Omphacite

At temperatures germane to melting and erystallizrng mantle magmas, Li isotopes do not show permil-level mass fractionation (Fig. 4 Tomascak et al. 1999b). This has since been corroborated by examination of bulk rocks and olivine separates from basaltic lavas, which yield consonant isotopic values (Chan and Frey 2003). Also, whole rocks and omphacite mineral separates from alpine eclogite with metamorphic peak temperatures approximately 650°C (Zack et al. 2003) show no consistent Li isotopic difference. 

Figure 8. Plot of Li isotopic composition vs. Na O content of eclogites from Trescolmen, Swiss Alps (Zack et al. 2003). Data from pairs of bulk rock ( ) and omphacite mineral separate (O) samples, indicating that bulk rocks generally preserve isotopic systematics of these rocks. Well-preserved, high pressure metamorphosed basaltic rocks were interpreted to retain much of their elemental character, and as such were good recorders of the residue remaining after subduction dehydration. The data indicated that originally isotopically heavy altered sea floor basalt could be transformed during subduction into some of the isotopically lightest materials in the Earth system.

Ganguly (1973). Activity-composition relation of jadeite in omphacite pyroxenes Theoretical deductions. Earth Planet. Set Letters, 19 145-153. 

Mineral Quartz Phengite Omphacite Garnet Rutile... 

Figure 12-7. X-ray intensities as 4 function of time t for omphacite according to 1M. A. Carpenter, et al. (1989)].

The Fe2+ —> Ti4 IVCT transition has been identified in the spectra of other titanian pyroxenes, including samples from the Moon (Bums et al., 1976 Dowty, 1978) and, perhaps, a blue titaniferous omphacite from Labrador (Curtis et al., 1975). An intense broad band in this omphacite centred around 15,000 cm-1 and showing a pressure-induced shift to lower energies was originally assigned to Fe2+ -> Fe3+ IVCT (Abu-Eid, 1976) but later it was reassigned as a Fe2+ - Ti4+ IVCT transition (Strens et al., 1982). 

APBs are essentially stacking faults and have been observed by TEM in a number of minerals that undergo order-disorder transformations. Examples include omphacite (Carpenter 1979 Champness 1973 Phakey and Ghose 1973), pigeonite (Bailey et al. 1970 Carpenter 1979 Christie et al. 1971 Fujino, Furo, and Momoi 1988), calcic plagioclase feldspars (Christie et al. 1971 Czank et al. 1973 Czank, Schulz, and Laves 1972 Heuer et al. 1972 McLaren 1973 McLaren and Marshall 1974 Muller et al. 1973 Muller, Wenk, and Thomas 1972 Nord, Heuer, and Lally 1974 ... 

Champness, P. E. (1973). Speculation on an order-disorder transformation in omphacite. Amer. Mineral., 58, 540-2. 

Direct observation of anti-phase domain structure in omphacite. Con-trib. Mineral. Petrol., 39, 239-45. 

E-T3fpe inclusions pyrope-almandine, high Na garnet (>0.1 wt.%), omphacite, coesite, low-Ni sulfide. 

Beran A., Danger K., and Andrut M. (1993) Single crystal infrared spectra in the range of OH fundamentals of paragenetic garnet, omphacite and kyanite in an eklogitic mantle xenohth. Mineral. Petrol. 48, 257-268. 

Holland T. J. B. (1990) Activities of components in omphacite solid solutions. Contrib. Mineral. Petrol. 105, 446—553. 

Pan-African Belt, Mali, W. Africa Coesite in omphacite from marble E 690-750 °C E >27 kb 620 Ma Caby (1994) Jahn et al. (2001a)... 

Tie-lines between coexisting garnet and omphacite plotted as 8Nd(0 versus S Osmow are shown in Figure 7 (redrawn from Zheng et al., 2002). In such a plot, values of SNd(0 calculated for the age of UHP metamorphism are expected to be identical for minerals experiencing isotopic equilibration under UHP conditions. The differences between values of for minerals equilibrated... 

Figure 7 Tie-lines between coexisting garnet (Gt) and omphacite (Omp) in eclogites from the Su-Lu terrane, China, plotted on a 8Nd(0 versus diagram. Samples identified by names of localities where collected. See text for discussion (after Zheng et al., 2002).

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