Distribution clinopyroxene

Figure 4.11 Monte-Carlo simulation (100 trials) of error propagation for La/Yb fractionation in residual melts by clinopyroxene-garnet removal from a basaltic parent magma (see text for parameter description and distributions used). Top mineral-liquid partition coefficients for La and Yb. Bottom variations of the La/Yb ratio as a function of the fraction F of residual melt.

This model of liquid extraction is symmetrical to fractional crystallization and has attracted renewed interest after the demonstration by Johnson et al. (1990) that REE distributions in abyssal peridotite clinopyroxene cannot be accounted for by equilibrium melting processes. The solid is supposed to maintain its chemical homogeneity while liquid is continuously extracted. Only the last drop of liquid is supposed to be in equilibrium with the residue. 

Figure 5,65 Garnet-clinopyroxene geothermometric exchange (Fe +-Mg +). Thermodynamic constant K is compared with distribution coefficient K the difference between the two terms represents the effect of interactions in mixtures. Reprinted from J. Ganguly, Geochimica et Cosmochimica Acta, 43, 1021-1029, copyright 1979, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

We have already noted that the double conversion from activity ratio to weight concentration ratio implicit in trace element geochemistry may involve complexities that must be carefully evaluated in the interpretation of natural evidence. Let us consider, for instance, the distribution of Ni between clinopyroxene and silicate liquid. Eigure lO.lOA shows the effect of temperature on the conven-... 

Figure 10.10 Arrhenius plot of conventional partition coefficient (A) and thermodynamic constant (B) for clinopyroxene/liquid distribution of Ni.

Dal Negro A., Carbonin S., Molin G. M., Cundari A., and Piccirillo E. M. (1982). Intracrystalline cation distribution in natural clinopyroxenes of tholeiitic, transitional and alkaline basaltic rocks. In Advances in Physical Geochemistry, vol. 1, S. K. Saxena (series ed.), New York Springer-Verlag. 

Ganguly J. (1979). Garnet and clinopyroxene solid solutions and geothermometry based on Fe-Mg distribution coefficient. Geochim. Cosmochim. Acta, 43 1021-1029. 

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. 

Duke, J. M. (1976) Distribution of the period four transition elements among olivine, calcic clinopyroxene and mafic silicate liquid experimental results. J. Petrol.,... 

Witt-Eickschen G. and Harte B. (1994) Distribution of trace elements between amphibole and clinopyroxene from mantle peridotites of the Eifel (western Germany) an ion microprobe study. Chem. Geol. 117, 235—250. 

McKay G. A., Wagstaff J., and Yang S. R. (1986) Clinopyroxene REE distribution coefficients for shergottites the REE content of the Shergotty melt. Geochim. Cosmochim. Acta 50, 927-937. 

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.

Williams CT, Potts PJ (1988) Element distribution maps in fossil bones. Archaeometry 30 237-247 Wittmers LE, Aufderheide AC, Wallgren J, Rapp G, Alich A (1988) Lead in Bone IV Distribution of lead in the human skeleton. Arch Environ Health 43 381-391 Wood BJ, Blundy ID (1997) A predictive model for rare earth element partitioning between clinopyroxene and anhydrous silicate melt. Contrib Mineral Petrol 129 166-181 Wright J, Seymour R S, Shaw HF (1984) REE and Nd isotopes in conodont apatite variations with geologic age and depositional enviromnent. In Conodont Biofacies and Provincialism. Clark DL (ed) Geol Soc Am Spec Paper, p 325-340... 

Fig, 20. Lanthanide mineral/mell distribution coefficient values for clinopyroxenes from volcanic rocks, showing the extreme dependence on the composition of the bulk rock. For example, the values for La extend over three orders of magnitude. (Data by courtesy of Dr. A. Ewart, University of Queensland, Australia.)... 

This process describes the effects of irrrpact on the chromium phase distributed in clinopyroxene. In this process, clinopyroxene is deformed and partially decomposed. Increasing the interrsity of exposure leads to transformatiorrs in clinopyroxene (which forms a sohd solution with carbon). As a resrrlt, pure carbon is allocated to the periphery of the crystallites. 

Despite the apparently over-sin lified form of the Berthelot-Nernst distribution coefficient, it has been extensively employed in trace element geochemistry often with considerable success. In this respect it is instructive to compare D. with the true equilibrium constant for a given reaction. As an example the partition of Ni between olivine and clinopyroxene (Hakli and Wright, 1967 Broecker and Oversby, 1971 Banno and Matsui, 1973 Carmichael Consider the following exchange reaction ... 

Ideally a trace element geothermometer should be based upon a reaction with a large AH and small AV and vice versa for a geobarometer. A number of experimental studies have now demons trated the temperature dependence of Berthelot-Nernst distribution coefficients. Noteworthy amongst these are the experiments of Shimizu (l97 ) on the distribution of Sr, Ba, K, Rb, and Cs between clinopyroxenes and liquid and those of Drake and Weill (1973) on the partitioning of Sr, Ba and Eu between plagioclase and liquid. 

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