Low temperature phase equilibria in the Fe-Ni and Fe-Ni-P systems: application to the thermal history of metallic phases in meteorites

The solubility limits of the α (kamacite) and γ (taenite) phases in the Fe-Ni and Fe-Ni-P phase diagrams have been measured at low temperatures, 700-300°C. The predicted $\text{α}\text{α}\text{+ γ}$ retrograde solubility below 500°C was demonstrated experimentally for the first time in the Fe-Ni system. The minimum solubility of Ni in γ at the $\text{γ}\text{α}\text{+γ}$ boundary increases with decreasing temperature to as much as 54 wt% at 300°C. The addition of P increases the maximum solubility of Ni in α by as much as 1.6 wt% and decreases the minimum solubility of Ni in γ by as much as 7 wt% at 300°C.The solubility limits of kamacite and taenite were also obtained from heat-treated samples of the Grant and Cape York iron meteorites. The data indicate that in iron meteorites minor and trace elements other than P do not significantly shift the Ni solubility limits of the Fe-Ni and Fe-Ni-P phase diagrams. The measured phase diagrams can be used to explain the Agrell effect and the differences in maximum Ni content of taenite among irons and chondrites. The formation of plessite and the influence of the measured solubility limits on the cooling rate simulation method are also considered.     

Experimental determination of the effects of pressure and temperature on the stoichiometry and phase relations of wüstite

When quenched metastable wüstite (Fe_.924O and Fe_.947O) is held at 300°C at pressures up to 200 kbar in a diamond anvil cell, a mixture of magnetite, metallic iron and wüstite is found. We interpret this to indicate that magnetite plus metallic iron constitute the stable phase assemblage at pressures and temperatures below this boundary is stoichiometric FeO ($\text{a}{}_0\text{= 4.332 ± 0.001}\text{A}\text{?}$) at pressures below 110 kbar at 300°C. However, just below the boundary in the pressure range 110 kbar to 200 kbar at 300°C, the residuál wüstite is non-stoichiometric ($\text{a}{}_0\text{< 4.332}\text{A}\text{?}$). Data collected at pressures and temperatures above the boundary indicate that non-stoichiometric wüstite (Fe_xO) plus metallic iron constitute the stable phase assemblage and that the value of x in Fe_xO increases as pressure is increased isothermally to 100 kbar and then decreases as pressure is increased above 100 kbar.     

Analytical electron microscopy study of the plessite structure in four IIICD iron meteorites

Electron optical techniques were employed to investigate the plessite structure and composition of four IIICD fine octahedrites. These meteorites have a similar thermal history and differences in plessite structure can be ascribed to varying bulk Ni content and/or localized differences in carbon content. Microdiffraction patterns from regions as small as 20 nm dia. were obtained for the first time from plessite structures. It was established that transformation twins in clear taenite I have the conventional fcc twin relationship, individual kamacite and taenite cells in the cloudy zone have the Kurdjumov-Sachs orientation and fine γ rods in the decomposed martensite zone display both the Nishiyama and Kurdjumov-Sachs relation with the matrix-α. All the IIICD irons contain cloudy zone and martensitic plessite. Except for Dayton, martensitic plessite shows further decomposition into α + λ at low temperatures. Using STEM X-ray microanalysis with a spatial resolution of ~ 50 nm, Ni composition profiles in taenite from all the IIICD irons showed a maximum of ~48 wt% Ni. The structural and compositional data indicate that plessite formation occurs at quite low temperatures (~ 200–300°C) during the cooling history of the IIICD irons.     

Fission track studies of xenolithic chondrites: implications regarding brecciation and metamorphism

We have studied fission tracks in phosphates from one gas-poor chondrite and three gas-rich chondrites to determine their thermal history and brecciation time scales. More than 70 percent of the tracks in whitlockites in these meteorites are due to the decay of extinct Pu²⁴⁴.Whitlockites separated from Bhola, a gas-poor chondrite, have $\text{ρ}{}_{\mathrm{Pu}}\text{ρ}{}_{\mathrm{U}}\text{= 2.6–5.2}$ and a model fission track age of 4.0 Gyr for a $\text{(}\text{Pu}\text{U}\text{)}{}_{\mathrm{4.55Gyr}}\text{= 0.045}$. Brecciation of the Bhola meteorite must have occurred at ?4.3 Gyr to account for the metal data (Scott and Rajan, 1981). A minimum cooling rate of $\text{0.9}{}_{\mathrm{–0.2}}{}^{0.3}\text{K}\text{Myr}$ in the temperature interval 800 to 300 K obtained from the track data is a factor of seven higher than the metallographic cooling rate ($\text{0.1}\text{K}\text{Myr}$).For the gas-rich chondrites, the $\text{ρ}{}_{\mathrm{Pu}}\text{ρ}{}_{\mathrm{U}}$ in whitlockites are: Weston, 32–148; Fayetteville, 21–227; and St. Mesmin, 26–137. Whitlockites from all these meteorites give model fission track ages of 4.4 Gyr assuming a $\text{(}\text{Pu}\text{U}\text{)}{}_{\mathrm{4.55\; Gyr}}\text{= 0.045}$. The final brecciation event definitely did not reset the track clock in phosphates of St. Mesmin. Our data suggest that it is also true for Weston and Fayetteville. We conclude that our observed fission track ages date the end of metamorphic cooling in the meteorite parent bodies and support the planetesimal model for the formation of xenolithic chondrites.     

Experimental hydrogen isotope studies—I. Systematics of hydrogen isotope fractionation in the systems epidote-H2O,zoisite-H2O and AlO(OH)-H2O

$\text{H}\text{D}$ Fractionation factors between epidote minerals and water, and between the AlO(OH) dimorphs boehmite and diaspore and water, have been determined between 150 and 650°C. Small water mineral ratios were used to minimise the effect of incongruent dissolution of epidote minerals. Waters were extracted and analysed directly by puncturing capsules under vacuum. Hydrogen diffusion effects were eliminated by using thick-walled capsules.$\text{H}\text{D}$ Exchange rates are very fast between epidote and water (and between boehmite and water), complete exchange taking only minutes above 450°C but several months at 250°C. Exchange between zoisite and water (and between diaspore and water) is very much slower, and an interpolation method was necessary to determine fractionation factors at 450 and below.For the temperature range 300–650°C, the $\text{H}\text{D}$ equilibrium fractionation factor (α^e) between epidote and water is independent of temperature and Fe content of the epidote, and is given by 1000 In α_epidote-H2O^e = ?35.9 ± 2.5, while below 300°C 1000 In , with a ‘cross-over’ estimated to occur at around 185°C. By contrast, zoisite-water fractionations fit the relationship 1000 In .All studied minerals have hydrogen bonding. Fractionations are consistent with the general relationship: the shorter the O-H -- O bridge, the more depleted is the mineral in D.On account of rapid exchange rates, natural epidotes probably acquired their H-isotope compositions at or below 200°C, where fractionations are near or above 0%.; this is in accord with the observation that natural epidotes tend to concentrate D relative to other coexisting hydrous minerals.     

A systematic deviation from Na-K-Ca geothermometer below 75°C and above 10−4 atm Pco2

If the temperature of ground water is below 75°C and the partial pressure of CO₂ in the aquifer is above 10^?4 atm, a chemical steady-state between water and felsic rocks (rather than chemical equilibrium) may be maintained. The temperature of water in the aquifer may be estimated using a modified form of the Na-K-Ca geothermometer from, . where the departure of the steady-state from equilibrium, $\text{I}$, is a function of : .     

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.     

Stenols and stanols in the oxic and anoxic waters of the Black Sea

Water samples collected from a slope station and two deep stations in the western basin of the Black Sea were analyzed for stenols and stanols by glass capillary gas chromatography. These results were used in conjuction with hydrographic, particulate organic carbon, and chlorophyll a data to better understand sterol sources and their transport and transformation mechanisms in anoxic basins.The total free sterol concentrations found in the surface waters were 450–500 ng/l dropping rapidly to values well below 100 ng/l at depths below the $\text{O}{}_2\text{H}{}_2\text{S}$ interface. In the upper 200 m of the water column a strong association of sterols with particulate matter is suggested. Structural elucidation by a gas chromatograph-mass spectrometer-computer system revealed the presence of at least sixteen different stenols and stanols in the surface waters of the Black Sea. Cholesterol, 24-methylenecholesterol and 24-methylcholesta-5,22-dien-3β-ol were the major sterols in the surface waters. Cholesterol and 24-ethylcholesterol both exhibited a subsurface maximum at the $\text{O}{}_2\text{H}{}_2\text{S}$ interface. In the anoxic deep waters (200–2000 m) only cholesterol and 24-ethylcholesterol were found. Two stenols were found that have not been reported in seawater: a C₂₆ stenol with a saturated C₇H₁₅ side chain (presumably 24-norcholesterol) and 24-ketocholesterol. At least six 5α-stanols could be identified in the surface samples, each of them comprising about 10–20% of the concentration of the corresponding Δ5-stenol. From these comparatively high surface values the stanol concentrations drop rapidly to values near zero at the $\text{O}{}_2\text{H}{}_2\text{S}$ interface. Except for very low concentrations of 5α-cholestanol (< 4ng/l) no other stanols could be detected in the anoxic zone.From this data it appears that no detectable stenol → stanol conversion is occurring at the $\text{O}{}_2\text{H}{}_2\text{S}$ interface or in the deep anoxic waters of the Black Sea.     

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.     

Some mineral stability relations in the system CaOMgOSiO2H2OHCl

Mineral-aqueous solution equilibria for the assemblages talc-quartz, tremolite-talc-quartz, diopside-tremolite-quartz, wollastonite-diopside-quartz and wollastonite-quartz have been studied at 2 kb total pressure, 500° to 700°C and chloride concentrations from 0.03 to 6.0 molal. Most work was at 1 m chloride. Both buffered and unbuffered data were obtained and a recalibration of the Ag-AgCl buffer is presented. Log equilibrium quotients at 500°, 600° and 700°C are respectively: Ta-Qz () 2.57, 1.71, 0.73; Tr-Ta-Qz and Di-Tr-Qz () 4.98, 3.99, 2.21 and 7.29, 5.30, 3.56; WoDi-Qz () 3.30, 3.00, 2.79: Wo-Qz () 5.15, 3.95, 2.68. Mineral stability fields plotted in terms of these concentration data more tangibly represent the compositional character of real systems and the mass transfer capabilities of their fluids than do the analogous theoretical activity diagrams.Overall dissociation constants of MgCl₂ and CaCl₂ were calculated from the experimental data using the calculated ionic activity constants for the reactions and the established dissociation constants of HCl. The negative log values are respectively: 3.88. 6.63, 9.20 for CaCl₂ and 4.60, 7.54, 10.37 for MgCl₂ at 500°, 600° and 700°C, 2 kb. The Ca values are about an order of magnitude more positive than the conductance-derived values by Frantz and Marshall (1982).The phase relations developed in this study have application to the genesis of talc, tremolite, and diopside-bearing assemblages in some regional metamorphic rocks, but more specifically to the calcsilicate skarn assemblages of many metasomatic aureoles. The equilibrium fluids are characterized by high concentrations of Ca relative to Mg and increasing $\text{Ca}\text{Mg}$ ratios with decreasing temperatures. The stability fields of talc, tremolite, and quartz expand relative to those of diopside and wollastonite with decreasing temperature, hence their more common appearance as retrograde products in skarn systems.     

Ammonia-ammonium leaching of deep-sea manganese nodules

Ammonia-ammonium leaching of samples of nodules from several different locations was carried out after reduction of the nodules under $\text{CO}\text{CO}{}_2$ gas mixtures at 400, 600, and 800°C. In accordance with thermodynamic analysis, nickel, copper and cobalt oxides in the nodules are preferentially reduced with a $\text{60}\text{40}$ gas mixture of $\text{CO}\text{CO}{}_2$. After an initial reduction step with $\text{CO}\text{CO}{}_2$ at 600°C, leaching at room temperature and atmospheric pressure with aqueous ammonia-ammonium carbonate and ammonia-ammonium sulfate solutions yielded high extractions of copper and nickel (> 80%), and close to 50% for cobalt. The nature of the pores in nodules from different locations appears to affect the extraction process. A lower reduction temperature is required to obtain the same extraction of nickel, copper and cobalt in a sulfate system than is necessary in a carbonate system. However, a higher manganese content results in the sulfate leaching solutions as compared to the carbonate system, where essentially none of the manganese and iron are extracted.     

Use of Pitzer's equations to determine the media effect on the formation of lead chloro complexes

The spectrophotometric measurements of chloro complexes of lead in aqueous HCl, NaCl, MgCl₂ and CaCl₂ solutions at 25°C have been analyzed using Pitzer's specific interaction equations. Parameters for activity coefficients of the complexes PbCl⁺, PbCl₂⁰ and PbCl₃^? have been determined for the various media. Values of $\text{K}{}_1\text{= 30.0 ± 0.6}$, $\text{K}{}_2\text{= 106.7 ± 2.1}$ and $\text{K}{}_3\text{= 73.0 ± 1.5}$ were obtained for the cumulative formation constants. [$\text{Pb}{}^{\mathrm{2+}}\text{+ nCl}{}^{\mathrm{?}}\text{→ PbCl}{}_{\mathrm{n}}{}^{\mathrm{2?n}}\text{)}$]. These values are in reasonable agreement with literature data. The Pitzer parameters for the PbCl ion pairs in various media were used to calculate the speciation of Pb²⁺ in an artificial seawater solution.     

40Ar-39Ar dating of inclusions from IAB iron meteorites

Silicate and troilite from IAB iron meteorites were dated by the ⁴⁰Ar-³⁹Ar technique. Silicate from four IAB meteorites gave well-defined apparent-age plateaus which accounted for 71–99% of the released ³⁹Ar. At low temperatures, only Copiapo showed appreciable loss of ⁴⁰Ar, while Mundrabilla and Woodbine released excess ⁴⁰Ar. The plateau ages are: 4.50 Byr for Copiapo, 4.57 Byr for Mundrabilla, 4.57 Byr for Woodbine, 4.54 Byr for unetched Pitts, and 4.57 Byr for etched Pitts; the 1σ error in each case is ± 0.03 Byr. A poorly-defined age plateau for Landes gives an age of 4.48 Byr, while the total K-Ar age (4.55 Byr) is significantly higher. The average $\text{(}{}^{40}\text{Ar/}{}^{36}\text{Ar)}$_trapped ratio for all silicate samples is 0.4 ± 0.4.Simple and undisturbed K-Ar systems are rare for meteorites, yet it appears to be a common feature for IAB silicates. In addition, plateau ages of IAB silicates are as old or older than the mean age of unshocked chondrites (4.47 Byr).Troilite samples yielded complex patterns which were evaluated via ⁴⁰Ar/³⁶Ar vs ³⁹Ar/³⁶Ar plots. Data for Pitts troilite are consistent with silicate and troilite retaining Ar at about the same time initially, but then 4.25 Byr ago nearly all the Ar in troilite was redistributed. The 700–1000°C points for Mundrabilla troilite define a line which gives an age of 6.2 Byr and $\text{(}{}^{40}\text{Ar/}{}^{36}\text{Ar)}{}_{\mathrm{trapped}}\text{= 42}$. This line may be an artifact, perhaps produced by homogenization of Ar and K.Approximate estimates of cosmic-ray exposure ages are 240 Myr for Landes, 130 Myr for Copiapo, 190 Myr for Woodbine, 170 Myr for Mundrabilla troilite, and 60 Myr for Pitts troilite.The I-Xe study of these same samples revealed a good correlation between well-defined I-Xe ages of silicates and Ni contents of metal (Niemeyer, 1979). The poorer resolution of the ⁴⁰Ar-³⁹Ar technique hampers a similar evaluation; nevertheless, plateau ages of the silicates suggest a systematic trend with Ni contents.     

Total individual ion activity coefficients of calcium and carbonate in seawater at 25°C and 35%. salinity,and implications to the agreement between apparent and thermodynamic constants of calcite and aragonite

We have calculated the total individual ion activity coefficients of carbonate and calcium, and , in seawater. Using the ratios of stoichiometric and thermodynamic constants of carbonic acid dissociation and total mean activity coefficient data measured in seawater, we have obtained values which differ significantly from those widely accepted in the literature. In seawater at 25°C and 35%. salinity the (molal) values of and are $\text{0.038 ± 0.002}$ and $\text{0.173 ± 0.010}$, respectively. These values of and are independent of liquid junction errors and internally consistent with the value . By defining and on a common scale (), the product is independent of the assigned value of and may be determined directly from thermodynamic measurements in seawater. Using the value and new thermodynamic equilibrium constants for calcite and aragonite, we show that the apparent constants of calcite and aragonite are consistent with the thermodynamic equilibrium constants at 25°C and 35%. salinity. The demonstrated consistency between thermodynamic and apparent constants of calcite and aragonite does not support a hypothesis of stable Mg-calcite coatings on calcite or aragonite surfaces in seawater, and suggests that the calcite critical carbonate ion curve of Broecker and Takahashi (1978, Deep-Sea Research25, 65–95) defines the calcite equilibrium boundary in the oceans, within the uncertainty of the data.     

Rates of reaction of pyrite and marcasite with ferric iron at pH 2

The relative reactivities of pulverized samples (100–200 mesh) of 3 marcasite and 7 pyrite specimens from various sources were determined at 25°C and pH 2.0 in ferric chloride solutions with initial ferric iron concentrations of 10^?3 molal. The rate of the reaction:

$\text{FeS}{}_2\text{+ 14}\text{Fe}{}^{\mathrm{3+}}\text{+ 8}\text{H}{}_2\text{O}\text{= 15}\text{Fe}{}^{\mathrm{2+}}\text{+ 2}\text{SO}{}^{\mathrm{2?}}{}_4\text{+ 16}\text{H}{}^{\mathrm{+}}$

was determined by calculating the rate of reduction of aqueous ferric ion from measured oxidation-reduction potentials. The reaction follows the rate law:

where m_Fe³⁺ is the molal concentration of uncomplexed ferric iron, k is the rate constant and $\text{A}\text{M}$ is the surface area of reacting solid to mass of solution ratio. The measured rate constants, k, range from 1.0 × 10^?4 to 2.7 × 10^?4 sec^?1 ± 5%, with lower-temperature/early diagenetic pyrite having the smallest rate constants, marcasite intermediate, and pyrite of higher-temperature hydrothermal and metamorphic origin having the greatest rate constants. Geologically, these small relative differences between the rate constants are not significant, so the fundamental reactivities of marcasite and pyrite are not appreciably different.The activation energy of the reaction for a hydrothermal pyrite in the temperature interval of 25 to 50°C is 92 kJ mol^?1. This relatively high activation energy indicates that a surface reaction controls the rate over this temperature range. The BET-measured specific surface area for lower-temperature/early diagenetic pyrite is an order of magnitude greater than that for pyrite of higher-temperature origin. Consequently, since the lower-temperature types have a much greater $\text{A}\text{M}$ ratio, they appear to be more reactive per unit mass than the higher temperature types.     

Interstitial water chemistry in the sediments of Saanich Inlet

Measurements of nutrients and trace metals are used to examine the processes controlling their distributions in the interstitial waters of Saanich Inlet. Samples were collected using both in situ and squeezing techniques with excellent agreement. Additional measurements of porosity, organic carbon and sedimentation rate by ²¹⁰Pb are used in conjunction with the nutrient measurements to test the equation for the diagenesis of organic matter in fine-grained, organic-rich and rapidly-accumulating sediments.Organic carbon and sulfate decrease with depth in the sediment whereas ammonia and alkalinity increase. In the zone of sulfate reduction (0–20 cm) the rate constants for sulfate reduction ($\text{k}{}_{\mathrm{s}}$), ammonia production ($\text{k}{}_{\mathrm{N}}$) and organic carbon decomposition ($\text{k}{}_{\mathrm{c}}$) agree within a factor of two. Our calculations indicate, however, that this is fortuitous since the observed decrease in paniculate organic carbon is insufficient to account for the sulfate consumption. Sulfate must also be consumed by reaction with methane diffusing up from the underlying sediments. The rate constant for sulfate reduction using particulate organic carbon is lower than a modelled rate encompassing all organic species, including methane.The rate constant for ammonia production ($\text{k}{}_{\mathrm{N}}$) decreases by an order of magnitude when sulfate is completely depleted and methane production dominates.Thermodynamic calculations suggest that the interstitial waters are saturated or supersaturated with respect to all forms of iron ‘monosulfides’, apatite and rhodochrosite.     

Etude des inclusions fluides du système N2-CO2 de dolomites et de quartz de Tunisie septentrionale. Données de la microcryoscopie et de l'analyse à la microsonde à effet Raman

Optical and analytical studies were performed on 400 N₂ + CO₂ gas bearing inclusions in dolomites and quartz from Triassic outcrops in northern Tunisia. Other fluids present include brines (NaCl and KCl bearing inclusions) and rare liquid hydrocarbons. At the time of trapping, such fluids were heterogeneous gas + brine mixtures. In hydrocarbon free inclusions the $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}$ mole ratio was determined using two different non-destructive and punctual techniques: Raman microprobe analysis, and optical estimation of the volume ratios of the different phases selected at low temperatures. In the observed range of compositions, the two methods agree reasonably well.The N₂ + CO₂ inclusions are divided into three classes of composition: (a) $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{> 0,57}$: Liquid nitrogen is always visible at very low temperature and homogenisation occurs in the range ?151°C to ? 147°C (nitrogen critical temperature) dry ice (solid CO₂) sublimates between ?75°C and ?60°C; (b) $\text{0,20 <}\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{? 0,57}$: liquid nitrogen is visible at very low temperature but dry ice melts on heating; liquid and gas CO₂ homogenise to liquid phase between ?51°C to ?22°C; (c) $\text{N}{}_2\text{(N}{}_2\text{+ CO}{}_2\text{)}\text{? 0,20}$: liquid nitrogen is not visible even at very low temperature (?195°C) and liquid and gas CO₂ homogenise to liquid phase between ?22°C and ?15°C. The observed phases changes are used to propose a preliminary phase diagram for the system CO₂-N₂ at low temperatures.Assuming additivity of partial pressures, isochores for the CO₂-N₂ inclusions have been computed. The intersection of these isochores with those for brine inclusions in the same samples may give the P and T of trapping of the fluids.     

Changes in the content and composition of pedogenic iron oxyhydroxides in a chronosequence of soils in southern California

Studies of the pedogenic iron oxyhydroxides in suites of latest Holocene to middle Pleistocene soils formed on fluvial deposits of the transverse ranges, southern California, indicate that the content and composition of iron oxyhydroxide change in a systematic manner. Analysis of total secondary free iron oxides (dithionite extractable, Fe₂O₃d) and ferrihydrite (oxalate extractable, Fe₂O₃o) shows that (1) a single-logarithmic model (Y = a + b log X) or double logarithmic model (log Y = a + b log X), where Y is the total mass of pedogenic Fe oxides (g/cm²-soil column) and X is soil age, describes the rate of increase in Fe₂O₃d with time; (2) the Fe₂O₃d content correlates linearly with soil reddening and clay content; (3) the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio, which indicates the degree of Fe oxide crystallinity, is moderately high to very high (0.22–0.58) in middle Holocene to latest Pleistocene soils and progressively decreases to less than 0.10 in older soils; (4) the value of the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio also appears to be infuenced by climate; and (5) temporal changes in Fe oxide content and mineralogy are accompanied by related, systematic changes in clay mineralogy and organic matter content. These relationships are attributed to a soil environment that must initially favor ferrihydrite precipitation and/or organic matter-Fe complexation. Subsequent transformation to hematite causes increasingly intense reddening and a concomitant decrease in the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio. The results demonstrate that iron oxide analysis is useful for numerical age studies of noncalcic soils and shows potential as an indicator of paleoclimates.     

Liquidus relationships in the system CaCO3Ca(OH)2CaS and the solubility of sulfur in carbonatite magmas

CaCO₃Ca(OH)₂CaS serves as a model system for sulfide solubility in carbonatite magmas. Experiments at 1 kbar delineate fields for primary crystallization of CaCO₃, Ca(OH)₂ and CaS. The three fields meet at a ternary eutectic at 652°C with liquid composition (wt%): CaCO₃ = 46.1%, Ca(OH)₂ = 51.9%, CaS = 2.0%. Two crystallization sequences are possible for liquids that precipitate calcite, depending upon whether the liquid is on the low-CaS side, or the high-CaS side of the line connecting CaCO₃ to the eutectic liquid. Low-CaS liquids precipitate no sulfide until the eutectic temperature is reached leading to sulfide enrichment. The higher-CaS liquids precipitate some sulfide above the eutectic temperature, but the sulfide content of the melt is not greatly depleted as the eutectic temperature is approached. Theoretical considerations indicate that sulfide solubility in carbonate melts will be directly proportional to and inversely proportional to ; it also is likely to be directly proportional to melt basicity, defined here by . A strong similarity exists in the processes which control sulfide solubility in carbonate and in silicate melts. By analogy with silicates, ferrous iron, which was absent in our experiments, may also exert an important influence on sulfide solubility in natural carbonatite magmas.     

Estimation of the dielectric constant of H2O from experimental solubilities of quartz,and calculation of the thermodynamic properties of aqueous species to 900°C at 2 kb

Experimental quartz solubilities in H₂O (Anderson and Burnham, 1965, 1967) were used together with equations of state for quartz and aqueous species (Helgesonet al., 1978; Walther and Helgeson, 1977) to calculate the dielectric constant of H₂O () at pressures and temperatures greater than those for which experimental measurements (Heger, 1969; Lukashovet al., 1975) are available ($\text{0.001 ? P ? 5}\text{kb}$ and $\text{0 ? T ? 600°C}$). Estimates of computed in this way for 2 kb (which are the most reliable) range from 9.6 at 600°C to 5.6 at 800°C. These values are 0.5 and 0.8 units greater, respectively, than corresponding values estimated by Quist and Marshall (1965), but they differ by <0.3 units from extrapolated values computed from Pitzer's (1983) adaptation of the Kirkwood (1939) equation. The estimates of generated from quartz solubilities at 2 kb were fit with a power function of temperature, which was then used together with equations and data given by Helgeson and Kirkham (1974a,b, 1976) Helgesonet al. (1981), and Helgeson (1982b, 1984) to calculate Born functions, Debye Hückel parameters, and the thermodynamic properties of Na⁺, K⁺, Mg⁺⁺, Ca⁺⁺, and other aqueous species of geologic interest at temperatures to 900°C.