Clinopyroxenes composition

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...

Figure 17 Summary fields of chondrite-normalized REE patterns for whole-rock peridotites and cUnopyroxenes for peridotite xenoliths. Noncratonic whole-rock peridotites are either LREE-depleted (type lA least common) or LREE-enriched (type IB most common). Data sources from Stosch and Seek (1980), Stosch and Lugmair (1986), Menzies et al (1985). Clinopyroxenes from these rocks also show LREE enrichment or depletion. Cratonic peridotite whole rocks are ubiquitously LREE-enriched. Low-T (granular) suite show greater LREE/HREE compared to high-T (sheared) suite and this is reflected in the more LREE-enriched clinopyroxene compositions in the low-T suite. Data sources from Shimizu (1975), Nixon et al. (1981), and Irvine (2002). Low-T whole-rock suite includes 19 samples...

Figure 17 Comparison of model output for distribution of residue compositions with abyssal peridotites and harzburgites from Oman, (a) Distribution of clinopyroxene compositions in equilibrium with melts at the top of the melting column corresponding to Figure 15. This distribution is sampled uniformly by area (as if we were sampling residues from the top of the column) and is dominated by depleted inter-channel samples, (b) Clinopyroxene compositions in harzburgites from the mantle section of the Oman ophiohte (Kelemen et al., 1995a) superimposed on the range of cpx compositions from abyssal peridotites (Johnson et al., 1990 Johnson and Dick, 1992), showing the predominance of highly depleted samples.

These diagrams should not be used with a single clinopyroxene analysis rather, Leterrier et al. (1982) recommend that a minimum of 10 but if possible more than 20 analyses should be plotted before a meaningful result can be obtained. The method is thought to work in metabasites up to greenschist facies thereafter clinopyroxene compositions are modified by metamorphic reactions. 

Leterrier J., Maury R.C., Thonon P., Girard D, and Marchal M., 1982, Clinopyroxene composition as a method of identification of the magmatic affinities of palaeo-volcanic series. Earth Planet. Sci. Lett., 59, 139—154. 

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 ... 

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 ... 

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 ... 

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. 

Lundstrom CC, Shaw H, Ryerson F, Phinney D, Gill J, Williams Q (1994) Compositional controls on on the partitioning of U, Th, Ba, Pb, Sr and Zr between clinopyroxene and haplobasaltic melts implications for uranium series disequilibria in basalts. Earth Planet Sci Lett 128 407-423 Lnndstrom CC (2003) Uranium-series disequilibria in mid-ocean ridge basalts observations and models of basalt genesis. Rev Mineral Geochem 52 175-214... 

Hirose and Kushiro (1993) have determined the composition of basaltic melts segregated from peridotite at 10-30 kbars. Some of the data are listed as molar fractions in Table 5.4. Making the rather crude assumption that the clinopyroxene Kcpx solubility product can be formed as... 

From the concentration of each element in the liquid and in the source, we can retrieve the degree of melting and the residual mineralogy. We assume that the liquid contains 676 ppm Ni, 426 ppm Cr, 0.763 ppm Yb and 0.1 ppm Rb, whereas its source composition y vector is (2500, 1500, 0.2, 0.01) in ppm. We will test the assumption that the residuum is composed of olivine and clinopyroxene with the partition coefficients given above. Phase abundances Jt will be ordered as liquid, olivine and clinopyroxene. Let us compute, as an example, the element in the third row and second column of the matrix A... 

Figure 14. Inter-mineral Fe isotope fractionations among olivine and clinopyroxene from spinel peridotite mantle xenoliths. Data are from Zhu et al. (2002) ( ) and Beard and Johnson (2004) ( ). In the study by Beard and Johnson (2004), the difference in the Fe isotope composition between clinopyroxene and olivine is larger as a function of their 5 Fe values, suggesting disequilibrium fractionation.

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