Orthopyroxenes 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. 

Fet us now consider an initial composition C2. We are within the two spinodal points on the Gibbs free energy coherent curve G decomposition takes place spontaneously throughout the entire phase, with modulated compositional fluctuations whose amphtudes increase as the process advances. We always obtain pigeonite plus diopside, although theoretically the orthopyroxene plus diopside paragenesis, also in this case, corresponds to the minimum energy of the system. 

Figure 5.68 Fe +-Mg + intracrystalline distribution between Ml and M2 sites in natural orthopyroxenes of composition near a binary mixture (Fe,Mg)Si03. From Saxena and Ghose (1971). Reprinted with permission of The Mineralogical Society of America.

As figure 5.70 shows, with increasing T the miscibility gap shrinks toward the iron-free join moreover, on the Ca-poor side of the quadrilateral, miscibility relations are complicated by the formation of pigeonite over a restricted P-T-X range (cf section 5.4.6). Once the compositions of coexisting clinopyroxene and orthopyroxene are known, figure 5.70 is a practical geothermometer. 

Figure 5.71A shows univariant equilibrium curves for various molar amounts of ferrous component in the orthopyroxene mixture. The P-T field is split into two domains, corresponding to the structural state of the coexisting quartz (a and j3 polymorphs, respectively). If the temperature is known, the composition of phases furnishes a precise estimate of the P of equilibrium for this paragenesis. Equation 5.277 is calibrated only for the most ferriferous terms, and the geobarometer is applicable only to Fe-rich rocks such as charnockites and fayalite-bearing granitoids. 

Ross M. and Huebner J. S. (1975). A pyroxene geothermometer based on composition temperature relationship of naturally occurring orthopyroxene, pigeonite and augite. In International Conference on Geothermometry and Geobarometry, The Pennsylvania State University. 

The bedrock source of the 10s to 100s of Cr-diopside grains in tiii across the region has not yet been found, in spite of the predominance of pyroxenites and peridotites in the TNB, remnants of primary Cr-diopside are scarce because most rocks have been metamorphicaiiy and metasomaticaiiy aitered such that serpentine has repiaced oiivine and orthopyroxene and amphiboie, chlorite, talc and carbonate has replaced clinopyroxene. Be-cause the Cr-diopside source is unknown, their compositional range and relationship to Ni-mineralization cannot be determined. 

Figure 2-4 Kjj values for the Fe-Mg order-disorder reaction in orthopyroxene as a function of temperature for selected compositions. The line is a fit to the data for Xps = 0.011 and 0.162 (Equation 2-57). Data sources are as follows = 0.011and0.162 (Wang et al.,...

Ganguly J., Yang H., and Ghose S. (1994) Thermal history of mesosiderites quantitative constraints from compositional zoning and Fe-Mg ordering in orthopyroxenes. Geochim. Cosmochim. Acta 58, 2711-2723. 

ALHA 84001 is a unique Martian meteorite with an ancient age of 4.5 billion years. It consists mostly of orthopyroxene crystals that accumulated in basaltic magma. Its most distinctive feature, however, is the occurrence of small globules of carbonates with unusual compositions and textures. The controversial (now largely discredited) hypothesis that the carbonates contain evidence of extraterrestrial life (McKay et al., 1996) made this the most famous meteorite on Earth. ALHA 84001 has also experienced intense shock metamorphism. 

Petrological and geochemical data are available for a few dredged samples from some seamounts. Rock compositions range from tholeiitic basalt to alkali basalt, hawaiite and basanite (Calanchi et al. 1989 and references therein Fig. 8.16). Tholeiites have been found at Tetide volcano. These rocks are sparsely porphyritic, with phenociysts of orthopyroxene rimmed... 

The spectra illustrated in fig. 5.15 show that absorption maxima of all spin-allowed CF bands move to longer wavelengths with increasing iron content of the orthopyroxene, forming the basis of composition determinative curves (Hazen et al., 1977b Adams, 1974) and enabling this pyroxene structure-type to be identified in telescopic reflectance spectra of surfaces of the Moon (Pieters etal., 1985 Bums, 1989a). 

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