Pressure dependence of partition coefficients between olivine and peridotite melt
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In order to understand the crystallization process of global magma ocean, precise knowledge on the element partitioning at various pressures is essentially important. We have determined PC-IR (partition coefficient, ionic radius) diagram for 32 elements between olivine and peridotite melt at 2, 5, 10, and 14 GPa. Two types of starting materials were prepared from a fertile peridotite KLB-1 [1] with different levels of trace elements. High-pressure melting experiments were performed using a piston-cylinder apparatus and a multi-anvil apparatus at the Magma Factory, Tokyo Inst. of Technology. Chemical analyses were performed using EPMA (JEOL-8800) for major and minor elements and LA-ICP-MS for trace elements (ArF excimer laser and a quadrupole mass spectrometer, 30 μm laser beam, NIST610 glass standard). Partition coefficients were calculated and were plotted on the PC-IR diagram. The results of this study are in general agreement with previous studies on similar compositions (analysis by EPMA [2]; by SIMS [3]) except that the shape of the peaks and pattern of the parabolas were tightly constrained in the present study. PC-IR diagram for trivalent cations between olivine and peridotite melt was tightly constrained for the first time with 14 elements (including 8 REE). The partition coefficient for Al increases with pressure (DAl= 0.012 at 2 GPa, 0.048 at 14 GPa) while that for all other trivalent cations decreases with increasing pressure (e.g., DY=0.0077 at 2 GPa, 0.0019 at 14 GPa). The pressure effect on the PC-IR diagram cannot be explained simply by the lattice strain model [4] but requires some additional factors. Increase in DAl may be explained by the combination of two types of substitutions (Mg, Mg) ↔ (Na, Al) and (Si, Mg) ↔ (Al, Al). Decrease in D values for other trivalent cations implies that the latter type of substitution is the dominant mechanism.Keywords:
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In this study geochemical processes and a geophysical parameter were investigated that are relevant to the crystallisation of a deep magma ocean, that likely existed during Earth’s accretion. The melting relations of potential magma ocean compositions, such as peridotitic and chondritic bulk compositions, were investigated using multianvil apparatus at pressures of 25-26 GPa and temperatures up to 2400°C. Compositional effects on the melting relations were investigated by varying bulk Mg/Si and Mg/(Mg+Fe) ratios (the latter is denoted as Mg-number, Mg#). At 26 GPa, peridotite liquids show a crystallisation sequence of ferropericlase (Fp) followed down temperature by Mg-silicate perovskite (MgPv) + Fp, which is in contrast to the sequence of MgPv followed by MgPv + Fp in chondritic composition. The melting relations along the different compositions depend primarily on the bulk Mg/Si ratio and not on the Mg#. Melting relations and eutectic compositions were studied in the simple binary MgO-SiO2 system between 10 and 26 GPa. Combining the new results with previously published data shows that the eutectic composition between Mg2SiO4 and MgSiO3, up to 20 GPa, moves towards MgO with increasing pressure. Between 20 and 23 GPa the direction in which the eutectic is moving with pressure reverses. At higher pressures, this trend is again reversed and the eutectic composition moves towards MgO. The multiple changes in the direction in which the eutectic is moving as a function of pressure explains qualitatively the differences in liquidus phase relations in the more complex peridotite and chondrite compositions. The effect of bulk chemical composition on the partitioning of major, minor and trace elements between MgPv and coexisting silicate melts was investigated using micro-beam techniques. MgPv/melt partition coefficients for Mg (DMg) and Si (DSi) are related to the melt Mg/Si ratio, such that DSi becomes smaller than DMg at chondritic Mg/Si melt ratios. This shows that the Earth’s upper mantle Mg/Si ratio is unlikely to be derived from chondrites by MgPv fractionation. Partition coefficients of tri- and tetravalent elements increase with increasing Al concentration of MgPv. A crystal chemical model indicates that Al3+ substitutes predominantly onto the Si-site in MgPv, but most other elements substitute onto the Mg-site. This is consistent with a charge-compensating substitution mechanism. A crystal fractionation model, based on refractory lithophile element ratios, is developed to constrain the amount of MgPv and Ca-silicate perovskite (CaPv) that could have fractionated in a magma ocean and could still be present as a chemical heterogeneity in the lower mantle today. It is shown that a fractionated crystal pile composed of 96% MgPv and 4% CaPv could comprise up to 13 wt% of the entire mantle. Fe3+/Fetotal ratios have been determined for MgPv, crystallised at temperatures below and above the peridotite solidus, using Mossbauer and electron-energy-loss spectroscopy. The amount of Fe3+ in MgPv is positively correlated to the…
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Abstract We have performed an experimental study of the influence of varying size and charge on cation partitioning between wollastonite and silicate-carbonate melt in the system CaCO 3 -SiO 2 . The experimental conditions (3 GPa, 1420°C) lie close to the wollastonite II tc/I tc phase boundary. Results for 1+, 2+, 3+ and 4+ partitioning show parabolic dependence of partition coefficients on ionic radius, which can be fitted to the elastic strain model of Blundy and Wood (1994), wherein partitioning is described using three parameters: site radius ( r 0 ), site elasticity (apparent Young's Modulus) and the ‘strain-free’ partition coefficient ( D 0 ) for an element with radius r 0 . The apparent Young's Modulus of the Ca site in wollastonite, obtained from modelling the 2+ partitioning data, is 99±3 GPa, similar to the bulk-crystal value for the polymorph wollastonite I tc. r 0 decreases with increasing charge on the substituent cation, while D 0 also shows an approximately parabolic dependence on charge, with a maximum for 2+ cations. Partition coefficients for divalent cations Zn, Co, Fe, Cd, Mn and Pb are lower than would be predicted from their ionic radii alone, indicating a preference for the melt. This may be a consequence either of cation-carbonate complexation in the melt, or of the more distorted nature of cation co-ordination environments in melts.
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