Partitioning orthopyroxenes

Orthopyroxene partitions nickel, cobalt, and manganese less than olivine and there are no clear correlations amongst these elements. Although low in abundance, orthopyroxene can be a significant reservoir for the trivalent cations vanadium, scandium plus tetravalent titanium, due to its high modal abundance, especially in depleted xenoliths with little or... 

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. 

Lanthanides are more incompatible in orthopyroxene than clinopyroxene, typically by a factor of 4-8 on the mantle solidus (Blundy and Wood 2003 Salters et al. 2002 McDade et al. 2003a,b). Not surprisingly there are fewer lanthanide partitioning data for... 

By analogy with clinopyroxene it is likely that Pa enters the orthopyroxene M2 site. In light of the fact that Du and Dxh in orthopyroxene are approximately ten times lower than in clinopyroxene, it is likely that Z)pa is also lower in orthopyroxene. However, this effect is offset to some extent by the smaller M2 site in orthopyroxene, which will tend to be more favourable to Pa than the M2 site in clinopyroxene. We have used the electrostatic model, applied to the two orthopyroxene-melt partitioning experiments of McDade et al. (2003a,b) to derive (Fig. 13). Both datasets, at 1.5 and 3 GPa, are... 

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... 

Jones JH (1995) Experimental trace element partitioning. In Ahrens TJ (ed) Rock physics and phase relations A handbook of physical constants Am Geophys Union Reference Shelf 3 73-104 Kennedy AK, Lofgren GE, Wasserburg GJ (1993) An experimental study of trace element partitioning between olivine orthopyroxene and melt in chondrales equilibrium values and kinetic effects. Earth Planet Sci Lett 115 177-195... 

Grover J. E. and Orville T. M. (1969). The partitioning of cations between coexisting single and multisite phases with application to the assemblage Orthopyroxene-clinopyroxene and orthopyroxene-olivine. Geochim. Cosmochim. Acta, 33 205-226. 

Medaris L. G. (1969). Partitioning of Fe + and Mg between coexisting synthetic olivine and orthopyroxene. Amer. Jour. Set, 267 945-968. 

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. 

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 observed relative enrichments of Fe2+ ions in coordination sites within individual silicate minerals were discussed in 6.7 and cation ordering trends shown by olivines and orthopyroxenes were summarized in table 6.5. These intersite partitioning patterns are partially explained by the relative CFSE s attained by Fe2+ ions in each coordination site of the mineral structures ( 6.8.3.1). 

Harley, S. L. (1984) An experimental study of the partitioning of Fe and Mg between garnet and orthopyroxene. Contrib. Mineral. Petrol., 86, 359-73. 

Kennedy A. K., Lofgren G. E., and Wasserburg G. J. (1993) An experimental study of trace element partitioning between olivine, orthopyroxene, and melt in chondrules. Earth Planet. Sci. Lett. 115, 177-195. 

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. 

Dobson P. F., Skogby H., and Rossman G. R. (1995) Water in boninite glass and coexisting orthopyroxene concentration and partitioning. Contrib. Mineral. Petrol. 118, 414-419. 

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