Orthopyroxene partition coefficients

Orthopyroxene has a Vl-fold Ml site and a Vl-fold M2 site. Both are predominantly filled by Mg and Fe. The smaller Ml site shares many characteristics with the clinopyroxene Ml site. It is therefore reasonable to assume that no U-series cations, including Pa (see above) enter that site. We will confine our discussion to the octahedral M2, which is smaller than the equivalent (Vni-fold) clinopyroxene site, even after allowing for the different coordination number. Consequently most of the U-series elements have very low orthopyroxene-melt partition coefficients. 

Figure 13. Electrostatic model fitted to partition coefficients for cations entering the M2-site in orthopyroxene, based on the experiments of McDade et al. (2003a,b). The curves are fits to Equation (7) and can be used to estimate and Do(m2) > from which D-p ui) can be calculated via the lattice strain model. The fit parameters are given in the legend.

Argon partition coefficients for olivine have recently been determined by Brooker et al. (1998). Their measured Z)at values are around 10 , which is higher than measured for clinopyroxene, but similar to the estimated values for orthopyroxene. We propose a value... 

Figure 6.3 Log-linear plot of the intersite partition coefficient, Kd, versus ionic radii of divalent cations in olivines and orthopyroxenes (modified from Rajamani et al., 1975).

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. 

Noting that magnesium is partitioned into both sites in orthopyroxene, the M2-melt partition coefficient D g for basaltic liquids is —1.0. Adopting this value implies, for the larger alkaline earth cations. 

A more minor consideration is the role of orthopyroxene. At 1.5 GPa orthopyroxene has Du an order of magnitude less than clinopyroxene, and the small lattice site strongly favors uranium over thorium (Blundy and Wood, 2003a Wood et al., 1999). The Z>u/Dxh for orthopyroxene is consequently high ( 2.5, Table 2) and may influence the bulk partition coefficient despite its low absolute partition coefficients, especially in highly depleted peridotites. 

Colson er al. (1988) in a detailed study of trace element partitioning between olivine and silicic melt, and orthopyroxene and silicic melt, have shown that many partition coefficients are strongly dependent upon temperature and melt composition. They show that these partition coefficients vary according to ionic size and they have modelled temperature and composition dependence as a function of these variables. On the basis of their equations it is possible to predict partidon coefficients between olivine and melt, and orthopyroxene and melt, for a wide range of tri- arid di-valent cations under a7yiriety of magmatic conditions. 

Beattie, P., C. Ford and D. Russell, Partition coefficients for olivine-melt and orthopyroxene-melt systems, Cont. Mineral. Petrol., 109, 212-224, 1991. 

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