H.St.C. O’Neill

Solubilities of Pt and Rh in a haplobasaltic silicate melt at 1300°C

The solubilities of Platinum (Pt) and Rhodium (Rh) in a haplobasaltic melt (anorthite-diopside eutectic composition) have been determined experimentally by using the mechanically assisted equilibration technique at 1300°C, as a function of oxygen fugacity (10 212 , fO2 # 1 bar), imposed by CO-CO2, N2-H2-H2O, Ar-O2, and air gas mixtures. Samples were analyzed by sample nebulization (SN) inductively coupled plasma-mass spectrometry and, using some of these samples as standards, also by laser ablation (LA) inductively coupled plasma-mass spectrometry. The latter is a true microanalytical technique that allows small-scale sample heterogeneity to be detected. At each oxygen fugacity step, a time-series of samples was taken, to demonstrate that the solubilities converge on a constant value. In addition, solubilities were measured after both increasing and decreasing the imposed fO2. The results fall into three groups, according to oxygen fugacity. At high fO 2s, (fO2

The solubility of rhenium in silicate melts: Implications for the geochemical properties of rhenium at high temperatures

10 22 bars), samples are homogenous at all sampling scales. Both Pt and Rh predominantly dissolve in the silicate melt as 21 species, with some evidence for Pt 41 and Rh 31 at the highest fO2s studied (air and pure O2). From these data, we obtained the following expressions for the solubilities of Pt and Rh: Pt/ppb 5 2100(fO2) 1 10980(fO2) 1/2 Rh/ppb 5 68630(fO2) 3/4 1 31460(fO2) 1/2 At fO2 , 10 25 bars, the true solubilities of Pt and Rh appear to be obscured by Pt-Rh micronuggets, which remain suspended in the melt despite stirring on time scales of 10 3 h, resulting in samples that are heterogenous on the laser sampling scale. Samples at intermediate fO2 (10 22 to 10 25 bars) are affected by the micronugget problem on the sampling scale of the conventional SN-inductively coupled plasma mass spectrometry, but these can be filtered out by analyzing on the laser sampling scale. Copyright

The influence of next nearest neighbours on cation radii in spinels: the example of the Co3O4–CoCr2O4 solid solution

The solubility of rhenium (Re) in a haplobasaltic melt (anorthite-diopside eutectic composition) has been experimentally determined using the mechanically assisted equilibration technique at 1400°C as a function of oxygen fugacity (10−12 < fO2 ≤ 10−7 bar), imposed by CO-CO2 gas mixtures. Samples were analysed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). This is a true microanalytical technique, which allows small-scale sample heterogeneity to be detected, while providing a limit of detection of 2 ppb Re. Time-resolved LA-ICP-MS spectra revealed the presence of suboptically sized micronuggets of Re in all samples, which, because they are present at the 0.5 to 10 ppm level, dominate the true solubilities of Re (<1 ppm at the conditions of the experiment) in bulk analyses of the samples. Nevertheless, the micronuggets could be filtered out from the time-resolved spectra to reveal accurate values of the true Re solubility. A number of time series of samples were taken at constant fO2 to demonstrate that the solubilities converge to a constant value. In addition, solubilities were measured after increasing and decreasing the imposed fO2. The results show that Re dissolves in the silicate melt as ReO2 (Re4+) and ReO3 (Re6+) species, with the latter predominating at typical terrestrial upper-mantle oxygen fugacities. The total solubility of Re is described by the following expression (fO2 in bars): [Re/ppb] = 9.7(±1.9) × 109 (fO2) + 4.2 (±0.3) × 1014 (fO2)1.5Assuming an activity coefficient for Re in Fe-rich metal of 1, this gives a value of DRemet/sil of 5 × 1010 at log fO2 = IW-2, appropriate for metal-silicate partitioning in an homogenously accreting Earth. Thus, Re is indeed very highly siderophile, and the mantle’s abundance cannot be explained by homogenous accretion.

Thermodynamic controls on element partitioning between titanomagnetite and andesitic–dacitic silicate melts

Abstract Lattice parameters and crystal structures of the synthetic spinels Co3O4, CoCr2O4, and solid solutions in the binary join Co3O4-CoCr2O4, have been determined by powder X-ray diffraction structural refinements. In all these spinels the cation distribution is completely normal at room temperature, and the tetrahedrally coordinated cation site is occupied only by Co2+. The ionic radius of Co2+(tet) increases from 0.556(3) in Co3O4 to 0.599(4) in CoCr2O4. In the spinel structure, the interatomic distance between the tetrahedral cations and oxygen are geometrically independent of those between the octahedral cations and oxygen; thus the variation in effective ionic radii is ascribed to next-nearest neighbour effects, induced by covalent tendencies in the low-spin Co3+-O bond. The results demonstrate that the assumption of constant ionic radii even within an isomorphic group such as the oxide spinels needs to be made with caution.

Effect of silica activity on OH - IR spectra of olivine: implications for low-aSiO 2 mantle metasomatism

Abstract Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO2) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg2+, Mn2+, Co2+, Ni2+, Zn2+) and Cu2+/Cu+ with increasing fO2 between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO2 implies a reciprocal relationship between a(Fe2+O) and a(Fe3+O1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO2 in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO2 in silicic, but less so in mafic bulk systems.