Clinopyroxenes M2-site

On the clinopyroxene M2 site, Pa will be discriminated against both on account of... 

Consider a trace ion entering the clinopyroxene M2-site. In general, the ion has a charge which is different from that of the major ion which it is... 

Figure 14 Plot showing the strain-free partition coefficient as a function of charge for ions entering the clinopyroxene M2 site. Line labeled AP = 0.195 corresponds to 15 kb experiment of Wood et al. (1999) line labeled Af = 0.016 corresponds to experiment 114c of Beattie (1993a) and from Chamorro et al.

Daf being in the range observed for partitioning of 4-f ions into the clinopyroxene M2-site. Furthermore, the experimental data of Chamorro et al. (2002) indicate little difference in partition coefficient between argon (atomic radius = 1.54 A) and krypton (1.69 A), which is in agreement with the observed linear dependence of Young s modulus on charge. 

Landwehr et al. (2001) extended the model of Wood and Blundy (1997) to include and Th". They measured experimentally Aj and Z)xh in a wide variety of synthetic clinopyroxene compositions in order to evaluate the crystal compositional dependence of U-Th fractionation. Their observations confirm the predictions of Wood et al. (1999), namely that as the M2 site becomes smaller, so D-m becomes smaller than Du (Figs. 1 and 11). The M2 site becomes smaller as the enstatite component of the clinopyroxene increases and Ca on M2 is replaced by Mg. Enstatite solubility in clinopyroxene increases with increasing temperature, consequently clinopyroxene coexisting with orthopyroxene will show higher Du/Dih at higher temperature. For this reason, DuIDjb increases with increasing pressure along the mantle solidus, as discussed above. 

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...

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. 

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... 

The Co2+, Ti3 and V3 ions are expected to prefer either distorted or small octahedral sites. Thus, Co2+ should be slightly enriched in the orthopyroxene M2 and cummingtonite M4 sites, favour the pseudo-tetragonally distorted olivine Ml site, and be randomly distributed over the amphibole Ml, M2 and M3 sites. The V3+ and Ti3+ ions are expected to occupy the orthopyroxene Ml and alkali amphibole M2 sites, and to be enriched in distorted epidote M3 sites. As noted earlier, the occurrence and stability of Ti3+ ions in lunar and mete-oritic clinopyroxenes ( 4.4.1) may be explained by the availability of the distorted octahedal Ml site in the calcic clinopyroxene structure. 

Only in calcic clinopyroxenes, in which Ca2+ ions completely fill the M2 sites and Fe2+ and other transition metal ions occur in the Ml sites alone, is ideal solution behaviour to be expected. This is because cation ordering is not possible in one-site atomic substitution in the pyroxene Ml site. Furthermore, there is an insignificant variation of the CFSE of Fe2+ across the diopside-hedenbergite series ( 5.5.3). 

In orthopyroxenes, the very intense Fe2+/M2 site bands near 11,000 cm-1 and 5,000 cm-1 in spectra measured at atmospheric pressure (cf. fig. 5.15), as well as the Fe2+/Ml site band located at 8,333 cm-1, show pressure-induced blue-shifts (Shankland et al., 1974 Mao and Bell, 1971). At elevated temperatures, the Fe2+/M2 site 1 micron (11,000 cm-1) band shows negligible thermal shifts (Sung et al., 1977 Singer and Roush, 1985). However, the 2 micron (5,000 cm-1) band shows a significant red-shift in orthopyroxenes and a blue-shift in clinopyroxenes (Singer and Roush, 1985). These effects, which have important applications in remote-sensed spectral measurements of hot planetary surfaces, are described in chapter 10 ( 10.7). 

Figure 8 Plot of bulk modulus versus cation charge (Zc) divided by metal-oxygen distance cubed (A ) for cation polyhedra in oxides (Hazen and Finger, 1979) and for 1-h, 2-h, and 3-h ions in the M2-site of clinopyroxene (Wood and Blundy, 1997) and the cation site in plagioclase (Blundy and Wood, 1994). Note that the linear correlation derived from compressibility data by Hazen and Finger holds reasonably well in our results independently estimated from the -dependence of cation partition coefficient.

Given Equation (16) we derived, by nonlinear least-squares fitting, values of r from a total of 82 experiments in which three or more clinopyroxene-liquid REE partition coefficients were measured and in which the compositions of crystal and liquid phases were well constrained. Adopted radii values for individual REE were those of Shannon (1976) for 3+ ions in VIII coordination. Resulting values of range from 0.979 A to 1.055 A. We performed stepwise linear regression of the derived values of r against all major compositional parameters, pressure and temperature. The result was that the only important parameters appeared to be the octahedral (Ml) A1 content and the calcium content of the M2-site. Fitting Tq to these parameters yields an equation which reproduces the 82 points with a standard deviation of 0.009 A (i.e., less than the difference in ionic radius between adjacent REE) ... 

Figure 9 Calculated (from Equation (17)) M2 site radius for experimentally produced clinopyroxenes in peridotite close to the solidus. Note that, despite scatter due to differences in bulk composition, the trend to lower radius with increasing pressure is clear and reproduced in all of the polybaric data sets. This leads to crossover from less than 1.0 at low pressure to...

In general, the partition coefficient of Ca, Dca, between clinopyroxene and melt is in the range 1-4. Given a reasonable estimate of Dca, partition coefficients of other 2+ ions, particularly strontium, barium, and radium, into the M2-site should be calculated from ... 

Orthopyroxene has a large sixfold site, M2 and the smaller sixfold Ml site, which is similar to the clinopyroxene Ml site. We will confine our discussion to the octahedral M2, which is smaller... 

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