D. J. Lindstrom

Partitioning of Ni2+ between basaltic and synthetic melts and olivines—an experimental study

Abstract We present experimental measurements of olivine/liquid distribution coefficients for Ni (KNi) in basaltic and simple synthetic melts over a temperature range of 1400° to 1070°C. Our results suggest that 1. (1) KNiand activity coefficients for NiO in basaltic melts are strongly dependent on temperature (e.g., KNi =wt. %NiOol/wt. %NiOliq ranges from 5.9 at 1400° to 29 at 1100°C); 2. (2) at constant temperature KNi is dependent upon melt composition and structure, but this dependence is small over the composition range of common basaltic magmas; 3. (3) KNi can be used as a geothermometer to predict olivine crystallization temperatures of basaltic magmas to within ±50°C; 4. (4) KNi is significantly greater than one at all natural magmatic temperatures, thus the presence of olivine as a cumulus phase during fractional crystallization or as a residual phase during partial melting has a significant effect on the content of Ni in the coexisting melt phase; 5. (5)KNi does not vary significantly as a function of Ni content of the melt up to at least one mole percent (i.e., Henrys law behavior is obtained for olivine and melt solutions of Ni).

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.

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 E∗ (Ni2+Ni0, diopside, 1500°C) = −0.32 ± .01 V, E∗ (Co2+Co0, diopside, 1500°C) = −0.45 ± .02 V, and E∗ (Zn2+Zn0, diopside, 1500°C) = −0.53 ± .01 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 E∗ (Ni2+Ni0, 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 E∗ (Ni2+Ni0, 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.

Kinetic effects on trace element partitioning

Kinetic effects on trace element partitioning have been measured for anorthite, forsterite, and diopside grown from synthetic compositions doped with REE. A seeding technique allowed determination of crystal growth rates and partitioning information was obtained from electron microprobe analyses. Compositional deviations from equilibrium values were sought in the crystals and as gradients in the quenched liquids adjacent to the crystals. The principal result is that large deviations in trace element distribution coefficients from equilibrium values do not occur because of a compensating effect. Rapid growth depletes the melt adjacent to the crystal in the elements of which the crystal is composed, leading to different values for apparent distribution coefficients. However, as the boundary layer melt becomes depleted in the components of the crystal, growth slows and the size of the compositional perturbations decreases. Crystals grown at very high rates (e.g., > 0.2 μm/sec for diopside) tended to be too small for accurate microprobe analyses, but are probably not compositionally extreme since the melts adjacent to the crystals did not acquire sizable compositional gradients. At moderately high growth rates (e.g., 0.02 μm/sec), crystals form in the presence of boundary layer compositions perturbed by as much as 10% from bulk melt values and, in diopside, attain concentrations for excluded trace elements about 70% higher than equilibrium values for crystals plus bulk melt. At the slower growth rates typical of igneous systems, kinetic effects on trace element partitioning are probably negligible.

Trace element identification of three chemically distinct very low titanium (VLT) basalt glasses from apollo 17

Abstract Lunar basaltic samples of VLT (Very Low Titanium) composition occur primarily as small glass fragments in the regolith at the Apollo 17 landing site. Electron microprobe analyses of glasses from the double drive tube 79001/2 (90–150 μm size fraction of soils from mean depths of 0.8–16.8 cm) indicate that 34 of 90 (38%) are of VLT composition. A subset of twenty glasses was removed from the thin sections using a micro-coring device and analyzed using special micro-INAA techniques to obtain major and trace element abundances. On the basis of the results, we delineate three distinct groups which have been named for their relative amounts of Co: HICo, MECo, and LOCo Apollo 17 VLTs. The most magnesian HICo basalt glasses (4 of 34, or about 12% of the VLT glasses) are probably pristine pyroclastics. Soil breccias 79135 and 70295 apparently contain only this type of VLT glass. The medium-Co MECo glasses (6 of 34, 18%) strongly resemble the largest sample of VLT composition, the olivine vitrophyre 78526, as well as all three of the Apollo 17 VLT lithic fragments that have been analyzed for trace elements. The LOCo VLT glasses (23 of 34, 68%) are more enigmatic. Trace element concentrations require that these three VLT compositions cannot be related by simple igneous processes. The distribution of the three types provides important information on the sequence of events that produced the regolith at the Apollo 17 site.

Ni partitioning between diopside and silicate melt: A redetermination by ion microprobe and recognition of an experimental complication

Abstract The ion probe is uniquely suited for measurement of element partitioning between phases in experimental and natural systems. A redetermination of the partitioning of Ni between diopside and quenched silicate melt using samples previously measured by β-track mapping gives 1.87 (weight ratio of 62Ni in diopside/melt), slightly lower than the β-track value of 2.05. Critical to the accurate determination of distribution coefficients are: (1) a secondary ion signal that is linear with concentration in the range measured, and (2) a calibration using known concentrations to correct for differential secondary ion yields from different phases. In the present case the secondary ion signal is linear with Ni concentrations below ~ 1 wt% in both diopside and glass, but nonlinear above. Differential yields were corrected by calibrating the secondary ion signal against compositions determined by electron microprobe. Partition coefficients measured using 58Ni and 60Ni, in contrast to 62Ni, are not constant with concentration in these samples probably due to Ni migration during crystallization. Measurements using these isotopes (or bulk Ni) show a change of partition coefficient with Ni concentration.