Pyroxene oxygen isotope fractionation

Matthews A., Goldsmith J.R. and Clayton R.N., 1983, Oxygen isotope fractionations involving pyroxenes the calibration of mineral-pair thermometers. Geochim. Cosmochim. Acta, 47, 631-644. 

The resulting temperature estimates in column (1) range from 613°C to 1126°C. Omitting the lowest and the highest values yields a mean of 872 112°C (let) The oxygen-isotope fractionation factors reported by Chiba et al. (1989) for feldspar-pyroxene are Albite-Diopside = 1.81, Anorthite-Diopside = 0.76. For labradorite (An = 60%, Ab = 40%) the numerical value of A is ... 

During metamorphism and recrystallization, oxygen isotopes are redistributed among mineral phases, according to the mass-dependent equilibrium fractionations corresponding to the peak metamorphic temperature. The measured mineral-pair fractionations (usually for major minerals olivine, pyroxene, and feldspar) can then be used for metamorphic thermometry, yielding temperatures of 600 °C for an L4 chondrite, and 850 50 °C for several type-5 and type-6 chondrites (Clayton et al., 1991). Isotopic equilibration, even in type-6 chondrites, involves oxygen atom transport only over distances of a few millimeters (Olsen et al., 1981). 

CIl chondrites also contain a small fraction of isolated olivine and pyroxene grains up to 400 p,m in size with chemical, oxygen isotopic compositions, and rounded inclusions of metallic Fe,Ni and trapped melt indicating that they were derived from chondrules (Leshin et al, 1997). Brearley and Jones (1998) estimate the abundance of olivine and pyroxene at <1 vol.%, but X-ray diffraction studies by Bland et al. (2002) indicate 7 wt.% olivine. The dilference probably reflects a high abundance of olivine crystallites embedded in phyllosilicates (P. A. Bland, private communication). CIl chondrites also contain rare refractory grains with oxygen isotopic compositions comparable to those in CAIs (see Scott and Krot, 2001). 

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