The Fe2+ ,Mg distribution in orthopyroxene: A critical assessment of its potential as a geospeedometer
Kroll, Herbert; Lueder, Tore; Schlenz, Hartmut; Kirfel, Armin; Vad, Thomas
European Journal of Mineralogy Volume 9 Number 4 (1997), p. 705 - 733
published: Jul 23, 1997
manuscript received: Feb 13, 1996
ArtNo. ESP147050904006, Price: 29.00 €
Extremely precise and accurate Fe2+,Mg distributions are required in order to derive meaningful cooling rates from the orthopyroxene geospeedometer. A discussion of X-ray data collection and refinement strategies, using orthopyroxene from the Acapulcoite meteorite ALHA81261 (X̅FS = 0.11) as an example, discloses that the standard practice of one single X-ray refinement can be inadequate. Therefore, a novel bivariate analysis is suggested that is based on both stepwise truncation of low order data and rejection of outliers (LOT analysis). Thereby, extinction and chemical bonding effects are reduced. Correlations between site occupancies and thermal vibration parameters are circumvented when a B(M2)/B(M1) ratio is imposed that provides invariance of the refined site occupancies with respect to the choice of data subsets.
For calculating cooling rates, the Mueller (1967, 1969) rate equation is used, which requires that the Fe ,Mg equilibrium distribution coefficient, KD, and the rate constant, kdis, for the (microscopic) disordering reaction are known as functions of both temperature, T, and composition, X̅FS. The coefficients of the lnKD function were calculated from our own and published KD values. Their analysis revealed internal inconsistencies related to the origin of the crystals (natural or synthetic) and to the data collection and site refinement strategies. From the total set of lnKD values we have chosen three subsets, each of which appeared to be rather consistent internally. The regression analyses of these subsets resulted in quite different parameters.
Rate constants kdis were (re)calculated from our own and published Fe2+,Mg jute occupancies determined in isothermal kinetic experiments. Coefficients for the dependence of kdis on T and X̅FS, assuming (1) a continuous, and (2)a stepwise behaviour of lnkdis(X̅Fs)T were derived. The results agree for 0 < X̅FS < 0.55, but disagree for 0.55