Partitioning clinopyroxenes

Harte and Kirkley, 1997). Differences in garnet-clinopyroxene partitioning behavior between the Roberts Victor and Koidu eclogite suites was attributed to the contrasting equilibration temperatures of the two suites by Barth et al. (2001). 

Mineral-Melt Partitioning ofU-Series Nuclides Clinopyroxene along mantle solidus... 

A further feature of the fit parameters obtained by Blundy and Wood (1994) for plagioclase and clinopyroxene was that the partitioning parabolae become tighter E increases) and displaced to lower as charge increases (Law et al. 2000 Blundy and Dalton 2001). These observations have since been confirmed by a large number of... 

Experimental clinopyroxene-melt (Brooker et al. 2003) and wollastonite-melt (Law et al. 2000) partitioning data can be fitted to the electrostatic model (see Fig. 9). Only species entering the large cation site (e.g., M2 in clinopyroxene) are considered. For each isovalent group (actinides, lanthanides etc.) derived as shown in Figure 4, is... 

Clinopyroxene is the mineral for which most partitioning data are available. It has... 

Wood and Blundy (1997) adapted the lattice strain model to describe lanthanide partitioning between clinopyroxene and melt as a function of crystal composition, pressure and temperature. In developing the model, they arrived at relationships between and respectively, crystal composition, and pressure and temperature ... 

Figure 11. The clinopyroxene-liquid partition coefficient for 4+ ions entering the M2-site shown as a function of the ionic radins of the trace cation. Changes in clinopyroxene composition along a solid solntion lead to small changes in the dimensions of M2 (ro Jw 2>), which can lead in turn to changes in the relative fractionation between 4+ ions of similar ioiuc radii, such as and Th" (shown as vertical lines). We contrast the partitioning behavior of a diopside-rich clinopyroxene = 1 044 A) and a...

Fitting 71 Djh values (27 from their study and 44 from the literature) and subsequently excluding 5 outliers, Landwehr et al. (2001) derived the following expression for Th partitioning between clinopyroxene and silicate melt as a function of temperature, pressure, crystal chemistry and the molar MgMi partition coefficient ... 

The denotes molar, rather than weight fraction, partition coefficient. The X terms denote the cation molar fractions of Ca, Al and Mg, while the X terms denote the molar fraction of diopside and enstatite components in clinopyroxene. Note that this expression contains no explicit P-f dependence. 

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... 

Only McDade et al. (2003b) report experimental data on the partitioning of Nb and Ta between olivine and melt. At 1.5 GPa they find Dm = 1 x lO " and Dia = 6 x 10 ", which indicates that is smaller than rNb (0.660 A). This will serve to exclude Pa (rpa = 0.78 A) from the M-sites in much the same way as it is excluded from Ml in clinopyroxene. Furthermore D h and Dxa are themselves two orders of magnitude lower in olivine than coexisting clinopyroxene (McDade et al. 2003b). For these reasons we suggest that Pa is substantially more incompatible in olivine than clinopyroxene. 

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... 

Van Westrenen et al. (2001a) present a model of lanthanide and Sc partitioning between the garnet X-site and melt. The model is a variant of the lattice strain model of clinopyroxene-melt partitioning of Wood and Blundy (1997), and is based on 160 experimental garnet-melt pairs in the pressure-temperature range 2.5-7.5 GPa and 1450-1930°C. The model includes composition-sensitive expressions for and accounts for the non-linear variation in with composition, as follows ... 

P(x) U(x) we see that the variation in Du/Dm with (Fig. 14) is remarkably similar to that for clinopyroxene (Fig. 1). The fact that the plotted garnet partitioning data derive from a wide range of pressures, temperatures and compositions is particularly encouraging. 

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