Clinopyroxene

Figure 1.32. The relationship between MgO concentration and other major constituents. Solid circle represents the sample of relatively fresh rock which contains original clinopyroxene. A H20(+) vs. MgO. B CaO vs. MgO. C Na20 vs. MgO. D K2O vs. MgO. E SiOz vs. MgO. F FeO vs. MgO. G Fe203 vs. MgO. H EFe vs. MgO. I AI2O3 vs. MgO. (Shikazono et al., 1995).

Figure 1.74. Zonal sequence of the propylitic alteration in E-W section of the Seigoshi-Toi mine area (Yug = yugawaralite Heu = heulandite Stil = stilbite Opx = orthopyroxene Mont = montmorillonite Mor = mordenite Lm = laumontite Wr = wairakite Chi = chlorite pr = prehnite ep = epidote Py = pyrite Kf = K-feldspar Cpx = clinopyroxene) (Shikazono, 1985a).

Mineral abbreviations Amph = amphibole Cpx = clinopyroxene Gt = garnet... 

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

Blundy and Wood (1994) found that values for the optimum site radius, were not necessarily the same as the radius of the cation normally resident at the site of interest. Thus in clinopyroxene is slightly smaller that the radius of Ca, the ion... 

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 ... 

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...

In order to derive activity-composition relationships for ThMgAl206 in clinopyroxene, Landwehr et al. (2001) used a mixing-on-sites model assuming complete short-range order between M and T sites. The activity of ThMgAl206 in clinopyroxene is thus given by ... 

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 ... 

Salters et al. (2002) have recently proposed an alternative expression for calculating D j and Dm in clinopyroxene as a function of crystal and melt composition. The expressions are calibrated on over 40 experimental determinations of Ehj and Dm- Salters et al. (2002) do not give values for the average absolute deviation. The full expressions (with 1 s.d. uncertainties in brackets) are ... 

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. 

We conclude that it is very unlikely that any Pa enters the clinopyroxene Ml site. 

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