Spinel partitioning

Spinels. There are limited experimental data on uranium and thorium partitioning between magnetite and melt (Nielsen et al. 1994 Blundy and Brooker 2003). Both studies find U and Th to be moderately incompatible. Blundy and Brooker s results for a hydrous dacitic melt at 1 GPa and 1025°C give Du and D h. of approximately 0.004. The accuracy of these values is compromised by the very low concentrations in the crystals and the lack of suitable SIMS secondary standards for these elements in oxide minerals. Nonetheless, these values are within the range of Djh of magnetites at atmospheric pressure 0.003-0.025 (Nielsen et al. 1994). It is difficult to place these values within the context of the lattice strain model, firstly because there are so few systematic experimental studies of trace element partitioning into oxides and secondly because of the compositional diversity of the spinels and their complex intersite cation ordering. 

The lack of systematic lanthanide partitioning data for spinels precludes estimating values for Z)ac, D i etc. Suffice to say that they will be vanishingly small, as will Z>Ra and Z)pb, given the low partition coefficients for Ca and Sr in spinels. 

Blundy JD, Wood BJ, Davies A (1996) Thermodynamics of rare earth element partitioning between clinopyroxene and melt in the system Ca0-Mg0-Al203-Si02. Geochim Cosmochim Acta 60 359-364 Blundy JD, Robinson JAC, Wood BJ (1998) Heavy REE are compatible in clinopyroxene on the spinel Iherzolite solidus. Earth Planet Sci Lett 160 493-504... 

Beattie P (1993a) The generation of uranium series disequilibria by partial melting of spinel peridotite constraints from partitioning studies. Earth Planet Sci Lett 117 379-391... 

Nishizawa O. and Akimoto S. (1973). Partitioning of magnesium and iron between olivine and spinel, and between pyroxene and spinel. Contrib. Mineral Petrol, 41 217-240. 

Ottonello G. Piccardo G. B., Mazzucotelli A., and Cimmino F. (1978). Clinopyroxene-orthopyroxene major and REE partitioning in spinel peridotite xenoliths from Assab (Ethiopia). Geochim. Cosmochim. Acta., 42 1817-1828. 

Capobianco, C.H. Drake, M. 1990. Partitioning of ruthenium, rhodium, and palladium between spinel and silicate melt and implications for platinum-group element fractionation trends, Geochimica et Cosmochimica Acta, 54, 869-874. 

Righter, K., Campbell, A.J., Humayun, M., Hervig, R.L 2004. Partitioning of Ru, Rh, Pd, Re, Ir, and Au between Cr-bearing spinel, olivine, pyroxene and silicate melts. Geochimica et Cosmochimica Acta, 68, 867-880. 

The diffusion couple discussed above consists of two halves of the same phase. If the two halves are two minerals, such as Mn-Mg exchange between spinel and garnet (Figure 3-5), there would be both partitioning and diffusion. Define the diffusivity in one half (x < 0) to be D, and in the other half (x > 0) to be D. Both and are constant. Let w be the concentration (mass fraction) of a minor element (such as Mn). The initial condition is... 

Broadhurst, C. L., Drake, M. J., Hagee, B. E., Bernatowicz, T. J. (1990) Solubility and partition of Ar in anorthosite, diopside, forsterite, spinel, synthetic basaltic liquids. Geochim. Cosmochim. Acta, 54, 299-309. 

The site preferences shown by cations in the spinel structure demonstrate that transition metal ions prefer coordination sites that bestow on them greatest electronic stability. In addition, certain cations deform their surrounding in order to attain enhanced stability by the Jahn-Teller effect. These two features suggest that similar factors may operate and cause enrichments of cations in specific sites in silicate structures, leading to cation ordering or intersite (intracrystalline ) partitioning within individual minerals which, in turn, may influence distribution coefficients of cations between coexisting phases. 

Partitioning of iron in post-spinel phases in the Lower Mantle... 

Rampone E., Piccardo G. B., Vannucci R., Bottazzi P., and Ottolini L. (1993) Subsolidus reactions monitored by trace element partitioning the spinel- to plagioclase-facies transition in mantle peridotites. Contrib. Mineral. Petrol. 115, 1-17. 

The Mg of orthopyroxene is similar or slightly greater than that of olivine, due to a relative Fe-Mg partition coefficient (Kd) of 1 that is independent of P and T (von Seckendorff and O Neill, 1993). The calcium content of orthopyroxene also varies depending on the temperature of equilibration of the sample and its bulk composition. In both the spinel- and garnet-facies, the CaO content increases with T and varies between 0.2 wt.% and 2.0 wt.%. Orthopyroxenes with very low CaO occur in harzburgites and/or in low-T samples in many kimberlite-borne xenolith suites. 

Chazot G., Menzies M. A., and Harte B. (1996) Determination of partition coefficients between apatite, clinopyroxene, amphibole and melt in natural spinel Uierzolites from Yemen implications for wet melting of the lithospheric mantle. Geochim. Cosmochim. Acta 60, 423-437. 

Horn L, Foley S. F., Jackson S. E., and Jenner G. A. (1994) Experimentally determined partitioning of high field strength and selected transition elements between spinel and basaltic melt. Chem. Geol. 117, 193-218. 

Principal experimental data on spinel-melt partitioning are those of Nielsen et al. (1994) for scandium, nickel, vanadium, zirconium, hafnium, niobium, tantalum, uranium, and thorium and Horn et al. (1994) for scandium, vanadium, gallium, zinc, cobalt, zirconium, hafnium, niobium, and tantalum. These are supplemented by data for hydrous melts by Nielsen and Beard (2000). 

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