Physics
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.305w&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 305
Physics
Scientific paper
Little is known about the temperature of accretion of carbonaceous chondrite parent bodies and their subsequent thermal history. The frequent indicators of thermodynamic disequilibrium present in Allende (e.g., chemical zoning of olivine and pyroxene) clearly demonstrate that parent body metamorphism was limited to low temperatures and/or short periods of time. In order to quantify maximum temperatures and timescales we have studied chemical zoning of olivine and Fe/Mg exchange between coexisting olivine and chromite (olivine/spinel thermometry). We have analyzed 43 olivine-chromite pairs in type II chondrules and Fe-rich single olivine grains from Allende with the electron microprobe and calculated equilibration temperatures between 1500 K and 800 K (uncertainties are in the order of 200 K depending on various calibrations of the exchange reaction). In a few grains high temperatures, between 1300 and 1500 K (depending on the calibration), are preserved, which may be interpreted as chondrule crystallization temperatures. Most grains, however, yield much lower temperatures between 800 and 1000 K, presumably representing closure temperatures for Fe/Mg interdiffusion. The lowest temperatures of about 800 K may be regarded as an upper limit for metamorphic peak temperatures. In the few cases where higher temperatures are preserved, chromite grains are extraordinarily large, preventing low-temperature equilibration. Chemical zoning of Allende olivine indicates that temperatures in the parent body are below 800 K. In some olivine grains steep Fe/Mg concentration profiles across the boundary between forsteritic cores and fayalite-rich rims were observed, with FeO contents increasing by about 30 wt% within 5 micrometers (Weinbruch et al., 1990). The maximum time such steep concentration profiles could be retained at a given temperature was calculated using Fe/Mg interdiffusion coefficients of Buening and Buseck (1973) and Misener (1974). In extrapolating the Misener (1974) data to low temperatures we assume that his experiments were buffered by QFM, resulting in faster diffusion at low temperatures compared to equations reported by Jones and Rubie (1991) and McCoy et al. (1991). At 800 K the observed Fe/Mg concentration profiles cannot be retained for more than 2 x 10^3 years (extrapolated from Buening and Buseck, 1973) to 2 x 10^5 years (extrapolated from Misener, 1974). At temperatures around 700 K and 600 K, respectively the observed concentration profiles could survive several million years. These temperatures may, thus, be regarded as an upper limit for peak metamorphic temperature in the Allende parent body. Extensive Fe/Mg interdiffusion between forsterite and fayalite leading to broad diffusion profiles frequently observed in Allende cannot be the result of parent body metamorphism but must have occurred at high temperatures in the solar nebula. References Buening D.K. and Buseck P.R. (1973) J. Geophys. Res. 78, 6852-6862. Jones R.H. and Rubie D.C. (1991) Earth Planet. Sci. Lett. 106, 73-86. McCoy T.J., Scott E.R.D., Jones R.H., Keil K. and Taylor G.J. (1991) Geochim. Cosmochim. Acta 55, 601-619. Misener D.J. (1974) In Geochemical Transport and Kinetics (ed. A.W. Hofmann), pp. 117-129. Carnegie Inst. Washington 634. Weinbruch S., Palme H., Muller W.F. and El Goresy A. (1990) Meteoritics 25, 115-125.
Armstrong Thomas J.
Palme Herbert
Weinbruch Stephan
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