Daniel F. Weill

Partition of Sr, Ba, Ca, Y, Eu2+, Eu3+, and other REE between plagioclase feldspar and magmatic liquid: an experimental study

Plagioclase feldspar/magmatic liquid partition coefficients for Sr, Ba, Ca, Y, Eu2+, Eu3+ and other REE have been determined experimentally at 1 atm total pressure in the temperature range 1150–1400°C. Natural and synthetic melts representative of basaltic and andesitic bulk compositions were used, crystallizing plagioclase feldspar in the composition range An35–An85. Partition coefficients for Sr are greater than unity at all geologically reasonable temperatures, and for Ba are less than unity above approximately 1060°C. Both are strongly dependent upon temperature. Partition coefficients for the trivalent REE are relatively insensitive to temperature. At fixed temperature they decrease monotonically from La to Lu. The partition of Eu is a strong function of oxygen fugacity. Under extreme reducing conditions DEu approaches the value of DSr.

An igneous plagioclase thermometer

An empirical approach has been taken to develop a geothermometer based on plagioclase-magmatic liquid equilibrium. Compositions of coexisting plagioclase and liquid (glass) obtained by electron microprobe analysis of quenched samples from equilibrium melting experiments of natural granitic rocks at water pressures of 0.5 and 1.0 kilobars have been used along with data from the equilibrium experiments of Bowen (1913, 1915), Prince (1943) and Yoder et al. (1957) to calibrate this geothermometer. Applications of this geothermometer to natural occurrences demonstrate that it can provide useful information on temperature of equilibration of coexisting plagioclase and liquid in rocks ranging in composition from basalt to rhyolite. The plagioclase geothermometer is in good general agreement with other geothermometers wherever these are applicable. Where temperatures are known from other sources it can be used to predict the equilibrium compositions of plagioclase in magmas as well as to provide a rough estimate of water pressure.

Europium Anomaly in Plagioclase Feldspar: Experimental Results and Semiquantitative Model

The partition of europium between plagioclase feldspar and magmatic liquid is considered in terms of the distribution coefficients for divalent and trivalent europium. A model equation is derived giving the europium anomaly in plagioclase as a function of temperature and oxygen fugacity. The model explains europium anomalies in plagioclase synthesized under controlled laboratory conditions as well as the variations of the anomaly observed in natural terrestrial and extraterrestrial igneous rocks.

Thermochemistry of glasses and liquids in the systems CaMgSi2O6-CaAl2Si2O8-NaAlSi3O8, SiO2-CaAl2Si2O8-NaAlSi3O8 and SiO2-Al2O3-CaO-Na2O

Enthalpies of solution in 2PbO· B2O3 at 712°C have been measured for glasses in the systems albite anorthite diopside, NaAlO2-SiO2, Ca0.5AlO2-SiO2 and albite-anorthite-quartz. The systems albite-anorthite and diopside-anorthite show substantial negative enthalpies of mixing, albite-diopside shows significant positive heats of mixing. For compositions up to NaAlO2 = 0.42 (which includes the subsystem albite-silica) the system NaAlO2-SiO2 shows essentially zero heats of mixing. A negative ternary excess heat of mixing is found in the plagioclase-rich portion of the albite-anorthite-diopside system. The join Si4O8-CaAl2Si2O8 shows small but significant heats of mixing. In albite-anorthite-quartz. ternary glasses, the ternary excess enthalpy of mixing is positive. Based on available heat capacity data and appropriate consideration of the glass transition, the enthalpy of the crystal-glass transition (vitrification) is a serious underestimate of the enthalpy of the crystal-liquid transition (fusion) especially when the melting point, Tf, is many hundreds of degrees higher than the glass transition temperature, Tg. On the other hand, the same heat capacity data suggest that the enthalpies of mixing in albite-anorthite-diopside liquids are calculated to be quite similar to those in the glasses. The enthalpies of mixing observed in general support the structural models proposed by Taylor and Brown (1979a, b) and others for the structure of aluminosilicate glasses.

Electrical conductivity of magmatic liquids: effects of temperature, oxygen fugacity and composition

Abstract The effects of temperature, f O2 and composition on the electrical conductivity of silicate liquids have been experimentally determined from 1200 to 1550°C under a range of f O2 conditions sufficient to change the oxidation state of Fe from predominantly Fe 2+ to Fe 3+ . Oxidation of ferrous to ferric iron in the melt has no measurable effect on the conductivity of melts with relatively low ratios of divalent to univalent cations. Under strongly oxidizing conditions a minor decrease of conductivity is detected inth high ΣM 2 /ΣM + ratios. It is concluded that for purposes of estimating the conductivity of magmatic liquids, f O2 may be ignored to a first approximation. Both univalent and divalent cation transport is involved in electrical conduction. Melts relying heavily on divalent cations for conduction, i.e. melts with relatively large ΣM 2+ /ΣM + ratios, show strong departures from Arrheenius temperature dependence with the apparent activation energies decreasing steadily as the temperature increases. Conductivities dominated by the univalent cations, in melts with relatively small ΣM 2+ /ΣM + ratios, show classical Arrhenius temperature dependence. These observations are discussed in terms of the general characteristics of the melt structure. Compositional variations within the magmatic range account for much less than an order of magnitude variation in electrical conductivity at a fixed temperature. This observation, combined with previous measurements of the conductivity of olivine (A. Duba, H.C. Heard and R. Schock, 1974) make it possible to state with reasonable confidence that melts occurring within the mantle will be more conductive by 3–4 orders of magnitude than their refractory residues. Potential applications to geothermometry are discussed.

Partitioning of transition metals between diopside and coexisting silicate liquids. I - Nickel, cobalt, and manganese

Abstract Distribution coefficients have been experimentally determined for the partitioning of nickel, cobalt and manganese between calcium-rich clinopyroxenes and coexisting silicate liquids. Temperatures ranged from 1110–1360°C and oxygen fugacities in the furnaces were controlled by gas mixtures at one atmosphere total pressure. Bulk compositions used include synthetic compositions in the system albite-anorthite-diopside and a natural basalt. Charges were doped with a few percent transition metal oxides and analyzed by electron microprobe. Measured clinopyroxene/liquid distribution coefficients range from 1.5–14 for Ni, 0.5–2.0 for Co and 0.3–1.2 for Mn. Diopside/liquid distribution coefficients for nickel are shown to be independent of Ni content over a range of from 3 ppm to 3 wt.% Ni in the liquid and to increase with decreasing temperature. From analyses of pyroxenes grown from experimental charges differing only in the amounts of transition metals present, nickel and cobalt are shown to occupy the M1 site of diopside while manganese occupies both M1 and M2. Ordinary weight ratio distribution coefficients are strongly dependent on liquid composition as well as temperature. For example, experiments on synthetic Ab-An-Di compositions give clinopyroxene/liquid distribution coefficients higher by about a factor of five than those from experiments at the same temperature on a natural basalt. For Ni and Co, which occupy only the M1 site of clinopyroxene, an equilibrium constant can be defined in terms of activities of components in the liquid and solid phases. Activities of components in the solid are approximated by their mole fractions. An activity/concentration model based on the viscosity model of BOTTINGA and WEILL (1972) is used for the liquid. This model approximates the activity of silica as its mole fraction among the network-forming components SiO 2 , TiO 2 , KAlO 2 , NaAlO 2 and Ca 0.5 AlO 2 . Activities of network modifiers such as CaO are approximated as their mole fractions among the network-modifying components CaO, MgO, FeO, FeO 1.5 , etc. When these estimated activities are used in the expression for the equilibrium constant, the effects of compositional differences on trace element distribution coefficients can be understood and the results of experiments on synthetic and natural compositions reconciled.

Mineralogy and petrology of Apollo 12 sample no. 12013 - A progress report

Abstract A mineralogical-petrological survey of Apollo 12 sample 12013 has been made with petrographic microscope and electron microprobe. The rock is extremely heterogeneous and consists of at least two separate fragmental units (light and dark) permeated by a once-fluid granitic component. The fragmental material includes a wide variety of lithic and crystal fragments some of which have not yet been reported from other lunar samples. The granitic component is essentially bimineralic, with dominant potassic feldspar plus silica.

High temperature heat contents and heat capacities of liquids and glasses in the system NaAlSi3O8-CaAl2Si2O8

Enthalpies and heat capacities of glasses and of stable liquids in the system NaAlSi3O8-CaAl2Si2O8 have been measured by drop and differential scanning calorimetry. Within experimental error, values of Cp and of HT300 of three intermediate compositions fall on straight line interpolations between the end members for both liquids and glasses, indicating that excesses in true and in mean heat capacities [(HT−H300)/(T−300)] are small or absent. A value for the heat capacity of the An100 liquid component can therefore be derived, and is probably a better estimate than that based on measurements on the pure substance alone. On the assumption of zero excess heat capacity in this system, heats of mixing in the stable liquids are equal to those measured in the glasses by solution calorimetry, and can be as negative as -2 kcal mol−1.Heat capacities of solids and glasses in the Ab-An system are similar and do not vary greatly with composition. The CPs of the liquids, however, increase strongly with An content, suggesting major structural changes take place across the binary.

The enthalpy of fusion of anorthite

The high-temperature enthalpies of liquid and glassy CaAl2Si2O8 were measured by drop calorimetry using a diphenyl ether drop calorimeter. These data are combined with published values of the high-temperature enthalpy of crystalline anorthite and the enthalpy of vitrification of anorthite to obtain the enthalpy of fusion of anorthite. Analysis of the data yields the following preferred values (enthalpy in kcal/mol, uncertainty limits correspond to two standard deviations):enthalpy of vitrification at 985 K, ΔvHv985=18.6±0.6; enthalpy of the liquid at 1,830 K, H1830l300g=130.4±1.2; enthalpy of the glass at 985 K, H985g-H300g=46.7±0.4; enthalpy of crystalline anorthite between 985 and 1,830 K, H1830c-H985c=69.9±1.4; calculated enthalpy of fusion of anorthite at 1,830 K, ΔfH1830= 32.4±2.1.The average heat capacity of supercooled liquid CaAl2Si2O8 between the glass transition (Tg≈ 1,086 K) and the melting point (Tf7=1,830 K) is 102 ± 2 cal/mol/K. The large difference between the enthalpy of fusion and the enthalpy of vitrification for the minerals anorthite and diopside is emphasized. The practice of assuming ΔfH≈Δ λvH should be discontinued for silicate compounds for which Tf≫Tg.

Thermodynamic Modeling of Silicate Melts

The chemical and physical properties of silicate liquids exhibit a strong dependence on chemical composition as well as temperature and pressure. Physical chemists in the glass, ceramic, and metallurgical industries investigate these dependences in order to increase the efficiency of industrial processes and to discover materials with new and interesting properties required for technological progress. Earth scientists presently constitute another significant segment of the science community with a deep interest in the properties of silicate liquids. There is now perhaps a greater realization than ever before that many features of our planet and some of its solar system neighbors must be understood in the context of igneous processes. Since silicate melts occur in nature in an almost infinite variety of chemical compositions as well as a very large range of pressure-temperature conditions, it is a special concern of the geosciences to achieve sound models for interpolating and eventually extrapolating a limited number of laboratory measurements.