Temperature dependence of Fe,Mg partitioning in Acapulco olivine

The temperature dependence of the intracrystalline Fe,Mg partitioning (K-D) in two olivine crystals (Fa(11)) separated from the Acapulco meteorite was determined by single-crystal X-ray structure analysis. The independent ato m model (IAM) was compared with a "bond model" which accounts for bond-indu ced charge accumulations on the Si-O bonds. Outliers in the set of structur e amplitudes observed when using the IAM disappeared upon introducing the b ond model. The crystals were equilibrated at 750, 650, and 500 degrees C. T he refined site occupancies yield the relation ln(K-D) = 0.345(60) -204(53) /T, where T is in K, which is in qualitative agreement with earlier work. C omparison of these data with the higher temperature data of Artioli et al. (1995) suggests an unusual temperature variation of the Fe,Mg distribution within two temperature regimes. Below 880 degrees C, Fe tends to order onto the M1 site with increasing temperature whereas it concentrates on the M2 site above 880 degrees C. In principle, olivine may serve as a geospeedomet er similar to orthopyroxene. However, at present its usefulness is restrict ed because (1) the relatively weak dependence of ln(K-D) on temperature nee ds to be even more tightly constrained than presented here and (2) low-temp erature extrapolation of the rate constants for the Fe,Mg site exchange, de rived from interdiffusion coefficients, is uncertain.