The solubility of calcite,strontianite and witherite in NaCl solutions at 25°C

The solubility of CaCO₃ (calcite), SiCO₃ (strontianite), and BaCO₃ (witherite) has been determined in NaCl solutions from 0.1 to 6 m at 25°C. Activity coefficients estimated from Pitzer's equations with higher order interaction terms (θ and Ψ) were used to extrapolate the results to infinite dilution. Thermodynamic values of $\text{pK}{}_{\mathrm{sp}}\text{= 8.46 ± 0.03,9.13 ± 0.03}$ and $\text{8.56 ± 0.04}$ were found, respectively, for CaCO₃, SrCO₃ and BaCO₃ at 25°C. These results are in reasonable agreement with literature data. Since Pitzer parameters for the interactions of CO₃^2? with Ca²⁺, Sr²⁺ and Ba²⁺ were not used, our results indicate that they are not necessary at low values of $\text{P}{}_{\mathrm{co2}}$.     

Diffusion of 40Ar in biotite: Temperature,pressure and compositional effects

The measured radiogenic ⁴⁰Ar loss from sized biotite (56% annite) samples following isothermalhydrothermal treatment have provided model diffusion coefficients in the temperature interval 600°C to 750°C, calculated on the assumption that Ar transport proceeds parallel to cleavage. These data yield an array on an Arrhenius plot with a slope corresponding to an activation energy 47.0 ± 2 kcal-mol^?1 and a frequency factor of 0.077^+0.21_?0.06 cm²-sec^?1. Together with previous diffusion data for micas in the annitephlogopite series, our results indicate a strong compositional effect, with increasing $\text{Fe}\text{Mg}$ ratio corresponding to an increase in diffusivity. An effective diffusion radius of about 150 μm for biotite is inferred from the experimental data which compares favorably with that estimated from geological studies. A pressure effect on activation energy corresponding to an activation volume of about 14 cm³-mol^?1 is observed. These data yield closure temperature estimates for this biotite composition cooling at rates of 100°C-Ma^?1, 10°C-Ma^?1 and 1°C-Ma^?1 of 345°C, 310°C and 280°C, respectively. ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age-spectrum analysis of a hydrothermally treated biotite yields a complex release pattern casting doubt on the general usefulness of such measurements for geochronological purposes.     

An electrochemical study of Ni2+, Co2+, and Zn2+ ions in melts of composition CaMgSi2O6

Cyclic voltammetry has been done for Ni²⁺, Co²⁺, and Zn²⁺ in melts of diopside composition in the temperature range 1425 to 1575°C. Voltammetric curves for all three ions excellently match theoretical curves for uncomplicated, reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain dissolved in the melt. The reference electrode is a form of oxygen electrode. Relative to that reference assigned a reduction potential of 0.00 volt, the values of standard reduction potential for the ions are $\text{E}{}^1\text{(}\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.32 ± .01}\text{V}$, $\text{E}{}^1\text{(}\text{Co}{}^{\mathrm{2+}}\text{Co}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.45 ± .02}\text{V}$, and $\text{E}{}^1\text{(}\text{Zn}{}^{\mathrm{2+}}\text{Zn}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.53 ± .01}\text{V}$. The electrode reactions are rapid, with first order rate constants of the order of 10^?2 cm/sec. Diffusion coefficients were found to be 2.6 × 10^?6 cm²/sec for Ni²⁺, 3.4 × 10^?6 cm²/sec for Co²⁺, and 3.8 × 10^?6 cm²/sec for Zn²⁺ at 1500°C. The value of $\text{E1}$ ($\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0$, diopside) is a linear function of temperature over the range studied, with values of ?0.35 V at 1425°C and ?0.29 V at 1575°C. At constant temperature the value of $\text{E}{}^1\text{(}\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0\text{, 1525°C}$) was not observed to vary with composition over the range CaO · MgO · 2SiO₂ to CaO·MgO·3SiO₂ or from 1.67 CaO·0.33MgO·2SiO₂ to 0.5 CaO·1.5MgO·2SiO₂. The value for the diffusion coefficient for Ni²⁺ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO₂ to CaO · MgO · 3SiO₂. This is consistent with a mechanism by which Ni²⁺ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni²⁺ species discharging at the cathode regardless of the SiO₂ concentration in the melt.     

Total nitrogen content of deep sea basalts

Various models have been suggested concerning the origin and evolution of the earth's atmosphere. An estimate of the nitrogen content of the mantle could further constrain atmospheric models. Total nitrogen content was determined by thermal neutron activation analysis via ¹⁴N(n,p)¹⁴C. The ¹⁴C was converted to carbon dioxide and counted in miniature low level proportional counters. The total nitrogen content of U.S.G.S. standards BCR-1 and G-2 as determined by different laboratories is variable, probably due to atmospheric adsorption by the finely ground samples. Total nitrogen content was determined in deep sea basalt glasses from three regions: East Pacific Rise ($\text{15 ± 4, 18 ± 4}$, and $\text{7 ±}\text{ppm}\text{2 N}$), Mid-Atlantic Rift (FAMOUS Region:$\text{22 ± 5, 18 ± 3}$, and $\text{10 ± 2}\text{ppm}\text{N}$) and the Juan de Fuca Ridge ($\text{17 ± 4}\text{ppm}\text{N}$). Matrix material from the same samples as the glasses was available from the East Pacific Rise ($\text{37 ± 6, 26 ± 4}$, and $\text{34 ± 6}\text{ppm}\text{N}$) and the Mid-Atlantic Rift ($\text{39 ± 4}\text{ppm}\text{N}$) which are about 50 to 100% greater than the associated glasses. The increased matrix abundance may be due to incorporation of chemically bound nitrogen from sea water rather than dissolved molecular nitrogen. The nitrogen content of the FAMOUS samples are inconsistent with the model of Langmuir et al. (1977) for petrogenesis based on trace element data. Factors which can affect the observed nitrogen content in the basalts and the interpretation in terms of the mantle nitrogen abundance are discussed (e.g. partial melting and degassing of the basalts). A lower limit of about 2 ppm N in the mantle can be estimated.     

Mineralogical changes occurring during the retrogression of Archaean gneisses from the Lewisian complex of NW Scotland

Gneisses, metamorphosed at granulite facies ca 2.7 Ga, were subsequently retrogressed to amphibolite facies during a prolonged period of retrogression, perhaps lasting as long as 200 m.y. The Scourie dykes were emplaced towards the end of this event. Localised Laxfordian shear zones further modified the mineral assemblages. The retrogression caused the production of a uniform $\text{plagioclase-hornblende}\text{- ±}\text{quartz}\text{±}\text{biotite}$ assemblage. A study of hornblende composition shows that it depends on metamorphic grade, host rock composition and paragenesis. The sequence of mineral assemblages suggests that retrogression took place on a falling temperature path, beginning at about $\text{650±50°}\text{C}$. Post-tectonic muscovite indicates that temperatures were still in excess of 500°C after the formation of Laxfordian shear zones. This indicates that the Lewisian complex was uplifted and cooled extremely slowly.     

Fluid-mineral equilibria in a hydrothermal system,Roosevelt hot springs,Utah

The availability of fluids and drill cuttings from the active hydrothermal system at Roosevelt Hot Springs allows a quantitative comparison between the observed and predicted alteration mineralogy, calculated from fluid-mineral equilibria relationships. Comparison of all wells and springs in the thermal area indicates a common reservoir source, and geothermometer calculations predict its temperature to be higher ($\text{288°C ± 10°}$) than the maximum measured temperature of 268°C.The composition of the deep reservoir fluid was estimated from surface well samples, allowing for steam loss, gas release, mineral precipitation and ground-water mixing in the well bore. This deep fluid is sodium chloride in character, with approximately 9700 ppm dissolved solids, a pH of 6.0, and gas partial pressures of O₂ ranging from 10^?32 to 10^?35 atm, CO₂ of 11 atm, H₂S of 0.020 atm and CH₄ of 0.001 atm.Comparison of the alteration mineralogy from producing and nonproducing wells allowed delineation of an alteration pattern characteristic of the reservoir rock. Theoretical alteration mineral assemblages in equilibrium with the deep reservoir fluid, between 150° and 300°C, in the system Na₂O-K₂O-CaO-MgO-FeO-Fe₂O₃-Al₂O₃-H₄SiO₄-H₂O-H₂S-CO₂-HCl, were calculated. Minerals theoretically in equilibrium with the calculated reservoir fluid at $\text{>240°C}$ include sericite, K-feldspar, quartz, chalcedony, hematite, magnetite and pyrite. This assemblage corresponds with observed higher-temperature ($\text{>210°C}$) alteration assemblage in the deeper parts of the producing wells. The presence of montmorillonite and mixed-layer clays with the above assemblage observed at temperatures <210°C corresponds with minerals predicted to be in equilibrium with the fluid below 240°C.Alteration minerals present in the reservoir rock that do not exhibit equilibrium with respect to the reservoir fluid include epidote, anhydrite, calcite and chlorite. These may be products of an earlier hydrothermal event, or processes such as boiling and mixing, or a result of errors in the equilibrium calculations as a result of inadequate thermochemical data.     

Propriétés thermodynamiques des solutions (Na,K)ClH2O entre 400 et 800°C, 1–2 Kb: une revue des équilibres d'échange avec les silicates sodipotassiques

Several independant determinations of the difference in Gibbs free energy of formation (from the elements at 25°, 1 bar) between NaCl⁰ and KCl⁰ in aqueous solutions (molality > 0.5) are derived from equilibrium data between alkali feldspars, feldspathoids (nepheline-kalsilite), micas (muscoviteparagonite) and hydrothermal (Na, K)Cl-H₂O solutions. These results along with other data from the literature are discussed. The relation: $\text{Δ}{}^0\text{?,}\text{KCl}{}^0\text{? Δ}{}^0\text{/tf,}\text{NaCl}{}^0\text{(}\text{J}\text{) = ?16500(± 2500) ? 18(± 4) T(}\text{K}\text{)}$. is proposed from 400 to 800°C and 1 to 2 Kbar.     

Armalcolite stability as a function of pressure and oxygen fugacity

The stability of synthetic armalcolite of composition (Fe_0.5Mg_0.5Ti₂O₅ was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (?O₂) at 1200°C and 1 atm total pressure. The high pressure experiments were carried out in a piston-cylinder apparatus using silver-palladium containers. At 1200°C, armalcolite is stable as a single phase at 10 kbar. With increasing pressure, it breaks down ($\text{dT}\text{dP}\text{= 20°C/}\text{kbar}$), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way ($\text{dT}\text{dP}\text{= 30°C/}\text{kbar}$) to a two-phase assemblage of rutile plus ilmenite solid solution.A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO₂ appears to saturate armalcolite at 1200°C and 1 atm. The breakdown of Zr-armalcolite occurs at pressures of 1–2 kbar less than those required for the breakdown of Zr-free armalcolite. The zirconium partitions approximately equally between rutile and ilmenite phases.The stability of armalcolite as a function of ?O₂ was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe_0.5Mg_0.5)-Ti₂O₅, is stable over a range ?O₂ from about 10^?9.5to 10^?10.5 atm. Below this range to at least 10^?12.8 atm, ilmenite plus a reduced armalcolite are formed. These products were observed optically and by Mössbauer spectroscopy, and no metallic iron was detected; therefore, some of the titanium must have been reduced to Ti³⁺. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites.     

A note on the chemistry of seawater in the range 350°–500°C

The chemistry of seawater at conditions of 350° to 500°C, 220 to 1000 bars (22 to 100 MPa) is controlled by reactions involving magnesium hydroxide sulfate (MHSH) and anhydrite. During progressive heating from 350° to 500°C at 1000 bars (100 MPa), MHSH with a $\text{Mg}\text{SO}{}_4$ ratio of 1.25 is formed via precipitation from solution and via reaction of solution with pre-existing anhydrite. During adiabatic expansion the MHSH extracts additional SO₄ from seawater and converts to a stoichiometry in which $\text{Mg}\text{SO}{}_4\text{= 1.16}$. These reactions control and greatly change the concentrations of Ca, Mg, SO₄ in solution and produce significant ionizable hydrogen, attaining 11.7 mmoles kg^?1 at maximum conditions.     

Pressure dependence ot SrSO4 solubility

The solubilities of SrSO₄ in seawater, 0.65 M NaCl and and distilled water were measured as a function of pressure at 2°C. The thermodynamic solubility product was determined from the distilled water measurements and stoichiometric solubility products were determined from the seawater and Nad measurements. The equilibrium quotient for SrSO₄ dissolution at ionic strength of 0.65 was calculated from the NaCl measurements, using the known NaSO₄^? ionpairing association constant. For each of the solubility products values of $\text{Θ V}$ were determined. These experimental values were all $\text{11.0 ± 0.3}\text{ml}$ mole_? lower than the theoretical values based on anhydrous SrSO₄. This difference may be due to the equilibrating solid phase being a hydrated form of SrSO₄.     

Solubility product of galena at 298°K: A possible explanation for apparent supersaturation in nature

The synthetic chelating agent ethylenediaminetetraacetic acid (EDTA) has been used to evaluate the stoichiometric solubility product of galena (PbS) at 298°K: $\text{K}{}_{\mathrm{s2}}\text{=}{}^{\mathrm{a}}\text{Pb}{}^{\mathrm{2+}}{}^{\mathrm{a}}\text{HS}{}^{\mathrm{?}}{}^{\mathrm{a}}\text{H}{}^{\mathrm{+}}$ This method circumvents the possible uncertainties in the stoichiometry and stability of lead sulfide complexes. At infinite dilution, $\text{Log K}{}_{\mathrm{s2}}\text{= ?12.25 ±0.17}$, and at an ionic strength corresponding to seawater ($\text{I = 0.7 M}$), $\text{Log K}{}_{\mathrm{s2}}\text{= ?11.73 ± 0.05}$. Using the value of $\text{K}{}_{\mathrm{s2}}$ at infinite dilution, and the free energies of formation of HS^? and Pb²⁺ at 298°K (literature values), the free energy of formation of PbS at 298°K is computed to be $\text{?79.1 ± 0.8 KJ/mol (?18.9 Kcal/mol)}$. Galena is shown to be more than two orders of magnitude more soluble than indicated by calculations based on previous thermodynamic data.     

Complex formation in the ternary system Mg(II)-CO2-H2O

The carbonato and hydrogencarbonato complexes of Mg²⁺ were investigated at 25 and 50° in solutions of the constant ClO₄^? molality (3 M) consisting preponderantly of NaClO₄. The experimental data could be explained assuming the following equilibria: Mg²⁺ + CO_2B + H₂O ag MgHCO⁺₃ + H⁺, $\text{log}{}^1\text{β}{}_1\text{= ?7.64}{}_4\text{± 0.01}{}_7\text{(25°), ?7.46}{}_2\text{± 0.01}{}_1\text{(50°)}$, Mg²⁺ + 2 CO_2g + 2 H₂Oag Mg(HCO₃)⁰₂ ± 2 H⁺, $\text{log}{}^1\text{β}{}_2\text{= ?15.00 ± 0.14 (25°)}$, ?15.37 ± 0.39 (50°), Mg²⁺ + CO_2g + H₂Oag MgCO⁰₃ + 2 H⁺, $\text{log}{}^1\text{k}{}_1\text{= ?15.64 ± 0.06 (25°)}$,?15.23 ± 0.02 (50°), with the assumption γ_MgCO3⁰ = γ_Mg(HCO3)⁰2, ΔG⁰(I = 0) for the reaction MgCO⁰₃ + CO_2g + H₂O = Mg(HCO₃)⁰₂ was estimated to be ?3.9₁ ± 0.8₆ and 0.6 ± 2.4 kJ/mol at 25 and 50°C, respectively. The abundance of carbonate linked Mg(II) species in fresh water systems is discussed.     

Analyse isotopique de l'air piégé dans la glace pour quantifier les variations de température

Isotopic measurements in polar ice core have shown a succession of rapid warming periods during the last glacial period over Greenland. However, this method underestimates the surface temperature variations. A new method based on gas thermal diffusion in the firn manages to quantify surface temperature variations through associated isotopic fractionations. We developed a method to extract air from the ice and to perform isotopic measurements to reduce analytical uncertainties to 0.006 and 0.020$\text{‰}$ for δ¹⁵N and δ⁴⁰Ar. It led to a 16±1.5 °C surface temperature variation during a rapid warming ($\text{?70}\text{000}$ yr). To cite this article: A. Landais et al., C. R. Geoscience 336 (2004).     

Sorption of noble gases by solids,with reference to meteorites. I. Magnetite and carbon

To simulate trapping of meteoritic noble gases by solids, 18 samples of Fe₃O₄ were synthesized in a noble gas atmosphere at 350–720 K by the reactions: 3Fe + 4H₂O → Fe₃O₄ + 4H₂ (Ne, Ar, Kr, Xe) 3Fe + 4CO → Fe₃O₃ + 4C + carbides (Xe only) Phases were separated by selective solvents (HgCl₂, HCl). Noble gas contents were analyzed by mass spectrometry, or, in runs where 36 d Xe¹²⁷ tracer was used, by γ-counting. Surface areas, as measured by the BET method, ranged from 1 to 400 m²/g. Isotopic fractionations were below the detection limit of 0.5%/m.u.Sorption of Xe on Fe₃O₄ and C obeys Henry's Law between $\text{1 × 10}{}^{\mathrm{?8}}$ and $\text{4 × 10}{}^{\mathrm{?5}}$ atm, but shows only a slight temperature dependence between 650 and 720 K ($\text{ΔH}{}_{\mathrm{sol}}\text{= ?4 ± 2}\text{kcal/mole}$). The mean distribution coefficient K_Xe is $\text{0.28 ± 0.09}$ cc STP/g atm for Fe₃O₄ and only a factor of $\text{1.2 ± 0.4}$ greater for C; such similarity for two cogenetic phases was predicted by Lewis et al. (1977). Stepped heating and etching experiments show that 20–50% of the total Xe is physically adsorbed and about 20% is trapped in the solid. The rest is chemisorbed with $\text{ΔH}{}_{\mathrm{s}}\text{? ?13}\text{kcal/mole}$. The desorption or exchange half-time for the last two components is >10² yr at room temperature.Etching experiments showed a possible analogy to “Phase $\text{Q}$” in meteorites. A typical carbon + carbide sample, when etched with HNO₃, lost 47% of its Xe but only 0.9% of its mass, corresponding to a ~0.6 Å layer. Though this etchable, surficial gas component was more thermolabile than $\text{Q}$ (release $\text{T}$ below 1000°C, compared to 1200–1600°C), another experiment shows that the proportion of chemisorbed Xe increases upon moderate heating (1 hr at 450°C). Apparently adsorbed gases can become “fixed” to the crystal, by processes not involving volume diffusion (recrystallization, chemical reaction, migration to traps, etc.). Such mechanisms may have acted in the solar nebula, to strengthen the binding of adsorbed gases.Adsorbed atmospheric noble gases are present in all samples, and dominate whenever the noble gas partial pressure in the atmosphere is greater than that in the synthesis. Many of the results of Lancet and Anders (1973) seem to have been dominated by such an atmospheric component; others are suspect for other reasons, whereas still others seem reliable. When the doubtful samples of Lancet and Anders are eliminated or corrected, the fractionation pattern—as in our samples—no longer peaks at Ar, but rises monotonically from Ne to Xe. No clear evidence remains for the strong temperature dependence claimed by these authors.     

Solubilities of some hydrous REE phosphates with implications for diagenesis and sea water concentrations

Solubility product determinations suggest that the hydrous phosphates of the rare earths, REPO₄ · $\text{xH}{}_2\text{O}$, are important in controlling the sea water REE concentrations. Two of these solids, rhabdophane, (P6₂22) and “hydrous xenotime”, (14₁/amd), have been synthesized at 100°C via the acid hydrolysis of the respective REE pyrophosphate. The solubility products at infinite dilution were determined to be $\text{pK}{}^0\text{= 24.5}$, (La at 25°C); 26.0, (Pr at 100°C); 25.7, (Nd at 100°C): and 25.5, (Er at 100°C). On the basis of calculations involving the reaction of RE³⁺ with apatite to form the hydrous phosphate, the lanthanum concentration in sea water is predicted to be about 140 pmol/L. Laboratory experiments support the hypothesis that apatite is a substrate for reactions with dissolved REE.     

Methane-producing bacteria: natural fractionations of the stable carbon isotopes

The ${}^{13}\text{C}{}^{12}\text{C}$ fractionation factors ($\text{CO}{}_2\text{CH}{}_4$) for the reduction of CO₂ to CH₄ by pure cultures of methane-producing bacteria are, for Methanosarcina barkeri at 40°C, 1.045 ± 0.002; for Methanobacterium strain M.o.H. at 40°C, 1.061 ± 0.002; and, for Methanobacterium thermoautotrophicum at 65°C, 1.025 ± 0.002. These observations suggest that the acetic acid used by acetate dissimilating bacteria, if they play an important role in natural methane production, must have an intramolecular isotopic fractionation ($\text{CO}{}_2\text{H}\text{CH}{}_3$) approximating the observed $\text{CO}{}_2\text{CH}{}_4$ fractionation.     

40Ar-39Ar ages and trace element contents of Apollo 14 breccias; an interlaboratory cross-calibration of 40Ar-39Ar standards

Rare gas data are presented from step-wise heatings of lunar breccias 14066 and 14318 and from an interlaboratory cross-calibration of five standards used in ⁴⁰Ar-³⁹Ar dating. Four samples of 14066 all show depressed ⁴⁰¹Ar/³⁹¹Ar ratios at high temperatures, thus making age interpretation uncertain. While different in detail, the Ar release patterns in the four samples yield indistinguishable plateau ages of $\text{3.93 ± 0.03}$ b.y. and > 400°C total ages of $\text{3.87 ± 0.06}$ b.y. Concentrations of K, Ca, Ba, Br, U and I are given for 14318 and 14066. We also present an updating of all of the ⁴⁰Ar-³⁹Ar ages and trace element concentrations previously published by this laboratory.⁴⁰Ar-³⁹Ar dating standards from Menlo Park, Pasadena, Stony Brook, Toronto and Berkeley are calibrated against each other and the internal homogeneity of their ⁴⁰¹Ar/K ratios is tested. The Berkeley standard (from the St. Severin meteorite) has an age of $\text{4.504 ± 0.020}$ b.y. from this intercalibration.⁸⁰Kr from capture of lunar neutrons is detected in 14318. A comparison of the release pattern of the ⁸⁰Kr produced by lunar neutrons with the ^80,82Kr produced by pile neutrons in 14318, indicates that 14318 has lost approximately 35 per cent of the ⁸⁰Kr produced by lunar neutrons.     

The metal content of the eucrite parent body: constraints from the partitioning behavior of tungsten

The solid metal/silicate melt partition coefficient for W has been determined experimentally to have a value of 25 ± 5 at 1190°C and an oxygen fugacity of 10^?13.4, the temperature and oxygen fugacity conditions at which eucritic basalts formed. Given this partition coefficient, scenarios for the metal content and evolution of the eucrite parent body (EPB) are constructed to explain the reduction by a factor of 30, relative to the chondrites, of the $\text{W}\text{La}$ ratio in the eucrites.A possible model for the early geologic history of the EPB begins with accretion of a parent body, chondritic in composition with respect to nonvolatile siderophile and lithophile elements. The solid metal content was between 2% and 10%, which is within the range observed in the ordinary chondrites. Subsequent heating of the EPB caused the metal phase to separate and become isolated from the silicate phases before the degree of partial melting of the silicates reached 4% to 5%. Equilibrium partitioning of most of the W into the solid metal phase at low degrees of partial melting reduced the $\text{W}\text{La}$ ratio in the remaining silicates. Continued partial melting of the silicates generated primary eucritic magmas which recorded the reduced $\text{W}\text{La}$ ratio.     

The thermal significance of potassium feldspar K-Ar ages inferred from 40Ar39Ar age spectrum results

${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectrum analyses of three microcline separates from the Separation Point Batholith, northwest Nelson, New Zealand, which cooled slowly (~5°C-Ma^?1) through the temperature zone of partial radiogenic ⁴⁰Ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages (~80 Ma) corresponding to the time that the samples cooled below about 100°C. The last 35 percent of ³⁹Ar released from the microclines yields plateau ages (103,99 and 93 Ma) which reflect the different bulk mineral ages, and correspond to cooling temperatures between about 130 to 160°C. Theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates ≤5°C-Ma^?1. Diffusion parameters calculated from the ³⁹Ar release yield an activation energy, E = 28.8 ± 1.9 kcal-mol^?1, and a frequency factor/grain size parameter, $\text{D}{}_0\text{l}{}^2\text{= 5.6}{}_{\mathrm{?3.9}}{}^{\mathrm{+14}}\text{sec}{}^{\mathrm{?1}}$. This Arrhenius relationship corresponds to a closure temperature of 132 ± 13°C which is very similar to the independently estimated temperature. From the observed diffusion compensation correlation, this $\text{D}{}_0\text{l}{}^2$ implies an average diffusion half-width of about 3 μm, similar to the half-width of the perthite lamellae in the feldspars. The range in microcline K-Ar ages from the Separation Point Batholith is the result of relatively small temperature differences within the pluton during cooling. Comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous K-feldspars (E = 40 to 52 kcal-mol^?1) reveals that Ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state. Previously published U-shaped age spectra are probably the result of the superimposition of excess ⁴⁰Ar upon diffusion profiles of the kind described here.