractionation factors of chloride between the saturated solution and halite, kainite, carnallite, and bischofite. From the results, the stable chlorine isotope fractionation during the formation of evaporite was calculated using a Rayleigh fractionation model. The model predicts that δ37Cl of the precipitate decreases systematically during the main phase of halite crystallization but increases again at the latest stage of evaporation. The chlorine isotope fractionation model was tested on a core from the upper Zechstein III salt formation. The salt core contains layers dominated by either halite or KMg salts. The KMg salts, which are formed during the final stages of evaporation, contain up to 75% carnallite (KMgCl3·6H2O) and bischofite (MgCl2·6H2O). The observed chlorine isotope fractionation in the salt core is in general agreement with the Rayleigh fractionation model. During the main crystallization phase of halite, δ37Cl decreases continuously, but this trend reverses during the final stages when Mg-salts begin to crystallize. It is concluded that δ37Cl can be used as an indicator of evaporation cycles. In addition, it provides quantitative information on the proportion of salt that has been deposited on the input of fresh seawater and on the disturbance by postdepositional processes.

BASELINE STUDIES OF THE CLAY MINERALS SOCIETY SOURCE CLAYS: THERMAL ANALYSIS

Thermal analysis involves a dynamic phenomenological approach to the study of materials by observing the response of these materials to a change in temperature. This approach differs fundamentally from static methods of analysis, such as structural or chemical analyses, which rely on direct observations of a basic property of material ( e.g. crystal structure or chemical composition) at a well-defined set of conditions ( e.g. temperature, pressure, humidity). Clay minerals are highly susceptible to significant compositional changes in response to subtle changes in conditions. For example, changes in the fugacity of water affect the stability of interlayer H2O in a clay mineral (see below). Therefore, care must be taken that all experimental conditions are known with accuracy and precision. Differential thermal analysis (DTA), thermal gravimetric analysis (TG or TGA), and derivative thermal gravimetric (DTG) analysis are reported for each of the eight Source Clay minerals using commonly available commercial instruments. The DTA curves show the effect of energy changes (endothermic or exothermic reactions) in a sample. For clays, endothermic reactions involve desorption of surface H2O ( e.g. H2O on exterior surfaces) and dehydration ( e.g. interlayer H2O) at low temperatures (<100°C), dehydration and dehydroxylation at more elevated temperatures, and, eventually, melting. Exothermic reactions are related to recrystallization at high temperatures that may be nearly concurrent with or after dehydroxylation and melting. Discriminating between desorption and dehydration or dehydration and dehydroxylation may be problematic. The TG curves ideally show only weight changes during heating. The derivative of the TG curve, the DTG curve, shows changes in the TG slope that may not be obvious from the TG curve. Thus, the DTG curve and the DTA curve may show strong similarities for those reactions that involve weight and enthalpy changes, such as desorption, dehydration and dehydroxylation reactions. In …

The distribution of Na, K, Rb, Sr, Al, Ge, Cu, W, Mo, La, and Ce between granitic melts and coexisting aqueous fluids

The distribution of Na, K, Rb, Sr, Al, Ge, Cu, W, Mo, La, and Ce between H2O, NaCl, NaCl + KCl, HCl, NaF, Na2CO3, or Na2CO3 + K2CO3 aqueous fluids and granitic melts was determined at 750–800°C and 1–4 kbar. The distribution coefficients DNa, DK, DRb, and DCu (Di = Civ/Cim, where Civ and Cim are the concentrations of element i in the aqueous fluid and the melt, respectively) increase linearly with the (Na, K)Cl concentration in the fluid, indicating the presence of (Na, K, Rb, or Cu)Cl complexes. DSr shows a quadratic relation with the chloride concentration, suggesting a SrCl2 complex in these fluids. With the (Na, K)Cl-bearing aqueous fluids, Na, K, and Rb, and especially Cu strongly partition toward the fluid. DK and DRb are about half of DNa at comparable Cl concentrations. In contrast, Al, Ge, Mo, W, La, and Ce strongly partition toward the melt. NaF has little effect on the partitioning of these elements, except for Al, W, and Mo. DAl increases with increasing NaF content. At low NaF concentrations, W and Mo are enriched in the aqueous fluid, but at higher NaF contents they partition toward the silicate melt. With (Na,K)2CO3, all elements except Mo and Cu partition strongly towards the silicate melt, although Ge is slightly more soluble in the carbonated aqueous fluid. The quenched glasses are highly peralkaline in the experiments with (Na,K)2CO3, slightly peralkaline with NaF, slightly peraluminous with pure H2O or (Na,K)Cl, and highly peraluminous with HCl. DAl and DGe increase slightly in peralkaline melts. In the experiments with highly peraluminous melts, the distribution coefficient for all the elements, except Al, Ge, and W, is ≫1. With an increase of the (Na + K)/Al ratio to 0.3, the distribution coefficients become <1, except for Cu and Mo. Raising pressure to 4 kbars does not significantly affect partitioning of these elements except for DGe and DMo, which show an increase.

Forsteritic olivine : Effect of crystallographic direction on dissolution kinetics

Abstract Directional dissolution along the three crystallographic axes of gem-quality olivine (Fo 91 ), San Carlos, Arizona was studied at pH 1 and pH 2 at 23, 50, 70, and 90°C and 1 atm. The rate constant of dissolution for olivine at pH 1 and 70°C down the a -, b -, and c -axis (based on space group Pbnm ) is 2.7 × 10 −4 , 5.6 × 10 −3 , and 8.1 × 10 −4 mm/h, respectively. At pH 2 and 70°C the dissolution rates are 1.3 × 10 −4 , 2.1 × 10 −3 , and 4.3 × 10 −4 mm/h, respectively. At 50°C and 90°C, these rates are ∼0.2 and 5 times the rates at 70°C. The much higher dissolution rate in the direction of the b -axis is attributed to preferential protonation of the oxygen atoms around the M(1) site, which would result in a higher dissolution rate of the SiO 2 -M(1) network. The activation energy of dissolution E a dis in the direction down the a -, b -, and c -axis is 114.5 ± 23 kJ/mol, 69.9 ± 8 kJ/mol, and 72.9 ± 15 kJ/mol, respectively. Because of differences in the directional E a dis , dissolution in the direction down the a -axis will become dominant at temperatures above ∼140°C. The bulk E a dis , based on the dissolution rate along the crystallographic axes, is 71.5 ± 12 kJ/mol at the temperature range of the study. Because of the larger E a dis perpendicular to the a -axis, bulk E a dis must increase with temperature. The results indicate that the weathering rate of olivine is more temperature dependent than was considered previously.

Dehydration of K-exchanged montmorillonite at elevated temperatures and pressures

The dehydration temperature of K-montmorillonite, obtained by ion exchange of a Na-mont-morillonite, was determined at pressures to 2 kbar, using high-pressure differential thermal analysis. Dehydration reactions were found at about 50° and 100°C above the liquid-vapor curve of water. At pressures above the critical point of water the dehydration temperatures increased only slightly. The temperature of the first dehydration reaction is 10°C higher than for Na-montmorillonite, indicating a slightly greater stability of the hydration shell around the potassium interlayer cation. The second dehydration reaction occurs at a slightly lower temperature. The data were used to determine the enthalpy of the dehydration ΔH(dh) and the bonding enthalpy of the interlayer water ΔH(iw) at 1 atm. The first dehydration reaction of the K-exchanged montmorillonite has a ΔH(dh) = 46.16 ± 0.06 kJ/mole and a ΔH(iw) = 7.8 ± 0.5 kJ/mole, whereas for the second reaction, ΔH(dh) = 56.7 ± 2 kJ/mole and ΔH(iw) = 19.8 ± 2 kJ/mole. These values compare with a ΔH(dh) = 46.8 ± 0.3 kJ/mole and a ΔH(iw) = 7.8 ± 0.5 kJ/mole for the first dehydration reaction of the Na-montmorillonite and a ΔH(dh) = 62.9 ± 2 kJ/mole and ΔH(iw) = 27.1 ± 2 kJ/mole for the second dehydration.

Diffusion of chlorine in granitic melts

Abstract The chemical diffusivity of Cl in granitic and haplogranitic melts was determined as a function of temperature (650–1400°C), pressure (1 bar–4.6 kbar), H 2 O content and NaCl concentration. Three series of experiments were made: (1) high temperature runs at 1 atm with a NaCl liquid, (2) runs at pressures to 2 kbar with a pure NaCl liquid or NaCl-rich brine and (3) H 2 O-rich runs with NaCl/HCl solutions at pressures to 4.6 kbar. Chlorine concentrations were determined by electron microprobe. Chlorine diffusion follows the Arrhenius equation in both high temperature (log ( D ) = −4.5−4502/ T (K)) and H 2 O-rich runs containing 10 wt% NaCl solution at 2 kbar (log ( D ) = −2.19−5780/ T (K)). The pressure effect at 850°C is moderate for both NaCl-rich (log ( D ) = −8.487−0.125 P ) and H 2 O - rich runs (log ( D ) = −7.26−0.103 P ). D Cl is related to the concentration of NaCl in the initial solutions for H 2 O-rich runs. At 850°C and 2 kbar, D Cl ranges from log( D ) = −7.24 (5.8 wt% NaCl solution) to log ( D ) = −7.59 (20 wt% NaCl solution), where D is in cm 2 /s. D Cl in runs with a 10 wt% HCl solution is several times higher than with a 10 wt% NaCl solution at the same PT conditions. Furthermore, at higher concentration of NaCl, D Cl is lower. It was found that in the NaCl-rich series D Cl increases very sharply with the addition of H 2 O to the glass to 2–3 wt%, further addition of H 2 O has a significantly smaller effect. This difference is interpreted as a result of the change in the melt structure. The relationship of D Cl and viscosity does not follow the Eyring equation in the high temperature runs. The results of this study, combined with other investigations suggest that diffusion rates of volatiles decrease as: CO 2 >H 2 O>Cl>F. This indicates that during magma evolution differentiation of the volatile constituents may occur.

Differential thermal analysis of the liquidus relations in the system NaCl-H2O to 6 kbar

Abstract The liquidus relations in the system NaCl-H 2 O were investigated between 300 and 925°C and between 0.4 and 6 kbar for compositions between 27.6 and 100 mol% NaCl by high-pressure differential thermal analysis. The PT slope of the liquidus increases from 10.5°C/kbar for a composition with 27.6 mol% NaCl to 20.8°C/kbar with 90 mol% NaCl to 21.8°C/kbar with pure NaCl. The PT slope of the vapor saturation surface at the composition with 27.6 mol% NaCl is linear over the pressure range investigated and has a slope of 294°C/kbar. On the basis of the experiments the composition of the liquids along the NaCl-rich part of the three-phase assemblage halite + liquid + vapor was determined: the liquid composition varies nearly linearly to within ±0.5 wt% NaCl at temperatures above 480°C, following the expression wt% ( NaCl ) = 0.1412 T (° C ) −12.96.

HIGH-PRESSURE DIFFERENTIAL THERMAL ANALYSIS (HP-DTA) I. Dehydration reactions at elevated pressures in phyllosilicates

High-pressure differential thermal analysis results are used to describe dehydroxylation reactions for the clay materials of kaolinite, Na-rich montmorillonite, and K-, Ca-, and Mg-exchanged montmorillonite. Sealed capsules may be used to contain fluids and it is possible to evaluate the role of H2O in these reactions. Furthermore, structural information is used with DTA data to develop atomistic models to understand these processes.ZusammenfassungErgebnisse der Hochdruck-DTA werden zur Beschreibung der Dehydroxylierungsreaktionen der Tonmaterialien von Kaolinit, Na-reichem Montmorillonit und K-, Ca- und Mg-ausgetauschtem Montmorillonit verwendet. Verschlossene Kapseln können verwendet werden, um Fluids zu enthalten und es ist möglich, die Rolle des Wassers in diesen Reaktionen aufzuzeigen. Weiterhin werden Strukturinformationen zusammen mit DTA-Daten verwendet, um atomistische Modelle zum Verständnis dieser Prozesse zu entwickeln.

The stability of methane hydrate intercalates of montmorillonite and nontronite: Implications for carbon storage in ocean-floor environments

Abstract Sodium-rich montmorillonite, Na-exchanged montmorillonite, and Na-exchanged nontronite form intercalate complexes with methane hydrate, identified by a characteristic d(001) value of ~2.2 nm. The upper stability of both Na-rich montmorillonite-methane-hydrate complexes is nearly identical to that of methane hydrate, whereas that of Na-exchanged nontronite-methane-hydrate complex is ~1 °C lower. The low-temperature stability of these complexes is controlled by dehydration reactions of the montmorillonite and nontronite. At temperatures of 2 °C, the d(001) value of the montmorillonite complex decreases step-wise with decreasing temperature from ~2.2 nm at 2 °C to 1.6 nm at ≤-5 °C, indicating that H2O is progressively expelled from the interlayer. All methane is probably expelled at ~0 °C. The d(001) value of the nontronite complex did not show a similar step-wise reduction and, consequently, the lower stability of this complex is not well established. We conclude that under conditions of reduced salinity, smectite may sufficiently swell and intercalate with methane hydrate in an intermediate to deep-ocean floor environment. Consequently, these smectite-methane-hydrate complexes in the sub-ocean-floor surface may store substantial quantities of carbon.

CHLORINE STABLE ISOTOPES IN CARBONATITES : EVIDENCE FOR ISOTOPIC HETEROGENEITY IN THE MANTLE

Abstract Eight carbonatite samples were analysed for δ 37 Cl and for δ 18 O and δ 13 C. The results show that unaltered carbonatites have δ 37 Cl ratios between −0.8 and +0.1‰. Because the whole mantle δ 37 Cl value is approximately +4.7‰, chlorine for carbonatite is anomalously light. Although it is possible that the source regions for carbonatites represent isotopic anomalous mantle and may reflect contamination by a crustal component, our preferred explanation is that the processes resulting in the accumulation of carbonatite magma cause fractionation of chlorine isotopes.